Nitrate–nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH<sub>3</sub>-SCR

Tyrsted, Christoffer; Borfecchia, Elisa; Berlier, Gloria; Lomachenko, Kirill A.; Lamberti, Carlo; Bordiga, Silvia; Vennestrøm, Peter N. R.; Janssens, Ton V. W.; Falsig, Hanne; Beato, Pablo; Puig-Molina, A.

doi:10.1039/c6cy01820c

Cited by 46 publications

(78 citation statements)

References 57 publications

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“…The reactivity of pre‐adsorbed nitrates was also followed in the presence of NH 3 or NO alone, to study the role of each reactant in the NO/NH 3 mixture. NO showed the same effect already observed at 200 °C, that is a gradual decrease of the adsorbed nitrate complexes, which was explained with a displacement in the nitrate/nitrite equilibrium (not reported) . The reactivity of nitrates with NH 3 is reported in Figure (right‐hand panel), where it is directly compared to the spectra obtained while dosing NH 3 alone on the fresh O 2 ‐activated catalyst (left‐hand panel).…”

Section: Resultssupporting

confidence: 61%

“…Before the exposure to 1000 ppm of NO and 1200 ppm of NH 3 at 50 °C, the Cu−CHA surface has been saturated with nitrates, formed by exposing the O 2 ‐activated catalyst to 1000 ppm NO and 10 % O 2 at 50 °C . The formation of adsorbed nitrates after this treatment is confirmed by the peaks related to chelating bidentate Cu II structures (1610–1575 cm −1 ) and monodentate ones (1500, 1310 cm −1 , indicated with arrows in Figure , right hand panel) . A small amount of NO + , ascribed to the reaction of NO/O 2 with residual Brønsted sites is also observed (2159 cm −1 ), resulting in the formation of small amounts of water (H 3 O + signals at 1380 cm −1 labelled with ø, see Negri et al .…”

Section: Resultsmentioning

confidence: 89%

“…3 Å in the phase‐uncorrected FT‐EXAFS spectrum implies the presence of Cu‐nitrates in chelating bidentate configuration. While a second‐shell peak is visible, underpinning the presence of framework‐coordinated species, its shape is perturbed with respect to what has been previously reported under comparable conditions at higher temperature . Consistently with the FTIR results discussed above, this could be due to the simultaneous presence of a small fraction of framework‐coordinated monodentate Z[Cu II (NO 3 )] species.…”

Section: Resultsmentioning

confidence: 98%

“…Furthermore, NO can directly reduce

{N H}_{4} {N O}_{3}

{N H}_{4} {N O}_{2}

which, as previously mentioned, easily decomposes to N 2 and H 2 O already at 100 °C . However, both standard and fast reactions can be explaining without the role of

{N H}_{4} {N O}_{3}

…”

Section: Introductionmentioning

confidence: 94%

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Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

et al. 2019

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The reactivity with a NO/NH3 mixture of Cu‐nitrate complexes formed on the surface of a Cu−CHA catalyst active in the Selective Catalytic Reduction of NOx with NH3 (NH3−SCR) was followed at 50 °C by in situ spectroscopic techniques. The catalyst (Si/Al=15; Cu/Al=0.5) was first exposed to NO/O2 (mimicking the SCR oxidative half‐cycle), mainly resulting in the formation of chelating bidentate framework‐interacting CuII‐nitrates. These intermediates were gradually detached from the framework in the presence of NO/NH3 (or NH3 alone), forming mixed‐ligand mobile [CuII(NH3)3(NO3)]+ complexes, with infrared bands at 1624 (δNH3), 1430 and 1325 cm−1 (monodentate nitrate νNO2asym and νNO2sym, respectively). X‐ray absorption and Diffuse Reflectance UV‐Vis spectroscopies showed that during this transformation the CuII/CuI reduction, observed under similar conditions at 200 °C, hardly occurred. DFT calculations confirmed the stability of nitrate ligands in the monodentate conformation in [CuII(NH3)3(NO3)]+ complexes when solvated by ammonia. The resulting structure was successfully used to fit the corresponding experimental EXAFS spectra. The gradual change of ligands in the CuII coordination sphere was confirmed by the blue shifts of both d-d and Ligand to Metal Charge Transfer bands in Diffuse Reflectance UV‐Vis spectra, with formation of features (27500, 32000 and 38000 cm−1) ascribable to the mixed‐ligand configuration.

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Section: Resultssupporting

confidence: 61%

Section: Resultsmentioning

confidence: 89%

Section: Resultsmentioning

confidence: 98%

“…Furthermore, NO can directly reduce

{N H}_{4} {N O}_{3}

{N H}_{4} {N O}_{2}

which, as previously mentioned, easily decomposes to N 2 and H 2 O already at 100 °C . However, both standard and fast reactions can be explaining without the role of

{N H}_{4} {N O}_{3}

…”

Section: Introductionmentioning

confidence: 94%

See 2 more Smart Citations

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

et al. 2019

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“…From Figure it is clear that the MCR derived components show a good resemblance to the reference spectra of Cu I (NH 3 ) 2 , Cu II (NH 3 ) 4 and Cu II (NO x ) y reference Cu complexes previously reported (black, red and blue respectively). The aqueous copper nitrate reference is labelled as Cu II (NO x ) y due to the negligible difference between the XANES spectra of Cu II (NO 3 ) 2 and Cu II (NO 2 ) x (H 2 O) 6‐x . Cu I (NH 3 ) 2 and Cu II (NH 3 ) 4 are thought to be related directly to solvated linear Cu I[13] and square planar Cu II[41] and may not be limited to ligands composing solely of NH 3 .…”

Section: Resultsmentioning

confidence: 99%

Selective Catalytic Reduction of NO with NH₃ on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments

et al. 2020

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Cu‐exchanged small‐pore SSZ‐13 catalysts have found wide use for the selective catalytic reduction (SCR) of nitrogen oxides from automotive exhaust gases. The transient working environment of the Cu−SSZ‐13 catalyst during NH3‐SCR requires studying the rate limiting steps under the different operation conditions this catalyst is exposed to. By exploiting time‐resolved operando X‐ray absorption spectroscopy in combination with multivariate analysis we followed the transient speciation of Cu during unsteady state conditions. The results reveal that depending on operating temperature two different rate limiting behaviours inhibit the reduction of NO. At temperatures below 283 °C, ammonia hinders reoxidation of solvated CuI species thereby inhibiting reduction of NO. Whilst at temperatures of 283 °C and above, the reduction of zeolite bound CuII(OH−) is the rate limiting step in the SCR reaction. The results also reveal the presence of two detrimental side reactions occurring, the direct oxidation of zeolite bound CuI at low temperatures and the oxidation of ammonia over Cu at temperatures in excess of 283 °C. Between 250 °C and 350 °C, both side reactions may be present and could explain the dip in the SCR activity typically denoted by the seagull shape.

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Site‐Specific Reactivity of Copper Chabazite Zeolites with Nitric Oxide, Ammonia, and Oxygen

et al. 2017

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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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Nitrate–nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH₃-SCR

Abstract: A multi-technique in situ study of the reaction between NO and Cu-nitrates in the Cu-CHA deNOx catalyst yields novel structural and spectroscopic insights into the equilibrium between Cu-nitrates and Cu-nitrites, which was proposed as key step in the NH3-SCR cycle.

Cited by 46 publications

References 57 publications

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

Selective Catalytic Reduction of NO with NH₃ on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments

Site‐Specific Reactivity of Copper Chabazite Zeolites with Nitric Oxide, Ammonia, and Oxygen

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Nitrate–nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH3-SCR

Abstract: A multi-technique in situ study of the reaction between NO and Cu-nitrates in the Cu-CHA deNOx catalyst yields novel structural and spectroscopic insights into the equilibrium between Cu-nitrates and Cu-nitrites, which was proposed as key step in the NH3-SCR cycle.

Cited by 46 publications

References 57 publications

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH3 at Low Temperature

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH3 at Low Temperature

Selective Catalytic Reduction of NO with NH3 on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments

Site‐Specific Reactivity of Copper Chabazite Zeolites with Nitric Oxide, Ammonia, and Oxygen

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Nitrate–nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH₃-SCR

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

Selective Catalytic Reduction of NO with NH₃ on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments