Rate expression, adsorption limiting Langmuir-Hinshelwood

Based on the Langmuir-Hinshelwood expression derived for a unimolecular reaction system (6) Rate =k Ks (substrate) /[I + Ks (substrate)], Table 3 shows boththe apparent kinetic rate and the substrate concentration were used to fit against the model. Results show that the initial rate is zero-order in substrate and first order in hydrogen concentration. In the case of the Schiff s base hydrogenation, limited aldehyde adsorption on the surface was assumed in this analysis. Table 3 shows a comparison of the adsorption equilibrium and the rate constant used for evaluating the catalytic surface. 

In kinetic studies of the hydrogenation of aromatic hydrocarbons, the dependence of rate upon reactant pressures has usually been expressed in Power Rate Law formulations, that is, by orders of reaction that are simple exponents of the pressures. These as we have seen (Section 5.2) are at best approximations to more fundamental expressions based on concentrations of adsorbed species," " although they may well represent results over the limited range in which measurements were made. The Langmuir-Hinshelwood formalism has however sometimes been used, and heats of adsorption of the reactants in their reactive states derived from the temperature-dependence of their adsorption coefflcients. ... 

Many catalyzed surface reactions can be treated as a two-step process with an adsorption equilibrium followed by one rate-determining step (diffusion, surface reaction, or desorption). The surface reaction kinetics are usually discussed in terms of two limiting mechanisms, the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. In the LH mechanism, reaction takes place directly between species which are chemically bonded (chemisorbed) on the surface. For a bimolecular LH surface reaction. Aawith competitive chemisorption of the reactants, the rate of reaction is given by the following expression ... 

The power law expression was widely adopted in the literature for CO oxidation [25-27]. This form is simplified from a Langmuir-Hinshelwood (L-H) expression and not suitable for small CO concentrations [30]. Therefore a full L-H expression for CO oxidation is necessary to account for a wide range of CO concentrations (Equation 27.4). The H2 oxidation was previously modeled using empirical power law rate expressions by others [29]. However, in PrOx in the presence of CO, the rate-limiting CO desorption strongly inhibits H2 and O2 adsorption and the subsequent H2 oxidation. Hence the incorporation of Pco in the H2 oxidation rate expression is necessary (Equation 27.5). The kinetics of the r-WGS reaction were well studied previously [31], in which an empirical reversible rate expression [32] is attractive due to its relative simplicity and its appropriateness in PrOx kinetic studies, as demonstrated previously [29]. 

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