Heterogeneous-homogeneous reactions steady-state approximation

Even from the very beginning it became clear that kinetic methods generally accepted in heterogeneous catalysis, which are based on the analysis of adsorption-desorption equilibria and surface reactions in the steady-state approximation, cannot be applied to the studies of heterogeneous-homogeneous processes. However, on the eve of 1980s no developed methodology applicable... 

In formulating the kinetic expression for a heterogeneous as well as a homogeneous reaction, this steady state approximation is widely used with regard to the rate of change of extremely reactive intermediates, which are the surface complexes. Mathematical justification of this assumption has been demonstrated for certain simple situations [70, 71]. Thus, on applying the steady state approximation, Eqs. (2.4.30) and (2.4.31) become... 

In this form of voltammetry, the concentration distributions of each species in the electrode reaction mechanism are temporally invariant at each applied potential. This condition applies to a good approximation despite various processes still occurring such as mass transport (e.g. diffusion), heterogeneous electron transfer and homogeneous chemical processes. Theoretically it takes an infinite time to reach the steady state. Thus, in a practical sense steady-state voltammetric experiments are conducted under conditions that approach sufficiently close to the true steady state that the experimental uncertainty of the steady-state value of the parameter being probed (e.g. electrode current) is greater than that associated with not fully reaching the steady state. The... 

It is not possible to cover all of the history or the theory of the chemical kinetics in the context of this chapter. However, the authors intention is to give the student an essential minimum in the theory of chemical kinetics to be able to follow the literature and to incorporate in the design of the chemical reaction units. This chapter is divided into two sections in the first part, the homogeneous kinetics will be covered in detail, covering the collision theory and the transition state theory for the determination of the rate constants and reaction rate expressions. Old but still valid approximations of pseudo-steady-state and pseudoequilibrium concepts will be given with examples. In the second part, the heterogeneous reaction kinetics will be discussed from a mechauis-tic point of view. 

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