SURFACE PROCESSES OF H-2 IN THE INITIAL ACTIVATION OF LANI5

The surface processes of H-2 molecules in the initial activation of LaNi5 are demonstrated quantitatively by systematic measurements of the rate of H-2 absorption by LaNi5 with various surface conditions prepared in ultrahigh vacuum, low vacuum and air in the H-2 pressure range 10(-10) mbar-10(1) bar. The reaction probability of H-2 with LaNi5 is reduced from 1 to 10(-3) at 298 K when the vacuum condition in the degassing treatment of LaNi5 is changed from 10(-10) to 10(-5) mbar. The dissociation of H-2 molecules is markedly inhibited by the oxides and hydroxides of lanthanum and nickel which are formed during exposure of LaNi5 to low vacuum or air. Increasing amounts of these surface layers reduce the reaction probability to 10(-6). However, the kinetic data indicate that the dissociation of H-2 molecules can take place on lanthanum oxides. The addition of a small quantity of nickel to lanthanum oxides markedly facilitates H-2 dissociation and subsequent hydrogen permeation through the lanthanum oxide layers. The presence of suboxides such as LaOx, La2O3-x or La-NiOx in the surface region seems to be responsible for the ready activation of LaNi5 exposed to low vacuum or air. The activation model proposed in this report does not require the presence of metallic nickel or nickel clusters in the surface region.