Interaction of H atoms with oxygen adsorbed on Ni(100) surfaces: Direct reactions towards OH and H2O

The reactions of oxygen adlayers on Ni(100) surfaces with gaseous thermal H and D atoms were studied with AES, TDS and reaction-product monitoring methods in the temperature range 120-400 K. Independent of temperature and coverage, adsorbed O atoms are hydrogenated to adsorbed OH (OD) by impinging H(D) atoms. A cross-section of sigma=0.3 Angstrom(2) suggests that this reaction proceeds via an Eley-Rideal (ER)-type direct mechanism. Independent of temperature, in a second ER step with a cross-section of sigma=4.5 Angstrom(2), adsorbed OH (OD) groups are hydrogenated to water. Well above the water desorption temperature (180 K) the product molecules desorb either through a thermally activated or an energetically mediated step. Below the water desorption temperature (at 120 K), 90% of the product molecules remain adsorbed on the surface and 10% desorb through the energy set free at reaction. an isotope effect (H/D) on the cross-sections was not observed. The processes were observed independent of the oxygen coverage and also at oxidized Ni surfaces, which could be reduced completely at sufficiently high H or D atom fluences. (C) 1997 Elsevier Science B.V.