N2O decomposition and reduction on Co-MOR, Fe-MOR and Ni-MOR catalysts: in situ UV–vis DRS and operando FTIR investigation. An insight on the reaction pathways

Abstract Co-, Fe- or Ni-exchanged Na-MOR (Si/Al = 9.2) were prepared by ion-exchange method. The catalytic activity for N 2 O decomposition in the absence or in the presence of NO and for N 2 O reduction with CH 4 in the absence of O 2 ( C R N 2 O ) , or in the presence of O 2 ( S C R N 2 O ) was investigated. The catalytic measurements were performed in a flow apparatus with GC analysis of reactants and products. On Fe-MOR, in situ FTIR and UV–vis characterization evidenced coordinatively unsaturated sites (c.u.s.) Fe 2+ arising from two families of Fe 3+ dimers with different reducibility. Characterization and catalytic results combined with operando FTIR experiments gave an insight into the transition metal ion (tmi) species working during N 2 O abatement and into the reaction pathways. For N 2 O decomposition the activity order was Co-MOR > Fe-MOR in the absence of NO and Fe-MOR ≥ Co-MOR in the presence of NO, whereas Ni-MOR was always inactive. The decomposition occurred via redox mechanism passing through the formation of activated surface oxygen species, O ads * . The quasi-oxidic character of this activated oxygen in the Fe 3+ -O (1+δ)− ---Fe (2+δ)+ intermediates with respect to the oxyl character of that in Co 3+ O ‒ intermediates accounted for the lower activity and for the activity enhancement by NO addition to the feed of Fe-MOR with respect to Co-MOR. In Ni-MOR, both isolated or dimeric Ni 2+ species were unable to be oxidized by N 2 O yielding O ads * . For N 2 O reduction, ( C R N 2 O ) and ( S C R N 2 O ) , the activity order was Fe-MOR > Ni-MOR > Co-MOR. On all catalysts operando FTIR experiments revealed CH x O y intermediates (methoxy, formaldehyde and two types of formate species). The investigation of surface species changing the addition order of the reactants evidenced that the formation of CH x O y intermediates was favored on Co-MOR by pre-saturation with N 2 O, that yielded Co 3+ O − able to activate CH 4 , whereas on Ni-MOR by pre-saturation with CH 4 , that reduced Ni 2+ dimers to Ni + dimers, able to activate N 2 O. On Fe-MOR, the Fe 2+ dimers formed during activation behaved as Co 2+ , whereas the Fe 2+ dimers formed by reduction with CH 4 behaved as Ni + dimers. The formation and the stability of O ads * surface species were key factors for N 2 O decomposition and reduction pathways. These factors were affected by the mobility of the tmi electrons, that depends on the oxidation state, nuclearity and location in MOR framework of tmi.