Determining surface composition of mixed oxides with pH

Abstract

In this study we have developed a 2-surface model with the EPHL ("Equilibrium pH at high oxide loading") method for mixed and composite oxides. Oxide charging parameters, namely the protonation and deprotonation constants and the hydroxyl surface densities, can be established from measurements of the pure oxides and used in the 2-surface model to predict the point of zero charges (PZC) of mixed and composite oxides. The accuracy of these predictions has been demonstrated with diverse control samples of known surface composition (physical mixtures of silica and alumina of high and low surface area). The EPHL method has been extended to composite materials (bound catalysts) and can explain correlations of reactivity with catalyst surface composition. The "apparent surface coverage" (ASC) of a particular mixed or composite oxide sample may then be determined by comparing the PZC of the material to either the 2-surface model results, using parameters of the individual oxides, or a calibration curve of EPHL measurements of physical mixtures of the individual ingredients.

Keywords: Mixed oxides; Point of zero charge; Surface characterization; Surface charge parameter; Surface coverage.