Kinetics of Heterogeneous Catalysis

Kinetics of Heterogeneous Catalysis [Santacesaria 1997, Santacesaria 1997a] 

The art of the catalyst specialist is to determine which is the rate-determining step. To this end the effective reaction rate must be measured (macrokinetics), and it must be determine, whether the actual chemical reaction rate (microkinetics) is inhibited by the external material transport. With the aid of this knowledge, the catalyst can be modified such that this limiting step is accelerated, which leads to an increase in the space-time yield and directly affects the reactor costs. The mechanisms of the individual transport steps are discussed in the following. 

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That was Van t Hoff s position. For modern kinetics of heterogeneous catalysis his words "... the effect of the medium on the transformation rate during transformation processes is the most important and the most real retain their significance. 

Apart from enzyme kinetics, this new trend had also appeared in the kinetics of heterogeneous catalysis. In the 1950s, Horiuti formulated a theory of steady-state reactions [11, 12], many of the concepts of which correspond to the graph theory. Independent intermediates, a reaction route, an independent reaction route, all these concepts were introduced by Horiuti. 

Kinetic analysis of concrete catalytic reactions. In the kinetics of heterogeneous catalysis, such reactions are usually model oxidation reactions (e.g. CO and H2 oxidation) over metals. 

Kinetics of heterogeneous catalysis has received much attention. Its mathematical theory is well advanced, largely thanks to extensive work of Boudart, Temkin, and others. On the other hand, heterogeneous catalysis has to deal not only with the same kind of difficulties homogeneous catalysis faces, but with the added complications of surface properties, adsorption/desorption equilibria and rates, and mass transfer to and from catalytic sites, phenomena whose effects often are more important than those of actual kinetics of the reaction on the surface. 

Adsorption isotherms. Adsorption is an important facet of kinetics of heterogeneous catalysis. Adsorption isotherms, showing the equilibrium concentration of a species on a solid as a function of the concentration in the contacting fluid at constant temperature, may have different shapes, usually classified as Types I through V [18] (see Figure 2.4). 

However, in the years after Ostwald, his former students and collaborators dominated catalysis in Germany. We can mention Bodenstein who put the study of the kinetics of heterogeneous catalysis essentially in its modern state. The Ostwald school made little contribution to mechanism, but it adhered to a view surprisingly close to that of Faraday. The famous paper of Bodenstein and Fink of 1907 interpreted the kinetics of the oxidation of sulfur dioxide on platinum in terms of the diffusion of sulfur dioxide or oxygen through a polymolecular layer of adsorbed material. In extreme cases, accord with the observed inhibition by sulfur trioxide would have required adsorbed layers so thick that they could have been pared with a razor. 

M. Boudart and G. Djega-Mariadassou, The Kinetics of Heterogeneous Catalysis (Prentice-Hall, 1980)... 

J.A. Dumesic, D.F. Rudd, L. M. Aparicio, J.E. Rekoske, A.A. Trevino, Micro Kinetics of Heterogenous Catalysis, American Chemical Society (1992). 

Budart M, Djega-Maxiadassou G. Kinetics of Heterogeneous Catalysis, Translated by Gao Z, Zhen SA, Li CR, et al. Shanghai Fudan University Press, 1988, 148. 

A few years ago the kinetics of heterogeneous catalysis of the aquation of the [Co(NHa)5Br] + cation were investigated - some of the catalysts employed [e.g. mercury(n) and silver(i) compounds] were closely related to the metal-ion catalysts discussed earlier in this section. A recent reinvestigation of some of these systems shows that part of the catalytic effect of these solids can be ascribed to redox processes operating in parallel to simple catalysed aquations. ... 

Eq. (4.179) contains (1) the kinetic apparent coefficient k+ (2) the potential term, or driving force related to the thermodynamics of the net reaction (3) the term of resistance, ie, the denominator, which reflects the complexity of reaction, both its multi-step character and its non-hnearity finally, (4) the non-linear term (N k,c)), a polynomial in concentrations and kinetic parameters, which is caused exclusively by nonlinear steps. In the case of a linear mechanism, this term vanishes. In classical kinetics of heterogeneous catalysis (LHHW equations), such term is absent. 

Volume 109 Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. 

Adsorption of reactants on the surface of the catalyst is the first step in every reaction of heterogeneous catalysis. Flere we focus on gases reacting on solid catalysts. Although we will deal with the adsorption of gases in a separate chapter, we need to discuss the relationship between the coverage of a particular gas and its partial pressure above the surface. Such relations are called isotherms, and they form the basis of the kinetics of catalytic reactions. 

D.A. Rudd, L.A. Apuvicio, J.E. Bekoske and A.A. Trevino, The Microkinetics of Heterogeneous Catalysis (1993), American Chemical Society, Washington DC]. Ideally, as many parameters as can be determined by surface science studies of adsorption and of elementary steps, as well as results from computational studies, are used as the input in a kinetic model, so that fitting of parameters, as employed in Section 7.2, can be avoided. We shall use the synthesis of ammonia as a worked example [P. Stoltze and J.K. Norskov, Phys. Rev. Lett. 55 (1985) 2502 J. Catal. 110 (1988) Ij. 

The examples of the model studies presented show how the meshing of modern surface techniques with reaction kinetics can provide valuable Insights Into the mechanisms of surface reactions and serve as a useful complement to the more traditional techniques. Close correlations between these two areas holds great promise for a better understanding of the many subtleties of heterogeneous catalysis. 

M. Neurock, in Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis, ed. G. F. Froment and K. C. Waugh, Elsevier, Amsterdam, 1997, 3. 

The application of heterogeneous catalysis plays a key role in technological processes. Engineering of the catalytic activities requires the study of the complex chemistry between absorbate and the catalyst at the surface. Static SIMS has been used to determine the surface composition and properties of solid catalysts before and after the catalytic actions by several groups.138-140 In addition, the dissociation kinetics of NO on Rh (111) surfaces have been studied by temperature programmed static SIMS.139... 

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