Probing the Surface Heterogeneity of Polycrystalline Zinc Oxide by Static Adsorption Microcalorimetry. 1. The Influence of the Thermal Pretreatment on the Adsorption of Carbon Dioxide

The adsorption of carbon dioxide on differently pretreated polycrystalline ZnO was studied by thermodynamic and kinetic methods. The uptake of CO 2 observed in a static Tian−Calvet microcalorimeter reached saturation at about 5 μmol/m 2, corresponding to about half of the exposed Zn 2+ sites after a thorough thermal pretreatment at 450 °C for 4 h. The saturation uptake was found to be correlated inversely with the amount of residual hydroxyl groups on the ZnO surfaces. At room temperature, the adsorption of CO 2 was found to occur in two steps. Initially, the adsorption was nonactivated, and the exposed surfaces were saturated at a very low equilibrium pressure ( p ≪ 1 Pa) with an initial differential heat of adsorption ( q diff ) of 100−120 kJ/mol, a standard entropy of −190 J mol −1 K −1, and an adsorption rate constant of 10 −5 Pa −1 s −1 . During the second stage, an inhibiting effect was observed; the equilibrium coverage increased slowly with increasing pressure, q diff decreased rapidly with increasing coverage, and the rate of adsorption was low. Temperature-programmed desorption measurements indicated the formation of strongly adsorbed polydentate carbonates at higher temperatures with an adsorption energy between 120 and 160 kJ/mol.