Peculiar activity of H-Na-mordenite in the N2O abatement with CH4: in situ and operando FTIR study

The catalytic reduction of N₂O with CH₄ in the absence (CR_N2O) and in the presence of excess O₂ (SCR_N2O) were studied on H-NaMOR catalysts (H-exchange degree within the range 0–100 %) prepared from Na-MOR (Si/Al = 9.2) by ion-exchange method. The catalytic measurements were performed in a flow apparatus with GC analysis of reactants and products. In situ FTIR characterization and catalytic results combined with operando FTIR experiments gave an insight into the active sites for N₂O abatement and into the reaction pathway.

H-NaMOR catalysts were active for both CR_N2O and SCR_N2O, but inactive for N₂O decomposition and CH₄ combustion. The activity shows a complex dependence on the H-exchange degree. The active sites involved in the N₂O reduction are very likely generated by dehydration of pairs of protonated acid sites. Active sites having a specific location are related to the catalytic activity, specifically those located in E sites to CR_N2O and those in D sites to SCR_N2O. For both CR_N2O and SCR_N2O the protonation of A sites lowers the activity. The catalytic results combined with operando FTIR characterization suggest that CH₄ and N₂O cannot be activated separately, but a synergetic activation of N₂O and CH₄ molecules occurs on a bifunctional active site. Operando FTIR indicated formaldehyde-like species as possible reaction intermediates, as well as the formation of other CH_xO_y species which poison the catalyst in CR_N2O conditions but are easily oxidized in the presence of O₂ during the SCR_N2O.