Catalyst platinum/ruthenium

In 1965, synergistic (nonadditive) catalytic effects were discovered in elecho-chemical reactions. It was shown in particular that the electrochemical oxidation of methanol on a combined platinum-ruthenium catalyst will occur with rates two to three orders of magnimde higher than at pure platinum even though pure ruthenium is catalytically altogether inactive. 

It was seen when studying mixed systems Pt-WOj/C and Pt-Ti02/C that with increasing percentage of oxide in the substrate mix the working surface area of the platinum crystallites increases, and the catalytic activity for methanol oxidation increases accordingly. With a support of molybdenum oxide on carbon black, the activity of supported platinum catalyst for methanol oxidation comes close to that of the mixed platinum-ruthenium catalyst. 

Corrosion (spontaneous dissolution) of the catalyticaUy active material, and hence a decrease in the quantity present. Experience shows that contrary to widespread belief, marked corrosion occurs even with the platinum metals. For smooth platinum in sulfuric acid solutions at potentials of 0.9 to 1.0 V (RHE), the steady rate of self-dissolution corresponds to a current density of about 10 A/cm. Also, because of enhanced dissolution of ruthenium from the surface layer of platinum-ruthenium catalysts, their exceptional properties are gradually lost, and they are converted to ordinary, less active platinum catalysts. 

A period of high research activity in electrocatalysis began after it had been shown in 1963 that fundamentally, an electrochemical oxidation of hydrocarbon fuel can be realized at temperatures below 150°C. This work produced a number of important advances. They include the discovery of synergistic effects in platinum-ruthenium catalysts used for the electrochemical oxidation of methanol. 

PEM fuel cells operate at relatively low temperatures, around 80°C. Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, they require that a noble-metal catalyst (typically platinum) be used to separate the hydrogen s electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO. 

In a reactor that is similar to a reformer, the reaction occurs in tubes that are heated externally to supply the endothermic heat of reaction129. Sintered corundum (a-Al203) tubes with an internal layer ( 15 microns thick) of platinum/ruthenium catalyst are used, hi some cases a platinum/aluminum catalyst may be used. To achieve adequate heat transfer, the tubes may be only % in diameter and 6V2 feet long. Selectivities of 90-91% for methane and 83-84% for ammonia are reached at 1200°C to 1300°C reaction temperatures. 

Figure 9-2 shows the discharge curve of borosilicate electrode versus the standard carbon electrode of the H-Tec fuel cell. The load across the fuel cell for this test is 10 Q, resulting in a discharge of 100 mA cm. The cermet electrode demonstrates minimal increases of impedance over the discharge period and the higher overall voltage. The maximum developed by borosihcate substrate is 0.3489 V. This demonstrates that Ag metallization with a platinum/ruthenium catalyst can be developed as a cathode structure in DMFCs. 

It is evident from the data in the figure that the highest efficiency in recombining hydrogen and oxygen to water is observed when palladium is used as a catalyst. The platinum—ruthenium catalyst exhibits the lowest catalytic activity. 

C.T. Hable and M.S. Wrighton, electrocatal3dic oxidation of methanol and ethanol A comparison of platinum-tin, and platinum-ruthenium catalyst particles in a conducting polyaniline matrix, Langmuir, 9, 3284—3290 (1993). 

For reasons that are discussed in Section 19.4, the catalyst for the hydrogen electrode in polymer electrolyte membrane fuel cells is a mixed platinum-ruthenium catalyst applied to carbon black, rather than pure platinum. The overall thickness of modern MEA is about 0.5-0.6mm (of which 0.1 mm for the membrane, for each of the two GDLs, and for each of the two active layers). The bipolar plates have a thickness of about 1.5 mm, the channels on both sides having a depth of about 0.5 mm. 

Other than monetary costs, one must also take into account the availability of raw materials needed for fuel-cell production. Assuming a total platinum content in both electrodes of 0.8 mg/cm and an optimistic value of 1 W/cm for the specific power, then one will need 0.8 g platinum/kW. With a price of platinum of 30 /g, this gives 24 /kW. Therefore, an electric car with a power of 50 kW would have a price tag of 1200 for only the platinum in the fuel cells. It must of course be taken into account here that with a production volume of 1 million electric cars per year, 40 tons of platinum metal were needed, representing about 20% of current world production. An even more difficult situation would arise in the mass production of fuel cells with mixed platinum-ruthenium catalysts. World reserves of ruthenium are very limited and would not admit mass production on such a scale. For this reason, it will be one of the prime tasks in further fuel-cell development to broadly search for ways to lower the platinum metal content of the catalytic layer and to find new nonplatinum catalysts. 

At present, most of the work toward building methanol fuel cells relies on technical and design principles, developed previously for polymer electrolyte membrane fuel cells. In both kinds of fuel cells, it is common to use platinum-ruthenium catalysts at the anode and a catalyst of pure platinum at the cathode. In the direct methanol fuel cells, the membrane commonly used is of the same type as in the hydrogen-oxygen fuel cells. The basic differences between these versions are discussed in Section 19.7. 

With the experience gathered in the development of direct methanol fuel cells, platinum-ruthenium catalysts were used for the anodic process in the first studies on direct formic acid fiiel cells. Then, it was shown that much better electrical characteristics can be obtained with palladium black as the catalyst. Importantly, with this catalyst, one can work at much lower temperatures. In particular, at a temperature of 30°C power densities of 300 mW/cm were obtained with a voltage of 0.46 V, and about 120 mW/cm with a voltage of 0.7 V. Considering all these special features, it will be very convenient to use formic acid as a reactant in fuel cells of small size, for power supply in portable equipment, ordinarily operated at ambient temperature. 

Rauhe BR, Mclamon FR, Cairns EJ (1995) Direct anodic-oxidation of methanol on supported platinum ruthenium catalyst in aqueous cesium carbonate. J Electrochem Soc 142(4) 1073-1084... 

Dubau L, Coutanceau C, Gamier E, Leger JM, Lamy C (2003) Electrooxidation of methanol at platinum-ruthenium catalysts prepared from colloidal precursors atomic composition and temperature effects. J Appl Electrochem 33 419-429... 

Dudfield et al. performed investigations of compact aluminium fin heat-exchanger reactors for the preferential oxidation of carbon monoxide [549]. The reactors had the dimensions of 46-mm height, 56-mm width and 170-mm length, which corresponded to a 0.44-L volume and 590-g weight. They contained 2 g of catalyst each [328]. Platinum/ruthenium catalyst formulations of various composition were incorporated into different reactors and tested. Reformate surrogate with a... 

The primary contaminants of a PEFC are carbon monoxide (CO) and sulfur (S). Carbon dioxide (CO2) and unreacted hydrocarbon fuel act as diluents. Reformed hydrocarbon fuels typically contain at least 1 percent CO. Even small amounts of CO in the gas stream, however, will preferentially adsorb on the platinum catalyst and block hydrogen from the catalyst sites. Tests indicate that as little as 10 ppm of CO in the gas stream impacts cell performance (35, 36). Fuel processing can reduce CO content to several ppm, but there are system costs associated with increased fuel purification. Platinum/ruthenium catalysts with intrinsic tolerance to CO have been developed. These electrodes have been shown to tolerate CO up to 200 ppm (37). 

Figure 4.14. Arrhenius plot for methanol electrooxidation at 0.5 V vs. RHE using colloidal PtRu catalyst supported on Vulcan XC72. Electrolyte 1 M CH3OH - 0.5 M H2SO4. Scan rate 1 mV s . Pt Ru atomic ratios 2.33 1, , o 4 1 and Pt/C [96]. (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Elecfrooxidation of methanol at platinum-ruthenium catalysts prepared from colloidal precursors atomic composition and temperature effects, 33, 2003, 419-49, Dubau L, Coutanceau C, Gamier E, Leger J-M, Lamy C, figure 11.)...

Munke et al. [196] reported a technique (Figure 10.31) of in situ electrochemical FTIR and used it to study a real carbon-supported platinum + ruthenium catalyst. Different adsorptions were observed when methanol was electrooxidized at bulk Pt, Pt particles, and carbon-black-supported Pt -I- Ru electrodes, particularly with regard to the nature of the adsorbed CO species (Figure 10.32). 

Duhau. L., Coutanceau. C., Gamier, E., etal. (2003). Electrooxidation of Methanol at Platinum-Ruthenium Catalysts Prepared from Colloidal Precursors Atomic Composition and Temperature Effects, J. Appl. Electrochem., 33, pp. 419-429. 

Huang, S.Y., Chang, C.M. Yeh, C.T. Promotion of platinum-ruthenium catalyst for electro-oxidation of methanol by ceria. J. Catal. 241 (2006b), pp. 400-406. 

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