REACTIVITY OF VANADIA WITH SILICA, ALUMINA, AND TITANIA SURFACES

Vanadia has been, supported (two monolayers) on silica, alumina, and titania (rutile) by melting V2O5 at 973 K or by impregnation of these oxides with oxalic aqueous solutions of NH4VO3 and further calcination at 773 or 973 K. Evolution of the phases thus formed and of the surface texture has been monitored by X-ray diffraction (XRD), electronic spectroscopy (ultraviolet-visible/diffuse reflectance, UV-vis/DR), and nitrogen adsorption at 77 K. Formation of A1V04 is observed on alumina after heating the impregnated samples at 973 K and, to a smaller extent, when starting from V2O5. Vanadia XRD peaks are detected on silica (well-defined peaks for sample calcined at 773 K and poorly defined peaks for both samples calcined at 973 K) and on titania (but only after calcining at 773 K); in this case these peaks vanish after calcination at 973 K, probably because of an improved dispersion after melting of the vanadia. While sintering of unloaded alumina and silica does not take place at 973 K, the presence of vanadia deeply favors such a process, especially in the case of silica; sintering is already observed with bare rutile after calcination at 973 K, and in the presence of vanadia the behavior differs depending on the way (impregnation or melting) it had been incorporated. The results have been interpreted on the basis of previous data with similar systems using anatase and MgO as supports and with an acidity scale for binary oxides, the acidity decreasing in the order V2O6 < SiO2 < TiO2 < Al2O3 < MgO. Thus reaction between extreme oxides (alumina and magnesia on one side and vanadia on the other) is favored. The different results observed between the rutile- and the anatase-supported samples are due to the crystallographic fit between vanadia and rutile structures. © 1990, American Chemical Society. All rights reserved.