Photo-induced enhancement of reverse water–gas shift over Mo-modified cerium oxide†

Daichi Takami^a, Naoto Doshita^a, Ryosuke Sugiura^a, Yasutaka Kuwahara*^ab and Hiromi Yamashita^a
aDivision of Materials and Manufacturing Science, Graduate School of Engineering, The University of Osaka, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan. E-mail: kuwahara@mat.eng.osaka-u.ac.jp
^bInnovative Catalysis Science Division, Institute for Open and Transdisciplinary Research Initiatives (OTRI), The University of Osaka, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

First published on 24th July 2025

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Introduction

The continuous expansion of industrial activities and the extensive use of fossil fuels have resulted in elevated atmospheric concentrations of carbon dioxide (CO₂), significantly contributing to global climate change.¹ To mitigate this pressing environmental issue, the development of efficient and sustainable technologies for converting CO₂ into valuable chemicals and fuels has become crucial.² Among the various possible transformations, the conversion of CO₂ to carbon monoxide (CO) is particularly significant due to CO's utility as an essential intermediate in established C1 chemical processes, such as the Fischer–Tropsch synthesis and methanol production.³ Thus, efficient CO production from CO₂ is instrumental for sustainable chemical manufacturing and energy storage.

The reverse water–gas shift (RWGS) reaction (eqn (1)) is widely recognized as a promising catalytic pathway for CO₂-to-CO conversion.⁴

	(1)

Extensive research has been conducted on RWGS catalysts, predominantly focusing on transition metals such as Cu, Ni, Fe, and Pt supported on various oxide materials (Al₂O₃, TiO₂, CeO₂, ZrO₂, etc.).^5–10 Despite their progress, critical barriers to practical application persist. Notably, the RWGS reaction is endothermic, necessitating substantial energy input. Furthermore, undesired side reactions, including CO methanation (eqn (2)) and the Sabatier reaction (eqn (3)), produce methane (CH₄), complicating selectivity control and catalyst optimization.¹¹

	(2)

	(3)

Recent studies have increasingly utilized renewable energy sources, particularly solar energy, to address the high energy requirements associated with the RWGS reaction.

The photo-enhancement of the reaction between CO₂ and H₂ was first reported by M. Grätzel et al. in 1987, with CH₄ as the main product.¹² They observed that catalytic activity under light irradiation exceeded that in the dark, even at moderately elevated temperatures, and attributed this enhancement to the electrons and holes generated through band-gap excitation (i.e. photocatalytic effect). Subsequently, T. Tanaka et al. conducted a series of investigations on the photocatalytic RWGS reaction.^13–19 They reported CO formation under light irradiation over catalysts such as ZrO₂, which exhibited negligible activity for the RWGS reaction in the dark even under heated conditions.¹⁴ Moreover, they found that photo-excited CO₂ species (CO₂⁻) played a key role in promoting the RWGS reaction.²⁰ G. A. Ozin et al. demonstrated the effectiveness of a photothermal (light-driven thermal) approach to the RWGS reaction.²¹ By combining In₂O_3−x(OH)_y, a photocatalyst with a bandgap of 2.9 eV (∼425 nm), with silicon nanowires as a photothermal material, they achieved high CO formation rates through efficient utilization of broad-spectrum light. Furthermore, their detailed analysis successfully distinguished the contributions of photo-generated charge-carriers and photothermal heating to the catalytic performance.

Recently, the number of researchers exploring light-assisted catalytic reactions including the RWGS reaction^21–36 has grown rapidly, leading to a blurring of the definition of the “photothermal effect” (e.g. light-driven thermal effect^37–41 vs. photocatalytic effect at high reaction temperature^42–46). The ambiguity is further compounded by the lack of sufficient discussion on how to distinguish thermal and non-thermal contributions, an issue highlighted in several studies.^47–52 Accordingly, researchers are required to clearly state their definition of “photothermal” and to clarify the mechanisms underlying light-induced enhancement.

In addition to solar energy, industrial waste heat represents another abundant but underutilized energy resource. One of the reports stated that 60% of industrial waste heat is emitted at temperatures below approximately 500 K,⁵³ providing substantial potential for driving endothermic catalytic reactions such as the RWGS reaction. Combining waste heat utilization with photocatalytic approaches offers synergistic benefits, allowing catalysts to operate at elevated temperatures. This synergistic approach reduces band-gap constraints, extending the usable spectral range into visible and near-infrared (NIR) regions, and potentially reduces recombination rates of photogenerated electrons by utilizing materials exhibiting indirect electronic transitions.

Given their favorable properties, cerium oxide (CeO₂)-based catalysts emerge as highly promising candidates for integrated RWGS applications. CeO₂ is well known for its surface basicity, robust redox characteristics^54,55 and exceptional thermal stability,^56,57 which make it advantageous for catalysis involving CO₂ conversion. Additionally, its moderate band-gap energy allows effective utilization in various photocatalytic processes.^58,59 Consequently, CeO₂-based materials have garnered significant research interest as both thermal catalysts and photocatalysts in RWGS reactions. Moreover, doping CeO₂ with selected elements enhances catalytic performance by modifying the surface chemistry^60–62 and tuning band-gap excitation properties.^63–66

In this study, we systematically investigated CeO₂-based catalysts doped with various elements and loaded with platinum (Pt) nanoparticles for the RWGS reaction under visible-NIR irradiation. Our primary objective is to achieve high catalytic efficiency through the combined utilization of solar irradiation and industrial waste heat for the RWGS reaction via band gap tuning and surface engineering of CeO₂.

Experimental

Reagents used in this study

The following reagents were utilized in this study without any further purification. Anhydrous citric acid, cerium(III) nitrate hexahydrate, chromium(III) nitrate nonahydrate, manganese(II) nitrate hexahydrate, cobalt(II) nitrate hexahydrate, nickel(II) nitrate hexahydrate, copper(II) nitrate trihydrate, zinc nitrate hexahydrate, and hydrogen hexachloroplatinate(IV) hexahydrate were purchased from Nacalai Tesque. Iron(III) nitrate nonahydrate, ammonium molybdate tetrahydrate, and barium sulfate were purchased from FUJIFILM Wako Pure Chemical. Ammonium metatungstate hydrate was purchased from Strem Chemicals, Inc.

Catalyst preparation

Metal-modified CeO₂ catalysts were prepared using a citric acid method based on a previously reported procedure.⁶⁷ Ce(NO₃)₃·6H₂O and the metal precursor (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W; typically in a molar ratio of 10 atm% M to total metal ions) were dissolved with 4.23 g (1.1 equivalents, relative to total metal ions of 20 mmol) of anhydrous citric acid in 100 mL distilled water. The solution was stirred at room temperature to ensure complete dissolution. Subsequently, water was evaporated at 343 K under reduced pressure using a rotary evaporator to obtain a gel. The obtained gel was dried at 373 K overnight and then ground into a fine powder using a mortar. The powder was calcined at 873 K for 4 h in air. The obtained powder was denoted as M_xCe_1−xO_y.

Pure CeO_y and MoO_y were also synthesized using the same method described above by employing only Ce(NO₃)₃·6H₂O and (NH₄)₆Mo₇O₂₄·4H₂O, respectively.

1.00 g of M_xCe_1−xO_y powder was dispersed in 100 mL of distilled water. The required amount of aq. H₂PtCl₆ was added dropwise, and the mixture was stirred at room temperature for 1 h. After removing water using a rotary evaporator at 343 K under reduced pressure, the resulting powders were calcined at 873 K for 4 h in air, denoted as Pt/M_xCe_1−xO_y. The nominal Pt loading was set to be 3 wt%.

Characterization

Powder X-ray diffraction (XRD) was performed using an Ultima IV (Rigaku) instrument with Cu Kα radiation. X-ray photoelectron spectroscopy was carried out on an ESCA-3400 (Shimadzu). The specific surface area (SSA) was determined with the Brunauer–Emmett–Teller (BET) method using a BELSORP-max system (MicrotracBEL). Ultraviolet-visible diffuse reflectance (UV-vis DR) spectra were recorded using a Shimadzu UV-2600 spectrophotometer equipped with an ISR-2600 integrating sphere attachment. Samples for these measurements were diluted by physically mixing with BaSO₄ at a weight ratio of 1:1. In situ UV-vis DR spectra were obtained using a V-750 (JASCO) equipped with an HISV-728 (JASCO) without any dilution. In situ and ex situ X-ray absorption fine structure (XAFS) spectra were measured in transmission mode at BL01B1 in the SPring-8 (Hyogo, Japan). CO₂-temperature-programmed desorption (TPD) was performed under He flowing at a ramp rate of 5 K min⁻¹ using a BELCAT-II (MicrotracBEL) after reduction pretreatment (20% H₂/Ar, 473 K) and CO₂ adsorption (20% CO₂/Ar, 223 K). Photocurrent measurements were conducted to evaluate the photoresponse of the catalysts under different wavelength regions using an HZ-5000 electrochemical measurement system (Hokuto Denko), with a platinum wire as the counter electrode and an Ag/AgCl electrode as the reference and a 500 W Xe lamp (SUPER BRIGHT 500: XEF-501S, equipped with a UXL-500A short arc lamp, Ushio). Optical filters were employed to control the irradiation wavelength (Fig. S1 and Table S1†). Further experimental details are provided in the ESI.†

Activity test for the RWGS

The procedure of the evaluation of the catalytic activity for the RWGS reaction is as follows: 25 mg of Pt/M_xCe_1−xO_y catalyst was placed in a fixed-bed type reactor equipped with a quartz window, as described in the previous report.⁶⁸ After a reduction pretreatment at 473 K for 1 h under a hydrogen flow (50% H₂/Ar, 20 mL min⁻¹), a reaction gas mixture (50% CO₂/H₂, 20 mL min⁻¹) was introduced to the reactor at 473 K. The outlet gases were analyzed using a gas chromatograph equipped with a flame ionization detector (GC-14B, Shimadzu) and a methanizer (MTN-1, Shimadzu).

For the photo-assisted RWGS reaction, a 500 W Xe lamp (SUPER BRIGHT 500: XEF-501S) equipped with a xenon short arc lamp (UXL-500A, Ushio, Japan) and an optical long-pass filter (λ > 450 nm, LV0450, Asahi Spectra) was used as a light source. Surface temperatures of the samples were measured with an infrared (IR) thermometer (TMHXSTM0050, JAPANSENSOR) with an effective wavelength range of 1.95–2.60 μm. Wavelength dependence of catalytic activity was assessed using optical filters (Fig. S1†). The reactions were conducted, in principle, under dark conditions for the first 1 hour, followed by 1 hour under light irradiation. The CO formation rates shown in bar graphs generally correspond to those measured at the end of the first 1 hour in the dark and at the end of the first 1 hour under light irradiation.

Due to the temperature limitation of the quartz window reactor, temperature-dependence of the CO formation rate was evaluated using a typical fixed-bed type reactor equipped with an electric furnace and a glass tube reactor (φ: 0.4 mm). A mixture of catalyst (25 mg) and quartz sand (100 mg) was packed into the glass tube.

Results and discussion

Effects of additive elements on activity for the RWGS reaction

The catalytic activities of a series of the Pt/M_0.1Ce_0.9O_y catalysts were evaluated in the RWGS reaction at 473 K (Fig. 1, see Fig. S2† for the XRD patterns of the catalysts). Most catalysts exhibited high CO selectivity (>95%), whereas Pt/Co_0.1Ce_0.9O_y formed CH₄, resulting in low CO selectivity (84.3%) due to the methanation activity of Co species.⁶⁹ Furthermore, visible-NIR light irradiation enhanced the CO formation rate for all catalysts, due to the energy input by light. Among these catalysts, the Pt/Mo_0.1Ce_0.9O_y catalyst showed a superior CO formation rate (15.2 mmol g_cat⁻¹ h⁻¹) compared to the unmodified Pt/CeO_y catalyst (13.5 mmol g_cat⁻¹ h⁻¹).

Effect of the amount of Mo doping on the activity

To elucidate the effects of Mo modification, a series of Pt/Mo_xCe_1−xO_y catalysts were evaluated (Fig. 2). All catalysts exhibited quite high CO selectivity (>99.5%). The CO formation rate increased upon the addition of a small amount of Mo (x = 0.01–0.1) compared to the Pt/CeO_y catalyst; however, further increases in the Mo content decreased their activities. Enhancement of the CO formation rate by visible-NIR light irradiation was observed for all Pt/Mo_xCe_1−xO_y catalysts. The Pt/Mo_0.03Ce_0.97O_y catalyst exhibited the highest CO formation rate under light irradiation (23.5 mmol g_cat⁻¹ h⁻¹) among these catalysts (see Fig. S3 for the time course of CO formation over Pt/Mo_0.03Ce_0.97O_y and Table S2 for a comparison with previous studies) and the RWGS reaction proceeded over 270 h (Fig. S4†).

Characterization of Pt/Mo_xCe_1−xO_y catalysts

XRD analysis revealed three distinct crystallographic phases (Fig. 3a). For Mo_xCe_1−xO_y catalysts with a low Mo content (x = 0–0.1), characteristic peaks attributed to cubic CeO₂ were maintained. With moderate Mo concentration (Mo_0.5Ce_0.5O_y), additional diffraction peaks corresponding to Ce₂(MoO₄)₃ emerged, whereas at higher Mo concentrations (x = 0.9 and 1) MoO₃-related peaks became dominant. After Pt loading and reduction treatment, the Mo_xCe_1−xO_y structures remained stable for x values up to 0.5 (Fig. 3b). However, for Pt/Mo_xCe_1−xO_y catalysts with a higher Mo content (x = 0.9 and 1), the peak intensities assignable to MoO₃ significantly decreased, suggesting the formation of oxygen vacancies in MoO₃-based materials to form amorphous Mo suboxide, being consistent with our previous report.⁷⁰ No Pt-related diffraction peaks were observed for all samples, indicating a high dispersion of Pt species on the catalyst surface. This result is consistent with the quite small average diameter (∼1.6 nm) of Pt NPs in Pt/Mo_0.03Ce_0.97O_y (Fig. S5†).

Based on the Ce L₃-edge XANES spectra of Mo_xCe_1−xO_y, the low Mo addition group (x ≤ 0.1) did not significantly alter the electronic state of CeO₂, whereas higher Mo additions (x = 0.5 and 0.9) changed the CeO₂ electronic state (Fig. 4a). Based on the observed shift in the absorption edge, the XANES spectra suggest the presence of Ce³⁺, being in line with the formation of Ce₂(MoO₄)₃ in Mo_0.5Ce_0.5O_y. This result also indicates that Ce species predominantly existed as Ce₂(MoO₄)₃ locally formed in Mo_0.9Ce_0.1O_y. Mo K-edge XANES spectra revealed that Mo was present as Mo⁶⁺ in all samples. The higher intensity of the pre-edge peak observed for Mo_0.5Ce_0.5O_y and Mo_0.9Ce_0.1O_y corresponds to the symmetric tetrahedral MoO₄²⁻ coordination environment⁷¹ (Fig. 4b).

Based on these structural characterization results and activity tests, the Pt/Mo_xCe_1−xO_y catalysts can be broadly categorized into two groups: those with lower Mo contents (x = 0.01–0.1) exhibiting higher activities and CeO_y-based structures, and those with higher Mo contents (x = 0.5–0.9) exhibiting lower activities and MoO_y-based crystal structures.

Specific surface areas (SSAs) of the reduced Pt/Mo_xCe_1−xO_y catalysts were measured using the BET method (Fig. 5a). The catalysts in the lower Mo content group retained relatively high SSAs (S_BET = 21.3–26.5 m² g⁻¹) comparable to that of the pristine Pt/CeO_y (S_BET = 24.0 m² g⁻¹), although a slight decrease was observed with increasing Mo concentration. However, significant decreases in the SSA occurred in the higher Mo content group. A comparison of the SSA and the CO formation rate in the dark RWGS reaction revealed that Pt/Mo_xCe_1−xO_y with a lower Mo content and with a higher SSA exhibited a higher CO formation rate (Fig. 5b). These results suggest that the high SSA originating from the CeO₂-based structure is one of the factors contributing to the superior catalytic activity of Pt/Mo_0.03Ce_0.97O_y.

Based on the experiments on partial pressure dependence (Fig. S6†), the reaction order (0.48) for the concentration of CO₂ over Pt/Mo_0.03Ce_0.97O_y was lower than that (0.54) over Pt/CeO_y (Table S3†), suggesting that Mo doping facilitates the reaction step involving CO₂. As previously reported, oxygen vacancies in metal oxides serve as favourable active sites for CO₂ activation.³⁶ To evaluate the amount of the oxygen vacancies in the catalyst after reduction at 473 K, we conducted XPS and XAFS measurements. Ce 3d XPS showed that the reducibility of Ce species remained unchanged upon Mo doping (Fig. 6a and Table 1). On the other hand, Mo 3d XPS analysis revealed that the average valence of Mo species in reduced Pt/Mo_0.03Ce_0.97O_y catalysts was 5.8, slightly lower than 6 (Fig. 6b and Table 2).

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Based on in situ Mo K-edge XAFS measurements (Fig. S7†) and linear combination fitting, the average valence of Mo species in the Pt/Mo_0.03Ce_0.97O_y catalyst was estimated (Fig. 7). The result reveals the redox capability of Mo species in Pt/Mo_0.03Ce_0.97O_y with valence changes from 4.8 in H₂ flow to 5.2 in CO₂ flow. Although the shift was smaller compared to the previously observed shift (0.6, from 4.1 to 4.7) for Pt/MoO_y,³⁶ and consistent with the XPS results (Table 2), these XPS and XAFS results still indicate that the Mo species doped in the CeO₂ lattice interact with CO₂ molecules.

Next, the quality of oxygen vacancies formed in Pt/Mo_0.03Ce_0.97O_y was evaluated. CO₂ and CO desorption profiles were estimated from the MS signals obtained during CO₂-TPD analysis (Fig. 8 and see Fig. S8† for the raw data of the MS signals and method). After CO₂ adsorption at 223 K, the CO₂ signal gradually decreased in all samples, likely due to incomplete removal of atmospheric CO₂ from the TPD reactor. Although no significant differences in CO₂ signals were observed among all samples until around 60 min, the CO signal intensity was significantly higher for Pt/MoO_y compared to the other samples, indicating that CO₂ dissociation to CO occurred on Pt/MoO_y even at 223 K. Upon raising the temperature, distinct CO₂ and CO desorption peaks emerged for Pt/Mo_0.03Ce_0.97O_y and Pt/CeO_y. Notably, both peak intensities for Pt/Mo_0.03Ce_0.97O_y were higher than those for Pt/CeO_y. These results suggest that doping Mo on CeO₂ enhances both the number of CO₂ adsorption sites and the ability to dissociate CO₂ to CO, which is consistent with previous reports.⁷²

Effects of light irradiation

To investigate the effects of light irradiation, the maximum surface temperature (T_surf) of each catalyst was measured using an IR thermometer. Upon irradiation, all catalysts showed increased T_surf due to their photothermal properties (Fig. 9a). Pt/MoO_y exhibited the highest temperature (T_surf = ca. 504.2 K) among the catalysts, likely due to its superior light absorption properties (Fig. 9b), as we previously reported.⁷³ In contrast, Pt/Mo_0.03Ce_0.97O_y exhibited a slightly lower temperature (T_surf = ca. 492.6 K) than Pt/CeO_y (T_surf = ca. 493.4 K), despite having slightly higher light absorption. This discrepancy will be discussed later.

Based on the enhanced CO formation rate (r_Light − r_Dark) by light irradiation (Fig. 10a), Pt/Mo_0.03Ce_0.97O_y demonstrated the most efficient utilization of the same amount of photon energy for the RWGS reaction. The Arrhenius plot indicated that Mo doping did not significantly affect the activation energy from Pt/CeO_y (Fig. 10b, see Fig. S10† for the CO formation rates). Fig. 10c shows the relationships between the natural logarithm of the CO formation rate (Fig. 2) and the inverse of the T_surf value. For Pt/MoO_y, the slope of the trend line in Fig. 10c was slightly lower (slope = −4.43) compared to that in Fig. 10b (slope = −4.93). This slight decrease can be attributed to a photo-induced temperature gradient in the catalyst, which caused lower surrounding temperatures than the T_surf values. Such temperature gradients have previously been shown to be ineffective for endothermic reactions (i.e. RWGS reaction).^36,74,75 In contrast, the significantly steeper slopes (slope = −10.03 and −8.89) observed for Pt/CeO_y and Pt/Mo_0.03Ce_0.97O_y in Fig. 10c compared to Fig. 10b (slope = −6.67 and −7.04) strongly suggest an additional, non-thermal contribution induced by light irradiation.

To evaluate the effects of Mo doping on the band gap energies of the catalysts, in situ UV-vis measurements of CeO_y and Mo_0.03Ce_0.97O_y were conducted during the RWGS reaction at 473 K (Fig. 11a). Tauc plots for indirect transitions revealed that the band gap energies for CeO_y and Mo_0.03Ce_0.97O_y were approximately 2.73 eV (∼454 nm) and 2.38 eV (∼521 nm), respectively (Fig. 11b). The observed narrowing of the band gap due to Mo doping is consistent with previous reports on metal-doped CeO₂.⁷⁶ After Pt loading, both catalysts exhibited enhanced absorption in the visible region, which can be attributed to the absorption of Pt NPs and the formation of oxygen vacancies (Fig. 11c). The corresponding Tauc plots showed further narrowing of the band gaps to 2.28 eV (∼540 nm) for Pt/CeO_y and 1.85 eV (∼670 nm) for Pt/Mo_0.03Ce_0.97O_y, suggesting the formation of defect-induced sub-bandgap states (Fig. 11d).^77,78 These reduced band gap energies indicate that band gap excitation is possible under the visible-NIR light irradiation (λ > 450 nm) employed in this study.

Furthermore, photocurrent measurements confirmed that visible light in the wavelength ranges of 450–500 nm and 490–590 nm effectively generated photocurrent, whereas light in the 590–810 nm range did not induce any noticeable photocurrent response (Fig. 12; for the measurement conditions, see Table S1†).

To investigate the wavelength-dependent behavior of catalytic activity, the CO formation rates were evaluated under the narrow-band light irradiation. To minimize the influence of photothermal effects, the light intensity was adjusted so that the T_surf value was maintained at 482 K (+9 K compared with the dark conditions) under both irradiation conditions (450–500 nm and 590–810 nm, see Table S4† for the details of the experiment). Under both conditions, photo-induced enhancement of the CO formation rate was observed (Fig. 13a). However, the enhancement was significantly greater under 450–500 nm irradiation compared to 590–810 nm.

Arrhenius-like plots (natural logarithm of the CO formation rate vs. 1000/T_surf) revealed the slopes of the trend line of −7.94 for 450–500 nm irradiation and −4.37 for 590–810 nm irradiation (Fig. 13b and c). The shallower slope observed under 590–810 nm irradiation, compared to the dark conditions (−6.67, Fig. 10b), might be attributed to a lower surrounding temperature of the catalyst, as noted in the discussion on Pt/MoO_y. In contrast, the steeper slope observed under 450–500 nm irradiation indicates a significant photocatalytic contribution. The inconsistency between the slopes under 450–500 nm irradiation (−7.94 in Fig. 13b) and the standard light (−10.03 in Fig. 10c) conditions might be attributed to a lower effective photon flux under 450–500 nm irradiation (for a brief estimation of photon flux values, see Table S5†).

Based on the results of the photocatalytic activity investigation, the absorbed photon energy in Pt/Mo_0.03Ce_0.97O_y contributed to electronic excitation rather than photothermal conversion. The lower T_surf value of Pt/Mo_0.03Ce_0.97O_y, despite its higher optical absorbance compared to Pt/CeO_y (Fig. 9), is likely attributed to its broader effective wavelength range for the photocatalytic mechanism. Consequently, Pt/Mo_0.03Ce_0.97O_y exhibited enhanced catalytic activity in the photo-assisted RWGS reaction, benefiting from facilitated electronic transitions induced by Mo doping.

Conclusions

In this study, we investigated the catalytic performance of Pt/metal-doped CeO_y (Pt/M_xCe_1−xO_y) catalysts for the reverse water–gas shift (RWGS) reaction at 473 K under dark and visible-NIR irradiation conditions. Moderate Mo doping (x = 0.01–0.1) significantly enhanced catalytic activity by maintaining a cubic CeO₂ structure with high surface areas and by facilitating CO₂ activation through oxygen vacancies. Pt/Mo_0.03Ce_0.97O_y exhibited the highest activity under visible-NIR irradiation, benefiting from efficient photon utilization both via photothermal conversion and electronic excitation due to its narrower band gap (1.85 eV) compared to Pt/CeO_y (2.28 eV). These findings demonstrate the potential of Pt/Mo-doped CeO_y catalysts for efficient photo-assisted RWGS reactions.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and/or its ESI.†

Author contributions

D. T. conducted the formal analyses and wrote the original draft with comments from all authors. N. D. and R. S. prepared the samples, performed the activity tests, and conducted the characterization studies and formal analysis. Y. K. conceived and directed the project, reviewed and edited the manuscript, and acquired funding. Y. K. and H. Y. co-supervised the project.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) (Project no. JPNP14004), JSPS KAKENHI (Grant no. JP24K01256), and Tokuyama Science Foundation. The synchrotron radiation experiments for XAFS measurements were performed at the BL01B1 beamline in SPring-8 with approval from JASRI (Proposal No. 2022A1171, 2023A1676, 2024A1769, 2024A1737). A part of the present experiments was carried out using a facility in the Research Center for Ultra-High Voltage Electron Microscopy, The University of Osaka.

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Footnote

† Electronic supplementary information (ESI) available: Reagent list, XRD data of Mo-modified CeO2, long-term activity data, reaction order for CO2 and H2, Mo-K XANES spectra, MS signal of CO2-TPD, and temperature dependence of CO formation. See DOI: https://doi.org/10.1039/d5cy00597c

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