The short-range structure of aluminophosphate oxynitride catalysts. An ab initio and experimental study

In this study we combine spectroscopic data (diffuse reflectance infrared F ourier transform spectroscopy, DRIFTS, and X-ray photoelectron spectroscopy , XPS) with ab initio SCF calculations on model systems in order to get ins ight into the short-range order of amorphous aluminophosphate oxynitride (A lPON) catalysts. The use of fixed P/Al = 1/1 atomic ratio and varying N/O a tomic ratio in the models, allowed us to consider stoichiometries with nitr ogen contents in the range 0-20 wt %, similar to those found experimentally . The observed trends in computed Koopman ionization potentials (IPs), vibr ational frequencies, and thermodynamic stability are compared with experime ntal XPS and DRIFTS data and available information of aluminophosphate and related systems. They are used to propose a structural model for the short- range structure of amorphous aluminophosphate oxynitrides. From the XPS dat a it can be concluded that first nitrogen enters the phosphorus coordinatio n sphere; this results in the formation step by step of [PO3N] and [PO2N2] polyhedra that coexist with [PO4] ones, their relative proportion depending on the total nitrogen content of the sample. On the basis of both experime ntal and ab initio SCF calculations, it can be concluded that the amorphous solids are formed by P-X-P (X = O, N) rings connected to each other by alu minum atoms. On the basis of the thermodynamic stability of the different b onds, nitrogen enters preferentially the phosphorus coordination sphere.