Synchrotron-based photoemission spectroscopy (C 1s and O 1s regions) as wel
l as C K-edge NEXAFS spectroscopy have been used to investigate the formati
on 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 comp
ared 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 press
ure (predicted to be in the range of 10(-9)-10(-6) Torr), the reaction of C
O2 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 a
nalysis chamber (10-11 Torr), which is consistent with equilibrium thermody
namic 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(1
00) surface was also investigated, and is clearly different from CO32-.