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6,pp 39-69.
(6) . . Mozhaev. V. V.: Mellk-Nubarov, N. S.; Slksnis, V.; Martlnek, K. B/oca-
ta/ysls 1990, 3 , 189-198. (7) (a) Sahi, S.;Hail, 0. F.; Downs, M. E. A.; Turner, A. P. F. Anal. Chlm. Act8 1901, 249, 1-17. (b) Zaie, S. E.; Kilbanov, A. M. Bbchemlshy 1988, 25, 5432-5444. (8) Ahren, T. J. Pufe Appl. Chem. 1091, 63, 1538-1540. (9) Zentgraf. B.; Ringpfeil, M. Pure Appl. Chem. 1991, 63, 1528-1533. (10) Geise, R. J.; Adams. J. M.; Barone, N. J.; Yacynych, A. M. Blosens. Bloelectron. 1991, 6 , 151-160. (11) Guisen, J. M.; Bastlda, A.; Cuesta, C.; Fernandez-Lafuente, R.; Roseii, C. M. Blotechnol. Bioeng. 1991, 38, 1144-1152. (12) Lenders, J.-P.; Crichton, R. R. Bbtechnol. Bioeng. lg88, 37, 287-277. (13) . . Yabushlta. Y.: Suvama, K.: Takaai. K. Chem. Pharm. Bull. 1988. 36, 954-956. (14) Kennamer, J. E.; Usmani, A. M. J. Appl. Po/ym. Scl. l W l , 42, 3073-3074 - - . - - - . .. (15) Mosbach, K.. Ed. Methods In Enzynwlcgy; Academic Press: New York, 1987; Voi. 135. (16) Katchalski, E.; Sliman, I.; Goldman, R. In Advaces in Enzymology; Nord, F. F., Ed.; Marcel Dekker: New York, 1971; Vol. 34, pp 445-536.
(17) Amine, A,; Kauffmann, J.-M.; Patriarche. 0. J.; Guiibauit, G. G. Anal. Lett. inan. 22, 2403-2411. (18) Caiabrese,.G. S.; O’Connell, K. M. Medical Appilcatbns of Electrochemical Sensors and Techniques. I n Topics In Current Chem/sby; Sprlnger, Berlin. 1988; Voi. 143, pp 51-78. (19) Guibault, 0. 0. Enzyme Electrode Probes. I n Methods in Enzymology; Mosbach, K. Ed.; Academic Press: San Diego, 1988 Voi. 137, pp 14-29. (20) Speclflcatlons and Crlferla for B i O c h e ” 1 Compounds, 3rd ed.; National Academy of Sciences: Washington, 1984. (21) Cantor, C. R.; Schimmel, P. R. Bbphysicel Chemlshy: Freeman: San Francisco, 1960; Part 2, p 554. (22) Narinesingh. D.; Mungai, R.; Ngo, T. T. Anal. Blochem. 1990, 788, 325-329. (23) Dixon, M.; Webb, E. Enzymes; Academic Press: New York, 1964. (24) Przybyt, M.; Sugier, H. Anal. Chim. Acte 1900, 239, 269-276. (25) Thavarungkul, P.; Hakanson, H.; Hoist, 0.; Mattiasson, B. Blosens. Bioelectron. 1891, 6 , 101-107. (26) Cserhati, T.; Szogyi, M. Int. J. Blochem. Wgl, 23, 131-145.
RECEIVED for review October 2, 1991. Accepted January 6, 1992.
Quantitative Electron Spectroscopic Analysis of Argonne Premium Coals Cara L. Weitzsacker and Joseph A. Gardella, Jr.* Department of Chemistry, University at Buffalo, State University of New York, Buffalo,New York 14214
The eight Argonne Premium Coal Samples were analyzed by electron spectroscopy for chemical analysk (ESCA or XPS). The surface elemental compositions were determined. Quantmes of dements are reported as percent surface atomlc concentrations. Analyses were performed upon opening the sample vials and 3-24 h after opening to evaluate oxidation due to air exposure. Two samples were also analyzed 10 months after opening and after severe oxidation by radiofrequency glow discharge (RFGD). Two methods are discussed for quantifying organic oxidation products present at the surface of the coal. One method uses curve Wing of the C 1s envelope to determine types and amounts of carbon functional groups. The second method involves the stoichlometric subtraction of the inorganic oxygen content from the total oxygen content to quantify the organic oxygen present at the surface. The oxygenkarbon ratios were compared between the two methods, and It was concluded the dolchlometric technique is the more precise method to quantify the degree of oxidation of coal.
INTRODUCTION Coal is a complex material consisting of organic and inorganic components. Although there are models and theories of the structure of some of its components,l-s coal itself does not have a well-defined structure. There is general agreement in the literature regarding the origin and formation of c0al.5~899 The heterogeneous nature of coal makes it difficult to draw conclusions about something as fundamental as its structure. It is difficult to describe even the simplest reactions, in particular oxidation, without a good basic knowledge of the
* To whom correspondence should be addressed. 0003-2700/92/0364-1066$03.00/0
structure of coal. Coal from the same seam may give radically different results even when analyzed under very similar conditions.1° In studies in which two groups used coals from the Balmer 10 and Moss 3 seams, both groups did similar oxidation studies but got differing results.l1J2 Wachowska” concluded the oxidation resulted in ether cross-links. Ignasiak12concluded hydroxyl groups formed during oxidation. The Argonne Premium Coal Sample Program exists to fill the need for a standardized set of coals for the coal science community. The Argonne Program maintains a bibliography of all research published about the suite of coals. This collection of literature becomes a database of basic coal research.13J4 The goal of the Premium Coal Sample Program was to provide a small, homogenized suite of samples where all samples within a seam were collected and treated alike. Sufficient samples were prepared to ensure a long-term sup~ l y . ’ ~Samples J~ were homogenized as much as possible to allow direct comparison of results between different laboratories. Such a set of samples should eliminate differences seen in coals obtained a t different times. This should minimize conflicting resulta in research due to differences w i t h a seam. Electron spectroscopy for chemical analysis (ESCA), also known as X-ray photoelectron spectroscopy (XPS), gives chemical information about the surfaces of materials nondestructively. This is advantageous over many other analyses of coal, such as pyr01ysis~~J~ or derivatization which destroy the coal during analysis. With ESCA, the form of coal analyzed is similar to the form used industrially. Quantitation is also possible with ESCA. The elements present at the coal surface can be quantitated.wz Functional groups can also be quantitated from ESCA data using curve fitting.22 Oxidation of coal is of great concern due to negative effects on fuel content. Coking value is also adversely affected by o x i d a t i ~ n . Plasticity, ~ ~ ~ ~ * ~fluidity ~ and free-swelling ability measure the quality of a coking coal. The plastic range is the 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 84, NO. 9, MAY 1, 1992
temperature range between the softening and the resolidification temperature. Oxidized coal has a higher softening temperature, decreasing the plastic range. Maximum fluidity occurs in the 350-450 "C range; then the coal begins to resolidify after approximately 450 "C. Fluidity decreases with oxidation, therefore decreasing the quality of the coke from that coal. Free swelling generally increases with oxidation. Free swelling is determined by measuring the size of a known amount of coal after heating. The free swelling index ranges from 1to 9. Good coking coal has a free-swelling index of 4 or less. Increased free swelling decreases the quality of the coke. Oxidation of coal may begin at the air (gas)/coal interface. Therefore, analysis of the surface as it oxidizes can be of great importance in understanding the mechanisms of oxidation. A good understanding of coal oxidation could lead to development of methods for inhibiting or preventing undesirable oxidation. Oxidation of coal is thought to begin immediately after the coal is mined from the seam. This occurs as it is removed from a reducing environment to the air where oxidation is pcwible.25 Often, aging and weathering of coal are treated synonymously with oxidation of c ~ a l . This ~ . ~is due to the common practice of storing coal in air for some length of time previous to its use. This long storage time results in loss of fuel content. However, weathering and oxidation are not necessarily the same.23,26Weathering is what occurs to coal due to natural forces, such as exposure to air, weather and wind. Oxidation may occur due to the presence of air or oxygen, as well as alkali treatment,lg chromic acid, potassium ~ermanganate,'~ and many other reagents used to study the oxidation of coal.18~*~26 Precisely how oxidation occurs is a complex question. Much literature has appeared about the study of coal aging or oxidation.10J7-19v2s-30m In some studies, oxidative degradation was ' ~ was oxidized in used to study the structure of ~ o a l . ' ~ -Coal humidified air and then derivatized to facilitate analysis by secondary ion mass spectrometry (SIMS) and laser mass spectrometry (LMS) by Hercules.l8 Many methods of oxidizing coal were discussed by Hayatsu,17with the products being analyzed by mass spectrometric methods. From the analysis of oxidation products, the precursors making up the structure of the coal could be inferred. In the work of Balt r ~the, treatment ~ ~ of coal with alkali followed by mild heat treatment caused some forms of organically bound sulfur to become oxidized. This sulfur was subsequently removed by washing. Much literature reports investigations on coal as it weathers, often specifically addressing the question of what happens as coal oxidizes.10**ps33 Experimental conditions used to study oxidation vary widely in the literature. Lynch32oxidized coal at 55 "C at 9 MPa for 3-196 h, Michaelian31oxidized coal at 100 "C for 2 weeks, and Clemens30oxidized coal from 30 to 180 "C after initially drying the coal for 18 h at 105 "C. In the work by Liotta,l0 coal was exposed to weathering outdoors for up to 56 days, being sampled periodically. In the review by specific experimental conditions were not mentioned. Jakab's= review also reported results of that author's work on coal after low-temperature oxidation. Oxidation results in the uptake of oxygen during weathering. The above studies have followed the behavior of the carbonyl group in infrared studies (FTIR,los*DRIFT,26* PAFP1932). Except for Liotta,lothe other studies found changes in the carbonyl absorption in the infrared. Michaeliansl used differing frequencies in photoacoustic FTIR (PAS-FTIR) analysis to vary sampling depths. Michaelian concluded coal exposed to air up to 2 weeks has carbonyl functionality spread uniformly throughout the top 12 pm of the powdered coal surface. Clemens,mhug gin^,^ and Jakab= reported increased carbonyl
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group concentration with increased temperature as well as increased exposure time. Huggins%and Clemensmalso noted a decrease in absorption of the C-H stretch in the infrared spectrum as the oxygen functionality showed increases. A mechanism for the oxidation of coal has been proposed in several papemloam* There is a consensus that the reaction of oxygen with stable carbon radicals found in coal results in peroxide formation and propagation. There is spectroscopic evidence supporting oxidation mechanisms where peroxide formation propagates a chain reaction resulting in carbonyl, ether, aldehyde, and acidic functionality. Liotta,Io Jakab,25 Huggins,26and Clemens30 report infrared evidence of the products of such a mechanism. Nelson33reports direct infrared evidence of a peroxide feature at 1050 cm-'. Lynch32 also reports evidence of peroxides from infrared spectra after a workup of coal involving methylation, reduction, and acetylation. Both Nelson and Lynch report infrared spectra with hydroxyl, carbonyl, ether, and carboxylic acid functionalities. Evidence of radicals and peroxides has also been seen in electron spin resonance s p e c t r o ~ c o p y . ~ ~ ~ ~ ~ Other methods used for investigating oxidized coal and oxidation products are gas chromatography,29electron spin resonance (ESR),=V~~ and NMR spectroscopy.1° Many other techniques, beyond what will be discussed here, can be used to investigate coal weathering and oxidation. General studies of coal using ESCA have been published*% where the information obtained was an elemental analysis of the surface of coal and the identification of functional groups of carbon, sulfur, nitrogen, and mineral-associated elements. ESCA has also been used to investigate oxidation of the surface of c0al.26J743Papers addressing specific functionality have been published: pyrite and pyrite oxidation,**45sulfur,- sulfur o x i d a t i ~ n , ' and ~ , ~nitr0gen.5~"~ ~~~ For the most part the coal was analyzed as a powder (ground under various conditions). A few studies used solid coal p i e ~ e s . ~ ~ * ~ ~ , ~ ~ s ~ At present there is much interest in determining and understanding what reactions take place as coal ages or oxidizes. To describe the problems of surface chemistry of coal oxidation, some basic questions need to be considered: (a) What are the reactions involved in aging or oxidation? (b) What are the products of oxidation reactions? (c) How does the functionality of coal change? (d) How can the degree of oxidation be determined? (e) Does oxidation initiate at the surface? In the present study quantitative resulta of the analysis of the Argonne Premium Coal Samples are reported. Reproducibility of results, sensitivity, accuracy, and precision of the ESCA experiment will be described. An important application of quantitatively analyzing coal is demonstrated in being able to quantitate the degree to which coal has oxidized. In this study two methods to address degree of oxidation have been developed. One model involves the carbon functionality and specifically addresses how the functionality of the carbon changes. The other method stoichiometrically calculates the surface organic oxygen by subtracting the surface inorganic oxygen contribution from the total surface oxygen content.
EXPERIMENTAL SECTION Argonne Premium Coal Samples were obtained from the Argonne Premium Coal Sample Program.13 The collection and treatment of the coal samples before arrival in our laboratory was described previously.13J4The Argonne coals were opened in air immediately before introduction to the ESCA spectrometer for the f i t sample run. The initial analyses of all eight samples were performed during the period July 20-24,1990. The amount of air exposure €or each Argonne sample previous to ESCA analysis is listed in Table I. A sample of the lignite (Beulah-Zap Seam, APCS no. 8) was opened, mounted, and transferred to the ESCA under dry nitrogen. This was to compare the effects of handling the samples in air versus under an inert atmosphere.
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ANALYTICAL CHEMISTRY, VOL. 84, NO. Q, MAY 1, lQ92
Table I. Times of Air Exposure and Surface Compositions" UPFR
WYAN
ILLI
PITT
POCA
BLCA
LEST
1
2
3
4
5
SUB
HVB
HVB
HVB
7
MVB
LVB
6
HVB
X X
X X
X X
X X
X X
X X
24 h; 1 24 h; 2 48 h 13 days; 1 13 days; 2 295 days; 1 295 days; 2
X
X
X
X
X
X X X
X X X X X
carbon oxygen nitrogen sulfur aluminum silicon iron sodium calcium
X X X X X X
name sample no. rank
BZAP 8 LIG
Aging Time 5-10 min 1-2 H
4-5 days
X X X X X
X X X X
296 days RFGD 296 days
X X
Composition X X X X X X
X X X X X X
X X X X X X
X
X
X
X X X X X X
X X X
X X X
X X
X X
X X X X X X X X
Key: UPFR = upper freeport, WYAN = Wyodak-Anderson, ILLI = Illinois no. 6, PITT = Pittsburgh, POCA = Pocahontas, BLCA = Blind Canyon, LEST = Lewiston-Stockton, BZAP = Buelah-Zap.
Coals used to develop the C 1s curve-fit model was obtained from Otisca Industries,Ltd. (Syracuse,NY)." The collection and treatment of the coal from mine site to analysis was previously de~cribed.~~ The development and presentation of the C 1s curve-fit model was previously described.55 For further development of the C 1s curve fit, Otisca samples from the Illinois no. 6 seam were reanalyzed after storage at r c " temperature for 18 months. These samples were also treated by radio-frequency glow discharge (RFGD) and analyzed by ESCA as examples of severely oxidized coal. Modifications of the original C 1s curve fit were used to curve fit the Argonne results and are reported in the present study. The RFGD chamber used was a Harrick PDC-23G plasma cleaner utilizing a 60-W, 13.56-MHz radio-frequency-induced plasma.% The sample was placed into the chamber. The chamber was pumped by rotary mechanical pump to approximately 30 mTorr for 10 min. The sample was discharged in the chamber for 10 min on the HI setting. All coal samples were analyzed in the ESCA as dry powder mounted by double-sided adhesive tape. There were no artifacts from the tape. The coal showed no signs of degradation under the X-ray within the analysis time. Three repetitionsof the Otisca samples were done to assess reproducibility of analyzing coal by ESCA. Two to three repetitions of the Argonne samples were done after the initial opening of the vials. The ESCA spectrometer used for these analyses was a Perkin-Elmer Physical Electronics (PHI) 5100 ESCA spectrometer using a hemispherical analyzer and single-channel detector. The excitation source was the Mg Kal,zsource (hv = 1253.6 eV) from a dual anode, run at 300 W, 15 kV, and 20 mA. The base pressure of the system was 2 X 10" Torr with an operating pressure of 7 X lo-* Torr. The pass energy of 35.5 eV was used for high-resolution experiments. Signal was collected from a spot size of 3 X 10 111111. Under these conditions,the Ag 3d5/2peak at 367.9 eV had a full width at half-maximumof 1.0 eV and 210000 counts/s. Samples were under the nonmonochromatic X-ray for 60-75 min with no evidence of degradation. ESCA software V2.0 was used to collect and manipulate the data. Charge corrections for the organic components of coal samples were to 285.0 eV for the C Is CH,. The inorganic components were corrected to the Si 2p peak at 103.2 eV, corresponding to SOz.
RESULTS AND DISCUSSION ElementalResults. In general, ESCA can detect elements
present at greater than 0.1-0.3% atomic concentration at the surface of the coal samples. The sampling depth is approximately 50 A.57 Carbon, oxygen, nitrogen, aluminum, and silicon were detected at the surface of all eight samples. Table I lists all elements detected in the eight samples. Sulfur was below the limits of detection in both the Blind Canyon and Lewiston-Stockton samples. In three of the nine runs of the Beulah-Zap samples, sulfur was below limits of detection. Where sulfur was detected in the Beulah-Zap samples, the surface atomic composition was less than 0.25%. This is near the limits of detection for ESCA (0.1-0.3%). Variations in the coal where sulfur was present at such a low level resulted in sulfur not being detected in all the runs. Iron was below the limits of detection for all the samples. It has previously been noted in the literature that iron in coal powders is difficult to detect by ESCA.37 This would be due to the small amounts of iron present at the surface of coal at the depths at which ESCA measures.59 Sodium and calcium were detected in some samples. Other inorganic elements were detected in the Argonne samples by atomic absorption spectroscopy (AAS)methodsFg Aluminum, calcium, iron, potassium, magnesium, sodium, phosphorw, and titanium were detected. These elements were reported as major elements (
