Preparation of Nitrogen-Enriched Activated Carbons from Brown Coal

Laboratory of Coal Chemistry and Technology, Faculty of Chemistry, Adam ... Nitrogen-enriched activated carbons were prepared from a Polish brown coal...
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Energy & Fuels 2006, 20, 1275-1280

1275

Preparation of Nitrogen-Enriched Activated Carbons from Brown Coal Robert Pietrzak,* Helena Wachowska, and Piotr Nowicki Laboratory of Coal Chemistry and Technology, Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan˜ , Poland ReceiVed December 14, 2005. ReVised Manuscript ReceiVed March 4, 2006

Nitrogen-enriched activated carbons were prepared from a Polish brown coal. Nitrogen was introduced from urea at 350 °C in an oxidizing atmosphere both to carbonizates obtained at 500-700 °C and to activated carbons prepared from them. The activation was performed at 800 °C with KOH in argon. It has been observed that the carbonization temperature determines the amount of nitrogen that is incorporated (DC5U, 8.4 wt % Ndaf; DC6U, 6.3 wt % Ndaf; and DC7U, 5.4 wt % Ndaf). X-ray photoelectron spectroscopy (XPS) measurements have shown that nitrogen introduced both at the stage of carbonizates and at the stage of activated carbons occurs mainly as N-6, N-5, and imine, amine and amide groups. On the other hand, the activation of carbons enriched with nitrogen results in the formation of pyridonic nitrogen and N-Q. The introduction of nitrogen at the activated carbon stage leads to a slight decrease in surface area. It has been proven that the most effective way of preparing microporous activated carbons enriched with nitrogen to a considerable extent and having high surface area (∼3000 m2/g) is the following: carbonization f activation f reaction with urea.

1. Introduction The development of many branches of industry and the use of modern technologies that are accompanied by tougher and tougher requirements for environmental protection justify the ceaseless search for cheap adsorbents. Currently, microporous carbon adsorbents of different surface areas are widely used in the form of activated carbons, activated carbon fibers, and carbon nanotubes, as well as carbons of honeycomb structure or synthetic carbon sorbents.1-5 Raw materials for the manufacture of activated carbons are different materials, mostly of organic origin. The most important of them are wood, peat, lignin, cellulose, fruit stones, nut shells, and plastics.6-9 Prospects in this field are particularly bright for coals, beginning from brown coals and ending with anthracites.10-12 The main reason for the application of bituminous coals as raw materials for the manufacture of adsorbents * Author to whom correspondence should be addressed. Tel.: +486186291476. Fax: +48-618658008. E-mail: [email protected]. (1) Bagreev, A.; Menendez, J. A.; Dukhno, I.; Tarasenko, Y.; Bandosz, T. Carbon 2004, 42, 469-476. (2) Martin-Gullon, I.; Andrews, R.; Jagtoyen, M.; Derbyshire, F. Fuel 2001, 80, 969-977. (3) Grande, C. A.; Silva, V. M. T. M.; Gigola, C.; Rodrigues, A. E. Carbon 2003, 41, 2533-2545. (4) Zuttel, A.; Nutzenadel, Ch.; Sudan, P.; Mauron, Ph.; Emmenegger, Ch.; Rentsch, S.; Schlapbach, L.; Weidenkaff, A.; Kiyobayashi, T. J. Alloys Compd. 2002, 330-332, 676-682. (5) Gadkaree, K. P. Carbon 1998, 36, 981-989. (6) Diaz-Teran, J.; Nevskaia, D. M.; Fierro, J. L. G.; Lopez-Peinado, A. J.; Jerez, A. Microporous Mesoporous Mater. 2003, 60, 173-181. (7) Hayashi, J.; Kazehaya, A.; Muroyama, K.; Watkinson, A. P. Carbon 2000, 38, 1873-1878. (8) Hu, Z.; Srinivasan, M. P. Microporous Mesoporous Mater. 1999, 27, 11-18. (9) Wu, M.; Zha, Q.; Qiu, J.; Guo, Y.; Shang, H.; Yuan, A. Carbon 2004, 42, 205-210. (10) Martyniuk, H.; Wie¸ ckowska, J. Fuel 1995, 74, 1716-1718. (11) Carrasco-Marin, F.; Alvarez-Merino, M. A.; Moreno-Castilla, C.; Fuel 1996, 75, 966-970. (12) Perrin, A.; Celzard, A.; Albiniak, A.; Kaczmarczyk, J.; Mareche, J. F.; Furdin, G. Carbon 2004, 42, 2855-2866.

is the ease of formation of well-developed porous structure in these materials. This fact is a result of the presence of primary pores in bituminous coals. However, the pores mainly have very small sizes; therefore, they are inaccessible for many adsorbates. This is why the coals must be subjected to carbonization and activation. The activation can be performed either by physical methods (using steam or carbon dioxide)13-15 or by chemical methods (using KOH, NaOH, ZnCl2, H3PO4, MgCl2, AlCl3, K2CO3, etc.).16-19 The properties of activated carbons are dependent, to a large extent, on the raw material, surface structure, and pore size distribution. However, the greatest effect on physicochemical properties of activated carbons is exerted by heteroatoms that are built into their structure (mainly, oxygen,20 sulfur,21 nitrogen,22 boron,23 phosphorus,24 and halogens25). Recently, nitrogen-containing activated carbons are the subject of particular interest to researchers, because the aforementioned carbons can find application in the removal of contaminants (13) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Gonzalez, M. T. Carbon 1995, 33, 15-23. (14) Molina-Sabio, M.; Gonzalez, M. T.; Rodriguez-Reinoso, F.; SepulvedaEscribano, A. Carbon 1996, 34, 505-509. (15) Arenas, E.; Chejne, F. Carbon 2004, 42, 2451-2455. (16) Lillo-Rodenas, M. A.; Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2001, 39, 751-759. (17) Lozano-Castello, D.; Lillo-Rodenas, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2001, 39, 741-749. (18) Hsu, L. Y.; Teng, H. Fuel Process. Technol. 2000, 64, 155-166. (19) Hayashi, J.; Horikawa, T.; Takeda, I.; Muroyama, K.; Ani, F. N. Carbon 2002, 40, 2381-2386. (20) Biniak, S.; Szyman´ski, G.; Siedlewski, J.; SÄ wia¸ tkowski, A. Carbon 1997, 35, 1799-1810. (21) Jankowska, H.; SÄ wia¸ tkowski, A.; Starostin, L.; ŁawrienienkoOmiecyn´ska, J. Adsorption of Ions on ActiVated Carbon (in Pol.); PWN Publishing House: Warsaw, Poland, 1991. (22) Jansen, R. J. J.; Bekkum, H. Carbon 1995, 33, 1021-1027. (23) Shirasaki, T.; Derre, A.; Menetrier, M.; Tressaud, A.; Flandrois, S. Carbon 2000, 38, 1461-1467. (24) Puziy, A. M.; Poddubnaya, O. I. Carbon 1998, 36, 45-50. (25) Perez-Cadenas, A. F.; Maldonado-Hodar, F. J.; Moreno-Castilla, C. Carbon 2003, 41, 473-478.

10.1021/ef0504164 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/21/2006

1276 Energy & Fuels, Vol. 20, No. 3, 2006

from gas26,27 and liquid28 phases, in the environmental protection industry as catalysts or catalyst supports,29 and in electrochemistry for the manufacture of electrodes for electrochemical capacitors, cells, and batteries to upgrade their capacity parameters.30,31 The reagents most frequently used for the introduction of nitrogen to the structure of carbonaceous materials are ammonia, amines, and urea.30,32,33 Depending on the reagent that is applied, the process is conducted either in a liquid phase (the case of urea and amines) or in a gas phase (the case of ammonia). Jansen and Bekkum32 subjected activated carbons to the reaction with ammonia (amination) or with a mixture of ammonia and air (ammoxidation) in the temperature range of 200-420 °C. They have found that amination results in the formation of lactams and imides on the surfaces of activated carbons, whereas ammoxidation leads mainly to amides. Bimer et al.33 performed the reaction between coal and ammonia and its derivatives at elevated temperature and under increased pressure. They have concluded that the amount of nitrogen that is built into the carbon structure is approximately proportional to the carboxyl group content. On the other hand, products of the reaction between coal and urea contain a large quantity of thermally stable nitrogen-containing functional groups, which is a result of interactions between urea and the carbonaceous structure. Bagreev et al.1 have introduced nitrogen to activated carbon by impregnating the latter with an alcoholic solution of melamine and urea, followed by carbonization of the obtained samples at 650 and 850 °C. Carbons prepared in such a way contained considerable quantities of incorporated nitrogen; however, their structural parameters deteriorated by 25%, compared to unmodified carbon. The reaction between coal and urea was also investigated by Burg et al.34 These authors performed their study under increased pressure at 350 °C, at a weight ratio of urea to carbon equal to 1:1. The product was characterized by a considerable nitrogen content (13.3 wt %), which was present in the form of functional groups typical of amines, amides, nitriles, and lactams. The aforementioned reports show that there are many efficient methods of activated carbon enrichment with nitrogen. However, in most cases, the studies we reference were conducted under increased pressure and/or at elevated temperature. In our study, we have made an attempt to prepare nitrogen-enriched carbons by the reaction with urea in an oxidizing atmosphere under atmospheric pressure. 2. Experimental Section 2.1. Samples. The starting raw sample (R) was prepared from a Polish brown coal (from the Konin mine). It was milled and sieved to a grain size of 0.2 mm. All measurements were conducted on samples that had been demineralized using concentrated hydrochloric and hydrofluoric acids, according to the Radmacher and (26) Raymundo-Pinero, E.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2003, 41, 1925-1932. (27) Huang, M. C.; Teng, Carbon 2003, 41, 951-957. (28) Otowa, T.; Nojima, Y.; Miyazaki, T. Carbon 1997, 35, 1315-1319. (29) Jankowska, H.; SÄ wia¸ tkowski, A.; Choma, J. ActiVe Carbon (in Pol.); WNT Publishers: Warsaw, Poland, 1985. (30) Jurewicz, K.; Babeł, K.; Zio´łkowski, A.; Wachowska, H.; Kozłowski, M. Fuel Process. Technol. 2002, 77-78, 191-198. (31) Jurewicz, K.; Babeł, K.; Zio´łkowski, A.; Wachowska, H. J. Phys. Chem. Solids 2004, 65, 269-273. (32) Jansen, R. J. J.; Bekkum, H. Carbon 1994, 32, 1507-1516. (33) Bimer, J.; Sałbut, P. D.; Berłoz˘ ecki, S.; Boudou, J. P.; Broniek, E.; Siemieniewska, T. Fuel 1998, 77, 519-525. (34) Burg, P.; Fydrych, P.; Cagniant, D.; Nanse´, G.; Bimer, J.; Jankowska, A. Carbon 2002, 40, 1521-1531.

Pietrzak et al. Table 1. Characteristics of Raw and Demineralized Coal Content [wt %] Proximate Analysis coal

moisture

asha

volatile matter, VMa

R D

14.5 0.0

9.7 0.2

53.6 50.2

a

Ultimate Analysis Ca

Ha

Na

Sa

Oa,b

64.8 62.3

5.7 5.4

0.6 0.6

1.7 1.4

27.2 30.3

Dry-ash-free (daf) basis. b Determined by difference.

Mohrhauer method.35 A characterization of the coal sample that was studied is given in Table 1. The ashless coal (D) was carbonized, activated, and mixed with urea. 2.2. Carbonization. Carbonization (C) was performed in a horizontal furnace under an argon flow (using a flow rate of 170 mL/min). The samples were heated (5 °C/min) from room temperature to final carbonization temperatures of 500 °C (DC5), 600 °C (DC6), and 700 °C (DC7). At the final temperatures, samples were maintained for 1 h and then cooled in an inert-gas atmosphere. 2.3. Activation. Activation (A) was applied to carbonization products (DC) or nitrogen-enriched carbonizates (DCU). KOH was directly mixed at room temperature with samples at a weight ratio of 4:1. After the physical mixing, the samples were activated at 800 °C for 45 min. This process was conducted in a laboratory furnace in an argon flow (330 mL/min) to ∼50% burnoff. The obtained activated carbons (DCA and DCUA) were washed first with 5% HCl solution and then with distilled water until it was free of chloride ions. The washed activated carbons were dried at 110 °C for 24 h. 2.4. Incorporation of Nitrogen. Urea was applied as a reagent, introducing nitrogen functions into the carbon structure. The carbonizate (DC) was mixed with urea at a weight ratio of 1:1 and then oxidized with oxygen from air at 350 °C. The activated carbon prepared from the aforementioned carbonizate (DCA) was subjected to an analogous treatment. The reaction proceeded in a glass reactor under atmospheric pressure for 3 h. The obtained nitrogen-enriched carbons (DCU and DCAU) were washed with hot distilled water to remove the unreacted urea portion and dried. 2.5. Analytical Procedures. Proximate analysis of the initial coal and elemental analysis (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). Characterization of the pore structure of activated carbons was performed on the basis of low-temperature nitrogen adsorptiondesorption isotherms measured on a sorptometer (model ASAP 2010, manufactured by Micromeritics Instrument Corp., Norcross, GA). Surface area and pore size distribution were calculated using the Brunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) methods, respectively. The total pore volume and average pore diameter also were determined. Micropore volume and micropore area were calculated using the t-plot method. The chemical state of selected elements and surface composition of the samples were determined by X-ray photoelectron spectroscopy (XPS), using a VSW spectrometer (Vacuum Systems Workshop, Ltd., England) that was equipped with an Al KR source and 18-channel 2-plate analyzer. The spectra were taken in a fixed analyzer transmission mode (∆E ) constant) with a pass energy of 22 eV. They were smoothed, and the Shirley background was subtracted. The calibration was performed 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). (35) Radmacher, W.; Mohrhauer O. Brennst.-Chem. 1956, 37, 353358. (36) Scofield, J. H. J. Electron. Spectrosc. 1976, 8, 129-137.

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Energy & Fuels, Vol. 20, No. 3, 2006 1277

Table 2. Elemental Analysis of Carbonizates and Nitrogen-Enriched Products of Carbonization and Activation

Table 3. Elemental Analysis Activated Carbons Untreated and Treated with Urea

Content [wt %]

Content [wt %]

coal

asha

Ca

Ha

Na

Sa

Oa,b

coal

asha

Ca

Ha

Na

Sa

Oa,b

DC5 DC5U DC5UA DC6 DC6U DC6UA DC7 DC7U DC7UA

1.9 2.0 2.0 1.9 3.9 1.0 2.2 2.9 3.5

81.9 62.6 84.9 85.1 73.9 93.5 92.1 81.3 93.7

3.2 2.5 1.3 2.4 2.1 0.4 1.6 2.0 0.3

1.0 8.4 2.0 1.0 6.3 0.6 0.9 5.4 0.7

1.4 0.7 0.7 1.3 0.6 0.1 1.9 1.5 0.1

12.5 25.8 11.1 10.2 17.1 5.4 3.5 9.8 5.2

DC5 DC5A DC5AU DC6 DC6A DC6AU DC7 DC7A DC7AU

1.9 0.9 1.5 1.9 0.9 1.3 2.2 1.0 0.5

81.9 94.3 87.9 85.1 94.4 86.1 92.1 94.0 86.6

3.2 0.3 0.5 2.4 0.2 0.6 1.6 0.2 0.6

1.0 0.4 4.8 1.0 0.3 5.4 0.9 0.2 5.6

1.4 0.5 0.2 1.3 0.3 0.2 1.9 0.4 0.2

12.5 4.5 6.6 10.2 4.8 7.7 3.5 5.2 7.0

a

Dry-ash-free (daf) basis. b Determined by difference.

3. Results and Discussion 3.1. Precursor Characterization. The first stage of the study consisted of proximate and elemental analyses of the raw and demineralized coal. The results of those analyses, which are presented in Table 1, show that the treatment of coal samples with hydrochloric acid, followed by treatment with hydrofluoric acid, causes a considerable reduction in ash content. Ash contents in raw coal and demineralized coal samples served as a basis for calculating the degree of demineralization, which amounts to 97.9%. Despite such a considerable demineralization, the volatile matter (VM) content in demineralized coal is only slightly lower than that in raw coal. The elemental analysis data, given in Table 1, indicate a slight decrease in Cdaf, Hdaf, and Sdaf (the superscript “daf” indicates that the measurements were obtained on a dry ash-free basis), as a result of the demineralization process, while the oxygen content (calculated by difference) increases. The treatment of coal with HCl and HF does not affect the nitrogen content. The obtained demineralized coal was subjected to carbonization at three temperatures (500, 600, and 700 °C) in argon. 3.2. Characterization of Carbonizates. The data presented in Table 2 show that carbonization causes an increase in ash content, compared to the demineralized coal sample (Table 1), and results in a considerable increase in Cdaf content and reduction in Odaf content. A comparison of the carbonizates obtained (DC5, DC6, and DC7) shows that an increase in temperature does not affect their ash content significantly, whereas it results in an increase in Cdaf and a decrease in Hdaf and Odaf. The nitrogen content in carbonizates does not change with increasing temperature, which is an indication of the thermal stability of the nitrogen-containing functional groups present in the structure of coal starting material. The obtained carbonizates were subjected to the activation process, as well as to the treatment with urea, with the objective of enriching them with nitrogen. The reaction with urea was conducted at the stage of both carbonizates and activated carbons. 3.3. Reaction of Carbonizates with Urea. At the first stage of our study, the carbonizates were treated with urea under oxidizing conditions at 350 °C, at atmospheric pressure, and then activated with the use of KOH at 800 °C. The results of the analyses of the products obtained are presented in Table 2. The obtained data indicate that the reaction of carbonizates with urea enables the introduction of a considerable amount of nitrogen. All samples obtained are characterized by a decrease in carbon content and an increase in oxygen content, compared to initial carbonizates, which is a result of the conditions of the reaction discussed. The aforementioned reaction also causes a

a

Dry-ash-free (daf) basis. b Determined by difference.

reduction in sulfur content. The content of Hdaf changes, depending on the carbonization temperature. When comparing carbonizates prepared at different temperatures, one can notice that the highest amount of nitrogen was incorporated into the carbonizate obtained at 500 °C (DC5U), followed by those obtained at 600 °C (DC6U) and 700 °C (DC7U). However, even in the case of DC7U, which is characterized by the lowest nitrogen content, we have managed to introduce as much as 5.4 wt % Ndaf. Data presented in Table 2 also show that the oxygen content (calculated by difference) is the highest in DC5U and the lowest in DC7U. Based on the data listed in Table 2, one can conclude that the amount of nitrogen incorporated into carbonizates is dependent on the oxygen content in the starting material. In addition, a similar observation was made by Bimer et al.,33 who introduced nitrogen to coal using ammonia as a source of nitrogen. The obtained results clearly show that the greatest amount of nitrogen was incorporated into DC5 (8.4 wt % Ndaf in the resulting DC5U sample), which has the highest oxygen content (12.5 wt % Odaf), whereas the smallest amount was incorporated into DC7 (5.4 wt % Ndaf in the DC7U sample), which contained 3.5 wt % Odaf. Although the DC7 carbonizate sample has the lowest oxygen content, one can notice that the extent of oxidation was the greatest only in the case of DC7 (compared to the initial carbonizate). Products obtained as a result of the reaction between carbonizates and urea were subjected to activation with the use of KOH at 800 °C. Elemental analysis of the activated carbons prepared in this way (i.e., DC5UA, DC6UA, DC7UA; see Table 2) has shown that carbon content clearly increased in all the samples, both in relation to the urea-treated sample and to the initial carbonizate. The contents of other elements decreased in all activated carbons discussed. It can be concluded that the activation leads to a drastic reduction in nitrogen content by 80%-90%, compared to nitrogen-enriched carbonizates. This is probably a result of a low thermal stability of nitrogencontaining functional groups. The greatest loss of nitrogen (by ∼90%) was observed in the case of DC6UA and the smallest (by ∼80%) was observed in the case of DC5UA. 3.4. Reaction of Activated Carbons with Urea. At the second stage of our study, carbonizates were subjected to activation, followed by the reaction with urea. Data presented in Table 3 show that the activation of carbonizates (DC5A, DC6A, and DC7A), which was aimed at removing amorphous carbon formed during the carbonization process and the creation of new pores, results in an increase in carbon content and a clear reduction in hydrogen and sulfur contents. To see the changes more clearly, the results of analyses of the initial carbonizates were shown in the aforementioned table once again. The nitrogen content also decreases, and this is caused by poor stability of the nitrogen-containing functional groups at high

1278 Energy & Fuels, Vol. 20, No. 3, 2006

Pietrzak et al.

Table 4. Surface and Bulk Nitrogen Content in Carbons and the Contribution of Nitrogen Species to the N 1s Peak Nitrogen Content [at. %]

Contribution of Nitrogen Species to N 1s Peak [%]

coal

bulk

surface

imines, amines, amides

pyridine N-6

pyrrole, pyridone N-5

in graphene layers N-Q

pyridine N-oxides, ammonia

N-Ox

DC5U DC7U DC5UA DC7UA DC5AU DC7AU

8.1 4.9 1.8 0.6 4.3 5.0

11.5 8.2 1.8 0.8 4.9 5.2

23.9 24.4 0.0 5.0 20.9 24.4

37.5 33.1 23.1 14.2 28.5 29.7

25.7 28.6 42.5 54.0 31.4 27.1

8.0 8.4 17.1 9.6 9.7 9.5

3.2 3.4 9.8 4.6 6.0 5.9

1.7 2.1 7.5 12.6 3.5 3.4

temperatures. An increase in the oxygen content is observed only in the case of DC7A. The greatest changes in elemental composition, when comparing the activated carbon to the initial carbonizate, were observed in DC5. The DC5A sample, obtained as a result of activation, is characterized by the greatest increase in carbon content and a decrease in the contents of other elements (Table 3), compared to the starting material. The smallest changes are observed in the case of DC7A. The obtained activated carbons were subject to reaction with urea at 350 °C in an oxidizing atmosphere. Data given in Table 3 indicate that the aforementioned reaction leads to a decrease in the carbon content in all samples (in relation to activated carbons) and to an increase in the hydrogen, oxygen, and nitrogen contents; this fact is a consequence of the reaction conditions that have been applied. In the case of activated carbons, the highest nitrogen content was observed in the DC7AU sample and the lowest was observed in DC5AU (see Table 3). The greatest amount of nitrogen was incorporated into activated carbon of the highest oxygen content sample (DC7A) and the lowest amount of nitrogen was incorporated into activated carbon of the sample with the lowest content of oxygen-containing functional groups (DC5A), similar to that observed in the case of carbonizates. This indicates that, in the case of the introduction of nitrogen with the use of urea to activated carbon, the amount of incorporated nitrogen also is dependent on the oxygen content in the starting sample. 3.5. X-ray Photoelectron Spectroscopy (XPS) Studies. X-ray photoelectron spectroscopy (XPS) was applied to determine the changes that occur on the surfaces of carbonizates and activated carbons subjected to the reaction with urea, as well as to characterize the nitrogen-containing species in samples of the highest (DC5U, DC7AU) and the lowest (DC5AU, DC7U) nitrogen contents (see Tables 2 and 3). The nitrogen content on the surface of carbons, compared to that in the bulk of carbons investigated (given in atomic percent, at. %) and the contribution of different nitrogen species to the N 1s peak (given as a percentage) are given in Table 4. Figure 1 shows a typical XPS spectrum of N 1s for all samples. The fitting of the N 1s peaks gave the following binding energies: 398.1 ( 0.1 eV for imine, amide, and amine; 399.2 ( 0.2 eV for N-6 (pyridinic); 400.3 ( 0.2 eV for N-5 (pyrrolic and pyridonic); 401.4 ( 0.3 eV for N-Q (nitrogen substituents in aromatic graphene structures-quaternary nitrogen); 402.8 eV for pyridine-N-oxide; and 405.0 ( 0.5 eV for N-Ox (chemisorbed nitrogen oxides20,22,37-40). (37) Kapteijn, F.; Moulijn, J. A.; Matzner, S.; Boehm, H. P. Carbon 1999, 37, 1143-1150. (38) Stan´czyk, K.; Dziembaj, R.; Piwowarska, Z.; Witkowski, S. Carbon 1995, 33, 1383-1392. (39) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641-1653. (40) Jurewicz, K.; Babeł, K.; Zio´łkowski, A.; Wachowska, H. Electrochim. Acta 2003, 48, 1491-1498.

The data presented in Table 4 indicate that the nitrogen content on the surface is greater than that in the bulk for all samples, except for DC5UA, where there is no difference between the surface and bulk nitrogen content. Analysis of the N 1s peak has enabled us to make conclusions in regard to changes that occur in nitrogen species present on carbon surfaces during the processes that have been conducted. Relative quantities of surface nitrogen-containing groups (as a percentage of the N 1s maximum in groups of a particular type) were presented in Table 4. These data show that, in both DC5U and DC7U, nitrogen is present mainly in the form of N-6 and N-5, as well as imines, amines, and amides. Quantities of nitrogen present in the form of N-6 are higher than those present in the form of N-5 and are much higher than that present in the form of imines, amines, and amides. In the case of carbonizates, in addition to the aforementioned dominant forms of nitrogen, it occurs also as N-Q, pyridine-N-oxide, and N-Ox. Contents of the latter forms were significantly lower than those of forms previously mentioned; however, a general tendency was observed that their quantity decreases in the following order: N-Q > pyridine-N-oxide > N-Ox. Activation of nitrogen-enriched carbonizates causes a drastic reduction in the contents of imines, amines, and amides, which, in the case of the activated carbon DC5UA sample, disappear completely. This fact is in agreement with the literature,22 according to which the above species are unstable and decompose above 400 °C. The content of nitrogen present in the form of N-6 also decreases, whereas the contents of other species increase. Among the latter, a considerable increase in N-5 content is worth mentioning. It results from literature reports38,39 that an increase in temperature leads to a reduction in pyrrole structures and an increase in pyridine structures, which is contrary to the data shown in Table 4. However, in the case of carbons that we studied, the increase in N-5 content is probably a result of an increase in the content of pyridonic species that were formed by pyridine transformation, and the latter species, rather than pyrrole species, are the main contributors to the peak ascribed to N-5. Further analysis of the contribution of particular forms of nitrogen to the total nitrogen content on the surface brings into conclusion that pyridine rings, which are present on the edges of graphene layers in urea-treated carbonizates, break during activation and nitrogen is located in the center of graphene sheets, which is confirmed by the increase in N-Q nitrogen content. The increase in both pyridine-N-oxide and N-Ox species content corroborates our conclusion concerning the transformation of pyridinic nitrogen to nitrogen species linked to oxygen. In addition, the activation of nitrogen-enriched carbonizate obtained at 500 °C (DC5UA) causes a higher increase in N-Q nitrogen than that of nitrogen present in the form of N-5 and N-Ox. In the case of carbonizate obtained at 700 °C (DC7UA), this sequence is different, which is presumably a result of better ordering of the structure of the sample prepared at 700 °C and, as a consequence, higher stability of this structure during

Nitrogen-Enriched ActiVated Carbons from Brown Coal

Energy & Fuels, Vol. 20, No. 3, 2006 1279

Figure 1. N 1s peak for the carbons studied.

activation. Such a supposition is confirmed by the results that are collected in Table 2. Poorer ordering of the structure leads to the incorporation of a greater amount of nitrogen (N-Q form), whereas better ordering results in the transformation to nitrogen linked to oxygen on the surface of the structure. In the case of the reaction of urea with activated carbons (DC5AU and DC7AU), differences between the surface nitrogen content and the bulk nitrogen content are smaller than those observed when urea reacted with carbonizates (DC5U and DC7U). The distribution of particular forms of nitrogen in nitrogen-enriched activated carbons obtained in the aforementioned way is the same as in the case of initial nitrogen-enriched carbonizates; i.e., nitrogen present in the form of N-6 and N-5, as well as imines, amides, and amines, predominates. 3.6. Textural Studies of Carbons. Surface areas, pore volumes, and pore diameters calculated from low-temperature nitrogen adsorption data are listed in Tables 5 and 6. The results presented in Table 5 indicate an increase in surface area with increases in the carbonization temperature. The carbonizates obtained are microporous materials and the increase in temperature results in an increase in the micropore area contribution to the total surface area from 91.7% for DC5, through 95.6% for DC6, to 97.0% for DC7. The contribution of micropore volume to the total pore volume increases from 80% for DC5, to 88.2% for DC6, to 94.7% for DC7. The introduction of nitrogen to microporous carbonizates obtained using urea under oxidizing conditions increases the surface areas of DC5U and DC6U, i.e., those for which carbonization resulted in the mildest changes in their texture. The highest increase in the surface area (by a factor of 2) was

Table 5. Surface Area, Pore Volume, and Pore Diameter of Starting Carbonizates, Urea-Treated Carbonizates and Active Carbons Obtained from the Latter Ones Surface Area [m2/g] sample DC5 DC5U DC5UA DC6 DC6U DC6UA DC7 DC7U DC7UA

Pore Volume [cm3/g]

total surface micropore total pore micropore average pore area (BET) area volume volume diameter (nm) 192 383 2593 341 392 1876 395 363 3201

176 362 1582 326 374 1274 383 350 2913

0.10 0.19 1.80 0.17 0.20 1.26 0.19 0.18 1.73

0.08 0.17 0.92 0.15 0.18 0.75 0.18 0.16 1.44

2.1 2.1 2.8 2.0 2.1 2.7 2.0 2.0 2.2

observed for DC5U. In the case of the latter sample, contributions of micropore area (94.0%) and micropore volume (89.5%) to the total surface area and total pore volume, respectively, were also increased. The DC6U sample was characterized by a considerably smaller increase in contributions of micropore areas and micropore volumes. Contributions of micropore area to the total surface area of the latter sample and the starting carbonizate DC6 sample are almost identical. In the case of DC7U, reductions in the surface area and contributions of the micropore area and micropore volume are observed. This is caused, most likely, by temperature conditions of the reaction with urea (350 °C), the influence of which on the texture of carbonizate obtained at 700 °C is much weaker than that on carbonizates obtained at 500 and 600 °C. The activation of carbonizates enriched with nitrogen enables the preparation of activated carbons with a well-developed

1280 Energy & Fuels, Vol. 20, No. 3, 2006

Pietrzak et al.

Table 6. Textural Data of Starting Carbonizates, Active Carbons Prepared from These Carbonizates, and Urea-Treated Active Carbons Surface Area [m2/g]

Pore Volume [cm3/g]

sample

total surface area (BET)

micropore area

total pore volume

micropore volume

average pore diameter [nm]

DC5 DC5A DC5AU DC6 DC6A DC6AU DC7 DC7A DC7AU

192 3268 3042 341 3122 2838 395 2530 2209

176 2915 2739 326 2835 2613 383 2446 2135

0.10 1.86 1.72 0.17 1.72 1.54 0.19 1.24 1.08

0.08 1.52 1.42 0.15 1.44 1.32 0.18 1.15 1.00

2.1 2.3 2.3 2.0 2.2 2.2 2.0 2.0 2.0

surface area. The highest surface area was obtained for DC7UA, followed by DC5UA, and the lowest surface area was observed for DC6UA. An analogous sequence was observed in the case of a relative increase in surface area, when the aforementioned samples were compared to the starting materials. The surface area of DC7UA has increased by >880%, that of DC5UA increased by ∼675%, and that of DC6UA increased by ∼480%, in comparison to DC7U, DC5U, and DC6U, respectively. A possible explanation for the highest increase in surface area being observed when DC7U was transformed to DC7UA can be the opening of pores blocked by amorphous carbon that forms during carbonization, as well as the formation of new pores. The reason for a lower increase in surface area that occurred as a result of the activation of the DC5U and DC6U samples could be a greater extent of reactions proceeding during activation, i.e., in addition to the reaction of amorphous and elemental carbon, some of pore walls were also burned. Such an explanation is supported by values of the contribution of the micropore area to the total surface area (91.0% for DC7UA, 67.9% for DC6UA, and 61.0% for DC5UA) and those of the contribution of the micropore volume to the total volume of pores (83.2% for DC7UA, 59.5% for DC6UA, 51.1% for DC5UA). In Table 6, textural characterization of the samples is given, and these samples were subjected to the reaction with urea after their activation. (Values concerning the starting carbonizates were repeated in this table to facilitate notation of the changes that occur as a result of the above treatment.) The data presented in Table 6 show that the activation results in microporous samples, the surface areas of which decrease in the following order: DC5A > DC6A > DC7A. The direction of the changes is opposite, if the contribution of the micropore areas to the total surface areas are compared: DC7A (96.7%) > DC6A (90.8%) > DC5A (89.2%). An analogous situation is observed when the contribution of micropore volumes to the total pore volumes are compared: DC7A (92.7%) > DC6A (83.7%) > DC5A (81.7%). The reaction of activated carbon with urea results in a decrease in the surface area of all of the carbons studied. The reductions in surface area are 6.9% (DC5AU), 9.1% (DC6AU), and 12.7% (DC7AU). Moreover, the results obtained indicate that, despite the decrease in surface area, the contribution of the micropore area to the total surface area either increases (DC5AU and DC6AU) or remains almost unchanged (DC7AU).

A similar observation can be made in regard to the contribution of the micropore volume to the total volume of the pores. 4. Conclusions (1) The lower the temperature, the higher the quantity of nitrogen that is introduced using urea, as shown by the following order: DC5U, 8.4 wt % Ndaf; DC6U, 6.3 wt % Ndaf; and DC7U, 5.4 wt % Ndaf. Further activation of the nitrogen-enriched carbonizates results in a considerable loss of nitrogen already introduced: DC5UA, 2.0 wt % Ndaf; DC6UA, 0.6 wt % Ndaf; and DC7U, 0.7 wt % Ndaf. (2) Studies of the incorporation of nitrogen into activated carbons have shown that the highest quantity of nitrogen can be introduced to carbons obtained from carbonizates that have been prepared at higher temperatures. The nitrogen content changes in the following sequence: DC7AU, 5.6 wt % Ndaf; DC6AU, 5.4 wt % Ndaf; and DC5AU, 4.8 wt % Ndaf. (3) Based on the elemental analysis results, it has been concluded that the quantity of incorporated nitrogen is dependent on the oxygen content in the starting sample, irrespective of the stage at which the introduction of nitrogen occurred. (4) X-ray photoelectron spectroscopy (XPS) studies have proved that nitrogen incorporated both into carbonizates and activated carbons by means of urea is present mainly in the form of N-6 and N-5, as well as imine, amine, and amide groups. (5) The activation of nitrogen-enriched carbonizates results in the transformation of N-6 nitrogen to pyridonic nitrogen and the incorporation of nitrogen into the graphene structure (NQ). (6) The reaction of urea with activated carbons reduces their surface areas. Despite this fact, both ways of preparating the activated carbons (i.e., reactions of carbonizates and activated carbons with urea in an oxidizing atmosphere) result in products with a well-developed surface area. (7) The process, which proceeding according to the scheme

carbonization f activation f reaction with urea is an efficient way to prepare activated carbons that have high surface area and are considerably enriched with nitrogen. EF0504164