Influence of Structural Effects on Platinum Electrodes in the Preferential Adsorption of CO and Methanol in Acid Media

The complete methanol electro-oxidation on platinum single crystal electrodes produces CO₂, H₂O and 6 e^- in acidic medium. However methanol adsorption is sensitive to the platinum surface orientation, showing different reactivity for different platinum crystallographic planes.

In this work, it was studied the preferential adsorption and oxidation of carbon monoxide (CO) and methanol in acid media on polycrystalline and low index planes of platinum single crystals.

 In the experimental procedure carbon monoxide was bubbled in methanol solution with the platinum electrode polarized at 0.1 V vs. RHE. Then, the potential was scanned from 0.1 to 0.8 or 1.0 V vs ERH depending on platinum single crystal electrode.

From Figure 1a the carbon monoxide oxidation occurs at potential above 0.6 V vs. RHE for all platinum surfaces.  Besides data from Figure 1b methanol oxidation on Pt(111), Pt(100) and polycrystalline platinum electrodes exhibited high potential, above 0.8 V (RHE).

After adsorption of carbon monoxide on platinum electrodes, on polycrystalline platinum and Pt(100) the peak maximum occurred at more positive potential compared with methanol reaction. In this case methanol oxidation only was promoted after carbon monoxide oxidation reaction. On Pt(100) and Pt(111) the adsorption of CO contributed to reduce the potential for methanol oxidation.

Thus, these experiments suggest that carbon monoxide competes with methanol for adsorption on platinum electrode. For polycrystalline Pt and Pt(110) single crystals, the mixture CO+CH₃OH showed potential of oxidation larger than with pure reactants. On the other hand, on Pt (111) and Pt(100) the oxidation of methanol happens at potential above that for the mixture of methanol and carbon monoxide, so that, it is possible that the electrochemical reaction follows different pathways on these surfaces.

Figure 1. (a) first potential scan at platinum electrodes in 0.2 M CH₃OH + 0.1M H₂SO₄, obtained at 10 mV s^-1. Insert show CO peak between 0.2 – 0.4 V (RHE). (b) Potential peak maximum dependence for CO, CH₃OH and CO + CH₃OH oxidation.

Acknowledgedment:CNPq for financial support (process 150885/2012-1).

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