In chemistry, a catalyst is a substance that is added to speed the rate of a reaction without itself being consumed in the process. The catalyst often is added in a small quantity compared with the reactants and might be reused sequentially in future batches. A homogeneous catalyst is one that is part of the same phase as the reactants — whether solid, liquid or gas — during reaction. Often, the components are of different phases in the pure state but are dissolved in a common solvent. Under this definition, even a gas reacting with a liquid in the presence of an ordinarily solid catalyst is homogeneous if the three are all dissolved at the time.
Most industrial catalysis employs heterogeneous catalysts. In heterogeneous catalysis, there are two or more phases to the reaction, and it frequently involves a liquid or gas component exposed to a solid catalytic component attached to a carrier substrate for convenience and to prevent loss. This might be because the catalyst is costly, including a precious metal component. To increase efficiency, surface area may be maximized with the catalyst being very finally divided. One example is the catalytic converter found in most automobiles.
The use of a homogeneous catalyst in complex chemistry is of special interest, in part because of novel uses of organometallic complexes. Early applications of organomagnesium and organolithium compounds were largely as reaction ingredients, rather than as catalysts. Such compounds were unstable; their use required dissolving them in hazardous solvents such as ether or tetrahydrofuran (THF). Combining these with other liquid reactants placed those reactions, by definition, in the homogeneous category.
Today, there are far more organometallic compounds known. Some of them can be placed in the category of a homogeneous catalyst. They are often more stable and easier to handle. This type of compound affords a broader scope of uses and is often used as a homogeneous catalyst, rather than as a reactant.
Some of the new reagents are useful in polymerization reactions. Others are well suited for pharmaceutical manufacture because of their ability to impart chirality. This refers to the ability to control structural design so closely that polarized light rotates only one way.
A most noteworthy application is the attempt at mimicking the plant world by means of artificial photosynthesis. This is not to be confused another use of the term: the splitting of water into hydrogen for the production of fuel. Rather, artificial photosynthesis in this case refers to the conversion of carbon dioxide and water into carbohydrates and oxygen. For some years, organometallic catalysts have been studied with artificial photosynthesis in mind.