Hydrothermal synthesis of MnO2/CNT nanocomposite with a CNT core/porous MnO2 sheath hierarchy architecture for supercapacitors

ABSTRACT: MnO2/carbon nanotube [CNT] nanocomposites with a CNT core/porous MnO2 sheath hierarchy architecture are synthesized by a simple hydrothermal treatment. X-ray diffraction and Raman spectroscopy analyses reveal that birnessite-type MnO2 is produced through the hydrothermal synthesis. Morphological characterization reveals that three-dimensional hierarchy architecture is built with a highly porous layer consisting of interconnected MnO2 nanoflakes uniformly coated on the CNT surface. The nanocomposite with a composition of 72 wt.% (K0.2MnO20.33H2O)/28 wt.% CNT has a large specific surface area of 237.8 m2/g. Electrochemical properties of the CNT, the pure MnO2, and the MnO2/CNT nanocomposite electrodes are investigated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The MnO2/CNT nanocomposite electrode exhibits much larger specific capacitance compared with both the CNT electrode and the pure MnO2 electrode and significantly improves rate capability compared to the pure MnO2 electrode. The superior supercapacitive performance of the MnO2/CNT nancomposite electrode is due to its high specific surface area and unique hierarchy architecture which facilitate fast electron and ion transport.     

Porous MnO<Subscript>2</Subscript>/CNT catalysts with a large specific surface area for the decomposition of hydrogen peroxide

H₂O₂ vapor sterilization is an effective and safe method for removing various pathogens. To improve the efficiency of this technique, the time required for sterilization must be shortened. The aeration time constitutes a large portion of the total sterilization time; therefore, the development of a catalyst for H₂O₂ decomposition is necessary. Bulk MnO₂ is typically used in H₂O₂ decomposition, but it has a low specific surface area. To increase H₂O₂ decomposition activity, specific surface area and electron transfer ability of catalyst need improvement. In this study, MnO₂/CNT(x), where x denotes the weight ratio of CTAB to H₂O in the catalyst preparation, was synthesized using a soft template method with varying amounts of the template. Overall, the catalyst specific surface area remarkably increased to 190–200 m²/g from 0.043 m²/g for bulk MnO₂ and these increased surface areas resulted in superior H₂O₂ decomposition activity. Among the CNT-supported catalysts tested, MnO₂/CNT (1.0) exhibited the highest activity, which was 570 times that of bulk MnO₂. Aeration times were also calculated with some assumptions and the aeration can be finished within 1 hr (bulk MnO₂ needs about 25 hr).     

NiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel as a material for supercapacitors with a pseudocapacity effect

Nanopowders of the NiMn₂O₄ spinel of a grain size of about 30 nm are obtained by the method of coprecipitation. The specific capacity in an aqueous electrolyte (1 М LiNO₃) is determined by the method of cyclic voltammetry. Using computer simulation, the peculiarities of the mechanism of intercalation of Li⁺ into a NiMn₂O₄ spinel are studied by the method of crystal chemical analysis.     

Facile synthesis of TiO<Subscript>2</Subscript>/ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite by sol-gel auto combustion method for superior visible light photocatalytic efficiency

Photocatalytic composite materials having photon absorption capability in the range of visible light were synthesized by loading TiO₂ (5, 10, 15, and 20 wt%) on ferrite nanocomposites by sol-gel auto-combustion method. The synthesized nanocomposites were analyzed using X-ray diffraction, Transmission electron microscopy, diffuse reflectance spectroscopy and N₂ adsorption techniques. The generation of photo active hydroxyl radicals for all the synthesized composites was found higher under the irradiation of red LED (RLED irradiation) which was confirmed by degradation of rhodamine B dye under irradiation of RLED. Photocatalytic activity of the synthesized nanocomposites was also carried out under irradiation of ultraviolet (UVLED) and blue (BLED) light emitting diodes, which is comparatively less than for the reaction under red LED irradiation. The operational parameters like catalyst amount, pH and concentration of dye solution were studied and ESI-MS degradation pathway is proposed by analyzing the degraded samples.     

Study of electrochemical properties and the charge/discharge mechanism for Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>/MnO<Subscript>2</Subscript>-AC hybrid supercapacitor

Spinel Li₄Mn₅O₁₂ was prepared by a sol–gel method. The manganese oxide and activated carbon composite (MnO₂-AC) were prepared by a method in which KMnO₄ was reduced by activated carbon (AC). The products were characterized by XRD and FTIR. The hybrid supercapacitor was fabricated with Li₄Mn₅O₁₂ and MnO₂-AC, which were used as materials of the two electrodes. The pseudocapacitance performance of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor was studied in various aqueous electrolytes. Electrochemical properties of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor were studied by using cyclic voltammetry, electrochemical impedance measurement, and galvanostatic charge/discharge tests. The results show that the hybrid supercapacitor has electrochemical capacitance performance. The charge/discharge test showed that the specific capacitance of 51.3 F g⁻¹ was obtained within potential range of 0–1.3 V at a charge/discharge current density of 100 mA g⁻¹ in 1 mol L⁻¹ Li₂SO₄ solution. The charge/discharge mechanism of Li₄Mn₅O₁₂ and MnO₂-AC was discussed.     

Degradation kinetics of recalcitrant organic compounds in a decontamination process with UV/H<Subscript>2</Subscript>O<Subscript>2</Subscript> and UV/H<Subscript>2</Subscript>O<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> processes

In this study, degradation aspects and kinetics of organics in a decontamination process were considered in the degradation experiments of advanced oxidation processes (AOP),i.e., UV, UV/H₂O, and UV/H₂O,/TiO₂ systems. In the oxalic acid degradation with different H₂O₂ concentrations, it was found that oxalic acid was degraded with the first order reaction and the highest degradation rate was observed at 0.1 M of hydrogen peroxide. Degradation rate of oxalic acid was much higher than that of citric acid, irrespective of degradation methods, assuming that degradation aspects are related to chemical structures. Of methods, the TiO₂ mediated photocatalysis showed the highest rate constant for oxalic acid and citric acid degradation. It was clearly showed that advanced oxidation processes were effective means to degrade recalcitrant organic compounds existing in a decontamination process.     

Li<Subscript>2</Subscript>Cu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> + SiO<Subscript>2</Subscript>/Ti compositions for the catalytic afterburning of diesel soot

Two methods were used to obtain a catalytically active oxide coating on the surface of titanium for the catalytic afterburning of diesel soot: plasma electrochemical formation of an oxide film on the surface of titanium and extraction pyrolytic deposition of the Li₂Cu₂(MoO₄)₃ compound. The Li₂Cu₂(MoO₄)₃/TiO₂ + SiO₂/Ti compositions synthesized by the single-step extraction pyrolytic treatment of the oxidized surface of titanium ensured a high burning rate of soot of ∼300°C. The subsequent deposition of Li₂Cu₂(MoO₄)₃ lowers the activity of the catalyst, due probably to the growth of molybdate phase crystallites and the filling of open oxide film pores. Double lithium-copper molybdate is able to reduce appreciably the concentration of CO in the oxidation products of soot. The advantages of these methods are the possibility of forming high-cohesion durable coatings on surfaces of any complexity, the simplicity of their implementation, and high productivity and low cost. The obtained results can be recommended for use in developing methods for creating composite coatings on catalytic soot filters.     

Hydrocarbon synthesis from CO and H<Subscript>2</Subscript>: Promotion of Co/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts with noble metals

The effect of promotion of Co catalysts with noble metals on their activity and selectivity in the synthesis of hydrocarbons from CO and H₂ has been studied. Platinum at a content of (0.05–0.10)% has been found to be best promoter.     

Separation characteristics of tetrapropylammoniumbromide templating silica/alumina composite membrane in CO<Subscript>2</Subscript>/N<Subscript>2</Subscript>, CO<Subscript>2</Subscript>/H<Subscript>2</Subscript> and CH<Subscript>4</Subscript>/H<Subscript>2</Subscript> systems

Nanoporous silica membrane without any pinholes and cracks was synthesized by organic templating method. The tetrapropylammoniumbromide (TPABr)-templating silica sols were coated on tubular alumina composite support ( γ-Al₂O₃/ α-Al₂O₃ composite) by dip coating and then heat-treated at 550 °C. By using the prepared TPABr templating silica/alumina composite membrane, adsorption and membrane transport experiments were performed on the CO₂/N₂, CO₂/H₂ and CH₄/H₂ systems. Adsorption and permeation by using single gas and binary mixtures were measured in order to examine the transport mechanism in the membrane. In the single gas systems, adsorption characteristics on the α-Al₂O₃ support and nanoporous unsupport (TPABr templating SiO₂/ γ-Al₂O₃ composite layer without α-Al₂O₃ support) were investigated at 20–40 °C conditions and 0.0–1.0 atm pressure range. The experimental adsorption equilibrium was well fitted with Langmuir or/and Langmuir-Freundlich isotherm models. The α-Al₂O₃ support had a little adsorption capacity compared to the unsupport which had relatively larger adsorption capacity for CO₂ and CH₄. While the adsorption rates in the unsupport showed in the order of H₂> CO₂> N₂> CH₄ at low pressure range, the permeate flux in the membrane was in the order of H₂≫N₂> CH₄> CO₂. Separation properties of the unsupport could be confirmed by the separation experiments of adsorbable/non-adsorbable mixed gases, such as CO₂/H₂ and CH₄/H₂ systems. Although light and non-adsorbable molecules, such as H₂, showed the highest permeation in the single gas permeate experiments, heavier and strongly adsorbable molecules, such as CO₂ and CH₄, showed a higher separation factor (CO₂/H₂=5-7, CH₄/H₂=4-9). These results might be caused by the surface diffusion or/and blocking effects of adsorbed molecules in the unsupport. And these results could be explained by surface diffusion. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

New oxygen evolution anodes for metal electrowinning: MnO<Subscript>2</Subscript> composite electrodes

Several modifications of manganese dioxide (MnO₂) were investigated for use in composite electrode materials for oxygen evolution, the target application being anodes for the industrial electrowinning of metals. It is demonstrated that the performance of this material depends strongly on the modifications of MnO₂. All modifications investigated were found to be more active than the usual anode of lead alloyed with silver (PbAg) used in zinc electrowinning. A composite sample containing chemical manganese dioxide (CMD) was found to give an oxygen evolution overpotential 0.25 V lower than the standard PbAg anode material. In the second part of the article, we investigate the effect of varying several parameters of the composite electrode assembly, including the size of the catalyst particles and percentage of the catalyst material used. A model is proposed where the performance of the material is proportional to the total length of the boundaries between the lead matrix material and the MnO₂ catalyst particles. Physicochemical processes contributing to the observed data are discussed.     

Novel Colloidal Nanothermite Particles (MnO<Subscript>2</Subscript>/Al) for Advanced Highly Energetic Systems

Nanothermites (metal oxide/metal) are tremendously exothermic and run with self sustaining oxygen content. Manganese oxide is one of the most effective oxidizers for nanothermite applications. This paper reports on the sustainable fabrication of different nanoscopic forms of colloidal manganese oxides including: MnO₂ nanoparticles of 20 nm average particle size and Mn₂O₃ nanorods of 50 nm diameter and 1 µm length. TEM micrographs demonstrated mono-dispersed particles and rods. XRD diffractograms revealed highly crystalline materials. MnO₂ nanoparticles (oxygen content 37 wt%) can offer high oxidizing ability compared with Mn₂O₃ nanorods (oxygen content 30 wt%). The integration of colloidal particles into energetic matrix can offer enhanced dispersion characteristics; consequently stoichiometric binary mixture of MnO₂ and Al nanoparticles were re-dispersed in organic solvent. The integration of developed colloidal nanothermite particles into tri-nitro toluene offered enhanced shock wave strength by 35% using ballistic mortar test. Thanks to nanotechnology which offered sustainable manufacture and subsequent integration of one of the most effective nanothermite particles into highly energetic system.     

EPR study of compounds in the LaAlO<Subscript>3</Subscript>-La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> system

The EPR spectra of compounds in the LaAlO₃-La_0.67Sr_0.33Mn _y O₃ system at a frequency of 9.4 GHz have been investigated at the temperatures T = 77 and 300 K as a function of the manganese concentration y (y = 0.015, 0.030, 0.080). It has been revealed that, in the paramagnetic state at y = 0.015, there exist isolated Mn²⁺ and Mn⁴⁺ ions, which has been confirmed by simulating the EPR spectra. The parameters of the EPR spectra have been determined. The effective magnetic moments μ_eff of the Mn²⁺ and Mn⁴⁺ ions have been calculated from the EPR spectra. It has been demonstrated that an increase in the Mn concentration leads to a decrease in the number of isolated ions and to the formation of new spin clusters. This manifests itself in the predominance of a broad line with weak traces of the hyperfine structure due to the isolated manganese ions.     

Hydrothermal synthesis of nanotubes based on (Mg,Fe,Co,Ni)<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>4</Subscript> hydrosilicates

Hydrosilicate nanotubes of the variable composition (Mg,Fe,Co,Ni)₃Si₂O₅(OH)₄ with a chrysotile structure have been synthesized under hydrothermal conditions at temperatures of 250–450°C and pressures of 30–100 MPa in media of different compositions. The conditions and ranges of formation of nanotubular hydrosilicates of the compositions under investigation have been determined. It has been demonstrated that the type of cation of the octahedral layer in the chrysotile structure has a decisive effect on the physicochemical conditions, mechanism, and rate of formation of nanotubes, as well as on their structure, morphology, and sizes.     

Synthesis of MIL-101@g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanocomposite for enhanced adsorption capacity towards CO<Subscript>2</Subscript>

MIL-101@g-C₃N₄ nanocomposite was prepared by solvothermal synthesis and used for CO₂ adsorption. The parent materials (MIL-101 and g-C₃N₄) and the MIL-101@g-C₃N₄ were characterized by X-ray diffraction, argon adsorption/desorption, Fourier transform infrared spectroscopy, thermal analysis (TG/DTA), transmission electronic microscopy, and Energy-dispersive X-ray spectroscopy. The results confirmed the formation of well-defined MIL-101@g-C₃N₄ with interesting surface area and pore volume. Furthermore, both MIL-101 and MIL-101@g-C₃N₄ were accomplished in carbon dioxide capture at different temperatures (280, 288, 273 and 298 K) at lower pressure. The adsorption isotherms show that the nanocomposite has a good CO₂ adsorption affinity compared to MIL-101. The best adsorption capacity is about 1.6 mmol g^?1 obtained for the nanocomposite material which is two times higher than that of MIL-101, indicating strong interactions between CO₂ and MIL-101@g-C₃N₄. This difference in efficacy is mainly due to the presence of the amine groups dispersed in the nanocomposite. Finally, we have developed a simple route for the preparation of an effective and new adsorbent for the removal of CO₂, which can be used as an excellent candidate for gas storage, catalysis, and adsorption.     

Unexpected Support Effect in Hydrotreating: Evidence of a Metallic Character for ReS<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and ReS<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> Catalysts

Abstract  

Rhenium sulfide based catalysts were prepared by the incipient wetness impregnation method over alumina and silica supports and evaluated for 4,6-dimethyldibenzothiophene hydrodesulfurization in a high-pressure stirred-tank reactor. The catalyst prepared over silica was about six times more active for hydrodesulfurization than the corresponding catalyst prepared over alumina and a NiMo/Al₂O₃ industrial reference catalyst. This surprising and positive SiO₂ support effect was explained by a metallic character of the supported sulfide, which was demonstrated using a kinetic approach of competitive hydrogenations and by XPS characterization.