Effect of temperature on the point of zero charge and surface dissociation constants of aqueous suspensions of γ-Al2O3

Abstract

The surface acidity constants, the point of zero charge (p.z.c.) and the concentration of the charged surface groups, AlOH⁺₂ and AlO^–, of γ-Al₂O₃ over a wide pH range have been determined at temperatures between 10 and 50 °C by potentiometric titrations. It was found that a precise regulation of the value of the p.z.c. can be achieved in the pH range 4.45–8.95 by varying the temperature of the impregnating solution between 10 and 50 °C. Increasing the temperature in this range caused an increase in the p.z.c. value. These results demonstrate that the increase (decrease) in the temperature of the impregnating suspension is a very simple method for achieving a marked extension of the pH range in which γ-Al₂O₃ can adsorb negative (positive) species. Moreover, it was observed that an increase (decrease) in the concentration of the AlOH⁺₂(AlO^–) groups, which are sites for adsorption of negative (positive) species, is obtained by increasing the temperature of the impregnating suspension. The p.z.c. values obtained experimentally were in excellent agreement with those calculated from the average of the two dissociation constants, K^int₁, and K^int₂, corresponding to the deprotonation–protonation equilibria of the surface AlOH groups.

The influence of temperature of the impregnating suspension on the p.z.c., (AlOH⁺₂) and (AlO^–) was interpreted assuming that the surface deprotonation reactions of the AlOH⁺₂ and AlOH groups are exothermic. For the first time values for the enthalpies for these surface reactions were obtained (ΔH^°₁=–180±10 kJ mol^–1, ΔH^°₂=–230±20 kJ mol^–1) by plotting ln K^int_{1 or 2}vs. T^–1, thus confirming the abovementioned assumption.