The surface phase structure evolution of the fcc MoC (001) surface in a steam reforming atmosphere: systematic kinetic and thermodynamic investigations

Abstract

The kinetic and thermodynamic aspects of the surface phase structure evolution of an fcc MoC (001) surface under a H₂O/H₂-rich atmosphere typically found during steam reforming processes were systematically studied via periodic density functional theory (DFT) and ab initio thermodynamic methods. The various stable configurations of surface species (H₂O*, OH*, O*, , and H*) at different coverages and their formation rates considering different coverage effects of certain species were explored. At a molecular H₂O* adsorption coverage (θ_H2O) ≤1/3 ML, the adsorption of H₂O mainly takes place through single Mo–O coordination, while the capture of H₂O above 1/3 ML relies on hydrogen bonds. H₂O* dissociation resulting in OH* formation is always facile at different H₂O* coverages, whereas it becomes unfavourable as the OH* coverage increases beyond 4/9 ML due to unavoidable strong hydrogen bond breaking. Surface O* can be easily formed via hydroxyl disproportionation with negligible energy barriers, and the protonation of O* by H₂O* is also facile. The dissociation of will easily generate surface Mo–H* and C–H* species, where Mo–H* can readily transform to C–H* with significant exothermicity. The average surface binding strengths of various species at 0 K follow the order: H₂O* > H* ≈ OH* > > O*, where the average binding strength of O* becomes positive when θ_O* ≥ 1/3 ML. At 473.15 K and over a wide H₂O pressure range, mixtures of H₂O*, OH*, and O* are the predominant species on the (001) surface, highlighting the role of the (001) surface in steam reforming reactions, while H* species only emerge at low H₂O pressure or high H₂ pressure. The proportion of O* species decreases and finally tends to zero as the H₂ pressure increases from 10⁻¹⁰ to 10⁻⁷ MPa, while the proportion of OH* species increases due to O* protonation. As the H₂ pressure increases from 10⁻⁷ to 10 MPa, the proportion of OH* species decreased, accompanied by an increase in the H₂O coverage. As the H₂O pressure decreased, the stable existence of surface H* species became increasingly more favorable, and the emergence of surface H* species was accompanied by the disappearance of surface O-containing species, changing the catalytic role of the (001) surface from catalyzing steam reforming processes to promoting hydrogenation reactions.