Understanding the Hydrophilicity and Water Adsorption Behavior of

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Understanding Hydrophilicity and Water Adsorption Behavior of Nanoporous Nitrogen Doped Carbons K. Vasanth Kumar, Kathrin Preuss, Zhengxiao Guo, and Maria-Magdalena Titirici J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06555 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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The Journal of Physical Chemistry

Understanding Hydrophilicity and Water Adsorption Behavior of Nanoporous Nitrogen Doped Carbons K. Vasanth Kumar,a* Kathrin Preuss, a Zheng Xiao Guo b and M. Magdalena Titirici a* a

School of engineering and Materials Science & Materials Research Institute, Queen Mary, University

of London, Mile End Road, London, E1 4NS, U.K. b

Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, U.K.

*Email: [email protected]; [email protected]; Phone: +44 (0)20 7882 5301

Supporting Information Placeholder ABSTRACT: Through molecular simulation we exposed the anomalous water adsorption behavior in slit-shaped disordered nitrogen doped carbons. Instead of Langmuirian, water adsorption proceeds via mechanisms analogue to crystal nucleation: birth and spread of (poly) nucleation sites in one, two and three dimensions forming water nanowires/pillars and water clusters before complete pore filling via capillary condensation. The adsorption of water and the adsorption hysteresis in N doped carbons are strongly influenced by the pore-size. The smaller the pore-size, the smaller is the pressure at which the above-mentioned process tends to occur in N doped carbons. The nucleation analogues are clearly visible in a pore that has a homogeneous pore structure, whereas in disordered pore structures, the complex and distributed pore-sizes disturb these nucleation analogue patterns especially at lower relative pressures. The effect of the adsorbed water molecules on the connectivity of the available pore volume is discussed. Adsorption at zero loading confirmed water molecules preferentially adsorb over specific zones, which corresponds to regions with a high local density of N atoms rather than specific sites or type of N (such as graphitic or pyridinic). Simulated adsorption isotherms showed the hydrophilicity introduced to the carbon pore via N doping is sensitive to impurities such as water, such that it can affect the ability of the carbon framework to host another guest molecule such as CO2, a prime fluid involved in flue gas. 1. INTRODUCTION Understanding the hydrophilicity of nitrogen (N) doped carbon surfaces is of prime importance for the design of adsorbents or catalysts in several essential applications such as gas storage/separation, oxygen reduction/evolution reactions, etc.

1–4

In gas storage, hydrophilicity assists the condensation of water 1

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molecules inside the carbon pores, thereby decreasing the total uptake of any guest molecule.

5–7

Like-

wise, during the oxygen reduction reaction, hydrophilicity can assist the transport of oxygen molecules and hydroxyl radicals towards the catalyst active centers via hydrogen bonding.

8

Also N doping can

assist in maintaining the proton exchange membrane in a hydrated state which can also show a significant influence on the performance of fuel cells. 9 Studies detailing the effect of N doping on the hydrophilicity of carbons remain thus far poorly understood. Few of the works that report on the hydrophilic properties of N doped carbons were often biased due to the practical difficulty in obtaining impurity free N doped carbon surfaces.

10

Characterizing the hydrophilicity of N doped carbons via experiments are

often riddled with the practical problems such as the lack of easy synthesis techniques to produce impurity free carbons. 8,10,11 Synthesis of N doped carbons often relies on high-temperature, strong oxidizing agents and nitrogen precursors that also contain other atoms (e.g. O). Thus final materials often contain other impurities, which can change the natural hydrophobic properties of pristine carbons and their reactivity towards water molecules. These issues make it difficult to probe the sole effect of N doping on the hydrophilic properties of functionalized porous carbons. Studies showing exclusive information on the adsorption/hydrophilic properties of such N doped carbons are highly essential and much needed for the characterization and design/screening of these materials as electro-catalysts, adsorbents or as energy storage medium such as hydrates. N contains one additional electron as compared to a C atom; conceptually N doping can change the electron density, increase the basicity of the carbon framework and depending on the type of N, it can also tune the structure or create defect sites. 12 In fact, this versatile property of N to engineer the electronic and structural properties of the carbon framework make them so unique and important to find applications in several areas ranging from catalysis, batteries, gas storage, gas separation, super capacitors, heat pumps and so on. 13 All of these processes involve water as a main guest molecule, as an interface, as an impurity or as an electron transport medium and thus information on the water adsorption properties and the mechanism involved are of tremendous importance. Studies that detail the hydrophilic properties of carbon materials usually focus on the influence of oxygenated groups on the water uptake properties.

14,15

No research has been carried out until now that exclusively details the N doping effect

on the hydrophilic properties and the adsorption mechanism involved. In this work, we use molecular simulations to unlock some of the core issues connected with the adsorption properties of N doped carbons towards water molecules and address for the first time several key issues such as: (i) how does the N doping shift the carbon behavior from hydrophobic to hydrophilic? (ii) the influence of N atoms on the mechanism and the total uptake of water at different pres2


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sures (iii) the influence of the N concentration on the water adsorption properties, (iv) the role of poresize and structure of N doped carbons on the water adsorption mechanism and finally as a case study (iv) how does the hydrophilicity of N doped carbons affect the selectivity towards CO2 gas, a prime fluid involved in the carbon capture process? Additionally, we also address the structural details of the adsorbed water molecules within the N doped carbon structures at different stages of adsorption. At some stage, based on the adsorption behavior in disordered carbon structures, we also address the issues related to pore connectivity due to the adsorbed water molecules. To address the above said issues, we performed Grand Canonical Monte Carlo simulations to obtain water adsorption isotherms at 298 K in five different in-silico obtained N doped carbon prototypes, which we labelled as SN1, SN2, SN3, SN3_micro and RPC. The carbon porosity in four of the prototypes (SN1-SN3 and SN3_micro) is modelled as slit-shaped structures, while the carbon porosity of RPC is designed to account two of the main experimental parameters, pore-size distribution and chemical heterogeneity. Three of the slit-pores, SN1, SN2 and SN3 have a characteristic pore-width of 2 nm. This pore-width is specifically chosen as it lies between micro- and mesopores as defined by IUPAC and thus can portray the water adsorption mechanism in typical micro/mesoporous carbons. 15 To study, exclusively the effect of N doping in microporous structures, we fixed the pore-width of SN3_micro to be equivalent to 0.86 nm. The RPC considered in this work contains several carbon crystallites of similar chemical and surface properties and can represent the pore structures in the experimentally realized carbons. Water adsorption simulations performed in these structures can give a reliable picture on the hydrophilicity and the behavior of water molecules within the nanopores of the N doped carbons. Comparison of the simulated water adsorption isotherms in SN1-SN3; SN3_micro and RPC can also give clear pictures on how the pore structure/size heterogeneity can change the water adsorption properties of N doped carbons. The physics involved and the adsorption properties are detailed based on the simulated absolute adsorption isotherms, isosteric heat and snapshots captured during the course of the simulations. Based on the simulation results we also show how the water adsorption in N doped structures follows nucleation theory analogues. The paper is organized as follows. In the next section we describe the simulation details, force fields used and the strategies implemented while assigning point charges to N heteroatoms. Followed by the adsorption behavior of water in different N doped slit-pore carbons that have a pore width of 2nm but different N concentrations. In the same section we also discuss the water adsorption behavior in an N doped small micropore/ultra-micropore that has a pore-width of 0.86 nm and in an N doped disordered porous carbon structure. We also discuss the possibility of pore-blockage due to water adsorption on N sites at lower relative pressures in a disordered carbon structure. Finally, 3



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before concluding, we also show how N doping can change the capacity of the carbon frameworks to host guest molecules such as CO2 that contains water as an impurity in the bulk fluid. The fundamental insights on the water adsorption behavior in N doped carbons that will be discussed in this work is highly important and much desired information and can have huge impact in adsorbent or catalyst design for different targeted applications such as gas storage, gas separations and electrocatalysts. 2. SIMULATION DETAILS Grand canonical Monte Carlo (GCMC) simulations were used to study the adsorption behavior of water in N doped carbon structures. The fluid–fluid interactions were modeled using the 12-6 LennardJones (LJ) potential and electrostatics using Wolf summation method. All LJ cross interactions, both between different types of fluids and the solid–fluid interactions, were taken to conform to Lorentz– Berthelot rules. Fluid–fluid Coulombic interactions between partial charges are calculated by applying a Wolf summation method that is based on a spherically truncated summation and for the solid-fluid interactions we applied the Ewald summation. We set the cutoff to 12 Å for both LJ and Coulomb interactions. Pre-tabulated LJ and Coulombic interaction grids were used to speed up the calculations. All the atoms in the carbon structures described in the next section are explicitly considered and treated as a rigid structure by placing the atoms frozen inside the simulation cell during the simulations. The simulation box contains one unit cell that has a surface area of approximately 50 nm2 for the case of slit-pores and 156 nm2 for the case of RPC. Larger system size does not show any significant influence on the statistics of the final results. In this work, we used a SPC/E model to represent the water molecules in the simulation process. 16 This model was specifically chosen as it has been successfully used in literature to detail the properties and behavior of water in bulk and confined media such as adsorption and crystalline hydrates.

17–22

Fugacity is used in the simulations, and for the case of water, at lower pressures we as-

sumed the component fugacity is equal to the bulk pressure, while the saturation vapor pressure, po, was predicted separately by running a bulk simulation. At 298 K, for SPC/E, the fluid reaches the saturation vapor pressure at 2.25 kPa. For the case of slit-pores, we applied the boundary conditions in a and b directions (and not in the c axis), whereas for RPC we applied the periodic boundary conditions in all three directions. For each point on the isotherm, 10 to 20 billion Monte Carlo steps were performed, depending on the pressure conditions. Such long runs were needed to ensure equilibrium especially when the fluid concentration in the pore approaches condensation. Each state consisted of insertion of a new molecule, deletion of an existing molecule, and translation or rotation of an existing molecule. The first 4


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half of the run was used to ensure equilibration, the second half was used to calculate the ensemble averages. All of the simulations were performed using multipurpose simulation code, MUSIC 4.0. 23 The solid structures described in the next section were considered as being composed of spherical sp2 hybridized carbon sites fixed within the simulation cell with LJ parameters of σcc = 3.4 Å and εcc/kB = 28.0 K. In this work, the chemical heterogeneity was introduced to the carbon framework by placing two different types of N and hydroxyl groups in the carbon framework. Both the graphitic and pyridinic state N atoms were treated as a spherical atom of σNN = 3.26 Å and εNN/kB = 38.9 K. 24 The O and H atom in the pseudo moiety –OH group was represented as spherical atoms with LJ parameters: σOO = 3.1 Å, ε/kB = 79.0 K and σHH = 1.3 Å and εHH/kB = 30.0 K, respectively.

25

The O atom is assigned with a

partial charge of -0.60 and the C and H atom connected to it were assigned a partial charge of 0.30.

25

The graphitic N atom in the carbon framework and the C atoms connected to it were given a partial charge (electron unit) of −0.75 and 0.25, respectively. Likewise, the pyridinic N atom and the connecting C atoms were assigned a partial charge of -0.689 and 0.3445, respectively. These charges were earlier obtained by Wu et al 26 using a DFT method with Becke's hybrid three-parameter nonlocal exchange functional combined with the Lee–Yang–Parr gradient-corrected correlation functional (B3 LYP) and 631G (d,p) basis set for the elements C and N. In this work, N doping was introduced to the carbon framework by creating carbon atom vacancies (to introduce pyrdinic N) and placing N randomly on both sides of the carbon surfaces. In the case of RPCs, the pore-size distribution was introduced by randomly placing an N doped carbon crystallite within the simulation cell. In the case of slit-pores, magnitude of the charge distribution around the neighboring carbon atoms and the surface reactivity are most likely to change depending on the local defects, concentration, and the type and position of N atoms on the carbon surface. In the case of RPC, all the above said parameters can change with the size of the carbon crystallite, density of carbon framework, the number of carbon crystallites introduced in to the unit cell, the orientation and the position of the carbon crystallites. Clearly, porous carbon is an illdefined adsorbent and obviously presenting a global model to represent the charge distribution of such large systems similar to the ones studied here is highly complex. Thus, in this work we take a simplistic approach that conceptually agrees with the charges assigned by the commonly employed charge equilibration methods that can capture long-range interactions between atoms. 27 Partial charges for the same structure can be obtained via different methods such as periodic-DFT/ab-initio calculations or other empirical or semi-empirical methods.28,29 Provided they are often performed with a limited number of atomic clusters and given the complexity and lack of periodicity or unsymmetrical energy landscape of the carbon pores, charges obtained by such methods do not warrant a unique solution. Assigning point 5



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charges always introduces ambiguity due to the number of choices available to define atomic charges even when the true electron density of the material is known. 30 The main objectives of this work are to answer some of the several key questions connected with water adsorption behavior in N doped carbon surfaces and also to show how N doping changes the hydrophilicity and gas adsorption properties. To do this, we made the carbon prototypes electroneutral simply by assuming all the carbon atoms that are connected to N atoms as -(-x)/Nc. Where x is the partial charge of the N atom, while Nc is the number of C atoms connected to each N atom. Despite the simplicity, this approach is easy to adapt in any other system with similar or different chemistry and thus also makes the calculations more tractable. In our earlier work we adapted a similar strategy to demonstrate exclusively the N doping effect on CO2 uptake and additionally such assumptions also successfully predicted the experimentally observed isosteric heat values and the theoretical limit of N doped carbons. 5 Such approach was also earlier used by other researchers to estimate the H2 uptake in oxygenated carbon surfaces, single and binary adsorption of CO2 and CH4 and their mixtures in oxygenated carbons, water adsorption in oxygenated surfaces and to obtain the kinetic and thermodynamic selectivity of CO2/CH4 of carbon materials.31–34 In this work water molecules are represented as SPC/E model. 16 This model was specifically chosen as it has been successfully used in literature to detail the properties and behavior of water in bulk and confined media such as adsorption and crystalline hydrates.

17–22

This model treats water as a spherical

LJ atom with O atom located at its center (with LJ parameters: σO_H2O = 3.16 Å and ε O_H2O/kB = 78.212 K), which accounts for the dispersive interactions. The two H atoms are located at a distance of 1.0 Å with a HOH bond angle of 109.47°. A partial charge of -0.8476e and +0.4238e is fixed to the O and H sites to capture the electrostatics. CO2−CO2 interactions were modeled using the TraPPE potential. 35 This model treats CO2 as a linear triatomic molecule with charges placed at the center of each LJ atom. The LJ parameters for the atom C and atom O separated by a bond length of 1.16 Å from the TraPPE force- field are given by σO_CO2 = 3.05 Å and ε O_CO2/kB = 79.0 K and σC_CO2 = 2.80 Å and εC_CO2/kB = 27.0 K. The CO2 molecule quadrupole moment was simulated on the basis of a point charge of +0.70 placed on the center of mass of the carbon atoms and charge of −0.35 placed on each oxygen atom. 3. CARBON NANOSTRUCTURES Convincing results from the experiments based on X-ray photoelectron spectra and elemental analysis reported by several researchers for carbons obtained via different synthesis routes from various precursors confirm that the nitrogen species in N doped carbons frequently appear in different bonding con6


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figurations: (i) pyridinic-N, (ii) pyrrolic-N, (iii) graphitic-N and also some oxides of N. 36–38 Out of these states, graphitic-N is considered to be the more stable form at high temperatures (>800 °C), whereas pyridinic-N and other forms of N are usually converted to graphitic-N at higher temperatures. Thus the high surface area carbons which are usually obtained under harsh conditions and high temperature often end up with an N content with a relatively high percentage of graphitic nitrogen, and some pyridinic nitrogen. 39 The oxides of nitrogen, which are the least stable to temperature also appear in some high surface area carbons but in very low concentrations. In addition, the oxidizing agents and the oxygen atoms present in the carbon precursor always leave oxygenated groups on the carbon surface. Considering these experimental observations, in this work we constructed N doped carbon prototypes with >70 % of the N being graphitic and the rest being pyridinic. We also placed a very low concentration of pseudo moiety OH groups on the carbon surface. This can be taken as a representative of the O in oxides of nitrogen or the oxygenated groups itself. The N doped slit-pore models were built by placing three graphitic sheets (ABA stacking, see Figure 1) on both sides of the carbon pores (six in total). Earlier studies confirmed that three graphene layers are sufficient to model the guest host interactions in both pristine and functionalized slit-pores. 40– 42

The planes of graphitic sheets were separated by an interlayer distance of 0.335 nm. For the case of

graphitic N, N doping was introduced via a substitutional doping technique, thereby retaining the planar sp2 hybridization of the graphene sheet. To introduce pyridinic N, we artificially created C atom vacancies on the carbon surface which is accessible to the target molecule and replaced one of the C atoms on the edges with N. N sites were placed randomly on both sides of the pore wall until a desired level of N wt % (3-5.2 wt %) was reached. Care was taken during the doping process to mimic graphitic and pyridinic nitrogen, such that every N atom is connected to three carbon atoms and two carbon atoms in the graphene sheet, respectively (Figure 1). In this work, we have assumed that replacing C with N does not alter the interlayer spacing between the graphene layers connected to the pore wall, or creates any surface undulations at the pore surface that can be accessible for the probe molecules. Theoretical and scanning tunneling microscopy studies reported in the literature;

39,43,44

confirm

2

that N atoms in graphitic N adapt the sp planar structure of graphene, the nearest neighbor C–C bonds remain intact, and they do not wrinkle the planar nature of the graphene sheet. However the C atom vacancies and pyridinic N can create surface undulations. Surface undulation can also be influenced by the concentration, number of C atom vacancies and even by the position of N atoms in the carbon framework. Making a tractable model while considering all of these features is practically difficult. Consider7



ing the low concentration of the pyridinic N in the carbon framework and for convenience, we assume that the C–N distances are equal to the C–C distances of an ideal graphene sheet assuming N doping does not cause any local curvature. All four slit-pore models are characterized by a pore width, H, defined as the distance between the surface of the carbon (or N) atoms between the opposing walls. The pore-width, H, typically represents the experimentally measured effective pore width.

(a)

(b)

PSD

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0.4 0.3 0.2 0.1 0

(c)

2

4

6

8 10 12 14 H, Å

Figure 1. Atomic representation of in-silico obtained carbon prototypes (a) SN3 – N doped slitpore model and (b) RPC – a disordered N doped carbon structure that has a pore-size distribution and (c) pore-size distribution of RPC (PSD – pore-size distribution). To gain a better understanding of the water adsorption behavior in N doped structures and on the influence of N, we considered four different carbon prototypes that differ from each other in terms of their N concentration or in terms of their pore width. We labelled these structures as SN1, SN2, SN3 and SN3_micro (See Figure S1 of supplementary file). In the case of SN1 and SN2, N moieties and OH group are introduced by randomly placing the N atoms (or creating C atom vacancies) on both sides of the wall until a desired level of N concentration was reached. For the case of SN3 (see Figure 1a), a 8


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structural symmetry was maintained while introducing N doping and OH groups, by placing them on both sides of the pore wall exactly at the same positions (i.e., the number of N atoms and OH groups on both sides of the wall are the same and their positions merely reflect each other). Of these four structures, SN1, SN2 and SN3 share a common property; all of these models contains pores of similar width, H = 2 nm. This pore width is specifically chosen as it lies between micro- and mesopores as defined by IUPAC and thus can portray the water adsorption mechanism in typical micro/mesoporous carbons.

15

To show the N doping effect in an ultra-microporous carbon, we fixed the pore-width, H, of SN3_micro to 8.6 Å. This pore width typically lies in between the border of ultra-micropores and micropores. Similar to SN3, a structural symmetry is maintained in SN3_micro while introducing N atoms and OH groups on their pore-wall. The pore walls of SN3 and SN3_micro share the same surface chemistry with the only difference being the pore-width. The concentration of N, O, C and H atoms involved in the constructed carbon prototypes are given in Table 1. Table 1: % of N, O, H, C in the different carbon prototypes Prototype

N(a)

O

H

C

SN1 3.639(71.42) 0.990 0.062 95.309 SN2 5.187(72.22) 0.988 0.062 93.763 SN3 5.281(78.26) 0.984 0.061 93.674 RPC 6.114(66.67) 2.329 2.475 89.083 (a) In this work we considered only two types of N, graphitic N and pyridinic N. The percentage of graphitic N is shown in parenthesis; e.g., the concentration of graphitic N in SN1 is = 3.639x71.42/100 = 2.598).

In order to mimic the real topology of the activated carbons, we also constructed an in-silico disordered carbon prototype, RPC, from a collective of flat circumcoronene shaped graphitic basis units made up of 51 carbon atoms and 3 N atoms (two graphitic N and one pyridinic N) and one OH group (see Figure 1b). RPC is simply obtained by randomly placing a number of these basis units within a simulation cell avoiding overlapping of basis units until a desired density (0.56 g/cm3) is reached. Formation of any bridges between the building units were avoided and thus the basis units were artificially fixed in the space inside the unit cell. Such protocol allows us to include the possibility of adsorption along the crystallite edges and mimic the experimentally realized layered graphene crystallites. This structure can also help to study the effect of pore topology and also to address issues related to poreconnectivity on water adsorption in N doped carbons. Earlier, we used such computer-generated structures to study the pore structure effect on methane adsorption, CO2/N2 selectivity, CH4/H2 selectivity 9



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and even to study the chemisorption of H2 in activated carbons. 5 Recently, Biase and Sarkisov

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suc-

cessfully derived an in-silico version of Maxsorb, a commercial activated carbon using this approach. We also show how this specific structure allows us to expose the influence of pore structure and poreblocking effects (if any) during water adsorption. Analysis of the pore properties of the RPC showed this structure has a surface area of 3520 m2/g and porosity of 73 %. The surface area and the porosity, the total pore volume, Vtot, and the accessible pore volume for RPCs were estimated using a geometrical method available in the atom volume and surfaces module of Material Studio. 46 This technique involves random insertion of probe molecules around each of the framework atoms and checks for overlap with other framework atoms. The fraction of the probe molecules that do not overlap with the framework atoms is then used to calculate the accessible surface area and accessible pore volume. To obtain the surface area and accessible pore volume, a probe molecule equivalent to the kinetic diameter of N2 was used and for pore volume, Vtot, we used a probe molecule of zero diameter. The pore-size distribution (Figure 1c) obtained using pore-blazer v2.0

47

confirmed this structure is essentially microporous and

contains pores that range from 0.4 nm – 1.3 nm. 4. RESULTS AND DISCUSSION 4.1 Water adsorption behaviour in N doped carbon sli-pore of pore-width, H = 2 nm Figure 2 shows the amount of water adsorbed versus the relative pressure in the three N doped carbon prototypes SN1, SN2 and SN3. The results are presented in terms of number of molecules adsorbed per unit cell. For comparison we also show water adsorption in a defect free pristine structure of similar pore width. In pristine structures, the isotherms clearly show that there is no water adsorption for p/po up to 1 bar and condensation occurs only at pressures (p/po > 1.33) much higher than the vapor pressure of the bulk water at 298 K. This behavior is well known and is mainly due to the low interactions between the water molecules with the carbon surface, which is essentially hydrophobic. In the case of N doped carbons, the simulation results show that chemical heterogeneity changes the water adsorption behavior. Another notable feature is that the condensation in SN2 and SN3 occurs at different pressures. In the case of SN1, a framework that has a relatively low N content, we noticed an adsorption trend similar to the adsorption in the pristine structure, with zero water adsorption at relative pressures up to 0.90 and fluid condensation at pressures almost near the saturation vapor pressure (p/po = 0.993). In SN2 and SN3, which possess a higher N content, we have noticed a progressive adsorption of water molecules for p/po up to 0.488-0.533 (see Figure 2), after which a sudden jump in the adsorption isotherm occurs and the entire pore volume is filled by water due to condensation effects. The discontinuous jump in adsorption and the obtained S-shaped isotherm agrees with some of the experimentally measured water 10


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adsorption isotherms. 11 At higher relative pressures p/po, the adsorbed phase seems almost incompressible as increasing the pressure in the bulk fluid does not show any significant increase in the amount of water in the adsorbed phase. This behavior also confirms the occurrence of the phase change (gas to liquid) within the carbon pores. We performed additional simulations with a carbon prototype having the same pore width (not shown here) but with a higher N content (N up to 7 wt%) and found that the pressure at which the pore reaches the percolation limit is not influenced by the N concentration if above 5.4 wt%. This pressure typically lies in the range of 1.1-1.2 kPa. 57 1800

56

1500

SN1 SN2 SN3 pristine Qst

1200 900

55 54 53

600

52

300

51

0

Qst, kJ/mol

, molecules/u.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


50 0

0.2

0.4

0.6

0.8

1

1.2

1.4

p/po

Figure 2: Water adsorption isotherms in a : SN1, : SN2, : SN3, ∆: pristine pore (desorption shown as open circles) and : the isosteric heat of adsorption in SN3 (there is no adsorption of water in pristine pore for p/po < 1.33). To understand the adsorption behavior in detail, we captured a series of snapshots (Figure 3) showing the equilibrium adsorption of water in SN3 at different relative pressures (p/po = 0.044, 0.088, 0.177, 0.222 and 0.266). At p/po = 0.044 and 0.088 (Figure 3a & b), we found that water molecules are preferentially adsorbed on N atoms through hydrogen bonding on both sides of the wall. Additionally, the snapshots show that if the N atoms are close enough, the water molecules adsorb on the nearest N sites and readily form hydrogen bonds with water molecules that are already adsorbed on to the N sites through a cooperative adsorption phenomena to form water nanowires. In other words, once a water molecule is adsorbed on to an N site, it creates a new local heterogeneity within the pore volume that promotes the uptake of water at higher pressures. This interesting detail observed at the initial stage of adsorption is practically difficult to trace via experiments as it is hard to maintain an equilibrium adsorption at such low pressures due to the sensitivity of the pressure sensors. Cooperative adsorption corresponds to the simultaneous adsorption of water onto the N sites and the formation of water nanowires with the pre-adsorbed water molecules (see the magnified image in Figure 3a and the green arrows in Figure 3). Another notable feature in Figure 3a-c is the presence of a few single water molecules adsorbed on the N sites. These molecules, at higher relative pressures (0.177 & 0.222) act as (poly) nu11



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cleation sites and promote the formation of water nanowires. In Figure 3, the (poly) nucleation sites and water nanowires occurring at different stages of p/po are indicated with red and green arrows, respectively. At higher relative pressures (0.177 and 0.222 – see Figure 3c & d) we can observe that the water molecules are preferentially adsorbed onto these (poly) nucleation sites as well as onto its N neighbors forming more 1D (one-dimensional) or 2D (two-dimensional) water nanowires. At p/po = 0.266, on each pore wall, all of these individual water nanowires tend to connect with each other via cooperative adsorption and end up in a water cluster with a sheet like topology formed out of water molecules connected with each other and spread over the N doped carbon surface (Figure 3e). (a)

(b)

(c)

(d)

(g)

(h)

(e) c-axis (f)

Figure 3: Snapshot showing water molecules adsorbed in SN3 at (a) p/po = 0.044, (b) p/po = 0.088, (c) p/po = 0.177, (d) p/po = 0.222, (e) p/po = 0.266, (f) p/po = 0.446, (g) p/po = 0.488; and (h) Snapshot showing the center-of mass of hydrogen (purple dots) in a water molecule accumulated on one side of the wall of SN3; red arrows show the nucleation sites, green arrows correspond to 1D/2D water nanowires. This specific phenomena occurring at atomic resolution on each pore wall resembles the ‘birth and spread’ of a nucleus either in one dimension or in two dimensions and can be taken as a process analogue of 1D or 2D nucleation – a process that is frequently encountered in the growth of organic crystals from solution. 48 The only difference here is that the birth and spread of a 1D/2D nucleus is aided by the presence of heteroatoms and cooperative adsorption phenomena via H-bonding, whereas the classical 2D nucleation is associated with a Gibbs free energy formation of stable nuclei. This 1D/2D growth of water observed on N doped structures is unique, as under bulk conditions, water molecules typically form up to four hydrogen bonds to arrange themselves in a tetrahedral fashion. In a pore that is small enough to distort this tetrahedral geometry or in a material like carbon nanotubes, where the pore structure itself can force the water molecules to arrange in one dimension via hydrogen bonding, the formation of such 1D or 2D nanowires can be expected. Despite the fact that the pore width considered in this work would be enough to accommodate > six layers of water molecules, the tendency of the water molecules to form a hydrogen bond with the N atoms or with the water molecules that are already ad12


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sorbed on the nearest N neighbors, favors the formation of 1D/2D nucleation analogues on the carbon surface at low pressures (p/po up to 0.266 in SN3). At p/po = 0.266, we also noticed that the 1D/2D water nanowires tend to form H-bonds with the O atom in the pseudo moiety ‘OH’. This obviously indicates that if the oxygenated groups are in the vicinity of the adsorbed water molecules, or if they are located near the N atoms, they can assist the spreading of 1D/2D water wires on the carbon surface. There is no experimental evidence to relate the 1D or 2D spreading of water molecules in carbon nanopores as the quasi-1D and 2D structures were never realized experimentally, probably due to the low pressure (at the limit of pressure sensors) at which such structure might appear in real systems; although 1D water chains were realized experimentally on the steps of Pt surfaces, but at cryogenic conditions. 49 The snapshot in Figure 3e also shows that at p/po = 0.266 the water molecules tend to adsorb on the water nanowires exhibiting a few three-dimensional water clusters (an analogue of 3D nucleation) on both sides of the pore wall. At very high relative pressures, p/po = 0.488, we have noticed that the adsorption is dominated by the growth of water clusters on each pore wall in three dimensions via 3D nucleation. The snapshot in Figure 3g shows that before reaching the percolation limit, these water clusters adsorbed on both sides of the pore wall connect with each other forming a water pillar with a liquid density at the middle of the pore. The realization of such water pillar like structures is important in the design of carbon adsorbents for gas storage applications. Carbons synthesized using oxidation techniques often contain interconnected pores made up of carbon crystallites of different physicochemical properties with a wide range of pores with different pore volume. Most of these materials contain a significant amount of edges where functional groups are likely to be located. For instance, the presence of water as impurity during adsorption of any other gas molecule can readily form such water pillars around the edge sites located near the pore opening that can virtually make the remaining pore volume useless to host other gas molecules. For the case of N doped structures, N functionalities such as pyridinic N are usually located at the edge sites and it is more likely that they can create such pore-blockage effects. This effect cannot be explained with the adsorption results obtained in slit-pores, due to the nature of the pore structure and the boundary conditions applied. The adsorption happening in this (slit) pore is rather independent (as it cannot sense the water adsorbed in neighboring pores of similar structure but might be of different pore width) as the pore wall is assumed to be a graphene like sheet of infinite size. Another notable feature that can be observed from Figure 3a-g is the existence of carbon surfaces and N sites free of water molecules on both sides of the wall. This clearly indicates that water adsorption is extremely site-specific and is dictated by the formation of 1D/2D and 3D nucleation due to the local heterogeneity created by N atoms and already adsorbed water molecules, rather than a Langmuirian type of adsorp13



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tion. The calculated isosteric heat (see Figure 2) confirmed that these multiple steps involved in the water adsorption process are associated with different levels of binding energies. The formation of (poly) nucleation sites, 1D/2D water nanowires and water pillar like structures via 3D nucleation are associated with isosteric heat values ranging from 58 - 52 kJ/mol. These values are relatively higher than the heat of vaporization of liquid SPC/E water (50 kJ/mol). The high isosteric heat can be attributed to a decrease in entropy as a consequence of high structural ordering of the water molecules either as nucleation sites or as 1D/2D nanowires at lower pressures. The formation of water pillars and the complete pore-filling was associated with an isosteric heat of 50 kJ/mol, which is equivalent to the enthalpy of condensation of SPC/E water.

50

This indeed confirms the liquid state of the water molecules at higher

relative pressures (p/po > 0.488). The desorption isotherm showed a big hysteresis, which signifies that the water phase formed at p/po > 0.488 remained stable for p/po up to 0.088 (see Figure 2). Large hysteresis observed in N doped carbons with GCMC simulations can be matched with the hysteresis obtained with water adsorption isotherms on hydrophilic and hydrophobic carbon materials (activated carbon, charcoal), MOFs, silica gel, montmorillonite and also with the simulation results of water adsorption in pristine carbons. 18,51–54 Several theoretical concepts can be put forward to explain this phenomena. Hysteresis can be expected in a carbon with pore-size distribution, if the liquid water cannot evaporate until a very low p/po is reached due to pore-blockage effects, which is least likely to occur in the studied pore structures. Considering the pore topology, one possible explanation can be obtained based on the phases and the interface involved within the carbon pore during desorption. The liquid water always tends to minimize its surface area and thus the net force involved in the system orients towards the liquid phase. Thus high energy is required to disturb this minimum energy configuration to overcome the attraction forces involved in the liquid phase. Another possible explanation for the hysteresis can be associated with the interfacial surface area involved between the liquid/gas phases during the adsorption and desorption process.

21

The interfacial surface area is less when a liquid phase grows within a vapor phase

than that when a vapor phase is grown in liquid phase. 21 The contact angle involved during the adsorption and desorption phase is completely different, as the contact angle during the adsorption of water molecules on N sites and already adsorbed molecules is relatively higher than the contact angle during the desorption from a completely filled pore. Following the work of Striolo et al,

21

we confirmed this

phenomena in N doped structures from the intermediate configurational snapshots (see Figure 4) collected during the desorption phase and simultaneously analysing the structure of the adsorbed phase based on the available pore volume at different stages of the desorption.

14


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Figure 4: Intermediate configuration snapshots captured during the desorption of water from SN3 at p/po = 0.1/2.25 and 298 K. The pore-volume accessible to N2 molecules are shown as mauve surface (mauve surface corresponds to the boundary of the bubble; starting configuration was a pore filled completely with SPC/E water which is obtained from a GCMC run performed at p/po = 1). The available pore volume in the adsorbed phase was obtained using a geometric technique and the estimated pore volume corresponds to the space available for N2 molecules. The accessible pore volume can give visual information on the formation of bubbles inside the adsorbed phase during desorption. As opposed to the adsorption mechanism, where the pore-filling occurs due to the growth of water molecules in two or three dimensions, Figure 4 clearly shows desorption occurs via a cavitation like process. Snapshots expose that initially the ordered liquid-like structure is lost and several free spaces (left panel in Figure 4) of different volume appear within the adsorbed phase. These bubbles burst into larger void spaces (right panel of Figure 4) due to density fluctuations until the pore is empty. If we compare this phenomena with the adsorption trend where adsorption is extremely site-specific and thus the interfacial surface area involved is relatively high when compared to the interfacial area involved during the desorption process. This phenomena explains the reason behind the hysteresis observed during the adsorption/desorption of water in the studied structure (SN3). Finally, to confirm the liquid state of the water (incompressible water) in carbon structures at higher pressure, we obtained the pair wise distribution of atoms (see Figure 5) that can give accurate information on the nature of water molecules confined within the pore volume. The oxygen atom pair distribution functions, gOO(r) show a near neighbor centered around the distribution at 2.75 Å, which overlaps with the distribution of tetrahedrally coordinated second nearest neighbors at 4.65 Å. This observation matches with the neutron diffraction data/x-ray diffraction data for water at 298 K, 55 which clearly exposes the tetrahedral arrangement of water molecules as liquid water inside the pore. These physical insights on the state and the structure of water molecules inside the N doped pore, the associated energies and the information on the percolation limit of the structure are important and essential to develop any fundamental theory to explain the water adsorption process in N doped carbon structures.

15


The Journal of Physical Chemistry 3

12

2.5

10

6

1.5

4

1

gOO(r)

8

2 gOO(r)

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2

0.5

0

0

-2 0

2

4

6

8

10

r, Å

Figure 5: Pair distribution function for water (O-O): Calculations obtained based on the average of at least 100 equilibrium configurations collected during the adsorption of water in SN3 (black line) and SN3_micro (red line) at p/po = 1. In SN2, we noticed a similar adsorption trend, although the formation and number of water nanowires involved as well as the pressure at which the percolation limit is reached slightly differs from SN3 (See Figure S2 of supplementary file). At p/po = 0.177 and 0.355, we noticed (poly) nucleation sites or 2D nanowires occurring only on one side of the pore wall, while the other side, which has less N sites, is free of water molecules (See Figure S1 of supplementary file). We observed the formation of 2D nanowires on both sides of the wall only at a very high p/po, of 0.489. Similar to SN3, in SN2 we noticed the formation of water pillars and condensation with liquid density in the middle of the pore volume, though at slightly higher p/po (at 0.533). This can be explained if we consider the position of N atoms on both sides of the pore-wall. In SN3, water molecules adsorb on both sides of the wall progressively as both sides contain similar energetic heterogeneity. At some stage (at p/po = 0.446), this allows the adsorbed water molecules facing each other to form a bridge made of water nanowires (in c-direction) connecting both sides of the pore wall (see Figure 3h). This particular step aids the formation of water pillars in this structure at much lower pressures. In SN2, this particular step is not involved, as the 2D/3D water clusters adsorbed on both sides of the slit-pore wall are located too far away from each other to form any nanowire bridge in c-direction. This also explains why the percolation limit is reached at relatively higher pressure in this type of pore. The isotherms of SN2 and SN3, altogether show that the water adsorption follows a unique trend depending on the concentration and position of N. This is a notable result as, if properly exploited, this property can be used to characterize and screen the heterogeneous property of N functionalized carbons. For instance, the number of 2D or 3D nucleation occurring within the pore can give a quantitative information on the active N sites that are desirable for material design in catalysis and gas storage applications.

16


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To expose the zone-specific adsorption of water in N doped carbons, we collected the center-of mass of water molecules accumulated over 500 equilibrium configurations during adsorption at 0.01 kPa in SN3, which is shown in Figure 3h. At 0.01 kPa this pore can take up only one molecule of water, which theoretically should bind to the site with the highest binding energy. Figure 3h shows that water molecules typically adsorb onto two specific sites, one being graphitic N and the other being pyridinic N. This finding suggests that water molecules preferentially adsorb over specific zones, which corresponds to regions with a high local density of heteroatoms rather than specific sites. For the case of SN3, this corresponds to a graphitic and a pyridinic N surrounded by 3 other N atoms separated by a distance that ranges from 5.67-6.5 Å. 4.2 Water adsorption behaviour in an N doped carbon slit-pore of pore-width, H = 0.86 nm Simulations performed in slit-pores of a pore-width H = 2nm, can demonstrate the water adsorption behavior in N doped micro/mesoporous carbons, but it cannot detail the water adsorption properties that contain ultra-micropores and smaller micropores (0.8 nm to 0.488. Whereas in SN3_micro we observed a progressive change in phase from gas to an incompressible liquid like state. Equilibrium configurations collected at different stages of the adsorption exposed the fact that in smaller pores the water molecules adsorbed on both sides of pore wall are located close enough to create a local heterogeneity inside the pore volume which favors the adsorption of a new water molecule. 17


The Journal of Physical Chemistry 700

, molecules/u.c.

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600 500 400 300 200 100 0 0

(b)

0.2

0.4

p/po 0.6

0.8

1

(c)

Figure 6: (a) Water adsorption isotherm (desorption shown as open circles) obtained in SN3_micro at 298 K and snapshots showing water molecules adsorbed on SN3_micro (H = 0.86 nm) at (b) p/po = 0.0133 and (c) p/po = 0.0355. In Figure 6, we show how water molecules adsorbed on both sides of wall form a water bridge to connect the water nanowires adsorbed at very low relative pressures (0.03-0.08 kPa). These results indicate how the pore structure plays a crucial role during the water adsorption process. The smaller the pore volume, the smaller is the pressure at which fluid condensation occurs. Also the adsorbed water molecules tend to confine within a restricted pore volume (the water molecules adsorbed on both sides of the wall are located closer to each other) and create a local energetic heterogeneity or strong energetic pockets within the pore volume. These adsorbed molecules along with the N sites favor the progressive uptake of water molecules rather than a sudden phase change as observed in SN3. We confirmed this by performing a desorption simulation. The desorption isotherm shows almost zero hysteresis, which proves the progressive filling of water (rather than condensation) during the adsorption phase. The influence of pore width on the width of hysteresis or the lack of hysteresis in hydrophilic pores are an experimentally observed phenomena. 51,56 Several reasons can be put forward to explain the lack of hysteresis observed in the N doped microporous carbon (like SN3_micro). To gain some insights on the adsorption/desorption process, we estimated the density of the adsorbed phase. Analysis of the density of water in SN3_micro confirms that at p/po = 1, water molecules are confined within this pore with a density of 0.0428 mol/cm3, which is slightly lower than the liquid density of water at this temperature (0.0553 mol/m3). This can be explained by considering the restricted pore-size of this carbon prototype. 18


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Snapshot investigations of the adsorbed molecules at different locations and angles (not shown here) confirmed that the smaller pore-size disturbs the quasi-tetrahedral geometry of water along the c-axis of the pore (adsorbed phase partially changes from three-dimensional to a combination of two and threedimensional); which in turn can dictate the final density of the adsorbed phase inside the pore and also the nature of desorption. The absence of the second peak in the oxygen atom pair distribution functions, gOO(r) (Figure 5) of the adsorbed water in this structure confirms the structure is more inhomegeneous. 57

The lack of quasi-tetrahedral geometry of liquid water within the pore also means, that the intermo-

lecular interactions are not uniform in all three directions. 56 Thus the energy involved to lose the minimum energy configuration of water from a less ordered structure (or a high entropy) to a gas phase is lower than the energy involved in desorption from quasi-tetrahedral water (that has a low entropy) to vapor phase.

Figure 7: Intermediate configuration snapshots captured during the desorption of water from SN3_micro at p/po = 0.088 and 298 K. The mauve surfaces correspond to the boundary of pore volume accessible to N2 molecules (mauve surface corresponds to the boundary of the bubble; starting configuration was a pore filled completely with SPC/E water which is obtained from a GCMC run performed at p/po = 1;carbon framework is not shown for clarity purpose; water molecules are shown in grey). Careful analysis of the intermediate configurations (Figure 7) confirmed the above stated arguments: pore emptying during the desorption from a pore that contains a less ordered liquid water structure is progressive (instead of cavitation as observed in larger pores) similar to adsorption. Water molecules tend to continuously desorb from the surface and also from the pore volume available in between the pore surface which explains the zero hysteresis observed in N doped SN3_mico. 19



The results observed through simulations are of great importance as they give some necessary information for the design of adsorbents for carbon capture/gas separation, energy storage as hydrates and catalysts in PEM fuel cells. For instance, in the area of carbon capture, N doped carbons containing such small pores are highly vulnerable to impurities such as water. Our results show, even a very low concentration of water in the bulk fluid can completely block the pores or the rest of the pore volume, making them useless to host other gases (CO2, CH4, H2). An advantage of such pores may be the capability to store hydrates, as it is easier to control the stoichiometry of hydrates simply by changing the partial pressure of the bulk fluid in water. In the case of PEM fuel cells, the overall performance is dictated by the hydration state of the proton exchange membrane, which is usually controlled by maintaining the water content of the electrode. 9 In that spirit, an N doped carbon with micropores can be an excellent candidate material to maintain the hydration state of proton exchange membranes. 4.3 Water adsorption behaviour in an N doped microporous disorderd carbon structure

1600 1400

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(a)

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(b)

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p/po

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(c)

Figure 8: (a) Water adsorption isotherm obtained in RPC at 298 K and snapshots showing water molecules adsorbed on RPC (b) p/po = 0.577 and (c) p/po = 0.622 (in Figure 8c, carbon frameworks are not shown for clarity purpose). Water adsorption results obtained in slit-pore structures clearly show how the water molecules evolve inside the pore volume of N doped carbons. Due to the boundary conditions applied during the simulation process, the crystallite size of the slit-pore wall that creates the porosity is assumed to be infinite. 20


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This deviates from the pore environment of experimentally realized carbons which contain a pore-size distribution created by the layered arrangement of graphene crystallites. Studying the water adsorption behavior in such realistic structures is important especially if the carbon crystallites are enriched with pyridinic N, which are located at the crystallite edges. To illustrate such effects, we performed GCMC simulations in a disordered carbon, RPC. The simulated isotherm and snapshots of the molecular configurations of the water adsorbed in this structure at different relative pressures are given in Figure 8. At lower relative pressures (p/po 0.533. Similar to the adsorption in a slit-pore, at p/po = 0.577, water molecules tend to bind on to water molecules that are already adsorbed on to the N sites as well as to the nearest N or OH sites trying to from small water clusters (see snapshot in Figure 8a). In Figure 8a we also show the specific locations (magnified images show water molecules bind to both graphitic and pyridinic N) where water molecules are preferentially adsorbed. These specific locations correspond to pore volume that is bounded by several N doped graphitic crystallites (basis structural unit). At p/po = 0.622, some water clusters adsorbed on to active sites connect with each other (Figure 8c). The complex pore topology of this structure disturbs the lateral growth of the already adsorbed molecules in a, b or c- axis at higher pressures. At higher relative pressures (> 0.622), we noticed a sharp transition in the water adsorption instead of continuous adsorption confirming the fluid condensation process. Due to the microporous nature (PSD shows this structure contains a combination of ultramicropores/smaller micropores) and the surface chemistry (higher N concentration when compared to SN3_micro), a continuous surface adsorption plus pore filling process, as observed in SN3_micro, could be expected. However, the simulated adsorption isotherm clearly shows a sudden phase change from gas to liquid (incompressible fluid) like state in RPC at p/po > 0.622. From the snapshots shown in Figure 8, it can be realized that water molecules adsorb specifically on to two specific locations rather than being site-specific; while a majority of N atoms doped to different carbon crystallites does not show any preferential uptake for water molecules. This structure reaches its percolation limit at p/po = 0.622 as practically there is no formation of any nucleation sites on any of the crystallite surfaces and any increase in the bulk pressure of water just leads to a change in the adsorbed phase density. These observations show that not only the N doping, N concentration in the carbon framework or the pore-size but also the pore structure itself can play a major role on the water uptake mechanism and thus on the hydrophilic property of the material. Recently N doped carbons were screened for their oxygen reduction capabilities based on their hydrophilic properties (the higher the water uptake at lower pressures, the higher is their hydrophilicity). 8 Such techniques might be useful if the 21



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Page 22 of 32

pore structure is rather homogeneous as in slit-pores. Such approach might produce biased results in N doped carbons with disorder pore structure unless we have an access to probe the position and structure of water molecules in detail at different pressures. This observation agrees with the recent experimental study of Huber et al;

13

their water adsorption isotherms showed there is no clear correlation between

the N concentration and the water uptake at different relative pressures. The results observed in this material are important in material design for the capture of atmospheric water.10 For instance, hydrophilicity is correlated with the presence of active sites and recently N doping is considered as a viable option to capture air from water. The simulation results suggest, for such applications, disordered microporous carbons might not be an interesting candidate as these materials practically do not show any appreciable amount of water uptake at lower or intermediate relative pressures. This also means it will be difficult to operate such material in cyclic adsorption mode as the process is controlled only by condensation rather than any surface adsorption.

Figure 9: Accessible pore volume (blue dots) for an N2 molecule on RPC that contains already adsorbed water molecules (100 molecules); carbon framework and the adsorbed water molecules are not shown for clarity. The RPC considered in this work has a pore-size distribution due to the randomly arranged carbon basis units within the simulation cell. It is more likely that adsorbed water molecules can introduce pore blockage effects to the accessible pore volume. In order to see this effect, we obtained the accessible pore volume of the RPC already adsorbed with water at p/po = 0.405/2.25. The accessible pore volume was measured with respect to the size of an N2 molecule using a geometric method described earlier. The available pore volume accessible to N2 molecules is shown in Figure 9. It can be seen that at least at the studied pressure and for this particular carbon structure, loading of water molecules does not create any blockage, rather we noticed a physically continuous pore volume. Earlier studies confirmed that wa22


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ter molecules/clusters adsorbed on to reactive sites can disturb the connectivity of the pore volume.

22

The absence of pore-blockage observed in the RPC due to the water adsorption does not imply N doping cannot introduce such effects. Pore connectivity is related with more than one parameter such as the length and the width of the graphitic crystallites that bound the pore volume, position of reactive sites on the graphitic crystallites, density of the carbon framework and the pore structure itself. The RPC constructed in this work can mimic the pore-size distribution and chemical heterogeneity that are frequently encountered in experiments, though the entire structure was made by a simple basis unit of the same size and similar surface chemistry. Such simplistic structure is still far from experimentally obtained carbons and introducing all these effects might show a different scenario on the connectivity of the pore volume due to water adsorption. Pore blockage effects can be detailed by performing water adsorption simulations in different carbon structures constructed from a combination of basis units with different surface chemistry as well as narrow and wider pore-size distributions. Due to the computational cost involved in such simulations, further investigations on this particular issue could not be carried out in this work, as for some pressure points in this study (near condensation), equilibrium was achieved only after ten billion GCMC trials. 4.4 Effect of N doping on CO2 selectivity during its uptake from its bulk fluid that contains water as impurity The hydrophilic properties in the N doped structure can disturb the guest-host interactions of other molecules such as CO2. To further investigate this issue, as a case study we obtained CO2 adsorption isotherms for SN3 at pressures, pCO2 up to 100 kPa from a fluid mixture diluted with different concentrations of water, pw = 0.4, 0.8, 1.2 and 2.0 kPa as shown in Figure 10. The terms pCO2 and pw, correspond to the partial pressure of CO2 and partial pressure of water in the bulk binary fluid mixture, respectively. For convenience the results are shown in terms of CO2 adsorbed versus its partial pressure. For comparison, we also showed the single component CO2 isotherm. Dilution of the bulk fluid with water results either in moderate or significant influence on the CO2 uptake properties. For low concentrations of water in the binary fluid mixture (pw = 0.4 and 0.8 kPa), we noticed only a moderate influence on the CO2 uptake. To gain some insights on this adsorption behavior we captured the snapshots of CO2 molecules adsorbed at 1 bar (the partial pressure of CO2) from a binary fluid mixture with different concentrations of water (see Figure 9b-9d). For pw = 0.4 and 0.8 kPa, though the water molecules occupy some of the active sites either as nanowires or as large 3D water clusters, the CO2 molecules still have enough carbon surface, N sites and large pore volume to adsorb or fill the pore. However, if the water partial pressure in the bulk fluid is increased to 1.2 (the percolation limit of this pore for water), 23



the CO2 uptake is significantly decreased when compared to the CO2 uptake from its pure component as a significant amount of the pore volume is occupied by water molecules (Figure 9d). When the partial pressure of the water in the bulk pressure is closer to its saturation vapor pressure, the pore is completely filled by water, leaving negligible space for any CO2 molecules. In a more recent work, Liu et al

58

showed the influence of CO2 with water, which ultimately increased the selectivity of the pristine carbon nanotube for CO2. However, in our work, no such effects could be realized, probably due to the low partial pressure of CO2 considered (CO2 pressure that is usually encountered in the compressed flue gas; the partial pressure of CO2 in real flue gas is even lower than the pressure considered). In addition the pore environment in nanotubes and pristine carbon surfaces will differ from the N doped carbons where the pore volume contains several electrostatic pockets. This observation can be explained also based on the heat of liquefaction of water which is much higher than the heat of vaporization of CO2 (16.5 kJ/mol), meaning that CO2 molecules are least likely to condense or form hydrates under the studied conditions. This observation agrees with the theoretical works of Billemont et al 59 who performed molecular simulations on CO2 adsorption in a pristine slit-pore decorated with water molecules. They showed the CO2 pore-filling process is influenced by the presence of water molecules and they found a linear relationship exist between the (artificially introduced) water molecules in the pore and the CO2 uptake. 35 (a) , molecules/u.c.

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Figure 10: (a) CO2 adsorbed versus pCO2 from a binary fluid mixture of CO2 and water; Snapshots showing the adsorption of CO2 at 1 bar from a binary fluid mixture of CO2 and water with water concentrations in the bulk fluid of (b) pw = 0.4 kPa, (c) pw = 0.8 kPa and (d) pw = 1.2 kPa 24


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5. CONCLUSION The effect of N doping on the water adsorption behaviour was studied in different in-silico obtained carbon prototypes. GCMC results show that hydrophilicity and the water uptake mechanism in N doped carbons are dictated by multitude of parameters such as pore-size, concentration of N, pore structure and the position of N atoms. For low concentration of N, adsorption was almost negligible at lower pressures and capillary condensation occurs almost near the saturation vapor pressure of water. Increasing the N concentration significantly improves the water uptake at lower and intermediate relative pressures. Snapshot investigations on the atomic structure of adsorbed phase confirm water adsorption in N doped carbon proceeds via a series of classical nucleation analogues such as the birth and spread of mono or (poly) nucleation sites via 2D and 3D nucleation before complete pore filling. These nucleation analogues are associated with adsorption of a new water molecule on to an active N site or due to cooperative adsorption (water molecule adsorption simultaneously adsorb on to N site and on to already adsorbed water molecule via H boding) and these effects get manipulated by pore-size, pore structure and the position of N atoms. Decreasing the pore-size triggers all the above mentioned processes. While the influence of pore-size on adsorption is a known phenomenon, our results indicate decreasing the pore size can stimulate these effects and can change the adsorption scenario by enhancing the water uptake at lower and intermediate relative pressures with zero hysteresis. These nucleation like scenario occurs partially in a carbon with disordered pore structure. Isosteric heat at low loadings confirmed that water molecules adsorb on specific locations rather than specific sites or type of N (such as graphitic or pyridinic N). Our results show water adsorption in N doped carbons is dictated by the formation of nucleation site and its spreading that depends on the unique properties of each framework such as pore size, concentration and position of N atom and pore structure or in the case of disordered carbons the position of graphitic crystallites that contains N atom, it will be difficult to make a global correlation between water uptake versus any one of these properties. Nevertheless, this observation is useful as it offer the possibility to characterize carbon materials with different surface chemistry using water as a probe molecule. GCMC results on the binary adsorption of CO2 and water show that the sensitivity of water to N doped carbon surface can alter the natural property of the carbon structure to host guest molecules such as CO2. Water is involved as a prime host or as an interface in several engineering applications such as electro/photo-catalysis, gas adsorption/separation, energy storage and hydrates. In fuel cells, the active N sites dictate the catalysts efficiency; our result show water adsorption exposes the active N sites and thus can be a useful technique for preliminary screening of catalysts. The ability of N doped carbons to host 25



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water as nanowires or water pillars can lay a new path to design different stoichiometric hydrates in these materials. The information obtained on the nucleation (analogue) of water within N doped carbon pores is an important milestone to develop a fundamental theory and characterization of the surface chemistry of functionalized materials based on water adsorption isotherms. The simulation results obtained with disordered carbons showed that, N doping does not introduce any pore-blockage effects due to water adsorption at lower pressures. However such effects are most likely to occur in realistic carbons. Such information are still essential to design adsorbents for the purpose of gas storage applications. In future work, we will address such issues in a series in-silico obtained structures obtained with a wide range of basis units that contains different type of N atoms. ASSOCIATED CONTENT Supporting Information Additional details on the carbon prototypes SN1, SN2, snapshots showing adsorption of water in different carbon prototypes are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected], [email protected] ACKNOWLEDGMENT We thank EU for the Intra European Marie Curie Research Fellowship (PIEF-GA-2013-623227) through the project BIOADSORB. REFERENCES (1)

Zhao, L.; Fan, L.-Z.; Zhou, M.-Q.; Guan, H.; Qiao, S.; Antonietti, M.; Titirici, M.-M. NitrogenContaining Hydrothermal Carbons with Superior Performance in Supercapacitors. Adv. Mater. 2010, 22 (45), 5202–5206.

(2)

Sevilla, M.; Falco, C.; Titirici, M.-M.; Fuertes, A. B. High-Performance CO2 Sorbents from Algae. RSC Adv. 2012, 2 (33), 12792.

(3)

Falco, C.; Sevilla, M.; White, R. J.; Rothe, R.; Titirici, M.-M. Renewable Nitrogen-Doped Hydrothermal Carbons Derived from Microalgae. ChemSusChem 2012, 5 (9), 1834–1840.

(4)

Chen, T.; Pan, L.; Loh, T. A. J.; Chua, D. H. C.; Yao, Y.; Chen, Q.; Li, D.; Qin, W.; Sun, Z. 26


Page 27 of 32

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


Porous Nitrogen-Doped Carbon Microspheres as Anode Materials for Lithium Ion Batteries. Dalton Trans. 2014, 43 (40), 14931–14935. (5)

Kumar, K. V.; Preuss, K.; Lu, L.; Guo, Z. X.; Titirici, M. M. Effect of Nitrogen Doping on the CO2 Adsorption Behavior in Nanoporous Carbon Structures: A Molecular Simulation Study. J. Phys. Chem. C 2015, 119 (39), 22310–22321.

(6)

Sevilla, M.; Valle-Vigón, P.; Fuertes, A. B. N-Doped Polypyrrole-Based Porous Carbons for CO2 Capture. Adv. Funct. Mater. 2011, 21 (14), 2781–2787.

(7)

Sevilla, M.; Falco, C.; Titirici, M.-M.; Fuertes, A. B. High-Performance CO2 Sorbents from Algae. RSC Adv. 2012, 2, 12792.

(8)

Hao, G.-P.; Sahraie, N. R.; Zhang, Q.; Krause, S.; Oschatz, M.; Bachmatiuk, A.; Strasser, P.; Kaskel, S. Hydrophilic Non-Precious Metal Nitrogen-Doped Carbon Electrocatalysts for Enhanced Efficiency in Oxygen Reduction Reaction. Chem. Commun. (Camb). 2015, 51 (97), 17285–17288.

(9)

Canivet, J.; Fateeva, A.; Guo, Y.; Coasne, B.; Farrusseng, D. Water Adsorption in MOFs: Fundamentals and Applications. Chem. Soc. Rev. 2014, 43 (16), 5594–5617.

(10)

Hao, G.-P.; Mondin, G.; Zheng, Z.; Biemelt, T.; Klosz, S.; Schubel, R.; Eychmüller, A.; Kaskel, S. Unusual Ultra-Hydrophilic, Porous Carbon Cuboids for Atmospheric-Water Capture. Angew. Chem. Int. Ed. Engl. 2015, 54 (6), 1941–1945.

(11)

Horikawa, T.; Sakao, N.; Sekida, T.; Hayashi, J.; Do, D. D.; Katoh, M. Preparation of NitrogenDoped Porous Carbon by Ammonia Gas Treatment and the Effects of N-Doping on Water Adsorption. Carbon. 2012, 50 (5), 1833–1842.

(12)

Ayala, P.; Arenal, R.; Rümmeli, M.; Rubio, A.; Pichler, T. The Doping of Carbon Nanotubes with Nitrogen and Their Potential Applications. Carbon. 2010, 48 (3), 575–586.

(13)

Huber, L.; Ruch, P.; Hauert, R.; Saucke, G.; Matam, S. K.; Michel, B.; Koebel, M. M. Monolithic Nitrogen-Doped Carbon as a Water Sorbent for High-Performance Adsorption Cooling. RSC Adv. 2016, 6 (30), 25267–25278.

(14)

Müller, E. A. Purification of Water through Nanoporous Carbon Membranes: A Molecular Simulation Viewpoint. Curr. Opin. Chem. Eng. 2013, 2 (2), 223–228. 27



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

(15)

Page 28 of 32

Müller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. Adsorption of Water on Activated Carbons: A Molecular Simulation Study. J. Phys. Chem. 1996, 100 (4), 1189–1196.

(16)

Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91 (24), 6269–6271.

(17)

Conde, M. M.; Vega, C. Note: A Simple Correlation to Locate the Three Phase Coexistence Line in Methane-Hydrate Simulations. J. Chem. Phys. 2013, 138 (5), 056101.

(18)

Ghosh, P.; Colón, Y. J.; Snurr, R. Q. Water Adsorption in UiO-66: The Importance of Defects. Chem. Commun. 2014, 50 (77), 11329–11331.

(19)

Ghosh, P.; Kim, K. C.; Snurr, R. Q. Modeling Water and Ammonia Adsorption in Hydrophobic Metal–Organic Frameworks: Single Components and Mixtures. J. Phys. Chem. C 2014, 118 (2), 1102–1110.

(20)

Wu, J.; Ning, F.; Trinh, T. T.; Kjelstrup, S.; Vlugt, T. J. H.; He, J.; Skallerud, B. H.; Zhang, Z. Mechanical Instability of Monocrystalline and Polycrystalline Methane Hydrates. Nat. Commun. 2015, 6, 8743.

(21)

Striolo, A.; Chialvo, A. A.; Cummings, P. T.; Gubbins, K. E. Water Adsorption in Carbon-Slit Nanopores. Langmuir 2003, 19 (20), 8583–8591.

(22)

Striolo, A.; Naicker, P. K.; Chialvo, A. A.; Cummings, P. T.; Gubbins, K. E. Simulated Water Adsorption Isotherms in Hydrophilic and Hydrophobic Cylindrical Nanopores. Adsorption 2005, 11, 397–401.

(23)

Gupta, A.; Chempath, S.; Sanborn, M. J.; Clark, L. A.; Snurr, R. Q. Object-Oriented Programming Paradigms for Molecular Modeling. Mol. Simul. 2003, 29 (1), 29–46.

(24)

Mayo, S. L.; Olafson, B. D.; Goddard, W. A. DREIDING: A Generic Force Field for Molecular Simulations. J. Phys. Chem. 1990, 94 (26), 8897–8909.

(25)

Tenney, C. M.; Lastoskie, C. M. Molecular Simulation of Carbon Dioxide Adsorption in Chemically and Structurally Heterogeneous Porous Carbons. Environ. Prog. 2006, 25 (4), 343– 354.

(26)

Wu, P.; Du, P.; Zhang, H.; Cai, C. Microscopic Effects of the Bonding Configuration of Nitrogen-Doped Graphene on Its Reactivity toward Hydrogen Peroxide Reduction Reaction. 28


Page 29 of 32

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


Phys. Chem. Chem. Phys. 2013, 15 (18), 6920–6928. (27)

Nistor, R. A.; Polihronov, J. G.; Müser, M. H.; Mosey, N. J. A Generalization of the Charge Equilibration Method for Nonmetallic Materials. J. Chem. Phys. 2006, 125 (9), 094108.

(28)

Martínez, J. I.; Cabria, I.; López, M. J.; Alonso, J. A. Adsorption of Lithium on Finite Graphitic Clusters. J. Phys. Chem. C 2009, 113 (3), 939–941.

(29)

Yu, Y.-X. Can All Nitrogen-Doped Defects Improve the Performance of Graphene Anode Materials for Lithium-Ion Batteries? Phys. Chem. Chem. Phys. 2013, 15 (39), 16819–16827.

(30)

Watanabe, T.; Manz, T. A.; Sholl, D. S. Accurate Treatment of Electrostatics during Molecular Adsorption in Nanoporous Crystals without Assigning Point Charges to Framework Atoms. J. Phys. Chem. C 2011, 115 (11), 4824–4836.

(31)

Jorge, M.; Schumacher, C.; Seaton, N. A. Simulation Study of the Effect of the Chemical Heterogeneity of Activated Carbon on Water Adsorption. Langmuir 2002, 18 (24), 9296–9306.

(32)

Gotzias, A.; Tylianakis, E.; Froudakis, G.; Steriotis, T. Theoretical Study of Hydrogen Adsorption in Oxygen Functionalized Carbon Slit Pores. Microporous Mesoporous Mater. 2012, 154, 38–44.

(33)

Gotzias, A.; Tylianakis, E.; Froudakis, G.; Steriotis, T. Adsorption in Micro and Mesoporous Slit Carbons with Oxygen Surface Functionalities. Microporous Mesoporous Mater. 2015, 209, 141– 149.

(34)

Trinh, T. T.; Vlugt, T. J. H.; Hägg, M.-B.; Bedeaux, D.; Kjelstrup, S. Simulation of Pore Width and Pore Charge Effects on Selectivities of CO2 vs. H2 from a Syngas-like Mixture in Carbon Mesopores. Energy Procedia 2015, 64, 150–159.

(35)

TraPPE:

Transferable

Potentials

for

Phase

Equilibria

Force

Field

http://www.chem.umn.edu/groups/siepmann/trappe/ (accessed Apr 2, 2015). (36)

Wang, L.; Yang, R. T. Significantly Increased CO 2 Adsorption Performance of Nanostructured Templated Carbon by Tuning Surface Area and Nitrogen Doping. J. Phys. Chem. C 2012, 116 (1), 1099–1106.

(37)

Sevilla, M.; Parra, J. B.; Fuertes, A. B. Assessment of the Role of Micropore Size and N-Doping in CO2 Capture by Porous Carbons. ACS Appl. Mater. Interfaces 2013, 5 (13), 6360–6368. 29



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

(38)

Page 30 of 32

Wang, J.; Senkovska, I.; Oschatz, M.; Lohe, M. R.; Borchardt, L.; Heerwig, A.; Liu, Q.; Kaskel, S. Highly Porous Nitrogen-Doped Polyimine-Based Carbons with Adjustable Microstructures for CO2 Capture. J. Mater. Chem. A 2013, 1 (36), 10951.

(39)

Kondo, T.; Casolo, S.; Suzuki, T.; Shikano, T.; Sakurai, M.; Harada, Y.; Saito, M.; Oshima, M.; Trioni, M. I.; Tantardini, G. F.; et al. Atomic-Scale Characterization of Nitrogen-Doped Graphite: Effects of Dopant Nitrogen on the Local Electronic Structure of the Surrounding Carbon Atoms. Phys. Rev. B 2012, 86 (3), 035436.

(40)

Liu, Y.; Wilcox, J. Molecular Simulation of CO2 Adsorption in Micro- and Mesoporous Carbons with Surface Heterogeneity. Int. J. Coal Geol. 2012, 104, 83–95.

(41)

Liu, Y.; Wilcox, J. Effects of Surface Heterogeneity on the Adsorption of CO2 in Microporous Carbons. Environ. Sci. Technol. 2012, 46 (3), 1940–1947.

(42)

Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 Capture technology—The U.S. Department of Energy’s Carbon Sequestration Program. Int. J. Greenh. Gas Control 2008, 2 (1), 9–20.

(43)

Zheng, Q.; Li, Z.; Yang, J. Effects of N Doping and NH2 Grafting on the Mechanical and Wrinkling Properties of Graphene Sheets. RSC Adv. 2013, 3 (3), 923–929.

(44)

Zhao, W.; Sun, M.; Zhang, H.; Dong, Y.; Li, X.; Li, W.; Zhang, J. Catalytic Dehydrochlorination of 1,2-Dichloroethane to Produce Vinyl Chloride over N-Doped Coconut Activated Carbon. RSC Adv. 2015, 5 (126), 104071–104078.

(45)

Di Biase, E.; Sarkisov, L. Systematic Development of Predictive Molecular Models of High Surface Area Activated Carbons for Adsorption Applications. Carbon. 2013, 64, 262–280.

(46)

Accelrys Materials Studio: http://accelrys.com

(47)

Sarkisov, L.; Harrison, A. Computational Structure Characterisation Tools in Application to Ordered and Disordered Porous Materials. Mol. Simul. 2011, 37 (15), 1248–1257.

(48)

Crystallization, 4th Edition, J.W. Mullin, Butterworth Heinmann, Oxford.

(49)

Morgenstern, M.; Michely, T.; Comsa, G. Anisotropy in the Adsorption of H2O at Low Coordination Sites on Pt(111). Phys. Rev. Lett. 1996, 77 (4), 703–706. 30


Page 31 of 32

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

(50)


Vega, C.; Abascal, J. L. F. Simulating Water with Rigid Non-Polarizable Models: A General Perspective. Phys. Chem. Chem. Phys. 2011, 13 (44), 19663–19688.

(51)

Inagaki, S.; Fukushima, Y. Adsorption of Water Vapor and Hydrophobicity of Ordered Mesoporous Silica, FSM-16. Microporous Mesoporous Mater. 1998, 21 (4-6), 667–672.

(52)

Inagaki, S.; Fukushima, Y.; Kuroda, K.; Kuroda, K. Adsorption Isotherm of Water Vapor and Its Large Hysteresis on Highly Ordered Mesoporous Silica. J. Colloid Interface Sci. 1996, 180 (2), 623–624.

(53)

Salame, I.; Bandosz, T. Study of Water Adsorption on Activated Carbons with Different Degrees of Surface Oxidation. J. Colloid Interface Sci. 1999, 210 (2), 367–374.

(54)

Bruschi, L.; Mistura, G.; Liu, L.; Lee, W.; Gösele, U.; Coasne, B. Capillary Condensation and Evaporation in Alumina Nanopores with Controlled Modulations. Langmuir 2010, 26 (14), 11894–11898.

(55)

Soper, A. K. The Radial Distribution Functions of Water and Ice from 220 to 673 K and at Pressures up to 400 MPa. Chem. Phys. 2000, 258 (2-3), 121–137.

(56)

Llewellyn, P. L.; Grillet, Y.; Schüth, F.; Reichert, H.; Unger, K. K. Effect of Pore Size on Adsorbate Condensation and Hysteresis within a Potential Model Adsorbent: M41S. Microporous Mater. 1994, 3 (3), 345–349.

(57)

Mosaddeghi, H.; Alavi, S.; Kowsari, M. H.; Najafi, B. Simulations of Structural and Dynamic Anisotropy in Nano-Confined Water between Parallel Graphite Plates. J. Chem. Phys. 2012, 137 (18), 184703.

(58)

Liu, L.; Nicholson, D.; Bhatia, S. K. Impact of H2O on CO2 Separation from Natural Gas: Comparison of Carbon Nanotubes and Disordered Carbon. J. Phys. Chem. C 2015, 119 (1), 407– 419.

(59)

Billemont, P.; Coasne, B.; De Weireld, G. An Experimental and Molecular Simulation Study of the Adsorption of Carbon Dioxide and Methane in Nanoporous Carbons in the Presence of Water. Langmuir 2011, 27 (3), 1015–1024.

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SYNOPSIS TOC


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Understanding the Hydrophilicity and Water Adsorption Behavior of

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