Reaction pathways in the selective catalytic reduction process with NO andNO2 at low temperatures

The low-temperature behavior of the selective catalytic reduction (SCR) pro cess with feed gases containing both NO and NO2 was investigated. The two m ain reactions are 4NH(3) + 2NO + 2NO(2) --> 4N(2) + 6H(2)O and 2NH(3) + 2NO (2) --> NH4NO3 + N-2 + H2O. The "fast SCR reaction" exhibits a reaction rat e at least 10 times higher than that of the well-known standard SCR reactio n with pure NO and dominates at temperatures above 200 degreesC. At lower t emperatures, the "ammonium nitrate route" becomes increasingly important. U nder extreme conditions, e.g., a powder catalyst at T approximate to 140 de greesC, the ammonium nitrate route may be responsible for the whole NOx con version observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typica l monolithic sample, the lower threshold temperature at which no degradatio n of catalyst activity with time is observed is around 180 degreesC. The am monium nitrate route is interesting from a standpoint of general DeNO(x) me chanisms: This reaction combines the features typical to the SCR catalyst w ith the features of the NOx storage-reduction catalyst, i.e., NOx adsorptio n to a basic site.