Energy & Fuels 2002, 16, 847-854
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Molecular Orbital Theory Calculations of the H2O-Carbon Reaction Z. H. Zhu,† J. Finnerty,† G. Q. Lu,*,† M. A. Wilson,‡ and R. T. Yang§ Department of Chemical Engineering, University of Queensland, Brisbane, 4072 Australia, Department of Chemistry, Materials and Forensic Science, University of Technology, Broadway NSW 2001, Sydney, Australia, and Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Received November 13, 2001. Revised Manuscript Received May 10, 2002
Carbon gasification with steam to produce H2 and CO is an important reaction widely used in industry for hydrogen generation. Although the literature is vast, the mechanism for the formation of H2 is still unclear. In particular, little has been done to investigate the potential of molecular orbital theory to distinguish different mechanism possibilities. In this work, we used molecular orbital theory to demonstrate a favorable energetic pathway where H2O is first physically adsorbed on the virgin graphite surface with negligible change in molecular structure. Chemisorption occurs via O approaching the carbon edge site with one H atom stretching away from the O in the transition state. This is followed by a local minimum state, in which the stretching H is further disconnected from the O atoms and the remaining OH group is still on the carbon edge site. The disconnected H then pivots around the OH group to bond with the H of the OH group and forms H2. The O atom remaining on the carbon edge site is subsequently desorbed as CO. The reverse pathway occurs when H2 reacts with the surface oxygen to produce H2O.
Introduction Carbon gasification reactions have been an integral part of our industrial economy for a long time, yet a fundamental understandings of chemical reactions involved still lags far behind their practical use. In particular, the gasification of carbon with steam is an important reaction to energy, chemical, metallurgical, and mining industries, and to other industries using processes yet still not fully understood. This reaction is used to convert synthesis (H2 + CO) to hydrocarbon fuels or organic chemicals. Active carbons are also produced almost entirely through the activation of carbonaceous materials with steam and /or air. Another important use for this reaction is as a reducing agent for the direct processing of oxide ores to their metals. The kinetics and mechanism of the H2O-carbon reaction has been well studied.1-11 They suggested that the mechanism involves three steps. The first step is oxida†
Department of Chemical Engineering, University of Queensland. Department of Chemistry, University of Technology. § Department of Chemical Engineering, University of Michigan. (1) Long, F. J.; Sykes, K. W. Proc. R. Soc. 1948, A193, 377. (2) Ingles, O. G. Trans. Faraday Soc. 1952, 48, 706. (3) Walker, P. L., Jr; Rusinko, F., Jr.; Austin, L. G. Adv. Catal. 1959, 11, 133. (4) Smith, R. N.; Young, D. A.; Smith, R. A. Trans. Faraday Soc. 1966, 62, 2280. (5) Yang, R.T.; Duan, R. Z. Carbon 1985, 23, 325-331. (6) Yang, R. T.; Yang, K. L. Carbon 1985, 23, 537-547. (7) Chen, S. G.; Yang, R. T. J. Catal. 1992, 138, 12-23. (8) Chen, S. G.; Yang, R. T. J. Catal. 1993, 141, 102-113. (9) Hu¨ttinger, K. J.; Merdes, W. F. Carbon 1992, 30, 883-894. (10) Lussier, M. G.; Zhang, Z.; Miller, D. R. Carbon 1998, 36, 13611369. (11) Marchon, B.; Carrazza, J.; Heinemann, H.; Somorjai, G. A. Carbon 1988, 26, 507-514.
tion of carbon with water to form hydrogen. The second step is the reaction of carbon with hydrogen and the third is the formation of carbon monoxide from surface oxygen formed in the first step. This mechanism is outlined below where Cf is a surface active site.
Cf + H2O / C(O) + H2
(1)
1 Cf + H2 / C(H) 2
(2)
C(O) f CO (g)
(3)
This pathway can be further delineated in three ways which we distinguish here as mechanisms I-III. In mechanism I,5 steam decomposes to two hydrogen atoms and a bound oxygen radical. Mechanism I:
Cf + H2O f C(O) + 2C(H)
(4)
2C(H) / Cf + H2
(5)
C(O) f CO
(6)
‡
In mechanism II,2 a carbon surface accelerates the water-gas shift reaction by acting as a chain initiator by releasing trace hydrogen as radicals, which then propagate themselves. Mechanism II:
C(H) f Cf + H(g)
(7)
H(g) + H2O(g) / OH(g) + H2(g)
(8)
OH (g) + CO (g) / CO2(g) + H(g)
(9)
10.1021/ef010267z CCC: $22.00 © 2002 American Chemical Society Published on Web 06/28/2002
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In mechanism III,1 the steam molecule decomposes at the carbon surface into a hydrogen atom and hydroxyl radical both of which chemisorb rapidly on adjacent carbon sites. This is followed by the hydrogen atom on the chemisorbed hydroxyl radical joining the hydrogen atoms on the adjacent carbon site and leaving as a hydrogen molecule. Mechanism III:
2Cf + H2O(g) / C(H) + C(OH)
(10)
C(H) + C(OH) f C(H2) + C(O)
(11)
C(O) f CO(g)
(12)
C(H2) f Cf + H2(g)
(13)
There has been no systematic theoretical approach to understanding how water approaches the carbon surface to form hydrogen, or whether all three of these possible mechanisms are energetically favorable. In this study, we used molecular orbital theory to study the H2Ocarbon reaction mechanism. It is shown that the above three mechanisms are not satisfactory in describing the reaction pathway. An energetically more favorable mechanism is proposed, in which the oxygen is attached to the edge sites followed by hydrogen atoms migration to OH group. Experimental Section The Gaussian 98 package12 was used to undertake the molecular orbital theory calculations of the approach of a water molecular to a graphite surface. The detailed methodology for calculation using this package are described in elsewhere.13 One possibility for modeling of adsorption on graphite, involves the use of the Hartree-Fock (HF) method. This can save computation cost but requires unrestricted density functional theory to overcome spin contamination.14 Therefore, for the relatively large graphite structures A-A, B-B, C-C, and D-D (shown in Figures 1 and 3), HF/3-21G(d) was employed for calculating geometric optimization and frequency, while B3LYP/ 6-31G(d) was used for the self-consistent field (SCF) energies. Such a configuration proves to present reasonable balance between computational cost and accuracy.15 For structures from E-E to J-J, we used the higher level configuration, i.e., B3LYP/3-21 g(d, p) for geometry optimization, frequency, and IRC calculations, and B3LYP/6-31 g(d, p) for SCF energies calculations. The reason is 3-fold. The first one is that these structures are relatively smaller thus can afford more computational cost, another one is that we are studying the reaction pathway using IRC thus need to be more accurate. Finally, p functions are specifically added to hydrogen atoms (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, A. 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.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; 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. (14) Montoya, A.; Truon, T. N.; Sarofim, A. J. Phys. Chem. A 2000, 104, 6108-6110. (15) Chen, N.; Yang, R. T. Carbon 1998, 36, 1061-1070.
Figure 1. Models for mechanisms I and II. in the basis set, thus 3-21 g(d, p) and 6-31 g(d, p) are used for the formation of H2. The heat of adsorption (∆H) was determined as the difference between the total energy of the optimized system and the sum of the energies of the corresponding carbon model and gas molecules. The bond energy for CO desorption was calculated according to the procedures introduced by Yang et al.16 That is, for a geometrically optimized molecular system, we equilaterally change the bond lengths of the two adjacent C-C bonds that hold that CdO molecule on the edge. Upon each bond length change, the single point energy is calculated. This calculation is performed until there is no more change in the total energy upon C-C bond changes. The bond energies of C-CO bond discussed below were calculated as half the change of the system energies. For the bond energy for H atom desorption from the carbon surface, we change the bond lengths of the C-H bonds, and the bond energy is the energy change between the optimized system and the structure at the constant energy level.
Results and Discussion Previous Mechanisms. To analyze the possibility of the mechanism I being correct, we have calculated the bond energies of H desorption from the carbon surface in the zigzag model A-A and armchair model B-B shown in Figure 1. The relationships between energies and bond lengths are shown in Figure 2a,b. It is seen that the bond energy of H desorption is extremely high, being 508.15 kJ/mol for zigzag model A-A and 453.99 kJ/mol for armchair model B-B. We note that this calculation has been performed by Pan and Yang17 using simplistic methods. Extended Huckel molecular orbital (EHMO) calculations showed 375.0 kJ/ mol for zigzag model and 354.8 kJ/mol for armchair model. Both results show that H is bonded to zigzag sites more tightly than to armchair sites. The mean desorption activation energy of H was measured to be 440 kJ/mol by Zhang18 using a TPD (temperatureprogrammed desorption) technique. Thus the experimental result is relatively close to the calculated energy reported here, giving confidence to our calculations. The relationships between energies and bond lengths for CO release from zigzag model A-A and armchair (16) Chen, N.; Yang, R. T. J. Phys. Chem. A 1998, 102, 6348-6356. (17) Pan, Z. J.; Yang, R. T. J. Catal. 1990, 123, 206-214. (18) Zhang, Z. G.; Lussier, M. G.; Miller, D. J. Carbon 2000, 38, 1289-1296.
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Energy & Fuels, Vol. 16, No. 4, 2002 849
Figure 2. (a) Bond energy (BE) for H desorption from zigzag model A-A. (b) BE for H desorption from armchair model B-B. (c) BE for CO release from zigzag model A-A. (d) BE for CO release from armchair model B-B.
model B-B are shown in parts c and d of Figure 2, respectively. The bond energy is 325.0 kJ/mol for the former and 352.0 kJ/mol for the later. It is known that the absolute values calculated by molecular orbital theory are not accurate, but the relative comparison based upon the same calculation method is reliable.16 As mentioned above, the bond energy is over 450 kJ/ mol for H desorption from both zigzag site armchair site. This is much higher than that for CO release from either zigzag or armchair sites (based upon the same model chemistry configuration HF/3-21 g(d)//B3LYP/6-31 g(d)). Therefore, it is impossible that the formation of H2 is mainly from the C(H) groups since CO is preferentially desorbed.7,8 This means that mechanism I is incorrect. As for the chain reaction mechanism II, one H(g) initiated from C(H) can only react with one H2O and produce no more than one new H(g). Therefore, the reaction rate will depend on how quickly H(g) can be
produced from C(H). Similarly, this mechanism is also ruled out as the main pathway for H2 formation because of the extremely high bond energy of H desorption from C(H). If mechanism III was true, H2O would be dissociatively adsorbed on the carbon surface at zigzag sites and armchair sites, or as model clusters C-C and D-D in Figure 3. However, Marchon et al.11 ruled out the existence of hydroxyl groups formed by H2O adsorption on clean polycrystalline graphite because of the absence of O1s signal at the corresponding bond energies in the XPS measurements. Smith et al.4 studied surface complexes in the H2O-carbon reaction using infrared spectroscopy. The 7.4 µ (1351 cm-1) band appears to be from water and carbonyl oxygen and not -OH. The absence of hydroxyl groups in water-carbon reaction means that reaction mechanism III is also incorrect. This is confirmed by the extremely high bond energy for H desorp-
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Figure 3. Models for mechanism III.
tion from carbon sites as calculated above. We have also investigated the possibility of hydrogen formation from the H and -OH groups produced by H2O decomposition in models C-C and D-D by trying to locate a transition structure, but finding that H2 can by no means be formed through such a pathway. New Mechanism. The semiempirical method of complete neglect of differential overlap (CNDO) was first used by Hayns19 to study H2O adsorption to the basal plane of carbon. However, water molecules are preferencially adsorbed on the edges of the basal planes of carbon.20 The experiment conducted by Yang21 also showed that the edge sites are much more active than basal plane sites. Therefore, we focus on the edge sites. We use density function theory (B3LYP) here to determine the approach of a water molecule to a zigzag site E-E in Figure 4a. The energy change is shown in Figure 4b. A minimum state is located as model F-F, in which H2O molecule and the graphite cluster model are not on the same plane and the two H atoms of H2O are asymmetrically across the graphite layer. The model cluster F-F is clearly a weak physical adsorption characterized by the slightly released heat being -3.92 kJ/mol without the basis set superposition error correction (BSSEC), which is in agreement with the experimental data being from -0.6 to -25 kJ/mol.22,23 Whereas, with BSSEC, the adsorption heat becomes +69 kJ/mol, relatively higher than the experimental values. Considering that functional groups or inorganic minerals could be contained in the carbon materials and activated carbons were used, the experimentally obtained adsorption heat in the literature22,23 could be a bit lower than that from water adsorption on clean graphite. Therefore, the results with BSSEC can be more reliable than those without BSSEC. Kim et al.24 studied the water dimer using the second-order MøllerPlesset perturbation theory (MP2), also finding that the binding energy of water dimer without BSSEC was in good agreement with experimental values, and with (19) Hayns, M. R. Theor. Chim. Acta (Berlin) 1975, 39, 61-74. (20) Mowla, D.; Do, D. D.; Kaneko, K. Chem. Phys. Carbon. Submitted for publication. (21) Yang, R. T.; Wong, C. J. Chem. Phys. 1981, 75, 4471-4476. (22) Stoeckli, H. F.; Kraehenbuehl, F.; Morel, D. Carbon 1983, 21, 589. (23) Kimura, T.; Miyawaki, J.; Merraoui, M.; Iiyama, T.; Suzuki, T.; Kaneko, K. Int. Symp. Carbon, Tokyo 1998, 612. (24) Kim, K. S.; Mhin, B. J.; Choi, U. S.; Lee, K. J. Chem. Phys. 1992, 97, 6649.
Figure 4. (a) Pathway from H2O adsorption to H2 formation. (b) Energy change during the reaction according to this pathway.
BSSEC, the calculated values were slightly off the experimental error bounds. The calculated bond length between O (22) and the carbon edge site C(19) in model F-F is 0.259 nm, which means that H2O is adsorbed to the carbon site by only weak Van de Waal forces. The results also show two adjacent bonds of the anchoring site C(19), C(18)-C(19) and C(19)-C(20), are not significantly affected by adsorption, and the change of the molecular structure of H2O is also negligible. Our calculations show that a minimum state of nondissociative adsorption is not present when H2O and the graphite cluster are of the same plane. It is well established that the 1s electrons of two H atoms are bonded to the unpaired electrons of the sp3 hybridized O atom, the two O-H bonds are pushed by the two double-electron pairs of the O atoms from the tetrahedral angle of 104°30′. Our calculations show this to be only 103° probably because of the relatively low basis set. Because the zigzag edge site is electrophilic,25 H2O adsorption takes place with one pole approaching the active edge sites and two H atoms stretching toward (25) Zhu, Z. H.; Radovic, L. R.; Lu, G. Q.; Wu, X. X. Carbon. To be submitted for publication.
H2O-Carbon Reaction
Figure 5. Electronic structure for H2O adsorption on model F-F.
both sides. It is noteworthy that, in model F-F, calculations show that the bond C(19)-O(22), which connects the water molecule and the edge site, is not straightly upward, but tilted to one side. This is determined by the direction of the dipole of the water molecule (shown in Figure 5). We have performed similar calculations to armchair sites and found similar results. Calculations also show that 98.0 kJ/mol is needed for model F-F to reach the transitional state model G-G. This is achieved with a significant change of the molecular structure. For the H2O molecule to be adsorbed as in model G-G, the O(22)-H(24) bond is lengthened to 1.23 Å from 0.97 Å, but there is little change in the O(22)-H(23) bond. The H-O-H bond angle is however, increased by ca. 2°. The bond C(19)O(22) is shortened to 1.45 Å, which shows there is much stronger adsorption compared with model F-F. The two adjacent carbon-carbon bonds, C(18)-C(19) and C(19)C(20), are also apparently lengthened. This structure has all the properties of a chemisorbed molecule of water. The further stretching of O(22)-H(24) results in another local minimum state shown as model H-H. In this model, H2O is nearly decomposed into one H atom and one hydroxyl group, and the later is on the same plane as the graphite layer. This looks like mechanism III, but in mechanism III, H2O is dissociatively chemisorbed on the carbon sites resulting in C(H) and C(OH), while in the mechanism proposed here, the “disconnected” H(24) from H2O is still weakly connected to the OH group, not attached to the neighboring carbon edge site. Therefore, H(24) atom can readily move around and approach the H of the OH group, resulting in the subsequent local transitional state model I-I. H2 is formed when the two H atoms get close enough, leaving the O atom on the carbon edge site shown as model cluster J-J. Table 1b shows that there is one imaginary frequency for transition structure model G-G. Bear it in mind that the graphite layer is mainly located in the XY plane (the standard orientation of model G-G given in Table 1a). From the associated normal model of the imaginary frequency, we can see that the majority of the motion involves the big shifting H(24) and the modest shifting O(22). The movements of atoms become even clearer when we examine the alternate version of this normal
Energy & Fuels, Vol. 16, No. 4, 2002 851
model included later in the output, labeled as the eigenvector of the Hessian (see Table 1c). The most significant values are for the bond lengths R23, R26, and R27. By examining the stand orientation, we find that R23 is for bond C(19)-O(22), R26 for bond O(22)H(24), and R27 for bond H(23)-H(24). Such motion clearly shows that model G-G is the correct transition structure connecting two minimum states F-F and H-H. Table 2b presents the associated normal mode of the imaginary frequency of the transition structure model I-I (the standard orientation given in Table 2a), in which the shifting H(23) and H(24) atoms are the majority of the motion. The corresponding eigenvector of the Hessian is in Table 2c, in which the most significant values are for bond length R25, R26 and R27. Similarly, from the standard orientation, R25 is for O(22)-H(23), R26 for O(22)-H(24), and R27 for H(23)H(24). Model I-I thus proves to be the right transition structure connecting models H-H and J-J. We have also run IRC calculation on the two transition structure models G-G and I-I, respectively, finding that the former exactly goes to models F-F and H-H, and the latter to models H-H and J-J. The relationship between the energies and the bond lengths for CO release from model J-J is shown in Figure 6. The corresponding bond energy is 258.54 kJ/ mol. Very interestingly, -112.59 kJ/mol of heat is released from model cluster E-E to J-J for H2 formation, plus the bond energy for CO release, the calculated ∆H is + 146.0 kJ/mol for the reaction H2O + C f H2 + CO. This is so close to the experimental value (129.0 kJ/mol) 3. In summary, we proposed the following mechanism for the H2O-carbon reaction:
Cf + H2O / C-OH2
(14)
C-H2 / C-OH-H
(15)
C-OH-H / C-O-H-H
(16)
C-O-H-H / C-O-H2
(17)
C-O-H2 / C(O) + H2
(18)
C(O) f Cf + CO
(19)
Equation 14 shows the physical adsorption from model E-E to model F-F. Equations 15-18 represent the process of H2 formation from models F-F to J-J. The desorption of CO is shown by eq 19. Equations 14-18 are all reversible reactions. The reverse pathway from eq 18 to 14, or from models J-J to E-E, explains how H2 reduces the surface oxygen on carbon to produce H2O.3 In the study of Fredericks and Jordan,26 water adsorption on benzene was calculated using DFT (Density Function Theory) and MP2 methods, both of them produced good results, although the latter could be a bit better. Our mechanism is based upon DFT method and in good agreement with all the experimental observations and results. First, it explains that H2O is first physically adsorbed. This process is slightly exothermic and with negligible change of the molecular structure. The located minimum state of the physical (26) Fredericks, S. Y.; Jordan, K. D. J. Phys. Chem. 1996, 100, 7801.
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Table 1. Standard Orientation of Model G-G, the First Three Frequencies of Transition State Model G-G, and Eigenvector of Hessian for Model G-G (a) Standard Orientation of Model G-G coordinates (angstroms) center number
atomic number
atomic type
X
Y
Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
6 6 6 6 6 6 6 6 6 1 1 1 1 1 1 1 6 6 6 6 6 8 1 1
C C C C C C C C C H H H H H H H C C C C C O H H
3.701 672 3.695 062 2.506 604 1.240 937 0.012 356 -1.223 931 -2.488 596 -3.679 680 -3.693 842 4.634 785 4.637 188 2.514 670 0.019 713 -2.494 376 -4.620 393 -4.628 936 2.490 189 1.241 734 0.002 169 -1.229 808 -2.481865 0.013 679 -0.797 050 0.066 963
0.532735 -0.896491 -1.596 765 -0.935 511 -1.627 648 -0.953 141 -1.617 456 -0.921 540 0.509 266 1.077 002 -1.424 819 -2.677 268 -2.708 763 -2.698 229 -1.452 352 1.049 569 1.152 574 0.520 274 1.175 567 0.501 280 1.126104 2.616 401 2.991 605 3.096 575
0.005743 0.051141 0.035 422 -0.021 207 -0.014 815 -0.026 507 0.027 442 0.051 707 0.017 856 0.019 447 0.100 431 0.079 095 0.022 052 0.064 579 0.097 882 0.039 652 -0.059 301 -0.076 840 -0.137 650 -0.070 305 -0.049 499 0.044 292 -0.351 200 1.174 589
(b) The First Three Frequencies of Transition State Model G-G frequencies -1644.3631
84.9429
128.6910
atom
AN
X
Y
Z
X
Y
Z
X
Y
Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
6 6 6 6 6 6 6 6 6 1 1 1 1 1 1 1 6 6 6 6 6 8 1 1
0.01 0.01 -0.01 0.01 0.00 -0.01 0.01 -0.01 -0.01 0.01 0.00 -0.01 0.00 0.01 0.00 -0.01 -0.01 0.02 0.01 -0.02 0.01 -0.01 0.01 -0.01
-0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 -0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -0.05 0.01 0.00 0.05 0.01 -0.46
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -0.02 0.01 0.00 0.06 -0.05 -0.88
-0.01 -0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.02 -0.02 -0.02 0.00 0.00 0.00 0.01 0.02 -0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00
0.01 0.01 0.00 -0.01 -0.01 0.00 0.00 0.01 0.01 0.02 0.01 0.00 -0.01 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 -0.01 -0.01 -0.01
0.25 0.19 -0.02 -0.14 -0.22 -0.14 -0.02 0.18 0.25 0.41 0.31 -0.05 -0.25 -0.06 0.30 0.42 0.08 -0.10 -0.15 -0.10 0.08 -0.15 -0.14 -0.14
0.00 -0.01 0.00 0.00 0.00 0.00 -0.01 -0.01 0.00 0.00 -0.01 0.00 -0.01 -0.01 -0.01 0.00 0.00 0.00 -0.01 0.00 0.00 0.02 -0.04 0.23
0.00 -0.01 0.00 0.00 0.00 0.00 -0.01 -0.01 0.00 0.00 -0.01 0.00 -0.01 -0.01 -0.01 0.00 0.00 0.00 -0.01 0.00 0.00 0.02 -0.04 0.23
-0.15 0.13 0.22 0.06 0.00 -0.06 -0.23 -0.14 0.15 -0.29 0.25 0.41 0.00 -0.41 -0.27 0.29 -0.22 -0.07 0.00 0.07 0.22 -0.01 0.12 -0.02
(c) Eigenvector of Hessian for Model G-G (Eigenvectors Required To Have Negative Eigenvalues) 1 1 1 1 1 1 a
R1 0.034 53 R6 0.019 65 R11 0.000 63 R16 0.035 42 R21 -0.009 32 R26a 0.859 35
R2 -0.000 03 R7 -0.000 27 R12 0.019 29 R17 -0.000 40 R22 -0.004 43 R27a 0.324 24
R3 -0.035 28 R8 -0.031 29 R13 -0.001 08 R18 0.000 19 R23a -0.180 09 A1 -0.006 14
R4 -0.029 93 R9 -0.005 62 R14 -0.032 57 R19 -0.037 52 R24 0.022 90 A2 0.006 02
R5 -0.000 42 R10 -0.028 77 R15 -0.000 04 R20 0.023 32 R25 -0.537 1 A3 0.000 13
R23 is for bond C(19)-O(22), R26 for bond O(22)-H(24), and R27 for bond H(23)-H(24) according to the standard orientation.
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Table 2. The Standard Orientation of Model I-I, The First Three Frequencies Of Transition State Model I-I, and (a) The Standard Orientation of Model I-I coordinates (angstroms) center number
atomic number
atomic type
X
Y
Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
6 6 6 6 6 6 6 6 6 1 1 1 1 1 1 1 6 6 6 6 6 8 1 1
C C C C C C C C C H H H H H H H C C C C C O H H
3.687 546 3.705 656 2.531 569 1.256 692 0.044 743 -1.188 107 -2.441 298 -3.639 236 -3.667 326 4.614 360 4.657 340 2.551 992 0.063 385 -2.428 524 -4.573 123 -4.610 798 2.472 831 1.230 299 -0.002 280 -1.208 467 -2.471 372 0.030 254 -0.791 987 -1.592 181
0.576 983 -0.859 266 -1.573 541 -0.919 999 -1.630 037 -0.961 288 -1.650 795 -0.973 881 0.455 455 1.132 256 -1.371 900 -2.654 651 -2.709 883 -2.730 705 -1.515 274 0.974 234 1.180 330 0.529 309 1.219 045 0.488 030 1.101 041 2.574 191 2.998 950 3.395 129
0.017 183 0.019 171 0.004 150 -0.014 235 0.011 881 -0.010 083 0.057 944 0.049 710 -0.069 706 0.030 598 0.032 397 0.004 100 0.077 472 0.113 559 0.107 143 -0.162 485 -0.002 632 -0.024 437 -0.055 435 -0.062 240 -0.089 511 -0.100 132 0.370 435 1.237 277
(b) The First Three Frequencies of Transition State Model I-I frequencies -765.1665
86.1544
119.2874
atom
AN
X
Y
Z
X
Y
Z
X
Y
Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
6 6 6 6 6 6 6 6 6 1 1 1 1 1 1 1 6 6 6 6 6 8 1 1
0.00 0.00 0.00 -0.01 0.00 0.01 -0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 -0.01 0.00 0.02 -0.02 0.03 0.18 -0.38
0.01 0.00 0.00 -0.01 0.00 -0.01 0.00 -0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 -0.05 0.00 -0.01 0.04 -0.19 0.30
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 -0.01 0.00 0.00 0.00 0.00 -0.01 0.01 0.01 -0.01 0.01 -0.61 0.56
-0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 -0.01 0.00 0.00 0.00 0.01 0.01 -0.02 -0.01 0.00 0.00 0.00 -0.01 0.01 0.02 0.06
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 -0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04
0.25 0.19 -0.02 -0.15 -0.22 -0.14 -0.02 0.19 0.24 0.42 0.31 -0.06 -0.25 -0.05 0.32 0.40 0.08 -0.10 -0.14 -0.10 0.07 -0.15 -0.15 -0.08
0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.01 -0.01 0.01 0.02 0.00 0.01 0.00 0.01 0.02 -0.02 -0.18 -0.58
-0.01 0.00 0.00 0.00 0.00 0.00 0.02 0.03 0.01 -0.01 -0.01 0.00 0.00 0.02 0.04 -0.01 0.00 0.00 0.00 -0.01 0.01 -0.01 -0.09 -0.25
0.08 -0.07 -0.12 -0.05 -0.01 0.01 0.15 0.13 -0.10 0.14 -0.13 -0.21 0.03 0.28 0.25 -0.23 0.11 0.03 0.00 -0.07 -0.16 0.08 -0.14 -0.37
(c) Eigenvector of the Hessian for Model I-I (Eigenvectors Required To Have Negative Eigenvalues) 1 1 1 1 1 1 a
R1 0.01434 R6 0.00891 R11 -0.00032 R16 0.02384 R21 -0.03823 R26a 0.37115
R2 0.00022 R7 0.00005 R12 0.01179 R17 -0.00021 R22 -0.03320 R27a 0.78614
R3 -0.01243 R8 -0.01169 R13 0.01554 R18 -0.00004 R23 0.09097 A1 -0.00260
R4 -0.00943 R9 0.00994 R14 -0.01548 R19 -0.02696 R24 0.01943 A2 -0.00043
R25 is for O(22)-H(23), R26 for O(22)-H(24), and R27 for H(23)-H(24) according to the standard orientation.
R5 0.00043 R10 -0.01035 R15 0.00011 R20 0.01790 R25a -0.31442 A3 0.00303
854
Energy & Fuels, Vol. 16, No. 4, 2002
Figure 6. BE for CO desorption from zigzag model J-J.
adsorption (model E-E) is in agreement with the electronic structure of H2O being insignificantly perturbed. Second, it shows good agreement with the experimentally obtained ∆H for the total reaction H2O + C f H2 + CO. Third, the mechanism is consistent with the reverse reaction, i.e., H2 + C(O) f H2O + C, being possible as demonstrated by other workers.3 Finally, our study can do further justice to the results by Yang et al.5,6 about the inhibition and anisotropy in H2O-carbon reaction. The hydrogen inhibition is apparently due to the much stronger adsorption of hydrogen adsorption on edge sites compared with H2O. The
Zhu et al.
anisotropy of reaction is due to the different affinities of H atoms (the H atoms may come from either H2O dissociation or product H2) on zigzag and armchair sites, C-H bond is stronger on the former than on the latter. Consequently, the hexagonal etched pits with zigzag edges were observed.5,6 For CO2-C reaction, the etched pits are round due to the absence of H atoms. We do not deny that a few H2O may be dissociatively adsorbed through the reactions such as those of mechanisms I-III. But these mechanisms can by no means be the main pathway for H2 formation. Our present work may also be useful in modeling water adsorption on various carbons.20 Various models, such as the BET, Dubinin-Astakhov, and DubininSerpinsky, Talu, and Meunier association models, have been used to investigate the process of water adsorption. According to the established mechanisms, water molecules are first adsorbed on the primary active sites on the edges of the basal planes of carbon. The adsorbed water molecules then act as secondary sites for further adsorption of water molecules to form a cluster. The primary active sites are generally supposed to be surface functional groups. However, there is no convincing evidence for such a mechanism and in many carbon preparations the number of functional groups is insignificant. Our present study shows that pure carbon edge sites are attractive enough to adsorb H2O without the requirement of hydrophilic functional groups. Surface functional groups would be helpful for water adsorption, but are not a necessity. More importantly, our calculation gives a clear picture on how H2O approaches the carbon edge sites, and what the adsorption state looks like. EF010267Z
