Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction

Borfecchia, Elisa; Lomachenko, Kirill A.; Giordanino, Filippo; Falsig, Hanne; Beato, Pablo; Soldatov, A. V.; Bordiga, Silvia; Lamberti, Carlo

doi:10.1039/c4sc02907k

Cited by 345 publications

(549 citation statements)

References 71 publications

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“…These two locations for the Cu ions are well known from structural analysis of Cu-CHA catalysts, although some controversy exists on which form actually constitutes the active site. 6,12,28,34,[39][40][41]44,52,72,73 However, when adsorbates are present, the Cu ion is lifted out of the 6-ring plane and the preference for the 6-ring location diminishes. 42,43,74 This is also observed for the formation of Cu 2+ −NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 (step 1), where the difference in energy for the Cu located in the 8-ring is only 0.08 eV lower compared to the Cu in the 6-ring, showing that there is no real preference for location of the Cu in the 6-ring or 8-ring in this case.…”

Section: Verification Of the Reaction Schemementioning

confidence: 99%

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

et al. 2015

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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.

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Section: Verification Of the Reaction Schemementioning

confidence: 99%

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

et al. 2015

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“…report that the loss of water during dehydration of Cu-CHA typically reaches a plateau at 220-250 °C. [1,39] This temperature is sufficiently low that disturbance of covalent framework bonds and mobility of copper and alumina species is negligible. Upon dehydration, the spin Hamiltonian parameters of all the copper species in the zeolites change substantially and the EPR intensity decreases.…”

Section: Dehydrated Cu-zeolitesmentioning

confidence: 99%

Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy

et al. 2016

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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.

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“…if all Cu(II) were reduced). A possible explanation for this could be that it has previously been shown that Cu-SSZ-13-a has two different Cu sites occupied in the crystal structure at 250 °C one in the six ring and one in the centre of the eight rings present in chabazite and it may be possible for the NH 3 activated Cu reduction to be occurring at only one of these sites [18].…”

Section: Further Operando Xas Experiments On Cu-ssz-13-amentioning

confidence: 99%

“…There are two highly preferable sites within the framework, in 8r window connecting cha cages and in the corners of the d6r cages, [17][18][19]. At low temperatures, the 8r site is preferable, enabling the formation of large Cu 2+ species such as [Cu(NH 3 )] 2+ when in the presence of NH 3 [20], however, upon a modest temperature increase (250 °C) the d6r window becomes favourable [21].…”

Section: Introductionmentioning

confidence: 99%

Operando Spectroscopic Studies of Cu–SSZ-13 for NH3–SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions

et al. 2018

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The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic reduction by ammonia (NH 3 ) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH 3 -SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH 3 -SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH 3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH 3 -SCR of NOx occurring at Cu six ring sites within the zeolite.

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Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction

Abstract: X-ray absorption and emission spectroscopy, FTIR and DFT unravel the major Cu species in the activated Cu-SSZ-13 catalyst for NH3-SCR.

Cited by 345 publications

References 71 publications

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

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

Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy

Operando Spectroscopic Studies of Cu–SSZ-13 for NH3–SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions

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