Impact of Support Interactions for Single-Atom Molybdenum Catalysts on Amorphous Silica

Amorphous silica is a commonly used catalyst support, yet there are relatively few experimental or computational studies on catalyst support interactions for this material. This is largely due to the inherent difficulty in modeling and experimentally characterizing amorphous silica. We used a recently developed surface model for amorphous silica surfaces to study the support effects on single-atom molybdenum catalysts. We 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 metallacycle rotation, which is related to ethene metathesis. We have compared site energies, reaction energies, and reaction barriers computed from simple cluster models with results from surface models. The cluster models show a clear relationship between Si-Si distances and the site energies and reaction energies. In contrast, the surface model shows no correlation between Si-Si distances and energetics. The reaction barriers clearly increase with increasing Si-Si distances in the cluster model, whereas there is only a qualitative trend in the surface model. Analysis of the surface results indicates that the reaction energetics are affected by neighboring hydroxyl groups and Si atoms in the surface that are not accounted for in the cluster models. We therefore conclude that the simple trends relating support atom geometries to reaction energetics observed in the cluster models are artifacts of the model.