The Comparison of Oxygen and Sulfur Species Formed by Coal

Poland, and Faculty of Chemistry, Adam Mickiewicz University in Poznan´, Grunwaldzka 6,. 60-780 Poznan´, Poland. Received August 13, 2003. Revised ...
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Energy & Fuels 2004, 18, 804-809

The Comparison of Oxygen and Sulfur Species Formed by Coal Oxidation with O2/Na2CO3 or Peroxyacetic Acid Solution. XPS Studies T. Grzybek,*,† R. Pietrzak,‡ and H. Wachowska*,‡ Faculty of Fuels and Energy, AGH University of Science and Technology, 30-059 Krako´ w, Poland, and Faculty of Chemistry, Adam Mickiewicz University in Poznan´ , Grunwaldzka 6, 60-780 Poznan´ , Poland Received August 13, 2003. Revised Manuscript Received January 28, 2004

Oxidation of coals of different rank and containing different amounts of sulfur was carried out using O2/Na2CO3. The surface chemical state of some elements (C, O, S) and their content was studied by X-ray photoelectron spectroscopy and compared to those of coal samples oxidized with peroxyacetic acid solution. The extent of surface oxidation depended on the rank of coal, bulk oxygen content of the starting sample, and the type of oxidizing agent. The oxidizing process led to the increase of the amount of oxidized forms of sulfurssulfoxides and sulfones. The changes in the amount of oxygen-containing species were also discussed.

1. Introduction Surface chemical nature of coal is of great importance from a technological point of view. To name two examples, the amount and nature of oxygen bonding to the surface influence hydrophobicity and thus flotability of coal,1,2 while the chemical forms of sulfur and nitrogen impact the reactivity and the utilization strategy of fossil fuels.3 Thus surface studies of coal are especially interesting, and XPS has a unique role to play in that respect. XPS has been used to determine forms of sulfur in different types of coals: Argonne Premium coals,3 Spanish coals of Mequinenza,4,5 Utrillas and Calaf basins,4 Australian coalssGreta and Whybrow seams, Sydney Basin,6 and Moranbah Measures in Bowen Basin,2 or Chinese coals from Guizhou Province.7 Both aliphatic and aromatic sulfur was found in coalssthe levels of the latter increasing directly as a function of increasing rank.3,8,9 It was also found that the aliphatic sulfur oxidized in air more rapidly than aromatic sulfur.3,10,11 Aliphatic sulfides were found to be converted to oxidized forms (sulfoxides, sulfones, sulfonic species) during * Corresponding authors. Tel.:+48-126172119; Fax: +48-126172066; E-mail: [email protected] (T. Grzybek), [email protected] (H. Wachowska). † AGH University of Science and Technology. ‡ Adam Mickiewicz University in Poznan ´. (1) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324. (2) Buckley, A. N.; Lamb, R. N. Int. J. Coal Geol. 1996, 32, 87. (3) Gorbaty, M. L.; Kelemen, S. R. Fuel Process. Technol. 2001, 71, 71. (4) Olivella, M. A.; Palacios, J. M.; Vairavamurthy, A.; del Rio, J. C.; de las Heras, F. X. C. Fuel 2002, 81, 405. (5) Shimizu, K.; Iwami, Y.; Suganuma, A.; Saito, I. Fuel 1997, 76, 939. (6) Gong, B.; Pigram, P. J.; Lamb, R. N. Fuel 1998, 77, 1081. (7) Buckley, A. N.; Riley, K. W.; Wilson, M. A. Org. Geochem. 1996, 24, 389. (8) Burchill, P. Fuel 1993, 72, 1571. (9) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5, 93. (10) Gorbaty, M. L.; Kelemen, S. R.; George, G. N.; Kwiatek, P. J. Fuel 1992, 71, 1255. (11) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 1065.

oxidation in air at 125 °C as shown by XPS and XANES for Rasa, Charming Creek, and Mequinenza coals.10 Although mercaptans and disulfides can oxidize to sulfonic acids, the analysis of Mequinenza coal showed that mercaptans present there did not decrease upon mild oxidation in air.10 Much work has been carried out on S functionalities by other methods, e.g., TPR, XANES, SEM, SIMS, etc.,3,4,6,12,13,14 but still no general picture is as yet available concerning the changes of S chemical state for coals of different nature under varying oxidizing conditions, the notable exception being mild oxidation with air. The latter oxidizing medium was often used, and its influence was studied by XPS in detail for different coals [e.g., refs 3,10,12]. In contrast to other carbonaceous materials such as active carbons or carbon fibers, XPS was not so often used to study carbon-oxygen functionalities for coals. Of the newer examples, works of Buckley et al.,2 Gong et al.,6 or Grzybek et al.15 may be mentioned. Buckley et al.2 studied carbon-oxygen groups for Australian coals oxidized by air at low temperature in connection with their hydrophobicity and flotation. Gong et al.6 described changes upon oxidation by wet air at 20-120 °C, while Grzybek et al. discussed the formation of the mentioned groups on Polish coals oxidized by solutions of H2O2 or KMnO4.15 To obtain a more general picture concerning surface changes for different oxidizing media, the study was undertaken for coals of different ranks with low and high sulfur content. The first work of the series dealt with the oxidation of lignites and highly volatile or medium volatile bituminous coals by peroxyacetic acid solution.16 The aim of the study reported is to determine (12) Huffmann, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991, 5, 574. (13) Kozłowski, M.; Pietrzak, R.; Wachowska, H.; Yperman, J. Fuel 2002, 81, 2397. (14) Maes, I. I.; Mitchell, S. C.; Yperman, J.; Franco, D. V.; Marinov, S. P.; Mullens, J.; Van Poucke, L. C. Fuel 1996, 75, 1286. (15) Grzybek, T.; Kreiner, K. Langmuir 1997, 13, 909.

10.1021/ef030153i CCC: $27.50 © 2004 American Chemical Society Published on Web 04/06/2004

Oxygen and Sulfur Species Formed by Coal Oxidation

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Table 1. Elemental Analysis of Demineralized and Oxidized Coals (wt %)a [17] types of sulfur speciesc coal

ashd

Cdaf

Hdaf

Ndaf

Sdaf org

daf b Odiff

Sdt

d SSO4

Sdp

Sorg

L dem L/O2/Na2CO3 L/PAA M dem M/O2/Na2CO3 M/PAA Cz dem Cz/O2/Na2CO3 Cz/PAA Sz dem Sz/O2/Na2CO3 Sz/PAA Z dem Z/O2/Na2CO3 Z/PAA

1.5 1.4 1.1 1.2 0.9 1.7 1.5 0.7 0.6 1.6 1.0 0.8 0.5 0.5 0.1

75.7 75.1 73.7 65.1 63.9 61.1 75.4 73.8 68.5 76.1 74.8 69.1 87.8 87.5 84.8

5.7 5.5 5.7 5.2 5.1 6.2 5.2 4.9 4.5 4.5 4.4 4.1 5.1 5.1 5.0

1.2 1.3 1.3 0.8 0.9 0.8 1.0 1.0 1.0 1.1 1.1 1.0 1.5 1.5 1.5

10.0 9.8 8.2 10.4 10.3 7.1 1.4 1.4 1.1 1.1 1.1 0.9 0.6 0.6 0.5

7.4 8.3 11.1 18.5 19.8 24.8 17.0 18.9 24.9 17.2 18.6 24.9 5.0 5.3 8.2

10.05 9.74 8.18 11.60 11.14 7.40 1.92 1.60 1.09 2.14 1.86 0.87 0.75 0.63 0.50

0.01 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.16 0.07 0.06 1.29 0.94 0.40 0.59 0.18 0.00 1.05 0.74 0.01 0.20 0.03 0.00

9.88 9.67 8.12 10.28 10.20 7.00 1.33 1.42 1.09 1.09 1.12 0.86 0.55 0.60 0.50

a

Ref 17. b By difference. c Sdt , SSO4, Sdp, Sorg are total content of sulfur and S in sulfates, pyrite, or organic S compounds, respectively. Table 2. Surface Composition of the Studied Samples (C + O + S + N ) 100 at. %) composition [at. %]

Figure 1. Mass loss for coals oxidized with O2/Na2CO3 or peroxyacetic acid solution (PAA).

the influence of the oxidation of the same coals with O2/ Na2CO3 solution on surface chemical forms of sulfur and oxygen functionalities. 2. Experimental Section 2.1. Samples and Oxidation Procedure. Coals from the following mines were studied: lignites Mequinenza (Spain) and Labin (Croatia), highly volatile bituminous coals Czeczot and Siersza, and medium volatile coal Zofio´wka (Poland), further designated as M, L, Cz, Sz, and Z, respectively. Samples were demineralized (designation “dem”) and oxidized with O2 which was passed through the suspension of coal in 0.5 N Na2CO3 solution. The reaction was carried out at 353 K for 6 h. More preparative details are given elsewhere.17 Oxidized samples are denoted by “O2/Na2CO3” added to the name of the sample. Oxidation was accompanied by mass loss17 as illustrated by Figure 1 given for coals oxidized with O2/ Na2CO3 solution, and for comparison, with peroxyacetic acid solution (PAA). The reactivity of coals toward the former oxidizing agent is much lower than toward the latter. Results of elemental analysis of demineralized and oxidized samples and the specification of sulfur forms are given in Table 1. For comparison, appropriate data for samples oxidized with PAA were added.16,17 2.2. Chemical State of the Selected Elements and Surface Composition. The chemical state of selected elements and surface composition of the samples were determined by X-ray photoelectron spectroscopy using a VSW spectrometer (Vacuum Systems Workshop Ltd., England) equipped with an Al KR source and an 18-channel 2-plate analyzer. The spectra were taken in a FAT mode (∆E ) const) with pass energy of (16) Grzybek, T.; Pietrzak, R.; Wachowska, H. Fuel Process. Technol. 2002, 77-78, 1. (17) Pietrzak, R.; Wachowska, H. Fuel 2003, 82, 705.

coal

C

O

S

N

O/C × 102

L dem L/O2/Na2CO3 L/PAA M dem M/O2/Na2CO3 M/PAA Cz dem Cz/O2/Na2CO3 Cz/PAA Sz dem Sz/O2/Na2CO3 Sz/PAA Z dem Z/O2/Na2CO3 Z/PAA

87.6 85.6 78.6 80.8 78.4 79.3 82.9 82.9 77.7 81.6 81.5 75.3 91.4 89.8 85.2

6.2 7.8 15.7 14.0 16.5 15.8 15.2 15.8 20.7 16.4 16.5 22.9 6.8 8.5 13.1

4.7 4.8 4.5 5.2 5.1 3.7 0.7 0.6 0.4 0.5 0.7 0.5 0.4 0.4 0.4

1.5 1.8 1.2 a a 1.0 1.2 1.2 1.1 1.5 1.3 1.3 1.3 1.2 1.2

7.08 9.11 19.97 17.33 21.05 19.92 18.34 19.06 26.64 20.10 20.25 30.41 7.44 9.47 15.38

a

Not measured

22 eV. They were smoothed, and Shirley background was subtracted. The calibration was carried out to the main C 1s peak at 284.6 eV. The concentration of elements was calculated using the intensity of an appropriate line and XPS cross sections (as given by Scofield18).

3. Results and Discussion 3.1. Surface Composition. Table 2 summarizes the surface composition of demineralized coals and samples oxidized by O2/Na2CO3. For comparison, appropriate data for samples oxidized with PAA were added.16 As it may be seen from Table 2, surface composition changes upon oxidation treatment to an extent depending on the rank of coal, bulk oxygen content of the starting sample, and the type of oxidizing agent. The amount of carbon decreases and the amount of oxygen increases upon oxidation. The effect is rather small for O2/Na2CO3 solution in comparison to PAA. The only exception is coal M where, even for highly oxidizing PAA solution, surface carbon content decreases only by ca. 1.5% in comparison to 5-9% for other studied samples. The surface changes are accompanied by the changes in appropriate bulk values, although no direct correlation between mass loss and C and O surface content or O/C surface ratio could be found in contrast to the oxidation with PAA solution.16 It may be understood taking into account the following facts: (i) XPS (18) Scofield, J. H. J. Electron Spectrosc. 1976, 8, 129.

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probes 2-20 monolayers,19 depending on the kinetic energy of a given peak and thus gives an average value for surface and subsurface concentrations. As oxidation will depend, apart from the type of oxidative agent and coal rank, also on the accessibility of internal surface, oxidation degree of the external part of a coal particle, where internal diffusion plays no important role, will be higher than that of the internal surface. (ii) Reactivity of the samples expressed as mass loss will be connected with either gasification of surface groups to CO2 and/or oxidative cleavage of the C-X bond, where X may be both C and a heteroatom. Both processes must be preceded by oxygen chemisorption, and thus surface oxygen content will express the equilibrium state between oxygen chemisorption and the removal of oxidation products leading to the exposure of an unoxidized surface, which in turn, will again have started to chemisorb O2. Thus if oxygen is only collected on the surface, its amount will increase. If it leads to gasification or cleavage reaction, it will end in the products, thus depleting the surface in O. The analysis of relative increase in surface oxygen (calculated as (OO2/Na2CO3 Odem)/Odem, where O surface oxygen content in at. % and subscript denotes the type of coal treatment) shows the following sequence:

L/O2/Na2CO3 (0.258) ≈ Z/O2/Na2CO3 (0.250) > M/O2/Na2CO3 (0.179) > Cz/O2/Na2CO3 (0.039) > Sz/O2/Na2CO3 (0.006) Thus the relative increase in surface oxygen content is not a function of coal rank alone, as bulk C content of demineralized samples (expressed as Cdaf) forms a sequence:

Z (87.8) > Sz (76.1) > L (75.7) > Cz (75.4) > M (65.1) However, apart from coal rank it was observed earlier that the initial bulk oxygen content influenced the reactivity of coal. This parameter seems to have a higher influence on the relative increase in surface oxygen content than rank alone. The samples fall into two groups: (i) those with less bulk O content (L and Z) slowly collecting (chemisorbing) oxygen content without simultaneous gasification or cleavage reaction carried out to a greater extent, which results in a higher relative increase in surface oxygen, and (ii) with higher bulk oxygen content (M, Cz, and Sz) which, due to their higher reactivity, have already undergone gasification or cleavage reaction and their surface O content thus corresponds to partly uncovered surface. Within comparable bulk oxygen contents, the relative increase in surface O content depends on the coal ranksthe lower the rank the higher the mentioned parameter: M > Cz > Sz. It must be stressed, however, that even with higher relative increase in oxygen content, the total amount of oxygen after the treatment with O2/Na2CO3, is ca. twice lower for L and Z in comparison to M, Sz, and Cz. As it may be seen from Table 1, the efficiency of desulfurization is low for the used oxidizing solution. (19) Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley and Sons: New York, 1983.

Grzybek et al.

Some bulk sulfur was removed, as proven by the decrease in Std, but it seems to be connected with the removal of inorganic (pyrite) rather than organic S. This is in marked contrast to the use of PAA where it was observed that both inorganic and organic content of S was diminished, the former by 60 to 100% and the latter by 20 to 30%.16 Surface content of sulfur does not change within experimental error for O2/Na2CO3. The changes in S caused by oxidation with PAA were more complicated as described in more detail by Grzybek et al.16 The analysis of surface S content suggested that there was some surface enrichment in this element for L, S, and Z. The origin of such an effect was speculated to be connected with the structure of coal viewed as a polymer-like skeleton with smaller molecules trapped in it by weak forces. Upon oxidation, S-containing small molecules might migrate to the surface, as it was observed earlier, for example, for plastisizers or dyes in polymers. The oxidizing treatment may enable the migration by opening and enlarging the porous system.16 The comparison of the two oxidizing agents proves that obviously oxidation of the carbon-containing framework and the above-mentioned process concerning S-containing molecules are interconnected. 3.2. SulfursChemical State. Figure 2 shows typical XPS spectra of S 2p for the studied coal samples: dem L and Cz, as well as L and Cz oxidized by O2/Na2CO3 or PAA which represent highly and moderately oxidized samples. The fitting of S2p peaks (without fixing either positions or half-widths) gave the following binding energies: 163.6 ( 0.1, 165.1 ( 0.2, and 168.0 ( 0.3 eV which correspond respectively to sulfides + thiophenes + thioethers + mercaptans,5,20 sulfoxide C-SO-C,20,21 or sulfones C-SO2-C.20 The first and the third peak are in a very good agreement with literature and the second in a reasonable one; however, it must be added that no constriction on the binding energy was used in the fitting program. This procedure makes it possible to draw conclusions without the necessity of taking the spectra of the model compounds on the same equipment, as the positions obtained are in reasonable agreement with the literature ones. No attempt at an independent fitting of sulfides and thiophene was undertaken, because the maximum at the unoxidized species position does not show an indication of two separate peaks. Additionally, we were rather interested in oxidized forms, as the fact that thiophenic forms are difficult to oxidize in contrast to disulfides has already been well established in the literature both by XPS10 and AP-TPR.13 For Z dem ,the observed value of the third peak is 168.7 eV, which is most probably a superposition of 168.0 and 169.4 peaks, the latter corresponding to sulfonate. Cz dem showed additionally a fourth peak at 169.8 eV. The positions of the peaks do not change after oxidation, but the relative ratio of oxidized to unoxidized species does change to an extent, depending mainly on the used oxidation agent and also somewhat on the type of coal sample which is illustrated by Table 3. It must be remembered, however, that due to the low contents of sulfur for Z, Cz and Sz and the XPS detection limit of ca. 0.1%, the appropriate numbers must be taken with (20) Fiedler, R.; Bendler, D. Fuel 1992, 71, 381. (21) A Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. H., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Perkin-Elmer: Eden-Praire, 1979.

Oxygen and Sulfur Species Formed by Coal Oxidation

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Figure 2. S 2p peaks for coal Labin and Czeczot; (- - -)alkylsulfides + thiophene + arylsulfides; (‚‚‚) sulfoxides; (-‚-)sulfones; (-‚‚-)sulfates. Table 3. The Relative Content of S in Oxidized and Unoxidized Forms (% of S 2p peak at appropriate binding energies) coal L dem L/O2/Na2CO3 L/PAA M dem M/O2/Na2CO3 M/PAA Cz dem Cz/O2/Na2CO3 Cz/PAA Sz dem Sz/O2/Na2CO3 Sz/PAA Z dem Z/O2/Na2CO3 Z/PAA

thiophene alkyl- and arylsulfides sulfoxides sulfones other 96 83 18 72 60 68 47 49 11 60 58 18 54 53 6

4 13 13 20 22 24 26 18 16 20 12 23 17 10 22

4 69 8 18 8 8 33 73 20 30 59 29 30 50

19

7 22

great care for these samples and the results, apart from M and L, should be treated rather as tendencies than numbers alone. From Table 3, the following observations can be made. (i) The number of unoxidized S species slightly decreases after the treatment with O2/Na2CO3 solution, while the effect is considerable for PAA. The only exception seems to be M/PAA sample which, as it was already discussed by Grzybek et al.,16 represents the unoxidized part of coal particle left after extensive removal of its matter during the oxidation process. For samples Cz, Sz, and Z, the effect on the number of thiophenic/sulfidic S species caused by mild oxidation (O2/Na2CO3) is within experimental error. (ii) For more easily oxidized coal M, more species are oxidized by O2/Na2CO3 toward sulfones than for coal L

where the process obviously stops at sulfoxides. For samples Cz, Sz, and Z, numbers given in Table 3 have high errors of ca. 15, 15, and 25%, respectively, due to the fact that S contents registered in XPS for these samples were ca. 0.6, 0.6, and 0.4 at. % and generally accepted accuracy of the determination of elemental content in XPS is ca. 0.1 at. %. However, even if the numbers are imprecise, the tendency toward the formation of higher oxidized species and the process leading from unoxidized f lower oxidized forms f higher oxidized forms is well illustrated by the change in the shape of the peaks, as shown, e.g., for Cz in Figure 2. Thus it may be speculated that oxidation of S species is a sequential process: unoxidized S f sulfoxides f sulfones. This is in good agreement with the concept of Pasiuk-Bronikowska et al.22 who proposed that oxidation of organic compounds containing sulfur follows Scheme 1. Similar conclusions were also drawn from IR spectra of the same coals oxidized with different oxidation agents.17 The main difference between this work and mild oxidation by air as discussed by Gorbaty et al.10 is that no sulfonic species were found for Mequinenza and Labin (the latter coal being very similar to Rasa samples studied by Gorbaty). It may be connected with the good solubility of sulfonic species in aqueous solutions,23 while no such process is possible when samples are oxidized in air.10 Our XPS results are (22) Pasiuk-Bronikowska, W.; Bronikowski, T.; Ulejczyk, M. Bull. Polish Acad. Sci. Chem. 1989, 37, 73. (23) Palmer, S. R.; Hippo, E. J.; Kruge, M. A.; Crelling, J. C. Characterization of Organic Sulfur Compounds in Coals and Coal Macerals. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429, American Chemical Society, Washington, DC, 1990; Chapter 18, p 296.

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Grzybek et al.

Figure 3. C 1s peak for coal Labin and Czeczot; (- - -)C-C or C-H groups; (‚‚‚)C-O groups (alcohol, phenol, or ether); (-‚-)CdO groups (carbonyl or chinone) or O-C-O; (-‚‚-)COO- group (carboxyl). Scheme 1

in good agreement with AP-TPR results for the same samples and the same oxidizing agents published by Kozłowski et al.13 who showed the absence of sulfonic species and interpreted it as their removal from the surface by solution in the used oxidizing aqueous media. 3.3. Oxygen-Containing Surface Groups. Figure 3 shows typical C 1s spectra for the studied coal samples: dem L and Cz, as well as L and Cz oxidized by O2/Na2CO3 or PAA which represent highly and moderately oxidized samples. Peaks at 284.6, 286.1, 287.6, and 289.1 eV correspond to carbon not connected to oxygen or connected to it with one, two, or three bonds, respectively. It may be thus interpreted as C-C or C-H groups both aromatic and aliphatic (284.6 eV), C-O groups (alcohol, phenol, or ether), CdO groups (carbonyl or chinone) or O-C-O, and COO- group (carboxyl). In this case, fitting was carried out by fixing positions of all peaks (C-C or C-H and oxidized forms). It was well established before for many model materials (among others graphite, active carbons, carbon fibers, etc.) that oxygen-containing surface groups shift the position of C 1s toward binding energies higher by +1.5 to +1.6 eV (in comparison to 284.6 eV) per each C-O bond. Table 4 summarizes the number of abovementioned species as well as parameter R corresponding to oxidation degree of coal surface expressed as the ratio of the number of carbon atoms in species containing oxygen (CC-O) and those containing no oxygen (CC-H).

Table 4. The Relative Number of Oxygen-Containing Surface Groups Expressed as a Percentage of C 1s Peak Corresponding to C-H, C-O, CdO (or O-C-O), and COO- Species and Oxidation Degree of Coal Surface r Equal to the Ratio of the Number of Carbon Atoms in Species Containing Oxygen CC-O and Those Containing No Oxygen CC-H coal L dem L/O2/Na2CO3 L/PAA M dem M/O2/Na2CO3 M/PAA Cz dem Cz/O2/Na2CO3 Cz/PAA Sz dem Sz/O2/Na2CO3 Sz/PAA Z dem Z/O2/Na2CO3 Z/PAA

R ) CC-O/CC-H C-H C-O CdO O-C-O COO0.15 0.15 0.23 0.22 0.20 0.28 0.39 0.30 0.37 0.37 0.33 0.35 0.19 0.18 0.37

87 87 81 82 83 78 72 77 73 73 75 74 84 85 73

12 12 14 11 8 17 22 18 17 21 16 14 15 14 21

1 0 4 5 6 3 6 4 6 5 7 8 1 1 4

0 1 1 2 3 2 0 1 4 1 2 4 0 0 2

Parameter R expressing the oxidation degree of surface for demineralized coal samples forms a sequence:

L dem (0.15) < Z dem (0.19) < M dem (0.22) < Sz dem (0.37) < Cz dem (0.39) This corresponds only roughly to the sequence of bulk oxygen content which is:

Z dem (5.0) < L dem (7.4) < M dem (18.5) ≈ Sz dem (17.2) ≈ Cz dem (17.0) Oxidation degree does not change too much after the use of O2/Na2CO3 solution and does so distinctly for PAA.

Oxygen and Sulfur Species Formed by Coal Oxidation

Samples oxidized by O2/Na2CO3 in comparison to demineralized ones may be divided into two groups: (a) with smaller bulk oxygen content (Z and L) where R virtually stays the same as for demineralized samples although bulk oxygen content increases by 0.3 and 0.9 wt %, and (b) with higher bulk oxygen content (M, Sz, and Cz) where R decreases although bulk oxygen content increases considerably (1.2 to 1.9 wt %). Similar division of samples into two groups was found when discussing the relative increase in surface oxygen content (cp. 3.1). After oxidation with PAA, parameter R forms a sequence:

Z/PAA (0.37) ) Cz/PAA (0.37) ≈ Sz/PAA (0.35) > M/PAA (0.28) > L/PAA (0.23) The value of R observed for Z, Cz, or Sz is most probably an upper limit of the number of oxygen-containing groups which may be formed on the surface. XPS probes several upper layers of a solid, depending on kinetic energy of appropriate electrons. According to Seah and Briggs,19 this would correspond to ca. 10 monolayers for kinetic energy of ca. 1200 eV (C 1s as measured with Al KR at 1486.6 eV). Thus subsurface layers which do not chemisorb oxygen directly would lead to an underestimation of the number of carbon atoms in oxygen functionalities present in the first layer. The values of R for M and L are lower than for Z, Sz, or Cz, especially for M which is about 2/3 that for Z. This must be in connection with much higher reactivity of M, leading to the domination of gasification rate over that of oxygen chemisorption. Thus due to high oxidizing properties of PAA, there is an equilibrium between chemisorption leading to gasification followed by further chemisorption and gasification and so on, resulting in mass loss. It must be observed, however, that R, which represents the extent of the formation of oxygen-containing surface groups, is not a function of mass loss. It was observed before for the same samples,16 that total O/C ratio was proportional to mass loss for all samples except of M. The mass loss, on the other hand, depended on coal rank. This may suggest that in case of the use of PAA not only gasification of oxygen functionalities leads to mass loss and thus there must be a serious competition between mass loss through this process and the cleavage of bonds. If it were not so, then the O/C ratio and R should be a similar function of mass loss (and thus both would form the same sequence corresponding to reactivity or more roughly to coal rank). Table 4 contains additionally the relative number of carbon atoms in C-H, C-O, CdO (or O-C-O), and COO- groups. From these data the following observations can be made. (i) The surface of demineralized coals always contains a considerable amount of C-O groups (ca. 10 to 20% of all carbon atoms) and a smaller amount of CdO (or O-C-O). In most cases no COO- was observed. (ii) The relative number of C-H groups changes only slightly for O2/Na2CO3. This is accompanied by no change in the number of oxidized groups (C-O, CdO, COO-), as well as R, for L and Z; and the decrease in

Energy & Fuels, Vol. 18, No. 3, 2004 809

the number of C-O groups with simultaneous increase in CdO and COO- for M, Sz, and Cz. (iii) The effect is more pronounced for PAA where the number of all oxidized species increases for all sampless especially for C-O, indicating the formation of additional new groups. This suggests a sequential process C-O f CdO (or O-C-O) f COO-. Such a sequential process of oxidation of carbon-oxygen functionalities was observed before by Grzybek et al.15 and by Gong et al.6 If most reactive coal M is compared with the least reactive one (Z), it may be observed that relatively fewer groups are on the surface in the former case. This may be explained by the fact that PAA leads to higher mass loss through gasification of surface groups and the observed surface of M is that of uncovered (or partly uncovered) unoxidized coal.

4. Conclusions 1. Surface composition of the studied coal samples changed after oxidation treatment to an extent depending on the rank of coal, bulk oxygen content of the starting sample, and the type of oxidizing agent. When O2/Na2CO3 was used, no direct correlation between reactivity expressed as a mass loss and C and O surface content or O/C surface ratio was found in contrast to the oxidation with PAA. 2. Surface content of sulfur did not change within experimental error for O2/Na2CO3 in contrast to the use of PAA where the enrichment in surface S species was detected. The efficiency of desulfurization of the studied coals was low for O2/Na2CO3 solution while it led to the removal of both inorganic and some organic S when PAA was used. 3. After oxidation, the increase in oxidized forms of sulfur (sulfoxides and sulfones) was observed on the surface. The extent of this process was highly dependent on the oxidizing medium. The registered changes allow the conclusion that the formation of oxidized S-containing organic species is a consecutive process: unoxidized S f sulfoxides f sulfones. The possible absence of sulfonic species which were observed for similar coals (M, Rasa) after mild oxidation in air may be explained by their removal from the surface due to their good solubility in aqueous solutions. 4. The formation of oxygen-containing surface groups depended on the coal sample and the type of oxidizing medium. O2/Na2CO3 solution led to the oxidation of already existing O-functionalities and their partial gasification for coals of lower rank (and higher bulk oxygen content) while no new groups were formed for coals of higher rank (and low bulk oxygen content). PAA solution led to the formation of new groups and subsequently their oxidation followed by gasification to an extent much higher than in case of O2/Na2CO3. 5. XPS data suggest that the formation of O-functionalities is a sequential process: C-O f CdO (O-C-O) f COO-, then followed by the formation of CO2. EF030153I