PNNL

Abstract

Periodic density functional theory calculations have been used to study the formation and stability of surface carbonate on BaO(001) upon CO2 exposures. CO2 adsorbs at Lewis basic Os site forming anionic surface carbonate ( ) species until one monolayer coverage (1ML). Certain amount of electrons has been transferred from the surface to CO2 after CO2 adsorption. The adsorption energy of CO2 decreases with the increasing coverage as a combinative result of the less electrons accepted by each adsorbed CO2 and the lateral repulsive interactions. At ?CO2 0.75 ML, dramatic surface reconstruction had been found for the parallel pattern of surface carbonates that initially arranged on BaO(001). Due to strong lateral repulsion between the surface carbonates, the surface reconstruction actually pulls the surface Ba atom out of the surface plane, suggesting a possible onset of phase transition from surface carbonate overlayer to crystalline bulk-like barium carbonate. Surface free energy calculations have been performed to study the stability of surface carbonate at different temperature and pressure conditions. Our calculations indicate that surface carbonates decompose at 850 K at low coverage. For the fully covered carbonate overlayer, surface carbonate will become unstable at about 600 K. This is in good agreement with previous experimental observations. Finally, the effect of surface hydroxyl on the stability of surface carbonate is investigated. At low hydroxyl coverage, the neighboring hydroxyl stabilizes surface carbonate. On the fully hydroxylated BaO surface, the chelating bicarbonate instead of surface carbonate is formed upon CO2 adsorption. This work, performed in the Institute for Interfacial Catalysis at Pacific Northwest National Laboratory (PNNL), was partially supported by a PNNL Laboratory Directed Research and Development (LDRD) project. Computing time was granted by the National Energy Research Scientific Computing Center (NERSC) under project No. m752, and also by the scientific user project (st30469) using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). The EMSL is a U.S. DOE national scientific user facility located at PNNL, and supported by the DOE’s Office of Biological and Environmental Research.