INTERACTION OF WATER WITH A PD(110) SURFACE, STUDIED BY THERMAL-DESORPTION SPECTROSCOPY, LEED AND DELTA-PHI

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 H2O adsorbed on Pd(110) at 100 K show two peaks, one (A2) at ∼ 200 K and the other (C1) at ∼ 150 K. The A2 state is interpreted as arising from molecularly chemisorbed H2O. Saturation of the A2 state causes a work function decrease of ∼ 780 mV. The chemisorbed water associated with the A2 state in TDS is associated with a c(2 × 2) LEED pattern. The c(2 × 2) structure of H2O on Pd(110) is probably analagous to that of H2O on Ni(110). No dissociation of H2O is found during adsorption and desorption of the A2 state. No equivalent of the A1 state observed on Ni(110) is detected on clean Pd(110). This difference between H2O/Ni(110) and H2O/Pd(110) is explained by a larger heat of formation of NiO than that of PdO. The C1 peak arises from desorption of multilayers of water (so-called ice layers). It exhibits zero order kinetics with an activation energy Ea = 38 kJ mol-1 H2O. H2O in the C1 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 H2O. One (A1) at low coverage of oxygen (θo) is believed to arise from dimer clusters of H2O which are stabilized by the preadsorbed O. The other (C2) at high θo is probably from an ice layer that is strongly influenced by the surface O. Preadsorption of deuterium causes a continuous shift of the A2 peak of H2O to lower temperature and induces a new peak (A3) at high deuterium coverages (θD) This shift to lower T is explained as due to the weakining of the surface-water bond by adsorbed D. © 1990.