Chemical Adsorption or Chemisorption

Chemisorption, first investigated in 1916 by the American chemist Irving Langmuir (1881-1957), occurs when gas molecules accumulate on a solid surface by forming genuine chemical bonds -- covalent or ionic -- with the adsorbent, rather than merely being held by weak van der Waals attraction. Because a real chemical bond is involved, chemisorption is highly specific: a given adsorbent will only chemisorb an adsorbate with which it can actually form a bond, unlike physisorption where essentially any gas will physisorb on essentially any solid to some extent. Chemisorption involving a gas and a solid is usually exothermic (heat is released), with the heat evolved per mole of adsorbate being of roughly the same order of magnitude as an ordinary chemical bond -- the one notable exception given is hydrogen adsorbing onto glass, which is endothermic because the process requires dissociating the H2 molecule first. Chemisorption also involves a substantial activation energy (it is sometimes called 'activated adsorption'), so it initially increases with rising temperature as more molecules acquire enough energy to overcome that barrier -- but past a certain temperature, chemisorption instead decreases with further heating, because the chemical bonds holding the adsorbate to the surface start to break. It is also common for physisorption …