Metal cluster catalysts

Recently, metal cluster catalysts composed of two different metallic elements are of interest from both scientific and technological points of view because of their interesting physiochemical properties [46, 47]. Bimetallic clusters are known to exhibit specific reactivity. Their catalytic efficiency is also controlled by their size. 

The use of highly dispersed catalysts from soluble salts of molybdenum is another approach to the reduction of catalyst amount because of their excellent activity despite their higher price. Recently, metal carbonyl compounds, such as Fe(CO)5, Ru3(CO)i2, and Mo(CO)6 have been investigated as metal cluster catalysts. Preparation involved their deposition and decomposition on catalyst support surfaces (71-73). 

Alexeev, O., and Gates, B. C., EXAFS characterization of supported metal-complex and metal-cluster catalysts made from organometallic precursors, Top. CataL 10,273 (2000). 

Gates, B. C., Metal cluster catalysts dispersed on solid supports, in Catalysis by Di- and Polynuclear Metal Cluster Complexes (R. D. Adams and F. A. Cotton, Eds.), p. 509. Wiley-VCH, New York, 1998. 

The experimental approach needed to understand supported metal cluster catalysts, is to prepare new catalysts based on systems of high uniformity e.g. supported clusters with identical size and composition. There are a number of ways of attaining... 

Basset J-M, Lefebvre F, Santini C (1998) Surface organometallic chemistry Some fundamental features including the coordination effects of the support. Coord Chem Rev 178-180 1703 Gates BC (2000) Supported metal cluster catalysts. J Mol Catal A Chem 163 55 Fierro-Gonzalez JC, Kuba S, Hao Y, Gates BC (2006) Oxide- and zeoUte-supported molecular metal complexes and clusters Physical characterization and determination of structure, bonding, and metal oxidation state. J Phys Chem B 110 13326... 

These samples offer a long-envisioned opportunity to vary metal cluster size and to determine how the catalytic properties depend on it. Results of some of the first experiments with zeolite-supported metal cluster catalysts are summarized below. 

Table 3, EXAFS results characterizing fresh and used supported metal cluster catalysts...

Gates, B. C. Supported Metal Cluster Catalysts. In Preparation of Solid Catalysts-, Ertl, G., Knoezinger, H., Weitkamp, J., Eds. Wiley-VCH Verlag GmbH Weinheim, 1999 pp 371-388. 

The Reppe carbonylation route to aldehydes and alcohols has been known for some time, where alkenes react with a metal carbonyl, CO, and water in the presence of base. In the original work [Fe(CO)s] was used as catalyst precursor, but recently precious metal cluster catalysts have been described by Pettit and Laine." The proposed mechanism is given in Scheme 2. Pettit screened a number of carbonyl clusters of Ru, Rh, Os, Ir, and Pt for the Reppe carbonylation of propene at 2.4 MPa CO and 1 MPa propene with... 

Unsupported Floating Metal Cluster Catalysts The term floating catalyst implies the incorporation of a catalyst precursor in the form of a vapor into the reaction chamber, where it decomposes and generates the active catalyst under reaction conditions (76). It was employed by Sen, Govindaraj, and Rao (77) in 1997 to prepare carbon nanotubes by decomposition of ferrocene, cobaltocene, and nickelocene under reductive conditions. In this case, the precursor provides both the carbon and the metal to catalyze the synthesis reaction, but later, benzene or hexane was added to the ferrocene precursor to improve the carbon yield (78). 

An interesting intermediate between homogeneous and heterogeneous catalysts are the metal cluster catalysts. In many reactions that require several active centers of the catalyst, it is found that heterogeneous catalysts are active, while homogeneous catalysts give zero conversion. The reason is that crystallites on a metal surface exhibit several active centers, while conventional soluble catalysts generally contain only one metal center. 

DFT Calculations of the Heterojunction Effect for Precious Metal Cluster Catalysts... 

Density functional theory (DFT) has been applied to the investigation of the heterojunction effect resulting from the introduction of heteroatoms into mono precious metal clusters and the interaction between precious metal clusters and stabilizing polymers in cluster model systems. In the present theoretical study, we attempted to explain the heterojunction effect for these model systems as a first step for understanding the heterojunction effect for catalytic reactions involving precious metal cluster catalysts. 

A preparation of designed catalyst is one of the interest subjects to understand the catalysis. Efforts have been paid for the development of unique preparation method[1] those are metal cluster catalysts derived from metal carbonyls, tailored metal catalysts through organometallic processor and ultra-fine metal particle catalysts prepared by metal alkoxides, etc. These preparation methods are mainly concentrated to design the active sites on support surfaces. However, the property of support itself is also a dominant factor in order to conduct smoothly the catalytic reaction. It is known that some supports are valuable for the improvement of selectivity. For example, zeolites are often used as catalysts and supports for their regular pore structures which act effectively for the shape selective reaction[2]. In order to understand the property of support, the following factors can be pointed out besides the pore structure structure, shape, surface area, pore size, acidity, defect, etc. Since these are strongly correlated to the preparation procedure, lots of preparation techniques, therefore, have been proposed, too. Studies have been still continued to discover the preparation method of novel materials as well as zeolites[3]. 

Carbon Monoxide Reduction and the Water Gas Shift Reaction A review of the role of metal cluster catalysts in solution and attached to supports for CO hydrogenation is optimistic about the use of cluster-derived heterogeneous catalysts on industrial processes.Selective reduction of CO to methanol occurs on Pt cathodes coated with K2Fe2(CN)6 in methanol solutions containing Fe(III) or Cr(III) complexes. 

A review on the industrial applications of homogeneous catalysts is particularly welcome. The proceedings of recent symposia and a general text have been published in addition to reviews on metal cluster catalysts, activation of saturated hydrocarbons by metal complexes in solution, catalysis by arene Group-VIB tricarbonyls, titanocene-catalysed reactions ofalkenes, transition-metal hydrides in catalysis, the mechanisms of the catalytic cyclization of aliphatic, hydrocarbons, asymmetric hydrosilylation and asymmetric synthesis. A n.m.r. study of the conformations of chelated Diop and a MO study of organo-metallic migration reactions are also of interest. Polymer supported catalysts have been reviewed and the relationship between cross-linking of the polymer and catalytic activity has been discussed. ... 

An important direction in the development of heterogeneous catalysts grafted onto mesoporous silicas is exploiting the rich chemistry of metal cluster catalysts bearing carbonyl or alkylphosphine ligands or both is exploiting a broad library of trialkylsilyl precursors with cluster functions, developed in particular by Bronstein et al. [28]. 

Identification of Binding and Reactive Sites in Metal Cluster Catalysts Homogeneous-Heterogeneous Bridges... 

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