The simultaneous Selective Catalytic Reduction of N2O and NO over Fe-exchanged zeolites

Iron-exchanged zeolites are industrial heterogeneous catalysts deployed to remediate anthropogenic emissions of both nitrous oxide (N2O) and nitrogen oxides (NOx = NO + NO2). Despite the extensive scientific attention received, limited knowledge is available on the structural and catalytic behavior of catalysts comprising complex industrial formulations. Furthermore, although N2O and NO are often present simultaneously in the exhaust of several industrial operations, their conversion has been largely studied independently. Thus, the aim of this thesis work is to further the understanding of Fe-exchanged zeolites deployed for remediation of N2O and NOx with the primary goals to focus on industrial catalysts, and especially to study and optimize the simultaneous conversion of N2O and NO in a single catalytic stage under NH3-SCR conditions (N2O-NO-SCR).The main limitation of Fe-exchanged zeolites is their tendency to undergo irreversible modification when they are exposed to hydrothermal conditions. In addition to working operations, these conditions can be found also during the catalyst calcination. As a case study, the effect of an Al2O3-based binder on the development of the structural and catalytic properties of an industrial Fe-ZSM-5 catalyst was investigated (Chapter 2). Additionally, the majority of the previous studies on hydrothermal aging focused on its effect on the NOx abatement over catalysts synthesized at lab-scale. Instead, the emphasis was set on the development of the N2O conversion properties exhibited by industrial Fe-ZSM-5 and Fe-FER catalysts employed in the exhaust system of HNO3 production plants (Chapter 3). Given the limited knowledge available on the N2O-NO-SCR reaction, its reaction mechanism was studied over industrial Fe-FER by monitoring the interaction between N2O and the working catalyst equilibrated under different atmospheres combining catalytic experiments with several operando spectroscopic tools and density functional theory calculations (Chapter 4). In order to determine whether this interpretation is valid also on additional Fe-exchanged zeolites and to understand how the properties of the catalyst affect the N2O-NO-SCR reaction, the concomitant conversion of N2O and NO was studied over a series of Fe-ZSM-5 catalysts via ex situ, in situ and operando characterization and catalytic tests (Chapter 5). Furthermore, since the operating conditions in which the exhaust stream undergoing N2O-NO-SCR technology is treated can vary significantly, numerous catalytic were performed with the aim to monitor the N2O and NO conversions while tuning the feed composition and the working variables (Chapter 6).Finally, the potential of operando EPR spectroscopy for the analysis of red-ox mediated reactions was showcased (Chapter 7).The results reported in this thesis demonstrates that the authentic formulation of industrial catalysts must be considered when their activity, stability and structural modifications are investigated. The N2O-NO-SCR reaction is promising after-treatment strategy and could contribute to the development of exhaust treatment technologies of new generation. Dynamic operando spectroscopic experiments are essential in order to extract valuable molecular insights and draw reliable mechanistic considerations. This thesis provides guidelines for proper exploration of red-ox mediated catalyzed reactions and encourage further investigation of the N2O-NO-SCR reaction to achieve complete optimization.