Explain how catalytic converters reduce NOx and CO emissions from automobiles, and identify a potential byproduct concern if they are not functioning properly.

• A. They catalyze oxidation of CO to CO2 and reduction of NOx to N2; implemented with precious metal catalysts; improper operation can lead to higher emissions of unburned hydrocarbons.
• B. They convert all pollutants into water vapor and oxygen; no byproducts occur.
• C. They only affect emissions of sulfur compounds.
• D. They contribute to ozone formation by releasing extra NOx.

Answer: (A)

Catalytic converters use a three-way catalyst with precious metals to speed up two types of reactions in the exhaust: oxidation and reduction. They oxidize CO and hydrocarbons to CO2 and H2O, and they reduce NOx to nitrogen (N2) and oxygen. This works best when the engine runs close to the ideal air–fuel ratio, so all three reactions happen efficiently in the exhaust stream. If the converter isn’t functioning properly, those reactions don’t proceed well, and pollutants aren’t fully converted. In particular, incomplete oxidation leads to higher emissions of unburned hydrocarbons, which are a notable byproduct concern when the system fails.