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. 2023 Aug 21;145(37):20485–20491. doi: 10.1021/jacs.3c06413

CH4 Synthesis from CO2 and H2O of an Electron Source over Rh–Ru Cocatalysts Loaded on NaTaO3:Sr Photocatalysts

Wasusate Soontornchaiyakul †, Shunya Yoshino †, Tomoki Kanazawa ‡, Rie Haruki ‡, Dongxiao Fan ‡, Shunsuke Nozawa ‡, Yuichi Yamaguchi †,§, Akihiko Kudo †,§,*
PMCID: PMC10515478  PMID: 37599601

Abstract

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CO2 reduction as an artificial photosynthetic system is a promising technology to produce green energies and chemicals because it uses light energy to convert H2O and CO2 into valuable products such as CO, HCOOH, CH3OH, CH4, and preferably higher hydrocarbons. In photocatalytic reduction, water should be used as hydrogen and electron sources for CO2 reduction. Moreover, CH4 formation is an attractive and challenging topic because of the eight-electron-reducing product of CO2. Herein, we report the development of a new Rh–Ru cocatalyst decorated on an alkaline earth-doped NaTaO3 surface for the photocatalytic CO2 reduction to form CH4 using water as an electron donor. CH4 was obtained by a photocatalytic “uphill” reaction of CO2 reduction using Rh–Ru cocatalyst-loaded NaTaO3:Sr, water, and CO2 in an aqueous suspension system. About 10% of a selectivity (electronic efficiency) was obtained for CH4 formation under ambient conditions accompanied with O2 evolution of the oxidation product of H2O.

Introduction

Photocatalytic CO2 reduction as artificial photosynthesis has attracted attention as a chemical technology for CO2 fixation aiming at a sustainable society. In artificial photosynthesis, solar energy and CO2 are directly converted to useful chemical products. Moreover, the reaction can be operated under ambient temperature and pressure, and abundant water functions as hydrogen and electron sources.1,2 To achieve photocatalytic CO2 reduction as artificial photosynthesis being an uphill reaction (ΔG > 0), CO2 reduction products must be obtained accompanied with reasonable O2 evolution by water oxidation as the counterpart but not using strong sacrificial electron donors. Moreover, employing semiconductor photocatalyst powders is desirable because of their simplicity for practical use.3 Recently, efficient and selective CO formation as two-electron-photocatalytic CO2 reduction has been reported since discovering a Ag cocatalyst even using a photocatalyst powder-dispersion system.1,4−8 On the other hand, photocatalytic CH4 formation is still a challenging topic because of a tough reaction accompanied with eight-electron reduction. Selective hydrocarbon formation such as CH4 has been achieved by electrochemical CO2 reduction using a Cu electrode under ambient condition9−12 and Fe, Co, and Ni electrodes under high pressure of CO2.13,14 In contrast, although there are a lot of reports to produce CH4 over semiconductor photocatalysts, mainly TiO2, by loading various cocatalysts such as Cu, Ru, and Rh, the amount of evolved CH4 is quite small compared with the amount of an employed photocatalyst, and no O2 evolution is observed.15−18 There are only a few papers in which an amount of O2 evolved was determined, though the activity was low.19−21 It is challenging to obtain a reasonable amount of CH4 of an eight-electron reduction product with a stoichiometric O2 evolution by photocatalytic CO2 reduction using water as an electron donor without any strong electron donors. The critical issue comes from “lack of an active site to convert CO2 to CH4 relating to eight photogenerated-electrons and hydrogens” and “low photocatalytic activity”. Moreover, the importance of the present research is to use water as an electron donor to reduce CO2 to CH4 accompanied with O2 evolution. For breaking through the present stage, it is indispensable to develop a new cocatalyst to convert CO2 to CH4 accompanied by obvious O2 evolution on a highly efficient photocatalyst. We have developed a NaTaO3:Sr photocatalyst showing high activity for water splitting and CO2 reduction to form CO using water as an electron donor.6,22 From such background, in the present report, we developed a new Rh–Ru cocatalyst and demonstrated photocatalytic CH4 formation using water as an electron donor and artificial photosynthesis using the NaTaO3:Sr photocatalyst.

Experimental Section

Photocatalyst Preparation

Doped and non-doped NaTaO3 powders were prepared by a solid-state reaction. The starting materials of Na2CO3 (Kanto Chemical; 99.8%), Ta2O5 (rare metallic; 99.99%), CaCO3 (Kanto Chemical; 99.5%), SrCO3 (Kanto Chemical; 99.9%), BaCO3 (Kanto Chemical; 99.0%), and La2O3 (Kanto Chemical; 99.99%) were mixed and grinded in an alumina mortar. A molar ratio of Na/Sr/Ta as 1.0395:0.01:1 for 1% of Sr-doping. An excess amount of Na (5 mol %) was required to compensate a volatilization. The mixtures were put in a Pt crucible and calcined at 1173 K for an hour. After being cooled to room temperature, the mixtures were grinded again in the mortar and calcined again at 1423 K for 10 h using a Pt crucible.

Rh and Ru cocatalysts were loaded on photocatalysts by a liquid-phase chemical reduction method using RhCl3 (Tanaka Kikinzoku, 36–39.08% as Rh in RhCl3·3H2O) and RuCl4 (Tanaka Kikinzoku, 36% as Ru in RuCl4·nH2O) as cocatalyst sources and NaPH2O2 (Kanto Chemical; 82.0–86.5% as NaPH2O2 in NaPH2O2·H2O) as a reducing reagent. A mixed aqueous RhCl3 and RuCl4 solution was added into an aqueous suspension containing the photocatalyst powder and stirred at 323 K for 2 h. The aqueous NaPH2O2 solution was added to the mixture matured by stirring at 343 K for 12 h. The amount of NaPH2O2 was set as 40 times more molar quantity against Rh and Ru sources. The obtained Rh–Ru cocatalyst-loaded photocatalysts were washed with water and dried overnight at room temperature.

X-ray diffraction (Rigaku; MiniFlex600) was used to confirm a single phase of obtained NaTaO3:M (M = non, Ca, Sr, Ba, and La). Surface components of Rh–Ru/NaTaO3:Sr were analyzed by X-ray photoelectron spectroscopy (XPS; JEOL; JPS-9010MC) with a Mg Kα anode. Rh–Ru/NaTaO3:Sr powder before and after photocatalytic CO2 reduction was loaded on a carbon tape for the XPS measurement. The loaded amounts of cocatalysts were also measured by X-ray fluorescence (XRF; Rigaku; NEXDE). Morphologies of photocatalysts and loaded cocatalysts were observed by transmission electron microscopy (TEM; JEOL; JEM-2100F). X-ray absorption near-edge structure (XANES) of Ru and Ru K-edge were measured at AR-NW10A of the Photon Factory Advanced Ring in Tsukuba, Japan. The incident X-ray was monochromatized by a Si(311) double-crystal monochromator.

Photocatalytic Reaction

Photocatalytic CO2 reduction reaction and water splitting were conducted using a gas-flow system equipped with an inner irradiation quartz cell. Photocatalyst powder (1.5 g) was dispersed in 350 mL of water without any additives. CO2 or Ar gas (1 atm) was bubbled into the aqueous suspension at a flow rate of 30 mL min–1 for photocatalytic CO2 reduction and water splitting, respectively. A 400 W high-pressure Hg lamp (SEN, HL-400EH-5) was employed as a light source. Amounts of evolved H2, O2, CO, and CH4 were determined using online gas chromatographs (Shimadzu GC-8A equipped with the MS-5A column, a TCD, and an Ar carrier for H2 and O2; Shimadzu GC-8A equipped with the MS-13X column, a FID with methanizer, and an Ar carrier for CO and CH4). The selectivity for CH4 formation, the stored O2 ratio, the reacted electron to hole ratio (e–/h+), and the turnover number (TON) were calculated according to following equations.

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Results and Discussion

We conducted photocatalytic CO2 reduction using Rh(0.5 wt %)–Ru(0.375 wt %) cocatalyst-loaded NaTaO3:Sr(1%) in an aqueous solution bubbling 1 atm of CO2, as shown in Figure 1 and Table 1. In the present experiment, the released gases from the suspension in the flow reactor were quantified even under dark after light irradiation was stopped because the released O2 after the photocatalytic reaction was observed, as shown in Figure 1. The amount of released O2 under dark after stopping UV irradiation was set at “stored O2” columns in Table1.

Figure 1.

Figure 1

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Photocatalytic CO2 reduction using (a) NaTaO3:Sr, (b) Ru(0.375 wt %)/NaTaO3:Sr(1%), (c) Rh(0.5 wt %)/NaTaO3:Sr(1%), and (d) Rh(0.5 wt %)–Ru(0.375 wt %)/NaTaO3:Sr(1%) photocatalysts under UV irradiation. Photocatalyst: 1.5 g, solution: water without any additives (350 mL, pH 4–5), flow gas: CO2 (1 atm), light source: 400 W high pressure Hg lamp, and cell: inner-irradiation quartz cell.

Table 1. Effects of Rh–Ru Cocatalysts and Alkaline Earth Metal Dopants on Photocatalytic CO2 Reduction Using NaTaO3:Sr(1%)a.

entry photocatalyst cocatalyst rates of gas evolution under UV/μmol h–1
 CH4 select % amounts of evolved gases/μmol (9 h)
 stored O2 ratio % e–/h+
      H2 CO CH4 O2   H2 CO CH4 O2 stored O2    
1 NaTaO3:Sr(1%) none 350 0.12 0 177 0 3118 1.29 0 1564 0 0 1.0
2 NaTaO3:Sr(1%) Ru 1339 0.20 0.04 737 0.01 10 603 2.31 0.36 5761 242 4.0 0.9
3 NaTaO3:Sr(1%) Rh 129 0.02 0.07 60 0.22 1171 0.44 0.76 557 62 10 1.0
4 NaTaO3:Sr(1%) Rh–Ru 54 0 1.44 14 9.64 443 0.08 9.10 130 44 25 1.4
5 NaTaO3 none 139 0.10 0 65 0 1186 1.02 0 527 0 0 1.1
6 NaTaO3 Rh–Ru 20 0.03 0.11 8 2.12 222 0.23 0.82 79 7 8.1 1.3
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a

Photocatalyst: 1.5 g, cocatalyst: Ru (0.375 wt %) and Rh (0.5 wt %) loaded by a liquid phase reduction, solution: water without any additives (350 mL, pH 4–5), flow gas: CO2 (1 atm, 30 mL min–1), light source: 400 W high pressure Hg lamp, cell: inner-irradiation quartz cell, and total irradiation time: 9 h. Rates of gas evolution were estimated after induction period of UV irradiation. CH4 selectivity was calculated using the rates of gas evolution. e–/h+ was calculated from amounts of evolved gases including stored O2.

Bare NaTaO3:Sr split water into stoichiometric amounts of H2 and O2, but gave quite low activity for CO2 reduction (Figure 1a; Table 1 entry 1), because non-loaded NaTaO3:Sr did not have an effective active site for CO2 reduction.6 On the other hand, a Ru cocatalyst enhanced not only water splitting but also CO2 reduction to form a small amount of CH4 (Figure 1b; Table 1 entry 2). A Rh cocatalyst slightly enhanced CH4 formation compared with the Ru cocatalyst, though water splitting was suppressed (Figure 1c; Table 1 entry 3), meaning that the Rh cocatalyst functioned as an active site for CH4 formation by reducing CO2. Coloading of Rh and Ru much enhanced the CH4 formation (Figure 1d; Table 1 entry 4). The enhancement of CH4 formation might be due to the efficient supply of adsorbed hydrogen and photogenerated electron from Ru to Rh which should be a critical issue for reducing CO2 to form CH4 being multi-proton and electron reaction. We also confirmed that formic acid was not detected with an ion chromatograph. Alcohol and formaldehyde compounds should not be stored in the reactant solution because they are smoothly oxidized by photogenerated holes in a photocatalyst as a sacrificial reagent. We also investigated the effect of loading amounts of Rh–Ru cocatalysts on photocatalytic CO2 reduction using NaTaO3:Sr(1%), as shown in Table 2. When the ratio of Rh to Ru was fixed and the loading amounts were varied, Rh(0.5 wt %)–Ru(0.375 wt %) was optimal for CH4 formation (entries 5–10). Then, when the amount of Ru was changed from 0 to 0.75 wt % with 0.5 wt % of Rh (entries 3, 4, 7, 11, and 12), the highest activity for CH4 formation was obtained by loading 0.375 wt % of Ru. Thus, the effect of the cocatalyst on CH4 formation was in order of Ru < Rh ≪ Rh–Ru. The apparent quantum yield was estimated to be about 0.016% at 270 nm for CH4 formation on the optimized photocatalyst judging from the efficiency for water splitting on a NiO/NaTaO3:La photocatalyst.23

Table 2. Effect of Loading Amounts of Rh–Ru Cocatalysts on Photocatalytic CO2 Reduction Using NaTaO3:Sr(1%)a.

entry loaded Rh/μmol (wt %) loaded Ru/μmol (wt %) Rh/Ru rates of gas evolution under UV/μmol h–1
 CH4 select % amounts of evolved gases/μmol (9 h)
 stored O2 ratio % e–/h+
        H2 CO CH4 O2   H2 CO CH4 O2 stored O2    
1 0 0   350 0.12 0 177 0 3118 1.29 0 1564 0 0 1.0
2 0 54.9 (0.375)   1339 0.20 0.04 737 0.01 10 603 2.31 0.36 5761 242 4.0 0.9
3 72.9 (0.5) 0   129 0.02 0.07 60 0.22 1171 0.44 0.76 557 62 10.0 0.9
4 72.9 (0.5) 37.2 (0.25) 1.96 60 0 0.72 17 4.58 434 trace 4.02 119 89 42.9 1.1
5 29.8 (0.2) 22.5 (0.15) 1.32 75 0.01 0.50 25 2.60 568 0.09 4.11 177 100 36.1 1.1
6 44.2 (0.3) 32.3 (0.225) 1.37 74 0.01 0.65 24 3.39 519 0.09 4.73 163 66 28.8 1.2
7 72.0 (0.5) 55.1 (0.375) 1.31 54 0 1.44 14 9.64 443 0.08 9.10 130 44 25.3 1.4
8 101.8 (0.6) 76.5 (0.45) 1.33 44 0 1.26 10 10.28 340 trace 7.06 80 61 43.3 1.3
9 144.8 (1) 110.6 (0.75) 1.31 41 0 1.01 9.8 8.97 404 trace 4.43 100 60 37.5 1.3
10 363.9 (2.5) 271.1 (1.87) 1.34 75 0.02 0.23 19 1.21 1046 0.31 1.77 245 89 26.6 1.6
11 72.6 (0.5) 74.0 (0.5) 0.98 54 trace 1.00 14 6.90 423 0.09 4.84 118 64 35.2 1.2
12 72.8 (0.5) 110.0 (0.75) 0.66 61 trace 0.66 15 4.15 393 0.06 2.76 93 72 43.6 1.2
13 30.1 (0.2) 56.2 (0.375) 0.54 835 0.03 0.26 394 0.12 4694 0.19 1.40 2105 139 6.2 1.0
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a

Photocatalyst: 1.5 g, cocatalyst: a liquid phase reduction (loaded amounts (/μmol) was determined by XRF), solution: water without any additives (350 mL, pH 4–5), flow gas: CO2 (1 atm, 30 mL min–1), light source: 400 W high pressure Hg lamp, cell: inner-irradiation quartz cell, and total irradiation time: 9 h. Rates of gas evolution were estimated after induction period of UV irradiation. CH4 selectivity was calculated using the rates of gas evolution. e–/h+ was calculated from amounts of evolved gases including stored O2.

In the photocatalytic CO2 reduction using the Rh–Ru/NaTaO3:Sr photocatalyst (Figure 1d′), small amounts of CO and CH4 were obtained accompanied with H2 and O2 evolved by water splitting in the induction period for 5 h of UV light irradiation. CH4 evolution gradually increased through the induction period, while CO evolution was suppressed. After that, the Rh–Ru/NaTaO3:Sr(1%) steadily produced CH4, H2, and O2. Gradual activation for CH4 formation was not observed in the second run being different from the induction period of the first run, indicating that the cocatalyst changed during the induction period of the first run. After light irradiation was stopped at 51 h, obvious O2 was released from the suspension for several hours under dark, while H2, CO, and CH4 were not so (Figure S1). In other words, a part of O2 was stored in the suspension under UV light irradiation, and the stored O2 was quickly released by stopping light irradiation. The O2 release was observed even 17 h after stopping UV light irradiation. The O2 storage behavior was also observed for Ru- and Rh-loaded NaTaO3:Sr (Table 1 entries 2 and 3), whereas it was not so over non-loaded NaTaO3:Sr (Table 1 entry 1). Therefore, loading of the Rh–Ru cocatalyst should be essential for the O2 storage behavior. The O2 storage was also seen for the second run, as shown in Figure 1d.

The e–/h+ estimated from the amounts of evolved gasses including stored O2 in the second run of Figure 1d was 1.05 being almost unity. The TON of the molar quantity of the reacted electrons for CH4 formation in the 1st + 2nd runs to the loaded cocatalyst was estimated to be 3.5. We also conducted a control experiment under Ar flow using the Rh–Ru/NaTaO3:Sr photocatalyst at pH 4 adjusted by H2SO4 because pH of water saturated with CO2 under 1 atm was 4 (Figure S2). CO and CH4 did not evolve under the Ar atmosphere, revealing that the origin of CO and CH4 obtained in Figure 1d was flowed CO2. In contrast to the CH4 formation, the O2 storage was observed under not only CO2 but also an Ar atmosphere in which water splitting proceeded (Figure S2). Thus, the Rh–Ru/NaTaO3:Sr(1%) photocatalyst has unique property to produce CH4 using water as an electron donor and store O2 under UV light irradiation.

We examined the effect of Sr dopants into the NaTaO3 photocatalyst on CH4 formation and O2 storage ability. Bare NaTaO3 did not show CH4 formation and O2 storage abilities. When the Rh–Ru cocatalyst was loaded, NaTaO3 without Sr dopant showed low activities for CH4 formation and O2 storage (Table 1 entries 5 and 6). In contrast to them, Sr doping much enhanced CH4 formation and O2 storage with the loading of Rh–Ru cocatalysts (entries 4 and 6). We have also investigated the effect of other dopants on CO2 reduction over NaTaO3:M (M = Ca, Sr, Ba, and La) loaded with/without the Rh–Ru cocatalysts, as shown in Table 3. All dopants gave CH4 formation and O2 storage when the Rh–Ru cocatalyst was loaded. Sr was the most effective dopant. We have reported that those dopants formed nano-step structures on the surface of a NaTaO3 photocatalyst particle and widened the band gap from 4.0 to 4.1 eV, resulting in enhancement of water splitting.22,23 Therefore, it is concluded that a synergetic effect by loading the Rh–Ru cocatalyst and Sr doping to create surface nano-step structure was a key factor for reasonable CH4 formation and O2 storage.

Table 3. Effects of Dopant M on Photocatalytic CO2 Reduction Using Rh–Ru/NaTaO3:Ma.

entry photocatalyst surface step structure cocatalyst rates of gas evolution under UV/μmol h–1
 CH4 select % amounts of evolved gases/μmol (9 h)
 stored O2 ratio % e–/h+
        H2 CO CH4 O2   H2 CO CH4 O2 stored O2    
1 NaTaO3 absent none 139 0.10 0 65 0 1186 1.02 0 527 0 0 1.1
2 NaTaO3 absent Rh–Ru 20 0.03 0.11 8 2.15 222 0.23 0.82 79 7 8.1 1.3
3 NaTaO3:Ca(1%) incomplete none 233 0.09 0.01 115 0.02 1951 0.08 0.97 960 0 0 1.0
4 NaTaO3:Ca(1%) incomplete Rh–Ru 44 0.01 0.8 12 6.78 312 0.1 3.9 84 82 49.4 1.0
5 NaTaO3:Sr(0.5%) absent Rh–Ru 39 0.01 0.81 11 7.67 319 0.1 3.8 85 48 36.1 1.3
6 NaTaO3:Sr(1%) incomplete none 350 0.12 0 177 0 3118 1.29 0 1564 0 0 1.0
7 NaTaO3:Sr(1%) incomplete Rh–Ru 54 0 1.44 14 9.64 443 0.08 9.10 130 44 25.3 1.4
8 NaTaO3:Sr(2%) complete Rh–Ru 41 0.01 1.3 12 11.25 351 0.05 6.77 77 79 50.6 1.2
9 NaTaO3:Sr(5%) complete Rh–Ru 33 0.01 1.1 9 11.76 299 0.05 6.49 70 86 55.1 1.0
10 NaTaO3:Ba(1%) incomplete none 233 0.04 0 114 0 2105 0.95 0 1015 0 0 1.0
11 NaTaO3:Ba(1%) incomplete Rh–Ru 12 0.01 0.23 4 7.12 150 0.15 1.4 36 26 41.9 1.3
12 NaTaO3:La(1%) complete none 483 0.09 0 228 0 4362 1.04 0 2094 0 0 1.04
13 NaTaO3:La(1%) complete Rh–Ru 52 0.01 0.5 15 3.7 419 0.12 4.33 131 73 35.7 1.05
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a

Photocatalyst: 1.5 g, cocatalyst: Ru (0.375 wt %) and Rh (0.5 wt %) loaded by a liquid phase reduction, solution: water without any additives (350 mL, pH 4–5), flow gas: CO2 (1 atm, 30 mL min–1), light source: 400 W high pressure Hg lamp, cell: inner-irradiation quartz cell, and total irradiation time: 9 h. Rates of gas evolution were estimated after an induction period of UV irradiation. CH4 selectivity was calculated using the rates of gas evolution. e–/h+ was calculated from amounts of evolved gases including stored O2.

The O2 storage might be due to adsorption of O2 molecule or intermediate of evolved O2 on the surface of Rh–Ru/NaTaO3 with and without dopant. The e–/h+ ratio estimated from the gas evolution rates approached 1 with an increase in the photocatalytic reaction time, as shown in Figures 1d and S1, implying that the stored O2 was gradually saturated. The molar quantity of stored O2 molecule (310 μmol) was larger than those of Ru (55.6 μmol) and Rh (72.9 μmol) calculated from first run of Figure 1d, indicating that the O2 storage occurred on not only the cocatalyst but also photocatalyst surface. Although photoadsorption of O2 would be one possibility, the adsorbed O2 should immediately have desorbed. Such slow release of O2 was not observed, when O2 was flowed in the suspension of the photocatalyst. Moreover, O2 adsorption on the Rh–Ru cocatalyst should not be a major process because the cocatalyst is a reduction site to form H2 and CH4, and the adsorbed O2 should easily have been reduced. We conducted the photocatalytic reaction using Rh–Ru/NaTaO3:Sr by flowing the mixture of CO2 and O2. However, the CH4 production rate was not improved by the addition of O2. However, the positive relationship between appearance of CH4 formation and the O2 storage is actually observed. No O2 storage induced no CH4 formation. Although formation of other oxidation product such as H2O2 and percarbonate in the reactant solution would be another possibility, they were not clearly detected. Therefore, we cannot conclude the firm mechanism for O2 storage at present. However, the cocatalyst is necessary to realize O2 storage, and then, synergetic effect or interaction between the cocatalyst and the photocatalyst should affect the O2 storage ability.

Rh–Ru/NaTaO3:Sr(1%) powders before and after CO2 reduction were characterized to reveal how the cocatalyst functioned. Figure 2 shows TEM images of the Rh–Ru cocatalyst loaded on NaTaO3:Sr(1%) before and after CO2 reduction by TEM (JEOL; JEM-2100F). A clear particle was not observed on NaTaO3:Sr before CO2 reduction in the present resolution, while there were aggregated particles after CO2 reduction. Although we would have checked the cocatalyst using TEM–EDS before and after photocatalytic CO2 reduction, unfortunately, we could not distinguish Ru from Rh nanoparticles because of limitation of energy and spatial resolution. It was not clear whether the Rh and Ru made an alloy or were separately deposited. The surface composition of Rh–Ru/NaTaO3:Sr(1%) was analyzed by XPS (JEOL; JPS-9010MC) with a Mg Kα anode, as shown in Table 4. The area ratios of Rh and Ru to Ta of the sample after CO2 reduction were smaller than those before CO2 reduction though the total amounts of Rh and Ru in these samples did not change before and after the reaction judging from XRF (Rigaku; NEXDE). The decrease in the area ratios of Rh/Ta and Ru/Ta that indicated relative coverage by the cocatalysts to the surface of the NaTaO3:Sr photocatalyst supported aggregation of the loaded cocatalysts during photocatalytic CO2 reduction. Dissolution of the cocatalyst by photooxidation and subsequent deposition by photoreduction and/or migration of the metal atoms on the surface of photocatalyst would induce the aggregation. Similar aggregation by photooxidation and subsequent deposition by photoreduction was observed for a Ag cocatalyst on the BaLa4Ti4O15 photocatalyst for CO2 reduction.4 Moreover, a larger area ratio of Ru/Rh in XPS through CO2 reduction indicated Ru-rich surface (Rh is buried) or more aggregation of Rh than Ru. Although the area ratio significantly changed even after 1 h of light irradiation, the change still occurred slightly judging from the result at 9 h. This alternation would bring the induction period of the CO2 reduction to form CH4 and CO, as shown in Figure 1d′.

Figure 2.

Figure 2

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TEM images of Rh(0.5 wt %)–Ru(0.375 wt %) cocatalyst-loaded NaTaO3:Sr (1%) photocatalyst (a) before and (b) after photocatalytic CO2 reduction (9 h).

Table 4. Elemental Analysis of XPS and XRF for a Rh(0.5 wt %)–Ru(0.375 wt %) Cocatalyst-Loaded NaTaO3:Sr(1%) Photocatalysta.

CO2 reduction peak area ratio from XPS
 molar ratio from XRF
  Rh/Ta Ru/Ta Ru/Rh Rh/Ta Ru/Ta Ru/Rh
before 1.74 0.52 0.30 0.012 0.0093 0.76
after 1 h 0.30 0.28 0.94      
after 9 h 0.14 0.18 1.24 0.012 0.0092 0.77
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a

Area of Rh 3d (Rh 3d3/2 + Rh 3d5/2), Ru 3p (Ru 3p1/2 + Ru 3p3/2), and Ta 4d (Ta 4d3/2 + Ta 4d5/2) were used for calculation of an area ratio. Rh(0.5 wt %)–Ru(0.375 wt %) corresponds to Rh(1.22 mol %)–Ru(0.94 mol %) to Ta.

The chemical states of the loaded Rh and Ru were analyzed by XANES for each K-edge using AR-NW10A of the photon factory advanced ring, as shown in Figure 3. The Rh before CO2 reduction was loaded as a mixture of oxidized Rh (and/or hydrolyzed Rh) and metallic Rh judging from the position of the absorption edge and the shape of the post edge (Figure 3a), even if NaPH2O2 was used as a reducing reagent in the present loading step. By carrying out the photocatalytic CO2 reduction, the absorption edge shifted to low energy direction, and the shape of the post edge changed to metallic Rh, but a part of oxidized Rh remained. The change of the chemical state would occur due to dissolution of the cocatalyst by photooxidation and subsequent position by photoreduction during the photocatalytic CO2 reduction, as mentioned in the previous paragraph. The Ru on NaTaO3:Sr before CO2 reduction was also a mixture of the oxide and metal. However, in the case of Ru, the shift to low energy direction of the absorption edge and the change of the shape of the post edge was small even after CO2 reduction, suggesting that Ru was mainly maintained as oxidized (and/or hydrolyzed) state being different from Rh. These particle size and chemical state alternation of loaded cocatalysts should contribute to the induction period observed in the first run of photocatalytic CO2 reduction (Figure 1d′). Then, metallic Rh + hydrolyzed and/or oxidized Rh and Ru particles with aggregation functioned as a cocatalyst for CO2 reduction on a NaTaO3:Sr photocatalyst.

Figure 3.

Figure 3

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Rh and Ru K-edge XANES spectra of Rh(0.5 wt %)–Ru(0.375 wt %) cocatalyst-loaded NaTaO3:Sr(1%) photocatalyst (a,e) before and (b,f) after photocatalytic CO2 reduction (9 h). (c) Rh metal, (d) Rh2O3, (g) Ru metal, and (h) RuO2 were measured as a reference sample.

Conclusions

We successfully demonstrated photocatalytic CO2 reduction to form CH4 using water as an electron donor by using Rh–Ru/NaTaO3:Sr. It is stressed that reasonable O2 evolution by water oxidation was observed as the counterpart of CO2 reduction to CH4, indicating that the uphill reaction proceeded photocatalytically. The present Rh–Ru cocatalyst led not only conversion of CO2 to CH4 but also unique O2 storage behavior. The Rh–Ru cocatalyst and Sr doping are necessary for the reasonable CH4 formation and O2 storage. The combination of strategies for loading cocatalysts and doping was a key factor to realize the reasonable CH4 formation.

Acknowledgments

This work was supported by the JSPS KAKENHI grant numbers 17H06433 and 17H06440 in Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I4LEC)” and 23H00248. The authors gratefully appreciate Prof. Y. Idemoto and Dr. T. Ichihashi for measurement of TEM images.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c06413.

  • Production rate for photocatalytic CO2 reduction and control experiment under an Ar atmosphere (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja3c06413_si_001.pdf (238.1KB, pdf)

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