Site‐Specific Reactivity of Copper Chabazite Zeolites with Nitric Oxide, Ammonia, and Oxygen

• Published in: ChemCatChem 2018-01, Vol.10 (2), p.366-370
• Main Authors: Godiksen, Anita, Isaksen, Oliver L., Rasmussen, Søren B., Vennestrøm, Peter N. R., Mossin, Susanne
• Format: Article
• Language: English
• Subjects: Ammonia; Catalysis; Copper; Dilution; Electron paramagnetic resonance; EPR spectroscopy; in situ spectroscopy; Nitric oxide; Oxidation; Reactivity; reduction; Selective catalytic reduction; Zeolites
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.201701357

In situ electron paramagnetic resonance (EPR) spectroscopy was applied to dilute copper chabazite (CHA) zeolites under gas flows relevant for the selective catalytic reduction of NO with ammonia (NH3‐SCR). Under both reducing and oxidizing conditions, we observed differences in reactivity between the different monomeric copper sites present: Upon reduction of Cu2+ with NO+NH3, the rate is seen to depend on the NH3 coverage. Subsequent oxidation with O2 resulted in a clean EPR spectrum of only one type of copper site, whereas oxidation with NO+O2 gave two types of copper sites. The rate of oxidation differed significantly between the reaction with O2 alone and with NO+O2 together. Thus, it was revealed that [Cu(NH3)2]+ complexes, which are regarded to be only weakly associated with the framework, nevertheless have different reactivity depending on the Al distribution in the proximity. The observed differences in reactivity of the copper sites have implications for the mechanistic understanding of NH3‐SCR with Cu zeolites. One or two: By using in situ electron paramagnetic resonance spectroscopy, it is possible for the first time to differentiate between the two Cu sites in the six‐membered ring of Cu‐exchanged chabazite zeolites. The composition of the oxidizing gas flow determines whether a single Cu site or both Cu sites are oxidized.