Elementary Surface Reactions

Unraveling catalytic mechanisms in terms of elementary reactions and determining the kinetic parameters of such steps is at the heart of understanding catalytic reactions at the molecular level. As explained in Chapters 1 and 2, catalysis is a cyclic event that consists of elementary reaction steps. Hence, to determine the kinetics of a catalytic reaction mechanism, we need the kinetic parameters of these individual reaction steps. Unfortunately, these are rarely available. Here we discuss how sticking coefficients, activation energies and pre-exponential factors can be determined for elementary steps as adsorption, desorption, dissociation and recombination. 

Once the kinetic parameters of elementary steps, as well as thermodynamic quantities such as heats of adsorption (Chapter 6), are available one can construct a micro-kinetic model to describe the overall reaction. Otherwise, one has to rely on fitting a rate expression that is based on an assumed reaction mechanism. Examples of both cases are discussed this chapter. 

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Finally, we will apply transition state theory and collision theory to some elementary surface reactions that are important in catalysis. 

By applying the machinery of statistical thermodynamics we have derived expressions for the adsorption, reaction, and desorption of molecules on and from a surface. The rate constants can in each case be described as a ratio between partition functions of the transition state and the reactants. Below, we summarize the most important results for elementary surface reactions. In principle, all the important constants involved (prefactors and activation energies) can be calculated from the partitions functions. These are, however, not easily obtainable and, where possible, experimentally determined values are used. 

Temperature-programmed reaction spectroscopy offers a straightforward way to monitor the kinetics of elementary surface reactions, provided that the desorption itself is not rate limiting. Figure 7.14 shows the the reaction CO -f O CO2 + 2. ... 

Figure 7.14. The temperature-programmed reaction and corresponding Arrhenius plot based on rate expression (21) enables the calculation of kinetic parameters for the elementary surface reaction between CO and O atoms on a Rh(lOO) surface.

Give at least two reasons why it is important to know the kinetic parameters of elementary surface reactions in catalytic mechanisms. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

Handling the experimental data for the temperature range of 470-700°C, we used theoretical expressions for preexponential factors from the theory of absolute rates of surface reactions (Section IV), assuming the elementary surface reactions under consideration to be adiabatic, and also the known K values. Computer calculations were performed by the method of minimization of the sum of the squares of relative deviations of calculated reaction rates from experimental values. 

The other examples of elementary surface reactions can be inspected the same way. Note that in the reaction between sorbates on the active phase sur face, the magnitudes of , and, as a consequence, traditional rate constants k and k may be considered in the first approximation as independent of r. 

Physical Chemistry of Elementary Surface Reaction Steps... 

PHYSICAL CHEMISTRY OF ELEMENTARY SURFACE REACTION STEPS... 

The activation energies of elementary surface reactions that proceed along the same reaction path on similar reaction sites are often found to be linearly related to the corresponding reaction energies, which depend on the nature of the metal. This parallel behavior is expressed in the BEP relationship (20,25), which we used earlier to construct Figure 2. The BEP relationship can be formulated as follows ... 

Elementary Surface Reaction Steps at Transition Metal Surfaces... 

In the period mentioned above the most known world centers of the studies of chemical reactions on silica and alumina surfaces were USSR, Germany, Bulgaria. But since 1977 after the patent [26] had been published and later in the eightieth as well as at present the number of publications in this field has increased in various countries [27-39]. It should be noted, that some authors use another names for the synthesis like the ML method for example, atomic layer epitaxy (ALE) [30], chemical surface coating [31], grafting [32],elementary surface reactions of CVD method [33]. 

MD simulations of a-Si H film growth from thermal SilI, precursors have revealed a wide range of elementary surface reaction mechanisms that occur on the growth surface (Ramalingam, 2000). Tliese mechanisms can be divided into broad classes that include... 

Examination of the above equilibria shows that three products appear H2, CO2 and CO. Each of these products will have its own mechanism and rate of formation. Each of these rates will be coupled to the rates of formation of the other products, but will not be directly proportional to them. A foil overall mechanism for this reaction is thought (Peppley, B.A. et al. (1999)) to consist of over twenty elementary surface reactions taking place on two different types of sites. It is assumed in the development of this mechanism that the two types of sites are energetically homogeneous within each type. 

The high-temperature oxidations of molybdenum [311—313] and of tungsten [314, 315] have been shown to be surface oxidation reactions, and detailed mechanisms have been proposed [312, 315]. In the case of tungsten [315] a number of rate coefficients for the elementary surface reactions have been deduced. 

Rate expressions of the form of Equation 5.153 are known as Hougen Watson or Langmuir-Hinshelwood kinetics [17, This form of kinetic expression is often used to describe the species production rates for heterogeneously catalyzed reactions. We complete the section on the kinetics of elementary surface reactions by returning to the methane synthesis reaction listed in Section 5.2. The development proceeds exactly as outlined in Section 5.2. But now it is necessary to add a site-balance expression (Equation 5,129) in Step 3. 

However, correlation of these reaction mechanisms (suggested by inspection of the macroscopic rate equations) with molecular-level studies of the elementary surface reactions remains one of the future challenges of catalysis. 

If the predominant reaction mechanism involves CO dissociation (as appears to be the case over nickel and most other transition-metal catalysts), methane formation may be expressed by writing the following elementary surface reaction steps ... 

All of the species are reacting in their adsorbed states. If enol species form as reaction intermediates as suggested for CO hydrogenation over molybdenum, the elementary surface reaction sequence may be expressed as follows ... 

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