Theoretical Studies on Ethanol Dissociation on Iron Nanoparticles in

Article pubs.acs.org/JPCC

Theoretical Studies on Ethanol Dissociation on Iron Nanoparticles in the Early Stage of SWCNT Growth Ying Wang,*,† Menggai Jiao,†,§ Zhijian Wu,*,† and Stephan Irle‡ †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China ‡ WPI-Institute of Transformative Bio-Molecules and Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan § Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Computational Centre for Molecular Science, Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin 300350, China S Supporting Information *

ABSTRACT: The ethanol dissociation process on an iron cluster was estimated by nonequilibrium quantum chemical molecular dynamics simulations and static potential energy surface calculations based on the density-functional tight-binding potential. The competition among reaction pathways related to C−H, O−H, C−C, C−O cleavage, and H2 formation were studied. The schematic of ethanol as a carbon source to grow singlewalled carbon nanotubes on iron clusters was also predicted. The simulations highlighted the C−O and O−H bond cleavage were more favorable on iron cluster than the other pathways due to the lower barrier and higher exothermicity and C2Hx (x = 4−6) were the major intermediates. The ethanol dissociation pathway on iron catalysts promised the two carbon atoms in ethanol were nearly equivalent and had similar contribution for the further single-walled carbon nanotube growth, consistent with the observation in previous experimental work. first process is the precondition of the SWCNTs growth and the decomposition pathways of carbon sources directly decide the mechanism of the further SWCNTs CVD synthesis. Lots of gaseous carbon sources, such as methane,18 acetylene,19 carbon monoxide,8 benzene,20 etc., were used in the synthesis of CNTs. However, these carbon sources have some disadvantages such as being inflammable, explosive, toxic, and expensive for the synthesis of CNTs. In contrast, ACCVD (alcohol CVD), taking ethanol as a carbon source, has been widely used for scale-up CNT synthesis21−24 owing to the lower reaction temperature, high-purity features, and harmless and easy-handling nature,21 as well as for the chiralitycontrolled synthesis of SWCNTs.17 Distinguished from the hydrocarbon carbon sources, in which only −CHx (x = 0−4) groups were involved, the significant advantage of alcohols is explained evidently by the role of a special −OH group. It turned out that OH radicals not only can efficiently remove the amorphous carbon in the purification process of SWCNTs using H2O225 but also can attack nearby carbon atoms with a dangling bond to form CO21 to remove the seeds of amorphous carbon in its very early stage, prohibiting the generation of an MWCNT. Furthermore, because the alcohol molecules themselves include O or OH, the ACCVD method does not need any oxidants to reactivate the catalysts for the

1. INTRODUCTION Since the discovery by Iijima and Ichihashi,1 carbon nanotubes (CNTs) have been the focus of much scientific interest due to their unique physical and chemical properties, as well as their potential technological applications, such as hydrogen storage,2 electronic devices,3 and chemical sensors.4 With this concern, a synthesis technique of single-walled carbon nanotubes (SWCNTs) with a higher quality and at a lower cost is an urgent priority at present. Many efforts have been focused on mass synthesis of CNTs, such as the arc discharge process,5 laser ablation,6 pyrolysis of hydrocarbon,7 and chemical vapor deposition (CVD).8−15 Upon comparison of all these methods, the CVD method has several advantages because it uses transition metal particles as the reaction catalyst. By varying the size and kinds of the metal catalyst, it is possible to obtain the radius- and chirality-controlled CNT synthesis. Many kinds of carbon sources, support materials, and synthesis conditions have been tested so far; unfortunately, the efficient approach has not been set up yet. The mechanism of SWCNT growth on transition metal catalyst particles is conventionally assumed to consist of the following three stages:16 (1) the feedstock gas (carbon sources) decomposed to active carbon species directly on the metal particle; (2) the active carbon dissolved into the metal particle, producing liquid or surface-molten metal carbide until supersaturation; (3) carbon precipitation on the surface of the metal carbide particle and formation of initial cap16,17 as a nucleus, followed by the SWCNT growth. It is obvious that the © 2017 American Chemical Society

Received: December 4, 2016 Revised: January 8, 2017 Published: January 9, 2017 2276

DOI: 10.1021/acs.jpcc.6b12207 J. Phys. Chem. C 2017, 121, 2276−2284

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The Journal of Physical Chemistry C

present DFTB/MD simulations, the self-consistent-charge DFTB (SCC-DFTB)32 method in combination with a finite electronic temperature (Te) approach33,34 with Te = 10000 K was employed to evaluate the quantum chemical potential on the fly. The electronic temperature allows the occupancy of each molecular orbital to change smoothly from 2 to 0 depending on its energy and effectively incorporates the openshell nature of the system due to near-degeneracy of iron d orbitals and carbon dangling bonds. This method of SCCDFTB with Te has been successfully applied to SWCNT growth and graphene growth.35−39 In the molecular dynamics simulations the velocity Verlet integrator40 was used with a short time step of 0.5 fs. The nuclear temperature (Tn = 1073 K) was controlled by connecting the Nose−Hoover chain thermostat41 to the degrees of freedom of the present model system. Periodic boundary conditions (PBC) were imposed with a cubic box size of 20 Å.3 In our MD simulations, first we cut out Fe13, Fe38, and Fe55 structures from a face-centered cubic (fcc) crystal structure, which is in analogy with γ-phase iron, a stable phase in bulk iron between 1184 and 1665 K; then we equilibrated this fcc Fe13, Fe38, and Fe55 icosahedral magic number clusters at 1073 K for 10 ps. Finally, ten structures were randomly chosen from the equilibration simulations between 5 and 10 ps. The total energy plot as a function of equilibration time and the ten Fe13, Fe38, and Fe55 structures randomly chosen from 5 to 10 ps equilibration were shown in Figure S1−S3. On the basis of these 30 initial structures, we shot a single C2H5OH molecule onto these metal clusters with the kinetic energy of C2H5OH being 1073 K, which is consistent with the nuclear temperature Tn. The minimum distance between the incident molecule C2H5OH and iron atoms is 3 Å. Incident molecules were distributed in the spherical shell with the inner and outer radius being 10.0 and 12.0 Å. The structures after the shooting process were named S for the Fe13 nanoparticle, M for the Fe38 nanoparticle, and L for the Fe55 nanoparticle with a subscript n (n = 1−10). In the following, we annealed the resulting clusters until 350 (or 135), 200, or 175 ps for the systems of Fe13, Fe38, or Fe55 at the same nuclear temperature. 2.2. Potential Energy Curves. The geometries and frequencies of all the stationary points including reactants, transition states (TSs), and products were optimized by connecting the Paderborn DFTB code to Gaussian 03,42 using its “external” functionality. At the same level, the minimum energy path (MEP) was obtained by the intrinsic reaction coordinate (IRC) theory to confirm that the TS really connected to the minimum along the reaction path. For simplified model reactions, PBC was switched off on the Mermin free energy (EM) surface, where EM was estimated as the sum of the total energy and Te × S (S = entropy). It should be noted that the reliability of the “external” DFTB method on the estimation of these potential energy curves had been proved in our previous studies,36 in which the PBE43 barrier heights agreed well with DFTB barriers within 2−4 kcal/mol.

supergrowth process. Contrarily, when the hydrocarbons were used, it was required to involve a small amount of water. Additionally, ethanol is the simplest molecule that contains both C−C and C−O single bonds. The ease of cleavage of these bonds on different metal surfaces determines the relative selectivity of products. Depending on the application, either of these scenarios may be desirable. Several experimental and theoretical studies have been carried out in an attempt to understand these complicated dissociation processes. Tomie et al. revealed that ethylene and acetylene were important products for the synthesis of SWCNTs from the dissociation of ethanol by in situ mass spectroscopic analysis.23 Xiang et al.24 studied the adsorption and dissociation of ethanol on three metals, Fe, Co, and Ni. They concluded that the behavior of C2H5OH on these transition metals was different, although their catalyst abilities were similar for the hydrocarbon, and further noted that metal effects played an important role for determining the SWCNT growth. Theoretically, Mavrikakis et al. modeled the ethanol decomposition on transition metals by combing application of scaling and Brønsted−Evans−Polyani relations.26 Oguri et al. estimated the ethanol dissociation process on nickel.27 They demonstrated that the C−C bonds only in CHxCO fragments were dissociated on the nickel cluster, whereas there was no preferential structure for C−O bond dissociation, which promised unequivalent contribution of carbon atoms, consistent with Xiang et al.’s observation.24 Other groups also showed that on Ni, Rh, and Ir C−C bond breaking was the preferred pathway.28−30 However, Weststrate et al. identified on the Co(0001) surface that ethoxy (CH3CH2O) was more thermodynamically stable than the other candidates and for the first time experimentally observed that C−O bond breaking was favorable and occurred at 350 K;31 after that, C2H2 and CH3C became the most stable C2Hx species. Although much effort has been done on this decomposition process, due to the inherent difficulties involved in the investigation on a short-lifetime intermediate in experiments and theoretical method limitation or high computational cost in simulations, various intermediate species during C2H5OH decomposition remain unclear. Therefore, it is urgent to understand the characteristics, properties, and contribution of these species to SWCNTs growth. In this paper, we have carried out a quantum chemical molecular dynamics (QM/ MD) simulations together with the static potential energy surface estimations on the dissociation pathway of ethanol on iron cluster. The geometries of ethanol and intermediates, along with the transition states, have been optimized and the activation barriers for C−O, O−H, C−H, and C−C cleavage have been calculated, as well as H2 formation. The effect of cluster size on the decomposition of ethanol is also investigated. A schematic of ethanol as carbon source for SWCNT growth is presented. This knowledge of structures and chemisorptions of ethanol on iron cluster will help us to understand the basic aspects of heterogeneous catalysis and other surface phenomena, as well as a deeper understanding on the mechanism of SWCNTs growth in the CVD method.

3. RESULTS AND DISCUSSION 3.1. Early Stage Ethanol Evolution. In the 1 ps ethanol supply simulation, as shown in Figure S1−S3, we found that some C2H5OH molecules either chemisorbed on the bridge position or “stuck” on the top position as a side-on complex, with the O−Fe bond length of 2.74 or 2.10 Å. With the chemisorption, the C−O bond in ethanol molecule was formally elongated from 1.43 (in vacuum) to 1.45 or 1.46 Å,

2. COMPUTATIONAL METHOD 2.1. QM/MD Simulation. The DFTB/MD (density functional tight binding molecular dynamics) approach is a Born−Oppenheimer molecular dynamics technique based on the DFTB electronic structure method and can bridge the gap between classical and the first principle MD simulations. In the 2277


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The Journal of Physical Chemistry C indicating the iron catalysts actived the C−O bonds. On the contrary, we also found some of ethanol molecules were reflected by the iron clusters because ethanol is a saturated molecule. This was in sharp contrast to acetylene as a carbon source, in which the triple bonds would change to double or single bonds by an addition reaction. Similarly to acetylene, concerning shooting of C2, the chemisorption was also nearly 100%, as the C2 molecule was a diradical with particularly high activity and could react with other carbon species (such as C2, C3, C4, and so on) or the metal particles.44 At the following annealing stages the adsorbed C2H5OH molecules were found to “walk” randomly on the iron surface, or to be reflected continually by the surface of the iron cluster, or undergo chemical dissociation reactions. To observe the effect of iron cluster size on the ethanol decomposition, Fe13, Fe38, and Fe55 were chosen as representative examples. The last snapshots after 135−350 ps annealing simulations were shown in Figure 1. Figure 1a showed that on small Fe13 nanoparticles both O−H and C−O bonds, as well as C−H bonds, in ethanol molecules dissociated, and subsequently various reaction products such as C2H5, C2H4, C2H6, OH, O, and H were created on iron catalyst surface. However, on the larger clusters of Fe38 and Fe55, only C−O broken and O−H broken, producing the radical C2H5, OH, O, and H, were found, as shown in Figure 1b,c. Regardless of cluster size, we found that the C−O bond broken was the most favorable process due to frequently occurred (100% in 30 trajectories) in our QM/MD simulations. It was noted that the early stage of ethanol dissociation process on iron was different from that on nickel, where C−C and C−O bond dissociation did not occur as often as C−H bond dissociation.27 These different dissociation processes would further lead to an almost equivalent contributions from two different carbon atoms on iron, whereas a no. 2 carbon (far from OH group) was more preferentially incorporated into SWCNT growth than a no. 1 carbon (connects to the OH group) on nickel. Our ethanol dissociation process on iron, producing equivalent carbon, was in good agreement with Xiang’s experimental observation.24 Below we discussed the formation mechanism of all intermediates involved in this dissociation process in detail. 3.1.1. C2H4 Formation Mechanism. In our QM/MD simulations, only for a small size iron cluster (Fe13) were C− H bond cleavage and C2H4 formation observed, as shown in Figure 1a. This probably resulted from the larger curvature and higher catalytic activity of the small cluster. Figure 2 depicted the formation mechanism of C2H4 occurred in trajectory S1 and S3. It can be seen that two C2H4 intermediates were both formed from the hydrogen transformation reaction, in which the hydrogen transferred to the metal surface from the −C2H5 radical. The difference was for the forming process of the precursor, i.e., the radical C2H5. Two reaction pathways were observed: one was C−O bond first cleavage, such as in trajectory S1, as shown in Figure 2a. We can see that the C−O bond cleavage occurred at 0.22 ps and at the same time the C2H5 radical was produced. This C2H5 radical continually “walked” around the iron cluster until 15.10 ps. And then the C−H bond in −CH3 of the CH3CH2 radical was broken and the H was transferred to the iron cluster, followed by C2H4 formation. Another pathway was O−H first cleavage, such as in trajectory S3, as shown in Figure 2b. The O−H cleavage occurred at 2.68 ps and simultaneously the C2H5O radical was formed. At 2.93 ps the C−O bond in the C2H5O radical was broken, producing a C2H5 radical. Following at 12.17 ps, a H

Figure 1. (a) Last snapshots of 350 (or 135) ps annealing simulations for Fe13. (b) Last snapshots of 350 ps annealing simulations for Fe38. (c) Last snapshots of 175 ps annealing simulations for Fe55.

transfer reaction, similar to that in trajectory S1, was observed and C2H4 was prepared. Because C2H4 possessed double bond property, it could stick stably on the Fe13 metal surface with the side-on structure, as shown in Figure 1a. 3.1.2. C2H6 Formation Mechanism. Two trajectories, S8 and S9, exhibited the C2H6 formation on the Fe13 nanoparticle, as depicted in Figure 1a. Because they featured exactly the same formation mechanism, here, taking S8 as a representative example, we evolved the dissociation process and depicted them in Figure 2c. It can be seen that after C−O cleavage, the separated −OH and −C2H5 radicals randomly walked on the surface of the iron cluster. Also, due to the higher electronegativity of O compared to that for C, the −OH radical sticks more closely to the surface than the −C2H5 radical, such that the Fe−O and Fe−C distances at 0.01 ps were 2.80 and 3.60 Å, respectively. Then, immediately, at 0.06 ps, the H in the −OH group reached the C2H5 radical by continually waving and C2H5 was quickly switched to the C2H6 molecule. This 2278


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Figure 2. (a, b) C2H4 formation mechanism in trajectory S1 and S3. (c) C2H6 formation mechanism in trajectory S7.

Compared to the dissociation energy of ethanol thermal decomposition for producing CH3CH2 + OH (94.8 kcal/ mol45) in a vacuum, the dissociation energy with the iron catalyst indicated a significant increase in the decomposing process of ethanol. Following C−O bond breaking, essentially three types of H transferring reactions were observed in our QM/MD simulations, as noted in eqs 1a, 1b, and 1c; see Figure 4a−c,

saturated C2H6 was reflected by the iron cluster, as shown in Figures 1a and 2c. 3.2. Potential Energy Curves. To clarify the ethanol dissociation process, the static potential energy curves for some typical elementary reactions were investigated. To save the computational time, the small model of Fe13 particle was chosen and single C 2H 5OH molecule adsorption was considered in this study. Four typical elementary reactions were performed, namely, (a) C−O bond cleavage (reaction 1, Figure 3a), (b) O−H bond cleavage (reaction 2, Figure 3b), (c) C−H bond cleavage (including C−H in −CH2 and −CH3 groups, noted as reactions 3a and 3b (Figure 3c,d), and (d) C− C bond cleavage (reaction 4, Figure 3e), as follows, CH3CH 2OH(ads) → CH3CH 2(ads) + OH(ads)

(1)

CH3CH 2OH(ads) → CH3CH 2O(ads) + H(ads)

(2)

CH3CH 2OH(ads) → CH3CHOH(ads) + H(ads)

(3a)

CH3CH 2OH(ads) → CH 2CH 2OH(ads) + H(ads)

(3b)

CH3CH 2OH(ads) → CH3(ads) + CH 2OH(ads)

CH3CH 2(ads) + OH(ads) → C2H6 + O(ads)

(1a)

→ C2H5(ads) + O(ads) + H(ads)

(1b)

→ CH 2CH 2(ads) + OH(ads) + H(ads)

(1c)

Figure 4a depicted the hydrogen disproportionation mechanism leading to C2H6 and O formation, corresponding to eq 1a. We found that in the reactant or TS the H of the −OH group was pointing to the CH3CH2− group and in the TS the broken O−H increased from 0.98 to 1.08 Å. The forward barrier of the hydrogen transfer was 29.71 kcal/mol, and it was a slightly exothermic reaction with a value of −2.77 kcal/mol. Although the barrier was slightly higher, at higher temperature (∼1000 K for SWCNT growth) this reaction was also feasible. And thus, in our QM/MD simulations two trajectories exhibited this process, as discussed in section 3.1.2. With respect to elementary eq 1b (Figure 4b), O−H directly decomposed to atomic H and O with a relative lower barrier of 18.14 kcal/mol, which explained why this reaction was frequently observed in our QM/MD simulations. Furthermore, the atomic oxygens produced by both hydrogen disproportionation reactions were potentially ready for removing the amorphous carbon during the further SWCNTs nucleation and growth processes to prohibit the generation of MWCNT and possibly improve the SWCNT quality or chirality.21,25 As to elementary eq 1c (Figure 4c), in the TS structure, the breaking bond C−H and the forming bond H−Fe were elongated by 75.45% and 56.10% compared to the C−H and H−Fe equilibrium bond length in the reactant and product. The elongation of the forming bond was less than that of the breaking bond, suggesting that the TS was product-like and proceeded via “late” transition states with an endothermic process. This late character in the TS satisfied Hammond’s postulate. The higher barrier (26.10 kcal/mol) and endothermic process (5.57 kcal/mol) promised that the C−H broken was difficult. However, at 1000 K, this process was also likely to occur, and two trajectories (S1 and S3, Figure 1a) showed C2H4 formation in our QM/MD simulations. Compared to the barrier of C2H4 formation in the thermal decomposition of ethanol in a vacuum (the barrier was 66.60 kcal/mol with a slight endothermicity of 6.50 kcal/mol),45 the dramatically decreased barrier indicated that C2H4 formation was more kinetically favorable on an iron catalyst, which was in good

(4)

For each reaction, the initial structures of the transition states (TSs) were abstracted from our QM/MD trajectories. Followed by optimization and vibrational normal-mode analysis, they were identified as the existence of a single imaginary frequency. Starting from TS, on the basis of the intrinsic reaction coordinate (IRC) theory, the reactants and products of these single-step reactions were then located with optimization and confirmed with all real frequencies. Schematic energy profiles and stationary points of the five elementary reactions were shown in Figure 3, as well as their forward barriers, reaction energies, and imaginary frequencies of the TSs. The energies of the reactants were regarded as reference. The negative or positive reaction energies indicated exothermic or endothermic reactions. 3.2.1. C−O Cleavage. For reaction 1, the TS was located at the DFTB level with only one imaginary frequency of 738i, as shown in Figure 3a. In the TS structure, the C−O breaking bond and the C−Fe forming bond were elongated by 21.4% and 59.5% compared to the C−O and C−Fe equilibrium bond lengths in the reactant and product, respectively. The elongation of the forming bond was greater than that of the breaking bond, indicating that this TS was reactant-like; i.e., this reaction was exothermic (−41.28 kcal/mol) and proceeded via an “early” transition state. This early character in the TSs was in keeping with Hammond’s postulate. With respect to the barrier, it possessed a lower barrier of 13.56 kcal/mol with ZPE (zero point energy) correction. Therefore, the C−O cleavage was preferred both kinetically and thermodynamically. This was consistent with the observation in our QM/MD simulations; i.e., 90% trajectories showed that C−O bond cleavage was the first step of C2H5OH dissociation on the iron nanoparticle. 2279


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Figure 3. Reaction energy profiles and optimized geometries of reactants, transition states (TSs), and products for the five key elementary reactions occurring in the first step of ethanol decomposition: (a) C−O cleavage reactions; (b) O−H bond cleavage reactions; (c ,d) C−H bond cleavage reactions (from −CH2 or −CH3 group); (e) C−C bond cleavage reactions. In the case of TS, energies correspond to the forward direction barrier from the reactant, and v indicates the only one imaginary frequency and its value was in (cm−1). Plain values correspond to DFTB total energies, values in parentheses include the ZPE correction at the DFTB level. The unit of energy was kcal/mol.

Figure 4. Reaction energy profiles and optimized geometries of reactants, transition states (TSs), and products at the DFTB level for the elementary reactions occurring in the second step of alcohol decomposition followed by (a−c) C−O cleavage reactions; (d−f) O− H bond cleavage reactions; (g, h) H2 formation reactions. The unit of energy was kcal/mol.

agreement with the mass spectral observation that C2H4 was the major intermediate during ethanol dissociation.23 Fur2280


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3.2.3. C−H and C−C Bonds Cleavage. In the case of the C−H bond cleavage in ethanol dissociation process, two kinds of C−H bond cleavage were observed, from the −CH3 and −CH2 groups. When we compared these two pathways, the barrier heights or reaction energies showed that breaking the C−H of the −CH2 group was easier than breaking that of the −CH3 group, as shown in Figure 3c,d. However, both of them were less popular than C−O and O−H bond cleavage due to the higher barrier and less exothermic, which was consistent with the observation that no C−H bond cleavage occurred in the first step of ethanol dissociation. The further C−H cleavage reactions in CH2CH2O were also investigated, as shown in Figure S4a,b. It showed that C−H cleavage from the −CH2 group was comparable to cleavage from the −CH2O group with higher barriers (20−25 kcal/mol), which were similar to the ones of C−H cleavage from ethanol (Figure 3c,d). Although the former were typical endothermic reactions (13−15 kcal/ mol), the latter were exothermic reactions, which indicated C− H cleavage from CH2CH2O was more difficult thermodynamically and C−H cleavage was the rate-determining process in the ethanol dissociation process. The C−C bond cleavage possessed a higher barrier of 35.40 kcal/mol (with ZPE correction), and it was a slightly exothermic reaction with a value of −6.80 kcal/mol (with ZPE correction). Therefore, it was both thermodynamically and kinetically unfavorable and in our QM/MD simulations we did not find the process of C−C bond cleavage. However, if we compared the dissociation energy of ethanol thermal decomposition (87.5 kcal/mol45) for producing CH3 + CH2OH, we declared that the iron catalyst could significantly promote the C−C decomposition process. 3.2.4. H2 Formation. In the process of SWCNTs synthesis, H2 can be formed either by the dissociated hydrogen atoms combining with each other on the catalyst surface or by the dissociated H abstracting the H in C2HxOy (x = 3, 5 and y = 0, 1) group. Here, two representative pathways were considered, as follows,

thermore, comparing reactions 1b and 1c, i.e., the hydrogen transferring from −OH and CH3CH2− to the iron cluster (Figure 4b,c), one clearly sees that reaction 1b was more energetically favorable with the lower barrier and higher exothermicity, suggesting the −OH group on the iron surface was not stable and easy to dissociate to atomic oxygen and hydrogen. It should be noted that, due to the classical Newtonian treatment of the nuclei in our QM/MD simulations, no effects of quantum tunneling were included. It can be expected that, in reality, tunneling might enhance the rates of hydrogen transferring reaction on the order of several tens of percent. 3.2.2. O−H Cleavage. In our QM/MD simulation one trajectory showed that the O−H bond cleavage was the first step for ethanol decomposition. Similar to C−O cleavage, this reaction also possessed a low barrier (11.67 kcal/mol with zero point energy correction) and with an exothermic energy of −21.47 kcal/mol, as shown in Figure 3b. However, compared with the C−O bond cleavage pathway, although its barrier was slightly lower by 1.89 kcal/mol, the O−H bond cleavage reaction energy was higher than C−O bond cleavage by 19.81 kcal/mol. It indicated that C−O bond breaking was more thermodynamically favorable, and O−H bond breaking was more kinetically favorable. In the CNT growth conditions, the temperature was ∼1000 K, which was high enough to overcome both barriers of these two reactions, so both reactions were observed in our QM/MD simulations. Furthermore, the more released heat can adequately compensate the slightly higher barrier of C−O bond cleavage than O−H bond cleavage. Therefore, the former was more favorable in the C2H5OH dissociation process and in our QM/ MD simulations most of trajectories showed the C−O bond cleavage first. Following O−H bond cleavage, two reaction pathways were feasible, as follows, CH3CH 2O(ads) + H(ads) → CH3CH 2(ads) + O(ads) + H(ads)

CH3CH 2O(ads) → CH 2CH 2O(ads) + H(ads)

(2a) (2b)

(5)

C2HxOy (ads) + H(ads) → C2Hx − 1Oy (ads) + H 2

(6)

The corresponding reactants, transition states, and products involved in these reactions were shown in Figure 4g,h and Figure S4c−e. It was obvious that all H2 formation reactions were endothermic, indicating these reactions were thermodynamically unfavorable. Note that H2 formation by the H abstraction from −OH of CH2CH2OH or CH3CHOH (see Figure 4g and Figure S4c) was kinetically favorable with the lower barrier of 23.76 or 22.07 kcal/mol. However, according to our above discussions, the C−O and O−H bonds had already broken on the iron surface, so this hydrogen abstraction reaction was not observed in our QM/MD simulations. In the case of H abstraction from the −CH2 or −CH3 group in C2HxOy (Figure S4d,e), the barrier height was increased to 38.73 and 53.09 kcal/mol, and these reactions were immensely endothermic reactions with a high value of ∼30 kcal/mol, suggesting these hydrogen abstraction reactions were extremely difficult to occur. Regarding the combination of two adsorbed H atoms on the surface of the iron nanoparticle, the estimated barrier was much higher with a value of 38.54 kcal/mol. Therefore, in our QM/MD simulations H2 formation by this pathway was not observed in the current limited simulation time. As we know, H2 formation was an entropy control

CH3CH 2(ads) + O(ads) + H(ads) → CH 2CH 2(ads) + O(ads) + 2H(ads)

2H(ads) → H 2

(2c)

Figure 4d,e showed the schematic pathways for C−O cleavage (reaction 2a) and C−H (reaction 2b) cleavage. It was clearly seen that CH3CH2O was bound to the nanoparticles through the oxygen atom with the C−O bond perpendicular to the surface and the −CH3 group being away from the surface, similar to the DFT calculations of ethoxy adsorption on Co(0001).31 The forward barrier for C−O bond cleavage was only 9.84 kcal/mol, even lower than C−O bond cleavage directly in reaction 1. In contrast, C−H bond breaking required us to overcome a particular higher barrier of 45.65 kcal/mol, as shown in Figure 4e, indicating the C−H bond cleavage from the CH3CH2O radical was extremely difficult. However, if the C−H bond cleavage was from the CH3CH2− radical (reaction 2c; see Figure 4f), the forward barrier of C−H cleavage was decreased to 25.55 kcal/mol, which was comparable to that for reaction 1c, and C2H4 formation was observed in our QM/MD simulations. 2281


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pathways on iron nanoparticles promised that the two carbons in ethanol provided equivalent contributions for the SWCNTs nucleation and growth, in good agreement with the experimental observation.24 However, the ethanol dissociation pathways were different from that on Co and Ni, in which CHCHO46 and CCOH/CCO27 were the important intermediates and C−O cleavage was not observed. The findings have direct implications on the growth of SWCNTs or other carbon-related materials by the CVD method using ethanol as a carbon source.

process. If the catalyst was large enough and more hydrogen atoms were adsorbed on the surface of nanoparticle (i.e., higher coverage of hydrogen), the H2 formation reaction might occur spontaneously. Here we chose a representative model of Fe38 + 20H, in which a larger iron nanoparticle (Fe38) was used and 20 atomic H were randomly adsorbed on the surface, to investigate the process of H2 formation. The snapshots were shown in Figure 5. In the beginning all the H atoms continually jumped

4. CONCLUSIONS In this study, the first direct nonequilibrium molecular dynamics (MD) simulations were performed to investigate ethanol dissociation mechanism on an iron nanoparticle. In the QM/MD simulations and energetic studies of typical reactions, we demonstrated that the first step of ethanol dissociation was C−O or O−H bond cleavage. The former was thermodynamically favorable and the latter was kinetically favorable. The decomposed radicals, such as C2H4, C2H5, and C2H6, by losing an −OH group from ethanol, were similar to pure hydrocarbon sources, promising these two carbon atoms had contributions similar to that of SWCNTs growth, which was consistent with the experimental observations. Furthermore, the C−C and C− H broken bonds possessed higher barriers and less exothermicity or endothermicity; thus they probably occurred at a later stage. The H2 formation possessed the highest barrier, and it was an entropy control process. When the cluster became large enough or the coverage of hydrogen was relative high on the iron surface, this reaction possibly occurred in a short simulation time. The findings of this study will help us to understand the basic aspects on the growth of SWCNT or other carbon-related materials by the CVD method with ethanol as a carbon source.

Figure 5. Snapshots of H2 formation on Fe38 catalyst. The blue and purple atoms indicate the two reactive hydrogen atoms.

on the surface. At 12.45 ps two H atoms suddenly crashed. And then they rotated on the iron surface (at 12.50 ps) and stood on the same iron atom (at 12.55 ps). Finally, they combined with each other and left the surface as a H2 molecule at 12.70 ps. This H2 formation process was totally different from the ethanol decomposition on Ni and Co, on which H2 formation from intrahydrogen, such as H of CH3 and H of OH, was more preferred.46 3.3. Schematic of Ethanol Dissociation on Fe Cluster for SWCNT Growth. The schematic of ethanol dissociation on Fe cluster for SWCNT growth was shown in Figure 6. According to the above discussion, the first step of ethanol dissociation was C−O or O−H bond cleavage, followed by O− H or C−O bond cleavage, and finally C−H bond breaking from hydrocarbon radicals, producing the important intermediates C2H4, C2H5, and C2H6. Following that, by continually C−H cleavage and H2 formation, C2 would be formed and ready for accumulation to form SWCNTs. These ethanol dissociation

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site at DOI: 10.1021/acs.jpcc The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12207. Figures S1−S3 showing the total energy plots as a function of equilibration time, the ten Fe13, Fe38, and Fe55 structures chosen randomly from 5 to 10 ps equilibration, and the snapshots of 1 ps ethanol supply simulation; Figure S4 showing the structures of reactants,

Figure 6. Schematic ethanol dissociation processes on Fe cluster for SWCNT growth. 2282


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Nanotubes Produced by Ccvd Method. Chem. Phys. Lett. 2001, 345, 11−17. (14) Alvarez, W. E.; Kitiyanan, B.; Borgna, A.; Resasco, D. E. Synergism of Co and Mo in the Catalytic Production of Single-Wall Carbon Nanotubes by Decomposition of Co. Carbon 2001, 39, 547− 558. (15) Zheng, B.; Li, Y.; Liu, J. Cvd Synthesis and Purification of Single-Walled Carbon Nanotubes on Aerogel-Supported Catalyst. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 345−348. (16) Jourdain, V.; Bichara, C. Current Understanding of the Growth of Carbon Nanotubes in Catalytic Chemical Vapour Deposition. Carbon 2013, 58, 2−39. (17) Miyauchi, Y.; Chiashi, S.; Murakami, Y.; Hayashida, Y.; Maruyama, S. Fluorescence Spectroscopy of Single-Walled Carbon Nanotubes Synthesized from Alcohol. Chem. Phys. Lett. 2004, 387, 198−203. (18) Colomer, J. F.; Stephan, C.; Lefrant, S.; Van Tendeloo, G.; Willems, I.; Kónya, Z.; Fonseca, A.; Laurent, C.; Nagy, J. B. LargeScale Synthesis of Single-Wall Carbon Nanotubes by Catalytic Chemical Vapor Deposition (Ccvd) Method. Chem. Phys. Lett. 2000, 317, 83−89. (19) Jeong, H. J.; An, K. H.; Lim, S. C.; Park, M.-S.; Chang, J.-S.; Park, S.-E.; Eum, S. J.; Yang, C. W.; Park, C.-Y.; Lee, Y. H. Narrow Diameter Distribution of Singlewalled Carbon Nanotubes Grown on Ni−Mgo by Thermal Chemical Vapor Deposition. Chem. Phys. Lett. 2003, 380, 263−268. (20) Benito, A. M.; Maniette, Y.; Muñoz, E.; Martínez, M. T. Carbon Nanotubes Production by Catalytic Pyrolysis of Benzene. Carbon 1998, 36, 681−683. (21) Maruyama, S.; Kojima, R.; Miyauchi, Y.; Chiashi, S.; Kohno, M. Low-Temperature Synthesis of High-Purity Single-Walled Carbon Nanotubes from Alcohol. Chem. Phys. Lett. 2002, 360, 229−234. (22) Murakami, Y.; Miyauchi, Y.; Chiashi, S.; Maruyama, S. Characterization of Single-Walled Carbon Nanotubes Catalytically Synthesized from Alcohol. Chem. Phys. Lett. 2003, 374, 53−58. (23) Tomie, T.; Inoue, S.; Iba, Y.; Matsumura, Y. In Situ Mass Spectroscopic Analysis of Alcohol Catalytic Chemical Vapor Deposition Process for Single-Walled Carbon Nanotube. Chem. Phys. Lett. 2012, 536, 104−108. (24) Xiang, R.; Hou, B.; Einarsson, E.; Zhao, P.; Harish, S.; Morimoto, K.; Miyauchi, Y.; Chiashi, S.; Tang, Z.; Maruyama, S. Carbon Atoms in Ethanol Do Not Contribute Equally to Formation of Single-Walled Carbon Nanotubes. ACS Nano 2013, 7, 3095−3103. (25) Kataura, H.; Maniwa, Y.; Kodama, T.; Kikuchi, K.; Hirahara, K.; Suenaga, K.; Iijima, S.; Suzuki, S.; Achiba, Y.; Krätschmer, W. HighYield Fullerene Encapsulation in Single-Wall Carbon Nanotubes. Synth. Met. 2001, 121, 1195−1196. (26) Ferrin, P.; Simonetti, D.; Kandoi, S.; Kunkes, E.; Dumesic, J. A.; Norskov, J. K.; Mavrikakis, M. Modeling Ethanol Decomposition on Transition Metals: A Combined Application of Scaling and BronstedEvans-Polanyi Relations. J. Am. Chem. Soc. 2009, 131, 5809−15. (27) Oguri, T.; Shimamura, K.; Shibuta, Y.; Shimojo, F.; Yamaguchi, S. Bond Dissociation Mechanism of Ethanol During Carbon Nanotube Synthesis Via Alcohol Catalytic Cvd Technique: Ab Initio Molecular Dynamics Simulation. Chem. Phys. Lett. 2014, 595−596, 185−191. (28) Weststrate, C. J.; Ludwig, W.; Bakker, J. W.; Gluhoi, A. C.; Nieuwenhuys, B. E. Ethanol Adsorption, Decomposition and Oxidation on Ir(111): A High Resolution Xps Study. ChemPhysChem 2007, 8, 932−937. (29) Vesselli, E.; Baraldi, A.; Comelli, G.; Lizzit, S.; Rosei, R. Ethanol Decomposition: C□C Cleavage Selectivity on Rh(111). ChemPhysChem 2004, 5, 1133−1140. (30) Gates, S. M.; Russell, J. N., Jr; Yates, J. T., Jr Bond Activation Sequence Observed in the Chemisorption and Surface Reaction of Ethanol on Ni(111). Surf. Sci. 1986, 171, 111−134. (31) Weststrate, C. J.; Gericke, H. J.; Verhoeven, M. W. G. M.; Ciobîcă, I. M.; Saib, A. M.; Niemantsverdriet, J. W. Ethanol Decomposition on Co(0001): C−O Bond Scission on a Close-Packed Cobalt Surface. J. Phys. Chem. Lett. 2010, 1, 1767−1770.

products, and transition states for C−H cleavage and H2 formation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Ying Wang. E-mail: [email protected]. *Zhijian Wu. E-mail: [email protected]. ORCID

Ying Wang: 0000-0002-5437-8741 Stephan Irle: 0000-0003-4995-4991 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 (21503210, 21521092, 21673220), Jilin Province Natural Science Foundation (20150101012JC), and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase). Part of the computational time is supported by the Performance Computing Center of Jilin University and Changchun Normal University.

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REFERENCES

(1) Iijima, S.; Ichihashi, T. Single-Shell Carbon Nanotubes of 1-Nm Diameter. Nature 1993, 363, 603−605. (2) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of Hydrogen in Single-Walled Carbon Nanotubes. Nature 1997, 386, 377−379. (3) Anantram, M. P.; Léonard, F. Physics of Carbon Nanotube Electronic Devices. Rep. Prog. Phys. 2006, 69, 507. (4) Wang, Y.; Yeow, J. T. W. A Review of Carbon Nanotubes-Based Gas Sensors. J. Sens. 2009, 2009, 1−24. (5) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; de la Chapelle, M. L.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Large-Scale Production of Single-Walled Carbon Nanotubes by the Electric-Arc Technique. Nature 1997, 388, 756−758. (6) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; et al. Crystalline Ropes of Metallic Carbon Nanotubes. Science 1996, 273, 483−487. (7) Cheng, H. M.; Li, F.; Su, G.; Pan, H. Y.; He, L. L.; Sun, X.; Dresselhaus, M. S. Large-Scale and Low-Cost Synthesis of SingleWalled Carbon Nanotubes by the Catalytic Pyrolysis of Hydrocarbons. Appl. Phys. Lett. 1998, 72, 3282. (8) Dai, H.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Single-Wall Nanotubes Produced by Metal-Catalyzed Disproportionation of Carbon Monoxide. Chem. Phys. Lett. 1996, 260, 471−475. (9) Yan, H.; Li, Q.; Zhang, J.; Liu, Z. Possible Tactics to Improve the Growth of Single-Walled Carbon Nanotubes by Chemical Vapor Deposition. Carbon 2002, 40, 2693−2698. (10) Kong, J.; Cassell, A. M.; Dai, H. Chemical Vapor Deposition of Methane for Single-Walled Carbon Nanotubes. Chem. Phys. Lett. 1998, 292, 567−574. (11) Kitiyanan, B.; Alvarez, W. E.; Harwell, J. H.; Resasco, D. E. Controlled Production of Single-Wall Carbon Nanotubes by Catalytic Decomposition of Co on Bimetallic Co−Mo Catalysts. Chem. Phys. Lett. 2000, 317, 497−503. (12) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Gas-Phase Catalytic Growth of Single-Walled Carbon Nanotubes from Carbon Monoxide. Chem. Phys. Lett. 1999, 313, 91−97. (13) Colomer, J. F.; Benoit, J. M.; Stephan, C.; Lefrant, S.; Van Tendeloo, G.; Nagy, J. B. Characterization of Single-Wall Carbon 2283


Article

The Journal of Physical Chemistry C (32) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Self-Consistent-Charge DensityFunctional Tight-Binding Method for Simulations of Complex Materials Properties. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 7260. (33) Weinert, M.; Davenport, J. W. Fractional Occupations and Density-Functional Energies and Forces. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13709−13712. (34) Wentzcovitch, R. M.; Martins, J. L.; Allen, P. B. Energy Versus Free-Energy Conservation in First-Principles Molecular Dynamics. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 11372−11374. (35) Jiao, M.; Qian, H.; Page, A.; Li, K.; Wang, Y.; Wu, Z.; Irle, S.; Morokuma, K. Graphene Nucleation from Amorphous Nickel Carbides: Qm/Md Studies on the Role of Subsurface Carbon Density. J. Phys. Chem. C 2014, 118, 11078−11084. (36) Wang, Y.; Gao, X.; Qian, H.-J.; Ohta, Y.; Wu, X.; Eres, G.; Morokuma, K.; Irle, S. Quantum Chemical Simulations Reveal Acetylene-Based Growth Mechanisms in the Chemical Vapor Deposition Synthesis of Carbon Nanotubes. Carbon 2014, 72, 22−37. (37) Wang, Y.; Page, A. J.; Li, H. B.; Qian, H. J.; Jiao, M. G.; Wu, Z. J.; Morokuma, K.; Irle, S. Step-Edge Self-Assembly During Graphene Nucleation on a Nickel Surface: Qm/Md Simulations. Nanoscale 2014, 6, 140−4. (38) Wang, Y.; Page, A. J.; Nishimoto, Y.; Qian, H.-J.; Morokuma, K.; Irle, S. Template Effect in the Competition between Haeckelite and Graphene Growth on Ni(111): Quantum Chemical Molecular Dynamics Simulations. J. Am. Chem. Soc. 2011, 133, 18837−18842. (39) Page, A. J.; Wang, Y.; Li, H.-B.; Irle, S.; Morokuma, K. Nucleation of Graphene Precursors on Transition Metal Surfaces: Insights from Theoretical Simulations. J. Phys. Chem. C 2013, 117, 14858−14864. (40) Swope, W. C.; Andersen, H. C.; Berens, P. H.; Wilson, K. R. A Computer Simulation Method for the Calculation of Equilibrium Constants for the Formation of Physical Clusters of Molecules: Application to Small Water Clusters. J. Chem. Phys. 1982, 76, 637− 649. (41) Martyna, G. J.; Klein, M. L.; Tuckerman, M. Nosé-Hoover Chains: The Canonical Ensemble Via Continuous Dynamics. J. Chem. Phys. 1992, 97, 2635−2643. (42) Frisch, M. J.; Trucks, T. G.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 03, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2004. (43) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (44) Ohta, Y.; Okamoto, Y.; Page, A. J.; Irle, S.; Morokuma, K. Quantum Chemical Molecular Dynamics Simulation of Single-Walled Carbon Nanotube Cap Nucleation on an Iron Particle. ACS Nano 2009, 3, 3413−3420. (45) Park, J.; Zhu, R. S.; Lin, M. C. Thermal Decomposition of Ethanol. I. Ab Initio Molecular Orbital/Rice−Ramsperger−Kassel− Marcus Prediction of Rate Constant and Product Branching Ratios. J. Chem. Phys. 2002, 117, 3224. (46) Inoue, S.; Maruyama, S. Mass Spectroscopy of Chemical Reaction of 3d Metal Clusters Involved in Chemical Vapor Deposition Synthesis of Carbon Nanotubes. Jan J. Appl. Phys. 2008, 47, 1931.

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Theoretical Studies on Ethanol Dissociation on Iron Nanoparticles in

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