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Jan 29, 2014 - CO2 Adsorption Thermodynamics over N-Substituted/Grafted Graphanes: A DFT Study. Jing Xiao*†, Siddarth Sitamraju‡, and Michael J. J...
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CO2 Adsorption Thermodynamics over N‑Substituted/Grafted Graphanes: A DFT Study Jing Xiao,*,† Siddarth Sitamraju,‡ and Michael J. Janik‡,§ †

Key Laboratory of Enhanced Heat Transfer and Energy Conservation of Education Ministry, and School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China ‡ EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: This work examines CO2 adsorption over various N-substituted/grafted graphanes to identify the promotional effects of various N-functionalities have on the adsorption characteristics using DFT. CO2 adsorbs weakly on a graphane surface functionalized with a single, isolated substituted N- or grafted NH2-sites. The presence of coadsorbed H2O on the surface promotes CO2 adsorption on both N- and NH2-sites, with highly exothermic adsorption energies (∼−50 kJ mol−1). Directly grafted −NH2 or −OH functional groups on C atoms adjacent to C atoms which have a −NH2 group grafted suffer from geometrical restrictions preventing dual stabilization of formed carbamate upon adsorption of CO2. CO2 adsorption can be greatly enhanced with the presence of a −OH group or second −NH2 group in the proximity of a −NH2 site on graphane, and only if a n(−CH2−) (n ≥ 1) linker is introduced between the −NH2 or −OH and graphane surface (adsorption energies of −58.8 or −43.1 kJ mol−1 at n = 2). The adsorption mechanistics provided by DFT can be used to guide the atomic-level rational design of N-based graphane and carbon adsorbents for CO2 capture. and tailored CO2-philic surface functionalities.14−16 Wang et al.7 prepared a carbon-based “molecular basket” sorbent, polyethylenimine (PEI)-modified activated carbon by wet impregnation. A high sorption capacity of 135 mg CO2/g sorb was achieved, which is significantly higher than non-Nfunctionalized porous materials, i.e., MgO, hydrotalcite, basic alumina, clay, and 4A zeolite. 9 Grafting/anchoring Ncontaining functionalities onto porous carbon supports or incorporating N-functionalities during carbon material synthesis forming chemical bonding can reduce the loss of Nfunctionalities during heat regeneration. Sevilla et al.1 prepared N-doped polypyrrole-based porous carbons using KOH as activating agent and polypyrrole as a carbon precursor and reported a high CO2 adsorption rate, good regenerability, and high selectivity for CO2/N2 separation of the sorbent. Wei et al.12 reported a controlled synthesis rich N-doped (up to 13.1 wt %) ordered mesoporous carbon sorbents, which exhibit a

1. INTRODUCTION The mitigation of carbon dioxide emissions from power plants is important because these emissions are the major anthropogenic contributor to climate change.1−5 Among the strategies for the abatement of CO2 emissions, carbon capture and storage (CCS)6 has the potential for alleviating these large volume emissions. Currently, liquid amine scrubbing is a commercial technology applied in conventional adsorber/ stripper systems for effective CO2 capture. This technology is limited by its severe energy requirement, corrosion of the equipment, and amine loss due to degradation and evaporation.7,8 Design and development of materials with tailored sorption characteristics for CO2 capture may alleviate some or all of these limitations. Solid porous sorbents have been proposed as alternatives and have been widely studied as potential materials for CO2 capture. Reported sorbents include carbon-, silica-, clays-, and alumina-based sorbents, metal−organic frameworks (MOFs), zeolites, and polymers.9−13 Among these sorbents, carbon materials, specifically N-functionalized carbon materials, are promising materials for CO2 capture due to their high porosity © 2014 American Chemical Society

Received: December 20, 2013 Revised: January 26, 2014 Published: January 29, 2014 1837

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Figure 1. Optimized graphane surfaces: (a) bare, (b) N-substituted, and (c) NH2-grafted. Color code: gray, C; blue, N; white, H.

CO2 sorption capacity of 123.2−140.8 mg CO2/g sorb. CO2 adsorption on N-functionalized carbon materials warrants further study because of the immense potential of these materials as capture sites for CO2. Graphane, an extended two-dimensional covalently bonded hydrocarbon, is an emerging carbon material with interesting properties.17 Graphane is composed of a monolayer of twodimensional (2-D) sheets of sp3-bonded carbon atoms.18,19 Maximized atomic utilization ratio and adjustable composition and functionalities20,21 provide new opportunities for the design of materials based on 2-D carbon for applications including adsorption and catalysis. Unlike graphene, hydrogen bound to a carbon atom on graphane can be replaced by functionalities without affecting the rest of the graphane structure. Some of the possible N-containing functional groups include an N-dopant (substituting a C on the graphane surface) or grafted primary and secondary amines, prepared by condensation1,12 or postsynthetic substitution reaction.22 The general underlying mechanism for CO2 capture using Nfunctionalized sorbents is the chemical interaction of CO2 with primary or secondary amine groups on solid sorbent materials. Carbamates are formed under dry conditions, while bicarbonates are formed under humid conditions.23,24 The effectiveness of various types of N-functionalities on the adsorption may depend on their surface density/proximity and the presence of H2O during the capture process. Critical factors like these are not yet understood, thus limiting the rationale for designing optimal adsorbents. Herein, density functional theory (DFT) calculations are used to examine CO2 adsorption over various types of Nsubstituted/grafted graphanes. CO2 adsorption energies over various amine-functionalized graphanes are compared. A series of DFT models are used to identify the possible interaction mode and calculate the energy of CO2 adsorption over these different types of N-sites. The effects of N-site density and position on CO2 adsorption are investigated. The effect of moisture addition to each adsorption system is studied. Furthermore, sensitivity of CO2 adsorption energy to the length of the alkyl chain of N-functional groups bound to graphane surface is analyzed.

forces on all atoms were below 0.05 eV Å−1, and all calculations were spin-polarized. To incorporate exchange and correlation energies, the Perdew−Wang (PW91) version of the generalized gradient approximation (GGA) was used.29 A 3 × 3 × 1 Monkhorst−Pack (MP)30 k-point mesh was used for all of the surfaces. 2.2. Model Construction. We follow the work of Sofo19 to build atomistic models of graphane. The unit cell of graphane was optimized as a = b = 2.53 Å, close to the reported unit cell values of graphane (a = b = 2.52 Å19). The chair conformation was chosen as it is more stable than the boat configuration.19 The system was modeled as a 4 × 4 supercell of graphane (32 C atoms plus 32 H atoms). Substitution of a single C atom in the lattice with a N atom led to a N concentration of 3.13 wt %. A single N-grafted functional group can be introduced by replacing one H in the graphane structure. A vacuum layer of 15 Å was added perpendicular to the single layer surface. All the atoms in the cell were relaxed. The isolated gas phase adsorbates (CO2 and H2O) were optimized separately in a 20 Å × 20 Å × 20 Å unit cell. 2.3. Adsorption Energy Calculations. The adsorption energy (BE) is calculated by subtracting the sum of the energy of the isolated adsorbent surface and the energy of the optimized gas-phase adsorbate from the energy of the optimized adsorbate−adsorbent system, which can be expressed by the following equations:

2. COMPUTATIONAL METHODOLOGY 2.1. Electronic Structure Method. The Vienna ab initio simulation program (VASP), an ab initio total-energy and molecular dynamics program developed at the Institute for Material Physics at the University of Vienna25−27 was used for all calculations. The wave functions of the core electrons were represented using the projector augmented wave (PAW) method.28 The cutoff energy of the valence plane waves was 450 eV. Structural optimizations were performed until the

3. RESULTS AND DISCUSSION 3.1. CO2 Adsorption over N-Substituted/Directly Grafted NH2-Graphanes. DFT was used to study CO2 adsorption thermodynamics over N-substituted/directly grafted NH2-graphanes. The optimized basic graphane surfaces(a) bare, (b) N-substituted, and (c) NH2-graftedare shown in Figure 1. The bare graphane structure is in a chairlike conformation with the hydrogen atoms alternating on both sides of the plane, which is a preferable conformation compared

BE = Eadsorbent − CO2 − Eadsorbent − ECO2

(1)

BE = Eadsorbent − H2O − Eadsorbent − E H2O

(2)

BE = Eadsorbent − CO2 − H 2O − Eadsorbent − ECO2 − E H 2O

(3)

A more negative BE indicates a stronger adsorption. Eadsorbent is the energy of the bare or functionalized graphane adsorbent, ECO2 is the energy of isolated CO2, EH2O is the total energy of isolated H2O, Eadsorbent−CO2 is the total energy of the adsorbent− CO2 system, Eadsorbent−H2O is the total energy of the adsorbent− H2O system, and Eadsorbent−CO2−H2O is the total energy of the adsorbent−CO2−H2O system.

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Figure 2. Optimized interaction configurations and adsorption energies (BEs, in kJ mol−1) for CO2 adsorption over graphanes: (a) a bare graphane; (b) a single N-substituted graphane; (c) graphane with 2 N-substitutions in meta- position; (d) graphane with 2 N-substitutions in para-position; (e) graphane with 2 N-substitutions in ortho-position. Color code: gray, C; red, O; blue, N; white, H.

3.1.2. CO2 Adsorption to Directly Grafted NH2-Graphanes. Primary amine (NH2-) grafted graphanes are expected to offer stronger basicity,32 leading to a stronger CO2 adsorption (an acidic molecule). This hypothesis was investigated using DFT by studying several modes of adsorption on these surfaces. The two interaction configurations that had the strongest adsorption were CO2 physisorption and CO2 chemisorption forming a carbamate. Previous studies reported that carbamate formation strengthens CO2 adsorption in most amine-functionalized adsorbents.11,33,34 Figure 3 lists the proposed interaction

to the boatlike conformer with the hydrogen atoms alternating in pairs.19 All the carbons form a hexagonal network, and the calculated C−C bond length is 1.52 Å, similar to the sp3 bond length of 1.53 Å of ethane, and much longer than the typical bond length (1.42 Å) of sp2 carbon. The C−C length is consistent with the DFT graphane structure reported by Sofo et al.19 N-substituted graphane is constructed by replacing a carbon atom in the graphane sheet with a nitrogen atom, as shown in Figure 1b. NH2-grafted graphane is constructed by replacing a hydrogen atom on one side of the graphane sheet with a −NH2 group, as shown in Figure 1c. In both modified cases, the basic graphane structures are well retained with no visible local distortion of the hexagonal lattice. 3.1.1. CO2 Adsorption to N-Substituted Graphanes. To study the effects of N-site density on N-substituted graphane on CO2 adsorption, CO2 adsorption thermodynamics over a 4 × 4 graphane unit cell surface with 0−2 units of N-sites (0, 3.13, and 6.27 wt % of N-site density) were studied. The optimized interaction configurations and adsorption energies are given in Figure 2. For the substitution of 2 N-sites per graphane cell, three configurations of N-sites in meta-, para-, and ortho-positions were considered. Supporting Information Figure S1 shows CO2 adsorption energy versus degree of Nsubstitution in a 4 × 4 graphane unit cell. Doping N atoms into a graphane unit cell strengthens the CO2 adsorption when compared to adsorption on an undoped surface (adsorption energy changes from −0.1 (undoped) to −2.1 (1-N) and −4.6 (2-N ortho-position) kJ mol−1, respectively. The result suggests that higher N-site density gives more exothermic CO2 adsorption. With 2 N atoms doped in the graphane unit cell, the CO2 adsorption energy varies with the placement of the N atoms, which increases in the order of ortho > para ∼ meta. Regardless of the concentration of N-doping, the adsorption of CO2 over N-substituted graphane is weak. DFT calculations underestimate van der Waals (vdW) contributions to interatomic interactions, which can be corrected using the DFT-D2 method of Grimme.31 The adsorption energy of CO2 adsorption over N-graphane obtained with/without DFT-D2 correction is calculated as −14.5 and −2.1 kJ mol−1, respectively. The noticeable difference suggests adding the dispersive interactions gives more exothermic adsorption.

Figure 3. Proposed interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over NH2-grafted graphanes: (a) by physisorption; (b) forming carbamate. Color code: gray, C; red, O; blue, N; white, H.

configurations and adsorption energies. CO2 adsorbs weakly on graphane surface-functionalized with single, isolated grafted NH2-sites, with an exothermic adsorption energy weaker than −5 kJ mol−1. The formation of carbamate (Figure 3b, BE = −1.0 kJ mol −1 ) is less favorable, being weaker than physisorption over a single NH2-functionalized surface (Figure 3a, BE = −3.0 kJ mol−1). Carbamate formation without stabilization by a second functional group is not observed to be favorable. These results are consistent with both the previous reported gas phase ammonia−CO2 1:1 reaction having a BE of +12.6 kJ mol−1 and the observation that carbamic acid is unstable to decomposition to ammonia and CO2.35 As the CO2−amine carbamate reaction is reported in a 1 CO2 to 2 amine ratio,5 a second −NH2 functional group proximate to the −NH2 site is added. Three interaction configurations were examined: CO2 adsorption by (a) physisorption, (b) forming cis-carbamate, and (c) forming 1839

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Figure 4. Proposed interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over 2NH2- grafted graphanes: (a) by physisorption; (b) forming cis-carbamate; (c) forming trans-carbamate. Color code: gray, C; red, O; blue, N; white, H.

trans-carbamate, with the optimized interaction configurations and adsorption energies listed in Figure 4. Physisorption occurs with an adsorption energy of −4.4 kJ mol−1, while adsorption energies for both cis- and trans-carbamate formation are positive (cis is more stable than trans, consistent with the results of Mindrup35). The positive values of BE in two −COOH formation configurations indicate that the structures are unstable relative to desorption. Bending the CO2 molecule or transferring a H to the CO2 creates a local minimum energy structure allowing location of this unstable species. Two proximate −NH2 functional groups on graphane does not promote carbamate formation, which is probably due to geometrical restrictions preventing the stabilization of the formed carbamate. By grafting NH2- onto a graphane surface, CO2 physisorption is strengthened slightly (adsorption energy of −0.1 with no −NH2, −3.0 with one −NH2, and −4.4 kJ mol−1 with two −NH2). Overall, similar to N-substituted graphane, the density of amine sites only slightly affects the CO2 adsorption energy. The adsorption of CO2 over NH2-grafted graphane is relatively weak, and carbamate formation is not favorable. Therefore, neither N-substitution nor directly grafted NH2-groups on graphane provide favorable sites for CO2 adsorption. 3.2. CO2 Adsorption over N-Substituted/Directly Grafted NH2-Graphanes in the Presence of H2O. In power plant flue gas, the water content is typically 8−20%.4 The effect of H2O on CO2 adsorption should hence be taken into consideration. Amidines react with CO2 to form bicarbonate in the presence of water.36 Therefore, bicarbonate formation is considered along with carbamate formation and physisorption when considering H2O effect. 3.2.1. CO2 Adsorption to N-Substituted Graphane with H2O. Figure 5 shows two proposed interaction configurations and adsorption energies for CO2 adsorption over N-substituted graphanes in the presence of H2O. In Figure 5a, the H2O molecule is weakly bonded to the N-site (the OH−N bond distance is 3.1 Å), with BE of −3.6 kJ mol−1 using eq 2. The

water molecule adsorbs CO2 strongly through a C−Owater electrostatic interaction. The total adsorption energy is calculated as −14.1 kJ mol−1 using eq 3. In Figure 5b, CO2 interacts with H2O to form bicarbonate, which is further bonded to the N-site on graphane. The OH−N bond distance is 1.5 Å, indicating a strong hydrogen bond. The adsorption energy calculated using eq 3 is −54.5 kJ mol−1. Comparing both configurations, binding CO2 with H2O in a 1:1 fashion forming bicarbonate is more exothermic. This mechanism is consistent with the experimental results of bicarbonate formation with CO2 absorption over tertiary amines in aqueous solution.37 H 2 O helps CO2 coadsorption through the promotion of bicarbonate formation. 3.2.2. CO2 Adsorption to NH2-Grafted Graphane with H2O. Figure 6 compares two proposed interaction configurations and adsorption energies for CO2 adsorption over NH2-grafted graphanes in the presence of H2O. In Figure 6a, CO2 reacts with a −NH2 site to form carbamate, which is further stabilized by a H2O molecule with two H−O bond distances of 1.8 Å. The total adsorption energy is calculated as −42.3 kJ mol−1. CO2 and H2O may form bicarbonate, which adsorbs on NH2grafted graphane possibly through hydrogen bonding, with a H−N bond distance of 1.5 Å. The total BE for bicarbonate formation calculated using eq 3 is −57.6 kJ mol−1. In both adsorption modes, the presence of H2O greatly strengthens CO2 adsorption over NH2-grafted graphane, and bicarbonate formation is more favorable. To understand the effect of NH2-site density on CO2 capture in the presence of H2O, a second −NH2 was introduced proximate to the −NH2 site on graphane as shown in Figure 6c. Bicarbonate forms and adsorbs on the N-atom of grafted amine through hydrogen bonding with the N−H bond distance calculated as 1.5 Å. The presence of the second −NH2 functional group weakens CO2 adsorption (adsorption energy increases from −57.6 to −50.7 kJ mol−1); this may be attributed to the repulsive force between the neighboring N and =O in formed bicarbonate. The presence of H2O strongly enhances CO2 adsorption over both N-substituted and NH2-grafted graphanes, which is advantageous since water is present in the flue gas, and the process can be completed without the introduction of additional water. However, H2O coadsorption leads to an impure CO2 stream upon desorption, which needs to be further dried. 3.3. Geometric Limitation on CO2 Adsorption with the Functionalization of Graphane Surfaces. Steric effects can influence binding to an adsorbent surface,38 especially when two or more collaborative sites on the adsorbent surface are involved. In the case of amine-functionalized graphane for CO2 adsorption, the spacing of carbon sites in the graphane sheet is fixed, and the spacing of collaborative amine sites is limited to

Figure 5. Proposed interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over N- substituted graphanes in the presence of H2O: (a) by physisorption; (b) forming carbamate. Color code: gray, C; red, O; blue, N; white, H. 1840

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Figure 6. Proposed interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over 1 (or 2) NH2-grafted graphanes in the presence of H2O: (a) over a NH2-grafted graphane forming carbamate, stabilized by a H2O molecule; (b) over a NH2-grafted graphane forming bicarbonate; (c) over a 2NH2-grafted graphane forming bicarbonate. Color code: gray, C; red, O; blue, N; white, H.

Figure 7. Optimized interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over 2(NH2-nCH2)-grafted graphanes: (a) over a 2(NH2−CH2)-grafted graphane forming carbamate; (b) over a 2(NH2-2CH2)-grafted graphane forming carbamate; (c) over a 2(CH3− NH−CH2)-grafted graphane forming carbamate. Color code: gray, C; red, O; blue, N; white, H.

Adsorption energies are also calculated on a secondary amine (−NH−) grafted graphane, with the optimized interaction configurations and adsorption energies shown in Figure 7c. Compared to primary amine (−CH2−NH2 and −CH2−CH2− NH2) grafted graphane, the CO2 adsorption over secondary amine (−CH2−NH−CH3) grafted graphane is less exothermic with an adsorption energy of −18.7 kJ mol−1, which may be due to the shorter −CH2 chain N-functionality linked to and the weaker H-bonding interaction between −COOH and secondary rather than primary amine. The CO2 adsorption energy can therefore be tuned with varied N-functionalities on graphane. 3.3.2. CO2 Adsorption over (NH2-nCH2)-(nCH2−OH)Grafted Graphanes. Generally, the types of functionalities on adsorbent surfaces govern the adsorption thermodynamics on CO 2 capture. The COOH stabilizers, including those participate in hydrogen bonding, may impact CO2 adsorption on functionalized carbons. As hydroxyl group could stabilize COOH through hydrogen bonding, the presence of an −OH group in the proximity of a NH2-site on graphane may impact CO2 adsorption thermodynamics. Figure 8 shows interaction configurations and adsorption energies for CO2 adsorption over (NH2-nCH2)- and (OH-nCH2)-grafted graphanes. Similarly to −NH2, a −OH group in the proximity of a NH2-group directly grafted to graphane does not promote CO2 adsorption, possibly due to geometrical restrictions. However, when 1 or 2 (−CH2−) linker (n = 2) is used for both the hydroxyl group and the proximate NH2-group on graphane, CO2 adsorption is strengthened, with the adsorption energies of −30.5 or −43.1 kJ mol−1, respectively, as shown in Figures 8c and 8d. Figure 9 summarizes the CO2 adsorption energy versus number of −CH2− linking −NH2 or −OH to graphane surface in NH2(OH)-grafted graphanes. The results suggest that the CO2 adsorption energy can be tuned in a substantial range (0− 60 kJ mol−1) with varied N-functionalities and the linker length of the introduced −NH2 and −OH groups bound to graphane. It should be mentioned that the CO2 adsorption efficiency or

configurations such as meta, ortho, or para. The spacing of binding sites can be tuned flexibly by introducing alkyl organic linkers between the amine functionality and the lattice carbon. 3.3.1. CO2 Adsorption over 2(NH2-nCH2)-Grafted Graphanes. Figure 7 shows the optimized interaction configurations and adsorption energies for CO2 adsorption over 2(NH2nCH2)-grafted graphanes, where n equals 1 or 2. Carbamate formation is exothermic and energetically viable when linkers are introduced, in contrast with just NH2-grafted graphane (n = 0) where endothermic carbamate formation was observed. When CH2 linkers (n = 1 or 2) are used, the adsorption energies are as strong as −30.2 and −58.8 kJ mol−1 as shown in Figures 7a and 7b, which are close to the interaction energy of gas phase CO2−NH3 in 1:2 ratio of (−63.5 kJ mol−1), and reported carbamic acid formation energy calculated by DFT (−52.8 kJ mol−1 39), and slightly less exothermic than the experimental heat of CO2 absorption in monoethanolamine (7140 and 88.9 kJ mol−1 41). The results suggest that CO2 adsorption can be tuned substantially with varied linker lengths to bind −NH2 functional groups to a graphane surface. Directly grafted −NH2 groups on graphane do not enhance CO2 adsorption, probably due to geometrical restrictions to stabilize formed carbamate. It has been widely accepted that primary and secondary amines react with CO2 in 2:1 ratio to form carbamate.24,42 Here, we report that on a rigid graphane surface CO2 adsorption can be greatly strengthened with the presence of the second −NH2 group in the proximity of a NH2-site on graphane, but only if a n(−CH2−) linker is introduced in between −NH2 and the graphane surface (n ≥ 1). Experimental studies of CO2−amine interactions show that isocyanate and urea can be formed at high temperatures and pressures or involving catalysts or dehydrating agents.43,44 These species (which dissociate a C−O) would require large activation barriers to form and a purely thermodynamic analysis (as done in this work) would be insufficient, and analysis of the kinetics of adsorption is beyond the scope of this study. 1841

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work in the temperature and pressure ranges of real flue gas to separate CO2.46 Therefore, too weak of an adsorption leads to low working capacities and sorbent efficiency, and too strong of an adsorption leads to greater energy consumption for sorbent regeneration. Interactions between CO2 and different species, such as zeolites (∼25 kJ mol−1),47 ionic liquids (0 to −100 kJ mol−1),48 acetate ion (−44.4 kJ mol−1),49 etc., by DFT calculations are predicted in the literature. Here, we show Nfunctionalized sorbents can be tuned to provide an ideal BE (0−60 kJ mol−1) to fit the operating conditions for CO2 capture.

4. CONCLUSIONS CO2 adsorption over various types of N-substituted/grafted graphanes was studied using DFT. N-substituted/grafted graphanes were designed by substitution of lattice carbon or surface hydrogen with N-functional groups. Not all types of Nfunctionalities on a graphane surface improve the adsorption of CO2 molecule. CO2 adsorbs weakly on a graphane surface functionalized with single substituted N- or grafted NH2-sites. The adsorption can be strengthened by the presence of coadsorbed H2O through bicarbonate formation, or by stabilization of the formed carbamate through hydrogen bonding, with exothermic adsorption energies stronger than −50 kJ mol−1. Under dry conditions, CO2 adsorption can be greatly enhanced with the presence of an −OH group or a second −NH2 group in the proximity of a −NH2 site on graphane and only if a n(−CH2−) (n ≥ 1) linker is introduced between the −NH2 or −OH functionality and the graphane surface. Without the alkyl linkers, directly grafting −NH2 or −OH functional groups does not enhance CO2 adsorption, probably due to geometrical restrictions preventing stabilization of formed carbamate. When a 2(−CH2−) linker (n = 2) is used for the additional −NH2 or −OH, the adsorption can be greatly strengthened, with adsorption energies of −58.8 or −43.1 kJ mol−1, respectively. The CO2 adsorption energy can be tuned in a substantial range with varied types and locations of functionalities on graphane. The interaction chemistries, H2O effects, and geometrical restrictions described in this study provide guidelines for the future atomic-level rational design of N-based graphane adsorbents for CO2 capture.

Figure 8. Interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over (NH2-nCH2)- and (OH-nCH2)grafted graphanes: (a) over a NH2- and OH-grafted graphane forming carbamate; (b) over a NH 2 - and OH-grafted graphane by physisorption; (c) over a (NH2−CH2)- and (OH−CH2)-grafted graphane forming carbamate; (d) over a (NH2-2CH2)- and (OH2CH2)-grafted graphane forming carbamate. Color code: gray, C; red, O; blue, N; white, H.

Figure 9. CO2 adsorption energy versus number of −CH2− linking −NH2 or −OH to graphane surface in NH2(OH)-grafted graphanes.

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capacity depends on not only the interaction strength between CO2 and adsorbents but also the availability of adsorption sites,45 which could be affected by textural properties of adsorbents. The introduction of varied amount or length of −CH2 linkers on graphane surface causes variation in the textural properties of graphane and thus impacts the CO2 capture performance in another fashion. The interaction chemistries of N-functionalities with CO2 and the effects of H2O addition on CO2 adsorption over N-grafted sorbents disclosed in this work can also be applied to the impregnated N-functional sorbents. Different from N-grafted sorbents, impregnated N-functional sorbents may not suffer geometrical restrictions to amine−CO2 interaction due to the flexible-chain structures of impregnated polymers. An ideal CO2 solid adsorbent must not only adsorb CO2 in the capture cycles but also release the CO2 during the regeneration cycle. The operating conditions, including temperatures and pressures, for sorption/desorption processes depend on their use in a pre- or postcombustion process.13 To minimize the energy consumption, “ideal” sorbents should

ASSOCIATED CONTENT

S Supporting Information *

CO2 adsorption energy versus degree of N-substitution in a 4 × 4 graphane unit cell (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to acknowledge the research grants provided by the National Natural Science Foundation of China (21306054), Guangdong Natural Science Foundation (S2013040014747), Specialized Research Fund for the Doctoral Program of Higher Education (20130172120018), and Fundamental Research Funds for the Central Universities (2013ZM0047). 1842

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