Effect of Nitrogen Doping on the CO2 Adsorption Behavior in

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Effect of Nitrogen Doping on the CO Adsorption Behaviour in Nanoporous Carbon Structures: a Molecular Simulation Study K. Vasanth Kumar, Kathrin Preuss, Linghong Lu, Zhengxiao Guo, and Maria-Magdalena Titirici J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06017 • Publication Date (Web): 07 Sep 2015 Downloaded from http://pubs.acs.org on September 12, 2015

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The Journal of Physical Chemistry

Effect of Nitrogen Doping on The CO2 Adsorption Behaviour in Nanoporous Carbon Structures: A Molecular Simulation Study K. Vasanth Kumar,*a,b Kathrin Preussa, b, Linghong Lu,c Zheng Xiao Guo,d M. Magdalena Titiricia,b (a) Queen Mary University of London, School of Engineering and Materials Science, Mile End Road, E14NS, London. E-mail: [email protected] (b) Queen Mary University of London, Materials Research Institute, Mile End Road, E14NS, London. (c) State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University, 210009, Jiangsu, China. (d) Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK.

ABSTRACT Nitrogen (N) doping is considered an effective design strategy to improve CO2 adsorption in carbon materials. However, experimental quantification of such an effect is riddled with difficulties, due to the practical complexity involved in experiments to control more than one parameter, especially at nanoscale level. Here, we use molecular simulations to clarify the role of N doping on the CO2 uptake and the CO2/N2 selectivity in representative carbon pore architectures (slit and disordered carbon structures) at 298 K. Our results indicate that N doping shows a marginal improvement on the CO2 uptake, although it can improve the CO2/N2 selectivity. CO2 uptake and CO2/N2 selectivity is predominantly controlled by the pore-architecture as well as ultra-micropores; the tendency of linear CO2 molecules to lie flat on the carbon surface favours the CO2 uptake in slit pore architectures rather than disordered carbon pore structures. We also demonstrated through molecular simulations that the N doping effect may be difficult to exemplify experimentally if the material has a disordered pore architecture and complex surface chemistry (such as the presence of other functional groups).

Key word: CO2 adsorption, carbon, N doping, CO2/N2 selectivity, isosteric heat, pore structure

1. INTRODUCTION

Climate scientists agree that CO2 capture and storage technology (CCS), together with improved energy conversion efficiency, is a near-term solution to reduce CO2 emissions from fossil fuel power generation on a massive scale. 1-3 Adsorption on porous carbon materials to remove CO2 from flue gas seems to be an interesting option due to its economic and environmental advantages. The ease of synthesis, availability, selectivity and their unprecedented stability to temperature and moisture makes carbon-based porous materials

ACS Paragon Plus Environment


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as prime adsorbents to use in pressure swing adsorption (PSA) units, a widely used technology in industries.

4-5

CO2 adsorption in carbon materials is essentially due to van der Waals type

physisorption or through electrostatic interactions as well as a combination of both depending on the surface chemistry. 6 Carbon porosity is usually associated with van der Waals forces, whereas the electrostatic or columbic forces are synthetically created by introducing impurities to the carbon lattice by substitutional chemical doping or through functionalization of the pore surface with functional groups. In essence, adding impurities changes the electron density and creates micro-domains with different electrostatic strengths within the available pore volume. Relying on this concept, considerable research is invested in recent years to incorporate nitrogen atoms into carbon framework and most of them targets on improving the CO2 adsorption capacity (also for other end uses) and CO2/N2 selectivity of the carbon materials, particularly for post-combustion CO2 capture.

7, 8–12

Theoretically introducing N will improve the electron density of the carbon

framework or in other words increase the basicity of the carbon framework which in turn will anchor the electron deficient carbon of the CO2 to the carbon pore surface by Lewisacid/Lewis-base (N atom) interactions. Nitrogen incorporation also poses certain advantages over the other established techniques of modifying the carbon surface with amines, which are limited by amine instability and leaching over regeneration cycles.

13

Despite the recent

advances in the synthesis strategies to obtain N doped carbons, understanding the role of N atoms doped within carbon frameworks on the CO2 adsorption capacity or CO2/N2 selectivity are still unclear and often controversial. For instance, the work of Wang and Yang 9 shows that both N doping and surface area play a major role on the overall CO2 uptake at 298 K. Their work shows that N doped carbons with lower surface areas adsorb more CO2 than pristine carbon materials that contain relatively larger surface areas at 298 K. These results are contested in the recent experimental work of Sevilla et al.;

14

their work confirms that

irrespective of the pore properties the presence of N functionalities does not show any significant improvement on the CO2 uptake. In recent years, several experimental results confirmed that high surface area carbons, with micropores ranging from 0.5 to 1 nm, can hold large masses of CO2 (> 4.0 mmol/g);

10,15-16

all of them exceed the CO2 uptake in most of the

reported N doped carbons (< 4.0 mmol/g). 8-9,17 The high CO2 uptake in these pristine structures was attributed to the presence of large surface areas and finely tuned micropores. Contrary to these results, an ultra-microporous carbon, Maxsorb, with a large surface area (> 3000 m2/g), could exhibit only a CO2 loading capacity of up to 2.7 mmol/g under ambient conditions. 9 All of the above mentioned results indicate that there is no clear understanding on the key


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parameter that influences the high CO2 uptake in the aforementioned materials. These controversial results reported in literature can be explained if the practical difficulties associated with the measurement of CO2 uptake in carbon based materials are considered/identified. Small angle scattering study performed by Wei et al.,

18

on Maxsorb,

suggests that its pore architecture is composed by randomly connected carbon units; whereas a carbon obtained via hard template sacrificial route (e.g. using silica as a sacrificial template) showed some definite two-dimensional pore symmetry. A fluid molecule will experience different level of binding energy in different pore-architecture and this will play a crucial role in the adsorption and fluid confinement properties and thus, performing comparison studies between those materials are too complicated. This scenario will be different and even more complex in the presence of any impurities, such as N or other functional groups within the pore architecture. The presence of impurities will create nano-domains with different attraction potentials and electrostatic strengths that will either promote or disturb the fluid adsorption/confinement properties within the pore volume. Microporous carbon materials are most likely to have a wide range of pore-architecture and studying the influence of N doping in such carbons is practically difficult to perform via experiments. Apart from this issue, the CO2 uptake measured experimentally will also be influenced by the concentration of N or other impurities and the presence of other functional groups. The difficulty of synthesizing well-characterized carbon adsorbents makes a parametric study, from an experimental point of view is practically difficult to perform. 19 As far as adsorption of CO2 and N2 mixture (the two prime fluids involved in the flue gas) is considered, the experimental selectivity is often calculated from their pure component adsorption isotherms, assuming negligible interactions between the fluid molecules. In a realistic approach, such assumptions might not be valid as in the case of mixtures, the adsorption behavior might be even more complex as steric effects and the interplay of the fluid-fluid interactions between the same components and different components because both dispersion and electrostatic forces become important. For instance, N doping will introduce electrostatic domains inside the pores, which might host CO2 and N2 with different electrostatic strengths. It is more likely that both CO2 and N2 will compete for the adsorption sites during the mixtures. Such effects until now are not well understood as experimental selectivity are often obtained ignoring the fluid-fluid interactions. In this work, we performed theoretical studies using molecular simulations to clarify some of the above-mentioned issues, while solely focusing on the influence of N doping in carbon structures on both CO2 uptake and CO2/N2 selectivity. To understand the effect of N doping in



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carbon micropores, we performed Grand Canonical Monte Carlo simulations to obtain CO2 and CO2+N2 (fluid mixture) adsorption isotherms in N doped carbon slit-pores (a well-established carbon prototype to define the porosity of carbon based materials) of different pore-widths (ranging from ultra-micropores to large micropores), and also in two different in-silico generated models of N doped disordered carbons with ultra-high surface areas. The latter models are considered here, as the slit-pore models are overly simplified to portray the complex pore architecture of the synthetic carbons that have a surface area exceeding the limit of an ideal graphene sheet, characteristic pore size distribution and energetic heterogeneity due to the randomly interconnected graphitic segments.

20-21

The two

disordered carbon structures constructed in this work are exclusively microporous and share some common properties with the slit-pore models used, so that a comparison of their adsorption properties could give some reliable picture on the pore-architecture effect on the CO2 adsorption or CO2/N2 selectivity in N doped high surface area carbons. The physics behind the adsorption was detailed in terms of adsorption excess and isosteric heat. At some stage we proposed a hypothesis that could explain the experimentally debated results on CO2 adsorption in high surface area and microporous (pristine or N-doped) carbon structures could plausibly due to the influence of pore architecture. To emphasize the role of the N doping effect, we compare the adsorption isotherms obtained from N doped carbon structures with their pristine iso-structures (N free). Before concluding as a case study, we also show how the presence of other functional groups can interfere with the N doping effect on CO2 uptake in N doped carbons using OH as an added impurity. Theoretical studies are usually focused on CO2 adsorption in oxygentated surfaces and very little has been done on the study of the N doping effect on the CO2 adsorption isotherms or CO2/N2 selectivity in porous carbons. 20,22–26 The results generated in this work will detail this issue. Understanding the exact influence of Nfunctionalities in porous carbons is highly important and will have a high impact on the adsorbent design for PSA unit or for process upscaling.

2. SIMULATION METHOD Grand Canonical Monte Carlo (GCMC) simulations were used to study the adsorption behavior of CO2 and CO2+N2 mixtures. The fluid-fluid interactions were modeled using the 12-6 LennardJones (LJ) potential. 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 σCC = 3.4 A and ε/kB = 28.0 K. The graphitic state N atom is treated as a spherical atom of σCC = 3.26 Å and ε/kB = 38.9 K 27. The N atom in the carbon framework and the C atoms


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connected to the N atom were given a partial charge (electron unit) of -0.94 and 0.3133 (these charges were obtained with B3LYP/6-31G(d) level of theory and is taken from the work of Tachikawa et al 28). The partial charge of 0.3133 assigned to the carbon atoms is slightly higher than the average carbon charges of the neighboring carbon atoms reported by Tachikawa et al. In this work, the N doped carbon prototype was obtained by placing nitrogen atoms randomly on both sides of the carbon surfaces. This will obviously change the reactivity of the carbon surface and therefore affect the magnitude of the charge distribution around the neighboring carbon atoms. Presenting a global model to represent the charge distribution of such large systems similar to the ones studied here (slit and RPCs) is highly complex, which is one of the limitations involved while studying an ill-defined material such as activated carbon. The charge distribution assumed in this work, strategically agrees with the charges assigned by the commonly employed charge equilibration (EQq) method, that usually uses empirical forcefield and can properly capture long-range interactions between atoms. Alternatively, the partial charges can be obtained using quantum chemistry calculations; however they are computationally demanding and thus are often performed in a system with a limited number of atomic clusters. In any case, for carbon materials, due to the lack of periodicity or structural symmetry, the assignment of point charges via any of these methods does not warrant any reliable solution. Even for periodic crystalline materials, methods to assign the point of charges to the atoms and their influence on the uptake of quadrupolar fluids is still unclear.

29-30

The

limitations and the practical difficulties associated with assigning point charges while studying the adsorption of quadrupolar fluids are detailed in some of the recent works; albeit with the other class of porous materials (metal organic frameworks). The objectives of this work are limited to exclusively portray the influence of the N doping effect on the CO2 uptake; to do this we kept the system electroneutral by (i) assuming the local redistribution of the charges around the C atoms which are connected to N atoms is 0.3133 and (ii) the local electronic redistribution to the next neighbouring carbon atoms is negligible. A similar strategy was successfully applied earlier to study the hydrogen adsorption properties of oxygen functionalized carbon materials,31 CO2 and CH4 single component and their binary mixture adsorption in different carbon nanopore models, 4 water adsorption in oxygen functionalized carbons

32

and to estimate the equilibrium and dynamic selectivity of CO2/H2 of carbon

structures. 33

All CO2-CO2 and N2-N2 interactions were modeled using the TraPPE potential.

34

This model

treats CO2 as a linear triatomic molecule with charges placed at the center of the each LJ atom.



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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= 3.05 Å and εO/kB=79.0 K and σC: 2.80 Å and εC/kB = 27.0 K. The CO2 molecule quadrupole moment was simulated based on 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 atoms. The LJ potential parameters for each N atom that are separated by a distance of 1.10 Å in N2 molecule is given by: σN = 3.310 Å; εN/kB = 36.0 K. For N2, TraPPE force-field treats the quadrupole moment by assigning a negative charge on each LJ atoms N atom (qN = -0.482) and a positive charge (qCOM = 0.964) at the COM (center of mass) site. All LJ cross interactions, both between different types of fluids and the solid-fluid interactions, were taken to conform to Lorentz-Berthelot rules. Ewald summation method is used to account for the long-range corrections to electrostatic interactions. We set the cut-off to 12 Å for both LJ and Coulomb interactions. In the smallest pore studied in this work, to maintain this cut-off, we placed an additional dummy layer of graphene treated as hard spheres on the boundaries (in x-direction) of the pore-wall. The size of the simulation cells are given in supplementary information file. 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 1 unit cell that has a surface area of 50 Å2. Larger system size does not show any significant influence on the statistics of the final results. Fugacity is used in the simulations, and the component fugacity of bulk phase was transformed to the component bulk phase pressure using Peng-Robinson equation of state.

35

In the case

of slit-pores, we applied the boundary conditions in a and b directions (and not in c axis), whereas in the case of disordered carbon structures, we applied the periodic boundary conditions in all the directions. For each point on the isotherm, 30 million Monte Carlo steps were performed. Each state consisted of insertion of a new molecule, deletion of an existing molecule and translation or rotation of an existing molecule. The first half of the run was used to ensure equilibration, and the last half was used to calculate the ensemble averages. All the simulations were performed using Multipurpose Simulation code, MUSIC 4.0. 36

3. NANOPOROUS STRUCTURES X-ray photoelectron spectra 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 configurations (see Fig 1): (i) pyridinic-N, (ii) pyrrolic-N and (iii) graphitic-N.

9, 16-17

Although the active sites for CO2 adsorption on these N doped

carbons are still not clear, vibrational studies performed using DFT simulations confirmed that,


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irrespective of the type of N, the N atoms in graphene enhances the electro-donor or the basic capacities of carbon materials with different charge densities. 37 The enhanced basicity can be related to the lone pair of electrons in pyridinic-N as well as to the increased electron density in the delocalized π-orbitals in case of graphene with graphitic-N. Scanning Tunnel microscopy measurements carried out by Kondo et al

38

confirmed that graphitic-N is stable at high

temperatures (> 800 oC), whereas pyridinic-N and other forms of N are converted to graphiticN. Preparation of high surface area carbons relies on high temperature annealing techniques, 39

thus typically large concentrations of graphitic-N are expected. For the above said reasons

and also for the sake of simplicity, we only considered graphitic-N in this study. Fig 1 Two different N doped carbon prototypes (and its pristine isostructures) were investigated, i.e. a slit-pore and a disordered carbon with high surface area. The first one (slit-pore), assumes a pore size homogeneity while the second considers a pore size heterogeneity that is usually encountered in the experimentally obtained carbons. Slit-pore model: The N doped slit-pore model was built by placing three graphitic sheets (ABA stacking, Fig 2a) on both sides of the carbon pores (six in total). Earlier studies confirmed that three graphene layers are sufficient to model the CO2 surface interactions in both pristine and functionalized slit-pores. The planes of graphitic sheets were separated by an interlayer distance of 0.335 nm. N doping was introduced via a substitutional doping technique, thereby retaining the planar sp2 hybridization of the graphene sheet. We randomly replaced the carbon atoms (in the graphitic sheets which are accessible to the target molecules) on both sides of the pore-wall with N atoms until we reached a desired level of N wt% (4.1 wt%). Care was taken during the doping process to mimic graphitic nitrogen, such that every N atom is connected to three carbon atoms in the graphene sheet (Fig 2b). 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. This assumption is supported by theoretical and scanning tunnelling microscopy studies reported in the literature;

38,40

these works confirm

that N atoms in graphitic N adapt the sp2 planar structure of graphene, the nearest neighbour C-C bonds remains intact and they do not wrinkle the planar nature of the graphene sheet. The differences in distance between the C-N and the C-C bond (C-C bond distance: 0.142 nm), in an ideal graphene sheet, are negligible (as they differ by only 0.002 nm). Such minor deviations are expected to have insignificant effects on the solid–fluid interactions and also on the



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interlayer spacing between the graphene layers and thus on the density of the adsorbed phase. Therefore in this current study we assume that the C-N distances are equal to the C-C distances of an ideal graphene sheet. The pore is characterized by a pore width, H, defined as the distance between centres of the carbon (or N) atoms between the opposing walls. To gain a better understanding of the CO2 adsorption behaviour in micropores, we considered four different pore-widths; one ultra-micropore: H = 0.8 nm and three larger micropores, H = 1.2 nm, 1.6 nm and 2.0 nm. The pore-width, H, differs from the experimentally measured effective pore width, H’, which is given by: H’ = H - σCC, where σCC is the Lennard Jones (LJ) diameter of the carbon atom. Unless mentioned otherwise, the term pore width used in this work corresponds to the parameter H. CO2 adsorption in these N doped pores and its pristine isostructures can emphasize the theoretical limit of CO2 adsorption and CO2/N2 selectivity as well as the fluid adsorption behaviour in these structures. The in-silico generated disordered carbon structures that can be taken as a representative model of real carbons were constructed from a collective of flat coronene-shaped graphitic basic units made up of 23 carbon atoms and one N atom (Fig 2c). The disordered carbon structures were obtained by placing a number of these carbon-building units in a simulation box avoiding overlapping of the carbon units until a desired level of density was reached. No bridges were formed between the building units, which were artificially fixed in space within the simulation cell. Two disordered carbon structures (see Fig 2d and 2 e), labelled as RPC1 (randomly placed carbon units) and RPC2, with a framework density of 0.3 and 0.6 g/cm3, respectively, were studied. Obviously, a number of realizations of these structures can be obtained by different placements of the basic units; however, the results (for the same density of basic units) seemed to be reasonably independent of the actual details. A unique realization, depicted in Fig 2, was selected for the adsorption studies presented here on the basis of attempting to broadly have similar morphological properties as other, better defined structures (slits). Each individual carbon centre has the same properties (C-C spacing and intermolecular potentials). Earlier, Segarra and Glandt

41

constructed similar models from a

different basis structural unit (cylindrical-shaped discs). Such models have been considered to represent accurately several classes of porous carbon materials, like BPL activated carbon. Fig 2 Recently Biase and Sarkisov 20 reproduced a high surface area carbon Maxsorb using a similar approach as we did. A brief review on different computer generated carbon models and their limitations/advantages can be found in some of the earlier works. 39,42 The artefact model with


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a disordered pore structure presented here can be taken as a limiting case of the model constructed by Segarra and Glandt. 41 We obtained the pristine structures of RPC1 and RPC2 by simply replacing the N with carbon atoms. This hypothetical strategy allows obtaining disordered carbon structures, both pristine and N doped that retain this complex disordered pore-architecture. This enables to elucidate the N doping effect on the CO2 adsorption performance in such complex pore-architectures, which are almost impossible to set-up via experiments. The cif files of the studied RPC structures are available as supplementary files.

4. CHARACTERIZATION OF NANOPOROUS STRUCTURES The physical pore properties, such as surface area, pore volume and pore size distribution, play a major role in the gas adsorption properties. Before performing adsorption studies, we characterized the pore properties of all the atomistic carbon prototypes. For the case of slitpores, this can be easily estimated from analytical expressions, whereas for RPCs, it is far from trivial. The surface area and pore volume for RPCs were estimated using a geometrical method available in the atom volume & surfaces module of Material Studio. 43 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. To obtain the surface area a probe sphere equivalent to the kinetic diameter of N2 was used; since experimentally the surface area is routinely deduced from N2 adsorption isotherms.

44-45

For

the case of pore volume (also referred to as free pore volume), Vtot, we used a probe molecule of zero diameter. In order to compare the total adsorption obtained via simulations with the experimentally measured excess adsorption, Vtot was used to convert total (calculated) adsorption isotherms to adsorption excess, Γexcess (molecules per unit pore volume), using the expression: 46

Γ

=

〈

〉

−

(1)

where is the number of molecules loaded inside the simulation cell, Vtot is the total pore volume (cm3) and ρbulk (molecules/cm3) is the bulk density of the fluid at a given temperature T. In Table 1 we show the calculated pore properties of the studied structures. The pore volume and surface area do not vary much between the N-doped pristine RPCs and those with slit



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pores. This can be expected, as the N atom is only slightly heavier than carbon and the van der Waals diameter of an N and a C atom differ only by 0.014 nm, thus the pore properties are only marginally modified by N doping. In the case of slit-pores we showed values of two slitpores, H = 0.8 and H = 1.2 nm, for comparison. Depending on the carbon framework density, the surface areas of RPCs are considerably larger than the ideal surface area limit of graphene (~2600 m2/g) or other high surface area carbons, which are typically obtained via experiments. High surface areas in this structures are expected due to the building unit coronene, which has a high surface area itself (> 13000 m2/g); 47 the large number of edges available in this structure contribute greatly to the surface area. The two RPCs are selected only to study the influence of surface area and pore architecture effect on CO2 adsorption. RPC2 with a surface area of around 2800 m2/g, can be taken as a limiting model version to represent some of the high surface area carbons that are frequently reported in the literature. RPC1, which has a tremendous surface area (>6000 m2/g), represents a case study model to illustrate CO2 adsorption in materials with such remarkable properties. Although such high surface area carbon based materials were never synthesized experimentally, it is well within the theoretical limit of the porous materials. 47 Table 1 To estimate the pore size distribution (PSD) of the RPCs we used poreblazer v3.0 that incorporates a Monte Carlo Integration procedure described elsewhere.

20, 48

As mentioned

earlier, since the Lennard Jones diameters of the N and C atom do not differ significantly, the pore properties of N doped and pristine structures are almost identical, which is why we present only the PSD of the N doped RPCs. The PSD of both RPCs are exclusively microporous. RPC1, which has a relatively low framework density, shows a tri-modal PSD and contains a large amount of pores with effective pore-widths of H’ = 1.3, 1.6 and 1.8 nm. The large framework density model RPC2 (0.6 g/cm3) exhibits a unimodal PSD that contains mostly micropores with an effective pore-width of H’= 0.76 nm. All of these pores frequently appear in microporous carbons obtained experimentally,

39

thus the CO2 adsorption performed with

these structures could give some ideas about the CO2 behavior in N doped or pristine carbons with disordered pore structures.

5. RESULTS AND DISCUSSION 5.1. CO2 adsorption in pristine and N-doped slit-pores It is well established, that in the case of CO2 adsorption at flue gas conditions (0.01-1 bar), strong adsorption occurs exclusively in micropores,

10,16

which is why we considered only


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micropores that can confound from one to three layers of CO2 (although mesopores can hold larger volumes of CO2 than micropores at near fluid condensation pressure, which can be useful for other applications). All the N doped slit-pores considered in this work contain 4.1 wt% of nitrogen, which is in agreement with concentrations frequently reported in carbons that were obtained via harsh conditions (physical and chemical activation). 8-10 Calculations for the N concentration are based on the percentage of N to C atoms on the innermost graphene layer that bound the porosity, the concentration of N will be lower if we consider the carbon atoms in all the graphene layers which are not accessible for the gas molecules. Fig 3 shows the amount of CO2 adsorbed in pristine and N doped carbon slit-pores of different pore-widths at 298 K. The adsorbent mass of a slit-pore is usually ill defined as the porosity is bounded by arbitrary numbers of graphitic sheets that do not contribute to the surface area for any target molecule. Thus, in this work the gravimetric capacity is obtained by assuming the mass of the adsorbent is equal to the mass of carbon (and N) atoms in each of the graphene layer that are accessible for the guest molecules (we ignore the two other graphitic sheets that are placed above the pore walls on both sides of the pore). Despite inflating the predicted CO2 uptake, this assumption can provide the theoretical upper limit of the gravimetric capacity of the considered carbon structures. It is clearly seen in Fig 3 that throughout the range of pressures studied, irrespective of the pore width, N doping shows only a minor influence on the CO2 uptake. The CO2 adsorption is always slightly higher in N doped carbons compared to their pristine structures. As expected, pore-width plays a crucial role on the CO2 uptake at 298 K. An ultra-micropore (H = 0.8 nm), containing only carbon atoms (pristine), exhibited a remarkable CO2 uptake of 4.4 mmol/g at 1 bar, the N doped version even showed a CO2 uptake of 4.6 mmol/g at 1 bar. These minor influences of N doping on the CO2 adsorption are almost impossible to predict experimentally, as it is affected by so many other factors mentioned earlier. In addition, it can be observed from Fig 3 that the CO2 uptake at 1 bar in the pristine pore of H = 0.8 nm, exceeds the CO2 adsorption excess in larger micropores, for both pristine and N doped structures by four to five times depending on the pore-width. These results expose the need of ultra-micropores to manage the low driving force of CO2 in the flue gases (0.1-1 bar) and reveal that N doping may be useful only if the carbon material contains ultramicropores. Fig 3 As far as the effect of N doping is considered, the N doping shows only a minor influence on the CO2 uptake at 1 bar and at 0.1 bar which is the partial pressure of CO2 in a flue gas. The CO2 uptake of N doped carbons is almost equal to its pristine structures (see Fig 3). Our results



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contradict some of the experimental values reported on a significant CO2 uptake in N doped carbons when compared to the pristine models.

49-52

For comparison, we showed the CO2

uptake of different N doped carbons at 1 bar and 298 K reported in literature in Table 2. The experimentally measured CO2 uptakes (at 1 bar 298 K) in N doped carbons (irrespective of the pore properties or synthesis method) are always around 4.0 mmol/g or less, which is close to the theoretical limit of the CO2 uptake in a slit pore of H = 0.8 nm (pristine) predicted in our simulations. This suggests that the high uptake observed in the reported experimental works might be due to the presence of ultra-micropores instead of the N doping effect, or may be due to the combination of both in such confined pores. Adsorption capacities significantly exceeding this theoretical limit (4.4 mmol/g) were reported, however their isosteric heat (see Table 2) is well within the theoretical limit for CO2 adsorption in pristine carbons. In addition, experimentally realized carbons are prone contain curved surfaces which theoretically can improve the binding energy and thus the total up take. In addition, in our work, the adsorption capacity is reported only for a fixed pore size that has a limited pore volume. It is more likely to expect a higher adsorption in a slightly bigger pores that does not exhibit any fluid confounding (adsorption on one wall disturb the adsorption on other wall) pattern. Such restricted confinement of fluid can lead to predict a lower theoretical limit of the carbon structure for CO2 uptake. Further the high adsorption capacities in the experimental values could also be due to several factors such as the presence of other oxygenated functional groups like -COOH and -SO3H, the acidic protons of which can form hydrogen bonds with the CO2 oxygen. In such scenarios, strong binding affinities of CO2 at infinite dilution can also be caused by a combination of electrostatics and H-bonding with a wide range of functional groups, aromatic H, local defects and surface curvature. Such scenarios can only be verified in detail via in-situ diffraction measurements or rather by individual theoretical approaches targeting each of this issues, which is beyond the scope of this work. To conclude, as far as the effect of N doping in carbon slit-pores is considered, no significant improvement to the CO2 adsorption could be shown. The high CO2 uptake of N doped carbons reported in literature might be due to the contributions of strong binding energy sites that only exist within the ultra-micropores which are bounded by one atom thick layer of carbon or may be due to the presence of other functional groups, local defects or a combination of these. Table 2 In order to understand the effect of N doping on the CO2 adsorption at higher pressures, we extended the CO2 adsorption isotherms for pressures up to 60 bar (Fig 4). Irrespective of the pore-width, N doping shows neither any notable influence on the CO2 uptake at higher


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pressures, nor on the maximum adsorption excess or the pressure at which the maximum adsorption occurs. At higher pressures, the adsorption isotherms in N doped structures almost overlap with the amount adsorbed in pristine slit-pores. This can be expected, as adsorption shifts from the surface chemistry regime to the pore volume regime at higher pressures. Thus, the higher the pore volume, the higher is the adsorption excess maximum, irrespective of the surface chemistry. Fig 4 Fig 5 To gain some insights on the fluid adsorption behaviour inside the carbon pores, we estimated the isosteric heat, Qst, associated with the adsorption process. Qst was obtained from the ensemble average fluctuations: 54

=

〈

〈

〉 〈 〉〈 〉 〉 〈 〉

(2)

Where is the average potential energy of the adsorbed phase and is the average number of molecules in the simulation system. In Fig 5 the isosteric heat versus the coverage of CO2, nabs (absolute adsorption) for all the slitpores is plotted. For convenience, Qst at lower coverage is given as a Figure inset. Both, the pristine and N doped, ultra-micropore, that reached the highest CO2 uptake at 1 bar, exhibited an isosteric heat of around 46 -51 kJ/mol at lower coverage (< 4mmol/g or p < 0.1 bar). This value is comparable with the experimental isosteric heat value reported for an N doped carbon (N-TC-EMC) that exhibited a CO2 uptake of 4 mmol/g at 1 bar and 298 K (see Table 2). If we compare the Qst of some of the other best performing carbon materials reported in literature with our theoretically obtained Qst value of an ultra-micropore, it can be seen that the Qst of these lab derived carbons are still far away from the theoretical limit. This indicates that there is still scope for improving the performance of those carbons in terms of CO2 uptake, by pushing their Qst closer to the ideal theoretical limit mentioned above, provided that the synthesis strategy allows pore size tuning without modifying the surface chemistry. In the case of Qst trend, in larger micropores of H = 1.2, 1.6 and 2 nm, the isosteric heat decreases with loading and again increases with further increase in loading. In larger pores, the initial decrease in Qst is typical at sub-monolayer coverage where the CO2 molecules tend to adsorb on high-energy binding sites (preferentially on N atoms) and proceed successively to the energetically weaker sites. The latter increase in isosteric heat at higher loadings is associated with adsorption sites with constant energy, while fluid-fluid interactions increase



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with coverage (liquid density). In the case of an ultra-micropore of H = 0.8 nm, the pressure inside the pore reaches the liquid density even at the lowest pressure studied (0.025 bar), thus the fluid-fluid interactions increase continuously with the increase in coverage. This can be expected as the attraction potential from both walls adds up to promote fluid condensation even at low pressures. Qst is relatively high in N doped carbons compared to its pristine structures. Theoretically, N doping creates a large amount of microdomains of different electrostatic field strengths within the pore volume, which contribute to the total isosteric heat. In larger micropores (H = 1.2, 1.6 and 2 nm) Qst of N-doped structures was expected to match with the Qst of the pristine structures, as the adsorption at higher pressures is controlled by a pore-filling process rather than by surface chemistry. Typically, at these conditions, the pressure inside the pores is related to the shortest probable distance between the fluid molecules and thus surface chemistry is least likely to play any crucial role on the heat of adsorption. However, our results show that Qst of N doped structures are always higher than Qst of the pristine structures for the entire range of pressures studied, typically by 0.5-2 kJ/mol depending on the pressure. Fig 6

To understand this phenomenon we captured some snapshots during the adsorption of CO2 at higher pressures in both pristine and N doped structures. In pristine structures, CO2 molecules tend to lie flat on the pore-wall (see Fig 6a), whereas in N doped structure (Fig 6b), the electrostatic fields within the pore tend to disturb this minimum energy configuration and CO2 molecules are slightly tilted away from the pore surface. Although such functionality induced CO2 adsorption pattern does not show any improvement on the fluid confinement behaviour, it is more likely to play a crucial role on the adsorption energy (as it alters the fluid-fluid interaction energy involved within the confined pore volume). Functionality induced packing patterns were reported by Liu and Wilcox 22 during the adsorption of CO2 in carbon structures that contain hydroxyl and carbonyl groups. The scenario of CO2 orientation within the confined pore volume was also studied experimentally using in-situ neutron diffraction experiments in nanoporous silica materials. 55 In this work, we confirmed that the CO2 orientation is influenced by the solid-fluid electrostatic gradients involved within the pore, by performing another set of simulations in all the N doped slit-pores considering only Lennard-Jones interactions (switching off the electrostatic interactions between solid and fluid). We found that the adsorption isotherms (not shown), the adsorption patterns (see Fig 6c-CO2 molecules tend to arrange in such a fashion that all the


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LJ atoms adsorb on the pore-wall as in pristine structures) and Qst for N doped structures (black trend line in Fig 5) overlap with the ones of the pristine structures.

5.2 CO2 adsorption in pristine and N doped RPCs In Fig 7 the amount of adsorbed CO2 versus the pressure in N doped and pristine microporous RPCs, RPC 1 and RPC2 is plotted. For clarity we show the adsorption at lower pressures (up to 1 bar) separately as a figure inset. Despite their high surface areas (RPC1: > 5000 m2/g and RPC 2: > 3000 m2/g) and large pore volumes (see Table 1), the adsorption is significantly low in these structures throughout the range of pressures studied. As expected, N doping does not show any significant improvements to the CO2 uptake. At 1 bar, N doped RPC 1 and RPC 2 can hold 0.81 and 0.53 mmol/g and its pristine structures adsorb up to 0.66 and 0.42 mmol/g, respectively. This is significantly lower than the CO2 adsorption in pristine or N doped slit-pore structures of H = 0.8-2.0 nm (see also Fig 3). The excess adsorption at higher pressures as well as the maximum adsorption excess in pristine and N doped structures are also considerably lower than the adsorption in slit-pores. Clearly, this phenomena exposes the limitation of the disordered pore architecture in RPCs and reveals the need for energetically homogeneous pore structures for CO2 adsorption. The random arrangement of the carbon basis unit in RPCs creates numerous weak van der Waals pockets/low energy binding sites (pore zone where the solid-fluid potential is weak); where CO2 can only establish a stable potential at the expense of high pressures. At higher pressures, this random arrangement also disturbs the confinement of linear CO2 molecules within the available pore in both pristine and N doped structures.

To confirm our hypothesis, in a case study we estimated the isosteric heat of RPC2 and compared this with the Qst of a slit-pore of H = 1.2 nm (see Fig1s SI) (which is comparable to the average pore-width of RPC2). The results show that up to 1 bar, Qst of RPC2 tends to decrease with increasing pressure, which is a characteristic feature of adsorption in structures that have a wide range of energetic pockets; these pockets interact with the target molecules, with different levels of binding energies depending on the pressure and temperature. In the case of Qst in a slit-pore, the pressure inside the pore reaches the fluid density even at a low pressure of 0.1 bar, which can be seen in the rise of Qst above this pressure. In Fig 8 snapshots showing CO2 molecules adsorbed in a slit-pore of H = 1.2 nm and RPC2 at 0.3 bar and 298 K are shown. The tendency of linear CO2 molecules to lie flat towards the carbon surface in a slitpore makes the arrangement of CO2 within the available pore space more efficient, whereas in RPC2, the random arrangement of the carbon basis unit disturbs this energetically



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favourable adsorption behaviour and thus the CO2 molecules are least confound inside the available pore. Introducing chemical heterogeneity in this structures via N doping does not change the above discussed adsorption scenarios; this can be expected, as N atoms cannot change the basicity of the carbon structure significantly to enhance electrostatic interactions by any significant amount at 298 K, although they might show a significant influence at lower temperatures. 37 Many experimental works highlight the role of N doping, high surface area and the presence of micropores in carbon materials for better CO2 adsorption. For instance, Wang and Yang,

9

showed that N doped carbons, despite their low surface area, outperformed pristine structures with a relatively high surface area at 298 K in terms of CO2 adsorption. They attributed this high CO2 adsorption to the N-doping effect. They also showed that the existence of a linear correlation between the surface area and the CO2 uptake in both pristine and N doped carbon structures. Our simulations reveal that CO2 adsorption cannot be correlated solely to chemical heterogeneity (as N-doping has minor effect and the sole effect of N may be even difficult to trace via experiments), the surface area or microporosity. Instead CO2 adsorption is predominantly controlled by the presence of ultra micropores (as discussed in earlier section) and the pore architecture itself, which should be taken into account while characterizing the experimentally derived carbons. In order to improve the CO2 adsorption density (at 1 bar), an energetically homogeneous carbon pore with optimum pore-size is required. This architecture essentially corresponds to an ultra-microporous slit pore with an ideal (defect free) surface area of 2600 m2/g. Fig 7 Fig 8

5.3. CO2/N2 selectivity in pristine and N doped slit-pores As the single component adsorption of CO2 in RPCs is relatively low, we only studied the N doping effect on the CO2/N2 selectivity of slit pore models. In Fig 9 the selectivity of the slit-pores for CO2 from a binary fluid mixture, which contains 15% CO2 and 85 % N2, (the anticipated concentration of CO2 in flue gas) for pressures up to 6 bar (typical operating condition of a PSA unit) is shown. The term selectivity, S in Fig 9 is used to estimate the adsorptive capability of the carbon pores to separate CO2 from the CO2+N2 mixture and is defined as the ratio of molar compositions x, of the components in the pore, to the bulk gas phase:


Page 17 of 38

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=

⁄

"

⁄

"

# $%&

(3)

'()*

Fig 9

Fig 9 clearly shows that the pore-size plays a crucial role on the selectivity, as it is remarkably high in both pristine and N doped ultra-micropores (H = 0.8 nm). N doping considerably improves the selectivity of the ultra-micropore structure. For example, at 0.1 bar, N doping increases the selectivity of the pristine ultra-micropore from 21.2 to 26.3 and at 6 bar, from 30.4 to 35.6, respectively. Increasing the pore-size severely penalizes the selectivity. Incorporation of N atoms in larger micropores enhances the selectivity moderately; however the selectivity is always lower compared to the pristine ultra-micropore. Our results show that although N doping does not affect the single component adsorption of CO2, it seems to be a useful design strategy to significantly improve the CO2/N2 selectivity, which is a desired property for carbon capture. The higher selectivity in N doped ultra-micropore can be correlated to micro domains with strong electrostatic fields that can segregate the components CO2 and N2 from the mixture, due to their greatly different electrostatic interactions (dipole induced dipole and dipole-quadrupole between CO2 or N2 with N atoms and quadrupole-quadrupole between CO2 interactions). Fig 10 To confirm this concept, we captured snapshots (Fig 10) for a single component adsorption of N2 at 5.1 bar as well as for a CO2+N2 mixture, containing 15% CO2, at 6 bar in N doped slit-pores of H = 0.8 nm. Fig 10 clearly shows that during the adsorption of binary mixtures, CO2 molecules push out the N2 molecules from the pore, whereas in the case of a pure component CO2 adsorption, the same pore can hold a relatively large amount of N2. These results also infer that predicting the selectivity from a single component adsorption might not be reliable and might underestimate the actual selectivity of a material. To conclude, N doping is a reasonable strategy to improve the CO2/N2 selectivity, which is a desired property for industrial applications. 5.4. Effect of other impurities on CO2 uptake in a N doped carbon pore High surface areas in carbons are usually obtained through experimental synthesis at high temperatures in the presence (or without) of certain oxidizing agents, such as KOH or NaOH. 39

At high temperatures (>800 oC) these oxidants alter the surface chemistry by creating



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Page 18 of 38

chemically adsorbed oxygenated groups, such as hydroxyl, carboxyl, peroxide, aldehyde etc. 56

. Electronic structure calculations performed by Torrisi et al 57 showed that the presence of

these oxygenated groups can enhance the binding affinity of the carbon materials for CO2 uptake. In general, these functional groups will alter the performance of any carbon material, including N doped carbon structures. Despite the expected phenomenon, due to the practical difficulty associated with experimental set-up it is difficult to specifically detail the role of these impurities on the CO2 adsorption properties of N doped carbon structures. To investigate this particular issue, we artificially introduced hydroxyl groups as an added impurity to the surface of N doped carbon slit-pore of H = 1.2 nm. Introducing OH groups on the graphitic surface via covalent bonds locally hybridizes the sp2 carbon to sp3 carbon. For simplicity, we assumed the oxidized N doped structures to be planar although OH groups theoretically impose surface undulations. The C–O bond length of the hydroxyl groups and the C-O-H bond angle are fixed to 1.47 Å and 107.9 o, respectively. Oxygen atoms lie on top of the carbon atoms normal to the graphitic plane. Additional details are given in the supplementary file. In Fig 11, the amount of CO2 adsorbed per unit surface area versus the pressure in N doped and OH functionalized N doped carbons is shown. The CO2 adsorption excess in pristine structures is also given for comparison. It is clear from Fig 11 that, the performance of OH functionalized N doped carbons overlaps with the performance of pristine structures. In fact at 1 bar, the CO2 uptake in OH functionalized N doped structures is slightly lower than the CO2 uptake in pristine structures. Incorporation of OH groups, although it can moderately improve the CO2 density inside the pore (not shown), it diminishes the pore volume and thus the CO2 loading, especially at higher pressures. In Fig 12 it can be seen that the incorporation of OH groups decreases the available pore space (green net shows the accessible pore surface) and that the CO2 molecule tends to confine within the restricted pore space. The presence of any other functional groups like COOH, SO3H, etc., especially in micropores, might alter the CO2 binding capacity, but will also reduce the excess adsorption due to pore volume restrictions. The purpose of this study was to show exclusively the complexity involved in detailing the effect of N doping on the CO2 uptake in microporous carbon structures in the presence of framework impurities. The reported scenario may be completely different in larger pores and can vary with temperature, 24,58

which are not considered in this work.

Fig 11 Our theoretical results reveal that an effect of N doping combined with the incorporation of oxygenated groups, if there is any, is difficult to detect with experimental studies. This is due to the low level of binding energy involved between the CO2 and the N atoms as well as to the


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influence of other functional groups on the adsorption excess and pore properties (such as OH which is demonstrated in this work).

Fig 12 6. CONCLUSIONS Molecular simulations were carried out to study the effect of N doping on carbon structures of different pore architectures for CO2 uptake and CO2/N2 selectivity. As far as the N doping (N treated as graphitic N) effect is considered, simulations performed in the carbon prototypes used in this work showed that N doping can only marginally improve the single component adsorption CO2, although it significantly improves the selectivity in smaller micropores. Our results also show that N doping affects the packing pattern of CO2 within the carbon pores. Pore architecture play a crucial role on CO2 adsorption; the tendency of CO2 molecules tend to lie flat towards the pristine carbon surfaces favours the CO2 uptake in this type of pore architecture. In disordered structures, CO2 molecules are least confined due to the basis structural units that form these architectures, which disturb the arrangement of CO2 molecules within the pore. A pristine slit-pore that exhibited an isosteric heat of ~46 kJ/mol at 0.1 bar seemed to be the ideal configuration for CO2 uptake as well as for the separation of CO2 from N2 at 298 K. The results obtained from this work are based on the carbon prototypes that is constructed from simple basis units or a structure that ignores pore size heterogeneity and ignores the presence of local defects. The carbon is one of the ill-defined adsorbent and the general limitations of the carbon prototypes used in simulation studies are described elsewhere 39. The realistic carbons that are frequently encountered in experiments often might contains local defects and the presence of sp3 hybridized carbon atoms and different type of oxygenated groups; all of them will either increase or alter the density of the fluid inside the pores and the binding affinity for the guest molecules. Despite the limitations, as far as the N doping effect or the influence of other impurity such as OH, which is considered in this work, the conclusions obtained based on the less refined carbon models can hold true. At 298 K, CO2 is supercritical with large thermal motions which will have a strong influence on the solid-fluid interactions involved between different atoms present in the carbon framework; thus studying the N doping effect exclusively through experiments are practically difficult due to the variety of parameters involved such as the presence of other functional groups as well as the pore architecture. In this work, the influence of other type of N moieties is not discussed. We are currently constructing more refined carbon structures that contain other functional groups



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and other type of N moieties. The limitations of the present work will be overcome in our future study which will detail the adsorption properties of CO2 and other gases and gas mixtures in such refined structures.

Acknowledgements We thank EU for the Intra European Marie Curie Research Fellowship (PIEF-GA-2013-623227), and the EPSRC (Grant No. EP/J020745/1) for part sponsorship. Supporting Information Isosteric heat of adsorption at 1 bar in different carbon structures and the unit cell lattice parameters of the carbon structures are given as supporting information. This information is available free of charge via the Internet at http://pubs.acs.org

REFERENCES (1)

Haszeldine, R. S. Carbon Capture and Storage: How Green Can Black Be? Science 2009, 325 (5948), 1647–1652.

(2)

Drage, T. C.; Snape, C. E.; Stevens, L. A.; Wood, J.; Wang, J.; Cooper, A. I.; Dawson, R.; Guo, X.; Satterley, C.; Irons, R. Materials Challenges for the Development of Solid Sorbents for PostCombustion Carbon Capture. J. Mater. Chem. 2012, 22 (7), 2815–2823.

(3)

Wilcox, J.; Haghpanah, R.; Rupp, E. C.; He, J.; Lee, K. Advancing Adsorption and Membrane Separation Processes for the Gigaton Carbon Capture Challenge. Annu. Rev. Chem. Biomol. Eng. 2014, 5, 479–505.

(4)

Lu, L.; Wang, S.; Müller, E. A.; Cao, W.; Zhu, Y.; Lu, X.; Jackson, G. Adsorption and Separation of CO2/CH4 Mixtures Using Nanoporous Adsorbents by Molecular Simulation. Fluid Phase Equilib. 2014, 362, 227–234.

(5)

Carbon Capture: J. Wilcox, Springer Science+Business Media, 2012.

(6)

Srinivas, G.; Guo, Z. X. Graphene-Based Materials: Synthesis and Gas Sorption, Storage and Separation. Prog. Mater. Sci. 2014, 69, 1–60.

(7)

Yang, Z.; Xia, Y.; Mokaya, R. Enhanced Hydrogen Storage Capacity of High Surface Area Zeolite-like Carbon Materials. J. Am. Chem. Soc. 2007, 129 (6), 1673–1679.

(8)

Ashourirad, B.; Sekizkardes, A. K.; Altarawneh, S.; El-Kaderi, H. M. Exceptional Gas Adsorption Properties by Nitrogen-Doped Porous Carbons Derived from Benzimidazole-Linked Polymers. Chem. Mater. 2015, 27 (4), 1349-1358.


Page 21 of 38

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


(9)

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.

(10)

Sevilla, M.; Falco, C.; Titirici, M.-M.; Fuertes, A. B. High-Performance CO2 Sorbents from Algae. RSC Adv. 2012, 2 (33), 12792-12797.

(11)

Srinivas, G.; Krungleviciute, V.; Guo, Z.-X.; Yildirim, T. Exceptional CO 2 Capture in a Hierarchically Porous Carbon with Simultaneous High Surface Area and Pore Volume. Energy Environ. Sci. 2014, 7 (1), 335–342.

(12)

Gadipelli, S.; Travis, W.; Zhou, W.; Guo, Z. A Thermally Derived and Optimized Structure from ZIF-8 with Giant Enhancement in CO2 Uptake. Energy Environ. Sci. 2014, 7 (7), 2232-2238.

(13)

Chen, Z.; Deng, S.; Wei, H.; Wang, B.; Huang, J.; Yu, G. Activated Carbons and Amine-Modified Materials for Carbon Dioxide Capture — a Review. Front. Environ. Sci. Eng. 2013, 7 (3), 326– 340.

(14)

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.

(15)

Sevilla, M.; Fuertes, A. B. Sustainable Porous Carbons with a Superior Performance for CO2 Capture. Energy Environ. Sci. 2011, 4 (5), 1765-1771.

(16)

Zhang, Z.; Zhou, J.; Xing, W.; Xue, Q.; Yan, Z.; Zhuo, S.; Qiao, S. Z. Critical Role of Small Micropores in High CO2 Uptake. Phys. Chem. Chem. Phys. 2013, 15 (7), 2523–2529.

(17)

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), 10951-10961.

(18)

Xing, W.; Liu, X.; Bai, P.; Yan, Z.; Lu, G. Synthesis of Novel Ordered Carbon Nanorods and Its Application in Electrochemical Double Layer Capacitor. Sci. China Ser. E Technol. Sci. 2006, 49 (4), 425–433.

(19)

Zhu, B.; Li, K.; Liu, J.; Liu, H.; Sun, C.; Snape, C. E.; Guo, Z. Nitrogen-Enriched and Hierarchically Porous Carbon Macro-Spheres – Ideal for Large-Scale CO 2 Capture. J. Mater. Chem. A 2014, 2 (15), 5481–5489.

(20)

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.

(21)

Xing, W.; Qiao, S. Z.; Ding, R. G.; Li, F.; Lu, G. Q.; Yan, Z. F.; Cheng, H. M. Superior Electric Double Layer Capacitors Using Ordered Mesoporous Carbons. Carbon. 2006, 44 (2), 216–224.

(22)

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

(23)

Mosher, K.; He, J.; Liu, Y.; Rupp, E.; Wilcox, J. Molecular Simulation of Methane Adsorption in Micro- and Mesoporous Carbons with Applications to Coal and Gas Shale Systems. Int. J. Coal Geol. 2013, 109-110, 36–44.



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

Page 22 of 38

(24)

Liu, Y.; Wilcox, J. Molecular Simulation Studies of CO2 Adsorption by Carbon Model Compounds for Carbon Capture and Sequestration Applications. Environ. Sci. Technol. 2013, 47 (1), 95–101.

(25)

Tenney, C. M. Molecular Simulation of Carbon Dioxide Adsorption for Carbon Capture and Storage; PhD Thesis, University of Michigan. 2009.

(26)

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.

(27)

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.

(28)

Tachikawa, H.; Iyama, T.; Azumi, K. Density Functional Theory Study of Boron- and NitrogenAtom-Doped Graphene Chips. Jpn. J. Appl. Phys. 2011, 50 (1S2), 01BJ03.

(29)

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.

(30)

Haldoupis, E.; Nair, S.; Sholl, D. S. Finding MOFs for Highly Selective CO2/N2 Adsorption Using Materials Screening Based on Efficient Assignment of Atomic Point Charges. J. Am. Chem. Soc. 2012, 134 (9), 4313–4323.

(31)

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.

(32)

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.

(33)

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.

(34)

TraPPE: Transferable Potentials for Phase Equilibria Force Field, http://www.chem.umn.edu/groups/siepmann/trappe/ .

(35)

Peng, D.-Y.; Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 1976, 15 (1), 59–64.

(36)

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.

(37)

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


Page 23 of 38

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)

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.

(39)

Marsh, H.; Rodriguez Reinoso, F. Activated Carbon; Elsevier Ltd: Oxford, U.K., 2006.

(40)

Zheng, Q.; Li, Z.; Yang, J. Effects of N Doping and NH 2 Grafting on the Mechanical and Wrinkling Properties of Graphene Sheets. RSC Adv. 2013, 3 (3), 923–929.

(41)

Segarra, E. I.; Glandt, E. D. Model Microporous Carbons: Microstructure, Surface Polarity and Gas Adsorption. Chem. Eng. Sci. 1994, 49 (17), 2953–2965.

(42)

Biggs, M. J.; Buts, A. Virtual Porous Carbons: What They Are and What They Can Be Used for. Mol. Simul. 2006, 32 (7), 579–593.

(43)

Accelrys Materials Studio: Homepage: http://accelrys.com

(44)

Frost, H.; Düren, T.; Snurr, R. Q. Effects of Surface Area, Free Volume, and Heat of Adsorption on Hydrogen Uptake in Metal-Organic Frameworks. J. Phys. Chem. B 2006, 110 (19), 9565– 9570.

(45)

Düren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Design of New Materials for Methane Storage. Langmuir 2004, 20 (7), 2683–2689.

(46)

Myers, A. L.; Monson, P. A. Physical Adsorption of Gases: The Case for Absolute Adsorption as the Basis for Thermodynamic Analysis. Adsorption 2014, 20 (4), 591–622.

(47)

Sarkisov, L. Accessible Surface Area of Porous Materials: Understanding Theoretical Limits. Adv. Mater. 2012, 24 (23), 3130–3133.

(48)

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

(49)

Sánchez-Sánchez, A.; Suárez-García, F.; Martínez-Alonso, A.; Tascón, J. M. D. Influence of Porous Texture and Surface Chemistry on the CO₂ Adsorption Capacity of Porous Carbons: Acidic and Basic Site Interactions. ACS Appl. Mater. Interfaces 2014, 6 (23), 21237–21247.

(50)

Yu, J.; Guo, M.; Muhammad, F.; Wang, A.; Zhang, F.; Li, Q.; Zhu, G. One-Pot Synthesis of Highly Ordered Nitrogen-Containing Mesoporous Carbon with Resorcinol–urea– formaldehyde Resin for CO2 Capture. Carbon. 2014, 69, 502–514.

(51)

Liu, Z.; Du, Z.; Song, H.; Wang, C.; Subhan, F.; Xing, W.; Yan, Z. The Fabrication of Porous NDoped Carbon from Widely Available Urea Formaldehyde Resin for Carbon Dioxide Adsorption. J. Colloid Interface Sci. 2014, 416, 124–132.

(52)

Chandra, V.; Yu, S. U.; Kim, S. H.; Yoon, Y. S.; Kim, D. Y.; Kwon, A. H.; Meyyappan, M.; Kim, K. S. Highly Selective CO2 Capture on N-Doped Carbon Produced by Chemical Activation of Polypyrrole Functionalized Graphene Sheets. Chem. Commun. 2012, 48 (5), 735–737.



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

(53)

Hao, G.-P.; Li, W.-C.; Qian, D.; Lu, A.-H. Rapid Synthesis of Nitrogen-Doped Porous Carbon Monolith for CO2 Capture. Adv. Mater. 2010, 22 (7), 853–857.

(54)

Nicholson, D. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press, 1982.

(55)

Steriotis, T. A.; Stefanopoulos, K. L.; Katsaros, F. K.; Gläser, R.; Hannon, A. C.; Ramsay, J. D. F. In Situ Neutron Diffraction Study of Adsorbed Carbon Dioxide in a Nanoporous Material: Monitoring the Adsorption Mechanism and the Structural Characteristics of the Confined Phase. Phys. Rev. B 2008, 78 (11), 115424.

(56)

Müller, E. A.; Gubbins, K. E. Molecular Simulation Study of Hydrophilic and Hydrophobic Behavior of Activated Carbon Surfaces. Carbon. 1998, 36 (10), 1433–1438.

(57)

Torrisi, A.; Mellot-Draznieks, C.; Bell, R. G. Impact of Ligands on CO(2) Adsorption in MetalOrganic Frameworks: First Principles Study of the Interaction of CO(2) with Functionalized Benzenes. II. Effect of Polar and Acidic Substituents. J. Chem. Phys. 2010, 132 (4), 044705.

(58)

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.


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Page 25 of 38

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


Table 1: Characteristic pore properties of the carbon prototypes.

slit (0.8 nm) N-slit (0.8 nm) slit (1.2 nm) N-slit (1.2 nm) RPC1 N-RPC1 RPC2 N-RPC2

SSA (m2/g) 1297 1289 1349 1303 5861 5823 2993 2910

Vt (cm3/g) 0.387 0.401 0.645 0.669 2.834 2.815 1.156 1.407

(SSA: specific surface area; Vt: total pore volume, ρ: carbon framework density)


ρ (g/cm3) 2.01 2.02 1.34 1.35 0.30 0.30 0.60 0.60


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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Table 2: CO2 uptakes and their isosteric heat (shown in parenthesis) reported for N doped and pristine carbon materials at 1 bar and 298 K. Sample

Types of N

CO2 uptake at 1 bar, 298 K, mmol/g

Reference

RFL carbon

N1, N2, N4

3.13

53

CPC-550

N1,N2,N3,N4

5.8(35.3 kJ/mol)

8

CPC-600

N1,N2,N3,N4

4.7(30.8 kJ/mol)

8

CPC-650

N1,N2,N3,N4

4.0(24.4 kJ/mol)

8

CPC-700

N1,N2,N3,N4

3.3(24.2 kJ/mol)

8

CPC-800

N1,N2,N3,N4

3.1(27.9 kJ/mol)

8

N-TC-EMC

N1,N2,N3,N4

4.0(50 kJ/mol)

9

N-TC-Y1

N1,N2,N3,N4

3.2

9

N-TC-Y2

N1,N2,N3,N4

2.6

9

NPC-600 N2, N3 3.04 NPC-650 N2, N3 3.10 NPC-700 N2, N3 2.46 NPC-750 N2, N3 2.15 (N1: Graphitic; N2, pyrollic, N3: pyridinic; N4: pyridine N-oxide)


17 17 17 17

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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


pyridinic-N

pyrollic-N

graphitic-N

Fig 1: Two dimensional illustration of the three different bonding configurations of N groups in graphene.



(a)

(b)

(c)

(d)

(e)

0.3 0.4

0.2 0.1

PSD

PSD

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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0 0

5

10 15 20

H', A

0.2 0 0

3

6

9

12

H', A

Fig 2: (a) atomic representation of a N doped (red) slit-pore of H = 1.2 nm, (b) model representing the graphitic state of N in a graphene layer, (c) basis structural unit of RPCs, (d) atomistic model of RPC1 with pore size distribution (PSD), (e) atomistic model of RPC2 with pore size distribution.


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5

4

n, mmol/g

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


3

2

1

0 0

0.2

0.4

0.6

0.8

1

p, bar

Fig 3: CO2 adsorption isotherms at 1 bar and 298 K in pristine (closed symbols) and N doped (open symbols) slit-shaped carbon pores of different pore-widths, H ( : H= 0.8 nm, : H = 1.2 nm, : H = 1.6 nm and : H = 2.0 nm).



20 18 16 14

n, mmol/g

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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12 10 8 6 4 2 0 0.01

0.1

1

10

100

p, bar

Fig 4: CO2 adsorption isotherms at 60 bar and 298 K in pristine (closed symbols) and N doped (open symbols) slit-shaped carbon pores of different pore-widths H ( : H= 0.8 nm, : H = 1.2 nm, : H = 1.6 nm and : H = 2.0 nm).


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53 51

51

49

46

47

qst, kJ/mol

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


41

45 43

36 0

41

1

2

3

4

39 37 35 0

10

20

30

40

50

60

nabs, mmol/g

Fig 5: Isosteric heat as a function of absolute adsorption, nabs in pristine (closed symbols) and N doped (open symbols) slit-shaped carbon pores of different pore-widths H ( : H= 0.8 nm, : H = 1.2 nm, : H = 1.6 nm and : H = 2.0 nm).



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Page 32 of 38

(a)

(b)

(c) Fig 6: Snapshots showing the CO2 adsorption patterns in a slit-pore of H = 1.2 nm for (a) pristine, (b) N doped and (c) N doped pore without any charges on the surface.


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35

1

30 0.5 25

n, mmol/g

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


20

0 0

0.5

1

15 10 5 0 0

10

20

30

40

50

60

p, bar

Fig 7: CO2 adsorption in pristine (closed symbols) and N doped (open symbols) RPCs ( RPC1= 0.8 nm, : RPC2 = 1.2 nm).


:


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Page 34 of 38

Fig 8: Snapshots showing CO2 adsorbed at 0.3 bar and 298 K in (a) RPC2 and (b) slit-pore of H = 1.2 nm (O: yellow; C in CO2: blue).


Page 35 of 38

40 35 30

selectivity

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


25 20 15 10 5 0 0

1

2

3

4

5

6

p, bar

Fig 9: CO2/N2 selectivity for pristine and N doped slit pores of different pore-width, H = 0.8 – 2.0 nm ( : H= 0.8 nm, : H = 1.2 nm, : H = 1.6 nm and : H = 2.0 nm) (open symbols: N doped carbons, close symbols: pristine structure).



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(a)

Page 36 of 38

(b)

Fig 10: Snapshots showing (a) adsorption of pure N2 at 5.1 bar and (b) adsorption of CO2+N2 mixture at 6 bar (partial pressure of N2: 5.1 bar) at 298 K (pore-walls are not shown for Fig clarity: N2: blue; CO2: blue+yellow). (snapshots are shown at different angles for clarity)


Page 37 of 38

60 50

n, mmol/nm2

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


40 30 20 10 0 0

0.2

0.4

0.6

0.8

1

p, bar

Fig 11: Effect of OH group incorporation on the CO2 uptake of pristine and N doped slit pores of H = 1.2 nm ( : pristine slit pore, : N doped slit pore, : N doped slit pore with OH functional groups).

Fig 12: Snapshot showing CO2 adsorption in N doped slit-pore (H = 1.2 nm) with OH impurity at 1 bar and 298 K (CO2 is shown as rods, green net corresponds to the surface area accessible for a CO2 molecule)



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

Graphical Abstract

N doped graphitic carbon

Disordered N doped carbon

CO2 uptake


Page 38 of 38

Effect of Nitrogen Doping on the CO2 Adsorption Behavior in

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