Surface chemistry of pure tetragonal ZrO and gas-phase dependence of the tetragonal-to-monoclinic ZrO transformation.

The surface chemical properties of undoped tetragonal ZrO and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO polymorph synthesized via a sol-gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO is defective and strongly hydroxylated with pronounced Lewis acidic and Brønsted basic surface sites. Combined in situ FT-infrared and electrochemical impedance measurements reveal effective blocking of coordinatively unsaturated sites by both CO and CO, as well as low conductivity. The transformation into monoclinic ZrO is suppressed up to temperatures of ∼723 K independent of the gas phase composition, in contrast to at higher temperatures. In inert atmospheres, the persisting structural defectivity leads to a high stability of tetragonal ZrO, even after a heating-cooling cycle up to 1273 K. Treatments in CO and H increase the amount of monoclinic ZrO upon cooling (>85 wt%) and the associated formation of either Zr-surface-(oxy-)carbide or dissolved hydrogen. The transformation is strongly affected by the sintering/pressing history of the sample, due to significant agglomeration of small crystals on the surface of sintered pellets. Two factors dominate the properties of tetragonal ZrO: defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO.