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Abstract

The adsorption of CO₂ on CaO(001) is investigated by density functional theory and infrared reflection absorption spectroscopy (IRAS). The calculations show that isolated CO₂ adsorbates on terraces as monodentate carbonates can freely rotate at room temperature, while rotation within carbonate aggregates has some hindrance. Rotation and other motions are important to facilitate oxygen atom exchange between the CO₂ adsorbate and CaO lattice. The calculated intrinsic barrier to oxygen scrambling is 114 kJ/mol for an isolated carbonate species and 148 kJ/mol within a long carbonate chain. However, due to the large adsorption energy for CO₂ on a defect-free CaO terrace site, the apparent barrier becomes −39 kJ/mol for an isolated carbonate. At lower coordinated sites with higher degrees of freedom, the calculated intrinsic barrier to oxygen atom exchange is 80 kJ/mol at filled monatomic step sites and 26.9 kJ/mol at corner sites. IRAS studies are performed by adsorbing C¹⁸O₂ on well-ordered Ca¹⁶O films grown on Mo and Ru substrates. The magnitude and splitting of the red shifts due to isotopic labeling are rationalized when considering oxygen scrambling, such that observed normal modes of surface carbonates involve both ¹⁶O–C and ¹⁸O–C vibrations. As previously assigned, the earliest observable infrared peaks are due to adsorption at step sites, and additional observable peaks are due to aggregation of carbonates on terraces.