Catalytic Converter Clogged, But with what?

i guess if there was a misfire before and the car was driven for a while without fixing the misfire, then the unburnt fuel had to go to the CAT.. ignited there since it is very hot in there. With the high temps the membrane could have been melted...

The catalyst support ( the ceramic you see) breaks up because of thermal shock. This was a big problem up to about 1980 ,then better support ceramics were developed that don't breakup. I emptied about three converters in vehicles older than 1980, just remove the large Allen nut on the bottom for a couple weeks. I could no longer get on the expressway because of lost power. My guess is that your auto manufacturer hired MBAs to calculate profit,. found old cheap ceramics for your converters( they take more than 50,000 miles to pulverize and plug the converter). The MBAs probably added $ 20 to the manufactures profits. An X-ray diffraction analysis would prove my analysis but would cost more than your new converters , unless you know someone in the trade.

Most cats get blocked with a combination of combustion products and burnt oil.

This happens due to two main causes:

1. a worn engine allowing oil into the exhaust (valve seals, rings etc)

2. the wrong oil being used - this is a common issue as people don't check for the correct grade of oil and, in many cases, opt for the cheapest.

Some cats can be cleaned but that is not always possible or easy.