Author: Rachel L. Behrens

Publisher:

Published: 2010

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In support of the recent interest in fuels from renewable resources for fuel cells, the intermediates of the oxidation of formic acid and ethanol to CO2 on platinum electrodes have been examined. The adsorbed carbon monoxide intermediate continues to be the cause for a decrease in efficiency and activity for platinum electrodes and is the leading surface poison (intermediate) in the most common oxidation reactions. We have studied the CO species generated from both fuel and CO-saturated media to observe site conversion of the CO species on polycrystalline platinum and platinum single crystal surfaces with respect to concentration, composition of electrolyte, and potential with electrochemical techniques and broad-band sum frequency generation spectroscopy (BB-SFG). In situ BB-SFG allows for the chemical analysis of electrode surfaces to examine details of surface electrochemical reactions. SFG is based on a second order nonlinear optical process that is forbidden in centrosymmetric media. Therefore, SFG is intrinsically interface-sensitive and enables surface chemical measurements without contribution from the bulk. With the aid of a femtosecond IR laser, we probe vibrational transitions of adsorbates in real time on the electrode surface as the potential at the surface is scanned at rates up to 5 mV/s. Studies of CO species, other reaction intermediates, and adsorbates will be discussed in relation to their poisoning of catalysis by single crystal Pt electrodes. These experiments demonstrate the sensitivity of the BB-SFG technique to the adsorbed species, and its capability to examine adsorption site conversions of the species on the electrode surface. In formic acid fuel solution, in situ BB-SFG was used to obtain vibrational spectra of CO adsorbates produced from formic acid oxidation on a Pt(100) electrode in sulfuric acid and perchloric acid media. The BB-SFG simultaneously monitored all forms of the CO intermediates, including steady-state, as the potential was scanned at 5 mV/s. Spectra were compared to those obtained from CO adsorbed from a CO-saturated electrolyte. While adsorbed from HCOOH, CO had a sharp atop transition near 2050 cm-1 and a broader multiply-bonded transitions in the 1700-1900 cm-1 range, which appear to result from bridge-like and higher-coordinated (possibly fourfold) CO. As the potential was scanned from -0.2 to 0.3 V (vs. Ag/AgCl), the bridge-like CO disappeared and the amount of atop CO increased. At potentials above 0.5 V, CO was in steady-state, being oxidized on the surface to CO2 and replenished by CO from HCOOH. These measurements show that BB-SFG can observe potential-dependent interconversion of different CO forms on the electrode surface and can measure steady-state reaction intermediates on a surface in real time. In situ BB-SFG was also employed to study ethanol on platinum electrodes as a means to elucidate the mechanism of this reaction on catalyst surfaces for fuel cell applications. Recently, the interest in ethanol has increased, not only as a renewable resource, but especially as a fuel source due to its high theoretical yield of 12 electrons released upon complete oxidation. Incomplete oxidation, a major setback with ethanol oxidation, forms byproducts and intermediates slowing the oxidation reaction or prohibiting it from occurring further. Among the byproducts are acetic acid and acetaldehyde, where the C-C bond is not yet broken, or the intermediate CO, where the C-C bond has been broken and is further oxidized to CO2. Using simultaneous electrochemical techniques and broad-band sum frequency generation, these byproducts and intermediates formed on platinum electrodes will be discussed in both acidic and basic media, as a function of electrolyte composition and ethanol concentration.