EFFECT OF PREPARATIVE PH AND AGING MEDIA ON THE CRYSTALLOGRAPHIC TRANSFORMATION OF AMORPHOUS TIO2 TO ANATASE AND RUTILE

A study of the various parameters affecting the crystallogrphic transformation of amorphous titania gels (prepared from TiCl3) to anatase and rutile has been conducted. Analysis of the effect of the preparative pH (3.0, 7.5 and 11.0) and of the ageing media (HCl, H2O, air and NH4OH) suggests that coordinated water as well as the presence of chloride ions (bulk or surface) play an important role in the transformation of amorphous titania gels to the anatase and rutile modifications. The formation or depletion of anion vacancies as well as the presence of interstitial ions seem to be the crucial parameters controlling the rate of transformation of anatase to rutile. A higher heating temperature as well as a higher preparative pH generally lead to an accelerated conversion. In most cases first-order kinetics are followed. A nucleation process with high k(n) predominates with NH4OH and H2O as ageing media, whereas with air a propagation-growth process in general controls the kinetics of the transformation. Ageing in HCl gives ill-defined kinetic curves. A notably low activation energy for nucleation (almost-equal-to 52 kcal mol-1) was observed in the case of the low pH gels aged in ammonia.