Catalysis is essential in chemical reactions because it speeds up the reaction. Catalysts are those substances that are added to the chemical reaction in order to accelerate it. There are two forms of catalysis based on the catalysts involved. The first one is referred to as homogeneous catalysis because of the fact that the catalyst and the reactants are of the same physical state. That means the reactants that are in liquid phase include a catalyst that is also in a liquid state. This is exemplified by organocatalysis and acid catalysis. In contrast, the heterogeneous catalysis is a form of catalysis in which the reactants and the catalysts in a chemical reaction have different physical states. For instance, the catalyst may be a solid agent that speeds up the chemical reaction comprised of reactants in a liquid or gaseous phase.1 A specific example is sulfuric acid synthesis, i.e. SO2 + O2, SO3 are the reactants whereas vanadium oxides act as the catalyst that speeds up the hydration of SO3 producing H 2 SO 4 .
1 Gadi Rothenberg, Catalysis: Concepts and green applications, Wiley-VCH: Weinheim.