The Role of NO2 in the Fast NH3-SCR of NOx: A Combined In Situ FTIR and EPR Spectroscopic Study

For elucidating the role of NO2 in the fast NH3-SCR of NOx combined in situ FTIR and EPR spectroscopic studies were carried out to correlate adsorbate formation at the catalyst surface (FTIR) with changes of the different iron sites (EPR). For this purpose interactions of the feed components NO, NO2, and NH3 with Fe sites in a commercial Fe-ZSM-5 were studied and reactivities of preadsorbed species towards other reactants (nitrates created by NO2 adsorption towards NO and NH3, and ammonia/NH4+ towards NO2 and NO/O2) were investigated. In addition, the behavior of this catalyst in standard and fast SCR was studied. NO2 in the feed gas mixture causes an accelerated formation of nitrates mainly adsorbed on β-Fe sites on which they are reduced by NH3 and NO, more effectively by the mixture of both. The adsorption of NO2 and NH3 at different but adjacent sites facilitate the formation of NH4NO2 which decomposes into N2 and H2O. The Fe sites in β and γ position are efficiently reoxidized by NO2 already at 150 °C and are thus kept in a redox-active state.