Magic angle spinning NMR investigations on amorphous aluminophosphate oxynitrides

Abstract

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 ³¹P 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 PO_4-xN_x, with x>1.