Analysis of the Reaction between O2 and Nitrogen-Containing Char

Jan 10, 2011 - Theoretical research on heterogeneous reduction of N 2 O by char. Zhengyang Gao , Weijie Yang , Xunlei Ding , Yi Ding , Weiping Yan...
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Energy Fuels 2011, 25, 670–675 Published on Web 01/10/2011

: DOI:10.1021/ef1014742

Analysis of the Reaction between O2 and Nitrogen-Containing Char Using the Density Functional Theory Xiuxia Zhang,† Zhijun Zhou,*,† Junhu Zhou,† Shudong Jiang,‡ Jianzhong Liu,† and Kefa Cen† †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, Zhejiang, People’s Republic of China, and ‡North China Power Engineering Company, Limited, of China Power Engineering Consulting Group, Beijing 100120, People’s Republic of China Received November 1, 2010. Revised Manuscript Received December 20, 2010

Quantum chemistry calculations using a reasonably simplified nitrogen-containing char model have been performed to clarify mechanisms for CO and NO desorption during the reaction between molecular oxygen and nitrogen-containing char. The density functional theory at the B3LYP/6-31G(d) level was used to optimize geometries of reactants, products, stable intermediates, and transition states in possible reaction pathways. The first step was chemisorption of molecular oxygen across two adjacent benzene rings on the char surface with 141.6 kJ mol-1 exothermic. The adsorbed molecular oxygen tends to dissociate in three different possible modes, resulting in CO and NO desorption. Computational results show that the formation of CO and NO can be achieved in three steps: (1) cleavage of corresponding bonds to form open-ring structures, (2) spontaneous ring contraction with the formation of species containing a ketene group or a nitroso group on a five-membered ring, and (3) desorption of CO or NO, regenerating free active sites for further gasification. The number of active sites available for reaction on the carbon surface fluctuates as a result of the open-ring reaction, cyclization reaction, and CO/NO desorption. The overall process for CO and NO desorption from oxidation of nitrogen-containing char by O2 was exothermic. Energy barriers for NO desorption were lower than those for CO desorption. Nitrogen in char could be preferentially oxidized because of the lower energy barriers.

nitrogen are relatively less well-understood. Until recently, it is believed that NO is released to the gas phase from a heterogeneous reaction between char nitrogen and oxygen. Different experimental techniques, such as temperature-programmed desorption (TPD),4 transient kinetics (TK),5 thermogravimetrymass spectrometry (TG-MS),6 and an entrained flow combustion reactor,7 have been employed to research heterogeneous oxidation mechanisms of carbon and nitrogen in char under combustion conditions. However, the drastic experimental conditions, heterogeneous reaction characteristics, and the complex gaseous environment surrounding char during combustion make it difficult to elucidate pathways for reactions between char and O2 at the molecular level.8,9 Quantum chemistry calculations provide a theoretical method to fill this gap, which have been proven to be of great utility in understanding reaction mechanisms. Quantum chemistry methods can obtain substantially accurate information that cannot be detected experimentally and reproduce the experimental phenomena observed in combustion.10-11 Molecular orbital calculations have significantly advanced the understanding of mechanisms for char gasification, as

1. Introduction The combustion and gasification of coal are important industrial processes. Voluminous literature has been concentrated on mechanisms of char gasification with oxygencontaining gases, such as O2, CO2, H2O, and NO. Despite the large amount of literature, these reactions are still not wellunderstood and have long been in dispute, particularly the char-O2 reaction during coal combustion. In general, pulverized coal is split into volatiles and residual char during devolatilization at the first stage of combustion. Elements, such as carbon and nitrogen, are distributed between volatiles and char. During combustion, a large part of nitrogen in fossil fuel evolves into nitric oxide, a well-known pollutant causing acid rain and photochemical smog. Therefore, the relationship between fuel nitrogen and NO released during coal combustion is a subject attracting increasing interest. Significant progress has been made in understanding how carbon and nitrogen in volatiles are oxidized to CO and NO by O2 through homogeneous reactions experimentally and theoretically.1 Several technologies have been developed to control conversion of volatile nitrogen to NO by combustion modification.2,3 In contrast, conversion mechanisms of char

(4) Muckenhuber, H.; Grothe, H. Carbon 2006, 44 (3), 546–559. (5) Radovic, L. R.; Jiang, H.; Lizzio, A. A. Energy Fuels 1991, 5 (1), 68–74. (6) Sta nczyk, K. Energy Fuels 1999, 13 (1), 82–87. (7) Coda, B.; Kluger, F.; Fortsch, D.; Splienthoff, H.; Hein, K. R. G.; Tognotti, L. Energy Fuels 1998, 12 (6), 1322–1327. (8) Molina, A.; Eddings, E. G.; Pershing, D. W.; Sarofim, A. F. Prog. Energy Combust. Sci. 2000, 26 (4-6), 507–531. (9) Zhu, Z. H.; Finnerty, J.; Lu, G. Q.; Yang, R. T. Energy Fuels 2002, 16 (6), 1359–1368. (10) Chen, N.; Yang, R. T. Carbon 1998, 36 (7-8), 1061–1070. (11) Radovic, L. R. J. Am. Chem. Soc. 2009, 131 (47), 17166–17175.

*To whom correspondence should be addressed. Telephone: þ86571-87952889. Fax: þ86-571-87951616. E-mail: [email protected]. (1) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust. Sci. 2003, 29 (2), 89–113. (2) Fan, W.; Lin, Z.; Li, Y.; Kuang, J.; Zhang, M. Energy Fuels 2009, 23 (1), 111–120. (3) Quang Dao, D.; Gasnot, L.; Marschallek, K.; El Bakali, A.; Pauwels, J. F. Energy Fuels 2010, 24 (3), 1696–1703. r 2011 American Chemical Society

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pioneered by the work of Yang’s group. Zhu et al. have comprehensively reviewed fundamental studies of gascarbon reactions using electronic structure methods in the last several years. Sendt and Haynes14,15 presented studies of O2 chemisorption on the armchair and zigzag surface of graphite, in which part of the potential energy surface was computed. The calculation results showed that the neighboring C-C bonds are weakened significantly because of chemisorbed oxygen atoms; hence, carbon is freed from the char structure. More recently, Wang16 investigated possible products during reactions of O2 with N-bounded char using quantum chemistry calculations. However, neither transition states nor intermediates connecting reactants and products were determined in his work. To the best of our knowledge, a definitive conclusion on pathways for NO formation during char N oxidation is absent from the literature. The purpose of this work is to clarify the interaction between O2 and nitrogen-containing char at molecular level. A reasonably simplified char model and possible reaction pathways were investigated using the density functional theory.17-19 The possible channels for NO and CO desorption were discussed in detail. 2. Computational Details

Figure 1. Optimized geometries of reactants, intermediates, and transition states for O2 chemisorption on nitrogen-containing char. Bond lengths are in angstroms.

2.1. Choice of Char Model. A reasonably simplified char model is important for substantially accurate results. The actual structure of char appears to be small graphite crystallites, which are irregularly stacked in size and orientation by three to seven aromatic rings. Furthermore, there exist numerous impurities in char particles. Thus, the graphitic structure modified by the effect of impurities can be employed as a simplified model system for char combustion.20,21 A basal plane structure C22H8 in armchair configuration has been shown to achieve a satisfying agreement with experimental results.15 X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) analyses have identified that, after severe pyrolysis, most nitrogen is present in six-membered rings located at edges of graphite layers as pyridinic nitrogen.22 Montoya et al.23 and Espinal et al.24 found that modeling studies using the graphite model with nitrogen in the pyridine group to represent nitrogen left in coal char provided reasonable theoretical explanations to experimental results. On the basis of the analysis above, the basal plane cluster C21H8N in armchair configuration [see structure C(N) in Figure 1] was selected to model nitrogencontaining char, in which a pyridinic nitrogen atom was located at the edge of char. Some of the carbon atoms were numbered for convenience.

2.2. Choice of Calculation Method. The theoretical level of calculation is another factor that can greatly influence the accuracy of results. The density functional theory (B3LYP)17-19 was used throughout this work with the 6-31G(d) basis set. Previous studies have shown that B3LYP/6-31G(d) can obtain a reasonable balance between the accuracy of final results and computational cost.9,12,14-16,25 In addition, it was shown that spin contamination at the B3LYP level of theory is reasonably small for carbonaceous models.26 Electronic ground states were used for all molecular systems in this study, which were the lowest energy states determined from single-point energy calculations at the B3LYP/6-31G(d) level for several electronic states. An unrestricted open-shell wave function was used in all openshell cases. All calculations include the following steps: geometric optimization for molecular structures to a minimum on the potential energy surface, frequency calculation to confirm the nature of all structures and obtain zero-point corrections, and intrinsic reaction coordinate (IRC) calculations to confirm the right connection between each transition state and corresponding intermediates.27 The Gaussian 03 package was used in this work.28

(12) Zhu, Z. H.; Finnerty, J.; Lu, G. Q.; Wilson, M. A.; Yang, R. T. Energy Fuels 2002, 16 (4), 847–854. (13) Zhu, Z. H.; Lu, G. Q.; Finnerty, J.; Yang, R. T. Carbon 2003, 41 (4), 635–658. (14) Sendt, K.; Haynes, B. S. Combust. Flame 2005, 143 (4), 629–643. (15) Sendt, K.; Haynes, B. S. J. Phys. Chem. C 2007, 111 (14), 5465– 5473. (16) Wang, S. Asia-Pac. J. Chem. Eng. 2010, 5 (3), 408–412. (17) Becke, A. D. J. Chem. Phys. 1992, 96 (3), 2155–2160. (18) Becke, A. D. J. Chem. Phys. 1992, 97 (12), 9173–9177. (19) Becke, A. D. J. Chem. Phys. 1993, 98 (7), 5648–5652. (20) Perry, S. T.; Hambly, E. M.; Flecher, T. H.; Solum, M. S.; Pugmire, R. J. Proc. Combust. Inst. 2000, 28 (2), 2313–2319. (21) Miessen, G.; Behrendt, F.; Deutschmann, O.; Warnatz, J. Chemosphere 2001, 42 (5-7), 609–613. (22) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Kwiatek, P. J.; Solum, M. S.; Hu, J. Z.; Pugmire, R. J. Energy Fuels 2002, 16 (6), 1507– 1515. (23) Montoya, A.; Truong, T. N.; Sarofim, A. F. J. Phys. Chem. A 2000, 104 (36), 8409–8417. (24) Espinal, J. F.; Truong, T. N.; Mondragon, F. Carbon 2007, 45 (11), 2273–2279.

(25) Montoya, A.; Mondragon, F.; Troung, T. N. Carbon 2002, 40 (11), 1863–1872. (26) Montoya, A.; Truong, T. N.; Sarofim, A. F. J. Phys. Chem. A 2000, 104 (26), 6108–6110. (27) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94 (14), 5523– 5527. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.

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Zhang et al. Table 1. Energy Barrier (ΔE) and Change of Enthalpy (ΔH) for Each Intermediate Reaction

Figure 2. Potential energy surface for O2 chemisorption on nitrogencontaining char.

3. Results and Discussion The interaction between O2 and nitrogen-containing char is assumed to take place in two distinct phases through multistep reactions. First the O2 molecule approaches and adsorbs on nitrogen-containing char. The next step is the extraction of nitrogen and carbon atoms in the form of nitric oxide and carbon monoxide, respectively. Theoretical and experimental evidence shows that the edge attack reaction is the most significant source of char gasification. Frankcombe et al. confirmed that oxygen adsorption onto the graphite basal plane surface will not significantly contribute to the gasification of well-ordered carbonaceous char.29 Zhu et al.9 found that molecular O2 tended to adsorb on the char surface in sideon mode. Therefore, this work is concentrated on NO and CO desorption from edge attack reactions occurring with O2 adsorbed in side-on mode. It is known that B3LYP functional cannot describe van der Waals interactions very well because of its repulsive long-range behavior. Studies about dispersion effects are currently in progress.30-32 Because our interest here is to investigate O2 chemisorption on the edge of nitrogencontaining char and the subsequent reactions and B3LYP can give excellent performance for thermochemistry during these processes,32,33 we would not pursue this issue further using a more accurate level of theory in this study. 3.1. O2 Chemisorption on the Active Sites. Optimized geometries of complexes for O2 chemisorption were shown in Figure 1. The symbols TSn and Mn (“n” is a number) were used to denote transition states and stable intermediates, respectively. O2 chemisorption on active sites of nitrogencontaining char took place via several low-barrier exothermic intermediate reactions, as shown in Figure 2. First of all, we found that one oxygen atom preferred to attack C3 with the formation of a peroxide structure M1 or M2. Energy barriers for intermediate reactions C(N) þ O2 f M1 and C(N) þ O2 f M2 were 33.9 and 25.9 kJ mol-1, respectively. Both reactions were exothermic processes. Intermediates M1 and M2 could transform to each other via TS3, with an energy barrier of 11.3 kJ mol-1. M1 and M2 would subsequently undergo ring closure to form a six-membered ring intermediate M3. Energy barriers to be overcome in this

reactions

ΔE (kJ mol-1)

ΔH (kJ mol-1)

C(N) þ O2 f M1 C(N) þ O2 f M2 M1 f M2 M1 f M3 M2 f M3 C(N) þ O2 f M3 M3 f a-M1 a-M1 f a-M2 a-M2 f a-M3 a-M3 f a-M4 a-M3 f P1 þ NO a-M4 f P1 þ NO C(N) þ O2 f P1 þ NO M3 f b-M1 b-M1 f b-M2 b-M2 f b-M3 b-M3 f P2 þ CO C(N) þ O2 f P2 þ CO M3 f c-M1 c-M1 f c-M2 c-M2 f c-M3 c-M3 f c-M4 c-M3 f P3 þ NO c-M4 f P3 þ NO c-M1 f c-M5 c-M5 f c-M6 c-M6 f P4 þ CO C(N) þ O2 f P3 þ NO C(N) þ O2 f P4 þ CO

33.9 25.9 11.3 16.5 18.3

-133.3 -134.6 -1.6 -8.3 -7.0 -141.6 -29.6 152.6 -215.9 -0.3 137.2 137.5 -97.2 -72.9 -151.6 -274.5 235.1 -405.6 -54.3 42.1 -189.4 -38.4 102.2 140.7 80.6 -309.2 234.3 -241.0 -190.2

26.6 248.6 13.5 54.7 215.2 205.1 20.3 255.6 42.0 269.5 9.0 111.8 156.9 46.7 180.8 207.9 93.2 12.4 327.2

process were 16.5 and 18.3 kJ mol-1 for M1 and M2, respectively. The chemisorption reaction C(N) þ O2 f M3 was 141.6 kJ mol-1 exothermic (Table 1), which was lower than that of the side-on chemisorption onto the armchair edge of pure graphite (275 kJ mol-1).15 It means that the sideon approach toward pyridinic nitrogen is a less favorable process than toward nitrogen-free carbon edge sites, which is in agreement with Kyotani and Tomita’s results.34 The heat released in initial O2 chemisorption was likely to provide energy to overcome barriers for other reactions, such as atom rearrangement, migration, open-ring, cyclization, and desorption reactions. It is also noted that O2 tends to dissociate after chemisorption because of the significant reduction of the O-O bond strength, as shown in Figure 1. The O-O bond was 1.216 A˚ before chemisorption and was elongated to 1.544 A˚ after chemisorption, indicating that the strength of the O-O bond was weakened because of chemisorption. The breakage of the O-O bond in the peroxide structure M3 occurred in three possible modes: (A) oxygen atom connected with C3 rotating around C3, away from the oxygen atom connected with the nitrogen atom, (B) oxygen atom connected with the nitrogen atom rotating around the N atom, away from the oxygen atom connected with C3, and (C) both oxygen atoms moving away from each other. The following sections will analyze subsequent reactions after O-O bond breakage in detail. Optimized geometries of intermediates, transition states, and possible products for subsequent reactions between O2 and nitrogen-containing char were shown in Figure 3. Potential energy surfaces for NO and CO desorption as a result of O2 dissociating in modes A, B, and C were displayed in Figures 4-6, respectively. The energy barrier (ΔE) and

(29) Frankcombe, T. J.; Bhatia, S. K.; Smith, S. C. Carbon 2002, 40 (13), 2341–2349. (30) Grimme, S. J. Comput. Chem. 2004, 25 (12), 1463–1473. (31) Grimme, S. J. Comput. Chem. 2006, 27 (15), 1787–1799. (32) Lill, S. O. N. J. Phys. Chem. A 2009, 113 (38), 10321–10326. (33) Espinal, J. F.; Montoya, A.; Mondrag on, F.; Truong, T. N. J. Phys. Chem. B 2004, 108 (3), 1003–1008.

(34) Kyotani, T.; Tomita, A. J. Phys. Chem. B 1999, 103 (17), 3434– 3441.

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Figure 3. Optimized geometries of intermediates, transition states, and products in NO and CO desorption pathways. Bond lengths are in angstroms.

enthalpy for M3 f a-M1 was -29.6 kJ mol-1, indicating that this was an exothermic process, as shown in Table 1. An energy barrier of 248.6 kJ mol-1 was needed to cleave the C5-N bond via a-TS2, and an open-ring intermediate a-M2 was produced with 152.6 kJ mol-1 endothermic. Intermediate a-M2 was not stable and would easily undergo cyclization to produce a closed-ring structure a-M3. About 13.5 kJ mol-1 was surmounted in this step. The reaction a-M2 f a-M3 released 215.9 kJ mol-1, which meant that a-M3 was more stable than a-M2. Intermediates a-M3 and a-M4 could

reaction enthalpy (ΔH) for each intermediate reaction were listed in Table 1. 3.2. NO Desorption with O2 Dissociating in Mode A. Possible pathways for NO desorption from M3 with O2 dissociating in mode A were shown in Figure 4. As mentioned above, M3 could undergo breakage of the O-O bond via a-TS1. The calculated energy barrier for this step was only 26.6 kJ mol-1. A three-membered ring structure comprised of oxygen and carbon was formed because of the attraction between the oxygen atom and C2. Change of 673

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oxygen atom connected with C3, which almost remained motionless, as shown in Figure 5. A three-membered heterocyclic structure (b-M1) containing N and O was formed. The energy barrier for this step was 20.3 kJ mol-1, with an exothermicity of 72.9 kJ mol-1. The structure b-M1 would undergo cleavage of the C3-C4 bond via b-TS2 and produce an open-ring intermediate b-M2. This step had an energy barrier of 255.6 kJ mol-1 and was 151.6 kJ mol-1 exothermic. The subsequent step was reorganization of the openring intermediate b-M2 into intermediate b-M3, which contained a chemisorbed CO on a closed five-membered ring. This reaction step involved the concomitant formation of a bond between C2 and C4 and breakage of the bond between N and O via the barrier b-TS3 of 42.0 kJ mol-1. About 274.5 kJ mol-1 was released in this process. The chemisorbed CO on b-M3 was desorbed via b-TS4, with a barrier of 269.5 kJ mol-1, leaving behind a carbon substrate with a defect consisting of a five-membered ring and a pyridone functional group (P2). Zhu et al.35 have experimentally verified the formation of a pyridone functional group when carbons with pyridinic groups were partially combusted, which proved that modeling calculations were reasonable in this study. The overall CO desorption reaction C(N) þ O2 f P2 þ CO was 405.6 kJ mol-1 exothermic. 3.4. NO and CO Desorption with O2 Dissociating in Mode C. Figure 6 showed possible pathways of NO and CO desorption from M3 with O2 dissociating in mode C. Cleavage of the O-O bond in M3 via c-TS1 readily occurred with an energy barrier of just 9.0 kJ mol-1 to produce intermediate c-M1, which included a ketone structure and a surface (-NO) group. Reaction M3 f c-M1 was 54.3 kJ mol-1 exothermic (Table 1). There were two different ring-opening ways for c-M1: breakage of the C5-N bond via c-TS2 to form c-M2 or cleavage of the C3-C4 bond via c-TS3 to produce c-M5. NO and CO were respectively released through these two different pathways. The following sections discuss the subsequent reactions in detail. Intermediate c-M1 could undergo a ring-opening reaction by cleaving the C5-N bond via c-TS2 to produce an openring intermediate c-M2, as shown in Figure 6. This occurred with an energy barrier of 111.8 kJ mol-1 and was 42.1 kJ mol-1 endothermic. An energy barrier of 156.9 kJ mol-1 was needed to form a bond between C5 and C7. Intermediate c-M3 containing a ketone structure and a nitroso group on a five-membered ring was produced during cyclization via c-TS4. The cyclization reaction was a highly exothermic process with 189.4 kJ mol-1 released. Intermediate c-M3 and c-M4 transformed to each other by a rotation of the oxygen atom around the C7-N bond via c-TS5. The energy barrier for transformation was 46.7 kJ mol-1. The nitroso groups on the edge of c-M3 and c-M4 were not stable. Energy barriers of 180.8 and 207.9 kJ mol-1 were needed to break the C7-N bond, producing NO and P3 for c-M3 and c-M4 via c-TS6 and c-TS7, respectively. After NO desorption from c-M3 and c-M4, the remaining char fragment (P3) could undergo rearrangement to produce a ketene group, which would desorb the CO molecule directly. In addition, the oxygen atom connected with C3 could migrate along the edge of char, which was studied by Sendt and Haynes.15 The overall reaction C(N) þ O2 f P3 þ NO was 241.0 kJ mol-1 exothermic.

Figure 4. Potential energy surface for reactions with O2 dissociating in mode A.

Figure 5. Potential energy surface for reactions with O2 dissociating in mode B.

Figure 6. Potential energy surface for reactions with O2 dissociating in mode C.

transform to each other by oxygen atom rotation about the C7-N bond via a-TS4, with an energy barrier of 54.7 kJ mol-1. Energy barriers of 215.2 and 205.1 kJ mol-1 were needed to break the C7-N bond and produce NO and P1 for a-M3 and a-M4, respectively. The change of enthalpy for the overall reaction C(N) þ O2 f P1 þ NO was -97.2 kJ mol-1. The oxygen atom left on P1 could migrate along the edge of char and further released CO, as discussed in Sendt and Haynes’ study.15 3.3. CO Desorption with O2 Dissociating in Mode B. The O-O bond in M3 was broken when the oxygen atom connected with nitrogen gradually rotated away from the

(35) Zhu, Q.; Money, S. L.; Russell, A. E.; Thomas, K. M. Langmuir 1997, 13 (7), 2149–2157.

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In addition to producing c-M2 by cleavage of the C5-N bond, c-M1 also evolved to an open-ring structure c-M5. The bond between C3 and C4 was broken via c-TS3 in this process. The energy barrier was 93.2 kJ mol-1 for this intermediate reaction. The open-ring structure c-M5 could undergo cyclization to form a stable intermediate c-M6. The transition state of this reaction, c-TS8, was just 12.4 kJ mol-1 above intermediate c-M5. Reaction enthalpy of cyclization c-M5 f c-M6 was -309.2 kJ mol-1. Because of the low relative energy, c-M6 may be a stabilized intermediate and have a significant lifetime. Because c-M6 contained a ketene group on a five-membered ring, the CO molecule was desorbed directly via c-TS9, with an energy barrier of 327.2 kJ mol-1 and endothermicity of 234.3 kJ mol-1. The overall reaction C(N) þ O2 f P4 þ CO released 190.2 kJ mol-1. We can see in Table 1 that the highest energy barriers for two CO desorption reactions, C(N) þ O2 f P2 þ CO and C(N) þ O2 f P4 þ CO, were 269.5 and 327.2 kJ mol-1, respectively, and the highest energy barriers for two NO desorption reactions, C(N) þ O2 f P1 þ NO and C(N) þ O2 f P3 þ NO, were 248.6 and 207.9 kJ mol-1, respectively. Energy barriers for NO desorption were lower than that for CO desorption. Nitrogen in char could be preferentially oxidized because of the lower energy barriers. Baxter et al.36 also observed this phenomenon experimentally. They attributed this to the thermal instability and the heterogeneous oxidative vulnerability of nitrogen-containing aromatic structures. 3.5. Reaction Mechanisms. The mechanism for CO and NO desorption during heterogeneous reactions between nitrogen-containing char and O2 can be described according to active site theory as follows:

from aromatic rings. Spontaneous ring contraction occurred after open-ring reaction and formed structures containing a ketene group or a nitroso group on five-membered rings. Desorption of stable molecules, CO and NO, was the final step with the regeneration of free active sites for further gasification. Open-ring and cyclization reactions were important steps during interaction between O2 and nitrogen-containing char, which meant cleavage of six-membered rings and the formation of five-membered rings. These processes were also observed by Jones et al.37 and Sendt and Haynes.15 One free active site was generated with the cleavage of a six-membered aromatic ring. The subsequent cyclization would result in ring closure with the formation of a five-membered ring and concomitant loss of the active site generated in the open-ring reaction. Desorption of CO/NO would regenerate a free active site. This means that the number of active sites available for the reaction on the carbon surface fluctuates as a result of the open-ring reaction, cyclization reaction, and CO/NO desorption.37 4. Conclusions A systematic theoretical study using a nonlocal hybrid B3LYP density functional theory was carried out to provide insight into mechanisms of CO and NO desorption from oxidation of nitrogen-containing char by O2. The chemisorption of molecular oxygen across two adjacent aromatic rings on the armchair surface of nitrogen-containing char was 141.6 kJ mol-1 exothermic and yielded stable oxygen complexes. Several possible pathways for CO and NO desorption were proposed. CO and NO desorption took place by a series of rearrangement reactions, including open-ring reactions and spontaneous ring contraction with the formation of structures containing a ketene group or a nitroso group. The number of active sites available for the reaction on the carbon surface fluctuates as a result of the open-ring reaction, cyclization reaction, and CO/NO desorption. No direct CO and NO desorption pathways were found. These pathways can result in a gasification reaction by removing O and N atoms at the edge of char during combustion. The overall processes for both CO and NO desorption were exothermic. Energy barriers for NO desorption were lower than those for CO desorption. Nitrogen in char could be preferentially oxidized because of the lower energy barriers. Desorption of NO2 and CO2 is not considered in this work and is left for future studies.

Cð Þ þ CðNÞ þ O2 f CðOÞ þ CðNOÞ CðOÞ f Cð Þ þ CO CðNOÞ f Cð Þ þ NO where species in parentheses refer to surface active sites. The first step was chemisorption of O2 on the surface of nitrogencontaining char with the formation of C(O) and C(NO). Surface species C(O) and C(NO) subsequently released CO and NO, respectively. Desorption of CO and NO from the oxygen-chemisorbed product took place not directly but by a series of rearrangement, open-ring, and cyclization reactions. The essential role of O2 in overall reactions was not only the supply of oxygen atoms but also the attack to surface carbon and nitrogen atoms, making them divorced

Acknowledgment. The work is supported by the National High Technology Research and Development Program of China (2008AA05Z304).

(36) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels 1996, 10 (1), 188–196.

(37) Jones, J. M.; Jones, D. H. Carbon 2007, 45 (3), 677–680.

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