Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

The performance of CNT as catalyst support on CO oxidation at low temperature

A carbon nanotube (CNT) was used as catalyst support impregnated with transition metal cobalt for CO oxidation at low temperature. Catalyst properties were analyzed by X-ray powder diffractometer (XRD), X-ray photoelectron spectrometer (XPS), and transmission electron microscope (TEM). Analytical results for TEM and XRD demonstrated that cobalt particles were highly dispersed on the carbon nanotube (20–30 nm) with nanosized cobalt particles (10–15 nm). These investigations indicated that Co/CNT generates about 99% of the high activity for CO conversion at 250 °C and thermally stability that is superior to Co/activated carbon (AC). The optimum reaction conditions for CO conversion were O₂ concentration 3%, operation temperature 250 °C, CO concentration 5000 ppm, and space velocity 156,000 h⁻¹. At 250 °C, CO may act as a reductant for NO reduction over Co/CNT in the presence of oxygen, whereas CO/NO = 2.5 showed that maximum NO reduction was 30%. Under H₂ rich conditions, the optimum reaction temperature for CO conversion was under 300 °C, and performance of CO₂ selectivity was better at 200 °C than 250 °C as the oxygen concentration increased.     

The deactivation of Co/SiO2 catalyst for Fischer–Tropsch synthesis at different ratios of H2 to CO

The deactivation of Co/SiO₂ catalyst for Fischer–Tropsch synthesis (FTS) at different H₂ / CO ratios was investigated by XRD, FTIR, BET, XPS, TPR and H₂ chemisorption. It was found that the deactivation rate of the catalyst increased with the rise of the H₂ / CO ratio. The generation of silicates and/or hydrosilicates species was evidenced by TPR and XPS, and their amounts were monotonously enhanced with increasing H₂ / CO ratio, which suggested that the deactivation was caused by the transformation of metallic cobalt into inactive silicates and the high partial pressure of H₂ facilitated the formation of the silicates. Moreover, the percentage loss of the surface cobalt was larger than that of bulk cobalt, suggesting that the cobalt silicates and/or hydrosilicates species were formed mainly on the surface of the catalyst or in the small crystallites. For the catalyst run at H₂ / CO ratio of 1, it was observed that the sintering also contributed to the catalyst deactivation, but it was a less important factor for the deactivation.     

Growth and surface engineering of vertically-aligned low-wall-number carbon nanotubes

A novel catalyst of cobalt supported by single crystal MgO was prepared by atomic layer deposition and used for carbon nanotube growth. With CO as carbon source, vertically-aligned carbon nanotubes with predominant double-walled (82%) were produced at 700 °C. Similar carbon nanotube array with a majority of single-walled tubes (62%) was produced at 900 °C using methane as the carbon source. Due to their high flexibility compared with multi-walled carbon nanotubes, the low-wall-number carbon nanotube array could form a 3-dimensional honeycomb-like network when being spread with acetone.     

Low temperature growth of carbon nanotubes on Si substrates in high vacuum

Carbon nanotube (CNT) growth was carried out on SiO₂/Si substrates using an alcohol gas source in a high vacuum without any carbon decomposition processes. In the Raman spectra of the grown CNTs, both the G/Si peak intensity ratio and G/D peak intensity ratio indicated that the optimum growth temperature became lower as the pressure decreased. By reducing the pressure to 1 × 10^− 4 Pa, CNTs could be grown at 400 °C, and the G/D ratio was about 16, indicating that the quality of the grown CNTs was good, taking into account the low growth pressure. In addition, the Raman spectra in the radial breathing mode (RBM) region showed that the diameter distribution of the grown CNTs was dependent on both the growth pressure and temperature, and the relative intensity of the RBM peaks from small-diameter CNTs increased as the growth pressure and/or temperature was reduced.     

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H₂/NG and Ar/NG atmospheres at 950 °C. The Cr_2 − xFe_xO₃ catalyst was capable to produce carbon nanotubes only in H₂/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO₃.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

CO removal from reformed fuel over Cu/ZnO/Al2O3 catalysts prepared by impregnation and coprecipitation methods

A composition of Cu/ZnO/Al₂O₃ catalysts prepared by the impregnation method was optimized for water gas shift reaction (WGSR) coupled with CO oxidation in the reformed gas. The optimum composition of the impregnated catalyst for high WGSR activity was 5 wt.% Cu/5 wt.% ZnO/Al₂O₃. The optimum loading amounts of Cu and ZnO in the impregnated catalyst were smaller than those in the coprecipitated catalyst. Its catalytic activity above 200 °C was comparable to that of the conventional coprecipitated Cu/ZnO/Al₂O₃ catalyst. However, the activity of the impregnated Cu/ZnO/Al₂O₃ catalysts was significantly lowered at 150 °C, whereas no deactivation was observed for the coprecipitated catalyst at the same temperature. It was found that deactivation occurred over impregnated catalysts with H₂O and/or O₂ in the reaction gas; it prevented CO adsorption on the surface.     

Hydrodechlorination of tetrachloroethene over Pd/Al2O3: influence of process conditions on catalyst performance and stability

The influence of process parameters (temperature, pressure, hydrogen flow rate, and nature of solvent) on both activity and stability of a 0.5% Pd on alumina catalyst used for tetrachloroethene (TTCE) hydrodechlorination in an organic matrix was studied. In the range of temperature studied (250–350 °C), higher temperatures lead to higher initial activity but faster deactivation. Increasing hydrogen flow rates, up to 0.8 L/min (STP), produce higher activity and stability of the catalyst, whereas pressure in the range 0.5–2 MPa has no significant effect. In all the cases, both hydrodechlorination and hydrogenation of the double bond take place, yielding ethane as the main product. Concerning to the solvent, there is no difference in the initial catalytic activity for either toluene or n-decane, but n-decane leads to faster catalyst deactivation.The effect of temperature and space time in TTCE conversion at the period of constant catalytic activity can be modelled by a kinetic model assuming first order for TTCE and zero-order for H₂.Finally, the performance of the Pd alumina-supported catalyst is compared with that of a Pd carbon-supported catalyst with the same metal load, used in previous works. Although the carbon-supported catalyst yields higher initial conversion, the alumina-supported catalyst is more resistant to deactivation.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Activity and deactivation of Au/Al2O3 catalyst for low-temperature CO oxidation

Au/Al₂O₃ catalyst was investigated with respect to its activity for low-temperature CO oxidation. The activity changes of the catalyst were examined after separate treatment in the following different atmosphere: (i) O₂ + N₂ + CO; (ii) O₂ + N₂ heated above 100 °C and (iii) O₂ + N₂ + H₂O vapor. The results show that each of the treatments above may deactivate the catalyst to the different degree. The deactivation by CO oxidation is mainly due to the accumulation of carbonate-like species on the catalyst surface. The addition of H₂O vapor may inhibit the deactivation effectively. The removal of hydroxyl groups at active sites during heating may be responsible for the deactivation by thermal treatment. These two kinds of deactivations are reversible. The irreversible deactivation by H₂O vapor treatment is mainly caused by the growth of gold particles size.     

Catalytic performance of Ag–Co/CeO2 catalyst in NO–CO and NO–CO–O2 system

A new bimetallic catalyst (Ag–Co/CeO₂) was studied for simultaneously catalytic removal of NO and CO in the absence or presence of O₂. CeO₂ prepared by homogeneous precipitation method was optimized as supports for the active components. The addition of Ag on CeO₂ greatly improved the catalytic activities in the lower temperature regions (⩽300 °C), and the introduction of Co on CeO₂ increased the activities at higher temperatures (⩾250 °C). The bimetallic Ag–Co/CeO₂ catalyst combined the advantages of the corresponding individual metal supported catalysts and showed superior activity due to the synergetic effect. The effect of support, temperature, loading amount, GHSV and oxygen on catalysis was investigated. NO and CO could be completely removed in the temperature range of 200–600 °C at a very high space velocity of 120 000 h⁻¹. No deactivation was observed over 4% Ag–0.4% Co/CeO₂ catalyst even after 50 h test.     

Trace precious metal Pt doped plate-type anodic alumina Ni catalysts for methane reforming reaction

A novel plate-type anodic alumina supported 17.9 wt% Ni/Al₂O₃/alloy showed a quick deactivation in daily start-up and shut down (DSS) steam reforming of methane (SRM) at 700 °C, because of the Ni oxidation reaction with steam. When 0.078 wt% Pt was doped, the catalyst exhibited self-activation and self-regeneration ability, while 3000 h continual and 500-time DSS stability was testified. Further, this Pt–Ni catalyst also showed excellent reactivity during carbon dioxide reforming of methane (CMR) and partial oxidation of methane reaction (POM). According to the TPR and XRD analyses, the H₂ spillover effect and the formation of Pt–Ni alloy were believed to be the main reason for the reactivity improvement of this catalyst.     

SO2 retention on CaO/activated carbon sorbents. Part III. Study of the retention and regeneration conditions

The retention of SO₂ on CaO/activated carbon sorbents is studied. The effect of several variables such as the reaction temperature, partial pressure of SO₂ for different calcium loads, and O₂ presence are analysed. Additionally, the regeneration and reutilization of spent sorbents is investigated. In all cases presence of well-dispersed CaO in the sorbents improves SO₂ retention in comparison with the activated carbon. In absence of O₂ in the gas mixture, the amount of SO₂ retained does not depend on the SO₂ partial pressure in the range of partial pressures studied and, as expected, SO₂ physisorption on the activated carbon support occurs at room temperature. SO₂ retention occurs in surface CaO between 100 °C and 250 °C, and in bulk CaO above 300 °C. The total calcium conversion is reached at 500 °C. Above 550 °C calcium-catalysed carbon gasification by SO₂ occurs. In presence of O₂ in the gas mixture, the studied sorbents are very effective for SO₂ removal. However, the SO₂ retention process in presence of oxygen must be carried out at temperatures lower than 300 °C to avoid carbon gasification by O₂. The thermal regeneration of the spent sorbents can be done under inert atmosphere (880 °C) with only 20% activity loss after the first regeneration cycle due to sintering and formation of CaS. No additional activity loss is detected in the subsequent cycles.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Self-assembled one-dimensional carbon nitride architectures

Three kinds of assembled one-dimensional carbon nitride architectures were realized in large scale by a simple solvothermal technique. Carbon nitride nanotube bundles were formed involving the reaction of cyanuric chloride (C₃N₃Cl₃) with sodium at 230 °C and 1.8 MPa in a stainless steel autoclave using NiCl₂ as a catalyst precursor. Without any catalyst, aligned nanoribbons were formed at 290 °C and 3 MPa, and microspheres consisting of hundreds of nanoribbons were formed at 260 °C and 3.5–4.5 MPa. The electron energy-loss spectroscopy (EELS) proved all these 1-D carbon nitride nanostructures are consistent with the stoichiometry of CN. The similarity and difference of their microstructures and optical properties were researched by FTIR, Raman, PL measurements and UV–vis absorption spectra. These assembled CN architectures with well-controlled dimensionality and luminescent property may have potential uses as component of optical nanoscale devices. Their formation mechanisms were briefly discussed.     

Carbon oxidation characteristics of yttrium manganate catalyst prepared via urea decomposition

YMnO₃ is a hexagonal crystal characterized by high carbon oxidation activity. In this study, carbon black powder has been directly oxidized at temperatures as low as 250 °C with the active oxygen species generated by YMnO₃ catalyst. The activation energies measured for the non-catalyzed and YMnO₃-catalyzed carbon oxidation reactions were 160 kJ mol⁻¹ and 131 kJ mol⁻¹, respectively. During combustion testing of particulate matter in a ceramic form coated with YMnO₃, the captured soot was continuously purified at a temperature of 350 °C.     

Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose

Gasification of glucose in near- and supercritical water was investigated at temperature and pressure ranges from 400 to 600 °C and 20 to 42.5 MPa with a reaction time of 1 h. Hydrothermal gasification of glucose was performed in the absence and presence of catalyst (K₂CO₃) in a batch reactor. The influences of temperature and pressure in the supercritical regimes of water, catalyst were examined in relation to the yield and composition of the gases and aqueous products. The product gases were analyzed by gas chromatography, and the aqueous products were analyzed by high performance liquid chromatography. The gases produced were carbon dioxide, methane, hydrogen, carbon monoxide, and C₂–C₄ hydrocarbons and there was significant production of aqueous products and residue. The aqueous products composed of oxygenated compounds, including carboxylic acids (glycolic acid, formic acid, acetic acid), furfurals (furfural, 5-hydroxymethyl furfural, 5-methyl furfural), phenols (phenol, methyl phenols, hydroxy phenols, methoxy phenols), aldehydes (formaldehyde, acetaldehyde, acetone, propionaldehyde), ketones (3-methyl-2-cyclo-pentene-1-one, 2-cyclo-pentene-1-one) and their alkylated derivatives. Carbon gasification efficiencies were improved by addition of K₂CO₃ into the reacting system. Carbon gasification efficiency reached maximum (94%) at 600 °C and 20 MPa. The yield of hydrogen among gaseous products increased with increasing temperature and decreasing pressure.     

Solubility and diffusion coefficient of supercritical-CO2 in polycarbonate and CO2 induced crystallization of polycarbonate

The solubility and diffusion coefficient of supercritical CO₂ in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO₂ decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO₂. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − T_g ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.     

Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.