The Chemistry of Clean Air: How SCR Precisely Targets NOx Emissions
Selective Catalytic Reduction (SCR) is a masterclass in chemical engineering applied to automotive technology. It doesn't just reduce pollution; it targets a specific pollutant with remarkable precision. The "Selective" in its name means the catalyst is designed to favor the reaction between ammonia (from AdBlue®) and nitrogen oxides (NOx), even in the presence of oxygen.
The core chemical reactions inside the SCR catalyst are:
Hydrolysis: The injected AdBlue® (aqueous urea) first thermally decomposes into ammonia (NH₃) and isocyanic acid (HNCO), which then hydrolyzes into more ammonia and carbon dioxide (CO₂).
Reduction: This ammonia then acts as a reducing agent. On the surface of the catalyst, it reacts with NOx gases in two primary ways:
Ideal Reaction: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O
NO₂ Reaction: 2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O
