Effects of structural charges on points of zero charge and intrinsic surface reaction equilibrium constants of Zn–Al and Zn–Al–Fe hydrotalcite-like compounds

Abstract

A series of Zn–Al and Zn–Al–Fe hydrotalcite-like compounds (HTlc) with different n(Zn)/ n(Al)/ n(Fe) molar ratios are synthesized by using co-precipitation method. The samples have the following general formula [Zn 1− x Al x (OH) 2] x+ [(OH, Cl) x ] x− or [Zn 1− x (Al, Fe) x (OH) 2] x+ [(OH, Cl) x ] x− . The points of zero net charge (PZNC) and the intrinsic surface reaction equilibrium constants (p K in 1-p K model and p K a 1 int , p K a 2 int in 2-p K model) of the samples were determined by applying potentiometric titration (PT). It is found that the values of the pH PZNC, p K, p K a 1 int and p K a 2 int decrease linearly with the increase of structural charge density ( σ st). The interrelations of the intrinsic equilibrium constants with the pH PZNC are also demonstrated. Similar to oxides without structural charges, the fairly linear dependence of the p K, p K a 1 int and p K a 2 int on pH PZNC for HTlc samples with structural positive charges is also observed.