| Calcium oxide is an exceptionally important material which is widely used as heterogeneous catalyst, adsorbent and additive in refractory and paint. In order to deeply investigate the microscopic mechanism of CaO as carbon dioxide adsorbent material, periodic first-principles DFT calculations are performed to study the configurations and electronic structures of CaO(001), (110) surface and different defected surfaces in this work. The interactions between CO2 and different kinds of CaO surfaces are also included. We employ MD/QM method to study the configurations and electronic structures of CO2 adsorption on the CaO(001) surface at different coverage, the high resolution electron energy loss spectroscopy for the most stable configuration is further simulated.Our research results for a series of alkaline earth metal oxides (001) surface show that the size of the metal ions has a significant influence on the CO2 adsorption configurations. For the CaO, SrO and BaO(001) surfaces, CO2 adsorbed on the surface by only one C-O bond, while another two Mg-O bonds formed on MgO(001) surfaces. Due to the specific topology of pristine CaO(110), the two O atoms of CO2 also bonded with the two neighbouring surface Ca atoms. Our calculated results indicate that the CO2 molecule tends to be parallelly adsorbed on the O atom at the surface through the carbon atom and the lengths of formed C-Os bonds vary with the structure and type of the substrate surface. The coordination number of O atom on the (110) surface is lower, which result the newly formed C-Os bond become shorter.CO2 exclusively adsorbs at surface Os sites, forming C-Os bond and Ca-Oc bond at defect surfaces except for the oxygen defect surfaces. For the same type of CaO surface, the adsorption of CO2 molecules on oxygen defect surface is the most stable, the adsorption of CO2 molecules on the step defect surface take the second. For the same type of defects, the adsorption of CO2 molecules on the CaO (110) defect surface is more stable than that on the CaO(001) defect surface. Considering the results of the adsorption of CO2 molecules on pristine surface, the defect enhances the combination ability between the CO2 and the (001) surface. However, the presence of the defect on the (110) surface confines the combination ability and the trend becomes more pronounced with the unsaturated degree increased. In addition, the defect state are partially or completely disappeared after the adsorption except for the (110) step surface.Based on the analysis of the most stable adsorption configuration at different coverage, we can conclude that:the adsorptions of CO2 molecules on the CaO(001) surface are more stable in parallel surface pattern than that in perpendicular surface pattern at low coverage (≤1/3 ML). Dramatic surface reconstruction is induced by strong repulsive interactions between oxygen atoms of the surface carbonates at medium coverage (4/9ML-7/9ML). At high coverage(≥8/9 ML), the structure of the CaO(001) surface is damaged seriously, and some CO2 molecules are adsorbed to the O atoms of the second layer. Therefore, the formation of carbonate could expand into the bulk of CaO and form the layered structure of CaCO3 eventually. With the coverage increased from 1/9ML to 1ML, the average adsorption energy decrease first and then increase... |
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