Heterogeneous catalysis rate-determining step

Using the concept of the rate-determining step significantly simplifies the overall rate expression. Therefore, it is widely used in the analysis of kinetie data, especially in the field of heterogeneous catalysis. 

Vijh (6) has suggested more recently that if one assumes the adsorbed species formed in this reaction to be a covalent one, the available data can be interpreted in terms of the Sabatier-Balandin views on heterogeneous catalysis. According to his interpretation, he has indicated a volcanic relationship between the catalytic activity (defind as the temperature at which the reaction first becomes appreciable) against the heat of formation per equivalent of the oxide catalyst, AHe. Based on this volcanic relationship, he has concluded that the rate-determining step (r.d.s.) of the reaction on the oxide catalysts such as CiO, NiO and CoO probably involves rupture of a M-0 bond. On the other hand, r.d.s. on oxides such as MgO,CaO and Ce02 would involve the formation of a M-0 bond. 

In general, the rate (A ) of heterogeneous catalysis in a gas-catalyst (and liquid-catalyst) reaction may be expressed as the product of the rate coefficient A o and a function of pressure (or concentration), p, i.e. k = kof(p) where p is the partial pressure of the reactant, ko depends on the reaction conditions and may involve reaction steps prior to the first rate-determining step of the reaction. A convenient method for determining A o is to use the Arrhenius equation ... 

Thus, the analysis of the rate-determining step, as analyzed for heterogeneous processes in Section 3.1.2, is equally applied in adsorption and ion exchange. The only difference is that the diffusion processes in the fluid film and in the particle are followed by physical adsoiption or ion exchange and not by a reaction step as in catalysis. 

The Rate-Determining Step. Determination of the step that decides the overall rate in a series of consecutive or parallel reactions in heterogeneous catalysis is the most significant part of mechanism determination. It is best to deal with the ideas here in a general way they will be exemplified in three reactions later on in the section. 

In surface-controlled catalyses, the rate-determining step involves the reaction on the surface of an adsorbed reactant or of a derived species. The resultant kinetics may be more or less complicated depending upon the circumstances, and treatments of various cases have been given in reviews dealing with the heterogeneous catalysis of gas reactions [9, 31, 42]. Several... 

Mass transport is much more likely to be rate-controlling in the heterogeneous catalysis of solution reactions than in that of gas reactions. The reason lies in the magnitudes of the respective diffusion coefficients [48] for molecules in normal gases at 1 bar and 300 K these are 10 5 to 10 4 m2s while, for typical solutes in aqueous solution, they are 10 10 to 10 9 m2 s. The rate-determining step in many solution catalyses has indeed been found to be external diffusion of reactant(s) to the outer surface of the catalyst and/or diffusion of product(s) away from it [3, 6]. Another possibility is internal diffusion within the pores of the catalytic solid, a step that often determines the rates of catalysed gas reactions [49-51]. It is clearly an essential part of a kinetic investigation to ascertain whether any of these steps control the rate of the overall catalytic process. Five main diagnostic criteria have been employed for this purpose ... 

Hence it has been concluded that the rate-determining step involves removal of the two phosphorus-bound protons. The energy of activation (24 kcal.mole ) decreases with increasing alkalinity, probably due only to a salt effect. The decomposition has also been examined in the presence of nickel and cobalt, which apparently introduce heterogeneous catalysis . 

In a series of papers, Suvorov et al. investigated heterogeneous catalysis of the cyclization of isolated aldehyde and ketone phenylhydrazones. y-Alumina was typically employed as catalyst in the vapor phase reaction at atmospheric pressure and at temperatures around 300 °C. A maximum yield of 60% was obtained from acetaldehyde phenylhydrazone as a result of thermal decomposition of the hydrazone [7] and the formation of benzene and aniline as by-products [8]. Kinetic studies indicated that the rate-determining step was desorption of product from the surface [9]. 

In Chapters 3 and 4 the Langmuir-Hinshelwood-Hougen-Watson approach to heterogeneous catalysis was discussed. Such an approach supposes that usually there is one rate determining step (adsorption, surface reaction or desorption) and that the other steps are in quasi-equilibria. 

Like homogeneous catalysis, the removal of a-hydrogen of the amino acid fragment by OH ions, the local concentration of which is apparently high in the polymer phase, is probably the rate-determining step of heterogeneous racemization. Under similar conditions, the rate of a-amino acid racemization decreases in the sequence Ala = Ser>Phe>Nva>Lys>Val, and correlates with the rate of substrate racemization in the presence of Schiff bases and transamination of amino acids by pyridoxal phosphate. 

The concept of a rate-determining step (RDS) is common and useful in heterogeneous catalysis. It has been used as an a priori assumption in the development of reduced rate expressions from experimental data [4]. Knowledge of the RDS can provide insights into how to improve a catalyst. The definition of the RDS has been discussed in the past two decades [51, 52]. Considering the definition proposed by Campbell [51], the degree of rate control by an elementary step is... 

Several factors can influence reaction rates. Concentration and reaction rate are related. niiaui Key Terms reaction rate chemical kinetics heterogeneous reaction lull nai catalysis homogeneous catalyst heterogeneous catalyst LC order rate-determining step... 

Out of the above four steps, steps (i) and (iii) are rapid and normally do not play any part in the overall rate determination of reaction. Mechanism of heterogeneous catalysis is discussed in Section 6.9. 

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