This paper describes the elution behavior of model compounds in a polystyrene-divinylbenzene SEC column with NMP as mobile phase, operating at high temperature (80°C). Model compounds covering polycyclic aromatic hydrocarbons, azaarenes, and other nitrogen and polar compounds have been studied. Most of the standard compounds eluted within one minute of the expected time indicated by the polystyrene calibration. The fractionation of a complex coal-derived sample (a coal tar pitch) using the same column has been achieved, with subsequent reinjection and analysis of the fractions by heated-probe mass spectrometry and UV-fluorescence. The probe-MS experiments were performed in order to show that the material of the excluded peak does not consist of small and polar molecular species, rather than larger-molecular mass material. All the fractions were reinjected and some of them gave small extra peaks in the SEC chromatogram. The earliest fractions showed very weak UV-fluorescence indicating the presence of very high molecular mass material. The later-eluting fractions showed relatively strong fluorescence intensities with the position of the fluorescence intensities shifting to shorter wavelengths as the SEC elution time increased, indicating that the smaller polynuclear aromatic ring systems elute in the late fractions. Probe mass spectra showed that only those fractions isolated from SEC at the long elution times gave signals characteristic of aromatic and nitrogen compounds; the molecular mass range decreased with increasing elution time. Since the structures of the material excluded from the column or even that near the exclusion limit are not known, it is impossible to select standard materials or standard polymeric compounds to represent the molecular mass range of coal-derived liquids. For this reason, we believe that the polystyrene calibration represents the most reasonable compromise for SEC in NMP solvent in our system.

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A coal tar pitch from the high temperature coking of coal has been fractionated by planar chromatography into fractions immobile in pyridine, mobile in pyridine but not mobile in acetonitrile, and mobile in both solvents. Fractions have been examined by pyrolysis‐gas chromatography/mass spectrometry at 770 and 1300 °C, and by nuclear magnetic resonance (NMR) methods (1H in solution and 13C (CPMAS TOSS) in the solid state). The fraction mobile in both solvents resembled the whole pitch in that the pyrolysis products were all polycyclic aromatics. The fraction mobile only in pyridine and the immobile fraction both gave alkene fragments as the most abundant pyrolysis products with polycyclic aromatics of low intensity only. The pyridine mobile and immobile fractions were shown by size exclusion chromatography with ultraviolet (UV) absorbance detection and by UV fluorescence spectroscopy to consist of large aromatic clusters and of large molecules. It is likely that the large aromatic clusters did not pyrolyse into fragments able to pass through the GC column. NMR methods confirmed the apparent trend of increasing aliphatic content with increasing immobility in thin layer chromatography. Copyright © 1999 John Wiley & Sons, Ltd.

The catalytic layers of a low temperature fuel cell must present a sufficient electronic conductivity for a high overall performance and, in this sense, the electrocatalyst support plays an important role. However, regarding carbon nanofibers as support, it is necessary to chemically treat their surface to improve both metal deposition and anchorage, which unfortunately affects negatively the electronic conductivity. The effects of functionalization of carbon nanofibers (CNFs) on their electronic conductivity, in addition with their physicochemical properties have been studied. Oxygen surface groups have been created on the surface of three CNFs with different properties, following three oxidation treatments with diverse severity. Whereas some important properties for their application as electrocatalyst support are not significantly modified after functionalization (texture, crystalline structure, etc.), other properties like the electronic conductivity are affected depending on the extent of the process. Refluxing at boiling temperature causes the largest reduction of electronic conductivity, of ca. 80% when using a mixture of nitric and sulfuric acids and of ca. 50-60% when using nitric acid. On the other hand, the electronic conductivity of certain CNFs is not negatively influenced after treatment at room temperature with the mixture of nitric and sulfuric acids.

Variations in molecular mass distributions observed by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry as a function of changes in the ion accelerating voltage and composition of the matrix have been investigated, using the pyridine-insoluble fraction of a coal tar pitch. With increasing ion extraction voltages (10, 20 and 30 kV) and in the absence of added matrix, spectra clearly showed increases in intensity, particularly at higher masses. The use of higher ion extraction voltages may be considered as providing a more complete inventory of ionized species, apparently significantly enhancing the kinetic energy imparted to larger molecular weight materials. Magnitudes of molecular masses at maximum ion intensity observed using a set of different matrices may be ordered as follows: m/z 1500 [dimethoxy-4-hydroxycinnamic (sinapinic) acid], m/z 1400 (2,5-dihydroxybenzoic acid), m/z 900 [2-(4-hydroxyphenylazo)-benzoic acid], m/z 900 (α-cyano-3-hydroxycinnamic acid), m/z 300 (9-anthracenecarboxylic acid) and m/z 300 (no matrix and a sample loading comparable with the with-matrix experiments). Maximum ion intensities, in the absence of matrix, were found at values which varied from about 300 m/z up to about m/z 1500 as sample loading varied from very low (i.e. suitable for with-matrix operation) up to a visibly black spot. In the absence of a quantitatively based criterion for distinguishing between signal and instrument noise at high mass, accurate high mass limits were not available. However, in each case, ion signals could clearly be observed at values greater than m/z 100 000. The present MALDI spectra do not necessarily show the highest molecular mass materials present in the sample. In view of the observed changes in the spectra with ion extraction voltage and matrix composition, it seems conceivable that higher voltages and/or specific matrices, other than those used in the study, may lead to the detection of larger molecules. © 1997 by John Wiley & Sons, Ltd. Received 23 December 1996; Revised 3 February 1997; Accepted 25 February 1997 Rapid. Commun. Mass Spectrom. 11, 638-645 (1997 Characterization by laser desorption mass spectrometry has considerably extended the range of molecular masses identified in coal-derived liquids. The work was initially undertaken in an attempt to provide independent confirmation for the identification of molecular masses (MMs) up to 4-6000 Da found by size-exclusion chromatography (SEC).1-5 During this preliminary stage of the study, MMs up to the limit of a LIMA (laser ionization mass analysis) instrument (m/z 12 000) were detected in liquefaction extracts and in a coal tar pitch. [6][7][8][9] In these experiments, the reflectron flight path caused loss of detection at high mass but the instrument could not be operated in linear mode. Experiments were conducted at an ion extraction voltage of 20 kV and with a defocused primary beam to prevent the breaking up of sample molecules due to the power of the laser. However, lacking a spectrum additio...

Pt-Sn catalysts supported on carbon, with different Pt-Sn atomic ratios (3:1, 1:1, 1:3), were prepared through the formic acid method (FAM) and supported on different carbon supports (nanofibers and carbon blacks) to study their behavior toward CO stripping and ethanol oxidation reaction (EOR). PtSn/C catalysts and supports were physicochemically characterized by XRD, EDX, XPS, TEM and TPD. Good particle dispersion onto the carbon support, similar particle sizes (around 4-5 nm) and the presence of tin oxides were observed in all cases. Sn insertion favored the development of the Pt 3 Sn 1 phase and the presence of higher oxidation states of both metals in the catalyst. Ethanol and adsorbed CO oxidation was studied at these materials both in acid and alkaline media, by linear sweep voltammetry and chronoamperometry. Higher EOR current densities were obtained with the increase in the amount of Sn in the samples, been those materials supported on CNF the ones with the best CO tolerance and catalytic activity toward this reaction. As expected, a great improvement of the EOR activity was found in alkaline media compare with sulfuric acid media, providing good expectative for these materials as catalysts for alkaline direct ethanol fuel cells.

The oxidations of carbon monoxide and formic acid at ultrathin Pd layers grown on Au nanoparticles were studied as a function of Pd thickness. Pd shells with thickness between 1 and 10 nm were grown on 19 nm Au nanoparticles by chemical reduction of H 2 PdCl 4 with ascorbic acid. High-resolution transmission electron microscopy and X-ray diffraction confirm the coreÀshell configuration of the nanostructures. While the synthesis of pure Pd nanostructures led to a rather amorphous material, Pd nanoshells exhibited a polycrystalline structure confirming that Au nanostructures act as templates for Pd growth. Three-dimensional assemblies of nanoparticles were generated by alternate electrostatic layer-by-layer adsorption steps, involving poly-L-lysine and colloidal dispersions. Electrochemical studies in H 2 SO 4 containing electrolyte solution demonstrate that CO coverage and anodic stripping potential are affected by the thickness of Pd nanoshells. In addition, the faradaic current density associated with HCOOH oxidation significantly increases with increasing Pd thickness. The thickness-dependent reactivity of Pd nanoshells is discussed in terms of lattice strain relaxation.

A sample of Athabasca bitumen was fractionated by preparative size-exclusion chromatography (SEC), and the molecular-weight distribution of the five fractions and the original sample were determined by SEC in NMP solvent using a calibration based on polystyrene standards, by 252 Cf plasma desorption mass spectrometry (PDMS), and by laser desorption and matrix-assisted laserdesorption mass spectrometry (LDMS and MALDI) and the average molecular weights were measured by vapor pressure osmometry (VPO). A comparison of the results shows that VPO, MALDI, and SEC gave similar results while PDMS showed the same trend. The molecular-weight distribution of this 525+ °C fraction is continuous up to and perhaps beyond a molecular weight of 15 000.