Catalysts For Low-temperature NH3-SCR Research Articles

Synthesis of low-temperature NH3-SCR catalysts for MnOx with high SO2 resistance using redox-precipitation method with mixed manganese sources

It remains a big challenge to enhance the SO2 resistance of catalyst in low-temperature NH3-SCR reduction. In this work, a series of MnOx-a%Mn (a = 0, 1, 3, 5, 10) catalysts were prepared by introducing manganese acetylacetonate during the reaction between lactic acid and KMnO4, based on the molar ratio of manganese acetylacetonate to KMnO4. Notably, MnOx-5 %Mn exhibited more than 95 % NO conversion and 100 % N2 selectivity at 80–240 °C. Moreover, the catalyst demonstrated excellent sulfur resistance by sustaining over 90 % NO conversion at 140 °C at the presence of 100 ppm SO2 for more than 6 h. Furthermore, XPS characterization indicated that adding manganese acetylacetonate enhanced electron transfer efficiency and improved the reduction capacity of the catalyst due to the interconversion between Mn3+ and Mn4+, which improved electron transfer efficiency. The enhanced reduction capacity of the catalyst promoted the formation of oxygen vacancies and surface-adsorbed oxygen species (Oα), avoiding over-oxidation of NH3. The results elucidated that manganese acetylacetonate could optimize the overall performance by modulating the synergistic effect between the oxidizing ability and surface acidity of the catalyst. Therefore, the MnOx-5 %Mn catalyst possessed a broad operational temperature window (40–240 °C) and SO2 resistance in the NH3-SCR reaction, indicating its potential for practical application.

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia.

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

Synergistic effect of Cerium and Niobium enhances the low-temperature NH3-SCR activity of Fe-containing solid waste

To develop environment-friendly low temperature NH3-SCR catalyst and realize the resource utilization of industrial solid waste, this research prepared a series NH3-SCR catalysts by using Fe-containing solid waste loading CeO2 and Nb2O5. It was found that the Ce2Nb1/Fe-Si catalyst exhibited the highest NH3-SCR activity, reaching about 93 % NOx conversion and 99 % N2 selectivity at 300 °C due to the synergistic effect between Ce and Nb species. The characterization results showed that the interaction between Ce and Nb was stronger than that between Nb and Fe species, promoting the NH3 adsorption and redox cycle of Ce4+ + Nb4+ ↔ Ce3+ + Nb5+, Ce3+ + Fe3+ ↔ Fe2+ + Ce4+, which increased the contents of Fe2+ and surface adsorbed oxygen. Moreover, the co-existence of Ce and Nb species would increase the NOx adsorption ability and promote the oxidation of NO to NO2, which was beneficial for “Fast SCR” reaction. Finally, the introduction of CeO2 and Nb2O5 did not changed the reaction mechanism, followed by a Langmuir-Hinshelwood mechanism.

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

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

Recycling of spent lithium-ion batteries into SO2‑tolerant low temperature NH3-SCR catalyst

Lithium-ion batteries will usher in a large-scale decommissioning wave, and how to resourcefully recycle spent lithium-ion batteries is a hot issue in today's environmental protection business. In this work, a catalyst with excellent catalytic activity and SO2 resistance was designed by modulating the morphology of the catalyst through the mixed solvent of water-polyethylene glycol using the spent lithium-ion batteries cathode materials, analyzed the impact of the ratio of water to polyethylene glycol on the anti-H2O, anti-SO2 and physicochemical properties of the catalysts. The NiCoMnOx-20PEG catalysts prepared when the polyethylene glycol: deionized water ratio was 1:3 showed a spherical structure assembled by needles and flakes, with smooth surfaces and uniform distribution, which resulted in stable catalysts and avoided pore clogging during the reaction process, and achieved near 100% NOx conversion at 80 ∼ 250 °C. The NOx conversion remained at 87.31% after 12 h of SO2 and H2O at 170 °C. The addition of polyethylene glycol can significantly delay the combination the sulfur dioxide and active sites and reduce the formation of sulfate, thus improving the catalyst's resistance to sulfur toxicity.

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

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

Performance and mechanism of low-temperature NH3-SCR denitrification over Mn/CePO4-SiO2 catalysts

Preparation of low-temperature NH3-SCR catalysts by Si and Mn modification and Si and Mn co-modification of CePO4 catalysts using mixed roasting and hydrothermal methods. XRD,BET,SEM and other characterisation tools were used to investigate the effect of Si and Mn modification on the catalyst structure. A series of different Si and Mn loadings and changes in catalyst denitrification performance, sulfur and water resistance after co-modification with Si and Mn were tested using NH3-TPD, H2-TPR and XPS, and the reaction pathways were investigated and the reaction mechanism of the catalysts was explored by FTIR detection in situ. The results show that CePO4-SiO2 catalyst at 250℃∼400℃, denitrification rate of about 90%; The optimum temperature for denitrification activity of the Mn/CePO4 catalyst was reduced, with a denitrification rate of 82% at 150℃; Mn/CePO4-SiO2 catalyst denitrification activity with significantly lower temperature and wider range. The denitrification rates of the catalysts were greater than 90% at 50℃∼300℃, with an optimum denitrification rate of 95 % at 150℃. The N2 selectivity of the catalyst is greater than 95 % in the low temperature range (below 200℃), good resistance to sulfur and water. The CePO4, SiO2 diffraction peaks on the surface of the sample decreased under the combined effect of Si and Mn. More acid sites appeared on the surface of the catalyst with elevated ratios of Ce3+ and adsorbed oxygen Oα, which promoted the catalyst's oxygen migration and conversion ability. After Si modification, the proportion of Mn4+ was significantly increased, which promoted the redox cycling of the catalyst in the low-temperature section. In situ IR results show that the catalyst follows both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism at 150℃. A variety of nitrate species produced by NO adsorption of the Si and Mn co-modified samples reacted rapidly with NH4+ species adsorbed at the Brønsted acid sites, which favoured the NH3-SCR reaction on the catalyst surface and improved the low-temperature denitrification efficiency.

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.

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.

The promotion effect of Pr doping on the catalytic performance of MnCeOx catalysts for low-temperature NH3-SCR

Using a one-step impregnation method, a series of Pr-modified MnCeOx catalysts with different Pr molar ratios were created to manufacture an efficient and effective catalyst for low-temperature NH3-SCR. It was demonstrated that the Pr modification could extensively boost the MnCeOx catalysts’ de-NOx activity and SO2 resistance and led to the formation of a unique broom-like morphology, especially when the molar ratio of Pr/Mn = 0.3, the MnCePrOx-0.3 exhibited above 90 % NO conversion at 250 °C with the existence of 100 ppm SO2. According to the characterization results, the enhancement of Pr doping for MnCeOx catalysts was mainly manifested by the expanded specific surface area, the improved dispersion of active components, the enhanced surface acidity and redox ability, and the generated more chemisorbed oxygen (Oads) species. All these characteristics allowed the NH3-SCR reaction over MnCePrOx-0.3 catalysts to proceed through the both Eley-Rideal (E-R) and Langmuir-hinshelwood (L-H) mechanism.

Promoting effect of Si on MnOx catalysts for low-temperature NH3-SCR of NO: Enhanced N2 selectivity and SO2 resistance

Manganese-based catalysts have excellent catalytic performance for the effective removal of NOx in low-temperature flue gas through selective catalytic reduction with NH3 (NH3-SCR). However, the catalysts suffer from poor SO2 resistance and N2 selectivity in the actual process, which still need to be solved. In this study, MnOx-SiO2 mixed oxide catalysts were designed and prepared by coprecipitation method to improve the SO2 resistance and N2 selectivity in NH3-SCR process. In the presence of 100 ppm SO2, MnOx-SiO2(0.72) catalyst showed excellent sulfur resistance with the conversion of NOx reached 95 % even after introducing SO2 for 8 h. The doping of SiO2 greatly improved the specific surface area and surface acidity of the catalyst, which was conducive to the adsorption of NH3. Meanwhile, the redox performance of the catalyst was decreased, improving the N2 selectivity. The study on the sulfur resistance mechanism revealed that the doping of SiO2 reduced the adsorption of SO2 on the surface of catalyst, protecting the active components and avoiding the accumulation of sulfates, thus achieving good sulfur resistance performance. This work provided an environmentally friendly NH3-SCR catalyst by inhibiting SO2 adsorption to have good catalytic performance and SO2 resistance.

Unraveling the synergy between MnOx and CeO2 in MnOx-CeO2 SCR catalysts based on experimental and DFT studies

MnOx-CeO2 catalysts have shown great potential as selective catalytic reduction of NOx by NH3 (NH3-SCR) at temperature below 300 °C. While the improved catalytic activity due to the synergy between MnOx and CeO2 in MnOx-CeO2 catalysts is widely acknowledged, the synergistic mechanism in NH3-SCR reaction remains elusive due to the lack of direct evidences. In this study, we report a comprehensive study of how MnOx and CeO2 interacted during NH3-SCR in two designed MnOx-CeO2 catalysts with different Ce4+ concentration through both experimental and theoretical approaches. Our results reveal that electron transfer from Mn2+ to Ce4+ induced the formation of surface oxygen vacancy on the CeO2, leading to the synergistic promotion of low-temperature catalytic performance by establishing Mn-redox and Ce-redox cycles to activate NH3 and O2, respectively. This work provides a distinct mechanism diagram of the synergy between MnOx and CeO2, which sheds light on the rational design of low-temperature NH3-SCR catalysts.

Unveiling the SO2 Resistance Mechanism of a Nanostructured SiO2(x)@Mn Catalyst for Low-Temperature NH3-SCR of NO.

Low N2 selectivity and SO2 resistance of Mn-based catalysts for removal of NOx at low temperatures by NH3-SCR (selective catalytic reduction) technology are the two main intractable problems. Herein, a novel core-shell-structured SiO2@Mn catalyst with greatly improved N2 selectivity and SO2 resistance was synthesized by using manganese carbonate tailings as raw materials. The specific surface area of the SiO2@Mn catalyst increased from 30.7 to 428.2 m2/g, resulting in a significant enhancement in NH3 adsorption capacity due to the interaction between Mn and Si. Moreover, the N2O formation mechanism, the anti-SO2 poisoning mechanism, and the SCR reaction mechanism were proposed. N2O originated from the reaction of NH3 with O2 and the SCR reaction, as well as from the reaction of NH3 with the chemical oxygen of the catalyst. Regarding improving the SO2 resistance, DFT calculations showed that SO2 was observed to preferentially adsorb onto the surface of SiO2, thus preventing the erosion of active sites. Adding amorphous SiO2 can transform the reaction mechanism from Langmuir-Hinshelwood (L-H) to Eley-Rideal (E-R) by adjusting the formation of nitrate species to produce gaseous NO2. This strategy is expected to assist in designing an effective Mn-based catalyst for low-temperature NH3-SCR of NO.

Strategies for designing hydrophobic MnCe-montmorillonite catalysts against water vapor for low-temperature NH3-SCR

The H2O resistance of low-temperature NH3-SCR catalysts is a key factor for practical applications. This work aims to strategically enhance the hydrophobicity of catalysts by intercalating surfactants into montmorillonite (M) via its cation exchange capacity. Various thermal pretreatment temperatures and chain lengths of surfactants have been used to enhance the hydrophobicity and interlayer spacing of montmorillonite, respectively. As a result, MnCe-M8-600 (with thermal pretreatment at 600 °C and the chain length of surfactant = 8) exhibits good low-temperature NH3-SCR performance and H2O resistance due to its labile oxygen, hydrophobicity, and structural stability. It is worth noting that the hydrophobicity of MnCe-M8-600 weakens the interaction between –OH and the catalyst, thereby continuously maintaining the redox ability and transferring electrons between Mn4+ and Ce3+. The results of the H2O resistance test showed that the NOX conversion of MnCe-M8-600 decreased by only 6% in the presence of 10% H2O, and its activity was fully recovered after turning off H2O. Hence, MnCe-M8-600 shows high activity and H2O resistance and is a promising catalyst for low-temperature NH3-SCR system applications.

Superior catalytic performance of Zr-incorporated MnCu/SBA-15 catalyst for low-temperature NH3-SCR of NO: Effect of support

High surface area mesoporous solid support has attracted much interest in the development of cost-effective catalysts for NOx abatement at low temperatures. SBA-15, a high surface area mesoporous material, was employed in the preparation of Al, Ti, and Zr incorporated SBA-15 through a direct synthesis approach under long hydrothermal reaction conditions (100 °C for 72 h). Heteroatom incorporation facilitated change in the morphology and textural properties of the material and was applied as a catalytic supports in order to facilitate the development of a MnCu bimetallic catalyst by a facile conventional co-impregnation method. At temperatures as low as 260 °C, the catalyst that was developed displayed superior deNOx activity for SCR of NO with NH3. There have been various physicochemical studies undertaken on the catalysts that were prepared, such as XRD, XPS, BET, HR-SEM, and HR-TEM. The deNOx optimization and deNOx activity over the catalysts are discussed. The exceptional deNOx activity at low temperatures may be attributed to the rod-like morphology, extensive oxygen vacancies, and high ratio of Mn3+ + Mn4+/Mn on 10Mn4Cu/Zr-SBA-15 catalyst. Furthermore, the catalyst also displayed a synergetic interaction between the Mn and the Zr species on its surface, which contributed to the catalyst's superior catalytic recycle ability and its long-term stability on stream stability. A useful approach of preparation is provided in order to accomplish the industrial use of a bimetallic catalyst consisting of manganese and copper at a low cost.

Influence of Y Doping on Catalytic Activity of CeO2, MnOx, and CeMnOx Catalysts for Selective Catalytic Reduction of NO by NH3

Novel yttrium-doped CeO2, MnOx, and CeMnOx composites are investigated as catalysts for low-temperature NH3-SCR. The study involves the preparation of unmodified oxide supports using a citrate method followed by modification with Y (2 wt.%) using two approaches, including the one-pot citrate method and incipient wetness impregnation of undoped oxides. The NH3-SCR reaction is studied in a fixed-bed quartz reactor to test the ability of the prepared catalysts in NO reduction. The gas reaction mixture consists of 800 ppm NO, 800 ppm NH3, 10 vol.% O2, and He as a balance gas at a WHSV of 25,000 mL g−1 h−1. The results indicate that undoped CeMnOx mixed oxide exhibits significantly higher deNOx performance compared with undoped and Y-doped MnOx and CeO2 catalysts. Indeed, yttrium presence in CeMnOx promotes the competitive NH3-SCO reaction, reducing the amount of NH3 available for NO reduction and lowering the catalyst activity. Furthermore, the physical-chemical properties of the prepared catalysts are studied using nitrogen adsorption/desorption, XRD, Raman spectroscopy, temperature-programmed reduction with hydrogen, and temperature-programmed desorption of ammonia. This study presents a promising approach to enhancing the performance of NH3-SCR catalysts at low temperatures that can have significant implications for reducing NO emissions.

The construction of MnOx with rich oxygen vacancy for robust low-temperature denitration

In the present study, manganese oxide (MnOx) with attractive feature of rich oxygen vacancy was constructed by a surfactant-assisted method and explored for NH3-SCR. The obtained MnOx-OV showed distinct performance, with full conversion of NO achieved at the low temperature window of 100–200 °C under a high space velocity of 120,000 mL·g−1·h-1. Besides, good water tolerance was exhibited, as evidenced by the minor drop (13%) of NO conversion under typical low temperature of 125 °C with 10% H2O. Through comprehensive characterization, it was found that the MnOx-OV with rich oxygen vacancy could promote oxygen diffusion and thus induce enhanced oxidative dehydrogenation capacity to NH3, which played a crucial role in facilitating the reaction between NOx and NH3. Regarding the reaction mechanism, Langmuir-Hinshelwood (L-H) pathway was proposed based on the results from in situ DRIFTS and reaction order measurement. The result of present study is expected to provide useful guidance for designing well performed catalysts for low-temperature NH3-SCR.

Optimization of high surface area VOx/TiO2 catalysts for low-temperature NH3-SCR for NOx abatement

We have prepared two series of VOx/TiO2 catalysts (V surface density in the range 1.6–7.7 V at/nm2) employing anatase TiO2 with different surface area (89 and 181 m2/g) to find the optimal composition for the low temperature NH3 Selective Catalytic Reduction and correlate their physico-chemical properties to performance. The same platelet-like morphology was observed for the two TiO2 supports by electron microscopies and X-ray diffraction, indicating that the different surface area is only related to particle size. A higher amount of defective surface sites (undercoordinated Ti4+ sites at edges, corner and steps positions) was found on the high surface area TiO2 by FTIR spectroscopy of CO probe molecule. The distribution of VOx species determined by FTIR and Raman spectroscopies is similar on the two series, reaching the monolayer coverage and maximum activity at ≈ 5 V at/nm2, which can be considered the optimal V density for these catalysts.

Denitrification performance and sulfur resistance mechanism of Sm-Mn catalyst for low temperature NH3-SCR

Denitrification performance and sulfur resistance mechanism of Sm-Mn catalyst for low temperature NH3-SCR