9 - Chemisorption

Summary

Fundamentals

At the beginning of this century, scientists generally believed that all adsorption phenomena were of the sort we have called physisorption. Some (unspecified) long range attractive force drew gas phase matter toward a solid and the increased concentration of the gaseous substance near the surface was thought to be analogous to the retention of the Earth's atmosphere by the gravitational field. The adsorbed layer was viewed as a ‘compressed vapor’ with little or no interaction with the atoms of the substrate. However, compelling experimental evidence soon accumulated that pointed to another, distinctly different, form of adsorption.

Langmuir (1916) introduced and extensively investigated the idea that there can exist strong, short range forces between adsorbates and a substrate. He regarded the arrangement of atoms at the surface of a solid as a sort of Chinese checkerboard that defines a specific density of potential adsorption sites. Foreign gas atoms that strike the surface may either bounce back into the gas phase or bind to one of these sites through formation of a surface chemical bond. The latter process is termed chemisorption and, in this view, it is not unreasonable to regard the adsorbate/substrate complex as an enormously large molecule.

Adsorption, and chemisorption in particular, lowers the free energy of any closed system that contains only a free surface and atoms or molecules in the gas phase.