Chemical reaction kinetics difference between heterogeneous

It was concluded on the basis of carbonylation reaction kinetics measurements (26) that in spite of the obvious physical differences between heterogeneous (3% Rh/C) and homogeneous (RhCl3 /CH3I) catalysts (5), a similar chemical reaction mechanism is probably operative (Fig. 1). 

Electrode reactions are heterogeneous chemical reactions in which stoichiometric transfer of electric charges takes place between the electrode and the electrolyte. The kinetics of such reactions depend not only on concentration and chemical structure, but also on the electrical conditions in and near the phase boundary. Semiconductor and metal electrodes present very different electrical conditions. Here we shall discuss current-potential curves which show the significance of these differences with respect to the mechanism of electrode reactions. 

In the above derivation a power-law kinetic expression was assumed for the intrinsic chemical reaction. For the systems where large concentration differences occur between the bulk fluid and the interior of the porous solid, the Langmuir-Hinshelwood type rate expression should be used because this provides a better description of the rate of heterogeneous reactions. 

Heterogeneously catalyzed reactions are usually studied under steady-state conditions. There are some disadvantages to this method. Kinetic equations found in steady-state experiments may be inappropriate for a quantitative description of the dynamic reactor behavior with a characteristic time of the order of or lower than the chemical response time (l/kA for a first-order reaction). For rapid transient processes the relationship between the concentrations in the fluid and solid phases is different from those in the steady-state, due to the finite rate of the adsorption-desorption processes. A second disadvantage is that these experiments do not provide information on adsorption-desorption processes and on the formation of intermediates on the surface, which is needed for the validation of kinetic models. For complex reaction systems, where a large number of rival reaction models and potential model candidates exist, this give rise to difficulties in model discrimination. 

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