Chemisorption

The chemisorption is a special form of adsorption in which, in contrast to the physisorption the adsorbate by stronger chemical bonds to the adsorbent is bound (substrate). The adsorbate and / or the adsorbent is chemically changed by the chemisorption.

Most of the time, physisorption is a precursor to chemisorption. In contrast to physisorption, chemisorption is not always reversible and often requires a high activation energy . The binding energy is typically around 800 kJ / mol (approx. 8 eV / atom) in contrast to physisorption with approx. 80 kJ / mol (approx. 0.8 eV / atom). A maximum of one monomolecular layer can be adsorbed.

A classification of sorption into chemisorption or physisorption cannot be based solely on the binding energy . The most important criterion for chemisorption is the chemical change in the adsorbate or adsorbent. As a result, chemisorption can be present in a few combinations even at low binding energies (e.g. 80 kJ / mol), while in other combinations there is still physisorption at 100 kJ / mol.

The direct transition of an adsorbate from the weak van der Waals bond in physisorption to the formation of a chemical bond was demonstrated with the aid of atomic force microscopy.

The strong binding of the adsorbate molecules to the substrate (mostly a metal) can lead to intramolecular bonds in the adsorbed molecules being dissolved ( dissociation ) or weakened. As a result, these molecules are then in a very reactive state. This is used in heterogeneous catalysis , the substrate is then called a catalyst .

The interaction of a catalyst with different adsorbate molecules can be very different. So-called catalyst poisons show a very high binding energy with the substrate and thus occupy the entire surface. In this way you prevent the adsorption of other substances whose reactions can then no longer be catalyzed.

literature

• Andrew Zangwill: Physics at surfaces , Cambridge University Press 1988, ISBN 0-521-34752-1 .

See also

• Surface chemistry

Web links

• Definition according to IUPAC

Individual evidence

1. ↑ Ferdinand Huber, Julian Berwanger, Svitlana Polesya, Sergiy Mankovsky, Hubert Ebert: Chemical bond formation showing a transition from physisorption to chemisorption . In: Science . September 12, 2019, ISSN  0036-8075 , p. eaay3444 , doi : 10.1126 / science.aay3444 ( sciencemag.org [accessed September 18, 2019]).