Unveiling the Structural and Electronic Properties of the B-Nb2O5

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Unveiling the Structural and Electronic Properties of the B-NbO Surfaces and Their Interaction with HO and HO 2

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Mirele Bastos Pinto, Antonio Lenito Soares Jr., Matheus Campos Quintão, Hélio Anderson Duarte, and Heitor Avelino De Abreu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11972 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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The Journal of Physical Chemistry

Unveiling the Structural and Electronic Properties of the B-Nb2O5 Surfaces and their Interaction with and Mirele B. Pinto, Antonio Lenito Soares Jr., Matheus Campos Quintão, Hélio A. Duarte, and Heitor A. De Abreu* GPQIT. Departamento de Química. ICEx. Universidade Federal de Minas Gerais. Belo Horizonte. 31.270-901. MG. Brazil

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Abstract Niobium pentoxide (Nb2O5) also known as niobia has been applied in several areas among others in heterogeneous catalysis. This is due to both its high acidity (Brönsted acid and Lewis acid sites) and its Lewis acid sites tolerant to water. The structure and morphology of these sites present tunable quantity and strength, however, little attention has been given to its polymorphic forms and reactivity. In this work, the surface properties of stoichiometric B phase (B-Nb2O5), including the cleavage surfaces, structural, energetic, electronic properties and chemical reactivity towards water (H2O) and hydrogen peroxide (H2O2), by means of periodic density functional theory (DFT), have been studied through DFT calculations. An initial investigation was carried out to determine cleavage surface of the B-Nb2O5. Our results show that the B-Nb2O5 (010)-2 surface is the most stable (surface energy 0.52 J m-2) of the surfaces studied. Projected density of state (PDOS) analysis showed that the niobium atom is a Lewis acid site. When H2O was adsorbed on the (010)-2 surface, the molecular adsorption is the most stable, under Nb site. However, the results showed that both dissociative and molecular mechanisms must be present on the surface, although the dissociative one to a lesser extent. When H2O2 was adsorbed on the (010)-2 surface, The calculated adsorption energies showed that the preferred site for H2O2 adsorption is the Nb, with adsorption energy of 1.63 eV, which resulted in the formation of a hydrosuperoxo (HO ) species. However, the HO , O and H O species may exist in equilibrium on the (010)-2

surface due to small difference between their adsorption energies (up to 0.14 eV).


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1.INTRODUCTION. Nb2O5 is a white, air-stable, water-insoluble solid. Experimentally, it is known that Nb2O5 presents several crystallographic phases depending on the temperature and pressure to which the materials are subjected and of the methods of preparation and the starting materials. The literature reports that the first study of polymorphs of Nb2O5 was around the beginning of the 1940s.1 According to Schafer et al.,2 the crystallographic phases are classified in the groups: low temperature phases (∼700 to 900 K), T and TT; medium temperature phases (∼900 to 1200K), B and M; and high temperature phase, H (1223 K). Polymorphic transitions may occur, however, slowly under conditions not very well defined and irreversible.3 Generally, Nb2O5 polymorphs are based on distorted octahedra (NbO6) and their degree of distortion depends on whether the octahedra are connected by edges and (or) by corners.

Initially, the main interest of niobia was to be applied in the construction of sensors, in the electrochromism and like catalysts.3-6 Nevertheless in the last few years, the use of niobia as a catalyst has received more attention.3, 7-10 When Nb2O5 is used with other metal oxides/metals, it may play the role of active phase and/or enhance the catalyst activity due to strong metal-support interaction.3,

7-8

In the context of heterogeneous

catalysis, an example of the catalytic application of the niobium oxide is the hydrogen storage. Transition metals such as Nb, V, and Ti are well known catalysts that can dissociate and adsorb H2, but Nb2O5 demonstrated a catalytic effect superior for both hydrogen absorption and desorption.11 The magnesium (Mg) is one of the attractive hydrogen storage materials, however the reaction kinetics of hydrogen absorption and desorption is too low, requiring high temperatures of 300−400 °C. Studies has showed that Nb2O5 reduce the dissociation energy of H2 molecules on the Mg surface. Takahashi et al.12 through DFT calculations studied H2 dissociation over the different surface planes of Nb, NbO, and Nb2O5 as well as NbO clusters. There were no activation barrier for H2, showing that those niobium oxides help develop better catalysts for H2 dissociation toward hydrogenation of Mg. Hanada et al.13 investigated valence state and a local structure of transition metals (Nb, V, and Ti) in MgH2 doped with metal oxides (Nb2O5,V2O5, and TiO2nano) by ball milling using X-ray absorption spectroscopy (XAS). They observed through XRD profiles that after dehydrogenation there are two new phases: MgO and Mg. The Nb, V, and Ti K-edge XANES spectra of 3 ACS Paragon Plus Environment


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the MgH2 showed that the Nb2O5, V2O5, and TiO2nano are reduced by MgH2 during ball milling to the metal oxides. However the authors believe that dehydrogenation and rehydrogenation treatment scarcely affect the oxidation state of the additives. This material also shows a high acid strength and stable activity.7, 14 Hydrated niobium pentoxide, Nb2O5.nH2O, which is usually called niobic acid is the most acidic form of the niobia, equivalent to the acidity of 70% sulfuric acid when calcined at moderate temperatures of 373−573 K. Its activity is manifested even in the presence of water vapour.14 The use of compounds and material containing niobium in reactions has attracted attention where the water participates or is eliminated during the catalytic process such as esterification, hydration, dehydration, hydrolysis, and others reactions.1517

This is due to the fact that many catalysts have their Lewis acidity character lost in the

presence of water, such as inorganic oxides including zeolites and metal oxides, which may be justified by water coordination to the Lewis acid sites.18-19 According to Nakajima et al.20 the most NbO4 tetrahedra (ca. 80%) on Nb2O5·nH2O in water are inactive to basic molecules, the remaining 20% of the NbO4 tetrahedral (Lewis acid) are the form of the NbO4-H2O adducts. They observed that the Lewis acid sites appear to be active for the reaction in water. According to Nakajima et al. the adducts formed can still be effective Lewis acid sites and may promote acid-catalyzed reactions even in the presence of water. On the other hand, Brönsted acid centers may be also present on the surfaces of the Nb2O5. According to Iizuka et al.21 after the mild heat treatment at 100 °C, acidic OH groups (Brönsted acid sites) were observed in the IR spectra of pyridine adsorbed on Nb2O5.nH2O. Furthermore, they also observed that when the temperature increases the amount of Brönsted acid sites decreases. Foo et al.22 studied the in-situ generation of Brönsted acid sites on the Nb2O5 surface calcinated at different temperatures. The authors showed through FTIR spectroscopy that it is possible to generate Brönsted sites from Lewis acid sites in the presence of water. However only part of the Lewis acid sites leads to the formation of Brönsted acid sites. H2O2 is often used as a powerful, environmentally friendly and safe oxidant, because its only degradation products are water and oxygen (2H O → 2H O + O ). Hydrogen peroxide interaction with the surface of catalysts is the key point to determine further transformations and reactions with the oxidized molecule. Niobia also has been used in oxidation reactions after treatment with hydrogen peroxide (H2O2) to generate highly 4 ACS Paragon Plus Environment

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oxidizing surface groups.23-28 Ventura et al.29 evaluated the catalytic activity of Nb2O5 nanoparticles in the liquid phase oxidation of aniline with aqueous H2O2 as oxidizing agent. The results evidenced high catalytic activity with total conversion of aniline at ambient condition. Astorga et al. 30 studied the regioselective oxidation of geraniol over niobium catalysts (Nb2O5 bulk, a mixed oxide of Nb2O5/SiO2 and Nb2O5/SiO2 mesoporous) using H2O2 as oxidant agent. Differences in selectivity were observed, which the authors believe to be due to inclusion of niobium on amorphous or mesoporous silica, changing the mode of coordination of niobium, modifying the activation of the oxidant. In addition to the application of this material, understanding chemical oxidation mechanisms is one of the most relevant issues. Different properties of the catalyst surface, may cause diverse types of O-O bond cleavage in H2O2 and, consequently, many types of oxidative species can be generated (OH ,•OH, O , O ).

Jorda et al.31 reported that when TiO2 nanoparticles interact with aqueous H2O2, hydroxyl radicals were generated and were active in the epoxidation of cyclohexene. Ziolek et al.32 used XRD, UV–vis, FTIR and ESR techniques to identify changes on the surface of Nb2O5 and Ta2O5 upon interaction with hydrogen peroxide and their activity in the oxidation of glycerol to glycolic acid. The authors observed that when amorphous Nb2O5 interacted with H O , peroxo radical species (O ) were formed and are active intermediate in the glycerol oxidation reaction. While Ta2O5 surface was treated with hydrogen peroxide, the radicals formed were poorly active in the oxidation of glycerol. Thus, different materials can present diverse types of radical species formed on surface.

Many theoretical and experiments studies have been conducted aiming to understand the adsorption of molecules on the Nb2O5 surface.22,

33-34

The understanding at a

molecular level is a challenge due to the extensive polymorphism that Nb2O5 presents. According to Foo et al.,22 the structural characteristics of niobia are directly related to the calcination temperature. In fact, studies about the different polymorphic phases of niobia are scarce, however due to the diversity of structures; one should expect that, depending on the Nb2O5 polymorph, some of the properties may be different. In our previous work, we performed an investigation using DFT calculations to better understand structural, electronic, and thermodynamic properties of the T and B phases of niobia.35 This study showed that the B phase is more stable at a low temperature than T phase, which is in accordance with what is observed experimentally. In this current work, the aim is to understand the interaction of the H O and H O molecules on the B5 ACS Paragon Plus Environment


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Nb2O5 surface through energetic, structural and electronic calculations. To be best of our knowledge, there is no information in the literature about the direction of preferential cleavage of polymorphic phases of niobia. Therefore, an investigation into the cleavage surface of the B-Nb2O5 is necessary in order to study the adsorption process on the preferential adsorption surface.

2.COMPUTATIONAL DETAILS. 2.1 Surface Models. The B-Nb2O5 phase is indexed at the space group C2/c, monoclinic crystal structure and it contains four unit formulas Nb2O5 per unit cell and a total of 28 atoms.36 The crystal structure is built up by blocks of distorted octahedral NbO6, arranged in strings composed of pairs of edge-sharing octahedral linked in a zigzag of corner-sharing octahedral. The unit cell parameters are: a = 5.560 Å, b = 12.740 Å and c = 4.883 Å (Figure 1a). In our previous study, the B-Nb2O5 bulk was optimized using firstprinciples calculations based on DFT and pseudopotential. Deviations of less than 2% for lattice parameters and bond lengths were found when compared with experimental data.35 The lattice parameters and internal coordinates optimized in the previous study were used as initial guess for the construction of all B-Nb2O5 surfaces. The proposed cleavage planes were those with low Miller index. The six different surfaces investigated were labeled: (100), (010)-1, (010)-2, (001), (110) and (111), Figure 1a. In order to determine the lowest energy cleavage plane a supercell model was created for all six slabs and are represented in Figure 1b. In the cleavage process two surfaces were generated and only plane (010)-1 presented asymmetrical surfaces. Convergence with respect to slab thickness was tested. A vacuum space of 15 Å was used to separate the slab from its periodic image along the surface normal direction. It was enough to avoid interactions between slabs. Similar vacuum has been used in other minerals studies.22, 25, 37 Slab thickness, i.e., number of atomic layers, was determined by the convergence test using the (100) surface. Four layers were enough to allow a variation less than 0.01 J m-2 and a thickness of 10 Å, see Figure S1. Thus, all studies were performed using a slab model (1x2x2), with a total of 112 atoms, stoichiometry Nb2O5 and with a thickness of four layers. The two upper atomic layers of the slab were


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allowed to fully relax, whereas the three bottom layers were kept frozen during ionic relaxation.

Figure 1: (a) Optimized structure of the B-Nb2O5 bulk, and the six different cleavage planes and (b) six different surfaces: (100), (010)-1, (010)-2, (001), (110) and (111), represented in blue, purple, green, brown, red and yellow, respectively. The green balls represent the niobium atoms and the red ones are the oxygen atoms.

2.2 Computational Method. All the calculations were carried out by using the Quantum Espresso package (pWscf),38 which is a first-principle software based on density-functional theory (DFT).39-40 The generalized gradient approximation (GGA) with parameterization due to Perdew, Burke e Ernzerhof (PBE)41 has been used for the exchange-correlation functional. The dispersion correction was included in the calculations by the D2 method developed by Grimme.42-43 The core electrons were described by ultrasoft pseudopotentials44 considering Nb 4s2 4p6 4d4 5s1 5p0 and O 2s2 2p4 as the valence configurations. The valence electrons were explicitly treated by plane waves energy cutoff 60 Ry (600 Ry used for the charge density). In the surface model, Monkhorst-Pack45 meshes of 4x4x1 and 8x8x2 k-points were used to sample the first Brillouin zone for the structural and PDOS calculations, respectively. The adsorption studies were performed at the Γ point to sample the Brillouin zone. The convergence criteria for electronic self-consistency and ionic relaxation cycles were 10-4 Ry and 10-3 Ry Borh-1, respectively. The other technical parameters were identical to those used in ref. 17. 7 ACS Paragon Plus Environment


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During the surface forming process simulation, two steps should be taken into account, (i) the first one is the breaking of the chemical bonds, and (ii) the second step is the relaxation or reconstruction of the surface. The cleavage energy (γnre) is calculated before the relaxation, in a single point calculation. In this study it was defined by equation (1): ESnre − E B 2A

γ nre =

(1)

where ESnre is the non-relaxed energy, EB is the energy of the fully optimized bulk structure and 2A is the area of the two surfaces formed during the cleavage process (top and bottom). The surface energy (γ) is defined as the formation energy of a surface from the bulk and can be calculated according to equation (2):46-47

γ =

E Sre − E B 2A

(2)

where, ESre is the energy of the relaxed surface. Finally, it is possible to calculate the surface relaxation energy (equation 3):

S

re

=

E

Sre

− E Snre 2A

(3)

In this work, the adsorption energy (Eads) quantity was achieved from equation 4:

E

ads

1 = −  E slab − nM − E slab − nE M  (4) n

where, Eslab-nM and Eslab are the total energies of the slab with and without the adsorbed substrate, respectively, and EM is the total energy of the substrate and n is the number of the substrate molecules adsorbed on the surface.

3. RESULTS AND DISCUSSION. 3.1 Stabilities of the Surfaces. The surface energies calculated for each slab as defined in equations 1, 2 and 3 are reported in Table 1, together with the coordination of each surface atom in the different cleavages. There are two possible terminations for the (010)-1 surface: the bridging 8 ACS Paragon Plus Environment

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oxygen terminated surface ((010)-1-Top) and the terminal oxygen surface ((010)-1Down). The relaxation of both surfaces were tested. The (010)-2 surface presented the lowest cleavage energy (1.37 J m-2) among the proposed planes. In addition, the (010)-2 surface presented lower surface and relaxation energy, estimated at 0.52 J m-2 and -0.43 J m-2, respectively. Surface energy ranges were from 0.52 to 1.42 J m-2 and increased along the series: (010)-2 < (110) < (100) < (010)-1 < (001). The γ of the (111) surface was not calculated due to its high cleavage energy (γnre). The relaxed surfaces are depicted in Figure S2. The literature reports that B-Nb2O5 presents a structure derived from the rutile structure (TiO2), but modified by the presence of a crystallographic shear plane.36 The (010)-2 surface of B-Nb2O5, which showed the lowest surface energy, presents the exact intersection of the shear plane compared to rutile crystal structure. Moreover, according to Ercit,36 there is equivalence between the (101) plane in the rutile48 and the (100) plane in the B-Nb2O5. In the (101) rutile plane, Ramamoorthy et al.49 estimated a value of 1.39 J m-2 in for the surface energy (using LDA/plane waves approximation) and Perron et al.48 (using the GGA/PW91 approximation) determined the surface energy equal to 1.08 J m-2. Our GGA/PBE approximation calculation presented surface energy equal to 1,15 J m-2 in the (100) plane of the B-Nb2O5. These results were in accordance with Perron et al. due to the similar arrangement and indicated good accuracy of our BNb2O5 modeling surfaces. Table 1. Surface energies and coordination of surface atoms of different B-Nb2O5 surfaces. Surface

γnre (J m-2)

γ (J m-2)

Bulk (100) 2.67 (010)-B 2.76 (010)-U (010)-2c 1.37 (001) 3.85 (110) 2.45 (111) 5.87 a (010)-1-B = bottom surface plane of the (010) surface

1.15

b

Sre (J m-2)

-0.76 -0.50a 1.28 -0.97b 0.52 -0.43 1.42 -1.22 1.03 -0.71 (010)-1-U = upper

Coordination Nb 6 5 6 5 5 5 3,6 4,5 surface c (010)-2

O 2 and 3 1 and 2 1 2 2 1 and 2 1 and 2 1 and 2 second cleavage



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3.2.1 Structure of B-Nb2O5 (010)-2 Surface. The surface geometry plays an important role in catalytic reactivity.50 In view of this, the structural and electronic properties and stability of (010)-2 cleavage surface (the most stable, see Table 1) were investigated. The supercell model created for (010)-2 surface has lattice parameters: a = 8.84 Å and b = 11.26 Å and α = β = γ = 90°. The (010)-2 surface has niobium and oxygen atoms exposed on it. Figure 2 shows the (010)2 optimized surface. In the B-Nb2O5 bulk structure the niobium cations have octahedra coordination (Nb6c) and the oxygen ions have coordination 2 and 3 (O2c and O3c, respectively). On the surface, the niobium cations are unsaturated with coordination number equal to five (Nb5c) and the oxygen ions have coordination number equal to two forming a bridge with the niobium atoms. There are two types of oxygen atoms: more exposed (Oup) and less exposed (Odown). The top view of this surface is also presented in Figure 2. After relaxation, some bonds reduced (down to 0.38 Å) and others increased (up to 0.25 Å), when compared to the B-Nb2O5 bulk structure. This surface exhibits a mesh type structure and the Nb-O-Nb angles in each square unit are between 177.3° and 130°. In a recent theoretical work, Xiao-Jing Zhuang et al.33 showed that the (001) surface of the niobia R phase also presented a mesh type structure. According to these authors, this mesh structure in which the Nb-O-Nb angles are quite large (158.4° and 155.2°) may result in better accommodation of the substrate on the surface, due to a strong tendency of the Nb-O bonds to break. Another indicator of slab stability is the Bader charge of the atoms.51-52 On stable surfaces, the terminating atoms present Bader charges similar to the charges found in the bulk. On the (010)-2 surface, the charge of the Nb atoms at the top is 2,69 e, which is very close to the bulk (2.72 e).35 The charges of the O atoms at the top are the same as the ones found in the bulk (1.07, 1.01 and 1.16 e).


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Figure 2: Optimized structure of B-Nb2O5 (010)-2 surface form: (a) side view and (b) top view. Different species are labeled. The green balls represent the niobium atoms and the red are the oxygen atoms. The electronic structure of the optimized (010)-2 surface is calculated in terms of the PDOS, Figure 3. The band gap (1.71 eV) near the Fermi level shows that the surface exhibits semiconductor property. One can note in Figure 3 that there are remaining occupied states upper the Fermi level, it is important to mention that this is an effect of the broadening adjust of the gaussian functions used in the plot. It means that the (010)2 surface keeps the semiconductor property of the B-Nb2O5 bulk (gap found is 2.55 eV), as was observed in our previous work.35 The Nb atoms are the one that most contribute to the states in the conduction band, indicating the possible Lewis acidic activity of these atoms on the surface of the material. Therefore, niobium atom is a Lewis acid site and it should be the preferred site for the adsorption of water, as reported in the literature.20 There is also a significant contribution in the valence band close to the Fermi Level of states from oxygen atoms, indicating their possible Lewis basic activity.



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Figure 3: PDOS over the atoms of (010)-2 surface. Inter = internal atoms and surf = atoms on the surface. 3.3 H2O adsorption on the (010)-2 B-Nb2O5 surface. Concerning the water adsorption, the behavior of each surface depends on the interaction strength and the preferred adsorption geometry. Different arrangements for the water adsorption on the relaxed (010)-2 surface were investigated. Although the PDOS analysis showed the niobium atom as a preferential adsorption center (Figure 3), the remaining adsorption sites (oxygen atoms – Oup and Odown) were also analyzed, using both molecular and dissociative adsorption modes. The adsorption configurations after optimization are shown in Figure 4. The adsorption energies (equation 4) are described in Table 2. The water prefers to adsorb molecularly on the niobium sites with adsorption energy of 1.10 eV. For the dissociated H2O, the OH fragment binds to the Nb atom on the surface and the H atom forms a hydroxyl group (OH) with the first and second neighbor oxygen atoms (Figure 4(d) to (g), respectively). The dissociative adsorption of the H atom on the Oup of the first neighbor is more favored than the second one (Table 2). The dissociative adsorption is about 0.17 eV less favorable than the molecular adsorption. Therefore, both dissociative and molecular mechanisms must be present on the surface, although the dissociative one to a lesser extent. This is in accordance with the results of Foo et al.22 The authors 12 ACS Paragon Plus Environment

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observed the in-situ generation of Brönsted acid sites (Nb-OH) when the calcined Nb2O5 at 500 °C was exposed to water vapor. In this process the number of lost Lewis acid sites exceeded the number of Brönsted acid sites formed. Therefore, the authors suggested that the adsorption of water vapor on Lewis acid sites does not always lead to the formation of a Brönsted acid site, indicating the molecular and dissociative adsorption mechanisms. The same behavior was reported for DFT calculations of water adsorption on the (111) surface of the CeO2 suggesting that dissociative and molecular water adsorption may occur in this system.53

Figure 4: Adsorption configurations of H2O molecule on the (010)-2 surface, molecular adsorption (a) to (c) and dissociative adsorption (d) to (g). Top view of the surfaces. The green balls represent the niobium atoms, the red are the oxygen atoms and white are the hydrogen atoms.



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Table 2. H2O adsorption energy (in eV) for the (010)-2 B-Nb2O5 surface. Type of Adsorption

Adsorption Site Nb (Fig.4(a))

Eads -1.10

Molecular (one H2O)

Oup (Fig.4(b))

-0.84

Odown (Fig.4(c))

-0.28

O (first neighbor, (Fig.4(d)))

-0.90

up

Dissociative (adsorption site for H+)

Molecular (eight H2O)

O

down

(first neighbor, (Fig.4(e)))

-0.31

up

O (second neighbor, (Fig.4(f)))

-0.85

Oup (first neighbor, (Fig.4(d)))

-0.59

Nb

-1.02

Figure 5 shows the most stable adsorption configuration of H2O on the (010)-2 surface. The water lone electron pairs are directed to the niobium atom, The Lewis acid niobium center readily receives the electron density from the water molecule. In addition, the molecular adsorption occurs through the formation of hydrogen bonding with the oxygen atom, Oup, of the surface. The bond length between the niobium and oxygen (Nb-O) is 2.27 Å which is about 0.03 Å greater than in bulk B-Nb2O5 (at the same level of calculation).35 One of the hydrogen atoms of the water molecule (OH1) is tilted downward and is pointing toward one oxygen atom, Oup, forming a hydrogen bond with a distance of 1.67 Å and an angle equal to 166° (O---H1-O). The Oup-Nb bond presents an increase of bond length from 2.08 to 2.15Å. The molecular adsorption of water on Nb causes a slight asymmetry in the O−H distances 0.97 Å (OH1) and 1.02 Å(OH2) (see Figure 5b).


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Figure 5: Water molecule adsorbed on the (010)-2 surface, (a) top view and (b) side view. The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms. Figure S3a and b shows the calculated PDOS for surface before and after H2O adsorption, respectively. Overall, the difference of the electronic properties of the surface after molecular adsorption is negligible. In Figure 6a only the PDOS of the atoms involved in the adsorption (Nb, Oup, H2O) is shown. The density of states of the p-orbitals of O presents a lower energy value than the density of states of the porbitals of Osurf of the surface in the valence band. This suggests that the empty d orbitals of niobium accommodate the non-bonding electron pair of the oxygen of the H2O. The topological analysis based on electron localization function (ELF) was applied. ELF values are between 0 and 1, where ELF value close to 1 suggests that the electrons are localized. In Figure 6b, it is possible to see the non-sharing of the cloud between the Nb O atoms, indicating an ionic character for the Nb O bond (range of 0.2), similar to that observed in the bulk in our previous work.35

Figure 6: (a) PDOS for Nb, Oup, H2O and (b) ELF map after H2O adsorption on the (010)-2 surface. The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms. Bader charge analysis of the surface before and after the molecular adsorption was performed (Figure 7). Bader charge analysis of the water adsorbed on the surface shows a slight increase in the charge of the Nb atom which the water molecule is adsorbed and also of the neighboring Nb atoms. Another change observed was the increase of the negative charge of the Oup which interact with the H atom. These results indicate that the Lewis acid and Brönsted base abilities of Nb and O atoms, respectively, are 15 ACS Paragon Plus Environment


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increased. Moreover, it can also be observed that the charge on the Nb atoms at the surface approached to the charge found in the B-Nb2O5 bulk.

Figure 7: Bader charges: a) pristine surface, b) H2O adsorbed (top view and side view). The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms. The slab model of the (010)-2 surface has 8 Nb sites on top. Eight water molecules were added to each niobium site, resulting in a monolayer of water. The adsorption structure was optimized using water molecule optimal geometry repeated on each Nb adsorption site on the surface as starting point. The optimized structure with the eight water molecules adsorbed is shown in Figure 8. After relaxation, seven water molecules remained adsorbed on the Nb atom, with bond distances Nb O ranging from 2.27 to 2.42 Å, almost 0.15 Å larger than the value with just one water molecule adsorbed. Just one H2O molecule was adsorbed by hydrogen bonding with two surface oxygen atoms, Oup, wherein the interaction distances were 1.77 and 2.27 Å. The energy calculation per water molecule shows that the most stable configuration of the full covered surface is about 0.09 eV less stable than the adsorption of one water molecule (Table 2). This slight increase in adsorption energy is probably due to change of the Lewis acid strength of the niobium adsorption sites because of the water adsorption on the surroundings as is shown in Figure 8a.


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Figure 8. Top and side view of the full covered (010)-2 surface with eight water molecules disposed in a symmetric way. The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms. 3.4 H2O2 adsorption on the (010)-2 B-Nb2O5 surface. Several studies reported the formation of peroxo (O ), superoxide (O ), hydrosuperoxo (HO ) and hydroxyl (HO ) species on the Nb2O5 surface using UV-vis, FTIR, Raman

and EPR techniques.32, 54-56 Depending on the exposed surface, different oxidant species can be formed when upon contact with H2O2. We studied the adsorption of the species observed experimentally on the (010)-2 surface. For the O species, we investigated two possibilities: the bridge formed on the same Nb atom and on two neighboring Nb atoms. The dissociated hydrogen species was adsorbed in different positions considering the distinct types of oxygen atoms, Oup and Odown. All possible adsorption sites and orientations were also considered. Figure 9 shows the most stable adsorption configurations for each species. The adsorption energies (Eads) were calculated considering equation 4 and are reported in Table 3. The most stable species was HO , with an adsorption energy of -1.63 eV. Ziolek et al.56 also observed the formation of the same species on the amorphous Nb2O5. According to the authors, this species formation occurs when H O interacts with the Brönsted acid site on the surface (H O NbV OH HO NbV H O). Figure 9a shows the formation of HO species on the (010)-2 surface.

The Nb O bond length is 2.02 Å, close to the Nb-O distance found in the B-Nb2O5 bulk.35 The bond length O-O is predicted to be 1.45 Å, which is slightly shorter than the calculated value for the free H2O2 (1.48 Å). The dissociated hydrogen atom was adsorbed on the surface (bond length H-Odown equal 0.99 Å) and, after optimization, it interacted with the most internal oxygen, at a distance H-Obulk of 1.60 Å (see Figure 17 ACS Paragon Plus Environment


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S4). The hydrogen atom of the HO species also interacted with oxygen of the surface forming a hydrogen bond (distance H-Oup equal 1.56 Å).

Figure 9: Top view of the optimized structures of H2O2 adsorption on the (010)-2 surface. The dissociative adsorption (a, b, d and e), and the molecular adsorption (c). The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms. The adsorption energies of the H O , HO species did not show large and O

differences (up to 0.6 eV, Table 3). Therefore, these species may exist in equilibrium on the (010)-2 surface. Figure 9c shows that H2O2 adsorbs molecularly on top of two Nb atoms with a symmetry in the Nb-O distances equal to 2.46 Å. The molecular plane is parallel to the surface and the O−H bonds are tilted downward, pointing toward the surface oxygen atoms (Oup), forming two hydrogen bonds. The O-O bond length found on the adsorbed H2O2 is equal to the free H2O2 (1.48 Å). Although some works have reported that the formation of the O22- species on the surface of the niobia occurs in a single niobium atom of the surface,25, 57 this study showed that the formation of the OO bridge between two niobium atoms (Figure 9b) is about 0.49 eV more favorable than the O-O bridge on only one niobium atom (Figure 9e), see Table 3. The O-O bond length of the bridge formed between two niobium atoms was slightly shorter than the O18 ACS Paragon Plus Environment

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O bond length of the bridge formed on one niobium atom, 1.41 and 1.46 Å, respectively. Table 3. H2O2 adsorption energy (in eV) for the (010)-2 B-Nb2O5 surface. Type of Adsorption

Adsorption Site

∆Eads

Molecular (one H2O2) Dissociative HO (adsorption site for H+)

Nb (Fig. 9c)

-1.49

Odown (Fig. 9a)

-1.63

Dissociative O22(adsorption site for 2H+)

Oup and Oup (Fig. 9b)

-1.52

Odown and Odown (Fig. 9e)

-1.03

Dissociative 2x(-OH)

Nb (Fig. 9d)

-1.23

Figure S3c shows the PDOS of surface before and after the HO species and H adsorption. After the adsorption of the HO and H species it was not observed new

states when compared with the PDOS surface. The PDOS of the (010)-2 surface interacting with the HO and H species was calculated, see Figure 10a. The PDOS

shows, at low energy, a stronger overlap between the Nb1-d and O1-p orbitals and Nb2d and O2-p orbitals (far from Fermi level, -6 to -4 eV). This confirms that the d orbitals of the Nb atoms and p orbitals of the O atoms play an important role in the Nb-O interaction. The region from -2 eV to Fermi level the Nb1-d and O1-p state presents the most localized states than the Nb2-d and O2-p states. This fact is in agreement with the shorter bond length of Nb1-O1 (2.02 Å) compared to Nb2-O2 (2.30 Å), as showed in Figure 10b. Even though this analysis confirms some observed results it still needs a future investigation to confirm it. The interaction between Oup-H1 and Odown-H2 also can be confirmed through of PDOS analysis. The p orbital of Odown significantly interacts with the s orbitals of H2 as shown by the overlap at approximately -4.0 and 3.1 eV, while the p orbital of Oup interacts with the s orbital of H1 as shown by the superposition at approximately -3.8 eV.



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+ Figure 10: (a) The PDOS of interactions between the HO and H species with the

(010)-2 surface (Nb1, Nb2, Odown, Oup , O1, O2, H1 and H2 atoms) and (b) label of atoms. The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms. The calculations of the Bader charges of the H2O2 dissociative adsorption were performed and their values were compared with the Bader charges of the surface, as showed Figure 11. The charges of the Nb atoms of the surface that interact with HO and H+ species presented a slight increase (are positively charged). Similar to what happened with the water adsorption, the dissociative adsorption (HO and H ) increases

the Lewis acidity of Nb atoms on the surface.


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Figure 11: Bader charges: a) pristine surface, b) adsorption of the HO and H

species (top view and side view). The green balls represent the niobium atoms, the red are the oxygen atoms and the white are the hydrogen atoms.

4. CONCLUSIONS Based on the B-Nb2O5 phase, the cleavage surfaces, structural, energetic and electronic properties as well as the adsorption of H2O and H2O2, on the (010)-2 B-Nb2O5 surface were studied by DFT calculations. The primary conclusions are summarized as follows: (1) In the study of the cleavage surfaces, the surface energies were calculated for each slab. Among the proposed cleavage planes, the (010)-2 surface is more likely to occur (1.37 J m-2). The (010)-2 surface has unsaturated niobium cations with coordination number equal to five (Nb5c) and oxygen ions with coordination number equal to two. The oxygen ions form a bridge with the niobium atoms. In addition, there are two types of oxygen atoms: more exposed (Oup) and less exposed (Odown). The Bader analysis shows that the charges found on the surface are similar to those found for the bulk, indicating the large stability of the (010)-2 surface. The electronic structure of the (010)-2 surface calculated in terms of the PDOS, shows that the niobium atom is a Lewis acid site. (2) The adsorption of the H2O on the (010)-2 B-Nb2O5 surface was investigated. This study reveals that the most stable adsorption site is the niobium atom. The water prefers to adsorb molecularly on the niobium sites with adsorption energy of 1.10 eV. However, the dissociative adsorption is about 0.17 eV less favorable than the molecular adsorption. Therefore, both dissociative and molecular 21 ACS Paragon Plus Environment


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mechanisms must be present on the surface, although the dissociative one to a lesser extent. The PDOS analysis of the surface does not present significant differences of the electronic states close to the Fermi level when compared with the (010)-2 surface. The topological analysis based on ELF was performed and suggests an ionic character for the Nb O bond. (3) The adsorption of the H O was also investigated. The dissociative (peroxo (O ), superoxide (O ), hydrosuperoxo (HO ) and hydroxyl (HO ) species) and

the molecular adsorption were studied. The HO2- species is the most stable adsorbed species with adsorption energy of -1.63 eV. The species O led to the formation of the species O on the (010)-2 surface. The energy adsorption decreased along the series: HO > O ≈ H O > OH. Small differences between the adsorption energies of the HO , O and H O species were found

(up to 0.14 eV). This suggests that these species may exist in equilibrium on the (010)-2 surface. These results were consistent with the experimental observation. The PDOS analysis showed an overlap density state between the d and p orbitals of the Nb1 and O1, respectively, indicating a stronger interaction between the Nb atom and the O atoms (HO species).

ASSOCIATED CONTENT Supporting Information available: Energy convergence to the number of layers of the slab; optimized B-Nb2O5 surfaces and PDOS over the atoms of (010)-2 surface before and after adsorption; structural details of the HO species adsorbed on the (010)-2 BNb2O5 surface. AUTHOR INFORMATION Corresponding Author * Heitor A. De Abreu: [email protected] Phone: +55(31)3409-5748 Notes 22 ACS Paragon Plus Environment

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The authors declare no competing financial interest. ACKNOWLEDGMENTS We would like to thank the support of the Brazilian agencies Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional para o Desenvolvimento

Científico

e

Tecnológico

(CNPq)

and

Coordenação

de

Aperfeiçoamento de Pessoal de Ensino Superior (CAPES). The National Institute of Science and Technology for Mineral Resources, Water and Biodiversity– INCTACQUA (http://www.acqua-inct.org) and PRONEX/CNPq/FAPEMIG (CEX – APQ03155-15 and CEX - APQ-01626-14) have also supported this work.

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TABLE OF CONTENTS


Unveiling the Structural and Electronic Properties of the B-Nb2O5

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