INTERACTION OF WATER WITH CLEAN AND OXYGEN PRECOVERED NICKEL SURFACES

The adsorption of water on a Ni(1 1 1) single crystal surface, clean a s well as precovered with oxygen, has been investigated with thermal d esorption spectroscopy (TDS) and measurements of the adsorption-desorp tion equilibrium combined with XPS (X-ray photoelectron spectroscopy). The measurements have been carried out with water pressures up to 10( -5) mbar on surfaces, which have been either clean or precovered with oxygen. On the clean Ni(1 1 1) surface the first adsorbate layer with a maximum coverage of 0.42 ML (monolayers) has a desorption energy of 52 kJ/mol and a preexponential factor of desorption of 10(16) s(-1). A second water layer adsorbs with the desorption energy of the ice mult ilayer but with first order kinetics. On Ni(1 1 1) precovered with che misorbed oxygen an additional state of molecular, more strongly bound water is found, but no dissociation. For higher oxygen precoverages wh ere NiO islands are formed on the surface, also the water dissociation product OH is found adsorbed. On a sample covered with a closed NiO l ayer, adsorbed OH and molecular water in an energetically not well-def ined state are found. High doses of water on oxygen-precovered Ni(1 1 1) induce a slow surface modification leading to water dissociation.