Interaction of hydrogen with RuO2(110) surfaces:: Activity differences between various oxygen species

The interaction of hydrogen with RuO2(110) surfaces was studied by means of thermal desorption and vibration spectroscopies. The stoichiometric surface exposes two types of coordinatively unsaturated atoms: double-bonded O-bridge and five-fold-bonded Ru-cus, while at the O-rich surface the Ru-cus atoms are covered with single-bonded O-cus. On the stoichiometric RuO2(110) surface at 90 K, H-2, either adsorbs molecularly on Ru-cus sites or dissociates and forms with O-bridge an H2O-like surface group. If, in addition, also O-cus is present at the surface, hydrogen interacts exclusively with this species forming H2O-Cus. This demonstrates that hydrogen reacts much more readily with O-cus than with O-bridge as expected from the reduced bond order and smaller binding energy of O-cus. It is furthermore shown that at surface temperatures below 90 K free coordinatively unsaturated Ru-cus sites are needed to activate the incoming H, molecules prior to any reaction with O-cus or O-bridge. Generally, Ru-cus sites play a key role for reactions of a number of molecules at the RuO2(110) surface. These findings are supported by recent DFT-based calculations but are at variance with other reports.