Study of the surface chemistry of modified carbon nanofibers by oxidation treatments in liquid phase.

Carbon nanofibers (CNF) were grown by thermocatalytic decomposition of methane. Their texture and surface chemistry were modified by different oxidation treatments with HNO3 at different concentrations or a mixture of HNO3-H2SO4 to optimise their ability of dispersing active metal particles, because this material will be used as electrocatalytic support for polymeric electrolyte fuel cells. The effect of liquid phase oxidation on the surface chemistry and the textural properties of the CNF was studied by temperature programmed desorption (TPD), scanning electron microscopy (SEM) and N2-physisorption. Moreover, their thermal stability was studied by temperature programmed oxidation (TPO). During oxidation treatments functional groups were created and their number was function of the oxidation treatment conditions. Results indicated that an increase in severity of the oxidation treatment produces an increase in the number of surface oxygen groups and in the thermal stability. However, a very severe treatment can destroy partially the structure of carbon nanofibers.

Characterization of carbon nanofibers grown over Ni and Ni-cu catalysts.

Carbon nanofibers were obtained by thermo-catalytic decomposition of methane at 700 degrees C over Ni and Ni-Cu catalysts prepared by different methods (co-precipitation, impregnation and fusion) and using either Al or Mg as textural promoter. Characterization of the carbon thus obtained was performed by N2 adsorption isotherms (BET surface area), temperature programmed desorption (TPD), temperature programmed oxidation (TPO), X-ray diffraction, Raman spectrometry, and electron microscopy SEM and TEM. The carbon obtained possesses high crystallinity and poor surface chemistry. The crystallinity is enhanced when using Mg as textural promoter and in the presence of copper. SEM and TEM examinations show that the fibers have fishbone structure and they grow generally from one nickel particle (tip growing) although there are some bidirectional growing. Copper-doping lead to the formation of thicker filaments and promotes the formation of bamboo-like structures. Catalyst particles higher than 100 nm do not promote the formation of nanofibers and the carbon deposits as uniform coatings.

Carbon nanofiber growth optimization for their use as electrocatalyst support in proton exchange membrane (PEM) fuel cells.

Carbon nanofiber (CNF) growth by catalytic decomposition of methane in a fixed-bed reactor was studied out to elucidate the influence of some important reaction conditions: temperature, space velocity and reactant partial pressure, in the morphological properties of the carbonaceous material obtained. The main objective is to synthesize a suitable carbonaceous nanomaterial to be used as support in platinum based electrocatalysts for Proton Exchange Membrane Fuel Cells (PEMFC) which improves current carbon blacks. High specific surface area is required in an electrocatalyst support since platinum dispersion is enhanced and so a cost-effective usage and high catalytic activity. Good electrical conductivity of carbon support is also required since the fuel cell power density is improved. With this proposal, characterization was carried out by nitrogen physisorption, XRD, SEM and TPO. The results were analysed by a factorial design and analysis of variance (ANOVA) in order to find an empirical correlation between operating conditions and CNF characteristics. It was found that the highest specific surface area and pore volume were found at 823 K and at a space velocity of 10 L gcat(-1) h(-1). The graphitic character of CNF, which is known to influence the electrical conductivity, presented a maximum value at temperatures between 923 K and 973 K. SEM images showed a narrow size distribution of CNF diameter between 40 and 90 nm and homogeneous appearance.

Pyrolysis-gas chromatography/mass spectrometry of fractions separated from a low-temperature coal tar: an attempt to develop a general method for characterising structures and compositions of heavy hydrocarbon liquids.

A low-temperature coal tar has been fractionated by column chromatography into acetonitrile, pyridine and 1-methyl-2-pyrrolidinone- (NMP) solubles. The tar and its fractions have been examined by pyrolysis-gas chromatography/mass spectrometry (GC/MS). Fractionation by planar chromatography was also carried out for purposes of comparison. Molecular masses of the fractions were estimated by size-exclusion chromatography (SEC), and bulk structural characterisation was carried out by (13)C-NMR and UV-fluorescence spectrometry. SEC showed that the fractions shifted to progressively shorter elution times (higher apparent masses) with diminishing solubility, i.e. from acetonitrile to NMP solubles. UV-fluorescence spectra showed parallel shifts to longer wavelengths and lower fluorescence quantum yields, indicating increasing sizes of aromatic ring systems and increasingly complex molecules. GC/MS analysis of the tar showed alkanes from C10 to C32 and extensive series of alkylated aromatics, phenols, indenes, naphthalenes, phenanthrenes and fluoranthenes. Pyrolysis-GC/MS results for the acetonitrile solubles closely resembled the data for the tar sample, with extensive series of alkylated benzenes, phenols and naphthalenes as well as alkanes from C16 to C28. The pyridine-soluble fraction showed no significant aromatic pyrolysis products and only relatively weak signals for alkanes between C16 and C27. The NMP-soluble fraction showed even less overall signal, with no significant aromatic components and weak signals for alkanes between C21 and C25, even though (13)C-NMR analyses showed that approximately half of the carbon detected was aromatic. The aliphatics are assumed to provide bridging structures between polycyclic aromatic (PCA) ring systems.

Study of the polymerization of anthracene oil with AlCl3 by chromatography and related techniques.

The structure and composition of products from the reaction of anthracene oil with anhydrous AlCl3 have been examined. Size-exclusion chromatography has been carried out using a column with polystyrene-polydivinylbenzene as stationary phase, 1-methyl-2-pyrrolidinone at 80 degrees C as eluent and variable-wavelength UV-absorption detection. This system provides a chromatogram of the sample with several peaks. Molecular masses corresponding to these peaks were estimated using a calibration curve obtained with polycyclic aromatic hydrocarbon standards, ranging from 92 to 532 u. The most abundant compounds of the substrate were dimers and trimers of the main anthracene oil components. These results are corroborated on a qualitative level by synchronous UV-fluorescence spectra.

Structural characterisation of Baltic amber and its solvent extracts by several mass spectrometric methods.

A sample of Baltic amber ( approximately 40 million yrs old) has been extracted using pentane, toluene and 1-methyl-2-pyrrolidinone (NMP). The relationship between solubility characteristics of the extracts in relation to molecular mass and chemical makeup has been investigated. The extracts were first characterised by (13)C-NMR spectrometry, size exclusion chromatography (SEC) and UV-fluorescence spectroscopy. The fractions differed less in terms of chemical structural features than they did in terms of molecular mass. This contrasts markedly with data on fractions of coal-derived liquids, but parallels results from petroleum-derived vacuum residues. In SEC, the toluene soluble/pentane insoluble fraction gave a peak for high mass material at about 67 000 u. Material excluded from the column porosity in this fraction and in NMP solubles eluted between 8 and 11 min, corresponding to polystyrene masses between 200 000 and several million u. A column with a larger pore size distribution was calibrated using polystyrene and polymethylmethacrylate standards with detection by a light-scattering evaporative analyser. The largest polystyrene standard (15.4 million u) eluted at 13.4 min, similar to that of the earliest eluting amber-derived material in the NMP solubles fraction. Results from probe-MS and pyrolysis-GC/MS have been used to confirm the similarity of chemical structures of the three solubility fractions. Broadly, low mass ions appear to correspond to the various monomeric units of structures present in the amber, the higher mass ions to dimer units and the molecular ions to the different combinations of three or more monomeric units. The main monomer groups have been identified in detail, showing a situation very different from that of coal-derived materials, where the sizes of aromatic ring systems increase with molecular size.

Pyrolysis-gas chromatography/mass spectrometry of a coal extract and its fractions separated by planar chromatography: correlation of structural features with molecular mass

The structural characterisation of a coal liquefaction extract and its three fractions separated by planar chromatography has been described. Size exclusion chromatography showed the molecular mass distributions to become progressively larger with decreasing mobility on the plate. UV-fluorescence spectroscopy of the fractions indicated parallel increases in the sizes of polynuclear aromatic ring systems. Analysis by probe-mass spectrometry of the 'whole' coal extract showed the expected array of small polynuclear aromatic groups extending to m/z 450. The probe mass spectra of the lightest fraction ('mobile in pyridine and acetonitrile') showed similar features, except for effects due to vacuum drying to remove solvent. In sharp contrast, the two heaviest fractions ('mobile in pyridine and immobile in acetonitrile' and 'immobile in pyridine') showed no significant ions other than those from residual NMP solvent (m/z 98 and 99). Pyrolysis-gas chromatography/mass spectrometry of these two heaviest fractions showed only traces of aromatic compounds or fragments. The aromatic pyrolysis products of these fractions were too large and involatile to pass through the GC column. The major components observed in the pyrolysis-gas chromatography/mass spectrometry of the two heavy fractions were alkanes and alkenes, ranging between C10-C25. Since none of the samples contained free alkanes, alkenes or cycloalkanes before pyrolysis, they were generated during the pyrolysis step. The shifts of UV-fluorescence spectral intensity to shorter wavelengths with decreasing size indicated by size exclusion chromatography (SEC) provide direct evidence of differences in structure with changing molecular mass. This evidence strongly suggests that species identified as being of large molecular mass in this extract sample are not composed of molecular aggregates. It remains difficult to establish whether and when it would be legitimate to invoke molecular aggregates to explain the large molecular masses (MMs) identified here and in other work. Copyright 2000 John Wiley & Sons, Ltd.