Surface design, of catalysts

Characteristics of a catalyst particle include its chemical composition, which primarily determines its catalytic activity, and its physical properties, such as size, shape, density, and porosity or voidage, which determine its diffusion characteristics. We do not consider in this book the design of catalyst particles as such, but we need to know these characteristics to establish rate of reaction at the surface and particle levels (corresponding to levels (1) and (2) in Section 1.3). This is treated in Section 8.5 for catalyst particles. Equations 8.5-1 to -3 relate particle density pp and intraparticle voidage or porosity p. 

The size and complexity of the motif is clearly important, single hehces are of limited use because of the long intra-residue distances on the helical surface (Fig. 2) and functions that require more than two-residue sites will most hkely have to depend on the functionahzation of supersecondary structures. The clever designs of catalysts for decarboxylation and hgation reactions are, on the other hand, good examples of how small motifs can be exploited for complex functions [11,12]. 

These results show that some correlations can now reasonably be made between the structure of the nanoparticles and their catalytic properties. This should lead, in the near future, to a more rational design of catalysts and to the application of metals modified by surface organometaUic chemistry to a larger number of reactions. 

Advanced Design of Catalyst Surfaces with Metal Complexes for Selective Catalysis... 

I 70 Advanced Design of Catalyst Surfaces with Metal Complexes for Selective Catalysis Table 10.6 Performances of Re/zeolite catalysts for direct phenol synthesis at 553 K". 

P. Ciambelli, G. Bagnasco, and P. Corbo, in Successful Design of Catalysts, Studies in Surface Science and Catalysis, Vol. 44, T. Inui, ed., Elsevier, Amsterdam, 1989, p. 239. 

To describe the surface state of catalysts during the course of reactions, one can also use new variables that are surface coverages. They have a simple distinct sense when active centres are of the same type. If the number of active centres and their concentration are designated as bz and c2 = bzjS, respectively, any compound on the surface A 8 can be written as... 

As the above examples illustrate, the kinetics of surface-catalytic events depend on complex structural and electronic considerations that, thus far, have not been understood at the level of detail that would permit predictive mathematical modeling and therefore rational design. For this reason, the molecular-level engineering of catalytic surfaces harbors perhaps the greatest future potential for chemical reaction engineering, at least from the standpoint of the design of catalysts. 

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