Ab Initio Framework for Uncovering Structure-Property Relationships of Amorphous Catalysts

Approaches for modeling catalytic sites on amorphous supports lag far behind approaches for modeling catalytic sites on crystalline materials, e.g., zeolites and metal surfaces. A typical approach to model amorphous supports is to use cluster models with arbitrarily chosen constraints to represent the rigid amorphous support, but these constraints arbitrarily influence catalyst site activity. An alternative strategy is to use no constraints, but this results in catalytic sites with unrealistic flexibility. We present a systematic ab initio method to model isolated active sites on insulating amorphous supports using small cluster models. A sequential quadratic programming framework helps us relate chemical properties, such as the activation energy, to active site structure. The algorithm is first illustrated on an empirical valence bond model energy landscape. We then use the algorithm to model an off-pathway kinetic trap in olefin metathesis by isolated Mo sites on amorphous SiO_2.