What is a Selective Catalytic Reduction (SCR) system and its purpose?

• A. A non-catalytic process that reduces NOx using ammonia at elevated temperatures.
• B. A catalytic process that oxidizes NOx to nitrogen dioxide.
• C. A catalyst-based process that reduces NOx to nitrogen and water in the presence of ammonia/urea at elevated temperatures.
• D. A filter that physically removes NOx from the exhaust.

Answer: (C)

Selective Catalytic Reduction is a catalyst-based process that lowers NOx emissions by reacting NOx with ammonia (usually from injected ammonia or urea) over a catalyst in the exhaust stream at elevated temperatures, converting NOx into nitrogen and water. The catalyst enables the NOx reduction to proceed efficiently within a specific high-temperature window (roughly a few hundred degrees Celsius), which is why the system must be tuned for temperature and ammonia dosing to avoid ammonia slip. This approach and chemistry distinguish it from non-catalytic reduction methods, which can operate at different temperatures but lack the solid catalyst that makes SCR highly selective for NOx reduction. It also differs from simply oxidizing NOx to NO2 or from physically filtering exhaust, which do not achieve the same NOx destruction.