Heterogeneous catalysis elementary reaction rate

In particular, reactions in heterogeneous catalysis are always a series of steps, including adsorption on the surface, reaction, and desorption back into the gas phase. In the course of this chapter we will see how the rate equations of overall reactions can be constructed from those of the elementary steps. 

Elementary reactions on solid surfaces are central to heterogeneous catalysis (Chapter 8) and gas-solid reactions (Chapter 9). This class of elementary reactions is the most complex and least understood of all those considered here. The simple quantitative theories of reaction rates on surfaces, which begin with the work of Langmuir in the 1920s, use the concept of sites, which are atomic groupings on the surface involved in bonding to other atoms or molecules. These theories treat the sites as if they are stationary gas-phase species which participate in reactive collisions in a similar manner to gas-phase reactants. 

Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify... 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

There is no generally accepted classification of elementary processes in heterogeneous catalysis. However, names for a few types of elementary processes are generally accepted and terminology for a partial classification [see M. Boudart, Kinetics of Chemical Processes, Chap. 2 (1968)] has received some currency. The particular reactions used below to exemplify this terminology are ones which have been proposed in the literature but some have not been securely established as occurring in nature at any important rate. 

If a chemical reaction is operated in a flow reactor under fixed external conditions (temperature, partial pressures, flow rate etc.), usually also a steady-state (i.e., time-independent) rate of reaction will result. Quite frequently, however, a different response may result The rate varies more or less periodically with time. Oscillatory kinetics have been reported for quite different types of reactions, such as with the famous Belousov-Zha-botinsky reaction in homogeneous solutions (/) or with a series of electrochemical reactions (2). In heterogeneous catalysis, phenomena of this type were observed for the first time about 20 years ago by Wicke and coworkers (3, 4) with the oxidation of carbon monoxide at supported platinum catalysts, and have since then been investigated quite extensively with various reactions and catalysts (5-7). Parallel to these experimental studies, a number of mathematical models were also developed these were intended to describe the kinetics of the underlying elementary processes and their solutions revealed indeed quite often oscillatory behavior. In view of the fact that these models usually consist of a set of coupled nonlinear differential equations, this result is, however, by no means surprising, as will become evident later, and in particular it cannot be considered as a proof for the assumed underlying reaction mechanism. 

It is generally accepted that heterogeneous catalysis represents a sequence of elementary reactions such as the adsorption of the reactant on the catalyst surface, atomic rearrangements of the adsorbed particles, and desorption of the products, the overall reaction rate being governed by the slowest step of these elementary reactions. The rate of the slowest... 

The central topic of the book is the rates of chemical reactions or elementary steps. A rate r states the number of moles of species i formed (if positive) or consumed (if negative) by a chemical reaction or reactions per unit volume and unit time (unit catalyst weight and unit time in heterogeneous catalysis). It is a process rate as distinct from a rate of change. The distinction is important. A rate of change is an observable phenomenon of nature, e.g., a change of concentration with time, and is the combined result of all contributing process rates. ... 

A mechanism of a catalytic reactions is a sequence of elementary steps, the rate of which can be described by for instance, transition state theory. Catalytic specie react with a catalyst forming complexes as described in Chapters 5-7 respectively for homogeneous, enzymatic and heterogeneous catalysis. From these rather complicated reaction sequences rate laws should be derived which could be then compard with experimental data. 

The aim is to induce changes in the reaction rates and/or in the selectivity of these reactions via potential or current application. The mechanism of these catalytic reactions is fairly well understood from the vast literature of heterogeneous catalysis. " The main elementary step is the dissociative chemisorption of NO which produces chemisorbed N and O. The desired product N2 is formed by reaction of two adsorbed N atoms. The undesired byproduct N2O comes from the reaction of coadsorbed N and NO. Further mechanistic aspects of these catalytic reactions are out of the scope of this review. As a general rule, the polarization of the catalyst does not modify the kinetic reaction scheme but affects the rate constants. ... 

Kinetic measurements show that the simple rate laws known from the last chapter are often not sufficient for a correct description of the temporal course of a reaction or the composition of a reaction mixture. Many reactions take place by mechanisms that involve several elementary steps. Three fundamental types of composite reactions are discussed in this chapter opposing or equilibrium reactions, parallel reactions, and consecutive reactions. Composite reactions not only play a large role in industrial applications (e.g., heterogeneous catalysis) but are also very important in nature (e.g., enzyme reactions). 

It can be seen that the energy of adsorption of the reagent produces two opposite effects on the reaction rate one through the activation energy of the elementary act and the other through the surface concentration of the reagent. When a < 1, the latter effect is predominant. These effects are well known in ordinary surface reactions, e.g., in heterogeneous catalysis. 

The aim of chemical kinetics is not only to measure reaction rates, but also to justify their expressions by a so-called mechanism A mechanism is a set of elementary steps into which the overall reaction decomposes Each step involves given intermediates Putting forward a mechanism amounts to making an hypothesis on the elementary steps and on the intermediates Heterogeneous catalysis is no long range interaction, and thus implies a kind of chemical combination between the reaction constituents and the solid surface Such a combination is an elementary step and its investigation is a prerequisite in any study of catalysis ... 

In heterogeneous catalysis, nth-order kinetics may be the result of adsorption on a nonideal catalyst surface. In homogeneous systems, nth-order kinetics may represent the overall rate of the underlying elementary reactions, e.g., the classical Rice-Herzfeld mechanism for thermal cracking of hydrocarbonsFor simplicity, n is assumed to be constant for all species. This is not a strong assumption for many petroleum processes. 

FIGURE 13.16. Effect of catalyst work function eO on the activation energy and catalytic rate enhancement ratio r/r for C2H4 oxidation on Pt (a) and CH4 oxidation on Pt (h). (Prom Vayenas, C.G, Electrochemical activation of catalytic reactions, in Elementary reaction steps in Heterogeneous Catalysis, Joyner, R.W. and van Santen, R.A., Eds., NATO ASI Series, Kluwer Academic Publishers, Dordrecht, 1993, 73. With permission.)... 

The microkinetic modeling procedure for catalyst screening will be illustrated for the CO oxidation example, which is the prototype reaction in heterogeneous catalysis. The same four-step mechanism used in the Sabatier example will be used here and the net rates of the elementary steps are given by ... 

An interesting further development in describing the kinetics of heterogeneously catalyzed reactions is the so-called microkinetics approach, whereby independent information about adsorbed species from temperature programmed desorption and spectroscopic studies are used to predetermine rate and equilibrium constants of elementary processes, thus enabling the prediction of the overall rate. Especially for metal catalyzed reactions this gives good results [9]. More information about reaction kinetics related to catalysis can be found in Refs. [10-13]. 

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