Low Temperature NOx Reduction Research Articles

Development of an Onboard Urea Electrolyser to Reduce NOx Emissions in Vehicles By Enhancing the Selective Catalytic Reduction with the Produced H2 and NH3

In the persistent pursuit of reducing nitrogen oxide emissions from diesel exhaust, challenges arise in achieving optimal performance at lower temperatures, especially in urban driving conditions. This research focuses on a pragmatic approach – development of an onboard urea electrolyser using diesel exhaust fluid (DEF) as the hydrogen-rich carrier. The primary goal is to enhance the NOx reduction efficiency at temperatures below 200oC by providing the electrolyser products – hydrogen and ammonia [1]. The reason behind this innovation lies in recognising DEF as a viable electrolyte, offering both safety and a promise of performance increase due to splitting of urea instead of water. Beyond the contribution to NOx reduction, this approach aligns with the broader societal and political imperatives of integrating cleaner energy solutions within the automotive sector. However, the development of a functional urea electrolyser involves certain complexities. The key considerations are the material stability under operating conditions, anode catalyst dissolution in DEF, mass transport limitations, and the reduction of relatively high cathode overpotentials. A notable aspect of our research involves addressing these challenges in scenarios both with and without potassium hydroxide alongside DEF, requiring a nuanced understanding of the electrochemical processes involved. Additionally, weight, packaging, and system integration play an important role in the development of an onboard urea electrolyser.This study systematically explores these challenges to devise a comprehensive understanding of the underlying electrochemical processes involved as well as the integration and operation strategy. The research methodology adopts single cell tests for which activation and measurement protocols have been established to achieve representative and comparable performance curve recordings and electrochemical impedance spectroscopy (EIS) data. Additionally, to various operating conditions of the electrolyte, the cell components screening adds to a considerable effort of this study. We explore the influence on the materials and design of the bipolar plates (BP) and porous transport layers (PTL); electrode performance and durability as well as the membrane selection. Within this study we consider anion exchange membranes (AEM) and bipolar membranes (BPM) for the performance comparison. Adopting both catalyst-coated membrane (CCM) and catalyst-coated substrate (CCS) approaches enables for examination of electrode preparation techniques and their influence on performance of the electrolysis cell. The initial results are promising, especially when employing 1 M KOH in the DEF electrolyte at 70oC, reaching 0.85 A cm-1 at 2 V. The respective performance curve is presented in Figure 1., together with a diagram of the electrolyser system integration into an existing Diesel exhaust line.This research represents a practical stride towards realising the denitrification of transportation sector especially in city cycle driving when the exhaust gases temperature is too low for the state-of-the-art SCR systems. By addressing identified challenges and optimising key parameters, our study lays a robust foundation for the scalable implementation of a functional urea electrolyser for transport applications and a basic understanding of the electrochemical processes behind the performance and stability of such a system.Figure 1.Depicts a recorded polarisation curve of a single cell using an anion exchange membrane (AEM) and nickel catalyst anode fed DEF and 1 M KOH as an electrolyte at 70oC. Below, the suggested integration of the electrolyser into latest generation Diesel exhaust aftertreatment system. It includes diesel oxidation catalyst (DOC), selective Diesel particulate filter (sDPF), hydrogen supported denitrification catalyst ‘H2-deNOx’ and finally a selective catalytic reduction (SCR) step.Reference: Esser, E., Kureti, S., Heckemüller, L., Todt, A. et al., "Low-Temperature NOx Reduction by H2 in Diesel Engine Exhaust," SAE Int. J. Adv. & Curr. Prac. in Mobility 4(5):1828-1845, 2022, https://doi.org/10.4271/2022-01-0538. Figure 1

Improved N2 selectivity for low-temperature NOx reduction over etched ZSM-5 supported MnCe oxide catalysts

Improved N2 selectivity for low-temperature NOx reduction over etched ZSM-5 supported MnCe oxide catalysts

Low-temperature NOx reduction over Cu-LTA and SmMnOx composite catalysts

Low-temperature selective catalytic reduction of NOx with ammonia (NH3-SCR) over zeolite catalysts remains a great challenge in diesel exhaust purification. Herein, Cu-LTA zeolite with excellent hydrothermal stability was composited with a small proportion of SmMnOx oxides, the composite catalytic system efficiently resolves the low-temperature activity challenge encountered by Cu-LTA. The promoting pathways revealed the presence of active nitrite intermediates formed on SmMnOx by activating NO, that were able to migrate to the Cu-LTA and can be further decomposed on the Brønsted acid sites. Compared to the continuous deposition of nitrates in Cu-LTA, the presence of SmMnOx in composite catalysts efficiently reduced the accumulation of inert nitrate and improved the nitrate deposition phenomena. This innovative research would provide a rational strategy to break the barrier of limited low-temperature performance faced by zeolite catalysts, effectively promoting the NOx removal in vehicles sources.

Hydrothermally stable metal oxide-zeolite composite catalysts for low-temperature NOx reduction with improved N2 selectivity

Development of hydrothermally stable, low-temperature catalysts for controlling nitrogen oxides emissions from mobile sources remains an urgent challenge. We have prepared a metal oxide-zeolite composite catalyst by depositing Mn active species on a mixture support of CeO2/Al2O3 and ZSM-5. This composite catalyst is hydrothermally stable and shows improved low-temperature SCR activity and significantly reduced N2O formation than the corresponding metal oxide catalyst. Comparing with a Cu-CHA catalyst, the composite catalyst has a faster response to NH3 injection and less NH3 slip. Our characterization results reveal that such an oxide-zeolite composite catalyst contains more acidic sites and Mn3+ species as a result of oxide-zeolite interaction, and this interaction leads to the generation of more NH4+ species bound to the Brønsted acid sites and more reactive NOx species absorbed on the Mn sites. Herein, we report our mechanistic understanding of the oxide-zeolite composite catalyst and its molecular pathway for improving the low-temperature activity and N2 selectivity for NH3-SCR reaction. Practically, this work may provide an alternative methodology for low-temperature NOx control from diesel vehicles.

Highly active alkali etching silica-modified Cu-Ce-Ti catalyst with superior water resistance for low temperature NOx reduction

In this study, Cu-Ce oxides supported on a silica modified TiO2 with post-alkali treatment (Cu-Ce/TS-am) were synthesized to enhance the catalytic activity and water resistance for low-temperature NOx reduction with ammonia. Experiment results revealed that the optimal Cu-Ce/TS-am sample possessed over 90% NOx conversion at 180–350 °C in the presence of 10 vol% water. Essentially, the formed Ti-Si strong interactions after Si addition could be responsible for that. On the one hand, the interactions increased specific surface area, total acidity and oxygen mobility, thus promoting SCR activity. On the other hand, it could enhance the hydrophobicity of the catalysts as there existed lower surface roughness, smaller particle size and enlarged mesoporous pore size after Si addition, alleviating the competitive adsorption of water with NH3. Additionally, the alkali treatment removed uncombined SiOx, which optimized the mesoporous pore structure and increased the exposure of active sites and acidic sites on the catalyst surface, further enhancing water resistance and catalytic activity. These findings could provide useful guidelines for the development of highly active SCR catalyst with well water resistance in real application.

Low-temperature selective catalytic reduction of NOx with NH3 over in-situ grown MnOx-Fe2O3/TiO2/Ti monolithic catalyst

The design and synthesis of highly efficient and low-temperature NOx reduction catalysts with long-term stability and good water resistance still remain a challenge in the field of gas catalysis. In this work, we demonstrated the integration of MnOx-Fe2O3/TiO2/Ti monolithic catalysts on flexible Ti mesh through plasma electrolytic oxidation technology, associated with hydrothermal reaction, and chemical bath deposition methods. The in-situ grown TiO2 nanosheet supports hold a very strong substrate adhesion with Ti mesh, and provide ideal nucleation sites for the following deposition of high density of MnOx-Fe2O3 active components. The brush-like nanostructure morphology of active MnOx-Fe2O3 catalysts enables the complete adsorption and reaction of gas molecules on the catalyst surface. The selective catalytic reduction of NOx with NH3 tests indicate that MnOx-Fe2O3/TiO2/Ti monolithic catalysts exhibit excellent low-temperature catalytic performance with 100% of NOx conversion at 137 °C, good stability and water resistance, showing great potential and broad application prospects in comparison with powder catalysts and honeycomb ceramic catalysts. The strategy of depositing multi-components active catalysts on in-situ grown ultra-thin TiO2 nanosheet supports on flexible Ti mesh paves a solid way toward the practical applications of novel low-temperature selective catalytic reduction catalysts in diverse fields.

Low-Temperature H2-deNOx in Diesel Exhaust

For NOx removal from diesel exhaust, the selective catalytic reduction (SCR) is the most common abatement technology. However, low engine load scenarios such as city driving and cold start phases demand efficient NOx reduction clearly below 200 °C, which is difficult to achieve with SCR. Hence, this work investigates the potential of the low-temperature NOx reduction with H2 in diesel exhaust. A monolithic Pt/WOx/ZrO2 catalyst, recently reported as highly active, was evaluated in synthetic and real diesel exhaust. The monolith demonstrated high deNOx activity between 130 and 215 °C in the synthetic exhaust including peak conversion of 90% with N2 selectivity up to 85%. CO/HC components were shown to inhibit the H2-deNOx conversion thus requiring a pre-oxidation catalyst in practice. Furthermore, studies performed in an optical reactor indicated strong heat evolution along the monolith. As a result, the reaction kinetics was accelerated with an enhanced consumption of H2 limiting the H2-deNOx efficiency above 200 °C. Stationary tests in diesel engine exhaust substantiated the low-temperature H2-deNOx performance of the monolith including NOx conversions up to 80% at temperatures as low as 135 °C.

Low-Temperature NOx Reduction by H2 on Mo-Promoted Pt/ZrO2 Catalysts in Lean Exhaust Gases

AbstractThis article aims to improve the low-temperature H2-deNOxperformance of the active Pt/ZrO2catalyst using MoOxas a promoter. For this purpose, a systematic series of Pt/ZrO2samples were prepared with a Pt content of 0.25 wt% and Mo loads from 0 to 10 wt%. The samples were physico-chemically characterized by means of powder X-ray diffraction, N2physisorption, temperature-programmed desorption of CO and NH3, Raman spectroscopy and diffuse reflectance infrared spectroscopy using NH3as probe molecule, while the H2-deNOxefficiency was investigated in a lean synthetic exhaust. The Pt/ZrO2catalyst with a Mo load of 3 wt% showed the best performance, including H2-deNOxbetween 80 °C and 150 °C, a maximum NOxconversion of 90% and N2selectivity up to 78%. Isolated MoOxspecies predominately present at Mo loads below 4 wt% were found to act as structural promoter by stabilizing the BET surface area, while also providing smaller Pt particles and more active Pt sites, respectively. By contrast, the aggregated Mo oxide moieties found at higher Mo loads exhibit a clearly weaker promotional effect. The structure–activity-selectivity correlations also suggest that the promoter additionally enables a SCR-related mechanistic pathway to be followed, including the spill-over of NHxspecies from the Pt sites to strong Lewis acid sites in the case of highly dispersed MoOxentities followed by reaction with NOx.

Low-temperature NOx reduction over hydrothermally stable SCR catalysts by engineering low-coordinated Mn active sites

Low-temperature selective catalytic reduction (SCR) of NOx over hydrothermally stable metal oxide catalysts is still a great challenge in heavy-duty diesel exhaust purification. Herein, hydrothermally stable MnOx/Al2O3 catalysts with highly dispersed low-coordinated Mn active sites were originally demonstrated for low-temperature NOx reduction. The catalyst exhibited greater than 90 % NOx conversion within 200–360 °C even after a 24 h aging treatment at 650 °C with a flow of 10 % H2O. A series of characterization revealed that the developed catalysts with more active low-coordinated Mn species possessed reactive redox sites and Lewis acid sites. Thus, the successive adsorption and activation of NH3 and NOx species would be efficiently accelerated and proceeded with the subsequent SCR reaction over hydrothermally stable MnOx/Al2O3 catalysts. This work discovered hydrothermally stable manganese oxide catalyst for low-temperature NOx reduction which would provide new opportunities for the application of metal oxide catalysts among diesel engine exhaust emission control.

Green synthesis of mesoporous MnNbOx oxide by a liquid induced self-assembly strategy for low-temperature removal of NOx.

Herein, we present a facile, sustainable, and highly efficient ionic liquid induced self-assembly protocol to construct a highly porous MnNbOx oxide with exceptionally high catalytic activity for NOx removal at low temperatures and excellent tolerance to H2O and SO2 poisoning.

SO2 Poisoning of the NH3-SCR Reaction over Cu-SAPO-34: Effect of Ammonium Sulfate versus Other S-Containing Species

SO2 poisoning of NH3-SCR over Cu-SAPO-34 was studied, specifically to evaluate the forms/states of stored S and the effect of such species on low-temperature NOx reduction activity. Two primary sulfur species types were observed and were found to be interchangeable depending on whether NH3 was available or not. In one case both ammonium sulfate and Cu sulfate species could be present and in the other only Cu sulfate species. Cu sulfate, in the absence of ammonia, was found in three different states/forms, identified by three desorption features during temperature-programmed desorption (TPD) experiments. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO adsorption was used to investigate the nature and accessibility of the Cu species before and after sulfate formation, without the interference of ammonium sulfate; these data revealed that the Cu2+ inside the six-membered rings was completely blocked by sulfur and that the nature of the [CuOH]+ close to the eight-membered ring chang...

Effect of H2 on deNOx performance of HC-SCR over Ag/Al2O3: Morphological, chemical, and kinetic changes

The enhanced deNOx performance of the Ag/Al2O3 catalyst by addition of H2 was investigated for HC-SCR using a mixture of simulated diesel and ethanol as the reductant. As H2 concentration increased in the feed-gas stream, the low-temperature NOx reduction activity was remarkably enhanced. Addition of H2 promoted the oxidation of both NO and hydrocarbons, resulting in an increased formation of nitrates, enolic, and isocyanate species regarded as reaction intermediates. Results of the UV–vis indicated that H2 promoted the transformation of Ag+ to Agnδ+ and Ag0 on the surface of Ag/Al2O3 catalyst, while those of H2-TPR, O2-TPD, in situ FTIR, and TGA revealed the formation of highly mobile and reactive oxygen species under the (O2+H2) flow. A reaction mechanism is proposed to elucidate the role of H2 in the enhancement of deNOx performance of HC-SCR in view of morphological, chemical, and kinetic changes of the Ag/Al2O3 catalytic system.

Lean NOx reduction on LNT-SCR dual-layer catalysts by H2 and CO

Lean reduction of NOx (NO & NO2) by H2 and CO was conducted over monolithic catalysts consisting of a selective catalytic reduction (SCR) catalyst layer deposited on top of a lean NOx trap (LNT) catalyst. An increase in the CO/H2 ratio decreased the cycle-averaged NOx conversion for the ceria-free LNT catalyst. CO poisoning was especially significant below 250°C. The low-temperature NOx reduction was increased either by use of an LNT-SCR dual-layer catalyst or deposition of CeO2 on the LNT catalyst. However, the ceria decreased the high-temperature reductive conversion of NOx due to promotion of the undesired NH3 oxidation. Ceria zoning enhanced the monolith NOx conversion. Downstream loading of ceria led to the highest NOx reduction at both low- and high- temperatures due to the beneficial interaction of the ceria and H2. The low-temperature NOx conversion of an aged dual-layer catalyst could be increased by a higher SCR catalyst loading. However, at high temperatures NOx reduction was independent of the SCR loading. The ratio of the lean to rich feed duration and the total cycle time were optimized to improve the NOx conversion in a temperature range from 150 to 400°C. The highest cycle-averaged NOx conversion was obtained with a 30s:5s lean-rich cycle containing 1.25% total reductant for all CO/H2 ratios for a lean feed containing 500ppm NO and 5% O2.

SO2 poisoning and regeneration of Mn-Ce/TiO2 catalyst for low temperature NOx reduction with NH3

SO2 poisoning and regeneration of Mn-Ce/TiO2 catalyst prepared by a novel co-precipitation method for low temperature selective catalytic reduction (SCR) of NOx with ammonia were investigated in this study. When 700 ppm SO2 was fed in, the Mn-Ce/TiO2 catalyst had good resistance to SO2, but the deactivation of Mn-Ce/TiO2 poisoned by SO2 still occurred. The NO conversion of Mn-Ce/TiO2 (the molar ratio of Ce to Ti is 0.075) catalyst decreased from 92.5% to 34.6% in 13 h. Characterizations of fresh and SO2-poisoned Mn-Ce/TiO2 catalysts were carried out by Brunauer-Emmett-Teller method (BET), ion chromatography (IC), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The characterized results indicated that the deposition of sulfates and nitrates on the surface made the catalyst deactivated. Water washing, thermal regeneration and reductive regeneration were used to regenerate the deactivated Mn-Ce/TiO2. And water washing showed best performance on the regeneration of poisoned catalysts, especially with ultrasonic vibration. The Mn-Ce/TiO2 catalyst showed high stability under a series of deactivation-regeneration experiments for ten cycles.

New “Enhanced NH3-SCR” Reaction for NOx Emission Control

NH3-selective catalytic reduction (SCR) is applied worldwide to DeNOx of combustion exhausts of stationary and mobile sources, but a greater DeNOx activity at low temperatures is desired to meet forthcoming restrictive legislations and to increase energy efficiency in specific applications. We have obtained high NO reduction efficiencies at 200−300 °C over commercial V-based and Fe-exchanged zeolite catalysts by reacting NO with both ammonia and nitrates (e.g., ammonium nitrate) according to a novel “enhanced SCR” reaction, 2NH3 + 2NO + NH4NO3 → 3N2 + 5H2O. This provides a new route for low-temperature NOx reduction wherein the DeNOx activity is maximized by dosing an aqueous solution of nitrates to the SCR reactor feed stream.

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250°C and could potentially deactivate low temperature NOx reduction catalysts by blocking active sites. It is shown that NO reduces neat NH4NO3 above its 170°C melting point, while acidic solids catalyze this reaction even at temperatures below 100°C. NO2, a product of the reduction, can dimerize and then dissociate in molten NH4NO3 to NO++NO3−, and may be stabilized within the melt as either an adduct or as HNO2 formed from the hydrolysis of NO+ or N2O4. The other product of reduction, NH4NO2, readily decomposes at ≤100°C to N2 and H2O, the desired end products of DeNOx catalysis. A mechanism for the acid catalyzed reduction of NH4NO3 by NO is proposed, with HNO3 as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNOx systems could help mitigate catalyst deactivation at low operating temperatures (<150°C).

05/00329 Catalytic exhaust gas fuel reforming for diesel engines — effects of water addition on hydrogen production and fuel conversion efficiency: Tsolakis, A. and Megaritis, A. International Journal of Hydrogen Energy, 2004, 29, (13), 1409–1419

In the presented work the selective catalytic reduction (SCR) of NOx in a real diesel engine exhaust gas (O2 present) from the engine operating at different conditions with and without H2 and CO additions were studied. The tests were carried out using real diesel engine exhaust gas over 1%Pt supported on alumina (Al2O3). The catalyst exhibits good NOx reduction activity at a narrow temperature range of 200 to 300 ∘C when there is only a HC present. The maximum NOx reduction of around 60% was achieved at temperature of 260 ∘C. Although, the engine operating with EGR improves the percentage of NOx converted in the SCR system due to increased HC:NOx ratio and reduced NOx concentration in the engine exhaust gas, the number of NOx-ppm reduced over the catalyst was reduced. The cause of this effect is not yet clear, but there are evidences that this attributes to (a) lower NOx coverage on the catalyst surface, which in turn makes its reduction by HC less probable and (b) the increased soot emissions which are blocking part of the catalyst active sites that are active in reducing NOx. Hydrogen addition expands the SCR activity window towards lower temperatures (100–300 ∘C) without affecting the maximum NOx conversion. In contradiction to H2 the CO addition is favourable to the H2 oxidation reaction and the poisoning of the catalysts active sites and the good low temperature NOx reduction activity cannot be seen. The incorporation of a mini-exhaust gas-reformer on-board a vehicle to provide the H2 in the SCR reactor will require catalyst design and reactor engineering to maximise H2 production and eliminate CO with the minimum penalty in the fuel economy.

Selective Catalytic Reduction of Nitric Oxide by Ammonia over Egg-Shell MnOx/NaY Composite Catalysts

A novel composite catalyst system for the selective catalytic reduction (SCR) of NOx by NH3 is described operating at temperatures lower than 470 K in the presence of water with NO conversions of 80–100% at space velocities of 30,000–50,000 h−1. The catalyst is prepared by egg-shell precipitation of MnO2 on the external surface of zeolite NaY. Structural and thermal stability of precipitated MnO2 as well as of the MnO2/NaY composite catalyst were characterized by N2 adsorption, X-ray diffraction, laser Raman spectroscopy, temperature-programmed reduction, and electron microscopy. MnO2 precipitated on zeolite NaY (15 wt% loading) retained its amorphous state up to calcination temperatures of 775 K. The zeolite component remained structurally intact. Calcination at higher temperatures destroyed the zeolite structure and transformed MnO2 into Mn3O4. DRIFT spectroscopic investigations revealed the presence of symmetric ON—O—NO species formally corresponding to N2O3 on the composite catalyst after contact with NO. Catalytic measurements under integral flow conditions showed that the catalyst performance is associated with a close coupling of nitrite formation and its drain off from equilibria with NO/NO2 and nitrate by ammonia. Several results are in line with the “diazotation” mechanism, including NH3 protonation to NH4+, whereas prevailing Lewis acid sites should enable NH3 activation via amide species, thus leading to a parallel “amide/nitrosamide” SCR reaction route. The activity-temperature profile fulfills the requirements of a low-temperature NOx reduction catalyst for mobile diesel engines if an ammonia supply is implemented “on board,” e.g., by urea decomposition.

An investigation of catalysts for the on board synthesis of NH3. A possible route to low temperature NOx reduction for lean-burn engines

Platinum group metal catalysts have been investigated for the formation of NH3 from NO + H2 at low temperatures in the absence and presence of CO. Although CO inhibits the formation of NH3, substantial amounts are still observed with a Pt catalyst. By combining Pt with a support (ceria–zirconia) that has low temperature NOx storage characteristics it has been shown in transient experiments that NH3 can be formed and stored in situ under rich conditions, and may then be used to reduce NOx under lean burn conditions.

Low-temperature NOx reduction processes using combined systems ofpulsed corona discharge and catalysts

In this paper, we will report NOx removal via reduction processesusing two types of combined system of pulse corona discharge and catalysts:the single-stage plasma-driven catalyst (PDC) system, and the two-stageplasma-enhanced selective catalytic reduction (PE-SCR) system. Severalcatalysts, such as γ-alumina catalysts, mechanically mixed catalysts ofγ-alumina with BaTiO3 or TiO2, and Co-ZSM-5 were tested. In thePDC system, which is directly activated by the discharge plasma, it was foundthat the use of additives was necessary to achieve NOx removal byreduction. Removal rates of NO and NOx were linearly increased as the molarratio of additive to NOx increased. The dependence of NO and NOx removalon the gas hourly space velocity (GHSV) at a fixed specific input energy (SIE)indicates that plasma-induced surface reaction on the catalyst plays animportant role in the PDC system. It was found that the optimal GHSV of thePDC system with the γ-alumina catalyst was smaller than 6000 h-1.Mechanical mixing of γ-alumina with BaTiO3 or TiO2 did notenhance NO and NOx removal and γ-alumina alone was found to be themost suitable catalyst. The dielectric constant of the catalyst onlyinfluenced the plasma intensity, not the NOx removal. In the PE-SCR system,plasma-treated NOx (mostly NO2) was reduced effectively with NH3 overthe Co-ZSM-5 catalyst at a relatively low temperature of 150 °C. Underoptimal conditions the energy cost and energy yield were 25 eV/molecule and21 g-N (kWh)-1, respectively.