Bimetallic catalyst systems

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

Vickerman and Ertl (1983) have studied H2 and CO chemisorption on model Cu-on-Ru systems, where the Cu is deposited on single-crystal (0001) Ru, monitoring the process using LEED/Auger methods. However, the applicability of these studies carried out on idealized systems to real catalyst systems has not been established. Significant variations in the electronic structure near the Eermi level of Cu are thought to occur when the Cu monolayer is deposited on Ru. This implies electron transfer from Ru to Cu. Chemical thermodynamics can be used to predict the nature of surface segregation in real bimetallic catalyst systems. 

Figure 9.6 Chirality-directed self-assembly of chiral bidentate SALs and bimetallic catalyst systems.

The catalytic silylative carbonylation of aryl iodides using a bimetallic catalyst system, [PdCl2(PPh3)2-Co2(CO)8], has been reported by Hidai and coworkers. The carbonylation of... 

The bimetallic catalyst system Ru3(CO)i2/Co2(CO)g catalyzes the reaction of terminal acetylenes with methyl iodide and 1 atm of CO under phase-transfer conditions to give y-oxocarboxylic add (Eq. 11.12) [44]. 

The results of this investigation have highlighted the existence of an unusual deactivation phenomenon when certain bimetallic catalyst systems are reacted in either a hydrocarbon or carbon monoxide environment. The activation/deactivation behavior of the respective bimetallic catalysts appears to be completely reversible in nature and is found to be highly sensitive to both reaction temperature and of the composition of the reactant gas. Deactivation is attributed to the preferential segregation of the respective metal components to either the solid carbon or gas phase interfaces of the catalyst particles. When the temperature is subsequently increased or lowered to a previously active regime then catalytic activity is restored to a level comparable to that observed with a fresh sample. 

A regioselective hydroformylation-amidocarbonylation process of a fluoroolefin is reported for cobalt-rhodium bimetallic catalyst systems With Rh6(CO),6 and Co2(CO)8 (1 50) in dioxane, 3,3,3-trifluoropropene (57) reacts with acetamide under hydroformylation conditions to give 58 with 94% selectivity (80-bar CO, 50-bar H2, 120°C, 10 hr, CT821) (226,275) ... 

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. 

Although bimetallic catalysts did not represent a totally new area of research in the early 1960s, my research emphasized entirely new aspects of this subject. Earlier work on metal alloy catalysts was dominated by efforts to relate the catalytic activity of a metal to its electron band structure. Very little attention had been given to other aspects of metal alloy catalysts, such as the possibility of influencing the selectivity of chemical transformations on metal surfaces and of preparing metal alloys in a highly dispersed state. These aspects were the basis for my work on bimetallic catalyst systems. 

While the use of a support or carrier stabilizes small metal crystallites against growth and surface area loss, reactions of support and metal may occur, especially if the catalyst is subjected to elevated temperatures The main factors affecting these phenomena are temperature, time of exposure and atmosphere. Most previously reported sintering studies were for supported platinum and particularly for Pt/AlgOj catalysts (1,2). Considerably less information is available on the sintering of other supported metal catalysts (3,4), and thus there is very little information on the behavior of bimetallic catalyst systems such as supported platinum-rhodium. 

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. 

Scheme 2.26 Catalytic asymmetric CDC reaction by a cooperative bimetallic catalyst system reported by Feng.

Abstract Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond. 

Also the activation energy was calculated as 56.3 2 kJ mol. Bimetallic alloy had Pt Ru metal ratio of 1 1. The particle size had a narrow distribution of 3.2 1.4 nm. It was foimd that the Pt-Ru PVP catalyst system had higher catalytic activity than the physically mixed Pt and Ru monometallic catalysts. This may arise from the synergistic effects of using Pt and Ru together or the smaller particle size relative to the monometallic Pt and Ru which were 4.6 nm and 4.2 nm, respectively. However, the bimetallic catalyst system lost 28% of its initial catalytic activity by the end of the fifth run [85]. 

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 hydroformylation of alkynes with palladium, in particular, has had limited success. A bimetallic catalyst system consisting of PdCl2(PCy3)2-C02(CO)8 was developed for the hydroformylation of internal acetylenes (Scheme Although the reaction would... 

Mohlala et al. [36] utilized a bimetallic catalyst system [FcH/W(CO)5(f-BuNC) and FcH/W(CO)5(f-BuNC)] to synthesize MWCNTs in 5% H2 in Ar in the temperature range 700-9(X) TEM analysis revealed the formation of large metal particles of Mo/Fe alloys rich in Fe. Under similar reaction conditions, FcH yielded MWCNTs and spheres while the W(CO)5(t-BuNC) complex yielded little carbonaceous material. It was observed that the diameters of the CNTs formed in the presence of FcH are smaller, while the diameters of CSs are larger relative to the diameters of CNTs and spheres produced by the bimetallic catalyst systems. 

In 1959 Bestian et d. 3,23,24) first used a bimetallic catalyst system of the Ziegler-Natta type (TiCl4 or an alkylated Ti(IV) compound in combination with an alkyl aluminumhalide) at extremely low temperatures ( —1(X) to —50°) for the oligomerization of ethylene. 

The polymer-supported bimetallic catalyst system PVP-PdCl2-NiCl2/TPPTS/ PPh3 (PVP = polyvinylpyrrolidone) was found to have good activity in the hydro-xycarbonylation of styrene under aqueous-organic two-phase condition and can be reused four times with little loss of catalytic activity. The effects of temperature, CO pressure, and reaction time were studied to obtain optimum reaction conditions (Equation 7.14) [139]. 

Scheme 7.14 Asymmetric CDC reaction of cyclic p-ketoesters using a bimetallic catalyst system.

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