Operando FTIR study of Fe-MOR, Co-MOR, and Ni-MOR as catalysts for simultaneous abatement of NOx and N2O with CH4 in the presence of O2. An insight on reaction pathway

Co-, Fe- or Ni-exchanged Na-MOR (Si/Al = 9.2) were prepared by ion-exchange method. The catalytic activity of Fe-MOR catalyst for the simultaneous selective catalytic reduction of NOx and N2O with CH4 in the presence of excess O2 (SCRsim) and for the related reactions (SCRN2O, SCRNO, N2O decomposition, N2O decomposition in the presence of NO and CH4 combustion) was investigated and compared with that previously measured for Co-MOR and Ni-MOR. The evolution of the surface species on Co-, Fe- or Ni-MOR (tmi-MOR) in the presence of flowing reactant mixtures during the SCRNO and the SCRsim were investigated by operando FTIR experiments and compared with that previously studied during SCRN2O. The correlation between the results of both operando FTIR and catalytic activity study gave an insight into the active sites and the pathway of the simultaneous abatement in tmi-MOR catalysts. Operando FTIR experiments under SCRNO feed allowed to identify on Co-MOR and Ni-MOR, both active for NO reduction, isocyanate species and monodentate tmi-nitrates, whereas on inactive Fe-MOR NO+ and bridged tmi-nitrates. The SCRNO pathway consisted of CH4 activation by NO+ yielding Al3+-isocyanates, that together with monodentate tmi-nitrates on nearby sites were involved in the pathway leading to the N-N pairing. Operando FTIR experiments under SCRsim feed on tmi-MOR identified the same NOy species as in SCRNO feed and no CHxOy species, intermediates of SCRN2O. The formation of CHxOy species was hindered by the active sites coverage due to nitrates. We concluded that the simultaneous NO and N2O abatement occurred via two independent processes (NO reduction and N2O decomposition). The zeolite MOR structure, allowing the location of working sites in different confined positions, guaranteed the simultaneous but independent pathways for the two abatements. Co-MOR alone was effective for the simultaneous abatement, because it combined the proper location with the suitable properties (nuclearity, coordination and electron charge mobility) of active sites. © 2019 Elsevier B.V.