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 CO2 on rutile TiO2(110) from 80 to 180 K. For low CO2 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 CO2 adsorbed atop 5-fold coordinated titanium surface atoms (Ti-5f), without binding preferentially near oxygen vacancies. CO2 desorption from Ti-5f occurs at similar to 140 K. The more strongly bound site involves molecular CO2 binding at bridging oxygen vacancies (V-O,V-br). We observe two distinct configurations of V-O,V-br bound CO2 molecules. Despite its being bound to the vacancy, CO2 does not dissociate thermally but remains intact up to the desorption temperature of similar to 175 K. At an elevated tunneling bias, the STM tip can selectively dissociate these CO2 molecules and thus trigger the healing of individual V-O,V-br. At higher coverage, CO2 adsorption occurs predominantly at the more abundant Ti-5f sites, with the distribution of CO2 molecules being determined by interactions both along the [001] and [1 (1) over bar0] directions.