The current 3 ways catalytic converters reduce unburned hydrocarbons - HC (unburned fuel), carbon monoxide - CO (lack of sufficient oxygen at the fuel molecule burning burning), and oxides of notrogen - NOx (combination of nitrogen and oxygen from the inlet air when exposed to high combustion temperatures).
http://www.catalyticconverter.org/index.htm http://www.howstuffworks.com/catalytic-converter.htm (use table of contents at bottom of page)
