Atomistic Simulation of Surface Selectivity on Carbonate Formation at

Article pubs.acs.org/JPCC

Atomistic Simulation of Surface Selectivity on Carbonate Formation at Calcium and Magnesium Oxide Surfaces Jeremy P. Allen,*,†,‡ Arnaud Marmier,§ and Stephen C. Parker*,† †

Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom Department of Engineering, University of Exeter, North Park Road, Exeter EX4 4QF, United Kingdom

§

ABSTRACT: We report atom-level simulations of the surface selectivity and the resulting surface phase diagrams for the {100}, {110}, {111}, and {310} surfaces of CaO and MgO as a function of varying CO2 and H2O partial pressures. This work extends the traditional approach based on ab initio calculations, which can be time-consuming and costly for large systems, by using semiempirical atomistic simulations. The advantage of this approach is that very large numbers of calculations can be performed, thereby allowing a more effective search of the configurational space. The resulting free energies are used to generate the surface phase diagrams. The results indicate that the {100} surfaces of MgO and CaO show different dominant phases at atmospheric concentrations of gaseous water and carbon dioxide. The CaO surface forms a carbonated phase, whereas the MgO surface contains associatively adsorbed water. On the other hand, both {111} surfaces show the dominance of surface hydroxylation, effectively forming a layer of mineral hydroxide, with carbonation only observed at very high carbon dioxide concentrations. Finally, the {310} surfaces show enhanced reactivity with carbonate, most likely a result of the steps on the surfaces. In general, we predict that the minimum CO2 partial pressure needed to carbonate these surfaces can be controlled by the water partial pressure. The effect of temperature is also considered, and the results show how the number of surface adsorbates decreases as temperature increases.

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Krischok et al.32 used metastable impact electron spectroscopy to show that the chemisorption of carbon dioxide on MgO and CaO surfaces occurs via reaction with a surface O−2 species to form a carbonate species. For CaO, this was seen to occur at regular lattice sites, but for MgO only low-coordinated sites resulted in this chemisorption. This was shown more recently by both Kadossov and Burghaus33 and Voights et al.34 to result in the formation of a layer of CaCO3 for the CaO (100) surface. As well as the difference in reactivity, both experimental and computational studies have considered the manner of adsorption and resulting orientation of the carbonate to the surface. Yanagisawa et al.35 used a mixture of temperature-programmed desorption, infrared (IR) spectroscopy, and ab initio modeling to study the carbonation of MgO powders, reporting two distinct carbonate environments, primarily due to monodentate bonding, with some partial bidentate coordination. The mode of carbonate adsorption was also studied previously by Jensen et al.13, through the examination of theoretical IR frequencies. The findings concurred with Yanagisawa, in that two adsorption modes were found for MgO, a monodentate mode on edge sites and a bidentate mode on corner sites. For CaO, however, both sites show monodentate bonding only. In our previous study,36 we showed that although strong surface interactions are seen for

INTRODUCTION In 1990, Seifritz postulated a process for removing anthropogenic carbon from the environment and industrial waste streams based on the natural weathering process of alkaline silicate materials.1 This process, usually termed as mineral or carbon sequestration, involves the adsorption, and subsequent reaction of, carbon dioxide with a mineral to form a stable carbonate phase, which can then be disposed of in a more environmentally sound manner. Since 1990, a wealth of research2−9 has gone into trying to optimize both the process and the minerals used. Some of the most promising materials thus far have been based on calcium- and magnesium-bearing minerals, namely, silicates and hydroxysilicates.10−12 An important requirement for a better exploitation of this process is to understand the surface selectivity at the atomic scale and how the surface structure and stability affect the reactivity and formation of the carbonate product. Furthermore, because water is ubiquitous in the environment, it is also of importance to understand how its presence can affect the carbon sequestration process. The majority of studies so far have mainly focused on the adsorption and reaction of CO2 with alkaline metal oxides and hydroxides, such as MgO 13−17 and Mg(OH)2,18−20 which can be used as simple models prior to the study of higher order oxide materials. The other advantage of studying these simple oxide materials is due to the wealth of knowledge in the literature using both atomistic simulations,21−23 ab initio modeling,24−28 and experiment.29−31 © 2012 American Chemical Society

Received: April 6, 2012 Revised: May 19, 2012 Published: May 22, 2012 13240

dx.doi.org/10.1021/jp303301q | J. Phys. Chem. C 2012, 116, 13240−13251

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monodentate bonding above the surface, direct incorporation of the carbonate species into the surface results in a greater degree of surface stabilization. For both materials, the presence of a step, while destabilizing the surface relative to a flat terrace, enhanced surface carbonation. In contrast to surface carbonation, the adsorption of water has been studied more extensively on the surfaces of these simple oxides. Studies have shown that water adsorption on the (100) MgO surface occurs via a primarily associative mode of adsorption but with a small amount of dissociation.37,38 Foster et al.39 reported results from IR spectroscopy which showed that only the physisorption of water was seen on freshly cleaved MgO (100) surfaces; however, hydroxyl groups were later observed after several days, mostly present at defect sites. Computational methodologies have also been used to understand both the associative and dissociative adsorption of water on other low index surfaces of MgO and CaO. For example, the dissociative adsorption of water is seen to be especially favorable on the {111} surface, and the presence of the step on the {310} surface directs adsorption onto the step edge prior to inside the step or on the terrace.40 de Leeuw et al.41,42 also studied the effect that the Mg coordination number has on the adsorption of water on MgO surfaces. Surfaces with highly coordinated Mg atoms were found to favor the associative, or physisorption, of water, whereas hydroxylation was seen to dominate for Mg atoms with a low coordination number. This was also seen by Costa et al.43 and Goniakowski and Noguera25 using ab initio modeling. Despite the individual knowledge of carbon dioxide or water on the surfaces of these materials, little work has been conducted to understand and examine the relationship of the adsorbates together or the resulting surface adsorption. However, the role of differing adsorbates on other surfaces has been examined by researchers. One method of studying this has been through the generation of surface phase diagrams to elucidate the effect adsorbate concentration has on both the surface structure and composition. For example, we44 used this method to consider the hydroxylation of α-alumina for a range of low-index surfaces. The results showed that surfaces were either fully stoichiometric or fully hydroxylated, except at extremes of oxygen or hydrogen partial pressures, with surface energy trends that were, in general, in agreement with experiment.45,46 Similarly, Kerisit et al.47 used this approach to consider the {101̅4} calcite surface and its interaction with water, with results showing the importance of nonstoichiometric surfaces in understanding the surface structure and chemistry. In addition, they reported that the relative humidity plays a significant role in defining the surface structure due to it being located on a boundary between hydrated stoichiometric and hydrated calcium-poor terminations. Stampfl48 used a similar approach to consider surface phase transitions of a range of different materials, including the adsorption of oxygen on a number of transition metal surfaces. Overall, these studies have demonstrated that surface phase diagrams are not only of use to predict phase stabilities but also to analyze complex or ambiguous experimental results. However, these studies have relied solely on electronic structure techniques to calculate surface energies. Such first principle methods are computationally expensive for large or complex systems, which, out of necessity, reduce the size of the surfaces and number of configurations modeled. Atomistic techniques based on semiempirical potentials are an attractive alternative due to their reduced computational cost, robustness, and transferability because they can more easily scan through the large numbers of different surface compositions required to fully explore configurational space.

The limitation of these models is that they cannot reproduce chemical reactivity, but with new developments in reactive force fields, for example the work of Gale et al.,49 even this will no doubt be overcome. In summary, understanding the interaction of both water and carbon dioxide with mineral surfaces is essential to developing and exploiting a mineral sequestration process. Although a number of studies have considered elements of this process for CaO and MgO surfaces, using both experiment and computational study, there is a lack of knowledge surrounding the interplay of these species and the surface compositions as a function of partial pressure. Therefore, this work presents a preliminary atomistic simulation investigation using interatomic potentials on the low-index CaO and MgO surfaces and demonstrates that it can be used to predict surface composition, in terms of water and carbon dioxide coverage, as a function of partial pressure of the component species.

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METHODOLOGY Modeling Adsorption and Calculating Free Energies. The surfaces in this study are simulated with potential-based methods using the METADISE code.22 This approach is based on the Born model of solids which assumes that the energy of the system can be calculated from the pairwise addition of the longand short-range interactions between the atoms. The short-range interactions are modeled using parametrized analytical functions, such as Buckingham potentials. A shell model50 is also included to account for the polarizability for the oxygen atoms. These interatomic potentials are well-characterized and have been used in numerous previous studies.36,51−56 They are based on simulations of CaO and MgO by Lewis and Catlow.57 The model for water is that of de Leeuw et al.,21 which gives calculated hydration energies of chemisorption that agree well with available experimental enthalpies.42 The carbonate model is that of Pavese et al.,58 which has also been shown to give a good account of the carbonate−water interaction.55 The simulation cells were generated by first cleaving the unit cell normal to the appropriate surface and generating two regions, I and II, which are periodic in two dimensions, by adding together images of the cleaved unit cell perpendicular to the surface. Region I contains those ions adjacent to the surface, and they are allowed to relax to their mechanical equilibrium. The ions in region II represent the rest of the crystal and are kept fixed at their bulk equilibrium positions. The depths of regions I and II were 2 and 10 nm, respectively, to ensure convergence, and the surface area was scaled 2 × 2 from the primitive unit cell; thus, for example, the {100} CaO periodic surface area was approximately 0.92 nm2. The different modes of adsorption for water and carbon dioxide used in this study are detailed schematically in Figure 1. To account for the possibility of water dissociation, following the adsorption of a water molecule, two models are used. Associative adsorption, or hydration, is the adsorption of a whole water molecule, initially placed above a surface metal atom, whereas dissociative adsorption, or hydroxylation, involves the adsorption of a hydroxide group above a surface metal atom and a hydrogen atom above a surface oxygen atom. The associative and dissociative modes of water adsorption are detailed in Figure 1a,b, respectively. The surface adsorption of carbon dioxide, however, is more difficult to model with potential-based methods as they do not take into account the reactions that occur at the surface. Therefore, this process is modeled by a postreacted surface with the adsorption of carbonate groups. To ensure the 13241



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Figure 1. Schematical representation of water and CO2 adsorption on CaO and MgO surfaces. Water adsorption is modeled in (a) an associative and (b) a dissociative manner. Carbon dioxide adsorption is modeled as the reaction of CO2 with a surface oxygen atom, with the carbonate bonding to the surface either (c) above a surface metal atom with a corresponding oxygen vacancy (colored black) or (d) into the surface, where carbonate bonds directly into the surface at the location of the lattice oxygen atom involved in the reaction with CO2. Metal atoms are colored green, carbon is gray, and oxygen atoms are red, blue, or pink depending on whether they are from the lattice, water, or the carbonate, respectively.

Once a number of different surface configurations had been simulated, the free energy can be calculated from the configurational entropy in a manner similar to our previous work.36 The energy of the surface calculation, ES, can be corrected according to the number of surface adsorbates, n, using

surface remains charge neutral, for each carbonate group (charge of −2) a lattice oxygen ion (also with a charge of −2) is removed. This model is consistent with the carbonate species being formed through reaction of a CO2 molecule with a lattice oxygen atom from the surface. Carbonate addition is modeled in two ways, as addition above a surface metal atom, with a bidentate coordination and a surface oxygen vacancy from the lattice oxygen atom involved in the reaction with CO2, or addition into the surface by insertion of one of the carbonate oxygen atoms directly into the oxygen vacancy location. These two modes are shown schematically in Figure 1c,d, respectively. The carbonate groups are modeled as being aligned in either the x- or y-direction, where the z-direction is defined as being perpendicular to the surface. Surfaces containing mixed water and carbonate are generated from the surfaces found after surface carbonation, where the lowest energy surface for each carbonate orientation and coverage is both hydrated and hydroxylated until all surface metal and oxygen atoms are covered by water. To allow the calculation of surface free energies at different surface coverages, the number of adsorbed molecules is varied from one adsorbed molecule up to monolayer coverage. This maximum coverage is considered to be reached at a 1:1 ratio of adsorbate molecules to surface metal ions. The results generated from surface carbonation, as well as those formed through the adsorption of water, are discussed in detail in our previous paper.36 Following this work, mixed surfaces were considered which contain both water and carbonate groups. To generate these surface configurations, the minimum energy carbonated surface, for each coverage and carbonate orientation/addition mode, was then either hydrated or hydroxylated on the basis of the number of surface metal and lattice oxygen species present, using the approach described above. The assessment of the effect of surface coverage for the different adsorption modes of water, carbon dioxide, and a mixture of the two requires the evaluation of a large number of surface calculations. In this study, where four surfaces of two different materials are used, in excess of 180 000 different surface configurations were generated and their energies minimized.

corr corr corr − n EScorr = ES − nCO32−ECO 2− H 2OE H 2O − nH − OHE H − OH 3

(1)

Ecorr S

where is the corrected energy. The H2O and H−OH subscripts are used to distinguish between associatively and dissociatively adsorbed water, respectively. The energy correction terms for carbonate and water, Ecorr, account for the selfenergies of the species, effectively representing the change in the second electron affinity of oxygen in the different species. For liquid water, EHcorr is simply the sum of the self-energy of an 2O isolated water molecule, −9.10 eV, and the heat of vaporization, −0.45 eV. However, for the dissociative adsorption of water and carbonation these terms represent the energy balance of the reactions in eqs 2 and 3, where M refers to either Mg or Ca, and are determined using calculated lattice energies and formation enthalpies.36,59 M2 +(g) + O2 −(g) + H 2O(g) → M2 +(g) + 2HO−(g) (2)

M2 +(g) + O2 −(g) + CO2 (g) → M2 +(g) + CO32 −(g) (3)

The free energy of the simulation cell, Asimul, can then be calculated from the minimum energy surface configuration, Ecorr Smin , using the partition function, Q, via eq 4: A simul = EScorr − RT ln(Q ) min

(4)

Q is the sum over all surface calculations with the same number of surface adsorbates, as defined by Q=

∑ e(−E S

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corr corr S − E Smin / RT )

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configurations to ensure that configurational space is explored effectively. Generating Surface Phase Diagrams. The surface phase diagrams are determined through the evaluation of the surface free energies, γ, for the different surface configurations at varying chemical potentials, which are related by γ=

1 simul (A − nMOμMO − nCO2ΔμCO − n H2OΔμH O) 2 2 S (6)

where S is the surface area, n is the number of species, and μ is the chemical potential, which is displayed in terms of the change in chemical potential for water and carbon dioxide. The change in chemical potential is used so that, in this work, a Δμ of 0 eV represents the activity at 298.15 K, allowing us to easily consider changes in the concentration relative to this value. MO refers to the mineral oxide, where M is either Ca or Mg. Because the surface is in equilibrium with the bulk material, its energy, EMO, can be used to calculate the chemical potential of the mineral oxide, assuming that the entropic contribution to bulk free energy GMO is negligible: GMO = EMO = mμMO

(7)

where m is the number of formula units in the bulk simulation. It is convenient to define the excess, Γ, for both carbon dioxide and water by, respectively,

Figure 2. Surface phase diagrams of the (a) CaO and (b) MgO {100} surfaces as a function of the change in chemical potential of gaseous H2O and CO2. Right and top axes indicate the pressure of the gas (bar) at 298 K and the black circle indicates atmospheric conditions.

ΓCO2 =

1 nCO2 S

(8)

ΓH2O =

1 nH O S 2

(9)

By combining eqs 6−9, we can obtain an equation for calculating the surface free energy as a function of the change in chemical potential of carbon dioxide and water, as shown by

Once the free energies have been determined for all combinations, they can be used to generate surface phase diagrams. This approach to calculating free energies using potential-based simulation requires that we consider many different surface

γ=

n ⎞ 1 ⎛⎜ simul AMO − MO EMO⎟ − ΓCO2ΔμCO − ΓH2OΔμ H O 2 2 ⎠ S⎝ m (10)

Figure 3. Top and side views of the {100} CaO surfaces designated regions (a) IX and (b) VII in Figure 2a. Surfaces have been expanded 2 × 2 for clarity. 13243



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Figure 4. Side and top views of the {100} MgO surfaces designated regions (a) I, (b) II, (c) V, (d) VI, and (e) VII in Figure 2b. Surfaces have been expanded 2 × 2 for clarity.

With this technique, the surface energy can be calculated for any temperature and vapor-phase composition. Although both electronic structure and potential-based models can be used with this procedure, potential models lead to a computationally cheaper exploration of configurational space. Therefore, as long as the configurational space is efficiently explored, we can have confidence that all of the significant local minima have been found.

Additionally, the change in chemical potential as a result of the temperature can be determined by ΔμH O(T ) = T°s H2O(T°) + Δh H2O(T° , T ) − Ts H2O(T ) 2

(11)

where s and h are the entropy and enthalpy, which can be obtained from experimental data,60 and T° refers to 298.15 K. Although eq 11 refers solely to water vapor, an identical expression can be derived for gaseous carbon dioxide. Additionally, assuming that water vapor and carbon dioxide act as ideal gases, the effect of the partial pressure p on the change in chemical potential can be calculated from ΔμH O(T ) = 2

⎛ pH O ⎞ 1 kBT log⎜ 2 ⎟ 2 ⎝ p° ⎠

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RESULTS Surface Phase Diagrams. The generated surface phase diagrams are plotted over a range of changes in the chemical potential of water and carbon dioxide. This can also be considered in terms of the partial pressure at 298 K, shown on the axes above and to the right of the surface phase diagrams. The color code for the surface phase diagrams is that blue represents water and red is used for carbon dioxide; surfaces with mixed water and carbon dioxide are shaded with different combinations

(12)

where p° is 1 bar. An identical expression can also be written for carbon dioxide. 13244



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The surface concentrations, and their corresponding structures, are reflective of the lowest free energies. If the phase boundary has a positive slope, then the water adsorption is acting as a poison to further CO2 adsorption, whereas if the slope is negative, then water enhances the adsorption. Exceptions to this are when moving between regions which are composed of purely water. The {100} Surface. The surface phase diagrams for the {100} CaO and MgO surfaces are detailed in Figure 2a,b, respectively. The phase diagrams indicate that CaO has a greater susceptibility to surface carbonation than MgO. At atmospheric conditions, the most dominant phase for the CaO surface (region IX) is formed purely through surface carbonation and corresponds to a coverage of 6.5 CO2 nm−2. The structure, shown in Figure 3a, indicates that the carbonates form ridges across the surface, with the resultant surface reminiscent of the pure {110} surface with its microfaceted appearance.40 The formation of this surface can be understood as the clustering of carbonates around a surface adsorption site.36 The formation of the first and second carbonates at this site cause the displacement of calcium ions out of the surface to increase the bonding with the carbonate groups, thus stabilizing the surface cluster. Subsequently, additional carbonates form around this site, aligning into ridges in one direction across this surface. Although this surface is shown to be the most stable, modest increases in the partial pressure of water will result in the coadsorption of water, in a dissociative manner, and a decrease in the amount of surface carbonate species. This mixed phase, labeled region VII in Figure 2a, possesses a surface composition of 6.5 H2O and 2.2 CO2 nm−2. The structure of this mixed surface, as seen in Figure 3b, comprises rows of CaCO3 units, with a structure similar to that seen in region IX, with hydroxide groups present across the remainder of the surface. In contrast, the MgO {100} surface, Figure 2b, shows a preference for water adsorption at atmospheric conditions. The most dominant surface under these conditions, region I, has a surface coverage of 7.1 H2O nm−2 and comprises primarily associatively adsorbed water molecules. This corresponds to a surface coverage of 0.62, which is comparable to the experimental monolayer coverage of 0.67, as reported by Xu and Goodman.31 A small increase in the concentration of water is predicted to increase the surface coverage of associatively adsorbed water to 10.0 H2O nm−2 (region II). The structures of these surfaces,

Figure 5. Surface phase diagrams of the (a) CaO and (b) MgO {110} surfaces as a function of the change in chemical potential of gaseous H2O and CO2. Right and top axes indicate the pressure of the gas (bar) at 298 K, and the black circles indicate atmospheric conditions.

of these base colors, with the intensity of the color varying from light to dark for low to high concentrations, respectively. The black circle on the surface phase diagrams represents atmospheric conditions of 3% CO2 and a vapor pressure of water of 26.7 Torr. The long-dashed, solid and small-dashed, vertical/horizontal lines on the diagrams represent unit activity (i.e., a change in chemical potential of 0 eV) at 0, 298, and 1000 K, respectively. Surface phase diagrams have been determined for the {100}, {110}, {111}, and {310} surfaces of CaO and MgO.

Figure 6. Side and top views of the {110} CaO surfaces designated (a) pure, (b) region VII, and (c) region IV in Figure 5a. Surfaces have been expanded for clarity (3 × 3). 13245



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Figure 7. Side and top views of the {110} MgO surfaces designated regions (a) I and (b) III in Figure 5b. Surfaces have been expanded for clarity (3 × 3). The surface structure of the pure surface is identical to the CaO equivalent.

Figure 4a,b for regions I and II, respectively, show that the water molecules coordinate together by hydrogen bonding. In region I, this occurs via chains of water molecules, similar to the clustering of water at sub-monolayer coverages previously seen by simulation.61 A disruption of the chain structure is seen in region II, due to the adsorption of water molecules on top of the chain, rather than directly on the surface. Interestingly, it can be seen that merely reducing the partial pressure of water would not result in carbonation. An increase in the partial pressure of carbon dioxide is also needed to achieve surface carbonation. This is consistent with experimental observations, where CO2 was not found to absorb on MgO under dry conditions at temperatures of less than 70 °C.62 The most accessible purely carbonated surface is region V (8.5 CO2 nm−2). However, modest increases in the water concentration will also give additional associatively adsorbed water molecules on this surface. The structures of these surfaces (regions V, VI, and VII) are shown in Figure 4c−e, respectively. As can be seen, the structures all possess the same basic carbonate structure, a disordered MgCO3 layer. Water then adsorbs to this surface, orientating to maximize coordination with the surface Mg atoms and the formation of hydrogen bonds to the carbonate oxygen atoms. The {110} Surface. The surface phase diagrams for the {110} CaO and MgO surfaces are shown in Figure 5. The phase diagrams show a similarity to those calculated for the {100} surfaces but with an increased amount of surface carbonation. For the calcium oxide surface, full carbonation is reached at atmospheric conditions, as designated by region VII. This surface, shown in Figure 6b, has a surface concentration of 6.1 CO2 nm−2 and shows a faceted surface similar to the pure surface, Figure 6a. The carbonate ions on this surface are located in two adsorption sites. One resides at the bottom of the facet and the second carbonate site is at the top of the facet, with the carbonate species leaning into the trench. A reduction in the partial pressure of CO2, or an increase in the amount of gaseous water, is seen to lead to the formation of a mixed phase, with water adsorption occurring in a dissociative manner, region IV. The surface structure, Figure 6c, is similar to region VII with carbonation seen at the top of the facet but with the carbonate at the bottom of the facet replaced by dissociated water.


The MgO surface is similar to that of CaO but with a reduction in selectivity for CO2, as may be expected.36 The dominant phase at atmospheric conditions is a mixed phase (region III) with dissociative water adsorption, Figure 7a, and is nearly identical to the CaO mixed phase labeled IV in Figure 5a, demonstrating the increased reactivity of the MgO surface with water over CO2. Moreover, as with the {100} MgO surface, a reduction in the partial pressure of H2O alone at atmospheric conditions is not sufficient to give the formation of a purely carbonated surface, 13246



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Figure 9. Structures of the most stable (a) hydroxylated CaO {111} and (b) Ca(OH)2 {0001} surfaces. Panel a has a surface water coverage of 5.0 H2O nm−2 and is designated region I in Figure 8a. Similar structures are seen for the magnesium surfaces, region I in Figure 8b (6.5 H2O nm−2) and the Mg(OH)2 {0001} surface.

with the pure surface dominating. This implies that some cooperation between the CO2 and water must occur to stabilize this mixed surface; to form a carbonated surface, a small increase in the CO2 concentration is also required. A reduction in the concentration of carbon dioxide, however, leads to the formation of a surface possessing a surface concentration of 8.0 H2O nm−2, region I, with the water adsorbed in a dissociative manner, Figure 7b. The {111} Surface. The {111} surfaces, Figure 8, show very different behavior. As expected for these polar surfaces, at ambient atmospheric concentrations there is a strong preference to water adsorption, namely, in the formation of a hydroxylated surface. This is in agreement with many previous studies indicating the stability of this hydroxylated CaO and MgO {111} surface29,63,64 and its preference to hydroxylation over carbonation.36 The surface structure of the most stable hydroxylated CaO surface, region I in Figure 8a, is shown in Figure 9a and corresponds to a surface water concentration of 5.0 H2O nm−2. As can be seen, the hydroxylated surface has a near identical structure to the lowest energy {0001} Ca(OH)2 surface (Figure 9b). Although not shown, the most stable MgO surface, region II in Figure 4b, has an identical surface structure and relationship to the {0001} Mg(OH)2 surface, with the surface fully hydroxylated. Due to the smaller surface area, the surface water concentration is increased to 6.5 H2O nm−2. The {310} Surface. The {310} surface phase diagrams, displayed in Figure 10, show similarities to the corresponding {100} surfaces but with an enhanced dominance of carbonated or mixed phases. The {310} surface is a stepped surface, where the steps are separated by {100} terraces, Figure 11a. There are two different length steps on the pure surface, which alternate from short to long. This nature of the surface allows the influence of the steps to be assessed through comparison to the pure {100} surface. For the CaO surface, the phase diagram, Figure 10a, shows considerably less mixed phases than the equivalent {100} surface phase diagram, Figure 2a. Maximum surface carbonation is achieved at standard conditions (region VI in Figure 10a), indicating that the adsorption is improved by this stepped surface. This surface also has a greater surface concentration of carbonate than the related {100} surface does at the same atmospheric conditions. This suggests that step defects on the surface may increase the concentration of carbon dioxide which can be sequestered. The surface structure, Figure 11b, shows that the surface still retains some of its stepped character, although the steps have a more symmetric trenchlike appearance than seen for the pure surface, suggesting that carbonation takes place on the steps in preference to the surface terraces. However, it is seen that increases in water partial pressure by ∼108 bar (which can be reduced when the CO2 partial pressure is lowered) give rise to the surface designated region III, which has a surface coverage of 9.6 H2O nm−2 and is composed of dissociated water. The surface


structure, Figure 11c, shows a small reorganization of the steps, where the small terrace is effectively removed, to give a hydroxide layer with one step. In addition, the direction of the elevation of the terrace across the surface is seen to reverse upon hydroxylation. The magnesium oxide {310} surface also shows a greater reactivity toward carbonation than the {100} surface, with a more complex surface phase diagram, Figure 10b. This complexity is to be expected from previous static lattice simulations reporting a competition between surface hydroxylation and carbonation.36 At standard conditions, the most stable surface, region I, has a surface coverage of 3.6 H2O nm−2. The surface structure, Figure 12a, shows that this water dissociates at the steps, in agreement with previous computational studies.43,63 This is also in agreement with our previous work suggesting the {310} steps preferentially hydroxylate over carbonation for single molecule adsorption. A large number of other surface configurations are close to this region of stability in the phase diagram, suggesting that the surface composition is highly susceptible to varying 13247



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Figure 11. Side and top views of the {310} CaO surfaces designated (a) pure, (b) region VI, and (c) region III in Figure 10a. For clarity, surfaces have been expanded 3 × 1 and 1 × 3 for the top and side views, respectively.

Figure 12. Side and top views of the {310} MgO surfaces designated regions (a) I, (b) IV, (c) IX, (d) VI, and (e) VII in Figure 10b. For clarity, surfaces have been expanded 3 × 1 and 1 × 3 for the top and side views, respectively. The surface structure of the pure surface is identical to the CaO equivalent.

original steps, with hydroxylation of the terraces, causes a restructuring of the surface, reducing the number of steps by half but with each step being two units high, instead of being only one unit high in the pure structure. A decrease in the partial pressure

partial pressures. Interestingly, a small increase in the partial pressure of water leads to the mixed surface, region IV in Figure 12b, rather than a surface consisting only of adsorbed water. This surface shows that the presence of the carbonate at one of the 13248



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this temperature range carbonate-rich phases will dominate, but there are a number of mixed carbonate/water phases nearby, and hence the composition will alter as a function of the water chemical potential.

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DISCUSSION OF MODEL As with all models, the assumptions used must be reviewed. First this model assumes no chemical reactivity between the surface adsorbates. However, reaction between adsorbed water and carbonate species is indeed possible, for example, leading to the formation of bicarbonate ions. This deficiency could be addressed in future work through the addition of other species to the model, such as the bicarbonate ion. Reactive force fields or electronic structure calculations would also be useful to examine surface reactions with carbon dioxide in more detail. Additionally, the adsorption of water is considered as occurring via either a hydration or hydroxylation process. However, a mixture of these processes is feasible. Indeed, previous studies of the {100} MgO surface indicates that surface sites of lower coordination will favor hydroxylation, for example, Duński et al.,66 allowing the formation of a mixed surface. This will obviously be important for surfaces which either are defective or have a structure that gives rise to a variation in coordination number. In addition, this model does not allow for any surface reconstructions, other than as a result of the minimization process. Finally, the associative physisorption of CO2 has also been neglected in this study. Recent computational studies by Besson et al.67 indicate that a low level of CO2 coverage would be expected at a range of temperatures and pressures on the CaO (100) surface. The phase diagram presented here does predict a carbonate-rich phase under standard conditions, albeit as carbonate units rather than physisorbed CO2 units, consistent with the affinity of this surface to carbon dioxide. Despite these additional factors, the work presented in this study not only demonstrates how atomistic models can be used to generate surface phase diagrams but also details preliminary results regarding the composition of CaO and MgO surfaces under CO2 and H2O atmospheres.

Figure 13. Surface phase diagram of the CaO {100} surface as a function of the change in chemical potential of gaseous H2O and CO2. Right and top axes refer to the pressure of that gas at 298 K (black) and 1000 K (gray), with units in bar.

of water leads to a carbonated surface, region VI, which is also very close to region VII, Figure 12d,e, respectively. These regions differ in the extent of surface carbonation, with region VII having a higher concentration. Both surfaces show that the presence of the carbonate group leads to a flatter surface without the presence of well-defined steps. An increase in the CO2 partial pressure from atmospheric conditions moves the surface from the purely hydroxylated surface to a mixed surface, region IX (Figure 12c), which possesses the same carbonate structure as region VII but with the addition of associatively adsorbed water. Variation of Partial Pressure with Temperature. The effect of temperature is illustrated by including the relationship between the change in chemical potential and partial pressure at 1000 K. The small-dashed lines on Figures 2, 5, 8, and 10 indicate this unit activity. For all surfaces, with the exception of the {111}, at unit activity and 1000 K the pure phase dominates, with the absence of any surface adsorbed carbonate or water. However, the {111} surfaces remain dominated by the hydroxylated layer at this high temperature. To consider the variation in partial pressure at 1000 K, we can display the corresponding scales. This is shown for the example of the CaO {100} surface in Figure 13. The figure illustrates that increasing the temperature not only leads to the surface being free of surface adsorbates but also that by increasing the temperature of the system the surface will adsorb less carbon dioxide. Recent experimental studies on the carbonation of CaO have indicated that at 400−450 °C water vapor will accelerate the conversion of CaO to CaCO3 but at 550 °C water vapor has a slightly retarding effect on carbonation.65 Although this study only considers surface adsorption effects on specific crystal planes and assumes no chemical reactivity between species, the results are consistent with experiment. For example, the CaO surface phase diagrams suggest that within

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CONCLUSIONS Overall, the predicted surface phase diagrams detailed above identify the selectivity of different surfaces to carbon dioxide incorporation as a function of temperature as well as carbon dioxide and water partial pressures. In addition, they can also explain the effects of different surface features, such as steps, suggesting that these simulations may play a role in predicting the optimum morphology for a given application. From the surface phase diagrams, the increased reactivity of CaO compared to MgO surfaces with regard to surface carbonation can be clearly seen. The {100}, {110}, and {310} surfaces all show that they are amenable to surface carbonation, particularly as the partial pressure of CO2 is increased. This is particularly important for the MgO surfaces, where the {100} and {110} surfaces indicate that a reduction in the partial pressure of water alone does not give rise to surface carbonation. One of the features of the incorporation of carbonate into the CaO surfaces is that the calcium ions show considerable relaxation out of the surface, forming a calcium carbonate overlayer. Although there is no direct evidence, this may provide the mechanism for the formation of a mineral carbonate shell, as observed in experiment.68 The coadsorption of water is also seen to give rise to areas of stability, many close to the stable surfaces found at atmospheric conditions. In general, for the mixed surfaces, surface 13249



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hydroxylation appears to be more stable than associative water adsorption, suggesting that there is a more favorable interaction between the hydroxide and carbonate species on the surface. However, the MgO surface phase diagrams do suggest that increasing water partial pressures will largely limit CO2 adsorption. This is particularly evident for the {111} surfaces of both materials, where, as expected, the surface phase diagrams are dominated by the stable hydroxylated surface, forming an effective layer of mineral hydroxide to stabilize the polar surfaces. In summary, we have described a methodology for generating thermodynamic surface phase diagrams from potential-based simulation, which had not been achieved previously without the use of electronic structure methods. This makes possible the generation of this type of diagram for complex or large systems which would either be very expensive or unfeasible using ab initio methods. This technique, although limited by the constraints of the model used in the simulation, not only allows predictions of surface stabilities and compositions to be further examined by experimental study or ab initio simulation but is also directly transferable to many applications such as concrete curing and reactions in flue gases. Moreover, by scanning through many different surfaces, it is possible to predict the optimum particle shape as well as composition and external conditions including temperature and chemical potentials that are best suited for incorporating carbon dioxide in this case. Finally, it is worth emphasizing that the chemical potentials in this study are relative to the gas-phase species, but could be extended to the liquid phase by including molecular dynamics explicitly, which would make this approach more applicable to geological carbon sequestration, where the mineral surfaces interact with fluid phases of compositions varying from dry CO2, through watercontaining CO2 to CO2-containing aqueous phases.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.P.A.); [email protected] (S.C.P.). Present Address ‡

School of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank David Price and Mark Read of AWE for their assistance and useful discussion relating to this study. Funding is also acknowledged from both AWE and EPSRC.

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Atomistic Simulation of Surface Selectivity on Carbonate Formation at

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