Bimetallic Au catalysts

A. Zwijnenburg, M. Saleh, M. Makkee, and J. A. Moulijn, Direct gas-phase epoxidation of propene over bimetallic Au catalysts, Catal. Today 72(1—2), 59—62 (2002). 

Scheme 11 (a) A )i -Ti Ti -acetylide bimetallic Au catalyst (24) used in the azide-alkyne cycloaddition, (b) a gem-diaurated catalyst (25) used for the cycloaddition of dialkynes, and (c) a trimetallic gold complex (26) used in the cycloisomerisation of allenynes... 

For example, the bimetallic Au catalyst 36 was shown to catalyse the intramolecular hydroalkoxylation of allene alcohols with exceptional enantios-electivity (>93% ee) (Scheme 15c). 

While effective bimetallic catalyst design has the potential to lead to an enhancement of the reaction rate, the use of chiral bimetallic catalysts has also been explored to enhance the enantioselectivity of a reaction. Such bimetallic chiral induction is excellently demonstrated by the use of digold catalysts for the hydroamination of prochiral substrates such as allenes and alkenes [59]. The bimetallic Au catalyst 66, for example, was shown to be an effective catalyst for the hydroamination of amino-allenes in the presence of a silver salt activator (Scheme 24) [106]. The highest enantioselective induction for this reaction was achieved with a 1 1 ratio of AgBp4 to 66 (51 % ee) suggesting that the monocationic... 

CO oxidation catalysis showed that, for all the supports, the bimetallic catalyst was more active at low temperatures than the corresponing monometallic and cometallic catalysts. Apparent activation energies for monometallic Pt and Au catalysts were very consistent, near 32 and 80 kJ/mole, respectively. The s)uiergism for PtieAuie catalysts also shows up in the apparent activation energies for these catalysts, which were consistently around 23 kJ/mole. 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

On the basis of the combined weight of the above results, we believe that bifunctional electrocatalytic properties may be operative for both MOR and ORR on the AuPt bimetallic nanoparticle catalysts depending on the nature of the electrolyte. For ORR in acidic electrolyte, the approaching of both the reduction potential and the electron transfer number for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is indicative of a synergistic effect of Au and Pt in the catalyst. For MOR in alkaline electrol)he, the similarity of both the oxidation potential and the current density for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is suggestive of the operation of bifunctional mechanism. Such a bifunctional mechanism may involve the following reactions ... 

Bimetallic particles with layered structures have opened fascinating prospects for the design of new catalysts. Schmid et al. [10m] have applied the classical seed-growth method [20] to synthesize layered bimetallic Au/Pd and Pd/Au colloids in the size range of 20-56 nm. The sequential reduction of gold salts and palladium salts with sodium citrate allows the gold core to be coated with Pd. This layered bimetallic colloid is stabilized by trisulfonated triphenylphosphane and sodium sulfanilate. More than 90% metal can be isolated in the solid state and is redispersable in water in high concentrations. 

Series of supported Ru, Pt, and Au catalysts have been tested for the selective oxidation of CO. Table 3 shows a comparison of three typical catalysts reported recently. Since Pt group metals are essentially more active for H2 oxidation than for CO oxidation, support materials [77,78] and bimetallic systems [79] were exploited to suppress H2 oxidation. Better selectivities to CO oxidation (around 50%) are... 

Also, Marsh and co-workers [145] showed that gold on cobalt oxide particles, supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania remains active in a gas stream containing 15 ppm SO2. Haruta and co-workers [207] found that although the low-temperature CO oxidation activity of Ti02-supported Au can be inhibited by exposure to SO2, the effect on the activity for the oxidation of H2 or propane is quite small. Venezia and co-workers [208] reported that bimetallic Pd-Au catalysts supported on silica/alumina are resistant to sulphur poisoning (up to 113 ppm S in the form of dibenzothiophene) in the simultaneous hydrogenation of toluene and naphthalene at 523 K. 

Scurrell and co-workers have studied bimetallic Au-Ru catalysts supported on Fe203 [225]. When compared with Au/Fe203 and Ru/Fe20s catalysts, the activity of the bimetallic system was found to be higher over all the reaction temperatures studied (373-513 K). TPR analysis indicated a shift in reduction temperature of the bimetallic system, when compared with the mono-metallics. 

Venezia and co-workers also used bimetallic Pd-Au catalysts supported on ASA for the hydrogenation of aromatic compounds [208]. The simultaneous hydrogenation of toluene and naphthalene in the presence of dibenzothiophene was studied. This type of catalyst was shown to be resistant to sulphur poisoning (at 523 K, until 113 ppm S in the form of dibenzothiophene). 

A series of monometallic Pd(5wt%)/C and bimetallic Au(5wt%)-Pd(5wt%)/C catalysts were then prepared by controlling the pH of the solutions in order to keep it at the optimal values (i.e. in the maximum adsorption range) throughout all synthetic steps. These catalysts were tested in glyoxal oxidation and representative catalytical results are listed in Table 2. The Pd/C catalysts were indeed sensitive to the pH values used in the preparation steps. For example, a catalyst prepared with an acidic starting pH (n°l) was more active... 

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