Changes In Radical Concentration Research Articles

The auto-oxidation characteristic of coal at different stages of the low-temperature oxidation process

The spontaneous combustion in the mining gob are often caused by the low-temperature oxidation of coal, and the low-temperature oxidation process has a long process leading to different characteristics in each stage, which brings great difficulties to the prevention and control of coal spontaneous combustion in the underground mining gob. Therefore, this article investigated the oxidation characteristics of coal at different stages of the low-temperature oxidation, mainly including: analyzing the changes in radical concentration and g factor based on electron spin resonance (ESR) experiments, studying the variations in coal temperature by using the programmed temperature controller and analyzing the characteristics of gas products and oxygen consumption by using the gas chromatograph. And the degree of auto-oxidation of coal was characterized by the difference between dry air and nitrogen conditions during the constant temperature period. The results showed that when the constant temperature reached 130 ℃, the coal began to exhibit auto-oxidation characteristics, while when the constant temperature reached 180 ℃, the auto-oxidation phenomenon of the coal significantly intensified, the types and content of radicals rapidly grew, the coal temperature significantly rose, and the oxygen consumption rapidly increased, while releasing a large amount of CO and CO2. And the relationship between the spontaneous oxidation ability of coal radicals and the temperature change of coal during constant temperature was obtained. The conclusions obtained provide theoretical guidance for the safety and clean production of coal resources in mines, as well as the prevention and control of coal spontaneous combustion fires.

Modeling Molecular Interactions with Wetting and Non-Wetting Rock Surfaces by Combining Electron Paramagnetic Resonance and NMR Relaxometry.

Three types of natural rocks─Bentheimer and Berea sandstones, as well as Liège Chalk─have been aged by immersion in a bitumen solution for extended periods of time in two steps, changing the surface conditions from water-wet to oil-wet. NMR relaxation dispersion measurements were carried out on water and oil constituents, with saturated and aromatic molecules considered individually. In order to separate the different relaxation mechanisms discussed in the literature, 1H and 19F relaxation times were compared to 2H for fully deuterated liquids: while 2H relaxes predominantly by quadrupolar coupling, which is an intramolecular process, the remaining nuclei relax by dipolar coupling, which potentially consists of intra- and intermolecular contributions. The wettability change becomes evident in an increase of relaxation rates for oil and a corresponding decrease for water. However, this expected behavior dominates only for the spin-lattice relaxation rate R1 at very low field strengths and for the spin-spin relaxation rate R2, while high-field longitudinal relaxation shows a much weaker or even reverse trend. This is attributed in part to a change of radical concentration on the pore surface upon coverage of the native rock surface by bitumen as well as by the change of surface chemistry and roughness. EPR and DNP measurements quantify the change of volume vs surface radical concentration in the rocks, and an improved understanding of the role of relaxation via paramagnetic centers is obtained. By means of comparing different fluids and nuclei in combination with a defined wettability change of natural rocks, a refined model for molecular dynamics in conjunction with NMR relaxation dispersion is proposed.

Elucidation of poly(vinyl alcohol) and iodide gel dosimeter characteristics by predicting changes in radical concentrations and measuring responsiveness to heavy ion beams

Three-dimensional (3D) gel dosimeters enable a more practical radiation measurement and are expected to be widely used as a new measuring instrument. Herein, we investigate a type of 3D gel dosimeter, the poly(vinyl alcohol) and iodide (PVA-I) gel dosimeter. This gel can perform 3D dose distribution measurements by forming a red-colored complex of I3− and PVA when irradiated. We previously developed a leuco crystal violet (LCV) micelle gel dosimeter and measured its reactivity to heavy ion beams. However, the amount of color developed near the Bragg peak decreased, and the dose was underestimated. In this study, we used a PVA-I gel dosimeter to measure the dose distribution of heavy ion beams and compared the results with those of the LCV micelle gel dosimeter. We also examined the gels’ reactivity to heavy ion beams by simulating changes in the radical concentrations in the gel, using Monte Carlo calculations. The findings of this study demonstrate that the PVA-I gel dosimeter has a higher responsiveness near the Bragg peak than the micelle gel dosimeter and can be used as a dosimetry device for heavy ion beams.

Radicals and coking behaviors during thermal cracking of two vacuum resids and their SARA fractions

Inhibition of coke formation in cracking of distillation residues, such as the vacuum residue (VR), has attracted extensive attention for the development of better technologies. However, the radical coking mechanism was seldom studied in the past. This work studies coking and radical concentration during cracking of two VRs and their SARA fractions in sealed micro-reactors at 400, 420, 440 and 460 °C and elaborates the results with respect to the chemical composition of the starting materials and the kinetics of radicals formation. It is found that most of the radicals in VR locates mainly in the asphaltene fraction (Asp). The agglomeration of Asp to coke at low temperatures does not involve significant change in radical concentration. The coke yield includes that formed from the agglomeration of Asp and that from the condensation of radical fragments generated in cracking of all the fractions. The coke itself also undergoes cracking and condensation, resulting in a higher radical concentration and a lower coke yield. The interactions between the Asp and other SARA fractions are relatively large in coke formation by agglomeration, but relatively small in cracking and condensation due to limited amounts of small radicals or hydrogen radicals from the cracking of other fractions. The VR with a higher Asp fraction and richer poly-aromatic structures (QD-VR) is higher in radical generation and lower in activation energy.

A comparison of the radiosensitisation ability of 22 different element metal oxide nanoparticles using clinical megavoltage X-rays

BackgroundA wide range of nanoparticles (NPs), composed of different elements and their compounds, are being developed by several groups as possible radiosensitisers, with some already in clinical trials. However, no systematic experimental survey of the clinical X-ray radiosensitising potential of different element nanoparticles has been made. Here, we directly compare the irradiation-induced (10 Gy of 6-MV X-ray photon) production of hydroxyl radicals, superoxide anion radicals and singlet oxygen in aqueous solutions of the following metal oxide nanoparticles: Al2O3, SiO2, Sc2O3, TiO2, V2O5, Cr2O3, MnO2, Fe3O4, CoO, NiO, CuO, ZnO, ZrO2, MoO3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Er2O3 and HfO2. We also examine DNA damage due to these NPs in unirradiated and irradiated conditions.ResultsWithout any X-rays, several NPs produced more radicals than water alone. Thus, V2O5 NPs produced around 5-times more hydroxyl radicals and superoxide radicals. MnO2 NPs produced around 10-times more superoxide anions and Tb4O7 produced around 3-times more singlet oxygen. Lanthanides produce fewer hydroxyl radicals than water. Following irradiation, V2O5 NPs produced nearly 10-times more hydroxyl radicals than water. Changes in radical concentrations were determined by subtracting unirradiated values from irradiated values. These were then compared with irradiation-induced changes in water only. Irradiation-specific increases in hydroxyl radical were seen with most NPs, but these were only significantly above the values of water for V2O5, while the Lanthanides showed irradiation-specific decreases in hydroxyl radical, compared to water. Only TiO2 showed a trend of irradiation-specific increase in superoxides, while V2O5, MnO2, CoO, CuO, MoO3 and Tb4O7 all demonstrated significant irradiation-specific decreases in superoxide, compared to water. No irradiation-specific increases in singlet oxygen were seen, but V2O5, NiO, CuO, MoO3 and the lanthanides demonstrated irradiation-specific decreases in singlet oxygen, compared to water. MoO3 and CuO produced DNA damage in the absence of radiation, while the highest irradiation-specific DNA damage was observed with CuO. In contrast, MnO2, Fe3O4 and CoO were slightly protective against irradiation-induced DNA damage.ConclusionsBeyond identifying promising metal oxide NP radiosensitisers and radioprotectors, our broad comparisons reveal unexpected differences that suggest the surface chemistry of NP radiosensitisers is an important criterion for their success.

Plant-Based Antioxidant Nanoparticles without Biological Toxicity.

Here, we present a function to derive non‐deteriorated nanoparticulated lignin as an antioxidant without biological toxicity that is supplied through the simultaneous enzymatic saccharification and comminution of plants. The lignin exhibits an oxygen radical absorption capacity, even in its macromolecular nature. The non‐deteriorated lignin nanoparticles never inhibit the biological activity of living things, despite their antioxidant nature. The oxygen radical absorption capacity of lignin is dependent on its botanical origin and monomeric structure. A stable organic radical in lignin is responsible for the antioxidant nature of non‐deteriorated lignin. The organic radical of non‐deteriorated lignin, which yields a distinct signal on electron spin resonance spectra, serves as a spin trap reagent that detects the emergence of short lifespan radicals as the change of radical concentration of the lignin. The presented discovery of non‐deteriorated lignin will induce not only the industrial utilization of plant biomass polymers in pharmaceuticals and reagents, but also advance our scientific understanding of the antioxidant function of native lignin.

Detecting and Visualizing Reaction Intermediates of Anisotropic Nanoparticle Growth

This paper describes a correlative approach to detect, visualize, and characterize intermediate species during a seedless, anisotropic nanoparticle synthesis. Changes in radical concentration as a function of time were correlated in situ to the optical properties and morphology of the particles. Depending on type and concentration of reaction precursors, either one or two increases in radical production occurred, corresponding to initial particle formation and increased branch length, respectively. Thus, changes in radical intensity can be considered as an indicator of nanoparticle structure and properties.

Pyrolysis of a lignite briquette – Experimental investigation and 1-dimensional modelling approach

A laboratory-scale shaft furnace was used for the pyrolysis of the lignite briquette together with thermogravimetric analysis to study the intrinsic pyrolysis kinetics under two different heating rates, 10 °C/min and 100 °C/min. Apart from coal conversion rate, the tar yield, quality and radical concentration in char were also measured to explore the difference between lignite briquette and the respective pulverised powder. Additionally, a 1-D model coupled with the chemical percolation devolatilisation (CPD) code was developed to quantitatively understand heat transfer using temperature dependent parameters; product distribution and yields, and the pyrolysis mechanism. It was discovered that heat transfer was the limiting factor for pyrolysis of the lignite briquette, which subsequently lowered the heating rate and led to increased cross-linking and decreased tar production compared to a coal particle. Simultaneously, the primary tar also underwent internal cracking and even deoxygenation to decompose into light aromatics and gases inside the briquette char matrix. Providing a hot gas environment was found to facilitate the cracking of tar species compared to a slow heating rate where tar is released at a lower temperature. The changes in radical concentration in the solid material were linked to the structural changes predicted by the CPD model including bridge-breaking, tar release and cross-linking phenomena.

Coke and radicals formation on a sulfided NiMo/γ-Al2O3 catalyst during hydroprocessing of an atmospheric residue in hydrogen donor media

Coking on catalyst during hydroprocessing of petroleum residues is an important issue that should be addressed for better catalyst performance. This paper studies reaction and coking behaviors of Maoming atmospheric residue (AR) over a sulfided commercial NiMo/γ-Al2O3 catalyst in the presence and absence of a hydrogen donor solvent, tetrahydronaphthalene (THN), in a micro-reactor at temperatures of 400–460°C and reaction time of 0–60min. It reports the changes in yields of CS2 soluble matter (CS2-S), CS2 insoluble matter (coke) and gas, as well as the changes in quantity of coke on the catalyst and in radical concentration of the reaction system, coke on catalyst and CS2-S. It is found that compared with the changes in yields, which are significant at temperatures higher than 420°C, the changes in radical concentration are significant at 400°C. The catalyst promotes hydrogen donation of THN, which lowers the amounts of coke formed in the liquid phase, on the catalyst, and the activation energy of coking on the catalyst.

Stable radicals formation in coals undergoing weathering: effect of coal rank

Once coal is excavated it comes into contact with atmospheric oxygen and begins to undergo low temperature oxidation. The mechanism by which the molecular oxygen interacts with the coal macromolecule is suggested to occur in several steps. These steps primarily involve O(2) diffusion to the surface where physical adsorption followed by chemical adsorption takes place. The chemical adsorption forms several types of oxides that can subsequently react to form several products, primarily CO(2). It has also been suggested that some of these oxidation mechanisms might involve radical reactions. As the previous studies were conducted under conditions where significant structural changes occur it is possible that in the low temperature range (T < 100 °C) the oxidation mechanism is different. Several different rank (lignite-subbituminous-bituminous) coals were isothermally heated at 95 °C in an air atmosphere for a period of up to 6 months and samples were collected at two week intervals. The radical concentration of each sample was measured by Continuous Wave Electron Paramagnetic Resonance (CW-EPR). It is apparent that there are distinct differences between the lower rank (lignite) and the higher rank (subbituminous, bituminous) coals. The lower rank coals exhibited only carbon centered radicals with an adjacent oxygen atom and the higher rank coals exhibited only carbon centered radicals. Interestingly, the lower rank coals exhibited no change in radical concentration due to the long term oxidation treatment while the higher rank coals showed a distinct increase in the radical concentration. These findings shed new light on the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.

The effects of a solar eclipse on photo-oxidants in different areas of China

Abstract. This study investigates the effects of the total solar eclipse of 22 July 2009 on surface ozone and other photo-oxidants over China. A box model was used to study the sensitivity of ozone to the limb darkening effect during an eclipse event, and to show that the impact on ozone is small (less than 0.5 %). In addition, the regional model WRF-Chem was applied to study the effects of the eclipse on meteorological and chemical parameters, focusing on different regions in China. Chemical and meteorological observations were used to validate the model and to show that it can capture the effects of the total solar eclipse well. Model calculations show distinct differences in the spatial distributions of meteorological and chemical parameters with and without the eclipse. The maximum impacts of the eclipse occur over the area of totality, where there is a decrease in surface temperature of 1.5 °C and decrease in wind speed of 1 m s−1. The maximum impacts on atmospheric pollutants occur over parts of north and east China where emissions are greater, with an increase of 5 ppbv in NO2 and 25 ppbv in CO and a decrease of 10 ppbv in O3 and 4 ppbv in NO. This study also demonstrates the effects of the solar eclipse on surface photo-oxidants in different parts of China. Although the sun was obscured to a smaller extent in polluted areas than in clean areas, the impacts of the eclipse in polluted areas are greater and last longer than they do in clean areas. In contrast, the change in radical concentrations (OH, HO2 and NO3) in clean areas is much larger than in polluted areas mainly because of the limited source of radicals in these areas. The change in radical concentrations during the eclipse reveals that nighttime chemistry dominates in both clean and polluted areas. As solar eclipses provide a natural opportunity to test more thoroughly our understanding of atmospheric chemistry, especially that governed by photolysis, a comprehensive experimental campaign during a future solar eclipse is highly desirable.

Pulsed radio frequency plasma deposition of a-SiNx:H alloys: Film properties, growth mechanism, and applications

In this work, we propose a fabrication process of a-SiNx:H alloys by pulsing the radio frequency (rf) signal in a low pressure plasma-enhanced chemical vapor deposition (PECVD) system. The characteristics of the films can be controlled simply by adjusting the duty cycle of the pulsed rf power, while keeping the N2∕SiH4 gas mixture constant. Spectroscopic ellipsometry analysis in the ultraviolet-visible-near infrared and far infrared ranges, atomic force microscopy, and elastic recoil detection reveal strong variations in the optical properties (1.88⩽n⩽2.75, 10−4⩽k⩽5×10−2 at 550nm), optical gap (4.01eV⩽Eg⩽1.95eV), microstructural characteristics (1.3nm⩽surfaceroughness⩽8.3nm), and chemical composition (0.47⩽x⩽1.35) of the coatings as a function of duty cycle. This behavior is interpreted in terms of radical concentration changes in the gas phase, as well as variation in the average ion bombardment energy at the film surface, leading to modifications of both chemical and physical mechanisms that sustain the film growth. Using the control of duty cycle, we fabricated two types of a-SiNx:H-based thin film devices, namely, (i) a model Fabry-Perot optical filter deposited on plastic substrate and (ii) a superlattice structure displaying a photoluminescence signal four times higher than the reference single layer. These two examples of applications point out the main advantages of this pulsed rf PECVD process, in particular, low deposition temperature, reproducibility, versatility, and ease of use.

Radical Polymerization of Methyl Methacrylate in Presence of Ferrocene. Monte Carlo Simulation

Monte-Carlo technique is used for the numerical simulation of radical polymerization kinetics of methyl methacrylate in the presence or absence of ferrocene and exposition of molecular-weight distribution during the process. The peculiarities of polymeric radical concentration changes at the initial stage of polymerization are shown. Satisfactory correspondence between calculated and experimental data is obtained. The data of numerical experiment have allowed to evaluate the influence of ferrocene on kinetic parameters of polymerization.

Radical polymerization of methyl methacrylate and styrene in the presence of ferrocene

The influence of ferrocene on the kinetic parameters of methyl methacrylate (MMA) or styrene polymerization is investigated. It is shown, that in the presence of ferrocene the growth of the initial rate of polymerization is accompanied with significant decrease of the polymerization degree of polymer obtained when peroxides are used as the polymerization initiators. The polymerization kinetics parameters are evaluated. Its demonstrated that the process of polymerization in the presence of a ferrocene has a low activation energy, that allows to carry out the polymerization at lower room temperatures. The influence of the ferrocene on the molecular-mass characteristics of poly(methyl methacrylate) (PMMA) was studied and it was found that the influence of ferrocene on PMMA molecular mass is determined by the nature of the initiator used. The decomposition temperature of PMMA increases with the increase of ferrocene concentration. Monte-Carlo technique was used for numerical simulation of MMA radical polymerization kinetics in the presence and in the absence of ferrocene and to follow the change of molecular-weight distribution during the polymerization. The peculiarities of polymer radical concentration changes at the initial stage of polymerization are shown. Satisfactory correspondence between calculated and experimental data was obtained.

Analysis of radical content on carbon black pigments by electron spin resonance: influence of functionality, thermal treatment and adsorption of acidic and basic probes

The radical content and its change under thermal treatment of different types of carbon black (CBs) are examined using electron spin resonance (ESR) through calibration via stable free radicals. In order to characterise a range of different CBs, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) have been used in an attempt to determine the nature of the functional groups present on their surface. A linear correlation was found between the oxygen content on the CB surface obtained by XPS analysis and the radical content for several grades of pigment. Increasing surface oxygen content resulted in minor changes in radical concentration between the different CB grades analysed. An overall decrease in radical content occurred with exposure of the CBs at higher treatment temperatures accompanied by a recovery between 450 and 750°C. Depending on the CB grade there were also some structural changes, which resulted in an increase in the specific surface area, as well as, a progressively more extensive localisation of the itinerant basal plane electron states associated with chemisorptions of atmospheric oxygen after the thermal treatment. The change of the radical content of the CBs with the thermal treatment was coincident with parallel FMC studies using basic and acidic probes, which revealed changes in acidic and basic character throughout the treatment. These features could be related to changes in surface morphology and chemistry of the modified blacks. Other techniques like thermogravimetric (TGA), BET analyses and Karl Fisher test were also performed in order to determine the differences in specific surface, water and volatile content of the CB pigments and their influence on the radical content. Inter-relationships are shown and discussed.

Formation of free radicals during drying and oxidation of a lignite and a bituminous coal

The changes in radical concentrations and species brought about by vacuum and gas flow drying of New Vale lignite and Stockton bituminous coal samples were studied. Vacuum drying produces a sudden increase in concentration, flow drying a more gradual but ultimately greater one. For both coals the changes depend on a balance between decarboxylation, which produces new radical sites, and removal of moisture from heteroatomic sites within the coal macrostructure. This balance is influenced by the drying method used. Exposure of dried samples to dry air produces changes in radical concentration consistent with the findings of previous oxidation studies of these coals.

A study of the process of pyrolysis in a water-vapor stream of activated carbons, prepared from agricultural by-products by some physico-chemical methods

The changes in the radical concentration and the development of the porous structure of the activated carbon obtained by pyrolysis in the presence of water vapor of various agricultural by-products are investigated by EPR spectroscopy and argon thermal desorption. It was found that they depend on the nature of the raw material and the final temperature of the process. A similarity was established between the dynamics of the change in radical concentration and the development of new surfaces of some of the activated carbons.

Cool flames and oscillations in hydrocarbon oxidation

Stabilized cool flames and oscillations of propane and butane have been studied. An experimental technique has been developed, which enabled for the first time the detection of free radicals in stabilized cool flames of hydrocarbons. For this purpose, a two-section flow reactor has been constructed. Cool flame appears in the second section and stabilizes there. In order to detect free radicals and examine their behaviour, a small part of the gas flow was sampled from each section continuously and directed at low pressure to a freezing pin at liquid nitrogen temperature inside the cavity of an electron paramagnetic resonance (EPR) spectrometer. It has been shown that when passing from the first section (preflame zone) to the stabilized cool flamezone, the concentration of radicals increases aruptly. For example, in the case of propane (C3H8:O2=1:1; P=270 torr; T=320°C), the radical concentration makes up 6·1012 molecules/cm3, whereas in the cool flame zone it is 3·1014 molecules/cm3 at the same temperature. This abrupt change of radical concentration is caused by the self-accelerated chain process. The temperature increase in the cool flame zone results in an increase of the radical quantity. Thefurther temperature increase leads to the decrease of radical quantity, and the flame fades at T=370°C to 380°C. Asymmetric profiles of temperature, radical concentration, and stable products appear in the cool flamezone along the vertical cross section of the reactor. It is shown that the regularities observed are the results of nonlinear processes with the participation offree radicals. The same results are obtained for other hydrocarbons and mixtures. By changing the process parameters, damped and stable oscillations are observed, which are correlatedto cool flame characteristics. Stabilized cool flames of propane and butane have been studied in a special two-section reactor. Direct experimental data have been obtained on the behaviour and nature of free radicals in the stabilized cool flame. It has been ascertained that the appearance and stabilization of the cool flame are accompanied by an abrupt increase of radical concentration caused by the rapid self-accelerated chain process. The temperature increase above 350°C to 360°C adversely affects the process development. The radical concentration is decreased, and the cool flame is damped. The oscillation regimes of the oxidative process have also been studied in the two-section reactor. A correlation between the cool flame phenomenon and the oscillations has been verified.

The Early Stage of Carbonization of Synthetic Pitches Derived from Aromatic Hydrocarbons Using Trifluoromethanesulfonic Acid as Catalyst

The early stage of carbonization of synthetic pitches derived from aromatic hydrocarbons such as naphthalene, anthracene, phenanthrene and 9, 10-dihydroanthracene, with trifluoromethanesulfonic acid as a catalyst were investigated by means of high temperature ESR.Temperature dependence of radical concentration of synthetic pitches observed with in situ ESR showed the following three distinct stages in common; 1st stage, from 100°C to 200°C, where radical concentration increases: 2nd stage, from 200°C to 400°C, in which decreases: and 3rd stage, above 400°C, where again increases. Especially, the change in radical concentration in the 2nd stage is closely related to the form of optical anisotropic texture of derived carbon.The radical concentration of pitches measured at room temperature increased with increasing of average molecular weight, and good correlation was found.

The effect of heating rate on radical concentration during coal pyrolysis

The effect of heating rate on the change in radical concentration during coal pyrolysis has been studied using high temperature ESR with a rapid scan unit. The maximum radical concentration of Yallourn coal decreased with an increase in the heating rate. The results were compared with those obtained from a theoretical treatment of the kinetics of radical formation. The effect of a catalyst on the change in radical concentration has also been monitored under rapid heating condition. The maximum radical concentration of the coal/ZnCl 2 system showed a significant shift to temperature about 100 K lower than that of coal alone.