Identification and stability of the nitrogenous species in zirconium phosphate oxynitride catalysts

A new family of basic phosphate oxynitride catalysts, ZrPON, has been synthesized. by nitridation of high specific surface area zirconium phosphate precursor under ammonia flow. The identification study of the different nitrogenous species has been realized by the combination of XPS and DRIFTS analysis. The presence of NH4+, NH3, -NH2 and -NH-groups has been evidenced on the solid surface while nitride N3- anions replaced bulk oxygen O2- species. The evolution of the concentration of the various species allows us to propose a mechanism for the nitridation of the precursor powder. The thermal stability of the ZrPON oxynitrides has also been studied by TPD-MS and DRIFTS. Nitrogen is removed from the ZrPON powder by a three-step desorption process depending on the temperature range considered. At low temperature (<200 degrees C), weakly physisorbed species are desorbed from the surface. In the 200-400 degrees C range, ammonia adsorbed on the Lewis acid sites is removed, and at higher temperature, terminal amino groups are attacked by gaseous water coming from hydroxyls condensation which results in the appearance of new hydroxyls bonded to phosphorus atoms.