Oxygen Scrambling of CO2 Adsorbed on CaO(001)

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 CO2 on well-ordered Ca O 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 O−C and O−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. ■ INTRODUCTION Carbon dioxide (CO2) is an abundant chemical feedstock with wide application in industry. Environmental concerns have driven much research into CO2 interactions with alkaline-earth oxides, such as calcium oxide (CaO), which have utility for carbon capture and catalysis. For sequestration application, the capacity for CO2 adsorption on CaO powders is dependent on particle size, where carbonation results in formation of outer calcium carbonate (CaCO3) layers. 6,7 Nucleation of CaCO3 formation, which is related to the partial pressure of gaseous CO2, occurs quickly, followed by slow growth of CaCO3, which is diffusion-controlled. Modeling of this nucleation process was designed to gain insight into catalyst regeneration in the carbon sequestration effort. Electron spectroscopic techniques indicated CaCO3 also forms a top layer upon CO2 adsorption on CaO(001) thin films and single crystals. However, the thin films were characterized as polycrystalline, and the single crystal was contaminated with water in the ultrahigh vacuum (UHV) system. In a molecular beam experiment on a CaO(001) crystal, a preponderance of defects, assigned to oxygen vacancies, led to the decomposition of CO2 to CO. Therefore, great interest is focused on the initial stages of CO2 adsorption before CaCO3 formation, especially on defect-free, monocrystalline CaO surfaces. Formation of surface carbonate (CO3 2−) species on CaO(001) powder has been proposed by an early infrared spectroscopy study, while numerous theoretical reports corroborated such an assignment on terrace, edge, step, and corner sites. Recently, we presented a joint experimentaltheoretical investigation on CO2 adsorption on CaO(001) in the low coverage regime. Thin films of CaO(001) were grown on a Mo(001) substrate, which have been shown to be nearly defect-free and exhibit properties virtually identical to the bulk. Infrared reflection absorption spectroscopy (IRAS) experiments of CO2 on CaO(001) showed vibrational modes consistent with surface carbonates. We assigned the bands in the IRA spectra as resulting from monodentate carbonate species that first adsorb at steps and other lowcoordinated sites, followed by surface islanding of monodentate carbonates adsorbed on terraces. The aggregation of adsorbates on terraces was consistent with previous computational results and microcalorimetry experiments that observed a coverage effect of CO2 adsorption. 21 The adsorption of CO2 onto metals and metal-oxide surfaces has been probed by experimental techniques to compare the properties of different surfaces and to study the effects of morphology and surface defects. Temperature-programmed CO2 desorption experiments on MgO and CaO utilized isotopically labeled CO2. 31−33 Both CO2 and mixed C OO were observed upon desorption at room temperature, indicating that oxygen atoms of the adsorbate can exchange with the oxide surface lattice. Single and double oxygen atom exchange were explained by formation of bidentate carbonate that can either migrate along the surface before dissociation upon heating, or react with nearby oxygen vacancies that are readily available at low-coordinated sites. Small cluster calculations were used to verify the plausibility of an oxygen scrambling mechanism of CO2 on MgO via lowReceived: May 31, 2017 Revised: July 21, 2017 Published: August 14, 2017 Article