Growth kinetics of MWCNTs synthesized by a continuous-feed CVD method

Unlike two-step chemical vapor deposition (CVD) methods using pre-deposited catalyst particles, in a continuous-feed CVD process, the liquid feed (consisting of catalytic precursor and hydrocarbon source) is continuously supplied into the reactor causing catalyst particle formation, nucleation of carbon nanotubes (CNTs) and CNT growth to occur simultaneously throughout the reaction period. In order to observe these processes, CVD experiments were conducted for different durations (30 s to 3 h) and the product multiwalled carbon nanotubes (MWCNTs) were characterized using scanning electron microscopy. It was found that the nanotubes did not grow in the vapor phase and that substrates played an important role in the growth by providing a place for them to anchor before growth took place. Based on transmission electron microscopy images, it has been suggested that MWCNTs grew by root-growth mechanism from the catalyst particles that were deposited on the substrate during the early stages. At long process times, continuously supplied feed gas produced additional catalyst particles which were deposited mostly on the growing nanotube mat. Due to weak catalyst-mat interaction, the additional nanotubes grew by tip growth. A comprehensive MWCNT growth model has been presented for the continuous-feed CVD.     

Experimental characterization of the role of hydrogen in CVD synthesis of MWCNTs

An experimental study was conducted to examine the role of hydrogen in the chemical vapor deposition (CVD) synthesis of multiwalled carbon nanotubes (MWCNTs) in a flow tube reactor using xylene as a carbon source and ferrocene as a catalyst. Ferrocene was introduced into the reactor by two methods. In a single step method, the catalyst was dissolved in xylene and the mixture was introduced using a syringe pump. A two step method was also used where the ferrocene powder was placed in the preheated zone for a certain time to deposit iron catalyst particles on the reactor wall prior to introducing the pure xylene into the reactor. CVD synthesis of carbon products was performed as a function of hydrogen input under constant flow conditions using both methods. SEM and TEM images of the carbon products were examined. The results revealed a competition between the formations of the different carbon products (soot, carbon fibers and CNTs) that altered by the addition of hydrogen. The role of hydrogen is suggested to reduce the rate of carbon production by dehydrogenation so that the more ordered and thermodynamically stable MWCNTs can be produced rather than less ordered and thermodynamically stable soot and carbon fibers.     

煤层气产出水水质特征及处理技术研究进展

论述了国内外煤层气产出水的水质特征及其处理技术的研究进展。煤层气产出水具有高钠度和高矿化度的特征。反渗透法普遍运用于处理煤层气产出水,并且已投入实际工程应用,通常和纳滤或超滤联用,处理后的产出水可进行再利用。而其他处理技术如离子交换法、电容去离子法、电絮凝法、吸附法和化学法等仍处于试点研究阶段。     

重力作用下甲烷催化部分氧化积炭特性数值分析

为了研究微尺度下甲烷催化部分氧化反应在重力作用下的反应特性,考察了定C/O比情况下各个壁面的积碳量随着壁温和催化剂活性位密度变化的特性。研究结果表明：在壁温和C/O比一定,催化剂的活性位密度为1.66 mgmol/m2时,甲烷转化率和出口氢气的质量分数达到最大值,分别为99.10%和12.39%;重力的存在,使得沿重力方向的壁面积碳量增加,重力反方向的壁面积碳量减少,在温度低于1500K时,沿壁面的积碳量随着催化剂活性位密度的增加而降低,当温度在1500K时,积碳量基本为零。     

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.     

Comparative study of the textural characteristics of oil palm shell activated carbon produced by chemical and physical activation for methane adsorption

The objective of this study is to relate textural and surface characteristics of microporous activated carbon to their methane adsorption capacity. Oil palm shell was used as a raw material for the preparation of pore size controlled activated carbon adsorbents. The chemical treatment was followed by further physical activation with CO₂. Samples were treated with CO₂ flow at 850 °C by varying activation time to achieve different burn-off activated carbon. H₃PO₄ chemically activated samples under CO₂ blanket showed higher activation rates, surface area and micropore volume compared to other activation methods, though this sample did not present high methane adsorption. Moreover, it was shown that using small proportion of ZnCl₂ and H₃PO₄ creates an initial narrow microporosity. Further physical activation grantees better development of pore structure. In terms of pore size distribution the combined preparation method resulted in a better and more homogenous pore size distribution than the conventional physical activation method. Controlling the pore size of activated carbon by this combined activation technique can be utilized for tuning the pore size distribution. It was concluded that the high surface area and micropore volume of activated carbons do not unequivocally determine methane capacities.     

阳离子交换树脂对煤层气产出水除盐的研究

采用离子交换的方法,通过静态法和动态法两种方法来降低煤层气产出水的矿化度,研究阳离子交换树脂在不同工艺条件下对矿化度的影响。结果表明,树脂活化时间为2 d,且连续使用3次后需更新,交换时间最佳为3 h。在静态法处理过程中,水与树脂的比例为1∶0.5时,连续处理3次后除盐效果最佳,可达94.8%。动态法需采用双柱串联,流速为0.85mL/min时,除盐率达到94.4%,本研究为工业上降低排出水的矿化度提供了有效的工艺依据。     

Carbon nanotubes produced by fluidized bed catalytic CVD: first approach of the process

Multi-walled carbon nanotubes have been produced with high yield on an iron supported catalyst by catalytic chemical vapor deposition in a fluidized bed reactor. The choice of such a technique allows to reach high selectivity towards the desired material. A remarkable feature of this process is the huge bed expansion observed during the nanotubes growth that affects the fluidization regime due to the evolution of the apparent density of the composite powder. The catalytic powder, the composite material and the purified nanotubes have been analyzed by SEM, TEM and BET nitrogen adsorption.     

Effect of laser intensity on yield and physical characteristics of single wall carbon nanotubes produced by the Nd:YAG laser vaporization method

We report on the effect of the Nd:YAG laser intensity on diameter distribution, yield and physical characteristics of single-wall carbon nanotubes (SWNTs) while comparing three different laser configurations (namely: (i) single 532 nm pulse; (ii) single 1064 nm pulse; and (iii) 532 nm followed by the 1064 nm double pulse). The carbon SWNTs were synthesized at a furnace temperature of 1150 °C and characterized by means of laser micro-Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Regardless of the laser configuration used, it is found that both the yield and the structural characteristics of the SWNTs are highly sensitive to the laser intensity. Indeed, by combining Raman analyses together with HRTEM observations we were able to point out the existence of an optimal laser intensity which leads not only to the highest yield of SWNTs and the largest bundles but also to the lowest level of amorphous and, or disordered sp² carbon in the deposits. While the optimal laser intensity was found to increase from 1.7 to 2.9×10⁹ W/cm² when the laser wavelength is changed from 1064 to 532 nm, the double pulse configuration offered a larger process latitude since high yield of SWNTs was obtained over the (0.8–3.5)×10⁹ W/cm² laser intensity range centered around the optimal value of 2.3×10⁹ W/cm². Moreover, it is shown that the increase of the laser intensity (from 0.5 to 5.6×10⁹ W/cm²) favors the growth of large nanotubes (1.4 nm-diam.) to the detriment of smaller ones (1.1 nm-diam.). A tendency to form larger nanotubes was also observed when increasing the furnace temperature from 1000 to 1150 °C. Finally, the laser intensity effect is interpreted in terms of near-surface or deep laser energy absorption in the graphite target.     

Raman characterisation of single-walled carbon nanotubes produced by the catalytic pyrolysis of methane

Bundles of single-wall carbon nanotubes (SWCNT) were synthesised using a chemical vapour deposition technique. This basic process was optimised over a wide range of process parameters. For the optimal results, ethane was decomposed at 950 °C over a catalyst material consisting of 2% by wt Fe chemically deposited on an MgO support. The samples were characterised using scanning electron microscopy and transmission electron microscopy to show the presence of nanotubes and also to measure their diameters and the size of amorphous carbon deposits. Raman scattering was also used to probe the electronic properties and hence derive the distribution of diameters of the SWCNT. Samples were measured in both the radial breathing mode and tangential mode ranges using three different laser lines. For comparison purposes, similar data have been reported for a standard commercial SWCNT material (HiPCO). From the data, we can conclude that the tubes in our sample are significantly different to those in the HiPCO sample. In particular, we conclude that samples produced by our method contain a much narrower distribution of tube diameters than does the commercial sample.     

Effects of the structural properties of solid fuels on their re-ignition characteristics

A set of small-scale experiments was carried out to study the effects of material structural properties on the re-ignition characteristics of solid fuels. The influence of other key parameters, such as the incident heat flux and pre-burn, was also carefully investigated. The experiments were conducted on specimens of wood and PMMA using a cone calorimeter. As expected, the effect of water on the re-ignition time was found to be significant. It was also found that the re-ignition characteristics of charring materials, such as wood, are quite different from non-charring materials, mainly due to the structural differences. Based on the experimental observations two different mathematical models were developed to analyse the data for both wood and PMMA samples. Calculations of the re-ignition time made using these models agree generally well with the measurements and confirm that the material structure plays a vital role in its re-ignition behaviour. © 1998 John Wiley & Sons, Ltd.     

Low-temperature total oxidation of methane by pore- and vacancy-engineered NiO catalysts

Designing methane combustion catalysts operated under low temperature (<400°C) remains a huge challenge, especially for noble-metal–free catalytic systems. With NaCl as a crystalline scaffold, NiO catalyst with abundant oxygen vacancies and an ultra-high–specific surface area of 181 m² g⁻¹ is obtained. The mesoporous NiO exhibits outstanding CH₄ combustion performance (T₉₀ = 370°C at the weight hourly space velocity (WHSV) = 20,000 mL g⁻¹ h⁻¹). X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (TPR), kinetic measurements, and O¹⁸ isotope-labeling experiments together disclose the key role of surface lattice oxygen and reaction mechanism by NiO catalysts. More importantly, the excellent stability of NiO by doping La was obtained (low-temperature thermal stability: 385°C, 400 h, 4 vol% H₂O).     

Characterisation of carbon blacks produced by solar thermal dissociation of methane

Solar methane dissociation appears as a possible route toward a hydrogen-based economy. The competitiveness of the process strongly depends on the carbon black properties and economic value. At CNRS-PROMES, a 50 kW tubular solar reactor was developed to produce carbon black and hydrogen from methane. The reaction was carried out in a graphite receiver crossed by seven graphite tubes heated up by concentrated solar radiations at the 1 MW CNRS solar furnace. The temperatures ranged between 1608 K and 1928 K and the methane flow-rates varied from 10.5 to 21 NL/min. Total methane dissociation was reached with hydrogen yield higher than 80% and the carbon yield was drastically affected by the acetylene by-product. The carbon samples were analysed in detail and their properties were compared to a commercial conductive grade carbon black. Transmission electron microscopy showed primary particles of 10–70 nm diameter. The crystallinity of the samples was characterised by Raman spectroscopy. It was also possible to correlate the specific surface area with the reaction temperature and with the concentrations of residual methane and of the acetylene by-product. The resistivity and the structure of the agglomerates were determined by simultaneous measurements of the conductivity and the density under compression.     

Preparation and surface functionalization of MWCNTs: study of the composite materials produced by the interaction with an iron phthalocyanine complex

Carbon nanotubes [CNTs] were synthesized by the catalytic vapor decomposition method. Thereafter, they were functionalized in order to incorporate the oxygen groups (OCNT) and subsequently the amine groups (ACNT). All three CNTs (the as-synthesized and functionalized) underwent reaction with an iron organometallic complex (FePcS), iron(III) phthalocyanine-4,4",4",4"-tetrasulfonic acid, in order to study the nature of the interaction between this complex and the CNTs and the potential formation of nanocomposite materials. Transmission electronic microscopy, N2 adsorption at 77 K, thermogravimetric analysis, temperature-programmed desorption, and X-ray photoelectron spectroscopy were the characterization techniques employed to confirm the successful functionalization of CNTs as well as the type of interaction existing with the FePcS. All results obtained led to the same conclusion: There were no specific chemical interactions between CNTs and the fixed FePcS.     

CO2 reforming and partial oxidation of methane

CO₂ reforming and partial oxidation of CH₄ were investigated on different supported noble metal and Ni catalysts. A detailed thermodynamic analysis was performed for both reactions. The observed reaction behaviour can be predicted by thermodynamics. Product selectivity is catalyst independent, the role of the catalyst is to bring the reactants to approach equilibrium. The partial oxidation is a two-stage process, total oxidation of CH₄ is followed by CO₂ and H₂O reforming of the remaining CH₄. A staged addition of O₂ to the reactor is tested and recommended. TPSR show that the catalyst surface for CO₂ reforming was highly covered with carbonaceous species of four different types; two were identified as reactive intermediates.     

Effects of partial acidulation on chemical and mineralogical characteristics of residual phosphate rocks

Original phosphate rocks (PR) and water insoluble residues (WIR) from mixtures of reactive PRs and single superphosphate, known commercially as longlife single superphosphate (LLSSP), and from partially acidulated PRs (PAPR), were compared in terms of their elemental content, chemical reactivity as indicated by the apatite unit cell a dimension and solubility. Phosphate rock reactivity is known to be inversely related to the a dimension. Partial acidulation (20%) with commercial grade phosphoric acid resulted in an increase in aluminium (Al), iron (Fe) and fluoride (F) concentrations in the WIRs. The apatite a dimensions of WIRs from LLSSPs were greater than those of the respective original North Carolina (NC), Khouribga (KR), Jordan (JR), Sechura (SE) and Arad (AR) PRs added to single superphosphate (SSP), made from Nauru PR (NR)) to produce the LLSSPs. This was attributed to the presence of the less reactive NR in the WIRs left-over from the SSP. Partial acidulation with phosphoric acid increased the apatite a dimensions of NC and ElHassa (EH) PRs. The increase in apatite a dimension of NC and EH was probably due to selective dissolution of a more reactive fraction of the PRs during partial acidulation. Changes in the apatite a dimension following partial acidulation with phosphoric acid were not significant for the other PRs studied, e.g. Gafsa (GF), KR and AR, although differential X-ray diffractograms (DXRD) indicated that the material dissolved during partial acidulation was more reactive than the WIRs and the original PRs. The apatite a dimension of NC PR was not affected by pretreatment with 2% or 4% citric acid (CTA). The contrasting response in a of NC PR to acidulation with phosphoric and citric acids may be related to differences in the strength of these acids, and/or to the differing environments under which the reactions took place.The 2% CTA and formic acid (FMA) solubilities of the WIRs from LLSSPs and PAPRs were markedly lower than those of the original PRs. This reduction in solubility of PRs following partial acidulation was probably related to changes in mineralogical and chemical composition of the WIRs as indicated by the increases in apatite a dimension of some residual PRs and shifts in peak positions in DXRD, to increases in the concentrations of Fe, Al and F compounds, and to coating effects of PR particles by Fe, Al and F compounds. This, in turn, may reduce the agronomic value of the residual PR component of PAPR and LLSSP fertilizers, particularly over the short-term.The solubility of residual PRs following pretreatment with 2% or 4% CTA was slightly lower than that of the original PRs. The pretreatment caused no significant change in the apatite a dimension of NC PR. The complexing effects of CTA and its lack of Fe and Al impurities may have prevented the formation of Fe, Al and F compounds. The effect of citric acid on PR reactivity is thus quite different from that of the mineral acids used to prepare LLSSPs and PAPRs.     

Effects of catalyst pre-treatment on the growth of single-walled carbon nanotubes by microwave CVD

Layers of carbon nanotubes were deposited by microwave CVD on oxidized silicon substrates coated with Al-Fe-Mo catalyst films. To achieve a tube growth at about 973 K, the ion bombardment of the catalyst surface has to be avoided. The appropriate pre-treatment of the substrates is essential for the deposition of single-walled carbon nanotubes. Annealing in air is preferable to the frequently used reducing pre-treatment prior to the deposition as a higher area density of the tubes and a better reproducibility of deposition can be obtained. To figure out this finding, selected samples were investigated by analytical transmission electron microscopy and Raman spectroscopy. It is shown that the pre-treatment has a strong effect on the size and distribution of the catalyst particles.     

Infrared chemiluminescence study of CO produced by partial oxidation of butane on platinum

The selective formation of CO and H₂ was observed by a molecular-beam catalytic reaction betweenn-C₄H₁₀ and O₂ on a Pt surface from around 1000 to 1500 K. The infrared emission of the product CO desorbed from the surface showed that the CO molecules are vibrationally substantially excited but rotationally very cool (rotational temperature;T _R = 360 K). The present molecular-beam study showed that CO and H₂ were formed directly from the hydrocarbon and O₂ without involving formation of CO₂ and H₂O as primary products. The implications of these results are discussed for the partial oxidation of methane (and other alkanes) to synthesis gas using practical supported metal catalysts.     

NO x -catalyzed partial oxidation of methane and ethane to formaldehyde by dioxygen

At 600 °C, NO_x catalyzes the partial oxidation of both methane and ethane by dioxygen to form formaldehyde. The yield of oxygenates from methane is over 11. The yield increases to over 16 when 0.7% of ethane is added to the gas mixture. The yield of oxygenates from ethane is over 24. A catalytic cycle involving NO₂ as the C–H activating species is proposed.