Reaction Pathways in the Selective Catalytic Reduction Process with NO and NO₂ at Low Temperatures

Abstract

The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO₂ was investigated. The two main reactions are 4NH₃ + 2NO + 2NO₂ → 4N₂ + 6H₂O and 2NH₃ + 2NO₂ → NH₄NO₃ + N₂ + H₂O. The "fast SCR reaction" exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200 °C. At lower temperatures, the "ammonium nitrate route" becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T ≈ 140 °C, the ammonium nitrate route may be responsible for the whole NO_x conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180 °C. The ammonium nitrate route is interesting from a standpoint of general DeNO_x mechanisms: This reaction combines the features typical to the SCR catalyst with the features of the NO_x storage-reduction catalyst, i.e., NO_x adsorption to a basic site.