Clusters as catalysts: advantages and challenges
Cluster science is a fascinating realm of extensive research activity. The experimental realization and computational studies of various types of clusters, starting from alkali atoms to transition metals, have hogged much attention from the scientific community since the last decades. Modelling these clusters helps one in gaining the insight of the various atomic level mechanisms involved in developing novel materials. The chapter provides an overview of some of the theoretical models applicable to cluster models along with their impediments and strengths. Theoretical formalisms have been well developed to study the structural transition from clusters to bulk phases, phase changes and quantum effects in clusters. Transition metal oxides have wide applicability as both catalysts and catalytic supports in industrial processes. However, the mechanisms by which these materials function as catalysts and the structure–reactivity relationships are not well understood. A valuable approach to identify the active sites of transition metal oxides is to study the chemistry of gas phase metal oxide clusters. We present our findings on reactions of group V and VI transition metal oxide clusters, with particular emphasis on the mechanism of oxygen transfer to hydrocarbons that provide valuable information about reaction intermediates, reaction mechanisms, structure–reactivity relationships and two state reactivity phenomena. It is stressed that experimental realization requires these clusters to be deposited on a substrate which not only changes the electronic and geometric structure of the cluster, but also brings about new relaxed energy pathways. The chapter ends with a discussion of the challenges ahead in realistic design of cluster based catalysts.