Sulfur vacancy-confined Co-Mo sites in MoS_2 for high-efficiency CO_2 hydrogenation to formate

Domain: chemistry
Published: 2026-02-24
Editorial outcome: minor_revision

Abstract
CO hydrogenation to formate is a significant process for converting greenhouse gas to high-value chemicals. MoS2 catalysts are emerging as low-cost candidates for this reaction, but their performance is constrained by the challenge of generating sufficient and high-activity sulfur vacancy-confined Mo sites as active sites. Here we report that confining Co atoms into MoS2 lattice to substitute Mo atoms (Co-MoS2) remarkably enhances the efficiency of CO2 hydrogenation to formate via enriching sulfur vacancies under practical reaction conditions. The Co-MoS2 achieves a high formate production rate of 17.0 mmol g−1 h−1 with a selectivity exceeding 99% at 200 °C, in which the reaction rate is nearly three times that of the pristine MoS2 catalyst and surpasses previously reported non-precious metal-based catalysts. Experimental characterizations combined with theoretical calculations demonstrate that the weakened bonding between the Co-Mo pair and its adjacent sulfur or oxygen facilitates the removal of sulfur or oxygen for the generation of sulfur vacancies during hydrogenation. The exposed sulfur vacancy-confined Co-Mo sites both at the edge and in the basal plane exhibit moderate CO2 adsorption strength, thus suppressing C-O bond cleavage and promoting selective CO2 hydrogenation to formate.

Introduction
Formate is an important chemical building-block that has been widely used in the fields of leather, textile, and dyeing industries, etc. Hydrogenation of CO2 to formate provides a promising route for converting this greenhouse gas to high-value chemicals, which is typically achieved over precious metal catalyst1–5. Over the past few decades, various precious metal catalysts based on Pd6,7, Au8, Ir9, and Ru10,11, were developed for this reaction with high activity, but are limited by their scarcity and high cost. In contrast, earth-abundant non-precious metal-based catalysts are more attractive for practical applications. To date, great efforts have been made to develop non-precious metal-based heterogeneous catalysts for this process, yet these catalysts usually suffer from insufficient reactivity12–20. Coordinatively unsaturated Mo sites at the sulfur vacancies (SVs) of MoS2, i.e., SV-confined Mo sites, were found possessing superior catalytic activity for CO2 hydrogenation. These sites enabled highly efficient hydrogenation of CO2 to methanol in a gas-solid phase reaction and to formate in aqueous reaction system20,21. However, the practical utilization of MoS2 is limited by the difficulty in generating highly active SV-confined Mo sites via removing sulfur atoms as well as their susceptibility to oxygen blockage upon air exposure22. The oxygen-blocked SVs are exceedingly difficult to regenerate due to the strong bonding between oxygen species with the SV-confined Mo sites20, which severely limit the amount of accessible active sites and thus the overall catalytic performance.

Herein, we report that by confining Co atoms into the MoS2 lattice to facilitate the formation of SVs, highly efficient CO2 hydrogenation to formate is achieved over this Co-MoS2 catalyst. It delivers a high formate production rate of 17.0 mmol g−1 h−1 with a selectivity of over 99% at 200 °C, in which the reaction rate is nearly three times that of the pristine MoS2 catalyst and surpasses previously reported non-precious metal-based catalysts. In addition, the activity of Co-MoS2 can be well-maintained for over 80 h in 8 reaction cycles, demonstrating the high durability of the catalyst. Multiple experimental characterizations combined with theoretical calculations reveal that the lower coordination number of Co than that of Mo leads to a weaker bonding interaction between the lattice-confined Co-Mo pair and surface sulfur or oxygen, which promotes the hydrogenation-driven removal of sulfur or oxygen species to generate SVs. The exposed SV-confined Co-Mo sites with moderate CO2 adsorption both at the edge and in the basal plane suppress the cleavage of C-O bond and promote the formation of formate, leading to the superior activity and selectivity of Co-MoS2 for the CO2 hydrogenation to formate.

Results
Co-MoS2 catalysts with different Co loadings were prepared using a one-pot hydrothermal method. High-resolution transmission electron microscopy (HRTEM) results show that these catalysts are all featured by flower-like thin-layer nanosheets with large lateral sizes, exposing abundant basal plane sites (Fig. 1a, b, and Supplementary Fig. 1). The Co loading has no obvious influence on the overall structure and morphology of the catalyst (Fig. 1a–c, and Supplementary Figs. 1, 2). To determine the chemical state of Co in the Co-MoS2, various spectroscopic and microscopic techniques were employed to analyze its geometric and electronic structures. No signs of Co-related nanoparticles or clusters were observed in HRTEM images and XRD patterns, indicating the well dispersion of Co in MoS2 (Fig. 1a–c, and Supplementary Figs. 1, 2), which was further demonstrated by energy-dispersive X-ray (EDX) mapping characterization (Fig. 1d). In addition, as indicated by the X-ray absorption near-edge structure (XANES) spectra, the Co coordination structure of Co-MoS2 differs from those of Co foil, CoS, and Co3O4 standard samples and its absorption edge falls between those of CoS and Co3O4, indicating an intermediate oxidation state of Co in Co-MoS2 between CoS and Co3O4 (Fig. 1e). The extended X-ray absorption fine structure (EXAFS) spectra of Co-MoS2 and its fitting result further reveal that Co-S coordination is dominant in Co-MoS2 with a coordination number of ca. 4.42 and no Co-Co coordination observed (Fig. 1f, Supplementary Fig. 3, and Supplementary Table 1). These findings suggest that Co atoms are atomically dispersed and covalently confined within the MoS2 lattice, rather than forming other phases like Co, CoS, or Co3O4 nanoparticles. The existence and location of Co single atom was further confirmed by the atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with electron energy loss spectroscopy (EELS). Intensity profile analyses of the HAADF-STEM image along the corresponding dashed lines in Fig. 1g show the replacement of bright spot (Mo atom) by relatively darker spot in the atomic lattice (Fig. 1h). The relatively darker spot was identified as Co atom based on the Co L2,3 feature peak in the EELS spectra (Fig. 1i). These results reveal that the Co atoms are confined into the MoS2 lattice by replacing Mo atoms.

Catalytic performances were evaluated by using stainless-steel autoclave. To enhance the solubility of H2 in the aqueous solution, a CO2/H2 ratio of 1/3 with a higher H2 partial pressure compared to the stoichiometric ratio of 1/1 was used for CO2 hydrogenation, enabling a significantly increased formate production rate (rformate, Supplementary Fig. 4). Compared to the pristine MoS2 catalyst, the introduction of Co significantly enhances the activity, achieving a maximum rformate of 12.1 mmol g−1 h−1 at a Co loading of 1.2 wt%, which is nearly three times that over the pristine MoS2 catalyst (Fig. 2a, b). The rformate was not increased at higher Co loadings, which may be due to the limitation of equilibrium. The superiority of the Co-MoS2 catalyst in rformate was further verified at different reaction temperatures and pressures (Fig. 2c, d). Notably, the Co-MoS2 catalyst delivers a high rformate of 17.0 mmol g−1 h−1 with a selectivity of over 99% at 200 °C and 3.5 MPa, in which the rformate markedly surpasses those of previously reported non-precious metal-based catalysts (Fig. 2e, Supplementary Figs. 5, 6, and Supplementary Table 2)12–20. In addition, the Co-MoS2 catalyst exhibits high durability in the reaction without obvious activity decay in 8 reaction cycles for 80 h (Fig. 2f). HRTEM and X-ray diffraction (XRD) characterizations of the used Co-MoS2 catalyst reveal the well-maintained morphology and 2H-MoS2 crystal phase after 8 reaction cycles (Supplementary Fig. 7), further demonstrating the structural stability of the catalyst.

Coordinatively unsaturated Mo sites at the SVs of pristine MoS2 were found possessing unique activity and selectivity for CO2 hydrogenation20,21. To investigate the effect of Co substituting Mo on the formation of SVs, in-situ NO pulse adsorption measurements were conducted to quantify the SVs23,24. NO adsorption was not detected on the as-prepared fresh 1.2% Co-MoS2 and pristine MoS2 catalysts, which could be due to the occupation of SVs by oxygen species after their exposure to air (Fig. 3a and Supplementary Fig. 8), as revealed by the presence of Mo-O vibrational peaks in the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectrum (Supplementary Fig. 9)21,25,26. After in-situ treatment of the samples with H2, the NO adsorption capacities are significantly increased, indicating the regeneration of SVs due to the removal of oxygen species (Fig. 3a, and Supplementary Figs. 8, 9). It should be noted that when the H2-reduced catalysts are exposed again to the O2 atmosphere even at room temperature, the SVs on MoS2 could be blocked again due to the formation of Mo-O species (Fig. 3b). Therefore, the in-situ removal of O* species from the SVs by H2 to regenerate the active sites during the reaction plays an essential role in boosting the catalytic activity of MoS2 catalysts for CO2 hydrogenation to formate.

To understand the role of lattice-confined Co single atoms in promoting the reaction, H2 temperature-programmed reduction (H2-TPR) characterizations were conducted to study the in-situ reduction process. With the consumption of H2 in the H2-TPR test, only signals of H2O were detected without H2S signal (Supplementary Fig. 10), indicating that the formation of SVs in the reduction atmosphere is mainly due to the removal of O* species occupying the SVs. Figure 3c shows that the reduction of the Co-MoS2 catalyst is accompanied with H2 consumption peaks at 146 and 196 °C, which are notably lower than the peaks at 171 and 218 °C for the pristine MoS2 catalyst. This suggests that the O* species on the Co-MoS2 catalyst are less stable and more readily reduced by H2 compared to those on the pristine MoS2 catalyst. NO adsorption DRIFTS spectra show that, in addition to the characteristic vibrational bands around 1789 and 1700 cm−1 arising from NO adsorption on Mo sites, an additional vibrational bond at 1871 cm−1 was observed on Co-MoS2 (Supplementary Fig. 11), which could be assigned to the weaker NO adsorption on Co sites27. It suggests Co site possesses a lower activity than the Mo site, which accounts for the weakened adsorption of O* species at the Co sites of Co-MoS2 catalyst. In addition, the NO adsorption capacity on the reduced Co-MoS2 catalyst is more than two times that on the reduced MoS2 catalyst (130 vs. 63 μmol g−1 at 200 °C, 47 vs. 23 μmol g−1 at 160 °C, Fig. 3d). According to the density functional theory (DFT)-optimized NO adsorption structures on double SVs of pristine MoS2 and Co-MoS2 (Supplementary Fig. 12)20, the SVs density on Co-MoS2 is estimated to be 2.8–4.1 times that of pristine MoS2. These results indicate that the lattice-confined Co single atoms are beneficial for the formation of more SVs on MoS2. The Arrhenius plots, derived from the formate production rate at various reaction temperatures (Fig. 2c), indicate that the apparent activation energies (Ea) for CO2 hydrogenation on both the Co-MoS2 and pristine MoS2 catalysts are nearly identical (Fig. 3e). However, the Co-MoS2 catalyst shows markedly higher CO2 hydrogenation activity than the pristine MoS2 catalyst, which could be attributed to the increased number of SVs on Co-MoS2 induced by the lattice-confined Co single atoms (Fig. 3f)28.

To elucidate the role of the lattice-confined Co single atoms in enhancing the catalytic activity of MoS2, we have carried out DFT calculations to simulate the evolution of SVs during hydrogenation and the reaction mechanism of CO2 hydrogenation to formate. Double-SV configurations were employed for studying the reaction process (Fig. 4a and Supplementary Fig. 13). The results show that both in-plane and edge SV-confined metal sites on Co-MoS2 or pristine MoS2 are highly active for the adsorption of O2 with dissociative adsorption energies (ΔGads) ranging from −1.90 eV to −7.97 eV (Fig. 4b and Supplementary Fig. 14), thus are susceptible to blockage by oxygen20. This aligns with the immediate formation of Mo-O species when O2 passes through the reduced catalysts, as mentioned above (Fig. 3b). The regeneration of in-plane SVs of pristine MoS2 requires a high activation energy of 2.71 eV, which is notably higher than that of 1.68 eV for the regeneration of edge SVs (Fig. 4b), which could be the reason for the low activity of the basal plane-dominated pristine MoS2 catalyst20. In comparison, for the Co-MoS2, the energy barriers for the regeneration of Co-decorated in-plane SVs and edge SVs are markedly decreased to 1.45 eV and 0.51 eV, respectively (Fig. 4b, and Supplementary Figs. 15, 16), indicating that Co substituting Mo can significantly promote the formation of SVs. This is resulted by the weakened interaction between the lattice-confined Co-Mo pair and oxygen at the SVs, as revealed by the decreased overlapping between the Co 3d (or Mo 4d) and O 2p orbitals compared with that between the Mo 4d and O 2p orbitals in the case of pristine MoS2 (Fig. 4c). In addition, the higher electron density of states near the Fermi level for the oxygen at the SV-confined Co-Mo sites suggests the increased activity of the adsorbed oxygen atom on Co-MoS2, which is favorable for the removal of oxygen by H2 to generate the SVs (Fig. 4c).

The reaction pathways for the CO2 hydrogenation were then studied on the SV-confined metal sites. At the in-plane SV-confined Co-Mo sites, the CO2 hydrogenation to COOH* intermediate is energetically more favorable with a lower energy barrier of 0.92 eV than that of 1.19 eV for the dissociation to CO* and O* (Fig. 4d). This is different from the case of pristine MoS2 where the dissociation of CO2* to CO* and O* is relatively more favorable than the COOH* pathway20, due to the relatively low activity of the SV-confined Co-Mo sites than the pristine Mo sites. The COOH* pathway is also more favorable than the HCOO* pathway which has a notably high energy barrier of 1.83 eV (Fig. 4d). In addition, the H2O-mediated mechanism was also investigated considering the water environment20. On the in-plane SV-confined Co-Mo sites, although H2O can be readily dissociated to OH* and H* with a low energy barrier of 0.43 eV, the subsequent hydrogenation of CO2 to HCOO* is very difficult with a high energy barrier of 2.71 eV (Supplementary Fig. 17a). In the case of edge SV-confined Co-Mo sites, the direct adsorption and hydrogenation of CO2 is also more favorable than the adsorption and dissociation of H2O* into OH* and H* with a high energy barrier of 1.51 eV (Fig. 4e, Supplementary Fig. 17b). These results suggest that the H2O-mediated mechanism is not likely at the SV-confined Co-Mo sites and the reaction proceeds via the direct hydrogenation of CO2* by adsorbed H* through the COOH* pathway.

Methods
Preparation of catalysts
The MoS2 catalyst was synthesized using the method reported previously20,21,29. The Co-MoS2 catalyst was synthesized through the similar method to MoS2 with the addition of Co(NO3)2·6H2O as Co source. Typically, for the synthesis of Co-MoS2, 900 mg of (NH4)6Mo7O24·4H2O and a specified amount of Co(NO3)2·6H2O were dissolved in 20 mL of deionized water. Then, the mixture and 10 mL CS2 were transferred into a 40 mL stainless steel autoclave in the glovebox and followed by maintaining at 400 °C for 4 h. The product was treated with 6 M KOH at 60 °C for 3 h and then washed with deionized water and dried.

Catalytic tests of catalysts in CO2 hydrogenation
The catalytic reactions were performed in a stainless-steel autoclave with a capacity of 78 mL. Typically, 20 mg of catalyst was added into 10 mL of 0.5 M KHCO3 aqueous solution. After flushing with reaction gas (23% CO2, 69% H2, and 8% Ar as internal standard, or 24% CO2, 72% H2, and 4% N2 as internal standard) for three times, the autoclave was pressurized to 2.5 MPa. Then the reaction proceeded under stirring at 200 °C for 10 h. After the reaction, the possible gas-phase products were detected via gas chromatography (GC-2014, Shimadzu or GC-8860, Agilent) with a thermal conductivity detector (TCD) and flame ionization detector (FID). The formate product was analyzed by high-performance liquid chromatography (HPLC) equipped with refractive index and UV detectors using a Shodex SUGAR SH-1011 (8 × 300 mm) column. The carbonate/bicarbonate was analyzed by ion chromatography (IC). Carbon balance was calculated based on the following formulas:

Carbon balance = (n(CO2,final) + n(CH4,final) + n(HCOO−,final) + n(CO32−/HCO3−,final)) / (n(CO2,initial) + n(KHCO3,initial)) (1)

Where n(CO2,final), n(CH4,final), n(HCOO−,final), and n(CO32−/HCO3−,final) denote the amount of carbon dioxide, methane, formate, and carbonate/bicarbonate in the post-reaction gas or solution, respectively; n(CO2,initial) and n(KHCO3,initial) denote the amount of carbon dioxide and potassium bicarbonate before the reaction, respectively. Here, n(CO2,final), n(CH4,final), and n(CO2,initial) are calculated based on the equation of pV=nRT. In the post-reaction gas, the partial pressures of carbon dioxide and methane are calculated based on the measured system pressure of 3.2 MPa and their molar content determined by GC analysis. The n(HCOO−,final) and n(CO32−/HCO3−,final) are quantified via HPLC and ion chromatography (IC), respectively, using the standard curves shown in Supplementary Figs. 5b, 18. On these bases, the carbon balance was calculated as following:

Carbon balance = (20.759 + 0.001 + 3.344 + 3.061) mmol / (23.457 + 5.024) mmol × 100% = 95.4%

The slightly lower carbon balance can be attributed to the residue of CO32− on the used catalyst as demonstrated by the Fourier Transform Infrared Spectroscopy (FT-IR) test (Supplementary Fig. 19). The reference reaction with KHCO3 and gases only exhibited a carbon balance of 97.8%, indicating a good sealing integrity of the autoclave.

Materials characterization
PXRD measurements were conducted on a Rigaku Ultima IV diffractometer with Cu Kα radiation at 40 kV and 30 mA. ICP-OES tests were conducted on Thermo Fisher Scientific iCAP 7400. X-ray absorption fine structure (XAFS) spectra, including XANES and EXAFS, were measured at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility. TEM was conducted on FEI Tecnai F20 Transmission Electron Microscope and the accelerating voltage was 200 kV. Atomic-resolution HAADF-STEM images and EELS mapping were performed on a spherical aberration-corrected Nion U-HERMES100. NO chemisorption measurements were performed on a Micromeritics Auto Chem II 2920 instrument. Typically, catalysts were in-situ pretreated with 10% H2/He at the specific temperature for 2 h and subsequently purged with He for 1 h. NO chemisorption experiments were then performed at 0 °C by pulsing 500 μL of 2% NO/He through the catalyst every 30 min. For the NO chemisorption of the untreated catalysts, experiments followed the same processes except for the H2 pretreatment. The double SVs of pristine MoS2 can adsorb four NO molecules20, whereas the Co-decorated double SVs at Mo-edge and in-plane of Co-MoS2 can only adsorb three and two NO molecules, respectively (Supplementary Fig. 12). Using the measured NO adsorption capacities (Fig. 3d), the SVs densities are estimated as 15.75 μmol g−1cat. for pristine MoS2 (63/4 μmol g−1cat.) and 43.3–65 μmol g−1cat. for Co-MoS2 (130/3–130/2 μmol g−1cat.). On these bases, although Co-MoS2 exhibits a twofold higher total NO adsorption capacity, its density of SVs is estimated to be 2.8–4.1 times that of pristine MoS2. In situ DRIFTS and ex situ FT-IR measurements were performed using a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700 for O2 adsorption tests and ex situ FT-IR tests; Vertex 70v for NO adsorption tests) equipped with a mercury cadmium telluride detector. For the O2 adsorption tests, the catalysts were in-situ pretreated with H2 at 200 °C for 2 h. Then, the catalysts were exposed to 2% O2/Ar at 25 °C after purging with He. For the NO adsorption test, the catalysts were in-situ pretreated with H2 at 200 °C for 2 h. Then, the catalysts were exposed to 2% NO/Ar at 25 °C after purging with He. Finally, the unabsorbed NO was swept with He. The H2 temperature-programmed reduction tests were performed on a Micromeritics Auto Chem II 2920 instrument. Typically, catalysts were in-situ pretreated with He at 80 °C for 1 h and subsequently switched feeding gas to 10% H2/He. The temperature was elevated to 240 °C with a rate of 4 °C min−1 and maintained for 2 h.

Computational details
The structure optimization and the pathways for CO2 hydrogenation to HCOOH on various catalysts were calculated using DFT with spin polarization, as implemented in the Vienna ab initio simulation package (VASP 5.4.4) code30–33. The projector augmented wave (PAW) method was used to describe the interactions between ion cores and valence electrons34,35. The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was used to describe the exchange-correlation interactions36,37. Dispersion interactions were considered by applying D3 corrections38,39. For the plane-wave expansion, a 400 eV energy cutoff was used. The convergence criterion for electronic structure iteration was set to be 10−5 eV and that for geometry optimization 0.02 eV/Å. To calculate the transition states (TS), the fixed bond length (FBL) method implemented in the Atomic Simulation Environment was used40. The free energies of adsorbed reaction intermediates were calculated as G = Etotal + ZPE + Hvib − TSvib, where Etotal is the total energy calculated by DFT, ZPE is the zero point energy, Hvib and Svib represent the enthalpy and entropy contributions of non-imaginary vibrations, which are derived from the harmonic approximation to the potential at the temperature T. The free energies of gas molecules were calculated as Etotal + ZPE + ∫0T CpdT − TS, where Etotal is DFT-calculated total energy, ZPE is the zero-point energy, T is temperature, ∫0T CpdT presents the integrated heat capacity from 0 K to T, and S is the entropy under ideal gas approximation. A tri-layer model of MoS2 with confined Co single atoms was built in a 6×6 supercell to simulate the in-plane structure of Co-MoS2 (Supplementary Fig. 13). A nanoribbon model with lattice-confined Co single atoms was constructed in an 8×8 supercell to simulate the edges with the Mo edge saturated with S monomers and the S edge saturated with a combination of S monomers and dimers (Supplementary Fig. 13)41,42. Different confining sites were discussed, as shown in Supplementary Fig. 13. The most stable confining structures were adopted to simulate the in-plane (s2) and Mo-edge (s2) structure of Co-MoS2. The vacuum spaces were set larger than 20 Å. For the Brillouin zones sampling, a Monkhorst-Pack 3×3×1 k-point mesh was selected for the in-plane structure of Co-MoS2 and a Monkhorst-Pack 1×1×1 k-point mesh was used for the Mo-edge structure of Co-MoS2, respectively43.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper describes the synthesis and characterization of a Co-substituted MoS2 with a sulfur vacancy, and application to\nthe selective CO2 hydrogenation into formate. The Co-MoS2 achieves a high formate production rate of 17.0 mmol gcat.-1 h-\n1 with a selectivity exceeding 99% at 200 ºC, in which the reaction rate is nearly three times that of the pristine MoS2\ncatalyst and surpasses previously reported non-precious metal-based catalysts. This paper also ºdemonstrated that the\nweakened bonding between the Co-Mo pair and its adjacent sulfur or oxygen facilitates the removal of sulfur or oxygen for\nthe generation of SVs during hydrogenation.\nThis paper has potential to be published, but several obscured issues were found. The following issue should be accurately\nconsidered before publication.\n1) Introduction: the research for the CO2 hydrogenation into formate is categorized into two topics. One is the precious metal-\nbased catalysis at lower temperature, and the other is the non-precious metal-based catalysis at higher temperature. The\nauthors should mention about the history of the selective CO2 hydrogenation into formate as well as the advantage of non-\nprecious metal-based catalysis.\n2) XANES spectra in Figure 1e: the edge position of Co-MoS2 is quite different from that of Co3O4 despite the similar\nXANES shapes. The authors should discuss the oxidation state and coordination environment of the Co species in Co-\nMoS2.\n3) FT-EXAFS spectra in Figure 1f: the main peak of Co-MoS2 at around 1.7 angstrom is quite different from those of Co3O4\nand CoS. The authors should discuss the coordination environment of the Co species in Co-MoS2 based on the curve-fitting\nanalysis.\n4) Reaction conditions of CO2 hydrogenation into formate: the reaction is performed under 2.5MPa of feed gas (CO2/H2\n=1/3). CO2/H2 ratio is different from the stoichimetry of this reaction (CO2 + H2 →HCOOH). The authors should comment on\nthis.\n5) H2-TPR in Fig. 3c: The authors mentioned that “the reduction of the Co-MoS2 catalyst is accompanied with H2\nconsumption peaks at 146 and 196 °C, which are notably lower than the peaks at 171 and 218 °C for the pristine MoS2\ncatalyst.” What does this result mean? The authors should discuss more details.\n6) Judging from comparison of energy activation, the increased number of SVs on Co-MoS2 induced by the lattice-confined\nCo single atoms plays an important role. Figure 3d shoes the adsorption capacity of Co-Mo3 is 2 times higher than that of\nthe MoS2. On the other hand, the reaction rate of Co-MoS2 is nearly three times that of the pristine MoS2 catalyst. The\nauthors should discuss this inconsistency.\n7) I relation to the above comment, the authors should measure the DRIFT spectra of NO adsorption for Co-MoS2 and\npristine MoS2 in order to investigate the differences in adsorption strength.", "reviewer_2": "Dear Authors,\nThank you for taking the time and care taken to write the manuscript. You have demonstrated that the modified Co-Mo2S are\npromising catalysts for CO2 hydrogenation to formate, an industrially and environmentally important chemical reaction. The\nwork presented here will surely lay foundation for further research on modified Mo2S catalysts. In line with the editor's\nguidelines, after reviewing your manuscript I tried to answer the following questions:\n> What are the noteworthy results?\nTPR studies on the presented catalysts show that the formation of sulfur vacancies (SVs) in Co-modified Mo2S in the\nreduction atmosphere is mainly due to the removal of O* species occupying the SVs. The DFT studies point that Co-Mo sites\nand the reaction proceeds via the direct hydrogenation of CO2* by adsorbed H2* through the COOH* pathway. A very\ncomprehensive DFT study is mostly hidden in the SI. The highly active catalyst is an interesting finding, but currently not\ndocumented well enough.\n> Will the work be of significance to the field and related fields? How does it compare to the established literature? If the\nwork is not original, please provide relevant references.\nYes, the work is significant as it explores the use of earth-abundant metal-derived catalysts for CO2 hydrogenation, and\nshows that they compete with industry standards.\n> Does the work support the conclusions and claims, or is additional evidence needed?\nCrucial information is missing, e.g. HPLC results which all productivity data is based on are not included. It is unclear how\nthe formate production is quantified, the volume of autoclave column used for catalyst testing is not provided (assumed to be\n40 ml as per the synthesis section), and the only presented piece of reactivity evidence is the lack of substrate gases from\ngas chromatography. Additional evidence is required to consider the following:\na) presence of carbonates on the solid catalyst post-reaction should be explored,\nb) HPLC results must be included in the SI alongside GC,\nc) carbon balancing from HPLC and GC combined must be included,\nd) a reference reaction procedure with KHCO3 and gases only should be provided with carbon balancing to exclude gas\nleaking from autoclave\ne) post-reaction autoclave pressure readings would be useful,\nf) GC should show both FID and TCD signals, the figure in the SI does not state if it is a combined signal or FID only (CH4\nsignal present).\nWithout the above evidence, it is unclear what happens to the reactants.\nIn addition, the FBL method used in computational part can be used to estimate reaction barriers, however it is unclear if TS\nwere confirmed by vibrational frequency calculations and are in fact true TS, casting doubt on derived activation energy\nbarriers. Computational works could (optionally) consider the adsorption of carbonates on Co-Mo2S.\n> Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nThere are flaws, mentioned above. The flaw require revision and once ambiguities are resolved, the work will be\npublishable. The introduction is about 50% abstract repeated and does not explore the problem SV in detail, the work's\nimpact should also be highlighter more.\n> Is the methodology sound? Does the work meet the expected standards in your field?\nThe methodology is sound, but at present is missing important evidence.\n> Is there enough detail provided in the methods for the work to be reproduced?\nThe characterisation is well documented, while reactivity testing is lacking. The computational work methodology is robust.\nI hope that you will find the comments above useful. Most of the comments can be resolved, by simply expanding the\nmanuscript and SI with data you likely already have. I can recommend the work for publication following revision pertaining\nto the points mentioned above.\nKind regards,\nIgor Kowalec"}, "author_response": "Responses to the reviewers’ comments\nWe are grateful for all reviewers’ in-depth comments and suggestions, which have helped us greatly to\nimprove the quality of this manuscript. We have considered the comments and suggestions seriously, and carried\nout the related experiments following the suggestions of the reviewers. We have made detailed and point to point\nresponse and revised the manuscript accordingly with all changes being highlighted in yellow in the revised\nManuscript and Supplementary Information.\nReviewer #1 (Comments for the Author):\nGeneral Comments: This paper describes the synthesis and characterization of a Co-substituted MoS with a\n2\nsulfur vacancy, and application to the selective CO hydrogenation into formate. The Co-MoS achieves a high\n2 2\nformate production rate of 17.0 mmol g -1 h-1 with a selectivity exceeding 99% at 200 oC, in which the reaction\ncat.\nrate is nearly three times that of the pristine MoS catalyst and surpasses previously reported non-precious metal-\n2\nbased catalysts. This paper also demonstrated that the weakened bonding between the Co-Mo pair and its adjacent\nsulfur or oxygen facilitates the removal of sulfur or oxygen for the generation of SVs during hydrogenation.\nThis paper has potential to be published, but several obscured issues were found. The following issue should be\naccurately considered before publication.\nResponse: We thank you very much for your valuable comments and suggestions for further improving the quality\nof our manuscript. We have carried out additional experiments and explanations according to your suggestions.\nThe point-by-point responses to the detailed comments and the corresponding revisions are described as follows.\nComment 1. Introduction: the research for the CO hydrogenation into formate is categorized into two topics.\ncatalysis at higher temperature. The authors should mention about the history of the selective CO hydrogenation\n2\ninto formate as well as the advantage of non-precious metal-based catalysis.\nResponse: Thank you for your valuable suggestion. We have included related discussion into the revised\nmanuscript. The related description has been revised as follows.\n“Hydrogenation of CO to formate provides a promising route for converting this greenhouse gas to high-\n2\nvalue chemicals, which is typically achieved over precious metal catalyst.1-5 Over the past few decades, various\nprecious metal catalysts based on Pd,6-7 Au,8 Ir,9 and Ru,10-11 were developed for this reaction with high activity,\nbut are limited by their scarcity and high cost. In contrast, earth-abundant non-precious metal-based catalysts are\n1\nmore attractive for practical applications.” (Page 2, lines 32-36).\nComment 2. XANES spectra in Figure 1e: the edge position of Co-MoS is quite different from that of Co O\n2 3 4\ndespite the similar XANES shapes. The authors should discuss the oxidation state and coordination environment\nof the Co species in Co-MoS .\n2\nbetween those of CoS and Co O , indicating an intermediate oxidation state of Co in Co-MoS between CoS and\n3 4 2\nCo O . As for the coordination environment of Co, the EXAFS spectra of Co-MoS and its fitting result reveal\n3 4 2\nthat Co-S coordination is dominant in Co-MoS with a coordination number of ca. 4.42 and no Co-Co\n2\ncoordination observed (Figs. 1f, R1 and Table R1), demonstrating the atomical dispersion of Co. This is further\nsupported by atomic-resolution HAADF-STEM combined with EELS in Figs. 1g-i, which directly show that Co\natoms are confined into the MoS lattice by replacing Mo atoms. We have added the related discussion into the\n2\nrevised manuscript (Page 3, lines 71-75; Supplementary Fig. 3, Table 1).\nTable R1. Co K-edge EXAFS fitting results of the 1.2% Co-MoS .\n2\nΔE R\n(eV) factor\n1.2% Co-MoS Co-S 4.42±0.54 2.23 0.0035±0.0015 2.24±1.44 0.0211\n2\nand the red curve is the fitting curve.\nComment 3. FT-EXAFS spectra in Figure 1f: the main peak of Co-MoS at around 1.7 angstrom is quite different\n2\nfrom those of Co O and CoS. The authors should discuss the coordination environment of the Co species in Co-\n3 4\nMoS based on the curve-fitting analysis.\nfor Co K-edge EXAFS of Co-MoS (Table R1). As mentioned in Response to Comment 2, the EXAFS spectra of\n2 2\nof ca. 4.42 and no Co-Co coordination observed (Fig. 1f and Table R1), demonstrating the atomical dispersion of\nCo. This is further supported by atomic-resolution HAADF-STEM combined with EELS in Figs. 1g-i, which\ndirectly show that Co atoms are confined into the MoS lattice by replacing Mo atoms. We have added the related\n2\ndiscussion into the revised manuscript (Page 3, lines 71-75; Supplementary Fig. 3, Table 1).\nComment 4. Reaction conditions of CO hydrogenation into formate: the reaction is performed under 2.5 MPa of\n2\nfeed gas (CO /H = 1/3). CO /H ratio is different from the stoichiometry of this reaction (CO + H → HCOOH).\n2 2 2 2 2 2\nThe authors should comment on this.\nResponse: Thank you for your suggestion. As shown in Fig. R2, the production rate of formate is significantly\nincreased when using a CO /H ratio of 1/3 compared with that using a CO /H ratio of 1/1, which should be\n2 2 2 2\nattributed to the enhance the solubility of H in the aqueous solution via increasing partial pressure of H . To\n2 2\nclarify this point and to avoid confusion, we have added an explanation for the selection of gas ratio in the revised\nmanuscript (Page 4, lines 92-95; Supplementary Fig. 4).\n2 2 2\nAll reaction tests were conducted using 20 mg of Co-MoS in 0.5 M aqueous KHCO solution with 3.5 MPa of\n2 3\nfeed gas at 200 °C for 10 h.\nComment 5. H -TPR in Fig. 3c: The authors mentioned that “the reduction of the Co-MoS catalyst is\n2 2\naccompanied with H consumption peaks at 146 and 196 oC, which are notably lower than the peaks at 171 and\n2\n218 oC for the pristine MoS catalyst.” What does this result mean? The authors should discuss more details.\n2\nwell with the formation of H O, indicating the removal of surface oxygen species during reduction. The observed\n2\nlower reduction temperature of the Co-MoS catalyst suggests that the surface oxygen species on the Co-MoS\n2 2\ncatalyst are less stable and more readily reduced by H compared to those on the pristine MoS catalyst. Therefore,\n2 2\nunder reductive reaction conditions, the Co-MoS catalyst can expose more SVs-confined Co and Mo sites as\n2\nactive sites than the pristine MoS catalyst. According to your suggestions, we have added related discussion into\n2\nthe revised manuscript (Page 6, lines 138-139).\nComment 6. Judging from comparison of energy activation, the increased number of SVs on Co-MoS induced\n2\nby the lattice-confined Co single atoms plays an important role. Figure 3d shows the adsorption capacity of Co-\nMoS is 2 times higher than that of the MoS . On the other hand, the reaction rate of Co-MoS is nearly three\n2 2 2\ntimes that of the pristine MoS catalyst. The authors should discuss this inconsistency.\n2\npristine MoS can adsorb four NO molecules (Angew. Chem. Int. Ed. 2023, 62, e202307086), whereas the Co-\n2\ndecorated double SVs at Mo-edge and in-plane of Co-MoS can only adsorb three and two NO molecules,\n2\nrespectively (Fig. R3). Using the measured NO adsorption capacities (Fig. 3d), the SVs densities are estimated as\n15.75 μmol·g -1 for pristine MoS (63/4 μmol·g -1) and 43.3~65 μmol·g -1 for Co-MoS (130/3~130/2\ncat. 2 cat. cat. 2\nμmol·g -1). On these bases, although Co-MoS exhibits a twofold higher total NO adsorption capacity, its density\ncat. 2\nof SVs is estimated to be 2.8~4.1 times that of pristine MoS . Therefore, the non-linear increase of the reaction\n2\nrate with NO adsorption capacity should be attributed to different types and distributions of SVs on the Co-MoS\n2\nand pristine MoS catalysts, which cause a non-linear relationship between NO adsorption and SV density on the\n2\ntwo catalysts. To further clarify these points, we have added these new results and related discussion into the\nrevised manuscript and Supplementary information (Page 6, lines 146-148; Page 12, lines 269-274;\nSupplementary Fig. 12).\nFigure R3. a-b, DFT-calculated adsorption structures of NO at the Mo-edge (a) and in-plane (b) SVs of Co-MoS .\n2\nc-d, DFT-calculated adsorption structures of NO at the Mo-edge (c) and in-plane (d) SVs of MoS .\nfor Co-MoS and pristine MoS in order to investigate the differences in adsorption strength.\n2 2\nResponse: Thank you for your kind suggestion. Accordingly, we have conducted NO adsorption DRIFTS for the\nCo-MoS and pristine MoS catalysts (Fig. R4). Compared to the pristine MoS catalyst, where NO adsorption\n2 2 2\n5\non Mo sites gives rise to characteristic vibrational bands around 1789 and 1700 cm-1, the Co-MoS catalyst\n2\nexhibits an additional vibrational band at 1871 cm-1, which could be assigned to the weaker NO adsorption on Co\nsites (Journal of Catalysis, 1983, 84, 386). This suggests that the Co site possesses a lower activity than the Mo\nsite, which accounts for the weakened adsorption of O* species at the Co sites of Co‑MoS catalyst, thus\n2\nfacilitating the formation of more SVs. We have added the new data with related discussions into in the revised\nmanuscript (Page 6, lines 139-144; Supplementary Fig. 11).\nFigure R4. DRIFT spectra of NO adsorption on Co-MoS and pristine MoS .\n2 2\nGeneral Comments: Dear Authors,\nThank you for taking the time and care taken to write the manuscript. You have demonstrated that the modified\nCo-MoS are promising catalysts for CO hydrogenation to formate, an industrially and environmentally\n2 2\nimportant chemical reaction. The work presented here will surely lay foundation for further research on modified\nMoS catalysts. In line with the editor’s guidelines, after reviewing your manuscript I tried to answer the following\n2\nquestions:\nResponse: Thank you very much for your valuable comments and suggestions for further improving the quality\nof our manuscript. We have added additional evidence and explanations according to your suggestions. The point-\nby-point responses to the detailed comments and the corresponding revisions are described as follows.\nComment 1. What are the noteworthy results?\nTPR studies on the presented catalysts show that the formation of sulfur vacancies (SVs) in Co-modified MoS\n2\nin the reduction atmosphere is mainly due to the removal of O* species occupying the SVs. The DFT studies point\nthat Co-Mo sites and the reaction proceeds via the direct hydrogenation of CO * by adsorbed H * through the\n2 2\nCOOH* pathway. A very comprehensive DFT study is mostly hidden in the SI. The highly active catalyst is an\ninteresting finding, but currently not documented well enough.\nResponse: We appreciate your insightful comments and your approval of our work. In the revised manuscript, we\nhave integrated the key DFT findings from the Supplementary information into the main text to further clarify the\nreaction mechanism (previous Supplementary Fig. 13 has been added into Fig. 4). We have provided a more\nrobust presentation of the catalyst’s high activity by including the carbon balance, HPLC results, and FID as well\nas TCD signals of GC tests in the manuscript and Supplementary information to fully support this interesting\nfinding (Pages 11-12, lines 240-257; Supplementary Figs. 5, 6, and 19).\nComment 2. Will the work be of significance to the field and related fields? How does it compare to the established\nliterature? If the work is not original, please provide relevant references.\nYes, the work is significant as it explores the use of earth-abundant metal-derived catalysts for CO hydrogenation,\n2\nand shows that they compete with industry standards.\nResponse: Thank you for this positive comment and for acknowledging the significance of our work. As you\nnoted, our study demonstrates that earth-abundant metal-derived catalyst (Co-MoS ) can indeed compete with\n2\n7\nstate-of-the-art catalysts for CO hydrogenation to formate, offering a more sustainable and cost-effective\n2\npathway for this important reaction.\nComment 3. Does the work support the conclusions and claims, or is additional evidence needed?\nCrucial information is missing, e.g. HPLC results which all productivity data is based on are not included. It is\nunclear how the formate production is quantified, the volume of autoclave column used for catalyst testing is not\nprovided (assumed to be 40 ml as per the synthesis section), and the only presented piece of reactivity evidence\nis the lack of substrate gases from gas chromatography. Additional evidence is required to consider the following:\n(a) presence of carbonates on the solid catalyst post-reaction should be explored,\n(b) HPLC results must be included in the SI alongside GC,\n(c) carbon balancing from HPLC and GC combined must be included,\n(d) a reference reaction procedure with KHCO and gases only should be provided with carbon balancing to\n3\nexclude gas leaking from autoclave,\n(e) post-reaction autoclave pressure readings would be useful,\n(f) GC should show both FID and TCD signals, the figure in the SI does not state if it is a combined signal or FID\nonly (CH signal present).\nIn addition, the FBL method used in computational part can be used to estimate reaction barriers, however it is\nunclear if TS were confirmed by vibrational frequency calculations and are in fact true TS, casting doubt on\nderived activation energy barriers. Computational works could (optionally) consider the adsorption of carbonates\non Co-MoS .\nprovided additional information needed to support our conclusions and claims. The HPLC and GC results used\nfor evaluating the catalytic performance have been included into Supplementary information (Supplementary Figs.\n5, 6). A detailed description of the formate production quantification methodology has been included in the\nexperimental section (Pages 11-12, lines 240-257). In addition, we have added the standard curve which was used\nto quantify the formate production in Supplementary Fig. 5b. The volume of autoclave used for catalyst testing is\n78 mL and has been included in the experimental section (Page 11, line 233). Detailed responses to your additional\nsuggestions have been described as follows.\n(a) Presence of carbonates on the solid catalyst post-reaction should be explored.\ncatalyst post-reaction and conducted the Fourier Transform Infrared Spectroscopy (FT-IR) test. Results show the\nemerging of a new absorption peak around 1402 cm-1 on the used Co-MoS compared to the fresh catalyst (Fig.\ncatalyst surface. This could also be the reason for the slightly lower carbon balance mentioned in the following\nResponse to Comment (c). We have added related results and discussions into the revised manuscript (Page 11,\nlines 254-255; Supplementary Fig. 19).\nFigure R5. Fourier Transform Infrared Spectroscopy (FT-IR) test of the fresh and used Co-MoS catalyst.\n2\n(b) HPLC results must be included in the SI alongside GC.\nAccording to your suggestion, we have included the HPLC result (Fig. R6a) and the standard curve (Fig. R6b)\nused to quantify the concentration of formate into the revised Supplementary information (Supplementary Fig. 5).\n2 2\ntest was conducted using 20 mg of catalyst in 0.5 M aqueous KHCO solution with 3.5 MPa of feed gas (24%\n2 2 2\nconcentration of formate.\n(c) Carbon balancing from HPLC and GC combined must be included.\nWe have calculated the carbon balance from HPLC and GC based on the following formulas:\nn(CO , final) + n(CH , final) + n(HCOO-, final) + n(CO 2- /HCO - , final)\nCarbon balance = 2 4 3 3\nn(CO , initial) + n(KHCO , initial)\n2 3\n(20.759 + 0.001 + 3.344 + 3.061) mmol\n= ×100% = 95.4%\n(23.457 + 5.024) mmol\nWhere n(CO , final) , n(CH , final) , n(HCOO-, final) , and n(CO2-/HCO-, final) denote the amount of\n2 4 3 3\ncarbon dioxide, methane, formate, and carbonate/bicarbonate in the post-reaction gas or solution, respectively;\nn(CO , initial) and n(KHCO , initial) denote the amount of carbon dioxide and potassium bicarbonate before\n2 3\nthe reaction, respectively. Here, n(CO , final) , n(CH , final) , and n(CO , initial) are calculated based on the\n2 4 2\nequation of pV = nRT. The volume of autoclave (V) is 78 mL. In the post-reaction gas, the partial pressures of\ncarbon dioxide and methane are calculated based on the measured system pressure of 3.2 MPa and their molar\ncontent determined by GC analysis. The n(HCOO-, final) and n(CO2-/HCO-, final) are quantified via HPLC\n3 3\nand ion chromatography (IC), respectively, using the standard curves shown in Figs. R6b, R7.\nWe have added the corresponding discussion in the revised manuscript (Page 11, lines 240-255) and\nSupplementary information (Supplementary Figs. 5, 6, and 19).\n(d) A reference reaction procedure with KHCO and gases only should be provided with carbon balancing to\n3\nexclude gas leaking from autoclave.\nWe have conducted the reference reaction procedure with KHCO and gases only and calculated the carbon\n3\nbalance based on the results of HPLC and GC using the formula mentioned in Response to Comment (c). The\ncarbon balance reached 97.8%, indicating a good sealing integrity of the autoclave. We have added the\ncorresponding discussion in the revised manuscript (Pages 11-12, lines 255-257).\nn(CO , final) + n(CH , final) + n(HCOO-, final) + n(CO 2- /HCO - , final)\nCarbon balance = 2 4 3 3\nn(CO , initial) + n(KHCO , initial)\n2 3\n(22.871 + 0 + 0 + 5.057) mmol\n= ×100% = 97.8%\n(23.509 + 5.035) mmol\n(e) Post-reaction autoclave pressure readings would be useful.\nThe autoclave pressure was measured to be 3.2 MPa following the reaction, starting from an initial value of\n3.5 MPa. These values have been used to calculate the carbon balancing as shown in Response to Comment (c).\n(f) GC should show both FID and TCD signals, the figure in the SI does not state if it is a combined signal or\nFID only (CH signal present).\nsignal shows characteristic peaks for CO , H , and N , where CO and H are the reactant gases and N is used as\n2 2 2 2 2 2\nthe internal standard (Fig. R8). CH was detected only by FID and not by TCD, as its concentration fell below the\n4\ndetection limit of the TCD. We have added the GC-TCD results in the revised Supplementary information\n(Supplementary Fig. 6b).\n2\nover the 1.2% Co-MoS catalyst. The reaction test was conducted using 20 mg of catalyst in 0.5 M aqueous\n3 2 2 2\nIn addition, to further verify the calculated TS and activation energy barriers, we have provided the\nvibrational frequencies of the TS structures (Tables R2-R7), which show that each TS has only one imaginary\nfrequency, thus confirming the accuracy of our computational results. These additional data have been included\ninto the revised Supplementary information (Supplementary Tables 3-8).\nTable R2. Calculated vibrational frequencies in cm-1 of transition states for CO hydrogenation to HCOOH on\n2\nin-plane SV-confined Co-Mo sites of Co-MoS via carboxyl (path 1) and formate (path 2) pathways, which\n2\ncorresponds to the DFT calculations in Fig. 4d. The frequencies in the brackets denote the imaginary frequencies\nof the transition states.\nTransition states Vibrational frequencies (cm-1)\n3606.23, 1523.30, 1266.47, 1124.03, 1012.62,\nTS 850.89, 681.81, 614.38, 502.53, 378.38,\n1\n320.10, 276.18, 213.20, 167.27, [763.61]\n3264.56, 1308.92, 1285.50, 1168.89, 1008.30,\nTS\n2\n902.43, 821.16, 577.03, 498.51, 324.94, 299.54, 244.28, 188.07, 315.51,\n12\n[859.35]\n2996.58, 1774.54, 1311.67, 1003.19, 955.27,\nTS 898.46, 678.28, 582.40, 396.64, 312.84,\n3\n224.91, 177.29, 130.77, 49.25, [802.37]\nTable R3. Calculated vibrational frequencies in cm-1 of transition states for CO hydrogenation to HCOOH on\n2\ncorresponds to the DFT calculations in Fig. 4e. The frequencies in the brackets denote the imaginary frequencies\nof the transition states.\nTransition states Vibrational frequencies (cm-1)\n2373.45, 1475.33, 1333.21, 1145.90, 882.79,\nTS 874.20, 755.08, 552.99, 395.51, 343.46,\n1\n300.27, 228.23, 190.43, 113.77, [627.05]\nTable R4. Calculated vibrational frequencies in cm-1 of transition states for the removal of oxygen from the in-\nplane SV-confined Co-Mo sites of Co-MoS , which corresponds to the DFT calculations in Supplementary Fig.\n2\n15. The frequencies in the brackets denote the imaginary frequencies of the transition states.\nTransition states Vibrational frequencies (cm-1)\n1607.72, 1080.08, 883.35, 782.81, 572.34,\nTS 502.43, 465.74, 460.66, 415.60, 322.43,\n1\n296.16, 283.14, 264.20, 179.71, [1519.50]\n3631.98, 1107.28, 919.80, 775.50, 671.52,\nTS 593.11, 521.05, 481.04, 411.85, 385.88,\n2\n331.76, 280.71, 253.09, 196.32, [466.71]\n1454.05, 1392.85, 1042.03, 786.80, 621.32,\nTS\n3\n450.87, 414.89, 289.71, [1192.64]\n3516.96, 1276.46, 882.88, 637.05, 603.97,\nTS\n4\n390.69, 369.03, 315.56, [153.50]\nedge SV-confined Co-Mo sites of Co-MoS , which corresponds to the DFT calculations in Supplementary Fig.\n2\n16. The frequencies in the brackets denote the imaginary frequencies of the transition states.\nTransition states Vibrational frequencies (cm-1)\n3594.08, 1004.51, 884.76, 813.93, 722.48,\nTS 497.68, 427.99, 365.51, 236.53, 171.66\n1\n171.28, [862.00]\n2004.32, 1560.78, 864.83, 839.17, 757.67,\nTS\n2\n419.77, 305.74, 223.85, [670.81]\n3669.18, 1093.13, 1015.59, 673.18, 446.97,\nTS\n3\n359.50, 241.76, 117.65, [1148.07]\nTable R6. Calculated vibrational frequencies in cm-1 of transition states for CO hydrogenation to HCOOH on\n2\nin-plane, which corresponds to the DFT calculations in Supplementary Fig. 17a. The frequencies in the brackets\ndenote the imaginary frequencies of the transition states.\nTransition states Vibrational frequencies (cm-1)\n3617.20, 1090.92, 792.01, 747.018, 455.10,\nTS\n1\n410.86, 377.26, 336.62, [471.46]\n1805.33, 1679.73, 1384.61, 1199.74, 1059.02,\n967.69, 735.68, 693.68, 660.46, 453.40,\nTS\n2\n325.67, 286.09, 250.60, 210.74, 174.97,\n157.09, 70.98, [258.59]\nTable R7. Calculated vibrational frequencies in cm-1 of transition states for CO hydrogenation to HCOOH on\ndenote the imaginary frequencies of the transition states.\nTransition states Vibrational frequencies (cm-1)\n3636.71, 956.80, 853.71, 766.69, 469.39,\nTS\n1\n457.70, 337.17, 180.47, [974.41]\n14\n3630.11, 1926.95, 1526.23, 1116.43, 1002.63,\nTS 818.86, 793.59, 570.46, 526.93, 416.30, 381.39, 281.42, 264.97, 160.24,\n2\n139.20, 115.07, 89.95, [114.76]\n2616.09, 1778.11, 1417.72, 1308.31, 1191.66,\n1070.27, 939.44, 868.77, 746.87, 651.87,\nTS\n3\n349.00, 273.45, 229.66, 177.28, 168.77,\n148.67, 50.11, [104.75]\n2506.82, 2275.05, 1777.42, 1273.82, 1019.12\nTS ' 798.68, 736.14, 722.33, 590.95, 562.59, 506.14, 386.40, 205.03, 126.76,\n2\n99.07, 63.43, 77.31, [256.43]\nFor your suggestion about optionally calculating the adsorption of carbonates on Co-MoS , since it remains\n2\na high challenge to accurately simulate charged ions using the DFT method which inevitably introduces errors,\nthis calculation was not conducted to avoid misleading. From a mechanistic perspective, in the reaction solution,\ncarbonate ions are surrounded and stabilized by water molecules via hydrogen bonding interactions, thus may not\nbe adsorbed on the catalyst surface, thus it is not further considered for simplicity.\nComment 4. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication\nor require revision?\nThere are flaws, mentioned above. The flaw require revision and once ambiguities are resolved, the work will be\npublishable. The introduction is about 50% abstract repeated and does not explore the problem SV in detail, the\nwork’s impact should also be highlighter more.\nResponse: We thank you for your constructive suggestions and for recognizing the potential of our work. To\naddress the flaws you identified, we have conducted additional experiments and have provided additional\ninformation in the revised manuscript, as mentioned above. We have also revised the introduction and added more\ndetails to explore the SVs in the revised manuscript (Page 2, lines 39-41, 43-45).\nComment 5. Is the methodology sound? Does the work meet the expected standards in your field?\nThe methodology is sound, but at present is missing important evidence.\nResponse: We thank you for your valuable comments. We have now provided the important evidence that was\n15\nmissing, as mentioned above, to ensure that the work meets the expected standards.\nComment 6. Is there enough detail provided in the methods for the work to be reproduced?\nThe characterization is well documented, while reactivity testing is lacking. The computational work\nmethodology is robust.\nResponse: We thank you for acknowledging the quality of the characterization and computational methodology.\nWe have now provided the additional details in the reactivity testing section to address your concern and ensure\nreproducibility (Pages 11-12, lines 233, 240-257; Supplementary Figs. 5, 6, and 19; Supplementary Tables 3-8).\nComment 7. I hope that you will find the comments above useful. Most of the comments can be resolved, by\nsimply expanding the manuscript and SI with data you likely already have. I can recommend the work for\npublication following revision pertaining to the points mentioned above.\nResponse: We sincerely appreciate your constructive comments and suggestions, which are very helpful for\nfurther improving the quality and robustness of our manuscript. Thank you very much for your positive and\nconstructive feedback, and we are appreciative that you can recommend our work for publication after full\nrevisions. We have thoroughly addressed all your comments by expanding the manuscript and Supplementary\nInformation with additional data and explanations as suggested.\n16"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have carried out further studies on the system and clarified the original questions. The manuscript reads well\nand now this paper is recommended to publish as revised version.", "reviewer_2": "Following the resolution of the previous comments, I can recommend the work for publication."}, "author_response": "Response: We are grateful to the reviewer’s recommendation for the acceptance of our manuscript.\nResponse: We are grateful to the reviewer’s recommendation for the acceptance of our manuscript."}]

Decision letter