Cluster heterogeneous catalysis

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

M. Kolb, Y. Boudeville. Kinetic model for heterogeneous catalysis Cluster and percolation properties. J Chem Phys 92 3935-3945, 1990. 

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

In 1982, Laine reviewed simple chemical tests and application of some techniques to distinguish between homogeneous and heterogeneous catalysis. He proposed five criteria for detecting homogeneous cluster catalysts ... 

The main issue of the book is application of nanosized particles in both homogeneous and heterogeneous catalysis. A variety of reactions catalyzed by metal colloids or supported nanosized metals is discussed. The most intriguing reaction seems to be ethane hydrogenolysis catalyzed by Pt clusters on porous carrier and studied by G. A. Somorjai and his group. Another challenging observation by this group is shape isomerization of Pt metal particles affected by the addition of silver ions. 

Henry CR, Chapon C, Giorgio S, Goyhenex C. 1997. Size effects in heterogeneous catalysis. In Lambert RM, Facchioni G, editors. Chemisorption and Reactivity on Supported Clusters and Thin Films. Dordrecht, Kluwer Academic, pp. 117-152. 

The novel T 2- X2-SiO bonding mode observed in these heterometallic complexes may have relevance to the stabilization of unsaturated, catalytically active heteronuclear systems. It also provides a new structural model for the bonding of small clusters or aggregates onto silica [1], a dominant theme in heterogenous catalysis. 

Reetz, M.T., Quaiser, S.A., Breinbauer, R., and Tesche, B., A New Strategy in Heterogeneous catalysis the design of cortex catalysts/catalysis/clusters/ immobilization/ surface chemistry, Angew. Chem. Int. Ed. Engl., 34, 2728,1995. 

OrganometaUic catalysts hydrodesulfurization, 42 413, 417-425, 432 OrganometaUic chemistry, role of phase-transfer catalysis, 35 398-400 OrganometaUic clusters heterogeneous catalysts, 38 283-288 precursors, 38 391-392... 

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Braunstein, P. and Rose, J. (1998) Heterometallic clusters for heterogeneous catalysis, in Catalysis hy Di- and Polynuclear Metal Cluster Complexes (eds R.D. Adams and... 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

The transfer of single electrons is observed on the current-potential dependence as a sequence of steps. This shows that clusters behave as redox reactants. There are many applications of gold clusters in various fields, such as preparation of new materials, electronics, heterogeneous catalysis and electrocatalysis, biosensors and others. 

Metal cluster compounds simulate surface species produced by the interaction of molecules with metal surfaces (Muetterties et al, 1979) and this is of value in understanding heterogeneous catalysis. The development of selective catalysts for the C, chemical industry employing CO (and possibly CO2) as the raw material has resulted in major efforts in metal cluster research. Criteria have been developed to distinguish between cluster catalysis and mononuclear catalysis. Typical of the catalysts investigated hitherto are [Ir4(CO),2. <(PPh3)J where Ph = phenyl and X = 1, 2 or 3. 

Several synthetic methods are now available for the preparation of mixed-metal clusters.1 However, when particular clusters are desired, two main points of concern for their synthesis often remain the availability and price of the precursors, and the yield of the reaction. Mixed-metal clusters containing ruthenium have attracted considerable interest mainly because of the variety of structural and bonding types encountered, and of their potential for homogeneous and heterogeneous catalysis.2... 

The Rh4(CO)12 complex finds extensive application in homogeneous as well as in heterogeneous catalysis where it is a very useful precursor. This compound is also a convenient starting material for the synthesis of various rhodium clusters.1... 

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