Elementary surface reaction steps

Physical Chemistry of Elementary Surface Reaction Steps... 

PHYSICAL CHEMISTRY OF ELEMENTARY SURFACE REACTION STEPS... 

Elementary Surface Reaction Steps at Transition Metal Surfaces... 

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 ... 

To summarize, the coupling of traditional methods for the characterisation of nanomaterials (TEM and WAXS) with simple methods of organometallic chemistry (NMR) provides useful information on the chemical reactivity of the surface of the nanoparticles and sheds light on elementary surface reaction steps such as substitution, oxidative addition and reductive eUmination. 

This subsection begins with a short summary of particle-size-dependence observations of chemical bond activation. Next, the Bronsted-Evans-Polanyi relation that relates activation energies of elementary surface reaction steps with the corresponding reaction energies is introduced. In the subsections that follow, the... 

Investigations in the first step define the surface structure and composition on the atomic scale and the chemical bonding of adsorbates. Studies in the second step, which are carried out at low pressures, reveal many of the elementary surface reaction steps and the dynamics of surface reactions. Studies in the third and fourth steps establish the similarities and differences between the model system and the dispersed catalyst under practical reaction conditions. 

In the subsections that follow we will focus on the factors that maximize the rate constant for elementary surface reaction steps. Again we will stress the need explicitly to include entropic contributions. According to transition-state reaction rate theoryl l, the rate of the elementary conversion step is defined as... 

Currently, the most straightforward way to bridge electronic structure and surface kinetics requires a decoupling of the time scales that govern electronic transfer processes that control elementary surface reaction steps from the overall catalytic cycle which proceeds at much longer times. Ab initio calculations are used first to calculate the kinetics, energetics and potential mechanisms necessary for an external database. The database could then be called in situ within the simulation algorithm. 

Boudart has discussed in detail the fact that the rate law derived from a complex catalytic cycle comprised of a number of elementary steps can frequently be represented by only two kinetically significant steps if the assumptions of a RDS and a MARI are invoked however, ambiguities can develop which prevent one from distinguishing among different reaction models [11,26]. In similar fashion, but with perhaps less dramatic results, a L-H-type or H-W-type model [27] invoking more than one elementary surface reaction step can be greatly simplified by the presence of quasi-equilibrated steps which precede the RDS or, if a RDS does not exist, the series of slow steps on the surface. The SSA may also be required in the latter case to eliminate all unknown surface reaction intermediates from the rate law. Significant simplification is achieved with the assumption of a RDS. 

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. 

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 ... 

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. 

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. 

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. 

Sabatier s principle provides a kinetic rmderstanding of the catalytic cycle and its corresponding elementary reaction steps which include adsorption, surface reaction, desorption and catalyst self repair. The nature of the catalytic cycle implies that bonds at the surface of the catalyst that are disrupted during the reaction must be restored. A good catalyst has the unique property that it reacts with the reagent, but readily becomes liberated when the product is formed. This will be further discussed in Section 2.2, where we describe the kinetics of elementary surface reactions and their free energy relationships. 

Based on a model proposed by Harris and Goodwin [36], an elementary surface reaction mechanism for the diamond (lOO)-surface reconstructed to the (100)-(2xl) H form is developed [11]. The mechanism assumes that diamond growth takes place at surface step sites as shown in Fig. 5. The principle features... 

It is mosdy accepted that CO oxidation on nohle metals occurs between the CO and O adsorbates (Karadeniz et al., 2013 Karakaya, 2013). The intrinsic kinetics of the CO oxidation over Rh/Al203 is taken here from the recent study of Karakaya et al. (2014) without any modification. This surface reaction mechanism is a subset of the kinetics of the water-gas shift reaction over Rh/Al203 catalysts given hy Karakaya et al. (2014). This direct oxidation of CO involves 10 elementary-hke surface reaction steps among 4 surfaces and 3 gas-phase species. The reaction rates are modeled by a modified Arrhenius expression as given in Eq. (2.6). The nominal values of the preexponential factors are assumed to he IO Na/T (cm /mol s), where is Avogadro s number (the surface site density was estimated to be 1.637 X 10 site/cm derived from a Rh(llO) surface). The nominal value of 10 is the value calculated from transition state theory k T/h) with being Boltzmann s constant and h Plank s constant (Maier et al., 2011). 

The models developed here account for unmeasurable intermediates such as adsorbed ions or free radicals. Microkinetic analysis, pioneered by Dumesic and cowokers"", is an example of this approach. It quantifies catalytic reactions in terms of the kinetics of elementary surface reactions. This is done by estimating the gas-phase rate constants from transition state theory and adjusting these constants for surface reactions. For instance, isobutane cracking over zeolite Y-based FCC catalysts has 21 reversible steps defined by 60 kinetic parameters." The rate constants are estimated from transition state theory... 

The dissociation of over a Cu surface discussed earlier is an example of an elementary surface reaction. Other types of elementary surface reactions are illustrated in Figure 2.4. After reactants are adsorbed and perhaps dissociated, they will diffuse and recombine to form new molecules before the product is desorbed into the surrounding gas or liquid phase. For each of these different elementary reaction steps, we can define a PES and a ID PED. 

Elementary surface reactions at an electrode involve electron transfer processes. Take as an example the last step in the formation of water in the oxygen reduction reaction (ORR) (or, in the reverse direction, the first step in the water splitting reaction) ... 

The pre-exponents of reactions have been chosen to have no activation entropy for most of the surface reactions steps [4], but large activation entropy differences are included for molecular adsorption or desorption reaction steps. Table 16.1 collects the basic elementary reaction rate data used. Figure 16.5 presents reaction... 

As with the other surface reactions discussed above, the steps m a catalytic reaction (neglecting diffiision) are as follows the adsorption of reactant molecules or atoms to fomi bound surface species, the reaction of these surface species with gas phase species or other surface species and subsequent product desorption. The global reaction rate is governed by the slowest of these elementary steps, called the rate-detemiming or rate-limiting step. In many cases, it has been found that either the adsorption or desorption steps are rate detemiining. It is not surprising, then, that the surface stmcture of the catalyst, which is a variable that can influence adsorption and desorption rates, can sometimes affect the overall conversion and selectivity. 

In most cases surface reactions proceed according to well-established elementary steps, as schematized in Fig. 1. The first one comprises trapping, sticking, and adsorption. Gaseous reactants atoms and/or molecules are trapped by the potential well of the surface. This rather weak interaction is commonly considered as a physisorbed precursor state. Subsequently, species are promoted to the chemisorbed state, that is, a much stronger... 

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. 

---