ON THE SURFACE THERMODYNAMICS OF GAMMA-AL2O3 DERIVED FROM THE REACTION WITH CHLORINE

In the case of gas-solid type reactions not only the bulk phase but also the surface energetics may govern the process of interaction. In the temperature range 300-1070 K, where bulk phase reactions of gamma-alumina with Cl2 are highly unfavoured for thermodynamic reasons nevertheless a surface type O/Cl exchange was observed. The present work deals with thermodynamic characteristics of the surface of gamma-alumina derived from this reaction with chlorine. The temperature-dependent exchange capacity of the surface was determined together with the stoichiometry of the reaction. The equilibrium constants obtained under isothermal conditions vary with chlorine coverage, giving proof of the energetical heterogeneity of the surface. The surface excess Gibbs free energy was found to be a nearly linear function of chlorine coverage. An attempt is made to derive a mathematical description of the energetical heterogeneity of the surface. The O/Cl exchange was found to be exothermic. At and above 870 K this exothermic process is followed by an endothermic surface rearrangement. On the basis of the experimental results surface Gibbs free energy and entropy changes were calculated.