J. Phys. Chem. C 2007, 111, 13103-13108
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Cyclic Cluster Study on the Formation of Brucite from Periclase and Water Karl Jug,*,† Bettina Heidberg, and Thomas Bredow‡ Theoretische Chemie, UniVersita¨t HannoVer, Am Kleinen Felde 30, 30167 HannoVer, Germany ReceiVed: April 13, 2007; In Final Form: June 28, 2007
Cyclic cluster calculations were performed with the quantum chemical method MSINDO to elucidate the mechanism of the formation of brucite from periclase and water. A model was developed that is based on the experimental observation of the epitaxy relation MgO(111)||Mg(OH)2(0001). A stepwise mechanism is proposed where Mg-OH groups of the hydroxylated MgO(100) surface are shifted into the adsorbate layer. Also Mg species diffuse from inside the crystal to the surface and attach themselves to the developing brucite (0001) plane. Additional water molecules from a second layer of molecularly adsorbed water molecules are necessary for a continuing growth of the brucite nuclei. The proposed mechanism explains the various experimental observations.
1. Introduction The conversion of periclase (MgO) to brucite (Mg(OH)2) is of particular significance in concrete construction.1 The volume increase of a factor of 2.2 in the reaction of periclase with water can have a damaging influence on structures built by concrete, because periclase is used as a component in cement. Such processes are usually very slow, and the damage is observed often only after years of destructive reaction processes. Already in 1968, Giovanoli et al.2 used gravimetry, X-ray photography, and electron microscopy to study the hydration of periclase to brucite at room temperature. With X-ray photography, the first nuclei of brucite could be detected. With electron microscopy, the orientation of the nuclei could be observed. Here, the epitactic relations MgO(111)||Mg(OH)2(0001)3 and MgO(11h0)||Mg(OH)2(112h0)4 play a role. The first epitaxy relation leads to nuclei parallel to the space diagonal of the cube. The second epitaxy relation allows also the growth of nuclei parallel to the edges of the cube. Obviously, the formation of the (0001) plane of brucite nuclei parallel to the (111) plane of periclase is preferred. Atomic force microscopy (AFM) experiments confirm the restructuring of the MgO(100) surface in water.5 Three-dimensional images show the formation of quadratic pits and elevations of nanometer size on the originally flat surface. The conversion process has also been observed on MgO single crystals.5 In recent work,6 we studied the stability of water structures on the perfect MgO(100) surface. Overlayers of c(4 × 2), p(3 × 2) and (1 × 1) translational symmetry were considered at 0 K. The role of partial dissociation for the stability of such overlayers was illustrated in the context of the literature on experimental7-10 and theoretical work11-17 on this subject. A related study of water molecule dissociation on ice was presented by Minot.18 This work was extended to acidic and basic media.19 Our previous study was the basis for the present investigation of the periclase (MgO) to brucite (Mg(OH)2) conversion described below. * Corresponding author. † Present address: Theoretische Chemie, Leibniz Universita ¨ t Hannover, Callinstr. 3A, 30167 Hannover, Germany. ‡ Present address: Institut fu ¨ r Physikalische und Theoretische Chemie, Universita¨t Bonn, Wegelerstr. 12, 53115 Bonn, Germany.
TABLE 1: Reaction Energy ∆Ehydrox (kJ/mol) of the Nucleation of Brucite from the Hydroxylated (MgO)144 Cluster and N Additional Water Molecules under Relaxation of m Planes step
starting structurea
1 2 3 4 5 6 7 8 9 10
1(Mg-OH) 2(Mg-OH) 2(Mg-OH) 2(Mg-OH) 2(Mg-OH) + 1Mg 2(Mg-OH) + 2Mg 2(Mg-OH) + 2Mg 2(Mg-OH) + 2Mg 2(Mg-OH) + 2Mg 2(Mg-OH) + 2Mg
a
N
m
optimized structure
Ehydrox
Figure 2a
1 2 2 2 3 4 5 6
2 2 2 2 3 3 3 3 3 3
-576 -807 -1035 -1115 -1483 -1566 -2086 -2982 -2813 -3112
Figure 2b Figure 3a Figure 3b Figure 4a Figure 4b
Characterized by the number of Mg-OH and Mg species
We extended our cyclic cluster work6 by molecular dynamics simulations of these overlayer structures at 300 K to understand the stability of the adsorbate structures.20 One of us21 was involved in work where the first steps of dissolution of the MgO(100) surface in acidic solution was studied. From this work, a pathway appeared possible where a (2 × 2) pit and two (2 × 1) elevations in its neighborhood were involved. In the present work, we present a topotactic model for the nucleation of brucite from periclase and water. This model is verified by simulations in which the hydroxylation process of MgO is considered stepwise on the atomic level. These steps comprise the formation of Mg-OH species and Mg ions moving out of the crystal. All steps are calculated by optimization of the corresponding structures. The properties of brucite are not the subject of the present study. Details on the structure and vibrational frequencies of brucite can be found elsewhere.22-26 2. Computational Details The calculations were performed with the semiempirical method MSINDO27 in the parametrization for first- and secondrow elements28 as in our previous studies.6,20 The accuracy of the method for structures and energies has been documented. For the description of hydrogen bonding, which is important in this study, additional 2p functions on H are included.29 There is good agreement of the calculated water dimerization energy
10.1021/jp072889c CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007
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Figure 1. Model of the topotactic nucleation of brucite on the periclase (100) surface: (a) hydroxylated MgO(100) surface; (b) MgOH units move out of the crystal surface into the adsorbate layer of the OH groups; (c) additional water molecules from the second water layer (not shown) are dissociatively bound; and (d) MgO species move in the Mg(111) plane out of the bulk to the surface.
with the experimental value. The systems were treated as cyclic clusters. The cyclic cluster model30 is implemented in MSINDO. It simulates periodicity in a finite system, and hence it is also suitable for the description of translational symmetry in monolayers or multilayers. An embedding procedure takes care of the long-range electrostatic interactions.31 We have used a (MgO)144 cyclic cluster of (6 × 4 × 3) shape for the surface simulations. Reference is the square Mg2O2 in vertical orientation that would represent the (1 × 1 × 1) shape. (l × m × n) means that this basic Mg2O2 unit is shifted in three perpendicular directions l times, m times, and n times, respectively. In the present case, the cluster has a (6 × 4) surface of 24 MgO units, which present a tetragonal cut of the surface, and 6 layers. 3. Model of the Expansive Topotactic Periclase Hydration On the basis of our previous work6,20 and the epitaxy relation from experiment,3 we developed the following model for the brucite nucleation process. Starting point was the hydroxylated MgO(100) surface where our molecular dynamics calculations at 300 K spontaneously showed an instability of the MgO(100) surface. We placed a second adsorbate layer of molecularly
bound H2O molecules on the adsorbate layer of the dissociated OH groups. In this way, the brucite nuclei can form and grow. We did not intend to describe the full conversion of periclase to brucite. Rather, we wanted to describe the initiation process of the nucleation and the continuing growth on the surface connected with a diffusion process inside the crystal. This is in line with the observation that the degree of hydration of periclase in cement does not reach 100%. Even after many years, a periclase nucleus exists surrounded by a porous, larger volume brucite shell. The epitaxy relation MgO(111)||Mg(OH)2(0001)3 and the observed intrusion of H atoms into the crystal4 led to the model presented in Figure 1. To start from the hydroxylated MgO(100) surface (Figure 1a), Mg-OH groups move from the crystal surface into the adsorbate layer of the OH groups (Figure 1b). Additional water molecules from the second layer of water molecules (not shown) are dissociatively bound; OH groups are bound to the moved Mg species, and hydrogens are bound to the oxygens in the vacancy of the second MgO plane (Figure 1c). Finally, Mg species diffuse in the MgO(111) plane from inside the crystal to the surface and attach themselves in the
Formation of Brucite from Periclase and Water
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Figure 2. Optimized structures of (MgO)144‚24H2O cyclic hydroxylated cluster from starting structures with (a) one Mg-OH and (b) two Mg-OH out of bulk into adsorbate and one additional H2O; for further details see text.
brucite [0001] plane. Hydrogens penetrate into the crystal. The brucite nucleus grows into the crystal and at the same time out of the crystal on the MgO(100) surface (Figure 1d). This results in the epitaxy relation MgO(111)||Mg(OH)2(0001). Every second layer of magnesium is replaced by hydrogen. For the formation of the brucite lattice, oxygen and magnesium layers have to be distorted. 4. Simulation Studies 4.1. Formation of Nuclei on the MgO(100) Surface. All starting structures are model structures based on the cyclic hydroxylated six-layer (MgO)144‚24H2O cluster. In this cluster, one or two surface magnesium atoms with their OH groups from the hydroxylation are placed into the adsorbate layer of the dissociated OH groups. Also, one or two Mg species are placed into the adsorbate layer, and between one and six additional water molecules are added to allow a stepwise conversion of the hydroxylated Mg(100) surface into brucite nuclei. All systems are optimized in the following way. All water molecules and the two upper MgO layers for the first five steps, and the three upper MgO layers for the other steps are relaxed. The whole scheme is given in Table 1. The optimized three upper MgO layers of the structures are shown in Figures 2-4.
In the first step, one surface Mg with its OH group is shifted into the adsorbate layer of the dissociated OH groups, and this starting structure is optimized (Figure 2a). This corresponds to Figure 1b of the topotactic model. The relevant MgO(111) plane with Mg atoms is indicated in light gray. The arrow marks the Mg vacancy in the MgO(100) surface. During the optimization process, two additional Mg species moved from the surface to the adsorbate layer. The next starting structure was one where a second Mg-OH group was placed adjacent to the first moved Mg-OH group. The dissociated hydrogens, which were vertically bound to the neighbor oxygens, were rotated into the vacancies. This corresponds to Figure 1c of the topotactic model. Then, one additional water molecule from the second adsorbate layer was added in such a way that the OH group was attached to the shifted Mg and the H was placed in the vacancy of the Mg and attached to an oxygen of the plane below the surface. The optimized structure is shown in Figure 2b. Step 4 treated a system with two additional H2O from the second adsorbate layer. Up to step 4, the relaxation of two MgO planes was sufficient. In the following steps, the diffusion of one (Figure 3a) or two (Figure 3b) Mg species from the plane below the surface to the surface was simulated. Here and in the following steps, three planes had to be relaxed. The Mg species were
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Figure 3. Optimized structures of (MgO)144‚24H2O cyclic hydroxylated cluster from starting structures with (a) two Mg-OH and one Mg and (b) two Mg-OH and two Mg out of bulk into adsorbate and two additional H2O in both cases.
attached to the OH groups of the nucleus in such a way to form brucite. Four hydrogens were placed into the magnesium vacancies. The OH groups stabilize the shifted Mg species in a brucite-like structure. These steps lead to Figure 1d of the topotactic model. This mechanism can be repeated until all Mg species are replaced by H. The process must occur parallel in alternating layers. Due to the epitactic relation, an exact brucite structure can develop on the surface. Inside the crystal, whole planes have to be moved slightly against each other. We show now the optimized systems for two Mg-OH and two Mg species with five (Figure 4a) and six (Figure 4b) additional water molecules. These structures are different from each other and different from the one in Figure 3b. The reaction energy of all optimized systems were calculated with reference to the hydroxylated (MgO)144 cyclic cluster and the N additional water molecules.
∆Ehydrox ) Enucleus - EMgOhydrox - NEH2O
(1)
Here Ehydrox is the binding energy of the nucleus (MgO)144‚ 24H2O with the nucleus structure optimized for each step in the way described above. EMgOhydrox is the binding energy of the initial hydroxylated cluster and EH2O the binding energy of
the water molecule. The results of the reaction energy for each step are listed in Table 1. Both the additional water molecules and the relaxation of planes stabilize the system with the former being more important. These conclusions are based on thermodynamics. No activation energies were calculated due to the complexity of the process. 4.2 Partial Periclase Hydration. To show the final product of a partially hydrated periclase, a complementary study on the cluster (MgO)48[Mg(OH)2]32 was undertaken. The cluster is shown in Figure 5. The degree of hydration of periclase is 40%. This is a realistic degree observed in experiments. Figure 5 shows the volume increase in the upper rectangle. From the binding energies of 32 H2O (-30432 kJ/mol), (MgO)80 (-81840 kJ/mol) and (MgO)48[Mg(OH2)]32 (-118445 kJ/mol), a hydroxylation energy Ehydrox of -193 kJ/mol per Mg(OH)2 unit was obtained. 4.3 Discussion. From the results of the adsorption calculations,6 the molecular dynamics simulation of H2O on the MgO(001) surface20 and the present nucleation of brucite, it can be concluded that the hydration of periclase with water vapor occurs in five steps: 1. Dissociative adsorption of a monolayer of water at the periclase (100) surface.6
Formation of Brucite from Periclase and Water
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13107
Figure 4. Optimized structures of (MgO)144‚24H2O cyclic hydroxylated cluster from starting structures with two Mg-OH and two Mg out of bulk into adsorbate and (a) five and (b) six additional H2O.
2. Molecular adsorption of a water layer on the first monolayer.20 3. Diffusion of magnesium through the solid through vacancies in the layer of dissociatively adsorbed water into the adsorbate (steps 1-4 of Table 1). 4. Topotactic nucleation in the multilayer adsorbate (steps 5-6 of Table 1). 5. Growth of brucite on periclase according to the epitaxy relation brucite (0001)||periclase (111) (steps 7-10 of Table 1). All reaction steps are exothermic. The theoretical reaction sequence is consistent with experimental findings. This is particularly true for the water uptake by MgO for the described degrees of wetting.32 The first step of the reaction is assigned to the multilayer adsorption based on the gravimetric measurements. The nucleation is stopped if there is no second layer of molecularly
adsorbed water molecules. Because larger time intervals are necessary for the diffusion of water vapor in concrete, the second layer contributes significantly to the long hydration times of periclase under ambient conditions. The transfer of Mg species from the stable MgO(001) surface into the water adsorbate proceeds through a vacancy in the dissociatively adsorbed monolayer. This is a new finding. Until now, only lattice defects in the solid surface have been considered for the nucleation. In this work, it is shown that the topotactic growth of brucite can occur on the defect-free surface. As shown in Figure 2-4, the nucleation is a complex process. Nevertheless, it is seen that a one-dimensional nucleus is formed in the periclase(111)-brucite(0001) plane. The brucite nucleus grows from the surface of the oxide cube parallel to the octahedral planes and forms an acute angle with the surface. In the course of the reaction, brucite grows out of the crystal. For a hydration degree of 40%, a hydration energy of -193
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Jug et al. by the corresponding calculations. The mechanism is in line with experimental observations. Acknowledgment. B.H. thanks Universita¨t Hannover for a graduate fellowship. She also acknowledges support by the Faculty of Architecture. References and Notes
Figure 5. (MgO)48 [Mg(OH)2]32 partially hydrated periclase to brucite with 40% degree of hydration; Veduct is the initial volume of the MgO cluster.
kJ/mol per Mg(OH)2 was obtained, compared with -217 kJ/ mol for a full conversion.22 5. Conclusion A quantum chemical study has shown that a (topotactic) model based on the observed epitaxy relaxation MgO(111)||Mg(OH)2(0001) can explain the nucleation process of brucite on a hydroxylated periclase surface. This model is verified by simulations of the various steps involved on the atomic level. For this purpose, cyclic cluster calculations on the system (MgO)144‚24H2O with a second layer of water molecules proved suitable. To understand the growth process of Mg(OH)2 on MgO, a mechanism was developed that starts with the dissociative adsorption of a monolayer of water molecules on top of which a second layer of molecularly adsorbed water molecules is arranged. The simulations allow us to explain how the diffusion of magnesium through vacancies in the layer of dissociatively adsorbed water into the adsorbate initiates the creation of brucite nuclei. Finally, the formation of a brucite (0001) plane parallel to the MgO(111) plane can be confirmed
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