Catalytic Oxidation of Niobium

In situ x-ray photoelectron spectroscopy was used to study the oxidation of niobium films covered by some monolayers of cerium. Significant amounts of Nb₂{O}₅are formed at pressures as low as 6.6 ×{10}^{{-}6}$ Pa, promoted by the cerium overlayer. This catalytic activity is related to the trivalent-to-tetravalent valence change of the cerium during oxidation. The kinetics of ${Nb}₂O₅ formation beneath the oxidized cerium shows two stages; the first is fast growth limited by ion diffusion, the second is slow growth by electron tunneling.