SURFACE AND SUBSURFACE ACIDITY OF FAUJASITE-TYPE ZEOLITES IN RELATION TO THEIR COMPOSITION - AN XPS AND TPD OF AMMONIA STUDY

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, Bronsted 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.