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

Abstract

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 Brønsted 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 (1010) 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.