ENTHALPIES OF REACTIONS AT THE METAL-OXIDE AQUEOUS INTERFACE

A detailed thermodynamic analysis of simultaneous surface reactions is presented, with a special emphasis on the definition of standard enthalpies. An experimental study of the surface reactions at the anatase/water interface is also reported. Generally, the reaction enthalpy is defined as the ratio of the reaction heat and its extent. The latter cannot be calculated unless the stoichiometric equations and corresponding extents of all processes taking place are known. According to the surface complexation model, the surface charge is a result of protonation and deprotonation of amphoteric surface sites. In the case of surface reactions, an additional problem is the electrostatic effect on the enthalpies. In order to solve the above problems, a new design of the calorimetric experiment is proposed: the electrostatic contributions to the enthalpy will cancel if the difference between initial pH of the suspension and point of zero charge (p.z.c.) equals the difference between final pH and p.z.c. In such a case, one obtains the difference in standard enthalpies DELTA(b)H-degrees-DELTA(a)H-degrees which, for anatase, was found to be 14.7 kJ mol-1. This value agrees well with those obtained from potentiometric measurements of the temperature dependence of p.z.c. (14.6 kJ mol-1). The point of zero charge is determined by the so-called ''mass titration method'', originally proposed by Noh and Schwarz. This method yields more accurate p.z.c. values with respect to other methods and consequently results in a more accurate value of enthalpy.