Breaking activity-selectivity-stability trade-offs in reverse water-gas shift reaction via high-energy micro-faceted Mo_2N nanocrystals

Domain: neuroscience
Published: 2026-02-02
Editorial outcome: minor_revision

Abstract
The reverse water-gas shift reaction (RWGSR) is essential for converting CO2 into fuels using renewable hydrogen, but it remains challenged by the difficulty of simultaneously maximizing catalyst activity, selectivity, and stability. These limitations stem from thermodynamic constraints – specifically, the Gibbs-Curie-Wulff theorem – which restricts the synthetic accessibility of high-energy micro-faceted nanocrystals via conventional methods. To address this, we introduce a near-surface “quasi-hyperbaric” ammonia strategy that integrates atmospheric-pressure processing with in-situ ammonia decomposition. This approach enables the controlled synthesis of molybdenum nitride nanocrystals with preferentially exposed high-energy (112) microfacets. These facets promote CO2 activation through a hydrogen-assisted redox mechanism, driven by geometrically confined and stabilized Mo-N/M-O hybrid active sites. The resulting catalyst outperforms the benchmark Pt/CeO₂, which typically suffers from CO selectivity below 92%. Our catalyst achieves near-equilibrium conversion (56%) at a space velocity (24000 ml/g_cat/h), with 100% CO selectivity and outstanding stability (≤1% deactivation over 250 hours).

Introduction
Catalytic conversion of CO₂ to CO via the reverse water-gas shift reaction (RWGSR) is a key pathway for sustainable fuel production and advancing carbon neutrality1–3. Despite its thermodynamic favorability at elevated temperatures, practical implementation of RWGSR is limited by the lack of catalysts that concurrently offer near-equilibrium conversion, perfect selectivity and industrial-grade stability4–6. This performance paradox arises from intrinsic trade-offs between the reactivity of catalytically active sites and their thermodynamic stability during synthesis and reaction. Although high-energy active sites, such as the high-energy microfacets of nanocrystals, are expected to enhance CO production7, the creation of these metastable states poses significant challenges. Unfortunately, they also accelerate catalyst sintering and side reactions, which compromise selectivity and stability.

Conventional catalyst systems exemplify this dilemma. While Pt-group catalysts are active and robust, they suffer from CO selectivity loss above 400oC due to unfavorable electronic state transitions under reducing conditions8–10. Conversely, Cu-based catalysts offer high selectivity but lack sufficient activity and thermal instability, limited by their low Tamman temperature and a tendency for excessive hydrogenation3,11,12. Emerging transition metal nitrides and carbides, particularly molybdenum nitride (Mo₂N) nanocrystals, show promise in bridging this gap by mimicking noble metal behavior while offering superior thermal stability1,5,13–18. However, conventional synthesis routes for Mo₂N nanocrystals constrained by Gibbs-Curie-Wulff thermodynamic principles, inevitably expose low-energy microfacets as global surface energy minima through the predictive Wulff reconstruction, compromising the activity towards the RWGSR reaction19,20. To address the thermodynamic constraint, it is crucial to implement a kinetic control strategy that stabilizes highly active metastable microfacets by overriding the surface termination configurations at global energy minima. A viable approach involves introducing an intermediate stage characterized by a non-equilibrium steady-state chemical environment, guiding unconventional crystal growth during chemical synthesis. This strategy facilitates the formation of high-energy metastable microfacets in nanocrystals under synthetic conditions, allowing them to be effectively maintained and stabilized upon quenching.

As a proof-of-concept study, we introduce a near-surface “quasi-hyperbaric” ammonia atmosphere as a non-equilibrium steady-state chemical environment that completely alters the growth kinetics of Mo₂N nanocrystals restricted by the Gibbs-Curie-Wulff theorem. This is achieved by imposing strong surface-adsorbate interaction onto the intermediate structures during crystal growth, which completely alters the micro-faceting behaviors of nanocrystals and leads to the formation of high-energy (112) microfacets instead of thermodynamically stable (001)/(111) microfacets as predicted by Wulff construction. The high-energy-micro-faceted molybdenum nitride nanocrystal (HEMN) performs as a catalyst and demonstrates exceptional CO₂ hydrogenation performance, achieving 56% near-equilibrium CO₂ conversion with 100% CO selectivity at 550°C and a space velocity of 24,000 mL·g⁻¹·h⁻¹, while maintaining excellent stability (<1% activity loss over 250+ hours). Notably, HEMN outperforms noble-metal benchmarks, sustaining 100% CO selectivity compared to <92% for Pt-based catalysts under equivalent conditions. The in situ spectroscopy, reaction kinetic analysis, and DFT calculations reveal that Mo-N/M-O hybrid structures on (112) microfacets act as active sites for CO₂ activation via an H₂-assisted redox mechanism.

Results
The creation of HEMN catalysts
The growth of Mo₂N nanocrystals is influenced by the point symmetry of the tetragonal crystal lattice and thermodynamic constraints defined by the Gibbs-Curie-Wulff theorem. Consequently, conventional chemical synthesis typically results in Mo₂N nanocrystals featuring low-energy microfacets, as predicted by Wulff construction. This characteristic can be assessed through the surface free energy plot, or γ-plot, illustrated in Fig. 1a for the Mo₂N crystal structure projected along the (110) direction under vacuum conditions (red contour). Notably, the γ-plot under vacuum displays multiple cusped minima at 0.1003, 0.1121, 0.0977, and 0.1009 eV/Å² along the normals of the (001), (112), (111), and (110) microfacets. These cusped minima emerge from the lateral growth of Mo₂N nanocrystals, following the classical terrace-ledge-kink model. The surface energies of these microfacets are ranked as (111)<(001)<(110)<(112) (Fig. 1 and Supplementary Figs. S1, S2). The Wulff construction under vacuum would therefore lead to an enclosure of the low-energy (001) and (111) microfacets, resulting in Mo₂N nanocrystals shaped like truncated elongated octahedra. To overcome the thermodynamic restrictions associated with these stable microfacets, a near-surface “quasi-hyperbaric” ammonia atmosphere was introduced as a non-equilibrium steady-state chemical environment, integrating both atmospheric and on-site decomposed ammonia (Supplementary Fig. S3a). This “quasi-hyperbaric” environment modulates the growth kinetics of Mo₂N nanocrystals by significantly enhancing the surface-adsorbate interaction. Investigating the γ-plot of Mo₂N surfaces bonded with ammonia adsorbates (purple contour) reveals that the interaction dramatically alters the surface energies of the nanocrystals. The chemical bonding of ammonia molecules decreases the surface energies for all microfacets at the cusped minima of the γ-plot, with a particularly pronounced effect on the (112) microfacets (approximately a 45% reduction in surface energy, from 0.1121 eV/Å² to 0.0648 eV/Å²). Consequently, the surface energies are re-ranked to (112)<(111)<(110)<(001), leading to the tentative stabilization of the high-energy (112) microfacets under synthetic conditions. Metastable nanocrystals in HEMN catalysts with exposed (112) microfacets and an octahedral shape can be synthesized by creating a near-surface “quasi-hyperbaric” ammonia atmosphere based on Wulff construction.

The experimental validation of HEMN catalysts can be achieved through combined high-resolution scanning transmission electron microscopy (HR-STEM) and electron tomography (ET). Multimodal STEM imaging, which incorporates high-angle annular dark-field (HAADF) and integrated differential phase contrast (iDPC) modes, enables the simultaneous visualization of both heavy and light elements within the Mo₂N nanocrystals. HAADF-STEM images captured along the [021] and [001] zone axes (Fig. 1d, h) clearly depict projected polyhedral nanocrystals measuring 5–10 nm (Supplementary Figs. S4, S5) with regular shapes. The angular gaps measured between the projected microfacets of these nanocrystals are 75° and 91°, respectively, aligning with the crystallographic interplanar angle for (112) microfacets. This observation supports the octahedral model predicted by Wulff construction (Fig. 1a). In addition, the explicit three-dimensional (3D) morphology of the nanocrystal can be reconstructed using the ET method based on tilt-series of STEM images, as illustrated in Fig. 1c and Supplementary Fig. S6. The Gaussian surface curvature of the 3D tomogram reveals that a randomly selected Mo₂N nanocrystal exhibits an octahedral shape, enclosed by microfacets shown in blue and connected by edges depicted in yellow. These microfacets can be indexed to the (112) group of crystal faces by comparing the projections of the reconstructed tomogram with the HAADF-STEM image along the [021] axis (Fig. 1b, c and Supplementary Fig. S5). Such observations confirm the (112) micro-faceted structure of HEMN catalysts stabilized under a “quasi-hyperbaric” ammonia atmosphere as a non-equilibrium steady-state. Furthermore, atomic-resolution structural elucidation of Mo₂N nanocrystals is facilitated through combined HAADF- and iDPC-STEM imaging, as demonstrated in Fig. 1d, h and Fig. 1e, i. An enlarged view of the HAADF-STEM image along the [021] axis reveals hexagonally arranged bright spots indicative of Mo atomic columns (Fig. 1f). The nitrogen atoms residing in the octahedral voids of the Mo sublattice can be visualized as atomic columns in the iDPC-STEM image (Fig. 1j). The phase contrast images align well with both the structural projections and the simulated projected potential of the Mo₂N crystal structure (Fig. 1g, k). In summary, these observations robustly corroborate the proposed concept, strategy, and structural model of HEMN catalysts.

Evaluation of the RWGSR Performance
Systematic optimization of calcination conditions, support materials, and loading levels identified 30wt% Mo₂N on silica, formed via ammonia calcination, as the optimal catalyst, exhibiting outstanding RWGSR activity (Supplementary Figs. S7, S8). We further conducted a comprehensive comparative analysis of the synthesized HEMN catalyst against representative RWGSR catalysts from the literature, evaluating key metrics including reaction rate (mol/g−1*h−1), CO selectivity, CO2/H2 ratio, deactivation rate, and catalyst synthesis complexity (Supplementary Table S1). The HEMN catalyst demonstrated superior performance across all dimensions (Fig. 2a and Supplementary Fig. S10)9,18,21,22. Notably, within the high-temperature range (450–600°C), the catalyst maintained nearly 100% CO selectivity across the full CO₂ conversion range (41.9%–62.6%). In contrast, Pt/CeO₂ exhibited a notable decrease in CO selectivity from 97.2% to 91.6% under the same reaction conditions (Fig. 2b and Supplementary Fig. S9). When evaluated under the same conditions, its overall performance exceeds that of other commonly reported catalysts (Supplementary Fig. S10). Additionally, the stoichiometric CO2/H2 ratio for the RWGSR is 1:1. However, in practical applications, a CO2/H2 ratio below 1:1 is often used to promote CO2 reaction and generate syngas compatible with downstream conversion processes. Increasing the H2 partial pressure thermodynamically favors the formation of methane formation23. To evaluate the effect of reactant composition, we investigated the influence of the CO2/H2 ratio on the catalytic performance. Increasing the H2 partial pressure significantly enhanced CO turnover rates while maintaining nearly 100% CO selectivity across both low (400oC, in Supplementary Fig. S11) and high (550oC, in Fig. 2c) temperature ranges. This behavior is a distinct feature of the Mo2N-based catalysts. Overall, the synthesized HEMN catalyst demonstrates robust performance over a broad temperature window (300–600oC), wide space velocity range (12000 to 48000 ml/g_cat/h, shown in Supplementary Fig. S12), and varied CO2/H2 ratios (1:1 to 1:4). Catalyst stability is a critical factor for its industrial application. To evaluate the stability of this HEMN catalyst in the RWGSR, we conducted long-term tests at both low (400oC, in Supplementary Fig. S13) and high (550°C, Fig. 2d) temperatures. For example, at 550oC with a space velocity of 24000 ml/g_cat/h and a CO2/H2 ratio of 1/3, the catalyst exhibited only a slight decrease in conversion (from 52.6% to 52.1%) over 250 h, corresponding to just ~1% deactivation, which is significantly lower than values reported for similar catalytic systems18. Moreover, the catalyst demonstrated excellent cyclic stability under repeated temperature cycling between 300oC and 550oC (Fig. 2e). XRD analysis of the catalyst after thermal cycling revealed no significant changes in the catalyst’s microstructure (Supplementary Fig. S14), further confirming its high-temperature stability. Moreover, the HEMN catalyst demonstrated exceptional high-temperature stability during a 12h test at 600°C, exhibiting no deactivation while maintaining consistent performance (Supplementary Fig. S15). Comprehensive characterization via in-situ and ex situ XRD (Supplementary Figs. S16, 17) and HRTEM (Supplementary Figs. S18, 19) confirmed that both the phase structure and the specific (112) crystal facet remained stable without detectable thermodynamic reconstruction. These results collectively affirm the catalyst’s robust performance and structural integrity under demanding operational conditions.

Active sites of the HEMN catalyst in the RWGSR
To elucidate the reasons for the high RWGSR activity of this HEMN catalyst, we performed surface structure analysis on the synthesized catalyst using XPS. As shown in Supplementary Fig. S20, the O 1s results revealed a peak at a binding energy of 530.6 eV, in addition to the characteristic peak of the support Si-O at 532.9 eV, which can be attributed to Mo-O species on the catalyst surface24. The Mo 3d results showed peaks at 228.9 eV and 231.8 eV, which can be attributed to Mo-N species, and peaks at 229.9 eV, 232.7 eV, and 232.5 eV, 235.2 eV, which correspond to two oxidation states of Mo-O species, Mo(IV) and Mo(VI)25. This further confirms the presence of a certain amounts of Mo-O species on the surface of this catalyst. To further investigate the redox properties of M-O species, we performed H2-TPR-MS tests on the HEMN catalyst (Supplementary Figs. S21, S22). The results showed that compared to pure-phase MoO3 (with a reduction onset temperature of ~600°C), the Mo-O species on the HEMN catalyst surface are more easily reduced (with a reduction onset temperature of ~350oC). This indicates that the Mo-O species on this HEMN catalyst are highly dispersed, consistent with the previously discussed HAADF-STEM structure (Supplementary Figs. S4, S5). These species have more active oxygen species, which can easily form a rich oxygen vacancy structure with high electron-donating capacity, acting as a scissor to directly cleave the C=O double bond of CO2 to form CO26.

To further explore the impact of Mo-O content on the catalytic performance of the RWGSR, we reduced the HEMN catalyst at different temperatures to construct a series of catalysts with varying Mo-O contents without destroying the MoN crystalline phase structure (Supplementary Figs. S23 and S24). Supplementary Fig. S25 indicates that as the reduction temperature increases, the catalytic performance of the RWGSR gradually increases, reaching an optimal level when reduced at approximately 500oC. When the reduction temperature is further increased, the CO conversion decreases significantly (from 21.5% to 14.5%). Supplementary Fig. S26 shows a volcano-like relationship between Mo-O content and CO yield, indicating that the catalytic activity is optimal only when the Mo-O content (~4%, as determined by quantitative XPS analysis) on this catalyst surface is moderate. The activation of the C−O bond is crucial in the CO₂ hydrogenation reaction27, and oxygen vacancies play a critical role28. At low reduction temperatures, the surface Mo-O concentration is high, resulting in weak CO adsorption and hindered C-O bond cleavage, which collectively lead to reduced catalytic activity. At higher reduction temperatures, the surface Mo-O concentration decreases, exposing more oxygen vacancies and enhancing CO adsorption. However, this also causes stronger adsorption of intermediate species, which hinders subsequent reactions29 and reduces the overall reaction rate (Supplementary Fig. S27). Therefore, the Mo-O/Mo-N species on the surface of the HEMN catalyst are the key determinants of its catalytic activity, a conclusion supported by studies of other similar systems30,31.

Reaction mechanism of the catalytic reaction
Depending on the interaction between CO2 and H2, the RWGSR can proceed via different mechanisms, including: the redox mechanism (where H* does not assist in C-O bond cleavage), the HCOO* pathway (H* directly hydrogenates C*), and the COOH* mechanism (H* directly hydrogenates O*)5. To investigate the reaction mechanism of the RWGSR on the HEMN catalyst, we tracked the intermediate species on the catalyst surface during the reaction using in situ IR experiments, as shown in Fig. 3a. The results showed that when the reaction gas (CO2/H2/N2 = 24/72/4) was introduced, signals at 2175 cm−1 and 2120 cm−1 gradually appeared as the reaction temperature increased. These characteristic peaks can be attributed to the asymmetric and symmetric stretching vibrations of CO, respectively, as shown in Fig. 3a. Throughout the temperature ramping process (300oC–600oC), we did not observe the formation of any species related to COOH* or HCOO* (Supplementary Fig. S28), indicating that the hydrogenation of CO2 on the HEMN catalyst does not follow the formate or carboxylate pathways. We hypothesize that the HEMN catalyst operates via the redox mechanism, in which CO2 adsorbs on active sites, such as oxygen vacancies, followed by C-O bond cleavage to form CO and O species. To test this hypothesis, we introduced a CO2/N2 (30/70) mixture over the HEMN catalyst. If the redox mechanism were operative, CO* intermediates should be observed. However, unexpectedly, no CO* signals were detected under any of the following conditions: i) gradual temperature increase under CO2/N2 (Supplementary Fig. S29a), ii) The experiments involved either temperature ramping in 5% H₂/Ar after CO₂ adsorption at room temperature, or H₂ pre-adsorption followed by CO₂/N₂ exposure at varying temperatures (Supplementary Fig. S29b and c), or iii) after high temperatures reduction at 600oC followed by similar in-situ experiments (Supplementary Figs. S30, S31). In addition, we complemented these studies with temperature-programmed surface reaction (TPSR) experiments and in-situ Raman measurements under CO₂/Ar atmosphere on the HEMN catalyst (Supplementary Fig. S32). Collectively, these results from both gas-phase and surface-sensitive characterization techniques consistently demonstrate that the reaction does not proceed via a conventional redox pathway, indicating a distinct mechanism deviating from the simple redox route.

Based on in situ experiments under reactive conditions (Fig. 3a), we conclude that CO* formation requires the presence of H2, indicating a likely H-assisted redox mechanism, which is consistent with previous reports on the Mo2C system5. Furthermore, throughout the reaction process (Fig. 3b and Supplementary Fig. S33), we observed no peaks forming or consuming in the range of 3500 cm−1 to 3200 cm−1, which is attributed to the stretching vibrations of hydroxyl groups32. This indicates that the consumption or generation of OH/H2O is a fast step in the elementary reactions and occurs after the rate-determining step33. During the H2-TPR-MS test of the HEMN catalysts, we observed a significant formation of the H2O peak before 300oC (Fig. 3c). Since the samples were pre-treated in an inert atmosphere at 300oC before the experiment, the adsorbed water on the samples was eliminated, ruling out the possibility that the H2O peak was from desorption of adsorbed water. Simultaneous online mass spectrometry detection revealed no signal peaks at m/z = 2 (H2 species), indicating that the H2O peak did not originate from the reduction of surface metal oxide species. Infrared transmission spectroscopy of the HEMN catalyst revealed distinct surface Mo-OH species (Supplementary Fig. S34), suggesting that the H2O peak detected by mass spectrometry likely originates from hydrogen-assisted dehydration of surface OH groups on the support.

To further verify this hypothesis, we conducted in situ IR experiments on the catalyst with H2/Ar (Fig. 3d). The results showed a significant consumption of OH in a temperature range similar to that observed in the H-TPR-MS experiment, which is consistent with our hypothesis. When the HEMN samples were pretreated with hydrogen reduction at temperatures above 300oC, there was no consumption of hydrogen observed in the in situ IR spectra. This finding confirms that surface Mo-OH species undergo hydrogen-assisted dehydration, forming Mo-O/Mo-N sites that constitute the active site structure, further supporting our earlier conclusions (Figs. S24–S27). Theoretical calculations further support this finding (Supplementary Figs. S35, S36), showing that the presence of H2 lowers the dehydration energy of Mo-OH from 0.76/0.74 eV to 0.45/0.46 eV on MoN (111) or (112) facets.

To further investigate the reaction mechanism of the RWGSR on the HEMN catalyst, we studied the kinetic dependences of CO2 and H2 at 300°C and atmosphere pressure with 50.7–76.0 kPa H2, and 10.1-35.5 kPa CO2, when maintaining the CO2 conversion below 10% and approach-to-equilibrium less than 10%. As shown in Fig. 4a, b, the turnover rate on HEMN catalyst gradually increased with the increasing partial pressure of CO2, demonstrating a reaction order of 0.37 (±0.003). The effect of H2 partial pressure on the reaction rate showed a similar trend, with a reaction order of 0.28 (±0.01). Combining the results from H2-TPR-MS, CO-TPD, and in-situ IR spectroscopy experiments, we proposed the elementary reaction steps of the RWGSR on the HEMN catalyst, as shown in Fig. 4c. The reaction begins with the adsorption of CO2 onto the Mo-O/N-Mo active site pair (denoted as *), forming adsorbed CO2* (Step 1). Concurrently, H2 undergoes dissociative adsorption at an adjacent site, producing an H* adatom and a Mo-OH* (Step 2). Subsequently, the H* species performs a nucleophilic attack on the oxygen atom of an nearby CO2* molecule, cleaving the C=O bond to generate OH* and CO* intermediates (Step 3). The CO* intermediate then desorbs into the gas phase as CO (Step 4). Finally, two OH* species combine to form an H2O* intermediate, which subsequently desorbs as a water molecule (Steps 5 and 6). Based on the above findings, together with the assumption of Step 3 as the rate-determining step and the application of the pseudo steady-state approximation, we can obtain the following turnover rate equation, with the detailed derivation process given in the Supporting Information:

r = k K_CO2 P_CO2 (K_H2 P_H2)^0.5 / (1 + K_CO2 P_CO2 + K_H2 P_H2^0.5) (1)

where k and K_j (subscript j = CO2 or H2) denote the rate constant of the kinetically relevant step (Step 3) and the equilibrium constant of the relevant elementary step (Step 1 or 2), respectively. The non-linear fitting of the experimental data with the turnover rate Eq. (1) determined k, K_CO2, and K_H2 to be 3.32(±0.30)×10-4 mol h−1 g_cat−1 kPa_CO2−0.37 kPa_H2−0.28, and 5.01 (±1.26)×10−3 kPa−1, respectively, as present in Fig. 4d. The predicted turnover rates closely match the experimental results, as confirmed by the parity plot (Fig. 4e). Under the operating conditions (50.7–76.0 kPa H2, and 10.1-35.5 kPa CO2), the coverage of unoccupied active site pairs on the catalyst surface varies from 33.3% to 52.1%, that of CO2* from 20.4% to 49.2%, and that of H* from 17.0% to 30.4%. We also investigated catalytic systems with lower M-O content on HEMN reduced at 600oC, as shown in Supplementary Fig. S37, S38. The results indicate that the different M-O contents do not affect the reaction kinetics, such as the reaction orders and the rate-determining step.

To gain deeper mechanistic insights into the RWGSR catalyzed by the HEMN system, we conducted comprehensive theoretical investigations of the hydrogen-assisted CO2 activation pathway, as illustrated in Fig. 5. Advanced electron microscopy coupled with 3D structural reconstruction unambiguously identified the preferential exposure of the (112) crystallographic facet in the HEMN catalyst, overriding the conventional thermodynamic stability of the (111) surface. Building on these experimental insights, we constructed DFT-optimized models of the dominant (112) and reference (111) surfaces of Mo₂N (Supplementary Fig. S39). To this end, we systematically probed the influence of Mo–O/N vacancy concentration on the H₂ binding energy, thereby identifying the Mo–O···H–Mo configuration as the initial active site for CO₂ activation (Supplementary Fig. S40). Comparative analysis of surface reactivity (Fig. 5 and Supplementary Fig. S41) revealed a striking reduction in the CO2 dissociation energy barrier on the (112) surface (0.66 eV vs 1.14 eV on (111) facet), providing fundamental rationale for the exceptional catalytic performance of our engineered material compared to conventional MoN systems.

To corroborate the hydrogen-mediated activation mechanism suggested by kinetic investigation and operando spectroscopic characterization, we systematically probed the hydrogen participation in CO2 activation thermodynamics. DFT calculations demonstrate that on the (112) facet, the spontaneous conversion of adsorbed CO2* to CO* and O* proceeds with an energy change of −1.20 eV under hydrogen-free conditions. Remarkably, the co-adsorption of H* species amplifies the thermodynamic favorability, lowering the energy change to −1.45 eV. Crucially, kinetic analyses revealed that hydrogen participation reduces the activation barrier from 0.66 eV to 0.45 eV, decisively confirming the viability of the H*-assisted activation pathway. In summary, our calculations demonstrate that the (112) facet is more active than the (111) facet for C–O bond cleavage, and that the H2-assisted pathway enables CO2 dissociation with a substantially lower energy barrier compared to direct dissociation. Furthermore, the catalytic cycle maintains sustainability through dynamic surface regeneration: The transient Mo-OH species formed during reaction undergoes rapid dehydrogenation in the H2-rich environment (Fig. 3 and Supplementary Fig. S35), reconstructing the active Mo-O/Mo-N configuration that perpetuates the catalytic cycle. The superior catalytic performance and stability of the HEMN system arise from a synergistic combination of favorable thermodynamics, lowered kinetic barriers, and self-repairing surface architecture. Based on the structural model of the (112) facet, we further evaluated alternative reaction pathways, namely, the HCOO* and *COOH routes, to theoretically exclude competing mechanisms. The results show that the H2-assisted pathway features a significantly lower activation barrier (0.45 eV) compared to the HCOO* (1.45 eV) and *COOH (1.45 eV) routes, as shown in Supplementary Fig. S28.

Methods
The synthesis of catalysts
The high-energy-micro-faceted molybdenum nitride (HEMN) catalysts. A precise amount of ammonium molybdate (H₈MoN₂O₄, Aladdin, AR grade) was dissolved in deionized water and uniformly dispersed onto silica or other supports using the incipient wetness impregnation method. The resulting slurry was dried at 100°C for 12 h to obtain the precursor. Subsequent calcination was performed in a tubular furnace under flowing NH₃ at 600°C (with a heating rate: 5°C/min) for 4 h to yield the corresponding supported HEMN catalysts.

Synthesis of reference catalysts. (i) The pure Mo₂N catalyst was synthesized via a co-precipitation method. First, 3.5g of ammonium molybdate (H₈MoN₂O₄, Aladdin, AR grade) and 3.6g of melamine (C₃H₆N₆, Sigma, 99%) were separately dissolved in 50mL deionized water. The solutions were combined under continuous magnetic stirring to induce complete precipitation. The resulting precipitate was isolated by centrifugation, dried at 120°C for 12 h, and subjected to temperature-programmed calcination in a tubular furnace under N₂ flow. The thermal profile included a ramp from ambient temperature to 800°C at 10°C/min for 7 h; (ii) The CeO₂ support was prepared by calcining 6g of cerium nitrate (Ce(NO₃)₃, Macklin, AR grade) in a muffle furnace at 350°C at 5°C/min for 2 h. Separately, 14.88mg of platinum tetra-ammine nitrate (H₁₂N₆O₆Pt, Aladdin, 99.9%) was dissolved in 525 μL deionized water and impregnated onto 1.5g of the pre-synthesized CeO₂ support using the incipient wetness impregnation method. The impregnated material was dried at 80°C for 12 h and activated in a muffle furnace under static air with a thermal ramp of 5°C/min to 800°C, where it was held for 1 h; (iii) The Ni/Al₂O₃ catalyst was prepared by dissolving 148.5mg of nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, Macklin, AR grade) in 1150 μL deionized water. The solution was impregnated onto 1.5g of γ-Al₂O₃ support via the incipient wetness method. The composite was dried at 100°C for 12 h and calcined in a muffle furnace with a 5°C/min ramp to 600°C for 4 h. (iv) The CuZnAl catalyst was prepared by co-precipitation. An aqueous solution of Cu, Zn, and Al nitrates and a separate K₂CO₃ solution were simultaneously added dropwise into a beaker at 80°C with vigorous stirring. The pH was maintained at approximately 9 during this process. The resulting suspension was aged at 80°C for 12 h, then the precipitate was collected, washed with deionized water, dried at 100°C for 12 h, and finally calcined at 550°C for 4 h in air. (v) The Ni/CeO₂ catalyst was prepared by incipient wetness impregnation of the CeO₂ support with an aqueous solution of nickel nitrate. The material was subsequently dried at 100°C for 12 h and calcined at 300°C for 2 h (ramp rate: 5°C/min). Prior to catalytic testing, the catalyst was reduced in a 5% H₂/Ar flow at 550°C for 2 h. (vi) The Fe/ZrO₂ catalyst with an iron loading of 1.43wt% was prepared by the incipient wetness impregnation of a ZrO₂ support with an aqueous solution of ferric nitrate. The precursor was dried at 100°C for 12 h and subsequently calcined at 450°C for 4 h in static air with a heating rate of 5°C/min. Prior to catalytic testing, the catalyst was reduced at 550°C for 2 h in a 5% H₂/Ar (30 mL/min). (vii) The Pt/SiO₂ catalyst was prepared by the incipient wetness impregnation of a SiO₂ support with an aqueous solution of tetra-ammine-platinum(II) nitrate. Following impregnation, the material was dried at 100°C for 12 h and subsequently calcined at 450°C for 2 h in static air with a heating rate of 10°C/min. (viii) The Ru/γ-Al₂O₃ catalyst was prepared by the incipient wetness impregnation of a γ-Al₂O₃ support with an aqueous solution of ruthenium(III) chloride. The resulting material was dried at 100°C for 12 h and subsequently calcined at 500°C for 2 h in static air with a heating rate of 10°C/min. Prior to catalytic testing, the catalyst was reduced in a 5% H₂/Ar flow (30 mL/min) at 550°C for 1 h.

The performance of catalysts
After synthesizing the HEMN catalyst under an ammonia atmosphere, we evaluated its performance of RWGSR. The RWGSR tests were conducted in a home-made fixed-bed reactor system, which consists of a quartz tube with an inner diameter of 8mm. The catalyst was prepared by pelletizing, crushing, and sieving to 60–80 mesh, followed by homogeneous mixing with quartz sand. The reaction was carried out at 1 atm with a feed gas mixture of CO₂, H₂, and N₂ in a volumetric ratio of 24:72:4. During the temperature-programmed process, the reaction gas mixture was continuously fed into the reactor. After reaching the target temperature, the system was stabilized for 30 minutes before data collection. Product analysis was performed using an online gas chromatograph (GC2060, Shanghai Ruimin Instrument Co., Ltd.) equipped with a flame ionization detector (FID; Rt-Q-BOND column) and a thermal conductivity detector (TCD; TDX-01 column). A high-temperature conversion furnace was integrated into the system, and all product lines were maintained at 150oC to ensure accurate online detection. CO₂ conversion and product selectivity were calculated via the carbon normalization method, with specific formulas provided as (2) and (3).

Con(%) = (A_i_in * f_i_in - A_i_out * f_i_out) / (A_i_in * f_i_in) * 100 (2)

Sel(%) = (A_pi * f_pi) / (sum(A_pi * f_pi)) * 100 (3)

Here, i denotes any gaseous species in the reaction system; N represents the number of carbon atoms in the component; A_i and f_i correspond to the chromatographic peak area and calibration factor of species i, respectively; and p denotes the products.

Detailed kinetic testing parameters are provided in Supplementary Tables S3, S4, with corresponding conversion and product selectivity data available in Supplementary Tables S7–S14.

The characterization of catalysts
X-ray diffraction (XRD) measurements were conducted on a Rigaku SmartLab 9kW Advanced powder X-ray diffractometer using Cu Kα radiation (λ=0.154 nm). The 2θ scanning range was 10° to 80° at a rate of 5°/min. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Scientific K-Alpha instrument. Binding energy results were calibrated by referencing the C1s peak to 284.6 eV. The specific surface area was determined using the Brunauer–Emmett–Teller (BET) equation by analyzing the adsorption branch of the isotherm in the pressure range of P/P₀=0.04–0.32. Pore size distribution was derived from the adsorption branch using the Barrett–Joyner–Halenda (BJH) method, and the total pore volume was measured at P/P₀=0.99. Temperature-programmed reduction and desorption analyses (H₂-TPR, H₂-TPD, CO₂-TPD) were conducted on a home-made system equipped with an online mass spectrometer (Tilon SRD200M). For all experiments, 100mg of catalyst was pretreated in Ar at 300°C for 2 h to eliminate surface impurities. In H₂-TPR, the sample was reduced with 5% H₂/Ar while heating from room temperature to 900°C at 10°C/min, monitoring H₂ (m/z=2) and H₂O (m/z=18). For H₂-TPD, after H₂ adsorption (5% H₂/Ar, 1 h) and Ar purging, H2 desorption was tracked (m/z=2) during heating to 800°C at 10°C/min. Similarly, CO₂-TPD involved CO₂ adsorption (30% CO₂/N₂, 1 h), Ar purging, and monitoring CO₂ (m/z=44) during heating to 800°C at 10°C/min under Ar flow. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast imaging (iDPC-STEM) were performed using a Thermo Scientific Spectra 300 double spherical aberration-corrected transmission electron microscope operated at 300 kV with a convergence semi-angle of 25 mrad. The sample was dispersed in anhydrous ethanol via ultrasonication, deposited onto a grid using a micropipette, dried, and loaded into the microscope. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were conducted using a Thermo Scientific Nicolet iS50 spectrometer equipped with an MCT detector. For RWGSR studies, the HEMN catalyst was pretreated in N₂ at 300°C for 1 h, cooled to room temperature, and background spectra were collected at 300–600°C under N₂. Reaction gas (CO₂/H₂/N₂=24/72/4) was then introduced into the in-situ cell to monitor intermediates and products. To investigate surface hydroxyl species interacted with H₂, the pretreated catalyst was exposed to 5% H₂/Ar from 25 to 300°C, with spectra recorded in the range of 4000–2500 cm⁻¹. In-situ XRD was performed using a PANalytical Empyrean diffractometer equipped with a Cu Kα radiation source (λ=1.5418 Å) at 40 kV and 40 mA. In-situ experiments were operated with a diffractometer equipped with a high-temperature reaction cell (XRK900, Anton Paar GmbH). All XRD patterns were scanned from 10° to 80° (2θ) at a rate of 5° min⁻¹. Temperature-programmed in-situ XRD measurements of the HEMN catalyst were performed under a mixed gas atmosphere of CO₂:H₂:Ar=72:24:4 (vol%) with a total flow rate of 40 mL min⁻¹. The catalyst was heated from 25°C to 50°C, 400°C, 500°C, 550°C, and 600°C sequentially at a ramp rate of 10°C min⁻¹, with one diffraction pattern collected at each target temperature for 60 min. Subsequently, the patterns were collected every 20 minutes (3 patterns in total). For the in-situ HRTEM characterization, the data were acquired using an FEI ETEM equipped with a GATAN K3 direct electron detection camera, combined with a DENS MEMS-based gas sample holder. First, a small amount of an aqueous solution of the HEMN catalyst was precisely drop-cast onto the silicon nitride membrane window of the gas chip and subsequently air-dried at room temperature. Finally, the gas chip loaded with the sample was carefully installed into the in-situ gas holder. At the beginning of the in-situ experiment, a mixed gas of CO2 and H2 was introduced, while the temperature was simultaneously and gradually increased to 600°C and held stable for 1 h. During this process, dynamic HRTEM images of the sample were acquired along the [021] zone axis. The TPSR experiment was conducted using a custom-built system equipped with an online mass spectrometer (Tilon SRD200M). Prior to the reaction, 100mg of the catalyst was pretreated in argon at 300°C for 2 h to remove surface impurities. The sample was then exposed to a 30% CO₂/N₂ gas mixture (flow rate: 20 mL/min) while heating from room temperature to 600°C at a rate of 10°C/min. The desorption signals of CO₂ (m/z=44) and CO (m/z=28) were continuously monitored. The In situ Raman spectra with CO2/Ar were recorded on a Renishaw in Via spectrometer with 532 nm laser excitation, coupled with a Linkam TS 1500 micro in-situ cell. The HEMN was exposed to a 10% CO₂/Ar flow (30 mL/min) during the heating process. A reference spectrum was first collected at room temperature. The sample was then heated at 10°C/min to 300°C to 600°C and then kept for 60 min. At each target temperature, the sample was stabilized, repositioned under the microscope, and two consecutive Raman scans were collected (60 s integration, spectral range: 100–1200 cm⁻¹).

Derivation of rate expression
The rate expression is derived based on the sequence of elementary steps proposed in Fig. 4c in the manuscript, considering the interaction between CO2* and H* as the kinetically relevant step (Step 3, main manuscript) and the pseudo steady-state approximation of all intermediates.

r = k * θ_CO2* * θ_H* (4)

where r and k represent the turnover rate and rate constant of Step 3, respectively. θ_j denotes the fractional coverage of surface species j (j=*, CO2*, H*). According to the Langmuirian adsorption model, the overall site balance is

θ_* + θ_CO2* + θ_H* = 1 (5)

The quasi-equilibrium assumption on CO2 adsorption (Step 1) and H2 dissociative adsorption (Step 2) gives

θ_CO2* = K_CO2 * P_CO2 * θ_* (6)

θ_H* = K_H2 * P_H2^0.5 * θ_* (7)

where K_i (i=CO2 or H2) is the equilibrium constant of CO2 adsorption or H2 dissociative adsorption. Substituting Eqs. 6–7 into Eq. 5 gives

θ_* + K_CO2 * P_CO2 * θ_* + K_H2 * P_H2^0.5 * θ_* = 1 (8)

Thus,

θ_* = 1 / (1 + K_CO2 * P_CO2 + K_H2 * P_H2^0.5) (9)

Substituting (6), (7), and (9) into (4) gives (1) in the main manuscript.

Density functional theory (DFT)
Density functional theory (DFT) calculations were conducted using the Vienna ab initio simulation package (VASP 5.4.4)34. The exchange-correlation energy was modeled with the Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA)35. Core electrons were described using projected-augmented wave (PAW) pseudopotentials, and a plane-wave cutoff energy of 450 eV was employed36. After conducting k-point convergence tests (as shown in Table S5), we ultimately determined that a Γ-centered 2×1×1 k-point grid was employed for the (111) facet, and a 2×2×1 grid was used for the (112) facet in the k-point sampling process. Convergence criteria were set to 10⁻⁵ eV for electronic relaxation and -0.05 eV/Å for ionic relaxation. Based on electron microscopy analysis, the synthesized Mo₂N material predominantly exposes the high-surface-energy (112) facet (Fig. 1 in the main manuscript). To explore its surface properties, we constructed a computational model of the Mo₂N(112) facet with lattice parameters of a=12.0366 Å, b=9.9712 Å, c=21.0606 Å and angles of α=90.0o, β=90.0o, γ=90o. For comparative purposes, a thermodynamically stable Mo₂N (111) supercell model was also established (Supplementary Figs. S39, 40), with lattice parameters of a=12.0366 Å, b=18.0350 Å, c=19.9984 Å, and angles of α=90.0o, β=90.0o, γ=109.5o. In the computational setup, the bottom atomic layers were fixed to simulate the bulk catalyst’s structural rigidity, while the top surface layers were fully relaxed to capture local atomic rearrangements during adsorption and surface reactions. This approach strikes a balance between computational efficiency and accuracy, ensuring the identification of the most stable configurations during adsorption-reaction processes. The climbing image nudged-elastic-band method (CI-NEB), including six replicas, was used to search the transition states (TS) for all of the elementary reactions. The TS was denoted by the highest image along the minimum energy path. The energy barrier (E_a) of each elementary reaction was calculated by the energy difference between the TS and the initial state (IS) without the inclusion of the zero-point energy. Vibrational frequency analysis was performed using the finite displacement method to determine the Hessian (force constant) matrix (Supplementary Table S6). The calculation was performed with IBRION=5, a displacement step of POTIM=0.015 Å, and the same exchange-correlation functional and pseudopotentials as used in the transition state optimization. NSW was set to 1 to prevent any further geometric optimization. The viability of the transition state was visualized using “Jmol” (a 3D structure viewer), confirming that the atomic displacements align with the expected reaction coordinate.

The adsorption energy (E_ads) of adsorbates on all slab models was calculated as below:

E_ads = E_total - (E_slab + E_g) (10)

where E_total, E_slab, and E_g represent the total energy of the adsorbed system, the pristine slab energy, and the gas-phase adsorbate energy, respectively. Correspondingly, the desorption energy (E_des) is defined as below:

E_des = (E_slab + E_g) - E_total (11)

The vacancy formation energy (ΔE_f) of HEMN catalysts was calculated as below:

ΔE_f = E_v - (E_slab + E_H2O(g)) (12)

where, E_v denotes the energy of the vacancy-containing surface, and E_H2O(g) represents the energy of an isolated water molecule in the gas phase.

The surface energy calculation involves three key steps: First, both the bulk material and the surface models (as illustrated in Supplementary Fig. S1) undergo geometric optimization to ensure they correspond to their lowest-energy stable configurations. Second, the bulk energy E_bulk is calculated, and the average energy per atom E_bulk,peratom is determined using the formula (E_bulk,peratom = E_bulk / N_bulk, where N_bulk is the number of atoms in the bulk material). Third, the total energy of the surface model E_slab is computed. The surface energy γ_surface is then calculated using the following formula:

γ_surface = 1 / 2A * (E_slab - N_slab * E_bulk,peratom) (13)

where: A is the surface area, N_slab is the number of atoms in the surface model. For the surface model with adsorbed ammonia, which simulates the surface energy under an ammonia atmosphere, the calculation formula is as follows:

γ_surface,NH3 = 1 / 2A * (E_slab,NH3 - N_slab * E_bulk,peratom - N_NH3 * E_NH3) (14)

where E_slab,NH3 represents the total energy of the surface model with adsorbed ammonia, N_NH3 is the number of ammonia molecules adsorbed on the surface, and E_NH3 is the energy of a single ammonia molecule in the gas phase.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "While this manuscript presents an interesting synthesis approach using a near-surface hyperbaric ammonia atmosphere to stabilize [112] facets of Mo2N nanocrystals, the work suffers from several critical limitations that significantly undermine its claimed advances. The comparison with only Pt/CeO2 as a benchmark, the lack of thermal stability data above 600°C, and insufficient mechanistic evidence raise serious questions about the practical significance and scientific rigor of this work.\n\nMajor Comments\n1. The authors compare their catalyst exclusively with Pt/CeO2, which is neither the most active nor the most practical RWGS catalyst in the 400-600°C range. This selective comparison appears designed to exaggerate the performance advantages. Numerous catalysts including Cu/ZnO/Al2O3, Ni/CeO2, other Mo2C systems, and Fe-based catalysts operate effectively in this temperature window, often with comparable or superior performance metrics. The authors must provide comprehensive comparisons with at least 5-10 relevant catalysts under identical conditions, including both noble and non-noble metal systems. Without such comparisons, claims of \"exceptional performance\" and \"outperforming benchmarks\" are unsubstantiated.\n2. The complete absence of data above 600°C represents a fundamental flaw in this work. The authors synthesize their catalyst at 600-800°C but only test stability up to 550°C, conveniently avoiding the temperature regime where [112] facets would likely reconstruct to thermodynamically stable configurations. This omission appears deliberate and raises questions about the true stability window of these metastable structures. For a catalyst claimed to break \"stability trade-offs,\" the failure to define its thermal limits is unacceptable. The manuscript requires extensive high-temperature characterization including in-situ XRD/TEM up to at least 800°C and kinetic analysis of facet transformation.\n3. The central concept of a \"near-surface hyperbaric NH3 atmosphere\" is poorly defined and lacks any experimental verification. The authors provide no measurements of actual NH3 partial pressures, no evidence of how this \"hyperbaric\" condition is maintained during high-temperature synthesis, and rely entirely on static DFT calculations that ignore the dynamic nature of 600-800°C synthesis conditions. The term \"hyperbaric\" implies pressures above atmospheric, yet the synthesis appears to occur under flowing NH3 at atmospheric pressure. This misleading terminology and lack of experimental validation of the key synthesis parameter undermines the credibility of the entire work.\n4. The mechanistic conclusions are based primarily on negative evidence from in-situ DRIFTS (absence of COOH* and HCOO* signals) rather than positive identification of proposed intermediates. The H2-assisted redox mechanism lacks direct spectroscopic confirmation and comparative DFT calculations of competing pathways. Moreover, the authors fail to explain why [112] facets specifically enhance this mechanism compared to other facets. The connection between structure and function remains speculative, and the role of Mo-O/Mo-N sites is supported only by indirect H2-TPR data without proper operando characterization.\n5. The computational work shows concerning limitations: inadequate k-point sampling (2×1×1), unspecified slab thickness and vacuum spacing, and no transition state calculations for the proposed mechanism. The authors calculate surface energies with NH3 but fail to model the actual synthesis conditions or reaction pathways on different facets. The selective presentation of only favorable results without showing competing pathways or temperature effects.\n\nMinor Comments\n1. Figure 2(c) uses incorrect symbol colors that don't match the plotted data. The radar chart in Figure 2(a) is misleading without proper normalization and inclusion of relevant catalysts.\n2. The manuscript mentions both 600°C and 800°C for NH3 calcination without clarification.\n3. Equation S8 is unnecessarily repeated, and the kinetic analysis lacks proper error propagation.\n4. No information on catalyst activation procedures, pretreatment conditions, or how the catalyst transitions from NH3 synthesis atmosphere to H2/CO2 reaction conditions.", "reviewer_2": "The manuscript reports a novel synthesis strategy enabling in situ formation of high-energy microfacetted molybdenum nitride nanocrystals (HEMN) with dominant [112]-oriented facets. The synthesized catalyst exhibits exceptional performance, with mechanistic studies revealing that Mo-N/M-O motifs function as bifunctional active sites, activating CO2 via a hydrogen-assisted redox mechanism. However, to consolidate these claims, the following revisions are required:\n1. I am interested in understanding the controlled synthesis of molybdenum nitride nanocrystals with preferentially exposed high-energy [112] microfacets observed in this system. Previous studies (ACS Catal. 2025, 15, 8337−8344; Nat. Commun. 2024, 15, 9126; Nat. Commun. 2024, 15, 3100) have reported that Mo2N exhibits a high RWGS reaction rate. A systematic comparison with existing literature on Mo2N catalysts is warranted, explicitly highlighting the novelty of this work through multi-dimensional performance evaluations.\n2. Regarding catalyst synthesis via impregnation of H8MoN2O4 onto SiO2, what specific functions does the SiO2 support provide? Furthermore, how would the catalytic performance compare if pure-phase Mo2N were used instead?\n3. Does the catalyst's surface structure undergo changes after stability testing? The reaction occurs under H2/CO2 atmosphere—particularly at elevated temperatures (>500°C)—which accelerates surface reduction and promotes carburization. Given these conditions, it is essential to investigate whether the Mo-O and Mo-N configurations maintain structural integrity throughout the reaction process. Notably, MoC also exhibits catalytic activity for this reaction. The authors should explicitly address: A rigorous examination of Mo2N structural integrity and potential surface reconstruction during catalytic operation is imperative.\n4. The authors must conduct meticulous proofreading of the manuscript to eliminate errors, exemplified by the correction needed on page 7, line 198: 'HADDF-STEM' should be revised to 'HAADF-STEM'; Additionally, the caption for Figure 3 is ambiguous and inconsistent with the presented data, requiring comprehensive restructuring for clarity and accuracy; Figure 4c exhibits inconsistencies between the illustrated procedural steps and the corresponding textual descriptions, requiring rectification to ensure accuracy.\n5. The authors synthesized HEMN via a near-surface hyperbaric ammonia atmosphere, specifically claiming that Mo-N/M-O hybrid structures on [112] microfacets act as active sites for CO2 activation. However, XRD analysis failed to detect the (112) diffraction peak characteristic of such facets, instead exclusively exhibiting patterns consistent with cubic γ-Mo2N (37.6° (111), 43.7° (200), 63.4° (220), and 76.2° (311)). This apparent discrepancy requires explicit explanation.\n6. Regarding the proposed Mo-N/Mo-O active sites: What is the precise chemical state of oxygen on the catalyst surface—is it present as amorphous MoOx or MoOxNy?\n7. To conclusively exclude conventional redox mechanisms—where CO2 undergoes direct C-O bond scission to form CO and atomic oxygen—the authors must supplement this study with temperature-programmed surface reaction (TPSR) experiments, excluding CO2 direct dissociation pathways.\n8. Quantitative assessment of surface hydroxyl (OH) content on hydrogen-treated catalysts is essential to demonstrate hydrogen-mediated OH depletion. Furthermore, given the proposed mechanism wherein 'surface Mo-OH species undergo hydrogen-assisted dehydration to form Mo-O/Mo-N active site structures', rigorous characterization of the pre- and post-H2-treatment surface architecture is required to validate structural reorganization into enriched Mo-O/Mo-N sites.\n9. According to the reaction pathway depicted in Figure 4, H2 dissociation generates two H* adatoms, with OH* subsequently reacting with a H* adatom to form H2O*. This elementary step nominally consumes one H2 molecule per reaction cycle. Why then does H2-TPR analysis show no detectable H2 consumption peak while simultaneously exhibiting clear H2O formation?", "reviewer_3": "Tian et al. demonstrate a near-surface hyperbaric ammonia strategy for controlled synthesis of molybdenum nitride nanocrystals with preferentially exposed high-energy [112] microfacets. The catalyst achieves near-equilibrium conversion (56%) at a high space velocity (24000 ml/gcat/h), with 100% CO selectivity and outstanding stability (≤ 1% deactivation over 250 hours), which outperforms the benchmark Pt/CeO₂, with CO selectivity below 92%. The Mo-N/M-O sites are identified as active sites, which promote CO2 activation through a hydrogen-assisted redox mechanism. The material exhibits excellent performance, but there are several fundamental issues in the theoretical calculations. The authors may want to revise the paper to address the following issues.\nMajor issues:​\n1. The adsorption energies of intermediates and activation energies of elementary reactions strongly depend on the selected models. Figure S32 shows: (A) 2OH and (B) 1O adsorbed on Mo2N(111), and (C) 1OH + 1H and (D) 2OH adsorbed on Mo2N(112). Figure B corresponds to the possible Mo-N/M-O active sites, while Figures A, C, and D represent possible structures after H2 dissociation on these active sites. Generally, H2 dissociation activity follows: two metal sites > one metal and one non-metal site > two non-metal sites. The authors should calculate activation energies for all three H2 dissociation pathways on Mo2N(112) to verify model reliability (why doesn't H2 dissociation form 2Mo-H bonds?). Additionally, why does Figure A show two OH groups tilted at Mo top sites forming H-bonds, while Figure D shows 2OH at hollow sites without H-bonds? H-bond formation should be more stable—why choose the Figure D configuration?\n2. In Figure 5A, why did the authors study direct CO2 dissociation on Mo2N(111) but H-assisted dissociation on Mo2N(112)? These are not comparable. The authors should supplement calculations with: (i) H-assisted CO2 dissociation on Mo2N(111) and (ii) direct CO2 dissociation on Mo2N(112), to identify favorable pathways on each surface before comparing their activity differences.\n3. Figure 5C: It is difficult to reconcile how one C-O bond breaking, two O-H bond breakings, and two O-H bond formations could occur in a single elementary step. Similarly, Figure 5D shows one C-O bond breaking, one Mo-H bond breaking, one O-H bond formation, and one Mo-O bond formation in one step. The main text and SI do not specify the transition state (TS) search method. The authors must provide: (i) imaginary frequency analysis for all TS calculations to validate TS reliability, and (ii) identification of corresponding initial and final states for each TS to confirm they belong to the target reactions.\n4. The authors cite Ref. 27, suggesting C-O bond activation is the rate-limiting step in CO2 hydrogenation. However, rate-determining steps depend on catalyst structure and reaction conditions—C-O bond cleavage may not be rate-limiting on Mo2N. The authors should systematically calculate all possible elementary steps in reverse water-gas shift reactions, including HCOO/COOH formation and their C-O/C-H bond cleavages. Importantly, energy diagrams are not state-of-the-art for mechanistic/kinetic studies, as they ignore coverage effects (often dominant in reaction rates), which may lead to misleading interpretations. DFT calculations combined with microkinetic modeling have been widely used to quantify competing pathways, identify rate-limiting steps, etc.\n5. In Figure S29, 2OH co-adsorption structures are similar with or without H2, but H2O and O co-adsorption structures differ significantly. H2 configurations also change pre-/post-reaction in the right panel. Are the energy differences between pathways (left vs. right panel) caused by structural disparities rather than H2 effects?\n​\nMinor issues:​\n1. Methods section: (i) Mo2N slab thickness (layers), (ii) tests for slab convergence, and (iii) unit cell size should be specified.\n2. Line 160 of SI: Clarify \"Figure Sxx.\"\n3. Many figures lack full data visibility (e.g., Figs. 3C-E, S9B, S10-S12)."}, "author_response": "Point-to-Point Responses to Reviewers’ Comments\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nWhile this manuscript presents an interesting synthesis approach using a near-surface\nhyperbaric ammonia atmosphere to stabilize (112) facets of Mo N nanocrystals, the work\n2\nsuffers from several critical limitations that significantly undermine its claimed advances.\nThe comparison with only Pt/CeO as a benchmark, the lack of thermal stability data above\n2\n600°C, and insufficient mechanistic evidence raise serious questions about the practical\nsignificance and scientific rigor of this work.\nResponse: Thanks for the reviewer’s insightful comments and valuable suggestions. We\nhave addressed all the points raised point-by-point in the revised manuscript, with the\nchanges highlighted in yellow.\nMajor Comments\n1. The authors compare their catalyst exclusively with Pt/CeO , which is neither the most\n2\nactive nor the most practical RWGS catalyst in the 400-600°C range. This selective\ncomparison appears designed to exaggerate the performance advantages. Numerous\ncatalysts including Cu/ZnO/Al O , Ni/CeO , other Mo C systems, and Fe-based catalysts\n2 3 2 2\noperate effectively in this temperature window, often with comparable or superior\nperformance metrics. The authors must provide comprehensive comparisons with at least\n5-10 relevant catalysts under identical conditions, including both noble and non-noble\nmetal systems. Without such comparisons, claims of \"exceptional performance\" and\n\"outperforming benchmarks\" are unsubstantiated.\nResponse: We sincerely thank the reviewer’s valuable suggestion. To address it, we have\nsynthesized a series of representative reverse water-gas shift (RWGS) catalysts that are\ncommonly used within the 400-600°C range, based on relevant references (Nat. Commun.,\n2025, 16, 7265; J. Catal., 2019, 374, 60-71; J. Catal., 2016, 343, 115-126; J. Catal., 2016,\n343, 185-195; ACS Catal. 2022, 12, 15451-15462), including Cu/ZnO/Al₂O₃, Ni/CeO₂,\nFe/ZrO₂, Pt/SiO₂, and Ru/Al₂O₃ catalysts. The performance of these catalysts was\nsystematically compared with that of the molybdenum nitride catalyst reported in our work\n(HEMN) under identical testing conditions, as shown in Figures R1.\nThe thorough comparison of these reference RWGS catalysts highlights the superior\ncatalytic performance of the HEMN catalyst reported in this study. While both\nCu/ZnO/Al₂O₃ and Mo C-based catalysts exhibit high CO selectivity (~99.9%), their CO₂\n2\nconversion remains limited to approximately 30% in the high-temperature regime,\naccompanied by noticeable stability issues. The Pt/SiO₂ system, though achieving high CO₂\nconversion, shows CO selectivity below 95%. For Ni/CeO₂, the conversion rate is 34.6%\nwith a selectivity of 93.9%. In comparison, Ru/Al₂O₃ and Fe/ZrO₂ achieve conversions of\n6.8% and 95.3%, along with selectivities of 36.4% and 99.0%, respectively—both inferior\nto the performance of our HEMN catalyst. The developed HEMN catalyst achieves a CO₂\nconversion of 51.3% while maintaining 99.95% CO selectivity, together with excellent\nlong-term stability under identical reaction conditions. Overall, the HEMN catalyst\ndemonstrates significantly superior performance compared to the various representative\ncatalyst systems evaluated in this study.\nAs suggested, we have moved Figure R1 to the Supporting Information, where it now\nappears as Figure S10. The relevant discussion has been added to the main text and\nSupporting Information (SI).\nRevised version (highlighted in yellow color)\nPage 5 Line 18 and Page 6 Line 1-8 in manuscript:\n“…Notably, in the high-temperature range (450-600oC), it maintained nearly 100% CO\nselectivity over the entire CO conversion range (41.9% to 62.6%), whereas Pt/CeO\n2 2\nshowed a marked decline in selectivity from 97.2% to 91.6% under identical conditions\n(Figure 2B and Figure S9). Notably, within the high-temperature range (450–600°C), the\ncatalyst maintained nearly 100% CO selectivity across the full CO₂ conversion range\n(41.9%–62.6%). In contrast, Pt/CeO₂ exhibited a notable decrease in CO selectivity from\n97.2% to 91.6% under the same reaction conditions (Figures 2B and S9). When\nevaluated under the same conditions, its overall performance exceeds that of other\ncommonly reported catalysts (Figure S10).”\nPage 2 Line 28-35 and Page 3 Line 1-16 in SI:\n“…a muffle furnace with a 5°C/min ramp to 600°C for 4 h; iv) The CuZnAl catalyst was\nprepared by co-precipitation. An aqueous solution of Cu, Zn, and Al nitrates and a separate\nK₂CO₃ solution were simultaneously added dropwise into a beaker at 80 °C with vigorous\nstirring. The pH was maintained at approximately 9 during this process. The resulting\nsuspension was aged at 80 °C for 12 h, then the precipitate was collected, washed with\ndeionized water, dried at 100 °C for 12 h, and finally calcined at 550 °C for 4 h in air; v)\nThe Ni/CeO₂ catalyst was prepared by incipient wetness impregnation of the CeO₂\nsupport with an aqueous solution of nickel nitrate. The material was subsequently dried at\n100 °C for 12 h and calcined at 300 °C for 2 h (ramp rate: 5 °C/min). Prior to catalytic\ntesting, the catalyst was reduced in a 5% H₂/Ar flow at 550 °C for 2 hours; vi) The Fe/ZrO₂\ncatalyst with an iron loading of 1.43 wt% was prepared by the incipient wetness\nimpregnation of a ZrO₂ support with an aqueous solution of ferric nitrate. The precursor\nwas dried at 100 °C for 12 h and subsequently calcined at 450 °C for 4 h in static air with\na heating rate of 5 °C/min. Prior to catalytic testing, the catalyst was reduced at 550 °C for\n2 h in a 5% H₂/Ar (30 mL/min); vii) The Pt/SiO₂ catalyst was prepared by the incipient\nwetness impregnation of a SiO₂ support with an aqueous solution of tetra-ammine-\nplatinum(II) nitrate. Following impregnation, the material was dried at 100 °C for 12 h and\nsubsequently calcined at 450 °C for 2 h in static air with a heating rate of 10 °C/min; viii)\nThe Ru/γ-Al₂O₃ catalyst was prepared by the incipient wetness impregnation of a γ-Al₂O₃\nsupport with an aqueous solution of ruthenium(III) chloride. The resulting material was\ndried at 100 °C for 12 h and subsequently calcined at 500 °C for 2 h in static air with a\nheating rate of 10 °C/min. Prior to catalytic testing, the catalyst was reduced in a 5% H₂/Ar\nflow (30 mL/min) at 550 °C for 1 h.”\nFigure R1. (A) Catalytic performance of various catalysts for the RWGS reaction under identical testing\nconditions; (B, C) Stability tests of the Cu/ZnO/Al₂O₃ and Mo₂C-based catalysts. Reaction conditions: 0.1\nMPa, 550 °C, feed gas composition: H₂/CO₂/Ar = 72/24/4, space velocity = 24,000 mL·gcat⁻¹·h⁻¹, catalyst:\n100 mg diluted with 200 mg quartz sand.\n2. The complete absence of data above 600°C represents a fundamental flaw in this work.\nThe authors synthesize their catalyst at 600-800°C but only test stability up to 550°C,\nconveniently avoiding the temperature regime where (112) facets would likely reconstruct\nto thermodynamically stable configurations. This omission appears deliberate and raises\nquestions about the true stability window of these metastable structures. For a catalyst\nclaimed to break \"stability trade-offs,\" the failure to define its thermal limits is\nunacceptable. The manuscript requires extensive high-temperature characterization\nincluding in-situ XRD/TEM up to at least 800°C and kinetic analysis of facet\ntransformation.\nResponse: Thanks for the reviewer’s constructive suggestions. The question regarding the\nhigh-temperature stability of the catalyst is indeed critical, and we fully acknowledge the\nconcern about possible reconstruction of crystal facets under such harsh conditions. To\naddress this point directly, we have carefully designed and performed additional\nexperiments.\ni）Clarification on synthesis vs. test temperature. We apologize for our statement,\nwhich may have caused a misunderstanding regarding the synthesis temperature. In our\nstudy, the optimal synthesis condition for the catalyst was actually calcination at 600 °C\nfor 4 hours under flowing NH₃, rather than a range of 600–800 °C. The initial stability test\nat 550 °C, which is well below the synthesis temperature, was conducted as a conservative\nevaluation to observe the catalyst's performance under \"sub-limiting\" conditions, rather\nthan to avoid higher temperatures. We fully agree that investigating its thermodynamic\nlimits is essential.\nii）Extended stability test at 600 °C. To directly define the stability window of the\ncatalyst, we carried out a stability test at 600 °C (equal to the synthesis temperature) for\nRWGS reaction. As shown in Figure R2, the CO₂ conversion remained stable at ~60%\nwith CO selectivity consistently above 99.9%. This result strongly demonstrates the\nexcellent stability of the catalyst even under harsher temperature conditions.\nFigure R2. RWGSR over As-Synthesized HEMN catalyst under harsh conditions. Reaction conditions:\n600 °C; H₂/CO₂/Ar = 72:24:4 (vol%); space velocity = 24,000 mL g⁻¹ cat h⁻¹; Total flow = 40 mL min⁻¹; 100\nmg catalyst + 200 mg quartz sand.\niii) Stability of crystal phase. To investigate whether the crystal phase structure of the\ncatalyst maintains at high temperature, we performed in-situ XRD during heating under\nreaction conditions and ex-situ XRD on post‑reaction samples (Figures R3–R4).\nThe in-situ XRD experiment monitored the structural evolution of the catalyst from\nroom temperature to 600 °C, together with a 60-minute isothermal procedure at 600 °C.\nCharacteristic peaks of the molybdenum nitride crystal phase were observed at the 37.4°\nand 43.5°, assigned to the (111) and (200) reflections of the molybdenum nitride phase,\nrespectively. Throughout both the heating process and the isothermal procedure at 600 °C,\nthere are no emergence of new reflection peaks or the disappearance of existing reflections\nobserved. This indicates that the crystal phase of the synthesized HEMN catalyst is well\nmaintained during the entire process, without any discernable structural evolutions\nobserved.\nFigure R3. In-situ X-Ray diffraction characterization of the HEMN catalyst (in-situ XRD) with different\nreaction temperature and times.\nFurthermore, ex-situ XRD was performed on the samples after 12 hours of catalytic\nreaction at different temperatures (500 °C, 550 °C, and 600 °C). The characteristic\ndiffraction peaks of molybdenum nitride remain visible at 2θ values of 37.4° and 43.5°,\nand there are no marked structural evolutions observed in any of the post-reaction samples\ncompared to the fresh catalyst (Figure R4). These results further confirm the outstanding\nstability of the crystal phase associated with the RWGS catalyst under reaction conditions.\nFigure R4. XRD patterns of the spent HEMN catalysts (reaction for 12 h) with different temperatures.\niv）Stability of the microfacets. To further address the reviewer’s concern about the\nstability of the (112) microfacets under high‑temperature reaction conditions, we\nconducted in-situ high‑resolution TEM (in-situ HRTEM) experiments and ex-situ HRTEM\non post‑reaction samples at atomic resolution (Figures R5–R6). This is achieved by\nincluding the state-of-the-art environmental electron microscopy coupled with a MEMS\nchip based in-situ heating holder. To minimize the beam induced structural damage during\nin-situ HRTEM imaging, a common challenge for conventional electron microscopy, low-\ndose imaging technique developed by our group is employed here. This technique allows\nthe real space molecular resolution imaging of extremely beam sensitive materials like\nMOFs, COFs and organic-inorganic hybrid materials (Nature 2025, 638, 418; Nat. Chem.\n2025, 17, 518; Nat. Commun. 2025, 16, 261; Nat. Chem. 2024, 16, 1906; Nat. Mater.\n2017, 16, 532).\nAs illustrated in Figure R5, the HEMN nanocrystals were progressively heated from\nroom temperature (RT) to 600°C under reaction conditions, of which the morphological\nand structural evolutions are continuously monitored at this elevated temperature for\nanother 60 min. The experimental results clearly show that when viewed along the [021]\nzone axis (consistent with the HAADF-STEM observation direction in the manuscript,\nFigure 1D/F), a randomly chosen Mo₂N nanocrystal consistently maintained their distinct\noctahedral morphology, adopting a rhombic shape in projection throughout the entire\nheating process and the isothermal stage at 600 °C. The angle between the two neighboring\nprojected microfacets sharing an identical vertex measures 75°. This value aligns perfectly\nwith the characteristic crystallographic angle for the (112) microfacets projected along the\n[021] axis, as observed in the previously mentioned ex-situ STEM analysis (Figure 1D in\nthe manuscript). This in-situ dynamic result robustly demonstrates that the Mo₂N\nnanocrystals remained terminated by the high-energy (112) microfacets, without any\ndiscernible surface reconstruction or phase transformation. This indicates that the HEMN\ncatalysts possess exceptional thermal stability, successfully locking their highly active (112)\nmicro-faceted structure under high-temperature reaction conditions.\nFigure R5. In-situ HRTEM image of the HEMN catalyst under reaction conditions with different reaction\ntemperature and times.\nTo further supplement and corroborate the exceptional thermal stability observed by in-\nsitu HRTEM, we also conducted a series of rigorous ex-situ tests under reaction condition.\nSamples of the HEMN catalyst were treated under a CO /H atmosphere at 500°C, 550°C,\n2 2\nand 600°C, respectively, for a prolonged duration of 12 hours. Microstructural analysis of\nthese samples revealed that even after enduring long-term, high-temperature operating\nconditions, the nanocrystals still maintained their well-defined octahedral morphology\nenclosed by (112) microfacets, and the corresponding angular gap remained at ~ 75°. High-\nresolution images confirmed that their (112) microfacets remained well exposed, with\nindividual atomic columns clearly visible without any discernable reconstruction.\nFigure R6. HRTEM Images of the spent HEMN catalysts (reaction for 12h) at different temperatures.\nIn summary, we have provided multi-level, mutually consistent evidence through\nextended stability testing at 600 °C, in-situ XRD monitoring of the phase structure, and\nmost critically, atomic-scale in-situ HRTEM analysis. Collectively, these data demonstrate\nthat the synthesized catalyst not only exhibits stable performance at temperatures up to\n600 °C but also maintains excellent stability in both its phase structure and the (112)\nmicrofacets, without any surface reconstruction observed. We believe these additional\nexperiments fully address concerns regarding the stability issues of the HEMN catalyst.\nAs suggested, we have moved Figure R2-6 to the Supporting Information, where it\nnow appears as Figure S15-19, respectively. The relevant discussion has been added to the\nmain text and SI.\nRevised version (highlighted in yellow color)\nPage 7 Line 15-21 in manuscript:\n“…stability. Moreover, the HEMN catalyst demonstrated exceptional high-temperature\nstability during a 12-hour test at 600 °C, exhibiting no deactivation while maintaining\nconsistent performance (Figure S15). Comprehensive characterization via in-situ and ex-\nsitu XRD (Figures S16-17) and HRTEM (Figures S18-19) confirmed that both the phase\nstructure and the specific (112) crystal facet remained stable without detectable\nthermodynamic reconstruction. These results collectively affirm the catalyst's robust\nperformance and structural integrity under demanding operational conditions.”\nPage 5 Line 9-29 in SI\n“…In-situ XRD was performed using a PANalytical Empyrean diffractometer equipped\nwith a Cu Kα radiation source (λ = 1.5418 Å) at 40 kV and 40 mA. In-situ experiments\nwere operated with diffractometer equipped with a high-temperature reaction cell (XRK\n900, Anton Paar GmbH). All XRD patterns were scanned from 10° to 80° (2θ) at a rate of\n5° min⁻¹. Temperature-programmed in-situ XRD measurements of the HEMN catalyst\nwere performed under a mixed gas atmosphere of CO₂:H₂:Ar = 72:24:4 (vol%) with a total\nflow rate of 40 mL min⁻¹. The catalyst was heated from 25 °C to 50 °C, 400 °C, 500 °C,\n550 °C, and 600 °C sequentially at a ramp rate of 10 °C min⁻¹, with one diffraction pattern\ncollected at each target temperature. Subsequently, the catalyst was isothermally held at\n600 °C for 60 minutes, during which diffraction patterns were collected every 20 minutes\n(3 patterns in total). For the in-situ HRTEM characterization, the data were acquired using\nan FEI ETEM equipped with a GATAN K3 direct electron detection camera, combined with\na DENS MEMS-based gas sample holder. First, a small amount of an aqueous solution of\nthe HEMN catalyst was precisely drop-cast onto the silicon nitride membrane window of\nthe gas chip and subsequently air-dried at room temperature. Finally, the gas chip loaded\nwith the sample was carefully installed into the in-situ gas holder. At the beginning of the\nin-situ experiment, a mixed gas of CO and H was introduced, while the temperature was\n2 2\nsimultaneously and gradually increased to 600°C and held stable for 1 hour. During this\nprocess, dynamic HRTEM images of the sample were acquired along the [021] zone\naxis. …”\n3. The central concept of a \"near-surface hyperbaric NH atmosphere\" is poorly defined\n3\nand lacks any experimental verification. The authors provide no measurements of actual\nNH partial pressures, no evidence of how this \"hyperbaric\" condition is maintained during\n3\nhigh-temperature synthesis, and rely entirely on static DFT calculations that ignore the\ndynamic nature of 600-800°C synthesis conditions. The term \"hyperbaric\" implies\npressures above atmospheric, yet the synthesis appears to occur under flowing NH at\n3\natmospheric pressure. This misleading terminology and lack of experimental validation of\nthe key synthesis parameter undermines the credibility of the entire work.\nResponse: Thanks for the reviewer’s valuable feedback. We fully agree with the reviewer\nthat the concept associated with the \"near-surface hyperbaric NH₃ atmosphere\" needs to be\nclearly elucidated and experimentally supported in the manuscript to avoid confusion for a\nbroader audience.\nIn this study, the term \"hyperbaric\" was intended to describe the locally hyperbaric and\nammonia-enriched environment that forms within the near-surface region of the catalyst\nprecursor (ammonium molybdate) during thermal decomposition (Figure R7). This\nhyperbaric environment originates from two types of NH₃ sources: i) the continuous\nfeedstock of NH₃ gas with an atmospheric pressure (Externally supplied NH ), and ii) the\n3\nadditional and on-site released NH₃ gas upon the decomposition of N–H species in the\nprecursor itself during heating (On-site released NH ). These two types of NH₃ sources\n3\ncollectively contribute to the hyperbaric NH₃ environment that facilitates the formation of\nHEMN Mo₂N nanocrystals.\nFigure R7. The schematic diagram for synthesizing supported HEMN catalysts with near-surface\nhyperbaric NH atmosphere\nand spatial mapping of spectral features (Figure R8). During the synthetic process,\nconsecutive decomposition of H₈MoN₂O₄ precursor and the nitridation of partially reduced\nMoO intermediate phases eventually lead to the formation of Mo N phase. To explicitly\nx 2\ninvestigate the critical pre-nucleation stage for Mo₂N, where the NH₃ atmosphere may\ninfluence structural evolution, a low concentration of approximately 5% NH₃ was\nintroduced under a flow of NH₃/Ar into the chamber for in-situ Raman spectroscopy\nexperiments. This setup was designed to impede the nitridation of MoOₓ intermediate\nphases, following the decomposition of H₈MoN₂O₄ precursor. We observe the\ncharacteristic N-H vibrational peaks (~ 3200 cm⁻¹) assigned to ammonium species in the\nH₈MoN₂O₄ precursor, which vanish at an elevated temperature due to the phase\ndecomposition and on-site release of NH gas (Figure R8A). Along with the externally\n3\nsupplied NH₃ gas flow at atmospheric pressure, the on-site decomposition creates a local\nnear-surface hyperbaric environment of NH₃. An interesting outcoming from the on-site\ndecomposition, even under much lower NH concentration than the experimental condition,\n3\nis the formation of a partially reduced quasi-liquid-like MoO intermediate phase. This is\nx\nevidenced by visualizing the spatial distributions of both the precursor and the intermediate\nphases through mapping the most prominent Mo-O stretching vibrational modes associated\nwith these phases (Figure R8B). Unlike many shape-preserving solid-state phase\ntransformation processes, the partial reduction of MoO intermediate phases under the\nx\nnear-surface hyperbaric NH environment leads to a quasi-liquid-like matrix featured with\n3\ndynamic spatial distributions upon heating from 25°C to 600°C. This process overcomes\nthe morphological restrictions of the micrometer-sized H₈MoN₂O₄ precursor crystals,\nfacilitating the consecutive nucleation and faceted growth of Mo₂N nanocrystals that\nemerge from the quasi-liquid-like matrix during nitridation. The preferential interaction\nbetween NH₃ molecules and the (112) microfacets of Mo₂N, as demonstrated by our\ntheoretical calculations, eventually results in octahedral-shaped nanocrystals terminated by\n(112) microfacets.\nCollectively, the local NH environment contributed by both externally supplied and on-\n3\nsite released NH plays a critical role creating a quasi-liquid-like matrix for crystal growth\n3\nand directing the formation of high-energy microfacets of Mo N phase. We fully agree\n2\nwith the reviewer that in order to avoid confusions with the concept of conventional\nhigh-pressure condition, we change the term \"hyperbaric\" to “quasi-hyperbaric”\nthroughout the main text and provide a clear definition of this concept in the revised\nmanuscript. This updated concept properly defines the local and near surface NH\n3\nenvironment that drives the formation of HEMN nanocrystals. We are grateful to the\nreviewer for prompting us to refine this concept and express it more rigorously.\nFigure R8. In-situ confocal Raman Spectroscopy of precursor decomposition under different\natmospheres. Temperature-dependent in-situ Raman spectra of H₈MoN₂O₄ (A) were acquired in a 5%\nNH₃/Ar atmosphere (flow rate: 20 mL/min). After collecting a reference spectrum at room temperature, the\ntemperature was increased at a rate of 10 °C/min to 500 °C, 550 °C, and 600 °C. At each target temperature,\nthe sample was repositioned under the microscope, and two consecutive Raman spectra were recorded using\na 532 nm excitation laser, with an integration time of 60 s and a spectral range of 50–3800 cm⁻¹. Raman\nmapping images (B) corresponding to the spectra shown in (A) at different temperatures. All measurements\nwere performed using a liquid-helium-temperature confocal Raman spectrometer (Alpha300R, Witec,\nGermany) equipped with a 532 nm TEM₀₀ single-frequency laser.\n4. The mechanistic conclusions are based primarily on negative evidence from in-situ\nDRIFTS (absence of COOH* and HCOO* signals) rather than positive identification of\nproposed intermediates. The H -assisted redox mechanism lacks direct spectroscopic\n2\nconfirmation and comparative DFT calculations of competing pathways.\nResponse: We sincerely thank the reviewer for raising these important points regarding the\nmechanistic aspects of our study. To address these comments, the multiple complementary\nexperimental techniques and theoretical calculations were employed to corroborate the\nproposed H₂-assisted redox mechanism, as summarized below.\ni) To probe the reaction mechanism of RWGS reaction over the HEMN catalyst, we first\nperformed in-situ IR spectroscopy under reactive conditions (CO₂/H₂/N₂). As shown in\nFigure 3A in manuscript, characteristic CO peaks (2175 cm⁻¹ and 2120 cm⁻¹) emerged\nwith increasing temperature, while no spectral features corresponding to COOH* or\nHCOO* species were detected (Figure S28), ruling out the formate and carboxylate\npathways. We then tested whether a conventional redox mechanism (without H₂\nparticipation) could explain CO formation. When only CO₂/N₂ was introduced over the\ncatalyst—under temperature ramping (Figure S29A), after CO₂ adsorption at room\ntemperature followed by H₂ temperature programming (Figure S29B), or under other pre-\ntreatment conditions (Figures S30–S31), no CO* intermediates were detected. This\nsuggests that CO₂ dissociation does not proceed via a simple redox route on this catalyst.\nFigure R9. (A) The TPSR experiment over the HEMN catalyst; (B) In-situ Raman spectroscopy of the\nHEMN catalyst under CO₂/Ar atmosphere.\nTo further exclude a conventional redox pathway, we conducted temperature-\nprogrammed surface reaction (TPSR) experiments (Figure R9A). After pretreatment, the\ncatalyst was exposed to CO₂/N₂ while monitoring CO and CO₂ via on-line mass\nspectrometry. No CO formation or CO₂ consumption was detected, which would be\nexpected if the redox mechanism were operative. Additionally, in-situ Raman spectroscopy\nunder CO₂/Ar at various temperatures and during extended holding at 600 °C (Figure R9B)\nshowed no change in Mo-O vibrational modes, indicating no significant redox activity\ninvolving molybdenum oxide species under CO₂ alone. Collectively, these results from\ngas-phase and surface-sensitive techniques consistently indicate that the reaction does\nnot follow a conventional redox pathway but rather proceeds via H₂-assisted\nactivation of CO₂.\nii) In addition, we performed DFT calculations to compare the energetics of the proposed\nH₂-assisted mechanism with two common competing pathways (*COOH and *OOCH\nformation). As shown in Figure R10, the H₂-assisted route not only is thermodynamically\nmore favorable-forming CO in a single step versus multiple steps for *COOH (two steps)\nand *OOCH (three steps)-but also exhibits a significantly lower activation barrier: 0.45 eV\nfor the H₂-assisted path, compared to 2.22 eV for the *COOH route and 1.45 eV for the\n*OOCH route. These computational results provide a solid theoretical foundation\nsupporting the H₂-assisted redox mechanism and are fully consistent with our experimental\nobservations.\nIn summary, through a combination of in-situ IR and Raman spectroscopy,\nTPSR\nexperiment, and DFT calculation, we have obtained consistent and positive evidence for\nthe H₂-assisted redox mechanism. As suggested, we have moved Figure R9 and R10 to\nthe Supporting Information, where it now appears as Figure S32 and S42, respectively.\nThe relevant discussion has been added to the main text and SI.\nRevised version (highlighted in yellow color)\nPage 9 Line 7-14 in manuscript:\n“…similar in-situ experiments (Figures S24-S25). These results suggest that the reaction\nmechanism on this catalyst deviates from a simple redox pathway. Additionally, we\ncomplemented these studies with temperature-programmed surface reaction (TPSR)\nexperiments and in-situ Raman measurements under CO₂/Ar atmosphere on the HEMN\ncatalyst (Figure S32). Collectively, these results from both gas-phase and surface-\nsensitive characterization techniques consistently demonstrate that the reaction does not\nproceed via a conventional redox pathway, indicating a distinct mechanism deviating from\nthe simple redox route.”\nPage 5 line 29-36 and Page 6 1-6 in SI\n“…The TPSR experiment was conducted using a custom-built system equipped with an\nonline mass spectrometer (Tilon SRD200M). Prior to the reaction, 100 mg of the catalyst\nwas pretreated in argon at 300 °C for 2 h to remove surface impurities. The sample was\nthen exposed to a 30% CO₂/N₂ gas mixture (flow rate: 20 mL/min) while heating from room\ntemperature to 600 °C at a rate of 10 °C/min. The desorption signals of CO₂ (m/z = 44)\nand CO (m/z = 28) were continuously monitored. The In-situ Raman spectra with CO /Ar\n2\nwere recorded on a Renishaw in Via spectrometer with 532 nm laser excitation, coupled\nwith a Linkam TS 1500 micro in-situ cell. The HEMN was exposed to a 10% CO₂/Ar flow\n(30 mL/min) during the heating process. A reference spectrum was first collected at room\ntemperature. The sample was then heated at 10 °C/min to 300 °C to 600°C and then kept\nfor 60 min. At each target temperature, the sample was stabilized, repositioned under the\nmicroscope, and two consecutive Raman scans were collected (60 s integration, spectral\nrange: 100–1200 cm⁻¹).”\nFigure R10. (A) Schematic illustration of the proposed reaction mechanism. (B) Calculated relative energy\nprofile and energy barriers for different reaction pathways on the HEMN catalyst with (112) facet. (C,\nD) Corresponding optimized geometric structures for the (C) COOH and (D) HCOO pathways.\n5. Moreover, the authors fail to explain why (112) facets specifically enhance this\nmechanism compared to other facets.\nResponse: Thanks for the reviewer’s valuable comment. To address it, we performed\ndensity functional theory (DFT) calculations to investigate the CO₂ adsorption energy\n(E ), a key descriptor for the RWSG reaction, on different Mo₂N crystal facets (Figure\nads\nR11A). Our results indicate that the (112) facet exhibits the strongest CO₂ adsorption\nstrength (–2.67 eV), significantly surpassing those of the (001) (–1.04 eV), (110) (–1.82\neV), and (111) (–1.73 eV) facets. To gain deeper insight into this behavior, we computed\nthe projected density of states (pDOS) of surface Mo and N atoms for the different facets\n(Figure R11B). The d-band centers of the Mo atoms shift progressively closer to the Fermi\nlevel (E = 0 eV) in the order: (001) < (110) < (111) < (112). This electronic structure\nf\nvariation plays a critical role in catalytic performance. According to d-band center theory,\noverlap between the frontier orbitals of an adsorbate and the metal d-band leads to σ or π\nhybridization, splitting the interaction into bonding and antibonding states. A higher d-\nband center (closer to E) raises the energy of the antibonding orbitals, reducing their\nf\nelectron occupancy and thereby weakening the adsorption bond. The resulting\nstabilization of CO₂ adsorption on the (112) facet facilitates subsequent reaction steps.\nFigure R11. (A) Comparative study of CO₂ adsorption energies on different facets of molybdenum nitride\ncatalysts. (B) Projected density of states (PDOS) of surface Mo and N atoms on various crystalline planes\nand corresponding schematic illustration depicting the influence of the d-band center on CO₂ adsorption.\n6. The connection between structure and function remains speculative, and the role of Mo-\nO/Mo-N sites is supported only by indirect H -TPR data without proper operando\n2\ncharacterization.\nResponse: Thanks for the reviewer’s insightful comments. We fully agree that more direct\nevidence is needed to clarify the structure-function relationship. As summarized in the SI\n(now in Figure R12A), H₂-TPR-MS measurements revealed that the Mo-O species on the\nHEMN surface are more readily reduced (onset temperature ∼350 °C) than those in pure-\nphase MoO₃ (onset temperature ∼600 °C, Figure S22 in SI). To systematically tune the\nsurface Mo-O content while preserving the Mo₂N crystal structure (Figure R12B), we\ntreated the HEMN catalyst at different reduction temperatures and evaluated its\nperformance in the RWGS reaction. Catalytic tests indicated that activity increases with\nrising reduction temperature, reaching an optimum at approximately 500 °C (Figure S25\nin SI). XPS analysis was further employed to quantitatively track the Mo-O species (Figure\nR12C), showing a gradual decrease in its signal intensity with increasing reduction\ntemperature. Notably, a volcano-type correlation was observed between the Mo-O content\nand CO yield (Figure R12D), demonstrating that optimal RWGS reaction activity is\nachieved only at an intermediate concentration of surface Mo-O species.\nTo further address the reviewer’s comment for proper operando characterization, we\ncarried out additional in-situ Raman spectroscopy coupled with online mass spectrometry\n(Figure R13). This setup enabled real-time monitoring of the Mo-O vibrational features\nand the corresponding CO production during reaction. The results confirm that the peak\nintensity of Mo-O species decreases progressively as the reduction temperature increases,\nwhile the catalytic activity follows a volcano-shaped trend, consistent with our earlier\nfindings, reaching a maximum after reduction at 500 °C (Figure S25 in SI). This behavior\nindicates that a balanced concentration of Mo-O species is crucial: an excess Mo-O species\nleads to weak CO₂ adsorption and impeded C–O cleavage, whereas excessive reduction of\nMo-O species, despite increasing oxygen vacancies and enhancing CO₂ activation, causes\noverly strong intermediate binding that hinders subsequent reaction steps and lowers the\noverall rate (Chem. Eng. J. 2024, 495, 153333; Nat. Chem. 2024, 16, 2044–2053). This\nconclusion is consistent with our finding presented on page 8, line 9-20 in manuscript.\nFigure R12. (A) H₂ temperature-programmed reduction (H₂-TPR) profile of the HEMN catalyst, with online\nMS signals for H₂ (m/z = 2) and H₂O (m/z = 18). (B) XRD patterns and (C) O 1s XPS spectra of HEMN\ncatalysts treated at different reduction temperatures. (D) Correlation between CO yield and the concentration\nof M–O species.\nFigure R13. In-situ spectroscopic characterization and structure-activity relationship of the HEMN\ncatalyst. (A) In-situ Raman spectroscopy under 5% H₂/Ar atmosphere at programmed temperatures.\n(B) Online mass spectrometry (MS) signal of CO (m/z = 28) after reduction at different temperatures.\n(C) Correlation between the spectral area of the M-O bond (from Raman spectra) and the CO yield.\n7. The computational work shows concerning limitations: inadequate k-point sampling\n(2×1×1), unspecified slab thickness and vacuum spacing, and no transition state\ncalculations for the proposed mechanism.\nResponse: Thanks for the reviewer’s valuable comments. We have addressed each of your\nconcerns point by point as follows:\ni) Slab model details. To ensure the reliability of the surface calculations, we first\nconstructed supercells for the Mo₂N (111) and (112) surfaces. The optimized supercell\nparameters are (111) facet with a = 12.0366 Å, b = 18.0350 Å, c = 19.9984 Å and (112)\nfacet with a = 12.0366 Å, b = 9.9712 Å, c = 21.0606 Å (Table R1). Each slab model\nconsists of 3 layers, with a vacuum layer of 15 Å applied along the c-axis.\nii) k-point sampling. To verify the adequacy of the k-point sampling, we systematically\ntested the energy convergence for the (111) and (112) surfaces by increasing the k-point\nmesh density (Table R1). Results confirm that all structural optimizations met the preset\nconvergence criteria, and the total energy changes with increasing k-points were negligible,\nwhere maximum energy differences for the (111) and (112) facets are 0.04 eV and 0.06 eV\nrespectively. These variations are within the acceptable error range for DFT calculations.\nTherefore, the original k-point settings (2×1×1 for the (111) facet and 2×2×1 for the (112)\nfacet) are reasonable and provide a balance between computational accuracy and efficiency.\niii) Transition state calculations. We confirm that transition state (TS) calculations\nwere performed for all proposed CO₂ activation pathways. Key TS properties, including\nimaginary frequencies (confirming the authenticity of each TS) and activation energies, are\nsummarized in Table R2. These results further support the feasibility of the proposed\nreaction mechanism.\nAll the above details, including supercell parameters, slab and vacuum settings, k-\npoint convergence data, and TS results, have been added to the revised manuscript and\nSupporting Information for clarity and transparency. As suggested, we have moved Table\nR1 and Table R2 to the Supporting Information, where it now appears as Table S5 and\nS6, respectively.\nRevised version (highlighted in yellow color)\nPage 7 Line 1-5 in SI\n“…(Physical review B, 1999, 59(3): 1758). A Γ-centered 2 × 1 × 1 grid for (111) facet and\n2×2×1 for the (112) facet was used for k-point sampling. After conducting k-point\nconvergence tests (as shown in Table S5), we ultimately determined that a Γ-centered 2\n× 1 × 1 k-point grid was employed for the (111) facet, and a 2 × 2 × 1 grid was used for\nthe (112) facet in the k-point sampling process. Convergence criteria were set to…”\nPage 7 Line 19-23 in SI\n“…processes. The climbing image nudged-elastic-band method (CI-NEB) including six\nreplicas was used to search the transition states (TS) for all of the elementary reactions.\nThe TS was denoted by the highest image along the minimum energy path. The energy\nbarrier (Ea) of each elementary reaction was calculated by the energy difference between\nthe TS and the initial state (IS) without inclusion of the zero-point energy.”\nPage 7 Line 23-31 in SI\n“…Vibrational frequencies were computed at the optimized transition state structure using\nthe finite-displacement method to determine the Hessian (force constant) matrix. The\ncalculation was performed with IBRION = 5, a displacement step of POTIM = 0.015 Å, and\nthe same exchange-correlation functional and pseudopotentials as used in the transition\nstate optimization. NSW was set to 1 to prevent any further geometry relaxation. The\nvibrational mode corresponding to the imaginary frequency was visualized using “Jmol”,\nconfirming that the atomic displacements align with the expected reaction coordinate.”\nTable R1. Summary of unit cell parameters and k-Point test results used in DFT Calculations\nTable R2. Imaginary frequencies and activation energies of transition states in different CO₂ activation\npathways over various Mo₂N facets\n8. The authors calculate surface energies with NH but fail to model the actual synthesis\n3\nconditions or reaction pathways on different facets. The selective presentation of only\nfavorable results without showing competing pathways or temperature effects.\nResponse: Thank you for your comment. As addressed in our responses to Comments #4\nand #5, we have provided a detailed DFT-based explanation for the selective focus on the\n(112) facet, as well as an analysis of competing pathways occurring on this specific surface.\nAs shown in Figure R14, we have now supplemented our study with a comparison of the\nactivation barriers for both direct and hydrogen-assisted CO₂ activation on the (001),\n(111), and (112) facets, along with an evaluation of competing routes (*COOH vs. *OOCH)\nspecifically on the (112) facet. The results clearly indicate that the (112) facet exhibits\nsuperior catalytic behavior for the target reaction compared to other facets.\nFurthermore, the hydrogen-assisted pathway is shown to be energetically more\nfavorable than alternative competing routes.\nFigure R14. (A) Energy profiles of the redox mechanism and the H₂-assisted redox mechanism on different\nfacets of Mo N. (B) Comparison of the reaction pathways on the (112) facet of Mo N.\n2 2\nMinor Comments\n1. Figure 2(c) uses incorrect symbol colors that don't match the plotted data. The radar\nchart in Figure 2(a) is misleading without proper normalization and inclusion of relevant\ncatalysts.\nResponse: Thanks for the reviewer’s for suggestions. We have carefully addressed all the\npoints raised and conducted a thorough review of all figures and tables throughout the\nmanuscript. Specifically, the symbol colors in Figure 2(C) have been corrected to\naccurately align with the corresponding data series. Regarding the radar chart in Figure\n2(A), to avoid potential misinterpretation, we have implemented appropriate data\nnormalization and incorporated additional relevant catalysts to facilitate more robust\ncomparisons. The original data table has also been included in the SI to enhance clarity and\nreproducibility (now renumbered as Table S1).\nTable S1. Performance comparison of catalysts for the RWGSR. Note: This table corresponds to\nthe radar chart presented in Figure 2A.\nCatalysts Rate CO Step of Deactivation Ratio of CO2 Space Temperature\nmol/g/h Selectivity synthesis rate CO2/H2 Conversion velocity （℃）\n(%) (%/h) (%) (ml/gcat/h)\nOur work 0.07156 99.9 1 0.01468 0.33 55.9 24000 550\nPt/CeO2 0.00981 93.5 1 \\ 0.33 54.2 24000 550\nγ-Mo2Na 0.078 99.5 3 \\ 0.33 44.0 360000 500\nMoOₓ/Mo₂Nb \\ 100 2 \\ 0.33 50.8 200000 600\nβ-Mo₂Nc \\ 92 3 \\ 0.33 43.1 30000 500\nγ-Mo2Nd 0.07232 100 2 0.18 0.33 40.8 720000 550\nCu/β-Mo2Ce 0.09598 99.2 3 0.375 0.5 35 300000 550\nN12P5-SiO2f 0.00059 84.5 3 0.25 0.25 39.1 12000 550\nPt/CeO2g 0.01688 98.0 1 0.12 0.33 35 43200 450\nMo2N 0.00994 99.1 1 0.02532 0.33 52.3 24000 550\nNi/CeO2 0.00134 96.8 1 \\ 0.33 5.2 24000 450\nCu/Al2O3 0.00766 99.7 1 \\ 0.33 29.8 24000 450\nC-Mo2C 0.00651 99.6 \\ \\ 0.33 47.9 48000 550\nC-MoO3 0.00249 99.6 \\ \\ 0.33 14.9 48000 550\na, ACS Catalysis 2025, 15, 8337; b, Nature Communications 2024,15, 9126; c, Nature Communications 2024, 15, 3100; d, Chinese Journal\nof Chemical Engineering, 2022, 43, 248-254; e, ACS Catalysis, 2016, 7, 912-918; f, Journal of CO2 Utilization, 2023, 77, 102606; g, Applied\nCatalysis B: Environmental, 2021, 291, 120101.\n2. The manuscript mentions both 600°C and 800°C for NH calcination without\n3\nclarification.\nResponse: Thanks for the reviewer’s suggestion. We apologize for the lack of clarity in\nour original manuscript, which led to the misunderstanding. The synthesis conditions are\nclarified as follows: i) The High-energy-micro-faceted molybdenum nitride (HEMN)\ncatalyst was prepared by calcination at 600 °C under flowing NH₃ for 4 h. ii) In\ncontrast, the reference Mo₂N catalyst was synthesized via a temperature-programmed\napproach under N₂ flow, ramping to 800 °C and holding for 7 h. The different temperatures\nand atmospheres were intentionally selected as the optimized synthesis routes for each\nspecific catalyst.\n3. Equation S8 is unnecessarily repeated, and the kinetic analysis lacks proper error\npropagation.\nResponse: We thank the reviewer’s comment. In the original manuscript, all data obtained\nfrom the kinetic study were plotted directly in Figures 4A, 4B, 4E, and S30 directly, rather\nthan using averaged values; therefore, error propagation was not applied. According to the\nreviewer’s comment, we have updated these figures with averaged turnover rates and\nincluded error bars. Please note that, due to the small deviation in turnover rates under\nidentical operating conditions (< 1%), the error bars are barely visible. Besides, Equation\nS8 has been removed from the relevant statement. The corresponding figures (Figures 4A,\n4B, 4E, and S30) and related statements have been revised accordingly.\nRevised version (highlighted in yellow color)\nPage 6 Line 29-31 in SI\n“…Substituting Eqns. S5, S6, and S8 into Eqn. S3 gives Eqn. 1 in the main manuscript.\n𝑟 =\n𝑘𝜔𝐾𝐶𝑂2,𝜔(𝐾𝐻2,𝜔)0.5[𝐶𝑂2 ][𝐻2]0.5\n(Eqn. S8)\n𝜔 2\n(1+𝐾𝐶𝑂2,𝜔 [𝐶𝑂2 ]+√𝐾𝐻2,𝜔[𝐻2])\nwhich appears as Eqn. 1 in the main manuscript.”\n4. No information on catalyst activation procedures, pretreatment conditions, or how the\ncatalyst transitions from NH synthesis atmosphere to H /CO reaction conditions.\n3 2 2\nResponse: Thanks for the reviewer’s comment. The catalyst preparation and pretreatment\nprocedures were as follows: The catalyst was first synthesized by calcination in a tubular\nfurnace under flowing NH₃ at 600 °C (heating rate: 5 °C/min) for 4 hours. The resulting\nsolid was subsequently pelletized, crushed, and sieved to a 60-80 mesh fraction. The\ncatalytic testing was performed in a custom fixed-bed reactor system, which utilized a\nquartz tube with an 8 mm inner diameter. For the test, approximately 100 mg of the sieved\ncatalyst was homogeneously mixed with quartz sand and loaded into the reactor. The\nactivity for the RWGS reaction was then evaluated at atmospheric pressure by introducing\na feed gas mixture of CO₂/H₂/N₂ (24:72:4 by volume) and initiating the temperature ramp\nprogram.\nRevised version (highlighted in yellow color)\nPage 3 Line 18-30 in SI\n“After synthesizing the HEMN catalyst under an ammonia atmosphere, we evaluated its\nperformance of RWGSR. The RWGSR tests were conducted in a home-made fixed-bed\nreactor system, which consists of a quartz tube with an inner diameter of 8 mm. The\ncatalyst was prepared by pelletizing, crushing, and sieving to 60–80 mesh, followed by\nhomogeneous mixing with quartz sand. The reaction was carried out at 1 atm with a feed\ngas mixture of CO₂, H₂, and N₂ in a volumetric ratio of 24:72:4. During the temperature-\nprogrammed process, the reaction gas mixture was continuously fed into the reactor. After\nreaching the target temperature, the system was stabilized for 30 minutes before data\ncollection. The RWGSR was evaluated in a home-made fixed-bed reactor system. The\nreactor comprised a quartz tube with an inner diameter of 8 mm. The catalyst, prepared\nby pelletizing, crushing, and sieving to 60-80 mesh, was homogeneously mixed with\nquartz sand. The reaction was conducted at 1 atm using a feed gas mixture of CO₂, H₂,\nand N₂ with a volumetric ratio of 24:72:4.”\nReviewer #2 (Remarks to the Author):\nThe manuscript reports a novel synthesis strategy enabling in situ formation of high-energy\nmicro-facetted molybdenum nitride nanocrystals (HEMN) with dominant (112)-oriented\nfacets. The synthesized catalyst exhibits exceptional performance, with mechanistic studies\nrevealing that Mo-N/M-O motifs function as bifunctional active sites, activating CO via a\n2\nhydrogen-assisted redox mechanism. However, to consolidate these claims, the following\nrevisions are required:\nResponse: We are grateful for the reviewers' constructive comments. All points have been\naddressed in a point-by-point manner in our response, and the associated changes in the\nmanuscript have been marked in yellow.\n1. I am interested in understanding the controlled synthesis of molybdenum nitride\nnanocrystals with preferentially exposed high-energy (112) microfacets observed in this\nsystem. Previous studies (ACS Catal. 2025, 15, 8337−8344; Nat. Commun. 2024, 15, 9126;\nNat. Commun. 2024, 15, 3100) have reported that Mo N exhibits a high RWGS reaction\n2\nrate. A systematic comparison with existing literature on Mo N catalysts is warranted,\n2\nexplicitly highlighting the novelty of this work through multi-dimensional performance\nevaluations.\nResponse: We sincerely appreciate the reviewer’s suggestion. We have systematically\ncompared our catalyst with previously reported Mo₂N-based catalysts for the RWGS\nreaction, and the compiled results are summarized in Table R4.\nMost reported Mo₂N catalysts are typically synthesized via a multi-step ammoniation\nroute, where Mo precursors are first oxidized to MoOₓ, followed by temperature-\nprogrammed nitridation in ammonia, and final stabilization under a low-concentration\noxidizing atmosphere (ACS Catal. 2025, 15, 8337; Nat. Commun. 2024,15, 9126; Nat.\nCommun. 2024, 15, 3100;). In contrast, we developed a novel in-situ one-step ammoniation\nstrategy, leveraging an ammonium molybdate precursor under a near-surface “pseudo-\nhyperbaric” NH₃ atmosphere to directly fabricate highly dispersed Mo₂N nanocrystals with\npreferentially exposed high-energy (112) facets. Synthetically, this approach eliminates the\nneed for multiple atmosphere-switching steps and intermediate treatments, enabling a more\nefficient and streamlined process. Notably, our strategy yields highly dispersed Mo₂N\nnanocrystals anchored on the support (see Figure 1 and Figures S4-S5), which differs\ndistinctly from the bulk Mo₂N phases typically obtained via previous methods.\nFurthermore, we have comprehensively compared key performance metrics,\nincluding space velocity, reaction atmosphere, reaction temperature, CO₂ conversion,\nproduct selectivity, and catalytic stability, as illustrated in Table S1. The results\ndemonstrate that the catalyst developed in our work exhibits exceptional competitiveness\nacross multiple performance indicators, featuring a near-equilibrium CO₂ conversion of\n56%, nearly 100% CO selectivity, and long-term cyclic stability of 250 h at 550 °C. These\nperformance advantages are directly associated with the unique (112) high-energy facets\nand the highly dispersed nanostructure achieved via our one-step ammoniated strategy,\nhighlighting the novelty and practical potential of the present work. As suggested, we have\nintegrated the data from Table R4 into Table S1 in the SI.\nRevised version (highlighted in yellow color)\nPage 5 Line 18 and Page 6 Line 1-8 in manuscript:\n“Notably, in the high-temperature range (450-600oC), it maintained nearly 100% CO\nselectivity over the entire CO conversion range (41.9% to 62.6%), whereas Pt/CeO\n2 2\nshowed a marked decline in selectivity from 97.2% to 91.6% under identical conditions\n(Figure 2B and Figure S9). Notably, within the high-temperature range (450–600°C), the\ncatalyst maintained nearly 100% CO selectivity across the full CO₂ conversion range\n(41.9%–62.6%). In contrast, Pt/CeO₂ exhibited a notable decrease in CO selectivity from\n97.2% to 91.6% under the same reaction conditions (Figures 2B and S9). When\nevaluated under the same conditions, its overall performance exceeds that of other\ncommonly reported catalysts (Figure S10).”\nTable R4 A systematic comparison with existing literature on Mo N catalysts through multi-dimensional\n2\nperformance evaluations\nCatalysts H /CO Temperature Conversion Selectivity Stability Ref.\n2 2\nRatio ℃ % % h\nγ-Mo2N 3:1 500 44.0 99.5 100 a\nMoOₓ/Mo₂N 3:1 600 50.8 100 300 b\nβ-Mo₂N 3:1 500 43.1 92 100 c\nγ-Mo₂N 3:1 600 45.9 100 50 d\nHEMN 3:1 550 56.0 100 250 This work\na, ACS Catal. 2025, 15, 8337; b, Nat. Commun. 2024,15, 9126; c, Nat. Commun. 2024, 15, 3100; d, Chin. J. Chem. Eng\n2022, 43, 248–254\n2. Regarding catalyst synthesis via impregnation of H MoN O onto SiO , what specific\n8 2 4 2\nfunctions does the SiO support provide? Furthermore, how would the catalytic\n2\nperformance compare if pure-phase Mo\nResponse: We thank the reviewer for their insightful comments. As shown in Figure R15,\ndirect calcination of H₈MoN₂O₄ at 600 °C under a flowing NH₃ atmosphere, without the\nSiO₂ support, led predominantly to the formation of MoOₓ, rather than the desired\nmolybdenum nitride phase. This finding indicates that the SiO₂ support facilitates the\nnucleation and growth of Mo N phase, from the intermediate phases derived by the\n2\ndecomposition of H₈MoN₂O₄ precursor during the nitridation process. Therefore, we\ninferred that SiO₂ plays a critical role in stabilizing key intermediates and enabling the\nsuccessful formation of Mo₂N nanostructures. Although the detailed mechanism behind\nthis effect has not been fully elucidated in the current study, it is likely tied to the\nMoOₓ/SiO₂ interfacial interaction that reduces the initial nucleation energy barrier for\nMo₂N. This will be a key focus of our future work.\nAdditionally, the support plays another critical role in effectively dispersing the\nmolybdenum nitride nanoparticles. As schematically illustrated in Figure R16, the process\ninvolves transforming a bulk structure into smaller entities with higher surface energy. The\nintroduction of the support then stabilizes these small structures by lowering their overall\nsurface energy. This strategy successfully enhances the dispersion of the active species, as\nconfirmed by the results of high-resolution TEM in Figure R17, which confirms the\npresence of uniformly dispersed molybdenum nitride particles with sizes predominantly\nbelow 10 nm, directly validating the enhanced dispersion conferred by the support.\nMoreover, the supported catalyst exhibits a specific surface area of 88.3 m²/g significantly\nhigher than the 9.8 m²/g measured for unsupported Mo₂N (Table S2).\nCompared to pure-phase Mo₂N, the SiO₂-supported molybdenum nitride catalyst\n(HEMN) exhibits a significantly higher CO₂ conversion rate, increasing from 0.004\nmol/g/h for the unsupported catalyst to about 0.016 mol/g/h for the supported system\n(Figure R18). This fourfold enhancement underscores the crucial role of the SiO₂ support\nin improving the accessibility of active sites and thereby promoting the catalytic\nperformance.\n§ SiO\n8 2 4\n)\n.\nu\n.\na\n(\ny\nt\ni s MoO -PDF#97-064-4064\nn 2\ne\nt\nn\nI §\nH MoN O -with SiO support\n8 2 4 2\nMo N-PDF#25-1366\n2\n10 20 30 40 50 60 70 80\n2q (degree)\nFigure R15. Comparison of XRD patterns for supported and unsupported H₈MoN₂O₄ precursors after\ncalcination at 600 °C under an ammoniating atmosphere.\nFigure R16. Schematic illustration of synthesizing the highly dispersed molybdenum nitride\nFigure R17. Atomic-scale analysis of HEMN catalyst. (A) iDPC-STEM image viewed along the [021] zone\naxis, (B) HAADF-STEM, and (C) close-up view of the region in the white box in (B). (D) Atomic structure\nmodel and (E) corresponding simulated potential.\nFigure 18. The comparison of reaction rate between pure-phase molybdenum nitride and the HEMN catalyst.\nReaction conditions: 1 atm, feed gas CO₂/H₂/N₂ (24/72/4), space velocity = 18000 mL·gₐₜ⁻¹·h⁻¹, reaction\ntemperature = 400 °C\n3. Does the catalyst's surface structure undergo changes after stability testing? The\nreaction occurs under H /CO atmosphere—particularly at elevated temperatures\n2 2\n(>500°C)—which accelerates surface reduction and promotes carburization. Given these\nconditions, it is essential to investigate whether the Mo-O and Mo-N configurations\nmaintain structural integrity throughout the reaction process. Notably, MoC also exhibits\ncatalytic activity for this reaction. The authors should explicitly address: A rigorous\nexamination of Mo N structural integrity and potential surface reconstruction during\n2\ncatalytic operation is imperative.\nResponse: Thanks for reviewer’s comments and suggestions. To address it, we conducted\na series of in-situ and post-reaction characterizations to systematically evaluate the\nstructural integrity of the catalyst, with particular focus on the stability of Mo-N/O\nconfigurations and potential surface reconstruction of (112) facet. The key findings are\nsummarized as follows:\ni) Phase structure’s stability. To assess whether the phase structure remains stable at\nhigh temperature under the H₂/CO₂ atmosphere, we performed in-situ XRD during heating\nfrom room temperature to 600 °C and kept at 600 °C for another 60 min (Figure R3). No\nemergence of new diffraction peaks or disappearance of existing ones was observed.\nFurthermore, XRD patterns of the catalyst after 12 h of reaction at 550 °C and 600 °C\n(Figure R4) showed no detectable changes compared to the fresh sample. These results\nconfirm the excellent phase structural stability of the HEMN catalyst under reaction\nconditions.\nii) Stability of high-energy (112) microfacets. To address the stability of exposed facets-\nespecially the (112) plane, at the atomic scale, we performed in-situ high-resolution TEM\nduring heating to 600 °C under reaction condition (Figure R5). The images revealed no\nreconstruction or transformation of the (112) plane throughout the heating process.\nMoreover, HRTEM of the spent catalyst after 12 h at 600 °C (Figure R6) clearly showed\nthat the lattice fringes corresponding to the (112) plane remained intact, confirming the\npreservation of the phase structure.\niii) Stability of surface Mo-O/N species. In-situ Raman spectroscopy was employed to\ntrack the evolution of surface species over the temperature range of 300–600 °C under\nreaction conditions. At 300 °C, where the RWGS reaction is negligible, prominent\nvibrational bands associated with Mo-O bonds were detected. Upon heating to 400 °C, the\nintensity of these Mo-O features decreased significantly, an observation consistent with the\nparticipation of surface Mo-O species in the reaction, wherein they are converted into\nshort-lived reactive intermediates that fall below the time resolution limit of Raman\nspectroscopy. Notably, with further heating and subsequent isothermal operation at 600 °C,\nno new vibrational bands emerged nor did existing features disappear, and the overall\nspectral profile remained stable. When cooling the system back to 300 °C, the original Mo–\nO vibrational characteristics were largely recovered, indicating that the surface Mo–O\nmotifs undergo reversible structural changes rather than irreversible degradation\nduring the reaction.\nIn summary, our multi-technique analysis consistently demonstrates that the HEMN\ncatalyst maintains its structural integrity, in both phase and surface regions, under high-\ntemperature H₂/CO₂ conditions, with no evidence of phase transformation, facet\nreconstruction, or irreversible loss of active Mo-N configurations. As suggested, we have\nmoved Figure R3-6 to the SI, where it now appears as Figure S16-19, respectively. The\nrelevant discussion has been added to the main text and SI.\nRevised version (highlighted in yellow color)\nPage 7 Line 15-21 in manuscript:\n“…stability. Moreover, the HEMN catalyst demonstrated exceptional high-temperature\nstability during a 12-hour test at 600 °C, exhibiting no deactivation while maintaining\nconsistent performance (Figure S15). Comprehensive characterization via in-situ and ex-\nsitu XRD (Figures S16-17) and HRTEM (Figures S18-19) confirmed that both the phase\nstructure and the specific (112) crystal facet remained stable without detectable\nthermodynamic reconstruction. These results collectively affirm the catalyst's robust\nperformance and structural integrity under demanding operational conditions.”\nPage 5 Line 9-29 in SI\n“…In-situ XRD was performed using a PANalytical Empyrean diffractometer equipped\nwith a Cu Kα radiation source (λ = 1.5418 Å) at 40 kV and 40 mA. In-situ experiments\nwere operated with diffractometer equipped with a high-temperature reaction cell (XRK\n900, Anton Paar GmbH). All XRD patterns were scanned from 10° to 80° (2θ) at a rate of\n5° min⁻¹. Temperature-programmed in-situ XRD measurements of the HEMN catalyst\nwere performed under a mixed gas atmosphere of CO₂:H₂:Ar = 72:24:4 (vol%) with a total\nflow rate of 40 mL min⁻¹. The catalyst was heated from 25 °C to 50 °C, 400 °C, 500 °C,\n550 °C, and 600 °C sequentially at a ramp rate of 10 °C min⁻¹, with one diffraction pattern\ncollected at each target temperature. Subsequently, the catalyst was isothermally held at\n600 °C for 60 minutes, during which diffraction patterns were collected every 20 minutes\n(3 patterns in total). For the in-situ HRTEM characterization, the data were acquired using\nan FEI ETEM equipped with a GATAN K3 direct electron detection camera, combined with\na DENS MEMS-based gas sample holder. First, a small amount of an aqueous solution of\nthe HEMN catalyst was precisely drop-cast onto the silicon nitride membrane window of\nthe gas chip and subsequently air-dried at room temperature. Finally, the gas chip loaded\nwith the sample was carefully installed into the in-situ gas holder. At the beginning of the\nin-situ experiment, a mixed gas of CO and H was introduced, while the temperature was\n2 2\nsimultaneously and gradually increased to 600°C and held stable for 1 hour. During this\nprocess, dynamic HRTEM images of the sample were acquired along the [021] zone\naxis. …”\nFigure R3. In-situ X-Ray Diffraction Characterization of the HEMN Catalyst (in-situ XRD) with different\nreaction temperature and times.\nHEMN-600℃ reaction after 12h\n)\n. u\n. a HEMN-550℃ reaction after 12h\n(\ny\nt\ni\ns\nn\ne\nt\nn\nI HEMN-500℃ reaction after 12h\nMo N-PDF#25-1366\n2\n10 20 30 40 50 60 70 80\n2q (degree)\nFigure R4. XRD patterns of the spent HEMN catalysts (reaction for 12 h) with different temperatures.\nFigure R5. In-situ HRTEM image of the HEMN catalyst under reaction conditions with different reaction\ntemperature and times.\nFigure R6. HRTEM Images of the spent HEMN catalysts at different temperatures.\nMo-O Mo-O Reaction condition: CO :H :Ar = 72:24:4\n2 2\nMo-O\nCooling to 300℃\n)\n. 600℃-60 min\nu\n. a 600℃-50 min\n(\n600℃-40 min\ny\nt i 600℃-30 min\ns\nn 600℃-20 min\ne\nt 600℃-10 min\nn\nI 500℃-30 min\n500℃-20 min\n500℃-10 min\n500℃\n400℃\nHeating to 300℃\n200 400 600 800 1000 1200\nRaman shift (cm-1)\nFigure R18. In-situ Raman spectra of the HEMN catalyst under reaction conditions from room temperature\nto 600 °C and kept for another 60 min. The In-situ Raman spectra experiment was conducted under a\ncontinuous flow (30 mL/min) of CO₂:H₂:Ar (72:24:4). A reference spectrum was first collected at room\ntemperature. The temperature was then raised from room temperature to 600 °C at a rate of 10 °C/min, with\nspectra acquired at intervals of 100 °C. Additionally, isothermal holds were performed at 500 °C for 30\nminutes and at 600 °C for 60 minutes. After thermal stabilization at each temperature point, the sample was\nrepositioned under the microscope, and two consecutive spectra were recorded using a 532 nm laser with a\n60 s integration time across the spectral range of 100–1200 cm⁻¹. All measurements were performed under\nconsistent positioning and spectral acquisition settings.\n4. The authors must conduct meticulous proofreading of the manuscript to eliminate errors,\nexemplified by the correction needed on page 7, line 198: 'HADDF-STEM' should be\nrevised to 'HAADF-STEM';\nResponse: We are grateful to the reviewer for pointing out this typographical error. We\nhave conducted a thorough proofreading of the entire manuscript. The specific term\n\"HADDF-STEM\" on page 7, line 35 has been corrected to \"HAADF-STEM\".\n5. Additionally, the caption for Figure 3 is ambiguous and inconsistent with the presented\ndata, requiring comprehensive restructuring for clarity and accuracy;\nResponse: Thanks for the reviewer’s comment. We have revised it in page 10, line 3-4, as\nfollows:\n“Figure 3. Study of the reaction pathway of the RWGSR over the HEMN catalyst. In-situ DRIFTS\nat different temperatures: A: 2250 cm⁻¹ to 2000 cm⁻¹, B: 4000 cm⁻¹ to 2500 cm⁻¹; C: H₂-TPR profile\nof HEMN catalyst, D: In-situ DRIFTS of the HEMN catalyst in H₂/Ar atmosphere at different\ntemperatures…”\n6. Figure 4C exhibits inconsistencies between the illustrated procedural steps and the\ncorresponding textual descriptions, requiring rectification to ensure accuracy.\nResponse: We are grateful to the reviewer for pointing out the inconsistencies between the\nillustrated steps in Figure 4C (now in Figure R19) and the corresponding textual\ndescriptions. We have carefully re-examined both the figure and the manuscript and\nidentified that the text did not precisely match the step order. The text has been revised on\npage 11, line 16-29 to accurately reflect the procedure in Figure 4C. The corrected passage\nis shown below:\n\"…the HEMN catalyst, as shown in Figure 4C. The reaction initiates with CO adsorption\n2\non the Mo-O/N-Mo active site pair (denoted as *) to form CO * (Step 1). Simultaneously,\n2\nattack the oxygen atom in the nearby CO * species, cleaving the C=O bond and forming\n(Step 4), while OH* reacts with another H* adatom to form H O* species (Step 5), which\n2\nsubsequently desorbs as H O (Step 6). The reaction begins with the adsorption of CO₂\n2\nonto the Mo-O/N-Mo active site pair (denoted as *), forming adsorbed *CO₂ (Step 1).\nConcurrently, H₂ undergoes dissociative adsorption at an adjacent site, producing an H*\nadatom and a Mo-OH* (Steps 2). Subsequently, the H* species performs a nucleophilic\nattack on the oxygen atom of a nearby CO₂* molecule, cleaving the C=O bond to generate\nOH* and CO* intermediates (Step 3). The CO* intermediate then desorbs into the gas\nphase as CO (Step 4). Finally, two OH* species combine to form an H₂O* intermediate,\nwhich subsequently desorbs as a water molecule (Step 5 and 6). Based on the above\nfindings, …\"\nFigure R19. Elementary steps on the HEMN catalyst, corresponding to Figure 4C in manuscript.\n7. The authors synthesized HEMN via a near-surface hyperbaric ammonia atmosphere,\nspecifically claiming that Mo-N/M-O hybrid structures on (112) microfacets act as active\nsites for CO2 activation. However, XRD analysis failed to detect the (112) diffraction peak\ncharacteristic of such facets, instead exclusively exhibiting patterns consistent with cubic\nγ-Mo N (37.6° (111), 43.7° [200], 63.4° [220], and 76.2° [311]). This apparent discrepancy\n2\nrequires explicit explanation.\nResponse: We sincerely appreciate the reviewer’s valuable comment, which addresses the\ncritical correlation between the material structure and characterization results. We have\ncarefully organized and supplemented the explanation as follows:\nWe would like to clarify that this work employs the polycrystalline XRD (PXRD)\ntechnique, which provides volume-averaged, randomly oriented, and long-range ordered\nstructural information of crystals. This means that diverse groups of parallel crystal planes\nfrom individual, randomly oriented γ-Mo₂N nanocrystals satisfy the Bragg condition,\nmaking their associated Bragg reflections visible in the PXRD pattern, even though these\nnanocrystals preferentially expose (112) microfacets.\nBy including a statistically significant number of nanocrystals with random orientations,\nthe intensities of these Bragg reflections (e.g., 37.6° (111), 43.7° (200), 63.4° (220), and\n76.2° (311)) are proportional to the square of the amplitudes of the crystallographic (hkl)\nstructural factors, predictable from the atomic coordinates of the γ-Mo₂N crystal structure.\nIn essence, PXRD cannot directly probe the microfacets of individual nanocrystals but\nreveals their spatially averaged crystallographic structure.\nIn contrast, electron microscopy enables real-space imaging at atomic resolution, allowing\nfor the tilting of individual nanocrystals and direct identification of the terminated crystal\nmicrofacets. Consequently, the results from XRD and HRTEM are highly complementary:\nXRD confirms the overall bulk crystal structure of the catalyst as cubic γ-Mo₂N, ensuring\nphase purity and key crystalline characteristics. Meanwhile, electron microscopy\naccurately identifies the (112) microfacets of individual nanocrystals, which serve as the\nstructural basis for the formation of Mo-N/M-O hybrid active sites.\n8. Regarding the proposed Mo-N/Mo-O active sites: What is the precise chemical state of\noxygen on the catalyst surface—is it present as amorphous MoO or MoO N ?\nx x y\nResponse: We sincerely appreciate the reviewer’s insightful comment regarding the\nchemical state of oxygen on the catalyst surface. Electron energy loss spectroscopy (EELS)\nis a powerful tool for directly probing these oxygen species with high spatial and energy\nresolution. It enables us to selectively investigate the site-specific chemical state of oxygen\nspecies from the subsurface to the external surface of these nanocrystals. Based on the\natomic-resolution STEM imaging presented in the manuscript (Figure 1), the surface of\nthe Mo₂N nanocrystals terminates without any reconstruction and is free from amorphous\nMoOₓ or MoOₓNᵧ layers. Additionally, as shown in Figure R20, line profiling the O-K\nedge structures from the subsurface to the external surface of a randomly chosen Mo₂N\nnanocrystal reveals a decrease in oxygen content from the external surface to the subsurface\nregion, after deconvoluting the zero-loss peak (ZLP) to eliminate thickness effects.\nMoreover, the oxygen species exhibit a similar chemical environment and K edge structure,\nwith energy loss peaks observed at 539.2 eV and 560.8 eV. The predominant peak at 539.2\neV arises from the transition of O 1s electrons to unoccupied O 2p states hybridized with\nMo s-p orbitals, corresponding to the σ∗ resonance. The secondary peak at 560.8 eV results\nfrom multiple scattering events. The identical σ → σ∗ energy loss peak in the O-K edge\nstructure for both external and subsurface oxygen species indicates that the chemical\nenvironment of surface oxygen closely resembles that of the bulk lattice oxygen in the\nMo₂N nanocrystal, acting as an anion dopant for nitrogen, consistent with our STEM\nobservations.\nCollectively, the absence of surface reconstruction allows for the creation of\nhomogeneous surface oxygen species, which exhibit a similar chemical environment to the\nsubsurface lattice oxygen as anion dopants in the Mo₂N phase. No amorphous\nMoOₓ/MoOₓNᵧ layers were observed on the surface of these Mo₂N nanocrystals.\nFigure R20. O K-edge Electron Energy Loss Spectroscopy (EELS) spectrum of the HEMN catalyst moving\nfrom the subsurface to the external subsurface. EELS analysis was performed using an FEI Spectra 300\naberration-corrected transmission electron microscope operating in STEM mode, with an energy resolution\nof approximately 0.8 eV. To obtain the data shown in Figure R20, a high-resolution EELS line scan analysis\nwas executed. We scanned the sub-nanometer electron probe starting from the subsurface of an Mo N particle\n2\nuntil it entered the external surface. Core-loss spectra of the O K-edge were acquired at each pixel along this\nline scan path.\n9. To conclusively exclude conventional redox mechanisms—where CO undergoes direct\nwith temperature-programmed surface reaction (TPSR) experiments, excluding CO direct\n2\ndissociation pathways.\nResponse: Thanks for the reviewer’s suggestion. To conclusively exclude conventional\nredox mechanisms, we have performed additional temperature-programmed surface\nreaction (TPSR) experiments (Figure R9A) and in-situ Raman spectroscopy (Figure R9B).\nFor the TPSR test (Figure R9A), the catalyst was first pretreated in Ar at 300 °C for 2 h\nto remove surface impurities. Afterward, a baseline was established under a CO₂\natmosphere (30% CO₂/N₂, 20 mL/min). The temperature was then increased to 600 °C at a\nrate of 10 °C/min, while the signals of m/z = 44 (CO₂) and m/z = 28 (CO) were monitored\nby online mass spectrometry. Throughout the heating process, no consumption of CO₂ or\nformation of CO was detected. If a conventional redox mechanism were operative, we\nwould expect to observe both CO evolution and CO₂ consumption in the mass spectra.\nFurthermore, we conducted in-situ Raman spectroscopy under a CO₂/Ar atmosphere to\ntrack changes in surface Mo-O species at different temperatures and during prolonged\nexposure at 600 °C (Figure R9B). Should the conventional redox pathway occur,\nnoticeable consumption or formation of Mo-O species would be anticipated. However, as\nshown in Figure R9B, no such changes in Mo–O species were observed throughout the\nheating and holding stages. Together, these results, from both gas-phase product analysis\nand surface species monitoring, provide consistent evidence that the HEMN catalyst does\nnot follow a conventional redox mechanism. As suggested, we have moved Figure R9 to\nthe SI, where it now appears as Figure S32. The relevant discussion has been added to the\nmain text and SI.\nRevised version (highlighted in yellow color)\nPage 9 Line 7-14 in manuscript:\n“…similar in-situ experiments (Figures S24-S25). These results suggest that the reaction\nmechanism on this catalyst deviates from a simple redox pathway. Additionally, we\ncomplemented these studies with temperature-programmed surface reaction (TPSR)\nexperiments and in-situ Raman measurements under CO₂/Ar atmosphere on the HEMN\ncatalyst (Figure S32). Collectively, these results from both gas-phase and surface-\nsensitive characterization techniques consistently demonstrate that the reaction does not\nproceed via a conventional redox pathway, indicating a distinct mechanism deviating from\nthe simple redox route.”\nPage 5 line 29-36 and Page 6 1-6 in SI\n“…The TPSR experiment was conducted using a custom-built system equipped with an\nonline mass spectrometer (Tilon SRD200M). Prior to the reaction, 100 mg of the catalyst\nwas pretreated in argon at 300 °C for 2 h to remove surface impurities. The sample was\nthen exposed to a 30% CO₂/N₂ gas mixture (flow rate: 20 mL/min) while heating from room\ntemperature to 600 °C at a rate of 10 °C/min. The desorption signals of CO₂ (m/z = 44)\nand CO (m/z = 28) were continuously monitored. The In-situ Raman spectra with CO /Ar\n2\nwere recorded on a Renishaw in Via spectrometer with 532 nm laser excitation, coupled\nwith a Linkam TS 1500 micro in-situ cell. The HEMN was exposed to a 10% CO₂/Ar flow\n(30 mL/min) during the heating process. A reference spectrum was first collected at room\ntemperature. The sample was then heated at 10 °C/min to 300 °C to 600°C and then kept\nfor 60 min. At each target temperature, the sample was stabilized, repositioned under the\nmicroscope, and two consecutive Raman scans were collected (60 s integration, spectral\nrange: 100–1200 cm⁻¹).”\nFigure R9. (A) The TPSR experiment over the HEMN catalyst; (B) In-situ Raman spectroscopy of the\nHEMN catalyst under CO₂/Ar atmosphere.\n10. Quantitative assessment of surface hydroxyl (OH) content on hydrogen-treated\ncatalysts is essential to demonstrate hydrogen-mediated OH depletion. Furthermore, given\nthe proposed mechanism wherein 'surface Mo-OH species undergo hydrogen-assisted\ndehydration to form Mo-O/Mo-N active site structures', rigorous characterization of the\npre- and post-H -treatment surface architecture is required to validate structural\n2\nreorganization into enriched Mo-O/Mo-N sites.\nResponse: Thanks for the reviewer’s constructive suggestion. To address the reviewer’s\nconcerns regarding quantitative assessment of surface hydroxyl (OH) content and rigorous\ncharacterization of H₂-induced surface restructuring, we have performed systematic, multi-\ntechnique investigations on the behavior of surface OH species and structural evolution of\nthe catalyst upon H₂ treatment.\ni) The analysis of H₂-Mediated OH depletion. As presented in Figure S34, the as-\nsynthesized HEMN catalyst exhibits characteristic infrared absorption bands centered at\n~3200 cm⁻¹, which are attributed to stretching vibrations of surface Mo-OH species-\nconfirming their initial presence. To quantitatively probe H₂-induced OH depletion, in-situ\ndiffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) experiments\nwere conducted. After pretreatment at 300 °C under inert N₂ atmosphere to eliminate physi-\nsorbed water, the Mo-OH signal remained stable during subsequent ramping to 600 °C,\nindicating no spontaneous dehydration in the absence of H₂ (Figure R21A). In contrast, a\nsignificant attenuation of the Mo-OH absorption band was observed under H₂ flow (Figure\nR21B), providing direct evidence for H₂-facilitated hydroxyl depletion. Complementary\nonline mass spectrometry (MS) measurements further verified this phenomenon (Figure\nR21C), where H₂O evolution (m/z = 18) was detected below 300 °C, with no measurable\nH₂ consumption (m/z = 2). Since physi-sorbed water was pre-removed via N₂ pretreatment,\nthe evolved H₂O is unambiguously derived from the dehydration of surface Mo-OH species.\nThese results collectively confirm that Mo-OH dehydration is strictly H₂-assisted and not\nfeasible in the absence of H₂-consistent with the proposed mechanism and addressing the\nkey requirement for validating hydrogen-mediated OH depletion.\nii) The characterization of pre- and post-H₂-treatment surface architecture. To\nvalidate H₂-induced structural reorganization into Mo-O/Mo-N-enriched active sites, in-\nsitu Raman spectroscopy was performed on catalysts before and after H₂ treatment (25 °C\nvs. 300 °C, as shown in Figure R22). The Raman bands at 190 cm⁻¹ and 240 cm⁻¹, assigned\nto Mo-O stretching vibrations, showed remarkable intensification after H₂ treatment at\n300 °C. This observation supports the transformation of Mo-OH species into Mo-O active\nsites, confirming the formation of Mo-O/Mo–N-coordinated active structures. Collectively,\nthese in-situ characterizations demonstrate that H₂ treatment efficiently depletes surface\nhydroxyls and triggers structural rearrangement to form Mo-O/Mo-N-enriched active sites,\nwhich is in good agreement with the proposed reaction mechanism (Figure 4-5 in\nmanuscript).\nFigure R21. (A) In-situ DRIFTS spectra of the HEMN catalyst in a N atmosphere at different\n2\ntemperatures. (B) In-situ DRIFTS spectra of the HEMN catalyst under the H₂/Ar atmosphere. (C)\nH₂-TPR profile of the HEMN catalyst.\nFigure R22. Monitoring the evolution of the catalyst via in-situ Raman spectroscopy upon H₂\ntreatment at room temperature and 300 °C.\n11. According to the reaction pathway depicted in Figure 4, H dissociation generates two\n2\nelementary step nominally consumes one H molecule per reaction cycle. Why then does\n2 2\nclear H O formation?\nopportunity to further clarify the surface processes of the catalyst. Figure R19 (reaction\nmechanism, corresponding to Figure 4C in manuscript), and Figure R21C (H₂-TPR\nexperiment, corresponding to Figure 3C in manuscript) reflect two distinct processes that\noccur at different stages. Figure R19 depicts the catalytic reaction cycle, illustrating the\nmicro-level mechanism of CO₂ hydrogenation taking place on the pre-generated Mo–O/N\nactive sites. In this steady-state catalytic process, H₂ is consumed as a reactant and H₂O is\nproduced as a product, consistent with conventional stoichiometric expectations.\nIn contrast, Figure R21C (H₂-TPR experiment) reveals the catalyst activation process,\nprobing the initial surface evolution of the fresh HEMN catalyst. A key observation is that\nH₂O formation is detected around 120 °C without corresponding H₂ consumption. We\npropose that the fresh HEMN catalyst surface contains a certain amount of hydroxyl groups\n(*OH). During H₂-TPR heating, H₂ does not participate as a directly consumed reactant;\ninstead, it significantly promotes dehydration between surface hydroxyl groups (i.e., 2*OH\n→ O* + H₂O). This interpretation is supported by the following evidence: i) The sample\nwas pre-treated in N₂ at 300 °C prior to testing, eliminating interference from physi-sorbed\nwater. In addition, In-situ IR spectroscopy under the same conditions as the H₂-TPR\nexperiment directly showed a decrease in surface hydroxyl concentration in the presence\nof H₂, as shown in in Figure R21B; ii) DFT calculations further confirmed that H₂\neffectively promotes the surface de-hydroxylation reaction, as shown in Figure S35.\nIn summary, the H₂O signal observed in H₂-TPR originates from H₂-promoted surface\nreconstruction, specifically, de-hydroxylation of Mo-OH species that forms the active Mo–\nO/N sites, rather than from the consumption of H₂. We sincerely thank the reviewer for\nprompting us to clarify this important distinction.\nReviewer #3 (Remarks to the Author):\nTian et al. demonstrate a near-surface hyperbaric ammonia strategy for controlled\nsynthesis of molybdenum nitride nanocrystals with preferentially exposed high-energy (112)\nmicrofacets. The catalyst achieves near-equilibrium conversion (56%) at a high space\nvelocity (24000 ml/gcat/h), with 100% CO selectivity and outstanding stability (≤ 1%\ndeactivation over 250 hours), which outperforms the benchmark Pt/CeO₂, with CO\nselectivity below 92%. The Mo-N/M-O sites are identified as active sites, which promote\nCO activation through a hydrogen-assisted redox mechanism. The material exhibits\n2\nexcellent performance, but there are several fundamental issues in the theoretical\ncalculations. The authors may want to revise the paper to address the following issues.\nResponse: We appreciate the reviewer's positive feedback and their insightful comments\nregarding the theoretical calculations. We have provided a point-by-point response to each\nissue below and have incorporated all corresponding revisions into the main text, with\nchanges highlighted in yellow.\nMajor issues:\n1. The adsorption energies of intermediates and activation energies of elementary reactions\nstrongly depend on the selected models. Figure S32 shows: (A) 2OH and (B) 1O adsorbed\non Mo N(111), and (C) 1OH + 1H and (D) 2OH adsorbed on Mo N(112). Figure S32B\n2 2\ncorresponds to the possible Mo-N/M-O active sites, while Figures A, C, and D represent\npossible structures after H dissociation on these active sites. Generally, H dissociation\n2 2\nactivity follows: two metal sites > one metal and one non-metal site > two non-metal sites.\nThe authors should calculate activation energies for all three H dissociation pathways on\n2 2\nResponse: We thank the reviewer for their insightful suggestion. The point raised, that H₂\ndissociation activity generally follows the trend of two metal sites > one metal and one\nnon-metal site > two non-metal sites, is indeed crucial, and we fully recognize its broad\nrelevance in catalytic reactions. To thoroughly examine how this applies to our specific\nsystem, we constructed a Mo N model with -Mo–O⋯Mo–O- sites and systematically\n2\ncomputed the dissociation energies of H₂ across surfaces with different vacancy\nconcentrations.\nOur calculation results reveal that, thermodynamically, configurations such as H–Mo–\nO⋯H–Mo–O, Mo–OH⋯H–Mo–OH, and Mo–OH⋯OH–Mo are indeed feasible. In\nparticular, under an ideal model with sufficient surface vacancies, the H–Mo–O⋯H–Mo–\nO species (i.e., the formation of two Mo–H bonds) is the most favorable, with an H₂\ndissociation energy of –1.56 eV (Figure R23A), consistent with the trend highlighted by\nthe reviewer. However, structure–activity relationships derived from our in-situ Raman\nspectroscopy coupled with online mass spectrometry (Figure R13) indicate that the\nmetallic Mo sites on the catalyst surface are partially covered by O/N species. Moreover, a\nvolcano-type correlation exists between the surface Mo-O content and catalytic\nperformance, suggesting that the Mo-O content should be neither too high nor too low. We\ntherefore infer that under realistic reaction conditions, the occupation of sites adjacent to\nmetallic Mo reduces the number of available vacancies for H₂ dissociation, thereby making\nthis process more challenging.\nTo verify this hypothesis, we further built a series of Mo–O⋯Mo–O surface models with\nvarying vacancy concentrations and computed the H₂ dissociation energetics. The results\nclearly show that as the number of surface vacancies decreases, the energy barrier for the\nH–Mo–O⋯H–Mo–O pathway gradually increases (from –1.56 eV to –0.10 eV; see the\nnewly added Figure R23B). This suggests that, under our actual reaction conditions, the\ncomplete formation of a full H–Mo–O⋯H–Mo–O structure (i.e., two Mo–H bonds) is\nkinetically less feasible.\nTherefore, based on a comprehensive consideration of both experimental structure-\nactivity relationships and theoretical calculations, we did not select the 2Mo–H model as\nthe primary pathway in this study. Instead, we adopted Mo–OH–H–Mo as the\nrepresentative site for H-assisted CO₂ activation, with an H₂ dissociation energy of –\n0.87 eV—intermediate between the other two types of sites—which better reflects the\nactual conditions in our system. To support the rationality of this choice, we have included\nthe additional computational results (Figure R23) as Figure S40 in the SI and added\ncorresponding explanations in the main text.\nRevised version (highlighted in yellow color)\nPage 13 Line 9-15 in manuscript:\n“…surface. Leveraging these experimental observations, we established DFT-optimized structural\nmodels for both the dominant (112) surface and the reference (111) facet of molybdenum nitride\n(Figure S32). Building on these experimental insights, we constructed DFT-optimized models of\nthe dominant (112) and reference (111) surfaces of Mo₂N (Figure S39). To this end, we\nsystematically probed the influence of Mo–O/N vacancy concentration on the H₂ binding energy,\nthereby identifying the Mo–O···H–Mo configuration as the initial active site for CO₂ activation\n(Figure S40). Comparative analysis of surface reactivity (Figure 5 and Figure S41) revealed…”\nFigure R23. (A) Calculated H₂ dissociation energies on the Mo–O⋯Mo–O model. (B) Effect of surface\nvacancies on H₂ dissociation over the Mo–O⋯Mo–O model.\nFigure R13. In-situ spectroscopic characterization and structure-activity relationship of the HEMN\ncatalyst. (A) In-situ Raman spectroscopy under 5% H₂/Ar atmosphere at programmed temperatures.\n(B) Online mass spectrometry (MS) signal of CO (m/z = 28) after reduction at different temperatures.\n(C) Correlation between the spectral area of the M-O bond (from Raman spectra) and the CO yield.\n2. Additionally, why does Figure S32A show two OH groups tilted at Mo top sites forming\nH-bonds, while Figure S32D shows 2OH at hollow sites without H-bonds? H-bond\nformation should be more stable—why choose the Figure D configuration?\nResponse: Thanks for the reviewer’s comments. To address it, we constructed an initial\nsurface model on the (112) facet with two OH groups positioned to potentially form H-\nbonds. After geometric optimization, the system relaxed to the configuration we\npreviously reported—specifically, two OH groups adsorbed at hollow sites without\nforming H-bonds. This confirms that our computational results and conclusions are\nrobust.\nIt is generally accepted that two hydroxyl groups can form a hydrogen bond when the\nO–O distance falls between approximately 2.2 Å and 3.5 Å, though the bond strength varies.\nFrom Figure S36 (now Figure R24), the O–O distances between the two adsorbed *OH\nspecies are 2.75 Å on the (111) facet and 2.95 Å on the (112) facet. Both values are within\nthe typical range for H-bond formation, suggesting that the (111) configuration forms a\nstronger H-bond, while a weaker one might be expected on the (112) surface. However, H-\nbond formation depends not only on O–O distance but also critically on the bond angle ∠\nO–H···O. We measured this angle to be 145° on the (111) surface and 97° on the (112)\nsurface. Since an ∠O–H···O below approximately 120° generally cannot support a stable\nH-bond, we conclude that a well-defined H-bond exists on the (111) surface, but not on the\n(112) surface.\nThis difference ultimately stems from the distinct adsorption site geometries on the two\nfacets. For example, on the (111) surface, OH groups preferentially adsorb at top sites,\nwhere the oxygen atom is coordinated to only one surface metal atom. This configuration\noffers high flexibility in adsorption orientation. To maximize stability, the OH groups tilt\nto optimize the H-bond angle, and the energy gained from forming a strong H-bond\noutweighs any penalty from deviating from the vertical adsorption geometry. In addition,\non the (112) surface, OH groups adsorb at hollow sites, where the oxygen is coordinated\nto three surface metal atoms in a triangular arrangement. This geometry severely restricts\norientational freedom. Any tilt attempting to form an H-bond would significantly weaken\nthe metal–oxygen coordination bonds, and the energy cost of doing so cannot be\ncompensated by the weak H-bond that might form. Thus, the OH groups remain nearly\nvertical to maintain optimal coordination, resulting in the non-H-bonded configuration.\nTherefore, the observed configurations are determined by the intrinsic adsorption site\ngeometries and the resulting trade-off between coordination strength and H-bonding\nenergy.\nFigure R24. O–O Distance, ∠O–H···O Angle, and Adsorption Sites for Two OH Groups\non the (111) and (112) Facets.\n3. In Figure 5A, why did the authors study direct CO dissociation on Mo N(111) but H-\n2 2\nassisted dissociation on Mo N(112)? These are not comparable. The authors should\n2\nsupplement calculations with: (i) H-assisted CO dissociation on Mo N(111) and (ii) direct\n2 2\nCO2 dissociation on Mo N(112), to identify favorable pathways on each surface before\n2\ncomparing their activity differences.\nResponse: We appreciate the reviewer's valuable suggestion. We have conducted the\nadditional calculations as recommended: i) H-assisted CO₂ dissociation on Mo₂N (111),\nand ii) direct CO₂ dissociation on Mo₂N (112). As illustrated in Figure R25, these results\nyield two key insights: i) For a given facet, the H-assisted pathway for CO₂ dissociation\nfeatures a substantially lower energy barrier than the direct dissociation pathway; ii) the\n(112) facet is more conducive to C-O bond activation than the (111) facet. As suggested,\nwe have replaced previous Figure 5 with Figure R25. The relevant discussion has been\nadded to the main text.\nRevised version (highlighted in yellow color)\nPage 14 Line 14-16 in manuscript:\n“…In summary, our calculations demonstrate that the (112) facet is more active than the\n(111) facet for C–O bond cleavage, and that the H₂-assisted pathway enables CO₂\ndissociation with a substantially lower energy barrier compared to direct dissociation.\nFurthermore, the catalytic...”\nFigure R25. Energy barrier diagrams and corresponding structural configurations for C–O bond activation\non the Mo₂N (111) and (112) facets in the presence and absence of H*. (A) Reaction energy profiles for\ndifferent pathways on the (111) and (112) facets. (B–E) Corresponding initial state, transition state, and final\nstate structures.\n4. Figure 5C: It is difficult to reconcile how one C-O bond breaking, two O-H bond\nbreakings, and two O-H bond formations could occur in a single elementary step. Similarly,\nFigure 5D shows one C-O bond breaking, one Mo-H bond breaking, one O-H bond\nformation, and one Mo-O bond formation in one step.\nResponse: We thank the reviewer for raising this important point regarding the\nplausibility of multiple bond-breaking and forming events occurring within a single elementary step in\nFigures 5C and 5D.\nWe agree with the reviewer that the original reaction step depicted in Figure 5C was not\nphysically realistic. To address it, we have thoroughly re-examined this pathway. The\ncalculations for Figure 5C have been revised using a Mo-O model instead of the previous\nMo-OH model, and now depict the direct CO₂ dissociation on the Mo₂N (112) facet (Figure\nR25). In this updated mechanism, the elementary step involves only the breaking of a\nsingle C–O bond, which is fully consistent with the standard definition of an elementary\nreaction as a single, concerted molecular event.\nRegarding Figure 5D, which illustrates the H₂-assisted CO₂ activation pathway, we\nwould like to clarify that the simultaneous breaking of the C–O and Mo–H bonds, alongside\nthe formation of O–H and Mo–C bonds, represents a concerted process. Such a mechanism\nis chemically reasonable and reflects the synergistic role of surface hydrogen in facilitating\nCO₂ scission. Splitting this into sequential steps would misrepresent the essence of the\nhydrogen-assisted pathway, reducing it to a simple redox sequence and failing to capture\nthe cooperative effect of H₂ in lowering the activation barrier.\n5. The main text and SI do not specify the transition state (TS) search method. The authors\nmust provide: (i) imaginary frequency analysis for all TS calculations to validate TS\nreliability, and (ii) identification of corresponding initial and final states for each TS to\nconfirm they belong to the target reactions.\nResponse: We appreciate the reviewer's valuable suggestion. To fully address it, we have\nsupplemented the required details on transition state (TS) calculations and validated their\nreliability as follows:\nFirst, we confirm that TS calculations were performed for all proposed CO₂ activation\npathways. To validate TS authenticity, imaginary frequency analysis has been conducted\nfor every TS, and all results confirm that each TS exhibits exactly one imaginary\nfrequency—an essential criterion for verifying that the identified structure corresponds to\na true transition state along the target reaction coordinate (Table R2). Additionally, the\ncorresponding initial states and final state for each TS have been explicitly identified and\nassigned to confirm their direct association with the intended elementary reactions (Figure\nR10 and R25). Regarding the TS search method, detailed descriptions have been\nsupplemented in both the revised manuscript and Supporting Information (SI) for clarity:\nPage 7 Line 26-36 and Page 8 Line 1-2 in SI:\n“…processes. The climbing image nudged-elastic-band method (CI-NEB) including six\nreplicas was used to search the transition states (TS) for all of the elementary reactions.\nThe TS was denoted by the highest image along the minimum energy path. The energy\nbarrier (E ) of each elementary reaction was calculated by the energy difference between\na\nthe TS and the initial state (IS) without inclusion of the zero-point energy. Vibrational\nfrequencies were computed at the optimized transition state structure using the finite-\ndisplacement method to determine the Hessian (force constant) matrix (Table S6). The\ncalculation was performed with IBRION = 5, a displacement step of POTIM = 0.015 Å, and\nthe same exchange-correlation functional and pseudopotentials as used in the transition\nstate optimization. NSW was set to 1 to prevent any further geometry relaxation. The\nvibrational mode corresponding to the imaginary frequency was visualized using “Jmol”\n(an open-source Java viewer), confirming that the atomic displacements align with the\nexpected reaction coordinate….”\nTable R2. Imaginary frequencies and activation energies of transition states in different CO₂ activation\npathways over various Mo₂N facets\n6. The authors cite Ref. 27, suggesting C-O bond activation is the rate-limiting step in CO\n2\nhydrogenation. However, rate-determining steps depend on catalyst structure and reaction\nconditions—C-O bond cleavage may not be rate-limiting on Mo N.\nsystematically calculate all possible elementary steps in reverse water-gas shift reactions,\nincluding HCOO/COOH formation and their C-O/C-H bond cleavages.\nResponse: We appreciate the reviewer's constructive comment. We fully agree that the\nrate-determining step (RDS) of CO₂ hydrogenation is highly dependent on catalyst\nstructure and reaction conditions, and C-O bond cleavage may not serve as the RDS\nuniversally, especially on Mo₂N with its unique surface properties. In response to the\nreviewer’s suggestion, we have systematically investigated and computed all possible\nelementary steps of the RWGS reaction, including the formation of *HCOO (formate)\nand\n*COOH (carboxyl) intermediates, as well as their subsequent C-O and C-H bond cleavage\nprocesses.\nAs summarized in Figure R10, the H₂-assisted CO₂ activation pathway exhibits two key\nadvantages over the *HCOO and *COOH-mediated routes: i) Thermodynamic favorability:\nThe H₂-assisted pathway produces CO in a single concerted step, whereas the *COOH\nroute requires two sequential steps (formation of *COOH followed by C-O bond cleavage)\nand the *HCOO route involves three steps (formation of *HCOO, C-H bond cleavage, and\nC-O bond cleavage). This streamlined process confers greater thermodynamic feasibility;\nii) Kinetic advantage: The H₂-assisted pathway features a significantly lower activation\nbarrier (0.45 eV) compared to the *COOH route (2.22 eV) and *HCOO route (1.45 eV).\nNotably, for the H₂-assisted pathway, C-O bond cleavage is indeed the rate-limiting step—\nbut its barrier is substantially lower than the RDS barriers of the other two pathways,\nconfirming its dominance under the reaction conditions.\nThese systematic computational results not only validate the rationality of the H₂-\nassisted redox mechanism but also provide robust theoretical support for its superiority\nover the *HCOO/*COOH-mediated routes on Mo₂N. Importantly, the theoretical findings\nare well-aligned with our experimental observations (Figure S28 and Figure 3A), further\ncorroborating the reliability of the proposed reaction pathway.\nAs suggested, we have moved Table R10 to the SI, where it now appears as Figure S42,\nrespectively.\nRevised version (highlighted in yellow color)\nPage 15 Line 6-11 in SI\n“…barriers, and self-repairing surface architecture. Based on the structural model of the (112)\nfacet, we further evaluated alternative reaction pathways, namely, the HCOO* and *COOH routes,\nto theoretically exclude competing mechanisms. The results show that the H₂-assisted pathway\nfeatures a significantly lower activation barrier (2.22 eV for the *COOH route and 1.45 eV for the\n*HCOO route, as shown in Figure S42), in good agreement with our in-situ DRIFTS measurements\n(Figure S28).\nFigure R10. (A) Schematic illustration of the proposed reaction mechanism. (B) Calculated relative energy\nprofile and energy barriers for different reaction pathways on the HEMN catalyst with (112) facet. (C,\nD) Corresponding optimized geometric structures for the (C) COOH and (D) HCOO pathways.\n7. Importantly, energy diagrams are not state-of-the-art for mechanistic/kinetic studies,\nas they ignore coverage effects (often dominant in reaction rates), which may lead to\nmisleading interpretations. DFT calculations combined with microkinetic modeling have\nbeen widely used to quantify competing pathways, identify rate-limiting steps, etc.\nResponse: We greatly appreciate the reviewer’s insightful and constructive comments.\nWe fully agree that integrating density functional theory (DFT) calculations with microkinetic\nmodeling is a powerful approach for kinetic quantification, enabling the prediction of\nreaction rates, the resolution of coverage-dependent reactivity, and the quantification of\ncompeting pathway contributions. However, quantifying competing reaction pathways\nusing this combined approach in complex catalytic systems, as discussed in the current\nwork, remains quite challenging. A reliable quantification of microkinetic modeling\nnecessitates a comprehensive elucidation and quantification of all catalytic-relevant\nmicrostructural features (e.g., faceting, defects, terminating groups) and their dynamics\nduring the reaction. Unfortunately, this is often beyond the capabilities of most\ncharacterization techniques for many practical catalytic systems. This limitation partly\nexplains why successful applications of microkinetic modeling are often linked to simpler,\nmore uniform model catalytic systems.\nThe primary focus of the current study is the identification of key structural features,\nspecifically the (112) microfacets in Mo₂N nanocrystals, which support the creation of\nactive sites. Additionally, we provide mechanistic insights to distinguish the dominant CO₂\nactivation pathway on Mo₂N (i.e., the H₂-assisted route) from competing mechanisms (such\nas direct dissociation and *COOH/*HCOO-mediated pathways) by verifying key\nintermediates, comparing relative energy barriers of elementary steps, and correlating\ntheoretical predictions with experimental spectroscopic observations. In this study, we\nemploy an integrated approach that combines DFT energy profile analysis with in-situ\nspectroscopic experiments to enhance the validity of our conclusions. These methods\nreliably confirm the existence of the H₂-assisted pathway, rule out alternative mechanisms,\nand establish a clear structural-activity relationship for CO₂ activation—central aims of our\nwork. Notably, this methodological choice aligns with widely adopted practices in recent\nhigh-impact studies within this field (e.g., Nat. Commun. 16, 7335 (2025); Nano Lett. 2025,\n25, 9, 3579–3587; Chinese J. Catal. 73 (2025) 186–195).\nWhile microkinetic modeling would undoubtedly add value to kinetic quantification—an\navenue beyond the current focus of this study—our existing data, including distinct energy\nbarriers between pathways, spectroscopic confirmation of key intermediates, and\nconsistent experimental reaction trends, already provide strong evidence for the proposed\nH₂-assisted mechanism. As the reviewer suggested, we plan to extend this work in future\nstudies by incorporating DFT-coupled microkinetic modeling to address kinetic\nquantification, coverage effects, and dynamics of rate-limiting steps, provided we can\ndevelop a more quantitative, statistical, and comprehensive structural model. We sincerely\nthank the reviewer for this valuable suggestion.\n8. In Figure S29, 2OH co-adsorption structures are similar with or without H , but H O\n2 2\nand O co-adsorption structures differ significantly. H configurations also change pre-\n2\n/post-reaction in the right panel. Are the energy differences between pathways (left vs.\nright panel) caused by structural disparities rather than H effects?\nimpact of structural disparities on the observed energy differences. To directly address your\nconcern—whether the energy discrepancies between the pathways (left vs. right panel)\narise from structural variations (e.g., differences in H₂O/O co-adsorption configurations)\nrather than the intrinsic effect of H₂—we have re-constructed and re-optimized the reaction\nmodels.\nSpecifically, we meticulously designed the models for the H₂-involved pathway to\nminimize structural discrepancies relative to the H₂-free system: the surface coordination\nenvironment, adsorbate binding sites, and overall lattice structure were intentionally\naligned with the H₂-free configuration prior to geometric optimization. After re-\noptimization, the energy variations between the newly optimized models and the original\nstructures were acceptable, and it is lower than that without the participation of H (0.74\n2\neV, as shown in Figure R26 and Figure S35). This confirms that structural disparities (e.g.,\ndifferences in H₂O/O co-adsorption or H₂ configurations pre-/post-reaction) do not\ncontribute significantly to the observed energy differences.\nFurthermore, our experimental data strongly support that the energy differences stem\nfrom the intrinsic effect of H₂: in-situ infrared spectroscopy clearly captures the\ndehydration of surface OH groups promoted by H₂, while online mass spectrometry\nconfirms the corresponding formation of H₂O as a byproduct (Figure 3). These\nexperimental observations directly corroborate the theoretical finding that H₂ modulates\nthe reaction thermodynamics/kinetics by facilitating OH dehydration, rather than structural\nvariations inducing the energy differences.\nCollectively, the re-optimized models and complementary experimental evidence fully\nrule out structural disparities as the origin of the energy differences, validating that the\nobserved pathway differences are indeed attributed to the H₂-assisted activation mechanism.\nFigure R26. Optimized Structures and Energy Changes for H₂-Assisted Hydroxyl Dehydration on Various\nSurfaces\nMinor issues:\n1. Methods section: (i) Mo N slab thickness (layers), (ii) tests for slab convergence, and\n2\n(iii) unit cell size should be specified.\nResponse: Thanks for the reviewer’s valuable comments, which has helped us improve the\nrigor of our computational work. We have addressed each of your concerns point by point\nas follows:\ni) Slab model details. To ensure the reliability of the surface calculations, we first\nconstructed supercells for the Mo₂N (111) and (112) surfaces. The optimized supercell\nparameters are (111) facet with a = 12.0366 Å, b = 18.0350 Å, c = 19.9984 Å and (112)\nfacet with a = 12.0366 Å, b = 9.9712 Å, c = 21.0606 Å. Each slab model consists of 3\nlayers, with a vacuum layer of 15 Å applied along the c-axis.\nii) k-point sampling. To verify the adequacy of the k-point sampling, we systematically\ntested the energy convergence for the (111) and (112) surfaces by increasing the k-point\nmesh density (Table R1). Results confirm that all structural optimizations met the preset\nconvergence criteria, and the total energy changes with increasing k-points were negligible,\nwhere maximum energy differences for the (111) and (112) facets are 0.04 eV and 0.06 eV\nrespectively. These variations are within the acceptable error range for DFT calculations.\nRevised version (highlighted in yellow color)\nPage 7 Line 1-5 in SI\n“… (Physical review B, 1999, 59(3): 1758). A Γ-centered 2 × 1 × 1 grid for (111) facet and\n2×2×1 for the (112) facet was used for k-point sampling. After conducting k-point\nconvergence tests (as shown in Table S5), we ultimately determined that a Γ-centered 2\n× 1 × 1 k-point grid was employed for the (111) facet, and a 2 × 2 × 1 grid was used for\nthe (112) facet in the k-point sampling process. Convergence criteria were set to…”\nTable R1. Summary of unit cell parameters and k-Point test results used in DFT Calculations\n2. Line 160 of SI: Clarify \"Figure Sxx.\"\nResponse: Thanks for the reviewer’s suggestion. The corresponding text has been\ncorrected and now reads \"…supercell model was also established (Figures S39-40), with\nlattice parameters…”\n3. Many figures lack full data visibility (e.g., Figs. 3C-E, S9B, S10-S12).\nResponse: We thank the reviewer for this suggestion. The mentioned figures (Figs. 3C-E,\nS9B, S10-S12) have been revised to ensure full data visibility.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\n1. The manuscript has been thoroughly revised according to the reviewers' feedback, and\nthe discussion has been substantially advanced overall. I support acceptance.\nResponse: We are pleased with the reviewer's positive final assessment and their\nrecommendation for acceptance. The reviewer’s expert feedback during the review process\nwas instrumental in guiding these improvements. We thank the reviewer once again for the\ntime and valuable contribution.\nReviewer #2 (Remarks to the Author):\n1.The manuscript has been modified with care. I suggest accepting it with very minor\nrevisions. In the abstract, the author mentioned that “Our catalyst achieves near-\nequilibrium conversion (56%) at a high space velocity (24000 ml/gcat/h)”. For RWGS\nreaction, the space velocity of 24000 ml/gcat/h is a very low value. I do suggest removing\nthe word “high”.\nResponse: We are grateful for the reviewer’s favorable recommendation to accept the\nmanuscript and for pointing out this important detail. In accordance with the reviewer’s\nsuggestion, the term \"high\" has been removed from the abstract in the revised version. We\nsincerely thank the reviewer’s scrutiny and feedback, which have further polished our work.\nReviewer #3 (Remarks to the Author):\nThe author has addressed most of my concerns, except for two minor points.\nResponse: We appreciate the reviewer’s recognition of our revisions and valuable\nsuggestions. The two remaining minor points will be addressed point-by-point in our\nresponse below, with all changes clearly marked in the revised manuscript.\n1. Why are the energies of CO*+OH* not equal across the three paths in Figure R10B?\nResponse: We are grateful to the reviewer for pointing out this inconsistency in the energies\nof CO*+OH*. We have carefully reviewed the calculation parameters and identified that\nthe minor discrepancy arises from a different k-point grid sampling, which may impact\nconvergence and energetics. To address this, we have completely recalculated the *COOH\nand HCOO* pathways using the same set of calculation parameters in the VASP software\nas those used for the H₂-assisted pathway. This ensures all data are derived from a uniform\ncomputational framework, resolving the discrepancy as shown in the revised Figure R1.\nThe reviewer’s careful review was invaluable in helping us correct this issue and uphold\nthe foundational rigor of our study.\nFigure R1. (A) Schematic illustration of the proposed reaction mechanism. (B) Calculated relative energy\nprofile and energy barriers for different reaction pathways on the HEMN catalyst with (112) facet. (C, D)\nCorresponding optimized geometric structures for the (C) COOH and (D) HCOO pathways.\n2. Surface activity and reaction mechanisms are strongly model-dependent. In this study,\nthe vacancy concentration is a critical factor, and experimentally, there should be\nquantitative data to justify why a model with a specific vacancy concentration is chosen,\nrather than a qualitative description of \"not too high, not too low.\"\nResponse: Thanks for the reviewer for raising this point and we totally agree that providing\na quantitative experimental measurement associated with vacancy concentration is\nimportant. Based on the fact that the surface concentration of Mo-O species should be\ninversely proportional to the vacancy concentration, it provides a reliable quantitative\nmetric to evaluate the structure-activity relationship. Accordingly, we have prepared\ncatalysts with varying reduction degrees and established a quantitative relationship\nbetween the concentration of Mo-O species and the CO yield. Based on the quasi-in situ\nXPS and performance tests, we have found that the maximum CO yield aligns with a Mo-\nO species concentration of approximately 4% as determined by quantitative XPS analysis\n(Figure R2, also Figures S21-16 in SI). This experimental optimum serves as the\nquantitative basis for selecting the specific vacancy concentration. We have revised the\nmanuscript accordingly and appreciate the reviewer’s suggestion.\nFigure R2. (A) H₂ temperature-programmed reduction (H₂-TPR) profile of the HEMN catalyst, with online\nMS signals for H₂ (m/z = 2) and H₂O (m/z = 18). (B) XRD patterns and (C) O 1s XPS spectra of HEMN\ncatalysts treated at different reduction temperatures. (D) Correlation between CO yield and the concentration\nof M–O species.\n3. Additionally, the authors did not provide the complete energy profiles or microkinetic\nsimulations. The current surface model shows very high activity, enabling direct CO2\ndissociation. However, on such active surfaces, might the recombination reaction become\nrate-limiting? Can the reaction cycle proceed rapidly? Microkinetic simulations could\neffectively address this. I disagree with the authors' statement that \"This limitation partly\nexplains why successful applications of microkinetic modeling are often linked to simpler,\nmore uniform model catalytic systems.\" In fact, microkinetic modeling has been widely\napplied to many complex reaction systems. I suggest the authors complete this part of the\nwork in future studies.\nResponse: We greatly appreciate the reviewer’s suggestion to conduct a comprehensive\nmicrokinetic modeling in our future work. We fully agree that microkinetic modeling offers\na robust framework for addressing many mechanistic questions, even within complex\nreaction systems. In the near future, we plan to undertake a detailed follow-up study\nfocusing on the microkinetic modeling of the current catalytic reaction. This will involve\nproviding complete energy profiles for all possible competitive reaction pathways,\nincluding both dissociation and recombination reactions that complete the reaction cycle,\nalongside various structural parameters such as facets, defects, and coverage. We believe\nthat this dedicated microkinetic study will yield deeper insights into the mechanistic\naspects of the RWGS reaction based on the new findings revealed in this work."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript has been thoroughly revised according to the reviewers' feedback, and the discussion has been substantially advanced overall. I support acceptance.", "reviewer_2": "The manuscript has been modified with care. I suggest accepting it with very minor revisions. In the abstract, the author mentioned that “Our catalyst achieves near-equilibrium conversion (56%) at a high space velocity (24000 ml/gcat/h)”. For RWGS reaction, the space velocity of 24000 ml/gcat/h is a very low value. I do suggest removing the word “high”.", "reviewer_3": "The author has addressed most of my concerns, except for two minor points: 1) Why are the energies of CO*+OH* not equal across the three paths in Figure R10B? 2) Surface activity and reaction mechanisms are strongly model-dependent. In this study, the vacancy concentration is a critical factor, and experimentally, there should be quantitative data to justify why a model with a specific vacancy concentration is chosen, rather than a qualitative description of \"not too high, not too low.\" Additionally, the authors did not provide the complete energy profiles or microkinetic simulations. The current surface model shows very high activity, enabling direct CO2 dissociation. However, on such active surfaces, might the recombination reaction become rate-limiting? Can the reaction cycle proceed rapidly? Microkinetic simulations could effectively address this. I disagree with the authors' statement that \"This limitation partly explains why successful applications of microkinetic modeling are often linked to simpler, more uniform model catalytic systems.\" In fact, microkinetic modeling has been widely applied to many complex reaction systems. I suggest the authors complete this part of the work in future studies."}, "author_response": "Reviewer #1 (Remarks to the Author):\n1. The manuscript has been thoroughly revised according to the reviewers' feedback, and\nthe discussion has been substantially advanced overall. I support acceptance.\nResponse: We are pleased with the reviewer's positive final assessment and their\nrecommendation for acceptance. The reviewer’s expert feedback during the review process\nwas instrumental in guiding these improvements. We thank the reviewer once again for the\ntime and valuable contribution.\nReviewer #2 (Remarks to the Author):\n1.The manuscript has been modified with care. I suggest accepting it with very minor\nrevisions. In the abstract, the author mentioned that “Our catalyst achieves near-\nequilibrium conversion (56%) at a high space velocity (24000 ml/gcat/h)”. For RWGS\nreaction, the space velocity of 24000 ml/gcat/h is a very low value. I do suggest removing\nthe word “high”.\nResponse: We are grateful for the reviewer’s favorable recommendation to accept the\nmanuscript and for pointing out this important detail. In accordance with the reviewer’s\nsuggestion, the term \"high\" has been removed from the abstract in the revised version. We\nsincerely thank the reviewer’s scrutiny and feedback, which have further polished our work.\nReviewer #3 (Remarks to the Author):\nThe author has addressed most of my concerns, except for two minor points.\nResponse: We appreciate the reviewer’s recognition of our revisions and valuable\nsuggestions. The two remaining minor points will be addressed point-by-point in our\nresponse below, with all changes clearly marked in the revised manuscript.\n1. Why are the energies of CO*+OH* not equal across the three paths in Figure R10B?\nResponse: We are grateful to the reviewer for pointing out this inconsistency in the energies\nof CO*+OH*. We have carefully reviewed the calculation parameters and identified that\nthe minor discrepancy arises from a different k-point grid sampling, which may impact\nconvergence and energetics. To address this, we have completely recalculated the *COOH\nand HCOO* pathways using the same set of calculation parameters in the VASP software\nas those used for the H₂-assisted pathway. This ensures all data are derived from a uniform\ncomputational framework, resolving the discrepancy as shown in the revised Figure R1.\nThe reviewer’s careful review was invaluable in helping us correct this issue and uphold\nthe foundational rigor of our study.\nFigure R1. (A) Schematic illustration of the proposed reaction mechanism. (B) Calculated relative energy\nprofile and energy barriers for different reaction pathways on the HEMN catalyst with (112) facet. (C, D)\nCorresponding optimized geometric structures for the (C) COOH and (D) HCOO pathways.\n2. Surface activity and reaction mechanisms are strongly model-dependent. In this study,\nthe vacancy concentration is a critical factor, and experimentally, there should be\nquantitative data to justify why a model with a specific vacancy concentration is chosen,\nrather than a qualitative description of \"not too high, not too low.\"\nResponse: Thanks for the reviewer for raising this point and we totally agree that providing\na quantitative experimental measurement associated with vacancy concentration is\nimportant. Based on the fact that the surface concentration of Mo-O species should be\ninversely proportional to the vacancy concentration, it provides a reliable quantitative\nmetric to evaluate the structure-activity relationship. Accordingly, we have prepared\ncatalysts with varying reduction degrees and established a quantitative relationship\nbetween the concentration of Mo-O species and the CO yield. Based on the quasi-in situ\nXPS and performance tests, we have found that the maximum CO yield aligns with a Mo-\nO species concentration of approximately 4% as determined by quantitative XPS analysis\n(Figure R2, also Figures S21-16 in SI). This experimental optimum serves as the\nquantitative basis for selecting the specific vacancy concentration. We have revised the\nmanuscript accordingly and appreciate the reviewer’s suggestion.\nFigure R2. (A) H₂ temperature-programmed reduction (H₂-TPR) profile of the HEMN catalyst, with online\nMS signals for H₂ (m/z = 2) and H₂O (m/z = 18). (B) XRD patterns and (C) O 1s XPS spectra of HEMN\ncatalysts treated at different reduction temperatures. (D) Correlation between CO yield and the concentration\nof M–O species.\n3. Additionally, the authors did not provide the complete energy profiles or microkinetic\nsimulations. The current surface model shows very high activity, enabling direct CO2\ndissociation. However, on such active surfaces, might the recombination reaction become\nrate-limiting? Can the reaction cycle proceed rapidly? Microkinetic simulations could\neffectively address this. I disagree with the authors' statement that \"This limitation partly\nexplains why successful applications of microkinetic modeling are often linked to simpler,\nmore uniform model catalytic systems.\" In fact, microkinetic modeling has been widely\napplied to many complex reaction systems. I suggest the authors complete this part of the\nwork in future studies.\nResponse: We greatly appreciate the reviewer’s suggestion to conduct a comprehensive\nmicrokinetic modeling in our future work. We fully agree that microkinetic modeling offers\na robust framework for addressing many mechanistic questions, even within complex\nreaction systems. In the near future, we plan to undertake a detailed follow-up study\nfocusing on the microkinetic modeling of the current catalytic reaction. This will involve\nproviding complete energy profiles for all possible competitive reaction pathways,\nincluding both dissociation and recombination reactions that complete the reaction cycle,\nalongside various structural parameters such as facets, defects, and coverage. We believe\nthat this dedicated microkinetic study will yield deeper insights into the mechanistic\naspects of the RWGS reaction based on the new findings revealed in this work."}]

Decision letter