Bimetallic catalysts silver

The first deals with small islands of silver on a ruthenium substrate. One may look at this sample as a, perhaps somewhat far-fetched, model of a supported catalyst or a bimetallic surface. As metal layers are almost never in perfect registry with the substrate, they possess a certain amount of strain. Goodman and coworkers [46] used these strained metal overlayers as model systems for bimetallic catalysts. Here we look first at the electronic properties of the Ag/Ru(001) system as studied by UPS. 

Hydrogenation of Acetylene on Bimetallic Palladium-Silver Catalysts... 

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. 

J. T. Jankowiak, M. A. Barteau, Ethylene epoxidation over silver and copper-silver bimetallic catalysts 1. Kinetics and selectivity, /. Catal. 236 (2005) 366. 

Key Words Ethylene epoxidation, Silver, Bimetallic catalyst design, Copper, Density functional theory, Microkinetic modeling. 2008 Elsevier B.v. 

As far as finding improved catalysts or operating conditions is concerned, a variety of experimental studies have been conducted. Promotion by alkali metal salts [11], especially cesium [12,13], has been shown to increase the EO selectivity. Similarly, it is observed that addition of chlorine to the catalyst results in higher EO selectivity [14-18]. Atmospheric pressure experiments in our laboratory have indicated that Cu-Ag bimetallic catalysts are more effective than pure Ag [19]. Furthermore, it is observed that after addition of Cs and Cl to the Cu-Ag bimetallic catalyst, it outperforms the corresponding promoted silver catalyst [20]. Published experimental studies have t)q)ically focused on a limited number of catalyst components, whereas one could potentially explore a wide variety of catalyst combinations using theoretical tools. 

Motivated by the idea of rational catalyst design, we take advantage of the computational efficiency of DFT methods to examine a variety of Ag-containing bimetallic catalysts. We focus initially on the branching reactions of the OME since these have been shown to control selectivity [9]. Initial screening based solely on OME reactions suggests several promising bimetallic combinations. However, if one considers a more complete microkinetic model, copper stands out for its ability to enhance the selectivity of silver-based bimetallic catalysts. 

Pd-Ag bimetallic catalysts supported on carbon xerogels have been used in the hydrodechlorination reaction of 1,2-dichloroethane [103,104], Pd and Ag were deposited by co-impregnation using a solution of palladium and silver nitrates. Metal particle size ranged from 2 to 5 nm in Pd catalysts but had a wider distribution (4 to 20 mn) in Ag catalysts. Bimetallic Pd-Ag catalysts showed small particle alloys of 3 to 4 nm. The bulk Ag content in this alloy was limited to about 50 wt%, which fixed the minimum Pd surface content of the alloy at about 10 wt%. Pd catalysts produced mainly ethane, whereas bimetallic Pd-Ag catalysts were selective for the production of ethylene. The ethylene selectivity increased with silver fraction at the alloy surface. 

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... 

Similar to the C-H/C-H coupling mentioned in Section 2, some A A -bidentate coordinating groups also work well in the C-H/N-H coupling. Benzamides bearing the quinoline moiety are directly aminated under the copper/silver bimetallic catalyst system, although the exact role of the silver salt is not clear (Eq. 31)... 

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... 

Some other systems were explored in the literature as alternatives to the standard silver systems. One notable set of studies looks at bimetallic Ag—Cu catalysts for increased epoxide selectivity. Studies were performed looking at the selectivity of EO formation both with and without promoters present (69-71). Selectivity is increased with these systems however, conversion is either worse or only slightly better. 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

Supported silver catalysts are relatively commonly used in gas phase oxidations of alcohols.74,75 Benzyl alcohol can be selectively oxidised to benzaldehyde using a 0.6% Ag/pumice catalyst76 with 100% selectivity, although its activity is less than a similar Pd material. However, a mixed Pd-Ag/pumice bimetallic increases the activity whilst retaining the 100% selectivity to benzaldehyde. The authors of this study concluded that the role of the Pd was to activate the substrate whereas the highly dispersed silver particles served to activate the oxygen. Hence, the mechanism was one of cooperation between the Ag° and Pd° sites, the alloy phase, detected by EXAFS, was considered not to play an important role. 

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