Catalysts For 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.

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.

Effect of SBA-15 Morphology in the Catalytic Performance of Pt/Ce-SBA-15 Catalyst for NOx Reduction

Effect of SBA-15 Morphology in the Catalytic Performance of Pt/Ce-SBA-15 Catalyst for NOx Reduction

Three-dimensional MOFs confined Anderson-type POMs clusters—Templated fabrication of porous Co-NiMo/Cr2O3 catalyst with superior NH3-SCR catalytic activity

Fabricating ecofriendly and high-performance NOx reduction catalysts is significant but challenging. This study introduces an innovative approach for creating homogeneous and porous Co-NiMo/Cr2O3 composites by employing a POMs@MOFs (POMs = polyoxometalates, MOFs = metal organic frameworks) core − shell template as a precursor and subsequent high-temperature treatment, in which the Anderson-type POMs are confined within the MIL-101 MOFs as secondary building units. The well-organized architecture of NiMo6@MIL-101 made it possible to provide a widely dispersed metal source and later transform into defect-rich catalyst. This confinement strategy allows the prepared Co-NiMo/Cr2O3 catalysts to be uniformly dispersed without aggregation and exposes abundant active sites. It also triggers more electron transfer, producing a strong synergistic effect that results in an increase in specific surface area, Olatt/Otol ratio and the number of acidic sites. Boosted by synergistically multidoping effect, the as-prepared Co-NiMo/Cr2O3 catalyst exhibits superior activity in NOx reduction (95 %, 150 °C), which is 3 times that of Cr2O3 derived from pure MOFs under the same condition. This work may shed the light on the design and fabrication of multifunctional nanoporous metal oxide composites materials and inspire the application of polyoxometalates in NOx removal processes.

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.

Design of Cu-based MIL-100(Fe) catalyst for propylene-selective catalytic reduction of NOx: Combination of activity, characterizations and mechanism

The hydrothermal technique was applied to prepare a new metal–organic framework, MIL-100(Fe), which was modified by ultrasonic impregnation with different wt % of copper. The results showed that the NOx conversion over 2.3Cu-MIL-100(Fe) was 100 % at 275 °C and maintained at ≥ 85 %, with N2 selectivity ≥ 90 % in the temperature range of 275–400 °C and high SO2 resistance. An increase in the Cu content up to 2.3 wt% showed the highest surface area (1206 m2/g) observed in N2 adsorption–desorption. The results show that the introduction of Cu increased the oxygen vacancies on the catalyst surface. Electron transfer and a synergistic effect were observed between Cu and Fe. The shift of the reduction peaks to lower temperatures after the addition of Cu confirmed that Cu improved the reducibility of the catalysts. The 2.3Cu-MIL-100(Fe) exhibited the highest Lewis content (44.97 μmol/g), which functions as an active site for the selective catalytic reduction (SCR) reaction. By precisely controlling the acid-base properties of the catalyst, the specificity for the desired products can be increased in complicated reaction systems, especially in hydrocarbon-SCR technology.

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.

Current State and Future Prospects of Environmentally Catalytic Zn‐NOx Batteries

AbstractZn‐based catalytic batteries, recognized as eco‐friendly alternatives, are attracting significant research interest for their applications in energy storage, conversion, pollutant degradation, and ammonia synthesis. This review compiles the latest developments in Zn‐nitrogen oxides (NOx) batteries, covering various types including Zn‐nitrate, Zn‐nitric oxide, and Zn‐nitrite batteries. This study explores the electrode reactions and structural evolutions of these batteries, emphasizing the different challenges posed by cathodic reactions. Advanced design strategies for cathode materials, such as inhibiting hydrogen production, utilizing tandem sites, and enhancing reactant enrichment, are presented and evaluated. These strategies have markedly improved NOx reduction performance and have driven significant progress in Zn‐NOx battery. The future directions for research are outlined, highlighting the need for more efficient NOx reduction catalysts, optimization of Zn anodes, development of alternative metal anodes, battery structure improvements, and exploration of charging reactions. Addressing these challenges is crucial for advancing the development of eco‐friendly and high‐energy‐density Zn‐NOx batteries.

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.

Revealing the boosting roles of sulfate groups on NOx selective catalytic reduction over V2O5/CeO2 catalyst

Ceria-supported vanadia (VOx/CeO2) catalyst has been extensively investigated in the field of NOx selective catalytic reduction with NH3 (NH3-SCR) but is limited in practical application owing to its poor sulfur resistance and high-temperature activity. Herein, a series of SO42− modified V2O5/CeO2 catalysts were fabricated to improve the overall catalytic performance. The 2 %SO42−-V2O5/CeO2 catalyst exhibited above 95 % NOx conversion, 90 % N2 selectivity in the temperature range of 225–400 °C and excellent SO2 + H2O tolerance, which was much better than V2O5/CeO2 catalyst. Characterization results showed that the introduction of SO42− over V2O5/CeO2 increased the Ce3+/(Ce3++ Ce4+) ratio to promote the formation of oxygen vacancies and promoted the generation of more conducive polymeric vanadyl species. The synergistic effect of the redox sites and acid centers provided moderate redox property and enhanced surface acidity, thus improving the adsorption of NH3 and inhibiting the over-oxidation of NH3 that occurs on V2O5/CeO2. In-situ DRIFTS indicated that the NOx reduction over V2O5/CeO2 and 2 %SO42−-V2O5/CeO2 followed the Langmuir-Hinshelwood and Eley-Rideal routes concurrently. This study can expound the reasonable design of vanadium-based catalysts for efficient NOx reduction in industry.

Sb-loaded MnOx/TiO2/CNTs catalysts for selective catalytic reduction of NOx: insight to the SO2 and H2O tolerance

PurposeThis paper aims to synthesize and test Mn-Sb/TiO2-CNT catalysts with different Sb/Mn molar ratios to be used in NH3-SCR reaction.Design/methodology/approachA series of Sb-loaded carbon nanotubes (CNTs)-based Mn/TiO2 catalysts were prepared by the incipient wetness co-impregnation method and tested for the selective catalytic reduction (SCR) of NOx with NH3.FindingsThe Sb-loaded Mn/TiO2-CNTs catalyst outperformed other catalysts and presented the highest activity in the temperature regime of 100–400°C. The catalyst loaded with Sb also showed good SO2 and H2O resistance and exhibited better thermal stability. A stepwise study of SO2 addition evidenced that Mn-Sb/TiO2-CNTs catalyst exhibited better SO2 resistance than the base catalyst Mn/TiO2, where the Sb doping greatly inhibited the sulphating of active phase of the catalyst.Originality/valueIn Sb-loaded catalysts, the formed SOx species fused with SbOx instead of MnOx. This favoured interaction of SO2 with SbOx successfully prevents the MnOx from being sulphated by SO2 which substantially improves the SO2 tolerance of Sb-loaded catalysts.

Insights into Na+/Ca2+ poisoning mechanisms of Zr-modified and unmodified La-Mn perovskite oxide catalysts for NH3-SCR reaction

The presence of alkali metal/alkaline earth metal in fuel gas is one of the important reasons for the deactivation of catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR). In this work, the Na+/Ca2+ poisoning mechanisms of Zr-modified and unmodified La-Mn perovskite oxides for NH3-SCR reaction were studied. It is found that the deposition of Na+ into LaMnO3 (LM) catalyst has a much larger effect than that of Ca2+, while such a result is opposite for the Na+/Ca2+-deposited La0.8Zr0.2MnO3 (ZLM) catalysts. The deposition of Na+/Ca2+ compounds can block the pores and affect the adsorption and activation of NH3 on the catalysts. After Zr modification, the catalyst with a higher Mn4+ content shows a better redox property and increased acid sites. These acidic sites can trap Na+, thereby delaying the catalyst poisoning and causing a better activity of ZLM catalyst than LM catalyst. The deposition of Ca2+ in the form of CaCO3 on the catalyst surface leads to a serious pore blockage, showing a poor resistance to Ca2+. After regeneration, the pores of the catalyst are dredged, and the active sites originally occupied by Na+/Ca2+ are released, resulting in an recovery of catalyst activity. The recovery of adsorbed oxygen and Mn4+ content is the key factor for returning the SCR activity.

Boosting Electrochemical Ammonia Synthesis via NOx Reduction over Sulfur‐Doped Copper Oxide Nanoneedle Arrays

AbstractThe electrochemical NOx reduction reactions, involving nitrate and nitrite reduction reactions (NO3−RR and NO2−RR), have emerged as promising approaches for both NO3− and NO2− removal, and ammonium (NH3) synthesis under ambient conditions. However, the incorporation and stabilization of sulfur dopants in the catalysts for efficient NOx reduction are rarely explored, leading to an unclear effect of sulfur on the NOx reduction mechanism. Herein, sulfur‐doped Cu2O (S‐Cu2O) nanoneedle arrays via in situ electrochemical treatment are synthesized. The S‐Cu2O catalyst possesses excellent durability and selectivity for NH3 over a wide range of potentials in NO3−RR, attaining a maximum NH3 Faradaic efficiency of 94% at −0.6 VRHE and a maximum NH3 yield as high as 1.06 mmol h−1 cm−2. In NO3−RR, the sulfur dopant can accelerate the step from NO2− to NH3, contributing superior performance in NO2−RR and assembled Zn−NO2− battery device. Density functional theory (DFT) calculations reveal that the presence of sulfur can enhance the initial step of *NO3 adsorption, lower the reaction barriers for the formation of *NHO intermediate, and activate the H2O dissociation process. The work sheds light on the role of sulfur in enhancing electrocatalytic performance and provides a unique perspective for understanding the NOx reduction mechanism.

Modulating the Activity and SO2 Resistance of α-Fe2O3 Catalysts for NH3-SCR of NOx via Crystal Facet Engineering.

The development of Fe-based catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) has garnered significant attention due to their exceptional SO2 resistance. However, the influence of different sulfur-containing species (e.g., ferric sulfates and ammonium sulfates) on the NH3-SCR activity of Fe-based catalysts as well as its dependence on exposed crystal facets of Fe2O3 has not been revealed. This work disclosed that nanorod-like α-Fe2O3 (Fe2O3-NR) predominantly exposing (110) facet performed better than nanosheet-like α-Fe2O3 (Fe2O3-NS) predominantly exposing (001) facet in NH3-SCR reaction, due to the advantages of Fe2O3-NR in redox properties and surface acidity. Furthermore, the results of the SO2/H2O resistance test at a critical temperature of 250 °C, catalytic performance evaluations on Fe2O3-NR and Fe2O3-NS sulfated by SO2 + O2 or deposited with NH4HSO4 (ABS), and systematic characterization revealed that the reactivity of ammonium sulfates on Fe2O3 catalysts to NO(+O2) contributed to their improved catalytic performance, while ferric sulfates showed enhancing and inhibiting effects on NH3-SCR activity on Fe2O3-NR and Fe2O3-NS, respectively; despite this, Fe2O3-NR showed higher affinity for SO2 + O2. This work set a milestone in understanding the NH3-SCR reaction on Fe2O3 catalysts in the presence of SO2 from the aspect of crystal facet engineering.

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

Effect of sulfation on the catalytic performance of Mn-Co composite oxide catalyst for the selective catalytic reduction of NOx by NH3

The effect of sulfation on the catalytic performance of Mn-Co composite oxide catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) has been investigated. Different concentrations of SO2 have been used to sulfate Mn-Co composite oxide catalyst. The catalyst sulfated with 30 ppm SO2 showed the widest activity temperature window, over which 80% NOx conversion was obtained in the temperature range of 80–350 °C. The sulfation leads to the appropriate redox property of Mn-Co composite oxide, thus improving the N2 selectivity. Meanwhile, the sulfation contributes to the generation of more Brønsted acid sites, which is crucial for the enhanced activity. In situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) demonstrated that the adsorbed NH3 species on both Lewis acid and Brønsted acid sites, monodentate nitrite, bidentate nitrate and cis-nitrite species are the reactive intermediates in the NH3-SCR reaction. This study provides an idea for the regulating the activity temperature window of NH3-SCR catalyst to adapt to its practical application.

Effect of samarium doping on the activity and sulfur resistance of Ce/MnFeOx catalyst for low-temperature selective catalytic reduction of NOx by ammonia

The self-made MnFeOx catalysts doped with cerium and samarium were prepared by impregnation method for low-temperature selective catalytic reduction by NH3. In this work, the surface properties of the series of MnFe-based catalysts were studied. The results indicate Sm-modified catalyst have superior low-temperature SCR activity; NOx conversion maintained at nearby to 100% at 90–240 °C. In addition, The N2 selectivity of Sm doping remains above 80% in the range of 60–150 °C. In SO2 poisoning test, the NOx conversion can be remained >90% after 10 h of reaction. The XPS, NH3-TPD and H2-TPR results show the catalyst with Sm doping enhances the acid sites and oxidation catalytic sites of mixed oxides serves for improving oxygen vacancies and transfer electrons. In situ DRIFTS results show that NOx is more easily adsorbed on the surface after Sm doping, which provided favorable conditions for the NH3-SCR reaction to proceed. The reaction at the catalyst surface will follow the L-H reaction mechanism by transient reaction test.

Analysis of the reaction mechanism of N/S co-doped carbon-based catalysts for low-temperature NH3-SCR reduction of NOx

In this paper a series of N/S co-doped carbon-based low-temperature denitrification catalysts were prepared using thiourea and coal-based activated carbon. Tests such as BET, NH3-TPD, XPS and DRIFTS were applied to characterize the performance of the prepared catalysts. It was found that the denitrification activity of NSAC-1.2 catalyst was 2.08 times higher than that of coal-based activated carbon. The denitrification activity of all N/S co-doped carbon-based catalysts was greater than 80 % in the temperature range of 150–260 °C. The presence of N and S functional groups creates more acidic sites and oxygen vacancies on the catalyst surface, which leads to the reduction of the reaction energy barrier, the promotion of the adsorption and activation of reactants and in turn the improvement of the denitrification activity of the catalyst. N/S co-doping for N-5 and N-6 groups as well as C-SO2-C species enhancement affect the catalyst denitrification activity improvement comprehensively. The reaction mechanism of NH3-SCR process on N/S co-doped carbon-based catalysts was investigated by in-situ DRIFTS.

Enhancement of low-temperature activity of γ-Fe2O3-modified V2O5-MoO3/TiO2 catalysts for selective catalytic reduction of NOx with NH3

In this study, a series of γ-Fe2O3-modified V2O5-MoO3/TiO2 (VMT) catalysts was prepared by wet impregnation method. The γ-Fe2O3-modified catalysts exhibited excellent catalytic performance, and 1.25 wt% γ-Fe2O3 content improved the deNOx efficiency at 150 °C from 54% (unmodified VMT catalyst) to 84% (modified catalyst). BET, XRD, XPS, NH3-TPD, H2-TPR, and in situ DRIFTs were employed to characterize the physicochemical properties and reaction mechanisms of the catalysts. The introduction of γ-Fe2O3 increased the V5+/V4+ ratio, surface chemisorbed oxygen content, redox ability, and a number of acid sites in the prepared catalysts. XPS confirmed the addition of γ-Fe2O3 introduced new charge transfer pathways (Fe3+/Fe2+ ↔ V5+/V4+). In situ DRIFTs suggested that the addition of γ-Fe2O3 to the catalyst significantly increased the adsorption capacity of catalyst for NH3 and NOx, while also exerting a strong activating effect on the adsorbed NH3. Furthermore, selective catalytic reduction over the VMT and 5PEG@1.25Fe-VMT catalysts followed the Eley-Rideal (E-R) mechanism. At temperatures above 200 °C, the 5PEG@1.25Fe-VMT catalyst also followed the Langmuir-Hinshelwood (L-H) mechanism.