Catalysts For Reduction Of NOx Research Articles

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.

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.

Exploring the role of Cu/Mn-BTC catalyst in the selective catalytic reduction of NOx by C3H6: Synthesis to in-situ DRIFTS study

The choice of reducing agents and their impact on catalytic performance are important aspects of selective catalytic reduction (SCR) technology. Marking the inaugural use of C3H6 as a reducing agent with the BTC catalyst, a bimetallic organic framework (xCu-Mn-BTC) was prepared and tested as a catalyst for NOx reduction under oxygen-enriched conditions. The results showed that NO conversion was influenced by the ratio of Cu and Mn metals, among which 3.2Cu-Mn-BTC exhibited the highest denitrification performance (80 % NOx conversion and 94 % N2 selectivity) at 300 °C. From the characterization results, Cu was successfully incorporated into the BTC framework and 3.2Cu-Mn-BTC became spherical grains with smaller particle sizes. The high catalytic activity of 3.2Cu-Mn-BTC was due to the presence of more adsorbed oxygen Oα (89.7 %) and Cu2+ (63 %) species than in the other catalysts. The TPR peak of xCu-Mn-BTC shifted to a lower temperature and showed better reducibility. The Py-FTIR study shows that xCu-Mn-BTC contained more Lewis acid but no Brønsted acid, because Cu increased the concentration of Lewis acid on the catalytic surface (42 to 46 mmol/g), and Lewis acid plays an important role in the C3H6-SCR of NOx.

Low-cost Co–Al spinel and M–Co–Al (M = Fe and Ce) catalysts for selective catalytic reduction of NOx with NH3

Low-cost Co–Al spinel and M–Co–Al (M = Fe and Ce) catalysts for selective catalytic reduction of NOx with NH3

Overview of mechanisms of Fe-based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature.

With the increasingly stringent control of NOx emissions, NH3-SCR, one of the most effective de-NOx technologies for removing NOx, has been widely employed to eliminate NOx from automobile exhaust and industrial production. Researchers have favored iron-based catalysts for their low cost, high activity, and excellent de-NOx performance. This paper takes a new perspective to review the research progress of iron-based catalysts. The influence of the chemical form of single iron-based catalysts on their performance was investigated. In the section on composite iron-based catalysts, detailed reviews were conducted on the effects of synergistic interactions between iron and other elements on catalytic performance. Regarding loaded iron-based catalysts, the catalytic performance of iron-based catalysts on different carriers was systematically examined. In the section on iron-based catalysts with novel structures, the effects of the morphology and crystallinity of nanomaterials on catalytic performance were analyzed. Additionally, the reaction mechanism and poisoning mechanism of iron-based catalysts were elucidated. In conclusion, the paper delved into the prospects and future directions of iron-based catalysts, aiming to provide ideas for the development of iron-based catalysts with better application prospects. The comprehensive review underscores the significance of iron-based catalysts in the realm of de-NOx technologies, shedding light on their diverse forms and applications. The hope is that this paper will serve as a valuable resource, guiding future endeavors in the development of advanced iron-based catalysts.

The effect of hydrothermal aging on Mn2O3/Cu-SSZ-39 composite catalysts for selective catalytic reduction of NOx by NH3

The latest generation of fuel-efficient engines produces exhaust at cooler temperatures, highlighting a need for enhancement in the low-temperature effectiveness of Cu-zeolite catalysts. The metal oxide/Cu-zeolite composite catalysts have demonstrated superior deNOx performance at lower temperatures compared to Cu-zeolite alone. However, the hydrothermal stability of these composite catalysts remains unexplored. In this study, we focus on the Mn2O3/Cu-SSZ-39 catalyst, chosen for its exceptional activity at low temperatures. The synergy between the nitrate groups on Mn2O3 and NH4+ ions on the Brønsted acid sites of Cu-SSZ-39 enhances NOx conversion efficiency. Nonetheless, the efficiency of Mn2O3/Cu-SSZ-39 diminishes following exposure to hydrothermal conditions at various temperatures (700 °C, 750 °C, and 800 °C), with the degradation degree intensifying at higher temperatures. Characterization studies revealed that hydrothermal treatment leads to the recrystallization of Mn2O3 particles, which reduces the content of active adsorbed oxygen. This reduction impedes the formation of active bridged nitrates in the aged catalysts and disrupts the SCR pathway through the interaction between bridged nitrates and NH4+ on the composite catalysts. Despite these challenges, some adsorbed oxygen remains on the Mn2O3/Cu-SSZ-39 after aging, ensuring that the aged composite catalyst still exhibits higher activity than the fresh Cu-SSZ-39 catalyst. This observation suggests that there is potential for the application of Mn2O3/Cu-SSZ-39 in practical scenarios.

Phosphorus poisoning and regeneration of the Cu-LTA catalyst for the selective catalytic reduction of NOx with NH3

In this study, the phosphorus poisoning and regeneration of the Cu-LTA catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated.

Theoretical insight into H2O impact on V2O5/TiO2 catalysts for selective catalytic reduction of NOx.

H2O in flue gas causes the deactivation of V2O5/TiO2 catalysts for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Developing water resistance requires understanding the theoretical mechanism of H2O impact on the catalysts. The aim of this work was to clarify the adsorption process of H2O and the deactivation mechanism induced by H2O through density functional theory (DFT). The process of H2O adsorption was studied based on a modeled V2O5/TiO2 catalyst surface. It was found that H2O had a strong interaction with exposed titanium atoms. Water adsorption on the catalyst surface significantly alters the electronic structure of VOx sites, transforming Lewis acid sites into Brønsted acid sites. Exposed titanium sites contribute to the decrease of Lewis acidity via adsorbed water. Ab initio thermodynamic calculations show that H2O adsorption on V2O5/TiO2 is stable at low coverage but less favorable at high coverage. Adsorption of NH3 is the most critical step for the SCR of NOx, and the adsorption of H2O can hinder this process. The H2O coverage below 15% of adsorption sites could enhance the NH3 adsorption rate and have a limited effect on the acidity, while higher coverage impeded the adsorption ability of VOx sites. This work provided electron-scale insight into the adsorption impact of H2O on the surface of V2O5/TiO2 catalysts, presented thermodynamic analysis of the adsorption of H2O and NH3, paving the way for the exploration of V2O5/TiO2 catalysts with improved water resistance.

Heteropoly acid-grafted iron oxide catalysts for efficient selective catalytic reduction of NOx with NH3

A series of heteropoly acid-grafted Fe2O3 catalysts (i.e., HPA/Fe2O3) for selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated.

Cu-SAPO-56, a potential catalyst for the selective catalytic reduction of NOx by NH3

A novel Cu-exchanged catalyst, Cu-SAPO-56 prepared by direct ion exchange method, was studied for NH3-SCR reaction. Cu-SAPO-56 exhibits excellent NH3-SCR activity and hydrothermal stability. The NOx conversion of Cu-SAPO-56 maintains more than 90 % at a wide temperature range between 250 °C and 400 °C even after a rigorous steam treatment (800 °C for 16 h or 50 °C for 24 h). The isolated Cu2+ ions located in double 6-ring of AFX structure are determined as the catalytic active sites. Both of Cu content and Si content in Cu-SAPO-56 play critical roles in NH3-SCR activity and hydrothermal stability. The excellent catalytic performance and strong hydrothermal stability exhibits the huge application potential of Cu-SAPO-56 for NH3-SCR reaction.

Oxalic acid-mediated regeneration strategy and mechanism of phosphorus poisoned CeO2/TiO2 catalysts for selective catalytic reduction of NOx with NH3

The development of the regeneration strategies can extend the service life of deactivated catalysts and improve resource efficiency. In this work, an acid treatment method was proposed to regenerate the P-poisoned CeO2/TiO2 (PCeTi) catalyst for ammonia selective catalytic reduction (NH3-SCR) of NOx. Catalytic activity test results showed that acid treatment could achieve the efficient regeneration of PCeTi catalyst, the regenerated PCeTi mediated with oxalic acid (PCeTi-OA) catalyst showed a 90% mineralization temperature (T90) of 236 °C (T90 for fresh CeTi was 232 °C) and retained the promotional effect of P species at the high temperature (>350 °C). The characterization results showed that most of the P species on the PCeTi catalyst were removed after regeneration, which restored the majority of the original Ce-O active structure, thereby promoting the Ce4+/Ce3+ redox cycling and inducing the formation of new oxygen vacancies. At the same time, the residual P-O structures and new oxygen vacancies could provide a large amount of acid sites, which was favorable for the adsorption and activation of NH3. The recovered redox properties and the increased surface acidity of the PCeTi-OA catalyst were responsible for its recovered low temperature activity and superior high temperature activity, respectively.

Generating silicon-rich AFX zeolite using rigid diquaternary alkylammonium structure directing agent towards stable selective catalytic reduction of NOx by NH3

New synthetic protocols to manipulate Si/Al ratios of small pore zeolites hold promise to find better selective NOx reduction catalyst using NH3 as reducing agent (NH3-SCR). AFX zeolite as NH3-SCR catalyst is currently impeded by the insufficient hydrothermal stability owing to the low Si/Al ratio. In this contribution, we report the synthesis of AFX zeolite using N,N-(1,4-phenylenebis(methylene))bis(N,N,N-triethylammonium) as organic structure directing agent (OSDA) to increase the framework Si/Al ratio to 7.8, giving rise to a marked enhancement of hydrothermal stability to withstand hydrothermal aging at 850 °C for up to 10 h. The increase in Si/Al ratio for AFX zeolite minimizes catalytic cycle driven by Brønsted acid site and broadens high temperature operation window (150–515 °C) for Cu-exchanged catalyst. Moreover, the hydrothermally aged Cu-AFX catalyst exhibits comparable catalytic activity to that of the fresh catalyst owing to Cu predominant distribution close to the 6-MR.

Enhancement of Fe-Nb interaction by acid modification over SO42--FeOx-Nb2O5 catalyst for NH3 selective catalytic reduction of NOx

A series of iron-based composite oxide catalysts, modified with acid and niobium, were produced for the purpose of selective catalytic reduction of NOx with NH3 (NH3-SCR) via co-precipitation. The obtained SO42--FeOx-Nb2O5 (S-FeNbOx) catalyst exhibited significantly better SCR performance than the unmodified FeOx catalyst, with a NOx conversion above 90% and N2 selectivity higher than 95% in the temperature range of 275–450 °C. The characterization results suggest that the addition of Nb not only modifies the electronic structure and alleviates the reducibility but also enhances the surface acidity of the catalysts. The sulfuric acid modification of FeOx-Nb2O5 catalyst promotes the formation of redox-acid sites. The in situ Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFTs) results revealed that the combining sulfuric acid and niobium modifications resulted in a significant increase in the amount of Brønsted acid sites, and also led to a blue shift of the band related to the Lewis acid sites, implying the existence of a synergistic effect of the two modification methods. The creation of redox-acid sites significantly enhances the adsorption and activation capacity of NH3 on the catalyst surface. These results suggest that the NH3-SCR reaction to primarily follow the Eley-Rideal (E-R) mechanism on the catalyst.

Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review

Low-temperature NH3-SCR is an efficient technology for NOx removal from flue gas. The carbon-based catalyst designed by using porous carbon material with great specific surface area and interconnected pores as the support to load the active components shows excellent NH3-SCR performance and has a broad application prospect. However, overcoming the poor resistance of H2O and SO2 poisoning for carbon-based catalysts remains a great challenge. Notably, reviews on the sulfur and water resistance of carbon-based low-temperature NH3-SCR catalysts have not been previously reported to the best of our knowledge. This review introduces the reaction mechanism of the NH3-SCR process and the poisoning mechanism of SO2 and H2O to carbon-based catalysts. Strategies to improve the SO2 and H2O resistance of carbon-based catalysts in recent years are summarized through the effect of support, modification, structure control, preparation methods and reaction conditions. Perspective for the further development of carbon-based catalysts in NOx low-temperature SCR is proposed. This study provides a new insight and guidance into the design of low-temperature SCR catalysts resistant to SO2 and H2O in the future.

Promotional effects of phosphotungstic acid on CePO4 catalyst for the selective catalytic reduction of NOx with NH3

CePO4 was prepared using the co-precipitation method. Recognizing the bifunctional nature of hetero-polyacid (salt), with both acidic and redox properties, it was modified by using phosphotungstic acid (HPW), and the HPW/CePO4 catalysts were synthesized using the impregnation technique. Various advanced techniques were employed to investigate aspects such as catalytic activity, microstructure, surface properties, and reaction mechanisms. These methods included NH3-SCR, XRD, XPS, SEM, BET, NH3-TPD, H2-TPR, and In-situ DRIFTs. Experimental findings indicated that the HPW-modified CePO4 catalysts exhibited enhanced SCR activity. Among them, the reaction activity window for NOx conversion of the W: Ce 0.05 catalyst was found to be 300∼400 °C, which represented the best activity temperature window. Following the incorporation of HPW, the HPW/CePO4 catalysts demonstrated an increased presence of Brønsted acidic sites, which concurrently led to a significant reduction in SO2 adsorption. The W: Ce 0.05 catalyst possessed the highest ratios of Oα/(Oα+Oβ+Oγ) and Ce4+/(Ce3++Ce4+). These ratios effectively regulated the sequestration and release of reactive oxygen species, thereby enhancing the activation of NH3. Consequently, this promoted the catalyst's low-temperature NH3-SCR reactivity. Lastly, the reaction process was examined using in-situ DRIFTs. The study revealed the generation of -NH2 intermediates at the Lewis acid sites of the W: Ce 0.05 catalyst. These intermediates subsequently underwent reactions with gaseous NO via the Eley-Rideal mechanism. This approach not only amplified the low-temperature catalytic efficacy of the catalysts but also bolstered their resilience to SO2, thereby fostering the long-term stability of the NH3-SCR reaction.

Shielding ceria based catalysts from SO2 poisoning in NH3-SCR reaction: Modification effect of acid metal oxides

Modifying CeO2 with acid sites is a common strategy for designing efficient catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx). However, the effect of acid metal oxide modification on the SO2 resistance of CeO2 has not been well revealed. In this work, it was found that the SO2 resistance of CeO2 modified with several acid metal oxides (i.e., MoO3, WO3 and Nb2O5), followed an order of Mo/CeO2 ≥ W/CeO2 > Nb/CeO2. Further systematic characterizations revealed that Mo-OH and W-OH on CeO2 as Brønsted acid sites could better inhibit the SO2 adsorption and the sulfation of active sites than highly dispersed NbOx mainly as Lewis acid sites because NH4+ coordinated to Brønsted acid sites could help trap SO2 to form ammonium sulfates rather than metal sulfates. The findings in this work provided important guidance for the design of efficient catalysts with superior SO2 resistance performance.