A gasoline engine equipped with a three-way catalytic converter—what effect does the reduction stage have on NOx emissions?

• A. Reduction of HC to Hydrogen and Carbon
• B. Reduction of NOx to nitrogen and oxygen
• C. Reduction of CO to carbon and oxygen
• D. Reduction of H2O to hydrogen and oxygen

Answer: (B)

In a three-way catalytic converter, the catalyst uses two kinds of reactions to clean exhaust: oxidation and reduction. The reduction stage specifically targets nitrogen oxides (NOx). When the engine runs with a bit more reducing agents in the exhaust—things like CO, H2, and hydrocarbons—the NOx molecules are converted by the catalyst into nitrogen and oxygen. This is the step that lowers NOx emissions, which is why the reduction stage is responsible for transforming NOx into nitrogen and oxygen.

The other parts of the catalyst handle oxidizing carbon monoxide and hydrocarbons into CO2 and H2O, but reducing NOx is the job of the reduction stage.