CO Disproportionation Research Articles

Decomposition of carbon-containing compounds on solid catalysts for single-walled nanotube production.

Different catalyst formulations and reaction conditions have been used to test the validity of a hypothesis that tries to elucidate the mechanism of single-walled nanotube (SWNT) formation by CO disproportionation over a highly selective cobalt-molybdenum catalyst. This model proposed an intrinsic dependence between the selectivity of the catalysts toward SWNT and the stabilization of Co species in a nonmetallic state, which in turn results from an interaction with Mo. The series of tests performed to examine this model include the doping of this highly selective catalyst with sodium, the substitution of molybdenum by tungsten in the original catalyst formulation, the variation on reaction temperature, and the introduction of hydrogen in the gas feedstock. All these modifications were carried out to modify the growth conditions in which the SWNTs are formed. The results are consistent with the hypothesis proposed by the authors.

Rietveld refinement and activity of CO oxidation over Pd/Ce 0.8Zr 0.2O 2 catalyst prepared via a surfactant-assisted route

A surfactant-assisted preparation method was successfully used to synthesize the Ce 0.8Zr 0.2O 2 nanophases that had homogeneous chemical composition, enhanced thermal stability and improved catalytic activity. Structural refinement by the Rietveld method confirmed that many cationic lattice defects were formed in the ceria-zirconia crystals due to incorporation of the cationic surfactant in the network of the solid. Zirconium dissolving into cubic structure of ceria improved its thermal stability but led to structure distortion. FT-IR in situ characterization showed that CO chemisorption on the palladium loaded ceria-zirconia catalyst formed linear and multibonded CO–metal complexes. It was also evidenced that CO 2 was produced in the CO chemisorption procedure from the pathways of CO disproportionation and CO reacting with lattice oxygen of the catalyst. Owing to high dispersion of the loaded metal palladium and defective structure of the ceria-zirconia support, the Pd/Ce 0.8Zr 0.2O 2 catalyst calcined at a temperature range between 873 and 1173 K not only exhibited a more stable catalytic activity for CO oxidation but also performed a lower light-off temperature at cool start below 373 K in comparison with Pd/CeO 2.

Uniform Field Emission from Carbon Nanotubes Fabricated by CO Disproportionation.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Growth of Single-Walled Carbon Nanotubes by Disproportionation of CO over Co-Re/SiO2 Catalysts

Growth of Single-Walled Carbon Nanotubes by Disproportionation of CO over Co-Re/SiO2 Catalysts

カーボンナノチューブの大量合成と触媒効果

Carbon nanotubes have potential applications in a variety of fields, such as composites, field-emission displays, electrodes, gas adsorbent, and catalyst supports. For putting these applications into industry, it is very important to establish the mass production of nanotubes. Here, we introduce our recent progress on the large-scale synthesis of carbon nanotubes, in particular, single-wall carbon nanotubes (SWNTs), based on the gas-phase flow reaction. By spraying the Co-Mo nanoparticles dissolved in toluene to the vertical furnace, we succeeded in selectively obtaining SWNTs and multi-wall carbon nanotubes (MWNTs). It was found that the structure and quality of nanotubes depend on the concentration of the thiophene additives, the hydrogen gas flow rate, and the injection rate of the colloidal solution. The growth mechanism of SWNTs and MWNTs are discussed in terms of catalytic effects. Other large-scale synthetic methods for SWNTs, such as high pressure CO disproportionation (HiPco) and the supported catalysts-based fluidized bed reactions are also introduced.

Carbon nanotubes based on anodic aluminum oxide nano-template

The effect of reaction gas and catalyst on the growth of carbon nanotubes (CNTs) in the anodic aluminum oxide (AAO) nano-template was investigated. A mechanism of CNT growth was proposed, which involves the competitive catalytic carbon deposition between on the Co catalyst particles electrodeposited at the bottom of the pores and on the AAO template itself. Presence of H2 in the reacting gas mixture significantly affected the morphology and the wall structure of synthesized CNTs: CNTs of high crystallinity grew out of pores with H2 while no CNTs overgrew in the absence of H2. CNT synthesis by CO disproportionation showed a lower growth rate and a higher degree of ordering than those grown by C2H2 pyrolysis. The unified mechanism of CNT growth on AAO template is also proposed.

Uniform Field Emission from Carbon Nanotubes Fabricated by CO Disproportionation

Field emission of carbon nanotubes (CNTs) fabricated by disproportionation of CO has been studied. CNTs fabricated on well-ordered Co nanowire arrays formed on the porous anodic aluminum oxide templates were well graphitized, uniform in diameter and aligned vertically with respect to the plane of the template, and showed a good field emission property. Very uniform emissions were observed from the CNTs fabricated at relatively low temperature, <TEX>$500-600^{\circ}C$</TEX>. Low fabrication temperature such as <TEX>$500^{\circ}C$</TEX> could make it possible to fabricate CNTs on soda lime glass, a low-cost substrate, for display panel.

A DFT Study of Interaction of Carbon Monoxide with Carbonaceous Materials

Reaction of CO with carbonaceous surfaces was investigated using B3LYP density functional theory level (DFT) with the 6-31G(d) basis set. It was found that CO can be adsorbed exothermically on the active sites of zigzag, armchair, and tip carbonaceous models to yield stable intermediates such as cyclic ether, carbonyl, lactone, ketone, carbonate, and semiquinone functionalities. The above reactions are important in carbon gasification processes as well as in carbon single-wall nanotubes formation from CO disproportionation reaction. In the case of gasification, adsorption of CO blocks the active sites of the carbonaceous material and thus can reduce the efficiency of the process. Furthermore, it was found that when CO is adsorbed in a carbonyl type structure (CCO), there is a reversible interconversion process by ring closure with a neighbor active site to produce a cyclic ether (furan type), a process that requires an additional neighboring active site. Consequently, the available number of active sites for gasification reaction is decreased and therefore the gasification reaction is inhibited. In addition, CO adsorption on oxidized surfaces can favor CO2 desorption. Such desorption can be either taking off an oxygen atom that potentially was going to be desorbed as CO or depositing a carbon atom on the surface due to the disproportionation reaction 2 CO = C + CO2. Both effects can inhibit or retard the gasification process. The results from the disproportionation reaction can also provide an insight into the mechanism for carbon single-wall nanotubes growth using CO as precursor.

Loss of single-walled carbon nanotubes selectivity by disruption of the Co–Mo interaction in the catalyst

In previous work, we reported the growth of well-structured single-walled carbon nanotubes (SWNT) by CO disproportionation on Co:Mo/SiO 2 catalysts. We have proposed that high SWNT selectivity can only be obtained when most of the Co in the sample is interacting with Mo and forming a cobalt–molybdate surface layer. In this contribution, we report further evidence to the hypothesis by an investigation of Na-doped Co:Mo/SiO 2 catalysts. The effect of the sodium addition has been investigated by Raman spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy, H 2 temperature-programmed reduction, and X-ray photoelectron spectroscopy. The results are compared to those obtained in a previous study on undoped CoMo/SiO 2 catalysts. Significant differences are observed between the undoped and Na-doped catalytic systems. It has been found that the presence of sodium favors the dispersion of molybdenum, which tends to form sodium molybdate species over the silica support. At the same time, the formation of these sodium molybdate structures hinders the interaction between cobalt and molybdenum, preventing the formation of the cobalt molybdate species. This disruption has been found to be detrimental to the SWNT selectivity of the catalyst, giving further evidence to the link between the presence of this surface species and the selective SWNT growth.

A first principles analysis of CO oxidation over Pt and Pt 66.7%Ru 33.3% (111) surfaces

Nonlocal gradient corrected DFT slab calculations were carried out to determine the overall reaction energies along with the barriers for the activation of water and the oxidation of CO over well-defined Pt(111) and PtRu(111) surfaces. We attempt to elucidate the features that control the bifunctional mechanism proposed for the oxidation of CO in solution. The addition of Ru to Pt along with the presence of solution helps to enhance the elementary steps that comprise the bifunctional mechanism. The activation of water over Pt(111) in the vapor phase is energetically unfavorable with an activation barrier of +142 kJ/mol and overall heat of reaction of +53 kJ/mol. Water will desorb before it will react on Pt(111). The presence of solution reduces the barrier to about 75 kJ/mol. The addition of Ru lowers the barrier for water activation in the vapor phase. Alloying Pt with Ru lowers the barriers for the homolytic as well as the heterolytic activation of water in solution. The presence of solution, however markedly favors the heterolytic activation which leads to the formation of an adsorbed OH* surface intermediate, a proton which migrates into solution and an electron. The energetics for this path over the Pt 66.7%Ru 33.3%(111) surface were calculated to be quite favorable with an activation barrier of +27 kJ/mol and an overall energy of reaction of +26 kJ/mol. Ab initio molecular dynamics results indicate that water can be activated over PtRu at 300 K. The resulting surface Ru-OH group that forms induces the adsorption and subsequent activation of water at a neighboring Pt site. The surface hydroxyl intermediate continues to diffuse across the surface via a sequence of proton transfer steps. The barrier for the subsequent oxidation of adsorbed CO by surface OH groups over Pt(111) in the vapor phase is 86 kJ/mol with an overall reaction energy of 21 kJ/mol. The barrier is reduced to 71 kJ/mol when carried out over the Pt 66.7%Ru 33.3% alloy in the vapor phase. The barrier for the homolytic disproportionation of CO and OH in solution (over Pt 66.7%Ru 33.3%) is 90 kJ/mol with an overall reaction energy of +24 kJ/mol. The heterolytic path which involves the oxidation of CO by OH to form CO 2+H +(aq)+e− is more favorable than the homolytic path. The activation barrier for the heterolytic path over Pt 66.7%Ru 33.3% is +60 kJ/mol with an overall reaction of −6 kJ/mol (exothermic). These results indicate that at potentials which are less than or equal to the potential of zero total charge, water activation over Pt(111) may be difficult. CO oxidation, however is more likely limiting over the Pt 66.7%Ru 33.3% alloy at these potentials. These results only hold for the ideal conditions studied herein including ideal single crystal surfaces, an ideal liquid water solution phase and the absence of an applied potential.

Interaction of probe molecules with active sites on cobalt, copper and zinc-exchanged SAPO-18 solid acid catalysts

Novel cobalt (Co), copper (Cu) and zinc (Zn) exchanged chabazite related SAPO-18 solid acid catalysts were prepared and characterised using techniques such as BET (nitrogen sorption), FTIR, ICP-OES, XPS and XRD. A detailed in situ FTIR investigation undertaken on the adsorption and disproportionation of NO and CO over Co-, Cu- and Zn-SAPO-18 molecular sieve catalysts indicated the formation of various NO/CO species or complexes with cationic Lewis acid and/or active metal sites. NO adsorption at room temperature leads to the formation of up to seven bands attributed to adsorbed nitrous oxide (N2O), chemisorbed nitrogen dioxide (NO2), nitrite (M-NO2) (M, metal), mononitrosyl (M-NO) and dinitrosyl [M-(NO)2] complexes. CO adsorption at room temperature leads to the formation of up to six bands attributed to physi-sorbed carbon dioxide (CO2), cationic Lewis acid carbonyl moieties (L-CO) and transition metal carbonyl complexes (M-CO). The concentration and distribution of NO/CO species varies with pressure, reaction temperature and evacuation.

Role of Co−W Interaction in the Selective Growth of Single-Walled Carbon Nanotubes from CO Disproportionation

A combination of characterization techniques has been employed to obtain a full description of the structure and chemical state of a series of Co−W/SiO2 catalysts used for the production of single-...

Raman spectroscopy of HiPCO single-walled carbon nanotubes at 300 and 5 K

Single-walled carbon nanotubes (SWNTs) produced by the high pressure CO disproportionation (HiPCO method) and purified by controlled thermal oxidation in air have been studied by Raman spectroscopy at 300 and 5 K. Raman spectra have been observed at λ exc=632.8 and 441.6 nm laser excitation in the range of 160–1800 cm −1. In the low-frequency part of the spectra (the radial breathing mode range) eleven narrow lines can be detected at low temperatures, enabling an estimation of nanotube diameters (0.8–1.3 nm) and chirality. The width at half-maximum intensity of these spectral lines is about 3–4 cm −1 at 5 K. The Stokes and anti-Stokes spectra are measured at λ exc=632.8 nm at room temperature. The most intense lines in these spectra are caused with the resonant Raman-scattering process. With increasing temperature from 5 to 300 K the shift (3–4 cm −1) of the most intense high-frequency component of the tangential mode (G mode) to lower frequency is observed. Based on the analysis of the Stokes/anti-Stokes spectra and the G band shape, the corresponding lines were identified with metallic or semiconducting type of nanotubes.

Uniform field emission from aligned carbon nanotubes prepared by CO disproportionation

We have fabricated aligned carbon nanotubes (CNTs) by disproportionation of CO on a well-ordered Co nanowire array prepared on a porous anodic aluminum oxide (AAO) template using overdeposition and etching techniques, and studied their field emission properties. CNTs fabricated at 600 °C were well graphitized, uniform in diameter and height, and aligned vertically with respect to the plane of the template; they also showed good field emission properties. The electron field emission was turned on at a field as low as 1.1 V/μm and the enhancement factor was 3520 by assuming a CNT work function of 5 eV. Also, very uniform emission was observed by applying 3.1 V/μm. These good field emission properties may be attributed to good graphitization, uniformity in diameter and height, and vertical alignment of the CNTs fabricated on an AAO template by CO disproportionation.

A Novel Complex Cobalt Gallium Oxide Ca 2Co 0.8Ga 1.2O 4.8: Synthesis and High-Temperature Electron Transport Properties

A new complex oxide with the cation ratio Ca:Co: Ga=2:0.8:1.2 has been synthesized in air at 1150 oC. The cobalt atoms adopt oxidation states 2+ and 3+ in equal amounts giving an oxygen content corresponding to the composition Ca 2Co 0.8Ga 1.2O 4.8. It crystallizes in F-centered cubic structure with a=15.0558 Å. Conductivity measurements performed at high temperatures revealed that the temperature increase gives a charge disproportionation of Co 3+ species resulting in a small concentration of Co 4+ species and thus a small p-type conductivity in the oxide. A decrease of the oxygen pressure promotes oxygen depletion from the oxide and a deterioration of the conductivity. The electric properties are interpreted within a small polaron conduction mechanism. An unusually large mobility activation energy of 0.45 eV can be explained by a large spatial separation of cobalt cations in the structure.

Growth of single-wall carbon nanotubes within an ordered array of nanosize silica spheres

Single-wall carbon nanotubes (SWCNTs), which were well-separated individuals or, small bundles of two to five tubes, were produced in a controllable manner by disproportionation of CO at 700 °C and 1 atm in the voids of an ordered array of silica spheres with embedded bimetallic Mo/Co catalytic particles. A combination of transmission electron microscopy, electron energy-loss spectroscopy, and Raman scattering excited with an Ar laser at λ=514.5 nm (2.41 eV), was used to characterize the nanotubes. The corresponding Raman spectra strongly indicate that these nanotubes are primarily chiral. This method offers a valid route for a high-quality single-step production of SWCNT composites for mechanical, electronic, and optoelectronic applications.

In situ X-ray diffraction study of carbon nanotubes and filaments during their formation over Co/Al 2O 3 catalysts

Depending on the experimental conditions, the CO disproportionation reaction over well-calibrated Co/Al 2O 3 catalysts leads to the formation of carbon with two distinct morphologies: carbon nanotubes (with cylindrical carbon layers parallel to the tube axis) and carbon filaments (constituted of carbon layers with a truncated cone shape). The structural differences between these two morphologies were investigated by X-ray diffraction using synchrotron X-ray radiation. In situ X-ray diffraction analyses performed during the carbon growth at 600 °C show that the (002) inter-layer spacing is larger by a few percents in carbon nanotubes than in carbon filaments. d(002) is, respectively, equal to 0.3486 and 0.3469 nm for the nanotubes and filaments. The variation of d(002) with the graphene layer stacking geometry was confirmed by additional measurements performed at room temperature on samples prepared ex situ. The evolution of the cobalt catalyst during the carbon growth was also investigated. The results of this study indicate that the catalyst deactivation is not the result of a bulk transformation of the cobalt particles.

Nucleation of Polypropylene Crystallization by Single-Walled Carbon Nanotubes

Nonisothermal and isothermal crystallization experiments were performed on polypropylene mixed with carbon nanotubes produced by disproportionation of CO on Co−Mo catalysts. Functionalization of the nanotubes with octadecylamine made the tubes hydrophobic and allowed the tubes to be solubilized in an organic solvent. Mixing of the nanotubes with the polymer was accomplished by adding the nanotubes to a Decalin solution that contained dissolved polypropylene, followed by evaporation of the solvent. Dynamic mechanical analysis indicated very little difference in the small-strain mechanical properties between filled and unfilled polymers at the very low solid levels that were tested. By contrast, the crystallization behavior of the filled and unfilled polymer was quite different. Nanotubes promoted growth of the less-preferred beta form of crystalline polypropylene at the expense of the alpha form. In nonisothermal crystallization, the total amount of crystalline material in the sample was the same for the filled and unfilled materials. However, for isothermal crystallization experiments, the percent crystallinity in the filled materials was slightly higher. Most importantly, the rate of crystallization was substantially higher in the filled system. The results presented in this paper clearly show that carbon nanotubes nucleate crystallinity in polypropylene.

Characterization of Single-Walled Carbon Nanotubes (SWNTs) Produced by CO Disproportionation on Co−Mo Catalysts

The disproportionation of CO over Co−Mo/SiO2 catalysts with low Co/Mo ratios results in a high selectivity to single-walled carbon nanotubes (SWNTs) when the reaction is conducted between 700 and 950 °C in a flow of pure CO at a total pressure between 1 and 5 atm. A detailed characterization involving Raman spectroscopy, temperature-programmed oxidation (TPO), and transmission electron microscopy (TEM) shows that the product of this process has a high concentration of SWNTs. Depending on the temperature of operation, the selectivity and nanotube diameter can be varied.

Crystal structure, thermal expansion and electrical conductivity of Nd 0.7Sr 0.3Fe 1− xCo xO 3 (0≤ x≤0.8)

The crystal structure, thermal expansion and electrical conductivity of the solid solution Nd 0.7Sr 0.3Fe 1− x Co x O 3 for 0≤ x≤0.8 were investigated. All compositions had the GdFeO 3-type orthorhombic perovskite structure. The lattice parameters were determined at room temperature by X-ray powder diffraction (XRPD). The pseudo-cubic lattice constant decreased continuously with x. The average linear thermal expansion coefficient (TEC) in the temperature range from 573 to 973 K was found to increase with x. The thermal expansion curves for all values of x displayed rapid increase in slope at high temperatures. The electrical conductivity increased with x for the entire temperature range of measurement. The calculated activation energy values indicate that electrical conduction takes place primarily by the small polaron hopping mechanism. The charge compensation for the divalent ion on the A-site is provided by the formation of Fe 4+ ions on the B-site (in preference to Co 4+ ions) and vacancies on the oxygen sublattice for low values of x. The large increase in the conductivity with x in the range from 0.6 to 0.8 is attributed to the substitution of Fe 4+ ions by Co 4+ ions. The Fe site has a lower small polaron site energy than Co and hence behaves like a carrier trap, thereby drastically reducing the conductivity. The non-linear behaviour in the dependence of log σT with reciprocal temperature can be attributed to the generation of additional charge carriers with increasing temperature by the charge disproportionation of Co 3+ ions.