Molecular and Atomic Hydrogen Interactions with Au-Ir Near-Surface Alloys

Using a combination of density functional theory and experimental electrochemical methods, we have investigated Au-Ir near-surface alloys and their ability to take up hydrogen (H). Despite the relative instability of H in the subsurface of pure Au(111) and Ir(111), H in the subsurface of a near-surface alloy made of a monolayer of Au over Ir(111) (Au*/Ir) is stabilized compared to surface H. While dissociative adsorption of H-2 is activated on this alloy surface, the presence of subsurface H stabilizes the transition state for further H-2 dissociation. This is explained by the upshift in the d-band center of the surface Au atoms induced by the presence of subsurface H in Au*/Ir. Submonolayers of Au on Ir(111) stabilize H at the Au-Ir interface while allowing for nonactivated H-2 dissociation at the exposed Ir atoms. Synthesis of submonolayer alloys of Au on Ir(111) shows that Au does not block hydrogen adsorption on the Ir surface, despite the inability of pure Au to take up hydrogen under these conditions. The possible existence of subsurface hydrogen in these alloys may explain this unexpected behavior.