Interaction of water with a Pd(110) surface, studied by thermal desorption spectroscopy, LEED and Δφ

Abstract

The interaction of water with a Pd(110) surface has been studied by thermal desorption spectroscopy (TDS), work function change (Δφ) and low energy electron diffraction (LEED). TD spectra of H 2O adsorbed on Pd(110) at 100 K show two peaks, one (A 2) at ∼ 200 K and the other (C 1) at ∼ 150 K. The A 2 state is interpreted as arising from molecularly chemisorbed H 2O. Saturation of the A 2 state causes a work function decrease of ∼ 780 mV. The chemisorbed water associated with the A 2 state in TDS is associated with a c(2 × 2) LEED pattern. The c(2 × 2) structure of H 2O on Pd(110) is probably analagous to that of H 2O on Ni(110). No dissociation of H 2O is found during adsorption and desorption of the A 2 state. No equivalent of the A 1 state observed on Ni(110) is detected on clean Pd(110). This difference between H 2O/Ni(110) and H 2O/Pd(110) is explained by a larger heat of formation of NiO than that of PdO. The C 1 peak arises from desorption of multilayers of water (so-called ice layers). It exhibits zero order kinetics with an activation energy E a = 38 kJ mol −1 H 2O. H 2O in the C 1 state contributes Δφ = −230 mV, which is m caused by the first layer of ice. A precoverage of oxygen on Pd(110) induces two peaks in TDS of H 2O. One (A 1) at low coverage of oxygen (θ o) is believed to arise from dimer clusters of H 2O which are stabilized by the preadsorbed O. The other (C 2) at high θ o is probably from an ice layer that is strongly influenced by the surface O. Preadsorption of deuterium causes a continuous shift of the A 2 peak of H 2O to lower temperature and induces a new peak (A 3) at high deuterium coverages (θ D) This shift to lower T is explained as due to the weakining of the surface-water bond by adsorbed D.