ABSTRACT
A catalyst is a chemical compound that has the ability to enhance the rate of a chemical reaction without being consumed by the reaction. The catalytic effect was discovered by the famous British physician, Michael Faraday, who observed that the presence of a metal powder enhanced oxidation reactions. The catalytic effect was first described by the Swedish chemist, Jöns Jacob Berzelius, in 1836. The Baltic chemist and the Nobel Prize winner, Wilhelm Ostwald, described homogeneous, heterogeneous, and enzymatic catalysis separately-a definition that is valid even today. Homogeneous catalysts, such as organic and inorganic acids and bases, as well as metal complexes, are dissolved in reaction media. They enhance the reaction rate, but their effect is limited to the reaction kinetics. Heterogeneous catalysts, on the other hand, form a separate (solid) phase in a chemical process. Mass and heat transfer effects thus often become prominent when using heterogeneous catalysts. Heterogeneous catalysts are typically metals and metal oxides, but many other materials have been found to possess a catalytic effect (e.g., zeolites and other alumina silicates). Heterogeneous catalysts are the dominant ones in the chemical industry, mainly because they are easier to recover and recycle. Enzymes are highly specific catalysts of biochemical systems. They are, for instance, used in the manufacture of alimentary products and pharmaceuticals. Although enzymes are homogeneous catalysts, they can be immobilized by several methods on solid materials, and a heterogeneous catalyst is thus obtained.
