A PHOTOEMISSION-STUDY OF THE COADSORPTION OF CO2 AND NA ON TIO2(110)-(1X1) AND TIO2(110)-(1X2) SURFACES - ADSORPTION GEOMETRY AND REACTIVITY

The coadsorption of CO2 and Na on TiO2(110)-(1 x 1) and -(1 x 2) surfa ces have been investigated by synchrotron-radiation based core-level a nd valence band photoemission. We find that the initially adsorbed Na exhibits a core-level shift of 1.15 eV when the two surfaces are compa red. From a simple adsorption model this binding energy shift is under stood 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 tha t the (1 x 2) surface facilitates a chemically more advantageous Na ad sorption ''adjacent to'' the bridging oxygen atoms. Valence band measu rements 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, th e relatively high resolution of the core-level emission allows in a di rect way the various features contributing to the Na 2p core-lever emi ssion 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 coverag e compared to 1 ML for the (1 x 1) surface, indicating that the Na cov erage required for saturation of CO2 uptake is proportional to the den sity of protruded oxygen rows present at the surface. The CO2 uptake, however, increases as the density of the oxygen rows decreases. Valenc e band photoemission data obtained from both interfaces show that a su rface carbonate species is formed. At lower coverages/exposures there are, however, indications of the presence of a CO2- species rather tha n carbonate, there by suggesting that the carbonate species is formed through the surface reaction: 2CO(2)(-) --> CO32- + CO.