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The Role of Adsorbed Formate and Oxygen in the Oxidation of Methanol at a Polycrystalline Pt Electrode in 0.1 M KOH: An In Situ Fourier Transform Infrared Study

Lookup NU author(s): Professor Paul Christensen, Douglas Linares Moya

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Abstract

The oxidation of methanol at a polycrystalline Pt anode was investigated with in situ external reflectance Fourier transform infrared (FTIR) spectroscopy in both nitrogen- and oxygen-saturated 0.1 M KOH. The data strongly suggest that adsorbed formate is either the, or at least an important, intermediate in the direct pathway, with attack on this intermediate (rather than its formation) being the rate-determining step. The HCOOads is formed via an Eley-Rideal-type reaction between solution methanol and OHads. Linearly adsorbed CO (COL) appears to be a spectator rather than a participant in the methanol oxidation reaction, at least up to relatively high potentials, i.e., 0.1 V. The primary products of the oxidation are formate up to -0.5 V, then bicarbonate as well as formate and finally only CO2 at potentials greater than or equal to -0.3 V. Clearly, the pH in the thin layer drops below 10.25 at potentials greater than -0.5 V and below 6.37 in the latter potential region, these being pKa,2 and pKa,1, respectively, of carbonic acid. This indicates the disadvantage of the external reflectance approach, where the thin layer causes very restricted diffusion, compared to those techniques such as surface-enhanced infrared spectroscopy (SEIRAS) that employ attenuated total reflectance. However, the external reflectance approach has a major advantage over SEIRAS of detecting solution-borne intermediates and products, which are inaccessible to the latter technique because of its lack of sensitivity beyond the near-electrode region. At potentials greater than -0.1 V, the CO2 solution band remained constant as excess CO2 was forced into bubbles, pushing electrolyte out of the thin layer. In oxygen-saturated solution, no COL was observed. The O2-reduction products clearly modified the Pt surface and altered the characteristics of the Pt oxide film formed at higher potentials, which, among other effects, removed the nucleation sites for CO 2 bubble formation. Because of the bubble formation in N2-saturated solution, it was difficult to judge whether more CO2 was formed in O2-saturated KOH, but the amounts of solution formate and bicarbonate formed in the latter were significantly higher than those in N 2-saturated solution. © 2010 American Chemical Society.


Publication metadata

Author(s): Christensen P, Linares-Moya D

Publication type: Article

Publication status: Published

Journal: Journal of Physical Chemistry C

Year: 2010

Volume: 114

Issue: 2

Pages: 1094-1101

ISSN (print): 1932-7447

ISSN (electronic): 1932-7455

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/jp908726f

DOI: 10.1021/jp908726f


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