Alloy catalysts, anodic methanol oxidation

The meehanism of methanol oxidation on the Pt surface has been investigated extensively for decades. According to a well-described mechanism [80], methanol oxidation is a dehydronation process involving the formation of the CO-like intermediates. These intermediates adsorb on the surface of Pt to reduce the catalytic activity. The Pt-Ru alloy has been found to be the most active antipoisoning anode catalyst for DAFCs. The enhanced activity of the Pt-Ru catalyst for methanol oxidation has been attributed to a bi-functional mechanism [81]. This bi-functional mechanism involves the adsorption of oxygen-containing species on Ru atoms at lower potentials, thereby promoting the oxidation of CO to CO2, which can be summarized as follows [82] ... 

According to long-lasting experimental efforts, the use of alloy catalysts that contain a less noble metal whose oxide exhibits low solubilities in acid electrolytes—in particular Sn and Bi are effective in this respect—enhance the catalytic activity of platinum. The rationale of this effect has been that the oxide of the nonnoble component at close atomic distance from the Pt surface atoms supplies by spillover the oxygen that is necessary to oxidize the adsorbed CO species. Today research and development turn more to Ru and lr or Rh, the more easily oxidizable platinum metals as alloying metals that seem to be at least as efficient as Bi and Sn and are certainly more stable than those in acidic environments—in particular if the anode potential becomes more anodic in cases of poor supply of fuel (158). The Pt-Ru anode exhibits a sizeable higher oxidation current for methanol and for adsorbed hydrogen than the Pt electrode, indication that a smaller part of the Pt electrode surface is blocked by CO adsorption. Still the catalytic activity is too low because the onset of the anodic peak of methanol oxidation is at a... 

Reddington et al. (66) reported the synthesis and screening of a 645-member discrete materials library L9 as a source of catalysts for the anode catalysis of direct methanol fuel cells (DMFCs), with the relevant goal of improving their properties as fuel cells for vehicles and other applications. The anode oxidation in DMFCs is reported in equation 1 (Fig. 11.12). At the time of the publication, state-of-the-art anode catalysts were either binary Pt-Ru alloys (67) or ternary Pt-Ru-Os alloys (68). A systematic exploration of ternary or higher order alloys as anode catalysts for DMFCs was not available, and predictive models to orient the efforts were also lacking. 

Platinum, ruthenium and PtRu alloy nanoparticles, prepared by vacuum pyrolysis using Pt(acac)2 and Ru(acac)3 as precursors, were applied as anode catalysts for direct methanol oxidation . The nanoparticles, uniformly dispersed on multiwaUed carbon nanotubes, were all less than 3.0 nm in size and had a very narrow size distribution. The nanocomposite catalysts showed strong electrocatalytic activity for methanol oxidation, which can... 

In all three MEAs the rate of methanol oxidation was facilitated by the platinum-ruthenium unsupported catalyst, which in the presence of CO as a byproduct of the reaction, exhibit an electrochemical activity higher than pure Pt. However, compared to Pt supported and unsupported catalysts, the electrochemically active surface area of PtRu alloys cannot be determined by hydrogen adsorption using cyclic voltammetiy due to the overlap of hydrogen and oxygen adsorption potentials, and the tendency for hydrogen to absorb in the ruthenium lattice [xvi]. However, under the same operation conditions, cyclic voltammetry can be used for qualitative estimation of the similarity in the PtRu anode layer properties. 

Presently, the oxidation of methanol on pure platinum has more academic interest than practical application once DMFC universally employs platinum based materials having two or more metals as an anodic catalyst In absence of methanoUc inteimediate readsorption, the maximum reactiOTi rate for CO oxidation is 100-fold smaller than maximum reaction rate for CO adsorption from methanol dehydrogenation steps [11]. Indeed, the mechanism of methanol oxidation on platinum is expected to be equal to that on its alloys despite different kinetics which would result in a selection of pathway. In terms of complex activation theory, alloyed Pt is intend to lower the Ea barrier for CO adsorption, thus driving methanol oxidation to completion. As previously established [3], there are several factors that affect the calculated activation energy for the MOR at a given potential, such as coverage of methanoUc intermediates and anion adsorption from the electrolyte as well as pH and oxide formation processes. 

For instance, DMFC uses frequently PtRu alloys as the anode catalyst to prevent fast poisoning of the catalyst surface by CO molecules produced during methanol oxidation. 

Platinum has been the most widely used catalyst, since it (and its alloys) is the only sufficiently efficient catalyst material for oxygen reduction in low temperature (< 120 °C) fuel cells. For fuel cell anodes, Pt-Ru alloys provide better tolerance to CO in the fuel stream (hydrogen from reformed methane or methanol) and have been found to be most effective for methanol oxidation. 

A DMFC is quife similar to a proton exchange membrane fuel (PEMFC) in stack structure and components. They both use a PEM for transporting the protons and Pt-based catalysts at the cathode. The anode catalyst for a DMPC is typically a Pt-Ru alloy that has higher CO tolerance than Pt alone, and this is similar to the PEMFC when H2 contains trace amounts of CO. In fhe infer-mediate sfeps during methanol oxidation, some CO-like species will form, which can seriously poison the anode catalyst. The presence of Ru helps fhe removal of fhe CO-like species from fhe Pt surface trough Reaction 7.6. 

Supported PtRu alloys are so far considered the best anodic materials for DMFC [109, 110], It is well known that the oxidation of methanol on platinum catalysts generates CO as an intermediate, which is a poison that adsorbs on the active sites of the catalyst. Ru forms oxygenated species at lower potentials than Pt and its presence in the catalyst promotes the oxidation of CO to CO2, through the so-called bifunctional mechanism [111, 112],... 

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