Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 118, Issue 40, Pages 23126-23138Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp5065616
Keywords
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Funding
- Danish Independent Research Council [DFF - 1335-00175, DFF - 09-070250]
- Carlsbergfondet is acknowledged for supporting the upgrade of the EPR instrument at Department of Chemistry, DTU
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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 informatiion is obtained for the different copper positions in dehydrated Cu-CHA. The wel-defined copper sites in the six membered ring of the CHA structure are found to be EPR active to give two distinct sets of signal 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 degrees 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.
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