Catalyst and technology for production of carbon nanotubes

Modifying the iron-aluminum catalyst with molybdenum oxide affords a marked increase in the yield of carbon nanotubes from 1,3-butadiene diluted with hydrogen. The optimum catalyst composition is 6.5% MoO₃-52% Fe₂O₃-Al₂O₃. With this catalyst, it is possible to obtain over 100 g of carbon nanotubes per gram of catalyst in a reactor fitted with a McBain balance. Replacing expensive 1,3-butadiene with the cheaper commercial propane-butane mixture (80 mol % propane + 20 mol % butane) leads to a sharp decrease in the nanotube yield because of the lower reactivity of the latter. Based on the concept of the catalytic decomposition of hydrocarbons and the formation of nanosized carbon materials via the carbide cycle mechanism, a new, efficient, CoO-MoO₃-Fe₂O₃-Al₂O₃ catalyst has been developed. The enhancement of the activity of the MoO₃-Fe₂O₃-Al₂O₃ catalyst by promoting it with cobalt oxide is achieved without a change in the rate-limiting step of the process. The design of a continuous reactor for carbon nanotube synthesis is suggested. The characteristics of the resulting nanotubes are presented.     

Preparation,Characterization, and Electrochemical Performance of the Hematite/Oxidized Multi-Walled Carbon Nanotubes Nanocomposite

In this research work, a hematite (α-Fe₂O₃) nanoparticle was prepared and then mixed with oxidized multi-walled carbon nanotubes (O-MWCNT) to form a stable suspension of an α-Fe₂O₃/O-MWCNTs nanocomposite. Different characterization techniques were used to explore the chemical and physical properties of the α-Fe₂O₃/O-MWCNTs nanocomposite, including XRD, FT-IR, UV-Vis, and SEM. The results revealed the successful formation of the α-Fe₂O₃ nanoparticles, and the oxidation of the MWCNT, as well as the formation of stable α-Fe₂O₃/O-MWCNTs nanocomposite. The electrochemical behaviour of the α-Fe₂O₃/O-MWCNTs nanocomposite was investigated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV), and the results revealed that modification of α-Fe₂O₃ nanoparticles with O-MWCNTs greatly enhanced electrochemical performance and capacitive behaviour, as well as cycling stability.     

Aligned MoOx/MoS2 Core–Shell Nanotubular Structures with a High Density of Reactive Sites Based on Self‐Ordered Anodic Molybdenum Oxide Nanotubes

The present work demonstrates the self‐organized formation of anodic molybdenum oxide nanotube arrays. The amorphous tubes can be crystallized to MoO₂ or MoO₃ and be converted fully or partially into molybdenum sulfide. Vertically aligned MoO_x/MoS₂ nanotubes can be formed when, under optimized conditions, defined MoS₂ sheets form in a layer by layer arrangement that provide a high density of reactive stacking misalignments (defects). These core–shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS₂ coating. Such structures are highly promising for applications in electrocatalysis (hydrogen evolution) or ion insertion devices.     

Recovery of MoO3 from spent catalysts of partial oxidation of methanol to formaldehyde

An effect of an ammonia solution concentration, temperature, l: s ratio on a MoO₃ extraction process was revealed in studying ammonia leaching of molybdenum oxide from spent iron-molybdenum catalyst of oxidation of methanol to formaldehyde The data obtained allow to optimize the extraction process of MoO₃ from the spent Fe-Mo catalysts.     

Carbon nanotubes decorated α-Al2O3 containing cobalt nanoparticles for Fischer-Tropsch reaction

A new hierarchical composite consisted of multi-walled carbon nanotubes (CNTs) layer anchored on macroscopic α-Al₂O₃ host matrix was synthesized and used as support for Fischer-Tropsch synthesis (FTS). The composite constituted by a thin shell of a homogeneous, highly entangled and structure-opened carbon nanotubes network and it exhibited a relatively high and fully accessible specific surface area of 76 m²·g^?1, compared with that of 5 m²·g^?1 of the original α-Al₂O₃ support. The metal-support interaction between carbon nanotubes surface and cobalt precursor and high effective surface area led to a relatively high dispersion of cobalt nanoparticles. This hierarchically supported cobalt catalyst exhibited a high FTS activity along with an extremely high selectivity towards liquid hydrocarbons compared with the cobalt-based catalyst supported on pristine α-Al2O₃ or on CNTs carriers. This improvement can attribute to the high accessibility of composite surface area comparing with the macroscopic host structure alone or to the bulk CNTs where the nanoscopic dimension induced a dense packing with low mass transfer which favoured the problem of reactants competitive diffusion towards the cobalt active site. In addition, intrinsic thermal conductivity of decorated CNTs could help the heat dissipating throughout the catalyst body, thus avoiding the formation of local hot spots which appeared in high CO conversion under pure syngas feed in FTS reaction. Cobalt supported on CNTs decorated α-Al₂O₃ catalyst also exhibited satisfied high stability during more than 200 h on stream under relatively severe conditions compared with other catalysts reported in the literature. Finally, the macroscopic shape of such composite easily rendered its usage as catalyst support in a fixed-bed configuration without facing problems of transport and pressure drop as encountered with the bulk CNTs.     

Decomposition of methane over alumina supported Fe and Ni–Fe bimetallic catalyst: Effect of preparation procedure and calcination temperature

Catalytic decomposition of methane has been studied extensively as the production of hydrogen and formation of carbon nanotube is proven crucial from the scientific and technological point of view. In that context, variation of catalyst preparation procedure, calcination temperature and use of promoters could significantly alter the methane conversion, hydrogen yield and morphology of carbon nanotubes formed after the reaction. In this work, Ni promoted and unpromoted Fe/Al₂O₃ catalysts have been prepared by impregnation, sol–gel and co-precipitation method with calcination at two different temperatures. The catalysts were characterized by X-ray diffraction (XRD), N₂ physisorption, temperature programmed reduction (TPR) and thermogravimetric analysis (TGA) techniques. The catalytic activity was tested for methane decomposition reaction. The catalytic activity was high when calcined at 500 °C temperature irrespective of the preparation method. However while calcined at high temperature the catalyst prepared by impregnation method showed a high activity. It is found from XRD and TPR characterization that disordered iron oxides supported on alumina play an important role for dissociative chemisorptions of methane generating molecular hydrogen. The transmission electron microscope technique results of the spent catalysts showed the formation of carbon nanotube which is having length of 32–34 nm. The Fe nanoparticles are present on the tip of the carbon nanotube and nanotube grows by contraction–elongation mechanism. Among three different methodologies impregnation method was more effective to generate adequate active sites in the catalyst surface. The Ni promotion enhances the reducibility of Fe/Al₂O₃ oxides showing a higher catalytic activity. The catalyst is stable up to six hours on stream as observed in the activity results.     

Preparation and characterization of supports with a synthesized layer of catalytic filamentous carbon: III. Synthesis of carbon nanofibers on nickel supported onto aluminum oxide

The synthesis of catalytic filamentous carbon (CFC) on catalysts prepared by supporting Ni²⁺ compounds onto the surface of various alumina modifications (macroporous α-Al₂O₃ and mesoporous ?-Al₂O₃ and δ-Al₂O₃) using two procedures (impregnation and homogeneous precipitation) was studied. The texture characteristics (specific surface area and pore structure) of the parent supports and adsorbents with a CFC layer were compared. The effect of the supporting procedure on the surface morphology of Ni/Al₂O₃ catalysts and the synthesized CFC layer was studied by scanning electron microscopy. It was found that the carbon yield on a macroporous catalyst prepared by homogeneous precipitation was higher than that on a catalyst prepared by impregnation by a factor of ～2. The CFC layer exhibited a mesoporous structure because of a chaotic interlacing of carbon nanofibers, and the synthesis of CFC on macroporous supports resulted in the formation of a bidisperse pore structure of the adsorbent. Active and stable heterogeneous biocatalysts were prepared by the adsorptive immobilization of enzymatically active substances (glucoamylase and nongrowing baker’s yeast cells) on CFC.     

Determination of the maximum monolayer dispersion and dispersed state for WO3 on γ-Al2O3

The maximum monolayer dispersion (the threshold) for WO₃ on γ-Al₂O₃ calcined at 500°, 550°, 600°, and 640°C has been determined quantitatively by XRD (amount of crystalline phase) and XPS (intensity ratios I_w4f/I_Al2). The results show that if the amount of WO₃ loaded is lower than the maximum monolayer dispersion, WO₃ will react with γ-Al₂O₃ to form surface compound due to mutual ionic interaction, and will be dispersed on γ-Al₂O₃ surface as monolayer then. In case the amount is higher than this value, the residual crystalline WO₃ will remain. The maximum monolayer dispersion (threshold) is 0.21 g and 0.20 g WO₃/100 m² γ-Al₃O₃ by XRD and XPS respectively. It agrees with the value (0.189 g WO₃/100 m² or 4.90 × 10^?18 W atoms/m²) calculated from the model on assumption that the WO₃ is dispersed as a closed-packed monolayer on γ-Al₂O₃ surface. Inasmuch as WO₃/γ-Al₂O₃ system is stable up to higher temperature, e.g. 700°C, than MoO₃/γ-Al₂O₃ system, WO₃ seems unfavorable to form new bulk compound with γ-Al₂O₃ at that temperature. However, Al₂(MoO₄)₃ forms perceptibly in MoO₃/γ-Al₂O₃ system at 500°C. Besides, the size of residual crystalline WO₃ in WO₃/γ-Al₂O₃ is much smaller than that of MoO₃ in MoO₃/γ-Al₂O₃. It might be the reason that WO₃/γ-Al₂O₃ catalyst is superior to MoO₃/γ-Al₂O₃ in hydrodesulfurization (HDS) or hydrodenitrogenation (HDN) in some cases.     

Hydrodeoxygenation of Anisole over Ni/α-Al2O3 Catalyst

Ni-based catalysts supported on di erent supports (α-Al₂O₃,γ-Al₂O₃, SiO2, TiO₂, and ZrO₂) were prepared by impregnation. Effects of supports on catalytic performance were tested using hydrodeoxygenation reaction (HDO) of anisole as model reaction. Ni/α-Al₂O₃ was found to be the highest active catalyst for HDO of anisole. Under the optimal conditions, the anisole conversion is 93.25% and the hydrocarbon yield is 90.47%. Catalyst characteriza-tion using H₂-TPD method demonstrates that Ni/α-Al₂O₃ catalyst possesses more amount of active metal Ni than those of other investigated catalysts, which can enhance the cat-alytic activity for hydrogenation. Furthermore, it is found that the Ni/α-Al₂O₃ catalyst has excellent repeatability, and the carbon deposited on the surface of catalyst is negligible.     

不同载体Ni基负载型催化剂对甲烷部分氧化制合成气催化行为研究

采用浸渍法制备了单一载体(Al₂O₃、ZrO₂、CeO₂)和ZrO₂、CeO₂改性的Al₂O₃复合载体的Ni催化剂,考察了在甲烷部分氧化制备合成气反应中的催化性能。通过N₂-物理吸附、H₂程序升温还原、X射线衍射、NH₃程序升温脱附和程序升温氧化等技术对催化剂进行了表征。结果表明,在单一载体催化剂中,Ni/Al₂O₃具有较大的比表面积,其初始反应活性较高,但该催化剂表面易形成大量的积炭而快速失活。Ni/ZrO₂和Ni/CeO₂催化剂比表面积较小,活性金属Ni在其表面分散性差,催化剂具有较低的CH₄转化率。而CeO₂和ZrO₂改性的Al₂O₃复合载体催化剂,具有较大的比表面积,反应活性明显高于单一载体催化剂。CeO₂-Al₂O₃复合载体催化剂具有最高的反应活性和较好的反应稳定性。同时表明,含CeO₂催化剂反应后表面积炭较少,CeO₂的储放氧功能增强了催化剂对O₂的活化,提高催化剂活性的同时,可以抑制积炭的生成。     

The Al2O3-Fe2O3 Mixed Oxidic System, I. Preparation and Characterization

A series of Al₂O₃-Fe₂O₃ mixed oxidic solids with composition ranging from 0 to 100% of Fe₂O₃ were prepared and examined for structural characteristics. XRD diagrams showed the presence of α-Al₂O₃ and hematite phases. The analysis of M?ssbauer spectra revealed the existence of two iron containing phases. The specific surface area of the mixture decreases by the addition of iron and depends on the crystal phases of the mixture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of Mo content in MoO3/g-Al2O3 on the catalytic activity for transesterification of dimethyl oxalate with phenol

The influence of molybdenum content on the catalytic performance in the transesterification of dimethyl oxalate (DMO) with phenol to methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) was investigated. The results indicated that the MoO₃/Al₂O₃ catalyst with 14 wt% Mo content gave maximal DPO yield with 6.1% and 75.1% DMO conversion. The component, structure and phase of MoO₃/Al₂O₃ catalysts were characterized by means of X-ray diffraction (XRD), BET specific surface area, temperature-programmed desorption of ammonia (NH₃-TPD), and FTIR analysis of adsorbed pyridine. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Mechanism of Action of a Multicomponent Co-Mo-Bi-Fe-Sb-K-O Catalyst for the Partial Oxidation of Propylene to Acrolein: II. Changes in the Phase Composition of the Catalyst under Reaction Conditions

The structures of a complex multicomponent Co-Mo-Fe-Bi-K-Sb-O catalyst for the oxidation of propylene to acrolein and simpler catalysts from which some catalyst components were absent were studied by X-ray diffraction analysis. The phases of α-CoMoO₄, β-CoMoO₄, Fe₂(MoO₄)₃, Bi₂O₃ ⋅ MoO₃, Bi₂O₃ ⋅ 2MoO₃, Bi₂O₃ ⋅ 3MoO₃, oxidized molybdenum oxide, and reduced molybdenum oxide are the main components of the catalyst. Ternary compounds were not detected. Under catalytic reaction conditions, the relative amounts of the phases changed; this change suggests the occurrence of redox transformations with the participation of these phases, probably, at the interface.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 580–584.Original Russian Text Copyright © 2005 by Shashkin, Udalova, Shibanova, Krylov.     

Phosphorus modified MoO<Subscript>3</Subscript>–Bi<Subscript>2</Subscript>SiO<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> catalyst for gas-phase epoxidation of propylene by molecular oxygen

The phosphorus (P) modified MoO₃–Bi₂SiO₅/SiO₂ catalyst was prepared by a simple co-impregnation method and investigated in the epoxidation of propylene by molecular oxygen. The catalyst was characterized by X-ray diffraction (XRD), N₂ adsorption–desorption analysis, NH₃-temperature-programmed desorption (NH₃-TPD), transmission electron microscopy, and Raman spectroscopy. It was found that the P-modified MoO₃–Bi₂SiO₅/SiO₂ catalyst with a P/Mo molar ratio of 0.5 exhibits the best catalytic performance for epoxidation of propylene by O₂, the TOFs for propylene oxide (PO) formation was four times higher than that of the unmodified one at 633 K. The modification by P could promote the dispersion of MoO₃ nanoparticles and increase the number of weak and moderate acid sites with respect to the phosphorus-free MoO₃–Bi₂SiO₅/SiO₂ catalyst, which were beneficial to the formation of PO. Moreover, the introduction of P also could protect the mesoporous structure by inhibiting the formation of Bi₂Mo₃O₁₂, which was beneficial to the dispersion of active species. We suppose that the phosphorus, bismuth and molybdenum species of P-modified MoO₃–Bi₂SiO₅/SiO₂ catalyst play important roles for propylene epoxidation by molecular oxygen.     

Deposition of NiO onto MoO3/γ-Al2O3 extrudates by slurry impregnation method

NiO-MoO₃/γ-Al₂O₃ catalysts were prepared by the reaction of γ-Al₂O₃ extrudates with an aqueous slurry of MoO₃, followed by the reaction of the MoO₃/γ-Al₂O₃ catalyst with an aqueous slurry of NiO, Ni(OH)₂, NiCO₃·2Ni(OH)₂·xH₂O, or 2NiCO₃·3Ni(OH)₂·4H₂O and by subsequent drying. The NiO deposition was examined with electron probe microanalysis. The deposited Ni efficiently increased the activity in benzothiophene hydrodesulfurization.     

Microwave synthesis of sulfur-doped α-Fe2O3 and testing in photodegradation of methyl orange

α-Fe₂O₃, as a promising photocatalyst in environmental aspects, was doped by a nonmetal to enhance the optical and electronic properties. Sulfur-doped hematite was synthesized by microwave irradiation. The samples were investigated by X-ray diffraction and energy dispersive X- ray-scanning electron microscopy. Nanostructure particles have a hexagonal structure that did not change after sulfur incorporation. The optical studies via UV–Visible spectroscopy proved the high absorbance of S/α-Fe₂O₃under the visible region. Moreover, the band gap of S/α-Fe₂O₃ was shifted to higher wavelengths. However, nonmetals may exhibit a negative effect and act as recombination centers as the photoactivity of undoped hematite was still higher in the photodegradation of methyl orange as a pollutant. H₂O₂ as an oxidant was fourfold better than O₂, leading to the formation of the active oxygen species. The preparation method plays a crucial role in the shaping of nanostructure particles and its photoactivity.     

Preparation of Nano-Sized γ-Al2O3 Supported Iron Catalyst for Fischer-Tropsch Synthesis by Solvated Metal Atom Impregnation Methods

Two types of small iron clusters supported onγ-Al2O3-RT(dehydroxylated at room temperature) andγ-Al2O3-800 (dehydroxylated at 800℃) were prepared by solvated metal atom impregnation (SMAI) techniques. The iron atom precursor complex, bis(toluene)iron(0) formed in the metal atom reactor, was impregnated intoγ-Al2O3 having different concentrations of surface hydroxyl groups to study the effect of surface hydroxylation on the crucial stage of iron cluster formation. Catalysts prepared in this way were characterized by TEM, Mossbauer, and chemisorption measurements, and the results show that higher concentration of surface hydroxyl groups ofγ-Al2O3-RT favors the formation of more positively charged supported iron cluster Fen/γ-Al2O3-RT, and the lower concentration of surface hydroxyl groups ofγ-Al2O3-800 favors the formation of basically neutral supported iron cluster Fen/γ-Al2O3-800. The measured results also indicate that the higher concentration of surface hydroxyl groups causes the rapid decomposition of precursor complex, bis(toluene)iron(0), and favors the formation of relatively large iron cluster. Consequently, these two types of catalysts show different catalytic properties in Fischer-Tropsch reaction. The catalytic pattern of Fen/γ-Al2O3-RT in F-T reaction is similar to that of the unreducedα-Fe2O3 and that of Fen/γ-Al2O3-800 is similar to that of the reducedα-Fe2O3.     

Formation of a supported cobalt catalyst for the synthesis of carbon nanofibers on aluminum oxide

Comparative studies of the effect of the physicochemical characteristics of a support (aluminum oxide) on the formation of a supported Co catalyst and its activity in the pyrolysis of alkanes (propane-butane) were performed. The effect of the crystalline modification of alumina on the yield of catalytic filamentous carbon (CFC) ((g CFC)/(g Co)) was studied. The surface morphologies of Co-containing catalysts and synthesized carbon deposits were studied by scanning electron microscopy. It was found that carbon deposits with a well-defined nanofiber structure were synthesized by the pyrolysis of a propane-butane mixture in the presence of hydrogen at 600°C on supported Co catalysts prepared by homogeneous precipitation on macroporous corundum (α-Al₂O₃). The yield of CFC was no higher than 4 (g CFC)/(g Co). On the Co catalyst prepared by homogeneous precipitation on mesoporous Al₂O₃, the intense carbonization of the initial support; the formation of cobalt aluminates; and, as a consequence, the deactivation of Co⁰ as a catalyst of FC synthesis occurred. The dependence of the yield of CFC on the preheating temperature (from 200 to 800°C) of Co catalysts before pyrolysis was studied. It was found that, as the preheating temperature of supported Co/Al₂O₃ catalysts was increased, the amount of synthesized carbon, including CFC, decreased because of Co⁰ deactivation due to the interaction with the support and coke formation.     

Preparation of bimetallic CoO-MoO3/γ-Al2O3 and NiO-MoO3/γ-Al2O3 hydrodesulfurization catalysts by deposition of Co, Ni and Mo onto α-AlOOH during paste processing

CoO-MoO₃/γ-Al₂O₃ and NiO-MoO₃/γ-Al₂O₃ catalysts were prepared by the reaction of α-boehmite (α-AlOOH) with MoO₃ in an aqueous paste, followed by the reaction of the MoO₃/α-AlOOH catalyst with Co(OH)₂·CoCO₃ or 2NiCO₃·3Ni(OH)₂·4H₂O in an aqueous paste, and by subsequent drying and/or calcination. The deposited MoO₃ functioned as a thermal stabilizer inhibiting the sintering of the Al₂O₃ phase during calcination. The deposited Co and Ni were efficient activity promoters in benzothiophene hydrodesulfurization.     

Effect of the support on cobalt carbide catalysts for sustainable production of olefins from syngas

Co₂C-based catalysts with SiO₂, γ-Al₂O₃, and carbon nanotubes (CNTs) as support materials were prepared and evaluated for the Fischer-Tropsch to olefin (FTO) reaction. The combination of catalytic performance and structure characterization indicates that the cobalt-support interaction has a great influence on the Co₂C morphology and catalytic performance. The CNT support facilitates the formation of a CoMn composite oxide during calcination, and Co₂C nanoprisms were observed in the spent catalysts, resulting in a product distribution that greatly deviates from the classical Anderson-Schulz-Flory (ASF) distribution, where only 2.4 C% methane was generated. The Co₃O₄ phase for SiO₂- and γ-Al₂O₃-supported catalysts was observed in the calcined sample. After reduction, CoO, MnO, and low-valence CoMn composite oxide were generated in the γ-Al₂O₃-supported sample, and both Co₂C nanospheres and nanoprisms were identified in the corresponding spent catalyst. However, only separated phases of CoO and MnO were found in the reduced sample supported by SiO₂, and Co₂C nanospheres were detected in the spent catalyst without the evidence of any Co₂C nanoprisms. The Co₂C nanospheres led to a relatively high methane selectivity of 5.8 C% and 12.0 C% of the γ-Al₂O₃- and SiO₂-supported catalysts, respectively. These results suggest that a relatively weak cobalt-support interaction is necessary for the formation of the CoMn composite oxide during calcination, which benefits the formation of Co₂C nanoprisms with promising catalytic performance for the sustainable production of olefins via syngas.