2006 Chevrolet Malibu Question: car idels rough and dies won't go over 20 mph

added fule cleaner that didn't help, took it to a shop and they said it was the coil pack and module then they said the plugs needed to be replaced and now the catalytic converter is bad. seriously?? is this possible?
the car is in good shape and had 175,000 highway miles
I thought when the catalytic converter went bad it smelled like sulphur.