Infrared spectroscopic surface characterization of zirconium oxide. Part 3.—The CO/CO2 and CO/H2O interactions at the surface of a high-area monoclinic preparation

Abstract

Carbon monoxide adsorbs at ambient temperature on microcrystalline monoclinic ZrO₂, vacuum activated at 100–600°C, yielding two reversible σ-coordinated species, whose spectral positions depend linearly on the overall CO coverage, and whose frequencies at zero CO coverage depend on the residual surface concentration of free (i.e. non H-bonded) hydroxyl groups. At the surface of ZrO₂ loaded with carbonate-like species, deriving from the irreversible adsorption of CO₂ at ambient temperature, CO adsorbs at ambient temperature still yielding two σ-coordinated species. The latter exhibit: (i) lower surface concentrations, owing to CO₂/CO site competition; (ii) higher frequencies, owing to inductive effects from the charge-withdrawing carbonates on the charge-releasing CO; (iii) modified frequencies at zero CO coverage, owing to a different role of the surface OH groups brought about by surface bicarbonate species and (iv) different frequency dependence on the overall CO coverage, owing to a modified mechanism and/or range of transmission of the surface inductive effects in the presence of surface carbonates. The stepwise rehydration at ambient temperature of a fully dehydrated monoclinic ZrO₂ also modifies severely its adsorptive behaviour towards CO: the number of coordinating sites declines fast with rehydration, whereas the spectral features of adsorbed CO and the mechanisms of transmission of the CO/CO inductive effects are different on a partly rehydrated material, from those observable during a stepwise dehydration of the same ZrO₂ preparation.