Selective Catalytic Reduction Activity Research Articles

Effect of surface coating CeO2 on the SO2 resistance of Cu-ZSM-5 with highly selective catalytic reduction activity

Effect of surface coating CeO2 on the SO2 resistance of Cu-ZSM-5 with highly selective catalytic reduction activity

Changes of Nitrate Activity and Byproduct Distribution Characteristics for Synergistic NOx and Dioxin Abatement over V2O5/AC Catalyst.

The simultaneous removal of NOx and dioxins has been considered an economical and effective technology of controlling multipollutant flue gas in the context of "carbon peaking and carbon neutrality". However, this technology has not yet been implemented in practical situations, because the interactive relationship between the selective catalytic reduction (SCR) reaction and dioxin catalytic oxidation lacks a deep understanding, especially on a carbon-based catalyst. In this research, the influence of NO and NH3 on the oxidation characteristics and byproducts distribution of dibenzofuran (DBF) was studied on V2O5/AC catalyst. Results indicated that NH3 has a stronger inhibition effect for DBF catalytic oxidation than NO due to obvious competitive adsorption between NH3 and DBF on the V2O5/AC catalyst. In addition, although both NO and NH3 inhibit the complete degradation of DBF, their effects on the byproduct distribution are not consistent. NO primarily affects the level of oxygen-containing byproducts, while NH3 primarily affects the level of alkane byproducts. Furthermore, the SCR reaction activity demonstrated a reduction when DBF was present. The occupation of V2O5 sites by DBF and its oxidizing intermediates has hindered the production of monodentate nitrate and the reactivity of bridged nitrate, resulting in a decrease in SCR activity via the L-H mechanism. This work aims to provide theoretical guidance for simultaneous removal of NOx and dioxins in industrial fumes.

Promoting effect of polyoxometallic acid modification on the NH3-SCR performance of Mn-based catalysts

The strong oxidation capabilities of Mn sites render Mn-based catalysts showing high activities of selective catalytic reduction (SCR) at temperatures < 250℃, but simultaneously lead to the generation of by-product N2O with strong greenhouse effect. The activity-N2-selectivity trade-off was a prevalent observation in the development of Mn-based SCR catalysts. Here, a polyoxometallic acid enhancement strategy was devised to simultaneously promote the N2 selectivity and low-temperature SCR (LT-SCR) activity of Mn-based metal oxide catalysts, where the introduction of uniformly dispersed phosphomolybdic acid (HPMo) tuned the oxidation capabilities and enhanced NH3 adsorption-storage capability of catalysts. The combination of transient reaction, in-situ DRIFTS analysis and DFT calculations revealed that the main origin of N2O on the Mn-based catalysts was the deep oxidation of NH3, which was inhibited by the interaction between HPMo and Mn sites. Additional NH3 molecules adsorbed by the abundant acid sites supplied by HPMo further facilitated the SCR reaction. This strategy could guide the design of high-activity and high-selectivity LT-SCR catalysts for industrial denitration.

Optimization of isolated copper species on the NH3-SCR performance over Cu/SSZ-39 modified by ammonia water

To further improve the activity of Selective Catalytic Reduction of nitrogen oxide (NOx) with Ammonia (NH3-SCR) and the hydrothermal stability at 900 °C on Cu-based small-pore zeolite catalyst, the interaction between ammonia water and Cu2+ is used to regulate the interaction between Cu2+ and Si-O-Al structures of the zeolite, and affect the migration of Cu2+ in the channels of zeolite. The characterization results show that the microchemical environment of isolated Cu2+ species is optimized, and more isolated Cu2+ species can stably exist over the surface of Cu/SSZ-39, exhibiting a strong redox property. Besides, the ability of acid sites to adsorb and activate NH3 is also enhanced, and the amount of H2O adsorbed in the pores of zeolite is reduced due to the stable surface isolated Cu2+ species, which is conducive to the stability of the catalyst after hydrothermal aging at 900 °C. When the mole ratio of ammonia and copper is 3, the NH3-SCR performance and the hydrothermal stability at 900 °C are both optimized on the modified catalyst. The temperature window that the NOx conversion is higher than 90% (ΔT90) on the unmodified catalyst is 200–475 °C, but the ΔT90 on the modified catalyst is broadened to 178–544 °C. More importantly, after hydrothermal aging at 900 °C, the ΔT90 on the modified catalyst is shifted to the higher temperature by 48 °C, with the maximum NOx conversion of 96%. This work would provide a new strategy for further optimizing Cu/SSZ-39 catalyst for deNOx emitted from diesel engines to meet higher emission standards in future.

Enhancing catalytic activity with less metal resource by regulating zeolite topology

Big-ring topology of zeolite is crucial to the catalytic activity of selective catalytic reduction (SCR) of nitrogen oxide (NOx). More works should explore how to increase the number of the big-ring topology. This work regulated ZSM-5 topologies by different acids and amino acids, exchanged Cu species into the regulated zeolites, and compared their SCR performances. L-aspartate- and L-cysteine-regulated ZSM-5 removed 67 ∼ 73 % of NO (1000 ppm, 15,000 h−1) at 150 °C. Their Cu contents were only 0.04 wt%. Under the same catalytic conditions, citric acid-regulated ZSM-5 only reached a conversion of 19 % with a Cu content of 0.15 wt%. After detailed characterizations, primary 10-membered ring (MR) on (200) plane of ZSM-5 was retained after mild treatment of the amino acids. At the same time, 5-MR on (351) and (352) planes were opened to form more 10-MR. 10-MR along (501) plane was reinforced. As a result, the 10-MR content was increased from 22 % (citric acid) to 32 % (L-aspartate and L-cysteine). The 10-MR-localized Cu was more effective than other species. Above results are in favor of understanding topology-activity relationship in zeolite. Synthesizing a more effective catalyst with less active metal is also helpful to resource saving.

Two easy methods to increase SO2-resistance ability of Mn-based catalyst

Manganese-based catalysts show high activities in selective catalytic reduction of nitrogen oxide (NO), but easily deactivated by sulfur dioxide. In this work, operation sequence and rare-earth-element doping were investigated to increase the anti-SO2 ability of Fe-doped manganese oxide (FM300). On one hand, when the catalyst pre-adsorbed NH3, SO2 only decreased the NO conversion from 100 % to 75 % at 200 °C. Otherwise, the conversion was decreased to 30 %. In-situ Diffuse Reflection Fourier Transform Infrared Spectroscopy proved that NH3 competed with SO2 and protected part of the Brønsted acidic sites. On the other hand, rare-earth-element doping increased the long-time anti-SO2 performance of FM300 from 150 min to 300 min (Sm-FM300) and > 700 min (Ho-FM300). Differential Scanning Calorimeter analysis indicated that Ho optimized ferromagnetism of FM300, which was in favor of NH3 adsorption. The main result of this work will help to increase applicability of the manganese-based catalyst.

Praseodymium effects on great improving low-temperature hydrothermal stability of Cu/SAPO-34 NH3-SCR catalysts

Based on the previous findings that the presence of hydroxyl groups on the outer surface is crucial for maintaining skeletal stability, we propose a strategy modified Cu/SAPO-34 using Pr ions in this study. Therefore, we conducted several measurements to investigate the effect of Pr ions on the low-temperature hydrothermal stability of Cu/SAPO-34. We find that Pr exists only on the surface of Cu/SAPO-34 as ions and oxides, with Pr3+ ions playing a protective role in occupying surface acidic sites. The addition of small amounts of Pr leads to the re-dispersion of Cu, resulting in improved low-temperature selective catalytic reduction (SCR) activity in the as-synthesized samples. Furthermore, it enhances the resistance to decomposition of the Si–(OH)–Al framework during low-temperature hydrothermal aging, thereby preserving the framework structure and allowing detached active Cu species to return to exchangeable positions, ultimately restoring SCR activity. However, as the Pr content increases, the enhanced acidity causes some structural damage, gradually weakening the protective effect. Our work demonstrates that Pr modification is a simple and effective solution to the issue of poor low-temperature hydrothermal stability in Cu/SAPO-34, providing a promising way for the application of light rare earth elements.

Puffy hierarchical nanonet structured multifunctional membranes for NO degradation and ultrafine dust filtration

Multifunctional nanofibrous catalytic membranes are used for comprehensive treatment of fine particulate matter (PM) and toxic gaseous contaminants. However, the need for high purification efficiency with low resistance presents a unique challenge. Herein, a method of optimizing the morphology of manganese oxide (MnOx) on Zr-TiO2 (TZ) nanofiber membranes was developed to remove ultrafine PM and nitric oxide (NO) efficiently with relatively low pressure drops. After tailoring the hydrothermal solution properties, the MnO2 ultrafine nanowires (≈22 nm) and Mn3O4 nano-octahedra assembled with scaffold nanofibers formed dual-network structures with high gas permeabilities, superior redox properties, and abundant surface acidities. The resultant M2TZ (2-MnOx/Zr-TiO2) catalytic membranes exhibited exceptional NH3 selective catalytic reduction (NH3-SCR) activity (T90 = 215 °C) with a gas hourly space velocity of 75,000 h−1, as well as outstanding SO2 and H2O tolerance and long-term stability. Meanwhile, owing to the puffy hierarchical nanonet structure, the M2TZ catalytic membranes also show a high filtration efficiency of 99.99% for PM0.3 removal with a low air resistance of 125 Pa, high air permeability of 1259 m3 m−2 h−1 kPa−1, and satisfactory PM capture at 300 °C. This work may shed light on the design of advanced multifunctional membranes for air pollutant purification.

Enhanced low-temperature selective catalytic reduction (SCR) activity and H2O and SO2 resistance of flower-like SmMnOx and SmMnOx-rGO catalysts

Reed flower-like SmMnOx and cauliflower-like SmMnOx-rGO catalysts were successfully synthesized by a coprecipitation-coupled solvothermal method for selective catalytic reduction (SCR) of NOx. The NOx conversion of the two catalysts is more than 90% in the temperature range of 75–200 °C, and the N2 selectivity of SmMnOx-rGO is above 90%. Moreover, both of them exhibit more than 62% of resistance to H2O and SO2 at a very low temperature of 100 °C, much superior than the fluffy spherical SmMnOx prepared by coprecipitation method. The strong synergy between Mn and Sm endowed by the flower-like structure contributes to a low degree of crystallization, a high ratio of (Mn3++Mn4+)/Mn, more chemisorbed oxygen species, strong redox ability, and more Lewis acid sites, hence effectively enhancing low-temperature SCR activity and resistance to H2O and SO2. In addition, the decreased ratio of Mn4+/Mn after rGO doping hinders the side reaction of NH3 oxidation, thus enhancing the N2 selectivity of rGO-doped SmMnOx catalyst.

3D-printing of attapulgite monoliths with superior low-temperature selective catalytic reduction activity: the influence of thermal treatment

This study provides a facile strategy for 3D-printing of attapulgite monoliths with superior low-temperature NH3-SCR performance under thermal treatment.

Copper/Nickel/Cobalt modified molybdenum-tungsten-titanium dioxide-based catalysts for multi-pollution control of nitrogen oxide, benzene, and toluene: Enhanced redox capacity and mechanism study

Previous studies have indicated the potential of monometallic-modified TiO2 catalysts in controlling nitrogen oxide (NOx) and volatile organic compounds (VOCs) in coal-fired flue gas. Unfortunately, increasing selective catalytic reduction (SCR) activity under complicated coal-fired flue gas status is tricky. In this study, modified Co-MoWTiO2 catalysts with multiple active sites were synthesized using the wet impregnation method, which exhibited excellent multi-pollution control ability of NO, benzene and toluene under low oxygen and high SO2 concentrations. The modification of Mo and Co achieved high dispersion and electron transfer. The interaction between W5+/W6+ and Co2+/Co3+ promoted gas-phase O2 adsorption on the catalyst surface, forming of reactive oxygen species (Oα). Density functional theory (DFT) calculations informed that the doping of Co effectively enhanced the NH3 and O2 adsorption capacity of the catalyst, and Co possessed the maximum adsorption energy for NH3 and O2. Possible pathways of multi-pollution control of NO, C6H6, and C7H8 were speculated. NH3/NH4+ on the Lewis/Bronsted acid site is reacted with intermediates of NO (e.g., NO2, nitrite, nitrate) via the Langmuir-Hinshelwood and Eley-Rideal mechanism. The introduction of NO and NH3 did not disrupt the oxidation pathways of benzene and toluene. Following the Mars-van Krevelen mechanism, C6H6 and C7H8 were progressively mineralized by Oα into CO2 and H2O.

Phase control of heterogeneous 1T/2H-MoS2 to improve the selective catalytic reduction activity of VMo/Ti

Phase control of heterogeneous 1T/2H-MoS2 to improve the selective catalytic reduction activity of VMo/Ti

Cu/BEA catalysts for the selective catalytic reduction of engine-out NOx emissions: Experimental and kinetic investigations

Selective catalytic reduction (SCR) technology using NH3 has shown to be the most promising method in the curbing of NOx and zeolite catalysts impregnated with copper has shown excellent SCR activity below 350 °C. However, most of the investigations are carried out with the artificial mixing combination of exhaust gases. The effect of real-time engine-out exhaust gas on NH3-SCR activity has been scantily reported. In this paper, Cu/BEA zeolite with varying copper loading from 0.83 to 4.31 wt% has been used and the effect of engine-out exhaust emissions has been investigated. The catalysts were prepared using ion-exchange method and characterized using SEM, XRD, and BET analyses. The highest NOx reduction efficiency of 62.5 % was observed for 1.77 wt% Cu for which the Cu/Al ratio was close to unity. The kinetic modeling of the SCR activity was also carried out to validate the experimental results with an average error of 3.85 %.

Zeolite Synthesis in the Presence of Metallosiloxanes for the Quantitative Encapsulation of Metal Species for the Selective Catalytic Reduction (SCR) of NOx.

Metal encapsulation in zeolitic materials through one-pot hydrothermal synthesis (HTS) is an attractive technique to prepare zeolites with a high metal dispersion. Due to its simplicity and the excellent catalytic performance observed for several catalytic systems, this method has gained a great deal of attention over the last few years. While most studies apply synthetic methods involving different organic ligands to stabilize the metal under synthesis conditions, here we report the use of metallosiloxanes as an alternative metal precursor. Metallosiloxanes can be synthesized from simple and cost-affordable chemicals and, when used in combination with zeolite building blocks under standard synthesis conditions, lead to quantitative metal loading and high dispersion. Thanks to the structural analogy of siloxane with TEOS, the synthesis gel stabilizes by forming siloxane bridges that prevent metal precipitation and clustering. When focusing on Fe-encapsulation, we demonstrate that Fe-MFI zeolites obtained by this method exhibit high catalytic activity in the NH3 -mediated selective catalytic reduction (SCR) of NOx along with a good H2 O/SO2 tolerance. This synthetic approach opens a new synthetic route for the encapsulation of transition metals within zeolite structures.

Promotion effect of Co addition on the activity and SO2 tolerance of CrCe catalysts for selective catalytic reduction of NOx with NH3 at middle–low temperatures

AbstractBackgroundNitrogen oxides (NOx) can result in a series of environmental issues (haze, photochemical smog and acid rain) that endanger human health. Currently, selective catalytic reduction (SCR) of NOx with ammonia (NH3‐SCR) has been considered to be one of the most efficient techniques to eliminate NOx.ResultsNovel Co‐modified chromium/cerium (CrCe) catalysts were synthesized by the citric acid method to improve their insufficient SCR activity and sulfur dioxide (SO2) resistance for NOx removal at middle–low temperatures. Cobalt (Co) addition could significantly increase the catalytic activity of CrCe catalysts, and the 5CoCrCe catalyst exhibited the highest catalytic activity with ≤95% NOx conversion and &gt;80% N2 selectivity within 180–280 °C. In addition, the 5CoCrCe catalyst exhibited outstanding SO2 resistance as well as reversibility, with &gt;90% NO conversion under 600 ppm SO2 for 510 min.ConclusionIt was determined by characterizations that Co addition can increase SSA, Cr6+ concentration, electron transfer between Co and Cr by redox cycle (Cr6+ + 4Co2+ ↔ 4Co3+ + Cr2+) and the redox properties of the CrCe catalyst. The 5CoCrCe catalyst surface consists mainly of Lewis acid and its NH3‐SCR reactions follows the Eley–Rideal (E–R) mechanism. The 5CoCrCe catalyst exhibited outstanding SO2 resistance due to higher Cr3+, Ce3+ and Oα concentrations and strong redox cycles (3Ce4+ + Cr3+ ↔ 3Ce3+ + Cr6+ and Cr6+ + 3Co2+ ↔ 3Co3+ + Cr3+). Furthermore, in situ DRIFTS results revealed that SO2 could induce more adsorbed NOx and NH3 species instead of bulk sulfate, which also was responsible for good SO2 resistance. © 2023 Society of Chemical Industry.

The improving effect of SO42− on environmental-friendly SO42−/α-Fe2O3 catalyst for NH3-SCR

For the selective catalytic reduction (SCR) of NOx by NH3, a variety of SO42−/α-Fe2O3 catalysts were created and utilized. The consequences manifest that the sulfation treatment signally boosted the NOx conversion of α-Fe2O3. With varying SO42− loadings, the T90 of SO42−/α-Fe2O3 catalysts ranges from 10% SO42−/α-Fe2O3 (301 °C) > 15% SO42−/α-Fe2O3 (306 °C) > 5% SO42−/α-Fe2O3 (314 °C) > 20% SO42−/α-Fe2O3 (321 °C). The optimum concentration of sulfate treatment samples was 10% wt., producing approximately 100% NOx conversion at 325–400 °C. Although the introduction of SO42− weaken the redox capacity of α-Fe2O3, the quantity and strength of acidic sites on SO42−/α-Fe2O3 was dramatically enhanced. Meanwhile, introducing SO42− to α-Fe2O3 hindered the ammonia oxidation process and increased the pore volume of the catalyst, enhancing SCR activity. In situ DRIFTS investigations demonstrated that NH3 adsorbed on Brønsted acid sites displayed better stability over SO42−/α-Fe2O3 catalysts. Due to the decreased capacity of the catalyst surface to adsorb NOx, the E-R route prevails on 10% SO42−/α-Fe2O3 catalyst.

Insight into the construction of multi-cavity micro-mesoporous SSZ-13: Dissolution, Al backfilling and framework reconstruction

The alkali leaching post-treatment method is a simple, economical and general route for introducing mesopores into SSZ-13, while the existing studies have rarely addressed the Al behavior between solution and zeolites framework during the alkali treatment. In this paper, a series of multi-cavity micro-mesoporous SSZ-13 were successfully prepared by using the alkali leaching post-treatment method at various treatment time. Extensive characterization illustrated that Al backfilling and zeolite framework reconstruction also existed during the NaOH treatment, besides the destruction of the zeolite framework due to the dissolution of Si and Al at the beginning of the reaction, and Al was preferentially backfilled into eight-membered ring (8 MR) to repair the zeolite framework and then filled into six-membered ring (6 MR) to reconstruct the zeolite framework. Concurrently, the amount of Brønsted acid (B acid) showed a decreasing-increasing-stabilizing trend during the progress. The catalytic activity and hydrothermal stability tests confirmed that multi-cavity micro-mesoporous SSZ-13 showed excellent catalytic activity for NH3 selective catalytic reduction (NH3-SCR) and good hydrothermal stability. This study provided basic guidance for the optimal design of the post-treatment alkali leaching route, which was highly significant for the development of hierarchically porous zeolites.

Elucidating the Effect of Ion Exchange Protocol on the Copper Exchange Efficacy, Copper Siting, and SCR Activity in Cu-SSZ-13.

The front cover artwork is provided by Professor Jean-Sabin McEwen at Washington State University. The image shows how ion exchanges prepared with different copper precursors influence how the copper ultimately sites relative to the zeolite framework, which ultimately impacts its catalytic reactivity for the selective catalytic reduction (SCR) of NOx in Cu-SSZ-13. Read the full text of the Research Article at 10.1002/cphc.202300271.

Performance characterization of a selective catalytic reduction on filter aftertreatment system in a heavy-duty diesel engine tested on a laboratory dynamometer

Increasingly stringent emission regulations call for improvement in the current emissions control technology in heavy-duty diesel engines. In the existing aftertreatment system utilized in diesel engines, there is already a challenge with space and volume, and an attempt to add more components would lead to the consumption of more space and impracticable. The Selective Catalytic Reduction on Filter (SCRF)-an integration of Selective Catalytic Reduction (SCR) and Diesel Particulate Filter (DPF) in a single component, offers space saving and favorable thermal inertia opportunities. However, literatures are conflicting on the effect of the SCR and DPF activities on the operations of the SCRF, as both require NO2 for fast SCR reaction, and passive soot oxidation respectively. This paper presents a detailed experimental characterization of a Silicon-Carbide (Si–C) coated with Copper-Zeolite (Cu-Ze) catalyst SCRF after-treatment component in a heavy-duty diesel engine, evaluated on a laboratory dynamometer. The effect of particulate matter loading on the SCRF activities in steady state, and transient operations was investigated, with emphasis on the simultaneous utilization of NO2 between the passive soot oxidation activities and SCR reactions-which is a determinant of the NOx conversion efficiency. The SCRF was loaded from 0 g/L up to 5 g/L while passive regeneration and NOx conversion activities were investigated over steady and transient cycles. The investigation has shown that both NOx conversion efficiency and passive soot oxidation increase with particulate matter loading in the SCRF system of a heavy-duty diesel engine.

Unveiling the temperature-dependent effect of Zn on phosphotungstic acid-modified MnOx catalyst for selective catalytic reduction of NOx: A poison at < 180 °C or a promoter at > 180 °C

MnOx-based catalysts are highly efficient for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures, but Zn contained in industrial flue gas may poison conventional SCR catalysts, and MnOx-based catalysts usually have low Zn tolerance. Herein, phosphotungstic acid-modified MnOx (denoted as Mn-HPW) was prepared to study the Zn tolerance. It was found that the deposition of Zn on Mn-HPW decreases the SCR activity at < 180 °C, but both NOx conversion and N2 selectivity are enhanced at 180–300 °C, and > 95% NOx conversion can be achieved on Zn/Mn-HPW. Relevant catalysts were characterized and the results reveal that the decline in the activity at < 180 °C is related to the decrease in the quantities of Mn4+, Mn3+, and chemisorbed oxygen as well as the decreased capacities for adsorbing and activating NH3 and NOx. The unusual improvement of SCR performance on Zn/Mn-HPW at 180–300 °C is mainly attributed to two aspects: (i) HPW can protect effectively the surface acid sites from Zn poisoning; (ii) The decrease in oxidation properties alleviates the over-oxidation of NH3, promoting the participation of more NH3 in SCR.