Chemisorption

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Chemisorption is a type of adsorption whereby a molecule adheres to a surface through the formation of a chemical bond, as opposed to the Van der Waals forces which cause physisorption. It is characterised by:

  • High temperatures.
  • Type of interaction: strong; covalent bond between adsorbate and surface.
  • High enthalpy: 50 kJ/mol <ΔH< 800 kJ/mol
  • Adsorption takes place only in a monolayer.
  • High activation energy
  • Increase in electron density in the adsorbent-adsorbate interface.
  • Reversible only at high temperature.

Uses

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.

Examples

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 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

See also

Physisorption

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