Kinetics and Reduction Mechanisms

The aim of the analysis of the TPR/TPO data is to derive kinetic parameters relating to the reduction/oxidation process. It is common to observe the reduced/oxidized fraction (o ) as a function of time for various temperatures and pressures/concentrations of reducing/oxidant gas. The reduction/oxidation of a sample with redox property or of part of it (in the case of supported phase system) is a bulk phenomenon and its degree of reduction/oxidation is interpreted in terms of mechanism by which the reaction occurs. Nucleation model or contracting sphere model are the most successful utilized kinetic models [2]. 

In synthesis, at constant temperature, the rate of the reaction MOx 4- H2 reduced products, may be expressed as  

If the rate constant k is expressed with its dependence on temperature by the classical Arrhenius equation (with A, the pre-exponential factor andEa, the activation energy) and the linear dependence of temperature on time is introduced by the linear 

If the H2 consumed is little (in accord with the criteria above cited in Sect. 5.2.3, the reaction rate can be considered independent of H2 concentration and Eq.(5.7) can be rewritten as  

The problem is now to define the correct expression for the variation of a versus t to solve Eq. (5.11) that is to describe the reduction process by a suitable model. 

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