María A. Callejas

Sensitivity of single wall carbon nanotubes to oxidative processing: structural modification, intercalation and functionalisation

Abstract The effect of oxidation on modification of single wall carbon nanotubes (SWCNTs) through successive purification steps has been studied. The efficient elimination of metal impurities has been followed by induced coupled plasma spectroscopy. Upon acid treatment, Raman spectroscopy clearly proofed that HNO3 molecules were intercalated into the bundles of SWCNTs. At the same time, SWCNTs also have suffered a high degree of degradation and defects were introduced. The subsequent thermal processes led to the removal of further defect carbon materials and to the almost complete de-intercalation of the HNO3 molecules. Changes in the structure of the SWCNT bundles have been observed by transmission electron microscopy. While bundles tend to separate upon acid treatment, after the complete purification process, the remaining SWCNTs tend to form thick bundles again. The existence of functional groups in the raw single wall carbon nanotubes material and their modification and almost complete removal after the final annealing step has been studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorption. Nitrogen adsorption isotherms analysed according to Brunauer–Emmet–Teller showed important changes in the pore volume and surface area through the purification steps.

Single-Walled Carbon Nanotubes as Electrodes in Supercapacitors

7 pages.-- PACS: 82.47.Uv; 82.45.Fk; 85.35.Kt; 68.43.Mn; 82.45.Yz; 82.45.Gj; 81.40.Gh; 78.30.Na

Synthesis and characterization of new polyaniline/nanotube composites

Abstract New polyaniline/nanotube (PANI/NT) composites have been synthesized by “in situ” polymerization processes using both multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) in concentrations ranging from 2 to 50 wt.%. Although no structural changes are observed using MWNTs above a concentration of 20 wt.%, the in situ synthesis results in electronic interactions between nanotubes and the quinoid ring of PANI leading to enhanced electronic properties and thus to the formation of a genuine PANI/MWNT composite material. On the other hand, using SWNTs favors the formation of inhomogeneous mixtures rather than of a homogeneous composite materials, independent of the SWNT concentration. X-ray diffraction, Raman and transport measurements show the different behavior of both classes of nanotubes in PANI/NT materials. The difficulties in the formation of a true PANI/SWNT composite are related to the far more complex structure of the SWNT material itself, i.e. to the presence of entangled bundles of SWNTs, amorphous carbon and even catalytic metal particles.

Modifications of single-wall carbon nanotubes upon oxidative purification treatments

A systematic characterization of single-wall carbon nanotube (SWCNT) material after successive purification steps, including reflux treatment with nitric acid, air oxidation, and annealing, has been performed. Inductively coupled plasma–optical emission spectroscopy shows that a considerable reduction of the metal impurities by up to 95% can be obtained by the nitric acid reflux treatment. During this process, Raman spectroscopy clearly proves that HNO3 molecules are intercalated into the bundles of SWCNTs. At the same time, SWCNTs have suffered a high degree of degradation and defects are being introduced. The subsequent thermal processes lead to the removal of further defect carbon materials and to the almost complete de-intercalation of the HNO3 molecules. Transmission electron microscopy reveals that the remaining SWCNT bundles tend to form thick bundles. Thus the applied purification process results in a high-purity SWCNT material with a drastically reduced content of metal nanoparticles and composed of large bundles of SWCNTs.

Thermal cracking of coal residues: Kinetics of asphaltene decomposition

Abstract An asphaltenic residue from a synthetic crude obtained by coal liquefaction was processed by thermal cracking. The kinetics of formation of oil + gas and coke (toluene-insoluble) from conversion of the asphaltenic fraction were determined. A three-lump model is proposed which considers parallel reactions for oil + gas and coke formation. Conversion data fitted second-order kinetics throughout for asphaltene conversion and oil + gas and coke formation. Delplot analysis indicated that under the most severe experimental conditions, asphaltenes and oils participated in secondary coke-forming reactions which are not included in the model. The activation energies ranged from 63.94 to 86.13 kJ mol −1 , the highest being that for asphaltene conversion. The highest asphaltene conversion, 55.9 wt%, was obtained at 475°C and 40 min reaction time. At short reaction times, the asphaltenic fraction converted to coke (∼30 wt%) was much less than that converted to oil (∼70 wt%) at the four temperatures used, whereas at long reaction times this trend was reversed, especially at 475°C. Structural analysis showed lower aromaticity and higher H/C and N/C ratios in the oil from the products than from the feed.

Hydrogen adsorption on a single-walled carbon nanotube material: a comparative study of three different adsorption techniques

Hydrogen adsorption measurements on mixed carbon material containing single-walled carbon nanotubes (SWNTs) obtained by the electric-arc method were carried out by three different techniques: a volumetric system, a gravimetric system and an electrochemical method. The found H2 gas adsorption capacity (volumetric and gravimetric) is very low, around 0.01 wt% at room temperature and pressure, increasing to 0.1 wt% at 20 bar. Electrochemical measurements show a slightly higher capacity (0.1–0.3 wt%) than volumetric and gravimetric data. The results obtained by the three different techniques are compatible within each other and they are also in good agreement with other previously reported data from different researchers.

Microwave single walled carbon nanotubes purification

A new single walled carbon nanotubes (SWCNTs) purification procedure has been developed; it consists in a combination of air treatment and acid microwave digestion leading to a high purity SWCNTs material; the procedure reaches high metal removal percentages and the operation time is drastically reduced compared to conventional acid reflux treatments.

Evolution of multiwalled carbon-nanotube/SiO2 composites via laser treatment

Multiwalled carbon nanotubes (MWNTs) were incorporated into SiO2 matrices for the first time by means of partial matrix melting caused by a Nd:YAG laser. Due to the rapid very short heating cycle of the laser MWNTs were detected being well incorporated in the silica matrix although temperatures well above 1200oC are reached. It was found that mainly the ability of heat absorption and heat conduction of the MWNTs are responsible for the sample heating. The composites were characterized using scanning electron microscopy, scanning transmission electron microscopy and high resolution electron energy-loss spectroscopy.

Kinetics of asphaltene hydroconversion: 2. Catalytic hydrocracking of a coal residue

Catalytic hydrocracking of an asphaltene-rich residue obtained by direct liquefaction of a Spanish subbituminous coal was carried out at different temperatures and reaction times. A kinetic study of asphaltene conversion was performed, and the chemical changes produced in the oil fraction during the process were studied by elemental analysis and 1H n.m.r. There was a high asphaltene conversion (max. 47 wt%) to oil + gas, and the data fitted first-order kinetics for asphaltene conversion and for oil and gas formation. The presence of catalyst and hydrogen considerably inhibited coke formation. Hydrogen was predominantly incorporated in the oil fraction, giving less aromatic structures, and cracking reactions mainly occurred in the asphaltenic fraction.

Kinetics of asphaltene hydroconversion: 1. Thermal hydrocracking of a coal residue

Abstract An asphaltenic residue from deasphalting a synthetic crude obtained by direct liquefaction of a Spanish subbituminous coal was submitted to several upgrading routes. This paper reports the results from thermal hydroprocessing. Solubility-based lumped kinetics with parallel reactions for oils, gases and coke formation are proposed. Conversion data fit second-order kinetics for asphaltene conversion and for oil, gas and coke formation. Coke formation was inhibited except under the harshest reaction conditions. Two pathways for coke formation are proposed: one as primary reaction product after an induction a period and the other as secondary product from a sequence of polymerization steps. The presence of hydrogen strongly inhibits the primary coke formation. Structural analyses show higher aromaticity of the oils from products obtained at 475°C than at 450°C, which supports the proposed condensation mechanism for coke production at high temperatures.