Magic angle spinning NMR investigations on amorphous aluminophosphate oxynitrides

Multinuclear magic angle spinning NMR spectroscopy has been applied to characterise the nitrogen species present in aluminophosphate oxynitrides, i.e. AlPONs, prepared by ammonolysis of high surface area aluminophosphates. At room temperature, the NH3 enters the coordination sphere of some tetrahedral Al in the amorphous AlPO4 to increase its coordination to 5 and 6. The IR spectra show bands characteristic of NH3 and NH4+, while the P-31 spectrum gives a new low field component that reflects the influence of adsorbed NH3 and/or NH4+. When the aluminophosphate precursor is treated with ammonia at temperatures within the interval 1073-1123 K, a deeper modification of the AlPO4 occurs giving rise to new nitrogen species, which remain after outgassing above room temperature, and are tentatively assigned to -P-NH2 terminal groups. When the ammonolysis is carried out at 1273 K an aluminophosphate trydimite phase appears, and nitrogen must be engaged in the formation of moieties of the type PO4-xNx, with x > 1.