Role of nitrogen on the acid–base properties of zirconophosphate (ZrPON) oxynitride catalysts Original Research Article

Activation of amorphous zirconium phosphate powder by high temperature treatment under NH3 allows to prepare series of zirconium phosphate oxynitrides. The adjustment of the N/O ratio allows the tuning of the acid–base properties of these high surface area and amorphous catalysts. It is evidenced that the substitution of oxygen by nitrogen induces a decrease of the number and strength of acid site while the basic character of the surface increases. By increasing the nitrogen content in the ZrPON powders, the strong acidic centres of ZrPO precursor are modified and a “soft” surface is obtained with good equilibrium between acid and basic sites of medium strength. In ZrPON with high N content, the basicity clearly dominates as numerous strong basic centres are identified. The basicity enhancement is directly connected to the presence of nitrogen atoms in the zirconophosphate structure. It is then proposed that in addition to nitride N3− anions and hydrogenated nitrogen species present at the surface and in the bulk, some oxygen species may be also responsible of the most strongest basic sites.