Model electrodes for the electrooxidation of simple alcohols

Abstract

<p>Pt is considered as a model for fuel cell electrocatalysts. In the present thesis, I stud-ied the electrooxidation mechanisms of methanol (chapter 3) and ethanol (chapter 4) on different Pt surfaces, using a dual thin-layer flow through cell combined with the mass spectrometer. In chapter 5, Ru quasi single crystal films on different bead Pt surfaces were formed using the resistive heating in a stream of nitrogen. The Ru films were exam-ined by cyclic voltammetry in sulfuric acid and by structure-sensitive underpotential deposition of Cu. Finally, in chapter 6, in order to use bead single crystal in the right ar-rangement (hanging meniscus) on DEMS, a new DEMS flow cell was manufactured and improved for that purpose. <br /> The electrooxidation of methanol proceeds via the dual pathway mechanism. The path involving the formation of soluble intermediates such as formaldehyde and formic acid is the direct pathway, while the dehydrogenation of methanol to adsorbed CO fol-lowed by its oxidation to CO₂ is referred to as indirect pathway. Methylformate is one of the volatile products formed during the electrooxidation of methanol at Pt surfaces. In all previous articles it is assumed that methylformate formation results from the reaction of formic acid and the excess of methanol, i.e. the detection of methylformate is an indirect way to determine the amount of formic acid produced during the oxidation reaction. However, the probability of esterification reaction is very small because the fast diffusion of the soluble products away from DEMS cell under effect of continuous electrolyte flow. A simple kinetic study of methanol esterification and methylformate hydrolysis in acid media was performed since literature data for the rate of this esterification reaction were not available. The reaction rate constant of methylformate formation was found to be far too low (τ ≈ 40 h at 0.1 mol L-1 methanol), while the time constant of dual-thin layer flow through cell at 1.6 µL s-1 is 5 s. Methylformate therefore is directly formed during oxida-tion of methanol at the electrode surface and not in the solution phase as believed before, with a current efficiency about 1%. <br /> The suggested mechanism for methylformate formation, is the nucleophilic attack of adsorbed methanol with another methanol molecule from the solu-tion; note that the nucleophilic power of the oxygen in methanol is higher than that in the water molecule.<br /> The current efficiency with respect to CO₂ and the surface coverage with methanol adsorbate (COad) have been shown to be independent of the electrolyte flow rate (from 1.6–30 µL s-1); this confirms the parallel pathway mechanism.<br /> Poisoning of the catalyst with adsorbed CO is one of the main problems in fuel cells. Ru as a catalyst with Pt promotes the electrooxidation of adsorbed CO according to bi-functional and the electronic mechanism. On such bimetallic surfaces, Ru is preferentially deposited at steps. Using deliberately stepped Pt surfaces as model electrodes, it could be shown that the complete coverage of the step sites with Ru has an inhibiting effect for methanol and ethanol oxidation due to the blockage of the most active sites, i.e. the free step sites are necessary for the first step of C1 and C2 alcohols adsorption and oxidation. <br /> For ethanol, the cleavage of C–C bond is the most difficult step in the complete oxi-dation of ethanol to CO₂. Also ethanol electrooxidation at Pt surfaces occurs according to different pathways depending on the surface structure. During the electrooxidation under controlled convection, where there is no further oxidation of soluble products at the sur-face, acetaldehyde is the main product at polycrystalline Pt and Pt stepped single crystal surfaces vicinal to the (100) plane. Acetaldehyde is formed at these surfaces over the po-tential range with a current efficiency close to 100%. <br /> At Pt stepped single crystals vicinal to the (111) plane, the formation of acetic acid proceeds at lower potentials than that of acetaldehyde production due to the direct reac-tion between adsorbed ethanol and adsorbed hydroxide species. At higher potentials, due the blockage of the surface with adsorbed anions, e.g. acetate and sulfate, only the dehydrogenation of ethanol takes place at (111) planes to produce acetaldehyde. <br /> In practical applications, the formation of acetic acid should be avoided because of its inertness whereas, in principle, acetaldehyde can be oxidized to CO₂. Therefore, it might be advantageous to use nanoparticles without a large degree of (111) facets as electro-catalyst in fuel cells. Another kind of model electrode would be Ru single crystals modified by Pt. How-ever, since Ru is oxidized by atmospheric oxygen very fast, the usual flame annealing method in air does not work. Attard and co-workers developed a new method for Ru quasi single crystal preparation by forced deposition of Ru multilayer on Pt single crystals followed by resistive heating in a nitrogen atmosphere. In order to characterize this Ru film on different Pt single crystals, Cu UPD is the suitable technique. <br /> For Pt(100), the charge density of Cu UPD stripping from Ru quasi-single-crystal electrode is in agreement with the charge density of Cu UPD stripping from clean Pt(100); this suggests the formation of an epitaxial Ru film on the Pt(100) electrode. For the Ru films formed on Pt(111) and Pt(110) surfaces, Cu UPD deposition is inhibited due to strongly adsorbed oxygen species. For Ru films deposited on stepped Pt single crystal vicinal to the (100) plane, it was found that: </p> <ul><li>Because the characteristic Cu UPD stripping peak related to the free Pt sites is absent, the stepped Pt single crystal surfaces are completely covered with Ru film. </li> <li> The charge density for the peak at 185 mV related to Cu UPD stripping from (100) terrace sites decreases linearly with increasing the step density of the Pt single crystal substrate, which confirms the formation of epitaxial Ru films on the Pt surfaces.</li> </ul><p>Preliminary results show that the deposition of a Pt sub-monolayer on the Ru film is pos-sible by galvanic replacement of Cu UPD.<br /> In order to be able to use bead single crystals in the hanging meniscus configuration, a new DEMS cell was constructed. The recorded cyclic voltammogrames for different bead Pt single crystals in supporting electrolyte, under hanging meniscus arrangement and at constant flow of electrolyte, are in agreement with literature profiles. This and the calibration constant and corresponding ionic signals for organic molecules oxidation sug-gest that this new cell is well suited for bead single crystals. Compared with the dual thin layer flow through cell, it has the advantage that cleanliness is easier achieved and that less expensive single crystals can be used. <br /> The K* values are reproducible and typical to that of the dual thin layer flow through cell under the same experimental conditions. For the electrooxidation of bulk methanol at polycrystalline Pt in the new flow cell, the current efficiency with respect to CO₂ is high-er than that in the previous cell design. The reason might be the further oxidation of the soluble intermediates because of a less efficient electrolyte flow in the thin layer between the electrode surface and the glass capillary. Also, the ionic signal of methylformate is not detected under the same experimental conditions due to the small surface area of the working electrode resulting in a small amount of product.</p>