20 research outputs found
Catalyst systems for selective catalytic reduction + NO: X trapping: From fundamental understanding of the standard SCR reaction to practical applications for lean exhaust after-treatment
No full textWe have developed chemical trapping techniques as a novel tool to assess the nature of unstable reaction intermediates in the standard SCR reaction at low temperatures (120-200 °C). For this purpose, we have conducted transient response experiments over mechanical mixtures of an SCR catalyst (Fe-ZSM-5 or Cu-CHA) and a NOx storage material (BaO/Al2O3), which is able to trap and stabilize highly reactive NOx species. The results conclusively confirm that NO oxidative activation forms a gaseous intermediate, which acts like a nitrite precursor (e.g. HONO/N2O3) and is eventually stored on BaO/Al2O3. Such a species is also able to react with ammonia to produce N2, and is therefore proposed as a key intermediate of the standard SCR mechanism. We have further demonstrated that the capability of chemical trapping mechanical mixtures to capture NO in O2 at low temperature can also be exploited in practical applications for NOx emission control during cold start transients of diesel vehicles. In fact, such systems (SCR catalyst + NOx storage material) are characterized by an intrinsic dual functionality, being able both to store NOx when urea cannot be injected (e.g. below 170 °C) and to reduce the stored NOx with ammonia at higher temperatures in a single device. Accordingly, these mixtures have been renamed adsorption + selective catalytic reduction (AdSCR) systems. This review will summarize the main results achieved when implementing mechanical mixtures of NOx adsorbers and SCR catalysts both for fundamental understanding of the standard SCR mechanism and for abatement of cold start emissions
AdSCR Systems (Adsorption + Selective Catalytic Reduction): Analysis of the Influence of H2O and CO2 on Low Temperature NOx Emission Reduction Performances
No full textThe removal of NOx from low-temperature diesel engine emissions still represents a big challenge in view of the upcoming more stringent worldwide regulations. In our previous studies, we proved the ability of novel AdSCR (Adsorption + Selective Catalytic Reduction) systems, based on the combination of a chemical trapping compound and a conventional SCR catalyst, to trap cold start NOx emissions and to desorb and simultaneously reduce them with ammonia at higher temperature. In the present work, we extend the investigation of Cu-CHA + BaO/Al2O3 systems under more realistic conditions, focusing on the impact of H2O and CO2. The experimental results reveal a reduction of the AdSCR system performances with respect to dry and CO2-free conditions. Despite this, the system is still able to store and reduce NOx. The NOx storage capacity on barium oxide is more affected by the presence of CO2 than by H2O. However, H2O hinders the NO oxidative activation in the zeolite cages, which is a fundamental step in order to be able to trap NOx on the storage material at low temperature. We further demonstrate that the detrimental effect of H2O can be mitigated by small amounts of NO2 in the gaseous feed or by including a 13X zeolite guard bed prior to the AdSCR bed
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
No full textLow-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
Abgasnachbehandlungseinrichtung für ein Kraftfahrzeug mit einem Katalysator, welcher wenigstens ein SCR-Material und wenigstens ein NOx-Speicher-Material aufweist, und Verfahren zum Betreiben einer solchen Abgasnachbehandlungseinrichtung
No full textUnexpected Low-Temperature deNOx Activity of AdSCR Systems for Cold Start NOx Abatement
No full textLow-temperature operation of urea selective catalytic reduction (SCR) aftertreatment systems for the abatement of NOx from diesel engines presents new challenges related to poor catalytic activity and the urea injection temperature threshold. Physical mixtures of a NH3-SCR catalyst (e.g., Fe-/Cu-zeolite) and of a NOx storage material (e.g., BaO/Al2O3, CeO2/Al2O3) have shown promising performances in terms of overall NOx removal efficiency, as they can operate both as NOx adsorbers, trapping NOx during the cold start transient, and as SCR catalysts, reducing at higher temperatures the previously stored NOx with NH3. Herein, we extend the investigation of new Cu-CHA + BaO/Al2O3 AdSCR systems (AdSCR = adsorption + selective catalytic reduction) focusing on the reactivity between NOx and ammonia in the low-temperature window (from room temperature up to 170 °C). We find that the selective reduction of NO by NH3 over Cu-CHA is surprisingly enhanced by the presence of BaO/Al2O3 when ammonia is preadsorbed, leading to the onset of nitrogen formation already at 40 °C
Analysis of AdSCR Systems for NOx Removal During the Cold-Start Period of Diesel Engines
No full textHerein, we extend the study of the AdSCR system (AdSCR = adsorption + selective catalytic reduction), consisting in a physical mixture of a conventional NH3-SCR catalyst and of a NOx storage material. We have shown in previous work that an AdSCR system can capture and store NOx at room temperature from Diesel engine exhausts, and directly reduce them with ammonia at higher temperatures in the same unit. The present work aims at optimizing the system composition, in order to minimize the release of NOx in the low temperature window. Results from cold start mimicking experiments show that the full storage time, i.e. the zero-emission period where the fed NO is completely adsorbed by the catalyst, is affected only by the amount and the composition of the storage material, whereas the NOx storage efficiency is controlled by amount and nature of both components of the physical mixture
Unraveling the Hydrolysis of Z2Cu2+to ZCu2+(OH)-and Its Consequences for the Low-Temperature Selective Catalytic Reduction of NO on Cu-CHA Catalysts
Get PDFAs the state-of-the-art catalyst for the selective catalytic reduction (SCR) of NOx, Cu-CHA has been extensively investigated in both its practical and fundamental aspects. Among the latter, how Z2Cu2+, an active site for SCR, participates in low-temperature (LT) SCR reactions remains debated. Here, we propose a scheme involving the hydrolysis of Z2Cu2+ to ZCu2+(OH)-, a thermodynamically and kinetically favorable process under LT-SCR conditions, based on multiple pieces of evidence from a probe reaction (transient CO oxidation), transient Cu2+ reduction kinetic runs, in situ FTIR spectroscopy, and first-principles calculations. Such an integrated investigation reveals unambiguously that the hydrolysis of Z2Cu2+ to ZCu2+(OH)- occurs facilely in the presence of NH3, which may thus reconcile the identical quadratic kinetics of Z2Cu2+/ZCu2+(OH)- reduction with the inactivity of Z2Cu2+ in the formation of Cu2+ pairs. Accordingly, we highlight that NH3-assisted hydrolysis plays a critical role in LT-SCR and should be taken into account especially when discussing SCR reaction details over Z2Cu2+
Transient Kinetic Analysis of Low-Temperature NH3-SCR over Cu-CHA Catalysts Reveals a Quadratic Dependence of Cu Reduction Rates on CuII
No full textWe combine gas phase Transient Response Methods with Transient Kinetic Analysis to investigate the reduction half-cycle (RHC: CuII → CuI) of the Standard SCR redox mechanism over Cu-CHA (chabazite) catalysts. The results confirm that NO + NH3 can readily reduce CuII at low temperatures (150-220 °C) according to a Cu:NO:NH3:N2 = 1:1:1:1 stoichiometry. The observed CuII reduction dynamics are invariant with the CuII speciation. Unexpectedly, the CuII reduction rates show a quadratic dependence on CuII, which is hardly compatible with the so far proposed single-site RHC mechanisms. The second order kinetics are found to apply under both dry and wet conditions (0% and 2% H2O v/v in the feed gas, respectively) across different temperatures, space velocities, and NO feed concentrations over two powdered Cu-CHA catalysts with different Cu loadings as well as over a commercial Cu-CHA washcoated honeycomb monolith catalyst. Another unprecedented finding is that H2O significantly inhibits the CuII reduction rate and lowers the RHC apparent activation energy. These findings provide for the first time a complete kinetic description of the low-temperature RHC reaction cascade and, from a mechanistic perspective, strongly suggest a dinuclear-CuII mediated RHC pathway, which may renew interrogations on the current mechanistic understanding of the CuII reduction pathway in the low-temperature NH3-SCR redox chemistry over Cu-CHA
An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst
No full textThe reactivity of Lewis and Brønsted ammonia in the reduction half-cycle (RHC) of the NH3-SCR low temperature redox mechanism was studied over a model Cu−CHA catalyst by transient kinetic tests involving reductive NO pulses. The CuII sites were reduced according to a 1:1:1:1 molar ratio with NO and NH3 conversion and N2 formation. The ammonia coordinated to Cu sites (Lewis ammonia) was preferentially consumed prior to that stored on the Brønsted acid sites. The catalyst was effectively re-oxidized by O2 in He at 150 °C even when the Cu-coordinated ammonia was depleted. A redox kinetic model assuming NO activation by CuII to a gaseous mobile intermediate (HONO) which reacts first with Lewis-NH3 and then with Brønsted-NH3 was successfully fitted to our transient data assuming the CuII reduction rate to be second order in the CuII sites. This suggests a possible role of CuII dimeric complexes in the RHC of Standard SCR
On the Redox Mechanism of Low-Temperature NH3-SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle
No full textCu-CHA is the state-of-the-art catalyst for the Selective Catalytic Reduction (SCR) of NOx in vehicle applications. Although extensively studied, diverse mechanistic proposals still stand in terms of the nature of active Cu-ions and reaction pathways in SCR working conditions. Herein we address the redox mechanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques, transient-response methods, operando UV/Vis-NIR spectroscopy with modelling tools based on transient kinetic analysis and density functional theory calculations. We show that the rates of the Reduction Half-Cycle (RHC) of LT-SCR display a quadratic dependence on CuII, thus questioning mechanisms based on isolated CuII-ions. We propose, instead, a CuII-pair mediated LT-RHC pathway, in which NO oxidative activation to mobile nitrite-precursor intermediates accounts for CuII reduction. These results highlight the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC reaction cascade
