Evolution of microstructure during the thermal processing of aluminum-modified titania and aluminum/vanadium co-modified titania gels

Abstract

Sol-gel materials of aluminum-modified TiO2 of nominal composition 5.7 wt% and 10.8 wt% aluminum and aluminum/vanadium co-modified TiO2 of nominal composition 5.7Al–3.5V wt% have been prepared by evaporation of aqueous colloidal sols obtained by the hydrolysis of aqueous solutions of titanium chloride with the appropriate amount of vanadyl oxalate and/or aqueous aluminum nitrate using aqueous ammonia followed by peptization of the resulting hydrated solids using nitric acid. The nature of the sol-gel materials and the behavior upon calcination at temperatures up to 1373 K have been investigated using x-ray fluorescence, x-ray powder diffraction, transmission electron microscopy, and electron diffraction. At 333 K, all the gels comprise small (approximately 5 ± 1 nm) particles of anatase together with traces of brookite and highly crystalline ammonium nitrate. The particle size changes little on thermal treatment at 573 K, but increases significantly at higher temperatures and is accompanied by transformation to rutile. Aluminum-modified gels stabilize the anatase phase from 923 K in unmodified TiO2 to 1023 K in the 6Al/TiO2 gel and 1173 K in the 11Al/TiO2 gel. The alumina in the co-modified gel has a dominating effect on stabilizing the anatase phase until 973 K. Only rutile is present at high temperatures, except for small amounts of phase-separated α-Al2O3 (Corundum). No substitutional incorporation of Al3+ ions in the tetragonal rutile lattice occurs at high temperatures.