What is the adsorption theory of heterogeneous catalysts?
As per the old adsorption theory of catalysis, the reactants in their gaseous state or in their dissolved state in any solution get adsorbed on the surface of a suitable catalyst in its solid state. As a result of the increase in the concentration of the reactants on the surface of the catalyst, availability, and probability of the occurrence of a reaction between two species increases, and thus the rate of the reaction increases. Also, as the reactants adhere to the surface of the catalyst some amount of energy is released as the process is exothermic in nature. This heat of combination is utilized in the reaction between the two species, thus enhancing the rate of the reaction.
As per the intermediate compound theory of catalysis, the reaction that is desired is achieved with the formation of an intermediate compound and the following decomposition of the same into the desired products. In this process, the catalyst is regenerated after the reaction is over.
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Heterogeneous Catalysis Steps
Now, as per the modern adsorption theory, which is a combination of the old theory of adsorption and the intermediate theory of compound formation, the process of heterogeneous catalysis takes place with these five steps,
- The reactants diffuse to the surface of the catalysts. In this process, the reactants first get in contact with the external surface, out of which some of them cross the barrier and enter the interior exposed surface that includes paths and cracks on the external surface.
- These molecules then get adhered to the suitable sites available for adsorption.
- The reactants, when bound to the surface have a higher probability of reacting with each other, and after the reaction, they form an intermediate compound.
- After this process, the intermediate compound gets desorbed from the surface, which again becomes available for adsorption for other molecules to come.
- The intermediate compound then disintegrates to form the final products, which then diffuse out of the internal pores and the external surface of the catalyst.
Here, we see that the catalyst remains unchanged and is obtained in its original form once the reaction is over. The mass and the chemical composition of the catalyst are not altered throughout the process. We cannot explain the concept of catalytic promoters and inhibitors through the modern theory of adsorption.
Examples of some heterogeneous catalyst
In the heterogeneous catalytic process reactant and the catalyst are in different phases. Some examples of heterogeneous catalysis are given below:
Oxidation of sulphur dioxide into sulphur trioxide in the presence of platinum metal or vanadium pentoxide as a catalyst. Here reactants are in gaseous form while the catalyst is in solid state.
Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in the Haber process.
Frequently Asked Questions – FAQs
What is adsorption theory?
The adsorption principle notes that adhesion is the product of molecular interaction between the two materials and the evolving surface forces. A bond emerges from the adsorption on the substratum of adhesive molecules and the resultant enticing forces, typically known as secondary or van der Waals forces.
Why are heterogeneous catalysts preferred?
In industrial chemical processing heterogeneous catalysts play a significant role. Because of their robustness and lower operating costs, they are favoured, particularly through easier recovery / separation from the products enabling streamlining of chemical processes.
What does heterogeneous mean?
Heterogeneous refers to a structure which appears abnormal or variegated with dissimilar components or elements. A dermoid cyst, for example, has a heterogeneous attenuation to CT. For homogeneous, it is the antonym, indicating a system of identical components. Heterogeneous refers to a system whose root is alien.
What are the two types of catalyst?
It is possible to separate catalysts into two major groups-heterogeneous and homogeneous. In a heterogeneous reaction the catalyst is in a phase other than the reactants. The catalyst is in the same step as the reactants, in a homogeneous reaction.
What are the most common types of heterogeneous catalysts?
The catalyst occurs at the same time as the reactants in homogeneous catalysis. The catalyst occurs in heterogeneous catalysis at a step distinct from that of the reactants. A solid catalyst is the most common sort of heterogeneous catalyst.
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