The transition of chemisorbed hydrogen into subsurface sites on Pd(311)

Abstract

The activated transition of chemisorbed hydrogen atoms into subsurface sites on Pd(311) has been investigated by means of He-atom scattering, high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and work function measurements. At 120 K, hydrogen exposure leads to the formation of (2×1)H, (2×1)2H, (2×1)3H and c(1×1) 2H phases, with coverages of 0.25, 0.50, 0.75, and 1 monolayers (ML), respectively. The TDS data show three desorption states: α at ∼170 K, β1 at ∼285 K and β2 at ∼310 K. Chemisorbed H atoms forming the ordered layers desorb in the β2 state, whereas the β1 is originated by H atoms located at subsurface sites. The α state is originated by decomposition of layers of Pd hydride near the surface. In all four phases, long-range order disappears at ∼170 K. Heating to 220 K leads to the migration of 0.25 ML H atoms into subsurface sites only if the coverage of the disordered layer is greater than 0.5 ML. The HREELS data demonstrate that this behavior is caused by the occupation of different adsorption sites as a function of coverage: only fourfold coordinated sites are occupied in the (2×1)H and (2×1)2H phases, whereas threefold coordinated sites are also occupied for Θ>0.5 ML. A surprising result is that the HREELS peaks of the surface hydrogen vibrations still exhibit significant changes once all surface sites are occupied, and saturate only after saturation of the subsurface sites. This effect presumably results from strong repulsion between H atoms adsorbed on threefold coordinated sites and subsurface H atoms located in octahedral sites.