Dual growth of graphene nanoplatelets and carbon nanotubes hybrid structure via chemical vapor deposition of methane over Fe–MgO catalysts

Abstract

In this study, Fe–MgO catalyst substrates with various Fe and MgO combinations were evaluated for the growth of different types of carbon nanostructure materials (CNMs), particularly graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) via chemical vapor deposition using methane as a carbon source. The hydrogen yield was also determined as a valuable by-product in this process. Therefore, a set of Fe–MgO catalysts with different iron loadings (30, 80, 85, 90 and 100?wt %) were prepared by the combustion method to realize this target. The physicochemical properties of freshly calcined Fe–MgO catalysts were investigated by XRD, TPR and BET, while the as-grown CNMs were studied by HR-TEM, XRD and Raman spectroscopy. The results verified that the morphology of as-grown CNMs as well as the H₂ yield was directly correlated to the iron content in the catalyst composition. The XRD and TPR results showed that various FeMgO_x species with deferent levels of interactions were produced with the gradual incorporation of MgO content. TEM images indicated that GNPs were individually grown on the surface of high loaded iron-containing catalysts (90–100?wt %) due to the presence of highly aggregated iron particles. While multi-walled carbon nanotubes (MWCNTs) with uniform diameters were grown on the low iron-loaded catalyst (30%Fe/MgO) due to the formation of highly dispersed FeMgO_x particles. On the other hand, GNPs/MWCNTs hybrid materials were grown on the surface of 80%Fe and 85%Fe/MgO catalysts. This behavior can be interpreted by the co-existence of highly aggregated and highly dispersed Fe₂O₃ particles in the catalyst matrix. The results demonstrated that the catalyst composition has a notable effect on the nature of CNMs products and H₂ yield.     

Preparation of Ce1?xFexO2 solid solution and its catalytic performance for oxidation of CH4 and CO

A series of Ce_1−x Fe _x O₂ (0 < x ≤ 0.5) catalysts were prepared by the co-precipitation method, and their catalytic performances were investigated for the total oxidation of CO and CH₄ as model reactions. X-ray diffraction (XRD) and Raman spectroscopy results show that Ce_1−x Fe _x O_2−δ solid solutions are formed with x ≤ 0.2. Ce_0.9Fe_0.1O₂ solid solution presents superior catalytic performance for CH₄ and CO oxidation, while Ce_1−x Fe _x O₂ with x > 0.2 shows less active for CO and CH₄ oxidation. The results of H₂-temperature programmed reduction (H₂-TPR), CH₄-temperature programmed surface reaction (CH₄-TPSR) and CO-TPSR reveal that, the surface oxygen of catalyst is relevant to CO oxidation, which was promoted by the oxygen vacancies formed in Ce–Fe–O solid solution, while the easier lattice oxygen migration property and the favorable reducibility of the catalysts is responsible for the promoted catalytic performance for CH₄ oxidation.     

Subsolidus phase relations of Mg-Ga-Fe-O spinel solid solutions

The phase relations of Mg-Ga-Fe-O spinel solid solutions have been analyzed in a topological approach using composition diagrams. Oxides with the nominal compositions Mg_{1 − x} Ga_{2 − 2x} Fe_3x O_{4 + δ} and Mg_{1 − x} Ga_2x Fe_{2 − x} O_{4 + δ} (with x varied in steps of 0.1) prepared by a pyrohydrolytic process and air-annealed at 1270 K have been studied by X-ray diffraction.     

Synthesis and photocatalytic behaviors of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–CNT/TiO<Subscript>2</Subscript> composite materials under visible light

Cr₂O₃–CNT/TiO₂ composites derived from chromium acetylacetonate, multi-walled carbon nanotubes (MWCNT) and titanium n-butoxide (TNB) were prepared, and the photocatalytic activity of the Cr₂O₃–CNT and CNT/TiO₂ composites was examined. The Cr₂O₃–CNT/TiO₂ composites were characterized by BET surface area measurement, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray analysis. The photocatalytic activity was determined from the decomposition of methylene blue (MB) under visible light irradiation. Methylene blue was photodegraded successfully in the presence of the Cr₂O₃–CNT/TiO₂ composite under visible light irradiation.     

Effect of Mg content on structure and properties of Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O:Al UV transparent conducting films

Mg _x Zn_1−x O:Al (0 ≤ x ≤ 0.6) UV transparent conducting films were deposited on quartz glass by radio frequency magnetron sputtering. Effect of Mg content on structure, optical and electrical properties of Mg _x Zn_1−x O:Al films was investigated. There is a single phase of basic wurtzite structure of ZnO in Mg _x Zn_1−x O:Al films at x ≤ 0.4, and of a basic structure of cubic structure of MgO at x ≥ 0.6. The band gap can be varied from 3.27 to 5.90 eV by controlling Mg contents. The resistivity of Mg _x Zn_1−x O:Al films increase with increasing Mg content x due to the decrease of Al-doping efficiency. The electrical conduction of Mg _x Zn_1−x O:Al films can be markedly improved by increasing the Al-doping level appropriately and annealing in argon atmosphere at over 500 °C. The maximum band gap of Mg _x Zn_1−x O:Al films with wurtzite structure was found to be 5.35 eV when Mg content x is 0.4, and the minimum resistivity of 5.4 × 10⁻⁴ Ω cm was obtained when the Al/(Zn + Mg + Al) is 0.03 and the annealing temperature is over 500 °C. The average transmittance of Mg _x Zn_1−x O:Al films was higher than 86% in the wavelength region from 300 (x ≥ 0.4) to 800 nm.     

Enhanced photocatalytic activity in Al-substituted Bi2Fe4O9 submicrocrystals

Bi₂(Fe_1−x Al _x )₄O₉ (x = 0, 0.1, 0.3, 0.5, and 0.7) samples were synthesized by a simple hydrothermal method. The samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier transformed infrared spectra, N₂-sorption, and UV–Vis diffuse reflectance spectra, and their photocatalytic activities were evaluated by the degradation of methyl orange under visible-light irradiation. The results reveal that the Al substitution can effectively improve photocatalytic performance, which is attributed to the increase of surface area, the improvement of energy band structure, and the distortion of the Fe–O octahedron in Al-substituted samples. In addition, it is found that the photocatalytic activity to decompose methyl orange under visible-light illumination increases monotonically as x increases from zero to 0.5 in Bi₂(Fe_1−x Al _x )₄O₉ and then decreases for x = 0.7, which is believed to be associated with the distribution of Fe³⁺ ions over the octahedral and tetrahedral sites.     

Synthesis and temperature dependent photoluminescence of Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O films grown by ultrasonic spray pyrolysis

Zn_1−x Mg _x O films were deposited on single crystal Si (100) substrates using ultrasonic spray pyrolysis under ambient atmosphere. A strong ultraviolet near-band-edge (NBE) emission was observed in the room temperature photoluminescence (PL) measurement for all the as-grown Zn_1−x Mg _x O films, while the deep-level emission was almost undetectable, suggesting that the obtained Zn_1−x Mg _x O-based films are well close to stoichiometry and of optically high quality. A distinct blue-shift of NBE emission peak from 386 nm to 358 nm was observed as the Mg concentration increases from 0% to 25 %. The photoluminescence spectra as a function of temperature were also investigated to examine the emission mechanism of Zn_1−x Mg _x O films.     

The effect of preparation method on the activities of Pd–Fe–O<Emphasis Type="Italic">x</Emphasis>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for CO oxidation

The Pd–Fe–Ox/Al₂O₃ catalysts were prepared by co-impregnation (co-Pd–Fe–Ox/Al₂O₃) and sol–gel method (sol–gel–Pd–Fe–Ox/Al₂O₃) and characterized by N₂ adsorption–desorption, X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The CO catalytic oxidation was investigated over Pd–Fe–Ox/Al₂O₃ catalysts prepared by different methods. The 100% conversion temperature (T ₁₀₀) over pre-reduced co-Pd–Fe–Ox/Al₂O₃ (co-Pd–Fe–Ox/Al₂O₃–R) and pre-reduced sol–gel–Pd–Fe–Ox/Al₂O₃ (sol–gel–Pd–Fe–Ox/Al₂O₃–R) is 90 and 25 °C when fed with the reaction mixture containing 1 vol.% CO and a balance of air, respectively. XRD results indicate that the sol–gel method is favorable for the high dispersion of PdO particles compared with co-impregnation method. H₂-TPR results suggest that the interaction between Pd and Fe is existent over both sol–gel–Pd–Fe–Ox/Al₂O₃ and co-Pd–Fe–Ox/Al₂O₃ catalysts, while the interaction in former catalyst is stronger than that in the latter. The XPS results show that the Pd species on the surface of both sol–gel–Pd–Fe–Ox/Al₂O₃–R and co-Pd–Fe–Ox/Al₂O₃–R catalysts are the mixture of oxide and metal state, leading to the high activity for CO oxidation. Furthermore, the different Pd²⁺/Pd⁰ ratio may be the reason for the different activity between sol–gel–Pd–Fe–Ox/Al₂O₃–R and reduced co-Pd–Fe–Ox/Al₂O₃–R catalysts.     

Microwave adsorption of core–shell structured Sr(MnTi)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>12−2<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>19</Subscript>/PANI composites

Polyaniline polymer-coated MnTi-substituted strontium hexaferrite (Sr(MnTi) _x Fe_12−2x O₁₉/PANI, x = 1.0, 1.5, 2.0) composites were synthesized by the oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate. The structure and morphologies of the products were characterized by X-ray diffraction, FT-IR, TGA, SEM, and TEM. In the magnetization for the Sr(MnTi) _x Fe_12−2x O₁₉/PANI composites, it was found that the saturation magnetization (M _s) and coercivity (H _c) decreased after polyaniline coating. The composite under an applied magnetic field exhibited hysteretic loops of ferromagnetic behavior, such as high saturation magnetization (M _s = 12.1–1.9 emu/g) and coercivity (H _c = 0.919–0.084 kG). The composite specimens of core–shell Sr(MnTi) _x Fe_12−2x O₁₉/PANI and thermal plastic resin had a band-width microwave absorption due to the reflection losses from −15 to −35 dB at frequencies between 18 and 40 GHz as observed by a high-frequency network analyzer.     

(2?x)MgO · x(MnO, FeO) · 2Al2O3 · 5SiO2 (x = 0–2) ceramic materials and heat effects of their formation

We have determined the main characteristics of ceramic materials prepared by modifying 2MgO · 2Al₂O₃ · 5SiO₂ with MnO and FeO. The formation of the ceramics was analyzed by detailed thermal analysis, X-ray diffraction, IR spectroscopy, and differential scanning calorimetry. We have determined the heat effects of formation of Mg_{1 − x} M _x Al₂O₄ (0.25 < x < 0.75) and Mg_{2 − y} M _y Al₄Si₅O₁₈ (0.5 < y < 1.5) solid solutions with M = Mn(II) and Fe(II) and calculated the standard heats of formation of the Mg_{1 − x} M _x Al₂O₄ and Mg_{2 − y} M _y Al₄Si₅O₁₈ solid solutions.     

Oxidation of ferritic steels dispersion strengthened with TiO2 and Y2O3 oxides in lead melt

We study the structure and composition of scales formed during the contact of Fe–13Cr–2Motype ferritic steels hardened with oxides TiO₂ and Y₂O₃ with oxygen-containing (10⁻³ mass% O) lead melt at 550°C for 1000 h. It is established that a Fe₃ O₄ – Fe (Fe_{1 − x} , Cr _x )₂ O₄ two-layer scale forms. Its upper layer (Fe₃ O₄) grows in the direction of the melt, and the internal layer (Fe (Fe_{1 – x} , Cr _x )₂ O₄) grows in the direction to the matrix. Oxide particles favor an increase in the porosity of the internal sublayer of the scale. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 38 – 44, September–October, 2008.     

Study of conduction mechanism in aluminium and magnesium co-substituted lithium ferrite

The conduction mechanism in Mg^{2 +} and Al^{3 +} substituted Li_0.5Fe_2.5O₄ with general formula Mg_xAl_2xLi_{0.5(1 − x)}Fe_{2.5(1 − x)}O₄ (x = 0.0, 0.2, 0.5, 0.6 and 0.7) has been studied by means of compositional and temperature dependent d.c. resistivity, thermoelectric power and I–V characteristics measurements. It is found that ferrites are electronic conductors. For x = 0.0 and 0.2 conduction is due to holes, while for x = 0.5, 0.6 and 0.7 it is due to electrons. Thermal variation of mobilities and activation energies determined through d.c. resistivity measurements confirm the formation of small polarons. The sample with x = 0.0 exhibits switching phenomena.     

The role of protons in ionic diffusion in (Mg, Fe)O and (Mg, Fe)2SiO4

The presence of hydrogen dissolved within iron-magnesium oxides and silicates results in an increase in the rate of Fe–Mg interdiffusion. Experimental data and point defect models suggest that the increased interdiffusivity is due to an increase in the total metal-vacancy concentration through stabilization of proton-vacancy defect associates in a hydrous environment. In the case of (Mg_1–x Fe _x )O, interdiffusion experiments under hydrothermal conditions at a fluid pressure of ∼0.3 GPa yield similar dependencies of interdiffusivity on Fe-content, oxygen fugacity, and temperature as under dry conditions, but interdiffusion coefficients are a factor of ∼3 larger. These data suggest that the increased interdiffusivities in (Mg_1–x Fe _x )O result from incorporation of defect associates formed between a metal vacancy and a single proton, For (Mg_1–x Fe _x )₂SiO₄, interdiffusion under hydrothermal conditions over a range of fluid pressures reveals a significant difference in the dependence of interdiffusivity on Fe content than obtained under dry conditions, combined with a strong dependence on water fugacity. These data indicate that the increased diffusivities in (Mg_1–x Fe _x )₂SiO₄ result from incorporation of defect associates involving a metal vacancy and 2 protons, It is anticipated that, at higher water fugacities, Fe–Mg interdiffusion in both materials will become dominated by these latter defects and that the interdiffusivity will increase linearly with water fugacity but will be independent of oxygen fugacity and iron concentration.

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Optical and electrical properties of Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O thin films by sol–gel method

Mg _x Zn_1−x O (0 ≤ x ≤ 0.35) thin films have been deposited by sol–gel technique and the composition related structural, electrical, and optical properties are investigated. All the films have hexagonal wurtzite structure and the separation of MgO phase occurs when x = 0.3 and 0.35. With the increase of Mg content, the densification of the films decrease and band gap values increase. The maximum band gap value reaches 3.56 eV when x = 0.15. After Mg doping the conductivities of the Mg _x Zn_1−x O films are reduced greatly and the electrical current–voltage (I–V) characteristics show nonlinearity for x > 0.15.     

Investigation on the synthesis of (Zn<Subscript>1−x</Subscript>Mg<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> and the modulation effect of CaTiO<Subscript>3</Subscript>

(Zn_1−xMg_x)TiO₃ (x = 0.1–0.5) solid solutions were synthesized by solid-state reaction using ZnO, (MgCO₃)₄·Mg(OH)₂·5H₂O and TiO₂ as raw materials. The influences of Zn: Mg ratio and calcining temperature on the properties of (Zn_1−xMg_x)TiO₃ were studied. By adding CaTiO₃ into (Zn_1−xMg_x)TiO₃, the microwave properties and sintering behavior were improved. The ceramics could be sintered at 1150 °C, and the ceramics with excellent microwave properties of τ_f ≈ ±10 ppm/°C, ε ≈ 24, Q × f > 45000 GHz (8 GHz) were obtained.     

Investigation on (100) face of KDP crystal poisoned by MoO4 2? impurity

Potassium dihydrogen phosphate (KDP) single crystals doped with molybdate (MoO₄ ²⁻) were grown via the conventional temperature cooling and rapid growth methods, respectively. MoO₄ ²⁻ made KDP crystals tapering for conventional temperature cooling method. When KDP crystals were grown by rapid growth method, MoO₄ ²⁻ could induce liquid inclusions and simultaneous crystals. The measurement on growth rates indicated that MoO₄ ²⁻ broadened the dead zone and decreased the growth rate of (100) face of KDP crystals. The growth kinetic analysis in terms of two-dimensional nucleus and screw dislocation models implied that the energetic parameter γ/kT decreased with an increase of MoO₄ ²⁻ concentration. The influence of MoO₄ ²⁻ growth steps on (100) face of KDP crystal was observed through ex situ AFM technique. It gave evidence that MoO₄ ²⁻ could postpone the step bunching and make the step edge curving and knaggy to reduce the edge free energy, which was in agreement with the growth kinetics calculations. Additionally, the poisoned mechanism of MoO₄ ²⁻ and Fe³⁺ on step morphologies was detailed contrasted. The interaction process was discussed according to electro negativity analysis, which indicated MoO₄ ²⁻ (actually were HMoO₄ ⁻ and H₂MoO₄) could be absorbed onto (100) face through charge-assisted hydrogen bonds and caused more Mo element distributed in prismatic sector.     

Magnetic and structural properties of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript>100−<Emphasis Type="Italic">x</Emphasis></Subscript> on MgO(110) substrates

Fe _x Pt_100−x (70.1 ≤ x ≤ 83.4) thin films with ordered Fe₃Pt phase were grown successfully onto MgO(110) substrates by electron beam evaporation. The unit cell of ordered Fe₃Pt phase is elongated along c-axis direction and the thin films become more chemically ordered with decreasing Fe content. The magnetization of thin films shows a decrease when Fe content is around 79 at.%. The relationship between magnetic anisotropy and structural parameters suggests that the change of magnetic anisotropy in ordered Fe₃Pt thin films with different compositions most likely stems from the magnetocrystalline origin.     

Preparation of high-yield multi-walled carbon nanotubes by catalytic decomposition of mixture of natural gas and propylene and their electrothermal properties

Abstract

Multi-walled carbon nanotubes (MWNTs) with high-yield were prepared by pyrolysis of mixture of natural gas (NG) and propylene (C₃H₆) over Fe-Ni/Al₂O₃-MgO catalyst. For C₃H₆/NG flow rate ratio ranging from 0 to 0.33, the carbon yield was increased from 903% to 4400%. The synthesized MWNTs after purification were dispersed by ball milling method and mixed with waterborne polyurethane to fabricate the electrothermal film. The mass fraction of CNT filler in the cured electrothermal film was controlled at 50%. The coating after drying was ca. 6?μm and the coating’s volume resistivity was 0.053 Ω·cm. The time-dependent temperature curves indicated that the heating rate of the electrothermal film was very fast under different low voltage and the steady-state temperatures were achieved within 100?s. The steady-state temperature reached 47.9?°C, 76.8?°C, and 102.8?°C, respectively at 10?V, 15?V, and 18?V.     

Preparation, proton conduction, and application in ammonia synthesis at atmospheric pressure of La0.9Ba0.1Ga1– x Mg x O3–α

La_0.9Ba_0.1Ga_1–x Mg _x O_3–α (0 ≤ x ≤ 0.25) was prepared by the microemulsion method. A single phase of LaGaO₃ perovskite structure was formed when x was ≥0.15. Electrochemical hydrogen permeation (hydrogen pumping) proved that La_0.9Ba_0.1Ga_1–x Mg _x O_3–α had proton conduction, and the proton conduction was measured by AC impedance spectroscopy method from 400 to 800 °C in hydrogen atmospheres. Among these samples, La_0.9Ba_0.1Ga_0.8Mg_0.2O_3–α has the highest proton conductivity with the values of 9.51 × 10⁻⁴ to 4.68 × 10⁻² S cm⁻¹ at 400–800 °C. Ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in an electrolytic cell using La_0.9Ba_0.1Ga_0.8Mg_0.2O_3–α as electrolyte. The rate of NH₃ formation was 1.89 × 10⁻⁹ mol s⁻¹ cm⁻² at 520 °C upon imposing a current of 1 mA through the cell.     

Carbon-coated nano-sized LiFe1?xMnxPO4 solid solutions (0?≤?x?≤?1) obtained from phosphate–formate precursors

LiFe_1−x Mn _x PO₄ solid solutions in the whole concentration range (0 ≤ x ≤ 1) are obtained at 500 °C by a phosphate–formate precursor method. The method is based on the formation of homogeneous lithium–iron–manganese phosphate–formate precursors by freeze-drying of aqueous solutions containing Li(I), Fe(II), Mn(II), phosphate, and formate ions. Thermal treatment of the phosphate–formate precursors at temperatures at 500 °C yields nano-sized LiFe_1−x Mn _x PO₄ coated with carbon. The structure and the morphology of the LiFe_1−x Mn _x PO₄ compositions are studied by XRD, IR spectroscopy, and SEM analysis. The in situ formed carbon is analyzed by Raman spectroscopy. The electrochemical performance of LiFe_1−x Mn _x PO₄ is tested in model lithium cells using a galvanostatic mode. All LiFe_1−x Mn _x PO₄ compositions are characterized with an ordered olivine-type structure with a homogeneous Fe²⁺ and Mn²⁺ distribution in the 4c olivine sites. The morphology of LiFe_1−x Mn _x PO₄ consists of plate-like aggregates which are covered by in situ formed carbon. Inside the aggregates nano-sized isometric particles with narrow particles size distribution (between 60 and 100 nm) are visible. The structure of the deposited carbon presents a considerable disordered graphitic phase and does not depend on the Fe-to-Mn ratio. The solid solutions LiFe_1−x Mn _x PO₄ deliver a good reversible capacity due to the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox-couples at 3.5 and 4.1 V, respectively.