Surface redox characteristics of mixed oxide catalysts used for selective oxidation

A number of mixed-metal oxides have been reduced in situ by H-2 and NH3 at pressures in the range 5 x 10(-5) to 1 mbar and temperatures of around 520 K. The oxides studied, FeSbO4, Bi2Mo2O9, VSbO4, Bi4V2O11, and USb3O10, were chosen as they represent model systems for the catalysts used in the selective oxidation and ammoxidation of alkanes and alkenes. The surface composition was monitored before and after reduction using X-ray photoelectron spectroscopy (XPS). The ease of reducibility of the oxides was Bi2Mo2O9 (easiest to reduce), FeSbO4, VSbO4, USb3O10, and Bi4V2O11 Following reduction either Bi or Sb metal was observed at the surface of the oxides containing these elements, together with a reduction in the oxidation state of some of the V, Mo, Fe, or U components to 3+, 4+, 2+, and 4+, respectively. Subsequent vacuum annealing between 770 and 520 K resulted in reoxidation of the surface so that the oxidation states of its components were returned to their original value, together with desorption of Bi or Sb. It is believed that diffusion of lattice oxygen to the surface was responsible for this process, with only a relatively thin surface layer of material having been reduced. In some cases the surface composition could not readily be returned to its original value by this annealing process, suggesting limited mobility of the cations at the annealing temperature. (C) 1998 Academic Press.