Structure and Acidic Properties of Phosphate-Modified Zirconia

Phosphate-modified zirconia was prepared by impregnation of Zr(OH)4 and of m-ZrO2 with an aqueous solution of (NH4)2HPO4. 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 31P-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 ZrO2/SO4. FT-IR spectroscopy with CO as a probe molecule was used to characterize the acidic properties of ZrO2/PO4. New hydroxyl groups, probably P−OH groups with enhanced protonic acidity as compared to pure zirconia but lower than that of ZrO2/SO4, were observed. In addition basic Zr−OH groups were observed. Furthermore, strong Lewis acidic centers (cus Zr4+), comparable to ZrO2/SO4, are formed by phosphate modification of zirconia.