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Selective catalytic reduction (SCR)

Johnson Matthey SCR catalysts control air pollution from industrial and power generating sources for cleaner industry and cleaner energy.

Selective Catalytic Reduction (SCR) of NOx with NH3

Selective Catalytic Reduction (SCR)
of NOx with NH3

The selective catalytic reduction (SCR) of NOx utilizing NH3 as the reductant is very effective in reducing NOx emissions and preventing air pollution from industrial and power generating  sources. The chemistry of NH3 SCR is well established:

  1. 4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O
  2. 4 NH3 + 2 NO2 + 2 NO → 4 N2 + 6 H2O
  3. 4 NH3 + 5 O2 → 4 NO + 6 H2O

Reactions 1 and 2 show that NOx and NH3 react in a 1:1 ratio to form N2 and water. Reaction 3 is the undesired oxidation of NH3 by O2. This reaction occurs above 425°C and creates NOx from NH3, lowering the overall NOx conversion of the system at higher temperatures.

Urea is often is often used in place of NH3 because it is easier to store and handle. Urea is injected into the hot exhaust and decomposes to NH3. The SCR system is designed to deliver NH3 to the catalyst at the optimum ammonia NOx ratio (ANR) to achieve maximum NOx reduction with minimum NH3 slip.

NOx conversions (solid) and corresponding NH3 slip (dashed)

NOx conversions (solid) and
corresponding NH3 slip (dashed)

Relationship between ANR, NOx conversion and NH3 slip:

  • At ANR < 1, Low NOx conversion and low NH3 slip
  • At ANR = 1.0,  Better NOx conversion but higher NH3 slip
  • At ANR > 1,  max NOx conversion and highest NH3 slip

Johnson Matthey SCR products: 


Johnson Matthey SCR catalysts are manufactured in standard dimensions as well as customized sizes to fit existing housings. The wide range of cell densities allow the highest NOx reduction at the lowest possible backpressure. There is a Johnson Matthey SCR catalyst to prevent air pollution from almost every application for cleaner industry, cleaner energy and cleaner air.