Adsorption and diffusion of hydrogen on Pd(211) and Pd(111): Results from first-principles electronic structure calculations

We have carried out first-principles calculations of H adsorption on Pd 211 using density-functional theory with the generalized gradient approximation in the plane-wave basis to find out that the most preferred is the threefold hollow site on the terrace of Pd 211 with an adsorption energy of 0.52 eV: the hcp and fcc sites being almost energetically equally favorable. For subsurface H adsorption on Pd 211 , the octahedral site with an adsorption energy of 0.19 eV is slightly more favorable than the tetrahedral site 0.18 eV . Our calculated activation energy barrier for H to diffuse from the preferred surface site to the subsurface one on Pd 211 is 0.33 eV, as compared with 0.41 eV on Pd 111 . Thus, there is an enhancement of the probability of finding subsurface hydrogen in Pd 211 . Additionally, we find the diffusion barriers for H on the terraces of Pd 211 to be 0.11 eV, while that along the step edge to be only 0.05 eV and that within the second layer subsurface to be 0.15 eV.