In situ fabrication of highly active γ-MnO2/SmMnO3 catalyst for deep catalytic oxidation of gaseous benzene, ethylbenzene, toluene, and o-xylene

Weakening Mn-O Bond Strength in Mn-Based Perovskite Catalysts to Enhance Propane Catalytic Combustion

Exploring highly efficient and robust non-noble metal catalysts for VOC abatement is crucial but challenging. Mn-based perovskites are a class of redox catalysts with good thermal stability, but their activity in the catalytic combustion of light alkanes is insufficient. In this work, we modulated the Mn-O bond strength in a Mn-based perovskite via defect engineering, over which the catalytic activity of propane combustion was significantly enhanced. It demonstrates that the oxygen vacancy concentration and the Mn-O bond strength can be efficiently modulated by finely tuning the Ni content in SmNixMn1-xO3 perovskite catalysts (SNxM1-x), which in turn can enhance the redox ability and generate more active oxygen species. The SN0.10M0.90 catalyst with the lowest Mn-O bond strength exhibits the lowest apparent activation energy, over which the propane conversion rate increases by 3.6 times compared to that on the SmMnO3 perovskite catalyst (SM). In addition, a SN0.10M0.90/cordierite monolithic catalyst can also exhibit a remarkable catalytic performance and deliver excellent long-term durability (1000 h), indicating broad prospects in industrial applications. Moreover, the promotional effect of Ni substitution was further unveiled by density functional theory (DFT) calculations. This work brings a favorable guidance for the exploration of highly efficient perovskite catalysts for light alkane elimination.

Enhanced adsorption of toluene on thermally activated ZIF-67: Characterization, performance, and modeling insights

The zeolitic imidazolate framework-67 (ZIF-67) has been explored for the dynamic adsorption of toluene vapor. We synthesized ZIF-67 through a straightforward room-temperature process and characterized it using XRD, FT-IR, DLS, and SEM techniques. The synthesized ZIF-67 possessed a Brunauer-Emmett-Teller (BET) surface area of 1578.7 m2/g and 0.76 μm particle size. Thermal activation under various conditions revealed that ZIF-67, activated in dry air at 250 °C, demonstrated optimal adsorption efficacy. Its adsorption capacity, time of breakthrough, and time of equilibration were 414.5 mg/g, 420 min, and 795 min, respectively. We investigated the impact of diverse operational parameters on adsorption through breakthrough curve analysis. An increase in the toluene concentration from 100 to 1000 ppm enhanced the adsorption capacity from 171 to 414 mg/g, while breakthrough time decreased from 1260 min to 462 min, respectively. Our findings show that increasing relative humidity from 0 to 70 % reduced 53.7 % in adsorption capacity and 46.3 % in breakthrough time. The competitive adsorption of toluene and ethylbenzene revealed that ZIF-67 had a higher selectivity for toluene adsorption. A 98 % adsorbent's regeneration efficiency at the first cycle reveals its reusability. The experimental data were successfully fitted to the Yan, Thomas, and Yoon-Nelson models to describe the adsorption process. The statistical validation of the model parameters confirms their reliability for estimating adsorption parameters, thus facilitating the design of fixed-bed adsorption columns for practical applications.

Selective removal of thiosulfate from coke oven gas desulfurization wastewater by catalytic wet air oxidation with manganese-based oxide from spent ternary lithium-ion batteries

Selective and efficient removal of thiosulfates (S2O32-) to recover high-purity and value-added thiocyanate products by fractional crystallization process is a promising route for the resource treatment of coke oven gas desulfurization wastewater. Herein, catalytic wet air oxidation (CWAO), with manganese-based oxide synthesized from spent ternary lithium-ion batteries (MnOx-LIBs), was proposed to selectively remove S2O32- from desulfurization wastewater. 98.0 % of S2O32- is selectively removed by the MnOx-LIBs CWAO system, which was 4.1 times that of the MnOx CWAO system. The synergistic effect among multiple metals from spent LIBs induces the enlarged specific surface area, increased reactive sites and formation of oxygen vacancy, promoting the adsorption and activation of O2, thereby realizing high-efficiency removal of S2O32-. The satisfactory selective removal efficiency can be maintained in the proposed system under complex environmental conditions. Notably, the proposed system is cost-effective and applicable to actual wastewater, in which 81.2 % of S2O32- is selectively removed from coke oven gas desulfurization wastewater. More importantly, compared with the typical processes, the proposed process is simpler and more environmentally-friendly. This work provides an alternative route to selectively remove S2O32- from coke oven gas desulfurization wastewater, expecting to drive the development of resource utilization of coke oven gas desulfurization wastewater.

PdPtVOx/CeO2-ZrO2: Highly efficient catalysts with good sulfur dioxide-poisoning reversibility for the oxidative removal of ethylbenzene

The PdPtVOx/CeO2-ZrO2 (PdPtVOx/CZO) catalysts were obtained by using different approaches, and their physical and chemical properties were determined by various techniques. Catalytic activities of these materials in the presence of H2O or SO2 were evaluated for the oxidation of ethylbenzene (EB). The PdPtVOx/CZO sample exhibited high catalytic activity, good hydrothermal stability, and reversible sulfur dioxide-poisoning performance, over which the specific reaction rate at 160°C, turnover frequency at 160°C (TOFPd or Pt), and apparent activation energy were 72.6 mmol/(gPt⋅sec) or 124.2 mmol/(gPd⋅sec), 14.2 sec-1 (TOFPt) or 13.1 sec-1 (TOFPd), and 58 kJ/mol, respectively. The large EB adsorption capacity, good reducibility, and strong acidity contributed to the good catalytic performance of PdPtVOx/CZO. Catalytic activity of PdPtVOx/CZO decreased when 50 ppm SO2 or (1.0 vol.% H2O + 50 ppm SO2) was added to the feedstock, but was gradually restored to its initial level after the SO2 was cut off. The good reversible sulfur dioxide-resistant performance of PdPtVOx/CZO was associated with the facts: (i) the introduction of SO2 leads to an increase in surface acidity; (ii) V can adsorb and activate SO2, thus accelerating formation of the SOx2- (x = 3 or 4) species at the V and CZO sites, weakening the adsorption of sulfur species at the PdPt active sites, and hence protecting the PdPt active sites to be not poisoned by SO2. EB oxidation over PdPtVOx/CZO might take place via the route of EB → styrene → phenyl methyl ketone → benzaldehyde → benzoic acid → maleic anhydride → CO2 and H2O.

Insight into modified CeMn based catalysts for efficient degradation of toluene by in situ infrared

Trace activated carbon (AC) and diatomaceous earth (DE) were used as structural promoters to be incorporated into Ce-Mn-based solid-solution catalysts by the redox precipitation method. The modified catalysts exhibit superior reducibility, with abundant Ce3+, Mn3+and reactive oxygen species, which are facilitated to the migration of oxygen and the generation of oxygen vacancies. In particular, the catalytic combustion temperatures of 90 % toluene (3000 ppm) on Ce1Mn3Ox-AC/DE were 84 °C (dry) and 123 °C (10 vol% H2O), respectively. The role of lattice oxygen and adsorbed oxygen was revealed by in situ DRIFTS. Additionally, in situ DRIFTS was employed to verify that the degradation of toluene by Ce1Mn3Ox-AC/DE satisfied the Langmuir-Hinshelwood (L-H) mechanism and the Mars-Van Krevelen (MvK) mechanism. The possible reaction pathway was elucidated (toluene → benzyl alcohol → benzoic acid → maleic anhydride → CO2 + H2O). Furthermore, final products attributed to toluene oxidation were detected by in situ DRIFTS at 50 °C in the absence of oxygen, confirming that the catalyst possessed outstanding performance at low temperatures beyond mere adsorption.

Thermally Inducing Viscous Fluids to Generate Co-Based Perovskites Enriched with Active Species for the Removal of VOCs

Various Co-based perovskites are synthesized through thermally driving viscous fluids. In this process, rare earth salts, cobalt salts, and citric acid do not require homogeneous mixing but only need to be heated until they melt into a molten viscous slurry. The physicochemical properties of cobalt-based perovskites were examined using techniques such as X-ray diffraction (XRD), electron paramagnetic resonance (EPR), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-Mapping-EDS), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), oxygen temperature-programmed desorption (O2-TPD), and N2 adsorption-desorption. The results indicate that the surface-active species can be controlled by altering the A-site elements of cobalt-based perovskites. All catalysts synthesized through the thermal treatment of viscous mixtures exhibited a low activation temperature and a low apparent activation energy for the catalytic oxidation of toluene. Among all cobalt-based perovskites, LaCoO3 demonstrated the most outstanding catalytic activity, primarily attributed to its capacity to expose a larger number of surface-active sites and oxygen species, as well as its superior reducibility. Furthermore, the formation process of optimal LaCoO3 was monitored using thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), and the byproducts of the low-temperature catalytic oxidation of toluene by the catalyst were identified using gas chromatography-mass spectrometry (GC-MS). The possible mechanism of toluene oxidation was inferred by in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). Moreover, LaCoO3 exhibits a predominant resistance to high-temperature hydrothermal conditions. This work provides a scalable and innovative approach to fabricating exceptionally effective catalysts for the efficient purification of VOCs.

Control of H-Related Defects in γ-MnO2 in a Hydrothermal Synthesis

Manganese dioxide is a good candidate for effective energy storage and conversion as it possesses rich electrochemistry. The compound also shows a wide polymorphism. The γ-variety, an intergrowth of β- and R-MnO2, has been extensively studied in several types of batteries (e.g., Zn/MnO2, Li-ion) and is a common electrode material for commercial batteries. It is well known that the insertion of protons thermodynamically stabilizes γ-MnO2 with respect to β-MnO2. Protons can enter the structure either by forming groups of 4 hydroxyls around a Mn4+ vacancy, called a Ruetschi defect, or by forming a hydroxyl group near a Mn3+ ion, called a Coleman defect. These defects differently affect the electrochemistry of manganese oxide, and tailoring their amount in the structure can be used to tune the material properties. Previous studies have addressed the proton insertion process, but the role of the synthesis pathway on the amount of defects created is not well understood. We here investigate how the parameters in a hydrothermal synthesis of γ-MnO2 nanoparticles influence the amount and type of H-related defects. Structural investigations are carried out using Pair Distribution Function analysis, X-ray absorption spectroscopy, thermogravimetric analysis, and inelastic neutron scattering. We demonstrate the possibility to control the amount and type of defects introduced during the synthesis. While the amount of Ruetschi defects increases with synthesis temperature, it decreases with extended synthesis time, along with the amount of Coleman defects. Moreover, we discuss the arrangement of the defects in the γ-MnO2 nanoparticles.

Fe(II)/LXQ-10 bifunctional resin materials for boosting synergistic adsorption/oxidation of benzene in industrial waste gas

A series of adsorption/oxidation bifunctional material with different Fe(II) loading amounts was prepared by using ultrahigh crosslinking adsorption resin (LXQ-10) as a carrier and FeCl2 as an impregnating solution. The bifunctional material was characterized by BET, SEM, XRD, XPS, and EPR. The effects of Fe loading, reaction temperature, and space velocity on benzene adsorption efficiency were investigated using self-made experimental equipment to explore the optimal reaction condition. The adsorption results were fitted and analyzed by using four typical models: the quasi-first-order kinetic model, the quasi-second-order kinetic model, Elovich's kinetic model, and the Weber and Morris kinetic model. The quasi-first-order kinetic model had the highest R2 value (0.998) and the best applicability. The fitting effect of the Freundlich equation (R2 = 0.997) was better than that of the Langmuir equation (R2 = 0.919). Furthermore, the effects of Fe loading, H2O2 concentration, benzene inlet concentration, and temperature on the catalytic oxidation efficiency of benzene were studied. The catalytic oxidation efficiency of 3-Fe(II)/LXQ-10 was maintained at about 95% at a temperature of 303 K and an H2O2 concentration of 150 mmol/L. Compared with the adsorption efficiency, the catalytic oxidation efficiency of bifunctional resin materials in a heterogeneous Fenton system was remarkably improved and had excellent stability. A possible migration and transformation path during benzene removal was proposed according to the results of the analysis of GC-MS intermediates. This study provided a novel process for the adsorption and oxidative degradation of VOCs.

Mutual inhibition effects on the synchronous conversion of benzene, toluene, and xylene over MnOx catalysts

Advancing the practical application of catalytic oxidation technology demands for illustrating the synchronous conversion behavior of various volatile organic compounds (VOCs) over catalysts. Here, the mutual effects of benzene, toluene and xylene (BTX) were examined for their synchronous conversion on the surface of the MnO₂ nanowire. Competitive adsorption of xylene (absorption energy (E_ads): -0.889 eV) facilitated its prior conversion and impeded the oxidization of toluene and benzene over the catalyst. The turnover frequencies were 0.52 min^-1 (benzene), 0.90 min^-1 (toluene) and 2.42 min^-1 (xylene) for mixed BTX conversion over the MnO₂. Doping MnO₂ with K⁺, Na⁺ and Ca²⁺ could enhance its ability to oxidize the individual VOCs but did not alter the conversion mechanism of mixed BTX over the catalyst. When reducing the competitive effects in the adsorption of BTX, the oxidation performance of catalysts would depend on their ability to oxidize toluene and benzene. K-MnO₂ showed superior properties, i.e. specific surface area, highly low-valent Mn species, high lattice oxygen content, and abundant oxygen vacancy, and then exhibited superior performance during long-term operation (90% conversion in 800 min). The present study uncovered the co-conversion mechanism of multiple VOCs and significantly leveraged the catalytic oxidization technology for VOCs removal in practical application.

Photo-Activated Direct Catalytic Oxidation of Gaseous Benzene with a Cu-Connected Serial Heterojunction Array of Co3 O4 /Cux O/Foam Cu Assembled via Layer upon Layer Oxidation

The foam copper (FCu) has been first used as a promising supporting material to prepare a photo-activated catalyst of Co₃ O₄ /Cu_x O/FCu, in which the fine Co₃ O₄ particles are inlayed on the Cu_x O nanowires to form a Z-type heterojunction array connected by substrate Cu. The prepared samples have been used as a photo-activated catalyst to directly decompose gaseous benzene and the optimized Co₃ O₄ /Cu_x O/FCu demonstrates a 99.5% removal efficiency and 100% mineralizing rate within 15 min in benzene concentration range from 350 to 4000 ppm under simulate solar light irradiation. To track the reactive mechanism, a series of MO_x /Cu_x O/FCu (M = Mn, Fe, Co, Ni, Cu, Zn) is prepared and a novel photo-activated direct catalytic oxidation route is proposed based on the comparative investigation of material properties. Moreover, the approach grew in situ via layer upon layer oxidation on FCu dedicates to the extra lasting reusability and the easy accessibility in the diverse situations. This work provides a novel strategy for the preparation of Cu connected series multidimensional heterojunction array and a promising application for the quick abatement of the high-leveled concentration gaseous benzene and its derivatives from the industrial discharged flow or the accident scene's leakage.

Catalytic removal of toluene using MnO2-based catalysts: A review

Volatile organic compounds (VOCs) have serious hazard to human health and ecological environment. Due to its low cost and high activity, the catalytic oxidation technology considered to be the most effective method to remove VOCs. Toluene is one of the typical VOCs, hence its catalytic elimination is crucial for the regulation of VOCs. Manganese dioxide (MnO₂) has been extensively studied for its excellent redox performance and low-temperature operation conditions. In this review, we summarize the research progresses in the toluene catalytic oxidation of MnO₂-based catalysts, which contain single MnO₂, metal-doped MnO₂ and supported MnO₂ catalyst. In particular, we pay much attention on the relationship between the chemical properties and toluene oxidation performance over MnO₂ catalyst, as well as the catalytic reaction mechanisms. Moreover, the effects of different crystal forms and morphologies on the catalytic toluene reaction were discussed. And the perspective on MnO₂ catalysts for the catalytic oxidation of toluene has been proposed. We expect that the summary of these important findings can serve as an important reference for the catalytic treatment of VOCs.

Catalytic Degradation of Toluene over MnO2/LaMnO3: Effect of Phase Type of MnO2 on Activity

Series of α, β, γ, δ type MnO2 supported on LaMnO3 perovskite was developed by a one-pot synthesis route. Compared with α-MnO2, β-MnO2, γ-MnO2, δ-MnO2 and LaMnO3 oxides, all MnO2/LaMnO3 showed promotional catalytic performance for toluene degradation. Among them, α-MnO2/LaMnO3 holds the best active and mineralization efficiency. By the analysis of N2 adsorption-desorption, XPS and H2-TPR, it can be inferred that the improved activity should be ascribed to the higher proportion of lattice oxygen, better low-temperature reducibility and larger specific surface area. Besides, the byproducts from the low-temperature reaction of toluene oxidation were detected by a TD/GC-MS, confirming the presence of the intermediates. Combined with the in-situ DRIFTS, the catalytic degradation path of toluene oxidation has also been discussed in depth.

Non-thermal plasma-enhanced catalytic activation of Mn-Zr-La/Al2O3 catalyst for meta-xylene degradation: Synergetic effects and degradation mechanism

The LaMnO₃ catalysts doped with transition metal (Zr, Co, Fe) were prepared. The influencing factors (the catalyst type, the initial concentration, the gas flow, and oxygen content) on the degradation efficiency by the non-thermal plasma synergistic the LaMnO₃ catalysts doped with Zr, Co and Fe were investigated systematically. The degradation mechanism of the meta-xylene degradation by the non-thermal plasma synergistic Mn-Zr-La/Al₂O₃ was researched. The results showed that the Mn-Zr-La/Al₂O₃ catalyst in the four catalysts had the best degradation efficiency for meta-xylene, which was 99.6% at the applied voltage of 44 kV. The by-product ozone concentration was low, and the NO_x was not detected. Meanwhile, the XPS characterization analysis study revealed that the proportion of Mn⁴⁺ element and the proportion of O_sur in the Zr-doped Mn-Zr-La/Al₂O₃ catalyst were both the highest. The degradation efficiency decreased with the increasing of the initial concentration and gas flow, but first increased and then decreased with the increasing of oxygen content. The fresh and used Mn-Zr-La/Al₂O₃ were characterized by SEM, XRD, BET, FT-IR, O₂-TPD, and the tail gas was treated by GC-MS. Then synergistic degradation mechanism for the meta-xylene by the non-thermal plasma over the Mn-Zr-La/Al₂O₃ catalyst are proposed.

Catalytic Oxidation of Toluene over Fe-Rich Palygorskite Supported Manganese Oxide: Characterization and Performance

A series of Fe–rich palygorskite supported manganese oxide (X%Mn–Pal) catalysts were prepared by co-precipitation method and used as catalysts for toluene oxidation. The components and structure of the as-prepared catalysts were characterized by XRD, Raman, TEM, XPS, and in situ DRIFTS. The results showed that the 15%Mn–Pal catalyst exhibited the highest catalytic activity (T90 = 227 °C) and excellent cycling stability for the oxidation of toluene compared with other catalysts. The characterization results indicated that remarkable activity of the 15%Mn–Pal catalyst for toluene oxidation should be ascribed to the abundant surface oxygen vacancies. In situ DRIFTS results elucidated that benzoate was the main intermediate, which can be further oxidized into H2O and CO2. The objectives of this study are to (i) investigate the synergistic effect between Fe and Mn for toluene oxidation, (ii) develop an efficient catalyst for toluene abatement with high activity and low–cost, and (iii) promote the application of natural Fe–rich palygorskite in the control of VOCs.

Mechanistic insights into benzene oxidation over CuMn2O4 catalyst

The spinel-type CuMn₂O₄ catalyst exhibits good catalytic activity towards benzene oxidation, but the catalytic oxidation mechanism is not established. Theoretical calculations were implemented to unearth the reaction mechanism of benzene catalytic oxidation over CuMn₂O₄ catalyst through density functional theory (DFT). The results indicate that benzene adsorption on both Cu-terminated and Mn-terminated surfaces are controlled by the chemisorption mechanism. The Cu-terminated surface is more active for benzene adsorption than the Mn-terminated surface. Cu atom is regarded as the primary active site. During benzene catalytic oxidation, benzene firstly undergoes dehydrooxidation reaction to generate phenoxy group (C₆H₆* → C₆H₅* → C₆H₅O*). Two reaction channels are responsible for the ring-opening and oxidation reactions of phenoxy group, including benzoquinone- and cyclopentadienyl-dominated channels. In the benzoquinone-dominated channel, C₆H₄O₂* is produced from phenoxy dehydrogenation and oxidation, and then decomposes into acetylene via the ring-opening reaction (C₆H₄O₂* → C₄H₂O₂* → C₄H₂O₄* → C₂H₂*). Compared with the benzoquinone-dominated channel, the cyclopentadienyl-dominated channel is dominant for phenoxy group oxidation. Phenoxy group decomposes to generate cyclopentadienyl. C₅H₅* is dehydrogenated and oxidized to form cyclopentadienone. Finally, C₅H₄O* is oxidized to form carbon dioxide through a nine-step reaction pathway. The ring-opening reaction (C₅H₄O* → C₃H₂O*) has the highest energy barrier of 283.45 kJ/mol, and is identified as the rate-determining step of benzene catalytic oxidation.

Effect of MnO2 Polymorphs' Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation

MnO₂ polymorphs (α-, β-, and ε-MnO₂) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO₂ shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T₉₀ = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO₂ (0.135 molecule_CO·nm^-2·s^-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO₂ and α-MnO₂, respectively. The specific reaction rate order (β-MnO₂ > ε-MnO₂ > α-MnO₂) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO₂ were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.

Catalytic stability enhancement for pollutant removal via balancing lattice oxygen mobility and VOCs adsorption

Manganese oxide supported Pt single atoms (Pt₁/MnO_x) are prepared by the molten salt method. Catalytic oxidation of toluene and iso-hexane, typical emissions from furniture paints industry, is tested. Pt₁/MnO_x shows poor and high catalytic stability for toluene and iso-hexane oxidation, respectively. Enhancement in the catalytic stability for toluene oxidation is observed after the hydrogen reduction treatment of Pt₁/MnO_x at 200 °C. The hydrogen treated catalyst possesses the weaker Mn-O bonds and lower coordination number of PtO, with superior mobility of lattice oxygen and appropriate toluene adsorption. Balancing lattice oxygen mobility and volatile organic compounds adsorption is important for the catalytic stability of Pt₁/MnO_x. For the oxidation of toluene and iso-hexane mixture, owing to the competitive adsorption, iso-hexane oxidation is greatly inhibited, while toluene oxidation is not influenced. The present Pt₁/MnO_x catalyst holds promising prospect in furniture paints industry applications because of high catalytic stability and water resistance ability.

Catalytic ozonation of VOCs at low temperature: A comprehensive review

VOCs abatement has attracted increasing interest because of the detrimental effects on both atmospheric environment and human beings of VOCs. The assistance of ozone has enabled efficient VOCs removal at low temperature. Thereby, catalytic ozonation is considered as one of the most feasible and effective methods for VOCs elimination. This work systematically reviews the emerging advances of catalytic ozonation of different VOCs (i.e., aromatic hydrocarbons, oxygenated VOCs, chlorinated VOCs, sulfur-containing VOCs, and saturated alkanes) over various functional catalysts. General reaction mechanism of catalytic ozonation including both Langmuir-Hinshelwood and Mars-van-Krevelen mechanisms was proposed depending on the reactive oxygen species involving the reactions. The influence of reaction conditions (water vapor and temperature) is fully discussed. This review also introduces the enhanced VOCs oxidation via catalytic ozonation in the ozone-generating systems including plasma and vacuum ultraviolet. Lastly, the existing challenges of VOCs catalytic ozonation are presented, and the perspective of this technology is envisioned.

In-situ regulation of acid sites on Mn-based perovskite@mullite composite for promoting catalytic oxidation of chlorobenzene

A series of Sm-Mn perovskite@mullite composites with different amounts of acid sites were successfully synthesized by regulating the level of in situ etched-surface modification. X-ray diffraction (XRD) test showed that the crystal structure of catalyst gradually changed from perovskite to perovskite@mullite composites and mullite. The characterization of temperature programmed desorption with ammonia (NH₃-TPD) confirmed the acid sites on the surface of catalyst can be deployed by the in-situ modification. The temperature-programmed reduction with hydrogen (H₂-TPR), and N₂ adsorption-desorption showed that the surface modification also increased the reducibility, surface area, and mesoporosity of catalyst. The catalytic activities were compared by a long-term catalytic oxidation of chlorobenzene evaluation for 20 h of uninterrupted reaction at a relatively low temperature of 300 °C, and the Sm-Mn perovskite@mullite composite (SMPM-1.2) possessed the best catalytic stability. The X-ray photoelectron spectroscopy (XPS) measurement determined that the high ratios of lattice oxygen and tetravalent manganese did not improve the stability of catalyst in the catalytic oxidation of chlorobenzene, but the activities trends of samples were consistent with the change of surface (Mn⁴⁺+Mn³⁺)/Mn²⁺ ratios. Meanwhile, the catalytic experiments for benzene, toluene, o-xylene and acetone showed that the as-prepared catalyst was also suitable for the efficient removal of the different types of VOCs. This work supplied a method for the further development of high activity catalysts for the removal of VOCs.

Pt/MnOx for toluene mineralization via ozonation catalysis at low temperature: SMSI optimization of surface oxygen species

The problem of deep oxidation of low concentrations of VOCs in industrial tail gas is exceptionally urgent. The preparation of VOCs ozonation catalyst with a high mineralization rate is still a challenge. In this paper, manganese oxide carriers with different morphologies were synthesized by simple methods and used to catalyze ozone mineralization of toluene after loading Pt nanoparticles efficiently. The conversion of toluene over Pt/MnO_x-T catalyst was more than 98 % at ambient temperature, and the mineralization rate of toluene was close to 100 % at 70 °C. Through a variety of characterization methods, the strong metal-support interaction (SMSI) between Pt nanoparticles and carriers was successfully constructed. It was found that SMSI successfully optimized the surface oxygen species and oxygen migration ability of the catalyst, and then realized the high degree of mineralization of toluene at low temperature. This paper guides the subsequent development of Pt-Mn catalysts for catalytic organic pollutants ozonation with high activity.

Insight into the pH effect on the oxygen species and Mn chemical valence of Co–Mn catalysts for total toluene oxidation

Mn-Based metal oxides have shown promising performance in catalytic oxidation of toluene due to the mixed Mn3+ and Mn4+ valences and in large numbers of oxygen vacancy clusters on the surface.

Improved catalytic oxidation of propylene glycol methyl ether over Sm-Mn and Sm-Co perovskite-based catalysts prepared by the recycling of spent ternary lithium-ion battery

The spent ternary lithium-ion batteries were utilized as the precursors to prepare Sm-Mn and Sm-Co perovskite oxides (SmMnO₃-spent ternary lithium-ion battery [STLIB] and SmCoO₃-STLIB) for the first time. Their catalytic activities were evaluated by catalytic oxidation of propylene glycol methyl ether. Compared with that of the catalysts synthesized by analytical reagents, the catalytic activities of SmMnO₃-STLIB and SmCoO₃-STLIB had been significantly enhanced. The analysis of X-ray photoelectron spectroscopy (XPS) showed that the molar ratios of Mn⁴⁺/Mn³⁺ and O_ads/O_latt of SmMnO₃-STLIB were higher than that of pure SmMnO₃ and the Co³⁺/Co²⁺ ratios of SmCoO₃-STLIB was much larger than that of pure SmCoO₃. The hydrogen temperature-programmed reduction (H₂-TPR) and N₂ adsorption-desorption tests determined that the reducibilities and specific surface areas of SmMnO₃-STLIB and SmCoO₃-STLIB were also superior to pure catalysts. Ultimately, the by-products of the catalytic oxidation of propylene glycol methyl ether over SmMnO₃-STLIB were also detected by gas chromatography-mass spectrometry (GC-MS). This work will provide a demonstration for the resource utilization of spent lithium ions batteries and the analysis of the increased activity obtained by using spent lithium ions batteries as the precursors to prepare catalysts.

Simple strategy for the construction of oxygen vacancies on α-MnO2 catalyst to improve toluene catalytic oxidation

A strategy simple, safe and suitable for large scale production of α-MnO₂ catalyst with high activity in VOCs oxidation is crucial for its application. The catalytic reactivity of α-MnO₂ catalyst is largely related with its oxygen vacancy. Herein, we report effective construction of oxygen vacancies on α-MnO₂ through simply adjusting precipitation temperature of a redox precipitation process. The key role of surface oxygen vacancies in toluene oxidation and the formation of different amount and distribution of the oxygen vacancies over the α-MnO₂ catalysts were revealed by characterizations together with DFT calculations. The best catalyst (α-MnO₂-60) exhibited significantly improved catalytic activity of α-MnO₂ catalyst in toluene oxidation (T₉₀ = 203 ℃) and excellent water resistance. The richest surface oxygen vacancies of α-MnO₂-60 contributed to its best catalytic activity, despite of its relatively lower specific surface area. This work may provide a new perspective for the rational design of high efficient VOCs catalysts.

Recent Progress of Thermocatalytic and Photo/Thermocatalytic Oxidation for VOCs Purification over Manganese-based Oxide Catalysts

Volatile organic compounds (VOCs) are one of the main sources of air pollution, which are of wide concern because of their toxicity and serious threat to the environment and human health. Catalytic oxidation has been proven to be a promising and effective technology for VOCs abatement in the presence of heat or light. As environmentally friendly and low-cost materials, manganese-based oxides are the most competitive and promising candidates for the catalytic degradation of VOCs in thermocatalysis or photo/thermocatalysis. This article summarizes the research and development on various manganese-based oxide catalysts, with emphasis on their thermocatalytic and photo/thermocatalytic purification of VOCs in recent years in detail. Single manganese oxides, manganese-based oxide composites, as well as improving strategies such as morphology regulation, heterojunction engineering, and surface decoration by metal doping or universal acid treatment are reviewed. Besides, manganese-based monoliths for practical VOCs abatementare also discussed. Meanwhile, relevant catalytic mechanisms are also summarized. Finally, the existing problems and prospect of manganese-based oxide catalysts for catalyzing combustion of VOCs are proposed.

Insight into the Crystal Structures and Surface Property of Manganese Oxide on CO Catalytic Oxidation Performance

α-MnO₂ nanorods and flower-like γ-MnO₂ microspheres were synthesized by facile and mild methods to illustrate the effect of crystal structures and surface features on catalytic performance with the help of carbon monoxide (CO) oxidation. It is revealed that the flower-like γ-MnO₂ microspheres possess better catalytic oxidation performance (CO complete conversion temperature at 120 °C and long-time stability for 50 h) than α-MnO₂ nanorods, which can be attributed to the obvious differences in the chemical bonds and linking modes of [MnO₆] octahedra due to the different crystal structures. γ-MnO₂ possesses lower Mn-O bond strength that enables γ-MnO₂ to present a large amount of surface lattice oxygen and superior oxygen mobility. The disordered random intergrowth tunnel structure can adsorb effectively CO molecules, resulting in excellent catalytic performance for CO catalytic oxidation. In addition, the MnO₂ catalyst probably occurred via a Mars-van Krevelen mechanism for CO oxidation. This work provides an insight into the effect of crystal structures and surface property of manganese oxide on catalytic oxidation performance, which presents help for the future design of promising catalysts with excellent catalytic performance.

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.

Boosting total oxidation of propane over CeO2@Co3O4 nanofiber catalysts prepared by multifluidic coaxial electrospinning with continuous grain boundary and fast lattice oxygen mobility

A one-dimensional (1D) core-shell of Co-Ce oxide has been prepared by multifluidic coaxial electrospinning method and evaluated for the total oxidation of propane (C₃H₈). Activity and morphological characterizations show that the CeO₂@Co₃O₄ nanofiber catalyst, of which the core is CeO₂ and the shell is Co₃O₄, exhibits excellent oxidation activity. The exposed Co₃O₄ grown on the outside of the fibers can rapidly react with C₃H₈ while CeO₂ with high oxygen storage capacity in the inside is conductive to the enhanced oxidation rate. Besides, the continuous grain boundary provides a fast mass transfer channel for lattice oxygen, and rich oxygen vacancies favor the mobility of active oxygen species. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) confirms that the CeO₂@Co₃O₄ catalyst have a faster rate of C₃H₈ adsorption and better oxidation activity with respect to the counterpart using a single-needle electrospinning method. Moreover, the CeO₂@Co₃O₄ catalyst displays excellent thermal stability, and strong resistance against 5 vol% H₂O and 5 vol% CO₂ at both 300 and 400 °C. Our strategy can give some new insights into morphological engineering to promote active oxygen mobility via the construction of one-dimensional core-shell of metal oxides for catalytic oxidation of VOCs.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

The growing world energy consumption, with reliance on conventional energy sources and the associated environmental pollution, are considered the most serious threats faced by mankind. Heterogeneous photocatalysis has become one of the most frequently investigated technologies, due to its dual functionality, i.e., environmental remediation and converting solar energy into chemical energy, especially molecular hydrogen. H2 burns cleanly and has the highest gravimetric gross calorific value among all fuels. However, the use of a suitable electron donor, in what so-called “photocatalytic reforming”, is required to achieve acceptable efficiency. This oxidation half-reaction can be exploited to oxidize the dissolved organic pollutants, thus, simultaneously improving the water quality. Such pollutants would replace other potentially costly electron donors, achieving the dual-functionality purpose. Since the aromatic compounds are widely spread in the environment, they are considered attractive targets to apply this technology. In this review, different aspects are highlighted, including the employing of different polymorphs of pristine titanium dioxide as photocatalysts in the photocatalytic processes, also improving the photocatalytic activity of TiO2 by loading different types of metal co-catalysts, especially platinum nanoparticles, and comparing the effect of various loading methods of such metal co-catalysts. Finally, the photocatalytic reforming of aromatic compounds employing TiO2-based semiconductors is presented.

Recovery of cathode materials from spent lithium-ion batteries and their application in preparing multi-metal oxides for the removal of oxygenated VOCs: Effect of synthetic methods

Due to the sustainable use of wastes, cathode materials of spent lithium-ion batteries are recovered and used as transition metal precursors to prepare metal oxides catalysts for the oxidation of VOCs. In this work, a series of manganese-based and cobalt-based metal oxides are synthesized via different preparation methods. Catalytic activities of the catalysts prepared are investigated through complete oxidation of oxygenated VOCs and the physicochemical properties of optimum samples are characterized. Evaluation results indicate that MnOx (SY) (HT) sample prepared via hydrothermal method and CoOx (GS) (CP) synthesized via co-precipitation method had better performance, because they have higher specific surface area, higher concentration of active oxygen species and high-valence metal ion, as well as better low-temperature reducibility compared to the other multi-metal oxides used in the study. In addition, TD/GC-MS results imply that further oxidation of by-products requires high reaction temperature during VOCs oxidation.

Enhanced catalytic oxidation of VOCs over porous Mn-based mullite synthesized by in-situ dismutation

Porous Mn-based mullite SmMn₂O₅ was synthesized by the in-situ dismutation of solid state Mn³⁺ in bulk SmMnO₃ perovskite to catalytic oxidation of benzene and chrolobenznen. The physicochemical property of catalyst was acquired by XRD, SEM, N₂ adsorption-desorption, XPS, O₂-TPD and H₂-TPR. Compared with that of bulk SmMnO₃ and bulk SmMn₂O₅, the porous SmMn₂O₅ mullite (SmMn₂O₅-ID) displayed higher molar ratios of Mn⁴⁺/Mn³⁺ and O_latt/O_ads, and better active oxygen desorption capacity, reducibility and larger specific surface, which promoted the preferable low-temperature catalytic oxidation of VOC. The increase in the content of Mn⁴⁺ on the surface of the Sm-Mn mullite reduced the surface defects and increased the proportion of its surface lattice oxygen, thereby promoting the attack of VOC molecules by more lattice oxygen. Combined with the analysis of reactant intermediate for benzene oxidation by in situ diffuse reflectance infrared Fourier transform spectroscopy, the catalytic mechanism of the catalyst was also explored. Moreover, SmMn₂O₅-ID also showed the excellent stability and the superior removal of mixed VOCs with different concentration ratios. This finding provides an efficient and practical method for exploiting highly active Mn-based mullite with a high efficiency and stability for the purification of air pollution.