Ammonia slip catalysts

Johnson Matthey's advanced ammonia slip catalyst (ASC) improves NOx reduction at low NH3 slip to improve the overall SCR system performance.

Even the best SCR catalyst will not achieve the maximum NOx reductions in a system with non-uniform NH3 distribution. In stationary applications with significant variation in engine load, exhaust flow rate and NOx concentration it can be difficult to deliver NH3 to the catalyst uniformly, in the 1:1 ammonia NOx ratio (ANR) required by the reaction stoichiometry. Non-uniform NH3 distribution can result in incomplete NOx conversion where localized ANRs < 1, and in NH3 slip where localised ANRs > 1.

To overcome the difficulty in attaining ideal mixing, Johnson Matthey's Research and Development team has developed ASC technology that couples highly active oxidation catalyst and SCR catalyst functions to achieve better NOx reduction at low NH3 slip. The ASC can compensate for non-uniform NH3 distribution, allowing continuous operation at higher ANR, boosting NOx conversion while maintaining low NH3 slip. And as an added bonus, the ASC delivers CO conversion which is not achieved over the SCR catalyst alone. SCR+ASC is installed in a single housing for low material costs.

ASC improves overall SCR system performance:

  • ASC allows operation at higher ANR with low NH3 slip
  • Better NOx and HC conversions with SCR+ASC
  • CO forms from HCs over the SCR catalyst alone
  • CO conversion achieved with SCR+ASC


The advanced ASC is very selective to N2 which means that almost all of the NH3 slip is converted to N2 rather than NOx. The ASC also converts hydrocarbons and CO to CO2. The overall performance of the SCR catalyst system is improved by addition of ASC. And better SCR performance can mean reduced catalyst volume which translates to lower material costs.


ASC converts NH3 to N2, not back to NOx:

  • Nearly complete conversion of NH3 slip
  • Formation of NOx, N2O very low
  • ASC is highly selective for N2
  • HC is converted to CO2, no CO formed

General enquiries

Joseph McCarney


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    Betty Ann Keen

    North America

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