A nickel gallium oxide chlorophyll mimic for green methanol synthesis

Domain: physics
Published: 2025-11-26
Editorial outcome: major_revision

Abstract
Light-driven methanol synthesis from CO2 provides a sustainable fuel source and approach to carbon neutralization. Mimicking natural photosynthesis could improve gas-solid photocatalytic efficiency, but it remains highly challenging due to the absence of well-organized mass and charge transfer networks in artificial materials. Herein, we report a chlorophyll-mimicking, nano-pigment nickel gallium oxide, which facilitates discrete light/dark reactions and proton-mediated charge transfer for efficient photocatalytic hydrogenation of CO2 to methanol. This nano-pigment features surface frustrated Lewis pairs, enabling heterolytic hydrogen splitting into H- and H+. The H- acts analogously to nicotinamide adenine dinucleotide phosphate in natural photosynthesis, with Ni(II)/Ni(III) and OH(-I) respectively serving as conduits for ion transport of H- and H+ to the Ni site, where they subsequently react with CO2, mimicking natural carbon fixation. This approach establishes a chlorophyll-mimetic structure for photocatalytic stepwise CO2 hydrogenation, achieving 3.0% quantum efficiency, 3.20 mmol·h-1·g-1 methanol activity, and 79.6% selectivity.

Introduction
Photocatalytic hydrogenation of CO2 into methanol offers a promising approach to address today’s energy and environmental issues and promote the global circular carbon-neutral energy system1–4. However, achieving high reaction activity and quantum efficiency remains challenging due to the inefficient separation, migration, and transport of photogenerated electrons and holes5–7. Additionally, limited light absorption and poor active site selectivity further hinder overall performance8–10. Thus, optimizing the spatial distribution of active sites and enhancing charge separation through advanced interface and material design are essential for improving photocatalytic efficiency.

Mimicking natural systems offers a potential strategy for enhancing quantum yield by spatially separating oxidation and reduction sites, similar to the division of light and dark reactions in plants11,12. Green leaves from plants are nature’s opto-chemical photosynthetic machines. Their hierarchical architectures are engineered to capture and transport incident sunlight to the chloroplasts, within which chlorophyll pigments photochemically transform carbon dioxide and water into carbohydrate energy vectors13,14. During sunlight exposure, chloroplasts split water to produce equivalents of [H] and oxygen [O2]15–17, which are subsequently used to produce reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) through sequential electron (e−) and proton (H+) transfers. Under dark, CO2 can be stepwise converted into carbohydrates with the aid of NADPH, H+, and ATP. Inspired by the leaf, artificial photosynthetic analogues have been proposed as a feasible approach to convert solar energy into storable chemical energy18,19. However, replicating the complex electron transport chain of natural systems, which facilitates stepwise electron transfer and energy conversion, is significantly challenging in gas-solid photocatalytic systems due to the lack of well-organized mass and charge transfer networks in artificial materials.

Surface frustrated Lewis pairs (SFLPs), composed of spatially adjacent yet sterically hindered Lewis acid and base sites, have emerged as highly effective active centers for activating small molecules including CO2, H2, alkenes, dienes, and alkynes. Typically, these bifunctional sites are engineered by creating isolated oxygen vacancies (Ov) or by designing O-OH configurations on metal oxide surfaces20. A prominent example is the In2O3-x(OH)y catalyst, where unsaturated In³+ cations (Lewis acid, generated by Ov) cooperatively interact with proximal In-OH groups (Lewis base) to enable efficient CO2 hydrogenation to methanol as validated by both theoretical and experimental studies21,22. However, the lower methanol production rate and lower quantum yield endow the challenge to find more efficient SFLP-based catalyst for photocatalytic CO2-to-methanol production. Thus, to find an alternative that circumvents this problem yet retains the attributes of defect engineered indium oxide, we turn to gallium oxide as it has been extensively studied as a promoter or support for methanol synthesis though it suffers from the poor light absorption due to its wide band gap.

Combining the natural process and the important role of OH and Ov during CO2 hydrogenation, we have developed a chlorophyll-mimicking green nano-pigment, nickel gallium oxide (NiGa2O4-x(OH)y), which consists of Ov and hydroxyl groups (OH) and mixed-valence Ni(II,III) sites within an inverse spinel structure (space group Fd3m; inversion parameter of 92%)23,24. Notably, edge-sharing Ni(II,III) and Ga(III) octahedra endow the material with a high electrical conductivity favorable for electron transfer and ion diffusion25,26. Notably, Ga(III) and adjacent Ni(II)/Ni(III) sites function as Lewis acid sites, while the O(-II) serve as Lewis base sites for the formation of surface frustrated Lewis pairs (SFLPs). In this situation, the nano-pigment NiGa2O4-x(OH)y not only mimics the optical response of chlorophyll, but also emulates the sequential dark and light reactions in natural photosynthesis (Fig. 1a, b). Specifically, light interacts with the SFLP sites to promote heterolytic splitting of hydrogen, forming hydride (H-) and proton (H+), where H- acts as a redox mediator similar to the role of NADPH. Ni(II)/Ni(III) and OH(-I) respectively serve as conduits for the ion-transport of H- and H+ to the Ni site, where they react with CO2, mimicking the dark reaction in nature photosynthesis. Consequently, the equivalent transfer of electrons (e-) and protons (H+) can occur within such a chlorophyll-mimicking structure, promoting the conversion of CO2 to methanol. This nano-pigment achieved an impressive methanol production rate of 3.20 mmol·h-1·g-1 and a methanol selectivity of 79.6% with the assistance of light. Additionally, a quantum efficiency of 3.0% was achieved during wavelength-dependent studies, with 92.3% photons contributing to methanol synthesis. These findings demonstrate significant progress in photocatalytic hydrogenation and provide a nature-inspired strategy for designing highly efficient photocatalysts. Ultimately, these insights form a solid foundation for the development of solar-driven refineries, paving the way for scalable and sustainable methanol synthesis.

Results
Design scheme and crystal structure characterization
The nano-pigment NiGa2O4-x(OH)y samples containing abundant oxygen vacancies (Ov) and hydroxyl groups (OH) were prepared using the hydrothermal method. To determine the effect of surface OH groups on the SFLP site, samples of NiGa2O4-x(OH)y calcined at 300 °C and not calcined were compared. The calcinated and uncalcined samples were denoted as NG-300 and NG, respectively. As displayed in Fig. 1c, the diffraction peaks of the as-prepared NG and NG-300 are identical and are consistent with those from standard cubic inverse spinel structure (Fd-3m, PDF no. 78-0546)27. High-resolution transmission electron microscopy (HRTEM) image also displays the same inverse spinel (311) planes over NG and NG-300, with a lattice parameter of 0.249 nm (Fig. 1d)24,27. Transmission electron microscopy (TEM) images (Supplementary Fig. 1a, b) revealed that NG has a particle size of ~9 nm, while NG-300 exhibits particles with an average diameter of 11 nm, resulting in a slight decrease in specific surface area for the latter (94.2 m2g-1 versus 83.0 m2g-1, Supplementary Fig. 2 and Table 1). The uniform distributions of Ni, Ga, and O in NG were confirmed from high angle annular dark-field (HAADF) imaging combined with energy dispersive X-ray (EDX) element mapping (Supplementary Fig. 3). The atomic ratio of Ni with Ga was measured to be 0.47 from inductively coupled plasma optical emission spectrometry (ICP-OES) (Supplementary Table 2), which is close to the theoretical value of 0.5 in the inverse spinel structure.

A series of characterizations were used to identify the oxygen defects and surface hydroxyls. X ray photoelectron spectroscopy (XPS) analysis on NG revealed a wide range of oxygen species conducive to SFLP-enabled hydrogenation. The relative percentages of lattice oxygens (Olatt), Ov, and OOH were 42.4%, 23.3% and 34.4% (Fig. 1e), respectively24. NG-300 was found to have lower OOH (20.5%), more Ov (27.5%), and more Olatt (52.0%) than that of NG (Fig. 1d and Supplementary Table 3). The decreased number of surface hydroxyls were further confirmed by the greater wettability of NG compared to NG-300 (Fig. 1f), more positive Zeta-potential (Supplementary Fig. 4)28, and higher FTIR peak intensity at 3420 cm-1 (OOH) (Supplementary Fig. 5). After calcination, NG-300 showed a slightly lower percentage of Ni(III), which consequently affects Ga oxidation state in the Ga-O-Ni sites, as shown in Fig. 1g and Supplementary Fig. 6. However, the electron energy loss spectroscopy (EELS) analysis (Fig. 1h, i) indicated a higher Ni oxidation state accompanied by more Ov following calcination. The valence states of Ni were determined by calculating its white-line ratio (L3/L2) (See Methods and Supplementary Table 4). The discrepancy between the XPS and STEM-EELS results may arise from variations in sampling depth between the two techniques and changes in Ni valence between the surface and subsurface layers29. Deprotonation likely releases H2O via coupling between the OH group of a bridging hydroxyl linked to Ni-Ga and the H atom of an adjacent terminal OH on Ni or Ga site, thereby generating additional Ov and exposing more Olatt, as supported by the Raman results (Supplementary Fig. 7)30,31. Consequently, different Ni valences were observed at the surface and in the subsurface. Based on the above analysis, the changes resulting from the reconstruction of NG to NG-300 are proposed as shown in Fig. 1j. Changes in the Lewis acidic and basic sites between NG and NG-300 were characterized by pyridine-IR spectroscopy and CO2-TPD, as shown in Supplementary Fig. 8.

The collected results demonstrate the formation of oxygen vacancies, hydroxide, and Ga(III) accompanied by redox-active Ni(II,III) sites in NG, providing rational and systematic control over the physicochemical properties of SFLPs. Light excitation of the SFLP system can tune the Lewis acidity and Lewis basicity of structures compared to the ground state32,33. The photogenerated electron can rapidly converse the Ni in the octahedral sites between Ni(II) and Ni(III) in the octahedral sites, which endows NG with high electrical conductivity that is favored for electron transfer, boosts the population of Ga-H under light, and promotes subsequent CO2 hydrogenation34.

Light-driven CO2 hydrogenation performance
The catalytic performance of nano-pigment NG for CO2 hydrogenation, with or without light irradiation, was investigated at different temperatures in a Harrick Cell reactor under 0.7 MPa pressure. Under irradiation, CO and CH3OH production rates increased with temperature from 260 °C to 300 °C, while CH3OH selectivity linearly decreased with increasing temperature for all tested conditions (Fig. 2a and Supplementary Table 5). CO and CH3OH production rates at 300 °C were 0.29 and 0.77 mmol·h-1·g-1, respectively, corresponding to a CH3OH selectivity of 72.8%. When the catalyst was irradiated with white LED light, significant increases in CO and CH3OH production were observed across the investigated temperature range. Remarkably, an high CH3OH production rate of 2.95 mmol·h-1·g-1 and a selectivity of 78.3% were achieved at 300 °C (Fig. 2a), demonstrating the advantage of light in methanol production. The performance of photocatalytic CO2 hydrogenation to methanol over NG was excellent compared to the prior state-of-the-art, both in thermal and photocatalytic reactivity at low pressure (Supplementary Table 6). The stability of NG was studied at 300 °C (Fig. 2b) for 150 h, during which the CH3OH production rate continuously increased over the first 60 h, reaching a maximum of 3.20 mmol·g-1·h-1 with the selectivity of 79.6%, and then decreased thereafter.

We speculate that the spinel structure of NG plays a vital role in its performance, which was verified by comparing the activities of NG with control samples including Ga2O3, a mixture of NiO/Ga2O3 (Ni:Ga molar ratio of 1:2) and reduced NiGa/SiO2 (see Supplementary Notes 1 for synthesis). From Supplementary Fig. 9, all control samples showed low methanol production and selectivity compared to the spinel structure of NG, confirming the latter’s key role in CO2 hydrogenation to methanol. This could be attributed to the edge-sharing nature between Ni and Ga sites in the crystal lattice, endowing the system with high electrical conductivity and favorable electron transfer dynamics35. Furthermore, to corroborate the efficacy of surface hydroxyls and multiple Ni oxidation states in catalysis, the activities of NG-300, NG-N (treated in N2), NG-H (treated in H2), and NG-H2O (treated in H2O) were also studied (Supplementary Figs. 9, 10). All CO and methanol production rates were substantially lower than that of NG, highlighting the importance of having Ov, OH, and/or multiple Ni oxidation states simultaneously during methanol synthesis. NG-N showed similar activity to NG-300, while NG-H and NG-H2O displayed comparable levels activity. This difference in activity can be attributed to the surface structural changes caused by different treatments. H2 and H2O atmospheres can partially retain OH. In contrast, N2 or air atmospheres may lead to the removal of OH, thereby reducing the density or effectiveness of SFLP sites and leading to lower methanol production. These findings support the hypothesis that OH acts as proton (H+) transport conduit, facilitating the hydrogenation of CO2 to methanol.

Further, to investigate the synergistic effect of SFLP sites on CO2 hydrogenation activity, quenching experiments were conducted using pyridine and pyrrole to selectively block the Lewis acid and base sites, respectively. The results presented in Supplementary Fig. 11 demonstrate that deactivation of either the acidic or basic site leads to a significant decrease in the CO2 hydrogenation activity. Specifically, methanol production activity decreased to approximately 75% upon acidic site poisoning and dropped to near zero upon basic site quenching. These observations indicate that both sites are required to work cooperatively for effective CO2 hydrogenation to methanol, with basic sites playing a more critical role in this process.

To distinguish the photochemical versus photothermal roles of illumination, the following experiments were conducted. First, the surface temperatures were measured using an infrared (IR) camera under both dark and light conditions. As shown in Supplementary Fig. 12 and Table 7, the surface temperature can increase about ~20 °C under light, confirming the photothermal contributions. Yet the CH3OH production rates under light remain higher compared to those in the dark even when the temperature was increased by 30 °C for the latter condition (Fig. 2a, Supplementary Table 4). This suggests a prominent photochemical contribution to CH3OH production from the excited carriers, distinct from the photothermal effect. The activation energies (Ea) were calculated using the surface temperatures under both dark and light conditions, as shown in Supplementary Fig. 13. A much lower Ea was measured under light than in the dark, indicating the contribution of solar light excitation in reducing energy barriers compared to the purely thermal ground-state pathway. Normally, light has been shown to have the capacity to serve dual roles, both in photothermal and photochemical processes32. Differentiating the contribution of photothermal and photochemical effects is essential in the photocatalytic systems.

Irradiation wavelength and photon flux can influence photocatalytic activity and products36,37. Thus, four LED light sources (UV, blue, green, and red lights with center peaks at 365 nm/3.4 eV, 460 nm/2.7 eV, 525 nm/2.4 eV, and 625 nm/2.0 eV) with tunable intensities were used to systematically explore the photochemical role of light in CO2 hydrogenation at 300 °C. The spectrum of each light source is shown in Supplementary Fig. 14. From Fig. 2c–f, Supplementary Fig. 15 and Table 8, an obvious increase of CO and CH3OH production was observed with increasing light intensity in the range of 0.1 to 0.83 W cm-2 for all four light sources. We were pleasantly surprised to find that the selectivity of CH3OH is not affected by light intensity, indicating that CH3OH production is independent of variations in both light intensity and wavelength. In addition, the CH3OH production rate is in the order of UV > red > blue > green for NG, which aligns with NG’s absorption spectra, suggesting a direct correlation between enhanced photoactivity and the light absorption properties of NG. Above results further confirm the existence of a consistent charge transfer route with photo-induced carriers under each light irradiation condition.

The near linear correlation between photocatalytic activity of methanol and irradiation intensity provides compelling evidence that excited charge carriers actively participate in the photochemical reaction rather than solely undergoing relaxation to produce photothermal heating in CO2-to-methanol. Next, the external quantum efficiency (EQE) of CO2 hydrogenation to form either CO or CH3OH was calculated using the rate difference between the dark and light reactions for different LED wavelengths (see Supplementary Note 3 for details). As shown in Fig. 2g and Supplementary Fig. 16, the EQE linearly increased with greater light intensity under UV and blue light but decreased with greater light intensity under green and red light. With an incident intensity of 0.83 W cm-2, measured EQEs are 2.7%, 1.6%, 1.1% and 1.4% under UV, blue, green, and red light, respectively. When these EQEs were separated into EQE CO and EQE methanol contributions, we found that the EQE methanol value exhibited a similar trend to that of total EQE, whereas a decreasing EQE CO was observed with increasing light intensity for all wavelengths used (Supplementary Fig. 17). By calculating the selectivity for EQE in EQE methanol at the light intensity of 0.83 W cm-2, as shown in Supplementary Fig. 18, it was found that the EQE methanol are 92.3%, 92.6%, 91.3%, and 92.6% at UV, blue, green and red LED lights, respectively. That is to say, a predominant fraction of the excited charge carriers actively engages in the production of methanol. Notably, the observed consistency between the optical absorption of NG and its EQE at different wavelengths originates from the absorption coefficient of NG, further suggesting that the CO2 hydrogenation reaction is driven by a photochemical process, as shown in Fig. 2h.

Conventionally, a positive correlation between EQE and incident photon flux is a distinct feature of light-induced, charge carrier-driven photocatalysis38. This is because increased carrier density at higher light intensities reduces recombination and enlarges the difference between electron and hole quasi-Fermi levels39. Furthermore, UV and blue light fall within NG’s interband absorption. Thus, increased CO and CH3OH productions with higher light intensity in this range further confirm the photochemical role of light in reactivity (Fig. 2h). Green light’s decreasing EQE with intensity can be attributed to NG’s lower absorption in the excitation wavelength centered at 525 nm. The pattern observed for red light, with an excitation wavelength centered at 625 nm and measured increase and subsequent decrease in EQE with light intensity, can be attributed to the saturation of NG’s long-wavelength photon utilization under higher intensity. Thus, the above results illustrate the important photochemical role of light for CO2-to-methanol production over NG.

Considering the linear increase of EQEs as the intensity increases under UV and blue light, we further increased the light intensity to 1.26 W cm-2 and found the highest EQEs are 3.0% and 1.9% for UV and blue light, respectively (Fig. 2h), which surpasses the majority of reported analogical photocatalytic CO2 hydrogenation to methanol using solar energy (Supplementary Table 6).

Identification of active sites
A series of spectroscopic characterizations were performed to probe the catalytic active site and its reaction pathway in the catalytic CO2 methanation process. The structure and composition of spent NG and NG-300 were first investigated with PXRD and HRTEM (Supplementary Figs. 19–21) and no significant changes were observed between the spent and fresh samples, indicative of the catalysts’ structural stability. These findings also necessitate in situ characterizations under reaction conditions. First, in situ X-ray photoelectron spectroscopy (in situ XPS) tests were conducted in the dark under Ar, H2, and CO2 + H2 reaction conditions to investigate the surface electronic properties specified in Fig. 3a–c. When moving from Ar to H2 atmosphere, an obvious decrease in the intensity of OH from 28.5% to 8.5% was observed, followed by an increase to 16.0% in a CO2 + H2 atmosphere. This behavior indicates that H2 undergoes heterolytic dissociation on the surface of NG and consumes surface OH. In the presence of CO2 this OH removal is partially inhibited. (Fig. 3a and Supplementary Table 9). Concurrently, the atomic contribution of Ni(II) peaks increased from 63.2% to 73.1% when switching from Ar to H2 and shifted to lower binding energies, and was partially restored under CO2/H2 gas compared to when under Ar (Fig. 3b and Supplementary Table 10). Meanwhile, the percentage of Ga-O-Ni changes from 36.7% (H2) to 39.0% (CO2 + H2), which is close to the 39.8% observed at RT under Ar. This small variation can be attributed to the CO2 reaction at Ga-O-Ni sites (Fig. 3c and Supplementary Table 11). These results above indicate that Ni contributed to H2 heterolytic splitting and the subsequent CO2 hydrogenation reaction along with Ga sites, which agrees with DFT results that both Ni and Ga donate electrons for H2 splitting (as further discussed in the following section).

The structure and oxidation changes during the reaction were further explored by in situ scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) in different gaseous environments, as shown in Fig. 3e, f. In H2 environment, the surface d-spacing of (311) plane expands to 2.59 Å compared to 2.49 Å at room temperature, and remains constant when CO2 is added to form a CO2 + H2 atmosphere. This lattice expansion can be attributed to Ni reduction from Ni(III) to Ni(II) and corresponding increased ionic radius40. Moreover, Ni L-edge and O K-edge EELS spectra confirmed the increase of Ni2+ and oxygen vacancies under H2 and the decrease under CO2, thereby further corroborating the formation of Ni-O bonding resulting from CO2 hydrogenation (Fig. 3e and Supplementary Table 12).

Elucidation of the role of light
The photo-thermal catalytic CO2 hydrogenation involves both photothermal and photochemical roles. Thus, we conducted a detailed investigation into their effects on the reaction. First, NG and NG-300 band structures were measured using the transformed Kubelka–Munk function from ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) and UV photoelectron spectroscopy (UPS) (Supplementary Figs. 22, 23). Conduction band (CB) and valence band (VB) edge potentials of NG and NG-300 were found to fulfill the thermodynamic requirements for converting CO2 to methanol41, as shown in Supplementary Fig. 24.

Subsequently, to investigate the hydrogen heterolytic splitting behaviors, we conducted the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements under dark and light conditions in H2 with varying temperature, as shown in Fig. 4a and Supplementary Figs. 25, 26. The prominent appearance of the peaks at 1825 cm-1 and 3682 cm-1 were attributed to Ga-H and Ga-OH+/Ga-OH species42. Notably, under light conditions, the intensities of these two species increased, demonstrating that light illumination enhances heterolytic H2 splitting at SFLP sites. To identify the hydrogen species from heterolytic H2 splitting in H2-DRIFTS tests, D2-DRIFTS was also conducted (Fig. 4b). Absorptions at 3682 and 1825 cm-1 were shifted to 2602 and 1185 cm-1, which matched the expected (νH2/νD2)0.5 ratio43,44. Moreover, H2 splitting on NG was confirmed by solid-state 1H nuclear magnetic resonance (NMR) spectroscopy by detecting the formation of H-related species (Fig. 4c). Without H2, only one peak, with a chemical shift at −2.98 ppm, assigned to surface OH was observed45. After exposing the sample to H2, a peak with the chemical shift at 4.72 ppm was observed and assigned to Ga-H46,47. Furthermore, the peak at −2.98 ppm showed a positive shift and increased area, as shown in Supplementary Table 13. This can be attributed to the formation of more Ga-OH+ or Ga-OH during H2 heterolytic splitting. The decrease in H2 desorption intensity under light compared to dark conditions in the H2 temperature-programmed desorption (H2-TPD) further confirmed light promotion of H2 splitting (Supplementary Fig. 27)48,49.

To study the dynamics of photoexcited electrons, in-situ photoconductivity measurements under vacuum, H2, CO2 and CO2 + H2 conditions were performed on NG, as shown in Fig. 4d. Accordingly, NG displayed photocurrent saturation and decay process lifetimes on the order of minutes. A slower increase in rate of photocurrent flow under H2 atmosphere was observed compared to that under CO2, which indicate the formation of hydrogen species (e.g., Ga-H) can trap more electrons and promote photocarrier excitation from the valence band to the conduction band. While the photocurrent under CO2 increased faster compared to that under vacuum conditions, their decay behaviors were similar, suggesting that CO2 adsorption removes pre-existing surface species which delay photoexcitation and relaxation. A similar increase in photocurrent between H2 + CO2 and vacuum conditions suggests that the reaction in H2 + CO2 proceeds quickly, which means that the photoelectrons are generated at the same rate as they are consumed during the CO2 hydrogenation reaction, and the surface is almost identical to that of the vacuum surface state. These results indicate that NG is sensitive to both acidic and basic gases as they influence photo-generated electron flow by either trapping or promoting it when adsorbed on the catalyst surface. An environment with both types of gases neutralizes their individual effects, restoring the catalytic surface to its original state and enhancing catalyst stability during repeated use. Additionally, the structure of SFLPs facilitates the formation of both Lewis acidic and basic adsorbates close to each other, which rapidly drive the desired chemical reaction and adsorbate removal under light. This forms the basis of the photochemical role of light in CO2-to-methanol production.

In order to further characterize the charge transfer dynamics and the changes to the Ni and Ga species in NG during the light illuminated reaction processes, in situ X-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted under dark and light conditions in H2 + CO2 (3:1) atmosphere (Fig. 4e–h). Upon illumination, the white line of the Ni L-edge in its XANES spectrum shifts to higher energy, while the Ga K-edge shifts to lower energy, indicating electron transfer from Ni to Ga. This electron transfer modulates the charge state of adsorbed intermediates and enhances catalytic activity, which could be attributed to a faster combination of ligand effects and the dissociation of reactants or reaction intermediates under light (Fig. 4e, f, Supplementary Fig. 28)50. The results are further verified by corresponding Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) (Fig. 4g, h). Peak I at ~1.5 Å was assigned to the metal-oxygen bond distance. Peaks II and III at 2.4 Å and 3.1 Å, respectively, were assigned to the radial distances of Moh (metal in octahedral site) and MTd (metal in tetrahedral site) to their neighboring metal atoms51. Strongly enhanced scattering at peak II was observed in Ni FT-EXAFS under light conditions compared to dark conditions, which was attributed to the faster adsorption and conversion of reactants at Ni sites in the presence of light30. Meanwhile, only minor changes were observed in the Ga FT-EXAFS between light and dark conditions, which is attributed to reaction intermediates bound to Ga transferring more quickly to Ni in the presence of light. The peak positions in our samples closely match those of NiO and Ga2O3 standards and no correspondence was observed with the spectra of their respective metal foils, further supporting the structural interpretation.

Next, in situ DRIFTS was employed in CO2 + H2 atmosphere to analyze the faster light promoted conversion of key intermediates during the reactions. As shown in Fig. 4i, j, formate bands52,53 at 1594, 1349, 1381, 2881, 2906, 2715, and 2739 cm-1 decreased under light irradiation compared to their intensity in the dark. Meanwhile, two bands at 2924 and 2843 cm-1, attributed to the δas(CH3) and δ(CH3) modes of methoxy species (*HCO3)54,55, increase under light. The downshift of the band at 2848 cm-1 under dark to 2843 cm-1 under light can be explained by the increased hydrogen bond strength of *HCO3 under light56. CO2 + D2 in situ DRIFTS experiments were conducted to further verify the surface intermediate species (Fig. 4k). No obvious peaks like the ones measured in CO2 + H2 experiments were observed between 2700 to 3000 cm-1. Two new peaks appeared at 2175 and 2142 cm-1 and increased moderately over time, which can be attributed to DCOO* and D3CO* species57, verifying the formation of formate and methoxy intermediates during CH3OH production. In situ DRIFTS results were constant with the in situ XAFS results and further support the enhanced conversion of the reaction intermediates under light.

DFT calculations
Density functional theory (DFT) calculations were performed to understand the H2 heterolysis and reaction pathway over NG. The (311) surface of NG, with one surface OH site and one Ov site, was used for the simulation, as shown in Fig. 5a, Supplementary Data 1. From the optimized configuration, Ga and Ni sites adjacent to the Ov and O sites were spatially separated by 3.330 and 3.337 Å, respectively. The sufficient distance between SFLP provides unquenched Lewis acid and base sites, allowing for flexible polarization of H-H bonds and heterolytically dissociation of H258. Bader charge analysis confirmed the SFLP property of Lewis acid-base (Ga/Ni)•••Ov sites, with the related Ga with Ni and O involving local atomic charges of +1.300e, +0.835e and −1.059e, respectively. The energy barriers of H2 splitting on the SFLP sites showed that ΔGbar = 0.43 eV for the formation of surface OH and (Ga/Ni)-H, as shown in Fig. 5b. After H2 splitting, one proton attaches to the surface OH and the other hydride bridges Ga and Ni. Analyzing the charge distribution, we see the differential charge density map of H+δ by the O and H-γ by the Ga/Ni (Fig. 5c), and the Bader charges show that the hydrogen in the OH moiety exhibited a charge of +0.658e, whereas in the metal hydride, it endowed a charge of −0.361e, thus reinforcing the evidence for heterolytic splitting of H2.

The projected density of states (PDOS) demonstrated that once H2 approaches the surface-active site, Gap sub-gap states disappear, supporting the role of Ga acting as a Lewis acid (Supplementary Fig. 29). In addition, noticeable shifts were also observed in the states around the Fermi level for Ni, Ga, and O (Supplementary Fig. 30a-c). As H2 splits to form a proton and hydride on the surface, a clear additional peak in the PDOS that can be attributed to the hydride bonding interaction. Additionally, some of the electron density from the hydride shifted up to the Fermi level, while there is a slight splitting in the peak for the O-H bond, indicating the presence of two oxygen-attached hydrogens. A new surface OH forms from the proton of H2, and the original surface OH remains (Supplementary Fig. 30d).

We also employed linear response time dependent DFT (LR-TDDFT) under a hybrid functional approach (details in the Methods) to study the role of light in facilitating H2 splitting. Figure 5d shows the computed absorption spectra for the bare NG, NG with H2 physical adsorption, and NG with the split H2 (named as NG, NG_H2, and NG_2H, respectively). The spectrum for bare NG, to a first approximation, is quite comparable to that from experiment (Fig. 1b), with major peaks in the correct regions. Upon H2 approach, the absorption intensity between 425 and 525 nm (a trough in the bare NG spectrum) and 600-700 nm increases significantly, indicating that H2 in proximity to the surface can be involved in electronic excitations coupled with the surface states of NG. Among all excitations with an oscillator strength greater than 0.1, we find several transitions that are conducive to charge polarization on H2, as demonstrated by difference densities between the relevant bands with the largest contribution to specific excitations (Fig. 5e, f). For example, we see an excitation of around 530 nm that involves an accumulation of electron density on the hydrogen closer to Ga and a depletion of electron density on the hydrogen closer to O. Another around 550 nm (Supplementary Fig. 31) that shows charge depletion around the Ga and charge accumulation around the O across the vacancy site (for the latter, the additional charge accumulation on an oxygen is consistent with previous findings)59. These results demonstrate that light can create a polarizing environment around the vacancy site that heterolytically splits H2 to produce H-γ by the Ga and H+δ by the O. In the absence of oxygen vacancies, one is typically limited to the dominant ligand to metal charge transfer from O to Ga. However, in the existence of oxygen vacancies, light can induce polarization around the vacancy site. With the atoms fixed in their lattice positions, this polarization contributes to the frustration of surface Lewis acid-base sites, facilitating the splitting of H2.

After H2 splits, we also see multiple transitions that lead to significant electron accumulation around the hydride and Ga, as demonstrated by the corresponding charge density difference plots (Supplementary Fig. 32). Since these transitions show up across the visible region (from around 400 to 570 nm) and are also among the ones with the largest oscillator strengths out of all transitions, this indicates that light can play a further role in making the hydride more hydridic following H2 splitting, thereby facilitating the hydrogenation of CO2.

Further, DFT calculations were conducted to determine the reaction pathway and associated energetics. As shown in Fig. 5g, a CO2 molecule is the physical adsorbed on the Ni near the Ga-H group with the ΔEad-CO2 of −0.228 eV. Subsequently, the transfer of hydride to the carbon of CO2 leads to HCOO* formation with a free energy barrier of 0.794 eV, which agrees with in situ DRIFTS and in situ XAFS results. For further hydrogenation, the HCOO* has to be flipped. Fortunately, this flipping is an exothermic process (−0.857 eV) with a small free energy barrier of 0.138 eV, indicating the favourable flipping process. After that, the second H diffuses from OH to the O terminal of HCOO* with the free energy barrier of 0.365 eV, resulting in the formation of HCOOH*. The C atom of the HCOOH* group could then be hydrogenated to H2COOH* group with a free energy barrier of 0.945 eV and a reaction free energy of 0.335 eV. The H2COOH* could further break the C–O bond and further hydrogenate to form H2CO* and a water molecule almost spontaneously with a free energy barrier of 0.049 eV. H2CO* could stepwise react with neighbouring hydrogen atoms with a third heterolytic H2 dissociation to form H3CO* and CH3OH*. The free energy barriers for the former hydrogenation steps were 0.650 eV, in which the last formation of CH3OH is spontaneous with the reaction energies being −0.854 eV. Note that all H+ species resulting from the heterolytic cleavage of H2 react with O atoms in reaction intermediates, while the H- species selectively attack the positively charged C atoms in these intermediates, as shown in Supplementary Figs. 33–38. It is not noteworthy that the preexisting OH group acts as a transporter for the subsequently split H+, which promotes the entire process of CO2 hydrogenation to methanol. This suggests that the designed SFLP primarily facilitates the heterolysis of H2, directing the generated H+ and H- to selectively bond with O and C atoms in the reaction intermediates, respectively. Such orchestrated bonding significantly accelerates the hydrogenation of CO2, ultimately resulting in the formation of CH3OH. Combining the experimental results, it can be illustrated that light interacts with the SFLP sites to promote the heterolytic splitting of hydrogen, forming a hydride (H-) and a proton (H+), where the hydride acts as a redox mediator, similar to the role of NADPH. Ni(II)/Ni(III) and OH(-I) respectively serve as conduits for the ion-transport of H- and H+ to the Ni site, where they react with CO2, mimicking the dark reaction in biologically-mediated photosynthesis. Consequently, the equivalent transfer of electrons (e-) and protons (H+) can occur within the nano-pigment and promote the photocatalytic conversion of CO2 to methanol. To investigate the influence of CO2 presence during the reaction process on NG, seven possible adsorption sites for CO2* on the NG surface were considered, as shown in Supplementary Fig. 39. Among these, the Ni-4 site exhibited the most stable adsorption energy, confirming it as the preferred configuration. Consequently, all subsequent calculations modeling CO2 co-adsorption utilized the Ni-4 site. Given that heterolytic H2 splitting and CO2 activation were the key steps in CO2 hydrogenation to methanol over NG, we focused our analysis on these processes under CO2 presence (Supplementary Fig. 40). The calculated reaction free energies for H2 splitting and the first CO2 hydrogenation step are 0.58 eV and 0.69 eV, respectively, with co-adsorbed CO2. These values are slightly different from those without CO2 (0.43 eV and 0.79 eV), indicating minor influence from CO2 adsorption. Notably, while CO2 presence modestly increases the energy barrier for H2 splitting (0.15 eV), it facilitates CO2 hydrogenation (−0.10 eV). This acceleration of the CO2 activation step is particularly advantageous for methanol synthesis.

Methods
Synthesis of NiGa2O4-x(OH)y photocatalyst
1.0 mmol of nickel nitrate (Ni(NO3)2·6H2O, Sigma ≥97.0%) and 2.0 mmol of gallium nitrate (Ga(NO3)3·xH2O, Sigma 99.9%) were dissolved in 15 mL deionized water and stirred for 30 min. Subsequently, ammonium hydroxide (NH3·H2O; Anachemia 28%) was added to adjust the pH to 8-9 under continuous stirring. After aging for 30 min, the obtained suspension was transferred into a 30 mL stainless-steel Teflon lined autoclave and kept at 180 °C for 16 h before being cooled to room temperature. Next, the obtained products were collected after being centrifuged and washed with deionized water several times and finally dried at 60 °C for 12 h in a vacuum oven. The NG-300 sample was prepared by treating the NG sample under air for 4 h at 300 °C with a heating rate of 5 °C min−1.

Material characterization
High-resolution transmission electron microscopy (HRTEM) image and electron energy loss spectroscopy (EELS) measurements were performed using a Hitachi HF-3300 environmental TEM/STEM. The EELS results were recorded using an EELS collection angle of 149 mrad. An energy dispersion of 0.25 eV/channel was used to acquire Ni L-edge, Ga L-edge and O K-edge, respectively. The energy resolution in EELS, as measured by the full-width at half-maximum of the zero-loss peak, was 1.0 to 1.3 eV for 0.25 eV/channel. All pre-peaks (labelled as peak a) of O-K edge are aligned to 532 eV. Gaussian function was used to fit the Ni-L edges for white-line ratio calculations. Powder X-ray diffraction (PXRD) was conducted on a Bruker D2-Phaser X-ray diffractometer, using Cu Kα radiation at 30 kV. X-ray photoelectron spectroscopy (XPS) was measured using a Perkin Elmer Phi 5500 ESCA spectrometer. The spectrometer uses an Al Kα X-ray source to generate X-rays with an energy of 1486.7 eV. Diffuse reflectance spectra (UV-Vis-NIR) of the samples were recorded using a Lambda 1050 UV/VIS/NIR spectrometer from Perkin Elmer equipped with an 150 mm diameter integrating sphere. FTIR spectra were collected on a Thermo Scientific iS50 series. The surface areas of samples were measured on an Autosorb-1 system (Quantachrome, USA) from N2 adsorption isotherms obtained via the BET method. The thermal properties of the samples were evaluated using a TA Instruments SDT Q600 thermogravimetric analyzer. The temperature was steadily increased from 30 °C to 800 °C at N2 a rate of 10 °C min-1. Contact angle (CA) measurements on pelletized samples were done by JC2000C1 goniometer to determine hydrophilicity. The zeta potential was measured by Malvern Zetasizer Nano ZS90 via dispersing samples in water. Raman spectra were acquired on a Bruker SENTERRA Dispersive Raman Microscope with an excitation wavelength of 532 nm. The H2 temperature-programmed desorption (H2-TPD) was recorded with an infrared spectrometer (ThermoFisher Nicolet iS50) equipped with a Deuterated Triglycine Sulfate (DTGS) detector. Before CO2-TPD, catalyst samples (~100 mg) were pretreated in Ar at 50 mL min-1 to 200 °C for 1 h with a ramp rate of 5 °C min-1. After the sample was cooled down to 40 °C, it was exposed to CO2 (40 kPa, diluted in Ar) for adsorption until saturation. Then, CO2-TPD was performed in 30 mL min-1 Ar with a heating ramp of 5 °C min-1 from 40 to 700 °C. Pyridine infrared absorption spectroscopy (pyridine-lR) was performed using a Bruker Vertex 70 spectrometer, equipped with CaF2 windows. Each spectrum was collected at a resolution of 2 cm-1 using an average of 64 scans. Before spectrum collection, catalyst was pressed into a self-supporting wafer (~30 mg cm-2) and placed into a customized cell. After heating under Ar (50 mL min-1) at 200 °C for 2 h, a background spectrum was acquired. Pyridine (~0.1 kPa) was then introduced until saturation. Finally, the sample was evacuated to remove weakly adsorbed pyridine before spectroscopic measurements61,62.

In situ photocurrent measurement
The photocurrent measurements were carried out in a modified cryostat with atmospheric pressure control and temperature control. 1 cm by 1 cm gold electrodes of 300 nm thicknesses were evaporated onto a glass slide with a 1 mm gap between the two electrodes. The catalytic powder was then deposited over the gap and pressed down firmly with a glass pipette. Tungsten probe needles were pressed in the gold electrodes on the areas where no powder was present, and current measurements were taken with a Kiethley 4200 instrument. The sample was heated to 200 °C for 3 h under a vacuum pressure of 4E-2 mtorr before current measurements were conducted. Photo-illumination was provided using a Xenon-mercury lamp with 250 W power input. After the first current measurement, the lamp was turned on for 100 minutes. Gases were added until to 1.5 atm pressure and the sample was allowed to sit for 2 h in the gas atmosphere before measurements began. For the mixed atmosphere, H2 and CO2 gases were introduced at a 3:1 pressure ratio.

In situ TEM test
In situ TEM was performed with a single-tilt Hitachi Blaze heating holder on a Hitachi HF-3300 environmental TEM operated at 300 kV with a cold field-emission gun. EELS spectra were acquired with a 149 mrad collection angle. An energy dispersion of 0.25 eV channel-1 was used to record the Ni-L, Ga-L, and O-K edges. The zero-loss peak full width at half maximum indicated an energy resolution of 1.0–1.3 eV under these conditions. Experiments began at room temperature and then proceeded at 300 °C under H2 before switching to CO2 at the same temperature.

In situ X-ray photoelectron spectroscopy
In situ XPS spectra were obtained using Thermo Scientific ESCALAB XI+ XPS Microprobe. A high pass energy (100 eV) and low point density (1 point eV-1) were used for photoelectron detection, and all results were calibrated to C 1s (284.8 eV). The tests were conducted at room temperature under vacuum, 300 °C under Ar, 300 °C under H2 and 300 °C under CO2 + H2 (H2/CO2 = 3/1), respectively.

In situ DRIFTS measurements
DRIFT spectra were performed using a Thermo Scientific iS50 FT-IR equipped with a Harrick Praying Mantis DRIFT accessory. Spectra were collected at 4 cm-1 resolution by averaging 32 scans. Undiluted powder samples were placed directly into the sample holder. Before measurements, the cell was purged with He at 300 °C for 2 h with the flow rate of 20 mL min-1 to obtain the background. Then, H2 or D2 was introduced for 20 min at different temperatures, followed by a 3:1 H2:CO2 or D2/CO2 for 20 min at 300 °C the total gas flow was maintained at 20 mL min-1.

DFT calculations
Spin-polarized density function theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) in this work. Projector-augmented wave (PAW) pseudopotentials were applied to treat the core electrons63. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof functional (PBE) was adopted in geometric optimization and exploration of reaction pathway for CO2 reduction reaction64. The kinetic energy cutoff for the wave-function calculations was set to 500 eV. The Fermi smearing function was applied with a smearing width of 0.05 eV. The (311) surface of NiGa2O4 was used as the active surface, including 84 atoms with the lattice of a = 10.11 Å and b = 11.68 Å. The vacuum gap of ~15 Å was used to avoid interactions between the system and its mirror images. The geometric relaxation was stopped when the incremental changes in total energy and forces were smaller than 1 × 10-5 eV and 0.05 eV/Å, respectively. The van der Waals interaction was considered through the DFT-D3 method proposed by Grimme65. All the transition states were obtained using the climbing image nudged elastic band (CI-NEB) method with a convergence force smaller than 0.05 eV/Å66. The reaction free energy (ΔG) was corrected by considering the zero-point energy and entropy, as shown below:

ΔG = ΔE + ΔZPE − TΔS (1)

where ΔE, ΔZPE, and TΔS denote the reaction energy (or energy barrier), zero-point energy change, and entropy change, respectively.

For studying the role of light in H2 splitting, spin-polarized electronic structure calculations for bare NG, NG_H2, and NG_2H were further carried out for the PBE-relaxed geometries using the Heyd–Scuseria–Ernzerhof (HSE) functional67, with dipole and dispersion corrections included. The latter was done at the DFT-D3(BJ) level68, with the parameters for Becke-Johnson damping selected based on those for PBE069. Density of states were evaluated at 1000 grid points with Gaussian smearing, and Bader charges were evaluated using the program of Henkelman70. The ground state HSE wave function was then used as the input for TD-DFT calculations. The frequency-dependent dielectric matrix was first calculated with a total of 800 bands (512 occupied, 288 unoccupied), and the result was used to evaluate the Casida equation under the Tamm-Dancoff Approximation. For the latter, 20 occupied and unoccupied bands were used to obtain the imaginary dielectric function (simulating the absorption coefficient) and the oscillator strengths (simulating the probability of excitations). For all calculations at the HSE level, the plane wave cutoff was set to 600 eV, SCF convergence achieved at 10-5 eV, and the Brillouin zone sampled at the Γ point only. The GW version of the PAW pseudopotentials with the extensions sv (i.e. the s and p semi-core states are treated as valence states) were used for all elements.

Excitations with an oscillator strength greater than 0.1 and between 1.5 and 3.5 eV were selected for further analysis. With in linear-response TDDFT71, each excitation (i.e. each oscillator strength that gets printed) corresponds to a linear combination of all possible 1-electron interband transitions, within the number of occupied and unoccupied bands specified-often, a few of these transitions contribute a lot and most do not contribute much (on the other hand, an individual transition can also contribute to multiple excitations across several wavelengths). The contribution of an individual transition from an occupied band/orbital i to an unoccupied band/orbital j is given by the interband coupling constant Xij for that pair of orbitals. For each selected excitation, we filtered out all origin and destination bands/orbitals with Xij < 0.1, and the remaining were selected for analysis. For the selected 1-electron interband transitions, we plotted their charge density differences by subtracting the partial charge density of the origin orbital from the destination orbital (i.e. destination orbital minus origin orbital), and after visualization, we down-selected the ones based on the oscillator strengths of the excitations they contribute to and/or ones that have visible shifts in electron density on the adsorbate and NG surface (at an isosurface level of 10-4 e/Bohr3 unless otherwise indicated). Filtering and parsing of the VASP TDDFT outputs were done using an in-house written code. Charge density differences between the relevant bands were generated using VASPKIT72 and visualized with VESTA73.

Catalytic tests
The catalytic activity of the NG for CO2 hydrogenation with and without light under different temperatures were investigated in a Harrick Cell reactor at different temperatures with a H2:CO2 ratio of 3:1 (6 sccm H2 + 2 sccm CO2). The reaction was conducted at a pressure of 0.7 MPa under white LED light irradiation at an intensity of 1.87 W cm-2. The reactor cell volume was ~13.0 mL, the catalyst loading was ~15 mg, and the photoirradiation area was ~28.3 mm2. Catalyst temperature was controlled by copper heating block under the catalyst bed, the temperature of which was controlled by a temperature controller equipped with a OMEGA k-type thermocouple, resistive heating cartridge, and water-cooling37. CO2 and H2 flow rates were set to 2 and 6 sccm by two mass flow controllers (Alicat MC-2SCCM-D/5M). The concentrations of the products in the effluent gas were periodically sampled and analyzed by gas chromatography equipped with a flame Ionization detector (GC-FID) using Ar as the carrier gas. The quenching experiments on the SFLPs were carried out with pyrrole and pyridine. 50 mg NG sample was dispersed in 2 mL of pyrrole or pyridine and kept in the dark for 1 h to achieve adsorption-desorption equilibrium. The treated samples were then evacuated and used for the activity tests under the same photocatalytic conditions as other samples.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Photocatalytic hydrogenation of CO2 into methanol represents a promising approach for the preparation of value-added chemicals, but it remains challenge due to the low productivity of target product. In this work, the authors proposed a chlorophyll-mimicking NiGa2O4-x(OH)y catalyst, which contains oxygen vacancies, hydroxyl groups and valence-tunable Ni2+/Ni3+ species. This material showed excellent performance in conversion of CO2 to methanol with the assistance of light. A high methanol production rate of 3242 μmol h-1g-1 and methanol selectivity of 77% were simultaneously achieved. The structure-performance relationship and the reaction mechanism were clarified using various in situ spectroscopy and DFT calculation. Overall, this work is well-organized and it could be possible published after solving the following issues.\n1. The true role of surface oxygen vacancies is not clearly elucidated. High-temperature calcination treatment can induce the formation of more surface oxygen vacancies, but the performance of NG-300 is significantly reduced, in comparison with that of the un-treated sample (NG).\n2. The authors highlighted that the increased Ov accompanied by a decreased percentage of Ni3+ was observed over NG-300 (e.g. high temperature calcination). However, the results of EELS analysis gave an opposite conclusion, in which a higher oxidation state of Ni species was observed after calcination. This inconsistence needs more comments.\n3. The result of zeta potential of various catalysts is interesting, but relevant illustration is lacking. Meanwhile, the zeta potential of NiO and Ga2O3 was not provided in Figure S4.\n4. The number of control samples is not sufficient. The catalytic performance of NiO, NG-treated by H2, NG-treated by Ar (or other inert atmosphere) and NG-treated by H2O should also be evaluated.\n5. Since the results obtained on NiGa2O4-x(OH)y is remarkable, the authors were expected to deeply clarify the significance of this work in the field of photocatalytic hydrogenation, and simultaneously provides some opportune prospect.\n6. In page 8, catalytic CO2 methanation process? Some typos still existed in current manuscript and they need to be carefully corrected.", "reviewer_2": "The manuscript introduces a chlorophyll-mimicking green “nano-pigment” NiGa2Ox(OH)y, designed for discrete light/dark reactions at neighboring sites and proton-mediated charge transfer, leading to efficient photocatalytic CO2 hydrogenation to methanol. The catalyst demonstrates exceptional methanol production performance. However, the manuscript contains significant issues that must be addressed before it is considered for publication. Below are the detailed concerns:\n1. Please clearly distinguish between the samples NiGa2O4-x(OH)y and NiGa2Ox(OH)y to avoid confusion. If NG and NG-300 are used to represent NiGa2Ox(OH)y without and with calcination, please consistently use these abbreviations throughout the main text and SI. Additionally, revise Figure 5d to Figure 5g in Line 419. Note that the figure referred to as 5h in Line 400 is missing and should be addressed.\n2. Include details about NiO and Ga2O3 in Supplementary Figure 4.\n3. The XPS peak positions of the three oxygen species of NG and NG-300 in Figure 1e are inconsistent.\n4. The Ga 2p peaks of samples NG and NG-300 in Supplementary Figure 6 should be fitted for both samples, not just NG, to better illustrate the variation. Additionally, the caption incorrectly refers to Ga 3d instead of Ga 2p. Ensure that the fitting in Supplementary Figure 6 and Figure 3c maintains consistent FWHM values across different Ga species for clarity and accuracy. In fact, for the XPS peak fitting of almost all elements, the FWHM for the same species is inconsistent, which makes it difficult to correctly assign and determine the formation of FLP.\n5. It is plausible that the increase in oxygen vacancies could lead to a decrease in the Ni valence state. Therefore, please clarify the statement: \"Unexpectedly, the increased Ov accompanied by a decreased percentage of Ni(III) was observed over NG-300, which consequently affects the Ga oxidation state in the Ga-O-Ni sites\" (Lines 128–130).\n6. In Figure 1h, NG-300 shows a shift toward higher energy loss, indicating an increase in Ni valence state, which contradicts XPS results.\n7. The R space data of EXAFS needs to provide comparison with standard samples to prove the accuracy of the corresponding bonds of its peak positions.\n8. In Fig. 2b, explain the observed fluctuations in activity during the light-on/off experiment.\n9. Line 192 notes that light increases the catalyst's temperature. When calculating the activation energy, has the heating effect of light been accounted for?\n10. The manuscript attributes higher reactivity to increased oxygen vacancies. However, NG-300, with more oxygen vacancies, is less reactive than NG. Provide a detailed explanation for this discrepancy.\n11. Please provide additional evidence for the synergistic effect of the SFLP sites. For example, test the activity of the catalyst after quenching the acidic or basic sites.\n12. Supplementary experiments, such as NH₃-TPD and pyridine infrared testing, are recommended to detect and identify the acid-base sites of NG and NG-300.\n13. Include the formula and calculation process for determining the valence band and conduction band positions.", "reviewer_3": "The paper describes a new photocatalyst concept based on NiGa2Ox(OH)y, and demonstrates CO2 hydrogenation to methanol at a production rate of 3242 μmol g-1 h-1 with a selectivity of methanol of 77%. These values exceed significantly those reported previously in the field of photocatalysis, including values reported at similar conditions by the same author(s). The manuscript is well-structured, and presents results from a range of in-situ experimental techniques combined with DFT and TD-DFT simulations to identify the active sites in NiGa2Ox(OH)y, establish the role of light irradiation, and reveal a potential mechanism of CO2 hydrogenation to methanol over the NiGa2Ox(OH)y catalyst.\nHowever, there are several issues which require attention before the article can be considered acceptable for publication in Nature Communications. There are also some areas of detail where the manuscript could be improved.\nThe first issue relates to the claim that the performance “endows NiGa2Ox(OH)y as a potential practical candidate for industrial photocatalytic CO2 hydrogenation to methanol”. Is the catalyst such a step forward? The photocatalyst is new, and the performance exceeds the best previous study in the field, but it is still well short of thermal catalysis which achieves 10x greater production rate sustained over extended periods of time with a selectivity greater than 95%. The authors need to:\n• demonstrate that the new photocatalyst is stable over a significant period of time\n• discuss how the production rate could be improved by catalyst development and/or process intensification\n• provide information allowing the reader to calculate values of CO2 conversion etc (e.g. volume of reactor, mass of catalyst etc).\nThe second issue relates to the presence of CO. The residence time for the reactor appears to be quite long. Therefore, the atmosphere in the reactor is likely to be close to the exit concentrations. The CO production and the influence or otherwise of the presence of CO need to be considered, in the DFT calculations and the mechanism.\nIn the Introduction the research gap in the field and the motivation for developing the NiGa2Ox(OH)y catalyst should be added.\nThe areas of detail where the manuscript could be improved are as follows:\n1. There are two greens in the title!\n2. The XPS analysis is rather unconvincing. The binding energy of the OOH is usually around 533eV, whereas here for 300-H2 it is 531eV, almost the same as the Ov. The deconvolution of the Ni features looks rather subjective – what is the FWHM of Ni2+ and Ni3+ based on? How does the data for Ga in Fig 1g fit with the supplementary Fig 6?\n3. Is any other interpretation of the data possible – for example can NiO and Ga2O3 form a kind of heterojunction?\n4. There are some minor errors relating to Figure references and descriptions", "reviewer_4": "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": "Responses to Referee\nPoint-by-Point Response\nReviewer #1 (Remarks to the Author):\nPhotocatalytic hydrogenation of CO into methanol represents a promising approach for the\n2\npreparation of value-added chemicals, but it remains challenge due to the low productivity of target\nproduct. In this work, the authors proposed a chlorophyll-mimicking NiGa O OH) catalyst, which\n2 4-x( y\ncontains oxygen vacancies, hydroxyl groups and valence-tunable Ni2+/Ni3+ species. This material\nshowed excellent performance in conversion of CO to methanol with the assistance of light. A high\n2\nmethanol production rate of 3242 μmol h-1g-1 and methanol selectivity of 77% were simultaneously\nachieved. The structure-performance relationship and the reaction mechanism were clarified using\nvarious in situ spectroscopy and DFT calculation. Overall, this work is well-organized and it could be\npossible published after solving the following issues.\nAuthor Reply: We greatly appreciate the reviewer’s positive comments on the novelty and significance\nof our work. and thanks for your constructive comments, which help to improve the quality of our\nmanuscript. According to your opinion, we added additional experimental results, analysis, explanatory\nsentences, and corrected typos to address the reviewer’s concerns. Actions taken are indicated in red\nwhile explanatory sentences are shown in blue.\nQ1.\nThe true role of surface oxygen vacancies is not clearly elucidated. High-temperature calcination\ntreatment can induce the formation of more surface oxygen vacancies, but the performance of NG-300\nis significantly reduced, in comparison with that of the un-treated sample (NG).\nReply to Q1: We thank the reviewer for the valuable comments. An optimal concentration of oxygen\nvacancies (O ) is pivotal for constructing surface‑frustrated Lewis pairs (SFLPs), which are composed of\nv\nspatially adjacent yet sterically hindered Lewis acid and base sites. These SFLPs have emerged as\nhighly effective active centers for activating small molecules including CO , H , alkenes, dienes, and\n2 2\nalkynes. Typically, such bifunctional sites are engineered by creating isolated oxygen vacancies (O ) or\nv\ndesigning O -OH configurations on metal oxide surfaces. A representative example is the In O (OH) ,\nv 2 3-x y\nin which unsaturated In3+ cations (Lewis acid, generated by O ) interact cooperatively with nearby In-\nv\nOH groups (Lewis base) to facilitate efficient CO hydrogenation to methanol, as validated by both\n2\ntheoretical and experimental studies [Proc. Natl. Acad. Sci., 2016, 113(50): E8011-E8020; Nat.\nCommun, 2019, 10(1): 2521; Phys. Chem. Chem. Phys., 2015, 17, 14623-14635].\nIn our NiGa O OH) , O play a crucial role in forming SFLPs by spatially separating Ga(III) and\n2 4-x( y v\nadjacent Ni(II)/Ni(III) Lewis acid sites from O(-II) Lewis base sites. These SFLP sites facilitate the\nheterolytic splitting of hydrogen into hydride (H-) and proton (H+). However, efficient transport of these\nions is critical to the overall reaction. In this system, the Ni(II)/Ni(III) and hydroxyl groups (OH) serve\nas conduits for of H- and H+ transport, respectively, to the Ni site where they subsequently react with\nCO to form methanol. Although NG-300 has a higher O density, it has a lower amount of OH, which\n2 v\ninduces the lower activity of NG-300. Thus, the higher activity of NG results from multiple factors,\nincluding O , OH, and the Ni(II)/Ni(III) ratio, rather than from O alone.\nv v\nActions taken: The corresponding contents were added in the revised manuscript.\n…Surface frustrated Lewis pairs (SFLPs), composed of spatially adjacent yet sterically hindered Lewis\nacid and base sites, have emerged as highly effective active centers for activating small molecules\nincluding CO , H , alkenes, dienes, and alkynes. Typically, these bifunctional sites are engineered by\n2 2\ncreating isolated oxygen vacancies (O ) or by designing O -OH configurations on metal oxide surfaces20.\nv v\nA prominent example is the In O (OH) catalyst, where unsaturated In³+ cations (Lewis acid, generated\n2 3-x y\nby O ) cooperatively interact with proximal In-OH groups (Lewis base) to enable efficient CO\nv 2\nhydrogenation to methanol as validated by both theoretical and experimental studies21,22. …\n(Highlighted in Page 3 of the revised manuscript)\nQ2.\nThe authors highlighted that the increased O accompanied by a decreased percentage of Ni3+ was\nv\nobserved over NG-300 (e.g. high temperature calcination). However, the results of EELS analysis gave\nan opposite conclusion, in which a higher oxidation state of Ni species was observed after calcination.\nThis inconsistence needs more comments.\nReply to Q2: Many thanks to the reviewer for the valuable suggestions. The discrepancy between the\nXPS and STEM-EELS results may arise from variations in sampling depth between the two techniques\nand changes in Ni valence between the surface and subsurface layers. The STEM-EELS spectra in Fig.\n1h were collected from the ~5 nm interior of the nanoparticles, representing the subsurface signal,\nwhereas Ni 2p XPS probes only from the outer ~2 nm (the surface layer). These two methods can\ntherefore complement each other. XPS shows a slightly lower percentage of Ni(III) in NG-300, while\nEELS shows a higher average Ni valence, indicating a lower and higher valence state at the surface\nand subsurface, respectively, after calcination. This difference in Ni valence state from the surface to\nbulk is also observed in previous reports [Nature communications 12.1 (2021): 2350]. This behavior can\nbe attributed to calcination induced deprotonation, which releases H O by removing bridging OH\n2\nbonded to Ni-Ga and the neighboring H of terminal OH on Ni or Ga, then generating additional O\nv\nand exposing more lattice O (evidenced from O 1s XPS results). Consequently, different Ni valences\nare observed by XPS and EELS.\nActions taken: The corresponding contents were added in the revised manuscript.\n… However, the Above results can be further supported by electron energy loss spectroscopy (EELS)\nanalysis (Fig. 1h,i) indicate that the L /L ratio decreases as the Ni L -edge shifts to higher energy\n3 2 2,3\nlosses, indicating a higher Ni oxidation state accompanied by more O following calcination. The valence\nv\nstates of Ni were determined by calculating its white-line ratio (L /L ) (See Methods and Supplementary\n3 2\nTable 4). Further, a higher peak ratio of a/b for NG-300 compared to NG in EELS spectra of O K-\nedge further revealed the production of more O in NG-300. The discrepancy between the XPS and\nv\nSTEM-EELS results may arise from variations in sampling depth between the two techniques and changes\nin Ni valence between the surface and subsurface layers29. Deprotonation likely releases H O via\n2\ncoupling between the OH group of a bridging hydroxyl linked to Ni-Ga and the H atom of an adjacent\nterminal OH on Ni or Ga site, thereby generating additional O and exposing more lattice O…\nv\nConsequently, different Ni valences were observed at the surface and in the subsurface. … (Highlighted\nin Page 5 of the revised Supporting Information)\nQ3.\nThe result of zeta potential of various catalysts is interesting, but relevant illustration is lacking.\nMeanwhile, the zeta potential of NiO and Ga O was not provided in Figure S4.\n2 3\nReply to Q3: We appreciate the reviewer for bringing up this point. To highlight that NG exhibits a\nhigher density of OH groups than NG-300 based on zeta-potential measurements, we initially removed\nthe “NiO and Ga O ” curves from Supplementary Fig. 4 but forgot to remove “NiO and Ga O ” in the\n2 3 2 3\ncaption. In the revised Supplementary Fig. 4, we have restored the zeta-potential results for NiO and\nGa O . In addition, we have also added more relevant illustrations in the revised Supporting Information.\n2 3\nActions taken: The corresponding contents were added in the revised Supporting Information.\nNG-300=42.7 mV NG\nNG-300\nNiO=37.8 mV NiO\nGa2O3\nNG=47.6 mV\nu.)\na.\nnts\n(\nGa2O3=41.7 mV\nu\no\nC\n0 10 20 30 40 50 60 70 80\nPotential (mV)\nSupplementary Fig. 4| Zeta potential of the NiO, Ga O , NGNiGa O (OH) and NGNiGa O (OH) -300 dispersed\n2 3 2 4-x y 2 4-x y\nin deionized water at pH 7.\n…Zeta potential analysis was conducted to further support the reduction of surface hydroxyl groups on\nNG-300 compared to NG after calcination. As shown in Supplementary Fig. 4, the zeta potential\nabsolute value decreased from 47.6 mV (NG) to 42.7 mV (NG-300). This reduction in surface charge\ndensity can be attributed to the dehydration of surface hydroxyl groups during heat treatment, which\nreduces the availability of protonated hydroxyl species. The correlation between hydroxyl loss and\ncharge density decline is consistent with the observed decrease in wettability4,5. … (Highlighted in Page\n8 of the revised Supporting Information)\nQ4.\nThe number of control samples is not sufficient. The catalytic performance of NiO, NG-treated by H ,\nGa O , NiO/Ga O , and NiGa /SiO (NiGa refers to the NiGa alloy supported on SiO and reduced\n2 3 2 3 2 2 2 2\nin H at 700 oC), we have now included the activity results of NiO and NG samples treated under\n2\ndifferent atmospheres (N , H , and H O) at 300 oC, denoted as NG-N , NG-H , and NG-H O,\n2 2 2 2 2 2\nrespectively. The corresponding activity results are presented in Fig. R1 and the revised Supplementary\nFig. 9. According to these results, NiO mainly produces CO and CH , consistent with previous reports\n4\n[Nature Energy 6.8 (2021): 807-814; Nature Communications 15.1 (2024): 3115]. All the treated NG\nsamples exhibit lower methanol production rates. Among them, NG-N shows a similar activity to\n2 2\nThis difference in activity can be attributed to the surface structural changes induced by different\ntreatments. Treatments in H and H O can partially preserve OH. In contrast, treatments in N or air\n2 2 2\nmay lead to the removal of OH, thereby reducing the density or effectiveness of SFLP sites and leading\nto lower methanol production. These findings support our hypothesis that OH acts as proton (H+)\ntransport conduits, facilitating the hydrogenation of CO to methanol.\n2 2 2 2\ninto Supplementary Fig. 9).\nActions taken: The corresponding contents were added in the revised Supporting Information.\n…NG-N showed similar activity to NG-300, while NG-H and NG-H O NG-H displayed comparable\n2 2 2 2\nlevels activity. This difference in activity can be attributed to the surface structural changes caused by\ndifferent treatments. H and H O atmospheres can partially retain OH. In contrast, N or air atmospheres\n2 2 2\nmay lead to the removal of OH, thereby reducing the density or effectiveness of SFLP sites and leading\nto lower methanol production. These findings support the hypothesis that OH acts as proton (H+)\ntransport conduits, facilitating the hydrogenation of CO to methanol. … (Highlighted in Page 6 of the\nrevised manuscript)\nQ5.\nSince the results obtained on NiGa O (OH) is remarkable, the authors were expected to deeply\n2 4-x y\nclarify the significance of this work in the field of photocatalytic hydrogenation, and simultaneously\nprovides some opportune prospect.\nReply to Q5: We sincerely appreciate the reviewer's recognition of the remarkable catalytic\nperformance obtained on NiGa O (OH) and the constructive suggestion to strengthen the significance\n2 4-x y\nof our findings in the field of photocatalytic CO hydrogenation. In the revised manuscript, we have\n2\nelaborated on the key advances achieved with NiGa O (OH) in Introduction and Conclusion.\n2 4-x y\nAdditionally, we have expanded the discussion on a broader prospect of this work.\nActions taken: The corresponding contents were added in the revised manuscript.\n…The unique design of “nano-pigment” nickel gallium oxide (NiGa O (OH) ) integrates surface SFLPs,\n2 4-x y\nmixed-valence Ni(II)/Ni(III) sites, and OH groups to achieve excellent methanol production rates. This\noutstanding performance stems not only from its ability to mimic the optical response of chlorophyll but\nalso from its ability to capitalize on the dark and light cycles found in nature. These findings demonstrate\nsignificant progress in photocatalytic hydrogenation and provide a nature-inspired strategy for\ndesigning highly efficient photocatalysts. Ultimately, these insights form a solid foundation for the\ndevelopment of solar-driven refineries, paving the way for scalable and sustainable methanol\nsynthesis. … (Highlighted in Page 3 of Introduction in the revised manuscript)\n…The outstanding performance of NiGa O (OH) Our findings opens exciting avenues for the\n2 4-x y\ndevelopment of green chlorophyll mimics using heterogeneous photocatalytic systems and provides\nvaluable opportunities for the light-driven synthesis of sustainable commodity chemicals from\ngreenhouse gases, furthering humanity’s quest to decarbonize emission intensive industrial processes.\nFuture studies are expected to benefit from more efficient photon-capturing photocatalyst designs\nthrough active site engineering, as well as from careful optical and chemical optimization of\nphotoreactors based on catalyst shape and morphology. These advancements aim to further enhance\nand improve the activity, selectivity, and quantum efficiency of photo-to-methanol conversion60. …\n(Highlighted in Page 16 of Conclusion in the revised manuscript)\nQ6.\nIn page 8, catalytic CO methanation process? Some typos still existed in current manuscript and they\n2\nneed to be carefully corrected.\nReply to Q6: We thank the reviewer for drawing our attention to this point. In page 8, it should be\n“catalytic CO hydrogenation process”. We have revised it and checked the whole manuscript carefully\n2\nto avoid any grammar or syntax errors.\nReviewer #2 (Remarks to the Author):\nThe manuscript introduces a chlorophyll-mimicking green “nano-pigment” NiGa O (OH) , designed for\n2 x y\ndiscrete light/dark reactions at neighboring sites and proton-mediated charge transfer, leading to\nefficient photocatalytic CO hydrogenation to methanol. The catalyst demonstrates exceptional\n2\nmethanol production performance. However, the manuscript contains significant issues that must be\naddressed before it is considered for publication. Below are the detailed concerns:\nAuthor Reply: We thank the reviewer for their insightful evaluation and constructive suggestions, which\nhave greatly strengthened our manuscript. According to your opinion, we have incorporated additional\nexperimental data, analysis, explanatory sentences, and corrected all typos to address the reviewer’s\nconcerns. Actions taken are indicated in red while explanatory sentences are shown in blue.\nQ1.\nPlease clearly distinguish between the samples NiGa O (OH) and NiGa O (OH) to avoid confusion.\n2 4-x y 2 x y\nIf NG and NG-300 are used to represent NiGa2Ox(OH)y without and with calcination, please\nconsistently use these abbreviations throughout the main text and SI. Additionally, revise Figure 5d to\nFigure 5g in Line 419. Note that the figure referred to as 5h in Line 400 is missing and should be\naddressed.\nReply to Q1: We appreciate the reviewer for pointing out these problems. We have corrected all of\n“NiGa O (OH) ” to “NiGa O (OH) ”throughout the manuscript. For clarity, the abbreviations of NG\n2 x y 2 4-x y\nand NG-300 have been used consistently to present NiGa O (OH) before and after calcination in\n2 4-x y\nboth the main text and Supporting Information. We have also revised “Fig. 5d” to “Fig. 5g”, removed\nFigure 5h from the main text, and carefully checked the whole manuscript to avoid any remaining\nspelling or labeling errors.\nQ2.\nInclude details about NiO and Ga O in Supplementary Figure 4.\n2 3\nReply to Q2: We appreciate the reviewer for bringing up this point. To highlight that NG exhibits a\nhigher density of OH groups than NG-300 based on zeta-potential measurements, we initially removed\nthe “NiO and Ga O ” curves from Supplementary Fig. 4 but forgot to remove “NiO and Ga O ” in the\n2 3 2 3\ncaption. In the revised Supplementary Fig. 4, we have restored the zeta-potential results for NiO and\nGa O . In addition, we have also added more relevant illustrations in the revised Supporting Information.\n2 3\nActions taken: The corresponding contents were added in the revised Supporting Information.\nNG-300=42.7 mV NG\nNG-300\nNiO=37.8 mV NiO\nGa2O3\nNG=47.6 mV\nu.)\na.\nnts\n(\nGa2O3=41.7 mV\nu\no\nC\n0 10 20 30 40 50 60 70 80\nPotential (mV)\nSupplementary Fig. 4| Zeta potential of the NiO, Ga O , NGNiGa O (OH) and NGNiGa O (OH) -300 dispersed\n2 3 2 4-x y 2 4-x y\nin deionized water at pH 7.\n… Zeta potential analysis was conducted to further support the reduction of surface hydroxyl groups\non NG-300 compared to NG after calcination. As shown in Supplementary Fig. 4, the zeta potential\nabsolute value decreased from 47.6 mV (NG) to 42.7 mV (NG-300). This reduction in surface charge\ndensity can be attributed to the dehydration of surface hydroxyl groups during heat treatment, which\nreduces the availability of protonated hydroxyl species. The correlation between hydroxyl loss and\ncharge density decline is consistent with the observed decrease in wettability4,5. … (Highlighted in Page\n8 of the revised Supporting Information)\nQ3.\nThe XPS peak positions of the three oxygen species of NG and NG-300 in Figure 1e are inconsistent.\nReply to Q3: We sincerely thank the reviewer for this pointing this out. We have double-checked and re-\nfitted the O 1s XPS spectra to ensure the consistent assignment of the O species in both NG and NG-\n300. The revised results are shown in Fig. R2, Table R1, Fig. 1e and Supplementary Table 3.\nFig. R2. XPS spectra of O 1s for NG andNG-300. (This Figure is also shown in Fig. 1e).\nTable R1. XPS results of NG and NG-300 samples (This Table is also shown in Supplementary Table 3).\nSample O\nOH\nO\nv\nO\nlatt\nNi2+ Ni3+ Ga-o-Ga Ga-o-Ni\nNG position 532.8 531.5 530.4 855.4 857.5 1117.4 1118.0\nratio 28.4%34.3% 36.4%23.3% 35.2%42.4% 47.2%68.6% 52.8%31. 60.2% 39.8%\n4%\nFWHM 1.41 1.59 2.69 2.05 2.54 1.53 3.04\nposition 533.0 531.8 530.5 855.6 857.4 1117.3 1117.8\nNG- ratio 11.6%20.47% 52.0%27.45% 36.4%52.08% 51.0%71.4% 49.0%28. 67.1% 32.9%\n300 6%\nFWHM 1.44 1.63 2.71 2.14 2.50 1.57 3.04\nQ4.\nThe Ga 2p peaks of samples NG and NG-300 in Supplementary Figure 6 should be fitted for both\nsamples, not just NG, to better illustrate the variation. Additionally, the caption incorrectly refers to Ga\n3d instead of Ga 2p. Ensure that the fitting in Supplementary Figure 6 and Figure 3c maintains consistent\nFWHM values across different Ga species for clarity and accuracy. In fact, for the XPS peak fitting of\nalmost all elements, the FWHM for the same species is inconsistent, which makes it difficult to correctly\nassign and determine the formation of FLP.\nReply to Q4: We thank the reviewer for pointing this problem out. In the revised version, we have\ncarefully re-fitted the Ga 2p, Ni 2p and O 1s spectra shown in Supplementary Fig. 6, Fig. 1e,g and\nFig. 3a-c using identical FWHM constraints and peak assignment principles based on the references\n[Applied Surface Science 257.7 (2011): 2717-2730; Materials Letters 70 (2012): 189-192; Journal of\nElectroanalytical Chemistry 449.1-2 (1998): 91-100; ACS Applied Nano Materials (2025); Optical\nMaterials 37 (2014): 854-861], thereby ensuring consistency within each element and enabling accurate\nidentification of SFLP formation. All of the results have been summarized in Supplementary Table 3, 9-\n11. We have also corrected the caption for Supplementary Fig. 6, which had mistakenly referred to\nGa 3d instead of Ga 2p.\nActions taken: The corresponding Figure and Table were added in the revised main manuscript and\nSupplemental Information.\nGa 2p\nGa-O-Ga\nu.) Ga-O-Ni\nnsity\n( a. NG-300\ne\nnt\nI\nNG\n1124 1122 1120 1118 1116 1114 1112\nBinding Energy (eV)\nSupplementary Fig. 6| XPS spectra of Ga 3d 2p for NGNiGa O (OH) andNGNiGa O (OH) -300.\n2 4-x y 2 4-x y\nFig. 3 | Identification of Active Sites. a-c, In situ XPS spectra of O (a), Ni (b) and Ga (c) of NGNiGa O (OH)\n2 4-x y\nobtained at 300 °C with different reaction conditions of Ar, H and CO +H (CO /H =1/3), respectively.\n2 2 2 2 2\nSupplementary Table 3| XPS results of NG and NG-300 samples.\nSample O\nOH\nO\nv\nO\nlatt\nNi2+ Ni3+ Ga-O- Ga-O-\nGa Ni\nPosition 532.8 531.5 530.4 855.4 857.3 1117.4 1118.0\n(eV)\nPercentage 28.4%34.3% 36.4%23.3% 35.2%42.4% 47.2%68.6% 52.8%31.4% 60.2% 39.8%\nNG\n(%)\nFWHM 1.41 1.59 2.69 2.05 2.54 1.53 3.04\n(eV)\nPosition 533.0 531.8 530.5 855.6 857.4 1117.3 1117.8\n(eV)\nNG- Percentage 11.6%20.5% 52.0%27.5% 36.4%52.0% 51.0%71.4% 49.0%28.6% 67.1% 32.9%\n300 (%)\nFWHM 1.44 1.63 2.71 2.14 2.50 1.57 3.04\n(eV)\nSupplementary Table 9| In situ XPS results of O 2p over NG under different reaction conditions.\nReaction\nO O O\nlatt v OH\nconditions\nPosition Percentage FWHM Position Percentage FWHM Position Percentage FWHM\n(eV) (%) (%) (%)\n(eV) (eV) (eV) (eV) (eV)\nRT-Ar 530.4 42.4% 1.41 531.5 23.3% 1.59 532.8 34.3% 2.69\n300-Ar 530.4 46.7% 1.43 531.6 24.8% 1.61 532.8 28.5% 2.60\n300-H 530.2 70.9% 1.42 531.5 20.6% 1.57 532.4 8.5% 2.66\n2\n300-CO +H 530.4 53.5% 1.44 531.6 30.5% 1.63 532.5 16.0% 2.69\n2 2\nSupplementary Table 10| In situ XPS results of Ni 2p over NG under different reaction conditions.\nReaction conditions Ni2+ Ni3+\nPosition Percentage FWHM Position Percentage FWHM\n(eV)\n(%) (eV) (eV) (%) (eV)\nRT-Ar 855.4 68.6% 2.05 857.3 31.4% 2.54\n300-Ar 855.3 63.2% 2.00 857.0 36.8% 2.53\n300-H 855.1 73.1% 1.94 856.9 26.9% 2.51\n2\n300-CO +H 855.3 66.1% 1.90 856.9 33.9% 2.52\n2 2\nSupplementary Table 11| In situ XPS results of Ga 2p over NG under different reaction conditions.\nReaction conditions Ga-O-Ga Ga-O-Ni\nPosition Percentage FWHM Position Percentage FWHM\n(eV)\n(%) (eV) (eV) (%) (eV)\nRT-Ar 1117.4 60.2% 1.53 1118.0 39.8% 3.04\n300-Ar 1117.4 57.5% 1.54 1118.0 42.5% 3.04\n300-H 1117.4 63.3% 1.60 1118.0 36.7% 3.04\n2\n300-CO +H 1117.3 61.0% 1.55 1117.9 39.0% 3.04\n2 2\nQ5.\nIt is plausible that the increase in oxygen vacancies could lead to a decrease in the Ni valence state.\nTherefore, please clarify the statement: \"Unexpectedly, the increased O accompanied by a decreased\nv\npercentage of Ni(III) was observed over NG-300, which consequently affects the Ga oxidation state in\nthe Ga-O-Ni sites\" (Lines 128–130).\nReply to Q5: We appreciate the reviewer for bringing up this question, we are sorry for the incorrect\nwriting of this sentence in the original manuscript. Yes, it is plausible that the increased oxygen vacancies\ncan lead to a decreased Ni valence, which can be confirmed by Ni 2p XPS results (Fig. 1g). We have\nrewritten this part as follows “Unexpectedly, the decreased OH resulting in increased O and O ,\nv latt\naccompanied by After calcination, NG-300 shows a slightly lower percentage of Ni(III) was\nobserved over NG-300, which consequently affects Ga oxidation state in the Ga-O-Ni sites, as\nshown in Fig. 1g and Supplementary Fig. 6.”\nQ6.\nIn Figure 1h, NG-300 shows a shift toward higher energy loss, indicating an increase in Ni valence state,\nwhich contradicts XPS results.\nReply to Q6: We appreciate the reviewer for bringing up this problem. The discrepancy between the\nXPS and STEM-EELS results may arise from variations in sampling depth between the two techniques\nand changes in Ni valence between the surface and subsurface layers. The STEM-EELS spectra in Fig.\n1h were collected from the ~5 nm interior of the nanoparticles, representing the subsurface signal,\nwhereas Ni 2p XPS probes only from the outer ~2 nm (the surface layer). These two methods can\ntherefore complement each other. XPS shows a slightly lower percentage of Ni(III) in NG-300, while\nEELS shows a higher average Ni valence, indicating a lower and higher valence state at the surface\nand subsurface, respectively, after calcination. This difference in Ni valence state from the surface to\nbulk is also observed in previous reports [Nature communications 12.1 (2021): 2350]. This behavior can\nbe attributed to calcination induced deprotonation, which releases H O by removing bridging OH\n2\nbonded to Ni-Ga and the neighboring H of terminal OH on Ni or Ga, then generating additional O\nv\nand exposing more lattice O (evidenced from O 1s XPS results). Consequently, different Ni valences\nare observed by XPS and EELS.\nActions taken: The corresponding contents were added in the revised manuscript.\n… However, the Above results can be further supported by electron energy loss spectroscopy (EELS)\nanalysis (Fig. 1h,i) indicate that the L /L ratio decreases as the Ni L -edge shifts to higher energy\n3 2 2,3\nlosses, indicating a higher Ni oxidation state accompanied by more O following calcination. The\nv\nvalence states of Ni were determined by calculating its white-line ratio (L /L ) (See Methods and\n3 2\nSupplementary Table 4). Further, a higher peak ratio of a/b for NG-300 compared to NG in EELS\nspectra of O K-edge further revealed the production of more O in NG-300. The discrepancy\nv\nbetween the XPS and STEM-EELS results may arise from variations in sampling depth between the\ntwo techniques and changes in Ni valence between the surface and subsurface layers29.\nDeprotonation likely releases H O via coupling between the OH group of a bridging hydroxyl\n2\nlinked to Ni-Ga and the H atom of an adjacent terminal OH on Ni or Ga site, thereby generating\nadditional O and exposing more lattice O…\nv\n…Consequently, different Ni valences were observed at the surface and in the subsurface. …\n(Highlighted in Page 5 of the revised Supporting Information)\nQ7.\nThe R space data of EXAFS needs to provide comparison with standard samples to prove the accuracy\nof the corresponding bonds of its peak positions.\nReply to Q7: We appreciate the reviewer for this valuable suggestion. In the revised manuscript, we\nhave added R-space EXAFS data of relevant standard compounds, including Ni foil, NiO, Ga foil, and\nGa O . As shown in the Fig. R3 and revised Fig. 4g,h, the peak positions in our samples closely match\n2 3\nthose of the NiO and Ga O standards, while no correspondence is observed with the metal foils. This\n2 3\nconfirms the accuracy of the bond assignments and further supports our structural interpretation of the\nEXAFS data.\nFig. R3. In situ EXAFS k2χ(k) Fourier transform (FT) spectra of Ni L-edge (g) and Ga K-edge (h) over NG. (This Figure is\nalso combined in Fig. 4g,h).\nActions taken: The corresponding contents and Fig. R3 were added in the revised manuscript.\n…The peak positions in our samples closely match those of the NiO and Ga O standards, while no\n2 3\ncorrespondence is observed with the metal foils, further supporting the structural interpretation. …\n(Highlighted in Page 12 of the revised manuscript)\nQ8.\nIn Fig. 2b, explain the observed fluctuations in activity during the light-on/off experiment.\nReply to Q8: We thank the reviewer for this insightful question. Two types of fluctuations can be\nobserved in Fig. 2b. (1) Lag between the CH OH rate increasing or decreasing slower than that of CO\n3\nwith light on and light off, respectively. (2) Irregular fluctuations in CH OH production rates, while the\npathways for CO and CH OH production. The CH OH proceeds by a multistep hydrogenation pathway\n3 3\ninvolving intermediates such as HCOO*, HCOOH*, H COOH*, H CO* and H CO*. This pathway\n2 2 3\nheavily relies on surface proton transfer facilitated by hydroxyl groups and is more sensitive to surface\nelectronic and structural changes. The intermittent CH OH production likely results from the dynamic\n3\nformation and stabilization of these intermediates, which can be influenced by transient charging, local\nsite availability, or competing surface reactions. The relatively steady CO rate compared to the\nfluctuating CH OH rate suggests that CO is not a rate-determining intermediate in the methanol\n3\nformation pathway and its desorption or diffusion is less sensitive to transient surface conditions.\nAs requested by another reviewer, we have revised the stability test for longer operating time (150\nh) without light-on/off, and the updated results are shown in the revised Fig. 2b.\nQ9.\nLine 192 notes that light increases the catalyst's temperature. When calculating the activation energy,\nhas the heating effect of light been accounted for?\nReply to Q9: We appreciate the reviewer’s insightful comment. In the original manuscript, we did not\naccount for the heating effect due to light-induced temperature changes during our activation energy\ncalculation. To address this oversight, we measured the surface temperature under both dark and light\nconditions using an IR camera and recalculated the activation energy (E ). The results shown in\na\nSupplementary Fig. 13 indicate a significant decrease in E for the light-driven reaction compared to\na\nthe dark condition. This decrease supports the synergistic role of photoexcitation in lowering the energy\nbarrier for CO and methanol production. The reduced activation energy under illumination might be\nattributed to the participation of photo-generated charge carriers in the reaction, consistent with\nprevious reports on photocatalysts such as In O [Nature Communications 13.1 (2022): 1512; Advanced\n2 3\nScience 5.6 (2018): 1700732], where light-induced E reduction has been observed due to\na\nphotoexcitation effectively complementing thermal activation.\nActions taken: The following contents have been added in revised Supporting Information.\n…The activation energies (E ) were calculated using the surface temperatures derived from Arrhenius\na\nplots of NG for the production of CO and CH OH under both dark and light conditions, as shown in\nQ10.\nThe manuscript attributes higher reactivity to increased oxygen vacancies. However, NG-300, with more\noxygen vacancies, is less reactive than NG. Provide a detailed explanation for this discrepancy.\nReply to Q10: We appreciate the reviewer’s valuable comment. In our work, NG achieved higher\nactivity, we attributed the higher activity to its unique design. First, NG features the surface-frustrated\nLewis pairs (SFLPs) sites. Thereinto, oxygen vacancies (O ) play a key role in forming SFLPs by spatially\nv\nseparating Ga(III) and adjacent Ni(II)/Ni(III) Lewis acid sites from O(-II) Lewis base sites. These SFLPs\nsites facilitate the heterolytic splitting of hydrogen into hydride (H-) and proton (H+) ions. However, the\nefficient transport of these ions to the activate sites is critical to the overall reaction. In our system, the\nNi(II)/Ni(III) and hydroxyl groups (OH) serve as conduits for of H- and H+ transport, respectively, to the\nNi site where they subsequently react with CO to form methanol. Thus, the decreased activity observed\n2\nin NG-300 can be attributed to its lower OH content, which hinders the effective transport of H- and H+\ndespite the presence of oxygen vacancies.\nActions taken: The following contents have been added in revised manuscript.\n…All CO and methanol production activity rates were substantially lower than that of when compared\nto NG, highlighting implying the importance of having O , OH, and/or multiple Ni oxidation states\nv\nsimultaneously during for methanol synthesis…. (Highlighted in Page 6 of the revised manuscript)\nQ11.\nPlease provide additional evidence for the synergistic effect of the SFLP sites. For example, test the\nactivity of the catalyst after quenching the acidic or basic sites.\nReply to Q11: We appreciate the reviewer’s valuable suggestions. To address this, complementary\nexperiments were conducted in which Lewis acidic and Lewis basic sites were selectively quenched using\npyridine and pyrrole, respectively [Nature Communications 14.1 (2023): 2000; Chem Catalysis 2.7\n(2022): 1775-1792]. The activity results are shown in Fig.R4 and Supplementary Fig. 11, methanol\nproduction activity decreased to approximately 75 % upon acid site poisoning and dropped to near\nzero upon basic site quenching. These observations indicate that both sites are required to work\ncooperatively for effective CO hydrogenation to methanol, with the basic sites playing the critical role\n2\nin this process.\n3500 90\nCO Dark\n- - 1 1 · ) g\nat\ne\n(\nμ m\nol· h\n1\n2\n2 3\n5\n0\n5 0\n0\n0\n0 0\n0\n0\n0 0 D\nL\nM C M\ni\na\ng\nO e e r\nh\nO O k\nt\nL H H ig h D L t i a g r h k t\n1\n7\n8\n0\n0\nel\necti\nvit\ny ( %)\nn\nr\nH\no O\nd u\ncti1000\nH3\no 500\nPr\nFig. R4. CO and CH OH production rates of NG with blocking Lewis acid or Lewis base sites, respectively. (This Figure\n3\nis also shown in Supplementary Fig. 11).\nActions taken: The following contents have been added in the revised manuscript.\n…Further, to investigate the synergistic effect of SFLP sites on CO hydrogenation activity, quenching\n2\nexperiments were conducted using pyridine and pyrrole to selectively block the Lewis acid and base\nsites, respectively. The results presented in Supplementary Fig. 11 demonstrate that deactivation of\neither the acidic or basic site leads to a significant decrease in the CO hydrogenation activity.\nand dropped to near zero upon basic site quenching. These observations indicate that both sites are\nrequired to work cooperatively for effective CO hydrogenation to methanol, with basic sites playing\n2\na more critical role in this process. … (Highlighted in Page 6 of the revised manuscript)\n…The quenching experiments on the SFLPs were carried out with pyrrole and pyridine. 50 mg NG\nsample was dispersed in 2 mL of pyrrole or pyridine and kept in the dark for 1 h to achieve an\nadsorption-desorption balance. The treated samples were then evacuated and used for the activity\ntests under the same the photocatalytic conditions as the other samples. … (Highlighted in Page 19 of\nthe revised manuscript)\nQ12.\nSupplementary experiments, such as NH -TPD and pyridine infrared testing, are recommended to detect\n3\nand identify the acid-base sites of NG and NG-300.\nReply to Q12: We appreciate the reviewer’s valuable suggestions. FT-IR of pyridine adsorption and\nCO -TPD analysis were carried out to identify the Lewis acidic and basic sites in NG and NG-300, the\n2\nresults are shown in Fig. R5. Pyridine-IR spectra reveal predominantly Lewis acid sites in both samples,\nwith NG exhibiting a higher acid sites density than NG-300. CO -TPD results show both NG and NG-\n2\n300 exhibit three distinct desorption peaks corresponding to weak (<150 °C), moderate (150-220 °C)\nand strong (>220 °C) basic sites. Notably, the CO desorption peaks for NG occur at higher\n2\ntemperatures and with greater intensity than those for NG-300, suggesting enhanced basic sites strength\nand density [Nature Communications 13.1 (2022): 2320]. Previous studies have shown that overall\nbasicity of metal oxides decreases with increasing calcination temperature [Green Chemistry 6.4 (2004):\n206-214]. These findings are therefore consistent with our observations that NG shows a comparatively\ngreater acidity and basicity than NG-300.\nFig. R5. Characterization of acid and base properties of NG and NG-300. a pyridine-IR spectra. b CO -TPD. (This\nActions taken: The following contents have been added in the revised manuscript and Supporting\nInformation.\n… Changes in the Lewis acidic and basic sites between NG and NG-300 were characterized by\npyridine-IR spectroscopy and CO -TPD, as shown in Supplementary Fig. 8. …. (Highlighted in Page 5\n2\nof the revised manuscript)\n…Pyridine-IR analysis revealed predominant Lewis acid sites in both NG and NG-300, with NG\nexhibiting stronger acidity. CO -TPD results show three distinct desorption peaks at weak (<150 °C),\n2\nmoderate (150-220 °C) and strong (above 220 °C) basic sites. Compared to NG-300, CO peaks in\nand density8. Moreover, previous studies have demonstrated that the overall basicity of metal oxides\ntends to decrease with higher calcination temperatures9. These findings are consistent with our\nobservations, further confirming that NG exhibits comparatively higher basicity than NG-300. …\n(Highlighted in Page 12 of the revised Supporting Information)\nQ13.\nInclude the formula and calculation process for determining the valence band and conduction band\npositions.\nReply to Q13: We appreciate the reviewer’s suggestions. The formula and calculation processes for\ndetermining the valence band and conduction band positions have been added in Supporting\nInformation.\nActions taken: The following contents have been added in Supplementary Note 4 of Supporting\nInformation.\n…Supplementary Note 4. The calculation of valence band and conduction band positions.\nFirst, the band gap energy (E ) can be calculated according to the equation (8)7:\ng\nhv= A(hv − E )n/2 (8)\ng\nwhere , h, v and A are the absorption coefficient, Planck constant, light frequency and proportionality,\nrespectively; n is decided from the properties of the transition in a semiconductor. NG and NG-300 are\ndirect band semiconductor and the value of n is 1.\nThus, according to the Tauc plots results in Supplementary Fig. 22, the E for NG and NG-300 can be\ng\ncalculated to be 3.82 eV and 3.49 eV.\nSecond, from UPS spectrum shown in Supplementary Fig. 23, the valence band (E ; vs Fermi) can be\nVB\ndetermined by fitting a straight line into the leading edge; the end energy level (EEL) can be determined\nby fitting a straight line into the leading edge8.\nThus, for NG, E (vs Fermi) and EEL are 1.84 and 16.3 eV; for NG-300, E (vs Fermi) and EEL are\nVB VB\n1.91 and 16.3 eV.\nThird, work function () can be considered as negative Fermi energy (E ) and can be calculated\nF\naccording to the equation (9):\n = 21.22 – EEL (9)\nwhere 21.22 is the light source used in the test. Thus,  for NG and NG-300 both are 4.92.\nThe valence band (E ; vs vacuum) can be calculated according to the equation (10):\nVB\nE (vs vacuum) = E - E (vs Fermi) (10)\nVB F VB\nThe E (vs vacuum) can be calculated according to the equation E =E +E\nCB CB VB g\nThen, for NG, E (vs vaccum) = -4.92-1.8=-6.76; E (vs vaccum) = -6.76+3.82=-2.94\nVB CB\nFor NG-300, E (vs vaccum) = -4.92-1.91=-6.83; E (vs vaccum) = -6.83+3.49=-3.34\nCB\n…\n(Highlighted in Page 3-4 of the revised Supporting Information)\nReviewer #3 (Remarks to the Author):\nThe paper describes a new photocatalyst concept based on NiGa O (OH) , and demonstrates CO\n2 x y 2\nhydrogenation to methanol at a production rate of 3242 μmol g-1 h-1 with a selectivity of methanol of\n77%. These values exceed significantly those reported previously in the field of photocatalysis, including\nvalues reported at similar conditions by the same author(s).\nThe manuscript is well-structured, and presents results from a range of in-situ experimental techniques\ncombined with DFT and TD-DFT simulations to identify the active sites in NiGa O (OH) , establish the\n2 x y\nrole of light irradiation, and reveal a potential mechanism of CO hydrogenation to methanol over the\n2 x y\nHowever, there are several issues which require attention before the article can be considered\nacceptable for publication in Nature Communications. There are also some areas of detail where the\nmanuscript could be improved.\nAuthor Reply: We greatly appreciate the reviewer’s positive evaluation and constructive comments,\nwhich help to improve the quality of our manuscript. According to your opinion, we added additional\nexperimental results, analysis, explanatory sentences, and corrected typos to address the reviewer’s\nconcerns. Actions taken are indicated in red while explanatory sentences are shown in blue.\nQ1.\nThe first issue relates to the claim that the performance “endows NiGa O (OH) as a potential practical\n2 x y\ncandidate for industrial photocatalytic CO hydrogenation to methanol”. Is the catalyst such a step\n2\nforward? The photocatalyst is new, and the performance exceeds the best previous study in the field,\nbut it is still well short of thermal catalysis which achieves 10x greater production rate sustained over\nextended periods of time with a selectivity greater than 95%. The authors need to:\n• demonstrate that the new photocatalyst is stable over a significant period of time\n• discuss how the production rate could be improved by catalyst development and/or process\nintensification.\n• provide information allowing the reader to calculate values of CO conversion etc (e.g. volume of\n2\nreactor, mass of catalyst etc).\nReply to Q1: We appreciate the reviewer for their valuable comments and suggestions. Following your\nsuggestion, the photocatalytic activity with longer time was supplied, as shown in the revised Fig. 2b.\nThe stability of NG was studied at 300 oC (Fig. 2b) for 200 h, during which the CH OH production rate\n3\ncontinuously increased over the first 110 h and then decreased thereafter.\nThe newly discovered NiGa O (OH) catalyst exhibits a promising methanol production rate, but it still\n2 4-x y\nhas room for improvement via catalyst engineering and photoreactor design. From the perspective of\ncatalyst design, strategies such as active site engineering through doping or the incorporation of single\natom catalytic (SAC) sites can enhance performance. In addition, optimizing the catalyst’s shape and\nmorphology, together with careful optical and chemical design of the photoreactor, can maximize\nphoton capture while minimizing losses caused by light scattering, reflection, transmission, and parasitic\nabsorption [Cell Rep. Phys. Sci., 2021, 2(1): 100300; Chem. Soc. Rev., 2020, 49(16): 5648-5663;\nJoule 2023, 7, 1347; CEJ, 2023, 452, 139204; Matter, 2022, 5(9), 2594]. Thus, our future studies\nwill focus on engineering both the photocatalyst and the photoreactor to further improve the\nperformance.\nAs the reviewer’s suggestions, more detailed information have been provided for activity rates’\ncalculation.\nActions taken: The following contents have been added in the revised manuscript.\n…. Future studies are expected to benefit from more efficient photon-capturing photocatalyst designs\nthrough active site engineering, as well as from careful optical and chemical optimization of\nphotoreactors based on catalyst shape and morphology. These advancements aim to further enhance\nand improve the activity, selectivity, and quantum efficiency of photo-to-methanol conversiton60. .…\n(Highlighted in Page 16 of Conclusion in the revised manuscript)\n…The reaction was conducted at a pressure of 0.7 MPa under white LED light irradiation with the\nintensity of 1.87 W·cm-2. (Pressure: of 0.7 MPa. Light: White LED light, 1.87 Wcm2. Other experimental\ndetails are shown in the Methods section). The reactor cell volume is 113.0 mm3, the catalyst loading is\n~15 mg, and the photoirradiation area is 28.3 cm2.… (Highlighted in Page 19 of the revised manuscript)\nQ2.\nThe second issue relates to the presence of CO. The residence time for the reactor appears to be quite\nlong. Therefore, the atmosphere in the reactor is likely to be close to the exit concentrations. The CO\nproduction and the influence or otherwise of the presence of CO need to be considered, in the DFT\ncalculations and the mechanism.\nReply to Q2: We appreciate the reviewer’s valuable suggestions. In response, we have investigated\nthe influence of CO presence during the reaction process on NG. First, we evaluated seven possible\nadsorption sites for CO* on the NG surface (Supplementary Figure 38). Among these, the Ni-4 site\nexhibits the most stable adsorption energy, confirming it as the preferred configuration. Consequently,\nall subsequent calculations modeling CO co-adsorption utilized the Ni-4 site. Given that heterolytic H\n2\nsplitting and CO activation are the key steps in CO hydrogenation to methanol over NG, we focused\n2 2\nour analysis on these processes under CO presence (Supplementary Fig. 39). The calculated reaction\nfree energies for H splitting and the first CO hydrogenation step are 0.58 eV and 0.69 eV,\n2 2\nrespectively, with co-adsorbed CO. These values are slightly different from those without CO (0.43 eV\nand 0.79 eV), indicating a small influence of CO adsorption. Notably, while CO presence modestly\nincreases the energy barrier for H splitting (ΔG = +0.15 eV), it facilitates CO hydrogenation (ΔG =\n2 2\n-0.10 eV). This acceleration of the CO activation step is particularly advantageous for methanol\n2\nsynthesis.\nActions taken: The following contents have been added in the revised manuscript.\n…To investigate the influence of CO presence during the reaction process on NG, seven possible\nadsorption sites for CO* on the NG surface were considered, as shown in Supplementary Fig. 38.\nAmong these, the Ni-4 site exhibits the most stable adsorption energy, confirming it as the preferred\nconfiguration. Consequently, all subsequent calculations modeling CO co-adsorption utilized the Ni-4\nsite. Given that heterolytic H splitting and CO activation are the key steps in CO hydrogenation to\n2 2 2\nmethanol over NG, we focused our analysis on these processes under CO presence (Supplementary\nFig. 39). The calculated reaction free energies for H splitting and the first CO hydrogenation step are\n2 2\n0.58 eV and 0.69 eV, respectively, with co-adsorbed CO. These values are slightly different from those\nwithout CO (0.43 eV and 0.79 eV), indicating minor influence from CO adsorption. Notably, while CO\npresence modestly increases the energy barrier for H splitting (0.15 eV), it facilitates CO\n2 2\nhydrogenation (-0.10 eV). This acceleration of the CO activation step is particularly advantageous for\n2\nmethanol synthesis. …\n(Highlighted in Page 15 of the revised manuscript)\nQ3.\nIn the Introduction the research gap in the field and the motivation for developing the NiGa O (OH) \n2 x y\ncatalyst should be added.\nReply to Q3: We appreciate the reviewer’s valuable suggestions. We have provided more information\nand motivation for developing our new catalyst NiGa O (OH) . The relative contents have been\n2 4-x y\nadded in Introduction and Conclusion part of our revised manuscript.\nActions taken: The following contents have been added in the revised manuscript.\n… Surface frustrated Lewis pairs (SFLPs), composed of spatially adjacent yet sterically hindered Lewis\nacid and base sites, have emerged as highly effective active centers for activating small molecules\nincluding CO , H , alkenes, dienes, and alkynes. Typically, these bifunctional sites are engineered by\n2 2\ncreating isolated oxygen vacancies (O ) or by designing O -OH configurations on metal oxide\nv v\nsurfaces20. A prominent example is the In O (OH) catalyst, where unsaturated In³+ cations (Lewis acid,\n2 3-x y\ngenerated by O ) cooperatively interact with proximal In-OH groups (Lewis base), enabling efficient\nv\nCO hydrogenation to methanol as validated by both theoretical and experimental studies21,22.\nmore efficient SFLP-based catalyst for photocatalytic CO -to-methanol production. Thus, to find an\n2\nalternative that circumvents this problem yet retains the attributes of defect engineered indium oxide,\nwe turn to gallium oxide-based as it has been extensively studied as a promoter or support for methanol\nsynthesis. However, it suffers from the poor light absorption due to its wide band gap. … (Highlighted\nin Page 3 of Introduction in the revised manuscript)\n…Combining the natural process and the important role of -OH and O during CO hydrogenation, we\nv 2\nhave developed a chlorophyll-mimicking green “nano-pigment”, nickel gallium oxide (NiGa O (OH) ),\n2 4-x y\nwhich consists of O and hydroxyl groups (OH) and mixed-valence Ni(II,III) sites within an inverse spinel\nv\nstructure (space group Fd3m; inversion parameter of 92%)23,24. Notably, edge-sharing Ni(II,III) and\nGa(III) octahedra endows the material with a high electrical conductivity favorable for electron transfer\nand ion diffusion25,26.… (Highlighted in Page 3 of Introduction in the revised manuscript)\n… The unique design of “nano-pigment” nickel gallium oxide (NiGa O (OH) ) integrates surface SFLPs,\n2 4-x y\nmixed-valence Ni(II)/Ni(III) sites, and OH groups to achieve excellent methanol production rates. This\noutstanding performance stems not only from its ability to mimic the optical response of chlorophyll but\nalso from its ability to capitalize on the dark and light cycles found in nature. These findings demonstrate\nsignificant progress in photocatalytic hydrogenation and provide a nature-inspired strategy for\ndesigning highly efficient photocatalysts. Ultimately, these insights form a solid foundation for the\ndevelopment of solar-driven refineries, paving the way for scalable and sustainable methanol\nsynthesis. … (Highlighted in Page 3 of Introduction in the revised manuscript)\n… The outstanding performance of NiGa O (OH) Our findings opens exciting avenues for the\n2 4-x y\ndevelopment of green chlorophyll mimics using heterogeneous photocatalytic systems and provides\nvaluable opportunities for the light-driven synthesis of sustainable commodity chemicals from\ngreenhouse gases, furthering humanity’s quest to decarbonize emission intensive industrial processes.\nFuture studies are expected to benefit from more efficient photon-capturing photocatalyst designs\nthrough active site engineering, as well as from careful optical and chemical optimization of\nphotoreactors based on catalyst shape and morphology. These advancements aim to further enhance\nand improve the activity, selectivity, and quantum efficiency of photo-to-methanol conversion60. …\n(Highlighted in Page 16 of Conclusion in the revised manuscript)\nThe areas of detail where the manuscript could be improved are as follows:\nQ4.\nThere are two greens in the title!\nReply to Q4: We thank for the reviewer for bringing up this point. We intentionally used two “greens”\nin the previous title of “Green NiGa O (OH) Chlorophyll Mimic for Green Methanol Synthesis”. The\n2 4-x y\nfirst green describes the color of our catalyst NiGa O (OH) , which exhibits green. The second green\n2 4-x y\nhighlights that the methanol synthesis process is environmentally friendly. Photocatalysis typically\nharnesses sunlight, which is a renewable and clean energy source, to drive chemical reactions. Light-\ndriven CO hydrogenation to methanol offers a sustainable fuel and a critical route to carbon neutrality.\n2 4-x y\nChlorophyll Mimic for Green Methanol Synthesis”.\nActions taken: The title has been revised as follows:\n…Green NiGa O (OH) Chlorophyll Mimic for Green Methanol Synthesis… (Highlighted in the\n2 4-x y\ntitle of the revised manuscript)\nQ5.\nThe XPS analysis is rather unconvincing. The binding energy of the OOH is usually around 533eV,\nwhereas here for 300-H it is 531eV, almost the same as the O . The deconvolution of the Ni features\n2 v\nlooks rather subjective – what is the FWHM of Ni2+ and Ni3+ based on? How does the data for Ga in\nFig 1g fit with the supplementary Fig 6?\nReply to Q5: We thank the reviewer for pointing this problem out. We have carefully re-fitted all of\nthe XPS data and the results are summarised in the revised Fig. 3a-c and Supplementary Table 9-11.\nIn our re-analysis, the position of OH located around 532.8 eV under RT and 300 oC in Ar atmosphere\nbut shifted to 532.4 eV under H atmosphere. This shift is consistent with previously reported effects of\n2\nhydrogen spillover and surface restructuring on reducible oxide surfaces [Journal of Materials Chemistry\nA 12.46 (2024): 31993-32013]. The FWHM values for Ni2+ and Ni3+ are approximately 1.85 and\n2.21 eV, respectively, which are in agreement with whose reported for spinel structures [Applied Surface\nScience 257.7 (2011): 2717-2730; Materials Letters 70 (2012): 189-192; Journal of Electroanalytical\nChemistry 449.1-2 (1998): 91-100]. Additionally, the Fig. 1g shows the XPS results for Ni 2p, while\nSupplementary Fig. 6 presents the Ga 2p, and thus these two figures correspond to different elements.\nActions taken: The revised figures and tables are as follows:\nSupplementary Table 9| In situ XPS results of O 2p over NG under different reaction conditions.\nReaction O O O\nlatt v OH\nconditions\nPosition Percentage FWHM Position Percentage FWHM Position Percentage FWHM\n(eV) (%) (eV) (eV) (%) (eV) (eV) (%) (eV)\nRT-Ar 530.4 42.4% 1.41 531.5 23.3% 1.59 532.8 34.3% 2.69\n300-Ar 530.4 46.7% 1.43 531.6 24.8% 1.61 532.8 28.5% 2.60\n300-H 530.2 70.9% 1.42 531.5 20.6% 1.57 532.4 8.5% 2.66\n2\n300-CO +H 530.4 53.5% 1.44 531.6 30.5% 1.63 532.5 16.0% 2.69\n2 2\nSupplementary Table 10| In situ XPS results of Ni 2p over NG under different reaction conditions.\nReaction conditions Ni2+ Ni3+\nPosition Percentage FWHM Position Percentage FWHM\n(eV) (%) (eV) (eV) (%) (eV)\nRT-Ar 855.4 49.4% 2.05 857.3 31.4% 2.54\n300-Ar 855.1 73.1% 1.94 857.0 26.9% 2.51\n300-H 855.2 66.1% 1.90 856.9 33.9% 2.52\n2\n300-CO +H 855.3 63.2% 2.00 857.0 36.8% 2.53\n2 2\nSupplementary Table 11| In situ XPS results of Ga 2p over NG under different reaction conditions.\nReaction conditions Ga-O-Ga Ga-O-Ni\nPosition Percentage FWHM Position Percentage FWHM\n(eV) (%) (eV) (eV) (%) (eV)\nRT-Ar 1117.4 60.2% 1.53 1118.0 39.8% 3.04\n300-Ar 1117.4 57.5% 1.54 1118.0 42.5% 3.04\n300-H 1117.4 63.3% 1.60 1118.00 36.7% 3.04\n2\n300-CO +H 1117.3 61.0% 1.55 1117.9 39.0% 3.04\n2 2\nQ6.\nIs any other interpretation of the data possible – for example can NiO and Ga O form a kind of\n2 3\nheterojunction?\nReply to Q6: We appreciate the reviewer's insightful comments. Theoretically, NiO and Ga O can\n2 3\nform a heterojunction structure, however, XRD and TEM results of the spent NG only show spinel phase,\nwith no evidence of separate NiO and Ga O phases. In the original manuscript, we prepared a\n2 3\nphysical mixture of NiO and Ga O with the same Ni/Ga ratio and tested its catalytic performance,\n2 3\nas shown in Supplementary Fig. 9. The results indicate that the activity of the physical mixture is\nsignificantly lower than that of NG. This suggests that simple physical mixing or possible heterojunction\nformation between NiO and Ga O is not sufficient to account for the high methanol production activity.\n2 3\nTherefore, we can make the conclude that the superior performance is mainly attributed to the unique\nstructure of NG, such as the formation of surface frustrated Lewis pairs (SFLPs), mixed valence of\nNi(II)/Ni(III) and OH group, rather than a simple heterojunction effect.\nQ7.\nThere are some minor errors relating to Figure references and descriptions.\nReply to Q7: We appreciate the referee’s careful review and constructive comments. We apologize\nfor the oversight and have thoroughly scrutinized the entire manuscript, correcting all identified errors\nto ensure clarity and accuracy in the figures, references and descriptions.\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of\nthe Nature Communications initiative to facilitate training in peer review and to provide appropriate\nrecognition for Early Career Researchers who co-review manuscripts.\nAuthor reply: We appreciate your diligent review of our manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This work can be accepted, since all the issues have been solved or improved.", "reviewer_2": "Most problems have been well addressed. Following peak deconvolution, the compositional ratios of NG and NG-300 showed minimal variation across different atmospheric conditions, with the observed differences potentially falling within the margin of experimental error.", "reviewer_3": "The authors have address all the points raised and provided detailed explanations. They have undertaken extensive manuscript revision where appropriate and added substantial new data to the Supplementary Information. Overall the revised manuscript appears satisfactory with the exception of the data given regarding the cell volume in Methods, Catalytic tests. Can the authors confirm the volume of 113.0 mm3 and irradiation area of 28.3 cm2.", "reviewer_4": "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": "Author Reply: We appreciate the reviewer’s recommendation for acceptance and the constructive\nfeedback, which strengthened our manuscript.\nAuthor Reply: We gratefully appreciate the reviewer for the valuable comments and apologize\nfor any ambiguity. XPS measurements under different atmospheres were performed only for NG,\nNG-300 was analyzed under ambient conditions. After rechecking the data, for Ni 2p the Ni(II)\npercentage increases from 63.2% (Ar) to 73.1% (H ) and partially recovers under CO +H (Fig.\n2 2 2\n3b and Supplementary Table 10), which is well beyond the fitting/measurement uncertainty. For\nGa 2p, the percentage of Ga-O-Ni changes from 36.7% (H ) to 39.0% (CO +H ), close to 39.8%\n2 2 2\nat RT. This small variation might be attributed to the CO reacting at Ga-O-Ni sites as indicated in\n2\nthe reaction pathway. These illustrations have been revised in the manuscript.\nAuthor Reply: We really appreciate the reviewer’s careful reading, which helped us improve the\nmanuscript. After re-evaluating with the COMSOL simulation, we identified a unit mislabeling. The\nvolume is ~13 mL (≈ 1.30 × 104 mm3), not 113.0 mm3. The illuminated area corresponds to a circle\n0.6 cm in diameter, with A=πr2≈28.3 mm2 (0.283 cm²), not 28.3 cm². These values and units have\nbeen corrected in the Methods.\nAuthor Reply: We are grateful to both the reviewer and the co-reviewer for their careful review\nand constructive suggestions."}]

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