MnO2 Catalyst Research Articles

Processes of Metal Oxides Catalyst on Conversion of Spent Coffee Grounds into Rich-Synthesis Gas by Gasification

Spent coffee grounds (SCGs), a waste product of the coffee industry, present a significant untapped resource for fuel production. This study aims to optimize the gasification of SCG using various metal catalysts (NiO, MnO2, Al2O3, and Fe2O3) to maximize syngas yield. SCG samples were gasified at different temperatures (800 °C, 900 °C, 1000 °C) and analyzed using Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TG-DTA), and Fourier Transform Infrared (FTIR) spectroscopy to evaluate catalyst performance and reaction mechanisms. The findings indicated that utilizing mixing techniques for physical contact to introduce catalysts led to a uniform distribution of catalyst particles throughout the sample. The decomposition rate of the gasification experiment after adding the catalyst was 24% faster than that of the pure SCGs. In the gasification experiment, the MnO2 catalyst showed the highest CO production, which was 71% higher than that of NiO under the same conditions. At this temperature, MnO2 generated around 171% more CO than at 800 °C, surpassing the yields observed with other catalysts. The study concludes that Mn emerged as the most promising catalyst, significantly improving both CO and CH4 yields. Selecting the appropriate metal catalyst and optimizing operational temperatures are crucial for enhancing the efficiency of SCG gasification.

Enhanced formaldehyde oxidation of plywood using MnO2 nanoflowers supported on porous hydrothermal carbon at ambient temperature

MnO2 nanoflowers were supported on porous hydrothermal carbon via in situ loading and used for the catalytic oxidation of formaldehyde from plywood manufactured with UF adhesive. The prepared MnO2 had a δ-type structure. The use of hydrothermal carbon (Hcs) improved the dispersion of active components in the materials and provided space for the catalytic reactions. Hcs/MnO2 exhibited catalytic activity and stability during formaldehyde oxidation at room temperature. The formaldehyde removal efficiency reached 90.43 % in 30 min. The Hcs/MnO2 had more oxygen vacancies than the MnO2 catalyst, providing more abundant active substances such as -OH, O2-, O- and other active substances to promote the catalytic reaction. The effects of Hcs/MnO2 on the formaldehyde emission and the mechanical properties of plywood were investigated. The MOR, MOE and bonding strength of the Hcs/MnO2-loaded plywood with 9 % Hcs were 51.1 MPa, 2994 MPa, and 0.8 MPa, respectively. After 28 d of exposure, the formaldehyde emitted from the plywood treated with Hcs/MnO2 decreased to 0.023 mg/m3, which fulfilled the requirement for ENF grading. The controlled release of free formaldehyde from plywood by Hcs/MnO2 was reflected by changes in mass transfer and catalytic oxidation. The manganese oxides in the micropores of the wood reduced the diffusion coefficient of free formaldehyde and hindered migration through the substrate. Once the concentration of formaldehyde decreased to a certain level, it was degraded by Hcs/MnO2, thus removing formaldehyde from the source.

Breaking the Passivation Effect for MnO2 Catalysts in Li-S Batteries by Anion-Cation Doping.

Transition metal oxides (TMOs) are recognized as high-efficiency electrocatalyst systems for restraining the shuttle effect in lithium-sulfur (Li-S) batteries, owing to their robust adsorption capabilities for polysulfides. However, the sluggish catalytic conversion of Li2S redox and severe passivation effect of TMOs exacerbate polysulfide shuttling and reduce the cyclability of Li-S batteries, which significantly hinders the development of TMOs electrocatalysts. Here, through the anion-cation doping approach, dual incorporation of phosphorus and molybdenum into MnO2 (P,Mo-MnO2) was engineered, demonstrating effective mitigation of the passivation effect and allowing for the simultaneous immobilization of polysulfides and rapid redox kinetics of Li2S. Both experimental and theoretical investigations reveal the pivotal role of dopants in fine-tuning the d-band center and optimizing the electronic structure of MnO2. Furthermore, this well-designed configuration processes catalytic selectivity. Specifically, P-doping expedites rapid Li2S nucleation kinetics by minimizing reaction-free energy, while Mo-doping facilitates robust Li2S dissolution kinetics by mitigating decomposition barriers. This dual-doping approach equips P,Mo-MnO2 with robust bi-directional catalytic activity, effectively overcoming passivation effect and suppressing the notorious shuttle effect. Consequently, Li-S batteries incorporating P,Mo-MnO2-based separators demonstrate favorable performance than pristine TMOs. This design offers rational viewpoint for the development of catalytic materials with superior bi-directional sulfur electrocatalytic in Li-S batteries.

MnO2 as efficient and robust catalyst for halogen-free synthesis of cyclic carbonate from CO2 and epoxides

Cyclic carbonate is an important fine chemical/intermediate and their synthesis from CO2 and epoxides is economical and green pathway, however, it is still a challenge to develop a efficient and robust catalyst for halogen-free synthesis of cyclic carbonate. Herein, we successfully prepared large-scale MnO2 catalysts by microfluidics method, and the MnO2 especially δ-MnO2 showed >99.9 % propylene carbonate yield under 393 K, 3 MPa CO2 and DMF solvent. Moreover, the initial TOF value over δ-MnO2 on the basis of total catalyst amount was 5.80 h−1, which was the maximum among all the reported heterogeneous metallic oxide catalysts for halogen-free synthesis of propylene carbonate from CO2 and epoxides, and the superior catalytic performance can be ascribed to the abundant moderate acidic–basic active sites on the δ-MnO2 surface and the synergistic effect of δ-MnO2 and DMF. Furthermore, the δ-MnO2 can be reused at least 4 times and exhibited wide substrate applicability.

Bio-templated micromotors for simultaneous adsorption and degradation of microplastics

Microplastic contamination has increasingly become a focus of public attention due to its ubiquity on earth. As the appearance of microplastics in the human body has raised great concerns about their effects on human health, it is imperative to develop efficient technologies to remove microplastics. In this work, taking advantage of the hollow structure of kapok fibers and low-cost MnO2 catalyst, a tubular self-driven micromotor was synthesized for simultaneous adsorption and degradation of microplastics. Functionalization with polydopamine endowed the micromotors with the ability of on-the-fly capture of microplastics. Attributed to the enhanced diffusion caused by the self-propulsion, the propelled micromotors exhibited an adsorption efficiency 3.41 times that of static micromotors, achieving a removal ratio of 73.57 % in 1 mg/mL polystyrene microspheres dispersion in 30 min without external stirring, which poses a prospective approach for in-situ removal of microplastics. In addition, coupling with photocatalytic CdS, the formed CdS/PDA heterojunction could generate photo-induced charge carriers, which together with the active species produced through the Fenton-like processes contributed to the degradation of microplastics. This work proposed a new platform for the simultaneous adsorption and degradation of microplastics and may provide implications for the removal of microplastics in the environment.

Transition metal-doped modification of lattice defects in formaldehyde catalysts − Controlling the specific surface area and mass transfer

Exploring room temperature treatment of formaldehyde (HCHO) is a crucial area of ongoing research, with transition metal catalysts showing promise for future development due to their cost-effectiveness. However, the challenge lies in the limited ability of these catalysts to activate oxygen efficiently and maintain stable catalytic oxidation of HCHO. The Mn3O4 catalyst doped with Cr, Cu, Zn, and Co was synthesized via hydrogen reduction in this study. Cr/Mn3O4 showed exceptional catalytic performance by completely converting HCHO at 30 °C and 200 ppm, while maintaining excellent stability during a 75-hour durability test. The characterization techniques revealed that the incorporation of transition metal Cr into the Mn3O4 lattice resulted in an increased number of oxygen vacancy defects on its surface, thereby significantly enhancing its ability to activate oxygen. Furthermore, Cr doping substantially augmented the specific surface area and abundance of mesoporous structures in the catalyst, which provided ample active sites and facilitated efficient mass transfer for substrate oxidation by promoting diffusion to these active sites. These enhancements ultimately led to a heightened activation of oxygen and H2O into reactive oxygen species, consequently reducing the dominance of side reaction pathways caused by intermediates occupying active sites.

Crystal-dependent doping effects over MnO2 for Fenton-like catalysis optimization

Heteroatom doping is one of the most important strategies to improve the Fenton-like activity of heterogenous catalysts by optimizing the coordination environment and electronic structure of active sites. However, the doping effect on the crystal structure that governs the atom orientation and chemical stability of catalysts remains largely elusive. Here, we report the rational design and synthesis of a series of fluorine-doped MnO2 catalysts with different crystal phases (denoted as F-α-, F-β-, F-γ-, and F-δ-MnO2), aiming to reveal the crystal-dependent disparity of doping effects in PMS activation. Results demonstrate that fluorine doping exhibits selectivity towards specific crystal structures of MnO2 catalyst. The γ-MnO2 with a lowest Mn average oxidation state (AOS) significantly enhances the adsorption and activation of PMS upon fluorine doping, outperforming other MnO2 counterparts (i.e., α, β, and δ-type). Interestingly, we identify a strong linear correlation (R2 = 0.99) between the degradation rate constant variation (Δk) and ΔAOS divergence of MnO2 catalysts before and after fluorine doping, revealing a novel structure–activity relationship for Fenton-like catalysis optimization. In addition, a nonradical pathway with synergism of 1O2 and active surface-complex contributes greatly to pollutant removal in the developed F-γ-MnO2/PMS system, thereby demonstrating outstanding degradation performance towards electron-rich organics. This work provides fundamental insights into the crystal structure–activity relationship in the disparity of doping effect, which may inspire the rational design of efficient Fenton-like catalysts for wastewater treatment.

Immobilization of MnO2 nanoflowers on coils using direct heating method for organic pollutant remediation

The immobilization of catalysts on supporting substrates for the removal of organic pollutants is a crucial strategy for mitigating catalyst loss during wastewater treatment. This study presented a rapid and cost-effective direct heating method for synthesizing MnO2 nanoflowers on coil substrates for the removal of organic pollutants. Traditional methods often require high power, expensive equipment, and long synthesis times. In contrast, the direct heating approach successfully synthesized MnO2 nanoflowers in just 10 min with a heating power of approximately 40 W·h after the heating power and duration were optimized. These nanoflowers effectively degraded 99% Rhodamine B in 60 min with consistent repeatability. The catalytic mechanisms are attributed to crystal defects in MnO2, which generate electrons to produce H2O2. Mn2+ ions in the acidic solution further dissociate H2O2 molecules into hydroxyl radicals (·OH). The high efficiency of this synthesis method and the excellent reusability of MnO2 nanoflowers highlight their potential as a promising solution for the development of supporting MnO2 catalysts for organic dye removal applications.

Catalytic selectivity in activating peroxymonocarbonate with a commercial MnO2 catalyst for water remediation

An in-situ formed peroxymonocarbonate (HCO4-) oxidation system from an equilibrium reaction of H2O2 and HCO3– was established for eliminating organic pollutants in water. With H2O2 as a co-existing oxidant in the system, a commercial available MnO2 was able to proceed selective catalytic activation of HCO4- to generate reactive oxygen species (ROS), such as ·OH and CO3·-. The MnO2 catalyst showed excellent catalytic ability, structural stability and recyclability in the HCO4--based oxidation system for removing tetracycline. The degradation rate of tetracycline reached 86 % and the catalyst could be used for at least 5 times with only 0.1 ppm of Mn leached in water. The kinetic study indicated that HCO4- was more related to the degradation of tetracycline and generated ·OH at the yield of 40 % was primarily from HCO4- activation rather than H2O2 activation. The evaluation of ROS and quenching experiments suggested that CO3·- was most responsible for tetracycline degradation. The catalyst characterization and reaction mechanism indicated that low-valent Mn species were the active sites for HCO4- activation. Finally, in-situ formation of HCO4- derived from CO2 in Na2CO3 solution with H2O2 was preliminarily explored, presenting a MnO2-catalzyed CO2-derived HCO4--based oxidation system for pollutant control. This study shows that a cheap and commercial available MnO2 can be an efficient, facile and selective catalyst for in-situ formed HCO4--based advanced oxidation processes.

Sustainable Utilization of Oak Bark for MnO2 Catalyst Synthesis

Sustainable Utilization of Oak Bark for MnO2 Catalyst Synthesis

High valent manganese-oxo species activation of peroxymonosulfate by different phases manganese oxides nanoplates for catalytic water decontamination

Advanced oxidation processes involving high-valent metal-oxo species have garnered increasing attention for their effective pollutant degradation in water. However, less is known about the mechanism of the species in the manganese oxides (MnOx) system. Therefore, it is crucial to design MnOx catalysts to systematically clarify the high-valent metal-oxo species reaction mechanism. In this study, MnOx nanoplates including Mn3O4, Mn5O8, and Mn2O3 were synthesized via a facile thermal treatment of a Mn(OH)2 precursor to activate peroxymonosulfate (PMS) for mechanism studies. The Mn3O4 catalyst exhibited superior catalytic performance and reusability owing to its unique spinel structure and rapid electron transfer properties. The first-order rate constant for Mn3O4 is 0.64 min−1, which is 3.7 and 1.5 times higher than those of Mn5O8 and Mn2O3, respectively. Scavenging experiments, electron paramagnetic resonance, and electrochemical analysis revealed that free radicals played a negligible role, while the formation of high-valent Mn-oxo species was promoted, facilitated by the Mn3O4-PMS* complex. These high-valent Mn-oxo species were highly reactive towards various pollutants. Moreover, the degradation intermediates of bisphenol A (BPA) in the Mn3O4/PMS system were identified, further elucidating the interactions between high-valent Mn-oxo species and BPA. Additionally, the Mn3O4 catalyst exhibited excellent catalytic performance under practical conditions and maintained superior stability over six cycles. This work provides crucial insights into the role of transition metal oxides in activating PMS for the catalytic degradation of emerging pollutants.

Zr‐doped β‐MnO2 for Low‐temperature Selective Catalytic Reduction of NO by NH3: Preparation, Characterization and Performance

Nanowire‐MnO2 and a series of Zr‐doped MnO2 catalysts with different Zr/Mn molar ratios were successively prepared by hydrothermal and impregnation methods. The Zr‐doped MnO2 catalyst with Zr/Mn molar ratio of 0.04 and calcination temperature of 400°C, proved to be the optimal that possessed the highest low‐temperature denitration efficiency. It showed the NO conversion of ~92% and N2 selectivity of ~80% at 150°C. Characterization results demonstrated that the main active phase of the catalyst was β‐MnO2, Zr atoms interacted with Mn atoms, and Zr doping increased the structural defects, oxygen vacancies and weak acid sites, which effectively enhanced the low‐temperature denitration activity and N2 selectivity of β‐MnO2, also improved the water and sulphur resistance to some extent.

Highly efficient Rh-doped MnO2 nanocatalysts for complete elimination of CO at room temperature.

Obliteration of carbon monoxide is significant due to its hazardous effect on human health and potential application in different fields. Catalytic CO oxidation at lower temperature is the most convenient method to diminish the toxicity of CO. The low-cost catalysts which are exhibiting higher activity at lower temperature with good stability are in demand. The nanosized Rh-doped MnO2 catalysts have been prepared by dextrose-assisted co-precipitation method. Catalytic CO oxidation reaction was carried out over these prepared nanocatalysts under environmentally suitable conditions. XRD confirms the phase formation of prepared catalysts. These samples exhibit rod-like morphology with thickness of rods of less than 10nm which is substantiated from electron microscopy images. XPS data reveals the oxidation state of Mn (+ 4) and Rh (+ 3). These catalysts are highly active for CO oxidation reaction at lower temperature, and one showed complete CO conversion at room temperature. The time-on-stream studies revealed that these catalysts are highly stable for CO oxidation for several hours. These catalysts are decidedly stable in moist condition and also showed higher activity in the presence of moisture, indicating participation of moisture in the oxidation reaction at above room temperatures.

Crystal phase-driven performance of MnO2 in aqueous phase low-temperature thermal catalysis: Synergistic interactions between Mn3+ and surface lattice oxygen

Catalytic oxidation at mild conditions is crucial for mitigating the high pressure and high temperature challenges associated with current catalytic wet air oxidation (CWAO) technologies in wastewater treatment. Among potential materials for catalytic oxidation reactions, polycrystalline MnO2 existed in natural minerals holds considerable promise. However, the relationships between different crystal phases of MnO2 and their catalytic activity sources in aqueous phase remain uncertain and subject to debate. In this research, we synthesized various MnO2 crystal phases, comprising α-, β-, δ-, γ-, ε-, and λ-MnO2, and assessed their catalytic oxidation efficiency during low-temperature heating for treatment of organic pollutants. Our findings demonstrate that λ-MnO2 exhibits the highest catalytic activity, followed by δ-MnO2, γ-MnO2, α-MnO2, ε-MnO2, and β-MnO2. The variations in catalytic activity among different MnO2 are attributed to variances in their oxygen vacancy abundance and redox activity. Furthermore, we identified the primary active species, which include Mn3+ and superoxide radicals (•O2–) generated by surface lattice oxygen of MnO2. This research highlights the critical role of crystal phases in influencing oxygen vacancy content, redox activity, and overall catalytic performance, providing valuable insights for the rational design of MnO2 catalysts tailored for effective organic pollutant degradation in CWAO applications.

Oxygen vacancy-mediated Mn2O3 catalyst with high efficiency and stability for toluene oxidation

Oxygen vacancy engineering in transition metal oxides is an effective strategy for improving catalytic performance. Herein, defect-enriched Mn2O3 catalysts were constructed by controlling the calcination temperature. The high content of oxygen vacancies and accompanying Mn4+ ions were generated in Mn2O3 catalysts calcined at low temperature, which could greatly improve the low-temperature reducibility and migration of surface oxygen species. DFT theoretical calculations further confirmed that molecular oxygen and toluene were easily adsorbed over defective α-Mn2O3 (222) facets with an energy of −0.29 and −0.48 eV, respectively, and corresponding OO bond length is stretched to 1.43 Å, resulting in the highly reactive oxygen species. Mn2O3-300 catalyst with abundant oxygen vacancies exhibited the highest specific reaction rate and lowest activation energy. Furthermore, the optimized catalyst possessed the outstanding stability, water tolerance and CO2 yield. In comparison with the fresh Mn2O3-300 catalyst, the physical structure and surface property of the used catalyst remained almost unchanged regardless of whether undergoing the stability test at consecutive catalytic runs as well as high temperature, and water resistance test. In situ DRIFTS spectra further elucidated that introducing the water vapor had little effect on the reaction intermediates, indicating the excellent durability of the defect-enriched catalyst.

Morphology and size modulation of Mn2O3 catalysts derived from MnBDC to enhance propane complete oxidation

Three Mn2O3 with different morphologies and particle sizes were synthesized using MnBDC as precursor. Interestingly, the morphology and size of MnBDC can be easily regulated by changing the reaction solvents slightly. The Mn2O3-L catalyst with large layers and small crystal size exhibited the best catalytic activity for complete oxidation of propane, with T50 of 266℃ and T90 of 311℃ at the WHSV of 120 L g−1 h−1. The multi-layered structure and small crystal size of Mn2O3-L allowed it to possess a high surface area and more interfacial oxygen defects that facilitated oxygen mobility, improving the catalytic activity. It seems that the large precursor size of MnBDC-L slowed down the decomposition rate of ligands, promoting the formation of large layered structure and small crystal size of Mn2O3-L. This study offers a simple strategy to control the structure and crystal size of Mn2O3 catalysts.

Mining the Carbon Intermediates in Plastic Waste Upcycling for Constructing C-S Bond.

Postconsumer plastics are generally perceived as valueless with only a small portion of plastic waste being closed-loop recycled into similar products while most of them are discarded in landfills. Depositing plastic waste in landfills not only harms the environment but also signifies a substantial economic loss. Alternatively, constructing value-added chemical feedstocks via mining the waste-derived intermediate species as a carbon (C) source under mild electrochemical conditions is a sustainable strategy to realize the circular economy. This proof-of-concept work provides an attractive "turning trash to treasure" strategy by integrating electrocatalytic polyethylene terephthalate (PET) plastic upcycling with a chemical C-S coupling reaction to synthesize organosulfur compounds, hydroxymethanesulfonate (HMS). HMS can be produced efficiently (Faradaic efficiency, FE of ∼70%) via deliberately capturing electrophilic intermediates generated in the PET monomer (ethylene glycol, EG) upcycling process, followed by coupling them with nucleophilic sulfur (S) species (i.e., SO32- and HSO3-). Unlike many previous studies conducted under alkaline conditions, PET upcycling was performed over an amorphous MnO2 catalyst under near-neutral conditions, allowing for the stabilization of electrophilic intermediates. The compatibility of this strategy was further investigated by employing biomass-derived compounds as substrates. Moreover, comparable HMS yields can be achieved with real-world PET plastics, showing its enormous potential in practical application. Lastly, Density function theory (DFT) calculation reveals that the C-C cleavage step of EG is the rate-determining step (RDS), and amorphous MnO2 significantly decreases the energy barriers for both RDS and C-S coupling when compared to the crystalline counterpart.

Insights into Na+/Ca2+ poisoning mechanisms of Zr-modified and unmodified La-Mn perovskite oxide catalysts for NH3-SCR reaction

The presence of alkali metal/alkaline earth metal in fuel gas is one of the important reasons for the deactivation of catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR). In this work, the Na+/Ca2+ poisoning mechanisms of Zr-modified and unmodified La-Mn perovskite oxides for NH3-SCR reaction were studied. It is found that the deposition of Na+ into LaMnO3 (LM) catalyst has a much larger effect than that of Ca2+, while such a result is opposite for the Na+/Ca2+-deposited La0.8Zr0.2MnO3 (ZLM) catalysts. The deposition of Na+/Ca2+ compounds can block the pores and affect the adsorption and activation of NH3 on the catalysts. After Zr modification, the catalyst with a higher Mn4+ content shows a better redox property and increased acid sites. These acidic sites can trap Na+, thereby delaying the catalyst poisoning and causing a better activity of ZLM catalyst than LM catalyst. The deposition of Ca2+ in the form of CaCO3 on the catalyst surface leads to a serious pore blockage, showing a poor resistance to Ca2+. After regeneration, the pores of the catalyst are dredged, and the active sites originally occupied by Na+/Ca2+ are released, resulting in an recovery of catalyst activity. The recovery of adsorbed oxygen and Mn4+ content is the key factor for returning the SCR activity.

Deactivation of layered MnO2 catalyst during room temperature formaldehyde degradation and its thermal regeneration mechanism

Layered δ-MnO2 prepared by a one-step redox method was shown to be deactivated during oxidation of formaldehyde at room temperature. Recovery of the formaldehyde degradation activity was investigated after thermal regeneration at different temperatures. XRD, SEM, TEM, H2-TPR, XPS, TGA and DRIFTS characterization were used to analyze the physical properties of fresh, deactivated and thermally regenerated catalysts. The results showed that deactivation of catalyst was caused by less oxygen vacancies due to increased Mn4+ content during formaldehyde degradation and formation of formate blocking active sites. Thermal regeneration helped to decompose formate at the catalyst surface, restoring some of the active sites. Interplanar spacing of MnO2 became wider and the number of Mn3+ on exposed crystal faces increased. More oxygen vacancies were formed. The activity of deactivated catalyst was restored. Formaldehyde degradation rate of catalyst regenerated at 200 °C remained above 80 % after 6 h, demonstrating the possibility of waste layered catalyst recycling.

Hydrogen peroxide enhancing the process of MnO2-modified ceramic membrane catalyzing micro-nano bubble

In this study, MnO2-modified ceramic membranes (MnO2@CMs) with different morphologies were prepared via hydrothermal method. The concentration of K+ and H+ in the solution determined the morphological evolution of the MnO2 on the CM. We then structured a H2O2/MNB/MnO2@CM system through adding H2O2 to the system of MnO2@CM coupling MNB. The performance analysis of the MnO2 catalyst, including loading, pH zero point charge, crystalline phase, and Mn(Ⅲ) content, determined that the sheet-like MnO2@CM was the most effective for removing pollutants in the H2O2/MNB/MnO2@CM system, achieving 99.45% decolorization and 73.07% TOC removal of methylene blue (model pollutant). The durability of the system was also confirmed through tests on membrane water flux, reuse, and manganese leaching. Mechanism analysis revealed that H2O2 could induce MNBs to generate reactive oxygen species (ROS) in the solution through reaction or hydrolyzing H+, which acted as a pretreatment for MnO2@CM catalytic filtration. In addition, a Fenton-like reaction was also formed on the surface of the MnO2@CM. These reactions greatly enhanced the process of MnO2@CM catalyzing MNB and its efficiency for removing pollutants. This work provides important insights for designing more efficient catalytic CMs and developing novel processes for wastewater treatment.