Hydride-induced palladium self-diffusion and surface restructuring on Pd(111)

Reaction conditions significantly affect the electrocatalyst surface but are often overlooked when evaluating catalytic performance. Pd forms hydride phases during proton reduction under cathodic potentials, which can lead to surface reconstructions and defect formation. However, the influence of hydride formation on Pd atom migration remains underexplored. Here, we show, using density functional theory and molecular dynamics simulations, the thermodynamic and hydride conditions associated with surface reconstruction in PdH_x/Pd(111) systems during the initial stages of hydride formation. Surface Pourbaix diagrams show the predicted stability of PdH_x/Pd phases under reaction conditions, while calculated Pd migration barriers decrease with hydride sublayers and become negligible at high hydrogen concentrations. Atomistic simulations further show that at least three subsurface hydride layers are necessary for observable reconstruction, dislocation and defect formation, during which Pd atoms predominantly migrate outward through strain relaxation while hydrogen atoms locally adjust to the reconstructed lattice. Overall, our results suggest an interplay between hydride formation and atom mobility, highlighting the potential importance of dynamic surface transformations when designing and evaluating Pd-based electroreduction catalysts.