Nitrogen oxides are formed in diesel engines and other combustion processes. The WHO estimated that in 2018 between 31,000-76,000 people died prematurely because of pollution from nitrogen oxides. Copper-exchanged zeolites and vanadium oxides supported on metal oxides can be used as catalysts to facilitate the reaction of toxic nitrogen oxides with added ammonia to form harmless nitrogen and water. It is important to gain insight into how these processes occur so better catalysts can be developed.
Using the unpaired electron on copper as a probe can provide such insights. This was done by using in-situ Electron Paramagnetic Resonance (EPR) spectroscopy. This technique is sensitive to small changes in the chemical surroundings of paramagnetic metal centers. Copper zeolite catalysts have different coordination of the active copper sites depending on the way the materials were synthesized. Therefore, it is important to know the history of the copper exchanged zeolites when comparing results. Our EPR investigation showed that the copper centers can move around inside the zeolite and that the copper can change its coordinating ligands rapidly when exposed to ammonia at temperatures below the normal operating temperature of the catalyst. When SO2 is present in the exhaust due to impurities in the fuel, it will bind to the copper centers and this hinders the mobility that is important for the SCR reaction. This effect persists even after a thermal regeneration of the catalyst.
The method was also applied on other SCR systems and we uncovered evidence of how the activity of a new vanadium-based catalyst system made by collaborators at DTU and in Germany was closely connected to the interactions with the metal oxide support materials. Using new in-situ EPR methodologies we also investigated the interaction of ammonia with a new type of metal-organic frameworks (MOF) developed by collaborators from the University of Southern Denmark, Australia, and Ireland. The new MOF materials were able to reversibly bind ammonia inside the porous structure. This stepwise reversible adsorption could be followed by EPR spectroscopy.