For the first time, the standard and fast selective catalytic reduction (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable molecules only. The standard SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidation of the catalyst by NO + O2 and a reduction by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the reduction, and finally, oxidation by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown experimentally for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and Fourier transform infrared spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the standard SCR cycle. The activation energy calculated by density functional theory (DFT) indicates that the oxidation of an NO molecule by O2 to a bidentate nitrate ligand is rate-determining for standard SCR. Finally, the role of a nitrate/nitrite equilibrium and the possible influence of Cu dimers and Brønsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidation to NO2.

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.

Recent quantitative electron paramagnetic resonance spectroscopy (EPR) data on different copper species present in copper exchanged in CHA zeolites are presented and put into context with the literature on other copper zeolites. The information were obtained using ex-situ and in-situ EPR on copper ion exchanged into a CHA zeolite with Si/Al = 14±1 to obtain Cu/Al = 0.46 ±0.02. The results shed light on the identity of different copper species present after activation in air. Since the EPR signal is quantifiable, the content of the different EPR active species has been elucidated and Cu 2+ in 2Al positions in the 6-membered rings (6mr) of the CHA structure has been characterized. sites of the CHA structure under the applied conditions.

In situ electron paramagnetic resonance (EPR)s pectroscopy was applied to dilute copper chabazite (CHA) zeolites under gas flows relevant for the selective catalytic reduction of NO with ammonia (NH 3 -SCR). Under both reducing and oxidizing conditions, we observed differences in reactivity between the differentm onomeric coppers ites present:U pon reduction of Cu 2 + with NO + NH 3 ,t he rate is seen to dependo nt he NH 3 coverage. Subsequent oxidation with O 2 resulted in ac lean EPR spectrum of only one type of copper site, whereas oxidation with NO + O 2 gave two types of copper sites. The rate of oxidation differed significantly between the reaction with O 2 alone and with NO + O 2 together.T hus, it was revealed that [Cu(NH 3 ) 2 ] + complexes,w hich are regarded to be only weakly associated with the framework, nevertheless have different reactivity depending on the Al distribution in the proximity. The observedd ifferences in reactivity of the coppers ites have implications for the mechanistic understanding of NH 3 -SCR with Cu zeolites.The characterization of copper-exchangedz eolitesh as been performed with al arge variety of spectroscopic methods. Efforts have been driven by the fact that copper-exchanged zeolites are excellent catalysts in the reduction of NO x (NO and NO 2 )t oN 2 by selective catalytic reduction with ammonia (NH 3 -SCR), which today is one of the fastest growing applications for zeolite materials. Specifically,s mall-pore zeolites, such as chabazite (CHA), show excellent activity,h ydrothermal stability, and hydrocarbon tolerance and low production of unwanted N 2 O. [1][2][3] Al arge selection of experimental methods (IR spectroscopy, X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), X-ray diffraction (XRD), and UV/Vis spectroscopy) [4][5][6][7] and theoretical approaches (DFT) [8][9][10] have been appliedi nr ecent years to elucidate the properties of Cu-CHA materials. EPR spectroscopy offersu nique opportunities be-cause of its high spectral resolution, its fundamentally different selection rules, and its applicability under in situconditions. [11,12] Recent studies on identicalC u-CHAm aterials revealed several well-defined Cu sites. [5-7, 13, 14] This complicates detailed mapping and kinetic analysiso ft he SCR reaction on chabazite zeolites, for which the discussion has mostly focusedo nt he difference between 2Al Cu sites and 1Al Cu sites. It is of vital importance to detailed kinetic studies [15] to be able to distinguish and characterize all sites. Herein, we present an experimental protocola nd as pectroscopicm ethod that are able to reveal significant differences between different types of 2Al Cu sites present.T he reactivity is followed under SCR-relevant in situ conditions, and the results give important insight into the reactivity at single Cu sites.After dehydration under an O 2 -rich atmosphere at temperatures above 200 8C, Cu 2 + loses coordinated water molecules and binds directly to the zeolite framework. The three most important monomeric si...

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