Kinetic of catalytic reactions

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. 

Micro-kinetic modeling represents the state of the art in describing the kinetics of catalytic reactions. It was pioneered by Stoltze and Norskov in the mid-1980s and was further explored by Dumesic and coworkers in the early 1990s [J.A. Dumesic,... 

What is the mean-field approximation in the kinetics of catalytic reactions, and when does it break down In such cases, is the rate larger or smaller than expected on basis of the mean-field approximation ... 

Djega-Mariadassou, G. and Boudart, M. (2003) Classical kinetics of catalytic reactions, J. Catal., 216, 89. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

A square concentration pulse flow technique has been developed to study the kinetics of catalytic reactions over catalysts which change their stoichiometry in response to the reaction conditions. The technique makes it possible to obtain hysteresis-free kinetics data while greatly reducing the time during which the catalyst is exposed to the reaction mixture. 

Megalofonos, S.K., Rapayaimakos, N.G. 1997. Kinetics of catalytic reaction of methane and hydrogen sulphide over MoS2. J Appl Catal A General 165 249-258. 

Vannice, M. A. Kinetics of Catalytic Reactions, Springer Heidelberg, 2005. 

Any type of reactor with known contacting pattern may be used to explore the kinetics of catalytic reactions. Since only one fluid phase is present in these reactions, the rates can be found as with homogeneous reactions. The only special precaution to observe is to make sure that the performance equation used is dimensionally correct and that its terms are carefully and precisely defined. 

Before we proceed, it may be useful to list some of the key problems in the study of the kinetics of catalytic reactions. 

The kinetics of catalytic reactions in a number of cases also agrees only qualitatively with the equations obtained on the basis of the model of an ideal adsorption layer experimental data often lead to fractional reaction orders (such as cannot be accounted for by dissociative adsorption). 

This source of multiplicity is probably the most intrinsic one in catalytic and biocatalytic reactors, for it occurs due to the nonmonotonic dependence of the intrinsic rate of reaction upon the concentration of reactants and products. Although a decade ago, nonmonotonic kinetics of catalytic reactions were considered the exceptional case, today it is clear that nonmonotonic kinetics in catalytic reactions are much more widespread than previously thought. The reader can learn more about various examples from the long list of catalytic reactions exhibiting nonmonotonic kinetics [71-76]. 

In a certain sense, the simplest class of reaction mechanism is that whose bipartite graphs do not contain cycles, i.e. are acyclic. The dynamic behaviour of the corresponding reactions is always extremely simple [7]. An example for such a mechanism can be Ax - A2 - A3 - . . . - A [see Fig. 2(a)]. The contribution of acyclic mechanisms to the kinetics of catalytic reactions is not of importance. The mechanisms of catalytic reactions always contain cycles and these cycles are oriented, the directions of all the arrows being matched [the end of the ith arrow is the beginning of the... 

G.S. Yablonskii, V.I. Bykov and T.A. Akramov, Proc. 2nd All-Union Conf. Kinetics of Catalytic Reactions (Kinetics-2), Vol. 3, Institute of Catalysis, Novosibirsk, 1975, pp. 43-49 (in Russian). 

Studies of hydrogenation, including destructive hydrogenation (Nemtsov, Prokopets, Dyakova), reported in the 1930 s, may have been utilized by now for industrial processes. A. V. Frost has been conducting research on the kinetics of catalytic reactions and on catalytic cracking. Frost and M. D. Tilicheev are co-editors of a series of publications on physical constants of hydrocarbons which may be used as a source of information on the synthesis of individual hydrocarbons. Other Russian groups have contributed (N. D. Zelinskii, A. D. Petrov) to this field. Some of this work involves catalytic reactions however, in this review mere mention of it may suffice. 

The kinetics of catalytic reactions on nonuniform surfaces have been discussed by Roginskii (330,331) certain general features of his discussion will be presented here. The rate of a complex multistage heterogeneous catalytic reaction is controlled by the rate of the slowest step. The slowest step may be the adsorption of the reactants, the chemical reactions on the surface, desorption of the products or diffusion of reactants or products through the gaseous phase near the surface of the catalyst. 

In this section we consider mechanically agitated reactors that are largely used to study intrinsic kinetics of catalytic reactions. It is clear that the nature of a reactor used for this purpose will depend on the type of kinetic data required, the nature of the reaction system, the nature and amount of catalyst,and safety considerations. Some of the mechanically agitated reactors used for kinetic measurements are illustrated in Table XI. In this section, we will not consider previously discussed conventional slurry reactors and their modifications. 

Transition metals exchanged into Y-zeolite offer a basis upon which to build an understanding of the important parameters involved in designing zeolitic redox catalysts. Y-zeolite was chosen for this study because it is the most thoroughly characterized catalytic zeolite. Thus, one can address such questions as what non-framework cation sites are occupied, whether the cations move between sites, whether interactions between the cations themselves are important and how these factors relate to the kinetics of catalytic reactions. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Vannice MA (2005) Kinetics of catalytic reaction. Springer, New York... 

In some instances of catalytic investigations /(c,) is a simple function of the reactant concentration, c, of the form /(c,) = c , so that dn/dt = fcjC". It is natural to try to fit the kinetics of catalytic reactions into such a simple picture. The exponent n is referred to as the order of reaction. However, the assumption of a definite order or exponent... 

Crystallite shape transformations, due to adsorbed reactants, may affect the steady-state kinetics of catalytic reactions. Such effects can be studied phenomenologically by employing the Wulff rule [39] in order to find the optimum crystallite shape. The use of this rule implies that the crystallite reshaping time scale is shorter than the experimental time scale. The latter can be very long in practical systems. 

The theoretical results presented in this review show that, due to purely kinetic factors, the kinetics of catalytic reactions occurring on nm-sized metal particles, exposing different crystalline facets, may be unique compared to those observed on poly- or single-crystal surfaces. Experimental studies focused on such factors are still rare but certainly will attract more attention in the near future. 

Historically, the use of microreactors dates back to the 1940s when they were developed to measure kinetics of catalytic reactions.One of the key early findings was Denbigh s 1965 observation that if a reactor were made small enough, temperature and concentration gradients with the reactor would be negligible, so that differential (i.e., gradientless) behavior would be observed. This allowed much more accurate kinetic... 

Another effect of the nanometer size in reaction kinetics of catalytic reaction is the disappearance of the bi-stability in the CO oxidation as recently evidenced by molecular beam experiments on supported model catalysts [26]. 

Although a decade ago, non-monotonic kinetics of catalytic reactions were considered to be exceptional cases, nowadays it is becoming clear that non-monotonic kinetics in catalytic reactions are much more widespread than previously thought (e.g. Nicholas and Shah 1976 Peloso et al., 1979 Satterfield, 1980 Cordova and Gau. 1983 Yue and Olaofe, 1984 Yue and Birk, 1984 Das and Biswas, 1986 Takahashi et al., 1986 Elnashaie et al. 1990). 

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