Water-Assisted Permeation of Gases in Carbon

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Water-Assisted Permeation of Gases in Carbon Nanomembranes Daniil Naberezhnyi, Armin Gölzhäuser, and Petr Dementyev J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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The Journal of Physical Chemistry Letters

Water-Assisted Permeation of Gases in Carbon Nanomembranes Daniil Naberezhnyi, Armin Gölzhäuser, and Petr Dementyev* Physics of Supramolecular Systems and Surfaces, Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany Corresponding Author *([email protected] )

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The Journal of Physical Chemistry Letters 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

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ABSTRACT

Planar nanomaterials finished with transverse ducts represent an intriguing avenue for exploring interfacial phenomena. Due to their small thickness, the kinetics of molecular diffusion across the channels is likely to be dominated by entrance events. Therefore, measuring transport rates in freestanding films can yield valuable information on surface processes. In this work, we study permeation of gases in carbon nanomembranes (CNMs) when accompanied by saturated water vapor. The experimental data reveal a manifold increase in transmembrane fluxes compared to dry conditions. Gas molecules are found to be trapped in adsorbed water which enhances their translocation likelihood. We demonstrate that the permeance correlates with the vapor relative pressure and discuss the observed crossing mechanism in terms of water condensation and Henry’s law. Our findings provide guidance for designing gas separation membranes upon twodimensional materials.

TOC GRAPHICS

KEYWORDS 2D materials, adsorption, membrane separation, solubility of gases


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Energy-efficient chemical separation is one of the most promising applications of 2D materials as they can serve for high throughput molecular sieving when equipped with uniform pores.1,2 Gas transport in nanomembranes has been widely addressed by computations, and, in addition to size exclusion, surface adsorption often appears to be an important factor in mixtures separation.3-8 In theory, particles accommodated on the membrane surface should have a greater chance of entering and passing the pores compared to their gas-phase counterparts. Although different types of gas-surface interactions have been proposed to affect molecular transport in single layer materials, the area remains largely unexplored from an experimental perspective. Wang et al. studied gas permeation in porous graphene as a function of molecular size and observed a significant deviation from the geometrical trend for CO2 and N2O.9 Graphene nanomembranes were also used to probe separation of multi-component mixtures and exhibited appreciable permselectivity to CO2 and CH4 over He and H2.10 These scarce examples confirm that surface properties might be relevant for tailoring membrane performance, and it is even more evident in permeation of vapors whose adsorption is governed by relative pressure. Thus, intrinsically porous carbon nanomembranes (CNMs) with a high density of channels were found to pass water vapor much faster than any other substance, featuring a pronounced humidity dependence.11 While condensed water is known to reveal spectacular dynamics in nanoconfinement,12-14 at low relative pressure water molecules are believed to diffuse through CNMs predominantly as separate adsorbates.15 As supported by experiments with mixtures, the membrane selectivity toward water in this nanometer-thick material is due to the higher probability of adsorbed species to penetrate into the channels. This letter reports on a significant increase in permeation rates of gases across CNMs upon combining with saturated water vapor.


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The basics of the transport measurements employed are illustrated in Figure 1a, while the complete experimental system is detailed elsewhere.15 A suspended CNM is secured between the three-section upstream compartment and the high-vacuum detection chamber with a quadrupole mass-spectrometer (QMS). The mixing chamber and the small cold finger are designed for preparing gaseous and vaporous mixtures, but there is always a pressure drop upon connecting with the membrane cell. Therefore, feed mixtures with saturated vapor were prepared in a different way with the valve 2 open. Specifically, an excess amount of water was first stored in the cold finger followed by admitting a gas of interest into the total volume of the mixing chamber and the sample channel. Water vapor was then released through the valve 1 to allow for intermixing with the gas and reaching the membrane. The equilibration was taking few hours depending on the gaseous species, and Figure 1b shows how the process was monitored by QMS. The signal of permeating water molecules grows rapidly as their concentration near the membrane increases, leveling off at relative pressure close to unity. Surprisingly, one can also see a simultaneous evolution of the signal corresponding to the gas particles, despite the fact that they were let to the sample long before and their partial pressure was constant. This observation strongly suggests that water vapor is able to promote permeation of molecular species through otherwise gas-tight CNMs.


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Figure 1. a) Schematic of the permeation experiments with gas-vapor mixtures. The volume ratio between the cold finger, the mixing chamber, and the sample channel is 1:80:100. b) Representative QMS output for a binary feed mixture. First, calibration signals for D2O and He were recorded; then, 25 mbar He was dosed into the mixing chamber and the sample channel; finally, D2O was released from the cold finger at saturation pressure of around 25 mbar. Previously, helium was reported to be the only gas whose cross-membrane flux was determined reliably under feed pressure of 130 mbar with a permeance in the range of 10-8 mol m-2 s-1 Pa-1.11 In order to facilitate the mixing, the above described experiments with saturated vapor were done at partial pressure as low as 25 mbar, and the He signal was revealed to stay at the background level until water was let in. Yet, its QMS intensity associated with the water-induced transport gives rise to a steady-state permeance value as great as 3×10-6 mol m-2 s-1 Pa-1 meaning a two orders of magnitude increase. To exclude diffusion anywhere else in the sample fixture, we checked permeation of the dry gas over the same time scale, and it is clear that the observed enhancement is caused by the presence of water. The measurements were conducted with multiple species, and the results obtained are shown in Fig. 2a. In addition to helium, the effect of water


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vapor on permeation was found for nitrogen, oxygen, and carbon dioxide, whereas no flow rate was measured for chloroform. Regardless of the mixtures, water permeation rate was proven to be the same as in the net state (~10-4 mol m-2 s-1 Pa-1) which is indicative of unimpeded condensation. At saturation pressure, water is expected to extensively cover the membrane surface, and its transmembrane flow is considered to be liquid-like in agreement with pervaporation measurements.11 It appears that condensed water takes up the gas particles and carries them away through the membrane channels. In comparison to anhydrous conditions, the trapped molecules exhibit much greater access to CNMs interior pointing to a steric hindrance at the pore entrances. To understand how water alters the residence time of gaseous species at the surface, one can recall Henry’s law which describes the amount of gases dissolved in liquids:16 𝑐 = 𝐻𝑝 (1) where c is the molar concentration, H is the Henry’s law solubility constant, and p is the partial pressure. According to this relation, the number of gas molecules absorbed by the water layer is in equilibrium with the gas phase, and hence the permeation process can be modelled by two steps: 𝐻

𝑘

𝐴𝑢𝑔 𝐴𝑢𝑑𝑖𝑠 𝐴𝑑𝑔 Here, the superscripts u and d denote particles on the upstream and the downstream, the subscripts g and dis are for the gas and the liquid phase, and k represents an effective rate constant. Under steady state, the flux of gas molecules crossing the membrane is expressed as follows: 𝐹 = 𝑘𝑐 = 𝑘𝐻𝑝 (2)


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Figure 2. a) Permeance of gaseous substances in CNMs upon mixing with saturated water vapor. The partial pressure of each component was 25 mbar (about 10% relative pressure for chloroform). The data represent mean values over five samples, and the error bars are standard deviation. LOD is the limit of detection as defined in [15]. The dashed line indicates the average permeation rate for water. b) Transmembrane flux of acetonitrile in CNMs as a function of its partial pressure in mixtures with saturated water vapor. The permeation measurements were done in the opposite order than shown in Fig. 1a, i.e. acetonitrile was frozen in the cold finger while water was dosed into the mixing chamber and the sample channel. This arrangement enabled shorter intermixing times. It is interesting that the linear pressure dependence was established even for condensable species (Fig.2b). Although solubility constants in water are known to vary in a broad range (Table 1),17


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the measured transport rates for gases are very similar (Fig.2a) indicating a trade-off between k and H in eq. 2. Indeed, the rate constant is likely to be size-dependent and should diminish for larger particles. For instance, carbon dioxide molecules are triatomic and can react with water yielding bicarbonate anions, albeit it is much more soluble than helium. Since vaporous substances behave as gases at low relative pressure, acetonitrile and chloroform were used to widen the scope of molecular dimensions and solubility. Thus, the former compound is fully miscible with water, while its molecules are bulkier than atmospheric gases. Similarly, the solubility of chloroform exceeds those for all the gases probed, but because of its size no permeation was observed in the experiments with water. In contrast, organic vapors are supposed to adsorb themselves when their partial pressure approaches saturation. We studied permeation of pure toluene, hexane, chloroform, and acetonitrile, and measurable flow rates were witnessed for all of them except hexane (Fig. 3a). These data support the idea that transmembrane channels in CNMs are tortuous and preclude translocation of inert gas particles.15 Moreover, it is clear that the ducts are narrower inside that their external appearance11 since the length of hexane molecules does not exceed 1 nm. Figure 3b summarizes the mechanisms of molecular transport in nanomembranes that merge from this study: i) direct impact transfer (seen only for He); ii) surface diffusion (possible for vapor molecules); iii) water-assisted permeation. Table 1. Henry’s law solubility constants in water [mol m-3 Pa-1].17 Helium

Nitrogen

Oxygen

Carbon dioxide

Chloroform

Acetonitrile

3.8×10-6

6.4×10-6

1.3×10-5

3.3×10-4

2.5×10-3

5.2×10-1


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Figure 3. a) Permeance of net substances in CNMs under the following feed conditions: p(He)=150 mbar; p(CH3CN)~115 mbar (sat.); p(C7H8)~35 mbar (sat.); p(CHCl3)=150 mbar; p(C6H14)~180 mbar (sat.). The data are averaged over 3-4 measurements, the error bars are standard deviation.

b) Phenomenological model for gas and vapor permeation across

nanomembrane channels. As evidenced from Fig. 1b, the gas flux increases gradually with time and follows the signal of water molecules implying a humidity-driven behavior. Given the experimental configuration, it is difficult to track the water relative pressure during the mixing. However, if the amount of water vapor is initially set to a precise value other than saturation, the experiments can be done at specific relative pressure. We watched the changes in permeation rates for helium and oxygen as a function of humidity and obtained very steep curves (Fig. 4). Since the transport of water vapor in CNMs is rationalized in terms of multilayer adsorption,15 surface clustering seems to be responsible for the trends revealed. Moreover, the coverage of water agglomerates predicted by the kinetic model is well in line with the measured data points. It means that the solvent-assisted permeation occurs


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in water nanodroplets which are formed at the membrane surface in the course of vapor condensation. This environment might result in interesting confinement effects, such as oversolubility,18,19 and requires further microscopic and theoretical investigation.

Figure 4. a) Permeance of gases in binary mixtures with water vapor at different humidity. The total pressure was kept at 50-60 mbar. The measurements were done with 3-5 samples, the errors bars indicate standard deviation. The solid line is the coverage of water clusters calculated by the kinetic model introduced in [15] with the parameter L0 equal to 100. In conclusion, we demonstrated a novel experimental approach to examine physicochemical events at solid-gas and liquid-gas interfaces. Upon exposing porous nanosheets to well-defined atmospheres and looking at the outside, we were able to get insights into complex adsorption, absorption and diffusion processes taking place on the inner surface. The procedure is readily extendable to any 2D material providing its perforation is properly fulfilled.20 Surface modification by chemical means turns out to be an effective route with respect to controlling gas permeability in nanoscale membranes. For example, ionic liquids could be used to overlay the membranes and impart variable affinity for certain molecular species. CNMs were proven to be selective to water vapor under high humidity, and the data presented display a clear cut-off of their pore size distribution. Filtration experiments with liquid water would shed more light on their separation


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performance as hydrophilic and especially ionic solutes can differ greatly in size due to hydration shells.

EXPERIMENTAL METHODS CNMs were prepared out of terphenylthiol (Sigma-Aldrich, 97%) and suspended over holey substrates (4-7 µm openings in 500 nm S3N4 windows, Silson Ltd) as specified formely.11 Seven defect-free samples were probed during this study. The experiments were performed at room temperature with a vacuum system described in ref. 15. He, N2, O2, CO2, were used as delivered by Linde. Acetonitrile (Merck, 99.9%), toluene (Fisher Scientific, 99.8%), chloroform (VWR, 99.8%), hexane (Fisher Scientific, 95%), and deuterium oxide (Sigma-Aldrich, 99.9% atom D) were degassed before use. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors disclose the following competing financial interest(s): A. G. is a co-founder and shareholder of a company aimed at commercializing carbon nanomembranes (CNM Technologies GmbH). ACKNOWLEDGMENT P. D. thanks the “Fonds der Chemischen Industrie” for financial support (Liebig fellowship). REFERENCES (1) Prozorovska, L.; Kidambi, P. R. State-of-the-Art and Future Prospects for Atomically Thin Membranes from 2D Materials. Adv. Mater. 2018, 30, 1801179.


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(2) Moghadam, F.; Park H. B. 2D Nanoporous Materials: Membrane Platform for Gas and Liquid Separations. 2D Mater. 2019, 6, 042002. (3) Du. H.; Li. J.; Zhang J.; Su G.; Li X.; Zhao Y. Separation of Hydrogen and Nitrogen Gases with Porous Graphene Membrane. J. Phys. Chem. C 2011, 115, 23261–23266. (4) Schrier J. Carbon Dioxide Separation with a Two-Dimensional Polymer Membrane. ACS Appl. Mater. Interfaces 2012, 4, 3745−3752. (5) Drahushuk, L. W.; Strano M. S. Mechanisms of Gas Permeation through Single Layer Graphene Membranes. Langmuir 2012, 28, 16671−16678. (6) Sun C.; Boutilier M. S. H.; Au H.; Poesio P.; Bai B.; Karnik R.; Hadjiconstantinou N. G. Mechanisms of Molecular Permeation through Nanoporous Graphene Membranes. Langmuir 2014, 30, 675−682. (7) Tian Z.; Mahurin S. M.; Dai S.; Jiang D. Ion-Gated Gas Separation through Porous Graphene. Nano Lett. 2017, 17, 1802−1807. (8) Sun C.; Bai B. Improved CO2/CH4 Separation Performance in Negatively Charged Nanoporous Graphene Membranes. J. Phys. Chem. C 2018, 122, 6178−6185. (9) Wang L.; Drahushuk, L. W.; Cantley L.; Koenig S. P.; Liu X.; Pellegrino J.; Strano M. S.; Bunch J. S. Molecular Valves for Controlling Gas Phase Transport Made from Discrete Ångström-Sized Pores in Graphene. Nature Nanotech. 2015, 10, 785-790. (10)

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Temperature-Dependent Gas Mixture Permeation and Separation through Suspended Nanoporous Single-Layer Graphene Membranes. Nano Lett. 2018, 18, 5057−5069. (11)

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X.; Beyer A.; Koch S.; Anselmetti D.; et al. Rapid Water Permeation through Carbon Nanomembranes with Sub-Nanometer Channels. ACS Nano 2018, 12, 4695−4701.


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and Temperature Dependence of the Anomalous State of Nanoconfined Water in Carbon Nanotubes. J. Phys. Chem. Lett. 2016, 7, 4433−4437. (13)

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V.; Katsiotis M. S.; Karagiannis T.; Fardis M.; Panopoulos N.; et al. Ultrafast Stratified Diffusion of Water Inside Carbon Nanotubes; Direct Experimental Evidence with 2D D−T2 NMR Spectroscopy. J. Phys. Chem. C. 2018, 122, 10600−10606. (14)

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Figure 1. a) Schematic of the permeation experiments with gas-vapor mixtures. The volume ratio between the cold finger, the mixing chamber, and the sample channel is 1:80:100. b) Representative QMS output for a binary feed mixture. First, calibration signals for D2O and He were recorded; then, 25 mbar He was dosed into the mixing chamber and the sample channel; finally, D2O was released from the cold finger at saturation pressure of around 25 mbar. 84x81mm (300 x 300 DPI)


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Figure 2. a) Permeance of gaseous substances in CNMs upon mixing with saturated water vapor. The partial pressure of each component was 25 mbar (about 10% relative pressure for chloroform). The data represent mean values over five samples, and the error bars are standard deviation. LOD is the limit of detection as defined in [15]. The dashed line indicates the average permeation rate for water. b) Transmembrane flux of acetonitrile in CNMs as a function of its partial pressure in mixtures with saturated water vapor. The permeation measurements were done in the opposite order than shown in Fig. 1a, i.e. acetonitrile was frozen in the cold finger while water was dosed into the mixing chamber and the sample channel. This arrangement enabled shorter intermixing times. 84x116mm (300 x 300 DPI)



Figure 3. a) Permeance of net substances in CNMs under the following feed conditions: p(He)=150 mbar; p(CH3CN)~115 mbar (sat.); p(C7H8)~35 mbar (sat.); p(CHCl3)=150 mbar; p(C6H14)~180 mbar (sat.). The data are averaged over 3-4 measurements, the error bars are standard deviation. b) Phenomenological model for gas and vapor permeation across nanomembrane channels. 84x93mm (300 x 300 DPI)


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Figure 4. a) Permeance of gases in binary mixtures with water vapor at different humidity. The total pressure was kept at 50-60 mbar. The measurements were done with 3-5 samples, the errors bars indicate standard deviation. The solid line is the coverage of water clusters calculated by the kinetic model introduced in [15] with the parameter L0 equal to 100. 84x51mm (300 x 300 DPI)



TOC graphic 50x50mm (300 x 300 DPI)


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Water-Assisted Permeation of Gases in Carbon

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