These phenomena occur throughout catalysis and describe how gas molecules “stick” to the catalyst surface.
If atoms, ions or molecules from gases or liquids adhere to a surface, this is called adsorption (note: Absorption means the volume). If you want to know more precisely, there is on the one hand chemisorption with strong, solid chemical bonds such as CO-Pt and on the other hand physisorption with weak bonds such as water on oxides, i.e. reversible. The binding energies of chemisorption vs. physisorption are approx. 800 to 80 kJ/mol (approx. 8 to 0,8 eV/atom). Now in more detail:
Chemisorption:
- chemical adsorption occurs when there is a “proper” chemical bond between the adsorbed species and the surface
- the bonds in chemisorption are strong: covalent or ionic
- Chemisorption is highly specific because it requires a high affinity between the surface and the adsorbed molecules
- it also occurs at higher temperatures, where the activation energy can be overcome
- the result of the above is the monolayer formation, since the adsorbate molecules form a strong bond to the surface molecules
- Application: Dispersion determination of finely distributed platinum in precious metal catalysts with CO titration; oxygen capacity of Pd catalysts; but also NiO reduction with hydrogen. C&CS uses the “Autochem 2930” from Micromeritics .
Physical sorption:
- physical adsorption is based on dipole or van der Waals forces, which are relatively weak compared to chemical bonds
- the bond strength is low, so it can also be “heated out”
- Physisorption is not very specific and can occur on a variety of molecules and surfaces; this leads to:
- Multilayer formation: Formation of several layers on top of each other on the surface.
- Physisorption occurs preferentially at lower temperatures because the molecules stick to the surface.
- Application: Determination of the internal surface area of catalysts using the BET method, expressed in m2/g. C&CS uses the “NOVAtouch” from Anton Paar Inc. .
