Elementary reaction steps on surfaces

It is convenient to subdivide the reactions discussed in this section into three groups  

In the substantial majority of cases the reaction takes place between the adsorbed molecule, or their fragments. This is known as the Langmuir-Hinshel-wood mechanism. For very few reactions a so-called Eley-Rideal mechanism has been postulated, in which an adsorbed molecule or its fragment reacts with a molecule impinging from the gas(liquid)phase. 

In many of these reactions a lattice constituent — oxygen or hydrogen — is brought into reacting molecules and the lattice defect is subsequently removed by a reaction with another reaction component, or by a reaction with another reactive centre of the same molecule. This is the so-called Mars and van Krevelen mechanism. With some oxides peroxide groups are formed on the surface (from adsorbed oxygen molecules). Radicals may form at high temperatures (1000 K). 

Examples are reactions initiated by the protonation, as for example carbenium ion formation from — for example—olefins (Bronsted acidic centres) or deprotonation reactions like base-catalysed aldol condensation. 

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FIGURE 2.4 lUnstration of the elementary reaction steps on surfaces. See insert for color representation of the figure.)... 

To rationally govern the selectivity of a catalytic process, the elementary reaction steps on real catalyst surfaces must be identified. The use of well-defined organometallic compounds (possible intermediates in surface reactions) can be very useful in the determination of these steps. The use of kinetic modelling techniques combined with statistical analysis of kinetic... 

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. 

The catalytic cycle of elementary reaction steps on a transition metal surface consists of the following reactions ... 

Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of the active species may be reduced by direct interaction with methane or from the reaction with H, Hg, C or CO. The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. A catalytic material and support which does not easily form or stabilise OH species is therefore desirable. Another essential property for the formation of Hg and CO as primary products is a low surface coverage of intermediates, such that the probability of O-H, OH-H and CO-O interactions is reduced. ... 

The latter concept is basic to the specific dependence of the activation energies of elementary reaction steps on different surfaces. There is not always an immediate relation with the coordinative unsaturation of the metal surface atoms. Ge and Neurock noted an exeptionally low barrier for the dissociation of NO adsorbed on the non-reconstructed Pt(lOO) surface. The corresponding transition state is shown in Fig. 3.37. The calculated activation energies for NO dissociation over the (111) and (110) surfaces are 160 and 105... 

Elementary steps on surfaces and in condensed phases are more complex because the environment for the elementary reactions can change as the composition of the reaction mixture changes, and, in the case of surface reactions, there are several types of reactive sites on solid surfaces. Therefore, the rate constants of these elementary steps are not really constant, but can vary from system to system. Despite this complexity, the approximation of a single type of reaction step is useful and often generally correct. 

Having chosen a particular model for the electrical properties of the interface, e.g., the TIM, it is necessary to incorporate the same model into the kinetic analysis. Just as electrical double layer (EDL) properties influence equilibrium partitioning between solid and liquid phases, they can also be expected to affect the rates of elementary reaction steps. An illustration of the effect of the EDL on adsorption/desorption reaction steps is shown schematically in Figure 7. In the case of lead ion adsorption onto a positively charged surface, the rate of adsorption is diminished and the rate of desorption enhanced relative to the case where there are no EDL effects. 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

Figure 6 shows the dependence of the rate of CO oxidation over single crystal catalysts on the partial pressure of CO. At conditions of relatively high partial pressures of CO, the reaction rate is observed to decrease linearly with increasing CO partial pressure reflecting the domination of reactant surface coverage by CO. For these reaction conditions on Rh, this behavior has been accurately modeled using individual elementary reaction steps established from surface science studies of the interactions of CO and Oj with Rh. 

The difference in H2 selectivity between Pt and Rh can be explained by the relative instability of the OH species on Rh surfaces. For the H2-O2-H2O reaction system on both and Rh, the elementary reaction steps have been identified and reaction rate parameters have been determined using laser induced fluorescence (LIF) to monitor the formation of OH radicals during hydrogen oxidation and water decomposition at high surface temperatures. These results have been fit to a model based on the mechanism (22). From these LIF experiments, it has been demonstrated that the formation of OH by reaction 10b is much less favorable on Rh than on Pt. This explains why Rh catalysts give significantly higher H2 selectivities than Pt catalysts in our methane oxidation experiments. 

An analysis of the kinetics of CO oxidation by 02 over Rh catalysts based on a model which accounts for the individual elementary reaction steps has been presented by Oh et al. [24]. A significant feature of the model is that the rate expressions used to describe the elementary steps and the rate parameters associated with these steps are all drawn from surface science... 

Catalytic reactions consist of a reaction cycle formed by a series of elementary reaction steps. Hence the rate expression is in general a function of many parameters. In heterogeneously catalysed reactions reactant molecules are adsorbed on the catalyst surface (characterized by equilibrium constants Kj), undergo chemical modifications on the surface to give adsorbed products with rate constants fc, and these products finally desorb. The overall catalyst activity and selectivity is determined by the composition and structure of its surface. Hence it is important to relate constants, such as fc and K with the chemical reactivity of the catalyst surface. 

To date the surface science approach and techniques such as those described above have been used to study structure of ceria surfaces, the adsorption and desorption of several molecular species on ceria and model ceria supported catalysts, and the co-adsorption and reaction of certain of these molecular species. The results provide a basis for clarifying the elementary reaction steps underlying catalytic processes occurring on ceria based catalysts. In this Chapter it is attempted to review and summarize this research. 

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