Structure and Acidic Properties of Phosphate-Modified Zirconia

Phosphate-modified zirconia was prepared by impregnation of Zr(OH) 4 and of m -ZrO 2 with an aqueous solution of (NH 4 ) 2 HPO 4 . Similar to sulfate and tungstate, phosphate shows a strong stabilizing effect on the specific surface area and the tetragonal phase of zirconia when using Zr(OH) 4 as the precursor material. The adsorbed phosphate species were characterized by Raman, DRIFT, and 31 P-MAS-NMR spectroscopies as pyrophosphates, chelate-bonded orthophosphates, and oligophosphates. Only a distortion of the geometry of the adsorbed phosphate species was found after dehydration of the materials in contrast to ZrO 2 /SO 4 . FT-IR spectroscopy with CO as a probe molecule was used to characterize the acidic properties of ZrO 2 /PO 4 . New hydroxyl groups, probably P−OH groups with enhanced protonic acidity as compared to pure zirconia but lower than that of ZrO 2 /SO 4, were observed. In addition basic Zr−OH groups were observed. Furthermore, strong Lewis acidic centers (cus Zr 4+ ), comparable to ZrO 2 /SO 4, are formed by phosphate modification of zirconia.