The engine dumps heat to the radiator and the exhaust. I think the catalyst does enable exothermic reactions, but it’s not using power, so it can’t get hotter than the outgoing exhaust (although that is pretty toasty). A computer adjusts the fuel/air ratio and timing to keep the incoming temperature and oxygen level in the catalyst’s effective range.
Some models even have a heater to get the catalyst working quickly after a cold start.
Some models even have a heater to get the catalyst working quickly after a cold start.