How can a catalytic converter change several exhaust pollutants at once?

A hot exhaust stream can pass through a catalytic converter and leave with a different mixture of molecules. In a gasoline engine's three-way converter, the main targets are carbon monoxide, unburned hydrocarbons and nitrogen oxides. Three-way refers to treating these three groups, not to splitting the exhaust into three pipes.

Inside is a ceramic or metal support with many small passages, resembling a honeycomb. Its surfaces carry catalytic materials. The passages expose flowing gas to a large area without making it pass through one thick solid block. Molecules interact with the catalytic surface, where reactions can follow pathways with lower activation-energy barriers: less of an initial energy hurdle must be overcome for rearrangement.

The two main chemical jobs handle oxygen differently. Oxidation converts carbon monoxide toward carbon dioxide and hydrocarbons toward carbon dioxide and water. Reduction converts nitrogen oxides toward nitrogen, removing their oxygen from those compounds. Atoms are rearranged into other molecules; pollutants do not simply disappear into the coating. The catalyst participates in the pathway and is regenerated rather than being consumed as a fuel for each reaction.

These jobs require a suitable balance in the incoming gas. Too much or too little available oxygen favors some reactions at the expense of others, so engine sensors and fuel control help manage the mixture. Temperature matters too. The catalytic pathway lowers the reaction barrier, but a cold converter does not immediately achieve the reaction rates of one at its intended operating temperature.

The converter is therefore both a material surface and part of a controlled system. Gas composition, flow and temperature affect what its reactions can accomplish. It reduces particular pollutants under suitable conditions; it does not eliminate the carbon dioxide associated with fuel use or make every exhaust stream harmless. Its useful work is rapid molecular rearrangement during the short time gas spends inside.