Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH3-Mediated Selective Catalytic Reduction

Negri, Chiara; Selleri, Tommaso; Borfecchia, Elisa; Martini, Andrea; Lomachenko, Kirill A.; Janssens, Ton V. W.; Cutini, Michele; Bordiga, Silvia; Berlier, Gloria

doi:10.1021/jacs.0c06270

Cited by 137 publications

(223 citation statements)

References 56 publications

(220 reference statements)

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“…Finally, particularly for the Standard SCR reaction (R4), the fundamental redox reaction mechanism has yet to be clarified, and this has led, in recent years, to a lively and prolific discussion in the specialised literature. More details concerning the latest development in SCR chemistry can be found in [16][17][18].…”

Section: Basic Principlesmentioning

confidence: 99%

An Overview of Lean Exhaust deNOx Aftertreatment Technologies and NOx Emission Regulations in the European Union

et al. 2021

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This paper reviews the recent advances in the management of nitrogen oxide (NOx) emissions from the internal combustion engine of light-duty and heavy-duty vehicles, addressing both technical and legal aspects. Particular focus is devoted to the often-virtuous interaction between new legislation imposing more restrictions on the permitted pollutant emission levels and new technologies developed in order to meet these restrictions. The review begins first with the American and then European directives promulgated in the 1970s, aimed at limiting emissions of pollutants from road transport vehicles. Particular attention is paid to the introduction of the Euro standards in the European Union for light- and heavy-duty vehicles, used as a legal and time frame reference for the evolution of emission aftertreatment systems (ATSs). The paper also describes governmental approaches implemented for the control of pollutant emissions in circulating vehicles, such as market surveillance and in-service conformity. In parallel, it is explained how the gradual introduction of small-scale devices aimed at the NOx control, such as lean NOx traps (LNTs) systems, and, most of all, the selective catalytic reduction (SCR) of NOx, permitted the application to road-transport vehicles of this ATS, originally designed in larger sizes for industrial usage. The paper reviews chemical processes occurring in SCR systems and their advantages and drawbacks with respect to the pollutant emission limits imposed by the legislation. Their potential side effects are also addressed, such as the emission of extra, not-yet regulated pollutants such as, for example, NH3 and N2O. The NOx, N2O, and NH3 emission level evolution with the various Euro standards for both light- and heavy-duty vehicles are reported in the light of experimental data obtained at the European Commission’s Joint Research Centre. It is observed that the new technologies, boosted by increasingly stricter legal limits, have led in the last two decades to a clear decrease of over one order of magnitude of NOx emissions in Diesel light-duty vehicles, bringing them to the same level as Euro 6 gasoline vehicles (10 mg/km to 20 mg/km in average). On the other hand, an obvious increase in the emissions of both NH3 and N2O is observed in both Diesel and gasoline light-duty vehicles, whereby NH3 emissions in spark-ignition vehicles are mainly linked to two-reaction mechanisms occurring in three-way catalysts after the catalyst light-off and during engine rich-operation. NH3 emissions measured in recent Euro 6 light-duty vehicles amount to a few mg/km for both gasoline and Diesel engines, whereby N2O emissions exceeding a dozen mg/km have been observed in Diesel vehicles only. The present paper can be regarded as part of a general assessment in view of the next EU emission standards, and a discussion on the role the SCR technology may serve as a NOx emission control strategy from lean-burn vehicles.

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Section: Basic Principlesmentioning

confidence: 99%

An Overview of Lean Exhaust deNOx Aftertreatment Technologies and NOx Emission Regulations in the European Union

et al. 2021

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“…Furthermore, there are differences in the energy position and shape of the white-line peak. The PC3 resembles a Z[Cu I (NH 3 )] species observed in the Cu K-edge XANES in Cu-CHA in a NH 3 -TPD procedure [60] and also in the reaction of NO with the mobile [Cu 2 (NH 3 ) 4 O 2 ] 2+ complexes [22]. Therefore, PC3 is assigned to a framework-coordinated quasi-linear Cu I species formed by thermal desorption of one ammonia ligand molecule from the mobile [Cu I (NH 3 ) 2 ] + complex, which we denote as Z[Cu I (NH 3 )] (Fig.…”

Section: Principal Component Analysis (Pca) and Multivariate Curve Rementioning

confidence: 65%

“…These mobile complexes are involved in the re-oxidation step at 200 °C in O 2 , forming oxygen-bridged diamino dicopper(II) complexes with general formula [Cu 2 (NH 3 ) 4 O 2 ] 2+ . According to DFT calculations [8], the structure of these dicopper(II) complexes is a side-on μ-η 2 ,η 2 -peroxo diamino dicopper(II), which has recently been confirmed by UV-Vis and X-ray spectroscopies [22,23]. In the reduction half cycle, the mobile [Cu I (NH 3…”

Section: Introductionmentioning

confidence: 85%

“…In agreement with previous reports, some differences are observed with respect to the spectrum of [Cu I (NH 3 ) 2 ] + at increasing temperature (from 200 °C upward), particularly in the shape of the white-line peak. This has been attributed to the formation of Z[Cu I (NH 3 )] species, where a Cu I center is coordinated to an oxygen atom of the zeolite framework and to an ammonia ligand molecule, in a quasi-linear fashion [22,60]. This means that the [Cu I (NH 3 ) 2 ] + complexes formed by reduction in NO/NH 3 release an NH 3 ligand above 200 °C and subsequently lose their mobility [13,14,62].…”

Section: Xanes Spectral Evolution During No-tprmentioning

confidence: 99%

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In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

et al. 2021

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Ammonia-mediated selective catalytic reduction (NH3-SCR) using Cu-exchanged chabazite zeolites as catalysts is one of the leading technologies for NOx removal from exhaust gases, with CuII/CuI redox cycles being the basis of the catalytic reaction. The amount of CuII ions reduced by NO/NH3 can be quantified by the consumption of NO during temperature-programmed reduction experiments (NO-TPR). In this article, we show the capabilities of in situ X-ray absorption near-edge spectroscopy (XANES), coupled with multivariate curve resolution (MCR) and principal component analysis (PCA) methods, in following CuII/CuI speciation during reduction in NO/NH3 after oxidation in NO/O2 at 50 °C on samples with different copper loading and pretreatment conditions. Our XANES results show that during the NO/NH3 ramp CuII ions are fully reduced to CuI in the 50–290 °C range. The number of species involved in the process, their XANES spectra and their concentration profiles as a function of the temperature were obtained by MCR and PCA. Mixed ligand ammonia solvated complexes [CuII(NH3)3(X)]+ (X = OH−/O− or NO3−) are present at the beginning of the experiment, and are transformed into mobile [CuI(NH3)2]+ complexes: these complexes lose an NH3 ligand and become framework-coordinated above 200 °C. In the process, multiple CuII/CuI reduction events are observed: the first one around 130 °C is identified with the reduction of [CuII(NH3)3(OH/O)]+ moieties, while the second one occurs around 220–240 °C and is associated with the reduction of the ammonia-solvated Cu-NO3− species. The nitrate concentration in the catalysts is found to be dependent on the zeolite Cu loading and on the applied pretreatment conditions. Ammonia solvation increases the number of CuII sites available for the formation of nitrates, as confirmed by infrared spectroscopy.

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“…Remarkable advances have been achieved towards the goal of understanding the rate-limiting steps of the SCR mechanism [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Cu(II) ions are present in Cu/zeolite materials (they were shown to be more active and hydrothermally stable than Cu(II)-OH ions [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]25]. The rate-limiting step for low-temperature SCR was shown to be re-oxidation of reduced Cu(I)(NH3)2 complexes via the formation of transient (NH3)2Cu(II)-O2-Cu(II)(NH3)2 dimers [17,18].…”

mentioning

confidence: 99%

Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

Khivantsev

Kwak²,

Jaegers³

et al. 2020

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Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH3)2 with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO+ in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N2H+ cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO+ reacts, “freed up” Bronsted acid site gets occupied by NH3 to form NH4+ – and so the catalytic cycle stops because NO+ does not form on NH4-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO+ by the reaction: Cu(II) + NO à Cu(I) + NO+ which we further confirm spectroscopically. The NO+ then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.

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Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH₃-Mediated Selective Catalytic Reduction

Cited by 137 publications

References 56 publications

An Overview of Lean Exhaust deNOx Aftertreatment Technologies and NOx Emission Regulations in the European Union

An Overview of Lean Exhaust deNOx Aftertreatment Technologies and NOx Emission Regulations in the European Union

In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

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