Godiksen, Anita1; Stappen, Frederick N.3; Vennestrøm, Peter N. R.7; Giordanino, Filippo5; Rasmussen, Søren Birk7; Lundegaard, Lars F.7; Mossin, Susanne6
1 Department of Chemistry, Technical University of Denmark2 Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark3 Technical University of Denmark4 Haldor Topsoe AS5 University of Turin6 Center for Hyperpolarization in Magnetic Resonance, Center, Technical University of Denmark7 Haldor Topsoe AS
Cu-CHA combines high activity for the selective catalytic reduction (SCR) reaction with better hydrothermal stability and selectivity compared to other copper-substituted zeolites. At the same time Cu-CHA offers an opportunity for unraveling the coordination environment of the copper centers since the zeolite framework is very simple with only one crystallographically independent tetrahedral site (T-site). In this study the results of an X-band electron paramagnetic resonance (EPR) investigation of ion-exchanged Cu-CHA zeolite with a Si/Al ratio of 14 ± 1 is presented. Different dehydration treatments and rehydration experiments are performed in situ while monitoring with EPR. The results are compared with recent literature evidence from temperature-programmed reduction, X-ray methods, IR spectroscopic methods, and UV–visible spectroscopy. On the basis of these findings quantitative information is obtained for the different copper positions in dehydrated Cu-CHA. The well-defined copper sites in the six-membered ring of the CHA structure are found to be EPR active, to give two distinct sets of signals in an approximate 1:1 ratio, and to add up to 19 ± 2% of the total copper in the material. The long-standing question of the EPR silent monomeric Cu2+ in copper-substituted zeolites is suggested to be copper species with an approximate trigonal coordination sphere appearing during the dehydration. After complete dehydration at 250 °C the majority of the EPR silent Cu2+ is suggested to exist as Cu2+–OH– coordinated to two framework oxygen atoms located in the microenvironment of an isolated Al T-site.
Journal of Physical Chemistry C, 2014, Vol 118, Issue 40