Towards Ultra-Low NOx Emissions within GHG Phase 2 Constraints: Main Challenges and Technology Directions

Seykens, Xander; Kupper, Frank; Mentink, Paul; Ramesh, Sudarshan

doi:10.4271/2018-01-0331

Cited by 11 publications

(5 citation statements)

References 12 publications

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“…Even in the 2020s, nitrogen oxides (NOx) emissions from diesel and lean-burn engines continue to represent a concerning environmental issue, and their control still attracts the interest of the scientific community. The well-established NH 3 -selective catalytic reduction (SCR) technology on Cu-CHA catalysts is considered the state-of-the-art DeNOx process, enabling a highly efficient NOx mitigation for a wide range of commercial applications. , As a result, the number of publications related to these materials has impressively increased in the last decade. In particular, great attention has been paid to the low-temperature (<250 °C) reaction mechanism under Standard SCR (STD-SCR) conditions () − 4 normalN normalH 3 + 4 normalN normalO + normalO 2 → 4 normalN 2 + 6 normalH 2 normalO …”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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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: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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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“…Given the upcoming stricter environmental regulations, internal combustion engine exhausts are still considered a critical concern, particularly for the automotive industry. The NH 3 –selective catalytic reduction (SCR) technology, which selectively reduces nitrogen oxides (NO x ) to harmless N 2 and H 2 O molecules according to standard SCR as the main reaction , is presently considered the leading after-treatment technique for denitrification of diesel exhausts. − 4 normalN normalH 3 + 4 N O + O 2 → 4 N 2 + 6 H 2 normalO …”

Section: Introductionmentioning

confidence: 99%

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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“…In this context, one possibility consists in positioning the SCR unit in proximity to the engine (e.g., by deploying closed-coupled SCR catalysts, cc-SCR, or SCR on filters, SDPF) in order to shorten the SCR warm-up time. 3,4 Nevertheless, the aforementioned upstream positioning, in combination with the limited catalytic performance at low temperature, may lead to HC (hydrocarbon) slip over the SCR component negatively affecting its DeNOx activity. 5−10 Furthermore, during the engine warm up, the aftertreatment system is not hot enough to allow a complete oxidation of HCs on the DOC unit.…”

Section: ■ Introductionmentioning

confidence: 99%

“…The NH 3 -SCR (selective catalytic reduction) process is nowadays regarded as the most important technology for NOx reduction from lean burn engines. , Nevertheless, albeit the devices have been implemented for many years, the enhancement of their performance at low temperature represents an actual challenge for the vehicle manufactory that has to achieve the current emission targets. In this context, one possibility consists in positioning the SCR unit in proximity to the engine (e.g., by deploying closed-coupled SCR catalysts, cc-SCR, or SCR on filters, SDPF) in order to shorten the SCR warm-up time. , Nevertheless, the aforementioned upstream positioning, in combination with the limited catalytic performance at low temperature, may lead to HC (hydrocarbon) slip over the SCR component negatively affecting its DeNOx activity. − Furthermore, during the engine warm up, the aftertreatment system is not hot enough to allow a complete oxidation of HCs on the DOC unit . Therefore, the impact of HCs on state-of-the-art SCR catalysts has been deeply explored and integrated in simulation models. ,− …”

Section: Introductionmentioning

confidence: 99%

Review of Hydrocarbon Poisoning and Deactivation Effects on Cu-Zeolite, Fe-Zeolite, and Vanadium-Based Selective Catalytic Reduction Catalysts for NOx Removal from Lean Exhausts

Gramigni

Iacobone

Nasello

et al. 2021

Ind. Eng. Chem. Res.

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Low-temperature operation of NH3-SCR (selective catalytic reduction) systems for NOx abatement in lean streams raises major challenges caused by the need to fix different problems related to poor catalyst activity and to urea injection handling. In this respect, an important issue is related to the presence of unburned hydrocarbons (HCs) in the exhausts that may damage irreversibly the activity of the DeNOx catalysts. The purpose of the present perspective is to summarize the most important effects of HCs on NH3-SCR performances of commercial SCR catalysts, as well as to present a comprehensive inventory of available kinetic models. In particular, the following main aspects will be discussed, according to recent literature indications: (i) competitive adsorption between HC and NH3; (ii) effect of the zeolite structure on HC deactivation; (iii) potential formation of surface intermediates and active site blocking; (iv) pore physical blocking due to large HC molecules or coke formation; (v) possible parasitic reactions between HCs and SCR reagents; vi) description of the available kinetic models able to account for these effects; and vii) design of improved catalysts with enhanced hydrocarbon poisoning resistance.

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Towards Ultra-Low NOx Emissions within GHG Phase 2 Constraints: Main Challenges and Technology Directions

Cited by 11 publications

References 12 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

Review of Hydrocarbon Poisoning and Deactivation Effects on Cu-Zeolite, Fe-Zeolite, and Vanadium-Based Selective Catalytic Reduction Catalysts for NOx Removal from Lean Exhausts

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Towards Ultra-Low NOx Emissions within GHG Phase 2 Constraints: Main Challenges and Technology Directions

Cited by 11 publications

References 12 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

Review of Hydrocarbon Poisoning and Deactivation Effects on Cu-Zeolite, Fe-Zeolite, and Vanadium-Based Selective Catalytic Reduction Catalysts for NOx Removal from Lean Exhausts

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