The role of niobium oxide as a surface catalyst for hydrogen absorption

The catalytic activity of niobium oxide for hydrogen absorption is determined using an optical indicator technique to measure hydrogen sorption kinetics. The catalytic activity of NbOx is studied as a function of the oxygen concentration determined from electron spectroscopy. STM measurements display a clustered NbOx surface, disproving a diffusion-controlled kinetics. The catalytic effect results from dissociation of hydrogen by niobium oxide. The measured activation energy depends strongly on the oxygen concentration and reaches a minimum of 0.6 eV at the highest oxygen content (x=2.5, i.e., Nb2O5). Pure Nb does not display a catalytic effect. Oxide surfaces with a high concentration of artificial oxygen vacancies are less catalytically active.