2.1 Introduction to Molecular Adsorption
The adsorption of molecules on to a surface is a necessary prerequisite to any surface mediated chemical process.
For example, in the case of a surface catalysed reaction it is possible to break down the whole continuously-cycling process into the following five basic steps :
The above scheme not only emphasises the importance of the adsorption process but also its reverse - namely desorption. It is these two processes which are considered in this Section.
Notes on Terminology :
Substrate- frequently used to describe the solid surface onto which adsorption can occur; the substrate is also occasionally (although not here) referred to as the adsorbent.
Adsorbate- the general term for the atomic or molecular species which are adsorbed (or are capable of being adsorbed) onto the substrate.
Adsorption- the process in which a molecule becomes adsorbed onto a surface of another phase (note - to be distinguished from absorption which is used when describing uptake into the bulk of a solid or liquid phase)
Coverage- a measure of the extent of adsorption of a species onto a surface (unfortunately this is defined in more than one way !). It is usually denoted by the lower case Greek "theta", θ
Exposure- a measure of the amount of gas which a surface has seen; more specifically, it is the product of the pressure and time of exposure (normal unit is the Langmuir, where 1 L = 10-6 Torr s ).