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 Brønsted–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.