Microkinetic model reversible reactions

Waugh [26], we thus assume an immobile transition state without rotation for all of the species, which results in a pre-exponential factor of 10 Pa s for adsorption/desorption reactions, and s for surface reactions. The resulting microkinetic model for the WGS reaction is summarized in Table 1. Fine-tuning of some of the pre-exponential factors of the adsorption/desorption reactions, however, was necessary in order to be consistent with the known thermodynamics of the overall reaction, i.e., for a RR, the activation energies of the forward and reverse reactions are related to the enthalpy change via... 

Microkinetic models are much more widely apphcable than LHHW traditional models that assume an RDS, since the RDS can change with reaction conditions. Because aU postulated elementary steps are included exphcidy, accurate rate parameters for aU of the forward and reverse reactions are needed to solve the equations comprising the model. This requirement gready increases the amount of information needed to create a microkinetic model, but this is also the power of the technique. Often, the information needed to create a microkinetic model cannot be determined from one set of experiments but requires the compilation of information from many different experiments and theoretical investigations (Fig. 3.24). 

The models developed here account for unmeasurable intermediates such as adsorbed ions or free radicals. Microkinetic analysis, pioneered by Dumesic and cowokers"", is an example of this approach. It quantifies catalytic reactions in terms of the kinetics of elementary surface reactions. This is done by estimating the gas-phase rate constants from transition state theory and adjusting these constants for surface reactions. For instance, isobutane cracking over zeolite Y-based FCC catalysts has 21 reversible steps defined by 60 kinetic parameters." The rate constants are estimated from transition state theory... 

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