Oxygen Deficiency and Reactivity of Spinel NiCo2O4 (001) Surfaces

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Oxygen Deficiency and Reactivity of Spinel NiCo2O4 (001) Surfaces Xiao Shi,† Steven L. Bernasek,†,‡ and Annabella Selloni*,† †

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States Science Division, Yale-NUS College, Singapore 138609, Singapore

‡

S Supporting Information *

ABSTRACT: We carried out density functional theory (DFT) calculations with on-site Hubbard U corrections to investigate the structure, defects, and reactivity of (001) surfaces of spinel NiCo2O4 (NCO), a promising catalyst for CO and methane oxidation. By examining surfaces with different Co/Ni compositions, we find that the formation of surface oxygen vacancies (VOs) on NCO(001) is strongly affected by the neighboring cation in the third layer, the computed formation energy being largest (∼1.2 eV) for O vacancies coordinated to third layer Co and smallest (∼0.5 eV) for VOs coordinated to a Ni neighboring another Ni ion. As a result, VO formation is generally much easier on NCO (001) than on Co3O4 (001) surfaces, suggesting that NCO may be a better catalyst than Co3O4 for oxidation reactions based on the Mars−Van Krevelen mechanism. Surface oxygen vacancies on reduced NCO surfaces can be healed through dissociative water adsorption at room temperature. In contrast, adsorption of molecular oxygen at VOs is energetically unfavorable under ambient conditions, suggesting that O2 adsorption may represent the thermodynamic limiting step for oxidation reactions on NCO(001) surfaces.

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INTRODUCTION Spinel cobalt oxide (Co3O4) has recently attracted attention as a highly active catalyst for various oxidation reactions.1−3 Interest in this material has also generated efforts aimed at tuning its catalytic activity through doping or substitution with selected transition metals. Among such substituted cobaltites, NiCo2O4 (NCO) has emerged as a particularly promising catalyst for low-temperature methane and CO oxidation4,5 as well as the oxygen evolution reaction.6,7 For instance, recent experiments have shown that NCO can completely oxidize methane at 350−550 °C, suggesting that in some cases NCO’s activity could be higher than that of precious-metal-based catalysts.8 NCO is a material with complex structural and electronic properties. It is generally considered to have an inverse spinel structure with mixed valence, where tetrahedral (Td) sites are occupied by Co2+ and Co3+ ions and octahedral (Oh) sites are occupied by Ni2+, Ni3+, and Co3+ ions.9−13 However, Ni(Oh) ↔ Co(Td) exchanges can take place rather easily,9 resulting in considerable cation disorder. NCO is also generally described as ferrimagnetic and metallic, with much higher conductivity compared to other cobaltites.12,14,15 It was indeed suggested that NCO is a more efficient water oxidation catalyst compared to pure Co3O4 due to its higher conductivity.16 Despite growing interest in the use of NCO in catalysis, understanding of its fundamental surface properties is still limited. Thus far, only a few experimental and theoretical studies on well-defined NCO surfaces have been reported.5,17−19 The aim of this work is to obtain insight into © 2017 American Chemical Society

NCO’s surface structure and reactivity through density functional theory (DFT) calculations on the (001) surface, which is one of the most common surfaces of spinel materials.20 Using DFT with the addition of on-site Coulomb repulsion U terms on Co and Ni 3d shells (DFT+U),21 we investigate surfaces with various Co/Ni ratios focusing on the formation of surface oxygen vacancies (VOs), which have been proposed to play a key role in the oxidation of CO and methane on NCO.4 We also investigate the adsorption of two typical probe molecules, water and O2, which are important for characterizing the surface structure under ambient conditions and the surface reoxidation process during catalytic reactions, respectively. Our results clearly show that Ni has a major influence on the formation of surface oxygen vacancies, leading to VO formation energies significantly lower than those found for Co3O4. On the other hand, O2 adsorption is more difficult and is likely to represent the thermodynamic limiting step of oxidation reactions on NCO(001).

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METHODS AND MODELS Spin-polarized DFT+U calculations were performed using the plane-wave-pseudopotential scheme as implemented in the Quantum Espresso package.22 Exchange and correlation terms were described using the Perdew−Burke−Ernzerhof (PBE)23 Received: November 30, 2016 Revised: January 31, 2017 Published: February 1, 2017 3929

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Figure 1. Side views of (a) NCO(001) and (b) NCO(100) slabs; blue, gray, and small red spheres represent Co, Ni, and oxygen atoms, respectively. Top views of (c) (001)Ni, (d) (001)Co, and (e) (100)mix surfaces; only the atoms of the first and second layers are represented by spheres. O1 and O2 are defined in the text.

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RESULTS AND DISCUSSION Pristine (001)/(100) Surfaces. Experimental control of the NCO stoichiometry is quite challenging, and in practice, NCO samples are always nonstoichiometric.14,17 Nonetheless, the perfectly stoichiometric NCO surface remains a convenient reference system for the study of surface properties and lattice defects. The structures of the defect-free NCO (001) and (100) slabs considered in our study are shown in Figure 1. We found an energy difference of less than 0.01 eV between the two slabs, which is consistent with the fact that they have the same overall stoichiometries (Ni12Co22O48). All investigated surfaces are terminated by oxygen anions and metal cations that are at Oh sites in the bulk: either Ni or Co(Oh) are present on the (001) surface (indicated as (001)Ni and (001)Co, Figure 1a, 1c, and 1d), while both Ni and Co(Oh) are present on the (100) surface (denoted as (100)mix, Figure 1b and 1e). In addition, two different types of surface oxygen anions exist: O1, bonded to two first layer metal cations and a third layer cation at an Oh site, and O2, bonded to two first layer metals and a second layer Co(Td). On the (100)mix surface, O1 atoms can be further distinguished in O1a and O1b, bonded to third layer Co and Ni cations, respectively. Average displacements of surface and second layer atoms from their ideal bulk positions are reported in Table 1, while inplane and out-of-plane Co−O and Ni−O distances are compared to computed and experimental10,11,17 bulk anion− cation bond lengths in Table S1 of the Supporting Information. Surface oxygen atoms tend to have large in-plane and smaller out-of-plane displacements, especially on the (001)Co surface, and O1 shows somewhat larger displacements compared to O2. The metallic cations (Ni, Co(Oh), and Co(Td)) exhibit smaller in-plane and larger out-of-plane displacements compared to oxygen ions. Co(Td) in the second layer relaxes outward,

functional with an on-site Coulomb repulsion U term on Co and Ni 3 d electrons. We used the values U(Co) = 3.0 eV and U(Ni) = 5.5 eV, which we recently found to provide a better description of NCO’s thermodynamic properties compared to the U values determined from linear response theory.24,25 Ultrasoft pseudopotentials26 were employed, and the valence electrons include the following: O 2 s, 2 p; Co 3 d, 4 s; Ni 3 d, 4 s states. Kinetic energy cutoffs used were 50 and 500 Ryd for wave function and augmented density, respectively. Structural optimizations were carried out by relaxing all atoms until forces were smaller than 1.0 × 10−3 au. Of the two possible symmetries, α-type and β-type, of inverse spinels, the latter is slightly more stable20 and was thus chosen for studying the surface properties. In this structure, the (100) and (001) surfaces are inequivalent, the latter exposing either Co or Ni only (Figure 1a) and the former exposing both Ni and Co cations (Figure 1b). These surfaces were modeled using slabs of 11 layers terminated by oxygen and octahedral Co and Ni ions, and a vacuum region of 20 Å was used to separate adjacent slabs. We kept the same chemical composition for the (100) and (001) slabs, resulting in two different terminations for the latter; nonetheless, the net polarization is zero in all cases because the system is metallic (see below). We used theoretical lattice parameters20 and considered a square surface unit cell of dimensions 8.209 × 8.209 Å2, exposing 4 octahedral cations and 8 oxygens in the outer layer and 2 Co(Td) in the second layer just below. We sampled the surface Brillouin zone using a 2 × 2 × 1 k-point grid. Oxygen vacancies and adsorbed molecule were introduced only on one of the surfaces of the slab. Dipole corrections were found to have only minor effects (about 0.01 eV) and were not included in our standard setup. 3930

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indicates that the NCO surface tends to be Ni rich, consistent with experiments showing NiO formation on the surface during thermal decomposition of NCO.9,29 In Figure 2 we compare the densities of states (DOS) of the (001) (Figure 2a) and (100) (Figure 2b) slabs to the DOS of bulk NCO (Figure 2c). Differences between the bulk and the slab DOS are larger for the minority spin states than for the majority spin ones. As shown by the layer-resolved DOS (Figure S1 of Supporting Information), these differences originate mainly from the atoms in the surface layers. For the majority spin states, a band gap of 1.4 eV, comparable to the bulk band gap, is still present on the (100) slab, whereas the majority spin states of the (001) slab are conducting due to surface states around the valence bands maximum. These surface states as well as those near the conduction band are mainly contributed by surface Co(Oh) ions on the (001)Co surface. On both (100)mix and (001)Co surfaces, Co(Oh) ions, while remaining 3+, become spin polarized, with their spin parallel to Ni cations and antiparallel to Co(Td). At the same time the second layer Co(Td) ions change their bulk fractional valence state to a 3+ high-spin state. Surface Oxygen Vacancies. The formation of surface oxygen vacancies (VOs) has a key role in the oxidation activity of metal oxide materials, often based on the Mars−van Krevelen (MvK) mechanism.4,30 We considered 1 and 2 VOs per surface unit cell, corresponding to a coverage of 1/8 and 1/ 4 monolayer (ML). respectively. The formation energies are calculated as

Table 1. Average Displacements of Surface Ni and Co(Oh) and Second Layer Co(Td) from Their Ideal Bulk Positionsa surface type (001)Ni

(001)Co

(100)mix

atom type

in-plane displacement (Å)

out-of-plane displacement (Å)

Ni Co(Td) O1 O2 Co(Oh) Co(Td) O1 O2 Ni Co(Oh) Co(Td) O1 O2

0.029 0.051 0.133 0.091 0.026 0.039 0.170 0.122 0.010 0.035 0.018 0.137 0.090

−0.087 0.146 −0.118 −0.046 −0.084 0.093 0.000 −0.049 −0.080 −0.118 0.106 −0.073 −0.084

a

Positive (negative) out-of-plane displacements correspond to outward (inward) displacements.

whereas surface atoms relax inward, i.e., toward the bulk. As a result, the distance between the first and the second layer is reduced by ∼0.2 Å. To estimate the relative stabilities of the (001)Ni, (001)Co, and (100)mix surfaces, we further considered symmetric (nonstoichiometric) slabs exposing those surfaces. We expressed their surface formation energies γ in terms of the chemical potentials of bulk NCO, NiO, CoO, and O227 and determined the formation energy differences Δγ by extrapolating the formation energies of slabs with different numbers of layers.28 We found that the (001)Ni surface is most stable, and the (100)mix and (001)Co surfaces have formation energies 0.229 and 0.515 J/m2 larger than (001)Ni, respectively. This

⎛ ⎞ n Eform = ⎜Edef + μO − E0⎟ /n 2 ⎝ ⎠ 2

where Edef and E0 are the total energies for the defected and pristine surfaces, respectively, n is the number of surface oxygen

Figure 2. Computed projected DOS for (a) NCO (001) slab, (b) NCO(100) slab, and (c) bulk NCO. The majority spin states of the (001) slab is conducting due to the surface states on the (001)Co surface. 3931

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The exchange of surface Co(Oh) and Ni ions has a computed energy cost of only 0.03 eV and is thus expected to occur quite frequently on NCO(100)mix. On the resulting (100)exch surface (see Figure 3d), four different types of O1 oxygen sites are present, characterized by 0/1/2/3 Ni neighbors and denoted as O1a/b/c/d, respectively. Our calculations show that the VO formation energy at O1b and O 1d is similar to that at O1b on the regular (100)mix surface, whereas O1a and O1c have formation energies similar to O1a on the (100)mix surface (Table 2). This indicates that third layer ions have a larger influence on the formation of surface oxygen vacancies than surface cations. In particular, the formation energy appears to be lower when the surface O1 is bonded to a third layer Ni ion. To better understand the role of third layer cations on Ovacancy formation energies, we performed calculations on (100) surfaces obtained by interchanging Co and Ni ions in the surface and third layer. The resulting surfaces with 25% and 75% Ni cations are shown in Figure S2 of SI. Consistent with our results for surface formation energies in the previous section, the 75% Ni surface is found to be 0.2−0.3 eV more stable in comparison to the (100)mix surface, while the 25% Ni surface is 0.2 eV less stable. This confirms that it is thermodynamically favorable for Ni to segregate at the surface, in agreement with the experimental observation that during high-temperature decomposition, Ni moves to the surface and forms NiO on top of spinel NCO.9,29 The average VO formation energies and standard deviations on the 75% and 25% Ni surfaces are reported in Table S2 of the Supporting Information. As shown in Table S2, standard deviations are very small when formation energies are grouped according to the third layer cations neighboring the vacancy, thus confirming that third layer cations have a major impact on VO formation. This result can be rationalized by considering that upon VO creation the less coordinated cations that are formed on the surface can undergo significant displacements to partially compensate for the reduced coordination, whereas displacements are more difficult for the third layer cations. In particular, our computed average VO formation energies are 1.26, 0.88, and 0.50 eV for O vacancies coordinated to third layer Co, Ni, and Ni neighboring another Ni ion, respectively. It is clearly more favorable to create a VO coordinated to a third layer Ni compared to a vacancy coordinated to a Co(Oh)3+, because the excess electrons associated with the vacancy can form a stable Ni2+ species from the original Ni fractional valence between +2 and +3. We further investigated the possibility of stronger surface reduction and examined models containing two O1 vacancies per unit cell. As sites for creating the second vacancy, we choose O1 sites not bonding to 4-coordinated Co(Oh) and Ni cations generated by the first vacancy in order to avoid the formation of 3-coordinated Co(Oh) and Ni ions. From Table 2, we can see that the first and second VO formation energies are similar on the (100)mix and (001)Ni surfaces, whereas Eform is much higher (1.19 eV) for the second vacancy than for the first one (0.4 eV) on the (001)Co surface. Finally, we combined the results in Table 2 with the temperature and pressure dependence of the oxygen chemical potential to determine the surface stability diagrams shown in Figure 4. In this figure, the range of variation of Δμ(O2) (the oxygen chemical potential referred to an isolated O2 molecule at T = 0 K) has been extended beyond the lower limit given by the conditions of thermodynamic equilibrium of bulk NCO

Table 2. Average Surface Oxygen Vacancy (VO) Formation Energies (in eV) for Different Surfaces and Different VO Concentrationsa surface type (001)Ni

(001)Co

(100)mix

(100)exch

vacancy site

formation energy (0 K)

formation energy (300 K, 0.2 atm)

O1 O2 O1−O1 O1 O2 O1−O1 O1a O1b O2 O1a−O1a O1a−O1b O1b−O1b O1a O1b O1c O 1d

1.38 1.73 1.41 0.40 1.91 0.79 1.18 0.82 1.70 1.31 1.13 0.88 1.11 0.87 1.25 0.81

1.10 1.44 1.12 0.11 1.62 0.51 0.90 0.54 1.42 1.02 0.85 0.60 0.82 0.59 0.96 0.53

a Both values at T= 0 K and under ambient conditions (T = 300 K and p(O2) = 0.2 atm, corresponding to the oxygen partial pressure in air) are reported. For the (100)mix and (100)exch surfaces, inequivalent O1 sites are denoted as “a/b/c/d” in the case of a single vacancy and “aa/ ab/bb” in the case of two vacancies per surface cell (see Figures 1e and 3d).

From Table 2, we can see that VOs at O1 sites (Eform ≈ 1.1 eV on average) are about 0.6 eV more favorable than at O2 sites (Eform ≈ 1.7 eV) in the case of one vacancy per unit cell (1/8 ML). This can be explained by the fact that a VO at O2 would result in a 3-cordinated Co(Td), which is quite unfavorable. Formation of an O1 vacancy has a particularly low energy cost on the (001)Co surface (0.40 eV), followed by O1b vacancies on the (100)mix surface (0.82 eV). For comparison, the computed VO formation energy is much larger for bulk NCO (∼2.3 eV)20 and for the Co3O4(100) surface (1.56 eV), whereas a somewhat smaller VO formation energy, 0.34 eV, was obtained for the (100) surface of NiFe2O4, a spinel catalyst with some similarity to NCO.24 From the structural point of view, O1 vacancies on the NCO(001)Co surface are characterized by a strong local relaxation, such that the closest O1 oxygen moves to the middle between two metal cations, resulting in the formation of a so-called “split vacancy”. The same effect is observed also for O1a vacancies on the (100)mix surface (Figure 3a). Figure 3b and 3c show the electronic structure changes induced by the formation of an oxygen vacancy on the (100)mix surface. Both O1 and, to a smaller extent, O2 vacancies generate new empty states in the majority spin band gap, which are primarily contributed by Co(Oh) and both Ni and Co(Oh) ions, respectively, with a significant contribution by oxygen. The spin states of Co(Oh) and Ni on the defected surface remain the same as on the pristine surface except for the case of a split vacancy, where Co(Oh) acquires a high-spin state. 3932

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Figure 3. (a) Top view of the O1a split vacancy on the (100)mix surface. (b) Projected DOS for the defected (100)mix surface with a VO at O1 and (c) O2. (d) Top view of the (100)exch surface, obtained after exchanging the Co(Oh) and Ni metal sites on (100)mix; inequivalent O1 sites are indicated.

oxidized from 3+ to 4+ and its magnetization is reduced. Water adsorption on Ni and Co sites is not favorable (Eads > 0) under ambient conditions, indicating that the pristine surface is very little affected by water. The introduction of van der Waals interactions (not included in our calculations) is not expected to change this conclusion, even though it may slightly strengthen the adsorption at T = 0 K. On oxygen-deficient surfaces, water adsorption is easier at VO sites, where it is thermodynamically favorable (Eads < 0) also at room temperature. For water at a VO, the hydrogen atoms tend to form H bonds of length 1.46−1.67 Å with neighboring lattice oxygens (Figure 5b). Proton transfer to one of these oxygens is facile, making dissociative adsorption at VOs energetically more stable by about 0.40 eV relative to molecular adsorption at the same site. Water adsorption is obviously less favorable at the reconstructed split-vacancy sites. For instance, even though water tends to remove the VO reconstruction and dissociate on the (001)Co surface, its adsorption energy is very small (−0.05 eV) under ambient conditions. By combining the results for VO formation (Table 2) and water adsorption (Table 3), we can also estimate the energetics of hydroxyl formation on NCO(100)/(001). For example, the formation energy of an O1b vacancy on the (100)mix surface is +0.54 (+0.82) eV under ambient conditions (at T = 0 K), while the dissociative water adsorption energy at VO1b is −0.49 (−1.15) eV under the same conditions. This indicates that the surface is likely to be partially hydroxylated at low T and becomes pristine under ambient conditions (after desorption of the hydroxyl hydrogens to form H2). A slightly different picture was obtained for the NiFe2O4(100) surface,24 for which the lower energy cost of VO formation makes hydroxylation highly favorable also under ambient conditions.

with O2 gas, Co3O4, and NiO (see Table S3 of the Supporting Information), because NCO nanoparticles are actually observed to be stable up to rather high temperatures.8 Note also that these diagrams account for the relative energies of the structures with 1/8 and 1/4 ML VOs, without considering the possibility that a lower energy 1/8 ML concentration could be created by phase separating into surface regions with no VOs and regions with 1/4 ML VOs. The behaviors of the three investigated surfaces are clearly quite different: VO formation is unlikely on the Ni-terminated (001)Ni surface, even under typical oxidation conditions (Figure 4a), whereas VOs (1/8 ML) can form easily, slightly above room temperature at ambient O2 pressure, on the (001)Co surface (Figure 4b). On the (100)mix surface, formation of 1/8 and 1/4 ML VOs becomes favorable around 700 and 800 K, respectively (Figure 4c), suggesting that this surface should be quite active in hightemperature oxidation reactions. Water Adsorption. Water adsorption free energies, Eads, on pristine and reduced NCO(001)/(100) containing one oxygen vacancy per unit cell (1/8 ML) are reported in Table 3. Values at T = 0 K and under ambient conditions (T = 300 K and water pressure pH2O = 0.02 atm) were determined using Eads = EH2O* − Esurf − μH2O + ΔZPE, where EH2O* and Esurf are the computed total energies of the surface with adsorbed water and without water, respectively, μH2O is the water chemical potential, and ΔZPE is the zero point energy difference between adsorbed water and an isolated water molecule. As shown in Table 3, the values of Eads are similar for the Ni and Co sites of pristine surfaces. However, water adsorbs in molecular form on Ni sites, whereas the adsorption is dissociative on Co (Figure 5a). Moreover, Ni’s electronic structure is barely influenced by the adsorption, whereas Co is 3933

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Figure 4. Phase diagram for VO formation on (a) (001)Ni, (b) (001)Co, and (c) (100)mix surfaces as a function of the oxygen chemical potential (referred to an isolated O2 molecule at T = 0 K). In all cases, the black horizontal line represents the pristine surface. Shaded regions indicate ambient conditions (p(O2) = 0.2 atm, T = 300−350 K; blue) and typical conditions for CO and methane oxidation (0.2 atm at 600−800 K; yellow).

Oxygen Adsorption. O2 adsorption is the first step of surface reoxidation in catalytic processes based on the MvK mechanism. Computed O2 adsorption free energies on reduced NCO(100)/(001) surfaces are reported in Table 4. These were 1 determined using Eads = EO2 * − Esurf − 2 μO , where EO2* and 2 Esurf are the total energies of the surface with and without adsorbed O2 and μO2 is the chemical potential of O2. From Table 4, it appears that O2 can adsorb at a VO but not at a surface metal site at low T, whereas adsorption is always unfavorable under ambient conditions. This suggests that surface reoxidation may be the thermodynamic limiting step for MvK oxidation reactions on NCO(100)/(001). For O2 adsorbed at a VO (Figure 5c), one oxygen atom of the molecule binds to a top layer metal atom while the other binds to both a metal atom at the surface and a metal of the third layer. The O−O bond length is 1.36 Å, suggesting the formation of a superoxide, O2−. We also examined whether the adsorbed molecule could dissociate (Figure S4i) but found the dissociated configuration to be less stable than the molecular one by 0.27 eV. Unlike water, O2 does not adsorb on split vacancies, notably on the (001)Co surface. The adsorption of an O2 molecule on a surface with higher oxygen deficiency (2 VOs per surface unit cell) is slightly more favorable than on a surface with only 1 VO per cell. However,

Table 3. Computed Water Adsorption Free Energies at Co and Ni Sites on Pristine (100)/(001) and at VO Sites on Reduced Surfacesa surface type

adsorption site

Eads (0 K) (eV)

Eads (air) (eV)

(001)Ni

Ni VO (M) VO (D) Co VO (M) VO (D) Ni Co VO1b (M) VO1b (D)

−0.42 −0.94 −1.57 −0.43 −0.63 −0.71 −0.44 −0.33 −0.79 −1.15

0.24 −0.28 −0.91 0.22 0.03 −0.05 0.22 0.33 −0.13 −0.49

(001)Co

(100)mix

figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure

S3a S3b S3c S3d S3e S3f S3g 5a 5b S3h

a

Both results at T = 0 K and under ambient conditions (T = 300 K and pH2O = 0.02 atm, corresponding to the water partial pressure in air) are reported. All values refer to 0.25 monolayer coverage (i.e., one adsorbed molecule per surface unit cell), and positive values indicate that adsorption is thermodynamically unfavorable. For water adsorbed at a VO, M and D indicate molecular and dissociative adsorption respectively; VO1b denotes a vacancy at an O1b site. Figures showing the various structures are listed in the last column.

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Figure 5. Water and O2 adsorption structures on the (100)mix surface (top views): (a) water dissociatively adsorbed at a Co site; (b) molecular water at a VO site; (c) O2 at a VO site; (d) O2 adsorbed on a surface with two VOs per unit cell. Oxygen atoms of adsorbed molecules are shown in orange; oxygen vacancy sites are indicated by a cyan dotted line.

1.37 Å. Breaking of the O−O bond to recover the pristine surface is highly favored thermodynamically and has a small activation barrier of only 0.25 eV, with an O−O distance of 1.39 Å at the transition state (Figure S5). This suggests that the mechanism of surface reoxidation by O2 would involve the diffusion of oxygen vacancies to form a close pair as ratelimiting step.

Table 4. Computed O2 Adsorption Free Energies at Co and Ni Sites on Pristine (100)/(001) and (non-split) VO Sites on Reduced (100)/(001) Surfacesa surface type (001)Ni

(001)Co (100)mix

adsorption site

Eads (0 K) (eV)

Eads (air) (eV)

Ni VO (M) VO (M) + VO Co VO (M) + VO Ni Co VO1b (M) VO1a (M) + VO1b VO1b (M) + VO1b

0.17 −0.40 −0.62 −0.01 −0.43 −0.03 0.05 −0.46 −0.55 −0.47

0.82 0.25 0.03 0.63 0.22 0.62 0.70 0.19 0.10 0.17

figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure

S4a S4b S4c S4d S4e S4f S4g 5c 5d S4h

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CONCLUSIONS In this work we studied the structure and chemistry of NCO(100)/(001) surfaces with different Co/Ni terminations using DFT+U calculations. Our results show that there is a thermodynamic driving force for Ni to segregate to the surface, which is consistent with the experimental observation of NiO formation on the surface during thermal decomposition.29 On the other hand, oxygen vacancy formation is considerably more difficult on the purely Ni-terminated NCO(001)Ni surface than on the Co-terminated (001)Co and mixed Ni- and Coterminated (100)mix surfaces (Figure 4). The latter are thus expected to represent the active surfaces in oxidation reactions. On these surfaces, VO formation is easiest at O1 sites, which are not bound to second layer Co(Td) and especially at O1 sites that are bound to third layer Ni atoms, while VOs at O1 sites with more Co(Oh) than Ni neighbors tend to reconstruct to form split vacancies. The computed formation energy of a regular (i.e., nonreconstructed) VO at O1 is approximately 0.8− 0.9 eV at T = 0 K (Table 2), which is essentially one-half the value (1.56 eV) that we find for a VO on the Co3O4(100) surface. Easier VO formation on NCO suggests that this material may be a better oxidation catalyst than Co3O4 under

a Both values at T = 0 K and under ambient conditions (T = 300 K and p(O2) = 0.2 atm, corresponding to the O2 partial pressure in air) are reported. All values refer to 0.25 monolayer coverage (i.e., one adsorbed molecule per surface unit cell), and positive values indicate that adsorption is not favorable. VO (M) and VO(M) + VO indicate O2 adsorption at a vacancy site of a reduced surface with 1 VO and 2 VOs per surface unit cell, respectively. Figures showing the various structures are listed in the last column.

O2 adsorption remains unfavorable at room temperature and atmospheric pressure. When the 2 vacancies are close to each other (e.g., on the (100)mix surface with O1a−O1b vacancies or on the (001)Co surface), O2 takes a different adsorption structure (Figure 5d), where only one of the two oxygens binds to a neighboring surface cation, whereas the other oxygen points toward the vacancy site, with an O−O bond length of 3935

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The Journal of Physical Chemistry C

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mild conditions (i.e., at temperatures below Ni segregation to the surface takes place). NCO’s surface reactivity has been further characterized by studying the adsorption of two typical probe molecules, water and O2. Both molecules preferentially adsorb at oxygen vacancy sites at low temperature. Under ambient conditions, however, VOs can be easily healed via dissociative water adsorption, whereas adsorption of O2 is not favorable. These results suggest that O2 adsorption is likely to represent the thermodynamic limiting step for oxidation reactions on NCO(001)/(100) surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12005. Layer resolved densities of states; surfaces with different Ni cation concentrations; adsorbed water and O2 on different surfaces; computed oxygen-cation bond lengths; surface oxygen vacancy formation energies on NCO(100) with 25% and 75% Ni surface concentrations; computed heats of formation of NCO, Co3O4, and NiO; limits of the oxygen chemical potential for the stability of bulk NCO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Annabella Selloni: 0000-0001-5896-3158 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DoE-BES, Division of Chemical Sciences, Geosciences and Biosciences under Award DESC0007347. SLB acknowledges support from NSFDMR1506989. We used resources of the National Energy Research Scientific Computing Center (DoE Contract No. DEAC02-05CH11231) and the TIGRESS High Performance Computer Center at Princeton University.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b12005 J. Phys. Chem. C 2017, 121, 3929−3937

Article

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DOI: 10.1021/acs.jpcc.6b12005 J. Phys. Chem. C 2017, 121, 3929−3937