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Monday, December 21, 2009

Types of adsorption

1. There are two main types of adsorption:
- physical or van der Waals' adsorption, and
- chemical or chemosorption.

2. In physical adsorption the molecules are held on the surface by van der Waals' forces which are very weak (20kJ/mol). It is easily reversed either by raising the temperature or lowering the pressure.

3. In chemosorption the molecules of the adsorbate are held by chemical bonds and it is characterised by higher heat of adsorption (40-80 kJ per mole of adsorbate). It is rarely reversible, and the thickness of the adsorbed layer is generally one molecule, for example, the adsorption of oxygen on tungten.

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