MnO2 Surface Research Articles

Enhanced removal of phenolic pollutants over MnO2 initiated by peracetic acid: In situ generation of a heterogeneous Mn(III)-hydroperoxo complex

Widely distributed manganese oxides are among the strongest oxidants in nature and effectively enhance the water decontamination by peracetic acid (PAA) activation. However, the precise mechanism requires further investigation. In this work, the intrinsic mechanism of PAA activation by MnO2 was systematically studied. Through in situ diffuse-reflectance UV–vis spectra and resonance Raman spectra, the active species was identified as a non-radical heterogenous Mn(III)-hydroperoxo complex. During the process, PAA drastically converted Mn(IV) in MnO2 into homogeneous Mn(II), which subsequently generated the active Mn(III) species in the aid of PAA on the surface of MnO2. By interacting with pollutants, the peroxide bond in Mn(III)-hydroperoxo broke, accompanied by the regeneration of Mn(IV). Eight phenolic compounds having para-substituent of –CH3, −H, −Cl, –COCH3, –CHO, –COOH, –COOCH3, and –NO2 were selected for degradation. Due to the electrophilic character of Mn(III)-hydroperoxo, the removal rates were 100 %, 100 %, 98.8 %, 69.8 %, 49.6 %, 30.2 %, 28.1 %, and 3.0 % respectively at the 45 min of experiment. The MnO2/PAA system was also readily coupled with a filtration membrane for continuous treatment of phenol in nearly 100 % removal efficiency. This study not only offered an efficient non-radical PAA activation protocol for water decontamination, but also elucidated the catalytic mechanism and redox cycle of MnO2 in PAA solution in the selective oxidation process.

Elucidating the diffusion and interaction mechanisms of Zn2+ and H+ in MnO2 for aqueous zinc-ion batteries: A DFT study

MnO2 is a promising cathode material for aqueous zinc-ion batteries (AZIBs) with potential applications in large-scale energy storage systems. However, its practical use is hindered by poor cyclic stability and slow diffusion kinetics. Despite considerable efforts, success has been limited due to an unclear understanding of the intrinsic properties of Zn2⁺ and proton (H⁺) insertion into MnO2. This study investigates the predominant phases of MnO2—α, β, δ, and γ—and elucidates their reaction mechanisms using first-principles computational methods based on density functional theory (DFT). Our research reveals that H⁺ ions exhibit a higher initial insertion voltage compared to Zn2⁺ ions, but their diffusion in MnO2 is significantly impeded by strong interactions with oxygen. Additionally, H⁺ insertion partially obstructs Zn2⁺ ion migration. Furthermore, the insertion of Zn2⁺/H⁺ and the adsorption of H⁺ on the MnO2 surface are intricately linked to the dissolution process of manganese. This work significantly enhances our fundamental understanding of MnO2 as a cathode material for AZIBs.

"All-in-One" MnO2@PtAuRu Nanoreactor for Self-Replenishing and Cascade Catalytic Therapy of Cancer.

Limited by the insufficient catalytic substrates such as H2O2 and O2 in the tumor microenvironment (TME), the continual propelling of nanozymes catalysis therapy of cancer remains a challenge. Here, an all-in-one MnO2@PtAuRu nanoreactor is constructed for self-propelled and cascade catalytic therapy of tumors. The MnO2@PtAuRu is constructed by using hollow MnO2 (≈150nm) as the core-carrier and ultrasmall PtAuRu nanoclusters (≈2nm) anchoring on the surface MnO2. According to the glucose oxidase (GOD)/catalase (CAT)/peroxidase (POD) mimic multienzyme activity of PtAuRu nanoclusters, cascaded nanocatalytic reactions can be self-replenishing to persistently produce •OH for superior chemodynamic therapy (CDT). Additionally, the MnO2 carrier can protect the ultrasmall PtAuRu nanoclusters during the circulation and the overexpressed glutathione (GSH) in the tumor can also be degraded by the MnO2 to synergy the CDT. The MnO2@PtAuRu displays obvious photothermal properties which further enhance the cascade catalytic ability and synergistic therapeutic effect. Therefore, this all-in-one nanozyme provides a promising strategy for the rational design of self-replenishment and self-replenishing cascade catalytic therapy of cancer.

Layered sodium manganese oxides for toxic nitrogen dioxide gas removal in ambient conditions

In this study, Na0.7MnO2 samples were successfully synthesized using a solid-state synthesis route with Na and Mn acetate precursors. The resulting Na0.7MnO2 crystallized in a hexagonal lattice with space group P63/mmc. These materials were investigated for their capacity to adsorb 100 ppm of NO2 gas at room temperature. Na0.7MnO2 exhibited an exceptional NO2 adsorption capacity of 163.3 mg/g, surpassing many previously reported porous and non-porous materials. Comprehensive experimental and spectroscopic analyses confirmed the formation of NO, NO2–, and NO3– species, following NO2 adsorption on the Na0.7MnO2 surface. Theoretical calculations further supported the chemisorption of NO2 gas molecules on the Na0.7MnO2 surface, with a maximum adsorption energy of –1.937 eV. Spectroscopic and Bader charge analyses confirmed that the NO2 gas molecule acted as a charge acceptor species on the oxide surface. This work represents the first detailed study validating Na0.7MnO2 as an effective room-temperature adsorbent for capturing and mineralizing NO2 gas. The exceptional NO2 adsorption capacity and the mechanistic insights into the adsorption process suggest the promising potential of Na0.7MnO2 and related oxides for applications in NO2 gas capture and conversion.

Insights into the transformation of dissolved organic matter and carbon preservation on a MnO2 surface: Effect of molecular weight of dissolved organic matter

Dissolved Organic Matter (DOM) is easily adsorbed and transformed by soil minerals and is an important redox-active component of soil and sediment. However, the effects of the molecular weight of DOM on the interface between MnO2 and DOM remain unclear. Herein, fulvic acid (FA) from peat was size-fractionated into four molecular weight fractions (FA>10kDa, FA5–10kDa, FA3–5kDa, and FA<3kDa) and then reacted with δ-MnO2 in this study. The affinity of FA for MnO2 varied significantly with different molecular weights, and large molecular weight FA was more easily adsorbed by MnO2. After 30 h of reaction, the highest mineralization rate was for FA>10kDa (42.39 %), followed by FA5–10kDa (28.65 %), FA3–5kDa (25.58 %), and FA<3kDa (20.37 %), consistent with the results of adsorption. The stronger reducing ability of the large molecular weight fraction of FA to MnO2 was mainly attributed to hydrophobic functional groups, promoting adsorption by MnO2 and the exposure of more active sites. The main active species involved in the mineralization of FA were •OH and Mn4+ through the quenching experiment. Our findings confirm that the large molecular weight fractions of FA play a crucial part in the adsorption and redox reactions of MnO2. These results may help evaluate the performance of different molecular characteristics of FA in the biogeochemical cycles of MnO2 in the soil environment.

Analysis of Microwave Effects on the MnO2-Catalyzed Toluene Oxidation Pathway

Microwave radiation has become an effective catalytic combustion method, especially in the degradation of volatile organic compounds (VOCs) such as toluene using catalysts like MnO2. In this study, a spine waveguide microwave reactor was designed to investigate the influence of different microwave processing conditions on the degradation of toluene catalyzed by MnO2. An experimental system for microwave-assisted catalytic degradation of toluene was established to explore the relationship between microwave power, catalyst conductivity, and toluene degradation rate. The results showed that the efficiency of MnO2 catalyzing toluene degradation had a nonlinear relationship with microwave power, first increasing to a peak and then decreasing. Additionally, the experiment found that the degradation rate of toluene was positively correlated with the conductivity of MnO2. Subsequent characterization analyses using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) further verified the changes in the microstructure and properties of MnO2 under microwave heating. The characterization results showed that with the increase in microwave power, the relative content of Mn3+ on the surface of MnO2 increased, and the relative content of adsorbed oxygen also increased accordingly. At a microwave power of 100 W, the treated MnO2 displayed the optimal ratio of manganese oxidation state and oxide, both close to 1:1, which was more conducive to the degradation of toluene. Based on these findings, this study hypothesized that the microwave-enhanced catalytic degradation of toluene by MnO2 may be attributed to changes in the surface electron transfer kinetics of MnO2, providing new insights into the field of microwave-enhanced catalysis.

Glycogen-assisted biosynthesis of MnO2 for adsorptive elimination of methylene blue from water

A novel biosynthesis of manganese dioxide (MnO2) NPs was achieved by oxidizing glycogen with KMnO4 in an alkaline medium. X-ray powder diffraction, Fourier-transform infrared spectroscopy, N2 sorption, and transmission electron microscope (TEM) techniques were utilized to assess the physicochemical characteristics of the prepared MnO2. The impacts of different adsorption parameters, including contact time, pH, concentration, temperature, and adsorbent dose, on methylene blue (MB) elimination utilizing the prepared MnO2 NPs, were investigated. It was found that the Langmuir model well describes how the dye molecules arrange themselves on the MnO2 surface compared to other models. This proposes a single layer of dye forms, with each molecule attaching to a specific site on the MnO2. Additionally, the results were well-matched with a pseudo-2nd-order model, demonstrating that chemical reactions had a significant role in the adsorption experiment. Under specific conditions (RT, natural pH of the dye solution, and 30-minute contact time), the MnO2 could hold up to 40.85 mg of dye per gram of MnO2 (mg/g). Further analysis reflected that the adsorption procedure was spontaneous, which means it happened favorably without needing extra energy input. This suggests that biosynthesized MnO2 has great potential as an eco-friendly material for cleaning up MB dye contamination in water.

Half-disturbed spin desalination: Asymmetric high-spin states in MnO2 accelerate charge transfer in capacitive deionization

Manganese dioxide (MnO2) has been recognized for its high theoretical capacitance. Recent advancements highlight the potential of modulating the spin state of Mn to elevate charge transfer kinetics. Nonetheless, the specific relationship between Mn spin distribution and desalination efficacy, alongside the underlying mechanism, remains elusive. This study introduces the novel concept of half-disturbed spin desalination, elucidating its complex mechanism via the strategic induction of oxygen vacancies on the MnO2 surface. Theoretical analyses reveal that electron transfer from the O py (up) and pz (up) to the Mn dz2 (up) orbitals alters MnO2's spin state from a symmetric low spin to an asymmetric high spin configuration. This transition disrupts spin up symmetry, bolstering dynamic processes and diminishing the ion diffusion energy barrier. Consequently, oxygen vacancy-enriched MnO2 (VO-MnO2) demonstrates an enhanced specific capacitance of 289 F g−1 at 1 A g−1. Remarkably, in the hybrid capacitive deionization (HCDI) process, it achieves a superior salt adsorption capacity (SAC) of 56.5 mg g−1 and a salt adsorption rate (SAR) of 2.5 mg g−1 min−1. This research pioneers the correlation between spin desalination mechanisms and spin state applications, paving the way for novel applications across diverse fields.

Seeding Atomic Silver into Internal Lattice Sites of Transition Metal Oxide for Advanced Electrocatalysis.

Transition metal oxides (TMOs) are widely studied for loading of various catalysts due to their low cost and high structure flexibility. However, the prevailing close-packed nature of most TMOs crystals has restricted the available loading sites to surface only, while their internal bulk lattice remains unactuated due to the inaccessible narrow space that blocks out most key reactants and/or particulate catalysts. Herein, using tunnel-structured MnO2, this study demonstrates how TMO's internal lattice space can be activated as extra loading sites for atomic Ag in addition to the conventional surface-only loading, via which a dual-form Ag catalyst within MnO2 skeleton is established. In this design, not only faceted Ag nanoparticles are confined onto MnO2 surface by coherent lattice-sharing, Ag atomic strings are also seeded deep into the sub-nanoscale MnO2 tunnel lattice, enriching the catalytically active sites. Tested for electrochemical CO2 reduction reaction (eCO2RR), such dual-form catalyst exhibits a high Faradaic efficiency (94%), yield (67.3mol g-1 h-1) and durability (≈48h) for CO production, exceeding commercial Ag nanoparticles and most Ag-based electrocatalysts. Theoretical calculations further reveal the concurrent effect of such dual-form catalyst featuring facet-dependent eCO2RR for Ag nanoparticles and lattice-confined eCO2RR for Ag atomic strings, inspiring the future design of catalyst-substrate configuration.

AsIII-enhanced oxidation by coexisting MnIII-phenolic complexes during arsenic contaminated groundwater treatment by MnO2

Manganese oxides (MnO2) are widely utilized in arsenic(AsIII)-contaminated groundwater remediation; however, As(III) oxidation is greatly affected by co-occurring substances. This study discusses the oxidation pathways of As(III) oxidation-adsorption in presence of co-solutes (e.g., Mn(II), Fe(II), and 2,4-dichlorophenol (DCP). Mn(II) species competed with As(III) for adsorption and oxidation sites at MnO2 surface, resulting in a decrease in As(III) removal. As(III) oxidation in Mn(II) and Mn(II)/MnO2 systems was primarily occurred via electron-transfer with Mn(III) intermediate, however, Mn(III) species were rapidly oxidized into the less reactive oxidant Mn(IV). In the presence of DCP, As(III) oxidation was enhanced due to stabilization of Mn(III) species by complexation with DCP. Fe(II) enhanced the adsorption of Arsenic and DCP by MnO2, as Fe-(hydr)oxides formed, creating extra adsorption sites on MnO2. Three DCP byproducts were also identified by means of LC/MS analysis in the Mn(II)/MnO2-DCP system, with reaction As(III) oxidation pathways are proposed. The enhanced As(III) oxidation by Mn(II)/MnO2 system, in presence of DCP resulted due to a series of chemical reactions with hydroxyl and phenolic radicals generated in system, in addition to electron transfer with DCP-Mn(III) complexes. These results provide new insights into the environmental behavior of As(III) within mixed water systems containing metals (Mn/Fe) and phenolics.

The adsorption mechanism of 5-fluorouracil on MnO2-based nanostructures by DFT calculation and experimental analysis

It is of great significance to explore the mechanism of drug molecules hiking on nanomaterials for targeted delivery. In this study, we evaluated the interaction potential of three manganese dioxide (MnO2) nanostructures with 5-fluorouracil (5-FU) using a density function theory (DFT) framework and dispersion correction (D3BJ) and conducted experimental verification. All possible adsorption modes and locations were predicted by electrostatic surface potential (ESP) and van der Waals force calculations. The charge transfer between molecular groups was estimated by electron density difference (EDD). The energy-favorable model of 5-FU interaction with distinct MnO2 crystals was investigated by using partial density of states (PDOS). Further, the quantum theory of atoms in molecules (QTAIM) calculation and independent gradient model based on Hirshfeld partition (IGMH) analysis were applied to reveal the interaction details. Our results suggest that 5-FU can be absorbed at different positions on the surface of MnO2, in which the O-adsorption mode is mainly dominated by hydrogen bond, while the Mn-adsorption mode is dominated by coordination interaction. Besides, the dispersion-dominated interaction cannot be ignored. DFT calculations show that MnO2 nanostructures are promising drug carriers, and β-MnO2 exhibits better adsorption behavior than the other two crystals. Experimental study showed that the 5-FU drug loading of β-MnO2 (28.82 %) was higher than that of α-MnO2 (16.72 %) and δ-MnO2 (21.69 %).

Amorphous MnO2 Lamellae Encapsulated Covalent Triazine Polymer-Derived Multi-Heteroatoms-Doped Carbon for ORR/OER Bifunctional Electrocatalysis.

The intelligent construction of non-noble metal materials that exhibit reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with bifunctional electrocatalytic performance is greatly coveted in the realm of zinc-air batteries (ZABs). Herein, a crafted structure-amorphous MnO2 lamellae encapsulated covalent triazine polymer-derived N, S, P co-doped carbon sphere (A-MnO2/NSPC) is designed using a self-doped pyrolysis coupled with an in situ encapsulation strategy. The customized A-MnO2/NSPC-2 demonstrates a superior bifunctional electrocatalytic performance, confirmed by a small ΔE index of 0.64V for ORR/OER. Experimental investigations, along with density functional theory calculations validate that predesigned amorphous MnO2 surface defects and abundant heteroatom catalytic active sites collectively enhance the oxygen electrocatalytic performance. Impressively, the A-MnO2/NSPC-based rechargeable liquid ZABs show a large open-circuit potential of 1.54V, an ultrahigh peak power density of 181mW cm-2, an enormous capacity of 816 mAh g-1, and a remarkable stability for more than 1720 discharging/charging cycles. Additionally, the assembled flexible all-solid-state ZABs also demonstrate outstanding cycle stability, surpassing 140 discharging/charging cycles. Therefore, this highly operable synthetic strategy offers substantial understanding in the development of magnificent bifunctional electrocatalysts for various sustainable energy conversions and beyond.

Self-Assembled Layer of Organic Phosphonic Acid Enables Highly Stable MnO2 Cathode for Aqueous Znic Batteries.

Manganese dioxide (MnO2) is an attractive cathode material for aqueous zinc batteries (AZBs) owing to its environmental benignity, low cost, high operating voltage, and high theoretical capacity. However, the severe dissolution of Mn2+ leads to rapid capacity decay. Herein, a self-assembled layer of amino-propyl phosphonic acid (AEPA) on the MnO2 surface, which significantly improves its cycle performance is successfully modified. Specifically, AEPA can be firmly attached to MnO2 through a strong chemical bond, forming a hydrophobic, and uniform organic coating layer with a few nanometers thickness. This coating layer can significantly inhibit the dissolution of Mn2+ by avoiding the direct contact between the electrolyte and cathode, thus enhancing thestructural integrity and redox reversibility of MnO2. As a result, the MnO2@AEPA cathode achieves a high reversible capacity of 223mAhg-1 at 0.5Ag-1 and a high capacity retention of 97% after 1700cycles at 1Ag-1. This work provides new insights in developing stable Mn-based cathodes for aqueous batteries.

Tunable Zn2+ de-solvation behavior in MnO2 cathodes via self-assembled phytic acid monolayers for stable aqueous Zn-ion batteries.

Sluggish ion diffusion kinetics at the electrode/electrolyte interface leads to insufficient rate capability and poor structural reversibility, which are mainly attributed to the hydrated Zn2+ migration process being inhibited due to its huge de-solvation energy barriers. Herein, a self-assembly method is proposed in which a multifunctional monolayer with phytic acid (PA) is coated on the surface of MnO2 strongly attaching to the substrate with the formation of chemical bonding to effectively prevent the dissolution of PA in the electrolyte. Due to the negative charge and inherent ultra-hydrophilicity of PA, modified MnO2 demonstrates stronger adsorption of positive ions and captures reactive water molecules, easily accelerating the de-solvation process of interfacial hydrated Zn2+, efficiently achieving reversible Zn2+ insertion/extraction. Meanwhile, the coating layers can protect the substrate from attack by active water molecules, thus inhibiting cathode dissolution during battery cycling. Experimental characterization studies reveal that the protected MnO2 cathode exhibits a remarkable specific capacity of 273 mA h g-1 with a zinc intercalation capacity contribution of more than 60% at a current density of 0.1 A g-1. Additionally, even at a high current density of 1 A g-1, it maintains a capacity of 197 mA h g-1, far exceeding that of pure MnO2. Furthermore, the Zn-ion pouch cell, serving as a proof of concept, achieves an impressive energy density of 300 W h kg-1 and exhibits remarkable capacity retention of 77% even after 100 cycles. This work offers a universal strategy for expediting the de-solvation process of hydrated Zn2+ on the surface of manganese-based cathodes in aqueous zinc-ion batteries.

Pseudocapacitance PPy coating modulation on the nano-MnO2 surface to improve zinc storage ability in neutral fiber battery

With the rapid development of wearable electronic technology, the research of fiber energy storage batteries, such as fiber Zn-MnO2 batteries, has also been promoted. However, mechanically mixing conductive carbon and MnO2 to improve charge transfer efficiency is still considered inefficient. In this work, PPy was used to coat the surface of nano MnO2. By regulating the structure of C-MnO2, an excellent charge transfer efficiency C-L27-MnO2 material with a suitable thickness and uniform PPy film was obtained. When the C-L27-MnO2 material was applied to a neutral fiber zinc-ion battery, it showed a significantly improved cycle and rate capacity. At a current density of 10 mA g−1, the specific capacity of the MnO2 electrode increased from 124.2 mAh/g to 316.0 mAh/g.

Oxidation of organic pollutants over MnO2 in cold water assisted by peroxydisulfate

MnO2 can be used in advanced oxidation processes to activate peroxydisulfate (PS) and generate reactive species such as sulfate radicals (SO4•-), hydroxyl radicals (•OH) and singlet oxygen (1O2) for the abatement of organic water contaminants. Alternatively, one can use MnO2 as particulate oxidant which, in contrast to PS, is active already at low temperatures. This low-temperature activity is particularly attractive for in situ groundwater treatment. The roles of PS and MnO2 as oxidant and catalyst, respectively, may invert: MnO2 functions as primary oxidant and PS works as re-oxidant of reduced Mn-oxides. In this way, the limited oxidation capacity of particulate MnO2 can be overcome. So far, such reaction systems have not been extensively explored.In the present study, the degradation of 4-hydroxybenzoic acid (HBA) by PS + MnO2 was studied under various experimental conditions, by varying reaction temperature (15–45 °C), reagent concentrations, and water matrix. Both PS and MnO2 alone have the ability to effect the oxidation of HBA. In the co-presence of PS and MnO2, a fast removal of HBA from water was achieved even at low temperatures (half-life of HBA about 0.5 h with 2.4 g L-1 MnO2 at 25 °C), owing to a synergic effect between the two components.Quenching experiments indicate that the primary HBA oxidation step is a heterogeneous reaction on the MnO2 surface rather than a homogeneous reaction driven by SO4•- or 1O2, as frequently reported in literature. This is in line with results from the PS driven oxidation of perfluorooctanoic acid which is not significantly enhanced in the presence of MnO2.

Surface Adsorption and Proton Chemistry of Ultra-Stabilized Aqueous Zinc-Manganese Dioxide Batteries.

Aqueous rechargeable Zinc (Zn) batteries incorporating MnO2 cathodes possess favorable sustainability properties and are being considered for low-cost, high-safety energy storage. However, unstable electrode structures and unclear charge storage mechanisms limit their development. Here, advanced transmission electron microscopy, electrochemical analysis, and theoretical calculations are utilized to study the working mechanisms of a Zn/MnO2 battery with a Co2+ -stabilized, tunnel-structured α-MnO2 cathode (Cox MnO2 ). It is shown that Co2+ can be pre-intercalated into α-MnO2 and occupy the (2×2) tunnel structure, which improves the structural stability of MnO2 , facilitates the proton diffusion and Zn2+ adsorption on the MnO2 surface upon battery cycling. It is further revealed that for the MnO2 cathode, the charge storage reaction proceeds mainly by proton intercalation with the formation of α-Hy Cox MnO2 , and that the anode design (with or without Zn metal) affects the surface adsorption of by-product Zn4 SO4 (OH)6 ·nH2 O on MnO2 surface. This work advances the fundamental understanding of rechargeable Zn batteries and also sheds light on efficient electrode modifications toward performance enhancement.

A novel catalytic filtration process using MnO2@sand and peroxymonosulfate for unselective removal of organic contaminants from water

Multiple catalytic oxidation processes involving new synthesized materials have recently been examined to replace conventional oxidative treatment methods for water purification, but upscaling and demonstration stages are mostly lacking, which hinders their practical implementation. In this study, we introduce a novel catalytic process where peroxymonosulfate (PMS) is activated by MnO2 surfaces that are attached on natural sand as part of a catalytic filtration column (CFC). PMS decomposition in the CFC was stable during steady-state filter operation with different natural waters (tap water and secondary effluent) and sulfate radicals were identified as main radical species. Complete oxidation (>99 %) of 10 mg/L rhodamine B in tap water could be achieved with PMS concentrations as low as 0.2 mM and a residence time of less than 3 min. Furthermore, unselective oxidation of various recalcitrant and environmentally-relevant trace organic chemicals (e.g., carbamazepine, sulfamethoxazole, and benzotriazole) was achieved with 0.6 mM PMS in tap water and secondary effluent, proving the robustness of the process in presence of multiple organic and inorganic constituents. Future investigations are needed to optimize the CFC process for specific applications and confirm its operation in real-world water matrices and to study sustainability aspects. Overall, this study demonstrates the potential of the CFC process to be implemented as a practical nano-enabled water treatment and offers a framework for next steps to be taken before upscaling. It provides important performance data that can be used as reference for future proposals of scalable catalytic oxidation water treatment technology.

Complementary Homogeneous Electrochemical and Photothermal Dual-Modal Sensor for Highly Sensitive Detection of Organophosphorus Pesticides via Stimuli-Responsive COF/Methylene Blue@MnO2 Composite.

Credible and on-site detection of organophosphorus pesticides (OPs) in complex matrixes is significant for food security and environmental monitoring. Herein, a novel COF/methylene blue@MnO2 (COF/MB@MnO2) composite featured abundant signal loading, a specific recognition unit, and robust oxidase-like activity was successfully prepared through facile assembly processes. The multifunctional composite acted as a homogeneous electrochemical and photothermal dual-mode sensing platform for OPs detection through stimuli-responsive regulation. Without the presence of OPs, the surface MnO2 coating could recognize thiocholine (TCh), originating from acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylthiocholine (ATCh), and exhibited a distinctly amplified diffusion current due to the release of plentiful MB; while the residual MnO2 nanosheets could only catalyze less TMB into oxidized TMB (oxTMB) with a typical near-infrared (NIR) absorption, enabling NIR-driven photothermal assay with a low temperature using a portable thermometer. Based on the inhibitory effect of OPs on AChE activity and OP-regulated generation of TCh, chlorpyrifos as a model target can be accurately detected with a low limit of detection of 0.0632 and 0.108 ng/mL by complementary electrochemical and photothermal measurements, respectively. The present dual-mode sensor was demonstrated to be excellent for application to the reliable detection of OPs in complex environmental and food samples. This work can not only provide a complementary dual-mode method for convenient and on-site detection of OPs in different scenarios but also expand the application scope of the COF-based multifunctional composite in multimodal sensors.

Enhanced formaldehyde oxidation over MnO2 and doped manganese-based catalysts: Experimental and theoretical Insights into mechanism and performance

Thermal catalytic degradation of formaldehyde (HCHO) over manganese-based catalysts is garnering significant attention. In this study, both theoretical simulations and experimental methods were employed to elucidate the primary reaction pathways of HCHO on the MnO2(110) surface. Specifically, the effects of doping MnO2 with elements such as Fe, Ce, Ni, Co, and Cu on the HCHO oxidation properties were evaluated. Advanced characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS), were employed to discern the physical properties and chemical states of the active components on the catalyst surface. The comprehensive oxidation pathway of HCHO on the MnO2(110) surface includes O2 adsorption and dissociation, HCHO adsorption and dehydrogenation, CO2 desorption, H2O formation and desorption, oxygen vacancy supplementation, and other elementary reactions. The pivotal rate-determining step was identified as the hydrogen migration process, characterized by an energy barrier of 234.19 kJ mol−1. Notably, HCHOO and *CHOO emerged as crucial intermediates during the reaction. Among the doped catalysts, Fe-doped MnO2 outperformed its counterparts doped with Ce, Ni, Co, and Cu. The optimal degradation rate and selectivity were achieved at a molar ratio of Fe: Mn = 0.1. The superior performance of the Fe-doped MnO2 can be ascribed to its large specific surface area, conducive pore structure for HCHO molecular transport, rich surface-adsorbed oxygen species, and a significant presence of oxygen vacancies.