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 ther mal H and D atoms were studied with AES, TDS and reaction-product moni toring methods in the temperature range 120-400 K. Independent of temp erature 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 di rect mechanism. Independent of temperature, in a second ER step with a cross-section of sigma=4.5 Angstrom(2), adsorbed OH (OD) groups are h ydrogenated 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 temperat ure (at 120 K), 90% of the product molecules remain adsorbed on the su rface and 10% desorb through the energy set free at reaction. an isoto pe 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.