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Theoretical study of methane reforming on molybdenum carbide
Applied Catalysis A: General 328 (2007) 35–42 www.elsevier.com/locate/apcata
Theoretical study of methane reforming on molybdenum carbide Hiroyuki Tominaga, Masatoshi Nagai * Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan Received 5 December 2006; received in revised form 24 April 2007; accepted 2 May 2007 Available online 7 May 2007
Abstract The adsorption of methane and the subsequent reaction of two dissociatively adsorbed CH3 species on a b-Mo2C (0 0 1) slab to form either ethylene or ethane were studied using density functional theory (DFT) calculations. Four methane-adsorption models of b-Mo2C (0 0 1) with different methane adsorption profiles were investigated. Following structural optimization for adsorption, methane was dissociated into CH3 and H at the three-fold site for the surface Mo atoms, without the underlying carbon atoms of the second layer and the two-fold site position for the surface Mo atoms before optimization. For the two adsorption positions, the adsorption energy of methane was identical at 289 kJ/mol. The other two adsorption positions were unstable relative to these two positions. After optimization of the two methanes for dissociation into 2CH3 and 2H, with one CH3 approaching the another CH3, C2H5 (Intermediate 2) was formed, together with the formation of ethylene through the release of one hydrogen. The first principle molecular dynamics (MD) of Intermediate 2 produced ethylene with one hydrogen, while the MD of Intermediate 2 with the addition of two hydrogen molecules produced ethane following the restricted attack of one hydrogen on the C2H5 species at approximately ˚ from the surface after its desorption. 2A # 2007 Published by Elsevier B.V. Keywords: DFT; Ethane; Ethylene; First principle molecular dynamics; Methane; Reforming
1. Introduction The activation of methane and its conversion to higher hydrocarbons has attracted much attention in recent years. The aromatization of CH4 to benzene and naphthalene on molybdenum (Mo) carbides supported on ZSM-5 [1–6] and MCM-22 [7] has been reported. During this reaction, ethylene and ethane are primarily formed on the molybdenum carbide catalysts [6]. Ethylene is reported to be produced as the first product by heterolytic splitting of the C–H bonds of methane on the zeolite catalysts [1–3,5,8,9] C–H bond activation on the Mo carbide clusters within the zeolite leads to the formation of ethylene and hydrogen. The free radicals of CH3 are formed from the breaking of the C–H bond of methane using MoOx, followed by coupling together of the methyl radicals to form ethylene, which aromatizes to benzene with the aid of protons from the HZSM-5 zeolite [2,9]. Solymosi et al. [10] have studied the
* Corresponding author. Tel.: +81 42 388 7060; fax: +81 42 388 7060. E-mail addresses: [email protected] (H. Tominaga), [email protected] (M. Nagai). 0926-860X/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.apcata.2007.05.011
adsorption and dissociation of CH2I2 and C2H5I2 on a Mo2C/Mo (1 1 1) surface and have reported that C2H5 undergoes hydrogenation and dehydrogenation to give ethane and ethylene at low temperature, while elevation of the adsorption temperature increased the amount of desorbed ethylene. On the other hand, Ding et al. [6] have reported that ethane formation precedes ethylene formation together with a hydrogen molecule. Although ethane is formed preferentially over unsupported Mo2C catalysts when a hydrogen molecule is present during the earlier stage, ethylene is produced subsequently together with benzene [11,12]. Furthermore, it has been reported that the principal route for the aromatization of CH4 to benzene is the formation of C2H2 over molybdenum carbide [7]. Although the conversion over molybdenum of methane to ethylene and ethane has been extensively studied, the detailed mechanism of the initial C–C bond formation and the formation of ethylene or ethane during CH4 conversion remain unclear. The density functional theory (DFT) calculation, which is often applied to the determination of catalysis mechanisms, that have not been elucidated experimentally, is difficult and has rarely been used to study the conversion of methane. There are a few reports regarding the mechanism of methane conversion and
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the initial step of benzene formation on Ni, Co, and Fe metals and carbides thereof. The calculation began with the CHx (x = 1–3) species, which has been widely studied theoretically [13–19]. The DFT calculation showed the adsorption of CH4 and the subsequent splitting of CH4 into the CHx species (x = 1–3) on Ni (1 1 1) [14,16,17] and on the Zn catalyst [18,20]. The C–H bond activation of methane on Ni was studied using the Gaussian 98 program, and the formation of a NiCH2 species with H2 was demonstrated [21]. Zhang and Bowers [13] studied the activation of methane in the presence of hydrogen using the MH+ cluster model (M = Fe, Co, and Ni) using the Gaussian 98 program, and reported that CH4 was adsorbed onto MH+ to form MH+–CH4, which then became MCH3+ and H2. A DFT study of methane adsorption onto Pt (1 0 0) has suggested that due to a weak interaction between CH4 and the Pt (1 0 0) surface methane is not dissociated into CH3 and H but is molecularly adsorbed [22]. However, few reliable investigations using DFT have been reported regarding molybdenum carbides and the formation of ethylene or ethane during the early stages. In the present study, we report the activation of methane on a b-Mo2C (0 0 1) slab using four different models of methane adsorption and DFT calculations. The production of ethylene and ethane was calculated by the MD of the adsorbed C2H5 species formed by two methane molecules on the slab. 2. Calculation methods Self-consistent, gradient-corrected, periodic DFT calculations were performed using CPMD [23]. The Perdew–Burke–
Ernzerhof [24] functional was used within the generalized gradient approximation to describe the exchange and correlation effects. The Fritz–Haber-Institute pseudopotential [25] was used for all the ions. For the plane wave set, the cut-off energy was 50 Ry (1 Ry = 13.6 eV). For our calculations, we selected surface Brillouin zone sampling restricted to the ’ point. The convergence criteria for the energy calculation and structure optimization were assigned an SCF tolerance of 1.0 107 and a maximum force tolerance of 5 104. The initial transition state structure was calculated by the Synchronous Transit-Guided QuasiNewton method, and then optimized using the partitioned rational function optimizer with the quasi-Newton method. The structures of the bulk b-Mo2C (0 0 1) surface and a selected slab have been described previously [26]. The periodically repeated orthorhombic super cell (9.000, 5.202, ˚ ) was used. Each slab included three atomic layers, 20.00 A with alternating layers of Mo and C. Thus, the top overlayer contained six Mo atoms (black balls in Fig. 1A), the second layer contained six C atoms (dark balls), and the third layer contained six Mo atoms (small black balls). The ViewerLite 5.0 (Accelrys Inc.) software was used to visualize the results of the computer graphics calculations. The lattice constant of b-Mo2C (0 0 1), as calculated by CPMD, was in good agreement with the reference at relative errors of 2.77% and 4.65%, and the relative error of the cell volume was calculated at 0.698%, which was within 1% [26]. This result was considered to be reliable for calculations using CPMD.
Fig. 1. (A) Adsorption structures on the b-Mo2C (0 0 1) slab with side and top views before optimization. (B) (a–d) Adsorbed CH4 probes with side and top views after optimization. Black ball, top layer Mo atom; dark ball, carbon atom; small black ball, third layer Mo atom; white ball, hydrogen atom.
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Table 2 The bond lengths of Mo atom with C, C–C and C–H bonds of adsorbed methane after methane adsorption
3. Results and discussion 3.1. Adsorption structure ˚ from the The carbon atom of methane was located 2.00 A top layer of b-Mo2C (0 0 1) before optimization (Fig. 1A). Four methane adsorption models were investigated, which corresponded to the following adsorption profiles, in which CH4 was placed in positions (a) over the three-fold site of the surface Mo atoms without the underlying carbon atoms of the second layer, (b) with the carbon atom of the second layer, (c) over the twofold site position of the surface Mo atoms, and (d) over the ontop position of the surface Mo atoms. The four structures were optimized (Fig. 1B and Table 1). The optimization suggested the dissociation of hydrogen from methane but with very little change in the position of the methane carbon atom for positions (a) and (b). For position (c), the methane carbon atom shifted to the three-fold position of the surface Mo atom along with hydrogen dissociation from the two-fold position before optimization. The bond lengths of C–H for position (a) were the same as those for position (c), except that the C1–H1 bond ˚ longer than that for position length for position (c) was 0.01 A (a). Furthermore, the Mo–C bond length differed only by ˚ in Mo1–C1 (2.44 A ˚ ) for position (c) and Mo2–C1 0.01 A ˚ (2.43 A) for position (a) over the three-fold site of the surface Mo atoms. Similarly, the bond length of Mo3–C1 for position ˚ longer than that of Mo4–C1 for position (c). (a) was 0.02 A Therefore, positions (a) and (c) exhibited the same adsorption structure. The adsorption energy of methane on the (c) position site was 289.0 kJ/mol, which was the almost the same value as that on the (a) position (289.1 kJ/mol). The adsorption of methane at the (a) and (c) positions gave the same structure with equal adsorption energy. For position (b), CH3 species was adsorbed on the three-fold position of the surface Mo atom. The adsorption energy for position (b) was 248.2 kJ/mol which was slightly less stable than positions (a) and (c). Regarding position (d), the location of methane was unchanged at the (d) position without the dissociation of methane. The structure of the (d) position was the least stable (23.42 kJ/mol). Consequently, the (a) and (c) models offer the most energetically Table 1 The bond lengths of Mo atom with C, C–H bonds of methane, and adsorption energy after the adsorption of methane Modela
Mo1–C1 Mo2–C1 Mo3–C1 Mo4–C1 C1–H1 C1–H2 C1–H3 C1–H4 Adsorption energy (kJ/mol) a
Position (a)
Position (b)
Position (c)
Position (d)
2.28 2.43 2.45 3.72 1.12 1.16 2.27 1.12
2.33 3.81 2.37 2.46 1.12 1.13 2.11 1.17
2.44 5.09 4.07 2.43 1.13 1.16 2.27 1.12
2.18 3.99 3.97 3.94 1.13 1.16 1.16 1.09
289.1
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248.2
The models of (a)–(d) are shown in Fig. 1.
289.0
23.42
˚) Bond length (A
Int.1a
TS1b
Int.2a
TS2b
Int.3a
Mo1–C1 Mo2–C1 Mo3–C1 Mo3–C2 Mo4–C2 Mo5–C2 C1–C2 C1–H1 C1–H2 C1–H3 C1–H4 C2–H5 C2–H6 C2–H7 C2–H8
2.28 2.40 2.49 2.49 2.28 2.40 3.24 1.16 1.10 2.30 1.17 1.10 1.16 2.30 1.17
2.21 2.26 2.29 3.96 3.65 4.10 3.39 3.08 1.12 2.25 1.21 2.03 1.04 3.46 1.05
2.31 2.40 2.43 3.64 5.30 6.05 1.55 3.16 1.15 2.29 1.17 1.14 1.10 4.51 1.10
2.32 2.46 2.42 3.59 5.25 6.03 1.52 3.43 1.14 2.27 1.15 3.56 1.10 4.54 1.09
2.30 2.40 2.41 3.54 5.29 6.07 1.51 3.25 1.13 2.33 1.17 3.26 1.10 4.58 1.09
a b
Int.1, Int.2, and Int.3 are Intermediates 1, 2, and 3 in Fig. 2, respectively. TS1 and TS2 are Transition states 1 and 2 in Fig. 2, respectively.
stable structures on b-Mo2C (0 0 1). Methane was dissociatively adsorbed to form CH3 and H, and was then stabilized on the three-fold Mo site without the underlying carbon atom of the second layer of the b-Mo2C (0 0 1) slab. For the adsorption of CH4 on Ni (1 1 1), CH3 showed a preference for chemisorption to the three-fold fcc and hcp sites of Ni (1 1 1) rather than with the one and two Ni atoms [16,17]. 3.2. Structural changes during methane decomposition The changes in the structure of methane during methane conversion are shown in Fig. 2 and Table 2. The carbon atoms ˚ above the surface Mo of the two molecules were located 2.00 A atoms of the three-fold sites, without the underlying carbon atoms of the second layer [model (a)]. After optimization of the two methanes, each methane was dissociated into CH3 and H in the same adsorption structure of methane (Intermediate 1), as previously described (ads denotes adsorbed): 2CH4 ! 2CH3 ads þ 2Hads Optimization was performed by the CH3 (C2H5H6H8) approaching another CH3 (C1H1H2H4), regardless of whether C2H4 or C2H6 was formed during the coupling of the two CH3 species. The C1 atom of CH3 (C1H1H2H4) was bonded with the C2 atom of CH3 (C2H5H6H8), concomitantly liberating one hydrogen atom (H1) and forming C2H5 (C1H2H4C2H5H6H8) together with two hydrogens (H3 and H7) (Intermediate 2): 2CH3 ads þ 2Hads ! C2 H5 ads þ 3Hads TS1 is the transition state from Intermediate 1 to Intermediate 2. The bond length of C1–H1 became longer than the bond length ˚ ) to Intermediate 2 (3.16 A ˚ ) through from Intermediate 1 (1.16 A ˚ TS1 (3.08 A) to desorb hydrogen from C1–H1. The bond length ˚ , which was longer of C2–H5 (C2H5H6H8) in TS1 was 2.03 A than those of Intermediates 1 and 2: H CH2 CH2 ads H þ 2Hads
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Fig. 2. (A, B) Reaction pathway for methane reforming on the b-Mo2C (0 0 1) slab. Some b-Mo2C (0 0 1) slab atoms are extended for the purpose of visualization.
These results imply that the CH2 (C1H2H4) species, which lacks the H1 of the CH3 species, reacts with the distorted CH2 (C2H6H8), which lacks the H5 of another CH3 species. The splitting of CH3 to CH2 and H and the subsequent reaction of CH2 may occur readily owing to the distorted structure of the adsorbed methane in TS1. The following experimental assumptions are made: CH3+ ! CH2 + H+ and 2CH2 ! C2H4 C2H4 for the H-galloaluminosilicate zeolite [27] and Mo/ HZSM-5 [1]. Intermediate 3 was obtained along with liberation of hydrogen (H5) from the Intermediate 2: C2 H5 ads þ 3Hads ! C2 H4 ads þ 4Hads
Thus, ethylene was formed through the adsorbed C2H5 during the combination of the two CH3 species on the b-Mo2C (0 0 1) surface in this step. A slight change was observed for the ˚ ), which represents the bond length of C2–H5 in TS2 (3.56 A ˚ ) to Intermediate 3 transition state from Intermediate 2 (1.14 A ˚ ). Ethylene was produced through Intermediate 2 with (3.26 A very low activation energy, as discussed below. The change in the location of H5 was important for the reaction process. For ˚ from C2 and formed Intermediate 1, H5 was located at 1.10 A ˚ from C2 and the CH3 species. At TS1, H5 was located at 2.03 A ˚ from C1. Therefore, H5 was located at a position at 2.21 A intermediate between C1 and C2. For Intermediate 2, the
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˚ . The C2H5 species was distance between H5 and C2 was 1.14 A ˚ from C2, while the formed. At TS2, H5 was located at 3.56 A ˚ height of H5 was 1.18 A from Mo1. From these results, we conclude that the H5 atom of the C2H5 species was about to dissociate and would then adsorb to the surface. Furthermore, ˚ from Mo1 for Intermediate 3, the height of H5 was at 1.02 A and it subsequently adsorbed to the surface. Ciobıˆcaˇ et al. [15] have reported that the successive dissociation of CHx ! CHx1 + H (x = 41) on a Ru (0 0 0 1) surface could be calculated using DFT. The activation energies for methyl and methylene decomposition were lower than that for methane decomposition. The lowest activation energy was found for methylene decomposition and the highest activation energy was found for CH decomposition. In the TS1 transition state, the structure of CH3 (C2H5H6H8) in C2H5 was distorted. Formation of ethane did not occur, which is in agreement with the results obtained for the reaction of CH3I on a Mo2C/ Mo (1 1 1) surface, as reported by Solimosi et al. [28]. For the C–C coupling stage and for oxidative coupling catalyzed by Lidoped MgO, the C–H bond-breaking step takes place in the gas phase: 2CH4 ! 2CH3 + 2H ! C2H6 + 2H [19,29]. As a result, these CH3 fragments condense on the surface of the Mo–C clusters to form C2H4 or C2H6 molecules, which can oligomerize and cyclize to form benzene through C2H4 and C3H8 [30] on the Mo carbide during the methane reaction. 3.3. Changes in energy during the reaction process The energy changes during methane conversion were calculated by DFT (Fig. 3). The activation energy for the step from Intermediate 1 to Intermediate 2 was 741.3 kJ/mol, which was higher than the expected value, i.e., the adsorption energy of Intermediate 1 from methane adsorption was 554.0 kJ/mol. This is because the step from Intermediate 1 to Intermediate 2 contains a mixture of processes, due to the formation of the C– C bond of C2H5 from two CH3 species, including the release of
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one hydrogen and distortion of the CH3 (C2H5H6H8) species. Furthermore, the energy difference between the b-Mo2C (0 0 1) slab + 2CH4 and TS1 was 187.3 kJ/mol. This value is not so high. The step from Intermediate 2 to the final reaction suggests the probability of two elemental reactions for the formation of ethylene to separate the adsorbed hydrogen atoms (C2H5 ads + 3Hads ! C2H4 ads + 4Hads) and the formation of ethane through the release of one hydrogen molecule (C2H5 ads + 3Hads ! C2H6 + H2). For the latter reaction, the energy change was 575.9 kJ/mol. For the former reaction, the activation energy was only 4.2 kJ/mol. Thus, ethylene was readily formed on the b-Mo2C (0 0 1) slab. This calculation is in accordance with the result obtained when ethylene and hydrogen are formed during C–H bond activation on Mo carbides [1–3,5,8,9,11,30]. For the formation of ethylene and two hydrogen molecules (C2H4ads + 4Hads ! C2H4 + 2H2), an energy of 795.4 kJ/mol was requited for the hydrogen release. This barrier is so high, since four desorbed hydrogens become two hydrogen molecules. 3.4. First principle molecular dynamics calculation during desorption The calculation of the structure optimization from Intermediate 1 to Intermediate 3 did not result in the formation of ethane on the b-Mo2C (0 0 1) surface, as mentioned above. In order to clarify whether or not ethane forms on the surface or in the gas phase, the first principle MD calculation was carried out with high temperature (973 K [31]). The desorption process of the reaction products during CH4 conversion on the b-Mo2C (0 0 1) slab was simulated by elevating the temperature of the Intermediate 2 state based on the first principle MD calculation. The first principle MD calculation was performed at the target temperature of 973 K using 2000 steps with time intervals of 5 a.u. (1 a.u. = 0.024 fs) under the fixed b-Mo2C (0 0 1) slab. Fig. 4 shows the change in the bond lengths of C1–H4 and
Fig. 3. Schematic potential energy for methane reforming on the b-Mo2C (0 0 1) slab.
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Fig. 4. The bond lengths of C1–H4 and C2–H5 and energy (1 Ha = 27.2 eV) obtained by MD starting from the configuration of Intermediate 2 (C2H5 and three hydrogens, 0 step) to that in the final state through the structure at the longest bond length of C1–H4. The energy was plotted relative to the energy at 0 fs. Some bMo2C (0 0 1) slab atoms are extended for the purpose of visualization.
C2–H5 with the structure at the longest distance of the C1–H4 and the final state. Because ethylene was observed in the final position at 973 K, the MD calculation for Intermediate 2 led to the formation of C2H4, with the release of one hydrogen (H5) at this reaction temperature. This result is consistent with the very small change in the activation energy of Intermediate 2 to Intermediate 3 leading to the formation of ethylene, as well as the experimental results reported for unsupported molybdenum carbides [30,32]. Thus, ethylene was produced through C2H5, which was produced by the combination of two methane molecules: C2 H5 ads þ 3Hads ! C2 H4 þ 4Hads : The MD trajectory showed that the C1–H4 bond length ˚ at a maximum at 28.2 fs and was about to extended to 1.77 A ˚ in the dissociate but was shortened to convergence at 1.12 A final stage (240 fs). The structure of C2H5 was distorted during the desorption process. Furthermore, the bond length of C2–H5 was monotonously enlarged and H5 was dissociated from C2. Finally, ethylene was formed. On the other hand, the MD calculations were used to study the changes in the structure of the Intermediate 2 state and the subsequent products by the addition of two hydrogen molecules, to confirm the experimental results [11,12] obtained for the formation of ethane on b-Mo2C (0 0 1) under higher coverage of hydrogens. The changes in the bond lengths of C1–H4 and C2–H5 with the structure at the longest distances of the C1–H4 and C2–H5 bonds and at the final state are shown in Fig. 5. This figure
shows that the C2H5 species was desorbed together with one hydrogen molecule. The other added hydrogen molecule was dissociated at the final position: C2 H5 ads þ 3Hads þ 2H2 ! C2 H5 þ H2 þ 5Hads : In the MD trajectory, the bond length of C1–H4 was ˚ at 6.96 fs in the presence of four extended maximally to 1.31 A added hydrogens, although this bond length was less than ˚ without the addition of hydrogens. This result shows that 1.77 A the addition of hydrogens depresses the extension of the C1–H4 bond length, probably due to the repulsion of hydrogens. Furthermore, the addition of hydrogens molecules did not increase the bond length of C2–H5 so much, and the new ˚ , and it did not cause maximum value is only 1.37 A dissociation. This result indicates that the addition depresses the dissociation of the C2–H5 bond. Next, the structures at 0 fs ˚ from the and 29.64 fs (with the C1 atom approximately 2 A surface) during the MD run were used to determine the formation of ethane. The optimized structure of the restricted ˚ is shown in approaching H9 to C1 as a distance of almost 1 A Fig. 6. At the beginning of the MD run, C1 was approximately ˚ from the surface. Optimization did not produce the 1.74 A formation of ethane after the approach of one hydrogen (H9) to C1 but induced the adsorption of the CH3–CH species with the liberation of the hydrogen (H2) that had been bonded to C1. On the other hand, the optimization produced the formation of ethane after the approach of one hydrogen (H9) to the C1 atom ˚ from the surface. From this result, it appears approximately 2 A
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Fig. 5. The bond lengths of C1–H4 and C2–H5 and energy (1 Ha = 27.2 eV) obtained by starting from the configuration of Intermediate 2 (C2H5 and three hydrogen’s and adding two H2, 0 step) to that in the final state through the structures at the longest bond lengths of C1–H4 and C2–H5. The energy was plotted relative to the energy at 0 fs. Some b-Mo2C (0 0 1) slab atoms were extended for the purpose of visualization.
that the formation of ethane on the surface is difficult to achieve, but it reacts with one hydrogen after desorption away from the surface. Thus, the forced approach of hydrogen to the C1 atom produced ethane, whereas the presence of excess
hydrogens led to the formation of ethane. This formation of ethane was promoted in the presence of excess hydrogen in the reactants and the formation of ethylene with decreasing hydrogen in the experimental results, as already mentioned.
˚ , using the structures of Intermediate 2 with added Fig. 6. Optimized structures for the restricted single hydrogen (H9) approach of C1 at approximately 1 A hydrogen’s at 0 fs and 29.64 fs during the MD run.
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Consequently, the first principle MD calculation supports the formation of ethylene from the C2H5 species in the absence of hydrogen and the formation of ethane in the presence of a large amount of desorbed hydrogen. A supply of energy, e.g., from hydrogen pressure and heat, would probably promote the formation of ethane.
References [1] [2] [3] [4] [5]
4. Conclusion The adsorption of methane onto a b-Mo2C (0 0 1) slab using four different models for the adsorption of methane and the formation of ethylene and ethane were studied on the basis of DFT calculations. Two of the four methane-adsorbed models appear to be stable: model (a) CH4 placed over the three-fold site of the surface Mo atoms without the underlying carbon atoms of the second layer, and model (c) CH4 placed over the two-fold site position of the surface Mo atoms before optimization. The energy of methane adsorption at the model (c) position site (289 kJ/ mol) was the same value as that at the model (a) position. Thus, methane is dissociatively adsorbed to form CH3 and H, and subsequently is stabilized on the three-fold Mo site without the underlying carbon atom of the second layer of the b-Mo2C (0 0 1) slab. After the optimization of the two methanes, each methane was dissociated into CH3 and H in the same adsorption structure of methane. The optimization of the dissociative adsorption of two methane molecules on the b-Mo2C (0 0 1) slab and one CH3 approaching the another CH3 resulted in the formation of H and C2H5, with the subsequent formation of C2H4. In the reaction scheme, the step from Intermediate 2 contained two elemental reactions: (1) the formation of ethane through the release of one hydrogen molecule, and (2) the formation of ethylene through the release of two hydrogen molecules. The first principle MD calculation predicted the production of ethylene and ethane from Intermediate 2 with and without the addition of hydrogens. Ethylene is produced through the C2H5 species by the combination of two methanes, while ethane is formed by the close approach of hydrogen to the C2H5 species that had been desorbed away from the surface in the presence of excess hydrogens. Acknowledgement The authors thank the Scientific Project program of NEDO for partial financial support.
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Theoretical study of methane reforming on molybdenum carbide Hiroyuki Tominaga, Masatoshi Nagai * Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan Received 5 December 2006; received in revised form 24 April 2007; accepted 2 May 2007 Available online 7 May 2007
Abstract The adsorption of methane and the subsequent reaction of two dissociatively adsorbed CH3 species on a b-Mo2C (0 0 1) slab to form either ethylene or ethane were studied using density functional theory (DFT) calculations. Four methane-adsorption models of b-Mo2C (0 0 1) with different methane adsorption profiles were investigated. Following structural optimization for adsorption, methane was dissociated into CH3 and H at the three-fold site for the surface Mo atoms, without the underlying carbon atoms of the second layer and the two-fold site position for the surface Mo atoms before optimization. For the two adsorption positions, the adsorption energy of methane was identical at 289 kJ/mol. The other two adsorption positions were unstable relative to these two positions. After optimization of the two methanes for dissociation into 2CH3 and 2H, with one CH3 approaching the another CH3, C2H5 (Intermediate 2) was formed, together with the formation of ethylene through the release of one hydrogen. The first principle molecular dynamics (MD) of Intermediate 2 produced ethylene with one hydrogen, while the MD of Intermediate 2 with the addition of two hydrogen molecules produced ethane following the restricted attack of one hydrogen on the C2H5 species at approximately ˚ from the surface after its desorption. 2A # 2007 Published by Elsevier B.V. Keywords: DFT; Ethane; Ethylene; First principle molecular dynamics; Methane; Reforming
1. Introduction The activation of methane and its conversion to higher hydrocarbons has attracted much attention in recent years. The aromatization of CH4 to benzene and naphthalene on molybdenum (Mo) carbides supported on ZSM-5 [1–6] and MCM-22 [7] has been reported. During this reaction, ethylene and ethane are primarily formed on the molybdenum carbide catalysts [6]. Ethylene is reported to be produced as the first product by heterolytic splitting of the C–H bonds of methane on the zeolite catalysts [1–3,5,8,9] C–H bond activation on the Mo carbide clusters within the zeolite leads to the formation of ethylene and hydrogen. The free radicals of CH3 are formed from the breaking of the C–H bond of methane using MoOx, followed by coupling together of the methyl radicals to form ethylene, which aromatizes to benzene with the aid of protons from the HZSM-5 zeolite [2,9]. Solymosi et al. [10] have studied the
* Corresponding author. Tel.: +81 42 388 7060; fax: +81 42 388 7060. E-mail addresses: [email protected] (H. Tominaga), [email protected] (M. Nagai). 0926-860X/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.apcata.2007.05.011
adsorption and dissociation of CH2I2 and C2H5I2 on a Mo2C/Mo (1 1 1) surface and have reported that C2H5 undergoes hydrogenation and dehydrogenation to give ethane and ethylene at low temperature, while elevation of the adsorption temperature increased the amount of desorbed ethylene. On the other hand, Ding et al. [6] have reported that ethane formation precedes ethylene formation together with a hydrogen molecule. Although ethane is formed preferentially over unsupported Mo2C catalysts when a hydrogen molecule is present during the earlier stage, ethylene is produced subsequently together with benzene [11,12]. Furthermore, it has been reported that the principal route for the aromatization of CH4 to benzene is the formation of C2H2 over molybdenum carbide [7]. Although the conversion over molybdenum of methane to ethylene and ethane has been extensively studied, the detailed mechanism of the initial C–C bond formation and the formation of ethylene or ethane during CH4 conversion remain unclear. The density functional theory (DFT) calculation, which is often applied to the determination of catalysis mechanisms, that have not been elucidated experimentally, is difficult and has rarely been used to study the conversion of methane. There are a few reports regarding the mechanism of methane conversion and
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the initial step of benzene formation on Ni, Co, and Fe metals and carbides thereof. The calculation began with the CHx (x = 1–3) species, which has been widely studied theoretically [13–19]. The DFT calculation showed the adsorption of CH4 and the subsequent splitting of CH4 into the CHx species (x = 1–3) on Ni (1 1 1) [14,16,17] and on the Zn catalyst [18,20]. The C–H bond activation of methane on Ni was studied using the Gaussian 98 program, and the formation of a NiCH2 species with H2 was demonstrated [21]. Zhang and Bowers [13] studied the activation of methane in the presence of hydrogen using the MH+ cluster model (M = Fe, Co, and Ni) using the Gaussian 98 program, and reported that CH4 was adsorbed onto MH+ to form MH+–CH4, which then became MCH3+ and H2. A DFT study of methane adsorption onto Pt (1 0 0) has suggested that due to a weak interaction between CH4 and the Pt (1 0 0) surface methane is not dissociated into CH3 and H but is molecularly adsorbed [22]. However, few reliable investigations using DFT have been reported regarding molybdenum carbides and the formation of ethylene or ethane during the early stages. In the present study, we report the activation of methane on a b-Mo2C (0 0 1) slab using four different models of methane adsorption and DFT calculations. The production of ethylene and ethane was calculated by the MD of the adsorbed C2H5 species formed by two methane molecules on the slab. 2. Calculation methods Self-consistent, gradient-corrected, periodic DFT calculations were performed using CPMD [23]. The Perdew–Burke–
Ernzerhof [24] functional was used within the generalized gradient approximation to describe the exchange and correlation effects. The Fritz–Haber-Institute pseudopotential [25] was used for all the ions. For the plane wave set, the cut-off energy was 50 Ry (1 Ry = 13.6 eV). For our calculations, we selected surface Brillouin zone sampling restricted to the ’ point. The convergence criteria for the energy calculation and structure optimization were assigned an SCF tolerance of 1.0 107 and a maximum force tolerance of 5 104. The initial transition state structure was calculated by the Synchronous Transit-Guided QuasiNewton method, and then optimized using the partitioned rational function optimizer with the quasi-Newton method. The structures of the bulk b-Mo2C (0 0 1) surface and a selected slab have been described previously [26]. The periodically repeated orthorhombic super cell (9.000, 5.202, ˚ ) was used. Each slab included three atomic layers, 20.00 A with alternating layers of Mo and C. Thus, the top overlayer contained six Mo atoms (black balls in Fig. 1A), the second layer contained six C atoms (dark balls), and the third layer contained six Mo atoms (small black balls). The ViewerLite 5.0 (Accelrys Inc.) software was used to visualize the results of the computer graphics calculations. The lattice constant of b-Mo2C (0 0 1), as calculated by CPMD, was in good agreement with the reference at relative errors of 2.77% and 4.65%, and the relative error of the cell volume was calculated at 0.698%, which was within 1% [26]. This result was considered to be reliable for calculations using CPMD.
Fig. 1. (A) Adsorption structures on the b-Mo2C (0 0 1) slab with side and top views before optimization. (B) (a–d) Adsorbed CH4 probes with side and top views after optimization. Black ball, top layer Mo atom; dark ball, carbon atom; small black ball, third layer Mo atom; white ball, hydrogen atom.
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Table 2 The bond lengths of Mo atom with C, C–C and C–H bonds of adsorbed methane after methane adsorption
3. Results and discussion 3.1. Adsorption structure ˚ from the The carbon atom of methane was located 2.00 A top layer of b-Mo2C (0 0 1) before optimization (Fig. 1A). Four methane adsorption models were investigated, which corresponded to the following adsorption profiles, in which CH4 was placed in positions (a) over the three-fold site of the surface Mo atoms without the underlying carbon atoms of the second layer, (b) with the carbon atom of the second layer, (c) over the twofold site position of the surface Mo atoms, and (d) over the ontop position of the surface Mo atoms. The four structures were optimized (Fig. 1B and Table 1). The optimization suggested the dissociation of hydrogen from methane but with very little change in the position of the methane carbon atom for positions (a) and (b). For position (c), the methane carbon atom shifted to the three-fold position of the surface Mo atom along with hydrogen dissociation from the two-fold position before optimization. The bond lengths of C–H for position (a) were the same as those for position (c), except that the C1–H1 bond ˚ longer than that for position length for position (c) was 0.01 A (a). Furthermore, the Mo–C bond length differed only by ˚ in Mo1–C1 (2.44 A ˚ ) for position (c) and Mo2–C1 0.01 A ˚ (2.43 A) for position (a) over the three-fold site of the surface Mo atoms. Similarly, the bond length of Mo3–C1 for position ˚ longer than that of Mo4–C1 for position (c). (a) was 0.02 A Therefore, positions (a) and (c) exhibited the same adsorption structure. The adsorption energy of methane on the (c) position site was 289.0 kJ/mol, which was the almost the same value as that on the (a) position (289.1 kJ/mol). The adsorption of methane at the (a) and (c) positions gave the same structure with equal adsorption energy. For position (b), CH3 species was adsorbed on the three-fold position of the surface Mo atom. The adsorption energy for position (b) was 248.2 kJ/mol which was slightly less stable than positions (a) and (c). Regarding position (d), the location of methane was unchanged at the (d) position without the dissociation of methane. The structure of the (d) position was the least stable (23.42 kJ/mol). Consequently, the (a) and (c) models offer the most energetically Table 1 The bond lengths of Mo atom with C, C–H bonds of methane, and adsorption energy after the adsorption of methane Modela
Mo1–C1 Mo2–C1 Mo3–C1 Mo4–C1 C1–H1 C1–H2 C1–H3 C1–H4 Adsorption energy (kJ/mol) a
Position (a)
Position (b)
Position (c)
Position (d)
2.28 2.43 2.45 3.72 1.12 1.16 2.27 1.12
2.33 3.81 2.37 2.46 1.12 1.13 2.11 1.17
2.44 5.09 4.07 2.43 1.13 1.16 2.27 1.12
2.18 3.99 3.97 3.94 1.13 1.16 1.16 1.09
289.1
37
248.2
The models of (a)–(d) are shown in Fig. 1.
289.0
23.42
˚) Bond length (A
Int.1a
TS1b
Int.2a
TS2b
Int.3a
Mo1–C1 Mo2–C1 Mo3–C1 Mo3–C2 Mo4–C2 Mo5–C2 C1–C2 C1–H1 C1–H2 C1–H3 C1–H4 C2–H5 C2–H6 C2–H7 C2–H8
2.28 2.40 2.49 2.49 2.28 2.40 3.24 1.16 1.10 2.30 1.17 1.10 1.16 2.30 1.17
2.21 2.26 2.29 3.96 3.65 4.10 3.39 3.08 1.12 2.25 1.21 2.03 1.04 3.46 1.05
2.31 2.40 2.43 3.64 5.30 6.05 1.55 3.16 1.15 2.29 1.17 1.14 1.10 4.51 1.10
2.32 2.46 2.42 3.59 5.25 6.03 1.52 3.43 1.14 2.27 1.15 3.56 1.10 4.54 1.09
2.30 2.40 2.41 3.54 5.29 6.07 1.51 3.25 1.13 2.33 1.17 3.26 1.10 4.58 1.09
a b
Int.1, Int.2, and Int.3 are Intermediates 1, 2, and 3 in Fig. 2, respectively. TS1 and TS2 are Transition states 1 and 2 in Fig. 2, respectively.
stable structures on b-Mo2C (0 0 1). Methane was dissociatively adsorbed to form CH3 and H, and was then stabilized on the three-fold Mo site without the underlying carbon atom of the second layer of the b-Mo2C (0 0 1) slab. For the adsorption of CH4 on Ni (1 1 1), CH3 showed a preference for chemisorption to the three-fold fcc and hcp sites of Ni (1 1 1) rather than with the one and two Ni atoms [16,17]. 3.2. Structural changes during methane decomposition The changes in the structure of methane during methane conversion are shown in Fig. 2 and Table 2. The carbon atoms ˚ above the surface Mo of the two molecules were located 2.00 A atoms of the three-fold sites, without the underlying carbon atoms of the second layer [model (a)]. After optimization of the two methanes, each methane was dissociated into CH3 and H in the same adsorption structure of methane (Intermediate 1), as previously described (ads denotes adsorbed): 2CH4 ! 2CH3 ads þ 2Hads Optimization was performed by the CH3 (C2H5H6H8) approaching another CH3 (C1H1H2H4), regardless of whether C2H4 or C2H6 was formed during the coupling of the two CH3 species. The C1 atom of CH3 (C1H1H2H4) was bonded with the C2 atom of CH3 (C2H5H6H8), concomitantly liberating one hydrogen atom (H1) and forming C2H5 (C1H2H4C2H5H6H8) together with two hydrogens (H3 and H7) (Intermediate 2): 2CH3 ads þ 2Hads ! C2 H5 ads þ 3Hads TS1 is the transition state from Intermediate 1 to Intermediate 2. The bond length of C1–H1 became longer than the bond length ˚ ) to Intermediate 2 (3.16 A ˚ ) through from Intermediate 1 (1.16 A ˚ TS1 (3.08 A) to desorb hydrogen from C1–H1. The bond length ˚ , which was longer of C2–H5 (C2H5H6H8) in TS1 was 2.03 A than those of Intermediates 1 and 2: H CH2 CH2 ads H þ 2Hads
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Fig. 2. (A, B) Reaction pathway for methane reforming on the b-Mo2C (0 0 1) slab. Some b-Mo2C (0 0 1) slab atoms are extended for the purpose of visualization.
These results imply that the CH2 (C1H2H4) species, which lacks the H1 of the CH3 species, reacts with the distorted CH2 (C2H6H8), which lacks the H5 of another CH3 species. The splitting of CH3 to CH2 and H and the subsequent reaction of CH2 may occur readily owing to the distorted structure of the adsorbed methane in TS1. The following experimental assumptions are made: CH3+ ! CH2 + H+ and 2CH2 ! C2H4 C2H4 for the H-galloaluminosilicate zeolite [27] and Mo/ HZSM-5 [1]. Intermediate 3 was obtained along with liberation of hydrogen (H5) from the Intermediate 2: C2 H5 ads þ 3Hads ! C2 H4 ads þ 4Hads
Thus, ethylene was formed through the adsorbed C2H5 during the combination of the two CH3 species on the b-Mo2C (0 0 1) surface in this step. A slight change was observed for the ˚ ), which represents the bond length of C2–H5 in TS2 (3.56 A ˚ ) to Intermediate 3 transition state from Intermediate 2 (1.14 A ˚ ). Ethylene was produced through Intermediate 2 with (3.26 A very low activation energy, as discussed below. The change in the location of H5 was important for the reaction process. For ˚ from C2 and formed Intermediate 1, H5 was located at 1.10 A ˚ from C2 and the CH3 species. At TS1, H5 was located at 2.03 A ˚ from C1. Therefore, H5 was located at a position at 2.21 A intermediate between C1 and C2. For Intermediate 2, the
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˚ . The C2H5 species was distance between H5 and C2 was 1.14 A ˚ from C2, while the formed. At TS2, H5 was located at 3.56 A ˚ height of H5 was 1.18 A from Mo1. From these results, we conclude that the H5 atom of the C2H5 species was about to dissociate and would then adsorb to the surface. Furthermore, ˚ from Mo1 for Intermediate 3, the height of H5 was at 1.02 A and it subsequently adsorbed to the surface. Ciobıˆcaˇ et al. [15] have reported that the successive dissociation of CHx ! CHx1 + H (x = 41) on a Ru (0 0 0 1) surface could be calculated using DFT. The activation energies for methyl and methylene decomposition were lower than that for methane decomposition. The lowest activation energy was found for methylene decomposition and the highest activation energy was found for CH decomposition. In the TS1 transition state, the structure of CH3 (C2H5H6H8) in C2H5 was distorted. Formation of ethane did not occur, which is in agreement with the results obtained for the reaction of CH3I on a Mo2C/ Mo (1 1 1) surface, as reported by Solimosi et al. [28]. For the C–C coupling stage and for oxidative coupling catalyzed by Lidoped MgO, the C–H bond-breaking step takes place in the gas phase: 2CH4 ! 2CH3 + 2H ! C2H6 + 2H [19,29]. As a result, these CH3 fragments condense on the surface of the Mo–C clusters to form C2H4 or C2H6 molecules, which can oligomerize and cyclize to form benzene through C2H4 and C3H8 [30] on the Mo carbide during the methane reaction. 3.3. Changes in energy during the reaction process The energy changes during methane conversion were calculated by DFT (Fig. 3). The activation energy for the step from Intermediate 1 to Intermediate 2 was 741.3 kJ/mol, which was higher than the expected value, i.e., the adsorption energy of Intermediate 1 from methane adsorption was 554.0 kJ/mol. This is because the step from Intermediate 1 to Intermediate 2 contains a mixture of processes, due to the formation of the C– C bond of C2H5 from two CH3 species, including the release of
39
one hydrogen and distortion of the CH3 (C2H5H6H8) species. Furthermore, the energy difference between the b-Mo2C (0 0 1) slab + 2CH4 and TS1 was 187.3 kJ/mol. This value is not so high. The step from Intermediate 2 to the final reaction suggests the probability of two elemental reactions for the formation of ethylene to separate the adsorbed hydrogen atoms (C2H5 ads + 3Hads ! C2H4 ads + 4Hads) and the formation of ethane through the release of one hydrogen molecule (C2H5 ads + 3Hads ! C2H6 + H2). For the latter reaction, the energy change was 575.9 kJ/mol. For the former reaction, the activation energy was only 4.2 kJ/mol. Thus, ethylene was readily formed on the b-Mo2C (0 0 1) slab. This calculation is in accordance with the result obtained when ethylene and hydrogen are formed during C–H bond activation on Mo carbides [1–3,5,8,9,11,30]. For the formation of ethylene and two hydrogen molecules (C2H4ads + 4Hads ! C2H4 + 2H2), an energy of 795.4 kJ/mol was requited for the hydrogen release. This barrier is so high, since four desorbed hydrogens become two hydrogen molecules. 3.4. First principle molecular dynamics calculation during desorption The calculation of the structure optimization from Intermediate 1 to Intermediate 3 did not result in the formation of ethane on the b-Mo2C (0 0 1) surface, as mentioned above. In order to clarify whether or not ethane forms on the surface or in the gas phase, the first principle MD calculation was carried out with high temperature (973 K [31]). The desorption process of the reaction products during CH4 conversion on the b-Mo2C (0 0 1) slab was simulated by elevating the temperature of the Intermediate 2 state based on the first principle MD calculation. The first principle MD calculation was performed at the target temperature of 973 K using 2000 steps with time intervals of 5 a.u. (1 a.u. = 0.024 fs) under the fixed b-Mo2C (0 0 1) slab. Fig. 4 shows the change in the bond lengths of C1–H4 and
Fig. 3. Schematic potential energy for methane reforming on the b-Mo2C (0 0 1) slab.
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Fig. 4. The bond lengths of C1–H4 and C2–H5 and energy (1 Ha = 27.2 eV) obtained by MD starting from the configuration of Intermediate 2 (C2H5 and three hydrogens, 0 step) to that in the final state through the structure at the longest bond length of C1–H4. The energy was plotted relative to the energy at 0 fs. Some bMo2C (0 0 1) slab atoms are extended for the purpose of visualization.
C2–H5 with the structure at the longest distance of the C1–H4 and the final state. Because ethylene was observed in the final position at 973 K, the MD calculation for Intermediate 2 led to the formation of C2H4, with the release of one hydrogen (H5) at this reaction temperature. This result is consistent with the very small change in the activation energy of Intermediate 2 to Intermediate 3 leading to the formation of ethylene, as well as the experimental results reported for unsupported molybdenum carbides [30,32]. Thus, ethylene was produced through C2H5, which was produced by the combination of two methane molecules: C2 H5 ads þ 3Hads ! C2 H4 þ 4Hads : The MD trajectory showed that the C1–H4 bond length ˚ at a maximum at 28.2 fs and was about to extended to 1.77 A ˚ in the dissociate but was shortened to convergence at 1.12 A final stage (240 fs). The structure of C2H5 was distorted during the desorption process. Furthermore, the bond length of C2–H5 was monotonously enlarged and H5 was dissociated from C2. Finally, ethylene was formed. On the other hand, the MD calculations were used to study the changes in the structure of the Intermediate 2 state and the subsequent products by the addition of two hydrogen molecules, to confirm the experimental results [11,12] obtained for the formation of ethane on b-Mo2C (0 0 1) under higher coverage of hydrogens. The changes in the bond lengths of C1–H4 and C2–H5 with the structure at the longest distances of the C1–H4 and C2–H5 bonds and at the final state are shown in Fig. 5. This figure
shows that the C2H5 species was desorbed together with one hydrogen molecule. The other added hydrogen molecule was dissociated at the final position: C2 H5 ads þ 3Hads þ 2H2 ! C2 H5 þ H2 þ 5Hads : In the MD trajectory, the bond length of C1–H4 was ˚ at 6.96 fs in the presence of four extended maximally to 1.31 A added hydrogens, although this bond length was less than ˚ without the addition of hydrogens. This result shows that 1.77 A the addition of hydrogens depresses the extension of the C1–H4 bond length, probably due to the repulsion of hydrogens. Furthermore, the addition of hydrogens molecules did not increase the bond length of C2–H5 so much, and the new ˚ , and it did not cause maximum value is only 1.37 A dissociation. This result indicates that the addition depresses the dissociation of the C2–H5 bond. Next, the structures at 0 fs ˚ from the and 29.64 fs (with the C1 atom approximately 2 A surface) during the MD run were used to determine the formation of ethane. The optimized structure of the restricted ˚ is shown in approaching H9 to C1 as a distance of almost 1 A Fig. 6. At the beginning of the MD run, C1 was approximately ˚ from the surface. Optimization did not produce the 1.74 A formation of ethane after the approach of one hydrogen (H9) to C1 but induced the adsorption of the CH3–CH species with the liberation of the hydrogen (H2) that had been bonded to C1. On the other hand, the optimization produced the formation of ethane after the approach of one hydrogen (H9) to the C1 atom ˚ from the surface. From this result, it appears approximately 2 A
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Fig. 5. The bond lengths of C1–H4 and C2–H5 and energy (1 Ha = 27.2 eV) obtained by starting from the configuration of Intermediate 2 (C2H5 and three hydrogen’s and adding two H2, 0 step) to that in the final state through the structures at the longest bond lengths of C1–H4 and C2–H5. The energy was plotted relative to the energy at 0 fs. Some b-Mo2C (0 0 1) slab atoms were extended for the purpose of visualization.
that the formation of ethane on the surface is difficult to achieve, but it reacts with one hydrogen after desorption away from the surface. Thus, the forced approach of hydrogen to the C1 atom produced ethane, whereas the presence of excess
hydrogens led to the formation of ethane. This formation of ethane was promoted in the presence of excess hydrogen in the reactants and the formation of ethylene with decreasing hydrogen in the experimental results, as already mentioned.
˚ , using the structures of Intermediate 2 with added Fig. 6. Optimized structures for the restricted single hydrogen (H9) approach of C1 at approximately 1 A hydrogen’s at 0 fs and 29.64 fs during the MD run.
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Consequently, the first principle MD calculation supports the formation of ethylene from the C2H5 species in the absence of hydrogen and the formation of ethane in the presence of a large amount of desorbed hydrogen. A supply of energy, e.g., from hydrogen pressure and heat, would probably promote the formation of ethane.
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4. Conclusion The adsorption of methane onto a b-Mo2C (0 0 1) slab using four different models for the adsorption of methane and the formation of ethylene and ethane were studied on the basis of DFT calculations. Two of the four methane-adsorbed models appear to be stable: model (a) CH4 placed over the three-fold site of the surface Mo atoms without the underlying carbon atoms of the second layer, and model (c) CH4 placed over the two-fold site position of the surface Mo atoms before optimization. The energy of methane adsorption at the model (c) position site (289 kJ/ mol) was the same value as that at the model (a) position. Thus, methane is dissociatively adsorbed to form CH3 and H, and subsequently is stabilized on the three-fold Mo site without the underlying carbon atom of the second layer of the b-Mo2C (0 0 1) slab. After the optimization of the two methanes, each methane was dissociated into CH3 and H in the same adsorption structure of methane. The optimization of the dissociative adsorption of two methane molecules on the b-Mo2C (0 0 1) slab and one CH3 approaching the another CH3 resulted in the formation of H and C2H5, with the subsequent formation of C2H4. In the reaction scheme, the step from Intermediate 2 contained two elemental reactions: (1) the formation of ethane through the release of one hydrogen molecule, and (2) the formation of ethylene through the release of two hydrogen molecules. The first principle MD calculation predicted the production of ethylene and ethane from Intermediate 2 with and without the addition of hydrogens. Ethylene is produced through the C2H5 species by the combination of two methanes, while ethane is formed by the close approach of hydrogen to the C2H5 species that had been desorbed away from the surface in the presence of excess hydrogens. Acknowledgement The authors thank the Scientific Project program of NEDO for partial financial support.
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