Field emission characteristics of CNFB-CNT hybrid material grown by one-step MPCVD

A hybrid material consisting of carbon nanotubes (CNTs) and carbon nanoflake balls (CNFBs) was successfully synthesized by microwave-plasma-assisted chemical vapor deposition using a H₂/CH₄/N₂ ratio of 4:1:2 at 80 Torr for 30 min. The precursor used was a sol-gel solution containing ferric nitrate, tetrabutyl titanate, and n-propanol. The carbon hybrid material (CNFB-CNT) exhibited excellent field emission properties, with its turn-on field being 1.77 V/μm. It also showed two field enhancement factors (1536 and 7932) for different electric fields. The emission current density of the hybrid remained higher than 0.65 mA/cm² for more than 50 h and was 0.82 mA/cm² even after 50 h of continuous emission. Further, the field emission properties of the CNFB-CNT hybrid were better than those of other single-structured carbon nanomaterials (CNTs, CNFs, or CNFBs). Therefore, the CNFB-CNT hybrid material should be a promising candidate for use in high-performance field emitters.     

Synthesis of CoFe2O4 powders with high surface area by solution combustion method: Effect of fuel content and cobalt precursor

High surface area cobalt ferrite (CoFe₂O₄) powders were synthesized by solution combustion method. The dependence of the adiabatic temperature and the released gases during combustion reaction on the fuel content and cobalt precursor type, cobalt nitrate and cobalt acetate, was thermodynamically calculated. Thermal analysis, infrared spectroscopy, X-ray diffractometry, nitrogen adsorption–desorption, electron microscopy and vibrating sample magnetometer were used for investigation of the phase evolution, surface areas, morphology and magnetic properties of the synthesized CoFe₂O₄ powders. The specific surface area decreased from 285.4 to 35.7 m²/g with increasing of fuel to oxidant molar ratio, ϕ, from 0.5 to 1.25 for the cobalt nitrate precursor, while the maximum surface area of 182.1 m²/g was attained at ϕ=1 for the cobalt acetate precursor. The synthesized CoFe₂O₄ powders from the cobalt nitrate precursor exhibited the higher saturation magnetization and coercivity on account of the higher purity and crystallinity.     

Porous carbon spheres as anode materials for sodium-ion batteries with high capacity and long cycling life

Porous carbon spheres (PCSs) with high surface area were fabricated by the reaction of D-Glucose monohydrate precursor with sodium molybdate dihydrate (Na₂MoO₄·2H₂O) via a facile hydrothermal method followed by carbonization and aqueous ammonia solution (NH₃·H₂O) treatment. The as-prepared PCSs exhibit a highly developed porous structure with a large specific surface area and show an excellent electrochemical performance as anode material of sodium-ion batteries (SIBs). A reversible capacity of 249.9 mA h g⁻¹ after 50 cycles at a current density of 50 mA g⁻¹ and a long cycling life at a high current density of 500 mA g⁻¹ are achieved. The excellent cycling performance and high capacity make the PCSs a promising candidate for long cycling SIBs.     

Fischer–Tropsch synthesis using Co/SiO2 catalysts prepared from mixed precursors and addition effect of noble metals

Fischer–Tropsch synthesis in Co/SiO₂ catalysts, which were prepared by mixed impregnation of cobalt (II) nitrate and cobalt (II) acetate, was studied under mild reaction conditions (Total pressure=1 MPa, H₂/CO=2, T=513 K). X-ray diffraction indicated that highly dispersed cobalt metal was the main active sites on the catalyst prepared by the same method. It was considered that the metallic crystallines, which were readily reduced from cobalt nitrate, promoted the reduction of Co²⁺ to metallic a state in cobalt acetate by H₂ spillover mechanism during the catalyst reduction process. The reduced cobalt, from cobalt acetate, was highly dispersed one and remarkably enhanced the catalytic activity. The addition of a small amount of Ru to this type of catalyst remarkably increased the catalytic activity and the reduction degree. Its turn over frequency (TOF) increased but the selectivity of CH₄ was unchanged. However, when Pt or Pd were added into catalysts, they exhibited a higher selectivity of CH₄. Although Pt and Pd hardly exerted an effect on cobalt reduction degree, they promoted cobalt dispersion and decreased the value of TOF. Characterization of these bimetallic catalysts suggested that a different contact between Co and Ru, Pt or Pd existed. Ru was enriched on the metallic cobalt surface but, Pt or Pd dispersed well in the form of Pt–Co or Pd–Co alloy.     

Microporous activated carbons from ammoxidised anthracite and their capacitance behaviours

The paper presents results of a study on obtaining N-enriched active carbons from anthracite mined in Siberia, and on its use as electrode material in supercapacitors. The anthracite was carbonised, activated with KOH and ammoxidised by a mixture of ammonia and air at the ratio 1:3 at 300 or 350 °C, at each stage of the activate production. The products were microporous N-enriched active carbon samples of well-developed surface area reaching from 1255 m²/g to 2011 m²/g and containing from 0.3 to 5.4 wt% of nitrogen. Capacity curves characteristics of the ammoxidised active carbon samples were determined by the galvanostatic and potentiodynamic methods, and by impedance spectroscopy for acidic and basic electrolyte solutions. The best capacity parameters in an acidic medium were obtained for the coal samples ammoxidised at the precursor stage (191 F/g), containing about 0.4 wt% of nitrogen, while in a basic medium—for the coal samples ammoxidised at the stage of active carbon (200 F/g), containing 4.0 wt% of nitrogen.     

Direct synthesis of carbon nanofibers on the surface of copper powder

A novel approach to synthesize carbon nanofibers (CNFs) directly on the surface of metal μm-sized particles to evenly disperse the carbon nanomaterials in a composite material was proposed. As a metal matrix, 5–10 μm copper particles were utilized. As a carbon source, C₂H₂, CH₄ and CO were examined. The best conditions were found to be in C₂H₂ (30 cm³/min) and H₂ (260 cm³/min) atmosphere at the temperature of 750 °C. The composites based on copper and CNFs prepared by vacuum hot pressing showed the increase in hardness from 35 to 60 kg/mm² almost retaining pure copper electrical properties.     

Solvothermal in situ synthesis of (Schiff-base)-containing dinuclear cobalt compound

A new dinuclear cobalt compound, namely Co₂(L)(H₂O)Cl₂ (1, H₂L = N,N′-o-phenylenebis(salicylide-neimine) was obtained by one-pot solvothermal self-assembly of CoCl₂, 1,2-phenylenediamine, and salicylaldehyde in C₂H₅OH. The magnetic studies suggest weak antiferromagnetic behavior and the magnetic data were interpreted by means of a dinuclear cobalt model with the parameters of g = 2.12, J = ?1.25 cm^?1, θ = ?3.12 K.     

Graphene supported Ru@Co core–shell nanoparticles as efficient catalysts for hydrogen generation from hydrolysis of ammonia borane and methylamine borane

Well-dispersed graphene supported Ru@Co core–shell nanoparticles were synthesized by one-step in situ co-reduction of aqueous solution of ruthenium(III) chloride hydrate, cobalt(II) chloride hexahydrate and graphite oxide (GO) with ammonia borane under ambient condition. The as-synthesized nanoparticles exert excellent catalytic activities, with the turnover frequency (TOF) value of 344 mol H₂ min^− 1 (mol Ru)^− 1 for catalytic hydrolysis of ammonia borane, which is the second highest value ever reported. The as-synthesized catalysts exert superior catalytic activities than the monometallic (Ru/graphene), alloy (RuCo/graphene), and graphene-free Ru@Co counterparts towards the hydrolytic dehydrogenation of AB. Moreover, the catalytic hydrolysis of MeAB at room temperature was also studied. These Ru@Co NPs are a promising catalyst for amine-borane hydrolysis and for developing a highly efficient hydrogen storage system for fuel cell applications.     

High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors

Partially graphitized ginkgo-based activated carbon (GGAC) is fabricated from ginkgo shells by pyrolysis, KOH activation and heat treatment using cobalt nitrate as graphitization catalyst. The graphitization temperature is 900 °C. The GGAC has a microporous structure and its specific surface area is 1775 m² g⁻¹. XRD patterns show that the carbon becomes more graphitic after heat treatment. The specific capacitance of the GGAC reaches to 178 F g⁻¹ at a potential scan rate of 500 mV s⁻¹, which is superior to that of commercial activated carbons and ordered mesoporous carbons. The high electrochemical performance of the GGAC is attributed to its good electronic conductivity and high surface area. Partially graphitized activated carbon is a promising electrode material for electrochemical supercapacitors with high rate performance.     

Hydrazine hydrate-induced hydrothermal synthesis of MnFe2O4 nanoparticles dispersed on graphene as high-performance anode material for lithium ion batteries

Herein, a MnFe₂O₄/graphene (MnFe₂O₄/G) nanocomposite has been synthesized via a facile N₂H₄·H₂O-induced hydrothermal method. During the synthesis, N₂H₄·H₂O is employed to not only reduce graphene oxide to graphene, but also prevent the oxidation of Mn²⁺ in alkaline aqueous solution, thus ensuring the formation of MnFe₂O₄/G. Moreover, MnFe₂O₄ nanoparticles (5–20 nm) are uniformly anchored on graphene. MnFe₂O₄/G electrode delivers a large reversible capacity of 768 mA h g⁻¹ at 1 A g⁻¹ after 200 cycles and high rate capability of 517 mA h g⁻¹ at 5 A g⁻¹. MnFe₂O₄/G holds great promise as anode material in practical applications due to the outstanding electrochemical performance combined with the facile synthesis strategy.     

Structure and catalytic performance of Pt-promoted alumina-supported cobalt catalysts under realistic conditions of Fischer–Tropsch synthesis

The structure of alumina-supported cobalt catalysts promoted with platinum and their catalytic performance in Fischer–Tropsch synthesis were investigated under realistic reaction conditions (P = 20 bar, T = 493 K) using in situ time-resolved X-ray diffraction with simultaneous analysis of reaction products. The catalysts were prepared via incipient wetness impregnation and characterized by a wide range of ex situ techniques. Direct in situ measurements were indicative of considerable versatility of alumina-supported cobalt catalysts during Fischer–Tropsch synthesis. Cobalt sintering occurred at the first hours of the reaction and resulted in a significant drop of the catalytic activity. In addition to sintering, partially oxidized catalysts containing smaller cobalt particles (mean particle size <5 nm) were slowly reducing during Fischer–Tropsch reaction. Treatment of cobalt catalysts in pure carbon monoxide led to selective transformation of cobalt metallic phases to Co₂C cobalt carbide. Cobalt carbidization followed by hydrogenation selectively led to cobalt hcp metallic phase, which seems to be more active in Fischer–Tropsch synthesis than cobalt fcc phase. Cobalt oxidation by water was not significant in the catalysts with metal particles larger than 5 nm even at high water concentrations.     

Potassium salt-assisted synthesis of highly microporous carbon spheres for CO2 adsorption

Highly microporous carbon spheres for CO₂ adsorption were prepared by using a slightly modified one-pot Stöber synthesis in the presence of potassium oxalate. Formaldehyde and resorcinol were used as carbon precursors, ammonia as a catalyst, and potassium oxalate as an activating agent. The resulting potassium salt-containing phenolic resin spheres were simultaneously carbonized and activated at 800 °C in flowing nitrogen. Carbonization of the aforementioned polymeric spheres was accompanied by their activation, which resulted in almost five-time higher specific surface area and total pore volume, and almost four-time higher micropore volume as compared to analogous properties of the carbon sample prepared without the salt. The proposed synthesis resulted in microporous carbon spheres having the surface area of 2130 m² g⁻¹, total pore volume of 1.10 cm³ g⁻¹, and the micropore volume of 0.78 cm³ g⁻¹, and led to the substantial enlargement of microporosity in these spheres, especially in relation to fine micropores (pores below 1 nm), which enhance CO₂ adsorption. These carbon spheres showed three-time higher volume of fine micropores, which resulted in the CO₂ adsorption of 6.6 mmol g⁻¹ at 0 °C and 1 atm.     

Production of hydrogen and MWCNTs by methane decomposition over catalysts originated from LaNiO3 perovskite

LaNiO₃ type perovskite was prepared by the “self-combustion” method and was used as catalyst precursor for the methane decomposition reaction at 600 and 700 °C. CH₄ conversion reaches 80% at 700 °C and 65% at 600 °C using pure CH₄. The yield of CNT and H₂ were 2.2 g_CNT g^?1 h^?1 and 8.2 L g^?1 h^?1 at 700 °C respectively after 4 h of reaction. When the reaction is prolonged to 22 h the catalytic activity decreases but the catalyst is still active, the production of hydrogen reaches 63.5 L (STP) per gram of catalyst and the production of MWCNT was equal to 17 g per gram of catalyst.Multi-wall carbon nanotubes were characterized by X-ray diffraction (XRD), surface area (BET), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Raman spectroscopy. TEM micrographs showed that MWCNT longer than 20 μm were formed with inner diameters ranging from 5 to 16 nm and outer diameters up to about 40 nm.The results obtained here clearly show that the use of the perovskite LaNiO₃ as catalytic precursor is very effective for the simultaneous production of carbon nanotubes and hydrogen.     

Controllable synthesis of α-MoC1-x and β-Mo2C nanowires for highly selective CO2 reduction to CO

Transition metal carbides are attractive catalysts because of their similar properties to precious metals. Here, we report the controllable synthesis of α-MoC_1-x and β-Mo₂C nanowires as highly active and selective catalysts for CO₂ reduction to CO (CO₂ + H₂ → CO + H₂O, reverse water-gas shift reaction, RWGS). CO₂ conversion of > 60% together with nearly 100% CO selectivity was achieved at 600 °C, H₂:CO₂ molar ratio of 4:1, and space velocity of 36,000 mL g^− 1 h^− 1. A formate decomposition mechanism for the RWGS reaction was proposed based on the in-situ DRIFTS results.     

Synthesis of nano-sized indium oxide (In2O3) powder by a polymer solution route

Indium oxide (In₂O₃) is a n-type semiconductor with various applications in thin film coatings, on the basis of its optical properties, and in gas sensing equipment, due to its high sensitivity to various oxides such as CO_x and NO_x. In this study, a synthesis process for obtaining In₂O₃ nanoparticles is examined. The precursor used is indium nitrate hydrate (InN₃O₉·H₂O) because of its high solubility in water. By dissolving the nitrate salt in a PVA (polyvinyl alcohol) solution, the precursor is dispersed homogeneously, which reduces the agglomeration of the resulting powder. Calcination at a low temperature of 200–250 °C burns out the organic materials of the PVA with NO_x gas emission and allows the oxidation of the indium, resulting in indium oxide nanoparticles. The influence of the PVA solution characteristics and the heat treatment temperature on the powder morphology and size was analyzed by using SEM, TEM, XRD, TGA/DSC, and four point BET for a specific surface area analysis. The measured specific surface area varies from 3 m²/g to 76 m²/g depending on the calcination temperature, and the particle size of the synthesized powders is under 10 nm for the samples heat treated at 300 °C.     

Synthesis,characterization and catalytic potential of MgNiO2 nanoparticles obtained from a novel [MgNi(opba)]n·9nH2O chain

We describe herein the synthesis of MgNiO₂ nanoparticles employing a new one-dimensional system [MgNi(opba)]_n·9nH₂O, with opba standing for ortho-phenylenebis(oxamato), as precursor. The MgNiO₂ nanoparticles could be obtained after heat-treatment at 800 °C for 5 h under air atmosphere, which was responsible for the elimination of water and organic precursor material leading the formation of nanoparticles with average size of 40±9 nm. To this end, we first described the synthesis of [MgNi(opba)]_n·9nH₂O chain, which was obtained using a pre-synthetized Na₂[Ni(opba)]·5H₂O and Mg²⁺ (molar ratio of 1:1) in aqueous media and then this chain was calcined to produce the desired MgNiO₂ nanoparticles. The obtained MgNiO₂ nanoparticles showed good catalytic performance towards ethanol decomposition achieving 100% of substrate conversion and producing acetaldehyde (56.8%) and hydrogen (24.8%) as the main gaseous products. Also, carbon based structures of great interest for technological applications, carbon nanotubes and onions were formed as valuable byproducts. Thus, we believe that our reported results may inspire the synthesis of catalysts with improved performances for applications in other gas-phase transformations.     

Mesoporous carbons synthesized by direct carbonization of citrate salts for use as high-performance capacitors

A simple one-step synthesis methodology for the fabrication of mesoporous carbons with an excellent performance as supercapacitor electrodes is presented. The procedure is based on the carbonization of non-alkali organic salts such as citrate salts of iron, zinc or calcium. The carbonized products contain numerous inorganic nanoparticles (i.e. Fe, ZnO or CaO) embedded within a carbonaceous matrix. These nanoparticles act as endotemplate, which when removed, leaves a mesoporous network. The resulting carbon samples have a large specific surface area up to ∼1600 m² g⁻¹ and a porosity made up almost exclusively of mesopores. An appropriate heat-treatment of these materials with melamine allows the synthesis of N-doped carbons which have a high nitrogen content (∼8–9 wt.%), a large specific surface area and retain the mesoporous structure. The mesoporous carbon samples were employed as electrode materials in supercapacitors. They exhibit specific capacitances of 200–240 F g⁻¹ in 1 M H₂SO₄ and 100–130 F g⁻¹ in EMImTFSI/AN. More importantly, the carbon samples possess a good capacitance retention in both electrolytes (>50% in H₂SO₄ and >80% in EMImTFSI/AN at 100 A g⁻¹) owing to their mesoporous structure which facilitates the penetration and transportation of ions.     

Tunable ceria–zirconia support for nickel–cobalt catalyst in the enhancement of methane dry reforming with carbon dioxide

Ceria–zirconia mixed oxides (CeZr) were glycol-thermally synthesised as nano-crystalline supports with tunable ratios for the anchoring of nickel–cobalt (Ni–Co) catalyst to enhance methane dry reforming (MDR) reaction with carbon dioxide. High conversion of methane (90%) and carbon dioxide (92%), good output (H₂ = 32%; CO = 44%), and selectivity and stability of syngas prove the effectiveness of the catalyst deposited on this support. 80:20 for Ce:Zr was identified as the optimal ratio to attain active and stable catalytic performance in MDR, with a low coking content of 0.47 wt.%.     

First macrocyclic oxamide Cu(II)–Co(II) complex: synthesis,crystal structure and magnetic properties

A oxamido-bridged binuclear complex, [Cu(L)Co(bpy)₂](ClO₄)₂·0.5C₂H₅OH·H₂O has been synthesized, and structurally and magnetically characterized, where L = 1,4,8,11- tetraazacyclotradecanne-2,3-dione and bpy = 2,2^′-bipyridine. In the complex, the copper atom from the macrocyclic oxamido-copper(II) precursor in a square-planar and cobalt atoms in a distorted octahedral environment is bridged by the oxamidate group, Cu⋯Co separation of 5.359 Å. The metal–metal separations are consistent with the sequence of decrease of the ion radius of the metal ion. The magnetic properties of the complex have been investigated. The χ_MT versus T plots is typical of antiferromagnetically coupled Cu(II)–Co(II).     

Electrogeneration of hydrogen peroxide in gas diffusion electrodes modified with tert-butyl-anthraquinone on carbon black support

Hydrogen peroxide (H₂O₂) is a versatile oxidizing agent that is synthesized commercially by the reduction of oxygen in organic medium. Electrochemical technology employing a modified gas diffusion electrode (MGDE) offers a viable alternative for the industrial-scale synthesis of the oxidant. Addition of 1% (w/w) of tert-butyl-anthraquinone (TBAQ) to carbon black deposited in the form of a microporous layer onto the disk of a rotating ring-disk electrode produced an increase in the ring current, which is directly related to H₂O₂ formation, and presented an efficiency of H₂O₂ generation of 89.6% compared with 76.6% for carbon black alone. No significant changes were detected in the number of electrons transferred in the presence of the catalyst suggesting an electrochemical/chemical mechanism for H₂O₂ formation. Analogous improvements in the generation of H₂O₂ were obtained with MGDEs comprising TBAQ on carbon black. The highest concentrations of H₂O₂ (301 mg L⁻¹) were produced at the fastest rate (5.9 mg L⁻¹ min⁻¹) with the lowest energy consumption (6.0 kWh kg⁻¹) when a potential of −1.0 V vs SCE was applied to a MGDE containing 1.0% of TBAQ on carbon black. It is concluded that the application of MGDEs comprising TBAQ on carbon black support offers considerable advantages in the electrogeneration of H₂O₂.