Coordination Environment of Copper Sites in Cu-CHA Zeolite Investigated by Electron Paramagnetic Resonance

Abstract

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 sixmembered 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 Cu 2+ 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 Cu 2+ is suggested to exist as Cu 2+ −OH − coordinated to two framework oxygen atoms located in the microenvironment of an isolated Al T-site.

A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

“…In EPR, exposure to NO alone leads to an EPR spectrum that is identical to that obtained with dehydrated Cu-CHA. 29 Upon oxidation with NO and O 2 , however, a difference is observed, indicating that an additional Cu species is formed in this case. This suggests that a fully oxidized Cu 2+ species in the 6-ring is obtained in both cases, but that oxidation in the mixture of NO and O 2 also produces a Cu 2+ species that is not located in the 6-ring.…”

Understanding the structure directing action of copper–polyamine complexes in the direct synthesis of Cu-SAPO-34 and Cu-SAPO-18 catalysts for the selective catalytic reduction of NO with NH3

“…The 27 Al MAS NMR spectra of as-prepared samples show two resonances: one between 7.1-7.9 ppm assigned to five-fold Al and one between 36.4-37.5 ppm assigned to tetrahedrally coordinated Al atoms. For Ni-SAPO-34 and Cu-SAPO-18, the five-fold environment is more abundant.…”

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…With the removal of H 2 O ligands, Cu ions now migrate to cationic exchange positions and bond to lattice O (O L ) of the zeolite framework. Such a change is well reflected by changes in unit cell parameters of the CHA substrate from X-ray diffraction (XRD) [27,35], -OH and H 2 O vibrations from FTIR [35], X-ray absorption and emission spectra (XAS and XES) [20] and hyperfine interactions between the unpaired electron and the nuclear spin of Cu(II) (I = 3/2) from electron paramagnetic resonance (EPR) [36]. It is now well-documented that in dehydrated Cu/SSZ-13, Cu 2+ -2Z with Cu ions located in windows of 6-membered rings (6 MR) are the energetically most favorable configuration.…”

“…At such Al abundance and low Cu loadings, it is expected that [Cu(H 2 O) 6 ] 2+ is the only Cu species in the hydrated form and it will be converted exclusively to Cu 2+ -2Z upon dehydration. However, at intermediate dehydration temperatures (e.g., 150 • C), the clear signal intensity loss suggests formation of EPR silent [Cu(OH)] + [36], even though it eventually converts to EPR active Cu 2+ -2Z as evidenced by the signal intensity recovery at higher temperatures. For catalysts with higher Si/Al and Cu/Al ratios, [Cu(OH)] + -Z can indeed be thermodynamically stabilized during dehydration.…”

“…Another interesting Cu ion transformation during dehydration is the so-called "auto-reduction"; that is, formation of Cu(I) moieties in vacuum or in an inert gas flow without addition of reductants [36]. Although not conclusive, a likely mechanism can be described as follows, involving formation of an OH radical:…”