Single atom iron promotes CS hydrogenation on interstellar grain analogues

Single atom iron promotes CS hydrogenation on interstellar grain analogues | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Single atom iron promotes CS hydrogenation on interstellar grain analogues Gerard Pareras This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9171400/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Understanding how sulphur-bearing molecules interact with catalytic grain analogues is relevant to the long-standing problem of sulphur depletion in dense interstellar environments. Here, periodic density functional theory, climbing-image nudged elastic band calculations, and kinetic modelling are used to investigate CS hydrogenation by a single-atom Fe⁰ site on amorphous silica. CS binds strongly to the supported Fe centre, while H₂ dissociation is assisted by the Fe–silica interface through a cooperative Fe–H–Si motif. This enables low-barrier hydrogenation to surface-bound H₂CS, which remains kinetically accessible at low temperature, including under conditions where tunnelling contributes to reactivity. By contrast, further hydrogenation to CH₃SH is strongly hindered by a high barrier and becomes favourable only at much higher temperature. Binding energies reveal strong retention of CS-derived intermediates and products. These findings highlight the importance of transition-metal-driven astrocatalysis in interstellar sulphur chemistry and provide new insight into the fate of sulphur in planet-forming environments. Physical sciences/Chemistry Physical sciences/Materials science Physical sciences/Physics Figures Figure 1 Figure 2 Figure 3 Introduction Sulphur chemistry in interstellar environments remains poorly understood, largely because observed abundances often deviate markedly from the cosmic elemental abundance. In the diffuse interstellar medium and photon-dominated regions, sulphur is observed at close to its cosmic abundance. 1 – 3 In dense molecular gas, however, sulphur is heavily depleted, with only ~ 0.1% of the cosmic abundance detected in the gas phase. 4 This implies a depletion factor of roughly three orders of magnitude, 5,6 while the identity of the main sulphur reservoirs remains unresolved. Consequently, the chemical processing and ultimate fate of sulphur in the interstellar medium continue to be intensively investigated. 7 – 10 Despite this pronounced depletion, only OCS and, tentatively, SO₂ have been detected in icy mantles towards high-mass protostars, 11,12 with abundances of ~ 10⁻⁷ accounting for less than 4% of the cosmic sulphur reservoir. How much of the interstellar sulphur inventory survives star formation and is inherited by planet-forming disks therefore remains uncertain, as does the extent to which sulphur chemistry is reset in these environments. Sulphur-bearing species are also present in remnants of our own protoplanetary disk, such as comets. 13 – 15 In particular, Rosetta observations of comet 67P/Churyumov–Gerasimenko suggest that at least part of the cometary sulphur inventory may be inherited from the parent interstellar cloud. 14 , 15 Molecular abundance patterns and isotopic ratios in comets may therefore provide valuable clues as to whether these species originated in the ISM or were chemically reprocessed within the protoplanetary disk. Perrero et al. investigated the binding energies of sulphur-bearing species on water-ice mantles, 16 and provided new insight into the sulphur depletion problem. Their results underscored the importance of dispersion interactions, particularly for S₂, whose low binding energy points to enhanced diffusivity and surface reactivity. In the same study, CS showed an unusual behaviour relative to CO: because of its larger dipole moment and stronger dispersion interactions, its adsorption on water ice was weaker than expected, suggesting that depletion on icy mantles may be only partially effective. In a subsequent study, Perrero et al. 17 demonstrated that CS adsorbs much more strongly onto bare dust-grain cores than onto icy mantles. Considering the porous nature of interstellar grains and the limited thickness of their ice coatings, CS depletion may therefore proceed preferentially on exposed grain-core surfaces, even at early stages of planetary-system formation. 18 Beyond surface binding, the reactivity of CS is another key factor in understanding its depletion. Experimental information on grain-surface reactions involving H \(\:{}_{n}\) CS species remains scarce, largely because of their chemical instability. It has nevertheless been proposed that CS can undergo successive hydrogenation on grain surfaces to form H 2 CS and CH 3 SH, 19–21 in analogy with CO hydrogenation, a process that has been experimentally validated and is known to play a central role in reproducing interstellar CH 3 OH abundances. 22 – 25 Despite this, the surface reactivity of CS and the mechanistic viability of its stepwise hydrogenation to CH 3 SH remain largely unexplored. Recent studies have shown that heterogeneous astrocatalysis at transition-metal-containing grain analogues, particularly Fe-based systems, can efficiently promote the formation of astrochemically relevant species through Fischer–Tropsch-type processes involving H 2 and CO. Extending this catalytic framework to interstellar sulphur chemistry, here we investigate the reaction of CS with H 2 at an isolated Fe site supported on amorphous silica, as a model of an Fe-containing dust-grain surface. We specifically assess the formation of H 2 CS and CH 3 SH, and show that strong CS binding at the Fe site is coupled to enhanced hydrogenation reactivity, identifying a plausible catalytic route for sulphur processing on dust-grain analogue surfaces. Results Activation and hydrogenation of CS at a single Fe site The formation of thioformaldehyde (H₂CS) and, subsequently, methyl mercaptan (CH₃SH) was investigated through two sequential hydrogenation steps, namely H₂ + CS and H₂ + H₂CS, both of which are part of the catalytic cycle shown in Fig. 1 . To elucidate the energetic requirements of this catalytic cycle, we computed the corresponding potential energy surfaces (PESs), shown in Figs. 2 and 3 . The pathway leading to H₂CS begins with adsorption of H₂ at the Fe centre, an exergonic step (− 86.2 kJ mol⁻¹) that yields intermediate B. Subsequent adsorption of CS is also highly exergonic, leading to intermediate C at − 347.1 kJ mol⁻¹. Interestingly, H₂ undergoes spontaneous homolytic cleavage upon adsorption, producing a structure in which one H atom bridges the Fe atom and a surface Si atom (Fe–H–Si). This interaction corresponds to a three-centre two-electron (3c–2e) bond, in which hydrogen is not fully transferred to Si but instead bridges the Fe and Si centres. Such cooperative bonding between the isolated Fe site and the SiO₂ support was reported in our previous studies on Fe⁰@SiO₂ and plays a key mechanistic role here, as the pre-dissociated H₂ configuration facilitates the subsequent hydrogenation steps. The first hydrogenation step leads to intermediate D (HCS). This step has a low intrinsic activation barrier of 2.9 kJ mol⁻¹ and is slightly endergonic, with an intrinsic reaction energy of + 1.5 kJ mol⁻¹. The second hydrogenation step, yielding H₂CS (intermediate E), proceeds through a similarly low barrier of 2.5 kJ mol⁻¹ and is strongly exergonic (− 134.7 kJ mol⁻¹). At this stage, the reaction may terminate at H₂CS formation or proceed through further hydrogenation toward CH₃SH. To explore this second regime, we computed the corresponding PES using the previously formed H₂CS intermediate as the starting point (Fig. 3 ). The pathway begins with adsorption of a second H₂ molecule at the Fe centre (− 5.62 kJ mol⁻¹), leading to intermediate F. The subsequent hydrogenation step, which forms intermediate G (CH₃S), proceeds through a much higher activation barrier (110.1 kJ mol⁻¹) than the preceding steps, although the reaction remains exergonic by − 16.5 kJ mol⁻¹. The final hydrogenation step, leading to CH₃SH (intermediate H), occurs through a barrier of 38.0 kJ mol⁻¹ and yields a final exergonic product with an intrinsic reaction energy of − 14.4 kJ mol⁻¹. All computational data is included in Tables S1 and S2 in the supporting information. Kinetic separation between H₂CS and CH₃SH formation The mechanistic analysis above identifies the relevant energetic features of the hydrogenation sequence, but does not by itself capture the role of quantum tunnelling or the temperature regime at which each step becomes kinetically viable. To address this, we complemented the energetic analysis with Rice–Ramsperger–Kassel–Marcus (RRKM) kinetic calculations. Table 1 summarizes, for each elementary step, the temperature at which the rate constant reaches 1 s⁻¹, which we take here as a threshold for a fast process. The data reveal that CS hydrogenation to H₂CS is highly accessible. The first hydrogenation step (TS-CD) reaches a rate constant of 2.9 s⁻¹ at only 15 K. Because this step is endergonic, tunnelling is expected to make a negligible contribution. The second hydrogenation step (TS-DE), leading to H₂CS, is even more kinetically favourable and reaches 1.8 s⁻¹ at 4 K when tunnelling is included. Even without tunnelling, the same step remains rapid, with a rate constant of 8.4 s⁻¹ at 12 K. Thus, once the first hydrogenation barrier is overcome, H₂CS formation proceeds efficiently under low-temperature conditions. By contrast, the kinetic picture changes drastically for further hydrogenation toward CH₃SH. The TS-FG step, which forms CH₃S, is associated with the highest barrier in the full mechanism and reaches a rate constant of 1 s⁻¹ only at 476 K, even when tunnelling is included. Although the final hydrogenation step (TS-GH) is substantially less demanding, it still requires 152–155 K to reach the same kinetic threshold. Overall, the kinetic analysis identifies a clear separation between two reactivity regimes: a low-temperature regime in which H₂CS formation is readily accessible, and a much higher-temperature regime required for further hydrogenation to CH₃SH. This behaviour indicates that H₂CS is a strongly stabilised intermediate and that the conversion to CH₃SH becomes the main kinetic bottleneck under interstellar conditions. Arrhenius plots are available in Figures S1 to S4 in the supporting information. Table 1 Calculated temperatures (T, K) at which each elementary step reaches a rate constant of 1 s⁻¹, together with the corresponding rate constants (k, s⁻¹), ZPE-corrected energy barriers (ΔU‡), and reaction energies (ΔU_Rx) in kJ mol⁻¹ for the hydrogenation pathways leading to H₂CS and CH₃SH. Values in parentheses indicate results obtained when tunnelling is included. Mechanism System T (K) k (s − 1 ) ∆U ‡ ∆U Rx H 2 CS TS-CD 15 2.9 2.89 1.48 TS-DE 12 (4) 8.4 (1.8) 2.48 -134.75 H 3 CSH TS-FG 480 (476) 1.0 (1.0) 110.13 -16.52 TS-GH 155 (152) 1.1 (1.0) 37.97 -14.38 Adsorption hierarchy and catalytic trapping of sulphur species The results above show that CS becomes highly reactive once adsorbed at the Fe centre, undergoing facile hydrogenation to H₂CS. This raises an additional mechanistic question: do the adsorbed intermediates and products remain bound to the catalytic site, or can they be readily released into the gas phase? To address this point, we calculated the binding energies of CS and of the key intermediates and products along the hydrogenation pathway, namely HCS, H₂CS, CH₃S and CH₃SH. The corresponding values are summarized in Table 2 . Table 2 Calculated binding energies (kJ mol⁻¹) of CS and the key intermediates and products involved in the hydrogenation pathways leading to H₂CS and CH₃SH at Fe⁰@SiO₂. System Binding Energy CS 384.79 HCS 260.89 H 2 CS 382.22 H 3 CS 280.20 H 3 CSH 184.99 Overall, all species bind strongly to the Fe site supported on amorphous silica. This indicates that, wherever such isolated metal sites are exposed to the gas phase, CS is expected to adsorb efficiently and remain available for subsequent reaction. The hydrogenated intermediates and products also remain strongly bound to the Fe centre. Their desorption is therefore expected to be strongly suppressed at low temperature, favouring retention of sulphur-bearing species at the catalytic site. At the same time, the binding strength decreases progressively with increasing hydrogenation along the sequence. In this respect, CH₃SH shows the lowest binding energy among the species considered, indicating that, if formed, it is the most likely product to desorb from the surface. Taken together, these results reveal an adsorption hierarchy in which the Fe site both activates CS and retains the resulting sulphur-containing intermediates and products. This behaviour is consistent with a catalytic trapping regime that may contribute to sulphur retention on grain-analogue surfaces. Discussion The present results identify the supported Fe⁰ site on amorphous silica as an efficient motif for CS activation and hydrogenation. The calculated binding energies show that CS interacts strongly with the exposed Fe centre, indicating that isolated transition-metal sites on grain analogues can act as efficient adsorption and activation centres for sulphur-bearing molecules. This behaviour contrasts with the weaker interaction previously reported for CS on water-ice mantles and supports the idea that exposed grain-core regions may play a distinct role in interstellar sulphur processing. Once adsorbed, CS becomes highly reactive. The reaction mechanism reveals that the Fe–silica interface not only stabilizes the adsorbate, but also promotes H₂ activation through formation of a cooperative Fe–H–Si motif. This support-assisted activation lowers the energetic cost of the first two hydrogenation steps and enables efficient formation of H₂CS. In this sense, the supported Fe site behaves as a genuine catalytic centre in which strong adsorption and low-barrier reactivity are intimately coupled. The kinetic analysis further reveals a marked separation between two reactivity regimes. The first two hydrogenation steps, leading to H₂CS, are accessible at very low temperatures, with the initial step becoming viable at 15 K and the second one proceeding even more readily, including under conditions where tunnelling contributes to reactivity. By contrast, further hydrogenation toward CH₃SH is associated with a much higher kinetic demand, with the TS-FG step representing the main bottleneck of the full mechanism. This sharp contrast indicates that H₂CS is not only a readily formed product, but also a kinetically favoured endpoint under cold interstellar conditions. The binding-energy analysis adds an additional mechanistic dimension to this picture. Not only CS, but also the hydrogenated intermediates and products remain strongly bound to the Fe site. As a result, the supported metal centre is expected to promote both activation and retention of sulphur-bearing species. The progressive decrease in binding strength along the hydrogenation sequence suggests that hydrogenation modulates product release, with CH₃SH being the most weakly bound species among those considered and therefore the most likely to desorb if formed. Taken together, these results point to a catalytic trapping regime in which isolated Fe sites can retain CS-derived species on the surface while selectively favouring early hydrogenation steps. From an astrochemical perspective, these findings support a scenario in which CS depletion does not arise solely from weak physisorption onto icy mantles, but also from strong chemisorption and catalytic transformation at transition-metal-containing grain surfaces. Such a mechanism may contribute to sulphur sequestration in dense environments and may also influence the temperature-dependent release of hydrogenated sulphur species during later stages of star and planet formation. More broadly, the present results highlight the importance of transition-metal-driven astrocatalysis in interstellar sulphur chemistry and provide new insight into the fate of sulphur in planet-forming environments. Methods Computational details All calculations were performed under periodic boundary conditions using the CP2K package. 26 Characterization of the potential energy surfaces (PESs) requires determination of the structures and energetics of the relevant stationary points. Geometry optimizations were carried out using the semi-local generalized gradient approximation (GGA) PBEsol functional, 27 together with the Grimme D3(BJ) dispersion correction. 28 A double-ζ basis set (DZVP-MOLOPT-SR-GTH) was employed for all atom types, in combination with a plane-wave auxiliary basis-set cutoff of 500 Ry. Goedecker–Teter–Hutter pseudopotentials 29 were used to describe the core electrons, while valence electrons were treated within the mixed Gaussian and plane-wave (GPW) framework. 30 To refine the energetics of the stationary points, single-point calculations were performed on the PBEsol-D3(BJ)-optimized geometries using the B3LYP hybrid functional 31 , 32 with the Grimme D3(BJ) correction 28 and a triple-ζ basis set (TZVP), as implemented in the CRYSTAL17 code. 33 , 34 The thresholds used for the evaluation of the Coulomb and exchange bi-electronic integrals (TOLINTEG keyword in CRYSTAL17) were set to 7, 7, 7, 7, and 14. Transition states were searched using the climbing-image nudged elastic band (CI-NEB) method implemented in CP2K 35 and their energies were evaluated at the B3LYP-D3(BJ)//PBEsol-D3(BJ) level of theory. Energy barriers were calculated as \(\:{\varDelta\:E}^{\ddagger}={E}_{TS}-{E}_{GS}\) Eq. 1 \(\:{\varDelta\:U}^{\ddagger}={\varDelta\:E}^{\ddagger}+\varDelta\:ZPE\) Eq. 2 \(\:{\varDelta\:G}_{T}^{\ddagger}={\varDelta\:E}^{\ddagger}+{\varDelta\:G}_{T}\) Eq. 3 where \(\:{\varDelta\:E}^{\ddagger}\) is the potential-energy barrier, and \(\:{E}_{TS}\) and \(\:{E}_{GS}\) are the absolute potential energies of the transition state and the preceding local minimum, respectively. \(\:{\varDelta\:U}^{\ddagger}\) denotes the zero-point-energy (ZPE)-corrected barrier, where \(\:\varDelta\:ZPE\) is the ZPE contribution to \(\:{\varDelta\:E}^{\ddagger}\) , whereas \(\:{\varDelta\:G}_{T}^{\ddagger}\) represents the Gibbs free-energy barrier at temperature T, where \(\:{\varDelta\:G}_{T}\) is the thermal Gibbs correction to \(\:{\varDelta\:E}^{\ddagger}\) . Binding energies (BEs) were computed using the counterpoise method, as implemented in CRYSTAL17, in order to correct for basis-set superposition error (BSSE). Adsorption energies were calculated as \(\:\varDelta\:{E}_{ads}=\:{E}_{cplx}-\left({E}_{sur}+{E}_{m}\right)\) \(\:\varDelta\:{U}_{ads}=\varDelta\:{E}_{ads}+\varDelta\:ZPE\) \(\:\varDelta\:{U}_{ads}=-BE\) (Eq. 2) where \(\:\varDelta\:{E}_{ads}\) is the potential adsorption energy, \(\:{E}_{cplx}\) , \(\:{E}_{sur}\) and \(\:{E}_{m}\) are the absolute potential energies for the adsorption complex, the isolated surface, and the isolated molecule, respectively, \(\:\varDelta\:{U}_{ads}\) is the ZPE-corrected adsorption energy (in which \(\:\varDelta\:ZPE\) refers to the contribution of the ZPE corrections to \(\:\varDelta\:{E}_{ads}\) ), and \(\:BE\) is the binding energy (which is \(\:\varDelta\:{U}_{ads}\) in opposite sign). The nature of the stationary points was confirmed by harmonic frequency calculations, identifying minima for reactants, intermediates, and products, and first-order saddle points with a single imaginary frequency for transition states. Harmonic vibrational frequencies were calculated at the PBEsol-D3(BJ)/DZVP-optimized structures using the finite-differences approach implemented in CP2K. 26 To reduce the computational cost, a partial-Hessian approach was adopted. Accordingly, vibrational frequencies were computed only for the subset of atoms directly involved in the reaction, including the reactive species and the surface atoms participating in the elementary step. The catalytic performance of the simulated processes was further analysed through kinetic calculations. For each elementary step, the rate constant was calculated using Rice–Ramsperger–Kassel–Marcus (RRKM) theory, 36 a microcanonical transition-state theory that assumes statistical population of the phase space. Tunnelling effects were included using an unsymmetrical Eckart potential barrier. 37 The calculated vibrational frequencies were used as degrees of freedom in the sum of states. Although only a partial Hessian matrix was computed, the included vibrational modes correspond to those directly involved in the reaction and are therefore expected to provide the dominant contribution to the rate constants. By contrast, the remaining modes, associated mainly with the inner layers of the surface, were assumed to have a negligible effect on the reaction kinetics. 38 These calculations were performed with a freely available in-house code in which RRKM algorithms were implemented for grain-surface processes. 39 Catalyst model The catalytic system considered in this study consists of a single Fe⁰ atom anchored to a periodic amorphous SiO₂ surface (Fe⁰@SiO₂). The amorphous silica model was taken from the work of Ugliengo et al., 40 in which the Fe⁰ centre is supported on a low-density silanol (SiOH) surface with a coverage of 1.5 SiOH nm⁻². This model has been used in our previous studies to investigate Fischer–Tropsch reactivity, and further details of its construction are given elsewhere. 41 , 42 The system adopts a high-spin electronic state, with triplet Fe⁰ identified in our previous work as the ground state. Declarations Competing Interests The author declares no competing interests. Author Contribution GP conceived the study, performed the calculations, analysed the data, acquired funding, and wrote the manuscript. Acknowledgement G.P. acknowledges funding from the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101105235 (CHAOS). G.P. also acknowledges financial support from the Spanish Ministry of Universities and the European Union’s NextGenerationEU fund through a Margarita Salas contract. The Spanish MICINN is acknowledged for funding projects PID2024-157971NB-C21 and CNS2023-144902. G.P. acknowledges RES and the Barcelona Supercomputing Center for access to MareNostrum under activity QHS-2023-10019, as well as the supercomputing facilities provided by CSUC. G.P. also acknowledges the EuroHPC Joint Undertaking through Regular Access project No. EU2025R01-014, hosted by the Ministry of Education, Youth and Sports of the Czech Republic through e-INFRA CZ (ID: EU-25-91). Data Availability All data generated or analysed during this study are available in the manuscript, the Supplementary Information, and the CORA repository at [https://doi.org/10.34810/data3126](https:/doi.org/10.34810/data3126) . References Rivière-Marichalar, P., Fuente, A., Goicoechea, J. R., Pety, J., et al. 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Supplementary Files SISingleatomironpromotesCShydrogenationoninterstellargrainanalogues1.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 29 Apr, 2026 Reviews received at journal 27 Apr, 2026 Reviews received at journal 15 Apr, 2026 Reviewers agreed at journal 30 Mar, 2026 Reviewers agreed at journal 30 Mar, 2026 Reviewers invited by journal 30 Mar, 2026 Editor assigned by journal 27 Mar, 2026 Submission checks completed at journal 24 Mar, 2026 First submitted to journal 19 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9171400","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":614679914,"identity":"242c5300-27bf-4f03-af5f-680ae7adecd7","order_by":0,"name":"Gerard Pareras","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9ElEQVRIiWNgGAWjYNACNjjLhoFBgkQtaVAtCcRrOUxYi7x778PPFWUMefyzDx/78OPP+cT+2Q2MH37+wK3F8MxxY8kz5xiKJc6lJc/sbbudOOPOAWbJHjy2GM5IY5BsbGNIbDjDY8zA23A7seFGAhsDDz4t858x/wRpmX+G/zPjnz/nEucDtTD+wecXCTY2sC0bzvAwM/OwHUjcANTCjM8WA540NsuGcxLFhmfYjJll25KNN9452Cwtk4bHlvZjzDcbymzy5M4wP2Z888dOdt7t5oMf39jgseUAmJJAdghjA271IFug0njcPgpGwSgYBSMeAACrvFGzWvtS8AAAAABJRU5ErkJggg==","orcid":"","institution":"Autonomous University of Barcelona","correspondingAuthor":true,"prefix":"","firstName":"Gerard","middleName":"","lastName":"Pareras","suffix":""}],"badges":[],"createdAt":"2026-03-19 15:54:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9171400/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9171400/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106402383,"identity":"d5c17960-dc05-4675-8a17-5c6d0e41dc17","added_by":"auto","created_at":"2026-04-08 09:11:56","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":42911,"visible":true,"origin":"","legend":"\u003cp\u003eProposed catalytic cycle for sequential CS hydrogenation on Fe⁰@SiO₂.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9171400/v1/e8dd5f1d40a42a63dd4d7c05.jpg"},{"id":106403900,"identity":"71153be5-ec86-4e6c-b1cf-1069b1602644","added_by":"auto","created_at":"2026-04-08 09:15:11","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":114972,"visible":true,"origin":"","legend":"\u003cp\u003eReaction pathway for H₂CS formation from CS hydrogenation at the supported Fe site. Top: elementary reaction steps. Bottom: corresponding potential energy surface (kJ mol⁻¹) calculated at the B3LYP-D3(BJ)//PBEsol-D3(BJ) level of theory.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9171400/v1/74202ad7f042e1ab0859aef3.jpg"},{"id":106188913,"identity":"6b4e806c-7dab-48bc-bdb1-f065887ff187","added_by":"auto","created_at":"2026-04-05 17:07:09","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":71275,"visible":true,"origin":"","legend":"\u003cp\u003eReaction pathway for CH₃SH formation through further hydrogenation of H₂CS at the supported Fe site. Top: elementary reaction steps. Bottom: corresponding potential energy surface (kJ mol⁻¹) calculated at the B3LYP-D3(BJ)//PBEsol-D3(BJ) level of theory.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9171400/v1/08b4aa0424e9a67db87fff8a.jpg"},{"id":106405782,"identity":"b5e8562a-75e4-4670-a6e0-a91d848bb17c","added_by":"auto","created_at":"2026-04-08 09:28:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":780893,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9171400/v1/8a4ae606-8496-46b1-be07-d43acf5be30e.pdf"},{"id":106188911,"identity":"b61ed961-9536-4b47-a0dc-3c18f27174fd","added_by":"auto","created_at":"2026-04-05 17:07:09","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":215052,"visible":true,"origin":"","legend":"","description":"","filename":"SISingleatomironpromotesCShydrogenationoninterstellargrainanalogues1.docx","url":"https://assets-eu.researchsquare.com/files/rs-9171400/v1/52cf8ecee37daa5a0133bdab.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Single atom iron promotes CS hydrogenation on interstellar grain analogues","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSulphur chemistry in interstellar environments remains poorly understood, largely because observed abundances often deviate markedly from the cosmic elemental abundance. In the diffuse interstellar medium and photon-dominated regions, sulphur is observed at close to its cosmic abundance.\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e In dense molecular gas, however, sulphur is heavily depleted, with only\u0026thinsp;~\u0026thinsp;0.1% of the cosmic abundance detected in the gas phase.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e This implies a depletion factor of roughly three orders of magnitude,\u003csup\u003e5,6\u003c/sup\u003e while the identity of the main sulphur reservoirs remains unresolved. Consequently, the chemical processing and ultimate fate of sulphur in the interstellar medium continue to be intensively investigated.\u003csup\u003e\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eDespite this pronounced depletion, only OCS and, tentatively, SO₂ have been detected in icy mantles towards high-mass protostars,\u003csup\u003e11,12\u003c/sup\u003e with abundances of ~\u0026thinsp;10⁻⁷ accounting for less than 4% of the cosmic sulphur reservoir. How much of the interstellar sulphur inventory survives star formation and is inherited by planet-forming disks therefore remains uncertain, as does the extent to which sulphur chemistry is reset in these environments.\u003c/p\u003e \u003cp\u003eSulphur-bearing species are also present in remnants of our own protoplanetary disk, such as comets.\u003csup\u003e\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e In particular, Rosetta observations of comet 67P/Churyumov\u0026ndash;Gerasimenko suggest that at least part of the cometary sulphur inventory may be inherited from the parent interstellar cloud.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Molecular abundance patterns and isotopic ratios in comets may therefore provide valuable clues as to whether these species originated in the ISM or were chemically reprocessed within the protoplanetary disk.\u003c/p\u003e \u003cp\u003ePerrero et al. investigated the binding energies of sulphur-bearing species on water-ice mantles,\u003csup\u003e16\u003c/sup\u003e and provided new insight into the sulphur depletion problem. Their results underscored the importance of dispersion interactions, particularly for S₂, whose low binding energy points to enhanced diffusivity and surface reactivity. In the same study, CS showed an unusual behaviour relative to CO: because of its larger dipole moment and stronger dispersion interactions, its adsorption on water ice was weaker than expected, suggesting that depletion on icy mantles may be only partially effective. In a subsequent study, Perrero et al.\u003csup\u003e17\u003c/sup\u003e demonstrated that CS adsorbs much more strongly onto bare dust-grain cores than onto icy mantles. Considering the porous nature of interstellar grains and the limited thickness of their ice coatings, CS depletion may therefore proceed preferentially on exposed grain-core surfaces, even at early stages of planetary-system formation.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eBeyond surface binding, the reactivity of CS is another key factor in understanding its depletion. Experimental information on grain-surface reactions involving H\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{}_{n}\\)\u003c/span\u003e\u003c/span\u003eCS species remains scarce, largely because of their chemical instability. It has nevertheless been proposed that CS can undergo successive hydrogenation on grain surfaces to form H\u003csub\u003e2\u003c/sub\u003eCS and CH\u003csub\u003e3\u003c/sub\u003eSH,\u003csup\u003e19\u0026ndash;21\u003c/sup\u003e in analogy with CO hydrogenation, a process that has been experimentally validated and is known to play a central role in reproducing interstellar CH\u003csub\u003e3\u003c/sub\u003eOH abundances.\u003csup\u003e\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e Despite this, the surface reactivity of CS and the mechanistic viability of its stepwise hydrogenation to CH\u003csub\u003e3\u003c/sub\u003eSH remain largely unexplored.\u003c/p\u003e \u003cp\u003eRecent studies have shown that heterogeneous astrocatalysis at transition-metal-containing grain analogues, particularly Fe-based systems, can efficiently promote the formation of astrochemically relevant species through Fischer\u0026ndash;Tropsch-type processes involving H\u003csub\u003e2\u003c/sub\u003e and CO. Extending this catalytic framework to interstellar sulphur chemistry, here we investigate the reaction of CS with H\u003csub\u003e2\u003c/sub\u003e at an isolated Fe site supported on amorphous silica, as a model of an Fe-containing dust-grain surface. We specifically assess the formation of H\u003csub\u003e2\u003c/sub\u003eCS and CH\u003csub\u003e3\u003c/sub\u003eSH, and show that strong CS binding at the Fe site is coupled to enhanced hydrogenation reactivity, identifying a plausible catalytic route for sulphur processing on dust-grain analogue surfaces.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eActivation and hydrogenation of CS at a single Fe site\u003c/h2\u003e \u003cp\u003eThe formation of thioformaldehyde (H₂CS) and, subsequently, methyl mercaptan (CH₃SH) was investigated through two sequential hydrogenation steps, namely H₂ + CS and H₂ + H₂CS, both of which are part of the catalytic cycle shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the energetic requirements of this catalytic cycle, we computed the corresponding potential energy surfaces (PESs), shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The pathway leading to H₂CS begins with adsorption of H₂ at the Fe centre, an exergonic step (\u0026minus;\u0026thinsp;86.2 kJ mol⁻\u0026sup1;) that yields intermediate B. Subsequent adsorption of CS is also highly exergonic, leading to intermediate C at \u0026minus;\u0026thinsp;347.1 kJ mol⁻\u0026sup1;.\u003c/p\u003e \u003cp\u003eInterestingly, H₂ undergoes spontaneous homolytic cleavage upon adsorption, producing a structure in which one H atom bridges the Fe atom and a surface Si atom (Fe\u0026ndash;H\u0026ndash;Si). This interaction corresponds to a three-centre two-electron (3c\u0026ndash;2e) bond, in which hydrogen is not fully transferred to Si but instead bridges the Fe and Si centres. Such cooperative bonding between the isolated Fe site and the SiO₂ support was reported in our previous studies on Fe⁰@SiO₂ and plays a key mechanistic role here, as the pre-dissociated H₂ configuration facilitates the subsequent hydrogenation steps.\u003c/p\u003e \u003cp\u003eThe first hydrogenation step leads to intermediate D (HCS). This step has a low intrinsic activation barrier of 2.9 kJ mol⁻\u0026sup1; and is slightly endergonic, with an intrinsic reaction energy of +\u0026thinsp;1.5 kJ mol⁻\u0026sup1;. The second hydrogenation step, yielding H₂CS (intermediate E), proceeds through a similarly low barrier of 2.5 kJ mol⁻\u0026sup1; and is strongly exergonic (\u0026minus;\u0026thinsp;134.7 kJ mol⁻\u0026sup1;).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt this stage, the reaction may terminate at H₂CS formation or proceed through further hydrogenation toward CH₃SH. To explore this second regime, we computed the corresponding PES using the previously formed H₂CS intermediate as the starting point (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe pathway begins with adsorption of a second H₂ molecule at the Fe centre (\u0026minus;\u0026thinsp;5.62 kJ mol⁻\u0026sup1;), leading to intermediate F. The subsequent hydrogenation step, which forms intermediate G (CH₃S), proceeds through a much higher activation barrier (110.1 kJ mol⁻\u0026sup1;) than the preceding steps, although the reaction remains exergonic by \u0026minus;\u0026thinsp;16.5 kJ mol⁻\u0026sup1;. The final hydrogenation step, leading to CH₃SH (intermediate H), occurs through a barrier of 38.0 kJ mol⁻\u0026sup1; and yields a final exergonic product with an intrinsic reaction energy of \u0026minus;\u0026thinsp;14.4 kJ mol⁻\u0026sup1;. All computational data is included in Tables S1 and S2 in the supporting information.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eKinetic separation between H₂CS and CH₃SH formation\u003c/h3\u003e\n\u003cp\u003eThe mechanistic analysis above identifies the relevant energetic features of the hydrogenation sequence, but does not by itself capture the role of quantum tunnelling or the temperature regime at which each step becomes kinetically viable. To address this, we complemented the energetic analysis with Rice\u0026ndash;Ramsperger\u0026ndash;Kassel\u0026ndash;Marcus (RRKM) kinetic calculations.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes, for each elementary step, the temperature at which the rate constant reaches 1 s⁻\u0026sup1;, which we take here as a threshold for a fast process.\u003c/p\u003e \u003cp\u003eThe data reveal that CS hydrogenation to H₂CS is highly accessible. The first hydrogenation step (TS-CD) reaches a rate constant of 2.9 s⁻\u0026sup1; at only 15 K. Because this step is endergonic, tunnelling is expected to make a negligible contribution. The second hydrogenation step (TS-DE), leading to H₂CS, is even more kinetically favourable and reaches 1.8 s⁻\u0026sup1; at 4 K when tunnelling is included. Even without tunnelling, the same step remains rapid, with a rate constant of 8.4 s⁻\u0026sup1; at 12 K. Thus, once the first hydrogenation barrier is overcome, H₂CS formation proceeds efficiently under low-temperature conditions.\u003c/p\u003e \u003cp\u003eBy contrast, the kinetic picture changes drastically for further hydrogenation toward CH₃SH. The TS-FG step, which forms CH₃S, is associated with the highest barrier in the full mechanism and reaches a rate constant of 1 s⁻\u0026sup1; only at 476 K, even when tunnelling is included. Although the final hydrogenation step (TS-GH) is substantially less demanding, it still requires 152\u0026ndash;155 K to reach the same kinetic threshold.\u003c/p\u003e \u003cp\u003eOverall, the kinetic analysis identifies a clear separation between two reactivity regimes: a low-temperature regime in which H₂CS formation is readily accessible, and a much higher-temperature regime required for further hydrogenation to CH₃SH. This behaviour indicates that H₂CS is a strongly stabilised intermediate and that the conversion to CH₃SH becomes the main kinetic bottleneck under interstellar conditions. Arrhenius plots are available in Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e to S4 in the supporting information.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCalculated temperatures (T, K) at which each elementary step reaches a rate constant of 1 s⁻\u0026sup1;, together with the corresponding rate constants (k, s⁻\u0026sup1;), ZPE-corrected energy barriers (ΔU\u0026Dagger;), and reaction energies (ΔU_Rx) in kJ mol⁻\u0026sup1; for the hydrogenation pathways leading to H₂CS and CH₃SH. Values in parentheses indicate results obtained when tunnelling is included.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMechanism\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSystem\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eT (K)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ek (s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e∆U\u003csup\u003e\u0026Dagger;\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e∆U\u003csub\u003eRx\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eH\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eCS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTS-CD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.48\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTS-DE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12 (4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8.4 (1.8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-134.75\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eH\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eCSH\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTS-FG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e480 (476)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.0 (1.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e110.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-16.52\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTS-GH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e155 (152)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.1 (1.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-14.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003eAdsorption hierarchy and catalytic trapping of sulphur species\u003c/h3\u003e\n\u003cp\u003eThe results above show that CS becomes highly reactive once adsorbed at the Fe centre, undergoing facile hydrogenation to H₂CS. This raises an additional mechanistic question: do the adsorbed intermediates and products remain bound to the catalytic site, or can they be readily released into the gas phase?\u003c/p\u003e \u003cp\u003eTo address this point, we calculated the binding energies of CS and of the key intermediates and products along the hydrogenation pathway, namely HCS, H₂CS, CH₃S and CH₃SH. The corresponding values are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCalculated binding energies (kJ mol⁻\u0026sup1;) of CS and the key intermediates and products involved in the hydrogenation pathways leading to H₂CS and CH₃SH at Fe⁰@SiO₂.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSystem\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBinding Energy\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e384.79\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHCS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e260.89\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eCS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e382.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eH\u003csub\u003e3\u003c/sub\u003eCS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e280.20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eH\u003csub\u003e3\u003c/sub\u003eCSH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e184.99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eOverall, all species bind strongly to the Fe site supported on amorphous silica. This indicates that, wherever such isolated metal sites are exposed to the gas phase, CS is expected to adsorb efficiently and remain available for subsequent reaction.\u003c/p\u003e \u003cp\u003eThe hydrogenated intermediates and products also remain strongly bound to the Fe centre. Their desorption is therefore expected to be strongly suppressed at low temperature, favouring retention of sulphur-bearing species at the catalytic site. At the same time, the binding strength decreases progressively with increasing hydrogenation along the sequence. In this respect, CH₃SH shows the lowest binding energy among the species considered, indicating that, if formed, it is the most likely product to desorb from the surface.\u003c/p\u003e \u003cp\u003eTaken together, these results reveal an adsorption hierarchy in which the Fe site both activates CS and retains the resulting sulphur-containing intermediates and products. This behaviour is consistent with a catalytic trapping regime that may contribute to sulphur retention on grain-analogue surfaces.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe present results identify the supported Fe⁰ site on amorphous silica as an efficient motif for CS activation and hydrogenation. The calculated binding energies show that CS interacts strongly with the exposed Fe centre, indicating that isolated transition-metal sites on grain analogues can act as efficient adsorption and activation centres for sulphur-bearing molecules. This behaviour contrasts with the weaker interaction previously reported for CS on water-ice mantles and supports the idea that exposed grain-core regions may play a distinct role in interstellar sulphur processing.\u003c/p\u003e \u003cp\u003eOnce adsorbed, CS becomes highly reactive. The reaction mechanism reveals that the Fe\u0026ndash;silica interface not only stabilizes the adsorbate, but also promotes H₂ activation through formation of a cooperative Fe\u0026ndash;H\u0026ndash;Si motif. This support-assisted activation lowers the energetic cost of the first two hydrogenation steps and enables efficient formation of H₂CS. In this sense, the supported Fe site behaves as a genuine catalytic centre in which strong adsorption and low-barrier reactivity are intimately coupled.\u003c/p\u003e \u003cp\u003eThe kinetic analysis further reveals a marked separation between two reactivity regimes. The first two hydrogenation steps, leading to H₂CS, are accessible at very low temperatures, with the initial step becoming viable at 15 K and the second one proceeding even more readily, including under conditions where tunnelling contributes to reactivity. By contrast, further hydrogenation toward CH₃SH is associated with a much higher kinetic demand, with the TS-FG step representing the main bottleneck of the full mechanism. This sharp contrast indicates that H₂CS is not only a readily formed product, but also a kinetically favoured endpoint under cold interstellar conditions.\u003c/p\u003e \u003cp\u003eThe binding-energy analysis adds an additional mechanistic dimension to this picture. Not only CS, but also the hydrogenated intermediates and products remain strongly bound to the Fe site. As a result, the supported metal centre is expected to promote both activation and retention of sulphur-bearing species. The progressive decrease in binding strength along the hydrogenation sequence suggests that hydrogenation modulates product release, with CH₃SH being the most weakly bound species among those considered and therefore the most likely to desorb if formed. Taken together, these results point to a catalytic trapping regime in which isolated Fe sites can retain CS-derived species on the surface while selectively favouring early hydrogenation steps.\u003c/p\u003e \u003cp\u003eFrom an astrochemical perspective, these findings support a scenario in which CS depletion does not arise solely from weak physisorption onto icy mantles, but also from strong chemisorption and catalytic transformation at transition-metal-containing grain surfaces. Such a mechanism may contribute to sulphur sequestration in dense environments and may also influence the temperature-dependent release of hydrogenated sulphur species during later stages of star and planet formation. More broadly, the present results highlight the importance of transition-metal-driven astrocatalysis in interstellar sulphur chemistry and provide new insight into the fate of sulphur in planet-forming environments.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eComputational details\u003c/h2\u003e \u003cp\u003eAll calculations were performed under periodic boundary conditions using the CP2K package.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e Characterization of the potential energy surfaces (PESs) requires determination of the structures and energetics of the relevant stationary points. Geometry optimizations were carried out using the semi-local generalized gradient approximation (GGA) PBEsol functional,\u003csup\u003e27\u003c/sup\u003e together with the Grimme D3(BJ) dispersion correction.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e A double-ζ basis set (DZVP-MOLOPT-SR-GTH) was employed for all atom types, in combination with a plane-wave auxiliary basis-set cutoff of 500 Ry. Goedecker\u0026ndash;Teter\u0026ndash;Hutter pseudopotentials\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e were used to describe the core electrons, while valence electrons were treated within the mixed Gaussian and plane-wave (GPW) framework.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eTo refine the energetics of the stationary points, single-point calculations were performed on the PBEsol-D3(BJ)-optimized geometries using the B3LYP hybrid functional\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e with the Grimme D3(BJ) correction\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e and a triple-ζ basis set (TZVP), as implemented in the CRYSTAL17 code.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e The thresholds used for the evaluation of the Coulomb and exchange bi-electronic integrals (TOLINTEG keyword in CRYSTAL17) were set to 7, 7, 7, 7, and 14.\u003c/p\u003e \u003cp\u003eTransition states were searched using the climbing-image nudged elastic band (CI-NEB) method implemented in CP2K\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e and their energies were evaluated at the B3LYP-D3(BJ)//PBEsol-D3(BJ) level of theory. Energy barriers were calculated as\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:E}^{\\ddagger}={E}_{TS}-{E}_{GS}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEq.\u0026nbsp;1\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:U}^{\\ddagger}={\\varDelta\\:E}^{\\ddagger}+\\varDelta\\:ZPE\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEq.\u0026nbsp;2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:G}_{T}^{\\ddagger}={\\varDelta\\:E}^{\\ddagger}+{\\varDelta\\:G}_{T}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEq.\u0026nbsp;3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:E}^{\\ddagger}\\)\u003c/span\u003e\u003c/span\u003e is the potential-energy barrier, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{TS}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{GS}\\)\u003c/span\u003e\u003c/span\u003e are the absolute potential energies of the transition state and the preceding local minimum, respectively. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:U}^{\\ddagger}\\)\u003c/span\u003e\u003c/span\u003e denotes the zero-point-energy (ZPE)-corrected barrier, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:ZPE\\)\u003c/span\u003e\u003c/span\u003e is the ZPE contribution to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:E}^{\\ddagger}\\)\u003c/span\u003e\u003c/span\u003e, whereas \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:G}_{T}^{\\ddagger}\\)\u003c/span\u003e\u003c/span\u003e represents the Gibbs free-energy barrier at temperature T, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:G}_{T}\\)\u003c/span\u003e\u003c/span\u003e is the thermal Gibbs correction to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:E}^{\\ddagger}\\)\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eBinding energies (BEs) were computed using the counterpoise method, as implemented in CRYSTAL17, in order to correct for basis-set superposition error (BSSE). Adsorption energies were calculated as\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Tabb\" border=\"1\"\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{E}_{ads}=\\:{E}_{cplx}-\\left({E}_{sur}+{E}_{m}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{U}_{ads}=\\varDelta\\:{E}_{ads}+\\varDelta\\:ZPE\\)\u003c/span\u003e\u003c/span\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{U}_{ads}=-BE\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(Eq.\u0026nbsp;2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{E}_{ads}\\)\u003c/span\u003e\u003c/span\u003e is the potential adsorption energy, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{cplx}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{sur}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{m}\\)\u003c/span\u003e\u003c/span\u003e are the absolute potential energies for the adsorption complex, the isolated surface, and the isolated molecule, respectively, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{U}_{ads}\\)\u003c/span\u003e\u003c/span\u003e is the ZPE-corrected adsorption energy (in which \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:ZPE\\)\u003c/span\u003e\u003c/span\u003e refers to the contribution of the ZPE corrections to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{E}_{ads}\\)\u003c/span\u003e\u003c/span\u003e), and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:BE\\)\u003c/span\u003e\u003c/span\u003e is the binding energy (which is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{U}_{ads}\\)\u003c/span\u003e\u003c/span\u003e in opposite sign).\u003c/p\u003e \u003cp\u003eThe nature of the stationary points was confirmed by harmonic frequency calculations, identifying minima for reactants, intermediates, and products, and first-order saddle points with a single imaginary frequency for transition states. Harmonic vibrational frequencies were calculated at the PBEsol-D3(BJ)/DZVP-optimized structures using the finite-differences approach implemented in CP2K.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e To reduce the computational cost, a partial-Hessian approach was adopted. Accordingly, vibrational frequencies were computed only for the subset of atoms directly involved in the reaction, including the reactive species and the surface atoms participating in the elementary step.\u003c/p\u003e \u003cp\u003eThe catalytic performance of the simulated processes was further analysed through kinetic calculations. For each elementary step, the rate constant was calculated using Rice\u0026ndash;Ramsperger\u0026ndash;Kassel\u0026ndash;Marcus (RRKM) theory,\u003csup\u003e36\u003c/sup\u003e a microcanonical transition-state theory that assumes statistical population of the phase space. Tunnelling effects were included using an unsymmetrical Eckart potential barrier.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e The calculated vibrational frequencies were used as degrees of freedom in the sum of states. Although only a partial Hessian matrix was computed, the included vibrational modes correspond to those directly involved in the reaction and are therefore expected to provide the dominant contribution to the rate constants. By contrast, the remaining modes, associated mainly with the inner layers of the surface, were assumed to have a negligible effect on the reaction kinetics.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e These calculations were performed with a freely available in-house code in which RRKM algorithms were implemented for grain-surface processes.\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCatalyst model\u003c/h3\u003e\n\u003cp\u003eThe catalytic system considered in this study consists of a single Fe⁰ atom anchored to a periodic amorphous SiO₂ surface (Fe⁰@SiO₂). The amorphous silica model was taken from the work of Ugliengo et al.,\u003csup\u003e40\u003c/sup\u003e in which the Fe⁰ centre is supported on a low-density silanol (SiOH) surface with a coverage of 1.5 SiOH nm⁻\u0026sup2;. This model has been used in our previous studies to investigate Fischer\u0026ndash;Tropsch reactivity, and further details of its construction are given elsewhere.\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e The system adopts a high-spin electronic state, with triplet Fe⁰ identified in our previous work as the ground state.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe author declares no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eGP conceived the study, performed the calculations, analysed the data, acquired funding, and wrote the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eG.P. acknowledges funding from the European Union\u0026rsquo;s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101105235 (CHAOS). G.P. also acknowledges financial support from the Spanish Ministry of Universities and the European Union\u0026rsquo;s NextGenerationEU fund through a Margarita Salas contract. The Spanish MICINN is acknowledged for funding projects PID2024-157971NB-C21 and CNS2023-144902. G.P. acknowledges RES and the Barcelona Supercomputing Center for access to MareNostrum under activity QHS-2023-10019, as well as the supercomputing facilities provided by CSUC. G.P. also acknowledges the EuroHPC Joint Undertaking through Regular Access project No. EU2025R01-014, hosted by the Ministry of Education, Youth and Sports of the Czech Republic through e-INFRA CZ (ID: EU-25-91).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data generated or analysed during this study are available in the manuscript, the Supplementary Information, and the CORA repository at [https://doi.org/10.34810/data3126](https:/doi.org/10.34810/data3126) .\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRivi\u0026egrave;re-Marichalar, P., Fuente, A., Goicoechea, J. R., Pety, J., \u003cem\u003eet al.\u003c/em\u003e Abundances of sulphur molecules in the Horsehead nebula: First NS+ detection in a photodissociation region. \u003cem\u003eAstron. Astrophys.\u003c/em\u003e 2019, 628, A16.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHowk, J. C., Sembach, K. R., Savage, B. D. 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Astrophys.\u003c/em\u003e 2024, 687, A230.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-computational-materials","isNatureJournal":false,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"npjcompumats","sideBox":"Learn more about [npj Computational Materials](http://www.nature.com/npjcompumats/)","snPcode":"41524","submissionUrl":"https://mts-npjcompumats.nature.com/","title":"npj Computational Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9171400/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9171400/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUnderstanding how sulphur-bearing molecules interact with catalytic grain analogues is relevant to the long-standing problem of sulphur depletion in dense interstellar environments. Here, periodic density functional theory, climbing-image nudged elastic band calculations, and kinetic modelling are used to investigate CS hydrogenation by a single-atom Fe⁰ site on amorphous silica. CS binds strongly to the supported Fe centre, while H₂ dissociation is assisted by the Fe\u0026ndash;silica interface through a cooperative Fe\u0026ndash;H\u0026ndash;Si motif. This enables low-barrier hydrogenation to surface-bound H₂CS, which remains kinetically accessible at low temperature, including under conditions where tunnelling contributes to reactivity. By contrast, further hydrogenation to CH₃SH is strongly hindered by a high barrier and becomes favourable only at much higher temperature. Binding energies reveal strong retention of CS-derived intermediates and products. These findings highlight the importance of transition-metal-driven astrocatalysis in interstellar sulphur chemistry and provide new insight into the fate of sulphur in planet-forming environments.\u003c/p\u003e","manuscriptTitle":"Single atom iron promotes CS hydrogenation on interstellar grain analogues","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-05 17:07:05","doi":"10.21203/rs.3.rs-9171400/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-29T13:16:55+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-27T09:49:11+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-15T14:16:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"320205562756893524237705533806233371558","date":"2026-03-30T14:45:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"72975594685903696916837833761046481694","date":"2026-03-30T12:58:47+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-30T12:24:35+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-27T16:04:26+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-24T12:05:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Computational Materials","date":"2026-03-19T15:45:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-computational-materials","isNatureJournal":false,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"npjcompumats","sideBox":"Learn more about [npj Computational Materials](http://www.nature.com/npjcompumats/)","snPcode":"41524","submissionUrl":"https://mts-npjcompumats.nature.com/","title":"npj Computational Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4c890d7d-99cc-47a7-aa5c-cbf4e308e30c","owner":[],"postedDate":"April 5th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":65406431,"name":"Physical sciences/Chemistry"},{"id":65406432,"name":"Physical sciences/Materials science"},{"id":65406433,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-05-19T11:24:03+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-05 17:07:05","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9171400","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9171400","identity":"rs-9171400","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}