Abstract: Users of Langmuir-Hinshelwood kinetics often neglect that it invokes the pseudo-steady state hypothesis (PSSH) for all adsorbed species, and they do not check that the PSSH is validly used. However, with solid catalyzed reactions, the validity of the PSSH must often be based on the concentration of active sites being very small. With a high site concentration catalyst, the PSSH would be incorrect for adsorbates with significant coverages, which would make the deduced kinetic parameters lose their physical meaning and be only the parameters of curve fitting. It is estimated that many catalysts do not meet the criterion for their site concentration to be less than one-tenth the highest reactant concentration.

Key words: Langmuir adsorption isotherm, Langmuir-Hinshelwood kinetics, adsorbed species, pseudo-steady state hypothesis, rate-determining step, microkinetics