REACTION OFCO2WITH 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 toCO2at pressures ranging from 0.8 to 260 Torr. These results are tentatively compared with a two-stage model for the reaction ofH2Oon MgO(100) [Liuet al.,Surf. Sci.412/413, 287 (1998)]. At p(CO2) values below a threshold pressure (predicted to be in the range of10-9–10-6Torr), the reaction ofCO2with MgO(100) is fairly limited andCO2is assumed to react with defect sites only. At pressures≥ 0.8Torr,CO2reacts with terrace sites, resulting in extensive formation of${\rm CO}_3^{2-}$groups on the MgO(100) surface. However, carbonate formed at "high" pressures (PCO2≥ 0.8Torr) begins to desorb at the low pressure of the analysis chamber (10-11Torr), which is consistent with equilibrium thermodynamic predictions of the "threshold" pressure for the reactionMgO+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 invesgtigated, and is clearly different from${\rm CO}_3^{2-}$.