Catalysts, bimetallic selectivity

In the preparation of faujasite zeolite-supported Pt-Re catalysts, bimetallic PtRe clusters have been reported to be predominantly formed when a carbonyl rhenium precursor (Re2(CO)io) is contacted with zeolite in which platinum has been previously introduced and reduced. The preexisting Pt clusters may act as nucleation sites. After reduction, these Pt-Re systems show a high selectivity to CH4 in the hydrogenolysis of n-heptane [58]. 

Since there are many problems with regards to establishing cluster/particle structure, studies reported often only deal with catalyst activity/selectivity experiments. A summary of recent work along this line will be given later. At this time, structural aspects of bimetallic particles will be summarized. By far the most widely used techniques for deducing cluster/particle structure are EXAFS and... 

Scheme 8.1 Modes of action of bimetallic catalysts in selective oxidations in A, C and D the squares and triangles represent atoms of either gold or the Group 10 metal involved in the reaction in a matrix of non-participating atoms (small circles) see text for explanation.

Activity, Selectivity and Coking of Bimetallic Ni-Co-Spinel Catalysts in Selective Hydrogenation Reactions. 

As far as the global selectivity, the ratio of the methylated products (3-MP-t2-MP) to the MCP consumption is concerned, the bimetallic catalysts showed selectivities much higher than the theoretically expected for a non selective behaviour, 0.6 (Figure 3). AD series shows a good selectivity towards isoparaffinic products, and ID series presents the lowest... 

Simple preparation of bimetallic palladium-copper catalysts for selective liquid phase semihydrogenation of functionahzed acetylenes and propargylic alcohols... 

Augustine and Sachtler, on reduction, rhenium oxides migrate over the support surface and become reduced in contact with a prereduced platinum particle. In this way, each platinum particle can generate a mixed platinum-rhenium cluster that has the beneficial effects of bimetallic catalysts on selectivity and stability. 

The relatively low reactivity or inertness of the carbon surface is also very useful in the preparation of bimetallic catalysts, since the low interaction between the carbon surface and the two metals or metal precursors facilitates their mutual interaction. This is especially interesting when the objective is the formation of bimetallic particles. One clear example is the preparation of bimetallic Pt-Sn catalysts for selective hydrogenations. The catalytic behavior of this system is determined by at least three aspects [29,30] that determine the catalytic activity and the selectivity toward the desired product (1) the oxidation state of tin in... 

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. 

For Pt-Au bimetallic catalysts the selectivity pattern shows that the addition of gold to the small particles (Al) leads to a large formation of oarvotanacetone. This means that their behaviour is similar to the one of the large particles. 

The inorganic cluster [Mo6Pt024] on MgO was used as precursor to a catalyst having much greater activity in the dehydrogenation of butane, isobutane, and propane than conventionally prepared bimetallic Mo-Pt or monometallic Pt/MgO and Mo/MgO catalysts. The selectivity to the corresponding alkene was typically above 97%. ° ... 

When a second metallic element is added to the common single metal-supported catalyst, bimetallic clusters of particles in the size range 1-50 nm are formed. About 50-95% of the metal atoms in the range of 1-3 nm are exposed to the surface. A particularly important effect of such microparticle clusters supported on silica or alumina is on selectivity, which can often be enhanced to significantly higher levels. 

Gasteiger, H.A. and Betim, R.. (2001) Bimetallic PtSn catalyst for selective CO oxidation in H2-rich gases at low temperatures. Phys. Chem. Chem. Phys., 3, 1123-1131. 

Supported bimetallic Re—Pt catalysts are important in selective reforming of petroleum. It is believed that sulhding the catalyst before use gives ReS units which act as inert diluents to reduce the size of a local ensemble of platinum atoms. Selectivity for desirable dehydrocyclization and isomerization reactions... 

Kinetic, spectroscopic, and enantioselectivity data provided strong evidence for a mechanism involving bimetallic catalysis. The configurational outcome depends upon the face selectivity of the enol approaching the Michael acceptor in 59 (Fig. 32). To differentiate between the enantiotopic faces, the catalyst has thus... 

Hydrogenolysis of esters to aldehydes or alcohols needs high temperatures and high pressures. Moreover, it leads to the formation of acids, alcohols, and hydrocarbons. In contrast, bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on sihca are very selective for the hydrogenolysis of ethyl acetate into ethanol [181]. For example while the selectivity to ethanol is 12% with Ru/Si02, it increases up to 90% for a Ru-Sn/Si02 catalyst with a Sn/Ru ratio of 2.5 [182]. In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The first step is the coordination of the ester to the alloy (Scheme 46), and most probably onto the tin atoms. After insertion into the M - H bond, the acetal intermediate decomposes into acetaldehyde and an ethoxide intermediate, which are both transformed into ethanol under H2. 

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