Selective catalytic reaction (SCR)

Selective catalytic reduction (SCR) systems work by chemically reducing NOx (NO and NO2) to nitrogen (N2).

Johnson Matthey can offer coated and extruded catalysts.

Selective catalytic reduction

In a lean gas stream, it is necessary to add a reductant such as ammonia to the system to enable this reaction. Ammonia-SCR systems react ammonia (NH3) with the NOx to form nitrogen (N2) and water (H2O). There are three reaction pathways:

  • 4NH3 + 4NO + O2 → 4N2 + 6H2O
  • 2NH3 + NO + NO2 → 2N2 + 3H2O
  • 8NH3 + 6NO2 → 7N2 + 12H2O

Any source of ammonia can be used but most commonly the source is an aqueous solution of urea. This decomposes in the exhaust stream in two stages to form ammonia and carbon dioxide (CO2):

  • NH2C(O)NH2 → HNCO + NH3

  •  HNCO + H2O → CO2 + NH3
Emission control catalyst

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SCRF® system

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