First-Principles Investigation of Surface and Subsurface H Adsorption on Ir(111)

Abstract

The atomic hydrogen adsorption on Ir (111) surface and in its subsurface with coverage from 0.11 monolayer (ML) to 2 ML is investigated by using density functional calculations with the generalized gradient approximation (GGA), the diffusion for hydrogen into the subsurface region at 0.25 ML coverage is also calculated. At all sites of our calculations, the most favorable site is hydrogen on top of Ir (111). The corresponding binding energy of a hydrogen atom at this site is 2.71 eV/H atom at the coverage of 0.25 ML, favored by about 0.05 eV over the next most stable site, the fcc site. For on-surface adsorption, the binding energy decreases very slowly for all of the sites as the hydrogen coverage increases from 0.25 to 1.00 ML, it indicates that there is only a weak repulsive interaction between the adsorbates. For the coverage range of 0.11−0.25 ML, the binding energy increases evidently, suggesting an attractive interaction between adsorbates. Compared to on-surface hydrogen adsorption, subsurface hydrogen adsorption is energetically unfavorable and takes place when on-surface hydrogen coverage is greater than full monolayer. For mixed structures at full monolayer both of surface and subsurface, it is found that the most favorable site is the hcp/octa site with a binding energy 2.35 eV/H atom. So when full monolayer hydrogen atoms adsorbed in the hcp site on the surface, H begins to adsorb on the subsurface in the octa site. Compared to hydrogen adsorption on other transition metals (Pt, Rh, Pd, Co, and Ni) with localized feature, it is found hydrogen adsorption on Ir (111) shows some delocalized nature. Meanwhile, at 0.25 ML coverage for H adsorption on Ir(111), the desorption energy for hydrogen from the Ir surface is found to be 0.96 eV, the calculated barrier for hydrogen from hcp to fcc is 0.02 eV, and penetration into the subsurface region from the on surface fcc site is 1.26 eV.