A kinetic study of the interaction of gaseous H(D) atoms with D(H) adsorbed on Ni(100) surfaces

The kinetics of reactions which occur upon subjecting D(H) covered Ni(100) surfaces with H(D) atom fluxes were investigated. At 120 K surface temperature in the H-->D-ad reaction HD and D-2 were observed as reaction products, in the D-->H-ad reaction HD and H-2 were reaction products. As the reaction temperature was well below the hydrogen desorption temperature, a direct reaction step, like in the Eley-Rideal (ER) mechanism, is suggested to operate for HD production. However, the characteristics of the HD formation kinetics observed in the present study contradict an essential element of the ER: mechanism the rate: of HD formation is not proportional to the surface coverage of the adsorbed reaction species D or H under impact of a flux of H or D atoms. Therefore, a modification of the mechanistic description of atom/surface reactions seems necessary. This modification should allow for reaction products which are completely unaccounted for in the ER picture: D-2 from H-->D-ad and H-2 from D-->H-ad reactions. The observed strong isotope effects in the reaction efficiencies support mechanisms in which the impinging gas phase atoms get trapped in the surface potential well prior to reaction. (C) 1997 American Institute of Physics.