Chromium ions in zeolites. Part 4.—X-ray photoelectron spectroscopic study of chromium valence states in the surface layers of CrY zeolites

Abstract

X-ray photoelectron spectroscopy has been used to elucidate the state of chromium in the surface layers of CrHY and stabilized CrSY zeolites. Values of both the binding energy (E_B) and the spin–orbit splitting (ΔE) of the Cr 2p level for chromium in the zeolite differ from those of bulk chromium oxide compounds or those of chromium supported on silica. The counter-Cr^III ions in the surface layers of Y zeolites are reduced by zeolite heat treatment both in carbon monoxide and in vacuo to Cr^II, unlike the Cr^III ions in the zeolite bulk, which are not self-reduced. The stable isolated Cr^V and Cr^VI ions present in the oxidized zeolite bulk were not detected by X.p.s. in the surface layers because of their reduction in vacuo and/or by X-ray irradiation in the ESCA system.