Catalytically active metals

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

The catalytic activity of micelles bearing catalytically active metal counterions (Lewis acid-surfactant combined catalysts, LASCs) on Diels-Alder reactions was recently investigated [72a, 76]. 

In the sixties of past century, a few patents issued to Bergbau Chemie [5,48,49] and to Mobil Oil [50-52], respectively described the use of CFPs as supports for catalytically active metal nanoclusters and as carriers for heterogenized metal complexes of catalytic relevance. For the latter catalysts the term hybrid phase catalysts later came into use [53,54], At that time coordination chemistry and organo-transition metal chemistry were in full development. Homogeneous transition metal catalysis was expected to grow in industrial relevance [54], but catalyst separation was generally a major problem for continuous processing. That is why the concept of hybrid catalysis became very popular in a short time [55]. 

Cross-linked functional polymers appear to be suitable supports for catalytically active metal(O) nanoclusters. 

In this chapter the potential of nanostructured metal systems in catalysis and the production of fine chemicals has been underlined. The crucial role of particle size in determining the activity and selectivity of the catalytic systems has been pointed out several examples of important reactions have been presented and the reaction conditions also described. Metal Vapor Synthesis has proved to be a powerful tool for the generation of catalytically active microclusters SMA and nanoparticles. SMA are unique homogeneous catalytic precursors and they can be very convenient starting materials for the gentle deposition of catalytically active metal nanoparticles of controlled size. 

The key to the successful development of homogeneous catalysts has been the exploitation of the effects that ligands exert on the properties of metal complexes by tailoring the electronic and steric properties of a catalytically active metal complex, activities and selectivities can be altered considerably. This especially holds for phosphorus based ligands, which are the most commonly encountered ligands as.sociated with organometallic compounds. 

The authors confirmed the formation of vinyl Ru-complex 21 by the reaction of [Cp Ru(SBu-t)]2 with methyl propiolate (Eq. 7.15). To my knowledge, this is the first observation of the insertion of an alkyne into the M-S bond within a catalytically active metal complex. In 2000, Gabriele et al. reported the Pd-catalyzed cycloisomerization of (Z)-2-en-4-yne-l-thiol affording a thiophene derivative 22 (Eq. 7.16) [26]. 

The creation of nanostructured surfaces is one thing, the study of electrochemical reactions on such nanostructures is another one. Especially in electrocatalysis, where size effects on reactivity are often discussed, there have been attempts to use the tip of an STM as a detector electrode for reaction products from, say, catalytically active metal nanoclusters [84]. Flowever, such ring-disk-type approaches are questionable,... 

The most catalytically active metals are Ni, Pd, Pt, and Rh. Nickel is used extensively in hydrogenation. It is frequently used in skeletal form as Raney nickel (Ra-Ni or RNi). The hydrogenation of almost all hydrogenatable functional groups can be accomplished over some form of Ra-Ni. Ra-Ni is also useful for desulfurization of organic compounds, but this is a stoichiometric reaction, not a catalytic reaction. 

Such a possibility has been recognized by early workers,9 but in spite of this intriguing possibility, only recently has such a metal surface been created. Chiral kink sites were created on Ag single crystal surfaces to produce the enantiomeric surfaces Ag(643)s and Ag(643)R however, no differences between (R)- and (S)-2-butanol were observed for either the temperature-programmed desorption from the clean surfaces or the dehydrogenation (to 2-butanone) from preoxidized surfaces.10 Unfortunately, Ag exhibits few catalytic properties, so only a limited array of test reactions is available to probe enantioselectivity over this metal. It would be good if this technique were applied to a more catalytically active metal such as Pt. 

Catalysis of supported metal ions is an area of interest. There are a number of advantages in depositing catalytically active metal ions on a support. The ion exchange method of catalyst immobilization is simple and the attractiveness of this method is further increased by providing stable inorganic ion exchangers of known structures as supports. 

Bonnemann, H. and Brijoux, W. (1996) Catalytically active metal powders and colloids, in Active Metals Preparation, Characterization, Applications (ed A. Fiirstner,), VCH Verlag GmbH, Weinheim, pp. 339-379. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

Because of a great practical importance of SMR as a major industrial process for manufacturing H2, the development of efficient steam reforming catalysts is a very active area of research. Nickel and noble metals are known to be catalytically active metals in the SMR process. The relative catalytic activity of metals in the SMR reaction (at 550°C, 0.1 MPa and steam/carbon ratio of 4) is as follows [12] ... 

The reaction mechanism of the SMR reaction strongly depends on the nature of the catalytically active metal and the support (the detailed discussion is provided in the review [14]). The kinetics and mechanism of the SMR reaction over Ni-based catalysts have been extensively studied by several research groups worldwide. For example, Xu and Froment [16] investigated the intrinsic kinetics of the reforming reaction over Ni/MgAl204 catalyst. They arrived at the reaction model based on the Langmuir-Hinshelwood reaction mechanism, which includes several reaction steps as follows ... 

Therefore, carbon nanofibers (CNFs) as well as carbon nanotubes (CNTs) were synthesized,18,19 functionalized (with the catalytic active metal Co), and finally... 

A cofactor is a nonprotein compound that combines with an inactive enzyme to generate a complex that is catalytically active. Metal ions are common cofactors for enzymatic processes. A cofactor may be consumed in the reaction, but may be regenerated by a second reaction unrelated to the enzymatic process. 

Another study on the preparation of supported oxides illustrates how SIMS can be used to follow the decomposition of catalyst precursors during calcination. We discuss the formation of zirconium dioxide from zirconium ethoxide on a silica support [15], Zr02 is catalytically active for a number of reactions such as isosynthesis, methanol synthesis, and catalytic cracking, but is also of considerable interest as a barrier against diffusion of catalytically active metals such as rhodium or cobalt into alumina supports at elevated temperatures. 

Alloying a catalytically active metal with one that is inert may be an efficient way to influence the selectivity of a catalytic reaction [62]. Besenbacher et al. 

The distinctive feature of the catalytically active metals is that they possess between 6 and 10 d-electrons, which are much more localized on the atoms than the s-electrons are. The d-electrons certainly do not behave as a free electron gas. Instead they spread over the crystal in well-defined bands which have retained characteristics of the atomic d-orbitals. 

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