Dopant-Centered versus Intersite Synergistic Mechanisms in H₂ Dissociation on Single-Atom Alloys

Six-dimensional (6D) quantum dynamics simulations on globally accurate neural network potential energy surfaces reveal two distinct mechanisms for H₂ dissociation on M₁/Au(111) (M = Cu, Pd, Ag) single-atom alloy (SAA) surfaces. Cu and Pd follow the conventional dopant-centered pathway, whereas Ag operates via a counterintuitive intersite synergistic mechanism, electronically activating neighboring Au atoms rather than serving as the active site itself. Despite doping reducing the static barrier from 1.25 eV on pristine Au(111) to 0.85, 0.26, and 1.02 eV for Cu, Pd, and Ag, respectively, a universal high-energy crossover emerges in the 6D dissociation probabilities, in which all SAA surfaces are outperformed by Au(111) at elevated collision energies. This crossover originates from dopant-induced steric and azimuthal constraints at hollow sites that restrict the reactive phase space at high energies. These findings establish a classification framework for SAA reactivity based on the spatial distribution of reactive regions induced by the dopant.