Lewis Acido-Basic Interactions between CO2 and MgO Surface: DFT and DRIFT Approaches

Combined experimental infrared (IR) and theoretical approaches have been carried out in an attempt to specify the actual structure of the CO2 species adsorbed on the magnesium oxide surface. The interaction of CO2 with regular sites of the MgO(100), (111), and (110) surfaces as well as MgO(100) defect sites (steps, corners, kinks, and divacancies) has been investigated by means of density functional theory study. Theoretical IR frequencies compared with IR experiments show distinguishable carbonate species, adsorbed on different planes and defects, vibrating in different IR-frequency ranges. In addition, by means of thermodynamic model, the stability of carbonates as a function of temperature have been calculated and compared to the experiment. Analyzing the nature of basic sites, the results show that the most active site versus CO2, which is a Lewis acid, is not the same as the strongest site for the deprotonating adsorption of Brensted acids. The present work revisits and improves the understanding of carbonate species that could exist on the magnesium oxide surface and gives a picture of the accessible planes of magnesium oxide as well as their surface Lewis basicity.