Platinum (and other precious metals) have a special effect when heated, they ignite hydrocarbons on contact. This converts HCs to less harmful elements. If your engine is running rich or you have antifreeze getting into the exhaust it will melt the ceramic base for the inside of the converter. Platinum is the only precious metal that can take that kind of heat. So yes, All cats have platinum.
Scrap prices are at an all time high. If you have old cats laying around, take them in. I've seen some go as high as $200 a piece. It is not legal to install a used converter so, scrap 'em!
There have been stories of whole fleets of vehicle having there cats stolen overnight. I forsee legislation stating you need a VIN number to sell a used cat.