Well, if that's a fruitful approach, it means that I'm wrong, and the PMs do get depleted (expelled) during the normal life of a catalytic converter. I really thought they were bound pretty tightly, though.
I'm not entirely sure how it comes out, but people have found trace amounts of PGMs in the dirt along heavily traffic'ed roads. Given how much they find, each car is likely losing some insignificant amount over its life. The overwhelming majority of CAT replacements likely come from other issues like physical damage from corrosion (road salt, or living near the ocean), running over something, manufacturer defect, broken welds, etc... Or... what they did find on the roads came from damaged CATs and not the erosion of the honeycomb itself... maybe?
Great resource But how many people actually know what is in a Catalytic Converter ? or even what one looks like ? do we offer classes ?