Abstract
We have previously proposed a model for the catalytic formation of Nb2O5 from niobium covered by some monolayers of cerium, involving a change in the valence state of the cerium oxides on the surface. This model is further evaluated by studying the oxidation of Nb covered by some monolayers of erbium, praseodymium, or terbium under ultrahigh vacuum conditions using x-ray photoelectron spectroscopy. The oxidation took place at room temperature in the 5×10−8 1 Torr pressure range. The experiments showed that Er, which has no other valence state than three, is not a catalyst, while Pr and Tb, which can be both tri- and tetravalent, are catalysts, but only at such high pressures of oxygen, where the tetravalent oxides form. Due to its very low dissociation pressure, CeO2 forms already at 5×10−8 Torr oxygen pressure, therefore, the catalytic action of cerium does not depend on pressure in this range.
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