Impact of surface adsorbed gases on hydrogen diffusion into Pd(1 0 0) subsurface from first principles

Abstract Hydrogen diffusion from metal catalyst surface into subsurface is the key step for the heterogeneous catalysis, hydrogen purification, and hydrogen-induced embrittlement in practical applications. However, it has been scarcely concerned by previous works, especially with distinct types of adsorbates covered on surface. To deeply understand the impact of adsorbates on the microscopic mechanism for hydrogen diffusion into subsurface, four representative types of gases (H, O, O2, and OH) were chose to serve as adsorbates with varying coverages, and all possible minimum energy pathways were globally searched out on base of an appropriate Pd(1 0 0) surface by the NEB method. The present calculation results imply that the barriers, the stable adsorption and absorption sites for the forward and inverse diffusion routes are greatly influenced by the main features for adsorbed gases, which include electronic property, spatial structure, the relative adsorption position, and the variation of coverages. The effect ranking for four types of adsorbed gases on lowering the barriers of hydrogen diffusion into subsurface is: O2 > O > OH > H. One exothermic process at 0.5 ML coverage particularly exhibits the good performance of oxygen gas in facilitating hydrogen diffusion. However, hydrogen is the special one that would increase diffusion barrier for the forward process at hydrogen coverages of 0.25 and 0.75 ML. Additionally, the substrate relaxation effect that plays a significant role in lowering diffusion barrier is also affected by the varying coverages of hydrogen and oxygen atoms.