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Silicon-Doped Carbon Nanotubes: Promising CO/ N Selective Agents for Sequestering Carbon Dioxide 2

Misaela Francisco-Marquez, and Annia Galano J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08641 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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The Journal of Physical Chemistry

Silicon-Doped Carbon Nanotubes: Promising CO2/N2 Selective Agents for Sequestering Carbon Dioxide Misaela Francisco-Marquez,1 Annia Galano 2* 1

Instituto Politécnico Nacional- UPIICSA. Té 950, Col. Granjas México, C.P. 08400, México D. F. México. 2

Departamento de Química. Universidad Autónoma Metropolitana-Iztapalapa. San Rafael Atlixco 186, Col. Vicentina. Iztapalapa. C. P. 09340. México D. F. México. Phone: (52) 5558044600

Abstract The potential ability of single-walled silicon-carbon nanotubes (SWSiCNT) as CO2 scavengers was investigated using density functional theory calculations and (5,5) SWSiCNT models with 2%, 33% and 50% Si. It was found that while the reactions between CO2 and pristine C tubes are endergonic, Si doped materials have exergonic adsorption routes. It was also found that 50-50 Si-C composition is not required for the SWSiCNTs to be able of sequestering CO2, which seems to be relevant since this is the maximum Si-C proportion allowed to maintain the SWSiCNT stability. The modeled SWSiCNT are predicted to be selective to CO2 over N2, which is a critical feature for materials with potential applications for CO2 capture. The rate constants for the SWSiCNT reactions with CO2 were found to be around 105 M-1 s-1, which suggests that they are fast enough to assure efficient CO2 capture at room temperature. In addition, for the SWSiCNT with 33% Si, the possibility of multiple CO2 adsorption was also investigated (up to 7 CO2 molecules). It was found that all the consecutive reactions are significantly exergonic, which indicates that one SWSiCNT is able of sequestering several CO2 equivalents. These findings suggest that SWSiCNT based materials are promising candidates for selectively, and efficiently, sequestering CO2 molecules, in particular SWSiCNT with intermediate (2% to 33%) Si amounts.

*

To whom correspondence should be addressed. E-mail: [email protected], [email protected]

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Introduction In the last decades carbon dioxide (CO2) emissions have raised major environmental concerns. The CO2 concentrations in the atmosphere are continuously increasing, mainly due to anthropogenic sources such as fossil fuel combustion, industrial processes and landuse. At the same time, CO2 is the primary greenhouse gas and it is directly involved in climate change. Therefore, it is crucial to find viable strategies for reducing the amounts of this gas in the atmosphere. Several options have been attempted to achieve this,1,

2

including sequestering CO2 by physical or chemical absorption. Different materials have been tested for that purpose. A few examples are: amines,3 metal–organic frameworks (MOFs),4,

5

mesoporous carbons,6,

7

calcium-functionalized carbon materials,8 carbon

foam,9 N-doped carbon architectures,10-18 hybrid silica membranes,19 amine functionalized silica,20,

21

microporous silicon carbide derived carbons,22 and nanoporous carbons with

nano-silica spheres.23 Carbon-based materials have attracted a great deal of attention, regarding CO2 capture and sequestration strategies. This is probably because of the appealing features of activated carbons. They have great porosity, large surface area and low production costs, and are easy to regenerate and unaffected by moisture. However, activated carbons have a rather poor performance for capturing CO2 under the ambient pressure. That is why many of the above mentioned materials are carbon-based, but functionalized with other elements, especially nitrogen. A critical feature for materials with potential applications for CO2 capture is their CO2/N2 selectivity, since post combustion gases typically contain 70% N2 and 15% CO2.9 Microporous silicon carbide derived materials are a particular class of materials that exhibit excellent CO2 capture performance, under ambient pressure, and at the same time have high CO2/N2 selectivity.22 Therefore, Si-doped carbon architectures seem to be promising candidates as CO2 sieves. Surprisingly, they are rather unexplored in this context, albeit they have been extensively studied for other purposes.24-29 To our best knowledge, there are four previous investigations on the CO2 sequestering ability of SiC materials.30-33 They all dealt with materials with 50% Si - 50% C composition, and were performed within the 2 ACS Paragon Plus Environment

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density functional theory (DFT) framework. The most important information gathered from these investigations is detailed next. Zhao and Ding30 studied CO2 adsorption on various zigzag, (n,0) with n = 6, 8, 10, 12, and 18, single-walled SiC nanotubes (SWSiCNT). They found that a CO2 molecule can be chemisorbed to the Si-C bonds of SWSiCNT with significant adsorption energy, which decreases as the tube diameter and the CO2 coverage increase. They also found that these tubes have the ability of adsorbing several CO2 molecules, and that there is a substantial charge transfer from the tube to the CO2 molecules. Based on these findings they proposed that SWSiCNT may be used as chemical sensors for detecting CO2. On the other hand, Mahdavifar et al.31 comparatively analyzed the CO2 adsorption on armchair (4,4) SWSiCNT with equivalent aluminum nitride (AlN) and boron nitride (BN) structures. Contrary to what was previously described,30 they reported that the interaction between a CO2 molecule and a SWSiCNT is weak, and corresponds to a physisorption process. Zhang et al.32 investigated the influence of the surface curvature on the adsorption of CO2, on SiC materials including sheets and SWSiCNT of different diameter, i.e., zigzag tubes from (3,0) to (12,0) and armchair tubes from (2,2) to (10,10). They found that while SiC sheets (no curvature) cannot adsorb CO2, SWSiCNT with significant curvature are efficient for that purpose with the adsorption energy lowering as the curvature decreases. On the other hand, Shi and coworkers33 proposed that SiC nanosheets, with or without preadsorbed O2 molecules, can behave as good CO2 adsorbents. They also propose that the most likely configuration of the adsorbed molecule is with its C atom bonded to a C atom in the SiC nanosheet, and one of its O atoms bonded to a Si atom in the nanosheet. This configuration agrees with those reported by Zhao and Ding30 and Zhang et al.32 Therefore, albeit it seems to be a consensus that SWSiCNT might be used as CO2 traps (especially those with significant curvature), there are still some contradictions. For example if the process corresponds to chemisorption or physisorption. This particular feature can become relevant depending on the final purpose of the adsorbent, i.e. for sequestering CO2 in pre-combustion or post-combustion processes.34 In addition, it has been pointed out that while standard DFT modeling of extended carbon-based materials may produce correct results, at the qualitative level, methods accounting for long range 3 ACS Paragon Plus Environment


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electron-electron exchange and correlations are required to properly describe CO2 capture by nanomaterials, at a quantitative level. Some of them are hybrid exchange–correlation DFT, or MP2, methods.34, 35 Accordingly, the above-mentioned disagreements might arise from the calculation method of choice. There are also −at least− two crucial aspects regarding such application that need to be explored. The first one, and probably the most important one, is whether they are selective for CO2 adsorption over N2 adsorption. The second one is if the 50% Si - 50% C composition is required for the SWSiCNT to be efficient for capturing CO2, or if other −lower amounts− of Si are enough to promote such activity This is particularly important since 50-50 has been identified as the maximum Si-C proportion allowed to maintain the SWSiCNT stability.36 Moreover, in the already synthesized SWSiCNT, the amounts of silicon atoms per carbon atoms are rather low.37 Interestingly, in the same work it was also found that higher dopants concentrations make narrower tubes more favored. Thus, the synthesis of SWSiCNT with the necessary curvature for them to be used as CO2 sieves does not seem to be a problem. Accordingly, the main goal of the present work is to investigate the two aspects previously mentioned. To that purpose (5,5) SWSiCNT structures with different Si/C proportion have been investigated using the DFT. Thermochemical and kinetic analyses were performed, and used to identify the most likely structures of the adsorption products, the role of the Si/C ratio on the CO2 capturing capacity of the SWSiCNT, and their CO2/N2 selectivity. In addition, based on the values of the binding energies, it will be proposed what their main application might be, i.e., as pre-combustion or post-combustion CO2 adsorbents. Hopefully, the data provided here for the first time would contribute to increase our knowledge on the potential use of SWSiCNT as CO2 sieves, and motivate future experimental researches on the subject.

Computational Details All the electronic calculations were carried out with the Gaussian 09 package of programs 38

. Geometry optimizations and frequency calculations were performed at the M05-2X/6-

31G(d) level of theory. No symmetry constraints were imposed in the geometry 4 ACS Paragon Plus Environment

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optimizations. The energies of all the stationary points were further improved by single point calculations with the 6-311++G(d,p) basis set and the same functional. Local minima were identified by the absence of imaginary frequencies, and transition states by the presence of a single imaginary frequency. In addition, intrinsic coordinate calculations (IRC) were carried out to verify that the imaginary frequency of the transition states actually corresponds to the expected motion along the reaction coordinate. Relative energies were calculated including thermochemical corrections at 298.15 K. The nanotubes were modeled as finite fragments of (5,5) armchair SWSiCNT, about 11 nm (5 hexagons) long, with the dangling bonds at the ends of the tube fragments saturated with H atoms to avoid unwanted distortions. A pristine carbon nanotube (SWCNT, Figure 1, T0,) with the same dimensions was also included in the investigation, for comparison purposes. SWSiCNT with two Si atoms (2% Si) were investigated using different isomers (T2a to T2e). All of them have both Si atoms in the same hexagon, because it was previously found that placing the Si atoms in different hexagonal rings leads to structures with higher energies.36 The arrangements of the Si atoms in the five T2 structures are: in ortho position, with the Si-Si bond perpendicularly (T2a) or diagonally (T2b) oriented with respect to the tube axis; in meta position (T2c); and in para position with the line connecting the 2 Si perpendicularly (T2d) or diagonally (T2e) oriented with respect to the tube axis. In addition two other models with higher proportion of Si (33% and 50%) were also included in the investigation (T33 and T50, respectively). For the latter we chose an arrangement of alternating Si and C, because it was previously demonstrated that it has lower energy than the alternative, i.e. with Si-Si bonds. 39



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Figure 1. Modeled pristine and Si-doped carbon nanotubes. The ability of the investigated nanotubes to capture CO2, was first analyzed in terms of Gibbs energies of reaction (∆G) for the adsorption processes. However, thermochemical feasibility is not the only relevant information for the present investigation, kinetics is also important. Therefore, for the reactions identified as exergonic the rate constants were also calculated, using the Conventional Transition State Theory (TST).40-42

Results and Discussion There are different arrangements in which the adsorbed CO2 molecule can be oriented on the surface of the investigated nanotubes. First, the binding of CO2 molecule to the tube can involve both O atoms, or an O atom and the C atom. Second, the formed bonds can involve only the Si atoms in the tube, or one Si and one C. Third, the line connecting the two binding sites can be diagonal or perpendicular to the tube axis. Accordingly, six different arrangements were considered for the CO2 adsorbed on the SWSiCNT surface (Figure 2):


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I-

O-Si, O-Si, with the OO line perpendicular to the tube axis

II-

O-Si, O-Si, with the OO line diagonal to the tube axis

III-

O-Si, C-Si, with the OC line perpendicular to the tube axis

IV-

O-Si, C-Si, with the OC line diagonal to the tube axis

V-

O-Si, C-C, with the OC line perpendicular to the tube axis

VI-

O-Si, C-C, with the OC line diagonal to the tube axis

Figure 2. Arrangements of the CO2 molecules on the adsorption sites. However, not all of them are possible for each SWSiCNT model. It depends on the particular location and number of the Si atoms in the C network. In addition, for the pristine SWCNT the I to IV arrangements are the equivalent to those in the SWSiCNT, but with the obvious difference that the atoms on the surface that are involved in the binding are always C. The corresponding binding energies are reported in Table 1. In addition to the above described arrangements, two other possibilities were tested for some selected systems: VII-

C-Si, O-C, with the CO line perpendicular to the tube axis

VIII- C-Si, O-C, with the CO line diagonal to the tube axis 7 ACS Paragon Plus Environment


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However, they were ruled out based on the finding that their energies are systematically higher than those corresponding to any of the I to VI configurations. Moreover, they always correspond to endergonic reactions, thus they are not expected to occur to a significant extent. Table 1. Gibbs free energies of reaction (∆G, kcal/mol) per adsorption site, at 298.15 K, for the reactions of SWSiCNT with one CO2 molecule.

T0 T2a T2b T2c T2d T2e T33 T50

I

II

III

IV

177.03 18.31

130.38

64.52 8.70

62.91

-3.37 30.96 17.25 21.53

-8.56 22.77

14.69

VI

-15.45

12.08 -17.98 -18.59

V

-29.81 -32.03

-10.13 -22.67 -10.48 -2.99

7.60 -17.44 -13.85 -7.02 -0.29

The reactions involving the pristine SWCNT were found to be endergonic, regardless of the bonding and arrangement tested. This indicates that Si doping is crucial for using this kind of nanostructures to sequester CO2 molecules. Different factors were identified to influence the thermochemical viability of the CO2 adsorption on the SWSiCNT surface. They are: (i)

The composition of the SWSiCNT, i.e., the Si/C proportion.

(ii)

The location of the Si dopants,

(iii)

The kind of atoms involved in the CO2 binding to the SWSiCNT surface.

However, regardless of these three features, all the investigated SWSiCNT present −at least− one thermochemical viable pathway in their reactions with CO2. Regarding the composition, probably the most important finding was that the 50% Si proportion is not mandatory for the SWSiCNTs to be able of sequestering CO2. Moreover, from a thermochemical point of view, (5,5) tubes with 2% Si and 33% Si proportion are predicted to be better for that purpose, based on the ∆G values (Table 1). However, it should be noted that the values reported in this table are per adsorption site, while a higher abundance of Si sites in the carbon network is expected to increase the efficiency of SWSiCNTs as CO2 8 ACS Paragon Plus Environment

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absorbents in terms of the number of CO2 molecules that can be sequestered by each tube. Considering both aspects together, SWSiCNTs with 33% Si are proposed as the optimum ones for sequestering CO2, among the tested ones. This is anticipated based on the finding that the adsorption route with the largest exergonicity corresponds to T33 (Table 1), and at the same time this structure might adsorb several equivalents of CO2. Based on the adsorption route with the largest exergonicity for each of the investigated SWSiCNT with 2% Si, the trend in reactivity towards CO2 was found to be T2e > T2d > T2c > T2a > T2b. Therefore, the location of Si pairs that seems to promote the most the CO2 adsorption is in para position, followed by meta and ortho, in that order. Regarding the kind of atoms involved in the CO2 binding to the SWSiCNT surface, the adsorption routes with largest exergonicity were found to correspond to arrangements IV to VI for all the investigated structures. This finding suggests, that at least for armchair SWSiCNTs the most abundant bonding should be O-Si, C-C; and O-Si, C-Si, with the OC line diagonal to the tube axis. However, adsorption products with all of the tested arrangements −except I and III− are expect to exist to some extent. In addition, it has been proposed that the ideal binding energies for pre-combustion and post-combustion gas adsorbents are around 2eV (∼54 kcal) and 0.2 eV (∼5 kcal) per adsorbed molecule, respectively.{Cazorla, 2015 #46} According to the values reported in Table 1, it seems that models T33, and probably, T2e should be better as pre-combustion adsorbents, while T50 and the rest of the SWSiCNT with 2% Si should be better in postcombustion processes. However, this hypothesis remains to be experimentally tested. The possibility of multiple CO2 adsorption by a single SWSiCNT was also investigated for the tubes with higher Si/C proportion, i.e., T33 and T50. To that purpose the ∆G values were calculated for sequential steps using the orientation that led to the most exergonic route for each tube (IV for T33 and V for T50). It was found that adsorption reactions −up to 7 CO2− are all exergonic for both SWSiCNT (Table 2). In addition, the ∆G values of the different adsorption steps were found to be systematically more negative for T33 than for T50. This finding supports the hypothesis that a 50% Si composition is not necessarily the best option for SWSiCNT in the context of CO2 adsorption ability. Albeit many other Si/C



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proportions remain to be tested, tubes with 33% Si seem to be very promising materials for that purpose. Table 2. Gibbs free energies of reaction (kcal/mol), at 298.15 K, for the adsorption of several CO2 molecules. T33-IV

T50-V

1CO2

-32.03

-2.99

2CO2

-47.19

-10.75

3CO2

-25.12

-4.25

4CO2

-22.28

-5.68

5CO2

-29.09

-3.80

6CO2

-18.11

-5.60

7CO2

-28.30

-4.78

To test the CO2/N2 selectivity of the modeled SWSiCNT we first analyzed different possible orientations of the N2 molecule on the surface of the T33 model. This tube was chosen for that purpose based on the above discussed finding that it is the one with the largest exergonicity when reacting with CO2. Because of the structure of T33 and that of the N2 molecule any possible binding necessary involves N atoms, and only three orientations are possible (II, III and IV). It was found that all the associated reactions have significantly positive values of ∆G (Table 3), with orientation II leading to the least endergonic process. Therefore N2 adsorption, if any, is expected to essentially involve N-Si binding. Accordingly, this was the only orientation tested for the other investigated tubes. In all the investigated cases, the SWSiCNT + N2 reactions were found to be endergonic (Table 3). Thus, based on the gathered results, the modeled (5,5) SWSiCNT are predicted to be selective to CO2 over N2. This finding supports the potential application of these materials for CO2 capture. However, contrary to what was found for the CO2 molecule, some of the N2 adsorption routes corresponds to physisorption instead to quimisorption processes. This classification was made based on the interaction distances and the charge transferred. Their values are also provided in Table 3. The charges on the N2 fragment were estimated using the Mulliken population analysis, and the reported interaction distances


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corresponds to the 2 shortest for each system. For the quimisorption reactions the shortest interaction distances were all found to be shorter than 2 Å, and the transferred charge ranges from 0.41 to 0.54. On the contrary, for the physisorption reactions the shortest interaction distances are systematically larger than 3 Å, and the transferred charge is lower than 0.03. Table 3. Gibbs free energies of reaction (kcal/mol), at 298.15 K, for the adsorption of one N2 molecule, interaction distances (d, Å), charge on the N2 fragment (q), and kind of adsorption. ∆G

d

q(N2)

Adsorption

T33-II

21.31

(N,Si)=1.992 (N,Si)=2.019

-0.513

quimisorption

T33-III

54.88

(N,C)=1.560 (N,Si)=1.831

-0.416

quimisorption

T33-IV

56.85

(N,C)=1.558 (N,Si)=1.860

-0.409

quimisorption

T2a-I

73.08

(N,Si)=1.870 (N,Si)=1.871

-0.539

quimisorption

T2b-II

54.03

(N,Si)=1.940 (N,Si)=1.981

-0.544

quimisorption

T2d-I

4.26

(N,Si)=3.506 (N,Si)=3.591

0.020

physisorption

T2e-II

6.19

(N,Si)=3.093 (N,Si)=3.275

0.019

physisorption

T2c-II

4.47

(N,Si)=3.483 (N,Si)=3.525

0.022

physisorption

T50-II

6.61

(N,Si)=3.598 (N,Si)=3.366

0.024

physisorption

However, as mentioned before thermochemical viability might not be enough for a chemical process to be successfully applied. In the particular case of CO2 adsorption by SWSiCNT, it is also important that they take place fast enough. Therefore, a kinetic study has also been performed in this work. Only the reaction routes that were found to be exergonic were included in the kinetic study. This is because endergonic reactions would be 11 ACS Paragon Plus Environment


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reversible to a significant extent, thus the corresponding products are not expected to be experimentally observed. The optimized geometries of the located transition states are provided as Supporting Information, while their imaginary frequencies are reported in Table 4. The Gibbs free energies of activation (∆G≠) were found to range from 10 to 31 kcal/mol (Table 4). The lowest barrier for each SWSiCNT corresponds to the most exergonic reaction route. This indicates that for the investigated systems the Bell–Evans–Polanyi principle is accomplished. Comparing the investigated tubes altogether the lowest barriers correspond to CO2 adsorptions by T2e and T33, which supports the idea that Si amounts significant lower than 50% are better than the maximum Si-C proportion allowed to maintain the SWSiCNT stability. Table 4. Imaginary frequencies of the transition states (νi, cm-1), Gibbs free energies of activation (∆G≠, kcal/mol), rate constants per reaction site (k, M-1 s-1), and rate constants including the reaction path degeneracy (k’, M-1 s-1).The values are reported at 298.15 K.

T2a-V T2b-II T2b-IV T2c-VI T2c-V T2d-V T2e-II T2e-IV T2e-VI T33-II T33-IV T33-V T33-VI T50-V T50-VI

k k' (a) νi ∆G≠ 456.1 13.83 4.48E+02 8.96E+02 333.0 30.11 5.21E-10 5.21E-10 383.0 14.41 1.69E+02 1.69E+02 467.4 12.96 1.97E+03 3.94E+03 420.3 15.40 3.17E+01 6.35E+01 471.1 13.48 8.12E+02 1.62E+03 538.3 27.83 2.48E-08 2.48E-08 222.0 10.06 2.63E+05 2.63E+05 454.7 14.06 3.05E+02 6.10E+02 597.9 31.22 8.07E-11 8.07E-10 352.7 11.90 1.18E+04 1.18E+05 487.1 17.88 4.89E-01 9.78E+00 438.8 23.51 3.61E-05 7.22E-04 403.3 14.57 1.30E+02 5.22E+03 376.5 15.87 1.44E+01 5.77E+02 (a) k’ = σ k, with σ = reaction path degeneracy


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To quantify how fast the SWSiCNT + CO2 reactions are, rate constants were also estimated (Table 4). They were calculated per reaction site (k), and also including the reaction path degeneracy (k’). While the first one is a measure of the site reactivity, the latter is expected to account also for the abundance of Si sites in the tubes network. For all the tested SWSiCNT models there is at least one reaction route with rate constant per reaction site larger than 102 M-1 s-1. The fastest adsorption pathways were identified to be T2e-IV and T33-IV, in that order (k = 2.63 × 105 and 1.18× 104, respectively). However, when considering also the multiple reaction sites, the corresponding rate constants become very similar (k’ = 2.63 × 105 and 1.18× 105, respectively). These values suggest that the target reactions are fast enough to assure efficient CO2 capture at room temperature. Considering the gathered data in this work, altogether, it is proposed that armchair SWSiCNT with 33% Si are very promising agents for efficiently sequestering CO2. This proposal is made based on three findings. These tubes present high CO2/N2 selectivity, their reactions with CO2 are spontaneous and fast at room temperature, and are able to capture multiple CO2 molecules.

Conclusions Several (5,5) SWSiCNT models (with 2%, 33% and 50% Si) were used to investigate the potential ability of single-walled SiC nanotubes as CO2 scavengers. It was found that while the reactions between CO2 and pristine C tubes are endergonic, Si doped materials have exergonic adsorption routes. Rate constants for the SWSiCNT + CO2 reactions were also estimated, and their values suggest that the target reactions are fast enough to assure efficient CO2 capture at room temperature. It was found that 50% Si proportion is not needed for the SWSiCNTs to be able of sequestering CO2. This is particularly important since 50-50 has been identified as the maximum Si-C proportion allowed to maintain the SWSiCNT stability. The CO2/N2 selectivity was also investigated, and all the tested SWSiCNT are predicted to be selective to CO2 over N2. This is a critical feature for materials with potential



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applications for CO2 capture because of the composition of post combustion gases (∼70% N2). For the SWSiCNT with 33% Si, the possibility of multiple CO2 adsorption was investigated, up to 7 CO2 molecules. It was found that all the consecutive reactions are significantly exergonic, which indicates that one SWSiCNT is able of sequestering several CO2 equivalents. Considering the gathered data, altogether, it is proposed that armchair SWSiCNT with 2% to 33% Si are very promising agents for efficiently sequestering CO2. The main findings supporting this proposal are: (i) they present high CO2/N2 selectivity, (ii) their reactions with CO2 are spontaneous and fast at room temperature, and (iii) they are able to capture multiple CO2 molecules. Supporting Information. Optimized geometries of the transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements We gratefully acknowledge the Laboratorio de Visualización y Cómputo Paralelo at Universidad Autónoma Metropolitana-Iztapalapa for computing time.

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Silicon-Doped Carbon Nanotubes: Promising CO2 ... - ACS Publications

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