2-mm Band and X-Band Electron Spin Resonance and Electron Spin

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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.ch023

2-mm Band and X-Band Electron Spin Resonance and Electron Spin-Echo Investigations of Some Carbonaceous Materials 1

Yuri D. Tsvetkov, Sergei A. Dzuba , and Victor I. Gulin Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Siberian Branch, Novosibirsk, 630090, Russia Argonne Premium coal samples were studied by using 2-mm band and X-band continuous-wave electron spin resonance (CW ESR) and X-band electron spin-echo (ESE) spectros copy. The line widths and g factors (Lande' g factor, spec troscopic splitting factor) were determined. The correlation between and the carbon content in coal sam ples was established. Paramagnetic centers in coals could be attributed to radicals with partial redistribution of spin densityfrompolycyclic π-system to peroxide-type structures. The degree of this redistribution depends on the degree of carbonization. Phase relaxation times, T , for these coals were determined by using ESE spectroscopy. 2

THE ELECTRON SPIN RESONANCE (ESR) SPECTRA of natural coals consist of singlet lines with widths from 0.1 to 10 G and with a g factor (Lande g factor, spectroscopic splitting factor) close to that of a free elec tron. High-resolution ESR spectroscopic methods are veiy important in interpreting these spectra because the latter have no hyperfine structure or other characteristic properties. 1

Corresponding author 0065-2393/93/0229-0443$06.00/0 © 1993 American Chemical Society

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

444

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

Experimental Details We studied the natural coals with a 2-mm band continuous-wave (CW) spectrom eter (1) and an X-band Bruker ESP-300 spectrometer. Electron spin-echo experi ments were made with the spectrometer described by Salikhov et al. (2). The g factors were measured by using standard samples containing M n ions in MgO and the l,l-diphenyl-2-picrylhydrazyl (DPPH) samples. The concen tration of the paramagnetic particles was determined by using single crystals of CuCl · 2H 0 as a reference. The coal sample measurements that were obtained with the X-band ESP-300 spectrometer were compared with standard samples by using a double resonator. For the 2-mm band measurements, the MgO powder was placed in the resonator together with a sample. ESR spectra were obtained at room temperature (X-band) and at 200 Κ (2-mm band). The internal diameter of the sample tube was 3 mm for the X-band measurements and 0.4 mm for the 2-mm band measurements. The coals were provided by K. S. Vorres (Argonne National Laboratory) within the framework of the Argonne Premium Coal Sample Program. Eight samples were supplied by different mines, and these samples included lignite and bituminous coals with different degrees of volatility and with other well-known properties described by Vorres (3) (Table I). In Table I, the samples are listed in order of increasing carbon content (degree of carbonization). The order in which the samples were supplied by the Argonne Program (3) is also indicated. The coal samples were finely dispersed powders held in an inert nitrogen atmosphere in sealed glass tubes. The coals were studied in open air both immedi ately after (type A samples) and more than an hour after (type Β samples) open ing the tube. 2 +


2

2

Results and Discussion The ESR spectra of the A samples at both bands are represented by two overlapping lines, one narrow and one broad (Figure 1). The narrow line vanishes with time after exposure to air. This phenomenon is known as the oxygen effect (4). The loss of the narrow line is accompanied by a broadening of the wide line in the Β samples. For example, the width of the broad line of sample 7, on transition from A to Β type, changes from 3.5 to 5.8 G at the X-band and from 5.3 to 10.8 G at the 2-mm band. The line widths for the Β samples are listed in Table II. The g factors of the broad line measured at the X-band at room temperature are shown in Table III for the Β samples. These results indicate that the g factor in creases with the decreasing degree of carbonization of the coal. The shape of the broad lines at the X-band was nearly symmetric for all of the Β samples (Figure 2a). Transition to the 2-mm band changes



Seam

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

Argonne No.

8 2 3 6 4 7 1 5

Rank L S HVB HVB HVB HVB MVB LVB

State ND WY IL UT PA WV PA VA

H 4.8 5.4 5.0 5.8 5.3 5.3 4.7 4.4

C 73 75 78 81 83 83 86 91

S 0.8 0.6 4.8 0.6 2.2 0.7 2.3 0.7

Ο 20 18 14 12 9 10 8 2

10 9 15 5 9 20 13 5

Ash

S O U R C E : Adapted from reference 3.

ABBREVIATIONS: L V B , low-volatile bituminous; M V B , medium-volatile bituminous; H V B , highvolatile bituminous; SB, subbituminous; and L, lignite.

1 2 3 4 5 6 7 8

Sample No.

Table I. Argonne Premium Coal Samples and Some Characteristics


446

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

X-band


2 mm

20 6

10 6

Figure 1. ESR spectra of sample 7 at 2-mm band and X-band ob tained just after exposure to air.

Table II. Paramagnetic Center Concentrations, Line Widths, and Phase Relaxation Times for Β Samples Sample No. 1 2 3 4 5 6 7 8

x l 0

[R] rl8 3.4 3.9 4.2 8.5 8.3 8.2 8.4 11.0

(g

-l)

X-band AH (G) 6.4 6.6 6.8 7.3 3.5 ,6.3 6.1 3.5*, 5.8 5.2 6

2-mm AH (G) a — — — 5.3* 14 5.3 ,10.8 14.4 6

T

2

O) 1.3 1.0 0.69 0.60 0.66 0.73 0.69 0.64

a

Dash indicates asymmetrical lines. ^Results obtained just after exposure to air (A samples).

the situation completely (Figures 2b, 2c, and 2d). Β samples with a low degree of carbonization display a noticeable axial anisotropy of their g fac tors in the 2-mm band ESR spectra. The experimental values for g y and g are given in Table III. The following correlation may be made from the data in Table III: the smaller the degree of carbonization, the greater the value. The value of g^ is also dependent on the degree of carbonization, but to a lesL


23.

TSVETKOV ET AL.

ESR & ESE Investigations

447

Table III. g Factor Values for B Samples Sample

X-band

2-mm

g

k

h

l/3(g|, + 2 )

xlO

2.0035 2.0035 2.0030 2.0031 2.0028 2.0027 2.00285 2.0028

2.00290 2.00290 2.00260 2.00261 2.00261 2.00261 2.00259 2.00267

2.00503 2.00490 2.00400 2.00371 a

2.00361 2.00357 2.00307 2.00298 — — — —

21.2 20 14 11

2-mm Band and X-Band Electron Spin Resonance and Electron Spin

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