Carbon-supported Pt-Ru ͑1:1͒ catalysts were synthesized from aqueous solutions of Pt͑IV͒ and Ru͑IV͒ salts by two different reductive methods and characterized in comparison to a commercial Pt-Ru/C catalyst purchased from E-TEK, Inc. The three catalysts differ in particle size, dispersion, and degree of alloying, as determined by X-ray diffraction and transmission electron microscopy. Cyclic voltammetry in different methanol concentrations and CO-stripping experiments were conducted to check their electrocatalytic activity. The results obtained are in good agreement with single-cell measurements using H 2 /CO mixtures with concentrations of 75 and 150 ppm CO. The synthesized catalysts show improved activities for low CO concentrations at 75°C cell temperature. In addition, for the synthesized catalysts only low CO coverages were found at the electrode surface by special in situ infrared reflectance techniques in contrast to the commercial one.Anode catalysts for polymer electrolyte membrane fuel cells ͑PEMFC͒ show a high sensitivity toward CO contamination, tolerating no more than 5-20 ppm CO in hydrogen under typical working conditions of 80°C and atmospheric pressure. Therefore the development of less CO-sensitive catalysts is still emphasized in fuel cell research. Various binary and ternary Pt catalyst systems, e.g., Pt-Ru, 1-3 Pt-Mo, 4 -6 Pt-Sn, 7,8 and Pt-Ru-W, 9,10 were tested, showing improved activity for the oxidation of H 2 /CO mixtures. So far, binary Pt-Ru systems seem to be the most promising catalysts for the electro-oxidation of both reformate and methanol. Therefore several studies deal with the investigation of the optimum Pt-Ru stoichiometry in dependence on temperature and concentration. Recent studies of Gasteiger and co-workers 11 obtained best activities for the oxidation of H 2 /CO mixtures on well-characterized Pt-Ru alloy electrodes at room temperature with 50 atom % Ru, whereas for higher CO concentrations (Ͼ1000 ppm) Ru-rich surfaces showed better performance. Pulsing techniques, such as the one developed by Carrette et al., 12 should even allow the direct feeding of reformate gas into the fuel cell without significant loss in performance. For direct methanol fuel cell ͑DMFC͒ applications, methanol electro-oxidation on well-characterized Pt-Ru bulk alloys 13 and Pt-Ru model systems 14 was studied and previous geometric considerations were confirmed 15 reporting an optimum Ru surface concentration of 10 atom % at room temperature. According to some authors and due to a change in the rate-determining step, the optimum Ru surface increased with increasing temperature up to approximately 30 ͑atom %͒ at 60°C. 15,16 In a more recent work, 17 optimum Ru concentrations for CO and methanol oxidation, respectively, were determined to be about 50 atom % Ru for reformate operation and 15-20 atom % in DMFC mode, showing no significant effect of the working temperature.However, whereas numerous results in the field of electrocatalysis produced a detailed knowledge of electrode processes and mechanism...
A special in-situ PEM fuel cell has been developed to make XAFS measurements during hydrogen and direct methanol operation possible. The chosen set-up in transmission allows for the on-line monitoring of changes in the catalysts oxidation states and their characteristic short-range order. In both operation modes, the catalyst is reduced during operation, as can be concluded from the XANES region of the spectra showing a decreased white-line intensity in E space.
Carbon-supported PtÈRu (1 : 1)-catalysts have been synthesized by two reduction methods in an aqueous phase and in a third way in an organic solution and characterized in comparison to a commercial PtÈRu/C catalyst purchased from E-TEK. X-ray di †raction and transmission electron microscopy were carried out on the di †erent samples in the as-synthesized state and after heat-treatment at 500 ¡C in nitrogen and air atmospheres respectively. Powder patterns of the di †erent catalysts in the as-synthesized state reveal a fcc pattern with d values matching or close to platinum. No evidence of metallic Ru or any oxide phases was found. After heat-treatment in nitrogen Ru reÑections occur in the synthesized catalyst samples, backing the supposition of separate platinum and ruthenium particles instead of a PtÈRu alloy.
Different Pt/Me/Pc and Pt/Me/Complex catalysts (with Me ¼ Metal: Co, Ni, and Pc ¼ phthalocyanine) were synthesized by an impregnation method. A commercially available platinum catalyst purchased from E-TEK. was impregnated with solutions of cobalt phthalocyanine (CoPc) and nickel phthalocyanine tetrasulfonic salt (NiPc). After the reaction, part of the catalyst was heat treated at 700°C under nitrogen atmosphere. The resulting catalysts were structurally and electrochemically characterized before (Pt/Me/Pc) and after heat treatment (Pt/Me/Complex). The Pt/Me/Pc had an average particle size of about 3 nm, while the average size after heat treatment increased to about 7 nm. The composition of the different catalysts was about 80 at% platinum and 20 at% of the second metal (Co or Ni), and was verified with EDXS. In single fuel cell tests the best electrocatalytic activity was observed for the Pt/Ni/Complex system.
Carbon-supported Pt-Ru (1 : 1) catalysts were synthesized from aqueous solutions of Pt IV and Ru IV salts by two different reduction methods and in an organic solvent according to a slightly-modified Boennemann synthesis. X-ray absorption spectroscopy was applied to characterize the in-house synthesized catalysts in comparison to a commercial carbon-supported Pt-Ru/C alloy catalyst purchased from E-TEK inc. Significant geometric differences were revealed by the conventional EXAFS analysis which are attributed to differences in particle size and alloy formation. In contrast to the commercial catalyst, which is at least partially alloyed, for the in-house synthesized catalysts a much smaller number of Pt-Ru nearest neighbours has been found pointing either towards a rather inhomogeneous alloy formation or to the formation of ruthenium oxide. These findings are in excellent agreement with the results of the '' Atomic '' XAFS; the lower the number of Pt-Ru nearest neighbour contributions the higher the R value at which the maximum of the AXAFS feature appears. The R value, however, is supposed to be directly reflective of the d band occupancy which decreases as the degree of alloy formation increases.
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