Catalytic converters on automobiles are designed to reduce the emission of?

Their real purpose is to clean unburned hydrocarbons out of the exhaust. But they do this by combining them with the oxygen in carbon monoxide, so they also reduce that, just as a side effect. They use carbon dioxide in the reaction too, but they also produce carbon dioxide, so it's not that.

carbon monoxide.

and nitrogen oxides

and hydrocarbons

Let me help you with that:

https://en.wikipedia.org/wiki/Catalytic_converter