Asymmetric charge-polarization tailoring active hydrogen transfer for selective photoreduction CO_2 to CH_4

Domain: chemistry
Published: 2026-04-10
Editorial outcome: major_revision

Abstract
Photocatalytic reduction of CO2 to natural gas using water vapor is a promising strategy for carbon recycling and renewable energy storage. However, the selectivity of current catalysts still remains a big challenge. Herein, we construct IrCu alloys on TiO2 nanosheets to promote photocatalytic CO2 to methane with 98.6% selectivity and 7.9% quantum efficiency at 365 nm under non-sacrificial ambient conditions. The performance is competitive with most other reported metal-based photocatalysts. Experimental and theoretical calculations demonstrate that the intensive H2O adsorption on Ir/TiO2 hinders *H transfer, inevitably generating the H2 by-product. Conversely, hydrophobic Cu effectively optimizes the interfacial hydrogen-bond network on IrCu/TiO2, predominantly in H-down configurations for H2O adsorption on the asymmetric charge-polarized Cuδ+-Irδ- structure, which facilitates the kinetic migration of dissociated *H to *CO-Cu sites, resulting in the reduced energy barrier for the key *CHO intermediate. This finding enables high CH4 selectivity on IrCu/TiO2, deepening our understanding of gas-solid interfacial water vapor in the enhanced natural gas synthesis.

Introduction
The photocatalytic conversion of carbon dioxide (CO2) via water (H2O) vapor into methane (CH4) is an emerging technology for synthetic natural gas (SNG), which is able to achieve a sustainable energy economy, as it closes the carbon cycle and enables the efficient transportation of solar energy to valuable fuels1-5. Currently, the solar methanation process from CO2 reduction under non-sacrificial ambient conditions remains challenging, due to the low selectivity associated with the multi-step process involving eight-electron reduction and hydrogen proton migration (Fig. 1a)6-7. Titanium dioxide (TiO2), a classical metal oxide semiconductor, is widely used in photocatalysis owing to its stability, non-toxicity and cost-effectiveness8-10. Nevertheless, the pristine TiO2 exhibits limited photocatalytic activity for CO2 reduction reaction (CO2RR) to CH4 11-12. Accordingly, many works focus on interfacial engineering to regulate the charge distribution, modification of CO2 reactant and key intermediates on the active sites for enhancing CH4 selectivity in TiO2-based catalysts13-19.

Water vapor (H2O) is an essential factor in CO2RR, serving as the hydrogen proton source and critically influencing product selectivity20-24. The hydrogen-bond network (HBN) formed from the interfacial water can affect intermediates and regulate the hydrogen proton transfer behavior25-26. For example, tuning water solvation could effectively modulate CO2RR performance25. And the co-assembly of molecular catalysts and HBN-rich polymers enhanced the *CO surface coverage, promoting C-C coupling to form C2+ selectivity27. However, achieving the selective reduction of CO2 to CH4 through the controlled behavior of interfacial water remains a significant challenge. Because proton-proton coupling to H2 is thermodynamically and kinetically favored over proton-to-CO hydrogenation, leading to the side reaction of the hydrogen evolution reaction (HER)28. Moreover, the reaction pathways and ultimate product selectivity are also affected by hydrogen proton coverage and distribution29, 30. In order to balance hydrogen proton concentration and facilitate available *H transfer, constructing active sites in photocatalysts is a potential way of activating interfacial water and achieving selective CO2RR methanation. Notably, Iridium (Ir)-based catalysts are widely used in the HER and oxygen evolution reaction (OER) due to their ability to adsorb and dissociate H2O31-34. This prominent role stems from its electronic structure, characterized by unfilled d-orbitals, which facilitates efficient hybridization with reactant molecular orbitals, enabling the adsorption of key intermediates such as *H and *O to be optimized32, 35. However, excessive H2O adsorption and strong HBN around the Ir active sites would hinder the active hydrogen proton transfer to carbon-containing intermediates, resulting in the reduced CO2RR efficiency36-37 and undesirable by-products, including H2 or CO38. Therefore, it is imperative to develop Ir-based photocatalysts with moderated H2O adsorption, active hydrogen transfer and intermediate binding for solar methanation.

Herein, we construct IrCu bimetallic nanoalloys supported on TiO2 nanosheets via an ethylene glycol-assisted reduction method. The dual-metal modulation strategy is proposed and illustrated in Fig. 1b, inferior CH4 generation efficiency can result from inappropriate HBN strength, such as localized hydrogen protons from the water dissociation at Ir sites on Ir/TiO2, or hydrophobic Cu sites on Cu/TiO2. In this work, we turn moderately HBN on the IrCu alloys to supply migratable hydrogen protons from the water dissociation at Ir sites to the adsorbed *CO intermediate at Cu sites, yielding high CH4 selectivity. Furthermore, the TiO2 support absorbs light for the photo-generated electrons and holes, and the alloy effect at the metal-support interface in IrCu/TiO2 generates oxygen vacancies (Ov), prolonging the lifetime of photoelectrons, which is critical for the sluggish eight-electron reaction in CH4 formation. The CH4 selectivity of 98.6% and quantum efficiency of 7.9% at 365 nm are achieved in IrCu/TiO2. In situ spectroscopic analysis and the density functional theory calculations reveal a Cuδ+-Irδ- structure increases Ov at the support interface, and inhibits the recombined photogenerated charges. Moreover, ab initio molecular dynamics simulations clarify the distinct adsorption structures of interfacial water molecules and the hydrogen proton transfer kinetics. Dual-functional IrCu/TiO2 effectively reconstructs the initial HBN, facilitating *H transfer and *CO reduction, which significantly improves CH4 selectivity. This work offers a fresh perspective on tuning the interaction between interfacial water molecules and the photocatalysts to achieve highly selective solar methanation.

Results
Rational design and synthesis of dual-metal IrCu/TiO2
As the different electronegativities of Cu (1.90) and Ir (2.20), constructing dual-metal IrCu catalytic site can introduce local polarization to regulate intermediate formation. Theoretical calculations were performed to verify the effectiveness of this strategy. Bader charge analysis indicates that the electrons transfer from Cu (+0.092 e) to Ir (-0.302 e) in IrCu/TiO2, leading to a local polarized Cuδ+-Irδ- structure (Fig. 2a). The d-band center energy (ɛd) of Ir in IrCu/TiO2 (-2.488 eV) is closer to the Fermi level (EF) than that of Ir in Ir/TiO2 (ɛd,Ir = -2.704 eV). Similarly, the d-band center of Ti in IrCu/TiO2 (-4.802 eV) also presents an upshift compared to Cu/TiO2 (-5.033 eV) and Ir/TiO2 (-4.899 eV), suggesting that dual-metal IrCu/TiO2 can introduce intensive electronic interaction (Fig. 2b-c). Combined with the reduced Bader charge of the oxygen atoms in IrCu/TiO2 (Supplementary Table 1-2), the Ov is easier to generate in IrCu/TiO2 due to the intensive electronic interaction in Cuδ+-Irδ-, resulting in the enhanced the photocatalytic CO2 reduction.

Under the guidance of dual-metal regulation strategy, IrCu nanoalloys supported on TiO2 nanosheets were prepared by an ethylene glycol-assisted reduction method (Fig. 2d)39. Inductively coupled plasma optical emission spectrometer (ICP-OES) proves successful introduction of Ir and Cu in a ratio of 1.1:1 in IrCu/TiO2 (Supplementary Table 3). Powder X-ray diffraction (XRD) patterns of IrCu/TiO2 only exhibit the anatase phase of TiO2 (JCPDS No. 21-1272), indicating that there is no significant agglomeration of IrCu nanoalloys on the TiO2 substrate (Supplementary Fig. 1). Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images suggest that IrCu nanoclusters are uniformly distributed on the TiO2 surface, with an average particle size of ca. 1.3 nm, and lattice spacings of 0.189 nm and 0.235 nm correspond to the TiO2 (200) and IrCu (111), respectively (Fig. 2e-g and Supplementary Fig. 2). In addition, energy dispersive spectroscopy (EDS) element mapping and color-coded images of the ordered structure further confirm of the successful preparation of IrCu nanoclusters on TiO2 nanosheets (Fig. 2g and Supplementary Fig. 3).

Electronic structure and oxygen vacancy modulation
The electronic structure of the IrCu nanoalloys supported on TiO2 was investigated by the X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) characterizations. The Ir 4f peak in the XPS spectra of IrCu/TiO2 shifts to a lower binding energy by 0.35 eV compared to that of Ir/TiO2, suggesting of electron-rich Ir species in the alloys (Fig. 3a). In the XPS spectra of Cu 2p (Fig. 3b), compared with Cu/TiO2, the Cu 2p1/2 and Cu 2p3/2 peaks of IrCu/TiO2 shift to a higher binding energy by 0.67 eV. The Cu LMM Auger electron spectra (AES) further confirms more Cu+ species in IrCu/TiO2 (Supplementary Fig. 4), indicating the electrons transfer from Cu to Ir to construct the Cuδ+-Irδ- structure. To further elucidate the electronic and coordination properties of IrCu structure, XANES spectroscopy was performed. The white line intensity and absorption edge of the IrCu/TiO2 is closely aligned to the metallic Ir foil (Fig. 3c), indicating metallic state of the Ir. The Cu K-edge XANES spectra shows that the absorption edge of the IrCu/TiO2 sample fell between the Cu foil and CuO (Fig. 3d), suggesting that the average valance state of Cuδ+ species is between 0 and +2 due to the electronic transfer from Cu to Ir. Furthermore, the coordination properties of Ir and Cu species were investigated by the extended X-ray absorption fine structure (EXAFS) spectroscopy, and the corresponding structural parameters (CN: coordination numbers; R: the distance between absorber and backscatter atoms) were analyzed. The Ir L3-edge Fourier-transform (FT)-EXAFS spectrum of IrCu/TiO2 exhibits three coordination peaks with the bond distances (R) of 2.01-2.72 Å (Supplementary Fig. 5a). Three coordination structures are identified: Ir-Cu metallic bonding (CN=2.3 at R=2.53 Å), Ir-Ir coordination (CN=4.9 at R=2.72 Å), and Ir-O interactions (CN=0.9 at R=2.01Å) (Supplementary Table 4). The lower coordination numbers of Ir-O (CN<2) are attributed to surface-adsorbed oxygen species or interfacial interactions with the support. The Cu K-edge FT-EXAFS analysis of IrCu/TiO2 confirms the formation of Ir-Cu metallic bonding, with the reduced Cu-Cu coordination numbers. Above all, these observations suggest that Cu modulates the electron density of Ir, resulting in the formation of a polarized Cuδ+-Irδ- structure, consistent with the calculated results. Moreover, the normalization XANES of Ti K-edge of of Cu/TiO2, Ir/TiO2, and IrCu/TiO2 were determined in comparisons. As shown in Fig. 3e, all the photocatalysts exhibit the TiO6 structure of TiO2 8. A reduced intensity of the white line peak indicates of lower content, and the weakest peak intensity in IrCu/TiO2 indicates a diminished TiO6 structure, due to more Ov 40-41. The changed local structure of Ti-O bond was further probed by Fourier transform (FT) and wavelet transform (WT) analysis. And the Ti-O bond length in IrCu/TiO2 is approximately 1.94 Å, longer than those in Cu/TiO2 and Ir/TiO2, likely due to the decreased Ti-O bond covalency (Fig. 3f and Supplementary Fig. 7). Moreover, the coordination numbers of the Ti-O bond follow the order IrCu/TiO2 < Ir/TiO2 < Cu/TiO2 (Supplementary Table 6), consistent with the trend in bond length and white line peak. These structural distinctions suggest that the IrCu nanoalloy effectively decreases the TiO6 structure and promotes Ov formation in TiO2. The blue-shifted and broadened Eg vibration peak in Raman spectra and negative shifted peak in the Ti 2p XPS spectra indicate more Ov and Ti3+ species in IrCu/TiO2 (Supplementary Fig. 8-10)42. The electron paramagnetic resonance (EPR) signal intensity is significantly stronger in IrCu/TiO2 than in Ir/TiO2 and Cu/TiO2, confirming a higher Ov concentration (Supplementary Fig. 11). In situ EPR spectroscopy was further employed to investigate the role of Ov. The signals at g = 2.003 and 1.994 are assigned to Ov and Ti3+ species, respectively43. Under CO2 atmosphere, the Ov signal decreased while the Ti³+ signal increased, confirming that Ov sites participate in the capture and activation of CO2, which is different from that under N2 condition (Supplementary Fig. 12-15). Furthermore, IrCu/TiO2 exhibits a higher transient photocurrent density response and a smaller impedance (Fig. 3g-h), suggesting that Ov facilitate the separation and migration of photogenerated carries44-45. In addition, according to the calculated band energies from the UV-vis diffuse reflectance spectroscopy (DRS) and the valence band maxima (VBM) from the VB-XPS results, all samples possess the suitable band-edge positions for CO2 reduction to CH4 or CO (Supplementary Fig. 16-18). Steady-state photoluminescence (PL) spectra shows that the emission peak intensity of IrCu/TiO2 is the weakest, compared to that of Cu/TiO2 and Ir/TiO2 (Supplementary Fig. 19), implying efficient charge recombination suppression and more electrons available for CO2RR. Time-resolved photoluminescence (TRPL) spectra elucidates the carrier recombination kinetics, confirming the longer carrier lifetime of 36.93 ns for IrCu/TiO2 than those of Ir/TiO2 (34.18 ns) and Cu/TiO2 (6.07 ns) (Fig. 3i and Supplementary Table 7) 46.

High selectivity of photocatalytic CO2 methanation
The photocatalytic performances of CO2 reduction for IrCu/TiO2, Ir/TiO2, and Cu/TiO2 were tested in a common gas-solid reactor under the conditions of CO2, H2O and full spectrum illumination at room temperature (25 ℃), without any additional photosensitizers and sacrificial agents. The performance of photocatalytic CO2 reduction is presented in Fig. 4a. With the increased Cu loadings, the CH4 yield and selectivity increase and then decrease. The IrCu/TiO2 photocatalyst affords the great CH4 yield (100.3 μmol g-1 h-1), which is nearly 33 and 100 times higher than that of Ir/TiO2 and Cu/TiO2, respectively. As shown in Fig. 4b, the IrCu/TiO2 sample exhibits a CH4 selectivity of 98.6 %, which was nearly 3.7 and 4.5 times higher than that of the Ir/TiO2 and Cu/TiO2 samples, respectively. In addition, the CH4 yield shows a linear relationship with the amount of catalyst (Supplementary Fig. 20). Moreover, the yield of H2 by-product during the process of CO2 photoreduction was detected (Supplementary Fig. 21a and Table 8), which was only observed on the Ir/TiO2 sample, confirming that the excessive *H on the Ir sites promoted the HER, but was inhibited on the IrCu/TiO2 sample. Meanwhile, we conducted a quantitative analysis of O2 production from the H2O oxidation, which was highest on IrCu/TiO2 (Supplementary Fig. 21b). The apparent quantum efficiencies (AQE) of CH4 evolution for IrCu/TiO2 were measured under various light wavelengths of 365, 420, 450, 475, and 520 nm (Fig. 4c). For comparison, we also measured AQE of CH4 evolution for Ir/TiO2 and Cu/TiO2 at 365 nm, respectively (Supplementary Table 9). The IrCu/TiO2 sample exhibits an AQE of 7.9% at 365 nm, which is one to two orders of magnitude higher than the AQEs of Ir/TiO2 and Cu/TiO2, and competitive with most reported metal-based photocatalysts (Supplementary Table 10).

To evaluate the reliability of the current photocatalytic system, a series of systematic blank experiments were conducted. As shown in Supplementary Fig. 22, in the absence of light irradiation or photocatalysts or CO2 reactant, there is no products to detect, demonstrating that the determined products originate from the photocatalytic CO2 reduction. In order to further verify that the gas products were only from the CO2 feed, the 13CO2 isotopic labelling experiments were carried out. In Fig. 4d, only 13CO (m/z=29) and 13CH4 (m/z=17) are detected, which clearly rules out the contributions from other carbon-containing species such as ethylene glycol. In addition, the IrCu/TiO2 photocatalyst also exhibits a good catalytic stability, as demonstrated by its stable high methane yield and selectivity throughout five cycling tests (Fig. 4e). Moreover, the XRD, EPR, XPS, and STEM analyses were employed to investigate the structural, morphological and electronic properties of IrCu/TiO2 photocatalysts before and after the reaction (Supplementary Fig. 23-25). These analyses confirm that the IrCu/TiO2 remains stable during the catalytic process. Furthermore, the photocatalytic performance of the IrCu/TiO2 photocatalyst is compared with the recently reported CO2-to-CH4 photocatalysts under the similar conditions (only CO2 and H2O reactants) (Supplementary Table 10)47-58. As a result, these findings suggest the great photocatalytic performance of CO2 methanation on IrCu/TiO2, where the IrCu dual-metal sites serve as the decisive active sites responsible for H2O dissociation and CO2 protonation.

In situ characterization and mechanism investigation
To deeply identify the active sites for H2O adsorption and dissociation, the water contact angle, H2O-TPD, and in situ DRIFTS experiments were carried out. The Ir/TiO2 presents the lowest water contact angles (<10°), suggesting of its hydrophilic characteristic, while Cu/TiO2 shows higher contact angle, indicating its decreased hydrophilicity59 (Fig. 5a and Supplementary Fig. 26). Clearly, the introduction of Cu modulates the surface hydrophilicity of IrCu/TiO2, indicating that the water adsorption on IrCu/TiO2 is moderate, compared with Ir/TiO2. The H2O-TPD profiles further display the distinct adsorption strength of H2O on Ir/TiO2 and IrCu/TiO2. In Fig. 5b, the desorption amount of H2O is significantly reduced on IrCu/TiO2, consistent with the water contact angles. In addition, three desorption peaks are observed, corresponding to physically and associatively adsorbed water, as well as structural hydroxyl clusters, and the associatively adsorbed water related to active *H and hydroxyl groups29. On IrCu/TiO2, the desorption peak of the associatively adsorbed water is shifted to a lower temperature, compared with Ir/TiO2, indicating that its active *H is easier to transfer, likely due to moderate hydrophilicity surface. In situ EPR conducted was used to further confirm this active *H around the IrCu/TiO2 photocatalyst interface. As shown in Fig. 5c, the EPR peaks belonging to DMPO-*H are observed, and the IrCu/TiO2 has a stronger signal intensity than the Ir/TiO2, indicating its higher abundance of available *H during the reaction process60. The active sites of *H migration on the photocatalysts were determined by in situ DRIFTS in dark and in light. The schematic diagram of in situ infrared reactor is displayed (Supplementary Fig. 27). As observed in Fig. 5d, the peaks at approximately 2050 and 2100 cm-1 correspond to the characteristic vibration peaks of the Ir-H and Cu-H species, respectively61. On the IrCu/TiO2 photocatalyst, a weak peak is observed in the dark. Once light is introduced, a distinct Cu-H stretching signal at about 2060 cm-1 is observed, suggesting that the adsorbed H2O is easier to donate the *H from Ir to Cu. In contrast, the Ir-H signal on Ir/TiO2 shows negligible enhancement in light. Furthermore, the formed HBN from adsorbed H2O around the photocatalyst surface were analyzed to reveal the different abilities of *H transfer. The -OH stretching mode (νOH*) signal centers at around 3200, 3350, and 3600-3700 cm-1 in Fig. 5e, f, which are assigned to strong HBN, intermediate HBN, and free water, respectively25, 62. An intensive and broad range of approximately 3400-3200 cm-1 is observed on the Ir/TiO2 surface, which may be attributed to the strong HBN. By contrast, for IrCu/TiO2, only the HBN (3450 cm-1) and free water (3590 cm-1) appear, but the strong HBN is absence, indicating that the strong HBN is broken down on IrCu/TiO2. These observations suggest that the moderate HBN on IrCu/TiO2 promotes H2O dissociation and accelerates *H migration compared to Ir/TiO2.

Furthermore, the reaction mechanism and active sites for CO2 protonation were revealed by in situ DRIFTS under CO2 and H2O conditions in the light. All samples exhibit the characteristic peaks for *COOH (1508, 1540, 1720 cm-1), *CH3O (1445 cm-1), m-CO32- (1455 cm-1) and b-CO32- (1280, 1298, 1310 cm-1) intermediates (Fig. 5g, h, and Supplementary Fig. 28)18, 63-64. Interestingly, on the IrCu/TiO2 sample, the intensity of *CH3O is more intensive than that of m-CO32- and b-CO32- intermediate, indicating that carbonate intermediates on IrCu/TiO2 are more prone to hydrogenation. The b-CO32- species on IrCu/TiO2 gradually convert to *CHO (1746 cm-1) and *CH3O in the process of reduction and protonation, while relatively stable on Ir/TiO2. Additionally, the normalized intensity of the *CH3O and b-CO32- species for Ir/TiO2 and IrCu/TiO2 are displayed (Supplementary Fig. 29). The formed *CH3O on IrCu/TiO2 is richer than that on Ir/TiO2, due to its sufficient *H for the protonation of b-CO32-, consistent with the in situ EPR results for *H species. The DFT calculations also confirm that the CH4 selectivity of IrCu/TiO2 is higher than that of Ir/TiO2, due to a lower energy barrier of key *CO-to-*CHO step on IrCu/TiO2 (0.467 eV) than that on Ir/TiO2 (0.906 eV) thermodynamically (Fig. 5i and Supplementary Fig. 30-33). Therefore, the selectivity mechanism is proposed that Ir sites dissociate H2O to provide a rich *H environment, and Cu sites reconstruct HBN on the catalytic interface, thereby facilitating *H transfer to *CO-Cu for highly selective CH4 production on IrCu/TiO2.

To elucidate the mechanism of migratable hydrogen proton by the modulated HBN on IrCu/TiO2, the ab initio molecular dynamics (AIMD) simulations and DFT calculations were further performed to investigate H2O adsorption orientation, thereby gaining insights into interfacial HBN microenvironments and dynamic proton transfer processes. AIMD simulations at room temperature (300 K) show the interfacial H2O molecules around Ir/TiO2 and IrCu/TiO2 (Supplementary Fig. 34-35). The representative snapshots reveal that the interfacial H2O is farther from the surface of IrCu/TiO2 (2.12 Å) than that of Ir/TiO2 (2.06 Å), indicating that the hydrophilic Ir/TiO2 exhibits a stronger affinity for H2O than IrCu/TiO2 (Supplementary Fig. 36). However, no significant differences in radial distribution function (RDF) of O-H bonds are observed between IrCu/TiO2 and Ir/TiO2 (Supplementary Fig. 37). Interestingly, the orientation of H2O molecules was investigated and identified as the key factor. AIMD simulations show the concentration profiles of H and O atoms along the surface orientation (Fig. 6a). The H peak (L1) appears before that of O (L2), and the H peak density is approximately twice that of the O peak, suggesting a predominant H-down configuration of interfacial H2O on IrCu/TiO2 (Supplementary Fig. 38). In contrast, Ir/TiO2 exhibits overlapping and broadened L1 and L2 peaks (Fig. 6b), indicating disordered H2O orientations with both H-down and O-down configurations. These differences in orientation were further confirmed by the dipole angle θ between the water dipole vector and surface normal. As displayed in Fig. 6c, a value of cosθ = -1 corresponds to H-down orientation, which is more pronounced in IrCu/TiO2, reflecting a highly ordered H-down arrangement. Whereas, Ir/TiO2 exhibits a broader distribution, consistent with a mixed H2O orientation. Slow-growth AIMD simulation was conducted to confirm the facilitated hydrogen proton transfer on H-down configurations of IrCu/TiO2, compared with Ir/TiO2 (Fig. 6d, e). In the reconstructed HBN-mediated pathway on IrCu/TiO2, the hydrogen proton from H2O dissociation rapidly jumps to an adjacent H2O molecule, triggering a hydrogen proton transfer within the interfacial H2O (Supplementary Fig. 39). The hydrogen protons with H-down configurations of the interfacial H2O on IrCu/TiO2 could transfer directly to Cu atoms with an energy barrier of only 0.54 eV, which is much lower than 0.63 eV of Ir/TiO2. DFT calculations further show that the adsorption energy of H2O on Ir/TiO2 is -0.63 eV, lower than that of -0.58 eV on IrCu/TiO2, suggesting that Ir/TiO2 is more likely to absorb H2O (Supplementary Fig. 40-43), but the stronger adsorption of *H hinders its migration to *CO intermediate. Therefore, the H-down configuration of H2O adsorption accelerates *H transfer to enable efficient CO2 methanation on IrCu/TiO2 owing to the polarized Cuδ+-Irδ- electronic structure and reconstructed HBN in IrCu/TiO2.

Based on in situ experiments, DFT calculations and AIMD simulations, the reaction mechanism of high CH4 selectivity mediated by interfacial H2O is proposed in Fig. 6f. And the specific contents are as follows: 1) the H2O molecules are adsorbed on the alloys of hydrophilic Ir sites, generating localized hydrogen protons. Meanwhile, the hydrophobic Cu sites of the alloys modulate the hydrogen-bond network for H-down configurations, which significantly weakens the *H binding ability and facilitates *H migration to the Cu sites. 2) CO2 molecules are then adsorbed at the IrCu/TiO2 interface (on the alloy and Ov), which are activated to form the *COOH intermediate. 3) Subsequently, the generated *CO intermediate on Cu sites undergoes continuous hydrogenation with its surface *H, ultimately producing CH4. From both kinetic and thermodynamic perspectives, the accelerated *H transfer to *CO-Cu reduces the energy barrier for key *CHO formation, promoting the continuous hydrogenation and reduction processes by utilizing the long-lived photogenerated electrons from the TiO2 semiconductor.

Methods
Materials and chemicals
All solvents and reagents purchased from commercial sources were used without further purification. Tetrabutyl titanate (C16H36O4Ti, 100%), Ethylene glycol (EG, 99%), Cupric chloride (CuCl2, 98%), Iridium trichloride (IrCl3, 99.8%) were purchased from Macklin. Hydrofluoric acid (HF, 50%), Sodium hydroxide (NaOH, 96%), Polyvinyl pyrrolidone (PVP, k-25) were purchased from Nanjing Chemical Reagent Co. Acetone (CH3COCH3, 99.8%) and n-hexane (C6H14, 99%) were purchased from Aladdin.

Preparation of catalysts
Anatase TiO2 nanosheets with (001) facet was prepared by a typical hydrothermal method. Tetrabutyl titanate was reacted with hydrofluoric acid solution in a Teflon-lined autoclave at 200 °C for 24 h. The resulting white precipitate was collected and washed alternately with ethanol and distilled water until the precipitate was pure white and dried at 110 °C for 24 h. Finally, the sample obtained by drying at 400 °C for 2 h in a muffle furnace. The resulting sample was denoted as TiO2.

The composition-tunable IrCu nanoclusters on TiO2 nanosheets was prepared with ethylene glycol (EG) as reducing agent through a one-step microwave reduction method, and the total metal loading was controlled to be 0.1 mmol. Depending on the real molar ratio of Ir and Cu by the ICP-OES analyses (Table S3), the catalysts were named as Ir/TiO2, IrCu/TiO2, Ir1Cu2/TiO2, Ir3Cu1/TiO2, and Cu/TiO2, respectively.

Synthesis of IrCu alloy samples: The 15 mL of glycol was placed in a three-necked flask with a magnetic stirring rotor, which was heated to 125 °C and held for 30 minutes in a microwave reactor. Then 1 mL of NaOH (0.25 M) and corresponding proportions of IrCl3 and CuCl2 were added to the three-necked flask. The mixture was held at 125 °C for an additional 30 minutes and then immediately immersed in an ice bath for 30 minutes. The colloid was then centrifuged at 8000 rpm for 15 min and the mixture was washed with acetone and n-hexane so that the PVP was completely washed out. The prepared colloid was redispersed in ethanol. A mixture of 300 mg prepared TiO2 NS and ethanol was added dropwise. To remove the ethanol, the mixture was dried at 70 °C for 6 h.

Synthesis of Cu monometallic sample: The mixture of 15 mL of glycol and 20 mg of PVP was placed in a three-necked flask with a magnetic stirring rotor. Since Cu itself was more difficult to be reduced by glycol, it is necessary to increase the temperature appropriately and PVP was needed to add. The microwave reactor was adjusted to a temperature of 175 °C and held for 30 min with a maintenance temperature of 175 °C. The following steps were the same as IrCu alloy samples. The obtained sample was denoted as Cu/TiO2.

Synthesis of Ir monometallic sample: This sample was prepared in the same way as the previous IrCu alloy, except for the maintenance temperature of 150 °C. The sample was denoted as Ir/TiO2.

Characterization
X-ray diffraction (XRD) patterns were recorded on Philips X’ Pert Pro diffractometer under Ni-filtered Cu Kα radiation (λ = 0.15418 nm, 40 kV, 40 mA). Atomic-resolution aberration-corrected high angle annular dark field scanning transmission electron microscopic (AC-HAADF-STEM) images were collected on a FEI Titan Cubed G2 60-300 aberration-corrected S/TEM instrument at 300 kV accelerating voltage. Imaging was conducted in HAADF mode to provide Z-contrast, with a probe convergence angle of 22 mrad and a detector inner angle of 79.5 mrad. Samples were prepared by dispersing the powder in ethanol and depositing it onto a TEM Mo grid. Elemental mapping was carried out via STEM-EDS using the SuperX detection system. Inductively coupled plasma optical emission spectrometer (ICP-OES) were recorded by analytik jena novAA350/ZEEnit650p (Analytik Jena). The catalysts were digested at 130 °C in a concentrated aqueous HCl/HNO3 and HF solution. After the solids were completely dissolved, concentrated sulfuric acid was added to remove the hydrofluoric acid. X-ray photoelectron spectroscopy (XPS) measurements were determined via PHI 5000 Versa Probe high performance electron spectrometer, on monochromatic Al Kα radiation (1486.6 eV). The binding energies were corrected by the adventitious carbon (C 1s peak at 284.6 eV). Raman spectra were collected on a Renishaw Laser Raman spectrometer (Renishanplc) with an Ar+ laser beam. The emission line was 532 nm and the output power was 10 mW. Samples were exposed to air at room temperature throughout the experiments. The in situ electron paramagnetic resonance (EPR) tests were recorded by ESR JES FA200 (JEOL) spectrometer at 77 K under the irradiation of a xenon lamp in N2 or CO2 atmosphere. In situ ESR of *H free radical using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to capture the instable hydrogen radical to form the DMPO-*H adduct to generate EPR signal. The X-ray absorption fine structure (XAFS) measurement analysis was performed with Si(111) crystal monochromators at the BL11B beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) (Shanghai, China). Before the analysis at the beamline, samples were pressed into thin sheets with 1 cm in diameter and sealed using Kapton tape film. The XAFS spectra were recorded at room temperature using a 4-channel Silicon Drift Detector (SDD) Bruker 5040. The EXAFS spectra were recorded in fluorescence mode. The spectra were processed and analyzed by the software codes Athena and Artemis. UV-Vis diffused reflection spectra were obtained on a Shimadzu UV-2401 spectrophotometer. BaSO4 was used for blank sample and the measurement ranged from 200 to 800 nm. Photoluminescence (PL) spectra were carried out on a F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) with 365 nm excitation and the slit in all measurements was 2 nm. Time-resolved photoluminescence (tr-PL) were measured by FLS1000 fluorescence spectrophotometer (Edinburgh Instruments, UK) at room temperature.

Photoelectrochemical measurements
Photoelectrochemical tests, including electrochemical impedance spectroscopy (EIS) and transient photocurrent response were performed on a three-electrode system electrochemical workstation (CHI 660, CHInstrument, Shanghai) at room temperature (25 ℃). Platinum wire electrode and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. The catalyst-modified ITO glass (1×1 cm2) by dip-coating method (5 mg of sample dispersed in 50 μL of ethanol) was used as the working electrode. A 300 W Xe lamp was used as the light source and Na2SO4 (0.25 M) as the electrolyte solution.

In situ diffuse reflectance infrared Fourier-transform spectra (DRIFTS)
In situ DRIFTS were determined by a Nicolet iS50FT-IR spectrometer (Thermo, USA) in a range of 3800-1000 cm-1, and the cell was equipped with a KBr window. The spectrometer was equipped with a specimen chamber and a highly sensitive mercury cadmium telluride (MCT) detector, cooled by liquid N2. In a typical process, 50 mg catalysts powder were put in the DRIFTS cell (PIKE Technologies DiffusIR), pressed and mounted. The catalysts were purged with N2 for 1 h at 300 ℃. Afterwards, the catalysts were cooled down step-wise to room temperature. During this process, the background spectra were collected at room temperature. For CO2 photoreduction experiments, the feed gas consisted of 15% CO2, respectively, using Ar as balance. For the CO2 photoreduction test, then the CO2 bubbles in water were switched into the system to collect adsorption signal interval of 2 min until equilibrium. Finally, the 300W Xe lamp was switched on to initiate the reaction, and the in suit DRIFTS spectra were collected by an MCT detector. The typical signals of diverse intermediates were detected subsequently under dark and light irradiation (0-30 min). All the spectra were collected from 1000 to 4000 cm-1 at a resolution of 4 cm-1 for 100 scans.

Photocatalytic test of CO2 reduction
The photocatalytic reduction of CO2 was carried out under the irradiation of a 300 W xenon lamp in a gas-solid reactor made of stainless steel with transparent quartz window at the top, and no additional photosensitizers and sacrificial agents were added. The procedures were followed: the sample power (50 mg) and 5 mL water were added into 100 mL Teflon-lined autoclave. The mixture was ultrasonicated for 20 min to make the sample fully disperse. Then, the container was sealed and purged by high-purity CO2 (0.4 MPa) for at least 30 min, afterward the gas in the container was analyzed by the gas chromatograph to confirm no air, and then the 300W Xe lamp (Light intensity: 500 mW cm-2) turned on. During the photoreaction process, the reactor temperature was maintained at 25 ℃ using a circulating water-cooling system. The mixture was irradiated for 8 h with stirring, and the gas products were analyzed using a gas chromatograph (GC-7920, CEAULIGHT), which was equipped with a thermal conductivity detector (TCD) for H2 and O2, and a flame ionization detector (FID) for CH4 and CO. The generation rate and selectivity of products were calculated using equations presented below.

Apparent quantum efficiency
The apparent quantum efficiency (AQE) was calculated as the ratio between the number of photogenerated electrons consumption and the number of incident photons, by taking into account the fact that eight electrons are required to produce on CH4 molecule. The wavelength-dependent apparent quantum efficiency was measured under the same photocatalytic reaction condition.

13CO2 isotopic labelling tests
In order to confirm the carbon source of products, the isotopic labelling experiments of the 13CO2 was used as the reactant to analyze the carbon source of CO and CH4 generated from the photocatalytic CO2 reduction. The tests were performed on SHIMADZU GCMS-QP2020 NX equipped with WAX chromatographic column (30 m, inner diameter 0.25 mm, film thickness 0.25 μm). The injection temperature is 280 ℃. The fragment of m/z = 16, 17 and 29 are assigned to 13•CH3, 13CH4, and 13CO, respectively.

Recycling test
To determine the stability of catalysts, the recycling testing was conducted as follows. After first reaction, the used catalyst was recovered by centrifugation. It was then used in the second reaction test, and then for the last test.

Density functional theory (DFT) calculations
All the first-principles spin-polarized calculations were performed by using the Vienna ab initio Simulation Program (VASP)65. The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) form and a cutoff energy of 500 eV for planewave basis set were adopted66. A 3 × 3 × 1 Monkhorst-Pack k grid was used for sampling the Brillouin zones at structure calculation, whereas a denser mesh of 5 × 5 × 1 was used for the electronic structure calculations. The ion-electron interactions were described by the projector augmented wave (PAW) method. A vacuum space greater than 20 Ǻ was employed to avoid the interaction between two periodic units. The convergence criteria of structure optimization were chosen as the maximum force on each atom less than 0.02 eV/Ǻ with an energy change less than 1 × 10-5 eV. The Grimme’s DFT-D3 scheme of dispersion correction was adopted to describe the van der Waals (vdW) interactions in these systems.

The standard hydrogen electrode model proposed by Nørskov and co-workers was employed to calculate the Gibbs free-energy change (∆G) for each elemental step67. The ∆G was defined as shown in Equation (1):
∆𝐺 = ∆E+∆ZPE−T∆S+∆𝐺U+∆𝐺pH (1)
in which ∆E was the adsorption energy based on DFT calculations, ∆ZPE was the zero-point energy correction, T was the temperature, ∆S was the entropy change, U was the applied electrode potential, and ∆G represented the free-energy correction of the pH (pH = 0 was assumed in the acidic medium in this study). The optimized computational model data are provided as a Supplementary data 1.

Ab initio molecular dynamics (AIMD) simulation
Both the AIMD and slow-growth approach simulations were sampled within the canonical (NVT) ensemble by Nosé-Hoover thermostats with a time step of 1.0 fs at a finite temperature of 300 K. First, an AIMD with a duration of 10 ps was applied to enable the system to reach a pre-equilibrium state. Then we mainly adopted the Slow-growth method to obtain the activation energy barrier68. In the Slow-growth method, a geometric parameter λ, i.e., the collective variable (CV), is constrained during the dynamics and varied from the value characteristic of the initial state (λ0) to that of the final state (λ1) with a velocity of transformation ʋ. The Helmholtz free energy difference ΔF can be computed as Equation (2) by collecting the derivative of the potential energy with respect to lambda during the simulation, and by integrating over lambda afterwards.
∆𝐹 =∫λ1λ0( 𝜕𝑉(𝑞) / 𝜕𝜆 ) ∙ʋ𝑑𝑡 (2)
In the limit of infinitesimally small ʋ, ΔF corresponds to the free energy difference ΔG between the final and initial state. The parameters of the method for different migration paths are presented in Supplementary Table 11. The initial, transitional, and final state configurations data are provided as a Supplementary data 1. Considering that the difficulties in modelling the exact material under catalytic conditions, both the DFT and AIMD simulations still have limitation.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors present a dual-functional alloy IrCu/TiO2 to enable highly selective photoreduction CO2 to CH4, through modulating the gas-solid interfacial water vapor. Furthermore, the “Cuδ+ -Irδ-” polarized structure in IrCu/TiO2 generates stronger interfacial electronic interactions. This process serves to enhance the covalent nature of the Ti-O bond, thereby promoting the generation of oxygen vacancies (Ov). This, in turn, facilitates carbon dioxide capture and the formation of long-lived charge carriers. The work is of high quality and is suitable for publication after minor revisions. The following issues should be addressed before final acceptance.\n1. In the two in-situ infrared spectra under consideration, certain absorption peaks exhibiting distinct trends (e.g. the peak near 1455 cm-1 around 1440 cm-1) lack explicit functional group assignments. The purpose of this study is to ascertain the vibrational origins of the unlabelled peaks.\n2. The authors construct IrCu bimetallic nanoalloys supported on TiO2 nanosheets via an ethylene glycol-assisted reduction method. The question of whether this affected the product formation remains to be answered. In addition to the primary gaseous products CH4 and CO that are generated during photoreduction, could liquid products also be formed?\n3. The authors stated that the Cu 2p XPS peaks of IrCu/TiO2 showed a slight shift to higher binding energy compared to the Cu/TiO2 nanosheets. However, based on the data, there is essentially no deviation.\n4. The five-cycle test in Figure 4f shows good catalytic stability, but the structural changes of the catalyst after cycling are not described. Have the structural integrity of the catalyst after cycling been verified by characterizations such as XRD, TEM, and XPS?\n5. In terms of structural characterization, the authors only performed Ti k-edge XANES. The characterization of the Ir-Cu alloy seems insufficient. XANES can be conducted on Ir and Cu to explain their electronic and coordination properties.\n6. In the Gibbs free energy diagram of Figure SI 24, Ir/ TiO2 exhibits a lower energy barrier, which should facilitate easier CO generation. However, its CO production performance is quite low in Figure 4a.\n7. Some relevant literatures may be discussed and cited (10.1007/s40843-025-3464-4; 10.1002/adma.202503021; 10.1002/anie.202421353; 10.1021/jacs.4c09841; 10.1039/d4cs00186a).", "reviewer_2": "This manuscript reports the construction of an IrCu/TiO2 bimetallic photocatalyst that achieves highly selective (CH4: 98.7%) and efficient (AQE = 7.9% at 365 nm) photocatalytic CO2 reduction to CH4 under sacrificial-agent-free conditions using water vapor as the proton source. The core innovation lies in the proposed concept of tailoring the hydrogen-bond network and proton-transfer kinetics of interfacial water molecules via an asymmetric charge-polarized “Cuδ+-Irδ-” structure, thereby overcoming the selectivity bottleneck between CO2 reduction and the hydrogen evolution reaction. The topic is timely and significant for advancing solar fuel synthesis and carbon cycling. However, the manuscript should be heavily improved and polished in terms of rigor in key data, depth of mechanistic elucidation, and completeness of control experiments. Thus, this work needs some major revision prior to be considered in Nature Communcations. The specific comments are as follows.\n1、Due to the difference in electronegativity between Ir and Cu, an Ir --Cu + polar structure could be formed. The question was then raised as to whether the same effect could be achieved with Ru and Pt. Furthermore, non-noble metals such as Fe, Co, and Ni also demonstrated excellent water dissociation ability, raising the possibility of whether they could replace noble metals like Ir.\n2、“In addition, the DOS results demonstrate that the wider Ti-O orbital overlap in IrCu/TiO2 indicates the higher Ti-O covalency degree.” However, the Ti K-edge R-space k3-weighted Fourier transform EXAFS results indicated a longer Ti-O bond length in CuIr/TiO2, suggesting a reduction in the Ti-O covalent character. The elongation of the Ti-O bond was indeed more conducive to the formation of oxygen vacancies, a point that required further explanation by the authors.\n3、“The peak of Ir 4f XPS spectra in IrCu/TiO2 shifts to a lower binding energy by 0.13 eV than Ir/TiO2, …… indicating that the electron transferred from Cu to Ir enables the formed structure of “Cuδ+-Irδ-”. A shift of 0.13 eV fell within the normal error range of XPS, and the valence state of Cu was Cu+, which could be fully attributed to electron transfer from Cu to O, thereby weakening the Ti-O bond.\n4、“As depicted in Fig. 3c , ……, whereas a lower wavenumber shift indicates a decreased electron density.” This phenomenon was essentially attributed to d→2π* backdonation. The data presented in Fig. 3c indicated that the valence state of Ir in IrCu/TiO2 should be Ir + rather than Ir -, which was opposite to the previously stated description. Moreover, the variation between 2130 cm-1 and 2128 cm-1 was too small and fell within the normal error margin of infrared spectroscopy. It was recommended that XAS be employed to conduct further analysis of Cu and Ir in order to support the polar structure of Ir --Cu +.\n5、Support was expressed for the authors' proposal that CuIr alloys could regulate the oxygen vacancy content. However, the experimental results did not fully support the Cuδ+-Irδ- structure; instead, they provided stronger evidence for the existence of a strong interaction between the CuIr alloy and TiO2. The need to consider how to better decouple the interface effect from the alloy effect was indicated. Attention was directed to the page numbers in the Supporting Information.\n6、 Please specify the light intensity used in the photocatalytic experiments. It should be clarified whether the light intensity was calibrated or standardized. We recommend conducting key photocatalytic performance tests under a standardized intensity of 100 mW/cm2 to enable a more direct and fair comparison with other reported photocatalysts. Additionally, in Supplementary Table S8, where performance comparisons are made with other photocatalysts, please supplement the corresponding light intensities and wavelength ranges used in those reference studies.\n7、The absence of detectable H₂ production over both IrCu/TiO2 and Cu/TiO2 within 4 hours of reaction, as indicated in Supplementary Fig. 19 and Supplementary Table 6, is an interesting observation. Given that the multi-step hydrogenation of CH4 requires protons derived from water dissociation, it is thermodynamically and kinetically highly challenging to completely suppress H2 evolution, as HER is often a dominant competitive pathway. For example, in a similar Cu/TiO2 system reported in PNAS (2023, 120, 44, e2307320120), H2 evolution was clearly detected. To rigorously rule out potential interference from the detection limit or instrument sensitivity, the authors should provide: (i) A calibration curve for H2 at various concentrations to confirm the detection threshold of the gas chromatograph used; (ii) Unprocessed, original chromatograms (raw peak data, without any post-processing, screenshot) for the product analysis during the photocatalytic tests.\n8、The photocatalytic CO2 reduction performance (yield and selectivity) presented in Fig. 4a, b, d, and f appears to be based on a single measurement. To ensure reproducibility and statistical reliability, it is recommended that the authors repeat these experiments and report the results with error bars derived from at least three independent tests.\n9、While the authors report good reproducibility and stability of IrCu/TiO2 for CH4 production in cycling tests in Fig. 4f , this conclusion relies on a single type of evidence. To more convincingly demonstrate the catalyst's stability, it is essential to provide complementary post-reaction characterizations. Please provide comparative TEM, XPS, and EPR data for the catalyst before and after testing to assess morphological changes, oxidation states, and oxygen vacancy concentration.\n10、The proposed role of Ir species in modulating proton transfer from water vapor to achieve high CH4 selectivity is impressive. However, the oxidation of water to O2, especially in the absence of any sacrificial agent, is kinetically challenging and often the rate-limiting step in such overall photocatalytic CO2 reduction. The manuscript mentions that air was introduced during manual injection and O2 was not quantified. To clarify the water oxidation pathway, it is suggested that the authors conduct and report a quantitative analysis of O2 production, for instance, by introducing an internal standard (e.g., N2) for calibration. This would help determine whether water dissociation primarily generates hydroxyl species (*OH) or leads to direct O2 evolution.\n11、While this work provides a detailed DFT analysis of the CO hydrogenation step to CHO, the free energy data of subsequent continuous hydrogenation steps from CHO to CH4 (e.g., *CHO → *CH2O → *CH3O) are lost. Presenting the complete reaction energy profile is crucial for elucidating the full eight-electron reduction pathway to CH4.\n12、 For the figures displaying atomic configurations (e.g., Fig. 2 and Fig. 5), it is recommended to include a legend indicating which atom each color represents. This would significantly enhance the readability and clarity of the presented structural information.\n13、Regarding the setting of active sites on the IrCu/TiO2 catalyst, the current model defaults to adsorbing key intermediates (CO2, COOH, CO, CHO) on Ir sites (see Supplementary Fig. 27 and 35), which may present a logical inconsistency with the \"*H migration to *CO-Cu\" mechanism proposed in the main text. It is suggested that the authors clarify whether they have considered analyzing the Cu sites as potential reaction sites. Additionally, does the co-adsorption configuration of *H and *OH on IrCu/TiO2 in Supplementary Fig. 37 correspond to the energy-optimized state? Given that Ir typically exhibits strong adsorption capacity for *H, whether *H is more inclined to adsorb on Ir sites rather than Cu sites also requires further clarification.", "reviewer_3": "The manuscript reported some interesting results. However, it is not suitable for publication in Nature Communications. The following problems have to be addressed.\n1. The English have to be polished carefully, and the tense should be consistent.\n2. Where was the catalytic active site? The authors have to give more discussions.\n3. The roles of every component of catalyst in the reaction have to be discussed detailedly.\n4. Catalysts after reaction have to be more detailedly analyzed, e.g. TEM, XPS and XRD.\n5. What was the fate of the O in CO2? Where did the O in CO2 go? The authors have to give more evidence.\n6. More references have to be added into Figure 4g, as there are a large number of papers about photoreduction of CO2 to CH4. In some references, the CH4 production rates and selectivity were higher than the results reported in the present work.\n7. The CH4 production rate was described by using μmol g-1 h-1. However, the photocatalyst used in experiments was only 50 mg. Thus, the authors enlarged the CH4 production rate by 20 times. The authors have to provide evidence which demonstrates that CH4 production rate is linearly related to photocatalyst amount.\n8. Reaction mechanism should be more detailedly discussed."}, "author_response": "Point-by-Point Response to Reviewers' Comments\nWe appreciate the reviewers for their thoughtful suggestions and comments concerning\nour manuscript entitled \"Asymmetric charge-polarization tailoring active hydrogen\ntransfer for selective photoreduction CO to CH \" (ID: NCOMMS-25-95618). Those\n2 4\ncomments are all valuable and very helpful for revising and improving our paper, as\nwell as the important guiding significance to our research. We have studied each\ncomment carefully and made revisions which we hope to achieve a satisfactory level\nfor publication in Nature Communications. Black represents the question, while Blue\nrepresents the response, all the changes in the content were highlighted in yellow\nbackground in the revised manuscript. The following are point-to-point responses to\nreviewers' comments:\nReviewer #1:\nIn this manuscript, the authors present a dual-functional alloy IrCu/TiO to enable\n2\nhighly selective photoreduction CO to CH , through modulating the gas-solid\n2 4\ninterfacial water vapor. Furthermore, the “Cuδ+ -Irδ-” polarized structure in IrCu/TiO\n2\ngenerates stronger interfacial electronic interactions. This process serves to enhance the\ncovalent nature of the Ti-O bond, thereby promoting the generation of oxygen vacancies\n(Ov). This, in turn, facilitates carbon dioxide capture and the formation of long-lived\ncharge carriers. The work is of high quality and is suitable for publication after minor\nrevisions. The following issues should be addressed before final acceptance.\nResponse: We sincerely appreciate the reviewer’s comments on our manuscript. We\nhave made revision based on the reviewer’s suggestions.\n1. In the two in-situ infrared spectra under consideration, certain absorption peaks\nexhibiting distinct trends (e.g. the peak near 1455 cm-1 around 1440 cm-1) lack explicit\nfunctional group assignments. The purpose of this study is to ascertain the vibrational\norigins of the unlabelled peaks.\nResponse: Thanks for your comment. According to the literature (Angew. Chem. Int.\nEd., 2024, 63, e202404660; Adv. Mater., 2022, 34, 2200057), the in situ DRFITS was\nreanalyzed and showed in the revised manuscript (Fig. 5g, h, and Supplementary Fig.\n28). Characteristic peaks for *COOH (1508, 1540, 1720 cm-1), *CH O (1445 cm-1), m-\n3 3\nCu/TiO , Ir/TiO , and IrCu/TiO samples. Interestingly, on the IrCu/TiO sample, the\n2 2 2 2\nintensity of *CH O was more intensive than that of m-CO 2- and b-CO 2- intermediate,\n3 3 3\nindicating that carbonate intermediates on IrCu/TiO are more prone to hydrogenation.\n2 2 2 2\nlight irradiation.\n2. The authors construct IrCu bimetallic nanoalloys supported on TiO nanosheets via\n2\nan ethylene glycol-assisted reduction method. The question of whether this affected the\nproduct formation remains to be answered. In addition to the primary gaseous products\nCH and CO that are generated during photoreduction, could liquid products also be\n4\nformed?\nResponse: Thanks for your comment. To evaluate whether the ethylene glycol affected\nthe product formation in the photocatalytic CO reduction, a series of systematic blank\n2\nexperiments were conducted in Fig. 4d. In the absence of light irradiation or\nphotocatalysts or CO reactant, there were no products detected, demonstrating that the\n2\ndetermined products originate from the CO reactant in the photocatalytic reduction\n2\nprocess. In order to further verify that the gas products were only from the CO feed,\n2\nthe 13CO isotopic labelling experiments were carried out. In Fig. 4e, only 13CO\n2\n(m/z=29) and 13CH (m/z=17) were detected, which clearly rules out the contributions\n4\nfrom other carbon-containing species, such as ethylene glycol. Therefore, it is\nconcluded that the ethylene glycol-assisted reduction method has no influence on the\nproduct formation. In addition, no liquid products are detected. We have provided more\ndiscussion in the revised manuscript (Page 10).\nFig. 4. d, Control blank experiments. e, The GC-MS spectra of the products from the\nphotocatalytic 13CO reduction.\n2\n2\n3. The authors stated that the Cu 2p XPS peaks of IrCu/TiO showed a slight shift to\n2\nhigher binding energy compared to the Cu/TiO nanosheets. However, based on the\n2\ndata, there is essentially no deviation.\nResponse: Thanks for your comment. We re-conducted the XPS measurements and\nsubsequently analysis. The Ir 4f peak in the XPS spectra of IrCu/TiO shifted to a lower\n2\nbinding energy by 0.35 eV compared to the Ir/TiO spectra, suggesting of the electron-\n2\nrich Ir species in the alloys (Fig. 3a). In the XPS spectra of Cu 2p (Fig. 3b), compared\nwith Cu/TiO , the Cu 2p and Cu 2p peaks of IrCu/TiO shifted to a higher binding\n2 1/2 3/2 2\nenergy by 0.67 eV. This result indicated that the electrons transfer from Cu to Ir to\nconstruct the “Cuδ+-Irδ-” structure. We have provided more discussion in the revised\nmanuscript (Page 6, Fig. 3a, b).\nFig. 3. a, The Ir 4f XPS spectra of Ir/TiO and IrCu/TiO . b, The Cu 2p XPS spectra of\n2 2\nCu/TiO and IrCu/TiO .\n2 2\n4. The five-cycle test in Figure 4f shows good catalytic stability, but the structural\nchanges of the catalyst after cycling are not described. Have the structural integrity of\nthe catalyst after cycling been verified by characterizations such as XRD, TEM,\nand XPS?\nResponse: Thanks for your comment. We conducted XRD, EPR, XPS, and STEM\nanalyses to study the structural, morphological and electronic properties of IrCu/TiO\n2\nphotocatalysts before and after the reaction. The XRD analysis confirmed that the\nanatase crystal structure of TiO was retained throughout the catalytic process\n2\n(Supplementary Fig. 23a). The EPR results showed that the signals of the fresh and\nused IrCu/TiO photocatalysts were similar (Supplementary Fig. 23b). The Ti 2p XPS\n2\nspectral results also indicated that there was no change in TiO supports before and after\n2\nthe reaction. Moreover, the Ir 4f XPS spectra, the Cu 2p XPS spectra, and the Cu LMM\nAES also demonstrated that the Ir and Cu species remained unchanged before and after\nthe reaction for IrCu/TiO photocatalysts (Supplementary Fig. 24). Aberration-\n2\ncorrected high-angle annular dark-field scanning transmission electron microscopy\n(AC-HAADF-STEM) images confirmed that the IrCu nanoclusters and TiO sheets\n2\nwere stable throughout the catalytic process (Supplementary Fig. 2, Fig. 25).\nTherefore, these results demonstrate its good catalytic stability. We have provided more\n3\ndiscussion in the revised manuscript (Page 10).\nSupplementary Fig. 23. a, XRD patterns, and b, EPR results of IrCu/TiO catalysts\n2\nbefore and after reaction.\nSupplementary Fig. 24. a, The Ti 2p XPS spectra, b, Ir 4f XPS spectra, c, Cu 2p XPS\nspectra, and d, Auger electron spectra of Cu LMM for IrCu/TiO catalysts before and\n2\nafter reaction.\ndistributions for the fresh IrCu/TiO catalyst.\ndistributions for the used IrCu/TiO catalyst.\n2\n5. In terms of structural characterization, the authors only performed Ti k-edge XANES.\nThe characterization of the Ir-Cu alloy seems insufficient. XANES can be conducted\non Ir and Cu to explain their electronic and coordination properties.\nResponse: Thanks for the reviewer’s comment. To better explore the electron\ninteraction in IrCu alloys, we further employed X-ray absorption near-edge structure\n(XANES). The white line intensity and absorption edge of the IrCu/TiO was closely\n2\naligned to the metallic Ir foil (Fig. 3c), indicating metallic state of the Ir. The Cu K-\nedge XANES spectra showed that the absorption edge of the IrCu/TiO sample fell\n2\nbetween the Cu foil and CuO (Fig. 3d), suggesting that the average valance state of\nCuδ+ species is between 0 and +2. This is due to the electronic transfer from Cu to Ir,\nbased on the XPS results. Furthermore, the coordination properties of Ir and Cu species\nwere investigated by the extended X-ray absorption fine structure (EXAFS)\nspectroscopy, and the corresponding structural parameters were analyzed. The Ir L -\n3\nedge Fourier-transform (FT)-EXAFS spectrum of IrCu/TiO exhibited three\n2\n5\ncoordination peaks with the bond distances (R) of 2.01-2.72Å (Supplementary Fig.\n5a). Three coordination structures were identified: Ir-Cu metallic bonding (CN=2.3 at\nR=2.53 Å), Ir-Ir coordination (CN=4.9 at R=2.72 Å), and Ir-O interactions (CN=0.9 at\nR=2.01Å) (Supplementary Table 4). The lower coordination numbers of Ir-O (CN<2)\nare attributed to surface-adsorbed oxygen species or interfacial interactions with the\nsupport (Nat. Commun., 2026, 17, 1084). Additionally, the Cu K-edge FT-EXAFS\nanalysis of IrCu/TiO confirmed the formation of Ir-Cu metallic bonding, with the\n2\nreduced Cu-Cu coordination numbers. Therefore, the IrCu alloys with the “Cuδ+-Irδ-”\nstructure are present in the IrCu/TiO catalyst. We have provided more discussion in\n2\nthe revised manuscript (Pages 6, and 7).\nFig. 3. c, Normalized Ir L -edge XANES spectra, and d, normalized Cu K-edge\n3 2\nand corresponding references.\nSupplementary Table 4. Ir L -edge EXAFS fitting results of IrCu/TiO .\n3 2\nSamples Shells CN R (Å) △E (eV) σ2 (10-2 Å) R factor\nIr-O 2 1.90 0.003\nIrO 9.80 0.011\nIrCu/TiO Ir-Cu 2.3 2.53 9.36 0.005 0.004\nCN, coordination number; R, the distance between absorber and backscatter atoms; ΔE ,\n0\ninner potential correction to account for the difference in the inner potential between\nthe sample and the reference compound; σ2, the Debye Waller factor value; R factor\nindicates the goodness of the fit. S 2 was fixed to 0.90, according to the experimental\nfixed during EXAFS fitting, based on the known structure of Ir. Fitting conditions: k\nrange: 3-10; R range: 1-3 (Ir sample). A reasonable range of EXAFS fitting parameters:\n0.800 < S 2< 1.000; CN > 0; σ2 > 0 Å2; |ΔE | < 10 eV; R factor < 0.02.\n0 0\nSupplementary Table 5. Cu K-edge EXAFS fitting results of IrCu/TiO .\nCu O Cu-O 2 1.87 9.31 0.004 0.018\nCu-Ir 2.6 2.64 0.009\nIrCu/TiO 1.83 0.006\nCN, coordination number; R, the distance between absorber and backscatter atoms; ΔE ,\n0\ninner potential correction to account for the difference in the inner potential between\nthe sample and the reference compound; σ2, the Debye Waller factor value; R factor\nindicates the goodness of the fit. S 2 was fixed to 0.90, according to the experimental\nconditions: k range: 3-10; R range: 1-3 (Cu sample). A reasonable range of EXAFS\nfitting parameters: 0.800 < S 2< 1.000; CN > 0; σ2 > 0 Å2; |ΔE | < 10 eV; R factor <\n0 0\n0.02.\n6. In the Gibbs free energy diagram of Figure SI 24, Ir/TiO exhibits a lower energy\n2\nbarrier, which should facilitate easier CO generation. However, its CO production\nperformance is quite low in Figure 4a.\nResponse: Thanks for the reviewer’s comment. The CO production performance is\nrelated to the free energy barrier of *CO-to-CO. Furthermore, the detailed free energy\nbarrier of *CO-to-CO on Ir/TiO was calculated to be 4.420 eV. This relatively high\n2\nenergy barrier indicates that it is hard to generate CO product on Ir/TiO\n2\n(Supplementary Fig. 30), consistent with the photocatalytic activity.\n2\n7. Some relevant literatures may be discussed and cited (10.1007/s40843-025-3464-4;\n10.1002/adma.202503021;10.1002/anie.202421353;10.1021/jacs.4c09841;10.1039/d4\ncs00186a).\nResponse: Thanks for your comment. These relevant literatures (10.1007/s40843-025-\n3464-4; 10.1002/adma.202503021; 10.1002/anie.202421353; 10.1021/jacs.4c09841;\n10.1039/d4cs00186a) have been added in the revised manuscript (Ref. 63, 17, 15, 4, 6).\nThis manuscript reports the construction of an IrCu/TiO bimetallic photocatalyst that\n2\nachieves highly selective (CH : 98.7%) and efficient (AQE = 7.9% at 365 nm)\n4\nphotocatalytic CO reduction to CH under sacrificial-agent-free conditions using water\n2 4\nvapor as the proton source. The core innovation lies in the proposed concept of tailoring\nthe hydrogen-bond network and proton-transfer kinetics of interfacial water molecules\nvia an asymmetric charge-polarized “Cuδ+-Irδ-” structure, thereby overcoming the\nselectivity bottleneck between CO reduction and the hydrogen evolution reaction. The\n2\ntopic is timely and significant for advancing solar fuel synthesis and carbon cycling.\nHowever, the manuscript should be heavily improved and polished in terms of rigor in\nkey data, depth of mechanistic elucidation, and completeness of control experiments.\nThus, this work needs some major revision prior to be considered in Nature\nCommuncations. The specific comments are as follows.\nResponse: We would like to express our sincere appreciation for the reviewer’s\ncomments on our manuscript. Further experimentation (XANES, XRD, TEM, XPS and\nEPR) and DFT calculations have been carried out, in order to analyze the photocatalysts’\nstructures and reveal the reaction mechanism. Each comment has been given serious\nconsideration. We hope that our revised manuscript will be accepted for publication.\n1. Due to the difference in electronegativity between Ir and Cu, an Ir--Cu+ polar\nstructure could be formed. The question was then raised as to whether the same effect\ncould be achieved with Ru and Pt. Furthermore, non-noble metals such as Fe, Co, and\nNi also demonstrated excellent water dissociation ability, raising the possibility of\nwhether they could replace noble metals like Ir.\nResponse: We would like to thank the reviewer for raising this insightful point. In our\nwork, excellent CH generation efficiency on IrCu/TiO is mainly resulted from the\n4 2\nalloy effect on the available generation and transfer of hydrogen protons. Furthermore,\na strongly polar structure of “Cuδ+-Irδ-” in IrCu/TiO interface prolongs photoelectron\n2\nlifetimes, which is critical for the sluggish eight-electron reaction in CH formation.\nis a contributing factor to CH product, and the generation and transfer of active\n4\nhydrogen from water dissociation to inhibit HER is also important.\nFor other metals, such as Pt or Ru, or even non-noble metals like Fe, Co, and Ni,\nthe electronegativity order is as follows: Pt (2.28) > Ir (2.20) ≈ Ru (2.20) > Ni (1.91) ≈ Cu\n(1.90) ≈ Co (1.88) > Fe (1.83). Similar to Ir, the Pt and Ru metals have the excellent\nwater-splitting ability. However, Pt is very near the top of the hydrogen volcano (ΔG\nH\n= -0.09 eV), while the ΔG of Ir is only 0.12 eV. Thus, Pt is the best-performing catalyst\nH\nfor the HER (Science, 2017, 355, 146; Nat. Mater., 2023, 22, 619-626). Additionally,\nit has been reported that the H is generated as a by-product on PtCu-based\n2\nphotocatalysts, alongside CH (ACS Catal., 2025, 15, 828-840). Ru is facilitated to the\n4\nadsorption of key oxygen intermediates (*OH, *O, and *OOH) (Science, 2025, 387,\n48-55; Nat. Commun., 2025, 16, 579; Nat. Commun., 2025, 16, 6716). The adsorption\nbehavior of *CHOCO intermediates can be optimized using a RuCu-based catalyst, and\nthe C H product, not CH is obtained (Nat. Commun., 2026, 17, 1084). For non-noble\n2 4 4\n9\nmetals such as Fe, Co, and Ni, although they show promising water-splitting activity,\ntheir electronegativities are similar to that of Cu, which could not replace Ir.\n2. “In addition, the DOS results demonstrate that the wider Ti-O orbital overlap in\nIrCu/TiO indicates the higher Ti-O covalency degree.” However, the Ti K-edge R-\n2\nspace k3-weighted Fourier transform EXAFS results indicated a longer Ti-O bond\nlength in CuIr/TiO , suggesting a reduction in the Ti-O covalent character. The\n2\nelongation of the Ti-O bond was indeed more conducive to the formation of oxygen\nvacancies, a point that required further explanation by the authors.\nResponse: We thank you for this valuable suggestion. Based on EXAFS fitting results,\nthe elongation of the Ti-O bond and the reduction in the coordination number indicated\na decrease in Ti-O covalency in the IrCu/TiO catalyst (Supplementary Table 6),\n2\nwhich facilitates the Ti-O bond breaking and oxygen vacancy formation. This\nconclusion is consistent with the EPR data (Supplementary Fig. 11). We consider that\nthe Ti-O orbital overlap obtained from the width measurement in the DOS is not\nrigorous enough. The following description: “In addition, the DOS results demonstrate\nthat the wider Ti-O orbital overlap in IrCu/TiO indicates the higher Ti-O covalency\n2\ndegree” has been deleted in the revised manuscript.\n3. “The peak of Ir 4f XPS spectra in IrCu/TiO shifts to a lower binding energy by 0.13\n2\neV than Ir/TiO , …… indicating that the electron transferred from Cu to Ir enables the\n2\nformed structure of “Cuδ+-Irδ-”. A shift of 0.13 eV fell within the normal error range of\nXPS, and the valence state of Cu was Cu+, which could be fully attributed to electron\ntransfer from Cu to O, thereby weakening the Ti-O bond.\nResponse: Thanks for your comment. We re-conducted the XPS measurements to\ninvestigate the peak shift. The Ir 4f peak in the XPS spectra of IrCu/TiO shifted to a\n2\nlower binding energy by 0.35 eV compared to the Ir/TiO spectra, suggesting of the\n2\nelectron-rich Ir species in the alloys (Fig. 3a). In the XPS spectra of Cu 2p (Fig. 3b),\ncompared with Cu/TiO , the Cu 2p peaks of IrCu/TiO shifted to a higher binding\n2 2\nenergy by 0.67 eV, indicating that the electron transfer is attributed to the “Cuδ+-Irδ-”\nstructure (Page 6, Fig. 3a-b).\nFurthermore, the Bader charge analysis confirmed that the formed Cu+ species is\nattributed to electron transfer from Cu to Ir. In Supplementary Table 2, the average Ir\ncharges of IrCu/TiO and Ir/TiO were -0.302 e and -0.033 e, respectively. It is\n2 2\nsuggested that the introduce of Cu significantly increases the electrons of Ir, which is\ncoming from the electron transfer from Cu to Ir. If the electrons are transferred from Cu\nto O in IrCu/TiO , its average charge on O atom should be the lowest among the three\n2\ncatalysts. However, the calculated average charges of O atom followed the order:\nCu/TiO (-0.990 e)＜Ir/TiO (-0.982 e) ＜IrCu/TiO (-0.978 e), indicating of less\n2 2 2\nelectron transfer from Cu to O in IrCu/TiO . Therefore, the Cu+ species is resulted from\n2\nthe “Cuδ+-Irδ-” structure.\n2 2\nfor Cu/TiO and IrCu/TiO .\n2 2\n4. “As depicted in Fig. 3c, ……, whereas a lower wavenumber shift indicates a\ndecreased electron density.” This phenomenon was essentially attributed to d→2π*\nbackdonation. The data presented in Fig. 3c indicated that the valence state of Ir in\nIrCu/TiO should be Ir+ rather than Ir-, which was opposite to the previously stated\n2\ndescription. Moreover, the variation between 2130 cm-1 and 2128 cm-1 was too small\nand fell within the normal error margin of infrared spectroscopy. It was recommended\nthat XAS be employed to conduct further analysis of Cu and Ir in order to support the\npolar structure of Ir--Cu+.\nResponse: Thanks for your comment. As the reviewer mentioned, in the in situ DRIFTS\nof CO adsorption, the peak of the adsorbed CO species is attributed to the interaction\nbetween d-electrons (metal) to the LUMO (2π*) orbital of CO molecules. However, the\nvibration was too small and fell within the normal error margin of infrared spectroscopy.\nThus, in situ DRIFTS of CO adsorption was replaced by XAS spectra. The XANES and\nEXAFS were then analyzed for the polarized “Cuδ+-Irδ-” structure.\nIn the normalized Ir L -edge XANES spectra, the white line intensity and absorption\n3\nedge of the IrCu/TiO was closely aligned to the metallic Ir foil (Fig. 3c), indicating\n2\nmetallic state of the Ir. The Cu K-edge XANES spectra showed that the absorption edge\nof the IrCu/TiO sample was between the Cu foil and CuO (Fig. 3d). This indicates that\n2\nthe average valance state of Cuδ+ species is between 0 and +2, which is due to the\nelectronic transfer from Cu to Ir, and the polarized “Cuδ+-Irδ-” structure is formed,\naccording to the XPS results.\nFurthermore, the coordination properties of Ir and Cu species were investigated by\nthe extended X-ray absorption fine structure (EXAFS) spectroscopy, and the\ncorresponding structural parameters were analyzed. The Ir L -edge Fourier-transform\n3\n(FT)-EXAFS spectrum of IrCu/TiO exhibited three coordination peaks with the bond\n2\ndistances (R) of 2.01-2.72Å (Supplementary Fig. 5a). Three coordination structures\nwere identified: Ir-Cu metallic bonding (CN=2.3 at R=2.53 Å), Ir-Ir coordination\n(CN=4.9 at R=2.72 Å), and Ir-O interactions (CN=0.9 at R=2.01Å) (Supplementary\nTable 4). The lower coordination numbers of Ir-O (CN<2) are attributed to surface-\nadsorbed oxygen species or interfacial interactions with the support (Nat. Commun.,\n2026, 17, 1084). Additionally, the Cu K-edge FT-EXAFS analysis of IrCu/TiO\n2\nconfirmed the formation of Ir-Cu metallic bonding, with the reduced Cu-Cu\n11\ncoordination numbers. Therefore, the IrCu alloys with the “Cuδ+-Irδ-” structure are\npresent in the IrCu/TiO catalyst. We have provided more discussion in the revised\n2\nmanuscript (Pages 6, and 7).\nFig. 3. c, Normalized Ir L -edge XANES spectra, and d, normalized Cu K-edge\n3 2\nand corresponding references.\nSupplementary Table 4. Ir L -edge EXAFS fitting results of IrCu/TiO .\n3 2\nSamples Shells CN R (Å) △E (eV) σ2 (10-2 Å) R factor\nIr-O 2 1.90 0.003\nIrO 9.80 0.011\nIr-O 0.9 2.01 0.003\nIrCu/TiO Ir-Cu 2.3 2.53 9.36 0.005 0.004\nCN, coordination number; R, the distance between absorber and backscatter atoms; ΔE ,\n0\ninner potential correction to account for the difference in the inner potential between\nthe sample and the reference compound; σ2, the Debye Waller factor value; R factor\n12\nindicates the goodness of the fit. S 2 was fixed to 0.90, according to the experimental\nfixed during EXAFS fitting, based on the known structure of Ir. Fitting conditions: k\nrange: 3-10; R range: 1-3 (Ir sample). A reasonable range of EXAFS fitting parameters:\n0.800 < S 2< 1.000; CN > 0; σ2 > 0 Å2; |ΔE | < 10 eV; R factor < 0.02.\n0 0\nSupplementary Table 5. Cu K-edge EXAFS fitting results of IrCu/TiO .\nCu O Cu-O 2 1.87 9.31 0.004 0.018\nCu-Ir 2.6 2.64 0.009\nIrCu/TiO 1.83 0.006\nCN, coordination number; R, the distance between absorber and backscatter atoms; ΔE ,\n0\ninner potential correction to account for the difference in the inner potential between\nthe sample and the reference compound; σ2, the Debye Waller factor value; R factor\nindicates the goodness of the fit. S 2 was fixed to 0.90, according to the experimental\nconditions: k range: 3-10; R range: 1-3 (Cu sample). A reasonable range of EXAFS\nfitting parameters: 0.800 < S 2< 1.000; CN > 0; σ2 > 0 Å2; |ΔE | < 10 eV; R factor <\n0 0\n0.02.\n5. Support was expressed for the authors' proposal that CuIr alloys could regulate the\noxygen vacancy content. However, the experimental results did not fully support the\nCuδ+-Irδ- structure; instead, they provided stronger evidence for the existence of a strong\ninteraction between the CuIr alloy and TiO . The need to consider how to better\n2\ndecouple the interface effect from the alloy effect was indicated. Attention was directed\nto the page numbers in the Supporting Information.\nResponse: Thanks for your comment. The XPS measurements were re-conducted, with\nthe results in Fig. 3a-b, and the XAS characterization was carried out to further confirm\nthe polarized “Cuδ+-Irδ-” structure in Fig. 3c-d. The white line intensity and the\nabsorption edge of the Ir L -edge XANES in the IrCu/TiO are closely located to the\n3 2\nmetallic Ir foil, indicating that Ir exists predominantly in a metallic state (Fig. 3c). For\nthe Cu K-edge XANES spectra, the absorption edge of the IrCu/TiO sample is between\n2\nthe Cu foil and CuO, confirming the average valance state of Cuδ+ species is between 0\nand +2 (Fig. 3d). Therefore, based on the results of XAS, XPS and Bader charge, a\npolarized “Cuδ+-Irδ-” structure is formed in IrCu/TiO .\n2\nof interfacial oxygen vacancies (O ) in TiO , prolonging photoelectron lifetimes, based\nv 2\non the results of EPR, electrochemical impedance, photocurrent-time, and TRPL\nspectra. The interface effect enhances the sluggish eight-electron reaction in CH\n4\nformation. Additionally, the interfacial O -assisted alloys promotes CO adsorption and\nv 2\nactivation.\nThe role of alloys significantly affects the CH selectivity. Ir promotes H O\n4 2\n13\ndissociation, while hydrophobic Cu restructures the interfacial hydrogen-bond network.\nThis IrCu alloys synergistically alter the configuration of interfacial H O and facilitate\n2\nthe migration of *H along the reconstructed HBN, ultimately lowering the energy\nbarrier for the key intermediate *CHO generation, and thereby significantly boosting\nCH selectivity. Therefore, the interface effect increases the number of photoelectrons\n4\navailable for the reaction activity, and the alloy effect is important for CH selectivity.\nThe page numbers in the Supporting Information have been carefully revised.\n6. Please specify the light intensity used in the photocatalytic experiments. It should be\nclarified whether the light intensity was calibrated or standardized. We recommend\nconducting key photocatalytic performance tests under a standardized intensity of 100\nmW/cm2 to enable a more direct and fair comparison with other reported photocatalysts.\nAdditionally, in Supplementary Table S8, where performance comparisons are made\nwith other photocatalysts, please supplement the corresponding light intensities and\nwavelength ranges used in those reference studies.\nResponse: Thanks for your comment. The light intensity used in the photocatalytic\nexperiments was 500 mW cm-2 of the 300W Xe lamp source. We are sorry for the\nabsence of photocatalytic activities under the standard light intensity, and the used light\nintensity (500 mW cm-2) have been added in the method section in the revised\nmanuscript (Page 18). Additionally, the corresponding light intensities and wavelength\nranges used in those reference studies have been added in revised Supplementary\nTable S10, in order to a more direct and fair comparison with other reported\nphotocatalysts.\nSupplementary Table 10. Comparison of the CH evolution rate, selectivity and AQE\n4\nwith other recent metal-based catalysts for photocatalytic CO reduction.\n4 4\nWavelength AQE\nCatalysts Light intensity (μmol selectivity Ref.\nrange (%)\n(mW cm-2) a g-1 h-1) (%)\n7.91\nIrCu/TiO 500 100.3 98.6 (365\nnm) This\nwork\nIr Cu /TiO 500 38.2 100.0 N. M.\n1 2 2\nXe spectrum\nZZ-Ag-Pt 320-780 nm N. M. 16.23 98.9 N. M 10\nXe\nCoRu-HCNp N. M. 2.1 N. M. N. M 11\nXe spectrum\n14\nAu Cd S 600 11.3 N. M. N. M 12\nSA 1-x\nXe spectrum\nCu/CZS-Vs λ≥400 nm 200 38 N. M. N. M 13\nXe\nEquipped\nPd/Cu 300 W\n3 with an AM N. M. 29.9 100 N. M 14\n(HITP) /TiO Xe\n2 2\n1.5 G filter\nPd Cu /TiO λ＜400 nm 2 19.6 95.9 N. M. 15\n7 1 2\nXe\nCeCo-PTI N.M. 45.4 88.3 N.M. 16\nXe spectrum\nRu-Ov/TiO N.M. 50.1 81 N.M. 17\n1.03\nCu /BT N.M. 19.63 98 (365 18\n1+NCs\nXe spectrum\nnm)\nMn/Co O 320-780 nm 80 23.4 94.23 N.M. 19\n3 4\nXe\nPd-HPP-TiO 325-780 nm N.M. 22.0 59 N.M. 20\n1.66\nAu/TiO -V N.M. 156.5 100 (300 21\nXe spectrum\nnm)\n2.36\nPd-ZnO-Vo N.M. 257.6 95.3 (280 22\nXe spectrum\nnm)\n0.11\nPt Cu 300 W Full\n0.5 0.5 N. M. 12.4 97.0 (365 23\n/TiO Xe spectrum\n2\nnm)\n0.98\nPtRu/TiO 320-780 nm N. M. 38.7 93.7 (380 24\nnm)\n(PdCu) -TiO N. M. 18.1 98.7 N. M 25\n2 2\nXe spectrum\nRuCu/CRP N. M. 19.7 96.0 N. M 26\nXe spectrum\na: N. M. means not mentioned\n15\n7. The absence of detectable H₂ production over both IrCu/TiO and Cu/TiO within 4\n2 2\nhours of reaction, as indicated in Supplementary Fig. 19 and Supplementary Table 6, is\nan interesting observation. Given that the multi-step hydrogenation of CH requires\n4\nprotons derived from water dissociation, it is thermodynamically and kinetically highly\nchallenging to completely suppress H evolution, as HER is often a dominant\n2\ncompetitive pathway. For example, in a similar Cu/TiO system reported in PNAS\n2\n(2023, 120, 44, e2307320120), H evolution was clearly detected. To rigorously rule\n2\nout potential interference from the detection limit or instrument sensitivity, the authors\nshould provide: (i) A calibration curve for H at various concentrations to confirm the\n2\ndetection threshold of the gas chromatograph used; (ii) Unprocessed, original\nchromatograms (raw peak data, without any post-processing, screenshot) for the\nproduct analysis during the photocatalytic tests.\nResponse: Thanks for your comment. The calibration curve for H at various\n2\nconcentrations is showed in Table R1 and Fig. R1, and the detection threshold of the\ngas chromatograph used was 0.06 μmol. The original chromatograms for H analysis\n2\nduring the photocatalytic CO reduction on Ir/TiO , IrCu/TiO , and Cu/TiO samples\n2 2 2 2\nare displayed in Fig. R2.\nIn the PNAS study cited in the comment, the Cu species in the Cu/TiO catalyst and\n2\nphotocatalytic testing conditions differ from those in our work, which may affect the\nproduct.\nTable R1. The calibration peak area (A) for H at various concentrations (C).\n1 0.5 20999.7\n2 1 41497.8\n3 2 82423.5\n4 3 123069.3\n5 4 160651.4\n2 2 2\nCu/TiO , respectively.\n2\n8. The photocatalytic CO reduction performance (yield and selectivity) presented in\nreproducibility and statistical reliability, it is recommended that the authors repeat these\nexperiments and report the results with error bars derived from at least three\nindependent tests.\nResponse: Thanks for your comment. The photocatalytic CO reduction performance\n2\n(yield and selectivity) were repeated, and the corresponding error bars were added in\nFig. 4a, b, d, and f.\n2 x 2 2\nin a gas-solid reactor under CO and H O at room temperature without additional\n2 2\nphotosensitizers or sacrificial agents. b, The product selectivity for Cu/TiO , Ir/TiO\n2 2\nand IrCu/TiO . d, Control blank experiments, f, cycling tests for five runs on IrCu/TiO .\n2 2\n9. While the authors report good reproducibility and stability of IrCu/TiO for CH\n2 4\nproduction in cycling tests in Fig. 4f, this conclusion relies on a single type of evidence.\nTo more convincingly demonstrate the catalyst's stability, it is essential to provide\ncomplementary post-reaction characterizations. Please provide comparative TEM, XPS,\nand EPR data for the catalyst before and after testing to assess morphological changes,\noxidation states, and oxygen vacancy concentration.\nResponse: Thanks for your comment. We conducted XRD, EPR, XPS, and STEM\nanalyses to study the structural, morphological and electronic properties of IrCu/TiO\n2\nbefore and after the reaction. The XRD analysis confirmed that the anatase crystal\nstructure of TiO was retained throughout the catalytic process (Supplementary Fig.\n2\n23a). The EPR results showed that the Ti3+ and oxygen vacancy concentration of the\nfresh and used IrCu/TiO photocatalysts were comparable (Supplementary Fig. 23b).\n2\nafter the reaction. Moreover, the Ir 4f XPS, the Cu 2p XPS, and the Cu LMM AES also\ndemonstrated that the Ir and Cu species remained unchanged before and after the\nreaction for IrCu/TiO photocatalysts (Supplementary Fig. 24). Aberration-corrected\n2\nhigh-angle annular dark-field scanning transmission electron microscopy (AC-\nHAADF-STEM) images confirmed that the IrCu nanoclusters and TiO sheets were\n2\n18\nstable throughout the catalytic process (Supplementary Fig. 25). Therefore, the\nmorphological structure, valence states, and oxygen vacancy concentration of the\nIrCu/TiO photocatalysts before and after reaction demonstrate its good catalytic\n2\nstability. We have provided more discussion in the revised manuscript (Page 10).\nSupplementary Fig. 23. a, XRD patterns, and b, EPR results of IrCu/TiO catalysts\n2\nbefore and after reaction.\nSupplementary Fig. 24. a, The Ti 2p XPS spectra, b, Ir 4f XPS spectra, c, Cu 2p XPS\nspectra, and d, Auger electron spectra of Cu LMM for IrCu/TiO catalysts before and\n2\nafter reaction.\ndistributions for the fresh IrCu/TiO catalyst.\ndistributions for the used IrCu/TiO catalyst.\n2\n10. The proposed role of Ir species in modulating proton transfer from water vapor to\nachieve high CH selectivity is impressive. However, the oxidation of water to O ,\n4 2\nespecially in the absence of any sacrificial agent, is kinetically challenging and often\nthe rate-limiting step in such overall photocatalytic CO reduction. The manuscript\n2\nmentions that air was introduced during manual injection and O was not quantified.\nreport a quantitative analysis of O production, for instance, by introducing an internal\n2\nstandard (e.g., N ) for calibration. This would help determine whether water\n2\ndissociation primarily generates hydroxyl species (*OH) or leads to direct O evolution.\nchromatograph (GC-7920, CEAULIGHT), which was equipped with a thermal\nconductivity detector (TCD) for H and O , and a flame ionization detector (FID) for\n2 2\nCH and CO. Furthermore, we conducted a quantitative analysis of O production in\n4 2\nphotocatalytic CO reduction. The O evolution rates were determined to be 5.36, 6.61,\n2 2\nand 10.56 μmol g-1 h-1 on Cu/TiO , Ir/TiO and IrCu/TiO , respectively, in the order of\n2 2 2\n2 2 2\ndiscussion and experimental content in the revised manuscript (Pages 10 and 18).\nSupplementary Fig. 20. The yield of O during the process of CO photoreduction.\n2 2\n11. While this work provides a detailed DFT analysis of the CO hydrogenation step to\nCHO, the free energy data of subsequent continuous hydrogenation steps from CHO to\nCH (e.g., *CHO → *CH O → *CH O) are lost. Presenting the complete reaction\n4 2 3\nenergy profile is crucial for elucidating the full eight-electron reduction pathway to CH .\ncalculations of subsequently continuous hydrogenation steps from *CHO to CH\n4\n(Supplementary Fig. 30). In particular, IrCu/TiO exhibits the lowest ΔG (0.467 eV)\n2\nfor *CO-*CHO, compared to Ir/TiO and Cu/TiO . This thermodynamic preference for\n2 2\n*CHO formation promotes the subsequent continuous hydrogenation to the CH\n4\nproduct on IrCu/TiO .\n2\n12. For the figures displaying atomic configurations (e.g., Fig. 2 and Fig. 5), it is\n21\nrecommended to include a legend indicating which atom each color represents. This\nwould significantly enhance the readability and clarity of the presented structural\ninformation.\nResponse: Thanks for your comment. To improve the readability and clarity of the\nstructural information (e.g., Fig. 2 and Fig. 5), we have added the detailed descriptions\nof the atomic species corresponding to each color in the figure captions of the revised\nmanuscript. Pink atom: Ir; blue atom: Cu; green atom: Ti; light yellow atom: O; black\natom: C; white atom: H. We have provided more discussion in the revised manuscript\n(Pages 5, and 14).\n13. Regarding the setting of active sites on the IrCu/TiO catalyst, the current model\n2\ndefaults to adsorbing key intermediates (CO , COOH, CO, CHO) on Ir sites (see\nmigration to *CO-Cu\" mechanism proposed in the main text. It is suggested that the\nauthors clarify whether they have considered analyzing the Cu sites as potential reaction\nsites. Additionally, does the co-adsorption configuration of *H and *OH on IrCu/TiO\n2\nin Supplementary Fig. 37 correspond to the energy-optimized state? Given that Ir\ntypically exhibits strong adsorption capacity for *H, whether *H is more inclined to\nadsorb on Ir sites rather than Cu sites also requires further clarification.\nResponse: Thanks for your comment. To clearly illustrate the adsorption site for the\nkey intermediates, the original space-filling models were replaced with ball-and-stick\nrepresentations. In Supplementary Fig. 33, the black, white, light yellow atoms\ncorrespond to carbon (C), hydrogen (H), and oxygen (O), respectively. For the key\nintermediates of *COOH, *CHO, and *CH O, the Cu sites interact with H and O atoms,\n2\nwhile the Ir sites adsorb the C atoms. This adsorption configuration is associated with\nthe “Cuδ+-Irδ-” structure due to their electron distribution. In addition, the structural\nmodels further show that H O* is adsorbed on the IrCu alloy in the IrCu/TiO sample\n2 2\n(Supplementary Fig. 41). Therefore, the proposed mechanism of *H migration to\n*CO-Cu is reliable.\n*CHO, d, *CH O, e, *CH O over IrCu/TiO model (side view).\n2 3 2\nSupplementary Fig. 41. H O* adsorption configuration on a, Ir/TiO and b, IrCu/TiO ,\n2 2 2\nrespectively. Pink atom: Ir; blue atom: Cu; green atom: Ti; light yellow atom: O; white\natom: H.\nTo determine the optimal adsorption configurations of *H and *OH on the IrCu/TiO\n2\nsurface, we performed additional DFT calculations on *H adsorption at Ir sites. In Fig.\nR3a, when *H is adsorbed on the Ir site, the total adsorption energy of *H and *OH is\n0.09 eV, which is indeed lower than that of *H on Cu sites (0.35 eV) (Fig. R3b).\nand is reluctant to migrate. For the CH\n4\nproduct, the process involves multiple electron-\nproton transfer. Therefore, the calculation model for *H adsorption must provide\navailable *H to generate CH . When *H is adsorbed on the Cu site and *OH is on the\nmore favorable for the subsequent hydrogenation steps towards CH .\n2\na, *H\nand *OH on Ir sites. b, *H on Cu site and *OH on Ir site.\nThe manuscript reported some interesting results. However, it is not suitable for\npublication in Nature Communications. The following problems have to be addressed.\nResponse: Thank you for your valuable comments. We have carefully considered all\nof the points you raised and addressed them in the revised manuscript. In response to\nyour comments, we have provided a more detailed clarification of the active sites, the\nstability of the catalysts and the reaction mechanism. We hope that the revised\nmanuscript is suitable for publication in Nature Communications.\n1. The English have to be polished carefully, and the tense should be consistent.\nResponse: Thanks for your comment. The English has been polished and the tense has\nbeen made consistent in the revised manuscript.\n2. Where was the catalytic active site? The authors have to give more discussions.\nResponse: Thanks for your comment. The catalytic active sites are as follows: (a) Ir\nsites are used for the adsorption and dissociation of H O, generating localized hydrogen\n2\nprotons. (b) Hydrophobic Cu sites modulate the hydrogen-bond network with H-down\nconfigurations for the migratable hydrogen protons, and the *CO intermediate on Cu\nsites undergoes continuous hydrogenation to produce CH . (c) Oxygen vacancies at the\n2 2\nThe Ir sites for the H O adsorption were investigated using the water contact angle\n2\nmeasurements, H O-TPD, and in situ adsorption DRIFTS (Fig. 5a, b, e and f). The\n2 2\nadsorption, indicating that H O is more easily adsorbed on Ir sites.\nwere investigated using in situ DRIFTS in light and DFT calculations (Fig. 5d, g, h and\ni). In situ DRIFTS under illumination revealed a distinct Cu-H stretching band at 2060\ncm-1, indicating that adsorbed H O readily donates *H from Ir to adjacent Cu sites (Fig.\n2\n5d). This is due to the moderately restructured HBN with H-down configurations on\nIrCu/TiO promoting *H migration. DFT calculations showed that IrCu/TiO has a\n2 2\nlower thermodynamic energy barrier (0.467 eV) for the *CO-to-*CHO step than Ir/TiO\n2\n(0.906 eV).\nOxygen vacancies (O ) at the IrCu/TiO interface are associated with the alloys for\nv 2\nCO activation, and this was confirmed by in situ EPR. Under a CO atmosphere, the\n2 2\nO signal decreased compared to the Ti³⁺ signal, confirming that O sites participate in\nv v\nthe capture and activation of CO , which is different to the N condition\n2 2\n(Supplementary Fig. 14-15). We have added the discussion in the revised manuscript\n(Pages 3, 12, 13, and 17).\n2 2 2 2 2\nTPD profiles of Ir/TiO and IrCu/TiO . c, In situ ESR signal of *H free radical under\n2 2\nCO , H O, and light conditions using DMPO as the *H-trapping reagent. d, In situ\n2 2\nDRIFTS under CO and H O conditions for Ir/TiO and IrCu/TiO photocatalysts at a\n2 2 2 2\nrange from 2200 to 1950 cm-1, and e, Ir/TiO , f, IrCu/TiO at a range from 3800 to 3000\n2 2\ncm-1 in dark, and g, Ir/TiO , h, IrCu/TiO under light irradiation. i, Energy barriers for\n2 2\n*CO→*CHO on Ir/TiO and IrCu/TiO . Pink atom: Ir; blue atom: Cu; green atom: Ti;\n2 2\nlight yellow atom: O; black atom: C; white atom: H.\n3. The roles of every component of catalyst in the reaction have to be discussed\ndetailedly.\nResponse: Thanks for your comment. The IrCu/TiO catalyst is composed of an IrCu\n2\nalloy and a TiO support. Their respective roles are summarized bellow:\n2\n(a) The Ir sites serve for the adsorption and dissociation of H O for the localized\n2\nhydrogen protons (Fig. 5a-b). Meanwhile, the hydrophobic Cu sites modulate the\nhydrogen-bond network for H-down configurations, providing the migratable hydrogen\nprotons (Fig. 5e-f). The *CO intermediate on the Cu sites facilitates continuous\nhydrogenation to produce CH (Supplementary Fig. 33). In addition, the polarized\n4\n26\n“Cuδ+-Irδ-” structure promotes the formation of interfacial O in the TiO support.\nv 2\n(b) The TiO support absorbs light for the photo-generated electrons and holes, which\n2\nare used for CO reduction and H O oxidation, respectively. The generated O in the\n2 2 v\nTiO support prolong the lifetime of photoelectrons, as demonstrated by a series of\n2\nphotoelectrochemical characterizations. This is critical for the sluggish eight-electron\nreaction in CH formation. In addition, O associated with the alloys are used for CO\n4 v 2\nactivation, as confirmed by in situ EPR results (Supplementary Fig. 14-15).\nOverall, the combination the IrCu alloy and TiO support enhances the performance\n2\nof photocatalytic CO reduction to CH . We have added the discussion in the revised\n2 4\nmanuscript (Page 3).\n4. Catalysts after reaction have to be more detailedly analyzed, e.g. TEM, XPS and\nXRD.\nResponse: Thanks for your comment. We conducted XRD, EPR, XPS, and STEM\nanalyses to study the structural, morphological and electronic properties of IrCu/TiO\n2\nbefore and after the reaction. The XRD analysis confirmed that the anatase crystal\nstructure of TiO was retained throughout the catalytic process (Supplementary Fig.\n2\n23a). The EPR results showed that the the fresh and used IrCu/TiO photocatalysts had\n2\ncomparable signals (Supplementary Fig. 23b). The Ti XPS results also indicated that\nthere was no change in TiO supports before and after the reaction. Moreover, the Ir 4f\nremained unchanged before and after the reaction for IrCu/TiO photocatalysts\n2\n(Supplementary Fig. 24). Aberration-corrected high-angle annular dark-field scanning\ntransmission electron microscopy (AC-HAADF-STEM) images confirmed that the\nIrCu nanoclusters and TiO sheets were stable throughout the catalytic process\n2\n(Supplementary Fig. 25). Therefore, the morphological structure, valence states, and\noxygen vacancy concentration of the IrCu/TiO photocatalysts before and after the\n2\nreaction show its excellent catalytic stability. We have provided more discussion in the\nrevised manuscript (Page 10).\nSupplementary Fig. 23. a, XRD patterns, and b, EPR results of IrCu/TiO catalysts\n2\nbefore and after reaction.\nspectra, and d, Auger electron spectra of Cu LMM for IrCu/TiO catalysts before and\n2\nafter reaction.\nSupplementary Fig. 2. AC-HAADF-STEM image and the average particle size\ndistributions for the fresh IrCu/TiO catalyst.\n2\ndistributions for the used IrCu/TiO catalyst.\n2\n5. What was the fate of the O in CO ? Where did the O in CO go? The authors have to\n2 2\ngive more evidence.\nResponse: Thanks for your comment. According to the literature (J. Am. Chem. Soc.,\n2017, 139, 9739-9754; ACS Catal., 2022, 12, 7300-7316), the O atoms in CO are\n2\nconverted into CO and H O in our reaction system.\n2\nphotoreduction as CO →*CO →*COOH→CO(g)/*CHO→*CH O→*CH O→\n2 2 2 3\nCH +H O. This indicates that the O atoms in CO are converted into the CO and H O\n4 2 2 2\nproducts. Moreover, in situ DRIFTS were carried out under different gas atmospheres\n(CO 2 /N 2 and H 2 O) under light irradiation to investigate the fate of the O atoms in CO 2\n(Fig. R4). According to the literature (Nat. Catal., 2024, 7, 689-701; Nat. Commun.,\n2023, 14, 6926), the signal in the range of 3200-3800 cm-1 is assigned to adsorbed water\non the catalyst surface, while the peak around 3600-3700 cm-1 corresponds to free water.\nWhen N was introduced instead of CO , the vibrational peaks at around 3600-3700 cm-\n2 2\n1 attributed to the free water were only observed under the CO flow, indicating that the\n2\noxygen atoms form CO 2 combine with protons to from H 2 O. Together with the CO\nproduct in the activity result (Fig. 4a), it is proposed that the O atoms in CO are\n2\nconverted into CO and H O within our reaction system.\n2\n2 2 2 2\nphotocatalysts in light.\n6. More references have to be added into Figure 4g, as there are a large number of\npapers about photoreduction of CO to CH . In some references, the CH production\n2 4 4\nrates and selectivity were higher than the results reported in the present work.\nResponse: Thanks for your comment. Other recent metal-based photocatalysts for CH\n4\nproduction under the similar conditions involving only CO and H O reactants have\n2 2\nbeen added into Fig. 4g (Adv. Mater., 2025, 37, 2411813; J. Am. Chem. Soc., 2024,\n146, 15538-15548; Nat. Commun., 2022, 13, 1400; Adv. Mater., 2022, 34, 2200929).\nWe note that the metal-based photocatalysts of Au/TiO -V and Pd/ZnO-V nanosheets\nhave higher CH production rates of 156.5 and 257.6 µmol g-1 h-1, respectively (Adv.\ne202508259). However, their apparent quantum yields (AQY) of 1.66% (300 nm) and\n2.36% (280 nm) are lower than our work (7.91% at 365 nm). The performance of the\ntwo photocatalysts is summarized in Supplementary Table S10. Furthermore, the light\nintensity and wavelength range have been provided in Table S10. This enables for a\nmore direct and fair comparison with other reported photocatalysts.\nFig. 4. g, Comparison of the CH evolution rates with other recent metal-based catalysts\n4\nfor photocatalytic CO reduction with only CO and H O reactants.\n2 2 2\nSupplementary Table 10. Comparison of the CH evolution rate, selectivity and AQE\n4\nwith other recent metal-based catalysts for photocatalytic CO reduction.\n2\n4 4\nWavelength AQE\nCatalysts Light intensity (μmol selectivity Ref.\nrange (%)\n(mW cm-2) a g-1 h-1) (%)\n7.91\nIrCu/TiO 500 100.3 98.6 (365\nnm) This\nwork\nIr Cu /TiO 500 38.2 100.0 N. M.\n1 2 2\nXe spectrum\nZZ-Ag-Pt 320-780 nm N. M. 16.23 98.9 N. M 10\nXe\nCoRu-HCNp N. M. 2.1 N. M. N. M 11\nXe spectrum\nAu Cd S 600 11.3 N. M. N. M 12\nSA 1-x\nXe spectrum\nCu/CZS-Vs λ≥400 nm 200 38 N. M. N. M 13\nXe\nEquipped\nPd/Cu 300 W\n3 with an AM N. M. 29.9 100 N. M 14\n(HITP) /TiO Xe\n2 2\n1.5 G filter\nPd Cu /TiO λ＜400 nm 2 19.6 95.9 N. M. 15\n7 1 2\nXe\nCeCo-PTI N.M. 45.4 88.3 N.M. 16\nXe spectrum\nRu-Ov/TiO N.M. 50.1 81 N.M. 17\n1.03\nCu /BT N.M. 19.63 98 (365 18\n1+NCs\nXe spectrum\nnm)\nMn/Co O 320-780 nm 80 23.4 94.23 N.M. 19\n3 4\nXe\nPd-HPP-TiO 325-780 nm N.M. 22.0 59 N.M. 20\n1.66\nAu/TiO -V N.M. 156.5 100 (300 21\nXe spectrum\nnm)\n31\n2.36\nPd-ZnO-Vo N.M. 257.6 95.3 (280 22\nXe spectrum\nnm)\n0.11\nPt Cu 300 W Full\n0.5 0.5 N. M. 12.4 97.0 (365 23\n/TiO Xe spectrum\n2\nnm)\n0.98\nPtRu/TiO 320-780 nm N. M. 38.7 93.7 (380 24\nnm)\n(PdCu) -TiO N. M. 18.1 98.7 N. M 25\n2 2\nXe spectrum\nRuCu/CRP N. M. 19.7 96.0 N. M 26\nXe spectrum\na: N. M. means not mentioned\n7. The CH production rate was described by using μmol g-1 h-1. However, the\n4\nphotocatalyst used in experiments was only 50 mg. Thus, the authors enlarged the CH\n4\nproduction rate by 20 times. The authors have to provide evidence which demonstrates\nthat CH production rate is linearly related to photocatalyst amount.\n2\nreduction experiments by varying the amount of photocatalyst used (Supplementary\nFig. 20). The CH yield exhibits a linear relationship with the amount of catalyst. We\n4\nhave provided more discussion in the revised manuscript (Page 9).\nSupplementary Fig. 20. a, The photocatalytic performance of CO reduction on\n2\ndifferent amounts of IrCu/TiO photocatalysts. b, The linear relationship between the\n2\nphotocatalyst amount and CH evolution rate.\n4\n8. Reaction mechanism should be more detailedly discussed.\nResponse: Thanks for your comment. Based on in situ experiments, DFT calculations\nand AIMD simulations, the reaction mechanism for high CH selectivity mediated by\n4\ninterfacial H O is illustrated in Fig. 6f. (1) First, the H O molecules are adsorbed on the\n2 2\n32\nalloys of hydrophilic Ir sites, generating localized hydrogen protons. Meanwhile, the\nhydrophobic Cu sites of the alloys modulate the hydrogen-bond network for H-down\nconfigurations, which significantly weakens the *H binding ability and facilitates *H\nmigration to the Cu sites. (2) CO molecules are adsorbed at the IrCu/TiO interface (on\n2 2\nthe alloy and O ), which are activated to form the *COOH intermediate. (3)\nV\nSubsequently, the generated *CO intermediate on Cu sites undergoes continuous\nhydrogenation with its surface *H, ultimately producing CH . From both kinetic and\n4\nthermodynamic perspectives, the accelerated *H transfer to *CO-Cu reduces the energy\nbarrier for key *CHO formation, promoting the continuous hydrogenation and\nreduction processes by utilizing the long-lived photogenerated electrons from the TiO\n2\nsemiconductor. Therefore, efficient CO methanation is achieved on the IrCu/TiO\n2 2\nphotocatalyst. A more detailed discussion of this mechanism has been included in the\nrevised manuscript (Page 17).\nFig. 6. f, Schematic illustration of interfacial H O mediated high CH selectivity of\n2 4\nIrCu/TiO in photocatalytic CO RR.\n2 2\n33"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my questions, and I recommend this manuscript for publication in Nature Communications.", "reviewer_2": "The authors have addressed my concerns with detailed revisions, and I could recommend the publication.", "reviewer_3": "The manuscript has been revised, and can be published."}, "author_response": ""}]

Decision letter