High growth of SWNTs and MWNTs from C2H2 decomposition over Co-Mo/MgO catalysts

A series of MgO supported catalysts having Co and Mo metals 5-40 wt.% in a ratio of 1:1 was prepared by impregnation method. Carbon nanotubes (CNTs) were grown over the catalysts by decomposition of C₂H₂ at 800 °C for 30 min. It was found that 5 and 10 wt.% Co-Mo/MgO catalysts produced single-wall nanotubes (SWNTs), whereas 20, 30 and 40 wt.% Co-Mo/MgO catalysts produced multi-wall nanotubes (MWNTs). The catalyst Mo/MgO was inactive in growing CNTs. In Co-Mo/MgO catalysts, however Mo generated a favorable environment to grow SWNTs. The growth of SWNTs was strongly dependent on the formation of small clusters of cobalt, which may generate from the decomposition of CoMoO₄ species during the nanotube growth. MWNTs were produced over comparatively larger cobalt clusters generated from Co₃O₄ phase during the nanotube growth stage. The yields of SWNTs were about 6% and 27% over 5 and 10 wt.% Co-Mo/MgO catalysts, respectively. MWNTs yield (576%) was observed over 40 wt.% Co-Mo/MgO catalyst. Carbon yield (%) highly varied with acetylene concentration.     

Synthesis of single-wall carbon nanotubes and long nanotube ribbons with Ho/Ni as catalyst by arc discharge

The effect of a new bimetallic catalyst Ho/Ni for synthesis of single-walled carbon nanotubes (SWNTs) by arc discharge has been studied. Long ribbons consisting of roughly-aligned SWNT bundles were obtained by a modified arc discharge apparatus. Ribbon lengths can reach as much as 20 cm. Both elements Ho and Ni play important roles in the synthesis of SWNTs with high yield and purity. Changes in the Ho and Ni concentration in the catalyst hardly affect the diameter distribution of SWNTs, but the yield and purity of SWNTs are very sensitive to the concentration. An optimal range of Ho/Ni compositions for synthesis of SWNTs with relatively high purity and yield is given.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.     

Synthesis of multiwalled carbon nanotubes supported on M/MCM-41 (M = Ni,Co and Fe) mesoporous catalyst by chemical vapour deposition method

Aim of this research is to develop an effective way in producing multi-walled carbon nanotubes (MWNTs) via chemical vapour deposition method using acetylene as carbon source and synthesized mesoporous M/MCM-41 (M?=?Ni, Co and Fe) as catalyst. The mesoporous MCM41 is synthesized by using sodium metasilicate as silica source of frameworks and cetyltrimethylammonium bromide as template. The effect of temperatures and growth times are investigated to produce MWNTs with high yield and high quality. The low-angle X-ray diffraction (XRD), Fourier transform infrared spectroscopy and scanning electron microscopy results verified the formation of MCM41. Meanwhile, high-angle XRD, Raman spectra and transmission electron microscopy results confirmed the synthesized carbon nanotubes at 600?°C and growth time for 30 min are multi-walled. The yield obtained is 1353 mg in 30 min. It shows that the MCM-41 has a promising potential to produce MWNTs at low cost and shorter duration.     

Catalytic growth of semiconductor micro- and nano-crystals using transition metal catalysts

The catalytic reaction concept was introduced in the growth of semiconductor micro- and nano-crystals. It was found that gallium nitride (GaN) micro- and nano-crystal structures, carbon nanaotubes, and silicon carbide (SiC) nanostructures could be efficiently grown using transition metal catalysts. The use of Ni catalyst enhanced the growth rate and crystallinity of GaN micro-crystals. At 1,100 ‡C, the growth rate of GaN micro-crystals grown in the presence of Ni catalyst was over nine times higher than that in the absence of the catalyst. The crystal quality of the GaN microcrystals was almost comparable to that of bulk GaN. Good quality GaN nanowires was also grown over Ni catalyst loaded on Si wafer. The nanowires had 6H hexagonal structure and their diameter was in the range of 30–50 nm. Multiwall nanotubes (MWNTs) were grown over 20Fe : 20Ni : 60Al₂O₃ catalyst. However, single wall nanotubes (SWNTs) were grown over 15Co : 15Mo : 70MgO catalyst. This result showed that the structure of CNTs could be controlled by the selection of catalysts. The average diameters of MWNTs and SWNTs were 20 and 10 nm, respectively. SiC nanorod crystals were prepared by the reaction of catalytically grown CNTs with tetrametysilane. Structural and optical properties of the catalytically grown semiconductor micro- and nano-crystals were characterized using various analytic techniques. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Multi-step purification of carbon nanotubes

An efficient purification procedure for multi-walled carbon nanotubes (MWNTs) synthesized by the floating catalyst method was discussed. The process involves ultra-sonication, heat treatment in hot water, bromination, oxidation and acid treatment. Most of amorphous carbon, multishell carbon nanocapsules as well as metal particles were successfully removed from the MWNT product. With this procedure, MWNTs with purity of more than 94% were obtained and the yield could approach 50%. It was found that bromination took an important role in the purification of MWNTs. Transmission electron microscopy, XPS and thermo-gravimetric analysis were used to evaluate the purification process of MWNTs. The mechanism of bromination on purification of the MWNTs was also discussed.     

Nanotubes and nanofilaments from carbon monoxide disproportionation over Co/MgO catalysts: I. Growth versus catalyst state

A catalyst prepared from the pyrolysis of Co and Mg nitrates and citric acid after their co-dissolution in water was used for carbon deposition from CO. Good yields of nanotubes or nanofilaments were obtained over catalysts which had been reduced by H₂ without preliminary treatment at high temperature. Nanotubes with 10 or more cylindrical carbon layers were obtained from pure CO or from CO+CO₂ mixtures. Nanofilaments with truncated conical layers were obtained from CO+H₂ mixtures in the 500–600 °C range. In both cases, high shape selectivity was obtained and almost all MgO could be eliminated by HCl treatment. The only significant impurities were embedded cobalt particles. This process is therefore suitable for preparing nanotubes or nanofilaments with good shape selectivity and 98 wt% purity. Lowering the Co content of the catalyst produces thinner nanotubes but reduces the yield.     

Ultraviolet irradiated ozone treatment of a metal catalyst for the large-scale synthesis of single-walled carbon nanotubes with small, uniform diameters

Large-scale synthesis of single-walled carbon nanotubes with small diameters and narrow distribution was performed using catalytic decomposition of C₂H₂ at 800 °C by introducing an ultraviolet irradiated ozone (UV-ozone) treatment on an as-prepared Fe-Mo/MgO catalyst (APC). The UV-ozone treatment effectively suppressed metal migration and the agglomeration of the Fe-Mo catalyst on the MgO support material at high temperature (800 °C). During UV-ozone treatment, active oxygen species were adsorbed onto the APC and generated hydroxyl groups. The hydroxyl groups prevented the formation of large catalytic metal nanoparticles at 800 °C by acting as a surfactant. We also investigated whether the Mo species prevented the metal sintering of iron species into the MgO lattice.     

Hydrogen adsorption/desorption behavior of multi-walled carbon nanotubes with different diameters

Multi-walled carbon nanotubes (MWNTs) with different mean outer diameters in the range of 13-53 nm, synthesized by the catalytic decomposition of hydrocarbons using a floating catalyst method, were purified and pretreated with the same procedure for volumetric hydrogen adsorption/desorption measurements. It was found that the hydrogen storage capacity of the purified and pretreated MWNTs was proportional to their diameter, and that hydrogen in all types of MWNTs measured could not be completely desorbed at room temperature and ambient pressure. A possible mechanism for the above behavior was proposed based on the results of cryogenic nitrogen adsorption analysis and high-resolution transmission electron microscopy observations. It was considered that small “carbon islands” might be the main hydrogen adsorption site in MWNTs. The effects of metal catalyst as well as an etched cavity on the surface of MWNTs on the hydrogen adsorption/desorption of MWNTs were also discussed.     

MgO-supported Mo, CoMo and NiMo sulfide hydrotreating catalysts

The most common preparation of high surface area MgO (100–500 m² g⁻¹) is calcination of Mg(OH)₂ obtained either by precipitation or MgO hydration or sol–gel method. Preparation of MoO₃/MgO catalyst is complicated by the high reactivity of MgO to H₂O and MoO₃. During conventional aqueous impregnation, MgO is transformed to Mg(OH)₂, and well soluble MgMoO₄ is easily formed. Alternative methods, that do not impair the starting MgO so strongly, are non-aqueous slurry impregnation and thermal spreading of MoO₃. Mo species of MoO₃/MgO catalyst are dissolved as MgMoO₄ during deposition of Co(Ni) by conventional aqueous impregnation. This can be avoided by using non-aqueous impregnation. Co(Ni)Mo/MgO catalysts must be calcined only at low temperature because Co(Ni)O and MgO easily form a solid solution. Literature data on hydrodesulfurization (HDS) activity of MgO-supported catalysts are often contradictory and do not reproduced well. However, some results suggest that very highly active HDS sites can be obtained using this support. Co(Ni)Mo/MgO catalysts prepared by non-aqueous impregnation and calcined at low temperature exhibited strong synergism in HDS activity. Co(Ni)Mo/MgO catalysts are much less deactivated by coking than their Al₂O₃-supported counterparts. Hydrodenitrogenation (HDN) activity of Mo/MgO catalyst is similar to the activity of Mo/Al₂O₃. However, the promotion effect of Co(Ni) in HDN on Co(Ni)Mo/MgO is lower than that on Co(Ni)Mo/Al₂O₃.     

Thin film metallic catalyst coatings for the growth of multiwalled carbon nanotubes by pyrolysis of xylene

Thin film metallic coatings applied to alumina and silicon substrates are investigated for their use as a catalyst to help grow high-quality multiwall carbon nanotubes (MWNTs). Substrate coatings examined include Fe, Tb:Fe, Ni, Cu, and Ni:Fe, with xylene used as the hydrocarbon source. Coating the substrate with Tb₉₀Fe₁₀ and Ni₈₀Fe₂₀ facilitated dense and uniform growth of MWNTs without graphitic particles; Ni and Fe substrate coatings produced graphitic particles in addition to the MWNTs, while Tb and Cu were found to be completely inactive with no MWNT growth. Many of the MWNTs grown over Ni:Fe have a helical appearance, while the MWNTs grown over Tb₉₀Fe₁₀ did not contain catalyst particles     

Ni-deposited multi-walled carbon nanotubes by electrodeposition

Multi-walled carbon nanotubes (MWNTs), having a diameter around 100-200 nm, were used as host material for electrodeposition of Ni using a Ni plating bath containing homogeneously dispersed MWNTs. Ni-deposited or coated MWNTs with a skewered dumpling shape accumulated on the Cu cathode electrode. A model representing the growth process of these electrodeposits, which possess such a skewered dumpling shape, is presented. These electrodeposits were separated easily from the cathode electrode by ultrasonic irradiation in an acetone bath to yield a Ni-deposited or coated MWNTs powder. The amount of Ni deposited on the MWNTs can be controlled by selecting the appropriate electroplating conditions.     

Co—Mo／MgO作催化剂CVD法制备碳纳米管

研究了催化剂的制备方法对合成碳纳米管的影响,分别采用溶胶凝胶法、浸渍法和燃烧法制备了Co-Mo／MgO催化剂,并以乙炔为碳源,Ar气为保护气,在750-950℃常压下生长碳纳米管。采用TEM对所得产物进行了表征,结果表明,对于Co-Mo／MgO体系,相对浸渍法和燃烧法,溶胶凝胶法是很好的选择,可以得到数量与质量均较好的碳纳米管,并讨论了溶胶凝胶法制备碳纳米管的过程中工艺参数的择优。     

Preparation of short carbon nanotubes by mechanical ball milling and their hydrogen adsorption behavior

Short multi-wall carbon nanotubes (MWNTs) with open tips were obtained by mechanical ball milling. The microstructure characteristics of MWNTs before and after ball milling were checked by transmission electron microscopy (TEM). The effect of ball milling on the hydrogen adsorption behavior of the MWNTs was studied. The hydrogen adsorption experiments were carried out at room temperature under a pressure of 8-9 MPa. The hydrogen adsorption capacity of carbon nanotubes milled for 10 h was 0.66 wt%, which was about six times that of MWNTs without milling. For the carbon nanotubes milled with MgO for 1 h, a hydrogen adsorption capacity of 0.69 wt% was obtained. The enhancement of hydrogen adsorption might result from the increase of defects and surface area of the MWNTs caused by ball milling.     

The effect of phase separation in Fe/Mg/Al/O catalysts on the synthesis of DWCNTs from methane

Double-walled carbon nanotubes (DWCNTs) were prepared by methane decomposition on Fe/Al/Mg/O catalysts with a fixed iron loading of 1.5%. Increasing Al/Mg ratio in the catalyst resulted in the formation of a new MgAl₂O₄ phase, which was characterized by XRD. The size of the MgO crystallites in the support was decreased, due to the phase separation, from 35 nm to 20 nm in the Al/Mg ratio range of 0:1-4:1. At an Al/Mg ratio of 1:200, this effect prevented the sintering of iron on the MgO support and resulted in the synthesis of high-purity DWCNTs in high-yield. Very high-Al/Mg ratio induced the formation of the MgAl₂O₄ phase, which became another catalyst support material. This had a negative effect on the synthesis of DWCNTs due to its acidity and hardness. Simultaneously maintaining MgO as the dominant catalyst support and decreasing its particle size by the phase separation effect are important for good metal dispersion and, consequently, the yield and purity of DWCNTs.     

Platelet-like catalyst design for high yield production of multi-walled carbon nanotubes by catalytic chemical vapor deposition

We investigated the effect of catalyst design on the synthesis of multi-walled carbon nanotubes (MWCNTs) by chemical vapor deposition (CVD). A set of highly active supported sol–gel Co–Mo/MgO and Ni–Mo/MgO catalysts was prepared systematically modifying the calcination temperature. First, the evolution of catalysts’ crystallographic phases and their morphology were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning electron (SEM) and transmission electron (TEM) microscopy. Second, the catalysts were used for the CVD growth of MWCNTs. The resulting materials were analysed by SEM and TEM, Raman and XRD to establish a relation between catalyst design and MWCNT yield. We show that our catalyst synthesis route leads to the formation of laminar non-porous catalyst systems, which at a calcination temperature of 800 °C stabilize in a crystallographic phase of Me_xMg_1−xMoO₄ (Me = Co or Ni). We give evidence that increased MWCNT yields of more than 3000 wt.% with respect to the catalysts are directly related to the aforementioned crystallographic phase. Finally, we propose a growth model based on the continuous exfoliation of platelet-like catalyst systems. This consistently explains the high catalytic activity towards MWCNT production using a non-porous catalyst. Our findings provide important insights for catalyst design strategies towards large-scale MWCNT production.     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

Novel hierarchical Ni/MgO catalyst for highly efficient CO methanation in a fluidized bed reactor

A facile synthesis of the hierarchical Ni/MgO catalyst is reported, with extremely fine dispersion of Ni nanoparticles (NPs) and high surface oxygen mobility. The hierarchical Ni/MgO catalyst exhibits higher activity for CH₄ formation than that prepared by the impregnation method. The enhanced activity and thermal stability of the hierarchical Ni/MgO catalyst is attributed to hierarchical MgO particles with a multilayer structure and high surface oxygen mobility. This induces better metal‐support interactions, high Ni dispersion to prevent Ni NPs sintering, and the high surface oxygen mobility provides a high resistance to carbon deposition. Compared to the impregnated Ni/MgO catalyst, the hierarchical Ni/MgO catalyst exhibits a better fluidization quality and a higher attrition‐resistance in a fluidized‐bed reactor. This approach to improve the catalytic activity by creation of hierarchical Ni/MgO particles is encouraging for the design of novel catalysts for synthetic natural gas production, especially from the perspective of matching catalysts with fluidized‐bed reactors. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2141–2152, 2017     

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I_G/I_D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Co-synthesis, purification and characterization of single- and multi-walled carbon nanotubes using the electric arc method

We report the simultaneous synthesis of single-walled (SWNT) and multi-walled (MWNT) carbon nanotubes using the electric arc method. Two distinct deposits are formed when an electric arc is struck between a Co/Ni impregnated carbon electrode (anode) and a graphite electrode (cathode). The cobweb-like deposit harvested from the walls of the growth chamber (chamber deposit) contained SWNT bundles while the MWNTs were present in the deposit formed at the tip of the cathode (cathode deposit). Energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and Raman spectroscopy of each deposit confirmed the presence of respective carbon nanotubes along with carbonaceous impurities. A systematic purification procedure for separating SWNT bundles from the rest of the impurities is discussed. The optimal yield of purified SWNT bundles is ∼25 wt.% of the as-prepared chamber deposit.