Investigations of the selective population of hydrogen subsurface sites on Pd(110) using He diffraction and thermal-desorption spectroscopy

Abstract

Surface-structure models for the 2×1 and 1×2 hydrogen chemisorption phases formed on Pd(110) at 100 K have been derived from He-diffraction data. The respective coverages correspond to 1 and 1.5 monolayers (ML). Upon heating to 200 K, the 1×2 saturation phase transforms back into the 2×1, and 0.5 ML hydrogen moves subsurface. Based on structural arguments, we suggest that only the first available subsurface sites, i.e., the octahedral interstitials between topmost and second layers are populated by thermal activation. The subsurface movement is eased since H-chemisorption sites on top of the second Pd layer can be occupied in the 1×2 owing to substrate reconstruction. Structural considerations also explain that exactly 1 ML H can be accommodated subsurface by thermal cycling. New TDS measurements corroborate these notions: only the α2 desorption state, probably associated with H in subsurface sites between first and second Pd layers, is selectively filled by the thermal-cycling processes. The α1 state remains empty upon thermal cycling, and is very likely connected with hydrogen deeper in the bulk. © 1986 Springer-Verlag.