Density Functional Theory Study of Surface Carbonate Formation on BaO(001)

Periodic density functional theory calculations were used to study the formation and stability of surface carbonate on BaO(001) at CO2 exposure. CO2 adsorbs at Lewis basic O-s sites, fort-ning anionic surface carbonate (CO3-delta) species. Upon adsorption, a portion of electrons is transferred from the surface to CO2. Although the adsorption strength of CO2 decreases with the increasing coverage (theta(CO2)), the surface carbonate overlayer in the perpendicular pattern is stable up to one monolayer (1 ML). Dramatic surface reconstruction is found for the surface carbonate overlayer in the parallel pattern at theta(CO2) > 0.75 ML. The BaO(001) Surface is reconstructed such that the surface Ba atoms are actually pulled out of the surface plane into the carbonate overlayer, suggesting a possible onset of phase transition from surface carbonate to crystalline bulklike carbonate when the coverage is higher than 0.75 ML. At ambient CO2 pressure, the calculated surface free energies indicate that the surface carbonate monolayer becomes unstable at 600 K, while the isolated surface carbonate is still stable at 800 K. This is in good agreement with previous experimental observations. The effect of surface hydroxyl on the stability of surface carbonate was also investigated in this work. At low hydroxyl coverage, the neighboring hydroxyls stabilize the surface carbonate. On the fully hydroxylated BaO(001) surface, chelating bicarbonate, instead of surface carbonate, is formed upon CO2 adsorption.