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

The interaction of hydrogen with RuO(2)(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 RuO(2)(110) surface at 90 K, H(2) either adsorbs molecularly on Ru-cus sites or dissociates and forms with O-bridge an H(2)O-like surface group. If, in addition, also O-cus is present at the surface, hydrogen interacts exclusively with this species forming H(2)O-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(2) 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 RuO(2)(110) surface. These findings are supported by recent DFT-based calculations but are at variance with other reports.