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Heterogeneous Catalyst

A chemical reaction involves the conversion of reactant molecules to product molecules with cleavage and formation of some chemical bonds. This conversion requires some amount of energy which comes from the collision of reactant molecules. It means reactant molecules must have some energy that induces collision between them and results the formation of product molecules. Sometimes reactant molecules do not have enough kinetic energy to collide with each other. Hence the reaction will not furnish and requires some energy that can provide in terms of heat.

Some of the reactions require large amount of energy that cannot be provided by external sources. In such cases, we can use catalyst which is a substance that reduces the activation energy for the reaction and quickly form the product. Do you know what activation energy is? Activation energy is the minimum amount of energy requires for effective collision between reactant molecules.

The presence of catalyst reduces the activation energy by bonding with reactant molecules. It regenerates in the chemical reactions at the end of reaction. Hence we can say that catalyst does not take part in the reaction and not changed by the reaction. They can be a transition metal or transition metal oxide or an enzyme. Catalyst provides a convenient surface which enables a different route for a chemical reaction with low activation energy.

The surface of catalyst makes the reacting particles more active results more of the collisions that lower the activation energy. In a chemical reaction, there are cleavage and rearrangement of chemical bonds which results the formation of transition state. Transition state is a high energy structure with incomplete chemical bonds. Due to high energy, it readily decomposes to product. The energy difference between reactant and transition state is called the Energy of Activation, or Ea. Catalyst w provides an alternate route with low activation energy. Remember catalysts cannot affect the chemical equilibrium shift and equilibrium constant. 

Heterogeneous Catalyst Activation Energy

 

Heterogeneous Catalyst Definition

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On the basis of phases of reactant, products and catalyst, they can be classified in two types; homogeneous and heterogeneous catalyst. Homogenous catalyst is of the same phases as of reactants of the chemical reaction.

On the contrary, heterogeneous catalyst can be defined as the substances which accelerate the chemical reaction and have different phases from reactant molecules. Two different phases contains a separate boundary between them like a solid and a liquid consists of two phases.

On the other hand, a mixture with various chemicals in a single solution has only one phase as there is no boundary between them. Heterogeneous catalysis involves the use of a catalyst which is in a different phase from the reactant molecules of reaction system. Solid catalysts are best examples of heterogeneous catalyst.

Heterogeneous Catalyst Example

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Let’s discuss few examples of heterogeneous catalyst. Catalyst of Haber process for the manufacture of ammonia from nitrogen and hydrogen gas is iron with molybdenum. Here Iron acts as catalyst and remains in solid state whereas both reactants; nitrogen and hydrogen are in gaseous state. Molybdenum is promoter here that enhances the activity of catalyst.

Another example of heterogeneous catalyst is use of transition metals like Ni in hydrogenation of olefins that results the formation of saturated hydro hydrocarbons. Similarly Palladium on activated charcoal is used in the hydrogenation of nitro groups to form amine groups is also an example of heterogeneous catalysis. Oxidation of ammonia to nitric oxide also involves the use of Pt as catalyst in which Pt is in solid state whereas both reactants remain in gaseous state. Contact process for the manufacture of sulpohuric acid requires solid catalyst that is vanadium oxide with gaseous reactants; sulphur dioxide gas and oxygen. 

Heterogeneous Catalyst Mechanism

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Mechanism of heterogeneous catalyst involves certain steps. It involves adsorption, interaction and desorption. First step can be done as physical or chemical adsorption.
As name suggested, physical adsorption involves physical bonding between catalyst and reactant molecules whereas chemical adsorption involves the formation of chemical bonds between catalyst and reactant molecules.

Interaction of nickel catalyst in hydrogenation of alkenes is an example of chemical adsorption. The cleavage of H-H bond requires 435 kJ/mol and it leads to formation of free hydrogen atoms which further bonded with double bonded carbon atoms. 

Heterogeneous Catalyst

The activation energy for this step is around 435 kJ. In the presence of Nickel catalyst, the cleavage of H-H bond and adsorption over catalyst surface occurs simultaneously.

$2 Ni + H_2 \xrightarrow[H_2]{2 Ni} 2 Ni-H$


 
It lowers the activation energy and increases the rate of reaction. Another common example of heterogeneous catalyst is use of catalytic converters which change poisonous molecules such as CO, NO in to harmless molecules like $CO_{2}$, $N_{2}$# in car exhausts.

Platinum, palladium and rhodium are common heterogeneous catalyst which acts as catalytic converter.

$2CO + 2NO\xrightarrow[]{Pt/Pd/Rh}2CO_{2} + N_{2}$

How Does a Heterogeneous Catalyst Work?

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In the heterogeneous catalysis, reactant molecules are adsorbed on active sites of the catalyst. The active site of the catalyst is good at adsorbing molecules. The interaction of catalyst and reactant molecules makes the reactant molecules more active therefore some of the chemical bonds become weak in same molecules.

It converts the reactant molecules to product molecules. Later product molecules are desorbed from the active sites. Adsorption and desorption are main characteristics of a good catalyst. For example; silver is a good catalyst as it adsorbs and desorbs the molecules easily. Similarly platinum and nickel are also showing same characteristics.
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