Coverage Effect of the CO2 Adsorption Mechanisms on CeO2(111) by First Principles Analysis

The adsorption of carbon dioxide on CeO2(111) has been studied using density functional theory. At low coverage (1/9 monolayer), CO2 is found to preferably adsorb in a monodentate configuration forming a carbonate species with a surface O atom. In this configuration, the CO2 molecule is bent with an O-C-O angle of 129 degrees and a remarkable elongation (to 1.27 angstrom) of the C-O bond length compared to the gas phase molecule, indicating a high degree of CO2 activation. A similar activation is observed when the CO2 molecule adsorbs as bidentate carbonate; however, this configuration is less stable. Linear configurations are found to adsorb very weakly at low coverage by physisorption. Increasing the coverage leads to a decrease of the stability of mono- and bidentate configurations which can be attributed to repulsive interactions between adjacent adsorbates and the limited capacity of the CeO2(111) surface to donate electrons to the adsorbates. In contrast, the binding energy of linearly adsorbed CO2 is shown to be coverage independent. At coverages >1/4 monolayer, we have also addressed the stability of mixed configurations where monodentate, bidentate, and linear species are present simultaneously on the surface. The most stable configurations are found when 1/3 monolayer CO2 is bound as monodentate species, and additional molecules are physisorbed forming partial layers of linear species. Analysis of the projected density of states has shown that the orbitals of linear species in the first partial layer lie at lower energies than the ones of the second partial layer suggesting stabilization of the former through interactions with preadsorbed monodentate species. These findings provide fundamental insight into the CO2 adsorption mechanism on CeO2 and potentially assist the design of new Ce-based materials for CO2 catalysis.