Langmuir 1997, 13, 909-912
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Surface Changes in Coals after Oxidation. 1. X-ray Photoelectron Spectroscopy Studies† Teresa Grzybek* and Krystyna Kreiner Faculty of Fuels and Energy, University of Mining and Metallurgy, Al.Mickiewicza 30, 30-059 Krako´ w, Poland Received November 29, 1995. In Final Form: April 12, 1996X X-ray photoelectron spectroscopy was used to determine the influence of liquid phase oxidation on the surface structure of coals. The surface composition changes (increase in sulfur and carbon content and decrease in mineral matter amount), together with the content ratio of N/C, prove the applicability of the two-component model to describe the structure of coal. The mobile phase contains sulfur but no nitrogen compounds. Analysis of the C 1s peak shape enabled detailed discussion of the formation of oxygencontaining surface groups, depending on the starting coal rank and concentration and type of the oxidizing solution.
Introduction Initial oxidation of coal influences greatly its properties, as well as those of carbonaceous materials using coals as precursors. It was found that selective oxidation removed pyritic sulfur and some organic sulfur from coal1 and additionally enhanced the reactivity of sulfur towards desulfurisation.2 The pyrolysis process,3 as well as structure and texture of carbonaceous materials prepared from coal, was found to be influenced by such pretreatments.4 The oxidation degree of the starting coal determined also the sulfur dioxide sorption capacity of the prepared active carbons.5 Much has been done on several aspects of oxidation itself (reaction parameters, chemicals, etc.) but the knowledge of surface changes introduced by such processes, as well as the experimental methods to investigate them, is still inadequate. The aim of this work was to study the influence of liquid phase oxidation on the surface heterogeneity of coals of different ranks: the organic/inorganic surface composition changes, and the type and number of oxygen-containing surface groups. This article also deals with sorption studies and the adsorption/absorption model as used for the determination of surface changes. Experimental Section Samples. Two coals from the Polish mines Borynia and Makoszowy (designated B and M, respectively) were studied. The proximate and elemental analysis of the starting materials is given in Table 1. Oxidation Procedure. B and M were oxidized with 1 or 3% H2O2 aqueous solutions, thus giving respectively B1 and B2 and M1 and M2. Additionally, M was treated with 0.035 mol/dm3 or concentrated potassium permanganate solutions, resulting in M3 and M4. In each case 7 g of coal was stirred with 0.5 dm3 of an appropriate solution at 298 ( 0.1 K for 10 h, then the mixture was left † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) Alvares Rodriguez, R.; Clemente, C.; Go´mez Limo´n, D.; Pena, E. Proc. Int. Conf. Carbon 1994, 48. (2) Palmer, S. R.; Hippo, E. J.; Dorai, X. A. Fuel 1995, 74,193. (3) Kaji, Ryuichi; Hishinuma, Yukio; Nakamura, Yoichi Fuel 1985, 64, 297. (4) Norton, F. J.; Hall, P. J.; Love, G. D. Proc. Int. Conf. Carbon 1994, 404. (5) Eur. Pat. 0042174 A1, 1981.
S0743-7463(95)01089-4 CCC: $14.00
Table 1. Characteristics of the Coal Samples Studied parameter proximate analysis moisture Wa (wt %) ash Aa (wt %) volatile matter Vdaf (wt %) elemental (wt %) C H N S Oa Roga index a
B
M
1.0
2.5
6.8
8.8
27.3
36.3
87.3 5.2 1.8 0.8 4.9 35.1
82.4 5.2 1.5 0.9 10.0 32.1
Calculated from the difference.
at ca. 295 K overnight and again mixed for 1 h at 298 K. Subsequently, the oxidized samples were filtered, washed with distilled water, and dried, first at room temperature in a crystallizer and then at 378 K, until no more change in the sample’s mass was determined. Surface Structure. The surface structure of the starting and pretreated coals was studied by X-ray photoelectron spectroscopy using a Leybold AG spectrometer equipped with an Al KR and Mg KR source and a cylindrical analyser. The spectra were taken in FAT mode with pass energy of 100 eV. They were smoothed, and a nonlinear background was subtracted except for N 1s and S 2p when linear background was used. The peaks were fitted with the convolution of 90:10% Lorentz and Gauss curves. The content of the elements was calculated using the area of S 2p, N 1s, Si 2s, C 1s, O 1s, Mn 2p, and K 2p peaks and sensitivity factors of Wagner et al.6 As K 2p and C 1s partly overlap, the peak area determination for M3 and M4 included the measurement of the area of the whole peak, containing species of both elements, after the subtraction of nonlinear background and the fitting of the obtained envelope with C 1s for CH2, CO-, CdO, and COOand K 2p3/2 and 2p1/2; subsequently, the fitting parameters (peak hights and half-widths) were used to calculate the area fraction of the whole peak connected with potassium and carbon. C 1s at 284.6 eV was used as an internal standard to calibrate binding energies of the studied peaks. (6) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
© 1997 American Chemical Society
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Table 2. Binding Energies for the Studied Coals before and after Oxidation binding energy (eV)a O 1s sample B
treatment none
B1
1% H2O2
B2
3% H2O2
M M1 M2
none 1% H2O2 3% H2O2
M3b
0.035 mol/dm3 KMnO4 saturated KMnO4
M4b
inorganic oxides
OH, SiO2
530.4
530.4
535.5 532.1 533.5 532.3 533.2 532.2 532.9 532.4 533.1
529.7
533.0
529.7
533.0
CdO, inorganic hydroxides
COOH
N 1s
S 2p
Si 2s
534.7
398.8 400.4 398.8 400.4 400.2
163.9
155.4 154.9
155.3 d 154.8
531.7
399.9 d 400.1 403.4c 399.4
164.1 169.3 164.2 168.9 169.3 d 169.3 168.4
153.9
531.5
399.5
168.7
154.0
531.0
534.7
531.7
534.4
531.7 531.1 531.7
534.4 533.5 534.3
154.5
a Calibrated to C 1s 284.6 eV. b Additional peaks of K 2p c 3/2 and Mn 2p3/2 at 292.6 and 642.1 eV, respectively. Very small, only ca. 10% of the whole peak area. d Not measured.
Figure 1. XPS spectrum for coal M.
Results and Discussion Chemical State of the Studied Elements. The XPS spectrum for C 1s, O 1s, S 2p, N 1s, Si 2s, and Fe 2p is given, as an example, for M in Figure 1. The binding energies for coals before and after oxidation are summarized in Table 2, except for Fe 2p which will only be related to in the text. From Table 2, one can see the following: (i) There are two N 1s peaks for B at 398.8 and 400.4 eV connected, respectively with pyridinic and pyrrolic nitrogen, in good agreement with literature.7 For M, only pyrrolic N was registered. No change in peak position was found when H2O2 solutions were used while a shift toward lower binding energies was observed for KMnO4(7) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100.
treated samples. The BE is in good agreement with pyridinium ion,8 but the matter needs some additional experimental data as no positive proof of the formation of such species under the used oxidation conditions has been given in the literature. (ii) Sulfur is present as thiophene for B (163.9 eV, in good agreement with the literature9) while two peaks at ca. 164.1 and 169.1 eV were registered for B1 and B2, which may be interpreted as thiophene and sulfonic acid/ sulfate (literature values respectively 163.9 and 169 eV9,10). For comparison, B was oxidized with KMnO4 (under the same conditions as M) and similarly two peaks at 164 and 168.6 eV were registered. Although Kelemen et al.9 suggest that the peak at ca. 164 eV may consist of S 2p in aliphatic sulfides as well as in thiophene, no attempt at this type of fitting was made because the amount of registered sulfur for B is at the detection limit. Only oxidized forms of sulfur were found for M-M4. It is difficult to say why no thiophenic forms, which according to Kelemen et al.9 are difficult to oxidize in air, were not found for the starting sample, perhaps the amount on the surface was under the detection level. (iii) There are several forms of oxygen present on the studied samples; they are connected with organic matter (OH-, CdO, and COO- groups respectively at ca. 533, 531.4, and 534.4 eV in good agreement with Desimoni et al.11) and overlapped with inorganic oxides/hydroxides at ca. 530.1 and 531.5 eV (Fe2O3, Fe(OH)3, Al2O3, SiO2 show BE 530.0, ca. 531.5, 531.5, and 533.0 eV, respectively12), or aluminosilicas exhibiting BE depending on Si/Al ratio (for zeolites of different silica to alumina ratio, BE is 530.4-532.2 eV13). (iv) Si 2s was found at 155.4 eV for B and M and it was shifted to lower values for B1, B2, and M2-M4. No literature data on Si 2s changes were found, but taking into account the research on the behavior of Si 2p binding energy in aluminosilicates13 it may be speculated that a certain depletion in surface aluminium caused the ob(8) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley and Sons: Chichester, New York, Brisbane, Toronto, Singapore, 1992; Chapter 3. (9) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939. (10) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 1065. (11) Desimoni, E.; Casella, G. I.; Morone, A.; Salvi, A. M. Surf. Interface Anal. 1990, 15, 627. (12) Paparazzo, E. Surf. Interface Anal. 1988, 12, 115. (13) Corma, A.; Formes, V.; Palleta, D.; Cruz, J. M.; Ayerba, A. J. Chem. Soc., Chem. Commun. 1986, 333.
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Table 3. Bulk and Surface Elemental Analysis of Organic Matter for Coals under Study elemental content (atom %) B
sample
M
element
bulk
surface
bulk
surface
C N S
97.9 1.8 0.3
98.1 1.7 0.2
98.1 1.5 0.4
98.1 1.6 0.3
Table 4. Surface Composition for the Studied Coals before and after Oxidation composition (atom %)a
B B1 B2 M M1 M2 M3 M4
sample
C
O
S
N
Si
B B1 B2 M M1b M2 M3 M4
64.2 62.1 62.4 59.4 58.5 48.6 23.9 17.0
29.2 32.1 32.9 33.7 41.5 44.4 70.3 78.4
0.1 0.3 0.3 0.2 c 0.2 0.6 0.3
1.2 1.3 1.2 1.0 c 1.2 0.5 0.3
5.3 4.2 3.2 5.7 c 5.6 4.7 4.0
a
Table 5. Changes in Surface Content of Oxygen, Sulfur, Nitrogen, Manganese, and Potassium in Comparison to Carbon as a Result of the Oxidation Process
C + O + N + S + Si ) 100%. b C + O ) 100%. c Not measured.
served change. In order to prove this hypothesis Al 2p, Si 2p, and C 1s peaks were measured for B-B2. The binding energies of Si 2p and Al 2p decrease in good agreement with the previous measurements (104.6 and 76.3 eV (B), 104.0 and 77.8 eV (B1), and 103.9 and 75.6 eV (B2), respectively. The tendency in Si/Al ratio is, however, less clear, although some decrease was found (B, 1.47; B1, 1.31; B2, 1.43 atom %/atom %). It may be therefore concluded that most probably two effects are presentssome depletion in aluminum and, possibly, the removal of cations bound with aluminosilicates, which may also lead to BE changes, as has been shown before for zeolites.13 (v) Considerable amounts of iron were registered for all M samples. Binding energies are rather high (between 711.7 and 713.5 eV) which is connected with a so-called double charging caused by high heterogeneity, i.e., “bigger islands” of iron oxide in the studied material. (vi) After oxidation, additional peaks appear for M3 and M4, corresponding to MnO2 (Mn 2p3/2 at 642.1 eV14) and K+ (K 2p3/2 at 292.6 eV15). Surface Composition. Table 3 compares the bulk and surface content of organic matter (carbon, nitrogen, and sulfur) for the starting coals and shows that their amounts in the bulk and on the surface are comparable. The fitting of N 1s as pyridinic and pyrrolic nitrogen for all B-derived samples gives the ratio of 2:1, respectively, in good agreement with Burchill and Welch7 who showed that pyridinic/pyrrolic N ratio depends on the rank of coal. For ranks similar to that of M, the ratio should be ca. 1:6; pyrridinic content was low and, therefore, reasonable fitting was obtained for N 1s only in pyrrolic form. No similar dependence was noted in literature for sulfur, which seems to be dependent on the source of coal rather than on its rank.16 The content of all studied elements for unoxidized as well as pretreated coals is given in Table 4, while the relative changes in relation to the surface carbon content are summarized in Table 5. Upon oxidation the changes in organic matter are as follows: (14) Wagner, C. D.; Gale, L. H.; Raymond, R. H. Anal. Chem. 1979, 51, 466. (15) Dwyer, D. J.; Hardenbergh, J. H. Appl. Surf. Sci. 1984, 19, 14. (16) Haenel, M. W. Fuel 1992, 71, 1211.
a
treatment 1% H2O2 3% H2O2
O/C S/C × 103 N/C Mn/C K/C K/Mn 0.46 0.52 0.53 0.57 0.71 0.91 2.94
1% H2O2 3% H2O2 0.035 mol/dm3 KMnO4 concn of 4.61 KMnO4
1.6 4.8 4.8 3.4 a 4.1 21.7
0.02 0.02 0.02 0.02 a 0.02 0.02
1.09
0.16
0.15
17.2
0.02
2.17
0.57
0.26
Not measured.
(i) Carbon content decreases while the amount of oxygen strongly increases. (ii) The amount of nitrogen changes similarly as carbon, which is proven by the constant value of N/C ratio throughout the treatments. (iii) Sulfur content increases strongly for B-derived samples while for M coals the trend is negligible for H2O2 oxidation (the C content decreases while that of S does not change) and pronounced when KMnO4 was used. The fitting of S 2p peak for B1 and B2 shows that the amount of thiophenic sulfur is the same as before H2O2 treatment while an additional amount of S is in the form of sulfonic/ sulfate groups. It may be interpreted as aliphatic sulfides bound in small molecules loosely packed in the coal structure which enables their move to the surface upon oxidation. This trend would be comprehensible if coal is viewed as a so-called two-component system.16-18 As no S in unoxidized form was registered for unpretreated M it may be assumed that either a very small number of mobile sulfur species are present or they do not become mobile unless the sample undergoes drastic oxidation. The amount of inorganic matter (as viewed by silicon and for B-B2, additionally aluminum content) is depleted on the surface after pretreatment in comparison to organic matter (carbon). This seems to be a universal trend as similar observations were made by Gonzalez-Elipe19 when brown coals were oxidized in air at 383 or 463 K. This may be due to mobile organic molecules moving toward the surface, as suggested by Gonzales-Elipe,19 with the condensation between hydroxyl groups of organic and inorganic phases and/or the transfer of protons from organic to mineral matter. The decrease in the organic matter content (C, N) upon MnO2/KOH introduction as compared to smaller changes in inorganic matter (cf. the surface amount of silicon, Table 4) leads to a conclusion that both discussed substances are deposited mainly on the organic part of the sample. Taking into account the model of Kerkhof and Moulijn,20 high intensity ratios of IMn/IC (14.4 and 22.6 respectively for M3 and M4) point to MnO2 present as large crystallites predominantly on the outer surface of coal particles. The K/Mn ratio of 0.15 (M3) and 0.26 (M4) may be caused either by washing the samples with water after the oxidation process or potassium distribution different from that for Mn, i.e., either Mn species are much more strongly bound to the surface or some potassium unlike manganese is introduced into the porous system of the studied coal, or both. (17) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986, 65, 155. (18) Given, P. H.; Marzec, A. Fuel 1988, 67, 242. (19) Gonzalez-Elipe, A. R.; Martinez-Alonso, A.; Tascon, J. M. D. Surf. Interface Anal. 1988, 12, 565. (20) Kerkhof, F. P. J. M.; Moulijn J. A. J. Phys. Chem. 1979, 83, 1612.
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Oxygen-Containing Surface Groups. The analysis of oxygen-containing surface groups was carried out by fitting a C 1s envelope with peaks of BE, 284.7, 286.2, 287.6, and 289.4 eV, which are connected with carbon in CH-type, C-O-, CdO, and COO- groups, respectively. It was assumed that all carbon peaks are symmetrical. Although Desimoni et al.11 showed that the C 1s peak connected with CH groups is asymetrical for both graphite and active carbons and the difference in half-widths of the left- and right-hand side of the peak depends, to a certain extent, on the size of graphitic units, a similar assumption does not seem reasonable for coals which, in their structure, are totally unlike graphite and, in the discussed respect, should be rather treated as an organic “macromolecule” or a set of those. As no information is available on differences in half-widths of C 1s for different types of groups, we assumed that they are the same for CH, CO-, CdO, and COO- species. Additionally, no analysis is quoted here for M3 and M4 because K 2p, whose intensity is ca. 3 times higher than that of C 1s, partly overlaps the spectrum of carbon species, and therefore we feel that fitting is rather unreliable. Only samples treated with H2O2 are fully described. There was no attempt at O 1s peak deconvolution because apart from the spectra of organic oxygen species it contains also oxygen connected with oxides and hydroxides of inorganic elements in coal (Si, Al, Fe, etc.) which are present in high amounts as shown by ash content in Table 1. Parameters characterizing the oxidation degree (calculated from C 1s peak analysis), the ratio of carbon atoms in oxygen-containing to CH-type groups (CO/CCH) and the ratio of total oxygen to carbon (O/C), are given in Table 6a while relative amounts of carbon atoms in the studied groups are summarized in Table 6b. From Table 6 the following may be seen: (i) the O/C ratio increases depending on: (a) the rank of the starting coal (higher oxidation for lower rank) cf. M1 vs B1 and M2 vs B2), (b) the type of the oxidizing solution (more pronounced effects were observed for KMnO4 than for H2O2), and, (c) the concentration of the oxidizing solution; (ii) the number of carbon atoms in oxygen-containing to CH-type groups first increases and then decreases, which means that the oxidation process does not stop for B2 and M2 as COO- groups but ends with the formation of CO2 and lower CO/CCH, (iii) the main products of oxidation by H2O2 are CdO and to some extent COO- groups, the number of carbon atoms in alcohol/ phenol-like species is, apart from B1, always smaller than that for the starting coals, and (iv) comparing the type of oxidized groups present after each pretreatment, it may be seen that the main difference in the state of surface oxidation for 1 and 3% H2O2 pretreated samples is the ratio of alcohol to carbonyl groups; for B1 and B2 the total amount of CO- + CdO is approximately the same (ca. 16.5%) and the amount of surface carbon is similar (cf. Table 3) but the ratio C(as CO-)/C(as CdO) is ca. 1.7 for the former sample and ca. 1.0 for the latter. The tendency is even more pronounced for M1 and M2.
Grzybek and Kreiner Table 6 (a) Parameters Characterizing Surface Oxidation of Organic Matter in the Studied Samples CO / CCH - the ratio of the number of carbon atoms in oxygen-containing (CO) and CH2 - type (CCH) groups, O/C - atomic ratio of oxygen to carbon sample
CO / CCHa
O / Cb
B B1 B2 M M1 M2 M3c M4c
0.15 0.29 0.27 0.18 0.27 0.22 d d
0.46 0.52 0.53 0.57 0.71 0.91 1.23c 1.78c
(b) Relative Amount of Carbon Atoms in CH2, CO-, CdO, and COO- Groups (CH2 + CO- + CdO + COO- ) 100 atom %) groups content (% of C 1s peak) sample
CH2
CO-
CdO
COO-
peak half-width (eV)
B B1 B2 M M1 M2
87.3 77.7 78.7 84.5 78.9 82.0
7.7 11.2 8.0 10.8 7.6 5.4
3.7 6.6 8.3 3.6 7.9 8.0
1.3 4.4 5.0 1.1 5.6 4.6
2.35 2.28 2.35 2.39 2.66 2.74
a Ratio of the number of carbon atoms in oxygen-containing (C ) O and CH2-type (CCH) groups. b Atomic ratio of oxygen to carbon.c The portion of the peak connected with inorganic oxide (529.7 eV) not taken into account. d Not analyzed.
Conclusions The process of liquid phase oxidation of coals leads to changes in the surface composition: the increase in elemental sulfur and carbon amount and decrease in mineral matter content. Nitrogen undergoes the same changes as the whole organic matrix. Therefore, surface changes may be understood when coal is described as a two-component system in which nitrogen, in the form of pyrrole or pyridine, is bound to the immobile carbon network and the mobile molecules are organic species containing C, O, H, or in some cases additionally sulfur, most probably aliphatic sulfides. The increase in oxygen amount is connected with the formation of CO-, CdO, and COO- groups, as studied by C 1s peak shape changes. The surface oxygen-containing groups are formed by oxidation of aliphatic chains and/or already present phenol/alcohol-like species. The oxygenated groups population is different depending on coal rank and type and the concentration of the used solution. For higher rank coals the process leads mostly to the formation of CdO. Lower rank coals are more easily oxidized which, when higher concentrations of appropriate solutions are used, results in the decrease in the number of oxygencarbon species on the surface. Acknowledgment. The support of this work by Polish State Committee for Scientific Research (KBN) Contract No. 11.210.27 has been greatly appreciated. The authors express their gratitude to Mr. S. Marschmayer from Leipzig University for carrying out additional XPS analysis on B, B1, and B2 samples. LA9510893
