SURFACE AND SUBSURFACE ACIDITY OF FAUJASITE-TYPE ZEOLITES IN RELATIONTO 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 dif ferent Si:Al molar ratios (ca. 2.7, 5.8, 24). The N 1s binding energie s of the probe molecule showed two types of sites, Bronsted and Lewis, whose relative proportions varied with pretreatment temperature, as w ell as dissociative adsorption of NH3 following dehydroxylation. The m easurement of the NH3 concentration in zeolites as a function of desor ption temperature enabled the site distribution to be determined in te rms of strength, for the external surface of crystals and for a subsur face which is partially free of the very slightly acidic extraframewor k alumina and silica species that cover it.