Langmuir 1997, 13, 2149-2157
2149
Determination of the Fate of Nitrogen Functionality in Carbonaceous Materials during Pyrolysis and Combustion Using X-ray Absorption Near Edge Structure Spectroscopy Q. Zhu, S. L. Money, A. E. Russell, and K. M. Thomas* Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K. Received October 22, 1996. In Final Form: December 17, 1996X X-ray absorption near edge structure (XANES) was used to investigate the fate of nitrogen functional groups in carbons derived from acridine, carbazole, and polyacrylonitrile during carbonization. Acenaphthylene and poly(vinyldene chloride) carbons with nitrogen incorporation by ammonia treatment at elevated temperatures were also studied. In general, the nitrogen XANES data provided more detail than the corresponding X-ray photoelectron spectroscopy (XPS) results and confirm the overall picture obtained from XPS involving the greater stability of pyridinic and quaternary nitrogen functionalities with increasing severity of pyrolysis. Partial gasification of the carbons in 20% oxygen/argon produced a new peak or greatly increased the intensity of an existing nitrogen XANES peak at ∼401.5 eV which was independent of the nitrogen functionality originally present in the carbon. The oxygen XANES spectra showed that carbonyl and carboxylic acid/acid anhydride functionality were also present on the surface of carbons partially gasified with oxygen. The XANES studies have also been compared with the temperatureprogrammed combustion and temperature programmed desorption studies of the carbons. The results are consistent with the presence of pyridone functionality in the partially gasified carbons. The overlap of bands may obscure the interpretation of data from X-ray absorption spectroscopy, and information is required from a number of techniques in order to provide a more detailed view.
1. Introduction During coal combustion the nitrogen and sulfur are released as NOx and SOx, which forms acid rain and has a major impact on the environment. Therefore, the relationship between the fuel nitrogen chemistry and NOx release during coal combustion is a subject attracting increasing interest. The coal combustion process may be divided1 into the following stages: (1) very rapid devolatilization; (2) rapid combustion of the volatiles; (3) the slower combustion of the char. The char nitrogen is the major contributor to NOx emissions in both fluidized bed combustion and low-NOx burners.2,3 Hence the combustion of char nitrogen is of major significance from an environmental perspective. The possible relations between nitrogen functionality present in the coal and its conversion to nitrogen functionality in the char need to be considered as these may influence both the release of nitrogen into the volatiles and also the combustion of char nitrogen. Until very recently, the most useful nondestructive technique applied to the analysis of nitrogen functionality in coal/carbons has been X-ray photoelectron spectroscopy (XPS). This technique provides strong evidence that the major nitrogen constituents in coals are present as pyrrolic, pyridinic, and quaternary4-14 func* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Wen, C. Y.; Dutta, S. Coal Conversion Technology; Wen, C. Y., Lee, E. S., Eds.; Addison-Wesley: Reading, MA, 1979; pp 57-170. (2) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 61. (3) Kramlich, J.; Linak, W. P. Prog. Energy Combust. Sci. 1994, 20, 149. (4) Bartle, K. D.; Perry, D. L.; Wallace, S. Fuel Process. Technol. 1987, 15, 351. (5) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 1450. (6) Attar, A.; Hendrickson, G. G. Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 155-162. (7) Davidson, R. M. Nitrogen in Coal. IEA PER/08; IEA Coal Research: London, U.K., 1994. (8) Wo´jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1995, 74, 507. (9) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (10) Pels, J. R.; Wo´jtowicz, M. A.; Moulijn, J. A. Fuel 1993, 72, 373.
S0743-7463(96)01027-X CCC: $14.00
tionalities. The exact nature of quaternary nitrogen in coals has not been established unequivocally. Buckley has reviewed12 the possible structural assignments of quaternary nitrogen in coals and concluded that it represents either protonated pyridinic-N, or an N-oxide of pyridinic-N. The latter assignment is unlikely because the XPS peaks due to N-oxides of pyridinic-N in organic reference compounds occur ∼1.5 eV higher in energy, i.e., at 403.2 eV. There does not appear to be a strong systematic variation in nitrogen functionality composition with coal rank. The nitrogen functionalities in coals are usually in the following ranges: pyrrolic 50-80%, pyridinic 20-40%, and quaternary 0-20%. The signal-tonoise ratios for the XPS data on coal available in the literature vary considerably, and it is possible that there are some weak trends in nitrogen functionality with rank, for example, pyridinic functionality increasing with increasing rank. However, there is considerable scatter in the data. The lack of a large variation in functionality with rank is consistent with the results obtained from pyrolysis and oxidation experiments.15-18 However, XPS studies of carbons suffer from the lack of spectral resolution; the spectra have broad overlapping bands which sometimes lead to difficulties in identification of the different forms of nitrogen. In addition, XPS is very sensitive to surface effects. Mitra-Kirtley et al.19-22 first reported studies of nitrogen chemistry of fuels using X-ray absorption near-edge (11) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993, 7, 1013. (12) Buckley, A. N. Fuel Process. Technol. 1994, 38, 165. (13) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896. (14) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100. (15) Hauck, R. D. Prepr.sAm. Chem. Soc. Div. Fuel Chem. 1975, 20, 85. (16) Hayatsu, R.; Scott, R. G.; Moore, L. P.; Studier, M. H. Nature 1975, 257, 378. (17) King, S. B.; Brandenburg, C. F.; Lanum, W. J. Prepr.sAm. Chem. Soc. Div. Fuel Chem. 1975, 20, 131. (18) Davies, C.; Lawson, G. J. Fuel 1966, 46, 127. (19) Mitra-Kirtley, S.; Mullins, O. C.; Elp, J. V.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252.
© 1997 American Chemical Society
2150 Langmuir, Vol. 13, No. 7, 1997
structure (XANES) spectroscopy. The results showed that the nitrogen XANES spectra of coals20,21 exhibit wellresolved resonances which are associated with different nitrogen chemical structures. XANES methodology appears to be a powerful and promising technique for identifying the nitrogen functionality in coals. In addition, XANES methods provide information on the bulk properties of samples. In this study carbons have been used as models for coal chars to avoid possible effects due to the presence of minerals and to provide a suite of carbons derived from a precursor with a specific nitrogen functionality. The initial objective of this study was to explore the application of XANES in identifying nitrogen functionality in carbons by comparing the results with the XPS spectra obtained from the same carbons. The second objective was to perform a systematic study of the changes in the chemical structure of nitrogen functionality during heat treatment and combustion. In this study, carbons were prepared by carbonization of polynuclear aromatics and polymer precursors with well-defined nitrogen functionality. Ammonia treatment was also used to incorporate nitrogen into both anisotropic and isotropic carbons. These carbons were also used for the study of the nitrogen chemistry during carbon combustion. 2. Experimental Section 2.1. Materials. Acridine, carbazole, 1-aminoanthracene, polyacrylonitrile (PAN), 2-hydroxypyridine, 9-cyanoanthracene, 1,3,5-tribenzylhexahydro-1,3,5-triazine, phthalic anhydride, and anthrone were supplied by Aldrich Chemical Co. The polyl(vinylidene chloride) (PVDC) was supplied by ICI. The reference compounds were used without further purification. All gases were supplied by BOC Ltd. 2.1.1. Carbon Synthesis. Carbazole and acridine were carbonized, separately, under an atmosphere of argon in an autoclave at an initial pressure of 4.1 MPa. The materials were heated to 873 K at a heating rate of 1 K min-1, and the soak time at the maximum heat treatment temperature (HTT) was 1 h. The carbons obtained were subjected to further heat treatments to 1073 and 1273 K under a flow of argon at a heating rate of 4 K min-1. CB and AD denote the carbazole and acridine carbons, respectively, followed by the HTT. PAN was carbonized to HTT’s of 573, 673, 773, 873, 1073, and 1273 K at a heating rate of 1 K min-1 with a soak time at the HTT of 1 h. The carbonization was carried out in a horizontal tube furnace under a flow of argon (∼100 cm3 min-1) at atmospheric pressure. These carbons are designated by PAN followed by HTT (in K). For example, PAN1073 represents the PAN carbon obtained by carbonization to 1073 K. The same procedure was used in the carbonization of PVDC and acenaphthylene at 873 K. The PVDC carbon (HTT 873 K) was heat-treated in a flow of ammonia gas at 4 K min-1 to 1073 K to incorporate nitrogen into the carbon structure. Various soak times were used to introduce different amounts of nitrogen into the carbon. P-N denotes the ammonia-treated PVDC carbon. The number after N denotes the soak time at the maximum HTT (in minutes) used in ammonia treatment. Two procedures were used in ammonia treatment of acenaphthylene carbon. In one case, the acenaphthylene carbon was heated at 4 K min-1 to the HTT in a flow of ammonia gas throughout the treatment. This carbon is referred to as A-N. In the other case, the carbon was heated to 1073 K under a flow of argon at 4 K min-1 and was held at the temperature for 1 h before the atmosphere was switched to ammonia. The resulting carbon is designated as A-Ar-N. A treatment time of 1 h at the HTT was used for ammonia treatment of acenaphthylene carbon. 2.2. Partial Gasification of the Chars. Some of the carbons were gasified in 20% O2/Ar to a specific carbon conversion level at 823 K (PAN carbons), 873 K (carbazole and acridine carbons), (20) Mitra-Kirtley, S.; Mullins, O. C.; Branthaver, J. F.; Cramer, S. P. Energy Fuels 1993, 7, 1128. (21) Mullins, O. C.; Mitra-Kirtley, S.; Elp, J. V.; Cramer, S. P. Appl. Spectrosc. 1993, 47, 1268. (22) Mitra-Kirtley, S.; Mullins, O. C.; Elp, J. V.; Cramer, S. P. Fuel 1993, 72, 133.
Zhu et al. Table 1. Analytical Data for the Carbons (wt % as received) A-Ar-N A-N AD873 AD1073 AD1273 CB873 CB1073 P-N10 P-N60 PAN573 PAN673 PAN773 PAN873 PAN1073 PAN1273
N
C
H
O
total
2.87 7.46 7.53 7.24 6.02 7.43 5.41 3.85 4.83 23.55 20.55 19.46 18.84 16.89 8.22
94.47 87.45 88.42 94.84 94.49 88.47 92.54 87.68 90.75 68.42 68.58 68.70 70.25 74.36 86.31
0.67 0.69 2.53 0.60 0.11 3.40 0.68 0.29 0.37 4.76 3.80 2.83 2.02 1.07 0.48
0.57 2.08 1.99 0.52 0.00 2.28 1.98 1.40 1.73 8.56 8.20 8.94 8.56 7.36 2.41
98.58 97.68 100.47 103.20 100.62 101.58 100.60 93.22 97.68 102.29 101.13 99.96 99.67 99.68 97.42
and 923 K (acenaphthylene carbons) using the following procedure. The sample was placed in the thermogravimetric analyzer and heated in an atmosphere of Ar to the combustion temperature at a heating rate of 50 K min-1. The atmosphere was then switched to 20% O2/Ar, and the weight change of the sample was recorded. When the desired carbon conversion level was reached, the atmosphere was switched back to argon and the system was allowed to reach equilibrium before being cooled down to room temperature. The gas flow rate was 50 cm3 min-1. The burn-off char is designated as the name of the carbon followed by -B and a number representing the extent of weight loss (carbon conversion in %). For example, A-N-B48 represents the partially gasified char of A-N (derived from acenaphthylene by ammonia treatment during the carbonization procedure) corresponding to a weight loss (burn-off) of 48 wt %. 2.3. Elemental Analysis. The C, H, and N contents of the carbons were determined using Carlo Erba 1106 and O content obtained using a Carlo Erba 1108 elemental analyzer. The results are shown in Table 1. 2.4. Temperature-Programmed Combustion Studies. Temperature-programmed combustion (TPC) of the carbons was carried out using a thermogravimetric analyzer/mass spectrometer (TG-MS). Approximately 5 mg of sample (particle size 3875 µm) was heated from room temperature at a heating rate of 15 K min-1 in 20% O2/Ar mixture. The gas flow rate was 50 cm3 min-1. The gas evolution profiles were monitored for the following mass/charge (m/z) values, 14, 18, 27, 28, 30, and 44, which correspond to N22+, H2O, HCN, CO, NO, and CO2. A more detailed description of the TG-MS and the procedure used is given elsewhere.23 2.5. Temperature-Programmed Desorption Studies. The temperature-programmed desorption (TPD) studies were carried out using the same TG-MS used for the TPC studies. The carbon was partially gasified as described above. This carbon was heated in argon at 15 K min-1 from ambient to 1473 K. The following m/z values were monitored: 14, 18, 26, 27, 28, 30, 44, and 52 which correspond to N22+, H2O, CN, HCN, CO, NO, CO2, and C2N2 ions, respectively. 2.6. XANES. The nitrogen XANES K edge measurements were performed using beamline 1.1 of the Synchrotron Radiation Source (SRS) at Daresbury Laboratory. The SRS storage ring was operated at an energy of 2 GeV with ring currents in the range 100-300 mA. The monochromator was a high-energy spherical grating monochromator with a Au grating and a slit width of 0.1 mm.24 Detection was by total electron yield. The samples were ground to a very fine particle size, dispersed in carbon tetrachloride, and applied dropwise to high-purity aluminum plates, and the solvent was allowed to evaporate. All measurements were made at room temperature under vacuum, 10-6-10-8 bar, in order to minimize the interference of atmospheric nitrogen. The vacuum of the beamline was separated from that of the sample compartment by an aluminum window, thickness 150 µm. The oxygen XANES K edge measurements were also performed using beamline 1.1. However in these measurements platinum (23) Zhu, Q.; Grant, K.; Thomas, K. M. Carbon 1995, 33, 35. (24) Surman, M.; Cragg-Hine, I.; Singh, J.; Bowler, B. J.; Padmore, H. A.; Norman, D.; Johnson, A. L.; Walter, W. K.; King, D. A.; Davis, R.; Purcell, K. G.; Thornton, G. Rev. Sci. Instrum. 1992, 63, 4349.
Nitrogen Functionality in Carbonaceous Materials
Figure 1. Nitrogen XANES spectra of reference compounds. plates were used to avoid interference from oxide species on the surface of the plate. The experimental procedure was identical in all other respects to that describe above. The oxygen XANES K edge spectra are reported on energy axes (eV) relative to each other because of the lack of a suitable absolute standard. A suite of organic compounds containing well-defined nitrogen functionality were studied as standards for identifying nitrogen functional groups in the carbons. The suite consists of acridine, carbazole, PAN, 1-aminoanthrancene, 9-cyanoanthracene, 2-hydroxypyridine, and 1,3,5-tribenzylhexahydro-1,3,5-triazine. Similarly, a series of organic compounds were used as references for oxygen functionality. The oxygen XANES K edge spectra of anthrone and phthalic anhydride are reported in this paper, while the full set of reference compounds are reported elsewhere.25
3. Results 3.1. Nitrogen K Edge XANES Spectra. 3.1.1. Reference Standards. The XANES spectra of the organic compounds used as standards for the various types of nitrogen functionality are shown in Figure 1. It is evident that there is a narrow energy range for the leading edge peaks for each type of nitrogen functionality. Where the same compounds were used, the data agree well with results published previously.19-22 The XPS data for nitrogen present in coals and carbons often require bands at 401.3 and 403.3 eV to obtain an acceptable curve fitting of the XPS spectrum. These bands are designated quaternary (N-Q) and N-X functionalities, and these (25) Turner, J. A.; Thomas, K. M.; Russell, A. E. Carbon, in press.
Langmuir, Vol. 13, No. 7, 1997 2151
represent real functionalities. There were no suitable reference compounds available for the N-Q and N-X functionalities. In coal the peak at ∼401.5 eV has been assigned to either protonated pyridinic nitrogen or an N-oxide of pyridinic nitrogen. The former assignment is the more likely of the two alternatives. The XPS nitrogen peak at 401.5 eV was proposed26 to be due to nitrogen included in the graphene layer.9,26 Thus the XPS peaks at 401.5 eV in coal and carbon are assigned to different structures. The XANES spectra of the reference compounds used in this study and those reported in the literature show that compounds with the same functionality have lowest energy π* resonances at similar energies. The aromatic nitrogen-containing reference molecules have both π*- and σ-shaped resonances. Saturated amines show only σ-shaped resonances. The pyridines which have higher pKb values have π* resonances at the lowest energies. The less basic pyrroles have resonances at higher energies. In the case of pyridone, which may exist in keto and enol form, infrared evidence indicates that it exists mainly in the keto form, and the relative position of the XANES resonances compared to pyridinic and pyrrolic resonances indicates that the pyridones are in the keto form.21 Therefore the positions of the lowest energy π* resonances can usually be used as a characteristic energy for the identification and quantification of nitrogen functionality present in carbons. In the case of aromatic amines the XANES spectrum of 1-aminoanthracene is very similar to that of 2-aminofluorene21 with several weak peaks at lower energy than the main peak at ∼405 eV. The XANES spectra reported in this study were obtained using electron yield detection whereas those reported in the literature were mainly fluorescence spectra. The electron yield data differ from the fluorescence data in the following respects: (1) the electron yield data are sensitive to surface effects; (2) fluorescence emission is anisotropic whereas electron yield measurements are isotropic; (3) the ratio between Auger electron emission and fluorescence emission is affected by excitation of (quasi) bound resonances.19 However, in general, there is good agreement between the spectral data obtained by the two techniques and the data are compared with the literature values19-22 in Table 2. 3.1.2. Acridine Carbons. Figure 2 shows the nitrogen XANES spectra of acridine carbons. AD873 exhibits a well-resolved, strong peak at 399.5 eV followed by a broad peak at 403.7 eV with a very weak shoulder at 402.5 eV. The first peak coincides with the resonance of pyridinic nitrogen. The XPS studies indicated9 that this carbon had 20% pyridinic, 58% quaternary, and 21% N-X nitrogen. The XANES spectra change with increasing HTT; the broad peak was resolved into peaks at 401.2 and 402.5 eV while the peak at 403.7 eV does not change significantly. The XPS results for AD1273 indicated9 that this carbon contained 21% pyridinic, 61% quaternary, and 19% N-X nitrogen functionality, which is a similar composition to AD873. The XPS studies indicated that there was little or no effect of heat treatment on the distribution of nitrogen functionalities in acridine carbons over the temperature range 873-1273 K. XANES spectra also suggested that relatively small changes in nitrogen functionality occur during heat treatment over this temperature range. It should be noted that there is no suitable standard available for XANES spectra of nitrogen incorporated in the graphene rings in carbons. It is most likely that the resonances are broad and diffuse making it difficult to quantify this functionality. The reason for this is the slightly different structural environments for the nitrogen (26) Grant, K.; Zhu, Q.; Thomas, K. M. Carbon 1994, 32, 883.
2152 Langmuir, Vol. 13, No. 7, 1997
Zhu et al.
Table 2. A Comparison of XANES and XPS Energies (eV) for Nitrogen Functionality in Carbonaceous Materials XANESa pyridine amine cyano (aromatic) cyano (aliphatic) nitroso pyridone pyrrole aromatic amines saturated amine quaternary (N-Q) pyridinium ammonium pyridine N-oxide N-X nitro
399.7 399.8 and 401 401.4 401.9 403.5 and 402.6 405 406.2 and 407.5
XPS9 398.9 ( 0.3 399.4 399.5 400.1 400.6 400.6 ( 0.3 401.4 401.2 401.5 403.2 403.5 406.1
a XANES reference compounds: (this work) acridine, 9-cyanoanthracene, 2 hydroxypyridine, carbazole, PAN, 1-aminoanthrancene, 1,3,5-tribenzylhexahydro-1,3,5 triazine. (References 19-22) Pyridinic: acridine, poly(4-vinylpyridine-co-styrene), phenanthridine, 4,7-phenanthroline, 2,6-di-p-tolylpyridine. Pyridone: 6-(2,2-diphenyl-2-hydroxylethyl)-2(1H)-pyridone, 2-hydroxypyridone, 1 hydroxisoquinoline, 1-methyl-4-pentadecyl-2(1H)-quinoline. Pyrrole: carbazole, tetrahydrocarbazole, poly(9-vinylcarbazole), 2-phenylindole. Aromatic amines: 5-aminofluorene, aminochrysene. Saturated amines: 1,3,5-tribenzylhexalhydro-1,3,5-traizine, 1,12diaminododecane.
Figure 3. Nitrogen XANES spectra of carbazole carbons.
Figure 2. Nitrogen XANES spectra of acridine carbons.
incorporated in the graphene layers. It is also likely that the peak is partially obscured by the strong σ resonance at ∼408 eV. The AD1273 carbon shows an additional
peak at 401.2 eV, which is possibly a pyridone functionality arising from surface oxidation. 3.1.3. Carbazole Carbons. Figure 3 shows the nitrogen XANES K edge spectra of carbons derived from carbazole with heat treatment temperatures of 873 and 1073 K. The carbazole carbon CB873 has an intense narrow peak at 403.8 eV which is assigned to pyrrolic functionality and two much weaker peaks at 399.6 and 401.5 eV indicating the existence of small amounts of pyridinic and pyridone nitrogen in the carbon structure. The latter is possibly due to a small amount of surface oxidation. XPS studies indicated that this carbon contained 100% pyrrolic functionality.9 When the carbon underwent heat treatment to 1073 K, the intensity of the peak at 403.8 eV decreased and the peak at 399.5 eV increased significantly in intensity. The peak at 401.5 eV was still visible as a weak shoulder. These observations are consistent with the conversion of pyrrolic functionality to pyridinic functionality during heat treatment and in agreement with the previous XPS studies which showed that CB1073 contained 29% pyridinic, 10% pyrrolic, 43% quaternary, and 18% N-X functionality.9 The peak at 401.5 eV is consistent with the presence of a small amount pyridone functionality possibly due to surface oxidation. However the XANES spectrum of CB1073 does not exhibit features which are readily associated with the quaternary and N-X functionalities indicated from the XPS data. A possible explanation is that the N-Q functionality is associated with the broad XANES peak at 403.5 eV, which is also observed in the XANES spectra of other carbons
Nitrogen Functionality in Carbonaceous Materials
Langmuir, Vol. 13, No. 7, 1997 2153
Figure 4. Nitrogen XANES spectra of PAN carbons.
where significant amounts of quaternary functionalities have been observed, for example, AD873 and AD1073. However comparison of the relative energies of the XPS and XANES peaks show that the latter are usually ∼1 eV higher, which suggests that the quaternary nitrogen should have bands at higher energy. 3.1.4. PAN Carbons. Figure 4 shows the nitrogen XANES spectra of PAN carbons. The changes in the chemical structure of nitrogen in PAN carbons with HTT are apparent. The PAN573 carbon shows two peaks at 400.4 and 401.4 eV. Comparison with the XANES spectra of PAN and 9-cyanoanthracene suggests that these peaks are due to CN functionality. The XPS results for this carbon indicated that it contained 100% pyridinic functionality.9 However, it is difficult to distinguish between pyridinic (398.9 eV) and CN (399.5 eV) functionality using XPS because of the broad peaks, and the alternative interpretation of the XPS data is that CN functionality is also present. The XANES data support the presence of CN species. With increasing HTT, the XANES spectra are consistent with the conversion of CN functionality to pyridinic and pyrrolic functionality. A peak at 403.7 eV is barely visible in the XANES spectrum of PAN573 but well resolved in the spectrum of PAN673, and this is consistent with the formation of pyrrolic functionality. In addition the lowest energy peak shifts to 399.8 eV, which is consistent with the conversion of CN functionality to pyridinic functionality. The XPS studies of PAN773 indicate that it contained 60% pyridinic, 19% pyrrolic, 8% quaternary, and 4% N-X functionalities.9 The XANES spectrum of PAN773 shows a strong peak at 399.8 eV due to pyridinic functionality while the peak at 403.7 eV due to pyrrolic functionality is clearly visible but considerably weaker. Further heat treatment to higher temperatures results in decrease in the relative intensity of
Figure 5. Nitrogen XANES spectra of ammonia-treated acenaphthylene carbons.
the 399.8 eV peak due to pyridinic functionality. The XPS data for PAN1073 indicate that it contains 40% pyridinic, 29% pyrrolic, 23% N-Q, and 9% N-X functionalities.9 Hence there is general agreement between the XANES and XPS results. However the increase in the N-Q and N-X functionalities indicated from the XPS results is not clearly associated with specific features in the XANES spectra. 3.1.5. Ammonia-Treated Carbons. The nitrogen XANES K edge spectra of ammonia-treated acenaphthylene and PVDC carbons are shown in Figures 5 and 6, respectively. The two ammonia treatment procedures used to produce two acenaphthylene carbons exhibited similar nitrogen XANES spectra with different relative peak intensities. The A-Ar-N showed a well-resolved intense peak at 399.7 eV and three weak peaks at 401.5 (shoulder), 402.5, and 404 eV. The XANES spectrum of A-N also gave a peak at 399.5 eV and three very weak peaks at 401.6, 402.6, and 403.8 eV, respectively. These peaks are possibly due to surface oxidation (401.6 eV) and pyrrolic nitrogen (402.5 and 403.8 eV). The XANES spectra of A-N and A-Ar-N are quite similar. The XANES spectrum of A-N is also very similar to that of AD873, and this suggests that pyridinic and quaternary nitrogen functionalities are present. The PVDC carbons which are highly microporous gave three XANES peaks which are identifiable with pyridinic (∼399.6 eV), pyridone or CN (401.9 eV), and pyrrolic (403.7 eV) nitrogen, respectively (see Figure 6). The pyridone functionality may originate from some surface oxidation
2154 Langmuir, Vol. 13, No. 7, 1997
Zhu et al.
Figure 6. Nitrogen XANES spectra of ammonia-treated PVDC carbons.
of the carbon samples, and this is expected to be marked in highly reactive microporous carbons, for example, PVDC carbons. 3.1.6. Partially Gasified Carbons. The combustion of acridine char to 56% carbon conversion gave a nitrogen XANES K edge spectrum (see Figure 2) with a new peak at 401.5 eV of higher intensity than the pyridinic peak at 399.7 eV. This new peak has a very similar energy to the pyridone reference compound 2-hydroxpyridine. After the CB873 carbon was gasified at 873 K to a carbon burn-off level of 66 wt %, there was a large decrease in the peak intensity at 403.8 eV (pyrrolic nitrogen) and increase at 399.9 eV (pyridinic nitrogen). However, the most marked change took place in the intensity of the peak at 401.2 eV, which was the most intense XANES peak for the partially gasified carbazole char (see Figure 3). These results indicated that the pyrrolic functionality has been converted to pyridinic and pyridone functionality during gasification. All the PAN partially gasified chars showed a small increase in the intensity of the 401.5 eV peak compared with the original PAN char (see Figure 7). The partially gasified acenaphthylene chars A-Ar-N and A-N also showed an intense peak at ∼401.4 eV, similar to the partially gasified, acridine, carbazole and PAN carbons (see Figure 5). The carbons studied had different functionalities prior to gasification, CB873 (100% pyrrolic), AD873 (pyridinic, quaternary and N-X), and PAN 1073 (a mixture of pyrrolic, pyridinic and quaternary functionalities). Nitrogen was incorporated by ammonia treatment of an acenaphthylene carbon whereas the other carbons were prepared by
Figure 7. Nitrogen XANES spectra of partially gasified PAN carbons.
carbonization of a nitrogen-containing organic precursor. Gasification of all the carbons studied in 20% oxygen/ argon, however, leads to the development of a peak at 401.5 eV, which is consistent with the presence of pyridone or CN functionality. The energy of the peak is intermediate between the standards for CN (399.8 and 401 eV) and pyridone (401.9 eV) functionalities. 3.2. Oxygen XANES K Edge Spectra. The oxygen XANES K edge spectra of A-Ar-N-42.3, AD873-B56.2, CB873-B65.7, and PAN1273-B56 were recorded to establish the presence of specific forms of oxygen functional groups present in partially gasified carbons. The spectra are shown in Figure 8 together with the XANES spectrum of two organic reference compounds. The oxygen XANES spectra of a suite of organic reference compounds have been studied,25 and the spectra of anthrone and phthalic anhydride shown are representative of carbonyl and carboxylic acid/acid anhydride functional groups. These results show that carbonyl groups are present in the partially gasified carbon. However, there is a similar amount of carboxylic acid/acid anhydride groups present. The first oxygen XANES peak in the partially gasified char samples is broad and has a shoulder on the highenergy side. The latter has a similar energy to the lowest energy absorption band of acid anhydride/carboxylic acid groups. The higher energy peak is also close in energy to the corresponding peak in the oxygen XANES spectra of
Nitrogen Functionality in Carbonaceous Materials
Langmuir, Vol. 13, No. 7, 1997 2155
Figure 9. Temperature-programmed combustion profiles of the ammonia-treated acenaphthylene carbons.
that the char nitrogen in the partially gasified chars was mainly released as NO. The NO profiles started at ∼950 K and reached a maximum at ∼1200 K. HCN and C2N2 were not detected during TPD suggesting that -CN species are not present in the partially gasified samples. These results are consistent with the desorption of char nitrogen in carbons partially gasified in oxygen mainly as NO resulting from the decomposition and reaction of carbon nitrogen and carbon oxygen surface species.23,26,27 4. Discussion
Figure 8. Oxygen XANES spectra for partially gasified carbons.
the reference compound (see Figure 8). The presence of carbonyl groups is consistent with the observed nitrogen XANES peak in the partially gasified carbons being similar to the keto form of pyridone. 3.3. Temperature-Programmed Combustion (TPC) Studies. The TPC gas evolution profiles of ammoniatreated acenaphthylene carbon, Ar-N, are given in Figure 9, and these profiles are clearly bimodal indicating the presence of species with different reactivities. The corresponding untreated carbons did not exhibit the strongly bimodal characteristics.26 It appears that treating the carbon in ammonia gas throughout the heating cycle produced a more reactive carbon with sites of different reactivity. It was proposed that these TPC peaks represented the gasification of different forms of nitrogen, i.e., nitrogen substituted on the graphene layers and nitrogen incorporated in the graphene layers.26 However the TPC profiles are subject to mass transfer effects, which complicate the interpretation of the results. As mentioned previously, the XANES spectra of both samples A-Ar-N and A-N are quite similar but with different relative peak intensities and also similar to the spectra of AD873 and AD1073. In addition, differences in nitrogen functionality as a function of carbon burn-off need to be considered. The results do not show marked differences, and presumably this is due to overlap of bands and the 401.5 eV pyridone peak obscuring any differences. 3.4. Temperature-Programmed Desorption (TPD) Studies. The temperature-programmed desorption profiles for the partially gasified, A-N-B51.8 and CB873-B74.5 chars are shown in Figure 10. These studies revealed
The types of nitrogen functionality present in carbon are shown in Figure 11. The forms of nitrogen present in carbons with the exception of the nitrogen incorporated in the graphene layer are similar to those found in organic chemistry, and therefore it is reasonable to use aromatic nitrogen-containing compounds as references for the absorption of the functional groups. In the case of quaternary nitrogen where the nitrogen is incorporated within the graphene layer,9,26 no suitable reference compound of similar structure could be obtained. XANES has some advantages over XPS in terms of additional detail and better resolution which allow more precise interpretation. The disadvantage of XANES is that problems exist in estimating the baseline which make it difficult to quantify the amounts of various functionality present in the sample. A comparison between XPS and XANES is useful because of the greater amount of information available in the literature on the interpretation of XPS. The effect of heat treatment on the type of nitrogen functionality present in the carbon has been studied previously by XPS.9 The XPS determination of the relative concentrations of nitrogen functionalities in coals and carbons is complicated by the requirement to include components at ∼401.4 and ∼403.5 eV to achieve an acceptable curve fitting of the spectra. The binding energy of the former corresponds to that of quaternary nitrogen which represents nitrogen in six-membered rings in the interior of graphene layers in carbons.9,26 Supporting evidence for this proposed structure comes from studies of the electronic structure and bonding in 3,5,11,13-tetraazacycl[3.3.3]azine using theoretical calculations and XPS.28 The results from both approaches (27) Jones, J. M.; Harding, A. W.; Brown, S. D.; Thomas, K. M. Carbon 1995, 33, 833. (28) Boutique, J. P.; Verbist, J. J.; Fripiat, J. G.; Delhalle, J.; PfisterGuillouzo, G.; Ashwell, G. J. J. Am. Chem. Soc. 1984, 106, 4374.
2156 Langmuir, Vol. 13, No. 7, 1997
Zhu et al.
Figure 11. Nitrogen forms in carbons and partially gasified carbons.
Figure 10. Temperature-programmed desorption profiles for A-N-B51.8 and CB873-B74.5.
indicated the more positive nature of the central nitrogen atom and the delocalization of its lone pair. In addition, in the case of the carbons used in this study the N:C atomic ratios were typically 1:15 and there is a limit to the number of nitrogens which can be located on the periphery of the graphene layers. The N-X functionality has been tentatively assigned9 to an oxidized form of nitrogen since nitro-N has been observed at high binding energies. These XPS results showed that increasing heat treatment temperature led to increase in N-Q and N-X functionalities. Above 800 °C there was little or no difference in the nitrogen functionality in the carbon irrespective of the starting functionalities. A comparison of the XANES spectra of the acridine, carbazole, and PAN chars suggests that the differences in nitrogen functionality are small and that the N-Q band is broad and possibly at ∼403-404 eV. However, this is a tentative assignment because overlap of bands may occur. Furthermore, the XANES bands are 1 and 3 eV higher than the corresponding XPS bands for pyridinic and pyrrolic nitrogen functionalities. It is likely that similar shifts occur for other functionalities. Hence, the quaternary nitrogen observed at 401.3 in the XPS is expected to occur at ∼0.7 eV higher than the XANES band due to pyrrolic nitrogen
(403.7 eV), i.e. at 404.4 eV, and this overlaps with the very strong broad σ* resonance. Hence it is most likely that the quaternary nitrogen absorption is broad and obscured by other resonances. The XANES and previous XPS results9 indicate that the nitrogen functionality in the precursor has little influence on the nitrogen functionality in carbons with heat treatment temperatures greater than ∼800 °C. Hence it is reasonable to conclude that small differences in the distribution of nitrogen functionalities in the original coals do not influence the formation of nitrogen oxides during char combustion, and this confirms the results of previous XPS studies.9 The origin of the N-Q and N-X bands in the XPS spectra is also an important consideration since under severe pyrolysis conditions these bands are found to represent the major part of the nitrogen functionality present in the carbons. However, the observation of broad bands in the XPS of carbons suggests that the corresponding XANES bands will also be broad. It is unlikely that the classic quaternary nitrogen (ammonium ion) is present in carbons, but it may be either a protonated pyridinic nitrogen or described as a nitrogen with a formal charge of +1 where the nitrogen is incorporated in the graphene layer. The origin of the broad peak at 402-405 eV (N-X) in the XPS of coals and chars is unclear and it may be an oxidized form of nitrogen.9 Comparisons of XANES spectra for carbons with XPS data which indicate high N-Q and N-X contents are consistent with the presence of broad XANES peaks. However, the data do not provide any additional definitive information for identification of these species other than the identification of surface oxidation in the cases of some of the more reactive chars. The improved resolution of XANES compared with XPS has allowed identification of a new type of nitrogen associated with the combustion process. The assignment of the new XANES feature at 401.5 eV introduced by the combustion process which behaves independently of other XANES peaks needs to be considered in relation to the following observations: (1) The XANES peak due to gasification in oxygen occurs at a similar energy, ∼401.5 eV, irrespective of the initial functionality in the carbon starting material, for example, the initial nitrogen functionality is pyrrolic in CB873, and pyridinic/pyrrolic/quaternary mixtures are present in the PAN and acridine carbons.
Nitrogen Functionality in Carbonaceous Materials
(2) Gasification of CB873 in 20% oxygen/argon converts pyrrolic to pyridinic functionality in addition to the development of the new peak observed at ∼401.5 eV. The latter peak is observed in all the carbon samples partially gasified in oxgyen. (3) Ammonia decomposes at high temperatures to form NH2, NH, and H radicals and these radicals react with the carbon surface to form methane, HCN, and cyanogen as well as incorporating nitrogen into the carbon structure.29,30 Therefore the formation of functional groups such as -NH2, -CN, pyridinic, and pyrrolic can be expected. The XANES spectra of these carbons are similar to AD873 and AD1073. Sto¨hr et al. have studied31 the nitrogen functionalities introduced into carbons by treatment with ammonia and HCN. The XPS results are equivocal with two main types of functionality being identified (1) at binding energies of 400-401 eV, corresponding to amine groups and (2) at 398-399 eV, corresponding to pyridinic and/or nitrile functionalities. Heat treatment to 1373 K reduced the intensity of the low binding energy peak (pyridinic and/or nitrile) relative to the high energy peak corresponding to amino and/or amide groups. However the heat treatment may also increase the amount of quaternary nitrogen which occurs at 401.4 eV in XPS.9 In the case of the ammonia-treated carbons, temperature-programmed combustion profiles are bimodal (see Figure 9). Previous work has suggested that the lowtemperature peak was due to primarily substitution on the edges of the graphene layers rather than incorporation into the graphene layers. However, the 399.7 eV peak may have contributions from both pyridinic and nitrile groups and the latter are gasified preferentially. Partial combustion of the ammonia-treated carbons leads to the development of the peak at 401.5 eV. (4) The TPC of the carbons with gas sampling directly above the sample has shown that HCN and C2N2 can be detected providing unequivocal evidence for the presence of CN surface species during char combustion at the temperatures used in these studies.27 It is a debatable point whether or not these species would survive as CN groups bonded to graphene after cooling the sample down to room temperature for the spectroscopic measurements. The TPD results showed that NO was the major nitrogencontaining desorption product, indicating that the nitrogen in the partially gasified chars was probably associated with oxygen. The lack of significant amounts of HCN and C2N2 in the desorption products suggests that it is unlikely that major amounts of CN groups are present in the carbons. (5) Oxygen XANES spectra of partially gasified pure acenaphthylene carbons show that the oxygen functionalities present are carbonyl and carboxylic/anhydride groups. Carbonyl and acid anhydride/carboxylic acid functionalities are observed in the partially gasified PAN, acridene, carbazole, and ammonia-treated acenaphthylene chars. The carbonyl functionality is observed in partially gasified chars irrespective of whether or not nitrogen functionality is present in the sample.25 (6) The carbons used in the partial gasification studies had low carbonization and heat treatment temperatures. As a consequence, these carbons had relatively high hydrogen contents. Typically carbons with HTTs of 873 (29) Tereckzi, B.; Kurth, R.; Boehm, H. P. Carbon’80, Int. Carbon Conf., Prepr. 1980, 218. (30) Boehm, H. P.; Mair, G.; Sto¨hr, Th.; de Rincon, A. R.; Terekzi, B. Fuel 1984, 63, 1061. (31) Sto¨hr, B.; Boehm, H. P.; Schlo¨gl, R. Carbon 1991, 29, 717.
Langmuir, Vol. 13, No. 7, 1997 2157
and 1073 K have hydrogen contents of 2-3 and 0.5-1 wt %, respectively (see Table 1). In addition, temperatureprogrammed combustion studies of ammonia-treated acenaphthylene carbons have shown26 that the H2O profiles reach a maximum toward the end of the combustion. Hence, it is likely that hydrogen is available for the formation of pyridone functionality during combustion. (7) The two low-energy peaks at 399.8 and 401 eV in the nitrogen XANES spectra of 9-cyanoanthracene are similar to the two low-energy peaks in the suite of partially gasified carbons. However, as shown in Table 2, overlap of XPS and XANES bands occurs for amine, cyano, and pyridinic groups. The assignment of the nitrogen and oxygen K edge XANES peaks of the partially gasified carbons suggests that some of the nitrogen is present as pyridone functionality. The 401.5 eV XANES peak is most likely similar to pyridone functionality rather that aromatic -CN groups bearing in mind the similarity in the XANES spectra of carbons derived from PAN which initially contains the -CN group and carbons derived from precursors with pyridinic and pyrrolic functional groups. It is unlikely that -CN groups survive at high temperatures and -CN species found from combustion are fragments of the decomposition of ring systems rather than CN functional groups. It can be envisaged that the decomposition of pyridone structures by cleavage of the C-N bond will lead to the production of -CO and -CN surface species with the latter being subsequently oxidized to -C(NO) surface species. The desorption and reaction of surface species then generate active sites for further gasification. Resonance structures outlined in Figure 11 are probable and may lead to the stabilization of the pyridone species in the graphene layers of chars/carbons. 5. Conclusions Comparison of XANES with XPS shows that the resolution is better, and this allows clearer identification of the functionalities present in the carbon in some instances. However XANES suffers from difficulties in the assessments on the background and, hence, quantification of the species present. Furthermore it is less accessible and more costly. In general, the XANES results for changes in nitrogen functionality as a function of heat treatment temperature confirm the XPS, in particular, the conversion of pyrrolic nitrogen to pyridinic nitrogen with increasing severity of pyrolysis. There was some evidence for the presence of surface oxidation in the more reactive carbon samples. Partial gasification of the char samples which contained various types of functionality gave rise to a new peak or a dramatic increase in the peak intensity at ∼401.5 eV in the nitrogen XANES spectra. The energy of the peak is similar to pyridone functionality. The oxygen XANES spectra show that carbonyl and carboxylic acid/anhydride groups are present in these carbons. NO was observed in the TPD of the partially gasified carbons. The pyridone species present in partially gasified carbons is probably stabilized by electron delocalization in the graphene layer. Acknowledgment. The research was supported by the European Coal and Steel Community. The beam time was allocated by the CCLRC. The authors thank M. Surman, I. Kirkman, and A. Porter of Daresbury Laboratory and A. Harding, S. Roy, and J. Turner of NCRL for assistance with the XANES measurements. LA961027S
