Unraveling Multiscale Kinetics over Subnanometer Cluster Catalysts: H<sub>2</sub> Desorption from Pt<sub>3</sub>(-H)<sub>2</sub>/γ-Al<sub>2</sub>O<sub>3</sub>(110)
George Yan, Dionisios G. Vlachos
Abstract
Despite the attractiveness of highly dispersed supported metal catalysts due to the efficient usage of the active metal component, the structural complexity of subnanometer metal cluster active sites and the interconnectedness of reaction networks over many active site configurations elude detailed understanding. Here, we perform density functional theory (DFT) calculations and state-based kinetic simulations of the desorption of H 2 from Pt 3 (-H) 2 clusters supported on dehydroxylated γ-Al 2 O 3 (110), serving as a prototype of such coupled reaction networks. Different from ideal low Miller index metal surfaces and highly symmetric gas-phase clusters, we find many unique H binding sites on the supported Pt 3 clusters, resulting in an ensemble of metastable Pt 3 (-H) 2 cluster configurations interwoven within a network of H diffusion, active site restructuring, and H 2 desorption elementary steps. Simulations and spectral analysis show that the catalyst and chemistry expose three principal time scales, corresponding to the diffusion of H, restructuring of Pt 3 (-H) 2, and desorption of H 2 . Free energy span-based interpretations of the reaction pathways and sensitivity analysis of the eigenvalues uncover favorable Pt 3 (-H) 2 restructuring and H 2 desorption processes as being kinetically relevant at intermediate and long times. Interestingly, H 2 desorption implicates catalyst restructuring as a prerequisite for forming more favorable desorption channels. We introduce simplified ensemble-based models and effective rate constants for the modeling of such multiscale reaction processes.