Energetics of CO₂ and H₂O adsorption on zinc oxide

Adsorption of H₂O and CO₂ on zinc oxide surfaces was studied by gas adsorption calorimetry on nanocrystalline samples prepared by laser evaporation in oxygen to minimize surface impurities and degassed at 450 °C. Differential enthalpies of H₂O and CO₂ chemisorption are in the range150 ±10 kJ/mol and 110 ±10 kJ/mol up to a coverage of 2 molecules per nm². Integral enthalpy of chemisorption for H₂O is96.8 ±2.5 kJ/mol at 5.6 H ₂O/nm² when enthalpy of water condensation is reached, and for CO₂ is96.6 ±2.5 kJ/mol at 2.6 CO₂/nm ² when adsorption ceases. These values are consistent with those reported for ZnO prepared by other methods after similar degas conditions. The similar energetics suggests possible competition of CO₂ and H ₂O for binding to ZnO surfaces. Exposure of bulk and nanocrystalline ZnO with preadsorbed CO₂ to water vapor results in partial displacement of CO₂ by H₂O. In contrast, temperature-programmed desorption (TPD) indicates that a small fraction of CO₂ is retained on ZnO surfaces up to 800 °C, under conditions where all H₂O is desorbed, with adsorption energies near200 kJ/mol. Although molecular mechanisms of adsorption were not studied, the thermodynamic data are consistent with dissociative adsorption of H₂O at low coverage and with several different modes of CO₂ binding.