Thermal Stability of Carbon-Centered Radicals Involved in Low-Temperature Oxidation of Bituminous and Lignite Coals as a Function of Temperature.

Coal is intensively used worldwide as a main fuel source. However, it may undergo oxidation processes [i.e., low-temperature oxidation (LTO)] when stored under an air atmosphere in piles post-mining at low temperatures ranging from 300 to 425 K, specifically, a surface gas/solid reaction with molecular oxygen. Therefore, it is of major importance to prevent or appreciably slow down such reactions, which result in a loss in the energy content (calorific value) of coal. Previously, we showed that radicals are formed during the LTO process. In this work, the dependence of radical formation on coal rank as a function of heating (temperature) and the presence of oxygen gas were studied using electron paramagnetic resonance spectroscopy. It was shown that lignite coals are more sensitive than bituminous coals to the atmospheric environment (i.e., molecular oxygen and nitrogen content) and to temperature, as reflected by the formation of surface carbon-centered radicals. Moreover, this is the first publication showing the effects of LTO on micro- and macro-pores by assessing how these structures affect O2 diffusion. The LTO process blocks the micro-pores, such that radicals form mainly at the surface of the coal macromolecules, in both bituminous and lignite coals.

Introduction

Low-Temperature Oxidation of Coal

Coal piles stored for long periods under atmospheric conditions at temperatures from RT to 150 °C undergo a chemical process of degradation known as low-temperature oxidation (LTO). LTO is a gas/solid reaction, in which atmospheric oxygen adsorbed physically to the surface of the coal macromolecules reacts with the different functional groups of coal, resulting in a change in the chemical content of the coal.[1−4] The LTO process combines different types of chemical chain reactions that eventually emit not only gaseous products, mainly carbon dioxide (CO2) and water (H2O), but also (as secondary products) polluting and fire hazardous gases, such as carbon monoxide (CO), low-molecular weight hydrocarbons CH (n = 1–5), and flammable H2.[1−5] It was found[3,5−7] that the process occurs from the moment that coal is exposed to atmospheric oxygen at room temperature, and as the reactions are exothermic, the temperature of the coal increases, such that from 150 °C, the rate is fast and may cause fire to erupt. Such self-heating is autocatalytic; it accelerates with temperature and occurs when the heat dissipation from the coal is slower than heat production via the LTO process.[2−4] Thus, the self-heating and formation of hot spots in coal piles occur at maximal depths of up to 3 m because no diffused atmospheric oxygen can penetrate the pile any deeper.[4,8−10] As a result of the LTO process, the calorific value of coal decreases due to structural changes. Likewise, the chemical composition of the coal macromolecule is modified during the process.[2,3,11] These changes are due to the oxidation processes, which also act as a weathering process of the coal piles under atmospheric storage conditions.[8,12−14]

The LTO process can be sub-divided into three main steps:[15,16]

Coal + O2(g) → O2(adsorbed physically)

O2(adsorbed physically) → O2(chemisorbed)

O2(chemisorbed) → CO2 + H2O(major products) and CO + CH + H2(minor products) + SO

It has been shown that the gas/solid reactions occur on the outer surface of the coal particles and in macro-pores, namely, pores wide enough to allow diffusion of atmospheric oxygen. The gases produced can diffuse out of the pores, although in weathered coals, the micro-pore entrance is blocked by chemisorbed oxygen.[17]

Whereas the mechanism underlying the LTO process is very complicated and has not yet been fully defined, it is essential to inhibit or reduce the rate of LTO so as to mitigate damage to coal quality resulting from a degradation in calorific value and to be able to prevent the occurrence of hot spots in coal piles, which can lead to fires in coal storage areas.[10,14,18,19] It is clear, however, that step (iii) of the LTO process involves the formation of carbon-centered radical species that are active in the chain reactions that make LTO a major concern for the coal industry.[1]

Free Radical Formation

Recently, we have studied[1,16,20] the coal LTO process using two types of coal, namely bituminous coal (Baily mine, denoted as BA) and lignite coal (Hambach mine, denoted as HA), as these two coals are used for power production.[2,12,21,22] These coals can be differentiated by the chemical composition of the coal macromolecule, a property which affects many aspects of the oxidation process.[1,5,16,20,23] Our earlier studies revealed that carbon-centered radicals are formed during the LTO process. The structure of these radicals is dependent on the coal rank, as the type of radical formed is dependent on the chemical environment and changes according to the chemical content of the coal.[16,21,24,25] The radicals were studied using electron paramagnetic resonance (EPR) spectroscopy.[16,20] Such studies also found that the precursors of the radicals are hydroperoxide groups, coal–OOH, formed at the coal surface when decomposed chemisorbed oxygen reacts with nearby active carbon–hydrogen groups in the coal macromolecule.[1] Carbon-centered radicals of aromatic character are of higher concentration than are aliphatic-based radicals.[1] Moreover, the carbon-centered radicals of aromatic character are more stable than are carbon-centered radicals with an adjacent oxygen atom (which are formed at much lower concentrations) although both radicals are much more stable than are the carbon-centered radicals of aliphatic character.[1] The characterization of these radicals and their stabilities via EPR spectroscopy provided more information on their involvement in the LTO process and revealed that the ratio of the different types of radicals is affected by the chemical environment of the coal.

Further EPR-based studies could thus further elucidate the mechanism underlying radical formation. It has been shown[16,20] that coals that have undergone simulated weathering processes (e.g., heating at 368 K under an air atmosphere for weeks) contain much higher concentrations of carbon-centered radicals than do fresh coals, thus indicating that these radicals are produced as a result of the LTO process. In addition, the type of carbon-centered radical is directly influenced by the chemical composition of the coal (element ratio, functional groups, etc.)[1,16]

Mechanism of Coal Oxidation

The primary mechanism[1] suggested to occur during the reaction of the solid coal surface with atmospheric oxygen is presented in Scheme .

Scheme 1

Mechanism Underlying the Formation of Radicals in the LTO Process; The Source of H+, Reaction 3, can be Functional Groups in the Coal, Such as Alcohols or Aldehydes

This mechanism describes the chemisorption of molecular oxygen to the coal macromolecule [reactions 1, 2, which are parts of steps (i) and (ii), delineated above] and cleavage of the hydroperoxide group, forming two types of “O”-centered radicals [via reactions 3, 4, which are a part of step (iii), delineated above]. These finally react with aliphatic hydrogen in the coal macromolecule, yielding a carbon-centered radical [reactions 5, 6, which are also parts of step (iii), delineated above].

To better understand how temperature affects the overall LTO process (important for devising different methods to reduce or inhibit this undesired event), it is essential to measure the activation parameters involved in producing the final gaseous products within the carbon-centered radicals. It is, moreover, of interest to characterize differences between bituminous coal, which contains an appreciable aromatic hydrogen content, and lignite coal, which contains a much higher aliphatic hydrogen content and oxygen. These coals serve as the main fuels used by utilities worldwide.

Energetic Aspects

All information reported to date[26,27] has discussed activation energies related to those aspects of the LTO process involving reactivity of molecular oxygen with coal or the emission of carbon dioxide and carbon monoxide as final products. None, however, has dealt with radical species. For example,[26,27] the Ea for O2 consumption was reported[26,27] to be 54–61 kJ/mol for bituminous coal and 63–69 kJ/mol for lignite coal. For CO2 emission, the Ea was 73–75 kJ/mol for bituminous coal and 67–77 kJ/mol for lignite coal, whereas for CO release, the Ea was 106–110 kJ/mol for bituminous coal and 76–83 kJ/mol for lignite coal.

Still, this information does not reflect how temperature affects the adsorption or chemisorption of molecular oxygen by coal nor the pattern with which O2 binds to the surface of the macromolecule and undergoes chemisorption.[26,28] An earlier report[16] described the physical structure of the pore system of coal, which is dependent on pore width. Those pores on the surface, that is, on the edge of the coal’s particle, absorb O2 better than pores found elsewhere because of their location and size. This finding supports the above conclusions concerning the activation energies, Ea, of the LTO process and the O2 absorption process.

In the present study, we addressed how temperature affects the carbon-centered radicals created by these LTO processes by analyzing EPR spectra recorded in situ as a function of temperature to better understand radical stability at different coal ranks and to thus obtain better understanding of the LTO process.

Results and Discussion

To evaluate the involvement and thermal stability of carbon-centered radicals in the LTO process, two coals were studied, namely, high-rank bituminous coal and low-rank lignite coal. In addition, activated coal served as a reference material (carbon content: 98% of the aromatic type, no hydrogen content). The bituminous coal (BA) was studied from the USA (Bailey Co., Pittsburgh no. 6 coal), the lignite coal (HA) came from Germany (Hambach coal from the Baden-Württemberg area, denoted as HA), and the activated carbon (AC) was produced by Merck (CAS no. 7440-44-0).

BA coal serves as fuel in pulverized coal-fired power plants in Israel and the USA while HA coal is used in German utilities. AC is used as an effective adsorbent for cleaning pollutants (in water) or for filters, in such as gas masks, as it is very porous and has a high surface area of 800–1200 m2/g.[27] Activated charcoal is highly purified (steam-activated and acid-washed) and comes in powder form. The hydrogen content, water content, and the aliphatic/aromatic content of the three coals (qualitative data) are listed in Table .

Table 1

Qualitative Data: Coal Hydrogen, Water, and Carbon Contents[20]a

	coal component properties
coal type	hydrogen content	water content	carbon content
HA	high content of aliphatic hydrogen	∼50%	mainly aliphatic
BA	high content of aromatic hydrogen	∼8%	mainly aromatic
AC	no hydrogen	none	only aromatic

Reprinted (Adapted or Reprinted in part) with permission from [Phys. Chem. Chem. Phys.2013,15, 6182/DOI: 10.1039/c3cp50533b]. Copyright [2013] [Green Uri, Zeev Aizenshtat, Sharon Ruthstein and Haim Cohen]. Green, U., Aizenshtat, Z., Ruthstein, S. & Cohen, H. Reducing the spin–spin interaction of stable carbon radicals. Phys. Chem. Chem. Phys. 15, 6182–6184 (2013).

BA coal, with the highest calorific value (29,258 J/g),[29] is exported overseas by ship to be used in utilities. It can be stored in large piles under open air conditions in utility yards of the utilities.[20] Anthracite coal, the highest graded coal, is not economically feasible for use as fossil fuel in power plants. HA coal has a lower calorific content (25,323 J/g)[29] and has a much higher aliphatic content[20] (i.e., a lower coal rank) and thus undergoes LTO processing much faster than does BA coal. As such, HA coal is used immediately after being mined, requiring utilities to be located adjacent to the mining area. AC coal is of an aromatic nature (100%) yet has no aromatic hydrogen structure[30,31] and thus, does not undergo the LTO process.

Energetic Aspects of the LTO Process in Coal

The different carbon-centered radicals were measured in fresh coal (BA fresh or HA fresh, which had not been exposed to the LTO process) and in weathered coal (BA 5 w or HA 5 w, 5 weeks of simulated weathering at 368 K in an oven under an air atmosphere) to determine how the LTO process affected the carbon-centered radical content and stability.

According to the Boltzmann equation, temperature affects the number of paramagnetic centers in the low-energy state as followswhere A is the pre-exponential factor (the number of radicals when T = 0), h is Planck’s constant, ν is the microwave frequency that radiates on the samples in the EPR unit (∼9.5 GHz), R is the gas constant 8.314 (J/mol K), and T is the temperature (kelvin). Thus, at low temperatures, the signal-to-noise ratio (SNR) is expected to be greater than at high temperatures because more paramagnetic centers in the lower energy state will be excited upon exposure to the microwave photons. In the temperature range of 300–365 K (∼30–95 °C), changes in the EPR signal, owing to the Boltzmann effect, are usually negligible. Therefore, any effects seen on the EPR signal are mainly due to different processes that occur within the sample.

The oxygen molecule, O2, is paramagnetic, so that when it interacts with paramagnetic centers, namely, carbon-centered radicals, the intensity of the EPR signal will be reduced because of exchange interactions between the two moieties. Moreover, the SNR might be reduced. When coal samples are measured under an air atmosphere (21% O2) or pure oxygen (>99% O2) using an EPR spectrometer, the effects of the exchange interaction between O2 and the paramagnetic carbon-centered radicals in the coal sample at various temperatures (568–638 K) were revealed. Figure shows the effects of temperature on the signal of the carbon-centered radicals in the EPR spectra for fresh and 5 w (weathered by 5 weeks of LTO at 368 K) BA and HA coals under an O2 gas atmosphere.

Figure 1

CW-EPR spectra collected in an O2 environment at various temperatures (300–365 K) for (A) BA fresh coal (no LTO process), (B) BA 5 w coal (after 5 weeks of the LTO process at 368 K), (C) HA fresh coal (no LTO process), and (D) HA 5 w coal (after 5 weeks of the LTO process at 368 K).

Whereas for BA 5 w there was almost no change in the signal, an appreciable change in the signal was observed for HA 5 w coal as a function of temperature. This result is surprising because it is contrary to the expectation that the radical concentration should be reduced upon an increase in temperature (see above). To better understand the effects of the temperature and atmospheric environment on the spin concentration, the natural logarithm of the number of spins (ln(# spins)) detected as a function of 1 over the temperature (1/T) was plotted. The resulting graph suggests exponential growth/decay of the signal with temperature (Figure ).

Figure 2

Natural logarithm of the number of spins, calculated according to a standard sample,[1] as a function of 1/T (K–1) for (A) BA fresh and BA 5 w coal in an air environment, (B) BA fresh and BA 5 w coal in an O2 environment, (C) HA fresh and HA 5 w coal in an air environment, and (D) HA fresh and HA 5 w coal in an O2 environment.

In an earlier report,[7] it was shown that the LTO process increased the number of radicals. Specifically, a large increase in spin concentration was detected for BA coal, as compared with HA. Interestingly, whereas the detected spin concentration in 5 w coals after 5 weeks of LTO was greater both for BA and HA coals than for fresh coal under air, under O2, the detected number of spins was lower in 5 w coals. Moreover, the number of spins increased for fresh HA and BA coals in an oxygen environment and decreased after 5 weeks of LTO in both cases. EPR analysis was used to determine the number of radicals in the total volume of the coal samples. However, the LTO process, which is a solid/gas reaction, occurs at the surface of the pores of a coal sample. Thus, carbon-centered radicals, which are products of the LTO process, are found on the coal pore surface. The 5 w (weathered coal) and fresh coal samples differ in that in 5 w coal, no O2 molecules can enter the micro-pores, which are blocked by chemisorbed oxygen molecules as a result of the weathering process. Instead, only the macro-pore surfaces are available for interaction with oxygen molecules. Because the surface area within the macro-pores is much smaller than that of the micro-pores in the two 2 coals studied (in BA and HA coals, the macro-pore surface area was 3.61 and 1.41 m2/g, respectively, while for micro-pores, the surface area was 100.54 and 109.53 m2/g, respectively[16]), the concentration of carbon-centered radicals that cannot interact with O2 in the weathered coals predominated. This experiment suggests that new carbon-centered radicals are formed at the micropores of fresh coals under an oxygen environment. However, after 5 weeks of the LTO process, the micro-pores are blocked. Therefore, the interaction between the macro-pore surface carbon radicals and the paramagnetic oxygen molecules resulted in fewer detected carbon-centered radicals in the weathered 5 w coals. It is of note that the EPR spectrum did not detect any new species forming in an oxygen environment, owing to the relatively broad line width. Consequently, the g-values of any such species cannot be determined. The g-value is a parameter which is affected by the chemical environment near the free radical, and it was found that when there is an oxygen atom adjacent to the free radical, the g-value of the free radical rises.[16] Therefore, a carbon-centered radical in an aromatic chemical environment has a different g-value (2.0032) than does a carbon-centered radical with an adjacent oxygen atom (2.0039–40).[16,32]

EPR analysis also detected different behaviors in the various coals as a function of temperature. In addressing temperature-dependence of the radical, values measured at 298 K represent the initial state. For BA coals (both fresh and BA 5 w), there was a small increase in the SNR at lower temperatures (in both atmospheric environments) as expected, owing to the Boltzmann equation (eq ). For HA coal in an air environment, a trend similar to that seen with BA coal was detected, namely, a small increase in the SNR at lower temperatures. However, in an oxygen environment, the change in the ln(# spins) as a function of 1/T was much more pronounced, with the value of the slope suggesting that an additional chemical process had occurred as a function of temperature in an oxygen environment. Moreover, at high temperatures, the SNR was higher for HA fresh coal yet was lower for HA 5 w coal. Finally, the errors calculated in three different experiments were higher for HA 5 w than for HA fresh coal under an air/O2 atmosphere.

The micro-pore surface area of HA coal is higher than that of BA coal.[16] Moreover, in HA coals, there is more aliphatic carbon on the coal surface, whereas BA is characterized by many more aromatic functional groups. Aliphatic carbon is more sensitive to changes in the environment. The EPR experiment suggests that at high temperatures, the aliphatic carbon in the micro-pores of HA coal undergoes an additional oxygenation process, which results in the formation of more carbon-centered radicals.

Previously, we suggested that the LTO process induced the formation of carbon-centered radicals in the aromatic macro-pore region.[1] During the initial days of the LTO process, an increase in the concentration of surface radicals in HA coals, resulting from interactions between oxygen and aliphatic C–H groups, was observed. However, after two weeks of LTO at 368 K, a more pronounced increase in macro-pore aromatic carbon radical levels was detected. The higher concentrations of radicals detected for fresh HA coal at a higher temperature, as compared with such increases seen at low temperature in the presence of oxygen, suggest that more surface radicals are exposed after two weeks of LTO at 368 K. This observation also suggests faster kinetics of surface radical formation at 368 K under oxygen. After 5 weeks of the LTO process at 368 K, the concentration of aliphatic C–H groups on the surface of HA coal decreased. Therefore, at high temperatures, interactions between oxygen molecules and the macro-pore aromatic radicals resulted in a decrease in the detected number of spins and a lower SNR.

The results shown in Figure also suggest that the rate of the change in the number of spins with respect to temperature differs for the various samples in different environments, as manifested by the different slope values. The slope value may be related to the energy, hν, as described in eq . For BA coal, it is clear that the slope increases when measured under air, containing 21% O2 (Figure A), than under O2 gas (Figure B). Specifically, the reaction occurs at a slower rate under a lower concentration of molecular oxygen (generating a larger hν value). In view of the periods of the LTO process (Figure B), there is apparently no difference in the hν values under O2, whereas under air (Figure A), this difference is significant. This could imply that under 21% O2, not all oxidative sites are occupied by physically adsorbed O2 molecules (see step (i) described in the Introduction). Therefore, oxidative sites may be affected by temperature, reflected as a change in the hν value. Thus, it can be concluded that N2 molecules in air are not inert from the point of view of their interaction with carbon-centered radicals. Indeed, it is possible that the N2 molecules are affected by physical adsorption at sites adjacent to carbon-centered radicals. This would prevent the entry of O2 molecules that undergo an exchange interaction with the carbon-centered radicals. In other words, a masking interaction occurs, which prevents O2 molecules from interacting with the carbon-centered radicals, which are protected by N2 molecules in air.

When investigating HA coal, it was interesting to find a negative slope for a fresh HA sample under O2 (Figure D). In contrast, the opposite trend was found for the HA 5 w sample. This may reflect the fact that a fresh sample of HA has more oxidative sites (i.e., a higher percentage of aliphatic C–H entities) than a weathered sample. Hence, changes in temperature would have a greater effect, translated as an appreciable increase in the reaction rate, with fresh HA coal than with HA 5 w coal, which has almost no available oxidative sites. Under an air atmosphere (Figure C), HA 5 w coal has a greater slope value, which implies a higher hν value, indicative of a decreasing rate of carbon-centered radical production. These observations indicate that HA lignite coal is more sensitive in terms of calorific value losses, as changes in the chemical structure were larger than in BA coal due to faster oxidation. Accordingly, the significance of avoiding energy loss with HA coal is greater, and as such, storage conditions are problematic. It is also difficult to compare the effects of the two atmospheres because under an O2 atmosphere, fresh HA coal (Figure D) shows the opposite trend (negative). This surprising result indicates that the reaction of fresh HA coal at higher temperatures results in the appearance of carbon-centered radicals, masking the usual effect of higher temperatures, namely the appearance of a lower concentration of carbon-centered radicals with a lower energy level. For the HA 5 w samples (Figure C,D), it is clear that under an air atmosphere, the slope is significantly smaller (4.65 orders of magnitude). This may imply that at a lower concentration of O2, the influence of temperature is much smaller. Thus, the loss of calorific value will be reduced when the coal pile is in an inert environment.

Conclusions

The radical content produced in fresh coal via the LTO process was compared to such content in weathered coal 5 w (subjected to 95 °C for 5 weeks in air), in the 300–365 K range.

It was found that in weathered coals, micro-pores are blocked and do not participate in the LTO process with atmospheric oxygen. The difference in the LTO process impact of high-ranking BA bituminous coal and low-ranking HA lignite coal stems from the different reactivity patterns of the process in the two coals. Such a difference reflects the high aromatic content of bituminous coal and the high aliphatic C–H content of lignite. The results are important for understanding radical involvement in the LTO process and may help elucidate measures to reduce self-heating in coal piles stored under air for long terms in the yards of coal mines or utilities.

Experimental Section

Coals

Bituminous and lignite coals were studied. As a reference, AC was used. These coals differ in their coal rank, with the carbon content appreciably affecting the LTO process. All coals were prepared by grinding and sieving to a particle size of 74 μm ≤ X ≤ 250 μm. The coal samples were then dried under vacuum in a vacuum oven (Heraeus model VT6060) for 24 h at 333 K.

Guidelines for characterizing the coals were as follows: bituminous coal from the USA (denoted as BA, for Bailey mines) is a Pittsburgh no. 6-bituminous coal. Lignite coal was from Germany (denoted as HA, for Hambach mines) and is a lignite coal. AC was produced via pyrolysis of peat as a biological source (Merck, CAS no. 7440-44-0) at high temperature (673–873 K). AC is very porous and presents a large surface area. The Bailey bituminous coal studied here serves as a fossil fuel for coal-fired power plants in Israel and the USA. The lignite coal is used in German utilities. AC served as a reference due to its high carbon content (Table ). The chemical characterization and properties of all three coals are presented in Table . The data are based on an elementary analysis, measured for each coal type.

As shown in Table , HA coal has a lower carbon content, is much richer in oxygen and in aliphatic hydrogen contents, and is lower in aromatic content, as compared with BA coal. Thus, because aliphatic C–H groups are much more active, the LTO process is much faster in lignite HA coal than in the bituminous BA coal (Table ).[1,29]

Table 2

Properties of Coalsa

	analytical data
	wt %			wt %, db
sample	moisture	ashwf	VMdb	C	H	Ob	S	CV (J·g–1)
HA	34.53	5.09	52.39	66.12	4.32	23.65	0.16	25,323
BA	5.87	7.78	37.20	78.07	5.18	5.84	1.50	29,258
AC	2	6	<3	>98	<2	<2

VM = volatile matter; CV = calorific value; db = dry basis, wf = water free. BA = Bailey, USA; HA = Hambach, Germany.

% O content was calculated according to the equation 100 – % C – % H – % S – % ash.

To simulate the effects of the LTO process on carbon-centered radicals, the coals were aged under atmospheric conditions at 368 K. The sieved coals were transferred to a glass vial at 368 K in an oven (MRC model MF3000) under an air atmosphere for 5 weeks. The oxygen concentration in the oven was maintained at ∼21% throughout the oxidation periods to simulate the weathering process. Effects on the heat-treated coal were compared to those on the fresh coal.

Chemicals

Measurements were taken under air or O2 (CP purity level) atmospheres. The water used throughout this study was mineral-free treated water with a conductivity of >14 MΩ/cm.

EPR Spectroscopy

Stable carbon radicals were characterized for different coals using EPR spectroscopy. EPR spectra were recorded using an E500 Bruker Elexsys spectrometer operating at 9.0–9.5 GHz. The spectra were recorded at a microwave power of 2.0 mW, a modulation amplitude of 1.0 G, and a time constant of 60 ms (WILMAD). The samples were measured in 3.0 mm quartz tubes. g-values were measured using bis-diphenylene-phenylallyl (BDPA) and diphenyl-picrylhydrazyl (DPPH) standards. The information recorded during measurements of BDPA and DPPH was used in a precise calibration method to determine the g-value using Xepr software in the Elexsys spectrometer, which takes into account any shift in frequency. The described setup, in a controlled gas atmosphere system, was used to implement a series of EPR in situ temperature scale measurements. The system, described in Scheme , included Tygon tubes connected to air and O2 by a high-vacuum tube faucet. To allow equal pressure measurements for all samples, the system was connected to a pressure meter and a flow meter.

Scheme 2

Description of the Experiment Setup That Includes a Controlled Gas System

Using the same gas system, each coal sample was measured in situ as the temperature was changed, using a heating element connected to a Dewar (cavity) in the range of 293–365 K under air or O2 conditions (each at atmospheric pressure). EPR analysis and the spin concentration, which were used to calculate activation energies, were determined by performing double integration using the Origin program.