Elementary Steps in Heterogenous Reactions

Chemical kinetics, either for homogenous reactions or for heterogenous reactions, rests on a basic postulate (see section 7.2.1), which affirms that all real transformations are combinations of a small number of transformations, each one causing minimum modifications of the system, which we call elementary steps. The notion of elementary steps can be introduced in two ways  

We will choose the second way that has the advantage of leading to a definition, which remains valid for both homogenous and heterogenous reactions. 

Macroscopic definition of elementary steps Consider a thermal reaction of the form - products 

This reaction is known as elementary if it meets all the following conditions  

In this relation. Pi is the partial order of the reaction with respect to T, and k is the voluminal speed constant, which depends only on temperature. The voluminal speed constant varies roughly with temperature according to the Arrhenius law  

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Scheme 3.2 Pictorial illustration of the elementary steps in heterogeneous catalysis (i) adsorption, (ii) reaction, and (iii) desorption.

Elementary Steps in Heterogeneously Catalyzed Reactions. A catalytic cycle from bulk phase reactant to bulk phase product on a porous heterogeneous catalyst essentially comprises seven steps (see Fig. 1), that is. 

In reactions involving crystallized solids, the intermediates are frequently structure elements among which the point defects play a major role. A branch of chemistry, called quasi-chemistry has thus been developed by taking into accoimt these defects in quasi-chemical reactions that feature thermodynamic and kinetic properties. It is often such reactions that constitute the elementary steps of heterogeneous reactions. 

Compelling molecular models also include complexes of early transition metals in higher oxidation states. These complexes mimic oxide surface structure at a molecular level, including extended oxide structures. These models, described by KLEMPERER, can also be used to mimic some elementary steps in heterogeneous oxidation reactions. 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

Y. Iwasawa, Elementary Reaction Steps in Heterogeneous Catalysis (R. W. Joyner and R. A. van Santen Eds.), NATO ASI Series, Kluwer, the Netherlands, 1993 pp. 287-304. ... 

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. 

While many techniques have evolved to evaluate surface intermediates, as will be discussed below, it is equally important to also obtain information on gas phase intermediates, as well. While the surface reactions are interesting because they demonstrate heterogeneous kinetic mechanisms, it is the overall product yield that is finally obtained. As presented in a text by Dumesic et al. one must approach heterogeneous catalysis in the way it has been done for gas phase systems, which means using elementary reaction expressions to develop a detailed chemical kinetic mechanism (DCKM). DCKMs develop mechanisms in which only one bond is broken or formed at each step in the reaction scheme. The DCKM concept was promoted and used by numerous researchers to make great advances in the field of gas phase model predictions. 

There are many more types of elementary processes in heterogeneous catalysis than in gas phase reactions. In heterogeneous catalysis the elementary processes are broadly classified as either adsorption-desorption or surface reaction, i.e., elementary processes which involve reaction of adsorbed species. Free surface sites and molecules from the fluid phase may or may not participate in surface reaction steps. 

The 1 1 correspondence of the elementary chemical steps in these reactions provides the underlying basis for the functional identity of the historically distinct kinetic equations for heterogeneous and enzymatic reactions. 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

In this chapter, we will review the reaction dynamics studies which has been performed on supported model catalysts in order to unravel the elementary steps of heterogeneous catalytic reactions. In particular we will focus on the aspects that cannot be studied on extended surfaces like the effect of the size and shape of the metal particles and the role of the substrate in the reaction kinetics. In the first part we will describe the experimental methods and techniques used in these studies. Then we present an overview of the preparation and the structural characterization of the metal particle. Later, we will review the adsorption studies of NO, CO and 02. Finally, we will review the two reactions that have been investigated on the supported model catalysts the CO oxidation and the NO reduction by CO. 

Figure 1.1.11 Steps of heterogeneous reactions. The individual processes comprising sequences of elementary step reactions are linked to a process sequence. The microscopic part is described by microkinetics, and the observable macroscopic performance by macrokinetics. A typical relative dimension of energy changes associated with the individual steps is indicated. In homogeneous reactions, the transport parts are often ignored.

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

Hydrogenation is an important industrial reaction that often requires the presence of a heterogeneous catalyst to achieve commercial yields. Ethylene, C2H4, is the smallest olefin that can be used to investigate the addition of hydrogen atoms to a carbon-carbon double bond. Even though many experiments and theoretical studies have been carried out on this simple system, the reaction is still not completely understood. Microkinetic analysis provides insights into the relevant elementary steps in the catalytic cycle. 

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