Effect of Zirconia Formation Temperature on the State of Surface Hydroxyl Groups and the Nature of Carbonate Species as Revealed by FTIR Spectroscopy

FTIR spectroscopy was employed to investigate the evolution of surface hydroxyl groups, carbonate species, and the crystalline structure of zirconium dioxide (ZrO2) synthesized by a topotactic method at calcination temperatures of 150, 200, 400, and 600 °C. At low temperatures, the broad absorption in the 3650–3300 cm-1 region is attributed to adsorbed water together with bridging ≡Zr2–OH (Type IIa) and tribridging ≡Zr3–OH (Type III) hydroxyls as classified by the Knözinger–Ratnasamy scheme, whereas at 400–600 °C isolated terminal ≡Zr–OH groups of Type Ia (3775–3760 cm-1, pKa ≈ 2–4) predominate. Drawing on recent DFT calculations by Scafuri et al. (2025), the carbonate bands observed at 1520, 1460, 1400, and 1360 cm-1 are assigned predominantly to polydentate surface carbonate species—bridging (BC) and polydentate (PC) types—formed through adsorption of atmospheric CO2 during drying rather than through a solid-state reaction. Monodentate carbonates are thermodynamically unstable on the m-ZrO2 surface and instantaneously rearrange into polydentate forms. The CO2 adsorption energy and the magnitude of Δν3 splitting are governed by the coordination number of surface Zr4+ ions. This conclusion is supported by the increase in carbonate band intensities from 150 to 200 °C in parallel with the growth of specific surface area, and by their disappearance at 400 °C, a temperature well below the decomposition temperature of bulk carbonate phases (480–530 °C). Analysis of the 800–400 cm-1 region confirms the structural evolution from the amorphous state (FWHM > 200 cm-1) through crystallization of t-ZrO2 (bands at ~740 and ~585 cm-1) to a mixture of t- and m-ZrO2 at 600 °C (~500 and ~455 cm-1).