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What role does adsorption play in heterogeneous catalysis?

What role does adsorption play in heterogeneous catalysis?

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Suraj Prasad IIT Patna
askIITians Faculty 286 Points
9 years ago
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts are solids and the great majority of reactants are gases or liquids. Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries. Heterogeneous catalysis has attracted Nobel prizes for Fritz Haber and Carl Bosch in 1918, Irving Langmuir in 1932, and Gerhard Ertl in 2007With catalyst supports, the reaction that occurs often occurs on the surface of either the catalyst or the support. In terms of surface reactions there are three mechanisms.

Langmuir-Hinshelwood mechanism. The two molecules A and B both adsorb to the surface. While adsorbed to the surface, the A and B "meet," bond, and then the new molecule A-B desorbs.
Rideal-Eley mechanism. One of the two molecules, A, adsorbs to the surface. The second molecule, B, meets A on the surface, having never adsorbed to the surface, and they react and bind. Then the newly formed A-B desorbs.
Precursor mechanism. One of the two molecules, A, is adsorbed on the surface. The second molecule, B, collides with the surface, forming a mobile precursor state. The molecule B then collides with A on the surface, they react, bind and the new molecule desorbs.Any surface reaction can be described as following one of these mechanisms, or some combination of these mechanisms. In addition, all of these above mechanisms can occur in reverse. In general, the pathway for a reaction on a surface is as follows. First the reactants adsorb onto the surface. Through a series of bonds being formed and being broken, adsorbed intermediates are produced and destroyed. Then the final product(s) is produced and it desorbs from the solid. Most metal surface reaction occur by chain propagation.

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