Theoretical Cluster Studies on the Catalytic Sulfidation of MoO₃

Density-functional theory together with large surface clusters is applied to study elementary processes of the catalytic sulfidation of the MoO 3 (010) surface. For all sites, surface oxygen is found to bind more strongly with its substrate environment than the corresponding sulfur substitute with binding distances that are shorter for oxygen than for sulfur. Sulfur−oxygen exchange reactions are energetically preferred over sulfur adsorption at MoO 3 (010). The first and second sulfur substitution takes place preferentially at the terminal oxygen site O(1) where the two steps are energetically similar. Further, sulfur binding is found to be facilitated by the existence of surface oxygen vacancies where sulfur substitution takes place preferentially at the terminal oxygen sites O(1) and O(1)′. On the basis of the theoretical results, different sulfidation schemes are considered. They indicate that sulfidation of the MoO 3 surface is facilitated by hydrogen participating in the reaction.