Heterogeneous catalysis desorption step

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. 

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 mechanism of heterogeneous catalysis is often complex and not well understood. Important steps, however, involve (1) attachment of reactants to the surface of the catalyst, a process called adsorption, (2) conversion of reactants to products on the surface, and (3) desorption of products from the surface. The adsorption step is thought to involve chemical bonding of reactants to the highly reactive metal atoms on the surface with accompanying breaking, or at least weakening, of bonds in the reactants. 

In addition to the chemical steps, which are the only steps involved in stoichiometric or in homogeneous catalysis reactions, heterogeneous catalysis reactions involve also physical steps, i.e. transport (transfer) of organic molecules (and heat) from the reaction mixture to the active sites of the solid catalyst and vice versa.113-151 Another difference deals with the chemical steps, which do not occur in the fluid phase, but for part of them involve both fluid and solid phases (chemisorption and desorption), the other part occurring at the surface of the catalyst.113-151... 

The mechanisms by which CVD occur are very similar to those of heterogeneous catalysis discussed earlier in this chapter. The reactant(s) adsorb on the surface and then react on the surface to form a new surface. This process may be followed by a desorption step, depending on the particular reaction. 

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... 

Of all of these steps, only the adsorption of the reactants, the reaction between the adsorbed species, and the desorption of the product are chemical reactions. The rest are physical processes that can strongly influence not only the apparent rate of the reaction but also the nature of the products obtained. To fully utilize heterogeneous catalysis as a viable synthetic procedure, then, it is important to recognize not only how these different transport steps can influence... 

Since the adsorption of reactants and desorption of products are unavoidable and fundamental steps of heterogeneous catalysis, there is a need of understanding the kinetics of adsorption-desorption phenomena on heterogeneous surfaces. In practically all situations the reaction involves two or more reactants, so that chemically interesting kinetics are multicomponent. This is immediately understood either considering the Rideal mechanism ... 

In Chap. 8 heterogeneous catalysis was explained by postulating a three-step process (1) chemisorption of at least one reactant on the solid, (2) surface reaction of the chemisorbed substance, and (3) desorption of the product from the catalytic surface. Now our objective is to formulate rate and equilibrium equations for these steps. We shall consider the kinetics and equilibrium of adsorption and then examine rate equations for the overall reaction. 

A general formalism for single-step surface reactions of heterogeneous catalysis has been developed by Hougen and Watson [3]. The rate may be controlled by the surface reaction, adsorption of a reactant, or desorption of a product. Explicitly covered in tabulations are reactions with the following stoichiometries ... 

Heterogeneous catalysis is most commonly modeled as a surface reaction with adsorption and desorption as additional steps. Following Langmuir, adsorption is taken to be a reversible "reaction" of a molecule in the contacting fluid with a vacant catalytic site... 

In a series of papers, Suvorov et al. investigated heterogeneous catalysis of the cyclization of isolated aldehyde and ketone phenylhydrazones. y-Alumina was typically employed as catalyst in the vapor phase reaction at atmospheric pressure and at temperatures around 300 °C. A maximum yield of 60% was obtained from acetaldehyde phenylhydrazone as a result of thermal decomposition of the hydrazone [7] and the formation of benzene and aniline as by-products [8]. Kinetic studies indicated that the rate-determining step was desorption of product from the surface [9]. 

In Chapters 3 and 4 the Langmuir-Hinshelwood-Hougen-Watson approach to heterogeneous catalysis was discussed. Such an approach supposes that usually there is one rate determining step (adsorption, surface reaction or desorption) and that the other steps are in quasi-equilibria. 

Often in heterogeneous catalysis, the reaction mechanism is rather complex and cannot be represented by a one-route multistep reaction sequence, as there are several routes leading to a variety of products. An example of the rate derivation for a multi-route mechanism of butadiene hydrogenation was presented in Chapter 4. Often for the derivation of kinetic equations in such a case it is assumed that the adsorption/desorption steps are in quasiequilibria. The rate of formation of a certain component in the reaction mixture is then defined as... 

Adsorption and desorption processes are also important elementary steps in the compUcated chain of reactions that make up a typical mechanism in heterogeneous catalysis. Before the reaction can start at all, the reactant atoms or molecules must adsorb at the solid surface. Then they will diffuse along the surface, react with each other or with the surface to generate reaction intermediates, and eventually, in most cases after several reaction steps, the final products have to desorb from the surface. 

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

In heterogeneous catalysis the overall reaction consists of a sequence of consecutive steps, such as diffusion, adsorption of reactants and products, surface reaction and desorption of the products. The heterogeneous reactions take place on the catalyst surface where some adsorbed... 

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