The behaviour and poisoning effects of the CO adsorbed at 50 and 500 mV (vs. RHE) on Pt were studied in 1 M HClO4 at room temperature. The 500 mV CO(a) is much more resistive to electrochemical oxidation than the 50 mV CO(a), although both are linearly bonded CO. Neither migrates on the electrode surface, being inactive to hydrogen but very reactive to O2. For the hydrogen evolution reaction, 50 mV CO(a) acts basically as a third body in the entire range of CO coverage but increases the Tafel slope above a coverage of 0.7. The hydrogen ionization reaction was completely blocked up to 0.2-0.3 and 0.65-0.7 V by 50 and 500 mV CO(a), respectively. At more positive potentials the reaction takes place at free sites provided by the initiation of CO(a) oxidation and attains a diffusion limiting rate far beyond the complete removal of CO(a). On the other hand, NO-2 undergoes reduction in the potential range where hydrogen ionization is completely blocked at the 50 mV CO(a) surface. The reaction takes place at the expense of bridged CO on contact of NO-2 with the surface.
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