Surface chemistry of pure tetragonal ZrO₂ and gas-phase dependence of the tetragonal-to-monoclinic ZrO₂ transformation

Abstract

The surface chemical properties of undoped tetragonal ZrO₂ and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO₂ polymorph synthesized via a sol-gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO₂ 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 CO₂, as well as low conductivity. The transformation into monoclinic ZrO₂ 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 ZrO₂, even after a heating-cooling cycle up to 1273 K. Treatments in CO₂ and H₂ increase the amount of monoclinic ZrO₂ 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 ZrO₂: defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO₂.