RESUMEN
Recyclable aggregates of mesoporous titania with different anatase-rutile ratios have been prepared by thermal treatments of either amorphous or peptized precursors. These last two have been obtained by hydrolysis of either Ti(OC2H5)4 or of Ti(OC2H5)4 in mixture with 5 mol % Zr(OC3H7)4 at room temperature in the presence of NH4OH as a catalyzing agent. The anatase-rutile ratio, the recyclable aggregates of the nano-sized particles, the mesoporosity, the surface area and the crystallinity of the resulting crystallized products of titania can be controlled by the synthesis parameters including: concentration of ammonia catalyst, stirring time and concentration of the peptizing HNO3, drying method of peptized precursors, calcination temperature, and finally the ramp rate up to the titania crystallization temperature. A broad range of synthesis parameters control the crystal sizes of titania particles produced. This allows catalyst preparation with very different crystal size, surface area, anatase to rutile crystal ratio and various mesoporous structures. Drying by lyophilization of precursors reduce the aggregation of the primary particles giving micro-/macroporous structures.
RESUMEN
Experiments have been performed to measure the effect of additives on the crystallization temperature of concentrated LiBr solutions cooled at a rate of 20 degrees C/h. The measured crystallization temperatures correspond not to the temperatures of equilibrium solubility but to the critical temperature for heterogeneous nucleation of the hydrated LiBr salt on the glass wall of the test tube containing the sample solution. Various additives at concentrations from 250 to 1500 ppm have been investigated. Some soluble additives further decreased the experimental crystallization temperature by as much as 13 degrees C, corresponding to 22 degrees C below the equilibrium solubility. Large decreases in the crystallization temperature can be correlated with large values of complexation constants of the additive for either the Li(+) or the Br(-) ion in solution. Solution complexation, however, is not sufficient to explain the magnitude of the decrease in the crystallization temperature. The only phenomenon capable of quantitatively explaining the magnitude of the decrease in the crystallization temperature is the change in the crystal/solution interfacial energy due to adsorption of the additive on the surface of the prenucleation embryos. A quantitative model of the crystal/solution interfacial energy due to adsorption has been developed using both the Langmuir and Gibbs adsorption equations, allowing the quantitative prediction of crystallization temperatures with additive concentration. Copyright 2001 Academic Press.