The simultaneous selective catalytic reduction of N2O and NOX with CH4 on Co- and Ni-exchanged mordenite

Co- or Ni-exchanged Na-MOR (Si/Al = 9.2) prepared by ion-exchange method were characterized by in situ UV-vis DRS and FTIR. We studied the Selective Catalytic Reduction with CH4 in the presence of O2 (CH4-SCR) for the simultaneous abatement of NO and N2O (CH4-SCRsim) and the related reactions: (i) abatement of N2O (CH4-SCRN2O), (ii) abatement of NO (CH4-SCRNO), (iii) N2O decomposition, and (iv) CH4 combustion. The catalytic measurements were performed in a flow apparatus with GC analysis of reactants and products. FTIR characterization with CO of Ni-MOR identified isolated Ni2+ and [Ni2+-O-Ni2+] dimers, both mainly located in alpha-sites. In Ni-MOR, the amount of isolated Ni2+ and of dimers was comparable, whereas in Co-MOR isolated Co2+ were more abundant than [Co2+-O-Co2+]. Transition metal ion (tmi) dimers were easily reduced by CO to [tmi+-?-tmi+] yielding CO2. In situ UV-vis DRS characterization indicated that by heating in N2O, Co2+ oxidized to Co3+-O-, whereas Ni2+ did not. Catalytic results for CH4-SCRsim showed that Co-MOR was active, whereas Ni-MOR was ineffective, because it did not abate N2O. Both catalysts were active for CH4-SCRN2O and for CH4-SCRNO. Whereas Co-MOR was highly active for N2O decomposition and poorly active for CH4 combustion, Ni-MOR was inactive for N2O decomposition and active for CH4 combustion. The NO abatement in CH4-SCRsim on both Co-MOR and Ni-MOR occurred via CH4-SCRNO, and the active sites were isolated tmi2+ in alpha-sites. The N2O abatement in CH4-SCRsim on Co-MOR occurred in beta-sites via N2O decomposition, and the active sites were isolated Co2+, that formed Co3+-O- intermediate (UV-vis DRS evidence). The N2O abatement in CH4-SCRsim on Ni-MOR did not occur, because Ni2+, that formed no Ni3+-O-, were inactive in N2O decomposition. Redox behavior of [Ni2+-O-Ni2+] accounted for Ni-MOR activity in CH4-SCRN2O and CH4 combustion.