Adsorption describes the association of a fluid-phase molecule with a solid substrate/surface. There are two main types of adsorption, physi- and chemi-sorption. For surface chemical reactions, chemi-sorption is the event/phenomenon with which we are concerned. Chemisorption implies a formal chemical reaction between the fluid-phase species and the surface, such that a chemical bond has been formed. This bond is formed between the fluid-phase species and a portion of the surface known as the active site. The active site is a surface atom or a collection of surface atoms with a propensity for forming energetically/electronically favorable interactions/bonds/configurations with the fluid-phase species. Often times, the chemisorption event is written as a chemical reaction, such as
where A is a fluid-phase species, S is the active site, and A•S is the chemisorbed complex of A on the active site, S. The reaction above is an example of molecular adsorption, which is the adsorption of a molecule on a catalytic surface.
In addition to molecular chemisorption, dissociative adsorption is also possible. In dissociative adsorption, the fluid-phase species dissociates as it bonds to the catalyst surface. This type of behavior is common among diatomic fluid-phase species, such as H2. Molecular, gas-phase hydrogen dissociatively adsorbs readily on catalysts such as platinum. Such as dissociative adsorption reaction looks as follows:
where H2 is a gas-phase, molecular hydrogen molecule, S is an active site, and H•S is complex formed between a radical H atom and an individual active site on the catalyst surface. In the above reaction, two such adsorbed complexes form.