Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface (e.g. corrosion, metallic oxidation). The strong interaction between the adsorbate and the substrate surface creates new types of electronic bonds - ionic or covalent, depending on the reactive chemical species involved.
Note: not to be confused with physical adsorption which leaves the chemical species of the adsorbate and surface intact. It is conventionally accepted that the energetic threshold separating the binding energy of "adsorption" from that of "chemisorption" is about 0.5 eV per atom or molecule.
It is characterised by:
Due to specificity, the nature of chemisorption can greatly differ from system to system, depending on the chemical identity and the surface structure.
The main way in which most chemists utilise the effect of chemisorption is in catalysed reactions. The process of chemisorption is actually pivotal to the role of heterogeneous catalysis where the catalyst is in a solid phase—particularly transition metal catalysts. In many cases the chemical reagents will both bind to the catalytic surface. The chemical bonds then form and draw electrons away from the chemisorption bonds. The molecule then desorbs and is free to leave the surface.
Self-assembled monolayers (SAMs) are often formed by chemisorbing thiols (RS-H) onto gold surfaces forming Au-SR bonds.
O2 on carbon at high temperatures.
Research is ongoing on the adsorption of hydrogen onto carbon nanotubes with the aim of producing a fuel cell that can eventually replace our dependence on fossil fuels.
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