GEOMETRIC AND ELECTRONIC-STRUCTURE OF AMORPHOUS ALUMINOPHOSPHATES - AB-INITIO AND EXPERIMENTAL STUDIES

A combination of experimental (diffuse reflectance infrared Fourier tr ansform spectroscopy, DRIFTS, and X-ray photoelectron spectroscopy, XP S) and ab initio studies of model clusters is used to understand the g eometric and electronic structure of aluminophosphate (AlPO) systems u sed as catalytic materials. The presence of an intense IR band in the DRIFTS spectra, around 1300 cm(-1), together with literature data sugg ested to represent the system using cluster models based on metaphosph ate-like structures. By varying the P/Al atomic ratio the basic featur es of AlPO4-Al2O3 catalysts are modeled. The calculated geometrical pa rameters (bond distances and associated stretch force constants) are d iscussed in relation to the structure of the catalyst and, despite the inherent approximations in modeling solids with cluster models, fits quite well-the experimental X-ray data for aluminum metaphosphate. The computed ionization potentials (IF), O-KVV transitions, and IR spectr a are in reasonable agreement with experimental data. The observed tre nds in XPS, continuous decrease of Al-2p, and P-2p and O-1s binding en ergies on decreasing the P/Al ratio are explained in terms of the alte ration of the electronic density of the O atoms induced by the presenc e of Al as second neighbor of the P atom. A similar effect of this sec ond neighbor lets us explain the modification of the O-KVV and DRIFTS data. As a result of these studies a model for the short-range structu re of amorphous AlPO systems is proposed based on metaphosphate anions connected by layers rich in aluminum with gamma-Al2O3-like structure.