Characterisation, surface hydrolysis and nitrogen stability in aluminophosphate oxynitride (AlPON) catalysts Original Research Article

Solid amorphous aluminophosphate oxynitride (AlPON) catalysts with a nitrogen content ranging from 0% up to 20% N (w/w) have been obtained by reaction between AlPO4 and NH3 at 800°C. Samples are analysed by X-ray photoelectron (XPS) and diffuse reflectance Fourier transform infrared (DRIFTS) spectroscopies, mass spectrometry (MS) and thermogravimetry (TG). Similar to that of AlPO4, AlPON local structure is pictured as a network of PO4 and AlO4 tetrahedra in which nitrogen preferentially replaces oxygen bonded to phosphorous, although some contribution from Al–N bonds in an AlON phase is also proposed. Nitridation stages yielding [PO3N] and [PO2N2] building units are characterised below 7.2% N and above 11% N (w/w), respectively. Also terminal –PNH2 groups are detected from early nitridation stages. Freshly prepared AlPON catalysts readily undergo hydration (10–15% by weight) and hydrolysis at atmospheric conditions. Hydration is more intense on low nitrogen containing samples and sharply decreases above 7.2% N (w/w). Ammonia is the product of both surface hydrolysis and bulk decomposition. Hydrolysis is almost independent on nitrogen content while bulk decomposition grows parallel to nitrogen percentage. Bulk decomposition below 500°C is more intense than hydrolysis and always involves terminal –PNH2 reaction either as a condensation around 230–240°C or through a reaction with water around 320–340°C. Condensation depends on –PNH2 coverage and is the preferred decomposition mechanism in high nitrogen containing AlPON.