Interaction of CO with Electron-Rich Defects on MgO(100)

Electron-rich defects are believed to be important for the reactivity of simple oxides such as MgO. We use density functional theory for embedded cluster models to unravel the reactivity differences toward CO between neutral F-0 centers with paired electrons and single excess electron sites, represented by singly charged, paramagnetic F+ centers and by MgO divacancies with one trapped electron. On neutral F-0 centers, adsorption of CO into the most stable state ((3)A '', 1.3 eV binding for a step site) is prevented by a significant barrier (0.5 eV). This explains that the predicted strongly red-shifted IR band of 1365 cm(-1) is not observed. This shift is due to transfer of more than one electron from the defect site into the antibonding CO orbitals. In contrast, single excess electron sites readily react with a CO molecule. On a F+ step site, the binding energy is similar to 1.2 eV and the predicted CO stretching frequency is 1734 cm(-1) within the range of observed values. Paramagnetic divacancies interact with CO in a similar fashion as F+ centers The picture emerging from the calculations supports the conclusions reached from IR and EPR observations of CO in contact with electron rich MgO surfaces.