Studies on Water and Ammonia Programmed Thermodesorption of Mixed M(III)-vanadyl Phosphates

Abstract

Hydrated M(III)-vanadyl phosphates (M (III)=Mn, Fe, Ga, Al) have been prepared and studied for water and ammonia adsorption properties by TG/DTA, NH₃ TPD, FTIR and XRD techniques. The compounds have the same tetragonal layered structure of VOPO₄ ⋅2H₂ O, but shorter interlayer distances. Ammonia adsorption leads to intercalation of large amounts (0.19–0.39 mol/mol) of base between the layers of the materials, without displacement of water. The ammoniated phases obtained from these compounds have interlayer distances shorter than that of the corresponding precursors. In this connection an interaction mechanism NH₃ -host is proposed. Treated at 450°C the materials adsorb ammonia only on the external surface because of the large decrease of the interlayer distance that prevents NH₃ from entering the interlayer space. All M(III)-vanadyl phosphates present a wide distribution of strength of ammonia adsorbing sites.