INVESTIGATIONS OF ZEOLITES BY PHOTOELECTRON AND ION-SCATTERING SPECTROSCOPY .3. CATION DEPLETION AT THE EXTERNAL SURFACE OF HNA-FAUJASITES

Zeolites containing both Na and H ions (HNa-X, HNa-Y) have been prepar ed by thermal decomposition of NH4+ in ammonia-exchanged precursors or by reduction of transition-metal ions (Ag+) previously introduced by ion exchange. These materials, together with other zeolites (Na-A, RuK -A, Na-MOR, Na-ZSM-5), have been studied by surface-spectroscopic tech niques [X-ray photoelectron spectroscopy (XPS), ultraviolet photoelect ron spectroscopy (UPS) and ion-scattering spectroscopy (ISS)]. After t he thermal treatment required to obtain the H forms (calcination, redu ction), the near-surface region of the zeolite crystals is depleted of Na cations, probably due to their migration into the interior of the crystallite, This is demonstrated by the relative intensities of XPS a s well as of ISS signals. In UPS, the characteristic changes of the si gnals ascribed to the influence of the anionic excess charge on the el ectronic structure of the framework, which reflect the transformation of the ionic Na form into the largely covalent H form, occur at low bu lk exchange degrees.