Surface-Reaction Steps

Central to surface catalysis are reaction steps involving one, or more than one, surface-bound (adsorbed) intermediate species. We consider three types. 

The rates (and rate constants) can be expressed on the basis of catalyst mass (e.g., mol kg-1h 1), or of catalyst surface area (e.g., mol m-2 s-1), or as a turnover frequency (molecules site-1 s-1), if a method to count the sites exists. 

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Combinations of several adsorption and surface reaction steps are usually not felt to be necessary, since so many alternatives are available individually. Single steps in combination with diffusion to the surface are usually adequate, as in the case leading to Eq. (7-52). 

As our first approach to the model, we considered the controlling step to be the mass transfer from gas to liquid, the mass transfer from liquid to catalyst, or the catalytic surface reaction step. The other steps were eliminated since convective transport with small catalyst particles and high local mixing should offer virtually no resistance to the overall reaction scheme. Mathematical models were constructed for each of these three steps. 

Determine the form of the pseudohomogeneous, intrinsic kinetics for each of these cases. Assume that the surface reaction step, as shown above, is rate limiting. 

Since early in this century the concept of the active site in catalysis [1] has been a focus of attention in this area of chemistry. This was proposed to be that ensemble of surface atoms/reactants which is responsible for the crucial surface reaction step involved in a catalytic conversion. Since those days much work has been done in the area, which cites the concept of the active site. However, no such ensemble has been positively identified due to the lack of availability of techniques which could image such a structure, which is of atomic dimensions. 

Figure 3.3. Schematic representation of the adsorption, surface diffusion, and surface reaction steps identified by surface-science experiments on model supported-palladium catalysts [28]. Important conclusions from this work include the preferential dissociation of NO at the edges and defects of the Pd particles, the limited mobility of the resulting Nads and Oads species at low temperatures, and the enhancement in NO dissociation promoted by strongly-bonded nitrogen atoms in the vicinity of edge and defect sites at high adsorbate coverages. (Figure provided by Professor Libuda and reproduced with permission from the American Chemical Society, Copyright 2004).

Hint. For the single-site mechanism assume (as Mathur and Thodos did) that the surface reaction step can be written as ... 

The commonest multiple step control mechanism in use is that of diffusion to the surface of the catalyst combined with one of the adsorption or surface reaction steps. Mass transfer by diffusion is proportional to the difference between partial pressures in the bulk of the gas and at the catalyst surface,... 

This reaction is sufficiently fast that it proceeds at low enough temperature to be useful to eliminate NO in the automotive catalytic converter. We assumed that the surface reaction step is... 

Surface reaction steps are frequently very important in controlling chain reactions. [Catalytic reactions again ] For example, the termination steps by which free-radical intermediates are removed will frequently occur readily on surfaces simply by adsorption. We can write this reaction as... 

For CFD modeling a detailed chemical mechanism for the relevant gas phase and surface reaction steps is necessary. Due to the difficulty involved in determining kinetic and thermodynamic parameters for the elementary steps, these are often based on empiricism and even guessing. Here, theoretical first-principles methods can be very helpful. 

In some electrode reactions, there are intermediates of a different kind, species more uncertain as to what they want to do. They do not always bond sufficiently to the substrate to remain there and undergo a consecutive surface reaction step, as does H that combines to H2. This latter type of intermediate comes loose" from the electrode surface. It may then contact the electrode again and react further, or quit the scene and diffuse off into the bulk of the solution, remaining lost to any continued reaction sequence that would be possible if the radical had stayed on the electrode surface for further consecutive reaction steps. 

Detailed microkinetic models are available for CO, H2 and HC oxidation on noble metal(s) (NM)/y-Al203-based catalysts (cf., e.g. Chatterjee et al., 2001 Harmsen et al., 2000, 2001 Nibbelke et al., 1998). The model for CO oxidation on Pt sites includes both Langmuir-Hinshelwood and Eley-Rideal pathways (cf., e.g., Froment and Bischoff, 1990). Microkinetic description of the hydrocarbons oxidation is more complicated, particularly due to a large number of different reaction intermediates formed on the catalytic surface. Simplified mechanisms, using just one or two formal surface reaction 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. 

In the case of the dual site model the initial rate even passes through a maximum if the surface reaction step is rate determining (cf. Eqn. (3.24)). 

Let us consider the extreme cases and first lump together the surface reaction steps and also the individual bulk hopping steps and write the whole process in a two step form... 

So far we considered the surface process in the limit ofproximity to equilibrium which is, for the relaxation experiments discussed, often a sufficient approximation. In order to extend the validity range but also to highlight the situation from a different point of view, let us apply chemical kinetics to the surface process which we now decompose into the individual steps of the reaction sequence.23,24 172 248 The rate determining step is supposed to be one of these surface reaction steps. 

Physical Chemistry of Elementary Surface Reaction Steps... 

The equations that we consider in this section were deduced on the basis of the additional simplifying assumption that only one type of "Ci species is formed. In principle, hydrogenation of "Ci" species proceeds via consecutive surface reaction steps in which different "CH. " species are subsequently formed. In Section 3.3, we discuss the consequences of the presence of various "CH " species on the surface for the kinetics of the Fischer-Tropsch reaction. Considering this more complex scenario is relevant to understanding and calculation of the chain-growth parameter a. 

PHYSICAL CHEMISTRY OF ELEMENTARY SURFACE REACTION STEPS... 

The over-all process of crystal growth in a seeded solution is analogous to other mass transfer situations encountered in chemical engineering and may be treated as a diffusional step in series with a surface reaction step. Solution supersaturation provides the driving force required for each step, as portrayed schematically in Fig. 12. First, solute molecules or ions diffuse through the solution to the growing crystal. Second, upon reaching the surface, the molecules or ions must be accepted and incorporated into the crystal lattice. 

In this model, the steps can be classified into two categories, mass transport and surface reaction steps. The slowest of these steps determines if the process is mass transport or surface reaction limited. At lower temperatures the deposition rate is generally surface reaction limited. As the temperature increases, the surface reaction rate rises exponentially, resulting in a mass transport limited because transport becomes the slowest step in the series of deposition steps. Reaction resistances are often used to predict rate-limiting steps in CVD process. 

The rate law for the surface reaction step producing adsorbed benzene and piropylene in the gas phase,... 

Assume a rate-limiting step. Choose the surface reaction firsf since more than 75% of all heterogeneous reactions that are not Affusion-limited are surface-reaction-limited. The rate law for the surface reaction step is... 

At first it may appear that a site has been lost when comparing the right- and left-hand sides of the surface reaction step. However, the newly formed germanium atom on the right-hand side is a site for the future adsorption of H2(g) or GeCl2(g) and there are three sites on both the right- and left-hand sides of the smface reaction step. These sites are shown schematically in Figme 10-17. 

Product distributions can be evaluated for reaction probabilities of elemental surface reaction steps with the model of non trivial surface polymerisation [2]. Specific inhibition of desorption of a chemisorbed organic species has been postulated to be the intrinsic principle of the FT-synthesis [5]. A chemisorbed species can react further by linear chain prolongation or chain branching or it can desorb as a paraffin, olefin or an organic oxygen compound. Growth probabilities pg, that contain a similar information as the Anderson-Schulz-Flory parameter a. 

Finally, the kinetics approximations cannot be forgotten. Usually, in order to put in evidence structure-activity relationships, a simple parameter, the TOF, is used. The TOF, which reflects the rate per accessible site, contains the combination of all the adsorption and surface reaction elementary steps. Each of these steps is dependent on adsorption and/or rate constants. For that reason, the significance of TOF dependence as a function of structural parameters, e.g., the particle size, is not obvious since the rate equation can be particle-size-dependent [17]. Moreover, the adsorption and surface reaction steps may exhibit very different sensitivities to electronic and geometrical features. 

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