Concentration Of Unpaired Electrons Research Articles

The Role of Vacancies in a Ti2CTx MXene‐Derived Catalyst for Butane Oxidative Dehydrogenation

AbstractMXenes are a new family of 2D carbides or nitrides that have attracted attention due to their layered structure, tunable surface groups and high electrical conductivity. Here, we report for the first time that the Ti2CTx MXene catalyses the selective oxidative dehydrogenation of n‐butane to butenes and 1,3‐butadiene. This catalyst showed higher intrinsic activity compared to a commercial TiC and TiO2 samples in terms of C4 olefin formation rate. We propose that the stabilisation of structural vacancies and the change in composition (from a carbide to a mixed phase oxide) in the MXene causes its higher catalytic activity. These vacancies can lead to a higher concentration of unpaired electrons in the MXene‐derived material, enhancing its nucleophilic properties and favouring the production of olefins.

Creation and research of antistatic carbon coatings on fabric 05С8-КВ, modified by metal clusters

Application modes have been developed and carbon coatings modified with metal clusters on 05S8-KV fabric (manufacturer OJSC Mogotex) were created. The surface morphology of this coated fabric was investigated. It was shown that during the deposition of coatings from 12X18H10T steel targets a droplet phase is present on the coatings (the maximum droplet size reaches 3.5 μm). The paramagnetic absorption of the system carbon coating – clusters from the 12X18H10T steel target is investigated. It was found that the EPR spectrum contains a spectral line 94.4 mt wide, which indicates a high concentration of magnetic resonance centers, the effective value of the g-factor is 2.26 ± 0.01. In addition, a narrow spectral line with an effective g-factor of 2.04 ± 0.01 is recorded in the spectrum, the linewidth is 4.3 mt, which is associated with a high local concentration of unpaired electrons in carbon clusters with a weak nonresonant absorption at UHF (ultra-high frequrncy). The result of measuring the specific surface resistance on fabrics with this coating (side 1 / side 2) is 2.3.109 and 4.8.109 ohms, respectively, which is slightly lower than for a pure carbon coating.

Synthesis and Characterization of Silver-Containing Nanocomposites Based on 1-Vinyl-1,2,4-triazole and Acrylonitrile Copolymer

Insoluble silver-containing nanocomposites based on 1-vinyl-1,2,4-triazole and acrylonitrile copolymer was synthesized and characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, atomic absorption, X-ray diffraction, and thermogravimetric data analysis. The silver content of the nanocomposites varied from 8.0 to 23.5% (the silver content depends on the ratio of the copolymer-silver salt). Synthesis of silver nanocomposites was carried out by thermal treatment of a polymer complex of copolymer-silver salt without the use of an additional reducing agent. X-ray diffraction was used to control the formation of silver nanoparticles at different temperatures (210°С, 220°С, and 250°С). Synthesis of nanocomposites shows a developed porous structure with a channel diameter in most cases of 1-4 microns. The EPR monitoring of the nanoparticle germination and growth reveals that the generation of the nanocomposites proceeds via a stage of complex formation between the polymeric matrix and silver ions. The synthesized nanocomposites possess paramagnetic properties and show a narrow EPR symmetric signal with the concentration of unpaired electrons of 1020 spin/g, a g-factor 2.0050, and signal width 5-6 Gs. Thermal stability of silver-containing nanocomposites is up to 270-320°C.

Protective Carbon Overlayers from 2,3-Naphthalenediol Pyrolysis on Mesoporous SiO₂ and Al₂O₃ Analyzed by Solid-State NMR.

Hydrothermally stable carbon overlayers can protect mesoporous oxides (SiO2 and Al2O3) from hydrolysis during aqueous-phase catalysis. Overlayers made at 800 °C by pyrolysis of 2,3-naphthalenediol deposited out of acetone solution were analyzed by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Power absorption due to sample conductivity was prevented by diluting the sample in nonconductive and background-free tricalcium phosphate. While pyrolysis on SiO2 produced a predominantly aromatic carbon film, at least 15% of nonaromatic carbon (sp3-hybridized C as well as C=O) was observed on γ-Al2O3. These species were not derived from residual solvent, according to spectra of the same material treated at 400 °C. The sp3-hybridized C exhibited weak couplings to hydrogen, short spin-lattice relaxation times, and unusually large shift anisotropies, which are characteristics of tetrahedral carbon with high concentrations of unpaired electrons. Moderate heat treatment at 400 °C on SiO2 and Al2O3 resulted in yellow-brown and nearly black samples, respectively, but the darker color on Al2O3 did not correspond to more extensive carbonization. Aromatic carbon bonded to hydrogen remained predominant and the peaks of naphthalenediol were still recognizable; however, some of the chemical shifts differed by up to 5 ppm, indicating significant differences in local structure. On SiO2, additional sharp peaks were detected and attributed to 1/3 of the 2,3-naphthalene molecules undergoing fast, nearly isotropic motions.

Thermolysis of brown coal in the presence of alkali metal hydroxides

The thermolysis (≤500°C) of brown coal impregnated with alkali metal hydroxides MOH was studied. Alkalis cause the cleavage of a three-dimensional coal framework into fragments similar to humic acids. The initial rate of the process increases with the diameter of a cation. For the brown coal–KOH compound, an increase in the temperature from 110 to 400°C increased the rate of fragmentation by a factor of ~200 to cause the almost complete (≥90 wt %) decomposition of coal. The promotion of condensational reactions that cause a decrease in the yield of low-molecular-weight volatile organic products is a competing process. In the presence of MOH, the buildup of radicals is changed: a local maximum (at 200–250°C) is formed in the temperature dependence of the concentration of unpaired electrons; the temperature of this maximum and the values of [e –] depend on the nature of MOH. In the series of alkalis from LiOH to CsOH, the EPR signal broadened from 0.63 to 0.84 mT due to the formation of semiquinone radical anions and the interaction of an unpaired electron with a cation. A reaction scheme was proposed for the processes of the thermolysis (to 400°C) of brown coal with MOH; it includes the cleavage of a coal framework by the simultaneous heterolysis and homolysis of C–O and C–C bonds with the subsequent formation of a secondary lattice due to polycondensation and radical polymerization.

Obtaining of water soluble microbial melanin and study of its some properties

The effective sorption method for melanin isolation and purification from fermentation solutions of Bacillus thuringiensis serovar galleriae K1 has been elaborated, the principle process flowsheet is presented. The identification of obtained pigment with the samples of natural and synthetic melanin was done by IR-spectroscopy, and the intensity ratio of optical absorption at 650 and 500 nm allows to refer the isolated melanin to eumelanin class. By thermal treatment it was determined, that melanin's amorphous sediment is steady at up to 120 degrees C temperatures, at that the concentration of paramagnetic centers is changed from 0.053 x 10(18) spin/g (48 degrees C) to 0.25 x 10(18) spin/g (120 degrees C). The rising of the temperature of treatment up to 210 degrees C brings to substantial increase of the concentration of unpaired electrons, and at 280 degrees C its sharp growth is observed. At 350 degrees C growth stops, then sharp decrease is observed. The obtained results were confirmed by methods of IR spectroscopy and derivatographic analysis.

Synthesis and properties of a microporous carbon material as a catalyst support for fuel cells

It is stated that one-dimensional conductivity in amorphous microporous carbon material (AMCM) samples is associated with the considerable imperfection of graphene fragments in the carbon material rather than the presence of unshared electrons. It is likely that the graphene fragments are formed upon the carbonization of a carbon precursor accompanied by the partial or complete removal of precursor heteroatoms. It is hypothesized that the presence of localized unpaired electrons, which give EPR spectra, is due to the formation of local defects in carbene fragments. Thus, the effects of the value of conductivity and the concentration of unpaired electrons on the power output of a fuel cell cannot be distinguished based on the experimental data with the use of an AMCM as a catalyst support. The interaction of localized paramagnetic centers with electron gas can be interpreted in terms of the C-S relaxation model.

Comparison of electron spin relaxation times measured by Carr–Purcell–Meiboom–Gill and two-pulse spin-echo sequences

Electron spin relaxation times obtained by two-pulse spin-echo and Carr–Purcell–Meiboom–Gill (CPMG) experiments were compared for samples with: (i) low concentrations of nuclear spins, (ii) higher concentrations of nuclear spins and low concentrations of unpaired electrons, (iii) higher concentrations of nuclear spins and of electron spins, and (iv) dynamic averaging of inequivalent hyperfine couplings on the EPR timescale. In each case, the CPMG time constant decreased as the time between the refocusing pulses increased. For the samples with low concentrations of nuclear spins (the E ′ center in irradiated amorphous SiO 2) the limiting value of the CPMG time constant at short interpulse spacings was similar to the T m obtained by two-pulse spin echo at small turning angle. For the other samples, the time constants obtained by CPMG at short interpulse spacings were systematically longer than T m obtained by two-pulse spin echo. For most of the samples, the CPMG time constant decreased with increasing electron spin concentration, which is consistent with the expectation that the CPMG sequence does not refocus dephasing due to electron–electron dipolar interaction between resonant spins. Dynamic processes that average inequivalent hyperfine couplings contributed less to the CPMG time constant than to the spin-echo decay time constant. The impact of nuclear echo envelope modulation on CPMG time constants also was examined. For a Nycomed trityl radical in glassy D 2O:glycerol-d 8 solution, the CPMG time constant was up to 20 times longer when the time between pulses was approximately equal to integer multiples of the reciprocal of the deuterium Larmor frequency than when the time between pulses was an odd multiple of half the reciprocal of the deuterium Larmor frequency.

Improved Conducting States Induced by an Electrochemical Charging Process in Polyaniline Film Doped with New Dopants

We study the electrode characteristics of Li//polyaniline cells by using polyaniline films as positive electrodes. Two mechanisms for charging in a Li//polyaniline-Li salt cell are suggested; charge separation mechanism and intercalation mechanism. In the charge separation mechanism, the electrical conductivity, the peak area of site in X-ray photoelectron spectroscopy (XPS) data, and the intensity of the electron spin resonance (ESR) signal increase as the concentration of unpaired electrons increases. We compare XPS and ESR signals, and dc conductivity for the polyaniline before and after charging. From the results of XPS experiments, the polyaniline samples after electrochemical treatment have more state or more polarons, resulting in higher conductivity and larger ESR signal. The and counterions exist deep inside the Pani- and Pani- samples by charging process. Based on these results, we propose the charge separation mechanism in Li//polyaniline cells by using the polyaniline films as the positive electrodes. © 2002 The Electrochemical Society. All rights reserved.

Solid-State 29Si, 113Cd, 119Sn, and 31P NMR Studies of II-IV-P2 Semiconductors

Solid-state 29Si, 113Cd, 119Sn, and 31P MAS NMR spectra are reported on a series of II-IV-P2 compounds. In favorable cases (e.g., high degree of crystallinity, low concentration of unpaired electrons), well-defined spectra, with sharp lines for each specific nearest-neighbor configuration, are observed; in such cases, expected J coupling patterns are also seen. High-resolution solid-state NMR studies of this type provide useful information on structure (disorder), doping, and electron-mediated coupling in semiconductor systems.

EPR characteristics of petrographically complex coal samples from durain and clarain

Electron paramagnetic resonance (EPR) spectroscopy was used to analyse properties of paramagnetic centres systems of the samples separated from two lithotypes of Polish coal (85.6wt% C). The EPR spectra of the durain samples with a content of 2–22vol.% exinite, 73–86vol.% vitrinite and 5–17vol.% inertinite, and the clarain samples with a content of 3–14vol.% exinite, 65–90vol.% vitrinite and 4–32vol.% 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.

Sulfurization of polymers

Polyethylene exhaustively sulfurized with elemental sulfur shows paramagnetic (spin concentration 2.7–9.7·1019 sp g−1,g=2.0041–2.0045, ΔH=0.53–0.62 mT) and redox properties, which was demonstrated by both voltammetric and chemical methods (sodium reduction in liquid ammonia). The high concentration of unpaired electrons, the character of the electrochemical activity, and the chemical properties are in agreement with the presence in the polymers of polyconjugated ladder polythiophene and parquet polynaphtho-thienothiophene structures along with polyene-polysulfide blocks. The use of the polymers under consideration as an active cathode material in lithium batteries enables their repeated cycling with a specific charge capacitance of 150–340 mA hg−1.

Novel Solid Nano Diamond/Pyrocarbon Semiconductor Materials

AbstractPorous solids made of nano diamonds cemented by pyrocarbon show a semiconductor behavior where the properties are controlled by the factor γ = mass ratio of sp2/sp3 phases. The volume electrical conductivity μ (T) was measured in the temperature range 77 - 290 K as a function of γ. By controlling the ratio γ from 0 to 50 %, the electrical conductivity will change by 12 orders of magnitude. The semiconductor activation energy depends on γ-ratio and has values between 0.03 - 0.3 eV. The variable-range hopping conduction was checked by measuring μ against T and to look for a T1/4 dependence. A positive thermoelectric power showed that NDC was p-semiconductor with a narrow band gap. The paramagnetic nanodiamonds, g = 2.0027(1) and δH = 0.86(2) mT, will still have this property in the NDC material and both the g-value and line width are not dependent on γ-ratio and temperature. The concentration of unpaired electrons is 41019 spin/g that equals to one dangling bond per nanodiamond particle. Absences of magnetoresistance and Hall potential were observed for all NDC materials. This was interpreted as resulting from a Tamm - Lifshitz - Pekar electronic conduction mechanism within onedimensional band of surface states.

Correlation of electron spin concentration and conductivity in the ladder polymer BBL as a function of electrochemical potential

We present results of electron spin resonance (ESR) experiments on benzimidazobenzophenanthroline (BBL) samples in which the relative concentration of unpaired electrons was monitored as a function of electrochemical doping potential and the number of electrons per repeat unit (ERU). An increasing spin concentration along with increasing conductivity at low doping levels (less than 1/2 ERU) indicates that the conducting species have spin 1/2. An increasing spin concentration along with decreasing conductivity at intermediate doping levels (between 1/2 and 1 ERU) indicates that conduction in dry BBL involves a redox hopping process. The polymer has 2 ERU when fully doped.

Adsorption and adsorbate interaction at soot particle surfaces

Previously reported research, demonstrating a role of adsorbed molecular oxygen in the reaction of ozone with soot particles and in their hydration, has been extended to explore the interaction between O2 and SO2, O3, H2O, NH3, NO2 and NO at the soot surface as well as the effect of these adsorbates on each other. Electron paramagnetic resonance (EPR) and microgravimetry have revealed the extent of interactions between some competing reactants on soot. EPR has been used as a probe of the surface behavior of paramagnetic species (O2, NO2, NO) through their effect on the intensity of a large signal which is the result of the high concentration of unpaired electrons in soots. The effects of diamagnetic reactants (H2O, NH3, SO2) also are measured through their influence on the paramagnetic adsorbates and thus on the EPR signal. Microgravimetric measurements of adsorption and changes in the mass resulting from surface reactions also have enabled interpretations of the behavior of reactants on soot particles. Details of the modes of adsorption of SO2, NO2 and NH3 on n-hexane soot have emerged from these measurements as well.

Dynamic nuclear polarization of organic compound doped with free radicals

For those organic compounds which lack free radicals, a doping approach named melting-liquid-nitrogen-quench was developed in order to perform dynamic nuclear polarization(DNP)-nuclear magnetic resonance experiments. By using this method, the H-1, C-13, N-15 DNP enhancements were observed with free-radical-doped dibenzofuran and benzamide. The enhancement mechanism and relation between the concentration of unpaired electrons and the maximum DNP enhancement are discussed.

96/05657 E.p.r. studies of thermal decomposition of exinite and inertinite

Electron paramagnetic resonance (EPR) spectroscopy was used to analyse properties of paramagnetic centres systems of the samples separated from two lithotypes of Polish coal (85.6 wt% C). The EPR spectra of the durain samples with a content of 2–22 vol.% exinite, 73–86 vol.% vitrinite and 5–17 vol.% inertinite, and the clarain samples with a content of 3–14 vol.% exinite, 65–90 vol.% vitrinite and 4–32 vol.% 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.

The relationship between the pore structure of coal and gas-dynamic behaviour of coal seams

An explanation of the process of methane emission from coal and associated strata from the general viewpoint of recent British and Soviet physicochemical research, concerning the pore structure, sorptive capacity and sorption-kinetic properties of these materials, is attempted. The control exerted by the geological environment of a coal seam, particularly the degree of structured disturbance and the distribution of mineral matter, on the gas-dynamic behaviour of coal and associated strata in both the laboratory and in mine workings is examined. The temperature effects accompanying methane emission are also discussed. Experimental results show that the mineral matter content of a sample determines its sorptive capacity, whilst the rate of methane emission depends on the number and distribution of the macropores and fractures present in a unit volume of the sample. A strong relationship between the degree of molecular damage suffered by coal, expressed in terms of the concentration of unpaired electrons (paramagnetic centres), and its sorption-kinetic characteristics is demonstrated. It is shown that these characteristics are strongly related to other phenomena such as the rule of methane emission from deep boreholes and the resulting reduction in temperature of the walls of coal boreholes. Large variations in such characteristics of coal are shown in occur over short distances as the geological structure of the seam changes, both vertically and horizontally. Consequently, they may be used to quantify the risk of outburst inherent to disturbed areas of coal and to predict the general level of methane emission from the seam.

Thermal degradation of cellulose model compounds in inert atmosphere

AbstractThermal degradation of cellulose model compounds was studied using thermogravimetry (TG), electric spin resonance (ESR) spectroscopy, and gas chromatography–mass spectrometry (GS–MS). The molecular weight of samples did not influence the activation energy of gasification. In the case of catalytic influence of NaOH on cellulose thermolysis formation of gaseous products in the temperature range 250–350°C increased and a greater amount of residue was formed at 350–400°C compared to noncatalytic thermolysis. Residues of compounds with hemiacetal groups showed higher concentration of unpaired electrons than residues obtained from models with the blocked hemiacetal group.

E.s.r. spectrometric study of directly prepared potassium-coal adducts

E.s.r. spectral parameters (concentrations of paramagnetic centres, g values and linewidths) of two vitrains of different rank (C, 89.2 and 75.8 wt%) and of their potassium adducts prepared by a direct method, as well as of the reductively alkylated high-rank vitrain are reported. The potassium adduct of the high-rank vitrain is characterized by the accumulation of a rather high concentration of unpaired electrons, 7.7 spins per 100 C atoms (a 120-fold enhancement). With the adduct of the low-rank vitrain the concentration is 0.14 spins per 100 C atoms (a 17-fold enhancement). The adduct of the low-rank vitrain is negligibly alkylated by alkyl halides in THF, unlike the adduct of the high-rank vitrain which can be significantly alkylated. The g values and linewidths of the spectra of the vitrains and the corresponding potassium adducts are very similar. The experimental results are interpreted mainly with respect to the formation of ionic potassium-coal adducts of coals of different rank and their alkylation.