Adsorbate Site Hopping—More Facts and Fiction: Driving Forces and Relevance for Heterogeneous Catalysis

Abstract

Adsorbate site hopping, defined here as surface-confined migration of adsorbed species or adsorbate equivalents between distinct sites, is a widespread phenomenon in heterogeneous catalysis and includes spillover between different surface phases. Because it enables communication between spatially separated functionalities, site hopping has often been viewed as a route to bifunctional catalytic synergy and, in some cases, to performance beyond constraints imposed by Brønsted–Evans–Polanyi relationships. However, its catalytic value is conditional rather than universal. Downhill migration to a stronger binding site may increase local barriers for subsequent transformation or introduce site-regeneration penalties, while kinetically accessible hopping may remain irrelevant if it does not reduce the energetic span of the catalytic cycle. In addition, apparent rate enhancements assigned to site hopping can arise from other modes of intersite communication, including desorption–readsorption, subsurface or bulk diffusion, ion–electron intersite coupling, catalyst restructuring, or changes in active-site identity. This Perspective develops a thermodynamic and kinetic framework for distinguishing these processes and assessing when site hopping becomes catalytically consequential. We discuss driving forces, hopping barriers, coverage and dynamic surface effects, kinetic signatures, and diagnostic pitfalls, with the aim of clarifying when site hopping can be used as a catalyst design principle.