STRUCTURE AND DISPERSION OF VANADIA SUPPORTED ON GAMMA-ALUMINA

A series of alumina-supported vanadia catalysts with V2O5 loadings ran ging from 3 to 20% were investigated by X-ray diffraction, V-51 solid State NMR, electron spin resonance, and oxygen chemisorption. V-51 NMR results under ambient conditions suggest that two types of vanadium o xide species are present on the surface: one due to a dispersed vanadi a phase at lower V loadings and the other due to a crystalline vanadia phase at higher V loadings, Dispersion of vanadia was determined by t he oxygen chemisorption method at 640 K and by a static method on the samples prereduced at the same temperature. At low loadings, i.e., bel ow 9.8%, vanadium oxide is found to be present in a highly dispersed s tate, A comparison of the dispersion of vanadia determined by low-temp erature oxygen chemisorption (samples reduced at 773 K for 6 h and sub sequent oxygen chemisorption at 195 K) with that determined at 640 K s hows that the latter method provides a better measure of the dispersio n of vanadia. ESR spectra obtained under ambient conditions for the ca talysts reduced at 640 K show the presence of V4+ ions in axial symmet ry. The results are discussed in terms of the possible surface V speci es present on Al2O3.