MIES and UPS study of O-predosed nickel surface reduction by H2 deposition

The present study was aimed at investigating the reduction by hydrogen of an oxygen‐predosed nickel surface at high temperature (530 K) using metastable impact electron spectroscopy (MIES) and UPS (with He(I)) experimental techniques. Both techniques have allowed us to follow the evolution of the electronic structure of the valence band during H2 deposition. The work function variation seems to be related directly to the oxygen coverage of the surface and exhibits two stages: an induction period during which no reduction reaction occurs; a rapid reaction period corresponding to the reduction of oxygen. The reaction mechanism entails the formation of hydroxyl compounds but, owing to their thermal desorption at lower temperature in the form of water, no evident OH features have been observed in the MIES and UPS spectra. The 3d band intensity, measured at 0.5 eV under the Fermi level using UPS, increased linearly during H2 deposition. This evolution could correspond to that of bare Ni atoms in the three or four surface layers. On the other hand, evolution of the MIES signal taken at 5.7 eV under the Fermi level provided us with more accurate information about the evolution of the outermost layer of the predosed nickel surface during H2 exposure. Copyright © 2002 John Wiley & Sons, Ltd.