Surface chemistry of AlPO4—a mixed oxide of Al and P

The surface chemistry of pure AlPO4 was studied by infra-red spectroscopy in an attempt to explain why this material shows low catalytic activity despite its high acidity. Two major bands (3680 and 3800 cm–1) in spectra of dry AlPO4 were shown to represent chemically distinct types of surface OH groups. The band at 3800 cm–1 is assigned to Al—OH and that at 3680 cm–1 to P—OH. Study of adsorbed NH3 and pyridine showed both Lewis and Bronsted acid sites on the surface. Chemisorption of NH3 also produced NH2 and OH groups, showing the presence of “strained” oxide links. Adsorption of CO2 and HCI revealed very few “α-sites” or reactive surface oxide ions. The evidence suggests that the surface largely resembles a prism face (10text-decoration:overline10) of tridymite-form AlPO4, holding vicinal pairs of OH groups, with one group attached to Al and the other to P. Condensation produces acidic Al—O—P links in which O is held primarily by the P atom. The inactivity of AlPO4 probably reflects the inadequacy of PO groups as base sites rather than a lack of suitable acid sites.