NOx emissions – Selective Catalytic reduction SCR

NOx emissions are very dangerous. They are responsible for acid rain, Smog, breathing related problems and much more. They are also responsible for global warming. There are many sources of NOx mainly being burning of fossil fuels in power plants and automobiles. One interesting thing to be noted here is in a combustion chamber, NOx forms only at elevated temperatures as the reaction is endothermic.   

Selective Catalytic Reduction aka SCR helps in pollution control especially in gasoline engines. As the name suggests, its function is selective and concentrates on NOx emissions. Using catalyst such as aqueous ammonia, urea, anhydrous ammonia, etc this converts NOx into nitrogen (N₂) and water (H₂O) and this reaction occurs at elevated temperatures (~700 ^oC). They were developed to reduce the oxides of nitrogen in boilers which was adopted to automobiles later. 

The reactions can be written as,

4NO + 4NH₃ + O₂ = 4N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ = 3N₂ + 6H₂O

NO + NO₂ + 2NH₃ = 2N₂ + 3H₂O

These reactions are done using ammonia.

4NO + 2NH₂CONH₂ + O₂ = 4N₂ + 4H₂O + 2CO₂ (Urea)

Note that using urea produces CO₂ as its byproduct.

Moving on to catalysts. SCR uses catalysts such as zeolites, base metal catalysts, precious metals etc. Due to cost effectiveness, metal catalysts are used in the industries. But it is not very common in automobiles because, these catalysts lack high thermal stability this especially creates problems when the vehicle is equipped with https://jetter.in/wordpress/wordpress/diesel-particulate-filter-dpf/  as this works at higher exhaust gas temperatures mainly during regeneration. This is the reason why zeolites are more common.

The picture shows the setup of SCR where the exhaust gas reaches the <a href = https://jetter.in/post.php?id=11> DPF </a> and particulate matter is filtered out there and reaches the SCR where Diesel Exhaust Fluid (DEF) is splashed. DEF is nothing but aqueous urea solution with deionized water which is used to reduce NOx. Then using catalyst in the chamber NOx is reduced to water and nitrogen gas.