Reaction of CO2 with MgO(100) surfaces

Synchrotron-based photoemission spectroscopy (C 1s and O 1s regions) as well as C K-edge NEXAFS spectroscopy have been used to investigate the formation of carbonate species on the surface of MgO(100) after exposure to CO2 at pressures ranging from 0.8 to 260 Torr. These results are tentatively compared with a two-stage model for the reaction of H2O on MgO(100) [Liu et al., Surf. Sci. 412/413, 287 (1998)]. At p(CO2) values below a threshold pressure (predicted to be in the range of 10(-9)-10(-6) Torr), the reaction of CO2 with MgO(100) is fairly limited and CO2 is assumed to react with defect sites only. At pressures greater than or equal to 0.8 Torr, CO2 reacts with terrace sites, resulting in extensive formation of CO32- groups on the MgO(100) surface. However, carbonate formed at "high" pressures (P-CO2 greater than or equal to 0 0.8 Torr) begins to desorb at the low pressure of the analysis chamber (10-11 Torr), which is consistent with equilibrium thermodynamic predictions of the "threshold" pressure for the reaction MgO + CO2 = MgCO3.(.) Results from polarization-dependent C K-edge NEXAFS spectroscopy suggest that the carbonate molecules formed on the surface are structurally disordered. The chemical nature of adventitious carbon formed on the MgO(100) surface was also investigated, and is clearly different from CO32-.