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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.
