Operando Effects on the Structure and Dynamics of Pt<i>_n</i>Sn_<i>m</i>/γ-Al₂O₃ from Ab Initio Molecular Dynamics and X‑ray Absorption Spectra

Abstract

Alumina-supported Pt–Sn nanocluster catalysts are widely used in reforming processes, yet a theoretical understanding of their structure and function is far from complete. In an attempt to elucidate their behavior under operando conditions, we have carried out a detailed investigation of nanoscale bimetallic clusters of Pt and Sn supported on γ-Al₂O₃ using a combination of finite temperature ab initio molecular dynamics and theoretical X-ray absorption spectroscopy (XAS). Our simulations reveal a rich nonequilibrium structure over several time scales, with vibrational and anomalous structural disorder and fluctuating charge transfer to the support. In contrast with bulk Pt–Sn materials, the clusters are found to be markedly inhomogeneous, with substantial differences between surface and internal structure. The Sn atoms are preferentially segregated to the surface and fluctuate between different Pt bonds over a picosecond time scale. Importantly, these properties show how an improved XAS analysis of these systems should take into account both their inhomogeneity and dynamic structural disorder. Although our study is limited to small nanoclusters due to the limitations of ab initio molecular dynamics, we argue that their unusual dynamical structure also has important implications for catalytic behavior of these systems, which is briefly illustrated by the adsorption and dissociation reactivity of H₂