Particle sites, heterogeneous catalysts

In contrast to a mixture of redox couples that rapidly reach thermodynamic equilibrium because of fast reaction kinetics, e.g., a mixture of Fe2+/Fe3+ and Ce3+/ Ce4+, due to the slow kinetics of the electroless reaction, the two (sometimes more) couples in a standard electroless solution are not in equilibrium. Nonequilibrium systems of the latter kind were known in the past as polyelectrode systems [18, 19]. Electroless solutions are by their nature thermodyamically prone to reaction between the metal ions and reductant, which is facilitated by a heterogeneous catalyst. In properly formulated electroless solutions, metal ions are complexed, a buffer maintains solution pH, and solution stabilizers, which are normally catalytic poisons, are often employed. The latter adsorb on extraneous catalytically active sites, whether particles in solution, or sites on mechanical components of the deposition system/ container, to inhibit deposition reactions. With proper maintenance, electroless solutions may operate for periods of months at elevated temperatures, and exhibit minimal extraneous metal deposition. 

Various works has pointed out the role of the nanostructure of the catalysts in their design.18-26 There is a general agreement that the nanostructure of the oxide particles is a key to control the reactivity and selectivity. Several papers have discussed the features and properties of nanostructured catalysts and oxides,27-41 but often the concept of nanostructure is not clearly defined. A heterogeneous catalyst should be optimized on a multiscale level, e.g. from the molecular level to the nano, micro- and meso-scale level.42 Therefore, not only the active site itself (molecular level) is relevant, but also the environment around the active site which orients or assist the coordination of the reactants, may induce sterical constrains on the transition state, and affect the short-range transport effects (nano-scale level).42 The catalytic surface process is in series with the transport of the reactants and the back-diffusion of the products which should be concerted with the catalytic transformation. Heat... 

The hydrogenation of unsaturated polymers like polyisoprene is based on the mobility of a soluble catalyst in the reaction medium. In the hydrogenation of such unsaturated polymers the soluble catalyst brings its active site to the C=C bonds in the polymer chain. In contrast, a heterogeneous catalyst requires that the polymer chain unfold to gain access to a catalytically active site on the surface of a metal particle. 

As already mentioned, the first step in any heterogeneous catalytic reaction is the adsorption of a gas molecule onto a solid surface. Adsorption heat measurements can provide information about the adsorption process not available using other surface analytical tools. For example, differential heat measurements can provide valuable insights into sites distribution on the catalyst surface as well as quantitative information on the changes in catalyst particle surface chemistry that result from changes in particle size or catalyst support material [148-150],... 

For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

Intraparticle Resistance. The Koros/Nowak (ref. 7) or Madon/Boudart (ref. 8) criterion states that, in the absence of mass transfer influences, the activity of a heterogeneous catalyst should be proportional to the number of active sites. In other words, the observed turn-over frequency (TOF) should be independent of the particle size if there is negligible intraparticle resistance since all active sites are fully effective. 

Nevertheless, many vanadium-based catalysts and polymerisation systems comprising them have received much academic attention in the hope that they might provide models for heterogeneous catalysts and polymerisation systems, since the problems connected with surface properties and particle size were believed to have been overcome. It must be noted, however, that homogeneous vanadium-based catalysts appeared to be more complex than was thought. There is no decisive evidence on the structure of catalytic sites formed by reaction between the procatalyst and activator. 

The heterogeneous catalyst particles in the reactor are surrounded by a boundary layer of gas or liquid, which can be considered as a static him around the particle. A reactant molecule has to diffuse through this boundary layer via film diffusion (1). As most catalysts have pores, the reactant molecule also has to diffuse through the pores in order to approach the active site, the pore diffusion process (2). Inside the pores, the reactant molecules adsorb at or near the active center and react (3, 4). The resulting product molecules desorb (5) and return back into the fluid phase via pore diffusion (6) and film diffusion (7). Further details on this can be found in general textbooks [1-3]. 

For heterogeneous catalysts it is more difficult to study the metal sites and their interaction with reactants under catalytic conditions. Many solid catalysts consist of metal crystallites or oxide particles in a variety of sizes and forms which themselves contain a number of different environments for the metal. Much of the routine characterization of these catalysts takes place before and after, rather than during, the catalytic reaction. Other studies probing surface bound species may have to be carried out under high vacuum rather than under the typical working conditions of the catalyst (see Appendix B for a summary of some relevant aspects of surface science). 

The mesoporous solids developed by Mobil group in 1991 were found to be catalytically inactive and have attracted a considerable interest from researchers throughout the world to introduce catalytically active sites within these materials. For example, doping of aluminium into the silica generates Bronsted acid sites and the resulting materials can be used as solid acids in acid-catalysed reactions. It is also possible to deposit metal particles within the pores and to use these materials as redox catalysts in many chemical reactions. Another avenue for catalytic functionalisation is to tether metal complexes within pores in order to prepare heterogeneous catalysts. It has been observed in the process of functionalisation that MCM materials can lose mesoporosity, surface area and pore volume as shown by nitrogen... 

Fig. 15.1. The principal surface and particle sites for heterogeneous catalysts.

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