Energy Fuels 2010, 24, 1197–1206 Published on Web 12/08/2009
: DOI:10.1021/ef900932g
X-ray Photoelectron Spectroscopy Study of Nitrogen-Enriched Active Carbons Obtained by Ammoxidation and Chemical Activation of Brown and Bituminous Coals Piotr Nowicki, Robert Pietrzak,* and Helena Wachowska Laboratory of Coal Chemistry and Technology, Faculty of Chemistry Adam Mickiewicz University, Grunwaldzka 6, 60-780 Pozna n, Poland Received August 27, 2009. Revised Manuscript Received November 18, 2009
The study reported has been undertaken to characterize the changes taking place on the surface of the nitrogen-enriched active carbons. The method applied for this purpose was X-ray photoelectron spectroscopy (XPS), and the samples studied were active carbons prepared from Polish brown and bituminous coals. An ammonia-air mixture was applied as a reagent introducing nitrogen functions into the carbon structure. Nitrogen was introduced both to demineralized coals and to active carbons obtained by carbonization and chemical activation of nonmodified precursors. The activation was performed at 700 °C with KOH, in an argon atmosphere. Results of XPS measurements have shown that in the active carbons enriched in nitrogen at the stage of precursor the dominant nitrogen species are the N-5 and N-6 groups; while in the samples ammoxidized after chemical activation, the dominant nitrogen species are the surface groups of imines, amines, amides and nitriles, accompanied by lactams and N-5 and N-6 groups.
melamine,5 formamide,13 and nitrogen oxides.14 In the second method, the plastics, containing in their structure nitrogen groups (e.g., polyacronitrile, polyamides, and polyimides15-17), are carbonized and activated. The active carbons obtained in this way are characterized with a high content of nitrogen mainly in the form of thermally stable functional groups. Other advantages of this method are the possibility of getting materials of desirable shape, high mechanical strength, and controlled chemical composition and pollutant content18. In the third commonly used method, amines or imines of different order (e.g., polyethyleneimines,19 ethylenediamines, hexamethylenediamine,20 or diethylene tramines21) are deposited on the carbon surface. The carbons obtained by this method have a few percent content of nitrogen and basic character of the surface but relatively low surface area. For the last two decades, much attention has been put on obtaining nitrogen-enriched active carbons by thermal treatment of precursor or active carbon in the presence of N-reagent. Interesting work has been reported by Jansen and Bekkum22 who have proved that modification of active carbon at 200-420 °C by ammonia or a mixture of ammonia and air is an effective method of enrichment in nitrogen.
1. Introduction Active carbons enriched in nitrogen have recently been the subject of great interest because of the wide range of their possible applications. In particular, they can find applications related to protection of the natural environment, e.g., for lowtemperature selective catalytic reduction of NO1,2, for removal of pollutants of acidic character such as SO2, H2S, NOx, and CO2 from the gas phase,3-6 for adsorption of metal ions (e.g., Cu2þ) from the liquid phase,7 and in electrochemistry for production of electrodes in electrochemical capacitors (to improve their performance).8,9 The three most often used methods for obtaining active carbons enriched in nitrogen are as follows. In the first of them, coal (carbonaceous material) is subjected to thermal treatment in the presence of nitrogen donating agent; the most popular to be used in this method is ammonia and urea,10,11 although there are reports on the use of aqua ammonia,12 *To whom correspondence should be addressed. Phone:þ48618291476. Fax: þ48-618291505. E-mail:
[email protected]. (1) Huang, M. C.; Teng, H. Carbon 2003, 41, 951–957. (2) Grzybek, T.; Klinik, J.; Samojeden, B.; Suprun, V.; Papp, H. Catal. Today 2008, 137, 228–234. (3) Boudou, J. P.; Chehimi, M.; Broniek, E.; Siemieniewska, T.; Bimer, J. Carbon 2004, 41, 1999–2007. (4) Maroto-Valer, M. M.; Tang, Z.; Zhang, Y. Fuel Process. Technol. 2005, 86, 1487–1502. (5) Bagreev, A.; Menendez, J. A.; Dukhno, I.; Tarasenko, Y.; Bandosz, T. J. Carbon 2004, 42, 469–476. (6) Bimer, J.; Salebut, P. D.; Berleo_zecki, S.; Boudou, J. P.; Broniek, E.; Siemieniewska, T. Fuel 1998, 77, 519–525. ) tkowski, A. Lang(7) Biniak, S.; Pakulea, M. G.; Szyma nski, S.; Swia muir 1999, 15, 6117–6122. (8) Pietrzak, R.; Jurewicz, K.; Nowicki, P.; Babele, K.; Wachowska, H. Fuel 2007, 86, 1086–1092. (9) Jurewicz, K; Pietrzak, R.; Nowicki, P.; Wachowska, H. Electrochim. Acta 2008, 53, 5469–5475. (10) Jansen, R. J. J.; Bekkum, H. Carbon 1994, 32, 1507–1516. (11) Pietrzak, R.; Wachowska, H.; Nowicki, P. Energy Fuels 2006, 20, 1275–1280. (12) Przepi orski, J.; Skrodzewicz, M.; Konno, H.; Morawski, A. W. Karbo 2004, 3, 127-130 [in Polish]. r 2009 American Chemical Society
(13) Cossarutto, L.; Zimny, T.; Kaczmarczyk, J.; Siemieniewska, T.; Bimer, J.; Weber, J. V. Carbon 2001, 39, 2339–2346. (14) Garcia, P.; Coloma, F.; de Lecea, C. S. M.; Mondragon, F. Fuel Process. Technol. 2002, 77-78, 255–259. (15) Laszlo, K.; Tombacz, E.; Josepovits, K. Carbon 2001, 39, 1217– 1228. (16) Boudou, J. P.; Parent, P.; Suarez-Garcia, F.; Vilar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Carbon 2006, 44, 2452–2462. (17) Pollak, E.; Salitra, G.; Soffer, A.; Aurbach, D. Carbon 2006, 44, 3302–3307. (18) Lahaye, J.; Nanse, G.; Bagreev, A.; Strelko, V. Carbon 1999, 37, 585–590. (19) Yin, C. Y.; Aroua, M. K.; Ashri Wan Daud, W. M. Colloids Surf., A 2007, 307, 128–136. (20) Tamai, H.; Shiraki, K.; Shiono, T.; Yasuda, H. J. Colloid Interface Sci. 2006, 295, 299–302. (21) Plaza, M. G.; Pevida, C.; Arenillas, A.; Ubiera, F.; Pis, J. J. Fuel 2007, 86, 2204–2212. (22) Jansen, R. J. J.; Bekkum, H. Carbon 1995, 33, 1021–1027.
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They have also shown that the annealing of carbon in a stream of ammonia leads to formation of lactam and imide groups on the carbon surface, while the reaction with a mixture of NH3/air generates the formation of amide groups. They have also proved that during treatment at increasing temperatures lactams, imides, and amides are converted into pyridine and pyrrole groups incorporated in the carbon structure. Of great importance is the study by Starck et al.23 who studied the influence of the methods of demineralization and the ammoxidation process (at 250 °C) on the final characteristics of the active carbons, especially on their surface chemistry and porous structure. They have shown the demineralization has a positive effect on the nitrogen-enrichment by ammoxidation relative to the raw material and that the modifications of surface chemistry by the demineralization and the nitrogenenrichment induce changes in adsorptive properties toward volatile organic carbons (VOCs). Another much interesting work is that by J.P. Boudou et al.3 who studied the influence of ammonia treatment of the surface on the activity of a viscose-based activated carbon cloth (ACC) for the oxidative retention of H2S and SO2. They have shown that ammonia/steam treatment improved ACC performance the most, not only by introducing nitrogen surface groups but also by extending the microporosity and by modifying the distribution of surface oxygen groups. The study of ammoxidation of cellulose (at 250 °C) by Cagniant et al.24 should also be mentioned. They have proved that the presence of ammonia permits avoiding the oxidative degradations of cellulose as well as that in the first step of the reaction between cellulose and ammonia-air mixture conjugated imines are formed. This study also implied that the products of cellulose ammoxidation contain two types of nitrogen groups; that is imines and pyridine nuclei (b.e. 399 ( 0.1 eV) as well as amides, lactams, and nitriles (b.e. 400.2 ( 0.1 eV). From among other methods proposed, recently, particular attention has been directed to the method proposed by Kodama et al.,25 based on carbonization of melamine foam and giving active carbons of high content of nitrogen. Another interesting method has been proposed by Kim et al.,26 who obtained nitrogen-enriched carbon by carbonization of silk fibers followed by activation with steam. The sources of nitrogen were the peptide bonds present in fibroine, a component of silk fibers. Shalagina et al.27 proposed a method of synthesis of nitrogen-enriched carbon nanofibers giving carbon fibers containing about 7% of nitrogen by a catalytic decomposition of a mixture of ethylene and ammonia in the presence of metallic catalysts. Nitrogen species present in the structure and on the surface of nitrogen-enriched active carbon can be investigated by direct (destructive) methods such as pyrolysis, extraction, oxidation, or hydrogenation and indirect (nondestructive) methods such as X-ray photoelectron spectroscopy (XPS),
X-ray absorption near-edge structure, and Fourier transform infrared spectroscopy. Up to this time, studies on preparation and characterization of nitrogen-enriched active carbons, the most often used methods, have been XPS and FTIR. The data obtained by these methods permit determination of the type and content of nitrogen functional groups present in the structure and on the surface of active carbon but also identification of the conversions they undergo upon thermal treatment28-30 and chemical modifications.22,31-33 In the majority of works, XPS measurements were performed on active carbon obtained by activation with steam (physical activation) or with carbon dioxide.22,28-33 Much less is known on the XPS results collected for active carbon obtained by activation with KOH or H3PO411. The main aim of this study was to prepare nitrogen-enriched active carbon samples of different contents and types of nitrogen functional groups by ammoxidation and chemical activation of two types of fossil coal of a different degree of metamorphism and by characterization of the changes taking place on the surface of these carbon samples upon ammoxidation, carbonization, and chemical activation by the XPS method. 2. Experimental Section Samples. The starting raw samples were prepared from a Polish brown coal (L) (Konin mine; moisture = 12.5 wt %; ashd = 28.9 wt %; volatile matter (VM)daf = 53.6 wt %; Cdaf = 63.4 wt %; Hdaf = 4.9 wt %; Ndaf = 0.7 wt %; Sdaf = 2.0 wt %; Odaf = 29.0 wt %) and low volatile bituminous coal (B) (JasMos mine; moisture = 0.8 wt %; ashd = 2.5 wt %; VMdaf = 19.4 wt %; Cdaf = 89.6 wt %; Hdaf = 4.6 wt %; Ndaf = 1.4 wt %; Sdaf = 0.4 wt %; Odaf = 4.0 wt %). Precursors were milled and sieved to the grain size of 0.2 mm. All measurements were carried out on samples demineralized by concentrated hydrochloric and hydrofluoric acids according to the Radmacher and Mohrhauer method.34 The demineralized coals were carbonized, activated, and enriched with nitrogen in different sequences. Carbonization (C). The carbonization process was performed in a horizontal furnace under argon flow at a temperature of 700 °C for 1 h. Activation (A). KOH was directly mixed at room temperature with samples at the weight ratio of 4:1. The samples were activated at 700 °C for 45 min in argon flow. Incorporation of nitrogen (N). Ammoxidation was performed with the mixture of ammonia and air at ratio 1:3 (the flow ratio 0.25 L/min:0.75 L/min). This process was carried out in a rotary horizontal glass reactor, at the temperature of 350 °C for 5 h. The active carbon samples were prepared by the procedures differing in the order of the technological processes: (a) ammoxidation of the precursor followed by carbonization and activation (NCA) and (b) ammoxidation after carbonization and activation (CAN). The sample preparation, detailed procedure of their carbonization, activation, ammoxidation, and characterization of the initial samples and products is given in our earlier work.35 (28) Kapteijn, F.; Moulijn, J. A.; Matzner, S.; Boehm, H. P. Carbon 1999, 37, 1143–1150. (29) Sta nczyk, K. Energy Fuels 2004, 18, 405–409. (30) Cagniant, D.; Gruber, R.; Boudou, J. P.; Bilem, C.; Bimer, J.; Salebut, P. D. Energy Fuels 1998, 12, 672–681. -tkowski, A.; Nefie, S. Carbon 2002, (31) Pakulea, M.; Biniak, S.; Swia 40, 1873–1881. -tkowski, A. Car(32) Biniak, S.; Szyma nski, G.; Siedlewski, J.; Swia bon 1997, 35, 1799–1810. (33) Wachowski, L.; Sobczak, J. W.; Hofman, M. Appl. Surf. Sci. 2007, 253, 4456–4461. (34) Radmacher, W.; Mohrhauer, O. Brennstoff-Chemie 1956, 37, 353–358. (35) Nowicki, P.; Pietrzak, R.; Wachowska, H. Energy Fuels 2009, 23, 2205–2212.
(23) Starck, J.; Burg, P.; Muller, S.; Bimer, J.; Furdin, G.; Fioux, P.; Guterl, C. V.; Begin, D.; Faure, P.; Azambre, B. Carbon 2006, 44, 2549– 2557. (24) Cagniant, D.; Magri, P.; Gruber, R.; Berlozecki, S.; Salbut, P. D.; Bimer, J.; Nanse, G. J. Anal. Appl. Pyrolysis 2002, 65, 1–23. (25) Kodama, M.; Yamashita, J.; Soneda, Y.; Hatori, H.; Kamegawa, K. Carbon 2007, 45, 1105–1107. (26) Kim, Y.; Abe, Y.; Yanagiura, T.; Park, K. C.; Shimizu, M.; Iwazaki, T.; Nakagawa, S.; Endo, M.; Dresselhaus, S. Carbon 2007, 45, 2116–2125. (27) Shalagina, A. E.; Ismagilov, Z. R.; Podyacheva, O. Y.; Kvon, R. I.; Ushakov, V. A. Carbon 2007, 45, 1808–1820.
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Table 1. Elemental Composition of the Surface and in the Bulk of the Precursors [C þ O þ N þ S = 100 at. %] Content of Carbon [at. %] sample
bulk
surface
CdC, C-H
C-O, C-N
CdO
COO-
L B
74.6 96.2
64.1 83.4
57.4 71.5
3.7 8.7
1.6
1.4 3.2
CO32-
Content of Nitrogen [at. %] sample L B
bulk
surface
0.7 1.3
1.1 2.9
N-6, imines
amines, amides, nitriles
0.8
N-5, lactams
N-Q
1.1 0.9
1.2
pyridine N-oxide, NH3
Content of Sulfur [at. %] sample L B
bulk
surface
thiophene, alkylsulfides, arylsulfides,
sulfoxides,
sulfones,
sulfates
0.8 0.2
1.7 0.5
1.5 0.2
0.2 0.1
0.1
0.1
Content of Oxygen [at. %] sample
bulk
surface
CdO
C-OH
C-O-C, CdN-O
L B
23.9 2.4
33.0 13.3
5.0 9.3
22.7
5.3 3.6
COO-
H2Oads 0.4
of demineralized coal samples the elemental composition of their surface is different from that in the bulk. These two samples show much lower content of carbon and much higher content of oxygen on the surface than in the bulk. These differences are probably a result of partial oxidation of coal upon demineralization with concentrated acids and drying after the demineralization. The content of nitrogen and sulfur is greater on the surface than in the bulk. The initial coals differ in the contents and types of particular species of carbon, nitrogen, oxygen, and sulfur. In the two types of demineralized coal samples, carbon occurs mainly in the form of graphene structures and groups with C-O and/or C-N bonds, and the sample of bituminous coal shows greater contents of these groups. The two types of coal also contain some carbon in the form of carboxyl groups, and brown coal contains 1.6 at.% of carbonyl and quinine groups. The type of nitrogen species and their contributions strongly depend on the degree of carbonization of precursor. As follows from Table 1, the nitrogen contained in brown coal occurs in the form of pyrrole and pyridone groups (N-5); while in bituminous coal, there are additionally pyridinium species (N-6) and quaternary nitrogen (N-Q). Depending on the degree of coalification, the two types of coal studied have different contents of sulfur. Much more sulfur (1.7 at. %) occurs on the surface of brown coal mainly in the thiophene and sulfide species with a small contribution of sulfoxides. Bituminous coal has on the surface 0.5 at.% of sulfur in the form of small amounts of thiophenes and sulfides and oxidized sulfur species such as sulfoxides, sulfones, and sulfates. According to our earlier studies,35 the process of ammoxidation has no effect on the content of sulfur in the coals studied and the majority of active carbon samples obtained from them do not contain sulfur, so we have resigned from presenting the data on this heteroatom. The XPS data imply that brown coal contains almost 3 times more oxygen on the surface (33.0 at. %) than bituminous coal (13.3 at. %). The demineralized coals studied have different contents of oxygen species. Brown coal has oxygen mainly in hydroxyl groups making up almost 70% of all
Analytical Procedures. The elemental analyses (C, H, N, S) of the products obtained at each stage of the processing were performed on an elemental analyzer CHNS Vario EL III (Elementar Analysensysteme GmbH, Germany). XPS. The chemical state of selected elements and surface composition of the samples were determined by X-ray photoelectron spectroscopy using VSW spectrometer (Vacuum Systems Workshop Ltd., England) equipped with Al KR source and 18-channel 2-plate analyzer. The spectra were taken in a Fixed Analyzer Transmition mode (ΔE=const) with pass energy of 22 eV. They were smoothed, and the 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 Scofield36). The binding energies corresponding to appropriate peaks are as follows:11,22-24,28-33,37,38 C 1s: 284.5 ( 0.1, graphitic carbon (CdC) and C-H groups; 286.1 ( 0.1, phenolic, alcohol, and ether (C-O) or C-N groups; 287.4 ( 0.2, carbonyl or quinone groups (CdO); 289.3 ( 0.1, carboxyl groups (COO-); 290.5, carbonate groups (CO32-). N 1s: 398.7 ( 0.3, pyridinic (N-6) and imine; 399.4 ( 0.4, amine, amide, and nitrile; 400.5 ( 0.3, pyrrolic, pyridonic (N-5), and lactam; 401.3 ( 0.3, quaternary nitrogen (N-Q); 402.8 ( 0.4, pyridine N-oxide and chemisorbed ammonia; 404-406, chemisorbed nitrogen oxides (N-Ox). O 1s: 531.3 ( 0.2, carbonyl (CdO); 532.6 ( 0.3, hydroxyl (C-OH); 533.5 ( 0.2, ether and CdN-O groups; 534.3 ( 0.2, carboxyl (COO-); 536.3 ( 0.4, adsorbed water molecules. S 2p: 163.7 ( 0.4, thiophene, alkylsulfides, and arylsulfides; 165.2 ( 0.1, sulfoxides; 167.9 ( 0.4, sulfones; 170.0 ( 0.5, sulfates.
3. Results and Discussion Surface Composition of Demineralized Coals. Analysis of the data collected in Table 1 indicates that for the two types (36) Scofield, J. H. J. Electron. Spectrosc. 1976, 8, 129–137. (37) Grzybek, T.; Pietrzak, R.; Wachowska, H. Fuel Process. Technol. 2002, 77-78, 1–7. (38) Pietrzak, R.; Grzybek, T.; Wachowska, H. Fuel 2007, 86, 2616– 2624.
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Table 2. Content of Nitrogen and the Contributions of Its Particular Species [% at.] in the Samples Subjected First to Ammoxidation and Then Carbonization and Activation content of N sample
bulk
surface
L LN LNC LNCA
0.7 22.3 15.2 3.4
1.1 20.8 14.6 12.3
N-6, imines 9.4 4.9 3.2
B BN BNC BNCA
1.3 7.3 4.3 1.1
2.9 34.9 18.8 2.7
0.8 20.4 6.4 0.2
amines, amides, nitriles
N-5, lactams
N-Q
7.3 2.4
1.1 1.4 4.1 5.9
2.5 2.4 1.3
0.9 13.4 4.3 1.7
content of C bulk
surface
CdC C-H
C-O C-N
CdO
COO-
L LN LNC LNCA
74.6 66.6 79.6 89.2
64.1 65.4 78.4 67.0
57.4 55.1 56.0 42.1
3.7 5.1 13.2 10.3
1.6 5.2 6.5 7.8
1.4
B BN BNC BNCA
96.2 86.1 92.1 93.4
83.3 52.3 69.6 89.7
71.5 24.4 31.5 39.9
8.7 18.2 21.1 27.6
6.0 11.0 13.3
3.4
N-Ox 0.3 0.9 1.9
1.2 6.1
1.2 2.0 0.8
content of nitrogen in the form of lactam, pyridonium, and pyrrole species and quaternary nitrogen. The surface composition of BN sample obtained by ammoxidation of bituminous coal is much different (Table 2, Figure 1). The sample contains nitrogen mainly in the form of imine and pyridine groups (398.7 ( 0.3 eV), lactam, pyrrole, and pyridonium groups (400.5 ( 0.3 eV) and a small amount of N-oxide of pyridine (402.8 ( 0.4 eV). No maxima were found near the binding energy 399.4 ( 0.4 eV assigned to amine, amide, and nitrile groups and 401.3 ( 0.3 eV assigned to quaternary nitrogen (N-Q). This observation means that upon ammoxidation of bituminous coal, nitrogen is built mainly in the form of groups directly related to the graphene structure, i.e., imine and lactam groups and N-6 or N-5 type groups. The different behavior of these two types of coal upon ammoxidation is probably related to significant differences in the degree of ordering of their carbon structure and much lower content of oxygen in bituminous coal. Analysis of Table 3 and Figure 2 reveals that ammoxidation of brown coal leads to total removal of carboxyl groups, which confirms the earlier reports that some amount of nitrogen builds in the carbon structure in the form of amide groups. Besides, a small decrease in the content of carbon in the form of graphene species is noted, which is interpreted as a result of the building of nitrogen functional species into the carbon structure and an increase in the number of groups containing the bonds C-O, C-N, and CdO, appearing upon oxidation of brown coal in the reaction with airammonia. Changes in the content of carbon species in BN sample have different character. The content of carbon in graphene structures decreases by over 65%, which confirms that the nitrogen species introduced upon ammoxidation are built directly in the carbon matrix. The content of ether, hydroxyl, and carbonyl groups significantly increases, which means that the surface of bituminous coal is considerably oxidized. The ammoxidation of bituminous coal (B samples) causes a small increase in the content of carboxyl groups, which can be treated as the evidence that amide species are not formed in this reaction. In brown coal (L samples), the ammoxidation causes a decrease in the content of carbonyl species. Besides the above, the different mechanisms of reaction of these two types of coal with a mixture of air-ammonia are also indicated by the changes in the total content of carbon. According to Table 3, for brown coal, a large decrease in the content of carbon in the bulk and a small increase in the content of this element on the surface is observed. For bituminous coal, a 10% decrease in the content of carbon in the bulk and almost 40% decrease in its content on the
Table 3. Content of Carbon and the Contributions of Its Particular Species [% at.] in the Samples Subjected First to Ammoxidation and Then Carbonization and Activation
sample
pyridine N-oxides, NH3
CO32-
2.7 3.4
3.2 3.8 6.0 8.9
oxygen species on its surface and some ether and carbonyl groups, while bituminous coal has only carbonyl and ether groups accompanied with a small amount of chemisorbed water. The content of surface oxygen groups can be analyzed on the basis of the O1s maximum and C1s maximum. The bands from the O1s range are expected to come from both organic and inorganic species; moreover, the total oxygen peak is usually smooth and rather symmetric.39 Therefore, further analysis of the type and content of oxygen groups was made only on the basis of the C1s maximum. Surface Composition of Samples Ammoxidized at the Precursor Stage. As follows from the data presented in Tables 2 and 3 and Figures 1 and 2, the ammoxidation of the demineralized coal samples brings significant changes in the elemental composition on their surface. The amount of nitrogen increases considerably on the surface and for brown coal also in the bulk (Table 2), while the content of carbon decreases (Table 3) because of the large amounts of nitrogen species built into the coal structure. The scale of changes in the content of individual elements depends significantly on the degree of metamorphism of the initial coal. Much greater increase in the content of nitrogen on the surface, by about 32.0 at. %, was observed in sample BN from bituminous coal; while for LN sample obtained from brown coal, the content of nitrogen increased by 19.7 at. %. Ammoxidation of coal of a low degree of coalification (L) introduces on its surface considerable amounts of nitrogen in the form of imine and pyridine species along with amine, amide, and nitrile groups (Table 2, Figure 1). Sample LN shows a small _ ea, M. The application of XPS (39) Grzybek, T.; Kreiner, K.; Zyl method for examination of the surface of oxidized coals. In The methane-coal system with regard to methane desorption and recovery _ ea, M. Ed.; Uczelniane Wydawnictwa Naukowofrom mine gases; Zyl Dydaktyczne: Krak ow, 2000; pp 35-52 [in Polish].
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Figure 1. N 1s peak for initial and ammoxidized coals.
surface is noted. The difference is most probably a consequence of significant differences in the structure of carbon in brown and bituminous coal and their different susceptibility to modifications. Surface Composition of Chars and Active Carbons Obtained from Ammoxidized Precursors. To characterize the changes in the surface composition taking place upon carbonization, also, chars of LNC and BNC obtained from the precursors ammoxidized at 350 °C were subjected to the XPS study. Results are presented in Tables 2 and 3 and Figures 3 and 4. According to the data from Tables 2 and 3, carbonization of the samples (irrespective of the degree of metamorphism of the initial coal) leads to a significant decrease in the content of nitrogen at simultaneous increase in the content of carbon, relative to their values in LN and BN. The decrease in the content of nitrogen indicates low thermal stability of the majority of nitrogen groups built into the coal structure upon ammoxidation. For the BNC sample, the content of nitrogen in the form of imine species, N-6 and N-5 type structures and lactam groups, decreases by almost 70%. For LNC, the content of nitrogen in the form of imine and pyridine groups decreases by about 50%, but the content
of lactam groups, pyridonium, and pyrrole groups notably increases. The LNC carbonizate (or char) shows almost a 5 at. % decrease in the content of nitrogen in the form of amine, amide, and nitrile species relative to their content in LN sample. The increase in the content of nitrogen in the form of lactams and N-5 type structures is probably caused by transformation of some of the nitrogen species introduced upon ammoxidation (e.g., imine or amide groups) to more thermally stable nitrogen species. In the same way, we can explain the appearance of quaternary nitrogen (N-Q) and an increase by over 65% in the content of pyridine N-oxide in BNC char. Carbonization of ammoxidized coal at the stage of precursor leads to qualitative changes in the content of carbon (Table 3). The content of carbon in the form of graphene species increases, probably as a result of progressed aromatization of the coal structure upon exposure to high temperature. Carbonization also brings significant increase in the content of hydroxyl, ether, and carbonyl groups and systems containing a C-N bond. Sample LNC shows also some increase in the content of carbon in the form of carbonate groups, while sample BNC shows an increase in the content of carboxyl groups. 1201
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Figure 2. C 1s peak for initial and ammoxidized coals.
To check the effect of activation with potassium hydroxide on the coal samples’ surfaces, the activated samples LNCA and BNCA, ammoxidized prior to carbonization, were also subjected to XPS measurements. Results are collected in Tables 2 and 3 and Figure 3 and 4. The activated samples have a lower content of nitrogen than the corresponding carbonizates LNC and BNC, both on the surface and in the bulk, Table 2, and a decrease in the content of this heteroatom significantly depends on the type of precursor used for obtaining the activated samples. In the activated sample LNCA, the content of nitrogen is only 15% lower than in the carbonizate LNC, while in BNCA, the nitrogen content on the surface is over 85% lower than in BNC char. A significant decrease in the content of nitrogen on the surface of BNCA testifies to low chemical resistance of nitrogen species toward potassium hydroxide. This observation also confirms the earlier reports that upon ammoxidation of bituminous coal the majority of nitrogen is built into the surface structures and can be easily decomposed upon further thermal and/ or chemical treatment.
As mentioned earlier, activation also causes a considerable reduction of the nitrogen content in the bulk. The changes are particularly well seen in the LNCA sample in which the content of nitrogen is almost 80% lower than in LNC char. This drastic decrease in the nitrogen content in the bulk upon activation of LNC, most probably follows from much lower resistance of this char to high temperature and KOH. Because of this lower resistance, the process of gasification of particular grains of the carbonizate takes place deeper and with greater intensity, which results in removal of the rich in nitrogen external layers and a considerable decrease in the content of nitrogen in the bulk. The uncovering of the deeper layers of carbon structure that also are enriched in nitrogen (in contrast to the sample obtained from bituminous coal) makes the active carbon LNCA show only a little lower content of nitrogen on the surface than the LNC char (Table 2). Upon chemical activation, not only the total content of nitrogen but also the contributions of its particular species change. Chemical activation of BNC leads to a drastic decrease in the content of all nitrogen species present in its 1202
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Figure 3. N 1s peak for chars and active carbons obtained from ammoxidized precursors.
structure and in particular imine and pyridinium groups and quaternary nitrogen. The situation looks different on activation of LNC. Under the effect of KOH and high temperature, the less thermally stable nitrogen groups such as amine, amide, and nitrile are fully decomposed and the contents of N-6 type nitrogen and quaternary nitrogen (of higher thermal stability) significantly decrease. This decrease is much smaller than upon activation of BNC char obtained from bituminous coal. The LNCA sample reveals over 40% higher content of nitrogen in lactam, pyrrole, and pyridonium groups and over 2 times higher content of chemisorbed nitrogen oxides than the initial carbonizate. The increase in the content of the above-mentioned nitrogen species and a smaller decrease in the other nitrogen species is probably a result of uncovering of the deeper layers of coal structure (much enriched in nitrogen) in the process of activation. The activation with KOH also gives significant changes in the contents of particular species of carbon (Table 3). The extent of these changes, similarly as those in the content of nitrogen, depends substantially on the type of precursor used for obtaining the active carbon. The activation of LNC char leads to a significant increase in the content of carbon in the
bulk at a simultaneous decrease in its content on the surface. The activation of BNC char brings about an insignificant increase in the content of carbon in the bulk (by about 1.5%) and a great increase in its content on the surface by almost 30%. The difference is probably caused by a different course of gasification of the grains of LNC and BNC carbonizates in the process of activation. As the carbonizate obtained from brown coal is less resistant to temperature and potassium hydroxide, the process of gasification takes place on the surface and inside the grains, which leads to a decrease in the carbon content on the surface. In BNC showing much more ordered coal structure, most probably only the surface of the grains underwent gasification. Activation of LNC and BNC chars leads to different changes in the contents of different carbon species. LNCA shows lower content of carbon in the form of graphene species than LNC char, whereas in BNCA the content of graphene species is greater than in BNC char. As to the contents of the groups containing C-O and C-N bonds, in LNCA, they decrease by over 20% with respect to that in LNC char, while in BNCA, the content of carbon in the form of phenol, alcohol, and ether groups and C-N bonds 1203
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Figure 4. C 1s peak for chars and active carbons obtained from ammoxidized precursors. Table 4. Content of Nitrogen and Contributions of Its Particular Species [% at.] in the Samples Subjected First to Activation and Then to Ammoxidation content of N sample
bulk
surface
LCA LCAN
0.4 5.8
1.3 10.2
BCA BCAN
0.5 6.4
0.8 16.4
N-6, imines
amines, amides, nitriles
N-5, lactams
1.9
4.0
0.8 2.4
4.6
3.9
0.8 4.4
increases by about 30%. These changes can be related to a different character of changes in the content of nitrogen on the surface and in the bulk taking place upon activation, as mentioned earlier. Activation of the two types of carbonizates also leads to an increase in the content of oxidized carbon species (carbonyl and carboxyl groups) and in LNC sample also an increase in the content of carbonate groups. Surface Composition of Samples Ammoxidized at the Active Carbon Stage. XPS measurements were also performed for unmodified active carbon samples (LCA and BCA) and the samples obtained after their ammoxidation
N-Q
pyridine N-oxides, NH3
N-Ox
0.7
0.5 0.7
0.5
3.5
(LCAN and BCAN) (Tables 4 and 5 and Figures 5 and 6). As inferred from the data in Table 4, the active carbon samples not subjected to ammoxidation show very low content of nitrogen, both on the surface and in the bulk. The amount and types of nitrogen species found on the surface depends to some degree on the type of precursor used to produce active carbon, although the differences are not so pronounced as for the samples discussed above. The LCA sample obtained by carbonization and activation of brown coal contains nitrogen in the form of pyrrole and pyridone (N-5) structures (N-5) and pyridine N-oxide, while BCA sample from bituminous coal contains nitrogen only in the form of N-5 1204
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type species. According to Table 5, LCA and BCA samples contain carbon mainly in the form of graphene structures. The two types of active carbon contain significant amounts of oxidized carbon species in the form of ether and hydroxyl groups (15.1 and 20.0 at. %) and carbonyl groups (6.1 and 8.4 at. %). LCA contains a significant amount of carboxyl groups, whereas BCA contains carbon in the form of carbonates. Ammoxidation of LCA and BCA leads to essential changes in the surface composition. Similarly as for modification of precursors (Table 2 and 3), reaction with the air-ammonia mixture gives a substantial increase in the
content of nitrogen at a simultaneous decrease in the concentration of carbon on the surface and in the bulk (Tables 4 and 5). Ammoxidation of the two types of active carbon gives first of all a significant increase in the content of nitrogen in the form of amine, amide, and nitrile species, imine groups, and N-6 type nitrogen. BCAN sample also shows a substantial increase in the content of lactam groups and/or N-5 type species, pyridine N-oxide, and chemisorbed ammonia. In LCAN, also some small amounts of quaternary nitrogen and chemisorbed nitrogen species were detected. Upon ammoxidation of active carbon obtained from brown coal and bituminous coal in both samples, we find in them amine, amide, and nitrile groups, which means that the mechanism of reaction of these samples with the airammonia mixture is very similar. This observation also suggests that the process of ammoxidation carried out at the stage of precursor (Surface Composition of Demineralized Coals) significantly differs from the mechanism of ammoxidation at the stage of active carbon, as is also indicated by different character of changes in the contents of particular nitrogen species. Ammoxidation of active
Table 5. Content of Carbon and Contributions of Its Particular Species [% at.] in the Samples Subjected First to Activation and Then to Ammoxidation content of C sample bulk surface CdC C-H C-O C-N CdO COO- CO32LCA 93.5 LCAN 88.6
80.9 77.1
52.0 49.0
15.1 14.3
6.1 6.2
BCA 93.7 BCAN 88.4
91.9 73.8
57.2 33.9
20.0 23.1
8.4 10.3
7.7 4.5
3.0 6.3 6.6
Figure 5. N 1s peak for samples ammoxidized at the active carbon stage.
1205
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Figure 6. C 1s peak for samples ammoxidized at the active carbon stage.
carbon leads to considerable changes in the content of individual carbon species (Table 5), especially upon ammoxidation of BCA. The most pronounced differences are noted in the content of graphene species. Upon ammoxidation, the content of graphene species decreases in LCA by about 6%, while in BCA by over 40%. It is most probably caused by the fact that ammoxidation of BCA leads to the building of a greater amount of nitrogen in the species bonded directly to the carbon matrix, i.e., imine, lactam groups, and N-5 and N-6 type nitrogen on the sample surface. Moreover, the BCAN sample shows a greater content of groups containing C-O, C-N, and CdO bonds than the BCA sample. In LCAN, the content of carbon in the form of carboxyl groups decreases and that of carbon in the form of carbonates increases relative to their content in LCA.
of nitrogen. The mechanism of ammoxidation was proved to depend to a substantial degree on the type of precursor used and the stage of active carbon production at which the process of ammoxidation was performed. Ammoxidation of brown coal (of low degree of coalification) led to formation of significant amounts of imine, pyridine, amine, amide, and nitrile groups, whereas modification of bituminous coal (of higher degree of coalification) brought the formation of imine, pyridine, pyrrole, pyridone, and lactam groups. Ammoxidation of the active carbon samples gave a considerable increase in the content of amine, amide, nitrile, imine, pyridine, and lactam groups and N-5 type nitrogen species. According to the results of spectral studies, upon carbonization and in particular upon chemical activation, the content of nitrogen decreases considerably, so in the active carbon obtained, only the most thermally and chemically resistant nitrogen species built directly into the graphene planes remain.
4. Conclusions The above-discussed results of the XPS measurements have shown that nitrogen enrichment of fossil coal at different stages of active carbon production leads to getting carbon materials containing different amounts of different species
Acknowledgment. This work was supported by The Polish Ministry of Science and Higher Education project No. N N204 056235. 1206
