22: Adsorption Methods

The adsorption of molecules on a surface at constant temperature and pressure results in a decrease in the system entropy; thus, the enthalpy of adsorption must be negative (exothermic) to be spontaneous (i.e. Gibbs free energy ≤ 0). Gas surface collisions are either elastic, with no interaction, or inelastic, the latter being exothermic via energy transfer from the adsorbate to the adsorbent. If the energy loss is sufficient to forbid spontaneous desorption, yet no reaction occurs to form chemical bonds, adsorption is classed as physisorption. Chemisorption involves the formation of a chemical bond between the surface and the adsorbate; thus, as this involves the formation of a molecular orbital between the surface atom and the incoming molecule/atom, the resulting chemisorbed species will saturate at the monolayer. This chapter will cover both physisorption and chemisorption and the techniques employed in their analysis.