Materials Chemistry A

View Article Online View Journal

Journal of

Materials Chemistry A Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: J. H. Lee, J. Ryu, J. Y. Kim, S. Nam, J. Han, T. Lim, S. Gautam, K. H. Chae and C. W. Yoon, J. Mater. Chem. A, 2014, DOI: 10.1039/C4TA01133C.

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

www.rsc.org/materialsA

Page 1 of 5

Journal Name

Journal of Materials Chemistry A

Dynamic Article Links ► View Article Online

DOI: 10.1039/C4TA01133C

ARTICLE TYPE

Published on 14 April 2014. Downloaded by Pohang University of Science and Technology on 25/04/2014 05:17:52.

www.rsc.org/xxxxxx

Carbon Dioxide Mediated, Reversible Chemical Hydrogen Storage Using a Pd Nanocatalyst Supported on Mesoporous Graphitic Carbon Nitride Jin Hee Lee,a Jaeyune Ryu,a Jin Young Kim,a Suk-Woo Nam,a,b Jong Hee Han,a,b Tae-Hoon Lim,a,b c c a,d 5 Sanjeev Gautam, Keun Hwa Chae, Chang Won Yoon* Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x

10

15

A reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C3N4). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C3N4 with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h-1 even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C3N4 support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H2-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C3N4 also proves to catalyze the regeneration of formic acid via CO2 hydrogenation. Governing factors on CO2 hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity.

Introduction 20

25

30

35

40

45

Efficient and sustainable technologies are being extensively studied to address urgent concerns about energy and environmental issues for future energy production and storage. Utilization of hydrogen via fuel cell is one of the promising alternatives to carbon-based fuels for current power generation. One of the key issues for achieving hydrogen economy is to develop reliable hydrogen storage systems that store large amounts of hydrogen in a secure manner. For the purpose, metal hydrides,[1] metal-organic frameworks,[2] and chemical hydrides[3] have been extensively explored as potential hydrogen storage materials over the last decade. Among these candidates, sodium borohydride and ammonia borane have attracted particular attention as chemical hydrogen storage materials since they can release H2 with high hydrogen storage densities.[4] To make the storage of hydrogen in a molecular form practical, it is important that the hydrogen needs to be economically transported to a desired site. Since the low transportability of solid materials may compromise high hydrogen storage densities, liquid chemicals seem to offer the greatest potential as hydrogen carriers because the storage and transport of such compounds are relatively straightforward. Formic acid (FA), a nontoxic liquid readily obtainable from biomass processing,[5] has recently been recognized as a safe and reversible hydrogen storage material.[6] The chemically stored H2 from FA can be catalytically liberated even at room temperature for polymer electrolyte membrane fuel cells (PEMFCs) (HCOOH This journal is © The Royal Society of Chemistry [year]

Scheme 1 The CO2 mediated hydrogen energy cycle

50

55

60

65

→ CO2 + H2). From the perspective of H2 storage, FA can be regenerated from the hydrogenation of carbon dioxide (CO2 + H2 → HCOOH). The use of CO2 as a hydrogen carrier (Scheme 1) could be achievable economically in conjunction with the utilization of renewable power sources.[6] Catalyst design would thus be one of the key processes to realize a CO2-mediated hydrogen energy cycle. The dehydrogenation of FA has recently been studied using a number of homogeneous Ru,[7] Ir,[8] and Fe[9] catalysts, as well as heterogeneous Pd,[10] Au,[11] and Pd-based alloy or core-shell[12] catalysts. Many of these catalytic systems employed external bases to promote gas production from FA.[7-8,9a,10a,11,12c-d] Specifically for heterogeneous catalysts, metallic alloy or coreshell nanoparticles (NPs) have been synthesized to avoid use of bases. For example, Tsang et al. reported various M@Pd coreshell catalysts (M=metal) and found that Ag@Pd showed the highest activity for FA decomposition due to the high electron donating ability of Ag.[12e] In the same vein, Zhang et al. developed AgPd alloy catalysts that also exhibited an high initial [journal], [year], [vol], 00–00 | 1

Journal of Materials Chemistry A Accepted Manuscript

Cite this: DOI: 10.1039/c0xx00000x


Page 2 of 5 View Article Online

Scheme 2 Pd/mpg-C3N4 catalyzed hydrogen production and storage

5

10

15

20

25

30

35

turnover frequency (TOF) of 382 h-1 at 50 °C in the absence of bases.[12a] Furthermore, the AuPd/C catalyst for FA dehydrogenation was also reported by the same research group.[12f] Moreover, combination of Co and PdAu also enabled FA dehydrogenation without bases.[12b] Very recently, Cai et al.[13] designed a Pd-based photocatalyst for FA dehydrogenation. The authors employed carbon nitride as the semiconductive support for photo-assisted H2-releases. Despite these steps forward for FA dehydrogenation, only a few catalysts that can promote both FA dehydrogenation and CO2 hydrogenation have been reported. One of these catalysts was reported by Hull et al. who recently demonstrated the reversibility of CO2-based hydrogen storage, catalyzed by a homogeneous Ir-based system.[8a] In addition, Beller and co-workers also showed that a Ru homgoenerous catalyst proved to catalyze both FA dehydrogenation and CO2 hydrogenation.[7c] However, to the best of our knowledge, no viable heterogeneous catalytic system catalyzing both directions for reversible catalysis has been established to date. We report here on Pd NPs supported on mesoporous graphitic carbon nitride (mpg-C3N4) for FA based reversible hydrogen storage. This Pd/mpg-C3N4 material was first demonstrated as a catalyst completing the CO2-based hydrogen storage cycle at a single heterogeneous catalyst (Scheme 2). The as-developed Pd/mpg-C3N4 material was shown to be a superior catalyst for efficiently promoting FA dehydrogenation under ambient conditions, and afforded a TOF of 144 h-1, which is comparable to the highest value (158 h-1) ever reported among heterogeneous catalysts for base-free FA dehydrogenation at room temperature.[12a] Nitrogen atoms located at mpg-C3N4 were suggested to play dual roles in providing adsorption sites for Pd ions to form highly dispersed, small-sized Pd nanoparticles without any stabilizing ligands as well as in enhancing FA deprotonation to accelerate dehydrogenation. In addition, this monometallic Pd/mpg-C3N4 catalyst was also capable of catalyzing CO2 hydrogenation to yield FA with high selectivity.

50

55

60

65

70

75

Results and Discussion 40

45

Metallic nanoparticles have shown to possess high activities due to the large surface area and the presence of catalytically active sites such as edges and steps. However, large quantities of ligands are typically needed to prevent catalysts aggregation, which could produce significant amounts of chemical wastes during a synthetic procedure. In this study, Pd nanoparticles (Pd NPs) were immobilized onto mpg-C3N4 by stirring of an aqueous solution containing Pd(NO3)2•2H2O (32 mM) in the presence of 2 | Journal Name, [year], [vol], 00–00

80

85

Fig. 1 HRTEM analyses: a) and b) Images of Pd/mpg-C3N4, c) HADDFSTEM image of Pd/mpg-C3N4, and d) particle size distribution of Pd/mpg-C3N4

the support, followed by reducing the adsorbed Pd2+ using H2 (g) without any additional stabilizing agent (Fig. S1). The X-ray diffraction (XRD) patterns, pore diameters, and surface areas of mpg-C3N4 and Pd/mpg-C3N4 are compared in the supporting material (Fig. S2 and S3; Table S1). Resulting catalyst contains 9.5 wt% Pd as evidenced by inductively coupled plasma mass spectrometry (ICP-MS). The Pd NPs were found to be uniformly distributed over the support with an average size of 1.7 nm without any agglomeration, as indicated by high-resolution TEM (HRTEM) and scanning transmission electron microscopy (STEM) (Fig. 1a-d). The highly dispersed, small-sized NPs likely resulted from nitrogen atoms located at mpg-C3N4 via initial intermolecular interactions with the Pd2+ ions; i.e., the cationic Pd2+ precursors preferentially interacted with the nitrogen atoms at mpg-C3N4. Moreover, the described simple synthetic procedure enabled large-scale preparation (> 3 g) of the catalyst. Upon Pd deposition onto the support, the observed graphitic stacking of mpg-C3N4 was clearly maintained in the XRD pattern at a 2θ of 23.5°. However, peaks corresponding to metallic Pd NPs appeared very broadly around 40°, presumably due to the small size of the Pd NPs (Fig. S2). Catalytic activity of Pd/mpg-C3N4 for FA dehydrogenation was evaluated under ambient conditions. Nearly no promotion was observed for the dehydrogenation of an aqueous FA (1.0 M), with mpg-C3N4 alone (Fig. 2, blue). In contrast, significant acceleration with the Pd/mpg-C3N4 catalyst was clearly found even in the absence of any bases. In addition, this catalyst exhibited much faster rate of H2-release compared to a commercial, well dispersed Pd/C with the size of ca. 2 nm (10% Pd, Fig. S4) under the same conditions. These results strongly suggest that (i) Pd NPs are catalytically active species, and (ii) basic nitrogen atoms located at mpg-C3N4[14-15] helped to enhance FA deprotonation that was proposed as an important initial step for FA dehydrogenation. Both the Pd metals and the support thus played a synergetic role in enhancing the rate of H2-release from This journal is © The Royal Society of Chemistry [year]



DOI: 10.1039/C4TA01133C

Page 3 of 5

Journal of Materials Chemistry A View Article Online

DOI: 10.1039/C4TA01133C

350

Pd/C mpg-C3N4

volume (mL)


250 200 150 100 50 0 0

20

40

60

80

100 120 140 160 180

time (min)

Fig.2 Gas production profiles for FA dehydrogenation catalyzed by Pd/mpg-C3N4, Pd/C, and mpg-C3N4. The material (50 mg) was employed to dehydrogenate FA (1.0 M, 10 mL) at 25 °C 5

10

15

20

25

30

FA. In addition, FA dehydration (HCOOH → CO + H2O) which yields CO, detrimental to PEMFCs, as a byproduct did not found in analysis of gaseous products by FT-IR spectroscopy (Fig. S5). The TOF of the Pd/mpg-C3N4 catalysts was calculated to be 144 h-1 (initial TOF at 10 min), which is higher than the recently reported highly active CoAuPd/C (TOF of 80 h-1)[12b] and comparable to the AgPd/C catalyst (TOF of 158 h-1).[12a] Furthermore, our system showed even higher TOF than those with a recently developed Pd@CN utilizing light during the same period.[13] The TOFs of the recently developed heterogeneous catalysts are compared in Table S2. The observed superior performance of Pd/mpg-C3N4 again likely originated from the synergetic functions of the well-dispersed small-sized Pd NPs as well as basic sites contained in mpg-C3N4.[14-15] Moreover, the Pd/mpg-C3N4 catalyst further demonstrated its high stability, and upon repeated dehydrogenation processes (5 times), no catalytic deactivation occurred (Fig. S6). Controlling factors for the dehydrogenation process were further identified by varying FA concentration. The rates of H2release increased as FA concentration increased from 0.50 M to 3.0 M; however, higher concentrations (> 4.0 M) were found to have a negative effect on dehydrogenation kinetics (Fig. 3), affording a volcano-shaped curve with the highest value being 150 h-1 at 3.0 M (Fig. 3). Despite the reduced activities at higher concentrations of > 4.0 M, the catalyst still exhibited enhanced activity compared to previous results even at a high concentration

Fig. 3 Gas production profiles of FA dehydrogenation at various concentrations and a plot of initial TOFs as function of FA concentrations (inset).

This journal is © The Royal Society of Chemistry [year]

35

Fig. 4 a) Gas production profiles of FA dehydrogenation at various temperatures, and b) an Arrehenius plot obtained from the rate data.

40

45

50

55

of 12 M, which likely provides advantages for practical applications. The rate of H2-release was further found to increase as temperature increased, and the calculated TOF at 55 °C reached 324 h-1 (Fig. 4a). Moreover, an Arrehnius plot obtained from the rate data gave apparent activation energy of 29.1 kJmol-1 (Fig. 4b), which is consistent with previous results ranging from 22 to 49 kJmol-1.[11a, 12a, 12e] To gain insight into the potential interactions between Pd and C3N4, we performed density functional theory (DFT) calculations using three melm units (Fig. 5a, red circle). The resulting optimized structure (1) contained three different types of nitrogen sites (N, N΄, and N˝) inside C3N4 (Fig. 5b). The optimized geometry showed a corrupted structure with a dihedral angle of N-C1-N΄-C2 of ca. 22° (Fig. S7), which is consistent with a

Fig. 5 a) Molecular structure of graphitic carbon nitride (red circle, a model unit for mpg-C3N4), b) a DFT-optimized structure (1) using a truncated C3N4 unit, and c) the calculated HOMOs of 1

Journal Name, [year], [vol], 00–00 | 3


Pd/mpg-C3N4 300


Page 4 of 5 View Article Online

DOI: 10.1039/C4TA01133C

Pd/mpg-C3N4, Et3N +

H2

HCOOH•NEt3

H2O

Scheme 3 FA synthesis catalyzed by Pd/mpg-C3N4


Table 1 CO2 reduction catalyzed by Pd/mpg-C3N4.a Entry 1 2 3 4 5 6 7 8d 9e Fig. 6 The XANES spectra of mpg-C3N4 and Pd/mpg-C3N4, a) N K-edge and b) C K-edge 50

5

10

15

20

25

30

35

40

45

previous study employing repeated melamine and melem building blocks.[16] Natural bond orbital (NBO) charge analyses revealed that compared to that of N΄ (δ; -0.422 and -0.429), the sp2 hybridized N and N˝ sites (δ; -0.517, -0.525, and -0.529) were more negatively charged (Fig. S8). In addition, molecular orbital analyses further indicated that HOMOs localized over the sp2hybridized nitrogen atoms located inside the three melm units (Fig. 5c). Notably, the sp2 N (or N˝) sites appear to have more diffused orbitals than the N΄ sites, implying that Pd2+ ions from the precursor or acidic proton of FA likely interacted with N or N˝ sites. These nitrogen sites may have induced the formation of small-sized Pd NPs which then stabilized the obtained particles, even in the absence of surfactants. Moreover, the reduced Pd(0) NPs can likely interact with the nitrogen atoms at C3N4, and the electron density of the nitrogen atoms could then be transferred into Pd NPs To validate this hypothesis, potential interactions between Pd and N atoms at Pd/mpg-C3N4 was further elucidated by X-ray absorption near edge structure (XANES). In the N K-edge spectra, the sharp and intense π* peaks of mpg-C3N4 corresponding to the pyridinic and graphitic N species[17] were observed at 399.2 and 402.1 eV while those of Pd/mpg-C3N4 were appeared at 399.4 and 402.3 eV (Fig. 6a). In the σ* region, a broad peak at > 405 eV were observed, which is attributed to CN conjugated double bonds (407-410 eV) and/or C-N double bonds (411-415 eV).[17a] Increases in peak intensities after Pd deposition indicate that charge transfer from N sites to Pd occurred via orbital hybridization between N and Pd.[17c] In addition, the photon energies of Pd/mpg-C3N4 were found to be shifted into high energies, which also supports the electron transfer from N into Pd.[17b] Likewise, the C K-edge spectra reveal that the peak intensities associated with carbon species increased following Pd deposition in both π* and σ* region, suggesting C orbital perturbation resulted from electronic interactions between Pd and C and/or between Pd and N (Fig. 6b).[17c-d] These results are consistent with the improved activity of Pd/mpg-C3N4 for FA dehydrogenation compared to the commercial Pd/C catalyst (Fig. 2); i.e., the enhanced charge density at Pd/mpg-C3N4 facilitated the rate of H2-release from FA.[12b-e] For this catalytic system to be useful for practical applications, CO2 hydrogenation should be achieved for reversible hydrogen 4 | Journal Name, [year], [vol], 00–00

55

60

65

70

75

80

85

90

Pressure (bar)b CO2 H2 20 20 13 27 27 13 20 20 13 27 10 30 5 35 13 27 13 27

Temp. (oC)

FA (mmol)c

100 100 100 150 150 150 150 150 150

1.74 1.70 1.22 3.62 4.74 4.26 3.44 2.05 n.d.

a Mixtures of D2O (10 mL), triethylamine (2.5 mL) and Pd/mpg-C3N4 (50 mg) were stirred for 24 h. b Pressure at 298 K. c Determined by 1H NMR using acetone as an internal standard. d 10% commercial Pd/C was used as a catalyst. e mpg-C3N4 was used as a catalyst..

storage. Only a few studies related to FA synthesis via the hydrogenation of CO2 have been reported using homogeneous and heterogeneous catalysts.[18] In the case of heterogeneous catalysts, they still suffered from low yield and selectivity or from the requirements of high pressure.[18f-g] The C3N4 material has been employed for CO2 capture and CO2 activation to catalyze chemical reactions such as benzene or cyclic olefin oxidation and cycloaddition to epoxide.[19] Given these results, we considered another avenue for the potential reversibility of FA dehydrogenation by reacting CO2 with H2 using a Pd/mpg-C3N4 catalyst. Since FA can further react with H2 to give highly reduced products such as methanol and methane, triethylamine was employed to trap the formed FA (Scheme 3). The total pressure of gases (CO2 and H2) was regulated to 40 bar, and upon heating the mixture at 100 °C and 150 °C, this pressure increased to 51 bar and 61 bar, respectively. The formation of FA was found to be dependent upon reaction conditions (Table 1). In particular, the relative ratio of CO2 and H2 influenced the productivity of FA (entry1-3). For example, a decrease in CO2 pressure did not notably affect the quantity of FA (entry 2), while activity decreased under a lower H2 pressure, although CO2 pressure was higher (entry 3). These results are likely due to the enhancement of CO2 solubility by the added Et3N, and this hypothesis was supported by a 13C NMR experiment that showed the formation of the Et3N•CO2 adduct upon pressurizing CO2 in a reactor containing Et3N and D2O (Fig. S9). The obtained results imply that high pressure of hydrogen is required to attain high yields. Temperature also influenced the FA formation, and upon CO2 hydrogenation at 150 °C, FA yield increased significantly (entry 1 vs 4 and 2 vs 5). In this preliminary study, the best activity was observed upon utilization of the CO2 and H2 pressures of 13 bar and 27 bar, respectively, to give 4.74 mmol of FA (entry 5). Further decreases in CO2 pressure, however, showed slightly reduced activities (entry 6-7). Compared to the commercial Pd/C catalyst, an increased affinity of Pd/mpg-C3N4 toward CO2 resulted in > 2 times higher catalytic activity (entry 8). In the absence of the Pd, no products except Et3N•CO2 were formed (entry 9). Notably, in all cases, our catalytic system produced only FA without any byproduct, as evidenced by 1H and 13C NMR spectra (Fig. S10). This journal is © The Royal Society of Chemistry [year]


CO2

Page 5 of 5

Journal of Materials Chemistry A View Article Online

Conclusions


5

10

15

20

In conclusion, we established an efficient process for immobilizing Pd NPs on mpg-C3N4. This green and convenient procedure requires no ligands and is easily applicable to largescale preparation. The resulting small-sized catalyst, Pd/mpgC3N4, showed superior activity for hydrogen production from FA without any additives under ambient conditions. The obtained excellent activity likely originated from the large number of nitrogen functionalities in mpg-C3N4, which played key roles as both stabilizers for Pd nanoparticles as well as basic sites for FA activation. In addition, the developed catalyst also demonstrated its capability to synthesize FA selectively with the aid of triethylamine. The basic sites of the support could also induce initial interaction with CO2 for the synthesis of FA. Relevant works associated with the exploration of plausible mechanisms for FA dehydrogenation, as well as the optimization of reaction conditions for FA synthesis, are now under investigation. Further improvement of catalytic activity and durability resulting from this catalyst represents a real opportunity to attain a CO2mediated, reversible hydrogen storage system to meet future energy requirements.

60

65

70

75

80

85

Acknowledgements

25

This research was funded by the Center of Excellence (COE) program of the Korea Institute of Science and Technology. The part of this research was also supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) of the Ministry of Knowledge Economy (MEST) (the New Renewable Energy Program, NO.20113030040020).

90

95

Notes and references 30

35

40

a Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,Seongbuk -gu, Seoul 136-791, Republic of Korea; Email: [email protected] b Green School, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-701, Republic of Korea c Advanced Analysis Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,Seongbuk -gu, Seoul 136-791, Republic of Korea d Deparment of Clean Energy and Chemical Engineering, University of Science and Technology, Daejeon, Republic of Korea † Electronic Supplementary Information (ESI) available: Detailed experimental procedures, TEM, XRD, BET, IR and DFT calculation data. See DOI: 10.1039/b000000x/

1. 45

2.

50

3.

55

4.

(a) P. Adelhelm and P. E. de Jongh, J. Mater. Chem. 2011, 21, 24172427; (b) S.-i. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel and C. M. Jensen, Chem. Rev. 2007, 107, 4111-4132. (a) K. Sumida, D. Stück, L. Mino, J.-D. Chai, E. D. Bloch, O. Zavorotynska, L. J. Murray, M. Dincă, S. Chavan, S. Bordiga, M. Head-Gordon and J. R. Long, J. Am. Chem. Soc. 2013, 135, 10831091; (b) M. Dincă and J. R. Long, Angew. Chem. Int. Ed. 2008, 47, 6766-6779; (c) N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe and O. M. Yaghi, Science 2003, 300, 1127-1129. (a) Y. Song, Phys. Chem. Chem. Phys. 2013, 15, 14524-14547; (b) R. B. Biniwale, S. Rayalu, S. Devotta and M. Ichikawa, Int. J. Hydrogen Energy 2008, 33, 360-365. (a) U. Sanyal, U. B. Demirci, B. R. Jagirdar and P. Miele, ChemSusChem 2011, 4, 1731-1739; (b) H.-L. Jiang and Q. Xu, Catal. Today 2011, 170, 56-63; (c) A. Staubitz, A. P. M. Robertson and I. Manners, Chem. Rev. 2010, 110, 4079-4124; (d) U. B. Demirci, O. Akdim, J. Andrieux, J. Hannauer, R. Chamoun and P. Miele, Fuel

100

105

110

115

120

125

130

This journal is © The Royal Society of Chemistry [year]

Cells 2010, 10, 335-350; (e) U. B. Demirci and P. Miele, Energy Enviorn. Sci. 2009, 2, 627-637. 5. (a) J. Albert, R. Wӧlfel, A. Bӧsmann and P. Wasserscheid, Energy Enviorn. Sci. 2012, 5, 7956-7962; (b) R. Wӧlfel, N. Taccardi, A. Bӧsmann and P. Wasserscheid, Greem. Chem. 2011, 13, 2759-2763. 6. (a) A. Boddien and H. Junge, Nat. Nano. 2011, 6, 265-266; (b) S. Enthaler, J. von Langermann and T. Schmidt, Energy Enviorn. Sci. 2010, 3, 1207-1217; (c) S. Enthaler, ChemSusChem 2008, 1, 801-804. 7. (a) B. Loges, A. Boddien, H. Junge and M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3962-3965; (b) C. Fellay, P. J. Dyson and G. Laurenczy, Angew. Chem. Int. Ed. 2008, 47, 3966-3968; (c) A. Boddien, C. Federsel, P. Sponholz, D. Mellmann, R. Jackstell, H. Junge, G. Laurenczy and M. Beller, Energy Enviorn. Sci. 2012, 5, 8907-8911. 8. (a) J. F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, J. T. Muckerman and E. Fujita, Nat. Chem. 2012, 4, 383388; (b) S. Fukuzumi, T. Kobayashi and T. Suenobu, J. Am. Chem. Soc. 2010, 132, 1496-1497. 9. (a) A. Boddien, D. Mellmann, F. Gärtner, R. Jackstell, H. Junge, P. J. Dyson, G. Laurenczy, R. Ludwig and M. Beller, Science 2011, 333, 1733-1736; (b) A. Boddien, B. Loges, F. Gärtner, C. Torborg, K. Fumino, H. Junge, R. Ludwig and M. Beller, J. Am. Chem. Soc. 2010, 132, 8924-8934. 10. (a) Z.-L. Wang, J.-M. Yan, H.-L. Wang, Y. Ping and Q. Jiang, Sci. Rep. 2012, 2, 598; (b) D. A. Bulushev, L. Jia, S. Beloshapkin and J. R. H. Ross, Chem. Commun. 2012, 48, 4184-4186. 11. (a) M. Yadav, T. Akita, N. Tsumori and Q. Xu, J. Mater. Chem. 2012, 22, 12582-12586; (b) Q.-Y. Bi, X.-L. Du, Y.-M. Liu, Y. Cao, H.-Y. He and K.-N. Fan, J. Am. Chem. Soc. 2012, 134, 8926-8933. 12. (a) S. Zhang, Ö. Metin, D. Su and S. Sun, Angew. Chem. Int. Ed. 2013, 52, 3681-3684; (b) Z.-L. Wang, J.-M. Yan, Y. Ping, H.-L. Wang, W.-T. Zheng and Q. Jiang, Angew. Chem. Int. Ed. 2013, 52, 4406-4409; (c) K. Mori, M. Dojo and H. Yamashita, ACS Catal. 2013, 3, 1114-1119; (d) X. Gu, Z.-H. Lu, H.-L. Jiang, T. Akita and Q. Xu, J. Am. Chem. Soc. 2011, 133, 11822-11825; (e) K. Tedsree, T. Li, S. Jones, C. W. A. Chan, K. M. K. Yu, P. A. J. Bagot, E. A. Marquis, G. D. W. Smith and S. C. E. Tsang, Nat. Nano. 2011, 6, 302-307; (f) O. Metin, X. Sun, S. Sun, Nanoscale 2013, 5, 910-912. 13. Y.-Y. Cai, X.-H. Li, Y.-N. Zhang, X. Wei, K.-X. Wang and J.-S. Chen, Angew. Chem. Int. Ed. 2013, 52, 11822-11825. 14. F. Goettmann, A. Fischer, M. Antonietti and A. Thomas, Angew. Chem. Int. Ed. 2006, 45, 4467-4471. 15. (a) Z. Tang, X. Chen, H. Chen, L. Wu and X. Yu, Angew. Chem. Int. Ed. 2013, 52, 5832-5835; (b) F. Su, M. Antonietti and X. Wang, Catal. Sci. Technol. 2012, 2, 1005-1009. 16. J. Sehnert, K. Baerwinkel and J. Senker, J. Phys. Chem. B. 2007, 111, 10671-10680. 17. (a) Y. Liang, H. Wang, J. Zhou, Y. Li, J. Wang, T. Regier and H. Dai, J. Am. Chem. Soc. 2012, 134, 3517-3523; (b) J. Zhou, J. Wang, H. Fang and T.-K. Sham, J. Mater. Chem. 2011, 21, 5944-5949; (c) P. Leinweber, J. Kruse, F. L. Walley, A. Gillespie, K.-U. Eckhardt, R. I. R. Blyth and T. Regier, J. Synchrotron Radiat. 2007, 14, 500-511; (d) J. M. Ripalda, E. Román, N. Díaz, L. Galán, I. Montero, G. Comelli, A. Baraldi, S. Lizzit, A. Goldoni and G. Paolucci, Phys. Rev. B 1999, 60, R3705-R3708. 18. (a) E. V. Kondratenko, G. Mul, J. Baltrusaitis, G. O. Larrazabal and J. Perez-Ramirez, Energy Enviorn. Sci. 2013, 6, 3112-3135; (b) C. Ziebart, C. Federsel, P. Anbarasan, R. Jackstell, W. Baumann, A. Spannenberg and M. Beller, J. Am. Chem. Soc. 2012, 134, 2070120704; (c) S. Wesselbaum, U. Hintermair and W. Leitner, Angew. Chem. Int. Ed. 2012, 51, 8585-8588; (d) W. Wang, S. Wang, X. Ma and J. Gong, Chem. Soc. Rev. 2011, 40, 3703-3727; (e) T. Schaub and R. A. Paciello, Angew. Chem. Int. Ed. 2011, 50, 7278-7282; (f) D. Preti, C. Resta, S. Squarcialupi and G. Fachinetti, Angew. Chem. Int. Ed. 2011, 50, 12551-12554; (g) C. Hao, S. Wang, M. Li, L. Kang and X. Ma, Catal. Today 2011, 160, 184-190. 19. (a) M. B. Ansari, B.-H. Min, Y.-H. Mo and S.-E. Park, Green. Chem. 2011, 13, 1416-1421; (b) Q. Li, J. Yang, D. Feng, Z. Wu, Q. Wu, S. S. Park, C.-S. Ha and D. Zhao, Nano Res. 2010, 3, 632-642; (c) F. Goettmann, A. Thomas and M. Antonietti, Angew. Chem. Int. Ed. 2007, 46, 2717-2720.

Journal Name, [year], [vol], 00–00 | 5


DOI: 10.1039/C4TA01133C