Selective Catalytic Reduction Activity Research Articles

Revealing insights into the unexpected effects of ZnO on selective catalytic reduction (SCR) activity of the CeOx-WO3 catalyst

This study delved into the intriguing impact of zinc oxide (ZnO) on the selective catalytic reduction (SCR) performance of a CeOx-WO3 catalyst with a Ce/W molar ratio of 3:2 (denoted as CW). The introduction of ZnO, at low concentrations of 1 wt% or 3 wt%, resulted in a diminution of the catalyst’s surface acid sites, which hampered the adsorption of NH3 and negated the benefits of improved NH3 activation due to the enhanced oxidative capacity. Consequently, the proceeding of the SCR reactions was significantly impeded, with a marked decrease in NOx conversion to approximately 42 % at 200 °C, which was nearly half that of the virgin CW catalyst. In contrast, increasing ZnO loading to 5 wt% facilitated the formation of ZnWO4, which introduced additional strong acid sites and bolstered NH3 adsorption at moderate and high temperatures. This, in conjunction with the heightened reactivity of surface Ce4+, resulted in a resurgence of deNOx performance, with a NOx conversion increase of ∼ 13 % at 250 °C compared to the catalyst with 3 wt% ZnO. Conversely, escalating ZnO loading to 7 wt% disrupted the formation of ZnWO4 and the excess ZnO neutralized the catalyst’s acid sites, thereby hindering NH3 adsorption and reducing NOx conversion. These insights were instrumental for the prospective application of the CeOx-WO3 catalysts under heavy metal-rich conditions.

Basic understanding of Cu‐SSZ‐13 in catalyzing low‐temperature selective catalytic reduction of NOx by ammonia

AbstractThe development of Cu‐SSZ‐13 represents a significant breakthrough in the field of NOx removal from mobile exhaust, demonstrating excellent selective catalytic reduction (SCR) activity across a broad temperature range (200–500°C). However, the limited reactivity of Cu‐SSZ‐13 at temperatures below 200°C poses a challenge for the removal of NOx emitted during vehicle cold starts. To stimulate new efforts to enhance low‐temperature SCR performance, this review provides a fundamental understanding of the nature of Cu‐SSZ‐13 in catalyzing the SCR reaction. The structures, coordination environments, and oxidation states of Cu2+ during the SCR reaction, as well as the redox cycle pathway of Cu2+ ions and the characteristics of the rate‐determining step, were discussed. Based on these insights, several strategies for improving the low‐temperature activity of Cu‐SSZ‐13 are proposed. Finally, the review offers a perspective on the future of low‐temperature SCR of NOx over Cu‐SSZ‐13.

The nature of synergy effects between VOx and TiO2 in low temperature NH3-SCR reaction

In this study, we investigated the low-temperature activity of selective catalytic reduction of NO by NH3 (NH3-SCR) with the effect of different vanadium oxide (VOx) loading in the VOx/TiO2 catalyst. By calculating the specific rate of activity, it was found that the fastest specific rate of activity was achieved when the loading of V2O5 reached 10 wt%, and the nitrogen oxides (NOx) conversion of the above optimal catalyst reached over 80 % in the temperature range of 180–465 °C. The effects of preparation method, calcination temperature, and support on the aggregation state of the vanadium species and the SCR activity of the catalyst were also investigated on the basis of the ideal ratio. Combining the activity and characterization results, it is found that the impregnation method and calcination at 410 °C are more favorable for the moderate dispersion of the vanadium species on the TiO2 surface, producing more highly reactive polymorphic VOx, which is more readily dispersed on the surface of the TiO2 support and interacts more strongly with the TiO2. The interaction between the two can promote the transfer of electrons, resulting in the production of more active O species and low-valent vanadium. The aforementioned species can enhance the catalyst's low- temperature denitrification activity.

Atom Pairing Enhances Sulfur Resistance in Low-Temperature SCR via Upshifting the Lowest Unoccupied States of Cerium.

Environmentally benign cerium-based catalysts are promising alternatives to toxic vanadium-based catalysts for controlling NOx emissions via selective catalytic reduction (SCR), but conventional cerium-based catalysts unavoidably suffer from SO2 poisoning in low-temperature SCR. We develop a strongly sulfur-resistant Ce1+1/TiO2 catalyst by spatially confining Ce atom pairs to different anchoring sites of anatase TiO2(001) surfaces. Experimental results combined with theoretical calculations demonstrate that strong electronic interactions between the paired Ce atoms upshift the lowest unoccupied states to an energy level higher than the highest occupied molecular orbital (HOMO) of SO2 so as to be catalytically inert in SO2 oxidation but slightly lower than HOMO of NH3 so that Ce1+1/TiO2 has desired ability toward NH3 activation required for SCR. Hence, Ce1+1/TiO2 shows higher SCR activity and excellent stability in the presence of SO2 at low temperatures with respect to supported single Ce atoms. This work provides a general strategy to develop sulfur-resistant catalysts by tuning the electronic states of active sites for low-temperature SCR, which has implications for practical applications with energy-saving requirements.

Selective catalytic oxidation of DMF over Cu-Ce/H-MOR by modulating the surface active sites

Selective catalytic oxidation of the hazardous DMF exhaust gas presents a significant challenge in balancing oxidation activity and products selectivity (CO, NOx, N2, etc.). It is found that Cu/H-MOR demonstrates superior performance for DMF oxidation compared to CuO on other supports (γ-Al2O3, HY, ZSM-5) in terms of product selectivity and stability. The geometric and electronic structures of CuO active sites in Cu/H-MOR have been regulated by CeO2 promoter, leading to an increase in the ratio of active CuO (highly dispersed CuO and Cu+ specie). As a result, the oxidation activity and stability of the Cu/H-MOR catalyst were enhanced for DMF selective catalytic oxidation. However, excessive CuO or CeO2 content led to decreased N2 selectivity due to over-high oxidation activity. It is also revealed that Ce3+ species, active CuO species, and surface acid sites play a critical role in internal selective catalytic reduction reaction during DMF oxidation. The 10Cu-Ce/H-MOR (1/4) catalyst exhibited both high oxidation activity and internal selective catalytic reduction activity due to its abundance of active CuO specie as well as Ce3+ species and surface acid sites. Consequently, the 10Cu-Ce/H-MOR (1/4) catalyst demonstrated the widest temperature window for DMF oxidation with high N2 selectivity. These findings emphasize the importance of surface active sites modification for DMF selective catalytic oxidation.

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.