Surface characterization of yttria-stabilized tetragonal ZrO 2 . Part 3.—CO 2 adsorption and the CO 2 –CO interaction

The surface chemical features and sintering behaviour of two preparations of tetragonal zirconia (t-ZrO2) stabilized with 3 mol % Y2O3 have been examined, and have been compared with those of a 2 mol% t-ZrO2 preparation and a monoclinic zirconia (m-ZrO2) preparation.The adsorption of CO2 indicates that the most peculiar features of the genuine t-ZrO2 phase are: (i) the almost complete absence of surface hydrogencarbonates, 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 monodenate carbonate species, characterized by a very small spectral separation between the coupled CO oscillators (and observed only on all t-ZrO2 preparations). In some stages of the thermal sintering process, some of the characteristics of t-ZrO2 become modified, and the surface chemical behaviour becomes similar to 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 observed on most crystalline oxides no longer form.When CO2 is preadsorbed onto t-ZrO2, the overall capacity for CO uptake is decreased to a variable extent (15–50%), whereas the IR band corresponding to adsorbed CO is shiffed to higher wavenumbers by surface inductive effects only when CO2 yields appreciable amounts of surface hydrogencarbonate species.