Heterogeneous catalysis supported metal catalysts

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]. 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

In heterogeneous catalysis, transition metal nanoparticles are supported on different substrates and are utilized as catalysts for different reactions [57], such as hydrogenations and enantioselective synthesis of organic compounds [58], oxidations and epoxidations [59], and reduction and decomposition [57],... 

The complexity of many heterogeneous systems used in multi-phase reactions, the use of a solid support, the difficulty in analyzing highly dispersed active sites and the bifunctional nature of many solid supported metal catalysts, make a detailed and complete study challenging. The simpler homogeneous systems teach many of the principles of catalysis active sites, reaction mechanisms, reaction kinetics and catalytic cycles, which can often be applied elsewhere. 

The thermal substitution of CO is catalyzed by several supported transition metals and their oxides, for example, PdO (see Heterogeneous Catalysis by Metals and Oxide Catalysts... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

Heterogeneous Catalysis by Metal Clusters. - Present activity in this area reflects the view that supported metal clusters could form the basis of a new generation of heterogeneous catalysts. Characterization of such materials, especially under reaction conditions, will be difficult. Such catalysts will be valuable if they exhibit activities and selectivities that differ from those afforded by conventionally prepared catalysts unfortunately, it is a deficiency of much of the work so far reported that such comparisons have not been made. However, evidence has recently become available that shows that cluster-derived heterogeneous catalysts may exhibit distinctive behaviour. 

Earlier in this section we stated that in many respects transition metal catalysis in ionic liquids is better regarded as heterogeneous catalysis on a liquid support than as conventional homogeneous catalysis in an alternative solvent. As in heterogeneous catalysis, support-ionic liquids and can lead to catalyst activation (see Section 5.3.1.2 for more details). Product separation from an ionic catalyst layer is often easy (at least if the products are not too polar and have a significant vapor pressure) as in dassical heterogeneous catalysis. However, mass transfer limitation problems (when the chemical kinetics... 

In the case of homogeneous catalysis, in which well-defined catalyst molecules are generally present in solution, the TON can be directly determined. For heterogeneous catalysts, this is generally difficult, because the activity depends on the size of the catalyst surface, which, however, does not have a uniform structure. For example, the activity of a supported metal catalyst is due to active metal atoms dispersed over the surface. 

Claus et al. concluded that an ionic liquid is able to modify the active sites facilitating limited reactant diffusion to the active sites which may affect the selectivity comparable to egg-shell-type catalysts. Thus, modification of supported metal catalysts by ILs results in a novel class of solid catalysts on the frontier between homogeneous and heterogeneous catalysis [57]. 

---