(323g) Role of Metastable Active Sites in C-H Bond Activation of C1 and C2 Molecules

Since the discovery of Au nanoclusters as active centers for CO oxidation, catalysis discussions are intrigued by the reactivity of nanoclusters. Prominent among them are transition metal nanoclusters, which are well-known to exhibit size and morphology dependent catalytic properties from quantum confinement at nanoscale. In this development, ab-initio level theoretical studies have focused on the reactivity of most stable nanocluster structures, as a representative, to understand overall turnover rates in a catalytic cycle. However, this paradigm has shifted a bit by recent theoretical findings, wherein metastable structures are evaluated for their contributions in measured reaction rates. A comprehensive method to calculate reaction rates is essential to account for the reactivity of metastable active sites. This developing understanding on reaction rates and active sites is serving as a prelude to identify and study heterogeneous catalytic systems. Herein an attempt is made to study reactivity of metastable molybdenum carbide (Mo₂C₆) clusters for C-H bond activation of methane, ethane and ethylene and comparing its activity with the lowest energy cluster. In order to screen the structures of Mo₂C₆ clusters, cascade genetic algorithm based search is applied to scan the potential energy surface. Interestingly, the metastable isomer is observed to show a reduction in C-H bond activation by half for respective alkane and alkene dehydrogenation. Mechanistic elucidation to the reduction in energy barrier is provided by quasi restricted orbital (QRO) analysis in which a unique co-planar orbital overlap between the reactant molecule and active center of the metastable cluster is observed. Motivated from these high reactivity for C-H bond activation observed on the metastable species, a contrasting framework to access the rate contributions of the elementary steps is carried out by constructing a statistical ensemble expression for rate constant. In this framework, metastable isomers are observed to provide a significant contribution for alkane dehydrogenation.