Catalytic mechanism adsorption-controlled reactions

In the presence of an oxidant, e.g., chlorate or bromate ions, the electrode reaction is transposed into an adsorption coupled regenerative catalytic mechanism. Figure 2.85 depicts the dependence of the azobenzene net peak current with the concentration of the chlorate ions used as an oxidant. Different curves in Fig. 2.85 correspond to different adsorption strength of the redox couple that is controlled by the content of acetonitrile in the aqueous electrolyte. In most of the cases, parabolic curves have been obtained, in agreement with the theoretically predicted effect for the surface catalytic reaction shown in Fig. 2.81. In a medium containing 50% (v/v) acetonitrile (curve 5 in Fig. 2.85) the current dramatically increases, confirming that moderate adsorption provides the best conditions for analytical application. 

Consider the heterogeneous catalytic mechanism described by (14-124) where adsorption of reactant A on a single active surface site is rate controlling in the five-step mechanism. Qualitatively outline a sequence of experiments and calculations that will allow you to determine the equilibrium constant for the chemical reaction on the catalytic surface ... 

Catalysis is the study of materials that can accelerate reactirms and control reaction mechanisms. After a catalytic cycle, namely adsorption of reactants, reactions and desorption of products, the catalyst is restored to its initial state. An ideal catalyst... 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (/ ), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium. 

The Pd-O bond also varies with the extent of oxidation of Pd. During the methane combustion reaction, the catalyst surface is a non-equilibrium, kineti-cally controlled structure. The oxygen concentration profile in the particle results from a combination of particle reconstruction, oxygen adsorption, bulk diffusion, and oxygen removal. This concentration profile varies as a function of time, and as the oxygen content increases, the Pd-O bond strength decreases. This increase is accompanied by an increase in the specific activity. The most widely accepted reaction pathway is the Mars and van Krevelen redox mechanism, which involves lattice oxygen and uneoordinated Pd centers as active species. Inhibition by products (H2O and CO2) and impurities (SO2) is a major drawback for low temperature combustion. The effect of sulfur is particularly important for catalytic converters for NGV applications because it drastically reduces the methane combustion activity. 

Answer In agreement with statements (1) and (2), it is reasonable to propose the following four-step mechanism. Since A2 requires two sites for adsorption after dissociation, and B requires one site, the catalytic reaction between one molecule of A2 and two molecules of B involves four sites. Hence, the quadruple-site chemical-reaction-rate controlling pathway is... 

The feed stream is stoichiometric in terms of the two reactants. Diatomic A2 undergoes dissociative adsorption. Components B, C, and D experience single-site adsorption, and triple-site chemical reaction on the catalytic surface is the rate-controlling feature of the overall irreversible process. This Langmuir-Hinshelwood mechanism produces the following Hougen-Watson kinetic model for the rate of reaction with units of moles per area per time ... 

Reactant equilibrium constants Kp and affect the forward kinetic rate constant, and all Ki s affect die adsorption terms in the denominator of the Hougen-Watson rate law via the 0, parameters defined on page 493. However, the forward kinetic rate constant does not appear explicitly in the dimensionless simulations because it is accounted for in Ihe numerator of the Damkohler number, and is chosen independently to initiate the calculations. Hence, simulations performed at larger adsorption/desorption equilibrium constants and the same intrapellet Damkohler number implicitly require that the forward kinetic rate constant must decrease to offset the increase in reactant equilibrium constants. The vacant-site fraction on the internal catalytic surface decreases when adsorption/desorption equilibrium constants increase. The forward rate of reaction for the triple-site reaction-controlled Langmuir-Hinshelwood mechanism described on page 491 is proportional to the third power of the vacant-site fraction. Consequently, larger T, s at lower temperature decrease the rate of reactant consumption and could produce reaction-controlled conditions. This is evident in Table 19-3, because the... 

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