Rich running and misfires will damage a CAT, when too much unburnt fuel goes in there, it can actually even melt the internal structure.
When there are misfires, there is raw fuel going inside the cat.
one simple thing you can do is, take the car for a ride, once engine is at operating temperature, run it in lower gear and hold rpm near 5k for a few minutes. This is done in other countries on cars when they don't meet emissions requirements, it restores the CAT efficiency.
It can be done every once in a while, just to keep it from clogging.
Another thing, people using API SG oil in cars with CAT Con, that is bound to go wrong
