ENERGY-DISTRIBUTION OF SURFACE ACID SITES OF METAL-OXIDES

The acid site strength distributions of gamma-Al2O3, BeO, Nb2O5, TiO2, WO3, and ZrO2 surfaces were evaluated starting from volumetric and calorimetric experimental data of ammonia adsorption collected at 80 and 150-degrees-C. A mathematical model was employed to describe the behavior of the adsorbate-adsorbent system. The model took into account a discrete inhomogeneity of the surface. A fitting procedure applied to both the isotherms and the integral heat curves of ammonia adsorption, at the two above temperatures, permitted the determination of the thermodynamic parameters characteristic of the different types of site. The number, n(max,i), the adsorption equilibrium constant, b(i), as well as a new parameter termed the ''half-coverage temperature at unit pressure,'' T-degree1/2,i, of each type of acid site were obtained. All the oxides showed a significant amount of acid sites with ammonia enthalpy of adsorption DELTA(a)H(i) at -40 kJ/mol corresponding to hydrogen-bonded ammonia. Three types of more energetic sites, with DELTA(a)H(i) ranging from -280 to -160 kJ/mol, were found for gamma-Al2O3, Nb2O5, TiO2, and ZrO2. Sites with DELTA(a)H(i) of -280 and -200 kJ/mol were found for BeO and WO3. Mean molar adsorption entropies were also determined, and the values obtained are typical of immobile or localized adsorption. (C) 1994 Academic Press, Inc.