Methanol oxidation bimetallic catalysts

Waszczuk et al., 2001b Tong et al., 2002]. Because Ru is deposited as nanosized Ru islands of monoatomic height, the Ru coverage of Pt could be determined accurately. In that case, the best activity with regard to methanol oxidation was found for a Ru coverage close to 40-50% at 0.3 and 0.5 V vs. RHE. However, the structure of such catalysts and the conditions of smdy are far from those used in DMFCs. Moreover, the surface composition of a bimetallic catalyst likely depends on the method of preparation of the catalyst [Caillard et al., 2006] and on the potential [Blasini et al., 2006]. 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

Until now, for methanol oxidation the best bimetallic catalyst was found to be Pt-Ru. Several papers deal with the electro-oxidation of methanol at Pt-Ru bimetallic system dispersed in polyaniline [33,46]. From results with bulk alloys, the optimum Pt/Ru ratio of around 6 1 to 4 1 was found [49] and confirmed [50]. The electroactivity of Pt-Ru-modified polyaniline is much better than that displayed by pure Pt particles dispersed into the PAni film. The optimum composition of the Pt-Ru bimetallic system was confirmed from these results [33]. The decrease of the poisoning phenomenon is the consequence of a low coverage in adsorbed CO resulting from the chemisorption of methanol. This was checked by considering the oxidation of CO at the same Pt-Ru/PAni-modified electrode [34], which occurs at low overvoltages (150 mV) in the presence of Ru. 

Zhong and co-workers [530] described recent results of an investigation of the electrocatal3dic oxidation of methanol using carbon-supported An and Au-Pt nanoparticle catalysts. The exploration of the bimetallic composition on carbon black support was aimed at modifying the catalytic properties for the methanol oxidation reaction at the anode in direct methanol fuel cells (DMFCs). An and Au-Pt nanoparticles of 2-3 nm sizes encapsulated in an organic monolayer were prepared, assembled on carbon black materials and treated thermally. The results have revealed that these Au-Pt nanoparticles catalysts are potentially viable candidates for use in fuel cells under a number of conditions [530],... 

Reviewing the work on tire Pt-Ru electrocatalysts is beyond the scope of this article. We will briefly comment on some key advances in this area. Although early discovery by Petrii, and Bockris and Wrob Io wa established the catalytic activity of Pt-Ru alloys for methanol oxidation, despite of active investigation that followed, even the optimum composition of Pt-Ru is yet to be firmly settled. An early explanation for the mechanism by which bimetallic catalysts improve upon the performance of pure Pt, that is, the bifunctional mechanism proposed by Watanabe and Motoo, was recently challenged. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

Carbon-supported bimetallic or even ternary catalysts are of increasing interest in electrocatalytic reactions such as methanol oxidation. In this sense, the preparation routes are of pristine importance in determining the catalytic performance. As an example, it has been shown that Pt/Rn/Ni = 5 4 1 nanoparticles have a higher catalytic activity for methanol electrooxidation than does Pt/Ru = 1 1... 

Much research has been carried out on catalysts for methanol oxidation (see also Section 9.3.4) to find a catalyst which can avoid the poisoning effect of the CO species. Several promoters have been found to increase the activity of the Pt catalyst. One of the most important and most investigated promoter is Ru. A bimetallic alloy consisting of Pt and Ru supported on carbon has thus far been one of the major research interests in DMFC technology. The action of Ru can be explained as follows. The adsorption of H2O molecules at Ru surfaces takes place with lower overvoltages (Eq. 9-35). 

At present, there is a general consensus that PtRu offers the most promising results, and has been the benchmark bimetallic catalyst since the mid-1960s. The reason for the enhanced rate of methanol oxidation on PtRu is often invoked by the bifunctional mechanism (see Section 9.3.1), where the first step of the reaction is adsorption of methanol ... 

Hepel M, Kumarihamy 1, Zhong CJ (2006) Nanoporous Ti02-supported bimetallic catalysts for methanol oxidation in acidic media Electrochem Commun 8 1439-1444... 

Studies of methanol oxidation with polyaniline supported catalysts have focused on the use of bimetallic catalysts. As for the bulk metals and carbon supported metals, Pt-Ru and Pt-Sn are both more effective than Pt for methanol oxidation vAien supported on polyaniline (26, 27). Pt-Ru also provides more complete oxidation of methanol to CO2 (27). 

The high stability of the block copolymer-colloid approach was also illustrated by the use of poly(A-vinyl-2-pyrrohdone) protected rhodium colloid (Rh-PVP) that was used as a catalyst for methanol carbonylation under elevated temperature (140 °C) and high pressure (5.4 MPa). During the reaction, the catalyst was still in a colloidal state as verified by TEM observations, even after repeated uses and a total TON reaching 19 700 cycles per atom of rhodium. Toshima and Shiraishi also demonstrated the possibility to enhance the catalytic activity of silver colloids (Ag-PVP) in the oxidation of ethylene by the addition of alkali metal ions such as cesium. Bimetallic catalysts in colloidal dispersions composed of two distinct metals also appeared in the literature with often better activity... 

Nitroaromatics have been reductively converted into aryl carbamate with methanol under CO atmosphere catalyzed by palladium(II) anchored onto montmorillonite clay. Conversely, the same carbamates can also be obtained via oxidative carbonylation of amines with carbon monoxide and oxygen in the presence of a bimetallic catalyst PdCl2-MnCl2 supported on poly( V-vinyl-2-pyrrolidone).f ... 

To lower the potential at which dissociation of water conunences, a number of bimetallic Pt-M catalysts (with M = Ru, Mo, Sn, Fe, Ni, etc.) have been investigated. With Pt - Ru catalysts, which are the best bimetallic catalysts for methanol oxidation, it appears clearly from the literature, and this was fully confirmed by IR reflectance spectroscopic stodies, that the presence of adsorbed -(OH)ads on ruthenium sites at low... 

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