Surface Catalysis

Many industrial chemical reactions are carried out in the presence of solid catalyst e.g., Fe-catalyzed synthesis of NH₃ from N₂ and H₂, SiO₂/Al₂O₃ catalyzed cracking of high molecular weight hydrocarbons to gasoline. Reactions involving catalysts of a phase different from reactants are known as heterogeneous catalysis. Besides other usual applications, heterogeneous catalysis is extremely important and will be discussed in detail.

Heterogeneous catalysis, like its homogeneous counterpart, changes the rate of a reaction by providing an alternative reaction mechanism. Solid catalysts lower activation energies by much greater extend than homogeneous catalysts. For the reaction, 2HI I₂ + H₂ without catalyst, the activation energy is 44 kcals/mol; 25 kcals/mol when catalyzed by Au, and is only 14 kcals/mol when catalyzed by Pt. Uncatalyzed reaction 2H₂O (aq) 2H₂ + O₂ has activation energy 17 kcal/mol, 12 kcal/mol when catalyzed by colloidal Pt and only 2 kcal/mol when catalyzed by enzyme catalase.

Similarly, decomposition of N₂O in gas phase without a catalyst has an activation energy 250 kJ/mol, and with catalysts Au and Pt, is about 125 kJ/mol.

The substances most often studied and used as heterogeneous catalysts are transition metals, alloys and semiconducting oxides and sulfides. The effectiveness of a catalyst can be measured by the amount of product formed per unit time per unit surface area of the catalyst. Activity of a catalyst depends on the nature of active sites present on the surface e.g., point defects, lattice distortions of % d-character of the metallic bond within the solid metal. A catalyst with a different surface property may lead to the formation of a different product from the same starting reactant. For example, isopropanol undergoes dehydrogenation on a surface of ZnO while dehydration occurs on Al ₂O₃(s). ZnO is n-type semiconductor whereas Al₂O₃ is an ionic insulator and acts as a Lewis acid. The probable mechanism may be given as follows:

In a surface catalyzed reaction, activation energy is normally lowered due to adsorption of reacting molecules on the surface of the catalyst. After adsorption, surface molecules are not allowed to perform translational motion but are able to perform vibrational motion.