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J. Phys. Chem. C 2009, 113, 8320–8328
Atomistic Simulation of the Surface Carbonation of Calcium and Magnesium Oxide Surfaces Jeremy P. Allen,† Stephen C. Parker,*,† and David W. Price‡ Department of Chemistry, UniVersity of Bath, ClaVerton Down, Bath, BA2 7AY, United Kingdom, and AWE, Aldermaston, Reading, Berkshire, RG7 4PR, United Kingdom ReceiVed: December 10, 2008; ReVised Manuscript ReceiVed: March 4, 2009
Computer modeling techniques using well-tested potential models to describe the interatomic interactions have been used to study the surface carbonation of the low index surfaces of magnesium and calcium oxides. We begin by studying the {100} surface and related, stepped {310} surface of these model oxides. Our results indicate that carbonation is indeed a favorable process, particularly enhanced by the inclusion of the step on the surface, and proceeds via incorporation into the surface, rather than mono- or bidentate adsorption above the surface. As the amount of surface carbonation is increased, surface energy lowers to a minimum, with calculated vibrational frequencies indicating the formation of a layer of carbonate material. In comparison to water adsorption, the majority of calcium oxide surfaces are predicted to compete favorably, whereas the magnesium equivalents show a greater stability from water. This is particularly apparent for the higher energy, polar {111} surface which forms a very stable hydroxylated surface. Introduction An important challenge for atomistic simulations of mineral surfaces is to generate a reliable description of the structure and composition of surfaces in different environments, not least because the properties of the mineral surfaces are determined by their surface structure and composition. In an attempt to obtain a better representation of the surfaces in the natural environment, most atomistic simulation studies have modeled the interactions with water. For example, de Leeuw et al. (1995),1 using a similar approach to that adopted in this paper, considered the effect dissociative adsorption of water has on the structure and stability of MgO and CaO surfaces. Their results highlighted a number of points, including that the chemisorption of water is particularly favorable for the {111} surface and that adsorption on the {310} surface will initially favor the top of the step edge rather than the terrace or inside the step. Additionally, de Leeuw et al. (1996)2 used atomistic modeling to study the effect that the Mg coordination number of MgO surfaces has on water adsorption. They found that surfaces with highly coordinated cations favor the associative adsorption of water, whereas low-coordinated cations will preferentially adsorb water in a dissociative manner. The interactions of calcium and magnesium oxides with water have also been studied using density functional theory (DFT), for example, Halim and Shalabi (2004),3 de Leeuw et al. (2000),4 and Scamehorn et al. (1994).5 In each case the results are similar to those from the potential-based studies. In contrast to the studies of water adsorption, other environmentally important species, such as CO2, have received much less attention, with selected modeling studies including the effect of the surface adsorption of CO,6 CO2, SO2,7 and methanol.8 Studies of such interactions also help to answer the questions asking to what extent the surfaces exist as a stochiometric oxide, hydroxide, or carbonate layer. The growing concerns associated * Corresponding author. E-mail:
[email protected]. † University of Bath. ‡ AWE.
with the increasing concentrations of atmospheric CO2 give an additional impetus for understanding its reactivity with mineral surfaces. Thus in this paper we begin to address this issue by considering carbonate formation at the surfaces of the model oxides, CaO and MgO. These have been chosen not least because there is a wealth of information on these materials9,10 but also because our models can more easily be tested against experiment and ab initio simulations, as well as representing a good starting place before considering more complex materials, for example, cements and silicate minerals. The majority of modeling work of CO2 adsorption on MgO and CaO surfaces has focused on the bonding of carbon dioxide. Jensen et al. (2005)11 reported on a study of theoretical IR frequencies using cluster-based DFT calculation of carbon dioxide adsorption on MgO and CaO. The main finding was that CO2 binds as a monodentate carbonate ion on edge sites of MgO but as a bidentate on corner sites, whereas both sites have monodentate bonding for CaO. These findings have supported other modeling and experimental studies in the area. Fukuda and Tanabe (1973)12 reported experimental IR studies showing that only monodentate bonding is seen for CaO at room temperature, with bidentate bands appearing at 350 °C. Bidentate bands were also seen for MgO, which decreased with increased loading but increased with increasing outgassing temperature. Other TPD and IR studies by Yanagisawa et al. (1995)13 also found two distinct carbonate groups on MgO surfaces, with the least stable assigned to the monodentate carbonate. Additionally, Dumesic et al. (1994)14 provided TPD results indicating adsorption energies from 80 to 170 kJ mol-1 depending on surface coverage. An MEIS study by Krischok et al. (2002)15 confirmed that CO2 chemisorption takes the form of a carbonate, through reaction with O2- surface anions. Further results suggest that on CaO surfaces chemisorption will take place at regular oxygen sites, whereas for MgO this occurs at low-coordinated oxygen ions only. More recent studies include that of Kadossov and Burghaus (2008)16 and Voigts et al. (2009)17 on the adsorption of CO2 on CaO (100) surfaces. Their results again
10.1021/jp810885m CCC: $40.75 2009 American Chemical Society Published on Web 04/15/2009
Surface Carbonation of CaO and MgO Surfaces confirm that carbon dioxide adsorption on this surface occurs via carbonate formation, leading to a layer of CaCO3. The carbonation of brucite, magnesium hydroxide, has also been reported in a range of studies. Studies suggest that the carbonation of brucite proceeds via an initial dehydroxylation step. Be´aret et al. (2002)18 have studied the dehydroxylation/ carbonation reaction using a range of experimental techniques, including XRD and thermogravimetric analysis. Their results suggest that the dehydroxylation to MgO is generally found to precede carbonation as a distinct but interrelated process. They also considered the effects of CO2 and temperature on this process. DFT studies of this reaction by Churakov et al. (2004)19 concluded that the partial dehydroxylation of the brucite surface to the simple oxide is a necessary precursor for the carbonation reaction. This study will not only continue the study of forming a single carbonate group but will also begin to consider the effect of coverage on the adsorption energies and bonding modes, thus allowing comparison to be made to the adsorption of water, and hence allowing the competition between these different adsorbates to be assessed. The materials simulated continue from previous work in this area, comprised of magnesium and calcium oxides. The surfaces which are considered are the “perfect”, flat {100} surface, the polar {111} surface, and the stepped {310}. The most stable of these surfaces is known to be the {100} surface.10 However, real surfaces will not consist of an ideal flat surface; there will be steps, kinks, vacancies, and other surface features present. Therefore, the stepped {310} surface is included to provide a model for a more realistic surface and to gain information on the effect steps will have on the carbonation process. The polar {111} surface will also provide information on the effect of different surface configurations, despite its relatively high surface energy. A further reason to study the {111} surface is due to the extremely stable hydroxylated {111} surface, which is effectively a layer of magnesium or calcium hydroxide. Therefore, this will provide further insight into the carbonation of metal hydroxide materials. A final consideration was that as we did not want to make prior assumptions of the coordination or concentration, we therefore needed to model many thousands of possible configurations; hence, we have initially considered atom-based simulations rather than using ab initio methods. The main focus of this study is to understand the carbonation of low index CaO and MgO surfaces in more detail. This will initially consider the single molecule formation, in terms of both energetics and the bonding mode of the carbonate, through the vibrational frequencies. The formation of a second carbonate molecule on the {100} surfaces will then be considered, before increasing the surface concentration of carbonate to consider the effect coverage has on surface stability. Finally, a comparison of the surface energies and adsorption energies from the carbonation and adsorption of water will be used to compare the competitiveness of the different species. Methodology The static simulation code used throughout this work was METADISE,20 which is designed to model dislocations, interfaces, and surfaces. METADISE uses a two-region approach, considering the crystal as being comprised of two blocks, each consisting of two regions, I and II, which are periodic in two dimensions. Thus, bulk material is modeled using the two blocks together, whereas a surface is considered as just one block. Region I represents all ions close to the extended defect, in this case a surface, whereas region II represents the rest of the crystal. The
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8321 inclusion of region II ensures that all the ions in region I experience the forces associated with the rest of the crystal and its size is chosen to ensure that the energies are fully converged. Region I ions are allowed to relax to their mechanical equilibrium, whereas region II ions are kept fixed at their bulk equilibrium positions. To achieve energy convergence, the number of ions in region I and II has to be sufficiently large. The surface energy, γ, of a crystal face is defined as the excess in energy of a surface simulation over the energy of a bulk system containing the same number of atoms per unit area, S. This can easily be calculated from the energy of the surface and bulk blocks, US and UB, respectively, as described by eq 1:
γ)
US - UB S
(1)
The energies of the blocks are calculated through consideration of the long- and short-range interactions between the ions, and Newton-Raphson minimization methods are used to find the minimum energy structures. All simulations carried out in this study were modeled using atomistic simulation techniques, based on the Born model of ionic solids.21 This model assumes that interactions between atoms consist of long-range electrostatic forces and short-range forces, which can be described using parametrized analytical functions. These analytical functions can be tested against both experimental observations and electronic structure calculations to ensure reliability and accuracy. The short-range forces consist of both a repulsive part and a van der Waals attractive term. Electronic polarizability is included using the shell model introduced by Dick and Overhauser.22 This model represents the polarizable ion as a core, containing the mass of the ion, which is connected to a massless shell by a spring. The longrange Coulombic interactions are determined using the Parry method,23,24 which is adapted from the Ewald method for twodimensional periodic systems. The potentials used in this work are now well-established and have been shown to reproduce a range of experimental properties effectively. They have also been developed so that they are transferable. The calcium carbonate potential is from the work of Pavese et al. (1996)25 on calcite and was fitted to structural properties, elastic constants, and vibrational frequencies. The interaction of magnesium with carbonate was derived by de Leeuw and Parker (2000)26 and fitted to reproduce the structure and energy of magnesite. The MgO and CaO potentials come from the work of Lewis and Catlow (1985);27 however, interactions of the ions with water (both hydrated and hydroxylated) come from other sources.2,28,29 The water molecule potential is a modified version of the de Leeuw and Parker potential (1998),30 with the introduction of the Lennard-Jones interaction made by Kerisit and Parker (2004),31 which was to combat the problem of the water freezing during molecular dynamics simulations. The hydroxide potentials are a modification of the Baram and Parker (1996)32 potential and have been successfully used to model the surfaces of goethite by Kerisit et al. (2005).33 As noted above, we need to consider many possible configurations, and each simulation needs to be energy minimized. Thus we have a large number (many thousands) of static lattice energy minimization calculations to complete; although each one is not prohibitively expensive, when considered together they represent a lot of simulation time. We used the eMinerals grid tools34 and in our case the UCL Condor pool, which comprised ∼1400 desktop PCs in teaching laboratories, etc., and the simulations were carried out when the machines were not in use.
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Allen et al. TABLE 1: Lattice Energies and Enthalpies of Formation35 Used To Calculate the Values of Ecorr(CO32-) and Ecorr(H2O) for Hydroxylation
Figure 1. Schematic showing different methods of carbonate placement: (a) above a metal atom with a surface oxygen removed and (b) replacement of a surface oxygen by a carbonate.
lattice energy/kJ mol-1
enthalpy of formation/kJ mol-1
-3984.80 -3378.26 -6175.04 -3468.63 -2931.15 -5898.94 -
-601.70 -924.50 -1095.80 -635.10 -986.10 -1206.9 -393.510 0 -241.82
MgO(s) Mg(OH)2(s) MgCO3(s) CaO(s) Ca(OH)2(s) CaCO3(s) CO2(g) + H(aq) H2O(g)
Calculating Surface Energy Only the lowest energy surface termination for each Miller index was considered for carbonation. The justification for this is that, due to the stability of the lowest energy surface, the contribution of higher energy surfaces would be minimal and would have a minor effect on calculated adsorption energies. We then modeled the incorporation of carbonate into the oxide surfaces as a function of surface coverage. The {100} surfaces were also grown 2 × 2 to allow a larger range of surface coverage to be explored. On introducing each carbonate, we considered four different carbonate orientations. The surface carbonation was modeled in two ways, illustrated by Figure 1. First was the addition of a carbonate group to a surface with a vacant oxygen site, to ensure charge neutrality, where the oxygen-metal distance was 2.0 Å, Figure 1a. Second was a “postcarbonated” surface, where carbon dioxide has reacted to form a carbonate group with one of the surface oxygen ions, Figure 1b. The carbonate was additionally modeled as being aligned in either the x- or y-planes. The addition of water to generate hydrated and hydroxylated surfaces is simpler to that of carbonation, with only one surface configuration modeled for each process. Generation of hydrated surfaces involved the placement of a water molecule with the oxygen 1.7 Å above a surface metal atom, with the hydrogens pointing away from the surface. Surface hydroxylation was carried out by addition of a hydroxide, aligned in the z-direction, 1.7 Å above a surface metal atom and a hydrogen placed 1.0 Å above an oxygen atom. Carbonated surface energies can be calculated directly from the “pure” and “modified” surface energies. The surface energy, γS, for a given configuration is given by eq 2
1 1 γS ) [ES - EB] - nCO2Ecorr(CO23 ) 3 S S
(2)
where S is the surface area, EB is the energy of the bulk, ES is the energy of the surface, n is the number of carbonate ions added, and Ecorr is an energy correction based on the selfinteraction of the species and accounts for the formation of the carbonate from reaction of gaseous carbon dioxide with the surface. The energy of carbonation, E(ads)(CO2), can then be calculated from eq 3
Eads(CO2) )
ES(modified) - ES(pure) + Ecorr(CO23 ) (3) nCO23
where ES(pure) is the energy of the stoichiometric surface before carbonate addition and ES(modified) is the energy of carbonation. The calculation of free energies is estimated by calculating the energies of many configurations and is expressed relative to the minimum energy for each coverage, ESmin. The free energy, AS, can be calculated using eq 4
AS ) ESmin - RT ln(Q)
(4)
where Q is the total partition function summed over all surfaces at a particular coverage and carbonate orientation, given by eq 5:
Q)
∑ e(-E -E S
Smin⁄RT)
(5)
S
The free energies can then be substituted into eqs 2 and 3, allowing the surface free energy and free adsorption energies to be calculated, respectively. Additionally, the average surface energies can be estimated using the usual statistical mechanical expression shown by eq 6.
∑ γSe(-E -E S
〈γ 〉
)
Smin⁄RT)
S
Q
(6)
This requires only that we sample sufficient numbers of different configurations, which is possible using the simple interatomic potentials. For hydration and hydroxylation, the calculation of surface and adsorption energies is identical to the above method, except that nCO32- becomes nH2O and Ecorr(CO32-) is changed to Ecorr(H2O). The correction term used in eqs 2 and 3 accounts for the self-energies of the species, which is different for each species added to the surface. For surface carbonation, it represents the following reaction, allowing energies to be calculated relative to gaseous CO2 2+ 22+ 2M(g) + O(g) + CO2(g) f M(g) + CO3(g)
where M2+ can be either magnesium or calcium. For the MgO calculations, a correction factor of 21.66 eV was used, whereas for CaO the value was 23.34 eV, as was determined using the lattice energies and enthalpies of formation given in Table 1. Thus, this correction term enables us to evaluate the surface and adsorption energies for removing an oxygen atom from the surface to form the carbonate species from gaseous CO2. When considering liquid water, Ecorr(H2O) includes the selfenergy of an isolated water molecule, -9.1 eV, and the heat of vaporization, -0.45 eV, whereas for hydroxylation, the selfenergy component of Ecorr(H2O) is obtained from the following the following reaction: 2+ 22+ M(g) + O(g) + H2O(g) f M(g) + 2HO(g)
It is again calculated using the data in Table 1 and is equal to -7.15 eV for MgO and -6.70 eV for CaO. The evaluation of self-energy is made complicated because the ionic model has to account for the different charge states of oxygen (i.e.,
Surface Carbonation of CaO and MgO Surfaces
Figure 2. Schematic showing different surface bonding modes for carbonate groups on the {100} CaO and MgO surfaces by (a) replacement of a surface oxygen, with a angle of 90°; (b) replacement of a surface oxygen, with a dihedral angle of 45°; (c) in a bidentate and (d) monodentate manner above a surface metal atom, with an oxygen removed to maintain charge neutrality.
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Figure 3. Plane and side views showing the lowest energy-minimized surface configurations of the surface carbonated by one carbonate ion onto the (a) CaO {100} and (b) MgO {100} surfaces.
-0.8 in water, -1.4 in the hydroxide, and -2 in the oxide) and effectively accounts for the oxygen electron affinity directly. Results and Discussion Single Carbonate Adsorption. The ability of low index CaO and MgO surfaces to chemisorb carbon dioxide is assessed by first considering the formation of a single carbonate molecule which then interacts with the surface. As the gaseous carbonate ion has a number of characteristic vibrational frequencies, which vary on adsorption, this allows the mode of surface binding to be characterized. These characteristic frequencies include a Raman-active asymmetric CO stretch, ν1, an IR-active out-ofplane deformation, ν2, and a doubly degenerate IR-active symmetric CO stretch, ν3. Upon adsorption and loss of symmetry, ν1 becomes IR-active and the doubly degenerate ν3 splits into high and low frequency peaks, ν3h, and, ν3l, respectively. These frequencies were initially evaluated by optimizing a carbonate ion using an MP2 Gaussian36 calculation, with a 6-311+g(d,p) basis set, giving 1012, 854, and 1327 cm-1 for ν1, ν2, and ν3, respectively. However, the calculation of the vibrational frequencies of a carbonate ion using the potential model produced modes at higher frequencies. This is most likely due to the rigidity enforced on the carbonate by the potential model itself, as well as other errors resulting from a lack of description of anharmonicity. However, as all frequencies calculated using potentials are going to suffer from identical constraints and we are only concerned with the change in force constants, a scaling factor of 0.711 was introduced, giving frequencies of 1046, 795, and 1507 cm-1 for ν1, ν2, and ν3, respectively. This scaling factor can then be used on all remaining potential-based calculations. The binding mode of adsorption of the carbonate frequencies for four reference bonding modes on the {100} surfaces of CaO and MgO were then calculated and are shown schematically in Figure 2. These consisted of direct replacement of a surface oxygen with a carbonate ion with an angle between the carbonate and the surface of either 90° or 45°, Figure 2a and b, respectively, and addition of a carbonate ion above a surface metal atom with the bonding in either a bidentate or monodentate manner, Figure 2c and d, respectively. Charge neutrality was maintained by removing an oxygen atom for the latter two cases, with the position of the oxygen varied and the IR frequencies summarized as a range. The calculated frequencies of these different modes are detailed in Tables 2 and 3 for {100} CaO and MgO surfaces, respectively. These frequencies were calculated using the PARAPOCS code,37 where the surface was first minimized with the carbon of the carbonate held fixed, and then the vibrational frequencies were determined after this constraint was removed.
Figure 4. Plane and side views showing the lowest energy-minimized surface configurations of the surface carbonated by two carbonate ions onto the (a) CaO {100} and (b) MgO {100} surfaces.
We can compare the frequencies found for the different modes of adsorption. The results indicate that the incorporation of the carbonate into the surface gives rise to higher frequencies than adding a carbonate aboVe the surface. In addition, the energy of adsorption for these modes shows that, for CaO, positioning of the carbonate at both angles to the surface is energetically favorable. However, for MgO, the carbonate perpendicular to the surface is the only exothermic process, in agreement with previous work by Jensen et al. The adsorption of carbonate molecules above the surface was found to be energetically unfavorable for both materials. However, the monodentate adsorption is shown to give the stronger surface bonding, indicated by the higher vibrational frequencies. The lowest energy minimized {100} structures following the single molecule carbonation were characterized by comparing the vibrational frequencies with those of the reference modes, given in Tables 2 and 3 for CaO and MgO, respectively. The structures of these minimized surfaces are also shown in Figure 3. The magnesia surface shows that the adsorption is almost identical to that of the replacement of a surface oxygen, with the carbonate ion being perpendicular to the surface. This is to be expected as rotation of the carbonate about the z-axis would result in weaker bonding, due to the smaller size of the magnesium ions. The CaO surfaces, however, do not show the same behavior. The vibrational frequencies here lie somewhere between the values for the replacement of a surface oxygen with the carbonate creating an angle with the surface of 90° and 45°. Further analysis of the surface structure shows that the carbonate has rotated about the z-axis, while remaining perpendicular to the surface, so that the uppermost carbonate oxygen atoms were now pointing diagonally toward two oxygens, though far enough away not to repel, thus resulting in stronger bonds to two calciums per carbonate oxygen rather than one, as seen for MgO. The energetic feasibility of this adsorption can be considered by analysis of carbonated surface energies and adsorption energies, given in Table 4. The results show that the surface carbonation should be favorable on all of the studied surfaces, showing reduction in surface energies and exothermic adsorption energies. Adsorption onto the step of the {310} is more favorable than on the flat terrace of the {100} surface. However, as the {100} surface energy is initially lower, upon carbonation it remains the most stable surface, indicating that the presence of the step continues to destabilize the surface. Adsorptions onto
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TABLE 2: Calculated Vibrational Frequencies for the Surface Carbonation of the CaO {100} Surface, Detailing Results of Different Modes of Adsorption as Well as Frequencies from the Lowest Energy Surfaces with One Carbonatea carbonate adsorption
ν1/cm-1
ν2/cm-1
ν3l/cm-1
ν3h/cm-1
replacement of O angle 90° replacement of O angle 45° monodentate above Ca bidentate above Ca {100} CaO with 1 CO23
1083 1095 1070-1078 1052-1070 1096
787 791 774-785 780-787 785
1485 1531 1415-1452 1400-1471 1513
1666 1652 1548-1592 1611-1637 16640
a
ν1 is the asymmetric CO stretch, ν2 is the out-of-plane deformation, and ν3l and ν3h are the doubly degenerate symmetric CO stretch which splits into high and low frequency peaks on loss of symmetry in the carbonate molecule.
TABLE 3: Calculated Vibrational Frequencies for the Surface Carbonation of the MgO {100} Surface, Detailing Results of Different Modes of Adsorption as Well as Frequencies from the Lowest Energy Surfaces with One Carbonate carbonate adsorption
ν1/cm-1
ν2/cm-1
ν3l/cm-1
ν3h/cm-1
replacement of O angle 90° replacement of O angle 45° monodentate above Mg bidentate above Mg {100} MgO with 1 CO23
1089 1101 1080-1087 1045-1075 1092
785 786 767-790 774-776 785
1468 1515 1371-1377 1350-1447 1478
1703 1691 1501-1623 1622-1677 1699
TABLE 4: Results of the Lowest Energy Surface Carbonated by a Single Carbonate Molecule onto the {100}, {111}, and {310} CaO and MgO Surfacesa material CaO surface dry γ/J m-2 change in γ upon carbonation/J m-2 Eads(CO2)/eV a
{100} 0.78 -0.22 -1.24
MgO
{111} 2.47 -1.21 -3.03
{310} 1.15 -0.37 -1.67
{100} 1.29 -0.05 -0.20
{111} 4.16 -2.09 -3.97
{310} 1.92 -0.21 -0.71
Note: results represent lowest energy surfaces only so are not free energies.
{100} and {310} calcium surfaces shows an increased reactivity when compared to the magnesium equivalents, with the {100} MgO showing only a tentative increase in surface stability of 0.05 J m-2 and a relatively small adsorption energy. Indeed, the adsorption energy is so small that it is likely to be similar to, or smaller than, the physisorbed value. The higher energy {111} surface remains the most unstable surface, although a large decrease in surface energy is seen upon carbonation, with a corresponding large adsorption energy, indicating the instability of this polar surface. Previous work by Jensen et al. showed that bonding to the {100} occurs in a monodentate manner, and this is also seen in this study. Comparable adsorption energies are also seen for terrace sites. Jensen et al. found adsorption energies of -1.04 and +0.13 eV for CaO and MgO terrace sites, respectively, using cluster-based calculation. Our energetically more favorable energies are not unexpected as there will be a contribution from van der Waals interactions intrinsically present in empirically derived potentials but not included in the quantum mechanical calculations. The values of the adsorption energies for the carbonation of the studied surfaces, with the exception of the MgO {100}, are relatively large due to the inclusion of the reaction energy of carbonate formation from carbon dioxide. The Surface Carbonation of Two Molecules on the {100} Surface. The surface carbonation by two molecules can again be assessed through comparison to the vibrational frequencies of the reference modes described above. The vibrational frequencies resulting from the adsorption of two carbonates on the {100} CaO and MgO surfaces are detailed in Table 5. The CaO results show that the energetically preferred adsorption is a result of the replacement of a surface O with the calculated values lying between the reference values for when the angle with the surface is 45° and 90°. Analysis of the surface structure, Figure 4a, shows that although the carbonate ions sit next to
TABLE 5: Calculated Vibrational Frequencies for the Surface Carbonation of Two Carbonate Units on the CaO and MgO {100} Surfaces surface CaO MgO
ν1/cm-1
ν2/cm-1
ν3l/cm-1
ν3h/cm-1
1094-1095 1102-1104
775-795 767-798
1522-1527 1517-1522
1646-1651 1680-1682
each other, one remains in the perpendicular position, while the other angles toward the surface. It is also seen that the calcium ions which are sandwiched between the two carbonate ions have lifted away from the surface to bond with the carbonates in a more efficient manner. However, the size of these ions causes efficient bonding to the perpendicular carbonate but forces the other carbonate to lean away from it, creating coordination to surface atoms instead. The magnesium oxide surface shows a similar structure, with the carbonate ions next to each other and the two magnesium ions situated between them rising up from the surface. However, as the magnesium ions are smaller, this actually results in the carbonate ions leaning toward the magnesiums rather than what is seen for the CaO surface; see Figure 4b. This results in the formation of stronger bonds from both of the carbonates to the magnesiums, holding the whole [MgCO3]2 unit in a relatively rigid manner. This enforced rigidity results in the vibrational frequencies being higher than those found for a single carbonate being perpendicular to the surface. This increased flexibility of bonding motifs seen for the carbonation of CaO may lead to the formation of different bonding configurations, thus allowing the formation of different polymorphs of the calcium carbonate which is not seen for magnesium carbonates. Variation of Surface Coverage. The number of surface carbonates was then varied from a single carbonate through to monolayer coverage, and the carbonated surface free energies and free adsorption energies were calculated. Figure 5 shows
Surface Carbonation of CaO and MgO Surfaces
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Figure 5. Plots of (a) average surface free energy and (b) average free adsorption energies for CaO as a function of surface carbonate coverage; plots (c) and (d) show the same respective plots for MgO.
plots of both the average surface energy and the average Aads(CO2) against surface coverage for CaO and MgO. The results are comprised of a combination of 78 862 separate energy minimization calculations. There are a number of similarities between the plots of CaO and MgO, as expected. The energies for CaO are lower for all surfaces, agreeing with its increased reactivity, causing all its surfaces to be more reactive toward the formation of a carbonated surface in an exothermic reaction. The increased reactivity of the CaO surface is likely due to the lower Madelung potential of CaO, caused by the increased lattice constant, which effectively destabilizes the O2- ion. The calculated surface Madelung potential for CaO is 20.2 eV, which compares with 23.1 eV for MgO. The exception to this is the 50% carbonated {111} surface, where adsorption on MgO is more exothermic. However, this large energy is most likely caused by the higher energy uncarbonated surface, which is much more unstable than its calcium equivalent. In accordance with “pure” surface energies, the {111} surface is less stable than the {100} and {310} surfaces at all levels of coverage. However, there is a crossover in surface energy of the {100} and {310} surfaces at approximately 25-30% carbonate coverage, with the {310} becoming the most stable. The stability of the {310} surface is then seen to continue to drop until the surface reaches its maximum coverage with the energy remaining lower than the {100}. The most likely cause of the crossover in energies for the {100} and {310} surface energies is the presence of the {310} step. Further analysis of the carbonation of the {310} surface shows that carbonation will preferentially form on the step edges prior to carbonation of the {100} terrace. This is highlighted by the minimum energy MgO surfaces at 12.5% and 25% coverages, Figure 6a and b, respectively. A similar case is also seen for the CaO surfaces. Interestingly, for the {310} surfaces, the minimum energy surface is seen where the step splits into two steps, each with a height of 2 Å. This surface structure is retained when the surface has a surface coverage of 12.5%, Figure 6a. However,
Figure 6. Surface structure of minimum energy carbonated {310} MgO surfaces with a surface coverage of (a) 12.5% and (b) 25% carbonation. Note: the step on the 12.5% surface has a partial dislocation, comprised of two separate 2 Å steps, whereas the 25% surface has a step of 4 Å depth.
when the surface carbonation is increased to 25%, the higher step of 4 Å is stabilized by the carbonate groups, Figure 6b. This prediction may well be able to be tested by surface sensitive techniques, such as AFM, that would be able to follow the change in step height. Carbonation of the flat surface creates a lower energy surface than the carbonation of a step. However, once the step has been removed by the carbonation, creating a flatter surface, the {310} becomes more stable. Conversely, the adsorption energies of carbonation for the {310} surface are all lower than those of the {100} surface, indicating that, energetically, surface carbonation is most favorable on the stepped surface at all coverages. Comparison of calculated adsorption energies with the bulk reaction energy, calculated from the energies of formation listed in Table 1, shows a good correlation with the {310} surface for both minerals. In particular, the closest match is with MgO, and as the {310} represents the closest surface to a real surface, it indicates that carbonation of the surface is indeed favorable. In addition to the increase in reactivity, the {100} and {310} CaO surfaces are seen to have negative surface energies, at approximate coverages of 55% and 40%, respectively. This indicates the large stabilization that the carbonate provides, relative to the formation of the surface, as has also been seen for θ-alumina.38,39 According to Tasker et al. (1985),40 the negative surface energy also indicates a thermodynamic barrier to sintering, hence inhibiting growth of the crystal. Therefore, at these surface coverages crystal growth would stop with a resultant layer of the calcium carbonate present on the surface;
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TABLE 6: Calculated Vibrational Frequencies of the Lowest Energy Carbonated CaO Surfaces, from the Variation of Carbonate Coverage Study and a 1-Layer Slab of {10.4} Calcite, CaCO3a surface
ν1/cm-1
ν2/cm-1
ν3l/cm-1
ν3h/cm-1
{100} CaO surface (87.5% carbonation) {111} CaO surface (100.0% carbonation) {310} CaO surface (87.5% carbonation) 1-layer {10.4} CaCO3 slab
1065-1085 1064-1076 1087-1087 1071
771-79 779-788 765-802 794
1516-1548 1472-1510 1516-1557 1540
1558-1591 1587-1590 1559-1609 1549
a Surface coverage for the lowest energy surfaces is also indicated. ν1 is the asymmetric CO stretch, ν2 is the out-of-plane deformation, and ν3l and ν3h are the doubly degenerate symmetric CO stretch which splits into low and high frequency peaks on loss of symmetry in the carbonate molecule.
TABLE 7: Calculated Vibrational Frequencies of the Lowest Energy Carbonated MgO Surfaces, from the Variation of Carbonate Coverage Study and a 1-Layer Slab of {10.4} Magnesite, MgCO3a
a
surface
ν1/cm-1
ν2/cm-1
ν3l/cm-1
ν3h/cm-1
{100} MgO surface (75.0% carbonation) {111} MgO surface (100.0% carbonation) {310} MgO surface (75.0% carbonation) 1-layer {10.4} MgCO3 slab
1055-1093 1057-1073 1081-1096 1078
734-805 754-807 755-797 791
1486-1540 1440-1459 1520-1564 1556
1545-1607 1572-1631 1588-1638 1563
Surface coverage for the lowest energy surfaces is also indicated.
however, adsorption of carbonate would still continue up to the surface coverage resulting in the lowest surface energy. An additional point of interest was seen on comparison of the different modes of carbonation with surface coverage. The minimum energy surfaces resulting from the increase in surface concentration were formed by addition of the carbonate unit above a surface metal atom. This differs from the results of the single molecule carbonation where incorporation into the surface resulted in the most stable configurations. This therefore implies that the adsorption of individual molecules will be preferentially incorporated into the surface. However, as the coverage of carbonate increases, approaching the formation of a monolayer, the carbonates will form a layer of carbonate mineral rather than a layer of surface-adsorbed carbonates. Additionally, the position of the oxygen vacancies, from the removal of an oxygen atom to balance the charges, prior to minimization is adjacent to the carbonate units. However, the extensive surface reconstruction around the vacancies results in a disordered carbonate layer that cannot be clearly identified as a particular carbonate polymorph. In addition, the lack of a minimum energy surface at 100% coverage is most likely a result of the size of the carbonate ion which will result in steric repulsion. Overall, it can be seen that carbonation of the low index surfaces is energetically favorable, with the most reactive surface being the {310} surface which resulted in the lowest energy surfaces and reasonably exothermic adsorption energies. As the {310} was observed to be more stable, the presence of steps can be inferred to be used to increase reactivity of both the CaO and MgO surfaces. In addition, the CaO surfaces are all seen to be more reactive than the MgO equivalents, through the more exothermic adsorption energies. This mimics what is observed for silicate minerals, where wollastonite, CaSiO3, is found to be more effective at sequestering carbon dioxide than the olivine and serpentine structured magnesium silicates.41 Lowest Energy Surfaces. The most stable surfaces resulting from the variation of surface carbonation can also be characterized by considering the vibrational frequencies. Tables 6 and 7 give the calculated frequencies for the lowest energy configurations for each of the CaO and MgO surfaces studied, respectively. These can be compared to both the frequencies of single molecule adsorption, Tables 2 and 3, and the frequencies for a thin, 1-layer {10.4} slab of the pure carbonate material, using the calcite structure for the calcium carbonate and magnesite for the magnesium equivalent. The results show a favorable
comparison with the results of the carbonate slabs, thus indicating that the surface is no longer a carbonated oxide surface but has become an oxide surface coated by a layer of the metal carbonate. This is also evident by the relaxation of the surface metal ions away from the surface to interact with the carbonate units allowing the formation of the mineral carbonate structure. In contrast, the 25% coverage {100} surfaces are clearly characterized by the adsorption of carbonate, indicating that the surface remains as a clean oxide surface. As the modeled surface represents the lowest energy structure, it indicates that not only is carbonation of these surfaces possible but also in vacuum conditions carbonation will result in the transformation to pure carbonate material, acting as an effective agent for carbon sequestration. Slight variations between the vibrational frequencies of the slabs and those of the carbonated oxide surfaces most likely result from the actual surface present on the carbonate. The {10.4} surface is the most stable surface for the carbonate materials, but there is no structural evidence that this is what is actually forming on the oxide surfaces. The same applies for the structure of the carbonate. Calcium carbonate can exist in a number of polymorphs, calcite being the most stable, respectively; however, there is again no structural evidence that this polymorph is indeed being formed. The surface structures of the lowest energy surfaces, not shown here, show disordered structures, therefore making identification of structural motifs unfeasible. However, the surface configurations of the carbonated CaO surfaces are more structured than their magnesium equivalents. This further indicates the flexibility of carbonation of calcium oxide forming more structural bonding patterns. Comparison of the Adsorption of Water and Carbonate. In real systems, however, pure vacuum conditions would not exist, particularly when used in conjunction with industrial processes. Therefore, the presence and competition of other species in the system must also be considered. To begin to assess this we can compare the surface carbonation of a single molecule with the adsorption of a single water unit, either in a hydrated or hydroxylated manner, as shown in Table 8. A number of observations can be drawn from these results. First, the surfaces of CaO all show a preference for surface carbonation. Although the stabilization of the surface as a result of the carbonation is much greater for the {100}, the other surfaces also show a similar stabilization from the hydroxylation. In addition, hydroxylation of these surfaces has larger adsorption
Surface Carbonation of CaO and MgO Surfaces
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TABLE 8: Adsorption Energies and the Changes in Surface Energy Resulting from the Adsorption of a Single Carbonate, Water Molecule, or Hydroxide and Proton onto the {100}, {111}, and {310} MgO and CaO Surfaces material CaO surface change in 〈γ〉/J m-2 Aads(CO2)/eV change in 〈γ〉/J m-2 Aads(H2O)/eV change in 〈γ〉/J m-2 Aads(H-OH)/eV
{100} -0.23 -1.32 +0.03 -0.26 -0.07 -0.83
{111} -1.21 -3.07 -0.38 -1.40 -1.18 -3.84
MgO {310} -0.36 -1.70 -0.15 -1.14 -0.34 -2.02
{100} -0.06 -0.29 +0.01 -0.43 +0.10 -0.03
{111} -2.08 -4.00 -0.82 -2.01 -2.22 -4.68
{310} -0.20 -0.74 -0.13 -0.90 -0.43 -1.94
TABLE 9: Adsorption Energies and the Changes in Surface Energy Resulting from the Lowest Energy Carbonated, Hydrated, and Hydroxylated {100}, {111}, and {310} MgO and CaO Surfaces and Their Associated Surface Coverage material CaO surface carbonate coverage/% change in 〈γ〉/J m-2 Eads(CO2)/eV water coverage/% change in 〈γ〉/J m-2 Eads(H2O)/eV hydroxide coverage/% change in 〈γ〉/J m-2 Eads(H-OH)/eV
{100} 87.5 -1.29 -1.06 87.5 -0.36 -0.97 87.5 -0.57 -0.92
{111} 100 -2.05 -2.57 100 -1.26 -1.24 100 -2.71 -3.84
energies than for carbonation, indicating that there will be strong competition between these processes. The surfaces of MgO, however, show surface hydroxylation to be the favored process, with the exception of the {100}. The results also indicate a certain amount of hydrophobicity on the {100} surfaces, indicated by the small positive change in surface energy but a negative value for the adsorption energy. As the adsorption energy is less than the heat of vaporization, 0.45 eV, this indicates that the adsorbed water would be more stable as liquid water than as an adsorbed gaseous molecule. The stabilization of the surfaces as a result of surface adsorption of more than one molecule can also be used to indicate differences between the systems under study. Table 9 shows the change in surface free energy resulting from the most stable surface found for each adsorption process, with its associated surface coverage. Similar trends are seen for the lowest energy surfaces as for the single molecule adsorption with a few exceptions. First, for the CaO {111} surface, hydroxylation dominates at higher coverage levels rather than carbonation. The MgO {100} and {310} surfaces also show differences. For the {100} surface, hydration is seen to give rise to the lowest energy surface, whereas the surface carbonation was more favorable for the single molecule. This change in stability is most likely a result of the formation of a stable hydrogen bonding network, which is facilitated by the small surface area of the MgO surface. The process giving rise to the minimum energy structures on the {310} MgO surface is also seen to change, from hydroxylation for the single molecule adsorption to carbonation. This change is directly related to the surface structure. For the adsorption on the step, a single dissociated water can provide greater stabilization. However, once the step has been removed, carbonation of the {100} terraces produces more surface stabilization than hydroxylation, in line with the {100} surface results. This results in the carbonation dominating and giving rise to more stable surfaces. Conclusions The use of potential-based static lattice minimizations has allowed the effect of surface carbonation to be examined in
MgO {310} 87.5 -2.40 -1.57 87.5 -0.91 -1.14 87.5 -1.48 -1.42
{100} 75 -0.85 -0.62 87.5 -1.08 -1.13 75 -0.22 -0.61
{111} 100 -2.13 -1.66 100 -2.94 -1.85 100 -4.21 -4.46
{310} 75 -1.69 -0.98 87.5 -1.20 -1.01 75 -1.07 -1.07
detail through examination of the vibrational frequencies as well as the adsorption and surface energies. The results indicate that surface carbonation, used as a model for carbon dioxide adsorption, is energetically favorable for the low index MgO and CaO surfaces. Carbonation is seen to preferentially occur in a monodentate manner, with the carbonate being part of the surface resulting in multiple monodentate bonds, rather than a single interaction to a metal atom. There are some common trends for MgO and CaO; for example, the results suggest that the minimum energy surfaces resulting from the single molecule carbonation on the {100} surfaces will occur via insertion into the surface, whereas the {310} surface will carbonate at the step before the {100}-like terrace. In addition, on the continued carbonation, a second molecule on the {100} surface will bond in an adjacent position to the first, causing the surface metal atoms lying between the carbonates to move away from the surface giving further stabilization. As the surface carbonate coverage is increased, the surface energies continue to decrease as a result of the surfaces being stabilized further until steric interactions between the carbonate ions prevent further adsorption. Once the {310} steps have been removed by the carbonation, this becomes the most stable surface, as the terrace begins to carbonate. Additionally, this surface is favored over the other surfaces due to it having lower adsorption energies at each coverage. Therefore it can be concluded that the presence of surface features, such as steps, can be used to enhance the reactivity at the surfaces. The vibrational frequencies for the minimum energy surfaces indicate that the surface carbonation results in the formation of a mineral carbonate layer, which would be the desired end result for its use in carbon sequestration. However, competition with water would occur. Initial results suggest that this is of less consequence on the CaO surface, with carbonation, in general, leading to the lowest energy surface, whereas for MgO, the lowest energy {100} surface is found when hydrated and the {111} when hydroxylated. Future work is required on computing the predicted low energy configurations with electronic structure methods, and also in investigating these effects in the presence of water, which is likely to need dynamical methods.
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In summary, the use of potential-based simulation methods has been shown to be invaluable for the carbonation of magnesium and calcium oxide surfaces thanks to their ability to model a large number of surface configurations. Simulations of the single molecule carbonation show that it will proceed via incorporation of the carbonate into the surface, in a monodentate manner, rather than bidentate or monodentate adsorption aboVe the surface. Results show that both magnesium and calcium oxide carbonated surfaces will readily form a layer of the mineral carbonate on the surface, with the calcium oxide surface in particular competing favorably with the adsorption of water. The increased reactivity of CaO shows a more energetically favorable process, but with both minerals showing maximum adsorption at similar coverage levels. Acknowledgment. The authors would like to thank Fei Zhang and Katharine Simpson for their initial work on the carbonation of MgO and CaO surfaces, respectively, which acted as a basis for this study. Funding is also acknowledged from both AWE and EPSRC. Supporting Information Available: Details of all the potentials used in this study and the parameters used in the parametrized equations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) de Leeuw, N. H.; Watson, G. W.; Parker, S. C. J. Phys. Chem. 1995, 99, 17219. (2) de Leeuw, N. H.; Watson, G. W.; Parker, S. C. J. Chem. Soc., Faraday Trans. 1996, 92, 2081. (3) Halim, W. S. A.; Shalabi, A. S. Appl. Surf. Sci. 2004, 221, 53. (4) de Leeuw, N. H.; Purton, J. A.; Parker, S. C.; Watson, G. W.; Kresse, G. Surf. Sci. 2000, 452, 9. (5) Scamehorn, C. A.; Harrison, N. M.; McCarthy, M. I. J. Chem. Phys. 1994, 101, 1547. (6) Halim, W. S. A. Appl. Surf. Sci. 2007, 253, 8974. (7) Pacchioni, G.; Ricart, J. M.; Illas, F. J. Am. Chem. Soc. 1994, 116, 10152. (8) Gay, I. D.; Harrison, N. M. Surf. Sci. 2005, 591, 13. (9) Colbourn, E. A.; Mackrodt, W. C.; Tasker, P. W. J. Mater. Sci. 1983, 18, 1917. (10) Tasker, P. W.; Duffy, D. M. Surf. Sci. 1984, 137, 91. (11) Jensen, M. B.; Pettersson, L. G. M.; Swang, O.; Olsbye, U. J. Phys. Chem. B 2005, 109, 16774. (12) Fakuda, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1973, 46, 1616. (13) Yanagisawa, Y.; Takaoka, K.; Yamabe, S.; Ito, T. J. Phys. Chem. 1995, 99, 3704. (14) Shen, J.; Kobe, J. M.; Chen, Y.; Dumesic, J. A. Langmuir 1994, 10, 3902. (15) Krischok, S.; Ho¨fft, O.; Kempter, V Nucl. Instrum. Methods Phys. Res., Sect. B. 2002, 193, 466. (16) Kadossov, E.; Burghaus, U. J. Phys. Chem. C 2009, 112, 7390.
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