From what I've read online the current price per 1 ounce of platinum that is used in a catalytic converter is $845.00 per ounce but you would need a lot of catalytic converters to sell or to get the platinum out of them to sell the platinum direct to a refiner.
Metal refiners need around 500 catalytic converters or 1,000 lbs minimum of platinum before they'll purchase it from someone.
So if you only have one or two catalytic converters it would be best to just sell them to someone buying scrap catalytic converters unless you plan to sit on the catalytic converters until you get more of them.