BI-DIMENSIONAL CO CLUSTERS AT THE SURFACE OF POLYCRYSTALLINE MONOCLINIC ZRO2

High Resolution Transmission Electron Microscopy (HRTEM) and high resolution FTIR spectroscopy were used to characterize unsintered and sintered monoclinic ZrO2. HRTEM indicates that: i) the crystallites of the unsintered material (10-20 nm) terminate with small coin-like patches of regular crystal planes, among which the (111) plane is abundantly exposed; ii) in the sintered material (50-80 nm) the crystal edges are sharper, the regular patches of the (111) planes are larger, and also the (001) crystal planes become abundantly exposed. CO adsorption at 300 K yields two sigma-coordinated carbonyl-like species, whose IR spectral position depends on CO coverage (theta-CO). The species at higher nu approximately is ascribed to CO adsorbed on Zr4+ centres in structurally defective configurations, which are selectively annealed with sintering. The species at lower nu approximately is CO adsorbed on Zr4+ centres located on regular patches of the (111) crystal planes. CO adsorption at 78K allows higher coverages. On the predominant (111) planes CO adsorption from theta-CO almost-equal-to 0 to theta-CO almost-equal-to 1 corresponds to the transition from a one-band spectrum (almost-equal-to 2188 cm-1) to another one-band spectrum (almost-equal-to 2167 cm-1), and the transition occurs through the reversible formation/destruction of discrete bands at intermediate nu approximately i.e., to the formation/destruction of discrete two-dimensional CO clusters. The graphical representation of the (111) plane and a preliminary hypothesis on the surface dehydration pattern agrees with the variety of surface CO clusters suggested by the spectra.