Photo-thermal coupling to enhance CO2 hydrogenation toward CH4 over Ru/MnO/Mn3O4

Upcycling of CO2 into fuels by virtually unlimited solar energy provides an ultimate solution for addressing the substantial challenges of energy crisis and climate change. In this work, we report an efficient nanostructured Ru/MnOx catalyst composed of well-defined Ru/MnO/Mn3O4 for photo-thermal catalytic CO2 hydrogenation to CH4, which is the result of a combination of external heating and irradiation. Remarkably, under relatively mild conditions of 200 °C, a considerable CH4 production rate of 166.7 mmol g−1 h−1 was achieved with a superior selectivity of 99.5% at CO2 conversion of 66.8%. The correlative spectroscopic and theoretical investigations suggest that the yield of CH4 is enhanced by coordinating photon energy with thermal energy to reduce the activation energy of reaction and promote formation of key intermediate COOH* species over the catalyst. This work opens up a new strategy for CO2 hydrogenation toward CH4.

The current arficle approaches photothermal methanafion of CO<sub>2</sub> over Ru/MnOx catalysts.The results obtained in this work shows high methane producfion rate (166 mmol g<sup>-1</sup> h<sup>-1</sup>), with a remarkably selecfivity (>99 %).In addifion, this arficle provides extensive characterizafion of the catalysts as well as of the metal-free MnOx.In general terms, this arficle is clearly wriften, and easy to read, although some parts may require some rewrifing for sake of clarity.Nevertheless, in my opinion, there are important aspects of this work that cast doubts about the soundness of the experimental results, whereas some interpretafion of the characterizafion appears to be flawed.In parficularly, the following points require further jusfificafion and elaborafion: -In the first secfion regarding characterizafion of the catalysts, it is stated that Ru does not modify the XRD paftern.However, Fig S2 shows clearly that following Ru addifion the low angle reflecfion at about 12º as well as a smaller one around 25 º almost disappear.Then, during Ru incorporafion some structural changes occur.Furthermore, a clear idenfificafion of phases should be given, and ideally the XRD paftern should be analyzed by Rietveld method to ensure that all contribufions are accounted.
-Although the methanafion acfivity reported here is notable, befter results have been reported recently (see 10.1038/s41929-023-00970-z) using Au/Ce<i>0.95</sub>Ru<sub>0.05</sub>O<sub>2</sub> as catalyst and working in confinuous-flow reactor.In contrast, in the present case a batch reactor with very low rafio catalyst mass/volume (15 mg/180 mL) is used.This configurafion is far from ideal because gas diffusion and water vapor accumulafion is likely to have a significantly influence on the measured acfivity and, therefore, isolafing the real contribufion of the catalyst can be difficult.In addifion, other experimental details of the catalyfic tests require further clarificafion: o Experimental secfion does not clarify if heafing is achieving exclusively by irradiafion with the Xe lamp or if it requires an addifional heafing system.This is key aspect for understanding the acfivity tests, parficularly the blank experiments of o As the stoichiometry of the Sabafier reacfion requires a CO<sub>2</sub>/H<sub>2</sub> rafio of 1:2 it is surprising the authors decided to test lower concentrafion of H<sub>2</sub> that can favor reverse water gas shift reacfion.What is the rafionale for tesfing those condifions?How is the selecfivity to methane affected?o Catalyfic tests are performed under pressure.This surely promotes methane formafion, but the authors should briefly jusfify the selecfion of these condifions with regards to other works in the literature.
-Since under operafion condifions the real composifion of the catalyst is Ru/MnO/Mn<sub>3</sub>O<sub>4</sub> it would be rather informafive to test catalysts with composifion Ru/MnO and Ru/Mn<sub>3</sub>O<sub>4</sub>, as to ascertain the possible role of these Mn oxides phases.
-The assignment of the different components obtained by deconvolufion of XPS, as it displayed in Fig S7 and S9, should be given in these graphs for quick reference.
-It is not clear what are exactly "semi-in situ" condifions for XPS and FTIR analyses.In any case, for these last experiments as presented in Fig 3H and 3I, they appear to be dominated by gas phase contribufion, providing liftle informafion about surface species.In fact, the band at 1305 cm<sup>-1</sup> is very likely due to gas phase methane.Furthermore, I think that fin order to show clear indicafions of the presence of formate, background should be subtracted, and the relevant spectral range should be zoomed.
-DFT analysis show that methane formafion is favored on Ru/Mn<sub>3</sub>O<sub>4</sub> with some oxygen vacancies, but these calculafions should be compared with the case of MnO, as this phase is also presented in the working catalyst.In addifion, If I understand correctly these calculafions consider only thermal processes, and therefore the role of photonic acfivafion is not clearly considered in these calculafions.Due to these reasons, I cannot recommend this paper to be published in Nature Communicafion.Nevertheless, I encourage the author to revise this contribufion considering the above-menfioned issues and resubmit an enhanced version to a more specialized journal.
Reviewer #2 (Remarks to the Author): In this manuscript, the authors synthesized a new photothermal catalyst Ru/MnO/Mn3O4 to convert CO2 + H2 into CH4, with a superior selecfivity 99.5% and a CO2 conversion rate of 66.8%.The author characterized the structure of catalysts by XRD, XPS, FTIR, and Raman.The reacfion results were analyzed under the influence of different factors such as Ru content, CO2:H2 rafio, temperature, light intensity, and irradiafion fime.They proposed some reacfion pathways by DFT calculafions, together with the experimental evidence from FTIR.Interesfingly, they observed the Ru-mediated H-spillover effect and found the formafion of COOH* is easier on Ru/Mo3O4-x, by both experiment and calculafion.This work is surely of interests to the community of photothermal catalyfic conversion of CO2.I would suggest the manuscript to be accepted to the journal of Nature Communicafions, under the condifion that the authors completed the following minor revisions.
1. Could the authors provide the specific morphology and size of Ru parficles on the support?For example, are they nanoparficles, atomic cluster, or single atoms? 2. Please clarify in the manuscript what is your definifion of photo-thermal catalysis.By "photo-thermal" catalysts, do you mean tradifional photocatalysis under the condifion of external heafing?If so, this might be an "extended" definifion of photothermal catalyst, which needs to be further clarified and proved in the manuscript.For photothermal catalysts defined in the literature [see Chem Catalysis, 1, 52-83, 2022; Chem Catalysis, 1, 272-297, 2021], phothothermal effect (e.g., photon energy is converted to heat) is the feature of photothermal catalysts.If the Ru/MnO/Mn3O4 catalyst has photothermal effect, which component (e.g., MnOx, Ru, or both) makes the major contribufion to the photothermal effect?To answer this quesfion, the authors should show the maximum temperatures can be reached by the irradiafion of Ru/MnOx and MnOx, respecfively, with increasing irradiafion fime.2C, how do you control the temperature of photothermal catalyst?In principle, reaching a steady (or maximum) temperature of photothermal catalyst depends on the nature of photothermal effect of materials.If external heafing is used to control the photothermal temperature, how may that influence the photothermal effect?In another word, will the photothermal effect of catalysts be influenced by the condifion of external heafing? 4. On Line 111, which vibrafional peak is the blue shift of 7 cm-1 relafive to? 5.For the proposed reacfion pathways in Figure 4, please comment on which step(s) can be the rate determining step (RDS) for the overall reacfion?Energefically, the formafion of HCO* seems to the RDS, the diagram is lack of transifion states.Is the formafion of COOH* the RDS?If not, why do you think that the difference in COOH* formafion between Ru/Mo3O4 and Ru/Mo3O4-x can cause significant difference in their overall reacfion kinefics?If possible, the authors should provide the transifion search results.

In Figure
6. Please clarify if the Ru-mediated H-spillover effect in thermal catalysis remains the same under the photothermal condifion.Can the light irradiafion influence the H-spillover effect, why or why not?
Reviewer #3 (Remarks to the Author): In this work, Zha and collaborators present a new catalyst based on Ru sites supported on MnOx for the low-temperature photo-thermal methanafion of CO2.The as-prepared catalyst displayed a remarkable catalyfic acfivity and CH4 selecfivity under reacfion condifions owing to the synergy between thermal and non-thermal contribufions of light.Mechanisfic studies indicate a decrease in the apparent acfivafion energy and an enhancement in the formafion of COOH* intermediates under illuminafion, thus favoring the methanafion reacfion.
The producfion of solar fuels and chemicals using CO2 as feedstock has raised as an interesfing alternafive to both tackle carbon dioxide emissions and energy crisis.In this context, photo-thermal catalysis overcomes the limitafions of tradifional photo-catalysis by synergisfically combining thermal and non-thermal contribufions of sunlight, thus becoming a very dynamic and promising field of research.The results presented by Zha and collaborators seem reassuring, however, given the vast amount of works on photo-thermal catalysis for CO2 methanafion using Ru-based catalyst, I cannot perceive any significant advance in the field.In addifion to this, the role of light and heat in the overall reacfion mechanism has not been completely discussed and this can lead to misinterpretafions in the overall reacfion pathway.Furthermore, authors did not provide any stability test to evaluate the longterm acfivity of the catalyst under reacfion condifions.
For these reasons, I cannot recommend the publicafion of this work in Nature Communicafions in the present form.Detailed comments to support this decision and suggesfions to improve the quality of this work can be found below: 1) Authors should provide an analysis on the parficle size distribufion of Ru on the surface of MnOx.From the available images it is impossible to have an idea of the size of the Ru parficles.
2) Authors report remarkable methane producfions in the order of hundreds of mmol g-1 h-1.Are these catalyfic rates normalized by the amount of catalyst (15 mg) or the total amount of Ru present in the sample?
3) When it comes to the photo-thermal experiments, could the authors explain in detail the posifion of the thermocouple in the setup?Is it in contact with the catalyst bed or inserted in the reactor wall?Imprecise temperature measurements can lead to misinterpretafions in the contribufions of photon and thermal energy to the overall catalyfic performance, for instance, in the calculafion of apparent acfivafion energy.4) In Fig. 2D, authors studied the effect of the light intensity on the catalyfic acfivity.Was the temperature constant at 200 °C throughout all the intensifies?Do the authors aftribute the improvement in the performance only to pure non-thermal effects?It is hard to imagine a scenario in which the temperature of the catalyst does not increase upon increasing light intensity, specially taking into account its broad light absorpfion across the visible and infrared.5) In Fig 2F, authors represented the CH4 producfion as a funcfion of the irradiafion fime.Why did the authors stop the experiment after 4 hours?Longer reacfion fimes would show if higher conversions are achievable.6) Results show a very high methane selecfivity in most of the experiments assuming a total reacfion fime of 4 hours.What happens at shorter reacfion fimes?Is sfill CH4 the main product?7) Authors did not provide any stability test of the catalyst, so it is not possible to assess if the material is stable upon consecufive reuses.This type of study is vital to evaluate the pracfical applicafion of the catalyst, so I encourage authors to perform a series of (at least) five consecufive runs to study the catalyst recyclability.8) Both steady-state and fime-resolved PL suggest a charge transfer from MnOx to Ru sites under irradiafion.Is this electronic transfer thermodynamically favored?Could the authors provide a band diagram showing the corresponding potenfials of MnOx and metallic Ru? Furthermore, authors did not clarify the specific role of these electrons in the overall reacfion pathway.9) In Table S2 in SI, please include the amount of Ru in all the samples.For a fair comparison and to avoid misleading conclusions, results should clearly indicate that the methane producfion rate has been normalized per total mass of catalyst or total mass of Ru.TOF calculafions should be also included in the table.

Response Letter
Manuscript ID: NCOMMS-23-30118 Title: "Photo-thermal coupling to enhance CO2 hydrogenation toward CH4 over Ru/MnO/Mn3O4" We are very grateful to the referees for the critical comments and the constructive suggestions, which helped us to improve the quality of the manuscript.We have carefully responded to all the questions point-by-point, and have revised the manuscript thoroughly.The changes have been highlighted by yellow background in the revised manuscript.

Reviewer #1 (Remarks to the Author):
The current article approaches photothermal methanation of CO2 over Ru/MnOx catalysts.The results obtained in this work shows high methane production rate (166 mmol g -1 h -1 ), with a remarkably selectivity (>99 %).In addition, this article provides extensive characterization of the catalysts as well as of the metal-free MnOx.In general terms, this article is clearly written, and easy to read, although some parts may require some rewriting for sake of clarity.Nevertheless, in my opinion, there are important aspects of this work that cast doubts about the soundness of the experimental results, whereas some interpretation of the characterization appears to be flawed.In particularly, the following points require further justification and elaboration: Response: We thank the referee for the encouraging comments, and we have addressed the critical questions and concerns by the referee thoroughly.
Comment 1.In the first section regarding characterization of the catalysts, it is stated that Ru does not modify the XRD pattern.However, Fig S2 shows clearly that following Ru addition the low angle reflection at about 12º as well as a smaller one around 25 º almost disappear.Then, during Ru incorporation some structural changes occur.Furthermore, a clear identification of phases should be given, and ideally the XRD pattern should be analyzed by Rietveld method to ensure that all contributions are accounted.

Response 1:
We thank the referee very much for bringing this question to our attention.Based on the referee's comment, we have carefully reevaluated our data, and it is found that the decrease in diffraction peaks corresponds to the MnO2 phase.Furthermore, we have also performed simulations on the XRD pattern by using Rietveld method, which support the notion that the decrease in diffraction peaks is attributed to a reduction in the MnO2 phase content.These changes may occur during the mild reduction of the MnO2 in the photo-deposition process.Regarding the identification of phases and the analysis of the XRD pattern, we acknowledge the importance of providing a clear identification of phases.In the revised manuscript, the discussion has been updated as following: "The Rietveld refinement of X-ray powder diffraction (XRD) results in Supplementary Fig. 3 and Supplementary Table   Response 2: We thank the referee again for the critical comment.
1) Firstly, we appreciate the referees' attention to recent advancements in the field, specifically the work published in "10.1038/s41929-023-00970-z," which demonstrates improved results using Au/Ce0.95Ru0.05O2catalyst in a continuous-flow reactor. 1 It is an insightful research and has been included in the revised manuscript as Ref. 12.
2) Secondly, we highly agree with the referee that the limitations of batch reactor setup and the factors of gas diffusion and water vapor accumulation have potential influence on the observed activity.However, despite these limitations, it is believed that this study still provides valuable insights into the field of photothermal methanation of CO2 research.The highlight of this study was to investigate an efficient catalyst, which allowed us to explore certain aspects of the reaction mechanism and open up a new strategy for photo-thermal CO2 hydrogenation toward CH4.Meanwhile, as reported by He and co-workers (Green Chem., 2021, 23, 5775), Liu and co-workers (Adv.Energy Mater., 2022, 12, 2201009) and Zhong and co-workers (Nature catalysis., 2023, 6, 519-530) etc, the explored catalysts that demonstrated good results in the batch reactor setup can also show good performance in continuous-flow setup 1,2,3 .According to the comment, the photothermal catalytic performance of the Ru/MnOx catalyst was also assessed in a fixed-bed reactor, and achieved excellent results.It further shows that our strategy is viable.As illustrated in Supplementary Fig. 13-14, under the conditions of 200 °C and 2.5 W cm -2 irradiation, the catalytic activity of Ru/MnOx remained stable after 20 hours at a high gas hourly space velocity (GHSV) of 40000 mL g -1 h -1 .A CO2 conversion of 29.5% was achieved with an excellent selectivity of 99.5% and a high space time yield (STY) of 95.8 mmolCH4 g -1 h -1 .According to the reviewer's suggestion, in the revised manuscript, we have elaborated the description as following, including the experimental setup: "Furthermore, the photothermal catalytic performance of the Ru/MnOx catalyst was also assessed in a fixed-bed reactor.As illustrated in Supplementary Fig. 13-14, under the conditions of 200 °C and 2.5 W cm -2 irradiation, the catalytic activity of Ru/MnOx remained stable after 20 hours at a high gas hourly space velocity (GHSV) of 40000 mL g -1 h -1 .A CO2 conversion of 29.5% was achieved with an excellent selectivity of 99.5% and a high space time yield (STY) of 95.8 mmolCH4 g -1 h -1 ."and "The photothermal CO2 conversion are also performed in the fixed-bed reactor (CEL-GPPCM, Beijing China Education Au-Light Co., Ltd.) at 200 °C.150 mg of catalyst and CO2/H2 mixed flow (20 mL min -1 /80 mL min -1 ) were used.A 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) was used as the light source for the reaction (light intensity: 2.5 W cm -2 ).The products in the effluent gas were periodically analyzed by using a gas chromatograph (GC-7920, Beijing China Education Au-Light Co., Ltd.).STY of CH4 (molCH4 g -1 h -1 ), was calculated according to the following equation ×   where FCO2, in is the volumetric flow rate of CO2, XCO2 is the CO2 conversion, SCH4 is the CH4 selectivity, Wcat is the overall mass of catalyst (g), and Vm is the ideal molar volume of CO2 at standard temperature and pressure.

" (Please see Page 7and Page 15 in the revised manuscript).
Supplementary Fig. 13 The images of (a) the photo-thermal catalytic performance evaluation process carried out in the flow reaction system and (b) the fixed-bed quartz tube reactor.Supplementary Fig. 14 The photothermal catalytic performance of Ru/MnOx catalyst in a fixed-bed reactor.Reaction conditions: 150 mg of catalyst, full-arc 300 W UV-xenon lamp, 2.5 W cm -2 , 200 °C, initial pressure 0.1 MPa, CO2/H2 mixed flow (20 mL min -1 /80 mL min -1 ).
3) Thirdly, we would like to clarify that the temperature of 200 °C was achieved by the combined effect of external heating and irradiation from the Xe lamp.As illustrated in Supplementary Fig. 9, in order to ensure accurate temperature measurement and to maintain uniform temperature throughout the entire reaction system, the thermocouple was positioned at a distance of 1 cm above the catalyst, in the middle of the reactor.This was done to avoid any contact between the thermocouple and the bottom of the reactor, which could result in inaccurate temperature readings.By doing so, the temperature measurement can accurately reflect the temperature of the entire catalytic reaction system.To provide a clear understanding of the experimental conditions and avoid any misleading information, in the revised manuscript, we have elaborated the description as following: "The catalytic performance of Ru/MnOx was evaluated at 200 °C in the batch reactor setup by feeding CO2/H2 mixed gas (the desired temperature was achieved by a combination of external heating and irradiation from the Xe lamp) and CH4 was identified as the dominant products, with no liquid products produced (Supplementary Fig. 9)" and "Then, the external heating and the 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) with an intensity of 2.5 W cm - 4) Fourthly, with respect to the CO2/H2 stoichiometry, we agree that the Sabatier reaction typically requires a ratio greater than 1:2.However, we intentionally selected a lower H2 concentration (CO2/H2 ratio is 1/1) when testing the catalytic activity of MnOx with varying Ru contents, with the aim of not only to investigate the optimal Ru loading, but also to explore Ru's impact on the selectivity towards CH4.As shown in Fig. 2a, the results demonstrate that even at lower Ru loadings, the catalysts consistently exhibit exceptional CH4 selectivity, emphasizing Ru's outstanding methanation capabilities.Furthermore, as shown in Fig. 2b, at a Ru content of 7.3 wt%, the Ru/MnOx catalyst still exhibits 94.7% CH4 selectivity even at a lower H2 concentration (CO2/H2 ratio is 4/1), further highlighting the superior methanation performance of the Ru/MnOx catalyst.5) Fifthly, on the basis of the referee's comment, we have investigated the catalytic performance at different total pressure on CH4 evolution.It was validated that 1 MPa was the optimal total pressure and the catalyst displayed a decent CH4 activity of 166.7 mmol g -1 h -1 .By further increasing the total pressure, the CH4 activity increased, but slowly.For better reading, the following statement has been added in the revised manuscript: "Furthermore, as shown in Supplementary Fig. 11, we studied the influence of the total pressure on the reaction at high H2/CO2 ratio (4/1).The activity was enhanced markedly with the elevating total pressure, but became slowly when the pressure exceeded 1 MPa." (Please see Page 6 in the revised manuscript) Supplementary Fig. 11 Influence of total pressure on CH4 evolution rate over Ru/MnOx; Reaction conditions: 15 mg of catalyst, full-arc 300 W UV-xenon lamp, 2.5 W cm -2 , 200 °C, irradiation time 4 hours, H2/CO2 =4/1.Comment 3. Since under operation conditions the real composition of the catalyst is Ru/MnO/Mn3O4 it would be rather informative to test catalysts with composition Ru/MnO and Ru/Mn3O4, as to ascertain the possible role of these Mn oxides phases.

Response 3:
We thank the referee very much for bringing this important question to our attention.On the basis of the referee's comment, we have investigated the catalytic properties of different manganese oxide supports for CH4 evolution.As shown in Supplementary Fig. 22, all the tested manganese oxide supports exhibit certain catalytic activity.Notably, the commercially available Mn3O4 support outperforms other commercially available supports like MnO2, Mn2O3 and MnO, indicating the significance of the Mn3O4 phase in the reaction.Furthermore, due to the advantages of multiple valences (Mn 2+ /Mn 3+ /Mn 4+ ) and reducible effect, the MnOx support exhibits the highest catalytic activity, further indicating that Ru-mediated H-spillover effect on the MnOx can efficiently transfer dissociated H to the support, thereby promoting the hydrogenation reaction. 4,5 ccording to the reviewer's suggestion, in the revised manuscript, we have devoted one paragraph to elaborate the data: "Furthermore, as shown in Supplementary Fig. 22, the catalytic activity of MnOx supports surpasses that of the other specific manganese oxide alone.It indicates that the H-spillover effect in Ru/MnOx can effectively transfer dissociated H to the support due to the multivalent states (Mn 2+ /Mn 3+ /Mn 4+ ) with varied reducibility, thereby promoting the hydrogenation reaction."(Please see Page 10 in the revised manuscript).Supplementary Fig. 22 Influence of various manganese oxide on CH4 evolution rate.Reaction conditions: 15 mg of catalyst, full-arc 300 W UV-xenon lamp, 2.5 W cm -2 , 200 °C, irradiation time 4 hours, initial pressure 1 MPa (H2/CO2 =1/1).
Comment 4. The assignment of the different components obtained by deconvolution of XPS, as it displayed in Fig S7 and S9, should be given in these graphs for quick reference.Response 4: We thank the referee again.Based on the referee's suggestion, the assignment of the different components obtained by deconvolution of XPS was conducted and the results are shown in Supplementary Fig. 8, Supplementary Fig. 15 and Supplementary Fig. 25 for quick reference as suggested by the referee.In any case, for these last experiments as presented in Fig 3H and 3I, they appear to be dominated by gas phase contribution, providing little information about surface species.In fact, the band at 1305 cm -1 is very likely due to gas phase methane.Furthermore, I think that fin order to show clear indications of the presence of formate, background should be subtracted, and the relevant spectral range should be zoomed.Response 5: We thank the referee for the comment.
1) In this study, the term "semi in-situ" refers to the experimental method we employed to characterize the catalyst under certain external conditions.Due to the limitations of the instrumental, it was beyond our capability to conduct completely in-situ characterization of photothermal catalysis by XPS or FT-IR.Instead, we implemented a semi in-situ approach where the reactor was simultaneously illuminated and externally heated for a specified time, followed by rapid collection of the relevant XPS and FT-IR data after the removal of the light illumination.The viability of such a testing method has been validated by other researchers (see references 6-7). 6,7  We highly agree with the referee on that "the band at 1305 cm -1 is very likely due to gas phase methane".The observed band at 1305 cm -1 can indeed be attributed to the ν(C-H) vibration of CH4.Meanwhile, as shown in Supplementary Fig. 26-27, due to the high catalytic activity of Ru/MnOx, the high intensity of the characteristic peaks of CH4 makes it difficult to capture the peaks of the intermediates, thus limiting the information available about surface species.In order to discuss the important peak more clearly and provide a clear indication of COOH*, the enlarged spectral range has been implemented in the Fig. 3i as suggested by the review.Meanwhile, the description has been updated in the revised manuscript as following: "For thermocatalysis, the typical peaks of monodentate carbonates (m-CO3 2-, 1509 cm -1 ) and ν(C-H) vibration of CH4 (1305 cm -1 ) were apparently strengthened by increasing the reaction temperature" (Please see Page 11 in the revised manuscript) Fig. 3i Spectra of semi in-situ FT-IR study of Ru/MnOx at different conditions.Comment 6. DFT analysis show that methane formation is favored on Ru/Mn3O4 with some oxygen vacancies, but these calculations should be compared with the case of MnO, as this phase is also presented in the working catalyst.In addition, If I understand correctly these calculations consider only thermal processes, and therefore the role of photonic activation is not clearly considered in these calculations.
Response 6: We appreciate the referee for the constructive comment.1) Firstly, as the suggested by the referee, we have conducted DFT calculations on the models of Ru/MnO (200) slabs.Through comparison of the Gibbs free energy (ΔG) in rate determining step of the Ru/MnO slabs, Ru/Mn3O4 slabs and Ru/Mn3O4-x slabs, it is found that the Ru/Mn3O4-x have a more negative ΔG, which is conducive to CH4 formation.Hence, in the revised manuscript, we have elaborated the description as following: "As shown in Fig. 4 and Supplementary Fig. 29- 2) Secondly, in order to verify the role of photonic activation, we conducted DFT calculations about the densities of states (DOSs) on the models of Ru/Mn3O4-x (321) slabs that simulated dark state or light state.As shown in Supplementary Fig. 32, it can be seen that the conduction band that simulated light state moves to the low energy region compared with that of dark, indicating the electron density increases in light state, which is conducive to the electron transfer. 8,9,10 I the revised manuscript, the description has been updated as following: "Moreover, as shown in Supplementary Fig. 32, compared with Ru/Mn3O4-x (321) slabs that simulated dark state, the conduction band that simulated light state moved to the low energy region, indicating that the involved photons were conducive to electron transfer, which is favorable to CO2 hydrogenation toward CH4." (Please see Page 13 in the revised manuscript) Supplementary Fig. 32 The calculated densities of states (a) and projected densities of states (b) for Ru/Mn3O4-x under dark and light conditions.Fermi levels are at 0 eV.

Reviewer #2 (Remarks to the Author):
In this manuscript, the authors synthesized a new photothermal catalyst Ru/MnO/Mn3O4 to convert CO2 + H2 into CH4, with a superior selectivity 99.5% and a CO2 conversion rate of 66.8%.The author characterized the structure of catalysts by XRD, XPS, FTIR, and Raman.The reaction results were analyzed under the influence of different factors such as Ru content, CO2:H2 ratio, temperature, light intensity, and irradiation time.They proposed some reaction pathways by DFT calculations, together with the experimental evidence from FTIR.Interestingly, they observed the Ru-mediated H-spillover effect and found the formation of COOH* is easier on Ru/Mn3O4-x by both experiment and calculation.This work is surely of interests to the community of photothermal catalytic conversion of CO2.I would suggest the manuscript to be accepted to the journal of Nature Communications, under the condition that the authors completed the following minor revisions.Response: We thank the referee very much for the encouraging comment and constructive suggestions.We have addressed the critical questions and concerns by the referee.
Comment 1: Could the authors provide the specific morphology and size of Ru particles on the support?For example, are they nanoparticles, atomic cluster, or single atoms?
Response 1: We thank the referee very much for the comment.Based on the referee's comment, the morphology and size of the Ru particles on the support were characterized using high angle annular dark-field scanning transmission electron microscope (HAADF-STEM).As shown in Supplementary Fig. 2, the obtained results clearly indicate that the Ru species were nanoclusters with an average size of 1.07 ± 0.26 nm.To address the referee's concern, in the revised manuscript, the discussion about the size of Ru particles has been supplemented: "The morphology of MnOx did not change considerably after the addition of Ru species and the average size of the deposited Ru nanoclusters is about 1.07 ± 0.26 nm" and "The high angle annular dark-field scanning transmission electron microscope (HAADF-STEM) was operated by EM-ARM300F".

(Please see Page 3 and 16 in the revised manuscript).
Supplementary Fig. 2 The high angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the Ru/MnOx catalyst.
Comment 2: Please clarify in the manuscript what is your definition of photo-thermal catalysis.By "photo-thermal" catalysts, do you mean traditional photocatalysis under the condition of external heating?If so, this might be an "extended" definition of photothermal catalyst, which needs to be further clarified and proved in the manuscript.For photothermal catalysts defined in the literature [see Chem Catalysis, 1, 52-83, 2022; Chem Catalysis, 1, 272-297, 2021], phothothermal effect (e.g., photon energy is converted to heat) is the feature of photothermal catalysts.If the Ru/MnO/Mn3O4 catalyst has photothermal effect, which component (e.g., MnOx, Ru, or both) makes the major contribution to the photothermal effect?To answer this question, the authors should show the maximum temperatures can be reached by the irradiation of Ru/MnOx and MnOx, respectively, with increasing irradiation time.

Response 2:
We thank the referee very much for the comments again.
1) Firstly, we are very pleased to clarify that photo-thermal catalysis in the manuscript refers to photothermal co-catalysis, which was achieved by the combined effect of external heating and irradiation from the Xe lamp.As suggested by the referee, to provide a clear understanding, in the revised manuscript, we have elaborated the description as following: "In this work, we report an efficient nanostructured Ru/MnOx catalyst composed of well-defined Ru/MnO/Mn3O4 for photo-thermal catalytic CO2 hydrogenation to CH4, which is the result of a combination of external heating and irradiation."and "A prominent CO2 conversion of 66.8% was achieved with a superior selectivity of 99.5% and a CH4 production rate of 166.7 mmol g -1 h -1 at relatively mild temperature of 200 ℃ (normalized by the amount of catalyst (~ 15 mg)), which is the result of a combination of external heating and irradiation."and "The catalytic performance of Ru/MnOx was evaluated at 200 °C in the batch reactor setup by feeding CO2/H2 mixed gas (the desired temperature was achieved by a combination of external heating and irradiation from the Xe lamp) and CH4 was identified as the dominant products, with no liquid products produced" and "Then, the external heating and the 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) with an intensity of 2.5 W cm -2 were both contributed to maintain the reactor temperature at 2) Secondly, in order to investigate the photothermal effect of the Ru/MnOx catalyst, we conducted a series of experiments to compare the maximum temperatures reached by irradiating Ru/MnOx and MnOx separately, with increasing irradiation time.As shown in Supplementary Fig. 16, under 2.5 W cm -2 illumination, the measured average temperature of Ru/MnOx reached 137.9 °C, higher than that of MnOx (115.4 °C), indicating that both Ru and MnOx contributed to the photothermal effect.In the revised manuscript, the description has been updated as following: "Meanwhile, due to the broadening of the wavelength range of light absorption, a strong photothermal effect was expected. 11,12 s shown in Supplementary Fig. 16, under 2.5 W cm -2 illumination, the measured average temperature of Ru/MnOx reached 137.9 °C, higher than that of MnOx (115.4 °C), indicating that both Ru and MnOx contributed to the photothermal effect."and "The temperature of samples was recorded by an infrared thermal imaging camera (Fotrfic 315, Shanghai Thermal Imaging Technology Co., Ltd.Comment 3. In Figure 2C, how do you control the temperature of photothermal catalyst?In principle, reaching a steady (or maximum) temperature of photothermal catalyst depends on the nature of photothermal effect of materials.If external heating is used to control the photothermal temperature, how may that influence the photothermal effect?In another word, will the photothermal effect of catalysts be influenced by the condition of external heating?Response 3: We thank the referee very much for bringing this important question to our attention.
1) Firstly, it is clarified that the temperature of 200 °C was achieved by the combined effect of external heating and irradiation from the Xe lamp.As illustrated in Supplementary Fig. 9, in order to ensure accurate temperature measurement and to maintain uniform temperature throughout the entire reaction system, the thermocouple was positioned at a distance of 1 cm above the catalyst, in the middle of the reactor.This was done to avoid any contact between the thermocouple and the bottom of the reactor, which could result in inaccurate temperature readings.By doing so, the temperature measurement accurately reflects the temperature of the entire catalytic reaction system.To provide a clear understanding of the experimental conditions and avoid any misleading information, in the revised manuscript, we have elaborated the description as following: "The catalytic performance of Ru/MnOx was evaluated at 200 °C in the batch reactor setup by feeding CO2/H2 mixed gas (the desired temperature was achieved by a combination of external heating and irradiation from the Xe lamp) and CH4 was identified as the dominant product, with no liquid products produced (Supplementary Fig. 9)" and "Then, the external heating and the 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) with an intensity of 2.5 W cm -2 were both contributed to maintain the reactor temperature at 200 °C.

" (Please see Page 5 and Page 15 in the revised manuscript).
Supplementary Fig. 9 (a 2) Secondly, regarding the influence of external heating on the photothermal effect of the catalyst, we acknowledge that it is an important consideration.The photothermal effect of materials depends on their intrinsic properties and the nature of the photothermal mechanism.In our experimental configuration, although the Ru/MnOx catalyst exhibits the photothermal effect, leading to the generation of heat during irradiation, the measured average temperature of Ru/MnOx reached 137.9 °C under 2.5 W cm -2 illumination, which is lower than the expected set temperature (as mentioned in our response to Comment 2).Therefore, the photothermal effect of the catalyst may be affected by external heating, which serves as a regulator to maintain consistency between the catalyst temperature and the reaction system temperature.
Furthermore, we also utilized an infrared thermal camera and thermochromic temperature indicator to measure the surface and bottom temperatures of the catalyst during photothermal catalytic reactions.As recorded by an infrared thermal camera, the average temperature of catalyst surface approached 203 °C (Supplementary Fig. 10a).Additionally, as shown in Supplementary Fig. 10b, we employed a commercially available thermochromic temperature indicator to measure the temperature at the bottom of the catalyst, which is lower than 210 °C.These results further validate that the external heating can effectively balance the temperature influence caused by the photothermal effect, thus maintaining consistency with the set temperature.
Supplementary Fig. 10 ( Comment 4. On Line 111, which vibrational peak is the blue shift of 7 cm-1 relative to?Response 4: We thank the referee very much for the critical comment.To address the referee's concern, in the revised manuscript, the blue shift of the vibration peak 7 cm -1 has been clarified by the following statement: "As illustrated in Supplementary Fig. 5, compared with the pristine MnOx, the introduction of Ru species led to a blue shift of ~ 7 wavenumbers, and the main peak at 637 cm -1 is assigned to A1g mode of crystalline Mn3O4, validating the strong metalsupport interaction between Ru and MnOx."(Please see Page 4 in the revised manuscript) Comment 5.For the proposed reaction pathways in Figure 4, please comment on which step(s) can be the rate determining step (RDS) for the overall reaction?Energetically, the formation of HCO* seems to the RDS, the diagram is lack of transition states.Is the formation of COOH* the RDS?If not, why do you think that the difference in COOH* formation between Ru/Mn3O4 and Ru/Mn3O4-x can cause significant difference in their overall reaction kinetics?If possible, the authors should provide the transition search results.
Response 5: We thank the referee very much for the comment.Based on the referee's comment, we conducted the search for the transition state in theoretical calculations and the results are shown in Fig. 4. For Ru/Mn3O4-x, the step of formation COOH* is the ratedetermining step of the reaction.Together with the FT-IR spectroscopic characterization above, it was rationalized that the synergy between photon energy and thermal energy favored the formation of COOH*, thus exerting a positive impact on the CO2 methanation over Ru/MnOx.According to the reviewer's suggestion, in the revised manuscript, we have elaborated the description as following: "As shown in Fig. 4 and Supplementary Fig. 29-31, Ru/Mn3O4-x has a more negative Gibbs free energy (ΔG) than both Ru/Mn3O4 and Ru/MnO during the adsorption of CO2, indicating a strong CO2 adsorption capacity, which is beneficial for CO2 hydrogenation (ΔG = −0.914eV, Ru/MnO; ΔG = −1.475eV, Ru/Mn3O4; ΔG = −1.651eV, Ru/Mn3O4-x).Afterwards, notable variations for the subsequent CO2 hydrogenation were observed among Ru/MnO, Ru/Mn3O4 and Ru/Mn3O4-x.The formation of COOH* from CO2* is a rate determining step (RDS) for CO2 hydrogenation over Ru/Mn3O4-x and Ru/MnO, which requires 1.232 and 1.544 eV, respectively.The protonation and subsequent dehydration of COOH* results in the generation of the intermediate of CO*, which is the RDS for the Ru/Mn3O4, (ΔG= 1.918 eV for Ru/Mn3O4).Notably, compared to HCO* formation, the CO* desorption from the catalytic surface as CO is relatively difficult for all the samples.As a result, it is favorable to yield CH4 via further hydrogenation.It is worth mentioning that in the process of CO2 hydrogenation, ΔG of RDS over Ru/Mn3O4-x (1.232 eV) is obviously lower than that on Ru/Mn3O4 (ΔG= 1.918 eV) and Ru/MnO (ΔG= 1.544 eV), thus facilitating the subsequent hydrogenation steps toward CH4." (Please see Page 12 in the revised manuscript) Fig. 4 Gibbs free energy pathway for the formation of HCO* and CO from CO2 over Ru/Mn3O4 (321), Ru/Mn3O4-x (321) and Ru/MnO (200).The blue, red, purple, yellow, and green spheres represent the Mn, O, Ru, C, and H atoms, respectively, in the calculation model.Comment 6. Please clarify if the Ru-mediated H-spillover effect in thermal catalysis remains the same under the photothermal condition.Can the light irradiation influence the H-spillover effect, why or why not?Response 6: We thank the referee very much for bringing this important question to our attention.Based on the referee's comment, we have conducted a series of experiments to investigate the influence of irradiation on the H-spillover effect under photothermal conditions.Firstly, we employed WO3 as a means to quantify the extent of H-spillover effect, by which the spillover hydrogen can migrate and readily react with yellow WO3, resulting in a dark coloration. 13,14 dditionally, to ensure that the temperature induced by the photothermal effect remains below the designated temperature, we conducted the tests at 80 °C with a light intensity of 0.3 W cm -2 under 1 MPa H2 (Supplementary Fig. 23).As depicted in Fig. 3f, it was revealed that when exposed to a H2 atmosphere, the color of WO3 remained unchanged under both photothermal and thermal conditions.In contrast, the mixture of Ru/MnOx and WO3 exhibited a darker color under photothermal conditions compared to pure thermal conditions.This observation suggests that under photothermal catalysis, photons irradiation can enhance the Hspillover effect, thereby promoting CO2 hydrogenation reaction.According to the reviewer's suggestion, in the revised manuscript, we have elaborated the description as following: "In addition, to investigate the impact of photons on the H-spillover effect under photothermal conditions, we employed WO3 as a means to quantify the extent of H-spillover effect, by which the spillover hydrogen can migrate and readily react with yellow WO3, resulting in a dark coloration.The experiment was conducted at 80 °C with a light intensity of 0.3 W cm -2 under 1 MPa H2 to ensure that the temperature induced by the photothermal effect remained below the designated temperature (Supplementary Fig. 23).As shown in Fig. 3f, it was revealed that the color of WO3 remained unchanged under both photothermal and thermal conditions.In contrast, the mixture of Ru/MnOx and WO3 exhibited a darker color under photothermal conditions compared to thermal conditions.This observation suggests that under photothermal catalysis, the irradiation can enhance the H-spillover effect, thereby promoting the subsequent CO2 hydrogenation reaction."and "In a typical experiment, a mixture containing 1 g of WO3 and 0.015 g of catalyst was placed in a quartz glass culture dish.Then the quartz glass culture dish was placed in stainless steel reactor of 180 mL (CEL-MPR, Beijing China Education Au-Light Co., Ltd.).Prior to photo-thermal reaction, the reactor was sealed and the air was replaced by H2 for three times, followed by filling with H2 (1 MPa).Then, the external heating and the 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) with an intensity of 0.3 W cm -2 were both contributed to maintain the reactor temperature at 80 °C.After the desired reaction time, the color change of the powder samples was recorded."(Pleasesee Page 11 and Page 17 in the revised manuscript) Fig. 3f Photographs of WO3 and the mixture of Ru/MnOx and WO3 samples after treatment with H2 at 80 °C with a light intensity of 0.3 W cm -2 for 20 min.

Reviewer #3 (Remarks to the Author):
In this work, Zhai and collaborators present a new catalyst based on Ru sites supported on MnOx for the low-temperature photo-thermal methanation of CO2.The as-prepared catalyst displayed a remarkable catalytic activity and CH4 selectivity under reaction conditions owing to the synergy between thermal and non-thermal contributions of light.Mechanistic studies indicate a decrease in the apparent activation energy and an enhancement in the formation of COOH* intermediates under illumination, thus favoring the methanation reaction.
The production of solar fuels and chemicals using CO2 as feedstock has raised as an interesting alternative to both tackle carbon dioxide emissions and energy crisis.In this context, photo-thermal catalysis overcomes the limitations of traditional photo-catalysis by synergistically combining thermal and non-thermal contributions of sunlight, thus becoming a very dynamic and promising field of research.The results presented by Zhai and collaborators seem reassuring, however, given the vast amount of works on photo-thermal catalysis for CO2 methanation using Ru-based catalyst, I cannot perceive any significant advance in the field.In addition to this, the role of light and heat in the overall reaction mechanism has not been completely discussed and this can lead to misinterpretations in the overall reaction pathway.Furthermore, authors did not provide any stability test to evaluate the long-term activity of the catalyst under reaction conditions.For these reasons, I cannot recommend the publication of this work in Nature Communications in the present form.Detailed comments to support this decision and suggestions to improve the quality of this work can be found below: Response: We thank the referee very much.The critical comments and advices by the referee are highly helpful for us to improve the quality of the manuscript.We have further conducted extensive computational and experimental investigations to well address all the referee's concerns.
Comment 1. Authors should provide an analysis on the particle size distribution of Ru on the surface of MnOx.From the available images it is impossible to have an idea of the size of the Ru particles.
Response 1: We thank the referee very much for the comment.Based on the referee's comment, we have conducted high angle annular dark-field scanning transmission electron microscope (HAADF-STEM) characterization to identify the size of Ru particles on the surface of MnOx.As shown in Supplementary Fig. 2, it was discovered that the average size of the deposited Ru nanoclusters was about 1.07 ± 0.26 nm.In the revised manuscript, the discussion about the size of Ru nanoclusters has been supplemented: "The morphology of MnOx did not change considerably after the addition of Ru species and the average size of the deposited Ru nanoclusters exhibit is about 1.07 ± 0.26 nm" and "The high angle annular dark-field scanning transmission electron microscope (HAADF-STEM) was operated by EM-ARM300F".(Please see Page 3 and 16 in the revised manuscript).Supplementary Fig. 2 The high angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the Ru/MnOx catalyst.Comment 2. Authors report remarkable methane productions in the order of hundreds of mmol g - 1 h -1 .Are these catalytic rates normalized by the amount of catalyst (15 mg) or the total amount of Ru present in the sample?Response 2: We thank the referee again.In fact, the catalytic rates were normalized by the amount of catalyst (~ 15 mg).In the revised manuscript, we have clarified the description by the following statement: "A prominent CO2 conversion of 66.8% was achieved with a superior selectivity of 99.5% and a CH4 production rate of 166.7 mmol g -1 h -1 at relatively mild temperature of 200 ℃ (normalized by the amount of catalyst (~ 15 mg))."(Please see Page 3 in the revised manuscript) Comment 3. When it comes to the photo-thermal experiments, could the authors explain in detail the position of the thermocouple in the setup?Is it in contact with the catalyst bed or inserted in the reactor wall?Imprecise temperature measurements can lead to misinterpretations in the contributions of photon and thermal energy to the overall catalytic performance, for instance, in the calculation of apparent activation energy.Response 3: We thank the referee very much for bringing this important question to our attention.As suggested by the referee, a detailed explanation regarding the position of the thermocouple in the experimental setup was provided.As illustrated in Supplementary Fig. 9, in order to ensure accurate temperature measurement and to maintain uniform temperature throughout the entire reaction system, the thermocouple was positioned at a distance of 1 cm above the catalyst, in the middle of the reactor.This was done to avoid any contact between the thermocouple and the bottom of the reactor, which could result in inaccurate temperature readings.By doing so, the temperature measurement can accurately reflect the temperature of the entire catalytic reaction system, while also ensuring the correct calculation of the apparent activation energy of the overall reaction.Furthermore, we also utilized an infrared thermal camera and thermochromic temperature indicator to measure the surface and bottom temperatures of the catalyst during photothermal catalytic reactions.As recorded by an infrared thermal camera, the average temperature of catalyst surface approached 203 °C (Supplementary Fig. 10a).Additionally, as shown in Supplementary Fig. 10b, we employed a commercially available thermochromic temperature indicator to measure the temperature at the bottom of the catalyst, which is lower than 210 °C.This further validates that the actual temperature closely aligns with the set temperature.To provide a clear understanding of the experimental conditions and avoid any misleading information, in the revised manuscript, we have elaborated the description as following: "The catalytic performance of Ru/MnOx was evaluated at 200 °C in the batch reactor setup by feeding CO2/H2 mixed gas (the desired temperature was achieved by a combination of external heating and irradiation from the Xe lamp) and CH4 was identified as the dominant product, with no liquid products produced (Supplementary Fig. 9 2) Secondly, in order to provide a comprehensive understanding of the role of electrons in the overall reaction pathway, we employed WO3 as a means to study the influence of electrons on H-spillover effect. 13,14 he experiment was conducted at 80 °C with a light intensity of 0.3 W cm -2 under 1 MPa H2 to ensure that the temperature induced by the photothermal effect remains below the designated temperature (Supplementary Fig. 23).As depicted in Fig. 3f, the photo reveals that when exposed to a H2 atmosphere, the color of WO3 remained unchanged under both photothermal and thermal conditions.In contrast, the mixture of Ru/MnOx and WO3 exhibited a darker color under photothermal conditions compared to thermal conditions.This observation suggests that the introduction of photo-generated carriers can enhance the H-spillover effect, thereby promoting CO2 hydrogenation reaction.In addition, as shown in Fig. 2e and Fig. 3i, the Arrhenius plot and the FT-IR spectroscopic characterization indicate that the introduction of photo-generated carriers can reduce the activation energy of the reaction and promote the generation of COOH*.Furthermore, as shown in Fig. 4, for Ru/Mn3O4-x, the step of formation COOH* is recognized as the rate-determining step of the reaction.Therefore, it was rationalized that the introduction of photo-generated carriers can enhance the H-spillover effect, reduce the activation energy of the reaction and promote the generation of COOH*, thus exerting a positive impact on the CO2 methanation over Ru/MnOx.
In the revised manuscript, we have elaborated the description as following: "In addition, to investigate the potential impact of photons on the H-spillover effect under photothermal conditions, we employed WO3 as a means to quantify the extent of H-spillover effect, by which the spillover hydrogen can migrate and readily react with yellow WO3, resulting in a dark coloration.The experiment was conducted at 80 °C with a light intensity of 0.3 W cm -2 under 1 MPa H2 to ensure that the temperature induced by the photothermal effect remained below the designated temperature (Supplementary Fig. 23).As shown in Fig. 3f, it was revealed that the color of WO3 remained unchanged under both photothermal and thermal conditions.In contrast, the mixture of Ru/MnOx and WO3 exhibited a darker color under photothermal conditions compared to thermal conditions.This observation suggests that under photothermal catalysis, the irradiation can enhance the H-spillover effect, thereby promoting the subsequent CO2 hydrogenation reaction."and "As shown in Fig. 4 and Supplementary Fig. 29-31, Ru/Mn3O4-x has a more negative Gibbs free energy (ΔG) than both Ru/Mn3O4 and Ru/MnO during the adsorption of CO2, indicating a strong CO2 adsorption capacity, which is beneficial for CO2 hydrogenation (ΔG = −0.914eV, Ru/MnO; ΔG = −1.475eV, Ru/Mn3O4; ΔG = −1.651eV, Ru/Mn3O4-x).Afterwards, notable variations for the subsequent CO2 hydrogenation were observed among Ru/MnO, Ru/Mn3O4 and Ru/Mn3O4-x.The formation of COOH* from CO2* is a rate determining step (RDS) for CO2 hydrogenation over Ru/Mn3O4-x and Ru/MnO, which requires 1.232 and 1.544 eV, respectively.The protonation and subsequent dehydration of COOH* results in the generation of the intermediate of CO*, which is the RDS for the Ru/Mn3O4, (ΔG= 1.918 eV for Ru/Mn3O4).Notably, compared to HCO* formation, the CO* desorption from the catalytic surface as CO is relatively difficult for all the samples.As a result, it is favorable to yield CH4 via further hydrogenation.It is worth mentioning that in the process of CO2 hydrogenation, ΔG of RDS over Ru/Mn3O4-x (1.232 eV) is obviously lower than that on Ru/Mn3O4 (ΔG= 1.918 eV) and Ru/MnO (ΔG= 1.544 eV), thus facilitating the subsequent hydrogenation steps toward CH4." and "In a typical experiment, a mixture containing 1 g of WO3 and 0.015 g of catalyst was placed in a quartz glass culture dish.Then the quartz glass culture dish was placed in stainless steel reactor of 180 mL (CEL-MPR, Beijing China Education Au-Light Co., Ltd.).Prior to photo-thermal reaction, the reactor was sealed and the air was replaced by H2 for three times, followed by filling with H2 (1 MPa).Then, the external heating and the 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) with an intensity of 0.  Response 9: We thank the referee very much for the suggestion.As suggested by the referee, we have revised Supplementary Table 3 by including the amount of Ru in all the samples.Meanwhile, TOF calculations has also been included in the table.Of note, the CH4 production rate was normalized per total mass of catalyst rather than per total mass of Ru.Please see the changes in the Supplementary Information.
Supplementary Table 3 The summarized CH4 yields for recently reported photo-thermo-catalysts.
Fig S8 and those of Fig 2D.
2 were both contributed to maintain the reactor temperature at 200 °C."(Please see Page 5 and Page 15 in the revised manuscript).Supplementary Fig. 9 (a) Photograph of the apparatus setup for photo-thermal CO2 experiments in the batch reactor; (b) Schematic illustration of the photo-thermal reactor.

Supplementary Fig. 8
(a-b) High-resolution Mn 2p XPS spectra of MnOx and Ru/MnOx; (c)Highresolution Ru 3p XPS spectra of Ru/MnOx; (d) XPS survey spectrum of Ru/MnOx.Supplementary Fig. 15 XPS spectra of Ru/MnOx after reaction in 4 h at 200 ℃in the batch reactor: (a) High-resolution of Mn 2p XPS spectra; (b) High-resolution of Ru 3p XPS spectra.Supplementary Fig. 25 Semi in-situ XPS spectra of Ru/MnOx after reacting at 200 ℃ for 4 h in a 20% CO2/H2 atmosphere: (a) High-resolution of Mn 2p XPS spectra; (b) High-resolution of Ru 3p XPS spectra.Comment 5.It is not clear what are exactly "semi-in situ" conditions for XPS and FTIR analyses.
)." (Please see Page 8 and Page 17 in the revised manuscript) Supplementary Fig. 16 Infrared thermal images captured for (a) MnOx and (b) Ru/MnOx under 2.5 W cm -2 illumination.
) Photograph of the apparatus setup for photo-thermal CO2 experiments in the batch reactor; (b) Schematic illustration of the photo-thermal reactor.
a) Infrared thermal images captured for the catalyst surface temperature under 2.5 W cm -2 irradiation, 0.1 MPa and external heating (Set temperature: 200 °C); (b)The temperature at the bottom of the catalyst, measured using a commercially available thermochromic temperature indicator.
-10)" and "Then, the external heating and the 300W UV-Xe lamp (Beijing China Education Au-Light Co., Ltd) with an intensity of 2.5 W cm -2 were both contributed to maintain the reactor temperature at 200 °C."(Please see Page 5 and Page 15 in the revised manuscript).
Fig. 3f Photographs of WO3 and the mixture of Ru/MnOx and WO3 samples after treatment with H2 at 80 °C with a light intensity of 0.3 W cm -2 for 20 min.

Supplementary Table 2 Crystal parameters and reliability factors of the refinement for MnOx and Ru/MnOx.
water vapor accumulation is likely to have a significantly influence on the measured activity and, therefore, isolating the real contribution of the catalyst can be difficult.In addition, other experimental details of the catalytic tests require further clarification: Experimental section does not clarify if heating is achieving exclusively by irradiation with the Xe lamp or if it requires an additional heating system.This is key aspect for understanding the activity tests, particularly the blank experiments of Fig S8 and those of Fig 2D.As the stoichiometry of the Sabatier reaction requires a CO2/H2 ratio of 1:2 it is surprising the authors decided to test lower concentration of H2 that can favor reverse water gas shift reaction.What is the rationale for testing those conditions?How is the selectivity to methane affected?Catalytic tests are performed under pressure.This surely promotes methane formation, but the authors should briefly justify the selection of these conditions with regards to other works in the literature.
Comment 2. Although the methanation activity reported here is notable, better results have been reported recently (see 10.1038/s41929-023-00970-z) using Au/Ce0.95Ru0.05O2as catalyst and working in continuous-flow reactor.In contrast, in the present case a batch reactor with very low ratio catalyst mass/volume (15 mg/180 mL) is used.This configuration is far from ideal because gas diffusion and