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Presented are initial, S₀, and coverage, Θ, dependent, S(Θ), adsorption probability measurements, respectively, of CO₂ adsorption on a hydrogen precovered, polar, Zn-terminated surface of ZnO, parametric in the impact energy, E_i, and atomic hydrogen precoverage, Θ_H. Furthermore, CO₂ Thermal Desorption Spectroscopy has been used to estimate Θ_H as well as the binding energy of CO₂ on H/Zn-ZnO. The S(Θ) curves are below E_i=0.56 eV, consistent with precursor-mediated adsorption (S~const), and above that impact energy with adsorbate-assisted adsorption (S increases with Θ). Although a decrease in the CO₂ binding energy from 32.5 to 28.8 kJ/mol with Θ_H is present, S(Θ, Θ_H) curves are consistent with a physical site blocking, as demonstrated by Monte Carlo Simulations.