We have used cyanide-modified Pt(111) electrodes, in combination with cyclic voltammetry (CV), Fourier transform infrared spectroscopy (FTIRS), and differential electrochemical mass spectrometry (DEMS), to investigate the oxidation of formic acid and methanol on Pt electrodes. Since CO is the poison intermediate formed during the oxidation of both formic acid and methanol, we have previously characterized the CO adlayer on cyanide-modified Pt(111) electrodes. Poison formation on cyanide-modified Pt(111) is nearly completely inhibited in the case of formic acid and methanol, the corresponding electro-oxidation reaction proceeding, hence, exclusively through the reactive intermediate pathway. These results suggest that, in the oxidation of formic acid and methanol, the formation of adsorbed CO would require the presence of at least three contiguous Pt atoms.

The rate of methanol adsorption and oxidation on carbon supported Pt nanoparticles and polycrystalline Pt has been studied by differential electrochemical mass spectrometry (DEMS) under continuous flow. At room temperature, the maximum adsorption rate at polycrystalline Pt is about 0.06 monolayer per second, whereas at Pt nanoparticles it is 0.04 monolayer per second. On polycrystalline Pt it amounts to 56% of a full CO monolayer, obtained by adsorbing CO from a CO saturated electrolyte. On the nanoparticles only 28% of a full CO monolayer can be obtained. The oxidation rate of the adsorbate is of zero order at polycrystalline Pt, whereas at nanoparticles it is of first order with respect to the coverage. Oxidation of methanol starts below the onset of CO2 formation. At polycrystalline Pt and Pt(111), the rate of CO2 formation is determined by the oxidation rate of the adsorbate. At Pt(332) and the nanoparticle electrodes, the rate of CO2 formation from bulk methanol is higher than that from methanol adsorbate, due to the heterogenity of these electrodes.Similarly to Ru modified Pt single crystal electrodes, Ru modified polycrystalline Pt and Pt nanoparticles offer different adsorption sites for CO, which can be selectively populated.

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The electrochemical oxidation of acetic acid in 1 M HClO 4 was studied on boron-doped diamond electrode using differential electrochemical mass spectrometry ͑DEMS͒. DEMS was used to identify products and intermediates of the reaction, monitored online during the potential sweep. The measurements showed that the main product of acetic acid oxidation is CO 2 , which is formed without showing any evidence for the direct discharge of acetic acid resulting in the Kolbe reaction. In fact, only a small amount of ethane was detected by DEMS. In addition to ethane, methanol was identified as a main intermediate of acetic acid oxidation. Based on these results, a model of indirect oxidation of acetic acid via hydroxyl radicals has been proposed.Acetic acid is one of the most refractory organic compounds toward oxidation. 1,2 In fact, the oxidation of acetic acid with O 2 ͑air͒ is often difficult even at a high temperature ͑300°C͒ and pressure ͑200 bar͒ in the wet air oxidation process. As a consequence, acetic acid appears as a final product of oxidation of many organic compounds. 3 Anodic oxidation of acetate ions on platinum electrodes results in the Kolbe reaction 4-7 ͑Eq. 1͒This reaction consists of several steps involving the discharge of acetate ions to acetate radical ͑Eq. 2͒, its decomposition to methyl radical ͑Eq. 3͒, and recombination of the resulting methyl radicals to ethane ͑Eq. 4͒According to the literature, 7,8 in aqueous solutions the Kolbe reaction requires potentials that are higher than the potential of water decomposition. At low current densities, the acetate radicals can also react with hydroxyl radicals to give methanol and carbon dioxide. Under these conditions, the oxygen evolution reaction predominates and the contribution of the Kolbe reaction is very small. At sufficiently high current densities, the electrode surface is occupied mainly by acetate ions which displace water molecules from the surface, hindering the formation of hydroxyl radicals. Under these conditions, the surface concentration of acetate radicals is sufficiently high to favor ethane formation. The critical current density for ethane formation is reached where the rate of formation of acetate radicals equals the rate of consumption of adsorbed acetate radicals with hydroxyl radicals. In strongly acidic media, the electro-oxidation of acetic acid is more difficult as mainly undissociated molecules are present in the solution. 9 As reported by Wadhawan et al., 10 the Kolbe reaction proceeds also on boron-doped diamond ͑BDD͒ electrodes. The authors studied the electrochemical oxidation of aliphatic carboxylic acids in 1 M NaOH under power ultrasound conditions at BDD and Pt electrodes. They found that the type and the yield of products obtained from the biphasic Kolbe electrolysis were identical at both studied electrodes.In this work, the oxidation of acetic acid is investigated in 1 M HClO 4 on a BDD electrode. BDD is a well-known anode of high oxidation power, on which highly reactive hydroxyl radicals are formed during water d...

The electrochemical oxidation of methanol and formic acid in 1 M HClO4 is studied on boron-doped diamond electrode using differential electrochemical mass spectrometry (DEMS). DEMS is used to identify products and intermediates of the reaction monitored online during the potential sweep. The measurements show that the oxidation of both methanol and formic acid proceeds close to the potential of the oxygen evolution reaction, resulting in the appearance of an oxidation wave. During the oxidation, the oxygen evolution reaction shifts toward a higher potential, i.e., becomes a secondary reaction. Depending on the potential, either the formation of intermediates (partial oxidation) or the complete mineralization of organics, simultaneous with oxygen evolution, is observed. Diffusion limitation of the oxidation is clearly observed in parallel to the increase in normalO2 formation. Methylformate is formed from methanol at potentials at which the local concentration of OH radicals is not sufficient for a complete mineralization. Formic acid oxidation proceeds in two waves; before the onset of oxygen evolution, an intermediate (probably oxalic acid) is produced in parallel to CO2 formation. Only in parallel to oxygen evolution is mineralization to CO2 is complete.