Oxidation of Methanol and Carbon Monoxide on Platinum Surfaces

Abstract

Despite of its fairly simple chemical structure, the oxidation of methanol at electrode surfaces follows a fairly complicated mechanism with parallel reaction paths and several reaction steps. Within the context of the search for better catalysts for fuel cells, the influence of various parameters such as potential, temperature, catalyst composition and structure on the single reaction steps were elucidated with the help of CO oxidation and the adsorption and oxidation of methanol on carbon supported Pt nanoparticles, polycrystalline Pt and Pt(665) as well as the influence of foreign metals such as Ru and Mo on these reactions has been studied by differential electrochemical mass spectrometry (DEMS) under continuous flow conditions. The rates and activation energies of the single reaction steps as well as the influence of the catalyst composition and structure was determined by measuring of the adsorption rate of methanol, the oxidation of the methanol adsorbate as well as the rate of the bulk methanol oxidation, where the ion current of the CO₂ was detected in parallel to the faradaic current. <br /> Catalysts, and this also includes Ru and Se modified catalyst which are important as cathode material for O₂ reduction in the fuel cells, were characterized by stripping of adsorbed CO and of underpotential deposition of Cu (Cu_UPD). It was shown that the active surface area, determined from the CO stripping, agrees with that calculated from the particle size assuming a narrow distribution and a spherical shape of the colloid particles. On the Se modified nanoparticle surfaces, it is shown that Se blocks the adsorption sites for CO, the free adsorption sites can be detected by the Cu_UPD).<br /> The maximum methanol adsorption rate found at room temperature at polycrystalline Pt is ca. 0.06 ML s^-1, whereas at Pt nanoparticles it is ca. 0.04 ML s^-1. At 50°C the rate constants of the methanol adsorption increases markedly on both electrodes. On polycrystalline Pt the maximum coverage of the methanol adsorbate 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 methanol adsorbate was found to be of zero^th order at polycrystalline Pt, whereas at nanoparticles it is of first order with respect to the coverage. At polycrystalline Pt and Pt(665), the rate of CO₂ formation is determined by the oxidation rate of the adsorbate. At the nanoparticle electrodes, the rate of CO₂ formation from bulk methanol is higher than that from the methanol adsorbate, due to the roughness of these electrodes.<br /> At Ru containing Pt surfaces the oxidation of the methanol adsorbate was shifted to lower potentials and a higher adsorption rate was observed in comparison to the pure Pt surface. It was observed that the Ru ad-atoms promote the reaction path via adsorbed CO in the low potential region. At higher potentials the Ru loses its co-catalytic activity towards methanol oxidation; possibly due to the formation of inactive anhydrous Ru oxide at higher potentials.<br /> Using isotopic labelling, the interaction of methanol and carbon monoxide with its adsorbed species on Pt and platinum based electrodes was also studied. It was shown that pure Pt surfaces modified by 0.2 ML of Ru offer different adsorption sites for CO, which can be selectively populated. On these sites, adsorption and oxidative desorption of CO can be selectively performed and ¹²CO at the sites with lower adsorption enthalpy can be replaced by ¹³CO.<br /> The additional deposition of Mo onto the Pt nanoparticle surfaces modified with 0.2 ML of Ru showed that the co-catalytic effect of Ru and of Mo in CO oxidation can be combined in a synergetic sense. On the other hand, no significant improvements on methanol oxidation were found.<br /> An enhancement of the overall methanol oxidation reaction for bulk methanol by the elevating temperature was observed and the overall apparent activation energies, determined from the faradaic current, as well as the apparent activation energies for theindirect oxidation pathway via adsorbed CO, determined from the ion current of CO₂, were calculated. The comparison of both kinds of activation energies confirmed that the Ru containing Pt surfaces have a positive effect on the catalytic activity towards methanol oxidation via adsorbed CO.