Catalytic Activity of Nonstoichiometric Niobium Oxide with Controlled Composition, NbO2.488–2.500, for Butene Isomerization

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Abstract
The composition of niobium oxide was controlled between NbO2.488 and NbO2.500 by treating with mixture of water vapor and hydrogen gas at 873K. X-ray diffraction (XRD) study of the treated niobium oxide showed that only a phase of nonstoichiometric T-Nb2O5 was found in the range of NbO2.488–2.500 and a phase of NbO2 coexisted with the T-Nb2O5 phase by further reduction. The catalytic activity of the oxide for cis-2-butene isomerization had a maximum value at the O/Nb ratio of 2.494. The electron spin concentration of Nb4+ ion, and the amount of nitrogen monoxide adsorbed on the niobium oxide also showed a maximum value at the O/Nb ratio of 2.496 and 2.494, respectively. Coincidence in the dependences, on the composition of the reduced niobium oxide, of the catalytic activity, the spin concentration of Nb4+ ion, and of the amount of adsorbed nitrogen monoxide shows that isolated Nb4+ ion with unsaturated coordination is a catalytically active site.