The catalyst is a substance that accelerates the reaction rate of chemical reactions at certain temperatures, without alteration or use by the reaction itself. Catalysts play a role in the reaction but not as a reactant or product.

The catalyst allows the reaction go faster or allow reaction at lower temperatures due to the changes triggered against reagent. The catalyst provides a path option with lower activation energy. Catalysts reduce the energy required for the course of the reaction. 

The catalyst can be divided into two main categories: homogeneous catalysts and heterogeneous catalysts. Heterogeneous catalysts are catalysts that exist in different phases of the reactants in the reaction, whereas homogeneous catalysts are in the same phase. 

One simple example to heterogeneous catalysis is that the catalyst provides a surface on which the reagents (or substrate) for a while. Bond in the substrates to be weak enough such that the formation of new products. The bond between the product and the catalyst becomes weaker, so that eventually detached. 

Homogeneous catalysts are generally reacted with one or more reactants to form a chemical intermediate that subsequently reacts to form the final product of the reaction, in a process to recover the catalyst. 

The following is a general scheme of the catalytic reaction, where C represents the catalyst:

A + C → AC (1) 
B + AC → AB + C (2) 

Although the catalyst (C) consumed by reaction 1, but subsequently regenerated by the reaction of 2, so for the overall reaction becomes, 

A + B + C → AB + C 

Catalyst is not consumed or created. The enzyme is a biocatalyst. Use of the term "catalyst" in a broader cultural context is analogous to this context. 

Some well-known catalysts that had developed among Ziegler-Natta catalysts are used for mass production of polyethylene and polypropylene. The most well known catalytic reactions are the Haber process for ammonia synthesis, which uses ordinary iron as a catalyst.

Source from: Wikipedia

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