Design of meso-TiO2@MnO(x)-CeO(x)/CNTs with a core-shell structure as DeNO(x) catalysts: promotion of activity, stability and SO2-tolerance

Iterative Approach of Experiment-Machine Learning for Efficient Optimization of Environmental Catalysts: An Example of NOx Selective Reduction Catalysts

An iterative approach between machine learning (ML) and laboratory experiments was developed to accelerate the design and synthesis of environmental catalysts (ECs) using selective catalytic reduction (SCR) of nitrogen oxides (NOx) as an example. The main steps in the approach include training a ML model using the relevant data collected from the literature, screening candidate catalysts from the trained model, experimentally synthesizing and characterizing the candidates, updating the ML model by incorporating the new experimental results, and screening promising catalysts again with the updated model. This process is iterated with a goal to obtain an optimized catalyst. Using the iterative approach in this study, a novel SCR NOx catalyst with low cost, high activity, and a wide range of application temperatures was found and successfully synthesized after four iterations. The approach is general enough that it can be readily extended for screening and optimizing the design of other environmental catalysts and has strong implications for the discovery of other environmental materials.

Insights into Nb doping effects on the catalytic activity and SO2 tolerance of Mn-Cu/BCN catalyst for low-temperature NH3-SCR reaction

Biochar-based supported denitration catalysts have shown tremendous potential in reducing NOx, while improving low-temperature NH3-SCR catalytic activity and SO2 tolerance still faces great challenges. In this work, Mn7-Cu3/BCN and Mn7-Cu3-Nbx/BCN catalysts were prepared by one-step wet impregnation. The enhanced effect of Nb doping on the catalytic performance and SO2 tolerance over the Mn7-Cu3/BCN catalyst was evaluated in the temperature range of 75-275 °C. The denitrification activity test showed that the introduction of an appropriate amount of Nb increased the catalytic activity and N2 selectivity of the catalyst. The NO conversion of Mn7-Cu3-Nb0.05/BCN with an optimum doping ratio of 0.05 wt% Nb was higher than 94% at 150-275 °C. The characterization results indicated that the introduction of Nb enhanced the interaction between the active components MnOx and CuOx, accelerated the electron transfer between elements, and thus improved the Mn4+/Mnn+ and Oα/(Oα+Oβ+Oγ) proportions and redox performance. On the other hand, Nb modification increased the number of weakly acidic sites, which was beneficial for the adsorption and activation of the reducing agent NH3 under low-temperature conditions. Meanwhile, Nb could significantly improve the SO2 poisoning resistance of the Mn7-Cu3/BCN-S catalyst when SO2 was added to the reaction system. The NO conversion of Mn7-Cu3-Nb0.05/BCN remained above 75% after a 13.5 h reaction under 100 ppm SO2 and 5 vol% H2O at 225 °C. By combining experimental characterization results with DFT calculation results, we effectively confirmed that Mn7-Cu3-Nb0.05/BCN had good sulfur resistance, mainly because Nb could effectively inhibit the formation of manganese sulfate and promote the decomposition of ammonium bisulfate.

Recent advances in core-shell structured catalysts for low-temperature NH3-SCR of NOx

Ammonia selective catalytic reduction (NH₃-SCR) of nitrogen oxides is an effective and well-established technology for NO_x removal, but current commercial denitrification catalysts based on V₂O₅-WO₃/TiO₂ have some obvious disadvantages, including narrow operating temperature windows, toxicity, poor hydrothermal stability, and unsatisfied SO₂/H₂O tolerance. To overcome these drawbacks, it is imperative to investigate new types of highly efficient catalysts. In order to design catalysts with outstanding selectivity, activity, and anti-poisoning ability, core-shell structured materials have been widely applied in the NH₃-SCR reaction, which exhibits numerous advantages including the large surface area, the strong synergy interaction of core-shell materials, the confinement effect, and the shielding effect from the shell layer to protect the core. This review summarizes recent developments of core-shell structured catalysts for NH₃-SCR, including basic classification, synthesis methods, and a detailed description of the performance and mechanisms of each type of catalyst. It is hoped that the review will stimulate future developments in NH₃-SCR technology, leading to novel catalyst designs with improved denitrification performance.

The Resistance of SO2 and H2O of Mn-Based Catalysts for NO x Selective Catalytic Reduction with Ammonia: Recent Advances and Perspectives

The treatment of NO _x has become an urgent issue due to it being difficult to degrade in air and its tremendous adverse impact on public health. Among numerous NO _x emission control technologies, the technology of selective catalytic reduction (SCR) using ammonia (NH₃) as the reducing agent (NH₃-SCR) is regarded as the most effective and promising technique. However, the development and application of high-efficiency catalysts is severely limited due to the poisoning and deactivation effect by SO₂ and H₂O vapor in the low-temperature NH₃-SCR technology. In this review, recent advances in the catalytic effects from increasing the rate of the activity in low-temperature NH₃-SCR by manganese-based catalysts and the stability of resistance to H₂O and SO₂ during catalytic denitration are reviewed. In addition, the denitration reaction mechanism, metal modification, preparation methods, and structures of the catalyst are highlighted, and the challenges and potential solutions for the design of a catalytic system for degenerating NO _x over Mn-based catalysts with high resistance of SO₂ and H₂O are discussed in detail.

Phosphotungstic Acid-Modified MnOx for Selective Catalytic Reduction of NOx with NH3

H3PW12O40-modified MnOx catalysts (denoted as Mn-HPW) were used for NOx elimination with co-fed NH3. The optimal Mn-HPW0.02 catalyst exhibited over 90% NOx conversion at 90–270 °C. The incorporation of HPW increased the amount of Lewis acid sites of the catalyst for adsorbing NH3, and accelerated the reaction between the adsorbed NH3 species and gas-phase NOx, thus, increasing the low-temperature catalytic activity. The oxidation ability of the Mn catalyst was decreased due to the addition of HPW, thus, mitigating the overoxidation of the adsorbed NH3 species and improving the de-NOx activity and N2 selectivity in the high-temperature region. DRIFT results revealed that the NH3 species on Lewis and Brønsted acid sites, bridged nitrate, and bidentate nitrate were important species/intermediates for the reaction. NH3-SCR over the Mn and Mn-HPW0.02 catalysts obeyed the Eley–Rideal and Langmuir–Hinshelwood mechanisms, simultaneously, at 120 °C.

Manganese oxide nanorod catalysts for low-temperature selective catalytic reduction of NO with NH3

MnO_x nanorod catalysts were successfully synthesized by two different preparation methods using porous SiO₂ nanorods as the template and investigated for the low-temperature selective catalytic reduction (SCR) of NO with NH₃. The catalysts were characterized by scanning electron microscopy, transmission electron microscopy, nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, and NH₃ temperature-programmed desorption. The results show that the obtained MnO_x-P nanorod catalyst prepared by redox precipitation method exhibits higher NO removal activity than that prepared by the solvent evaporation method in the low temperature range of 100–180 °C, where about 98% NO conversion is achieved over MnO_x(0.36)-P nanorods. The reason is mainly attributed to MnO_x(0.36)-P nanorods possessing unique flower-like morphology and mesoporous structures with high pore volume, which facilitates the exposure of more active sites of MnO_x and the adsorption of reactant gas molecules. Furthermore, there is a lower crystallinity of MnO_x, higher percentage of Mn⁴⁺ species and a large amount of strong acid sites on the surface. These factors contribute to the excellent low-temperature SCR activity of MnO_x(0.36)-P nanorods.

Compared with MnO_x(0.36)-E nanorods, MnO_x(0.36)-P nanorods possess unique flower-like morphology and mesoporous structures with high pore volume, contributing to the excellent low-temperature SCR activity of MnO_x(0.36)-P nanorods.

Stability Mechanism of Low Temperature C2H4-SCR with Activated-Carbon-Supported MnO x -Based Catalyst

Manganese-based catalysts have shown great potential for use as a hydrocarbon reductant for NO _x reduction (HC-SCR) at low temperatures if their catalytic stability could be further maintained. The effect of CeO₂ as a promoter and catalyst stability agent for activated carbon supported MnO _x was investigated during low temperature deNO _x based on a C₂H₄ reductant. The modern characterization technology could provide a clear understanding of the activity observed during the deNO _x tests. When reaction temperatures were greater than 180 °C and with ceria concentrations more than 5%, the overall NO conversion became stable near 70% during long duration testing. In situ DRIFTS shows that C₂H₄ is adsorbed on the Mn₃Ce₃/NAC catalysts to generate hydrocarbon activated intermediates, R-COOH, and the reaction mechanism followed the E-R mechanism. The stability and the analytical data pointed to the formation of stable oxygen vacancies within Ce³⁺/Ce⁴⁺ redox couplets that prevented the reduction of MnO₂ to crystalline Mn₂O₃ and promoted the chemisorption of oxygen on the surface of MnO _x -CeO _x structures. Based on the data, a synergetic mechanism model of the deNO _x activity is proposed for the MnO _x -CeO _x catalysts.

Recent advances and perspectives in the resistance of SO2 and H2O of cerium-based catalysts for NOx selective catalytic reduction with ammonia

This review emphasizes the aspects related to cerium-based catalysts at different levels: metal modification, preparation methods, structures, and reaction mechanisms.

O3 oxidation combined with semi-dry method for simultaneous desulfurization and denitrification of sintering/pelletizing flue gas

With the vigorous development of China's iron and steel industry and the introduction of ultra-low emission policies, the emission of pollutants such as SO₂ and NO_x has received unprecedented attention. Considering the increase of the proportion of semi-dry desulfurization technology in the desulfurization process, several semi-dry desulphurization technologies such as flue gas circulating fluidized bed (CFB), dense flow absorber (DFA) and spray drying absorption (SDA) are briefly summarized. Moreover, a method for simultaneous treatment of SO₂ and NO_x in sintering/pelletizing flue gas by O₃ oxidation combined with semi-dry method is introduced. Meantime, the effects of key parameters such as O₃/NO molar ratio, CaSO₃, SO₂, reaction temperature, Ca/(S+2N) molar ratio, droplet size and approach to adiabatic saturation temperature (AAST) on denitrification and desulfurization are analyzed. Furthermore, the reaction mechanism of denitrification and desulfurization is further elucidated. Finally, the advantages and development prospects of the new technology are proposed.

A review of Mn-based catalysts for low-temperature NH3-SCR: NOx removal and H2O/SO2 resistance

The development of high-efficiency catalysts is the key to the low-temperature NH₃-SCR technology. The introduction of SO₂ and H₂O will lead to poisoning and deactivation of the catalysts, which severely limits the development and application of NH₃-SCR technology. This review introduces the necessity of NO_x removal, explains the mechanisms of H₂O and SO₂ poisoning on NH₃-SCR catalysts, highlights the Mn-based catalysts of different active metals and supports and their resistance to H₂O and SO₂, and analyses the relationship between metal modification, selection of support and preparation method, morphology and structure design and SO₂/H₂O resistance. Given the current problems, this review points out the future research focus of Mn-based catalysts and also puts forward corresponding countermeasures to solve the existing problems.

FeVO4-supported Mn–Ce oxides for the low-temperature selective catalytic reduction of NOx by NH3

Iron vanadate (FeVO4) nanorods are used as a carrier to support manganese (Mn) and cerium (Ce) oxides for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with NH3 for the first time.

Recent advances in layered double hydroxides (LDHs) derived catalysts for selective catalytic reduction of NOx with NH3

In recent years, layered double hydroxides (LDHs) derived metal oxides as highly efficient catalysts for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) have attracted great attention. The high dispersibility and interchangeability of cations within the brucite-like layers make LDHs an indispensable branch of catalytic materials. With the increasingly stringent and ultra-low emission regulations, there is an urgent need for highly efficient and stable low-medium temperature denitration catalysts in markets. In this contribution, we have critically summarized the recent research progress in the LDHs derived NH₃-SCR catalysts, including their ability for NO_x removal, N₂ selectivity, active temperature window, stability and resistance to poisoning. The advantages and defects of various types of LDHs-derived catalysts are comparatively summarized, and the corresponding modification strategies are discussed. In addition, considering the importance of the catalyst's resistance to poisoning in practical applications, we discuss the poisoning mechanism of each component in flue gases, and provide the corresponding strategies to improve the poisoning resistance of catalysts. Finally, from the perspective of practical applications and operation cost, the regeneration measures of catalysts after poisoning is also discussed. We hope that this work can give timely technical guidance and valuable insights for the applications of LDHs materials in the field of NO_x control.

Improved activity of Ho-modified Mn/Ti catalysts for the selective catalytic reduction of NO with NH3

A series of Ho-modified Mn/Ti catalysts with various content of Ho were prepared by impregnation method, and the low-temperature catalytic performance was tested. Techniques of BET, SEM, XRD, H₂-TPR, and XPS were carried out to research the effects of Ho modification on the physicochemical properties of Mn/Ti catalysts. Results showed that appropriate Ho addition could reduce the starting temperature of Mn/TiO₂ catalyst to 100 °C. 0.2HoMn/Ti exhibiting a wider temperature range of 140~220 °C with nearing 100% NO_x conversion. It was found that the 0.2HoMn/TiO₂ catalyst possessed a better dispersion of active component, enhanced redox capacity, a higher concentration of Mn⁴⁺ species, and a larger amount of O_β content on the catalyst surface, which are all likely predominant factors related to the excellent SCR activity. Additionally, Ho improved the Lewis acid sites and enhanced the adsorption and activation ability of NH₃, as well as the NO to NO₂ oxidation ability. The selective catalytic reduction with ammonia (NH₃-SCR) deNO_x mechanism over HoMn/Ti catalysts obeyed both the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms under low-temperature reaction conditions.

Selective catalytic reduction of NOx by NH3 over CeVO4-CeO2 nanocomposite

In this study, it was found that the CeVO₄-CeO₂ nanocomposite possessed remarkably selective catalytic reduction (SCR) performance and wider active temperature scope. And, the promotion principle was explored based on BET, XRD, XPS, H₂-temperature-programmed reduction, NH₃-temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform (DRIFT) techniques. The characterization outcomes manifested that the CeVO₄-CeO₂ nanocomposite could inhibit its crystallinity and enhance the concentrations of chemisorbed oxygen species and Ce³⁺, which was advantageous to the SCR process. Moreover, the in situ DRIFT technique manifested that the NH₃-SCR reaction over Ce_0.75V_0.25O_y was enhanced effectively through the mechanism of L-H.

The positive effect of siderite-derived α-Fe2O3 during coaling on the NO behavior in the presence of NH3

Siderite is a naturally occurring mineral that can be found extensively in coal. The structural evolution of siderite in the process of coaling and its performance in the transformation of NO in the presence of NH₃ were investigated in this work. In addition, the effects of the coexisting component, including vapor, SO₂, and the alkali metal K, were also discussed. Heat treatment was performed at 450, 500, 550, 600, and 700 °C to obtain siderite-derived α-Fe₂O₃, which was then evaluated in de-NO_x via the selective catalytic reduction (SCR) of NO with NH₃ in a fixed bed. The X-ray diffraction (XRD), the X-ray fluorescence spectrometer (XRF), N₂ adsorption-desorption (BET), the X-ray photoelectron spectrometer (XPS), the scanning electron microscope (SEM), and the transmission electron microscope (TEM) were used to investigate the variations in the morphology and structure of the thermally treated siderite. The results showed that siderite was gradually oxidized and decomposed into α-Fe₂O₃ with a nanoporous structure and large surface area of 27.27 m² g^-1 after calcination under an air atmosphere. The α-Fe₂O₃ derived from siderite at 500 °C (H500) exhibited an excellent SCR performance, where the NO conversion rate was great than 90% between 250 and 300 °C due to the pore structure and high specific surface area, additional adsorbed oxygen states, abundant oligomeric Fe oxide clusters, and large amount of acid sites. Regardless of the vapor content, SO₂ concentration, and reaction temperature, the α-Fe₂O₃ derived from siderite at 500 °C (H500) still favored the conversion of NO. When the reaction temperature was lower than 350 °C, H500 favored the conversion of NO even in the presence of an alkali metal (K). The experimental data demonstrated the positive effect of siderite-derived α-Fe₂O₃ in SCR technology and provided insight into NO behavior in coaling flue gas after NH₃ injection.

Promotion effect of urchin-like MnO x @PrO x hollow core-shell structure catalysts for the low-temperature selective catalytic reduction of NO with NH3

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃. The structural properties of the catalyst were characterized by FE-SEM, TEM, XRD, BET, XPS, H₂-TPR and NH₃-TPD analyses, and the performance of the low-temperature NH₃-SCR was also tested. The results show that the catalyst with a molar ratio of Pr/Mn = 0.3 exhibited the highest NO conversion at nearly 99% at 120 °C and NO conversion greater than 90% over the temperature range of 100–240 °C. Also, the MnO_x@PrO_x catalyst presented desirable SO₂ and H₂O resistance in 100 ppm SO₂ and 10 vol% H₂O at the space velocity of 40 000 h⁻¹ and a testing time of 3 h test at 160 °C. The excellent low-temperature catalytic activity of the catalyst could ultimately be attributed to high concentrations of Mn⁴⁺ and adsorbed oxygen species on the catalyst surface, suitable Lewis acidic surface properties, and good reducing ability. Additionally, the enhanced SO₂ and H₂O resistance of the catalyst was primarily ascribed to its unique core–shell structure which prevented the MnO_x core from being sulfated.

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃.

Mn2NiO4 spinel catalyst for high-efficiency selective catalytic reduction of nitrogen oxides with good resistance to H2O and SO2 at low temperature

Mn-Ni oxides with different compositions were prepared using standard co-precipitation (CP) and urea hydrolysis-precipitation (UH) methods and optimized for the selective catalytic reduction of nitrogen oxides (NO_x) by NH₃ at low temperature. Mn₍₂₎Ni₍₁₎O_x-CP and Mn₍₂₎Ni₍₁₎O_x-UH (with Mn:Ni molar ratio of 2:1) catalysts showed almost identical selective catalytic reduction (SCR) catalytic activity, with about 96% NO_x conversion at 75°C and ~99% in the temperature range from 100 to 250°C. X-ray diffraction (XRD) results showed that Mn₍₂₎Ni₍₁₎O_x-CP and Mn₍₂₎Ni₍₁₎O_x-UH catalysts crystallized in the form of Mn₂NiO₄ and MnO₂-Mn₂NiO₄ spinel, respectively. The latter gave relatively good selectivity to N₂, which might be due to the presence of the MnO₂ phase and high metal-O binding energy, resulting in low dehydrogenation ability. According to the results of various characterization methods, it was found that a high density of surface chemisorbed oxygen species and efficient electron transfer between Mn and Ni in the crystal structure of Mn₂NiO₄ spinel played important roles in the high-efficiency SCR activity of these catalysts. Mn₍₂₎Ni₍₁₎O_x catalysts presented good resistance to H₂O or/and SO₂ with stable activity, which benefited from the Mn₂NiO₄ spinel structure and Eley-Rideal mechanism, with only slight effects from SO₂.

Preparation of Mesoporous Mn-Ce-Ti-O Aerogels by a One-Pot Sol-Gel Method for Selective Catalytic Reduction of NO with NH3

Novel Mn-Ce-Ti-O composite aerogels with large mesopore size were prepared via a one-pot sol-gel method by using propylene oxide as a network gel inducer and ethyl acetoacetate as a complexing agent. The effect of calcination temperature (400, 500, 600, and 700 °C) on the NH3-selective catalytic reduction (SCR) performance of the obtained Mn-Ce-Ti-O composite aerogels was investigated. The results show that the Mn-Ce-Ti-O catalyst calcined at 600 °C exhibits the highest NH3-SCR activity and lowest apparent activation energy due to its most abundant Lewis acid sites and best reducibility. The NO conversion of the MCTO-600 catalyst maintains 100% at 200 °C in the presence of 100 ppm SO2, showing the superior resistance to SO2 poisoning as compared with the MnOx-CeO2-TiO2 catalysts reported the literature. This should be mainly attributed to its large mesopore sizes with an average pore size of 32 nm and abundant Lewis acid sites. The former fact facilitates the decomposition of NH4HSO4, and the latter fact reduces vapor pressure of NH3. The NH3-SCR process on the MCTO-600 catalyst follows both the Eley-Rideal (E-R) mechanism and the Langmuir-Hinshelwood (L-H) mechanism.

Spinel-structured Mn–Ni nanosheets for NH3-SCR of NO with good H2O and SO2 resistance at low temperature

High specific surface area, more NH₃ adsorption ability and efficient electronic interaction over Mn–Ni spinel nanosheet leaded to good SCR activity, and Ni-outside with active Mn-inner spinel configuration and nanosheet morphology relieved SO₂-poisoning.

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh as novel monolithic catalysts for NOx reduction

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh were developed as novel monolithic catalysts for NO_x reduction.

Promotional effects of modified TiO2- and carbon-supported V2O5- and MnOx-based catalysts for the selective catalytic reduction of NOx: a review

This review presents the promotional effects of transition metal modification over TiO₂- and carbon-supported V₂O₅- and MnO_x-based SCR catalysts.

Facile fabrication of nanosheet-assembled MnCoOx hollow flower-like microspheres as highly effective catalysts for the low-temperature selective catalytic reduction of NOx by NH3

A series of MnCoO_x flower-like hollow microspheres with various molecular proportions of reactant were prepared through simple solvothermal method for the ammonia selective catalytic reduction (SCR) at low temperatures. The as-prepared samples have been applied by various characterization techniques to explore the formation process of the morphology and physicochemical properties. The Mn(1)Co(1)O_x presented the optimal intrinsic catalytic performance (95% NO_x conversion at 75 °C), favorable thermal stability, and strong SO₂ resistance. The excellent properties mainly related to its higher specific surface area and abundant active sites originated from hollow microsphere special structure consists of abundant nanosheets, robust redox properties beneficial for the strong interaction between the manganese and cobalt, larger number of acidic sites and stronger acid strength, etc., which collaboratively dominate its catalytic properties of NH₃-SCR at low temperatures.

Manganese oxides with hierarchical structures derived from coordination polymers and their enhanced catalytic activity at low temperature for selective catalytic reduction of NOx

Hierarchical manganese oxides with enhanced catalytic performance have been successfully synthesized via simple thermal annealing of manganese coordination polymer precursors, which is a facile, cost-effective, and environmentally benign preparation method. The resultant manganese oxide particles formed hierarchical structures with a starfish-like morphology and exhibited enhanced low-temperature SCR performance below 200 °C without dopants or supporting materials. In addition, the morphology, chemical states, crystal structure and acidity of manganese oxide catalysts prepared at different calcination temperatures were investigated. It is elucidated that enhanced SCR catalytic performance was strongly dependent on the hierarchical morphology of the catalysts.

Active Complexes on Engineered Crystal Facets of MnOx-CeO2 and Scale-Up Demonstration on an Air Cleaner for Indoor Formaldehyde Removal

Crystal facet-dominated surfaces determine the formation of surface-active complexes, and engineering specific facets is desirable for improving the catalytic activity of routine transition-metal oxides that often deactivate at low temperatures. Herein, MnO_x-CeO₂ was synthetically administered to tailor the exposure of three major facets, and their distinct surface-active complexes concerning the formation and quantitative effects of oxygen vacancies, catalytically active zones, and active-site behaviors were unraveled. Compared with two other low-index facets {110} and {001}, MnO_x-CeO₂ with exposed {111} facet showed higher activity for formaldehyde oxidation and CO₂ selectivity. However, the {110} facet did not increase activity despite generating additional oxygen vacancies. Oxygen vacancies were highly stable on the {111} facet, and its bulk lattice oxygen at high migration rates could replenish the consumption of surface lattice oxygen, which was associated with activity and stability. High catalytically active regions were exposed at the {111}-dominated surfaces, wherein the predominated Lewis acid-base properties facilitated oxygen mobility and activation. The mineralization pathways of formaldehyde were examined by a combination of in situ X-ray photoemission spectroscopy and diffuse reflectance infrared Fourier transform spectrometry. The MnO_x-CeO₂-111 catalysts were subsequently scaled up to work as filter substrates in a household air cleaner. In in-field pilot tests, 8 h of exposure to an average concentration of formaldehyde after start-up of the air cleaner attained the Excellent Class of Indoor Air Quality Objectives in Hong Kong.

Fe-modified Ce-MnOx/ACFN catalysts for selective catalytic reduction of NOx by NH3 at low-middle temperature

A series of MnO_x/ACF_N, Ce-MnO_x/ACF_N, and Fe-Ce-MnO_x/ACF_N catalysts on selective catalytic reduction (SCR) of NO_x with NH₃ at low-middle temperature had been successfully prepared through ultrasonic impregnation method, and the catalysts were characterized by SEM, XRD, BET, H₂-TPR, NH₃-TPD, XPS, and FT-IR spectroscopy, respectively. The results demonstrated that the 15 wt% Fe₍₁₎-Ce₍₃₎-MnO_x(7)/ACF_N catalyst achieved 90% NO_x conversion (100~300 °C), good water resistance, and stability (175 °C). The excellent catalytic performance of the Fe₍₁₎-Ce₍₃₎-MnO_x(7)/ACF_N catalyst was mainly attributed to the interaction among Mn, Ce, and Fe. The doping of Fe promoted the dispersion of Ce and Mn and the formation of more Mn⁴⁺ and chemisorbed oxygen on the surface of a catalyst. This work laid a foundation for the successful application of active carbon fiber in the field of industrial denitrification, especially in the aspect of denitrification moving bed. Graphic abstract.

Effect of SO2 on the Selective Catalytic Reduction of NOx over V2O5-CeO2/TiO2-ZrO2 Catalysts

The effect of SO₂ on the selective catalytic reduction of NO_x by NH₃ over V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalysts was studied through catalytic activity tests and various characterization methods, like Brunner−Emmet−Teller (BET) surface measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray fluorescence (XRF), hydrogen temperature-programmed desorption (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The results showed that the catalyst exhibited superior SO₂ resistance when the volume fraction of SO₂ was below 0.02%. As the SO₂ concentration further increased, the NO_x conversion exhibited some degree of decline but could restore to the original level when stopping feeding SO₂. The deactivation of the catalyst caused by water in the flue gas was reversible. However, when 10% H₂O was introduced together with 0.06% SO₂, the NO_x conversion was rapidly reduced and became unrecoverable. Characterizations indicated that the specific surface area of the deactivated catalyst was significantly reduced and the redox ability was weakened, which was highly responsible for the decrease of the catalytic activity. XPS results showed that more Ce³⁺ was generated in the case of reacting with SO₂. In situ DRIFTS results confirmed that the adsorption capacity of SO₂ was enhanced obviously in the presence of O₂, while the SO₂ considerably refrained the adsorption of NH₃. The adsorption of NO_x was strengthened by SO₂ to some extent. In addition, NH₃ adsorption was improved after pre-adsorbed by SO₂ + O₂, indicating that the Ce³⁺ and more oxygen vacancy were produced.

Tunable preparation of highly dispersed Ni x Mn-LDO catalysts derived from Ni x Mn-LDHs precursors and application in low-temperature NH3-SCR reactions

A series of Ni x Mn bimixed metal oxides (Ni x Mn-LDO) were prepared via calcining Ni x Mn layered double hydroxides (Ni x Mn-LDHs) precursors at 400 °C and applied as catalysts in the selective catalytic reduction (SCR) of NO x with NH3. The DeNO x performance of catalysts was optimized by adjusting the Ni/Mn molar ratios of Ni x Mn-LDO precursors, in which Ni5Mn-LDO exhibited above 90% NO x conversion and N2 selectivity at a temperature zone of 180-360 °C. Besides, Ni5Mn-LDO possessed considerable SO2 & H2O resistance and outstanding stability. Multiple characterization techniques were used to analyze the physicochemical properties of the catalysts. The analysis results indicated that all catalysts had the same active species Ni6MnO8, while their particle sizes showed significant differences. Notably, the uniform distribution of active species particles in the Ni5Mn-LDO catalyst provided the rich surface acidity and suitable redox ability which were the primary causes for its desirable DeNO x property.

The Characterization and SCR Performance of Mn-Containing α-Fe2O3 Derived from the Decomposition of Siderite

In this work, a nano-structured iron-manganese oxide composite was prepared by calcining natural manganese-rich siderite at different temperatures (450, 500, 550, 600 °C, labeled as H450, H500, H550, H600, respectively), and their performances of selective catalytic reduction (SCR) of NO by NH3 were investigated. XRD, XRF, BET, XPS, SEM, and TEM were used to investigate the morphology, composition, and surface characteristics of the catalyst. The results showed that the decomposition of siderite occurred from 450 °C to around 550 °C during the calcination in air atmosphere; moreover, the siderite could be converted into nano-structured α-Fe2O3. The specific surface area of the material increased, and Mn2+ was transformed into Mn4+, which were beneficial to the SCR. Among these catalysts, H550 had the best SCR performance, with NO removal of 98% at a temperature window from 200 to 250 °C. The presence of water vapor and sulfur dioxide can inhibit the SCR performance of the catalysts, but this inhibition effect was not obvious for H550 at the optimum reaction temperature (250 °C). The findings presented in this study are significant toward the application of the Mn-rich siderite as a precursor in preparing the Fe-Mn oxides for catalytic de-NOx by SCR.