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Trends in Chemical Engineering   Volumes    Volume 3 
Multiplicity of reaction-path hidden behind the apparent first order kinetics of heterogeneous catalysis
Masayoshi Kobayashi, Tohru Kanno, Junya Togawa
Pages: 145 - 157
Number of pages: 13
Trends in Chemical Engineering
Volume 3 

Copyright © 1996 Research Trends. All rights reserved

The validity of first order kinetic approximation in heterogeneous catalysis is evaluated by using the transient response method. As an example of solid catalyzed reactions, the oxidation of carbon monoxide is studied of four different metal oxides MnO2, Cr2O3, PbO4, and ZnO The steady state rate analysis apparently meets a first order kinetics with respect to the concentration of CO for all the catalysts. The Arrhenius plots of the apparent first order rate constants obtained clearly give a good straight line and estimates the apparent activation energy of the reaction on each catalyst. Two different models, a simple first order kinetic model (FM) due to surface reaction controlling and a transient state model (TM), are proposed to explain the transient state and the steady state of reaction. Based on the models proposed, the transient response curves of carbon dioxide caused by the stepwise change in the concentration of CO are simulated by using personal computers. The first order kinetic model exhibits a large deviation of the calculated curves from the experimental ones especially at the transient state whereas the steady state values of the both are in good agreement. For the transient state model, four different dual path models have separately been proposed for each of the four catalysts. The transient state model can consistently explain the characteristic mode of the transient response curves obtained for all the catalysts and under wide experimental conditions. The activation energy of the slowest elementary step is evaluated for all catalysts, and some of the values deviate largely from those estimated from the apparent first order kinetic model. It is clearly shown that the turnover frequency of the catalysts gives extraordinary difference depending on the models used, diverging from 0.035 to 2.7 of the ratio FM/TM.
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