MnO2 Catalyst Research Articles

Synthesis of xCe-MnO2 with three-dimensional ultra-thin nanosheet structure and its excellent low-temperature reducibility for toluene catalysis.

A series of xCe-MnO2 (x = 0-1) catalysts were synthesized using ammonium oxalate as a precipitator via the redox precipitation method and hydrothermal synthesis method. The results indicate that 0.25Ce-MnO2 exhibited the highest catalytic activity for toluene oxidation, with the T99 of 240°C. Characterization results from XRD, Raman, SEM, TEM, EDS-mapping, BET, and other techniques reveal that the 0.25Ce-MnO2 catalyst exhibited a three-dimensional multistage ultrathin nanosheet structure by adjusting the introduction amount of Ce, with abundant active sites, and effectively formed Ce-Mn homogeneous dispersion. The larger pore size and volume of 0.25Ce-MnO2 catalyst lead to it excellent toluene transfer ability. Furthermore, compared with MnO2, the crystal pattern of 0.25Ce-MnO2 shifted to the tetragonal cryptomelane type α-MnO2 phase and exposed more crystal planes which are beneficial to catalyze toluene. H2-TPR, O2-TPD, and XPS characterization further confirmed the strong interaction between Ce and Mn oxides, which exhibited better low-temperature reducibility and oxygen migration, along with abundant Ce3+ and Mn3+ species, where lattice oxygen played a major role. Moreover, in situ DRIFTS revealed that the 0.25Ce-MnO2 catalyst showed higher adsorption and desorption capacity for toluene than the MnO2 catalyst, and benzoate species were the key intermediates for catalytic oxidation. Additionally, benzoate and surface phenolic species were the key intermediates for catalytic oxidation of MnO2. Because 0.25Ce-MnO2 possesses better ability of converting toluene to benzoate species, it exhibits better activity.

Low temperature catalytic elimination of dichloromethane over manganese oxides catalysts with ozone assistance

This paper synthesized MnO2 catalysts with different crystal structures including α-, β-, γ-, and δ-phase for catalytic ozonation of CH2Cl2. Among them, δ-MnO2 exhibited superior activity (100 % mineralization rate for 100 ppm of CH2Cl2 at 120 °C under O3/DCM of 12.5) as well as decent stability and potential of multi-pollutants collaborative degradation, which was attributed to its abundant oxygen vacancies and mobile lattice oxygen species. Compared with traditional catalytic oxidation, O3 assistance facilitated the contact between reactants and catalyst and suppressed the accumulation of products, thus effectively reducing energy barrier of reaction. The evolution of intermediates from transient reaction studies during catalytic ozonation and oxidation further proved O3 introduction facilitated dechlorination. This study presents an efficient crystalline-engineering MnO2 for catalytic ozonation and verifies the superiority of catalytic ozonation.

Regulating the surface local environment of MnO2 materials via metal-support interaction in Pt/MnO2 hetero-catalysts for boosting methanol oxidation

Rational use of the strong metal-support interaction (SMSI) is an important way to achieve the complete destruction of volatile organic compounds (VOCs). In order to investigate the influence of SMSI on the catalytic oxidation of VOCs, three Pt/MnO2 hetero-catalysts, i.e., Pt/α-MnO2, Pt/γ-MnO2 and Pt/δ-MnO2, were synthesized and then applied to the catalytic oxidation of methanol. The results showed that, compared with their corresponding pure MnO2, the temperatures (T90) at which 90% of methanol is converted to CO2 for the above three synthesized catalysts were reduced by 54 °C, 67 °C and 32 °C, respectively. The Pt/α-MnO2 catalyst has the best reducibility at lower temperature (T90 = 91 °C). Multiple characterization analyses confirm that the interaction between Pt and MnO2 support can increase the amount of Mn4+ and surface lattice oxygen on the catalyst surface. Thanks to the optimization of local reaction environment on the catalyst surface, Pt/MnO2 hetero-catalysts have better low temperature reduction capacity and stronger VOC deep oxidation capacity than corresponding pure MnO2 catalysts.

In situ construction of heteroatom F-doped Mn3O4 spinel catalysts with robust activity and SO2 resistance for NH3-SCR at low temperature

Herein, we propose a novel in situ construction method of F-doped Mn3O4 catalysts through a crystallization-pyrolysis-oxidation strategy. The F-Mn3O4-3 % exhibits a NO conversion above 90 % from 150° to 310°C at an ultra-high GHSV of 400,000 h−1. The SO2 resistance is also significantly improved compared to pure Mn3O4. Doping of Mn3O4 with F increases the surface acidity and tunes the electronic properties of the catalysts. The increase of acid sites not only facilitates the adsorption of NH3 species, thereby improving NH3-SCR activity, but also inhibits SO2 adsorption on the catalyst surface. The electron conduction between SO2 and Mn3+ can also be effectively inhibited after F replaces O, resulting in the F-doped Mn3O4 catalysts can selectively oxidizing NO to NO2 rather than oxidizing SO2 to SO3, thereby causing the decrease of sulfate species. Our work provides a novel idea for the design of SCR catalysts with high activity and SO2 resistance.

Asymmetric Epoxidation of Alkenes Catalyzed by a Cobalt Complex.

Asymmetric epoxidation of alkenes catalyzed by nonheme chiral Mn-O and Fe-O catalysts has been well established, but chiral Co-O catalysts for the purpose remain virtually undeveloped due to the oxo wall. Herein is first reported a chiral cobalt complex to realize the enantioselective epoxidation of cyclic and acyclic trisubstituted alkenes by using PhIO as the oxidant in acetone, wherein the tetra-oxygen-based chiral N,N'-dioxide with sterically hindered amide subunits plays a crucial role in supporting the formation of the Co-O intermediate and enantioselective electrophilic oxygen transfer. Mechanistic studies, including HRMS measurements, UV-vis absorption spectroscopy, magnetic susceptibility, as well as DFT calculations, were carried out, confirming the formation of Co-O species as a quartet Co(III)-oxyl tautomer. The mechanism and the origin of enantioselectivity were also elucidated based on control experiments, nonlinear effects, kinetic studies, and DFT calculations.

Nitrogen-coordinated cobalt embedded in hollow carbon polyhedron for catalytic ozonation of odor CH3SH at ambient temperature

Metal − organic framework (MOF) derived carbon-based nanocatalysts have received great attention in environmental remediation applications. Herein, a hollow carbon polyhedron with nitrogen-coordinated cobalt embedded has been synthesized by thermal transformation of the core–shell structure ZIF-8@ZIF-67 (marked as ZIF-8@ZIF-67-C). Remarkably, the ZIF-8@ZIF-67-C catalyst could remove almost 98% of CH3SH after running for 60 min, exceeding the reported values (about 30 times higher than that of commercial MnO2 catalysts). Elimination performance experimental results demonstrate that ZIF-8@ZIF-67-C has better anti-interference ability and anti-toxicity properties. The excellent catalytic performance can be attributed to the following facts: unique structure, high conductivity, abundant hierarchical porous structure, and accessible active sites. Theoretical calculation reveals the mechanism is that the Co-N electron bridge promotes Co nanoparticles adsorb O3, providing electrons to O3 and producing 1O2 to further oxidize CH3SH into CO2/SO42−. Also, the ZIF-8@ZIF-67-C catalyst supported on Al2O3 still shows excellent catalytic activity and durability, which deepened the practical application. This work provides a strategy for the development of MOF-on-MOF-derived metal/carbon hybrid materials in environmental applications.

Defect-Engineered MnO2 as Catalyst for the Chemical Mechanical Polishing of Silicon Carbide Wafer

Chemical mechanical polishing (CMP) of SiC wafer is challenging due to its extreme hardness and inertness. Catalyst assisted CMP is a cost-effective approach to increase material removal rate (MRR) without sacrificing surface quality. Herein, oxygen-deficient α-MnO2 was prepared by mechanochemical synthesis and the effect of catalyst physiochemical structure on the CMP performance of Si-face SiC wafer was systematically studied. The addition of 1% α-MnO2 catalyst increased MRR by 38.8% to 1.11 μm h−1, much higher than commercial γ-MnO2. The synergy of phase structure, oxygen vacancy and surface area & porosity contributed to the high catalytic activity. α-MnO2 is an outstanding oxidation catalyst due to its stable framework, large tunnel size, rich surface area and porosity, which can facilitate the adsorption, activation and transfer of guest species and intermediates and therefore affects the reaction pathway and reaction kinetics. Mechanochemical synthesis generates nano MnO2 particles with rich oxygen vacancies. The presence of more surface oxygen vacancies can improve oxidizing activity of MnO2 catalyst, facilitating the oxidation of C species on wafer surface. The use of defect-engineered α-MnO2 catalyst is promising for overcoming the present bottlenecks of long processing time and high cost of current CMP of SiC wafer.

Catalysts for highly water-resistant catalytic decomposition of ozone: Hausmannite Mn3O4 on exposed (101) crystal surface

Recently, ozone pollution has gradually replaced PM2.5 as the main pollutant affecting air pollution. In this study, we synthesized a series of Mn3O4 catalysts by hydrothermal method changing the precursors and tested their activities at different relative humidity, gas volume space velocity of 150,000 h−1 and 5 ppm ozone. Remarkably, Mn3O4-SO4 prepared with MnSO4 as precursor showed excellent catalytic ozone decomposition activity, almost completely converting 5 ppm of ozone at different relative humidity ranges. Finally, the most active Mn3O4-SO4 catalyst was tested for its usability limit at RH= 90%, after 28 h of testing under high humidity conditions, it had retained successfully the complete decomposition of low concentrations of ozone. The catalysts were characterized by XRD, Raman, HRTEM, XPS, BET, H2O-TPD and in situ IR NH3 adsorption. The characterization analysis revealed that the Mn3O4-SO4 surface could exposed a highly active (101) crystalline surface with high specific surface area, excellent hydrophobicity as well as ozone adsorption capacity, which were highly favorable for ozone decomposition under high humidity conditions. In this work, Mn3O4 exhibits good catalytic activity, which provides an additional option for future studies of manganese oxides.

Oxidation of soot in gasoline exhaust on Mn2O3 catalyst

A Mn2O3 catalyst was investigated for soot oxidation in gasoline exhaust. Catalytic studies performed in model exhaust with Mn2O3/soot mixtures in tight contact showed efficient soot oxidation above 250 °C. This high performance was additionally confirmed on a gasoline engine test bench using a Mn2O3-coated particulate filter. The mechanism of the oxygen transport from the gas phase to soot was investigated by kinetic modeling of O2 adsorption and desorption as well as catalytic studies using 18O2. As a result, it was suggested that O2 dissociatively adsorbs on Mn2O3 followed by surface migration and transfer of oxygen to the soot at physical contact points between the two solids. The surface, sub-surface and bulk oxygen of the catalyst were also found to play a role. Enhanced participation of bulk oxygen appeared with declining O2 content due to partial reduction of Mn2O3 and formation of oxygen vacancies driving the oxygen mobility.

Toluene and 2-propanol mixture oxidation over Mn2O3 catalysts: Study of inhibition/promotion effects by in-situ DRIFTS

Three Mn2O3 catalysts were synthetized by different methods, characterized and tested in toluene and 2-propanol (iPrOH) oxidation in single and binary component mixture. In single VOC media, the catalytic activity was strongly influenced by the specific surface area, low-temperature reducibility and active surface-oxygen species. In the binary VOCs mixture oxidation, light-off curves were shifted to lower temperatures, suggesting a promoting effect on the catalytic activity for toluene and iPrOH oxidation. The acetone production in binary mixture was also shifted to lower temperatures. The obtained results were rationalized based on the different polarity of the VOCs, the available adsorbed oxygen and the exothermic character of the VOCs combustion. These promoting effects were pointed out by in-situ DRIFTS measurements, in which the pre-adsorption of any of these two VOCs affected the adsorption/reaction of the other. For the first time, in-situ DRIFTS has been applied to elucidate the reaction mechanisms of the oxidation of toluene/2-propanol mixture on Mn2O3, which allowed to correlate the surface phenomena and the catalytic performance.

Graphene Oxide Addition Induces the Improvement of ORR Catalysis Properties of Low-Cost Mn-Based Catalyst Used for Al-Air Battery

To further improve the oxygen reduction reaction (ORR) activity of low-cost Mn based catalyst, graphene oxide (GO) was added in the preparation of one dimensional (1D) α-MnO2 nanorod using KMnO4-MnSO4 system via hydrothermal method. Experimental results showed that the GO addition (20 wt%) could induce the formation of MnO(OH) nanorod. The Mn based@GO catalyst had more surface defects and oxygen vacancies compared with pure α-MnO2. The onset potential, half-wave potential (E1/2) and limiting current density were significantly enhanced from 0.86 V/0.66 V/3.56 mA cm-2 to 0.91 V/0.77 V/5.41 mA cm-2, indicating that GO addition could greatly improve the catalytic activity of Mn based catalyst. Furthermore, the discharge voltage, power density, mass energy density of Al-air battery using Mn based@GO catalyst were greatly improved comparing with the usage of pure MnO2 catalyst, and it was also found that the application effect of Mn based @GO catalyst in the Al-air battery was almost comparable to the commercial 20% Pt/C catalyst. Our research revealed for the first time the commercial potential of the novel and low-cost MnO2/MnO(OH)@GO nanocomposite in the Al-air battery.

Enhanced n-propanol oxidation coupled with hydrogen evolution by electrocatalytic membrane structured reactor via flow-through operation

An electrocatalytic membrane structured reactor (EMSR) fabricated by loading MnO2 catalyst in Ti membrane pores for oxidation of n-propanol coupled with hydrogen evolution. During the process of n-propanol oxidation coupled with hydrogen evolution by MnO2@Ti EMSR via flow-through mode, the conversion rate of n-propanol was increased by 44%, the selectivity of propionic acid was increased by 33%, and the Faraday efficiency was increased by 59%, compared with those of MnO2/Ti membrane with MnO2 immobilized on the membrane surface. Enhanced contact and mass transfer are two contributors for the catalytic performance improvement by MnO2@Ti EMSR via flow-through mode. Besides, the cell voltage during n-propanol oxidation coupled with hydrogen production is decreased by 305 mV with energy consumption reduced by 0.73 kWh NmH2-3, compared with those of hydrogen evolution by water splitting with oxygen evolution reaction. The achievements of this work provide a promising strategy for coproduction of propionic acid and hydrogen.

Highly Active Manganese Oxide from Electrolytic Manganese Anode Slime for Efficient Removal of Antibiotics Induced by Dissociation of Peroxymonosulfate.

In this paper, high-activity manganese oxide was prepared from electrolytic manganese anode slime to realize the efficient removal of antibiotics. The effects of sulfuric acid concentration, ethanol dosage, liquid-solid ratio, leaching temperature and leaching time on the leaching of manganese from electrolytic manganese anode slime were systematically studied. Under the optimal conditions, the leaching rate of manganese reached 88.74%. In addition, a Mn3O4 catalyst was synthesized and used to activate hydrogen persulfate (PMS) to degrade tetracycline hydrochloride (TCH). The synthesized Mn3O4 was characterized by XRD, XPS, Raman, SEM and HRTEM. As a result, the prepared Mn3O4 is spherical, with high purity and crystallinity. The catalytic activity of Mn3O4 for PMS to degrade TCH was increased to 82.11%. In addition, after four cycles, the performance remained at 78.5%, showing excellent stability and recyclability. In addition, O2- and 1O2 are the main active species in the degradation reaction. The activity of Mn3O4 is attributed to it containing Mn(II) and Mn(III) at the same time, which can quickly realize the transformation of high-valence and low-valence manganese, promote the transfer of electrons and realize the degradation of organic pollutants.

Effects of impure ions in precursors on the formation of active oxygen in MnO2 for promoting VOCs oxidation

Impure ions in the precursor can cause the structural transition of prepared MnO2 catalysts and influence their activity in toluene catalytic oxidation. Here, the effects of K+ and SO42- on the catalytic activity of MnO2 are examined. Removing absorbed SO42- by repeating water washing facilitates the exposure of active sites on the surface of MnO2. The temperature for toluene conversion reaches 90% (T90) of catalysts decreases from 247 ºC to 238 ºC. While the loss of K+ during the water washing leads to the inferior performance of MnO2. Re-impregnating of K+ can stimulate the generation of active oxygen species on the surface of catalysts (K/MnO2), which showed elevated catalytic activity in toluene conversion (T90, 229 ºC). In addition, K/MnO2 performs good stability for toluene combustion in long-term operation. Density Function Theory calculations reveal that the formation energy of oxygen vacancy and the adsorption energy of toluene on the surface of catalyst decrease from 6.996 eV and − 0.079 to 5.482 eV and − 0.468 eV as the introduction of K+, respectively. Our results provide a complete understanding of the effects of impurity ions on the performance of manganese oxide for catalytic oxidation of toluene.

Tailoring Oxygen Vacancies and Active Surface Oxygen Species in Copper-Doped MnO2 Catalysts for Total Catalytic Oxidation of VOCs

Catalytic oxidation has been considered an essential route to tackle volatile organic compounds (VOCs)-driven pollution. To this point, surface engineering has emerged as a robust pathway to promote the efficiency of MnO2-based materials toward VOC oxidation. However, regulating such surface properties has been a critical challenge. This investigation attempts to tailor surface oxygen vacancies and active surface oxygen for the total catalytic oxidation of various VOCs (e.g., formaldehyde, isopropanol, and toluene) via doping copper into cryptomelane and birnessite-type MnO2 catalysts. The catalysts are synthesized by a novel facile successive redox precipitation method between KMnO4 and an ethanol solution containing copper nitrate precursors, followed by annealing. It turns out that the low amount of Cu dopants, corresponding to the copper-to-manganese ratio of 0.17 (RCu/Mn = 0.17), generates a tunnel-structured cryptomelane-type MnO2 with strong bulk oxygen mobility. Such a crystalline structure exhibits the best catalytic performance for the total catalytic oxidation of toluene. The higher copper loading (RCu/Mn = 0.27–0.34) drives the formation of Cu-doped layered birnessite-type MnO2 materials containing rich oxygen vacancies and high active surface oxygen species. Those catalysts promote the oxidation of isopropanol and formaldehyde, respectively. To this end, oxygen vacancies and surface oxygen species crucially steer the molecular oxygen activation and interactions toward adsorbed oxygen species, significantly promoting the total oxidation of isopropanol and formaldehyde, respectively. The explored finding could disclose an outstanding platform toward highly selective and efficient oxidation of VOCs.

Unlocking efficient and robust ozone decomposition with CNT-confined manganese oxide via synergistic electronic modulation

Manganese oxides (MnO2)-based catalysts are prominent candidates for ozone elimination, which always bear the accumulation of the intermediate oxygen species and water, resulting in unsatisfactory catalytic efficiency. To overcome this critical defect, we propose carbon nanotubes confined MnO2 catalysts (MnO2-in-CNT) for efficient and stable ozone decomposition over a wide relative humidity range (RH, 15–90%). Strikingly, the catalyst exhibits an efficient and sustainable ozone conversion (98%) over 100 h under a gas hourly space velocity (GHSV) of 600,000 mL g−1 h−1 and RH of 15%, and durability under high RH (77% over 50 h, RH 70%), well beyond its unconfined analog and most reported works. This excellent catalytic performance can be attributed to the facilitated intermediate desorption on active sites and the confined structure alleviated the effect of water on inner MnO2. This discovery is expected to drive great progress in the applications of confined-structure catalysts for air purification.

Investigation on removal of multi-component volatile organic compounds in a two-stage plasma catalytic oxidation system - Comparison of X (X=Cu, Fe, Ce and La) doped Mn2O3 catalysts

Mn2O3-X catalysts (X = Cu, Fe, Ce and La) were prepared based on γ-Al2O3 for the mixture degradation of muti-component volatile organic compounds (VOCs) composed of toluene, acetone, and ethyl acetate. The catalysts were characterized, and the density functional theory (DFT) simulation of ozone adsorption on Mn2O3-X were carried out to investigate the influence of adsorption energy on catalytic performance. The results showed that the removal efficiency (RE) of each VOC component was similarly improved by Mn2O3-X catalysts, and the greatest increase in VOCs' removal efficiency was obtained (7.8% for toluene, 86.2% for acetone, and 82.5% for ethyl acetate) at a special input energy (SIE) of 700 J L−1 with Mn2O3–La catalyst. Characterization results demonstrated that Mn2O3–La catalyst had the highest content of low valence Mn elements and the greatest Oads/Olatt ratio, as well as the lowest reduction temperature. Mn2O3–La catalyst also presented superior catalytic effect in improving carbon balance (CB) and CO2 selectivity (SCO2). The CB and SCO2 were increased by 47.7% and 12.61% respectively with Mn2O3–La at a SIE of 400 J L−1 compared with that when only γ-Al2O3 was applied. The DFT simulation results of ozone adsorption on Mn2O3-X catalysts indicated that the adsorption energy of catalyst crystal was related to the catalytic performance of the catalyst. The Mn2O3–La/γ-Al2O3 catalyst, which had the highest absolute value of adsorption energy, presented the best performance in improving VOCs’ RE.

Non-Thermal Plasma Incorporated with Cu-Mn/γ-Al2O3 for Mixed Benzene Series VOCs’ Degradation

In this work, a coaxial dielectric barrier discharge (DBD) reactor was constructed to degrade the mixture of toluene and o-xylene, two typical benzene series. The Cu-MnO2/γ-Al2O3 series catalysts prepared by redox and impregnation methods were filled into the plasma device to degrade VOCs synergistically and explore the degradation effect. The experimental results showed that the introduction of a Cu-doped MnO2 catalyst significantly improved the pollutants’ removal efficiency and CO2 selectivity, and greatly inhibited the formation of by-products. Among them, Cu0.15Mn/γ-Al2O3 showed the highest removal efficiency (toluene was 100% and o-xylene was 100%), and the best CO2 selectivity (92.73%). The XRD, BET, XPS and SEM results confirmed that the synergistic effect between Cu and Mn in the Cu-Mn solid solution could promote the amount and reducibility of the surface active oxygen species, which improved the catalytic performance. Finally, the toluene and o-xylene decomposition pathways in the NTP catalytic system were speculated according to the detected organic matter. This work provides a theoretical and experimental basis for the application of DBD-catalyzed hybrid benzene series VOCs.

Enhancement of the redox reactions of the La0.8Sr0.2MnO3 catalyst by surface acid etching: A simple synthesis strategy to high-performance catalysts for methane combustion

Methane emission and treatment is an extremely important environmental problem, and the key and core treatment method are the application and development of catalysts in the after-treatment system. In this study, a new non-noble metal catalyst, which named OALSM, was fabricated by etching La0.8Sr0.2MnO3 with oxalic acid for the removal of methane. Related experiments and theoretical explorations by scholars had shown that acid etching of perovskite surfaces could achieve effective optimization of the catalytic oxidation properties and microscopic physical and chemical properties of the materials. And the oxalic acid reacted with high-valence state Mn to generate abundant coordination unsaturated Mn3+ sites, which could produce more active oxygen species. In addition, OALSM showed a higher methane catalysis capability than the samples that were not subjected to acid etching. Among all the as-prepared catalysts, OALSM-0.01 M demonstrated the optimal methane catalytic activity with the T50 value of 336 °C, the T90 value of 417 °C, the activation energy of 47.02 kJ·mol−1, and the favorable structural stability and toxicity resistance. This integration strategy can rationalize an innovative protocol to into the design of composite catalysts for methane oxidation, to broaden the application of acid etching in VOC oxidation.

Low-Temperature NH3-SCR Performance and In Situ DRIFTS Study on Zeolite X-Supported Different Crystal Phases of MnO2 Catalysts

In this study, a series of zeolite-X-supported different crystal phases of MnO2 (α-MnO2, β-MnO2, γ-MnO2, and σ-MnO2) catalysts were prepared via a solid-state diffusion method and high-heat treatment method to explore their low-temperature NH3-SCR performance. All of the catalysts featured typical octahedral zeolite X structures and manganese dioxides species of various crystal types dispersed across the support surface. Throughout the entire temperature range of the reaction, γ-MnO2/X catalyst had the highest NO conversion. Additionally, β-MnO2/X, γ-MnO2/X, and σ-MnO2/X catalysts had nearly 100% of N2 selectivity, whereas the α-MnO2/X catalyst had the lowest N2 selectivity (about 90%) below 125 °C. Moreover, the γ-MnO2/X catalyst demonstrated superior acidity capacity and reduction ability compared with the other three catalysts. All the catalysts contained the essential intermediates NH2NO and NH4NO3 species, which are essential to the SCR reaction. More acid sites and nitrate species existed on the γ-MnO2/X catalyst than on the other catalysts, thereby boosting the SCR reaction.