HREELS Studies of Hydrogen Adsorption on Pd(311)

Abstract

The adsorption of hydrogen on Pd(311) at 100 K has been investigated by high resolution electron energy loss spectroscopy (HREELS). The observed 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(311) and Rh(311), hydrogen adsorption is found to begin at the fourfold-coordinated instead of the threefold-coordinated sites. The latter sites are occupied only at coverages greater than 0.5 ML. The hydrogen adsorption behavior on Pd(311) seems to be ruled by the possibility of occupying threefold and fourfold sites and by the H–H repulsion.

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

“…This behavior was also observed for other electron energies. 16 The change in the slope at 0.1 L may well be due to completion of an ordered H phase and is in accord with the observation of the ͑2ϫ1͒H phase with HAS. The much slower change in intensity observed above 0.6 L suggests that the surface is saturated, which is again in agreement with the HAS data, where from 0.5 L on the saturated c(1ϫ1) structure was observed.…”

“…A full discussion of the site assignment for hydrogen on the surface can be found in Ref. 16. Although a simple comparison of vibration energies between different surfaces to determine adsorption sites can be erroneous, in the present case the comparison with the low index surfaces ͑100͒, ͑111͒, ͑110͒ and the recent investigations of Muschiol et al 33 for Pd͑210͒ leads to a conclusive assignment of the peaks.…”

“…Although a simple comparison of vibration energies between different surfaces to determine adsorption sites can be erroneous, in the present case the comparison with the low index surfaces ͑100͒, ͑111͒, ͑110͒ and the recent investigations of Muschiol et al 33 for Pd͑210͒ leads to a conclusive assignment of the peaks. 16 In Table I the vibration energies for hydrogen on Pd͑311͒ from our previous investigation 16 are compared with the energies found on other Pd surfaces. In Fig.…”

“…8 the peak at 80-85 meV can be attributed ͑see Table I and Ref. 16͒ to the vibration parallel to the fourfold microfacet, and the peaks at 94 and 120-126 meV to the parallel and perpendicular vibrations in threefold sites. 16 The surface exhibits C s symmetry so that for each adsorption site two dipole allowed vibrations are expected. The second dipole active vibration of hydrogen in the fourfold site is visible as a shoulder at 56 meV and is better resolved in the scattering geometry with the scattering plane arranged perpendicular to the closed packed rows ͑not shown, see Ref.…”

“…15 Since no evidence for any substrate reconstruction was found, it was suggested that this behavior may well be caused by the occupation of different adsorption sites at 100 K. Preliminary high resolution electron energy loss spectroscopy ͑HREELS͒ experiments performed in our group support this picture, since hydrogen was found to adsorb on the fourfold hollows at ⌰Ͻ0.5 ML, whereas both threefold and fourfold coordinated sites are occupied at ⌰Ͼ0.5 ML. 16 In this work we present a detailed study on the migration of chemisorbed hydrogen into subsurface sites on Pd͑311͒ by means of He-atom scattering ͑HAS͒, thermal desorption spectroscopy ͑TDS͒, high resolution electron energy loss spectroscopy, and work function measurements. Our main goal will be to better understand the physics involved in the activated population of subsurface sites on Pd͑311͒.…”