Bimetallic catalysts alloy composition

In acidic electrolyte, the catalytic activity is found to be highly dependent on the alloy composition. For example, while the AugiPtig/C catalyst showed little activity, the Aug8Pt32/C catalyst showed clear catalytic activity. A detailed correlation between the electrocatalytic activity and the bimetallic composition and the phase properties is part of our on-going work. 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

The early XPS studies, including those from our laboratory, revealed that the tin is present only in an oxidized state (10,16). These results were consistent with those for the Pt-Re bimetallic catalysts where only oxidized Re was observed (9,22). Li et al. (23,24) reported that a portion of the tin in Pt-Sn-alumina catalysts was present in the zero valence state furthermore, it appears that the composition of the Pt-Sn alloy, based upon the amount of Pt in the catalyst and the Sn(0) detected by XPS, increases with increasing ratios of Sn/Pt. 

Supported bimetallic catalysts can be made by adsorption of a bimetallic precursor such as molecular cluster compounds, colloidal particles or dendrimer-stabilised particles. In several cases, homogeneous bimetallic particles have been found where the compositions lie within the miscibility gap of the bulk alloy (e.g. with PtAu particles). This suggests that when the particles are small enough and do not possess metallic properties, the normal rules do not apply. 

The activity and selectivity of heterogeneous catalysts generally depend on the state of metal dispersion (particle size), structure (shape and morphology), metal composition, and metal-support interactions. If the catalytic centers include many metallic atoms, the electronic and geometric distributions of the constituents elements may be strongly related to the chemical reactivities and catalytic performances of the bimetallic and alloy catalysts (5). 

A preparation and characterization of new PtRu alloy colloids that are suitable as precursors for fuel-cell catalysts have been reported [43cj. This new method uses an organometallic compound both for reduction and as colloid stabilizer leading to a Pt/Ru colloid with lipophilic surfactant stabilizers that can easily be modified to demonstrate hydrophilic properties. The surfactant shell is removed prior to electrochemical measurements by reactive annealing in O2 and H2. This colloid was found to have nearly identical electrocatalytic activity to several other recently developed Pt/Ru colloids as well as commercially available Pt/Ru catalysts. This demonstrates the potential for the development of colloid precursors for bimetallic catalysts especially when considering the ease of manipulating the alloy composition when using these methods. 

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. 

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. 

Nickel-copper alloys provide a good example of a bimetallic catalyst system in which the variation of catalytic activity with composition depends markedly on the type of reaction, thus leading to substantial selectivity effects. The catalysts to be considered here are alloy powders with a surface area of approximately 1 m2/g (6). Approximately one atom out of a thousand is a surface atom in such catalysts. 

In the oxidation of several secondary alcohols an optimum in the catalyst composition Bi/Pts rf 0-50 was found. The formation of a two-dimensional alloy on the surface efficiently sup, sses the by-product formation and increases the conversion by a factor of 3 to 66. Some examples on the beneficial influence of Bi promotion are shown in Table 1. A three-component Degussa catalyst was used as a reference. This catalyst is, to our knowledge, the only commercially available alcohol oxidation catalyst, which has been developed for the selective oxidation of glucose to gluconic acid [20]. We suppose that the superior behavior of our bimetallic catalyst in the reactions studied is due to the more homogeneous distribution of Bi on Pt. 

Surface Composition. - Bimetallic catalyst systems have received much interest because variation in alloy composition offers a ready method of altering the metallic properties of the catalyst. For a range of Fe-Ni catalysts, Matsuyama et al have attempted to answer a question fundamental to such systems, namely, how does the surface composition compare with that of the bulk Powdered Fe-Ni catalysts were prepared from a solution of Fe(N03)2 and Ni(N03)2- The mixture also contained radioactive [63-Ni] which emits 3 radiation with an f max of 67 keV and a penetration of about 200 layers of heavy metal. It was possible to measure the amount of Ni which existed in the surface layers of the alloy, since 3-cmission from the underlayers of the metal was weakened by self-absorption. 

The interaction of CO with alloy or bimetallic surfaces is of special interest because of the importance of bimetallic catalysts in both the electrochemical and gas-phase oxidation of CO. Platinum-ruthenium alloys have long been known to be superior catalysts for the electrochemical CO oxidation, but the details of their catalytic action are still disputed. We will have more to say about the mechanism of the electrocatalytic CO oxidation on both metals and alloys in section III.8, in particular about the relevant ab initio quantum-chemical studies. Here, we will simply discuss how the chemisorption properties of CO on PtRu depend on the stmcture and composition of the bimetallic surface. 

In the search for information on the composition of active centres, and for materials of improved catalytic performance, very much use has been made of bimetallic catalysts (see Further Reading at the end of the chapter). The term is preferred to alloy as in many cases the degree of intimacy of the components is uncertain, and in some cases interesting behaviour is found with systems exhibiting... 

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