NO oxidation over Fe-based catalysts supported on montmorillonite K10, γ-alumina and ZSM-5 with gas-phase H2O2

Fe-ZSM-5 zeolite catalyst for heterogeneous Fenton oxidation of 1,4-dioxane: effect of Si/Al ratios and contributions of reactive oxygen species

Heterogeneous Fenton oxidation using traditional catalysts with H2O2 for the degradation of 1,4-dioxane (1,4-DX) still presents challenge. In this study, we explored the potential of Fe-ZSM-5 zeolites (Fe-zeolite) with three Si/Al ratios (25, 100, 300) as heterogeneous Fenton catalysts for the removal of 1,4-DX from aqueous solution. Fe2O3 or ZSM-5 alone provided ineffective in degrading 1,4-DX when combined with H2O2. However, the efficient removal of 1,4-DX using H2O2 was observed when Fe2O3 was loaded on ZSM-5. Notably, the Brønsted acid sites of Fe-zeolite played a crucial role during the degradation of 1,4-DX. Fe-zeolites, in combination with H2O2, effectively removed 1,4-DX via a combination of adsorption and oxidation. Initially, Fe-zeolites demonstrated excellent affinity for 1,4-DX, achieving adsorption equilibrium rapidly in about 10 min, followed by effective catalytic oxidative degradation. Among the Fe-ZSM-5 catalysts, Fe-ZSM-5 (25) exhibited the highest catalytic activity and degraded 1,4-DX the fastest. We identified hydroxyl radicals (·OH) and singlet oxygen (1O2) as the primary reactive oxygen species (ROS) responsible for 1,4-DX degradation, with superoxide anions (HO2·/O2·-) mainly converting into 1O2 and ·OH. The degradation primarily occurred at the Fe-zeolite interface, with the degradation rate constants proportional to the amount of Brønsted acid sites on the Fe-zeolite. Fe-zeolites were effective over a wide working pH range, with alkaline pH conditions favoring 1,4-DX degradation. Overall, our study provides valuable insights into the selection of suitable catalysts for effective removal of 1,4-DX using a heterogeneous Fenton technology.

Preparation, Characterization, and Magnetic Resonance Imaging of Mn@SiO2 Nanowires

The deposition time was controlled to prepare Mn nanowires of different lengths and diameters on templates of anodic aluminum oxide (AAO) with different pore sizes. The surface of as-prepared Mn nanowires was modified with SiO2 using the sol-gel method to improve their dispersion in aqueous solution. The effects of the diameter and length of the as-prepared Mn nanowires coated with SiO2 on the relaxivity were investigated. It was found that the Mn@SiO2 nanowires have smaller diameters and a higher longitudinal relaxivity (r1) with an increased length. Mn3@SiO2 nanowires had the highest r1 value of 5.8 mM-1 s-1 among the Mn@SiO2 nanowires (Mn3 nanowires have a diameter of about 30 nm and a length of about 0.5 μm length). Additionally, the biocompatibility and in vivo imaging ability of the Mn3@SiO2 nanowires were evaluated. The Mn3@SiO2 nanowires had good cytotoxicity and biocompatibility, and the kidney of SD rats showed a positive enhancement effect during small animal imaging at 1.5 T. This study showed that the Mn3@SiO2 nanowires could potentially become contrast agents (CAs) of longitudinal relaxation time (T1).

Enhanced gaseous acetone adsorption on montmorillonite by ball milling generated Si-OH and interlayer under synergistic modification with H2O2 and tetramethylammonium bromide

Montmorillonite (Mt) is a potential adsorbent for volatile organic vapor removal from contaminated soils because of its rich reserves and porous nature, but its inertia surface property has limited its application for polar compounds. In this study, modifications of Mt were carried out by high energy ball milling with H₂O₂ and tetramethylammonium bromide (TMAB) to obtain adsorbents with both high porosity and abundant Si-OH groups (BHTMt). The microporous structure produced by TMAB insertion as well as the silanol (Si-OH) groups formed by H₂O₂ oxidation improved the adsorption of acetone by the modified material. The adsorption capacity of BHTMt for acetone was increased by 80% compared to the original Mt. The effect of H₂O₂ dosage on the adsorption performance for gaseous acetone was investigated by dynamic adsorption experiments. The adsorption kinetic results demonstrated that the adsorption of acetone by the modified material was subject to both physical and chemical adsorption. Density functional theory calculations indicated that there was no obvious interaction between TMAB and acetone, and the materials adsorbed acetone mainly through hydrogen bonding interaction of Si-OH as well as pore filling effects.

Oxidation-absorption process for simultaneous removal of NOx and SO2 over Fe/Al2O3@SiO2 using vaporized H2O2

3% Fe/Al₂O₃ and 3% Fe/Al₂O₃@SiO₂ were prepared to investigate the performance in simultaneous removal of NO_x and SO₂ using vaporized H₂O₂. Certain paraments were changed to explore the activity of catalysts, including temperature, H₂O₂ concentration, GHSV and coexistence gases component. A 24-h durability test was conducted on 3% Fe/Al₂O₃@SiO₂. Moreover, a series of characterizations were employed to analyze the physical and chemical properties of catalysts, including XRD, BET, SEM, TEM, FTIR and XPS. Compared with 3% Fe/Al₂O₃, 3% Fe/Al₂O₃@SiO₂ exhibited more excellent catalytic activity, which could achieve the peak removal efficiency of 100% for SO₂ and 93.76% for NO_x. Moreover, 3% Fe/Al₂O₃@SiO₂ kept stable simultaneous removal efficiency in a 24-h test. The characterization results indicated that the BET area was greatly improved and the core-shell structure was synthesized with the formation of more micropores and mesopores by the coating of SiO₂, which could improve the activity of catalyst at high temperature and high SO₂ concentration. Besides, the mechanism of SO₂ molecules on simultaneous removal was investigated. On one hand, a part of H₂O₂ was consumed by SO₂ molecules without catalyst, which resulted in the drop of NO_x removal by the decrease of oxidants. The main products were sulfites and bisulfites, which were broken down into SO₂ over the catalyst. On the other hand, the presence of SO₂ was beneficial for NO_x removal by increasing oxygen vacancies on the catalyst surface and facilitating the absorption of NO₂ by NaOH solution.

Mn-based catalysts supported on γ-Al2O3, TiO2 and MCM-41: a comparison for low-temperature NO oxidation with low ratio of O3/NO

Mn-Based catalysts supported on γ-Al2O3, TiO2 and MCM-41 synthesized by an impregnation method were compared to evaluate their NO catalytic oxidation performance with low ratio O3/NO at low temperature (80-200 °C). Activity tests showed that the participation of O3 remarkably promoted the NO oxidation. The catalytic oxidation performance of the three catalysts decreased in the following order: Mn/γ-Al2O3 > Mn/TiO2 > Mn/MCM-41, indicating that Mn/γ-Al2O3 exhibited the best catalytic activity. In addition, there was a clear synergistic effect between Mn/γ-Al2O3 and O3, followed by Mn/TiO2 and O3. The characterization results of XRD, EDS mapping, BET, H2-TPR, XPS and TG showed that Mn/γ-Al2O3 had good manganese dispersion, excellent redox properties, appropriate amounts of coexisting Mn3+ and Mn4+ and abundant chemically adsorbed oxygen, which ensured its good performance. In situ DRIFTS demonstrated the NO adsorption performance on the catalyst surface. As revealed by in situ DRIFTS experiments, the chemically adsorbed oxygen, mainly from the decomposition of O3, greatly promoted the NO adsorption and the formation of nitrates. The Mn-based catalysts showed stronger adsorption strength than the corresponding pure supports. Due to the abundant adsorption sites provided by pure γ-Al2O3, under the interaction of Mn and γ-Al2O3, the Mn/γ-Al2O3 catalyst exhibited the strongest NO adsorption performance among the three catalysts and produced lots of monodentate nitrates (-O-NO2) and bidentate nitrates (-O2NO), which were the vital intermediate species for NO2 formation. Moreover, the NO-TPD studies also demonstrated that Mn/γ-Al2O3 showed the best NO desorption performance among the three catalysts. The good NO adsorption and desorption characteristics of Mn/γ-Al2O3 improved its high catalytic activity. In addition, the activity test results also suggested that Mn/γ-Al2O3 exhibited good SO2 tolerance.

Recent advances in selective catalytic oxidation of nitric oxide (NO-SCO) in emissions with excess oxygen: a review on catalysts and mechanisms

Nitric oxides (NO_x, which mainly include more than 90% NO) are one of the major air pollutants leading to a series of environmental problems, such as acid rain, haze, photochemical smog, etc. The selective catalytic oxidation of NO to NO₂ (NO-SCO) is regarded as a key process for the development of selective catalytic reduction of NO_x by ammonia (via fast selective catalytic reduction reaction) and also the simultaneous removal of multipollutant (pre-oxidation and post-absorption). Until now, scholars have developed various types of NO-SCO catalysts, dividing the main groups into noble metals (Pt, Pd, Ru, etc.), metal oxides (Mn-, Co-, Cr-, Ce-based, etc.), perovskite-type oxides (LaMnO₃, LaCoO₃, LaCeCoO₃, etc.), carbon materials (activated carbon, carbon fiber, carbon nanotube, graphene, etc.), and zeolites (ion-exchanged ZSM-5, CHA, SAPO, MCM-41, etc.) in this review. This paper summarizes the recent progress of the above typical catalysts and mostly analyzes the catalytic performance for NO oxidation in terms of the H₂O and/or SO₂ resistances and also the influencing factors, and their reaction mechanisms are described in detail. Finally, this review points out the key problems and possible solutions of the current researches and presents the application prospects and future development directions of NO-SCO technology using the above typical catalysts.

The roles of Brønsted acidity in low-temperature catalytic oxidation of NO over acidic zeolites with H2O2

In this study, low-temperature catalytic NO oxidation with H₂O₂ over Na- and H-exchanged Y and ZSM-5 zeolites was investigated at 140 °C which is the average exhaust temperature of coal-fired power plant. Fast catalytic NO oxidation rates were observed over H-zeolites, and catalytic activity was proportional to the amount of Brønsted acid sites. HZSM-5 and HY zeolites show 65% and 95% NO removal efficiency, respectively, but the catalytic stability of HY was lower than HZM-5 due to partial dealumination during the reaction. In-situ DRIFTS analysis showed that NO⁺ species coordinated at framework sites played a direct role in the catalytic NO oxidation. Moreover, the possible reaction pathway was proposed to elucidate the mechanism of NO oxidation with H₂O₂ catalyzed over Brønsted acid sites. The effect of reaction temperature, H₂O₂ concentration, H₂O₂ flow and SO₂ concentration on NO oxidation were investigated over H-zeolites. The experimental results indicated that the NO removal efficiency was increased with the increase of H₂O₂ concentration, but decreased with the increase of SO₂ concentration. The NO removal efficiency first increased and then decreased with the increase of H₂O₂ flow and reaction temperature.

Simultaneous removal of NOx and SO2 with H2O2 over silica sulfuric acid catalyst synthesized from fly ash

Considering that the utilization of fly ash in the removal of flue gas pollutants not only provide a way of high value-added utilization of fly ash, but also greatly reduce the cost of removing flue gas pollutant, the synthesis of silica sulfuric acid catalyst from fly ash and its application in simultaneous removal of NOx and SO₂ with H₂O₂ were investigated in this work. Circulating fluidized bed boiler (CFB) fly ash and pulverized coal boiler (PC) fly ash were selected as raw material to prepare silica sulfuric acid catalyst by H₂SO₄ activation. PC fly ash was difficult to be activated by H₂SO₄ due to its dense structure, while CFB fly ash could be treated with H₂SO₄ to promote dealumination, thereby increasing the silica content. Moreover, the -SO₃H withdrawing groups were detected on the silica surface by XPS and Py-FTIR technologies, indicating the formation of silica sulfuric acid. Silica sulfuric acid showed higher activity in catalyzing the NO oxidation by H₂O₂, and a possible reaction mechanism was proposed. Combined with alkali absorption, 99% SO₂ and 92% NOx removal efficiencies can be achieved. The effects of activation conditions such as activation temperature, activation time and calcination temperature and removal experimental parameters such as H₂O₂ concentration, SO₂ concentration and simulated flue gas temperature on the catalytic performance were studied. Finally, the catalyst was not found to be deactivated for ten hours in the stability test.

Insight into structure defects and catalytic mechanism for NO oxidation over Ce0.6Mn0.4Ox solid solutions catalysts: Effect of manganese precursors

The effects of Mn precursors on structure defects and NO catalytic mechanism over Ce_0·6Mn_0.4O_x catalysts were fully investigated. The Ce_0·6Mn_0.4O_x-Ac catalyst, synthesized by using MnAc₂ as a Mn precursor, showed the best catalytic activity for NO conversion (86.9%) at 250 °C under high space velocity (40,000 mL g^-1 h^-1). Detailed structure-activity relationship reveals that the abundant oxygen vacancies and the highly migratory oxygen species formed on Ce_0·6Mn_0.4O_x are the crucial factors that leading to the better NO oxidation activity than that of the other Ce_0·6Mn_0.4O_x-Y (YNO₃, SO₄, Cl) catalysts. In situ DRIFTS technique confirms that the differences in formation mode and desorption ability of N-based (nitrates, nitrites, and dimer nitroso) intermediate species are the vital factors for NO high-efficiency catalytic oxidation. The highly reactive surface intermediate species, like monodentate nitrates, were observed particularly on Ce_0·6Mn_0.4O_x-Ac catalyst, so that the NO oxidation performance on Ce_0·6Mn_0.4O_x-Ac catalyst was more active comparing with other Ce_0·6Mn_0.4O_x-Y catalysts. This study can broaden the horizons for understanding NO catalytic oxidation mechanism on serial Ce_0·6Mn_0.4O_x catalysts and serve as a reference guide in design of structure defects for functional materials by modulating precursor species.

Reduction of FeII(EDTA)-NO by Mn powder in wet flue gas denitrification technology: stoichiometry, kinetics, and thermodynamics

Conversion of Fe^II(EDTA)-NO or Fe^III(EDTA) into Fe^II(EDTA) is a key process in a wet flue gas denitrification technology with Fe^II(EDTA) solution. In this work, the stoichiometry, kinetics, and thermodynamics of Fe^II(EDTA)-NO reduction by Mn powder were investigated. We first studied the Fe^II(EDTA)-NO reduction and product distribution to speculate a possible stoichiometry of Fe^II(EDTA)-NO reduction by Mn powder. Then, the effects of major influencing factors, such as pH value, temperature, and Mn concentration, were studied. The pseudo-second-order model was established to describe the Fe^II(EDTA)-NO reduction. Simultaneously, according to Arrhenius and Eyring-Polanyi equations, the reaction activation energy (E_a), enthalpy of activation (∆H^‡), and entropy of activation (∆S^‡) were calculated as 23.68 kJ/mol, 21.148 kJ/mol, and - 149.728 J/(k mol), respectively. Additionally, simultaneous reduction of Fe^III(EDTA) and Fe^II(EDTA)-NO was investigated to better study the mechanism of Fe^II(EDTA) regeneration, suggesting that there was a competition between the two reduction processes. Finally, a simple schematic mechanism of NO absorption by Fe^II(EDTA) combined with regeneration of manganese ion and ammonium was proposed. These fundamental researches could offer a valuable guidance for wet flue gas denitrification technology with Fe^II(EDTA) solution.