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

The kinetic and thermodynamic aspects of the surface phase structure evolution of an fcc MoC (001) surface under a H2O/H-2-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*, H-2* and H*) at different coverages and their formation rates considering different coverage effects of certain species were explored. At a molecular H2O* adsorption coverage (theta(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* approximate to OH* > H-2* O*, where the average binding strength of O* becomes positive when theta(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 H-2 pressure. The proportion of O* species decreases and finally tends to zero as the H-2 pressure increases from 10(-10) to 10(-7) MPa, while the proportion of OH* species increases due to O* protonation. As the H-2 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.