Molecular Insight into the Growth of Hydrogen and Methane Binary

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Molecular Insight into the Growth of Hydrogen and Methane Binary Hydrates Zhengcai Zhang, Peter G. Kusalik, and Guang-Jun Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00842 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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The Journal of Physical Chemistry

Molecular Insight into the Growth of Hydrogen and Methane Binary Hydrates Zhengcai Zhang,†‡§ Peter G. Kusalik, ‡ Guang-Jun Guo†§¶* †

Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics,

Chinese Academy of Sciences, Beijing 100029, China. ‡

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary,

T2N 1N4, Alberta, Canada.

§

Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China.

¶

College of Earth and Planetary Sciences, University of Chinese Academy of Sciences,

Beijing 100049, China.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. G.)

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ABSTRACT H2 is considered as the ideal fuel, however, the storage and transportation of H2 limit its usage. Clathrate hydrates are candidate materials for H2 storage and transportation. Due to the extreme conditions necessary to stabilize pure H2 hydrate, additives are proposed to stabilize a mixed H2 hydrate. Compared to the widely studied H2 + THF binary hydrates, H2 + CH4 binary hydrates contain a higher energy density. In this study, we study the growth of H2 + CH4 binary hydrates for two sets of temperature and pressure conditions by using molecular dynamics simulations with atomic models. Our results show that CH4 acts as a thermodynamic promoter for H2 hydrate formation while H2 acts as a kinetic promoter for H2 + CH4 hydrate growth at some of our working conditions. We find that there is a maximum growth rate of H2 + CH4 binary hydrates at 250 K when the pressure is 50 MPa, and at fixed temperature the growth rate of H2 + CH4 binary hydrates shows a positive correlation with pressure. We also find that adding H2 in the gas phase, decreasing temperature (not smaller than 240 K), or increasing pressure all can dramatically reduce the percentage of empty cages in the grown hydrate. Moreover, with increasing temperature, the occupancy of 512 and 51264 cages by H2 decreases, and inversely the occupancy of cages by CH4 increases when the temperature is above 240 K. With increasing pressure, there is an increase in the percentage of 512 cages occupied by H2, where the ratio of H2 and CH4 occupied cages in grown hydrate can be 3:1 at 250 K and 80 MPa. However, the occupancy of 5 1264 cages by H2 and CH4 remains relatively constant with increasing pressure. In addition, at our working conditions, 51264 cages can be double-occupied by H2 and several 51264


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cages can be occupied by H2 and CH4 or triple- H2. Our simulations show that the solubility and diffusivity of guest molecules, especially CH4, in solution dominate the growth process.



1. INTRODUCTION H2 is regarded as the ideal fuel because it is environmentally clean and has high energy density.1-3 To use it in vehicles, H2 needs to be condensed to a liquid at 20 K or compressed at high pressure. However, due to the practical and safety concerns, these methods cannot be widely used.1-3 To achieve the efficient storage of H2, molecular adsorption on porous solids and H2 containing molecular crystals are developed.1, 3 One of the materials for H2 storage is clathrate hydrates. Clathrate hydrates are nonstoichiometric crystalline inclusion compounds composed of hydrogen-bonded water formed cages and small guest molecules, such as CH4, CO2, and H2.4 H2 hydrate was firstly synthesized by Dyadin et al.63 In 2002, Mao et al found that H2 and water mixtures form sII clathrate at 250-600 MPa and 240-249 K with a maximal H2 storage capacity of 5.3 weight percent.5 H2 can occupy the small 512 cages and also the larger 51264 cages, where the larger cages can be multi-occupied.611

In order to decrease the impractical pressure to stabilize the H2 hydrate, other

additives, i.e. tetrahydrofuran (THF) is used to synthesize the hydrate,12-19 where H2+THF binary hydrates can form at ~10 MPa and 280 K.12 However, THF molecules occupy all the larger 51264 cages, H2 molecules only occupy the small 512 cage. To achieve a high H2 occupancy, researchers reported that H2 molecules can occupy 51264 cages when there is a small amount of THF (below 5.6 mol %) in water. 13-14 Nevertheless, some researchers claimed that the concentration of THF cannot tune the H2 occupancy of 51264 cages.20-23 Recently, Matsumoto et al. proposed that CH4 can also serve as a thermodynamic


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promoter for the formation of H2 hydrate.24 On the one hand, as CH4 is relatively small, H2 and CH4 can both occupy 512 and 51264 cages. As a result, this may enhance the overall number of H2 in hydrates. On the other hand, both CH4 and H2 are fuel gases, H2+CH4 binary hydrates should have high energy density. However, there are few studies on H2+CH4 binary hydrates and most of them are on thermodynamic properties.24-28 As the binary hydrates grown at high thermodynamic driving force do not resemble the composition of the most stable crystal,29 we conjecture that growth kinetics of hydrates can affect the overall energy storage, the details of which are difficult for experimental studies to probe due to the temporal and spatial limitation. It should be noted that the only molecular dynamics study on binary hydrate growth was performed by Song et al.29 They reported the growth of binary hydrates with the mW water model and two highly soluble guest molecules (H2 and THF), where the guest model parameters were chosen to resemble the size of H2 and THF. As the mass transfer of guest molecules is of great importance to the growth process of hydrate,30-33 we believe that the growth of binary hydrates deserves investigation with atomic models. Hence, in this study, we use molecular dynamics simulations with atomic models to determine the effects of pressure and temperature on the growth of H2+CH4 binary hydrates. The growth mechanism of H2+CH4 binary hydrates is also analyzed to yield molecular level insights. This study will provide guidance on how to enhance the energy storage efficiency of gas hydrates.

2. SIMULATION DETAILS



2.1. Molecular dynamics simulations. Gas-liquid-hydrate-liquid-gas three-phase systems (the initial configuration is shown in Figure 1A) were used to evaluate the growth of CH4, H2, and H2+CH4 binary hydrates. The (001) crystal plane of a 2 ×2 × 2 sII hydrate crystal fully filled with CH4 faces the liquid. sII hydrate was chosen because H2+CH4 binary hydrates according to previous experimental studies

Note that the growth process of pure sII CH4 hydrate is used as a reference system (i.e. limiting case) against which to compare the behavior of H2 + CH4 binary hydrates and that sII CH4 hydrate is metastable for our conditions. The initial liquid phase contains 1088 water molecules and the gaseous phase contains a total of 384 gas molecules (for H2+CH4 binary hydrates, the ratio between H2 and CH4 is 1:1, while for H2 or CH4 hydrate, pure H2 or CH4 were used). The TIP4P/ICE water model was used and for CH4 the OPLS-UA model was employed,35 which can well describe the phase equilibria of methane hydrate.36 As for the H2-H2 and H2-Water interactions, we used the parameters of Alavi et al., 7 which can well describe the structure of pure H2 hydrate and H2-THF mixed


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hydrate,7, 37-38 the dynamics of H2 in hydrate,37 the H2-diffusion behavior in hydrate,8-9, 11, 37, 39-40

and also the measured incoherent neutron scattering data for various

transitions.41-42 Other cross-interactions were generated with Lorentz–Berthelot combining rules. Hence the models used in this work should provide a good description of the growth of H2+CH4 binary hydrates. Molecular dynamics simulations were performed with the GROMACS package.43 The NPT ensemble was used, where the temperature and pressure were maintained with a Nosé–Hoover thermostat44-45 (with a time constant of 2 ps) and a Parrinello–Rahman barostat46 (with a time constant of 4 ps), where the pressure is anisotropically controlled in the xy directions and z direction, respectively. Simulations were performed for the temperature series 230 K, 240 K, 250 K, 260 K, and 270 K and 50 MPa, while the pressure series 20 MPa, 50 MPa, 80 MPa, and 110 MPa was performed at 250 K. After 2 ns equilibration for each starting configuration, 1.0 µs simulations were performed. Four parallel simulations with different initial molecular velocities at each condition were performed to evaluate the reproducibility of the results. 2.2 Growth rate. Different from previous methods which calculated the growth rate by counting the water molecules added to the hydrate phase,47 or tracking the position of the hydrate-liquid interface,48-50 we used the number of 512 (S) and 51264 (L) cages to track the growth rate. The time windows used to get the growth rates are 100-250 ns at 110 MPa and 250 K, 100-300 ns at 80 MPa and 250 K, and 100-500 ns for other conditions. The first 100 ns was used to achieve steady state growth. We choose these time windows because the growth rate remains roughly constant and reasonable



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amounts of gas and liquid phase remain. For example, for the latter conditions, the rate 𝑅=

𝑁500 𝑛𝑠 −𝑁100 𝑛𝑠 𝐿𝑥 ×𝐿𝑦 ×400 ns

(1)

where N100 ns and N500 ns are the numbers of cages at 100 ns and 500 ns, respectively. Lx and Ly are the system box length in the x and y directions, respectively. The cages were counted with the face-saturated incomplete cage analysis (FSICA) method, which can find all filled or empty face-saturated cages in the simulation system.51 We calculated five specific values of growth rate, namely RS[all], RS[H2], RS[CH4], RL[H2], RL[CH4], where RS[all] represents the growth rate of the number of 512 cages, L and S denote 512 and 51264 cages, respectively. [X] means X guest occupies the cage, where X can be H2 or CH4. Since only pure sII hydrate grows at the interface, the RS[all] and RL[all] give same results. Hereafter, we only use RS[all] to indicate the growth rate of the hydrate slab.

Figure 1. (A) A typical initial configuration with a simulation box 3.45×3.45×11.4 nm3 and (B) a snapshot at 600 ns at 250 K and 50 MPa. Water molecules are shown as red spheres. Hydrates are shown in red tubes. CH4 and H2 molecules are cyan and purple


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spheres, respectively. Images were generated with VMD.52

3. RESULTS AND DISCUSSION 3.1. Growth rate. The growth rate of H2+CH4 binary hydrates was investigated at 50 MPa and at 230 K, 240 K, 250 K, 260 K, and 270 K. We also determined the effect of pressure at 250 K and 20 MPa, 50 MPa, 80 MPa, and 110 MPa. Hydrates grow for all these working conditions and only sII hydrate is observed to form. A typical snapshot at 600 ns at 250 K and 50 MPa is shown in Figure 1B. Figure 2 shows the time evolution of the numbers of 512 and 51264 cages at 240 K and 50 MPa. One can see that both 512 and 51264 cages can be occupied by H2 and CH4, and there is only a small number of triple-H2 occupied 51264 cages (Figure 2D). The temperature and pressure dependence of the average growth rates are summarized in Table S1 (supporting information) and Figure 3, and Table S2 (supporting information) and Figure 4, respectively. One can see from Table S1 that H2 hydrate does not grow at 250 K and 50 MPa, which is reasonable because this condition is not in the formation region for pure H2 hydrate.33, 53 However, H2 + CH4 binary hydrates grow with many filled H2 cages at all the conditions examined. Clearly, CH4 is a promoter for H2 hydrate formation, which was also observed by previous experimental studies.24, 26 At the same time, the growth rate of pure CH4 hydrate is slightly smaller than that of H2 + CH4 binary hydrates at 250 K and 50 MPa.



Figure 2. Time evolution of the numbers of (A) all 512 cages, (B) filled 512 cages, (C) filled 51264 cages, and H2 filled 51264 cages at 240 K and 50 MPa. Shown in (A) are results from the four independent runs. Only newly formed cages from 100 ns to 500 ns were used to get the growth rate.

Figure 3 shows the temperature dependence of the growth rate of H2 + CH4 binary hydrates at 50 MPa. We find that the growth rates of H2 filled 512 and 51264 cages have a maximum at 250 K and dramatically decreases above 250 K, while the growth rate of CH4 filled cages stays relatively constant above 250 K. In general, the growth rate of H2 + CH4 binary hydrates has a maximum of 250 K and decreases above 250 K, which is similar to the temperature dependence of the growth of ice,54 and ethylene oxide hydrate.55 It means that temperature can have a significant impact on the growth of H2+CH4 binary hydrates.


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Figure 3. Temperature dependence of growth rates of H2+CH4 binary hydrates at 50 MPa. (A) is for the growth rate of 512 cages; (B) is for the growth rate of H2 or CH4 filled 512 cages; (C) is for the growth rate of H2 or CH4 filled 51264 cages. The blue points are the growth rates of pure CH4 hydrate at 250 K.

Figure 4. Pressure dependence of growth rates of H2+CH4 binary hydrates at 250 K. (A) is for the growth rate of 512 cages; (B) is for the growth rate of H2 or CH4 filled 512 cages; (C) is for the growth rate of H2 or CH4 filled 51264 cages. The blue points are the growth rates of pure CH4 hydrate at 50 MPa. Figure 4 illustrates the pressure dependence of growth rates of H2+CH4 binary hydrates at 250 K. Interestingly, the growth rate of H2+CH4 binary hydrates increases almost 300 % from 20 MPa to 110 MPa, which is larger than that determined for CH 4 hydrate (176% from 10 MPa to 100 MPa at 260 K for sI hydrate).30 One can see from Figure 4B that the growth rate of CH4 filled 512 cages stays almost constant, while that of the H2 filled 512 cages increases six-fold from 20 Mpa to 110 MPa. The growth rate for filled 51264 cages shows a similar behavior, where the growth rate of H2 filled 51264 cages increases 450% compared to the 200% increase for that of CH4 filled 51264 cages.



Clearly, increased pressure can increase the H2 storage in the hydrate and H2 is a kinetic promoter for the mixed hydrate growth in this scenario.

Figure 5. Temperature and pressure dependence of the occupancy of cages for the H2+CH4 binary hydrates grown. The occupancy data are the 10-ns averaged results at the ending part of the measured growth time window. All the data are collected from the newly formed hydrate cages.

3.2. Cage occupancy. Figure 5 shows the occupancy of hydrate cages by H2 and CH4. One can find that 23% of 512 cages and 6% of 51264 cages are empty at 250 K and 50 MPa. However, from the pure CH4 hydrate growth simulations, there are 44% empty 512 cages and 31% empty 51264 cages. In this sense, the overall storage of guest molecules in the hydrate structure increases when H2 is added to the system. It should be noted that the percentages of empty cages at 270 K and 50 MPa are 11 % for 512 and 2 % for 51264, which are close to the naturally formed CH4 hydrate.56 The occupancy of cages by H2 (Figure 5A and B) stays constant for 230-250 K and decreases when the


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temperature is above 250 K. At the same time, when the temperature is above 230 K, more and more cages are occupied by CH4 with increasing temperature. As for the pressure dependence, the occupancy of 512 cages by H2 increases from 20 MPa to 80 MPa (Figure 5C), while the occupancy of 512 cages by CH4 decreases. A similar phenomenon is observed for the occupancy of 51264 cages. When the pressure is above 80 MPa, the occupancy of cages changes little except for the double-H2 filled 51264 cages. It should be noted that as H2 is small enough, it can easily pass through the 6membered rings connecting the 51264 cages.6-7, 9, 11, 39-40, 57-59 In this study, we also find some H2 can hop between 51264 cages without the distortion of the cages (see the short movie in the supporting information), even where both of 51264 cages are filled with CH4. We conjecture that H2 hoping events are more common between H2 filled 51264 cages than for CH4 filled. Certainly, we find several 51264 cages filled with H2 and CH4 (Figure 6), especially at high temperature and pressure. Amano et al. once reported the possibility of this phenomenon in their experimental study on H2+Ar binary hydrates,64 and the hopping dynamics of H2 between hydrate cages deserves to be explored in the future work.



Figure 6. A configuration at 150 ns at 250 K and 110 MPa. The H2 molecules (purple spheres) in the dashed blue circles are those of H2 and CH4 filled 51264 cages. Molecules appear as in Figure 1.

3.3 Growth mechanism. Our simulations show that the growth rate of H2+CH4 binary hydrates correlates positively with pressure. Meanwhile, when H2 is added to the system, the growth rate of H2+CH4 binary hydrates is slightly above the growth rate of pure CH4 hydrate and more cages are filled by guest molecules at 250 K and 50 MPa. Therefore, we conjecture that the growth process may be controlled by the mass transfer process (i.e. the solubility and diffusivity of guest molecules). In fact, as reported by previous studies, the growth of CH4 hydrate is controlled by the solubility of CH4 in solution, the diffusivity of CH4 in water, and the adsorption of CH4 at the hydrate-liquid interface.30, 48 However, Yagasaki et al. proposed that the range of attractive force at the hydrate-liquid interface is sub-nanoscale, which has no effect on the mass transfer of guest molecules.60 That’s to say, it is the flux of CH4 in solution, which will depend on its solubility and diffusivity in water, that is the dominant limiting factor in the hydrate growth process. The experimental study on H2 hydrate formation also showed that the growth of hydrate is a diffusion-limited process.33 These studies support our conjecture.


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Figure 7. The dependence of the concentration of H2 and CH4 in solution with (A) temperature and (B) pressure. The concentration data are from Lx×Ly×1nm3 central solution volumes averaged over the period of 180-220 ns to avoid the influences of the solid-liquid and liquid-gas interfaces. The concentration is defined as NH2 or CH4 /NH2O. (C) and (D) show the dependence of self-diffusion coefficients of H2, H2O, and CH4 upon the temperature and pressure, respectively. The diffusivity data are from several independent simulations at each condition considered in the work (where each system contained 2944 H2O). The same concentration of H2 and CH4 with those from (A) and (B) were used to allow for accurate determination of the self-diffusion coefficients at each condition.

In order to provide insight into the mass transfer process, we show temperature and pressure dependence of the concentration and self-diffusion coefficient of H2 and CH4 in Figure 7. One can find that the temperature dependences of concentration and selfdiffusion coefficients of H2 and CH4 show opposing trends (Figure 7A and C). This interplay of concentration and diffusivity of guest molecules in solution results in a



maximum growth rate at 250 K at a pressure of 50 MPa (Figure 3). It should be noted that due to the slow dynamics of water at 230 K, the binary hydrates grow very slowly (Figure 3) and more 512 cages are occupied (Figure 5). As shown in Figure 7B and D, both of concentration and diffusivity increases with pressure, which can benefit the growth of hydrate. That’s why our growth rate results show the overall growth rate of H2+CH4 binary hydrates increasing with the pressure (Figure 4). Clearly, mass transfer behaviors can explain the hydrate growth results. That’s to say, the mass transfer is the controlling factor in the H2+CH4 binary hydrate growth process.


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Figure 8. Snapshots from one of the simulations at 250 K and 50 MPa. R is defined as the ratio of the number of H2 to the number of CH4 in the specific area (i.e. the light blue and light green area). Only hydrate phase and guests near the solid-liquid interface are shown. Molecules appear as in Figure 1.

At the same time, one finds that H2 has much larger concentration and diffusivity relative to CH4 (see Figure 7), suggesting that the growth rate of H2+CH4 binary



hydrates should be much larger than that of pure CH4 hydrate. However, the growth rate of H2+CH4 binary hydrates is slightly above the growth rate of pure CH4 hydrate at 250 K and 50 MPa (Figure 3 and 4). Certainly, other factors may affect the growth of H2+CH4 binary hydrates. We find some simulations exhibit growth rates that couple to the guest concentration at the solid-liquid interface (i.e. shown in Figure 8). The right interface with a larger ratio of H2 to CH4 compared to the left interface grows slower (Figure 8). In order to promote growth of H2+CH4 binary hydrates, smaller ratios of H2 to CH4 (1.3, Figure 8C) is needed. It means that CH4 is needed to stabilize the hydrate structure, which is reasonable considering that CH4 is a thermodynamic promoter to the formation of H2+CH4 binary hydrates. In this scenario, the growth of H2+CH4 binary hydrates at the working conditions is controlled by CH4. This can be further verified by the growth rate of CH4 filled cages (Figure 3), when the temperature is above 240 K; with increasing temperature more CH4 is required to stabilize the hydrate, and consequently less H2 enters the hydrate phase, and as a result we see a decline of growth rate of hydrate. It should be noted that at least 10% of 512 cages are empty in our simulations, and moreover, when the pressure is above 80 MPa, the growth rate of H2 filled cages stays constant. This behavior may be due to the 1:1 gas mixture we used in this study. We conjecture that if enough H2 is supplied, more 512 cages would be filled and increased pressure would make more H2 enter the hydrate phase even when the pressure is above 80 MPa. Consequently, with a high percentage of H2 in the gas phase, one can store more H2 in the mixed hydrate, which has been predicted by theoretical evolution with


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the van der Waals and Platteeuw statistical thermodynamic model from Belosludov et al.28

4. CONCLUSIONS To the best of our knowledge, this work is the first to study the growth of H2+CH4 binary hydrates at a molecular level. By using a large number of microsecond simulations at 50 MPa and 230-270 K, and at 250 K and 20-110 MPa, we investigated the growth rate, cage occupancy, and the growth mechanism of H2+CH4 binary hydrates from a 1:1 gas mixture. We find that CH4 can be a thermodynamic promoter to the formation of H2+CH4 hydrate and H2 can be a kinetic promoter to help the growth of the mixed hydrate. The results show that with increasing temperature, the growth rate of H2+CH4 binary hydrates shows a maximum of 250 K at a pressure of 50 MPa. The growth rate of H2+CH4 binary hydrates shows a positive correlation with pressure at a temperature of 250 K. The impacts on cage occupancy were also examined. We find that adding H2 can dramatically reduce the percentage of empty cages in grown H2+CH4 binary hydrates in comparison with pure CH4 hydrate. Increasing pressure and temperature can also decrease the percentage of empty cages in grown crystals. A temperature increase was found to increase the occupancy of cages by CH4 while decreasing that for H2 when the temperature is above 250 K. Higher pressures increase the cage occupancy by H2 while slightly decreasing the occupancy of cages by CH4. This indicates that by tuning the temperature and especially pressure, we can strongly influence the amount of H2 in



grown H2+CH4 binary hydrates, where the occupancy of cages by H2 can be several times larger than those occupied by CH4. In addition, for the conditions considered here, 51264 cages can be double-occupied by H2 and only a small number of 51264 cages are occupied by triple-H2, or H2 and CH4 molecules. We found that the growth mechanism of H2+CH4 binary hydrates is reminiscent of that of methane hydrate, where the solubility and diffusivity of guest molecules control the growth process.30, 48, 60 Due to the larger concentration and diffusivity of H2 in solution in comparison with CH4 when the pressure is above 20 MPa at 250 K, H2 tends to occupy more cages than CH4. The interplay between solubility and diffusivity with increasing temperature results in growth rate and cage occupancy maxima at 250 K at a pressure of 50 MPa. In comparison with the growth of coarse-grain models for THF +H2 hydrate,29 of THF+CH4 binary hydrates,61 and of THF hydrate,50, 62 the behavior observed for the growth of H2+CH4 binary hydrates is different in part because H2 and CH4 both can enter the 512 and 51264 cages in the hydrate structure. However, as CH4 is needed to stabilize the hydrate structure, an enhanced H2 composition at the hydrateliquid interface was found to slow down the growth process. Our results provide key insights into the growth of binary hydrates, especially those hydrates containing guests with lower solubility and distinct diffusivity. Our results show the temperature and pressure dependence of the growth rate of H2+CH4 binary hydrates and provide details of the growth mechanism. We also identified the importance of the dynamics of guest species in the growth process. To understand the formation mechanism of binary H2+CH4 hydrates uncovered here,


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simulation studies into the nucleation process are warranted. Moreover, experimental studies on the local composition of guests in the hydrate and solution phase and the local structure of formed hydrates will provide further insights. As guest mole fraction may affect the structure of H2+CH4 binary hydrates,24 the composition dependence of H2+CH4 hydrate formation process deserves investigation.

ASSOCIATED CONTENT Supporting Information. The growth rate for the temperature series (Table S1) and the pressure series (Table S2). Short movie from one of the trajectories at 270 K and 50 MPa to show the hopping behavior of H2 between 51264 cages. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS We thank the Computer Simulation Lab at IGGCAS for the allocation of computing time. This work was supported by National Natural Science Foundation of China (Grant No. 41602038 and 41372059), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB10020301) and the China Scholarship Council (CSC). PK is grateful for the financial support of the Natural Sciences and Engineering Research Council of Canada (RGPIN-2016-03845).



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Molecular Insight into the Growth of Hydrogen and Methane Binary

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