In situ electrical conductivity study of titania-supported vanadium-niobium oxide catalysts used in the oxidative dehydrogenation of propane

The electrical conductivity of a series of TiO2-supported vanadia-niobia ca talysts was measured (i) in pure oxygen to characterize the supported activ e phases and (ii) in reacting atmospheres to determine their redox properti es. In vanadia/titania catalysts, minute amounts of pentavalent V5+ ions we re dissolved in the sub-lattice of titania by substitution to Ti4+ sites, t he amount increasing proportionally to the loading of vanadia. Similarly, N b5+ ions also dissolved in the titania sub-lattice, but when niobia were co deposited with vanadia, only V5+ ions dissolved in titania. The variations of the electrical conductivity under oxygen and propane as well as in oxyge n-propane mixture showed a redox behavior of the catalysts with the consump tion of surface O2- anions during the oxidative dehydrogenation of propane and their replacement by gaseous oxygen. A permanent electron transfer exis ts from the reduced supported oxidic phase to titania. Niobium sites are no t easily reduced and slightly affect the redox properties of vanadium sites which are the active sites, whereas niobium acts as an additive favoring t he selectivity to propene. (C) 1999 Elsevier Science B.V. All rights reserv ed.