Low-temperature Selective Catalytic Reduction Research Articles

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.

New Insights into the Effect of Ce Doping on the Catalytic Performance and Hydrothermal Stability of Cu-USY Zeolite Catalysts for the Selective Catalytic Reduction of NO with NH3

5Cu-USY and Ce-doped 5Cu8Ce-USY zeolite catalysts were prepared by the conventional impregnation method. The obtained catalysts were subjected to the hydrothermal ageing process. The catalytic performance of the selective catalytic reduction of NOx with NH3 (NH3-SCR) was evaluated on both fresh and aged catalysts. Physical/chemical characterizations such as X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) were performed, along with detailed in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) experiments including CO adsorption, NH3 adsorption, and NO + O2 reactions. Results showed that, for the 5Cu-USY catalyst, hydrothermal ageing treatment could somehow improve the low-temperature SCR activity, but it also led to significant formation of unfavorable byproducts NO2 and N2O. Such an activity change can be attributed to hydrothermal ageing inducing the migration of isolated Cu+ species in the sodalite cavities towards the super cages of the USY zeolites. The increased content of Cu+ species in the super cages was beneficial for the low-temperature activity improvement, but, at the same time, it also facilitated ammonia oxidation at high temperatures. Ce doping after hydrothermal ageing has a “double-edged sword” effect on the catalytic performance. First of all, Ce doping can inhibit Cu species migration by self-occupying the internal cage sites; thus, the catalytic performance of 5Cu8Ce-USY-700H remains stable after ageing. Secondly, Ce doping introduces a CuOx–CeO2 strong interaction, which facilitates lattice oxygen mobility by forming more oxygen vacancies so as to increase the concentration of surface active oxygen. These changes, on the one hand, could help to promote further oxidative decomposition of nitrate/nitrite intermediates and improve the catalytic performance. But, on the other hand, it also causes the byproduct generation to become more severe.

Mechanistic Insight into the Promotion of the Low-Temperature NH3-SCR Activity over NiMnFeOx LDO Catalysts: A Combined Experimental and DFT Study.

Mn-based catalysts have attracted much attention in the field of the low-temperature NH3 selective catalytic reduction (NH3-SCR) of NO. However, their poor SO2 resistance, low N2 selectivity, and narrow operation window limit the industrial application of Mn-based oxide catalysts. In this work, NiMnFeOx catalysts were prepared by the layered double hydroxide (LDH)-derived oxide method, and the optimized Ni0.5Mn0.5Fe0.5Ox catalyst had the best denitration activity, excellent N2 selectivity, a wider active temperature range (100-250 °C), higher thermal stability, and better H2O and/or SO2 resistance. A transient reaction revealed that Ni0.5Mn0.5Fe0.5Ox inhibited the NH3 + O2 + NOx pathway to generate N2O, which may be the main reason for its improved N2 selectivity. Combining experimental measurements and density functional theory (DFT) calculations, we elucidated at the atomic level that sulfated NiMnFeOx (111) induces the adjustment of the acidity/basicity of up and down spins and the ligand field reconfiguration of the Mn sites, which improves the overall reactivity of NiMnFeOx catalysts. This work provides atomic-level insights into the promotion of NH3-SCR activity by NiMnFeOx composite oxides, which are important for the practical design of future low-temperature SCR technologies.

Low-temperature selective catalytic reduction of NOx with NH3: Exploring the mechanism of enhancing H2O tolerance through methylation functionalization and structural regulation in IPAx-Mn-BTC

In the selective catalytic reduction (SCR) of NOx with NH3, it is crucial to design low-temperature de-nitrification (de-NOx) catalysts with high H2O tolerance. Here, we developed a type of low-temperature de-NOx catalyst (IPAx-Mn-BTC) with strong H2O tolerance through methyl functionalization. In the presence of the optimal catalyst (IPA50-Mn-BTC), the NOx conversion only decreased from 97% to 90% after introducing 6% H2O for 6 h at low temperature of 150 °C. The results of in-situ DRIFTS and density functional theory calculations confirm that the de-NOx reaction process and the generation of intermediate states that occur on the surface of the IPA50-Mn-BTC are not affected by H2O. The elimination of the small pore size (< 4.3 nm) and the diminished H2O adsorption energy resulted from methyl functionalization are the key factors behind the improved H2O tolerance. This work provides a new approach for designing low-temperature de-NOx catalysts with high H2O tolerance.

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.

Synthesis of MnO(OH) nanorods via hydrothermal method as efficient low-temperature SCR catalysts

We conducted a hydrothermal reaction using lactic acid as a reducing agent and potassium permanganate. This reaction successfully produced MnO(OH) with a one-dimensional nanorod structure, and its XRD spectrum perfectly matched the standard reference. Despite the excellent low-temperature ammonia selective catalytic reduction (NH3-SCR) performance observed in manganese-based catalysts, there was no reported performance data for any type of MnOOH in NH3-SCR prior to our research. Our test results now reveal that MnO(OH) nanorods, synthesized using this method, consistently maintain an impressive 98.8 % NO conversion rate even at the low temperature of 80 °C. This finding is of significant importance for the advancement of low-temperature NH3-SCR catalysts.

Boosting the low-temperature NH3-SCR performance via metals co-doping with inequality Mn/Ce ratios in the carbon-based catalyst prepared by Cr-containing leather waste

The resource utilization of Cr-containing leather waste is still a great challenge. In this work, waste from leather production as raw material was attempted to be used to synthesize activated carbon, a series of MnxCe1-xZr0.5/AC composite catalysts were prepared via loading three metal elements onto activated carbon by impregnation method, the NH3-SCR performance was measured in a low temperature range of 50 ∼ 200 °C. Among them, the catalyst Mn0.7Ce0.3Zr0.5/AC displays the optimal low-temperature SCR activity, excellent resistance to GHSV, H2O and SO2. The characterization results indicated that increasing the Mn/Ce inequality ratios within a certain range would promote the electron transfer between the active metal components, thereby enhancing the synergistic effect and the redox capacity of the catalysts, and it also could significantly improve the concentration of Lewis acid sites to accelerate the adsorption and activation of NH3, resulting in different catalytic activities. The in situ DRIFTS results prove that the Lewis acid site plays a vital role in the “fast SCR” reaction, and the whole reaction process conforms to the Langmuir-Hinshelwood (L-H) mechanism.

Hierarchical structured Ti-doped CeO2 stabilized CoMn2O4 for enhancing the low-temperature NH3-SCR performance within highly H2O and SO2 resistance

Developing effective and stable catalysts for low-temperature selective catalytic reduction (SCR) of NOx remains challenging. Herein, we constructed a hierarchical structure by loading CoMn2O4 onto Ti-doped CeO2, that CoMn2O4/CeTiOx catalyst has shown superior deNOx activity (>95% at 100–225 °C), prominent reaction activation energy (28.8 ± 0.9 kJ mol−1) and outstanding stability (>75% at 100–200 °C within H2O and SO2). The “low-temperature active sites” and “dual anti-poisoning sites” contribute to excellent activity and stability. Firstly, the hierarchical structure boosts generation of active metal-support interface, which is conducive to oxygen migration (including adsorbed oxygen (Oads), lattice oxygen (Olat) and oxygen vacancy (Ov)) and metal charge transfer (Mn2+/3++Ce4+↔Mn3+/4++Ce3+, Ti4++Ce3+↔Ce4++Ti3+). This is the key to breaking through the limits of catalytic activity stability. Secondly, enhanced surface acidity favors NH3 adsorption and activation, which accelerates -NH2/-NH concatenate with NOx through Eley-Rideal mechanism to generate N2 and H2O. Thirdly, the dual strong SO2 affinity sites by Ti-induced CeO2 crystal reconstruction retard the active center affected by the sulfate species, which contributes to striking stability. This work highlights the importance of design of isolated active sites to improve SO2 and H2O endurance.

The effect of dopants (Fe, Al) on the low-temperature activity and SO2 tolerance in solvothermally synthesized MnOx NH3-SCR catalysts

Fe-doped MnOx catalyst prepared by a preferred solvothermal method displayed noticeably better low-temperature (LT) NH3-SCR performance and water stability than an analogously prepared Al-doped MnOx catalyst. The SCR activity of both catalysts decreased markedly when exposed to SO2, but the resultant MnFeOx-S catalyst retained higher LT activity than MnAlOx-S and recovered significantly more of its original activity after thermal regeneration (400 °C). Comprehensive characterization confirmed that the deactivation of the catalysts was governed by formation of stable metal sulfates, which only decomposed to a minor extent upon thermal treatment although Al doping lowered the thermal stability of the adsorbed sulfur species. Additionally, Fe doping was found to facilitate electron transfer between Mn and Fe ions and weaken the interaction between active sites and deposited sulfates during the heating procedure, which promoted re-oxidation of Mn2+ to catalytically active Mn3+/Mn4+. Altogether, the altered redox properties resulted in improved LT SCR performance, enhanced water stability, higher SO2 tolerance and superior regeneration of the MnFeOx catalyst.

Engineering CeZrOx-Cu/SSZ-13 coupled catalysts to synergistically enhance the low-temperature NH3-SCR activity

Improving the low-temperature (<200 °C) NOx conversion of Cu/SSZ-13 NH3-SCR catalysts is crucial for achieving zero NOx emissions from diesel engines. This is primarily due to the limited oxidation capacity of active Cu species limits NOx conversion at low temperatures, particularly following the hydrothermal treatment at high temperatures (>650 °C) and high water vapor (10 vol.%). In this study, CeZrOx-Cu/SSZ-13 (CZ/CSZ) coupled catalysts were synthesized to construct additional active sites for NO oxidation on the CSZ surface, thus improving the low-temperature NOx conversion through synergistic interaction. The results reveal that when the CSZ and CZ mass ratio is 1:3, the NOx conversion at 150 °C is improved by 11% after hydrothermal treatment at 800 °C for 16 h. The coupled CZ oxidizes NO at low temperatures and generates abundant HONO species on CZ/3CSZ. These HONO species formed on CZ transfer to CSZ and rapidly react with BAS-NH3 to produce easily decomposable NH4NO2. This efficient synergistic mechanism circumvented the rate-limitation caused by Cu species, thus enhancing the low-temperature activity. Additionally, the coupled CZ obviously reduces the amount of H2O adsorption on CZ/3CSZ and effectively mitigates the damage of Si-OH-Al sites by H2O molecules during the hydrothermal treatment. The improved structural stability enables the maintenance of isolated Cu2+ and stable acid sites, ensuring the rapid response of synergistic reactions, ultimately resulting in a significantly enhanced in the low-temperature SCR activity even after the hydrothermal treatment. This work would provide a novel strategy for designing the next generation of commercial NH3-SCR catalysts.

Revealing different lead species of PbCl2, Pb(NO3)2, PbSO4 and PbCO3 poisoning effects on Mn-Ce/CuX catalyst for low-temperature NH3-SCR of NO

For the sintering flue gas in steel industry, selective catalytic reduction (SCR) of NOx using NH3 at low temperature in the presence of heavy metals is still a challenge. In this study, the toxicity effects of lead salt species (PbCl2, Pb(NO3)2, PbSO4, PbCO3) on Cu-exchanged zeolite X doped with Mn-Ce catalysts (MCCX) were investigated. All catalysts exhibited poor low-temperature SCR performance within 75–225 °C after Pb poisoning, and the poisoning effects of PbCl2, Pb(NO3)2, PbSO4 and PbCO3 on MCCX catalyst varied. The PbSO4 had the most serious poisoning on MCCX catalyst, while PbCO3 had poisoning effect on catalysts even at 300 °C. Lead species poisoning had bad impact on the structure of zeolite X on MCCX catalyst, and caused some agglutination of Cu species. Likewise, the Pb species might primarily occupy the active site of Mn and Cu species and influence the redox property of catalysts, thereby obstructing the redox and oxidation cycles on catalysts surface. In addition, the PbCl2 and PbNO3 species predominantly affected the Brønsted acid sites and occupied the Cu or Mn active sites, inhabiting the absorption and conversion of nitrites and nitrates on the catalyst surface. Moreover, PbSO4 and PbCO3 species mainly impacted the adsorption of NO + O2 and the conversion of nitrites and nitrates on the catalysts.

Insights into the enhanced activity and SO2 resistance of air oxidation treated Mn-Fe doped biochar catalyst in the low-temperature catalytic reduction of NOx with NH3

In this study, a sustainable two-stage air oxidation (TAO) was employed to prepare Fe modified MnOx-biochar catalyst (C-O2-MF-O2) instead of high temperature treatment and/or chemical activation for selective catalytic reduction of NO with NH3 (NH3-SCR). The collaborative effect of Fe modification and TAO on the physicochemical properties and catalytic performance of catalysts was systematically studied. Compared with counterpart catalysts treated with two-stage N2 (C-N2-MF-N2) or one-stage O2 (C-O2-MF-N2, C-N2-MF-O2), C-O2-MF-O2 showed obvious enhanced physicochemical properties (SBET, pore properties, high-valance metal ratio, chemisorption oxygen (Oα) content), superior catalytic performance, achieving 91.3% NO conversion with nearly 100% N2 selectivity at 150 °C, and much better H2O and SO2 resistance. It is worth noting that TAO facilitated the redox performance of MnOx after Fe modification to generate more dispersed high-valance metal (Mn4+ and Fe3+), thereby favoring the low-temperature SCR activity of the catalyst. Further, the in situ DRIFTS studies revealed TAO produced remarkable effects on the adsorption behaviors of NH3, NO and SO2, and the corresponding mechanisms of catalyst activity enhancement and SO2 resistance were proposed. Generally, TAO could be a favorable avenue to prepare catalysts with binary active metals for improving performance of NH3-SCR.

Enhanced low-temperature SCR performance of metal Oxide/Cu-SSZ-13 composite catalysts: The role of oxygen species in metal oxide

A composite catalyst composed of metal oxide and Cu-SSZ-13 components (OXZEO) presents a promising approach to solve the current challenge of improving the low-temperature efficiency of SCR catalysts. The diverse forms of oxygen present within the metal oxide components impact the formation of nitrate, which plays a critical role in the deNOx pathway, thereby influencing the overall SCR reaction in OXZEO. Despite this, the relationship between the various types of oxygen in metal oxide and their catalytic role in OXZEO catalysts remain not fully understood. To clarify this relationship, the activities of Cu-SSZ-13 combined with MnOx, CeOx, and ZrOx, respectively, were compared and analyzed. The low-temperature pathway (below 200 °C) for the three catalysts is primarily ascribed to the interplay between oxide-derived nitrate precursors and NH3 bound to Brønsted acids within the zeolite. However, MnOx/Cu-SSZ-13 demonstrated a stronger synergistic effect, achieving around 50 % higher NOx conversion than Cu-SSZ-13 alone at 200 °C, which was superior to the other two composite catalysts. Characterizations reveal that MnOx with surface oxygen vacancies can supply a significant quantity of chemisorbed oxygen, thus producing more reactive bridged nitrates and bolstering the reaction between nitrate and NH4+ in MnOx/Cu-SSZ-13. On the contrary, for CeOx/Cu-SSZ-13 and ZrOx/Cu-SSZ-13 catalysts, the lattice oxygen of the metal oxide encourages the formation of less active bidentate and monodentate nitrates, resulting in inferior low-temperature SCR performance.

Improving low-temperature NH3-SCR denitration activity and resistance to H2O and SO2 over Nb-modified NbaSn0.3CeOx catalysts by enhancing acidity to compensate for its weak oxidation ability

In this paper, a series of Nb-modified NbaSn0.3CeOx (a = 0.2, 0.4, 0.5) catalysts for denitrification were prepared using a facile co-precipitation method. In order to explore the various individual properties of the samples, the catalysts were characterized by XRD, Raman, N2 adsorption and desorption, TEM, NH3-TPD, H2-TPR, XPS and in-situ DRIFTS tests to understand the influences of the Nb introduction on the NH3-SCR reaction. Among these catalysts, the Nb0.4Sn0.3CeOx catalyst exhibit excellent NH3-SCR performance in the temperature range of 155–450 °C with more than 90 % NO conversion, 100 % N2 selectivity, and possess good resistance to H2O and SO2. Compared with the Sn0.3CeOx catalyst, we believe that the improvement of the low-temperature NH3-SCR performance of the Nb0.4Sn0.3CeOx catalysts is mainly due to the change in the amount of acid, special for the increase in B acid content, which compensates for the effect of the weaken oxidizing ability on the low-temperature activity. Beyond that, the good H2O and SO2 resistance at low temperature after the introduction of Nb into Nb0.4Sn0.3CeOx catalysts was analyzed by using in-situ DRIFTS characterization in detail, and the results indicate that the incorporation of Nb can inhibit the oxidation of SO2 to form sulfate on the catalysts, and prevent the competing adsorption of H2O and NOx, but the adsorption of NH3 is not affected. This work proves that the acidity regulation can improve low-temperature SCR activity by the synergistic effect between oxidation capacity and acidity.

Hydrothermally stable metal oxide-zeolite composite catalysts for low-temperature NOx reduction with improved N2 selectivity

Development of hydrothermally stable, low-temperature catalysts for controlling nitrogen oxides emissions from mobile sources remains an urgent challenge. We have prepared a metal oxide-zeolite composite catalyst by depositing Mn active species on a mixture support of CeO2/Al2O3 and ZSM-5. This composite catalyst is hydrothermally stable and shows improved low-temperature SCR activity and significantly reduced N2O formation than the corresponding metal oxide catalyst. Comparing with a Cu-CHA catalyst, the composite catalyst has a faster response to NH3 injection and less NH3 slip. Our characterization results reveal that such an oxide-zeolite composite catalyst contains more acidic sites and Mn3+ species as a result of oxide-zeolite interaction, and this interaction leads to the generation of more NH4+ species bound to the Brønsted acid sites and more reactive NOx species absorbed on the Mn sites. Herein, we report our mechanistic understanding of the oxide-zeolite composite catalyst and its molecular pathway for improving the low-temperature activity and N2 selectivity for NH3-SCR reaction. Practically, this work may provide an alternative methodology for low-temperature NOx control from diesel vehicles.

Cobalt-modified limonite catalyst for low temperature selective catalytic reduction of NOx in ore sintering flue gas by NH3

Natural limonite possesses characteristics of rich reserves and low sintering efficiency. In this study, the Co/Limonite composites were prepared through a one-step impregnation method by using limonite as a precursor and cobalt oxide as a modifier. The influence of cobalt doping amount and calcination temperature on the denitrification efficiency over the composite were investigated. The results showed that the Co/Limonite composite was the optimum over Co/Fe mass ratio of 0.06 and the calcination temperature of 350 °C, it exhibited an excellent low-temperature catalytic activity, stability and good resistance to H2O(g) and SO2. When NO initial concentration was 500 ppm, gas hourly space velocity (GHSV) was 36,000 h−1 and reaction temperature was 100 ℃, the denitrification efficiency reached 100%. Adding of 10 vol% H2O(g) or 50 ppm SO2, respectively, the denitrification efficiency was about 90% at reaction temperature was 150 ℃, while it could approach to 80% with H2O(g) and SO2 were added simultaneously. The micromorphology and physicochemical properties of the catalysts were analyzed by systematic characterization techniques. The Limonite could obtain more surface acid sites and larger adsorption capacity by cobalt modification. More Lewis acids and weak Brønsted acids could greatly contribute to improve the low temperatures denitrification catalytic activity.

Facile H2O-Contributed O2 Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2 Poisoning.

Mn-based catalysts preferred in low-temperature selective catalytic reduction (SCR) are susceptible to SO2 poisoning. The stubborn sulfates make insufficient O2 activation and result in deficient reactive oxygen species (ROS) for activating reaction molecules. H2O has long been regarded as an accomplice to SO2, hastening catalyst deactivation. However, such a negative impression of the SCR reaction was reversed by our recent research. Here, we reported a H2O contribution over Mn-based SCR catalysts to counteract SO2 poisoning through accessible O2 activation, in which O2 was synergistically activated with H2O to generate ROS for less deactivation and more expected regeneration. The resulting ROS benefited from the energetically favorable route supported by water-induced Ea reduction and was actively involved in the NH3 activation and NO oxidation process. Besides, ROS maintained high stability over the SO2 + H2O-deactivated γ-MnO2 catalyst throughout the mild thermal treatment, achieving complete regeneration of its own NO disposal ability. This strategy was proven to be universally applicable to other Mn-based catalysts.