The interactions of various anatase and rutile samples with V2O5, and their products, have been studied by thermal analysis, by ir and ESR spectroscopy and by X-ray diffraction. The V2O5 was introduced either by mechanical admixture or by impregnation of the TiO2 with NH4VO3 and subsequent calcination. With anatase samples containing only low levels of P2O5 and K2O (< 0.6%) or containing SO4 = (6.3%), heating to 450 to 600 °C leads only to supported V2O5: but heating at 750 °C produces (i) a change in color to black, (ii) an irreversible loss of oxygen, (iii) a transformation of the anatase into rutile, and (iv) a substantial decrease in surface area. The results of experiments performed with various V2O5 concentrations (1 to 60% w w) suggest that about 8% w w V2O5 becomes incorporated as V4+ ions into the rutile lattice during the transformation, through the formation of a compound having the composition V0.04Ti0.96O2. Corresponding processes with rutile take place only at higher temperatures. Anatase containing Na+ (1.4%) behaves quite differently, and there is evidence for the formation of sodium vanadium bronzes on heating to 450 or 750 °C. Catalysts containing supported V2O5 oxidize butadiene to maleic anhydride at 260 °C with a selectivity which increases with V2O5 contents between 1 and 10%, and thereafter remains constant (S ∼- 0.45). Those containing the compound V0.04Ti0.96O2 are less active but more selective (S ∼- 0.57 for 10 to 30% V2O5). With both types, the selective reaction is zero order in butadiene and in O2, and both show activation energies for selective and for nonselective oxidation of 20 to 25 kcal · mol-1. Selective oxidation is believed to require lattice oxygen, and selectivity correlates with difficulty of reduction by CO.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry