Facet sensitivity of iron carbides in Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) is a structure-sensitive reaction of which performance is strongly related to the active phase, particle size, and exposed facets. Compared with the full-pledged investigation on the active phase and particle size, the facet effect has been limited to theoretical studies or single-crystal surfaces, lacking experimental reports of practical catalysts, especially for Fe-based catalysts. Herein, we demonstrate the facet sensitivity of iron carbides in FTS. As the prerequisite, {202} and {112} facets of χ-Fe5C2 are fabricated as the outer shell through the conformal reconstruction of Fe3O4 nanocubes and octahedra, as the inner cores, respectively. During FTS, the activity and stability are highly sensitive to the exposed facet of iron carbides, whereas the facet sensitivity is not prominent for the chain growth. According to mechanistic studies, {202} χ-Fe5C2 surfaces follow hydrogen-assisted CO dissociation which lowers the activation energy compared with the direct CO dissociation over {112} surfaces, affording the high FTS activity.

In this work, the authors constructed Fe3O4@Fe5C2 core-shell with different facets which exhibited comparable activity and catalytic mechanism.However, the reconstruction of Fe5C2 during Fischer-Tropsch reaction has been systematically investigated in early reported works.(J.Phys.Chem.C 2017, 121, 9, 5154-5160; ACS Catal.2017, 7, 9, 5661-5667) The synthesis of Fe3O4@Fe5C2 coreshell for Fischer-Tropsch synthesis has also been reported.(ACS Catal. 2016, 6, 6, 3610-3618) The catalytic mechanism has also been reported.(Applied Energy 2015, 160, 15, 982-989).Therefore, I think the authors should really differentiate their works in novelty from the literature reports before getting publishing on Nat.Commun.. Some specific points: 1.In page 8 line 221, the authors claimed that "the solid octahedra collapsed after the reaction."However, In Fig. S16, it is easy to find the collapsed nanocubes while the CO conversion was nearly constant.The difference of stability of two samples could not be only contributed to the collapsed morphology.2.Schematic diagram for different Fischer-Tropsch mechanisms of Fe3O4@Fe5C2 with different facets should be provided to help the readers to understand the molecular catalytic mechanisms.
Point-by-point response to reviewers' comments Manuscript ID: NCOMMS-23-62544 Title: Facet sensitivity of iron carbides in Fischer-Tropsch synthesis Reviewer #1 "This work perpares varied tailor-made FeOx@FeCx capsule-like catalysts for Fischer-Tropsch synthesis (FTS), where surface layer is limited to one type of Fe carbide phase or facet.As in general, many kinds of Fe carbide and facet co-exist in conventional FTS catalysis, it was rather difficult to make clear the different role and function of each Fe carbide phase and facet type, until now.This paper determines and realizes clear catalyst structure being oriented to solve this vital FTS problem.The findings such as different facets exhibiting varied activity but similar chain-growth probability are interesting and important.I recommend the publication of this paper after suitable revision." We sincerely thank this reviewer's valuable comments on our work.We have clarified the comments raised by this reviewer as follows.
" (1) What is the driving force to regulate or limit one type of facet or phase at the surface layer?Please provide more insights on the conformal growth here." We sincerely thank this reviewer for his/her valuable comment.To achieve the specific facet and phase, our approach began with the synthesis of Fe3O4 templates with uniform size and facet.We finely controlled the shape by selectively stabilizing crystal facets through capping ligands and modulating growth kinetics.When 4-biphenylcarboxylic acid was used to bind {100} surfaces, Fe3O4 nanocubes were obtained.When oleylamine and stepped temperature programs were adopted, Fe3O4 octahedra were formed.After the in-situ reduction process, we obtained Fe nanocubes and octahedra.
The driving force to regulate one type of phase at the surface layer of Fe3O4@χ-Fe5C2 nanocrystals is the carbon chemical potential of reaction conditions.It was reported that the activation energy for carbon diffusion in Fe (43.9-69.0kJ mol -1 ) was lower than that for the FTS reaction (89.1±3.8 kJ mol -1 ) [Chem.Soc.Rev. 37, 2758-81 (2008)].Consequently, surface carbon atoms cleaved from CO exhibit a pronounced affinity to Fe atoms, thereby instigating a phase transition from iron to FeCx during the initial stage of the FTS reaction.Upon the formation of active FeCx on the surface, the FTS reaction is facilitated, resulting in the release of oxygen species, predominantly in the form of H2O, into the gas phase.This influx of H2O induces Fe oxide formation and impedes further carburization.As the concentration of oxygen species diminishes in the gas phase, a greater quantity of carbide accumulates on the surface, pushing the reaction condition back to the equilibrium position and vice versa.As the inner core of the catalyst oxidizes to Fe3O4, it becomes more resistant to carbon permeation and carburization compared to metallic Fe.When the reaction reaches equilibrium, the catalysts ultimately transform into a core-shell structure consisting of bulk Fe3O4 with surface FeCx.According to carbon chemical potential theory, carbon-rich χ-Fe5C2 is the thermodynamically stable phase under our reaction condition (20 bar, CO:H2 = 1:2, 270 o C) [J.Am.Chem.Soc.132, 14928-14941 (2010)].As such, other Fe carbides present in the surface layers will evolve into χ-Fe5C2 as the reaction progresses.The uniform size and facets of Fe3O4 templates ensure a consistent rate of evolution.Consequently, only a single type of phase was observed at the surface layer over Fe3O4@χ-Fe5C2 nanocrystals under equilibrium reaction conditions.
The small lattice mismatch results in only one type of facet at the surface layer of Fe3O4@χ-Fe5C2 nanocubes.In the case of nanocrystals with a core-shell structure, a slight lattice mismatch (f < ∼5%) is required for the epitaxial surface layer to form over the inner core [Chem.Rev. 120, 2123-2170 (2020)].This epitaxial relationship allows for the maintenance of orientation between the growth layer and the substrate within the first few atomic layers.The spacing of Fe3O4(400) planes (0.21 nm) in the core was approximately equal to that of χ-Fe5C2(202) planes (0.22 nm) in the shell (Fig. R1a).The lattice mismatch of Fe3O4@χ-Fe5C2 nanocubes was calculated as 4.65% (equation 1), ensuring the preservation of the epitaxial orientation relationship.
In equation 1, dshell and dcore refer to the lattice spacings of the shell and the core, respectively.Due to the close match of lattice constants, atomic smooth Fe3O4@χ-Fe5C2 nanocubes can be readily accessed via conformal growth, where the (202) facet of χ-Fe5C2 forms over the (400) facet of Fe3O4.
Besides lattice matching, the epitaxial orientation can also be preserved via domain matching, where the spacing of m lattice planes in the epilayer is approximately equal to n in the substrate [Nano Lett.10, 3028-3036 (2010); Nanoscale 10, 9862-9866 (2018)].For Fe3O4@χ-Fe5C2 octahedra, we observed that three surface unit cells of the Fe3O4(111) planes (0.48 nm) in the core exhibit good alignment with seven surface unit cells of χ-Fe5C2(112) planes (0.21 nm) in the shell (Fig. R1b).Such periodicity leads to a commensurate epitaxial relationship with a low mismatch value of 2.06% according to equation 2. Therefore, the domain match allows the conformal growth for the (112) facet of χ-Fe5C2 over the (111) facet of Fe3O4." (2) The catalyst structure before and after the reaction should be compared in detail.Does thickness of the layer change?" As suggested, we have characterized Fe3O4@χ-Fe5C2 nanocubes and octahedra after the reaction in detail.The cubic morphology was preserved after 100 h on stream (Fig. R2, a and b).We have measured the thickness of the shell layer for Fe3O4@χ-Fe5C2 nanocubes after the reaction.The thickness of the shell increased from 2.0 nm to 2.6 nm after the reaction (Fig. 1c and Fig. R2c).The lattice parameter of the Fe3O4 inner core was measured as 0.21 nm, which was indexed as the (400) facet of Fe3O4 (Fig. R2d).The lattice parameter of the χ-Fe5C shell was 0.22 nm, which was assigned to the (202) facet of χ-Fe5C2 (Fig. R2e).The exposed χ-Fe5C2 facets of Fe3O4@χ-Fe5C2 nanocubes were preserved after 100 h on stream.The EELS image implied that the core region mainly comprised Fe and O elements while the shell region contained Fe and C elements (Fig. R2f).We have also conducted Mössbauer spectroscopy to characterize the compositions of the iron phase in the used Fe3O4@χ-Fe5C2 nanocubes after 100 h on stream (Fig. R3a).The content of χ-Fe5C2 increased from 33.2% to 39.8% after the reaction (Table .R1).
With respect to Fe3O4@χ-Fe5C2 octahedra, the inter core collapsed after the reaction (Fig. R4, a and b).The thickness of the shell increased from 1.7 nm to 3.5 nm after the reaction (Fig. 3c and Fig. R4c).The lattice parameters of the inner core and the outer shell were measured as 0.48 nm and 0.21 nm, which were assigned to the (111) facet of Fe3O4 and (112) facet of χ-Fe5C2, respectively (Fig. R4, d and e).The exposed χ-Fe5C facet of Fe3O4@χ-Fe5C2 octahedra was preserved after the reaction.The distribution of Fe3O4 at the core and χ-Fe5C2 at the shell was supported by the EELS image (Fig. R4f).The content of χ-Fe5C2 in Fe3O4@χ-Fe5C2 octahedra increased from 29.5% to 40.4% after 100 h on stream (Fig. R3b and Table.R1).Therefore, the thickness of the shell layer for both Fe3O4@χ-Fe5C2 nanocubes and octahedra increased after the reaction.We have added relevant discussion in the revised manuscript (p. 10, lines 9-20 and 25-31, p. 11, lines 1-3, Supplementary Figs.17-19 and Supplementary Table 4, highlighted in yellow color).

"(3) Is the surface layer (FeCx) porous?"
Thanks for raising this issue.To investigate the textural properties, we have carried out N2 physisorption characterizations (Fig. R5, a and b).The pore-diameter distributions were analyzed using the Barrett-Joyner-Halenda (BJH) method.As shown in Figure R5, c and d), the surface layer of both Fe3O4@χ-Fe5C2 nanocubes and octahedra were not porous.The absence of pores at the surface layer was also confirmed by HAADF-STEM image (Fig. R5, e and f).We have also measured the textural properties of spent Fe3O4@χ-Fe5C2 nanocubes and octahedra after 100 h on stream (Fig. R6, a and b).The surface layer of spent Fe3O4@χ-Fe5C2 nanocubes remained nonporous after 100 h on stream (Fig. R6, c and e).In contrast, spent Fe3O4@χ-Fe5C2 octahedra contained mesopores as revealed by the pore-diameter distribution and HAADF-STEM image (Fig. R6, d and f).The average mesopore diameter was determined as 17.9 nm by the BJH method (Fig. R6d).
We have added relevant discussion in the revised manuscript (p.11, lines 4-14, Supplementary Figs. 4 and 20, highlighted in yellow color).

"(4) TOFs of nanocubes and octahedra are not the same?"
After being reminded, we have calculated the TOF numbers based on the moles of CO converted per mole of surface Fe atoms per hour (equation 3).TOF = CO conversion  moles of CO in syngas  gas-flow rate ÷ moles of surface Fe atoms The moles of Fe atoms on the surface of Fe3O4@χ-Fe5C2 nanocrystals were determined by CO pulse chemisorption measurement.CO pulse chemisorption measurements were performed using a Micromeritics Autochem 2920 chemisorption analyzer with an active loop volume of 0.1 mL.In a typical measurement, 100 mg of Fe3O4@χ-Fe5C2 nanocrystals/SiC were packed into a reactor with a quartz tube.Prior to the test, the samples were cleaned in He with a gas-flow rate of 100 mL min -1 at 270 o C for 5 h.After cooling down to 50 o C under He flow, CO/He pulses (10 vol% CO and 90 vol% He) were injected until adsorption reached saturation.The amount of adsorbed CO was calculated on the difference between the total amount of CO injected and the amount measured at the outlet from the sample.The metal dispersion was calculated by assuming the ratio of CO to surface metal atom as 1:1.The moles of Fe atoms on the surface of Fe3O4@χ-Fe5C2 nanocubes/SiC was 24.1 µmol g -1 , higher than that (19.5 µmol g -1 ) of Fe3O4@χ-Fe5C2 octahedra/SiC (Fig. R7, a and b).
(5) More info on the products is expected (i.e.olefinic hydrocarbon selectivity).
" (6) Some sentences are need to be improved.For example, "0.113g of CTAB" should be "CTAB of 0.113g"." As suggested, we have improved the sentences in the revised manuscript.

Reviewer #2:
"The authors successfully fabricated two core-shell catalysts with {202} and {112} facets of χ-Fe5C2 as the outer shell through the conformal reconstruction of Fe3O4 nanocubes and octahedra, as the inner cores.The sensitivity of the facet of iron carbides to performance were explored.The different types of CO dissociation led to different FTS activity.Although some interesting results have been got, there are still some problems need to be addressed." We appreciate the reviewer's comments regarding the structural characterizations.As suggested by this reviewer, we have conducted additional experiments and DFT calculations in the revised manuscript.The details are listed in the following responses.
"1.First and most importantly, as we know, the carbon permeation, diffusion and carburization occur.Therefore, the Fe-based catalysts undergo a long activation period before the structure is stable.In this work, the authors correlated the activity to the different facets.But no solid evidence confirms that the stable structure has been achieved.The carbon balance data were not provided.So, we can confirm that the different CO conversions are assigned to different activity or continuous carburization or structure evolution.And the CO/H2 ratio during activation and reaction is different.Even the TEM images for the used catalysts were provided, the Fe3O4@χ-Fe5C2 octahedra collapsed, and the nanocrystals seem to be more stable.But is the thickness of the carbide shell the same？Is the exposed facet maintained after reaction?And also, the compositions of iron phase of the used catalysts are missed.This is very critical for the main conclusion of this work.Because it is very common that the "apparent activity" keeps a long time, but the structure of carbides changes a lot." As suggested by this reviewer, we calculated the carbon balance value (Table R1).For catalytic tests, the products were detected via an offline method.There is an ice trap to separate liquid and gas products.The gaseous products were monitored by online gas chromatographs.The liquid products were analyzed using an offline chromatograph.The carbon balance value was calculated via equation 1, where CnHm and CO2 represent the moles of the produced hydrocarbons and CO2, respectively.COinlet and COoutlet are moles of CO at the inlet and outlet, respectively.
The carbon balance value of Fe3O4@χ-Fe5C2 nanocubes/SiC was 98.7%, similar to that (96.5%) of Fe3O4@χ-Fe5C2 octahedra/SiC (Table R1).The high carbon balance value indicated that the different CO conversions were not caused by continuous carburization.We characterized Fe3O4@χ-Fe5C2 nanocubes and octahedra in detail after the reaction.The cubic morphology was preserved after 100 h on stream (Fig. R1, a and b).We counted the thickness of the shell layer for Fe3O4@χ-Fe5C2 nanocubes after the reaction.The thickness of the shell was increased from 2.0 nm to 2.6 nm before and after the reaction (Fig. 1c and Fig. R1c).The lattice parameter of the Fe3O4 inner core was measured as 0.21 nm, which was indexed as the (400) facet of Fe3O4 (Fig. R1d).The lattice parameter of the χ-Fe5C shell was 0.22 nm, which was assigned to the (202) facet of χ-Fe5C2 (Fig. R1e).The exposed χ-Fe5C facet of Fe3O4@χ-Fe5C2 nanocubes maintained after 100 h on stream.The electron energy loss spectroscopy (EELS) image implied that the core region mainly comprised Fe and O elements while the shell region contained Fe and C elements (Fig. R1f).We also conducted Mössbauer spectroscopy to characterize the compositions of the iron phase in the used Fe3O4@χ-Fe5C2 nanocubes after 100 h on stream (Fig. R2a).The content of χ-Fe5C2 increased from 33.2% to 39.8% before and after the reaction (Table.R2).With respect to Fe3O4@χ-Fe5C2 octahedra, the inter core collapsed after the reaction (Fig. R3, a and b).The thickness of the shell increased from 1.7 nm to 3.5 nm before and after the reaction (Fig. 3c and Fig. R3c).The lattice parameters of the inner core and the outer shell were measured as 0.48 nm and 0.21 nm, which were assigned to the (111) facet of Fe3O4 and (112) facet of χ-Fe5C2, respectively (Fig. R3, d and e).The exposed χ-Fe5C facet of Fe3O4@χ-Fe5C2 octahedra preserved after the reaction.The distribution of Fe3O4 at the core and χ-Fe5C2 at the shell was supported by the EELS image (Fig. R3f).The content of χ-Fe5C2 in Fe3O4@χ-Fe5C2 octahedra increased from 29.5% to 40.4% before and after 100 h on stream (Fig. R2b and Table .R2).The exposed χ-Fe5C facet maintained for both Fe3O4@χ-Fe5C2 nanocubes and octahedra, while the thickness of the shell layer increased after the reaction.Moreover, The CO conversion of Fe3O4@χ-Fe5C2 nanocubes/SiC was 43.1%, much higher than that (20.1%) of Fe3O4@χ-Fe5C2 octahedra/SiC at the initial reaction (Fig. R4).Therefore, we assigned different CO conversions to the intrinsic activity of exposed facets of χ-Fe5C2.We have added relevant discussion in the revised manuscript (p. 8, lines 29-30, p. 9, lines 1-2, p. 10, lines 9-20 and 25-31, p. 11, lines 1-3, p. 20, lines 15-19, Supplementary Figs.17-19, Supplementary Tables 3 and 4, highlighted in yellow color).
"2.The authors claimed that the facets are not sensitive for carbon-chain growth.But the CH4 production is obviously improved.Is there any explanation?" We apologize for the not rigorous summary of the facet effect on carbon-chain growth.
"3.The pressure of DRIFTS is lower than the reaction evaluation.More important, the H2 is induced after CO adsorption in DRIFTS experiments.This is very different from the real reaction that the CO and H2 are co-fed.The hydrogen-assisted CO dissociation usually occurs in the co-existence of CO and H2.And even in the results of this work, once CO adsorbed, the dissociation occurred on both octahedra and nanocube.So, how do the authors discriminate the two kinds of CO dissociation?" We extend our gratitude to the reviewer for his/her insightful comment, which prompted us to further elucidate the CO dissociation mechanisms in our study.Acknowledging the importance of co-feeding CO and H2 in DRIFTS experiments, we aimed to discern between direct CO dissociation and hydrogen-assisted CO dissociation pathways.To distinguish between these mechanisms, we have conducted in-situ DRIFTS experiments under 20 bar of syngas, simulating realistic reaction environments.Prior to testing, thorough cleaning of the samples in He at 270 o C for 1 h ensured the elimination of any contaminants.Background spectra were acquired under He flow, followed by exposure to a 20-bar syngas mixture (CO/H2) at 270 o C for 30 min.
Our analysis of the resulting spectra revealed distinctive features indicative of each dissociation pathway.Specifically, the appearance of gaseous CO2 peaks at 2360 and 2336 cm -1 provided evidence for direct dissociation occurring on both Fe3O4@χ-Fe5C2 nanocubes and octahedra (Figure R11, Table R3).Moreover, the presence of CHO* species, as indicated by a peak at 1741 cm -1 , was observed solely on the Fe3O4@χ-Fe5C2 nanocubes catalyst and absent on the octahedral counterpart (Figure R11, Table R3).
These findings unequivocally demonstrate the discrimination between the two types of CO dissociation.While direct CO dissociation generates CO2, hydrogen-assisted CO dissociation leads to the formation of oxygen-containing species, specifically CHO*, which was uniquely observed on the nanocube catalyst.This selective occurrence of CHO* species provides compelling evidence for the preferential involvement of the hydrogen-assisted CO dissociation mechanism on the nanocube catalyst.We have added relevant discussion in the revised manuscript (p.13, lines 11-21, p. 21, lines 13-22, Supplementary Figs.29-35, highlighted in yellow color).

Reviewer #3:
"In this work, the authors constructed Fe3O4@Fe5C2 core-shell with different facets which exhibited comparable activity and catalytic mechanism.However, the reconstruction of Fe5C2 during Fischer-Tropsch reaction has been systematically investigated in early reported works.(J.Phys.Chem.C 2017, 121, 9, 5154-5160; ACS Catal.2017, 7, 9, 5661-5667) The synthesis of Fe3O4@Fe5C2 core-shell for Fischer-Tropsch synthesis has also been reported.(ACS Catal. 2016, 6, 6, 3610-3618) The catalytic mechanism has also been reported.(Applied Energy 2015, 160, 15, 982-989).Therefore, I think the authors should really differentiate their works in novelty from the literature reports before getting publishing on Nat.Commun.." We genuinely thank this reviewer for the insightful comment.We apologize for any lack of clarity in articulating the novelty of our work.Allow us to elaborate on the distinctive contributions of our research compared to the referenced literature.
Facet sensitivity in practical iron carbides.While previous studies have extensively explored iron carbides' role in FTS, they primarily focused on synthesis methods and general catalytic performance.In contrast, our work delves into the facet sensitivity of practical iron carbides.By synthesizing Fe3O4@χ-Fe5C2 nanocrystals with specifically exposed surfaces, we provide concrete evidence supporting facet sensitivity in FTS reactions, a crucial factor only proposed in theoretical calculations or studies on single-crystal surfaces.
Novel synthesis of uniformly exposed surfaces.While the synthesis of Fe3O4@χ-Fe5C2 core-shell nanoparticles has been reported previously, our approach stands out for the preparation of nanocrystals with uniformly exposed χ-Fe5C2 facets.Unlike previous methods resulting in irregular spherical particles, our technique utilizes a highly symmetrical Fe3O4 template to stabilize uniform facet exposure.This achievement represents a significant advancement in the synthesis of iron carbide catalysts, offering precise control over surface characteristics crucial for catalytic performance.
Insights into catalytic mechanisms.While existing literature has explored the transformation of iron phases and their role in FTS, our study goes beyond by elucidating the CO dissociation pathways on different facets of χ-Fe5C2.Through detailed DRIFTS measurements, we identify distinct dissociation routes over Fe3O4@χ-Fe5C2 nanocubes and octahedra, shedding light on the intricate relationship between facet types and catalytic behavior.This novel insight enhances our understanding of FTS mechanisms and offers valuable guidance for future catalyst design strategies.
In summary, our work significantly advances the field by providing experimental evidence of facet sensitivity in practical iron carbides for FTS, synthesizing nanocrystals with uniformly exposed surfaces, and elucidating distinct catalytic mechanisms based on facet types.We believe these contributions distinguish our research from previous literature reports and underscore its significance in advancing catalysis science.
The differences between our work and the references mentioned by the reviewer are discussed in detail as follows: The reference (J.Phys.Chem.C 2017, 121, 9, 5154-5160; ACS Catal.2017, 7, 9, 5661-5667) highlighted how Fe5C2 nanoparticles were evolved from Fe(CO)5 reagent through in-situ observation.These articles primarily investigated the synthesis of pure-phase iron carbides.
In contrast, our study elucidated the fabrication of Fe3O4@χ-Fe5C2 nanocrystals with surfaces terminated in {202} and {112} facets of χ-Fe5C2 shells, achieved by utilizing cubic and octahedral Fe3O4 as templates, respectively.Our focus lies in the methodology of constructing χ-Fe5C2 with uniformly exposed surfaces from Fe3O4 via a conformal reconstruction method.Thus, the research content of the references and our work diverges significantly.In short, these works provided mechanistic understandings of the active phase, whereas our work takes a step further by delving into the active facets.
In the reference (ACS Catal.2016, 6, 6, 3610-3618), the authors synthesized Fe3O4@χ-Fe5C2 nanoparticles by pyrolyzing iron-containing metal-organic frameworks, presenting a novel synthetic route for preparing FTS catalysts with the χ-Fe5C2 phase.However, the obtained Fe3O4@χ-Fe5C2 nanoparticles were irregular spherical particles.Although core-shell nanoparticles of Fe3O4@χ-Fe5C2 have been synthesized, the preparation of uniform χ-Fe5C2 facets is rare, to the best of our knowledge.χ-Fe5C2 nanocrystals tend to assume spherical shapes under pressures up to tens of atmospheres and temperatures up to several hundred o C to minimize surface energy.To stabilize the uniformly exposed facets of χ-Fe5C2, we utilized a highly symmetrical Fe3O4 template as the core to support the χ-Fe5C2 shell.Hence, one of the novelties of our work lies in the first-time synthesis of χ-Fe5C2 nanocrystals with uniformly exposed surfaces.
The reference (Appl.Energy 2015, 160, 15, 982-989) reported the positive role of the transformation of reduced iron phases to iron carbides in promoting the formation of hydrocarbon species.The authors characterized the microstructures at different FTS stages via DRIFTS.In our study, we reported the conformal reconstruction of well-defined Fe3O4 nanocrystals to generate χ-Fe5C2 with specifically exposed surfaces.Regarding the catalytic mechanism, the reference observed an increase in the amounts of hydrocarbon species absorbed on the catalyst as the temperature rose.In contrast, our work explored CO dissociation pathways for different facets of χ-Fe5C2 through DRIFTS measurements.By analyzing the intermediates generated on uniform χ-Fe5C2 facets, we distinguished between direct and hydrogen-assisted CO dissociation routes over Fe3O4@χ-Fe5C2 nanocubes and octahedra.Thus, our study provides clarity regarding the different roles and functions of different χ-Fe5C2 facets.
Some specific points: 1.In page 8 line 221, the authors claimed that "the solid octahedra collapsed after the reaction."However, In Fig. S16, it is easy to find the collapsed nanocubes while the CO conversion was nearly constant.The difference of stability of two samples could not be only contributed to the collapsed morphology.
We sincerely thank this reviewer for his/her constructive suggestions.To investigate other underlying mechanisms for catalyst deactivation, we have employed Raman spectroscopy to analyze the surfaces of Fe3O4@χ-Fe5C2 nanocubes and octahedra after 100 h.The presence of peaks at 1330 cm -1 indicated the existence of disordered carbon (D band), while those at 1592 cm -1 signified graphite (G band) (Fig. R1, a and b).Significantly higher intensities of these peaks were observed on the surface of spent Fe3O4@χ-Fe5C2 octahedra compared with spent nanocubes, suggesting a greater accumulation of deposited carbon on the octahedral surface (Fig. R1, a and b).
For a more precise comparison, we have conducted thermogravimetric analysis (TGA) under the N2 atmosphere on both Fe3O4@χ-Fe5C2 nanocubes and octahedra after the reaction.The weight loss between 200 and 500 o C was attributed to the removal of long-chain hydrocarbons from the surface, while the weight loss beyond 500 o C was associated with the loss of deposited carbon.In the case of spent Fe3O4@χ-Fe5C2 nanocubes, a weight loss of 6.2 wt% was observed (Fig. R1c).Conversely, during TGA testing, spent Fe3O4@χ-Fe5C2 octahedra exhibited a total weight loss of 13.5 wt% due to long-chain hydrocarbons and deposited carbon (Fig. R1d).The higher residual weight of long-chain hydrocarbons and deposited carbon on the octahedral structure suggests a more pronounced blockage of active sites compared to nanocubes.Hence, we hypothesize that carbon deposition also contributes to the deactivation of Fe3O4@χ-Fe5C2 octahedra.
We have added relevant discussion in the revised manuscript (p. 12, lines 25-28, Supplementary Fig. 23, highlighted in yellow color).
To verify the reliability of the results, we have re-conducted H2-pulse experiments by increasing the concentration of H2 in the feed gas.The proportion of H2 in H2/Ar pulse was increased from 10% to 30%.As shown in Figure R3, the amount of adsorbed H2 over Fe3O4@χ-Fe5C2 nanocubes/SiC and Fe3O4@χ-Fe5C2 octahedra/SiC were calculated as 7.3 µmol g -1 and 11.0 µmol g -1 , just equal to the original values (7.3 µmol g -1 and 11.0 µmol g -1 ).This result implies the reliability of our calculated H2 adsorption amounts.

Figure R3
. H2 pulse profiles of (a) Fe3O4@χ-Fe5C2 nanocubes/SiC and (b) Fe3O4@χ-Fe5C2 octahedra/SiC."6.Table R2, when the GHSV was changed from 2400 to 800 mL h 1 gcat-1, the CO conversion increased from 21.2% to 42.6%.It means longer residence time resulted in lower turnover number, why?" Thanks for raising this point.I guess that the reviewer might refer to the turnover frequency (TOF) number which reflects the intrinsic activity.Since the intrinsic activity of a catalyst refers exclusively to its ability to facilitate chemical transformations, catalysts must be evaluated at conditions under which rates are not impacted, let alone controlled, by mass and heat transport.A common practice is to operate the reaction in the kinetic control typically with a CO conversion of less than 5%.The conversions of 21.2% and 42.6% largely deviate from the kinetic control interval.As such, we cannot use the current conversion values and GHSV to calculate the TOF number.Based on these values, we can only obtain the apparent mass activity of 5.5 mmol h -1 gcat -1 at 2400 mL h -1 gcat -1 and 3.7 mmol h -1 gcat -1 at 800 mL h -1 gcat -1 .In generaly, the instrinsic activty is the theoretical maximum apparent activty, since the activity can be restrained by the mass and heat transport under a high conversion level.In other words, the lower the conversion, the higher the apparent activity.Thus, the longer residence time resulted in a lower apparent mass activity due to the limitation of mass and heat transport.

Reviewer #3
"Because the authors have well addressed the reviewers' comments in the revised paper by performing additional experiments as well as providing rational interpretations, I would like to support the acceptance of this work for publication on the journal in its present content." We genuinely thank the reviewer's careful reading of our manuscript.

Figure R2 .
Figure R2.Structural characterizations of Fe3O4@χ-Fe5C2 nanocubes after 100 h on stream.(a) TEM image of Fe3O4@χ-Fe5C2 nanocube/SiC.(b) HAADF-STEM image of an individual Fe3O4@χ-Fe5C2 nanocube.(c) Magnified HAADF-STEM image of the region marked by the corresponding boxes in panel b.(d) Intensity profile recorded from the area indicated by the rectangular box in panel c.(e) Intensity profile recorded from the area indicated by the rectangular box in panel c.(f) EELS spectra of a Fe3O4@χ-Fe5C2 nanocube in panel c.

Figure R4 .
Figure R4.Structural characterizations of Fe3O4@χ-Fe5C2 octahedra after 100 h on stream.(a) TEM image of Fe3O4@χ-Fe5C2 octahedra/SiC.(b) HAADF-STEM image of an individual Fe3O4@χ-Fe5C2 octahedron.(c) HAADF-STEM image of another Fe3O4@χ-Fe5C2 octahedron.(c) Magnified HAADF-STEM image of the region marked by the corresponding boxes in panel b.(d) Intensity profile recorded from the area indicated by the rectangular box in panel d.(e) Intensity profile recorded from the area indicated by the rectangular box in panel d.(f) EELS spectra of a Fe3O4@χ-Fe5C2 octahedron in panel d.

Figure R1 .
Figure R1.Structural characterizations of Fe3O4@χ-Fe5C2 nanocubes after 100 h on stream.(a) TEM image of Fe3O4@χ-Fe5C2 nanocube/SiC.(b) HAADF-STEM image of an individual Fe3O4@χ-Fe5C2 nanocube.(c) Magnified HAADF-STEM image of the region marked by the corresponding boxes in panel b.(d) Intensity profile recorded from the area indicated by the rectangular box in panel c.(e) Intensity profile recorded from the area indicated by the rectangular box in panel c.(f) EELS spectra of a Fe3O4@χ-Fe5C2 nanocube in panel c.

Figure R3 .Figure R4 .
Figure R3.Structural characterizations of Fe3O4@χ-Fe5C2 octahedra after 100 h on stream.(a) TEM image of Fe3O4@χ-Fe5C2 octahedra/SiC.(b) HAADF-STEM image of an individual Fe3O4@χ-Fe5C2 octahedron.(c) HAADF-STEM image of another Fe3O4@χ-Fe5C2 octahedron.(c) Magnified HAADF-STEM image of the region marked by the corresponding boxes in panel b.(d) Intensity profile recorded from the area indicated by the rectangular box in panel d.(e) Intensity profile recorded from the area indicated by the rectangular box in panel d.(f) EELS spectra of a Fe3O4@χ-Fe5C2 octahedron in panel d.

" 2 .
Schematic diagram for different Fischer-Tropsch mechanisms of Fe3O4@Fe5C2 with different facets should be provided to help the readers toe understand the molecular catalytic mechanisms."