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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Early Events of the Carburization of Fe Nanoparticles in Ethylene Pyrolysis: Reactive Force Field Molecular Dynamics Simulations Xiangyang Wang, Xianggui Xue, and Chaoyang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01481 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry

Early Events of the Carburization of Fe Nanoparticles in Ethylene Pyrolysis: Reactive Force Field Molecular Dynamics Simulations Xiangyang Wang,†‡ Xianggui Xue,*‡ and Chaoyang Zhang*‡ †

College of Material Science and Engineering, Southwest University of Science & Technology, Mianyang, Sichuan 621900, China.

‡

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-327, Mianyang, Sichuan 621900, China.

Abstract: The carburization of transition metals in hydrocarbons pyrolysis is a common corrosion phenomenon in the petrochemical industry. Nevertheless, early events of carburization mechanism remain still unclear. The present work reveals the details at earlier stages of the Fe nanoparticles carburization in ethylene (C2H4) pyrolysis with reactive ReaxFF force field molecular dynamics simulations. Our results show that the chemisorption and dissociation of C2H4 on Fe surfaces are crucial steps to the carburization corrosion. First of all, C2H4 molecules are chemically adsorbed on the surface of the Fe nanoparticle. Afterwards, continuous dehydrogenation of C2H4 occurs by C-H bonds break to form C2Hx (x=0~3). And finally, an amorphous C-rich carbide FeC3.39 is obtained. The carbide formation proceeds in four sequential and repetitive stages, including chemisorption and dehydrogenation of C2H4 on the surface of the Fe nanoparticle, diffusion and polymerization of C2Hx to form short C chains on the surface and in the bulk of the particle; growth and branching of the short chains; and chain crosslinking to form longer and more branched chains. Our finding provides deep insight into the fundamental process of carburization corrosion of Fe nanoparticles in the hydrocarbon pyrolysis, as a common phenomenon observed in practice.

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1. INTRODUCTION Ethylene (C2H4) is known as a good C source to produce carbonaceous materials and a good H source, with the aids of transition metals catalysis. During the past decade or more, the growth of C nanomaterials on iron or stainless steel (SS) alloy catalysts with hydrocarbons as precursor gases have attracted a lot of interest.1-10 For example, the direct growth of C filaments (nanotubes and nanofibers) on SS using C2H4 as a C source was reported.7-10 Due to the elevated temperatures and saturated carbonaceous atmospheres, carburization or metal dusting is accompanied by the formation of C filaments. In this process, the graphite coverage on the metal surfaces breaks bulk metallic structures into powders, which may be composed of carbon, carbides, and metal nanoparticles.11,12 These nanoparticles can act as catalysts for further C deposition,11 in the forms of graphite and CNTs.13 Carburization of transition metals in hydrocarbon environment at high temperature is a common corrosion phenomenon. It has been a long-standing problem in petrochemical, syngas, and other industries, as it poisons the catalysts, impairs the heat transfer efficiency, damages the structural materials, and even threatens personal safety.14-16 Therefore, it is crucial to understand the corrosion mechanism to benefit applications and reduce hazards. Lots of work concerning the carburization of pure iron as well as various alloy steels have been studied.16-22 As to the reaction mechanism of the carburization, the most widely accepted one was proposed by Hochman.23 Subsequently, the mechanism was confirmed by Grabke to include with four steps: (1) C from the carburizing atmosphere attacks the metal surfaces and is dissolved into the metals; (2) instable carbide Fe3C is formed as an intermediate; (3) the metastable Fe3C is further decomposed into Fe and graphite; and (4) deposition of additional C from the atmosphere on the surfaces of the fine metal particles occurs by repeating the former three steps. 21-22

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Although various experimental techniques including Raman scattering, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have widely been used to reveal the mechanism of metal dusting corrosions, it has so far not been fully understood.13,24,25 In fact, the understanding of the micro-processes and the detailed mechanism of this reaction can be advanced through computer simulations. Up to now, many theoretical studies focused on the adsorption and decomposition of carbon-containing gasses on metal surfaces or particles.26-35 First principle simulations are accurate enough to identify carburization and metal dusting mechanisms. Nevertheless, this approach remains extremely computationally demanding, even with massively parallel high-performance computer systems, preventing its application to large-scale issues. A recently developed molecular reactive force field, ReaxFF,36 provides an opportunity to overcome these difficulties. It has successfully been applied to investigate the catalytic activities of C, H and O on metal surfaces with good accuracy.31-35 For example, using ReaxFF force field, Sahputra et al studied carbon adsorption and diffusion through the surface of Fe and in bulk. Their studies indicated that the preferred C adsorption sites, the surface to subsurface diffusion of C atoms and their migration paths over the Fe (110) surface are in good agreement with density functional theory (DFT) simulation results.33 Recently, Bentria et al reported the effect of grain boundaries emerging at free surfaces on C adsorption and CO dissociation using both DFT and ReaxFF force field simulations.35 Their work showed that grooves possess preferential binding sites, and explained why emerging grain boundaries are more severely attacked by the metal dusting corrosion. Although the theoretical work has provided a general understanding of the very early stages of the metal dusting corrosion initiation at very small time and length scales, more insight into the details associated with corrosion is still required. We in the present work focus on the Fe carburization mechanism in C2H4 pyrolysis on an iron

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nanoparticle. Reactive ReaxFF force field molecular dynamics (MD) simulations were performed on the C2H4 decay with a catalyst of the iron nanoparticle. As a result, an amorphous C-rich carbide FeC3.39 was achieved from our simulations. And the formation mechanism of the iron carbide is proposed as following: chemisorption and dehydrogenation of C2H4 on the Fe nanoparticle to dissociate H atoms and form (C2Hx, x=0, 1, 2 or 3), diffusion and polymerization of C2Hx to form short carbon chains on the surface and bulk of the particle, chain growth and branching of the short chains, and chain crosslinking of long and branched chains. Moreover, the detailed structure of carbon-rich carbide FeC3.39 was investigated by radial distribution functions (RDFs) analyses, consistent with other theoretical and experimental results. Wholly, this work is expected to make progress in understanding the carburization corrosion of Fe in hydrocarbons pyrolysis. 2. METHODOLOGIES

Figure 1. Cubic box containing 800 C2H4 molecules and a Fe nanoparticle for modeling. (a) d = 0.01 g/cm3 and (b) d = 0.1 g/cm3. C, H and Fe atoms are represented in black, green and orange, respectively. These representations are employed in the following figures.

In this study we constructed two cubic cells for simulations, each with 800 C2H4 molecules and a Fe nanoparticle. The Fe nanoparticle was established by encompassing Fe atoms in the perfect Fe bulk into a sphere with a radius of 10 Å, that is, 339 Fe atoms were included. The two cubic cells

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with lengths of 71.96 and 155.04 Å contain C2H4 molecules with densities of 0.1 and 0.01g/cm3, respectively. The initial molecular configurations in the cells were constructed randomly by Amorphous Cell module in Materials Studio 6.0, as illustrated in Figure 1. Compared to the density at standard state of 0.00117 g/cm3, these C2H4 molecules in the simulation box are greatly compressed. Due to the higher pressure and concentration of C2H4 caused by compression, reaction velocities will be increased and equilibration time of reactions will also be decreased at the same temperature in terms of chemical kinetic theory. That is, the compression that leads to a higher pressure and a higher concentration of reaction gas will accelerate reaction velocities and shorten the time to reach same result at the same temperature. It facilitates to achieve more information within the time limitation of our simulations. All MD simulations were carried out using LAMMPS software package.37,38 A recent version of the ReaxFF potential for C/H/O/Fe/Cr/S developed by Shin et al

39

was adopted. The reliability of

this ReaxFF parameter was evaluated (see S3 of Supporting Information (SI), including Figures S5 and S6). The principle of ReaxFF can be referred to Ref. 36. In a typical procedure, ReaxFF is an improved force field derived from earlier reactive empirical force fields, such as those by Brenner, which are parameterized to reproduce the density functional theory (DFT) results for selected systems and properties.40,41 Moreover, ReaxFF has been found to accurately capture the reaction dynamics of a variety of molecular systems in catalytic environments.31-35 Before MD simulations, the conjugate gradient algorithm was employed to relax the local unfavorable structures of the systems using the common nonreactive COMPASS force field in the Materials Studio software, to keep the initial molecules unreacted. Thereafter, energy minimizations were performed again using conjugate gradient algorithm with the ReaxFF reactive force field. Afterwards, MD simulations were performed in canonical (NVT) ensemble with a timestep of 0.1 fs.

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The equations of motion were integrated using a velocity Verlet algorithm. Each cell was relaxed using an NVT MD simulation at 300 K for 5 ps, and no C2H4 decay was found prior to heating to the assigned temperatures, which were controlled by Nosé-Hoover chain (chain length n=3) thermostat with a relaxation time of 20 fs. Thereby, four independent simulations were performed, and the atomic positions and velocities were collected every 1 ps, namely, 10000 frames were used for trajectory analyses. For the cell with a density of 0.1 g/cm3, three temperatures of 1500, 2000 and 2500 K were adopted in the simulations. While, for the cell of 0.01 g/cm3, the simulation at 2500 K was conducted alone for a comparison purpose. The MD simulation lasted for 14 ns in the case of 0.1 g/cm3 and 2500 K, while 10 ns for the remaining cases. To identify the chemical species and their distributions in the dynamics evolutions, we used FindMole procedure42 to analyze dynamic trajectories, which is a FORTRAN program based on Strachan et al’s proposal.43 3. RESULTS AND DISCUSSION

Figure 2. Snapshots of the configurations of the Fe nanoparticle during C2H4 pyrolysis on it; the top of each plot exhibits a carbide, and the bottom shows only the carbon atoms in the carbide for clarity. 6


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As described above, four reactive MD simulations. Figure 2 shows the typical reaction process of ethylene pyrolysis on the iron nanoparticle. After carefully comparing the pyrolysis products, we find that the model with a density of 0.1 g/cm3 at 2500 K proceeds fastest due to the highest density and the highest temperature as expected. In this case, pyrolysis nearly reaches equilibrium at ~10 ns, i.e., a stable carbide is formed this time. In addition, by comparing snapshots in Figures S1-S3 in SI with those in Figure 2, we find they exhibit a similar evolution mechanism with different reaction rates. It is desirable for C2H4 to rapidly decompose within the first few ns at a high density of 0.1 g/cm3 and a high temperature of 2500 K high temperature so that carbon chain growth can better be studied. Therefore, to more comprehensively understand the process of Fe carburization, we mainly discuss the simulation results in the case of 2500 K as follows, unless otherwise specified. 3.1 Chemisorption and Decomposition 1.0

C2H4

0.9

Ratio of unreacted C2H4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C2H4+Fe

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

200

400

600

800 1000 1200 1400 1600 1800 2000

Time,ps

Figure 3. Comparison in the ratio of unreacted C2H4 with and without Fe nanoparticle.

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Figure 4. Snapshots of configurations observed during C2H4 pyrolysis on the surface of the Fe nanoparticle: (a) C2H4, (b) C2H3, (c) C2H2, (d) C2H and (e) C2 chemisorbed on the particle surface, and (f) C2 in the region of the subsurface.

To clarify the effect of the Fe nanoparticle on the C2H4 decay, we first performed a similar simulation without the particle. As demonstrated in Figure 3, the Fe nanoparticle evidently accelerates the C2H4 decay. As a matter of fact, the C2H4 decay begins at ~800 ps without the particle. The rapid C2H4 reduction with the particle is resulted from the chemical adsorption of C2H4 on the particle surface. The snapshots of atomic configurations of C2H4 chemisorbed on the surface of the catalytic iron particle are shown in Figure 4. Firstly, several C2H4 molecules chemisorbed on the iron particle without fragmentation are observed at ~1 ps (Figure (4a)). The π bonds of C2H4 molecules are broken to form σ bonds as it is chemisorbed to the surface, which is in good agreement with the experimental investigations for the chemisorption of acetylene and C2H4 adsorbed on Fe (110) and (111) surfaces.44,45 Our result is also consistent with a ReaxFF MD simulation by Goddard and co-workers, in which C2H4 chemisorption on the Ni particle surfaces with the π bond break to form σ bond was observed.32 The C-C bonds length of absorbed molecules was calculated to be 1.49-1.62 Å, in agreement with experimental and theoretical values of 1.455-1.52 Å for chemisorption of C2H4 on Fe46 and Pt clusters.27,47 The calculated C-C bond length in our work is close to that in ethane (1.54 Å), much longer than that of C2H4 (1.35 Å).48 It indicates 8


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the C-C bond rehybridization from sp2 to sp3. C2H4 is stable for a few picoseconds. Afterwards, a hydrogen atom is dissociated from C2H4 to form C2H3 at ~11 ps (Figure 4(b)). And the remaining three hydrogen atoms are step-by-step dissociated to form C2H2, C2H, and C2 intermediates (Figures (4c) to (4f)). These intermediates were also experimentally observed in the past.44,45,49 In our simulations, dehydrogenation is generally completed to form C2 before the C-C bond breaks. This result is in agreement with the preference for ethyne and C2H4 adsorption on the Ni particles, in which both dehydrogenates are completed before the C-C bonds partition.32 3.2 Chemical species and their evolution 1200

1200

(b)

(a) H H2 C2H4

800

T=1500K d=0.1g/cm3

1000

Amount of Particles

T=2500K 3 d=0.1g/cm

1000

Amount

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

400

H H2 C2H4

800

600

400

200

200

0

0 0

2000

4000

6000

8000

10000

12000

0

14000

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Time,ps

Time,ps

Figure 5. Evolution of the amounts of C2H4, H and H2 at 2500 (a) and 1500 K (b).

For the details of chemical reactions, attention should be paid to the evolution of related chemical species. As mentioned above, C2H4 is firstly adsorbed on the surface of the catalyst, subsequently, continuous dehydrogenation takes place with breaking C-H bonds to form C2H3, C2H2, C2H and C2, as displayed in Figure 4. The evolution of C2H4, H2 and H at temperatures of 2500 and 1500 K is shown in Figure 5. It is clearly seen that the rapid growth of H2 accompanied by the C2H4 reduction at 2500 K in Figure 5(a). while, the number H atoms is constant during the simulation. As a matter of fact, the pyrolysis of hydrocarbons is a way to H2 production.50-53 Besides, to obtain the formation mechanism of H2, the detailed reaction paths were analyzed. Table S1 of Supporting 9



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Information shows the primary reactions during the first two ns at 2500 K. As demonstrated in the table, the H2 is produced by the reactions including C2H4+H→C2H3+H2, C2H5→C2H3+H2, C2H4→C2H2+H2, CH4+H→CH3+H2 and C4H5→C4H3+H2. The decay of the reactant C2H4 is finished at ~8 ns at 2500 K; while, only ~230 C2H4 are decayed at 1500 K in Figure 5(b). With respect to atomic H, it reaches a dynamic equilibrium amount of 25 in a short time at temperature of 2500 K, only ∼500 ps. Although these atomic H possess a low population, they play an important role in the decay of hydrocarbon as they take part in many reactions in combination with other molecules or radicals.54,55 Table 1. Early Chemical Events on the Fe Nanoparticle at 2500 and 1500 K. Temperature

Serial numbers

Primary reactions

Time, ps

2500 K

1

Fe339 + C2H4 → C2H4Fe339

3

2

C2H4Fe339 + 14C2H4 → C30H60Fe339

5

3

C30H60Fe339 + 4C2H4 → C34H68Fe339 + 2C2H4

6

4

C34H68Fe339 + 6C2H4 → C38H77Fe339 + 3C2H4 +C2H3

7

5

C38H77Fe339 + 7C2H4 + C2H3 → C48H97Fe339 + 2C2H4 +C2H3

9

6

C48H97Fe339 + 7C2H4 → C60H119Fe339 + C2H4 + H2

11

7

C60H119Fe339 + C2H4 → C60H119Fe339 + C2H2 + H2

12

8

C60H119Fe339 + 3C2H4 + C2H2 → C66H129Fe339 + C2H4

13

9

C66H129Fe339 + 3C2H4 → C68H129Fe339 + C2H4 + C2H6 + H2

14

10

C68H129Fe339 + 3C2H4 → C72H133Fe339 + C2H4 + 2H2

15

11

C72H133Fe339 + 6C2H4 → C80H150Fe339 + C2H4 + C2H3

16

12

C80H150Fe339 + 4C2H4 → C82H146Fe339 + 2C2H4 + C2H5 + 3H2 + H

17

1500 K

…

……

1

Fe339 + C2H4 → C2H4Fe339

3

2

C2H4Fe339 + 7C2H4 → C16H32Fe339

5

3

C16H32Fe339 + 2C2H4 → C14H28Fe339 + 3C2H4

6

4

C14H28Fe339 + 2C2H4 → C18H36Fe339

7

5

C18H36Fe339 + 6C2H4 → C26H52Fe339 +2C2H4

8

6

C26H52Fe339 + 2C2H4 → C28H56Fe339 + C2H4

9

7

C28H56Fe339 + C2H4 → C30H60Fe339

11

8

C30H60Fe339 + 2C2H4 → C34H68Fe339

13

9

C34H68Fe339 + 2C2H4 → C38H76Fe339

14

10

C38H76Fe339 + C2H4 → C40H80Fe339

15

11

C40H80Fe339 →C36H72Fe339 + 2C2H4

16

12

C36H72Fe339 + 3C2H4 → C42H84Fe339

17

13

C42H84Fe339 + C2H4 → C44H88Fe339

18

…

……

10


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In addition, we pay attention to the early events of the nanoparticle. As listed in Table 1, at 2500 K, the chemical absorption of one C2H4 molecule on the surface of the nanoparticle starts at 3 ps, and more C2H4 molecules at 5 ps; subsequently, two absorbed C2H4 molecules are dissociated from the particle while four are attached to it; at 7 ps, it shows a C-H bond break as C2H3 appears as a product, and the dissociated H atom is bonded with the particle; afterward, the isolated decay products of C2H2, H2, H, C2H6 and C2H5 are partitioned from the particle. And till 17 ps, all the Fe atoms are clustered and the C atoms contained in the particle continuously increase. In the case of the relatively low temperature of 1500 K, it only shows more absorption and less desorption of C2H4, without any C-H bond dissociation. Compared with the configurations in S1 of SI, it exhibits no evidently polymerized C atoms at 1500 K and 10 ns; while, most of C atoms are clustered at 2500 K for 10 ns; and the case of 2000 K is moderated between those of 1500 and 2500 K. 3.3 Iron carbide formation

Figure 6. Typical structures showing the formation mechanism of the Fe carbide at different stages: (a) to (f) chemisorption and dehydrogenation; (g) diffusion and polymerization; (h) chain growth and branching; and (i) and (j) chain crosslinking.

The snapshots in Figure 2 show the details of the carbide formation from reactions of C2H4 with Fe nanoparticle. To be clearer, we abstracted a few typical structures in Figure 6 to show the stages

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of the carbide formation. As indicated in the figure, the whole process can be divided into the following stages. The first stage is chemisorption and dehydrogenation, as illustrated in Figures 6 (a) to (f). At this stage, several C2H4 molecules are chemisorbed on the surface of the Fe nanoparticle. Thereafter, continuous dehydrogenation takes place by breaking C-H bonds to form step by step C2H3, C2H2, C2H and C2. The second stage is diffusion and polymerization (Figure 6 (g)), with more and more C2Hx (x=0-3) formed from the C2H4 dehydrogenation. Meanwhile, some C2Hx penetrate into the bulk of the Fe nanoparticle, with diffusion on the surface or in bulk of the nanoparticle. Initially, the C2Hx monomers are diffused alone due to their low concentration. As time goes on, the concentration of C2Hx is elevated, with some of C2Hx polymerized into short carbon chains at ~0.1 ns (Figure 6(g)). At the stage of chain growth and branching (Figure 6(h)), some short chains continue to be grown into longer ones. Meanwhile, some short chains are connected with one another to form branched ones at ~1 ns. This stage lasts till 3 ns. The final stage is chain crosslinking (Figures 6(i) and 6(j)). At this stage, there are many longer and more branched chains in the Fe nanoparticle, which are easy connected with one another to form a crosslinking network, as demonstrated in Figures 2 (f) to 2(l) and Figures 6(i) and 6(j). The formation of the carbide is consistent with the first two steps of the corrosion mechanism proposed by Hochman23 and Grabke et al

21-22

: (1) carbon from the carburizing atmosphere attacks the metal surface and is

dissolved into the metal; and (2) instable carbide forms as an intermediate. In our simulations, more details can be achieved. The cyclization is found in our simulation, as found in our previous work of small molecular hydrocarbon pyrolysis without catalyst.54,55 A six membered ring is formed at ~6 ns (Figure (2h); thereafter, the cycle condensation takes place. Eventually, a polycyclic structure (a six-membered ring with two five-membered rings) (Figure (2l)) appears, which can be regarded as a seed to grow carbon filaments in experiments.7-10

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1200

1000

Number of C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

600

400

200

0 0

2000

4000

6000

8000

10000

12000

14000

Time,ps

Figure 7. Evolution of the number of C contained in the Fe particle.

In order to determine the size of the carbide, we count the number of carbon atoms adsorbed on and in the particle. Figure 7 shows the evolution of the number of C atoms contained in the particle. It is seen that the number increases sharply to ~600 during the first 0.5 ns, implying that ~300 C2H4 molecules are adsorbed on the surface of the particle. Then the number increased mildly until it reaches equilibrium of ~1150 at the end of 10 ns. Consequently, we obtain Fe carbide with a chemical formula of FeC3.39, showing the richness of carbon in the carbide. Additionally, from Figure 2, we find that the Fe particle is filled with carbon atoms. It should be reasonable, as Fe-C can exist as an amorphous solid with a random ratio at such high temperature of 2500 K, in terms of the Fe-C phase diagram. 56

Figure 8. Snapshots of the initial and final configurations of the particle with only Fe atoms, (a) 0 ns and (b) 14 ns.

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Because of the carburization, the size of the particle increases. Figure 8 shows the snapshots of the initial and final increased configurations of the particle with Fe atoms alone. Carbon pickup increases the metal volume and results in internally induced stresses, leading to the failures in pyrolysis coils for ethylene cracking.16

Figure 9. Snapshots of the configurations of the particle only with Fe and C atoms for clarity. Top and bottom are overall and section views, respectively.

Furthermore, the section views at some typical time points in Figure 9 show the step-by-step carburization from the outside to the center of the particle. During this period, Fe atoms are more and more separated with the carburization enhancement.

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140

(b) 0.05ns

(a) 0ns

(d) 2ns

(c) 1ns

1.19

1.19

120 100

1.35

80

1.35

60 1.35

1.37

40 1.19

20

2.63 3.71

2.61 3.83

0 140

(e) 3ns

(g) 5ns

(f) 4ns

(h) 6ns

120

g(r)

100

1.19

1.19

1.19

80

1.19

60

1.41

1.41

1.39 1.35

40 20 2.61

2.59

2.61 3.87

3.75

0 140

(i) 8ns

2.61

3.87

(j) 10ns

3.77

(l) 14ns

(k) 12ns

120 100 80

1.17

1.43

1.45

1.19 1.43

1.43

60

1.19

1.19

40 2.61

20

2.61

3.79

2.61

3.83

2.63 3.79

3.85

0 0

1

2

3

4

5

6

7

8

9

10 0

1

2

3

4

5

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8

9

10 0

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2

3

4

5

6

7

8

9

10 0

1

2

3

4

5

6

7

8

9

10

r, angstrom

Figure 10. Radial distribution functions (RDFs) of C-C at different time for system density of 0.1g/cm3 at 2500 K. (a) to (l) correspond to the time of 0, 0.05, 1, 2, 3, 4, 5, 6, 8, 10, 12 and 14 ns, respectively.

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


55 50 45 40 35 30 25 20 15 10 5 0 55 50 45 40 35 30 25 20 15 10 5 0 55 50 45 40 35 30 25 20 15 10 5 0

(a) 0ns

(b) 0.05ns

(c) 1ns

(d) 2ns 2.03

2.01

1.95 3.07

3.09 2.85

(e) 3ns

2.05

2.05

(f) 4ns

(g) 5ns

(h) 6ns

2.07

2.09

3.05

2.99

(i) 8ns

2.01

3.15

(j) 10ns

2.09

3.25

3.15

(k) 12ns

2.07

2.93

3.11

(l) 14ns

2.05

3.17

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10

r, angstrom

Figure 11. Radial distribution functions (RDFs) of C-Fe at different times for system density of 0.1g/cm3 at 2500 K. (a) to (l) correspond to the time of 0, 0.05, 1, 2, 3, 4, 5, 6, 8, 10, 12 and 14 ns, respectively.

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250 225 200 175 150 125 100 75 50 25 0 250 225 200 175 150 125 100 75 50 25 250 0 225 200 175 150 125 100 75 50 25 0

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.55

(a) 0ns 3.63

(b) 0.05ns

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(c) 1ns

(d) 2ns

4.43

2.57 2.57 2.57

4.41

4.15

(f) 4ns

(e) 3ns

2.57 4.25

2.57

(h) 6ns

2.57 3.99

4.02

(k) 12ns

(j) 10ns

(i) 8ns

2.57 4.01

(g) 5ns

2.57

2.57 3.99

3.99

2.57 3.91

3.99

(l) 14ns

2.57

3.91

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10

r, angstrom

Figure 12. Radial distribution functions (RDFs) of Fe-Fe at different times for system density of 0.1 g/cm3 at 2500 K. (a) to (l) correspond to the time of 0, 0.05, 1, 2, 3, 4, 5, 6, 8, 10, 12 and 14 ns, respectively.

In order to further confirm the evolution of ethylene and iron in the reactions, we focus on the evolution of the C-C and C-Fe bond types. A statistical method of radial distribution function (RDF, g(r)) that can provide assigned interatomic distances in any complex system within a cutoff was adopted.57 Figures 10 to12 show the RDFs of C-C, C-Fe and Fe-Fe at different times in the case of 0.1 g/cm3 and 2500 K, respectively. The dominant C-C distance at 0 ns is 1.35 Å (Figure 10), corresponding to C=C bond of C2H4.48 Then the first peak is split into two ones at 1.19 and 1.35 Å at 0.05 ns, respectively. The distance of 1.19 Å corresponds to a triple C≡C bond.58 It is consistent with the result of species analysis in Figure S4 of SI. As time proceeds, the peak of 1.19 Å becomes dominant and the peak of 1.35 Å disappears, corresponding to the increase of C2H2 and decay of C2H4. Meanwhile, a new peak of 1.41-1.43 Å appears at 5 ns, corresponding to the C-C bond in the carbide. This peak is in good agreement with a selected area electron diffraction (SAED) result for carbon-rich carbide Fe1−xCx (x=0.718), in which a peak at 1.4 Å was found.59 Figure 11 shows that

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the first peak of C-Fe RDF appears at 1.95 Å, corresponding to C2H4 and its derivant chemisorbed on the nanoparticle. Afterwards, this peak becomes stronger and stronger and shifts within 2.03~2.09 Å as time goes on, suggesting the formation of iron carbide. Meanwhile, with respect to the RDFs of Fe-Fe in Figure 12, it is dominant at 1.57 Å as an Fe-Fe bond and is weakened as time proceeds, due to the C separation to form numerous C-Fe, which can also be concluded from Figure 8. Our result is consistent with another simulation on the Fe3C crystal (1.95 Å)

60

and a SAED

experimental result of the amorphous structure of Fe carbide (2.0 Å).59 4. CONCLUSIONS Reactive MD simulations combined with ReaxFF force field have been performed to investigate the Fe metal carburization from ethylene pyrolysis on an iron nanoparticle. Our work exhibits the detailed carburization mechanism of an iron nanoparticle. Firstly, ethylene molecules are adsorbed on the surface of the Fe catalysts; subsequently, continuous ethylene dehydrogenation through the C-H bond break to form C2Hx (x=0~3). In addition, an amorphous carbon-rich carbide FeC3.39 is obtained from our simulation. The formation mechanism of the iron carbide is proposed to follow the four steps, including chemisorption and dehydrogenation of C2H4 on the Fe nanoparticle, diffusion and polymerization of C2Hx to form short carbon chains on the surface and in bulk of the particle, growth and branching of short chains, and chain crosslinking to longer chains and more branched chains. Moreover, a polycyclic structure is formed on the surface of the particle, as a seed to grow carbon filaments. Our work provides deep insight into the earlier stages of the metal carburization. We hope our work may be helpful to control the hazards of carburization and benefit its application to produce carbonaceous materials. ■ ASSOCIATED CONTENT Supporting Information 17



Comparison of pyrolysis evolutions under various conditions, and the evolutions of the ReaxFF parameters and of C species in the case of 2500 K and 0.1 g/cm3. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author X. G. Xue, email: [email protected]; Tel: 86-816-2488489. C. Y. Zhang, email: [email protected]; Tel: 86-816-2493506. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT The authors gratefully acknowledge the support of the National Natural Science Foundation of China (11602241 and 21673210).

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