ABSTRACT
You may be familiar with a deƒnition of a catalyst as something that changes the rate of a reaction, but is not consumed by it, and may be recovered unchanged at the end. For many purposes, this is satisfactory. However, things can be a little more complex-some species act as both catalyst and reagent; sometimes it is not possible to recover the catalyst, at least not in the same form as the material you put in; and in some cases, the product of a reaction may also act as a catalyst for it. Note that catalysts are distinct from the initiators that we have used in some polymerization and radical reactions. Catalysts are involved for each molecular event, whereas initiators simply start the process going, and it is then in principle self-sustaining, with the initiator being consumed. Homogeneous catalysts operate in a single homogeneous phase, nearly always a liquid. Heterogeneous catalysts usually involve a solid catalyst with the reactants in either liquid or vapor phase.
