Metal Surfaces and Heterogeneous Catalysis

A wide variety of solid surfaces is used as catalysts in an even wider assortment of industrial processes (see, for example, Richardson 1989 and Somorjai 1994) we limit our discussion to metal catalysts. While these represent only a fraction of all catalytic systems, they do include a number of industrially important examples. Table 9.6 lists some metals used as commercial catalysts and indicates briefly the types of reactions for which they are employed. In this section we emphasize the effect on catalytic activity of the chemical and crystallographic properties of metal surfaces. 

It should be emphasized here that there is a vast difference between the microenvironment of the catalyst surface as examined by the type of analytical techniques mentioned in Section 9.1 and the overall surface that influences commercial processes. Until the modern techniques became available, however, catalyst preparation was mostly a matter of trial and error we have now entered an era in which science has a chance to catch up with technology. It seems fairly safe to predict that a greatly increased understanding of heterogeneous catalysis will emerge as modern surface chemistry matures. 

TABLE 9.6 Examples of Some Metal Catalysts and the Reactions They Catalyze 

Haber synthesis of ammonia from N2 and H2 Hydrogenation of vegetable oils Oxidation of hydrocarbons and CO and reduc- 

In our discussion of micellar catalysts in Chapter 8, we noted that effective catalysts have two features the ability to accelerate the rate of a reaction and the ability to do so selectively. Chemistry students are familiar with the general notion that catalysts modify the mechanistic path of a reaction in such a way as to lower the activation energy and make the conversion of reactants to products more probable. One of the easiest places to see this is in reactions of diatomic gas molecules. In the gas phase the mechanism for the reaction of hydrogen and oxygen to form water involves the following steps, among others  

---