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 NH 3 at pressures in the range 5 x 10(-5) to 1 mbar and temperatures of a round 520 K. The oxides studied, FeSbO4, Bi2Mo2O9, VSbO4, Bi4V2O11, an d USb3O10, were chosen as they represent model systems for the catalys ts used in the selective oxidation and ammoxidation of alkanes and alk enes. The surface composition was monitored before and after reduction using X-ray photoelectron spectroscopy (XPS). The ease of reducibilit y of the oxides was Bi2Mo2O9 (easiest to reduce), FeSbO4, VSbO4, USb3O 10, and Bi4V2O11 Following reduction either Bi or Sb metal was observe d at the surface of the oxides containing these elements, together wit h a reduction in the oxidation state of some of the V, Mo, Fe, or U co mponents to 3+, 4+, 2+, and 4+, respectively. Subsequent vacuum anneal ing between 770 and 520 K resulted in reoxidation of the surface so th at the oxidation states of its components were returned to their origi nal value, together with desorption of Bi or Sb. It is believed that d iffusion of lattice oxygen to the surface was responsible for this pro cess, with only a relatively thin surface layer of material having bee n reduced. In some cases the surface composition could not readily be returned to its original value by this annealing process, suggesting l imited mobility of the cations at the annealing temperature. (C) 1998 Academic Press.