Catalysis: Types Of Catalysts

A substance which when used in small amount in reactions modifies the rate of a reaction without being consumed in the reaction itself is known as a catalyst and this phenomenon is known as Catalysis. By using catalyst the rate of reaction is increased and we require lesser activation energy.



Types of catalyst:

This process can be divided into following classes:

  • Homogeneous catalysis: If the phase of this process and the reactant is same, then the catalyst is known as the homogeneous catalyst and the phenomenon is known as a homogeneous catalytic process. For example:

When CO is oxidized CO2 by O2 in the presence of NO as a catalyst,

i.e.,   \( 2CO ~(g) ~~+~~ O_2~ (g)~~~\underrightarrow{NO~(g)}~~~ 2CO_~(g)\)

  • Heterogeneous catalysis: If the phase of the reactant and the catalyst is not same, then that type of catalyst is known as a heterogeneous catalyst. In general, the catalyst is in solid state and the reactant is in the gaseous state.

For example Hydrogenation of ethylene (unsaturated hydrocarbon) in the presence of catalysts like Ni, Pd, and Pt at 250⁰C. This is an example of heterogeneous catalytic reaction

  • Positive catalysis: When the rate of reaction is increased by the external use of other substances, then this phenomenon is known as positive catalytic process and the substance is known as a positive catalyst. For example, Ammonia is synthesized from N2 and H2 using iron as a positive catalyst and molybdenum as a promoter.

\(N_2~(g)~ ~+~~ 3H_2~(g) ~~~\underrightarrow{Fe(s), MO}~~~  2NH_3~ (g)\)

  • Negative catalysis: A substance that reduces the rate of a reaction without undergoing a chemical reaction is known as a negative catalyst. And the phenomenon is known as the negative process. For example Presence of glycerin in the decomposition of hydrogen peroxide.

\(2H_2O_2~(l)~~~\underrightarrow{Glycerin~ (l)}~~~ 2H_2O ~(l)~~ +~~ O_2~(g)\)

  • Auto catalysis: In some types of reaction the product itself acts as a catalyst and alters the rate of reaction. This kind of substance is known as an auto catalyst and the phenomenon is known as the auto catalysis.

For example: In the hydrolysis of ethyl acetate, ethyl alcohol and acetic acid is formed. The formation of acetic acid increases the rate of reaction.

\(CH_3COOC_2H_5~~ +~~   H_2O~ ~~ \rightarrow~~~ CH_3COOH ~~  +~ ~  C_2H_5OH\)

  • Induced catalysis: When some reactions do not occur under normal condition but it occurs under the influence of another reaction then this phenomenon is known as an induced mechanism.

This was just a brief layout about the process and the different types of catalytic process. To know more about the catalysis, its type in detail and its various theories, please visit BYJUS.’

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