Fischer–Tropsch synthesis over cobalt dispersed on carbon nanotubes-based supports and activated carbon

Carbon nanotubes (CNTs) and the ones grown on MgO and alumina are used as supports for cobalt catalyst in Fischer–Tropsch (FT) synthesis. Carbon nanotubes were synthesized by chemical vapor deposition of methane on 5.0 wt.% iron on MgO or alumina at 950 °C. The carbon nanotubes were characterized by SEM and TEM microscopy and Raman spectroscopy. Cobalt nitrate was impregnated onto the supports by impregnation, and the samples were dried and reduced in-situ at 400 °C for 12 h, and then FT synthesis was carried out in a fixed-bed reactor. The catalysts were characterized by BET surface area measurement, TPR and TPD. The effect of carbon nanotubes as cobalt support on CO conversion, product selectivity, and olefin to paraffin ratio of FT synthesis was investigated and compared with activated carbon as well as Al₂O₃, as a traditional support. The results revealed that the activity of the Co/CNT catalyst was improved by 22%, compared to the conventional Co/alumina catalysts. Also the cobalt supported on CNTs grown on MgO (Co/CNT–MgO) shows the highest selectivity to C₅₊ as the most desired FTS products. The C₅₊ selectivity enhancement was about 37, 34, 17, and 77% as compared to the Co/CNT, Co/alumina, Co/CNTs-alumina, and Co/activated carbon, respectively. Also the olefin/paraffin ratio on the Co/CNTs-MgO catalyst is about 7.7 times higher than the conventional cobalt catalysts. It seems that the degree of reduction of cobalt is higher when supported on CNTs than on alumina. This leads to higher FTS activity. Also, the particle size distribution of the cobalt is affected by the CNT–MgO support leading to higher C₅₊ selectivity.     

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.     

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.     

Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst

A new catalyst (Ni/Mo/MgO) is reported, with which one can synthesize multi-walled carbon nanotube (MWNT) bundles with a yield of more than 45 times the amount of the pristine catalyst, using a methane-hydrogen mixture as precursor. Powder X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis show that the purity of the as-prepared MWNTs is over 97%. The diameter of the carbon nanotubes is 9-20 nm, measured by high-resolution electron microscopy on 421 individual MWNTs. The high purity of the as-prepared MWNTs allows us to omit the usual complex purification process required for carbon nanotubes synthesized by chemical vapor deposition. Because of its durable high activity, the Ni/Mo/MgO catalyst in its pristine state is ideal for mass production of high-quality MWNTs. The synergism of nickel and molybdenum is considered the main reason for the high yield of carbon nanotubes.     

Synthesis of carbon nanostructures by arc evaporation of graphite rods with Co-Ni and YNi2 catalysts

Yields of single-walled carbon nanotubes (SWNTs) produced from electric arc evaporation of graphite electrodes with 3Co/Ni and YNi₂ catalysts differ substantially. For instance, with YNi₂ catalyst, the SWNT yield is ∼30-50 wt.% in ‘collar’ soot and ∼10-15 wt.% in ‘wall’ soot, while with 3Co/Ni catalyst, the yields are ∼15-20 wt.% and 2-3 wt.%, respectively. According to Raman spectroscopy data, the average dimension of SWNTs is ∼1.2 nm for 3Co/Ni and ∼1.4 nm for YNi₂ catalyst. Optimum conditions for synthesis also differ for catalysts compared; namely, for 3Co/Ni: current intensity is 93 A, helium pressure is 650 Torr, the electrode gap is 2.5-3 mm; for YNi₂: current intensity is 98 A, helium pressure is 500 Torr, the electrode gap is 1-2 mm.     

Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation

We discuss growth of high-quality carbon nanotube (CNT) films on bare and microstructured silicon substrates by atmospheric pressure thermal chemical vapor deposition (CVD), from a Mo/Fe/Al₂O₃ catalyst film deposited by entirely electron beam evaporation. High-density films having a tangled morphology and a Raman G/D ratio of at least 20 are grown over a temperature range of 750-900 °C. H₂ is necessary for CNT growth from this catalyst in a CH₄ environment, and at 875 °C the highest yield is obtained from a mixture of 10%/90% H₂/CH₄. We demonstrate for the first time that physical deposition of the catalyst film enables growth of uniform and conformal CNT films on a variety of silicon microstructures, including vertical sidewalls fabricated by reactive ion etching and angled surfaces fabricated by anisotropic wet etching. Our results confirm that adding Mo to Fe promotes high-yield SWNT growth in H₂/CH₄; however, Mo/Fe/Al₂O₃ gives poor-quality multi-walled CNTs (MWNTs) in H₂/C₂H₄. An exceptional yield of vertically-aligned MWNTs grows from only Fe/Al₂O₃ in H₂/C₂H₄. These results emphasize the synergy between the catalyst and gas activity in determining the morphology, yield, and quality of CNTs grown by CVD, and enable direct growth of CNT films in micromachined systems for a variety of applications.     

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。     

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron-nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al₂O₃, 40Ni:60Al₂O₃ and 20Fe:20Ni:60Al₂O₃ catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al₂O₃ catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al₂O₃ and 40Ni:60Al₂O₃ catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al₂O₃ catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al₂O₃ catalyst are discussed.     

Low cost growth route for single-walled carbon nanotubes from decomposition of acetylene over magnesia supported Fe-Mo catalyst

A large amount of single wall carbon nanotubes (SWNTs) was successfully produced by thermal decomposition of C₂H, at 800 °C over magnesia supported Fe-Mo bimetallic catalysts in a tubular flow reactor under an atmosphere of hydrogen flow. The growth density of SWNTs increased with increasing the weight percent of the catalyst metals (wt% ratio of two metals: 50 : 50) supported on magnesia (MgO) from 5 to 30 wt%. The yield of SWNTs reached 144.3% over 30 wt% metal-loaded catalyst. Raman measurements showed the growth of bundle type SWNTs with diameters ranging from 0.81 to 1.96 nm. The growth of SWNTs was also identified by thermal gravimetric analysis (TGA) and Raman spectroscopy.     

Characterization of single-wall carbon nanotubes by N2 adsorption

N₂ adsorption isotherms at 77 K of single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), and mixtures of these carbon nanotubes (CNTs) were analyzed for differences in their pore size distributions (PSDs). The PSDs, calculated in the microporous region by the Horvath–Kawazoe method and in the mesoporous region by the BJH method, are in agreement with the structures of both types of CNTs deduced from high-resolution transmission electron microscopy. A characteristic peak in the microporous region in the PSD of SWNTs is not present in the PSDs of MWNTs and impurities such as amorphous carbon, metal residues of catalysts, etc. The evaluation of this peak is proposed as a convenient tool for the quantitative characterization of SWNT purity in carbon nanotube-containing samples.     

Fischer–Tropsch synthesis over carbon nanotubes supported cobalt catalysts in a fixed bed reactor: Influence of acid treatment

The influence of acid treatment on carbon nanotubes (CNT) supported cobalt catalysts for Fischer–Tropsch synthesis (FTS) is discussed in this paper. CNTs were first treated with 30 wt.% HNO₃ at 25 and 100 °C for 14 h. Cobalt catalysts supported on fresh and acid treated carbon nanotubes were prepared using the incipient wetness impregnation method with a cobalt loading of 10 wt.%. The catalysts were extensively characterized by BET, XRD, TPR and TEM, and Raman spectroscopy. The TEM analyses of the acid treated support catalysts showed that the major parts of the cobalt particles were homogenously distributed inside the nanotubes. Raman I_D/I_G band intensity ratios as an indication of the quality of carbon nanotubes for catalyst supports, increased with acid treatment. The FTS activity (g HC produced/g cat./h) and selectivity (the percentage of the converted CO that appears as a hydrocarbon product) of the catalysts were assessed and compared with the as-prepared CNT supported 10 wt.% cobalt catalyst using a fixed bed micro-reactor. The acid treatments at 25 and 100 °C respectively, (a) increased the BET surface area by 18% and 25%; (b) decreased the cobalt particle size and increased the cobalt dispersion; (c) increased by 10 and 50% the reducibility of the catalysts and (d) increased the FTS activity and %CO conversion by 36 and 114%. Finally, the product selectivity showed a distinct shift to lower molecular weigh hydrocarbons.     

Transparent conductive film based on carbon nanotubes and PEDOT composites

Single-walled nanotubes (SWNTs), thin multiwalled carbon nanotubes (t-MWNTs) and multiwalled carbon nanotubes (MWNTs) were treated with H₂SO₄–HNO₃ acid solution, under different chemical conditions. The acid-treated CNTs were dispersed in DI water and in poly (3,4-ethylenedioxythiophene) (PEDOT) solution. Furthermore, the finely dispersed CNTs/PEDOT solutions were employed to a simple method of bar coating to obtain the transparent conductive films on the glass and polyethylene terephthalate (PET) film. A sheet resistance of 247 Ω/sq and a transmission of 84.7% were obtained at a concentration of the acid-treated CNTs of 0.01 wt.%.     

A study of the electrical properties of carbon nanotube-NiFe2O4 composites: Effect of the surface treatment of the carbon nanotubes

Novel carbon nanotube-NiFe₂O₄ composite materials have been prepared for the first time by in situ chemical precipitation of metal hydroxides in ethanol in the presence of carbon nanotubes (CNTs) and followed by hydrothermal processing. The obtained composite powders were characterized using XRD, TEM and EDS. The effect of surface oxidation treatment of CNTs on their properties was investigated by FTIR, zeta potential and hydrodynamic radius distribution characterization. Electrical conductivity measurements show that surface oxidation treatment of CNTs can improve the electrical conductivity of the composites more pronouncedly than pristine CNTs do. With 10 wt.% addition of surface treated CNTs, the electrical conductivity is increased by 5 orders of magnitude. The surface oxidized CNTs are crucial for this significant increase in electrical conductivity, which provides strong adhesion between the nanotubes and the matrix to give a homogeneous carbon nanotube-NiFe₂O₄ composite.     

Co-production of hydrogen and multi-wall carbon nanotubes from ethanol decomposition over Fe/Al2O3 catalysts

The co-production of hydrogen and carbon nanotubes (CNTs) from the decomposition of ethanol over Fe/Al₂O₃ at different temperatures and feeding rates of ethanol was investigated systematically. The results indicated that Fe/Al₂O₃ was a quite active catalyst for the co-production of hydrogen and CNTs and that its activity and stability depended strongly on the Fe loading. Among all catalysts tested, 10 mol% Fe/Al₂O₃ was the most effective catalyst based on the ratio of hydrogen production, the total H₂ yield, and the quality of the CNTs formed. The efficiency of hydrogen production from ethanol decomposition over 10 mol% Fe/Al₂O₃ reached a maximum of 80% at 800 °C and the yield of CNTs with well-oriented growth and uniform diameter was 141%. In addition, the reaction of hydrogen and CNTs co-produced from ethanol decomposition was proposed.     

High yield preparation of tubular carbon nanofibers over supported Co-Mo catalysts

In the search for high yield synthesis of carbon nanotubes (CNTs) at lower temperatures, Co-Mo catalysts on carbon black were investigated with ethylene and CO as carbon sources in catalytic gas-phase pyrolysis in comparison to that on TiO₂. The carbon black support was expected to be advantageous because of the feasibility of a CNT/carbon black composite possibly fabricated for several applications without removal of the support. Depending on the catalyst support, the catalytic activity toward CO and ethylene showed great differences. Co-Mo (9:1) catalysts on titania or carbon black provided a high carbon yield from CO and ethylene at the rather low temperatures of 450-530 °C.     

Improved thermal shock performance of Al2O3-C refractories due to nanoscaled additives

Carbon bonded alumina refractories with approximately 30 wt.-% residual carbon after coking are widely used as functional components such as submerged entry nozzles, monobloc stoppers and ladle shrouds in steel casting operations. Compositions with less residual carbon after coking based on nanoscaled magnesium aluminate spinel (MgAl₂O₄), alumina nanosheets (α-Al₂O₃) and carbon nanotubes (CNTs) either as single additives or combinations have been investigated according to their physical, mechanical and thermo-mechanical properties. The combination of nanoscaled powders based on carbon nanotubes and alumina nanosheets lead to superior thermal shock performance with approximately 30% less residual carbon in comparison to commercial available material compositions.     

Selectivity enhancement of Co-Mo/Al2O3 FCC gasoline hydrodesulfurization catalysts via incorporation of mesoporous Si-SBA-15

Mesoporous Si-SBA-15 was applied to enhance the FCC gasoline selective hydrodesulfurization (HDS) performance of conventional Co-Mo/Al₂O₃ catalysts and the physicochemical properties of the resulting catalyst were compared with those of Co-Mo/Al₂O₃ catalysts incorporated with macroporous kaolin, mesoporous Si-MCM-41 and microporous Si-ZSM-5. The selective HDS performances of all the catalysts were assessed with different FCC gasolines as feedstocks. The results showed that the HDS selectivity of the catalysts was closely related to the Mo sulfidation that depends on catalyst surface area and metal-support interaction. With the superior Mo sulfidation, the Co-Mo/Si-SBA-15-Al₂O₃ catalyst had the optimal HDS selectivity for not only the full-range FCC gasolines but also the heavy fractions thereof. The present article demonstrates the significance of enhancing Mo sulfidation in improving HDS selectivity and thus sheds a light on the development of highly selective HDS catalysts.     

Strong influence of buffer layer type on carbon nanotube characteristics

Carbon nanotubes (CNTs) have been grown by chemical vapor deposition in a vacuum chamber equipped with in situ photoelectron spectroscopy technique that allows for precise characterization of the chemical state of substrate and catalyst before the CNTs growth. The CNTs were grown onto Si wafers covered with thin buffer layers of Al, Al₂O₃, TiN, and TiO₂, using Fe as catalyst. Marked differences were observed both in growth rate and nanotube characteristics, as determined by SEM, TEM and micro-Raman spectroscopy, depending on the choice of buffer layer.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

Prediction of carbon nanotube growth success by the analysis of carbon-catalyst binary phase diagrams

The growth of carbon nanotubes (CNTs) was attempted using a vapor-phase CVD method, and a wide variety of transition metals, in the form of metallocenes and chlorides, were used as potential growth catalysts. Well-aligned mats of multi-walled carbon nanotubes were observed in samples catalyzed with iron, nickel, and cobalt. Nanotube lengths could be varied between 3 μm and 3 mm, and diameters ranged from 20 nm to 60 nm. X-ray diffraction, energy dispersive X-ray spectroscopy, and Mossbauer spectroscopy were used to determine the compositions of the nanotube products. None of the other elements studied, consisting of Cr, Mn, Zn, Cd, Ti, Zr, La, Cu, V, and Gd, were able to successfully catalyze the growth of CNTs. A study was performed of the respective metal-carbon binary phase diagrams to understand why the catalytic ability of different elements varied. Successful catalysts had carbon solubility limits of 0.5 wt.% to 1.5 wt.% carbon, followed closely by nanotube growth through graphite precipitation. Unsuccessful catalysts were found to have either nearly zero carbon solubility, or to form numerous intermediate carbides, making it difficult for the diffusion required for graphite precipitation to occur. These findings were then used to look for other potential CNT catalysts by examining the phase diagrams of a wide variety of metals.