Theoretical Insights on the C2Hy Formation Mechanism During CH4

Article pubs.acs.org/JPCC

Theoretical Insights on the C2Hy Formation Mechanism During CH4 Dissociation on Cu(100) Surface Kai Li,†,‡ Chaozheng He,†,§,∥ Menggai Jiao,‡ Ying Wang,*,‡ Jingyao Liu,*,§ and Zhijian Wu*,‡ ‡

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China § State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China ∥ College of Physics and Electronic Engineering, Nanyang Normal University, Nanyang 473061, People’s Republic of China S Supporting Information *

ABSTRACT: The possible C2Hy (y = 2−6) formation reactions (CHx + CHz → C2Hy (y = x + z)) and activated second-order CHx+1 + CHz−1 reactions (CHx + CHz → CHx+1 + CHz−1) during CH4 dissociation on Cu(100) surface have been investigated by using the density functional theory. Our results show that C2Hy (y = 2, 4) formation reactions are favorable both kinetically and thermodynamically, compared with the direct dehydrogenation of CH4 (CHx → CHx−1 + H) and second-order CHx+1 + CHz−1 reactions. The second-order CHx+1 + CHz−1 reactions are less competitive compared with the direct dehydrogenation of CHx. Both DFT calculations and microkinetic model demonstrate that the reaction CH + CH → C2H2 is a major channel to produce C2Hy at a temperature of 860 °C, followed by CH3 + CH → C2H4. When the H2 influence is introduced, the major intermediate changes from CH to CH3 on Cu(100) surface with the increase of H2 partial pressure, while the coverage difference between CH and CH3 is not significant. This means that both species will have a large influence on the graphene growth mechanism. dimer (C2Hy), carbon rectangles (C22H22), zigzag-like (size ∼ 0.63 nm) and armchair-like chains (size ∼ 0.71 nm).17 On the theoretical side, the mechanisms of CHx formation by CH4 direct dehydrogenation on Cu surface have been investigated.18−20 It was found that CH4 dissociation (CH4 → C) on Cu(100) and Cu(111) surfaces were successive endothermic reactions with the overall energy barriers of 3.40 and 4.09 eV, respectively.18,19 Although the energy barrier was lowered to 2.22 eV on Ni adsorbed Cu(100) surface, the CH4 dissociation was still a successive endothermic reaction.20 Therefore, the atomic C produced by CH4 direct dehydrogenation on Cu surface was unfavorable thermodynamically, while the active species for graphene growth should mainly be CHx (x ≠ 0).18 This would suggest that the large carbon clusters could probably be formed from CHx species rather than atomic C.21 Thus, the formation of C2Hy by CHx is important for the graphene growth. However, studies are rare about the mechanism of C2Hy formation on Cu surface. It is known that the hydrocarbon molecule dissociation, the diffusion of C atoms or clusters on the catalyst surface, the formation of the nucleation graphene islands, and the growth of graphene islands are the main four steps for graphene CVD growth.22−28 It is obvious that the first dissociation process is the precondition for the graphene growth and the decom-

1. INTRODUTION Graphene has recently attracted intense research interest because of its remarkable physical and chemical properties.1−3 For its applications, a scalable synthesis route of high-quality graphene is the prerequisite. Since Cu has negligible carbon solubility4 and restricts the growth processes on its surface, it can produce a high yield of monolayer graphene compared with precipitation of graphitic carbon from supersaturated metals.5 Therefore, methane catalytic cracking on Cu through chemical vapor deposition (CVD) represents a promising and scalable approach to achieve a reasonably high quality graphene for device applications.6−13 It is widely accepted that CH4 offers C adatoms for graphene growth, while many experiments found that besides C adatoms, some small carbon clusters, such as C2, C5, and so on, were also the important intermediates for graphene growth. Therefore, further understanding and controlling the intermediates are crucial to improve the quality of graphene. Up to now, many experimental studies were carried out to investigate the influence of the intermediates on graphene growth.14−17 It was found that on Ru(0001) and Ir(111) surfaces, the graphene can be grown by adding rare clusters of C5 rather than adding C adatoms.14,15 For Cu, similar results were obtained. C5 cluster can be stably attached to the zigzag edge of θG/Cu =0 (θG/Cu is the angle between the edge and the Cu chain), which was an important step in the growth of sizable graphene islands.16 Recent research further found more carbon cluster species on Cu surface during the graphene growth process, such as C © 2014 American Chemical Society

Received: April 27, 2014 Revised: July 20, 2014 Published: July 21, 2014 17662

dx.doi.org/10.1021/jp5041025 | J. Phys. Chem. C 2014, 118, 17662−17669

The Journal of Physical Chemistry C

Article

position products directly determine further possible graphene growth mechanisms. Since the formation of the C2 (C2Hy) is the beginning of the large cluster and graphene growth, it is important to understand how C2 is formed. Therefore, in order to further understand how C2 is formed during the CH4 dissociation on Cu surface, a fundamental insight into the mechanism of C2Hy formation is necessary. Motivated by this idea, a systematic study on the possible reaction mechanism for C2Hy formation on Cu(100) surface has been conducted in this work by employing the density functional theory (DFT). Meanwhile, the activated second-order CH x+1 + CH z−1 reactions based on the experiment29 are also considered in this study.

2. COMPUTATIONAL DETAILS 2.1. Method. The calculations were performed using Vienna ab initio simulation package (VASP).30−33 The interactions between the valence electrons and ion cores were treated by Blöchl’s all-electron-like projector augmented wave (PAW) method.34,35 The exchange-correlation functional was the generalized gradient approximation with the functional PW91.36 The wave functions at each k-point were expanded with a plane wave basis set and a cutoff energy of 400 eV. The electron occupancies were determined according to Fermi scheme with an energy smearing of 0.1 eV. Brillouin zone integration was approximated by a sum over special selected kpoints using the Monkhorst−Pack method37 and they were set to 4 × 4 × 1. Geometries were optimized until the energy was converged to 1.0 × 10−4 eV/atom and the force was converged to 0.05 eV/Å. Because of the existence of magnetic atoms, spin polarization was considered in all calculations. The transition states (TS) and the reaction pathways were located using the climbing image nudged elastic band (CINEB) method.38 The minimum energy pathway was optimized using a force-based conjugate-gradient method33 until the maximum force is less than 0.05 eV/Å. The harmonic vibrational frequency calculations were performed to characterize the nature of all the stationary points and to obtain zero point energy (ZPE) corrections. The transition states were confirmed by having only one imaginary frequency. The ZPE corrected energies were used for constructing the potential energy profiles and carrying out microkinetic analysis. The rate constants for elementary steps were calculated using the harmonic transition state theory.39,40 The calculated vibrational frequencies for the stationary points (Tables S1 and S2) and pre-exponential values (Table S3) were shown in Supporting Information. 2.2. Model. Both Cu(100) and Cu(111) surfaces are studied in graphene growth in experiments,11,12 in which the graphene growth on Cu(100) is at a low CH4 pressure.13 The energy profiles of the dehydrogenation process of methane on Cu(100) and Cu(111) surfaces are very similar, and the total endothermal energy for methane dehydrogenation on Cu(100) surface is smaller than that on Cu(111) surface.18 Therefore, the Cu(100) surface is selected in this work. The surface is obtained by cutting a fcc Cu along [100] direction (the structure is shown in Figure 1) and a three-layer slab is chosen. The atoms in the bottom are fixed in their bulk positions, while the other two layers are allowed to relax. A vacuum layer as large as 12 Å is used along the direction normal to the surface to avoid periodic interactions. A 3 × 3 supercell is employed. The chemisorption energy of CHx (x = 1−4) and C2Hy (y = 2−6) on Cu(100) surface, Eads, is defined as follows

Figure 1. Top view of the structures Cu(100) and possible adsorption sites on the surface. BR indicates bridge site, HO for hollow site, and T for top site.

Eads = Eadsorbates/slab − (Eslab + Eadsorbates)

where Eadsorbates/slab is the total energy of the adsorbate adsorbed on Cu(100) surface, Eslab is the total energy of the isolated slab, and Eadsorbates is the total energy of the isolated adsorbate. The first two terms are calculated with the same parameters. The third term is calculated by setting the isolated adsorbate in a box of 12 × 12 × 12 Å3. The negative Eads indicates the exothermic chemisorption, and positive values suggest the endothermic chemisorption. In addition, to investigate the interaction between CHx species coadsorbed on the surface, we define the Hc as follows, Hc = (E(adsorbate1 + adsorbate2)/slab + Eslab) − (Eadsorbate1/slab + Eadsorbate2/slab)

where Eadsorbate1(or2)/slab is the total energy of the first (or second) adsorbate on Cu(100) surface. Eslab is the total energy of the isolated slab. The negative Hc indicates that the interaction between the absorbate 1 and absorbate 2 is attractive, while the positive one indicates a repulsive interaction.

3. RESULTS AND DISCUSSION 3.1. Adsorption of CHx (x = 0−4) and C2Hy(y = 2−6) on Cu(100) Surface. As shown in Figure 1, there are three possible adsorption sites, that is, bridge (BR), hollow (HO), and top (T). For CHx, the adsorption structures and energies are shown in Figure 2. The results indicate that CH4 is a weak physisorption due to the very small adsorption energy (−0.14 eV, Figure 2a). For CHx (x = 1−3), the adsorptions are all exothermic chemisorptions (Figure 2b−f). The most stable adsorption site is HO for CH2 and CH, while it is BR for CH3. This is in agreement with the previous studies.18,20 The adsorption energy of CHx (x = 1−3) reduces from −5.50 to −1.45 eV from CH to CH3. For C2Hy (y = 2−6), only one stable structure is found for C2H6, C2H3, and C2H2 (Figure 3a, h, and i, respectively). For C2H5, two stable structures are located (Figure 3b,c), in which the adsorption at BR site (−1.13 eV) is slightly favored compared with that at T site (−0.99 eV). For C2H4, there are 17663



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Figure 4. Reaction profiles of CH4 dissociation on Cu(100). The black and red lines denote the data in this work and ref 20, respectively. Figure 2. Possible configurations for CHx (x = 1−4) adsorption on Cu(100) surface. Eads denotes the adsorption energy. The gray and white balls denote C and H atoms, respectively.

barriers. Thus, the formation of C on Cu(100) is unfavorable both thermodynamically and kinetically. 3.3. C2Hy Formation Reaction and the Activated Second-order CHx+1 + CHz−1 Reaction. According to the speculations in the previous experimental study,29 there are two possible reactions between CH x species, that is, C 2 H y formation, reaction 1, and the activated second-order CHx−1 + CHz−1, reaction 2, as follows: CHx + CHz → C2Hy

(y = x + z )

CHx + CHz → CHx + 1 + CHz − 1

(1) (2)

Since the previous theoretical studies on single wall carbon nanotubes (SWCNTs) CVD growth showed that the H atom would transfer from CHx with less H atoms to CHz with more H atoms,41 the condition of x > z in the activated second-order CHx+1 + CHz−1 reaction 2 are considered in this work. In addition, the case of x = z is also examined. 3.3.1. Reactions between CH3 and CHx (x = 1−3). CH3 + CH3 → C2H6 and CH3 + CH3 → CH4 + CH2 (Figure 5a). For the two CH3 species coadsorbed at the nearest neighboring BR sites, the Hc is calculated to be 0.25 eV, suggesting that they are repulsive. From Figure 5a, it is seen that for TSf1, the C−C distance is 2.170 Å. The energy barrier is calculated to be 1.56 eV and the reaction is exothermic by 1.03 eV. For TSso1, a H atom is transferred from one CH3 to the other. The energy barrier is 1.68 eV with exothermic of 0.18 eV. Compared with the direct dehydrogenation of CH3 (CH3 → CH2 + H, Figure 4, with an energy barrier of 1.18 eV and endothermic process), C2H6 formation and the second-order reaction are favorable thermodynamically due to their exothermic reaction, in particular for C2H6 formation. However, the higher barriers for C2H6 formation and the second-order reaction indicate that the direct dehydrogenation process is favored kinetically. CH3 + CH2 → C2H5 and CH3 + CH2 → CH4 + CH (Figure 5b). The interaction of CH2 with preadsorbed CH3 needs to overcome the repulsion energy of 0.26 eV. In TSf2, the C−C distance is 2.324 Å and the energy barrier is 0.86 eV. For TSso2, a H atom in CH2 moves toward CH3 with the Cf−H bond of 1.562 Å and the energy barrier of 1.22 eV. This means that the formation of C2H5 is favored compared with the second-order reaction. Furthermore, the barrier in C2H5 formation is lower than that CH3 direct dehydrogenation (CH3 → CH2 + H, Figure 4, with an energy barrier of 1.18 eV), indicating that

Figure 3. Possible configurations for C2Hy (y = 2−6) adsorption on Cu(100) surface. Eads denotes the adsorption energy. The gray and white balls denote C and H atoms, respectively. x and xy denote the C−C along x and xy directions, respectively.

two isomers available, that is, CH2 + CH2 and CH3 + CH. For convenience, we shall use C2H4′ to represent the CH3 + CH reaction in the following discussion. Our calculation shows that three stable structures are found for C2H4 (Figure 3d−f), while only one stable structure is located for C2H4′ (Figure 3g, at hollow site). In the following study, the most stable adsorption structures will be set as initial state (IS) and final state (FS) in the subsequent reaction study. 3.2. Direct Dehydrogenation of CH4 (CH4 → C) on Cu(100) Surface. The direct dehydrogenation of CH4 is the beginning of the dissociation reaction. Our study shows that the dehydrogenation of CH2 is a rapid step with the lowest energy barrier (0.63 eV) in the whole process (CH2 → CH + H, Figure 4), which is in agreement with the previous studies (∼0.76 eV).18,20 The rate-determining step is the dehydrogenation of CH4 to CH3 with a barrier of 1.42 eV (CH4 → CH3 + H). From Figure 4, it is seen that CH4 dissociation to C on Cu(100) surface is a successive endothermic process with high 17664



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Figure 5. Potential energy profiles of C2Hy formation (black lines) and the second-order CHx+1 + CHz−1 reactions (red lines) on Cu(100) surface. TSf and TSso denote the transition states of C2Hy formation and the second-order reaction, respectively. CHx···CHz indicates that there is no interaction between the two species. C−C denotes the newly formed C−C bond in the transition state. Cb−H and Cf−H denote the breaking C−H bonds and newly formed C−H bond in the transition state, respectively, when a H atom is transferred from one species to another in the secondorder reaction.

CH2 + CH → C2H3 and CH2 + CH → CH3 + C (Figure 5e). In the transition state of C2H3 formation (TSf5), the C−C bond distance is 2.113 Å. The reaction is exothermic by 0.69 eV with an energy barrier of 0.83 eV. In TSso5, the breaking H atom in CH moves to the CH2 with the Cf−H bond distance of 1.556 Å. The reaction is endothermic by 0.16 eV with an energy barrier of 1.61 eV. Compared with the direct dehydrogenation of CH2 (CH2 → CH + H, Figure 4, with an energy barrier of 0.63 eV and endothermic process), C2H3 formation reaction is favored thermodynamically but not kinetically due to the higher energy barrier. This situation is similar to C2H6 formation reaction. 3.3.3. Reactions between CH and CH. CH + CH → C2H2 and CH + CH → CH2 + C (Figure 5f). In TSf6, the C−C bond distance is 2.036 Å. Similar to the other C2Hy formation reactions, the reaction is an exothermic process (−0.62 eV) with an energy barrier of 0.66 eV. In TSso6, the breaking H atom in CH moves to the other CH with the Cf−H bond distance of 1.471 Å. Contrary to the other second-order reactions, the reaction CH + CH → CH2 + C is endothermic (1.12 eV), with a significantly higher energy barrier (1.72 eV), which is also higher than that in the direct dehydrogenation of CH (CH → C + H, Figure 4, with an energy barrier of 1.35 eV). Thus, this second-order reaction is unlikely to occur, while C2H2 formation is favorite, in agreement with the results from both the previous experiment13 and theoretical studies.18 From the discussion above, it is seen that the C2Hy (y = 2−6) formation is a favorite compared with the second-order reactions. Compared with the direct dehydrogenation reactions, the C2H4 (including C2H4′) and C2H2 formation reactions have obvious advantages both kinetically (lower energy barriers) and thermodynamically (large exothermic process). Thus, C2H4 and

CH3 prefers to react with CH2 to form C2H5 rather than forming CH2 by the direct dehydrogenation. However, the dehydrogenation barrier of CH2 (CH2 → CH + H, Figure 4, with an energy barrier of 0.63 eV) is lower than C2H5 formation (0.86 eV), suggesting that CH2 direct dehydrogenation process is favored kinetically. CH3 + CH → C2H4′ and CH3 + CH → CH4 + C (Figure 5c). For C2H4′ formation, the C−C distance is 2.191 Å in TSf3. The reaction is exothermic by 0.25 eV with an energy barrier of 0.80 eV. In TSso3, the breaking H atom in CH moves to the CH3 with an energy barrier of 1.39 eV, and the reaction is exothermic by 0.23 eV. Comparing with the direct dehydrogenation barrier of CH3 (CH3 → CH2 + H, Figure 4, with an energy barrier of 1.18 eV) and CH (CH → C + H, Figure 4, with an energy barrier of 1.35 eV), C2H4′ formation is favored. 3.3.2. Reactions between CH2 and CHx (x = 1 and 2). CH2 + CH2 → C2H4 and CH2 + CH2 → CH3 + CH (Figure 5d). For the two CH2 species, the Hc value of 0.45 eV suggests that they are repulsive, and the repulsion is the largest among the studied reactions in Figure 5. In TSf4, the C−C bond distance is 2.500 Å, the longest among the transition states in the studied C2Hy formation reactions (Figure 5). The energy barrier is 0.61 eV with an exothermic energy of 0.95 eV. In TSso4, the breaking H atom in CH2 moves to the other CH2 with the Cf−H bond distance of 1.585 Å. This hydrogen transfer reaction is less competitive than the C2H4 formation reaction due to the high energy barrier (1.00 eV) and less exothermic process (−0.34 eV). Compared with the direct dehydrogenation of CH2 (CH2 → CH + H, Figure 4, with an energy barrier of 0.63 eV and endothermic process), C2H4 formation has almost the same barrier (0.61 eV), but with a large exothermic process (−0.95 eV). This would imply that C2H4 formation is favored. 17665



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Table 1. Calculated Rate Constants (s−1) at 860 °C on Cu(100) Surfacea R1 R2 R3 R4 R5 R6

reaction

rate equation

k (forward)

k−1 (reverse)

CH4 + * ↔ CH4* H2 + 2* ↔ 2H* CH4* + * ↔ CH3* + H*

k3θCH4θ* − k−1 3 θCH3θH

6.91 × 105

1.16 × 1010

CH3* + * ↔ CH2* + H*

k4θCH3θ* −

k−1 4 θCH2θH

1.00 × 10

8

5.35 × 1011

CH2* + * ↔ CH* + H*

k5θCH2θ* −

k−1 5 θCHθH

2.11 × 10

9

3.54 × 1010

CH* + * ↔ C* + H*

k6θCHθ* − k−1 6 θCθH

3.61 × 107

2.46 × 1010

−1

The k and k denote the forward and reverse rate constants in the direct dehydrogenation reaction. The θ denotes the coverage of the adsorbed species on the Cu(100) surface. The * denotes the free Cu surface sites.

a

Table 2. Calculated Rate Constants k (s−1) of C2Hy Formation and Activated Second-Order CHx+1 + CHz−1 Reactions at 860 °C on Cu(100) Surfacea

a

reaction

rate equation

k

reaction

rate equation

k

R7

CH3 + CH3 → C2H6

k7θCH3θCH3

1.70 × 105

R13

CH3 + CH3 → CH2 + CH4

k13θCH3θCH3

3.93 × 105

R8

CH3 + CH2 → C2H5

k8θCH3θCH2

5.68 × 107

R14

CH3 + CH2 → CH + CH4

k14θCH3θCH2

6.67 × 106

R9

CH3 + CH → C2H4′

k9θCH3θCH

5.09 × 108

R15

CH3 + CH → C + CH4

k15θCH3θCH

2.05 × 107

R10

CH2 + CH2 → C2H4

k10θCH2θCH2

3.88 × 10

9

R16

CH2 + CH2 → CH + CH3

k16θCH2θCH2

2.02 × 108

R11

CH2 + CH → C2H3

k11θCH2θCH

1.10 × 10

9

R17

CH2 + CH → C + CH3

k17θCH2θCH

9.93 × 105

R12

CH + CH → C2H2

k12θCHθCH

1.16 × 10

10

R18

CH + CH → C + CH2

k18θCHθCH

2.11 × 106

The θ denotes the coverage of the adsorbed species on the Cu(100) surface.

direct dehydrogenation, the reverse rate constants are much larger than the corresponding forward rate constants. This indicated that hydrogenation is easier compared with dehydrogenation, consistent with the fact that CH4 direct dehydrogenation is an endothermic process. On the other hand, the forward rate constants of direct dehydrogenation (R4−6 in Table 1) are larger than the corresponding secondorder reactions (R13−18 in Table 2). For instance, for reactions CH3 + CHx → CHx−1 + CH4 (x = 1−3, R13−R15 in Table 2), the largest rate constant is 2.05 × 107 (R15), while the smallest one in direct dehydrogenation (R4−R6 in Table 1) is 3.61 × 107 (R6). For other reactions, the difference is even about 1−3 orders of magnitude larger. This conclusion is in agreement with the DFT calculations. For C2Hy formation reaction, however, we noticed that the reaction of CH3 + CH → C2H4′ shows a large rate constant (5.09 × 108, R9) compared with the direct dehydrogenation of CH3 (1.0 × 108, R4) and CH (3.61 × 107, R6). A similar trend is observed for reactions of CH2 + CH2 → C2H4 (R10) and CH + CH → C2H2 (R12), in particular, for the later (1.16 × 1010, R12) in which the rate constant is about 2 orders of magnitude larger than the direct dehydrogenation of CH (3.61 × 107, R6). These results are in agreement with the lower energy barriers in C2H4 and C2H2 formation reactions compared with the corresponding direct dehydrogenations from the above DFT calculations. This result indicates that C2H2 should be the most important intermediate on the Cu(100) surface in the first step of graphene growth (carbon source from CH4 dissociation), which is in agreement with the previous theoretical study.18 The previous studies45,46 showed that different H2 flux will change the major intermediate from CH to CH3 in the dissociation process of CH4 on the Cu surface. Therefore, the influence of H2 on the rates of C2Hy (y = 2−6) formation are also investigated here. Since the large endothermic CH4 direct dehydrogenation into atomic C has a very high energy barrier (Figure 4), it can be assumed that C by the dehydrogenation is hardly formed, that is, the coverage (θC) of the free C atom is zero. The estimated coverage of CHx under different H2/CH4

C2H2 are more likely to be the major intermediates during CH4 dissociation reaction on Cu(100) surface. The previous theoretical studies indicated that C2H was the key growth species for carbon nanotubes and graphene.41,42 Therefore, the formation of C2H is also studied by considering the following two reactions: CH + C → C2H and C2H2 → C2H + H. The results are given in Figure S1 of Supporting Information. Our calculations indicated that for both reactions, the reaction barriers of CH + C → C2H (0.96 eV) and C2H2 → C2H + H (1.20 eV) are higher than that of C2H2 formation (0.66 eV; Figure 5f). Furthermore, the atomic carbon is difficult to form from the direct CH4 dehydrogenation due to the large endothermic and high energy barrier of CH4 → C (Figure 4). In this sense, the density of atomic carbon would be low and the reaction to produce C2H, that is, C + CH → C2H will be minor. The above discussion indicates that C2H is not the major intermediate on Cu(100) surface. Two possible reasons would lead to the different observations (i.e., C2H in ref 41 and C2H2 and C2H4 in this work): (1) the carbon source is different from the previous study, that is, C2H2 in ref 41, while CH4 in this work; (2) the catalyst is different, that is, Fe in ref 41, while Cu in this work. On the other hand, it is noted that the reactions between CHx species depend highly on the coverage of species. Low coverage of CHx might reduce the possibility of reactions. Thus, in the following section, the reaction rate based on the coverage of CHx will be discussed. 3.4. Microkinetic Model. To further investigate the reaction mechanism, we performed a microkinetic analysis43,44 based on the DFT studies. The description of the microkinetic model and related references are described in Supporting Information. The reaction rate of each elementary reaction is estimated under the experimental temperature of 860 °C.13 The partial pressure of CH4 is fixed at the maximum level of 0.95 Pa in the experimental condition.13 The rate constants of the elementary steps involved in the direct dehydrogenation reaction of CH4, C2Hy formation and the second-order reaction are listed in Tables 1 and 2, respectively. It is seen that for 17666



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Table 3. Coverage of CHx (x = 1−4) (/m2) under Different H2/CH4 Ratios on the Cu(100) Surface at 860 °C H2/CH4 ratio 0.0 CH4 CH3 CH2 CH

1.2 8.2 3.7 2.3

× × × ×

0.005 10

10 107 106 108

1.1 8.5 5.0 2.1

× × × ×

0.05 10

10 107 106 108

1.1 1.0 6.4 1.7

× × × ×

1.0 10

10 108 106 108

5.8 6.6 2.9 3.7

× × × ×

18.0 9

10 107 106 107

6.1 6.1 2.1 2.1

× × × ×

186.0 8

10 106 105 106

6.1 6.1 2.0 2.0

× × × ×

107 105 104 105

calculations. The second reaction is C2H4′ formation, which contributes less than 0.2 to the total rate of C2Hy formation at broad H2/CH4 ratios. In addition to the experimental temperature at 860 °C, the dependence of the branching rate as a function of H2/CH4 ratio at 600 and 1000 °C is also studied. The results are given in Figures S2 and S3 of Supporting Information. It is seen that, at 600 °C, C2H4′ is the major species at high H2 partial pressure (H2/CH4 ≥ 1), followed by C2H2 (Figure S2 in Supporting Information), while at 1000 °C, the results are similar to those at 860 °C, that is, C2H2 is the major species at high H2 partial pressure (H2/CH4 ≥ 1), followed by C2H4′ (Figure 6 and Figure S3 in Supporting Information). This suggests that, on the Cu(100) surface, although C2H2 and C2H4′ are still the two major species at the studied temperatures, the branching rate is different.

ratios is shown in Table 3. For H2/CH4 = 0, the coverage of CH3 and CH is about 1 order of magnitude larger than that of CH2, indicating that CH and CH3 are the major intermediates, with CH slightly favored during the CH4 dissociation. As the H2 partial pressure increases, the coverage of CH and CH3 decreases. From H2/CH4 = 1.0, the coverage of CH3 is larger than that of CH, and the difference becomes large with the increase of H2/CH4 ratios. This suggests that CH3 becomes the major intermediate at high H2 partial pressure, while the coverage difference between CH and CH3 is not significant (only three times larger than CH at H2/CH4 = 186.0, Table 3). Recalling that in our previous study on the Cu(111) surface45 the major intermediate is CH at H2/CH4 = 0 (it is about 3 orders of magnitude larger than CH3), but it changes to CH3 once H2 is introduced, even at a much smaller H2/CH4 ratio of 0.005 (it is about 2 orders of magnitude larger than CH). This means that the partial pressure of H2 has less influence on the major intermediate on Cu(100) than on the Cu(111) surface during CH4 dissociation, while Cu(111) is more sensitive to H2 partial pressure than the Cu(100) surface during the methane decomposition. This suggests that the graphene growth process might be different on Cu(111) (with dramatic change of the coverage by different H2/CH4 ratio) and Cu(100) (with a relatively slow change of the coverage by different H2/CH4 ratios) due to the different major intermediates. Based on the CHx coverage on Cu(100) at different H2/CH4 (Table 3) ratios, the possible C2Hy (y = 6−2) formation reaction rates at 860 °C are calculated. The dependence of the branching rate as a function of H2/CH4 ratio is shown in Figure 6. It can be seen that, although the increased H2 pressure changes the major intermediate during the CH4 dissociation, the C2H2 formation reaction is the most important channel in all the C2Hy formation reactions, supporting again our DFT

4. CONCLUSIONS C2Hy (y = 2−6) formation reactions (CHx + CHz → C2Hy (y = x + z)) have been found to play a dominant role in CH4 dissociation on the Cu(100) surface, while the activated second-order reactions are less competitive. This is supported by both DFT calculations and microkinetic model analysis. For experimental temperature (860 °C), C2H2 is the major intermediate in C2Hy formation reactions, followed by C2H4. The influence of H2 on the rates of C2Hy formation reactions indicates that the major intermediate changes from CH to CH3 on Cu(100) surface with the increase of H2 partial pressure, but the coverage difference between CH and CH3 is not significant. This means that both species will have a large influence on the reaction mechanism at 860 °C. This is different from the results on the Cu(111) surface in which only CH3 plays an important role in the reaction mechanism when H2 is introduced, in particular, at high H2 partial pressure. This means that the CHx polymerization process might be different between Cu(100) and Cu(111) surfaces, and possibly affect the further graphene growth process.

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ASSOCIATED CONTENT

S Supporting Information *

First, the brief description of the Microkinetic Model and relevant references are presented. After that, Figure S1 shows the potential energy profiles of C2H formation. Figures S2 and S3 show the branching rate of C2Hy(y = 2−6) formation on Cu(100) surface at 600 and 1000 °C, respectively. Tables S1 and S2 give the vibrational frequencies of stable states and transition states, respectively. Table S3 gives the preexponential values (s−1) for the forward and reverse reactions involved in microkinetic model. Finally, the Cartesian coordinates for the most stable structures are provided. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 6. Branching rate of C2Hy (y = 2−6) formation on Cu(100) surface at 860 °C. 17667



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Phone: +86-431-85682801. Author Contributions †

These authors contributed equally to this work (K.L. and C.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos: 21221061 and 21203174), Jilin Province Youth Fund (Grant No: 20130522141JH), Jilin Province Computing Center (Grant Nos: 20130101179 JC-08 and 20130101179 JC-07), and Nanyan Normal University Science Foundation (No. ZX2014088). The authors also thank the financial support from Department of Science and Technology of Sichuan Province. Part of the computational time is supported by the Performance Computing Center of Jilin University.

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Theoretical Insights on the C2Hy Formation Mechanism During CH4

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