NO_2 REACTION PATHWAYS WITH NH_3 ON AN FE-ZEOLITE SCR CATALYST

摘要

One approach for nitrogen oxides (NO_x) emission control of medium duty diesel engines is through the use of a combination Lean NO_x Trap and Selective Catalytic Reduction (LNT-SCR) catalyst system. In this system, part of the NO_x conversion occurs via an NH_3 SCR catalyst that is dependent on the NO_2 to NO_x ratio of the feed gas with NO_2 being a more advantageous oxidizer. One benefit of using this system is the conversion of NO to NO_2 over the LNT which increases the NO_2:NO_x ratio of the feed gas to the SCR catalyst. An experimental study has been performed to investigate the NO_2-NH_3 reaction for an Fe-based zeolite SCR catalyst using a bench top flow reactor. The increase in NO_2 concentration at the inlet of the SCR results in the formation of large quantities of N_2O from 200°C to 400°C. Further experiments determined that N_2O and NH_3 react above 350°C. This has led to a hypothesis that one primary SCR reaction (Slow SCR) can be replaced with two reaction steps featuring NH_3, NO_2, and N_2O. As a result, this paper proposes five NO_x reduction reactions as part of a global mechanism, which would account for the observed experimental behavior.