In situ measurements of the electrical conductivity of bismuth molybdate catalysts in operation for oxidative dehydrogenation of butene

In situ measurement of electrical conductivities were performed on MoO3, Bi2Mo2O9, and Bi2MoO6 (koechlinite), the latter either as such or doped with small amounts of Mo or Bi; the reaction applied was the oxidative dehydrogenation of butene to butadiene. The reaction was performed in continuous flow or by pulsing butene over the catalyst, often followed by the addition of O2 pulses. The catalysts were characterized by surface area, XRD, and XPS. The latter measurements showed that surface Bi/Mo ratios were often different from those in the bulk; reduction by butene at temperatures around 673 K sometimes led to considerable changes in the surface Bi/Mo ratio that ran parallel with changes in activity. Pulsing butene in a He carrier stream over the catalyst strongly increased the electrical conductivity while pulsing O2 decreased it again; the conductivity is almost entirely located in the surface layers. The koechlinite-type catalysts, when heated in He, showed a considerably higher conductivity than the other catalysts. This is ascribed to a dissociation of O2 presumably from the Bi2O2 layers. Subsequent pulsing of butene first increased the conductivity but for catalysts with Bi/Mo bulk ratios >1, the conductivities became constant while reduction continued. The first process is supposed to be connected with a stripping of the oxygen in the surface layer, the second with a migration of O2- from bulk to surface and of electrons from surface to bulk. A model was presented for the surface band.