Pure modes with interface rate determining step

Pure modes with interface rate determining step 

Kinetic laws due to interface rate determining steps 

We can now apply the method of resolution of the pure kinetics, with one of the preceding reactions as the rate determining step in pseudo-steady state mode. For each type of solid MG, we will obtain three solutions according to whether the determining step is adsorption, the reaction at internal interface i or at the external interface e (we exclude the modes limited by the heart reaction that we have never encountered). Table 15.4 provides the results obtained in conditions far from equilibrium. To obtain the expression of the reactivity in the opposite case, closed to equilibrium conditions, we have to multiply the preceding relations by the term  

Remark.- Very often the experimental conditions are very far away from the equilibrium conditions. Thus, for example, at 1,000°C, the equilibrium pressures for nickel oxide and copper oxide are 10 and 10 atm, respectively, if the studies are undertaken for usual atmospheric pressures. We are obviously veiy far from equilibrium. 

The variations with tenperature are included in the rate constants that follow the Arrhenius law and the equilibrium constants that obey the vant Hoff law. 

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If we irutially consider the growth reaction of ABO2, assuming that, for one reason or another (one-process model with instantaneous nucleation or fast smface diffusion), interface AO/BO is immediately entirely covered with a fine layer of ABO2, it is possible to determine the kinetic laws relating to the pure modes that will be either with a constant rate if an interface reaction is the rate determining step or a function of time if diffusion is the rate determining step. 

Table 15.4. Reactivities in pure mode with adsorption or chemical interface reaction as the rate determining step...

We will only use pure mode with internal interface as rate-determining step and inward development. These modes are most frequent because the concerned reactions do not often use matter transport for growth. 

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