There is no catalytic converter cleaning process and no such detergent or acid exists to help. If you think your car is choking due to exhaust soot in the cat, just run it at high load and power on the highway for a little bit, it will clean it thoroughly. (what is being called italian tune up by @dms664 )
However if you have melted the cat (driving with misfire) or have poisoned it (coolant leak into combustion chamber or used cheap silicone RTV on the engine, specially the intake or exhaust manifold) there is no remedy to fix those two. Its water under the bridge.
TL DR
do not drive with a misfire or coolant leaking into combustion chamber or have mechanic use the wrong silicone sealant on the engine, the results of being super miser will cost you tens of thousands or even hundreds of thousands.