A photoemission study of the coadsorption of CO2 and Na on TiO2(110)-(1x1) and -(1x2) surfaces: Adsorption geometry and reactivity

The coadsorption of CO2 and Na on TiO2(110)-(1 x 1) and -(1 x 2) surfaces have been investigated by synchrotron-radiation based core-level and valence band photoemission. We find that the initially adsorbed Na exhibits a core-level shift of 1.15 eV when the two surfaces are compared. From a simple adsorption model this binding energy shift is understood in terms of a difference in initial Na adsorption site on these surfaces. While the (1 x 1) surface seems to favor Na adsorption in a hollow site ''between'' bridging surface oxygen atoms, it is found that the (1 x 2) surface facilitates a chemically more advantageous Na adsorption ''adjacent to'' the bridging oxygen atoms. Valence band measurements support this model since Na adsorption on the (1 x 2) surface leads to emission characteristic of alkali-oxygen-like compounds while this is not the case for the Na/TiO2(110)-(1 x 1) system. Finally, the relatively high resolution of the core-level emission allows in a direct way the various features contributing to the Na 2p core-lever emission to be determined. With respect to adsorption of CO2 we find for the (1 x 2) surface that CO2 uptake saturates around 0.5 ML Na coverage compared to 1 ML for the (1 x 1) surface, indicating that the Na coverage required for saturation of CO2 uptake is proportional to the density of protruded oxygen rows present at the surface. The CO2 uptake, however, increases as the density of the oxygen rows decreases. Valence band photoemission data obtained from both interfaces show that a surface carbonate species is formed. At lower coverages/exposures there are, however, indications of the presence of a CO2- species rather than carbonate, there by suggesting that the carbonate species is formed through the surface reaction: 2CO(2)(-) --> CO32- + CO.