The low-temperature interaction of NH3/NO/NO2 + O2 with Fe-ZSM-5 + BaO/Al2O3 and H-ZSM-5 + BaO/Al2O3: Influence of phase separation and relevance for the NH3-SCR chemistry

In an effort to elucidate mechanism and intermediates of Standard SCR over metal-zeolite catalysts, we apply Transient Response Methods (TRM) to identify the mediating species in the low-temperature (120 °C) interaction of NO2 + O2, NO + O2 and NO + NO2 + O2 (NO/NO2 = 10/1 v/v) with a composite Fe-ZSM-5 (Fe = 1% w/w) + BaO/Al2O3 system in different configurations (physical mixture versus double-bed), corresponding to different degrees of separation of the two component phases. The results clearly indicate for the first time that the strong interaction between the two system components, already demonstrated in previous work, survives their complete segregation, proceeds via the gas phase, and is mediated by stable gaseous NOx species. The nature of the NOx species trapped on the BaO phase is identified by TPD experiments: in line with previous data, they include primarily nitrates for NO2 adsorption, and nitrites for NO + O2 adsorption at short exposure times. A new, striking finding is that formation of nitrites on BaO upon exposure of Fe-ZSM-5 + BaO/Al2O3 to NO + O2, which involves the oxidative activation of NO on Fe-sites, is fully equivalent to the formation of nitrites observed upon exposing only BaO/Al2O3 to NO2 in excess NO. This suggests that NO2 (possibly in the form of N2O3) may play the role of mediating gas-phase species generated by the oxidative activation of NO on Fe centers. The reactivity with NH3 of nitrites trapped on BaO is probed by Temperature Programmed Surface Reaction (NH3-TPSR) runs, which show rapid dinitrogen formation from low temperatures when Fe-ZSM-5 is not only mixed with, but also placed downstream from BaO/Al2O3, thus confirming the stability of the NOx intermediate formed on Fe-centers, and linking it to the Standard SCR reactivity. Finally, in order to study the role of the metal redox sites in the reactivity of nitrites stored on BaO with ammonia, we compare NH3-TPSR experiments over a Fe-ZSM-5 catalyst and over a parent H-ZSM-5 zeolite with a drastically reduced Fe content (Fe ≅ 0.02% w/w). Results show that nitrites on BaO react with NH3 to dinitrogen equally well on Fe- and on H-ZSM-5, which questions the role of the metal sites and therefore of the oxidative activation of NH3 in such a step. The present data emphasize the bifunctional (redox + acid) nature of the NH3-SCR catalytic chemistry at low temperatures, and should be considered in the development of comprehensive mechanisms for the Standard SCR reaction over Fe-zeolite catalysts.