The origin of the site preference of H adsorption on Pd(100)

Spin-polarized density functional theory (DFT) calculations are carried out to determine the site preference of H adsorption on Pd(100) surface and subsurface. We carefully scrutinize the energy difference between different patterns at theta=0.50 ML and confirm the LEED observation that surface adsorption can form c(2x2) ordering structure. On the contrary, we disclose that p(2x1) structure become more favorable than c(2x2) for subsurface adsorption. These site preferences are rationalized via an analysis of the layer and orbital resolved density of states. Furthermore, we propose that the interstitial charge as a key factor determining the preferred H adsorbed site.