Rate expression, adsorption limiting reversible reaction

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]. 

The basic premise of the original kinetic description of inhibition was that, for a reaction to proceed on a surface, one or more of the reactants (A) must be adsorbed on that surface in reversible equilibrium with the external solution, having an equilibrium adsorption constant of KA, and the adsorbed species must undergo some transformation involving one or more adsorbed intermediates (n) in the rate-limiting step, which leads to product formation. The product must desorb for the reaction cycle to be complete. If other species in the reaction mixture (I) can compete for the same adsorption site, the concentration of the adsorbed reactant (Aad) on the surface will be lower than when only pure reactant A is present. Thus, the rate of conversion will depend on the fraction of the adsorption sites covered by the reactant (0A) rather than the actual concentration of the reactant in solution, and the observed rate coefficient (fcobs) will be different from the true rate coefficient (ktme). In its simplest form the kinetic expression for this phenomenon in a first-order reaction can be described as follows ... 

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