Nitrogen oxides removal by catalytic methods - Springer Link

Clean Products and Processes 1 (1999) 237–247 Q Springer-Verlag 1999

Nitrogen oxides removal by catalytic methods M. Wojciechowska, S. Lomnicki

237 Abstract Nitrogen oxides are harmful compounds, dangerous to the natural environment. This review reports on progress in studies of catalytic processes to promote nitrogen oxide decomposition as well as catalytic NO reduction from mobile exhaust sources. The activities of catalysts receiving attention in the technical literature range from metallic ones to transition metal oxides supported on zeolites, metal oxides and magnesium fluoride. Also the current ideas on the mechanism of catalytic decomposition and reduction of nitrogen oxides are reviewed.

1 Introduction NOx emitted to the atmosphere with exhaust gases are harmful to the environment and much effort has been made to eliminate them. The so far proposed methods meeting this purpose can be divided into: pre-combustion, combustion, and post-combustion. The first two allow about 50% reduction of the harmful NOx, while the post-combustion methods permit even their 100% elimination. The post-combustion methods include the catalytic reactions of which the catalytic reduction of NOx to N2 is one of the most important. The technologies described lead to obtaining clean combustion. The paper presented gives a review on the progress made in catalytic methods of nitrogen oxides elimination from the mobile exhaust sources. The main source of NOx emission is the combustion of fossil fuels such as coal in electrical power plants or petroleum in vehicles. However, the combustion gases resulting from fossil fuels contain NOx pollutants consisting mainly of NO and NO2, with NO representing 90 to 95% of the total NOx. Three types of NOx can be distinguished in flue-gas: fuel NOx

– formed by the oxidation of the nitrogencontaining compounds in the fuel thermal NOx – formed by fixation of atmospheric nitrogen prompt NOx – formed by the oxidation of intermediate HCN (Bosch et al. 1987)

Received: 11 February 1999 / Accepted: 1 June 1999 M. Wojciechowska (Y), S. Lomnicki Adam Mickiewicz University, Faculty of Chemistry, ul. Grunwaldzka 6, 60-780 Poznan´, Poland

NO or NO2 formation depends on temperature, oxygen concentration, residence time or light intensity. At temperatures above 1300 7C, the reaction between molecular oxygen and nitrogen introduced with air prevails. In excess of oxygen the NO formation goes according to the mechanism established by Zeldovich (Bosch et al. 1987). The first step of this mechanism requires thermal dissociation of molecular oxygen and the following reactions:

N2cO]NOcN NcO2]NOcO In fuel-rich flames another reaction occurs:

NcOH]NOcH Lower temperatures depress the formation of thermal NOx and fuel and prompt NOx prevail. As the presence of NOx in the atmosphere results in the formation of acid rain, photochemical smog and atmospheric degradation, the control of its emission is of utmost importance. Nitrogen oxides can react with atmospheric components leading to unfavorable changes in their state of equilibrium (Fig. 1). Ozone or peroxide radicals in reaction with NO produce NO2:

O3cNO]NO2cO2 or HO20 cNO]NO2cOH0 Consequently, the NO produced in supersonic air transport plays a role in the destruction of the stratospheric ozone layer. Nitrogen dioxide is a toxic substance which affects the respiratory system of humans and animals, and can produce dizziness and abdominal pain. In the extreme, it can be lethal. Photochemical decomposition of NO2 gives NO and atomic oxygen:

NO2chn (l~415 nm)]NOcO the latter reacting with O2 to form ozone. The formation of ozone in lower parts of the atmosphere (troposphere) is unwelcome because of its effect on human health and its destructive effect on green plants. The reaction between NO2 and hydrocarbons leads to peroxyacetylnitrate (PAN) which is more harmful than ozone and inhibits photosynthesis. Additionally, in the pressure of water, NO2 converts to nitrous and nitric acid. These two compounds are components of acid rain. High concentration of NOx in densely populated regions that have appropriate topography and climate is one of the causes of smog formation.

Clean Products and Processes 1 (1999)

238

Fig. 1. Chemical transformations of atmospheric NOx: (r) photochemical processes, (—J) thermal gas-phase processes, (––J) heterogeneous reaction, (....J) dry deposition (Bosch et al. 1987)

water into the combustion chamber or partial flue-gas recirculation achieves this goal and results in reduction of NOx emissions. Pre-combustion/combustion procedures are not very expensive, they generally reduce NOx by less then 50%. Post-combustion methods are much more effective, they can control NOx to 100%, but are very expensive. Some of the post-combustion methods include selective catalytic reduction, selective non-catalytic reduction and scrubbing in which nitrogen oxides are removed from exhaust gases (Fig. 2). To control the nitrogen oxides concentration in the atmosphere, many countries have developed maximum standards for allowed NOx emissions in exhaust gases. In particular, vehicles standards are very restrictive. In the beginning, such standards concerned only gasoline engine vehicles (since 1982) requiring the car industry to equip cars with catalytic exhaust converters. In 1988, standards for Diesel engines were introduced in Europe. These standards were sharpened in 1993. In 2000, cars with Diesel engines in Europe must reduce NOx emission to 0.45 g/km (Fritz et al. 1997). In gasoline engine vehicles, three-way catalysts are being used. The catalysts contain Pt, Rh and Pd supported on stabilized alumina. The name three-way is derived from the three main reactions that proceed over the catalyst:

Emission of nitrogen oxides can be reduced by the application of pre-combustion, combustion and postcombustion methods (Fig. 2). The aim of pre-combustion methods is to decrease the emission of nitrogen oxides by lowering the content of nitrogen compounds in the fuel. Combustion modifications can be introduced by controlling temperature, optimal air/fuel ratio and resi2NOc2CO]N2c2CO2 dence time in the combustion zone by designing furnaces COc1/2O2]CO2 and burners. As mentioned above, lower temperatures of flame limit the thermal NO formation. Injection of steam, CxHyc(xcy/4)O2]xCO2cy/2H2O

NOx CONTROL

Pre-combustion Control

N content of fuel

Primary Measures Combustion Control Clean Techniques

Furnance

Burner

WET SYSTEM

Gas-phase Oxidation + Absorption Gas-phase Oxidation Absorption Reduction EDTA systems

Secondary Measures Post-combustion Control Clean-up Techniques Flue-gas Treatment

DRY SYSTEM

Selective Catalytic Reduction

Selective Non-catalytic Reduction

Adsorption

Nonselective Catalytic Reduction

Non-selective Non-catalytic Reduction

Radiation

Fig. 2. Available techniques to decrease emission of NOx (Bosch et al. 1987)

M. Wojciechowska, S. Lomnicki: Nitrogen oxides removal by catalytic methods

adsorption of oxygen and sulfur atoms, which poison the catalysts. It is generally believed, that in the case of transition metal oxides, the surface oxygen defects are the active sites in this reaction. Recently, Hamada et al., when studying the activity of cobalt oxides, found favourable influence of silver additives on them. Namely, they reported that the oxide catalysts bonded the oxygen atoms formed during NO decomposition, which resulted in catalysts deactivation (Hamada et al. 1990). The promoting effect of silver is related to the inhibition of oxidation due to the low silver affinity to oxygen. Many researchers studied the possible use of perovskite systems as catalysts for NO decomposition. The advantage of such systems is their extreme thermal Fig. 3. Temperature dependence of decomposition of NO over stability, but they have low surface area. Cu-ZSM5 catalyst (Iwamoto at al. 1994) The most promising results for the NO decomposition were obtained on the high-silica zeolite catalysts containing copper ions (Fig. 3, Iwamoto et al. 1994). The Under the limitations of meeting the automotive emis- first to study Cu-ZSM 5 catalysts for NO decomposition was Iwamoto (Iwamoto 1982). Unfortunately, despite sions requirements, the highest conversion of the three much research of the Cu-zeolite systems, there is still key pollutants occur at the stoichiometric air-to-fuel much controversy as to what the active site for this reacratio. In excess of oxygen at air-to-fuel ratio 1 14.6 (so tion is. Namely, the catalytic activity of the Cu-zeolite called lean exhaust gas), the oxidation reactions for CO and HC are dominant. Under rich exhaust gas conditions, system containing Cu 2c ions is very high, while that of Cu-nonzeolite is very low. Besides, the activity of the i.e., air-to-fuel ratio ~14.6, the reaction of NOx occurs zeolites increases with the amount of Cu 2c ions. It is with high conversion rates (Kreuzer et al. 1996). Only in concluded that the catalytic activity is related only to the a very limited range of air-to-fuel ratio close to stoiCu 2c ions or desorbed lattice oxygen of copper oxide chiometry, the so-called window, all three major pollutants are converted at high levels. The air-fuel mixture is located in the zeolite. Also, Shelef (1992) suggested that the active sites for NO decomposition are coordinatively regulated by a closed-loop control with electronic injection system in combination with an oxygen sensor that is unsaturated Cu 2c ions and that the redox cycle of the catalysts does not take place. However, Valyon and Hall needed to regulate the fuel injection system. Addition of (1993) indicated that Cu 2c and Cu c can act together as cerium oxide (Kreuzer et al. 1996) and/or other rare a coupled active center. The majority of researchers earth oxides into the catalyst (which are responsible for agree, that the decomposition of nitrogen oxide proceeds the oxygen storage) broadens the window to guarantee in accordance with the redox cycle of the catalyst. high three-way activity. However improvements are needed to the present day catalysts to reach a new stand- Iwamoto et al. (1994) reported, that Cu c ions participate in the reaction as the active centres while NO – groups act ards for pollutants reduction in gasoline engine exhaust. as intermediates: There is an immediate need to develop catalysts which can operate with excess oxygen (lean NOx control) conditions to meet the NOx standards for Diesel engine exhaust.

2 Catalytic decomposition of NO At low temperatures, nitric oxide is a thermodynamically unstable molecule when compared with N2 or O2 (Iwamoto et al. 1994). Thus, its catalytic decomposition seems to be the simplest and cheepest method to remove NOx from exhaust gases. However, the high activation energy of NO decomposition (364 kJ/mol) makes it necessary to use a catalyst (Fritz et al. 1997). Many papers have been devoted to the catalytic decomposition of nitrogen oxide over noble metals, transition metal oxides (Lee et al. 1994, Iwamoto 1996), and zeolites. Among the supported noble metals the platinum catalysts were found to be the most active. Other studied Also other researchers (Centi et al. 1993) indicated systems were supported Pt, Rh, Au, Pt-Rh, Pt-Ni and Ptthat nitric oxide adsorbed on Cu c ions acts as an active Au. However, in the case of noble metal catalysts the activity decrease with time on stream, due to irreversible intermediate. In the above speculations, it was assumed

239


240

3 Reduction of NO by various reducing agents In general, reducing agents for NOx can be divided in two groups: – the selective reducing agents for instance ammonia or hydrocarbons – non-selective agents: H2, CO. The difference between these two groups stems from the fact that for the second group oxygen present in reacting gases competes with NO and reacts favourably with the reducing agent resulting in its oxidation. Thus, when choosing a catalyst for nitrogen oxides removal one should establish conditions in which (in the presence or absence of oxygen) it will have to work. In the case of gasoline engines, where the reaction mixture is near the stoichiometric oxidants/reductants ratio, the NOcCO process is favourable (low amounts of oxygen), whereas for Diesel or lean-burn engines (presence of excess oxygen) the reaction should be conducted with selective reductants.

3.1 Selective reduction of NO by ammonia One of the first and most effectively applied catalytic methods of NOx emission reduction in conventional electric power plants is the Selective Catalytic Reduction process (SCR) (Shelef 1995). In this reaction ammonia is used as the reducing agent and the overall stoichiometric reactions are as follows:

100 500ppm 0ppm 1000ppm 80

NO conversion (%)

that the redox cycle of the catalyst takes place during the reaction: Cu cpCu 2cce –. The unique activity of the Cu-zeolite catalysts is a consequence of the method of copper deposition. Namely, it is introduced into the system by ion exchange which allows copper ions to be obtained at atomic dispersion. However, the systems based on zeolites were not commercialised because of the rapid decrease of their activity in the presence of traces of water vapour or SO2 in the reacting gases.

60

40

20

0 150 200 250 300 350 400 450 500 550 Temperature (°C)

Fig. 4. NO conversion in NOcNH3 reaction versus temperature for various O2 feed concentration on V2O5/TiO2 catalyst (1.3% of active phase) (Went et al. 1992).

1970s, when it was applied for the first time. However, the SCR process has many drawbacks: the size of the installation limits its application to stationary sources of NOx emission and can produce additional environmental pollution as a result of ammonia leaks. High costs of the SCR installations are also a disadvantage. Thus, it is very important to find alternative effective methods of nitrogen oxides reduction. Much promising is the method involving the substitution of ammonia with other reducing agents like CO or hydrocarbons. The advantage of this method is that both CO and hydrocarbons are present in exhaust gases, and there is no need for introducing additional reactants.

3.2 Reduction of NO by CO

4NH3c4NOcO2p4N2c6H2O

3.2.1 Reduction of NO by CO over supported metals

4NH3c2NO2cO2p3N2c6H2O

As was mentioned earlier, much of the catalytic research devoted to NO reduction involves supported noble metals. The very early works (Schlatter et al. 1977) indicated that the catalysts containing ruthenium and rhodium show the best properties for the reduction of nitrogen oxide with carbon monoxide. These authors established the sequence of activity of noble metals in NOcCO reaction: Ru 1 Rh 1 Pd 1 Pt (Fig. 5). Although the ruthenium systems are the most active, their activity is unstable which is due to the formation of volatile ruthenium oxides during the reaction, and a decrease of the amount of the active phase. This is the reason why the catalysts applied most often in NOcCO reaction are the rhodium ones. Among the rhodium catalysts, the systems supported on alumina, silica and titania attracted much attention. A distinct relation between the selectivity to various nitrogen oxides and the support used, was observed. (The reaction can lead to the formation of N2, N2O and NO2,

Titania promoted with tungsten, molybdenum and vanadium oxides makes a typical SCR catalyst (Bosch et al. 1987).The above reactions require the presence of oxygen (Fig. 4). The presence of gas-phase oxygen enhances the catalyst activity and this effect is more pronunced at higher vanadia loading. The active sites seem to be the (VpO) 2c groups in the case of vanadia catalysts. The reaction mechanism is thought to proceed via direct reaction between the adsorbed ammonia molecule and gaseous NO:

NH3*cNO]H*cN2cH2O or by the intermediate nitrosoamine:

NOcNH2*H]NO–NH2*H]H*cN2cH2O The SCR by the ammonia method has found many applications in Japan, Germany and USA since the early


that these catalysts are generally used as 3-way automotive catalysts. In such systems, the formation of surface alloys of Pd and Rh play an important role. It is very important for this system to maintain appropriate air-tofuel ratio that was discussed in section 1. Palladium can be the basis for other bimetallic systems. Introduction of the second metal can boost the selectivity of the system in the nitrogen oxide reduction process. The research by Hamdaoui et al. (1994) on the bimetallic Pd-Cr catalysts supported on silica revealed that chromium modified the structure of palladium. As a consequence, the activity of Pd-Cr/SiO2 system increased when compared with that of Pd/SiO2 catalyst. In conclusion, noble metal catalysts are very efficient in the NOcCO reaction. However, their high cost stimulates further search for other, much cheaper systems. Fig. 5. Activity of some supported noble metals in NOcCO reaction. Reaction conditions: 0,5% NO; 2,0% CO; 97,5% Ar; 24,000 GHSV. (Kobylinski et al. 1973)

which is discussed in section 3.2.3, of course only N2 is the desired product.) While the catalysts supported on alumina revealed almost 100% selectivity in a wide temperature range, the systems deposited on silica were found to be non-selective. Belton and Schmieg (1993), when studying the NOcCO reaction over the Rh {111} monocrystal, proved that these differences corresponded to the size of Rh clusters formed during the preparation, which was a consequence of different metal-support interactions. Similarly, Schwartz et al. (1994) indicated a close relationship between the active phase distribution and activity. They found, that a decreasing metal dispersion (or coverage) on the surface resulted in a decrease in the activity for the NOcCO reaction. For rhodium catalysts, a better ratio of the mass of the active phase to its surface area can be achived by adding trace amounts of cerium (Krause et al. 1993). Such catalysts show higher activities when compared with those not containing Ce. On the other hand, the admission of cerium causes a decrease of selectivity to N2 (Schwartz et al. 1994), as the presence of the Ce 3c ions promotes N2O formation. Pretreatment conditions also significantly affect the activity of rhodium catalysts. The activity of rhodium catalysts depends on the activation conditions – slightly oxidative conditions caused an increase in, and stabilization of, activity. The increase of the reduction level of nitrogen oxide is connected with the formation of cationic rhodium sites during the activation process. Namely, oxygen atoms penetrate inside the metal clusters, making the catalyst resistant to reduction. The presence of oxygen greatly affects the properties of the surface Rh atoms. Bimetallic noble metal catalysts are often more active, selective, stable and resistant to poisoning than the systems containing only one metal. The systems containing both Pd and Rh supported on oxides have been the subject of frequent study. It should be noted,

3.2.2 NO reduction by CO over oxide catalysts Transition metal oxides are the subject of much research because of their possible application for nitrogen oxides reduction by carbon monoxide. Shelef et al. (1968) has already indicated different behaviour of particular metal oxides supported on alumina in the above reaction. They suggested the following order of metal oxides activity:

Fe 1 Cu 1 Cr 1 Ni 1 Co 1 Mn 1 V Others, like Kobylinski and Taylor (1973) reported that the temperature at which 90% of NO is converted can be a measure of activity. They observed that the temperature for Cr2O3/Al2O3 catalyst is 325 7C while for Fe2O3/Al2O3 it was only 280 7C. Admission of water vapour into the reacting mixture reduced the activity of both systems, and in this case the temperature for 90% conversion increased to 520 7C and 485 7C for chromium and iron oxide catalysts, respectively. The adsorbed water would be slowly desorbed and its catalytic activity would return to its original level. The activities of unsupported Cr2O3 and NiO were also studied (Gassan-Zade et al. 1987). The chromium oxides were found to be less active than NiO. In the case of the latter, molecular nitrogen started to form at 325 7C, and 100% yield of N2 was observed at 470 7C. For Cr2O3 the corresponding temperatures were 430 7C and 500 7C. The presence of molecular oxygen decreased the activity of both systems. This effect was explained by the competitive interaction between O2–CO and NO–CO on NiO catalyst. Halasz et al. (1993) performed experiments of NOcCO reaction on MoO3 and PdO supported on alumina. The molybdena preparations showed low activity (F25% of NO conversion to N2 at 550 7C) while palladium oxide converted 99% of NO at 100 7C. However, the activity of these systems decreased in the presence of small amounts of oxygen. This can be avoided by the preparation of mixed oxide system, PdO–MoO3/Al2O3. Such catalysts reveal activity similar to PdO/Al2O3 system under anaerobic conditions and their activity in the presence of oxygen is much higher relative to single metal oxide systems.

241


Interesting results of NOcCO reaction on CuO/MgF2 and Cr2O3/MgF2 were reported by Wojciechowska et al. (1997). They found almost 100% activity and selectivity for these catalysts in NOcCO process at low temperatures (100–200 7C). However, a decrease of their activity was observed with time on stream (Fig. 6).

3.2.3 The NOcCO reaction mechanism 242

To find new catalysts, which would be very active and selective in NOcCO process it is necessary to know the reaction mechanism. Despite much research devoted to this problem, the reaction mechanism is still not fully understood. Actually, the scientists agree that the first and necessary stage of the reaction is the adsorption of NO on the surface of the catalyst (Wojciechowska et al. 1997):

species do not participate in the formation of the reaction products; they are the by-products (Boccuzzi et al. 1994). This was confirmed by Krause et al. (1993), who did not detect the presence of isocyanate species on Cu/ TiO2 system. They explained it by their low stability on such a system. The stability of isocyanate species is determined by the supports used. For example these species quite quickly decompose on the Pt/TiO2 system while in the case of Pt/SiO2 they are extremely stable. On the basis of the above, the next steps of the NOcCO reaction can be as follows:

NO*cS]N*cO*

(3)

O*cCO*]CO2c2S

(4)

N*cCO*]NCO*cS

(5)

NOcS]NO*

(1)

The bimolecular mechanism cannot be excluded, although it may not the main path of the reaction:

COcS]CO*

(2)

NO*cCO*]N*cCO2cS

Controversies appear in the description of the next stages. Some authors suggest that the reaction proceeds via a unimolecular mechanism, while others propose a bimolecular mechanism. In the first case, the adsorbed NO molecule dissociates on the surface, forming oxygen atom which reacts with CO. In the bimolecular mechanism the adsorbed NO reacts directly with the adsorbed CO molecule. Kinetic studies of this reaction seem to substantiate the dissociative unimolecular mechanism. Namely, after the dissociation of adsorbed NO, adsorbed nitrogen and oxygen atoms appear. The adsorbed nitrogen atoms are responsible for the formation of the surface isocyanate species (Kobyli0 n˜ski et al. 1973). However, it should be noted that the surface isocyanate

(6)

It appears, that properties of particular catalysts cause the reactions to follow the unimolecular or bimolecular path and thus determine the reaction rate. The faster the surface dissociation of adsorbed NO, the less probable is reaction (6), and the more probable is the unimolecular mechanism for the main path of reaction. When the surface dissociation of adsorbed NO is slow, the probability of reaction (6) is high and its contribution to this reaction path increases. Thus, it can be suggested, that the surface dissociation of NO (reaction 3) determines the reaction rate, i.e., it is the rate determining step. The rate of dissociation on the surface of solids is affected by the type of catalyst, i.e., the ability of the surface to give electrons (Kobylinski et al. 1973). For example, for Pt/ ZnO catalyst NO already dissociates on some platinum sites at room temperature. Fink et al. (1991) found, that the formation of active phase islands and surface vacancies is necessary for NO dissociation on platinum catalysts. Others (Boccuzzi et al. 1994) indicated a faster rate and better yield of NOcCO reaction on reduced catalysts. How does the dissociation of NO proceed? Cleavage of the N–O bond requires that it should be first weakened. The NO molecule has one electron at the antibonding orbital p*. Its orbital energy is higher than that of the electrons on atomic orbitals of nitrogen and oxygen. Thus, donation of this electron lowers the energy of the molecule which results in its higher stability. However, the introduction of another electron on the antibonding orbital p* will make the molecule unstable and enable its dissociation. The dissociation of NO on the surface of catalyst will proceed as follows:

NOce –]NcO – Fig. 6. Catalytic activities of various metal oxides supported on magnesium fluoride (M/Mgp0,1; MpV, W, Cu, Cr, Mo) in the NOcCO process expressed as TOF. Reaction conditions: NOp1% in He; COp1% in He and O2p1% in He; total flowrate of 60 ml/min. (Wojciechowska et al. 1997)

It is clear from the above that the dissociation of NO can take place at the sites capable of donating an electron, which is consistent with the finding that the reaction rate was faster on reduced catalysts. Apart from N2, which is the main NOcCO reaction product, also N2O can be formed.


systems in NOcHC reaction: ferrierite, modernite, Y, and others. For some of them the observed conversions N2O*]N2OcS (8) were very high – the Co-Zeolite systems seem to be the N2O*]N2cO* (9) most reactive catalysts among zeolites – however, also in this case, the unfavourable influence of water vapour on Thus, the formation of N2O is determined by the rate its activity is reported. of N2O* dissociation and desorption. Interesting results were presented by Yokoyama et al. The last stage of the NOcCO reaction is a recombina- (1994), who used the Ce-ZSM5 catalyst mechanically tion of the adsorbed nitrogen atoms giving a nitrogen mixed with Mn2O3 for the NOcC3H8cO2 reaction. They molecule: reported that this mechanical admixture improved the N*cN*]N2c2S (10) activity, though pure Mn2O3 was completely inactive itself. Such mixed catalyst revealed the activity of over It is also very important to elucidate the reasons for 80% NO conversion at low temperatures (F250 7C). the deterioration of the catalysts activity in the NOcCO Unfortunately, the authors did not provide the informaprocess, despite their high initial activity. Wojciechowska tion on the influence of moisture on the activity. et al. (1997) indicated, that apart from the commonly Summing up, the majority of the NOcHC catalysts known reasons of their deactivation, the strongly reported are the metal-containing zeolites such as Madsorbed nitrates and nitrites on the surface of catalysts ZSM5. The series of activity in the NOcHC reaction can are also important. They performed in situ IR experibe established. In the case of the NOcC2H4cO2 over Mments of NOcCO reaction on highly active CuO/MgF2 ZSM5 such a series will be as follows (M is the ion system and detected the appearance of intensive bands exchanged) (Iwamoto et al. 1994): characteristic of these groups. These bands are marked in the spectra even after vacuum treatment at 200 7C for AgpCopZnpCu 1 Hp 12 h. Such nitrate and nitrite groups are formed as a Ni 1 Pt 1 Mn 1 FepCapLapPd 1 Cr 1 Na result of the surface reaction between NO and the surface However, for every zeolite system the moisture present oxygen atoms. in the reacting gases significantly reduces the activity. Currently there is a tendency to look for mixed or dotted 3.3 zeolite catalysts in order to prevent the unfavourable Reduction of NO by hydrocarbons (HC) impact of moisture and also increase their activity. Among these the interesting catalysts are the ones 3.3.1 containing small quantities of alkaline-earth metal Reduction of NO by HC over zeolite catalysts cations (Stakheev et al. 1996) or Fe-ZSM5 catalysts (Chen Among the zeolite catalysts examined in the NOcHC et al. 1998) which show high SCR activity and durability process the greatest interest has been aroused by the c both in the absence of H2O and in its presence. At low systems of H exchanged with copper and cobalt ions. temperatures, the presence of H2O even enhances the NO The early research of Iwamoto (1982) indicated the desirconversion. able properties of Cu-ZSM5 catalysts for this reaction. One of its most desirable properties is the ability to conduct the reaction in the presence of oxygen (lean NOx 3.3.2 control). However this system is not perfect despite its Reduction of NO by HC over metallic catalysts great catalytic properties. In the presence of water vapour Among metallic systems promoting the NOcHC reacor SO2, its activity in NOcHC reaction drops drastically. tion, traditionally the most attention is paid to noble Since the Iwamoto’s discovery, the search for new metal catalysts. Obuchi et al. (1993) report, that for the systems more resistant to the above mentioned agents Pt, Ru and Rh catalysts supported on alumina, the has been continued. One of the most promising are the conversion of NO changes with the reaction temperature cobalt-zeolite systems. Stakheev et al. (1996) reported with a maximum of about 50% at 250 7C for Pt/Al2O3 and at 300 7C for Ru/Al2O3 and Rh/Al2O3 (Fig. 7). Obuchi et that for the NOcC3H8cO2 reaction over the Co-ZSM5 catalysts the dependence of NO conversion on the reacal. (1996) studied the activity at various space velocities tion temperature has the volcano-type shape with the and found that the higher GHSV gave better results with maximum shifting with the cobalt content; the lower the olefins for both Pt and Rh supported on alumina, in cobalt content the lower is the temperature of maximum particular in the presence of hydrocarbons containing conversion. The highest conversion (F50%) was oxygen atom(s). The advantage of platinum catalysts is recorded at 450 7C. their resistance to moisture. On the other hand, during A comparison of the hydrothermal stability of Cothe NOcHC reaction, apart from N2, N2O is also formed ZSM5 and Cu-ZSM5 catalysts benefits the first one, which (up to 65% of NO is converted to nitrous oxide (Burch et al. 1997)). is explained by the presence of Co 2c ions stabilized in the zeolite lattice (de Correa et al. 1996). The thermal Other metallic catalysts for the NOcHC process treatment of Cu-ZSM5 above 500 7C results in the forma- described in literature are supported silver and gold tion of Cu 2c ions, inactive in NOx reduction. ZSM5 is systems. For the Ag/Al2O3 the maximum activity in not the only zeolite which was used as a cobalt ion NOcC2H5OHcO2cwater vapour reaction was found for support; many authors have studied various cobalt-zeolite the silver content of 4.0% wt. at a low temperature

N*cNO*]N2O*cS

(7)

243


244

Fig. 7. Activity of various noble metals supported on g-Al2O3 for the selective reduction of NO by C3H6 as a function of temperature. Reaction conditions: NOp1000 ppm; C3H6 p870 ppm; O2p5%; total flow-rate of 160ml/min. (Obuchi et al. 1993)

(F350 7C) and of 1.5–2.0% wt. Ag at higher temperatures ( 1 400 7C). Sumiya et al. (1998), report that in simulated Diesel engine exhaust, 80% conversion of NOx was reached at F500 7C over Ag/Al2O3 catalyst in the presence of moisture and SO2. The gold catalysts supported on various oxides (Ueda et al. 1997) were found to be very active in the NOcC3H6cO2 reaction in the presence of moisture. The interaction between Au and a support plays an important role. Similar conclusions were drawn by Bamweda et al. (1997). They suggest, that the maximum conversion of NO for different gold catalysts is determined by the support used. The highest conversion of NO on Au/Al2O3 catalyst was over 90%. It should be stressed the these catalysts do not deactivate in the presence of moisture. Hamada (1994) presents the activity of various metals supported on alumina in NOcpropane reaction which can be set in the following activity series:

samples containing 4.4% wt. of vanadia. They suggest that the surface polymeric vanadate species are responsible for the activity. Other interesting results were presented by Zhang et al. (1994), who described the La2O3/Al2O3 system, in which lanthanum oxide was promoted by strontium. The catalytic properties of such systems in the NOcHC process were better than those of Cu-ZSM5 catalyst – their activity was higher by 40%. The field of application of oxide catalysts for the NOcHC process is still to be explored. In particular the latest results indicate that the oxide catalysts can be as active as zeolite systems in the process described. For example, Shimizu et al. (1998) revealed that the activity of Ga2O3 supported on alumina, in SCR process with methane, is similar to that of Ga-ZSM5 and its selectivity is even better than that for Co-ZSM5. Besides, Ga2O3/ Al2O3 is highly resistant to the presence of moisture in the reacting gases. Also mixed oxide catalysts: Cu, Ni and Co-Al2O3 with spinel-type structure show high activity and selectivity for SCR of NO by C3H6 in excess oxygen. (Shimizu et al. 1998, Fig. 8). Cu-Al2O3 exhibits higher activity and hydrothermal stability than Cu-ZSM5.

3.3.4 Mechanism of the NOcHC reaction There is much controversy on the reaction mechanism of NO reduction by hydrocarbons. In general, the following main ideas can be discerned (Pârvulescu et al. 1998): 1. Oxidation of NO to reactive NO2 that reacts with hydrocarbons. 2. Oxidative conversion of hydrocarbons forming intermediates by reaction with NOx (e.g. isocyanate radical, oxygen-containing compounds, etc). 3. Redox mechanism involving the successive oxidation and reduction of the catalyst surface by NO and the hydrocarbons.

Co 1 Fe 1 Ni 1 Pt 1 Cu 1 Mn In concluding, one can state that supported metallic catalysts are promising systems for the process described, although their effective use still requires much research. The disadvantage, similarly to the three way catalysts, is their high cost. It seems that a reasonable way is the construction of mixed catalysts to lower their costs without the loss of activity.

3.3.3 Reduction of NO by HC over oxide catalysts Fewer papers are devoted to the application of oxide catalysts in the NOcHCcO2 reaction. An interesting work concerns the V2O5/TiO2 catalyst (Zegaoui et al. 1996). The authors report that the best activity of such a system (about 90% of NO conversion) is observed for the

Fig. 8. Activity of some transition metal oxides supported on g-Al2O3 in NOcC3H6 reaction as a function of temperature. Reaction conditions: NOp1000 ppm; C3H6p2000 ppm; O2p6,7%; total flow-rate of 100 ml/min. (Shimizu et al. 1998)


Some authors suggest that the mechanism of NOcHC reaction can be similar to that of the NOcCO process, where at first the surface reduction of NO to N2 occurs and the adsorbed oxygen atom thus formed oxidizes the hydrocarbon. However, such a mechanism seems to be less probable since it does not explain the favourable effect of the presence of oxygen on the catalytic activity. For each of the reaction mechanisms presented above, the presence of different intermediates is postulated. Matyshak et al. (1997) indicated the presence of Cu 2c–O–NpO complex on the surface of Cu-ZSM5, which is thought to be the intermediate species in the NOcHC process. They propose that in this case the early stage is the reaction between NO and O2 resulting in the formation of surface nitrite group. Satsuma et al. (1998) also support this idea and suggest that in the NOcC3H8 reaction over Na-H-modernite system the intermediate is the NO 3– group adsorbed. Also, Ueda et al. (1997) propose that the necessary stage of NOcHC process is the oxidation of NO to NO2 and it is the slowest reaction stage. Interesting conclusions about this reaction mechanism are presented by Yokoyama et al. (1994). Using the mechanical mixture of Ce-ZSM5 and Mn2O3, the authors revealed that the increase of activity of such a mixed system relative to Ce-ZSM5 catalyst results from the fact that on Mn2O3, the oxidation of NO to NO2 is much more effective than over Ce-ZSM5, and the next stages, i.e., reaction of NO2 with HC take place on cerium sites. Miyadera (1997) indicate, that the adsorbed isocyanate NCO groups are the intermediates in the NOcHC process. They suggest that in such a reaction mechanism N2 molecules are formed in the reaction of NO with the adsorbed NCO group. According to Takeda and Iwamoto (1995), the hydrocarbon molecules after adsorption can be divided into two groups of hydrocarbon species: those active in the reaction with NO and those unreactive. The latter, after the reaction with oxygen, became active in the reaction with NO. The adsorbed hydrocarbons during the reaction with NO form nitrogen containing surface species and undergo subsequent reactions with NO and O2 to give surface isocynate groups. These NCO groups reacting with NO produce N2 and CO2 (Hayes et al. 1996, Pognant et al. 1996). The example of the redox variant is proposed by Burch and Millington (1994). According to them, the hydrocarbon reduces a copper site, giving a coordina-

Lean

NO

O2

NO3− (nitrate)

tively unsaturated Cu c ion site. In the presence of NO, the site generates a dinitrosyl species, which by inversion converts into gaseous N2O and Oad. The nitrous oxide is then decomposed into N2 and another copper ion site. Hamada et al. (1994) suggests that the reaction mechanism is determined by the catalyst used. Thus, he proposes that over H-ZSM5 and Al2O3 the early stage of the reaction is the oxidation of NO to NO2 and in the case of Cu-ZSM5 the partial oxidation of hydrocarbon proceeds at first. He also indicates that the conditions of the reaction, such as temperature or oxygen concentration, also affect the reaction mechanism.

4 NOx storage-reduction catalyst (NSR) A new class of prospective catalysts for the removal of nitrogen oxides from vehicle exhaust are storage-reduction catalysts (Matsumoto 1996). In lean-burn conditions, where oxygen exists in high concentration in exhaust gasses, the oxidation of NOx followed by the accumulation of nitrates thus formed on the surface of NSR catalysts takes place. When the conditions change to a stoichiometric redox or reducing, the adsorbed nitrates are reduced by CO, HC and other reducing components of exhaust gasses and take off the surface in the form of N2. In NSR catalyst the basis of the active phase is platinum modified with alkaline earth metals and metal oxides supported on alumina. The mechanism of NOx reduction is shown schematically in Fig. 9. The conversion obtained is 60% and higher. However, these catalysts are still under study.

5 Concluding remarks There is still much to be done in the field of catalysts for decomposition and reduction of nitrogen oxides emitted to the atmosphere by mobile and stationary sources. It should be stressed that in the studies of these reactions, the components of exhaust gases other than CO and HC, affecting the catalytic process, must be taken into account. In particular these are SO2, water vapour and soot. All these compounds can cause poisoning and deactivation of catalysts. Sulphur dioxide can combine with the surface of a catalyst to form sulphates or can rearrange the surface structure. Water vapour combines

Rich

CO2, H2O N2

HC, CO H2 NO3− (nitrate)

NO2 Pt

Pt

Al2O3

Al2O3

Stored as nitrate

Reduced to nitrogen

Fig. 9. NOx storage-reduction mechanism (Matsumoto 1996)

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