We report here about an investigation on the role of Nb5+ when used as a promoter for vanadyl pyrophosphate, catalyst for the oxidation of n-butane to maleic anhydride. The effect of Nb was very complex, a function of both its amount and the reaction temperature used. The optimal catalytic behavior was shown for very low Nb contents, i.e., for a V/Nb atomic ratio as low as 150. The main role of Nb was that of accelerating the formation of a limited amount of \uf064-VOPO4 on the surface of vanadyl pyrophosphate, by accelerating the oxidation of V4+ into V5+ under reaction conditions
