Efficient low temperature lean NOx reduction over Ag/Al2O3—A system approach

This study focuses on lean NOx reduction (LNR) by n-octane using silver–alumina based catalysts, with the addition of hydrogen. The work takes a system approach, where parameters such as temperature, reformate gas composition, fuel penalty and realistic monolith samples are considered. The LNR catalyst samples were prepared by impregnation and sol–gel methods and the NOx reduction performance was characterized by flow-reactor experiments, where realistic engine-out gas compositions were used. The hydrogen feed over the LNR catalyst samples was determined via data achieved by autothermal reforming experiments over a rhodium based catalyst, using real diesel as feedstock. The LNR catalyst samples generally show an enhanced NOx reduction when hydrogen is added to the gas feed. In particular, a 2 wt% silver–alumina sample with the addition of minute amounts of platinum, shows a high increase in NOx reduction when hydrogen is added to the feed. The addition of CO, a potential poison in the reaction and a by-product from the reforming, did not show any significant effect on the LNR catalyst performance at the conditions used. This is beneficial, since it renders a CO clean-up step in the reformer system unneeded. Ammonia formation is discussed in terms of a possible dual-SCR system. Finally, the fuel penalty for hydrogen production and hydrocarbon addition is taken into consideration. It is found that an addition of 1000 ppm H2 leads to unacceptable fuel penalties.

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► A system approach for a possible on-board system, combining ATR and LNR.
► High NOx conversion at low temperatures using Ag/Al2O3.
► Pt doping of a 2%Ag sample increases the NOx conversion at high C/N ratios.
► NH3 formation over high loaded Ag samples may decrease fuel penalty.
► Fuel penalty is considered.