The effect of catalyst and heat treatment on the properties of carbon–metal composites

Carbon gel and carbon–nickel–palladium doped gels (C–Ni–Pd) were prepared by carbonising resorcinol–formaldehyde (RF) hydrogel and resorcinol–formaldehyde–nickel–palladium (RF–Ni–Pd) hydrogels at 900 °C in a nitrogen atmosphere. RF and RF–Ni–Pd hydrogels were synthesized through sol–gel polycondensation followed by ambient drying. The aim of this study was the determination of the effect of heat treatment in air at 450 °C on the properties of C–Ni–Pd gels prepared using different Pd salts. In the present work, Ni was added as acetate whereas Pd was added as acetate (CA–Ni–Pd) and as chloride (CB–Ni–Pd). Samples were examined by scanning electron microscopy and X-ray diffraction. Surface area was characterized by N₂ adsorption at ?195.5 °C. Thermogravimetric analysis was carried out in order to determine the thermal characteristics of carbon gel and nickel–palladium composites in air atmosphere. CA–Ni–Pd composite had a higher activity and two-phase reaction compared to the CB–Ni–Pd composite. Further improvement of the electrolyte diffusion into the particles of nickel and palladium was obtained by oxidative thermal treatment. During this process a structural modification of the material took place, consequently leading to changes in the electrochemical properties of the composites.     

Growth and optical properties of ZnO–In2O3 heterostructure nanowires

ZnO–In₂O₃ heterostructure nanowires were grown on a Si (111) substrate using the thermal evaporation method. Scanning electron microscopy results showed that the ZnO nanowires had spherical caps. The X-ray diffraction (XRD) pattern and energy-dispersive X-ray (EDX) spectrum indicated that these caps were In₂O₃. An analysis of the early growth process revealed that indium oxide might have played a self-catalytic role. Therefore, it was plausible that the vapor–liquid–solid mechanism (VLS) was responsible for the growth of the ZnO–In₂O₃ heterostructure nanowires. The optical properties of the products were characterized using a photoluminescence (PL) technique. The PL results for the ZnO–In₂O₃ heterostructure nanowires showed a strong peak in the ultraviolet region as a result of the near band emission and a negligible peak for the visible emissions that occurred as a result of the defects. Based on these PL results, it was found that the In₂O₃ nanostructures not only introduced the caps at the tips of the ZnO nanowires but also partially passivated the nanowire surfaces, leading to an improved near band edge emission and the suppression of the defect luminescence.     

In situ formation of noble metal nanoparticles on multiwalled carbon nanotubes and its implication in metal–nanotube interactions

A simple method to decorate multiwalled carbon nanotubes (MWCNTs) with Au, Ag and Cu nanoparticles is illustrated. The method consists in directly depositing the selected metals by thermal evaporation on the carbon nanotubes. Comparative measurements carried out on samples that differ in the quantity and type of the deposited metal, reveal that isolated discrete particles form on the nanotube outer wall for all three metals. The CNT-based composites have been investigated by scanning and transmission electron microscopy to determine the size, shape and distribution of the nanoparticles. The results indicate that the quantity of evaporated metal only affects the nanoparticle size and not the average particle density. Particle composition was determined by X-ray photoelectron spectroscopy study. The results are discussed in terms of metal nanoparticle–tube interactions, an important issue for the fundamental and practical applications of similar MWCNT based composites.     

Synthesis of a graphene–carbon nanotube composite and its electrochemical sensing of hydrogen peroxide

Graphene, whose structure consists of a single layer of sp²-hybridized carbon atoms, provides an excellent platform for designing composite nanomaterials. In this study, we have demonstrated a facile process to synthesize graphene–multiwalled carbon nanotube (MWCNT) composite. The graphene–MWCNT composite material is endowed with a large electrochemical surface area and fast electron transfer properties in Fe(CN)₆^3?/4? redox species. A graphene–MWCNT composite modified electrode exhibits good performance in terms of the electrocatalytic reduction of H₂O₂; a sensor constructed from such an electrode shows a good linear dependence on H₂O₂ concentration in the range of 2 × 10^?5 to 2.1 × 10^?3 mol L^?1. The detection limit is estimated to be 9.4 × 10^?6 mol L^?1. This study provides a new kind of composite modified electrode for electrochemical sensors.     

Fabrication and microwave absorption properties of magnetite nanoparticle–carbon nanotube–hollow carbon fiber composites

Absorbents with “tree-like” structures, which were composed of hollow porous carbon fibers (HPCFs) acting as “trunk” structures, carbon nanotubes (CNTs) as “branch” structures and magnetite (Fe₃O₄) nanoparticles playing the role of “fruit” structures were prepared by chemical vapor deposition technique and chemical reaction. Microwave reflection loss, permittivity and permeability of Fe₃O₄–CNTs–HPCFs composites were investigated in the frequency range of 2–18 GHz. It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances. The bandwidth with a reflection loss less than −15 dB covers a wide frequency range from 10.2 to 18 GHz with the thickness of 1.5–3.0 mm, and the minimum reflection loss is −50.9 dB at 14.03 GHz with a 2.5 mm thick sample layer. Microwave absorbing mechanism of the Fe₃O₄–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe₃O₄ and CNTs–HPCFs.     

Young's modulus and adhesion coefficient of amorphous Ni–P coatings on a metal substrate

In the present paper the Young's modulus and adhesion coefficient of amorphous Ni–P coatings obtained from aqueous solutions were determined. The measurements were carried out using a vibrating reed apparatus. In the temperature range 550–590 K, crystallization of Ni and formation of nickel phosphide Ni₃P were observed. The Young's modulus of Ni–P amorphous layers on stainless steel at room temperature was found to be about 112 GPa. The adhesion coefficient γ of the examined layers depends on the layer thickness a _f and strongly decreases for a _f > 8 μm. This dependence corresponds to the change of the relative adhesion coefficient of about 40% for 8 μm < a _f < 15 μm. It was also shown that the adhesion coefficient does not depend on the temperature, at least in the range 300–550 K.     

Effect of SiC nanowires on the thermal shock resistance of joint between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics

In order to improve the bonding property of joint between SiC modified carbon/carbon (C/C) composites and Li₂O–Al₂O₃–SiO₂ (LAS) glass ceramics, SiC nanowires were attempted as the reinforcement materials in the interface region of SiC transition layer and Li₂O–MgO–Al₂O₃–SiO₂ (LMAS) gradient joining interlayer. The C/C–LAS joint with SiC nanowire-reinforced interface layer was prepared by a three-step technique of pack cementation, in situ reaction and hot-pressing. The microstructure and thermal shock resistance of the as-prepared joints were examined. The average shear strength of the joined samples with SiC nanowires increased from 24.9 MPa to 31.6 MPa after 40 thermal cycles between 1000 °C and room temperature, while that of the joined samples without SiC nanowires dropped from 21.4 MPa to 8.3 MPa. The increase of thermal shock resistance of the C/C–LAS joints was mainly attributed to the toughening mechanism of SiC nanowires by pullout, bridging and crack deflection.     

Synthesis,microstructure and electrical conductivity of carbon nanotube–alumina nanocomposites

Carbon nanotube–alumina (CNT–Al₂O₃) nanocomposites have been synthesized by direct growth of carbon nanotubes on alumina by chemical vapor deposition (CVD) and the as-grown nanocomposites were densified by spark plasma sintering (SPS). Surface morphology analysis shows that the CNTs and CNT bundles are very well distributed between the matrix grains creating a web of CNTs as a consequence of their in situ synthesis. Even after the SPS treatment, the CNTs in the composite material are still intact. Experimental result shows that the electrical conductivity of the composites increases with the CNT content and falls in the range of the conductivity of semiconductors. The nanocomposite with highest CNT content has electrical conductivity of 3336 S/m at near room temperature, which is about 13 orders of magnitude increase over that of pure alumina.     

Effect of boric oxide doping on the stability and activity of a Cu–SiO2 catalyst for vapor-phase hydrogenation of dimethyl oxalate to ethylene glycol

Stable and efficient B–Cu–SiO₂ catalysts for the hydrogenation of dimethyl oxalate (DMO) to ethylene glycol were prepared through urea-assisted gelation followed by postimpregnation with boric acid. Auger electron spectroscopy and CO adsorption by in situ Fourier transform infrared spectroscopy revealed that the Cu⁺ species on the catalyst surface increased together with an increase in the amount of boric oxide dopant. X-ray diffraction and N₂O chemisorption indicated that a suitable amount of boric oxide doping tended to improve copper dispersion and retard the growth of copper particles during DMO hydrogenation. Catalytic stability was greatly enhanced in the B–Cu–SiO₂ catalyst with an optimized Cu/B atomic ratio of 6.6, because of the formation and preservation of appropriate distributions of Cu⁺ and Cu⁰ species on the catalyst surfaces. The effect of boric oxide was attributed to its relatively high affinity for electrons, which tended to lower the reducibility of the Cu⁺ species.     

Fabrication and characterization of multiwalled carbon nanotube–loaded interconnected porous nanocomposite scaffolds

Novel nanocomposite porous scaffolds based on poly(?-caprolactone) (PCL) and multiwalled carbon nanotubes (MWCNTs) were manufactured by a compression-molding/polymer-leaching approach utilizing cryomilling for homogeneous dispersion of nanotubes and blending of polymers. Addition of MWCNTs to PCL and PCL/polyglycolide (PGA) blends resulted in significant changes to scaffold morphology compared to control samples despite persistent interconnected porosity. Several structures exhibiting rough and nanotextured surfaces were observed. Mean pore sizes were in the range of ～3–5?µm. The nanocomposites presented good mechanical and water uptake properties. The results of this research provide significant insight into a strategy for producing nanocomposite scaffolds with interconnected porosity.     

Influence of catalyst pretreatment on catalytic properties and performances of Ru–Re/SiO2 in glycerol hydrogenolysis to propanediols

Bimetallic Ru–Re/SiO₂ and monometallic Ru/SiO₂ catalysts were prepared by impregnation method and their catalytic performances were evaluated in the hydrogenolysis of glycerol to propanediols (1,2-propanediol and 1,3-propanediol) with a batch type reactor (autoclave) under the reaction conditions of 160 °C, 8.0 MPa and 8 h. Ru–Re/SiO₂ showed much higher activity in the hydrogenolysis of glycerol than Ru/SiO₂, and the pretreatment conditions of the catalyst precursors had great influence on the catalytic performance of both Ru–Re/SiO₂ and Ru/SiO₂ catalysts. The physicochemical properties of Ru–Re/SiO₂ and Ru/SiO₂, such as specific surface areas, crystal phases, morphologies/microstructures, surface element states, reduction behaviors and dispersion of Ru metal, were characterized by N₂ adsorption/desorption, XRD, Raman, TEM–EDX, XPS, H₂-TPR and CO chemisorption. The results of XRD, TEM–EDX and CO chemisorption characterizations showed that Re component had an effect on promoting the dispersion of Ru species on the surface of SiO₂, and the measurements of H₂-TPR revealed that the co-existence of Re and Ru components on SiO₂ changed the respective reduction behavior of Re or Ru alone. High pre-reduction temperatures would decrease the activities of Ru–Re/SiO₂ and Ru/SiO₂ catalysts, compared with the corresponding calcined catalysts (without pre-reduction), which actually went through an in-situ reduction during the reaction. XPS analysis indicated that Ru species was in Ru⁰ metal state, while Re species was mostly in Re oxide state in the spent Ru–Re/SiO₂ sample. Re component was probably in rhenium oxide state rather than Re⁰ metal state to take part in the reaction via interaction with Ru⁰ metal.     

Effect of carbon black on corrosion resistance of Al2O3–SiC–C castables to SiO2–MgO slag

Metallurgical solid waste recycling is the shape of things to come in green development of Chinese iron and steel industry. Utilization of ironworks slag for producing mineral wool at high temperature is an important approach. However, refractory lining is seriously corroded by the SiO₂–MgO based slag at 1600 °C during the production process. Different production steps need different atmospheres, the changeable service atmospheres (air and reducing atmosphere) put forward high requirements for slag resistance. The Al₂O₃–SiC–C castables containing carbon black are usually used in iron runner, which faces high-temperature service condition of 1450 °C–1500 °C. Nevertheless, the function of carbon black in the Al₂O₃–SiC–C castables at 1600 °C is till essentially unknown. In the current study, the carbon black was introduced to tabular alumina based Al₂O₃–SiC–C castables to improve corrosion resistance to SiO₂–MgO based slag at 1600 °C. The result showed that 0.4 wt% carbon black was suitable for the castables, which the slag resistance of castables was significantly improved. The carbon black had contributed to block slag by wettability resistance. By comparison with the castables without carbon black, the corrosion index and penetration index had been reduced by 20.2% and 28.0%, respectively, under air atmosphere. And there were little corrosion or penetration under reducing atmosphere for castables with 0.4 wt% carbon black. For the mechanical properties, the Al₂O₃–SiC–C castables with 0.4 wt% carbon black could serve production process although the carbon black impaired the physical properties.     

Fabrication of multilayer porous structured TiO2–ZrTiO4–SiO2 heterostructure towards enhanced photo-degradation activities

Multilayer porous structured TiO₂–ZrTiO₄–SiO₂ photocatalyst with built-in TiO₂/ZrTiO₄ heterojunction and oxygen vacancies was synthesized by sol-gel method combined with template method using colloidal polystyrene spheres as templates. Results show that the multilayer porous structure can be fabricated by controlling the calcination system and the amount of template, and the fabrication of which can also contribute to the generation of oxygen vacancies by creating an anoxic environment. During the photodegradation process, high efficiency of visible light utilization can be achieved due to the slow photon effect of the multilayer porous structure, which can also increase the probability of the contact between Rhodamine B (RhB) with the active sites of the catalyst. Also, the synergistic effect of the generated TiO₂/ZrTiO₄ heterojunction and built-in oxygen vacancy defects jointly promoted the separation of photogenerated carriers. Thus excellent adsorption rate (75.6%, 60 min) and photodegradation rate (95%, 90 min) of the catalyst were obtained. Furthermore, the up-shifted conduction band and valence band maximum positions are beneficial for the mobility of photogenerated holes and inhibit their reaction with H₂O to generate ·OH, while the photogenerated electrons can react with O₂ to form ·O₂⁻, resulting that the holes and ·O₂⁻ participated in the photodegradation of RhB over the as-prepared catalyst.     

Fabrication and cell morphology of a microcellular poly(ether imide)–carbon nanotube composite foam with a three-dimensional shape

The preparation of microcellular poly(ether imide) (PEI) based foams with three-dimensional geometry remains a great challenge worldwide. In this study, we fabricated microcellular PEI–carbon nanotube (CNT) bead foams with a batch rapid depressurization method in a self-designed mold with supercritical carbon dioxide (scCO₂) as a blowing agent. The effects of the saturation time, foaming temperature, foaming pressure, and depressurization rate on the microcellular structures of the PEI foam were analyzed by the Taguchi approach to determine the optimum foaming conditions, and the influence of the CNT content on the cell structure was analyzed. The results show that the depressurization rate and foaming temperature were the key factors influencing the cell size and cell density (N _f); that is, the high depressurization rate and low foaming temperature favored a small cell size and high N _f. The foaming temperature also influenced the foaming ratio (ϕ), and a high ϕ was obtained at a high foaming temperature. Under optimal foaming conditions, PEI with 2.0 wt % CNTs presented the best cell structure; N _f, cell size, and ϕ were 6.14 × 10¹⁰ cell/cm³, 2.43 μm, and 2.08, respectively. The mechanical properties of the final parts were related more to the foaming time and CNT concentration, and the maximum tensile and compression strength were reached at 3 h foaming time and 2.0 wt % CNT, that is, at 2.75 and 15.1 MPa (10% strain), respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47501.     

Interplay of spin–orbit coupling and Zeeman effect probed by Kondo resonance in a carbon nanotube quantum dot

The spin orbit interaction in a carbon nanotube is probed by Kondo resonance. Spin orbit interaction lifts level degeneracy and leads to additional satellite current peaks along with the zero-bias Kondo resonance peak. From the positions of satellite peaks, the spin–orbit interaction energy Δ_so is determined to be 0.38 meV. In the presence of a parallel magnetic field, the satellite peaks split into two; one follows spin-Zeeman dependence with a g-factor of 1.91 and the other one follows orbital-Zeeman dependence with a 2μ_orb value of 170 μeV/T. The zero-bias Kondo resonance peak also displays a spin-Zeeman dependence but has an offset in magnetic field of 2.25 T, consistent with the value of Δ_so/2μ_orb obtained independently.     

The oxidative dehydrogenation of methanol over a novel low‐loading Ag/SiO2–TiO2 catalyst

Catalytic oxidation of methanol to formaldehyde was carried out over Ag/TiO₂-SiO₂ catalysts prepared by chemical reduction. The catalytic activity was measured at the temperature interval 820-920 K, O₂/CH₃OH molar ratio between 0.35 and 0.50 and at the space velocity of 1.2 × 10⁵ h^-1. The optimal content of silver determined by chemical analysis was about 1.7 wt%. The yield of formaldehyde over this catalyst was ~13% higher than that of the industrial pumice-supported silver catalyst and even ~3.5% higher than that of electrolytic silver. The XRD patterns for silver particles supported on TiO₂-SiO₂ are corresponding to Ag(111), (200) and (220), respectively. SEM was used to determine its morphology and particle size. Isolated silver particles were observed on the surface of the catalyst. O₂ chemisorption by using the pulse technique was carried out to determine the free silver surface areas. The average silver particle size from the calculation of selective oxygen chemisorption was found to be in good agreement with that observed from SEM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improved electrical conductivity of a non-covalently dispersed graphene–carbon nanotube film by chemical p-type doping

Using a high-pressure air spray we developed a method to deposit electrically-conducting thin films consisting of non-covalently dispersed graphene and carbon nanotubes. The graphene–carbon nanotube film was immersed in a nitric acid and followed by exposure to fuming nitric acid. The acid treatment induced an increased concentration of atomic nitrogen on the graphene basal plane and carbon nanotube sidewall. This result indicates chemical p-type doping of the graphene oxide–carbon nanotube film. After the two acid treatments, the spray coated graphene oxide–carbon nanotube films on a glass substrate exhibit a low sheet resistance of 171 Ω/sq, and a high transmittance of 84% at a wavelength of 550 nm.     

CO2 reforming of methane over coke-resistant Ni–Co/Si3N4 catalyst prepared via reactions between silicon nitride and metal halides

A series of Ni–Co/Si₃N₄ catalysts with different Ni/Co ratios were prepared via reactions between commercial silicon nitride (Si₃N₄) and metal halides (i.e., NiCl₂ and CoF₃) at high temperature (930 °C). By using X-ray diffraction and electron microscopy, it was shown that this method of catalyst preparation leads to formation of bimetallic Ni–Co nanoparticles encapsulated by a SiN_x layer (Ni–Co@SiN_x) on supporting Si₃N₄ material. The 4.0Ni–3.6Co/Si₃N₄ catalyst was highlighted by showing highly stable catalysis for stoichiometric CO₂ reforming of methane under widely varied reaction conditions, and was found completely free of coke formation after CRM reaction for 100 h.     

Fischer–Tropsch synthesis over a Co/SiO2 catalyst modified with Mn- and Zr under practical conditions

Fischer–Tropsch (FT) synthesis activity and the stability of a Co/SiO₂ catalyst modified with Mn- and Zr were examined under various practical conditions. Dependence of FT synthesis on reaction pressure and bench-scale FT synthesis were investigated. Evaluating catalyst lifetime during continuous FT reactions was conducted. The Co + Mn + Zr/SiO₂ catalyst exhibited relatively greater activity and stable reactivity for 168 h. Sulfur resistance of catalysts were investigated and results showed that the presence of 4 ppm H₂S drastically affected catalytic activity. The Co + Mn + Zr/SiO₂ catalyst exhibited greater activity even with H₂ presence and the sulfur poisoning rate was almost similar on both Co + Mn + Zr/SiO₂ and Co/SiO₂ catalysts.