Chemical Properties of Metal-Silicates Rendered by Metal Exchange

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Chemical Properties of Metal-Silicates Rendered by Metal Exchange Reaction Roberto C. Longo, Franz Königer, Alexei Nefedov, and Peter Thissen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00157 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Sustainable Chemistry & Engineering

Chemical Properties of Metal-Silicates Rendered by Metal Exchange Reaction Roberto C. Longo1, Franz Königer2, Alexei Nefedov2 and Peter Thissen2, *

Department of Materials Science and Engineering, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas, 75080, United States 1

Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2

*corresponding author: [email protected]

Abstract Calcium-Silicates and Calcium-Silicate-Hydrates (CS and CSH) are well known as the most important building material, cement. Both CS and CSH phases react fast with CO2 from the atmosphere. Due to the porosity of cement and concrete, such reaction goes deep into the material, producing phase transformations, crack formation and propagation. The aim of this work is twofold. In the first part, we compare the reaction of CO2 with CSH phases and with Magnesium-Silicate-Hydrates (MSH). Surprisingly, MSH did not show any contamination of carbonates in the infrared spectra. While the reaction of CO2 with CSH has been well studied and explained, there is currently no explanation about the resilience of MSH to the interaction with CO2. For the first time, the atomistic details of the reaction of CO2 with MgSiO3 are shown, and the chemical resistance of MgSiO3 against CO2 and other relevant chemicals for corrosion of cement and concrete is explained. Secondly, we demonstrate that Mg and other metals can undergo an exchange in situ process in CS and CSH phases. Depending on the type of metal exchanged, a completely new platform for rendering the properties of cement and concrete surfaces against corrosion is developed.

Keywords: Calcium-Silicate-Hydrate; Magnesium-Silicate-Hydrate; Infrared Spectroscopy; Carbonate; Climate Change; Density Functional Theory;

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Introduction Climate change is continually modifying the atmosphere of the earth.1-2 Particularly, the concentration of CO2 is especially affected, increasing monotonically. Certain materials, like cement and concrete, are particularly sensitive to such changes. Cement is currently the most important building material for technical infrastructure,3 and nearly 10% of the CO2 is emitted to the atmosphere during cement production. In contradiction to that, the lifespan of technical infrastructure is greatly reduced if the cement reacts with CO2 from the atmosphere without further protection. Such reactions, namely the interaction with CO2 and H2O combined, must be examined critically.4 Typically, the origin of chemical corrosion has so far been attributed only to the interaction with pure H2O5-12. As a result, most of the present solutions to extend the lifetime of the technical infrastructure protect cement-based materials only against H2O as a corrosion agent. In this context, the interaction with both CO2 alone and H2O and CO2 combined is ignored. During the production of cement, a great amount of CO2 is inevitably released. Therefore, different alternatives to decrease the CO2 footprint such as storage of the gas through sequestration via carbonation, are under active development.13-16 Carbonates can also show detrimental effects on life, for instance by inhibiting the transfer of metals from soil to plants.17 The presence of carbonate impurities in many minerals is well-known, and its presence in Calcium-Silicate phases (CS, the building block material for cement and concrete) has been well characterized. CS phases are considered promising compounds for CO2 storage, using the so-called supercritical CO2 conditions. However, the discussion about the use of hydrated cement for CO2 storage remains controversial,18-20 because the impact of such process on the mechanical properties of concrete is still unknown. Our own previous work has shown the influence of the chemical potential of water on the carbonation reaction of wollastonite (CaSiO3), as a model surface for cement and concrete.21 By means of firstprinciples calculations and kinetic studies, it was demonstrated that the exposure of water-free wollastonite surface to CO2 results in carbonation with no energy barrier. CO2 reacts with the surface oxygen and forms carbonate (CO32−) complexes, as well as promotes a major reconstruction of the surface. The reaction comes to a standstill after one carbonate monolayer has been formed. If the wollastonite surface is already hydrated, the carbonation process needs to overcome a kinetic barrier, yet ending in a localized monolayer. But if the surface is heavily hydrated, that is, covered with multilayers of water, the thermodynamic ground state of the wollastonite completely changes. Due to a metal-proton exchange reaction (also called early-stage hydration), Ca2+ ions are partially removed from the solid phase into the H2O/wollastonite interface. Such mobile Ca2+ ions react again with CO2 and form carbonate complexes, ending in a delocalized layer. These results were confirmed by means of high-resolution time-of-flight secondary-ion mass spectrometry images and low-energy ionscattering spectroscopy. Very interesting research on dynamic storage has been performed by Klaus Lackner. For instance, he showed that Na- or Ca-based sorbents generally have very high heats of adsorption and regeneration temperatures, namely 1200 to 1500 K for the Ca-based calcination process.22 Sorbents in dynamic systems are amine-based (NH4+), and they capture CO2 from ambient air when dry and release it when wet (the so-called moisture swing).23 In this context, a key result for mineral science has been recently published: contrary to metal-oxide systems, exchange of metals without degrading the silicate structure was demonstrated, the so-called metal-metal exchange reaction.24 This finding opens the door to future design of silicates with incorporated ions for specific applications. In this work, Calcium-Silicate-Hydrates (CSH) and Magnesium-Silicate-Hydrates (MSH) on a silicon wafer have been synthesized. Both CSH and MSH phases are well characterized by means of Environmental-Scanning Electron Microscopy (ESEM), Fourier Transform Infrared Spectroscopy (FTIR), ACS Paragon Plus Environment

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and X-Ray Photoelectron Spectroscopy (XPS). MSH is a novel cementitious material that may have a role to play in construction, but must be extensively assessed prior to usage. Surprisingly, MSH did not show any carbonate contamination. After examining the resistance of MSH phases by first-principles calculations, we combine the new understanding of such process with the metal-metal exchange reaction concept. Finally, we demonstrate that it is possible to spray a (Mg2+)-containing solution on cement-based infrastructure to make it more resistant against CO2 contamination25-29.


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Methods A. Sample Preparation A 3cm x 1cm Si(111) crystal (thickness 500 µm, CZ production, double side polished) was chemically cleaned with a 30 min exposure at 80 °C to a 1:3 solution of aqueous H2O2: 18M H2SO4 (piranha solution). Afterward, the crystal was rinsed thoroughly with deionized (DI) water with a resistivity of 0.055 µS/cm. Following the cleaning, the wafer was immersed in a solution of 5 mM Ca(OH)2 at different temperatures (60, 100, 150, and 200 °C) in 100 mL reactors over 6 h. According to the temperature of preparation (T), the samples are later called CSH_T. Aqueous solutions of Ca(OH)2, as bought from Sigma-Aldrich, were prepared in an N2(g) atmosphere to avoid any effects due to CO2 contamination. After the synthesis, the samples were rinsed with water and dried with argon inside the N2(g) atmosphere. The same procedure was performed with Mg(OH)2 to synthesize MSH_T samples30.

B. Sample Characterization Environmental Scanning Electron Microscopy (ESEM) SEM micrographs were recorded using a NEON 40 (Carl Zeiss SMT AG, Germany) field emission gun environmental scanning electron microscopy (FEG-ESEM), operated using electron energies between 15 kV and 30 kV at a chamber pressure between 0.7 Torr and 1 Torr.

C. Fourier Transform Infrared Spectroscopy (FTIR) FTIR measurements were performed in an N2(g) purged glovebox with a Bruker Vertex 70 and recorded with a nominal 4 cm-1 resolution. Spectra were collected from 400 cm-1 to 4000 cm-1 in transmission mode with an angle of incidence of 64o with respect to the silicon surface normal. A room temperature pyroelectric detector (DTGS) was employed for data collection. In all experiments, 1024 scans were detected.

D. X-ray photoelectron spectroscopy (XPS) XPS experiments were carried out at the HE-SGM beamline at the synchrotron radiation facility BESSY II, which is a part of the Helmholtz-Zentrum Berlin (HZB). The photon energy of the exciting radiation has been chosen, (i) to keep the kinetic energy of the emitted photoelectrons between 200 eV – 300 eV, in order to increase surface sensitivity, and (ii) to avoid the mixture core level and Auger lines in the obtained spectra. The acquisition of the XPS spectra was carried out in normal emission geometry with an energy resolution of 0.3 eV at an excitation energy of 300 eV. The spectra were fitted by symmetric Voigt functions and a Shirley-type or linear background.

E. Computational details The calculations were performed using density-functional theory (DFT) within the generalized gradient approximation (GGA), as implemented in the Vienna ab initio simulation package (VASP).31 The electron-ion interaction was described within the projector-augmented wave (PAW) scheme.32 The electronic wave functions were expanded into plane waves up to a kinetic energy of 400 eV. In this ACS Paragon Plus Environment

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work, we considered the (010) surface of Enstatite, MgSiO3, as a surface model for MSH (see the details below), by means of a 2x2 supercell containing two layers of oxygen-terminated MgSiO3, for a total number of 120 atoms, plus adsorbed molecules and a vacuum region of approximately 20 Å, large enough to avoid interactions between the surface and adsorbed molecules and their replica images. All the atoms and degrees of freedom, except the bottom Si layer, were allowed to relax via a conjugate gradient technique until the forces on the atoms were below 1 meV/Å. The Brillouin zone integration was performed using a 4x1x4 mesh within the Monkhorst-Pack scheme.33 The PBE functional was used to describe the electron exchange and correlation energies within the GGA.34 The kinetic energy barriers for carbonate formation were obtained with the climbing image nudged elastic band (CI-NEB) method,35 using a string of geometric configurations to describe the reaction pathways of the CO2 molecule adsorbed on the hydrated E(010) surface under different H2O chemical potential conditions. A spring interaction between every configuration ensured continuity of the reaction pathway. For the ab initio Molecular Dynamics (AIMD) simulations, we used the velocity Verlet algorithm to solve the equations of motion and a time step of 1 fs. The adsorption energy of the CO2 molecules was calculated as the difference between the total energy of the system with respect to the individual energies of the slab and the molecules.


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Results and Discussion PART 1: Experimental Observation Samples of ultra-thin CSH phases were synthesized on silicon wafers at different reaction temperatures, while keeping the reaction time constant. The reactions were performed in closed systems. Hence, the pressure increases with temperature. We begin our analysis by examining the grown CSH phases as a function of temperature with the help of ESEM images.

Figure 1: ESEM images of ultra-thin CSH phases grown on silicon wafers at different temperatures. Note that similar images have been published in [24].

Figure 1 clearly shows the strong influence of the temperature and pressure on the thickness of the CSH phases, as well as their nature. This behavior can be connected directly to the effect of the temperature on the solubility of the two main reaction products, namely Ca(OH)2 and Si(OH)4. While the solubility of Ca(OH)2 decreases with rising temperature, the solubility of Si(OH)4 rises. This conflicting role leads to a supersaturated solution of Ca(OH)2 at high temperature.36


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Figure 2: ESEM images of ultra-thin MSH phases, grown on silicon wafers at different temperatures.

A representative set of images of the as-synthesized MSH phases is depicted in Figure 2. The silicon wafer immersed in the Mg(OH)2 solution followed a similar wet chemical reaction to that of Ca(OH)2, except for two prominent features: i) MSH phases are much thinner, and ii) while the solubility of Ca(OH)2 in water decreases with rising temperature, the solubility of Mg(OH)2 in water increases with temperature. In contrast to the reaction of Ca(OH)2, we do not see a supersaturation of the solution at 200 °C. However, the precipitated phases (the spherical deposits observed at 200 °C) still need to be identified in future research. The obtained CSH and MSH phases can be very well characterized with the help of FTIR spectroscopy measurements. Figure 3 shows the IR spectra of ultra-thin CSH and MSH phases, grown on silicon wafers at different temperatures and referenced to the clean silicon wafer. The characteristics of the vibration modes allow the understanding of the growth changes as a function of temperature.


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Figure 3: IR spectra of ultra-thin, A) CSH and B) MSH phases, grown on silicon wafers at different temperatures, referenced to the clean silicon wafer.

The vibrational modes corresponding to molecular and dissociated water can be easily observed in Figure 3A. Two features related to the presence of OH are also present: a broad peak between 3100 and 3500 cm-1 corresponding to the adsorption of water, whereas the sharp peaks at 3691 cm-1 and 3736 cm-1 confirm the presence of dissociated water, through the stretching vibrational modes of CaOH and Si-OH, respectively 17. A very similar behavior for the MSH phases can be noted in Figure 3B. First, again two sharp peaks at 3677 cm-1 and 3735 cm-1, corresponding to the stretching vibration modes of Mg-OH and Si-OH, confirm the presence of dissociated water. Second, a broad peak between 3100 and 3500 cm-1 remarks the presence of molecular water in MSH phases. One can differentiate between CSH and MSH phases due to the shift of the stretching mode of the OH group bound to the metal ion of the corresponding silicate material.


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Figure 4: IR spectra at low wavenumbers of ultra-thin, A) CSH and B) MSH phases, grown on silicon wafers at different temperatures, referenced to the clean silicon wafer.

The FTIR spectra of the CSH phases at low wavenumbers show that (see Figure 4A), besides the vibrational modes associated to the presence of silicates, there are also modes related to carbonates (in νas(C-O), with a maximum at 1487 cm-1 and δ(O-C-O) at 875 cm-1).17 Furthermore, vibration modes corresponding to ν(Si-O) and δ(O-Si-O) are also observed, which evidences the formation of the CSH phases.18 The formation of such CSH phases constitutes the starting point for all the techniques applied in this work to passivate the silicate surfaces. It has already been proposed that carbonates do not stop the corrosion of CSH phases, but experimental confirmation is also important. In contrast to the CSH phases, nearly no carbonates were found on the MSH phases (see Figure 4B). This is highly surprising, because the standard formation enthalpies of CaCO3 and MgCO3 are relatively similar (1207 kJ/mol for CaCO3 and 1113 kJ/mol for MgCO3). Therefore, such effect cannot be explained only by thermodynamics. While ESEM and FTIR measurements already provide a good understanding of the surface chemistry and topology of CSH and MSH phases, the surface sensitivity of XPS is more suitable to extract information about the thinnest films (obtained by reaction at the lowest temperature, 60 °C). Figure 5 shows the XPS data obtained for ultra-thin CSH phases.


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Figure 5: XPS of ultra-thin CSH phases grown on silicon wafers at 60 °C.

The Ca 2p spectrum shown in Figure 5 is dominated by a strong Ca 2p3/2,1/2 doublet at 356.2 eV (Ca 2p3/2) and 352.65 eV (Ca 2p1/2). The doublets were fitted by using two peaks with the same full width at half-maximum (FWHM), the standard spin-orbit splitting of 3.55 eV (verified by fit), and a branching ratio of 2 (Ca 2p3/2/Ca 2p1/2). The normalized O 1s spectrum exhibits two component peaks at 536.04 eV and 537.3 eV, associated with two different chemical states in the sample (carbonates and silicates). Finally, the Si 2p spectrum obtained is characteristic of the silicon oxide. The strong (EB~5 eV) shift of the binding energies of all lines with respect to the tabulated values clearly demonstrates the sample charging during the XPS measurements. The small variation of this shift for different peaks is explained by the different kinetic energy of the emitted electrons. Since the calibration of the spectra position could not be done for such samples, we present all spectra in “as measured” fashion. It is also necessary to mention that the samples were introduced in the UHV chamber directly from ambient conditions. Thus their surface could not be cleaned in the UHV chamber without destroying the sample itself. This results in a strong C 1s peak due to carbon contamination of the sample surface, which does not allow us to obtain detailed C 1s spectra and, therefore, it is not presented in this work. The results of the quantitative analysis of the XPS data obtained for CSH phases are summarized in Table 1. Table 1: XPS measurements of the atomic composition of ultra-thin CSH grown on silicon wafers at 60 °C.

Element

Orbital

Position [eV]

FWHM

ASF *

Area

Atomic-%

Si

2p

105.9

1.90

0.283

58737

12.9

Ca

2p1/2

352.65

1.94

1.634

55655

9.19

2p3/2

356.2

1.94

1.634

27827

4.6

1s

536.04

1.88

0.711

76112

25.12

1s

537.30

1.88

0.711

145327

47.97

O

*ASF: atomic sensitivity factor


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Figure 6: XPS of ultra-thin MSH phases grown on silicon wafers at 60 °C.

The XPS spectra of MSH phases are shown in Figure 6. The Mg 2p spectrum is presented as an unresolved doublet at 352.65 eV. A very interesting feature is that the normalized O 1s spectrum exhibits only one dominant component at 534.47 eV, associated with only one chemical state of oxygen in the sample (silicates). The negligible (~2%) additional contribution (carbonates) can be explained by surface contamination. The Si 2p spectrum presents both the peak corresponding to silicon oxide at 103.46 eV and a typical Si 2p doublet from the silicon substrate. The latter allows us to conclude the small thickness of the MSH phase layer. A shift of only 2 eV in the binding energies was observed, which can also be correlated with the lower thickness of the MSH layer. The quantitative analysis of the XPS data spectra obtained for MSH phases is summarized in Table 2. We can then conclude this section by saying that IR and XPS measurements show that carbonates are ubiquitous on the surface of CSH phases. In contrast, nearly no carbonates are found on the surface of MSH phases.

Table 2: XPS measurements of the atomic composition of ultra-thin MSH phases grown on silicon wafers at 60 °C.

Element

Orbital

Position [eV]

FWHM

ASF *

Area

Atomic-%

Mg

2p

51.53

2.23

0.252

7540

3.83

Si

2p3/2

99.63

0.47

0.283

8541

5.15

2p1/2

100.25

0.47

0.283

4270

2.58

2p

103.46

1.81

0.283

72881

43.95

1s

532.88

1.85

0.711

7378

2.66

1s

534.47

1.85

0.711

116196

41.84

O

*ASF: atomic sensitivity factor


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PART 2: Atomistic Analysis

A. CO2 adsorption on E(010). Enstatite (E, MgSiO3) is a mineral formed by tilted Si-O tetrahedra, connected along the a and b (inplane) directions by Mg-O octahedra (see Figure 7). In the out-of-plane direction, the structure contains apex-to-apex joining Si-O tetrahedra, forming zigzag chains along the c direction. The adsorption properties and chemical reactivity of any compound also depend on the facet exposed to the adsorbing molecular species. Surface energy, polarity, and atomic termination determine to a large extent the chemical reactivity of a specific facet. In this study, we are basically interested in the carbonate formation under different water chemical potential conditions, that is, the adsorption of CO2 on Enstatite surfaces subject to different OH terminations. There are three main options to cleave the bulk system, those ending up in (001), (100) and (010) surfaces (see Figure 7). As can be seen in the figure, the three surfaces are nonpolar. They are type I surfaces according to Tasker's classification, which is defined by zero charges (q = 0) and dipole moment (µ = 0). The surface energy is defined as the difference in total energy between the bulk and the surface per unit area and, for a stoichiometric surface, can be obtained by the following equation: (1) γ=(Etot-Ebulk)/2A where Etot is the total energy of the surface, Ebulk is the bulk energy, and A is the surface area. For nonstoichiometric surfaces (with, for instance, additional OH termination groups), the chemical potential (µi) of the elements in excess (ni) must also be considered. The obtained surface energies of E(100), E(010) and E(001) are 0.224, 0.075, and 0.101 eV/Å2, respectively. Therefore, E(010) not only shows the lowest surface energy (that is, it requires “less energy” to cleave the bulk to create the surface) but also presents a more suitable atomic arrangement for CO2 adsorption. Indeed, E(100) undergoes a strong Si dimer reconstruction (the two end Si atoms are at the same height before relaxation), whereas E(001), although relatively similar to E(010), has a more pronounced Mg-Si surface step, thus complicating the study of their respective roles on CO2 adsorption and subsequent carbonate formation. Therefore, the E(010) surface is considered throughout this work. The first remark that needs to be mentioned is that, unlike wollastonite(001), water-free E(010) is not only oxygen-terminated, but it also shows three different types of reactive sites: Mg, Si, and O itself (see Figure 8).


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Figure 7. Surface energies of the different facets of Enstatite (MgSiO3). Red, orange and blue balls represent oxygen, magnesium, and silicon atoms, respectively.

If a CO2 molecule reacts with water-free E(010), there exist three different possibilities for adsorption, as depicted in Figure 8. The first two involve the formation of an Mg-O bond, at the top Mg site and at the valley of the E(010) surface, respectively. The third adsorption structure corresponds to carbonate formation with one of the surface oxygens. Any other structure evolves to one of these three. As Figure 8 shows, the formation of an Mg-O bond is the most likely thermodynamic reaction product, although the binding energies are all relatively small and similar to each other. Due to the configuration of the E(010) surface, the CO2 can adsorb in the Mg-valley site, forming a structure almost perpendicular to the surface itself due to the proximity of the Mg-surface site. The formation of a bridge Mg-O-C-O-Mg structure is unlikely, due to the distance between both Mg surface sites (4.92 Å). The adsorption of CO2 in the Mg-valley is the most thermodynamically stable adsorption structure, with a binding energy of -0.23 eV, with the formation of an Mg-O bond on a top Mg site and the CO32- carbonate structure displaying similar binding energies, -0.13 and -0.11 eV. The formation of the CO32- carbonate structure takes place over the surface Si-O tetrahedra, and the calculated binding energy (-0.11 eV) is weaker than that of calcium-based systems (-0.24 eV), due to the larger C-surface oxygen distance, ~2.2 Å. The reason is the presence here of Mg active sites. If one performs AIMD simulations of CO2 adsorption on the E(010) surface, the outcome is always the formation of an Mg-O bond, either in the top of or on the Mg valley sites. We can then conclude that carbonate formation on water-free E(010) is a weak physisorption process.


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Figure 8: CO2 adsorption on water free E(010). Red, orange and blue balls represent oxygen, magnesium, and silicon atoms, respectively.

CO2 adsorption is a spontaneous process with no kinetic energy barrier, see Figure 8. However, some changes can be noticed in the local structure. For instance, Mg-O bonds are slightly stretched (from 1.91 to 1.95 Å), whereas the O-C-O/Mg bond distance is 2.35 Å. Obviously, there is a small charge transfer from the bonding oxygen to the surface Mg, which makes the CO2 molecule slightly asymmetric and polar, with C-O distances of 1.16 and 1.18 Å.

B. CO2 adsorption on E(010) covered by a monolayer of water. If the E(010) surface is covered by water with different coverages, that is, water under different chemical potential conditions, CO2 adsorption characteristics differ greatly. We have considered two separate cases: below one H2O monolayer and a full H2O monolayer coverage. For the E(010) surface with less than a monolayer of water coverage, the H2O molecule can be adsorbed either on the top or on the valley Mg reactive site (see Figure 9). If the water molecule is adsorbed on the top Mg site, the impinging CO2 molecule undergoes a very weak physisorption process (Figure 9). The physisorption energy is 0.05 eV, and the O-H distance is about 2 Å. On the contrary, water adsorption on the Mg valley site and subsequent CO2 physisorption gives an extra 0.37 eV of binding energy, but this is due to the superior thermodynamic stability of the valley site, and not to a stronger physisorption. The important remark is that AIMD simulations did not produce spontaneous carbonate formation. Indeed, ACS Paragon Plus Environment

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H2O dissociation to form CO3H- always results in CO2 separation and H2O re-adsorption. The changes in the local structure of the surface can easily explain this. As can be seen in Figure 9, water adsorption results in the rupture of the surface Mg-O-Si bridge bond, to produce Mg-O and Si-O terminations, thus conferring an extra degree of stability due to oxygen-passivation of those surface species.

Figure 9: CO2 adsorption on E(010) covered with less than- and one monolayer of water. Red, orange and blue balls represent oxygen, magnesium, and silicon atoms, respectively.

With full water monolayer coverage, the physisorption distance is a bit longer, with an O-H distance of 2.3 Å, due to the presence of additional H2O molecules on the surface (see Figure 9). However, the physisorption energy is -0.11 eV, that is, two times larger than that of the E(010) with less than a full monolayer of water coverage. No additional changes in the local surface structure are observed (besides the aforementioned rupture of an Mg-O-Si bond and the formation of two separate Mg-OH and Si-O groups). The same considerations apply with respect to AIMD simulations, no spontaneous carbonation is obtained after 20 ps. Therefore, carbonation formation must be accompanied by other processes with large thermodynamic energy requirements or kinetic energy barrier. Indeed, as can be seen in Figure 9, CO3H- formation results in partial reconstruction of the Mg-O-Si termination, forming now an OH- bond which stands out from the surface. Besides, the process is strongly endothermic, with a final reaction energy of 2.02 eV. The kinetic energy barrier is 2.36 eV, not very high with respect to the final state (only 0.36 eV), which indicates the ease with which water can be re-adsorbed again on the E(010) surface during carbonate formation. The reaction path is shown in Figure 9. It can be seen in the figure (in the intermediate configurations of the reaction path) that, after the initial ACS Paragon Plus Environment

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physisorption, the water OH group is detached from the surface while at the same time the reconstruction of the Mg-O-Si bond begins, to end with the Mg-OH-Si termination aforementioned. The CO2 molecule does not lose its linear configuration until the interaction with the additional OHstarts, thus implying that this process of carbonate formation is not affected by neighboring Mg-H2O or Si-O termination structures (only the temporary formation of a hydrogen bond can be noticed). Finally, the resulting structure adopts the characteristic CO3H- planar shape. Therefore, the endothermic character of the reaction and the moderately high kinetic energy barrier demonstrate the low-speed formation of carbonates when the E(010) surface is covered by less than- or a full monolayer of water.

C. CO2 adsorption on E(010) covered by more than one monolayer of water. Finally, we would like to remark that, as has been shown previously by Oelkers et al. and our own work,37 metal-proton exchange reactions are strongly mineral-specific.38-40 As such, the number of protons consumed in each exchange reaction can differ notably, even if the nominal valence state of the metal present in the mineral is the same (as it happens for Ca2+ and Mg2+). We have shown that nearly no carbonates were found on the surface of MSH phases, that is, in the presence of water the E(010) surface does not form a nonstoichiometric thermodynamic ground state in equilibrium with a hypothetical Mg(OH)2 solution or, at least, that is an extremely slow process.

Figure 10: CO2 adsorption on E(010) covered with more than one monolayer of water. Red, orange and blue balls represent oxygen, magnesium, and silicon atoms, respectively.

In order to model such process, we can consider an E(010) surface covered with more than a monolayer of water, that is, having additional OH- groups on some of the surface Mg sites (see Figure 10). The CO2 adsorption path is then as follows. First, as the CO2 molecule approaches the surface, a weak physisorption is produced, due to the interaction between the carbon atom and one of the surface oxygens. The physisorption energy is -0.11 eV, of the same magnitude as the previous case (surface covered by a monolayer of water), because the atomic interaction is the same. The C-O distance is a bit longer, 2.52 Å, due to the specific surface configuration, with tilted OH- groups due to the hydrogen bonds formed between the hydrogens and neighboring, non-passivated surface oxygen atoms (see Figure 10). After this initial CO2 physisorption, again AIMD simulations show neither carbonate formation nor Mg detachment from the surface to form a hypothetical Mg(OH)2 solution in ACS Paragon Plus Environment

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equilibrium with the surface. CO3H- formation is an endothermic process, with a reaction energy of 1.48 eV and a kinetic energy barrier of 1.81 eV (Figure 10). Both the reaction energy and the kinetic barrier are lower than that of the previous case (E(010) surface with less than a monolayer of water coverage). The reason is that the availability of additional OH groups keeps the surface structure more or less unchanged, with no reconstruction after the CO3H- formation. Therefore, we can conclude that, although slowly, increasing the chemical potential of water could still result in some carbonate formation. Therefore, our results show that the exposure of the water-free Enstatite surface to CO2 results in barrier-less carbonation. CO2 reacts with the surface oxygen and forms carbonate (CO32–) complexes, together with a major reconstruction of the surface. The reaction comes to a standstill after one carbonate monolayer has been formed. If the surface is covered by a monolayer of water, the carbonation is no more a barrier-less process, yet ending in a localized monolayer. Finally, if covered with multilayers of water, the thermodynamic ground state of the Enstatite changes due to a metal– proton exchange reaction (found as MPER in literature) and Mg2+ ions are partially removed from the solid phase into the H2O/Enstatite interface. The main difference between the reactivity of CSH and MSH phases is not the reaction with CO2 itself, but the slowdown of the MPER. This is experimentally proved by a higher pH value (~10) at the MSH/water interface, as compared to the CSH/water interface (pH ~ 12).


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PART 3: Future scenarios and implications for the industry. Finally, we would like to discuss possible applications of the results shown so far, in order to emphasize the advantages of the new knowledge. To achieve that, the following experiment was devised and performed: first, carbonate was turned into separated H2O and CO2 and,41 then, the recovering process was tracked by means of IR spectroscopy. Second, a strip of cement was decarbonated, followed by passivation through immersion into a solution of Mg(OH)2 for 1h at room temperature. As expected, we do not find any carbonation on the substrate after this passivation step. The details of the experiment are now discussed on the base of FTIR spectra measurements.

Figure 11: A) FTIR of a CSH phase grown on a silicon wafer, referenced to the clean silicon wafer. B) FTIR of the sample immersed in HCl (4%, 10s), referenced to the CSH phase grown on the silicon wafer. C) FTIR of the sample 1h after immersion in HCl, referenced to sample immersed in HCl.

Figure 11A shows the IR spectrum of CSH phases grown on a silicon wafer, referenced to the clean silicon wafer. Afterward, the sample was dipped in HCl (concentration 4%) for 10 s, rinsed with H2O, dried with nitrogen and again FTIR measurements were performed. The result is depicted in Figure 11B (red line), this time referenced to the CSH phase before immersion in the HCl. At 1474 cm-1 one ACS Paragon Plus Environment

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can see the loss of carbonates bound to the surface. Without further surface treatment, carbonates will grow again in a short period of time. This chemical behavior is mainly explained by the metalproton exchange reaction, that drives out Ca2+ ions and makes them freely available for the reaction with CO2. In Figure 11C (upper part, red line) IR measurements of the HCl-treated sample already show the regenerated carbonates.

Figure 12 A) FTIR of a CSH phase grown on a silicon wafer, referenced to the clean silicon wafer. B) FTIR of the sample immersed in HCl (4%, 10s), referenced to the CSH phase grown on a silicon wafer. C) FTIR of the sample immersed in Mg(OH)2 (10mmol, 1h), referenced to the sample immersed in HCl. D) FTIR of the sample 1h after immersion in Mg(OH)2, referenced to the sample immersed in Mg(OH)2.

If the chemical behavior of the sample after immersion in HCl is altered, the IR spectra changes dramatically. Now, after removal of carbonates from the surface through immersion in HCl, the sample was dipped into a solution of Mg(OH)2. Based on the metal-metal exchange reaction (Mg vs Ca), the underlying idea was to passivate the surface against further chemical reactions with CO2. Figure 12C shows the FTIR spectra of the sample immersed in Mg(OH)2 (10mmol, 1h), referenced to the sample ACS Paragon Plus Environment

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immersed in HCl, and Figure 12D shows the FTIR of the same sample 1h after immersion in Mg(OH)2, referenced to the sample immersed in Mg(OH)2. In contrast to the untreated sample, the figure shows no carbonate contamination. These results allow us to conclude: (1) The Metal-Metal Exchange Reaction has been successfully performed for Mg vs Ca on cement. (2) MSH phases passivate the cement surface against CO2 corrosion (here passivate stands for slowing down the carbonation kinetics). (3) The reason for such passivation is given by a kinetic hindering through water molecules adsorbed on the CSH surface.

Conclusions To sum up, in this work we have shown the synthesis of CSH and MSH phases, which are important building blocks of today's infrastructure industry. ESEM, FTIR and XPS measurements show that carbonates are always present on the surface of CSH phases. Contrarily, almost no carbonates are found on the surface of MSH phases. DFT results explain that carbonate formation on the surface of MSH is a thermodynamically endothermic process with a moderately high kinetic energy barrier. Although slowly, increasing the chemical potential of water could still result in some carbonate formation on the MSH surface. Finally, as an example of the importance of our findings, we show that the use of an Mg-containing solution passivates CSH surfaces effectively against CO2 contamination. This scenario can be important for several industrial applications, especially in regards to coating cement surfaces against corrosion, thus opening a new door to atomically design new coatings and protective layers for cement-based infrastructure.

Notes The authors declare no competing financial interest.

ORCID Roberto Longo 0000-0003-4353-841X Peter Thissen 0000-0001-7072-4109 Alexei Nefedov 0000-0003-2771-6386

Acknowledgements Peter Thissen acknowledges the DFG for financial support. The authors acknowledge the Texas Advanced Computing Center (TACC) for computational resources. We thank Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime at BESSY II.


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Table of Contents

Synopsis: A Metal-Metal Exchange Reaction has been successfully performed for Mg vs Ca on cement and Magnesium-Silicate-Hydrate phases passivate the surface against CO2 corrosion.


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Chemical Properties of Metal-Silicates Rendered by Metal Exchange

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