Bimetallic catalysts alloys

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 subject of heteronuclear cluster compounds of the transition metals remains an active area of research interest, and was reviewed in the early 1980s by Geoffroy el al. (1,2). Clusters with novel architectures, exemplified by the star clusters of Stone and co-workers (5), continue to be synthesized. Whereas there is undoubtedly strong academic interest in the structure, bonding, and chemical reactivity of heteronuclear clusters in their own right, additional impetus to this field is given by the important relationship between heteronuclear clusters and bimetallic alloy catalysts. This relationship was the subject of a published symposium (4). 

Abstract Thermally stable, ordered surface alloys of Sn and Pt that isolate threefold Pt, twofold Pt, and single-Pt atom sites can be produced by controlled deposition of Sn onto Pt single crystals and annealing. The strnctnre was established by characterization with several techniques, including ALISS, XPD, LEED, and STM. Chemisorption and catalysis studies of these well-defined, bimetallic surfaces also define the overall chemical reactivity of Pt-Sn alloys, clarify the role of a second-metal component in altering chemistry and catalysis on Pt alloys, and develop general principles that describe the reactivity and selectivity of bimetallic alloy catalysts. 

In this chapter, we will illustrate with a few selected examples how well-defined, ordered Pt-Sn surface alloys have been used to elucidate the overall chemical reactivity of Pt-Sn alloys, clarify the role of Sn in altering this chemistry and catalysis, and develop general principles for understanding the reactivity and selectivity of bimetallic alloy catalysts. Most studies have involved chemisorption under UUV conditions, but the use of these alloys as model catalysts for investigating catalysis at pressures up to one atmosphere will also be discussed. 

Stamenkovic VR, Markovic NM. Oxygen reduction on platinum bimetallic alloy catalysts. Handbook of fuel cells 2010. 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Bimetallic (98) and alloy catalysts (97), of interest for hydrogenation reactions, have been investigated in in situ characterizations of methanol synthesis from CO and H2 in the presence of novel Cu-Pd alloy catalysts supported on carbon the results show surface segregation of palladium on the catalyst particles in CO atmospheres, but surfaces with equal amounts of copper and palladium when the atmosphere is H2 (97). 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

Cobalt-promoter alloy formation. Metal alloying or bimetallic alloy formation may also influence the activity and selectivity of Co F-T catalysts. 

Silica- or alumina-supported Pd-Fe mono- or bimetallic dehalogenation catalysts were prepared by conventional and microwave-assisted calcination. The catalysts synthesised under microwave conditions gave a higher conversion in the dechlorination of chlorobenzene. The effects were attributed to an enhanced crystallite size, a lower susceptibility to alloy formation and differences in the Pd morphology73,74. 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

High selectivity was also observed on a silica-supported Fe-Cu catalyst prepared by coprecipitation (333 K, 10 atm H2, ethanol)286 and over polymer-protected colloidal Pd-Pt cluster catalysts (303 K, 1 atm H2, ethanol)287,288. In contrast with the above observation, the activity of the bimetallic alloy was 1.4-3 times higher than that of the monometallic Pd cluster reaching the maximum activity at a composition of Pd/Pt = 4 1. 

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