SURFACE CHARACTERIZATION OF YTTRIA-STABILIZED TETRAGONAL ZRO2 .3. CO2ADSORPTION AND THE CO2-CO INTERACTION

The surface chemical features and sintering behaviour of two preparati ons of tetragonal zirconia (t-ZrO2) stabilized with 3 mol% Y2O3 have b een examined, and have been compared with those of a 2 mol% t-ZrO2 pre paration and a monoclinic zirconia (m-ZrO2) preparation. The adsorptio n of CO2 indicates that the most peculiar features of the genuine t-Zr O2 phase are: (i) the almost complete absence of surface hydrogencarbo nates, related to the virtual absence of monodentate surface OH groups ; (ii) the presence of various types of bidentate carbonates; and (iii ) the formation of a peculiar monodentate carbonate species, character ized by a Very small spectral separation between the coupled CO oscill ators (and observed only on all t-ZrO2 preparations). In some stages o f the thermal sintering process, some of the characteristics of t-ZrO2 become modified, and the surface chemical behaviour becomes similar t o that of m-ZrO2. When metastable t-ZrO2 specimens start to segregate out an amorphous phase, which coats the crystallites, the adsorptivity towards CO2 declines sharply, and the usual carbonate-like species ob served on most crystalline oxides no longer form. When CO2 is preadsor bed onto t-ZrO2, the overall capacity for CO uptake is decreased to a variable extent (15-50%), whereas the IR band corresponding to adsorbe d CO is shifted to higher wavenumbers by surface inductive effects onl y when CO2 yields appreciable amounts of surface hydrogencarbonate spe cies.