Transient Redox Behavior of a NH3-SCR Cu-CHA SCR Catalyst: Effect of O2 Feed Content Variation

Nasello, Nicole Daniela; Gramigni, Federica; Nova, Isabella; Tronconi, Enrico; Hofmann, Frank; Dieterich, Simone; Crocoll, Marcus; Weibel, Michel

doi:10.1007/s11244-022-01715-1

Cited by 4 publications

(6 citation statements)

References 37 publications

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“…In all cases, independently from the oxygen feed content, a monotonic increase of the fraction of oxidized Cu sites was observed with increasing O 2 exposure time. Furthermore, the increase of O 2 concentration had a positive effect on the extent of Cu reoxidation, in line with previous findings . Above all, full reoxidation of the Cu-CHA25 catalyst was achieved after 1 h of exposure at all tested oxygen concentrations.…”

Section: Resultssupporting

confidence: 91%

Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

et al. 2023

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The reduction (RHC: Cu II → Cu I ) and oxidation (OHC: Cu I → Cu II ) half-cycles of low-temperature (150−200 °C) NH 3 -selective catalytic reduction (SCR) were investigated by transient response methods over a model Cu-CHA catalyst (Cu loading = 1.8% w/w; silica to alumina ratio = 25). In line with previous findings: i) RHC proceeded via second-order kinetics in Cu II and with an equimolar stoichiometry between Cu II reduced and NO consumed; ii) complete reoxidation of reduced NH 3solvated Cu sites was achieved by exposing the catalyst to O 2 + H 2 O only, no other species (e.g., NO) were found to be relevant to the OHC pathway. Dedicated kinetic tests highlighted a secondorder dependence of the OHC rate on Cu I and a first-order dependence on O 2 . Coupling the RHC and OHC kinetic models, independently developed from transient tests, resulted in an accurate description of Standard SCR turnovers, predicting steady-state NO conversion, N 2 formation, and bed-average Cu-oxidation states, as well as light-off and extinction transients closely consistent with experimental measurements. These results demonstrate the rational dissection of the complex steady-state SCR reaction network into two redox half-cycles and provide a stoichiometrically and kinetically consistent closure of the Standard SCR redox cycle over Cu-CHA catalysts.

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Section: Resultssupporting

confidence: 91%

Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

et al. 2023

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“…Notably, the same outcomes were achieved replicating the experimental protocol at different levels of oxygen (O 2 = 5% v/v in Figure B; O 2 = 1% v/v in Figure C). Specifically: (i) for each oxygen feed concentration, complete Cu I sites reoxidation was accomplished; (ii) in line with previous findings, , at fixed time, the lower the O 2 concentration, the lower the Cu I reoxidation extent; (iii) experimental OHC profiles collected under wet conditions were always faster than their dry counterparts, with an observed gap between dry and wet data increasing when decreasing the oxygen feed concentration.…”

Section: Resultssupporting

confidence: 84%

“…To evaluate the mentioned bed-average Cu II fraction, the O 2 feed was then cutoff to start a second reduction phase (Figure E,F). As discussed in Section , by integration of NO and N 2 dynamic profiles, we were able to estimate the standard SCR Cu II –Cu I partitioning through eq (see refs and . for additional details).…”

Section: Resultsmentioning

confidence: 99%

“…Once stable conditions were achieved, the remaining O 2 was removed from the feed, and a second reduction phase started. As discussed in Section , by integration of the NO and N 2 transient profiles collected in the second RHC step, and applying eq , we were able to indirectly estimate the bed-average Cu I –Cu II partitioning achieved at the STD-SCR steady-state. , …”

Section: Methodsmentioning

confidence: 99%

“…As discussed in Section 2.2.1, by integration of the NO and N 2 transient profiles collected in the second RHC step, and applying eq 1, we were able to indirectly estimate the bedaverage Cu I −Cu II partitioning achieved at the STD-SCR steady-state. 29,58 2.3. Kinetic Model.…”

Section: Testing Proceduresmentioning

confidence: 99%

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Investigation of Low-Temperature OHC and RHC in NH₃–SCR over Cu-CHA Catalysts: Effects of H₂O and SAR

Nasello,

Iacobone,

Usberti

et al. 2024

ACS Catal.

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A transient kinetic approach was applied to independently investigate the oxidation half-cycle (OHC) and reduction half-cycle (RHC) of NH 3 −selective catalytic reduction (SCR) at low temperature (150−200 °C). Three model Cuexchanged chabazite (Cu-CHA) samples with fixed Cu loading (∼1.8% w/w) and different silica-to-alumina ratios (SARs = 10-17-25) were investigated under dry and wet conditions. We confirmed the following: (i) OHC proceeds via second-and first-order kinetics in Cu I and O 2 , respectively, with O 2 (+H 2 O) alone able to completely reoxidize Cu I sites; (ii) RHC proceeds via second-and first-order kinetics in Cu II and NO, respectively, according to a Cu/NO = 1:1 stoichiometry. Notably, coupling RHC and OHC rates resulted in an accurate prediction of the steady-state standard SCR conditions, providing the consistent closure of the SCR redox chemistry. Unexpectedly, we revealed the impact of H 2 O to vary depending on the catalyst formulation. At high SAR, water inhibits the RHC and promotes the OHC. As a result, a limited impact was observed on steady-state deNO x activity, while the Cu-oxidation state was significantly enhanced by H 2 O. With decreasing SAR, however, the H 2 O effect on RHC gradually shifts from inhibition to promotion, while the OHC is always promoted. At fixed water content, we revealed the RHC rate to decrease with decreasing Al density, with minor influence observed on the OHC; as a result, a lower deNO x activity was observed upon increasing SAR. Remarkably, the application of transient kinetic analysis to decouple RHC and the OHC greatly facilitated the identification of complex H 2 O and SAR effects on the global SCR redox chemistry.

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A Strategic NH3-Dosing Approach for the Minimization of N2O Production During NH3-SCR Reactions over Cu-SSZ-13 Catalysts

Nasello,

Usberti,

Nova

et al. 2024

Emiss. Control Sci. Technol.

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The undesired production of N2O during NH3-SCR reactions is investigated over a reference commercial Cu-CHA catalyst. Steady-state experiments performed in the 150–500 °C temperature range exhibit a bimodal trend in the N2O formation profile, confirming the existence of two different reaction mechanisms occurring at low and high temperatures. Focusing on a low-to-medium T-range, N2O production, usually ascribed to NH4NO3 formation and decomposition, increases with the NO2/NOx ratio. However, an excess of NO2 leads to a decrease in the N2O release due to ammonium nitrate deposition and catalyst clogging phenomena. Steady-state and dynamics experiments show the promoting effect of both NH3 feed concentration and NH3 storage on N2O production at T > 200 °C. Surprisingly, N2O decreases with increasing NH3/NOx ratio at lower temperature. A novel approach based on the strategic injection of NH3 is also applied to mitigate the N2O formation while maintaining high deNOx activity. Remarkably, complete NOx conversion and ~ 11% N2O saving are achieved (with inlet NO2/NOx = 0–0.5) at temperatures exceeding 200 °C; in addition, a peculiar behavior is observed in the N2O profile, which increases and decreases when adding and removing NH3 from the feed, respectively. Notably, the opposite trend is observed in the N2O profile at 200 °C. When under Standard SCR conditions, this so far unreported observation challenges the NH4NO3 formation route for N2O and suggests the existence of different controlling phenomena at different temperature regimes: i) the Cu/redox chemistry at T ≤ 200 °C and ii) the NH3 storage at higher temperature, ideally up to 300 °C.

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Transient Redox Behavior of a NH3-SCR Cu-CHA SCR Catalyst: Effect of O2 Feed Content Variation

Cited by 4 publications

References 37 publications

Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

Investigation of Low-Temperature OHC and RHC in NH₃–SCR over Cu-CHA Catalysts: Effects of H₂O and SAR

A Strategic NH3-Dosing Approach for the Minimization of N2O Production During NH3-SCR Reactions over Cu-SSZ-13 Catalysts

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Transient Redox Behavior of a NH3-SCR Cu-CHA SCR Catalyst: Effect of O2 Feed Content Variation

Cited by 4 publications

References 37 publications

Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

Investigation of Low-Temperature OHC and RHC in NH3–SCR over Cu-CHA Catalysts: Effects of H2O and SAR

A Strategic NH3-Dosing Approach for the Minimization of N2O Production During NH3-SCR Reactions over Cu-SSZ-13 Catalysts

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Investigation of Low-Temperature OHC and RHC in NH₃–SCR over Cu-CHA Catalysts: Effects of H₂O and SAR