Interaction of CO2 with oxygen adatoms on rutile TiO2(110).

The interactions of CO2 with oxygen adatoms (Oa's) on rutile TiO2(110) surfaces have been studied using scanning tunneling microscopy. At 50 K CO2 is found to adsorb preferentially on five-coordinated Ti sites (Ti5c's) next to Oa's rather than on oxygen vacancies (VO's) (the most stable adsorption sites on reduced TiO2(110)). Temperature dependent studies show that after annealing to 100-160 K, VO's become preferentially populated indicating the presence of a kinetic barrier for CO2 adsorption onto the VO's. The difference between the CO2 binding energy on VO's and Ti5c sites next to the Oa's is found to be only 0.009-0.025 eV. The barrier for CO2 diffusion away from Oa's is estimated to be ~0.17 eV. Crescent-like features of the images of CO2 adsorbed on Ti5c's next to Oa's are interpreted as a time average of terminally bound CO2 molecules switching between the configurations that are tilted towards Oa and/or towards one of the two neighbouring bridging oxygen (Ob) rows. In the presence of VO defects, the Ti5c bound CO2 is found to tilt preferentially away from the VO containing Ob row. If another CO2 is present on the neighbouring Ti5c row, both CO2 molecules tilt towards the common Ob row that separates them.