Heterogeneous catalysis, solid state mechanism

The book comprises 20 chapters that focus on state-of-the-art understanding of solid state mechanisms in heterogeneous catalysis and the relationship between catalytic behavior and solid state structure. The volume contains expanded and updated versions of papers presented on this subject at the ACS symposia in Washington, D.C. (1983) and Las Vegas (1982), and of written contributions from invited participants who could not attend these... 

Much of the understanding of the solid state mechanism of heterogeneous catalysis stems from fundamental studies of single phase model compounds (1-5). In many cases, the role of a metal component in a catalytic process has been discerned through its incorporation into solid solutions of relatively inert host matrices (O. In the case of the selective oxidation and... 

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

The selected EM studies described here illustrate this point and the fact that point and extended defects are inextricably linked to the process of catalysis, affecting both chemisorption and reaction mechanisms. The EM studies have resulted in an improved mechanism for the formation of CS planes and their role in heterogeneous catalysis (Gai 1981, 1982). They have led to a new understanding of defects and their role in solid state heterogeneous catalytic progress. 

This reaction has been studied in considerable detail and is mentioned here as providing a link between the nucleation and growth mechanisms characteristic of solid state decompositions and surface processes of the type discussed in heterogeneous catalysis. 

J. Flaher, Molecular mechanism of heterogeneous oxidation— organic and solid state chemists views. Studies in Surface Science and Catalysis, vol. 110, pp. 1-17, 1997. 

Bielanski, A. and Haber, J. Oxygen in Catalysis. Marcel Dekker, New York, 1991. Haber, J. Molecular mechanism of heterogeneous oxidation — Organic and solid state chemists views. Stud. Surf. Sci. Catal 1997,110, 1-17. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

Traditionally, the same overall mechanisms of acid catalysis invoking carben-ium ions have been assumed to prevail both in heterogeneous (2) and in liquid homogeneous (3) systems. But these mechanisms do not adequately take into account the fact that adsorbed, rather than free, carbenium ions are formed in the pores of solid catalysts. Consequently, a quantum-chemical model that demonstrates how the interaction of carbenium ions with the sites of their adsorption can influence the reaction mechanism has been formulated by Kazansky (4), taking double-bond-shift reactions in olefins as a particular example. According to this view, adsorbed carbenium ions are best regarded as transition states rather than reaction intermediates, a notion that had also been proposed earlier by Zhidomirov and one of us (5). 

---