Catalysis:Reactions and Mechanisms
Heterogeneous catalytic chemical reactions involve the surface as part of the governing chemical mechanism. In addition to adsorption and desorption events, discrete individual chemical reactions may occur on the surface. These reactions may involve both fluid-phase and surface bound species, and may take several of the following forms:
In the first reaction example listed above, fluid-phase species A reacts with B adsorbed on the surface to form the new chemical species AB, which desorbs readily from the surface, leaving behind an empty active site. In the second reaction example above, surface bound species A and B react to form the new chemical species AB, which desorbs readily from the surface, here leaving behind two empty active sites. In the third reaction example, we observe a fluid-phase species A reacting with adsorbed B to form an adsorbed AB complex on the surface. In reaction example four, an isomerization reaction is taking place, with an adsorbed a molecule of A being transformed into an adsorbed molecule of species B. Both of these last two example reactions would require subsequent desorption steps to remove the product species from the catalyst surface.
The reaction examples above do not constitute an exhaustive list of all possible types of surface chemical reactions, but rather give representative examples of the types of reaction events possible. It should also be noted that these reactions may be irreversible or reversible, depending on the associated thermodynamics.
The individual reaction steps, combined with any relevant adsorption and desorption events, comprise the complete reaction mechanism. With knowledge of this mechanism, and the associated thermodynamics and kinetics of each mechanistic event, one can determine the outcome of any reaction-engineering problem (assuming one has a suitable mole balance).
For more information on catalytic reaction mechanisms, see chapter 10 of Elements of Chemical Reaction Engineering, by H. S. Fogler.