Toward Understanding the Kinetics of CO2 Capture on Sodium

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Energy, Environmental, and Catalysis Applications 2

Toward Understanding the Kinetics of CO Capture on Sodium Carbonate Tianyi Cai, J. Karl Johnson, Ye Wu, and Xiaoping Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20000 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Materials & Interfaces

Toward Understanding the Kinetics of CO2 Capture on Sodium Carbonate Tianyi Cai, †,δ J. Karl Johnson, *,δ Ye Wu, ‡,§ and Xiaoping Chen, *,†

†Key

Laboratory of Energy Thermal Conversion and Control of Ministry of Education,

School of Energy & Environment, Southeast University, Nanjing 210096, People’s Republic of China

δDepartment

of Chemical and Petroleum Engineering, University of Pittsburgh,

Pittsburgh, Pennsylvania 15261, United States

‡MIIT

Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and

Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

§ Advanced

Combustion Laboratory, School of Energy and Power Engineering, Nanjing

University of Science and Technology, Nanjing 210094, People’s Republic of China

ACS Paragon Plus Environment

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*E-mail:

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[email protected] (J.K.J.), *E-mail: [email protected] (X.C.)

ABSTRACT: Sodium carbonate (Na2CO3) has been widely studied as a promising candidate for CO2 capture from humid flue gas because of its low cost, high abundance, reusability, and moderate operating temperatures. However, the slow kinetics of CO2 capture on unmodified Na2CO3 make it an unacceptable choice for practical applications. If the reaction kinetics could be dramatically improved, then Na2CO3 could be a viable material for large-scale carbon capture applications. The first step to systematic improvement of kinetics is to understand the rate-limiting steps in the uncatalyzed system. We have therefore investigated the structural, mechanistic, and energetic properties of CO2 capture on the (001) and (-402) surfaces of Na2CO3 using density functional theory in order to identify the origin of the slow kinetics observed in experiments. We have


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identified reaction pathways for co-adsorbed CO2 and H2O that lead to bicarbonate formation on the (001) and (-402) surfaces having activation energies of 0.40 and 0.34 eV, respectively. We modeled surface carbonation reactions under conditions of high surface loading of water by performing ab initio molecular dynamics simulations at typical operating temperatures. Multiple reactions were observed on picosecond time scales. Our results indicate that the Na2CO3 carbonation reaction is not controlled by the kinetics of the reaction at the surface, but is likely controlled by diffusion limitations. We propose two possible scenarios that could result in diffusion control of the reaction rate.

KEYWORDS: CO2 capture, sodium carbonate, surface carbonation barrier, density functional theory, ab initio molecular dynamics

INTRODUCTION

Carbon dioxide (CO2) has been identified as contributing to global warming.1 The Paris Agreement resolved that countries should pursue efforts to hold global warming to an upper limit of 1.5 °C.2 Despite the increase in the use of renewable energy sources, such as wind power,3 solar energy4 and biomass derived energy,5 carbon emissions grew by


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about 1.5% in 2017 after staying relatively flat from 2014 to 2016.6 Currently, more than 40% of the global energy-related CO2 emissions are attributable to emissions from electricity and heat production.7 The International Energy Agency predicts that fossil fuels will still account for 60% of electricity generation under the current policy scenario by 2040.8 Therefore, a short-term approach to aid in the control of CO2 levels should involve various CO2 reduction technologies for fossil-fueled power plants, including precombustion CO2 capture,9 O2/CO2 combustion,10 chemical looping combustion11 and post-combustion capture.12 Currently, amine scrubbing technology for CO2 separation is considered the dominant technology for large-scale post-combustion CO2 capture13 and monoethanolamine (MEA) is the favored solvent due to its commercial availability, fast absorption rate and prior use in industrial applications.14 However, the cost of electricity would be increased by 70100%15 with the implementation of MEA technology because of its high capital cost and parasitic energy consumption.16 In contrast, solid sorbents can greatly reduce the parasitic energy consumption by avoiding the heating large amounts of solvent.17 To illustrate the magnitude of the problem, a 1000 MW ultrasupercritical pulverized coal


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combustion plant can release 776.5 tons of CO2 per hour.18 The high flowrate of CO2 and short contact times require very large quantities of sorbents, making exotic and expensive materials unfavorable for these kinds of large-scale applications. In contrast, Na2CO3 is deemed to be both cost effective and energy efficient in capturing CO2 from humid flue gas19-20 because of its low cost, high abundance, and moderate operating temperatures (50-70 °C for sorption and 110-130 °C for desorption).21-22 The main reversible reaction for CO2 capture and regeneration cycles is: Na2CO3(s) + CO2(g) + H2O(g)↔2NaHCO3(g)

(R1)

Duan et al.23 used first-principles density functional theory (DFT) combined with phonon density of states calculations to investigate the phase diagrams of M-C-O-H (M=Li, Na, K) systems and identified Na2CO3 as having good thermodynamics for CO2 capture under post-combustion conditions. However, the kinetics of CO2 capture on unmodified Na2CO3 is too slow to be practical in industrial use.24 Therefore, the key to developing this technology is to improve the kinetics and the crucial point to improving the kinetics is identifying the rate limiting steps. We seek to provide a first principles approach toward a


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complete picture of the kinetics of CO2 capture on Na2CO3; this paper is the first step in this process. To the best of our knowledge, no atomic-level mechanistic studies have been performed for CO2 capture on Na2CO3, however, some work has done on carbon capture with K2CO3. Gao et al.25 investigated the adsorption behavior of single CO2 and H2O molecules on the K2CO3(001) surface and reported a formation barrier of surface bicarbonate species of about 0.30 eV. Liu et al.26 further expanded the investigation of surface carbonation processes on K2CO3 by considering various low-index facets. They found bicarbonate formation barriers varying from 0.19 eV to 0.50 eV. They also confirmed a two-step mechanism via H2O dissociation followed by the OH group reacting with a gas-phase molecule of CO2 on the hexagonal K2CO3(010) surface. These previous studies provided key insights into the mechanism of carbonation reaction on K2CO3 surfaces from the atomic level. However, there are two improvements that can easily be made to the modeling discussed above. First, high-index facets could be taken into consideration because they may be much more chemically active, thereby dominating the kinetics of reaction, even though they comprise a minority of the surface.27 Second, the


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surface carbonation reaction very likely takes place under conditions of high surface loading of water. We here employ these improvements for studying the carbonation reaction on Na2CO3 surfaces. The goal of this work is to determine if the kinetics of CO2 capture on Na2CO3 is controlled by the surface carbonation reaction. As part of this work, we identified multiple low-energy structures for bulk Na2CO3 and determined that the equilibrium shape of Na2CO3 crystals consists of four low-index and five high-index facets. Surface carbonation reactions on two typical facets, including one low-index facet, Na2CO3(001), and one high-index facet, Na2CO3(-402), were investigated. The equilibrium water coverage on both Na2CO3(001) and (-402) were calculated and ab initio molecular dynamics (AIMD) simulations were performed to simulate surface carbonation reaction under conditions of high surface loading of water. Finally, we propose two possible scenarios for diffusion processes in Na2CO3 carbonation reactions. METHODS


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DFT calculations. This study utilized DFT calculations to evaluate the adsorption of CO2 and H2O on the periodic Na2CO3 slab models. Calculations were performed using the Vienna Ab Initio Simulation Package (VASP).28-31 The generalized gradient approximation of Perdew, Burke and Ernzerhof32 (PBE) was applied to represent the exchange– correlation energy. The Projector augmented wave pseudopotentials, with a 500 eV energy cutoff, were used to represent the electron–ion-core interactions.32-33 The electron occupancies were determined according to Fermi scheme with an energy smearing of 0.05 eV. The bulk and surface Brillouin zones were sampled in a 3×5×4 and 2×4×1 Monkhorst-Pack k-point meshes,34 respectively. The DFT-TS approach with iterative Hirshfeld partitioning correction35 was applied to describe the structure and the energetics of ionic solids and to handle dispersion interactions in our systems. Dispersion interactions are important for accurate description of the physisorbed states of CO2 and H2O on the carbonate surfaces. Geometries were optimized until the energies and forces were converged to 1.0×10−5 eV/atom and 0.02 eV/Å, respectively. The accuracy of the computational settings have been tested and reported in Figure S1 of the Supporting Information.


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The transition state (TS) structures and the reaction pathways were located using the climbing image nudged elastic band (CI-NEB) method.36 The force tolerance for the TS calculations was 0.02 eV/Å. Fully relaxed seven-layer (1×1) slab models containing 28 Na2CO3 formula units were used to calculate surface energies and the Wulffman program37 was used to compute the theoretical equilibrium shape (Wulff shape) of Na2CO3. Three-layer (1×2) slab models with the bottom layer fixed were used to compute adsorption energies of CO2 and H2O, and also for the CI-NEB calculations. A vacuum region of 15 Å was added above these slab models to ensure negligible interaction between periodic replicas. The surface energy (𝛾surf) was calculated from 𝛾surf =

𝐸slab ― 𝑁𝐸bulk

(1)

2𝐴

where 𝐸slab is the energy of slab model, 𝐸bulk is the energy of a portion of bulk crystal containing the same number of atoms in each slab layer, 𝑁 is the number of layers and 𝐴 is the surface area. The surface energies calculated by the method of Boettger38 and


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the method of Fiorentini and Methfessel39 are reported and discussed in the Supporting information. The adsorption energy (𝐸ads), co-adsorption energy (𝐸co ― ads), and interaction energy (𝐸inter) were determined from the following equations: 𝐸ads = 𝐸𝐴/surf ― (𝐸𝐴 + 𝐸surf)

(2)

𝐸co ― ads = 𝐸𝐴/𝐵/surf ― (𝐸𝐴 + 𝐸𝐵 + 𝐸surf)

(3)

𝐸inter = 𝐸𝐴/𝐵/surf + 𝐸surf ― (𝐸𝐴/surf + 𝐸𝐵/surf)

(4)

where 𝐸𝐴/surf is the energy of the surface with one adsorbate and 𝐸𝐴/𝐵/surf is the energy of the surface with two adsorbates. 𝐸𝐴 or 𝐸𝐵 are the energies of the isolated CO2 or H2O molecules. AIMD calculations. Born−Oppenheimer AIMD simulations were performed with the CP2K software package,40 using the QUICKSTEP method41 with a time step of 0.5 fs. The

PBE

generalized

gradient

functional32

with

Goedecker-Teter-Hutter

pseudopotentials42-43 were used. Grimme’s D3 dispersion correction44 was included as well. The periodic systems containing a three-layer (2×2) full-relaxed slab with multiple


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water and CO2 molecules at a ratio of 1:1 above the surfaces and 15 Å of vacuum at the top were used to perform isothermal-isochoric ensemble simulations of about 20 ps at 323 K. Characterization & materials. The X-ray diffraction (XRD) measurements were carried out on a Rigaku Smartlab X-ray diffractometer using a Cu Kα (λ = 1.5406 Å) radiation at 40 kV and 30 mA and operated in a continuous scan mode. The X-ray diffraction was recorded in a range of 5-90° at a scan rate of 8°/min with a step of 0.02°. The simulations of powder diffraction patterns were performed using Mercury 3.10.3.45 Analytical reagent Na2CO3 was obtained from Shanghai Jiuyi Fine Chemical Co., Ltd. Before measurements, Na2CO3 powders were pre-exhausted at 473 K for 2 hours and ground to 300 mesh (48 µm in diameter). RESULTS AND DISCUSSION

Na2CO3 bulk structure revision. Accurate calculations of surface energies, adsorption energies, and reaction barriers requires accurate relaxed structures for the bulk material. Bulk Na2CO3 belongs to the C2/m(12) space group. We initially optimized the atomic


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positions and lattice constants starting from the reported experimental structure.46 The resulting relaxed structure, identified as Str. 0, is shown in Figure 1a. The optimized lattice parameters and experimental values46 are listed in Table 1 and all the relative errors are within 1.3%. Table 1. The experimental and optimized lattice parameters for Na2CO3 from two different initial structures along with percent errors in the lattice constants and angels

a (Å)

b (Å)

c (Å)

Exp.*

8.90

5.24

6.04

Str. 0

8.83

5.29

5.99

δ%

-0.79

0.92

-0.88

8.88

5.26

6.00

-0.24

0.32

-0.65

Str. 17 δ%

β (°)

V (Å3)

101.3

276.3

5

4

102.6

272.9

1

7

1.24

-1.22

102.1

273.9

1

9

0.75

-0.85

*Experimental values from Ref.46 However, the simulated XRD patterns of Str. 0, shown in Figure 1c-2, exhibits some significant differences from our experimental results, seen in Figure 1c-1. First, it overestimates the intensities of the (310) and (112) peaks. Second, the (-112) peak is


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missing, but a spurious peak (see Peak 1 in Figure 1c-2) located at 𝜃 = 33.44° is generated. Third, the (202), (-401) and (-221) peaks appear at wrong diffraction angles. These differences indicate that the optimized Na2CO3 bulk structure may be further revised to better represent the real material. We generated 22 additional candidate starting structures by either perturbing the locations of one or more Na ions or rotating a number of carbonate ions (details of the method for generating structures are given in Table S1 of the Supporting Information). We performed geometry optimizations for the 22 candidate structures (Str. 1 to Str. 22). The relative tolerance of optimized lattice parameters and bulk energies are listed in Table S2 of the Supporting Information. The 22 revised bulk structures are divided into three groups according to their relative energy differences (see Figure S2 of the Supporting Information). Group 1 contains structures that relax back to the same geometry as Str. 0 and consists of two structures obtained from perturbing the Na ion positions. The other two groups were started from configurations with the carbonates reoriented. The optimized structures from these groups are all energetically more favorable than Str. 0. Group 2 consists of 12 structures with energies lower than Str. 0 by about 5 to 7 meV per formula unit. Group 3 structures


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are lower in energy than Str. 0 by about 11 to 13 meV per formula unit. The existence of a number of different structures that are nearly isoenergetic means that the bulk material likely consists of domains having different orientations of the carbonate ions at room temperature and above. We generated the average XRD pattern of the structures in Group 3 and plotted this average pattern in Figure 1c-3. We note that some aspects of the averaged XRD pattern match the experimental data better than the experimentally determined structure, Str. 0. This strengthens our hypothesis that Na2CO3 has many different polymorphs, defined by slightly different orientations of the CO32- groups. Among Group 3, Str. 17 (see Figure 1b) was chosen for further calculations for the following reasons: (1) it correctly predicts the (020) and (-112) peaks, as shown in Figure 1c-4 and (2) it describes the (202), (-401) and (-221) peaks better than Str. 0 in both diffraction angles and peak intensities. Moreover, Str. 17 is in better agreement with the experimental patterns than Str. 0 in describing three major diffraction peaks, including (002), (310) and (112), as listed in Table S3 of the Supporting Information. However, our revised structure is not a perfect match for the experimental XRD pattern. Specifically,


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the spurious Peak 1 is still observed in Str. 17 (see Figure 1c-4) and the (112) peak is split into two peaks.

Figure 1. Na2CO3 bulk structure and XRD patterns. Bulk structure of Str. 0 (a) and Str. 17 (b). XRD patterns of experimental data (c-1), Str. 0 (c-2), the average of all structures in Group 3 (c-3) and Str. 17 (c-4). Red, purple and grey spheres represent oxygen, sodium and carbon, respectively.

Na2CO3 Wulff Construction. We identified a total of fourteen facets to include in Wulff construction calculations: (001), (100), (101), (011), (110), (111), (010), (112), (310), (-


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222), (-112), (-402), (201) and (-221). These were selected because they were observed in experimental XRD intensities (see Figure. S3 of the Supporting Information). All of these surfaces can have multiple surface terminations. To the best of our knowledge, the relative energetics of various surface terminations of Na2CO3 have not been reported, either from experimental or theoretical studies. The terminations of each of the facets listed above were examined (up to 12 terminations per facet) and these terminations were all stoichiometric. We computed the surface energies and the relative probabilities of observing each termination based on a Boltzmann distribution (see the Supporting Information). The surface energies and probabilities are reported in Table S4 of the Supporting Information. We found that in each case the termination with the lowest surface energy dominated the probability distribution and therefore the lowest energy termination was selected as the exposed surface in our calculations (see Figure S4 of the Supporting Information). The surface energies calculated from Eq. (1) as a function of the number of layers are shown in Figure 2 for each of the surfaces studied. The surface energies all converge after seven layers for both low-index (see Figure 2a) and high-index (see Figure 2b) facets. The values of the surface energies (listed in Table S5 of the


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Supporting Information) for all seven-layer slab models were used to perform Wulff construction calculations. The stability order (lowest to highest surface energies) of the relaxed surfaces is (101) > (112) > (001) > (010) > (201) > (111) > (-112) > (011) > (100) > (310) > (-222) > (-402) > (-221) > (110). Surprisingly, some high-index facets possess relatively low surface energies, indicating that these high-index surfaces comprise a significant proportion of the exposed surface area of the particles. We also computed surface energies using the methods of Boettger38 and Fiorentini and Methfessel,39 as discussed in the Supporting Information and shown in Figure S5 of the Supporting Information.

Figure 2. Surface energies of (a) low-index and (b) high-index facets.


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The relative surface areas for the different facets as computed from Wulff construction are reported in Figure 3a and the corresponding crystal shape is displayed in Figure 3b. The simulated Na2CO3 crystal is unfolded onto a 2D map to aid in visualizing the results and is shown in Figure 3c. Five facets, (110), (011), (111), (201) and (-221), do not appear in the final Wulff shape. The remaining low-index facets account for about 64.5% of total surface area, with 23.0% from (010), 16.2% from (001), 15.2% from (101) and 10.2% from (100). High-index surfaces contribute the remaining 35.5% of the total surface area, with 11.1% from (112), 9.7% from (310), 7.5% from (-222), 5.2% from (-402) and 1.9% from (112). This suggests that high-index surfaces should be included in simulating the carbonation reaction on the surface of Na2CO3.


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Figure 3. (a) The equilibrium shape of Na2CO3 from Wulff construction. (b) The proportion of various facets in the total surface area. (c) A 2D projection map of the Na2CO3 particle surfaces.

Screening CO2 adsorption on various facets of Na2CO3. The CO2 adsorption energy may be a key parameter in determining the carbonation reaction rate, according to Gao et al.25 They claimed that the lower adsorption energy of CO2 relative to H2O on the K2CO3 surface likely limits the rate of bicarbonate formation on dry potassium carbonate. We have therefore computed the adsorption energies of a single CO2 molecule on each of the nine facets of Na2CO3 existing in our Wulff construction model. This screening was


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carried out on a small (1×1) surface, mimicking half monolayer coverage of CO2. In accord with Gao et al.25 and Liu et al.26 for K2CO3, we found that CO2 prefers to adsorb on the surface oxygen sites with C-O bond distance varying from 2.5 Å to 2.8 Å. For all but one of the surfaces, the CO2 adsorption energy varies from 0.24 eV to 0.44 eV and no significant change of the O-C-O bond angle is observed (less than 10°), indicating that CO2 is physically adsorbed on the surfaces of Na2CO3. However, CO2 shows strong affinity towards the (-402) facet with an adsorption energy over 0.7 eV, a O-C-O bond angle of 134° and C-O bond lengths of 1.24 Å (relative to 1.17 Å in the gas phase). This indicates strong chemical interaction for CO2 on the (-402) facet. We therefore investigated reactions on the (-402) and (001) facets as representative surfaces. We selected the (001) facet because the adsorption energy of CO2 on Na2CO3(001) is higher than on any other low-index surface. The detailed geometric and energetic information of all configurations considered are summarized in Table S6 of the Supporting Information. Coverage effects and co-adsorption of H2O and CO2 on Na2CO3(001) and (-402) facets. Coverage effects are known to affect the adsorption energy and geometry of gas phase molecule on solid surface.47-48 Therefore, three-layer (1×2) slab models were used to


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investigate CO2 coverage effects (quarter monolayer) on (001) and (-402) facets. To be consistent, H2O single adsorption and co-adsorption of H2O and CO2 were carried out on (1×2) models as well. We identified several different local minimum energy adsorption configurations for CO2 on Na2CO3(001) and (-402). Only the configurations with the strongest adsorption energies for each facet are shown in Figure 4a and Figure 4c while the corresponding optimized structural parameters and adsorption energies calculated from Eq. (2) are given in Table 2. The geometric and energetic information of other configurations are listed in Figure S6 and Table S7 of the Supporting Information. Table 2. Adsorption structures and energies (Eads, eV) for CO2 and H2O adsorbed on Na2CO3(001) and (-402) facets using (1×2) slab models

Facet

Adsorptio

Eads

n species

(eV)

CO2 (001) H 2O

0.58 0.65

dC-Osa dH-

Bond distance in

Bond angle of

b Os

adsorbate

adsorbate

(Å)

(Å)

(°)

1.63

1.22/1.23

138.06

1.88/1.88

0.99/0.99

98.98


21


CO2 (-402) H 2O ad

C-Os

0.61 1.08

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1.58

1.23/1.24

137.31

1.68/1.76

1.00/1.00

102.36

is the distance between the C atom of CO2 and the closest O atom of the surface,

dH-Os is the distance between an H atom of H2O the closest and O atom of the surface.

Figure 4. Selected configurations of CO2 (a1, a2, c1, c2) and H2O (b1, b2, d1, d2) adsorption on Na2CO3(001) and (-402) facets. The left column is the top view and the right column is a close-up of the side view. Red, purple and grey spheres represent oxygen, sodium and carbon atoms, respectively. Light blue, pink and white spheres


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represent carbon, oxygen and hydrogen atoms, respectively, of the adsorbates. To illustrate the configurations more clearly, only the outermost atoms are shown.

Surprisingly, CO2 chemisorbs on Na2CO3(001) at quarter monolayer coverage, which is not observed in the (1×1) half monolayer coverage calculations. This means that CO2 can only chemisorb on the (001) facet when the surface coverage of CO2 is relative low. The adsorption energy is 0.58 eV, which is very close to the value on the Na2CO3(-402) (1×2) surface (0.61 eV in Table 2). The geometric parameters of the chemisorbed CO2 are similar as well. Chemisorption of CO2 on Na2CO3 surfaces is likely influenced by the surface morphology. Both the (001) and (-402) facets have Na atoms protruding from the surface, with carbonate groups situated lower on the surface. This morphology provides a cavity-like feature where CO2 can adsorb, forming bonds with the Na+ ions and an oxygen from the CO32- group directly underneath, resulting in a bent CO2 structure that is part of a C2O52- moiety, (see in Figure 4 a2 and c2). Our calculations have indicate a low barrier for CO2 chemisorption on these sites (< 0.1 eV), with the transition state occurring around 2.1 to 2.3 Å away from the surface.


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More than 20 initial H2O adsorption configurations (including the H2O orientational degrees of freedom) were identified on Na2CO3(001) and (-402) facets. These configurations can be generally divided into two categories: (1) H2O having one hydrogen interacting with a surface oxygen atom and (2) H2O having both hydrogen atoms pointing towards surface oxygen atoms. We display the configurations that are most relevant for bicarbonate formation in Figure 4 b1, b2, d1 and d2. The corresponding optimized structural parameters and adsorption energies are listed in Table 2. The geometric and energetic information of other configurations can be found in Figure S6 and Table S7 of the Supporting Information. As shown in Figure 4 b1 and d1, the adsorbed H2O on Na2CO3(001) and (-402) facets both form bidentate binding structures with surface oxygen atoms. However, the binding energy of H2O on the (001) is 0.43 eV less favorable than on the (-402) facet. The smaller binding energy on the (001) surface is mainly due to H2O strain energy, with a smaller component being due to the 0.2 Å larger distance between H2O and the (001) surface compared with the (-402) surface. The surface oxygen atoms to which H2O is hydrogen bonded are about 1.2 Å closer together on the (001) surface than on the (-402) surface. This induces


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the H-O-H bond angle to decrease from the gas phase value of 104.5° to 98.98° (see Table 2), resulting in strain energy of the adsorbed H2O molecule. In contrast, the water bond angle on the (-402) surface is 102.36°, which is only slightly smaller than the gas phase value. Both CO2 and H2O bind to surface O atoms, indicating competitive adsorption, as reported for the K2CO3(001) surface.25-26 We have computed co-adsorption and interaction energies from Eq (3) and Eq (4), respectively. The energies and key structural quantities for co-adsorption are reported in Table 3, with the configurations shown in Figure 5 as the initial states of the surface carbonation reaction. CO2 molecules in these co-adsorption cases are no longer chemisorbed, as seen from their roughly linear structure and larger dC-Os values (distance between the C atom of CO2 and the closest O atom in the surface) compared with Table 2. In contrast, H2O is strongly adsorbed on both facets, as seen from smaller dH-Os in Table 3 compared with Table 2. The interaction energies (Table 3) are only slightly positive, which is unexpected given that CO2 appears to be only physisorbed in the co-adsorption case. Table 3. Co-adsorption of CO2 and H2O on Na2CO3(001) and (-402)


25


EcoFacet

ads

(eV)

Einter

dC-Osa

dH-Osb

dC-Oc

(eV)

(Å)

(Å)

(Å)

Page 26 of 40

CO2 bond

H2O bond

angle

angle

(°)

(°)

(001)

-1.13

0.03

2.97

1.75/1.86

2.57

172.35

98.94

(-402)

-1.48

0.14

3.07

1.66/1.72

2.92

176.34

102.54

ad bd cd

C-Os

H-Os

C-O

is the distance between the C atom of CO2 and the closest O atom of the surface,

is the distance between an H atom of H2O and the closest O atom of the surface,

is the distance between the C atom of CO2 and the O atom of H2O.

Bicarbonate formation on Na2CO3(001) and (-402) facets. We considered a one-step mechanism for bicarbonate formation involving OH transfer from H2O to the C atom of CO2. The potential energy diagram for bicarbonate formation on Na2CO3(001) and (-402) are plotted in Figure 5, along with the optimized geometries of the initial state (IS), transition state (TS) and final state (FS). The structural parameters for the TS and FS are listed in Table S8 of the Supporting Information. Along the reaction coordinate, CO2 continuously approaches H2O. The bond angle in CO2 decreases from around 170° to about 130° with an enlarged bond length from 1.18 Å to around 2.26 Å. Simultaneously, one OH bond in H2O elongates and ultimately H2O splits, donating a proton to form a HCO3- group with the surface CO32- group. The resulting OH group forms another HCO3-


26

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group with the coming CO2. The reaction barriers for bicarbonate formation on Na2CO3 (001) and (-402) facets are 0.39 eV and 0.34 eV, respectively (without zero-point energy corrections). These values are comparable with the barriers for bicarbonate formation on K2CO3 surfaces obtained by Gao et al.25 (0.30 eV ) and Liu et al.26 (0.19 eV to 0.50 eV). Our calculations therefore indicate that bicarbonate formation should be about equally facile on the Na2CO3 and K2CO3 surfaces. This finding is inconsistent with experimental observations that carbonation of Na2CO3 is kinetically much slower than for K2CO3.24, 49

Figure 5. Potential energy diagram for bicarbonate formation on Na2CO3(001) and (-402). Atom colors are defined in Figure 4.


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Surface carbonation reaction under conditions of high surface loading of water. Real flue gas from fossil-fueled power plants consists of 8-12% CO2 and 8-10% H2O50 and the reported operating temperature for Na2CO3-based sorbents ranges from 45 to 70 °C (318 to 343 K).21, 51-52 We have estimated the equilibrium coverage of H2O on Na2CO3(001) and (-402) by computing the change in Gibbs free energy on adsorbing n water molecules, ∆𝐺(𝑃, 𝑇, 𝑛H2O), for various values of n. Details of the calculation method are given in the Supporting Information. The free energies as a function of temperature and fractional water coverage are plotted in Figure 6a and 6b for the Na2CO3(001) and (-402) facets, respectively, at a partial pressure of H2O of 0.1 bar. The estimated fractional coverages over the operating temperature range of 318 to 343 K are θ = 1 and 1.3, for (001) and (-402) facets, respectively. Hence, we deduce that water coverage is very high under typical operating conditions.


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Figure 6. Gibbs free energy of water adsorption as a function of temperature at a partial pressure of H2O of 0.1 bar on Na2CO3(001) (a) and (-402) (b).

We modeled the impact of high coverages of H2O on the surfaces of Na2CO3 by carrying out AIMD simulations with 21 CO2 and 21 H2O molecules in the gas phase (vacuum) above (001) and 32 CO2 and 32 H2O on (-402). These simulations resulted in CO2 capture reactions occurring on the surfaces after only a few ps of simulation time. We observed multiple reaction events (movies are provided in the Supporting Information), some following the basic mechanism shown in Figure 5, as seen in Figure 7a. We also identified an additional pathway, shown in Figure 7b, which involves a proton shuttling event on Na2CO3(-402). In this case, two H2O molecules are involved in the carbonation reaction.


29


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The mechanism is shown in a series of snapshots from the simulation in Figure 7b, where the H atoms on two H2O molecules involved in the reaction are colored white and green. These two molecules adsorb on two adjacent surface O atoms, followed by CO2 attacking the H2O shown with green colored H atoms to form a bicarbonate group. Subsequently, the bicarbonate group exchanges a proton with the adjacently adsorbed H2O molecule, as seen in Figure 7b. All these steps happen within 1 ps of simulation time, indicating that surface carbonation reactions are very facile. This means that the initial reaction rate is kinetically very fast at reaction conditions, in contrast to expectations based on the overall apparent reaction rate observed in experiments.


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Figure 7. AIMD simulation snapshots showing the possible pathways of surface carbonation reaction on (a) Na2CO3(001) and (b) (-402). Only atoms of interest are shown in color for clarity and nonreacting molecules are shown as stick models. Atom colors are defined in Figure 4 except that green represents H atoms from a second water molecule.

Hypothesis on the rate-limiting step of Na2CO3 carbonation reaction. As we have discussed above, the initial steps of bicarbonate formation on the Na2CO3 surface is both thermodynamically favored and kinetically facile. We therefore conclude that carbonation of the Na2CO3 surface is not the rate limiting step; we assume that the experimentally observed slow kinetics is due to diffusion limitations that develop as a relatively thick layer of NaHCO3 grows on top of Na2CO3. We have identified two possible rate-limiting mechanisms associated with diffusion of reactants. One option is that the carbonation reaction happens at the interface between NaHCO3 and Na2CO3, with the CO2 and H2O reactants diffusing through NaHCO3 shell as represented schematically in Figure 8a. Another possibility is that the carbonation reaction occurs on the NaHCO3 surface, facilitated by diffusion of Na+ from the Na2CO3 core accompanied by counter diffusion of


31


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H+ from the NaHCO3 surface to the interface between NaHCO3 and Na2CO3, as illustrated in Figure 8b. These possibilities will be explored in a future publication.

Figure 8. Two possible mechanisms for diffusion control of the Na2CO3 carbonation reaction.

CONCLUSION

We have shown that the surface carbonation reaction on Na2CO3 is facile through DFT calculations of reaction barriers and AIMD simulations. The barriers are similar to those reported for carbonation of K2CO3, which was an unexpected outcome, because it is known experimentally that the apparent reaction rate is much slower on Na2CO3 than K2CO3. Our results strongly indicate that the kinetics of CO2 capture on Na2CO3 is not controlled by the surface carbonation process. It is reasonable to assume that the reaction


32

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diffusion controlled under reaction conditions. We proposed two possible scenarios that could result in diffusion control of the Na2CO3 carbonation reactions. We are in the process of investigating these scenarios through calculation of vacancy formation energies and diffusion barriers. We point out that the kinetic results predicted in this paper could be tested experimentally by measuring the initial carbonation rates on clean Na2CO3 surfaces and perhaps also by comparing initial carbonations rates on Na2CO3 and K2CO3 surfaces.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org/. Computational details, revised bulk structures, Wulff shape information, lowest energy surface terminations, surface energies, CO2 adsorption energies, CO2 and H2O adsorption configurations and energies, structural parameters for transition and final states, details of water coverage calculations, reaction pathway movies. AUTHOR INFORMATION


33


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Corresponding Authors *E-mail

for J.K.J.: [email protected]

*E-mail

for Xiaoping Chen: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

This work was supported by National Key R&D Program of China (2017YFB0603300), the Natural Science Foundation of China (51476030, 51506095), the Jiangsu Graduate student scientific research innovation projects (KYCX17_0077), and the China Scholarship Council. We thank Abhishek Bagusetty for help with the CI-NEB calculations, and Lin Li and Minh Nguyen Vo for fruitful discussion on gas-solid surface reactions. Calculations were carried out at the Center for Research Computing at the University of Pittsburgh and on resources provided by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562.


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Toward Understanding the Kinetics of CO2 Capture on Sodium

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