Assessing the Role of Surface Carbon on the Surface Stability and Reactivity of ..Beta..-Mo2C Catalysts

Transition-metal carbides, e.g., Mo₂C, promote industrially important catalytic reactions with activities comparable to precious-metal catalysts. Yet, the nature of the catalytically active sites remains unclear. Herein, density functional theory calculations are applied to systematically assess the stability of low-Miller-index facets of β-Mo₂C as a function of carbon chemical potential (μ_C) and surface-carbon coverage (n_C). When reconstruction of surface carbon is considered to characterize β-Mo₂C under varying environments, the (1 1 1) and (1 0 1) surfaces are predicted to be most stable, dominating the Wulff particle surface area across a broad range of μ_C. The surface-carbon coverage is predicted to steadily increase for all low-index facets under more carburizing reaction conditions, leading to distinct effects on the adsorption energetics for atomic hydrogen and oxygen. Whereas H binding is only slightly affected by surface carbon, O is systematically destabilized. The findings reported indicate that consideration of only bulk-terminated surfaces, and the common focus of theoretical studies on the bulk-truncated, close-packed (1 0 0) surface, is likely insufficient to capture the reactivity of the working β-Mo₂C catalyst. Similar to metal oxides, β-Mo₂C catalysts are predicted to be dynamic materials with surface compositions and reactivities that evolve in response to the environment.