Exemptions occur due to several reasons including:
Poisoning: When a catalyst loses its activity due to the presence of impurities that bind irreversibly to its surface. Deactivation: Over time, catalysts can lose their effectiveness due to sintering, fouling, or coking. Selectivity: In some cases, catalysts may favor the formation of side products over the desired product. Inhibition: Certain molecules can inhibit the catalytic activity by blocking active sites temporarily.