Electron Paramagnetic Resonance and Electron Spin-Echo

29

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch029

Electron Paramagnetic Resonance and Electron Spin-Echo Spectroscopy of Argonne Premium Coals Bernard G. Silbernagel, L. A. Gebhard, Marcelino Bernardo, and H. Thomann Corporate Research, Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801 The series of Argonne Premium coals was examined by using electron paramagnetic resonance (EPR) and electron spin-echo (ESE) techniques. EPR measurements indicate the presence of both carbon radical species and transition metal ions, principally iron and manganese, in the samples. The carbon radical densities and the ease of saturation of the radical signal do not increase with increasing coal rank, as was observed in previous studies of demineralized, isolated coal macerals. ESE techniques discriminate between the narrow (inertinitic) and broad (vitrinitic) components of the carbon radical signal, and the line widths and relaxation rates have been determined for both components. Instantaneous diffusion was observed for all bituminous coals but not for the lignite coal. The data suggest that transition metal species significantly affect the carbon radical resonance properties of these coals. V ^ A R E F U L SELECTION AND EXTENSIVE DOCUMENTATION (1) have made the Argonne Premium coals a benchmark for the future study of coals. This chapter presents a comprehensive survey of electron paramagnetic resonance (EPR) properties of the Argonne Premium coals. The results discussed here pertain to the "as received" coals, that is, before any 0065-2393/93/0229-0539$06.25/0 © 1993 American Chemical Society

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


540

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

other treatment. The properties of these materials are compared with previous results obtained on isolated coal macérais that were demineralized and separated by density-gradient centrifugation techniques (2). E P R studies of those isolated vitrinite macérais revealed a systematic variation of the resonance properties with increasing coal rank (?). Subsequent electron spin-echo (ESE) studies of the same samples demonstrated that these variations were associated with an increase in the carbon radical density and, for vitrinite macérais with carbon contents exceeding 82%, the onset of local order of the aromatic molecules serving as hosts for the carbon radicals (4). One goal of this study was to determine if a similar systematic behavior occurs for the Argonne Premium coals. A second goal was to identify the resonance properties of individual maceral types in these coal samples in which the macérais occur as a mixture.

Electron Magnetic Resonance Properties Although E P R spectroscopy has a venerable history in coal science (5), E S E spectroscopy is a much more recent technique (6). This section describes the parameters of the paramagnetic species that can be determined by the two techniques and indicates their interrelationship.

EPR Spectroscopy. In EPR spectroscopy, the sample is exposed to a continuous, relatively weak microwave field, and the absorption of microwave energy is observed as the strength of an external magnetic field is varied. The resonance position, the magnetic field strength required to produce absorption of microwave energy at a given frequency, is usually defined in terms of the g-value: g = hi/Jp^I , where h is Planck's constant, u is the Larmor frequency, μ is the Bohr magneton, and H is the external magnetic field. The width of the line, expressed in terms of the full width of the microwave absorption at half maximum, ΔΗ , or the splitting between the maxima of the absorption derivative, Δ Η , and the shape of the line, which can be described in terms of the ratio, ΑΗ /ΑΗ^ are important parameters that are determined by the chemical and physical homogeneity, the physical state, and the molecular and spin dynamics. A general discus sion of this topic is given in Chapter 30. In addition, the number of paramagnetic species in the sample can be determined by integrating the microwave absorption. The carbon radical density is defined as the number of radicals per gram of carbon in the sample. In most coals, these radical densities are ~ 1 0 - 1 0 spins per gram of carbon. The response of the paramagnetic species to the applied microwave field can be determined by 0

Q

Β

Q

1/2

ρ ρ

1/2

18

19


29.

SILBERNAGEL ET AL.

541

EPR & ESE Spectroscopy

measuring the strength of the microwave absorption as a function of applied microwave field strength (7). At low microwave power, the absorption is proportional to the field strength, which by definition is pro portional to the square root of the applied microwave power. At higher powers, this response becomes sublinear as the microwave field equili brates populations in the spin states of the paramagnetic species, a phenomenon known as saturation. One means of quantifying the saturation process is to plot the absorption intensity, J, divided by the square root of the power: I/(P) . In the linear region this ratio is constant, and it falls off at higher Ρ values. The power needed to accomplish significant saturation is defined by the quantity P , the value of the microwave power for which J / ( P ) = 0.5. 1/2


1/2

1 / 2

ESE Spectroscopy. In E S E spectroscopy, the transient response to a short, intense ρμΐββ of microwave power is observed. This pulse prepares a predefined microscopic magnetization and imposes a phase coherence on the ensemble of carbon radical spins. As described in detail in the classic paper on the topic (S), the Fourier transform of the evolving magnetization after the application of a π 12 pulse (which tips the magneti zation by 90° from the direction of the applied field) is equivalent to the microwave absorption obtained by E P R techniques. The return of the applied magnetization to equilibrium is called electron spin—lattice relaxa tion and occurs at a rate defined as 1 / Γ . Loss of phase memory in the system occurs at a rate 1 / Γ , which is related to spin-spin interactions (described by a rate of and instantaneous-diffusion effects (9), the latter arising from changes in the local field resulting from tipping of the magnetization during the application of the microwave pulse. ( Γ is the phase memory decay time, and T is the spin-spin relaxation time.) As demonstrated in ref. 9, these processes can be described as 1Ε

Μ

Μ

2

— *M

= — *2

2

+ A 2

(1)

where θ is the angle through which the magnetization is tipped by the microwave pulse, and A is a measure of the local dipolar field strength. The ability to determine the relaxation parameters 1 / Γ , 1 / Γ , and 1/T is a key strength of the E S E technique. These data are related to the EPR microwave saturation experiments because Μ

2

l

[l + aPT T y 1E

2


m

(2)

542



Preparation and Properties Sealed ampules of each of the Argonne Premium coals were kept in a nitrogen dry box. Portions of each sample were transferred to 4-mm quartz EPR tubes while in the box, and a pump-out valve was attached to the E P R tube before removing it from the dry box to ensure no contact with the atmosphere. The sample-filled EPR tubes were then transferred to a vacuum line and were subjected to a series of evacuation and backfilling steps before being sealed in a partial pressure of helium gas. The details of the evacuation process are important because the as-received coal samples have an appreciable amount of water in the coal pores. We have used proton N M R spectroscopy to estimate the amount of residual water in the pores after this evacuation process. In the most extreme case, the Beulah-Zap lignite, the water content was ~5 wt%. For most of the other samples, the water content is would correspond to g-values of 4.2 and 5.7 and are typical of a number of iron ion species commonly encountered in minerals. Although g-values and line shapes can provide information about the valence and site symmetry of the ion, they can not provide unambiguous information about the chemical compound in which the ion occurs. The relative peak intensities, inferred from peak heights of the 4.8-K spectra, are presented in Table II. Wide variations occur in the three types of transition metal peaks from sample to sample. 2 +

2 +

Especially striking are the large amounts of the g = 4.2 iron species in the Beulah-Zap lignite, a very likely cause for the high P values in that sample. As seen in Table II, the saturation properties do not obviously correlate with a specific paramagnetic type. 1/2

Electron Spin-Echo Properties As mentioned previously, E S E experiments can distinguish between the sources of broadening and relaxation for carbon radicals. In addition, E S E can discriminate between the radicals in the narrow and broad E P R




548

Figure 3. The transition metal EPR signals seen in (a) Pocahontas No. 3, (b) Blind Canyon, and (c) Wyodak—Anderson coals. Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


0.12 0.20 2.87 0.75 6.16 1.30 0.60 3.00

LVB MVB HVB HVB HVB HVB SB L

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

1.44 1.84 0.58 7.25 10.0 10.2 36.8 134.0

Fe (g = 4.2) 9.44 5.64 0.79 0.88 2.08 33.6 n/a 12.0

2+

Mn

2

3.0 4.0 7.2 6.5 2.0 8.1 10.0 19.0

Vitrinite

P,/

— —

44

—

19 21 71 48

Inertinite

(mW)

ABBREVIATIONS : L V B ,

low-volatile bituminous; M V B , medium-volatile bituminous; H V B , high-volatile bituminous; S B , subbituminous; and L , lignite.

N O T E : The values listed are relative intensities from peak heights at 4.8 K .

(& = 5.7)

Rank

Coal

Fe

Table IL Transition Metal Species and Carbon-Radical Saturation


550


lines because of their different resonance properties. The broad EPR line exhibits strong inhomogeneous broadening, and the free induction decay (FID) of this line falls rapidly to zero. By contrast, the narrow line is nearly homogeneously broadened, and its FID decays much more slowly. Therefore the line-shape and relaxation properties of the narrow line can be probed by observing the FID at times long enough that the the contri bution from the broad signal has already decayed. Conversely, the strong inhomogeneous broadening found in the broad line can be capitalized upon by forming a spin echo (10) and determining 1 / Γ and 1/T for that component from the echo measurements because the nearly homo geneous line will not form a spin echo. Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch029

Μ

m

Narrow-line Results.

As shown in Figure 4, taking the complex Fourier transform of the narrow-component FID yields the absorption spectrum. The derivative is obtained by digitally differentiating the absorption. Resulting values of the half-width Δ ω and Δ ω for the five samples containing a narrow component are shown in Table III. The magnitudes of the widths are in good agreement with the corresponding E P R values. The form of the line shape can be inferred by examining the ratio Δ α ^ / Δ ω . For a Gaussian line shape this ratio should be 1.18 (2 In 2), and for a Lorentzian line shape it would be y/~3 (11). For the higher rank coals, the ratio lies quite close to the Lorentzian limit, and the ratio increases in the lower rank coals. This increased ratio reflects a distribu tion of radical types. Similar results were obtained from previous analyses of the EPR spectra of inertinites in isolated macérais (3). As shown in Figure 5, the spin-lattice relaxation can be determined by measuring the inversion recovery of the FID. The resulting data are fit to a two-exponential recoveiy model; the data and the residuals from the fitting process are shown in the figure. The longer relaxation times are the true spin-lattice relaxation times, Γ . The shorter relaxation times may indicate the presence of additional radical types, but are more likely the result of cross-relaxation phenomena. These relaxation values are summarized in Table IV. Qualitatively, the short Γ values are con sistent with the higher P values observed for the narrow-line component as compared to the broad-line component. 1 / 2

ρ ρ

1 Ε

1 Ε

1/2

Broad-Line Results. Decay of the spin echo as a function of echo-pulse spacing is used to trace the loss of phase memory. As men tioned in the preceding section, this phase-memory loss can occur for two reasons: loss of phase memory from spin-spin coupling (the usual T process) and instantaneous-difftision effects associated with the applica tion of the refocusing pulse that forms the spin echo. These two processes

2



0.0

1.0

1.5

Time (μβ)

2.0

2.5

-40

-20

0

Frequency (MHz)

20

40

Figure 4. Selective FID detection of the narrow line. The Fourier transform of the FID is the EPR absorption.

0.5


GO

ν©

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

Table III. Fourier Transform Line Widths for the Narrow Carbon-Radical Component 1/2

Δω„„

Coal

Rank

(MHz)

(MHz)



2.2 2.2 3.5 3.1 — 3.2 — —

1.4 1.2 1.8 1.5 — 1.2 — —


Αω

PP

Αω /Αω 1/2

ρρ

1.6 1.8 1.9 2.1 — 2.7 — —

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

Table IV. Spin—Lattice Relaxation of the Narrow Carbon-Radical Component A le (V)

Τ· Β

le (με)

I

0.93 0.50 0.59 0.52

6.54 1.92 1.93 2.12

0.09 0.50 0.56 0.55

Τ L

Coal

Rank



—

L

—

0.56

7.17

0.11

— —

— —

— —

N O T E : A two-component recovery is observed, with characteristic times T and T , and an intensity ratio (I) of the two components. A

l e

B

l e

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


29.

SILBERNAGEL ET AL.


π

553

π/2

μλ pulses

—t—. receiver signal


r boxcar gate

Figure 5. The measurements for the narrow component using FID inversion recovery. can be differentiated by examining the variation of the echo decay for pulses of different tipping angles, β = ΊΗ^, where 7 is the gyromagnetic factor of the spin, H is the microwave field strength, and tj is the width of the ith pulse. The instantaneous-diffusion effect is a maximum for θ = 180°. 1

t


554


Figure 6 shows the results for the high-rank Pocahontas No. 3 lowvolatile bituminous coal and for the low-rank Beulah—Zap lignite. The instantaneous-diffusion effect is clearly evident for the high-rank coal and not observable for the lignite. A plot of the ratio of 1 / Γ [θ = 9 0 ° ] / ( 1 / Γ ) [θ = 29°] is shown in Figure 7. With the exception of the Μ


Μ

0

1

2

3

4

τ (με)

Beulah-Zap (lignite)

0

1

2

3

4

τ (lis) Figure 6. Probing instantaneous diffusion by using microwave pulses of different tipping angles.


SILBERNAGEL ET A L


555


29.

Lewiston-Stockton coal, which has a peculiar maceral mixture, the other coals group into three categories: (1) low- and medium-volatile bituminous coals, with strong instantaneous diffusion; (2) high-volatile bituminous coals, with intermediate instantaneous diffusion; and (3) the lowrank coals, in which the instantaneous-diffusion effect is either small or absent. This trend is qualitatively consistent with the trend previously observed for isolated vitrinite coal macérais: The magnitude of the instantaneous-diffusion effect decreases with increasing rank for bituminous vitrains. However, the magnitude of the intrinsic phase-memoiy loss, 1 / T , defined as the limit 1 / Γ ( 0 ) as 0 —• 0, does not scale with the spin density, in contrast to the result seen for the isolated macérais. M L

Μ


556


Spin-lattice relaxation can be measured by using the three-pulse sequence shown in Figure 8. The results are again fit with a twoexponential model (the residuals from the fit are also seen in Figure 8), and the values for 1 / Γ are presented in Table V. Clearly, the magnitude of 1 / Γ is much smaller than for the narrow line, and the components 1 Ε

1 Ε

π/2

π

π


μλ pulses receiver signal

Y—XT boxcar gate τ

0

1

800

1

1

1600 t + 2τ (μβ)

1

1

Γ

2400

Figure 8. Spin-echo inversion-recovery technique to measure Ύ for the broad carbon radical component. 1Ε


557

29. SlLBERNAGEL ET AL. EPR & ESE Spectroscopy Table V. Spin-Lattice Relaxation of the Broad Carbon-Radical Component

Coal

Rank

f/XiJ

Β le (p)

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


70 116 47 94 102 66 33 28

304 517 136 423 354 271 156 182

nr l


le

A

τ

L

I 0.87 2.13 0.53 1.36 0.49 1.46 0.53 0.29

N O T E : A two-component recovery is observed, with characteristic times T and T , and an intensity ratio (I) of the two components. A

B

l e

l e

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

are comparably represented. On the basis of the magnitudes, we propose that relaxation occurs in two stages: a cross-relaxation step, which is responsible for the short-term recovery, followed by an approach to equi librium with the spins at a common spin temperature (11). The short relaxation times for the low-rank coals are particularly apparent and are consistent with the high values of P measured by saturation EPR spec troscopy. 1/2

Conclusions These E P R and E S E measurements provide a consistent picture of the Argonne Premium coals. E P R properties such as g-values, line widths, and line shapes are consistent with the changes in chemistry expected for coals of varying rank and are in agreement with previous observations on isolated coal macérais (3). Carbon radical densities vary widely and do not track rank, but previous isolated maceral observations suggest that such variability might be expected. The major departures from earlier work are in the behavior of the relaxation-related phenomena: microwave saturation and transient relaxation measurements.


558


For the isolated macérais, both the phase-memory decay and the spin-lattice relaxation were well fit by single exponential functions. The necessity for two exponential fits in the present case suggests a greater chemical and physical heterogeneity of the samples. Second, the magnitudes of the relaxation rates and P values are significantly different than they were for the isolated macérais. Although the values of l / r M L are comparable to those observed previously, no values of l/T comparable to the rapid rates observed for the high-rank macérais are seen here. Conversely, the low-rank coals have significantly higher rates than previously observed. Both of these results and the comparatively low value of the measured paramagnetic spin density could be explained by invoking enhancement of the relaxation process by paramagnetic impurities. For the low-rank coals, this paramagnetic impurity effect is manifested as an increase in 1 / Γ . For the high-rank coals, the relaxation can be so strong that the absorptions are no longer observable. This result would imply that we are seeing some nontypical components of the coal. The test of this hypothesis will rest in examination of demineralized and ion-ex changed samples of the Argonne Premium coals. [Note added in proof: This speculation has, in fact, been confirmed (12)]. lj2


1E

1 Ε

Acknowledgments We thank A . R. Garcia for preparation of the samples and for the pro ton N M R determination of the residual water content in the coals after sample sealing.

References 1. Vorres, K. S. Energy Fuels 1990, 4, 420-426. 2. Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982, 61, 3-12. 3. Silbernagel, B. G.; Gebhard, L. Α.; Dyrkacz, G. R.; Bloomquist, C. A. A. Fuel, 1986, 65, 558-65. 4. Thomann, H.; Silbernagel, B. G.; Jin, H.; Gebhard, L. Α.; Tindall, P.; Dyr kacz, G. R. Energy Fuels, 1988, 2, 333-9. 5. See (e.g.) Retcofsky, H. R. In Coal Structure; Gorbaty, M. L.; Ouchi, K., Eds.; Advances in Chemistry 192; American Chemical Society: Washington, DC, 1981; pp 37-58. 6. See (e.g.) Pulsed EPR: A New Field of Applications; Keijers, C. P.; Reijerse, E. J.; Smidt, J., Eds; North Holland: Amsterdam, Netherlands, 1989. 7. Bloch, F. Phys. Rev. 1946, 70, 1-15.


29.

SlLBERNAGEL ET AL. EPR & ESE Spectroscopy

559

8. Lowe, I. J.; Norberg, R. E. Phys. Rev. 1957, 107, 46-61. 9. Salikov, K. M.; Tsvetkov, Yu. D. In Time Domain Electron Spin Resonance; Kevan, L.; Schwartz, R. N., Eds.; John Wiley and Sons: New York, 1979; pp 232-77. 10. Hahn, E. L. Phys. Rev 1950, 80, 580-94. 11. Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford, Eng land, 1961;p107. 12. Silbernagel, B. G.; Gebhard, L. Α.; Flowers, R . Α.; Larsen, J. W. Energy Fuels, 1991, 5, 561-568.


RECEIVED for review June 8, 1990. ACCEPTED revised manuscript December 17, 1990.


Electron Paramagnetic Resonance and Electron Spin-Echo

Recommend Documents