HREELS STUDIES OF HYDROGEN ADSORPTION ON PD(311)

The adsorption of hydrogen on Pd(311) at 100 K has, been investigated by high resolution electron energy loss spectroscopy (HREELS). The obs erved vibration energies are similar to former values reported on low index Pd surfaces: which allowed relating the vibration modes with the different adsorption sites on the (311) surface. In contrast to Ni(31 1) and Rh(311), hydrogen adsorption is found to begin at the fourfold- coordinated instead of the threefold-coordinated sites. The latter sit es are occupied only at coverages greater than 0.5 ML. The hydrogen ad sorption behavior on Pd(311) seems to be ruled by the possibility of o ccupying threefold and fourfold sites and by the H-H repulsion.