The effect of framework structure on N2O formation over Cu-based zeolites during NH3-SCR reactions

Licentiatavhandling, 2021

Anthropogenic nitrous oxide (N2O), which is generally formed as a byproduct of industrial chemical processes and fossil fuel combustion, has attracted significant attention in recent years due to its destructive role in global warming and ozone layer depletion. Among different developed technologies applied for lean NOx reduction, the selective catalytic reduction (SCR) of NOx with ammonia (as a reducing agent) is presently the most used method. Hence, the development of catalysts for efficient lean NOx reduction that do not form N2O (an unwanted by-product in the SCR reaction) in the process, or only form N2O to a very small extent from the exhaust gases is of crucial significance. One of the types of catalysts that today are used for this purpose are zeolite-based catalysts. These have been extensively investigated owing to their extraordinary catalytic performance under practical reaction conditions such as high thermal stability and high N2 selectivity. Among all zeolites, Cu ion-exchanged zeolites, with MFI framework structure, like ZSM-5 and BEA framework structure, like Beta, represent high activity and N2 selectivity. Besides, Cu ion-exchanged zeolites with CHA Framework structure, like SSZ-13, show even higher hydrothermal stability, N2 selectivity and better hydrocarbon poisoning resistance compared to Cu-based ZSM-5 and Beta zeolites.

This work aims at investigating the effect of the zeolite framework structure on N2O formation during NH3-SCR reactions over three Cu-based zeolites ranging from small-pore to large-pore structures. Since Cu ions at the exchange sites play a crucial role in SCR reactions and promote different SCR pathways, the formation of different copper species has been investigated. In the zeolite framework, Cu exists in two cationic forms, which can catalyze the SCR reaction by activating NO to form NO+ and/or surface nitrate species. The nitrate species can subsequently react with NH3 to form another intermediate, ammonium nitrate (AN), which seems to be one source for N2O formation at low temperatures. These results are supported by kinetic studies in flow reactor and H 2 -TPR measurements. Furthermore, in situ infrared spectroscopy data revealed that in various NO/NO2 ratio in the inlet gas composition, higher formation of N2O is observed at low temperature which can be due to the higher formation and decomposition of ammonium nitrate. The investigations in flow reactor, are in line with the in situ infrared spectroscopy results.