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New Insights into NO-Carbon and N2O-Carbon Reactions from Quantum Mechanical Calculations Z. H. Zhu*,† and G. Q. Lu‡ Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, 6845 Australia, and Department of Chemical Engineering, University of Queensland, Brisbane, 4072 Australia Received September 17, 2002. Revised Manuscript Received April 8, 2003
Density functional theory calculations were used to investigate the mechanisms of NO-carbon and N2O-carbon reactions. It was the first time that the importance of surface nitrogen groups was addressed in the kinetic behaviors of the NO-carbon reaction. It was found that the offplane nitrogen groups that are adjacent to the zigzag edge sites and in-plane nitrogen groups that are located on the armchair sites make the bond energy of oxygen desorption even ca. 20% lower than that of the off-plane epoxy group adjacent to zigzag edge sites and in-plane o-quinone oxygen atoms on armchair sites; this may explain the reason why the experimentally obtained activation energy of the NO-carbon reaction is ca. 20% lower than that of the O2-carbon reaction over 923 K. A higher ratio of oxygen atoms can be formed in the N2O-carbon reaction, because of the lower dissociation energy of N2O, which results in a higher ratio of off-plane epoxy oxygen atoms. The desorption energy of semiquinone with double adjacent off-plane oxygen groups is ca. 20% less than that of semiquinone with only one adjacent off-plane oxygen group. This may be the reason why the activation energy of N2O is also ca. 20% less than that of the O2-carbon reaction. The new mechanism can also provide a good qualitative comparison for the relative reaction rates of NO-, N2O-, and O2-carbon reactions. The anisotropic characters of these gascarbon reactions can also be well explained.
1. Introduction Extensive experimental findings on NO-carbon and N2O-carbon reactions have shown an inflection point on the Arrhenius plot, at ca. 923 and 750 K, respectively.1-4 In these two temperature regimes, these two reactions have comparable activation energies: both are ca. 20% less in the higher-temperature regime (also shown in Table 1) and ca. 40%-60% lower in the lowertemperature regime than those of the O2-carbon reaction.2-6 A satisfactory explanation in regard to this inflection point is still unavailable. Yang et al.7 recently suggested that this inflection point is caused by cdirection (i.e., the direction perpendicular to the graphite layer) attack by NO at higher temperature,7 which results in the higher activation energy. However, such an explanation is made unsatisfactory by the lack of an inflection point in the O2-carbon reaction, because c-direction attack by O2 will also increase at higher temperature.8 We propose that the inflection point may * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Curtin University of Technology. ‡ University of Queensland. (1) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-244. (2) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475. (3) Teng, H.; Lin, H. C.; Hsieh, Y. S. Ind. Eng. Chem. Res. 1997, 36, 523-529. (4) Li, Y. H.; Radovic, L. R.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1999, 54, 4125. (5) Moulijn, J. A.; Kapteijn, F. Carbon 1995, 33, 1155-1165. (6) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. Adv. Catal. 1959, 11, 133. (7) Chen, N; Yang, R. T. J. Catal. 1998, 180, 245-257.
Table 1. Comparison of the Experimentally Obtained Activation Energies and the Calculated Bond Energies of Oxygen Desorption in Various Gas-Carbon Reactions
reaction
experimental activation energy, Ea (kJ/mol)
calculated bond energy (kJ/mol)
C + 2O2 f CO C + CO2 f 2CO C + H2O f CO + H2 C + N2O f CO + N2 C + NO f 1/2N2 + CO
208-242a ca. 358a ca. 333a 166-208a comparable to Ea,N2Ob
212-217 305-316 305-316 183 177-187
1/
a
From ref 6. b From refs 2-4.
be related to the ratio of products CO/CO2. Our suggestion is supported by the experimental results of the NO-carbon reaction by Furusawa et al.,1 which showed that, from 775 K to 910 K, CO2 was the only product and its concentration decreased rapidly as the temperature increased. Similar experimental results were reported for the N2O-carbon reaction.9,10 According to our recent molecular orbital theory calculations,11 the more oxygen that is adsorbed on the carbon edge sites, the weaker the adjacent C-C bonds. Apparently, CO2 desorption requires a lower energy than CO and the activation energy in the lower-temperature regime is (8) Chu, X.; Schmidt, L. D. Ind. Eng. Chem. Res. 1993, 32, 13591366. (9) Noda, K.; Chambrion, P.; Kyotani, T.; Tomita, A. Energy Fuels 1999, 13, 941. (10) Madley, D. G.; Strickland-Constanble, R. G. Trans. Faraday Soc. 1953, 49, 1312-1324. (11) Zhu, Z. H.; Finnerty, J.; Lu, G. Q.; Yang, R. T. Energy Fuels 2002, 16, 1359.
10.1021/ef0202079 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/07/2003
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Figure 1. (a) Influence of off-plane oxygen atoms in the O2-carbon reaction and off-plane nitrogen in the NO-carbon reaction on oxygen desorption from carbon zigzag edge site. (b) Influence of in-plane o-quinone oxygen atoms in the O2-carbon reaction and in-plane nitrogen groups in the NO-carbon reaction on oxygen desorption from armchair sites.
also lower for both NO-carbon and N2O-carbon reactions. In the O2-carbon reaction, the CO/CO2 ratio increases as the temperature increases, following the equation reported by Walker:6
(
CO/CO2 ) 103.4 exp CO/CO2 ) 0.3
12 400 RT
)
(for T > 790 K) (1)
(for T < 790 K)
(2)
This may be why no apparent inflection point was reported in the O2-carbon reaction, because the CO/ CO2 ratio is constant below 790 K. The formation and desorption of CO2 from the carbon edge sites are much more complicated than those of CO; thus, the former is not discussed in the present study. We only concentrate on the high-temperature regime, over which CO is the major product, and determine the kinetic behavior of reaction. The mechanismswhy the activation energies of the NO-carbon and N2O-carbon reactions are both ca. 20% lowerswill be elucidated. 2. Calculation Details The Gaussian 98 package12 was used in this study. More theoretical background can be found in the literature.13 The calculations were conducted on the SGI Origin 2000 64 MIPS R10000 high-performance computer at the University of Queensland. In the previous study,14,15 the unrestricted Hartree-Fock (UHF) method with the basis set of 3-21G(d) was employed for the geometric optimization of the carbon models and the self-consistent field (SCF) energies were calculated at the higher level, B3LYP/6-31G(d). The aforementioned selection of the molecular system and model chemistry has been proven to yield a reasonable balance between the final results and computation cost. However, the spin contamination
could be significant from the Hartree-Fock theory method in the investigation of a graphite reaction, and density functional theory (DFT) can effectively overcome the spin contamination in a graphite reaction.16 Therefore, in our present study, DFT B3LYP with a relatively lower basis set (3-21 g(d)) is used for geometric calculation that is free of spin contamination, and the higher-level model chemistry B3LYP/6-31G(d) is used to obtain accurate energy values. All calculations include the following steps: geometric optimization for optimizing the molecular system to a minimum on the potential energy surface, checking of the stability of wave functions, and higherlevel single-point calculations with B3LYP/6-31G(d) model chemistry for more-accurate total energies, according to the well-established conventional procedure.17 For a typical structure such as model A-1a (as shown in Figure 1), ca. 40 h is needed for the aforementioned evaluation. Therefore, the computational cost is also reasonable at our model chemistry configuration B3LYP/6-31G(d)// B3LYP/3-21G(d). The bond energy was calculated according to the procedure that was used by Yang et al.;17 that is, for a geometrically (12) 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, Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (13) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian: Pittsburgh, PA, 1996; p 64. (14) Chen, N.; Yang, R. T. Carbon 1998, 36, 1061-1070. (15) Kyotani, T.; Tomita, A. J. Phys. Chem. B 1999, 103, 3434. (16) Montoya, A.; Trong, T. N.; Sarofim, A. F. J. Phys. Chem. A 2000, 104, 6108-6110. (17) Chen, N.; Yang, R. T. J. Phys. Chem. A 1998, 102, 6348.
NO-C and N2O-C Reactions from Quantum Mechanics optimized molecular system, we equilaterally change the bond lengths of the two C-C bonds that hold that CdO molecule on the edge. Upon each change in bond length, the single-point energy is calculated. This calculation is performed until there is no more change in the total energy when the C-C bond changes. Finally, half of the total constant energy is regarded as the bond energy. It should be noted that the calculated bond energy (BE) of oxygen desorption from the carbon edge sites in the form of CO is different from the reaction activation energy; however, their relative positions have been shown to be very close (see Table 1). We therefore followed the method of Yang et al.17 and studied the activation energy of the NOx-carbon reaction, on the basis of the relative comparison of oxygen desorption bond energy. The oxygen desorption energy in H2O-carbon, CO2-carbon, and O2-carbon reactions have already been discussed in our previous studies,11,18,19 and NO-carbon and N2O-carbon reactions will be discussed in this study.
3. Results and Discussion The off-plane epoxy group was first proposed by Yang and co-workers17,20 in carbon reactions with oxygencontaining gases, which was suggested to be mainly formed in the O2-carbon reaction, whereas little can be produced in CO2-carbon and H2O-carbon reactions, because of the weak adsorption on the carbon sites. The formed epoxy oxygen atom weakens the adjacent C-C bond on the edge sites by 30%; thus, this finding well explains the 30% lower activation energy of the O2carbon reaction, compared to that of the CO2-carbon and H2O-carbon reactions. In our recent studies,11,19 in-plane o-quinone oxygen groups (i.e., two oxygen atoms connected on the same C-C bond on the armchair edge sites) were found to form on the armchair edge sites more readily than off-plane oxygen atoms in the O2-carbon reaction, but hardly form in either the CO2carbon or H2O-carbon reactions. In the latter two reactions, semiquinone oxygen groups are mostly formed on both zigzag and armchair sites. The CO release from o-quinone oxygen groups requires an energy that also is 30% less than that from semiquinone oxygen atoms, according to our calculations. Therefore, we concluded that the real reason for the 30% lower activation energy is the in-plane o-quinone oxygen groups, and the latter increases the number of active sites but does not further decrease the activation energy by another 30%. Our new mechanism can explain more experimental observations better. According to Kyotani’s calculations15 and our recent work,21 NO adsorption on a carbon surface is not only more thermodynamically favorable than O2, but also, with the nitrogen atom, more preferentially attached to the carbon site, rather than to the oxygen site. Previous reports have shown that a large amount of nitrogen-containing groups remain on the carbon surface, which are even harder to desorb than oxygen groups.22 We thus propose that nitrogen-containing groups make a contribution to the lower activation (18) Zhu, Z. H.; Finnerty, J.; Lu, G. Q.; Wilson, M. A.; Yang, R. T. Energy Fuels 2002, 16, 847-857. (19) Zhu, Z. H. Ph.D. Thesis, University of Queensland, Brisbane, Australia, 2002. (20) Chen, S. G.; Yang, R. T.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1993, 32, 2835-2840. (21) Zhu, Z. H.; Finnerty, J.; Lu, G. Q.; Yang, R. T. J. Phys. Chem. B 2001, 105, 821-830. (22) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840.
Energy & Fuels, Vol. 17, No. 4, 2003 1059
energy of the NO-carbon reaction. In Figure 1a, CO desorption energy from model cluster A-1a is 306 kJ/ mol. This mostly happens in the CO2-carbon or H2Ocarbon reactions. The CO desorption energy is weakened by 29% by the attached off-plane epoxy groups (shown as model cluster B-1a); this phenomenon occurs in the O2-carbon reaction. In the NO-carbon reaction, the offplane nitrogen groups are modeled here as model cluster C-1a, from which the bond energy of oxygen desorption is calculated to be 14% lower than that from model B-1a. We have also studied armchair sites that are shown in Figure 1b. According to our previous studies,11,19 O2 can adsorb on the armchair sites of model A-1b to form o-quinone oxygen groups, as shown in model B-1b. If NO dissociatively adsorbs on the same armchair sites, the nitrogen and oxygen atoms are connected on the same C-C bond, as shown in model C-1b. The CO desorption energy from the latter is 16.5% lower than that from the former. Our calculations are consistent with the experimental results that the NO-carbon reaction has an activation energy that is ca. 20% less than that of the O2-carbon reaction.2-6 The off-plane nitrogen group in model C-1a and the in-plane nitrogen group connected to the same C-C bond with the oxygen atom on the armchair sites (model C-1b) are the reason for the lower activation energy of the NO-carbon reaction. The in-plane nitrogen group that is connected to the same C-C bond with an oxygen atom on the armchair sites in model C-1b can be formed by the dissociative NO adsorption, which has been shown by Kyotani and Tomita.15 The big challenge is that the existence of an off-plane nitrogen group in model C-1a has never been reported experimentally. Table 2 lists the standard orientation of model cluster C-1a in the calculation, in which most of the atoms are in the X-Z plane, except that the nitrogen group is off-plane. Table 3 shows that the calculated IR bands of the symmetric off-plane nitrogen and CdO groups are located at 700 and 1728 cm-1, respectively. The latter is very close to the experimental value (1779 cm-1).23 The vibration direction of the off-plane nitrogen group is off-plane along the Y-axis, and the vibration of the CdO bond is inplane, along the X-direction. The intensity of the offplane nitrogen vibration is only 10, which is almost negligible, in comparison to the CdO group vibration intensity (245). Therefore, the reason why the off-plane nitrogen group has not been observed in the experiments may be that the existing instrument is not sensitive enough to detect the off-plane nitrogen group. In the study of nitrogen-rich carbon nitride, Miller et al.24 presented the IR spectra of four carbon nitride samples (see Figure 3 of ref 24), from which one can observe a very weak peak near 750 cm-1. Such a band may be related to the off-plane nitrogen group but is yet to be confirmed. It is a very challenging and also interesting research topic to find further experimental evidence for the existence of the off-plane nitrogen group. If an off-plane oxygen atom is added over C(5) and C(6) on model B-1b (in the O2-carbon reaction) and an (23) Zawadzki, J.; Wis´niewski, M.; Skowron´ska, K. Carbon 2003, 41, 235-246. (24) Miller, D. R.; Wang, J.; Gillan, E. G. J. Mater. Chem. 2002, 12, 2463.
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Table 2. Standard Orientation of Model C-1a center atomic atomic number number type 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 1 1 1 1 1 1 1 1 1 6 6 6 6 6 6 6 8 8 7
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
X
coordinates (Å) Y
-1.239384 0.151673 0.853754 2.284080 2.948702 2.238139 2.927745 2.238139 2.948702 2.284080 0.853754 0.151673 -1.239384 0.125454 0.827381 0.096149 0.827381 0.125454 -1.778958 0.716252 2.831593 4.032008 4.011545 4.032008 2.831593 0.716252 -1.778958 -1.895391 -1.309896 -2.153761 -1.419299 -2.153761 -1.309896 -1.895391 -3.376522 -3.376522 -0.740792
-0.240514 -0.218989 -0.115335 -0.128133 -0.069850 0.027987 0.048464 0.027987 -0.069850 -0.128133 -0.115335 -0.218989 -0.240514 -0.012110 0.078530 0.299717 0.078530 -0.012110 -0.322333 -0.289933 -0.199297 -0.098524 0.023444 -0.098524 -0.199297 -0.289933 -0.322333 -0.161797 -0.047770 -0.005344 0.251021 -0.005344 -0.047770 -0.161797 -0.159456 -0.159456 1.638960
Z
4.973893 4.946275 3.721249 3.672832 2.482321 1.239032 0.000000 -1.239032 -2.482321 -3.672832 -3.721249 -4.946275 -4.973893 2.487826 1.262531 0.000000 -1.262531 -2.487826 5.909118 5.869994 4.606007 2.453671 0.000000 -2.453671 -4.606007 -5.869994 -5.909118 3.753771 2.520807 1.303571 0.000000 -1.303571 -2.520807 -3.753771 1.355967 -1.355967 0.000000
off-plane nitrogen group is added in the same position on model C-1b (in the NO-carbon reaction), the bond energies of oxygen desorption in both cases will decrease, but the relative comparison should be the same; i.e., the bond energy of oxygen desorption in model C-1b should still be ca. 15%-20% less than that in model B-1b. Similarly, if one more off-plane oxygen atom is added over C(32) and C(17) on model B-1a (in the O2carbon reaction), and one more off-plane nitrogen group is added in the same position on model C-1a, the bond energies of oxygen desorption both will decrease but their relative comparison will remain the same. This can only happen when the O2 or NO concentration is very high, because the formation of more off-plane groups requires a higher gas pressure.11 Apparently, in reality, the NO concentration in flue gas is normally
