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 (ΔB_pp: 0.49-0.84 mT), broad Lorentz 1 (ΔB_pp: 0.18-0.35 mT), and narrow Lorentz 3 (ΔB_pp: 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.     

Quantitative in-situ EPR spectroelectrochemical studies of doping processes in poly(3,4-alkylenedioxythiophene)s: Part 1: PEDOT

Quantitative in-situ EPR spectroelectrochemical studies of poly(3,4-ethylenedioxythiophene) (PEDOT) have been carried out with an aim to gain new insights into the doping processes taking place in this polymer. Corroborating the findings made during previous studies of this polymer, absolute measurements conducted in this study provided new detailed information regarding some of the basic parameters characterising the doping process of this conjugated polymer. It was found that concentrations of paramagnetic centres in PEDOT vary from 0.02 spin per mer in the dedoped state up to a maximum of 0.12 spin per mer at 0.15 [e⁻/mer] doping level, corresponding to 1 spin per ca. 8.5 meric units. Such notable concentration values indicate that polarons represent a numerous charge carrier group in PEDOT, contrary to observations made for other members of polythiophene family. Furthermore polarons do not disappear at high doping levels of PEDOT but rather decrease their numbers gradually down to 0.08 spins per mer at a maximum doping level of 0.55 [e⁻/mer] attained in this study. Based on information about concentrations of spins and polymer doping charges, concentrations of bipolarons have been evaluated as a function of doping level. Results indicate that bipolaron formation starts at ca. 0.06 [e⁻/mer] doping level when spin generation efficiency begins to deviate from 1 and interspin interactions emerge as evidenced by doping level dependency of EPR signal linewidth (ΔB_pp). Decomposition of complex EPR spectra of PEDOT in its doped state corroborated the presence of two groups of paramagnetic centres in this polymer. Based upon doping level dependencies of their spectroscopic parameters (concentration, ΔB_pp linewidth and g-factor), the identity of these centres has been redefined compared to our previous reports, linking their properties with the type of polymer phase (crystalline or amorphous) they reside in.     

An EPR investigation of conductive polyaniline–poly(methyl methacrylate‐co‐acrylic acid) blends

A conductive blend of polyaniline (PANI) with sodium form poly(methyl methacrylate‐co‐acrylic acid) copolymer (PMMAA) was prepared by in situ dispersion polymerization. The PANI‐PMMAA blends were characterized with UV–vis, FTIR, and electron paramagnetic resonance (EPR) spectra. The effects of PANI and temperature on the electronic properties of the PANI‐PMMAA blends were studied with EPR technique. The PANI and temperature affect the intermolecular interaction between PANI and PMMAA and then determines the EPR parameters (A/B ratio, g factor, T₂, ΔH_pp, N_s, and lineshape). POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

EPR and optical studies on binary mixed alkali-alkaline earth oxide borate glasses doped with Cu<Superscript>2+</Superscript> ions

Mixed alkali alkaline earth oxide borate glasses of the composition (25 – x)Li₂O–xK₂O–12.5BaO–12.5MgO–49B₂O₃–1CuO (x = 0, 5, 10, 15 and 20 mol %) were prepared by the melt quenching technique. The X-ray diffractograms of all the glass samples were recorded at room temperature. Peak free X-ray spectra revealed the amorphous nature of all the prepared glasses. Modulated differential scanning calorimetry (MDSC) was used to determine the glass-transition temperature (T _g ). The probable mixed alkali effect was investigated using experimental techniques like density, molar volume, MDSC, electron paramagnetic resonance (EPR), and optical absorption studies. From the EPR spectra the spin-Hamiltonian parameters were evaluated. The spin-Hamiltonian parameter values indicated that the ground state of \(C{u^{2 + }}is{\kern 1pt} {d_{{x^2} - {y^2}}}\) orbital (²B_1g) and the site symmetry around Cu² is tetragonally distorted octahedral. The variation of g _|| and A _|| as a function of Li₂O content was found to be nonlinear. A broad optical absorption band was observed in all the glasses containing Cu² ions corresponding to ²B_1g → ²B_2g transition. From the optical absorption studies the values of the optical band gap (E _opt) for indirect, direct transitions and Urbarch energy (ΔE) have been evaluated. By co-relating the EPR and optical absorption data, bonding parameters α², β² and β ₁ ² were evaluated.     

Effect of electroreduction on the EPR spectra due to charge carriers in carbon blacks

The electrochemical reduction of carbon black particles dispersed in methylene chloride was monitored in situ with electron paramagnetic resonance (EPR) spectroscopy. The broad EPR signal is converted to a signal with intermediate linewidth (ΔH_pp ~ 23 G) in both CSX-99 and BP-1300 carbon blacks. This narrowing was coincident with the manifestation of the narrow EPR signal (ΔH_pp ~ 5 G, which has been previously shown to arise during electroreduction. In addition, changes in the cavity Q were also observed under electro-reduction and subsequent electrooxidation. This decrease in the dielectric loss and the narrowing of the EPR linewidth observed upon electroreduction are discussed.     

Influence of iron on kaolin whiteness: An electron paramagnetic resonance study

Well characterized kaolin samples from different deposits of Brazil were studied by electron paramagnetic resonance. Three paramagnetic species attributed to iron in different chemical environments were identified in raw samples. A very intense and broad line with 0.2 mT linewidth was typical of large particles of iron oxides/hydroxides. A species at g = 2.0 with 0.04 mT line width was associated with superparamagnetic particles adsorbed to kaolinite surfaces or trapped between its lamellae. An isotropic and/or orthorhombic species in the g = 4.2 region belongs to structural Fe³⁺ replacing Al³⁺ in sites with different crystal-field symmetries. It was verified that large iron oxide particles were responsible for keeping the kaolin whiteness at levels below 50%. Superparamagnetic iron particles mainly affect whiteness in the range of 50%< whiteness < 86%. They may be totally removed by magnetic separation followed by chemical treatment using sodium dithionite. The kaolin high whiteness levels (whiteness > 86%) were determined by Fe³⁺ iron content in the kaolinite structure. However, only a small part of this species can be removed from the kaolinite structure by traditional industrial processing techniques. These results showed that EPR is a powerful technique to assist in controlling kaolin quality in industrial processes.     

A Thermogravimetric Analysis Study of the Kuzbass Coals of Different Ranks

The coals of different ranks from the Kuznetsk Coal Basin were studied by thermogravimetric analysis in inert and oxidizing atmospheres. Correlations between thermal decomposition characteristics [maximum decomposition temperatures (T_max, °C), maximum rates of decomposition (Vmax, %/min), and sample weight losses in different temperature ranges (Δm, %)] and proximate analysis [yields of volatile substances (Vdaf, %)] and petrographic analysis [vitrinite reflectance (R_o)] data and NMR-spectroscopic data [aromaticity factor ( fa)] as the characteristics of the degree of maturity of the test coals were established.     

A study of the CCl stretching region of the Raman spectrum of PVC

The C-Cl stretching regions of the Raman spectra of a number of samples of PVC prepared by various methods which lead to differences in tacticity, conformational content and crystallinity have been studied. It is shown that the spectra of all samples can be fitted well with a set of nine Lorentzian peaks whose positions and widths are the same for each spectrum. Two of these peaks are attributable to the A_g and B_3g species vibrations of crystalline material and the remaining seven may be assigned with reasonable confidence to specific structures in the amorphous regions, three of which are associated with isotactic placements. The area under the A_g and B_3g peaks may be used to estimate the crystallinity of the samples and the area under the peaks assigned to isotactic material may be used to estimate the tacticity of the polymer.     

EPR studies of a nickel–boron centre in synthetic diamond

Possible models are discussed for a new EPR centre (NOL1) observed in a diamond grown from a nickel-containing solvent catalyst with a titanium nitrogen getter. Nickel and boron are the most prevalent impurities in the diamond. The centre has trigonal (C_3v) symmetry about 〈111〉, S=1, g_∣∣=2.0235(5), g_⊥=2.0020(10) and D=−171(1) GHz. The EPR lineshape is unusual and we tentatively attribute this to partially resolved ¹¹B hyperfine structure. This centre may be the same as NIRIM-5, observed in boron doped synthetic diamond. The most likely structure for this centre is an interstitial Ni²⁺ with a substitutional B⁻ ion at the nearest neighbour site along 〈111〉.     

Synthesis of sequential poly(1,3-phenyleneethynylene)-based polyradicals and through-space antiferromagnetic interaction of their solid state

Some 1,3-PPE-based polyradicals with pendent phenoxy radicals whose repeating unit consisted of four 1,3-phenyleneethynylene units were synthesized via polymerization between monomers and trimeric monomers in the presence of the Pd(PPh₃)₄ complex catalysts. The ESR spectra of the polyradicals in dichloromethane showed unimodal broad signals. For the solid samples which were prepared from the dichloromethane solution by evaporating the solvent and by drying in vacuo, the signal intensity decreased with broadening of peak-to-peak line-width (ΔH_pp). However, the ESR spectra almost recover to the initial intensity and shapes due to redissolving in dichloromethane. In particular, the decrease of doubly integrating the ESR signal (I_ESR) was more remarkable for the polyradical bearing galvinoxyl and phenoxyl residues than others. This behavior suggests that strong antiferromagnetic interaction partially arose for the polyradical.     

Resonance paramagnetique electronique des pyrocarbones

The spin resonance of pyrocarbons prepared at 2100°C and treated at various temperatures ranging from 2300°C up to 2900°C has been investigated. The line is anisotropic. It is found that the g anisotropy of crystallites A = g₃ — g₁ = Δg, varies in the same way as the anisotropy of the diamagnetic susceptibility. It is higher than that of graphite for the pyrocarbon deposited at 2100°C, decreases to a minimum at H.T.T. of about 2700°C and then, increases towards the anisotropy of graphite. The results of measurements of paramagnetic susceptibility xp allow to demonstrate the existence of a quantitative relation between Δg, Δχd and χp.     

Electronic active defects and local order in doped ZnO ceramics inferred from EPR and 27Al NMR investigations

Doped ZnO based ceramics were fabricated by using a solid state reaction of ZnO co-doped by TiO_2, Al₂O₃ and MgO and sintered in different atmospheres (Air, N₂, N₂ + CO). The crystalline structures consist in wurtzite ZnO and a minor spinel phase Zn₂TiO₄. The electrical conductivity is modulated by the sintering conditions with the highest value (˜10⁵ S m⁻¹) obtained in the reducing atmosphere (N₂ + CO). The role of defects and vacancies on the electrical behavior was exhaustively investigated by Raman, electron paramagnetic resonance (EPR) and solid state NMR methods. The paramagnetic centres inferred from EPR studies show a Pauli-like spin susceptibility. Their origin was assigned to shallow donors from interstitial defects (Zn_i) favored by substitutional Al ions (Al_Zn). The NMR spectral features with a characteristic 185 ppm line which correlates with the electrical conductivity are presumed to be caused by the Knight shift effect from the conduction electrons and the involved paramagnetic centres.     

Spectroscopic investigations of Mn ions doped polyvinylalcohol films

Electron paramagnetic resonance (EPR), luminescence and infrared spectral studies have been carried out on Mn²⁺ ions doped in polyvinylalcohol (PVA) films. The EPR spectra at room temperature exhibit sextet hyperfine structure (HFS), centered at g_eff≈2.0 characteristic of Mn²⁺ ions in octahedral symmetry. The zero-field splitting parameter (D) at room temperature has been evaluated from the intensities of allowed hyperfine lines. The EPR spectra exhibit a marked concentration dependence. The EPR spectra have also been recorded at various temperatures. The number of spins participating in the resonance is measured as a function of temperature and the activation energy (E_a) is calculated. The paramagnetic susceptibility (χ) is calculated from the EPR data at various temperatures. From the plot of 1/χ versus T, the Curie constant and Curie temperature have been evaluated. The emission spectrum of Mn²⁺ ions doped PVA film exhibits three bands centered at 390, 448 and 465 nm. The band at 448 nm is attributed to transition of Mn²⁺ ions. The bands at 390 and 465 nm are attributed to the recombination of free charge carriers. The excitation spectrum exhibits two bands at 250 and 216 nm, which are attributed to host lattice absorption bands. The FT-IR spectrum exhibits few bands, which are attributed to O-H, C-H, CC and C-O groups of stretching and bending vibrations.     

Synthesis and EPR studies of a near infrared absorbing tetrakis(2-naphthoxy)vanadylphthalocyanine

Vanadylphthalocyanine with four naphthoxy pendant groups on the periphery was prepared by cyclotetramerization of 4-(2-naphthoxy)phthalonitrile. The new compound was characterized by elemental analyses, Mass, IR and UV–Vis spectral data. The electronic spectra exhibit an intense absorption peak at 703 nm indicating the C_4v symmetry of VOPc. VO²⁺ ion in this structure was studied using electron paramagnetic resonance (EPR) technique at room temperature. The covalent bonding parameter for VO²⁺ ion and Fermi contact term were calculated using the principal axis values of the g and hyperfine splitting obtained from powder EPR spectrum.     

Thermogravimetric investigations in prediction of coking behaviour and coke properties derived from inertinite rich coals

The coal quality, temperature, pressure, heating rate, various processes and reactor type affect coking behaviour of coal and resulting coke properties. Several petrographic and chemical methods were proposed earlier for prediction of coking behaviour of coals. Inertinite rich coal samples (I^mmf>30 vol%) having different petrographic compositions were selected for thermogravimetric investigations (DTA, DTG and TGA) and their coking behaviour was studied. The petrographic build up, micro-structural properties (porosity and cell wall thickness) and mechanical strength of the resulted cokes were also investigated. ΔH and ΔH_max (the main endothermic area of heat absorption and fast absorbing main endothermic area, respectively) were distinguished in DTA curves. ΔA and ΔA_max (the main decomposition area and fast disintegrating main decomposition area) under DTG curves were identified. ΔH_max/ΔA_max shows good correlation with Roga's indices (RI, caking properties) as well as with petrographic caking ratio (PCR). The coarse mosaic content of cokes seem to depend on LΔT_max/ΔT_max (ratio of weight loss during fast decomposing main reaction to temperature difference) under DTG. L^mΔT/ΔT (ratio of weight loss during main decomposing reaction to temperature difference) under DTG exhibits correlation with p₁ (mean pore size) and t₁ (mean cell wall thickness) of cokes. ΔA_max/(L^mΔT_max) also indicates good relationship with mechanical strength of cokes. (L^mΔT_A−T_B)/(L^mΔT) (i.e. ratio of weight loss during main endothermic reaction under DTA to weight loss during main decomposing reaction) appears to have relationship with DD (compactness) of cokes. The course of main endothermic reaction/main decomposition reaction under DTA, DTG and TGA seems to govern coking behaviour and the resulting coke strength, which in turn is controlled by microlithotypes.     

Microstructure and enhanced magnetic properties of low-temperature sintered LiZnTiMn ferrite ceramics with Bi2O3-Al2O3 additive

Bi₂O₃ and Al₂O₃ were proven to separately have an impact on grain growth of LiZn ferrite ceramics. In this study, we synthesized LiZnTiMn ferrite ceramics under low temperature (<920 °C) using rationally designed formula of Bi₂O₃-Al₂O₃ as sintering agents. Microstructures, densities, and ferromagnetic performances were systematically investigated. Results show that appropriate amount of Bi₂O₃-Al₂O₃ leads to successful sintering of LiZnTiMn ferrites even below 900 °C. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) results show that spinel structure ferrite ceramics were obtained and that the addition of Bi₂O₃-Al₂O₃ accelerated solid-state reaction. Corresponding ferromagnetic properties, including saturation induction (B_s), remanence square ratio, coercivity (H_c), ferromagnetic resonance linewidth (ΔH), and M_s, were also investigated. At 920 °C, with addition amount of x=1 wt%, we obtain excellent performance: B_s=306 mT, B_r/B_s=0.849, H_c=2.34Oe, and ΔH=275 Oe. Results indicate that Bi₂O₃-Al₂O₃ is promising sintering agent for low-temperature sintering of LiZnTiMn ferrite ceramics.     

Excited triplet states in bituminous coal

The intensities of the broad and narrow lines constituting the e.p.r. spectrum of coal depend on the temperature of measurement. The system of paramagnetic centres related to the broad line does not obey the Curie law and consists of free radicals ($\text{S =}\text{1}\text{2}$) and of centres with singlet ground state (S = 0) and lowest-excited triplet state (S = 1). The singlet-triplet excitation energy, J, is comparable with thermal energy, kT, of lattice vibrations. The system of paramagnetic centres related to the narrow line obeys the Curie law ($\text{S =}\text{1}\text{2}$).     

In situ EPR spectroelectrochemistry of single-walled carbon nanotubes and C60 fullerene peapods

The first in situ electron paramagnetic resonance (EPR) spectroelectrochemical study of C₆₀ fullerene peapods (C₆₀@SWCNT) as well as that of single-walled carbon nanotubes (SWCNTs) in different electrolyte solutions describes the formation of spin states by charge transfer reactions. Electrochemical reduction of peapods at high negative potentials causes the production of spins at the SWCNT site, while the intratubular fullerene is unchanged.Slightly anisotropic EPR signals were detected during electrochemical reduction of single-walled carbon nanotubes and fullerene peapods in the potential region from −1.75 to −2.15 V vs. decamethylferrocene/decamethylferrocinium couple. They are centered at g = 2.0038 and exhibit a hyperfine structure indicating the presence of functional groups containing N, O, H atoms in neighborhood. They differ from the EPR signals of chemically (potassium) doped SWCNT and C₆₀@SWCNT. As the EPR signal is influenced by the electrolyte counter ions a reaction with electrolysis products of tetraalkylammonium cations is taken into consideration. No EPR lines of fullerene anions were found in electrochemically treated peapods, but these anions are detectable, if a free C₆₀ in solution is cathodically reduced on a SWCNT electrode.     

EPR study of solid solutions La1 ? 0.33yBa0.33yMnyAl1 ? yO3 (y = 0.02, 0.04, 0.10)

Solid solutions La_{1 − 0.33y} Ba_0.33y Mn _y Al_{1 − y} O₃ (y = 0.02, 0.04, 0.10) prepared using the ceramic technique have been investigated by the EPR method at temperatures of 77 and 300 K. By comparing with the calculated spectra, the experimental spectra have been assigned to Mn²⁺ ions (g = 2.04 and line width ΔH _pp = 64 × 10⁻⁴ T at T = 77K) and Mn⁴⁺ ions (g = 1.97 and ΔH _pp = 76 × 10⁻⁴ T at T = 77 K) and the broad line (ΔH _pp = 500 × 10⁻⁴ T at T = 77 K) has been attributed to clusters. The dilution of manganese ions as a result of the dissolution of La_0.67Ba_0.33MnO₃ in LaAlO₃ has allowed one to trace the cluster formation and the spin dynamics of Mn ions. It has been established that, at room temperature, compared to the system in which the Sr²⁺ ion with a smaller ionic radius is the replacing element, the localized states are more stable at all three manganese concentrations (the EPR lines are more intense). The La_0.67Ba_0.33MnO₃ clusters dissolved in LaAlO₃ retain some properties of a concentrated compound even upon dilution to y = 0.02.     

The behaviour of minerals in Cold Lake oil sands under dry combustion conditions

In the in situ or underground combustion oil recovery process, ignition is initiated at an air injection well and a combustion zone is propagated underground from this well to the production well. Under these conditions, the reservoir minerals undergo structural changes, new mineral phases are synthesized and their grain sizes are altered. These reactions control the porosity and the permeability of the reservoir. Cold Lake whole tar sand was subjected to air flow thermal conditions encountered under fire flood. The very fine-grained minerals were separated from the burnt oil sand as well as from the sand fraction from which bitumen had been removed with organic solvents. These mineral fractions were examined by 9.4 and 34 GHz electron paramagnetic resonance (EPR), Fourier transform infra-red (FTIR) and X-ray diffraction (XRD) spectroscopy. The EPR results indicate the presence of Mn²⁺ impurity ions in carbonates and Fe³⁺ impurity ions in kaolinite and illite clays. A broad transition at g ≈ 2 is attributed to the presence of Fe₂O₃ and a very broad transition at g 12 is attributed to the presence of Fe₃O₄. In addition, the EPR and FTIR results indicate that the bitumen is adsorbed on the mineral fraction. XRD shows that combustion produces a significant amount of Fe₂O₃ and reduces the quantity of kaolinite, illite and smectite clay. Treatment of both mineral fractions with dilute H₂SO₄ and HCl acids showed that the organic fraction adsorbed on minerals is immune to the acid attack while Fe₂O₃ and Mn²⁺ are removed.