Behavior of hydrided and dehydrided LaNi 5H x as an hydrogenation catalyst

Abstract

The catalytic properties of varying phase compositions of LaNi 5H x were studied using the hydrogenation of 1-undecene as a model reaction. All experiments were carried out with the catalyst in liquid suspension at 308 K under varying pressures of hydrogen. The H content of the catalyst particles was tracked during the course of each experiment. Four types of catalytic behavior were observed and are catalogued in turn. While the metal solid solution phase (α-LaNi 5H x ) exhibits catalytic properties, catalytic activity is significantly enhanced by the presence of the hydride phase (β-LaNi 5H x ) in the particle bulk even though the surface composition of the particle is likely unchanged. It was concluded that several factors contribute to this enhancement: (1) the hydride phase acts as a reservoir of H atoms which communicate with the surface via microcracks and grain boundaries; (2) the substantial increase in bulk volume upon the conversion of the metal to the hydride phase enlarges these conduits and facilitates said communication; (3) weak chemisorption of hydrogen on the surface, due to the high hydrogen atom activity when the hydride phase is present, increases the amount of labile hydrogen.