Computational Study of Single-Site Molybdenum Catalysts on Amorphous Silica

Amorphous silica is an extremely commonly used catalyst support yet there are very few experimental or computational studies on catalyst-support interactions for this material. This is largely due to the inherent difficulty in modeling and experimental characterization of amorphous silica. We have used a recently developed slab model for amorphous silica surfaces to study the support effects on single-site molybdenum catalysts. We have found that the local structure of the silica support in the vicinity of the Mo site has a profound effect on the energetics and kinetics of ethene metathesis. We have compared site energies, reaction energies, and reaction barriers computed from simple cluster models with results from the slab models. The cluster models show a clear relationship between Si-Si distances and the site energies and reaction energies. In contrast, the slab model shows no correlation between Si-Si distances and energetics. Likewise, the reaction barriers increase with increasing Si-Si distances in the cluster model but show no trends in the slab model. We have found that simple cluster models of the surface are not able to capture the rich heterogeneity of an extended surface model containing hundreds of atoms. As a result, trends that appear in the cluster models do not appear in the larger extended models.