Heterogeneous catalysts

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 2007.

Adsorption is commonly an essential first step in heterogeneous catalysis. Adsorption is when a molecule in the gas phase or in solution binds to atoms on the solid or liquid surface. The molecule that is binding is called the adsorbate, and the surface to which it binds is the adsorbent. The process of the adsorbate binding to the adsorbent is called adsorption. The reverse of this process (the adsorbate splitting from adsorbent) is called desorption. In terms of catalyst support, the catalyst is the adsorbate and the support is the adsorbent.

Two types of adsorption are recognized in heterogeneous catalysis, although many processes fall into an ambiguous range between the two extremes. In the first type, physisorption, induces only small changes to the electronic structure of the adsorbate. Typical energies for physisorption are from 2 to 10 kcal/mol. The second type is chemisorption, in which the adsorbate is strongly perturbed, often with bond-breaking. Energies for typical chemisorptions range from 15 to 100 kcal/mol.

For physisorption, adsorbate is attracted to the surface atoms by van der Waals forces. A mathematical model for physisorption was developed by London to predict the energies of basic physisorption of non-polar molecules. The analysis of physisorption for polar or ionic species is more complex.

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