Hydrocarbon Selective Catalytic Reduction Using a Silver-Alumina Catalyst with Light Alcohols and Other Reductants

Previously reported work with a full-scale ethanol-SCR system featuring a Ag-Al₂O₃ catalyst demonstrated that this particular system has potential to reduce NO_x emissions 80-90% for engine operating conditions that allow catalyst temperatures above 340°C. A concept explored was utilization of a fuel-borne reductant, in this case ethanol “stripped” from an ethanol-diesel micro-emulsion fuel. Increased tailpipe-out emissions of hydrocarbons, acetaldehyde and ammonia were measured, but very little N₂O was detected. In the current increment of work, a number of light alcohols and other hydrocarbons were used in experiments to map their performance with the same Ag-Al₂O₃ catalyst. These exploratory tests are aimed at identification of compounds or organic functional groups that could be candidates for fuel-borne reductants in a compression ignition fuel, or could be produced by some workable method of fuel reforming. A second important goal was to improve understanding of the possible reaction mechanisms and other phenomena that influence performance of this SCR system. Test results revealed that diesel engine exhaust NO_x emissions can be reduced by more than 80%, utilizing ethanol as the reductant for a space velocity near 50,000/h and catalyst temperatures between 330 and 490°C. Similar results were achieved for 1-propanol, 2-propanol and 1-butanol, with a (desirable) shift to a lower temperature range seen for the primary alcohols. Heavier alcohols and other oxygenated organics were also tested as reductants showing a range of less successful results. Non-oxygenated hydrocarbons and the selected secondary and tertiary alcohols proved to be very poor reductants for this system. Some discussion concerning the possible mechanisms behind the results is offered.