The Use of Bimetallic Catalysts

In the search for information on the composition of active centres, and for materials of improved catalytic performance, very much use has been made of bimetallic catalysts (see Further Reading at the end of the chapter). The term is preferred to alloy as in many cases the degree of intimacy of the components is uncertain, and in some cases interesting behaviour is found with systems exhibiting 

The critical question now addressed was whether the results could be understood simply in terms of the effect of the inert metal on the mean surface ensemble size of the active partner, or whether there was some movement of electrons between the components, modifying the properties of the more active one. Once again, effort was concentrated on bimetallics formed from Groups 10 and 

Calculation of the number of pairs, triplets or larger ensembles of one kind of atom randomly dispersed on a plane surface containing two kinds is a simple application of binomial theorem. Use of the results in real systems is however predicated on a number of assumptions, and conditions that have to be met. These may be enumerated as follows. 

The bimetallic system must be homogeneous, comprising a single phase phase separation may occur below a critical temperature as with the Ni-Cu/Si02 catalysts. Ordered superlattices may occur at certain compositions (e.g. PtsCu). 

There must be no short-range ordering, i.e. no preferred formation of clusters of one component in the bulk or on the surface. This condition is quite well met with Ni-Cu and perhaps other Group 10-11 bimetallics, but the greater the disparity in electronic structure the more likely it is to occur. 

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Introduction of zeolites into catalytic cracking improved the quality of the product and the efficiency of the process. It was estimated that this modification in catalyst composition in the United States alone saved over 200 million barrels of crude oil in 1977. The use of bimetallic catalysts in reforming of naphthas, a basic process for the production of high-octane gasoline and petrochemicals, resulted in great improvement in the catalytic performance of the process, and in considerable extension of catalyst life. New catalytic approaches to the development of synthetic fuels are being unveiled. 

The use of bimetallic catalysts is a direct consequence of the improved catalytic performance with respect to monometallic systems.1 The localised view mentioned above crystallises in a theory which supports such improved performance in the so-called geometric or ensemble,... 

The use of bimetallic catalysts in hydrocarbon reactions have extensively been studied because increased activity, selectivity and stability of the catalyst can be attained with the addition of a second metal. The disadvantage of studying catalytic phenomena on bimetallic catalysts prepared by a conventional coimpregnation method is that the catalyst surfaces are often heterogeneous, which makes it difficult to the catalytic systems. The use of bimetallic clusters as precursors has great advantages for preparation of relatively uniform bimetallic reaction sites well dispersed on oxide surfaces. 

Numerous studies were devoted to the use of bimetallic catalysts, promoted either by an active metal such as rhenium or iridium, or an inactive one such as tin or germanium. These different catalysts were generally prepared by co-impregnation or successive impregnations. Moreover, most of the catalysts prepared by redox reactions were only evaluated in model reactions. For example, Corro et al. [87] prepared Pt-Sn/Al203-Cl catalysts by catalytic reduction or co-impregnation and compared their resistance to coking under cyclopentane feed. Thus, catalysts prepared by the surface redox reaction were less sensitive than the others to deactivation. This result was explained in terms of a more effective interaction between... 

One of the major themes of basic research having a practical orientation has been the use of bimetallic catalysts for improved selectivity. Palladium is the... 

Little work has been reported on the use of bimetallic catalysts.Addition of copper to nickel as powder lowered the overall rate, without major change to the propane selectivity. Activation energies varied irrationally between 40 and 93 kJmol- ... 

Studies of methanol oxidation with polyaniline supported catalysts have focused on the use of bimetallic catalysts. As for the bulk metals and carbon supported metals, Pt-Ru and Pt-Sn are both more effective than Pt for methanol oxidation vAien supported on polyaniline (26, 27). Pt-Ru also provides more complete oxidation of methanol to CO2 (27). 

Related to this work, Seller and Geissler [109] advocated the use of bimetallic catalysts, one for the isomerization and the other for the hydroformylation. Indeed, with a catalytic system comprising a rhodium complex based on a chelating phosphine-phosphite ligand and Ru3(CO)j2 (0.1-0.5 mol%), almost a reversal of the regioselectivity in the reaction with -2-butene in comparison to the monometallic rhodium catalyst l/b = 42 58 TOP = 700 h ) was achieved. 

Since the first catalytic reformers were used in the 1950s the hydrogen/ hy-drocaibon mole ratio and the reformer operating pressure have both been gradually decreased. These developments resulted from improvements in the alumina support, the use of bimetallic catalysts, and finally the introduction of the low-pressure, continuous catalyst regeneration processes. The trends in operation are shown in Table 6.18. 

In this chapter, SOMC/M will be used to study the reactivity of organometallic complexes with the surface of supported metals. In 1984, Travers [31] and Margit-falvi [32] simultaneously described this application of SOMC for the preparation of bimetallic catalysts. 

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc catalysts. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Supported palladium and copper catalysts are usually used. A serious problem of this reaction is that palladium forms a complex with vinylacetylene below 100°C. This complex is soluble in the hydrocarbon mixture undergoing hydrorefining and, consequently, palladium is eluted from the catalyst. Operating at temperatures above 100°C or the use of bimetallic palladium catalysts310 solves this problem. 

For a further discussion of the structure and properties of bimetallic systems, see Sections 2.6 and 3.2.3 for the preparation of bimetallic catalysts, see Section 4.6 and for the mechanisms by which they work in oxidations, see Section 8.2.2. Most textbooks of physical chemistry have sections on adsorption and catalysis, but they frequently focus on studies made under ultra-high vacuum conditions with single crystal surfaces. While this work produces beautiful pictures, it has limited relevance to the more mundane world of practical catalysis. Other introductory treatments of about the level of this chapter, or slightly more advanced, are available,5,7,11 as are deeper discussions of the kinetics of catalysed reactions.12 14 Industrial processes using catalysts have also been described in detail.15,16... 

The preparation of a successful supported bimetallic catalyst is quite a difficult proposition. The main problem is to ensure that the two components reside in the same particle in the finished catalyst, and to know that it is so. The main physical techniques to characterise bimetallic particles are hydrogen chemisorption, XRD, TEM, EDX, XPS, XAFS,197Au Mossbauer (Section 3.3) and CO chemisorption coupled by IR spectroscopy (Section 5.3). The characterisation of bimetallic catalysts is not always thoroughly done, and there is the further complication of structural changes (particularly of the surface) during use. In situ or post-operative characterisation would reveal them, but it is rarely done. 

In practice the preparation of bimetallic catalysts using direct redox reactions can be extensively used for depositing a noble metal with a high standard electrochemical potential onto a non-noble metal with a lower standard electrochemical potential (eq 3). 

An example of the use of direct redox reactions in the preparation of bimetallic catalysts is the modification of copper catalysts by the addition of ruthenium, platinum, gold, or palladium [11-14], Assuming the metallic state for copper atoms on the surface, the redox reaction with the noble metal salts is... 

In summary, the direct redox reactions can be largely used in the preparation of bimetallic catalysts with a close interaction between the metallic constituents. In that case a metal with a high electrochemical potential is deposited on a metal with a lower potential. The applicability of the technique can be extended significantly by using different ligands which, by chelating metallic ions, modify the standard electrochemical potentials. 

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. 

In summary, the technique of catalytic reduction for the preparation of bimetallic catalysts can be extensively used with a variety of parent metals and re-ductants. However, some structure sensitivities of the reduction reactions become apparent and the modifying metal can be selectively deposited on specific sites of the parent-supported metal. Furthermore, such structure sensitivity depends on the nature of the re-ductant, and a given modifier can be deposited, according to the reductant used, selectively onto different parts of the metallic surface. In fact, a bimetallic catalyst can be tailored to provide the optimum activity, selectivity and lifetime for a given reaction. 

Although nickel catalysts have served as examples, articles dealing with other metals show that the same concepts apply. This is the case for Co deposition on silica or alumina [82], Cu/y-AhOj [124], and Ag/ /-AhOj and Ag/Ti02 [125], for example. The preparation of bimetallic catalysts is more complicated because of possible preferential reduction of one metal before the other, a phenomenon well known with the bulk oxides [3]. A few studies suggest that approaches similar to those mentioned above can also be used in these cases [122],... 

In this section, we discuss two topics that are not direct applications of bimetallic catalysts but for which a number of theoretical studies have appeared that also give useful insights into the properties of bimetallic catalysts. 

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