Elementary surface reaction steps adsorption energies

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. 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

Once the thermodynamic parameters of stable structures and TSs are determined from quantum-chemical calculations, the next step is to find theoretically the rate constants of all elementary reactions or elementary physical processes (say, diffusion) relevant to a particular overall process (film growth, deposition, etc.). Processes that proceed at a surface active site are most important for modeling various epitaxial processes. Quantum-chemical calculations show that many gas-surface reactions proceed via a surface complex (precursor) between an incident gas-phase molecule and a surface active site. Such precursors mostly have a substantial adsorption energy and play an important role in the processes of dielectric film growth. They give rise to competition among subsequent processes of desorption, stabilization, surface diffusion, and chemical transformations of the surface complex. 

Whenever an activation energy is known for one metal surface, then the activation energies of the same elementary reaction step on other metals can be deduced from the differences in the adsorption energies of C and O on the two metals. This procedure has been used to generate the activation barriers presented in Table 1. 

A heterogeneous catalytic reaction involves adsorption of reactants from a fluid phase onto a solid surface, surface reaction of adsorbed species, and desorption of products into the fluid phase. Clearly, the presence of a catalyst provides an alternative sequence of elementary steps to accomplish the desired chemical reaction from that in its absence. If the energy barriers of the catalytic path are much lower than the barrier(s) of the noncatalytic path, significant enhancements in the reaction rate can be realized by use of a catalyst. This concept has already been introduced in the previous chapter with regard to the Cl catalyzed decomposition of ozone (Figure 4.1.2) and enzyme-catalyzed conversion of substrate (Figure 4.2.4). A similar reaction profile can be constructed with a heterogeneous catalytic reaction. 

The kinetic model comprises the following elementary steps irreversible adsorption of oxygen and reversible adsorption of ethene on the noble metal surface, followed by a surface reaction between adsorbed ethene and oxygen. The values of the kinetic parameters, i.e. preexponential factors and activation energies, were estimated by non-linear regression of the ethene conversion and found to be physically meaningful. 

It is assumed that the composite catalytic reaction involves several elementary steps, e.g., adsorption, surface reaction, and desorption, which may individually be treated according to TTST, i.e., each step is assumed to possess its own transition state. For example, for the adsorption of A, the forward step is represented by, A + S [X ] - A S. The free energy changes of activation associated with each step may, of course, be substantially different providing justification of the common assumption of the rate determining step (rds). The rate of the forward elementary step i is proportional to the universal frequency, V = kgT jh [2], a transmission coefficient, K, varying between 0 and 1 [3], and the concentration of the transition-state complex (TSC)... 

It appears that a significant amount of energy is required for CH activation, the primary elementary step of the hydrocarbon conversion reaction. If one chemisorbs methane at low temperature on a transition metal surface, it desorbs before reaction can occur. Aliphatic hydrocarbons can dissociate from a preadsorbed state, if they contain enough carbon atoms in their chain to induce a high heat of adsorption whereby CH dissociation can take place at a rate large compared to the desorption rate. So far this has only been found for surfaces containing highly active metal atoms, such as the reconstructed Ir (110) surface or stepped surfaces l. As we will see, both electronic and steric effects may play a role. 

We note that the scaling parameter is independent of the surface. The cutoff, in Equation (6.1), on the other hand, depends on the surface structure. Hence, in order to obtain information about how weU a given structure binds an adsorbate, given that one knows how all the base elements (C, O, N, S, etc.) bind, one needs to do only a single value measurement or calculation of the adsorption energy, and the rest can be scaled from the binding of the base elements. This enables us to write a simple expression for the reaction energy of an elementary step as... 

FIGURE 11.11 Calculated free energy diagram for the full ORR over a Pt(lll) surface at C/=0.9V. The different elementary reaction steps included are as follows, in order from left to right diffusion of from the bulk electrolyte to the region (double layer) just outside the surface (the effective free energy barrier shown is deduced from the diffusion rate) adsorption of Oj (note that this involves electron transfer to the molecule, but not a whole electron, and the electron transfer is there also in the absence of the potential since a metal surface has a large pool of electrons available at the Fermi level), followed by four coupled electron-proton transfers to form water and recreation of the adsorption site A. Adapted from Hansen et al. (2014). 

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