Energy & Fuels 1996,9,841-848
841
Quantification of Organic Oxygen Species on the Surface of Fresh and Reacted Argonne Premium Coal S. R. Kelemen" and P. J. Kwiatek Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received February 17, 1995. Revised Manuscript Received June 5, 1995@
X-ray photoelectron spectroscopy (XPS) was used to determine the kinds of organic and inorganic oxygen species present in Argonne Premium coal. In most cases the XPS results for organic oxygen with fresh coal compare favorably with other methods of analysis. The evolution of COZ, CO, and HzO during pyrolysis a t 400 "C was quantified, and their appearance was associated with the loss of hydroxyl and carboxyl groups in fresh coal. Oxidation of fresh coal at 125 "C in air resulted in significant increases in the level of iron and inorganic sulfate on the coal surface. Oxidation of subbituminous and lower rank coal resulted in increases in the level of ,carboxyl species but decreases in the level of hydroxyl species. The levels of carboxyl, carbonyl, and other species increase upon oxidation of bituminous and higher rank coal. The advantages and limitations of the XPS approach for quantifying organic oxygen functionalities in coal are discussed.
I. Introduction Reactions involving organic oxygen can determine how a coal behaves in a particular application. For example, in coal liquefaction the thermolysis of labile chemical bonds is thought to initiate a complex series of reactions that lead t o either hydrogen addition reactions and lower molecular weight products or retrograde cross-linking reactions and heavier The chemical reactions of organic oxygen functionalities initially present in coal have been implicated as important factors. The chemical and physical mechanisms involved are not yet completely understood. Coal weathering (low-temperature oxidation) is another example where complex reactions involving and oxygen are believed to influence coal f l ~ i d i t f - ~ convertibility to liquids during liquefaction.8 Oxygen is the most abundant heteroatom in coal; nevertheless, the direct quantification of the organic oxygen species remains a formidable analytical challenge. There are well-known problems associated with different techniques for determining both total organic oxygen and different oxygen functionalities. Fast neutron activation analysis (FNAA) results for oxygen corrected for inorganic forms has beesexamined as an alternate to ASTM oxygen by difference method and other modified by difference f o r m u l a ~ . ~ Indirect J~ Abstract published in Advance ACS Abstracts, August 1, 1995. (1)Weiser, W. H. Fuel 1969,47,475. (2)Gorin, E.Chemistry of Coal Utilization; Wiley Interscience: New York 1981;Suppl. Vol. 2,Chapter 27. (3)Whitehurst, D. D.;Mitchell, T. 0.;Farasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (4)Neavel, R. C. Fuel 1976,55, 237. ( 5 ) Larsen, J . W.; Lee, D.; Schmidt, T.; Grint, A. Fuel 1986,65,595. ( 6 )Huffman, G. P.; Huggins, F. E.; Dunmyre, G. E.; Pignocco, A. J.; Lin, M. Fuel 1985,64, 849. (7)Ignasiak, B. S.; Clugston, D. M.; Montgomery, D. S. Fuel 1972, 51,76. (8)Cronauer, D.C.; Ruberto, R. G.; Silver, R. S.; Jenkins, R. G.; Ismail, I. M. R ; Schlyer, P. Fuel 1983,62,1116. (9)Ehmann, W. D.; Koppenall, D. W.; Hamrin, C. E., Jr.; Jones, W. C.; Prasad, M. N.; Tian, W. Z. Fuel 1986,65, 1563. (10)Mahajan, 0. P. Fuel 1986,64,973. @
chemical methods have been combined with by difference methods for total oxygen t o provide functional group information.'l These results have been compared with results from a pyrolysis based approach for organic oxygen analysis.ll 13C NMR analysis of Argonne Premium coal has yielded insight into the kinds of organic functionalities present.lZ XPS,13-19infrared,8,20-25and secondary ion mass spectrometry (SIMS)26g27 have been used to follow changes in oxygen forms in coal after oxidation. The set of Argonne Premium coal samples provides an excellent baseline for oxidation and pyrolysis studies because they can be reliably obtained in their pristine initial state. The present work examines XPS methods for determining the kinds of organic oxygen functionalities present on the surface of coal and compares these results t o other studies on the same set of coal samples.lZJ7 The XPS methods were then used t o selfconsistently follow the changes in the types of organic oxygen after simple oxidation and pyrolysis. (11)Jung, B.; Stachel, S. J.; Calkins, W. H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991,36, 869. (12)Solum, M. S.;Pugmire, R. J.; Grant, D. M. Energy Fuels 1989,
"
-on J, LOI.
(13)Clark, D.T.;Wilson, R. Fuel 1983,62,1034. (14)Grint. A.:Perrv. D. L. Fuel 1983.62. 1029. (15)Wu, hi.M.;Rogbins, G. A.; Winschel, R. A,; Burke, F. P. Energy Fuels 1988,2,150. (16)Gonzalez-Elipe, A. R.; Martinez-Alonso, A.; Tascon, J. M. D. Surf Interface Anal. 1988,12,565. (17)Weitzsacker, C. L.:Gardella, J . A., Jr. Anal. Chem. 1992,64. 1068. (18)Kelemen, S.R.;Freund, H. Energy Fuels 1989,3, 498. (19)Kelemen, S.R.;Freund, H. Energy Fuels 1990,4,165. (20)Painter, P. C.;Snyder, R. W.; Pearson, D. E.; Kwong, J. Fuel 1980,59, 282. (21)Rhoads, C. A.;SenRle, J. T.; Coleman, M. M.; Davis, A,; Painter, P. C. Fuel 1983,62, 1387. (22)Fredericks, P. M. Moxon, N. T. Fuel 1986,65,1531. (23)Gethner, J . S.Appl. Spectrosc. 1987,41,50. (24)Gethner, J. S.Fuel 1987,66,1091. (25)Lynch, B.M.; Lai-Im, I.; MacPhee, J. A. Fuel 1987,66, 979. (26)Martin, R. R,:McIntvre. N. S.; Macphee, J. A.: Aye, K. T. Energy -Fuels 1988,2,118. (27)Martin. R. R.: MacPhee.. J . A.:. Workinton,. M.:. Lindsav, E. Fuel 1989,68, 1077. '
0887-0624/95/2509-0841$09.00/00 1995 American Chemical Society
Kelemen and Kwiatek
842 Energy & Fuels, Vol. 9, No. 5, 1995 Table 1. List of Argonne Premium Coals wt % carbon
coal
rank
(dmmf)2a
Beulah Zap Wyodak Illinois No. 6 Blind Canyon Pittsburgh No. 8 Lewiston-Stockton Upper Freeport Pocahontas
lignite subbituminous high-volatilebituminous high-volatilebituminous high-volatilebituminous high-volatilebituminous medium-volatile bituminous low-volatile bituminous
74.05 76.04 80.73 81.32 84.95 85.47 88.08 91.81
11. Experimental Section All coal samples were -100 mesh. They were used immediately after opening the original sealed ampules. All oxidation's were carried out in the dark, in air (ambient relative humidity 60%) in an oven kept at 125 "C for 5 days. Previous work has shown that these oxidation conditions could be kinetically related to ambient oxidation over much longer times. The pyrolysis-GC apparatus for product analysis has been described e l s e ~ h e r e . ~Briefly, ~ ~ ' ~ thermolysis reactions were done at 400 "C for 300 s in a quartz reactor contained within a closed recirculation loop under helium. Gas samples for analysis were taken from the recirculation loop via an evacuated gas sample valve. GC analysis was accomplished using a thermal conductivity detector. Elemental analyses on the oxidized coals and pyrolysis char samples were obtained from Galbraith Analytical Laboratories, Knoxville, TN. Other elemental analytical data were obtained from the Users Handbook for the Argonne Premium Sample Program.28 A listing of Argonne Premium coals is given in Table 1. The X P S spectra were obtained on a Vacuum Generators (VG)ESCA Lab system using a five-channel detection system. Briefly, coal samples were mounted to a metallic sample nub using Scotch double-sided nonconducting tape. An energy correction was made to account for sample charging based on the carbon (Is) peak at 284.8 eV. All spectra were obtained at an analyzer pass energy of 20 eV and a constant analyzer transmission mode. Other procedures for obtaining XPS spectra have been previously d e s ~ r i b e d . ' ~ JData ~ , ~ acquisition ~ and analysis was accomplished using the VG 5000 software package. Elemental concentrations are reported relative to carbon, calculated from the area of the XPS peaks after correcting for differences in atomic sensitivity. The sensitivity factors were obtained from VG sensitivity tables and checked against experimental results from standard samples. Standard Si02 and A1203 samples gave silicon and aluminum values relative to oxygen that were 27 and 19% higher, respectively, than those expected based on the sensitivity tables. Therefore, the experimental derived sensitivity factors for silicon and aluminum were used. The amount of organic oxygen was derived from the total XPS oxygen (Is) signal by taking into account inorganic contributions. The amount of inorganic oxygen associated with silicon and aluminum were taken as Si02 and 4 0 1 . 5 . Inorganic sulfate sulfur was present as the dominant oxidized sulfur form following oxidation. The amount of oxygen associated with all oxidized sulfur forms was taken as Sod. Oxidized organic sulfur was absent in fresh and pyrolyzed coal. A relatively small amount of oxidized organic sulfur species will contribute to the oxidized sulfur (2p) signal in oxidized coal and the correction for all oxidized sulfur as so4 will result in a slight (0.1 oxygens per 100 carbons for most coal) underestimation of the total organic oxygen level. The difference between the amount of non pyritic iron signal and sulfate sulfur was associated with oxygen as Fe01.5. The following methodology was used to curve resolve the carbon (Is) spectrum. A nonlinear background was subtracted from the spectrum to account for inelastic electron energy loss (28) Vorres, K. S. Energy Fuels 1990,4, 420. (29) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J. Energy Fuels 1991,5,720.
and shake-up processes. The starting and ending points for the background subtraction were 281 and 290 eV, respectively. The choice of the end point for the background subtraction was based on a detailed investigation of the properties of electron emission from shake-up processes (I1t o 11*signals). A method for determining the level of aromatic carbon at the coal surface was developed based on the I1 to 11* signal intensity and is described in a separate report.30 A mixed 70% Gaussian-30% Lorentzian line shape and a peak width at half-maximum of 1.80 eV for each peak was used to curve resolve the carbon (IS)spectrum. Five peaks were used in curve resolution that occur at 284.8,285.3, 286.3, 287.5, and 289.0 (k0.1)eV. The 284.8 eV peak represents contributions from both aromatic and aliphatic carbon. The 286.3 eV peak represents carbon bound to oxygen by a single bond (e.g., ethers, hydroxyls, etc.), the 287.5 eV peak corresponds to carbon bound to oxygen by two oxygen bonds (carbonyl), and the 290.0 eV peak corresponds to carbon bound to oxygen by three bonds ( ~ a r b o x y l ) . ~ The ~-~~ 285.3 eV peak will have contributions from the carbon adjacent to the carboxyl carbon (beta peak) or carbon bound to nitrogen (i.e., pyrrole, p y r i d i n i ~ ) .The ~ ~ 285.3 eV peak intensity is therefore fixed to the sum of the intensity of the 289.0 eV peak and the intensity level of carbon adjacent to nitrogen (i.e+,twice the total nitrogen level relative t o total carbon). Oxygen species are determined by analyzing their effect on the XPS carbon (Is)signal of adjacent carbon atoms. Different oxygen species will influence a different number of adjacent carbon atoms. For example, an ether oxygen will influence two carbon atoms while the two oxygen atoms in a carboxyl group will influence only one carbon atom. The levels of carboxyls and carbonyls are obtained directly from the XPS carbon (Is) curve-resolved spectrum. A breakdown in terms of the number of ethers and hydroxyls that contribute to the carbon oxygen single bond peak is made based on the assumption that they are the only major contributors. The intensity of the carbon oxygen single bond peak is then equal to the hydroxyl concentration plus twice the ether concentration. The sum of ethers and hydroxyls is obtained by subtracting the amount of carbonyl and carboxyl oxygen from the amount of total organic oxygen. Solving these two simultaneous equations gives the amount of ethers and hydroxyls. XPS carbon (1s) difference curves were used, in addition to direct curve resolution of the XPS carbon (Is) spectrum of oxidized coal, for substantiating the changes in the oxygen functional groups. The difference curves were made by subtracting the area-normalized carbon (Is) signal of fresh coal from the corresponding signal of the oxidized coal. Changes in the carbon (Is) spectrum will therefore result in positive and negative going peaks in the difference curve. The positive peaks in the difference curves were resolved using methods similar to those applied t o the carbon (Is) spectrum. Peaks at 286.3,287.5, and 289.0 eV were used in the curve resolution process. Uniform charging of the organic matter in the coal matrix is a necessary prerequisite for a detailed analysis of the carbon (Is) line shape. It has been observed that when coal surfaces have relatively low levels of mineral matter there is less of a tendency for non uniform charging of the organic matrix. Table 2 shows the amount of silicon and aluminum found by XPS for fresh -100 mesh coal and the elemental data. Illinois No. 6 has a particularly high relative level of silicon and aluminum at the surface. In general, coal particles with high mineral matter content, being more insulating, will charge to a greater extent that organic-rich particles. If the surface of (30) Kelemen, S. R.; Rose, K. D.; and Kwiatek, P. J. Appl. Surf. Sci. 1993,64, 167. (31) Clark, D. T. Adv. PoZym. Sci. 1977,24, 125. (32) Clark, D. T. J . Polym. Sci.,Polym. Chem. Ed. 1978,16, 791. (33) Clark, D. T.; Cromarty, B. J.; Dilks, A. J . Pobm. Sci., Polym. Chem. Ed. 1978,16,3173. (34) Clark. D. T.: Dilks. A. J . Polvm. Sci., PoZvm. Chem. Ed. 1979, 17, 957.
(35) Thomas, H. R. Ph.D. Thesis, University of Durham, U.K., 1977.
Organic Oxygen on the Surface of Argonne Premium Coal
Energy & Fuels, Vol. 9, No. 5, 1995 843
Table 2. Comparison of the Amount of Silicon and Aluminum per 100 carbons Found in Fresh Argonne Premium Coal Determined by XPS and Elemental Analysis amount per 100 carbons silicon aluminum elemental elemental XPS analysis28 XPS analysisz8 Beulah Zap 2.03 2.13 3.28 Wyodak 2.67 1.00 0.47 14.76 Illinois No.6 11.64 2.07 1.02 Blind Canyon 3.20 0.56 3.30 0.24 Pittsburgh No. 8 3.29 2.45 1.13 0.73 Lewiston-Stockton 9.64 8.53 Upper Freeport 1.60 1.74 1.75 1.01 Pocahontas 0.35 0.26 0.50 0.38
Table 3. Comparison of the Amount of Organic Oxygen per 100 Carbons Found in Fresh Argonne Premium Coal Determined by XPS, by Elemental Analysis (Dmmf Basis)," and by Fast Neutron Activation Analysis (FNAA)l total organic oxygen per 100 carbons XPS elemental analysisz8 FNAA Beulah Zap 18.8 19.4 17.4 Wyodak 16.9 16.7 14.1 Illinois No.6 10.9 9.4 6.9 Blind Canyon 10.0 10.0 9.0 8.0 Pittsburgh No.8 6.1 6.0 Lewiston-Stockton 7.8 5.0 5.9 Upper Freeport 4.5 2.5 4.0 Pocahontas 3.2 1.4 1.3
~~~
an individual coal particle is particularly high in mineral matter, it is likely that the organic matter associated with mineral-rich particle surface will experience a similar level of enhanced charging. This situation may result in nonuniform sample charging that can cause a broadened XPS signal. The behavior of Argonne Premium coal toward sample charging was examined by further grinding of samples in an attempt t o make a chemically more uniform coal sample. Smaller particle size samples were prepared by grinding using a WigL-bug in a nitrogen drybox. The particle size distribution was shifted to smaller particle size by roughly 1order of magnitude upon grinding. After grinding, the silicon and aluminum levels remained close to those reported in Table 2 for all of the coals that had values close to the bulk. The carbon (Is)line shape was identical before and after grinding for all of these coals. After grinding Illinois No. 6 coal, however, the XPS silicon and aluminum signals were roughly 50% lower than the levels reported in Table 2. The Illinois No. 6 carbon (1s) line shape was slightly (0.3eV) narrower after grinding. The appearance of a broadened carbon (Is) peak is believed to be a result of enhanced charging by some of the organic matter in mineralrich coal particles.36 In these situations, additional grinding followed by analysis is one way to check for the presence of this experimental artifact in the original sample. Because of this concern for Illinois No. 6 coal, we only report carbon (Is) curve resolution results from samples that underwent additional grinding prior to analysis. Similar checks were made for other oxidized and pyrolyzed samples to help assure uniform sample charging during XPS analysis.
Table 4. Amount of Organic Sulfur and Nitrogen Found in Fresh Argonne Premium Coal Determined by X P S and Elemental Analysis amount Der 100 carbons organic sulfur total nitrogen elemental elemental elemental XPS analysis28 XPS analysis28 analysis" Beulah ZaD 0.30 0.36 1.5 1.4 1.5 Wyodak 0.21 0.34 1.3 1.3 1.4 Illinois No.6 1.20 1.15 1.2 1.5 1.7 Blind Canyon 0.20 0.17 1.6 1.7 1.7 PittsburghNo. 8 0.46 0.40 1.2 1.7 1.3 Lewiston-Stockton 0.34 0.29 1.0 1.6 1.7 Upper Freeport 0.34 0.32 1.5 1.6 1.6 Pocahontas 0.25 0.21 1.2 1.3 1.3
" Galbraith Laboratories.
15
111. Results
The amount of organic oxygen determined by XPS for fresh Argonne Premium coal was compared to the amount obtained from elemental analysis (dmmf basis) and FNAA." The results in Table 3 show that there is reasonably good agreement between the XPS results and other data. Since the XPS data pertain to the coal surface while the elemental and FNNA data are for the bulk, the result indicates that the amount of organic oxygen at the coal surface is nearly t h e same as in the bulk. It is well-known that the amount of carbon present in the organic matter of coal increases with increasing coal rank. It is possible to compare t h e carbon content of the organic matter present at the coal surface to the amount in the bulk using XPS data for organic sulfur and nitrogen. The data for nitrogen is presented in Table 4 and is close to t h e level found in t h e bulk. The differences between the XPS and elemental data for nitrogen were already d i s c ~ s s e d A . ~direct ~ (36) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8,896.
75
80
85
90
Wt% Carbon (XPS) Figure 1. Comparison of the weight percent of carbon derived from XPS data and bulk elemental data on a dmmf basis for Argonne Premium coals.
determination by XPS of the level of organic hydrogen is not possible; however, it was shown that t h e aromatic carbon content at t h e fresh coal surface follows the bulk p a t t e r n of increasing aromatic carbon content with increasing coal rank.30 It is reasonable t o assume that t h e hydrogen content at t h e coal surface is close t o that found for the bulk. Figure 1 shows the relationship between the weight percent carbon in the bulk (dmmf basis) and t h e weight percent carbon at the coal surface based on the XPS d a t a for organic oxygen, sulfur and nitrogen together with elemental hydrogen data. The data scatter about t h e parity line. This result shows that the carbon content of the organic matter at the fresh coal surface is nearly t h e same as in t h e bulk.
Kelemen and Kwiatek
844 Energy & Fuels, Vol. 9, No. 5, 1995
Table 6. Evolution of COz, CO, and HzO during Thermolysis of Fresh Argonne Premium Coals for 5 min at 400 "C amount per 100 initial carbons total oxygen CO COz HzO in thesegases 0.21 1.58 4.07 7.47 Beulah Zap 5.38 Wyodak 0.40 0.84 3.30 0.96 C0.05 0.20 0.56 Illinois No. 6 1.06 Blind Canyon 0.08 0.22 0.54 1.86 10.05 0.26 1.34 Pittsburgh No. 8 0.41 0.11 0.19 Lewiston-Stockton c0.05 0.24 Upper Freeport ~ 0 . 0 5 0.06 0.12 0.05 0.10 0.20
