ACIDITY OF THE OUTER SURFACE OF DEALUMINATED MORDENITES AS STUDIED BYX-RAY PHOTOELECTRON-SPECTROSCOPY OF CHEMISORBED AMMONIA

Chemisorption of ammonia on the acid sites located at the outer surfac e of dealuminated mordenites has been investigated by x-ray photoelect ron spectroscopy. The N 1s level of chemisorbed ammonia results from t he overlapping of three components occurring near 402.4, 400.5 and 399 .1 eV. From the N 1s binding energy of NH4+-exchanged mordenite and va lues reported in the literature for pyridine adsorbed on Bronsted and Lewis acid sites, the components at 402.5 and 399.1 eV are assigned to ammonia in interaction with Si-OH-Al framework groups and with Lewis acid sites, respectively. Based on XPS N/Al ratios and infrared result s, the component at 400.5 eV is attributed to ammonia chemisorbed on s ilanol groups. The surface concentration of the three types of acid si tes is estimated from the decomposition of the N 1s peaks. Evidence fo r the presence of surface extra-framework Al in dealuminated mordenite s is provided and the influence of these Al species on the outer surfa ce acidity is discussed.