On the differences in the reaction mechanism for CO and CO/H{sub 2} electrooxidation on PtRu and PtSn alloy electrodes

Electrooxidation kinetics of mixtures of carbon monoxide and hydrogen were studied on well-characterized surfaces of Pt and alloys of PtRu and PtSn in 0.5 M H{sub 2}SO{sub 4} at room temperature and 60 C. The alloy electrode surfaces were prepared in UHV by sputter/anneal cycles and their surface compositions were determined via low energy ion scattering. Subsequently, the electrodes were transferred contamination-free from UHV into a rotating disk electrode (RDE) configuration in a conventional electrochemical cell and their activity was measured both by CO stripping voltammetry and under the continuous flow of CO and CO/H{sub 2} gas mixtures in RDE-experiments. The overpotential for the continuous oxidation of pure CO on PtSn electrodes with a Sn surface composition of x{sub Sn,s} {approximately} 0.2 is significantly smaller than on PtRu alloys (x{sub Ru,s} {approximately} 0.5) and on pure Pt. The reaction order with respect to solution phase CO is negative on PtRu alloys due to the competition between OH{sub ads} nucleation and CO adsorption on Ru surface atoms. Owing to the lack of CO adsorption on OH{sub ads}-providing Sn surface atoms, the reaction order with respect to CO is positive on PtSn electrodes. Therefore, the activity enhancement of PtSn electrodes versus PtRu and Pt electrodes is most pronounced in pure CO and decreases with the CO concentration in CO/N{sub 2} and CO/H{sub 2} mixtures