Elementary steps of syngas reactions on Mo₂C(0 0 1): Adsorption thermochemistry and bond dissociation

Density functional theory (DFT) and ab initio thermodynamics are applied in order to investigate the most stable surface and subsurface terminations of Mo₂C(0 0 1) as a function of chemical potential and in the presence of syngas. The Mo-terminated (0 0 1) surface is then used as a model surface to evaluate the thermochemistry and energetic barriers for key elementary steps in syngas reactions. Adsorption energy scaling relations and Bronsted-Evans-Polanyi relationships are established and used to place Mo₂C into the context of transition metal surfaces. The results indicate that the surface termination is a complex function of reaction conditions and kinetics. It is predicted that the surface will be covered by either C₂H₂ or O depending on conditions. Comparisons with transition metals indicate that the Mo-terminated Mo₂C(0 0 1) surface exhibits carbon reactivity similar to transition metals such as Ru and Ir, but is significantly more reactive toward oxygen. This explains some previous observations for Mo₂C catalysts and suggests that Mo₂C may exhibit unique and potentially useful reactivity or selectivity patterns.