The Influence of Ammonia Slip Catalysts on Ammonia, N2O and NOX Emissions for Diesel Engines

The use of urea-based selective catalytic reduction (SCR) is a promising method for achieving U.S. Tier 2 diesel emission standards for NO_x. To meet the Tier 2 standards for Particulate Matter (PM), a catalyzed diesel particulate filter (CDPF) will likely be present and any ammonia (NH₃) that is not consumed over an SCR catalyst would pass over the CDPF to make nitrous oxide (N₂O) emissions and/or oxides of nitrogen (NO_x), or exit the exhaust system as NH₃. N₂O is undesirable due to its high greenhouse gas potential, while NO_x production from the slipped NH₃ would reduce overall system NO_x conversion efficiency. This paper reviews certain conditions where NH₃ slip past an SCR system may be a concern, looks at what would happen to this slipped NH₃ over a CDPF, and evaluates the performance of various supplier NH₃ slip catalysts under varied space velocities, temperatures and concentrations of NH₃ and NO_x. Current SCR technology shows that the greatest concern over NH₃ slip comes at low temperatures during cold starts, and at moderate exhaust temperatures (250 to 350%C) with both moderate and very high space velocities. Slipped NH₃ that passes over a CDPF is shown to form N₂O at temperatures below around 350%C, and to remake form NO_x at temperatures above approximately 350%C. A critical issue is size for an NH₃ slip catalyst, since diesels equipped with SCR will likely also have a diesel oxidation catalyst and a CDPF (thus making addition of an NH₃ slip catalyst difficult for both system backpressure and packaging space). Current state of the art NH₃ slip catalysts are shown to be capable of removing nearly all NH₃ at temperatures above 225%C at low (80,000 h^-1) space velocities (SV) with good selectivity to N₂. The “light-off” for high NH₃ oxidation shifts to higher temperatures as the SV is increased for NH₃ slip catalysts. At high space velocities and temperatures below about 250%C, NH₃ slip catalysts remove some but not all of the slipped NH₃. Thus, work should be done to minimize NH₃ slip under these conditions. Use of an NH₃ slip catalyst can selectively remove most slipped NH₃ at medium and high temperatures, thus minimizing undesired NH₃ reactions over a CDPF.