Surface characterization of yttria-stabilized tetragonal ZrO2. Part 2. - Adsorption of CO
Two preparations of tetragonal zirconia (t-ZrO2) stabilized with 3 mol% Y2O3 have been characterized, and their features compared with those of a 2 mol% t-ZrO2 preparation and of a monoclinic zirconia (m-ZrO2) preparation. At ambient temperature, CO adsorption occurs only on surface cationic Lewis acid sites located in crystallographically defective configurations. The various t-ZrO2 preparations present several types of such defective sites, depending on the temperature of the sintering stage. The spectral features of the ambient-temperature interaction of t-ZrO2 with CO are complemented, on quantitative grounds, by gas-volumetric and microcalorimetric data. At high firing temperatures, amorphous phases start segregating from the t-ZrO2 crystallites, causing the ambient-temperature adsorptive capacity towards CO to decline. Upon CO adsorption at low temperaure (ca. 77 K), weaker adspecies form prevalently. These are ascribed to CO adsorption on cationic Lewis acid sites located in regular patches of low-index crystal planes, and to CO H-bonded to surface OH groups. Different t-ZrO2 preparations and differently sintered samples exhibit different particle sizes and various proportions of crystal surface defect sites vs. regular-face terminations: these differences are reliably monitored by CO adsorption patterns observed via IR spectroscopy, both at 300 and ca. 77 K.