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.
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.
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