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Electrocatalysts with constant metallic composition, consisting of carbon-supported platinum and molybdenum phases, were synthesized following the thermolysis (thr) and borohydride (bhr) reduction methods and using different metallic precursors. The obtained electrocatalysts were characterized by X-ray energy-dispersive spectrometry, X-ray diffraction and high resolution transmission electron microscopy. Their activities were studied by cyclic voltammetry. Different surface structures were obtained and the electrochemical activities toward methanol oxidation were compared. Pt, MoO 2 and MoO 3 phases were well identified with the characterization techniques used. However, the electrochemical responses obtained from both sample series were considerably different, suggesting that the arrangement and relationships between active phases strongly depend on the synthesis method and aggregation sequence of the metallic precursors, and being the cause of different catalytic activities and stabilities of molybdenum oxide phases. The bhr method offered higher activity than the thr method. Among the sample series obtained by bhr method, the catalyst obtained by platinum deposition on the previously synthesized molybdenum on carbon, led to the highest overall activity. Fuel Cells represent one of the most promising options for generating electricity with high efficiency and low environmental impact. The Direct-Methanol Fuel Cell (DMFC) is a low-temperature cell where methanol is directly supplied as fuel to the anode compartment, without any previous reforming. Furthermore, methanol can be stored and transported in liquid phase, it can be obtained from biomass, and its complete oxidation can yield a high energy density.1 Platinum has been recognized as the most active catalyst for the methanol oxidation reaction and it has been employed in the formulation of such materials. However, the formation of different organic intermediates, adsorbed on Pt surfaces during methanol electrooxidation, causes the poisoning of active sites and decreases the catalyst efficiency. Several studies indicate that the modification of the Pt catalyst with other transition metal can lead to good results in oxidizing methanol at a low potential (lower than in pure Pt). 2On the other hand, it is well known that bimetallic materials show increased activity toward the CO electro-oxidation reaction, yielding CO 2 . Studies on catalytic effects that take place on bimetallic surfaces, obtained from pure metals and modified by the deposition of a second metal, are closely related to the technological development of low temperature fuel cells. Some researchers have emphasized the use of CO as a test molecule to show the electronic effect associated with this reaction. The CO desorption energy is apparently related to strong intermetallic bonds and mixed orbitals of the active phases. Hence, a catalyst with high activity toward methanol oxidation must also have high activity for CO oxidation.3 This activity is explained by some authors in terms of the so-cal...

A series of carbon-supported bimetallic catalysts with different metallic loadings was synthesized, using platinum as the principal active phase and molybdenum or tungsten as promoting phases. The materials were prepared by organometallic precursor thermolysis and characterized by direct current electrochemical methods, transmission electron microscopy, scanning electron microscopy and x-ray diffraction. Electrodes were elaborated with each catalyst and their electrochemical performances were studied by cyclic voltammetry. These results show an increased activity of the catalysts with small amounts of Mo or W, towards oxidation of methanol with respect to the catalyst containing only platinum. XRD results reveal the presence of molybdenum or tungsten bronzes (HxMoO3, HxWO3) that are responsible for the increase in activity. It is believed that the bronzes participate in a spillover effect by promoting the removal of protons from the platinum surface. It was found that the presence of molybdenum in this type of catalyst prevents the platinum phase from sintering during the thermal treatment and allows them to keep platinum particles with mean sizes between 2 and 8 nm. The proposed catalysts are adequate for methanol oxidation in liquid-fuel alcohol fuel cell systems, since it was found that oxidation potentials are lower than those observed with platinum catalysts.

The activities of a series of carbon-supported bimetallic catalysts, with different active phases loadings, were tested towards the oxygen reduction reaction (ORR). Pt was used in all materials and its loading was kept constant. Mo, W and Ru were used as promoting phases. Rotating Disk electrode experiments revealed that Pt-Ru displayed the best performance in oxygen reduction among the studied materials. The materials with the highest loadings of the second metal revealed the highest activities. X-Ray Diffraction studies (XRD) and Transmission Electron Microscopy (TEM) revealed the presence of homogeneously dispersed metallic ruthenium and different tungsten or molybdenum oxides in the corresponding catalysts. No evidence of alloying was found, and thus the catalytic performances appear to be related to the distribution and interaction of the active phases.

Different carbon-supported Pt-Mo and Pt-Ru materials were synthesized and a systematic study was carried out in order to evaluate their catalytic activity towards methanol oxidation. Direct current methods were applied in sulfuric acid and methanol - containing electrolytes, in order to evaluate the electrochemical response of the studied electrodes. Pt-Mo catalysts reveal similar performances and, in some cases, higher than Pt-Ru materials. For both catalysts series, it was found that low loadings of the promoting metal (Ru or Mo) improve the methanol oxidation activity. Characterizations by means of transmission electron microscopy and X-Ray Diffraction allowed to measure mean particle sizes below 10 nm for all phases. The Pt-Ru catalysts consist of metallic Pt and metallic ruthenium, while in the The Pt-Mo materials platinum is present in its metallic state and MoO3 is the predominant molybdenum species.