Geometric and electronic structure of amorphous aluminophosphates. Ab initio and experimental studies

A combination of experimental (diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS, and X-ray photoelectron spectroscopy, XPS) and ab initio studies of model clusters is used to understand the geometric and electronic structure of aluminophosphate (AlPO) systems used as catalytic materials. The presence of an intense IR band in the DRIFTS spectra, around 1300 cm(-1), together with literature data suggested to represent the system using cluster models based on metaphosphate-like structures. By varying the P/Al atomic ratio the basic features of AlPO4-Al2O3 catalysts are modeled. The calculated geometrical parameters (bond distances and associated stretch force constants) are discussed 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 spectra are in reasonable agreement with experimental data. The observed trends in XPS, continuous decrease of Al-2p, and P-2p and O-1s binding energies on decreasing the P/Al ratio are explained in terms of the alteration of the electronic density of the O atoms induced by the presence of Al as second neighbor of the P atom. A similar effect of this second 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 structure of amorphous AlPO systems is proposed based on metaphosphate anions connected by layers rich in aluminum with gamma-Al2O3-like structure.