Selective Catalytic Reduction of Oxides of Nitrogen with Ethanol/Gasoline Blends over a Silver/Alumina Catalyst in Lean Gasoline Engine Exhaust

Ethanol is a very effective reductant for nitrogen oxides (NO_X) over silver/alumina (Ag/Al₂O₃) catalysts in lean exhaust environments. With the widespread availability of ethanol/gasoline-blended fuel in the U.S., lean gasoline engines equipped with Ag/Al₂O₃ catalysts have the potential to deliver higher fuel economy than stoichiometric gasoline engines and to increase biofuel utilization while meeting exhaust emissions regulations. In this work a pre-commercial 2 wt% Ag/Al₂O₃ catalyst was evaluated on a 2.0-liter BMW lean burn gasoline direct injection engine for the selective catalytic reduction (SCR) of NO_X with ethanol/gasoline blends. The ethanol/gasoline blends were delivered via in-pipe injection upstream of the Ag/Al₂O₃ catalyst with the engine operating under lean conditions. A number of engine conditions were chosen to provide a range of temperatures and space velocities for evaluation of catalyst performance. High NO_X conversions were achieved with ethanol/gasoline blends containing at least 50% ethanol; however, as the ethanol content of the fuel blend decreased, higher reductant dosing was needed to maintain NO_X conversions of greater than 90%, which also resulted in significant hydrocarbon (HC) slip. Temperature and HC dosing were important in controlling selectivity to NH₃ and N₂O. At high temperatures, NH₃ and N₂O yields increased with increased HC dosing. At low temperatures, NH₃ yield was very low, but N₂O levels became significant. The ability to generate NH₃ under lean conditions creates the potential for application of a dual SCR approach (HC SCR + NH₃ SCR) that could reduce the fuel penalty associated with NO_X reduction and/or increase NO_X conversion; this strategy is discussed in this work.