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A catalyst was defined by J. J. Berzelius in 1836 as a compound, which accelerates the rate of a chemical reaction and is itself unchanged and not consumed by the overall reaction.

The definition allows for the possibility that minor amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly lost.

A catalyst does not make the reaction happen. The presence of a catalyst provides a lower energy pathway between the reactants and the products. The catalyst lowers the energy of the transition state of an intermediate which cannot be formed without the catalyst. However, it does not change the free energy of the chemical reaction.

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In silicone chemistry two types of catalysts are commonly used: Heterogenous and homogenous catalysts.

Heterogeneous Catalysts 

In heterogeneous catalysis, the catalyst is in a different phase than the reactants and the products. Usually, the catalyst is a solid and the reactants and products are liquids or gases.

The copper-based catalyst of the Müller-Rochow synthesis operates on the reaction of elemental silicon and methylchloride which takes place on the catalyst's surface. In the heterogenous catalysis, the acting species are held on the surface of the catalyst by a physical attraction called adsorption while the reaction takes place. The adsorption occurs primarily at certain favourable locations called active sites. After reaction, the formed methyl chlorosilane products must desorb from the catalyst's surface and diffuse away.

Homogenous Catalysts

The catalyst and the reactants are in a homogenous phase, usually liquid. It frequently happens that the concentration of the catalyst intervenes in the rate law, which demonstrates the influence of the catalyst on the reaction mechanism. However, its formula does not appear in the chemical equation.

Catalysts, which are commonly used in silicone chemistry are platinum, tin and titanium complexes.