Visualization of Atomic Processes on Ruthenium Dioxide using Scanning Tunneling Microscopy

The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO 2 (110) and RuO 2 (100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO 2 . CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO 2 at the oxide surface. In contrast to the RuO 2 (110) surface, the RuO 2 (100) surface stabilizes also a catalytically inactive c(2×2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO 2 (100)‐c(2×2) phase may play an important role in the deactivation of RuO 2 catalysts in the electrochemical Cl 2 evolution and other heterogeneous reactions.