Decline of Surface Activity Catalyst Deactivation

One particular aspect of reactions on surfaces that is not encountered in reactions that are not catalytic is a progressive decrease in the activity of the surface with its time of utilization. The reasons for this are numerous, but we will divide them into three general categories. 

Poisoning loss of activity due to strong chemisorption of a chemical impurity on the active sites of the surface, denying their access by reactant molecules. This should not be confused with inhibition as expressed by adsorption terms in the denominator of LH rate expressions. 

Coking or fouling loss of activity, normally in reactions involving hydrocarbons, due to reactant or product degradation, producing a carbonaceous residue on the surface which is inactive for catalysis. 

Sintering loss of activity due to a decrease in active surface per volume of catalyst, normally the result of excessively high temperatures. 

A vast effort has been expended over the years in investigation of these types of deactivation as they are encountered in catalytic reactions and catalysts of technological importance. The uninitiated are often amazed at the fact that many reaction-system process designs are dictated by the existence of catalyst deactivation, as are process operation and optimization strategies. In some cases the deactivation behavior is so pronounced as to make detailed studies of intrinsic kinetics of secondary importance. 

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