1-pK multisites description of charge development at the aqueous alumina interface. Adsorption of PdII–amine complexes

Abstract

Potentiometric titration data for γ-Al₂O₃ have been converted to proton-binding curves and deconvoluted, which gave the pH-dependent number and strength of surface groups with different proton acceptor/donor properties. The deconvoluted isotherm was reconstructed based on the assumption of a 1-pK multisites description of the amphoteric γ-Al₂O₃ surface.

We find that our picture of the oxide surface as consisting of structurally non-equivalent oxo and hydroxo groups allows for assignment of proton-binding processes to hydroxy groups with different acidic/basic properties, as determined independently from infrared spectra. Furthermore, we find that, based on models of the γ-Al₂O₃ surface consisting of individual low-index planes, the development of surface charge as a function of pH has completely different characteristics on different surface planes.

Adsorption of [Pd(NH₃)₄]²⁺ cations was measured at fixed ionic strength and varying pH and initial concentration. Although the pH dependence of adsorbed amounts could be predicted on the basis of the 1-pK multisites description, the effects of concentration could not be described exactly. In conjunction with results from temperature-programmed reduction and UV–VIS spectroscopy, we propose the concept of geometrical constraints during adsorption–impregnation of precursor ions on the manifold of charged sites revealed by proton-transfer reactions.