Dynamic In Situ 9-GHz Electron Paramagnetic Resonance Studies of

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Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch026

Dynamic In Situ 9-GHz Electron Paramagnetic Resonance Studies of Argonne Premium Coal Samples H. A. Buckmaster and Jadwiga Kudynska Department of Physics and Astronomy, The University of Calgary, Calgary, Alberta, T2N 1N4, Canada This chapter describes the preliminary determination of the spectroscopic parameters and a d Argand diagrams at 20 °C for as-received, dried, and moisture-saturated samples of the eight Argonne Premium coals using 9-GHz continuous -wave electron paramagnetic resonance (CW EPR) spectroscopy. The Argand line-shape diagrams characterize these coals according to rank. The low-temperature oxidation of as-received, dried, and moisture-saturated samples of the Wyodak-Anderson subbituminous and Blind Canyon high -volatile bituminous coals were studied by using dynamic, in situ 9-GHz CW EPR spectroscopy. The spectroscopic parameter changes below 100 °C are due primarily to one free radical species, and those above 100 °C are due to the other species and attain broad maxima near 120 °C. Exposure to nitrogen causes the maximum spin concentration to occur at a lower temperature in the dry Wyodak-Anderson coal but nitrogen exposure has the opposite effect in the Blind Canyon coal, a result suggesting that storage of coals in nitrogen may have altered the chemical properties of the Wyodak-Anderson coal. 0

0

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CONTINUOUS-WAVE electron paramagnetic resonance (CW EPR) measurements have been performed on coal samples that have been 0065-2393/93/0229-0483$07.00/0 © 1993 American Chemical Society

Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


484

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

contained in evacuated and sealed containers. The objective was to remove the air and water from the sample to avoid contamination effects, with particular emphasis on the presence of oxygen (1-5). Recently, Buckmaster, Kudynska, and co-workers (6—9) have performed exploratory studies of selected Alberta coals; these studies were directed to an assessment of the feasibility of using CW E P R spectroscopy to determine the susceptibility of Alberta coals to spontaneous combustion. A methodology was developed to study the low-temperature oxidation processes in these coals at temperatures below 250 °C to determine those factors that might play an important role in the sequence of events leading to spontaneous combustion. The industrial importance of this knowledge is self-evident. For controlled low-temperature oxidation, the relative total spin concentration increases by a factor of 6 between 25 and 100 °C for a sample of a subbituminous (SB) coal but by a factor of only 2 for a sample of a highvolatile bituminous (HVB) coal, and the presence of air is essential for these changes to occur (6). Demineralization of the H V B coal sample reduced the relative total spin concentration increase in this temperature interval by 25% (7). A study of the exinite-depleted and -enhanced as well as the intact samples of an H V B coal revealed that the low-temperature process occurred in the vitrinite maceral, if the contribution of the inertinite maceral to the change in the relative total spin concentration was assumed to be either negligible or temperature independent (8). The role of moisture content in the H V B sample was also investigated. The moisture content is the most important factor determining the maximum percentage increase in the relative total spin concentration (9). As a result of this experience, we decided to repeat the same type of measurement methodology on samples of the eight Argonne Premium coals. Preliminary measurements were made on all eight samples. In addition, comprehensive measurements have been completed on samples of SB (Wyodak-Anderson) and H V B (Blind Canyon) coals. These two coals were selected because their physical and chemical properties were similar to those of the Alberta coals that have been studied previously (6-9). This chapter describes the results of those measurements.

Experimental Details Sample Preparation. The sealed glass vials containing the samples were opened in a small, dry-argon-atmosphere glove box. Each vial was divided into three equal parts, which were then placed in a miniature desiccator jar. The sample tube assembly described in the next section was loaded with 50 mg of the asreceived sample. This assembly was then removedfromthe glove box and inserted into the tapered ground-glass joint in the high-sample-temperature resonant Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


26. BUCKMASTER & KUDYNSKA

485

In Situ 9-GHz EPR Studies

cavity glassware. The glove box was prepared for another sample transfer when the first EPR measurement sequence was completed. Samples of the Wyodak-Anderson, Pocahontas No. 3, and Blind Canyon Argonne Premium coals were measured on the day that the vials were opened. All eight coals were then measured 4 days later. All of the coals had dehydrated slightly when these latter measurements were made because the glove box contained dry argon. The glove box also contained two larger desiccators. One, which contained phosphorus pentoxide, was used to dry one fraction of each sample, and the other, which contained a saturated potassium sulfate solution, was used to moisture-saturate another fraction of each sample. Experience indicated that 3 days was sufficient time for the samples to attain their new states of moisture equilibria. The physical and chemical characterization of the eight Argonne Premium coals samples have been studied by many researchers and have been summarized elsewhere and in the references therein (10).

CW EPR Measurements. The 9-GHz CW EPR spectrometer used synchronous demodulation at the microwave and magnetic-field modulation frequencies (10 kHz). A Stanford Research SR 530 lock-in amplifier was used at 10 kHz because it could be operated under computer control via an IEEE-488 bus. The EPR data acquisition was controlled by a Zenith Z-158 computer, and the data sets were transferred to a SUN 3/160 workstation for analysis. Software was developed to facilitate the calculation of the CW EPR spectral parameters and for the creation and plotting of the radial-difference atft Argand diagrams (RDAQÎÎQAD). (The Fourier absorption component is a ; the Fourier dispersion component is d .) The relative total spin concentration, N(T), at temperature Τ was calculated by double integration of thefirstFourier absorption coefficient, a The value of a was calculated by single integration of a and d was obtained from a by using a Hubert transform (6). No attempt was made to determine the absolute spin concentration because this study was concerned with only relative changes in the total spin concentration. The values and their errors in the spectral parame ters were determined by repeating the computer spectral analysis 10 times. The errors were confirmed independently by repeating the low-temperature oxidation experiment with new samples of the same coal. The following values were obtained: the spectroscopic splitting error, Ag, = ±0.00002; the error in effective line width at half maximum amplitude, Δ(ΔΖ? ), = ±10 μΤ; and the error in relative spin concentration, ΔΛΓ /# , = 0.05. The cylindrical TE -mode Bruker ER 4114 MT high-sample-temperature 9-GHz resonant cavity was used with a new, laboratory-designed sampletemperature controller that incorporated the Bruker B-TC 80115 power supply used previously. This controller features direct sample-temperature measurement, 1-K accuracy, long-term temperature stability of ~0.1 K, temperature-set-point repeatability of ~0.1 K, very fast temperature-response time (a few seconds for a 5-K set-point change), and near-critical damping response. Q

Q

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486

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

The EPR measurements were made in constant temperature-time incre ments to ensure that all measurement sequences had the same thermal "history" and to enable those measurement sequences on the same sample source to be repeated and compared reliably (II). A special sample holder that was designed permitted various gases to flow through the sample at a rate of 200 mL/min and enabled the temperature of the sample to regulate the temperature controller feedback loop. The total elapsed time between setting the temperature and the end of recording the EPR spectrum at each temperature was 15 min. Measure ments were made in 5 °C stepsfrom25 to 150 °C and in 20 °C steps from 150 to 250 °C. At each temperature, the 50-mg sample, of which 35 mg was inside the resonant cavity, was exposed to either 15 min of dry air flow or 5 min of dry air flow followed by 10 min of dry nitrogen flow. The EPR spectrum was recorded during the last 5 min of either sequence.

Radial-Difference a d Argand Diagrams*

The measurement of Q 0 is a very sensitive diagrammatic technique that can be used to deter mine the nature of the magnetic resonance line-shape distortion (12). This tech nique is based on the fact that the graph of a as a function of d is a circle for a Lorentzian line-shaped resonance. This graph is called an a^d Argand diagram ( A ^ Q A D ) . Tlie presence of a distortion mechanism will cause the graph to devi ate from a circle, and the details of this deviation are characteristic of the mechanism. Buckmaster and Duczmal (12) modeled the effect of two overlapping resonances under two conditions, and Shams Esfandabadi (II) has studied experi mentally the effect of modulation broadening. A method of increasing the sensi tivity of this diagrammatic technique is to modify the A Q ^ Q A D obtained from experimental data by subtracting the A ^ Q A E ) for a Lorentzian line shape (i.e., a circle) normalized to the maximum amplitude. The new graph is referred to as a radial-difference α^φΑΟ (RDa^QAD). The changes in these RDfl^gAD graphs with temperature can be used to determine the portion of the resonance that has changed. This information can be interpreted in terms of a two-resonance model, and this interpretation is known as the Larsen-Marzec model for a coal (13-15). In an independent study (II), the RDa^QAD was reproduced with an accu racy better than ~5%, but only when the output noise from the microwave syn chronous demodulator was accurately minimized by extremely careful adjustment of the microwave reference power phase into this demodulator when the sample resonant cavity was critically coupled and precisely tuned. The necessary use of magnetic-field modulation to display the Fourier coefficients (usually a ) of the resonance also modulation-broadens this resonance. Consequently, the R D U Q ^ Q A D for a modulation-broadened Lorentzian line-shape resonance exhibits a characteristic symmetric shape. This component can be eliminated by subtract ing the RDaôAD obtained at one temperature from that obtained at the next higher temperature. This approach assumes that the line-width change is suffi ciently small to be neglected in this temperature interval. The data analysis in this chapter makes use of these incremental RDa^QADs. RDAQ^QAD

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26.

BUCKMASTER & KUDYNSKA

487


Results 20 °C Measurements of the Eight Argonne Premium Coal Samples. Table I gives the 20 °C, 9-GHz CW EPR effective g-factor, effective peak-to-peak line width AB , effective line width at half max imum amplitude Δ 2 ? , relative peak-fo-peak amplitude ^4 , and relative total spin concentration N for samples of the Wyodak-Anderson SB, Blind Canyon H V B and Pocahontas No. 3 low-volatile bituminous (LVB) Argonne Premium coals (202, 601, and 501), respectively, that were meas ured within 10 min of their sealed glass vials being opened (AR—F). It also gives the values of these parameters when these samples were remeasured 4 days later (AR-S). The samples were at a higher relative humidity when stored in the flame-sealed vials than after they were opened and stored in diy argon. The effective g-faetors all increased by 0.02% in the 4 days, Δ Β and Δ 2 ? both increased in Pocahontas No. 3 (LVB) and Blind Canyon (HVB) but decreased in the Wyodak-Anderson (SB) coal. The relative peak-to-peak amplitude, ^4 , increased in the Wyodak-An derson and Blind Canyon coals but decreased in the Pocahontas No. 3 coal. The relative total spin concentration, N , increased in all three of these coals. The ratios of the values of Δ Β to those for Δ Β are sig nificantly larger than those that would have been obtained tor a pure Lorentzian line shape (1.15), and the ratio increases with the rank of the coal. These measurements support the hypothesis that the resonances observed for coals are the composite of two, and in some cases three, overlapping resonances (13-15) and that the overlap decreases with rank. 1/2

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Table I. CW EPR Spectral Parameters at 20 °C for Samples of Three Argonne Premium Coals Δ Β

„„

Δ Β

2/2

Sample" Seam

Rank

EP g-factor (mf)

(mT)

202 F 202 S

WA

SB

2.00298 2.00351

0.78 0.76

1.22 1.19

0.03 0.04

0.214 0.263

601 F 601 S

BC

HVB

2.00269 2.00316

0.58 0.74

1.03 1.17

0.03 0.05

0.127 0.341

501F 501 S

POC

LVB

2.00244 2.00284

0.31 0.41

0.65 0.85

0.14 0.09

0.192 0.264

b

a

T h e sample designation refers to the Argonne Premium coal sample number, and F and S indicate as-received samples and as-received samples that have been stored 4 days under argon, respectively. ABBREVIATIONS: WA, Wyodak-Anderson; BC, Blind Canyon; and POC, Pocahontas No. 3.

6


488

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

Figures 1-3 show graphs of the CW EPR α spectra for A R - F and A R - S samples of these three Argonne coals (a) and the corresponding RDfl d ADs (b) for these spectra. The changes in the spectral parameters given in Table I can be correlated with the changes shown in Figure 3a. The line shape of the Wyodak-Anderson SB (Argonne 202) coal was not effected by storage, but it has significant symmetric distortion. This result shows that the relative number of free radicals in each species was not affected by drying due to storage. The line shape of the Blind Canyon H V B (Argonne 601) coal becomes more symmetric upon storage, and the low-field half becomes less distorted. The number of free radicals in the high-field (lower g-factor) species decreased as a result of drying due to storage. The line shape of the Pocahontas No. 3 L V B (Argonne 501) coal also becomes more symmetric upon storage. The distortion is probably due to three rather than two free radical species, and the result of drying due to storage is a decrease in the relative number of free radicals in the intermediate g-factor species. Table II lists the effective g-factor, Δ 5 , and iST at 20 °C for sam ples of the eight Argonne Premium coals (in increasing rank order) as received stored 4 days (AR-S), moisture saturated (MS), and dried (D). The values of these parameters are in good agreement with those reported previously (16, 17). The measurements reported in this chapter were obtained with the samples exposed to dry air and in various states of hydration. Both Δ 2 ? and N increase dramatically in the Beulah-Zap lignite (L) (Argonne 801) and Wyodak-Anderson SB (Argonne 202) sam ples when they are dried, and these parameters change the least in the Pocahontas No. 3 L V B (Argonne 501) sample. The intermediate-rank samples are intermediate in this effect. Moisture-saturation caused the inverse behavior: N increases as the rank increases, and Δ Β decreases as the rank increases, with the exception of the H V B (Argonne 601) sam ple. The effective g-factor is a linear function of the carbon content of the coal sample and is in agreement with the well-known decreasing relation ship (16,17). The effects of drying and moisture-saturation do not change this relationship, but the slope is less for the latter than for the former. Figures 4-7 show the RDu d ADs for as-received (AR-S), moisture-saturated (MS), and dried (D) samples of the eight Argonne Premium coals. The RDa d ADs for the lignite and subbituminous coals given in Figure 4 are similar because the effect of drying and moisture saturation is very small and the distortion is greater on the low-magneticfield side of the resonance, which implies that the spin concentration in the high-field component is less than the spin concentration in the lowfield component, that is, N(g ) < N(g ). The diagrams for the four H V B coals can be divided into two groups of two. The diagrams in Figure 5 for the Illinois No. 6 and Pittsburgh No. 8 H V B coals are similar although the low-magnetic-field distortion is greatχ


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BUCKMASTER & KUDYNSKA In Situ 9-GHz EPR Studies

Figure 1. Graphs of the 20 °C CW EPR ^ spectra (a) and RDdL d AD (b) for as-received (dashed line) and as-received stored 4 days under argon (solid line) samples of Wyodak-Anderson SB coal Q

0


490


a


0.05 i

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180

270

360

Angle Figure 2. Graphs of the 20 °C CW EPR a spectra (a) and RDaL d AD (b) for as-received (dashed line) and as-received stored 4 days under argon (solid line) samples of Blind Canyon HVB coal 2

0

Q


491



Figure 3. Graphs of the 20 °C CW EPR a spectra (a) and RDa d AD (b) for as-received (dashed line) and as-received stored 4 days under argon (solid line) samples of Pocahontas No. 3 coal. i

0

Q



Beulah-Zap Wyodak-Anderson Illinois No. 6 Pittsburgh No. 8 Blind Canyon Lewiston—Stockton Upper Freeport Pocahontas No. 3

Seam

0

L SB HVB HVB HVB HVB MVB LVB

Rank 2.00354 2.00351 2.00316 2.00293 2.00316 2.00298 2.00281 2.00284

b

AR-S 2.00316 2.00314 2.00272 2.00263 2.00282 2.00264 2.00253 2.00251

C

MS 2.00344 2.00345 2.00299 2.00265 2.00296 2.00278 2.00257 2.00256

d

D 1.15 1.19 1.08 0.91 1.17 1.02 0.88 0.85

AR-S

ΔΒ

1.07 1.11 1.04 0.96 1.16 1.01 0.94 0.86

MS

1,2 (mT> 1.38 1.43 1.26 1.01 1.23 1.08 0.98 0.91

D 0.212 0.263 0.154 0.153 0.341 0.383 0.150 0.264

AR-S

0.113 0.134 0.131 0.151 0.209 0.256 0.194 0.434

MS

c

0

ABBREVIATIONS : LVB, low-volatile bituminous; MVB, mid-volatile bituminous; HVB, high-volatile bituminous; SB, subbituminous; and L, lignite. As-received samples that were stored 4 days under argon. Moisture-saturated samples. ^Dried samples.

Û

801 202 301 401 601 701 101 501

Sample

Effective g-factor

Table H. CW EPR Spectral Parameters at 20 °C for Various Samples of Eight Argonne Premium Coals


0.717 1.000 0.318 0.234 0.514 0.430 0.228 0.439

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BUCKMASTER & KUDYNSKA In Situ 9-GHz EPR Studies 0.04

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Figure 4. Graphs of the 20 °C 9-GHz RD^à^s for as-received (solid line), moisture-saturated (dashed line), and dried (dotted line) samples of Beulah-Zap lignite (a) and Wyodak-Anderson SB (b) coals. Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

494



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Angle Figure 5. Graphs of the 20 °C 9-GHz RDâgADs for as-received (solid line), moisture-saturated (dashed line), and dried (dotted line) samples of Illinois No. 6 HVB (a) and Pittsburgh No. 8 HVB (b) coals. Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

BUCKMASTER & KUDYNSKA



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Angle Figure 6. Graphs of the 20 °C 9-GHz RD2L a ADs for as-received (solid line), moisture-saturated (dashed line), and dried (dotted line) samples of Blind Canyon HVB (a) and Lewiston-Stockton HVB (b) coals. 0

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Angle Figure 7. Graphs of the 20 °C 9-GHz RDÛQADS for as-received (solid line), moisture-saturated (dashed line), and dried (dotted line) samples of Upper Freeport MVB (a) and Pocahontas No. 3 LVB (b) coals. Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

26.


er in the former, particularly in the as-received sample. In this case three rather than two free radical species may be present. The diagrams in Fig ure 6 for the Blind Canyon and Lewiston-Stockton H V B coals are also very similar, with the distortion slightly greater on the low-magnetic-field side of the resonance except when the Blind Canyon sample was dried, a result that implies that N(g ) < iST(g ). The diagrams in Figure 7 for the Upper-Freeport M V B and Pocahontas No. 3 L V B coals are very simi lar, and the distortions may also indicate the existence of three rather than two free radical species (6). The rank of coal correlates with the shape of the R D ^ d ^ A D and hence with the relative proportions and number of different free radical species present. The measured line shapes from which the RDfl d ADs are derived are not very sensitive indicators of this structure because the deviations from a pure Lorentzian line shape are generally very small. LF


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Low-Temperature Oxidation Studies. Samples of both the Wy odak-Anderson SB (Argonne 202) and Blind Canyon H V B (Argonne 601) coals were studied as received with air and air—nitrogen flow as well as dried and moisture-saturated with air-nitrogen flow. Figures 8a and 8b are graphs of the effective g-factor as a function of the temperature from 20 to 250 °C for the SB (Argonne 202) and H V B (Argonne 601) coal samples, respectively. The effective g-factor for the dried samples of both coals exhibits a monotonie decrease with increasing temperature. This behavior was also observed in the Alberta SB and H V B coal samples studied previously under the same experimental regime (6 9). As expected, the values of the effective g-factor for the SB coal (Argonne 202) samples exceed those for the H V B coal samples under the same conditions. Figures 9a and 9b are graphs of the effective line width ( Δ Β ) as a function of the temperature from 20 to 250 °C for these same two SB and H V B coal samples, respectively. The behavior of the SB coal samples is distinctly different from that of the H V B coal samples. Initially the line widths for the SB coal samples that are subjected to various treatments decrease at about the same rate until a critical temperature is reached. The critical temperature is 70 °C for the as-received sample with air-nitrogen flow, 85 °C for the as-received sample with air flow, and 90 °C for the moisture-saturated sample, and the critical temperature decreased monotonically up to 250 °C for the dried sample. The line widths for the H V B coal samples subjected to various treatments in creased to attain a broad maxima centered at 120 °C before decreasing, except for the dried sample. This line-width behavior is similar to that observed for Alberta H V B coal samples (6, 9). The line-width behavior of the Alberta SB samples mirrors that for the Alberta H V B samples except y

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Figure 8. Graph of the temperature dependence of the effective g-factor for Wyodak-Anderson SB (a) and Blind Canyon HVB (b) coal samples as received, stored 1 day under argon, and exposed to airflow (solid line); as-receivedfresh(short dashes), moisture-saturated (dotted line), or dried (long dashes), and exposed to air—nitrogen flow.

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Dynamic In Situ 9-GHz Electron Paramagnetic Resonance Studies of

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