Stability, Structural, and Electronic Properties of Hausmannite

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Stability, Structural and Electronic Properties of Hausmannite (MnO) Surfaces and Their Interaction with Water 3

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Paulo Roberto Garcês Gonçalves, Heitor Avelino De Abreu, and Hélio Anderson Duarte J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06201 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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Stability, Structural and Electronic Properties of Hausmannite (Mn3O4) Surfaces and Their Interaction with Water Paulo Roberto Garcês Gonçalves Jra,b, Heitor Avelino De Abreua and Hélio Anderson Duartea* a

GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Belo

Horizonte-MG, 31270-901, Brasil b

Instituto Federal de Educação, Ciência e Tecnologia do Maranhão, Campus São Luís-

Maracanã, São Luís-MA, 65095-460, Brasil

* e-mail: [email protected] Phone: +55(31)3409-5748

Abstract Hausmannite (Mn3O4) is the stable phase of manganese oxides and has attracted interest due to their technological applications such as molecular adsorption, ion exchange, catalysis and water treatment. Hausmannite is a normal spinel structure that presents the formula A2+B23+O4 where tetrahedral sites are occupied by Mn2+ and the octahedral sites by Mn3+ cations. The present work intend to fulfill the lack of information about the electronic and structural properties of the hausmannite surfaces and their interaction with water. This is an important aspect for understanding the reaction mechanism occurring at the hausmannite/water interface. Density functional calculations have been employed to investigate the structural, electronic properties of hausmannite. Energy surfaces have been estimated for the low Miller indexes and the water adsorption on the most favored surface was investigated in detail. The results indicate that the (001) cleavage surface is the most favored in good agreement with the experimental results with an estimated surface energy of 1.40 J m–2. The water prefers to adsorb molecularly at the Mn2+ adsorption sites with an estimated adsorption energy of –16.5 kcal mol–1. The dissociative mechanism is estimated to be at least 5.2 kcal mol–1 higher in energy. For the monolayer limit for the water adsorption the energy is estimated to be –10.9 kcal mol–1 and the dissociative mechanism is only 4.7 kcal mol–1 higher in energy. This is an important step to understand the chemical reactivity of the hausmannite surfaces in the solid/water interface.

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1. INTRODUCTION Materials based on manganese oxides have incited great scientific interest due to their technological applications, such as molecular adsorption, ion exchange, catalysis and water treatment1-3. Manganese oxides are found in several structures with varying proportions of manganese in different oxidation states (2+, 3+ and 4+). The different minerals containing manganese are found as fine particles or coatings in the Earth's crust and in the aquatic environment with significant reactivity due to their high surface area and ability to act in oxidative processes of organic and inorganic compounds4-6. Hausmannite is the most stable phase of manganese oxides7. It belongs to spinel class with I41/amd space group (# 141) and tetragonal arrangement. It is represented by the formula Mn2+[Mn3+]2O4, where the divalent cations occupy tetrahedral sites and the trivalent cations are located in the octahedral sites8-10 (figure 1). This oxide is a semiconductor11-13 material and its unit cell contains four Mn3O4 formulas. The structure has manganese and oxygen bonded in a tetrahedral environment (Mn2+Td...O) and in an octahedral distorted arrangement with D4h symmetry hence containing Mn+3Oh...Oaxial and Mn3+Oh...Oequatorial, respectively. In the literature14, 15 it is reported that the cleavage surface of hausmannite leads indistinctly to the (001) oriented surface. Nevertheless, a study of thin films formation of Mn3O4 oriented in other crystallographic directions, such as the (110) SrTiO3 substrate, has also been reported16. The mixed valence mineral hausmannite is involved in many processes of environmental importance mostly at the mining regions17,

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. Its adsorptive and

oxidative properties are subject of investigation envisaging its technological application. However, the chemical properties of the hausmannite surfaces and their adsorptive properties towards water are still not understood at a molecular level. In the present work, the structural and electronic properties of the hausmannite bulk and low-index cleavage surfaces have been investigated to fulfil the lack of information about this system. The water adsorption on Mn3O4 (001) surface has been also investigated aiming to provide insights about its chemical reactivity and the solid/water interface.


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Figure 1.

Hausmannite cell unit with 28 atoms. (a) Mn2+ tetrahedral and Mn3+ octahedral sites. (b) Detailed structure of the different Mn sites.

2. COMPUTATIONAL DETAILS According to Jarosch´s crystallographic refinement data9, Mn3O4 exhibits tetragonal arrangement (I41/amd space group) with the following cell parameters: ߙ = ߚ = ߛ = 90°, a = b = 5.762Å and c = 9.442Å. Density functional theory (DFT)/plane waves methodology with periodic boundary conditions was used to calculate the electronic structure as implemented in the Quantum Espresso package19. The PBE exchange/correlation (XC) functional20 were used 5

2

0

2

with the following valence

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configurations: Mn (3d 4s 4p ) and O (2s 2p ). The core electrons were described by PAW (Projector-Augmented Wave) potential21. For the bulk, different kinetic cutoff energies, k-point meshes and spin magnetization were evaluated to assure the accuracy of the calculations (for more information see Supporting Information – figure S1 and table S1). The spinels are materials which have complex magnetic characteristics, as disorders in the distribution of cations on the tetrahedral sites (type A = Mn2+, the AB2O4 chemical formula) and octahedral (type B = Mn3+) can influence such magnetic properties of the system such as magnetization saturation, spin coupling and ferrimagnetic interactions22,

23

.

According to Lazaro et al.24 antiferromagnetic oxides such as Mn3O4 have spin couplings that are mediated by oxygen atoms coordinated with Mn(II) and Mn(III). The geometric and electronic properties of these cations are a factor of change in the magnitude of these interactions. Therefore, the predominant couplings of hausmannite are JA-A, JA-B and JB-B, where A and B are the cations distributed in the tetrahedral and octahedral sites, respectively. We performed spin polarization tests on the oxide,



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considering the open shell system consistent with antiferromagnetic properties. Our results indicated that the most stable system is spin compensated, with absolute magnetization of the order of 48µB (Bohr magnetons) per unit cell and total magnetization is equal to zero. The spin polarization at the Mn2+ and Mn3+ centers is about 4.3µB and 3.6µB per atom. In the bulk cleavage process along the plane (001), two asymmetric surfaces (bottom surface (001)-1 and top surface (001-2)) were generated exposing Mn3+ in the bottom, and the Mn2+ in the top. The cleavage therefore altered the spin compensation at the surfaces. At the (001)-1 surface the spin polarization on the Mn2+ is about 4.3µB per atom and, at the (001)-2 surface is about 4.6µB. At the Mn3+ cations the spin polarization is about 3.6µB for (001)-1 and 3.5µB for (001-2). The Kohn-Sham electronic states were expanded in a plane-wave basis set using the kinetic cutoff energy of 80 Ry and the charge density cutoff of 800 Ry. The Brillouin-zone integration has been performed based on the Monkhorst-Pack scheme25 using 6x6x6 k-point mesh and Gaussian smearing of 0.02 eV. These parameters lead to accuracy of 1 mRy atom-1 in the total energy calculation. The atomic positions, as well as the cell parameters, were fully optimized. Bader’s QTAIM (Quantum Theory of Atoms In Molecules) method26 was used to understand the nature of the chemical bonding in the hausmannite bulk. Band structure, density of states (DOS), electron localization function (ELF) plots and critical point analysis were calculated from single point calculations of the optimized geometry using larger 12x12x12 k-points mesh. The first Brillouin zone for band structure calculations is described according to the set of points suggested by Setyawan et al.27 for a tetragonal system. For both bulk and slabs all calculations were done with spin compensated condition. Geometry optimization was carried out using the BFGS method28 keeping the force tolerance criteria at the 10-3 Ry Bohr-1. Several Mn3O4 surfaces were investigated using 2x2x1 supercell slab model, built from the optimized bulk structure, with 12Å of vacuum to avoid interaction between slab layers. The kinetic energy cutoff of 40 Ry and 3x3x1 k-point mesh were used and the energy was converged to 10-6 Ry. The cleavage energies (γcleavage) were determined by equation (1), where ESnre is the non-relaxed surface energy estimated by single point calculation of the slab with the atoms positions fixed at the bulk value. We analyze the breaking of chemical bonds and the relaxation or reconstruction of the system through equation (2), in which Sxre (x=1,2 represents the relaxed bottom and top


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surfaces, respectively) is the surface relaxation energy, ESre-x refers to the total energy of the relaxed surface. Ebulk is the total energy of the fully optimized bulk structure and the term 2A denotes the area of two formed surfaces (bottom (1) and top (2)), which are formed during the cleavage process29, 30.

ߛ௖௟௘௔௩௔௚௘ = ௫ ܵ௥௘ =

‫ܧ‬ௌ௡௥௘ − ‫ܧ‬௕௨௟௞ 2‫ܣ‬

‫ܧ‬ௌ௥௘ି௫ − ‫ܧ‬௕௨௟௞ 2‫ܣ‬

(1) (2)

We adopt the same methodology of Soares Jr. et al.30 for calculating the surface energies (γsurface) using the equation (3), which ESre-1 and ESre-2 are the energies of the relaxed surfaces (bottom and top) computed separately.

ߛ௦௨௥௙௔௖௘ =

‫ܧ‬ௌ௥௘ିଵ + ‫ܧ‬ௌ௥௘ିଶ − ‫ܧ‬ௌ௡௥௘ − ‫ܧ‬௕௨௟௞ ଵ ଶ = ܵ௥௘ + ܵ௥௘ − ߛ௖௟௘௔௩௔௚௘ 2‫ܣ‬

(3)

The definition of the appropriate thickness of the slab of this surface was evaluated in order to ensure that the inner part of the slab behaves as the bulk (figures S6 and S7), the surface formation energy was studied by varying from 3 to 7 the number of atomic layers of the slab along the c axis, maintaining the same stoichiometric ratio of 2Mn2+:4Mn3+:8O2-. For the water adsorption simulation we used the slab (001) containing 4 stoichiometric layers with 112 atoms, 16 tetrahedral cations Mn2+, 32 octahedral cations Mn3+ and 64 anions O2-. The cutoff energy was set to 40 Ry at the gamma k-point. All adsorption energies were calculated using equation (4), where Eads is the adsorption energy, n is the number of water molecules on the surface, Esurface+water is the total energy of the conjugated system formed by the Mn3O4 relaxed surface and the water adsorbed, Esurface is the energy of the Mn3O4 relaxed surface and Ewater is the total energy of the water isolated molecule calculated in a box with identical dimensions of that used for the surface.

‫ܧ‬௔ௗ௦ =

1 (‫ܧ‬ − ‫ܧ‬௦௨௥௙௔௖௘ − ݊‫ܧ‬௪௔௧௘௥ ) ݊ ௦௨௥௙௔௖௘ା௪௔௧௘௥

3. RESULTS AND DISCUSSION 3.1. Bulk Analysis


(4)


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The calculated lattice parameters and interatomic distances of the Mn3O4 crystal structure are listed in table 1 with the respective hausmannite crystallographic data. They are in good agreement with the experimental crystallographic parameters determined by Aminoff8, Jarosch9 and Baron et al.10. The greatest structural differences are in the c axis of the tetragonal system with respect to the experimental and the values are not larger than 1.5%. The crystallographic parameters estimated by HF23, DFT/PBE0 and DFT/B3LYP24 levels of theory are also reported in table 1. It is clear that the GGA-PBE XC functional provides geometrical parameters in better agreement with the experimental values than those obtained with the hybrid XC functionals.

Table 1: Experimental and calculated structural parameters of Mn3O4 bulk.

Lattice parameters (Å) References

a

b

c

Bond distance (Å) MnTd-O MnOh-Oaxial MnOh-Oequatorial

Experimental Aminoff1 5.762 5.762 9.439 1.862 Jarosch2 5.765 5.765 9.442 2.040 Baron3 5.757 5.757 9.424 2.044 This work PBE/US 5.771 5.783 9.542 2.022 PBE/PAW 5.760 5.774 9.580 2.019 PW91/PAW 5.757 5.772 9.594 2.020 PZ/PAW 5.705 5.796 8.093 1.930 Other theoretical Works HF4 5.919 5.919 9.311 2.098 DFT/PBE05 5.794 5.794 9.470 2.049 DFT/B3LYP5 5.901 5.901 9.394 2.065 1 Ref.[8]; 2 Ref.[9]; 3 Ref.[10]; 4 Ref. [27]; 5 Ref. [28]

Volume (Å3)

2.360 2.282 2.284

2.037 1.930 1.932

313.38 313.81 312.34

2.331 2.345 2.367 1.948

1.941 1.946 1.928 1.928

318.42 318.59 318.80 267.61

2.247 2.299 2.307

1.956 1.935 1.959

326.21 317.91 327.12

The difference of the MnTd...O, MnOh...Oaxial and MnOh...Oequatorial bond lengths compared to the most recent experimental data9,

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is about 0.02, 0.06 and 0.02 Å,

respectively. Considering the standard deviations, these differences in the bond lengths are less than 3%. The band structure calculation shows that hausmannite presents an indirect bandgap, from point M to A, with a value of 0.8 eV, shown in figure 2 (red arrows). Experimental bandgap values of Mn3O4, obtained through absorption measurements in the ultraviolet and visible region, vary from the order of 2.07 eV for nanoparticles and 2.51 eV for thin films of the oxide12, 13. Larbi et al.31 synthesized catalytic hausmannite ACS Paragon Plus Environment

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by the spray pyrolysis method and estimated a bandgap of 2.23 eV. Recent theoretical studies24, 32, 33, using hybrid XC functional (B3LYP, PBE0 and HSE) and ferromagnetic models with spin polarization, presented hausmannite bandgap values very close to the experimental results. It is known that the GGA XC functionals underestimate the band gap energies while hybrid XC functionals frequently overestimate them34. Actually, Morales-García et al.35 showed that overall behavior of the hybrid XC functionals depends on the amount of Fock exchange included in the XC functional.

Figure 2.

DOS and band structure of Mn3O4, considering spin up and down. The Fermi level is shifted to zero eV.

The PDOS on the hausmannite atoms are shown in figure 3a and 3b. It is possible to observe symmetrical curves for spin up and down (upper and lower parts of figure 3, respectively) presenting curves with similar profiles due to the antiferromagnetic model simulated in this work. As it can be seen in figure 3b, there is an effective contribution to the valence band and to the conduction band from the (Mn3+)Oh and (Mn2+)Td sites, respectively. In fact, this is coherent with the presence of Mn(II) in the tetrahedral sites, since their contribution to the valence band is larger. The octahedral Mn(III) sites contribute mostly to the conduction band. Therefore, it is expected that in an oxidation process the Mn(II) cation will be preferentially oxidized and Mn(III) will be preferentially reduced.



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Figure 3.

DOS: (a) total and projected on the Mn3O4 atoms, and (b) PDOS normalized per atom on the Mn3O4 orbitals, considering spin up and down. The Fermi level is shifted to zero eV.

The PDOS on the atomic orbitals of the bulk Mn3O4 is represented in the figure 3-b. The valence band is predominantly composed of 3d states of manganese and 2 of oxygen, which suggests possible charge transfer between the metallic centers and the oxygen, evidenced in the conduction band with strong presence of d electrons of


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octahedral Mn3+. These results are close to those obtained by Larbi et al. in a theoretical and experimental study on the properties of magnetically ordered Mn3O4 oxide using DFT/PBE0 method31, 33. The structure and nature of the chemical bonds present in Mn3O4 were investigated based on the QTAIM theory. The position of the critical points is shown in figure S3 and the data of the bond critical points (BCPs) are shown in table S4 (see Supporting Information). According to the topological analysis, there are 4 Mn2+Td...O BCPs, typical of the tetrahedral environment, 2 Mn3+Oh...Oaxial and 4 Mn3+Oh...Oequatorial, the 6 characteristics of octahedral environment, as can be evidenced in Figure S3. All Mn3O4 BCPs have positive Laplacians and low electron density values (≤ 0.1106 e ao-3), similar to the values found by Morales-García et al. in a DFT study of covellite, which suggests ionic character of the metal bonds36, 37. The calculated topological charge and volume values derived from the QTAIM calculation of hausmannite are shown in table 3. The Bader’s charge, Q (Ω), calculations show the presence of Mn2+, with charge of 1.46 e, at the tetrahedral sites and Mn3+, with Q of 1.64 - 1.65 e at the octahedral sites. The two sites of Mn(II) an Mn(III) are clearly characterized affecting the volume of the basin for each of the atoms. Table 3: Bader´s charge and basin volume of manganese and oxygen present in the hausmannite unit cell.

Χ1

OS (Ω)2

Q (Ω)

Volume (Bohr3)

+2

1.46

81.53

1.64

58.92

1.65

58.63

MnOh

1.64

58.38

Otetrahedral

-1.18

84.42

-1.19

85.24

-1.19

84.60

-1.19

84.37

Atoms MnTd MnOh

1.55 MnOh

Oaxial Oequatorial

+3

3.44

-2

Oaxial

2149.79 1

Pauling Electronegativity Scale

38 2

Oxidation state.



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The ionicity of the crystal can be estimated by the c parameter and its value for the hausmannite is estimated to be 0.60 suggesting a solid with a 60% of ionic character (see SI for details). The ELF along the (001) plane of the bulk shown at figure 4-a exhibits strong electron localization and low electronic density in the interatomic Mn…O bonding, with well-defined atomic basin contour lines highlighting its ionic character.

Figure 4.

Electron Localization Functions (ELF) of hausmannite Mn...O bonds. (a) ELF along the (001) plane, highlight the Mn3+Oh...O octahedral bond. (b) ELF along the (010) plane, especially for the Mn2+Td...O tetrahedral bond.

Figure 4-b shows the ELF (010) plane, highlighting the chemical bonds between Mn3+Oh...Oaxial and Mn2+Td...O. The topological analysis of the electron density based on the ELF and QTAIM corroborates with the ionic model of the hausmannite with the formula (Mn2+)(Mn3+)2O4.The non-directional character of the ionic bonding favors the shearing of cleavage planes in the places where these BCPs are present. In the case of hausmannite, we can highlight the presence of eight connection points, as illustrated in figure S4, which are more susceptible to bond breaking between the manganese and oxygen atoms. This explains the preferential (001) cleavage plane due to a smaller number of chemical bondings to be broken.


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3.2. Surface formation and water adsorption From the optimized tetragonal structure I41/amd of hausmannite bulk, different surfaces were generated as shown in figure 5. The surfaces are identified by the following Miller indexes: (001), (100), (010), (101), (110), (110), (111) and (112). It is noteworthy that the most commonly observed crystalline surfaces are those with lower indexes, since they require less energy to be created. Table 2 shows the values of ଵ ଶ cleavage energy (γcleavage), relaxed surfaces energies (ܵ௥௘ and ܵ௥௘ ) and surface energy

(γsurface) of the slabs.

Figure 5.

Cleavage planes proposed from the hausmannite optimized bulk. The surfaces (100) and (010) are equivalent, as well (101) and (011). Mn2+ in green, Mn3+ in purple, and O2- red.

Table 2: Cleavage energies (γcleavage), relaxed surface energies (bottom and top) and surface energies (γsurface) of the differents Mn3O4 slabs. The values are in J m-2. Surfaces

γcleavage

ଵ ܵ௥௘ (Bottom relaxed)

(001) (100) = (010) (101) = (011) (110) (111) (112)

2.33 5.07 3.38 6.49 3.54 3.14

1.81 3.44 2.76 4.26 2.65 2.44

ଶ ܵ௥௘ (Top relaxed)

γsurface

1.92 3.56 2.48 4.09 2.57 2.48

1.40 1.93 1.96 1.85 1.68 1.78



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Large differences between cleavage energy and surface energy indicate large relaxation of the surface. Figures S8 and S9 at the supporting information show the relaxed surfaces. The (001) surface presented less relaxation in the atomic positions of the system, making this plane the most favorable to the cleavage in relation to the other investigated crystallographic planes, with estimated value of γcleavage equal to 2.33 J m-2 and γsurface equal to 1.40 J m-2. The MnOh...Oequatorial bond lengths presented variation lower than 0.05Å, whereas the MnOh...Oaxial bond lengths ranged about 0.07Å, and the MnTd...O bond lengths about 0.03Å. Kim and et al.40 studied cleavage planes of LiMn2O4 spinel-like system through DFT/GGA/PBE calculations, similar to the Mn3O4 structure, and found surface energy values for the plane (001) about 0.90 J m-2. Bayer et al.14 also showed the high stability of the hausmannite surface (001) based on experimental and theoretical investigations of the nanofilm interface of Mn3O4 on MnO substrate. X-ray absorption and photoemission spectroscopies (XPS), highresolution electron energy loss spectroscopy (HREELS), spot profile analysis low energy electron diffraction (LEED), and DFT calculations were employed to study the growth of (001) oriented Mn3O4 surfaces on a Pd (100)-supported MnO (001) substrate, with the hausmannite planar lattice constants aligned along the (110) direction of the underlying MnO (001) support. This work showed that abrupt interfaces may exist between rocksalt MnO and Mn3O4, arguing that this process was facilitated by the relatively low computed strain energy – about 22 meV atom-1. Finally, spot profile analysis LEED experiments showed that the Mn3O4/MnO conversion reaction proceeds easily in the (001) direction, indicating the reversible redox character of the transition. The (001) surface is the most favored cleavage plane, which exposes the cations Mn2+ and Mn3+ (respectively green and purple spheres of figure 6), located in the tetrahedral and octahedral sites. Therefore, the (001) Mn3O4 surface was an adequate model for investigating the surface reactivity of hausmannite.


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Figure 6.

The atomic structure of asymmetric Mn3O4 (001) surface.

Water adsorption on (001) surface The adsorption of water on (001) surface was investigated based on the molecular and dissociative mechanisms. The molecular adsorption can lead to the concomitant formation of hydrogen bonding with the oxygen sites on the surface. Figures 7 and 8 show the most stable structures of water adsorption on (001) Mn3O4 surface. Different starting positions were tested, see supporting information (figure S13). The water adsorption energies are shown at Table 3 and S5 (see Supporting Information). In figures 7a and 7b considering molecular adsorption, in the (001)-1 surface, a water molecule was attached to Mn3+ site with estimated adsorption energy of -7.0 kcal mol-1. Figures 7c and 7d shows that structure of the water adsorbed and forming hydrogen bond with the oxo group at the surface and the estimated energy is -7.4 kcal mol-1. The dissociative process with adsorption energy of -0.2 kcal mol-1 is shown at figure 7e and 7f. In the (001)-2 surface, one H2O molecule was placed at the Mn2+ sites, following the same adsorption scheme in the (001)-1. Thus, the most stable structures for molecular adsorption (8a and 8b) has estimated adsorption energy of -16.5 kcal mol-1. The water adsorbed and forming hydrogen bond with the oxo group at the surface has an estimated adsorption energy of -15.4 kcal mol-1 (Figures 8c and 8d) and the dissociative process has estimated adsorption energy of -11.3 kcal mol-1 (Figures 8e and 8f).



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It is important to note that the inclusion of London dispersion effects is unlike to change the results as it was shown by Grimme41 and Klimeš et al.42. In fact, they have shown that the effect on the reaction energies is not larger than 1.0 kcal mol–1.

Table 3: Water adsorption energy on Mn3O4 (001) surface. H2O adsorption energy / (kcal mol-1) Adsorption Site Molecular1 H-bond Dissociative1 (001)-1 Mn3+ -7.0 (-11) -7.4 -0.2 (-3.2) (001)-2 Mn2+ -16.5 (-10.9) -15.4 -11.3 (-6.2) 1 Values in parenthesis are for monolayer. Layer

Figure 7.

The most stable structures of water adsorption Mn3O4 (001)-1 surface. (a) and (b) molecular adsorption. (c) and (d) Hydrogen bond adsorption. (e) and (f) dissociative adsorption.


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Figure 8.

The most stable structures of water adsorption Mn3O4 (001)-2 surface. (a) and (b) molecular adsorption. (c) and (d) Hydrogen bond adsorption. (e) and (f) dissociative adsorption.

The molecular adsorption on the Mn3+ adsorption sites is the most favorable for the (001)-1 surface. The formation of hydrogen bonding between the adsorbed water with the oxygen surrounding the Mn3+ sites stabilizes just 0.5 kcal mol-1. The steric hindrance seems to play an important effect since the relaxed (001)-2 Mn3O4 surface present the Mn3+ and oxygen centers in the surface. Geometrical arrangement does not favor the hydrogen bond formation. The (001)-2 surface presents the Mn2+ exposed facilitating the water adsorption and, concomitantly, its hydrogen bonding with the oxygen surrounding leading to about -1 kcal mol-1 of stabilization. The water adsorption on (001)-2 surface is more favored with respect to the (001)-1 surface with estimated adsorption energy of -16.5 kcal mol-1. Besides the geometrical factor, the electronic effect has also to be evaluated. Figures S11 and S12 (at Supporting Information) show the integrated PDOS in the range of 2.9-3.5 eV at the conduction band. It is clear that the unoccupied states mostly prominent to receive the lone electron pair of the water is



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the Mn2+ adsorption sites. Therefore, the water adsorption on the top Mn2+ sites is more favorable due to the both geometrical and electronic effects. The dissociative mechanism for the water adsorption at the Mn2+ sites ((001)-2 surface) is also favored with estimated energy of -11.0 kcal mol-1, leading to the OHgroup adsorbed at the bridge sites with Mn-O bond distances of about 2.04 Å. This is an indication that both mechanisms are present at the top surface, with important consequences for the chemical reactivity of that surface. The water adsorption on the (001)-1 surface (Mn3+ adsorption) is predicted to follow the molecular mechanism. The hausmannite (001)-1 surface has 8 Mn3+ sites, and 4 Mn2+ sites in the (001)2. Thus, the first one could adsorb up to 8 water molecules in a monolayer coverage. On the other hand, the (001)-2 surface could adsorb up to 4 water molecules to form a monolayer, as one can see at the figure 9. The structures were optimized using the same geometry of the one water molecular adsorption but replicated on the other surface Mn sites. The adsorption energy per water molecule (kcal mol-1) on the (001)-1 surface was estimated to be -11.0 kcal mol-1, about 4.0 kcal mol-1 lower in energy than the single water adsorption. For the (001)-2 surface, the estimated energy was -10.9 kcal mol-1, about 5.6 kcal mol-1 higher than the single one. The monolayer water adsorption on the (001) Mn3O4 surface leads to the molecular adsorption close in energy. The monolayer water adsorption following the dissociative mechanism leads to the adsorption energy per water molecule of about -3.2 kcal mol-1 to the (001)-1 surface, and -6.2 to the (001)2 one. Hence, the dissociative way adsorption is less favorable than the molecular one for both surfaces.


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Figure 9.

Full water adsorption Mn3O4 (001) surface. (a) monolayer of water on (001)-1 surface Mn3+ sites. (b) monolayer of water on (001)-2 surface Mn2+ sites.

4. SUMMARY The DFT/Plane Waves calculations indicated that the hausmannite presents ionic character coherent with the formula (Mn2+)(Mn3+)2O4 and spin compensated system with local spin polarization of 4.3 and 3.6 µB per atom for the Mn2+ and Mn3+ centers. The cleavage energy for 6 different low index planes were estimated indicating that the (001) cleavage surface is the most favored leading to the surface energy of about 1.4 J m-2. The (001) cleavage surface leads to two surfaces exposing (001)-1 Mn3+ centers and (001)-2 Mn2+ adsorption sites. The water adsorption on both sites using the molecular and dissociative mechanism were investigated. The water adsorption on the Mn3+ sites is preferentially molecular with the adsorption energy estimated to be about -7 kcal mol-1. However, the molecular water adsorption on the Mn2+ sites is the most favored with adsorption energy estimated to be about -16.5 kcal mol-1. The geometrical and electronic properties of those surfaces explain why the water prefers to adsorb on the Mn2+ adsorption sites. Furthermore, the dissociative mechanism is also favored, leading to the OH- groups



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adsorbed on the bridge sites. This is an important aspect that affects the chemical reactivity of the hausmannite surfaces towards oxidative processes. Using the monolayer water adsorption model, the results indicate that the molecular water adsorption energy on the (001)-1 and (001)-2 surfaces became closer in energy and this process is at least 4.7 kcal mol-1 more stable compared to the dissociative way.

Support Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. Convergence test for hausmannite bulk; structural parameters and spin test for hausmannite bulk; Topological analysis of the electron density; QTAIM properties of hausmannite; slabs models: number of layers of the slab (001); structural and electronic details (projected density of states) of the adsorbed species in the different adsorption sites on hausmannite (PDF).

Acknowledgments: We are grateful to the support of the Brazilian agencies Conselho Nacional para o Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES), Fundação de Amparo à Pesquisa do Estado do Maranhão (FAPEMA), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG). In addition, we thank to the Instituto Federal de Educação, Ciência e Tecnologia do Maranhão (IFMA), the National Institute of Science and Technology for Mineral Resources, Water and Biodiversity – INCT-ACQUA and the RENOVAMin (Proc. RED-00102-16).

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Table of Contents Graphic


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Hausmannite cell unit with 28 atoms. (a) Mn2+ tetrahedral and Mn3+ octahedral sites. (b) Detailed structure of the different Mn sites. 252x98mm (96 x 96 DPI)


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Figure 2. DOS and band structure of Mn3O4, considering spin up and down. The Fermi level is shifted to zero eV. 375x174mm (96 x 96 DPI)



DOS: (a) total and projected on the Mn3O4 atoms, and (b) PDOS normalized per atom on the Mn3O4 orbitals, considering spin up and down. The Fermi level is shifted to zero eV. 201x245mm (96 x 96 DPI)


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Figure 4. Electron Localization Functions (ELF) of hausmannite Mn...O bonds. (a) ELF along the (001) plane, highlight the Mn3+Oh...O octahedral bond. (b) ELF along the (010) plane, especially for the Mn2+Td...O tetrahedral bond. 179x131mm (96 x 96 DPI)



Figure 5. Cleavage planes proposed from the hausmannite optimized bulk. The surfaces (100) and (010) are equivalent, as well (101) and (011). Mn2+ in green, Mn3+ in purple, and O2- red. 145x129mm (96 x 96 DPI)


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Figure 6. The atomic structure of asymmetric Mn3O4 (001) surface. 303x150mm (96 x 96 DPI)



Figure 7. The most stable structures of water adsorption Mn3O4 (001)-1 surface. (a) and (b) molecular adsorption. (c) and (d) Hydrogen bond adsorption. (e) and (f) dissociative adsorption. 188x186mm (96 x 96 DPI)


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Figure 8. The most stable structures of water adsorption Mn3O4 (001)-2 surface. (a) and (b) molecular adsorption. (c) and (d) Hydrogen bond adsorption. (e) and (f) dissociative adsorption. 168x184mm (96 x 96 DPI)



Figure 9. Full water adsorption Mn3O4 (001) surface. (a) monolayer of water on (001)-1 surface Mn3+ sites. (b) monolayer of water on (001)-2 surface Mn2+ sites. 244x190mm (96 x 96 DPI)


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Graphical Abstract 240x111mm (96 x 96 DPI)


Stability, Structural, and Electronic Properties of Hausmannite

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