What causes a catalytic converter to become clogged?

If the converter is operated for a long period at high temperatures, the substrate material may melt down and form a solid mass inside. It is normally a result of a systems fault that allows a too-rich fuel mixture to reach the converter. 

How to know when your catalytic converter has become clogged?
The main symptom of a clogged catalytic converter is the loss of power of your engine, which can appear sluggish. You also may smell an unusual odor from the exhaust gases. 

Early diagnosis and correction will stop further deterioration in the converter.

Clogged catalytic converter