An introduction to heterogeneous catalysis

Because of page constraints, this section must be limited to only a handful of the many important processes in which heterogeneous catalysis plays an indispensable role. The coverage is merely intended as an introduction to the general features, e.g., feedstock(s), product(s), catalyst(s), and operating conditions. In addition to references cited under specific topics. Pines book deals with a wide variety of reactions. ... 

For this, hydrolytic enzymes will be used to present the concept of the active site. However, an introduction to the general concepts of catalysis, which are based on transition state theory is needed first. Proximity and orientation of chemical groups will also be illustrated as factors responsible for the magnitude of enzyme catalysis. This will eventually allow the bridging of nonen-zymatic heterogeneous catalysis and enzymatic catalysis. 

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

The present volume continues to provide an entire spectrum of interdisciplinary exposures to catalysis. As stated in the introduction to the first chapter by R. J. Madix, heterogeneous catalysis is a complex phenomenon to understand at the molecular level, and the key to understanding such processes lies in the ability to dissect the catalytic event into its separate components. This chapter describes physical and spectroscopic approaches to make the explanation of a variety of catalytic reactions on clean metal surface possible. 

To conclude this brief introduction, it appears that there may be some relation between semiconductivity and chemisorption on oxide semiconductors. In catalysis a much more complex situation is expected and generalizations will be hard to come by. Indeed, it is well known that chemisorption is a necessary condition in heterogeneous catalysis but it is by no means a sufficient one. Furthermore, as was illustrated above, a given molecule can be chemisorbed in different ways on an oxide surface, and it will prove necessary to decide in each particular case which mode of adsorption is important in a catalytic process. 

---