Multimetallic catalyst

Alloys or multimetallic catalysts are not only commercially important but they are also an attractive object for scientific studies. In the recent past these studies helped to identify some factors which determine the activity and - mainly - the selectivity of metal catalysts (1-5). 

The state of the knowledge is at the moment such that still in each particular case, the investigator should analyse and establish experimentally how important are, relative to each other, the effects 1) and 2) for the activity and the selectivity of a given multimetallic catalyst. It is an historical experience that when the explanation of the data is not immediately obvious, the authors offer as an "explanation" "an electronic (ligand) effect" (7). In this way it happened that the effect sub 1) has been most frequently proven to operate, while the effect sub 2) is the most frequently postulated... 

Since 1976 until present time Toshima-t5q)e nanocolloids always had a major impact on catalysis and electrocatalysis at nanoparticle surfaces [47,210-213,398-407]. The main advantages of these products lie in the efficient control of the inner structure and morphology especially of bimetallic and even multimetallic catalyst systems. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

Multimetallic catalysts X-ray absorption smdies of, 34 271 Multiphase homogeneous catalysis, 42 473-474, 501... 

The very complex phenomena of bi- and multimetallic catalyst are far beyond the scope of this review. Excellent recent reviews can be consulted for further data 45, 46). Only a few relevant results will be mentioned. 

Dual function catalysts contain metallic and acidic active sites. No distinction will be made here between mono- and multimetallic catalysts—all the more since our knowledge on the latter type of catalysts is far from perfect. 

Metal catalysts composed of more than two different metal elements are of interest from both technological and scientific viewpoints for improving the catalyst quality or properties (20). In fact, bimetallic (or multimetallic) catalysts have long been valuable for in-depth investigations of the relationship between catalytic activity and catalyst particle structure (21). Sinfelt et al. have made a series of studies on bimetallic nanoparticle catalysts supported on inorganic supports, for example,... 

We have developed a multimetallic catalyst for the large scale synthesis of sterically hindered mono-N-alkylanilines with very good selectivity and high catalytic activity. In contrast to copper chromite catalysts which allow the N-alkylation only with primary alcohols, the doubly promoted Pt/Si02 catalysts described here are useful for the reaction of ortho-substituted anilines with both primary and secondary alcohols. 

Previously, a wide variety of metal-mediated asymmetric two-center catalyses based on a multifunctional catalyst concept was developed [5]. Similar to an enzyme reaction, the synergistic functions of two or more active sites in multimetallic catalysts make substrates more reactive, and control their position in the transition state so that the functional groups are proximal to each other. In order to extend this concept to asymmetric organocatalysis, two ammonium salt moieties were... 

This operation is even more delicate with multimetallic catalysts, where the removal of additives sublimation or destruction must be prevented. 

Past efforts in developing coal liquefaction catalysts have focused on alumina-supported systems and, except for exploratory studies, little attention has been given to systematic development of novel formulations. A particularly promising approach to the development of new catalysts specifically designed lor coal liquefaction processes lies in the formulation of multicomponent systems that, in comparison to work on single or bimetallic systems, are essentially unexplored. Use of multimetallic systems offers the possibility of multifunctional catalysts that are needed to perform the many different reactions encountered in coal processing. Because of its versatility for the preparation of multimetallic catalysts, the HTO system is an excellent candidate for further development. 

Table 1. Examples of multimetallic catalyst systems used to catalyze olefin hydroformylation reactions.

However, some other aspects of the chemistry of ti bonded metal BT and DBT complexes could be of importance in connection with HDS. It is now known, for instance, that 4-methyl- and 4,6-dimethyldibenzothiophene are desulfurized mainly through prior hydrogenation of one or both of the arene rings [46, 112-115], and therefore the study of model ti -DBT complexes could be of use in developing efficient homogeneous, heterogeneous or hybrid catalysts for reducing the arene moieties in this type of molecule either as a pretreatment for conventional HDS, or as an added functionality in novel multimetallic catalyst formulations this possibility is further discussed in Chapter 3. 

Pt group metals can activate alcohols and molecular oxygen under close to ambient conditions, producing the corresponding carbonyl or carboxylic acids in high yields. Enhanced selectivity and activity has been obtained by the use of bi- and multimetallic catalysts.35-37 However, as was the case with selective hydrogenation, the optimum catalysts developed to date have... 

In this work, we compare the TWC behaviour of Pd, Pt, Rh, Pd-Pt, Pd-Rh, Pt-Rh, and Pd-Pt-Rh in a simulated stationary and/or cycled environment near that existing in automobile catalytic converters, trying to discover the relative merits of each metal on the overall performance of the catalyst. Comparison of activity obtained with multimetallic catalysts prepared by co-adsorption and those obtained with physical mixtures of monometallic catalysts wUl contribute significantly to our understanding of the impact of the interactions between metals on performance and could provide a valuable basis for designing new formulations with enhanced characteristics. 

Multimetallic catalysts (physical mixtures of monometallic catalysts) ... 

Concerning the light-off temperatures for CaHe removal, although big differences cannot be observed, the best behaviour corresponds to the Pt-Rh catalyst, followed by the multimetallic catalysts prepared by physical mixture, whose CsHe-conversion profiles are practically coincident. 

These three steps have been carefully studied now in particular for multimetallic catalysts which are now the most widely used (see for example ). The following examples will show that EXAFS experiments can help to understand the structure and... 

Bi- or multimetallic catalysts are of growing importance for the oil industry. We give here two cases where we have been able to build models for the role played by the metals versus the carrier or the reactant. These results rest on a comparison between a monometallic ex-H PtClg catalyst and two bimetallic Pt-Re and Pt-Rh systems. At the dried stage we have shown the existence of two fixation sites for the metal while the study of the reduced catalyst concludes an inhomogeneous repartition of the two metals inside the aggregate. 

The first one is based on ionic exchange, the second one is a forced impregnation (17-18). The multimetallic catalysts were prepared by successive impregnations. 

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