The contribution of X-ray photoelectron and X-ray excited Auger spectroscopies in the characterization of zeolites and of metal clusters entrapped in zeolites

Abstract

X-ray photoelectron spectroscopy (X.p.s.) and X-ray-induced Auger spectroscopy (XAES) can be considered feasible techniques for the characterization of the chemical nature and surface composition of zeolites. In this review, we discuss the Auger parameter concept, an experimental quantity obtained combining X.p.s. and XAES measurements. We show that the Auger parameter is useful to overcome the problem of charging and to obtain structural and electronic information of metal clusters entrapped in zeolites. A simple electrostatic model is developed to calculate Auger parameter shifts as a function of number, distance, electronic polarizability, and local geometry of the ligand atoms around the core-ionized atom. Using our model, we investigated Pd m and Cu m , clusters entrapped in Y and A zeolites, respectively. It is demonstrated that the Auger parameter shift of Pd m clusters depends not only on the nuclearity and geometry of the clusters, but also on the nature of the heteroatoms in its close vicinity. This result could be explained considering the influence of these heteroatoms on the electronic polarizability of the surface Pd atoms of the cluster.