CO2 Adsorption, Diffusion, and Electron-Induced Chemistry on Rutile TiO2(110): A Low-Temperature Scanning Tunneling Microscopy Study

Low-temperature scanning tunneling microscopy (STM) has been used to study the adsorption of CO 2 on rutile TiO 2 (110) from 80 to 180 K. For low CO 2 doses, two molecular adsorption sites with different binding energies are identified, which are effectively isolated from one another by an apparent activation barrier to their interconversion. We identify the less tightly bound adsorption site as CO 2 adsorbed atop 5-fold coordinated titanium surface atoms (Ti 5f ), without binding preferentially near oxygen vacancies. CO 2 desorption from Ti 5f occurs at ∼140 K. The more strongly bound site involves molecular CO 2 binding at bridging oxygen vacancies (V O,br ). We observe two distinct configurations of V O,br bound CO 2 molecules. Despite its being bound to the vacancy, CO 2 does not dissociate thermally but remains intact up to the desorption temperature of ∼175 K. At an elevated tunneling bias, the STM tip can selectively dissociate these CO 2 molecules and thus trigger the healing of individual V O,br . At higher coverage, CO 2 adsorption occurs predominantly at the more abundant Ti 5f sites, with the distribution of CO 2 molecules being determined by interactions both along the [001] and [11̅0] directions.