Surface and subsurface acidity of faujasite-type zeolites in relation to their composition: an XPS and TPD of ammonia study

Abstract

A combination of temperature-programmed desorption (TPD) of NH₃ and X-ray photoelectron spectroscopy (XPS) has been used for the qualitative and quantitative study of the surface acidity of HY zeolites with different Si : Al molar ratios (ca. 2.7, 5.8, 24). The N 1s binding energies of the probe molecule showed two types of sites, Brønsted and Lewis, whose relative proportions varied with pretreatment temperature, as well as dissociative adsorption of NH₃ following dehydroxylation. The measurement of the NH₃ concentration in zeolites as a function of desorption temperature enabled the site distribution to be determined in terms of strength, for the external surface of crystals and for a subsurface which is partially free of the very slightly acidic extraframework alumina and silica species that cover it.