5 Chemistry and Structure of Coals Diffuse Reflectance IR Fourier Transform (DRIFT) Spectroscopy of Air Oxidation N. R. SMYRL and E. L. FULLER, JR. Union Carbide Corporation, Oak Ridge Y-12 Plant, Nuclear Division, Oak Ridge, T N 37830
Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been proven to be an excellent means of characterizing coals and related materials. This report is devoted to the evaluation of the technique as a method for in situ monitoring of the chemical structural changes wrought in reactions of coal with fluid phases. This technique does not require a supporting medium (matrix) which can contain chemical artifacts which inherently serve as a barrier for access to the solid coal. The rapid response of the Fourier transform infrared technique is further beneficial for kinetic studies related to combustion, liquefaction, gasification, pyrolyses, etc. Experimental equipment and techniques are described for studies over wide ranges of pressure (10 Pa to ca 1.5 x 10 kPa) and temperature (298°K to 800°K). -5
2
Fourier transform infrared (FTIR) spectroscopy has been used extensively in the past several years in the study of coal and its related products (1-13). The majority of this work has been done by transmission spectroscopy utilizing the KBr pellet technique. In relation to coal, the method has several drawbacks including variable H O content in the KBr, high scattering background, difficulty in reproducing background for accurate subtraction studies, the unknown effect of pressure in fabricating the pellet, and the inability to perform in situ studies. Diffuse reflectance (DR) and photoacoustic (PA) detection as infrared (IR) sampling methods both possess the potential to alleviate most of the above mentioned problems; although, neither of these techniques is completely without its own pitfalls. In the field of IR spectroscopy, DR has only recently experienced a wave of renewed interest when coupled with FTIR instrumentation (14-20). There are also recent reports by Hattori, et a l , (21-24) of the design of an emissionless DR-IR grating 2
0097-6156/82/0205-0133$06.00/0 © 1982 American Chemical Society
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spectrometer along w i t h i t s a p p l i c a t i o n to c a t a l y t i c r e a c t i o n s at e l e v a t e d temperatures and t o adsorbed s p e c i e s . We have r e c e n t l y reported the development of a high-vacuum DR-IR sample c e l l having a temperature c o n t r o l l e d sample stage f o r use w i t h our FTIR equipment (25). The c a p a b i l i t i e s of t h i s c e l l were demonstrated by m o n i t o r i n g the i n s i t u r e a c t i o n s of L i H and LiOH w i t h H 0 and C0 * P h o t o a c o u s t i c d e t e c t o r s f o r use w i t h FTIR spectrometers have been developed and are p r e s e n t l y a v a i l a b l e f o r a number of commerc i a l instruments. Since·PA-FTIR represents a very recent area of i n t e r e s t , the published l i t e r a t u r e i s , t h e r e f o r e , r a t h e r l i m i t e d (26-34)· The photoacoustic and d i f f u s e r e f l e c t a n c e methods are complimentary and both have been shown to be a p p l i c a b l e to c o a l a n a l y s i s (16 ,_19,29,31,34) . Our i n t e n t i o n i n t h i s report i s to demonstrate the u t i l i t y of d i f f u s e r e f l e c t a n c e i n f r a r e d F o u r i e r transform (DRIFT) spectroscopy f o r c o a l a n a l y s i s , p a r t i c u l a r l y i n r e l a t i o n to monit o r i n g the i n s i t u o x i d a t i o n of c o a l , and to compare i t s r e l a t i v e m e r i t s to those of the KBr p e l l e t and PA sampling techniques. 2
2
Experimental The DR equipment used f o r the present s t u d i e s c o n s i s t e d of a Model DRA-SID accessory designed f o r D i g i l a b FTS-15 s i d e - f o c u s FTIR spectrometers by H a r r i c k S c i e n t i f i c C o r p o r a t i o n . The u l t r a high-vacuum (UHV) DR c e l l was a modified v e r s i o n of a c e l l a l s o developed by H a r r i c k S c i e n t i f i c . A d e t a i l e d d e s c r i p t i o n of the DR c e l l and the accessory are given i n a previous report ( 2 5 ) . The c o a l samples u t i l i z e d i n the present work were a subbituminous c o a l and a bituminous c o a l obtained from f r e s h l y opened mine faces i n the Wyoming Wyodak mine and Pennsylvania Bruceton mine, r e s p e c t i v e l y . The samples were stored under argon p r i o r to i n i t i a t i o n of any experimental work. The chemical analyses f o r these p a r t i c u l a r c o a l samples are given i n Table 1. The samples were prepared f o r the DR s t u d i e s by g r i n d i n g i n a b a l l m i l l under an argon atmosphere to pass a 200 mesh screen. The Wyodak sample was u t i l i z e d f o r the o x i d a t i o n s t u d i e s . A powdered sample of t h i s c o a l was placed i n the DR c e l l i n the neat form (no supporting matrix medium). The pressure was s l o w l y reduced from atmospheric pressure (~ 100 kPa) to 1 Pa f o l l o w e d by a gradual i n c r e a s e i n temperature from 27°C to 191°C i n order to f o l l o w the d e s o r p t i o n of moisture. The sample was then heated to 393°C and o x i d i z e d i n 2.7 kPa of a i r f o r ~ 24 h r s . Spectra were p e r i o d i c a l l y obtained throughout t h i s sequence of events. The s p e c t r a were scanned at 2 cm" r e s o l u t i o n on a D i g i l a b FTS-15C F o u r i e r transform i n f r a r e d spectrometer as the s i g n a l average of 100 i n t e r f e r o g r a m s . A few s e l e c t e d s p e c t r a were obtained at a higher s i g n a l to noise r a t i o by scanning f o r a longer p e r i o d . The s p e c t r a are d i s p l a y e d i n the o r d i n a t e format of [- l o g ( R / R ) l which i s r e f e r r e d to i n t h i s paper as r e f l e c tance (by analogy to absorbance i n normal t r a n s m i s s i o n work) 1
s
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where Rg i s the r e f l e c t i o n of the sample and RQ i s the r e f l e c t i o n of the r e f e r e n c e . The r e f l e c t i o n from a m i r r o r mounted at 45° i n the DR accessory was used f o r the reference which had only the e f f e c t of v e r t i c a l l y s h i f t i n g the o v e r a l l b a s e l i n e when compared t o a m a t e r i a l such as KC1 f o r a reference standard. The m i r r o r reference was p r e f e r r e d since i t could be assured that there would be no moisture c o n t r i b u t i o n to the s p e c t r a . R e s u l t s and
Discussion
I n order to perform i n s i t u r e a c t i o n s t u d i e s and to monitor s o r p t i o n - d e s o r p t i o n phenomena, i t i s very b e n e f i c i a l to have the c a p a b i l i t y to view a s o l i d m a t e r i a l as a powder i n the neat form. Both DR and PA i n f r a r e d sampling methods possess t h i s a b i l i t y . Although DR can sometimes y i e l d h i g h l y d i s t o r t e d band s t r u c t u r e when viewing h i g h l y c r y s t a l l i n e neat m a t e r i a l s (16,25), DR-IR would s t i l l appear to have the edge over PA-IR f o r i n s i t u powder r e a c t i o n s t u d i e s . The reason f o r t h i s i s the f a c t that the PA method r e q u i r e s a d i l u e n t gas f o r sound propagation which i s r e q u i r e d f o r d e t e c t i o n and, t h e r e f o r e , cannot be used to acquire s p e c t r a under vacuum c o n d i t i o n s . Photoacoustic IR i s not, however, r e s t r i c t e d to only powder samples and t h e r e i n l i e s one of the p r i n c i p a l advantages of the PA method. As Figure 1 demonstrates, DRIFT spectroscopy can be u t i l i z e d f o r the a n a l y s i s of powdered c o a l without the n e c e s s i t y of d i l u t i o n i n a supporting medium. This f i g u r e a l s o demonstrates the a i b l i t y to f o l l o w the moisture desorptlon process as the Wyodak m a t e r i a l i s both evacuated and heated. Curve A of Figure 1 e x h i b i t s a r a t h e r broad band extending from ~ 3700 cm" to 2100 cm" which can be a t t r i b u t e d to the 0-H s t r e t c h of H 0 and various hydroxy c o n t a i n i n g c o n s t i t u e n t s i n the c o a l i n a v a r i e t y of hydro gen bonded environments. Superimposed on t h i s broad band are the aromatic and a l i p h a t i c C-H s t r e t c h i n g bands of the organic c o a l c o n s t i t u e n t s . As the head space i s evacuated and the sample i s heated, the broad 0-H band i s observed to decrease i n i n t e n s i t y and become more s t r u c t u r e d w i t h s e v e r a l d i s t i n c t peaks being r e s o l v e d a t 3540, 3390, 3290 cm" as noted i n Curves Β and C. Curve Β i s the DRIFT spectrum at room temperature and ^ 1 Pa pressure w h i l e Curve C i s the spectrum of the m a t e r i a l a t the same pressure heated t o 191°C. Most of the changes occur on I n i t i a l evacuation w i t h very l i t t l e o c c u r r i n g i n the heating stage as noted i n Curves D and Ε which represent the r e s p e c t i v e d i f f e r e n c e s p e c t r a (A-B) and (B-C)· (These d i f f e r e n c e s p e c t r a were obtained by s u b t r a c t i o n of the C-H s t r e t c h i n g bands which presumes that no v o l a t i l e organic f r a c t i o n Is l o s t i n the evacuation and heating process.) The major p o r t i o n of the changes appear to r e s u l t from moisture d e s o r p t l o n . One of the problems observed i n KBr p e l l e t t r a n s m i s s i o n work i s the u n c e r t a i n t y a s s o c i a t e d w i t h d r y i n g the 1
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3,000
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2.000
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Figure 1. DRIFT spectra of Wyodak coal. Key: A, 27°C, 100 kPa; B, 27°C, 1 Pa; C, 191°C, 1 Pa; D, difference spectrum (A — B); and E, difference spectrum (B - C).
T a b l e 1. Chemical a n a l y s i s o f c o a l
Analysis
C H 0 (by d i f f e r e n c e )
samples.
Wyodak Composition (wt %)
Bruceton Composition (wt %)
72.5
82.5
5.45 20.5
5.5 7.7
Ν
1.01
1.3
S
0.53
3.0
Moisture Ash
28.9 5.98
1.6 14.3
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KBr and the p o s s i b l e c o n t r i b u t i o n of the moisture from t h i s m a t e r i a l t o the 0-H band i n the c o a l spectra (10). The p o t e n t i a l v a l u e of DR f o r o b t a i n i n g b e t t e r understanding of the r e l a t i v e c o n t r i b u t i o n s to the 0-H band from the water and the organic c o n s t i t u e n t s i s apparent. Another d e s i r a b l e c h a r a c t e r i s t i c which i s demonstrated i n the DRIFT spectra of c o a l i s the r e l a t i v e l y f l a t b a s e l i n e s . The advantage from t h i s p a r t i c u l a r aspect i s apparent when compared to the r a t h e r severe s l o p i n g b a s e l i n e s which appear i n the spectra of KBr p e l l e t s of c o a l due to l i g h t s c a t t e r i n g . FTIR has been used s u c c e s s f u l l y i n the q u a l i t a t i v e and quan t i t a t i v e a n a l y s i s of mineral matter i n c o a l (1,5,13). The success of these methods r e l i e d h e a v i l y on computer s u b t r a c t i o n techniques. Figure 2 i l l u s t r a t e s that s i m i l a r techniques may be a p p l i c a b l e to DRIFT s p e c t r a . Curve Β i n Figure 2 represents the DRIFT spectrum of k a o l i n , a common mineral component found i n c o a l , d i l u t e d i n KC1 at the 2% l e v e l . The absorption c o n t r i b u t i o n s due to k a o l i n i n the Bruceton c o a l can be removed by s u b t r a c t i o n of Curve Β from Curve A to y i e l d Curve C. The 3390 cm" band of k a o l i n was used to determine the proper degree of s u b t r a c t i o n . I t i s necessary i n t h i s case to d i l u t e the k a o l i n i n KC1 to avoid problems a r i s i n g from specular r e f l e c t i o n . Figure 2 was included f o r i l l u s t r a t i v e purposes o n l y , and no q u a n t i t a t i v e work has been attempted i n the present study. P a i n t e r , e t a l , have studied c o a l o x i d a t i o n both f o r samples obtained from v a r i o u s areas i n a v e i n e x p l o r a t i o n audit through a h i g h v o l a t i l e coking c o a l seam(8) and i n l a b o r a t o r y studies at low temperature (100°C) ( 9 ) . I t was concluded from t h i s work that a v a r i e t y of carbonyl, c a r b o x y l i c a c i d , and carboxylate c o n t a i n i n g products are formed i n the o x i d a t i o n process. Rockley and D e v l i n ( 2 9 ) have compared the surfaces of f r e s h l y cleaved c o a l with aged surfaces by PA-FTIR and have a l s o noted carbonyl species on the aged surfaces which were a t t r i b u t e d to o x i d a t i o n . Our i n s i t u s t u d i e s a t a somewhat higher temperature (393°C) e s s e n t i a l l y p a r a l l e l the observations by P a i n t e r ' s group, although, there were some d i f f e r e n c e s that are noted i n the f o l l o w i n g d i s c u s s i o n . The DRIFT spectra of an e s s e n t i a l l y unoxidized s t a t e of the Wyodak c o a l (Curve B) i s compared to that of the same sample a f t e r 1 hour of o x i d a t i o n i n 2.7 kPa of a i r at 393°C (Curve B) i n Figure 3. The most notable d i f f e r e n c e s i n these spectra are decreased a b s o r p t i o n of the a l i p h a t i c C-H s t r e t c h i n g bands at ~ 2900 cm" and the appearance of a carbonyl s t r e t c h i n g band at ~ 1700 cm" . These features are accentuated i n the d i f f e r e n c e spectrum (Curve C). Figures 4 and 5, which represent the s e r i e s of d i f ference s p e c t r a f o r the 3900-2500 cm" and 2000-1200 cm" r e g i o n s , r e s p e c t i v e l y , measured at successive time i n t e r v a l s i n the oxida t i o n , i l l u s t r a t e i n considerably more d e t a i l the changes which are o c c u r r i n g . The negative peaks are i n d i c a t i v e of species or func t i o n a l groups which are l o s t and the p o s i t i v e peaks to those which are formed. Since the study was done i n s i t u , there was l i t t l e 1
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3,000
2,500
2,000
1,500
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500
1
WAVE NUMBER [cm" ]
Figure 2.
DRIFT spectra. Key: A, Bruceton coal; B, kaolin in KCl (2.0%); and C, difference spectrum (A - 0.7404 X B).
1
WAVE NUMBER [cm" ]
Figure 3. DRIFT spectra of Wyodak coal. Key: A, sample after ~ 24 h of oxidation in 2.7 kPa of air at 393°C; B, dried unoxidized sample; and C, difference spectrum (A — B).
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3400
3Z00
30
139
2600
WAVENUMBERS
Figure 4. DRIFT difference spectra of in situ oxidized Wyodak coal (3990 - 2500 cm' ). Key to time elapsed: A, 1 min; B, 5 min; C, 15 min; D, 30 min; E, 45 min; F, 1 h; G, 1.5 h; H, 1.75 h; I, 2 h; and J, 19.5 h. 1
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Spectroscopy of Air Oxidation
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doubt about the proper weighting f a c t o r f o r the s u b t r a c t i o n (1:1). Problems a s s o c i a t e d w i t h v a r i a t i o n s between samples, matrix a f f e c t s , t r a n s p o r t , and handling, e t c . are nonexistent. The changes due to o x i d a t i o n are evident since the mathematical d i f ferences need not be adjusted f o r extraneous f a c t o r s . The l o s s of a l i p h a t i c C-H f u n c t i o n a l groups i s a measure of the degree of o x i d a t i o n as noted by the i n c r e a s i n g negative f e a t u r e s at 2950, 2925, and 2865 cm" (Figure 4 ) . The l o s s of a l i p h a t i c C-H s t r e t c h i n g i n t e n s i t y has been i n t e r p r e t e d as oxida t i v e a t t a c k a t a l i p h a t i c s i t e s ( p r i m a r i l y methylene groups i n the b e n z y l i c p o s i t i o n ) i n c o a l (9)· There a l s o appears to be a s l i g h t l o s s i n aromatic C-H s t r e t c h i n g i n t e n s i t y at 3035 cm" w i t h oxida t i o n . However, by comparing the spectrum of the unoxidized c o a l t o that of the f i n a l o x i d i z e d s t a t e i n Figure 6, the a l i p h a t i c and aromatic C-H s t r e t c h i n g bands are observed to be roughly the same i n t e n s i t y i n the f i n a l s t a t e i n d i c a t i n g that o x i d a t i o n has had o n l y a minor e f f e c t on the aromatic p o r t i o n of the c o a l . I t should be noted a t t h i s point that the d i f f e r e n c e i n Curves A and Β o f Figure 6 i s represented by Curve J i n both Figures 4 and 5. The formation of c e r t a i n o x i d a t i o n products are noted i n the s e r i e s of d i f f e r e n c e spectra d i s p l a y e d i n Figure 5. The complex band s t r u c t u r e from ~ 1650 cm" to ~ 1875 cm" represents a broad range of carbonyl species formed during o x i d a t i o n . There are a t l e a s t four d i s t i n c t bands which can be d i s t i n g u i s h e d at v a r i o u s stages i n the o x i d a t i o n . There i s also a band noted at lower wavenumbers (1550 cm" ). P a i n t e r , £t a l , assigned a band at 1575 cm" t o a carboxylate s a l t . The bands observed i n the present study at 1550 cm" might correspond to the frequency observed f o r the antisymmetric O-C-O s t r e t c h i n g i n c e r t a i n carboxylate s a l t s (35). We would, however, tend to discount carboxylate s a l t formation during o x i d a t i o n due t o l a c k of m o b i l i t y of both the organic and mineral phases of the c o a l a t these lower temperatures. At t h i s p a r t i c u l a r time we have no s t r a i g h t f o r w a r d a l t e r n a t i v e suggestion f o r the o r i g i n of t h i s band. The s l i g h t i n c r e a s i n g negative feature at 1455 cm" should a l s o be noted. This feature i s due t o the l o s s of absorption f o r the methylene bending band which should accompany the corresponding l o s s of methylene C-H s t r e t c h i n g absorption as pre v i o u s l y noted. I n Figure 5 a band at 1705 cm" i s observed which appears to predominate i n the e a r l y stages of o x i d a t i o n (note i n p a r t i c u l a r Curve B ) . I n the low temperature o x i d a t i o n work of P a i n t e r , et a l ( 9 ) , i n a higher ranked c o a l , a band near t h i s v a l u e , a t t r i buted to a r y l a l k y l ketones, predominated i n the spectra of both the e a r l y and l a t t e r stages of o x i d a t i o n . A prominent shoulder i s a l s o noted i n Curve Β of Figure 5 a t ^ 1745 cm" . This band along w i t h a band observed at 1775 cm" , which i s the dominant feature i n the l a t t e r stages of o x i d a t i o n (see Curve J ) , are perhaps due t o a v a r i e t y of e s t e r type f u n c t i o n a l groups (9,10,35). The obser v a t i o n of a prominent shoulder at ~ 1845 cm" i n Curve J might 1
1
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COAL AND COAL PRODUCTS
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1
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1
i n d i c a t e that the 1775-1750 cm"" r e g i o n might a l s o contain an a b s o r p t i o n c o n t r i b u t i o n from c e r t a i n c y c l i c anhydrides attached to aromatic or unsaturated r i n g s t r u c t u r e s (10,35). Another p o s s i b l e e x p l a n a t i o n f o r the o r i g i n of the 1845 c n f ^ f e a t u r e i s the forma t i o n of organic carbonates (35,36). C e r t a i n l y , the i n t e r p r e t a t i o n of the carbonyl r e g i o n of the d i f f e r e n c e spectra i s f a r from being s t r a i g h t f o r w a r d ; however, a general trend does appear throughout these s p e c t r a which would i n d i c a t e a l o g i c a l p r o g r e s s i o n of oxida t i v e productβ that might be expected from i n c r e a s i n g r e a c t i o n (36,37). The e f f e c t i s manifest i n two aspects: (1) enhanced a b s o r p t i o n at (2) higher wavenumbers. Conclusion A number of d i s t i n c t advantages over the standard KBr p e l l e t technique has been demonstrated f o r the a n a l y s i s of c o a l using DRIFT spectroscopy. The p r i n c i p l e advantage i s the a b i l i t y to monitor v a r i o u s r e a c t i o n processes i n s i t u where temperature and f l u i d phase environment can be a c c u r a t e l y c o n t r o l l e d . S p e c i f i c a l l y , data was presented d e s c r i b i n g moisture desorptlon and intermediate temperature a i r o x i d a t i o n of a powdered subbituminous c o a l . In comparison to i t s companion method, PA-IR, DRIFT spectroscopy would appear to be the technique of choice f o r the study of such r e a c t i o n processes i n v o l v i n g powdered samples s i n c e the temperature and environment of the sample are more con v e n i e n t l y c o n t r o l l e d . A l s o PA-IR i n general r e q u i r e s longer data a c q u i s i t i o n times than DRIFT to produce a s i m i l a r q u a l i t y S/N r a t i o (34). No e f f o r t has been made i n t h i s report to t r e a t i n any way the q u a n t i t a t i v e aspects which most s u r e l y at some point must be considered. Most q u a n t i t a t i v e work i n v o l v i n g DR spectra has u t i l i z e d the Kubelka-Munk Equation to mathematically t r e a t the d a t a . This Equation seems to apply mainly to species i n h i g h l y r e f l e c t i n g matrices at low d i l u t i o n . Therefore, i t remains to be determined what treatment may be required f o r DR s p e c t r a l data obtained from neat m a t e r i a l s such as c o a l . I t should be emphasized that the technique has been shown to work very w e l l i n d e f i n i n g the organic intermediates f o r a i r o x i d a t i o n of c o a l . The progressive dehydrogenation and subsequent oxygenation of the s o l i d substrate are quite w e l l defined; thus we have the c a p a b i l i t y to e l u c i d a t e , i n r e a l time, the intermediate s t a t e s ("activated s t a t e s " , "surface complexes", etc.) (38) that e x i s t i n the c o n t r o l l e d combustion the of powder where the f i n a l s t a t e s are carbon d i o x i d e , carbon monoxide, and water. Future s t u d i e s w i l l evaluate the k i n e t i c s of combustion and the r e l a t i v e c o n t r i b u t i o n of these o x i d i z e d e n t i t i e s i n the g l o b a l analyses r e l a t e d to r e l e v a n t parameters: (1) c o a l rank, (2) par t i c l e s i z e , (3) gas phase t r a n s p o r t , (4) c a t a l y t i c adducts, (5) c o a l p o r o s i t y , (6) temperature, (7) pressure, e t c .
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COAL AND COAL
PRODUCTS
Acknowl edgment T h i s work was c a r r i e d out at the Oak Ridge Y-12 P l a n t , operated f o r the U.S. Department of Energy by the Union Carbide C o r p o r a t i o n , Nuclear D i v i s i o n , under U. S. Government Contract W-7405-eng-26.
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