Vapor Decomposition Research Articles

Biotar Ironmaking Using Wooden Biomass and Nanoporous Iron Ore

This paper describes fundamental experiments of a new biomass ironmaking that employs low-grade iron ore and woody biomass for promoting the direct reduction, FeO + C = Fe + CO, in which dehydrated, porous limonite iron ore was filled with carbon deposited from the biomass tar, biotar. In our experiments, three types of iron ores containing different amounts of combined water (CW; 1.6, 3.8, and 9.0 mass %) were first dehydrated at 450 °C to make them porous and then heated with pine tree biomass at 500−600 °C for the gasification and the tar vapor generated was decomposed to deposit carbon within/on the porous ores. The dehydration treatment made the iron ores porous by removing CW and significantly increased their Brunauer−Emmett−Teller (BET) specific surface areas and porosities. In the second treatment of biomass gasification and decomposition of tar vapor, the biomass was changed into char, tar vapor, and reducing gas; the biotar was decomposed and carbonized within the porous ores. Interestingly, the...

Catalytic Vapor Decomposition of Methane over Nickle Catalyst: Growth Rate and the Corresponding Microstructures of Carbon Nanofibers

Time-resolved intrinsic reaction rate of catalytic vapor deposition of methane into carbon nanofibers (CNFs) and hydrogen on supported Ni catalysts was determined under different reaction temperatures (773–973 K) and feed compositions (PH2 = 0–0.29 atm, PCH4 = 0:28–0.71 atm) and the microstructure of the produced CNFs was characterized by Raman and TEM. The results showed that, increasing the reaction temperature, methane partial pressure and decreasing hydrogen partial pressure increased the initial reaction rate but generally led to fast catalyst deactivation. A high carbon yield per unit catalyst was obtained as a compromise between initial reaction rate and catalyst deactivation rate. The microstructures of the produced CNFs had a close relationship with the initial reaction rate. The CNFs produced with high initial rate tended to have a small angle between the graphene layers and the growth axis, thin diameter and a hollow core. This phenomenon can be used for monitoring and controlling the CNFs synthesis process.

Growth of nitrogen-doped filamentous and spherical carbon over unsupported and Y zeolite supported nickel and cobalt catalysts

Unsupported and Y zeolite supported nickel and cobalt catalysts have been tested for the growth of nitrogen-doped structured carbon via the chemical vapour decomposition (CVD) of acetonitrile where 550 °C ≤ T ≤ 1150 °C. A range of carbonaceous structures, including carbon filaments with different lattice structures and carbon nanospheres were obtained. The graphitic character of the carbon product has been evaluated by means of temperature-programmed oxidation (TPO) and XRD analyses, surface area and porosity measurements for both catalyst and carbon product are provided and structural/morphological features illustrated by scanning and transmission electron microscopy (SEM and TEM). Carbon yield and morphology were strongly dependent on reaction temperature, the nature of the active metal and the use of a support. Carbon yield increased with increasing temperature (up to 1050 °C) to give maximum values in the order Ni < Co < Ni/Y < Co/Y. The supported catalysts delivered significantly higher carbon yields (up to 35 gcarbon/gmetal) and initiated carbon growth at temperatures up to 300 °C lower than that observed for the bulk metals. Carbon generated over Ni/Y and Co/Y at 750 °C took the form of high aspect ratio nanofibres where the arrangement of graphene layers exhibited a periodic variation at higher reaction temperatures to ultimately result in a predominant production of nanospheres at 1150 °C, rather attributed to a thermal than a catalytic route. Based on TPO analyses, carbon grown from bulk and supported Ni showed a higher degree of structural order compared with the Co promoted growth. An increase in reaction temperature served to enhance graphitic character. XRD analyses were in all the cases consistent with a graphitic product. Elemental (CHN) analysis of the carbon product has revealed a nitrogen content of up to 5 mol%.

Reduced Carbon Solubility in Fe Nanoclusters and Implications for the Growth of Single-Walled Carbon Nanotubes

Fe nanoclusters are becoming the standard catalysts for growing single-walled carbon nanotubes via chemical vapor decomposition. Contrary to the Gibbs-Thompson model, we find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first-principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function of particle size, there are three scenarios compatible with steady state growth, limited growth, and no growth of single-walled carbon nanotubes, corresponding to unaffected, reduced, and no solubility of C in the particles.

Charging of Aerosol Products during Ferrocene Vapor Decomposition in N2 and CO Atmospheres

Decomposition of ferrocene vapor in CO and N2 atmospheres in the temperature range from 800 to 1150 °C leading to the formation of single-walled carbon nanotubes (CNTs) and maghemite (γ-Fe2O3) part...

Effect of Fe/Ni catalyst composition on nitrogen doping and field emission properties of carbon nanotubes

Nitrogen-doped carbon (CN x ) nanotubes have been synthesized by acetonitrile vapor decomposition over catalyst nanoparticles produced in the result of thermolysis of solid solutions of Fe and Ni bimaleates. X-ray photoelectron spectroscopy revealed the nitrogen content in CN x nanotubes grows from 0.4 to 1.2 at%, when Ni portion in catalyst increases. Nitrogen doping level of sample produced using bimaleates of Fe and Ni taken in a ratio of 7:3 is deviated from this dependence due to formation of two phases of metallic catalyst. N 1s-edge X-ray absorption spectra of samples showed three peaks, which by results of quantum-chemical calculation on nitrogen-containing carbon nanotube (CNT) model were assigned to pyridinic, graphitic, and molecular forms of nitrogen. Measurements of current–voltage characteristics of the samples found the electron emission threshold is reduced with amount of nitrogen incorporated into CN x nanotubes.

Synthesis and modification of micro-and nanorods of vanadium oxides

A method for the gas-phase synthesis of single-crystalline micro-and nanorods of vanadium pentoxide (V2O5) has been developed based on the thermal decomposition of vanadium chloride vapor. Using this technique, it is possible to obtain separate micro-and nanorods (with thicknesses ranging from 50 nm to 5 μm and a length of up to 7 mm), planar arrays, and three-dimensional structures. The temperature dependence of the resistance of individual microrods of vanadium oxides in various degrees of oxidation has been measured.

Preparation of Titania Film by Vapor Decomposition of Chelated Tetrabutyl Titanate

The vapor decomposition of chelated tetrabutyl titanate on glass substrate was used for the preparation of titania films. The influences of decomposition temperature, chelant concentration on the photocatalytic properties of titania film were investigated. The films were characterized by XRD, ATR and the precursor solution was characterized by DTA.

Filling Carbon Nanotubes with Co9S8 Nanowires through in Situ Catalyst Transition and Extrusion

We describe the synthesis of novel Co9S8-nanowire-filled carbon nanotubes (CNTs) by a simple method involving the pyrolysis of thiophene on cobalt catalyst in a conventional chemical vapor decomposition system. The encapsulated Co9S8 nanowires are single-crystalline, and their lengths are about 10 μm with their [110] direction parallel to the axis of the CNTs. Detailed investigation suggests that the filling of the Co9S8 nanowires results from the volume increase induced by a phase transition from cobalt to cobalt sulfide together with the in situ extruding action of CNTs as nanomolds. A new filling mechanism is thus found and proposed.

RDX flame structure at atmospheric pressure

The chemical structure of an RDX flame at a pressure of 1 atm was studied using probing molecular beam mass spectrometry. The flame was found to contain RDX vapor, and its concentration profile was measured in a narrow zone adjacent to the burning surface. In addition to RDX vapor, ten more species were identified (H2, H2O, HCN, N2, CO, CH2O, NO, N2O, CO2 and NO2), and their concentration profiles were measured. Two main chemical-reaction zones were found in the RDX flame. In the first, narrow, zone 0.15 mm wide adjacent to the burning surface, decomposition of RDX vapor and the reaction of NO2, N2O, and CH2O with the formation of HCN and NO occur. In the second, wide, zone 0.85 mm wide, HCN is oxidized by NO to form the final combustion products. The composition of the final combustion products was analyzed from an energetic point of view. The measured composition of the products near the burning surface was used to determine the global reaction of RDX gasification at a pressure of 1 atm. Values of heat release in the condensed-phase calculated by the global gasification reaction and by the equation of heat balance on the burning surface (using data of microthermocouple measurements) were analyzed and compared.

A Novel Catalyst for Synthesis of Styrene: Carbon Nanofibers Immobilized on Activated Carbon

Carbon NanoFibers (CNFs) with hierarchically structure have been immobilized onto Activated Carbon (AC) by impregnation with an aqueous solution of Fe(CH3COO)2, reduction and subsequent chemical vapor decomposition of ethylene. The morphology of the CNFs can be modulated by adjusting the pH of the Fe(CH3COO)2 solution used for impregnating the AC. A stable yield of 35% in the oxidative dehydrogenation of ethylbenzene to styrene was obtained at a temperature of 673 K, around 200 K lower than the current industrial process. The immobilized CNFs on AC catalysts combine the catalytic properties of the carbon nanofibers and the suprastructure of the AC host. The final material is an easy to handle active catalyst, with an open structure of immobilized CNFs avoiding the pressure drop problem, which is typically observed for fine powder forms of CNFs. The immobilized CNFs on AC are attractive for gas-phase fixed-bed industrial applications.

Synthesis and hydrogen storage properties of carbon nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapor decomposition of acetylene over rare-earth (RE) based AB 2 alloy hydride catalysts. The AB 2 alloy hydride catalysts have been prepared by hydrogen decrepitation technique and characterized by scanning electron microscopy. The advantage of this novel method of obtaining catalysts has been discussed. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermo gravimetric analysis and Raman spectroscopy. Hydrogen adsorption measurements were carried out on as-prepared and purified MWNT in the temperature range of 143–373 K and pressure range of 10–100 bar using a high pressure hydrogen adsorption setup and the results have been discussed. A maximum hydrogen storage capacity of 3.5 wt% is obtained for purified MWNT prepared with DyNi 2 alloy hydride catalyst at 143 K and 75 bar.

Composition of plasma-chemical silicon dioxide films as studied by IR spectroscopy

The results of a comparative analysis of the compositions of silicon dioxide films prepared by the decomposition of tetraethoxysilane vapor in a glow discharge and the deposition of the products onto a substrate are presented. The compositions of films prepared in gas mixtures containing argon, oxygen, tetraethoxysilane vapor, and NaCl were compared using IR spectroscopy. The electric discharge was excited at a frequency of 19 kHz and in a radiofrequency range at 81.36 MHz. It was found that the additives of oxygen and sodium-containing vapors exerted a noticeable effect on the composition of the films. The compositions of the films prepared at the low-frequency and high-frequency discharge excitation were also different.

Effect of Purity and Substrate on Field Emission Properties of Multi-walled Carbon Nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapour decomposition (CVD) of acetylene over Rare Earth (RE) based AB2(DyNi2) alloy hydride catalyst. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Thermo Gravimetric Analysis and Raman Spectroscopy. Fully carbon based field emitters have been fabricated by spin coating a solutions of both as-grown and purified MWNT and dichloro ethane (DCE) over carbon paper with and without graphitized layer. The use of graphitized carbon paper as substrate opens several new possibilities for carbon nanotube (CNT) field emitters, as the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon paper and reduces contact resistance. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. CNT field emitter prepared by spin coating of the purified MWNT–DCE solution over graphitized carbon paper shows excellent emission properties with a fairly stable emission current over a period of 4 h. Analysis of the field emission characteristics based on the Fowler–Nordheim (FN) theory reveals current saturation effects at high applied fields for all the samples.

A multi-step strategy for cutting and purification of single-walled carbon nanotubes

A multi-step procedure has been developed to cut and purify single-walled carbon nanotubes (SWCNTs) from the pristine soot synthesized by catalytic chemical vapor decomposition, which contains significant amount of multiwalled carbon nanotube (MWCNT) impurities. The procedure involves refluxing in dilute HNO 3 solution followed by (NH 4) 2S 2O 8/H 2SO 4 ultrasonication and a novel ammonia heat treatment step. The metal impurities are removed in the HNO 3 refluxing step. In the (NH 4) 2S 2O 8/H 2SO 4 treatment step, the SWCNTs are cut uniformly into micrometer long, while the MWCNTs are selectively cut into very short curled pieces. These short MWCNT pieces and the amorphous carbon impurities are partly removed in the (NH 4) 2S 2O 8/H 2SO 4 treatment step, and most of them are finally removed by the final ammonia heat treatment. The ammonia heat treatment step also recovers the structures of SWCNTs by healing defects on the sidewall. The final SWCNT products have purity higher than 95%, a length distribution between 1 and 2 μm, and very few defects.

Carbon-Coated Li4Ti5O12 as a High Rate Electrode Material for Li-Ion Intercalation

is a promising electrode material for high power density lithium-ion batteries and hybrid supercapacitors, but has the drawback of low electrical conductivity. We report a thermal vapor decomposition method to coat a uniform nanothickness graphitized-carbon on the particle surface. The resulting product coated at 800°C has a thick carbon layer and a electrical conductivity of , which is much higher than that of raw . As a result, it shows much better rate capability when used as a negative electrode for electrochemical supercapacitors. AC impedance measurements reveal that the carbon-coated has smaller charge-transfer resistance due to large effective interface reaction area.

Electron field emitters based on multiwalled carbon nanotubes decorated with nanoscale metal clusters

The present work describes the field emission characteristics of nanocrystalline metal clusters decorated multi walled carbon nanotubes (MWNT) field emitters fabricated over flexible graphitized carbon cloth. MWNT have been synthesized by catalytic chemical vapour decomposition (CCVD) of acetylene over Rare Earth (RE) based AB2 (DyNi2) alloy hydride catalyst. Fine powders of RE based AB2 alloy hydride catalysts have been prepared by hydrogen decrepitation technique. The as-grown carbon nanotubes (CNTs) are purified by acid and heat treatments and characterized using XRD, SEM and TEM. Purified MWNT have been decorated with nanocrystalline Pd, Pt and Pt–Ru metal clusters using a simple chemical reduction method. The characterization of metal decorated CNTs were done by using XRD, SEM, TEM, HRTEM and EDX. Nanocrystalline Pd, Pt and Pt–Ru metal clusters decorated MWNT field emitters have been fabricated over graphitized carbon fabric using spin coating method. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. The samples show excellent emission properties with a fairly stable emission current over a period of 4 h. As the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon cloth, the use of graphitized carbon cloth as substrate opens new possibilities for CNT field emitters.

A comparison of measured temperatures with those calculated numerically and analytically for an exothermic chemical reaction inside a spherical batch reactor with natural convection

When any exothermic chemical reaction occurs inside e.g., an unstirred spherical vessel, the heating effect of the reaction often induces temperature gradients and consequently natural convection. This work sets out to compare previously measured temperatures at different positions inside such a batch reactor with values computed numerically and analytically. It is the first such study for a reaction with an order greater than zero, occurring in a spherical reactor. The main reaction considered is the thermal decomposition of the gas, azomethane, which has often been used in experimental studies of thermal explosion. Other experimental results for the reaction between nitric oxide and oxygen, as well as between hydrogen and chlorine are also considered here. The measured temperatures at the centre of the vessel are first compared with analytical scales, derived by inspecting the governing equations. It is found that the temperature rise when diffusion is the dominant transport mechanism (i.e., at small values of the Rayleigh number) is directly proportional to the ratio of the characteristic timescales for diffusion and for the reaction. Similarly, when natural convection is the dominant transport mechanism, the temperature rise is proportional to the ratio of the timescales for convection and reaction. A numerical scheme was developed to simulate the thermal decomposition of azomethane vapour under the influence of natural convection, as well as the diffusion of both matter and heat ( via thermal conduction). The results of these simulations are compared with the temperature profiles measured along the vertical axis of the reactor. There is excellent agreement between experimental and numerical results. This confirms the computational procedures. The simulations indicate that the hottest point in the reactor moves upwards above the centre of the vessel when Ra is increased. In fact, three distinct types of temperature profile occur, depending on the value of Ra. For low Ra, the temperature profile is approximately spherically symmetric, as expected. When Ra is increased, the symmetry in the temperature profile is disrupted by the flow produced by natural convection. In that case, the temperature profile becomes skewed, with the maximum occurring on the axis in the top half of the reactor. Thirdly, under some circumstances at high Ra, a large, sharp peak in the temperature profile near the top of the reactor is produced. This resembles a stabilised flame front, but could be an explosion. These three types of temperature distribution have also been observed experimentally. Finally, the ratio of the notional temperature rise in a purely diffusive system to that measured experimentally is compared over a wide range of Ra. The experimental results for the three reactions considered all follow the same trend; the numerical and analytical solutions are in excellent agreement with the experimental results, confirming that the onset of natural convection inside a spherical vessel occurs at Ra ∼ 500 . This result seems to be true, regardless of the order of the reaction, and only depends on the shape of the reactor.

Laser-induced synthesis of nanostructured thin films

We present a number of important characteristics of laser techniques for preparation of nanostructured thin films with foci on laser ablation (LA) and laser-assisted chemical vapor decomposition (LCVD). Size-related features and structures of the deposited layers are associated with process parameters. Whenever possible, the performance of the deposits is compared to that of films prepared by other methods ( e.g., physical vapor synthesis such as gas deposition, sputtering, etc.). The LA and LCVD methods allow a wide variety of materials to be prepared. A number of promising applications emerge when one combines this versatility with the potential for making certain types of nanostructured layers. These possibilities are also discussed.

Synthesis of CNTs via ethanol decomposition over ball-milled Fe2O3 coated copper sheets

Carbon nanotubes (CNTs) were synthesized on ball-milled Fe2O3 coated copper sheets by the catalytic decomposition of ethanol vapor at 650°C. TEM, SEM, and EDX revealed the presence of 30–50 nm diameter multiwalled carbon nanotubes with catalytic particles at their tips. CNTs, α-Fe, and Fe3C were detected by XRD. Raman and TG analyses show that the product is CNTs with less than 10 wt % residues. The carbon yield was the maximum at 354 wt %.