Activation of Carbon Dioxide on ZnO Nanoparticles Studied by Vibrational Spectroscopy

The activation of CO2 on clean and hydroxylated ZnO nanoparticles has been studied by ultrahigh vacuum FTIR spectroscopy (UHV-FTIRS). Exposing the clean ZnO powder samples to CO2 at 300 K leads to the formation of a number of carbonate-related bands. A detailed assignment of these bands was carried out using isotope-substitution experiments with (CO2)-O-18. On the basis of vibrational and thermal stability data for ZnO single crystal surfaces, a consistent description of the interaction of CO2 with ZnO powder particles can be provided: (1) on the mixed-terminated ZnO(10 (1) over bar0) facets, a tridentate carbonate is formed; (2) on the polar, O-terminated (000 (1) over bar) facets, a bidentate carbonate species is formed via CO2 activation at oxygen vacancy sites; and (3) additional monodentate or polydentate carbonate species are formed at defect sites such as steps, edges, kinks, and vacancies. The formation of carbonate-related vibrational bands is observed at an exposure temperature as low as 100 K, thus demonstrating the high activity of ZnO nanoparticles with regard to CO2 activation.