FULL PAPER DOI: 10.1002/asia.201402303

Integration of [(CoACHTUNGRE(bpy)3]2 + Electron Mediator with Heterogeneous Photocatalysts for CO2 Conversion** Jinliang Lin, Yidong Hou, Yun Zheng, and Xinchen Wang*[a]

Abstract: An efficient chemical system for electron generation and transfer is constructed by the integration of an electron mediator ([CoACHTUNGRE(bpy)3]2 + ; bpy = 2,2’-bipyridine) with semiconductor photocatalysts. The introduction of [CoACHTUNGRE(bpy)3]2 + remarkably enhances the photocatalytic activity of pristine semiconductor photocatalysts for heterogeneous CO2 conversion; this is attributa-

ble to the acceleration of charge separation. Of particular interest is that the excellent photocatalytic activity of heterogeneous catalysts can be developed Keywords: carbon dioxide · electron transfer · heterogeneous catalysis · photochemistry · photosynthesis

Introduction

systems that are mainly based on semiconductors as solarenergy transducers. In homogeneous systems, some photosensitizers (e.g., Ru, Ir, or Re complexes) exhibit outstanding performance in CO2 reduction, including intensive visible-light absorption, high quantum efficiency, and good product selectivity.[6] However, these systems rely on expensive and rare late-transition-metal complexes and suffer from photobleaching of the dye molecules during prolonged operations.[7] Heterogeneous catalysts are typically preferable to homogeneous catalysts in terms of separation, stability, recycling, and environmental issues.[4a, 8] Since initial work reported by Halmann in 1978, photocatalytic transformation of CO2 has been achieved based on a variety of heterogeneous semiconductor catalysts.[9] Unfortunately, the efficiency and selectivity of the developed semiconductor photocatalysts are relatively low in the application of CO2 reduction in most cases.[10] Therefore, there is a need for the development of a new photocatalytic system for CO2 reduction, with good efficiency and high quantum yields. It is well known that a photosynthetic process involves three main steps, namely, light harvesting, charge generation and separation, and product formation.[11] Most of the photoinduced electrons are kinetically unfavorable to activate CO2 molecules owing to charge recombination.[12] Thus, accelerating charge separation by fabricating an electrontransfer chain between the semiconductor photocatalyst and a third transporter (an appropriate electron mediator) provides a feasible approach for the construction of a highly efficient CO2 reduction system. The electron mediator plays an important role in accelerating charge transfer and improving photocatalytic reactivity in many fields.[13] In plant photosynthetic processes, the electron mediator (e.g., nicotinamide adenine dinucleotide phosphate (NADP + /NADPH) plays a vital role in the construc-

Global warming and ocean acidification caused by anthropogenic emission of carbon dioxide (CO2) have attracted scientific attention, and has stimulated intensive investigations into the development of carbon-neutral or even -negative systems for chemical/energy conversion and production.[1] Artificial photosynthesis, which is a chemical process to convert sunlight, water, and carbon dioxide into energized carbohydrates, with the release of oxygen, is regarded as one of the most promising and ultimate solutions for addressing climate and energy problems.[2] As an important aspect of artificial photosynthesis, the photocatalytic conversion of CO2 into valuable chemical feedstocks (such as CH4, CH3OH, HCOOH, HCOH, and CO) is advantageous in terms of green and sustainable chemistry and has aroused extensive scientific interest.[3] However, CO2 conversion is not an easy task because of a high thermodynamic barrier for the activation of the linear CO2 molecule.[4] To date, the development of cost-acceptable photocatalytic systems that enable the efficient reduction/fixation of CO2 is still considered to be a difficult challenge. Photocatalytic CO2 conversion systems are generally classified into two categories.[5] One involves homogeneous reaction systems that primarily use transition-metal complexes as light harvesters, and the other involves heterogeneous

[a] J. Lin, Dr. Y. Hou, Y. Zheng, Prof. X. Wang State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry, Fuzhou University Fuzhou 350002 (P.R. China) Fax: (+ 86) 591-83779027 E-mail: [email protected] [**] bpy = 2,2’-bipyridine.

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as a universal photocatalytic CO2 reduction system. The present findings clearly demonstrate that the integration of an electron mediator with semiconductors is a feasible process for the design and development of efficient photochemical systems for CO2 conversion.

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tion of a cascade catalytic system to convert solar energy into chemical energy in the form of adenosine triphosphate (ATP).[14] In the field of artificial photosynthesis, the redox mediator has been widely utilized in a variety of light-driven methodologies, such as photocatalytic water splitting, photoelectrochemical cells, and photochemical CO2 conversion. For Z-scheme water splitting system, redox couples (e.g., Fe3 + /Fe2 + , IO3 /I , [CoACHTUNGRE(bpy)3]3 + /2 + and [CoACHTUNGRE(phen)3]3 + /2 + ; bpy = 2,2’-bipyridine, phen = phenanthroline) and reduced grapheme oxides act as electron mediators to shuttle photogenerated electrons from the O2-evolving catalyst to the H2evolving catalyst in a two-photon-type photocatalytic system.[13b, 15] Furthermore, many transition-metal complexes have been reported as redox couplers in dye-sensitized solar cells.[16] Grtzel et al. have found that the cobalt complexes with imine-group ligands are comparable electron mediators to a well-known I3 /I redox coupler in the dye-sensitized solar cell.[16a,c] Particularly in the field of CO2 conversion, Lehn and Ziessel reported that the photoreduction of CO2 to CO by a ruthenium complex could be significantly promoted in the presence of CoCl2 owing to the formation of an electrontransfer mediator, such as the [CoACHTUNGRE(bpy)n]2 + complex.[17] Tinnemans et al. also reported the beneficial use of cobalt macrocyclic complexes as electron mediators for promoting the photochemical reduction of CO2 under comparable conditions.[18] Calvin et al. investigated the electron mediation of the NiIIL1 complex for the same photosensitized CO2 reduction.[19] Yanagida et al. reported that the combination of the oxidative pentose phosphate (OPP)-3-catalyzed photosystem with CoIIIL (L = cyclam or related macrocycles) as an electron mediator in solution induced efficient and selective electron transfer for the photocatalytic reduction of CO2.[20] There are reports on the coupling of an electron transporter with molecular light harvesters for CO2 reduction; however, these investigations mainly center on homogenous photocatalytic systems. The construction of efficient heterogeneous photocatalytic CO2 reduction systems based on the integration of an electron mediator with semiconductor catalysts has received much less coverage.

Herein, we show that the combination of semiconductor (e.g., CdS, TiO2, mesoporous carbon nitride (MCN)) catalyzed photosystem with [CoACHTUNGRE(bpy)3]2 + as a cobalt complex redox shuttle induces highly efficient and selective electron transfer for the reduction of CO2. The electron mediator promotes the reaction rate of CO2 conversion by coupling an electron mediator with heterogeneous catalysis. The design of a catalytic system with [CoACHTUNGRE(bpy)3]2 + as an electron mediator not only facilitates charge separation but also provides the possibility of revealing mechanistic insights into photocatalytic CO2 reduction reactions. In addition, this work relates to a stratagem for using the room-temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]ACHTUNGRE[BF4]). The ionic liquid [Emim]ACHTUNGRE[BF4] has a dual role: it acts as both a green reactant medium and as a cocatalyst for CO2 reduction.[21] Herein, we mainly focus on reaction activity, product selectivity, catalyst stability, as well as the effects of various factors on the yield and selectivity of products. The reaction activity in the presence/absence of an electron mediator was carefully compared to reveal the function and inherent working mechanism of cobalt species as an electron mediator. Moreover, characterization methods, including UV/Vis absorption spectroscopy, photoluminescence (PL) tests, and photochemical assays, were utilized to evaluate the efficiency of charge transfer and to investigate the thermodynamic and kinetic behavior of CO2 conversion. Importantly, the mechanism of the photocatalytic CO2 conversion system and the schematic steps of electron transfer are discussed.

Results and Discussion Photocatalytic CO2 reduction was performed by using a catalyst combination of CdS as a photocatalyst and [CoACHTUNGRE(bpy)3]2 + as an electron mediator with triethanolamine (TEOA) as an electron donor and water as a hydrogen source. The reaction was performed in [Emim]ACHTUNGRE[BF4] under atmospheric CO2. As shown in Table 1, the activity of CdS is moderate, while [CoACHTUNGRE(bpy)3]2 + alone is inactive in the reaction (Table 1, entries 1 and 2). This result indicates that [CoACHTUNGRE(bpy)3]2 + does not directly initiate the reactive species. No clear CO evolution is obtained if the system is comprised of CdS with Co2 + or bpy (Table 1, entries 3 and 4). It is easily to be understood that the fragmentary component is unable to assemble

Abstract in Chinese:

Table 1. Study of the reaction conditions.[a] Entry

Conditions

CO [mmol]

H2 [mmol]

Selectivity[b]

1 2 3 4 5

CdS Co2 + + bpy CdS + Co2 + [c] CdS + bpy[d] CdS + Co2 + + bpy[e]

0.6 – 0.9 0.8 35.5

2.0 – 8.8 2.2 8.9

23.1 – 9.3 26.6 79.9

[a] Reaction conditions: [EMIM]ACHTUNGRE[BF4] (4 mL), TEOA (1 mL), H2O (1 mL), CO2 (1 bar), CdS (50 mg), l > 420 nm, 30 8C, 2 h. [b] Selectivity = nCO/nACHTUNGRE(CO+H2)  100. [c] Co2 + (1 mmol). [d] bpyACHTUNGRE(100 mmol). [e] Co2 + (1 mmol), bpy (100 mmol).

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into an electron mediator. This result was in accordance with the absorption spectra (see Figure 5, below). It should be noted that the generation of H2 is greatly enhanced by the addition of Co2 + alone; this is due to the formation of CoII/CoI hydride species, followed by the reduction of H + to H2.[22] In the catalytic system employing CdS and [CoACHTUNGRE(bpy)3]2 + , desirable production (35.5 mmol) and high selectivity (79.9 %) of CO are obtained. It should be noted that the reduction of CO2 does not proceed in such a system in the dark. To further reveal the promotional effect of an electron mediator on the catalytic system employing CdS, the products of CO and H2 as a function of reaction time were studied in the presence and absence of an electron mediator (Figure 1). With the extension of the reaction time, the accu-

Figure 2. CO/H2 evolution as a function of the amount of Co2 + and bpy added to the reaction system (after 2 h of reaction). a) 100 mmol bpy; b) 1 mmol Co2 + .

have been immobilized on the catalyst surface.[22] The performance of the [CoACHTUNGRE(bpy)3]2 + mediator and the yields of both H2 and CO strongly depended on the presence of a highly connected ligand between Co2 + and bpy. Thus, optimal catalytic reactivity and CO product can be pursued by the delicate tailoring of the amount of Co2 + and bpy. More importantly, we further proved that such an enhancement in the photocatalytic process could be successfully achieved by the integration of an electron mediator with other kinds of heterogeneous photocatalysts.[23] In addition to a metal sulfide semiconductor (i.e., CdS), a metal-free organic polymer (MCN) and metal oxide (TiO2, P25) have also been investigated herein. For reference, [RuACHTUNGRE(bpy)3Cl2] was also employed as a classical homogenous catalyst in an experiment under identical conditions.[24] As shown in Figure 3, in the absence of electron mediator, all of these catalysts exhibit low photocatalytic performance for CO and H2 production, and none of the heterogeneous catalysts are comparable to [RuACHTUNGRE(bpy)3Cl2] for the yield of (CO + H2). The

Figure 1. The production of CO (*) and H2 (&) in the presence of an electron mediator, and CO (*) and H2 (&) in the absence of an electron mediator, as a function of reaction time.

mulative amounts of CO and H2 continually increase. The yields of CO and H2 in the presence of electron mediator are 50- and 4-fold as high as those in the absence of [CoACHTUNGRE(bpy)3]2 + under visible-light irradiation (l > 420 nm) for 4 h. Additionally, the selectivity (CO/H2) for CO2 reduction is also significantly improved by adding the electron mediator. To better understand the intrinsic behavior of the electron mediator, CO/H2 evolution as a function of the molar concentration of Co2 + and bpy was investigated over the CdS catalyst (Figure 2). First, the amount of bpy (100 mmol) was constant as the quantity ratio of Co2 + changed. In the initial stages, the redox reaction over the photocatalyst occurred with little product generation. With increasing amounts of Co2 + , CO production increased and reached a maximum value of 70 mmol. However, when the amount of Co2 + was higher than 4 mmol, CO production decreased, whereas H2 evolution was enhanced continually. This indicates that the excessive addition of Co2 + increases the yield of H2, but it lowers the selectivity of CO generation. Subsequently, the Co2 + content was fixed at 1 mmol and the evolution of both CO and H2 were strongly promoted with increasing amounts of bpy (< 150 mmol). For a higher bpy content, the production of both CO and H2 clearly decreased, because excess bpy no longer completely coordinated with CoII and might

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Figure 3. The amounts of CO2 reduction products obtained in the catalytic systems employing various photocatalysts without (left) and with (right) electron mediators in [Emim]ACHTUNGRE[BF4] under visible-light irradiation for 2 h. The inset shows the selectivity of CO2 reduction in the absence (&) and in the presence of electron transfer (*).

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selectivity of CO formation is also low (< 30 %) for all of the catalysts (inset in Figure 3). Notably, when an electron mediator is involved in the reaction, CO production and H2 yield are improved by an order of magnitude and several times for all samples, respectively. Furthermore, with regard to the samples examined, the following sequences are obtained: CdS > [RuACHTUNGRE(bpy)3Cl2] > P25 > MCN for the yield of CO; CdS > P25 > [RuACHTUNGRE(bpy)3Cl2] > MCN for the selectivity of CO production. These findings reveal that the electron mediator plays dual roles: one role is to promote the photoreactivity of CO2 conversion and increase the yield of CO and H2, and the other one is to achieve a superior selectivity in the competitive reaction between CO2 reduction and production reduction. Interestingly, CdS gave a higher yield of CO and H2 than that of MCN, although the latter had a strong affinity for CO2 molecules; this may result from the narrower band gap of the CdS semiconductor.[25] Information obtained from an isotopic experiment with 13 CO2 instead of CO2 clarified that the obtained CO originated from CO2 rather than organic materials involved in the reaction system. As shown in the GC-MS spectra (Figure 4), the peak at 3.8–4.0 min and m/z 29 is assigned to 13 CO for both CdS or MCN. This result clearly demonstrates that the carbon source of CO stems from CO2.[26]

Figure 5. Absorption spectra of solutions of Co2 + , bpy, or a mixture of Co2 + and bpy in acetonitrile. Inset: Photographs of the corresponding solutions.

color, which is quite different from the colorless solution of bpy and the red one of Co2 + .[28] This color change was verified by a new absorption peak at l  458 nm in the UV/Vis spectrum; this was in accordance with results for [CoACHTUNGRE(bpy)3Cl2] previously reported.[29] These results are due to the formation of a new photoresponsive compound, [CoACHTUNGRE(bpy)3]2 + , when Co2 + and bpy are combined. It is acknowledged that the electron mediator is a conjunction of double or multiple molecules, or of diverse sections of one large molecule, in which a portion of electronic charge migrates between the molecular entities. The unique task of electron transfer is speculated to occur through the formation of [CoACHTUNGRE(bpy)3]2 + complexes. Chemical association in a charge-transfer complex is not a stable chemical bond and much weaker than covalent forces.[27] Metal-to-ligand charge transfer (MLCT), which has a great positive effect on charge transfer, appears for ligands with N-containing aromatic compounds.[31] However, it should be noted that, although strong optical absorption in the visible-light range was observed in the solution of [CoACHTUNGRE(bpy)3]2 + , the reaction of CO2 conversion could hardly occur without the participation of a semiconductor catalyst (Table 1, entry 2). PL spectra are widely used to investigate the migration, transfer, and recombination processes of the photogenerated electron–hole pairs in a semiconductor because PL emission arises from the recombination of free carriers.[30] Herein we used the PL technique to investigate the reaction systems and the results are shown in Figure 6. All of the semiconductor catalysts have a strong PL band assigned to their inherent recombination of photogenerated electrons and holes.[32] In the presence of [CoACHTUNGRE(bpy)3]2 + , there is a significant decrease in the PL intensity for all semiconductor photocatalysts. A weaker intensity of the PL peak represents a lower recombination probability of photogenerated charge carriers. Therefore, the integration of cobalt redox mediators with a semiconductor could effectively inhibit the recombination of photogenerated charge carriers and accelerate electron transfer/migration.

Figure 4. GC/GC-MS spectra of 13CO produced by using 13CO2 as the carbon source in the photochemical system employing CdS or MCN as the catalyst.

Figure 5 displays the optical absorption spectra of several solutions, including Co2 + , bpy, or a mixture of Co2 + and bpy. Compared with the blank sample (which only contains acetonitrile), the solution containing Co2 + or bpy shows an absorptive peak at l = 480 nm or a strong absorption band at wavelengths shorter than l = 320 nm, respectively. Remarkably, an absorption band in the region of visible light is found in the sample containing both Co2 + and bpy. It can also be clearly identified from the photographs that the solution composed of both Co2 + and bpy exhibits an orange

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During the photocatalytic process, electron–hole pairs are created in semiconductors upon light irradiation and the generation of labile intermediates.[33] Unstable intermediates are readily converted into end products. It is believed that desirable active performance can Figure 6. PL spectra of semiconductor photocatalysts without (dashed line) and with [CoACHTUNGRE(bpy)3]2 + (solid line). be accomplished only if the free energy (DG) is surmounted. The photocurrent is widely used to provide an identification of electron delivery. The typical role of electron diffusion in the case of CoII electrolytes could be further investigated by recording photocurrent transients under light illumination (ca. 100 mW cm 2). A time-resolved experiment (Figure 5) showed that all of the electrodes exhibited low constant photocurrent density (< 1.5 mA cm 2) without [CoACHTUNGRE(bpy)3]2 + . The observed behavior could suffer from limitations in electron transport.[34] When the electron mediator is introduced, an enhanced photocurrent is observed for every electrode. The high photocurrent Figure 7. Photocurrent behavior obtained over the photocatalysts in the absence (dashed line) and in the pres2+ represents an increase in the ence of CoACHTUNGRE[bpy]3 (solid line). charge-transfer rate of the system. These results strongly suggest that the bottleneck hindering charge transfer can be reduction of [CoIIACHTUNGRE(bpy)3]2 + , which accepts an electron from 2+ overcome by the aid of [CoACHTUNGRE(bpy)3] ; this is in good agreethe excited photocatalysts, to produce [CoIACHTUNGRE(bpy)3] + , which ment with the activity shown in Figure 3. Information shown preferentially reacts with CO2 to give the transition-state inin Figure 7 D indicates that no apparent photocurrent is gentermediate [CoIACHTUNGRE(bpy)3ACHTUNGRE(CO2)] + . [CoIACHTUNGRE(bpy)3ACHTUNGRE(CO2)] + thus erated without a catalyst; this is in accordance with the formed may result in electron transfer from CoI to CO2, above result that no photoreduction reaction took place in leading to the formation of [CoIIACHTUNGRE(bpy)3ACHTUNGRE(CO22 )] + . Protonathe absence of photocatalyst (Table 1, entry 2). Upon irradiation, the excited semiconductor catalysts liberate electrons from the valence band to conduction band.[28] Although directional charge transfer probably happens in the absence of [CoACHTUNGRE(bpy)3]2 + , massive electron–hole pairs are unsuccessful in reaching CO2 because of fast recombination, especially in the heterogeneous photocatalytic system. There is a dominant pathway designed for electron transfer towards the CO2 molecule through the electron mediator (Scheme 1), such as [CoACHTUNGRE(bpy)3]2 + .[16a,c] Once the electrons depart from the semiconductor, a cascade of electrontransfer events allows the confinement of the electron on Co3 + by virtue of variable redox species, [CoACHTUNGRE(bpy)3]2 + ; thus giving rise to very efficient electron recapture.[16c] Scheme 1. Mechanism of CO2 conversion in the photocatalysis system The interactions of CO2 with cobalt complexes can be decomposed of an electron mediator and photocatalysts under light irradiascribed as follows:[20, 35] The procedure is initiated with the tion.

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tion of [CoIIACHTUNGRE(bpy)3ACHTUNGRE(CO22 )] + and reduction with an electron from the semiconductor and/or CoI species may produce CO, OH , and [CoIIACHTUNGRE(bpy)3]2 + . The N H proton of tertiary amines may be involved in the catalytic process in the above coordination events by serving as a ligand. As a consequence, the majority of Co3 + created in the second electrontransfer step is free to diffuse to the acceptor, whereupon CO2 activation and cleavage occurs.

Product Detection The produced gases (CO and H2) were detected by using a gas chromatograph equipped with a packed molecular sieve column (TDX-1 mesh 42/ 10). Argon was used as the carrier gas. The identification of 13CO was made by using a GC/MS instrument consisting of an Agilent 6890N gas chromatograph and a Hewlett Packard 5973 quadrupole mass spectrometer.

Acknowledgements

Conclusion

This work was financially supported by the National Basic Research Program of China (2013CB632405) and the National Natural Science Foundation of China (21033003 and 21173043)

We presented an effective heterogeneous system for photocatalytic CO2 reduction by the synergetic combination of an electron mediator ([CoACHTUNGRE(bpy)3]2 + ) with semiconductor photocatalysts. This hybrid system exhibits high photocatalytic reactivity towards CO2-to-CO conversion owing to the addition of [CoACHTUNGRE(bpy)3]2 + as a promoter for charge-carrier separation, transfer kinetics, and interface interactions. This result provides access to the construction of efficient heterogeneous photochemical CO2 reduction systems under mild conditions. Although current research is still in the early stages, we believe that this finding will open up new opportunities in artificial photosynthesis, catalytic chemistry, and energy conversion by creating diverse photocatalyst–electron mediator couplers that could be expanded to other types of reactions.

[1] a) J. H. Mercer, Nature 1978, 271, 321 – 325; b) S. C. Doney, V. J. Fabry, R. A. Feely, J. A. Kleypas, Annu. Rev. Mater. Sci. 2009, 1, 169 – 192; c) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2009, 42, 1890 – 1898; d) M. Y. He, Y. H. Sun, B. X. Han, Angew. Chem. Int. Ed. 2013, 52, 9620 – 9633; Angew. Chem. 2013, 125, 9798 – 9812; e) M. Aresta, A. Dibenedetto, Dalton Trans. 2007, 2975 – 2992. [2] a) N. Armaroli, V. Balzani, Chem. Asian J. 2011, 6, 768 – 784; b) G. A. Olah, G. K. S. Prakash, A. Goeppert, J. Am. Chem. Soc. 2011, 133, 12881 – 12898; c) G. Centi, S. Perathoner, Catal. Today 2009, 148, 191 – 205; d) A. Dibenedetto, A. Angelini, P. Stufano, J. Chem. Technol. Biotechnol. 2014, 89, 334 – 353; e) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729 – 15735; f) S. Bensaid, G. Centi, E. Garrone, S. Perathoner, G. Saracco, ChemSusChem 2012, 5, 500 – 521. [3] W. C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Furler, S. M. Haile, A. Steinfeld, Science 2010, 330, 1797 – 1801. [4] a) M. Aresta, in Activation of Small Molecules: Organometallic and Bioinorganic Perspectives, Chapter 1 (Ed. W. B. Tolman), WileyVCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2006, pp. 3 – 5; b) A. Ueno, T. Sato, N. Todo, Y. Kotera, S. Takasaki, Chem. Lett. 1980, 1067 – 1070. [5] a) T. Yui, Y. Tamaki, K. Sekizawa, O. Ishitani, Top. Curr. Chem. 2011, 303, 151 – 181; b) G. Centi, S. Perathoner, Stud. Surf. Sci. Catal. 2004, 153, 1 – 8; c) P. G. Jessop, F. Joo, C. C. Tai, Coord. Chem. Rev. 2004, 248, 2425 – 2442. [6] a) V. Balzani, E. Fujita, B. S. Brunschwig in Organic Chemistry Vol. 3 (Ed.: V. Balzani), Wiley-VCH, Weinheim, 2008, pp. 98 – 103; b) D. S. Laitar, P. Muller, J. P. Sadighi, J. Am. Chem. Soc. 2005, 127, 17196 – 17197; c) S. Sato, T. Morikawa, T. Kajino, O. A. Ishitani, Angew. Chem. Int. Ed. 2013, 52, 988 – 992; Angew. Chem. 2013, 125, 1022 – 1026. [7] a) P. G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 1995, 95, 259 – 272; b) C. Federsel, R. Jackstell, M. Beller, Angew. Chem. Int. Ed. 2010, 49, 6254 – 6257; Angew. Chem. 2010, 122, 6392 – 6395. [8] J. Ma, N. Sun, X. Zhang, N. Zhao, F. Xiao, W. Wei, Y. Sun, Catal. Today 2009, 148, 221 – 231. [9] a) M. Halmann, Nature 1978, 275, 115 – 116; b) Y. Zheng, Z. M. Pan, X. C. Wang, Chin. J. Catal. 2013, 34, 524 – 535. [10] D. Q. Zhang, G. H. Li, H. X. Li, Y. F. Lu, Chem. Asian J. 2013, 8, 26 – 40. [11] E. S. Andreiadis, M. C. Kerlidou, M. Fontecave, V. Artero, Photochem. Photobiol. 2011, 87, 946 – 964. [12] A. L. Linsebigler, G. Q. Lu, J. T. Y. Jr, Chem. Rev. 1995, 95, 735 – 758. [13] a) F. Boussicault, M. Robert, Chem. Rev. 2008, 108, 2622 – 2645; b) Y. Sasaki, H. Kato, A. Kudo, J. Am. Chem. Soc. 2013, 135, 5441 – 5449. [14] J. D. Rochaix, Biochim. Biophys. Acta. 2011, 1807, 878 – 886. [15] a) A. Iwase, Y. H. Ng, Y. Ishiguro, A. Kudo, R. Amal, J. Am. Chem. Soc. 2011, 133, 11054 – 11057; b) R. Abe, T. Takata, H. Sugihara, K. Domen, Chem. Commun. 2005, 3829 – 3831.

Experimental Section Materials and Preparation Bpy (Sigma), CoCl2 (Sigma), tris(2,2’-bypyridyl)ruthenium(II) chloride hexahydrate (TBR, > 98.0 %, Tokyo Chemical Industry Co.), CdS (Sinopharm Chemical reagent Co.), P25 (Degussa, 99.8 %), and [EMIM]ACHTUNGRE[BF4] ( 98 % Shyfhx Co.) were used as received. N,N-Dimethylformamide (DMF, Sinopharm Chemical reagent Co.) was stored over molecular sieves. MCN was directly synthesized by using cyanamide as a raw material through thermal treatment by a hard temperate method.[36] Characterization Absorption spectra were obtained by using a UV/Vis spectrophotometer (Varian Cary 50 Conc.). Electrochemical measurements were conducted with a BAS Epsilon Electrochemical System in a conventional three-electrode cell by using a Pt plate as the counter electrode and an Ag/AgCl electrode (3 m KCl) as the reference electrode. PL spectra were observed by using a spectrophotometer (Varian Cary 50 Conc.) and the excitation wavelength were l = 400 (CdS and MCN) and 360 nm (P25). Photocatalytic Tests Photocatalytic tests was performed in a Schlenk flask (80 mL) under a CO2 atmosphere (1.0 bar). Catalyst powder (50 mg) was dispersed in a mixture containing [EMIM]ACHTUNGRE[BF4] (4 mL), TEOA (1 mL), H2OACHTUNGRE(1 mL), CoCl2 (1 mmol), and bpy (100 mmol). The Schlenk flask was degassed with a vacuum pump and then backfilled with pure CO2 gas. This process was repeated three times, and after the last cycle the flask was backfilled with CO2 (1.0 bar). The temperature of the reaction solution maintained at 30 8C was controlled by a flow of warm water during the reaction. Then the system was irradiated with a non-focus 300 W Xe lamp with a l = 420 nm cutoff filter under vigorous stirring. The labeling experiments were performed under the same conditions, but 13CO2 (98 %) was used instead of CO2.

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[16] a) H. Nusbaumer, J. Moser, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grtzel, J. Phys. Chem. B 2001, 105, 10461 – 10464; b) S. A. Sapp, C. M. Elliot, C. Contado, S. Caramori, C. A. Bignozzi, J. Am. Chem. Soc. 2002, 124, 11215 – 11222; c) H. Nusbaumer, S. M. Zakeeruddin, J. Moser, M. Grtzel, Chem. Eur. J. 2003, 9, 3756 – 3763; d) S. Nakade, Y. Makimoto, W. Kubo, T. Kitamura, Y. Wada, S. J. Yanagida, Phys. Chem. B. 2005, 109, 3488 – 3493. [17] J. M. Lehn, R. Ziessel, Proc. Natl. Acad. Sci. USA 1982, 79, 701 – 704. [18] A. H. A. Tinnemans, T. P. M. Koster, D. H. M. W. Thewissen, A. Mackor, Recl. Trav. Chim. Pays-Bas. 1984, 103, 288 – 295. [19] C. A. Craig, L. O. Spreer, J. W. Otvos, M. Calvin, J. Phys. Chem. 1990, 94, 7957 – 7960. [20] S. Matsuoka, K. Yamamoto, T. Ogata, M. Kusaba, N. Nakashima, E. Fujita, S. Yanagida, J. Am. Chem. Soc. 1993, 115, 601 – 609. [21] Y. Oh, X. Hu, Chem. Soc. Rev. 2013, 42, 2253 – 2261. [22] T. Abe, M. Kaneko, J. Mol. Catal. A 2001, 169, 177 – 183. [23] E. Lindner, T. Schneller, F. Auer, H. A. Mayer, Angew. Chem. Int. Ed. 1999, 38, 2154 – 2174; Angew. Chem. 1999, 111, 2288 – 2309. [24] K. Kalyanasundaram, Coord. Chem. Rev. 1982, 46, 159 – 244. [25] M. A. El-Sayed, Acc. Chem. Res. 2004, 37, 326 – 333. [26] J. L. Lin, Z. M. Pan, X. C. Wang, ACS Sustainable Chem. Eng. 2014, 2, 353 – 358. [27] C. Kaes, A. Katz, M. W. Hosseini, Chem. Rev. 2000, 100, 3553 – 3590. [28] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899 – 926.

Chem. Asian J. 2014, 9, 2468 – 2474

Xinchen Wang et al.

[29] a) H. A. Schwarz, C. Creutz, N. Sutin, Inorg. Chem. 1985, 24, 433 – 439; b) http://hdl.handle.net/10603/2689. [30] B. Beyer, C. Ulbricht, D. Escudero, C. Friebe, A. Winter, L. Gonzalez, U. S. Schubert, Organometallics 2009, 28, 5478 – 5488. [31] F. B. Li, X. Z. Li, Chemosphere 2002, 48, 1103 – 1111. [32] a) R. R. Prabhu, M. A. Khadar, Pramana 2005, 65, 801 – 807; b) J. Zhou, M. Takeuchi, A. K. Ray, M. Anpo, X. S. Zhao, J. Colloid Interface Sci. 2007, 311, 497 – 501; c) X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76 – 80. [33] F. Y. Wen, C. Li, Acc. Chem. Res. 2013, 46, 2355 – 2364. [34] a) P. W. Anderson, P. A. Lee, M. Saitoh, Solid State Commun. 1973, 13, 595 – 598; b) H. Akamatu, H. Inokuchi, Y. Matsunaga, Nature 1954, 173, 168 – 169. [35] a) F. R. Keene, C. Creutz, N. Sutin, Coord. Chem. Rev. 1985, 64, 247 – 260; b) T. Ogata, S. Yanagida, B. S. Brunschwig, E. Fujita, J. Am. Chem. Soc. 1995, 117, 6708 – 6716. [36] a) F. Z. Su, M. C. Smitha, L. Grzegorz, X. Z. Fu, M. Antonietti, S. Blechert, X. C. Wang, J. Am. Chem. Soc. 2010, 132, 16299 – 16301; b) A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv. Mater. 2005, 17, 1648 – 1652. Received: March 30, 2014 Published online: July 1, 2014

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Integration of [(Co(bpy)₃]²⁺ electron mediator with heterogeneous photocatalysts for CO₂ conversion.

An efficient chemical system for electron generation and transfer is constructed by the integration of an electron mediator ([Co(bpy)3](2+); bpy=2,2'-...
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