Magnetic Resonance of Carbonaceous Solids - American Chemical

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Downloaded by UNIV OF AUCKLAND on May 8, 2015 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch024

Electron Spin Resonance, Electron Nuclear Double Resonance, and Electron Spin-Echo Spectroscopic Studies of Argonne Premium Coals Xinhua Chen, Hugh McManus, and Larry Kevan

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Department of Chemistry, University of Houston, Houston, T X 77204-5641 Six coals provided by the Argonne Premium Coal Sample Program were studied by using electron spin resonance (ESR) and electron spin-echo (ESE) spectroscopy, and five of these coals were studied by using electron nuclear double resonance (ENDOR) spectroscopy. Measurements were conducted at various temperatures. Sample pretreatment was done by exposing to air, evacuating at room tempera ture, or evacuating at 673 Κ. Spin concentrations, g values, and relaxation parameters were all affected by the pretreatments. The average radius of the unpaired-electron wave function was determined from proton-matrix ENDOR re sults. ESE spectra show that C and H nuclei have signi ficant interactions with the unpaired electrons in some coals. 13

1

ELECTRON MAGNETIC RESONANCE SPECTROSCOPY

has proven to be a powerful means of studying paramagnetic species in coal (1-10). Conventional continuous-wave (CW) electron spin resonance (ESR) can

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Corresponding author

0065~2393/93/0229-0451$06.00/0 © 1993 American Chemical Society

In Magnetic Resonance of Carbonaceous Solids; Botto, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


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M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

be used to determine the geometrical and electronic structure and the concentration of the paramagnetic species in coal. A n ESR spectrum of coal shows a convolution of a number of unresolved hyperfine structures. The insufficient resolving power of ESR spectroscopy does not permit a determination of the identity of the interacting nuclei or of the hyperfine coupling. Resolution enhancement can be achieved by electron nuclear double resonance (ENDOR) spectroscopy (11). Analysis of matrix E N D O R line widths in solids gives the distance between the paramagnetic species and the interacting nuclei. The pulsed-electron-magnetic-resonance method, electron spin-echo (ESE) spectroscopy, also provides high sensitivity for probing weak hyperfine interactions. Analysis of the echo decay provides a direct determination of electron-spin-relaxation parameters (12), and simulation of the echo-modulation pattern yields the identity and number of interacting nuclei as well as the interaction distances (13). In the present study, ESR and E S E spectroscopy were employed in investigating six selected coals from the Argonne Premium Coal Sample Program, and E N D O R spectroscopy was employed in studying five of these coals.

Experimental Details Coal samples were provided by the Argonne Premium Coal Sample Program. These coals were collected from freshly exposed seams and were stored under nitrogen under carefully controlled conditions with minimum exposure to air. The coals as received had been sieved to 100 mesh and sealed under nitrogen. The coal names and the abbreviation codes are listed in the following table in order of decreasing coal rank. Name

Rank

Code

Pocahontas No. 3 seamfromVirginia Blind Canyon seamfromUtah Pittsburgh No. 8 seamfromPennsylvania Illinois No. 6 seamfromIllinois Wyodak-Anderson seamfromWyoming Beulah-Zap seamfromNorth Dakota

LVB HVB HVB HVB SB L

POC UT PITT IL WY ND

NOTE: The order of decreasing carbon content is POC > UT ~ PITT > IL - N D > WY > N D . ABBREVIATIONS: LVB, low-volatile bituminous; HVB, high-volatile bituminous; SB, subbituminous; and L, lignite.


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CHEN ET AL. ESR, ENDOR, & ESE Spectroscopic Studies

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Approximately 5 mg of coal was transferred to 2-mm i.d. χ 3-mm o.d. Suprasil quartz tubes in a glove box under a nitrogen atmosphere. The three pretreatments used to prepare individual samples from each coal were (1) exposing the coal to air overnight and sealing in air; (2) evacuating the coal at room tem perature (RT) overnight with a final residual pressure of approximately 1 χ 10~ torr (1.33 mPa) and sealing under vacuum; and (3) evacuating at RT, heating under vacuum to 673 Κ for ~2 h, and sealing under vacuum. ESR spectra were recorded at RT, 77 K, and 4 Κ with a Bruker ESP 300 ESR spectrometer operating at X-band (9 GHz) with 100-kHz magnetic-field modulation. The microwavefrequencyand the magnetic field were monitored with a Hewlett-Packard 5350Bfrequencycounter and a Bruker ER035 gaussmeter, respectively. The g values of the samples were measured by using 1,1-diphenyl-2-picrylhydrazyl (DPPH) as a standard with g = 2.0036, and the spin concentra tions were determined by comparing the sample results with a Bruker strong-pitch reference containing 3 χ 10 spins/cm. ENDOR spectra were recorded at RT and at 150 Κ with the ESP 300 ESR spectrometer interfaced with a Bruker ENDOR unit. Frequency modulation was performed without magnetic-field modulation, and the result was a first-derivative presentation of the ENDOR spectra. Typical experimental conditions were as follows: microwave power, 2 mW; microwave frequency, 9.46 GHz; radiofrequency power, 100 W;frequencymodulation, 12.5 kHz; and modulation depth, 158 kHz. For low-temperature ENDOR experiments, a Bruker ER411 variabletemperature unit was used. ESE spectra were recorded at RT, 77 K, and 4 Κ with a home-built X-band ESE spectrometer (14). Both two- and three-pulse experiments were performed. In the two-pulse experiments, pulse widths of 26 and 52 ns were used to generate 90°-r-180° pulse sequences where r is the interpulse time. In the three-pulse experiments, three 26-ns pulses were used to generate 90°-τ-90°-Γ-90° pulse sequences, and the phase sequence {(0 0 0) + ( Ϊ Γ 7 Γ Ο ) } - { ( 7 Γ 7 Γ 7 Γ ) + (0 0 7 Γ ) } was employed for successive pulse sequences to eliminate two-pulse interferences (25).


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Results and Discussion ESR Spectroscopy. The ESR spectra of the studied coals can be categorized into two groups: type-1 spectra, in which both narrow and broad components can be seen; and type-2 spectra, in which only a broad signal can be seen. The narrow component and the broad signal have peak-to-peak line widths of ~1 G and -3-11 G , respectively. The ESR results of all coal samples are sumarized in Table I, and an example of the ESR spectra from PITT coal is shown in Figure 1. The



Sample Code

22 20 7 17 14 6 13 8 7

1.3 5.9 6.0 5.5 4.9 5.0 5.6 5.6 5.4 6.9 6.9 5.3

2.0028 2.0029 2.0035 2.0028 2.0029 2.0029 2.0028 2.0023 2.0028 2.0027 2.0028 2.0027

In air R T evac 673-K evac




4K

RT

77 Κ

4K

11 5 5

98 5 5

1.3 5.5 5.9

2.0028 2.0029 2.0030


77 Κ

13 5 6

3.1 5.3 5.7

Line-Shape Parameter

2.0039 2.0030 2.0029

Line Width (G)

g Value


Pretreatment

RT

Measurement Temperature

Table I. Summary of ESR Parameters of Argonne Premium Coals


1.0 0.8 6.4

3.9 3.2 5.5

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Spin Concentration χ 10 spins/g

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CHEN ET AL. ESR, ENDOR, & ESE Spectroscopic Studies


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Magnetic Resonance of Carbonaceous Solids - American Chemical

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