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 localized ANRs > 1.

To overcome the difficulty in attaining ideal mixing, Johnson Matthey' 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.

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.