Reaction-induced modification of Co nanoclusters driven by Co-Mn interfacial sites to control selectivity in CO_2 hydrogenation

Domain: chemistry
Published: 2026-03-07
Editorial outcome: major_revision

Abstract
Controlling the selectivity of chemical products on small Co nanoparticles is crucial in many catalytic applications. Reaction-driven structural changes offer an alternative methodology to regulate their properties. Herein, carbon-induced surface restructuring occurs on a 2Co/MnOₓ catalyst (cobalt nanoclusters with 2% mass loading on manganese oxide) during thermal CO2 hydrogenation, driven by the formation of bridging Co-C-O-Mn interfacial sites. This leads to a shift in selectivity from methane to CO, with a remarkable enhancement of the CO/CH4 product ratio from 0.89 to 13.4. Such a Co/MnO system has unique interfacial properties, including strong carbonophilic and oxophilic characteristics. It chemisorbs reaction-derived CO and facilitates C-O bond breaking, promoting rapid CO dissociation and subsequent carbon coverage on Co nanoclusters. This restructuring of Co nanoclusters suppresses the hydrogenation of CO intermediates to methane. This effect is unique to 2Co/MnOₓ and absent at higher/lower Co loadings or other oxide supports. This insight shows how structural evolution during catalysis enables precise surface engineering, overcoming the structure-sensitivity limits of Co nanoclusters.

Introduction
Cobalt nanoclusters serve as active elements in numerous industrially relevant applications, including Fischer-Tropsch synthesis (FTS), CO methanation, alkane dehydrogenation, bioethanol conversion, and electrocatalytic nitrogen reduction, among others1-5. Often, these reactions are structure-sensitive. As exemplified in FTS, where the optimal cobalt particle size is around 6-8 nm6, the use of smaller nanoparticles would be preferable for metal utilisation. Co nanoclusters exhibit remarkable capabilities to activate H-H and C-O bonds and facilitate C-C coupling reactions, establishing them as widely utilized active centres in FTS for producing high-value-added multi-carbon compounds (CxHyOz) from synthesis gas (CO+H2)7,8. The methanation reaction is an undesired process and needs be suppressed 1,9. Carbonisation of Co during the FTS process modulates catalytic selectivity10,11. Carbon encapsulation of Co nanoparticles and the formation of cobalt carbide (CoxC) serve as methods to tune selectivity, particularly promoting olefin and oxygenate formation in FTS 9,12. However, these transformations simultaneously inhibit the water-gas shift reaction and exhibit instability under CO2/H2 feeds compared to syngas, rendering them unsuitable for direct CO2-to-olefins (FTO) production 13,14. Enhancing the CO/CH4 ratio during CO2 hydrogenation provides a crucial foundation for subsequent Fischer-Tropsch and methanol industries that utilize CO as a key platform molecule15,16. This strategy is particularly advantageous as it can be seamlessly integrated into existing industrial frameworks17. This depends on the controlled modulation of the catalyst’s surface microstructure.

Reaction-driven structural changes offer an alternative methodology with industrial potential for regulating catalyst structure and overcoming these limitations18. Unravelling the complex mechanisms driving these gas-induced structural changes represents a pioneering approach in catalyst design, offering new perspectives for the development of tailored catalysts19. A catalyst structure that is deactivated toward one reaction may serve as an excellent active site for another reaction. In other words, strategically designing the reaction atmosphere to induce the specific structure required for a target reaction represents a valuable catalyst design principle with industrial potential. In particular, the surface carbon species formed during CO/CO2 hydrogenation reactions have a complex impact on the reaction process, including phenomena such as carbon coating20, carbon infiltration21, and carburization22. The tunability of these fine structures is crucial for both catalyst design and advancing our understanding of the reaction mechanisms. It is necessary to clarify how structural changes depend on the inducing atmosphere and material properties.

By adjusting catalyst composition (support type and Co loading) and pretreatment conditions under various gases (such as pure CO2, CO and H2, as well as CH4, CO or CO mixed with H2), we unravelled the switches governing the structural evolution of Co-based catalysts during reaction and related selectivity modulation during CO2 hydrogenation (reverse water gas-shift reaction). Notably, the 2Co/MnOₓ catalyst, with 2 % cobalt on MnOₓ, exhibited a pronounced, rapid surface carbon species formation instead of carbide (CoxC) generation during CO2 hydrogenation, shifting the primary product from CH4 to CO. Comprehensive surface structure characterization and in-situ experiments revealed that the carbon-reconstructed Co surface decisively weakened CO adsorption and hydrogenation. This structural evolution critically depends on the strong oxophilicity of the MnO support and the carbonophilic nature of the Co metal, which facilitates CO chemisorption in the Co-C-O-Mn configuration at the Co/MnO interface. The disproportionation of adsorbed CO follows this to form C atoms that migrate across the Co nanoclusters’ surface. They thus represent a novel advance in inducing controlled gas-induced modifications for Co nanoclusters beyond current carbon-modified Co nanocatalysts (including Mn-promoted materials)23-25. This study employs a controllable gas-induced strategy to modify the surface properties of active metals. It elucidates how evolved structures mechanistically affect reaction performance and further highlights the critical significance of such gas-induced strategies in regulating catalyst surface structures at the nano- and even atomic levels.

Results
Reaction-induced Co/MnO catalyst changes modulate the RWGS behaviour

The catalytic performance of Co/MnO catalysts was evaluated in CO2 hydrogenation at ambient pressure (H2/CO2 ratio = 4). Figure S1 shows that the reactivity of the 2Co/MnO catalyst changes significantly between 300 and 500 °C, with CO selectivity rising from 52.6% to nearly 100% before stabilizing at ~88% at 500 °C. In the second run, high CO selectivity (e.g., 90.2% at 400 oC) is maintained across the entire temperature range tested. When the reaction temperature is maintained at 400 oC, the CO selectivity of the 2Co/MnO catalyst rises from 45.7% to 94.0% after 5 hours, while CO2 conversion decreases (Figures 1a and S2). The initial-to-final reaction stage selectivity shift (CO/CH4 ratio) for the 2Co/MnO catalyst (Figure 1b) surged from 0.89 (initial, ~1 h) to 13.4 (final, ~18 h). The CO2 conversion trend aligns with the literature, indicating that the RWGS reaction rate is slower than the methanation reaction rate26 due to the higher thermodynamic equilibrium conversion for methanation27 and CO poisoning of active sites28. Despite these changes, the RWGS performance of 2Co/MnO remains superior to other Co-based catalysts (Table S1). This suggests that the catalyst underwent structural evolution during the reaction, transforming methanation sites into CO-generation sites.

The Co-loading significantly impacts the rate of this structural evolution (Figures 1a and S2). For the 1Co/MnO catalyst (1wt% Co), CO selectivity increases from 89% to 96% within 20 - 40 minutes of reaction. In contrast, the 5Co/MnO catalyst (5wt% Co) shows low CO selectivity even after 18 hours on stream. Other conventional oxide-supported cobalt catalysts (2Co/TiO2 and 2Co/SiO2) were further synthesized and compared with 2Co/MnO in RWGS at 400 °C (Figures 1a, 1b, S2, and S3). These catalysts display minimal changes in CO2 conversion and product selectivity (CO/CH4 ratio), highlighting that the MnO support and moderate Co loading are crucial for this transformation.

During CO2 hydrogenation, the catalyst undergoes structural changes, but the specific gas species driving this transformation was unclear due to complex gas interactions. To better understand the performance transition, simplified gas mixtures were used to treat the 2Co/MnO catalyst (Figures 1c, S4, and S5). Figure 1c compares the CO2 hydrogenation performance of the 2Co/MnO catalyst after various gas treatments, including pure CO2, CO, and H2, as well as CH4, CO, or CO mixed with H2. Notably, CO selectivity exceeded 90% after treatment with CO or CO2-containing gases (CO2), whereas H2 or CH4 + H2 treatment failed to trigger rapid, selective transformation. A CO2:H2 ratio of 1:4 was optimal, balancing high CO selectivity with good RWGS reactivity (Figure 1c). The catalytic performance of the 2Co/MnO catalyst varies with time on stream at 300 °C, 400 °C, and 500 °C (Figure S6). This is likely due to intrinsic reaction differences from thermodynamic and kinetic variations at different temperatures. However, when the catalysts pretreated at these three temperatures for 18 hours were tested at a constant 400 oC, they showed similar catalytic performance (Figure 1d). Thus, the surface in-situ restructuring between 300 - 500 °C is temperature-independent but kinetically influenced by temperature.

Identification of the in-situ changes during the reaction

The selectivity in CO2 hydrogenation is often associated with the type of surface intermediates, particularly formate and CO species29,30. CO2+H2-DRIFTs analysis of the 2Co/MnO catalyst before and after (denoted as 2Co/MnO-U) reaction gas treatment revealed identical intermediate formation. The gas treatment did not significantly alter the surface structure of MnO support related to these intermediates (Figure S7)30. Thus, the change in catalyst selectivity is attributed to alterations in the Co species or the Co-MnO interface, which affect the subsequent hydrogenation pathways of the intermediates (i.e., formate or CO). The 2Co/MnO sample exhibited complex CO adsorption configurations (Figure 2a), with infrared vibrations at ~2022 cm⁻¹ corresponding to linear CO adsorption and three peaks below 2000 cm-1 indicating bridge CO adsorption. This suggests that MnO enhances bridge-bonded CO formation on Co-based catalysts31. In contrast, no CO adsorption signals were detected on the surface of the 2Co/MnO-U sample (Figure 2b), indicating a structural transformation of the Co species during the reaction that suppressed CO adsorption.

Temperature-programmed CO-DRIFTs on the 2Co/MnO sample revealed a gradual decrease in surface CO species (Figure 2c). Notably, the linear CO adsorption peak (>2000 cm-1) experienced a blue shift (towards higher wavenumbers) during heating, indicating weakened CO adsorption. However, reduced CO coverage would typically be expected to cause a red shift (toward lower wavenumbers) due to altered chemisorption bond strength, as demonstrated by Bertolini et al.32 and Kalhara et al.33 through experiments and DFT calculations. The anomalous results from the temperature-programmed CO-DRIFT experiment suggest that the cobalt active centres undergo an intrinsic structural change during CO treatment, progressively weakening CO adsorption. Temperature-programmed reaction (TPR) profiles of CO2+H2 and CO+H2 over the 2Co/MnO catalyst (Figure 2d) showed a rapid decline in CO methanation activity at 417 oC. In comparison, two separate peaks emerged at 329 oC and 497 oC for CO+H2-TPR. The results indicate that the restructured cobalt sites increased the energy barrier for CO hydrogenation. This strongly suggests that CO may be a key species in driving the catalyst’s structural evolution.

XRD patterns of 2Co/MnO and 2Co/MnO-U catalysts exhibited only MnO diffraction lines with no detectable Co species (Figure S8). At the same time, TEM images showed moiré fringes adjacent to Co nanoclusters (Figures 2e, 2g) due primarily to the overlap of lattice fringes between Co and MnO34. Though the fresh 2Co/MnO catalyst exhibited Co nanoparticles with an average diameter of ~4.0 nm, a modest increase to 4.7 nm is observed after 18h of time on stream at 400 oC (Figure 2e-2h). The 2Co/MnO catalyst maintains Co species in nanoparticle form before and after the reaction, as confirmed by EDS mapping of elemental distribution (Figure S9-S10). Cobalt-based catalysts exhibit significant structure sensitivity in CO hydrogenation1, with small Co nanoparticles (single atoms or nanoclusters) favoring RWGS35, while larger Co nanoparticles promote CH4 production36. This analysis clearly indicates that the increased CO selectivity is not due to variations in Co particle size.

Furthermore, semi-in-situ XPS analysis of 2Co/MnO and 2Co/MnO-U (before and after reaction gas treatment) detected no CoxC species (at 283.1 eV)14 (Figure S11a). As indicated in the introduction, particle size and the formation of a Co-carbide species are two key factors to alter CO hydrogenation selectivity when the catalyst composition remains unchanged. Even with the enhanced surface elemental resolution afforded by synchrotron NAP-XPS, no formation of CoxC species is discernible under CO2+H2 atmospheres (Figure S11b). This fact is justified by the pretreatment temperature (400 oC) used in this study, which exceeds the stable temperature of CoxC (260 oC)14,37. Thus, neither changes in particle size nor carbide formation account for the evolution of selectivity, highlighting an alternative restructuring mechanism. Sun et al.38 reported that CO dissociation into surface carbon species inhibited subsequent CO adsorption on the Pt/FeOx catalyst due to the excessively strong CO adsorption, which led to a low reactivity39. Analogously, the fresh 2Co/MnO catalyst exhibited strong CO adsorption ability with a bridged configuration (Figure 2a). In contrast, temperature-programmed CO treatment revealed a reduced CO adsorption strength (Figure 2c), suggesting that reactant gas exposure induced a notable suppression of CO adsorption capacity on the 2Co/MnOx catalyst, consistent with the deposition of surface carbon species.

Further temperature-programmed hydrogenation (TPH) and temperature-programmed oxidation (TPO) analyses of the catalyst after reaction at 400 oC for 5 and 18 h (Figures 3a and S12) provided definitive evidence of surface carbon deposition (identified as polymeric carbon species), with TPH showing three distinct peaks of CH4 formation at 197 oC (highly reactive surface atomic carbon species), 325 oC (metal-carbon species), and 550-590 oC (polymeric carbon species)40,41. Analogously, the TPO analysis revealed three CO2 evolution peaks at 116 oC, 237 oC, and 363-378 oC, which sequentially align with the carbon species identified in the TPH analysis42. The lower temperatures observed in the TPO compared to TPH suggested a higher sensitivity of surface carbon species to oxidation by O2. The comparison of results for samples after 5 h and 18 h of catalytic testing shows that surface atomic carbon and metal-carbon species increased with time on stream, while polymeric carbon species tended to accumulate progressively over time (Figure 3a). In contrast, the 2Co/TiO2 and 5Co/MnO catalysts significantly reduced abundances of surface carbon species after 5 h of reaction (Figure 3b).

The similar Mn 3s and O 1s spectra of semi-in-situ XPS before and after the reaction over a 2Co/MnO catalyst suggest that the valence state of Mn (Mn2+) in the MnO support and the surface oxygen species remain largely unaffected by carbon deposition (Figures S13a-13b). There is also a similar Co 2p spectra at 780.3 eV after carbon deposition (Figure S13c). However, due to the minimal number of interfacial Co-C-Mn sites, the sensitivity is insufficient to prove that the average charge of Co atoms decreases after C deposition. Thereby, DFT calculations were employed to demonstrate that surface charge transfer in Co species after carbon deposition inhibits subsequent CO adsorption (Figures S14-S15). Bader charge analysis reveals that the Co atoms in the C-deposited Co cluster have lower average charges compared to the pristine Co cluster, indicating electron transfer from Co to the carbon species (Figure S15a). This electron depletion weakens Co's adsorption of CO molecules. The projected density of states (PDOS) in Figure S15b also shows that the d-states of Co in the clean cluster are closer to the Fermi level, allowing stronger interaction and easier electron donation to the antibonding orbitals of CO, thereby enhancing CO activation. In Figure S15c, the calculated adsorption energies indicate that CO adsorption is weaker on the C-deposited Co cluster than on the pristine Co cluster. We also examined CO adsorption at next-nearest neighboring sites of the deposited carbon. While adsorption is stronger at these sites than at the nearest sites, it is still weaker than on the clean Co cluster. These results confirm that the charge transfer caused by carbon deposition inhibits CO adsorption. The surface carbon species content was maintained at a moderate, low level (Figure S16), with no evidence for the formation of carbon nanotubes or carbon fibre-like species (Figure S17)43. However, a layered graphitic carbon species was observed on the surface of 2Co/MnO catalyst (Figures 3(c1-c2), and S18-S19) after the reaction, confirming the occurrence of surface carbon deposition. In addition, the activity tests indicated that after ~5 h of reaction, CO selectivity and CO2 conversion over 2Co/MnO catalyst remained largely stable, suggesting that only initial surface carbon deposition during the early stages of the reaction influenced the reactivity of Co-based catalyst. The activity also remains nearly stable after the abrupt change in selectivity (Figure S3), indicating that this layered graphitic carbon is sufficiently porous to guarantee reactant access to the Co surface while altering surface reactivity. Thus, our results rule out conventional fouling and confirm a carbon-induced surface restructuring of Co nanoclusters. This mechanism fundamentally differs from C-encapsulated Co or Co-carbide species9,37,44,45, which are formed under different conditions, establishing a novel restructuring pathway.

TPH results confirmed surface carbon species (particularly polymeric carbon) could be eliminated via H2 treatment at elevated temperatures, leading to the generation of CH4, with atomic carbon and metal-C bond hydrogenated below 400 oC. In contrast, surface polymeric carbon species require higher temperatures for elimination. The 2Co/MnO catalyst, after 18 h of reaction followed by 4 h of H2 treatment at 400 oC, exhibited high CO selectivity and unaltered catalytic performance (Figures 3d and S20), demonstrating that atomic carbon and metal-carbon species were not selectivity-determining factors, whereas increasing the post-reaction H2 treatment temperature of the 2Co/MnO catalyst to 500 oC rapidly restores fresh-catalyst selectivity (Figures 3e and S21), proving polymeric carbon deposition drives the catalytic behavior shift. This interpretation aligns with results from multiple reaction-reduction-reaction cycles, confirming that surface polymeric carbon species critically modulate catalytic selectivity. The CO2 hydrogenation atmosphere promotes the formation of carbon-modified Co active sites on the 2Co/MnO catalyst, thereby shifting selectivity from CH4 to CO. At the same time, H2 treatment at 500 oC effectively eliminates surface carbon species, thereby restoring the catalyst's high CH4 selectivity (Figure 3f).

The Co/MnO interface role in the in-situ restructuring

The above results indicate that surface carbon deposition significantly affects the reaction performance of 2Co/MnO. In contrast, analogous catalysts, such as 2Co/TiO2 and 5Co/MnO, exhibited a markedly slower surface carbon deposition rate under identical treatment conditions, indicating a unique mechanism specific to 2Co/MnO. CO-TPD experiments on the 2Co/MnO, 2Co/TiO2, and 5Co/MnO catalysts (Figure S22) revealed not only the desorption of CO (m/z = 28) but also substantial generation of CO2 (m/z = 44). The CO2 may originate from either the disproportionation of CO (2CO → CO2 + C) 46-48 or the direct oxidation of CO (CO+O*→CO2) 49. However, the catalyst undergoes high-temperature hydrogen reduction followed by inert gas purging, which removes surface-active oxygen species. The amount of CO2 formed is about half of that of CO converted, as expected from the Boudouard reaction, and different from what would occur in the case of CO oxidation.

Additionally, there is an enhanced presence of surface carbon species on the 2Co/MnO catalyst (as shown by TPH, TPO, TEM, etc.), compared to 5Co/MnO and 2Co/TiO2 cases. All these indications strongly support the occurrence of CO disproportionation. The CO2 production in CO-TPD experiments enables quantification of surface carbon deposition. Specifically, 2Co/MnO and 5Co/MnO catalysts exhibited pronounced CO2 desorption peaks below 400 oC, whereas 2Co/TiO2 primarily desorbed CO with only minor CO2 desorption above 600 oC. Temperature-programmed CO-DRIFTs data confirm CO disproportionation on the 2Co/MnO through emerging gaseous CO2 bands at 2300-2400 cm-1 during the heating process (Figure 2c)50. Figures 4a and S22 comparatively analyze the integrated areas of CO and CO2 desorption peaks from CO-TPD experiments across three catalysts, using the desorbed CO2 quantity to quantify CO disproportionation capability or carbon deposition propensity. Both 2Co/MnO and 5Co/MnO catalysts exhibit significantly higher CO2 desorption than 2Co/TiO2, underscoring the role of MnO in enhancing CO disproportionation; while 5Co/MnO exhibits greater CO2 desorption than 2Co/MnO alongside a lower disproportionation onset temperature of ~57 oC (Figure S22d)-attributed to higher cobalt loading and larger nanoparticles sizes (Figure S23), which elevating the number of CO disproportionation sites. The 5Co/MnO catalyst exhibited a slower selectivity shift than 2Co/MnO catalyst (Figure S24), indicating a delayed in-situ restructuring process of Co nanoclusters, which is reasonably associated with its initially low CO selectivity of the 5Co/MnO catalyst during CO2 hydrogenation (Figure 1a), which limited the initial CO partial pressure and thus also the initial CO disproportionation kinetics. The 5Co/MnO catalyst has a lower initial CO concentration under reaction conditions than the 2Co/MnO catalyst, which was attributed to its higher Co loading and larger metal particle size, both of which facilitated CO methanation 36. For longer times on stream and amount of surface carbon deposition, the 5Co/MnO catalyst shows high CO selectivity comparable to that of the 2Co/MnO catalyst. This highlights the importance of synergistically controlling the gas composition and material properties in governing the structural restructuring process36.

TPH analysis confirmed that deposited surface carbon species (particularly polymeric carbon) underwent hydrogenation by gas-phase H2, establishing a dynamic equilibrium during carbon-modified Co active site formation between carbon-generating CO disproportionation and hydrogenation-mediated carbon removal. The 5Co/MnO catalyst, with a higher Co loading, provides more H2 adsorption and dissociation sites compared to the 2Co/MnO catalyst. The much larger desorption peak area observed in the H2-TPD results for the 5Co/MnO catalyst supports this (Figures 4b and S25), thereby enhancing the consumption of deposited carbon species and reducing the overall carbon deposition efficiency, consistent with the mechanistic framework. Furthermore, the HR-TEM analysis of the 5Co/MnO catalyst surface after reaction reveals only a small amount of onion-like carbon species (Figure S26), primarily consisting of some amorphous carbon layers (Figure S27). The carbon amount after the same reaction time (5 h) is significantly lower on the 5Co/MnO catalyst compared to the 2Co/MnO catalyst (Figures S18-S19). The H/C balance on this catalyst determines the carbon deposition efficiency, similar to how the Co nanocluster surface favors H dissociation, while the peripheral edge sites drive CO dissociation and disproportionation processes51.

In addition, the catalyst underwent notable structural evolution only in the presence of a CO2 pretreatment atmosphere (see Figure 1c), reinforcing the notion that in-situ gas-induced selectivity modulation stems from the sequential transformation pathway (CO2) → CO → C, with the CO disproportionation reaction constituting the fundamental mechanism driving the observed structural evolution of Co-based catalysts.

CO chemisorbs at the Co/MnO interfacial sites on the 2Co/MnO catalyst and readily dissociates into C atoms, which migrate onto the Co nanocluster surface. The O atoms from CO remain bound to MnO. These O atoms can either undergo disproportionation with another CO molecule (triggered by the interfacial sites) or react with H spillover generated from H2 activation on Co nanoclusters. These two mechanisms coexist, with their relative dominance depending on the reaction conditions. Both mechanisms maintain the catalytic activity of the interfacial sites and enable efficient C atom migration to Co nanocluster sites. This leads to the restructuring of Co nanoclusters, explaining the different behavior compared to other supports, such as TiO2, and the effect of Co loading on MnO. This behavior was evidenced by CO adsorption at 1884 and 1848 cm-1 on the 2Co/MnO catalyst (Figure 4c), attributed to the formation of Co-C-O-Mn μ2-bridging carbonyls at the Co/MnO interface, which is consistent with CO stretching frequencies of binuclear complexes52-54. These features are absent in unsupported MnO or 2Co/TiO2. The strong metal-support interaction (SMSI) leads to a weak CO adsorption vibration peak for the 2Co/TiO2 catalyst55. However, the bare Co surface observed in the EDS elemental mapping suggests that no SMSI effect is present before or after the reaction for the 2Co/MnO catalyst (Figures S9-S10), consistent with our previous results29,30. The μ2-bridging carbonyl can rearrange into a Co-C-O-Mn precursor structure, facilitating CO dissociation due to the synergistic effect of the carbon affinity of Co and the oxygen affinity of Mn. This suggests that the Co/MnO interfacial sites play a crucial role in driving the in-situ restructuring of Co nanoclusters.

The well-established carbon affinity of Co metal provides a preferred site for CO adsorption9 and a substrate for carbon material growth56. Co itself can undergo CO disproportionation to generate carbon57,58, while the formation of the Co/MnO interface significantly promotes this process due to the enhanced bridged Co-C-O-Mn adsorption configuration59. NO chemisorption DRIFT (NO-DRIFT) (Figures 4d,e and S28) and O2-H2 titration (Figure 4f) studies confirmed the oxygen affinity of MnO species. NO-DRIFTs revealed two distinct infrared vibrational peaks in the 1700-1900 cm-1 region, corresponding to linear (NO+) and bent (NO-) adsorption at the oxygen and metal sites, respectively60. With increasing NO coverage, both 2Co/MnO and 5Co/MnO catalysts exhibited varying degrees of blue shifts (Figures 4d and S28). In contrast, no shift was observed on the 2Co/TiO2 catalyst, directly demonstrating the oxygen-binding capability of MnO. Figure 4f presents O2-H2 titration results for 2Co/MnO and 2Co/TiO2 catalysts. After reduction at 500 oC, followed by O2 pretreatment at 50 oC and subsequent temperature-programmed H2 exposure, the 2Co/MnO catalyst displayed significantly more surface O adsorption sites than the 2Co/TiO2 catalyst. This confirmed the enhanced oxophilicity of MnO, which facilitated partial oxidation of Co species, thereby weakening NO adsorption and causing the observed blue shifts in NO-DRIFT experiments.

Density functional theory (DFT) calculations investigated the CO conversion path during the in-situ reaction process (Figures 5a-f and S29-S35) , revealing CO dissociation energy barriers on the Co nanoclusters (Co13), the MnO support with an oxygen vacancy (MnO), and the interface of Co13/MnO (Figure 5a). The results provided compelling evidence that the Co/MnO interface facilitates the formation of a Co-C-O-Mn structure, exhibiting the lowest activation energy barrier for C-O bond dissociation (2.05 eV), indicating that the interfacial sites critically enable efficient CO activation. At the Co13/MnO interface, the dissociation barrier for a second CO molecule is remarkably low (0.92 eV), much lower than the coupling barrier of 1.70 eV (Figure 5b). In contrast, the C-C coupling barrier on the isolated Co surface is 0.93 eV (Figure 5c). This indicates that CO dissociates rapidly at the interface into C* (surface-chemisorbed carbon atoms). With migration barriers of only 0.82 eV (interface-to-Co) and 0.21 eV (on Co surface), the diffusion of C* atoms is kinetically facile compared to the more demanding steps of CO dissociation and coupling (Figures S33-S34). This efficient diffusion provides a mechanistic basis for how interface-generated carbon atoms can populate the Co surface and, ultimately, form polymeric carbon species and trigger the restructuring of Co nanoclusters. Meanwhile, the O* from CO dissociation remains on the MnO surface. It is efficiently removed either by reacting with CO to form CO2 through a near-barrierless process (0.06 eV, Figure 5d) or via spillover H from Co nanoclusters. The latter pathway likely dominates under high-temperature, high-H2-pressure conditions.

The above DFT calculations are consistent with our above experimental observations and describe the in-situ transformation of CO molecules on the 2Co/MnO catalyst. Furthermore, the C-modified Co/MnO interface (C@Co13/MnO) was studied for CO dissociation and CO hydrogenation. Compared to unmodified interfaces (Co13/MnO), C@Co13/MnO exhibits higher energy barriers for CO dissociation (2.36 eV vs. 2.00 eV, Figure 5e) and CO hydrogenation (1.46 eV vs. 0.87 eV, Figure 5f), respectively. This shows carbon modification inhibits both CO dissociation and hydrogenation, aligning with the observed shift from CH4 to CO as the main catalyst product during CO2 hydrogenation. The pronounced oxophilicity of MnO served as the foundation for the Co-C-O-Mn generation, facilitating carbon deposition during CO2 hydrogenation (Figure 5g). Meanwhile, CO2 and H2 simultaneously remove excess surface oxygen species and maintain a dynamic equilibrium between carbon deposition and methanation. As the reaction progresses, carbon-modified Co active centres gradually emerge. Here, deposited carbon species reduce the adsorption of small molecules, such as CO and H2, on the metallic Co surface, thereby suppressing methanation in CO2 hydrogenation and shifting selectivity toward CO hydrogenation products. Conventional supports such as TiO2 exhibit weaker CO dissociation ability, enabling Co-dominated methanation at reaction temperatures.

In contrast, the properties of these reconstructed Co nanoclusters differ from those reported in the literature (i.e., Co-carbide or C-encapsulated Co nanoparticles12,44,61-64) as they uniquely suppress methanation activity while preserving CO conversion activity and stability. Previous studies have proposed that enhancing CO adsorption improves methane selectivity65,66. Despite inconsistent findings in the literature39,49, this study reveals an alternative mechanism for selectivity control in CO2 conversion. The Co/MnO interface simultaneously enhances CO adsorption and promotes in-situ generation of C atoms. This dual action triggers the dynamic restructuring of Co nanoclusters, suppressing methanation activity while maintaining key catalytic functions.

Methods
Synthesis of the catalysts

Preparation of MnO support. MnO was synthesised by the carbonate precipitation method 30. Specifically, a 1 M solution of manganese sulfate was slowly added dropwise to a 1 M solution of ammonium carbonate. After continuous stirring for 2 h, the mixture was filtered and washed. The obtained solid was then dried overnight at 60 oC and subsequently calcined at 450 oC in a muffle furnace for 3 h to obtain the MnO support.

Preparation of Co/support catalysts. The Co/support catalysts were synthesised by the impregnation method, with commercial SiO2, TiO2 (P25), and synthesised MnO serving as the supports, respectively. Taking the preparation of 2Co/MnO with 2 wt.% Co loading as an example, 0.980 g of MnO support was mixed with 0.099 g of cobalt nitrate hexahydrate in 30 mL of deionised water in a beaker. The mixture was continuously stirred in a water bath at room temperature for 1 h, then heated to 50 oC and stirred until no visible moisture remained. The resulting mixture was dried overnight at 60 oC and calcined at 350 oC (5 oC·min-1) for 3 h to obtain 2Co/MnO. Unless otherwise specified, the theoretical Co loading was 2 wt.%. For loading levels of 1 wt.% and 5 wt.%, the amount of cobalt nitrate hexahydrate was adjusted to 0.049 g and 0.247 g, respectively. The above-mentioned catalysts were labeled as 1Co/MnO, 2Co/MnO, 5Co/MnO, 2Co/SiO2, and 2Co/TiO2, respectively.

Characterisation

The crystalline structure of the samples was analysed by X-ray powder diffraction (XRD) on the X'pert-Pro diffractometer from PANalytical (Holland), using Cu Kα radiation at 40 kV and 40 mA. High-resolution transmission electron microscope (TEM) images were obtained using a JEM-2100 operated at 200 kV. The surface element valence analysis was conducted using semi-in-situ XPS on the Thermofisher Escalab 250 Xi+ with monochromated Al Kα radiation. The used catalyst was pretreated with CO2 + H2 for 5 h at 400 oC. In-situ synchrotron radiation XPS was performed at the BL02B beamline station of Shanghai Synchrotron Radiation Facility (SSRF). The 2Co/MnO catalyst was first reduced and reacted for 5 h in the reactor, then transferred to the XPS equipment and treated with a CO2 + H2 reaction gas at 400 °C for 1 h. The X-ray energy was 480 eV for surface analysis of C species. In-situ Raman spectroscopy measurements of 2Co/MnO during CO2 hydrogenation were conducted on a Renishaw Raman microscope with a 532 nm laser.

The adsorption and reaction properties of the catalysts were investigated using DRIFTs on a VERTEX 80V (Bruker). The catalysts were pre-reduced at 500 oC for 1 h in H2 and then purged in Ar for 30 minutes. Treatment process of CO2+H2-DRIFTs: Stabilised the reaction at room temperature, then programmed the temperature to rise and stabilised at 350 oC, followed by switching to H2 treatment, observing the characteristic changes in the infrared spectrum. Both H2 and Ar were highly pure, and the reacting gas was composed of 5% CO2, 15% H2, and 80% Ar.

The CO-DRIFTs and NO-DRIFTs experiments underwent the same pretreatment strategy, followed by cooling to room temperature. The corresponding spectral data were collected during CO and 5% NO/Ar adsorption for 30 minutes, respectively. The CO-DRIFTs were made by adsorption at room temperature, followed by a linear temperature increase to 350 oC. The TPH and TPO experiments on the catalysts were conducted by heating the fresh catalysts or those after reaction in 5% H2/Ar or 5% O2/He, and the CH4 or CO2 signals were analysed by mass spectrometry (MS). CO or H2 temperature-programmed desorption (CO-TPD or H2-TPD) was carried out using an AutoChem 2920 from Micromeritics Instrument Corp. with a mass spectrometer (MS) detector. The catalyst was reduced at 500 °C for 1 h, then swept with Ar for 30 minutes and cooled to room temperature. It was then pre-adsorbed with CO or H2, switched to Ar, and the CO, CO2, or H2 signals were recorded at programmed temperatures. For the O2-H2 titration experiments, 30 ml·min-1 of 5% O2/He was pre-adsorbed at 50 °C for 60 minutes, then switched to 5% H2/Ar for 60 minutes until the baseline stabilised. The MS signals of m/z=2 and m/z=18 were used to record the consumption of H2 and the formation of H2O as the reaction temperature increased linearly (at a rate of 10 °C·min ¹) from 50 °C to 600 °C.

DFT calculation

DFT calculation methods are described in the Supporting Information.

Catalytic evaluation

The catalyst performance was evaluated in a fixed-bed reactor with a 8 mm inner diameter quartz reaction tube. A 100 mg sample of the catalyst with a particle size of 40-60 mesh was pre-reduced at 500 oC for 2 h in H2. The reactant gas flow rate was set at 100 ml·min⁻¹, and the reactant gas composition, unless otherwise specified, was CO2 : H2 : N2 = 19%: 76%: 5%. The reaction products were CH4 and CO, and the gas-phase composition was detected using an Agilent gas chromatography (7820A) equipped with a TCD detector. The CO2 conversion (XCO2) and CO selectivity (SCO) were calculated as follows:

XCO2 = (FCO2,in - FCO2,out) / FCO2,in * 100% (1)

SCO = FCO,out / (FCO2,in - FCO2,out) * 100% (2)

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The work prepared by Kang et al. detailed investigated the Co/MnOx system in CO2 hydrogenation reaction. It is observed that 2%Co/MnOx underwent structural change due to carbon deposition during the reaction and the CO selectivity significantly increased along with the reaction. This is an interesting research which potentially provides some new insights about structure-function relationship and providing a potential method to alter the catalyst selectivity in CO2 hydrogenation reaction. However, there are still some issues in the manuscript and need to be addressed before the work can be considered for publication.\nHere are the detail comments:\n1. On page 9 “Though the fresh 2Co/MnOx catalyst exhibited Co nanoparticles with an average diameter of ~4.0 nm, a modest increase to 4.7 nm is observed after 18 h of time on stream at 400 oC (Figure 2e-2h)” It is questionable that nanoparticles with average diameter of 4~5 nm cannot be detected by XRD. In general the detection limit of the XRD is around 2~3 nm. I would suggest authors to measure/report the Co dispersion based on TPD results to make sure the Co dispersion is matching with the nanoparticle size observed by TEM.\n2. On page 13 “underscoring the role of MnOx in enhancing CO disproportionation; while 5Co/MnOx exhibits greater CO2 desorption than 2Co/MnOx alongside a lower disproportionation onset temperature of ~57 oC”. How to confirm the CO2 is only generated from Boudouard reaction? Is it possible that MnOx also provide oxygen and convert some of CO to CO2?\n3. On page 14, the DRIFTS results show that Co/TiO2 doesn’t have CO adsorption. This might come from the strong metal support interaction between Co and TiO2, which leads to the encapsulation of Co by TiO2. This might also be the reason of Figure 4f that Co/TiO2 has much weaker signal intensity compared to Co/MnOx.\n4. On page 14, “H2-TPD results (Figures 4b and S20) for the three catalysts revealed that 5Co/MnOx catalyst exhibited a much larger desorption peak area than 2Co/MnOx, indicating higher surface H concentration during the in-situ reactions,” This may originate from the larger Co surface area on 5Co/MnOx than 2Co/MnOx rather than “higher surface H concentration” on 5Co/MnOx.\n5. On page 14, “which enhances the consumption of deposited carbon species and reducees the overall carbon deposition efficiency” Evidence such as HRTEM should be provided to support the argument that 5Co/MnOx has less carbon deposition.\n6. The authors claim that “CO disproportionation reaction” (Boudouard reaction) occurred at the interface between Co and MnOx. However, this reaction can easily occur on Co nanoparticle without support (Faraday Discussions, 197, 337-351; Surface Science, 201(1-2), 185-194). Thus the authors may overemphasise the Co-MnOx interface for this reaction.\n7. The most significant concern is the structure of Co nanoparticle after reaction is explained insufficiently. After carbon deposition, does the Co nanoparticle chemical composition changed (i.e. forming CoCx)? If the Co nanoparticle is just covered by porous graphite without chemical composition change and the Co site is still accessible by CO2 and H2 molecules, then the mechanism behind the CO selectivity increase, especially the structure-function relationship of Co nanoparticle and Co/MnOx interface, needs to be detailed explained.", "reviewer_2": "This manuscript reports on the surface restructuring of 2Co/MnOx during CO₂ hydrogenation and the consequential increase in the CO/CH₄ ratio by approximately fifteen-fold. The study addresses an important catalytic problem, and the results are valuable. I recommend the manuscript for publication after major revision.\n  Clarity and Writing:\no The overall writing can be improved for conciseness and consistency. Clearer explanation of the mechanisms and terminology would make the manuscript more accessible to a broader audience.\n  Terminology – “Restructuring” vs. “Reconstruction”\no The manuscript currently uses the term reconstruction to describe changes in the catalyst surface. However, in surface science, reconstruction typically refers to the rearrangement of surface adsorbates on single crystals, particularly under UHV conditions.\no In contrast, restructuring is more appropriate when describing changes in the surface atomic structure under reaction conditions. I strongly recommend replacing reconstruction with restructuring throughout the manuscript to align with established terminology in catalysis and surface science.\n  Catalytic Performance and Economic Relevance:\no The authors report a significant change in catalytic performance, with a large increase in the CO/CH₄ ratio after surface restructuring. However, the manuscript does not adequately discuss the economic and practical implications of this change.\no If the major product becomes CO instead of CH₄, the main by-product is presumably H₂O. This raises the question of the value proposition: what is gained by converting CO₂ to CO using H₂? Since syngas generation often requires a balance between CO and H₂, the authors should justify the practical benefit of increasing the CO/CH₄ ratio.\no A short discussion on whether this shift is desirable from an application or industrial perspective would strengthen the impact of the work.\n  Spectroscopic Analysis and Terminology:\no In Figure S9, the term “Quasi-in-situ” is used. A more accurate description would be “Semi-in-situ”. Please adopt this terminology and define clearly what is meant by semi-in-situ.\n  Similarly, the technique described as NAP-AXPS should instead be referred to as AP-XPS, which is the more commonly accepted term in the literature,\no Additionally, please correct “Quais” to “In-situ.”\n  Relation to Prior Work:\no This study successfully confirms gas-induced surface restructuring at the nanoscale and even at the atomic scale. The authors may wish to strengthen the discussion by highlighting how their observations correlate with previous reports in the literature on surface restructuring\n  Limitations of the Current Study:\no One limitation is the absence of temperature-dependent AP-XPS data to directly monitor the surface/interface evolution during CO₂ hydrogenation over 2Co/MnOx. Such data could provide critical insights into the mechanism. The authors should explain why these experiments were not performed.\no Do the authors also have time-on-stream AP-XPS data? If available, these results would significantly strengthen the mechanistic claims. If not, please clarify this limitation in the discussion.", "reviewer_3": "The authors synthesized the Co/MnOx catalysts for CO2 hydrogenation and found the unique Co-C-O-Mn interfacial sites for selective CO production. It is interesting that the catalyst was dynamically changed with the reaction conditions especially the atmosphere. The reaction-driven structure was found to be prefer for CO rather than CH4. However, some questions need to be addressed, the details are as follows:\n1. In the text, why 5Co/MnOx has lower selectivity than 2Co/MnOx? How about the stability of these two catalysts? Will the dynamic change of the catalyst structure change every time after its production and reuse? (Fig3e)\n2. The manuscript attributes the high CO selectivity to the dynamic changes in catalyst structure, but according to the description in the article, the main reason should be the different pretreatment atmospheres of the catalyst (Fig 1c).\n3. Why Co13 was selected in the DFT calculation?\n4. In the Discussion part, it said “the 2Co/MnOx catalyst exhibited a pronounced selectivity shift from CH4 to CO in CO2 hydrogenation compared to other catalysts …”, However, From FigS19, it seems that 5Co/MnOx has the similar activity after 1500 min, and From Fig S2, 5Co/MnOx has higher CO2 conversion than 2Co/MnOx. Please explain this.\n5. On Page 14, line 328-329, Fig 4b and S20 can not explain “Co nanocluster surface favor methanation while peripheral edge sites drive CO dissociation and disproportionation”"}, "author_response": "Point-by-point response and action to the reviewers' comments.\nReviewer #1\nThe work prepared by Kang et al. detailed investigated the Co/MnO system in CO\nx 2\nhydrogenation reaction. It is observed that 2%Co/MnO underwent structural change\nx\ndue to carbon deposition during the reaction and the CO selectivity significantly\nincreased along with the reaction. This is an interesting research which potentially\nprovides some new insights about structure-function relationship and providing a\npotential method to alter the catalyst selectivity in CO hydrogenation reaction.\nthe work can be considered for publication.\nResponse: Thank you for recognizing our work. The carbon deposition phenomenon is\ngenerally undesirable in many reactions, as it can cover the active sites of catalysts and\nlead to catalyst deactivation. In this study, we discovered that carbon-modified active\ncenters effectively inhibit the adsorption and dissociation of CO. However, we show\nthat this would promote the selective transformation of CO hydrogenation. Therefore,\n2\nthe role of carbon deposition is as a selective poison that alters the catalytic behavior.\nThis finding deepens our understanding of the fundamental processes involved in CO\n2\nhydrogenation and also gives a new perspective on the effect of carbon deposition.\nAs detailed below, we have significantly improved the manuscript to address the\npoint raised. Additionally, we systematically investigated the effects of factors such as\nthe gas composition, material properties, and energy conditions on carbon deposition\nefficiency, enabling a controllable carbon deposition process. Our research also\nprovides insights for future studies to employ gas-induced strategies for nanostructure\nand atomic-level surface control of catalysts.\nWe have addressed all your concerns point by point and made revisions throughout\nthe manuscript to enhance its readability and scientific quality.\nComment 1: On page 9 \"Though the fresh 2Co/MnO catalyst exhibited Co\nx\nnanoparticles with an average diameter of ~4.0 nm, a modest increase to 4.7 nm is\nobserved after 18 h of time on stream at 400 oC (Figures 2e-2h)\" It is questionable that\nnanoparticles with average diameter of 4~5 nm cannot be detected by XRD. In general,\nthe detection limit of the XRD is around 2~3 nm. I would suggest authors to\nmeasure/report the Co dispersion based on TPD results to make sure the Co dispersion\nis matching with the nanoparticle size observed by TEM.\nResponse: Thank you for your comments. The intensity of the XRD diffraction peaks\nis highly dependent on factors such as metal particle size, crystallinity, and metal\nloading content. We re-prepared the 2Co/MnO catalyst and performed XRD testing\nx\n(Figure R1, R represents the figures for reviewing only), but still did not observe any\ndistinct Co species diffraction peaks, which further confirms the reliability of our\nexperimental results.\nFigure R1 (for reviewing only). XRD pattern of re-tested reduced 2Co/MnO catalyst\nx\n(Scan Speed: 5 o/min)\nAdditionally, H and CO are commonly used as probe molecules to evaluate the\n2\ndispersion of catalysts. However, after the reaction, the catalyst surface is modified by\ncarbon, which, as is well known, inhibits the adsorption process, especially for CO\nmolecules. Therefore, it's unreliable to use probe molecule adsorption to analyze the\nmetal particle size in this work. Similarly, for the TPD method.\nIn this study, electron microscopy is the only effective method for evaluating\nparticle size. Additionally, we have incorporated EDS elemental distribution analysis,\nwhich reveals that Co species consistently exist in nanoparticle form (Figures S9-S10).\nTogether with the TEM particle size distribution results (Figures 2e-2h), this further\nsuggests that the observed changes in the selectivity of Co/MnO catalysts are not\nx\ngoverned by the structure sensitivity of Co (Nature Catalysis, 2022, 5, 1051-1060).\nAction: The EDS elemental mappings of the 2Co/MnO catalyst before and after the\nx\nreaction were added, and the manuscript has been revised accordingly:\n\"The 2Co/MnO catalyst maintains Co species in nanoparticle form before and after the\nx\nreaction, as confirmed by EDS elemental mapping (Figures S9-S10). Cobalt-based\ncatalysts exhibit significant structure sensitivity in CO hydrogenation (Nature\nnanoclusters) favoring RWGS (Applied Catalysis B: Environmental, 2023, 324:\n122298), while larger Co nanoparticles promote CH production (Angewandte Chemie\nincreased CO selectivity is not due to variations in Co particle size.\"\nFigure S9. Different regions of STEM-HAADF images and corresponding EDS\nelements mapping of fresh 2Co/MnO catalyst.\nx\nFigure S10. Different regions of STEM-HAADF images and corresponding EDS\nelements mapping of 2Co/MnO catalyst after 5 hours' reaction.\nx\nComment 2: On page 13 \"underscoring the role of MnO in enhancing CO\nx\ndisproportionation; while 5Co/MnO exhibits greater CO desorption than 2Co/MnO\nx 2 x\nalongside a lower disproportionation onset temperature of ~57 oC\". How to confirm the CO is only generated from Boudouard reaction? Is it possible that MnOx also\n2\nprovide oxygen and convert some of CO to CO ?\n2\nhigh-temperature Ar, which reduced MnO to MnO and removed the surface-active\nx\noxygen. This fact evidences that the oxygen from MnO is not responsible for the\noxidation of CO to CO . The amount of CO is about half of the amount of CO\n2 2\nconverted, as expected from the Boudouard reaction, and different from what would\noccur in the case of CO oxidation. Additionally, an enhanced carbon deposition is\ndetected on the surface of the 2Co/MnO catalyst (as shown by TPH, TPO, TEM, etc.),\nx\ncompared to 5Co/MnO and 2Co/TiO₂. All these indications strongly support the\nx\noccurrence of CO disproportionation.\nAction: Additional discussion added to the revised manuscript:\n\"The CO may originate from either the disproportionation of CO (2CO → CO + C)\n2 2\nor the direct oxidation of CO (CO+O*→CO ). However, the catalyst undergoes high-\n2\ntemperature hydrogen reduction followed by inert gas purging, which removes surface-\nactive oxygen species. Additionally, the amount of CO formed is about half of that of\noccur in the case of CO oxidation.\nAdditionally, there is an enhanced presence of surface carbon species on the 2Co/MnO\nx\ncatalyst (as shown by TPH, TPO, TEM, etc.), compared to 5Co/MnO and 2Co/TiO\nx 2\ncases. All these indications strongly support the occurrence of CO disproportionation.\nThe CO production in CO-TPD experiments enables quantification of surface carbon\n2\ndeposition.\"\nComment 3: On page 14, the DRIFTS results show that Co/TiO doesn't have CO\n2\nadsorption. This might come from the strong metal support interaction between Co and\nTiO , which leads to the encapsulation of Co by TiO . This might also be the reason of\n2 2\nFigure 4f that Co/TiO has much weaker signal intensity compared to Co/MnO .\n2 x\nResponse: Indeed, there is an SMSI effect between Co/TiO , as reported in many\n2\nstudies. Due to this effect, CO does not adsorb and dissociate well on this catalyst.\nAccordingly, there is a significantly weaker CO adsorption on 2Co/TiO in Figure 4c\n2\ncompared to 2Co/MnO . However, when MnO is used as the support instead of TiO ,\nx x 2\nthe case is different. The additional EDS mapping results indicate that reduction and\nreaction do not induce the SMSI effect in the Co/MnO catalyst (Figures S9-S10).\nThe\nx\nweak CO adsorption is attributed in this case to the surface carbon modification\nproposed in this study. The additional DFT calculations indicate that strongly adsorbed\nCO dissociates on Co, forming a strong Co-C bond, which results in electron transfer\nfrom Co to C, thereby inhibiting the back-donation process during subsequent CO\nadsorption (Figure S15). Therefore, the results clearly demonstrate the different case of\n2Co/MnOx and why the behavior is not related to the SMSI effect.\nAdditionaly, Figure 4f shows a series of O -H titration experiments, where the pre-\n2 2\nreduced samples were treated with O at 50 oC, followed by a switch to an H\n2 2\natmosphere with a programmed temperature ramp. The oxygen affinity of the catalyst\nwas assessed by observing the generation of H O (or consumption of H ). The results\n2 2\nshow that the 2Co/MnO catalyst exhibited significantly higher H O generation and\nx 2\nhigher reaction temperatures compared to 2Co/TiO , indicating a stronger oxygen\n2\naffinity. Therefore, it further proves the substantial difference between the two catalysts.\nBased on our previous research, this strong oxygen affinity originates from the MnO\nsupport (Applied Catalysis B: Environment and Energy, 2024, 352: 124010.). This is\nthe reason for the Co-C-O-Mn bridge adsorption structure.\nAction: Additional discussion added to the revised manuscript:\n\"The strong metal-support interaction (SMSI) leads to a weak CO adsorption vibration\npeak for the 2Co/TiO catalyst (Nature Communications, 2018, 9: 4459). However, the\n2\nbare Co surface observed in the EDS elemental mapping suggests that no SMSI effect\nis present before or after the reaction for the 2Co/MnO catalyst (Figures S9-S10),\nx\nconsistent with our previous results (Nature Catalysis, 2023, 6: 1062-1072; Applied\nCatalysis B: Environment and Energy, 2024, 352: 124010)\"\nComment 4: On page 14, \"H -TPD results (Figures 4b and S20) for the three catalysts\n2\nrevealed that 5Co/MnO catalyst exhibited a much larger desorption peak area than\nx\n2Co/MnO , indicating higher surface H concentration during the in-situ reactions,\"\nx\nThis may originate from the larger Co surface area on 5Co/MnO than 2Co/MnO\nx x\nrather than \"higher surface H concentration\" on 5Co/MnO .\nx\nResponse: Our previous statement regarding the higher surface H concentration was\nindeed not precise. The correct explanation is that a higher Co loading provides more\nH₂ adsorption sites, leading to a higher surface H content. We have revised this\ndiscussion accordingly.\nAction: The revised manuscript has corrected the relevant statements:\n\"The 5Co/MnO catalyst, with a higher Co loading, offers more H adsorption and\nx 2\ndissociation sites than the 2Co/MnO catalyst. The larger desorption peak area observed\nx\nin the H -TPD results for this catalyst (Figures 4b and S25) is due to this effect. The\n2\nenhanced H activation causes the consumption of deposited carbon species and/or\n2\ninhibits overall carbon deposition, with both mechanisms contributing to the net lower\ncarbon deposition compared to the 2Co/MnO catalyst. \"\nx\nComment 5: On page 14, \"which enhances the consumption of deposited carbon\nspecies and reduces the overall carbon deposition efficiency\" Evidence such as\nHRTEM should be provided to support the argument that 5Co/MnO has less carbon\nx\ndeposition.\nResponse: In Fig. 3b, we conducted a temperature-programmed hydrogenation (TPH)\nexperiment on the post-reaction catalysts, evaluating surface-deposited carbon species\nby detecting CH signals. The results prove that the 5Co/MnO catalyst has a slower\n4 x\ncarbon deposition rate. The balance between CO disproportionation and H -induced\n2\ncarbon removal (or inhibition of CO disproportionation sites) determines the actual\ncarbon deposition rate. Fig. 4a shows that the 5Co/MnO catalyst has the best CO\nx\ndisproportionation activity, but the higher amount of H species on the surface\ndetermines a fast carbon removal. We have added Additional HR-TEM results for the\n5Co/MnO catalyst (Figure S18-S19) after the reaction, which proves this result. Under\nx\nthe same reaction time, there is clearly a lower amount of surface carbon species\ncompared to the 2Co/MnO catalyst.\nx\nAction: HR-TEM results for the 5Co/MnO catalyst after reaction were added in the\nx\nrevised manuscript, and text modified accordingly:\n\"Furthermore, the HR-TEM analysis of the 5Co/MnO catalyst surface after reaction\nx\nreveals only a small amount of onion-like carbon species (Figure S26), primarily\nconsisting of some amorphous carbon layers (Figure S27). The carbon amount after the\nsame reaction time (5 h) is significantly lower on the 5Co/MnO catalyst compared to\nx\nthe 2Co/MnO catalyst (Figures S18-S19).\"\nx\nFigure S19. TEM image and false-color graph of reacted 2Co/MnO (5 h).\nx\nFigure S26. TEM images and false-color graph of reacted 5Co/MnO (5 h) in different\nx\nregions.\nFigure S27. TEM images of reacted 5Co/MnO (5 h) in different regions.\nx\n6. The authors claim that \"CO disproportionation reaction\" (Boudouard reaction)\noccurred at the interface between Co and MnO . However, this reaction can easily\nx\noccur on Co nanoparticle without support (Faraday Discussions, 197, 337-351;\nSurface Science, 201(1-2), 185-194). Thus the authors may overemphasise the Co-\nMnO interface for this reaction.\nx\nResponse: CO can strongly adsorb onto the surface of Co nanoparticles and further\ndissociate into carbon species. In our previous manuscript, we referenced similar\nstudies that highlighted the strong carbon affinity of Co, making it widely applicable in\nCO conversion (ACS Catalysis, 2017, 7: 5289-5293) and carbon nanomaterial catalytic\ngrowth (CCS Chemistry, 2021, 3, 154-167). However, this process is also influenced\nby factors such as temperature, gas-phase composition, and the properties of the catalyst\nmaterial (e.g., crystal facets, interfaces, etc.) (ACS Catalysis, 2023, 13: 8551-8560). On\nthe other hand, if CO disproportionation occurs on the bare Co surface, rather than at\nthe interface with the support, it should be independent of the support and particle size.\nWe observed that under CO hydrogenation conditions, the Co/MnO interface\n2 x\nexhibited significant differences in selectivity compared to Co/TiO , Co/SiO , and other\n2 2\nsurfaces. By combining temperature-programmed methods, spectroscopy, and\ntheoretical simulations, we have proven that the strong Co-C-O-Mn adsorption\nstructure at the Co/MnO interface plays a positive role in C-O bond cleavage. Previous\nstudies on metal-support interfaces that enhance CO adsorption and dissociation also\nsupport this finding (ACS Catalysis, 2020, 10: 7894-7906; ACS Catalysis, 2015, 5:\n5888-5903).\nThis result does not conflict with previous literature (Faraday Discussions, 2017,\n197: 337-351; Surface Science, 1988, 201(1-2): 185-194), but rather represents an\nenhancement, emphasizing the critical role of specific interface characteristics in\npromoting carbon deposition. In contrast to conventional Co-based catalysts that\nexhibit stable methanation performance in CO hydrogenation reactions (Applied\nthe low-loading 2Co/MnO catalyst in this study demonstrates a unique shift in\nx\nselectivity, further highlighting the critical role of the interface.\nAction: Additional discussion added to the revised manuscript:\n\"Metallic Co itself can undergo CO disproportionation to generate carbon (Faraday\nDiscussions, 2017, 197: 337-351; Surface Science, 1988, 201(1-2): 185-194), while the\nformation of the Co/MnO interface significantly promotes this process due to the\nenhanced bridged Co-C-O-Mn adsorption configuration (ACS Catalysis, 2020, 10:\n7894-7906)\"\n7. The most significant concern is the structure of Co nanoparticle after reaction is\nexplained insufficiently. After carbon deposition, does the Co nanoparticle chemical\ncomposition changed (i.e. forming CoC )? If the Co nanoparticle is just covered by\nx\nporous graphite without chemical composition change and the Co site is still accessible\nby CO and H molecules, then the mechanism behind the CO selectivity increase,\n2 2\nespecially the structure-function relationship of Co nanoparticle and Co/MnO\nx\ninterface, needs to be detailed explained.\nResponse: No evidence for the formation of CoC was observed using various\nx\ncharacterization techniques. Particularly, the C species at 283.1 eV in the XPS spectrum\nare absent (Science Advances, 2023, 9: eadg0167). CoC is known to be an excellent\nx\ncatalyst for CO to olefin conversion, but its stability is a significant concern (Journal of\nCatalysis, 2020, 382: 86-96). As discussed in the manuscript, CoC typically\nx\ndecomposes around 260 oC (Science Advances, 2023, 9: eadg0167; ACS Catalysis,\n2021, 11: 809-819).\nThe very strong CO adsorption on Pt/FeO catalysts leads to CO dissociation into\nx\ncarbon (which exhibits low reactivity), thereby inhibiting the adsorption of subsequent\nmolecules (Surface Science, 2009, 603: 3099-3103; Chemical Society Reviews, 2023,\n52: 3627-3662). In this work, the Co-MnO interface exhibits similar behavior, where\nthe strong adsorption of CO leads to carbon modification of the Co active sites, which\nin turn suppresses the adsorption of both CO and H . This facilitates the rapid\n2\ndesorption of CO during the reaction and weakens the hydrogenation capacity, which\ncontributes to the change in CO hydrogenation selectivity. This, in turn, reduces the\n2\nnumber of active sites, as evidenced by the catalytic activity tests, where an evident\ndecline in conversion rate was observed. This result shows a classic trade-off dilemma\nin catalytic reactions - how to achieve both high conversion and high selectivity for the\nRWGS reaction. Nevertheless, despite the evident decrease in performance, our catalyst\nstill outperforms others reported in the literature (Table S1).\nYou have raised an excellent question regarding the interaction between carbon\nand cobalt and how it weakens the adsorption and dissociation of CO. DFT calculations\nwere employed to demonstrate that surface charge transfer in Co species after carbon\ndeposition inhibits subsequent CO adsorption. As shown in Figure S15a, the Bader\ncharge analysis indicates that the average charge of Co atoms in the C-deposited Co\ncluster is lower than that of the pristine Co cluster, suggesting partial electron transfer\nfrom Co atoms to the deposited carbon species. This electron depletion weakens the\nadsorption ability of Co toward CO molecules. Moreover, the projected density of states\n(PDOS) reveals that the d-states of Co in the clean cluster are closer to the Fermi level,\nimplying stronger orbital overlap and easier electron donation to the antibonding\norbitals of CO, thereby enhancing CO activation (see Figure S15b). Similarly, as shown\nin Figure S15c, the calculated adsorption energies show that CO adsorption on the C-\ndeposited Co cluster is weaker than that on the pristine Co cluster. We also evaluated\nCO adsorption on the next-nearest neighboring sites of the deposited carbon. Although\nthe adsorption at these sites is stronger than that at the nearest sites, it is still weaker\nthan that on the clean Co cluster. These results further confirm that surface charge\ntransfer induced by carbon deposition inhibits subsequent CO adsorption.\nAction: We added a series of statements to explain the above comments.\n\"Sun et al. (Surface science, 2009, 603: 3099-3103) reported that CO dissociation into\nsurface carbon species inhibited subsequent CO adsorption on the Pt/FeO catalyst due\nx\nto the excessively strong CO adsorption, which led to a low reactivity (Chemical\nSociety Reviews, 2023, 52: 3627-3662).\"\n\"There is also a similar Co XPS signal at 780.3 eV after carbon deposition (Figure S13c).\nHowever, due to the minimal number of interfacial Co-C-Mn sites, the sensitivity is\ninsufficient to prove that the average charge of Co atoms decreases after C deposition.\nThereby, DFT calculations were employed to demonstrate that surface charge transfer\nin Co species after carbon deposition inhibits subsequent CO adsorption (Figures S14-\nS15). Bader charge analysis reveals that the Co atoms in the C-deposited Co cluster\nhave lower average charges compared to the pristine Co cluster, indicating electron\ntransfer from Co to the carbon species (Figure S15a). This electron depletion weakens\nthe adsorption ability of Co toward CO molecules. The projected density of states\n(PDOS) in Figure S15b also shows that d-states of Co in the clean cluster are closer to\nthe Fermi level, allowing for stronger interaction and easier electron donation to\nantibonding orbitals of CO, thus enhancing CO activation. In Figure S15c, the\ncalculated adsorption energies indicate that CO adsorption is weaker on the C-\ndeposited Co cluster than on the pristine Co cluster. We also examined CO adsorption at next-\nnearest neighboring sites of the deposited carbon. While the adsorption is stronger at\nthese sites compared to the nearest sites, it is still weaker than on the clean Co cluster.\nThese results confirm that the charge transfer caused by carbon deposition inhibits CO\nadsorption.\"\nFigure S15. a) Bader charge analysis of pristine and C-deposited Co clusters. b)\nProjected density of states (PDOS) of Co atoms in pristine and C-deposited Co clusters.\nc) Calculated CO adsorption energies on pristine and C-deposited Co clusters.\nReviewer #2:\nThis manuscript reports on the surface restructuring of 2Co/MnO during CO\nx 2\nhydrogenation and the consequential increase in the CO/CH ratio by approximately\n4\nfifteen-fold. The study addresses an important catalytic problem, and the results are\nvaluable. I recommend the manuscript for publication after major revision.\nResponse: Thank you for your recognition of our work. We have provided point-by-\npoint responses to the issues you raised.\nComment 1: Clarity and Writing\n-The overall writing can be improved for conciseness and consistency. Clearer\nexplanation of the mechanisms and terminology would make the manuscript more\naccessible to a broader audience.\nResponse: Thank you for your feedback. We have made comprehensive revisions to\nthe entire paper to achieve a more accurate and scientifically rigorous presentation.\nComment 2: Terminology – \"Restructuring\" vs. \"Reconstruction\"\n- The manuscript currently uses the term reconstruction to describe changes in the\ncatalyst surface. However, in surface science, reconstruction typically refers to the\nrearrangement of surface adsorbates on single crystals, particularly under UHV\nconditions.\n- In contrast, restructuring is more appropriate when describing changes in the surface\natomic structure under reaction conditions. I strongly recommend replacing\nreconstruction with restructuring throughout the manuscript to align with established\nterminology in catalysis and surface science.\nResponse:\nWe intended to describe how the atmosphere induces the formation of carbon-modified\nCo active centers, thereby altering the local chemical environment of Co. However, we\nagree that this process does not involve atomic rearrangement of the catalyst itself, and\nthe use of \"Reconstruction\" was not entirely accurate. We have revised the relevant\nphrasing to atmosphere-induced Co structure \"Restructuring\".\nAction: A full-text replacement was performed.\nComment 3: Catalytic Performance and Economic Relevance:\n- The authors report a significant change in catalytic performance, with a large\nincrease in the CO/CH₄ ratio after surface restructuring. However, the manuscript does\nnot adequately discuss the economic and practical implications of this change.\n- If the major product becomes CO instead of CH , the main by-product is presumably\n2 2\nand H₂, the authors should justify the practical benefit of increasing the CO/CH ratio.\n4\n- A short discussion on whether this shift is desirable from an application or industrial\nperspective would strengthen the impact of the work.\nResponse: CH is an important fuel and a fundamental chemical (Nature Synthesis,\n4\n2025, 4, 288-302), while CO is a key platform molecule utilized in established\nprocesses such as Fischer-Tropsch synthesis (Science, 2025, 387: eadn9388).\nSpecifically, CO, as a platform molecule, integrates well with existing mature industries\nlike methanol and Fischer-Tropsch synthesis, facilitating the production of widely used\nchemicals, gasoline, diesel, aviation fuels, and more. Regardless of the route taken,\nthere is significant potential in various practical applications. However, the critical\nchallenge remains improving the selectivity of CO hydrogenation to reduce separation\n2\ncosts. We believe that improving selectivity is highly important, and the approach of\natmosphere-induced modulation of the catalyst's surface microenvironment also has\npractical significance. We have expanded on this topic in the revised Introduction.\nAfter selectivity improvement, the main by-product is H O, whose low boiling point\n2\nfacilitates rapid separation from the reactor outlet. In the reaction CO + H → CO +\n2 2\nH O, one molecule of CO reacts with one molecule of H to produce one molecule of\n2 2 2\nCO and one molecule of H O. By appropriately increasing the number of reaction\n2\ncycles, near-complete conversion can be achieved. Excess H not only drives the\n2\nreaction forward but also combines with CO to form syngas, thereby enhancing the\npotential industrial applicability of the process.\nAction: Additional discussion added to the revised manuscript:\n\"Enhancing the CO/CH ratio during CO hydrogenation provides a crucial foundation\n4 2\nfor subsequent Fischer-Tropsch and methanol industries that utilize CO as a key\nplatform molecule (Science, 2025, 387: eadn9388; Nature Synthesis, 2025, 4, 288-302).\nThis depends on the controlled modulation of the catalyst's surface microstructure.\"\nComment 4: Spectroscopic Analysis and Terminology:\n-In Figure S9, the term “Quasi-in-situ” is used. A more accurate description would be\n\"Semi-in-situ\". Please adopt this terminology and define clearly what is meant by semi-\nin-situ.\nResponse: Thank you for your comment. We carefully compared the differences\nbetween Quasi-in-situ and Semi-in-situ and found that there is indeed some confusion\nbetween the two in the literature. In our actual tests, to restore or analyze the catalyst\nafter reaction, we transferred the catalyst to the testing chamber via a vacuum transfer\nsystem, avoiding exposure to the external environment. This approach is more\nconsistent with the Semi-in-situ method. We sincerely appreciate your rigorous\nattention to scientific details.\nComment 4: Relation to Prior Work:\n-This study successfully confirms gas-induced surface restructuring at the nanoscale\nand even at the atomic scale. The authors may wish to strengthen the discussion by\nhighlighting how their observations correlate with previous reports in the literature on\nsurface restructuring.\nResponse: Thank you for your insightful comment. Our previous work has\ndemonstrated H -induced spontaneous dispersion of TiO on MnO (Nature Catalysis,\n2 2\n2023, 6: 1062-1072), CO hydrogenation gas-induced dispersion of In O on TiO\n2 2 3 2\n(Molecular Catalysis, 2025, 577: 114979), and other related unpublished studies,\nhighlighting the potential of using gas-induced structure evolution for catalyst design.\nWhile approaches such as encapsulation (Science, 2023, 380: 644-651), carbonization\n(Nature Communications, 2024, 15: 3100), and surface segregation (Nature Catalysis,\n2020, 3: 411-417) have been explored, a systematic understanding linking atmospheric\nconditions to structural evolution is still lacking.\nTo systematize this approach as a catalyst construction strategy, it is necessary to\nestablish a more comprehensive database to enable the controllability and predictability\nof structural evolution during atmosphere-induced processes. This requires a systematic\nunderstanding of the relationship between atmospheric conditions (e.g., gas\ncomposition, energy conditions, and material properties) and structural evolution.\nThis\nmanuscript serves as a demonstration of such an approach. This study demonstrates this\nconcept. For example, at 300-500 oC, the Co/MnO interface shows strong oxygen and\ncarbon affinity, promoting CO dissociation and carbonization to form C-modified Co\nactive sites. Developing a comprehensive database that correlates atmosphere-induced\nconditions with structural changes could establish a robust top-down strategy for\ncontrolling catalyst structures at the nano- and atomic scales.\nAction: Additional discussion added to the revised manuscript:\n\"A systematic study of the relationship between atmosphere-induced conditions (such\nas gas composition, energy condition, and material properties) and structural evolution\nis a fundamental basis for achieving controllable regulation of catalyst surface\nnanostructures through atmospheric conditions.\"\nComment 5: Limitations of the Current Study:\n- One limitation is the absence of temperature-dependent AP-XPS data to directly\nmonitor the surface/interface evolution during CO hydrogenation over 2Co/MnO .\n2 x\nSuch data could provide critical insights into the mechanism. The authors should\nexplain why these experiments were not performed.\n- Do the authors also have time-on-stream AP-XPS data? If available, these results\nwould significantly strengthen the mechanistic claims. If not, please clarify this\nlimitation in the discussion.\nResponse: We have previously conducted semi-in situ XPS (Figure S12a) and constant-\ntemperature synchrotron XPS (Figure S12b) experiments with the primary aim of\nobserving whether any additional species (such as metal carbides, 283.1 eV) are\ngenerated during the reaction. However, no significant signals were detected. Therefore,\nperforming temperature-dependent or time-evolution XPS experiments to verify the\nabsence of these species further would not provide additional insights. Moreover, the\ngas-phase pressure and flow rate during the atmospheric induction process have a\nsignificant impact on surface carbon deposition. Currently, the AP-XPS technique is\nunable to operate under conditions that closely resemble real reaction conditions (1 bar,\n100 mL/min), making it challenging to obtain results of significant reference value.\nWe also investigated the effect of reducing the pretreatment gas flow rate to 20\nmL/min for 6 hours (Figure S2, for reviewing only). After switching to a 100 mL/min\nreaction gas flow, the performance test revealed that the catalyst still exhibited low CO\nselectivity, indicating that the flow rate directly influences the surface carbon deposition\nprocess. This limitation hinders the effective execution of in situ experiments.\nFigure R2 (for reviewing only). Performance of the 2Co/MnO catalyst after 6 hours\nx\nof pretreatment with a low flow rate (20 mL/min) of reaction gas. The performance test\nconditions are as follows: flow rate of 100 mL/min; reaction gas composition: 19% CO ,\n2\n76% H , and 5% N .\n2 2\nReviewer #3:\nThe authors synthesized the Co/MnO catalysts for CO hydrogenation and found the\nx 2\nunique Co-C-O-Mn interfacial sites for selective CO production. It is interesting that\nthe catalyst was dynamically changed with the reaction conditions especially the\natmosphere. The reaction-driven structure was found to be prefer for CO rather than\nCH . However, some questions need to be addressed, the details are as follows:\nto each of the points you raised.\nComment 1: In the text, why 5Co/MnO has lower selectivity than 2Co/MnO ? How\nx x\nabout the stability of these two catalysts? Will the dynamic change of the catalyst\nstructure change every time after its production and reuse? (Fig3e)\nResponse: Cobalt-based catalysts show structure sensitivity in CO hydrogenation\n2\nreactions. Larger Co particle sizes are associated with higher CH selectivity\n4\n(Angewandte Chemie, 2023, 135(52): e202314274). Additionally, increased Co\nloading enhances the hydrogenation capacity (Figure 4), facilitating more efficient\nhydrogenation of CO to CH (Chem Catalysis, 2022, 2: 610-621). Therefore, the\n4\n5Co/MnO catalyst, with an average Co particle size of 4.8 nm (Figure S23),\nx\ndemonstrates higher CH selectivity (and lower CO selectivity) compared to the\n4\n2Co/MnO catalyst, which has an average Co particle size of 4.0 nm (Figure 2f).\nx\nIn fact, during continuous CO hydrogenation, both catalysts not only show\n2\nincreased CO selectivity but also a decrease in CO conversion. Persistent carbon\n2\ndeposition leads to the coverage of active sites, resulting in a loss of activity. To further\nillustrate this, we have added catalytic results of the 2Co/MnOx sample after 100 hours\nof continuous CO hydrogenation (Figure R3, for reviewing only). The results show a\n2\ncatalyst (Figure S24) shows significant deactivation after 50 hours of reaction, with a\nnotable decrease in conversion rate.\nThis manuscript does not claim that carbon deposition is inducing deactivation.\nStill, it shows that moderate carbon deposition may actually be beneficial for certain\nreactions (such as the RWGS reaction). Therefore, it proves that carbon deposition\nmodifies the active center for CO hydrogenation, while understanding of this effect\n2\nopens new possibilities for other reactions as well. For instance, previously observed\nstructural changes such as sintering, dispersion, carbonization, and sulfur poisoning can\nbe leveraged to build new catalysts, showing great potential. As you rightly pointed out,\nthe catalytic stability of using an atmosphere-induced structural evolution strategy for\nother target reactions is a critical issue that must be carefully considered. This will\nundoubtedly be a focus in our future work.\nFigure R3 for reviewing only. Catalytic performance of the 2Co/MnO catalyst over\nx\na continuous 100-hour period at 400 oC.\nIn Figure 3e, we observe that the freshly reduced catalyst (by H ) undergoes carbon\n2\nre-deposition during CO hydrogenation (or in a CO -containing atmosphere), which\n2 x\nleads to a change in selectivity. After high-temperature H re-treatment, the surface\n2\ncarbon is removed, restoring the catalytic performance, thereby forming a dynamic\ncycling process. This phenomenon highlights the critical role of modulating the\ncatalyst's surface microstructure by controlling the reactive atmosphere. It further\nsupports the potential for future systematic studies exploring the controlled induction\nof structural evolution under reactive gas conditions.\nAction: Additional discussion added to the revised manuscript:\n\"The 5Co/MnO catalyst has a lower initial CO concentration under reaction conditions\nx\ncompared to the 2Co/MnO catalyst, which was attributed to its higher Co loading and\nx\nlarger metal particle size, both of which facilitated the CO methanation process\n2\n(Angewandte Chemie, 2023, 135(52): e202314274). For longer times on stream and\namount of surface carbon deposition, the 5Co/MnO catalyst shows high CO selectivity\nx\ncomparable to that of the 2Co/MnO catalyst. This highlights the importance of\nx\nsynergistically controlling the gas composition and material properties in governing the\nstructural restructuring process.\"\nComment 2: The manuscript attributes the high CO selectivity to the dynamic changes\nin catalyst structure, but according to the description in the article, the main reason\nshould be the different pretreatment atmospheres of the catalyst (Fig 1c).\nResponse: We agree that we were unclear. The structural evolution induced by\natmospheric conditions should be considered as a top-down catalyst regulation\napproach, rather than a dynamic change of the catalyst itself. We indicated \"dynamic\"\nbecause it is a way to induce partially reversible changes through control of the reactive\natmosphere (thus, in a dynamic manner after preparation). Still, we agree that the term\nmay lead to misunderstanding. We have revised the manuscript accordingly.\nThe catalyst pretreatment is a crucial step in the CO hydrogenation process. The\n2\nstructural changes in the catalyst can be utilized as a regulatory approach. However,\nunderstanding this effect may allow us to use these gas-induced changes not only as a\npretreatment but also as a way to create in-situ controlled changes in the surface nature,\neven after the initial pretreatment. This is why we called it dynamic control. However,\nin this work, we used it only as a pretreatment and thus it is correct to limit the\ndiscussion to the presented results. In perspective, we believe that understanding the\nrelationship between atmospheric conditions (such as the type of atmosphere, energy\nconditions, and material properties) and the structural changes of the catalyst provides\na valuable tool for controlling the catalyst performances.\nAction: Optimized the expression in the revised manuscript.\nComment 3: Why Co was selected in the DFT calculation?\n13\none of the most stable motifs among the small Co clusters, containing both bulk-like\n(center atom, CN≈12) and low-coordinated surface sites (outer atoms, CN≈6). For\ninstance, Ranber Singh et al. reported that M (M = Fe, Co, Ni) clusters favor\n13\nicosahedral structures, where surface atoms exhibit nearly linear increases in local\nmagnetic moments while the central atom shows a nonmonotonic variation,\nhighlighting the coordination and topology dependent magnetic behavior (Physical\nReview B, 2008, 78(24): 245404). Li et al. demonstrated that the icosahedral Co\n13\ncluster, with complementary Co0 and Cox+ sites, enables ethanol dehydrogenation, H\n2\nformation, C-C cleavage, and coke oxidation in a synergistic manner; meanwhile, the\nunique structural-electronic features of Co , reinforced by strong Co-CeO\n13 2-x\ninteractions, were shown to be key to acetic acid formation, ESR activity enhancement,\nand coke resistance (ACS Catalysis, 2019, 9(3): 2355-2367).\nSecondly, the Co structure corresponds to a so-called \"magic-number\" cluster with\n13\nenhanced structural stability, which has been widely employed in the previous DFT\nstudies for transition-metal catalysis. For example, Zhou et al. employed the Co\n13\ncluster model to elucidate the electronic structure characteristics, geometric\ndeformation effects, and energy barrier polysulfide conversion, providing mechanistic\ninsights into the size-dependent activity observed experimentally (Small Methods, 2021,\n5(10): 2100571). Yang et al. further showed that Mo dopants modulate the electronic\nstructure of Co cluster based DFT calculations, optimizing *OOH adsorption and\n13\nsuppressing H O reduction, thereby guiding the design of other transition metal-doped\n2 2\nCo catalysts (Advanced Materials, 2025, 37(17): 2416401). In addition, Wang et al.\ninvestigated Au-coated transition-metal clusters. They demonstrated the crucial role of\ncore–shell interactions, with [Mn @Au ]⁻ exhibiting enhanced magnetism while\n13 20\n[Co @ Au ]⁻ showed reduced magnetism (Journal of the American Chemical Society,\n13 20\n2007, 129(14): 4110-4111).\nThirdly, the size of the icosahedral Co cluster provides a good balance between the\n13\ncomputational cost and calculated accuracy. It provides diverse coordination sites and\ncaptures cobalt's spin polarization effects, while remaining tractable for systematically\nstudying the species adsorption and reaction pathways.\nWe thus believe that the icosahedral Co cluster is a representative model for\n13\ninvestigating species adsorption and reaction processes over transition-metal\nnanoclusters and is well-suited to model our specific case.\nComment 4: In the Discussion part, it said \"the 2Co/MnO catalyst exhibited a\nx\npronounced selectivity shift from CH to CO in CO hydrogenation compared to other\n4 2\ncatalysts …\", However, From Fig S19, it seems that 5Co/MnO has the similar activity\nx\nafter 1500 min, and From Fig S2, 5Co/MnO has higher CO conversion than\nx 2\n2Co/MnO . Please explain this.\nx\nResponse: Thank you for your detailed reading and comments. The original manuscript\nwas not sufficiently precise. We intended to emphasize that, within the same reaction\ntime, the 2Co/MnO catalyst shows a more pronounced shift in selectivity compared to\nx\nother catalysts. Still, this point was not clearly stated in the original text. In fact, after\nextending the reaction time beyond 1600 minutes, the CO selectivity of the 5Co/MnO\nx\ncatalyst also increases significantly. Similar behavior was observed with Co/SiO and\neffectively accelerate the C-O bond cleavage and carbon deposition processes.\nAction: Revised text in the manuscript:\n\"We reported here the structural evolution and reactivity relationships of supported Co-\nbased catalysts during CO hydrogenation, where the 2Co/MnO catalyst exhibited a\n2 x\npronounced selectivity shift from CH to CO in CO hydrogenation at the same reaction\n4 2\ntime compared to other catalysts (5Co/MnO , 2Co/TiO , and 2Co/SiO ).\"\nx 2 2\nComment 5: On Page 14, line 328-329, Fig 4b and S20 can not explain \"Co\nnanocluster surface favor methanation while peripheral edge sites drive CO\ndissociation and disproportionation\"\nResponse: We agree that the original description was not clear enough. This statement\noriginally referenced findings from the literature, where DFT simulations revealed that\nthe edge sites of Co nanoclusters are capable of facilitating CO dissociation and\ndisproportionation, forming carbon and CO-covered sites, while the planar sites can\ndissociate H to hydrogenate surface carbon species (ACS Catalysis, 2017, 7: 5289-\n2\n5293). This is conceptually similar to our observations in this manuscript, where we\nsuggest that the Co-MnO interface enhances CO dissociation. In contrast, H₂\ndissociation on the Co metal surface maintains a dynamic equilibrium between CO\ndissociation and H -induced carbon removal. We revised to the corresponding phrasing\n2\nto improve clarity and accuracy.\nAction: Revised statement in the manuscript:\n\"The H/C balance on this catalyst determines the carbon deposition efficiency, similar\nto how the Co nanocluster surface favors H dissociation, while the peripheral edge\n2\nsites drive CO dissociation and disproportionation processes (ACS Catalysis, 2017, 7:\n5289-5293).\""}, {"round": 2, "reviewer_comments": {"reviewer_1": "I appreciate the efforts from the authros. All of my comments/concerns have been clearly addressed. I recommend this manuscript to be publishedn in Nature Communication.", "reviewer_2": "Authors have addressed my comments in the revised manuscript. I don't have further comments. I recommend publishing the manuscript.", "reviewer_4": "The manuscript by Kang et al. reports a combined experimental and DFT study on CO₂ hydrogenation over a 2 wt% Co/MnOₓ catalyst. The authors claimed that reaction-driven surface restructuring of Co nanoclusters caused the CO/CH₄ selectivity to shift from 0.89 to 13.4 under identical reaction conditions. The authors employed a suite of characterization techniques, including DRIFTS, TPH/TPO, TPD, microscopy, surface analysis, and DFT calculations, to justify that polymeric surface carbon species formed at Co–MnOₓ interfacial sites resulted in the reconstruction of the Co surface, which suppressed CO hydrogenation while maintaining CO₂ conversion.\nWhile the study is comprehensive, I do not think the quality and novelty of the manuscript justify its publication in Nature Communications.\n1. Why Co/MnOx catalyst? As the authors indicated at the beginning of their manuscript, Co-based catalysts are known for their activity in FTS and CO2 hydrogenation, producing long-chain hydrocarbons. It has been reported that the carbided Co surface during the reaction facilitated C-chain growth. In fact, the authors showed an accessible C-C coupling barrier (0.93 eV) under the reaction conditions. What is the advantage of quenching the reaction at CO on a catalyst that can catalyze carbon coupling to produce more valuable products? The previous reviewer raised similar concerns, but I do not think the authors’ responses addressed the concerns.\n2. The activity of Co-based catalysts for CO2 hydrogenation is generally low, and the authors acknowledged that the increased CO selectivity is at the expense of the decreased CO2 conversion. On the other hand, the Ni-based catalysts exhibited high activity for CO2 hydrogenation and can be engineered to have a high RWGS selectivity. There are abundant studies in the literature, but the author did not compare the activity of the Co/MnOx catalyst with that of state-of-the-art RWGS catalysts.\n3. The DFT calculation used MnO as the substrate to model MnOx, but did not offer a convincing justification. Furthermore, the p(2x2) supercell (~9x9 A in lateral direction) for the DFT calculations appears too small for the Co13 cluster on the MnO interface. The experiments also did not justify such a high Co loading.\n4. According to the authors, C atoms were generated at the interface and migrated to cover the Co surface. However, the authors did not quantify the amount of C on Co. What are the diffusion barriers for the C atoms to migrate to the metallic Co? Note that the Co particles in the experiment have an average size of ~4 nm. How feasible is it for C to reach a critical coverage to block further hydrogenation?"}, "author_response": "Response: We thank you for your review and for recognizing our work.\nResponse: We thank you for your review and for recognizing our work.\nResponse: Thank you for your careful review and for recognizing the study's comprehensiveness. The key point of the manuscript, and its novelty, lies in demonstrating that it is possible to manipulate catalytic performance through C-induced reconstruction of the Co surface, shifting from methanation to reverse water-gas shift. This is a key novel aspect that, to our knowledge, has not been proven before. It is essential not only for the specific reaction but also for catalysis in general, for example, in understanding and controlling performances of FT Co-based catalysts, where the reaction of methanation is a critical issue. The concept we presented and proved thus has general interest for the chemistry/catalysis community, and all the reviewers did not contest its novelty. Regarding quality, we think that the extensive characterization and modelling have proven the mechanism, and other reviewers agree. We thus redress the comment of insufficient quality and novelty of the manuscript, which, in our opinion, is not sustained by clear evidence.\nAs highly efficient CO /CO hydrogenation catalysts, Co-based catalysts offer\n2\nsignificant value for precisely controlling reaction processes through fine-tuning of\ntheir surface structures. Carbon deposition is a major cause of deactivation in industrial\ncatalysts, and it generally requires a rational catalyst composition and structural design\nto suppress it and enhance catalytic stability. In this work, by coupling reaction\natmospheres with tailored Co/MnO catalysts, we modulated the catalytically active\nx\nsites, transforming a conventional methanation catalyst into a reverse water-gas shift\ncatalyst. A novel configuration with carbon-modified Co active centers was established\nfor the RWGS reaction, providing catalyst support for the resource utilization of CO\n2\nvia CO platform molecules. Furthermore, through systematic investigation into the\neffects of gas-induced conditions, including gas composition, material properties, and\nenergy conditions, on the surface carbon deposition process, this study offers a research\nparadigm for controllable gas-induced structural evolution in catalyst design. This\napproach encourages researchers to better utilize structural changes in catalysts under\nreactive/atmospheric conditions (e.g., sintering/dispersion, carbon deposition/removal,\nencapsulation/decapsulation) and to adopt it as a top-down catalyst design strategy,\nrather than focusing solely on catalyst stability.\nComment 1. Why Co/MnO catalyst? As the authors indicated at the beginning of their\nx\nmanuscript, Co-based catalysts are known for their activity in FTS and CO\n2\nhydrogenation, producing long-chain hydrocarbons. It has been reported that the\ncarbided Co surface during the reaction facilitated C-chain growth. In fact, the authors\nshowed an accessible C-C coupling barrier (0.93 eV) under the reaction conditions.\nWhat is the advantage of quenching the reaction at CO on a catalyst that can catalyze\ncarbon coupling to produce more valuable products? The previous reviewer raised\nsimilar concerns, but I do not think the authors' responses addressed the concerns.\nResponse: We believe the comment is incorrect and stems from a misunderstanding of\nthe objectives. The key point of the manuscript is to demonstrate that Co nanoparticles\ncan alter their intrinsic reactivity through carbon deposition, distinct from carbide\nspecies formation. This concept of manipulating surface reactivity by forming carbon\nspecies at the interface with the support is of general interest for catalysis beyond the\nspecific example discussed before, such as in the case of FT catalysts. Thus, it is a\nwrong question to ask what the advantage of quenching the reaction at CO on a catalyst\nthat can catalyze carbon coupling. First, we shift the behavior from methanation (which\nis not a C-C reaction) to RWGS. Second, the aim is to understand how this can be\nachieved by the formation of special C-based sites at the interface. These are not carbide\nspecies. Thus, asking why Co/MnO catalyst is incorrect. The aim is to improve\nx\nunderstanding of these systems, which are relevant for a series of applications beyond\nthe specific application studied. Thus, not to develop novel catalysts.\nAs you have noted, cobalt-based catalysts are widely employed in CO /CO\n2\nhydrogenation reactions. However, the dynamic structural changes of these catalysts\nunder reaction conditions remain poorly understood, particularly how such structural\nvariation modulates product selectivity. The research focuses primarily on structure-\ndependent reaction behavior rather than on targeted product selectivity. We showed that\nreaction-driven carbon deposition suppresses CO hydrogenation while simultaneously\npromoting the reverse water-gas shift (RWGS) reaction. However, rather than merely\ntreating carbon accumulation as a conventional trigger for catalyst deactivation, we\nhave, through systematic characterization of its properties, repurposed it as an effective\ntool for structural modulation of catalysts. On this basis, we propose that a catalyst\nstructure deactivated toward one reaction may serve as an excellent active site for\nanother. This concept can be readily extended to other studies involving reaction- or\ngas-induced structural evolution. By gaining a deeper understanding of the reaction-\ninduced structural characteristics and corresponding reaction performance, the mutual\ncompatibility between structure and reactivity can be realized, an approach we define\nas the gas-induced structural evolution strategy. In addition, we are currently\nundertaking further in-depth investigations into the mechanisms and controllability of\nsuch structural evolution in related research fields.\nTo further address the reviewer's concerns, we identify CO as a key component in\ncarbon deposition through a series of controlled experiments performed under diverse\natmospheric conditions (see results in Figs. 1c, S4, and S5). Further results showed that\nthe strong carbon affinity of Co and the strong oxygen affinity of MnO promote the\nformation of a Co-C-O-Mn bridge-adsorption configuration (Fig. 4). This, in turn,\ndrives C-O bond cleavage and carbon-carbon coupling, leading to the formation of\npolymeric carbon species. Simultaneously, an appropriate cobalt loading fosters an\noptimal surface hydrogen concentration, enabling a dynamic equilibrium between CO\ndisproportionation into carbon and carbon removal by hydrogen etching, thereby\nenabling a more efficient carbon deposition process. This effectively weakens CO\nadsorption, thereby shifting selectivity. These results highlight the significance of the\n2wt%Co/MnO catalyst, as such phenomena were less pronounced on the\nx\n5wt%Co/MnO , 2wt%Co/TiO , or 2wt%Co/SiO catalysts. Although this process\nx 2 2\nsacrifices CO conversion, it effectively improves CO selectivity and still offers more\n2\nefficient RWGS performance than existing Co-based catalysts (Table S1). The efficacy\nof carbon deposition on a catalyst is determined by how it adsorbs and dissociates the\ninduced atmosphere (CO and H ), as well as by the consequent chemical potentials of\n2\nsurface carbon and hydrogen.\nAction: Additional discussion and references have been added in the Introduction\nsection.\n\"This strategy is particularly advantageous as it can be seamlessly integrated into\nexisting industrial frameworks (Joule, 2025, 9, 102163).\"\n\"A catalyst structure that is deactivated toward one reaction may serve as an excellent\nactive site for another reaction. In other words, strategically designing the reaction\natmosphere to induce the specific structure required for a target reaction represents a\nvaluable catalyst design principle with industrial potential.\"\nComment 2. The activity of Co-based catalysts for CO hydrogenation is generally low,\n2\nand the authors acknowledged that the increased CO selectivity is at the expense of the\ndecreased CO conversion. On the other hand, the Ni-based catalysts exhibited high\n2\nactivity for CO hydrogenation and can be engineered to have a high RWGS selectivity.\nof the Co/MnO catalyst with that of state-of-the-art RWGS catalysts.\nx\nResponse: As already mentioned in previous replies, the aim is not to develop superior\nRWGS catalysts, but to demonstrate a novel mechanistic aspect of catalyst in situ\ndynamics and to show how to manipulate their behavior. It is thus not relevant to prove\nthat they are the most active RWGS catalysts, even if their performance is excellent.\nThus, there is no need to investigate different catalysts. It is not only relevant to develop\nsuperior catalysts, but also to base their development on understanding and definition\nof new aspects relevant outside the specific reaction investigated.\nIt is worth noting that we mainly aim to uncover the distinctive role of carbon\nmodification in boosting CO selectivity over Co-based active sites and to develop a\nresearch paradigm for the rational design of catalysts through controlled gas-induced\nstructural evolution. This contrasts with the prevailing view that weakening CO\nadsorption is essential for promoting CO desorption and achieving high CO selectivity\nin CO₂ hydrogenation catalysts (e.g., Chem. Sci., 2021, 12, 14660-14673; J. Am. Chem.\nSoc., 2020, 142, 5184-5193). The reduction in CO conversion observed as product\n2\nselectivity transitions from CH to CO is fundamentally governed by the\n4\nthermodynamic properties of the reactions involved. The RWGS reaction is\nendothermic and thus favored by elevated temperatures, whereas the methanation\nreaction is exothermic and inherently characterized by a high conversion rate. At 400 °C,\nthe equilibrium conversion of the RWGS reaction is only 42%, whereas that of the CO\n2\nmethanation reaction is 85.2%.\nIndeed, the RWGS performance of the as-synthesized Co-based catalysts does not\ncompare favorably with that of previously documented Mo carbide-based catalysts or\nNi-based alloy catalysts. Nevertheless, we wish to highlight that this study prioritizes\nelucidating the mechanistic basis of structure evolution-mediated precise surface\nengineering of structure-sensitive active sites, rather than aiming to synthesize a class\nof high-performance RWGS catalysts.\nAction: Additional discussion have been added in the Discussion section.\n\"This research approach can be effectively extended to the top-down regulation of\nvalence states, interfaces, crystal phases, compositions, sizes, and other properties.\"\n\"These catalysts, along with insights into their in-situ restructuring mechanisms and\ndistinct CO reactivity characteristics relative to other Co-based catalysts, open new\n2\npossibilities for designing high-performance novel CO conversion catalysts and anti-\nComment 3. The DFT calculation used MnO as the substrate to model MnO , but did\nx\nnot offer a convincing justification. Furthermore, the p(2x2) supercell (~9x9 A in lateral\ndirection) for the DFT calculations appears too small for the Co cluster on the MnO\n13\ninterface. The experiments also did not justify such a high Co loading.\nResponse: We sincerely thank the reviewer for raising these critical points regarding\nour computational model. We have conducted extensive research on CO hydrogenation\n2\ncatalysts using MnO as a carrier in our previous work. In a reducing atmosphere, MnO\nx x\nwas indeed reduced to the divalent manganese (MnO) and afforded active sites for CO\n2\nadsorption and the transport of active H species (see our previous results: Nature\nCatalysis, 2023, 6(11): 1062-1072 / Applied Catalysis B: Environment and Energy,\n2024, 352: 124010). In this study, we employed MnO as the substrate model to support\nCo cluster.\n(~9×9 Å) was too small for the Co cluster. It could induce artificial periodic\n13\ninteractions between cluster images and does not correspond to a realistic, dispersed\ncatalytic interface (ref.: Chemical Reviews, 2021, 122(12): 10777-10820.). To\ndecisively address this issue, we have systematically enlarged the substrate model. All\nDFT calculations have been re-performed using a p(3×2) MnO(220) supercell (lateral\ndimensions ~13×13 Å). This ensures a minimum distance of >12 Å between periodic\nimages of the Co cluster, effectively eliminating any significant spurious adsorbate-\n13\nadsorbate interactions. This larger model represents a more realistic, lower-Co-\ncoverage scenario. A comparison of the results is presented in Table R1 (for reviewing\nonly). The size of the MnO substrate indeed has a significant impact on the stability\nenergies and also improves convergence during the calculations. In contrast, the energy\nbarriers involved in the reaction pathways are less affected. The revised results remain\nrobust and consistent with experimental observations, confirming the rapid dissociation\nof CO, the key molecule, at the Co/MnO interface, and its efficient coupling with Co\nto form carbon. The resulting carbon-modified Co sites effectively suppress further CO\nreactions.\nTable R1(for reviewing only). Effect of MnO substrate supercell size on key activation\nenergies (E ) and reaction energies (ΔE).\na\nE ∆E\na\nModel p(2x2) p(3x2) p(2x2) p(3x2)\nsupercell supercell supercell supercell\nCO→C*+O*\n2.05 2.00 -0.16 0.51\non Co /MnO\n2.26 2.36 1.96 0.55\non C@Co /MnO\n1.51 1.70 -2.01 -2.22\non Co /MnO\n0.29 0.92 -3.65 -0.33\non Co /MnO\n0.82 0.87 0.74 0.75\non Co /MnO\n1.41 1.46 0.96 -0.65\non C@Co /MnO\nbeen modified.\nFigure 5. DFT calculations of in-situ carbon deposition process. a) CO dissociation\nenergy barrier on the MnO, Co cluster, and Co /MnO catalysts. b) Second CO\n13 13\ndissociation and coupling energy barrier at the Co /MnO interface. c) C-C coupling\n13\nenergy barrier on the surface of Co cluster. d) Oxygen elimination energy barrier on\n13\nthe surface of MnO. e) CO dissociation energy barrier on the surfaces of Co /MnO and\n13 13\nC@Co /MnO. g) Scheme of CO hydrogenation reaction induced carbon-modified\n13 2\nCo-based for RWGS.\nO\nv\n(a) MnO (bulk) (b) MnO (c) Co (d) Co /MnO\n13 13\nFigure S14. a) Model of the bulk MnO. b) Top and side views of defective MnO(220)\nsurfaces with an oxygen vacancy. c) Structure of Co cluster. d) Top and side views of\nCo /MnO\nFigure S29. The structures of the initial state, transition state (TS), and final state\ninvolved in CO dissociation on the surfaces of Co , MnO and Co /MnO. Color-coding\n13 13\nin the ball-and-stick models: red, O; purple, Mn; blue, Co; and grey, C.\nCO* + C*+ O* TS C-C-O*\nCO* + C*+ O* TS 2C*+ 2O*\nFigure S30. The structures of the initial state, transition state (TS), and final state\ninvolved in the second CO dissociation and coupling at the Co /MnO interface. Color-\n13\ncoding in the ball-and-stick models: red, lattice O; purple, Mn; blue, Co; grey, C; and\nyellow, adsorbed O.\nFigure S31. The structures of the initial state, transition state (TS), and final state\ninvolved in the C-C coupling on the surface of Co. Color-coding in the ball-and-stick\nmodels: blue, Co; and grey, C.\nCO* + O* TS CO *\nFigure S32. The structures of the initial state, transition state (TS), and final state\ninvolved in the oxygen elimination on the surface of MnO. Color-coding in the ball-\nand-stick models: red, lattice O; purple, Mn; grey, C; yellow, adsorbed O; and white, H.\nC@Co /MnO\nC@Co /MnO\nCo /MnO\nFigure S35. The structures of the initial state, transition state (TS), and final state\ninvolved in CO dissociation on the surfaces of C@Co /MnO, as well as CO\n13\nhydrogenation on the surfaces of Co /MnO and C@Co /MnO. Color-coding in the\n13 13\nball-and-stick models: red, lattice O; purple, Mn; blue, Co; grey, C; yellow, adsorbed\nO; and white, H.\nComment 4. According to the authors, C atoms were generated at the interface and\nmigrated to cover the Co surface. However, the authors did not quantify the amount of\nC on Co. What are the diffusion barriers for the C atoms to migrate to the metallic Co?\nNote that the Co particles in the experiment have an average size of ~4 nm. How\nfeasible is it for C to reach a critical coverage to block further hydrogenation?\nResponse: The most common quantitative method for surface metal sites is probe\nmolecule adsorption experiments. However, in this manuscript, the inherently weak\nadsorption of CO molecules on Co leads to non-stoichiometric adsorption, which poses\nchallenges for quantification via this approach. As an alternative, we performed\nquantitative analyses of surface carbon on the spent catalysts using temperature-\nprogrammed hydrogenation (TPH) and temperature-programmed oxidation (TPO). The\nresults show that the 2%Co/MnO catalyst indeed exhibits a significantly higher surface\nx\ncarbon content compared to both 2%Co/TiO and 5%Co/MnO . This outcome can be\n2 x\nattributed to the dynamic balance between CO dissociation into surface carbon and\ncarbon removal by H .\naddressed in our initial calculations. This aspect has now been thoroughly considered\nin the revised manuscript. Our investigation is motivated by prior work identifying Co\nand Ni as effective active centers for converting CH₄ or CO₂ into carbon materials,\ncoupled with literature evidence suggesting relatively low energy barriers for C\nmigration on metal surfaces.\nTo address this, we have calculated the migration barriers for C atoms at two critical\nlocations: 1) from the Co/MnO interface onto the metallic Co surface, and 2) across the\nsurface of the Co nanoparticle itself. The calculated barriers are 0.82 eV and 0.21 eV,\nrespectively. Both values are significantly lower than the energy barriers determined\nfor the rate-determining steps of CO dissociation and C-C coupling. This key finding\nstrongly indicates that once atomic C is formed, its subsequent diffusion across the\ninterface and the Co surface is facile and does not constitute a rate-limiting step in the\noverall process.\nParticularly noteworthy is the exceptionally low migration barrier of 0.21 eV on the\nCo nanoparticle surface. This low barrier indicates the high feasibility of C atoms\nmigrating and eventually polymerizing on the surfaces of realistic Co nanoparticles\n(e.g., ~4 nm in size, as relevant to our experimental system), which aligns with and\nsubstantiates our original claims. The inclusion of this data and the accompanying\ndiscussion makes the mechanistic description in our work more systematic and\ncomplete.\nAction: Calculations for C migration barriers have been added, along with\ncorresponding discussion in the main text.\n\"With migration barriers of only 0.82 eV (interface-to-Co) and 0.21 eV (on Co surface),\nthe diffusion of C* atoms is kinetically facile compared to the more demanding steps\nof CO dissociation and coupling (Figures S33-34). This efficient diffusion provides a\nmechanistic basis for how interface-generated carbon atoms can populate the Co\nsurface and ultimately, forming polymeric carbon species and triggering the\nrestructuring of Co nanoclusters.\"\nFigure S33. Energy barrier and structure of interfacial carbon atom migrating to the Co\ncluster surface. Color-coding in the ball-and-stick models: red, lattice O; purple, Mn;\nblue, Co; and grey, C.\nFigure S34. Energy barrier and structure of interfacial carbon atom migrating on Co\ncluster surface. Color-coding in the ball-and-stick models: blue, Co; and grey, C.\nPoint-by-point response and action to the reviewers’ comments.\nReviewer #4\nWhile the authors have made notable efforts to address several technical concerns—\nparticularly by revising the DFT model and adding calculations of carbon migration\nbarriers—the fundamental scientific issues regarding motivation, catalyst choice, and\nsignificance relative to existing literature remain unsatisfactorily resolved.\nFirst, the authors merely asserted the key novelty aspect by stating their study\ndemonstrates “that it is possible to manipulate catalytic performance through C-\ninduced reconstruction of the Co surface, shifting from methanation to reverse water-\ngas shift”, and “has not been proven before”. How narrowly is the key novelty defined\nhere? In fact, reaction-induced Co surface reconstruction has been well documented in\ncatalysis, particularly in Fischer–Tropsch synthesis to control product selectivity. More\nrecently, reaction-induced reconstruction at the Cu(0)-Co2C interface has been\nexploited for CO hydrogenation to C hydrocarbons (Wang et al. JACS, 2025).\n2 2+\nSecond, the authors’ response to my question of why Co/MnOx is an appropriate and\nmeaningful system is dismissive, and their arguments are largely conceptual and not\nscientifically justified. My concern was not merely philosophical but practical:\ncobalt-\nbased catalysts are already known to promote C–C coupling and multi-carbon\nhydrocarbon formation, and the authors have not provided a compelling reason why\ndeliberately steering the reaction toward CO formation on such a catalyst represents\nan advantageous case study. The authors dismissed the question as “incorrect” because\ntheir goal is mechanistic understanding rather than catalyst development. However,\nthis does not adequately explain why this system provides unique mechanistic insight\ncompared to other better-performing RWGS or restructuring-prone catalysts (e.g., Ni-\nbased or carbide systems).\nThird, the lack of meaningful comparison with state-of-the-art RWGS catalysts remains\na major flaw. The authors repeatedly state that superior catalytic performance is not\ntheir objective. While mechanistic studies need not deliver the best-performing catalyst,\nthe absence of quantitative benchmarking (e.g., apparent activation barriers, turnover\nrates, or selectivity at comparable conversions) prevents the reader from assessing the\nbroader relevance of the reported phenomenon. Without such context, it is difficult to\njudge whether the observed selectivity shift is of general significance or specific to a\nnarrowly defined, relatively low-activity system.\nIn contrast, the authors’ revisions to the computational methodology (a larger MnO\nsupercell and recalculation of key barriers) and their addition of carbon migration\nbarrier calculations strengthen the internal consistency of the proposed mechanism.\nThese improvements address specific technical criticisms regarding model validity and\nthe feasibility of carbon diffusion on Co nanoparticles. However, resolving these\nmethodological issues does not overcome the central concern that the work’s\nconceptual advancement is overstated relative to existing literature on catalyst\nrestructuring, carbon deposition, and RWGS selectivity control.\nOverall, while the mechanistic analysis is more robust after revision, the manuscript\nstill lacks a compelling justification of why Co/MnOx and the presence of the\n“dynamical surface carbon species” steering the reaction toward RWGS constitute a\nsufficiently novel and impactful advance for publication in this journal. The authors’\narguments rely heavily on broad claims of a “new paradigm” and “general interest,”\nbut they provide little comparative or quantitative evidence to support their claims. In\nsummary, the scientific significance and the strength of the argument remain inadequate,\nand I cannot recommend publication of this work in Nature Communications.\nResponse: We sincerely appreciate your careful reading and rigorous comments on our\nmanuscript. In the previous revision, we addressed the technical concerns regarding\nDFT calculations, which has enhanced the scientific rigor and logical consistency of\nour work. While we acknowledge your concerns regarding the research topic of this\nmanuscript, we firmly believe that our research methodology and conclusions hold\nsubstantial significance for future studies. This is particularly true with respect to the\nfundamental understanding of catalytic active sites in CO hydrogenation, as well as\n2\nthe strategic design of catalyst structure evolution via atmosphere-controlled regulation.\nNevertheless, your insightful comments have prompted us to incorporate more in-depth\nconsiderations of industrialization prospects into our subsequent research endeavors.\nBelow are our detailed responses to your comments:\nOur work emphasizes the controlled atmosphere-induced regulation of the fine\nsurface structure of catalysts as a fabrication strategy, with a particular focus on\ncontrollability. This strategy relies on the systematic correlation between material\nproperties, gas composition, and energy condition with the direction of structural\nevolution, rather than merely focusing on new structural evolution phenomena. Beyond\nsurface carbon modification, other structural evolutions (e.g., sintering/dispersion,\nalloying/segregation, encapsulation/de-encapsulation) are not isolated structural\nchanges. Instead, they undergo nano-scale or even atomic-scale structural\ntransformations. By controlling these processes, we aim to ultimately achieve the\nprogrammability of nanomaterials.\nFurthermore, industrial catalysts attach great importance to stability, leading to a\ngeneral consensus in academia that catalyst deactivation is a problem to be avoided as\nmuch as possible. In this work, we utilize carbon deposition - traditionally regarded as\na key factor for deactivation - as a means to regulate catalytic performance. Our study\ninspires the in-depth utilization of previous catalyst structural changes, rather than\nmerely focusing on the phenomena themselves or deactivation. This aligns with the\ngoals of green chemistry, providing a new perspective for the rational design of\ncatalysts by turning “undesirable” structural changes into “controllable” performance-\ntuning tools.\nExtending the application of Co-based catalysts to the reverse water-gas shift\n(RWGS) reaction possesses unique value in the RWGS + Fischer-Tropsch (FT)\ncoupling process. Meanwhile, the core value of this work lies in the systematic\nelucidation of the adsorption, activation, dissociation, and coupling of CO molecules\non the Co/MnO catalyst, as well as the revelation of the structural dependence of each\nx\nprocess. These fundamental insights provide direct guidance for the rational design of\nsubsequent CO /CO hydrogenation catalysts, which is difficult to achieve through\n2\nstudies on traditional catalyst systems optimized solely for catalytic activity.\nAll in all, we highly appreciate your valuable comments and suggestions. As we\ncontinue to develop the most efficient catalyst system, we will integrate economic\nevaluations into our subsequent studies to further enhance the practical relevance of our\nresearch.\nAction: No further revisions have been made in the revised manuscript."}]

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