An Examination of the Langmuir-Hinshelwood Model Using Ion Exchange Catalysts

Abstract

The most frequently discussed question in the kinetics of heterogeneous catalytic reactions is whether, in deriving rate equations, the active centers of a catalyst should be regarded as uniform or nonuniform. The literature (cf. Refs. 1–3) shows that the kinetics of heterogeneous catalytic reactions is most frequently described by some tape of Langmuir-Hinshelwood rate equations (LHRE) which are based on the assumption of uniformity of active centers (the so-called ideal surface). In most cases these equations fit the experimental data very well. This, however, cannot be considered to prove their theoretical validity, since the agreement may result from the ability of the equations to successfully approximate more complex kinetics. The objection against the use of LHRE stems from the fact that for a majority of catalytically active solid substances, the nonideality of the surface, i.e. the nonuniformity of adsorption sites, has been established by adsorption measurements. On the other hand (cf. Refs. 2 and 4)it is assumed that of a variety of adsorptionally active sites, only a small fraction, representing quasi-ideal surfaces, might be active in a given reaction. As with most catalysts, we have no possibility of verifying whether catalytically (not adsorptionally) active sites are equivalent or not, and thus the controversy remains unresolved.