Electrosorption and electrooxidation of CO on Ru(0001)

The cyclic current-potential curve for a well-defined Ru(0001) surface in 0 .1 M HCIO4 solution clearly exhibits hydrogen and O/OH adsorption peaks at -0.15 and 0.25 V, respectively. The coulometric charge of the current peak at -0.15 V equals 120 muC cm(-2) corresponding to 0.5 monolayer (ML) H cove rage. Both voltammetric peaks disappear completely by the CO electrosorptio n at -0.1 V, demonstrating that the electrosorbed CO completely blocks H ad sorption. The disappeared H adsorption current peak due to the electrosorbe d CO corresponds to a CO saturated coverage of 0.5 ML on Ru(0001) in good a greement with the coulometric data determined by the current transient of C O electrooxidation. The coverage of the electrosorbed CO also agrees well w ith the data obtained for the coadsorption system CO + O/Ru(0001) under UHV conditions. At 0.4 V no CO electrosorption takes place on a (1 x 1)-O/Ru(0 001) surface in a CO-saturated HClO4 solution similarly as with CO adsorpti on on a (1 x 1)-O/Ru(0001) surface under UHV conditions. On the other hand, the coadsorption of COad with O-ad at 0 V gives rise to a well-ordered (2 x 2)-(0 + 2CO) structure similar to that observed from the coadsorption of CO and 0 on Ru(0001) under UHV conditions. No CO electrooxidation occurs at 0.45 V for the coadsorbed CO and 0 on Ru(0001) electrode surface. Up to 0. 55 V the CO oxidation rate increases markedly with increasing potential in good agreement with our previous in situ IR results. The driving force for the CO electrochemical reaction is attributed to the decrease in the activi ation barrier for CO oxidation by the polarization potential.