Heterogeneous catalysis conventions

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc 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]. 

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

A. Corma and H. Garcia, Lewis Acids From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis , Chem. Rev., 2003, 103, 4307-65. 

Biocatalysis is a rather special case, somewhere between homogeneous and heterogeneous catalysis. In most cases, the biocatalyst is an enzyme - a complex protein that catalyzes the reactions in living cells. Enzymes are extremely effident catalysts. An enzyme typically completes 1000 catalytic cycles in one second. Compared to this, conventional homogeneous and heterogeneous catalysts are slow and inefficient (100-10000 cydes per hour). Speed, however, is not the only advantage enzymes specialize in converting one specific reactant into another... 

The steady state experiments took a year to carry out and yielded useful information, but limited insight. The dynamic kinetic studies, performed in four months on a conventional fixed-bed reactor with minor equipment modification, revealed a wealth of information. These experiments made it possible to propose a reaction mechanism. The use of dynamic kinetic studies is strongly recommended for the study of heterogeneous catalysis. 

In conclusion, the doping of oxide materials opens promising new routes to change the morphology and electronic properties of supported metal particles as used in heterogeneous catalysis. Thin oxide films are ideally suited to elucidate such doping effects, as they can be explored by means of conventional surface science techniques at a fundamental level. The identified mechanisms can be transferred to real catalysts later, as the doping approach is not based on specific thin-film effects. 

It might seem likely that porous hybrid frameworks would be generally inferior to conventional zeolitic materials for applications in heterogeneous catalysis, given their relative instability, their lack of strong acidity, and their relatively costly synthesis. However, hybrids offer certain advantages... 

Chemical vapor deposition is a key process for thin film formation in the development and manufacture of microelectronic devices. It shares many kinetic and transport phenomena with heterogeneous catalysis, but CVD reactor design has not yet reached the level of sophistication used in analyzing heterogeneous catalytic reactors. With the exception of the tubular LPCVD reactor, conventional CVD reactors may be viewed as variations on the original horizontal reactor. These reactors have complex flow fields and it is consequently difficult to control and predict the effect of operating conditions on the film thickness and composition. 

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