TYPES OF CATALYST: Positive, Negative, Induced & Auto-Catalyst

A catalyst is any substance (or material), when it is added in the reaction then due to their presence in the reaction, the speed of the reaction is increased. And this reaction in which a catalyst is present, is called as catalytic reaction. This phenomena is called "Catalysis". 

Types Of Catalysts

Several types of catalysts are there which we used in a chemical reaction based on their need and requirement. These are;

(1) Positive catalysts 

(2) Negative catalysts

(3) Induced catalyst 

(4) Auto-catalyst

Let's see all these types of catalysts one by one;

(1)Positive Catalysts

......"The catalyst that reduces the activation energy by providing lower path and increases the rate of a reaction is known as Positive-Catalyst and this phenomenon is known as Positive-Catalysis"..... 

Remember that if we reduced the activation energy then the path through which a reactant is converted into product, also decreases. 

Let's have a look some examples related to positive catalysts;

➤Haber process in which iron oxide used as a positive catalyst for the production of Ammonia (mostly used method) 

      N₂+ 3H₂(Fe₂O₃ as catalyst) ➝ 2NH₃

➤Manganese oxide as a positive catalyst in the production of oxygen from potassium oxide,

      2KClO₃ (MnO₂) ➝ 2KCl + 3O₂

➤Platinum as a positive catalyst in the production of sulfur trioxide;

      2SO₂ + O₂ (Pt as catalyst) ➝ 2SO₃

(2)Negative Catalysts

......"The catalyst which reduces the rate of reaction by increasing the activation energy, is called Negative-Catalysts and this phenomena is called as Negative-Catalysis"........ 

Remember if we increased the activation energy then the path through which a reactant is converted into product, also increases. And thus, the rate of reaction decreases. 

Let's have a look some examples related to negative catalysts;

➤Phosphoric acid as negative catalysts in the definition of hydrogen peroxide;

         2H₂O₂ (H₃PO₄ as catalyst) ➝ 2H₂O + O₂

➤Oxidation of Na₂SO₃ in presence of alcohols used ethyl alcohol as negative catalysts;

         Na₂SO₃ (C₂H₅OH as catalyst)➝ Na₂SO₄

➤Acetanilide/glycerine also acts as negative catalyst in decomposition of H₂O₂.

➤T.E.L.(Tetra Ethyl Lead) an anti-knocking substance is added to petrol to decrease the ignition of petrol vapours. Here, T.E.L. behaves as negative catalyst.

(3)Induced Catalysts

Here, no foreign materials (means catalysts) are present. 

........"When one reaction affects another reaction, which does not happen under normal conditions. This means that, when an entire reaction changes the rate of another reaction, this reaction acts as a catalyst for that second reaction. This catalyst is called Induced-Catalyst and this phenomenon is called Induced-Catalysis".........

Let's have a look an example; 

(a) Sodium sulphite solution oxidises in the presence of air but Sodium arsenite solution doesn't oxidise by passing air. But when both solution are mixed then both solutions are oxidised in presence of air,

        Na₂SO₃, (air passing) ➝ Na₂SO₄ ------(1)

    Na₂AsO₃ (air passing) ➝ No reaction  ---------(2)

But, 

      Na₂SO₃ + Na₂AsO₃ ➝Na₃AsO₄+ Na₂SO₄

                  (after passing of air) 

Here,  

       Reaction -1 acts as a induced catalyst.

(4) Auto-catalyst

......."When one of the product of a reaction is behaves like a catalyst and influences the rate of reaction then it is called Auto-Catalyst for that reaction. And this phenomena is called Auto-Catalysis"....... 

No specific catalyst is required in this type of reaction mechanism. Let's have a look some examples; 

▶Arsene decomposition in which one of the product of reaction, As (arsenic) acts as an auto-catalyst and influences the rate of reaction; 

                2AsH₃ ➝ 2As + 3H₂

▶Hydrolysis of an Ester in which one of the product, CH₃-COOH acts as an auto-catalyst and influences the rate of reaction; 

      CH₃COO-C₂H₅+ H₂O ➝ CH₃-COOH+C₂H₅OH

➤ Point to be remembered that positive catalyst has a tendency to decrease the activation energy, hence reaction takes short pathway (or certain time period) for completion while negative catalyst has a tendency to increase the activation energy, hence reaction takes long pathway (or longer time period) for its completion.

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