Combined studies of DFT atomistic modelling and in-situ FTIR spectroscopy on surface oxidants and CO oxidation at Ru electrodes, Journal of Electroanalytical Chemistry (2012), doi: http://dx.doi.org/10. 1016/j.jelechem.2012.10.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Combined studies of DFT atomistic modelling and in-situ ABSTRACTWe report the combined studies of density functional theory (DFT) calculations and electrochemical in-situ FTIR spectroscopy on surface oxidants and mechanisms of CO oxidation at the Ru(0001) electrodes. It is shown that CO can co-adsorb with both O and OH species at lower potential region where a low coverage of the (2 x 2)-O/OH adlayer formed; the oxidation of CO adsorbates takes place at higher potentials where a high coverage of the (1 x 1)-O/OH adlayer formed. Surface O species are not the active oxidants under all coverages studied, due to the high reaction barriers between CO and O (> 1 eV). However, surface OH species with higher coverage are identified as the active oxidants, and CO oxidation takes place via a two-steps' mechanism of CO + 3OH  COOH +2OH  CO 2 + H 2 O + OH, in which three nearby OH species are involved in the CO 2 formation: CO reacts with OH, forming COOH; COOH then transfers the H to a nearby OH to form H 2 O and CO 2 , at the same time, another H in the H 2 O transfers to a nearby OH to form a weak adsorbed H 2 O and a new OH. The reaction barrier of these processes is reduced significantly to around 0.50 eV. These new results not only provide an insight into surface active oxidants on Ru, which is directly relevant to fuel cell catalysis, but also reveals the extra complexity of catalytic reactions taking place at solid/liquid electrochemical interface in comparison to the relatively simpler ones at solid/gas phase.