Effect of Co-Mo catalyst preparation and CH4/H2 flow on carbon nanotube synthesis

Abstract

Supported Co-Mo catalysts with a given ratio of metals were prepared from polyoxomolybdate Mo₁₂O₂₈(μ₂-OH)₁₂{Со(H₂O)₃}₄ using impregnation and combustion methods. Effects of the type of catalyst and the ratio and flow of methane and hydrogen gases on the structure of carbon nanotubes (CNTs) synthesized by catalytic chemical vapor deposition (CCVD) method were studied using transmission electron microscopy and Raman spectroscopy. The catalyst prepared by combustion method yielded mainly individualized CNTs, while the CNTs were highly entangled or bundled when impregnation method was used. In both cases, addition of hydrogen to methane led to reduction of the CNT yield. The samples synthesized using two different catalysts and the same CH₄/H₂ ratio and flow of gases were tested in electrochemical capacitors. A higher specific surface area of the CNTs grown over impregnation-prepared catalyst caused a better performance at scan rates from 2 to 1000?mV/s.     

Study of phase transformation and crystal structure for 1D carbon-modified titania ribbons

One-dimensional hydrogen titanate ribbons were successfully prepared with hydrothermal reaction in a highly basic solution. A series of one-dimensional carbon-modified TiO₂ ribbons were prepared via calcination of the mixture of hydrogen titanate ribbons and sucrose solution under N₂ flow at different temperatures. The phase transformation process of hydrogen titanate ribbons was investigated by in-situ X-ray diffraction at various temperatures. Besides, one-dimensional carbon-modified TiO₂ ribbons calcined at different temperatures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption isotherms, diffuse reflectance ultraviolet–visible spectroscopy, and so on. Carbon-modified TiO₂ ribbons showed one-dimensional ribbon crystal structure and various crystal phases of TiO₂. After being modified with carbon, a layer of uniform carbon film was coated on the surface of TiO₂ ribbons, which improved their adsorption capacity for methyl orange as a model organic pollutant. One-dimensional carbon-modified TiO₂ ribbons also exhibited enhanced visible-light absorbance with the increase of calcination temperatures.     

Controllable synthesis of SnO2 photocatalyst with superior photocatalytic activity for the degradation of methylene blue dye solution

SnO₂ photocatalyst was successfully synthesised by novel chemical route in hydrothermal environment and annealed at two different temperatures viz 550 and 600 °C, respectively. The crystal structure, optical properties, surface and bulk morphology have been characterised using various tools like X-ray diffraction (XRD), UV visible spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscope (TEM) and scanning electron microscope (SEM). Cubic, spheres and porous like morphology of SnO₂ photocatalyst was successfully confirmed using SEM micrographs and TEM. In addition to this the photocatalytic activity was evaluated towards the degradation of methylene blue dye solution. SnO₂ photocatalyst annealed at 600 °C exhibits excellent photocatalytic efficiency which may be attributed to the unique morphology, high crystalline nature and charge separation. The photocatalyst efficiency was further tested towards the concentration of dye, catalyst dosage and pH of the dye. The involvement of ?OH in the photocatalytic reaction was evidenced using trapping experiment by employing different scavengers. The photocatalyst was moderately active, stable upto its fifth usage and stability of the photocatalyst before and after the photocatalytic reaction was also been studied using XRD and SEM.     

Modelling and optimization of syngas production by methane dry reforming over samarium oxide supported cobalt catalyst: response surface methodology and artificial neural networks approach

The reforming of methane by carbon dioxide for the production of syngas is a potential technological route for the mitigation of greenhouse gases. However, the process is highly endothermic and often accompanied by catalyst deactivation from sintering and carbon deposition. Besides, the applications of dissimilar catalytic systems in methane dry reforming have made it difficult to obtain generalized optimum conditions for the desired products. Hence, optimization studies of any catalytic system often resulted in a unique optimum condition. The present study aimed to investigate optimum conditions of variables such as methane (CH₄) partial pressure, carbon dioxide (CO₂) partial pressure and reaction temperature that will maximize syngas yields from methane dry reforming over samarium oxide supported cobalt (Co/Sm₂O₃) catalyst. The Co/Sm₂O₃ catalyst was synthesized using wet-impregnation method and characterized by thermogravimetric analysis), field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction and nitrogen (N₂) physisorption. Syngas production by methane dry reforming over the synthesized Co/Sm₂O₃ catalyst was investigated in a stainless steel fixed-bed reactor. The process variables (CH₄ partial pressure, CO₂ partial pressure and reaction temperature) for the syngas production were optimized using response surface methodology (RSM). The RSM and artificial neural networks (ANNs) were used to predict the syngas production from the experimental data. The comparative analysis between the two models showed that the ANN model has better prediction of the syngas yields compared to the RSM model as evident from the good agreement between the observed and the predicted values. At maximum desirability value of 0.97, optimum CH₄ and CO₂ partial pressures of 47.9 and 48.9 kPa were obtained at reaction temperature of 735 °C resulting in syngas yield of ~79.4 and 79.0% for hydrogen (H₂) and carbon monoxide (CO), respectively.     

Ni/γ-Al2O3 catalyst from kaolinite for the dry reforming of methane

A catalyst consisting of a 5% Ni over a γ-Al₂O₃ novel support was evaluated under high-temperature reaction conditions. The γ-Al₂O₃-rich phase was obtained by selective dissolution of siliceous components of heat-treated kaolinite. The support was appropriate to prepare a catalyst with high surface area (170 m²/g) that showed to be active and stable for dry reforming of methane at 650 °C and a CH₄/CO₂=0.5 molar ratio. Compared to the conventional Ni/α-Al₂O₃ catalyst, this new material showed an improved sulfur resistance when it was reduced in hydrogen steam at the reaction temperature. In industrial conditions, CH₄/CO₂=1, deactivation by carbon deposition was not increased. The content and type of deposited carbon were analyzed by TPH and TPO techniques. As regeneration methods of the coked catalysts, hydrogen and oxygen carbon treatments were employed. After the treatment with hydrogen the catalyst reaches the initial activity, but with oxygen the activity only is partially regenerated.     

Boron Phosphide Nanoparticles: A Nonmetal Catalyst for High‐Selectivity Electrochemical Reduction of CO2 to CH3OH

Electrocatalysis has emerged as an attractive way for artificial CO₂ fixation to CH₃OH, but the design and development of metal‐free electrocatalyst for highly selective CH₃OH formation still remains a key challenge. Here, it is demonstrated that boron phosphide nanoparticles perform highly efficiently as a nonmetal electrocatalyst toward electrochemical reduction of CO₂ to CH₃OH with high selectivity. In 0.1 m KHCO₃, this catalyst achieves a high Faradaic efficiency of 92.0% for CH₃OH at ?0.5 V versus reversible hydrogen electrode. Density functional theory calculations reveal that B and P synergistically promote the binding and activation of CO₂, and the rate‐determining step for the CO₂ reduction reaction is dominated by *CO + *OH to *CO + *H₂O process with free energy change of 1.36 eV. In addition, CO and CH₂O products are difficultly generated on BP (111) surface, which is responsible for the high activity and selectivity of the CO₂‐to‐CH₃OH conversion process.     

Anatase–CMK-3 nanocomposite development for hydrogen uptake and storage

The nanometric carbon CMK-3 modified with TiO₂ in anatase phase was synthesized and applied to energy uptake and storage. TiO₂ nanoclusters are important for hydrogen energy harvesting. The creation of porous structures or large surface with TiO₂ nanoclusters inside can potentially face the challenge of improving their efficiency. In the present work, we report the synthesis and characterization of TiO₂–CMK-3 material assembled from anatase nanoparticles dispersed in the nanometric carbon CMK-3. The resulting nanocomposite was characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and N₂ adsorption–desorption analysis. The newly synthesized hybrid composites exhibited significantly enhanced H₂ storage, in which CMK-3-ordered porous carbon modified with anatase nanoclusters proved to be a material for hydrogen uptake. The nanoparticles of anatase (～5 nm) incorporated onto CMK-3 showed higher hydrogen uptake at low and high pressures (2.9 wt% of H₂ sorption at 10 bar and 77 K) than CMK-3. The approach includes a discussion of H₂ adsorption process and storage properties.     

Synthesis and characterization of Ni-Si mixed oxide nanocomposite as a catalyst for carbon nanotubes formation

Ni-Si mixed oxide nanocomposite was prepared by co-precipitation method with Ni(NO₃)₂ · 6H₂O and tetraethylorthosilicate (TEOS) at pH = 10.5 under reflux condition for 6 days. It was then used as a catalyst for the formation of carbon nanotubes (CNTs) by CVD procedure. Characterization of the catalyst and the CNTs was carried out using X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results showed that Ni-Si mixed oxides nanorods with the average diameter of 3 to 4 nm play a key role in CNTs formation.     

Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature

Vertically-aligned carbon nanotubes(CNTs) with multi-walled structure were successfully grown on a Fe-deposited Si substrate at low temperature below 330°C by using the microwave plasma chemical vapor deposition of methane and carbon dioxide gas mixture. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene ... etc. High quality carbon nanotubes were grown at lower temperature with CO₂ and CH₄ gas mixture than those used by the previous. After deposition, the microstructure morphology of carbon nanotubes was observed with scanning electron microscope and high-resolution transmission electron microscope. The characteristics of carbon nanotubes were analyzed by laser Raman spectroscopy. The results showed the variation of the flow rate ratio of CH₄/CO₂ from 28.5 sccm/30 sccm to 30/30 sccm and the DC bias voltage from –150 V to –200 V, at 300 W microwave power, 1.3–2.0 kPa range of total gas pressure, and substrate temperatures between 300°C and 350°C. Vertically aligned carbon nanotubes with the diameter of about 15 nm and multi-walled structure were illustrated by SEM and HRTEM. However, the highest yield of carbon nanotubes of about 50% was obtained at low temperature below 330°C by MPCVD for the CH₄/CO₂ gas mixture with properly controlled parameters.     

Synthesis and characterization of copper ions surface-doped titanium dioxide nanotubes

Copper ions surface-doped titanium dioxide nanotubes were prepared via an assembly process based on the reactions between Cu(NH₂CH₂CH₂NH₂)₂(OH)₂ and hydroxide radicals on the surface of TiO₂ nanotubes, followed by the heat treatment in air at 723 K. The as-prepared samples were characterized with infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and fluorescence spectroscopy (FL). The photocatalytic activity of the copper ions surface-doped titanium dioxide nanotubes was investigated by photodegradation of Rhodamine B. The results showed that copper ions were successfully introduced onto the surface of TiO₂ nanotubes. And two kinds of copper species of Cu(I) and Cu(II) were found on TiO₂ surface. Copper ions act as electron trappers facilitating the separation of electrons and holes on the surface of TiO₂ nanotubes, which allows more efficiency for the photodegradation of Rhodamine B.     

Synthesis and electrorheological performance of nanosized composite with polar inorganic compounds

To obtain an economical and applicable electrorheological (ER) material, a novel nanocomposite composed of polar inorganic compounds, NH₄Al(OH)₂CO₃, AlO(OH) and (NH₄)₂SO₄, has been synthesized using low-toxic and economical and facile starting materials by a simple chemical reaction process. The experimental result shows that this material has better ER performance. The static yield stress (τ_y) of the suspension (50 wt%) of the material in silicone oil reached 22.8 kPa at a DC electric field of 4 kV/mm, and the relative yield stress (τ_r) (the ratio of the yield stresses with to without an electric field) is also higher (12.7-33.3 for different concentration suspensions). The composition, grain size, dielectric and surface properties of the material have been studied by the elemental analyses, X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM), dielectric spectroscopy and determinations of the surface area and surface energy of the material. The influences of the grain size, dielectric and surface properties on ER performance of the material have been discussed.     

Etch characteristics of FePt magnetic thin films using inductively coupled plasma reactive ion etching

Etch characteristics of L10 FePt thin films masked with TiN films were investigated using an inductively coupled plasma (ICP) reactive ion etching in a CH₃OH/Ar plasma. As the CH₃OH gas was added to Ar, the etch rates of FePt thin films and TiN hard mask gradually decreased, and the etch profile of FePt films improved with high degree of anisotropy. With increasing ICP rf power and dc-bias voltage to substrate and decreasing gas pressure, the etch rate increased and the etch profile becomes vertical without any redepositions or etch residues. Based on the etch characteristics and surface analysis of the films by X-ray photoelectron spectroscopy, it can be concluded that the etch mechanism of FePt thin films in a CH₃OH/Ar gas does not follow the reactive ion etch mechanism but the chemically assisted sputter etching mechanism, due to the chemical reaction of FePt film with CH₃OH gas.     

Mechanistic aspects of formation of MgO nanoparticles under microwave irradiation and its catalytic application

This work reports a preparation of Mg(OH)₂ and MgO nanoparticles (NPs) using magnesium acetate in benzylamine and mechanistic study of its formation. The benzylamine acts as a solvent, base, promoter and capping agent in this reaction. The structure and morphology of particles were analyzed by X-ray diffraction pattern (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), selected area energy dispersion (SAED), energy-dispersive X-ray spectroscopy (EDAX), thermogravimetric analysis (TGA), FT-IR, CO₂–temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) surface area analysis techniques. The application of as prepared MgO NPs was used in catalysis as a catalyst for the formylation of amines with recyclability studies of nanocatalyst.     

Growth and characterization of bamboo-like multiwalled carbon nanotubes over Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

The growth of bamboo-like multiwalled carbon nanotubes (MWCNTs) over Cu/Al₂O₃ catalyst by chemical vapor deposition under atmospheric pressure using ethanol as the carbon source has been demonstrated. The obtained MWCNTs are dominant with bamboo-like morphology. The morphologies, graphitization degree, and microstructures of the products were characterized by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. The results show that the combination of Cu/Al₂O₃ catalyst and ethanol was critical for the growth of bamboo-like MWCNTs. The possible factors causing the formation of bamboo-like structures were also discussed.     

Constructing MAX dispersed Ni2P/TiO2 nanocomposite with investigating influential effect of parameters through design expert and kinetic study for photocatalytic H2 evolution

Well-designed Ni₂P/TiO₂ nanoparticles dispersed over 2D Ti₃AlC₂ MAX, were investigated for H₂ evolution with parameter optimization and kinetic modelling in a liquid phase slurry photoreactor. The highest H₂ production rate of 1300 µmol was obtained over MAX dispersed Ni₂P/TiO₂ nanocomposite. The H₂ production was observed to be 3.80 times more than the H₂ generated by pristine TiO₂, based on the inhibited charge recombination, improved visible light response, and good redox potential of TiO₂. The sacrificial reagents, catalyst loading, and reaction time were optimized through the design of experiment (DoE). The results revealed 10.5 CH₃OH concentration, 0.11 g loading, and a 3.59 h reaction time as the optimum conditions for maximum H₂ generation. Finally, for investigating the adsorption behaviour, a modified Langmuir-Hinshelwood (L-H) mechanism-based kinetic model was developed. According to the kinetic model, the lower CH₃OH adsorption constant suggested low adsorption at lower concentrations, but the higher CH₃OH value indicated more adsorption at higher reactant concentrations. Thus, structured photocatalysts with enhanced photoactivity and kinetic-model findings should help researchers to comprehend photocatalytic reaction engineering properly for solar energy applications.     

Thermodynamic properties for the quaternary system H2O + CH3OH + LiBr + ZnCl2

The thermodynamic properties (solubility, vapour pressure, density, viscosity, heat capacity and heat of mixing) of the H₂O + CH₃OH + LiBr + ZnCl₂ (9:1 H₂O:CH₃OH and 1:1 LiBr:ZnCl₂ by mass) system using H₂O + CH₃OH as the working media and LiBr + ZnCl₂ as the absorbents were measured. The solubility data were obtained in the temperature range from 270.35 to 389.55 K. The measurements of vapour pressure, density, viscosity and heat capacity were carried out at various temperatures and absorbent concentrations. The differential heat of dilution and differential heat of solution at 298.15 K were measured for solutionw with absorbent concentrations from 0 to 75.2 wt%. The integral heat of mixing data at 298.15 K were obtained from both sets of experimental data. The integral heats of mixing for this quaternary system showed exothermic behaviour. The vapour pressure data were correlated with an Antoine-type equation. An empirical formula for the heat capacity was obtained from experimental data. The experimental data for the basic thermodynamic properties of this quaternary system were compared with those of the basic H₂O + LiBr system.     

Friction force microscopy study of annealed diamond-like carbon film

In this paper we introduce mechanical and structural characteristics of diamond-like carbon (DLC) films which were prepared on silicon substrates by radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) method using methane (CH₄) and hydrogen (H₂) gas. The films were annealed at various temperatures ranging from 300 to 900 °C in steps of 200 °C using rapid thermal processor (RTP) in nitrogen ambient. Tribological properties of the DLC films were investigated by atomic force microscopy (AFM) in friction force microscopy (FFM) mode. The structural properties of the films were obtained by high resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The wettability of the films was obtained using contact angle measurement. XPS analysis showed that the sp³ content is decreased from 75.2% to 24.1% while the sp² content is increased from 24.8% to 75.9% when the temperature is changed from 300 to 900 °C. The contact angles of DLC films were higher than 70°. The FFM measurement results show that the highest friction coefficient value was achieved at 900 °C annealing temperature.     

Etch characteristics of IrMn thin films using an inductively coupled plasma of CH3OH/Ar

An inductively coupled plasma reactive ion etching of IrMn magnetic thin films patterned with Ti hard mask was studied in a CH₃OH/Ar gas mix. As the CH₃OH concentration increased, the etch rates of IrMn thin films and Ti hard mask decreased, while the etch profiles improved with high degree of anisotropy. The effects of coil rf power, dc-bias voltage to substrate and gas pressure on the etch characteristics were investigated. The etch rate increased and the etch profile improved with increasing coil rf power, dc-bias voltage and decreasing gas pressure. X-ray photoelectron spectroscopy revealed that the chemical reaction between IrMn films and CH₃OH gas occurred, leading to the clean and good etch profile with high degree of anisotropy of 90°.     

Effect of Mn precursors on the morphology and electrocatalytic activity toward water oxidation of micro-nanostructured MnO<Subscript>x</Subscript> films prepared by voltammetric deposition

MnO_x is a promising nonprecious metal-based water oxidation catalyst that is low in cost, abundant, and has low toxicity. The electrocatalytic properties of micro/nanostructured materials depend partially on the material’s morphology type. Here, we describe the morphology-dependent electrocatalytic activities toward water electrooxidation, i.e., the oxygen evolution reaction, of micro-nanostructured MnO_x films prepared by voltammetric deposition. Films were prepared under fixed deposition conditions using one of three water soluble Mn salt precursors: manganese nitrate (Mn(NO₃)), manganese acetate (Mn(CH₃COO)₂), and manganese chloride (MnCl₂). Each of the as-prepared MnO_x films was heated at 400 °C to tune the morphology, oxidation state of Mn and water electrooxidation efficiency. The as-prepared films lost significant amounts of their activity over several voltammetric water oxidation cycles. Heat treatment of the as-prepared films significantly improved the signal and stability of the water electrooxidation reaction. The water oxidation signal at 1.5 V, obtained using 400 °C-heated micro-nanostructured MnO_x films prepared from each of the three precursors, followed the order Mn(NO₃)₂?>?Mn(CH₃COO)₂?>?MnCl₂. The reason of these different activities was due to their different morphologies as the same phase, ɑ-Mn₂O₃, was found in the entire 400 °C-treated MnO_x films. The nanostructured MnO_x film prepared from Mn(NO₃)₂ was heated at different temperatures, and a temperature of 300 °C was found to be optimal for water electrooxidation efficiency. The high resolution transmission electron microscopic image and X-ray photoelectron spectra revealed that the optimal MnO_x was α-Mn₃O₄ phase, which might be a factor along with its special morphology for improving water oxidation reaction.     

Microscopy,thermal and structural properties of new synthesized 1,4-dihydroxybenzene modified titanium alkoxide

Homogenous, transparent, monolithic and red titanium hybrid gel was obtained by reaction of homogenous mixture of titanium butoxide Ti(OBuⁿ)₄ with the aromatic ring (1,4-dihydroxybenzene HO–C₆H₄–OH) (DO_1,4) in n-butanol at room temperature, without the addition of catalyst and water. Infrared spectroscopy was used to characterize the obtained material. The results show that 1,4-dihydroxybenzene reacted with the titanium alkoxide leading to the transparent monolithic and red gel in which both organic and inorganic –Ti–O–C₆H₄–O–Ti-bridges are formed. The thermal analysis of the xerogel was determined by TGA and DTA. The morphology, texture and structure of the material were studied by scanning electronic microscopy (SEM), Brunauer–Emmett–Teller (BET) method and X-ray powder diffraction (XRD). The X-ray diffraction pattern of the material calcined at 600 °C is consistent with the formation of TiO₂.