B. Pilawa

In situ EPR spectroelectrochemical studies of paramagnetic centres in poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-butylenedioxythiophene) (PBuDOT) films

In situ EPR spectroelectrochemical studies of two members of poly(3,4-alkylenedioxythiophene) family of polymers, namely poly(3,4-ethylenedioxythiophene) – PEDOT and poly(3,4-butylenedioxythiophene) – PBuDOT have been performed with an aim to investigate the effect of doping and subsequent dedoping on charge carriers in these polymers. The objectives of investigations were to characterise the type of paramagnetic centres appearing in the polymer and to observe the influence of doping on their concentration and other spectroscopic properties. It was found that the evolution of spins in both polymers upon doping follows a trend similar to that observed for other thiophene-based polymers. The characteristic points of these curves are different however for each polymer. The maximum spin concentration in PEDOT takes place at 0.2 V and for PBuDOT it is at 0.35 V. While generally similar in behaviour during doping, the behaviour of these two polymers upon dedoping is markedly different. For PBuDOT the concentration of spins follows a more or less reverse path of the doping curve whereas in PEDOT a hysteresis of concentration of spins appears. Similar trends are observed for the signals’ ΔBpp widths. Both polymers display distinct narrow EPR lines at the end of the reduction half-cycle suggesting that a noteworthy concentration of spins exist in them even in the fully reduced (dedoped) state. Analysis of the EPR spectra of the polymers in their doped states revealed their complex character. It was found that these complex spectra could be decomposed into two separate components – one Gaussian and one Lorentzian. Based on EPR microwave saturation studies the Gaussian component was attributed to highly mobile polarons which are confined in isolated spin packets and the Lorentzian one to delocalised less mobile polarons, homogeneously distributed within the material, which thus could be associated with the electrons of conductivity.

Oxidation of demineralized coal and coal free of pyrite examined by EPR spectroscopy

The influence of oxidation on complex paramagnetic centres system of demineralized coal and coal free of pyrite was compared. Polish orthocoking coal with a carbon content of 87.8 wt% was studied. This coal was oxidized with nitric acid (HNO3), peroxyacetic acid (PAA) and in O2/Na2CO3 system. Multi-component structure of X-band EPR spectra of the coal samples was numerically analysed. The lineshape and the parameters of the component lines: linewidths and g-factors, were determined. Concentrations of paramagnetic centres were measured. The three groups of paramagnetic centres belonging to different molecular units were found in the studied samples. They were responsible for broad Gauss (DBpp: 0.49 ‐ 0.84 mT), broad Lorentz 1 (DBpp: 0.18 ‐ 0.35 mT), and narrow Lorentz 3 (DBpp: 0.04‐ 0.08 mT) EPR lines. Properties of paramagnetic centres of the simple molecular units with broad Gauss and Lorentz 1 lines were changed during demineralization of coal and after pyrite removing from coal. g-Values of Gauss and Lorentz 1 lines increased, Lorentz 1 lines were broadened and concentrations of paramagnetic centres with Lorentz 1 lines increased. Oxidation of both demineralized coal and coal free of pyrite with nitric acid led to the highest decrease of the concentration of paramagnetic centres with Gauss lines, narrowing of these lines and increase of g-factor. The higher effects of oxidation on paramagnetic centres responsible for broad lines were observed for coal free of pyrite. Paramagnetic centres with narrow Lorentz 3 lines belonging to multi-ring aromatic units in demineralized coal and coal free of pyrite were not susceptible for oxidation. q 2002 Elsevier Science Ltd. All rights reserved.

Numerical analysis of EPR spectra of coal, macerals and extraction products

Abstract The multi-component structure of the EPR spectra of Polish medium metamorphosed coal, its macerals and their pyridine extractrion products were analyzed. The bes numerical approximation of the EPR spectra for exinite, vitrinite and their extraction residues was obtained when two Lorentz and one Gauss curve were chosen for calculation. The EPR spectra of inertinite and its residue are superposition of two Lorentz lines. The EPR spectra of the extracts are fitted by single Lorentz lines. Four groups of paramagnetic centres which give the EPR signals with different ranges of linewidths are present in the macerals and the extraction products. Paramagnetic centres with the narrow EPR lines are present not only in inertinites, but also in exinites and vitrinites. Each of the four types of paramagnetic centres is present in macromolecular and molecular part of the macerals. As was expected, four component lines were detected for the EPR spectra of the coal and its residue. Paramagnetic centres responsible for the broad lines are located on structures consisting of a few aromatic rings. Paramagnetic centres of large aromatic structures are responsible for the narrow lines.

Microwave saturation of e.p.r. spectra of vitrinite and products of its pyridine extraction

Abstract The influence of microwave saturation on e.p.r. lines of vitrinite and products of its pyridine extraction was investigated. E.p.r. spectra taken at higher microwave power have clearly revealed the complex structure of resonance absorption lines both for vitrinite and the residue. The best numerical approximation of e.p.r. spectra for both samples was obtained when two Lorentz and one Gauss curve were chosen for calculation. The values of the g -factor, the widths of the components and their fraction in the total spectrum were determined. The e.p.r. spectrum of the pyridine extract of vitrinite was fitted with a single line of Lorentz shape.

E.p.r. studies of thermal decomposition of vitrinite

Abstract Vitrinite from Polish medium-rank coal (85.6 wt% C) heated in an inert atmosphere at 300–650°C was investigated by electron paramagnetic resonance in the X-band (9.3 GHz). The e.p.r. spectra of the vitrinite, recorded using different microwave powers, were approximated by superposition of a Gauss line and two Lorentz lines. The g factor, linewidth of the components and their fraction in the total spectrum were determined. A sharp increase in total concentration of paramagnetic centres after heating above 550°C was observed. Strong quenching and production of paramagnetic centres with broad Gauss and broad Lorentz lines above 550°C were detected. The heating of the vitrinite only slightly influenced the amount of paramagnetic centres with narrow Lorentz lines. The saturation behaviour of the all measured e.p.r. lines depended on heat-treatment temperature.

Microwave saturation of EPR spectra of oxidised coal

Microwave saturation of multi-component EPR spectra of oxidized lignite Mequinenza (Spain) with a carbon content of 65.1 wt % and with a high sulphur content of 10.3 wt % was studied. The coal was oxidized with nitric acid (NHO3), peroxyacetic acid (PAA), and in O2/Na2CO3 system. Three different groups of paramagnetic centres exist in the coal samples analyzed. The EPR spectrum of the demineralised coal was a superposition of broad Gauss (ΔBpp = 0.75 mT), broad Lorentz 1 (ΔBpp = 0.42 mT) and narrow Lorentz 3 lines (ΔBpp = 0.08 mT). The three EPR components with linewidths: 0.58–0.77 mT (Gauss line), 0.30–0.39 mT (Lorentz 1 line) and 0.05–0.06 mT (Lorentz 3 line) were recorded for the oxidized coal. The g-values were obtained for the samples studied in the ranges 2.0043–2.0046 (Gauss lines), 2.0035–2.0038 (Lorentz 1 lines) and 2.0032–2.0034 (Lorentz 3 lines). The broad Gauss and Lorentz 1 lines saturate at low microwave powers. The narrow Lorentz 3 lines of demineralised coal were not saturated at microwave power from the range considered. After the coal oxidation with HNO3, PAA and in O2/Na2CO3 system, the microwave saturation of the narrow Lorentz 3 lines was also observed, which indicated a degradation of the multi-ring aromatic structures upon oxidation.

EPR characteristics of petrographically complex coal samples from durain and clarain

Abstract Electron paramagnetic resonance (EPR) spectroscopy was used to analyse properties of paramagnetic centres systems of the samples separated from two lithotypes of Polish coal (85.6xa0wt% C). The EPR spectra of the durain samples with a content of 2–22xa0vol.% exinite, 73–86xa0vol.% vitrinite and 5–17xa0vol.% inertinite, and the clarain samples with a content of 3–14xa0vol.% exinite, 65–90xa0vol.% vitrinite and 4–32xa0vol.% inertinite were compared. Three different groups of paramagnetic centres in each of the coal samples were found. The concentration of unpaired electrons and the parameters of the resonance absorption curves: linewidths and g-factor, were determined.

Groups of paramagnetic centres in coal samples with different carbon contents

Complex systems of paramagnetic centres existing in demineralised flame coal (71.4 wt% C), medium-rank coal (85.6 wt% C) and anthracite (94.9 wt% C) were analysed by electron paramagnetic resonance spectroscopy (EPR). Different groups of paramagnetic centres were searched in macerals (exinite, vitrinite, inertinite) of coals with carbon contents of 73.8 wt% C and 85.6 wt% C. Experimental EPR spectra were fitted by different superposition of theoretical lines. Total concentration of paramagnetic centres both in coal and macerals increases with carbon content in the sample. Total concentration of paramagnetic centres in macerals increases from exinite to inertinite. Four groups of paramagnetic centres were found in medium-rank coal, and only two groups exist in flame coal and anthracite. The EPR spectrum of medium-rank coal is a superposition of two broad and two narrow lines. Both broad and narrow, and two narrow lines were measured in EPR spectra of flame coal and anthracite, respectively. Two types of paramagnetic centres with broad lines and one group with narrow lines exist in exinite and vitrinite from medium-rank coal. Two narrow components were observed in EPR spectra of its inertinite. Two different broad electron paramagnetic resonance signals were detected for macerals from low-coalificated coal. Paramagnetic centres systems of medium-rank coal samples reveal the most complex character. Broad EPR lines were not observed for the higher coalificated samples, i.e., anthracite and inertinite from medium-rank coal. Narrow lines do not occur in EPR spectra of low-coalificated macerals. Strong dipolar interactions and fast spin-lattice relaxation processes are characteristic for paramagnetic centres with broad lines. Strong exchange interactions and short spin-lattice relaxation time were measured for paramagnetic centres with the narrowest EPR signals.

Paramagnetic centres in exinite, vitrinite and inertinite

Exinite, vitrinite and inertinite from durain, vitrain and clarain of Polish medium-rank coal with 85.6% C were investigated by X-band (9.3 GHz) electron paramagnetic resonance spectroscopy. Multicomponent structure of the EPR spectra of these macérais was analysed. The number of component lines, their lineshapes and parameters: linewidths andg factors, were determined. Total concentrations and concentrations of paramagnetic centres responsible for the component lines were measured. The broad Gaussian, broad Lorentzian and narrow Lorentzian lines were observed in the experimental spectra of exinite and vitrinite. The EPR spectra of inertinite are superposition of two narrow Lorentzian lines with different linewidths. The evolution of paramagnetic centres during heating of the macerais at 300–650°C was studied. Paramagnetic centres with broad Lorentzian lines are the most active ones in the thermal decomposition. The EPR results indicate reactions between individual macerais during thermal decomposition of coal. Thermally excited multiplet states were found in exinite and vitrinite.

Effect of reduction and butylation on coal: Application of microwave saturation of two-component EPR spectra

Changes in individual groups of paramagnetic centers after reduction and reductive butylation of Polish flame coal (70.8 wt.% C) were studied by electron paramagnetic resonance (EPR) spectroscopy. The modern method of reductive butylation of coal in a potassium-liquid ammonia system was used. This process increases the solubility of coal in organic solvents. Microwave saturation of EPR spectra was applied to test the spin-lattice relaxation in coal. The measured EPR spectra were a superposition of broad (ΔBpp, 0.42–0.49 mT) and narrow (ΔBpp, 0.09–0.13 mT) Lorentz lines. Paramagnetic centers located in simple and multiring aromatic structures were responsible for the broad and narrow lines, respectively. Microwave saturation indicates that slow and fast spin-lattice relaxation processes are characteristic for these two types of structures in the original coal. A decrease of the microwave power saturation of the broad Lorentz line after a single reduction of coal was observed. It increased for both 4 times reduced coal and reductively butylated coal. As the result of multiple reduction and butylation, spin-lattice relaxation processes in simple coal aromatic units were fastened. The narrow Lorentz lines of both 4 times reduced and reductively butylated coal were saturated and the spin-lattice relaxation time increased.