Unique Solvent Effects on Visible-Light CO2 Reduction over

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Unique Solvent Effects on Visible-Light CO2 Reduction over Ruthenium(II)-Complex/Carbon Nitride Hybrid Photocatalysts Ryo Kuriki, Osamu Ishitani, and Kazuhiko Maeda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11836 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016

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ACS Applied Materials & Interfaces

Unique Solvent Effects on Visible-Light CO2 Reduction

over

Ruthenium(II)-Complex/Carbon

Nitride Hybrid Photocatalysts Ryo Kuriki, Osamu Ishitani, Kazuhiko Maeda* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan E-mail: [email protected] KEYWORDS. Artificial photosynthesis, Carbon dioxide fixation, Heterogeneous photocatalysis, Light-energy conversion, Solar fuels

ABSTRACT. Photocatalytic CO2 reduction using hybrids of carbon nitride (C3N4) and a Ru(II) complex under visible light was studied with respect to reaction solvent. Three different Ru(II) complexes, trans(Cl)-[Ru(X2bpy)(CO)2Cl2] (X2bpy = 2,2′-bipyridine with substituents X in the 4-positions, X = COOH, PO3H2, or CH2PO3H2), were employed as promoters, and will be abbreviated as RuC (X = COOH), RuP (X = PO3H2), and RuCP (X = CH2PO3H2). When C3N4 modified with a larger amount of RuCP (>7.8 µmol g–1) was employed as a photocatalyst in a solvent having a relatively high donor number (e.g., N,N-dimethylacetamide (DMA), N,Ndimethylformamide (DMF) and dimethylsulfoxide (DMSO)) with the aid of triethanolamine

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(TEOA) as an electron donor, the hybrid photocatalyst exhibited relatively high performance for CO2 reduction, producing CO and HCOOH with relatively high CO selectivity (40–70%). On the other hand, HCOOH was the major product when RuC/C3N4 or RuP/C3N4 was employed regardless of the loading amount of Ru(II) complex and the reaction solvent. Results of photocatalytic reactions and UV-visible diffuse reflectance spectroscopy indicated that polymeric Ru species, which were formed in situ from RuCP on C3N4 under irradiation in a solvent having a high donor number, were active catalysts for CO formation. Nonsacrificial CO2 reduction using RuP/C3N4 was accomplished in a DMA solution containing methanol as an electron donor, which means that visible light energy was stored as chemical energy in the form of CO and formaldehyde (∆Gº = +67.6 kJ mol–1). This study demonstrated the first successful example of an energy conversion scheme using carbon nitride through photocatalytic CO2 reduction.

1. Introduction In order to address the depletion of fossil fuels and the concomitant global warming problem, CO2 reduction into useful fuels such as CO and HCOOH through an artificial photosynthetic scheme is one of the most important subjects in modern chemistry. Metal complexes, semiconductors, and their hybrids have been studied as photocatalysts for the reaction for many years.1–33 For practical applications in the future, there still remain a lot of challenges including the development of photocatalysts, which are highly active, durable, and cost-effective. However, none have yielded a satisfactory result to date. Therefore, fundamental studies for photocatalytic CO2 reduction are important.


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Our group has studied CO2 reduction using a hybrid photocatalyst containing a metal complex and a semiconductor.27–32 Among the semiconductors employed, carbon nitride polymers (abbreviated C3N4 for simplicity) are of particular interest.26,29–36 High photocatalytic performance has been demonstrated by optimizing the pore-wall structure of C3N4,30 a catalytic Ru(II) complex and reaction solvent.31 An apparent quantum yield of 5.7% at 400 nm and a catalytic turnover number (TON) for HCOOH production of greater than 1000 (with respect to the loaded complex) have been achieved using DMA–TEOA (4:1 v/v) and RuP/C3N4 as the solvent and photocatalyst, respectively, which are the highest records among the reported systems.31 From the standpoint of catalysis, controlling the selectivity of a given catalytic reaction is another important subject both in homogeneous and heterogeneous systems. CO2 reduction is an interesting reaction that can give different products depending on reduction potential and how many electrons are consumed as follows. CO2 + 2H+ + 2e– → HCOOH

Eº = –0.61 V (at pH 7 vs. NHE)

(1)

CO2 + 2H+ + 2e– → CO + H2O

Eº = –0.53 V

(2)

CO2 + 4H+ + 4e– → HCHO + H2O

Eº = –0.48 V

(3)

CO2 + 6H+ + 6e– → CH3OH + H2O

Eº = –0.38 V

(4)

CO2 + 8H+ + 8e– → CH4 + 2H2O

Eº = –0.24 V

(5)

Metal complexes such as Re, Ru, Co or Ni are known to catalyze CO2 reduction to give CO and/or HCOOH, with some of them showing very high selectivity.1–9,11–13,27–32,37–43

The


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selectivity of CO2 reduction over a metal complex catalyst in a homogeneous system can be controlled by changing reaction parameters including acidity/basicity37 and concentration of catalyst.11 On the other hand, little has been investigated in terms of controlling the selectivity of photocatalytic CO2 reduction under visible light using a heterogeneous photocatalyst. In this work, we report unique solvent effects on photocatalytic CO2 reduction using Ru(II)complex/C3N4 hybrids, which have strong influence on product selectivity and allow for nonsacrificial, energy-conversion type CO2 reduction. Factors affecting the product selectivity are discussed on the basis of the results of photocatalytic reactions and structural analyses. CO2 reduction systems studied in this work, which consist of a catalytic Ru(II) complex and C3N4, are illustrated in Scheme 1. The Ru(bpy)(CO)2Cl2-type complexes employed in this work are known to function as electrocatalysts for CO2 reduction.11,38–43

Scheme 1. Ru(II)-complex/carbon nitride hybrid photocatalyst for CO2 reduction under visible light. In this work, Ru(II) complexes with X = COOH, PO3H2, or CH2PO3H2 were employed, with abbreviations of RuC, RuP, and RuCP, respectively. Of course, parts of HCOOH generated would exist in the form of HCOO– because of the presence of basic triethanolamine (TEOA). However, we use “HCOOH” in this article for simplicity. 2. Results and Discussion


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2.1. Influence of the Reaction Solvents on the CO2 Reduction Products We first studied the effects of solvent on photocatalytic CO2 reduction products in the presence of triethanolamine (TEOA) as an electron donor. Figure S1A shows UV-visible spectra of RuC, RuP, and RuCP, along with a diffuse reflectance spectrum of C3N4. C3N4 is capable of absorbing visible light up to 600 nm. On the other hand, the tested Ru(II) complexes exhibit little absorption in the visible light region. The absorption profiles of hybrid materials (Ru(II)complex/C3N4) are very similar to that of C3N4 (Figure S1B). Under irradiation of visible light with wavelength longer than 400 nm, therefore, one can selectively photoexcite the C3N4 component.

Table 1. Effects of solvents and Ru(II) complexes on photocatalytic CO2 reduction over Ru(II)complex/C3N4 hybrid photocatalysts (λ > 400 nm)a Amount of products / µmol Entry

a

Photocatalyst

Selectivity / %

Solvent CO

HCOOH

H2

CO

HCOOH

1

RuP/C3N4

DMA

18

68

0.9

21

78

2

RuP/C3N4

MeCN

3.8

13

2.0

20

70

3

RuCP/C3N4

DMA

29

15

1.3

64

34

4

RuCP/C3N4

MeCN

1.2

6.3

0.9

13

75

5

RuC/C3N4

DMA

4.5

17

1.5

20

74

6

RuC C3N4

MeCN

2.1

7.2

1.8

19

65

Reaction conditions: Photocatalyst, Ru(II)-complex(7.8 µmol g–1)/C3N4 8.0 mg; solution, DMA

or MeCN (containing 20 vol% TEOA) 4.0 mL. Reaction time: 20 h.


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Table 1 displays CO2 reduction products using C3N4 modified with different Ru(II) complexes. Here, N,N-dimethylacetamide (DMA) and acetonitrile (MeCN) were employed as solvents. In most of the combinations, the main product was HCOOH. However, there was one exceptional case. When RuCP/C3N4 and DMA were used as the photocatalyst and solvent, respectively, CO was the main product of CO2 reduction, with 64% selectivity (entry 3). No reaction took place in the absence of either Ru(II) complex, C3N4, CO2, or visible light.

Table 2. Effects of the loading amount of RuCP on photocatalytic CO2 reduction over RuCP/C3N4 in a DMA/TEOA mixed solution (λ > 400 nm)a Amount of products / µmol

Selectivity / %

Amount of RuCP adsorbed / µmol g–1

CO

HCOOH

H2

CO

HCOOH

1

1.0

0.5

1.1

0.5

24

52

2

1.4

0.5

2.0

0.4

17

67

3

7.8

29

15

1.3

64

34

4

20.8

28

24

1.1

53

45

5

52.4

27

22

1.0

53

45

Entry

a

Reaction conditions: Photocatalyst, RuCP/C3N4 8.0 mg; solution, DMA (containing 20 vol%

TEOA) 4.0 mL. Reaction time: 20 h.

To achieve high selectivity to CO, a relatively large amount of RuCP needed to be loaded on C3N4. As listed in Table 2, more than 50% selectivity for CO formation was achieved when the loading amount of RuCP was larger than 7.8 µmol g–1. On the other hand, loading a small amount of RuCP (< 1.4 µmol g–1) resulted in lower CO selectivity (< 24%). These results clearly indicate that both reaction solvents and Ru(II) complexes (as well as the loading amount) are


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important factors that govern the selectivity of CO2 reduction. More concretely, the combination of DMA solvent and a large amount of RuCP were both necessary to improve the CO selectivity. We note that even when using a higher loading of RuCP (~40 µmol g–1), HCOOH was the main CO2 reduction product when MeCN was employed as the solvent, as indicated by our previous study.29

Figure 1. Amount of reaction products as a function of irradiation time using RuCP(7.8 µmol g– 1

)/C3N4 hybrid photocatalyst under visible light (λ > 400 nm). Reaction conditions: photocatalyst,

8.0 mg; solution, DMA (containing 20 vol% TEOA) 4.0 mL. (A) At the initial stage, and (B) the entire time course data. We observed an induction period of CO formation using RuCP(7.8 µmol g–1)/C3N4 during the CO2 reduction reaction, as shown in Figure 1A. On the other hand, HCOOH was produced almost linearly at the initial stage of the reaction with no induction period. These results suggest that the catalytically active species that form CO and HCOOH are different, and that in particular, the CO-evolving species are formed after a structural change of the initial RuCP complex. Additionally, the fact that TONs for CO and HCOOH generation both greatly exceeded 1 indicates that the reaction occurred catalytically. For the extended period of irradiation,


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however, the rates of HCOOH and CO generation decreased (Figure 1B). As discussed in our previous works,29,32 this is at least in part due to desorption of the initially adsorbed Ru complexes and unknown structural changes of the immobilized complexes. We note that a color change occurred in the reactant suspension after visible light irradiation, suggesting a change in the photocatalyst structure. As shown in Figure 2A, the suspension containing RuCP/C3N4 turned from yellow to black. This black suspension returned to the original yellow color when it was exposed to air for a day. This change is also evident from the result of UV-visible diffuse reflectance spectroscopy (Figure 2B), judging from a rise in absorption in the 600–800 nm region. This is attributed to polymeric Ru species generated though Scheme 2, as reported by Deronzier and Ziessel et al.38–43

+ 2ne–

+ 2nCl– n

Scheme 2. Formation of Ru polymer.

According to those studies, it was reported that polymeric Ru species serve as active catalysts for CO formation in electrochemical CO2 reduction. Furthermore, Ru polymers were reported as photocatalysts for CO formation in a homogeneous system.7 Therefore, it is most likely that the CO-evolving species in our case is Ru polymer, which is formed in situ during the CO2 reduction reaction using RuCP/C3N4 in a mixed solution of DMA and TEOA under visible light.


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Figure 2. (A) Photograph of reactant suspensions and (B) diffuse reflectance spectra of RuCP/C3N4 before and after irradiation under a CO2 atmosphere. Reaction conditions: photocatalyst, RuCP(81.4 µmol g–1)/C3N4 4.0 mg; solution, DMA (containing 20 vol% TEOA) 4.0 mL. Reaction time: 20 h. The “Exposed to air” sample was exposed to air after irradiation under a CO2 atmosphere, and was kept under ambient conditions for 24 h. The diffuse reflectance spectra of these samples were acquired just after the suspended sample was filtered off and washed with 2 mL of ether. It took about 3 minutes to measure the spectrum after filtration.

To investigate the correlation between selective CO generation and polymer formation, visiblelight CO2 reduction was conducted using various solvents. As shown in Figure 3, relatively high CO selectivity was obtained using DMA, DMF, and DMSO as reaction solvents. As expected, a similar color change was observed in these cases (Figure S2), which was further supported by UV-visible diffuse reflectance spectroscopy (Figure S3). On the other hand, the CO selectivity was relatively low when MeCN, tetrahydrofuran (THF), and methanol were employed, without


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noticeable change in color of the reactant solution. These facts strongly suggest that the Ru polymer species formed during the reaction acted as active catalysts for CO formation.

Figure 3. Product distributions of visible-light (λ > 400 nm) CO2 reduction using RuCP/C3N4 as a photocatalyst and various solvents under CO2 atmosphere. Reaction conditions: photocatalyst, RuCP(81.4 µmol g–1)/C3N4 4.0 mg; solution, DMA, DMF, DMSO, MeCN, THF or methanol (containing 20 vol% TEOA) 4.0 mL. Reaction time: 20 h.

It is worth mentioning that the total amount of CO2 reduction products obtained using these solvents (DMA, DMF, and DMSO) was greater than that using other solvents (MeCN, THF, methanol), as displayed in Figure 3. Unfortunately, however, we cannot fully uncover the molecular mechanism of CO2-to-CO conversion over Ru polymers formed on the surface of C3N4, due to the lack of a suitable methodology. DMA, DMF and DMSO have relatively high donor number as listed in Table S1, compared to the MeCN, THF and methanol, suggesting the donor number might relate to these reaction results. We are now pursuing this detail, and the results will be reported in our future work. When different Ru(II) complexes were employed, a connection between CO selectivity and the formation of polymeric Ru species could be identified as well. As listed in Table 1, CO was the


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main product when RuCP was used. On the other hand, using RuC and RuP allowed for HCOOH production with high selectivity (70~80%). As shown in Figure S4, UV-visible diffuse reflectance spectroscopy indicated that the absorption band in the 600–800 nm region, due to polymeric Ru species, was more pronounced only when RuCP was employed. This means that RuCP on C3N4 undergoes a structural change to become a polymer species, while no appreciable transformation occurs for RuC and RuP. Our group has previously reported photocatalytic CO2 reduction in a homogeneous system using a multinuclear Ru(II) complex that has light-harvesting and catalytic units in its structure.7 We found that formation of Ru polymers (and/or oligomers) occurred when the electron injection efficiency from the photosensitizer unit to the catalytic unit was slow, resulting in predominant CO formation.44 It is therefore speculated that the driving force for electron injection from the conduction band of C3N4 to the loaded Ru(II) complex governed the formation of polymeric Ru species and the concomitant CO generation. In order to investigate the LUMO level (i.e., reduction potential) of Ru(II) complexes, cyclic voltammetry was conducted. As shown in Figure S5, all of the Ru(II) complexes used in this work showed catalytic activity for electrochemical CO2 reduction. Under an Ar atmosphere, the first irreversible reduction wave started at –1.57 V (vs. Ag/AgNO3), attributable to reduction of the X2bpy ligand (X2bpy•–/X2bpy). Under a CO2 atmosphere, the first reduction wave was more pronounced, and a clear catalytic CO2 reduction current was observable at around –1.9 V for RuCP. RuC and RuP both showed similar behavior, but the onset potential of the first irreversible reduction current was –1.48 V in both cases. This is reasonable because RuCP has methylphosphonic acid groups in the diimine ligand, which increases the electron density and the reduction potential. Thus, the driving force for electron injection from the conduction band of


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C3N4 (–1.65 V vs. Ag/AgNO3) to the LUMO level of the loaded Ru(II) complex is the smallest in the case of RuCP. This could contribute to Ru polymer formation from RuCP on C3N4, and the resulting CO formation. Besides the difference between the conduction band potential of C3N4 and the reduction potential of RuCP, the longer distance between the two components would result in relatively inefficient electron injection. As shown in Scheme 1, RuCP has a methylene spacer between phosphoric acid anchors and diimine ligands, while RuC and RuP do not. 2.2. Nonsacrificial CO2 Reduction Using a RuP/C3N4 Hybrid Thus, a unique solvent effect was highlighted for the first time in a heterogeneous photocatalytic CO2 reduction system using Ru(II)-complex/C3N4 hybrids. A challenge in these systems is that they still rely on a strong electron donor (i.e., TEOA) for the operation, making the overall scheme “sacrificial” (∆G0 < 0). Therefore, development of a photocatalytic CO2 reduction system using C3N4 that works in the presence of a weak electron donor (ideally water) is highly desirable. As reported in our previous work, the RuP/C3N4 hybrid can efficiently photocatalyze CO2 reduction under visible light in the presence of TEOA as an electron donor.31 In this case, we observed a significant solvent effect on photocatalytic activity, and found that DMA (with TEOA) is the most effective solvent for the reaction. Based on this finding, we attempted to utilize methanol as an electron donor to accomplish “nonsacrificial” CO2 reduction according to the following equations. CO2 + CH3OH → HCOOH + HCHO

(∆Gº = +83.0 kJ mol–1)

(6)

CO2 + CH3OH → CO + H2O + HCHO

(∆Gº = +67.6 kJ mol–1)

(7)


12

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We conducted CO2 reduction using RuP/C3N4 in a DMA/methanol mixed solution, and the results are summarized in Table 3. In pure methanol or DMA, the photocatalyst exhibited little activity (entries 1 and 2). Using a mixed solution of DMA and methanol, however, catalytic CO formation (TONCO > 1) was observable, and the activity increased upon increasing relative concentration of DMA (entries 3–5). Control experiments in the absence of either light, CO2 or RuP, showed that no reaction took place (entries 6–8). RuP immobilized on Al2O3 produced CO upon visible light, but TONCO was low (entry 9). These results indicate that light, CO2, RuP, and C3N4 were all necessary to drive CO formation. In our previous reports using RuP/C3N4 hybrids, the main product of CO2 reduction conducted in a mixed solution of DMA and TEOA was HCOOH.31 By contrast, CO was the major product in the present case. Presumably, this difference was attributable to the acidity (or basicity) of the reactant solution. Ishida et al. have reported that the main product of CO2 reduction in a homogeneous system in basic solution was different from that in acidic solution.37 Specifically, formate was produced under basic conditions, while CO was the major product under acidic conditions. In our previous works, we used 20 vol% of TEOA as an electron donor with the proper solvent, which made the reactant solution basic. In the present case, however, we employed methanol alternatively with DMA, which should become acidic when subject to CO2 bubbling before reaction. This change in solution acidity would influence the product selectivity. We also investigated the possibility of the formation of a Ru polymer that acted as the active species for catalytic CO generation, as described in the previous section. We measured the UVvisible diffuse reflectance spectra of RuCP/C3N4 before and after irradiation in a DMA– Methanol (9:1 v/v) mixed solution. As shown in Figure S6, no appreciable rise in absorption at 600–800 nm region was observed, although there was a slight increase in the background. This


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result suggested polymeric Ru species were not formed efficiently in this situation. Therefore, the change in the main product is considered to result from the acidity of the reactant solution.

Table 3. Results of visible-light CO2 reduction using RuP/C3N4 in a mixed solution of DMA and methanol (λ > 400 nm)a Entry

Photocatalyst

Ratio of DMA/methanol

CO / nmol

TONCO

1

RuP/C3N4

0/1

65.3

1.1

2

RuP/C3N4

1/0

N.D.

-

3

RuP/C3N4

1/1

272

4.5

4

RuP/C3N4

4/1

455

7.5

5

RuP/C3N4

9/1

423

7.0

6b

RuP/C3N4

4/1

N.D.

-

7c

RuP/C3N4

4/1

N.D.

-

8

C3N4

4/1

N.D.

-

9d

RuP/Al2O3

4/1

235

1.5

a

Reaction conditions: Photocatalyst, RuP(15.1 µmol g–1)/C3N4 4.0 mg; solution, a mixture of

DMA and methanol 4.0 mL. Reaction time: 20 h. b Under Ar atmosphere.

c

In the dark. d 40.0

µmol g–1.


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Figure 4. Gas chromatograms of the photocatalytic reaction samples using mass spectrometry as a detector (m/z 28, 29). RuP(28.0 µmol g–1)/C3N4 (4 mg) was irradiated (λ > 400 nm) for 20 h in a DMA/methanol mixed solution (9:1 v/v, 3 mL) under 13CO2 (670 Torr).

To examine the origin of the formed CO, isotope tracer experiments using

13

CO2 were

conducted since RuP has two CO ligands that may become the source of CO formation. Figure 4 shows gas chromatograms of the gas phase in the reaction cell after 20 h of irradiation using RuP/C3N4 in a DMA/methanol (9:1 v/v) mixed solution. Both

13

CO and

12

CO were detected.

The amounts of 13CO and 12CO evolved were 301 and 211 nmol, corresponding to TONs of 2.7 and 1.8, respectively. This result indicates that CO2 was indeed the main source of CO, but part of the CO came from RuP. Considering the fact that RuP has two carbonyl ligands, the recorded TON for 12CO formation might be reasonable. CO ligand substitution in a Ru(II) complex might occur under irradiation, as observed in Re(I) complexes.45 However, we do not have the direct evidence of the ligand exchange of the immobilized RuP at present.


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At the same time, a very small amount of HCOOH (< 100 nmol), another possible two-electron reduction product of CO2, was detected in the liquid phase. No H2 was detected. We could not, however, detect H13COOH, but a small amount of H12COOH was detected under irradiation in the isotope-tracer experiments using

13

CO2. To investigate the possible production of HCOOH

via oxidation of HCHO, which is the two-electron oxidation product of methanol, photocatalytic CO2 reduction was conducted in a mixed solution of DMA and isopropanol (9:1 v/v). It is known that photocatalytic oxidation of isopropanol gives acetone, which may undergo further decomposition into CO2 and H2O, as the sole product.46 The result showed that CO (TON = 4.3) was detected, but no HCOOH production. This result suggests that the observed HCOOH in a DMA/methanol mixed solution originates from either oxidation of HCHO as follows, or from contaminants including DMA.28 HCHO + H2O + 2h+ → HCOOH + 2H+

(8)

Based on these results, the main reduction product of the reaction in a mixed solution of DMA and methanol was concluded to be CO. We also detected HCHO after the reaction. The amount of HCHO detected was 380 nmol, close to the total production of CO (300–500 nmol), regardless of the possible CO ligand substitution. This fact confirms reasonable electron/hole balance of the reaction. We thus concluded that photocatalytic CO2 reduction was achieved in a nonsacrificial manner (i.e., ∆Gº > 0) according to equations (6) and (7). This is the first example of a non-sacrificial energy conversion scheme using carbon nitride through photocatalytic CO2 reduction. With the use of a suitable solvent, the construction of an energy conversion type of CO2 reduction was demonstrated to be possible.


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3. Conclusions Although highly selective production of CO or HCOOH still remains a challenge, we found some important parameters that control the CO/HCOOH selectivity in a heterogeneous photocatalytic CO2 reduction system using Ru(II)-complex/C3N4 hybrids in the presence of TEOA as an electron donor. In particular, the selectivity to CO formation increases when (1) DMA, DMF and DMSO having a relatively high donor number are employed, and (2) electron injection efficiency from the conduction band of C3N4 to the loaded Ru(II) complex is low. We also succeeded in achieving nonsacrificial CO2 reduction with C3N4, converting visible light energy into chemical energy in the form of CO and HCHO from CO2 and methanol. 4. Experimental Section 4.1. Synthesis of Mesoporous Carbon Nitride Mesoporous carbon nitride (abbreviated as C3N4, specific surface area: 180 m2 g–1) was prepared by a method reported previously.35 C3N4 was prepared by heating a mixture of cyanamide and colloidal SiO2 (Ludox HS40; Aldrich, 12 nm, 40 vol %) with stirring at 333 K overnight. The mass ratio of silica/cyanamide was 1/1. The resulting mixture was heated at a rate of 2.3 K min–1 over 4 h to reach a temperature of 823 K and then kept at this temperature for another 4 h. The obtained powder was treated with a 4 M NH4HF2 aqueous solution for 24 h to remove the silica template. Note that special care should be taken when using NH4HF2. The treated solid was then centrifuged and washed three times with distilled water and ethanol. Finally, the sample was dried at 373 K in air overnight. The XRD pattern of the as-prepared C3N4 is shown in Figure S7. Nitrogen adsorption/desorption isotherms of C3N4 and the corresponding BJH pore size distributions are shown in Figure S8.


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4.2. Purification of Solvents DMA and DMF was dried over molecular sieves 4A (which was heated at 373 K under reduced pressure (< 1 Torr) overnight for several days), and distilled under reduced pressure (10–20 Torr). Methanol was used after distillation. MeCN was distilled over P2O5 twice, and then distilled over CaH2 prior to use. Both DMSO and THF are dehydrated one (>99.5 %) and were used without further purification. TEOA was distilled under reduced pressure ( 400 nm). Prior to irradiation, the suspension was purged with CO2 (Taiyo Nippon Sanso Co., > 99.995%) for 30 min. The gaseous reaction products H2 and CO were analyzed using a gas chromatograph with a TCD detector (GL Science, GC323), an activated carbon column, and argon as the carrier gas. The formic acid in the liquid phases was analyzed using a capillary electrophoresis system


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(Otsuka Electronics Co., CAPI–3300). Before analysis, the solution was filtered to remove any photocatalyst particles. After diluting the resulting solution with H2O (1:9 v/v), analysis was performed using capillary electrophoresis. HCHO was analyzed using colorimetric analysis following a published method.27 The reproducibility for CO, HCOOH, and H2 production were within ~20% under the same reaction conditions using different batch of samples. 4.7. Isotope-Tracer Experiments 13

CO2 (13C 99%) was purchased from Aldrich Co., and was introduced into the photocatalyst

suspension after degassing by freeze-pump-thaw cycling. After visible light irradiation, the gas phase was analyzed by GC/MS with a Molsieve5A capillary column.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:XXX Additional characterization data including UV-visible absorption spectra of Ru(II) complexes in methanol; UV-visible diffuse reflectance spectra of C3N4 and Ru-complex/C3N4 hybrids; Photographs of reactant suspensions containing RuCP/C3N4; Properties of some selected solvents; Cyclic voltammograms of Ru(II) complexes; XRD pattern and results of nitrogen adsorption/desorption measurements of C3N4. AUTHOR INFORMATION Corresponding Author [email protected]


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ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Area “Artificial Photosynthesis (AnApple)” (JSPS), the Photon and Quantum Basic Research Coordinated Development Program (MEXT, Japan), and a CREST program (JST). K.M. acknowledges The Noguchi Institute and The Murata Science Foundation for financial support. REFERENCES and NOTES (1) Fujita, E. Photochemical Carbon Dioxide Reduction with Metal Complexes, Coord. Chem. Rev. 1999, 185–186, 373–384. (2) Takeda, H.; Ishitani, O. Development of Efficient Photocatalytic Systems for CO2 Reduction Using Mononuclear and Multinuclear Metal Complexes Based on Mechanistic Studies. Coord. Chem. Rev. 2010, 254, 346–354. (3) Yamazaki, Y.; Takeda, H.; Ishitani, O. Photocatalytic Reduction of CO2 Using Metal Complexes. J. Photochem. Photobiol. C: Photochem. Rev. 2015, 25, 106–137. (4) Sahara, G.; Ishitani, O. Efficient Photocatalysts for CO2 Reduction. Inorg. Chem. 2015, 54, 5096–5104. (5) Tamaki, Y.; Watanabe, K.; Koike, K.; Inoue, H.; Morimoto, T.; Ishitani, O. Development of Highly Efficient Supramolecular CO2 Reduction Photocatalysts with High Turnover Frequency and Durability. Faraday Discuss. 2012, 155, 115–127. (6) Tamaki, Y.; Koike, K.; Morimoto, T.; Ishitani, O. Substantial Improvement in the Efficiency and Durability of a Photocatalyst for Carbon Dioxide Reduction Using a Benzoimidazole Derivative as an Electron Donor. J. Catal. 2013, 304, 22–28. (7) Tamaki, Y.; Morimoto, T.; Koike, K.; Ishitani, O. Photocatalytic CO2 Reduction with High Turnover Frequency and Selectivity of Formic Acid Formation using Ru(II) Multinuclear Complexes. Proc. Natl. Acad. Sci. USA 2012, 109, 15673–15678. (8) Nakada, A.; Koike, K.; Nakashima, T.; Morimoto, T.; Ishitani, O. Photocatalytic CO2 Reduction to Formic Acid Using a Ru(II)-Re(I) Supramolecular Complex in an Aqueous Solution. Inorg. Chem. 2015, 54, 1800–1807.


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However, one may think that the results shown in Table 2 are inconsistent with this idea.


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We think that this discrepancy is due to the difference between heterogeneous and homogeneous systems. It would be natural to expect that solid surfaces have different kinds of adsorption sites, which originate from inhomogeneity of the solid surfaces. This is also supported by our previous work,32 which suggests that there are at least two types of adsorption sites on C3N4: one can strongly adsorb Ru(II) complexes, while the other cannot. Taking into account this, the performance of catalytic Ru(II) complexes is expected to depend on their location on C3N4. As shown in Table 2 (entries 3–5), the CO formation activity remained almost unchanged even though the loading amount of RuCP was increased from 7.8 to 52.4 µmol g–1. This might be because occupation of active sites that can generate polymeric Ru species (working as CO-evolving catalysts) by RuCP had been achieved already at the loading mount of 7.8 µmol g–1. In other words, further increase in the loading amount of RuCP does not contribute to an increase in the selectivity to CO, because excessively loaded RuCP cannot accept electrons from C3N4. Another possible explanation for the unchanged performance is that the polymeric Ru species generated during the CO2 reduction reaction may hinder light absorption by C3N4, thereby lowering the performance. (45)

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Table of Contents Graphic and Synopsis


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e–

Visible Light

e– CB

X

Cl N

X

VB

h+

N

Ru

CO CO

Cl

Carbon Nitride

We found unique solvent effects on photocatalytic CO2 reduction using Ru(II)-complex/C3N4 hybrids under visible light (λ > 400 nm), presenting the first example of nonsacrificial CO2 reduction through the use of carbon nitride.


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Unique Solvent Effects on Visible-Light CO2 Reduction over

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