Selective Catalytic Reduction Of NOx Research Articles

Effect of modified Fe2O3/activated carbon catalysts for low-temperature CO selective catalytic reduction of NOx in underground diesel vehicle exhaust

The elimination of CO and NOx has practical significance for the control of exhaust pollutants from underground diesel vehicles. Catalytic reduction of NO by CO (CO-SCR) using activated carbon (AC) catalysts is considered a cost-effective and environmentally friendly denitrification method. In this study, Fe2O3/AC-based catalysts were prepared using an impregnation method and modified with different metals to investigate their impact on catalyst performance. The results demonstrated that the catalyst exhibited the highest denitrification efficiency when loaded with 10% Fe. The Mn-doped 10Fe/AC catalyst exhibited the best catalytic performance, with the highest NO conversion of 97.5% and CO removal rate of 83.3% at 240 °C. The effects of loading Mn, Ce, and La on the physical and chemical properties of the catalysts were analyzed using various characterization tools, and the reaction mechanism of the catalysts was investigated by in situ DRIFTS technique. The findings revealed that the strong synergistic interaction occurring between Mn and Fe led to an increase in the surface-adsorbed oxygen (Oα) and Fe3+, which produced more oxygen vacancies on the catalyst, thus improving the redox properties. The in-situ DRIFT results indicated that the addition of Mn enhanced the adsorption capacity of active NO and CO. For the CO-SCR reaction on Fe-Mn/AC catalyst, the E-R mechanism was followed at low temperature and the L-H mechanism was followed at high temperature.

Recent Advancements in Fe-Based Catalysts for the Efficient Reduction of NOx by CO.

The technology of CO selective catalytic reduction of NOx (CO-SCR) showcases the potential to simultaneously eliminate CO and NOx from industrial flue gas and automobile exhaust, making it a promising denitrification method. The development of cost-effective catalysts is crucial for the widespread implementation of this technology. Transition metal catalysts are more economically viable than noble metal catalysts. Among these, Fe emerges as a prominent choice due to its abundant availability and cost-effectiveness, exhibiting excellent catalytic performance at moderate reaction temperatures. However, a significant challenge lies in achieving high catalytic activity at low temperatures, particularly in the presence of O2, SO2, and H2O, which are prevalent in specific industrial flue gas streams. This review examines the use of Fe-based catalysts in the CO-SCR reaction and elucidates their catalytic mechanism. Furthermore, it also discusses various strategies devised to enhance low-temperature conversion, taking into account factors such as crystal phase, valence states, and oxygen vacancies. Subsequently, the review outlines the challenges encountered by Fe-based catalysts and offers recommendations to improve their catalytic efficiency for use in low-temperature and oxygen-rich environments.

The Catalytic Mechanisms and Design Strategies of Noble Metal Catalysts for Selective Reduction of NOx with CO

AbstractThe selective catalytic reduction of NOx by CO (CO‐SCR) is a viable method for simultaneously mitigating NOx and CO emissions in motor vehicle exhaust and industrial flue gases. Extensive research has been conducted on noble metal catalysts because of their superior catalytic activity and stability compared to non‐noble metals. Noble metal catalysts demonstrate efficient NOx conversion and high selectivity for N2. However, achieving high conversion, selectivity, and stability over a wide temperature range in the presence of excess O2, H2O, and SO2 remains a significant challenge. This review summarizes recent advancements in CO‐SCR over noble metal catalysts, providing insights for the development of CO‐SCR catalysts. This paper first examines the reaction mechanisms of CO‐SCR, followed by a discussion on the design and optimization strategies of noble metal catalysts, with a focus on composition and structural effects. This includes creating active metal sites, selecting appropriate supports, integrating catalytic additives, choosing monometallic nanoparticles, alloying, and using single‐atom catalysts. Finally, remaining challenges and potential avenues for future research have been suggested. The discovery of negatively charged noble metal single‐atom‐site catalysts presents new research opportunities.

Understanding the roles of Brønsted/Lewis acid sites on manganese oxide-zeolite hybrid catalysts for low-temperature NH3-SCR

Although metal oxide-zeolite hybrid materials have long been known to achieve enhanced catalytic activity and selectivity in NOx removal reactions through the inter-particle diffusion of intermediate species, their subsequent reaction mechanism on acid sites is still unclear and requires investigation. In this study, the distribution of Brønsted/Lewis acid sites in the hybrid materials was precisely adjusted by introducing potassium ions, which not only selectively bind to Brønsted acid sites but also potentially affect the formation and diffusion of activated NO species. Systematic in situ diffuse reflectance infrared Fourier transform spectroscopy analyses coupled with selective catalytic reduction of NOx with NH3 (NH3-SCR) reaction demonstrate that the Lewis acid sites over MnOx are more active for NO reduction but have lower selectivity to N2 than Brønsted acids sites. Brønsted acid sites primarily produce N2, whereas Lewis acid sites primarily produce N2O, contributing to unfavorable N2 selectivity. The Brønsted acid sites present in Y zeolite, which are stronger than those on MnOx, accelerate the NH3-SCR reaction in which the nitrite/nitrate species diffused from the MnOx particles rapidly convert into the N2. Therefore, it is important to design the catalyst so that the activated NO species formed in MnOx diffuse to and are selectively decomposed on the Brønsted acid sites of H-Y zeolite rather than that of MnOx particle. For the physically mixed H-MnOx+H-Y sample, the abundant Brønsted/Lewis acid sites in H-MnOx give rise to significant consumption of activated NO species before their inter-particle diffusion, thereby hindering the enhancement of the synergistic effects. Furthermore, we found that the intercalated K+ in K-MnOx has an unexpected favorable role in the NO reduction rate, probably owing to faster diffusion of the activated NO species on K-MnOx than H-MnOx. This study will help to design promising metal oxide-zeolite hybrid catalysts by identifying the role of the acid sites in two different constituents.

The enhanced SO2 resistance of Fe-Ti catalyst by W/SO42− co-modification for NH3-SCR: A combined experimental and DFT study

It remains a great challenge to improve the low-temperature SO2 resistance of catalysts applied for selective catalytic reduction of NOx with NH3 (NH3-SCR). This work develops an outstanding SO2-tolerant Fe-Ti (FT) catalyst by W/SO42− co-modification via a sol–gel method using Fe(NO3)3·9H2O, tetrabutyl titanate, ammonium tungstate and thiourea as raw materials. The physicochemical characterizations, in-situ diffuse reflectance infrared Fourier transform (DRIFT) measurements and density functional theory (DFT) calculations were combined to reveal the underlying mechanisms. Although W doping can effectively enhance the surface acidity, NH3 adsorption on Lewis acid sites can still be restrained by competitive adsorption of SO2, whereas SO42− modification can enhance the adsorption stability of NH3 without being affected by SO2. Simultaneously, the NO adsorption ability on Fe sites can be significantly enhanced by W/SO42− co-modification. Furthermore, the W doping or SO42− modification can induce conversion of Fe3+ to Fe2+ to affect the redox property of Fe species. A proper amount of Fe2+ suppressed the oxidizability of FT catalyst to inhibit NH3 overoxidation to enhance N2 selectivity, and created more oxygen vacancies to generate larger amount of chemisorbed oxygen to facilitate NH3 dehydrogenation and NO oxidation. More importantly, the inhibition effect of SO2 adsorption on Fe sites was strengthened by W/SO42− co-modification. Consequently, the W/SO42− co-modified FT catalyst exhibited high activity with >90 % of NO conversion and >95 % of N2 selectivity within a broad window of 275–450 °C and possessed superior SO2 + H2O tolerance at 275 °C.

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.

Liquid Nitrogen Exfoliation and Nanodispersion of WS2 for Thermocatalysts.

The efficient and clean exfoliation of single and/or few-layer nanosheets of WS2, a two-dimensional transition metal dichalcogenides, remains a significant challenge. In this study, a simple exfoliation method was proposed to produce ultrathin WS2 nanosheets by combining the liquid nitrogen exfoliation and nanodispersion techniques. This approach efficiently exfoliated WS2 into several layers of nanosheets via rapid temperature changes and mechanical stress without inducing defects or contamination. After five cycles of heating/liquid nitrogen and nanodispersion, the resulting WS2 nanosheets (WS2-5N ND) were confirmed to have been successfully exfoliated into 1-4 layers. When applied as a promoter in a thermocatalyst for the selective catalytic reduction of NOX using NH3, 2V3WS2/Ti (WS2-5N ND) exhibited excellent NOX conversion and N2 selectivity, along with excellent durability even in the presence of SO2. This result was greater than 2V3WS2/Ti (WS2-5N) subjected to only liquid nitrogen exfoliation, proving the importance of the simultaneous action of both methods. This method is expected to be an important contribution to ongoing research on high-performance WS2-based catalysts, thereby opening up potential opportunities for a wide range of applications.

Self-protected Chlorella@Mn catalyst with excellent resistance to alkali/alkaline earth metal for NOx reduction by NH3

In the selective catalytic reduction of NOx by NH3 (NH3-SCR), conventional Mn-based denitration catalysts often suffered from susceptibility to poisoning by alkali and alkaline earth metals, this paper presented an innovative self-protected Chlorella@Mn denitration catalyst. Remarkably, in the presence of high concentrations (2 wt%) of alkali and alkaline earth metal oxides, the Chlorella@Mn catalyst sustained a NOx conversion exceeding 96 % at 175 °C. At an even higher concentration (4 wt%), NOx conversion above 90 % at 175 °C, surface analysis revealed that POMn sites acted as sacrificial sites, binding to the alkali and alkaline earth metals, the Chlorella@Mn catalyst surface naturally carried a spectrum of acidic species (such as SO42−, PO3−, SiO32−), proficient in capturing alkali/alkaline earth metal effectively, elements such as S, P, and Si formed bonds with K, Na, Ca, and Mg. The synergistic protection of the active sites and the surface elements avoided the deactivation of the catalyst. The detrimental effects of high concentrations of alkali and alkaline earth metals were primarily due to promoting an excessively high valence state of Mn on the catalyst surface and the reduction or loss of NH3 adsorption and activation at Brønsted acid sites. This research provided valuable insights for advancing the development of low-temperature denitration catalysts with improved resistance to alkali and alkaline earth metal poisoning.

Accelerator or inhibitor? The effects of H2O on selective catalytic reduction of NOx by methanol over HZSM-5 catalysts with different Lewis/Brønsted ratios

HZSM-5 zeolites are widely regarded as promising catalysts for the selective catalytic reduction of NOx using methanol (CH3OH-SCR). However, the effect of H2O on CH3OH-SCR over HZSM-5 zeolites with varying Lewis and Brønsted acid sites (LAS and BAS) remains unclear. Herein, a series of HZSM-5 catalysts with varying LAS/BAS ratios were synthesized through Ga modification combined with calcination. Notably, these catalysts with varying LAS/BAS ratios exhibit diverse CH3OH-SCR activity. The HZSM-5 catalyst with a higher amounts BAS shows superior SCR performance (93 % NOx conversion at 300 ℃) under dry conditions, while Ga/HZSM-5 with abundant LAS demonstrate better De-NOx activity in the presence of H2O. In situ DRIFTS results, TPSR-MS coupled with DFT calculations demonstrate that the Si(OH)Al sites in HZSM-5 zeolites promote the generation of intermediate formamide under dry conditions. Whereas, the extra-framework Al-OH and Ga species in Ga/HZSM-5 catalysts lead to excessive oxidation of methanol to formate species. When water is present, intense competitive adsorption between CH3OH and H2O occurs at the Si(OH)Al sites, significantly decreasing the denitration activity of HZSM-5. However, H2O not only alleviates the over-oxidation of methoxy species, but also enables the Al-OH and GaO+ sites to adsorb and activate CH3OH to generate formamide species, thereby exhibiting preferable catalytic activity in Ga/HZSM-5 catalysts. Accordingly, a reaction mechanism based on the synergistic effect of BAS and LAS is proposed for Ga/HZSM-5 catalysts.

Facile synthesis of SSZ-16 nanoaggregates with excellent performance in NH3-SCR reaction

d6r units were proved to be crucial for the synthesis of SSZ-16 zeolite with AFX topology, and FAU zeolite owning plenty of d6r units was chosen as the essential raw material for constructing AFX frameworks. In this work, SSZ-16 zeolite was directly synthesized using colloidal silica and sodium aluminate as the raw materials in short crystallization time of 5 h. The corresponding crystallization process were carefully investigated by various characterizations, demonstrating that the key to this success was the formation of d6r units built up by a large amount of s4r units which were formed with the assistance of the appropriate organic structure directing agent (OSDA) and seeds in the synthetic media. Interestingly, the products of C-SSZ-16 zeolites presented spherical shapes consisted of relatively small particles with size range from 30 to 50 nm. Moreover, after ion-exchanged with Cu ions, the Cu-C-SSZ-16 products showed excellent performance in the selective catalytic reduction of NOx with NH3 (NH3-SCR) and better hydrothermal stability than conventional Cu-SSZ-16-con samples due to slightly higher Si/Al ratio of the Cu-C-SSZ-16. Therefore, remarkable catalytic performance and the use of low-cost raw materials as well as short period of synthesis time make AFX zeolites as potential applicants in NH3-SCR field, from an industrial viewpoint.

The preferential occupying effect regulated atomically dispersed Cu site over Cu-SSZ-13 for enhancing NH3-SCR performance

The development of highly active catalysts for selective catalytic reduction of NOx with ammonia (NH3-SCR) at broad temperature range are desirable but challenging. Herein, a preferential occupying effect of Cd regulates atomically dispersed Cu site is proposed for NH3-SCR. Benefiting from the modulation effect of Cd, the temperature of NO conversion above 90 % for Cu-Cd-SSZ-13 is broadened to 160–550 ℃, and hydrothermal stability is significantly improved. The Cd ions prefer to occupy eight-membered rings (8 MRs) favor the generation of Z2Cu2+ at 6 MRs, and the electronegativity of Z2Cu2+ increases due to the introduction of Cd, thus boosting NH3-SCR process by reducing formation energy of key intermediate (NH4NO2) at active Z2Cu2+ site. Noticeably, the preferential occupying of Cd enhances the intrinsic stability of Z2Cu2+ by inhibiting its transformation into inactive CuOx. This work sheds new insight for regulating atomically dispersed Cu site to enhance NH3-SCR performance.

A key and facile strategy for improving sulfur resistance of Ce-based SCR catalysts: Enhancing the dispersion of Fe species

Enhancing the sulfur resistance of ceria-based catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) has always been a major challenge. Herein, the NH3-SCR activity and sulfur tolerance of CeFeOx loaded with WO3 were improved by adjusting the precipitation sequence for Ce and Fe. W/Fe(f)-Ce catalysts (namely, first precipitate Fe and then Ce) presented the best sulfur tolerance due to their more uniformly dispersed Fe species, thereby giving rise to superior physicochemical properties, including smaller particle size, larger surface area, and stronger low-temperature redox capacity. Because of these factors, the contact between the catalyst and reactants was more thorough, the CeO2 sites were better protected, and the low-temperature reactivity for consuming NH4+ from ammonium bisulfate (ABS) was higher. This study suggests that doping with Fe can enhance the SO2 tolerance of Ce-based catalysts, and that the dispersion of Fe species is crucial, which provides guidance for preparing efficient and sulfur-tolerant NH3-SCR catalysts.

Ultra-low temperature selective catalytic reduction of NOx into N2 by micron spherical CeMnOx in high-humidity atmospheres containing SO2

The solvothermal synthesis of optimized micron-sized spherical Ce1Mn7Ox-350 yields remarkable results in ultra-low temperature NH3-SCR of NOx, with over 91 % NOx conversion achieved between 59 and 255 ℃. Notably, under 5 vol% H2O and 50 ppm SO2, the Ce1Mn7Ox maintains NOx conversion >99 % at 127 ℃ for extended periods, surpassing current ultra-low temperature deNOx standards. This superior performance is attributed to the material's unique characteristics: the regular and porous surface morphology enhances exposure to active sites, particularly Mn3O4(112) facets crucial for ultra-low temperature deNOx, while the rough and loose surface and high Mn2O3(222) exposure mitigate water vapor and SO2 poisoning. Furthermore, the thermal storage effect of the Mn2O3/Mn3O4 system within Ce1Mn7Ox facilitates rapid thermal dissipation and ammonium sulfite decomposition. This process is further augmented by the pores, which aid in the confinement of deNOx reaction heat and facilitate the flushing of flowing flue gas, thereby impeding the formation of ammonium bisulfate.

Effect of Tourmaline Addition on the Anti-Poisoning Performance of MnCeOx@TiO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx.

In view of the flue gas characteristics of cement kilns in China, the development of low-temperature denitrification catalysts with excellent anti-poisoning performance has important theoretical and practical significance. In this work, a series of MnCeOx@TiO2 and tourmaline-containing MnCeOx@TiO2-T catalysts was prepared using a chemical pre-deposition method. It was found that the MnCeOx@TiO2-T2 catalyst (containing 2% tourmaline) exhibited the best low-temperature NH3-selective catalytic reduction (NH3-SCR) performance, yielding 100% NOx conversion at 110 °C and above. When 100-300 ppm SO2 and 10 vol.% H2O were introduced to the reaction, the NOx conversion of the MnCeOx@TiO2-T2 catalyst was still higher than 90% at 170 °C, indicating good anti-poisoning performance. The addition of appropriate amounts of tourmaline can not only preferably expose the active {001} facets of TiO2 but also introduce the acidic SiO2 and Al2O3 components and increase the content of Mn4+ and Oα on the surface of the catalyst, all of which contribute to the enhancement of reaction activity of NH3-SCR and anti-poisoning performance. However, excess amounts of tourmaline led to the formation of dense surface of catalysts that suppressed the exposure of catalytic active sites, giving rise to the decrease in catalytic activity and anti-poisoning capability. Through an in situ DRIFTS study, it was found that the addition of appropriate amounts of tourmaline increased the number of Brønsted acid sites on the catalyst surface, which suppressed the adsorption of SO2 and thus inhibited the deposition of NH4HSO4 and (NH4)2HSO4 on the surface of the catalyst, thereby improving the NH3-SCR performance and anti-poisoning ability of the catalyst.

Excellent performance of Ce doped CoMn2O4/TiO2 catalyst in NH3-SCR of NOx under H2O&SO2 conditions

Mn-based catalysts have become a research hotspot for low-temperature NH3-SCR, and the improvement of anti-SO2 poisoning performance is crucial to promote their industrial application for NH3 selective catalytic reduction of NOx. In this work, the CoMn2O4/Ce-TiO2 catalyst was able to maintain 95 % NOx conversion even in the presence of 10 vol% H2O+50 ppm SO2 for 24 h at 250 °C.The excellent performance was attributed to the large mesoporous structure, specific surface area, and the enhancement of acid and redox properties associated with the catalysts. The Ce doped CoMn2O4/Ce-TiO2 catalyst maintained Mn3+, Mn4+ and Co3+ in high concentrations on the surface, which improved the lattice oxygen mobility, and the electron transfer and interaction. The presence of more Ce4+ provided higher SCR activity due to the strong oxygen storage migration and also release ability. The reaction process of NH3-SCR for NOx removal on the surface of CoMn2O4/Ce-TiO2 catalyst mainly followed the Eley-Rideal (E-R) mechanism. The NH3 adsorbed on the catalyst surface reacted mainly with NO/NO2 species, even under H2O&SO2. The E-R reaction mechanism on the surface of CoMn2O4/Ce-TiO2 catalyst was less restricted by the inhibition of H2O&SO2. This is one of the primary reasons for the superior anti-sulfur toxicity performance of the catalyst. This work opens a new avenue for the design of efficient and environmentally friendly NH3-SCR catalysts with good application prospects.

Low-silica chabazite zeolites for NOx reduction: Tailoring the active center of Cu2+-2Z and [Cu(OH)]+-Z through the regulation of CHA-cages opening

Low-silica chabazite (CHA) catalysts with varying configurations of Cu2+-2Z and [Cu(OH)]+-Z active centers are developed through the regulation of the opening of CHA-cages. The catalysts are subsequently applied to the selective catalytic reduction of NOx by NH3 (NH3-SCR) reactions. A series of characterizations and performance evaluations indicate that the density of [Cu(OH)]+-Z in eight-member rings (8MRs) is positively correlated with low-temperature performance. Moreover, the presence of partial Cu2+-2Z in six-member rings (6MRs) enhances activity and tolerance at high temperatures. The residual K+ ions in the CHA-cages after NH4+ exchange are retained in the 6MRs. Density functional theory calculations demonstrate that the residual K+ ions stabilize the 6MRs structure, ensuring the entry and stable presence of the active Cu2+-2Z centers. In situ analysis reveals the reaction intermediates involved in the NH3-SCR process on representative catalysts. These findings can further elucidate the active centers of CHA-type catalysts and guide the design of commercial catalysts.

Interface Induced by Hydrothermal Aging Boosts the Low-Temperature Activity of Cu-SSZ-13 for Selective Catalytic Reduction of NOx.

Hitherto, sulfur poisoning and hydrothermal aging have still been the challenges faced in practical applications of the Cu-SSZ-13 catalyst for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust. Here, we elaborately design and conduct an in-depth investigation of the synthetic effects of hydrothermal aging and SO2 poisoning on pristine Cu-SSZ-13 and Cu-SSZ-13@Ce0.75Zr0.25O2 core@shell structure catalysts (Cu@CZ). It has been discovered that Cu@CZ susceptible to 750 °C with 5 vol % H2O followed by 200 ppm SO2 with 5 vol % H2O (Cu@CZ-A-S) could still maintain nearly 100% NOx conversion across the significantly wider temperature region of 200-425 °C, which is remarkably broader than that of the Cu-SSZ-13-A-S (300-400 °C) counterpart. The experimental results show that the hydrothermal aging process results in the migration of highly active Cu species within the cage of Cu-SSZ-13 to the CZ surface, forming CuO/CZ with abundant interfaces, which significantly enhances the adsorption and subsequent activation of NO, leading to the generation of reactive N2O3 and HONO intermediates. Moreover, density functional theory (DFT) calculations reveal that the H of the HONO* species can function as Brønsted acid sites, effectively adsorbing NH3 to generate the active NH4NO2* intermediate, which readily decomposes into N2 and H2O. Furthermore, this pathway is the rate-determining step with an energy barrier of 0.93 eV, notably lower than that of the "standard SCR" pathway (1.42 eV). Therefore, the formation of the new CuO/CZ interface profoundly boosts the low-temperature NH3-SCR activity and improves the coresistance of the Cu@CZ catalyst to sulfur poisoning and hydrothermal aging.

Unveiling the mechanistic insights into the potassium resistance of 3.5 W-V-1 %K NH3-SCR catalysts: The dual functionality of V-O-W structure as acid and redox sites

The poisoning effect of alkali metals over Vanadium-based catalysts still faced a significant challenge in the selective catalytic reduction of NOx with NH3 (NH3-SCR). In order to improve the alkali (mainly potassium resistance) resistance of the Vanadium-based catalysts, WO3 was introduced into the vanadium oxide catalysts. The obtained 3.5WO3-V2O5(3.5 W-V) catalyst exhibited the excellent low-temperature NH3-SCR performance owing to the formation of V-O-W structures. Noteworthily, when 1 wt% K was introduced into the 3.5 W-V catalyst, vanadium oxide (V-O, V=O) species preferentially bonded to alkali metal K as an acidic sacrifice site, prompting more active V-O-W species to remain. Thus, the 3.5 W-V catalyst also possessed excellent alkali resistance. The NH3-TPD results showed that the 3.5 W-V-1 %K catalyst had a considerable acid site to ensure the adsorption capacity of NH3, while the XPS results showed that the 3.5 W-V-1 %K catalyst had excellent redox performance to ensure the activation capacity of the reactant NH3. Moreover, in-situ DRIFTS transient experiments revealed that the 3.5 W-V and 3.5 W-V-1 %K catalysts were more capable of adsorbing and activating NH3 species to rapidly react with gaseous NO, obeying the Eley-Rideal (E-R) pathway in NH3-SCR reaction. This work provides new insights into poisoning resistant of vanadium-based catalysts.

Selective catalytic reduction of NOx by NH3 at low temperature over Mn0.5Zr(0.5-x)Fex catalyst with different Fe/Zr ratios

ABSTRACT Through a basic co-precipitation technique, a series of Mn0.5Zr(0.5-x)Fex ternary oxide catalysts with different Fe/Zr ratios have been prepared and applied to the selective catalytic reduction of NOx. The results of catalytic activity test show that the Mn0.5Zr0.2Fe0.3 catalyst with a molar ratio of Fe/Zr of 3:2 has the most effective denitrification activity at low temperature and displays considerable tolerance to SO2 and H2O. The Mn0.5Zr0.2Fe0.3 catalyst exhibits a combination of physical and chemical attributes, including an elevated specific surface area and a homogeneously distributed active component, which collectively furnish additional points of interaction for the reacting gases, thus improving the catalytic performance. In addition, the high Mn4+/Mnn+ and Fe2+/Fen+ ratios, strong redox capacity, and more acidic sites on the surface of the catalyst also have positive effects on its denitrification performance. Meanwhile, the reaction process on Mn0.5Zr0.2Fe0.3 catalyst is driven by the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. This study provides an important reference for the design of Mn-based catalysts with high denitration performance and excellent resistance to SO2 and H2O.

Engineering In-Co3O4/H-SSZ-39(OA) Catalyst for CH4-SCR of NOx: Mild Oxalic Acid (OA) Leaching and Co3O4 Modification.

Zeolite-based catalysts efficiently catalyze the selective catalytic reduction of NOx with methane (CH4-SCR) for the environmentally friendly removal of nitrogen oxides, but suffer severe deactivation in high-temperature SO2- and H2O-containing flue gas. In this work, SSZ-39 zeolite (AEI topology) with high hydrothermal stability is reported for preparing CH4-SCR catalysts. Mild acid leaching with oxalic acid (OA) not only modulates the Si/Al ratio of commercial SSZ-39 to a suitable value, but also removes some extra-framework Al atoms, introducing a small number of mesopores into the zeolite that alleviate diffusion limitation. Additional Co3O4 modification during indium exchange further enhances the catalytic activity of the resulting In-Co3O4/H-SSZ-39(OA). The optimized sample exhibits remarkable performance in CH4-SCR under a gas hourly space velocity (GHSV) of 24,000 h-1 and in the presence of 5 vol% H2O. Even under harsh SO2- and H2O-containing high-temperature conditions, it shows satisfactory stability. Catalysts containing Co3O4 components demonstrate much higher CH4 conversion. The strong mutual interaction between Co3O4 and Brønsted acid sites, confirmed by the temperature-programmed desorption of NO (NO-TPD), enables more stable NxOy species to be retained in In-Co3O4/H-SSZ-39(OA) to supply further reactions at high temperatures.