A new approach for the determination of As, Cu, and Pb in seawater samples using manganese oxide octahedral molecular sieve as a sorbent for dispersive solid-phase microextraction

This study introduces a novel method for preconcentrating As, Cu, and Pb from seawater samples using manganese oxide octahedral molecular sieve (OMS-2), as a sorbent, and the analysis by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The OMS-2 nanomaterial was synthesized and characterized using X-ray diffraction and scanning electron microscopy, revealing a crystallite size of 20.9 nm and a typical needle-like morphology of cryptomelane structure. To optimize the ICP-OES operating conditions and the preconcentration process, a central composite design was used. The optimal conditions for ICP-OES analyses were 1200 W and 0.7 L min-1 for the levels of the radio frequency potential (RF) and nebulization gas flow rate, respectively. The optimal conditions for the adsorption process were achieved at a pH of 6.5, 30 mg of OMS-2, and 35 min of stirring time, in the presence of the sample matrix. The enrichment factors obtained were 66, 45, and 21, and a limit of detection of 0.3, 0.1, and 2.1 μg L-1 for As, Cu, and Pb, respectively. The recovery tests ranged from 80 % to 120 %. The method was successfully applied to determine As, Cu, and Pb in seawater samples.

Highly Active and Water-Resistant Cu-Doped OMS-2 Catalysts for CO Oxidation: The Importance of the OMS-2 Synthesis Method and Cu Doping

Porous cryptomelane-type Mn oxide (OMS-2) has an outstanding redox property, making it a highly desirable substitute for noble metal catalysts for CO oxidation, but its catalytic activity still needs to be improved, especially in the presence of water. Given the strong structure-performance correlation of OMS-2 for oxidation reactions, herein, OMS-2 is synthesized by solid state (OMS-2S), reflux (OMS-2R), and hydrothermal (OMS-2H) methods, aiming to improve its CO oxidation performance through manipulating synthesis parameters to tailor its particle size, morphology, and crystallinity. Characterization shows that OMS-2S has the highest CO oxidation activity in the absence of water due to its low crystallinity, high specific surface area, large oxygen vacancy content, and good redox property, but the presence of water can greatly reduce its CO oxidation activity. Doping Cu into an OMS-2 can not only improve its CO oxidation activity but also greatly improve its water tolerance. The Cu-doped OMS-2S catalyst with ∼4 wt % Cu can achieve a T90 of 49 °C (1% CO/10% O2/N2 and WHSV = 60,000 mL·g-1·h-1), ranking among the lowest reported T90 values for Mn oxide-based CO oxidation catalysts, and it can maintain nearly 100% CO conversion in the presence of 5 vol % water for over 50 h. In situ DRIFTs characterization indicates that the good water resistance of Cu-doped OMS-2S can be attributed to the significantly suppressed surface hydroxyl group generation because of Cu doping. This work demonstrates the importance of the synthesis method and Cu doping in determining the CO oxidation activity and water resistance of OMS-2 and will provide guidance for synthesizing highly active and water-resistant CO oxidation catalysts.

Enhanced degradation and mineralization of estriol over ZrO2/OMS-2 nanocomposite: Kinetics, pathway and mechanism

Although remarkable progresses have been achieved in the exploration of new and efficient catalytic systems for efficient degradation of estriol, there are only very few available reports providing high mineralization of estriol. Hence, it is still a serious challenge to develop the novel and efficient methods for enhanced degradation and mineralization of estriol due to its serious threat to environment and human health. Herein, this study proposes a series of ZrO₂ modified manganese oxide octahedral molecular sieve (ZrO₂/OMS-2) nanocomposites as efficient catalysts for enhanced degradation and mineralization of estriol via PMS activation at 30 °C. Among them, ZrO₂/OMS-2-27% provided the highest degradation efficiency (95%) and mineralization degrees (70.1%), which exceeded most reported catalytic systems, in the catalytic degradation of estriol. These quenching tests and EPR analysis had confirmed that O₂•^- and ¹O₂ were primary reactive oxygen species (ROS) in the ZrO₂/OMS-2-27%/PMS system, contrary to the OMS-2/PMS system for which SO₄•^- and OH• are primary ROS. This might be due to the abundant O-containing surface functional groups of ZrO₂/OMS-2-27%. This work not only provides a facile and high-efficiency methodology for the construction of Mn-based nanomaterial, but also proposes a new and efficient nano-catalyst for estriol removal.

Insights into selective hydrogenation of levulinic acid using copper on manganese oxide octahedral molecular sieves

Selective hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) was studied using copper on manganese oxide octahedral molecular sieve (OMS-2) as catalysts. A range of copper supported on OMS-2 catalysts was prepared using the modified wet-impregnation technique and characterized thoroughly using powder X-ray diffraction, inductively coupled plasma optical emission spectroscopy metal analysis, Fourier transform infrared, high-resolution transmission electron microscopy and N₂ sorption analyses. Process parameters for selective hydrogenation of LA to GVL were optimized using the design of experiment (DoE) approach with response surface methodology comprising a central composite design. Using the optimized conditions (190°C reaction temperature, 20 bar H₂ pressure and 20 wt% Cu loading on OMS-2), up to 98% yield of GVL could be achieved in water as a solvent. Based on DoE, H₂ pressure had the most influence on GVL selectivity followed by catalyst loading used for the hydrogenation of LA. The response surface methodology model also showed synergistic effect of reaction temperature and H₂ pressure on the yield of GVL. 20 wt% Cu/OMS-2 catalysts were re-used up to four cycles and showed noticeable loss of activity after the first cycle due to observed leaching of loose Cu species, thereafter the activity loss diminished during subsequent recycles.

Boosting Oxygen Reduction Activity of Manganese Oxide Through Strain Effect Caused By Ion Insertion

Transition-metal oxides with a strain effect have attracted immense interest as cathode materials for fuel cells. However, owing to the introduction of heterostructures, substrates, or a large number of defects during the synthesis of strain-bearing catalysts, not only is the structure-activity relationship complicated but also their performance is mediocre. In this study, a mode of strain introduction is reported. Transition-metal ions with different electronegativities are intercalated into the cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2) structure with K ions as the template, resulting in the octahedral structural distortion of MnO₆ and producing strains of different degrees. Experimental studies reveal that Ni-OMS-2 with a high compressive strain (4.12%) exhibits superior oxygen reduction performance with a half-wave potential (0.825 V vs RHE) greater than those of other reported manganese-based oxides. This result is related to the increase in the covalence of MnO₆ octahedral configuration and shifting down of the e_g band center caused by the higher compression strain. This research avoids the introduction of new chemical bonds in the main structure, weakens the effect of e_g electron filling number, and emphasizes the pure strain effect. This concept can be extended to other transition-metal-oxide catalysts.

The post plasma-catalytic decomposition of toluene over K-modified OMS-2 catalysts at ambient temperature: Effect of K+ loading amount and reaction mechanism

The present work is devoted to study the post plasma-catalytic (PPC) degradation of toluene using packed-bed discharge (PBD) plasma over K-modified manganese oxide octahedral molecular sieve (OMS-2) catalysts at ambient temperature. Compared to plasma alone, PPC can significantly improve the toluene degradation and mineralization performance simultaneously, and the generation of discharge byproducts and organic intermediates is suppressed. The catalytic capacity of OMS-2 for toluene degradation is greatly promoted by tuning potassium ions (K⁺) content in OMS-2 tunnel, which might be owing to the formation of more surface active oxygen species derived from weak Mn-O bonds, plenty of oxygen vacancies, as well as more superior low-temperature reducibility. Highest toluene degradation efficiency (89.4%) and CO_x selectivity (88.9%) can be achieved in plasma-catalysis system over K-modified OMS-2 sample with K/Mn molar ratio of 2 at the SIE of 658 J/L. A long-term stability test has also been successfully carried out to evaluate the stability of K-modified OMS-2 with the assistance of plasma. Possible reaction mechanism for plasma-catalytic degradation of toluene on K-modified OMS-2 catalyst has been proposed based on the plasma diagnosis, catalysts characterization, and organic intermediates identification. This work aims to gaina deeperunderstandingof plasma-catalytic degradation mechanism and provides an environmentally friendly and energy-efficient method for practical volatile organic compounds (VOCs) abatement in PPC process.

The remarkable effect of alkali earth metal ion on the catalytic activity of OMS-2 for benzene oxidation

Cryptomelane-type octahedral molecular sieve (OMS-2) is one of the most promising catalysts for VOCs oxidation, and introduction of metal ions in OMS-2 tunnel is widely used for tailoring its catalytic activity. Here, different types of alkali earth metal ions with the same X/Mn atomic ratio of approximately 0.012 (X represents Mg²⁺, Ca²⁺, and Sr²⁺) were successfully introduced into OMS-2 tunnel by a one-step redox reaction. The catalytic test showed that introducing alkali earth metal ions into tunnels had a considerable effect on the catalytic performance of OMS-2 for benzene oxidation. The Sr²⁺ doped OMS-2 catalyst exhibited the better catalytic activity compared with those of Mg-OMS-2 and Ca-OMS-2 samples, and was also superior to a commercial 0.5% Pt/Al₂O₃ catalyst, as evidenced by its low reaction temperatures of T₅₀ = 200 °C and T₉₀ = 223 °C (corresponding to benzene conversions at 50% and 90%, respectively). The origin of the considerable effect of alkali earth metal doping on the catalytic activity of OMS-2 catalysts was experimentally and theoretically investigated by an ¹⁸O₂ isotopic labeling experiment, CO temperature-programmed reduction, O₂ temperature-programmed oxidation, and density functional theory calculations. The greatest catalytic activity of Sr-OMS-2 compared with those of Mg-OMS-2 and Ca-OMS-2 samples was attributed to its highest lattice oxygen activity as well as its largest surface area. By introducing alkali earth metal ions into the OMS-2 tunnel, we developed a low-cost and highly efficient catalyst that could be used as alternative to noble metal catalysts.

Lewis acids promoted organic pollutants degradation in aqueous solution with peroxymonosulfate and MnO2: New insights into the activation mechanism

Nonredox metal ions have been widely recognized to be important in a wide range of biological and chemical oxidations as Lewis acids (LA). However, the role of LA in peroxymonosulfate (PMS) activation for wastewater treatment has not been considered until now. This study shows that oxidizing power of PMS can be promoted after binding nonredox metal ions such as Ca²⁺ as LA, leading to the easier reduction of the oxidant to radicals and substantial enhancement of dye degradation by employing manganese oxides OMS-2 as model catalysts. Increased with Lewis acidity of the metal ion, the rate of PMS decomposition enhanced linearly, while the dye degradation rate first increased and then declined due to the formation of a larger amount of dioxygen. The interactions between Ca²⁺ and PMS were further investigated by Raman, cyclic voltammetry and XPS; and the detailed mechanism of PMS activation was proposed. The performance of Ca²⁺+OMS-2/PMS system under different conditions was also studied. The findings indicate the importance of LA in PMS activation reaction and their role must be considered in other transition metal oxides/PMS systems. It will be also helpful to design new and highly active catalysts for the reactions.

Enhancing Water Resistance of a Mn-Based Catalyst for Low Temperature Selective Catalytic Reduction Reaction by Modifying Super Hydrophobic Layers

OMS-2 catalysts exhibit excellent selective catalytic reduction (SCR) activity at low temperature but weak H₂O resistance restricts its industrial application. To remarkably improve the water resistance of Mn-based catalysts is a key technical problem. In this work, the H₂O endurance and self-cleaning properties of OMS-2 catalysts are remarkably improved by the facile process, construction of hydrophobic coating. The performance of the hydrophobic layer on the bulk OMS-2 catalyst surface could be effectively controlled by adjusting the polydimethylsiloxane (PDMS) vapor deposition temperature. It is discovered that the 200 °C catalyst obtained super hydrophobic properties and formed with a contact angle of 160.3°, which not only exhibited satisfactory NH₃-SCR activity at low temperatures (140-300 °C) but also dramatically improved H₂O endurance and self-cleaning performance. Moreover, the mechanism of improving H₂O resistance and stability of the 200 °C catalyst was investigated in detail through a series of characterizations. Although the SCR activity of the 200 °C catalyst decreased slightly because of the combination of some active species (O_α and Mn³⁺) with PDMS, the H₂O passivation of the active species was eliminated. The advantage of self-cleaning was confirmed by the analysis of surface species and simulation experiments, which could avoid the accumulation of intermediates on the surface and promote the stability of the OMS-2 catalyst for NH₃-SCR at low temperature. This method of constructing special coating might be a huge step to remarkably improve the H₂O endurance properties of the catalyst and provided a new concept for future industrial application.

Tuning manganese (III) species in manganese oxide octahedral molecular sieve by interaction with carbon nanofibers for enhanced pollutant degradation in the presence of peroxymonosulfate

The development of environmental-benign, efficient, and durable Mn based catalysts is of great importance for remediation of organic pollutants. In this study, a series of cryptomelane-type octahedral molecular sieve (OMS-2) and carbon nanofibers (CNFs) nanocomposites (OC) were synthesized by a facile refluxing approach under different conditions, and the catalytic activity was evaluated for Acid Orange 7 degradation under neutral conditions via peroxymonosulfate activation. It was revealed that the composites were more efficient than the pure OMS-2 and CNFs, due to the higher amounts of Mn(II) and Mn(III) species and the synergistic effects between OMS-2 and CNFs. The OC catalysts presented long-term stability without the leaching of Mn ions during seven consecutive cycles. Radical scavenger and EPR experiments indicated that the low valent Mn species in the composites were oxidized by PMS to produce sulfate radicals to degrade dyes. However, the structure and performance of OC were influenced significantly by CNFs dosage and refluxing time. Under a high CNFs ratio or a long refluxing time, OMS-2 was damaged and lost its catalytic activity completely. This study provides important implications for turning of the valence of Mn species in manganese oxides, and widens the practical applications of the manganese oxides/ carbon materials in wastewater treatment.

Room-temperature synthesis of OMS-2 hybrids as highly efficient catalysts for pollutant degradation via peroxymonosulfate activation

Due to low cost and low toxicity, manganese oxides have been extensively explored to reduce organic pollutants in wastewater via peroxymonosulfate (PMS) activation; but the development of manganese-based catalysts with facile synthesis process at low temperatures and high efficiency is still of significant practical interest. In this paper, a simple room temperature method has been successfully developed to synthesize cryptomelane-type manganese octahedral molecular sieves (OMS-2) from KMnO₄ and MnSO₄ in the presence of carbon nanotubes (CNTs). The redox reaction between amorphous manganese oxide and CNTs results the decrease of manganese valences and the formation of OMS-2 phase at a low temperature of 25 °C. The changes of synthesis time, temperature and CNTs dosage altered the characteristics of the prepared materials. Compared with CNTs, OMS-2 and other manganese oxides hybrids, the synthesized catalysts demonstrated a remarkable efficiency for PMS activation to degrade organic dyes and a superior reusability during ten successive cycles. Sulfate radicals were formed as the active species in the system from the oxidation of low valent Mn species by PMS. This study not only provides a simple method to synthesize OMS-2 at room temperature, but also improves the understanding of PMS activation on manganese-based catalysts for pollutants degradation.

Enhanced oxidation of arsenite to arsenate using tunable K+ concentration in the OMS-2 tunnel

Cryptomelane-type octahedral molecular sieve manganese oxide (OMS-2) possesses high redox potential and has attracted much interest in its application for oxidation arsenite (As(III)) species of arsenic to arsenate (As(V)) to decrease arsenic toxicity and promote total arsenic removal. However, coexisting ions such as As(V) and phosphate are ubiquitous and readily bond to manganese oxide surface, consequently passivating surface active sites of manganese oxide and reducing As(III) oxidation. In this study, we present a novel strategy to significantly promote As(III) oxidation activity of OMS-2 by tuning K⁺ concentration in the tunnel. Batch experimental results reveal that increasing K⁺ concentration in the tunnel of OMS-2 not only considerably improved As(III) oxidation kinetics rate from 0.027 to 0.102 min^-1, but also reduced adverse effect of competitive ion on As(III) oxidation. The origin of K⁺ concentration effect on As(III) oxidation was investigated through As(V) and phosphate adsorption kinetics, detection of Mn²⁺ release in solution, surface charge characteristics, and density functional theory (DFT) calculations. Experimental results and theoretical calculations confirm that by increasing K⁺ concentration in the OMS-2 tunnel not only does it improve arsenic adsorption on K⁺ doped OMS-2, but also accelerates two electrons transfers from As(III) to each bonded Mn atom on OMS-2 surface, thus considerably improving As(III) oxidation kinetics rate, which is responsible for counteracting the adverse adsorption effects by coexisting ions.

Controlled growth of γ-MnO2 nanoflakes on OMS-2 for efficient decomposition of organic dyes in aqueous solution via peroxymonosulfate activation

The development of green and efficient catalysts for peroxymonosulfate (PMS) activation and abatement of organic pollutants in wastewater is of significant practical interest. In this paper, the three-dimensional mixed manganese oxides of OMS-2 and γ-MnO₂ were fabricated through a simple refluxing method from KMnO₄ and MnSO₄. It was found that growth of γ-MnO₂ nanoflakes on OMS-2 can be controlled by the concentration of MnSO₄. The catalysts not only have many excellent structural properties such as interconnected network and highly exposed active sites, but also show the high ratio of low valent manganese species. In particular, the catalysts exhibited much higher efficiency for Acid Orange 7 degradation in the presence of PMS than pure OMS-2 or γ-MnO₂. The oxidation of Mn(III) species by PMS occurs in the system with the formation of sulfate and hydroxyl radicals contributed to the dye degradation. Moreover, the catalysts showed good stability and reusability during four consecutive cycles. Thus, the environmental friendly mixed manganese oxides of γ-MnO₂ and OMS-2 with low cost, facile synthesis process and high efficiency are very promising catalysts for PMS activation and pollutants degradation.

Catalytic degradation of Acid Orange 7 by manganese oxide octahedral molecular sieves with peroxymonosulfate under visible light irradiation

In this paper, the photodegradation of Acid Orange 7 (AO7) in aqueous solutions with peroxymonosulfate (PMS) was studied with manganese oxide octahedral molecular sieves (OMS-2) as the catalyst. The activities of different systems including OMS-2 under visible light irradiation (OMS-2/Vis), OMS-2/PMS and OMS-2/PMS/Vis were evaluated. It was found that the efficiency of OMS-2/PMS was much higher than that of OMS-2/Vis and could be further enhanced by visible light irradiation. The catalyst also exhibited stable performance for multiple runs. Results from ESR and XPS analyses suggested that the highly catalytic activity of the OMS-2/PMS/Vis system possible involved the activation of PMS to sulfate radicals meditated by the redox pair of Mn(IV)/Mn(III) and Mn(III)/Mn(II), while in the OMS-2/PMS system, only the redox reaction between Mn(IV)/Mn(III) occurred. Several operational parameters, such as dye concentration, catalyst load, PMS concentration and solution pH, affected the degradation of AO7.

Enhanced Photocatalytic Degradation of Methyl Orange Dye under the Daylight Irradiation over CN-TiO₂ Modified with OMS-2

In this study, CN-TiO₂ was modified with cryptomelane octahedral molecular sieves (OMS-2) by the sol-gel method based on the self-assembly technique to enhance its photocatalytic activity under the daylight irradiation. The synthesized samples were characterized by X-ray diffraction (XRD), UV-vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and porosimeter analysis. The results showed that the addition of OMS-2 in the sol lead to higher Brunauer-Emmett-Teller (BET) surface area, pore volume, porosity of particle after heat treatment and the specific surface area, porosity, crystallite size and pore size distribution could be controlled by adjusting the calcination temperature. Compared to the CN-TiO₂-400 sample, CN-TiO₂/OMS-2-400 exhibited greater red shift in absorption edge of samples in visible region due to the OMS-2 coated. The enhancement of photocatalytic activity of CN-TiO₂/OMS-2 composite photocatalyst was subsequently evaluated for the degradation of the methyl orange dye under the daylight irradiation in water. The results showed that the methyl orange dye degradation rate reach to 37.8% for the CN-TiO₂/OMS-2-400 sample under the daylight irradiation for 5 h, which was higher than that of reference sample. The enhancement in daylight photocatalytic activities of the CN-TiO₂/OMS samples could be attributed to the synergistic effects of OMS-2 coated, larger surface area and red shift in adsorption edge of the prepared sample.