2.1: Introduction to Molecular Adsorption
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- 25367
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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 catalyzed reaction it is possible to break down the whole continuously-cycling process into the following five basic steps:
- Diffusion of reactants to the active surface
- Adsorption of one or more reactants onto the surface
- Surface reaction
- Desorption of products from the surface
- Diffusion of products away from the surface
The above scheme not only emphasizes the importance of the adsorption process but also its reverse - namely desorption. It is these two processes which are considered in this chapter.
- 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 ).