What emission did the Three-Way Catalytic Converter help reduce that was not previously controlled?

• A. HC.
• B. NOx emissions.
• C. Particulate matter.
• D. Ozone.

Answer: (B)

The main idea is that a Three-Way Catalytic Converter can tackle NOx emissions as well as CO and hydrocarbons. Historically, catalytic systems mainly oxidized CO and HC, but NOx was difficult to control with catalysts alone. The three-way design uses different catalysts (including rhodium) to reduce NOx to nitrogen and oxygen while platinum/palladium oxidize CO and HC to CO2 and H2O. This dual action works best when the engine runs near the stoichiometric air–fuel ratio, allowing both oxidation and reduction reactions to occur in one converter. So, the emission that this converter helped reduce that wasn’t previously controlled is nitrogen oxides. Ozone is a secondary pollutant formed in the atmosphere, not a direct tailpipe emission, and particulate matter is managed by other systems, not the primary function of the three-way converter.