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[Display omitted] • The morphology evolution of Hägg carbide Fe 5 C 2 under hydrogen conditions was explored. • Carbon-rich facets expose more area surface in Wulff construction with decreasing hydrogen chemical potential, while iron-rich facets are in contrast. • The reduction of surface carbon to methane for (1 1 1) and (11-1) facets is more facile than other facets. In heterogeneous catalysis, the interaction between adsorbates and substrate surface plays an important role on surface structure evolution and even the reaction mechanism. Herein, unified DFT calculation and ab initio atomistic thermodynamics were adopted to explore the interaction between hydrogen and multiple Fe 5 C 2 surfaces. The computation indicates that the dissociation of hydrogen molecule is both thermodynamically and kinetically favorable on Fe 5 C 2 surfaces. The adsorption energy of hydrogen atoms is related to the summed H bond valance (SBV H), a higher SBV H corresponding to a more stable adsorption strength. Based on calculated surface free energy with different hydrogen coverage, the equilibrium crystalline morphology evolution of Fe 5 C 2 under dynamic conditions was constructed with Wulff theorem. C-rich facets expose more area in Wulff construction with decreasing hydrogen chemical potential. The generation of carbon vacancy through reducing to methane has also been investigated, and the reduction of (1 1 1) and (11-1) facets is more facile than other facets. We anticipate this work could complement some knowledge to help understanding the surface structure and performance of iron carbide catalysts under experimental conditions. [ABSTRACT FROM AUTHOR]

• On (2 1 0) surface, the hydrogen dissociation is spontaneous. • The chemical bonding analysis is employed to elucidate the interaction between adsorbate and substrate. • At high coverage, the hydrogen adsorption configuration is surface-dependent. • On (2 1 0) surface, the surface sulfur atoms are easily to be removed by hydrogen. Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is of great importance in rationally modulating the catalytic performance of hydrogenation processes. Herein, we have investigated the interaction between hydrogen and four typical surfaces, (1 0 0), (2 1 0), (2 1 1), and (3 1 1) of pyrite FeS 2 through density functional theory calculations. On (2 1 0) surface, the hydrogen dissociative adsorption on unsaturated-coordination sulfur atoms is favorable both in thermodynamics and kinetics. The hydrogen activation barrier is 0.83 eV with slight exothermic of 0.12 eV on (3 1 1). While on (1 0 0) and (2 1 1) surface, the hydrogen dissociation is unfavorable due to the high activation barriers and remarkable positive reaction energies. For high adsorption coverage, the pure molecule adsorption mode is favorable on (1 0 0) facet, opposed to the other surfaces which have temperature and pressure dependence. The saturated coverage sequence is (1 0 0) > (2 1 0) > (2 1 1) > (3 1 1) for a wide range of temperature and pressure. The remove of sulfur atoms most likely occurs on (2 1 0) surface. Our atomistic insights might be useful in engineering hydrogen-involved processes catalyzed by iron sulfide. [ABSTRACT FROM AUTHOR]