Higher Activity Leading to Higher Disorder: A Case of Four Light

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Higher Activity Leading to Higher Disorder: A Case of Four Light Hydrocarbons to Variable Morphological Carbonaceous Materials by Pyrolysis Jian Liu,*,† Hao Ke,†,‡ Kai Zhong,†,‡ Xudong He,† Xianggui Xue,† Linyuan Wang,‡ and Chaoyang Zhang*,†,§

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†

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-311, Mianyang, Sichuan 621999, China ‡ School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China § Beijing Computational Science Research Center, Beijing 100048, China S Supporting Information *

ABSTRACT: The subtle and efficient manufacture of high-quality carbonaceous materials dominates their extensive applications. Meanwhile, revealing the underlying mechanism in the formation of carbonaceous materials is crucial to improving their manufacture efficiency. In the present work, we focus upon the pyrolysis mechanism for four light hydrocarbons including methane, CH4, ethane, C2H6, ethylene, C2H4, and acetylene, C2H2, to carbonaceous materials, combined with reactive molecular dynamics (RMD) simulations. The carbonaceous materials with various morphologies are observed in our simulations, and the morphologies are strongly dependent on the initial reactants; i.e., a disorderly C cluster, a crossed C multilayer, and an orderly C monolayer are made from C2H2, C2H4, and C2H6 and CH4, respectively, as ascertained partly in experiments. Tracing the RMD trajectories, we confirm that the pyrolysis of all four light hydrocarbons undergoes three stages, including the C chain elongation with generation of new small carbonaceous molecules or radicals, the formation and growth of polycyclic aromatic hydrocarbons, and the stable growth of C clusters. The morphologic difference of the final C clusters is attributed to the reactant activity and C growth styles. That is, the higher activity and the faster growth by the C2 addition facilitate the more disorderly arrangement of C atoms, and vice versa. Typically, the dense C2H2 tends to form disorderly C black, while the thin CH4, to orderly C nanotubes. It shows that selecting the reactants in terms of their activities is a key to preparing orderly carbonaceous materials. These findings are expected to be useful to understand the formation mechanism and design techniques for efficiently manufacturing high-quality carbonaceous materials.

1. INTRODUCTION Up to the present, the knowledge of carbonaceous materials has been enhanced from the traditional amorphous C, graphite, and diamond to the new morphologies like fullerene, graphene, and C nanotubes (CNTs).1−3 These new carbonaceous materials have been proven to possess extraordinary properties and performances to meet the requirements of advanced applications.4−9 Nevertheless, their applications remain still limited due to the inefficient yield of high-quality products.10 The yield and quality of carbonaceous materials are governed by many factors, such as initial reactants, catalysts, dispersed phases, temperature, and so forth. Reasonably, it is difficult to combine efficiently with these factors together to produce satisfied carbonaceous materials. Revealing the underlying mechanisms for these factors responsible for carbonaceous material formation becomes crucial, as it is the base for improving manufacture techniques and enhancing the quality and yield of products. Nevertheless, most of these mechanisms remain still unclear.11 Sometimes, similar syn© XXXX American Chemical Society

thesis processes with a subtle change alone may give rise to different products. For instance, CNTs were an unexpected discovery by elevating arc discharge current, in contrast to an original intention to produce fullerene.12 Using methane (CH4) as an initial reactant, both graphene13 and CNTs14 can be produced by chemical vapor deposition (CVD) at >1200 K, and the product morphologies are partly dependent on the geometric adjustment of the reactor and careful control of the growth substrate. These cases indicate that it is difficult to control the morphologies of carbonaceous materials, and it is in principle necessary to clarify the related reaction mechanisms to implement the controllable and efficient manufacture.15,16 Usually, the temperature for manufacturing carbonaceous materials ranges from 1000 to 4000 K, in the cases of arc discharge,17 CVD,18 laser ablation,19 pyrolysis,20,21 and Received: August 9, 2018 Revised: November 24, 2018 Published: December 4, 2018 A

DOI: 10.1021/acs.jpcc.8b07762 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Snapshots of C2H2 (a), C2H4 (b), C2H6 (c), and CH4 (d) pyrolysis with time proceeding under the condition of 0.1 g/cm3 and 3500 K. Only carbon atoms are exhibited for clarity.

combustion.2,19 The high temperature contributes to the atomic C formation from initial reactants. Thus, the nucleation and growth mechanisms of carbonaceous materials at high temperature can simply be seen as the self-assembly of atomic C. For this, many kinetic models have been developed to describe the pyrolysis of hydrocarbons.22−27 Nevertheless, these models were employed to compare and fit the elementary reactions with experimental observations to determine dominant reactions in the total conversions alone and cannot provide any dynamic detail at the atomic level. Using reactive molecular dynamics (RMD), our recent work indicated that the pyrolysis product of CH4 is an orderly cavity-like carbonaceous material as a possible precursor of CNTs,28 while that of acetylene (C2H2) is a disorderly C black.29 It shows that pyrolysis of different light hydrocarbons can result in variable morphological carbonaceous materials. Thus, it provokes us to focus upon what happens to the other light hydrocarbons like ethane (C2H6) and ethylene (C2H4) and whether there is a rule for selecting reactants of carbonaceous material formation with respect to morphologies, via direct pyrolysis of light hydrocarbons. These concerns push us to extend our work to other light hydrocarbons. In the present work, the RMD simulations were also applied to C2H6 and C2H4, and the analyses were performed in combination with our previous results. Interestingly, we find that the initial pyrolysis of all C2H6, C2H4, C2H2, and CH4 undergoes three successive stages, including the C chain elongation with generation of new small carbonaceous molecules or radicals (CMs) (I), the formation and growth of polycyclic aromatic hydrocarbons (PAHs) (II), and the stable growth of C clusters (III). More importantly, we find the morphologies of the final carbonaceous materials are

strongly dependent on the activity of primary reactants; i.e., the higher activity leads to the higher disorder of C arrangement in carbonaceous materials under pyrolysis. This is the root for why the final products of the four hydrocarbons are different from one another in morphology, i.e., a disorderly C cluster, a crossed multilayer, and an orderly monolayer for C2H2, C2H4, and C2H6 and CH4, respectively. These results are partly supported by previous experimental works.13,30,31 The initial pyrolysis of hydrocarbons is mainly attributed to the homolytic fission of CH,31 and the subsequent growth of C clusters of C2H6 and C2H4 is found to be enlarged uniformly with a step of two C atoms (C2 addition), due to the high stability of C C and CC bonds to be dissociated into single C atoms or radicals.30 All of these findings will facilitate the understanding of the formation mechanism of carbonaceous materials and help to improve manufacture efficiency in the future.

2. METHODOLOGIES To simulate the pyrolysis of C2H2, C2H4, C2H6, and CH4, we established four cubic cells separately containing the hydrocarbon molecules with the same density of 0.1 g/cm3. Because the C source usually governs the growth of carbonaceous materials, it is necessary to keep a same level of C sources for all four cells, for example, 2000 C atoms, in our simulation. That is, the CH4 cell contains 2000 molecules and the other three cells each contain 1000 molecules. All of the RMD simulations were performed using the LAMMPS package with the ReaxFF-lg force field.32 ReaxFF33−37 is a reactive force field based on the bond-order (BO) principle, and it is parametrized to reproduce the density functional theory (DFT) results for selected systems and properties. ReaxFF allows us to recognize the processes of bond breaking and forming in a simulation B


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Figure 2. Evolutions of MCMs in the pyrolysis of C2H2 (a), C2H4 (b), C2H6 (c), and CH4 (d). The C and H atoms are denoted in black and green, respectively.

frequency of the thermostat to nuclear motion was composed of 200 steps. In order to relax the molecules in the cells, each cell underwent a NVT RMD simulation at 300 K for 10 ps, without decomposition found in this period. Afterward, the temperature was set to 3500 K for performing pyrolysis simulations for 10 ns each. The files recording atomic information, including positions, velocities, and forces, were exported every 1 ps, namely, 10000 frames. For detailing primary reactions responsible for the decay and the subsequent formation of larger molecules, a series of FORTRAN scripts were implemented. The script Getreac can search and compare all of the identifications of the atoms of each reaction and can expunge all of the reversible reactions and collect the net reactions and their frequencies.45,46

system by calculating BOs at each MD step, and thus, it is capable of describing the structures, energy surfaces, and reaction barriers for a variety of materials. It provides more details of complex chemical processes under a given condition, especially in larger space and time scales, relative to usual firstprinciples calculations.38,39 ReaxFF-lg32 is an improved version of ReaxFF, proposed by Liu and Goddard using low-gradient (lg) to improve the description of London dispersion as in standard DFT methods. Three kinds of atomic pairs of C−C, C−H, and H−H were present in this work, whose BO cutoffs were assigned to 0.55, 0.4, and 0.55, respectively. ReaxFF has successfully been applied to study the pyrolysis of hydrocarbons28,29,40−43 at temperatures ranging from 773 to 3500 K, showing its reliability already and no additional validation required. The practical pyrolysis of light hydrocarbons like CH4 can take place at ∼1000 K at ambient pressure. Within the time scale limit of available RMD simulations, we could not observe any chemical reaction when the simulation conditions were assigned to be similar to the practical ones. Thereby, the simulation temperature in this work was elevated to 3500 K to overcome the time scale limit, as we did in recent work.28,29 Though there will be temperature and pressure enhancement in these simulations, it has extensively been proven that the enhancement will not give rise to any artificial feature. That is, it is a common way of various MD simulations to increase temperature significantly to overcome the time scale limit. The constant particle, temperature, and volume ensemble, also referred to as the canonical ensemble (NVT), was employed for our simulations, with the simple Nosé−Hoover thermostat method44 applied to control the thermodynamic temperature. The time step and effective relaxation time were assigned to 0.1 and 20 fs, respectively. Specifically, the coupling

3. RESULTS AND DISCUSSION After analyzing the dynamic bonds of the four systems, we illustrate in Figure 1 the typical snapshots to show the main stages of the pyrolysis, in which the evolutions of the maximal carbonaceous molecules with the most C atoms (MCMs) are paid much attention, including morphology, size, and bonding characteristic. Figure 1 shows that the four hydrocarbons undergo a similar process, from a large quantity of small CMs initially to a single large C cluster finally. Despite this, the final morphologies of the clusters in Figure 2 are different from one another. As time proceeds to 1 ns, the PAHs of all four cases are evidently emerged by the continuous addition of C atoms, and the MCMs of C2H4 (Figure 1b2) and C2H6 (Figure 1c2) are much larger than those of C2H2 (Figure 1a2) and CH4 (Figure 1d2). It shows that reactions in the former two cases proceed faster due to the faster fusion of small CMs into MCMs. C


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As simulation runs to 2 ns, the four light hydrocarbons are evolved differently. That is, the enlarged chains of C2H2 are still distributed evenly (Figure 1a3), without any remarkable ordered structure. It shows that the dense, small, and disorderly CMs are produced in C2H2 in the primary 2 ns. Unlike C2H2, the other three hydrocarbons of C2H4, C2H6, and CH4 (Figure 1b3−d3) possess large CMs with ordered structures. Afterward, all of the CMs are fused to a huge cluster, a MCM, at 3 ns for C2H2 and C2H4 and at 6 ns for C2H6 and CH4 (Figure 1a4−d4). For the MCM of C2H2, the C atoms are disorderly assembled as a precursor for the C black (Figure 2a),28 while, for those of C2H4, C2H6, and CH4, the C atoms are orderly arranged with one or several layers (Figure 2c,d). Meanwhile, we find that CH4, as well as C2H6, tends to produce CNTs, as the MCM seems to be a precursor of a CNT, which possesses an ordered nanocavity structure (Figure 2c).29 This ordered nanocavity structure can just be seen as a part of an integrated closed single-wall CNT, and it will be grown to a perfect one once the C source is sufficient. In more detail, the MCMs at the earlier stage of the pyrolysis of the four light hydrocarbons were examined, with morphologies shown in Figure S1 of the Supporting Information. We can deduce from the figure that the longest C chains (MCMs) are enlarged much faster in the pyrolysis of C2H2 and C2H4 than in that of CH4 and C2H6 in the first 500 ps, showing a higher activity of C2H2 and C2H4 at 3500 K and 0.1 g/cm3. Besides, the appearance or not of small carbonaceous molecules Cn (n ≤ 4) in the early period of 500 ps shows that most types of small carbonaceous molecules are exhibited in the pyrolysis of CH4, followed by C2H6, C2H4, and C2H2 (Table 1). These small carbonaceous molecules may be neutral, radical, or charged according to their stoichiometry of C and H atoms and can play an important role in the following chain elongation to big cluster formation, which will be discussed later. A further statistical analysis based on the size of CMs facilitates revealing the underlying mechanism for generation and enlargement of CMs at the early stage of the pyrolysis. To do this, we define the population percentage of C atoms in a CM (P) relative to the total of a system by the equation of P = 100% × ∑jn=i n × NCn/Ntot, where Cn, NCn, and Ntot represent a CM with n C atoms, the amount of the CM, and the total C atoms in a system, respectively. In the equation, the size of the CM is limited as i < n < j. Thereby, the amount and size of CMs are detailed in Figure 3. Because C3−C9 generated from the primary hydrocarbons are smaller than the minimal PAH containing 10 C atoms, that should be concerned as a base for subsequent molecular enlargement. Meanwhile, it is widely accepted that a PAH unit usually contains ≥10 C atoms;46−48 thus, tracing Cn (n ≥ 10) is feasible to evaluate the PAH generation. A PAH is thought be a lighter one if 10 ≤ n ≤ 19 and a heavier one when n ≥ 20. Figure 3a shows that, at the earlier stage, C2H2 produces C3− C9 the most rapidly, followed by C2H4, C2H6, and CH4, and the cases of C2H4 and C2H6 are very close to each other. It shows a decreasing order of reactivity of C2H2, C2H4, C2H6, and CH4. The maximal P of C3−C9 occurs at ∼100, ∼200, ∼200, and ∼400 ps for C2H2, C2H4, C2H6, and CH4, respectively, also suggesting the aforementioned reactivity order. Figure 3b indicates the case of C10−C19. At first, the C10−C19 in the C2H2 increase fast, followed by C2H4, C2H6, and CH4, also implying the aforementioned reactivity order.

Table 1. Appearance (√) or Not (×) of Small Carbonaceous Molecules Cn (n ≤ 4) at the Early Stage of the Pyrolysis of the Four Light Hydrocarbons species

C2H2

C2H4

C2H6

CH4

CH4 CH3 CH2 C2H6 C2H5 C2H4 C2H3 C2H2 C2H C2 C3H4 C4H2 C4H3 C4H4

× × × × × × √ √ √ √ √ √ √ √

× × × × × √ √ √ √ √ √ √ √ √

× √ √ √ √ √ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √ √ √ √ √ √

Figure 3. Population percentage sums of Cn in the initial 2500 ps in the cases of 3 ≤ n ≤ 9 (a), 10 ≤ n ≤ 19 (b), and n ≥ 20 (c), representing small molecules and lighter and heavier PAHs, respectively.

D


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Figure 4. Evolution of Cn (3 ≤ n ≤ 10) in the initial 500 ps.

attributed to the high stability of CC and CC bonds to be broken into single C atoms or radicals as clarified in previous work.28−30,49,50 With respect to the evolution of CH4 (Figure 4d), the dominance of Cn decreases with increasing n in a oneby-one manner. Reasonably, the Cn is enlarged by the C1 addition in this case. The case of C2H6 (Figure 4c) is more complex. C3, C4, C5, and C6 govern the intermediates at the earlier stage, and their amounts decrease in an order of C4, C3, C5, and C6, instead of C3, C4, C5, and C6. That the successiveness of n appears dominantly in this case suggests that the CC bond is ruptured and the C1 addition occurs to enlarge Cn. Meanwhile, along with the possible H dissociation, it makes the saturated CC bonds unsaturated, setting a base for the C2 addition. The highest abundance of C4 should be resulted from such a kind of addition. Thus, the C n enlargement in the pyrolysis of C2H6 is implemented by both the C1 and C2 additions. Besides, the molecular enlargement style can influence the final morphology of the MCMs. Even though it has been accepted for decades that the C2H2 pyrolysis undergoes the C2 addition, the influence of the C addition style on the morphology of carbonaceous materials remains still unclear.51,52 However, after examining all of the trajectory files of the present work, we find a very common phenomenon in Figure 5, which presents a good explanation for the mechanism of the C2 addition. As shown in Figure 5, the C2 addition can lead to a disorderly arrangement of C atoms, while the C1 addition usually makes the C atom arranged orderly. Thus, the pyrolysis product of C2H2 was soot or carbon black with high disorder.29,52 The mechanism of the C1 addition explains well a recent finding by Das and Drucker, in which the carbon black in graphene synthesis is significantly reduced by mixing CH4 and H2 in the CVD processes and the graphene size increases with increasing H2:CH4 ratio.13 In this case, CH4 molecules are dissipated by abundant H2, and the self-collision possibility of CH4 molecules is largely reduced, facilitating the formation of ordered carbonaceous materials by the C1 addition, instead of the C2 one.

Figure 5. Structures generated from the C2 additions. The red ball stands for an added C atom, and the blue dashed lines stand for the two potential pathways to link a second C atom to saturate it.

Afterward (after ∼160 ps), the C10−C19 in C2H2 reduce first to the lowest P. Following C2H2, the C10−C19 in C2H4, C2H6, and CH4 decrease too, with P in an opposite reverse order. It shows that, in the case of C2H2, most C10−C19 are converted into heavier PAHs the fastest, while the conversion in CH4 proceeds the slowest. Figure 3c exhibits that Cn (n ≥ 20) emerge at ∼100, ∼200, ∼200, and ∼350 ps for C2H2, C2H4, C2H6, and CH4, respectively, showing a decreasing order of reactivity too. It can also be seen that the Cn (n ≥ 20) in all four cases undergo a rapid increase at earlier stages, at the cost of the consumption of C3−C9 and C10−C19, and the period for the increasing of C3−C9 just corresponds to that for the decreasing of Cn (n ≥ 20). The evolutions of Cn (3 ≤ n ≤ 10) in the primary 500 ps are concerned about further revealing the initial reaction mechanisms. C2H2 and C2H4 are evolved in a similar manner, as both parts a and b of Figure 4 exhibit that C4, C6, C8, and C10 (n is even) dominate the intermediates, and their amounts decrease with increasing n. It suggests that the C2 addition is a dominant style to enlarge Cn, as n of these dominant Cn changes uniformly with a step of two C atoms. The small amounts of Cn (n is odd) in parts a and b of Figure 4 are E


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Figure 6. Evolution of the dominant bond length of C−C (rC−C) for C2H2 (a), C2H4 (b), C2H6 (c), and CH4 (d). There is no C−C bond for CH4; thus, its rC−C is void at 0 ps. The dashed lines mark the mutation points of rC−C and divide the chemical reaction of each system into three stages, I, II, and III.

Figure 7. Evolutions of the sizes of MCMs during the pyrolysis of C2H2 (a), C2H4 (b), C2H6 (c), and CH4 (d). n represents the number of C atoms of the MCM.

Å, respectively. At first, we pay attention to the case of C2H2 in Figure 6a and Figure S2 of the Supporting Information. The dominant rCC increases from an original value of 1.26 Å (the CC bond length of C2H2) to 1.34 and 1.44 Å and maintains 1.44 Å afterward. Correspondingly, the pyrolysis of C2H2 is divided into three stages of I, II, and III. At stage I, the C chains are elongated by self-addition reactions with the intensity of g(r) gradually bcomes weak, as is shown in Figure S2 of the Supporting Information. The rCC increase at stage

Learning the bonding features of CMs is helpful to reveal the chemical structure evolution in the simulations. To do this, we apply the radial distribution function (RDF, g(r)) to analyze the bond types between C atoms (Figures S2−S5 of the Supporting Information). We abstract the dominant rCC, the positions of closest peaks of RDFs at each time step, to evaluate the bond types in Figure 6, because the CC bond types in hydrocarbons can be distinguished by rCC, i.e., rCC of single, double, and triple C/C bonds are 1.54, 1.34, and 1.22 F


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Figure 8. Evolutions of the sizes and amounts of Cn during the pyrolysis of C2H2 (a), C2H4 (b), C2H6 (c), and CH4 (d). The amounts of Cn are described logarithmically with an equation of lg(1 + NCn) in the color charts.

abstraction. Both the CC to CC and CC to CC transitions were explained as a H-abstraction mechanism, as it has been proven to generate abundant CC bonds as active sites to trigger nucleation of large C clusters.47 For CH4 containing one C atom only, it requires a forehand C1 to C2 by dimerization, which will be discussed later. As demonstrated by Figure 6d and Figure S5 of the Supporting Information, a g(r) peak of rCC at 1.22 Å exhibits a small shape first and then becomes stronger and stronger. Afterward, rCC is evolved similarly to the case of C2H6. From above the rCC evolution during the pyrolysis of all four hydrocarbons, we can know that, regardless of any type of hydrocarbons, rCC is finally dominant at ∼1.45 Å, featuring a typical C/C bond length in PAHs. Thus, as illustrated in Figure 6, all of the hydrocarbon pyrolysis can be partitioned into three stages: C chain elongation with generation of new small carbonaceous molecules or radicals (I), the formation and growth of PAHs (II), and the stable growth of C clusters (III). At stage I, it tends to form CC bonds (1.22 Å of rCC) by original C2H2, H abstraction from C2H4 and C2H6, or dimerization of C1 radicals from CH4. Aromatic C/C bonds (1.45 Å of rCC) are formed with PAHs at the second stage, and they keep until the end of the simulations. In comparison, no H abstraction and C1 dimerization is required for C2H2 to form PAHs; thus, stage II is significantly shorter and a higher activity is exhibited for C2H2. Next, we concern ourselves about the size evolution of MCMs. There is some discontinuity in each plot of Figure 7, showing the fluctuant fusion and dissociation of the MCMs at stage II. For C2H2 (Figure 7a) or C2H4 (Figure 7b), the MCM is remarkably enlarged with a much higher variation slope, relative to stages I and III. It is caused by more CMs forming PAHs, as shown in Figures 1−3. For C2H6 (Figure 7c) or CH4 (Figure 7d), the MCM increases mildly, suggesting that C growth occurs always based on a single big PAH, with an

Figure 9. Decay evolutions of the four light hydrocarbons.

II represents that the dominance of CC is continuously weakened with other types of C/C bonds formed. Finally, that rCC maintains 1.44 Å at stage III suggests the bond types in this period keep constant. In the case of C2H4, the rCC are shortened from 1.34 to 1.22 Å in the initial 100 ps, implying a CC to CC transition by H abstraction (Figure 6b). Subsequently, a similar process to C2H2 occurs for C2H4, as the rCC increases to 1.45 Å and maintains it afterward (Figure S3 of the Supporting Information). The whole process is partitioned into three stages too. Figure 6c and Figure S4 of the Supporting Information exhibit a similar evolution tendency of rCC for C2H6. The rCC is shortened from 1.57 to 1.22 Å, implying a CC to CC transition by H G


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Figure 10. Main reaction pathways in the initial 0.5 ns with reverse reactions expunged. The red dashed ellipses denote paths to C2H.

the rates of reactant consumption are in a reducing order of C2H6, C2H4, C2H2, and CH4. It exhibits an inconsistency in the evolution rates between the reactant decay and the MCM size increasing. It has been proved that the initial pyrolysis of hydrocarbons is mainly attributed to the C−H homolytic fission.31,57−59 The bond dissociation energies (BDEs) of C− H bonds in these hydrocarbons (Table S1 of the Supporting Information)57 increase in the order of C2H6 (423), CH4 (439), C2H4 (464), and C2H2 (556 kJ/mol). Roughly, a large BDE suggests a small decay rate, with the exception of CH4. This exception results from a side reaction of C2H4 + CH3 → C2H3 + CH4 (Table S2 of the Supporting Information), which generates plenty of CH4 to compensate its consumption at the initial stage of the pyrolysis. For C2H6, the BDE of the C−C bond (347−356 kJ/mol) is smaller than that of the C−H bond (Table S1 of the Supporting Information); thus, the homolytic fission of both C−H and C−C bonds contributes to its highest decay rate. Also, we traced the primary steps for the initial decay of all four light hydrocarbons (Tables S2−S5 of the Supporting Information). By these steps, we deduce the main reaction pathways in Figure 10. For C2H2, as proved by a kinetic analysis, its pyrolysis follows chain radical reactions at high temperature, where H and C2H drive the polymerization path to form polyynes.52 The generation of C2H is also detected from the main reaction pathways of the C2H2 pyrolysis in this study. In the decay of other light hydrocarbons, the reaction of C2H2 → C2H is also observed in combination with Habstraction. According to the mechanism of H-abstraction and C2H2-addition (HACA), the reaction C2H2 → C2H would trigger the nucleation to form MCMs.59,60 Thus, it is reasonable for the rapid generation of CMs in C2H2 during pyrolysis, which further leads to the highest disorder of the product among all hydrocarbons, as shown in Figure 2. The CC homolysis, C2H6 → 2CH3, occurs with a high frequency during the initial pyrolysis of C2H6 (Table S5 of the Supporting Information), while it is seldom in the others. The CH3, as an important intermediate of the pyrolysis of

almost invariable variation slope in the time scale of the simulation, despite a little fluctuation at stage II. Figure 7 also indicates that the mild fusion of C atoms facilitates the orderly C arrangement. Moreover, comparing the four plots in Figure 7, we find that the MCM enlargement proceeds the most slowly, also favoring the orderly C arrangement to a monolayer. It agrees with the above analysis from the snapshots in Figure 1. These findings provide successfully a mechanistic explanation for a recent synthesis53−56 that reducing the activity of reactant will increase the size of carbonaceous materials with ordered morphologies. Also, the evolutions of the sizes and amounts of all Cn are concerned. As exhibited in Figure 8, for each case, at the earlier stage of pyrolysis, most of the Cn possess small sizes; i.e., they each possess a large quantity but small n. As time proceeds, reasonably, the size (n) increases with the quantity reduction. Similar to the MCM evolutions in Figure 7, the discontinuousness of n can also be seen in Figure 8, where the fusion among small clusters results in a decrease in the amount of total C clusters. Furthermore, disordered structures are generated if the C clusters are not grown in a pure C1 style only. We deduce that it is difficult to achieve highly arranged carbonaceous materials like CNTs through the pyrolysis of dense hydrocarbons with high reactivity. It is true, as no such technique has been employed to produce highly arranged carbonaceous materials. In more detail, we clarify the consumption details of Cn with small sizes (n ≤ 100). Comparing parts a and b of Figure 8 with parts c and d, we can readily find that the small Cn in C2H2 and C2H4 are consumed faster than those in C2H6 and CH4, suggesting the faster growth of MCMs. Overall, C2H2 and C2H4 are decayed faster than C2H6 and CH4, together with a much more disorderly C arrangement. We learn from the above discussion that the MCM sizes in C2H2 and C2H4 pyrolysis increase faster than those of C2H6 and CH4; in particular, that in CH4 increases the most slowly. The decay rates of reactants can be used to analyze the reactivity of the four systems. It can be seen in Figure 9 that H



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C2H6, as well as of CH4, makes C atoms arranged orderly in MCMs by the C1 addition. Contrarily, the unsaturation of the CC bond in C2H4 and the CC bond in C2H2 makes the C growth implemented through the C2 addition, disfavoring the orderly C arrangement. Meanwhile, numerous radicals produced at high temperature (Table 1 and Figure 10) play an important role in the MCM formation due to their high reactivity. This has also been clarified very recently, as the radicals react with other hydrocarbon species to form covalently bound complexes that promote further growth and clustering by regenerating resonance-stabilized radicals.61 Thereby, the mechanism for the pyrolysis of light hydrocarbons to form variably morphologic carbonaceous materials can be understandable.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07762.

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REFERENCES

(1) Kroto, H. W.; Heath, J. R.; Obrien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (2) Arikawa, M. Fullerenes an Attractive Nano Carbon Material and Its Production Technology. Nanotechnol. Perceptions 2006, 2, 121− 128. (3) Deng, J.; Wang, C.; Guan, G.; Wu, H.; Sun, H.; Qiu, L.; Chen, P.; Pan, Z.; Sun, H.; Zhang, B.; et al. The Deformations of Carbon Nanotubes under Cutting. ACS Nano 2017, 11, 8464−8470. (4) Tegos, G. P.; Demidova, T. N.; Arcila-Lopez, D.; Lee, H.; Wharton, T.; Gali, H.; Hamblin, M. R. Cationic Fullerenes Are Effective and Selective Antimicrobial Photosensitizers. Chem. Biol. 2005, 12, 1127−1135. (5) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based Composite Materials. Nature 2006, 442, 282−286. (6) Park, R. S.; Hills, G.; Sohn, J.; Mitra, S.; Shulaker, M. M.; Wong, H.-S. P. Hysteresis-Free Carbon Nanotube Field-Effect Transistors. ACS Nano 2017, 11, 4785−4791. (7) Peng, B.; Locascio, M.; Zapol, P.; Li, S.; Mielke, S. L.; Schatz, G. C.; Espinosa, H. D. Measurements of Near-Ultimate Strength for Multiwalled Carbon Nanotubes and Irradiation-Induced Crosslinking Improvements. Nat. Nanotechnol. 2008, 3, 626−631. (8) Filleter, T.; Bernal, R.; Li, S.; Espinosa, H. D. Ultrahigh Strength and Stiffness in Cross-Linked Hierarchical Carbon Nanotube Bundles. Adv. Mater. 2011, 23, 2855−2860. (9) Lahiri, J.; Lin, Y.; Bozkurt, P.; Oleynik, I. I.; Batzill, M. An Extended Defect in Graphene as a Metallic Wire. Nat. Nanotechnol. 2010, 5, 326−329. (10) Lin, X.; Zhao, W.; Zhou, W.; Liu, P.; Luo, S.; Wei, H.; Yang, G.; Yang, J.; Cui, J.; Yu, R.; et al. Epitaxial Growth of Aligned and Continuous Carbon Nanofibers from Carbon Nanotubes. ACS Nano 2017, 11, 1257−1263. (11) Wang, L.; Pumera, M. Residual Metallic Impurities Within Carbon Nanotubes Play a Dominant Role in Supposedly ″metal-free″ Oxygen Reduction Reactions. Chem. Commun. 2014, 50, 12662− 12664. (12) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56−58. (13) Das, S.; Drucker, J. Nucleation and Growth of Single Layer Graphene on Electrodeposited Cu by Cold Wall Chemical Vapor Deposition. Nanotechnology 2017, 28, 105601. (14) Seah, C.; Chai, S.; Ichikawa, S.; Mohamed, A. R. Parametric Study of Methane Catalytic CVD into Single-Walled Carbon Nanotubes Using Spin-Coated Iron Nanoparticles. Chem. Vap. Deposition 2013, 19, 53−60. (15) Matsukawa, Y.; Ono, K.; Dewa, K.; Watanabe, A.; Saito, Y.; Matsushita, Y.; Aoki, H.; Era, K.; Aoki, T.; Yamaguchi, T. Reaction Pathway for Nascent Soot in Ethylene Pyrolysis. Combust. Flame 2016, 167, 248−258. (16) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Structural Evolution during the Reduction of Chemically Derived Graphene Oxide. Nat. Chem. 2010, 2, 581−587. (17) Zhao, T.; Ji, X.; Jin, W.; Yang, W.; Li, T. Hydrogen Storage Capacity of Single-Walled Carbon Nanotube Prepared by a Modified Arc Discharge. Fullerenes, Nanotubes, Carbon Nanostruct. 2017, 25, 355−358. (18) Red’kin, A. N.; Kipin, V. A.; Sedlovets, D. M.; Knyazev, M. A. Chemical Vapor Deposition of Carbon Nanotube Layers on Aluminum Foil. Inorg. Mater. 2017, 53, 148−153. (19) Eatemadi, A.; Daraee, H.; Karimkhanloo, H.; Kouhi, M.; Zarghami, N.; Akbarzadeh, A.; Abasi, M.; Hanifehpour, Y.; Woo, J. S. Carbon Nanotubes: Properties, Synthesis, Purification, and Medical Applications. Nanoscale Res. Lett. 2014, 9, 393. (20) Moothi, K.; Simate, G. S.; Falcon, R.; Iyuke, S. E.; Meyyappan, M. Carbon Nanotube Synthesis Using Coal Pyrolysis. Langmuir 2015, 31, 9464−9472. (21) Hou, G.; Chauhan, D.; Ng, V.; Xu, C.; Yin, Z.; Paine, M.; Su, R.; Shanov, V.; Mast, D.; Schulz, M.; et al. Gas Phase Pyrolysis

4. CONCLUSIONS We apply RMD simulations with the ReaxFF-lg force field to reveal the underlying mechanism for the pyrolysis of the four light hydrocarbons, C2H6, C2H4, C2H2, and CH4, to form variably morphological carbonaceous materials. We find that the reactant activity or decay rate and the C growth style by the C1 or C2 addition are responsible for the morphological variety; i.e., the higher activity and the faster growth by the C2 addition facilitate the most disorderly C arrangement, and vice versa. Thereby, the root for the final products of the four hydrocarbons being different in morphology from one another, i.e., a disorderly C cluster, a crossed multilayer, and an orderly monolayer for C2H2, C2H4, and C2H6 and CH4, respectively, can be understandable. Typically, the dense C2H2 tends to form disorderly C blacks, and the thin CH4, orderly CNTs. Moreover, the processes of the generation of small CMs and C chain elongation, formation and growth of PAHs, and stable growth of the MCMs to final carbonaceous materials with certain morphologies are also found in the pyrolysis of all four light hydrocarbons. Thereby, this work is expected to be helpful to deepen the insight into mechanisms for manufacturing carbonaceous materials and to guide the technical design for manufacturing carbonaceous materials efficiently.

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Article

Morphologies of MCMs in the primary 500 ps, structures generated by the C1 and C2 additions, BDEs of the four hydrocarbons, and high frequency reactions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chaoyang Zhang: 0000-0003-3634-7324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by Science Challenge Project (TZ2018004) and the NSFC (11572296). I


Article

The Journal of Physical Chemistry C Synthesis of Carbon Nanotubes at High Temperature. Mater. Des. 2017, 132, 112−118. (22) Fau, G.; Gascoin, N.; Gillard, P.; Steelant, J. Methane Pyrolysis: Literature Survey and Comparisons of Available Data for Use in Numerical Simulations. J. Anal. Appl. Pyrolysis 2013, 104, 1−9. (23) Kahle, L. C. S.; Roussière, T.; Maier, L.; Delgado, K. H.; Wasserschaff, G.; Schunk, S. A.; Deutschmann, O. Methane Dry Reforming at High Temperature and Elevated Pressure: Impact of Gas-Phase Reactions. Ind. Eng. Chem. Res. 2013, 52, 11920−11930. (24) Dors, M.; Nowakowska, H.; Jasiński, M.; Mizeraczyk, J. Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at Atmospheric Pressure. Plasma Chem. Plasma Process. 2014, 34, 313− 326. (25) Saldana, M. H.; Bogin, G. E.; Wang, K.; Dean, A. M. Comparative Kinetic Analysis of Ethane Pyrolysis at 0.1 and 2.0 MPa. Energy Fuels 2016, 30, 9703−9711. (26) Saldana, M. H.; Bogin, G. E., Jr. Investigation of n-Pentane Pyrolysis at Elevated Temperatures and Pressures in a Variable Pressure Flow Reactor. J. Anal. Appl. Pyrolysis 2016, 118, 286−297. (27) Tsolas, N.; Yetter, R. A. Kinetics of Plasma Assisted Pyrolysis and Oxidation of Ethylene. Part 1: Plasma Flow Reactor Experiments. Combust. Flame 2017, 176, 534−546. (28) Xue, X.; Meng, L.; Ma, Y.; Zhang, C. Molecular Reactive ForceField Simulations on the Carbon Nanocavities from Methane Pyrolysis. J. Phys. Chem. C 2017, 121, 7502−7513. (29) Zhang, C.; Zhang, C.; Ma, Y.; Xue, X. Imaging the C Black Formation by Acetylene Pyrolysis with Molecular Reactive Force Field Simulations. Phys. Chem. Chem. Phys. 2015, 17, 11469−11480. (30) Richter, H.; Howard, J. B. Formation and Consumption of Single-Ring Aromatic Hydrocarbons and Their Precursors in Premixed Acetylene, Ethylene and Benzene Flames. Phys. Chem. Chem. Phys. 2002, 4, 2038−2055. (31) Tsang, W.; Hampson, R. F. Chemical Kinetic Data Base for Combustion Chemistry. Part I. Methane and Related Compounds. J. Phys. Chem. Ref. Data 1986, 15, 1087−1279. (32) Liu, L.; Bai, C.; Sun, H.; Goddard, W. A., III. Mechanism and Kinetics for the Initial Steps of Pyrolysis and Combustion of 1,6Dicyclopropane-2,4-hexyne from ReaxFF Reactive Dynamics. J. Phys. Chem. A 2011, 115, 4941−4950. (33) Van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A., III ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105, 9396−9409. (34) Chenoweth, K.; van Duin, A.; Goddard, W. ReaxFF Reactive Force Field for Molecular Dynamics Simulations of Hydrocarbon Oxidation. J. Phys. Chem. A 2008, 112, 1040−1053. (35) Naserifar, S.; Liu, L.; Goddard, W. A.; Tsotsis, T. T.; Sahimi, M. Toward a Process-Based Molecular Model of SiC Membranes. 1. Development of a Reactive Force Field. J. Phys. Chem. C 2013, 117, 3308−3319. (36) Naserifar, S.; Goddard, W. A.; Liu, L.; Tsotsis, T. T.; Sahimi, M. Toward a Process-Based Molecular Model of SiC Membranes. 2. Reactive Dynamics Simulation of the Pyrolysis of Polymer Precursor To Form Amorphous SiC. J. Phys. Chem. C 2013, 117, 3320−3329. (37) Naserifar, S.; Goddard, W. A.; Liu, L.; Tsotsis, T. T.; Sahimi, M. First Principles-based Multiparadigm, Multiscale Strategy for Simulating Complex Materials Processes with Applications to Amorphous SiC Films. J. Chem. Phys. 2015, 142, 174703. (38) Ding, J.; Zhang, L.; Zhang, Y.; Han, K. A Reactive Molecular Dynamics Study of n-Heptane Pyrolysis at High Temperature. J. Phys. Chem. A 2013, 117, 3266−3278. (39) Zhang, C.; Wen, Y.; Xue, X. Self-Enhanced Catalytic Activities of Functionalized Graphene Sheets in the Combustion of Nitromethane: Molecular Dynamic Simulations by Molecular Reactive Force Field. ACS Appl. Mater. Interfaces 2014, 6, 12235−12244. (40) Wang, Q.; Hua, X.; Cheng, X.; Li, J.; Li, X. Effects of Fuel Additives on the Thermal Cracking of n-Decane from Reactive Molecular Dynamics. J. Phys. Chem. A 2012, 116, 3794−3801.

(41) Cheng, X.; Wang, Q.; Li, J.; Wang, J.; Li, X. ReaxFF Molecular Dynamics Simulations of Oxidation of Toluene at High Temperatures. J. Phys. Chem. A 2012, 116, 9811−9818. (42) Wang, Q.; Wang, J.; Li, J.; Tan, N.; Li, X. Reactive Molecular Dynamics Simulation and Chemical Kinetic Modeling of Pyrolysis and Combustion of n-Dodecane. Combust. Flame 2011, 158, 217− 226. (43) Castro-Marcano, F.; Duin, A. C. T. Comparison of Thermal and Catalytic Cracking of 1-Heptene from ReaxFF Reactive Molecular Dynamics Simulations. Combust. Flame 2013, 160, 766− 775. (44) Hoover, W. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A: At., Mol., Opt. Phys. 1985, 31, 1695−1697. (45) Plimpton, S. J. Fast Parallel Algorithms for Short-range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1−19. (46) Aktulga, H. M.; Fogarty, J. C.; Pandit, S. A.; Grama, A. Y. Parallel Reactive Molecular Dynamics: Numerical Methods and Algorithmic Techniques. Parallel Comput 2012, 38, 245−259. (47) Dillstrom, T.; Violi, A. The Effect of Reaction Mechanisms on the Formation of Soot Precursors in Flames. Combust. Theory Modell. 2017, 21, 23−34. (48) Sánchez, N. E.; Callejas, A.; Millera, A.; Bilbao, R.; Alzueta, M. U. Polycyclic Aromatic Hydrocarbon (PAH) and Soot Formation in the Pyrolysis of Acetylene and Ethylene: Effect of the Reaction Temperature. Energy Fuels 2012, 26, 4823−4829. (49) Wang, H.; Frenklach, M. A Detailed Kinetic Modeling Study of Aromatics Formation in Laminar Premixed Acetylene and Ethylene Flames. Combust. Flame 1997, 110, 173−221. (50) Richter, H.; Grieco, W. J.; Howard, J. B. Formation Mechanism of Polycyclic Aromatic Hydrocarbons and Fullerenes in Premixed Benzene Flames. Combust. Flame 1999, 119, 1−22. (51) Gay, I. D.; Kistiakowsky, G. B.; Michael, J. V.; Niki, H. Thermal Decomposition of Acetylene in Shock Waves. J. Chem. Phys. 1965, 43, 1720. (52) Saggese, C.; Sánchez, N. E.; Frassoldati, A.; Cuoci, A.; Faravelli, T.; Alzueta, M. U.; Ranzi, E. Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons and Soot Formation in Acetylene Pyrolysis. Energy Fuels 2014, 28, 1489−1501. (53) Vlassiouk, I.; Fulvio, P.; Meyer, H.; Lavrik, N.; Dai, S.; Datskos, P.; Smirnov, S. Large Scale Atmospheric Pressure Chemical Vapor Deposition of Graphene. Carbon 2013, 54, 58−67. (54) Zhou, H.; Yu, W. J.; Liu, L.; Cheng, R.; Chen, Y.; Huang, X.; Liu, Y.; Wang, Y.; Huang, Y.; Duan, X. Chemical Vapour Deposition Growth of Large Single Crystals of Monolayer and Bilayer Graphene. Nat. Commun. 2013, 4, 2096. (55) Lee, Y.; Bae, S.; Jang, H.; Jang, S.-E.; Zhu, S. H.; Sim, Y. I.; Song, B. H.; Hong, J.-H. Wafer-Scale Synthesis and Transfer of Graphene Films. Nano Lett. 2010, 10, 490−493. (56) Bae, S.; Kim, H.; Lee, Y.; Xu, X. F.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I. Roll-to-Roll Production of 30-in. Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (57) Hamid, Z. A.; Azim, A. A.; Mouez, F. A.; Rehim, S. S. A. Challenges on Synthesis of Carbon Nanotubes from Environmentally Friendly Green Oil Using Pyrolysis Technique. J. Anal. Appl. Pyrolysis 2017, 126, 218−229. (58) Blanksby, S. J.; Ellison, G. B. Bond Dissociation Energies of Organic Molecules. Acc. Chem. Res. 2003, 36, 255−263. (59) Benson, S. W. The Mechanism of the Reversible-Reaction: 2C2H2 ⇌ Vinyl Acetylene and the Pyrolysis of Vutadiene. Int. J. Chem. Kinet. 1989, 21, 233−243. (60) Frenklach, M. Reaction Mechanism of Soot Formation in Flames. Phys. Chem. Chem. Phys. 2002, 4, 2028−2037. (61) Johansson, K. O.; Head-Gordon, M. P.; Schrader, P. E.; Wilson, K. R.; Michelsen, H. A. Resonance-stabilized Hydrocarbonradical Chain Reactions May Explain Soot Inception and Growth. Science 2018, 361, 997−1000.

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Higher Activity Leading to Higher Disorder: A Case of Four Light

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