Platinum Metals Review - Johnson Matthey Technology Review

VOLUME 52 NUMBER 3 JULY 2008

Platinum Metals Review

www.platinummetalsreview.com E-ISSN 1471–0676

E-ISSN 1471–0676

PLATINUM METALS REVIEW A Quarterly Survey of Research on the Platinum Metals and of Developments in their Application in Industry www.platinummetalsreview.com

VOL. 52 JULY 2008 NO. 3

Contents Platinum Metals Review and Sustainability

132

An editorial by N. A. P. Carson and Barry W. Copping

A New Palladium-Based Catalyst for Methanol Steam Reforming in a Miniature Fuel Cell Power Source

134

By Oleg Ilinich, Ye Liu, Christopher Castellano, Gerald Koermer, Ahmad Moini and Robert Farrauto

Noble Metal Catalysts for Mercury Oxidation in Utility Flue Gas

144

By Albert A. Presto and Evan J. Granite

“Highly Efficient OLEDs with Phosphorescent Materials”

155

A book review by R. J. Potter

Practical New Strategies for Immobilising Ruthenium Alkylidene Complexes: Part II

157

By Ileana Dragutan and Valerian Dragutan

Novel Lipophilic Platinum(II) Compounds of Salicylate Derivatives

163

By Wei-Ping Liu, Qing-Song Ye, Yao Yu, Xi-Zhu Chen, Shu-Qian Hou, Li-Guang Lou, Yong-Ping Yang, Yi-Ming Wang and Qiang Su

CRC International Symposium: Cross Coupling and Organometallics

172

A conference review by Thomas Colacot

Electrochemical Water Disinfection: A Short Review

177

By Alexander Kraft

Processing of Iridium and Iridium Alloys

186

By E. K. Ohriner

“Platinum 2008”

198

Abstracts

200

New Patents

203

Final Analysis: Accurate and Precise Determination of Platinum in Solution by ICPES

205

By Peter Ash

Communications should be addressed to: The Editor, Barry W. Copping, Platinum Metals Review, [email protected]; Johnson Matthey Public Limited Company, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K.

DOI: 10.1595/147106708X325169

Platinum Metals Review and Sustainability A Johnson Matthey Initiative Johnson Matthey has announced a major initiative on sustainability, with commitment to the principles of sustainable development and a quest for outstanding resource efficiency and carbon neutrality. The company will also encourage its suppliers and customers to embrace similar values and will work with them to exploit the opportunities presented by the development of more sustainable products. The initiative is being implemented against a background of increasing global concern about the environment and the need to make the most efficient use of natural resources, including hydrocarbons. In addition, health issues remain high on the sustainability agenda and we continue to see the progressive tightening of worldwide emissions control legislation. Platinum Metals Review (PMR) is well placed to contribute to this initiative. Johnson Matthey began publication of the Journal in 1957, to disseminate, free of charge, knowledge of the science and technology of the platinum group metals (pgms) to a worldwide readership, to

Coverage of Sustainability in P l a t i n u m Metals Review One of PMR’s earliest published papers, in 1957, was a conference review dealing with the importance of pgms for catalysing industrial reactions (1). Since then, many ‘classic’ articles have become landmark papers in their fields, including autocatalysts for reducing harmful vehicle emissions (2, 3), fuel cells as an alternative energy technology (4) and the use of catalysis for improving the efficiency of industrial processes (5). Further themes have included hydrogen as an alternative fuel (6), solar energy (7), the recovery of pgms from spent catalysts (8) and the use of

Platinum Metals Rev., 2008, 52, (3), 132–133

support the platinum industry and to encourage research and development. PMR has followed the growth of pgm technologies as they have moved from laboratory to industrial scale, and has always covered advances in sustainable technology. The carbon footprint of PMR’s publishing operation was significantly reduced in July 2004, when the electronic-only format superseded printed copy distribution. I am delighted that the PMR editorial team will be actively taking Johnson Matthey’s sustainability initiative forward in commissioning articles, as well as conference and book reviews. It is clear that PMR’s contributors and readers have a fundamental interest in achieving a more sustainable world, and this will have increasingly important implications for the future N. A. P. CARSON development of pgm science. Neil Carson became the Chief Executive of Johnson Matthey PLC in July 2004. He joined Johnson Matthey in 1980, becoming Managing Director, Catalysts & Chemicals in 1999. In 2002 he assumed board level responsibility for the Precious Metals Division. He is Chairman of the U.K. Government Business Taskforce on Sustainable Consumption and Production.

pgms in environmental remediation (9). Recent coverage takes many of these themes forward. Further improvements to industrial process catalysis (10, 11), chemical reactions (12) and ‘green’ chemistry (13) have all featured in the last two years. There is continuing interest in alternative fuels (14), and autocatalysts contribute crucially in the drive to reduce vehicle emissions (15). In the current issue of PMR, we present articles on fuel cells (16), chemical catalysis (17) and emissions abatement for industrial processes (18). This proves that research and development on the use of pgms for sustainable technologies are alive and well in the 21st century.

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With the Journal hosted on the dedicated website www.platinummetalsreview.com, features have been added in the PGM Science Mine to actively engage the pgm science and technology community in addressing key issues such as sustainability. Within the PGM Science Mine, a range of organisations involved in sustainability issues are now listed in the PMR Organisation Directory, including: – Environment, Sustainability & Energy, Royal Society of Chemistry, U.K. – Good Practice, Sustainable Development in the Mining and Metals Sector

– Roundtable on Sustainable Platinum Group Metals – SusChem, Belgium – Sustainable Development Commission (SDC), U.K. – U.K. Government Sustainable Development We hope that our readers and authors will continue to make use of the PMR journal and its website as valuable resources to further their work on sustainable pgm science and technology into the future. BARRY W. COPPING, Editor

References 1 ‘The Platinum Metals in Catalysis’, Platinum Metals Rev., 1957, 1, (1), 24 2 ‘Automobile Emission Control Systems’, G. J. K. Acres and B. J. Cooper, Platinum Metals Rev., 1972, 16, (3), 74 3 ‘Twenty-Five Years of Autocatalysts’, M. V. Twigg, Platinum Metals Rev., 1999, 43, (4), 168 4 ‘Fuel Cell Energy Generators’, D. S. Cameron, Platinum Metals Rev., 1978, 22, (2), 38 5 ‘The CativaTM Process for the Manufacture of Acetic Acid’, J. H. Jones, Platinum Metals Rev., 2000, 44, (3), 94 6 ‘Progress in Hydrogen Energy Systems’, P. A. Sermon, Platinum Metals Rev., 1978, 22, (4), 130 7 ‘Highly Efficient Nanocrystalline Photovoltaic Devices’, M. Grätzel, Platinum Metals Rev., 1994, 38, (4), 151 8 ‘Precious Metal Recovery from Spent Catalysts’, P. Grumett, Platinum Metals Rev., 2003, 47, (4), 163 9 ‘The Dechlorination of Hydrocarbons’, N. Korte, L. Liang, R. Muftikian, C. Grittini and Q. Fernando, Platinum Metals Rev., 1997, 41, (1), 2 10 ‘Enhancement of Industrial Hydroformylation Processes by the Adoption of Rhodium-Based Catalyst: Part I’, R. Tudor and M. Ashley, Platinum Metals Rev., 2007, 51, (3), 116

Platinum Metals Rev., 2008, 52, (3)

11 ‘Enhancement of Industrial Hydroformylation Processes by the Adoption of Rhodium-Based Catalyst: Part II’, R. Tudor and M. Ashley, Platinum Metals Rev., 2007, 51, (4), 164 12 ‘Practical New Strategies for Immobilising Ruthenium Alkylidene Complexes: Part I’, I. Dragutan and V. Dragutan, Platinum Metals Rev., 2008, 52, (2), 71 13 ‘Green Chemistry and Catalysis’, D. Macquarrie, Platinum Metals Rev., 2008, 52, (2), 83 14 ‘Alcoholic Fuels’, G. Acres, Platinum Metals Rev., 2007, 51, (1), 34 15 ‘Diesel Engine Emissions and Their Control’, T. Johnson, Platinum Metals Rev., 2008, 52, (1), 23 16 ‘A New Palladium-Based Catalyst for Methanol Steam Reforming in a Miniature Fuel Cell Power Source’, O. Ilinich, Y. Liu, C. Castellano, G. Koermer, A. Moini and R. Farrauto, Platinum Metals Rev., 2008, 52, (3), 134 17 ‘Practical New Strategies for Immobilising Ruthenium Alkylidene Complexes: Part II’, I. Dragutan and V. Dragutan, Platinum Metals Rev., 2008, 52, (3), 157 18 ‘Noble Metal Catalysts for Mercury Oxidation in Utility Flue Gas’, A. A. Presto and E. J. Granite, Platinum Metals Rev., 2008, 52, (3), 144

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DOI: 10.1595/147106708X324403

A New Palladium-Based Catalyst for Methanol Steam Reforming in a Miniature Fuel Cell Power Source By Oleg Ilinich*, Ye Liu, Christopher Castellano, Gerald Koermer, Ahmad Moini and Robert Farrauto BASF Catalysts LLC, 25 Middlesex-Essex Turnpike, Iselin, NJ 08830-0770, U.S.A.; *E-mail: [email protected]

A novel palladium-based catalyst has been developed for use in a miniature fuel cell power source for portable applications, incorporating a polymer electrolyte membrane (PEM) fuel cell. Hydrogen, which is the fuel for the cell, is produced in a ceramic microreactor via the catalytic reaction of methanol steam reforming: CH3OH + H2O → 3H2 + CO2. The need for a new catalyst in this application is driven by the limitations of traditional particulate catalysts based on copper oxide, zinc oxide and alumina (Cu-Zn-Al catalysts), which have low thermal stability and high sensitivity towards air and condensing steam. These features result in a declining activity and mechanical integrity of Cu-Zn-Al catalysts under the frequent start-stop conditions typical of the operational mode of the miniature power source. The new Pdbased catalyst has activity and selectivity similar to those of Cu-Zn-Al catalysts, but is more durable and stable under the duty cycle conditions of a portable power source. In the microreformer, the catalyst is washcoated directly on the walls of the steam reforming section, providing favourable conditions for efficient heat transfer between the heat-generating catalytic combustion section of the microreformer and its heat-consuming steam reforming section.

1. Introduction Current trends in energy demand for portable electronics show that the power consumption in devices such as cell phones, personal digital assistants (PDAs), notebook computers and digital cameras continues to rise. Consumers demand small size, light weight, but long battery life. The batteries most commonly used in these devices are of the rechargeable (secondary) type. However, in certain applications such as those used by exploratory expeditions, first responders and the military, recharging a battery in the field is often difficult, if possible at all. Therefore a heavy load of disposable (primary) batteries must be transported to remote locations. One solution to this problem is to use a small portable battery charger powered by a fuel cell. Due to the low energy density of compressed hydrogen, using it as a fuel for a portable fuel cell charger is not a viable option. Liquid fuel, in particular methanol, has a much higher energy density and is easier to transport and handle, which makes


it practically attractive for this application. Methanol is used as a fuel in two different types of fuel cells. In the direct methanol fuel cell (DMFC) (Figure 1), methanol is fed directly to the anode where it reacts with water, generating electrons which travel through the external circuit as electric current, Reaction (i): CH3OH + H2O → CO2 + 6H+ + 6e–

(i)

Protons travel through the proton-conducting polymer electrolyte membrane (for example, Nafion®) to the cathode where they react with oxygen from the atmosphere, to produce water. The subject catalyst of this article was developed for use in a reformed methanol fuel cell, shown in Figure 2. In essence this is a classic type of fuel cell, invented in 1839 by William Grove. Here the fuel is hydrogen which is fed to the anode, where it splits electrocatalytically into protons and electrons, Reaction (ii): H2 → 2H+ + 2e–

(ii)

134

CH3OH

Fig. 1 Schematic of a direct methanol fuel cell

Anode reaction: 2CH3OH + 2H2O → 2CO2 + 12H+ + 12e–

H+

e–

NAFION® membrane 70ºC

Overall reaction: 2CH3OH + 3O2 → 2CO2 + 4H2O

Cathode reaction: 12H+ + 12e– + 3O2 → 6H2O Air e

–

Hydrogen production via methanol steam reforming: CH3OH + H2O → 3H2 + CO2

Fig. 2 Schematic of a reformed methanol fuel cell

Anode reaction: 2H2 → 4H+ + 4e–

H+

PBI/H3PO4 (Polybenzimidazole) 180ºC

Overall reaction: 2H2 + O2 → 2H2O

e– Cathode reaction: + – 4H + O2 + 4e → 2H2O e–

Air

In contrast to Grove’s cell, however, in our application the hydrogen is produced by the catalytic steam reforming of methanol, Reaction (iii): CH3OH + H2O → 3H2 + CO2


(iii)

This reaction has received much attention in the last decades as an attractive route to hydrogen supply. An excellent review on methanol steam reforming (MSR) for hydrogen production has recently been published by Palo et al. (1).

135

The MSR reaction can be efficiently catalysed by copper-based catalysts (2–7), including the CuZn-Al particulates otherwise used in methanol synthesis (8) and the water-gas shift reaction (9). These catalysts are commercially produced, and have been used successfully in industry for many years. By their nature they are quite sensitive to the process conditions. In particular, they are prone to sintering at temperatures above about 280 to 300ºC, which results in a significant decline in activity, and also deteriorate both mechanically and in performance if steam condenses on them. Besides, a Cu-Zn-Al catalyst can develop dangerously strong exotherms if in its oxidised state it is exposed to a reducing environment, or, in its reduced (active) state, to an oxidising environment, such as ambient air (10). Therefore in industrial settings Cu-Zn-Al catalysts are operated under carefully controlled conditions. The operating cycle includes a lengthy start-up with slow reduction in syngas (H2/CO) or hydrogen heavily diluted with nitrogen to minimise the reduction exotherm. The reduced and activated catalysts normally operate under steady-state conditions. By contrast, the duty cycle anticipated for the MSR catalyst in the miniature fuel cell power source is much more demanding. The miniature power source will be operated with frequent starts and stops, during which liquid (reformate) will condense and may even freeze on the catalyst. Besides, slow reduction in dry gas with low concentrations of a reductant will clearly be unavailable, and the catalyst will have to be activated (reduced) upon direct contact with the methanol/water feed mixture. The properties of Cu-Zn-Al catalysts are incompatible with these requirements, and therefore a new applicationspecific catalyst had to be developed. This catalyst is a further improvement over the family of palladium-zinc-based catalysts which have been developed for fuel cell applications in recent years (11–16).

2. Experimental The novel MSR catalyst has been developed using a combination of rapid catalytic screening and detailed parametric studies simulating the duty cycle


of the miniature power source. The catalyst consists of Pd and Zn on an oxide support with proprietary additives. In the present paper this catalyst is designated as ‘Pd-Zn/oxide support’. It is fabricated in a powder form, then slurried and used as a washcoat on the channels of the microreformer (Figure 3) to be incorporated into a miniature fuel cell power source. The catalytic performance of a MSR catalyst is determined by two characteristics: activity and selectivity. A good MSR catalyst should provide high rates of conversion of methanol and water into hydrogen and carbon dioxide, while side reactions should be minimised. In the MSR process, in addition to the target steam reforming Reaction (iii), several undesired side reactions can occur. In particular, carbon monoxide can be formed via methanol decomposition, Reaction (iv): CH3OH → CO + 2H2

(iv)

and/or reverse water-gas shift, Reaction (v): CO2 + H2 → CO + H2O

(v)

(a)

(b)

Pd-Zn/oxide support Fig. 3 (a) The prototype microreformer (with U.S. quarter dollar coin for scale); (b) cross section of the microreformer with Pd-Zn/oxide support washcoated in the microchannels (Reproduced with permission from Motorola Energy Technologies Lab)

136

Carbon monoxide, a known catalytic poison, can only be tolerated by the fuel cell catalysts if its concentration in the reformate is low. Even for the fuel cells based on polybenzimidazole (PBI) membranes, which operate at elevated temperatures (~ 180ºC) and are more CO-tolerant than the fuel cells with Nafion® membranes which operate at ~ 80ºC, the CO level in the reformate must not exceed 1–2%. Another possible side product is methane, which could be generated via methanation, Reactions (vi) and (vii), that consumes considerable amounts of hydrogen, while generating large amounts of heat: CO2 + 4H2 → CH4 + 2H2O

(vi)

CO + 3H2 → CH4 + H2O

(vii)

In the context of this article, the selectivity is understood as the ratio of the concentration of a given product at certain conversion of methanol to the sum of concentrations of all carbon-containing gas-phase products (in our catalytic tests no coke formation was observed), for example Equation (viii): SCO2 = 100 × [CO2]/([CO2] + [CO] + [CH4]) (viii)

where SCO2 (%) is the selectivity towards CO2, and [CO2], [CO] and [CH4] are the concentrations of the corresponding species.

3. Results

3.1 Performance Testing of the Cu-Zn-Al and ‘Pd-Zn/Oxide Support’ Catalysts in the Simulated Start-Stop Mode The anticipated duty cycle of the miniature power source will include frequent starts and stops, with variable periods of steady-state operation. The catalysts to be used in the microreformer must be thoroughly tested under conditions simulating such operation, and a special laboratory test procedure has been developed for this purpose. The test consisted of the following elements as illustrated in Figure 4: (a) initial heat-up of the reactor to the reaction temperature, followed by starting the flow of the methanol/water feed mixture and (b) steadystate operation for approximately sixty hours; (c) multiple start-stop cycles consisting of (d) cooling down to 40ºC with the feed flow stopped (simulated shutdown of the power source); (e) heating up to the reaction temperature; (f) restarting the flow of the methanol/water feed mixture with steadystate operation for approximately two hours.

300 300 (c) Start-stop cycles

200 200 (a) Initial heat-up 150 150

(b) Steady-state

Temp

Reactor temperature, ºC

250 250

100 100

50 50

(d) Stop feed flow and cool down (e) Ramp up to operating temperature

0 0 02/07/05

02/07/05

1 02/08/05

02/08/05

2 02/09/05

3 02/10/05 Time, Time days

02/09/05

(f) Restart CH3OH + H2O feed 02/10/05

4 02/11/05

02/11/05

5 02/12/05

Fig. 4 Experimental temperature profile in methanol steam reforming reactor during the catalyst performance test


137

In the catalytic performance test using the above protocol, the commercial Cu-Zn-Al catalyst, initially run under steady-state conditions, showed stable activity for a period of about two weeks. However, soon after the onset of the start-stop temperature cycling the activity began decreasing, which continued for another two weeks of operation, showing no signs of stabilisation. However the CO2 selectivity remained very high (in excess of 99%) throughout the test. For the catalyst ‘Pd-Zn/oxide support’, tested using the same protocol, the activity under the steady-state conditions was also stable and close to that of Cu-Zn-Al catalyst. At the beginning of the temperature cycling, a slight drop in the methanol conversion was registered but the activity remained stable thereafter. The CO2 selectivity was also stable throughout the test at 98 ± 0.6%. The temperature dependence of the initial CO2 selectivity (i.e. SCO2 at low conversions of methanol) was further investigated in the range 230 to 320ºC, and was found to decrease with the temperature from 98.2 to 94.2% (Figure 5). It was also observed that for the given reaction conditions CO2 selectivity is fairly constant over a broad range of methanol conversions; however it decreases at high conversions (above ~ 95 to 97%).

3.2 Mechanical Strength of the Cu-Zn-Al and ‘Pd-Zn/Oxide Support’ Catalysts It is known that exposure to a liquid can cause mechanical deterioration of particulate Cu-Zn-Al catalysts (9). This practically important aspect has not been sufficiently addressed in the open literature. Therefore in addition to catalytic performance we also analysed the Cu-Zn-Al catalyst for its mechanical strength before and after sixty temperature cycles, and found that the catalyst pellets lose about 80% of their initial strength as a result of the temperature cycling. The number of starts and stops of the future miniature power sources will certainly be much greater, and therefore a more significant negative impact on the mechanical strength and finally on the integrity of the catalyst should be anticipated. This is yet another reason why particulate Cu-Zn-Al catalysts cannot be used in the miniature fuel reformer, and why a new and more robust catalyst had to be developed. The ‘Pd-Zn/oxide support’ catalyst was deposited on the walls of the prototype microchannel reformer, with the hydraulic diameter of a channel measuring a few hundred microns (Figure 3). The catalyst was successfully tested in the microreformer, producing hydrogen-rich reformate via MSR. The same catalyst on different support

100 100 COCO % (% 2 selectivity, 2 selectivity

95 90 90 85 80 80 75 70 70 65 60 60 55 50 50 220

230

240

250

260

270

280

290

300

310

320

330

oo TeTemperature, m pe ra ture (C C)

Fig. 5 Initial CO2 selectivity versus the methanol steam reforming reaction temperature for the catalyst ‘Pd-Zn/oxide support’. Test conditions: molar ratio of feed CH3OH:H2O = 0.88; gas hourly space velocity (GHSV) = 230,000 h–1 (powder catalyst); CH3OH conversions within 11%


138

structures is now used commercially in other applications.

3.3 ‘Pd-ZnO/Oxide Support’ Compared to ‘Pd-ZnO/Al2O3’, Pd/Al2O3 and Cu/CeO2 Catalysts High MSR activity with high CO2 selectivity is well documented for Pd-Zn catalysts, which were first discovered by Iwasa et al. (17), and are now being extensively investigated for hydrogen generation in portable fuel cell power systems. The literature information pertaining to the MSR performance of the Pd-Zn catalytic system, and available to the authors, deals with catalysts composed of Pd supported on zinc oxide and also Pd-ZnO compositions supported on alumina (12–17). Driven by a continuing interest in inexpensive non-precious metal catalysts for MSR applications, new copper-based catalytic compositions are also being developed (18, 19). To better understand the strengths and possible limitations of our Pd-based catalyst it is important to compare performance of this catalyst with that of other Pd- and Cu-based catalysts. To that end, following a procedure similar to those described in References (14) and (16), we prepared two Pd-ZnO catalysts on different supports: alumina and an oxide support material used in preparation of the new Pd-based catalyst, both catalysts having equivalent contents of Pd and ZnO. These catalysts are hereinafter referred to as ‘Pd-ZnO/Al2O3’ and ‘Pd-ZnO/oxide support’, respectively. For comparison, alumina-supported palladium catalyst (Pd/Al2O3) with the same amount of palladium as in Pd-ZnO samples was also prepared via the same procedure. In addition, a copper/ceria catalyst with 20 wt.% CuO (Cu/CeO2), similar to that used in (18), was prepared by incipient wetness impregnation of ceria with aqueous copper nitrate solution followed by drying and calcining. All catalysts were compared in terms of their activities and selectivities, now at elevated temperatures typical for certain advanced applications. The samples (3 g of each) were tested in a laboratory flow reactor as granules 250 to 710 μm in size. This particle size ensures test conditions free of pore diffusion. The feed mixture


consisted of a reagent grade methanol (Aldrich) and deionised water in volume ratio MeOH:H2O = 1:1 (molar ratio 0.88:1). The feed flow rates (from 0.3 cm3 min–1 to 2.0 cm3 min–1) and the catalyst temperatures (375ºC to 475ºC) were programmed and controlled throughout the test as shown in Figures 6 to 9. The automated gas chromatographic analysis was conducted at twenty minute intervals by sampling the gas mixtures exiting the reactor. Performance of all of the catalysts described above is analysed below. Methanol conversion and CO2 selectivity for the catalyst ‘Pd-ZnO/oxide support’ are plotted in Figure 6 for temperatures 375ºC, 425ºC and 475ºC and a range of flow rates. It can be seen that the catalyst has stable activity (100% methanol conversion in the first and the last segments of the run, these segments having identical experimental conditions) with CO2 selectivity ranging from around 70% at full conversion of methanol to around 82% at lower conversions. Under the same experimental conditions the sample ‘Pd-ZnO/Al2O3’ is less active, and has much lower CO2 selectivity (Figure 7). This catalyst ages, partially losing activity in the course of the run. This is illustrated by lower methanol conversion in the last segment of the run as compared with the first. The main product with this sample is CO; under the experimental conditions of the test, methane is also produced with selectivity ranging from 1 to 6%. This implies significantly less efficient production of hydrogen with ‘Pd-ZnO/Al2O3’, and also different reaction pathways for the two similar catalysts – ‘Pd-ZnO/oxide support’ and ‘Pd-ZnO/Al2O3’. The Pd/Al2O3 sample is even less active than ‘Pd-ZnO/Al2O3’, is more prone to ageing and has very poor CO2 selectivity (Figure 8). CO and methane are the dominant carbon-containing products with the Pd/Al2O3 catalyst under the experimental conditions employed, rendering this catalyst composition unsuitable for hydrogen generation. The Pd-free sample with Cu impregnated on ceria has high initial activity and relatively high CO2 selectivity (up to about 75%); however it ages rapidly with a significant loss in activity (Figure 9).

139

MeOH conversion/CO CO selectivities, selectivities (%) % CH3OH conversion, CO2 and CO 2 and

CH conversion 3OH MeOH con versio n

100 100 90 90

80 80

70 70

–1 0.30 .3mlml/m min in, , 375ºC 3 75 C

–1 0.8 ml min , 0 .8 ml/m in, 425ºC 4 25 C

2.0 ml min–1, 2.0 m l/min, 47 5C475ºC

1.2ml m l/min, 1.2 min–1, 42 5C 425ºC

0.3 ml min–1, 0 .3 m l/m in, 375ºC 3 75 C 2.0 ml min–1, 2.0 m l/min, 425ºC 42 5C

2.0 ml min–1, 2.0 ml/min, 425ºC 42 5C

C O22 se lectivity CO selectivity

60 60 50 50 40 40

CO CO selectivity se lectivi ty

30 30 20 20 10 10 0 0

4

2 /8 /08 5 :45 PM

2/8/08 10:33 PM 4

2/9/08 12 8:09 AM 12 8 Tim eon on stream (hourhours s) Time stream,

16

2/9/08 83:21 AM

20

2/9/0 8 12:57 16 PM

2/9 /0 820 5 :4 5 PM

Fig. 6 Methanol steam reforming performance test of the catalyst ‘Pd-ZnO/oxide support’

77

110

90 90 80 80

66 0 .3 ml/m in,–1

0.3 ml min , 3 75 C 375ºC

–1 0.8 m l/min,

0.8 ml min , 42 5C 425ºC

2.0 ml/min, –1

2.0 ml min , 47 5C 475ºC

70 70 60 60

0 .3ml ml/m in,–1, 0.3 min 3 75 C 375ºC

1 .2 ml/min, 1.2 ml min–1, 4 25 C 425ºC –1 2.0 min 2.0 ml m l/m in, , 425ºC 42 5C

CO selectivity CO selectivity

55 44

0 .6 m l/mmin in, –1, 0.6 ml 3 75 C 375ºC

50 50

33

CH44 selectivity selectivity CH

40 40

30 30

CH4 selectivity (%)

MeOH conversion/CO2 and CO selectivities (%)

100 100

CH4 selectivity, %

CH3OH conversion, CO2 and CO selectivities, %

CH conversion 3OHconversion MeOH

22

CO CO2 2selectivity selectivity

20 20

11

10

10 0 039519.62

39519.72

44

39519. 82

39520.02 12 8 1239520.12 str eam (hour s) TimeTime ononstream, hours

39519.92 8

39520. 22

16 16

39520.32

20

20 39520.42

00

Fig. 7 Methanol steam reforming performance test of ‘Pd-ZnO/Al2O3’ catalyst

The ageing in this case is probably due to the thermal sintering of Cu (20). The fresh catalyst at high temperatures also produces small amounts of


methane, but methanation stops as the catalyst ages. The results of our comparative study show that the catalyst ‘Pd-Zn/oxide support’ has an

140

CH3OH conversion MeOH conversion

100 100 90 90

MeOH conversion/Selectivities (%)

CH3OH conversion, CO2, CO and CH4 selectivities, %

110

80 80

COselectivity selectivity CO

70 70 60 60

–1 0.80.8ml min m l/m in, , 425ºC 425 C

0.30.3ml min–1, m l/min, 37 375ºC 5C

50 50

–1 2.02.0ml min m l/m in, , 475ºC 475 C

0 2 l/m in N2

0.4 ml m l/mmin in, –1, 0.4 37 5C 375ºC

2.0 2ml min–1, .0 ml/min, 4425ºC 25 C

1.21.2ml min–1, m l/min, 42425ºC 5C

40 40 30 30

CH CH44 selectivity selectivity

20 20

10 10

CO2 selectivity CO2 selectivity

00 39498. 7

39498. 8

4

4

39498. 9

39499 88 39499. 1 Time on stream (hours) Time on stream, hours

1239499. 2

12

39499. 3

16

16

39499. 4

Fig. 8 Methanol steam reforming performance test of Pd/Al2O3 catalyst

1.1 1.1

110

CH3OHconversion conversion MeOH 1.0 1.0

0.3 0.3 mlml/min, min–1, 375ºC 375C

80 80

0.8 ml/min, 0.8 ml min–1, 425C 425ºC

1.2 ml min–1, 1.2 ml/min, 425C425ºC

2.02.0 mlml/min, min–1, 475ºC 475C

2.0 ml min–1, 2.0 ml/min, 425C425ºC

2.0 min–1, 2.0 ml ml/min, 425ºC 425C

0.3 min–1, 0.3 ml ml/min, 375ºC 375C

0.9 0.9 0.8 0.8 0.7 0.7

70 70

selectivity CO22 selectivity 60 60

0.6 0.6

50 50

0.5 0.5 0.4 0.4

40 40 30 30

CO selectivity selectivity

0.3 0.3

20 20 10 10

CH4 selectivity (%)

90 90

CH4 selectivity, %

CH3OH conversion, CO2 and CO % MeOH conversion/CO COselectivities, selectivities (%) 2 and

100 100

0.2 0.2

CH44 selectivity selectivity

00

39490.58

6

39490.83 6

0.1 0.1

12 Time hours Timeon on stream, stream (hours) 39491.08 12

18

39491.33 18

24

00.0

39491.58 24

Fig. 9 Methanol steam reforming performance test of Cu/CeO2 catalyst

optimum composition for the MSR reaction. It possesses high and stable activity, as well as the highest CO2 selectivity over a broad range of MSR


process conditions. The catalyst ‘Pd-Zn/oxide support’ is hence the most efficient among the catalysts tested in this study for hydrogen generation.

141

4. Conclusions

Acknowledgements

A new active Pd-based MSR catalyst has been developed for use as a washcoat in a microchannel reformer integrated to a miniature fuel cell power source. The catalyst ‘Pd-Zn/oxide support’ with proprietary additives shows stable performance under the frequent start-stop operating conditions typical of a portable fuel cell power source. The new catalyst is also thermally stable, which enables its extended operation over a broad range of temperatures and simplifies fabrication of the miniature power source.

This article contains information produced in partnership with the Motorola Energy Technologies Lab within the National Institute of Standards and Technology (NIST) Advanced Technology Program: “Hydrogen Generator for a Miniature Fuel-Cell Power Source”. Part of this information was presented at the 31st Annual Conference of the International Precious Metals Institute (IPMI) (21). The authors gratefully acknowledge the kind permission of the IPMI to publish this article.

References 1 D. R. Palo, R. A. Dagle and J. D. Holladay, Chem. Rev., 2007, 107, (10), 3992 2 H. Kobayashi, N. Takezawa and C. Minochi, Chem. Lett., 1976, 5, (12), 1347 3 K. Takahashi, H. Kobayashi and N. Takezawa, Chem. Lett., 1985, 14, (6), 759 4 K. Takahashi, N. Takezawa and H. Kobayashi, Appl. Catal., 1982, 2, (6), 363 5 N. Takezawa, H. Kobayashi, A. Hirose, M. Shimokawabe and K. Takahashi, Appl. Catal., 1982, 4, (2), 127 6 E. Santachesaria and S. Carrá, Appl. Catal., 1983, 5, (3), 345 7 K. Miyao, H. Onodera and N. Takezawa, React. Kinet. Catal. Lett., 1994, 53, (2), 379 8 C. J. Jiang, D. L. Trimm, M. S. Wainwright and N. W. Cant, Appl. Catal. A: Gen., 1993, 93, (2), 245 9 J. C. Amphlett, M. J. Evans, R. F. Mann and R. D. Weir, Can. J. Chem. Eng., 1985, 63, (4), 605 10 L. Lloyd, D. E. Ridler and M. V. Twigg, in “Catalyst Handbook”, 2nd Edn., ed. M. V. Twigg, Wolfe Publishing, London, 1989, pp. 283–338 11 Y. Wang, J. Zhang and H. Xu, Cuihua Xuebao, 2006, 27, (3), 217; Chem. Abstr., 145:441089

12 P. Pfeifer, A. Kölbl and K. Schubert, Catal. Today, 2005, 110, (1–2), 76 13 C. Fukuhara, Y. Kamata and A. Igarashi, Appl. Catal. A: Gen., 2007, 330, 108 14 C. Cao, G. Xia, J. Holladay, E. Jones and Y. Wang, Appl. Catal. A: Gen., 2004, 262, (1), 19 15 N. Iwasa and N. Takezawa, Top. Catal., 2003, 22, (3–4), 215 16 A. Karim, T. Conant and A. Datye, J. Catal., 2006, 243, (2), 420 17 N. Iwasa, S. Kudo, H. Takahashi, S. Masuda and N. Takezawa, Catal. Lett., 1993, 19, (2–3), 211 18 Y. Men, H. Gnaser, R. Zapf, V. Hessel, C. Ziegler and G. Kolb, Appl. Catal. A: Gen., 2004, 277, (1–2), 83 19 P. Clancy, J. P. Breen and J. R. H. Ross, Catal. Today, 2007, 127, (1–4), 291 20 M. V. Twigg and M. S. Spencer, Top. Catal., 2003, 22, (3–4), 191 21 IPMI, 31st Annual Conference of Precious Metals, 9th–12th June, 2007, Miami, Florida, U.S.A.: http://www.ipmi.org/seminars/conf_detail.cfm?id =17

The Authors Dr Oleg Ilinich is a Senior Chemist at BASF Catalysts LLC in Iselin, New Jersey, U.S.A. He received his M.S. in Chemical Engineering from the St. Petersburg Institute of Technology in Russia, which was later followed by Ph.D. and D.Sci. degrees in Catalysis from the Boreskov Institute of Catalysis (Novosibirsk, Russia). His recent work includes catalyst development as well as kinetic and mechanistic studies for the water-gas shift reaction, methanol steam reforming and direct methanol fuel cells. Prior to joining BASF Catalysts, Dr Ilinich was involved in fundamental and applied research in selective heterogeneous catalysis, and catalytic and gas separation membranes.


Dr Ye Liu is a Senior Research Engineer at BASF Catalysts LLC. He received his B.S. and M.S. in Chemical Engineering from the Dalian University of Technology, China, and his Ph.D. in Material Science and Engineering from the Pennsylvania State University, U.S.A. His current work involves catalyst development for three-way catalysis, partial oxidation, steam reforming and the autothermal reforming of hydrocarbons. His past research involvement has included catalytic gasification, combustion, sulfur removal in flue gases, and product and process development for carbonaceous materials – activated carbons and carbon blacks.

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Christopher Castellano is a Chemist at BASF Catalysts LLC. He received his B.S. in Ceramic Engineering from Rutgers University, U.S.A.; and is currently working on a M.S. in Material Science and Engineering from Columbia University, U.S.A. His current work involves combinatorial catalyst development for environmental applications, as well as flexible fuel catalysts. His previous research involvement has included methanol steam reforming catalysis, NH3selective catalytic reduction technology, light-duty diesel catalysts, ozone conversion and high-throughput zeolite synthesis. He is also currently serving as the President of the Rutgers Engineering Society.

Dr Ahmad Moini is a Senior Research Associate at BASF Catalysts LLC. He obtained his Ph.D. in Chemistry from Texas A&M University, U.S.A., followed by a postdoctoral appointment at Michigan State University, U.S.A. Dr Moini started his career at Mobil Research & Development Corporation, where he conducted research in microporous materials. He joined Engelhard Corporation (now BASF) in 1996. His research focus is on the synthesis and development of novel catalysts and materials.


Dr Gerald Koermer received a Ph.D. from the University of Wisconsin, U.S.A. in Physical Organic Chemistry and a B.A. in Chemistry from Rutgers College. He has over twenty-four years of experience in catalysis and currently leads the Materials Science and Enabling Technology group at BASF Catalysts LLC’s Research and Development.

Dr Robert J. Farrauto is a Research Fellow at BASF Catalysts LLC. He obtained a B.S. in Chemistry from Manhattan College, New York City and a Ph.D. in Electrochemistry from Rensselaer Polytechnic Institute, Troy, New York, U.S.A. His major responsibilities have included the development of advanced automobile emission control catalysts and process catalysts for the chemical industry. He managed an Engelhard research team that developed and commercialised diesel oxidation catalysts for the European, North American and Asian markets for passenger cars and heavyduty trucks. Currently he manages a research team developing new catalyst technology for the hydrogen economy, including hydrogen refueling stations and fuel cells for stationary, portable power and vehicular applications. He is also Adjunct Professor in the Earth and Environmental Engineering Department of Columbia University, in the City of New York, where he teaches a course in catalysis.

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DOI: 10.1595/147106708X319256

Noble Metal Catalysts for Mercury Oxidation in Utility Flue Gas GOLD, PALLADIUM AND PLATINUM FORMULATIONS By Albert A. Presto and Evan J. Granite* National Energy Technology Laboratory, United States Department of Energy, PO Box 10940, MS 58-106, Pittsburgh, PA 15236-0940, U.S.A.; *E-mail: [email protected]

The use of noble metals as catalysts for mercury oxidation in flue gas remains an area of active study. To date, field studies have focused on gold and palladium catalysts installed at pilot scale. In this article, we introduce bench-scale experimental results for gold, palladium and platinum catalysts tested in realistic simulated flue gas. Our initial results reveal some intriguing characteristics of catalytic mercury oxidation and provide insight for future research into this potentially important process.

1. Introduction Coal-fired utility boilers are the largest anthropogenic emitters of mercury in the United States, accounting for approximately one third of the 150 tons of mercury emitted annually (1, 2). In 2005, the U.S. Environmental Protection Agency (EPA) announced the Clean Air Mercury Rule, to limit mercury emissions from coal-fired utility boilers to 15 tons annually, approximately 30% of 1999 levels, by 2018 (3). At the time of publication (July 2008) this measure is under legal dispute. Of alternative legislative proposals to regulate mercury along with other pollutants, most would require a 90% mercury reduction, with deadlines for control varying from 2011 to 2015. Mercury exists in three forms in coal-derived flue gas: elemental (Hg0), oxidised (Hg2+) and particle-bound (Hg(p)) (4). During combustion, mercury is liberated from coal as Hg0. As the flue gas cools, some of the Hg0 is oxidised, presumably to mercury(II) chloride (HgCl2) because of the large excess of chlorine present in coal. Both Hg0 and Hg2+ can enter the particulate phase by adsorption onto fly ash particles (5). Hg2+ and Hg(p) are relatively easy to remove from flue gas using typical air pollution control devices. Hg(p) is captured, along with fly ash particles, in the particulate control device. Hg2+ is soluble in water, and is therefore removed with high efficiency by wet flue gas desulfurisation equipment (6). Hg0, on the other hand, is difficult


to capture. It is insoluble in water and is therefore not removed during flue gas desulfurisation. Activated carbon injection will remove both Hg0 and Hg2+, and currently this is the best method for removing Hg0 from flue gas (6). In addition to the Clean Air Mercury Rule, the U.S. EPA also enacted the Clean Air Interstate Rule, which requires reductions in NOx and SO2 emissions in twenty-eight states (7). An expected consequence of this law is increased use of wet flue gas desulfurisation for SO2 removal (8). Among the technologies being considered for mercury abatement in coal-fired boilers is therefore the combination of a catalyst and a wet scrubber; the catalyst oxidises Hg0 to Hg2+, and the oxidised mercury is subsequently absorbed by the scrubber solution. Catalysts capable of significant conversion (> 80%) of Hg0 to Hg2+ could have tremendous value because the oxidised mercury can be removed concurrently with acid gases during flue gas desulfurisation. Mercury oxidation catalysts can be employed in either of two configurations. In the ‘co-benefit’ application, selective catalytic reduction (SCR) catalysts with sufficient activity for mercury oxidation are installed upstream of a flue gas desulfurisation scrubber. The primary function of the SCR catalyst is to reduce NOx concentration in flue gas, and some power plants will need to install SCR to achieve compliance with the Clean Air Interstate

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Rule (9). During operation, NO is reduced by NH3, which is injected upstream of the SCR, at temperatures above 300ºC. When the NH3 is consumed (catalysts are typically oversized to prevent NH3 slip), the catalyst is available for mercury oxidation. Mercury oxidation catalysts can also be installed specifically for mercury control. In this case the catalyst is located downstream of the particulate control device, where the flue gas temperature is approximately 150ºC. The lower temperature favours Hg0 adsorption, and may therefore lead to more efficient mercury oxidation. Catalysts tested in this configuration include gold, palladium and vanadium-tungsten (10). The catalytic oxidation of mercury to mercury(II) chloride typically assumes an overall reaction between Hg0 and HCl, for example, Reaction (i): Hg + 2HCl → → HgCl2

(i)

Cl2 may also play a role in the formation of HgCl2, but the equilibrium concentration of Cl2 is only ~ 1% of the HCl concentration. Several key questions exist regarding this reaction. Specifically, the reaction mechanism is uncertain. The bimolecular reaction between two species adsorbed to a surface can be described by a LangmuirHinshelwood mechanism (11), Reactions (ii)–(v):

catalysts to confirm adsorption of specific reactants such as mercury and HCl. Additionally, pre-exposure of the catalyst to an oxidant, followed by mercury oxidation in the absence of the oxidant, would suggest either a Langmuir-Hinshelwood reaction or an Eley-Rideal reaction with the oxidant as the adsorbed species. A Langmuir-Hinshelwood mechanism can also be identified via chemical kinetics, though the relative adsorption behaviour of the reacting species may complicate analysis. In some cases, a Langmuir-Hinshelwood mechanism is characterised by a reaction that is first-order in each of the reactants (for example, Hg0 and HCl). However, if one species saturates the surface, the reaction order with respect to the saturating species can be –1 (11). Granite et al. (12) proposed that mercury oxidation could occur via a Mars-Maessen (13) mechanism. In this mechanism, adsorbed Hg0 would react with a lattice oxidant (either O or Cl) that is replenished from the gas phase. Reactions (viii)–(xii) show the Mars-Maessen mechanism for the reaction of an adsorbed species (for example, Hg0) with lattice oxygen: A(g) ' A(ads)

(viii)

A(ads) + MxOy → AO(ads) + MxOy–1

(ix)

MxOy–1 + ½O2 → MxOy

(x)

A(g) ' A(ads)

(ii)

AO(ads) → AO(g)

(xi)

B(g) ' B(ads)

(iii)

AO(ads) + MxOy → AMxOy+1

(xii)

A(ads) + B(ads) → AB(ads)

(iv)

AB(ads) → AB(g)

(v)

For this mechanism, the rate of reaction is dependent on the concentrations of reactants A and B, the adsorption equilibrium constant (Ki), and the rate constant for the surface reaction (ksurf). Mercury could also react via an Eley-Rideal mechanism, which is the reaction between a surface-bound species and a gas-phase (or weakly adsorbed) species, Reactions (vi)–(vii): A(g) ' A(ads)

(vi)

A(ads) + B(g) → AB(g)

(vii)

Eley-Rideal and Langmuir-Hinshelwood mechanisms can be inferred by surface analysis of used


The Mars-Maessen mechanism can be confirmed by the observation of mercury oxidation in the absence of gas-phase oxygen or chlorine, respectively (through variations of Reaction (ix)). Medhekar et al. postulated that catalytically active mercury(II) chloride forms on the surfaces of many materials (14). They observed the reaction between elemental Hg and Cl2 catalysed by Inconel® (an austenitic nickel-based alloy), quartz, stainless steel and Teflon®-coated stainless steel. Medhekar et al. found that many surfaces can catalyse the reaction between Hg and Cl2 and that the surfaces are difficult to passivate with oxygen or fluorine. This suggests that the adsorbed HgCl2 product is the actual catalyst. Ariya et al. observed

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that the Hg + Cl2 reaction proceeded faster when the reactor surface was covered with the reaction products of Hg + Br2 than with a clean surface, suggesting a similar effect (15). To date, none of the above mechanisms has been verified as the dominant mechanism for catalytic mercury oxidation. Mercury appears to react from an adsorbed state (10), but the phase of the HCl is uncertain. Furthermore, it is unknown whether Hg and HCl react directly, or if another species, such as mercury(II) oxide (HgO), is formed first (16, 17). The role of other flue gas species, specifically NO and SO2, is unclear, and the behaviour of these species in mercury oxidation may depend strongly upon the nature of the catalyst. The deactivation mechanisms for the various mercury catalysts are also unknown. In a previous article (10), we asserted that further research into the fundamental aspects of catalytic mercury oxidation is required to answer these significant questions. The information presented in this article is part of an ongoing effort toward that goal. We present initial results for mercury oxidation over three noble metal catalysts, Au, Pd and Pt. We envision that these materials could be used downstream of particulate control devices as mercury-specific catalysts. The results reveal several important aspects of the catalysts, and highlight some of the differences between Au and the platinum group metals (pgms) in mercury oxidation.

2. Experimental Noble metal catalyst samples were exposed to mercury in a bench-scale packed bed reactor that has been described previously (18) and is shown schematically in Figure 1. The bench-scale assembly consisted of a quartz tube reactor, 22 mm internal diameter and 61 cm long, contained in a clamshell tube furnace. A catalyst bed containing approximately 0.5 g of catalyst was placed in the reactor and was supported by a quartz frit. Alumina beads were placed above the catalyst bed to ensure plug flow. A mass spectrometer was located downstream of the packed bed to monitor potential side reactions such as the formation of flue gas halides.


Gas inlet

Adjustable thermocouple

Heated quartz reactor

Alumina beads Catalyst Quartz frit

Gas out to mercury monitor Fig. 1 Schematic diagram of the packed bed reactor

The catalysts used in this study were 1 wt.% Au, Pd and Pt, respectively, supported on 2 mm alumina beads (Johnson Matthey PLC). The BET surface area of the alumina beads was approximately 200 m2 g–1. The catalysts were air calcined at elevated temperatures, thereby decomposing the precursor salts. It is very unlikely that the Au particles are of nanometre size because they sinter at the calcination temperatures. Therefore, the Au catalyst should have typical properties of bulk Au. The mercury concentration and speciation exiting the packed bed were measured using a P S Analytical model 10.525 ‘Sir Galahad’ continuous mercury monitor. A wet conditioning system with two channels for determining elemental and total mercury was placed upstream of the mercury monitor. The elemental mercury channel used an impinger filled with KCl solution to remove Hg2+ from the sample, and the total mercury channel used a SnCl2/HCl solution to reduce Hg2+ to Hg0. Both the KCl and SnCl2/HCl impingers were followed by impingers containing NaHCO3 solution that captured the acid gases SO2 and HCl. The mass of catalyst was selected to provide a small (10 to 50%) conversion of Hg0 to Hg2+. Very high or very low fractional conversions are unfavourable because they complicate the

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interpretation of the experimental results. The precision of the mercury monitor used in this study is approximately 10 to 20% (1σ); therefore gas mixtures containing less than 10%, or more than 90%, oxidised mercury are statistically indisinguishable from gas mixtures that contain 0% or 100% oxidised mercury, respectively. The instrument precision is summed in quadrature for kinetics measurements, which require both elemental and oxidised mercury concentrations. The total uncertainty for the kinetics measurements is therefore 15 to 30% (Figure 2), and small fractional mercury conversions are nearly indistinguishable from the noise. Difficulty in obtaining consistent mercury measurements in real or simulated flue gas at levels of parts per billion by volume is a common problem. For example, results from field studies indicate significant variability in mercury capture efficiency during activated carbon injection. Specifically, during long-term injection tests, individual measurements of mercury capture efficiency (timescale of minutes to hours) can differ significantly from the long-term results (timescale of months) (19). When all of the potential sources of experimental uncertainty are considered, it is our opinion that the precision presented here is appropriate for our experimental system, and is consistent with previous work from this laboratory (20). The catalysts were exposed to simulated flue gas containing O2, CO2, HCl, SO2, Hg0 and N2. Each catalyst sample was tested using the ‘baseline’ simulated flue gas detailed in Table I. The baseline

Hg oxidation conversion, %

20

Baseline

[HCl] = 75 ppm

[HCl] = 0 ppm

Table I

Typical Simulated Flue Gas Conditions for the Experiments Conducted in This Study Parameter

Units

Baseline value

Range

[O2] [SO2] [NO] [CO2] [HCl] [CO] [Hg] Temperature Flow rate

% ppm ppm % ppm ppm μg Nm–3 ºC slpm

5.25 500 0 12.5 50 0 10 149 8

0–5.25 0–1000 500 – 0–100 0–35 6–18 138–160 8–10

conditions are roughly consistent with previous work from this laboratory, with the exception that NO was excluded from all but one experiment in this study because its presence interferes with mercury detection by the mercury monitor. The baseline conditions serve two purposes: first, they provide the basis for a like-for-like comparison for each of the catalysts tested. We will refer to the mercury oxidation rate measured in the presence of the baseline simulated flue gas as the ‘baseline reaction rate’. The effects of excursions from the baseline gas composition are measured as deviations from the baseline reaction rate. Second, because the baseline conditions are used at the start of each experiment, they provide a way to measure catalyst deactivation over time. Excursions from baseline conditions were undertaken in order to gain a more complete understanding of the reaction order with respect to [HCl] = 50 ppm [O2] = 0%

15

10

Fig. 2 Fractional mercury oxidation conversion across a gold catalyst as a function of time, from a typical experiment. The open symbols, , indicate the average conversion for the baseline and [HCl] = 75 ppm time periods. Mercury conversion is expected to be nominally constant during these phases of the experiment. The error bars, I, show the 1σ level of precision

5

0

9:00

11:00


Time

13:00

15:00

147

mercury and/or HCl, potential side or interfering reactions, and apparent activation energy. The effects of other flue gas species, such as SO2 and CO, were also considered. These species can possibly bind to and deactivate the catalyst, or can participate in parallel reactions, such as flue gas halide formation, that may inhibit mercury oxidation. The catalyst samples were tested for approximately six hours per day for several days. This procedure contrasts with packed bed experiments conducted by this group using mercury sorbents. In those experiments (18) the sorbent was exposed for six hours and removed from the packed bed reactor. The procedural difference between catalyst and sorbent experiments is intended to partially mimic the application of the two technologies in power plants: sorbents are typically injected and subsequently disposed of, whereas catalysts need to stay in place for months or years in order to be an economically viable option for mercury control (21). Exposing the catalysts for multiple six-hour experiments also allows for an initial investigation into the flue gas species and/or processes that can deactivate the catalyst.

3. Results The data presented in this article were analysed according to the chemical kinetics framework previously outlined by this group (22). The catalysts are compared by considering the overall reaction rate for Hg2+ formation, measured in (mol Hg2+) (g catalyst)–1 s–1. The results presented in this section focus specifically on the roles of HCl and oxygen in mercury oxidation.

A time series from a typical experiment is shown in Figure 3. In this experiment, the HCl and O2 concentrations were changed in successive steps, and the mercury oxidation rate was measured following each change in simulated flue gas composition. Please note that the total concentration of mercury exiting the reactor bed, [HgT], is equal to the concentration entering the reactor. This steady state, with no net adsorption of mercury, is typically referred to as ‘complete breakthrough’. As with other studies of mercury oxidation catalysts, oxidation rate measurements were only made under conditions of complete breakthrough (22, 23). At the start of the test series for each catalyst, the samples adsorbed mercury for two to six hours before reaching complete breakthrough. During most of the subsequent experiments, the catalyst sample adsorbed mercury for a short period, typically one hour, prior to reaching complete breakthrough. The mercury adsorbed during this start-up period likely replaced mercury that was desorbed during the cooldown period of the previous experiment. During the first experiment for each catalyst, Hg/CO2/N2 and Hg/CO2/O2/N2 gas mixtures were passed prior to the baseline simulated flue gas. All subsequent experiments were initiated with the baseline simulated flue gas (Table I). Complete mercury breakthrough occured quickly in the Hg/CO2/N2 atmosphere, and no mercury oxidation was evident in either of the mixtures. The onset of mercury oxidation coincided with the use of the baseline simulated flue gas.

[Hg] at reactor bed exit, μg Nm–3

12 10 8

Baseline

6

[HCl] = 75 ppm

[HCl] = 0 ppm

[HCl] = 50 ppm [O2] = 0%

4 [Hg0] [HgT]

2 0

9:00

11:00


Time

13:00

Fig. 3 Mercury monitor data showing mercury oxidation across a gold catalyst from a typical experiment. The simulated flue gas composition was changed from baseline conditions to high HCl (75 ppm), low HCl (0 ppm) and O2-free conditions in successive steps. The system was allowed to reach an (apparent) steady state after each change to the simulated flue gas composition before another change was imposed. The oxidation rate was measured during the steady state portion of each time interval. [HgT] = total mercury concentration

15:00

148

3.1 Gold Catalyst

baseline rate after 1.5 hours and ~ 45% of the baseline rate after 2.5 hours. Time limitations prevented further testing, though we assume that mercury oxidation would have eventually stopped in the absence of an HCl source. The mercury oxidation rate was also dependent on the presence of O2. Removing O2 from the simulated flue gas produced a similar effect to removing HCl: mercury oxidation continued at a reduced rate. Upon stopping O2 flow, the Hg2+ formation rate fell to < 50% of the baseline rate. Mercury oxidation in the O2-free simulated flue gas was only monitored for approximately 45 minutes, and as shown in Figure 2, the fractional mercury conversion was trending downward at the conclusion of the experiment. However, we are uncertain whether the reaction rate would have continued to decline as in the case of 0 ppm HCl. In a separate experiment the temperature was varied from 138 to 160ºC. An apparent activation energy of 40 kJ mol–1 for the global reaction (i):

The Au catalyst exhibited a consistent baseline rate of Hg2+ formation, as shown in Figure 4. The baseline reaction rate remained constant over a period of seven experiments, suggesting that there was no apparent catalyst deactivation. The mean baseline reaction rate was (2.2 ± 0.3) × 10–10 (mol Hg2+) (g catalyst)–1 s–1. The baseline reaction rate remained constant when the flow rate was raised from 8 standard litres per minute (slpm) to 10 slpm with no change in simulated flue gas composition. This result suggests that the mercury oxidation reaction is not limited by mass transfer under the conditions tested here. The HCl concentration was varied from the baseline level of 50 parts per million (ppm) to 75 and 0 ppm. Raising the HCl concentration to 75 ppm had no effect on the reaction rate (Figure 2). Even though the reaction rate, and therefore the fractional conversion to oxidised mercury, were nominally constant during the baseline and elevated HCl portions of the experiment, the data in Figure 2 exhibit considerable scatter. As noted above, the precision for the kinetics measurements is approximately 15 to 30%. Due to the scatter in the data and the difficulty in making precise kinetic measurements with our current system, reaction rates reported here often represent time averages over periods of relative consistency (for example, the diamond symbols in Figure 2). When the HCl concentration was set to 0 ppm, mercury oxidation continued, but the reaction rate slowed. The reaction rate fell to ~ 85% of the

Hg + 2HCl → → HgCl2

was calculated for this temperature range. This is consistent with the apparent activation energy of ~ 30 kJ mol–1 measured by Zhao et al. (24) for the reaction of mercury with Cl2 across a Au catalyst.

3.2 Palladium Catalyst The baseline reaction rate across the Pd catalyst (Figure 4) declined over the course of the test period, falling from 1.6 × 10–10 to 3.3 × 10–11 (mol Hg2+) (g catalyst)–1 s–1. Because of the rapid decline Fig. 4 Baseline reaction rate across the gold (squares) and palladium (circles) catalysts for the experiments presented here. Experiments were conducted on consecutive days (excluding weekends), with one experiment per day. — = the mean baseline reaction rate for the gold catalyst; – – – = one standard deviation

1010 × Baseline reaction rate, (mol Hg2+) (g catalyst)–1 s–1

3.0 2.5 2.0 1.5 1.0

Au

§ Pd

0.5 0

(i)

1

2

3 4 5 Experiment number


6

7

149

in the baseline oxidation rate, all comparisons with the baseline rate are limited to a particular experiment. The baseline reaction rate was measured at the start of each experiment, and we assume that the baseline rate is roughly constant during a given experiment. The decrease in the baseline reaction rate with time suggests that the Pd catalyst is deactivated or fouled more readily than the Au catalyst. The Pd catalyst exhibited similar responses to changes in HCl concentration to those of the Au catalyst. Raising the HCl concentration from 50 ppm to 100 ppm had no impact on the reaction rate. Lowering the HCl concentration to 0 ppm slowed, but did not halt, mercury oxidation. The oxidation rate fell to ~ 45% of the baseline rate after 80 minutes, and ~ 42% of the baseline rate after 150 minutes. NO (500 ppm) was added for one experiment near the end of the test period. The baseline rate for this experiment was 3.3 × 10–11 (mol Hg2+) (g catalyst)–1 s–1, and the mercury was approximately 10 to 20% oxidised downstream of the catalyst bed. Adding NO to the simulated flue gas reduced the sensitivity of the mercury monitor, and the observed total mercury concentration fell from 10 μg Nm–3 to 4 μg Nm–3. With NO present, the mercury downstream of the catalyst bed was 90% oxidised. The reaction rate is not reported here because the measurement for the total Hg concentration is biased low. Regardless of the actual reaction rate, adding NO to the simulated flue gas resulted in significantly greater fractional mercury oxidation downstream of the catalyst bed.

3.3 Platinum Catalyst As with the Pd catalyst, the baseline reaction rate observed with the Pt catalyst decreased over the course of the test period. The baseline rate fell from an initial maximum of 4.1 × 10–10 to 1.5 × 10–11 (mol Hg2+) (g catalyst)–1 s–1. As with the Au and Pd catalysts, increasing the HCl concentration to 75 ppm and 100 ppm had no impact on the mercury oxidation rate. Reducing the HCl concentration to 0 ppm yielded an immediate halt to mercury oxidation. This behaviour is in contrast to that of the Au and Pd catalysts, both of which continued oxidising mercury at reduced rates when HCl flow stopped.


Prior to one experiment the Pt catalyst was reduced at 165ºC with 60 ppm CO in N2. The catalyst was then exposed to a mixture of Hg/CO2/HCl/SO2/N2 at 150ºC. No oxidised mercury formation was observed. When O2 was added to the gas mixture to form the baseline simulated flue gas, mercury oxidation was evident. The HCl flow was then stopped, with the expectation that mercury oxidation would again stop. Instead, oxidation continued at a reduced rate, consistent with the behaviour observed for the Au and Pd catalysts. The apparent activation energy for mercury oxidation across the Pt catalyst was measured for the temperature range 140 to 157ºC. The measured activation energy was ~ 120 kJ mol–1, significantly higher than that measured for the Au catalyst.

4. Discussion As stated in the Introduction, previous investigations of mercury oxidation over a variety of catalysts indicate that mercury reacts from a bound state, for example, Hg(ads). It is well established that mercury adsorbs to Au, Pt and Pd surfaces. Gold (25, 26), palladium (27) and iridium (28) have all been used as modifiers for improving mercury capture in graphite tube atomic absorption spectrometry. The mercury monitor used in this study removes mercury vapour from the sample gas using Au/sand traps. The captured mercury is thermally desorbed during the analysis step of the instrument cycle. Mercury is also known to adsorb to Pt and form a solid solution with it (29). Therefore, we are confident in assuming that mercury adsorbs to the catalyst surface prior to reacting. The dependence of the mercury oxidation rate on the presence of HCl in the simulated flue gas suggests that the oxidised mercury species observed by the mercury monitor is indeed HgCl2. Mercury oxidation was not observed when the catalyst was exposed to the Hg/CO2/O2/N2 gas mixture used prior to the simulated flue gas, indicating that the observed oxidised mercury species is unlikely to be HgO. Mercury(I) chloride (Hg2Cl2) is also a possibility, as it can form via the BolidenNorzink reaction (30), Reaction (xiii):

150

Hg(g) + HgCl2 → Hg2Cl2 (xiii) However, themodynamic calculations indicate that Hg2Cl2 is not stable under typical flue gas conditions (31). For all three noble metal catalysts tested here, increasing the HCl concentration above 50 ppm had no impact on the reaction rate. In a previous study, we made an initial assumption that the mercury oxidation rate can be described by r = k[Hg][HCl] (22). This assumption is obviously false for HCl concentrations > 50 ppm. The assumption above required Hg + HCl as participants in the rate-limiting step. At this point, the rate-limiting step is unclear. When HCl was removed from the simulated flue gas during the tests with the Au and Pd catalysts, the reaction rate immediately fell, suggesting that the lack of a chlorine source to replenish the surface reduced the reaction rate. Mulla et al. suggested that the adsorption of O2 to an empty surface site was the rate-limiting step for NO oxidation over a Pt/Al2O3 catalyst (32). Perhaps this step is also rate-limiting for the formation of the presumed HgO intermediate product detailed below. The data for the Au and Pd catalysts suggest that mercury reacts with HCl that is bound to the catalyst surface. This explains why mercury oxidation continues in the absence of gas-phase HCl, but with a declining reaction rate. Cl2 can chemisorb to Au surfaces and form AuCl3 (33). HCl dissociatively adsorbs to Pt surfaces (34), and similar behaviour might be expected for Pd. Thus, surface-bound Cl should be available for reaction on the Au and Pd surfaces. The Pt catalyst exhibited different behaviour in the absence of HCl, and mercury oxidation stopped. This might be evidence of an Eley-Rideal mechanism for mercury oxidation across the Pt catalyst, with adsorbed Hg (or an intermediate such as HgO, described below) reacting with gas-phase HCl. Eley-Rideal kinetics would not suggest the zero-order dependence on [HCl] for concentrations greater than 50 ppm, but the overall reaction could exhibit a zero-order dependence on [HCl] if the Hg(ads) + HCl(g) step is not rate-limiting. In a chlorine- and sulfur-free flame, Schofield (16, 17) observed HgO deposition on Pt and stain-


less steel surfaces. When HCl was added to the flame, the HgO desorbed as HgCl2. The implication of this observation is that Hg and HCl do not react directly to form HgCl2, but rather form via a HgO intermediate. Pt is an effective adsorber of oxygen (32, 35–37), hence surface-bound oxygen should be present in excess for the conversion of Hg0(g) to HgO(ads). In the absence of HCl(g), the HgO remains bound to the surface because of its low vapour pressure. When HCl is present, the HgO is converted to HgCl2, which desorbs from the surface and allows more Hg to react. The vapour pressure of HgCl2 is sufficiently high (1 Torr at 136ºC), and the concentration is sufficiently low, that the simulated flue gas stream can hold HgCl2 as a vapour even at temperatures well below the sublimation point. We tested the Schofield hypothesis in the experiment that used the reduced Pt catalyst. This test yielded two important results: (a) O2 and HCl (or possibly Cl2) are required for mercury oxidation across a Pt catalyst, possibly because HgCl2 formation is preceded by HgO; (b) the Pt catalyst can display a significant history effect. In the initial series of experiments, the catalyst was exposed to an O2-containing gas mixture prior to the introduction of HCl. In the experiment with the reduced Pt, the catalyst was exposed to HCl prior to O2. We hypothesise that in the initial tests, O(ads) greatly outnumbers Cl(ads) to the point of exclusion. Thus, HgO is easily formed on the surface, and HCl reacts with the adsorbed HgO from the gas phase. Without HCl, there is no chlorine source for HgCl2 formation. In the experiment using the reduced Pt, the initial exposure to HCl allows for an ample concentration of adsorbed chlorine that is joined by adsorbed oxygen when O2 is introduced. When HCl is removed from the simulated flue gas, there is sufficient surfacebound chlorine to sustain HgCl2 formation at a reduced rate. The reaction mechanism remains unclear. The initial experiments suggest the possibility of an Eley-Rideal mechanism, and the experiments with the reduced Pt catalyst might suggest a Langmuir-Hinshelwood mechanism. The data suggest that mercury oxidation across the Au catalyst is dependent on the presence of O2.

151

This behaviour, while puzzling, may indicate the formation of the HgO intermediate. HgO binds to Au surfaces, and density functional theory calculations indicate that the binding energy for mercury species on Au(001) decreases in the series: HgO > Hg0 > HgCl2 (38). The predicted energy of binding of HgCl2 to the Au(001) surface is only 17.2 kJ mol–1, suggesting that this species could easily desorb from the catalyst surface at the temperatures tested here. The mechanism governing the oxygen dependence of mercury oxidation across the Au catalyst is unknown at this time. Unlike with Pt, oxygen is not expected to efficiently adsorb to the Au surface (39), suggesting that adsorbed oxygen for HgO formation is not readily available at the surface. One could postulate an Eley-Rideal reaction between bound Hg and O2(g) to form HgO, but this reaction requires accounting for the second oxygen atom, suggesting either a ternary reaction (xiv): 2Hg + O2 → 2HgO

(xiv)

which is unlikely, or the migration of an oxygen atom to the Au surface, which is also unlikely. Further research is required to elucidate this behaviour. The nature of the bonding of mercury, chlorine and oxygen species to the catalyst surface is unknown at this time. One possibility is that each species adsorbs to the surface individually – Hg(ads), Cl(ads), HgO(ads), etc. Mercury is known to interact with other metals to form a variety of oxides and halides (40) that could participate in the surface reactions which lead to the formation of HgCl2. Surface analysis of fresh and used catalyst samples will be conducted in the future in order to gain more insight. The most significant difference between the performance of the Au and the pgm catalysts, was the loss of catalytic activity for the Pd and Pt over the course of the test period. The presence of adsorbed oxygen on the catalyst surface may offer an explanation for the observed drop in catalyst activity. While testing Pt/Al2O3 catalysts for the oxidation of NO to NO2, Mulla et al. observed that the catalyst deactivated during cooldown and other changes to process conditions (32). The


researchers proposed that deactivation may have been the result of the oxidation of the Pt surface. Olsson et al. also observed deactivation of Pt/Al2O3 and Pt/BaO/Al2O3 catalysts during the same reaction and attributed the loss of catalytic activity to the formation of unreactive PtO (41). The formation of Pt and Pd oxides, especially while exposed to O2 during cooldown, may also explain the deactivation observed here. The poor reactivity of PtO might also suggest that the possible HgO intermediate product is not formed via a Mars-Maessen reaction. Au, on the other hand, is typically a poor adsorber of oxygen and is likely not subject to this deactivation mechanism (39). Mulla et al. found that their Pt/Al2O3 catalyst could be regenerated with CO or H2 (32). This observation agrees well with the data presented here; Hg oxidation proceeded at nearly the initially observed baseline reaction rate after the Pt catalyst was exposed to CO. The enhanced extent of Hg oxidation observed across the Pd catalyst in the presence of NO also suggests that surface oxygen inhibits Hg oxidation. When NO was added to the simulated flue gas used here, it is possible that surface oxygen was removed, allowing for increased conversion of mercury (32, 35–37, 41). It is unclear at this time whether the Pt and Pd catalysts were deactivated by the formation of oxides (for example, PtO and PdO), surface-bound oxygen, or both. While NO may have removed adsorbed oxygen from the Pd surface, the presence of PtO has been observed to inhibit NO oxidation across Pt catalysts (41). Thermal regeneration of Pt catalysts requires temperatures of 600 to 650ºC, significantly higher than the temperatures used here. Surface analysis will be required to confirm the nature of the catalyst surface following exposure to the simulated flue gas. Of concern with all potential mercury oxidation catalysts are the unwanted side reactions (xv) and (xvi): NO + ½O2 → NO2

(xv)

and SO2 + ½O2 → SO3

(xvi)

152

Stack NO2 concentrations as low as 15 ppm (42) can lead to the formation of a brown plume and enhanced local ozone production. SO3 is captured poorly in most wet flue gas desulfurisation systems, and can lead to sulfuric acid mist in the downstream plume. In each case, a small concentration can produce a significant impact – 15 ppm NO2 corresponds to only 3% NO conversion for flue gas containing 500 ppm NO. To date, both Au and Pd catalysts have been tested at pilot scale, and neither SO3 nor NO2 formation was observed (23). However, formation of these unwanted byproducts remains a concern. The results introduced here may have several implications for the use of noble metal catalysts in mercury abatement schemes. Perhaps most important is the observation that Au exhibits superior resistance to deactivation than Pd and Pt. Contrary to our short-term, bench-scale results, pilot-scale testing of Au and Pd showed similar performance and deactivation over time (23). A likely explanation is the presence of NO in the real flue gas; as shown here, NO appears to regulate the surface oxygen concentration on the Pd catalyst, leading to improved Hg oxidation versus a NO-free simulated flue gas. The oxygen resistance of Au may come into play during periods of down time, when the catalyst beds come into contact with air at ambient temperature. This condition favours oxygen adsorption onto Pt and Pd. However, a preference for Au remains to be seen, and future, longer-term testing may be warranted.

5. Conclusions The results presented here provide an initial investigation into the mechanisms behind mercury oxidation across noble metal catalysts, and more work is needed. Investigations are needed into the role of other potential flue gas catalyst poisons such as arsenic, selenium and SO3. Selenium is suspected to deactivate Au catalysts tested at pilot scale (23), and initial experiments conducted in our laboratory suggest that high concentrations of SO3 can deactivate Au catalysts. Future work should also focus on the possible beneficial role of different promoters, alloys and supports. At the size and timescale presented here, both Au and pgms show


promise for use as mercury oxidation catalysts. As we begin to understand mercury oxidation across these catalysts – specifically the roles of O2 and HCl and the apparent deactivation mechanisms – improvements can be made that will help mercury oxidation catalysts become economically competitive as part of a mercury abatement strategy.

Acknowledgements The authors thank Johnson Matthey PLC for providing the catalyst samples. Hugh Hamilton of the Johnson Matthey Technology Centre, U.K., provided excellent insight into the catalyst preparation and properties. The comments of the reviewers are greatly appreciated. Albert Presto acknowledges the support of a postdoctoral fellowship at the U.S. Department of Energy (DOE) administered by the Oak Ridge Institute for Science and Education (ORISE). Funding support from the DOE Innovations for Existing Power Plants (IEP) Program is greatly appreciated. We thank Gregson Vaux, Power/Energy Engineer for the Science Applications International Corporation (SAIC), for his kind help in understanding the regulatory status for mercury emissions as of April 2008.

Disclaimer References in this paper to any specific commercial product, process, or service are to facilitate understanding, and do not necessarily imply its endorsement by the U.S. DOE.

References 1 “Mercury Study Report to Congress”, U.S. EPA, U.S. Government Printing Office, Washington, D.C., December, 1997: http://www.epa.gov/mercury/report.htm 2 “Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units – Final Report to Congress”, U.S. EPA, U.S. Government Printing Office, Washington, D.C., February, 1998: http://www.epa.gov/ttn/oarpg/t3/reports/eurtc1.pdf 3 ‘Clean Air Mercury Rule’, U.S. EPA, 15th March, 2005: http://www.epa.gov/camr/ 4 K. C. Galbreath and C. J. Zygarlicke, Environ. Sci. Technol., 1996, 30, (8), 2421 5 C. L. Senior and S. A. Johnson, Energy Fuels, 2005, 19, (3), 859 6 J. H. Pavlish, E. A. Sondreal, M. D. Mann, E. S. Olson,

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7 8 9

10 11

12 13 14 15 16 17 18 19

20 21

22

23

K. C. Galbreath, D. L. Laudal and S. A. Benson, Fuel Proc. Technol., 2003, 82, (2–3), 89 ‘Clean Air Interstate Rule’, U.S. EPA, 10th March, 2005: http://www.epa.gov/cair/ R. K. Srivastava, N. Hutson, B. Martin, F. Princiotta and J. Staudt, Environ. Sci. Technol., 2006, 40, (5), 1385 R. K. Srivastava, R. E. Hall, S. Khan, K. Culligan and B. W. Lani, J. Air Waste Manage. Assoc., September, 2005, 55, (9), 1367 A. A. Presto and E. J. Granite, Environ. Sci. Technol., 2006, 40, (18), 5601 M. J. Pilling and P. W. Seakins, “Reaction Kinetics”, 2nd Edn., Oxford University Press, Oxford, U.K., 1995 E. J. Granite, H. W. Pennline and R. A. Hargis, Ind. Eng. Chem. Res., 2000, 39, (4), 1020 P. Mars and J. G. H. Maessen, J. Catal., 1968, 10, (1), 1 A. K. Medhekar, M. Rokni, D. W. Trainor and J. H. Jacob, Chem. Phys. Lett., 1979, 65, (3), 600 P. A. Ariya, A. Khalizov and A. Gidas, J. Phys. Chem. A, 2002, 106, (32), 7310 K. Schofield, Chem. Phys. Lett., 2004, 386, (1–3), 65 K. Schofield, Proc. Combust. Inst., 2005, 30, (1), 1263 A. A. Presto and E. J. Granite, Environ. Sci. Technol., 2007, 41, (18), 6579 S. Nelson, “Brominated Sorbents for Small Cold-Side ESPs, Hot-Side ESPs, and Fly Ash Use in Concrete”, Progress Report to U.S. DOE/NETL, Cooperative Agreement No. DE-FC26-05NT42308, Sorbent Technologies Corporation, 2007 E. J. Granite and H. W. Pennline, Ind. Eng. Chem. Res., 2002, 41, (22), 5470 G. Blythe, “Pilot Testing of Mercury Oxidation Catalysts for Upstream of Wet FGD Systems”, Quarterly Technical Progress Report to U.S. DOE/NETL, U.S. Department of Energy Agreement No. DE-FC26-01NT41185, URS Corporation, 2003 A. A. Presto, E. J. Granite, A. Karash, R. A. Hargis, W. J. O’Dowd and H. W. Pennline, Energy Fuels, 2006, 20, (5), 1941 G. Blythe, K. Dombrowski, T. Machalek, C. Richardson and M. Richardson, “Pilot Testing of Mercury Oxidation Catalysts for Upstream of Wet FGD Systems”, Final Report to U.S. DOE/NETL, U.S. Department of Energy Agreement No. DEFC26-01NT41185, URS Corporation, 2006

24 Y. Zhao, M. D. Mann, J. H. Pavlish, B. A. F. Mibeck, G. E. Dunham and E. S. Olson, Environ. Sci. Technol., 2006, 40, (5), 1603 25 Z. Hladky, J. Rísová and M. Fisera, J. Anal. At. Spectrom., 1990, 5, (8), 691 26 J. P. Matousek, R. Iavetz, K. J. Powell and H. Louie, Spectrochim. Acta Part B.: At. Spectrom., 2002, 57, (1), 147 27 X.-P. Yan and Z.-M. Ni, Anal. Chim. Acta, 1993, 272, (1), 105 28 B. Radziuk and J. Kleiner, Spectrochim. Acta Part B: At. Spectrom., 1993, 48, (14), 1719 29 F. L. Fertonani, A. V. Benedetti and M. Ionashiro, Thermochim. Acta, 1995, 265, 151 30 T. Allgulin, Boliden AB, U.S. Patent 3,849,537; 1974 31 F. Frandsen, K. Dam-Johansen and P. Rasmussen, Prog. Energy Combust. Sci., 1994, 20, (2), 115 32 S. S. Mulla, N. Chen, L. Cumaranatunge, G. E. Blau, D. Y. Zemlyanov, W. N. Delgass, W. S. Epling and F. H. Ribeiro, J. Catal., 2006, 241, (2), 389 33 N. D. Spencer and R. M. Lambert, Surf. Sci., 1981, 107, (1), 237 34 F. T. Wagner and T. E. Moylan, Surf. Sci., 1989, 216, (3), 361 35 J. Després, M. Elsener, M. Koebel, O. Kröcher, B. Schnyder and A. Wokaun, Appl. Catal. B: Environ., 2004, 50, (2), 73 36 R. Marques, P. Darcy, P. Da Costa, H. Mellottée, J.M. Trichard and G. Djéga-Mariadassou, J. Mol. Catal. A: Chem., 2004, 221, (1–2), 127 37 M. F. Irfan, J. H. Goo, S. D. Kim and S. C. Hong, Chemosphere, 2007, 66, (1), 54 38 E. Sasmaz and J. Wilcox, ‘The Binding of Flue Gas Components on CaO, TiO2, Pd, Au and PdAu Alloy’, Paper No. 409, in “Proceedings of the Air & Waste Management Association’s 100th Annual Conference and Exhibition”, Pittsburgh, PA, June, 2007, A&WMA, Pittsburgh, PA, 2007 39 X. Deng, B. K. Min, A. Guloy and C. M. Friend, J. Am. Chem. Soc., 2005, 127, (25), 9267 40 J. W. Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Longmans, Green and Company, London, New York, 1952, Vol. IV 41 L. Olsson and E. Fridell, J. Catal., 2002, 210, (2), 340 42 A. S. Feitelberg and S. M. Correa, J. Eng. Gas Turbines Power, 2000, 122, (2), 287

The Authors Dr Albert A. Presto is currently a Research Scientist and Laboratory Manager for the Air Quality Laboratory in the Center for Atmospheric Particle Studies at Carnegie Mellon University. Prior to his current position he was an ORISE postdoctoral fellow at the U.S. Department of Energy’s National Energy Technology Laboratory (NETL) in Pittsburgh. His research interests include mercury removal from coal-derived flue and fuel gas, atmospheric secondary organic aerosol formation, the atmospheric processing of organic aerosol, and atmospheric radical chemistry.


Dr Evan J. Granite is a Research Group Leader at the NETL. His research has focused on mercury and carbon dioxide removal from flue and fuel gases. Dr Granite is the principal investigator for three projects on the capture of mercury, arsenic and selenium from coal-derived flue and fuel gases, and carbon dioxide separation from flue gas. His research interests are in catalysis and surface chemistry, pollution clean-up, electrochemistry and photochemistry.

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DOI: 10.1595/147106708X332215

“Highly Efficient OLEDs with Phosphorescent Materials” EDITED BY HARTMUT YERSIN (University of Regensburg, Germany), Wiley-VCH, Weinheim, Germany, 2007, 458 pages, ISBN 978-3-527-40594-7, £100.00, €135.00, U.S.$190.00

Reviewed by R. J. Potter Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.; E-mail: [email protected]

In January 2008 Sony announced the release of the first organic light-emitting diode (OLED)-based television (Figure 1) into the U.S. consumer market (1). This was a milestone in the development of materials that for over twenty years have been heralded as the successor to liquid crystal displays. Fig. 1 Side view of Sony XEL-1 television (Courtesy of OLED Display.net)

This new monograph on OLEDs joins a small but growing number of books on the subject, including a 2006 publication from Wiley-VCH entitled “Organic Light Emitting Devices: Synthesis, Properties and Applications” (2), which was reviewed in this Journal (3). The new book reviewed here concentrates on recent advances in the materials characterisation and development aspects of OLEDs. There is a strong emphasis on the special role played by the platinum group metal (pgm) complexes as dopants to improve critical light-emitting characteristics; see Figure 2 for examples. It is revealing that of the thirty-three contributors, over a quarter are from China, with the rest spread evenly between Japan, the U.S.A., Germany, The Netherlands and Switzerland; there are no U.K. contributors.


(a)

(b)

Fig. 2 Examples of structures of iridium electrophosphorescent dopant complexes: (a) fac-tris(phenylpyridine) iridium, Ir(ppy)3; (b) bis(2-(2'-benzothienyl)-pyridinato-N,C)iridium(acetylacetonate), Ir(btp)2(acac)

The reader of the book is required to be familiar with basic concepts in photophysics and coordination chemistry. Although much of the work is clearly very specialised, it should prove accessible to those seeking an introduction to the field. Complexes of the pgms (platinum, palladium, iridium, rhodium, osmium and ruthenium are all represented) dominate eleven out of twelve chapters, with in-depth reviews of the design, synthesis, modelling and behaviour of a fascinatingly diverse range of molecules. For the important Ir complexes, ligands include (as their anions): acetylacetonate (acac), 2-benzo[b]thiophen-2-ylpyridine (btp), 1-phenylisoquinoline (1-piq) and 2-phenylpyridine (ppy). (See Figure 2(a) for ppy and Figure 2(b) for acac and btp.) There is much less emphasis on OLED device architectures and the practical side of converting the science into working technology, such as material processing and durability. The monograph is very well packaged, with a remarkably high standard of English and very few editing errors. It contains prolific, clear figures of molecular structures and performance data. Most of the chapters include a useful summary and conclusions at the end of the section, followed by

155

references. The indexing and overall layout are exemplary, as you would expect for the price! The occasional repetition of the basics in some of the introductions will probably be useful to newcomers, like myself, who read this in the hope of quickly learning about the field. Conversely, those already knowledgeable should find plenty of detail to engage with. Gripes are minor; one or two of the photographs would have benefited from colour, a list of acronyms would have helped, and perhaps a summary of the pros and cons of small-molecule versus polymer materials in OLED devices would have been useful. To conclude, this is an excellent book for physicists and chemists alike interested in the role of pgm complexes in OLEDs, and the challenge of building nanometre-thick multilayers of functional materials.


References 1

2

3

‘Sony bringing super-thin high-quality organic LED television to US this month’, International Herald Tribune, 7th January, 2008: http://www.iht.com/articles/ap/2008/01/07/business/NA-TEC-US-Sony-OLED-TV.php “Organic Light Emitting Devices: Synthesis, Properties and Applications”, eds. Klaus Müllen and Ullrich Scherf, Wiley-VCH, Weinheim, Germany, 2006 J. A. G. Williams, Platinum Metals Rev., 2007, 51, (2), 85

The Reviewer Rob Potter has worked in electrochemistry and photoelectrochemistry for over thirty years. Having joined Johnson Matthey, U.K., in 1986, he has spent much of his time since in research and development on fuel cells, electrochemical synthesis and, more recently, photovoltaics.

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DOI: 10.1595/147106708X322720

Practical New Strategies for Immobilising Ruthenium Alkylidene Complexes: Part II IMMOBILISATION VIA SCHIFF BASES, ARENES, AND ANIONIC AND TAGGED LIGANDS By Ileana Dragutan* and Valerian Dragutan** Institute of Organic Chemistry “Costin D. Nenitescu”, Romanian Academy, 202B Spl. Independentei, PO Box 35-108, 060023 Bucharest, Romania; E-mail: *[email protected]; **[email protected]

Following Part I, previously published in this Journal (1), the present paper discusses new routes for immobilisation of ruthenium alkylidene complexes through their ligands. Systematically addressed as suitable participants for immobilisation are Schiff bases, arenes, anionic ligands and specifically tagged (ionic liquid tag, fluoro tag) substituents.

1. Immobilisation via the Schiff Base Ligand Detailed studies by Verpoort et al. (2–5) have been directed towards the design, synthesis and progressive development of homogeneous and immobilised N,O-bidentate ruthenium complexes bearing Schiff base ancillary ligands, as an attractive alternative to N-heterocyclic carbenes (NHCs) (6–15) for applications in ring-closing metathesis (RCM), Kharasch addition, ring-opening metathesis polymerisation (ROMP), atom transfer radical polymerisation (ATRP) and vinylation reactions (16, 17). Structurally robust and effective supported catalysts have been devised (for example,

H Cll C Ru Ru CHPh O CHPh PCy33 PCy

Si Si O O

Si Si O N N

Cl Cl Ru O O CHPh C HPh PCy33 PCy

32

O

O O

H H

O N

32–34) in which the homogeneous Ru complex was anchored to the carrier by a non-labile tether, bound to the Schiff base ligand yet imposing little or no steric hindrance at the reactive site (18–20) (Scheme I). (Structures 1–31 are given in Part I (1).) From all potential inorganic supports, the mesoporous silica gel MCM-41 (mobile crystalline material) was selected as most appropriate because of its advantages (21–24): (i) retention of a constant exposed surface area, in contrast to conventional polymer beads that typically swell and shrink variably in different media, resulting in unpredictable effects on the catalyst activity; (ii) the greater robustness of MCM-41 than organic

N Cl Cl Ru Ru

Scheme I Immobilised Schiff base ruthenium complexes 32–34

O

33

O Si Si O O O

34


157

polymers and inorganic solids and, particularly for types with a structured surface, a considerably larger area; (iii) anchoring the active catalytic species on a larger surface area would help to overcome the activity loss (due to an inefficient interfacial mass transfer between the liquid phase and the solid), currently encountered in passing from homogeneous to heterogeneous catalysis; (iv) the MCM-41 solid support consists of an ordered array of hexagonal channels with the pore diameter in the mesoporous region, allowing lower resistance to diffusion of the reactant molecules accessing the metal active sites that are located within the channels (vs. nanoporous zeolite supports). The methodology followed in preparing the MCM-41 supported catalyst systems 32–34 implies

H

H

OEt N

O

the prior synthesis of the immobilised Ru precursors 35–37 endowed with an anchorable tris(alkoxy)silyl functionality, followed by tethering onto the mesoporous silica surface (Schemes II–IV). Chemical tethering, as employed for 32–34, is one of the best strategies for anchoring a homogeneous catalyst to a solid support, in view of its minimising leaching behaviour. Structural measurements on the above immobilised complexes carried out by X-ray diffraction (XRD), N2-adsorption analysis, Raman spectroscopy, X-ray fluorescence (XRF) spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy evidenced that in all cases, the homogeneous catalyst is anchored to MCM-41 via a spacer having two or three covalent bonds.

CHPh Cl PCy3

THF, THF, 40ºC, 40 oC,2424h h

OEt

Ru

N

MCM-41

Si OEt

O

O

35

Ru

Si O CHPh

Cl PCy3

O

32

Scheme II Synthesis of immobilised ruthenium complex 32

OEt

H

Si O

Si OEt N

O

O

H N

OEt

Ru CHPh Cl PCy3

MCM-41

THF, 2424h h THF,40ºC, 40 o C,

O

36

Ru CHPh Cl PCy 3

O

33

Scheme III Synthesis of immobilised ruthenium complex 33

Cl O

OEt

O

OEt3 Si Si(OEt)

Si O

OEt

N

MCM-41

MCM-41

Ru

THF, 24 h, RT

THF, RT, 24 h 37

Cl O

O

N

Ru Ru

34

Scheme IV Synthesis of immobilised ruthenium complex 34


158

2. Immobilisation via the Arene Ligand

support. Using this method, they replaced the chloride ligand from the first-generation Grubbs catalyst Cl2(PCy3)2Ru(=CHPh), 2, with a carboxylate group attached to a polystyrene (PS) resin via a strongly electron-withdrawing tether, finally gaining access to the immobilised Ru complex 39 (Scheme VI).

Arene ligands coordinated to Ru have also been employed to immobilise homogeneous complexes onto solid supports. This approach has been used by Akiyama and Kobayashi (25) to prepare the polystyrene-bound Ru-allenylidene complex 38 (Scheme V). Surprisingly, catalyst 38 showed wideranging activity, for example in RCM, hydrogenation and cyclisation reactions of olefins.

Cl Cl Cl Cl

3. Immobilisation via Anionic Ligands

H H

PR PR33

2

Complex 39 was very active and versatile in selfmetathesis of internal olefins (trans-decene and methyl oleate) and RCM of α,ω-dienes (diethyl diallylmalonate); it could easily be separated from the reaction products, resulting in virtual freedom from Ru contamination. Additionally, catalyst 39 could be recycled for at least six reaction cycles and

CO2 Et Cl Ru Cl PPh3

nn Ph

Ph

Ph Ph

Ru

R = Cy

A totally different protocol for immobilisation of Ru complexes has been devised by Mol et al. (26). Taking advantage of the capacity of anionic ligands to create strong bonds with Ru which remain intact throughout the entire catalytic cycle, the authors used such ligands to generate a permanent covalent link between the Ru centre and the

nn

PR PR33

OH Ph Ph

nn Ph PCy 3 Ru | PF | 6 Cl C

PCy3 , NaPF6 Cl

Ru

Cl

C

PPh3

C

38

Ph

Ph

Scheme V Synthesis of immobilised ruthenium arene complex 38 O OH

PS Resin

+

F F

O

O

F F

F

F

(a)a.THF THF

(CF2) 3

O

1.5 1.5 hh

O

OH O

(b) b.(Me (Me3Si) THF, 2NNa, 3Si) 2 NNa, THF 2 2h h c.AgNO 12 h12 h (c) AgNO 3,THF/EtOH, 3, THF/EtOH, (CF2)3

O O

OAg O

(d) Catalyst 2 d. 2(0.5 (0.5equiv.) equiv.) THF/Hexane, THF/Hex 15 15 h h

(CF2)3

O O

O

PCy3

Ru CHPh O Cl PCy3

39

Scheme VI Synthesis of immobilised ruthenium benzylidene complex 39 ((Me3Si)2NNa = sodium bis(trimethylsilyl)amide)


159

be stored for more than six months, under nitrogen, retaining its initial activity intact. Functionalised, partially fluorinated Hoveyda and Grubbs-Hoveyda catalysts supported on silica gel (SG 60), were obtained by Blechert et al. (27) from their homogeneous counterparts; these catalysts performed conventional RCM with activity comparable to that of similar heterogeneous systems (28–31).

4. Tagged Ruthenium Alkylidene Complexes A fairly recent technique for non-covalent immobilisation of homogeneous Ru metathesis catalysts onto liquid supports focuses on the advantages provided by room temperature ionic liquids (RTILs). Applied first in reactions involving RTILs merely as reaction media, in particular in earlier work by Dixneuf et al. using allenylideneRu precatalysts (32–34), the technique was further extended to ionic liquid-tagged catalyst precursors. Thus, several NHC-Ru complexes, in particular the IL-tagged counterparts of the Hoveyda or Hoveyda-Grubbs catalysts, such as 40 (35), 41 (36, 37) and 42 (38) or 43 (39), have been used with improved results in various metathesis reactions

Cl

PCy PCy33 ClCl Ru

|

PF6

N Me

|N

conducted in ILs or IL/organic solvent mixtures (biphasic catalysis) (Scheme VII). Complexes 40–43 demonstrated convenient recyclability, combined with high reactivity and extremely low residual Ru levels in the products. Two new Hoveyda-type catalysts, containing an IL-tag linked either to the ortho-oxygen substituent (44) or to the meta-position (45) of the styrenylidene ligand have recently been reported (40). In catalyst 44, the IL-tag is innovatively attached through the Ru-chelated oxygen atom. The catalysts were evaluated for RCM of N,N-diallyltosylamide and dimethyldiallylmalonate, conducted in an IL medium, showing moderate recyclability, yet good activity for the first cycle (Scheme VIII). As an alternative for tagging Ru complexes, the ‘light fluorous’ versions, 46 and 47, of the firstand second-generation Grubbs-Hoveyda metathesis catalysts have also been proposed (41) (Scheme IX). Catalysts 46 and 47 exhibit the expected reactivity profile, are readily recovered from reaction mixtures by fluorous solid-phase extraction, and can routinely be recycled five or more times. They can be used in a stand-alone fashion, or supported on fluorous silica gel.

Mes

H

N

Cl

O

N Cl Cl Mes Ru

|

PF 6

H O

40

Cl

41

PCy PCy33 Cl Cl Ru

Mes

|

PF6 O

N Me

|N

O N

|

N

Cl

N Cl Mes Cl Ru

|

PF6 O

O

N Me

42

N

|

N Me

43

Scheme VII Representative ionic liquid-tagged PCy3- and NHC-ruthenium complexes 40–43


160

Cl

PCy PCy33 Cl Ru

Cl

Scheme VIII Representative ionic liquid-tagged PCy3-ruthenium complexes 44 and 45

PCy PCy33 Cl Cl Ru

O

O

O O

N N Me N | Me N

Me Me

N N

PF6 | PF6

PCy PCy33 Cl Cl Cl

Me N | Me N Me Me PF6 | PF6

44

Mes

N

Ru

45

Scheme IX Fluoro-tagged first- and secondgeneration GrubbsHoveyda catalysts 46 and 47

N Cl Mes Cl Ru

Cl O

(CH2 )n C8F17

O

(CH2) 2C8 F17

46

47

5. Conclusion As described here and in Part I (1), immobilisation of well defined homogeneous Ru alkylidene complexes by means of their anionic and coordinative ligands is now a readily accessible, efficient technique, providing active catalysts for metathesis reactions. Immobilisation is achieved on a broad range of solid supports ranging from organic polymers (polystyrene, vinyl polystyrene) to inorganic matrices (silica, mesoporous silica gel, zeolites) and the fashionable ionic liquids. So far immobilisation through NHCs appears to be the most popular approach. Synthetic applications of immobilised catalysts in the metathesis field

provide important advantages, among which the most valued are a high catalytic activity and stereoselectivity, simpler, clean and recyclable processes, and low impurity content in the reaction products. Future scale-up to industrial exploitation of the immobilised Ru catalysts is to be envisaged, allowing promotion of sustainable technologies within the framework of ‘green’ chemistry protocols. An attractive aspect of immobilisation is that it holds great promise for elaborate syntheses of fine chemicals, pharmaceuticals, nutraceuticals and speciality polymers, where a very low residual metal content from the catalyst is a requirement.

References 1 I. Dragutan and V. Dragutan, Platinum Metals Rev., 2008, 52, (2), 71 2 R. Drozdzak, B. Allaert, N. Ledoux, I. Dragutan, V. Dragutan and F. Verpoort, Coord. Chem. Rev., 2005, 249, (24), 3055 3 B. Allaert, N. Dieltiens, C. Stevens, R. Drozdzak, I. Dragutan, V. Dragutan and F. Verpoort, in “Metathesis Chemistry: From Nanostructure Design to Synthesis of Advanced Materials”, eds. Y. Imamoglu and V. Dragutan, NATO Science Series II: Mathematics, Physics and Chemistry, Vol. 243, Springer Verlag, Berlin, Heidelberg, 2007, pp. 39–78 4 R. Drozdzak, N. Ledoux, B. Allaert, I. Dragutan, V.


5

6 7 8 9

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161

10 “N-Heterocyclic Carbenes in Synthesis”, ed. S. P. Nolan, Wiley-VCH, Weinheim, 2006 11 “N-Heterocyclic Carbenes in Transition Metal Catalysis”, ed. F. Glorius, Topics in Organometallic Chemistry, Vol. 21, Springer-Verlag, Berlin, 2007 12 S. Díez-González and S. P. Nolan, Coord. Chem. Rev., 2007, 251, (5–6), 874 13 “Metathesis Chemistry: From Nanostructure Design to Synthesis of Advanced Materials”, eds. Y. Imamoglu and V. Dragutan, NATO Science Series II: Mathematics, Physics and Chemistry, Vol. 243, Springer Verlag, Berlin, Heidelberg, 2007 14 V. Dragutan, I. Dragutan, L. Delaude and A. Demonceau, Coord. Chem. Rev., 2007, 251, (5–6), 765 15 I. Dragutan, V. Dragutan, L. Delaude, A. Demonceau and A. F. Noels, Rev. Roumaine Chim., 2007, 52, (11), 1013 16 V. Dragutan and I. Dragutan, J. Organomet. Chem., 2006, 691, (24–25), 5129 17 V. Dragutan and F. Verpoort, Rev. Roumaine Chim., 2007, 52, (8–9), 905 18 K. Melis, D. De Vos, P. Jacobs and F. Verpoort, J. Mol. Catal. A: Chem., 2001, 169, (1–2), 47 19 H. I. Beerens, W. Wang, L. Verdonck and F. Verpoort, J. Mol. Catal. A: Chem., 2002, 190, (1–2), 1 20 B. De Clercq, T. Opstal, K. Melis and F. Verpoort, in “Ring Opening Metathesis Polymerisation and Related Chemistry: State of the Art and Visions for the New Century”, eds. E. Khosravi and T. Szymanska-Buzar, NATO Science Series II: Mathematics, Physics and Chemistry, Vol. 56, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002, p. 451 21 “Heterogeneous Catalysis and Fine Chemicals IV”, eds. H. U. Blaser, A. Baiker and R. Prins, Studies in Surface Science and Catalysis, Vol. 108, Elsevier, Amsterdam, 1997 22 S. Ernst, R. Gläser and M. Selle, in “Progress in Zeolite and Microporous Materials, Proceedings of the 11th International Zeolite Conference”, eds. H. Chon, S. Kilhm and Y. S. Uh, Studies in Surface Science and Catalysis, Vol. 105, Part 2, Elsevier, Amsterdam, 1997, pp. 1021–1028

23 B. De Clercq, F. Lefebvre and F. Verpoort, New J. Chem., 2002, 26, (9), 1201 24 B. De Clercq, F. Lefebvre and F. Verpoort, Appl. Catal. A: Gen., 2003, 247, (2), 345 25 R. Akiyama and S. Kobayashi, Angew. Chem. Int. Ed., 2002, 41, (14), 2602 26 P. Nieczypor, W. Buchowicz, W. J. N. Meester, F. P. J. T. Rutjes and J. C. Mol, Tetrahedron Lett., 2001, 42, (40), 7103 27 K. Vehlow, S. Maechling, K. Köhler and S. Blechert, J. Organomet. Chem., 2006, 691, (24–25), 5267 28 D. Fischer and S. Blechert, Adv. Synth. Catal., 2005, 347, (10), 1329 29 L. Li and J.-l. Shi, Adv. Synth. Catal., 2005, 347, (14), 1745 30 X. Elias, R. Pleixats, M. Wong Chi Man and J. J. E. Moreau, Adv. Synth. Catal., 2006, 348, (6), 751 31 M. R. Buchmeiser, New J. Chem., 2004, 28, (5), 549 32 D. Sémeril, H. Olivier-Bourbigou, C. Bruneau and P. H. Dixneuf, Chem. Commun., 2002, (2), 146 33 S. Csihony, C. Fischmeister, C. Bruneau, I. T. Horváth and P. H. Dixneuf, New J. Chem., 2002, 26, (11), 1667 34 R. Castarlenas, C. Fischmeister, C. Bruneau and P. H. Dixneuf, J. Mol. Catal. A: Chem., 2004, 213, (1), 31 35 N. Audic, H. Clavier, M. Mauduit and J.-C. Guillemin, J. Am. Chem. Soc., 2003, 125, (31), 9248 36 H. Clavier, N. Audic, M. Mauduit and J.-C. Guillemin, Chem. Commun., 2004, (20), 2282 37 H. Clavier, N. Audic, J.-C. Guillemin and M. Mauduit, J. Organomet. Chem., 2005, 690, (15), 3585 38 Q. Yao and Y. Zhang, Angew. Chem. Int. Ed., 2003, 42, (29), 3395 39 Q. Yao and M. Sheets, J. Organomet. Chem., 2005, 690, (15), 3577 40 C. Thurier, C. Fischmeister, C. Bruneau, H. OlivierBourbigou and P. H. Dixneuf, J. Mol. Catal. A: Chem., 2007, 268, (1–2), 127 41 M. Matsugi and D. P. Curran, J. Org. Chem., 2005, 70, (5), 1636

The Authors Ileana Dragutan is a Senior Researcher at the Institute of Organic Chemistry “Costin D. Nenitescu” of the Romanian Academy. Her interests lie in the synthesis of stable organic radicals, EPR spin probe applications in organised systems and biological environments, late transition metal complexes with radical ligands, ruthenium catalysis in organic and polymer chemistry, iminocyclitols and prostaglandin-related prodrugs.


Valerian Dragutan is a Senior Researcher at the Institute of Organic Chemistry “Costin D. Nenitescu” of the Romanian Academy. His research interests are homogeneous catalysis by transition metals and Lewis acids; olefin metathesis and ROMP of cycloolefins; bioactive organometallic compounds; and mechanisms and stereochemistry of reactions in organic and polymer chemistry. He is a member of several national and international chemical societies, and has contributed significant books, book chapters, patents and papers to the scientific literature.

162

DOI: 10.1595/147106708X324845

Novel Lipophilic Platinum(II) Compounds of Salicylate Derivatives RESEARCH, DEVELOPMENT AND LIPOSOMAL FORMULATION By Wei-Ping Liu*, Qing-Song Ye, Yao Yu, Xi-Zhu Chen and Shu-Qian Hou Platinum-Based Drug Lab, Kunming Institute of Precious Metals, Kunming, Yunnan 650021, P.R. China; *E-mail: [email protected]

Li-Guang Lou and Yong-Ping Yang Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, P.R. China

and Yi-Ming Wang and Qiang Su Tsinghua University, Department of Chemistry, Beijing 100084, P.R. China

A series of novel lipophilic platinum(II) compounds containing salicylate derivatives as the leaving group have been designed, synthesised and characterised. Most of the platinum compounds exhibit high solubility and have a partition coefficient suited to liposomal encapsulation. Some of the compounds are more pharmacologically active and/or less toxic than carboplatin and oxaliplatin. The liposomal formulation of the most promising compound has been successfully prepared with long stability and high encapsulation rate, showing great potential to be developed as a new tumour-target drug.

In developed countries, such as the U.S.A., Canada, Australia and European countries, about 25% of deaths are related to malignant diseases. Chemotherapy is a central component in the fight against cancer. It is based on various classes of compounds, among which platinum-based drugs are a unique class. cis-Diamminedichloroplatinum(II) (cisplatin), first approved for clinical use in 1978 in the U.S.A., is one of the most effective anticancer drugs currently available for the treatment of testicular, lung and bladder carcinomas. Driven by the impressive impact of cisplatin on cancer therapy, numerous analogues have been prepared and evaluated in a search for alternative active agents, leading to the discovery of another important Pt drug, carboplatin, cis-diammine(1,1cyclobutanedicarboxylato)platinum(II) in 1986. Today, carboplatin has become one of the most successful anticancer drugs after cisplatin and has received worldwide approval for treating ovarian and small lung cancers (1, 2). Carboplatin shows a spectrum of activity identical with that of cisplatin, but is much less nephrotoxic and emetic. However carboplatin is not effective in treating


cancer cells resistant to cisplatin, possibly due to the same diammine carrier, suggesting that crossresistance exists between the two Pt drugs (3, 4). Therefore, the search for new potent Pt complexes possessing high antitumour activity and lack of cross-resistance continues. The so-called ‘third generation’ Pt drug, oxaliplatin, (trans-1R,2Rcyclohexane-1,2-diamine)oxalatoplatinum(II), was approved in 1999 as the first line therapy for metastatic colorectal cancer in combination with 5-fluoroural. Oxaliplatin has also shown potency in many cancer cell lines, including some cells resistant to cisplatin and carboplatin (5). Further Pt-based drugs, nedaplatin, lobaplatin and eptaplatin, have gained regionally limited approval, respectively, in Japan, China and South Korea, for the treatment of certain kinds of cancers (6). Ptbased drugs currently in clinical use are shown in Figure 1. In the meantime, the rational design of Pt anticancer compounds with specific characteristics has led to the invention of the orally available drugs satraplatin (JM216) and picoplatin (AMD473, a sterically hindered complex), see

163

O H3N H3N

Cl

H3N

Cl

H3N

N HH33N

Pt Pt

NH22

O

C

NH NH22

O O

NH NH22 Pt Pt

O O

NH NH22

O O

O

O C C

O

Oxaliplatin Oxaliplatin

O

O O

O

NH22

O

C

CH CH22

Pt

CH CH33

Lobaplatin Lobaplatin

Nedaplatin Nedaplatin

O Pt

O Carboplatin Carboplatin

O O

O O

C

Pt

Pt

Cisplatin Cisplatin

N HH33N

O

O

NH22 Eptaplatin Eptaplatin

O C O

Fig. 1 Platinum-based drugs currently in clinical use

Figure 2. Satraplatin and picoplatin are able to circumvent some drug resistance. They have recently shown promising clinical activity, respectively, in hormone-refractory prostate cancer and in smallcell lung cancer, and are strongly anticipated to receive clinical approval (7, 8). The clinical use of Pt-based drugs is frequently limited by severe toxic side effects such as nephrotoxicity, neurotoxicity and meylosuppression, as well as drug resistance. One of the most intriguing strategies to overcome these drawbacks is to encapsulate the agent in a liposome (9). Some anticancer drugs such as doxorubicin have been approved in their liposomal formulations (doxil in the case of doxorubicin) for the treatment of AIDS-related Kaposi’s sarcoma (AIDS-KS) and relapsed ovarian cancer in the U.S.A. and Europe

H3C C

Fig. 2 Chemical structures of satraplatin (JM216) and picoplatin (AMD473)

O

O

H3 N

Cl

Cl

H3N Pt

Pt NH 2

(10). Several different liposomal formulations of cisplatin have also been prepared and biologically evaluated. Among them, SPI-77 and lipoplatin are currently in Phase I and II clinical trials (11–14). To date, none of the liposomal formulations of cisplatin have been approved for clinical use. The key reasons for this are the poor water solubility and low lipophilicity of cisplatin (other Pt anticancer drugs show similarly poor lipophilicity), which makes it difficult to efficiently encapsulate the drug in a liposome. An alternative approach is to synthesise lipophilic Pt complexes. NDDP (cis-bis-neodecanoato-trans-R,R-1,2-diaminocyclohexane platinum(II)) is an example of such a complex, and its liposomal formulation L-NDDP (aroplatin) has entered Phase II clinical trials (15, 16). Unfortunately, NDDP is intraliposomally

O

Cl

O C

CH3

Satraplatin JM216 (JM216)


N

Cl CH3

Picoplatin (AMD473)

164

unstable due to the presence of two monodentate carboxylate leaving groups (17, 18). Furthermore, in order to improve the liposolubility of NDDP, highly branched aliphatic carboxylate groups were used, greatly increasing the molecular weight and making passive diffusion through the cell membrane difficult. It is therefore important to identify lipophilic Pt complexes using chelating bidentate ligands of small molecular weight.

All the compounds can be synthesised as precipitates from aqueous solution by the general method shown in Scheme I, owing to their low water solubility. Potassium tetrachloroplatinate (K2PtCl4) was first converted to potassium tetraiodoplatinate (K2PtI4) in situ by reaction with potassium iodide (KI). K2PtI4 was then treated with ammine/diamine (‘A2’) to form diam(m)inediiodoplatinum(II) complexes, which were reacted with silver nitrate (AgNO3), giving rise to [PtA2(H2O)2](NO3)2. The addition of sodium salicylate derivatives (‘Na2X2’) to the solution of [PtA2(H2O)2](NO3)2 precipitated the target compounds. Purification was carried out by re-precipitation from an ethanol or acetone solution of the compound after adding water.

Design and Synthesis On the basis of the above findings, we designed a series of novel lipophilic platinum(II) compounds of salicylate derivatives (19) including 3,5-diiodosalicylate (DISA), 3-isopropyl-6-methylsalicylate (o-thymotate) and 3,5-diisopropylsalicylate (DIPSA) as the leaving groups. DISA is a food additive used as an iodine source and o-thymotate is derived from plants of the genus Thymus. Salicylate and its derivatives are important nonsteroidal anti-inflammatory agents. Their capacity to block metastasis of cancer cells by inhibiting synthesis of prostaglandin is well known, as well as their reduction of the ototoxic and nephrotoxic side effects caused by cisplatin (20). This is a further reason for selecting salicylate derivatives as leaving groups in the target Pt complexes. As for non-leaving groups, the diammines of cisplatin, oxaliplatin and eptaplatin were used. The design strategy in our research is to develop Pt complexes providing higher liposolubility and chemical stability, along with higher antitumour activities and lower systemic toxicity. The compounds we designed are illustrated in Figure 3.

HH33N HH33N N

O O

O O

Pt Pt

H H H H22 N N

22 R R 11 R R

O R R

Pt Pt N N H2 H H 2

O O

Characterisation and Lipophilicity The compounds were characterised by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy and positive ion fast atom bombardment mass spectrometry (FAB+-MS). The elemental analysis data for each compound were in good agreement with the empirical formula proposed. All the compounds developed [M + H]+ peaks in the FAB+-MS spectra, corresponding to their molecular weights. The binding of the salicylic acid derivatives to Pt(II) atoms as a bidentate ligand was confirmed by the shift of the C=O absorption frequency, νC=O, to lower values and the absence of νO–H absorption in the IR spectra in the resulting compounds. 1H NMR spectral peaks were compatible with the chemical

O O

2 2 R R 11 R R

O O R R

H H

H H22 N N

H H

N N H2 H 2

O O

O O

O O

Pt Pt

O O

2 2 R R 1 1 R R

O R R

Compounds 1–3: R = R1 = I, R2 = H Compounds 4–6: R = CH3, R1 = H, R2 = CH(CH3)2 Compounds 7–9: R = R1 = CH(CH3)2, R2 = H Fig. 3 Structures of platinum(II) complexes of salicylate derivatives


165

K2PtCl4

KI

K2PtI4

A2

cis-[Pt(II)A2I2]

AgNO3

H A 2NH33 , A22==2NH

,

H –O O

O O

X X22== –O O

N NH H22

cis-[Pt(II)A2(H2O)2]2+ H H

,

N H2 NH ,

O O O O

HH

Na2X2

cis-[Pt(II)A2X2]

NH NH 22 NH 22 NH

2 R R2 1 R R1

R R

Scheme I General procedure for the synthesis of the platinum(II) compounds of salicylate derivatives

structures given in Figure 1. In order to further explore the chemical structures, we attempted to prepare single crystals suitable for X-ray crystallography, but failed. So we resorted to the “Gaussian 03” computer software (21, 22) and constructed the chemical structures of two representative Pt compounds 6 and 8, as shown in Figure 4. The Pt(II) compound had the expected square planar geometry exhibiting the usual structural parameters. Pt-N1, Pt-N2, Pt1-O1, and Pt1-O2 distances were in the normal range, and bond angles of O2-Pt1-O1, N1-Pt1-N2 were also within the normal values for other diaminedicarboxylatoplatinum(II) complexes (23–25). The solubilities of the compounds 1–9 in both water and organic solvents such as ethanol, acetone and ether were determined. All the

N1-N2-O1-O2: 0.44º

compounds except for compounds 1 and 4 had low solubility in water but high solubility in the organic solvents (> 20 mg cm–3). Partition coefficients in an octanol/water system were measured for the lipophilic Pt compounds 2, 3, 5, 6, 7, 8 and 9. The partition coefficients and solubility in water are listed in Table I. The lipophilic complexes were stable in the organic solvents for five days at room temperature, as indicated by monitoring their ultraviolet (UV) spectra. Presumably this stability results from the chelation effect of the leaving groups. From Table I it appears that both the carrier and leaving group influenced the lipophilicity of the compounds. For the same leaving group the order of lipophilicity was: DACH > BAMID > NH3, and DIPSA > thymotate > DISA when the carrier is the same. (DACH = trans-1R,2R-

N1-N2-O1-O2: 0.56º

Fig. 4 The steric structures of platinum compounds 6 and 8


166

Table I

Partition Coefficients and Solubility of the Platinum Compounds Compound Solubility in water, – μg cm 3 Partition coefficient, log P

1

2

3

4

5

6

7

8

9

Cisplatin

Carboplatin

300

12

25

250

12

7.3

5.9

5.7

3.5

1000

17,500

–

3.3

3.1

–

3.4

4.3

4.1

4.4

4.3

–

–

diaminocyclohexane; BAMID = (4R,5R)4,5-bis-(aminomethyl)-2-ispropyl-1,3-dioxolane.)

Biological Evaluation The target Pt compounds 1–9 were assayed in vitro against several human cancer cell lines, including A549 (human lung carcinoma) and SGC-7901 (human gastric carcinoma). Cellular survival was evaluated by the MTT (3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide) method (26). The median inhibitory concentration (IC50) values were

calculated from plots of cell survival (%) versus compound concentration (in μM). Surprisingly, all the compounds were more active against A549 and SGC-7901 cell lines with lower IC50 values than the parent drugs carboplatin, oxaliplatin and SKI-2053R (cis-malonato(4,5-bis(aminomethyl)-2-isopropyl1,3-dioxolane)platinum(II)) (Table II). Among the lipophilic compounds 2, 3 and 5–9, compounds 6 and 8 were the most active. No clear structureactivity relationship could be established from in vitro activity.

Table II

In Vitro Cytotoxicity of the Platinum Compounds Against Selected Human Tumour Cell Lines Compounds

Carriers (non-leaving groups)

IC50, μM

Leaving groups A549

SGC-7901

1

2NH3

DISA

1.54 ± 0.08

2.65 ± 0.16

2

DACH

DISA

1.05 ± 0.05

2.93 ± 0.13

3

BAMID

DISA

2.16 ± 0.11

2.93 ± 0.15

4

2NH3

thymotate

0.89 ± 0.03

1.83 ± 0.09

5

DACH

thymotate

1.49 ± 0.04

6.95 ± 0.62

6

BAMID

thymotate

1.27 ± 0.07

2.64 ± 0.06

7

2NH3

DIPSA

0.33 ± 0.03

7.63 ± 0.82

8

DACH

DIPSA

0.28 ± 0.03

1.09 ± 0.06

9

BAMID

DIPSA

1.80 ± 0.07

2.54 ± 0.34

Carboplatin

2NH3

CBDCA*

9.26 ± 0.25

16.34 ± 0.69

Oxaliplatin

DACH

oxalate

3.54 ± 0.18

7.77 ± 0.56

SKI-2053R

BAMID

malonate

3.56 ± 0.20

2.36 ± 0.07

*CBCDA = cyclobutane-1,1-dicarboxylate


167

Compounds 6 and 8 were therefore evaluated for their in vivo antitumour activity using conventional methods (27). Sarcoma S180 tumourbearing mice and NCI-H460 (human lung cancer) xenograft mice were established and used as the in vivo models, as described in previous studies (28, 29). The potency of the antitumour effects was measured in terms of the ratio of tumour weights for treated (T) and control (C) animal groups (T:C), expressed as a percentage. Tables III and IV show the antitumour activity of compound 6 in solid S180 tumour-bearing mice, and of compound 8 in mouse NCI-H460 xenograft following intraperitoneal (i.p.) administration. The results indicate that compound 6 exhibited in vivo activity comparable to carboplatin in treating animals with S180. However compound 6 was much more active in vitro than carboplatin, probably due to its different pharmacokinetic behaviour. Nevertheless

compound 8 was more effective on mouse NCIH460 xenograft than carboplatin and oxaliplatin. We also tested the preliminary toxicity of compound 8. A range of doses (mg kg–1) of the test compound were administered i.p. to healthy Institute of Cancer Research mice in volumes of 0.1 cm3 per 10 g body weight (n = 10, 12–22 g, in standard environmental conditions). After fourteen days, the median lethal dose (LD50) was calculated by the Bliss method (30). The data in Table V show the LD50 value to be 230 mg kg–1 (95% confidence limit 207 to 258 mg kg–1) by i.p. administration to ICR mice, much larger than that of carboplatin (150 mg kg–1) and oxaliplatin (19.8 mg kg–1), indicating that compound 8 was less toxic. Antitumour activity of the two compounds on other animal models is being assayed at the Shanghai Institute of Materia Medica.

Table III

Antitumour Activity of Platinum Compound 6 in Mice with S180 Group

Dose, mg kg–1

Treatment scheme

Number of mice

Tumour weight, g, x ± SD

T:C, %

Control

–

–

12

1.42 ± 0.25

–

Compound 6

30

i.p., d 1, 4

6

1.26 ± 0.34

83.4

60

Carboplatin

6

0.74 ± 0.51

53.1*

90

i.p., d 1

6

0.96 ± 0.29

67.6*

60

i.p., d 1, 4

8

0.62 ± 0.29

43.7*

Note: The compound was dissolved in arachis oil before administration; SD = standard deviation; d = day(s);*P < 0.01 vs. control

Table IV

Antitumour Activity of Platinum Compound 8 in Mouse NCI-H460 Xenograft Group

Dose, mg kg–1

Scheme

Number of mice

RTV, x ± SD

T:C, %

Control

–

–

12

18.5 ± 5.4

–

Compound 8

15

i.p., d 0, 4, 8

6

11.5 ± 3.8

62.2

6

5.7 ± 2.0

38.8*

30 60

i.p., d 0

6

7.1 ± 1.9

30.4*

Carboplatin

60

i.p., d 0, 4, 8

8

8.5 ± 4.1

45.9*

Oxaliplatin

9

i.p., d 0, 4, 8

6

11.2 ± 7.3

60.5*

Note: The compound was dissolved in arachis oil before administration; *P < 0.01 vs. control; RTV = relative tumour volume


168

Table V

Acute Toxicity of Platinum Compound 8 Following Intraperitoneal Administration in Mice Dose, mg kg–1

Group

Number of mice

Death number

1

197.0

10

2

2

226.0

10

6

3

260.0

10

7

4

299.0

10

7

5

344.0

10

10

Liposomal Platinum Compound A liposomal formulation of compound 8 has been successfully prepared in our laboratory by an evaporation-lypophilisation method. The compound, lipoid Emblica officinalis (Indian gooseberry) polyphenol fraction (EOP) and cholesterol were mixed and dissolved in chloroform. After removal of chloroform at 37ºC in a rotary evaporator, tertbutanol was added to form a clear solution. The solution was freeze-dried, yielding lyophilised preliposomal powder from which the final liposomal Pt compound can be obtained by reconstitution in aqueous solution. The liposomal entrapment efficiency (EE) was greatly influenced by pH, as shown in Table VI. EE exceeded 95% when pH was below 4.0, indicating good compatibility between compound 8 and the lipids used. The optimal pH was 3.4 to 4, since the compound would undergo dissociation under more acid conditions. The average size of the liposome reconstituted in saline varied from 100 to 300 nm with a distribution index of 0.1 to 0.2 (Figure 5). As observed by transmission electron microscopy (TEM), the particles were of elliptical or ellipsoidal form, containing about 5% of Pt compound (Figure 6). The liposomal formulation of compound 8 so prepared was determined by high-performance liquid chromatography

LD50, mg kg–1

230.9

(HPLC) to be stable for ninety days when kept at 4ºC in a sealed, nitrogen-filled container.

Conclusion The three principal Pt drugs, cisplatin, carboplatin and oxaliplatin, along with other Pt drugs including nedaplatin, lobaplatin and eptaplatin, continue to have a major role in contemporary medical oncology. Should other Pt drugs such as satraplatin and picoplatin receive approval for clinical use, they would further broaden the applicability of Pt compounds to prostate cancer and small-cell lung cancer. However, reducing toxicity and increasing activity are still the most important goals for Pt drug development. These will be achieved only by targeting tumours or tumour cells either via liposomal formulations or with new tumour-specific Pt compounds. Therefore an effective liposomal formulation affording antitumour activity must be developed, while preserving the chemical stability of Pt compounds within the liposomes up to the point of administration to cancer patients. Lipophilic Pt complexes with chelating bidentate ligands as the leaving group are required for such a liposomal formulation. Our studies show that the Pt(II) compounds with salicylate derivatives as the leaving group are

Table VI

Effect of pH on Liposomal Entrapment Efficiency (EE) pH EE, %


2.6–3.0

3.5

4.0

4.5

5.0

6.0–7.0

99

95–97

93–95

90

85

80

169

Proportion of sample of given diameter, %

Fig. 5 The size distribution of the liposomal formulation of platinum compound 8

40

20

5

10

50 100 Diameter, nm

500 1000

distribution are being carried out in our laboratories. Of course, there are many hurdles to be overcome and much research to be done before clinical testing can begin.

Acknowledgements We are grateful to the Yunnan Provincial Government for financial support for this research and development (No. 2004KFZX-17, 2006C0070M). 100 nm Fig. 6 Transmission electron microscope image of the liposome

lipophilic with partition coefficients of 3 to 4, and are stable as a result of the chelation effect of the leaving groups. Among them, Pt compound 8 shows greater antitumour activity and less toxicity than carboplatin and oxaliplatin. Its liposomal formulation has the advantages of high liposomal entrapment efficiency, high drug content and long stability, showing great potential for further development as a novel liposomal Pt drug which will directly target tumours. Further biological evaluation for compound 8 and its liposomal formulation including antitumour activity, toxicities and drug


References 1 E. Wong and C. M. Giandomenico, Chem. Rev., 1999, 99, (9), 2451 2 Z. Guo and P. J. Sadler, Angew. Chem. Int. Ed., 1999, 38, (11), 1512 3 Y.-P. Ho, S. C. F. Au-Yeung and K. K. W. To, Med. Res. Rev., 2003, 23, (5), 633 4 M. A. Jakupec, M. Galanski and B. K. Keppler, ‘Tumour-Inhibiting Platinum Complexes – State of the Art and Future Perspectives’, in “Reviews of Physiology, Biochemistry and Pharmacology”, eds. S. G. Amara, E. Bamberg, M. P. Blaustein, H. Grunicke, R. Jahn, W. J. Lederer, A. Miyajima, H. Murer, S. Offermanns, N. Pfanner, G. Schultz and M. Schweiger, Springer, Berlin, Heidelberg, 2003, Vol. 146, pp. 1–53 5 S. van Zutphen and J. Reedijk, Coord. Chem. Rev., 2005, 249, (24), 2845 6 G. Momekov, A. Bakalova and M. Karaivanova,

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Curr. Med. Chem., 2005, 12, (19), 2177 7 L. Kelland, Expert Opin. Investig. Drugs, 2007, 16, (7), 1009 8 L. Kelland, Nature Rev. Cancer, 2007, 7, (8), 573 9 R. Langer, Nature (London), 1998, 392, (6679, Suppl.), 5 10 D. W. Northfelt, B. J. Dezube, J. A. Thommes, B. J. Miller, M. A. Fischl, A. Friedman-Kien, L. D. Kaplan, C. Du Mond, R. D. Mamelok and D. H. Henry, J. Clin. Oncol., 1998, 16, (7), 2445 11 K. N. J. Burger, R. W. H. M. Staffhorst, H. C. de Vijlder, M. J. Velinova, P. H. Bomans, P. M. Frederik and B. de Kruijff, Nature Med., 2002, 8, (1), 81 12 E. S. Kim, C. Lu, F. R. Khuri, M. Tonda, B. S. Glisson, D. Liu, M. Jung, W. K. Hong and R. S. Herbst, Lung Cancer, 2001, 34, (3), 427 13 G. P. Stathopoulos, T. Boulikas, M. Vougiouka, G. Deliconstantinos, S. Rigatos, E. Darli, V. Viliotou and J. G. Stathopoulos, Oncol. Rep., 2005, 13, (4), 589 14 G. J. Veal, M. J. Griffin, E. Price, A. Parry, G. S. Dick, M. A. Little, S. M. Yule, B. Morland, E. J. Estlin, J. P. Hale, A. D. J. Pearson, H. Welbank and A. V. Boddy, Brit. J. Cancer, 2001, 84, (8), 1029 15 R. Perez-Soler, D. M. Shin, Z. H. Siddik, W. K. Murphy, M. Huber, S. J. Lee, A. R. Khokhar and W. K. Hong, Clin. Cancer Res., 1997, 3, (3), 373 16 C. Lu, R. Perez-Soler, B. Piperdi, G. L. Walsh, S. G. Swisher, W. R. Smythe, H. J. Shin, J. Y. Ro, L. Feng, M. Truong, A. Yalamanchili, G. LopezBerestein, W. K. Hong, A. R. Khokhar and D. M. Shin, J. Clin. Oncol., 2005, 23, (15), 3495

17 H. Insook, A. R. Khokhar and R. Perez-Soler, Cancer Chemother. Pharmacol., 1996, 39, (1–2), 17 18 D.-K. Kim, J. Gam, H.-T. Kim and K. H. Kim, Bioorg. Med. Chem. Lett., 1996, 6, (7), 771 19 Q.-S. Ye, L.-G. Lou, W.-P. Liu, Y. Yu, X.-Z. Chen, S.-Q. Hou, W.-Q. Gao and Y. Liu, Bioorg. Med. Chem. Lett., 2007, 17, (8), 2146 20 G. Li, S.-H. Sha, E. Zotova, J. Arezzo, T. Van De Water and J. Schacht, Lab. Invest., 2002, 82, (5), 585 21 H. J. Zhu, J. X. Jiang, S. Saebo and C. U. Pittman, Jr., J. Org. Chem., 2005, 70, (1), 261 22 “Gaussian 03”, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., Gaussian, Inc., Wallingford, Connecticut, U.S.A., 2004: http://www.gaussian.com/ 23 K. Meelich, M. Galanski, V. B. Arion and B. K. Keppler, Eur. J. Inorg. Chem., 2006, (12), 2476 24 M. Galanski, C. Baumgartner, V. Arion and B. K. Keppler, Eur. J. Inorg. Chem., 2003, (14), 2619 25 M. Galanski, W. Zimmermann, C. Baumgartner and B. K. Keppler, Eur. J. Inorg. Chem., 2001, (5), 1145 26 T. Mosmann, J. Immunol. Meth., 1983, 65, (1–2), 55 27 Y. Morinaga, Y. Suga, S. Ehara, K. Harada, Y. Nihei and M. Suzuki, Cancer Sci., 2003, 94, (2), 200 28 T. Tsubomura, S. Yano, K. Kobayashi, T. Sakurai and S. Yoshikawa, J. Chem. Soc., Chem. Commun., 1986, (6), 459 29 Y. Yu, L.-G. Lou, W.-P. Liu, H.-J. Zhu, Q.-S. Ye, X.-Z. Chen, W.-G. Gao and S.-Q. Hou, Eur. J. Med. Chem., in press 30 M.-J. Xie, W.-P. Liu, L. Li and Z.-H. Chen, Acta Chim. Sinica, 2002, 60, (5), 892

The Principal Author Professor Wei-Ping Liu was born in 1963 in the People’s Republic of China and received his Bachelor’s degree in Chemistry in 1983 from Wuhan University. He obtained his Master’s degree in the chemistry of the precious metals in 1986 at the Kunming Institute of Precious Metals, where he is currently a research professor and the Head of the Chemistry and Pharmacy Department. He specialises in research and development on platinum-based antitumour compounds.


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DOI: 10.1595/147106708X333818

CRC International Symposium: Cross Coupling and Organometallics Reviewed by Thomas Colacot Johnson Matthey, Catalysis and Chiral Technologies, West Deptford, New Jersey 08066, U.S.A.; E-mail: [email protected]

The third CRC International Symposium on Cross Coupling and Organometallics (1) was conducted on 25th September 2007 at the Université de Lyon, ESCPE Lyon, France; the institution where Victor Grignard was a Chemistry Professor. Although it was a one-day gathering, about 250 participants attended from all over the world. Out of the nine speakers, I was the only one from industry. In addition to the talks there were a poster session and vendor booths. The details of the meeting are summarised in this report. The opening address was given by Professor Tamotsu Takahashi who emphasised that CRC stands for Catalysis Research Center of Hokkaido University, Japan, which is the main sponsor of the Symposium. Professor Takahashi strongly believes that the area of cross-coupling will be recognised by the award of a Nobel Prize in the near future, as no other field of chemistry has grown like this one over the past two decades. He thanked agencies such as the Japan Interaction in Science and Technology Forum (JIST) and various fine chemicals companies for their sponsorship. The purpose of this review is to provide a background about the speakers and their original contributions to the area of coupling. In 1966 Professor E. Negishi joined H. C. Brown’s group to work on C–C bond forming reactions using organoboron compounds. From 1976 to 1978 he published about ten papers describing palladium- and nickel-catalysed crosscoupling reactions using magnesium, zinc, boron, aluminium and tin, while he was working at Syracuse University as an Associate Professor. Currently he occupies the chair of the H. C. Brown Distinguished Professor of Chemistry at Purdue University, U.S.A. Negishi is known for his work on coupling using organozinc reagents;


these allow cross-coupling under milder conditions than is the case using organomagnesium reagents. During his talk Negishi praised Grignard for initiating a new era in organometallic chemistry relevant to organic synthesis. However, “Grignard left a big hole”, he said, which Negishi and others are trying to fill currently. The developments in this area are summarised in a recent review (2). Negishi has been very successful in using [1,1'-bis(diphenylphosphino)ferrocene]dichloropalladium (dppfPdCl2), bis(2-diphenylphosphinophenyl)ether dichloropalladium (DPEphosPdCl2) and bis(tri-tert-butylphosphine)palladium (Pd(tBu3P)2) catalysts for various organozinc-based transformations to make natural products such as xerulin, carotenes, scyphostatin, γ -bisabolenes, 6,7-dehydrostipiamide, O-methylmyxalamide D, amphotericin B, mycolactone A/B and epolactaene, by simple and efficient ways. He has also successfully completed asymmetric syntheses of various molecules. One of the recent examples from his group is the stereoselective synthesis of (+)-epolactaene, 1, a microbial metabolite isolated from fungal strain, Penicillium. Professor A. Suzuki spent his entire academic life at Hokkaido University, starting from 1954 as a B.S. student, except for his postdoctoral studies at Purdue with Professor H. C. Brown from 1963 to 1965. He is currently a Professor Emeritus at Hokkaido University. Of the various coupling reactions, Suzuki coupling has received the most attention both in industry and academia, with the greatest number of publications/patents since 1990. Part of the reason for this growth is due to the non-toxicity of readily available boron reagents such as boronic acid, pinacolboronate and potassium salts of trifluoroboronate for

172

O

O

HO

COOMe N H

O

1(+)-epolactaene (+)-Epolactaene

coupling reactions with both alkenyl- and arylbased electrophiles. These reactions can be carried out under both homogeneous and heterogeneous conditions in the presence or absence of water. The conditions are typically milder. Because of the wider applications of his work in industry, Suzuki should be the first name to be considered for a Nobel Prize in the area of cross-coupling. In his talk, Suzuki highlighted the developments in this area from 1979 onwards. Some of the important generations of catalysts which were mentioned in his talks were tetrakis(triphenylphosphine)palladium (Pd(Ph3P)4), dppfPdCl2, Pd(tBu3P)2 and Buchwald ligands in conjunction with Pd. The electron-rich and bulky ligand-based catalysts were responsible for coupling aryl chlorides (3) and hindered substrates (4). Suzuki also touched upon recent progress on alkyl halide coupling, a very challenging area due to the propensity of alkyl substrates to undergo β-hydrogen elimination. Interestingly, Professor Suzuki

2

had published a paper in 1992 (5) on the coupling of n-hexyl iodide with n-octyl 9-borabicyclo[3.3.1]nonane (n-octyl-9-BBN) to produce tetradecane in 64% yield using the catalyst Pd(Ph3P)4. Fu’s reinvestigation of the reaction using n-dodecyl bromide yielded 93% product when the catalyst system was changed to palladium acetate/tricyclohexylphosphine (Pd(OAc)2/Cy3P) (6). Suzuki’s talk also pointed towards the Suzuki coupling step in Kishi’s synthesis of palytoxin, 2, an incredibly complex marine natural product containing 71 stereochemical elements (7). This is still considered by many to be the greatest synthetic accomplishment ever, due to its structural complexity. Professor K. Tamao has been the Director of RIKEN’s Advanced Science Institute since 2005. Earlier, he spent most of his career at Kyoto University. His lecture was dedicated to M. Kumada who died on 28th June 2007. Tamao started his lecture by stating that

Palytoxin, C129H223N3O54 (Molecular weight = 2678.6)


173

Ni-catalysed cross-coupling was reported independently by Corriu (8) from France and Kumada (9) from Japan in 1972. Applications of Ni chemistry were different from those of Pd chemistry even in the 1970s, in the sense that some Ni catalysts were useful for aryl chloride and aryl fluoride coupling. Although his lecture gave a brief account of what has happened in the area of Ni- and Pd-catalysed Kumada type coupling since 1972, Tamao’s major focus was on communicating some characteristic features of Ni-catalysed coupling, in addition to providing insights into the mechanistic understanding of Ni-catalysed coupling vs. Pd-catalysed coupling. Tamao also emphasised the common role of Lewis acids in activating aryl–fluoride and aryl–nitrogen bonds in cross-coupling reactions (10). Professor G. Fu is currently a full professor at the Massachusetts Institute of Technology (MIT), U.S.A. He worked with Professor Evans (Harvard University, U.S.A.) and Professor Grubbs (California Institute of Technology (Caltech), U.S.A.) for his Ph.D. and postdoctoral studies, respectively. Fu’s contributions are very significant in revitalising the area of coupling by establishing the importance of bulky electron ligands (such as t Bu3P) in aryl chloride coupling, although Koei from Japan has also earlier worked on the bulky tBu3P. In his lecture Fu described his latest work in one of the yet challenging areas in coupling namely, coupling reactions between two sp3-hybridised carbon centres (Csp3–Csp3 coupling). His recent contributions in developing Ni- and Pd-based methodologies for coupling unactivated primary and secondary alkyl electrophiles that bear β-hydrogen were discussed in detail. In his lecture he also suggested that Kochi’s radical mechanism might be more suited to explaining the Ni-catalysed coupling in comparison to the oxidative addition, transmetallation and reductive elimination steps in the Pd-catalysed coupling. His work on asymmetric Negishi coupling was also discussed in some detail. Professor T. Hiyama has been a full professor at Kyoto University since 1997. Prior to this he held various academic and industrial positions in Japan.


The Hiyama coupling involves the coupling of aryl, alkenyl or alkyl halides or pseudohalides with organosilanes. To drive silicon-based cross-coupling, formation of a pentacoordinated silicate intermediate is required in the catalytic cycle. Therefore this reaction requires an activating agent such as a base or fluoride ions, for example tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-nbutylammonium fluoride (TBAF). The reaction rate is typically increased by using silanes containing R groups such as fluoro or alkoxy instead of alkyl. In his talk Hiyama discussed stable and easy-to-handle tetraorganosilicon reagents with 2-hydroxymethylphenyl groups on Si as reagents to deliver organic groups to palladium, copper and rhodium during catalysis, Scheme I. In general, organosilanes are stable and easily prepared compounds, with lower toxicity than their Sn analogs. Hiyama’s talk also focused on some of his recent published results; see References (11–13). Professor P. Dixneuf is a Professor of Chemistry at the Université de Rennes, France. He has published over 350 papers in international journals including several reviews; also book chapters and patents. Unlike conventional cross-coupling processes, Dixneuf ’s focused on making C–C bonds using ruthenium-based organometallics. The use of Ru(II) complexes such as cyclopentadienyl 1,5-cyclooctadienyl ruthenium chloride (Cp*Ru(COD)Cl) in regio- and stereoselective oxidation was highlighted with several examples. His approach is to develop novel greener pathways to organic molecules in an atom economic way. The details of the work are already published; see References (14–16). Professor G. Balme has been a Research Director first class (DR1) at the Centre National de la Recherche Scientifique (CNRS), Université Claude Bernard Lyon 1, France, since 2005. Her talk was centred on ‘cyclo-functionalising’ unactivated C–C multiple bonds using Pd catalysts. Her work has practical applications such as one-pot synthesis of substituted furans,

174

Scheme I O

R-R' PdCl2 CuI catal. Ligand

Me2Si

RMgX/H2O

K2CO3 DMSO

R'-I

{ O

HO

Me Me

R

R

Si Me2

lactone lignans and cyclopropanes. Professor Balme has published several articles in this area. Professor O. Baudoin has been an Associate Professor of Chemistry at the Université Claude Bernard Lyon 1 and ESCPE Lyon, France, since 2006. In his talk Professor Baudoin discussed the Pd-catalysed C–H activation, also called direct arylation. This area has received much attention recently as it is one step ahead of the conventional C–C cross coupling chemistry such as that of Suzuki, Hiyama and Negishi. The other significant players in this area are Fagnou, Samford, Sames and Lautens. Some of the important publications in this area are References (17–19). Thomas J. Colacot has been working as a Research and Development Manager in Homogeneous Catalysis at Johnson Matthey Catalysis and Chiral Technologies (CCT) with

{

K

Si

global responsibilities since 2004, in addition to a part-time assignment as a visiting graduate professor at Rutgers, the State University of New Jersey, U.S.A. The work is mainly aimed at developing and commercialising Pd-based catalysts, as well as providing practical catalytic solutions to challenging coupling reactions in the fine chemicals and pharmaceuticals arena. In addition to developing new catalysts, research involves understanding the roles of the catalyst, substrate and reaction conditions. One of the air-stable, highly active and versatile catalysts that has been developed recently is [1,1'-bis(di-tert-butylphosphino)ferrocene]dichloropalladium (dtbpfPdCl2), 3. An example of a supported Pd catalyst is structure 4. A few typical publications in this are given in References (20–23).

Cl P

Ph

3

dtbpfPdCl2


P

Cl

Fe Fe Ph

Ph

Pd Pd

Ph Ph

4

Q-Phos based FibreCat

175

Conclusion Although work on coupling dates back to the 1970s, the last two decades have seen major breakthroughs in the area. These have been driven by applications in the fine chemical and

pharmaceutical industries for the construction of complex organic molecules. This area will grow continuously. Remaining challenges include Csp3–Csp3 coupling, asymmetric coupling and metal residues in the product.

References 1 CRC International Symposium: Cross Coupling and Organometallics, 25th September, 2007, Lyon, France: http://www.cpe.fr/crcsymposium/; Catalysis Research Centre, Hokkaido University: http://www.cat.hokudai.ac.jp/ 2 E. Negishi, Bull. Chem. Soc. Jpn., 2007, 80, (2), 233 3 A. F. Littke, C. Dai and G. C. Fu, J. Am. Chem. Soc., 2000, 122, (17), 4020 4 T. E. Barder, S. D. Walker, J. R. Martinelli and S. L. Buchwald, J. Am. Chem. Soc., 2005, 127, (13), 4685 5 T. Ishiyama, S. Abe, N. Miyaura and A. Suzuki, Chem. Lett., 1992, 21, (4), 691 6 M. R. Netherton, C. Dai, K. Neuschütz and G. C. Fu, J. Am. Chem. Soc., 2001, 123, (41), 10099 7 R. W. Armstrong, J.-M. Beau, S. H. Cheon, W. J. Christ, H. Fujioka, W.-H. Ham, L. D. Hawkins, H. Jin, S. H. Kang, Y. Kishi, M. J. Martinelli, W. W. McWhorter, Jr., M. Mizuno, M. Nakata, A. E. Stutz, F. X. Talamas, M. Taniguchi, J. A. Tino, K. Ueda, J. Uenishi, J. B. White and M. Yonaga, J. Am. Chem. Soc., 1989, 111, (19), 7525 8 R. J. P. Corriu and J. P. Masse, J. Chem. Soc., Chem. Commun., 1972, (3), 144a 9 K. Tamao, K. Sumitani and M. Kumada, J. Am. Chem. Soc., 1972, 94, (12), 4374 10 N. Yoshikai, H. Mashima and E. Nakamura, J. Am. Chem. Soc., 2005, 127, (51), 17978 11 Y. Nakao, Y. Hirata and T. Hiyama, J. Am. Chem. Soc., 2006, 128, (23), 7420

12 Y. Nakao, T. Yukawa, Y. Hirata, S. Oda, J. Satoh and T. Hiyama, J. Am. Chem. Soc., 2006, 128, (22), 7116 13 Y. Nakao, A. Yada, S. Ebata and T. Hiyama, J. Am. Chem. Soc., 2007, 129, (9), 2428 14 “Ruthenium Catalysts and Fine Chemistry”, eds. C. Bruneau and P. H. Dixneuf, Topics in Organometallic Chemistry, Vol. 11, Springer, Berlin, Heidelberg, 2004 15 C. Bruneau, S. Dérien and P. H. Dixneuf, in “Metal Catalyzed Cascade Reactions”, ed. T. J. J. Müller, Topics in Organometallic Chemistry, Vol. 19, Springer, Berlin, Heidelberg, 2006, pp. 295–326 16 C. Bruneau and P. H. Dixneuf, Angew. Chem. Int. Ed., 2006, 45, (14), 2176 17 K. Goldula and D. Sames, Science, 2006, 312, (5770), 67 18 D. Alberico, M. E. Scott and M. Lautens, Chem. Rev., 2007, 107, (1), 174 19 L.-C. Campeau and K. Fagnou, Chem. Commun., 2006, (12), 1253 20 G. A. Grasa and T. J. Colacot, Org. Lett., 2007, 9, (26), 5489 21 G. A. Grasa and T. J. Colacot, Org. Proc. Res. Devel., 2008, 12, (3), 522 22 T. J. Colacot and H. A. Shea, Org. Lett., 2004, 6, (21), 3731 23 Y. Wang and D. R. Sauer, Org. Lett., 2004, 6, (16), 2793

The Reviewer Thomas Colacot is a Research and Development Manager in the Catalysis and Chiral Technologies Division at Johnson Matthey, West Deptford, U.S.A. with global reports and responsibilities. He holds a Ph.D. in Chemistry and an MBA in Strategic Management. He is involved in the development of new organometallic compounds for catalysis, process development, supported homogeneous catalysts and high throughput screening of catalysts for organic reactions, such as C–C couplings and C–heteroatom couplings. He is also a visiting faculty member of Rutgers University in the Graduate School of Chemistry, where he teaches an applied organometallic chemistry course relevant to the fine chemicals and pharmaceutical industries. He has published over fifty papers, in addition to giving numerous lectures all over the world.


176

DOI: 10.1595/147106708X329273

Electrochemical Water Disinfection: A Short Review ELECTRODES USING PLATINUM GROUP METAL OXIDES By Alexander Kraft Gesimat GmbH, Köpenicker Str. 325, D-12555 Berlin, Germany; E-mail: [email protected]

Electrochemical water disinfection is a rarely used but convenient and highly efficient way to produce germ-free water. The technique works without the addition of chemical compounds to the water to be treated, but is nevertheless based on the biocidal action of various chemical substances. Electrodes with platinum group metals (pgms) or their oxides as active coatings are generally the best suited to electrochemical water disinfection. In special cases, novel doped diamond electrodes can be applied. A short historical and technical overview of the process is given, augmented by some application examples.

1. Background Electrochemical water disinfection can be defined as the eradication of microorganisms by using an electric current passed through the water under treatment by means of suitable electrodes. At the phase boundary between the electrodes and the water, the electric current leads to the electrochemical production of disinfecting species from the water itself (for example, ozone), or from species dissolved in the water (for example, chloride is oxidised to free chlorine). Attempts to clean or disinfect water by direct electrolysis had been reported as early as the nineteenth century (see, for example, Reference (1)). It has even been speculated that the electrical elements (the so-called ‘Baghdad battery’), which were discovered in 1936 in the ruins of a Parthian city (inhabited from about 300 BC to 300 AD) near Baghdad in Iraq, were in use for the electrochemical preparation of germ-free water (2). Since the end of the nineteenth century there have been frequent attempts to use electrochemical disinfection (for example, References (3–6)). Until recently none have been successful, at least not for long-term practical use. Different terms are or have been in use to describe this type of water treatment process or the water produced by this process, such as ‘electrolytic disinfection’, ‘electrochemical


disinfection’, ‘anodic oxidation’, ‘functional water’ and ‘electrochemically activated water’ among others. There are three reasons why electrochemical water disinfection has arrived at technical maturity only recently, rather than earlier in the (possibly) 2000 years since its discovery: (a) Sufficiently stable and efficient electrode materials for electrochemical water disinfection have been developed and optimised only in the last forty years. These are titanium electrodes with mixed oxide coatings based on iridium and/or ruthenium oxide (7–9), and doped diamond electrodes (10). (b)The functional interrelationships between chloride concentration in the water, current, current density, electrode material, water quality, electrochemical production of free chlorine and disinfecting action have been investigated in detail only recently (11–15). (c) Development work on electrochemical water disinfection has often been undertaken by amateurs in both electrochemistry and water chemistry, and this remains somewhat true today. Only a few electrochemists have been interested in this topic, mostly only for a short period in their career. This has resulted in mistakes in device dimensioning and in unscientific explanations of the mechanism of the process.

177

2. General Processes for Water Disinfection Conventional disinfection methods may be divided between chemical and physical processes. In chemical processes, disinfecting substances such as ozone, chlorine, sodium hypochlorite or chlorine dioxide are added to the water to be treated. These processes are reliable, and have proven their efficiency over many decades. They not only kill microorganisms, but also provide a disinfection reservoir which protects the water against recontamination for a certain time. A frequent drawback of the chemical processes is unwanted side reactions of the disinfectants with substances present in the water. These reactions lead to disinfection byproducts, some of which are considered dangerous. There are also hazards in producing, transporting and handling large amounts of such substances as chlorine and ozone. In physical disinfection processes the microorganisms are removed or killed by means of irradiation with ultraviolet or ionising radiation, heating to elevated temperatures, ultrasound, or separation through membrane filtration. The main drawback of the physical disinfection methods is the lack of a reservoir effect. These processes are only effective in the immediate surroundings of their operating devices. As compared with other chemical disinfection methods, the advantages of electrochemical water disinfection are obvious: no transport, storage and dosage of disinfectants are required. The disinfecting effect can be adjusted according to the on-site demand. Electrochemical water disinfection shows a reservoir effect and is often more cost-effective and requires less maintenance than other disinfection methods. Photovoltaic power supply makes it possible to use electrochemical water disinfection far from the electrical supply grid. This may be important for its application to drinking water in developing countries. Electrochemical water disinfection can also be used in conjunction with other disinfection methods. In electrochemical water disinfection, electrodes (at least one cathode and one anode) are inserted either directly into the volume of water to be disinfected, or into a bypass pipe. A DC voltage


is applied between the electrodes, leading to the electrolysis of the water. At the anode the main product is oxygen (Equation (i)): 2H2O → O2 + 4H+ + 4e–

(i)

accompanied by an acidification of the water in the vicinity of the anode. At the cathode, hydrogen is formed (Equation (ii)): 2H2O + 2e– → H2 + 2OH–

(ii)

and the water near the cathode becomes alkaline. Since the evolved hydrogen is generally unwanted, it must be separated from the water stream. Because only small amounts are formed at normal currents (about 0.4 litres of hydrogen is produced per amp-hour), this is possible without problems in most cases. In most practical applications, simple undivided electrochemical reactors employing parallel-plate, monopolar electrode stacks are inserted into the reactor pipe. The electrode plates may be configured as unperforated or perforated plates, or as expanded metal. Recently, an electrochemical disinfection process which completely avoids hydrogen production has been developed. Atmospheric oxygen is reduced to hydroxyl ions at a gas diffusion cathode (16) (Equation (iii)): O2 + 2H2O + 4e– → 4OH–

(iii)

Here the cathodic reaction (Equation (iii)) replaces the hydrogen producing reaction (Equation (ii)). The gas diffusion electrodes are composed of a porous graphite-polytetrafluoroethylene (PTFE) layer, in contact with a metal mesh as current collector, and backed by an oxygen-permeable PTFE layer to prevent water leakage. The graphite carries a manganese oxide catalyst which eliminates unwanted hydrogen peroxide.

3. Production of Free Chlorine from the Chloride Content of the Water If electrochemical disinfection is applied to drinking water, industrial water, seawater or other solute-containing water, its effect is mainly based on the electrochemical production of hypochlorite

178

In the nomenclature of water disinfection, the sum of hypochlorous acid and hypochlorite concentrations is usually termed ‘free chlorine’ or ‘active chlorine’. The disinfecting effect of free chlorine is based on the release of atomic oxygen according to Equations (vii) and (viii):

and/or hypochlorous acid from the chloride content of the water. The effectiveness of this method has always been accepted for water which contains higher concentrations of chloride ions (17), such as seawater with about 19 g l–1 chloride (18), or where large amounts of sodium chloride have been added, for instance to swimming pool water (chloride concentrations here are usually about 2–5 g l–1). For the disinfection of drinking water and other waters with much lower chloride content, the effectiveness of the method was not clear for a long time (19). It was eventually demonstrated that even at very low chloride concentrations (less than 100 mg l–1) sufficient free chlorine can be produced to efficiently disinfect water (11–15). The disinfectant hypochlorous acid/hypochlorite is produced at the anode in a side reaction to oxygen evolution. The following simplified reaction mechanism is proposed. First, chlorine is produced electrochemically from chloride ions dissolved in the water (Equation (iv)): 2Cl– → Cl2 + 2e–

(v)

Hypochlorous acid and the hypochlorite anion form a pH-dependent equilibrium (Equation (vi)):

(viii)

(vi)

10

125 IrO2

100

8 6

75

IrO2/RuO2 4

50 Doped-diamond

Pt

25

0

20

40

60 80 100 120 140 160 180 Chloride concentration, mg l–1


2

Current efficiency, %

Free chlorine production efficiency, mg Ah–1

ClO → O + Cl

–

During the disinfection, chloride ions which have been consumed by electrochemical free chlorine production are reformed. Thus there is no overall change in the chemical composition of the water during electrochemical water disinfection. Where there is a low chloride concentration in the water to be treated (as in drinking water) the current efficiency of the electrode material for the production of free chlorine is crucial; it should be as high as possible. Very great differences have been found in the efficiency of free chlorine production between different electrode materials at low chloride concentrations (11–13, 15). Figure 1 shows the dependence of the free chlorine production efficiency on chloride concentration for two dimensionally stable anode (DSA®) type electrodes (using active coatings of IrO2 or IrO2/RuO2), platinum and boron-doped diamond electrodes. It can be seen that DSA® type electrode materials clearly outperform diamond and Pt electrodes, which are therefore not generally applicable as anodes for water disinfection based on the electrochemical

Chlorine hydrolyses in water and hypochlorous acid (HClO) is formed (Equation (v)):

HClO ' ClO– + H+

(vii)

–

(iv)

Cl2 + H2O → HClO + HCl

HClO → O + Cl– + H+

Fig. 1 Dependence of the electrochemical free chlorine production efficiency on the chloride content of the electrolysed water under standard conditions using four different anode materials (iridium oxide, mixed iridium/ruthenium oxides, platinum, doped-diamond)

0

179

production of free chlorine. Diamond electrodes have another disadvantage in this application area. Because of their high overvoltage for both oxygen and chlorine evolution, they may further oxidise hypochlorite to chlorate and perchlorate (20). Only very low levels of these disinfection byproducts are permissible in potable water. Chlorate and perchlorate are not formed at DSA® type and Pt electrodes (11, 13). Earlier devices for electrochemical water disinfection employed anode materials such as carbon (1) or Pt (3, 4) with very low production efficiency for free chlorine. Another important aspect in choosing the appropriate electrode material is electrode lifetime. Although electrode lifetime has been improved where electrode polarity remains constant, frequent polarity change between anode and cathode is still problematic in this regard. Because of the formation of calcareous deposits at the cathode during electrolysis in water containing calcium and magnesium ions, polarity reversal is necessary to clean the cathode surface of these deposits at regular intervals (12, 13). The alkaline pH in the vicinity of the cathode (see Equation (ii)) leads to the precipitation of calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2). On reversing the polarity, the former cathode acts as an

0

1

anode and the scaling is removed due to the acidic anodic pH (see Equation (i)). But polarity reversal reduces electrode lifetime. This is especially true for IrO2 or mixed IrO2/RuO2 electrodes (12, 13). Figure 2 shows the results of a long-term experiment (still running) on the lifetime of electrodes of various materials under periodic reversal of polarity. A steep rise in cell voltage indicates the total stripping of the active coating at the end of the electrode lifetime. The shortest lifetime of the electrodes tested was observed for the RuO2-coated Ti electrodes, followed by IrO2-coated electrodes with a lifetime of about three months. Mixed IrO2/RuO2-coated electrodes had a lifetime of nearly one year under our experimental conditions. These materials are all clearly outperformed by platinised Ti electrodes, which are still running seemingly unaffected after nearly eight years. Other means for cathode cleaning have been investigated, such as ultrasonication (21), and continuously rotating brushes (22) or vanes (23) which move over the cathode surface. Another method is the imposition of a current pulse, which increases the formation of gas bubbles (24). None of these techniques have been successful in long-term operation, so that polarity reversal remains the best automatic cathode cleaning technique to date.

Electrolysis time, years 6

7

8

30

Cell voltage, V

25

IrO2

IrO2/RuO2

20

15

10

5

RuO2

0

100 200 300 400 500 600 700 2250 2500 Electrolysis time, days


Pt

Fig. 2 Cell voltage versus electrolysis time in Berlin tap water (conductivity about 0.8 mS cm–1) for titanium electrodes coated with different active electrode materials (ruthenium oxide, iridium oxide, mixed iridium/ruthenium oxides, platinum): polarity reversal every 30 minutes, current density 20 mA cm–2, electrode distance 4 mm

2750

180

An example of an application of electrochemical water disinfection based on free chlorine production is the ‘AQUADES-EL’ device. This is optimised for the cyclic operation of hot water recirculating systems in larger residential and commercial buildings. Potable water, especially in such systems, is especially prone to microbial contamination. This is due to high water temperatures which favour the growth of microorganisms such as Legionella. From time to time the media report on legionellosis outbreaks due to contaminated drinking water systems in hotels, hospitals etc. However, Legionella is only the most notorious genus of germs. Others are also dangerous, such as Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus, to name but a few. The AQUADES-EL system has been produced and distributed since 1998 by the German

companies AquaRotter GmbH and G.E.R.U.S mbH. More than 400 systems have been sold and installed in hotels, hospitals, barracks, retirement homes and other buildings. Figure 3 shows the AQUADES-EL device, which is usually located in a bypass to the water recirculation system. It employs Ti electrodes coated with mixed IrO2/RuO2. An advanced control system measures the free chlorine concentration in the water at several locations in the distribution system at regular time intervals by amperometric detection. In many industrial areas, large amounts of water are used for cooling purposes, as evidenced by the cooling towers dominating the landscape. Microbial contamination of cooling water, whether in cooling towers or air conditioning systems, currently poses a major problem. Large concentrations of disinfectant are usually added to Fig. 3 Electrochemical disinfection device AQUADES-EL (AquaRotter GmbH and G.E.R.U.S mbH, Germany) equipped with mixed iridium/ruthenium oxidecoated titanium electrodes: right-hand side: reactor pipe with inserted electrode stack; upper middle: power supply and control unit; upper left: amperometric free chlorine sensor equipment


181

the cooling water. The two commonest biocides for cooling towers are free chlorine and quartenary ammonium compounds. New biocides continue to be developed, but electrochemical water disinfection is a promising alternative for this application. Electrochemical disinfection has been successfully implemented in the cooling water recirculation system of a German paper mill (Figure 4). The ‘Hypocell® B4’ device from G.E.R.U.S mbH employs four parallel reactor pipes. The titanium electrodes carry a mixed IrO2/RuO2 coating. About 130 m3 of water is treated. The water has a typical pH of 8.3, a conductivity of 1.9 mS cm–1 and a chloride concentration of about 280 mg l–1. The flow rate in the treatment bypass is 1000 l h–1.

4. Ozone Production If water with low or zero chloride concentration is required, the addition of sodium chloride is not acceptable and free chlorine cannot be produced in situ. Disinfection must therefore be based on other electrogenerated species. By using anodes with a high oxygen overvoltage, a high current

®

Fig. 4 Hypocell B4 electrochemical disinfection device (G.E.R.U.S. mbH, Germany), with four pipe reactors equipped with mixed iridium/ruthenium oxide-coated titanium electrodes in a cooling water system, installed at a German paper mill


density and a low water temperature it is possible to produce ozone directly from the water according to Equation (ix): 3H2O → O3 + 6e– + 6H+

(ix)

Electrochemical ozone production has been known since the nineteenth century (25). Electrolysis was the first production method for ozone (25), but for most applications ozone is now produced by corona discharge. The disadvantages of electrolytic production include too low a current efficiency, complicated production systems, unstable electrode materials (such as lead oxide (PbO2) anodes) and/or difficult-to-handle electrolytes. Electrolytic production may become more attractive using a new simple electrode assembly (26) of a ‘sandwich’ configuration: diamond anode/solid polymer electrolyte (SPE)/cathode sandwich. By using such a diamond-SPE sandwich or similar device in deionised water, ozone can be produced with a high current efficiency (up to 47%) (27, 28). These devices can easily be inserted into water pipes or reservoirs to produce the required quantity of ozone directly from the water to be treated. The Nafion® ion exchange membrane may be employed as the SPE material. If the doped diamond anode is replaced by a Pt- or IrO2-coated anode, a much lower ozone production rate is measured. On the other hand, if Pt is used as the cathode material, the overvoltage for hydrogen production can be lowered, minimising the cell voltage. For the electrochemical disinfection of ultrapure water, or other waters with very low conductivity (such as rain water), electrochemical ozone production with diamond-SPE sandwich electrodes is the method of choice. Figure 5 shows a small stack of two diamond electrode/SPE/diamond electrode sandwiches. With this technology, water volume rates from 60 l h–1 up to 5 m3 h–1 can be treated using different sized electrode stacks. In recent years several pilot projects have been equipped with this technology. An improved ozone generating device is in development. This produces no hydrogen, and the cathode consumes oxygen.

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Fig. 5 Electrode stack with two diamond-SPE-electrode sandwiches for ozone production in deionised water (Gerozon device by G.E.R.U.S. mbH, Germany)

5. Disinfection or Germ Minimisation by Electrochemically Produced Oxygen In some applications, electrolytically produced oxygen, the main anodic reaction product, shows some germicidal activity. This is especially true if anaerobic bacteria are the disinfection target. An example of this type of application is the wash water cycle of car wash stations. Here, the formation of anaerobic digestion products often leads to bad odours. Anaerobic conditions are eliminated via the fine dispersion of bubbles of electrolytically produced oxygen in the water. This is a highly effective mode of dissolution. For this application, Pt-coated electrodes are the most suitable anodes, because the main germicidal effect is based on electrolytically produced oxygen and not on free chlorine. Figure 6 shows the Hypocell® B4 device for germ minimisation in car wash stations, based on this technology. About twenty-five systems have been successfully installed.

6. Disinfection by Cathodically Produced Hydrogen Peroxide

7. Conclusion

While most of the possible disinfectants in electrochemical water treatment are produced at the anode, hydrogen peroxide may also be produced at the cathode. This process has been used by Dhar et al. (29) and Drogui et al. (30, 31) for water disinfection (Equation (x)): O2 + 2H2O + 2e– → H2O2 + 2OH–

(x)

Oxygen dissolved in the water may serve as the reactant in Equation (x). The maximum


concentration of oxygen in water which is in equilibrium with air at 25ºC is about 10 mg l–1 (0.3 mmol l–1). The oxygen produced by the anodic half reaction according to Equation (i) can also be used for the cathodic production of hydrogen peroxide. In this case, higher concentrations of dissolved oxygen are possible, because the water is in contact with pure oxygen, and not merely with air. It is also possible to use a gas diffusion cathode on which the oxygen from the surrounding air is reduced to H2O2. In terms of energy efficiency, the electrode material best suited to H2O2 production is graphite. This material (without additional catalysts) is also the core component in gas diffusion electrodes for H2O2 production. Because of its lower oxidation potential, H2O2 is a less effective disinfectant than free chlorine or ozone. Therefore, higher concentrations and/or longer disinfection times are necessary, limiting its applicability. Hydrogen peroxide has the advantage that its disinfectant action produces neither byproducts nor residues.

Electrochemical water disinfection has many advantages compared with conventional disinfection technologies. It has proven its reliability in several practical applications, mainly for the disinfection of drinking water, swimming pool water and industrial cooling water. Electrochemical water disinfection has also been used or tested for the reduction of bacterial contamination in dental water supplies (32), and for the disinfection of contact lenses (33) and ion exchange

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resins (34) etc. However, only a few electrochemical water disinfection products are currently available on the market. This is due to the relative unfamiliarity of the technology, and to fierce market competition with other technologies. Eventually, the cost and performance advantages of electrochemical technology should lead to its wider use. RuO2 and/or IrO2-coated electrodes are the best suited to disinfection based on hypochlorite generation. This is due to their high production efficiency for hypochlorite from water with a very low chloride content. Pt is the favoured electrode material for oxygen production from natural waters. Pt electrodes are also the most stable. For the production of ozone and hydrogen peroxide, pgm electrodes are not the first choice, being outperformed by carbon electrodes, i.e. doped diamond for ozone, and graphite for hydrogen peroxide production. ®

Fig. 6 Hypocell B4 electrochemical disinfection device (G.E.R.U.S. mbH, Germany) equipped with platinised titanium electrodes, mounted in a car wash station

This article is a translated and revised version of Reference (35).

References 1 J. Chisholm, British Patent 1,499; 1858 2 W. König, “Neun Jahre Irak”, Rudolf M. Rohrer Verlag, Brünn, München, Wien, 1940, p. 167 3 C. F. Burgess, U.S. Patent 1,200,165; 1916 4 P. M. R. Salles, British Patent 271,721; 1927 5 W. Juda and W. A. McRae, Ionics, Inc., U.S. Patent 2,752,306; 1956 6 A. Reis, N. Kirmaier and M. Schöberl, Institut für Biomedizinische Technik, U.S. Patent 4,188,278; 1980 7 P. C. S. Hayfield, Platinum Metals Rev., 1998, 42, (1), 27 8 P. C. S. Hayfield, Platinum Metals Rev., 1998, 42, (2), 46 9 P. C. S. Hayfield, Platinum Metals Rev., 1998, 42, (3), 116 10 A. Kraft, Int. J. Electrochem. Sci., 2007, 2, (5), 355 11 A. Kraft, M. Stadelmann, M. Blaschke, D. Kreysig, B. Sandt, F. Schröder and J. Rennau, J. Appl. Electrochem., 1999, 29, (7), 859 12 A. Kraft, M. Blaschke, D. Kreysig, B. Sandt, F. Schröder and J. Rennau, J. Appl. Electrochem., 1999, 29, (8), 895 13 A. Kraft, M. Wünsche, M. Stadelmann and M. Blaschke, Recent Res. Devel. Electrochem., 2003, 6, 27


14 N. Nakajima, T. Nakano, F. Harada, H. Taniguchi, I. Yokoyama, J. Hirose, E. Daikoku and K. Sano, J. Microbiol. Methods, 2004, 57, (2), 163 15 M. E. H. Bergmann and A. S. Koparal, J. Appl. Electrochem., 2005, 35, (12), 1321 16 M. Wünsche, A. Kraft, W. Kirstein, M. Blaschke and H. Petzer, AquaRotter GmbH, European Patent 1,326,805; 2005 17 A. T. Kuhn and R. B. Lartey, Chem. Ing. Tech., 1975, 47, (4), 129 18 A. F. Adamson, B. G. Lever and W. F. Stones, J. Appl. Chem., 1963, 13, (11), 483 19 G. Patermarakis and E. Fountoukidis, Water Res., 1990, 24, (12), 1491 20 S. Palmas, A. M. Polcaro, A. Vacca, M. Mascia and F. Ferrara, J. Appl. Electrochem., 2007, 37, (11), 1357 21 A. Kraft, M. Blaschke and D. Kreysig, J. Appl. Electrochem., 2002, 32, (6), 597 22 M. Blaschke, A. Kraft, H. Petzer and M. Suchi, G.E.R.U.S. mbH, German Patent Appl. 19,836,431; 1999 23 M. A. Silveri, BioQuest, U.S. Patent 5,885,426; 1999 24 Y. Gao, T. Gao, B. Han, D. Zhang and J. Li, The National Engineering Research Center for Urban Pollution Control, European Patent 0,862,538; 1999 25 M. B. Rubin, Bull. Hist. Chem., 2001, 26, (1), 40

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26 M. Stadelmann, M. Blaschke, M. Wünsche, H. Petzer, A. Kraft, T. Matthee and M. Fryda, CONDIAS GmbH and G.E.R.U.S. mbH, European Patent Appl. 1,730,080; 2006 27 A. Kraft, M. Stadelmann, M. Wünsche and M. Blaschke, Electrochem. Commun., 2006, 8, (5), 883 28 K. Arihara, C. Terashima and A. Fujishima, J. Electrochem. Soc., 2007, 154, (4), E71 29 H. P. Dhar, J. O. Bockris and D. H. Lewis, J. Electrochem. Soc., 1981, 128, (1), 229 30 P. Drogui, S. Elmaleh, M. Rumeau, C. Bernard and A. Rambaud, J. Appl. Electrochem., 2001, 31, (8), 877

31 P. Drogui, S. Elmaleh, M. Rumeau, C. Bernard and A. Rambaud, Water Res., 2001, 35, (13), 3235 32 L. Jatzwauk and B. Reitemeier, Int. J. Hyg. Environ. Health, 2002, 204, (5–6), 303 33 G. R. Holland and B. E. Cloud, Allergan, Inc., U.S. Patent 5,129,999; 1992 34 D. Napper and L. Rubner-Petersen, Adept Technologies A/S, World Patent Appl. 02/22,506; 2002 35 A. Kraft, M. Wünsche, M. Stadelmann and M. Blaschke, Wasserwirtschaft Wassertechnik, 2006, (9), 36

The Author Alexander Kraft is co-founder and Managing Director of Gesimat GmbH, Berlin, Germany, a company which develops ‘smart’ switchable electrochromic glazing. Before starting Gesimat in 1998, he developed electrochemical water disinfection technologies and devices at G.E.R.U.S. mbH, Germany, from 1994. He continued to work for G.E.R.U.S. as a scientific adviser until 2006. Alexander Kraft received a Ph.D. in Physical Chemistry (semiconductor electrochemistry) from Humboldt University, Berlin, in 1994.


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DOI: 10.1595/147106708X333827

Processing of Iridium and Iridium Alloys METHODS FROM PURIFICATION TO FABRICATION By E. K. Ohriner Oak Ridge National Laboratory, Materials Science and Technology Division, PO Box 2008, Oak Ridge, TN 37831, U.S.A.; E-mail: [email protected]

Iridium and its alloys have been considered to be difficult to fabricate due to their high melting temperatures, limited ductility, sensitivity to impurity content and particular chemical properties. The variety of processing methods used for iridium and its alloys are reviewed, including purification, melting, forming, joining and powder metallurgy techniques. Also included are coating and forming by the methods of electroplating, chemical and physical vapour deposition and melt particle deposition.

Introduction The metal Ir has a unique combination of properties, including a high melting temperature, strength at high temperature, oxidation resistance and corrosion resistance, that are useful in a range of applications, particularly at elevated temperature. However Ir is also one of the more difficult materials to process into finished products, in many cases due to the same properties that recommend its use. This was highlighted by the review ‘A History of Iridium’ published twenty years ago in this Journal with the subtitle ‘Overcoming the Difficulties of Melting and Fabrication’ (1). The purpose of the present review is to summarise advances in the processing of Ir during the past two decades. In some cases processing methods have been refined and improved, while in others entirely new processing methods have been developed to serve new applications of Ir. The scope of this review is limited to the processing of pure Ir and Ir alloy materials in which Ir constitutes the majority of the composition. The processing methods that are reviewed include purification, melting, powder processing, forming, joining and coating.

Purification The purity of Ir has been shown to have important effects on the mechanical properties of both the nominally pure metal and its alloys (2, 3). Ir is separated from platinum group metal (pgm) concentrates and purified either by conventional chemical refining methods or by a solvent


extraction process (4). In the conventional methods Ir oxide is dissolved in aqua regia (a mixture of concentrated nitric and hydrochloric acids) and precipitated with ammonium chloride. After a series of dissolutions and precipitations the salt is heated in a hydrogen atmosphere to produce Ir sponge. In the solvent extraction method, a series of organic liquids are used to concentrate various pgms from aqueous solutions. The Ir can then be precipitated and heated in hydrogen to produce Ir sponge. Solvent extraction methods may offer cost and environmental benefits over conventional chemical precipitation methods. The purity of the sponge is sensitive to both the details of the processing and the starting materials. A method of chemical purification of Ir compounds following dissolution is reported to achieve removal of platinum, rhodium, ruthenium and palladium to levels below 5 parts per million by weight (ppm) (5). A method of scrap purification, as well as fabrication, uses electrodeposition of Ir from molten salt solution (6). Purification is effective in removing many base metals, but less so for some others including pgms. A pyrometallurgical method for purification of Ir scrap employs induction melting in a crucible of magnesium oxide with an air atmosphere, to remove both volatile impurities through vaporisation and oxideforming impurities as a vapour or as a slag (7). This is a relatively low-cost method, although some impurities, including iron, are not easily removed by it.

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Electron beam melting has been used for purification and is effective for removal of most impurities with the exception of refractory metal elements (8, 9). Recent work treated removal of a large number of impurity elements by electron beam melting (10). It showed that titanium, vanadium and zirconium were not removed by electron beam melting, due to negative deviations from ideal solution behaviour (10). In addition, several other elements were only partially removed even under optimal conditions of melt stirring. The impurity ratio, defined as the ratio by weight of the final impurity content to the initial impurity content, was measured for electron beam melting of Ir buttons under conditions producing Ir vaporisation of 5.6% by weight. Low ratios were observed for iron, aluminium and chromium of 0.004, 0.014 and 0.06, respectively. Intermediate ratios were obtained for the impurities platinum, silicon and carbon of 0.11, 0.2 and 0.3, respectively. The results were shown to be consistent with ideal mixing and vaporisation. The observed purification behaviour for the impurity elements Fe, Al, Cr and Si was explained by their low activity coefficients.

Melting Melting of Ir is performed by induction melting, electron beam melting and arc melting methods. Induction melting, frequently used for initial melting (11), is performed in air with zirconia or magnesia crucibles. Due to excessive volatilisation of the crucible material at the Ir melting temperature (5), the crucibles are not suitable for use in vacuum. Ceramic inclusions are avoided by the use of electron beam, plasma or arc melting with water-cooled copper crucibles. Plasma melting of Ir is performed at a moderate pressure of about 50 Pa and provides for some purification by vaporisation of impurities, although to a lesser extent than with electron beam melting. In some cases, the final melting operation prior to deformation processing (12) is electron beam melting (discussed above under purification). Button arc melting is used for relatively small quantities of Ir and Ir alloys, particularly those with alloying additions subject to vaporisation during melting.


Vacuum arc remelting (VAR) is applicable to larger-size melts, and provides ingots with little or no internal porosity (13). The VAR ingots have a relatively large grain size with some degree of directional solidification. Melting of 27 mm diameter electrodes to produce 63 mm diameter ingots of an Ir alloy is performed with a direct current (DC) of 3000 A at 31 V in a vacuum of about 13 mPa with a melt rate of about 3 kg min–1. Control of porosity is important even in the case of ingots to be forged or rolled, since porosity in the melted ingot may never be completely sealed even with extensive hot deformation, such as by hot extrusion (14).

Powder Metallurgy The powder metallurgy methods of pressing and sintering can be used to prepare billets for subsequent hot working to produce a fully dense product (15), but progress in producing a fully dense near-net shape by these means has been limited. In a study of powder metallurgy methods for pgms and their alloys, including pure Ir and Ir-Pt alloys, neither pure Ir nor Ir-30 wt.% Pt were amenable to pressing and sintering, nor to hot isostatic pressing (HIP) (16). The problems with Ir include its high melting temperature, which precludes standard gas atomisation methods, and contamination from can materials during HIP. An Ir-50 wt.% Pt alloy was processed to powder by a plasma rotating electrode process and brought to near theoretical density by HIP (16). Ir powder or sponge typically consists of agglomerates of Ir crystals, with crystal sizes of the order of 1 μm and agglomerates in the size range 10 μm to 150 μm. Sintering of Ir powder with subsequent hot pressing has been used for the fabrication of Ir crucibles (17). An Ir composite with 15% by weight of yttrium oxide, selected for the necessary combination of high melting temperature and electrical conductivity together with a low electron work function, was developed as an electrode material for plasma cutting (18). The material was prepared by pressing and sintering in hydrogen at 2273 K for 30 minutes to obtain a density of 94%. Ir-base alloys containing up to 15% alloy additions of niobium, titanium, zirconium or hafnium have

187

been consolidated on a laboratory scale from prealloyed powders using a pulse electric current sintering method (19). Homogeneity was improved over earlier work with elemental powders, and densities close to 98% were achieved. The use of elemental powders was also found unsuitable for preparing quaternary alloys of Ir because the desired microstructures could not be obtained in melted alloys (20). Powder metallurgy methods have been used to produce porous Ir for use as filters or in other applications. Conventional pressing and sintering methods have been used to produce a porous Ir metal filter which is bonded to Ir alloy components (21). The bonding and final sintering at 2173 K were performed in a vacuum furnace under an applied load of 22 N over an area of 0.5 cm2 using graphite tooling. A density of 47% of theoretical was achieved. Compression at room temperature subsequently increased the density to 67% in order to obtain the desired flow rates for the filter. Slurry casting has been investigated as a method for producing porous Ir components with controlled density and pore size (22). After removal of the wax binder the samples were presintered in argon for one hour at 1273 K to achieve a density of 33%. Sintering in vacuum for one hour at 1473 K and 1573 K resulted in densities of 35% and 46% respectively.

Deformation Processing Ir and its alloys can be processed using standard metalworking methods including forging, extrusion, rolling and drawing, but only with some difficulty (23). In general deformation is performed at elevated temperature in order to avoid crack formation and propagation (15). The deformation behaviour depends on impurity content, impurity distribution, microstructure and texture. Annealing at temperatures of 2273 K and higher was shown to result in homogenisation of some potentially deleterious impurities and to decrease the tendency for grain boundary fracture (24). In contrast, small additions of elements such as thorium and cerium have been shown to segregate to grain boundaries and to improve ductility (2, 25). The very coarse grain structure of Ir and Ir alloys


in cast ingot form makes the material particularly sensitive to cracking at relatively small tensile strains. Preheat temperatures for the initial deformation of cast Ir by extrusion, forging or rolling are 1500 K or higher (26). Initial working temperatures for Ir ingots can be as high as 2075 K (27). Hot extrusion minimises tensile stresses during the initial deformation processing. Canning in molybdenum for extrusion minimises cooling of the Ir during the extrusion process and permits preheat temperatures in the range of 1600 to 1700 K with an extrusion ratio of 6.4:1 (28). (The extrusion ratio is defined as the ratio of the container bore area to the total cross-sectional area of extrusion.) Hot rolling of Ir and its alloys following initial ingot breakdown is generally performed with preheat temperatures in the range 1100 to 1500 K. In order to minimise chill during rolling, covers of Mo have been used (29). In-process recrystallisation is also used to minimise cracking during rolling. Ir alloy sheet of about 0.5 mm thickness or less can be rolled to foil at room temperature with cold work levels of more than 80%. A schematic flow diagram of the production of the DOP-26 alloy sheet material from Ir powder through multiple melting and deformation processing steps is shown in Figure 1. (DOP-26 contains (by weight) 3000 ppm tungsten, 60 ppm thorium and 50 ppm aluminium.) Ir wire is normally produced from a melted Ir stock with deformation performed at elevated temperature. An enriched 191Ir isotope was forged square at about 1775 K and then rod rolled beginning at 1675 K preheat temperature (12). The final circular cross section of about 0.6 mm was achieved by swaging. Another method uses repeated hot extrusion of a melted ingot followed by warm drawing, and is applicable for initial ingots as small as 30 g (30). Trials of upset forming of annealed Ir wire determined that limited cold deformation of up to 25 to 30% could be achieved, but forming of complex geometries was unsuccessful (27). Forming of sheet or plate of Ir and its alloys is generally performed at elevated temperature. Warm drawing, that is deformation below the recrystallisation temperature, has been successfully

188

Fig. 1 Schematic diagram of the processing of iridium powder to a DOP-26 iridium alloy forming blank. Note: photographs are not to the same scale

DOP-26 Iridium Alloy Processing

Iridium powder

Compact

Electron beam button

Arc-melted ingot

Electrode

Drop-cast segment

Extruded bar

Sheet

Blank

performed on Ir and its alloys. Hemispherical cups of an Ir alloy were hydroformed using tooling preheated to 775 K (31). Prior to forming, the blanks were encapsulated in evacuated stainless steel covers that were preheated to 1175 K. Similar encapsulation in stainless steel was used for deep drawing with preheated steel tooling (32). The encapsulation and subsequent removal of the capsule materials does add processing steps and potentially decreases dimensional control. Crackfree cups may be formed from similar Ir DOP-26 alloy without encapsulation, using preheated tooling, isothermal forming temperatures of 825 K to 875 K and a draw ratio of about 2 (33). Cracking occurred at a draw temperature of 775 K. An earlier work on deep drawing of Ir cups reported wrinkling of the drawn cups (34), and incorrectly concluded that drawing below the recrystallisation temperature was not possible. Increasing the holddown pressure minimises wrinkling, and improved lubrication minimises loads on the cup wall. By


way of example, blanks of 51 mm diameter and 0.65 mm thickness, in stress-relieved conditions, were drawn with a hold-down force of 65 kN using flexible graphite sheet as a lubricant (33). The advantages and disadvantages of various deep drawing methods for DOP-26 Ir alloy have been evaluated (35). No substantial differences were found between the inspection yields of formed cups with and without encapsulation. Additional processing steps associated with encapsulation and later removal of the encapsulation material increase processing costs, but the potential for surface contamination of the Ir alloy is minimised. Also, as reported elsewhere (36), the tendency for fracture near the cup radius might possibly be minimised by maintaining the punch at a lower temperature to increase the strength of the material in that region. Hot drawing of Ir has also been reported, using a preheat temperature up to 1625 K with tooling heated to about 650 K. The blanks were of 2 mm thickness and the draw ratio

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was 1.1 (27). In general, Ir sheet material has been formed by deep drawing, spinning or pressing at temperatures in the range 1175 to 1675 K (26). The minimum working temperature increases with increased material thickness and increased applied tensile strain levels.

Joining Ir is weldable by a number of methods, as are some Ir alloys. The applications for welding Ir and its alloys include the fabrication of spark plug electrodes, nuclear fuel containers and crucibles for crystal growth. Arc welding 0.63 mm thick Ir metal was performed in a helium atmosphere with a 3.2 mm diameter thoriated tungsten electrode, an arc gap of 1 mm, a weld velocity of 5 mm s–1 and a current of 41 A DC with straight polarity (37). The weld metal along the centreline exhibited an unfavourable microstructure, with single grains extending through the thickness of the weld. An increase in weld velocity, with corresponding increases in welding current, reduced the size of grains along the centreline. Oscillation of the arc across the weld centreline by magnetic deflection, equivalent to a square wave at a frequency of 6.25 Hz, produced a microstructure with smaller and more equiaxed grains along the weld centreline. Some modifications to these methods were required for welding of the DOP-26 Ir alloy containing 60 ppm Th (38). The alloy was found to be more susceptible to hot cracking during welding, a phenomenon associated with grain boundary separation during the solidification process. A weld current of 83 A was used and hot cracking was minimised by using long initial and final tapers, and a short arc length. The use of arc deflection with a four-pole oscillator tended to further reduce hot cracking and gave a smaller average grain size in the weld (39). The grain orientations and grain sizes in the weld were associated with changes in the shape of the weld pool from teardrop to elliptical (40). This improvement in grain structure with four-pole oscillation resulted in increased tensile elongation from 4% to 14%, as measured in tests at 923 K. The effect of weld width on the DOP-26 alloy


was studied under similar conditions at a weld speed of 12.5 mm s–1, with current adjusted to give weld bead widths of 3.7 mm or 2.5 mm (41). The narrow welds exhibited a finer grain size, and a tensile impact elongation more than double that for the wider bead as measured at 5000 s–1 and temperatures of 1253 and 1373 K. Scanning electron microscopy showed equiaxed particles of Ir-Th intermetallic along grain boundary fracture surfaces within the narrow weld, whereas the wider weld gave aligned intermetallic precipitates similar to a eutectic structure. The cracking at grain boundaries during solidification was attributed to segregation of Th and local melting point depression, resulting in strain concentration at weld grain boundaries. Arc welding of the Ir alloys has been automated. Autogenous, full joint penetration gas tungsten arc (GTA) girth welds were made to join two DOP-26 Ir alloy shells over a plutonium oxide fuel pellet using computer-based control (38). This process was later updated to use a PCbased commercial controller that continuously monitors and controls welding current, rotational speed, and the composition and flow of the torch gas (42). The use of precision tooling and controls for location, rotation and joint loading allowed tack welding to be eliminated and improved yields of defect-free welded components (43). Electron beam welding provides some advantages in the welding of Ir alloys that are difficult to join by arc welding. An alloy of Ir with 200 ppm Th was successfully welded without cracking by electron beam, but was not weldable by arc welding (44). At low speed, successful electron beam welds could only be made over a narrow range of beam focus conditions, whereas at high speeds welds could be made over a wide range of focus conditions. This behaviour at high welding speeds was associated with fusion zone grain structure and positive segregation of Th at the fusion boundary under conditions leading to cracking. High heat input led to more rapid solidification, finer grain size and lower levels of segregation (45). The micrograph in Figure 2 shows the alignment of grain boundaries in the weld metal along the centreline of a weld obtained at the relatively

190

(a)

Welding direction

5 mm

300 μm

(b)

200 μm Fig. 2 Optical micrographs of an electron beam weld in iridium alloy sheet, showing regular grain alignment along the weld centreline associated with a relatively low weld traverse speed (Reproduced with permission from (45))

low travel speed of 2.5 mm s–1. At higher speeds the grain boundaries become less uniformly oriented. The shape of the weld pool during electron beam welding was also found to influence the orientation of grains in the weld pool (46), as discussed above for arc welding of Ir. Electron beam welding of Ir alloy components has been practiced, although control of heat input and tooling is essential to obtaining acceptable welds (47). Figure 3 shows an example of an electron beam weld in a DOP-26 Ir alloy cup assembly. Laser welding has shown benefits in welding an Ir alloy that is subject to hot cracking (48). An alloy containing 200 ppm Th was welded with a continuous-wave high-power carbon dioxide laser system without hot cracking. This was explained in a manner similar to that for electron beam welding, a highly concentrated heat source and a relatively fine fusion zone microstructure. Laser welding has been used to join Ir wire segments to nickel or nickel alloy for use in spark plug applications (49). Electric resistance welding, a potentially lower-cost alternative for this joining process, was also evaluated using a programmable high-frequency power supply. Direct bonding of Ir to the


Fig. 3 DOP-26 iridium alloy cups with electron beam welded frit vent component. The light coloured area, seen as four semi-circular regions, is a porous iridium metal filter or frit (Photograph courtesy of G. B. Ulrich, Oak Ridge National Laboratory)

Ni was not achieved but metallurgical bonding was achieved with the use of an intermediate layer. The intermediate layers were chosen to assist in alloying between the dissimilar joint materials and to provide an orderly transition in the thermal expansion coefficient of the material across the joint. Ir alloys have been characterised with respect to weldability, defined as the capacity to produce crack-free welds. Since there are multiple mechanisms by which cracks may initiate and propagate, there are a variety of methods for measuring weldability of Ir and its alloys. The simplest method of determining whether autogenous welds can be made without cracks in sheet material under specific conditions was employed to show that Ir containing up to 100 ppm Th produced sound welds by arc welding, whereas alloys containing 200 ppm or greater did not produce crack-free welds (44). The more sensitive modified circular patch test employs a disk of material which is clamped to a test fixture and arc welded to produce two concentric autogenous welds in sequence (50). This test introduces thermal stresses from mechanical constraint as well as increased residual stresses from the multiple welds. The modified circular patch test was used

191

to characterise various individual lots of the DOP26 alloy with respect to hot cracking. Another test employed the detection of underbead cracks in arc-welded DOP-26 Ir alloy capsules (51). Here, a closure weld was made by arc welding the circumference of two mating Ir alloy cups, followed by additional circumferential welds and shorter arc welds in the same locations as the previous welds. The welded cups were examined for cracks by both non-destructive and destructive methods. This permitted the selection of improved weld parameters, in particular current ramp rates, as well as characterisation of various lots of Ir alloy sheet materials. In the Sigmajig weldability test for hot cracking, an initial tensile stress is applied to the test sample, and threshold cracking stress is determined; this is the maximum applied stress at which a crack-free weld can be made (52). This test has been used both to characterise the weldability of Ir alloy materials and to characterise the effects of varying some welding parameters (53). A study of the effect of Th concentration in Ir-0.3 wt.% W alloy showed that the threshold cracking stress decreased from 170 MPa at 37 ppm Th to 85 MPa at 94 ppm Th. Neither oxygen impurities up to 2000 ppm by volume nor water vapour up to 1000 ppm in the argon atmosphere of the glove box affected the threshold cracking stress. However, significant increases in weld width were observed with increased gas impurity levels, an effect attributed to changes in the surface tension of the liquid. The test was also used to evaluate the weldability of alloys with Ce or both Ce and Th at levels (in atom fraction) up to 100 ppm (54). An alloy with 50 ppm Ce and alloys with 40 ppm Ce and 10 ppm Th or 30 ppm Ce and 20 ppm Th all showed threshold cracking stresses of 170 MPa or greater. A number of other alloys with boron and Y additions exhibited low threshold cracking stresses. The performance of welded Ir crucibles has also been characterised. Crucibles fabricated by welding of electron beam melted Ir have shown longer service life than those made by powder metallurgy (55). Grain growth during service at temperatures of 1800 to 2400 K resulted in large grains, up to 10 mm in size. Crucible failures were


attributed to segregation of impurities at these boundaries. One unusual application is the autogenous welding of an entire wrought Ir crucible, with a goal of increasing crucible life (56). The grains in the welded structure, although large by normal standards, can impede further grain growth and delay crucible failure. Nondestructive examination of Ir welds and base metal has been performed using both dye penetrant and ultrasonic methods. Fluorescent dye methods have been used for deep drawn cups (57). Ultrasonic inspection methods initially used in the 1980s (38, 58) have been further developed to provide better sensitivity and diagnostic capability (59).

Deposition Processes Deposition processes for Ir include electrolytic deposition, chemical and physical vapour deposition and melt deposition. Each of these methods has advantages and limitations depending on the requirements of the application. The plating of Ir from aqueous solutions has been the subject of a recent review (60). Plating of Ir from Ir chloride solutions with sulfamic acid produces deposits up to 25 μm thick, although the deposits exhibit cracks. Plating of Ir from solution in hydrobromic acid produces crack-free deposits of up to 1 μm thick using a plating rate of about 1 μm per hour. Improved plating efficiencies and decreased cracking of the coating were reported for sodium hexabromoiridate(III) baths with additions of oxalic acid. While typical thicknesses of Ir plating of 1 μm or less can minimise corrosion and serve for many electronic applications, thicker coatings are necessary for use at elevated temperature. Electrodeposition from molten salts has been used to produce coatings of Ir up to 0.4 mm thick, and net shape components up to 3 mm thick. Initial work on electrodeposition demonstrated that Ir plating could be performed in a bath of fused sodium cyanide or a mixture of sodium cyanide and potassium cyanide under inert atmosphere at rates up to 10 μm h–1 (61). Coatings up to 0.125 mm thick were produced in a single coating cycle (62) and coatings up to 0.4 mm were produced in multiple cycles with intermediate surface removal (63). Later work demonstrated that

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deposition of Ir coating from fused chlorides was also possible, with thicknesses up to 350 μm deposited at temperatures of 800 to 920 K (64). Coatings have been made using a ternary eutectic molten salt bath of NaCl-KCl-CsCl containing 2 to 7 wt.% Ir (65). This bath composition has also been used to electroform crucibles by deposition of thick deposits on to a graphite mandrel that is later removed (66). Deposition rates up to 100 μm h–1 were reported. The grain size was 10 to 15 μm for direct current deposits with a thickness of 200 μm. The grain size was about 70 μm using intermittent current reversal. Electrodeposition has been used for a variety of other components, including crucibles, tubes, nozzles and jewellery (6, 67). Some examples are shown in Figure 4. The chemical vapour deposition (CVD) processing of Ir has progressed significantly in recent years, particularly for very high-temperature applications. Ir was first applied by CVD to rhenium rocket thruster chambers for high-temperature oxidation protection in 1986, and a 490 N chamber was flight-qualified in 1997 (68). Ir coatings are deposited by CVD to a thickness of about 50 μm onto a Mo or graphite mandrel of the appropriate shape, and Re, to a thickness of about 2 mm, is subsequently deposited over the Ir by CVD and machined (69), as shown schematically in Figure 5. Typical deposition rates are about 10 μm h–1 for Ir and 40 μm h–1 for Re at a temperature stated to be about 1475 K (70). Further performance improvements and weight savings are reported using a carbon/carbon composite outer shell and CVD Re

Coat mandrel with iridium

Mandrel

Fig. 4 Iridium products produced by electrodeposition from a molten salt bath (Reproduced with permission from (6))

inner liner with a CVD Ir coating (71). The range of methods reported for CVD of Ir was recently reviewed (72). Ir hexafluoride can be used to produce non-porous coatings at rates of up to 10 μm h–1, but the compound requires high deposition temperatures and is corrosive. Organic complexes of Ir offer the potential for lower deposition temperatures. Ir acetylacetonate has been used to produce Ir coatings up to 50 μm thick at rates up to 25 μm h–1, although these deposits contain up to 20% carbon by weight. Controlled additions of oxygen can produce essentially carbon-free coatings, but deposition rates are about 0.2 μm h–1. A variety of carbonyl, allyl and cyclooctadienyl complexes of Ir have been evaluated for CVD of Ir coatings. Temperatures

Overcoat with rhenium

Remove mandrel

Rhenium (structure)

Completed iridium/rhenium combustion chamber

Iridium (oxidation protection)


Fig. 5 Schematic diagram of the production of an iridium-coated rhenium nozzle by chemical vapour deposition (Reproduced with permission from (69))

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for decomposition are in the range of 400 to 760 K and oxygen or hydrogen is generally added to control the carbon content of the coating. A methylcyclopentadienyl complex of Ir, Ir(COD)(MeCp) (COD = 1,5-cyclooctadiene) has been used to produce coatings of 1 to 2 μm thickness, of good purity, at temperatures in the range 573 to 673 K, but with low deposition rates of 0.25 μm h–1 or less (73). Physical vapour deposition methods for Ir include both thermal evaporation and sputtering. Electron beam vapour deposition (EBVD) has been used to produce thin coatings on mirrors for infrared telescopes (74). Ir alloys have been deposited by EBVD for use as diffusion barriers on coated Ni-base superalloys (75). Pulsed laser vaporisation of Ir has also been studied (76). Studies of coatings of Ir for high-temperature oxidation protection of carbon materials showed that continuous coatings were produced via radio-frequency magnetron sputtering, but not with direct current sputtering methods (77). A number of melt deposition processes have been investigated for Ir component production. The production of near-net shape parts with Ir by directed light fabrication has shown some promise (78). In this method metal powder is transported in a stream of inert gas and fused to a surface in the focus of a high-power laser beam, to form fully fused near-net-shaped components. Initial work on this process indicates that porosity originating from gases during melting and solidification is an issue. Plasma spray, and, in particular, vacuum plasma spray or low-pressure plasma spray of Ir has been proposed as a method for achieving highdensity coatings. While there is little published

literature available on plasma spraying of Ir, it is expected to perform similarly to that of a number of refractory metals (79). The use of spherical and/or pre-alloyed powders of Ir may also offer advantages, as it does for other refractory metals (80).

Conclusions During the past twenty years improvements have been made in the processing of Ir and its alloys and also in the fundamental understanding of some processing methods. These advances have supported the use of Ir and its alloys in applications such as rocket combustion chambers, fuel containers for nuclear power in space, radiation sources for medical treatments, engine ignition devices and crucibles for the growth of electronic and photonic materials. Research on new methods for Ir processing, including novel powder metallurgy and metal deposition techniques, may facilitate future applications such as the production of Ir alloys as high-temperature structural materials.

Acknowledgements The author acknowledges the assistance of George B. Ulrich and Stan A. David, both of the Oak Ridge National Laboratory, in providing some of the illustrations and in reviewing the manuscript. This work was sponsored by the Office of Radioisotope Power Systems (NE-34) of the United States Department of Energy and performed at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, under contract DEAC05-00OR22725.

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ed. M. S. El-Genk, AIP Conference Proceedings, 1995, Vol. 324, pp. 245–51 K. V. Cook, R. A. Cunningham, Jr., W. A. Simpson, Jr. and R. W. McClung, J. Mater. Energy Syst., 1987, 8, (4), 356 M. W. Moyer, ‘Advanced Ultrasonic Inspection Techniques for General Purpose Heat Source Fueled Clad Closure Welds’, ORNL/TM-2001/9, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A., 2001 T. Jones, Met. Finish., 2004, 102, (6), 87 W. B. Harding, Plat. Surf. Finish., 1978, 65, (2), 30 D. Schlain, F. X. McCawley and G. R. Smith, Platinum Metals Rev., 1977, 21, (2), 38 R. L. Andrews, C. B. Kenahan and D. Schlain, ‘Electrodeposition of Iridium from Fused Sodium Cyanide and Aqueous Electrolytes. A Preliminary Study’, U.S. Bureau of Mines, Invest. Rep. No. 7023, Washington, DC, U.S.A., 1967 S. V. Plaksin, A. B. Smirnov, N. A. Saltykova and A. P. Baraboshkin, Sov. Electrochem., 1982, 18, (6), 645 N. A. Saltykova, S. N. Kotovskii, O. V. Portnyagin, A. N. Baraboshkin and N. O. Esina, Sov. Electrochem., 1990, 26, (3), 338 N. L. Timofeyev, V. E. Baraboshkin and N. A. Saltykova, ‘Production of Iridium Crucibles by Electrolysis of Molten Salts’, in “Iridium”, eds. E. K. Ohriner, R. D. Lanam, P. Panfilov and H. Harada, Proceedings of the international symposium held during the 129th Annual Meeting & Exhibition of The Minerals, Metals & Materials Society (TMS), 12th–16th March, 2000, Nashville, Tennessee, U.S.A., TMS, Warrendale, Pennsylvania, pp. 175–179 C. Volpe, K. Vaithinathan and R. Lanam, ‘The Use of Iridium for Jewelry’, in “Iridium”, eds. E. K. Ohriner, R. D. Lanam, P. Panfilov and H. Harada, Proceedings of the international symposium held during the 129th Annual Meeting & Exhibition of The Minerals, Metals & Materials Society (TMS), 12th–16th March, 2000, Nashville, Tennessee, U.S.A., TMS, Warrendale, Pennsylvania, pp. 227–237 R. H. Tuffias, Mater. Manuf. Process., 1998, 13, (5), 773

69 A. J. Fortini, R. H. Tuffias, R. B. Kaplan, A. J. Duffy, B. E. Williams and J. W. Brockmeyer, ‘Iridium/Rhenium Combustion Chambers for Advanced Liquid Rocket Propulsion’, in “Iridium”, eds. E. K. Ohriner, R. D. Lanam, P. Panfilov and H. Harada, Proceedings of the international symposium held during the 129th Annual Meeting & Exhibition of The Minerals, Metals & Materials Society (TMS), 12th–16th March, 2000, Nashville, Tennessee, U.S.A., TMS, Warrendale, Pennsylvania, pp. 217–225 70 J. C. Hamilton, N. Y. C. Yang, W. M. Clift, D. R. Boehme, K. F. McCarty and J. E. Franklin, Metall. Trans. A, 1992, 23, (3), 851 71 ‘Modifications of a Composite-Material Combustion Chamber’, NASA Tech Briefs, 31st May, 2005; http://www.techbriefs.com/content/view/781/34/ 72 J. R. Vargas Garcia and T. Goto, Mater. Trans., 2003, 44, (9), 1717 73 F. Maury and F. Senocq, Surf. Coat. Technol., 2003, 163–164, 208 74 H. Herzig, Platinum Metals Rev., 1983, 27, (3), 108 75 F. Wu, H. Murakami and A. Suzuki, Surf. Coat. Tech., 2003, 168, (1), 62 76 M. Galeazzi, C. Chen, J. L. Cohn and J. O. Gundersen, Nucl. Instr. Methods Phys. Res. A, 2004, 520, (1–3), 293 77 K. Mumtaz, J. Echigoya, T. Hirai and Y. Shindo, J. Mater. Sci. Lett., 1993, 12, (18), 1411 78 J. O. Milewski, D. J. Thoma, J. C. Fonseca and G. K. Lewis, Mater. Manuf. Process., 1998, 13, (5), 719 79 J. S. O’Dell, T. N. McKechnie and R. R. Holmes, ‘Development of Near Net Shape Refractory Metal Components Utilizing Vacuum Plasma Spray’, in “Tungsten, Refractory Metals and Alloys 4”, Proceedings of the 4th International Conference on Tungsten and Refractory Metals, 17th–19th November, 1997, Lake Buena Vista, Florida, U.S.A., eds. A. Bose and R. J. Dowding, Metal Powder Industries Federation, New Jersey, U.S.A., 1998, pp. 159–166 80 J. S. O’Dell, E. C. Schofield, T. N. McKechnie and A. Fulmer, J. Mater. Eng. Perform., 2004, 13, (4), 461

The Author Dr Evan K. Ohriner is a Distinguished Development Staff member in the Materials Science and Technology Division of the Oak Ridge National Laboratory, U.S.A. His main interest is in processing of refractory metals and alloys. In 2005 Dr Ohriner was honoured by ASM International as an ASM Fellow ‘for the development of iridium alloys and hightemperature and wear-resistant materials used in space exploration and energy generation and transmission’.


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Platinum 2008 Johnson Matthey published its latest market survey of the platinum group metals (pgms) in May 2008. “Platinum 2008” covers supply and demand for the whole calendar year 2007, and provides a short-term outlook on the status of the pgm market and pgm prices into 2008.

Platinum

Platinum Market Deficit for 2007

“Platinum 2008” reports a market deficit of 480,000 oz of platinum for 2007. Production in South Africa was disrupted by a series of unscheduled smelter closures, safety problems and a difficult industrial relations climate. As a result, South African supplies fell by 4.9 per cent to 5.04 million oz, and this drove global platinum supplies down to 6.55 million oz. At the same time, rising purchases of platinum for autocatalysts and industrial use caused demand to rise by 8.6 per cent to 7.03 million oz. In response, the price of platinum rose almost 35 per cent, hitting a series of record highs. Johnson Matthey’s expectations are for the market to remain in deficit in 2008, and for price volatility to persist. Record Demand for Autocatalysts

Production of light-duty diesel vehicles grew during 2007. Many of these vehicles were fitted with a platinum-based oxidation catalyst and a platinum-coated particulate filter to comply with emissions legislation, especially in Europe where

the Euro 4 legislation has been in place since 2006. Car production in Asia also increased, including both gasoline and diesel vehicles for domestic use or export, most of which were fitted with autocatalysts. There is an increasing requirement that heavy-duty diesel vehicles meet tightening emissions legislation around the world, leading to the fitment of platinum-based exhaust aftertreatment to many of these vehicles. Substitution of platinum by palladium continued in some gasoline and diesel catalysts, but purchases of platinum by the autocatalyst sector still rose by 8.2 per cent to 4.23 million oz in 2007. Strong Industrial Demand

Purchases of platinum for industrial applications rose to 1.94 million oz in 2007. Increasing demand for data storage for electronic devices led to higher production of hard disks. The average platinum content of a hard disk rose to increase storage capacity. However, the growing share of the inherently higher capacity perpendicular magnetic recording hard disk moderated the rise in platinum demand. Overall, platinum demand by the electronics sector rose to 425,000 oz. Increased demand for flat panel glass resulted in rising demand for platinum for LCD glass manufacture, particularly in Asia. High oil prices and high demand for oil products led to an increase in platinum use for petroleum refining, which rose by 13.9 per cent to 205,000 oz. Platinum requirements in the chemical sector fell slightly to 390,000 oz, although demand from the silicones industry remained steady, with increasing demand offsetting reduced levels of catalyst in individual products. From nitric acid producers, platinum demand rose. Net demand for the dental sector fell to 105,000 oz due to price sensitivity and increased recycling. Little Price Impact on Jewellery Demand

Strong growth in the flat panel display and fibre glass (shown above) markets in Asia boosted platinum demand last year (Courtesy of Owens Corning)


Jewellery demand for platinum fell only marginally to 1.59 million oz, although there was growth in some markets. European demand rose by 7.7 per cent to 210,000 oz, and in China demand rose by 20,000 oz to 780,000 oz.

198

Chinese manufacture of platinum Olympic memorabilia ahead of the Beijing games contributed to demand, and is expected to boost platinum demand in 2008. Demand in Japan was lower than previous estimates, at 280,000 oz, due to increased recycling; and in North America demand fell to 240,000 oz.

Palladium

Palladium Market in Surplus

Supplies of palladium rose to a total of 8.59 million oz, with slightly decreased primary production from Russia at 3.05 million oz, and substantial sales of Russian State stocks at 1.49 million oz. South African production was down to 2.77 million oz, while output from North America, Zimbabwe and elsewhere rose to 1.28 million oz. Demand reached a total of 6.84 million oz, up 3.5 per cent. Overall the market showed a surplus of 1.75 million oz. Rising Use of Palladium in Autocatalysts

Strong growth in vehicle production led to a rise in autocatalyst demand for palladium of 10.8 per cent to 4.45 million oz. Car manufacturers continued to use palladium in place of platinum in a typical gasoline autocatalyst, and as a minor component in some diesel autocatalysts. The total worldwide amount of palladium used in light-duty diesel catalysts was less than 300,000 oz. Strong Growth in Electronics Demand

Demand for palladium grew strongly in the electronics sector, increasing by 6.6 per cent to 1.29 million oz. This was largely due to the use of palladium in multilayer ceramic capacitors,

with more capacitors per device and increasing sales of electronic goods outweighing the effects of miniaturisation and the slowly increasing use of nickel. Use of palladium in the dental sector also rose by 15,000 oz to 635,000 oz, after several years of decline. Jewellery Demand Falling

Overall demand for palladium for jewellery fell to 740,000 oz in 2007. However, demand rose in some markets, notably Europe and North America where combined demand reached 95,000 oz. The Chinese jewellery requirement for new metal was down to 500,000 oz, with increased use of recycled metal, particularly Pd950 pieces which were being returned for remanufacture into higher purity Pd990 alloys.

Special Features “Platinum 2008” carries three Special Features: ‘South African PGM Production’, which includes a map showing the South African pgm mines; ‘The Russian PGM Industry’ and ‘Exchange Traded Funds’. The last describes the two new investment funds, backed by physical pgms, which were launched in 2007.

Availability and Contact Information “Platinum 2008” is available to download free of charge as a PDF file from the Platinum Today website: http://www.platinum.matthey.com/. Alternatively, a printed copy can be requested from Johnson Matthey PLC, Precious Metals Marketing, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K., or by E-mail: [email protected].

Johnson Matthey is the first western autocatalyst manufacturer to establish a plant in Russia. The facility shown is in Krasnoyarsk and started operations in the first half of 2008


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ABSTRACTS CATALYSIS – APPLIED AND PHYSICAL ASPECTS

A Versatile Iridium Catalyst for Aldehyde Reduction in Water

Kinetics of o-Chlorotoluene Hydrogenolysis in the Presence of 3%, 5% and 10% Pd/C Catalysts

2008, 1, (1–2), 71–74

T. JANIAK,

Appl. Catal. A: Gen., 2008, 335, (1), 7–14

The kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C (1) were studied in an alkaline–n-heptane–H2(g) system. The main product of hydrogenolysis was toluene. The extent of dechlorination increased with time and temperature, and depended on the amount of (1).

X. WU, C. CORCORAN, S. YANG and J. XIAO,

ChemSusChem,

Ir-N-tosyldiamine complexes catalysed the reduction of a wide range of aldehydes, including aromatic, aliphatic, heterocyclic and α,β-unsaturated aldehydes, in H2O. The hydrogenations were fast and chemoselective. The reactions proceeded without the need for added organic cosolvents.

EMISSIONS CONTROL

Gas-Phase Thermochemistry of Ruthenium Carbene Metathesis Catalysts

Role of Zeolite Structure on NO Reduction with Diesel Fuel over Pt Supported Zeolite Catalysts

S. TORKER, D. MERKI and P. CHEN, J. Am. Chem. Soc., 2008, 130, (14), 4808–4814

A. SULTANA, M. HANEDA, T. FUJITANI and H. HAMADA, Microporous Mesoporous Mater., 2008, 111, (1–3), 488–492

Quantitative energy-resolved collision-induced dissociation cross-sections by tandem ESI-MS gave absolute thermochemical data for phosphine binding energies in first- and second-generation Ru metathesis catalysts of 33.4 and 36.9 kcal mol–1, respectively. A study of RCM in the second-generation system to liberate norbornene by forming the 14-electron reactive intermediate from the intramolecular π-complex allowed an estimate of the olefin binding energy to the 14-electron complex of ~ 18 kcal mol–1, assuming a loose transition state.

CATALYSIS – INDUSTRIAL PROCESS Development of a Robust Ring-Closing Metathesis Reaction in the Synthesis of SB-462795, a Cathepsin K Inhibitor H. WANG, S. N. GOODMAN, Q. DAI, G. W. STOCKDALE and W. M. CLARK, Jr., Org. Process Res. Dev., 2008, 12, (2), 226–234

RCM with Hoveyda’s second-generation catalyst can be used to synthesise SB-462795. With pure diene precursor (1), very low loadings (0.1–0.2 mol %) of the Ru catalyst are required; however, the reaction conversion decreased when (1) is of reduced purity. Projection methods were applied to historical data to identify the main sources of variation in starting material quality, and to determine the main detrimental impurities.

CATALYSIS – REACTIONS Dendritic SBA-15 Supported Wilkinson’s Catalyst for Hydroformylation of Styrene P. LI and S. KAWI,

Catal. Today, 2008, 131, (1–4), 61–69

PAMAM dendrimers were grown in mesoporous SBA-15. RhCl(PPh3)3 precursor was then tethered on these supports. The silanols outside the SBA-15 pores could be passivated. The Rh catalysts supported in the pore channels of this passivated SBA-15 showed positive dendritic effects in enhancing the catalytic activity, regioselectivity and stability of the catalyst by minimising the leaching of the Rh complex catalyst.


The selective catalytic reduction of NO with diesel fuel over Pt/zeolites was investigated under simulated exhaust conditions. Pt/MOR was the most active, giving 90% NO conversion at 300ºC, however Pt/FER showed a desirable low temperature window, giving 77% NO conversion at < 260ºC. Over ZSM-5, BEA and Y with 3D pore structures, extensive carbonaceous deposits were observed. FER having a 1D pore structure did not allow extensive coke formation, resulting in low temperature NO conversion. It is suggested that NO reduction takes place mainly near the zeolite pore opening. A New Route for Degradation of Volatile Organic Compounds under Visible Light: Using the Bifunctional Photocatalyst Pt/TiO2–xNx in H2–O2 Atmosphere D. LI, Z. CHEN, Y. CHEN, W. LI, H. HUANG, Y. HE and X. FU,

Environ. Sci. Technol., 2008, 42, (6), 2130–2135

N-doped and Pt-modified TiO2 was used to obtain Pt/TiO2–xNx (1) by wet impregnation of TiO2 xerogel. Superior photocatalytic activity and catalytic stability of (1) for decomposing benzene were achieved under visible light in a H2–O2 atmosphere. (1) also successfully decomposed other VOCs such as toluene, ethylbenzene, cyclohexane and acetone. Modification of Pd/Al2O3 Catalyst to Improve the Catalytic Reduction of NO in Waste Incineration Processes J. C. CHEN, F. Y. CHANG and M.Y. WEY, Catal. Commun., 2008,

9, (6), 1106–1110

Na, Cu, Ni and Co were used to modify Pd/Al2O3 catalysts (1) for NO reduction with CO reductant in a simulated flue gas containing 6% O2. Na addition was very effective in promoting the NO conversion of (1) at 300–400ºC with the concentration ratio of CO:NO = 1. Adding Cu improved the NO conversion at 250–300ºC. Ni or Co slightly improved the NO conversion at 150ºC, but maintained good catalytic activity on the CO oxidation.

200

FUEL CELLS

METALLURGY AND MATERIALS

Crystallographic Characteristics of Nanostructured Thin-Film Fuel Cell Electrocatalysts: A HRTEM Study

Hydrogen Absorption in the Core/Shell Interface of Pd/Pt Nanoparticles

L. GANCS, T. KOBAYASHI, M. K. DEBE, R. ATANASOSKI and A. WIECKOWSKI, Chem. Mater., 2008, 20, (7), 2444–2454

The structure of nanostructured thin film (NSTF) electrocatalysts supported on PR149 (N,N-di(3,5xylyl)perylene-3,4:9,10-bis(dicarboximide))-crystalline organic whiskers was investigated by HTREM. Common trends in the electrocatalyst crystallography, morphology and surface characteristics were observed for Pt, PtRu and PtNiFe NSTFs. Specific details of the electrocatalyst particles’ growth mechanisms, morphology and support interactions were established. A High-Throughput Study of PtNiZr Catalysts for Application in PEM Fuel Cells J. F. WHITACRE, T. I. VALDEZ and S. R. NARAYANAN, Electrochim.

Acta, 2008, 53, (10), 3680–3689

Pure Pt or PtNiZr alloys (not exceeding 11 at.% Zr) were fabricated using cosputter deposition. A highthroughput fabrication approach was used wherein 18 thin film ORR electrocatalyst alloy samples were deposited onto a large-area substrate. A multichannel pseudo-potentiostat enabled the simultaneous quantitative study of catalytic activity for all of the electrodes in a single test bath. The best performing catalyst was Pt59Ni39Zr2.

H. KOBAYASHI, M. YAMAUCHI, H. KITAGAWA, Y. KUBOTA, K. KATO and M. TAKATA, J. Am. Chem. Soc., 2008, 130, (6),

1818–1819

From the results of H2 pressure-composition isotherm and solid-state 2H NMR measurements, it was shown that Pd/Pt bimetallic nanoparticles (1) with a Pd core/Pt shell structure can absorb H2. Most of the absorbed H atoms were situated around the interfacial region between the Pd core and the Pt shell. This indicates that the core/shell boundary plays a key role in the formation of the hydride phase of (1). Hydrogen Storage Properties of Pd Nanoparticle/Carbon Template Composites R. CAMPESI, F. CUEVAS, R. GADIOU, E. LEROY, M. HIRSCHER, C. VIX-GUTERL and M. LATROCHE, Carbon, 2008, 46, (2),

206–214

A C/Pd composite was prepared by chemical impregnation of an ordered porous C template (CT) with a H2PdCl4 solution followed by a reduction treatment. 10 wt.% of Pd clusters (2 nm in size) were introduced in the C porosity. At room temperature and moderate pressure (0.5 MPa), the filling of the CT with nanocrystalline Pd resulted in an H2 uptake eight times larger than that of the Pd-free CT.

APPARATUS AND TECHNIQUE

In Situ and Real-Time Visualisation of Oxygen Distribution in DMFC Using a Porphyrin Dye Compound

Gas Sensing Properties of Nano SnO2 Based Thick Films Prepared by Dip Coating Method with Effect of Molarity of PtCl2 Solution

J. INUKAI, K. MIYATAKE, Y. ISHIGAMI, M. WATANABE, T. HYAKUTAKE, H. NISHIDE, Y. NAGUMO, M. WATANABE and A. TANAKA, Chem. Commun., 2008, (15), 1750–1752

A. D. GARJE and R. C. AIYER,

A film of the luminescent dye [tetrakis(pentafluorophenyl)porphyrinato]platinum dispersed in poly(1-trimethylsilyl-1-propyne) was coated onto a transparent separator on the cathode side of a DMFC to visualise O distribution under operating conditions by analysing emission from the dye. Higher O consumption due to MeOH crossover occured for a fluorinated membrane than for a hydrocarbon membrane. Improved Performance of Pd Electrocatalyst Supported on Ultrahigh Surface Area Hollow Carbon Spheres for Direct Alcohol Fuel Cells F. P. HU, Z. WANG, Y. LI, C. LI, X. ZHANG and P. K. SHEN, J. Power

Sources, 2008, 177, (1), 61–66

Hollow C spheres (HCSs) were prepared using glucose as the C source and polystyrene spheres as the template. Combined methods of hydrothermal and intermittent microwave heating were employed. The addition of PEG-block-PPG-block-PEG during the hydrothermal process greatly increased the surface area of the HCS, mainly from the huge micropores. The catalytic activity of Pd/HCSs is 3 times higher than Pd/Vulcan XC-72 C at the same Pd loadings.


J. Mater. Sci.: Mater. Electron.,

2008, 19, (6), 547–552

The title films modified by dip coating in PtCl2 solutions (0.05–0.2 M) were tested for 400 ppm concentration of H2, CO and LPG. Sensors dip coated with 0.15 M solution of PtCl2 showed the highest sensitivity which is ten times higher than undoped SnO2 sensors. The sensors have fast response time of 10 s to all the gases with a minimum detection limit of 10 ppm. Micro Coulter Counters with Platinum Black Electroplated Electrodes for Human Blood Cell Sensing S. ZHENG, M. LIU and Y.-C. TAI,

Biomed. Microdevices, 2008,

10, (2), 221–231

Two designs of micro Coulter counter were fabricated using integrated parylene and soft lithography technologies. Pt black enhanced detection in the intermediate frequency range (~ 100 Hz to 7 MHz). Polystyrene beads were used to validate the operation of the devices, and using excitation frequency of 10 kHz, the signal magnitude was found to be correlated with the volume of the individual bead. Human blood cell sensing was then demonstrated with diluted whole blood and leukocyte rich plasma under the same excitation frequency.

201

CHEMISTRY

PHOTOCONVERSION

Structure of a Crystalline Vapochromic Platinum(II) Salt

The Influence of Platinum on UV and ‘Visible’ Photocatalysis by Rutile and Degussa P25

L. J. GROVE, A. G. OLIVER, J. A. KRAUSE and W. B. CONNICK,

T. A. EGERTON and J. A. MATTINSON,

Inorg. Chem., 2008, 47, (5), 1408–1410

J. Photochem. Photobiol. A: Chem., 2008, 194, (2–3), 283–289

Square-planar cations of the orange form of [Pt(Me2bzimpy)Cl](PF6)·DMF (Me2bzimpy = 2,6bis(N-methylbenzimidazol-2-yl)pyridine) stack along the b axis in a head-to-tail arrangement with short interplanar spacings (3.35 and 3.39 Å). The DMF solvent molecules line channels parallel to c, which may provide a channel for vapour absorption. Crystals were shown to be vapochromic, changing from orange to violet upon exposure to MeCN vapour.

The influence of Pt on the UV photocatalytic degradation of the dichloroacetate anion (DCA) by rutile and the P25 form of TiO2 was investigated. The Pt was deposited photochemically. Although the catalytic activity of rutile was much less than that of the P25, the effect of Pt addition was so much greater on rutile than on P25 that the activities of the Pt-treated titanias were similar. Visible light irradiation of Pt/rutile oxidised the DCA.

Synthesis and Dynamic Structure of Multinuclear Rh Complexes of Porphyrinoids

Single Dopant White Electrophosphorescent Light Emitting Diodes Using Heteroleptic TrisCyclometalated Iridium(III) Complexes

J. SETSUNE, M. TODA and T. YOSHIDA, Chem. Commun., 2008,

(12), 1425–1427

Multinuclear Rh complexes of the large porphyrinoids expanded rosarin and octaphyrin having the 1,4-phenylene spacers where the Rh(CO)2 group passes through the macrocycle were synthesised. The Rh3 complex of the expanded rosarin exists as the C3v-isomer in CH2Cl2 as well as in the crystal state. Relatively slow metal transposition passing through macrocycle was observed in toluene solution to cause interchange between the C3v-isomer and the Cs-isomer. Four metals are fixed in the Rh4 complex of the expanded octaphyrin.

ELECTRICAL AND ELECTRONICS Nanomechanical and Nanotribological Characterization of Noble Metal-Coated AFM Tips for Probe-Based Ferroelectric Data Recording M. PALACIO and B. BHUSHAN, Nanotechnology, 2008, 19, (10), 105705 (9 pages)

Nanoindentation experiments were carried out to evaluate the mechanical properties of Pt, Pt-Ni, AuNi and Pt-Ir deposited on AFM probes. The Pt-Ir coating exhibited the highest hardness, highest elastic modulus and lowest creep resistance. Nanoscratch studies revealed that the noble metal coatings are removed primarily by plastic deformation.

ELECTROCHEMISTRY Degradation Characteristics of IrO2-type DSA® in Methanol Aqueous Solutions J.-M. HU, X.-J. SUN, Y.-Y. HOU, J.-Q. ZHANG

and C.-N. CAO,

Electrochim. Acta, 2008, 53, (7), 3127–3138

A comparative study was done on the long-term stability and deactivation characteristics of Ti/IrO2-type dimensionally stable anodes (1) in acidic solutions in the absence and the presence of MeOH, respectively. The service life increased and then decreased as the calcination temperature of the as-prepared (1) was increased. The lifetime was shortened by the addition of MeOH into the testing solution.


J. H. SEO, I. J. KIM, Y. K. KIM and Y. S. KIM, Thin Solid Films, 2008, 516, (11), 3614–3617

A single dopant single emissive layer white organic electroluminescent (EL) device (1) was based on Ir(dfppy)2(pq) (dfppy = 2-(2,4-difluorophenyl)pyridine, pq = 2-phenylquinoline) as the guest and 1,4-phenylenebis(triphenylsilane) as the host. The maximum luminous and power efficiencies of (1) were 11.00 cd A–1 (J = 0.05 mA cm–2) and 5.60 lm W–1 (J = 0.001 mA cm–2), respectively. The CIE coordinates of (1) are (0.443, 0.473) and the EL spectrum of (1) shows emission bands at 473 and 544 nm, at the applied voltage of 12 V. Exploitation of the Dual-emissive Properties of Cyclometalated Iridium(III)-Polypyridine Complexes in the Development of Luminescent Biological Probes K. K.-W. LO, K. Y. ZHANG, S.-K. LEUNG and M.-C. TANG, Angew.

Chem. Int. Ed., 2008, 47, (12), 2213–2216

In polar and nonpolar media the title complexes show green and orange-yellow emission, respectively. The incorporation of biological substrates into this system results in luminescent probes that exhibit pronounced changes in their emission profiles upon binding to their specific receptors. Novel luminescent biological probes for avidin, oestrogen receptor α and human serum albumin have been developed.

SURFACE COATINGS The Electrodeposition and Electrocatalytic Properties of Copper–Palladium Alloys C. MILHANO and D. PLETCHER,

J. Electroanal. Chem., 2008,

614, (1–2), 24–30

The codeposition of Cu and Pd from CuSO4 and PdSO4 in HClO4 was investigated using microdisc voltammetry. Good quality coatings of CuPd were deposited. The composition of the coatings was controlled either through the deposition potential or the Cu(II):Pd(II) ratio in solution.

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NEW PATENTS CATALYSIS – APPLIED AND PHYSICAL ASPECTS

EMISSIONS CONTROL

Self-Adjusting Propellant Decomposition Catalyst

JOHNSON MATTHEY PLC

A. J. FORTINI et

al.

U.S. Appl. 2008/0,064,913

A self-adjusting catalyst for decomposing highenergy chemical propellants is formed from a Pt group metal, preferably Ir, Ru or their alloys, supported on a second catalyst selected from Ba oxide, metal chromites, metal hafnates, metal zirconates other than Ca zirconate, or hydrates or mixtures thereof. The Pt group metal catalyst is present in 15–30 wt.%. Platinum Polymerisation Catalyst WACKER CHEMIE AG

U.S. Appl. 2008/0,103,322

Pt 1,3-diketo compounds are prepared by stirring a dichloroplatinum compound containing an aliphatic or cyclic diolefin radical such as norbornadiene or cyclooctadiene, with a diketo compound in a keto solvent at < 10ºC for 5–90 minutes, then isolating the reaction product. Purity of the obtained Pt compound is > 95% and it is suitable for use as a catalyst for polymer preparation for the medical and food industries. Fibrous Protein-Supported Osmium Catalyst WAKO PURE CHEM. IND. LTD

Japanese Appl. 2008-006,349

The title catalyst is prepared by supporting Os on a fibrous protein such as silk fibroin, in which S-containing amino acid residues are present in ≤ 1 wt.%. The catalyst can be used for oxidation of alkenyl compounds, or for selective reduction of a carbonyl group in a compound containing a C=C or a C≡C bond in the presence of H2(g).

CATALYSIS – REACTIONS Rhodium Complexes as Hydrosilylation Catalysts UNIW. ADAMA MICKIEWICZA

World Appl. 2008/033,043

Heterogenised Rh(I) complexes [(≡SiO)(L)Rh(diene)] immobilised on a silica support are claimed, where diene = cyclooctadiene, norbornadiene or tetrafluorobenzobarrelene; L = a ≡SiOX unit on the silica substrate where X = H or Si, or alternatively L = PR3, where R = an alkyl, cycloalkyl or phenyl group. The complexes can be used to catalyse a hydrosilylation reaction between alkenes or functionalised alkenes having a terminal C=C bond and silanes, (poly)siloxanes or (poly)carbosiloxanes containing a Si=H bond. Iminosugar Glycoconjugates TECH. UNIV. GRAZ

European Appl. 1,903,034

The title compounds are N-alkylated 1,5-dideoxy1,5-iminohexitol or 1,5-dideoxy-1,5-iminopentitol derivatives with a very stable linkage between the carbohydrate and the peptide component. The compounds are synthesised by catalytic intramolecular reductive amination of dicarbonyl sugars with partially protected amino acids, using H2(g) and a catalyst selected from Pearlman’s catalyst (Pd(OH)2/C) and Pd or Pt on activated C, at atmospheric pressure or higher and room temperature in MeOH and/or H2O.


Thermally Regenerable Nitric Oxide Adsorbent World Appl. 2008/047,170

A method for reducing NOx in a lean gas stream includes adsorbing NO on an adsorbent containing Pd and a Ce oxide at < 200ºC, thermally desorbing NO at > 2000ºC, and catalytically reducing NOx on a catalyst other than the NO adsorbent using a hydrocarbon or nitrogenous reductant, H2 or a mixture. The NO adsorbent may optionally be combined with a thermally regenerable NOx adsorbent containing Pt and a metal oxide such as Al2O3, CeO2 or ZrO2. NOx Reducing Catalyst System British Appl. 2,441,623

FORD GLOBAL TECHNOL. LLC

An exhaust system includes a first emissions control device having two regions containing Pt, Pd, Rh, Ir, Ru, Os, Re, Ag or Au or a mixture, preferably Pt, and a NOx sorbent such as BaO. The second region is physically segregated from the first and partially downstream of it, and contains more NOx sorbent. A second emissions control device downstream of the first includes a selective catalytic reduction catalyst.

FUEL CELLS Gold-Platinum Nanoparticle Electrocatalysts BROOKHAVEN SCI. ASSOC.

World Appl. 2008/033,113

An O2-reducing electrocatalyst is formed from particles with an electrocatalytically active core and an atomically thin outer shell of Au or Au alloy, on a support. The core may contain one or more of Pt, Pd, Rh, Ir, Ru, Os and Re, with optionally Au, in a homogeneous or heterogeneous composition. Preferred core compositions are Pt or Pt and Pd, which may have an inner subcore of Pd and an outer subshell of Pt. Palladium Electrocatalyst SHANGHAI INST. MICROSYST. INFORM. TECHNOL.

Chinese Appl. 1,083,325

An electrocatalyst is formed of nanoparticulate Csupported Pd or Pd-Pt alloy containing 10–100 at.% Pd, prepared from an aqueous solution of a Pd salt and optionally a Pt salt. The C carrier is present in 20–80 wt.%. Particle size is controllable in the range 1.8–20 nm, with narrow size distribution. The catalyst can be used for the anode of a DFAFC or as a MeOH-tolerant cathode catalyst for a DMFC.

METALLURGY AND MATERIALS Ornamental Platinum Alloy KYOCERA CORP

Japanese Appl. 2007-291,492

A Pt alloy for ornamental use includes a first phase of Pt, a second phase of Cu and optionally a third phase between the first two and containing an intermetallic compound of Pt and Cu. Total content of Pt is 40–75 wt.%. Maximal reflectivity of light from the surface is in the range 560–640 nm, the alloy has a pink colour and has excellent corrosion resistance.

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APPARATUS AND TECHNIQUE

ELECTRICAL AND ELECTRONICS

Ruthenium-Containing Electrode

Iridium Encased Metal Interconnects


CHLORINE ENG. CORP LTD

U.S. Appl. 2008/0,045,013

INTEL CORP

An electrode for H2 generation can maintain low H2 overvoltage for a long time under conditions of both low and high current density. The electrode includes a coating layer containing Ru and La, prepared by thermal decomposition in an O2-containing atmosphere of a Cl-free material prepared from a nitric acid solution of a La carboxylate and Ru(NO3)2, on a conductive base member. Atomic ratio Ru:La is 30:70–90:10. The coating material may optionally include a Cl-free Pt compound with atomic ratio Pt:La ≥ 0.005.

A semiconductor substrate with a trench etched into a dielectric layer is cleaned and then a chelating group layer is deposited. An Ir species layer is deposited, activated and then a Cu seed layer is applied by an electroless deposition process. Finally a layer of bulk Cu is deposited using an electroplating process. The Ir species layer may be deposited by immersing the substrate in a solution containing Ir species or by an atomic layer deposition or CVD process.

Platinum Apparatus for Glass Manufacture

HITACHI MAXELL LTD

ASAHI GLASS CO LTD

U.S. Appl. 2008/0,050,609

A Pt or Pt alloy structure for use in a high-temperature environment is formed into a hollow tube body with a flange on its outer periphery, which is provided with a stress-strain absorbing structure. The flange may have a disc shape and may incorporate a concentric flexible portion. The structure can be used as a conduit tube for molten glass in a vacuum degassing apparatus for glass production.

Palladium-Containing Magnetic Recording Medium Japanese Appl. 2007-305,261

High output and excellent short wavelength recording characteristics are claimed for a magnetic recording medium made from a spherical or elliptical magnetic powder containing Pd, Fe and N. Content of each element is (in at.%): 0.1–10.0 Pd, 1.0–20.0 N, plus optionally 0.05–20.0 Y or Sa and/or 0.1–20.0 Si and/or Al, with the balance Fe. The average particle size is 5–30 nm and a Fe16N2 phase is present. Platinum Etching For Capacitor Manufacture

BIOMEDICAL AND DENTAL

HYNIX SEMICONDUCTOR INC

Osmium Compounds for Cancer Treatment

Pt can be etched using a mixed gas including a Fcontaining gas, preferably SF6(g), and an inert gas, preferably Ar(g), using an electron cyclotron resonance etching apparatus. Flow rate of SF6(g) is ≥ 50%. A capacitor is fabricated by etching Pt layers to form an upper and a lower electrode with a dielectric layer in between. The etched Pt is claimed to be free of fencing or tapering and to have suitable surface roughness for capacitor electrodes.

UNIV. WARWICK

World Appl. 2008/017,855

Os(II) compounds containing an arene moiety; a halogen or donor ligand; a bidentate ligand optionally linked to the arene moiety and containing O, N or S; and optionally a counter ion can be used in a pharmaceutical composition for the treatment of cancer. Solvates, prodrugs or physiologically active derivatives of the Os compounds are also claimed. Palladium-Cobalt Dental Alloys IVOCLAR VIVADENT AG


Alloys for dental articles such as crowns and bridges contain (in wt.%): 20–90 Pd, 10–80 Co, plus 0–20 Al, B, Cr, Ga, Li, Re, Ru, Si, Ta, Ti and/or W. Coefficient of thermal expansion is ~ 14.0–15.2 between 25–500ºC. Alternative compositions contain (in wt.%): 10–80 Pd, 80–10 Co, plus 0–30 Au, Pt, Cr, Mo, W, Fe, Al, Si, Mn, Ga, Ta, Ti, Ru and/or Re, with coefficient of thermal expansion 14.0–15.5 between 25–500ºC.

CHEMISTRY Ruthenium Compounds for Decontamination of Water U.S. EPA

U.S. Patent 7,335,307

Ru compounds selected from RuO2·xH2O or oxides, oxyhydroxides or hydroxides of Ru-Fe, Ru-Mn or Ru-Al can be used to remove biological and chemical contaminants from water, soil and sediments. Both positively and negatively charged ionic or polar contaminants can be sorbed, and the sorbed material is then removed. The Ru compound may optionally be coated onto or complexed with sand, silica, zeolites, nylon, polystyrene or cellulose.


Korean Appl. 2007-0,089,573

SURFACE COATINGS Electrodeposition of Palladium Layers ENTHONE INC

World Appl. 2008/023,339

Pd or Pd alloy can be deposited from an electrolytic solution containing a source of Pd such as PdCl2; a sulfonic or sulfuric acid or a mixture; and a surfactant. There may optionally be a source of alloying metal such as Cu. Electric current at a density of 0.25–1.0 A dm–2, preferably 0.3–0.8 A dm–2, is applied at 20–45ºC to deposit the Pd or Pd alloy layer on a substrate. The solution may further include a S-containing amino acid to enable the deposition of dark Pd layers. Rhodium Sulfate Plating Solution FORMFACTOR INC

U.S. Appl. 2008/0,063,594

A Rh salt cake (for example Rh(SO4)2) for preparation of a Rh plating bath is prepared by mixing a basic and an acidic solution containing Rh to form a colloidal suspension of Rh salt, then removing the liquid. Mixing is carried out at constant pH and temperature, and Rh polymers are < 1% of Rh in the cake. Increased shelf life is claimed for the plating bath and Rh platings are claimed to have low or no dendrites, lower internal stress and less susceptibility to cracking.

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DOI: 10.1595/147106708X333836

FINAL ANALYSIS

Accurate and Precise Determination of Platinum in Solution by ICPES It is widely recognised that Wendt and Fassel (1) and Greenfield et al. (2) were the first to describe the technique of inductively coupled plasma emission spectrometry as a means of analysis. The technique has a number of acronyms, with ICPES being favoured over ICP-OES (where the O stands for optical) and ICP-AES (where the A stands for atomic). In the forty or so years since the inception of the technique, ICPES instrumentation has developed in sophistication and usability and is now to be found in most laboratories where simultaneous compositional analysis of a number of elements is required.

Application for Elemental Analysis In recent years, the ICPES technique has been increasingly used as the method of choice for the quantitative determination of many metals, including platinum and the other platinum group metals (pgms). The reference methods remain in the domain of classical gravimetry, where precision in the order of 0.1% relative standard deviation (RSD) can be achieved. However, gravimetric methods are relatively slow and require complex sequential chemical separations before an element can be determined. In complex samples this degrades the optimum precision and accuracy obtainable. ICPES is capable of achieving precision of < 1% RSD with fast simultaneous determination of each of the elements present. To achieve the best precision and accuracy in determining Pt and the other pgms in samples, several factors must be considered.

Nature of the Sample Samples for Pt analysis are almost invariably in solid form when received by the laboratory. Pt loadings may range from low mg kg–1 (for example in emission control catalysts) to high percentage levels (for example in fuel cell catalysts). The sub-


ject of dissolution of the pgms for analysis has been covered in many other texts by many other authors (see, for example, (3)). To achieve optimum precision and accuracy in determination, the Pt concentration in the solution presented should fall in the range 10 to 1000 mg l–1, with not more than 50 g l–1 in total of dissolved solids.

Sample Introduction The sample is introduced into the plasma via nebulisation. In this process, a fine spray of the sample is carried to the plasma by the injector gas. In the experience of laboratories analysing Pt solutions, the concentric-type nebulisers give good precision when coupled with a tortuous path for the nebulised spray. The classical ‘double pass’ type and newer ‘Twister cyclonic’ nebulisers have both found utility within leading Pt assay laboratories. A range of concentric glass nebulisers are available on the open market, for example (4). The sample is delivered to the nebuliser by a peristaltic pump. Where poor precision and accuracy occur in ICPES determination, they are often attributable to the sample introduction system. In most expert laboratories the standard set-up consists of humidified injector gas, and a 30 rpm eight-roller peristaltic pump delivering the sample at a rate of 1 ml min–1 to a glass concentric nebuliser in a double pass spray chamber.

Calibration ICPES is often quoted as having a wide linear response range, covering five to seven orders of magnitude in concentration. Nevertheless, a good calibration strategy is still important in achieving ultimate accuracy and precision. Common practice is to limit the concentration range to 10 to 1000 mg l–1 via manipulation of the sample dissolution and dilution strategy. Under these circumstances a good match of the calibration solutions with the samples can be achieved.

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Matching the major base metal composition of the sample solution in the calibration solutions requires some planning, but can still allow simultaneous analysis of each of the sample constituents. Importantly, acid concentrations in solution should be matched between samples, standards and solutions used for flushing between intermediate samples. For ultimate accuracy and precision, a weightbased approach to solution preparation over standard volumetric glassware has begun to find favour. Good traceability and reliable calculation can be achieved through integration of laboratory balances, with instrumental systems reducing errors attributable to manual transcription and data entry (5).

Spectrometers, Detectors and Lines One major improvement in ICPES technology of the last fifteen years has been the introduction of solid-state detector systems. The low cost of the detector elements has allowed true simultaneous measurement of peak and background intensities. Line selection is the subject of much discussion in the standard texts. However, for Pt, the leading laboratories will select Pt lines at wavelengths of 265.9, 214.4, 299.8, 306.4 and 203.6 nm.

Internal Standardisation While internal standards can partially correct for matrix differences between sample and standard, their best use is in precision improvement. In common use are yttrium (321.7 nm), scandium (357.6 nm) and indium (451.1 or 303.9 nm). Measuring the ratios of intensities for the analyte and internal standard filters out imprecision caused by noise at mid-range frequencies (~ 1 kHz). An accurate match of the signal counting parameters is necessary to achieve the best performance. In the final analysis, optimising the analytical method according to the above parameters offers routine Pt determinations of high accuracy and PETER ASH precision.

References 1 2 3 4 5

R. H. Wendt and V. A. Fassel, Anal. Chem., 1965, 37, (7), 920 S. Greenfield, I. Ll. Jones and C. T. Berry, Analyst, 1964, 89, (1064), 713 J. C. Van Loon and R. R. Barefoot, “Determination of the Precious Metals”, Wiley, Chichester, 1991 and references therein Glass Expansion, ICP/ICP-MS Sample Introduction Systems: www.geicp.com L. R. Guy, Johnson Matthey, Analytical Laboratories, Brimsdown, U.K., personal communication

The Author Dr Peter Ash is manager of the Analytical Group at the Johnson Matthey Technology Centre, Sonning Common, U.K. Since joining Johnson Matthey in 1989, he has specialised in platinum group metals assaying method development and has been involved in a number of inter-laboratory assay comparison exercises.


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Platinum Metals Review Johnson Matthey Plc, Precious Metals Marketing, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K. E-mail: [email protected] http://www.platinummetalsreview.com/