MnO2-based Catalyst Research Articles

Enhancing catalytic oxidation of benzene with Cu-Doped MnO2: Insights into structural defects and electronic modifications

The present research focuses on the role of copper (Cu) doping in enhancing the catalytic efficacy of manganese dioxide (α-MnO2) for benzene oxidation, a representative volatile organic compound (VOC). Despite the abundance and low cost of MnO2, its pristine form requires improvement for industrial VOC oxidation processes. Utilizing a range of characterization methods, the study reveals that Cu doping generates oxygen vacancies and increases the Mn3+ content within the catalyst structure, which are crucial for enhancing the adsorption and activation of reactants. The findings indicate that Cu-doped α-MnO2 exhibits superior catalytic activity and lower activation energy for benzene oxidation compared to the undoped counterpart. The electronic interactions induced by the doping of Cu and the effect on the catalytic process are demonstrated and discussed in depth. This work emphasizes the strategic importance of dopant optimization and contributes to the advancement of MnO2-based catalysts for environmental remediation, aligning with goals of environmental protection and human health safety.

Defect engineering of copper-doped mesoporous manganese dioxide nanoflowers for enhancing electrocatalytic detection of hydrogen peroxide

Birnessite-type δ-MnO2 nanoflowers doped with copper ions (Cu-MnO2) can enhance the catalytic performance of MnO2-based catalysts for efficient electrochemical sensing of H2O2. There are no significant changes in the surface morphology and lattice structure of bulk MnO2 after Cu doping via engineered cation exchange. The catalytic Cu species are effectively incorporated into the tunnel entrance of δ-MnO2, the Mn site in the MnO6 octahedron, and surface defects. The Cu doping enhances the electrical and ionic conductivities, and generates numerous Mn3+ defects, oxygen vacancies, and Cu+/Cu2+ redox sites. Thus, the Cu doping facilitates the transport and adsorption of more H2O2 molecules to the catalytic sites, significantly increasing the catalytic rate. In contrast, pristine MnO2 exhibits negligible catalytic performance for H2O2 reduction because of its slow catalytic kinetics and low electrical conductivity. The Cu-MnO2 catalyst exhibits higher sensitivity (105.7 μA·mM−1·cm−2), a lower detection limit (0.6 μM), and a broader linear range (0.005 ∼ 4.2 mM) compared to pristine δ-MnO2.

Pyridinic and Pyrrolic-N induced carbon nanotubes support bimetallic oxide nanoparticles as high-performance electrocatalysts for oxygen reduction reaction

In-expensive MnO2-based nanostructure has been considered the ideal electrode material for oxygen reduction reaction (ORR) in fuel cell technology. Regardless of their highly active ORR performance, the fuel cross-over capacity and stability of MnO2-based catalysts are still far from satisfying. Herein, we report the highly active SnO2 nanoparticles (NPs) integrated on N-induced carbon nanotube (SnO2@NC) are combined with MnO2 nanoparticles through the electrodeposition to form effective nanostructured in which the MnO2 NPs and SnO2@NC in situ grown on a glassy carbon electrode (MnO2/SnO2@NC) with well-controlled reaction parameters, that exhibits superior ORR activity for the first time. We assessed the marginal role of pyridinic-N and pyrrolic-N in the electrocatalytic ORR activity, synthesis of MnO2/SnO2 bimetallic oxide electrocatalysts, and distribution behavior of NPs. Bimetallic MnO2/SnO2@NC electrode has unique synergistic interactions between MnO2/SnO2 NPs and NC support, which increases electro-active sites, and fast charge transfers towards ORR as compared to other as-prepared electrodes on account of the rational design of the electrode-electrolyte interface plays a significant role in expediting the ORR kinetics. Additionally, the surface nanostructure features, crystalline structure, as well as electronic chemical states of the MnO2/SnO2 NPs embedded in N-induced CNTs, were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Furthermore, bimetallic MnO2/SnO2@NC nanostructure leads an outstanding ORR performance in the 0.1 M KOH electrolyte, delivering a halfwave potential (-0.878 V) and remarkable limiting current density (-4.14 mA cm−2) and much better tolerance to the ethanol cross-over effect with high stability. Thus, the present strategy opens a new path to the design of a pyridinic-N and pyrrolic-N-oriented bimetallic oxide nanostructure as a high-active electrocatalyst for ORR application.

Mechanistic understanding of electron structure regulation by Co Ion-Doping on MnO2 nanocatalyst during optimizing peroxymonosulfate activation

Controlling the electronic structure of the active centre through ion doping modulation has been widely recognized as an effective means to enhance catalytic activity, particularly for MnO2-based catalysts. However, existing models fail to fully elucidate the underlying mechanism behind this phenomenon. Here, we utilizes the quenching route to synthesize a variety of MnO2-based catalysts (named as Co-MnO2-Q) with different Co content, aiming to investigate the catalytic mechanism of MnO2 in activating peroxymonosulfate (PMS) and establish a correlation between its electronic structure and catalytic activity. Notably, the optimal catalyst (Co-MnO2-Q3) exhibited a rate constant (kobs) for Levofloxacin (LEVO) removal up to 0.105 min−1, which is eight times higher than that of pure MnO2 (0.013 min−1). Experimental and density functional theory (DFT) investigations have demonstrated that the adsorption of PMS on the constructed catalyst models is an exothermic process, which occurs spontaneously and effectively elongates the O–O bond. Meanwhile, DFT calculations reveal a positive correlation between Co-doped catalyst activity and upward shift of Mn 3d and O 2p band centres. Essentially, Co doping enhances the covalency of Mn–O bonds and increases electron density levels near the Fermi energy, thereby facilitating electron transfer from Mn sites to adsorbed PMS molecules and accelerating both PMS activation and catalytic degradation reactions. This study successfully establishes a descriptor that effectively characterizes the intrinsic activity enhancement of MnO2-based catalysts, providing valuable guidance for designing more advanced catalytic systems based on MnO2.

Substitutional C and interstitial N in MnO2/NC catalysts enable high performance of formaldehyde oxidation at room temperature

Formaldehyde (HCHO) is a harmful gas that dangers the health of human kind. Catalytic oxidation provides a green approach converting HCHO to harmless CO2. In this work, we designed MnO2 catalysts by co-doping NC for HCHO catalytic elimination. The substitutional C and interstitial N in the MnO2 activate the surface lattice oxygen and weaken the Mn-O bond strength. The more active surface lattice oxygens and the more active adsorbed dioxygen species at the oxygen vacancies result in the greater HCHO oxidation performance over the MnO2/NC catalysts doping with NC than the MnO2 catalysts without doping. The performance enhancement is dependent on the crystal phase of MnO2, HCHO is fully converted at 30 ℃ over α-MnO2/NC catalyst, followed by 35 °C over the γ-MnO2/NC catalyst. The higher performance of the α-MnO2/NC catalyst is associated with its higher BET surface area, more active surface lattice oxygen and adsorbed dioxygen species than those of the γ-MnO2/NC catalyst. This work verifies the more effective HCHO elimination performance of MnO2-based catalysts by co-doping NC, and the strategy is applicable for other related oxidative reactions.

Boosting the total oxidation of toluene by regulating the reactivity of lattice oxygen species and the concentration of surface adsorbed oxygen

Surface absorbed oxygen (Oads) and surface lattice oxygen (Olatt) specie are critical for accelerating oxidative removal of toluene over MnO2-based catalyst. Herein, an excellent CuMnO2 catalyst has been successfully designed and fabricated. Doping Cu2+ with strong Jahn-Taylor effect into the lattice of MnO2 not only weakens the strength of Mn-O bonds but also induces the formation of oxygen defects, which enhances the reactivity of Olatt and promotes the adsorption of gaseous oxygen molecules. As a result, CuMnO2 reaches 50 % and 90 % of toluene conversion at 212 and 241 °C respectively, showing obviously better catalytic performance compared to CeMnO2, NiMnO2, CoMnO2 and δ-MnO2. Moreover, CuMnO2 exhibits outstanding stability, good cycling stability and excellent water resistance ability. The in situ DRIFTS demonstrate that ring opening of benzoate and the dehydrogenation of benzaldehyde are key processes that affect the deep oxidation of toluene.

Oxygen vacancy-engineered in ethylene glycol solvated α-MnO2 for low temperature catalytic oxidation of toluene

Synthesis of catalyst by the organic solvent is an attractive strategy for improving the oxygen vacancy concentration and catalytic activity. Due to the excellent reducibility of ethylene glycol (EG), the reduction strategy was designed to synthesize α-MnO2-EG, and for toluene oxidation. The results showed that EG could significantly increase the oxygen vacancy concentration and catalytic activity of α-MnO2-EG. The oxygen vacancy concentration of α-MnO2-EG-160 is 30% higher than that of α-MnO2-H2O, which leads to the significant low-temperature catalytic performance of the α-MnO2-EG-160 catalyst (T90 = 183 °C), much lower than the currently reported MnO2-based catalysts. The structure–activity relationship of the α-MnO2-EG-160 has been explained. To be specific, EG macromolecules could intrude into [MnO6] framework to exfoliate oxygen atoms and generate oxygen vacancies, which not only improves redox properties, but also promotes the adsorption of toluene. Furthermore, the toluene catalytic mechanism of α-MnO2-EG-160 based on oxygen vacancies was proposed.

Oxidation of Toluene over Mesoporous MnO2/CeO2 Nanosphere Catalysts: Effects of the MnO2 Precursor and the Type of Support.

Toluene is the most common volatile organic compound (VOC), and the MnO2-based catalyst is one of the excellent nonprecious metal catalysts for toluene oxidation. In this study, the effects of MnO2 precursors and the support types on the oxidation performance of toluene were systematically explored. The results showed that the 15MnO2/MS-CeO2-N catalyst with Mn(NO3)2·4H2O as the precursor and the mesoporous CeO2 nanosphere (MS-CeO2) as the support exhibited the most excellent performance. To reveal the reason behind this phenomenon, the calcination process of the catalyst precursor and the reaction process of toluene oxidation were investigated by in situ DRIFTS. It was found that the MnO2 precursor and the type of catalyst support could have a large effect on the reaction pathway and the produced intermediates. Therefore, the roles of the MnO2 precursor and the type of support should be key considerations when developing the high-performance MnO2-based toluene oxidation catalyst.

Morphology-steered synthesis of Ce-MnO2 catalysts for catalytic oxidation of ethane at low temperature

Ce-MnO2 catalysts with nanosheets-stacked flower-like architecture were prepared by hydrothermal method, and Ce0.5Mn1 sample exhibited excellent catalytic performance for ethane combustion, with T90 at 310 °C. The apparent activation energy of the Ce0.5Mn1 sample (70.7 kJ mol−1) was comparatively lower than that of pure MnO2 (112.4 kJ mol−1). The formation of CeMn solid solution reduced the sizes of the Ce-MnO2 crystallites, affording higher SBET. Electron transfer and redistribution in the CeMn solid solution enhanced the activation of surface oxygen (OA) and generated more oxygen vacancies. The increasing OA species and highly mobile lattice oxygen boosted the oxidation of ethane. Also, the Ce0.5Mn1 catalyst exhibited enhanced H2O resistance with an ethane conversion over 95 % at 350 °C. Moreover, the morphologies of the CexMn1 catalysts were significantly influenced by Ce doping. Variations in Ce3+ amount significantly altered the thickness of the nanosheets, and the structure displayed more surface defects with considerable active sites. This work provides insights into the design of a promising MnO2-based catalyst for the mitigation of light alkanes.

Regulating the Pt-MnO2 Interaction and Interface for Room Temperature Formaldehyde Oxidation.

Formaldehyde (HCHO) is a hazardous pollutant in indoor space for humans because of its carcinogenicity. Removing the pollutant by MnO2-based catalysts is of great interest because of their high oxidation performance at room temperature. In this work, we regulate the Pt-MnO2 (MnO2 = manganese oxide) interaction and interface by embedding Pt in MnO2 (Pt-in-MnO2) and by dispersing Pt on MnO2 (Pt-on-MnO2) for HCHO oxidation over Pt-MnO2 catalysts with trace Pt loading of 0.01 wt %. In comparison to the Pt-in-MnO2 catalyst, the Pt-on-MnO2 catalyst has a higher Brunauer-Emmett-Teller surface area, a more active lattice oxygen, more oxygen vacancy activating more dioxygen molecules, more exposed Pt atoms, and noninternal diffusion of mass transfer, which contribute to the higher HCHO oxidation performance. The HCHO oxidation performance is stable over the Pt-MnO2 catalysts under high space velocity and high moisture humidity conditions, showing great potential for practical applications. This work demonstrates a more effective Pt-dispersed MnO2 catalyst than Pt-embedded MnO2 catalyst for HCHO oxidation, providing universally important guidance for metal-support interaction and interface regulation for oxidation reactions.

Recent progress of manganese dioxide based electrocatalysts for the oxygen evolution reaction

This review discusses the OER reaction mechanism (AEM and LOM) and the research progress of MnO2-based OER catalysts. The optimization strategy of MnO2-based catalysts was summarized.

Experimental study on supported MnO2-based catalysts for NO oxidation

Experimental study on supported MnO2-based catalysts for NO oxidation

Steering the construction of amorphous/crystalline hierarchical structure for accelerating the oxygen evolution reaction rate

As electrochemical water splitting is an extremely promising direction for solving future energy problems and the oxygen evolution reaction (OER) is the key half-reaction in water splitting, it is necessary to develop a new type of highly active and stable non-noble metal catalyst. Herein, we construct a series of hierarchical structure by modifying ZIF-67 on clustered-MnO2 nanotubes which are used as precursor. The fruit@branch-shaped compounds, marked as X@MnO2 (X = CoSx, Co3O4 and CoP). Compared with pure MnO2 nanotubes, the obtained derivatives imaginably result in the unique hierarchical structure and introduce synergistic effect between MnO2 and Co-based species that result in higher performance. In Particular, owing to the abundant active sites, a facil mass transfer pathway and improved electrical conductivity in amorphous/crystallize hierarchical structure, CoSx@MnO2 possesses the highest electrocatalytic activity with a lower overpotential of 329 mV at the current density of 10 mA cm−2, which is superior to most MnO2-based catalyst previously reported. Most importantly, to expand this strategy is an effective way to synthesize more high-activity catalysts.

Investigation into the roles of interfacial H2O structure in catalytic oxidation of HCHO and CO over CuMnO2 catalysts

The rapid deactivation of cost-effective MnO2-based catalysts in humid air limits their application in practice, and the identification of the role of water in an oxidation process is significant for developing water-resistant MnO2-based catalysts. Here, CuMnO2 showed a 20.3% HCHO conversion in 10 hr at room temperature in humid air with relative humidity of 40%, but deactivated in 3 hr in dry air. The excellent activity and stability of HCHO oxidation in humid air were attributed to the positive effect of H2O on HCHO oxidation to the H2O-HOCH2OH supermolecule assemblies via hydrogen bonds formed on CuMnO2. H2O-HOCH2OH supermolecule assemblies tend to be oxidized to carbonate, which is further oxidized to CO2. Furthermore, CuMnO2 exhibited a much poorer activity of CO oxidation in humid air, but the CO conversion was still 100% in 10 hr in dry air. H2O showed a competitive adsorption effect to CO on CuMnO2. CuMnO2 could be applied in HCHO elimination in humid air and CO elimination in dry air.

Singlet oxygen-dominated activation of peroxymonosulfate by 3D hierarchical MnO2 nanostructures for degradation of organic pollutants in water: Surface defect and catalytic mechanism

Manganese dioxide (MnO2) is attracting significant attention in the activation of persulfate for the degradation of organic pollutants in water. But the factor determining the catalytic efficiency of MnO2, and the evolution of reactive oxygen species (ROS) remain equivocal and elusive. Herein, three-dimensional (3D) hierarchical MnO2 nanostructures are synthesized via the calcination of hydrothermal product and used to active peroxymonosulfate (PMS) for degradation of acid orange 7 (AO7) in water. The results show that the calcination temperature can tune the morphology of 3D hierarchical MnO2 nanostructures from flower to urchin-like as well as phase transformation from δ-MnO2 to α-MnO2. Owing to the presence of more surface defects and surface adsorbed oxygen species, the urchin-like α-MnO2 nanostructures prepared at 500 °C (MnO2-500) deliver the best catalytic activity in activation of PMS, and 98.3 % of AO7 is degraded in 300 s. Additionally, the MnO2-500 shows wide pH applicability (3.5–9.6). Quenching tests and electron paramagnetic resonance spectra reveal that the catalytic oxidation of AO7 in the MnO2-500/PMS system is mainly mediated by the non-radical pathway, and the dominated ROS is 1O2. Besides, electrochemical experiment confirms the presence of electron transfer in the catalytic process. Density functional theory calculations indicate that the CN and CS groups of AO7 with high electron density are easily attacked by electrophilic ROS, which is further confirmed by liquid chromatography-mass spectrometry. Finally, MnO2-500 exhibits gradually enhanced catalytic activity with the increased cycle number due to the increased surface defects and the surface adsorbed oxygen. This study provides new insight into the PMS activation by MnO2-based catalyst for the remediation of organic pollutants in water.

First-principles calculation on effects of oxygen vacancy on α-MnO2 and β-MnO2 during oxygen reduction reaction for rechargeable metal-air batteries

MnO2 is widely applied as oxygen reduction reaction (ORR) electrocatalysts in different metal-air batteries (MABs). Enhancing the ORR activity of MnO2-based catalysts is necessary for improving the performance of MABs. Defect-engineering of catalyst materials is a key approach for enabling the high performance of ORR. Here, the defect-engineering of α-MnO2 (211) and β-MnO2 (110) by oxygen vacancy (OV) is investigated using the first-principles density functional theory calculation. The geometric structure, adsorption, electronic conductivity, and oxygen reduction reaction (ORR) activity are studied. As a result, the OV induces the geometric structure that the Mn–Mn and Mn–O distances are closer when the catalysts lose the oxygen atom(s) on the top-layer surfaces. The presence of OV not only enhances the adsorption energy of *OOH, *O, and *OH, but also increases the electronic conductivity analyzed via the electron transfer. The Bader charge analysis demonstrates that the Mn(IV) can be altered to Mn(III) by the electron accumulation from OV. The volcano plot of ORR overpotential indicates that having the excess OV concentration on MnO2 surfaces cannot enhance the ORR activity. The excellent activity is yielded by 12.50 % OV α-(211) and 66.66 % OV β-(110) with the ORR overpotential of 0.31 V and 0.60 V, respectively. The results demonstrate that Ov is an essential parameter defining the existence of Mn(III) and Mn(IV) on the surface of MnO2-based catalysts. The optimal ratio of Mn(IV):Mn(III) is in challenge developing the α-MnO2 and β-MnO2 electrocatalyst cathode for metal-air batteries. This study provides a guideline for developing the potential cathode catalyst for MABs, used for harvesting energy.

Ti-modification enhanced the activity of flexible mesh-type Mn/γ-Al2O3/Al catalyst for indoor HCHO removal under humid conditions

Nowadays, MnO2-based catalysts have been widely applied in environmental fields, especially the removal of HCHO at room temperature. However, the moisture resistance of the catalyst is still a challenge. In this study, a series of MnTix/γ-Al2O3/Al catalysts were prepared by the methods of redox precipitation and impregnation, which showed excellent HCHO removal efficiency under humid conditions. When the catalyst accumulatively treated ca. 25 ppm of HCHO, there was still a HCHO conversion of 87.5% and the activity can be restored by washing and drying. DFT calculations, XPS, and TG results showed that Ti-modification could inhibit the adsorption of physisorbed water on the catalyst surface, thereby weakening the competitive adsorption between H2O and HCHO. This study provides a feasible strategy for the removal of HCHO under humid conditions.

Low-Temperature SCR Catalyst Development and Industrial Applications in China

In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NOx emissions in China. Currently, the most commonly used catalysts in industry are V2O5-WO3(MoO3)/TiO2, MnO2-based catalysts, CeO2-based catalysts, MnO2-CeO2 catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO2, moisture and alkali metals, which can affect the service life of SCR catalysts. This paper summarizes the mechanism of catalyst poisoning and aims to reduce the negative effect of NH4HSO4 on the activity of the SCR catalyst at low temperatures in industrial applications. It also presents the outstanding achievements of domestic companies in denitrification in the non-power industry in recent years. Much progress has been made in the research and application of low-temperature NH3-SCR, and with the renewed demand for deeper NOx treatments, new technologies with lower energy consumption and more functions need to be developed.

Constructing MnO2 alpha/amorphous heterophase junction by mechanochemically induced phase transformation for formaldehyde oxidation

Construction of heterophase junction has emerged as an effective approach for boosting the catalytic performance of diverse photoelectrocatalysts and electrocatalysts. But its potential in remediation of formaldehyde (HCHO) pollution was scarcely investigated. Herein, we report the fabrication, catalytic properties, and mechanistic study of α/A-MnO2 heterophase junction catalyst for HCHO oxidation. Our study found that mechanically milling δ-MnO2 could readily induce an δ to α phase transformation via an intermediate amorphous phase, thus providing a simple method for construction of MnO2 heterophase junction. The α/A-MnO2 catalyst prepared under the optimal condition exhibited an exceptionally high area specific reaction rate of 0.21 μmol m−2 min−1 at 80 °C and excellent stability towards HCHO oxidation, outperforming most reported MnO2-based catalysts. Based on the in situ diffused reflectance infrared Fourier transform spectroscopy and on-line gas chromatography analyses, a possible mechanism was put forward to describe the synergistic catalysis effect between α-MnO2 and amorphous MnO2.

Experimental Study on Supported Mno2-Based Catalysts for No Oxidation

Experimental Study on Supported Mno2-Based Catalysts for No Oxidation