NECESSITY OF CO-FREE PT SITES FOR THE ELECTROOXIDATION AT LOW POTENTIALS OF DISSOLVED CO ON POLYCRYSTALLINE PT

We have studied the effect of several experimental variables (CO admis sion time, bubbling rate of CO, and magnetic stirring of the electroly te) on the electrooxidation at low potentials of dissolved CO on Pt. A lthough this electrooxidation was affected by changes in the said vari ables, in ail cases it was always accompanied by the presence in the s ame potential range of a hump of chemisorbed CO (here designated as CO (1)) in parallel CVs recorded in the absence of dissolved CO and whose charge was about 15% of a monolayer of chemisorbed CO. Since this hum p of chemisorbed CO(1) preceded the main peak of electrooxidation of c hemisorbed CO, it is clear that electrooxidation of the dissolved CO t akes place only on this 15% of the Pt surface free from chemisorbed CO (1). The stationary currents, the peak currents, and the Cottrell plot s of dissolved CO electrooxidation were in good agreement with those c alculated for a wholly active surface. The only possible explanation o f this behavior is that the CO(1)-free Pt sites act as an ensemble of microelectrodes, so that the overlap of their hemispherical diffusion layers yields the same current density as a conventional electrode. Ac tually, both plots of peak potential vs the logarithm of the sweep rat e and of peak current density vs the square root of the sweep rate yie lded the linear behavior expected of conventional electrodes, which sh ows that the time necessary for both electrooxidation of the chemisorb ed CO(1) and for the said overlap was negligible for sweep rates up to 10 V s(-1). From this an upper value of about 3 mu m for the diameter of the CO-free patches was estimated.