Ab Initio Molecular Dynamics Study of Carbonation and Hydrolysis

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ab Initio Molecular Dynamics Study of Carbonation and Hydrolysis Reactions on Cleaved Quartz (001) Surface Jihui Jia, Yunfeng Liang, Takeshi Tsuji, Caetano Rodrigues Miranda, Yoshihiro Masuda, and Toshifumi Matsuoka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12089 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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The Journal of Physical Chemistry

Ab Initio Molecular Dynamics Study of Carbonation and Hydrolysis Reactions on Cleaved Quartz (001) Surface Jihui Jia,1,2 Yunfeng Liang,3* Takeshi Tsuji,1,4* Caetano R. Miranda,5 Yoshihiro Masuda,3 and Toshifumi Matsuoka6

1International

Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Fukuoka 819-0395, Japan

2

Unconventional Petroleum Research Institute, China University of Petroleum, Bejing, Beijing 102-249, China 3Center

for Engineering, Research into Artifacts (RACE), The University of Tokyo, Chiba 277-8568, Japan

4Department 5Instituto

of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan

de Física, Universidade de São Paulo, CP 66318, São Paulo, SP 05315-970, Brazil 6Fukada

Geological Institute, Tokyo 113-0021, Japan

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*Corresponding

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author: Yunfeng Liang

Postal address: The University of Tokyo Room 566, Kashiwa Research Complex 5F, Kashiwa Campus 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan Tel:

+81-3-7136-4271

E-mail:

[email protected]

*Corresponding

author: Takeshi Tsuji

Postal address: Kyushu University Room 321, International Institute for Carbon-Neutral Energy Research (I2CNER) 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Tel:

+81-9-2802-6875

Email:

[email protected]


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Abstract: Geochemical trapping (i.e. mineralization) is considered to be the most efficient way for long term CO2 storage in order to mitigate “Global Warming Effect” induced by anthropogenic CO2 emission. Common view is that the reaction process takes hundreds of years, however, recent field pilots have demonstated that it only took 2 years to convert injected CO2 to carbonates in reactive basaltic reservoirs. In this work, ab initio molecular dynamics (MD) simulations were employed to investigate chemical reactions between CO2, H2O and newly cleaved quartz (0 0 1) surface in order to understand the mechanisms of carbonation and hydrolysis reactions, which are essential parts of CO2 mineralization. It is shown that CO2 can react with undercoordinated Si and nonbridging O atoms on the newly cleaved quartz surface leading to formation of CO3 configuration that is fixed on the surface by Si-O bonds. Furthermore, these Si-O bonds can break under hydrolysis reaction, and HCO3 occurs simultaneously. Electron localization function and Bader charge analysis were used to describe the bonding mechanism and charge transfer during the two reaction processes. The result highlights the importance of the intermediate configuration of CO𝛾2 ― in the carbonation reaction process. Furthermore, it confirms the formation of CO23 ― and HCO3― . We conclude that CO23 ― and HCO3― in the formation water are not necessarily originated from dissociation of H2CO3 and these anions may accelerate CO2 mineralization process in the presence of required cations, such as Ca2+, Mg 2+ or Fe2+.



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1 Introduction Over decades, burning fossil fuel has greatly promoted economic development of human society however, it also leads to serious environmental problems. Carbon dioxide (CO2) is a major byproduct from this process. A tremendous amount of CO2 emissions into atmosphere will induce significant increases in global temperature. The phenomenon is known as “Global Warming Effect” and has become a major concern of climate change all over the world.1-3 Besides, dissolution of extra CO2 in seawater can induce “Ocean Acidification” which threatens marine life’s habitats and their normal activities.4 A worldwide effort to reduce the emission of the anthropogenic greenhouse gas is ongoing to accomplish goals of sustainable development and low carbon society. To cite just a few among these works, we have the geological storage CO2,5-9 geochemical trapping of CO2,10-19 conversion of CO2 to chemicals,20-22 replacement of CH4 hydrate by CO2,23-25 etc. In this setting, carbon capture and storage (CCS) has been proposed as an efficient countermeasure to prevent the release of large quantities of CO2 into atmosphere.3 In short, CO2 will be captured from their sources, then transported to suitable storage sites by pipelines or tanks, and finally injected into subsurface geological formations. As oil and natural gas can be held in pore structure of underground sediment and rock, similarly, it is expected that CO2 can be stored in the pore space. Besides, injected CO2 can displace oil in the pore space leading to improved and enhancing oil recovery processes during oil production.26 Furthermore, the temperature and pressure of formations enable CO2 to be liquid or supercritical state, which decreases the total volume for dozens of times compared with that of CO2 gas on earth surface. Therefore, the capacity of CO2 storage in geological formation is enormous.The trapping mechanisms that keep CO2 in the subsurface rock is categorized into four processes: (1) structural and stratigraphic, (2) residual CO2, (3) solubility and (4) mineral trappings.3,19 (1) and (2) are physical processes and usually predominant at the beginning of post-injection phase, which involves with folds, faults with low permeability seals and buoyant migration in pore space, respectively. In the longer term, CO2 will dissolve in formation water resulting in (3), and migrate with groundwater. (4) can convert CO2 to stable carbonate minerals. It is the longest and the most secure form 4 ACS Paragon Plus Environment

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of geological storage occurring posterior to (3). In this work, we will focus on CO2 mineralization because it is the most efficient way for long term CO2 storage.10 The conventional perceptions of reactive parthways are: CO2 firstly interacts with formation water, and this process can generate carbonic acid (H2CO3) which will dissociate into carbonate – 2– – + ion (CO2– 3 ) or bicarbonate ion (HCO3) and hydrogen ion (H ). Then, CO3 and HCO3 will react with

divalent cations (such as Ca2+, Mg2+, Fe2+) in formation water and form carbonate minerals in the pore space.10-13 CO2 mineralization is generally believed to be a slow process, which would take hundreds to thousands of years under natural environment.5 However, over 95% of the injected CO2 into basaltic rock at CarbFix site in Iceland has transformed to carbonate minerals within less than 2 years.12 Wallula Basalt Pilot Projet in USA provides field validation of remarkable rapid mineralization rates as well.13 Both results have demonstrated that CO2 mineralization can be far faster than previously assumed, but the underlying molecular mechanisms remain unexplored as detailed features for geochemical interactions are difficult to be captured by experiments. CO2 is chemically stable, however, it can interact with reactive solid materials like metal surfaces or metal oxide surfaces via either physisorption or chemisorption.27-29 Physisorption of CO2 is reported on less reactive metal surfaces30 in linear configuration whereas nonlinear CO2 and CO𝛾2 ― species have been reported on very reactive metallic and oxide surfaces. Accordingly, the interactions between CO2 and minerals are anticipated as well. There are mainly two relevant types of SiO2 surface consisting of cleaved surface and reconstructed surface.31,32 Generally, the former one is more reactive than the latter as it has dangling bonds on the surface resulting in relatively high surface energy.32 Carbonation and hydrolysis reactions are two fundamental processes of CO2 mineralization, extensive investigations have been done regarding chemical interactions between CO2, H2O and SiO2 systems,33-43 which are essential parts for theses processes. For interactions between SiO2 and H2O, Ledyastuti et al.33 and Goumans et al.34 reported that hydroxyls from dissociation of H2O molecules can lead to formation of silanol groups on the cleaved SiO2 surface by first-principles methods. By using the same method, Adeagbo et al.35 found that the 5 ACS Paragon Plus Environment


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hydroxylated SiO2 surface will lower the diffusion coefficient of residual H2O molecules. Regarding CO2 and H2O system, Pan et al.36 pointed out that carbon would occur in the form of rapidly interconverting CO23 ― and HCO3― rather than solvated CO2 molecules under extreme conditions, and P-T conditions will affect ion pairing between Na+ and CO23 ― /HCO3― . Glezakou et al.37 showed effect of water-cluster formation in the supercritical phase of CO2 as a function of H2O content and evidence of short-lived hydrogen bonds between CO2 and H2O molecules. Liu et al.38 suggested the bending vibrations of CO2 play the vital role in localizing the excess electrons interacting with CO2–H2O clusters and the angle of O–C–O is influenced by the trapped excess electrons. Concerning interactions between CO2 and SiO2 system that implies to carbonation process, Hermann et al.39 emphasized the importance of the vdW forces in such a complex system. Malyi et al.40 reported that CO2 molecule can react with reactive cleaved SiO2 surface and CO3-like structure can occur on the surface on the basis of first principles calculations. Threephase system of mineral-CO2-H2O have been mostly investigated by classical molecular dynamics (MD) simulations.41,42 The results are dependent on the predetermined potential fields and cannot model the processes of making and breaking of chemical bonds. Lee et al.43 successfully addressed carbonation process of CO2-H2O-anorthite system by using density functional theory (DFT) based ab initio MD simulation. Born-Oppenheimer dynamics has been employed based on the forces acting on the nucleus from electronic structure calculations, while the trajectories of particles are generated according to Newton's equation.44 This method can perfectly deal with the difficult tasks of breaking and making of chemical bonds, which are essential parts for chemical reactions. Inspired by foresaid studies, we employed ab initio MD simulations to investigate the mechanisms of carbonation and hydrolysis reactions on newly cleaved quartz (0 0 1) surface. Firstly interaction between CO2 molecules and newly cleaved SiO2 substrate was modelled to represent carbonation process. CO2 bents due to charge transfer from the surface to the adsorbate, and CO𝛾2 ― , as an important intermediate, is usually considered as precursor to carbonate formation.27-29 The relevant variations of charge 𝛾 value, and molecular configuration changes are quantitatively characterized during the transformation of CO2 to 6 ACS Paragon Plus Environment

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CO3 by the simulations. Meanwhile, variations of electronic structures with time lapse are mapped during the reaction process. Then H2O molecules were placed on the resultant surfaces that attached CO3 configurations. Because formation water can be dissociated into H+ and hydroxide ion (OH–), they would react with formed CO3 configuration on the surface and influence its stability. This process is so-called hydrolysis reaction, and there lacks of relevant study in this regard. The electronic properties including charge transfer analysis (based on Bader Charge analysis45,46) and electron localization function (ELF)47,48 of the hydrolysis reaction prcocess are also investigated in details. In general, hydrolysis reaction occurs in prior to carbonation reaction in natural environment. However, according to previous works,19,33 nonbridging O sites indeed can exist after hydrolysis reaction. In this paper, we studied the carbonation at first. The system is similar to that used in Malyi et al’s40 work, however, we have employed a larger solid slab with more CO2 molecules. In addition, geological P-T condition is used. As the nonbridging O and undercoordinated Si sites are massive, a “rare event” may occur in the simulation time scale.40 As a consequence, we have been able to reveal molecular and electronic insights of CO2 carbonation process, and the hydrolysis process of the carbonate species. In particular, we have been able to reveal the charge transfer during these chemical reactions. We will describe computational details in next section.

2 Methodology DFT Parameters. On the basis of DFT, ab inito MD simulations were performed as implemented in CP2K/QUICKSTEP program (2.6.2 version) with Gaussian and plane waves (GPW) method.49 Exchange and correlation were treated with Perdew, Burke and Ernzerhof (PBE) functional50. Grimme's third generation correction DFT-D351 was employed for dispersion corrections due to its superior performance for large-scale MD simulations with the computation of forces and combination with standard functionals.52 Inter-molecular interactions play an important role when CO2 molecules interact with the SiO2 substrate.39,40,53 The core electrons and valence part were addressed by norm-conserving pseudopotentials54 and double-zeta quality basis set,55 respectively. The charge density was expanded in 7 ACS Paragon Plus Environment


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plane waves with cutoff value of 340 Ry as the energy of the systems converged well (see Fig. S1). Similar to previous studies in a CO2-H2O-anorthite system43, the spin polarization was not considered. Periodic boundary condition was adopted as if the simulations occur in an infinite space. The time step was set as 0.5 fs. All of the simulations were carried out under canonical (NVT) ensemble and the temperature was set as 323 K, controlled by means of the Nosé-Hoover thermostat.56

Simulation details. In order to investigate chemical bonding process of carbonation reaction, α-quartz substrate was prepared by cleaving a primitive quartz model in the middle along (0 0 1) direction (see Fig. S2). The mineral model includes 72 silicon (Si) and 144 oxygen (O) atoms. All the Si atoms are fourfold coordinated with O atoms except those located on the surface, which link three O atoms. This surface structure is similar to those in previous studies.33-35,40 It has nonbridging O atoms on the surface as shown in Fig. S2. A bulk system with 300 CO2 molecules was equilibrated initially by classical MD simulations under 323 K and 9 MPa with isothermal-isobaric (NPT) ensemble by GROMACS version 4.5.5.57 In this case, EPM2 potential58 was employed to model CO2 molecules. Then, a slab of CO2 molecules (eight) were tailored from the equilibrated system to combine with the quartz substrate (see Fig. 1(a)). The lattice parameters of the ab initio MD simulation systems are: a=14.738 Å; b=17.018 Å; c=18.879 Å, α=β=γ=90°. The intercalated space is about 8.71 Å. When the simulation time of CO2 and SiO2 system comes to 20.5 ps, seven CO2 molecules were converted to CO3-like configurations and one linear shape CO2 molecule remains. By using the final configuration from carbonation reaction, we investigated the influence of hydrolysis reaction on the formed CO3 structures attached to the SiO2 surfaces as if they were immersed in formation H2O underground. The CO2 molecule that had not reacted with the SiO2 substrate was removed from the system. A slab with 48 H2O molecules was tailored from a bulk system of 300 H2O molecules, which was equilibrated under 323 K and 9 MPa with NPT ensemble by classical MD simulations. SPC/E potential59,60 was used to model H2O molecules. The thickness of the water slab is fit to the void space 8 ACS Paragon Plus Environment

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previously occupied by CO2 phase. The ab initio MD simulations with this system runs over 25 ps. Bader Charge analysis45,46 and ELF47,48 were performed in order to calculate the charge distributions of atoms during chemical reaction processes. VESTA61 was used to visualize the molecular configurations and electronic properties.

3 Results and Discussions 3.1 Carbonation reaction of CO2 molecules with the SiO2 surfaces At the initial stage, the adsorption of CO2 molecules on the reactive SiO2 surfaces resembles a physical process as the CO2 molecules remain within its linear configuration. Soon, chemisorption occurs: the linear shape CO2 molecules start to bend and are converted to CO3 configurations in the end, which are the same as the CO3-2 structure in Malyi et al’s40 paper. That is, CO3 is connected with two different Si sites forming greater ring instead of CO3-1 structure, which is a two-member ring formed by one C and one Si. All the CO2 molecules in the system are marked from number (No.) 1 to 8 in order, whereby the individual trajectory can be tracked with the simulation time. Fig. 1 shows selected snapshots of the simulation system during simulation process. At the very beginning (0 ps), all the CO2 molecules are placed in the void space between the two surfaces. With time lapses, some of the linear shape CO2 molecules move towards to the surface of the substrates, and we can observe that four CO2 (No. 3, 5, 6, 7) molecules are attached to undercoordinated Si atoms (0.8 ps) on the surface by sharing O atoms of CO2 molecules (so called "SiO4-C-O" in Malyi et al.40). Linear shape CO2 molecules start to bend, when C atoms approach to nearby nonbridging O atoms on the surfaces (No. 1, 7). Finally, they transform into trigonal planar arrangement (CO3) with one C atom at the center and three O atoms at the corners of an equilateral triangle (so called "CO3-2" in Malyi et al.40) (1.2 ps). We remark that the observation of CO3-2 structure in our simulations is in line with previous simulation work, where they reported that CO3-2 structure was formed with higher CO2 coverage and CO3-1 structure was formed when only one CO2 molecule was adsorbed.40 Six CO2 molecules transform into CO3-2 structures within the first 5 ps, and 9 ACS Paragon Plus Environment


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only one remains to be linear shape (the others are CO3-2 structure) not attaching to the surface after 20.5 ps. Interestingly, the CO2 molecules are not necessarily attached to the substrate sites that are more closer to them, such as No. 2 and 5, implying they may diffuse freely in the system at the initial stage prior to adsorption on the reactive SiO2 surfaces. It seems that the carbonation reaction preferentially occurs on the top surface over the lower one, presumably because CO2 molecules (except No. 4 and 8 CO2 molecules) are distributed closer to top surface. Both undercoordinated Si atoms and nonbridging O atoms on the surface are active to CO2 molecules. In the following two sub-sections, we will report the evolution process from two perspectives with regard to CO2 molecules: (1) changes of CO2 molecular configurations and (2) electronic properties and charge transfer of CO2 molecules as making and breaking of chemical bonds occur during the reactions.

3.1.1 Time evolution of CO2 molecular configurations Mean square displacement (MSD) of C atoms and angle of O–C–O for CO2 molecules were estimated in order to characterize the diffusive motions and the geometric shape of CO2 in the simulations. MSD is 1

𝑁

defined by MSD = 𝑁∑𝑛 = 1(𝑥𝑛(𝑡) ― 𝑥𝑛(0))2, N is the total number of CO2 molecules (which equals eight here), 𝑥n(𝑡) and 𝑥n(0) represent the positions of the nth particle at simulation time t and starting point 0 ps. The angle of O–C–O is expressed as inverse cosine function of 𝜃 (arccos 𝜃); and cos 𝜃 = 𝒆1 ∙ 𝒆2/

|𝒆1||𝒆2|, where

𝒆1 and 𝒆2 represent vectors of two C-O chemical bonds, respectively. The Cartesian

coordinates of C and O atoms at each step as predicted by ab inito MD simulations were tracked and used for analysis. Meanwhile, the total energy of each step is also obtained to illustrate the overall reaction process. In Fig. 2, the MSD curve in conjunction with total energy curve are displayed within the first 5 ps. It can be classified into 3 stages which are termed (1) free diffusion, (2) chemical reactions, (3) chemisorption. Fig. 3 shows the time evolution of the O–C–O angle for eight CO2 molecules in eight different panels, repectively. It can be employed to indicate the formation of trigonal planar CO3 species. 10 ACS Paragon Plus Environment

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When the configuration changes from linearshape CO2 molecules to CO3 configurations, a decrease of the O–C–O angle from ~180° to ~120° is observed. The transition point for each molecule is marked by vertical dash line in each panel. At the first stage (green shading) in Fig. 2, the MSD curve exhibits linear relationships from 0 to ~0.7 ps indicating CO2 molecules can freely diffuse in the pore space, the slope of the curve can be used to estimate self-diffusion coefficients. The tendency of energy curve is almost invariable with thermal fluctuations. In Fig. 3, the curves of angles of O–C–O are fluctuating around 180°, namely CO2 molecules are moving around with bending vibrations. When coming to the second stage from ~0.7 to ~3 ps (blue shading), the MSD curve immediately becomes nonlinear and the overall slope slows down. The total energy starts to decrease correspondingly. These indicate the beginning of the chemical reactions and the motions of CO2 molecules have borne restraints by covalent bonding attributed to the SiO2 substrate. Angles of O–C–O for six CO2 molecules (No. 1, 2, 3, 5, 6, 7) decrease to ~120° within this stage. Although the transition time is extremely short, the making and breaking of chemical bonds can be captured by the ab inito MD simulations (as shown in Fig. 1). In Fig. 1, at 0.8 ps, No. 3 CO2 molecule attached to the SiO2 substrate earlier than No. 1 CO2 molecule, however, the latter one formed CO3-like structure earlier than the former one at 1.2 ps. The formation of CO3-like configuration for No.7 CO2 is the earliest. We will focus on the chemical bonding processes within this stage by using electronic properties in the following subsection. In the third stage from ~3 to 5 ps, both the MSD and energy curves tend to level off though still fluctuate. This is mainly attributed to the motions of the two remaining linear shape CO2 molecules as they have not been adsorbed yet during this period. The C-O bond of linear shape CO2 molecule that predicted by simulation is ~1.18 Å (see Fig. 4(a)), which is in good consistent with experimental data of ~1.16 Å.62 When CO2 molecule is converted to CO3-like structure, the C-O bonds length increase to average value of 1.29 Å that is also in good agreement with experimental data of 1.31 Å.63



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3.1.2 Electronic properties and charge transfer in the carbonation reaction process Most of the linear shape CO2 molecules transformed into CO3-like configurations from 0 to 2 ps. Electronic properties including ELF and atomic charges were analyzed within this period. The purpose is understand how the chemical bond forms and how the electric charge transfers via the intermediate configuration of CO𝛾2 ― during the carbonation process. Generally, formation of a chemical bond is dependent on the distance between two interacting atoms. When the distance is shorter than a critical value, an electron in atomic orbital of one atom will pair with that of atomic orbital from the other atom, leading to formation of a covalent bond. ELF can describe the likelihood of finding such kind of electrons and characterize the formation of covalent bonds. Fig. 4 illustrates the ELF map throughout a CO2 molecule (No. 7), which is shown with molecular models represented by balls and sticks. The distributions of valence electrons surrounding O atoms are not spherically symmetric. Further, the electronegativity of O atom is apparently greater than that of central C atom, indicating the C-O bond is strongly polarized towards the oxygen. The carbonation process can be divided into four steps. In Fig. 4 (a), the CO2 molecule remains with linear shape while it is approaching to an under-coordinated Si atom on the reactive SiO2 surface. The length of C-O bond that is closer to Si atom (1.21 Å) is relatively longer than that on the opposite side (1.18 Å). This difference is attributed to the attractive force from SiO2. In Fig. 4 (b), the O atom is linked with the undercoordinated Si atom with bond length of 2.02 Å. Simultaneously, CO2 molecule starts to bend from the center as C atom is getting closer to a neighboring O atom on the SiO2 surface which is nonbridging. Later on in Fig. 4 (c), C atom and the neighboring O atom share a green area with value of 50%, suggesting the critical point for formation of covalent bonding. The Si-O bond becomes stronger expressed by shorter bond length of 1.75 Å. The trigonal planar arrangement of CO3 formed in Fig. 4 (d). All the 3 C-O bonds are longer than those of CO2 molecule shown in Fig. 4 (a) due to rearrangement of valence electrons, which is in good agreement with experimental results.62,63 Bader analysis was employed to unambiguously determine atomic charges of all the atoms for specific CO3-like configuration (related to No. 7 CO2) during the transition process and the results are shown in 12 ACS Paragon Plus Environment

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Fig. 5. The 𝛾 values of bent intermediate CO𝛾2 ― were quantitatively determined. Fig. 5 (a) is a zoom-in figure for angle variation of O–C–O from Fig. 3. From that, we can know that the transition finished at simulation time of ~1 ps. At this moment, the total charge 𝛾 ― of CO2 molecule suddenly drops to ~ – 0.45 e, and in the end maintains with average value about –0.40 e which is comparable to the sum of the Bader charges (–0.32 e) on the CO2 molecule according to previous DFT calculations.40 From Fig. 5 (f), it can be understood that those excess electrons are transferred from the neighboring O atom on the reactive SiO2 surface (which is denoted by OS) to the CO2 molecule since its atomic charge arises correspondingly with same quantity. Interestingly, the excess electrons do not directly transfer from OS to C atom, instead, the O atom plays a role as "bridge" in transfering the electrons to C atom as atomic charge of C atom (in Fig. 5 (c)) decreases (~1.15 ps) later than the uptake of excess electrons by CO2 molecule (in Fig. 5 (b)) (~1.0 ps). Because O1 atom is further away from the surface compared with O2 atom, the fluctuation of nominal atomic charge of O1 atom is not as large as that of O2 atom. These results indicates that the O2, OS and C all together play important roles during the electronic charge transfer process of carbonation reaction. Fig. 6 shows projected density of state (PDOS) of C atom and O atom occurring on the SiO2 surface before and after formation of CO3 configaruation of No. 7 CO2 molecule. All of the valence electrons on 2s and 2p orbitals of both C atom and O atom are relevant in bonding formation. Before the formation of CO3 configuration, density states of C atom are mainly distributed within -22~-25 eV, -6~-8 eV and 3~5 eV as shown in Fig. 6 (a), and density states of O atom from the reactive SiO2 surface are highly concentrated in -17~-16 eV and -5~0 eV as shown in Fig. 6 (b). This suggests that they are unpaired valence electrons as they do not have identical binding energy. Figs. 6 (c) and (d) illustrate the distributions of electron energy after formation of CO3 configuration. They evidently occupy the same area, which implies that they have mutually paired and shared the same molecular orbitals. Valence electrons on 2s in conjuction with some parts of 2p from C atom form 𝜎 bonds, which are dominant in low energy range of -20~-25 eV. The rest of 2p orbitals form 𝜋 bonds, which are distributed in higher 13 ACS Paragon Plus Environment


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energy ranges at -13~-3 eV. The bonding process has decreased the absolute values of density state and broadened energy distribution ranges for the two atoms. In addition, the effect of CO2 adsorption on electronic structure of SiO2 surfaces was investigated. Fig. S3 illustrates the PDOS of SiO2 surface with CO2 molecules (including the CO2 in the CO3) as well as that without CO2 molecules. The results show that CO2 molecules can suppress the surfaces states of SiO2, which is consistent with previous researches.64,65 Meanwhile, they can also activate new energy states via carbonation reaction. According to the frontier molecular orbital theory, the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO) can determine the reaction capability of molecules.66 Freund et al.27 and Taifan et al.29 have qualitatively reported that when angle of O–C–O decreases from 180° to 90°, binding energy of HOMO increases, whereas that of LUMO decreases. This will lead to reduction of LUMO-HOMO gap. Fig. 7 illustrates that variations of LUMO-HOMO gap versus corresponding changes of the O–C–O angle for reacted CO2 molecules (No. 1, 2, 3, 5, 6, 7) from Fig. 3. Individual molecule are investigated because their transformations occur at different time. As we can see, all of the fitting curves show that the energy gap reduces (with thermal fluctuations) while O– C–O angle decreases from ~180° to ~120°. As an entire system, angle geometries of most CO2 molecules change from 0.5 ps to 1.5 ps, therefore, the variations of LUMO-HOMO gap of system against average O–C–O angle of all CO2 molecules are also investigated (see Fig. S4), in which the same conclusion can be conveyed. In summary, during carbonation process, the electrons transfer from SiO2 substrate to CO2 molecules while the O–C–O angles decrease from 180˚ to 120˚. The occupation of LUMO of CO2 molecule is of particular importance for changing of linear-shape CO2 molecule to CO3 species since reduction of LUMO of CO2 molecule can favor charge transfer.

3.2 Hydrolysis reaction of CO3–like configuration in aqueous solution From now, we discuss the influence of hydrolysis reaction on formed CO3 configuration. The initial model was built by combination of the final configuration (20.5 ps) resulted from carbonation process 14 ACS Paragon Plus Environment

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and 48 H2O molecules in a liquid phase. H2O molecules are sandwiched by two surfaces (of the quartz substrate) with seven CO3 configurations. Fig. S5 shows selected snapshots from the ab inito MD simulations within the first 5 ps. It is expected that the reactive cleaved SiO2 surface can be hydroxylated via hydrolysis reaction as reported by Ledyastuti et al.33 and Goumans et al.34 As shown in Fig. 8, silanol groups including geminal silanol and vicinal silanol were observed during the simulations. The former one is defined by two silanol sites (‒Si(OH)2) occurring on the same Si atom, whereas the latter one has one single silanol (‒SiOH) on two neigbouring Si atoms, which are connected by a bridging O atom. All of the Si atoms are fourfold coordinated. Interestingly, we found that the morphology of CO3 configuration (No. 4) resulted from carbonation process changed at ~1.65 ps, i.e. it changed from CO3-2 to CO3-1 type (nomenclature called by Malyi et al.40) as shown in Figs. 8 (c) and (d). That is, CO3 configuration, which is firstly connected with two different Si sites (i.e. CO3-2 type), is changed into an intermediate state with two-member ring formed by one C and one Si (i.e. CO3-1 type). The detached Si atom is coordinated with a neighboring hydroxide ion after that. Besides, one of the Si-O bonds that restrains the free motion of CO3 (No. 3) breaks (shown in Fig. S5(b)) at ~1.8 ps and it is replaced by a newly formed Si-O bond generated by a free hydroxide ion in the vicinity. Meanwhile, HCO3-like configuration forms as hydrogen ion that dissociated from H2O attached with O atom of the CO3 configuration. We have tracked the O-H distance in the newly-formed HCO3 configuration over 25 ps. As shown in Fig. S6, HCO3 forms at ~1.8 ps and remains stable throughout the simulation in this study. These two processes correspond to two peaks on potential energy profile for hydrolysis reaction (see Fig. S7). The overall trend of potential energy decreases from the beginning to ~2.5 ps, then it levels off. The bump around ~1 ps is possibly due to dissociation of H2O into H+ and OH–, which increases the potential energy. Fig. 9 illustrates the detailed features regarding formation of HCO3 configuration during hydrolysis reactions by using ELF. In Fig. 9 (a), a H2O molecule attaches its O atom to a Si atom on the surface, acting as an intermediate species. In the next step, the water molecule dissociated into OH– and H+ ions. 15 ACS Paragon Plus Environment


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The former is attached to the Si atom forming a silanol site, while the dynamics of the proton undergoes Grotthus-type diffusion through a neighboring H2O molecule. Finally, the H+ ion (from the neighboring H2O molecule) attaches to the CO3 configuration. In detail, a free H2O molecule nearby is protonated and a hydronium (H3O+) formed as shown in Fig. 9 (c). The protonation of water results in displacement of a primitive H+ of the H2O molecule and the displaced H+ moves towards to a O atom of the CO3 configuration and finally form HCO3 configuration in Fig. 9 (e). Both Fig. S5 (b) and Fig. 9 (e) illustrate that Si-O bond limiting dynamics of CO3 configuration can be ruptured by hydrolysis process, and HCO3― can form during this process. Consequently, it is concluded that CO23 ― and HCO3― in the formation water are not necessarily originated from dissociation of H2CO3. The charge transfer during the formation process of HCO3 configuration on the basis of Bader analysis is shown in Fig. 10. In Fig. 10 (a), evidently, the total charge of CO3 configuration decreases in terms of absolute value before the transition point indicated by vertical dash-line, once HCO3 configuration formed, the total charge changes to -1 e, which is in good agreement with the nominal charge of HCO3. Remarkably, it is found that the charge of O atom in CO3 that connects to H+ (see Fig. 10 (d)) varies from ~ -2 e to ~ -1.7 e, while the other two O atoms remain to be ~ -2 e. Bicarbonate formation has been observed on TiO2, Al2O3, and Fe2O3 surfaces in CO2 adsorption or CO2 and H2O coadsorption experiments.28,29 Furthermore, it was shown that CO2 might react with surface hydroxyl groups on Al2O3, and Fe2O3 cluster surfaces to form bicarbonate.67 This implies the chemical reactions on mineral surfaces is a complicated process. To our knowledge, there is no detailed simulation study on the surface hydrolysis reaction process of a carbonate species as reported here.

4 Conclusions By using ab initio MD simulations, we firstly investigated the chemical reactions between CO2 and newly cleaved quartz, which represents the carbonation process. Then the hydrolysis reaction between the resultant quartz surface and H2O was studied. Although simulations of carbonation process and 16 ACS Paragon Plus Environment

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hydrolysis process were performed only 20.5 and 25 ps, respectively, some significant changes might occur beyond these two runs, it is confirmed in this study that free CO2 molecule can be immobilized by chemical bonding process on the reactive cleaved quartz surface and form CO3 configuration. Reduction of LUMO-HOMO gap of CO2 molecules is observed, when linear CO2 transforms into planar CO3 configuration. Hydrolysis reaction can rupture the Si-O bond that connected to CO3 configuration, meanwhile it can also convert the latter one to HCO3 configuration. ELF map and Bader analysis were employed in order to analyze the bonding mechanism and charge transfer of the two processes. Based on these results, it can be postulated that CO23 ― and HCO3― in the formation water are not necessarily originated from dissociation of H2CO3. Active undercoordinated Si and nonbridging O atoms located on the cleaved SiO2 surface are of great importance for formation of these CO3 and HCO3 configurations, which are usually protected by either cations or hydrogen bonding in natural environment.19,33 If we can successfully dissolve these cations into formation water or dehydrogenate SiO2 surface, CO2 mineralization can be accelerated in the presence of Ca2+, Mg 2+ or Fe2+, such as in basaltic rock. Similar reactions may occur, when CO2 is used as fracturing fluid in shale gas development.40 It is noted that both the carbonation and hydrolysis reactions are very complicated processes, and may be influenced by the mineral surfaces, fluid components, pH, temperature, and pressure.19 It has been reported that CO2 will be physically adsorbed on the reconstructed silica surface.40 That is, surface reconstruction of SiO2 may suppress the formation of CO3. This again indicates that the reaction reported here is complicated in nature. Our study confirms that the formation of CO3 should be observed on the reactive SiO2 surfaces. Future studies should be expanded to basalt minerals, to find the most probable reaction pathways, and to validate the kinetic mechanisms behind them.

Acknowledgements J.J. and T.T. are grateful for the support of the I2CNER sponsored by the World Premier International research Center Initiative (WPI), MEXT. This research is supported by Cross-ministerial Strategic 17 ACS Paragon Plus Environment


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Innovation Promotion (SIP) program of Japan, JSPS through a Grant-in-Aid for Science Research on Innovative Area (No.JP15H01143; JP17H05318). J.J. acknowledges the support provided by Science Foundation of China University of Petroleum, Beijing (No.2462017YJRC036). C.R.M. acknowledges the financial support provided by the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), and the Brazilian Ministry of Science and Technology for collaborative research between China and Brazil.

Supporting Information Supporting Information Available: total energy as function of cutoff for plane waves, detailed settings of newly cleaved quartz surface; effect of CO2 adsorption on electronic structure of SiO2 surface; variations of LUMO-HOMO gap during the carbonation reaction process; snapshots of hydrolysis reaction between carbonated SiO2 surface and liquid water; time evolution of H and O distance of the formed HCO3 configuration throughout our simulation; potential energy (i.e. the total energy of eletrons) as a function of time during the hydrolysis reaction process. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes: The authors declare NO competing financial interest.


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Figure 1. Snapshots of simulated system with ball and stick representation. The system consists of 72 SiO2 formula units and 8 CO2 molecules. The molecular configuration shown in the figure was duplicated on Y direction (therefore 144 SiO2 formula units and 16 CO2 molecules are displayed). Red, blue and brown color represent oxygen, silicon and carbon elements respectively. The numbers in each panel denote marked CO2 molecules (not for replica).


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Figure 2. Mean square displacement (MSD) of CO2 molecules (average value) in conjuction with the potential energy of simulation system (the total energy of electrons) within the first 5 ps. Red curve represents MSD while blue curve indicates potential energy. The light green shading on the left, light blue shading in the middle and light orange on the right side represent (1) free diffusion stage, (2) chemical reaction stage and (3) chemisorption stage, respectively.



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Figure 3. O–C–O angle variations of CO2 molecules in the system within the first 5 ps. CO2 molecules of No. 1, 2, 3, 5, 6 and 7 transform into CO3 configurations within the first 2 ps. The vertical dash lines indicate the transition points.


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Figure 4. Distribution of Electron Localization Function regarding transformation of linear CO2 (No. 7) molecule into planar CO3 configuration. The red color denotes that the occurrence probability of valence electrons is 100%, whereas blue color means that no electrons exist in the area. Green color means free electron-gas indicating border of covalent bonds.



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Figure 5. Variation of atomic charges during carbonation process on the basis of Bader analysis from 0.5 to 1.5 ps. (a) a zoom-in figure of Fig. 2(7), inset shows the relevant position of each atom, (b) 𝛾 ― value of bent intermediate CO𝛾2 ― (No. 7 CO2 molecule). (c) atomic charge of C from No.7 CO2 molecule. (d) atomic charge of O1 from No.7 CO2 molecule, (e) atomic charge of O2 from No.7 CO2 molecule, which links Si atom from the SiO2 substrate, (f) atomic charge of OS, which is from the SiO2 substrate.


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Figure 6. Projected Density of States (PDOS) of relevant C atom from CO2 molecule (No. 7) ((a) and (c)) and O atom from the reactive SiO2 surfaces ((b) and (d)) in prior to (0.8 ps) and posterior to (1.1 ps) chemical bonding. (a) and (b) are the density states before formation of C-O bond, whereas (c) and (d) are those after formation of C-O bond. In (c) and (d), the PDOS distributions of the two different atoms have the same range, it indicates that the electons from C and O are pairing, and they have the same binding energy, therefore the chemical bond forms.



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Figure 7. Changes of LUMO-HOMO gap against changes of angle of O–C–O for each CO2 molecule (No. 1, 2, 3, 5, 6, 7). The linear curves are obtained from corresponding data by using least square method, which represent the trend of LUMO-HOMO gap variations. Note: the energy gap reduces as angle of O– C–O decreases during the reaction process.


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Figure 8. Snapshots of hydrolysis effect induced by reaction between carbonated SiO2 surface (with CO3 configuration) and H2O. (a) Geminal silanol; (b) Vicinal Silanol; (c) CO3-2 structure (at 1.5 ps); (4) CO3-1 structure (at 1.8 ps).



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Figure 9. Distribution of Electron Localization Function regarding hydrolysis reaction with the formation process of HCO3 configuration (No. 3 CO2 molecule), which is with regard to Grotthus-type proton diffusion. The red color denotes that the occurrence probability of valence electrons is 100%, whereas blue color means that no electrons exist in the area. Green color means free electron-gas indicating border of covalent bonds.


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Figure 10. Variation of atomic charges during hydrolysis process on the basis of Bader analysis from 1.5 to 2.2 ps. CO3 configuration transforms into HCO3 configuration at ~1.8 ps. (a) Charge of CO3/HCO3 configuration, (b) charge of hydrogen atom related to HCO3 configuration, (c) atomic charge of C from No. 3 CO2 molecule, (d) atomic charge of O1 from No. 3 CO2 molecule, (e) atomic charge of O2 from No.3 CO2 molecule, which links Si atom, (f) atomic charge of OS, which is from the SiO2 substrate.



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TOC Graphic


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Figure 1. Snapshots of simulated system with ball and stick representation. The system consists of 72 SiO2 formula units and 8 CO2 molecules. The molecular configuration shown in the figure was duplicated on Y direction (therefore 144 SiO2 formula units and 16 CO2 molecules are displayed). Red, blue and brown color represent oxygen, silicon and carbon elements respectively. The numbers in each panel denote marked CO2 molecules (not for replica).

ACS Paragon Plus Environment


Figure 2. Mean square displacement (MSD) of CO2 molecules (average value) in conjuction with the potential energy of simulation system (the total energy of electrons) within the first 5 ps. Red curve represents MSD while blue curve indicates potential energy. The light green shading on the left, light blue shading in the middle and light orange on the right side represent (1) free diffusion stage, (2) chemical reaction stage and (3) chemisorption stage, respectively.


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Figure 3. O–C–O angle variations of CO2 molecules in the system within the first 5 ps. CO2 molecules of No. 1, 2, 3, 5, 6 and 7 transform into CO3 configurations within the first 2 ps. The vertical dash lines indicate the transition points.



Figure 4. Distribution of Electron Localization Function regarding transformation of linear CO2 (No. 7) molecule into planar CO3 configuration. The red color denotes that the occurrence probability of valence electrons is 100%, whereas blue color means that no electrons exist in the area. Green color means free electron-gas indicating border of covalent bonds.


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Figure 5. Variation of atomic charges during carbonation process on the basis of Bader analysis from 0.5 to 1.5 ps. (a) a zoom-in figure of Fig. 2(7), inset shows the relevant position of each atom, (b) γ^- value of bent intermediate CO_2^(γ-) (No. 7 CO2 molecule). (c) atomic charge of C from No.7 CO2 molecule. (d) atomic charge of O1 from No.7 CO2 molecule, (e) atomic charge of O2 from No.7 CO2 molecule, which links Si atom from the SiO2 substrate, (f) atomic charge of OS, which is from the SiO2 substrate.



Figure 6. Projected Density of States (PDOS) of relevant C atom from CO2 molecule (No. 7) ((a) and (c)) and O atom from the reactive SiO2 surfaces ((b) and (d)) in prior to (0.8 ps) and posterior to (1.1 ps) chemical bonding. (a) and (b) are the density states before formation of C-O bond, whereas (c) and (d) are those after formation of C-O bond. In (c) and (d), the PDOS distributions of the two different atoms have the same range, it indicates that the electons from C and O are pairing, and they have the same binding energy, therefore the chemical bond forms.


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Figure 7. Changes of LUMO-HOMO gap against changes of angle of O–C–O for each CO2 molecule (No. 1, 2, 3, 5, 6, 7). The linear curves are obtained from corresponding data by using least square method, which represent the trend of LUMO-HOMO gap variations. Note: the energy gap reduces as angle of O–C–O decreases during the reaction process.



Figure 8. Snapshots of hydrolysis effect induced by reaction between carbonated SiO2 surface (with CO3 configuration) and H2O. (a) Geminal silanol; (b) Vicinal Silanol; (c) CO3-2 structure (at 1.5 ps); (4) CO3-1 structure (at 1.8 ps).


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Figure 9. Distribution of Electron Localization Function regarding hydrolysis reaction with the formation process of HCO3 configuration (No. 3 CO2 molecule), which is with regard to Grotthus-type proton diffusion. The red color denotes that the occurrence probability of valence electrons is 100%, whereas blue color means that no electrons exist in the area. Green color means free electron-gas indicating border of covalent bonds.



Figure 10. Variation of atomic charges during hydrolysis process on the basis of Bader analysis from 1.5 to 2.2 ps. CO3 configuration transforms into HCO3 configuration at ~1.8 ps. (a) Charge of CO3/HCO3 configuration, (b) charge of hydrogen atom related to HCO3 configuration, (c) atomic charge of C from No. 3 CO2 molecule, (d) atomic charge of O1 from No. 3 CO2 molecule, (e) atomic charge of O2 from No.3 CO2 molecule, which links Si atom, (f) atomic charge of OS, which is from the SiO2 substrate.


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Ab Initio Molecular Dynamics Study of Carbonation and Hydrolysis

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