MODULATION OF THE SURFACE-PROPERTIES OF SULFATED ZIRCONIA BY CONTROLLED ADDITION OF CALCIUM-OXIDE

Several CaO-ZrO2 samples have been prepared by impregnation of sulfate d zirconia with calcium nitrate solutions followed by calcination at 1 000 K for 3 h. The addition of calcium oxide to the sulfated zirconia contributes to the stabilization of the tetragonal form of zirconium o xide, although the major phase present, in the CaO-ZrO2 samples is the monoclinic one. Calcium oxide causes the transformation of the surfac e sulfate groups of sulfated zirconia from covalent to ionic and the s hift of the O-H stretching bands to higher wavenumbers, reaching value s higher than those corresponding to pure ZrO2. in this way, calcium o xide modifies the acid character of sulfated zirconia; the strongest L ewis acid sites were observed to disappear progressively with increasi ng calcium content. Bronsted acidity was also found to disappear when the molar concentration of calcium oxide reaches 3%.