Concentrated-solar catalytic methane dry reforming with ultrahigh conversion and durability

Domain: earth_science
Published: 2025-10-30
Editorial outcome: minor_revision

Abstract
Methane dry reforming not only utilizes two potent greenhouse gases of methane and carbon dioxide, but also provides a valuable feedstock for the production of chemicals. However, this process has been heavily hindered by high operating temperature and coke formation with catalyst deactivation over the last century. Herein, we propose an approach whereby concentrated-solar catalytic methane dry reforming addresses these longstanding issues. By leveraging focused light as the sole energy source and utilizing a well-designed catalyst, the catalyst with Ni-O coordination active center achieves high conversion rates of 93.6% for CH4 and 93.7% for CO2, meanwhile sustaining stability for over 800 hours. Particularly noteworthy is the light-to-chemical energy conversion efficiency reaching 25.9%. This research represents a significant leap forward in integrating renewable energy sources with chemical production, offering a viable and sustainable alternative to traditional thermochemical processes for generating valuable chemicals.

Introduction
Methane dry reforming (DRM) emerges as a pivotal process by facilitating the transformation of methane (CH4) and carbon dioxide (CO2) into syngas, a versatile precursor for synthesizing chemicals and fuels, which underscores the dual benefits of environmental protection and resource optimization1. Given that the reaction is thermodynamically and kinetically unfavorable (CH4+CO2 → 2CO+2H2, ΔH298K = 247 kJ mol−1), conventional thermal DRM processes require high temperature (above 700 °C) to achieve desirable conversion rates (at least 50%)2,3. Elevated temperature facilitates the overcoming of energy barriers associated with these high-energy bonds in CH4 (C-H, 434 kJ mol−1) and CO2 (C=O, 750 kJ mol−1), thus enhancing conversion rates4. However, the requirement for elevated temperature initiates a cascade of problems that compromise the sustainability and efficiency of the process. High temperature is essential for enhancing catalyst performance by optimizing its surface state and facilitating the migration and consumption of oxygen species5. Initially, this improvement in catalyst efficiency is critical for accelerating the reaction rate. Yet, this high-energy demand leads to catalyst deactivation through mechanisms such as sintering and coking, ultimately hindering the long-term application and development of DRM technology.

Integrating renewable solar energy to initiate the catalytic reaction represents a viable approach for energy storage and offers a sustainable alternative to non-renewable energy sources6,7. This approach facilitates the catalytic reaction by using light-induced carriers to promote C-H/C=O bond activation during DRM through depositing energy into adsorbed reactants, which enables DRM at milder conditions than the thermal process8. Recent progress has broken the limitation of conventional DRM thermodynamics through exploring effective light-driven DRM catalysts, such as Rh/SrTiO3, Rh/CeWOx, Ru/TiO2, Ru/Mg-CeO2, and Rh/LaNiO39–13. However, this process still faces challenges, including low light utilization rates and carrier recombination, which diminish the activity and stability of catalysts. Thermo-assisted light-driven DRM catalysis has been proposed to further accelerate the dissociation of reactants by promoting mass transfer and reducing the redox potentials of semi-reactions (CO2/CO and CH4/CO). Despite these advancements, the conversion rates of this process generally remain below 50%, and/or the reactions often need to be carried out in a dilute atmosphere14–16. This is because the light-driven DRM also encounters catalyst deactivation caused by coking, similar to the thermal process. Specifically, following the initial C-H bond cleavage in CH4, an uncontrollable dehydrogenation process (CH4 → CH3 → CH2 → CH → C) can occur, resulting in the reaction active sites being covered by coke17,18. This rapid coke deposition is particularly prevalent at high CH4 coverage, leading to inevitable catalyst deactivation. Consequently, achieving a durable solar-driven DRM process poses a significant challenge. Many strategies, such as Ru single-atom-modified hierarchical porous TiO2-SiO2, Cu-Pd dual-reaction-sites, and Cu-Ru alloy antenna-reactor plasmonic catalysts, have been proposed to inhibit coking during the light-driven DRM process, but the reported stability under undiluted reactant gas is mainly less than 100 h19–21. Therefore, the main obstacle in this scenario is the lack of catalysts that can support active sites for anti-coking while achieving a high conversion of CH4.

In light of the limitations presented by DRM processes, our research introduces a concept that leverages concentrated solar energy with precisely designed catalysts to drive the DRM reaction efficiently at mild conditions, thereby addressing the critical issues of high energy demand and coke formation. This strategy utilizes Ni atoms anchored on different CeO2 surfaces, where precise control over Ni-O coordination plays a pivotal role in achieving exceptional catalytic performance and stability. Our findings reveal that the Ni-O4 coordination enables a breakthrough in conversion rates for CH4 and CO2, reaching 93.7% and 93.6%, respectively, at a concentrated-solar temperature of only 585 °C without additional heat input, leading to the light-to-chemical energy conversion efficiency as high as 25.9%. This performance is sustained over 800 h, marking a substantial advancement over traditional methods that require a higher temperature of 800 °C to achieve similar conversion, however, suffering from rapid degradation in 20 h. Through operando steady-state isotopic transient kinetic analysis and diffuse reflectance infrared Fourier transform spectroscopy (SSITKA-DRIFTS), we determined the critical role of Ni-O4 sites in leveraging the intensified solar energy to facilitate the formation of active intermediates. This occurs through the interaction of adsorbed CH4 and CO2 with coordinated oxygen on active metals, a process markedly accelerated by the photogenerated carriers’ effects inherent in concentrated-solar catalysis. In situ near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and in situ Raman analysis with C18O2 isotope labeling showed that Ni-O4 sites further aid in creating oxygen vacancies under concentrated-solar irradiation, which could not be achieved under thermal process. This dual function not only mitigates coking but also underscores the vital role of concentrated-solar energy in controlling the reactivity and availability of oxygen sites on catalysts, thereby directly enhancing both the efficiency of solar energy conversion and the durability of the catalysts in the concentrated-solar DRM process as compared to the thermal DRM process.

Results
The synthesis and structural characterization of the catalysts
The ceria (CeO2) is extensively employed in catalytic reactions, particularly for the conversion of CH4 and CO2, attributed to its superior redox properties, which are primarily due to oxygen vacancies22–25. Previous work has demonstrated that the processes of oxygen migration and activation, facilitated by these oxygen vacancies, are integral to the catalytic efficiency of CeO226. Moreover, these processes are influenced by the coordination environment of active metal species dispersed on the CeO2 surface27. Specifically, our density functional theory (DFT) calculations reveal that atomically dispersed Ni species, known as active species for DRM reaction, on the (110), (111), and (100) facets of CeO2, endow the catalysts with Ni-O2, Ni-O3, and Ni-O4 coordination environments, respectively (Fig. 1a–c). Notably, the Ni-O4 sites with an oxygen vacancy formation energy of −5.6 eV exhibit the most active capacity compared with Ni-O2 (+3.2 eV) and Ni-O3 (+3.5 eV). This disparity in active capacity can be attributed to the inherent structural differences of CeO2 surfaces; the (110) and (111) surfaces are non-polar, whereas the (100) surface is polar, resulting in a more formable Ni-O4 coordination on the (100) surface28. This analysis underscores the significant influence of the coordination number of active sites on the CeO2 surface for oxygen migration and activation. More importantly, effectively modulating the migration and activation of oxygen could be achieved by precisely controlling the coordination environment of active metals on the CeO2 surface. Consequently, a series of Ni-loaded CeO2 catalysts with varying exposed surfaces were synthesized through the atomic dispersion of Ni species on CeO2 nano-rods (Ni/CeO2(110)), CeO2 nano-octahedrons (Ni/CeO2(111)), and CeO2 nano-cubes (Ni/CeO2(100)) with exposed specific (110), (111), and (100) facets, respectively22,29,30.

The detailed procedures for preparing catalysts are described in the experimental sections. The analysis and description of the specific morphology and size of the obtained CeO2 nanostructures were investigated by transmission electron microscopy (TEM, Supplementary Fig. S1, and Supplementary Note S1). The actual loading of Ni on Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts was 0.67 wt%, 0.54 wt%, and 0.77 wt%, respectively, from the inductively coupled plasma optical emission spectroscopy (ICP-OES). No signals of NiO and Ni nanoparticles could be detected from the X-ray diffraction (XRD) patterns; moreover, ultraviolet-visible diffuse (UV-vis) absorption spectra exhibit no characteristic absorption peaks of NiO and Ni on Ni-loaded CeO2 (Supplementary Figs. S2 and S3, Note 2). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was conducted to further identify the Ni species dispersion. The STEM and corresponding EDS mapping images of Ni/CeO2(110) (Fig. 1d–g), Ni/CeO2(111) (Fig. 1h–k), and Ni/CeO2(100) (Fig. 1l–o) catalysts reveal that the Ni species exhibit high dispersion on CeO2. Moreover, no aggregate Ni particles or clusters can be observed from HR-HAADF-STEM, which indicates that Ni might be dispersed at the atomic scale on CeO2 (Fig. 1g, k, and o). Ni atom is difficult to directly observe due to its much lighter element than Ce, according to the STEM imaging simulation (Supplementary Fig. S4). The coordination structures of Ni for the Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts were further investigated through the in situ DRIFTS of CO chemisorption. Only the linearly CO bound to isolated Ni atoms (2060-2000 cm−1) peaks were observed at 2064 cm−1, 2017 cm−1, and 2084 cm−1, respectively, for Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts, respectively, without any signals attributing to the bridge or carbonyl C=O vibrational features (1900–2000 cm−1) from the Ni nanoparticles (Supplementary Fig. S5)31. This also demonstrated the high dispersion of the Ni species, consistent with the above results. The average coordination environments of Ni for the Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts were investigated by the X-ray absorption fine structure (XAFS) of Ni K-edge. As shown in Fig. 1p, the presence of Ni species displayed as Ni2+ in the Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(110) catalysts32,33. Furthermore, the corresponding k2-weighted Fourier transform extended X-ray absorption fine structure (EXAFS) spectra (Fig. 1q), confirmed that no Ni-Ni scattering observed in Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(110). The fitting of Ni K-edge EXAFS spectra over these catalysts was investigated. For Ni/CeO2(110), the main Ni-O scattering path (R ≈ 1.7 Å, CN ≈ 2, Ni-O) with an apparent Ni-Ce signal from Ni-O-Ce structures (Supplementary Fig. S6 and Supplementary Table S2). Different from Ni/CeO2(110), Ni/CeO2(111) possessed a Ni-O component (R ≈ 1.8 Å, CN ≈ 3, Ni-O) and an obvious Ni-Ce contribution. Moreover, a distinct Ni-O shell (R ≈ 1.8 Å, CN ≈ 4, Ni-O) and Ni-Ce contribution were observed in Ni/CeO2(100), consistent with the DFT calculations. From the above analysis, the nickel species dispersed on the (110), (111), and (100) surfaces of CeO2 as single atoms with Ni-O2, Ni-O3, and Ni-O4 coordination, respectively (denoted as Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100)).

The catalytic performance and anti-coking ability of the DRM reaction
The corresponding surface temperature of catalysts under concentrated-solar catalytic DRM conditions was recorded through the thermal camera (Fig. 2a and Supplementary Fig. S7). The conversion of CH4 and CO2 in all catalysts increased with the increasing intensity of concentrated solar light (Fig. 2b, c). The conversion of CH4 and CO2 over Ni/CeO2(100) catalyst reached 93.6% and 93.7%, respectively, under concentrated solar light of 580 °C, which is the most excellent performance compared with Ni/CeO2(110) and Ni/CeO2(111). Meanwhile, the light-to-chemical energy efficiency has reached 25.9%. Importantly, this significantly surpassed traditional DRM thermodynamics limitations, which could only be achieved below 40% at 580 °C. To further investigate the effect of crystallinity on DRM performance, we enhanced the crystallinity of Ni/CeO2(110) by calcining at higher temperatures (700 °C and 800 °C, Supplementary Fig. S8a) since all the previous samples were calcined at 600 °C. The results of concentrated-solar catalytic DRM showed that as the crystallinity increased, the catalytic performance showed a decreasing trend (the conversion of CH4 and CO2 over Ni/CeO2(100)-700 °C catalyst were 6.8% and 17.2%, respectively, and over Ni/CeO2(100)-800 °C catalyst were 2.5% and 4.1% (Supplementary Fig. S8b), respectively). Therefore, we conclude that crystallinity is not the reason why Ni/CeO2(100) exhibits more excellent performance than Ni/CeO2(110) and Ni/CeO2(111) catalysts.

Besides, a remarkable inactivation after 30 h was observed over Ni/CeO2(110) and Ni/CeO2(111) catalysts from the results of the long-term stability (Fig. 2e, f). Intriguingly, the Ni/CeO2(100) catalyst exhibits an impressively stable performance for more than 800 hours without deactivation (Fig. 2d). The turnover number (Supplementary Fig. S9) for the Ni/CeO2(100) catalyst is 8.1 × 106, which is one order of magnitude higher than that of the Ni/CeO2(110) catalyst (3.2 × 105) and the Ni/CeO2(111) catalyst (1.5 × 105). This demonstrates the excellent stability of the Ni/CeO2(100) catalyst during the concentrated-solar catalytic DRM process. Meanwhile, the H2/CO ratio remains close to the unity (0.96) of the theoretical number due to the production of H2O (Supplementary Fig. S10)34. On the contrary, Ni/CeO2(100) catalyst exhibits significant deactivation due to the coking (Supplementary Fig. S11) during the thermal DRM process from temperature ranges of 600–800 °C with stability less than 20 hours (Fig. 2g). The in situ Raman spectroscopy (Supplementary Fig. S12) was employed to investigate the dynamic evolution of coke deposition behavior during the reaction process. For Ni/CeO2(110), the no signs of coke deposition under light irradiation, while apparent coking was observed in the thermo-driven process (D band and G band). This demonstrates that light irradiation could prevent the coking deposition, and this also occurred in Ni/CeO2(100). No peak of carbon deposition occurs in the Ni/CeO2(111) catalyst under the conditions of thermal and light irradiation, which is due to the low catalytic performance of Ni/CeO2(111). Significant coke deposition was observed on Raman spectroscopy and TEM image (Supplementary Fig. S13) after extended reaction times to 70 hours over Ni/CeO2(111), indicating coke deposition was the main reason for its deactivation during concentrated-solar catalytic DRM.

To investigate the impact of Ni agglomeration on DRM durability, we performed XPS analysis on the Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts after the long-term stability reaction (Supplementary Fig. S14). The results indicated that Ni did not agglomerate (maintaining the oxidation state), thereby maintaining the atomic dispersion on CeO2. This suggests that the Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts retained good stability throughout the reaction process. The XAFS and EXAFS spectra of post-reaction Ni/CeO2(100) maintain features characteristic of Ni-O coordination (Supplementary Fig. S15a, b), indicating retained atomic dispersion. Atomic-resolution STEM mapping further verifies persistent Ni atom dispersion across CeO2 surfaces (Supplementary Fig. S15c), with discrete atomic columns visualized at ceria step edges (Supplementary Fig. S15d, e). These results revealed that ultrahigh activity of DRM with no coke under long turn reaction could be achieved over Ni/CeO2(100) catalyst.

Although the reported work achieved a high conversion of reactants, harsh conditions, such as the use of a diluent reactant gas, white laser light incident, or high reaction temperature (800 °C), were generally unavoidable, which is challenging for its implementation9,10,16,35. This is the first report to overcome the limitation of thermal reaction systems under an undiluted gas, making the impossible thermochemical DRM a reality under mild conditions. The converted reactants per unit time in Ni/CeO2(100) catalyst is approximately two orders of magnitude higher than in representative works (Supplementary Table S3)9,36. Meanwhile, the Ni/CeO2(100) catalyst was operated at 580 °C under reactive conditions for up to 800 h in a dilution-free stream without any external heat, which is far superior to the reported works (Fig. 2g). Although the reported thermal DRM could achieve high conversion and stability, high temperature is inevitable. Our strategy achieves ultrahigh conversion and durability with the temperature reduced to more than 150–200 °C, substantially lowering energy requirements and operational costs for industrial applications. In a word, we successfully achieved ultrahigh stability while obtaining high CO2 and CH4 conversions under mild conditions using the concentrated-solar catalytic technology with Ni/CeO2(100) catalyst.

The effects of catalyst evolution during concentrated-solar and thermal DRM
The superior performance of the catalyst under the concentrated-solar process compared to the thermal process highlight the importance of understanding the contribution of concentrated solar light during the reaction process. As a structure-sensitive reaction, the large differences in activity and stability in DRM could be attributed to the surface reaction process, including the adsorption of reactants, activation, and the subsequent desorption process of products. Specifically, CH4 activation occurs on the active metals to produce methyl (CHx*). According to DFT simulation, the dissociation of CH4* into CH3* at the Ni/CeO2(100) is thermodynamically favorable, with an energy change of −3.8 eV (Supplementary Fig. S16a). In comparison, the same dissociation process at the Ni/CeO2(110) results in a less favorable energy change of −2.8 eV (Supplementary Fig. S16b).

CH4* → CH3* + H* (CH4 dissociation) (1)

CH3* → C* + 3H* (CH3 dissociation) (2)

CH3* + Ov* → CH3O* + Ov (CH3O formation) (3)

CO2 + Ov → CO* + O*metal (CO2 activation) (4)

However, a rapid dehydrogenation would occur once the first C-H cleavage of the CH4, resulting in the ensemble active sites being covered by coking (1 and 2)17. The coordinated oxygen bonding with the active metals is believed to be the key to capturing the CHx* to form anti-coking intermediates (3), thereby avoiding the excessive dehydrogenation of CH4, eventually producing syngas (CO and H2)37,38. Moreover, once coordinated oxygen consumed, the formed oxygen vacancies could trap and activate CO2, generating CO and regenerating coordinated oxygen (4), and completed the cycle for the reaction1.

According to the above discussion, the generation and subsequent decomposition of CH3O* intermediates and the produced oxygen vacancies filled by CO2 are the key processes for the coordinated oxygen of the catalyst to participate in the reaction. The operando SSITKA-DRIFTS was conducted to distinguish the origin of the parallelly produced carbon-containing intermediates from CH4 or CO218. The results indicated that both concentrated-solar and thermal DRM processes showed the ability to form CH3O* intermediate, which is believed to be an active anti-coking intermediate for DRM. This demonstrates that CH3* would react with coordinated oxygen to produce CH3O* (Supplementary Fig. S17). However, the HCO3* and CO3* species originated from CO2 (Fig. 3a), which is important for CO2 activation, exhibit significant isotopic effects under concentrated-solar catalysis, while a similar phenomenon cannot be detected in the thermal process. This confirms that the concentrated solar process accelerates the conversion of CO3* to HCO3* compared to the thermal process. Besides, the concentrated-solar catalytic DRM process shows more prominent peaks assigned to the gaseous CO and linearly adsorbed CO (CO*) compared with the thermal process (Fig. 3c)39. This indicates that the concentrated-solar catalytic process promotes the conversion of reactant molecules to CO.

Furthermore, the isotope labeling experiment (C18O2/CH4) over Ni/CeO2(100) under thermal and concentrated-solar conditions, respectively, was performed to investigate the reconstruction process of oxygen from CO2 to catalysts. With light irradiation, the amount of C16O, deriving from the oxygen of Ni/Ce16O2(100) catalyst, exhibited a sharply increasing trend at the concentrated-solar temperature of 480 °C (Fig. 3b), which is much higher than that in thermal catalytic DRM (Fig. 3d). Therefore, the concentrated-solar catalytic process demonstrably enhanced coordinated oxygen reactivity for CO formation via dual pathways: (i) CO2 interaction with surface hydroxyls (OH*) or adsorbed oxygen (O*) generating transient carbonate (CO3*) or bicarbonate (HCO3*) intermediates, eventually producing CO, and (ii) direct CO2 dissociation at oxygen vacancies yielding CO.

The dynamic evolution of coordinated oxygen
We further employed in situ NAP-XPS and quasi in situ high-resolution XPS to investigate surface coordinated oxygen involved during concentrated-solar and thermal DRM processes over different catalysts (Fig. 4). After the introduction of the reaction gases (CH4 and CO2), CH4 will be preferentially adsorbed on Ni/CeO2 and consume part of the surface oxygen, so the oxygen vacancy concentration shows an increasing trend over Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts. The concentration of oxygen vacancy (531.8 eV, marked by red shadow) was increased from 37.5%, 39.8%, and 33.5% during the concentrated-solar DRM process to 56%, 42%, and 42.5% compared with the thermal DRM process for Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts, respectively (Fig. 4a-c)40,41. This demonstrates that the concentrated-solar DRM process accelerated surface oxygen involved. Interestingly, fewer oxygen vacancies were detected in Ni/CeO2(100) (42.5%) than in Ni/CeO2(110) (56%) through the NAP-XPS (Fig. 4e). However, from the quasi in situ high-resolution XPS when the catalysts were transferred to the vacuum atmosphere after the concentrated-solar process, more amounts of oxygen vacancies could be observed in Ni/CeO2(100) (41.3%) than that of Ni/CeO2(110) (26.8%) after reaction, indicating that the oxygen cannot be replenished in time under a vacuum atmosphere (Fig. 4d, f). This was further supported by that Ni/CeO2(100) shows more interfacial Ce3+ defect sites than Ni/CeO2(110) in Ce 3d XPS (Supplementary Fig. S18)35,42. Moreover, more shift assigning to Ce-O was observed in Ni/CeO2(100) (36 cm−1) than that in Ni/CeO2(110) (27 cm−1) from the in situ Raman combined with the C18O2 isotope labeling experiments during the concentrated-solar catalytic DRM process (Fig. 4g, h)43,44, highlighting its superior oxygen migration ability during the DRM process. The above analysis demonstrates that the oxygen in Ni/CeO2(100) exhibited a more excellent ability to participate in the concentrated-solar process than that of the thermal process.

The investigation of surface reaction mechanism
The significant differences in oxygen involved during concentrated-solar catalysis over different coordination numbers of Ni-O demonstrate the superior oxygen migration ability and the abundance of coordinated oxygen in the Ni/CeO2(100) catalyst during the concentrated-solar DRM process. The operando SSITKA-DRIFTS over Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100), respectively, were conducted to elucidate the underlying mechanisms contributing to the enhanced performance of Ni/CeO2(100) as compared to Ni/CeO2(110) and Ni/CeO2(111) catalysts (Supplementary Figs. S19 and S20). The result indicated that the poor DRM performance over Ni/CeO2(111) was attributed to the limitation of adsorption and further activation of CO2 under concentrated solar light irradiation (Supplementary Fig. S21 and Supplementary Note S3). On the other hand, both Ni/CeO2(110) and Ni/CeO2(100) showed the ability to form CH3O* intermediate (path 2), which demonstrates that CH3* would react with coordinated oxygen to produce CH3O* (Supplementary Fig. S22), and then produce CO and H2. However, the Ni/CeO2(100) showed much more excellent DRM performance than Ni/CeO2(110). Therefore, CH4 activation to CH3O* is not the limitation for achieving high activity over Ni/CeO2(100). For the DRM process, the activation of CO2 is also one of the decisive factors determining the overall reaction performance. The activation of CO2 mainly includes two pathways in such process, path 3 and path 4, and the HCO3* intermediate is the key intermediate of path 4.

CO2 + O*metal = OH* + CO2* = HCO3* = HCO3* (CO2 hydrogenation) (5)

HCO3* → CO* + OH* (CO2 hydrogenation) (6)

Interestingly, in the case of HCO3* intermediates (Fig. 5a–c), a more apparent isotopic effect was found in Ni/CeO2(100) than in Ni/CeO2(110) (Fig. 5d–f) after switching the gas from 12CO2/12CH4/Ar to 13CO2/12CH4/Ar31,45. The amounts of 13CO from 13CO2 ads over Ni/CeO2(100) that was more abundant compared with Ni/CeO2(110) (Supplementary Fig. S23). This further demonstrates that, over Ni/CeO2(100), path 3 is more than path 4 for the contribution to produce CO. Meanwhile, from the results of the DFT calculations (Supplementary Fig. S24), the energy barrier of Path 3 over Ni/CeO2(100) was lower than that of Path 4, while an opposite trend can be observed over Ni/CeO2(110). Consequently, we can conclude that path 3 in Ni/CeO2(100) dominates the DRM activity more than that of Ni/CeO2(110), which significantly accelerated the regeneration of oxygen during concentrated solar irradiation.

In summary, for the CH4 activation, the coordinated oxygen reacts with the CH3* stemming from dissociation (5), leading to the formation of the CH3O* intermediate (6) on the Ni/CeO2(100). In this process, the sufficient coordinated oxygen (Ni-O4) in Ni/CeO2(100) reacts with CH3* groups in a timely and effective manner to avoid the direct cleavage of CH3* to produce coke deposition (Path 1-1). Over Ni/CeO2(110), CH3* also reacts with coordinated oxygen to form CH3O. However, this process is relatively slower on Ni/CeO2(110) than that of Ni/CeO2(100) due to the limited amount of coordinated oxygen (Ni-O2) on Ni/CeO2(110). On Ni/CeO2(111), the insufficient reactivity of oxygen limits the formation of CH3O*, resulting in lower overall reaction performance. For CO2 activation, the formation of oxygen vacancies aids in the capture and activation of CO2 on Ni/CeO2(100), promoting the generation of CO (Path 3). The reaction rapidly refills the vacancies with CO2 due to the strong mobility of oxygen, further driving the reaction forward. In comparison, Ni/CeO2(110) exhibits weaker CO2 activation capabilities, with lower efficiency in vacancy generation and refilling, limiting the overall reaction activity.

Furthermore, the time-resolved PL (TRPL) decay spectra indicate that the average photoluminescence (PL) lifetime of Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(100) catalysts were 2.25 ns, 3.20 ns, and 2.39 ns, respectively (Supplementary Fig. S25). This demonstrates that the more photo-induced carriers participate in the surface reaction process over the Ni/CeO2(111) catalyst, while the poor adsorption capacity for reactant molecules limit the overall DRM activity. More importantly, to better elucidate the distinct catalytic performance of the three samples, we investigated their charge carrier dynamics following light excitation using transient absorption spectroscopy (TAS, Supplementary Fig. S26). In our analysis of the steady-state absorption characteristics, all three samples exhibited similar absorption band edges, indicating identical optical band gaps (~3.4 eV). However, it’s worth noting that Ni/CeO2(110) and Ni/CeO2(111) displayed more pronounced sub-bandgap absorption tails, attributed to excitonic defect states commonly observed in metal oxide semiconductors. To gain deeper insights, we altered the excitation energy, employing wavelengths both above the optical band gap (360 nm, 3.4 eV) and within the absorption tail region (400 nm, 3.1 nm) to monitor the evolution of excited state charge carriers situated either within the semiconductor band or at defect sites.

As shown in Supplementary Fig. S26C, Ni/CeO2(100) exhibited a distinctive broad negative ground state bleach (GB) band between 440 nm and 500 nm, peaking at 470 nm, following excitation above the band edge. Since Ni/CeO2(100) samples displayed minimal sub-bandgap absorption, this bleach band likely arises from the electron population in Ni atoms embedded within the CeO2 lattice. In addition, the GB signal decayed swiftly, with a lifetime of 1.5 picoseconds. This lifetime corresponds to the conventional hot electron lifetime of interband transitions in metals46. The near-instantaneous rise of the GB signal within the response time of the transient absorption setup (0.15 ps) suggests rapid charge transfer from CeO2 to Ni within the sub-picosecond timeframe. Subsequently, the injected electrons from CeO2 maintained elevated carrier temperature until 1.5 ps when they gradually cooled down. In contrast, the absence of the GB band in the other two samples indicates that the charge transfer process in these samples is either hindered or suppressed.

When the samples were excited below the band edge of CeO2, the TA spectra of Ni/CeO2(110) and Ni/CeO2(111) displayed relatively long-lived GB signals between 450 nm and 525 nm. This phenomenon can be attributed to the direct excitation of defects following excitation. The lifetime of these GB bands could be fitted within their decay kinetics, measuring 78 ps for Ni/CeO2(110) and 65 ps for Ni/CeO2(111), respectively (Fig. 5g, h). The slightly faster depopulation of defect states in the Ni/CeO2(111) sample may be due to the deeper levels of these defects, as evidenced by the red-shifted GB band maximum position (510 nm) compared to that of Ni/CeO2(110) (500 nm). This implies a narrower energy gap between the excited defect state and the ground state, facilitating phonon emission for non-radiative recombination. Conversely, no transient absorption signal was detected in Ni/CeO2(100) in this spectral region, primarily because of the absence of absorption transitions. Also, the projected density of states (PDOS) of Ni/CeO2(100) indicates that the conduction band of CeO2(100) is positioned lower in energy compared to that of Ni. This energy alignment suggests that upon excitation, electrons can readily move from the valence band of CeO2(100) into the conduction band and then migrate toward the Ni active sites (Supplementary Fig. S27a).

Therefore, the DFT calculation combined with TA characterization confirmed that, with above-bandgap excitation, efficient hot charge transfer occurs from CeO2 to catalytic Ni sites in Ni/CeO2(100) (Supplementary Fig. S27b), whereas this transfer is hindered in Ni/CeO2(110) and Ni/CeO2(111). With sub-bandgap excitation, even though defects in Ni/CeO2(110) and Ni/CeO2(111) can partially absorb light, the lifetimes of the excited charge carriers remain relatively short, limiting their contribution to catalytic reactions.

To further investigate the influence of carriers generated by CeO2 bandgap excitation on the reaction performance, we conducted experiments using a cutoff filter with a wavelength distribution of 420~800 nm. The results (Supplementary Fig. S28) showed that when the wavelength distribution below 420 nm was removed, the reactant conversions experienced a significant decline (CH4 conversion dropped from 91.3% to 56.2%, and the CO2 conversion decreased from 93.5% to 70%). In addition, we used a bandpass filter to compare different wavelength distributions with the experimental performance. When the wavelength distribution was limited to 380 nm, the CH4 and CO2 conversions were 2.4% and 7.5%, respectively. When the wavelength distribution was limited to 500 nm, the conversions of CH4 and CO2 were 2.8% and 8.7%, respectively. These low conversions in both cases may be attributed to the reduced light intensity from the narrow bandpass, which limit overall photothermal effects rather than clearly isolating bandgap contributions. While these results indicate that narrow-band illumination yields poor performance regardless of wavelength, the cutoff filter data support the importance of shorter wavelengths for optimal activity.

Based on the above evidence, we proposed the concentrated-solar catalytic DRM process (Fig. 6a). Firstly, the first C-H dissociation step of CH4 takes place to generate CH3* at Ni sites. In the next step, the CH3* reacts with adjacent oxygens to give rise to CH3O* species and oxygen vacancies, subsequently generating CO* and H2 under both thermal and concentrated-solar processes. The concentrated solar energy plays a vital role in the catalytic process for facilitating the creation of oxygen vacancies, followed by the formation of active intermediates through the photogenerated carriers’ effects. The accompanying oxygen vacancy is refilled by CO2, simultaneously producing CO under concentrated-solar catalytic DRM, while it could not be achieved at the same temperature under thermal catalytic DRM. Besides, the pathway from carbonate/bicarbonate (CO3*/HCO3*) also contributes to the formation of CO, thereby completing the consumption and regeneration of active sites during the DRM process under the concentrated-solar process. The abundant coordination of oxygen from Ni-O4 coordination on CeO2 creates this dynamic cycle of Ni-O4 active sites efficiently. However, such a process cannot be realized with catalysts such as CeO2 with Ni-O2 of Ni/CeO2(110) and Ni-O3 of Ni/CeO2(111). As a result, the precise control of the Ni-O4 coordination environment on the CeO2(100) surface, coupled with the integration of renewable concentrated-solar energy, results in a highly efficient and stable catalytic process. This innovative approach achieves remarkable conversion rates of CH4 and CO2 at a relatively mild temperature, addressing the critical issues of high energy demand, catalyst deactivation, and coke formation that have long hindered the development of DRM processes. Importantly, the dish-engine solar concentrating device has the advantages of a small footprint and a desirable concentrating effect. To further evaluate the DRM activity under sunlight, we designed and fabricated a dish-engine solar concentrating system with the function of autonomously tracking light capabilities, which mainly consists of a solar concentrator with autonomous light tracking and a solar-concentrating catalytic reactor (Fig. 6b). Furthermore, the solar-concentrated catalytic DRM and performed using sunlight without any external energy (Fig. 6c). The conversion of reactants reached maximum value (CO2 and CH4 for 15.9% and 15.6%, respectively) at 13:00 under a diluent-free atmosphere.

Methods
Catalyst preparation
The synthesis of CeO2 with varying morphologies was carried out using the reported methods, and employed an impregnation approach for Ni species loading47,48. The sodium hydroxide (NaOH) was ≥98%. CeO2 nano-rods and CeO2 nano-cubes were obtained with the hydrothermal method. The sodium hydroxide solution (CNaOH = 0.01 mol L−1, 35 mL) was added to cerium nitrate solution (0.87 g of Ce(NO3)3·6H2O in 5 mL of deionized H2O) by drops and stirred for 30 min. Then, the white precipitates were transferred into a 100 mL Teflon-lined stainless-steel autoclave and hydrothermally heated for 24 h at 100 °C and 180 °C to obtain CeO2 nano-rods and CeO2 nano-cubes, respectively. After centrifugation and drying, the obtained samples were calcined at 600 °C for 2 h in an air atmosphere. For CeO2 nano-octahedrons, the sodium phosphate dibasic solution (CNa2HPO4 = 0.02 mol L−1, 35 mL) was added to cerium nitrate solution (0.87 g of Ce(NO3)3·6H2O in 5 mL of deionized H2O) by drops and stirred for 30 min. After centrifugation and drying, the obtained samples were calcined at 600 °C for 2 h in an air atmosphere. The Ni was loaded on the surface of CeO2 using an incipient wetness impregnation method. In more detail, 1.0 g of CeO2 was added to 50.0 mL of an aqueous solution of 0.049 g of Ni(NO3)2·6H2O to obtain a Ni/CeO2 catalyst, respectively. Then, the extra water was evaporated at 80 °C until dryness. After that, the mixture was calcined at 400 °C for 2 h under air.

Catalytic measurements
The thermal catalytic DRM and concentrated-solar catalytic DRM were carried out in a quartz tube with a diameter of 1.5 mm. All the gases (CH4 and CO2) used in this study were 99.99% with a flow rate of 10 mL min−1 controlled by a mass flow controller (Vögtlin mass flow controller). The catalyst powder was pressed under 20 MPa pressure and ground into granules with a diameter of 1.5 mm, 200-40 μm (40–70 mesh), and no pretreatment was required before the reaction. The used light was a 300 W Xe lamp focused by a homemade concentrator system. The reaction gases and production gases were analyzed by using gas chromatography (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID), and a mass spectrometer (OmniStar GSD-320, Pfeiffer Vacuum).

Catalytic characterization
The operando SSITKA-DRIFTS measurements were carried out on a Bruker infrared spectrometer (Tensor II) equipped with a liquid nitrogen-cooled mercury-cadmium-telluride (MCT) detector in a three-window diffuse-reflectance cell (Harrick Praying Mantis) with ZnSe windows. The in situ Raman spectra were performed by a high-resolution Raman spectrometer system (Horiba LabRam HR) equipped with 473 nm laser excitations in a Harrick Raman high-temperature reaction chamber with a 25 mm thick mica window facing the incoming laser. Detailed information about the structural characterization (XRD, TEM, XAS) and in situ/operando measurements (operando SSITKA-DRIFTS, the DRIFTS of CO probe experiments, the NAP-XPS, in situ Raman spectra) can be found in the Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper reports the DRM properties of Ni-Ceo2 catalysts under light irradiation. As the conversions of CH4 and CO2 are remarkable, it is worth to be published in Nature Communications after the revision. The following issues should be addressed by the authors.\n1. Reference No.6-10, 12, 13, 17, and 19 are missing in the introduction section.\n2. As the authors indicate that the bandgap excitation (electron-hole pair) contributes to the reaction, the authors should investigate the action spectrum using a monochromatic light source. The action spectrum analysis is useful to discuss the effect of the bandgap excitation of CeO2.\n3. In the XRD patterns of Figure S2, the peaks of Ni/CeO2(110) were broader than those of (100) and (111). The reviewer wonders if the different crystallinity would affect their catalytic activities. The authors should discuss the effect of the crystallinity on their catalytic performance.\n4. In line 234, Ni/CeO2(100) would be Ni/CeO2(110).\n5. In Figures 3 c,d, the authors described the setting temperature in the graphs. However, the authors should use the same temperature profiles for both light-irradiated and dark conditions to elucidate the light irradiation effect.\n6. In the isotope experiments, the reviewer supposes the following process,\nCO2 reduction\nC18O2 + Ov → C18O + 18O(in catalyst) (1)\nC18O + 16O(lattice) → C16O18O (2)\nmethane oxidation\nCH4 + 16O(lattice) → 2H2 + C16O (3)\nCH4 + 18O(in catalyst) → 2H2 + C18O (4)\nFor the stoichiometric DRM reaction, (1) + (2) should be relevant to (3) +(4). The authors should check the material balance of these four reactions.\n7. The authors should mention their turnover number rather than the irradiation time (durability).", "reviewer_2": "The manuscript describes a very interesting approach for achieving efficient dry reforming of methane using a combination of reactor and catalyst design. By using either simulated or actual solar light, the authors described the characteristics of the light-driven reactor and demonstrated the relevance of Ni-O4 sites for the reaction. Here, 3 catalyst materials were used, each showing a particular orientation of the CeO2 crystal. The study investigated the surface sensitivity and the metal (Ni) coordination effect on the reaction. The authors showed that the Ni-O4 system operating with concentrated solar energy (~580°C) outperformed the other catalyst technologies including traditional thermocatalytic reactors working at much higher temperatures (~700-800°C).\nI commend the authors for their achievements and recall the noteworthy results of their work:\n1. The study discloses important design aspects for direct harvesting of solar energy into chemical forms.\n2. The study highlights the importance of the catalytic substrate structure leading to tuned interactions of the active metallic component with the coordination environment.\n3. The solar-driven system was shown to potentiate the formation of oxygenate intermediaries (specifically CO3-, CO2H-) thereby reducing the coking tendency of the reaction for over 800h of operation in a simulated environment. Furthermore, they claimed that no sintering of the Ni component occurred. These aspects are very important since the traditional thermocatalytic DRM suffers of strong deactivation by both coking and sintering, hindering its applicability in industry.\n4. The energy-to-product efficiency and high reaction selectivity are among the best values reported in the literature for the DRM reaction.\n5. The study used a combination of operando, in situ, quasi in situ and ex situ characterization techniques to decipher the catalytic principles governing the process.\nI think that the manuscript is well structured and shows significant advancements in the field, the experiments are well described, and enough information is given for reproducing them. After a literature survey in the field, I consider that the work consolidates many of the previous findings concerning solar-driven activation of C1 and the important roles of reactor design and catalyst structure.\nIn conclusion, I recommend the publication of this manuscript in Nature Communications.\nHowever, there are some aspects that should be improved before the manuscript is published.\n1. The color setting and font size of many of the figures does not allow for clear reading. Using different shades of the same tone (see Fig. 1p, Fig. 3a-3c., etc) and small fonts makes it very difficult for the reader. Also, the 3D dispersion plot of Fig. 2g is very hard to understand, even with a zoom of 500% into the text. I recommend removing it and making a table instead.\n2. It is hard to tell from the (S)TEM images of Fig. 1 if the synthetized catalysts represent an atomic dispersion of the Ni component. The images lack the spatial resolution for this claim. Furthermore, the XANES and EXAFS spectra (Fig. 1p, 1q) indicate an extended coordination environment on the Ni atom that does not fit the calculated coordination for monoatomic dispersions (see Fig. S6). This may indicate clustering of the Ni from synthesis. The authors further discuss that (line 380): “Our findings were corroborated by theoretical calculations, which affirmed the surface plasmon resonance (SPR) absorption band for Ni species smaller than 10 nm, located at energies greater than 3 eV.” Hence, the SPR and presumably other observations would basically remain the same for single atoms and for nanoclusters smaller than 10nm. It is also claimed that the Ni does not agglomerate after the solar-driven reaction, although only the XPS is shown to support this. It is needed to show the XANES-EXAFS and/or (S)TEM characterization of the post-reaction samples to demonstrate that the Ni dispersion did not change. Please clarify these points.\n3. Line 151. The authors say that: “As shown in Fig. 1p, all the catalysts displayed the prevailing presence of Ni species in an oxidation state across the Ni/CeO2(110), Ni/CeO2(111), and Ni/CeO2(110) catalysts”. Do they mean the same oxidation state across the samples? What is this oxidation state? Please, clarify these points.\n4. Line 241. In the last mechanistic step, the authors used the word “restructuring” to describe the refilling of the oxygen vacancy. Is this the correct term? Restructuring usually means a change in the structure, which may not be the case here.\n5. The DRIFTS data in Fig. 3 represent the formation of intermediate species from CH4 and CO2 activation, showing as well the marked isotopic effect in the solar-driven reaction, specifically for CO2 activation. I personally had a hard time trying to figure out the information contained in the IR data. I recommend changing the layout of the plots, and make the text clearly readable, explaining which conditions apply in each of the plots. This is a minor improvement that can make a difference when the article is used for further referencing.\n6. What is the involvement of water and other byproducts in the reaction? In thermal DRM, water is usually found as a by product during the starting of the reaction. Given that the authors discussed the CO2 activation involving -OH/CO2H- species (paths 4 and 4-1), it would be interesting to corroborate the effect of water in the reaction. I suspect the formation of byproducts (water, coke, methanol?) because the reactant consumption is not completely balanced, i.e., in a 1:1 ratio, during the lighting up of the reaction (see Fig. 2b).\n7. For matters of implementing this work in actual solar light technologies, I recommend that the authors include a few lines about “what to do” when solar light is not available (cloudy days, nights, etc) to keep the reaction running. 5h of operation per day is certainly something to be addressed.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Manuscript ID: NCOMMS-25-44988\nTitle: “Concentrated-solar catalytic methane dry reforming with ultrahigh conversion and\ndurability”\nResponse to Reviewer’s Comments\nReviewer 1\nThis paper reports the DRM properties of Ni-CeO catalysts under light irradiation. As the\n2\nconversions of CH and CO are remarkable, it is worth to be published in Nature\n4 2\nCommunications. The following issues should be addressed by the authors.\nResponse 1: We are deeply appreciative of the reviewer's insightful encouragement and\nconstructive feedback, which have significantly enhanced the quality of this work. With the\nsuggestion from the reviewer, we conducted the following experiments:\n(1) We performed activity tests of Ni /CeO (100) under different wavelengths to investigate\n1 2\nthe effect of carriers generated from the bandgap excitation of CeO on the DRM reaction\n2\nperformance.\n(2) To explore the impact of crystallinity on the reaction performance, the concentrated-solar\ncatalytic DRM reaction experiments over Ni /CeO (110) catalysts with varying crystallinity\n1 2\nwere performed.\n(3) The turnover number of the concentrated-solar catalytic DRM reaction over Ni /CeO (110),\n1 2\nNi /CeO (111), and Ni /CeO (100) catalysts has been calculated to evalute the durability of\n1 2 1 2\nDRM over Ni /CeO (110), Ni /CeO (111), and Ni /CeO (100) catalysts.\n1 2 1 2 1 2\nThese revisions and changes are discussed in detail below. We hope that these alterations could\nmeet the expectations of the reviewer.\n1.1. Reference No.6-10, 12, 13, 17, and 19 are missing in the introduction section.\nResponse 1.1: We apologize for any misleading information to the reviewers. Reference\nformatting updates were inadvertently omitted during revision. We have now reviewed and\nreformatted all references to comply with journal requirements.\n1.2. As the authors indicate that the bandgap excitation (electron-hole pair) contributes to the\nreaction, the authors should investigate the action spectrum using a monochromatic light source.\nThe action spectrum analysis is useful to discuss the effect of the bandgap excitation of CeO .\n1 2\ncatalyst under different wavelengths. Reaction conditions: 200 mg catalyst, CH /CO =1/1, a\n4 2\ntotal flow rate of 10 mL min-1.\nResponse 1.2: We thank the reviewer for the insightful suggestion regarding action spectrum\nanalysis to elucidate the role of bandgap excitation in CeO . To address this, we conducted\nperformance. Based on the UV-vis absorption spectra showing the absorption edge of\nNi /CeO (100) at approximately 400 nm, we first used a cutoff filter (>420 nm) to exclude\n1 2\nwavelengths below 420 nm. As shown in Fig. R1, this resulted in a notable decrease in\nconversions (CH from 91.3% to 56.2%, CO from 93.5% to 70%), suggesting that light below\n4 2\n420 nm, likely involving bandgap excitation, contributes significantly to the enhanced\nperformance under full-spectrum conditions.\nAdditionally, we tested bandpass filters at 380 nm and 500 nm for comparison. At 380 nm,\nCH and CO conversions were 2.4% and 7.5%, respectively; at 500 nm, they were 2.8% and\n4 2\n8.7%. These low conversions in both cases may be attributed to the reduced light intensity from\nthe narrow bandpass, which limits overall photothermal effects rather than clearly isolating\nbandgap contributions. While these results indicate that narrow-band illumination yields poor\nperformance regardless of wavelength, the cutoff filter data supports the importance of shorter\nwavelengths for optimal activity.\nThese findings align with the role of bandgap-generated carriers in synergizing with\nphotothermal effects, though a more comprehensive action spectrum would be valuable for\nfuture studies. We have incorporated this discussion into the revised manuscript and\nhighlighted it in yellow (Page 25).\n2\n1.3. In the XRD patterns of Figure S2, the peaks of Ni/CeO2(110) were broader than those of\n(100) and (111). The reviewer wonders if the different crystallinity would affect their catalytic\nactivities. The authors should discuss the effect of the crystallinity on their catalytic\nperformance.\nFigure R2. (a) The XRD patterns of Ni/CeO (110) calcining at 600 °C, 700 °C, and 800 °C,\n2\nrespectively. The concentrated-solar catalytic DRM reaction performance over Ni /CeO (110)-\n1 2\n600 °C, Ni /CeO (110)-700 °C, and Ni /CeO (110)-800 °C. Reaction conditions: 200 mg\n1 2 1 2\ncatalyst, CH /CO =1/1, a total flow rate of 10 mL min-1.\n4 2\nResponse 1.3: We thank the reviewer for the valuable comments. To assess whether\ncrystallinity influences catalytic activity, we systematically varied the calcination temperature\nof the CeO (110) support to modulate its crystallinity. Specifically, Ni/CeO (110) samples\n2 2\nwere prepared by calcining the support at 600 °C, 700 °C, and 800 °C, followed by loading 1.0\nwt% Ni (Figure R2a).\nThe results of concentrated-solar catalytic DRM showed that as the crystallinity increased, the\ncatalytic performance showed a decreasing trend (the conversion of CH and CO over\n4 2\nNi/CeO (100)-700°C catalyst were 6.8% and 17.2% (Fig. R2b), respectively, and over\n2\nthat of Ni /CeO (110)-600 °C (91.3% for CH conversion, 93.5% for CO conversion).\n1 2 4 2\nTherefore, we conclude that crystallinity is not the reason why Ni /CeO (100) exhibits more\n1 2\nexcellent performance than Ni /CeO (110) and Ni /CeO (111) catalysts.\n1 2 1 2\nWe have added these discussions to the revised manuscript and marked them in yellow (Page\n11).\n3\n1.4. In line 234, Ni/CeO2(100) would be Ni/CeO2(110).\nResponse 1.4: We thank the reviewer for pointing out the errors in our manuscript. We have\ncorrected “Ni/CeO (100)” to “Ni/CeO (110)” and highlighted the changes in yellow in the\n2 2\nrevised manuscript (Page 14).\n1.5. In Figures 3 c, d, the authors described the setting temperature in the graphs. However, the\nauthors should use the same temperature profiles for both light-irradiated and dark conditions\nto elucidate the light irradiation effect.\nFigure R3. The (a) operando SSITKA-DRIFTS and (b) C18O isotope exchange experiment\n2\nduring concentrated-solar catalytic DRM processes over Ni /CeO (100). The (c) operando\n1 2\nSSITKA-DRIFTS and (d) C18O isotope exchange experiment during thermal catalytic DRM\n2\nprocesses over Ni /CeO (100).\n1 2\n(1) To elucidate the light irradiation effect on the surface reaction process during the light-\ndriven and heat-driven DRM, respectively, we compared the operando SSITKA-DRIFTS of\nDRM driven by light and heat at the same reaction temperature. Therefore, we used the\ncorresponding temperature values marked on the curves in the figure. Following the reviewer's\nsuggestions, we re-plotted and re-labeled the spectra to compare those from the same\ntemperature curve under both light and dark conditions, thereby revealing the role of light.\nSpecifically, for the operando SSITKA-DRIFTS, we collected and labeled the spectra under\nboth light and dark conditions for the corresponding temperatures (Fig. R3a and c).\n(2) For the C18O isotope exchange experiment, we also annotated the light and dark conditions\n2\nat the same temperature (480 °C) (Fig. R3 b and d).\nWe have updated Fig. 3 in the revised manuscript. We hope that these alterations could meet\nthe expectations of the reviewer.\n1.6. In the isotope experiments, the reviewer supposes the following process,\nCO reduction\n2 v\nC18O + 16O (lattice) → C16O18O (2)\nmethane oxidation\nCH + 16O (lattice) → 2H + C16O (3)\n4 2\nCH + 18O (in catalyst) → 2H + C18O (4)\n4 2\nFor the stoichiometric DRM reaction, (1) + (2) should be relevant to (3) +(4). The authors\nshould check the material balance of these four reactions.\nResponse 1.6: We thank the reviewer for this insightful suggestion and have carefully\nexamined the material balance of the proposed reactions (1)-(4). In a strictly stoichiometric\nDRM process (CH + CO → 2CO + 2H ), the oxygen atoms from CO reduction [(1) + (2)]\n4 2 2 2\nshould ideally balance those consumed in methane oxidation [(3) + (4)], with lattice oxygen\nacting as an intermediary. However, our isotope data reveal a deviation, where more CO is\n2\n5\nconsumed relative to the expected balance, leading to additional incorporation of ¹⁸O into the\nlattice without a proportional increase in C18O products.\nThis discrepancy arises from supplementary CO activation pathways that extend beyond\n2\nsimple oxygen vacancy (O ) filling. Specifically, CO can interact with surface hydroxyl\nv 2\ngroups (OH*) or adsorbed oxygen (O*) to form transient carbonate (CO *) or bicarbonate\n3\n(HCO₃*) intermediates. A representative pathway is as follows:\nC18O + 16OH* → HC18O 16O* + 18O (incorporated into lattice via exchange)\n2 2\nHC18O 16O* → C16O* + H¹⁸O* (or 18OH* upon recombination)\n2\nprimarily labeled with 16O (from the lattice or surface), and it regenerates lattice oxygen sites\nthrough additional CO consumption. As a result, more CO equivalents are utilized to maintain\n2 2\nthe catalyst's redox cycle, explaining why the observed rates of (1) + (2) exceed those of (3) +\n(4) in terms of oxygen balance. We have added a discussion of this in the revised manuscript\nto clarify the isotope scrambling and material balance (Pages 17).\n1.7. The authors should mention their turnover number rather than the irradiation time\n(durability).\nFigure R4. The turnover number of concentrated-solar catalytic DRM reaction over\nNi /CeO (110), Ni /CeO (111), and Ni /CeO (100) catalysts. Turnover frequencies (s-1) and\n1 2 1 2 1 2\nTurnover number were calculated from Eq. 1 and Eq. 2, respectively; W represents the nickel\nNi\n6\nmetal loading (wt%), a dispersion of 100% for the atomically dispersed Ni catalyst was\nassumed.\nTOF = nCH\nNi\nTON= TOF × Lifetime of catalyst\nResponse 1.7: We thank the reviewer for his constructive comments. As suggested, we have\ncalculated the turnover number of the concentrated-solar catalytic DRM reaction over\nNi /CeO (110), Ni /CeO (111), and Ni /CeO (100) catalysts (Fig. R4). The turnover number\n1 2 1 2 1 2\nfor the Ni /CeO (100) catalyst is 8.1×106, which is one order of magnitude higher than that of\n1 2\nthe Ni /CeO (110) catalyst (3.2×105) and the Ni /CeO (111) catalyst (1.5×105). This confirms\n1 2 1 2\nthe exceptional operational stability of Ni /CeO (100) under concentrated-solar-driven DRM\n1 2\nconditions. Critically, the sustained performance meets industrial durability requirements (＞\n1×106), advancing commercial potential for solar-powered DRM. We have incorporated this\ndiscussion into the stability analysis section (Pages 11 and 12), with key findings highlighted\nfor emphasis.\nThe manuscript describes a very interesting approach for achieving efficient dry reforming of\nmethane using a combination of reactor and catalyst design. By using either simulated or actual\nsolar light, the authors described the characteristics of the light-driven reactor and\ndemonstrated the relevance of Ni-O sites for the reaction. Here, 3 catalyst materials were used,\n4\neach showing a particular orientation of the CeO crystal. The study investigated the surface\n2\nsensitivity and the metal(Ni) coordination effect on the reaction. The authors showed that the\nNi-O system operating with concentrated solar energy (~580°C) outperformed the other\n4\ncatalyst technologies including traditional thermocatalytic reactors working at much higher\ntemperatures (~700-800°C). I commend the authors for their achievements and recall the\nnoteworthy results of their work:\n1. The study discloses important design aspects for direct harvesting of solar energy into\nchemical forms.\n2. The study highlights the importance of the catalytic substrate structure leading to tuned\ninteractions of the active metallic component with the coordination environment.\n3. The solar-driven system was shown to potentiate the formation of oxygenate intermediaries\n(specifically CO3-, CO2H-) thereby reducing the coking tendency of the reaction for over 800h\nof operation in a simulated environment. Furthermore, they claimed that no sintering of the Ni\ncomponent occurred. These aspects are very important since the traditional thermocatalytic\nDRM suffers of strong deactivation by both coking and sintering, hindering its applicability in\nindustry.\n4. The energy-to-product efficiency and high reaction selectivity are among the best values\nreported in the literature for the DRM reaction.\n5. The study used a combination of operando, in situ, quasi in situ and ex situ characterization\ntechniques to decipher the catalytic principles governing the process.\nI think that the manuscript is well structured and shows significant advancements in the field,\nthe experiments are well described, and enough information is given for reproducing them.\nAfter a literature survey in the field, I consider that the work consolidates many of the previous\nfindings concerning solar-driven activation of C1 and the important roles of reactor design and\ncatalyst structure.\nIn conclusion, I recommend the publication of this manuscript in Nature Communications.\nResponse 2: We are grateful for your positive and constructive evaluation of our work. We\nespecially thank the reviewers for recognizing the significance of our catalyst design strategy\nfor efficient solar-driven DRM. We also acknowledge your point about the need for more\nthinking when solar light is not available to keep the reaction running. We have proposed our\nown solution to this problem and are working hard to implement this strategy.\nAs suggested by the reviewer, we have made revisions and explorations"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have reasonably answered all of the reviewers' comments and have properly revised their paper. This paper is publishable as is.", "reviewer_2": "The authors undertook thoughtful revisions. All my concerns have been addressed, and I find the current version suitable for publication.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter