Varied Ammonia:NOx Ratio and Ammonia Slip Catalysts for Urea SCR Systems

James  Girard, Ford Motor Company

Urea-based selective catalytic reduction (SCR) will likely be needed to meet U.S. Tier 2 diesel emission standards for oxides of nitrogen (NOX). The ideal ratio of ammonia (NH3) to NOX is 1:1 based on having NH3 available for reaction of all of the exhaust NOX without NH3 Slip. However, SCR efficiency can be less than ideal at low temperatures (potential low SCR activity), and at higher temperatures with high exhaust flow rates (high space velocities). Optimizing the NH3 to NOx ratio is shown to lead to potential improvements in overall NOx conversion efficiency with little additional NH3 slip. NH3 slip past an SCR catalyst can be selectively oxidized to N2 if an NH3 slip formulation is used in front of or on a DPF. Slipped NH3 that passes over a CDPF is shown to form N2O at temperatures below around 350C, and to remake NOX at temperatures above 350C. The selectivity and activity of state of the art NH3 slip catalysts are also presented here.