REACTION-KINETICS OF H2 ABSORPTION BY LANTHANUM WITH AND WITHOUT SURFACE OXIDE LAYERS

The kinetics of H-2 absorption by lanthanum with and without surfac oxide layers has been investigated at 77 and 298 K using a volumetric method. Lanthanum film samples carefully prepared under ultrahigh vacuum conditions exhibit an initial reaction probability of unity at 77 and 298 K in the formation of a hydrogen solution in lanthanum. The dihydride formation proceeds at an almost constant rate at 298 K until the sample becomes completely hydrided whereas the reactivity decreases as the hydride layer grows at 77 K. The pressure dependence of the reaction rate indicates that the rate-controlling step in the H2 absorption by lanthanum changes according to the surface conditions of lanthanum: for lanthanum with a clean surface, H-2 transport in the gas phase is rate controlling at H-2 pressures lower than 10(-9) mbar, and with increasing H-2 pressure or hydrogen surface concentration H-2 dissociation as a rate-controlling step is replaced by hydride formation at the hydride-lanthanum interface. For lanthanum with oxide layers, two rate-controlling steps, the H-2 dissociation on and the hydrogen permeation through the oxide layers, seem to be important.