Catalysts are chemical compounds that can increase the rate of a reaction by lowering the activation energy required to reach the transition state. Unlike reactants, a catalyst is not consumed as part of the reaction process.
Catalysts can be divided into two types depending on the reaction phase that they occupy: homogeneous and heterogeneous. Homogeneous catalysts are those that occupy the same phase as the reaction mixture (typically liquid or gas), while heterogeneous catalysts occupy a different phase. In general, heterogeneous catalysts are solid compounds that are mixed with liquid or gas reaction mixtures. Figure 1 shows an example of heterogeneous catalysis involving the absorption of ethylene onto a solid catalyst followed by hydrogenation of the double bond to yield ethane:
In general, heterogeneous catalysis requires the diffusion of one or more reactants to the catalyst surface, followed by the absorption onto a solid catalyst. The use of heterogeneous catalysts has both advantages and disadvantages. On one hand, diffusion to a binding site on a solid catalyst can often be the rate-limiting step in a reaction catalyzed by a heterogeneous catalyst. In addition, the available surface area of the heterogeneous catalyst will also affect the reaction rate, as it will dictate and potentially limit the number of reaction sites that are available. On the other hand, heterogeneous catalysis does have a number of benefits. Heterogeneous catalysts can be separated from a reaction mixture in a straightforward manner, such as by filtration. In this way, expensive catalysts can be easily and effectively recovered, which is an important consideration for industrial manufacturing processes.