Capture gaseous arsenic in flue gas by amorphous iron manganese oxides with high SO2 resistance

In non-ferrous metal smelting, the problem of gaseous arsenic in high-sulfur flue gas is difficult to solve. Now we have developed oxygen-enriched amorphous iron manganese oxide (AFMBO) based on the unique superiority of iron-manganese oxide for arsenic capture to realize the effective control of gaseous arsenic in the non-ferrous smelting flue gas. The experimental results show that the arsenic adsorption capacity of AFMBO is up to 102.7 mg/g, which has surpassed most of the current adsorbents. In particular, AFMBO can effectively capture gaseous arsenic even at 12% v/v SO2 concentrations (88.45 mg/g). Moreover, the spent AFMBO possesses pronounced magnetic characteristics that make it easier to separate from dust, which is conducive to reducing the secondary environmental risk of arsenic. In terms of mechanism study, various characterization methods are used to explain the important role of lattice oxygen and adsorbed oxygen in the capture process of gaseous arsenic. Moreover, the reason for the efficient arsenic removal performance of AFMBO is also reasonably explained at the microscopic level. This study provides ideas and implications for gaseous arsenic pollution control research.

Promotional effects of Nb5+ and Fe3+ co-doping on catalytic performance and SO2 resistance of MnOx-CeO2 low-temperature denitration catalyst

As we know, SO₂ can cause MnO_x-CeO₂ (MnCeO_x) catalyst poisoning, which seriously shortens the service life of the catalyst. Therefore, to enhance the catalytic activity and SO₂ tolerance of MnCeO_x catalyst, we modified it by Nb⁵⁺ and Fe³⁺ co-doping. And the physical and chemical properties were characterized. These results illustrate that the Nb⁵⁺ and Fe³⁺ co-doping can optimally improve the denitration activity and N₂ selectivity of MnCeO_x catalyst at low temperature by improving its surface acidity, surface adsorbed oxygen as well as electronic interaction. What's more, NbO_x-FeO_x-MnO_x-CeO₂ (NbFeMnCeO_x) catalyst possesses an excellent SO₂ resistance due to less SO₂ being adsorbed and the ammonium bisulfate (ABS) formed on its surface tends to decompose, as well as fewer sulfate species formed on its surface. Finally, the possible mechanism that Nb⁵⁺ and Fe³⁺ co-doping enhances the SO₂ poisoning resistance of MnCeO_x catalyst is proposed.

A novel strategy for efficient utilization of manganese tailings: High SO2 resistance SCR catalyst preparation and faujasite zeolite synthesis

Herein, a core-shell structure SiO₂@Mn catalyst was successfully synthesized from manganese tailings for NO removal through selective catalytic reduction with ammonia at low temperature. In the presence of 10 % H₂O and 100 ppm SO₂, the SiO₂@Mn catalyst exhibited excellent catalytic activity of 85 % NO conversion at 225 °C. This special core-shell was constructed by inert nano-SiO₂ particles, which grew and encapsulated on the surface of manganese oxide, inhibiting the reaction between SO₂ with MnOx and thus improving the SO₂ resistance. Furthermore, a filter residue was generated in the process of catalysts preparation. Under proper hydrothermal conditions, the residue comprising of Si, Al, and O was used to synthesize high-crystallinity X-type zeolite. This process fully utilized manganese tailings and inspired for Mn-based catalysts design and X-type zeolite preparation, realizing the dual benefits of atmosphere purification and solid waste disposal.

Effecting pattern study of SO2 on Hg0 removal over α-MnO2 in-situ supported magnetic composite

α-MnO₂ was in-situ supported onto silica coated magnetite nanoparticles (MagS-Mn) to study the adsorption and oxidation of Hg⁰ as well as the effecting patterns of SO₂ and O₂ on Hg⁰ removal. MagS-Mn showed Hg⁰ removal capacity of 1122.6 μg/g at 150 °C with the presence of SO₂. Hg⁰ adsorption and oxidation efficiencies were 2.4% and 90.6%, respectively. Hg⁰ removal capability deteriorated at elevated temperatures. Surface oxygen and manganese chemistry analysis indicated that SO₂ inhibited the Hg⁰ removal through consumption of adsorbed oxygen and reduction of high valence manganese. This inhibiting effect was observed to be counteracted by O₂ at lower temperatures. O₂ tended to compete with SO₂ for active sites and further create additional adsorbed oxygen sites for Hg⁰ surface reaction via surface dissociative adsorption rather than replenish the active sites consumed by SO₂. The high valence manganese was also preserved by O₂ which was essential to Hg⁰ oxidation. The intervention of O₂ in the inhibition of SO₂ on Hg⁰ removal was weakened at temperatures higher than 250 °C. Aa a result, Hg⁰ tends to be catalytic oxidized in the condition of low reaction temperatures and with the presence of O₂ over α-MnO₂ oriented composites.

Novel synthesis of reed flower-like SmMnOx catalyst with enhanced low-temperature activity and SO2 resistance for NH3-SCR

In this work, a novel co-precipitation coupled solvothermal procedure is proposed to prepare a SmMnO_x catalyst (SmMnOx-CP + ST) with a reed flower-like structure for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). Over 90% NO_x conversion and N₂ selectivity was achieved at a low temperature range (25-200 °C), and 96% NO_x conversion was achieved in the presence of 100 ppm SO₂ at 75 °C. While the NH₃-SCR of the SmMnO_x catalysts prepared by co-precipitation (SmMnO_x-CP) and solvothermal (SmMnO_x-ST) methods performed much poorer than the SmMnO_x-CP + ST catalyst. All catalysts were characterized by XRD, BET, SEM, XPS, H₂-TPR, NH₃-TPD, NO_x-TPD, and FT-IR. The results revealed that the superior performance of the SmMnO_x-CP + ST is due to the unique reed flower-like structure morphology, which endows the SmMnO_x-CP + ST with the largest surface area, the strongest synergistic reaction of Sm and Mn, abundant surface oxygen species and surface active sites, and significantly enhances the redox ability. Furthermore, the amorphous reed flower-like structure showed strong short-range ordered interaction between the active components and weaken the formation of sulfates species. In addition, the highest content of Mn⁴⁺ and Mn³⁺+Mn⁴⁺ greatly promotes the redox cycles of Sm²⁺↔Mn⁴⁺ and Sm²⁺↔Mn³⁺, and suppresses the production of sulfate species in the presence of SO₂.

Brønsted acid enhanced hexagonal cerium phosphate for the selective catalytic reduction of NO with NH3: In situ DRIFTS and DFT investigation

The possible effect of optimized acid sites on NH₃-SCR performance and the fundamental mechanism are barely illustrated. In this work, we report two model catalysts of hexagonal (h-CPO) and monoclinic (m-CPO) cerium phosphate with disparate acidity that show different NH₃-SCR activities under the same reaction conditions. Brønsted acid sites were found to be crucial for NH₃-SCR performance at both low and high temperature. The electron localization discrepancy of h-CPO was more pronounced as compared with m-CPO, leading to the enrichment of P-OH (Brønsted acid site) which could strongly absorb NH₃ and then generate NH₄⁺ to participate in fast SCR via Langmuir-Hinshelwood mechanism, resulting in good activity at low temperature. The zeolitic water stored in the open channels of h-CPO could be released as supplement for P-OH sites which prevent the depletion and non-selective oxidation of NH₃ thus maintaining its high activity at high temperature via the Eley-Rideal mechanism. Meanwhile, as DFT calculation revealed, cerium is the electron deficient center which can easily fix NO and NO₂ from the intake, generating active NO_2(ad) or nitrites and facilitating fast SCR by reacting with NH₄⁺ species. Hence, the superior protonation ability to form P-OH and low energy barrier to generate active nitrites of h-CPO led its T₈₀ NO_x conversion to a broaden temperature of 150-450 ^oC under high GHSV of 177,000 h^-1. Furthermore, experimental and DFT calculation also demonstrated that the enriched Brønsted acid sites over h-CPO have largely suppressed SO₂ adsorption_, thus significantly reducing the formation of metal sulfates and achieving great SO₂ resistance. The ammonium sulfate deposits can be storage of NH₃, supplying additional reductant to promote high temperature activity and selectivity.

Excellent adsorption performance and capacity of modified layered ITQ-2 zeolites for elemental mercury removal and recycling from flue gas

Adsorption is a superior method for removing and recycling high concentration of mercury from nonferrous metal smelting flue gas, especially adsorbents with good sulfur resistance and large adsorption capacity. In this study, Co and Mn oxide-modified layered ITQ-2 zeolites were designed to capture and recycle elemental mercury (Hg⁰). The physicochemical characteristics of the adsorbents were characterized using BET, XRD, FESEM, TEM, and XPS, and the results showed that Mn/ITQ-2 zeolite has a large specific surface area, and MnO_x was highly dispersed on ITQ-2 zeolite. The Hg⁰ removal efficiency and adsorption capacity of the 5%Mn/ITQ-2 zeolite at 300 °C were 97% and 2.04 mg/g in 600 min, respectively, much higher than those of the previously reported 5%Mn/MCM-22 zeolite. The 2%Co-2%Mn/ITQ-2 zeolite exhibited a higher SO₂ resistance performance. The mechanism of Hg⁰ removal was concluded to be driven by the primary catalytic oxidation of MnO_x, secondary oxidation of active chlorine, and concurrent chemisorption. However, the Hg⁰ adsorption capacity was determined by the specific surface area and pore structure of ITQ-2. The 2%Co-2%Mn/ITQ-2 zeolite exhibited a high SO₂ resistance performance. The Mn/ITQ-2 and Co-Mn/ITQ-2 zeolites have excellent regenerability and reusability, which can realize mercury recycling from flue gas.

Acid modification enhances selective catalytic reduction activity and sulfur dioxide resistance of manganese-cerium-cobalt catalysts: Insight into the role of phosphotungstic acid

Improving the SO₂ resistance of catalysts is crucial to driving commercial applications of Mn-based catalysts. In this work, the phosphotungstic acid (HPW) modification strategy was applied to improve the N₂ selectivity, SO₂ and H₂O resistance of the Mn-Ce-Co catalyst, and further, the mechanism of HWP modification on enhanced catalytic performance was explored. The results showed that HPW-Mn-Ce-Co catalyst exhibits higher NO_x conversion (~100% at 100-250 °C) and N₂ selectivity (exceed 80% at 50-350 °C) due to more oxygen vacancies, greater surface acidity, and lower redox capacity. In situ diffused reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) reveal that HPW changed the reaction path of Mn-Ce-Co catalysts, promoted the adsorption and activation of NH₃, and reduced the effect of SO₂ on the active bidentate nitrate species, and thereby exhibiting good SO₂ resistance. X-ray photoelectron spectrometer (XPS) and NH₃ temperature-programmed desorption of (NH₃-TPD) results show that HPW can inhibit the formation of metal sulfate, and SO₂ can be combined with Ce species more easily. The generated Ce₂(SO₃)₃ can not only protect Mn species but also increase the acid sites and weaken the poisoning effect of metal sulfate. This study provides a simple design strategy for the catalyst to improve the low-temperature catalytic performance and toxicity resistance.

A MnOx@Eu-CeOx nanorod catalyst with multiple protective effects: Strong SO2-tolerance for low temperature DeNOx processes

A novel MnO_x@Eu-CeO_x catalyst with multiple protective attributes was designed and fabricated using a chemical precipitation method and tested for its low temperature SCR activity. The subject MnO_x@Eu-CeO_x nanorod catalyst exhibited superior SCR performance and strong SO₂-tolerance. The formation of the composite-shell structure enhanced the catalysts' surface acidity and redox performance, which resulted in excellent SCR performance. Moreover, the TG results suggested that the protective effect of the EuO_x-CeO_x composite-shell effectively reduced the deposition of the surface sulphates. The XPS, XRD analysis results of the subject catalyst together with theoretical calculations provided strong evidence that there was a strong interaction between Mn and Ce in the MnO_x@Eu-CeO_x. This significant interaction could provide maximum protection to the core from the effect of SO₂, which also contributed to the high SO₂ resistance of the catalyst. In situ FT-IR results also indicated that the chemisorbed species on MnO_x@Eu-CeO_x were much more stable in the presence of SO₂ compared to Eu-CeO_x/MnO_x, which resulted in the deposition of significantly less sulphates. This low temperature SCR catalyst with multiple protective attributes, including composite shell, strong interaction and core-shell structure, is the key to long-term resistance to SO₂.

Insight into the interfacial stability and reaction mechanism between gaseous mercury and chalcogen-based sorbents in SO2-containing flue gas

Chalcogen-based materials have been confirmed to possess large adsorption capacities for gaseous elemental mercury (Hg⁰) from SO₂-containing flue gas. However, the interface reaction mechanisms and the interfacial stability are still ambiguous. Here, we selected some commonly used chalcogen-based sorbents (e.g., X, ZnX, CuX. X = S, Se) to investigate the in-depth reaction mechanisms. The adsorption capacities, structure effect on thermal and surface mercury stability, and interfacial reaction mechanism in the absence/presence of SO₂ were evaluated. The experimental results indicated that Cu-chalcogenide had higher Hg⁰ adsorption capacity and surface Hg-X bonding stability compared with zinc one, while they exhibited an opposite degree of thermal stability. Moreover, all the chalcogenides showed well SO₂ tolerance but with a slight difference. Chalcogenides with the same crystal structures, like ZnX or CuX, exhibited similar properties in stability and interfacial Hg⁰ and SO₂ reaction mechanism. X^- in chalcogenides have a better affinity to mercury, while in the Hg⁰ capture process, the existence of multivalent metal elements (like Cu²⁺ and Cu⁺) can faster the Hg⁰ oxidation for the further chemical-adsorption. This work provides a basic understanding of the application for efficiently enriching and recycling gaseous Hg⁰ from industrial SO₂-containing flue gas.

A superior Fe-V-Ti catalyst with high activity and SO2 resistance for the selective catalytic reduction of NOx with NH3

A series of Fe-V-Ti oxide catalysts were prepared by a co-precipitation method, among which the Fe_0.1V_0.1TiO_x catalyst showed the optimal NH₃-SCR performance and excellent SO₂ resistance. Fe_0.1V_0.1TiO_x achieved > 90% NO_x conversion at 225-450 °C under a GHSV of 200,000 h^-1. When introducing SO₂ and H₂O to the SCR reaction for 24 h, the NO_x conversion maintained a level above 93% at 250 °C. The Raman and Mössbauer spectra showed that FeVO₄ and Fe₂O₃ coexisted on the surface of TiO₂. In Fe-V-Ti catalysts, the charge interaction between Fe₂O₃ and FeVO₄ as well as the electronic inductive effect between Fe and V species resulted in the improvement of SCR activity and N₂ selectivity at high temperatures. The NH₃-SCR process on the Fe_0.1V_0.1TiO_x catalyst mainly followed the Eley-Rideal (E-R) reaction mechanism with gaseous NO reacting with adsorbed NH₃ adsorbed species.

Boosting the low-temperature activity and sulfur tolerance of CeZr2Ox catalysts by antimony addition for the selective catalytic reduction of NO with ammonia

In this paper, a series of Sb modified CeZr₂O_x mixed oxides (Sb_yCZ) were synthesized by citrate method for the selective catalytic reduction of NO with ammonia (NH₃-SCR). Experimental results exhibited that the Sb addition could bring a great improvement of SCR activity at 200-360 °C owing to the enhancement in surface area, redox ability and surface acidity. More importantly, the sulfur tolerance of the catalyst with proper Sb loading contents was dramatically improved. For instance, above 85% deNO_x efficiency was retained over Sb_0.5CZ catalyst after 24 h reaction in the presence of 100 ppm SO₂ and 5 vol.% H₂O. As for pure CeZr₂O_x and the catalysts with low Sb loading contents, the serious accumulation of ammonium sulfates resulted in the deactivation after SO₂ exposure. However, with excessive Sb addition, more labile oxygen readily reacted with SO₂ and the redox cycle was then disrupted, leading to the decrease of SCR activity. With an appropriate Sb loading contents, the sulfate species preferentially formed around Sb cations could restrain the further consumption of oxygen species in Ce-O-Ce or Ce-O-Zr mode by SO₂ via a space confinement effect. Thus, a certain amount of labile oxygen was preserved to drive the SCR reaction, thereby enhancing the sulfur tolerance of the catalyst.

Novel CeO2@TiO2 core-shell nanostructure catalyst for selective catalytic reduction of NOx with NH3

The CeO₂@TiO₂ core-shell nanostructure catalyst prepared by a two-step hydrothermal method was used for selective catalytic reduction (SCR) of NOx with NH₃ in this study. The catalyst presented the obvious core-shell structure, and the shell was amorphous TiO₂ which could protect the active center from the SO₂ erosion. The catalyst showed high activity and stability, excellent N₂ selectivity and superior SO₂ resistance and H₂O tolerance. Characterizations such as TEM, HR-TEM, XRD, BET, XPS, NH₃-TPD, and H₂-TPR were carried out. The results indicated that the catalyst had large surface area and the active sites were well dispersed on the surface. The NH₃-TPD, H₂-TPR and XPS results implied that its increased SCR activity might be due to the enhancement of NH₃ chemisorption and the increase of active oxygen species, both of which were conductive to NH₃ activation. The excellent catalytic performance suggests that it is a promising candidate for SCR catalyst.