Homogeneous MnO –CeO2 pellets prepared by a one-step hydrolysis process for low-temperature NH3-SCR

Effect of Indium Addition on the Low-Temperature Selective Catalytic Reduction of NO x by NH3 over MnCeO x Catalysts: The Promotion Effect and Mechanism

A MnCeInO _x catalyst was prepared by a coprecipitation method for denitrification of NH₃-SCR (selective catalytic reduction). The catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry, scanning electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, H₂ temperature-programmed reduction, and NH₃ temperature-programmed desorption. The NH₃-SCR activity and H₂O and SO₂ resistance of the catalysts were evaluated. The test results showed that the SCR and water resistance and sulfur resistance were good in the range of 125-225 °C. The calcination temperature of the Mn₆Ce_0.3In_0.7O _x catalyst preparation was studied. The crystallization of the Mn₆Ce_0.3In_0.7O _x catalyst was poor when calcined at 300 °C; however, the crystallization is excessive at a 500 °C calcination temperature. The influence of space velocity on the performance of the catalyst is great at 100-225 °C. FTIR test results showed that indium distribution on the surface of the catalyst reduced the content of sulfate on the surface, protected the acidic site of MnCe, and improved the sulfur resistance of the catalyst. The excellent performance of the Mn₆Ce_0.3In_0.7O _x catalyst may be due to its high content of Mn⁴⁺, surface adsorbed oxygen species, high specific surface area, redox sites and acid sites on the surface, high turnover frequency, and low apparent activation energy.

Selective catalytic reduction of NO x by low-temperature NH3 over Mn x Zr1 mixed-oxide catalysts

Mn_xZr₁ series catalysts were prepared by a coprecipitation method. The effect of zirconium doping on the NH₃-SCR performance of the MnO_x catalyst was studied, and the influence of the calcination temperature on the catalyst activity was explored. The results showed that the Mn₆Zr₁ catalyst exhibited good NH₃-SCR activity when calcined at 400 °C. When the reaction temperature was 125–250 °C, the NO_x conversion rate of Mn₆Zr₁ catalyst reached more than 90%, and the optimal conversion efficiency reached 97%. In addition, the Mn₆Zr₁ catalyst showed excellent SO₂ and H₂O resistance at the optimum reaction temperature. Meanwhile, the catalysts were characterized. The results showed that the morphology of the MnO_x catalyst was significantly changed, whereby as the proportion of Mn⁴⁺ and O_α species increased, the physical properties of the catalyst were improved. In addition, both Lewis acid sites and Brønsted acid sites existed in the Mn₆Zr₁ catalyst, which reduced the reduction temperature of the catalyst. In summary, zirconium doping successfully improved the NH₃-SCR performance of MnO_x.

Mn_xZr₁ series catalysts were prepared by a coprecipitation method.

Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review

Recently, manganese oxides (MnOx)/cerium(IV) oxide (CeO2) composites have attracted widespread attention for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), which exhibit outstanding catalytic performance owing to unique features, such as a large oxygen storage capacity, excellent low-temperature activity, and strong mechanical strength. The intimate contact between the components can effectively accelerate the charge transfer to enhance the electron–hole separation efficiency. Nevertheless, MnOx/CeO2 still reveals some deficiencies in the practical application process because of poor thermal stability, and a low reduction efficiency. Constructing MnOx/CeO2 with other semiconductors is the most effective strategy to further improve catalytic performance. In this article, we discuss progress in the field of MnOx/CeO2-based ternary composites with an emphasis on the SCR of NOx by NH3. Recent progress in their fabrication and application, including suitable examples from the relevant literature, are analyzed and summarized. In addition, the interaction mechanisms between MnOx/CeO2 catalysts and NOx pollutants are comprehensively dissected. Finally, the review provides basic insights into prospects and challenges for the advancement of MnOx/CeO2-based ternary catalysts.

Role of impurity components and pollutant removal processes in catalytic oxidation of o-xylene from simulated coal-fired flue gas

Volatile organic compounds (VOCs) emitted from coal-fired flue gas of thermal power plants have reached unprecedented levels due to lack of understanding of reaction mechanisms under industrial settings. Herein, inhibition mechanisms for catalytic oxidation of o-xylene in simulated coal-fired flue gas are elucidated with in-situ and ex-situ spectroscopic techniques considering the presence of impurity components (NO, NH₃, SO₂, H₂O). MnCe oxide catalysts prepared at Mn: Ce mass ratios of 6:4 are demonstrated to promote 87% o-xylene oxidation at 250 °C under gas hourly space velocities of 60,000 h^-1. Reaction intermediates on the catalyst surface are revealed to be o-benzoquinones, benzoates, and formate and they were stably formed under O₂/N₂ atmospheres. When either NO or NH₃ was introduced into the simulated flue gas, the formed species shifted toward formate in minutes, which indicated that changes in catalyst surface chemistry are directly related to impurity components. Presence of NH₃ in the simulated flue gas inhibited o-xylene oxidation by reducing Mn and lowering Brønsted acidity of the catalyst. Impurity components associated with pollutant removal processes (Hg⁰ oxidation and selective catalytic reduction of NO) lowered o-xylene removal efficiency. Presence of o-xylene in the flue gas had little effect on the efficiency of pollutant removal processes. Layered catalytic beds located downstream from Hg⁰/NO pollutant removal processes are proposed to lower VOC emissions from coal-fired flue gases of thermal power plants in industrial settings.

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.

Superior activity of iron-manganese supported on kaolin for NO abatement at low temperature

A series of Fe-Mn catalysts was prepared using different supports (kaolin, diatomite, and alumina) and used for NO abatement via low-temperature NH₃-selective catalytic reduction (SCR). The results showed that 12Fe-10Mn/Kaolin (with the concentration of Fe and Mn 12 and 10wt.%, respectively) exhibited the highest activity, and more than 95.8% NO conversion could be obtained within the wide temperature range of 120-300°C. The properties of the catalysts were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), thermogravimetry (TG), Brunner-Emmet-Teller (BET) measurements, X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR), NH₃-temperature programmed desorption (NH₃-TPD), X-ray photoelectron spectroscopy (XPS), scanning electron microprobe (SEM) and energy dispersive spectroscopy (EDS) techniques. The support effects resulted in significant differences in the components and structures of catalysts. The 12Fe-10Mn/Kaolin catalyst exhibited better dispersion of active species, optimum low-temperature reduction behavior, the largest amount of normalized Brønsted acid sites, and the highest Mn⁴⁺/Mn and Fe³⁺/(Fe³⁺+Fe²⁺), all of which may be major reasons for its superior catalytic activity.

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.

Mn-CeOx/MeOx(Ti, Al)/cordierite preparation with ultrasound-assisted for non-methane hydrocarbon removal from cooking oil fumes

Cooking oil fumes (COFs) which contains a variety of volatile organic compounds (VOCs) is noxious not only to the environment but also to human health. Among COFs, the Non-methane Hydrocarbon (NMHC) removal rate is an index of the removal rate of COFs in the latest COFs purification standard (DB 11/1488-2018). Ultrasonic treatment can assist the synthesis of catalysts by creating "cavitation", which can improve the surface microtopography of catalysts. The research results in this paper revealed that the NMHC conversion of catalysts with ultrasonic treatment increased significantly. Besides, the samples that coated TiO₂ had a higher conversion of NMHC than that coated Al₂O₃ because TiO₂ has better oxidation performance than Al₂O₃. According to the XPS, SEM, EDS and BET data, because of the enhancement of interaction of the active components of catalysts caused by ultrasonic intervention, T-UU-CTAB exhibited the best catalytic performance, which attributed to the high levels of Mn⁴⁺/Mn³⁺, Ce⁴⁺/Ce³⁺ and O_ads/O_latt, more developed pore structure owing to the smaller particle size, higher dispersion of active components, larger specific surface area and larger total pore volume produced by the ultrasonic treatment. Moreover, the conversion of NMHC over T-UU-CTAB is 93.6% at 400 °C.

Novel Homogeneous and Mesoporous MnOx-Doped Ceria Nanosheets as Catalysts for Low-Temperature Selective Catalytic Reduction

The development of a catalyst for the selective catalytic reduction (SCR) of NOx is essential for purifying air and the denitration of coal-burning exhaust. Herein, we prepare novel MnOx-CeO2 nanosheets with porous structures by a homogeneous coordination precipitation (HCP) method which exhibit a high NO removal efficiency above 90% in the SCR reaction at low temperature (150–240°C). The MnOx-CeO2(HCP) catalysts have a higher Brunauer–Emmett–Teller (BET) surface area and more homogeneous distribution of Mnx+ in the CeO2 lattice than those prepared by co-precipitation and precursor mixture combustion methods according to BET, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. Together with a higher ratio of Mn4+, Ce3+, and Oα, the above properties are responsible for the high catalytic performances of MnOx-CeO2(HCP) in the SCR of NOx.

Promoted dispersion and uniformity of active species on Fe–Ce–Al catalysts for efficient NO abatement

Fe–Ce–Al catalysts were synthesized by the co-precipitation method (labeled as Fe–Ce–Al–P), co-impregnation method (Fe–Ce–Al–I), and direct mixing method (Fe–Ce–Al–M), respectively, and used for effective removal of NO. The synthesized catalysts were characterized by many methods including N₂ physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NH₃-temperature programmed desorption (NH₃-TPD), H₂-temperature programmed reduction (H₂-TPR), high-resolution transmission electron microscopy (HR-TEM), and energy dispersive spectroscopy (EDS) mapping. The results show that the synthesis methods greatly influence the catalytic performance of catalysts. The Fe–Ce–Al–P catalyst prepared by the co-precipitation method yields the highest catalytic performance, while the Fe–Ce–Al–I and Fe–Ce–Al–M catalysts exhibit relatively low catalytic activity. The co-precipitation method can promote the accumulation and dispersion of more surface active species on the catalyst surface, and provide smaller particle size of active species and generate more uniform particle size distribution, while these characteristics can't be obtained by the co-impregnation method and direct mixing method. Moreover, the co-precipitation method could produce the highest surface area and enhanced redox ability and surface acidity of the catalyst, which resulted from the high dispersion and uniform distribution of surface active species. These may be the key factors to the superior catalytic performance of the Fe–Ce–Al–P catalyst.

Fe–Ce–Al catalysts were synthesized by the co-precipitation method (labeled as Fe–Ce–Al–P), co-impregnation method (Fe–Ce–Al–I), and direct mixing method (Fe–Ce–Al–M), respectively, and used for effective removal of NO.

High N2 selectivity in selective catalytic reduction of NO with NH3 over Mn/Ti–Zr catalysts

A series of Mn-based catalysts were prepared by a wet impregnation method for the selective catalytic reduction (SCR) of NO with NH₃. The Mn/Ti–Zr catalyst had more surface area, Lewis acid sites, and Mn⁴⁺ on its surface, and showed excellent activity and high N₂ selectivity in a wide temperature range. NH₃ and NO oxidation was investigated to gain insight into NO reduction and N₂O formation. The formation of N₂O was primarily dominated by the reaction of NO with NH₃ in the presence of O₂via the Eley–Rideal mechanism. An intimate synergistic effect existed between the Mn-based and the Ti–Zr support. It was demonstrated that the Ti–Zr support greatly promoted the catalytic performance of Mn-based catalysts.

The Ti–Zr support greatly promotes the catalytic performance of Mn-based catalysts.

A review of Mn-containing oxide catalysts for low temperature selective catalytic reduction of NOx with NH3: reaction mechanism and catalyst deactivation

The reactions over Mn-containing selective catalytic reduction (SCR) catalysts.

A comparative study of manganese–cerium doped metal–organic frameworks prepared via impregnation and in situ methods in the selective catalytic reduction of NO

Impregnation and in situ doping were applied for the preparation of MnCe loaded MOFs and the behavior of the catalysts in the SCR was investigated.

Simultaneous removal of elemental mercury and NO from simulated flue gas using a CeO2 modified V2O5–WO3/TiO2 catalyst

The redox cycle (V⁴⁺ + Ce⁴⁺ ↔ V⁵⁺ + Ce³⁺) over V₂O₅–WO₃/TiO₂–CeO₂ plays a key role in Hg⁰ oxidation and NO conversion.

Oxidation of elemental mercury by modified spent TiO2-based SCR-DeNOx catalysts in simulated coal-fired flue gas

In order to reduce the costs, the recycle of spent TiO2-based SCR-DeNOx catalysts were employed as a potential catalytic support material for elemental mercury (Hg(0)) oxidation in simulated coal-fired flue gas. The catalytic mechanism for simultaneous removal of Hg(0) and NO was also investigated. The catalysts were characterized by Brunauer-Emmett-Teller (BET), scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) method. Results indicated that spent TiO2-based SCR-DeNOx catalyst supported Ce-Mn mixed oxides catalyst (CeMn/SCR1) was highly active for Hg(0) oxidation at low temperatures. The Ce1.00Mn/SCR1 performed the best catalytic activities, and approximately 92.80% mercury oxidation efficiency was obtained at 150 °C. The inhibition effect of NH3 on Hg(0) oxidation was confirmed in that NH3 consumed the surface oxygen. Moreover, H2O inhibited Hg(0) oxidation while SO2 had a promotional effect with the aid of O2. The XPS results illustrated that the surface oxygen was responsible for Hg(0) oxidation and NO conversion. Besides, the Hg(0) oxidation and NO conversion were thought to be aided by synergistic effect between the manganese and cerium oxides.

Simultaneous Removal of NO and Hg(0) from Flue Gas over Mn-Ce/Ti-PILCs

A series of Mn-Ce/Ti-PILCs (PILCs, pillared interlayered clays) catalysts were prepared via impregnation method in simultaneous removal of NO and elemental mercury in simulated flue gas. The physicochemical properties of these catalysts have been examined by some characterization methods, such as H2-TPR, nitrogen adsorption, XRD and XPS. Mn(6%)-Ce(6%)/Ti-PILCs exhibited superior NO conversion (>95%) and Hg(0) removal efficiency (>90%) at low temperature (250 °C). The results indicated that the elemental mercury had little impact on NO removal efficiency, while the presence of NH3 and NO in SCR system inhibited the Hg(0) removal. NO and Hg(0) removal activity was strongly affected by the transform between surface adsorbed oxygen and lattice oxygen. The species ratio of Mn(4+)/Mn(3+) and Ce(4+)/Ce(3+) on the catalyst surface contributed to the NO conversions and Hg(0) removal. Mn-Ce/Ti-PILCs displayed a broad prospect for controlling the emission of NO and mercury. On the basis of the results obtained, a mechanism for the simultaneous removal of NO and Hg(0) was proposed for the Mn-Ce/Ti-PILCs catalysts: -NH2 + NO → N2 + H2O, -OH + 1/2 Hg(ad) →1/2 HgO + 1/2 H2O.

Promoting effect of Nd on the reduction of NO with NH3 over CeO2 supported by activated semi-coke: an in situ DRIFTS study

The morphology and NH₃-SCR performance of CeO₂-Nd/ASC catalysts.

Transition metal ions regulate the catalytic performance of Ti0.8M0.2Ce0.2O2+x for the NH3-SCR of NO: the acidic mechanism

The maximum activity is dependent on the number of Lewis acid sites, and the active temperature window is dependent on the acid strength.

Adsorption and oxidation of elemental mercury over Ce-MnOx/Ti-PILCs

A series of innovative Ce-Mn/Ti-pillared-clay (Ce-Mn/Ti-PILC) catalysts combining the advantages of PILCs and Ce-Mn were investigated for elemental mercury (Hg0) capture at 100-350 °C in the absence of HCl in the flue gas. The fresh and used catalysts were characterized by scanning electron microscopy (SEM), nitrogen adsorption-desorption, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The catalyst characterization indicated that the 6%Ce-6%MnOx/Ti-PILC catalyst possessed a large specific surface area and high dispersion of Ce and Mn on the surface. The experimental results indicated that the 6%Ce-6%MnOx/Ti-PILC catalyst exhibited high Hg0 capture (>90%) at 100-350 °C. During the first stage of the reaction, the main Hg0 capture mechanism for the catalyst was adsorption. As the reaction proceeded, the Hg0 oxidation ability was substantially enhanced. Both the hydroxyl oxygen and the lattice oxygen on the surface of the catalysts participated in Hg0 oxidation. At a low temperature (150 °C), the hydroxyl oxygen and lattice oxygen from Ce4+→Ce3+ and Mn3+→Mn2+ on the surface contributed to Hg0 oxidation. However, at a high temperature (250 °C), the hydroxyl oxygen and lattice oxygen from Mn4+→Mn3+ contributed to Hg0 oxidation. Hg0 oxidation was preferred at a high temperature. The 6%Ce-6%MnOx/Ti-PILC catalyst was demonstrated to a good Hg0 adsorbent and catalytic oxidant in the absence of HCl in the flue gas.