1-PK MULTISITES DESCRIPTION OF CHARGE DEVELOPMENT AT THE AQUEOUS ALUMINA INTERFACE - ADSORPTION OF PD-II-AMINE COMPLEXES

Potentiometric titration data for gamma-Al2O3 have been converted to p roton-binding curves and deconvoluted, which gave the pH-dependent num ber and strength of surface groups with different proton acceptor/dono r properties. The deconvoluted isotherm was reconstructed based on the assumption of a 1-pK multisites description of the amphoteric gamma-A l2O3 surface. We find that our picture of the oxide surface as consist ing of structurally non-equivalent oxo and hydroxo groups allows for a ssignment of proton-binding processes to hydroxy groups with different acidic/basic properties, as determined independently from infrared sp ectra. Furthermore, we find that, based on models of the gamma-Al2O3 s urface consisting of individual low-index planes, the development of s urface charge as a function of pH has completely different characteris tics on different surface planes. Adsorption of [Pd(NH3)(4)](2+) catio ns was measured at fixed ionic strength and varying pH and initial con centration. Although the pH dependence of adsorbed amounts could be pr edicted on the basis of the 1-pK multisites description, the effects o f concentration could not be described exactly. In conjunction with re sults from temperature-programmed reduction and UV-VIS spectroscopy, w e propose the concept of geometrical constraints during adsorption-imp regnation of precursor ions on the manifold of charged sites revealed by proton-transfer reactions.