From surface to subsurface: theoretical insights into hydrogen spillover dynamics

Hydrogen spillover, the migration of dissociated hydrogen from metal sites to supports, has long been studied as a surface-confined phenomenon. Observations of subsurface and bulk hydrogen in surface science suggest that spillover-generated hydrogen may dynamically access subsurface or bulk transport channels. However, the dynamics of this entry process remain poorly understood. In this opinion article, we argue that hydrogen spillover should be considered through surface migration, penetration, and subsurface transport. Machine-learning-accelerated simulations on single-atom near-surface alloys reveal two penetration pathways, collision-induced and thermally assisted, whose competition is governed by nonequilibrium conditions. Once subsurface, hydrogen may exhibit high mobility under specific energetic and structural conditions. This surface-to-subsurface view motivates subsurface accessibility, pathway branching, and postentry mobility as candidate descriptors for catalyst design.