Preliminary studies on the aromaticity of Australian coals: Solid state n.m.r. techniques

The aromaticities of samples from nine Australian coal seams, including pairs of hand-picked vitrains and durains, have been determined by ¹³C cross-polarization n.m.r. spectroscopy with magic angle sample spinning. The results clearly show that the aromaticity (f_a) of the coals increases with increase in vitrinite reflectance and carbon content and decrease in atomic H/C ratio. For a given coal seam, durain (inertinite-rich coal) has a higher f_a value than vitrain (vitrinite-tich coal). The trends for carbon content and atomic H/C are in good agreement with results from North American coals, although the aromaticities of Australian coals obtained in this study appear to be slightly lower than some of those reported for North American coals of similar carbon content.     

Solid state magnetic resonance spectra of Illinois No. 6 coal and some reductive alkylation products

Illinois No. 6 coal and its reductive methylation and butylation products have been studied by magnetic resonance techniques. Conventional CP-MAS¹³Cn.m.r. spectroscopy indicates that 62% of the carbon atoms in the coal are aromatic and that about 6% of the carbon atoms are carbonyl. Esters are more abundant than carboxylic acids. The resonances of methoxy groups and other novel etheric carbon atoms are apparent in the high field region. Dipolar dephasing experiments suggest that methyl carbon atoms constitute no more than 16% of the carbon, methylene and methine carbon atoms about 14% and quaternary aliphatic carbon atoms about 2%. The dipolar dephasing experiments in conjunction with previous work also permit estimates of the hydrogen atom distribution. The THF-insoluble products obtained in the reductive alkylation reactions with¹³C-enriched alkylating agents contain paramagnetic and ferromagnetic substances that adversely influence the solid state n.m.r. spectra. However, good ¹³C n.m.r. spectra were obtained after these substances were extracted with aqueous hydrochloric acid. The O:C alkylation ratios are 1.2 and 1.0 for methylation and butylation, respectively. Dipolar dephasing experiments establish that the decay constants of functional groups in the whole coal and of C- and O-methyl-¹³C and C- and O-butyl-1 -¹³C nuclei in the labelled coal molecules are very similar to those of reference compounds. These findings suggest that the decay constants measured for the ¹³C nuclei in coals and coal-derived solids provide reliable information about their degree of substitution.     

Structural changes in humic acids during the coalification process

Humic acids from four coals, varying in rank from peat to subbituminous coal, have been characterized by elemental analysis, acidic groups, molecular weight, electrophoresis and visible, FT-i.r. and CP/MAS ¹³C n.m.r. spectroscopy. The humic acids increase in carbon content, molecular weight, condensation degree and aromaticity (f_a) with increasing maturation of the parent coals, while the oxygen content decreases with a loss of oxygen functional groups. The presence of lignin-like polymers, poly-saccharides and peptidic materials in humic acids from peat was established using i.r. and ¹³C n.m.r. spectroscopy. The structural changes observed in humic acids are in agreement with the recognized coalification theory and tend to support the hypothesis of condensation of humic acids into insoluble humin of coal.     

Selective saturation as an aid for carbon-13 CP/MAS n.m.r. analysis of coals at high frequencies

A selective saturation experiment is described and applied to selective elimination of ¹³C high resolution solid state n.m.r. signals and of their spinning sidebands, as well for sharp lines as for broad bands. A proper adjustment of experimental conditions permits selective saturation of the aromatic carbon signals of coal samples and eliminates unwanted overlapping of aromatic sidebands with the aliphatic region of the spectrum at high working frequencies. The described procedure seems to be especially useful for qualitative and quantitative high frequency n.m.r. investigations of coals having a distribution of short T₂ relaxation times.     

Substrate temperature effects on the microhardness and adhesion of diamond-like thin films

Diamond-like films were deposited on silicon substrates by r.f. plasma-enhanced chemical vapor deposition from gas methane. In this study, the substrate temperature, T_S, was varied in a wide range from 20 to 370°C while maintaining fixed other important process parameters such as r.f. power (70 W) or pressure (2.5 Pa). The increase of T_S causes an increase of the sp²/(sp²+sp³) bonded carbon ratio and a decrease of the hydrogen content. These changes produce a great modification of the mechanical properties: microhardness, friction coefficient and adhesion. The variations of mechanical properties with T_S correlate well with the sp²/(sp²+sp³) bonded carbon ratio and the hydrogen content in the films showing a gradual transformation of the diamond-like structure into a more sp²-rich one.     

Quantitative FT 13C n.m.r. of solvent-refined coals using sym-triazine as solvent

Aromaticities (f_a) determined for several solvent-refined coals (SRC) by ¹³C n.m.r. in sym-triazine solution and by the Brown and Ladner ¹H n.m.r. technique have been found to be the same within experimental error (0.01). Use of sym-triazine to determine the f_a of the soluble and insoluble fractions of a SRC in several solvents showed that the soluble fraction reflected reasonably accurately the properties of the whole sample for dioxan, carbon disulphide, chloroform and benzene extracts. The properties of the insoluble fraction varied only slightly. A more detailed analysis of the aromatic region of the ¹³C n.m.r. spectrum has led to some useful structural relations.     

Structural dependence of corrosion resistance of amorphous carbon films against nitric acid

To investigate the structural dependence of the corrosion resistance of amorphous carbon (a-C:H) films, three different types of a-C:H films etched by nitric acid were evaluated using a surface plasmon resonance (SPR) device with a multilayer structure consisting of an a-C:H layer on Ag. Two non-hydrogenated amorphous carbon (a-C) films and one hydrogenated a-C:H film were synthesized to estimate the effects of the sp²/sp³ ratio and hydrogenation, respectively. A flow cell for the introduction of nitric acid solution was placed on the amorphous carbon layer of the multilayer structure. A 0.3 mM nitric acid solution was used in the etching tests. The Kretschmann configuration was used for SPR measurement, and the SPR angle was determined as the angle with minimum reflectivity. The SPR angle decreased with increasing duration of nitric acid injection into the flow cell, indicating that the film was corroded by the nitric acid. The thickness of the films was calculated from the SPR angle. The rates of decrease in the thickness were 2.2, 0.8, and 1.6 nm/h for the a-C films with lower and higher sp² contents and the 17 at.% hydrogenated a-C:H film, respectively. Although the hydrogen content had little effect on the rate of change in the film thickness, the film thickness clearly decreased with decreasing sp²/sp³ ratio. These results indicate that the sp²/sp³ ratio is an important factor determining the chemical resistance to nitric acid solution.     

Oil-shale analysis by CP/MAS-13C n.m.r. spectroscopy

Solid-state ¹³C n.m.r. (CP/MAS-¹³C n.m.r.) spectroscopy provides a direct method for estimating potential oil yields of oil shale formations. Relative aliphatic resonance areas correlate linearly with oil yield and provide a method for oil yield estimation that obviates the need to determine weight per cent organic carbon for each specimen. This direct measurement is performed using an internal area standard, the carbonyl resonance of N-(2-¹³C-propanonyl)-N,N,N-trimethylammonium chloride, to monitor spectrometer sensitivity. Oil shale samples obtained as a function of depth at a site in the Mahogany Zone of the Green River Formation show a near-constant aliphatic carbon fraction, f_al ≡ (1?f_a), and a twofold, nonlinear variation in oil yield over the vertical dimension of the sampling. Aliphatic carbon resonance band shape changes among these samples are interpreted qualitatively as reflecting a two component mixture composed of the condensed alicyclic structures which link together to form the kerogen matrix and an n.m.r.-distinct but not necessarily chemically distinct contribution from normal-long chain hydrocarbon residues.     

Oxidative dehydrogenation of ethylbenzene to styrene over ultra-dispersed diamond and onion-like carbon

The catalytic properties of sp³-hybridized ultra-dispersed diamond and sp²-hybridized onion-like carbon in the oxidative dehydrogenation of ethylbenzene to styrene were investigated, highlighting the structure sensitivity of the reaction. The sp³-carbon led initially to C-C cleavage and benzene formation, while a switchover of the main reaction pathway into the styrene formation occurred with time on stream due to the formation of surface sp² carbon, required for the selective styrene formation. This was confirmed by the behavior and the high stable styrene selectivity shown by onion-like carbons. High temperature oxygen pre-treatment created catalytically active species at the sp² carbon surface, confirming that a high thermal stability carbon-oxygen complex was the active surface site for forming styrene.     

Investigation of the chemical structures of coking and non-coking coals in original and reductively alkylated solid states via 13C n.m.r. and i.r. spectroscopies

Chemical structures of four Turkish coals in original and reductively alkylated forms were investigated in the solid state by CP/MAS ¹³C n.m.r. and i.r. spectroscopies. Dilatation properties of these samples were also determined. It was observed that, while the aliphatic parts of the coking coals are mainly composed of short and straight alkyl groups and alicyclic structures, the aliphatic parts of the non-coking coals are to a larger extent composed of branched chains and/or alicyclic structures. It was concluded that ether bonds linking aromatic units are more prominent in the coking coals, whereas in the non-coking coals aromatic-O-aliphatic and/or alicyclic ethers dominate. The degree of condensation of aromatic structures was found to be higher in the coking coals, and the extent of reductive alkylations was higher in comparison with the non-coking coals. Non-coking coals were found to be alkylated preferentially at their phenolic oxygens. Difference spectra were found to be very useful in following organic chemical structural changes that accompany reductive alkylation.     

On the structure of polybutadiene: 4. 13C n.m.r. spectrum of polybutadienes with cis-1,4-, trans-1,4- and 1,2-units

¹³C n.m.r. spectra of polybutadienes with different contents of cis-1,4- and 1,2-units are assigned in the resonance region of the cis and trans carbon double bond. The observed signals are assigned to configurational triads.     

The role of the sp2:sp3 substrate content in carbon supported nanotube growth

We report the growth of vertically-aligned nanotube forests, of up to 0.2 mm in height, on an 85:15 sp²:sp³ carbon support with Fe catalyst. This is achieved by purely-thermal chemical vapour deposition with the catalyst pretreated in inert environments. Pretreating the catalyst in a reducing atmosphere causes catalyst diffusion into the support and the growth of defective tubes. Other sp²:sp³ compositions, including graphite, tetrahedral amorphous carbon, and pure diamond, also lead to the growth of defective carbon morphologies. These results pave the way towards controlled growth of forests on carbon fibres. It could give rise to applications in enhanced fuel cell electrodes and better hierarchical carbon fibre-nanotube composites.     

Atmospheric plasma deposition of diamond-like carbon coatings

The atmospheric pressure plasma-enhanced chemical vapor deposition of diamond-like carbon (DLC) has been investigated. The DLC coatings were grown with a mixture of acetylene, hydrogen and helium that was fed through a linear plasma source. The plasma was driven with radio frequency power at 27.12 MHz. Deposition rates exceeded 0.10 µm/min at substrate temperatures between 155 and 200 °C. Solid-state carbon-13 nuclear magnetic resonance revealed that the coatings contained approximately 43% sp²-bonded carbon and 57% sp³-bonded carbon. Coefficient of friction values for the coatings were found to be 0.24 ± 0.02, which is within the range observed for vacuum deposited DLC.     

Studies on the structure of coals. 2. The valence state of carbon in coal

A specific oxidation method for estimating the valence states of carbon in coal is described. The method, which is also an important tool for more generally exploring the structure of coal, is based on the fact that sodium hypochlorite is a very selective oxidant capable of differentiating between Sp³-carbon and Sp²-aromatic carbon. Proceeding from this recognition it is estimated that between 55 and 60% of the total carbon in coal exists in the Sp³-valence state. This estimate is much higher than the accepted ‘hydroaromaticity’ of coals.     

Determination of ether oxygen in coals by 13C CP/MAS n.m.r. spectrometry and acetylation

The amount of aliphatic and aromatic ethers in coals with different carbon contents were determined by combined use of ¹³C CP/MAS n.m.r. spectrometry and the acetylation method. The role of etheric oxygen in the depolymerization of coals was re-evaluated. Etheric oxygens were distributed more abundantly in aromatic ether rather than in aliphatic ether and amounted to 3.7% relative to carbon. The cleavage of ether linkages in early stages of depolymerization of coal led to the formation of asphaltene or preasphaltene.     

Grain size dependent mechanical properties of nanocrystalline diamond films grown by hot-filament CVD

Nanocrystalline diamond (NCD) films with a thickness of ~ 6 µm and average grain sizes ranging from 60 to 9 nm were deposited on silicon wafers using a hot-filament chemical vapor deposition (HFCVD) process. These samples were then characterized in order to identify correlations between grain size, chemical composition and mechanical properties. The characterization reveals that our films are phase pure and exhibit a relatively smooth surface morphology. The levels of sp²-bonded carbon and hydrogen impurities are low, showing a systematic variation with the grain size. The hydrogen content increases with decreasing grain size, whereas the sp² carbon content decreases with decreasing grain size. The material is weaker than single crystalline diamond, since both stiffness and hardness decrease with the reduction in crystal size. These trends suggest gradual changes in the nature of the grain boundaries, from graphitic in case of 60 nm grain size material to hydrogen terminated sp³ carbon in 9 nm grain size material. The films exhibit low levels of internal stress and free-standing structures with a length of several centimeters could be fabricated without noticeable bending     

Chemical characterization of solid coals through magic-angle 13C nuclear magnetic resonance

Recently developed ¹³C n.m.r. techniques for high resolution in solids are applied to native coals. The quantity and distribution of hydroxyl groups in the samples are determined by combining chemical modification through acetylation with the n.m.r. measurements. Spectral subtraction isolates the reaction product resonances. Spectroscopic separation of protonated and non-protonated carbons through pulse programming techniques is also demonstrated. Finally, the susceptibility of the aliphatic carbons in coals to low-temperature oxidation is shown clearly by subtraction of spectra of the oxidized and unoxidized materials.     

Clustering in nanostructured carbon: Evidence of electron delocalization

The electronic properties of disordered carbon-based materials can be discussed in terms of the clustering of the sp² carbon phase and delocalization of the electron wave function. In smooth amorphous carbon thin films this results in a mixed phase material of conductive sp² clusters embedded in an electrically insulating sp³ matrix. The delocalization of the electron wave function associated with the sp² clusters is shown to play an important role in understanding many of the electronic and optical properties of the films. It is demonstrated that the extent of the electron delocalization and clustering can be estimated using magnetic resonance methods. Evidence for delocalization in a range of carbon-based materials such as diamond-like carbon thin films produced by chemical vapour deposition, nanostructured carbon produced by pulsed laser ablation and ultrananocrystalline diamond is presented.     

Field emission properties of screen-printed activated carbons

The electron emission property of various screen-printed activated carbons was studied. Each activated carbon was characterized with a thick film, which was made of photosensitive paste. The activated carbon made of coal showed the highest emission current density (J) of ∼350 (μA/cm²) and with the lowest threshold electric field (E_th) ∼ 4.2 (V/μm). These results were explained by graphitization factor and sp²/sp³ ratio. The data clearly show that the carbon content in the paste is one of the main factors affecting the morphology of the film surface and field emission properties. As the carbon increases, J increases and E_th decreases.     

Aromatic products of 340 °C supercritical-toluene extraction of two Turkish lignites: an n.m.r. study

Fractions of Elbistan and Seyitomer (Turkish) lignites, extracted with supercritical toluene at 340 °C and 8 MPa, have been separated by solvent extraction and silica-gel chromatography. Analyses by n.m.r. and i.r. spectroscopies and other methods have been combined in structural-analysis schemes to yield information about the average molecule in aromatic extracts. Carbon aromaticities, f_a, derived from 22.63 MHz ¹H-decoupled pulse Fourier-transform (PFT) ¹³C-n.m.r. are more widely spread for Elbistan (0.34–0.56) than for Seyitomer (0.40–0.43), and are lower than for supercritical-gas (SCG) products from bituminous coals. ¹³C-n.m.r. also reveals the presence of aromatic ether-O in polar fractions. Narrow aromatic signals in 100 MHz ¹H-n.m.r. spectra suggest the presence of single-aromatic-ring average structures. In the hexane-soluble aromatics, 27% (Elbistan) and 29% (Seyitomer) of the available sites are substituted by alkyI groups, some of which are at least eight carbon atoms long; the hexane-soluble polar and asphaltene/asphaltol fractions contain fewer such groups.