The effects of synthesis and pretreatment conditions on the bulk structure and surface properties of zirconia

An investigation has been conducted to establish the effects of synthesis conditions and subsequent calcination conditions on the bulk and surface properties of zirconia. Samples of zirconia were prepared by precipitation from an aqueous solution of zirconyl chloride at both high and low pH. Precipitation at high pH (10) produced an amorphous material with a surface area of 280 m(2)/g. Extended digestion of this material in a boiling NH4OH solution at 1 atm increased the surface area to 400 m(2)/g. When digestion was carried out at similar to3 atm, the amorphous zirconia became crystalline and its surface area reduced to 200 m(2)/g. Precipitation of zirconia at low pH (<1.5) with extended reflux produced monoclinic ZrO2 with a surface area of 110 m(2)/g. Calcination at temperatures in the range of 300-1200 K caused a loss in surface area with increasing temperature and induced the amorphous-tetragonal and tetragonal-to-monoclinic phase transformations. The surface concentration of hydroxyl groups on monoclinic ZrO2 is always higher than that on tetragonal ZrO2. In the former case the dominant species are tribridged hydroxyl groups and in the latter, a combination of bibridged and terminal hydroxyl groups. CO2 adsorbs on zirconia at 298 K to form bicarbonate species and a smaller amount of carbonate species. The surface concentration of bicarbonate species grows monotonically with the surface concentration of hydroxyl groups. A similar relationship is observed for the adsorption of CO as formate species. Consistent with these results, monoclinic zirconia adsorbs more CO and CO2 than tetragonal zirconia. (C) 2000 Elsevier Science B.V. All rights reserved.