Low-temperature Selective Catalytic Reduction Research Articles

Revealing the two distinctive roles of HY zeolite in enhancing the activity and durability of manganese oxide-zeolite hybrid catalysts for low-temperature NH3-SCR

Recently, metal oxide-zeolite hybrid systems have shown promise in enhancing catalytic performance in low-temperature selective catalytic reduction (SCR), but not much is known as to mechanistic details. The purpose of this study is to examine fluctuations in low-temperature activity contingent upon the presence or absence of zeolite and to elucidate a dual functional mechanism of mixed zeolite in improving the catalytic performance of MnOx, which is closely related to inter-particle diffusion of nitrite and nitrate species from MnOx to zeolite. Firstly, strong Brønsted acid sites on zeolite demonstrate a significantly faster reduction of diffused nitrite species compared to Brønsted/Lewis acid sites on MnOx. More importantly, we discovered that zeolite prevents ammonium nitrate deposition by absorbing and decomposing diffused nitrate species from MnOx. Such distinct interaction between zeolite and both nitrite and nitrate species clarified here will be of great promise in designing future hybrid catalytic materials for low-temperature NH3-SCR reactions.

Investigation of Co–Mn–Ce Ternary Composite Oxide Catalyst for Low-Temperature Selective Catalytic Reduction of NOx with NH3

Investigation of Co–Mn–Ce Ternary Composite Oxide Catalyst for Low-Temperature Selective Catalytic Reduction of NOx with NH3

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.

Significantly Improving N2 Selectivity and H2O Resistance by Confining Mn into TiO2 Nanotubes for Low-Temperature Selective Catalytic Reduction of NO with NH3

Significantly Improving N₂ Selectivity and H₂O Resistance by Confining Mn into TiO₂ Nanotubes for Low-Temperature Selective Catalytic Reduction of NO with NH₃

Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NOx with NH3.

The formation of dimer-Cu species, which serve as the active sites of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies on the mobility of CuI species in the channels of the Cu-SSZ-13 catalysts. Herein, the key role of framework Brønsted acid sites in the mobility of reactive Cu ions was elucidated via a combination of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance ultraviolet-visible spectroscopy. When the number of framework Al sites decreases, the Brønsted acid sites decrease, leading to a systematic increase in the diffusion barrier for [Cu(NH3)2]+ and less formation of highly reactive dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of Al sites is uneven, the [Cu(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage (e.g., cage with paired Al sites), which effectively accelerates the formation of dimer-Cu species and hence promotes the SCR reaction. These findings unveil the mechanism by which framework Brønsted acid sites influence the intercage diffusion and reactivity of [Cu(NH3)2]+ complexes in Cu-SSZ-13 catalysts and provide new insights for the development of zeolite-based catalysts with excellent SCR activity by regulating the microscopic spatial distribution of framework Brønsted acid sites.

Significantly enhanced low-temperature activity and SO2/H2O tolerance of Mn–Ce-Ox/TiO2 catalyst prepared by a facile citric acid assisted impregnation method

Two different Mn–Ce-Ox/TiO2 catalysts were prepared by ordinary impregnation (denoted as MCT) and citric acid assisted impregnation (denoted as MCT-CA) methods, respectively. Excellent NOx removal is achieved over MCT-CA for selective catalytic reduction with NH3 (NH3-SCR), and 100% NOx conversion is obtained at 125 °C under weight hour space velocity (WHSV) of 80000 mL/(gcat·h). Particularly, 100% NOx is converted on MCT-CA in the presence of 10 vol% H2O at 175 °C. As H2O and SO2 coexist in the reaction system for 9 h, NOx conversion can still be maintained >90%, much higher than that (22%) of MCT. A series of characterization results indicates that MCT-CA exhibits a larger BET specific surface area, pore volume, and pore size, which enhances the dispersion of Mn and Ce oxides and promotes the rapid adsorption of reactants and desorption of products. Additionally, MCT-CA possesses more Mn4+, Ce3+, chemisorbed oxygen species, and stronger reducibility, facilitating the conversion of NO to NO2. Specially, the amount of active NH3 species and active nitrate species on MCT-CA is much more than that over MCT. The combined effect of the aforementioned factors devotes to the excellent low-temperature SCR performance and tolerance to H2O/SO2 over MCT-CA.

Structure and activity relationships of AMn2O4 (A = Mg and Zn) spinels in selective catalytic reduction of NOx with NH3

Mn-based spinel as a catalyst for low-temperature selective catalytic reduction with ammonia (NH3-SCR) has been widely studied. However, most researchers have focused on double transition metal Mn-based spinel and the reports about doping with non-transition metal are not common. Herein, Mg2+ and Zn2+ with similar ionic radii were used to prepare high purity MgMn2O4 and ZnMn2O4 (named MMO and ZMO) to study their structure and activity relationships in NH3-SCR. It was found that the denitration performance of the two showed significant differences at low temperature despite have a very similar crystal structure. A series of characterization methods and first-principles calculations demonstrates that the relatively weak bonding strength and basic adsorption sites of MMO catalyst are the key factors contributing to its superior reactivity at low temperatures. This work provides novel insights and ideas in comprehending the catalytic function of non-transition metal-doped Mn-based spinels in NH3-SCR reactions.

Combined Experimental and Density Functional Theory Study on the Mechanism of the Selective Catalytic Reduction of NO with NH3 over Metal-Free Carbon-Based Catalysts.

Metal-free carbon-based catalysts are attracting much attention in the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR). However, the mechanism of the NH3-SCR reaction on carbon-based catalysts is still controversial, which severely limits the development of carbon-based SCR catalysts. Herein, we successfully reconstructed carbon-based catalysts through oxidation treatment with nitric acid, thereby enhancing their low-temperature activity in NH3-SCR. Combining experimental results and density functional theory (DFT) calculations, we proposed a previously unreported NH3-SCR reaction mechanism over carbon-based catalysts. We demonstrated that C-OH and C-O-C groups not only effectively activate NH3 but also remarkedly promote the decomposition of intermediate NH2NO. This study enhances the understanding of the NH3-SCR mechanism on carbon-based catalysts and paves the way to develop low-temperature metal-free SCR catalysts.

Modulating the acidity and reducibility of FeMn/TiO2 to boost the low-temperature selective catalytic reduction of NOx

The design and fabrication of selective catalytic reduction (SCR) catalyst with ultra-low-temperature activity is diligent, however, remains challenging hitherto. Herein, a variety of TiO2-based catalysts with various Fe/Mn ratios were fabricated using a coprecipitation technique and tested for SCR of NOx with ammonia. The experimental results demonstrate that the Fe/Mn ratio considerably affects the acidity, redox property and thereafter the NH3-SCR behavior of FeMn/TiO2 catalysts. Therein, Fe0.3Mn0.7/TiO2 exhibits the broadest activity temperature window from 75 to 210 °C because of its more reactive oxygen species and higher Mn4+ ratio on the catalyst surface, leading to improved oxidation performance and surface acidity, thereby promoting the generation of active NO2 and HONO intermediates, and thereafter enabling the catalyst to adsorb more NH3 to generate active nitrate species and increasing the NH3-SCR performance. Further increasing or decreasing the Fe/Mn ratio would downgrade the SCR performances of corresponding catalysts due to the mismatch between acidity and redox ability, which inhibits the formation of active NO2 and HONO intermediates. In-situ DRIFTS results further illustrate that the SCR of NOx over FeMn/TiO2 catalysts follow both the L-H and E-R mechanisms at the low temperature, however, the types of active nitrates flourish at high temperatures, primarily via the E-R mechanism.

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.

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.

Significant enhanced SO2 resistance of Ce-doped OMS-2 catalyst by building the CeO2 shell for the low-temperature SCR of NO with NH3

Developing highly SO2-resistance catalysts for the NOx reduction at low temperatures is a significant challenge. In this study, we fabricate a catalyst of Ce-doped manganese oxide octahedral molecular sieve by building the CeO2 shell (Ce-OMS-2@CeO2) to solve this problem. This catalyst showed high selective catalytic reduction with NH3 (NH3-SCR) activity and enhanced resistance to SO2-induced deactivation. Ce-OMS-2@CeO2 exhibited outstanding NH3-SCR activity and anti-sulfur performance at 200 °C. The mesoporous CeO2 shells exerted no influence on the transport of reactants to catalytic active sites. The NH3-SCR on Ce-OMS-2@CeO2 catalyst occurred via the Langmuir-Hinshelwood mechanism with adsorption of NH3 and NO on different sites, thus preventing competing adsorption. The CeO2 shell bound SO2 strongly in priority, thus protecting the Ce-OMS-2 core from SO2 poisoning. Moreover, a dynamic balance was established between NH4HSO4 formation and decomposition over our Ce-OMS-2@CeO2 catalyst at low temperatures, which also hindered the SO2 poisoning in NH3-SCR reaction.

A green approach for preparation of MnFeTi Low-temperature NH3-SCR catalysts: utilizing spent V2O5–WO3/TiO2 Catalysts

V2O5–WO3/TiO2 catalysts are commonly used in industry to remove nitrogen oxides. However, spent V2O5–WO3/TiO2 are hazardous solid waste, and their improper disposal can lead to water and soil pollution risks. This study proposes a waste-to-resource approach that promotes the safe disposal of spent catalysts while reducing the production cost of denitrification catalysts. Specifically, vanadium-free low-temperature selective catalytic reduction catalysts are synthesized using spent catalysts. The optimized catalyst, named MnFeTi-22, exhibits a broad temperature activity range, with NO conversion exceeding 96 % from 100 to 300 °C. Even with 5 % H2O, the catalyst maintains a 95 % NO conversion (at approximately 200 °C). Characterization results confirm that the loading of TiO2 recycled from spent catalysts onto MnFe significantly optimizes the distribution of active species. Furthermore, TiO2 can adjust the distribution of acidic sites and the quantity of surface-adsorbed oxygen, promoting electron transfer between Mn4+/Fe2+ and Mn3+/Fe3+.

Dy-Modified Mn/TiO2 Catalyst Used for the Selective Catalytic Reduction of NO in Ammonia at Low Temperatures.

A novel Mn/TiO2 catalyst, prepared through modification with the rare-earth metal Dy, has been employed for low-temperature selective catalytic reduction (SCR) denitrification. Anatase TiO2, with its large specific surface area, serves as the carrier. The active component MnOx on the TiO2 carrier is modified using Dy. DyxMn/TiO2, prepared via the impregnation method, exhibited remarkable catalytic performance in the SCR of NO with NH3 as the reducing agent at low temperatures. Experiments and characterization revealed that the introduction of a suitable amount of the rare-earth metal Dy can effectively enhance the catalyst's specific surface area and the gas-solid contact area in catalytic reactions. It also significantly increases the concentration of Mn4+, chemisorbed oxygen, and weak acid sites on the catalyst surface. This leads to a notable improvement in the reduction performance of the DyMn/TiO2 catalyst, ultimately contributing to the improvement of the NH3-SCR denitrification performance at low temperatures. At 100 °C and a space velocity of 24,000 h-1, the Dy0.1Mn/TiO2 catalyst can achieve a 98% conversion rate of NOx. Furthermore, its active temperature point decreases by 60 °C after the modification, highlighting exceptional catalytic efficacy at low temperatures. By doubling the space velocity, the NOx conversion rate of the catalyst can still reach 96% at 130 °C, indicating significant operational flexibility. The selectivity of N2 remained stable at over 95% before reaching 240 °C.

Stability and Enhanced Low-Temperature NH3-SCR Activity of Supported Vanadate Catalysts in the Microwave Field

The low-temperature selective catalytic reduction (SCR) of NO by NH3 over the supported NdV/Ti-x% (x = 1, 5, and 10) catalysts has been assessed in a microwave field. The NO...

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.

Crystal growth regulation of Cu-SSZ-13 zeolites for effective catalytic reduction of NOx from automotive exhausts

In this study, glucose was used in the synthesis of SSZ-13 zeolites; it acted as a zeolite growth regulator to successfully change the morphology of SSZ-13 zeolites. The particle size of the SSZ-13 zeolites reduced from 10 to 3 μm, and the morphology of the zeolite surface changed from rough and irregular spherical to smooth and uniform cubic. The glucose-modified SSZ-13 zeolites were ion-exchanged with Cu2+ ions (named as Cu-SSZ-13-Glu) and tested for the catalytic reduction of NOx with NH3 (NH3–SCR). The results demonstrated that the catalyst Cu-SSZ-13-Glu exhibited better NH3–SCR catalytic performance than that of conventional Cu-SSZ-13, with a wider effective reaction window, superior low-temperature SCR performance, favorable anti-SO2 poisoning ability, and excellent hydrothermal stability. The characterization results of the Cu-SSZ-13-Glu sample revealed an increase in crystallinity as well as acidic sites, and an improvement in the redox properties. After hydrothermal aging, compared with traditional Cu-SSZ-13, the Cu-SSZ-13-Glu zeolites exhibited a more stable structure and maintained higher crystallinity, larger specific surface area, and microporous volume. Moreover, the Cu2+ ions in the Cu-SSZ-13-Glu zeolites were more stable, and fewer CuOx species were generated after hydrothermal aging, retaining more Cu2+ ions species and acidic sites. To summarize, the simply prepared Cu-SSZ-13-Glu zeolites exhibited excellent catalytic activity and hydrothermal stability, showing broad application prospects in the field of nitrogen oxide removal from automotive exhausts.

Enhanced Low-Temperature Activity and Hydrothermal Stability of Ce-Mn Oxide-Modified Cu-SSZ-39 Catalysts for NH3-SCR of NOx

Cu-SSZ-39 zeolite with an AEI structure exhibits excellent hydrothermal stability and can be a potential alternative to Cu-SSZ-13 zeolite SCR catalysts for NOx removal in diesel vehicles. However, the inferior low-temperature performance of Cu-SSZ-39 leads to substantial NOx emissions during the cold-start period, impeding its practical application. In this study, Ce-Mn oxide-modified Cu-SSZ-39 catalysts (CeMnOx/Cu-SSZ-39) and references (CeO2/Cu-SSZ-39 and MnOx/Cu-SSZ-39) were prepared by the ion-exchange of Cu ions followed by impregnation of the oxide precursors, with the aim of enhancing the NH3-SCR performance at low temperatures. The modified catalysts exhibited improved low-temperature activity and hydrothermal stability compared to the unmodified counterpart. In particular, CeMnOx/Cu-SSZ-39 showed the highest activity among the three catalysts and achieved NOx conversions above 90% within the temperature range of 180 °C to 600 °C, even after undergoing hydrothermal aging at 800 °C. Experimental results indicated that the synergistic effect between Ce and Mn in CeMnOx improves the redox properties and acidity of the catalyst due to the presence of Ce3+, Mn4+, and abundant adsorbed oxygen species, which facilitate low-temperature SCR reactions. Furthermore, the interaction of CeMnOx with Cu-SSZ-39 stabilizes the zeolite framework and hinders the agglomeration of Cu species during the hydrothermal aging process, contributing to its exceptional hydrothermal stability. The kinetics and NO oxidation experiments demonstrated that CeMnOx provides access to fast SCR reaction pathways by oxidizing NO to NO2, resulting in a significant increase in low-temperature activity. This study provides novel guidelines for the design and preparation of Cu-SSZ-39 zeolite with outstanding SCR performance over a wide temperature range.

Strong metal oxide-zeolite interactions during selective catalytic reduction of nitrogen oxides

In response to the stricter EU VII emission standards and the "150 ℃ challenge", selective catalytic reduction by ammonia (NH3-SCR) catalysts for motor vehicles are required to achieve high NO conversion below 200 °C. Compounding metal oxides with zeolites is an important strategy to design the low-temperature SCR catalysts. Here, we original prepared Cu-SSZ-13 @ MnGdOx (Cu-Z @ MGO), which achieved over 90% NO conversion and 95% N2 selectivity at 150 ℃. It has been demonstrated that a uniform mesoporous loaded layer of MGO grows on Cu-Z, and a recrystallization zone appears at the MGO-Cu-Z interface. We discover that the excellent low-temperature SCR activity derives from the strong metal oxide-zeolite interaction (SMZI) effects. The SMZI effects cause the anchor and high dispersion of MGO on the surface of Cu-Z. Driven by the SMZI effects, the Mn3+/Mn4+ redox cycle ensures the low and medium temperature-SCR activity and the Cu2+/Cu+ redox cycle guarantees the medium and high temperature-SCR activity. The introduction of MGO improves the reaction activity of -NH2 species adsorbed at Mn sites at 150 ℃, achieving a cycle of reduction and oxidation reactions at low temperatures. This strategy of inducing SMZI effects of metal oxides and zeolites paves a way for development of high-performance catalysts.

Highly efficient MnCe/Ti catalyst for NH3-SCR of NO from sintering flue gas at low temperature: CO tolerance and reaction mechanism assessed by in situ DRIFTS

The effect of metal doping (Ce, Fe, and Co) on CO tolerance of Mn/Ti catalyst used for eliminating NO from sintering flue gas has been investigated. The best MnCe/Ti catalyst achieves a NOx conversion of 80 % from 125 to 250 °C at a GHSV of 80,000 h−1. NOx conversion dropped by only 4 % below 200 °C and N2 selectivity slightly increased after introduction of CO, indicating strong resistance of MnCe/Ti catalyst to CO. XPS results suggest that CO oxidation does not alter surface content of Mn4+ and Ce3+ sites of used MnCe/Ti catalyst while it reduces the surface adsorbed oxygen of used MnCe/Ti catalyst because surface adsorbed oxygen consumed by CO oxidation cannot be readily replenished owing to the insufficient capacity in oxygen and electron transport. This is primarily accountable for the slight decrease in NOx conversion. Furthermore, NH3 can completely restrain CO adsorption on MnCe/Ti catalyst while CO cannot hinder the adsorption of NH3 and NO on the catalyst because the interaction between CO and MnCe/Ti catalyst significantly weakened in the presence of NH3, leading to the strong CO tolerance of MnCe/Ti. On MnCe/Ti Eley-Rideal mechanism governs the NH3-SCR involving gaseous NO and adsorbed NH3, while Mars-van Krevelen mechanism describes the oxidation of CO, indicating its reaction with gaseous CO and surface adsorbed oxygen. The findings in this paper can provide modification strategy for low-temperature SCR catalysts which are expected to be applied for sintering flue gas.