Heterogeneous Catalysis of Solution Reactions

The heterogeneous catalysis of gas reactions has been extensively studied and indeed forms the subject matter of three previous volumes (19-21) of Comprehensive Chemical Kinetics. The heterogeneous catalysis of solution ractions has received far less systematic attention. This is surprising since the phenomenon has been known and utilised sporadically for almost 150 years. As long ago as 1845, Millon [1] found that the oxidation of oxalic acid by iodate 

The present review will begin by analysing various steps that can be rate-determining in heterogeneously catalysed solution reactions. These mechanisms can be distinguished in practice by the resulting kinetic behaviour and by other means that will be described. General stoichiometric and thermodynamic aspects will then be discussed. The later parts of this chapter will be devoted to a detailed survey of the specific types of catalysed reaction (substitution, isomerisation and redox) which have been studied in the literature. 

The present chapter is concerned only with catalysis at the solid/liquid interface and will not deal with microheterogeneous catalysis by enzymes, micelles and polyelectrolytes even though the resulting kinetics are closely similar [4], Moreover, little reference will be made to catalytic processes involving gases as these have been the subject of Vols. 19-21 of this series, nor to catalytic polymerisations which have been treated in Vols. 14, 14A, and 15. 

The function on the left-hand side is plotted against t to yield a value for k. Other correction equations can easily be developed to fit different circumstances. 

Heterogeneous catalysis must of necessity involve interaction between the surface and at least one of the reactants. The catalytic process therefore involves five distinct steps [8, 9]  

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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 ... 

The topics covered are as follows. The structure of the interfacial region and its experimental investigation are covered in Chapter 1. The following chapter reviews the mechanisms by which heterogeneous catalysis of solution reactions can take place. The third chapter is concerned with the mechanism and kinetics of crystal growth from solution and the final contribution deals with corrosion processes at the metal-solution interface. 

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