Oxide electronic polarizabilities and aluminum coordination at the outer surface of zeolites obtained by X-ray photoelectron spectroscopy

An extension of the simple electrostatic model of the Anger parameter developed a few years ago by Moretti [Surf. Interf. Anal. 16 (1990) 159; 17 (1991) 352] is presented and discussed. It is shown that the Anger parameter is site-sensitive and can bring very useful information about the local chemical environment of core-ionized atoms. In zeolites, the Si and AI Auger parameters permit the direct determination of the in-framework electronic polarizability of the two oxide species Si-O-Si(AI) and Si-OM-Al, where M (H,Na,...) is the counter-cation of the negative charge on Al. The knowledge of these quantities is fundamental to theoretical investigations of the adsorptive properties of zeolites. This approach was previously investigated by Pellenq and Nicholson [J. Chem. Sec. Faraday Trans. 89 (1993) 2499] who, however, due to the limitations of the simple electrostatic model, only obtained averaged oxygen electronic polarizability. The extended model also permits the direct identification of framework tetra- and tri- coordinated Al sites in zeolites (Bronsted and Lewis acid sites), as well as of extra-framework hexa-coordinated Al species in dealuminated zeolites. We applied our model to all the zeolites for which Si and Al Auger parameter data were reported in the literature.