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 (CRN2O) and in the presence of excess O2 (SCRN2O) 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 CRN2O and SCRN2O, 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 CRN2O and those in D sites to SCRN2O. For both CRN2O and SCRN2O 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 CHxOy species which poison the catalyst in CRN2O conditions but are easily oxidized in the presence of O2 during the SCRN2O.