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

Abstract

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 AlPO 4 and NH 3 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 AlPO 4, AlPON local structure is pictured as a network of PO 4 and AlO 4 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 [PO 3N] and [PO 2N 2] building units are characterised below 7.2% N and above 11% N (w/w), respectively. Also terminal –PNH 2 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 –PNH 2 reaction either as a condensation around 230–240°C or through a reaction with water around 320–340°C. Condensation depends on –PNH 2 coverage and is the preferred decomposition mechanism in high nitrogen containing AlPON.