Investigations of zeolites by photoelectron and ion-scattering spectroscopy. Part 3.—Cation depletion at the external surface of HNa-faujasites

Abstract

Zeolites containing both Na and H ions (HNa-X, HNa-Y) have been prepared 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 techniques [X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and ion-scattering spectroscopy (ISS)]. After the thermal treatment required to obtain the H forms (calcination, reduction), 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 as well as of ISS signals. In UPS, the characteristic changes of the signals ascribed to the influence of the anionic excess charge on the electronic structure of the framework, which reflect the transformation of the ionic Na form into the largely covalent H form, occur at low bulk exchange degrees.