Lean NOx Reduction Research Articles

Promotive effects of Ba addition on lean NOx reduction by CO over IrRu/Al2O3 catalyst

A series of IrRu/Al2O3 based catalysts, where Ba promoter was added in differing amounts were applied for the lean reduction of NOx using CO (CO-SCR), and the promotive effects of Ba addition were thoroughly analyzed. Below 225 °C, IrRu/Al2O3 catalyst exhibits high de-NOx activity, but at high temperature above 250 °C, suffers from drastic decline of NOx conversion. Promotion of Ba enhances the catalytic activity by suppressing NO2 production in the high temperature region, which expands the operation window. Ba species is capable of trapping the NOx species, following its traditional role as a NOx storage compartment. However, when compared to the Ba/IrRu/Al2O3 catalyst, physically mixing Ba compartment with IrRu/Al2O3 catalyst did not lead to substantial promotion. Thus, participation of Ba species during the reaction is not limited to NOx storage, where it is proposed that a continuous regeneration of NOx-trapping Ba sites takes place by direct reaction with the CO reductant catalyzed by IrRu species. It is also revealed that promotion of Ba led to fundamental alterations in IrRu species, by maintaining their active metallic states under oxidizing reaction conditions and modifying their intrinsic activity towards NO bond cleavage, thereby leading to enhanced activity at elevated temperatures. The utilization of Ba/IrRu/Al2O3 catalysts in lean NOx reduction using CO as a reductant is anticipated to enable the facile reduction of NOx over a wide temperature range with high efficiency and stability, while also eliminating the need to employ conventional urea-based reductant, even under oxidizing conditions.

Lean NOx Reduction by In-Situ-Formed NH3 under Periodic Lean/Rich Conditions over Rhodium-Loaded Al-Rich Beta Zeolites

Lean NO<i>x</i> Reduction by In-Situ-Formed NH₃ under Periodic Lean/Rich Conditions over Rhodium-Loaded Al-Rich Beta Zeolites

Unraveling the origin of extraordinary lean NOx reduction by CO over Ir-Ru bimetallic catalyst at low temperature

Ir-Ru/Al2O3 developed has shown an outstanding activity for reducing NOx by CO at low temperature in excess oxygen. However, it has remained elusive how the Ir-Ru catalyst derives this unique reactivity. Here, we tried to unveil the origin of the synergistic effect in the Ir-Ru catalyst on the lean deNOx by CO. Ir-Ru/Al2O3 showed superb NOx reduction activity along with high N2 selectivity compared to monometallic catalysts. Results of EXAFS and XRD studies indicated the formation of Ir-Ru alloy phases on the alumina. These alloy phases were likely to accelerate reaction steps required for the NOx reduction by CO, based on temperature programmed experiments together with DFT calculations. Ir-Ru/Al2O3 appeared to effectively dissociate NO on the bimetallic site, which could successively be reutilized by the facile CO-O reaction, believed to be the primary causes for the excellent deNOx activity of Ir-Ru/Al2O3 by CO under lean exhaust condition.

Kinetic and DRIFTS studies of IrRu/Al2O3 catalysts for lean NOx reduction by CO at low temperature

This study employs a series of bimetallic IrRu/Al2O3 catalysts with differing Ir:Ru compositions for lean NOx reduction by CO (CO-SCR).

Pd-doped or Pd impregnated 30% La0.7Sr0.3CoO3/Al2O3 catalysts for NOx storage and reduction

Here we report the effects of the incorporation of palladium into 30% La0.7Sr0.3CoO3/Al2O3 catalyst by different methods on the NOx storage and reduction behaviour. Several catalysts were prepared incorporating four palladium nominal loadings (0.75, 1.5, 2.25 and 3.0%) by doping the perovskite formulation (30% La0.7Sr0.3Co1-yPdyO3/Al2O3) or by wetness impregnation over alumina-supported perovskite (y% Pd-30% La0.7Sr0.3CoO3/Al2O3). The results of X-Ray diffraction, N2 adsorption-desorption at −196 °C, electron microscopy, temperature programmed techniques, Raman and X-ray photoelectron spectroscopies demonstrated that the wetness impregnation method induces the formation of small PdO particles homogenously distributed over the surface in strong interaction with the perovskite. Meanwhile, doping method leads to the formation of a mix between intraframework palladium species and surface agglomerated PdOx particles with weaker interaction with the perovskite phase. Therefore, palladium accessibility and the synergetic effects with the perovskite are lower for Pd-doped samples. The NO-to-NO2 conversion is similar irrespective of the palladium content and the incorporation method. This confirms the perovskite phase as the main active site for NO oxidation. However, NOx adsorption during lean conditions and NOx reduction to N2 during rich periods are significantly promoted after the incorporation of palladium, especially by impregnation method. This enhancement is assigned to better NOx adsorption sites regeneration and to a promotion of NOx reduction rate, respectively. Thus, the best De-NOx performance of Pd-impregnated catalysts is derived from the higher efficient use of the palladium active sites. Among the developed formulations, the 1.5% Pd-30% LSCO/Al2O3 sample shows the best balance between NOx removal efficiency and minimum palladium content. This sample shows a NO removal efficiency and nitrogen yield as high as 86.2% and 69.5%, respectively. Based on these results, the developed formulation is revealed as a promising alternative to the NSR model catalyst (1.5% Pt-15% BaO/Al2O3) for NOx removal in the automotive application.

Enhanced transport in washcoated monoliths: Application to selective lean NOx reduction and ammonia oxidation

Washcoated monolithic reactors are extensively used in vehicle emission control systems to reduce pollutants from engine exhaust. Selective catalytic reduction (SCR) with the downstream ammonia slip catalyst (ASC) are deployed in diesel emission control to remove NOx and prevent ammonia release. Earlier works have shown that washcoat diffusion can be rate controlling for both SCR and ASC. Here we use sacrificial agents (polymer, yeast) to enhance the conversion without compromising the selectivity for SCR and ASC. The agents are removed during calcination, creating inter-crystallite pores that increase the washcoat macroporosity. Steady state reaction studies of SCR on a Cu-SSZ-13 coated monolith show a noted increase in NOx conversion during standard SCR (NH3 + NO + O2). The improved performance is sustained for hydrothermally aged samples. Similar conversion enhancement is observed for NH3 oxidation over the single-layer Cu-SSZ-13 washcoat. Conversion increases are also encountered for the dual-layer ASC (top Cu-SSZ-13 and base Pt/Al2O3 layer) during the selective oxidation of NH3 to N2. For both SCR and ASC, the N2 product selectivity is negligibly impacted on the modified catalysts. Sharper NH3 desorption profiles are obtained during NH3 temperature programmed desorption (TPD) experiments with Cu-SSZ-13. The results show that modified washcoat may reduce the amount of catalyst to maintain a desired conversion. For the ASC up to a 70% reduction in the Pt loading is demonstrated by using the sacrificial agents to achieve the same conversion. The implication for both SCR and ASC catalyst design and performance is discussed.

Catalytic Influence of Water Vapor on Lean Blow-Off and NOx Reduction for Pressurized Swirling Syngas Flames

It has become increasingly cost-effective for the steel industry to invest in the capture of heavily carbonaceous basic oxygen furnace or converter gas, and use it to support the intensive energy demands of the integrated facility, or for surplus energy conversion in power plants. As industry strives for greater efficiency via ever more complex technologies, increased attention is being paid to investigate the complex behavior of by-product syngases. Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O on heavily carbonaceous syngas mixtures. Direct formation of CO2 from CO is slow due to its high activation energy, and the presence of disassociated radical hydrogen facilitates chain branching species (such as OH), changing the dominant path for oxidation. The observed catalytic effect is nonmonotonic, with the reduction in flame temperature eventually prevailing, and overall reaction rate quenched. The potential benefits of changes in water loading are explored in terms of delayed lean blow-off (LBO), and primary emission reduction in a premixed turbulent swirling flame, scaled for practical relevance at conditions of elevated temperature (423 K) and pressure (0.1–0.3 MPa). Chemical kinetic models are used initially to characterize the influence that H2O has on the burning characteristics of the fuel blend employed, modeling laminar flame speed and extinction strain rate across an experimental range with H2O vapor fraction increased to eventually diminish the catalytic effect. These modeled predictions are used as a foundation to investigate the experimental flame. OH* chemiluminescence and OH planar laser-induced fluorescence (PLIF) are employed as optical diagnostic techniques to analyze changes in heat release structure resulting from the experimental variation in water loading. A comparison is made with a CH4/air flame and changes in LBO stability limits are quantified, measuring the incremental increase in air flow and again compared against chemical models. The compound benefit of CO and NOx reduction is quantified also, with production first decreasing due to the thermal effect of H2O addition from a reduction in flame temperature, coupled with the potential for further reduction from the change in lean stability limit. Power law correlations have been derived for change in pressure, and equivalent water loading. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed for laminar flames are appraised within the challenging environment of turbulent, swirl-stabilized flames at elevated temperature and pressure, characteristic of practical systems.

Dissociative influence of H2O vapour/spray on lean blowoff and NOx reduction for heavily carbonaceous syngas swirling flames

Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O with heavily carbonaceous syngas mixtures. In this study, the potential benefits of these subtle changes in water loading and hence reaction pathways are explored in terms of delayed lean blowoff, and primary emission reduction in a premixed turbulent swirling flame (Ø = 0.6–0.8), scaled for practical relevance. Chemical kinetic models initially confirm that H2O has a substantial impact on the employed fuel behaviour; increasing flame speed by up to 60% across an experimental range representative of fluctuation in atmospheric humidity (∼1.8 mol%). OH* chemiluminescence and OH planar laser induced fluorescence (PLIF) were employed to analyse the changes in heat release structure resulting from the experimental addition of H2O vapour to the combustor. Equivalent concentrations of liquid H2O were introduced into the central recirculation zone of the premixed flame as an atomised spray, to investigate the influence of phase changes on the catalytic effect. Near the lean stability limit, H2O addition compresses heat release to shorten the elongated flame structure. Whereas with a stable and well-defined flame structure, the addition triggers a change in axial heat release location, causing the flame front to retract upstream toward the burner outlet. Higher quantities of two-phase flow were combined to explore the possibility of employing the spray as a stabilising mechanism, effectively dampening the observed influence of humidity. The chemical enhancement induced by the controlled supply was shown to reduce the lean blowoff stability limit, enabling an increase in additional air flow of almost 10%. However, the catalytic effect of H2O diminishes with excessive supply and thermal quenching prevails. There is a compound benefit of NOx reduction from the use of H2O as a flame stabiliser with the practically-relevant syngas: First NOx production decreases due to thermal effect of H2O addition, with potential for further reduction from the change in lean stability limit; leanest experimental concentrations reduced by up to a factor of four with two-phase flow at the highest rates of supply. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed in the laminar environment, is shown to translate into practical benefits in the challenging environment of turbulent, swirl-stabilised flames.

Lean NOx reduction over Ag/alumina catalysts via ethanol-SCR using ethanol/gasoline blends

This study focuses on the activity for lean NOx reduction over sol-gel synthesized silver alumina (Ag/Al2O3) catalysts, with and without platinum doping, using ethanol (EtOH), EtOH/C3H6 and EtOH/gasoline blends as reducing agents. The effect of ethanol concentration, both by varying the hydrocarbon-to-NOx ratio and by alternating the gasoline concentration in the EtOH/gasoline mixture, is investigated. High activity for NOx reduction is demonstrated for powder catalysts for EtOH and EtOH/C3H6 as well as for monolith coated catalysts (EtOH and EtOH/gasoline). The results show that pure Ag/Al2O3 catalysts display higher NOx reduction and lower light-off temperature as compared to the platinum doped samples. The 4wt.% Ag/Al2O3 catalyst displays 100% reduction in the range 340–425°C, with up to 37% selectivity towards NH3. These results are also supported by DRIFTS (Diffuse reflection infrared Fourier transform spectroscopy) experiments. The high ammonia formation could, in combination with an NH3-SCR catalyst, be utilized to construct a NOx reduction system with lower fuel penalty cf. stand alone HC-SCR. In addition, it would result in an overall decrease in CO2 emissions.

Lean NOx reduction over Rh/ZrP2O7 catalyst under steady-state and perturbation conditions

Catalytic lean NOx reduction performance was studied over Rh supported on ZrP2O7 and ZrO2 under steady-state and perturbation conditions. The ZrP2O7 support significantly enhanced the NOx purification efficiency at the air-to-fuel (A/F) ratio between 14.6 and 15.0 compared with the ZrO2 support, because of the higher activity toward C3H6 oxidation and the higher stability of metallic Rh against reoxidation. The lean NOx purification efficiency was further enhanced by switching two simulated gas feeds with stoichiometric (A/F=14.6) and lean (A/F=14.9) compositions with shorter intervals (∼1s). Considering faster reduction of Rh oxide in contrast to slower reoxidation of Rh on ZrP2O7, the perturbation effect could be rationalized by the stabilization of metallic Rh species, which is more active than Rh oxide for the catalytic NOx reduction.

Elementary Kinetic Mean Field Modelling of the Lean NOx Reduction by H2 on Pt/WO3/ZrO2 Catalyst

This paper deals with the elementary kinetic mean field modelling of the catalytic NOx reduction by H2 on Pt/WO3/ZrO2 under oxygen-rich conditions. Pt/WO3/ZrO2 exhibits outstanding deNOx activity at low exhaust temperatures as well as improved N2 selectivity as compared with classical Pt catalysts. The kinetic model was developed based upon a postulated reaction mechanism as well as kinetic examinations and implied a network of 48 reaction steps described by Arrhenius-based rate expressions. Kinetic parameters were taken from literature and were elucidated by fitting calculations, while pre-exponential factors of adsorption were estimated from kinetic gas theory. For validation, catalytic studies were simulated and compared with experiments and thermodynamic consistency was proven. As a result of the kinetic model, the formation of OH surface species was identified as the rate determining step of H2 oxidation, while the reduction of NO predominately occurs by dissociation of chemisorbed NO.

Characterization of the active species in the silver/alumina system for lean NOx reduction with methanol

Low-temperature activity and selectivity for lean NOx reduction over silver/alumina is strongly dependent on the composition of surface silver species. This motivates the present investigation of the role of the supported silver species for the lean NOx reduction with methanol. The catalyst samples, with different composition of silver species, are characterized by temperature programmed desorption of ammonia (NH3-TPD), temperature programmed reduction with hydrogen (H2-TPR) and temperature programmed desorption with NO (NO-TPD) in oxygen excess. The small differences in acidity do not significantly influence the lean NOx reduction with methanol. However, comparison of results from H2-TPR experiments with previous characterization by UV–vis spectroscopy shows that fairly small silver species are reduced by hydrogen, possibly small silver clusters. These small silver species are likely, in addition to others, involved in the lean NOx reduction reactions. NO-TPD experiments, in the presence of oxygen and hydrogen, reveal a shift in temperature for one of the desorption peaks from the different samples. This peak is likely related to the shift in temperature for NOx reduction during methanol-SCR conditions for the compared samples.

ChemInform Abstract: Recent Advances in Automotive Catalysis for NOx Emission Control by Small‐Pore Microporous Materials

AbstractReview: 211 refs.

Solid SCR용 암모니아 저장물질인 Magnesium Ammine Chloride의 합성방법 및 물질특성 연구

Among various ammonium salts and metal ammine chlorides used as solid materials for the sources of ammonia with solid SCR for lean NOx reduction, magnesium ammine chloride was taken up for study in this paper because of its ease of handling and safety. Lab-scale synthetic method of magnesium ammine chloride were studied for different durations, temperatures, and pressures with proper ammonia gas charged, as a respect of ammonia gas adsorption rate(%). To understand material characteristics for lab-made magnesium ammine chloride, DA, IC, FT-IR, XRD and SDT analyses were performed using the published data available in literature. From the analytical results, the water content in the lab-made magnesium ammine chloride can be determined. A new test procedure for water removal was proposed, by which the adsorption rate of lab-made sample was found to be approximately 100%.

Zr-SBA-15 supported Ni catalysts for lean NOx reduction

An efficient C3H8–SCR catalyst with remarkable surface acidity, high surface area and high hydrothermal stability has been developed using an innovative catalyst preparation technique. In this method, a combination of direct and post-synthesis methods (DS–PS) has been employed sequentially for the preparation of a highly Zr-loaded SBA-15 (32.1wt%). A series of novel catalysts have been developed by doping various contents of nickel on the prepared Zr-loaded support and have been used for NO reduction with propane in the presence of excess oxygen and water vapor in a fixed bed reactor. Among the various catalysts prepared, the catalyst with a nickel content of 2.5wt% exhibited the highest activity with the ability to fully convert NO into N2 in the presence of 10% oxygen, 1750ppm propane and 500ppm NO at 450°C. A satisfactory catalytic activity has been found in the presence of 7vol% of water vapor.

Methanol Assisted Lean NOx Reduction Over Ag–Al2O3—Influence of Hydrogen and Silver Loading

The influence of hydrogen and silver loading on lean NOx reduction with methanol over Ag–Al2O3 are investigated by flow-reactor experiments, H2-TPR, TEM and XPS. Reduction of silver at low temperatures over Ag–Al2O3 with high silver loading, suggests involvement of silver species/particles that are large (which is supported by the TEM analysis) and/or loosely bound to the support. The silver species reduced by hydrogen cannot be directly associated with the activity for NOx reduction under methanol-SCR conditions. Furthermore, the activity for NOx reduction increases slightly at low temperatures when hydrogen is added to the feed. However, also without co-fed hydrogen, a high low-temperature activity is achieved for methanol assisted lean NOx reduction over Ag–Al2O3.

Silver/alumina for methanol-assisted lean NOx reduction—On the influence of silver species and hydrogen formation

High low-temperature activity for lean NOx reduction can be achieved for silver/alumina by using an oxygenated reducing agent. In this system the catalytic reactions, including the H2 formation previously observed during methanol-SCR conditions, are strongly dependent on the composition of surface silver species. With the aim to increase the understanding of the role of supported silver species in combination with the methanol-SCR reactions, catalysts with the same silver loading but different composition of silver species are prepared by utilizing different preparation methods. The supported silver species are characterized by UV–vis spectroscopy and transmission electron microscopy, while the catalytic performance for lean NOx reduction with methanol is investigated in flow reactor experiments and surface species studied by DRIFT spectroscopy. The results indicate that hydrogen atoms are abstracted mainly from the methyl group during the conversion of surface species formed from methanol. The hydrogen atoms could contribute to reduction of the catalyst or affect the catalytic reactions in other ways, before they react to form H2 or H2O. Here, more H2O is formed over the samples containing more silver nanoparticles. The released hydrogen atoms are suggested to explain the high NOx reduction at low temperature associated with oxygenated reducing agents, rather than the subsequently formed gaseous H2. Furthermore, the results show that lean NOx reduction with methanol is determined by silver species in the size range from small silver clusters to small silver nanoparticles (<ca20nm). Small silver nanoparticles (<ca20nm) are concluded to catalyse formation of N2O. The role of silver ions and small clusters, on the other hand, is suggested to be promotion of N2 formation. The largest silver nanoparticles observed in this study (>ca20nm), however, do not have a significant impact on the catalytic reactions. Finally, in order to achieve both high low-temperature activity, as well as high selectivity to N2 a combination of small and somewhat larger silver species are needed.

Solid SCR용 암모니아 저장물질인 Calcium Ammine Chloride의 합성방법 및 물질분석 연구

Solid materials of ammonia sources with SCR have been considered for the application of lean NOx reduction in automobile industry, to overcome complex problems of liquid urea based SCR. These solid materials produce ammonia gas directly with proper heating and can be packaged by compact size, because of high volumetric ammonia density. Among ammonium salts and metal ammine chlorides, calcium ammine chloride was focused on this paper due to low decomposition temperature. In order to make calcium ammine chloride in lab-scale, simple reactor and glove box was designed and built with ammonium gas tank, regulator, and sensors. Basic test conditions of charging ammonia gas to anhydrous calcium chloride are chosen from equilibrium vapor pressure by Van't Hoff plot based on thermodynamic properties of materials. Synthetic method of calcium ammine chloride were studied for different durations, temperatures, and pressures with proper ammonia gas charged, as a respect of ammonia gas adsorption rate(%) from simple weight calculations which were confirmed by IC. Also, lab-made calcium ammine chloride were analyzed by TGA and DSC to clarify decomposition step in the equations of chemical reaction. To understand material characteristics for lab-made calcium ammine chloride, DA, XRD and FT-IR analysis were performed with published data of literature. From analytical results, water content in lab-made calcium ammine chloride can be discovered and new test procedures of water removal were proposed.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials.

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Effect of Cu Loading on the Structure and Catalytic Performance of the LNT Catalyst CuO-K&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;/TiO&lt;sub&gt;2&lt;/sub&gt;

A series of non-platinic lean NOx trap (LNT) CuO-K2CO3/TiO2 catalysts with different Cu loadings were prepared by sequential impregnation, and they showed relatively good performance for lean NOx storage and reduction. The catalyst containing 8% (w) CuO showed not only the largest NOx, storage capacity of 1.559 mmol.g(-1) under lean conditions, but also the highest NOx, reduction percentage of 99% in cyclic lean/rich atmospheres. Additionally, zero selectivity of NOx to N2O was achieved over this catalyst during NOx reduction. Multiple techniques, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), temperature-programmed desorption of CO2 (CO2-TPD), extended X-ray absorption fine structure (EXAFS), temperature-programmed reduction of H-2 (H-2-TPR), and in-situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS), were used for catalyst characterization. The results indicate that highly dispersed CuO is the main active phase for oxidation of NO to NO2 and reduction of NOx to N-2. The strong interaction between K2CO3 and CuO was clearly revealed, which favors NOx adsorption and storage. The appearance of negative bands at around 1436 and 1563 cm(-1), corresponding to CO2 asymmetric stretching in bicarbonates and -C=O stretching in bidentate carbonates, showed the involvement of carbonates in NO storage. After using the catalysts for 15 cycles of NOx storage and reduction in alternative lean/rich atmospheres, the CuO species in the catalysts showed little change, indicating high catalytic stability. Based on the results of in-situ DRIFTS and the other characterizations, a model describing the NOx storage processes and the distribution of CuO and K2CO3 species is proposed.