CO Separation from H2 via Hydrate Formation in Single-Walled

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CO Separation from H via Hydrate Formation in Single-Walled Carbon Nanotubes Wenhui Zhao, Joseph S. Francisco, and Xiao Cheng Zeng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02443 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry Letters

CO Separation from H2 via Hydrate Formation in Single-Walled Carbon Nanotubes

Wenhui Zhao†‡, Joseph S. Francisco§, and Xiao Cheng Zeng*,‡§ †Department of Physics, Ningbo University, Ningbo,Zhejiang 315211, China ‡Department of Chemical Physics, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China §Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA E-mail: [email protected]

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Abstract Hydrogen is an alternative fuel without generating green-house gas or other harmful emissions . Industrial hydrogen production, however, always contains a small fraction of carbon monoxide (CO) (~0.5-2%) which must be removed for usage in fuel cells. Here, we present molecular dynamics simulation evidence on facile separation of CO from H2 at ambient pressure via the formation of quasi-one-dimensional (Q1D) clathrate hydrates within single-walled carbon nanotubes (SWCNTs). At ambient pressure,Q1D CO (or H2) clathrates in SW-CNTs are formed spontaneously when the SW-CNTs are immersed in CO (or H2) aqueous solution. More interestingly, for the CO/H2 aqueous solution, highly preferential adsorption of CO over H2 occurs within the octagonal or nonagonal ice nanotubes inside SW-CNTs. These results suggest that the formation of Q1D hydrates within SWCNTs can be a viable and safe way for the separation of CO from H2, which can be exploited for hydrogen purification in fuel cells.

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The utilization of hydrogen as a fuel can be highly beneficial not only to the protection of environment but also to reducing dependency on fossil fuels. The industrial hydrogen production typically contains a small quantity of CO (0.5-2%), which must be removed for fuel-cell applications.1-3 This is because even a very small amount of CO can severely poison the fuel-cell catalyst due to CO agglutination on the surface of catalyst, resulting in its deactivation.2,3 Various approaches have been proposed to eliminate residual CO in H2 gas, such as methanation of residual CO, preferential CO oxidation (PROX), and membrane separation of CO from H2.4-12 PROX has been widely recognized as the cost-effective solution to removal of residual CO towards an acceptable level.1,5,8 However, additional monitoring and units for O2 dosing and additional maintenance cost of the PROX process make it limited in small-scale applications. Recently, gas hydrate-based separation has attracted increasing interests owing to its environmental benign implication and less energy intensive, compared to current separation methods.13-17 Clathrate hydrates are fascinating crystalline compounds composed of guest molecules, such as CH4, CO2, CO, and H2, encapsulated within cage-like network of hydrogen-bonded water molecules under certain temperature and pressure conditions.18 Given their natural ability to enclose guest molecules, clathrate hydrates have been of both high interest and concern in various scientific, technologies and global change-related issues, including energy recovery, carbon sequestration, gas storage, gas transportation, and gas separation.18-22 Although some water-soluble molecules can form clathrate hydrates,23,24 hydrate 3



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formers are typically small gas molecules. The conditions for the formation of bulk gas hydrates are usually at high pressure (>0.6 MPa). Particularly, H 2 hydrate is formed at very high pressure (> 180 MPa).19 Gas hydrate in porous medium is a widespread form in the natural world, where confinement and surface features show both promotion and inhibition effects on gas hydrate nucleation and growth.25-33 For example, confinement and hydrophilic surface of graphene oxide inhibits hydrate phase.30 Recently, based on inelastic neutron scattering experiments and synchrotron X-ray powder diffraction, Casco et al. 31 found that the confinement effects of carbon cavities and hydrophobic properties of carbon surface result in lower methane hydrate nucleation pressures (below 4 MPa) and faster growth kinetics. Later, they found that the activated carbons promote the formation of CH4 and CO2 hydrates, and the molecular change process of the two molecules is fully reversible with high efficiency at relevant pressure and temperature conditions.32 Also, importantly, the hydrate structures formed in nanopores are similar to natural bulk hydrates. However, when the pore size is reduced to subnanometer, the hydrogen-bonding network in water is disrupted by the highly confined environment, thereby affecting kinetics of crystallization.34-47 In 2D nanoconfinement, monolayer or bilayer gas clathrates can be formed under thousands of atmospheric pressure,38,39,48 while quasi-one-dimensional (Q1D) core-sheath n-gonal (n = 5-8) hydrogen hydrates can be formed spontaneously within (SW-CNTs).49 Also, the formation of Q1D heptagonal and octagonal hydrogen hydrates can be seen under ambient pressure when the SW-CNTs are immersed in the dilute H2 aqueous solution.49 4


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Can the formation of Q1D gas hydrates be used for separation of CO from H2 if mixed CO and H2 gases are dissolved in water? To address this question, we first study nucleation and growth of Q1D CO hydrates in SW-CNTs using both classical molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations. The classical MD simulations are carried out by using Gromacs 4.5 software package.50 Three finite-sized zigzag SW-CNTs with the indexes (17, 0), (18, 0) and (19, 0) (whose diameter is 1.33, 1.41 and 1.49 nm, respectively) are placed in a dilute CO aqueous solution. All SW-CNTs have the length of 5.112 nm and their length is fixed during the simulations. The aqueous solution includes 4858 water molecules and 15 CO molecules. Water molecules are described by the TIP3P model51 while CO molecules are described by the three-site “quadrupolar” model.52 Carbon atoms of SWCNTs are modeled as uncharged Lennard-Jones (LJ) particles with graphite parameters of εC = 0.3598 kJ/mol and σC = 0.34 nm.53 The SW-CNTs immersed in the dilute H2 aqueous solution are also studied (Figure S1). H2 molecules are described by a rigid two-center LJ model with a bond length of 0.074 nm and with the LJ parameters of εH = 0.1039 kJ/mol and σH = 0.259 nm.54 The system is initially equilibrated at 300 K and 1 bar, followed by stepwise cooling in temperature step of 10 K at ambient pressure. The heptagonal and octagonal CO hydrates can be formed spontaneously in (17, 0) and (18, 0) SWCNTs at 280 K, while the nonagonal CO hydrate can be formed in (19, 0) SWCNT at 270 K. Remarkably, as shown in Figure 1, CO molecules are trapped within the interior space of the polygonal ice nanotubes and form a single-file wire, akin to the formation of 5



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polygonal H2 hydrates in the same SW-CNTs (Figure S1). Note that both octagonal and nonagonal hydrates contain much more CO molecules than the heptagonal CO hydrate (Figure S2). In other words, the heptagonal CO hydrate under ambient pressure is almost guest-free due in part to the smaller diameter of the heptagonal ice nanotube. Higher pressure beyond 1 bar is needed to trap CO molecules within the heptagonal ice nanotube. The structures of Q1D polygonal hydrates in SW-CNTs at ambient pressure are dependent on the diameters of SW-CNTs, similar to ice nanotube in SW-CNTs.24

Figure 1. Snapshots of Q1D (A) heptagonal CO hydrate in (17, 0) SWCNT at 280 K, (B) octagonal CO hydrate in (18, 0) SWCNT at 280 K, and (C) nonagonal CO hydrate in (19, 0) SWCNT at 270 K. Big cyan and red spheres represent the carbon and oxygen atoms of CO 6


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molecules, small cyan spheres carbon atoms of SWCNTs, small red and white spheres oxygen and hydrogen atoms of water confined in SWCNTs, red lines water outside of SWCNTs (i.e., aqueous solution), and green dotted lines hydrogen bonds.

Furthermore, three independent AIMD simulations trajectories (Movies S1-S3) show that the polygonal CO hydrates are stable at 250 K when confined inside the (17, 0), (18, 0) and (19, 0) SW-CNTs (see method details in Supporting Information). Next, we perform three independent MD simulations of SWCNT immersed in a dilute CO/H2 aqueous solution. Here, the aqueous solution includes 4858 water molecules, 15 CO molecules and 15 H2 molecules. The system is initially equilibrated at 300 K and 1 bar. Then the temperature of the system is lowed in a step of 10 K, while the pressure is controlled at 1 bar.

Figure 2. Snapshots of Q1D CO/H2 (A) heptagonal hydrate in (17, 0) SWCNT at 280 K, (B) 7



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octagonal hydrate in (18, 0) SWCNT at 280 K, and (C) nonagonal hydrate in (19, 0) SWCNT at 270 K. The green spheres represent H2 molecules.

In (17, 0) SWCNT, the heptagonal hydrate is formed spontaneously at 280 K (Figure 2A). However, as in the case of pure heptagonal CO hydrate, few CO and H 2 molecules can enter into heptagonal nanochannel of the ice nanotube (Figure 3A). During the last 200 ns simulation, the heptagonal ice nanotube allows, on average, ca. 1.3 CO and 2.2 H2 molecules, although the number of CO and H2 molecules in the nanochannel fluctuates over time between 0 and 8 (Figure 3A). Hence, the heptagonal hydrate is unsuitable for separating CO and H2 at ambient pressure.

Figure 3. The number of CO and H2 molecules within SWCNTs during the formation of (A) 7-gonal hydrate at 280 K, (B) 8-gonal hydrate at 270 K, and (C) 9-gonal hydrate at 270 K.

In (18, 0) SWCNT, the octagonal hydrate is formed spontaneously at 270 K (Figure 2B). Here, the gas molecules can enter into the octagonal nanochannel much more easily and they form a CO-molecule wire. During the last 100 ns simulation (the first tens of nanoseconds of the simulation is for the nucleation and growth of the hydrate), the mean numbers of CO molecules within the octagonal nanochannel is 8


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about 8.4, while that of H2 molecules is only 0.8. So the ratio of CO/H2 in octagonal nanochannel is about 10. The number of CO in the octagonal nanochannel over time fluctuates between 5 and 11, whereas for most of the time, the number of H2 fluctuates between 0 and 1 (Figure 3B). It appears that the octagonal hydrate can entail high efficiency to separate the CO and H2 molecules in dilute CO/H2 aqueous solution. In (19, 0) SWCNT, the nonagonal hydrate is formed spontaneously at 270 K (Figure 2C). During the last 100 ns simulation, the average numbers of CO molecules in the nonagonal nanochannel is about 9.0, while that of H2 molecules is only 0.8. In other words, the ratio of CO/H2 in nonagonal nanochannel is about 11(Figure 3C). The number of CO in the hydrate fluctuates over time between 7 and 9, whereas for most of the time, the number of H2 fluctuates between 0 and 2 (Figure 3C). Hence, the nonagonal hydrage may entail higher efficiency to separate the CO and H2 molecules in dilute CO/H2 aqueous solution as well.

We note that both CO and H2 molecules can form single-file guest-molecule wire within octagonal and nonagonal nanochannels (Figure 1 and Figure S1). Why do the larger CO molecules tend to enter the nanochannels rather than the smaller H2 molecules? To address the issue of gas-molecule selectivity, we compute the potential of mean force (PMF), i.e., the free energy profile, for a gas molecule moving from the bulk solution into the polygonal ice nanotube. Note that this computation yields a single molecule PMF, while no other gas molecules are present in the nanochannel for the PMF computation (see Supporting Information).

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The PMF profiles for heptagonal hydrate in (17, 0) SWCNT show that the energy barrier (defined as the difference in PMF between the gas molecule outside and inside the ice nanotube) for H2 is about -4.5 kJ/mol, and 0.5 kJ/mol for CO (Figure 4A). The negative value of PMF for H2 indicates that H2 molecules are highly favored to stay inside the nanochannel, while the positive energy barrier for CO indicates that CO molecules are unlikely to enter the nanochannel, consistent with the results that the mean number of CO and H2 are very small (Figure S3). For the mixed CO/H2 solution, the steric hindrance of CO and H2 results in the lower numbers in heptagonal nanochannel (Figure 3A).

Figure 4. Potential of mean force (PMF) profiles for H2 and CO within the polygonal ice nanotubes. The green represent the end of the SWCNTs.

The PMF profiles for octagonal hydrate in (18, 0) SWCNT show that the energy barrier of H2 is about -7.5 kJ/mol, and that of CO is -12.2 kJ/mol (Figure 4B). The negative value of PMF for H2 and CO indicates both molecules would favor to enter 10


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nanochannel of the ice nanotube in (18, 0) SWCNT. More importantly, the lower energy barrier for CO indicates that CO is more preferred over H2 to be adsorbed in the nanochannel. This is why the number CO molecules in octagonal hydrate is much more than H2 molecules. For the (19, 0) SWCNT, the PMF profiles also suggest preference for CO molecules over H2 molecules to enter the nanochannel (Figure 4C).

Additional simulations at ambient pressure with different initial conditions (e.g., different gas concentrations and CO/H2 ratios) are also examined. No obvious changes are observed. Lastly, SW-CNTs with smaller or larger diameters are studied. The simulation results show that guest-free clathrates can be formed in SW-CNTs with smaller diameters (while few gas molecules occupy the nanochannels of ice nanotubes), but no clathrates are observed in SW-CNTs with larger diameters. In other words, the structures of Q1D polygonal hydrates within SW-CNTs are highly dependent on the SW-CNTs’ diameter. The high selectivity of CO over H2 is mainly due to much lower energy barrier for CO to enter into SW-CNTs. Overall, the formation of 1D gas hydrates within SW-CNTs and the high selectivity of CO over H2 are less affected by the gas concentration in the solution and SW-CNTs’ chirality. Confirmation of the simulation results in future experiments requires the fabrication of open-ended CNT bundles, preferably, with narrow variation of their diameters (in the range of 1.4-1.5 nm). The CNT bundles can be recycled once the trapped CO molecules and water are released to outside the system.

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In conclusion, by means of MD simulations, we show that the Q1D polygonal CO and H2 hydrates can be formed spontaneously at ambient pressure in SWCNTs. More importantly, preferential adsorption of CO over H2 molecules is found in octagonal and nonagonal ice nanotubes at ambient pressure due to much lower energy barrier of CO than H2 to enter the nanochannel. This property can be exploited for removal of CO from H2 when the mixed gases are dissolved in water, a potentially safe and benign way of hydrogen purification.

ACKNOWLEDGMENTS W.Z. is supported by the National Natural Science Foundation of China (21503205), Anhui Provincial Natural Science Foundation (1608085QB30) and K.C. Wong Magna Fund in Ningbo University. X.C.Z. is also supported by a grant from USTC for (1000plan) Qianren-B summer research and by a State Key R&D Fund of China (2016YFA0200604) to USTC. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.XXXX. Figures S1-S3, Movies S1-S3, and the details of AIMD and PMF calculation. REFERNCES (1) Saavedra, J.; Whittaker, T.; Chen, Z.; Pursell, C. J.; Rioux, R. M.; Chandler, B. D. Controlling Activity and Selectivity Using Water in the Au-Catalysed Preferential

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