Selective Catalytic Reduction (SCR) Catalyst

The SCR catalyst reduces nitrogen oxides (NOx) present in the exhaust stream to nitrogen (N₂ ) and water (H₂ O). The SCR catalyst contains a copper catalyst washcoated on a zeolite substrate. At the inlet of the SCR catalyst is a port for the reductant injector, followed by a louvered diffuser and a twist mixer. The reductant diesel exhaust fluid (DEF) is a solution of urea in deionized water. The urea solution percentage for correct SCR system operation is 28 - 35%. When DEF is introduced into the system, it finely atomizes in the louvered diffuser and mixes evenly with exhaust gases in the twist mixer. During this time, the heat of the exhaust gases causes the urea to split into carbon dioxide (CO₂ ) and ammonia (NH₃ ). As the ammonia and NOx pass over the SCR catalyst, a reduction reaction takes place and the ammonia and NOx are converted to N₂ and H₂ O. This reaction takes place at up to 95% efficiency and allows the engine to run leaner and more efficiently, since the high NOx levels that are produced under lean conditions are eliminated.

Fig 1: Typical SCR Catalyst (F-150)

Courtesy of FORD MOTOR CO.