Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into

Article pubs.acs.org/JPCC

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet Filip Zasada,* Witold Piskorz, and Zbigniew Sojka Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland S Supporting Information *

ABSTRACT: Periodic spin unrestricted DFT-PW91+U calculations together with ab initio thermodynamic modeling were used to study the structure, defects, and stability of different terminations of the (100) surface of cobalt spinel under various redox conditions imposed by different oxygen partial pressure and temperature. Three terminations containing under-stoichiometric (100)-O, stoichiometric (100)-S, and overstoichiometric (100)-R amount of cobalt ions were analyzed, and their atomic and defect structure, reconstruction, and stability were elucidated. For the most stable (100)-S and (100)-O facets, formation of cationic and anionic vacancies was examined, and a surface redox state diagram of possible spinel (100) terminations in the stoichiometry range from Co2.75O4 to Co3O3.75 was constructed and discussed in detail. The results revealed that the bare (100)-S surface is the most stable at temperatures and pressures of typical catalytic processes (T ∼ 200 °C to ∼500 °C, pO2/p° ∼ 0.001 to ∼1). In more reducing conditions (T > 600 °C and pO2/p° < 0.0001), the (100)-S facet is readily reduced by formation of oxygen vacancies, whereas in the oxidizing conditions (T < 200 °C and pO2/p° > 10), coexistence of (100)-S and (100)-O terminations was revealed. Formation of the oxygen vacancies involves reduction of the octahedral trivalent cobalt and is accompanied by migration of the divalent tetrahedral cobalt into empty, interstitial octahedral positions. It was also found that the constituent octahedral Co cation proximal to the interstitial cobalt adopts a low spin configuration in contrast to the distal one that preserves its surface high spin state. In the case of the Co depleted surfaces, the octahedral vacancies are thermodynamically disfavored with respect to the tetrahedral ones in the whole range of the examined T and pO2 values. The obtained theoretical results, supported by TPD-O2 and TG experiments, show that the octahedral cobalt ions are directly involved in the redox processes of Co3O4.

1. INTRODUCTION Owing to their tunable redox properties, well-defined structure, and easily controllable morphology spinel oxides of general formula AB2O4 have a remarkable record of widespread applications in heterogeneous catalysis.1−3 Among them, the bare and bulk or surface-doped cobalt spinels with complex, yet well-defined, structure and easily controlled morphology4 were found to be excellent redox-tunable model catalytic materials of great fundamental and practical interest. For example, Co3O4 is an active catalyst for CO,5,6 NH3,7,8 and hydrocarbons oxidation.9−11 It is also well-known for high activity in oxygen reduction reaction (ORR)12 and in low temperature decomposition of N2O.13 The redox state of the Co3O4 catalyst14,15 plays an important role in elementary step dynamics of the catalyzed reactions. It can be controlled readily by the spinel stoichiometry through introduction of cationic or anionic vacancies by appropriate heterovalent doping or by proper adjustment of the synthesis conditions. It should also be mentioned that the catalytic activity of cobalt spinel depends strongly on the grain size and morphology, including specific features of the exposed facets.16,17 Such factors have to be controlled strictly during © XXXX American Chemical Society

the catalyst preparation, for sensible comparison of the experimental and theoretical results. Careful inspection of the available experimental data reveals that Co3O4 specimens prepared by various methods are primarily faceted on the (100) and (111) planes with only a minor contribution of the (110) termination.18,19 Indeed, many reported computational results shows that in the wide range of temperatures and oxygen/water partial pressures, the (110) and (111) terminations are less stable than the (100) plane.20−22 Moreover, the (111) termination, due to the presence of highly unsaturated surface Co ions, is covered by water molecules even at elevated temperatures (>450 °C),21 which tempers its possible involvement in catalytic low-temperature processes in wet environments, leaving often only the (100) facet as a most abundant catalytically attractive termination. Whereas there is a number of papers dealing with the structure of the (110) and (111) surfaces and their interaction with small molecules,23−26 in the case of the (100) termination, Received: May 29, 2015 Revised: July 21, 2015

A

DOI: 10.1021/acs.jpcc.5b05136 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

and (100)-R terminations were further modified by the introduction of cationic and anionic vacancies to evaluate the role of defects. 2.3. First-Principles Thermodynamics. To describe redox behavior of the cobalt spinel surface under real temperature and oxygen pressure conditions, we used the well-established atomistic thermodynamic modeling following the scheme proposed elsewhere:40 within this approach the free energy, γ, of the surface containing NO oxygen atoms and NCo cobalt atoms relative to the bulk Co3O4, was calculated as

the literature is essentially confined to the exploratory studies of Montoya et al.20,22 However, despite the fact that the Co3O4 (100) facet is implicated in many sustainable catalytic processes,27−29 a comprehensive and in-depth description of its structural variants and stability of at various redox states is still lacking. Herein, we investigated by means of spin unrestricted DFTPW91+U calculations possible terminations and defect structure of the most abundant (100) surface of cobalt spinel of various stoichiometry (Co3−xO4−y). The DFT studies were complemented ab initio thermodynamic modeling to construct the surface state diagram of the (100) facet at different redox conditions, defined by temperature and oxygen partial pressure.

γ(T , pO ) = 2

2. COMPUTATIONAL METHODS 2.1. Geometry and Electronic Energy. For all DFT calculations, the Vienna ab initio simulation package (VASP)30 was used. The projector augmented plane wave (PAW)31 method was employed for treatment of the electron−ion interactions, whereas the electron exchange-correlation effects were accounted for within the generalized gradient approximation (GGA), using the PW91 exchange-functional.32 The effect of 3d electron correlation described by on-site Coulomb (U) and exchange (J) interactions was included within the formulation of Dudarev et al.33 We use Ueff = 3.5 eV since this value leads to adequate reproduction of the experimental oxidation energies of CoO to Co3O4 with good predictions of the band gap as well.34,35 All calculations were performed using a standard Monkhorst−Pack36 grid (5 × 5 × 5 sampling mesh for bulk calculations and 3 × 3 × 1 for slab calculations) with cutoff energy of 500 eV, and a Methfessel-Paxton37 smearing parameter σ = 0.1 eV. The Kohn−Sham SCF was solved in spin-unrestricted fashion, with the convergence criterion set to 10−6 eV. Bulk cobalt spinel unit cell was obtained by optimizing the cubic (1 × 1 × 1) cell (a = 8.08 Å) containing 56 ions (Co24O32). The optimal cell volume was calculated as a result of the E/V fit (applying the Birch−Murnaghan equation of state)38 with full optimization of all internal degrees of freedom (with the error of 10−4 eV). The atomic charges were calculated using the Bader population analysis.39 2.2. Surface Models. Surface (1 × 1) supercell slab models were obtained by cleaving the optimized spinel in the normal [100] direction. We employed a ∼ 16 Å thick slab composed of 17 atomic layers with a vacuum separation of 15 Å to avoid interactions of the periodically repeated oxide layers. The proper symmetry of the top and bottom slab terminations was assured to minimize the dipole moment development in the supercells. We considered three differently terminated models of the (100) surface of various composition: an oxidized (oxygen rich) termination (Co46O64 = Co2.87O4), a stoichiometric termination (Co48O64 = Co3O4), and a reduced (cobalt rich) termination (Co50O64 = Co3.12O4). They are denoted from here as (100)-O, (100)-S, and (100)-R, respectively. The labeling of all models used in this study can be found in Table S1. A comprehensive description of these terminations is discussed in the main manuscript, whereas detailed definition of the slab models used for DFT modeling can be found in Figure S1. Full relaxation of the atomic positions in the four top and four bottom layers was allowed to render the forces acting upon the ions below 1 × 10−3 eV Å−1. This number of layers is sufficient, since the changes in the ionic positions in the third (and deeper) sublayers are negligible. The (100)-O, (100)-S,

1 ⎡ slab ⎢GCo O (T , pO2 , NCo , NO) 2A ⎣ x y ⎛ ⎞ N 4 bulk (T , pO ) − ⎜NO − NCo⎟μO − Co GCo 3O4 2 ⎝ ⎠ 3 3 ⎤ (T , pO )⎥ 2 ⎦ (1)

In this formula, Gslab CoxOy represents the free energy of the bulk overall slab, GCo3O4 is the bulk free energy of the cobalt spinel per one formula unit, A denotes the surface area of the slab, and μO(T, pO2) the chemical potential of dioxygen. As the surface vibration terms do not vary upon oxygen extraction in a significant way, the vibrational contributions to the Gibbs free energies of the bulk and the slab cancel to the large extent and can as usual be neglected (i.e., Gslab Eslab, Gbulk, and Ebulk).41,40 The μO(T, pO2) potential can be factored into the T and p independent total energy component (including the zero point energy) of dioxygen, and a ΔμO(T, pO2) part that comprises all the T and p related contributions: μO(T , pO ) = 2

1 tot EO + ΔμO(T , pO ) 2 2 2

(2)

In this way the surface chemical state becomes a function of the oxygen chemical potential. Additionally, we can express the ΔμO(T, pO2) term as the sum of temperature (T)- and pressure (pO2)-dependent contributions: ⎛ pO ⎞ ΔμO(T , pO ) = μO′ (T , pO0 ) + kT ln⎜ 02 ⎟ 2 2 ⎝p ⎠

(3)

The μO′ (T,p0O2) term was computed by means of standard statistical thermodynamics as proposed by Hirano.42 The μO values were varied within the two limits imposed by the maximum value, μO = 0.0, corresponding to the state when oxygen condense on the surface (oxygen rich conditions), and a minimum value, μO = −2.4 eV, corresponding to decomposition of cobalt spinel into solid Co and gas phase oxygen (oxygen lean conditions).20

3. MATERIALS AND CHARACTERIZATION The Co3O4 spinel samples were prepared by pH-controlled precipitation method as described elsewhere in more detail.13 In order to obtain spinels of various defect concentration, the samples were calcined in the temperature range of 300−700 °C. The phase structure of the spinel samples was confirmed by X-ray diffraction, obtained by means of a Rigaku diffractometer using the Cu Kα radiation, and by micro-Raman spectra measured with a Renishaw InVia spectrometer using the excitation wavelength of 785 nm. Both the XRD and Raman B


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The Journal of Physical Chemistry C results (the presence of five characteristic peaks at 194 (F2g), 480 (Eg), 520 (F2g), 620 (F2g), and 690 (A1g) cm−1) confirm the cubic Fd3̅m spinel structure of the samples, regardless the temperature of their calcination (see Figure S2). HR-TEM survey pictures performed with an Osiris microscope (FEI) revealed that the obtained Co3O4 nanocrystal are exposed predominantly (100) facets. The temperature-programmed oxygen desorption (TPD-O2) measurements were performed in the range of 300−800 °C in a quartz flow reactor using 500 mg of the sample with the flow rate of 30 mL min−1 (1% O2 in He, 7000 h−1), and the heating rate of 10 °C min−1. The thermogravimetric studies (TG) were carried out by means of a Mettler Toledo instrument (TGA/DSC1) in the flow of helium.

Table 1. DFT+U Calculated Structural Parameters of Co3O4: Lattice Constant, a, Oxygen Parameter, u, and the MT−O, MO−O Bond Lengths, Along with the Corresponding Experimental Values DFT+U experiment

(4)

× CoCo → 2Co·iB′ + V″CoA A

(5)

u

CoT−O (Å)

CoO−O (Å)

8.149 8.08413

0.2638 0.263213

1.960 1.93548

1.932 1.92048

good agreement with the experimental distances of 1.920 and 1.935 Å. Inclusion of the Hubbard parameter was found to be crucial for proper description of the cobalt−oxygen bond lengths. Without Hubbard correction, the CoO−O distance of 1.931 Å is longer than the CoT−O bond length (1.920 Å), contrary to XRD data. The general overestimation (by about ∼0.5%) of the lattice constant and bond distances has been found previously in other GGA+U studies of oxide systems,49 and its reason was discussed in the literature.50 The calculated magnetic moments and Bader charges for the constituent ions are collected in Table 2. The value of 2.69μB

4. RESULTS AND DISCUSSION Tricobalt tetraoxide (Co3O4) is a magnetic p-type semiconductor characterized by the normal spinel structure (Fd3̅m, space group 227), and the unit cell contains eight formula units with 56 ions (Co24O32). The experimental lattice constant is equal to 8.084 Å.43 The oxygen ions form a fcc close-packed sublattice with the tetrahedral sites A (Wycoff position 8a), exhibiting one-eight occupancy, filled with Co2+(d7) cations (denoted CoT). The half-filled octahedral sites B (16d) accommodate Co3+(d6) cations, denoted hereafter as CoO. This ideal site occupancy may be perturbed, however, by spinel partial inversion or off-stoichiometry, favored at high temperatures. Partial inversion is essentially tantamount with the transfer of an electron from the divalent CoT to trivalent CoO sites, giving rise to appearance of a fraction of the Co3+ ions in the A sites and Co2+ in the B sites. The offstoichiometric spinels results, in turn, from the shift of the CoT ions to the empty octahedral 16c sites (called B′ hereafter), forming a cationic vacancy (VA) and an interstitial CoB′ cation.44 Such defects are induced by reduction of the CoO cations [e.g., upon oxygen vacancy formation (see below)]. With the use of the Kröger-Vink notation, their formation can be described as × × 2CoCo + OO → 2Co′CoB + V ··O + 1/2O2 B

a (Å)

Table 2. Calculated Electronic (Band Gap) and the Magnetic Properties of the Cobalt Spinel Constituents magnetization (μB)

DFT experiment52

Bader charge (|e|)

X→X (eV)

CoT

CoO

O

CoT

CoO

O

1.72 1.65

2.69 3.01

0.01 0.00

0.01 0.00

1.31 −

1.45 −

−1.05 −

for the divalent CoT ions is associated with their high spin e4t23 configuration, whereas for the trivalent CoO ions, the null magnetic moment is consistent with the closed shell (t2g6eg0) state. The electronic X → X band gap (1.72 eV) compares fairly well with previous calculations51 and the experimental value of 1.65 eV. As shown in Table 2, the qB for the formally trivalent bulk CoO and divalent CoT are equal to 1.45|e| and 1.31|e|, respectively. These bulk charges were used as reference points for monitoring changes in the oxidation states induced by the spinel surface and its nonstoichiometry. Good agreement between the calculated and experimental data revealed that both the applied model and the calculation scheme were adequate for sensible modeling of the cobalt spinel structure and energetics. 4.2. Surface Terminations. Perspective pictures of the three parent (100)-O, (100)-S, and (100)-R terminations of the cobalt spinel (100) facet are shown in the bottom panel of Figure 1. The visible parallel stripes of the blue-coded octahedral Co surface ions are bridged by recessed (green) and protruding (purple) tetrahedral ions. The central part of the images corresponding to the (1 × 1) surface element is magnified (top panel) to reveal the atomic structure in more detail. In Figure 2, the coordination of specific surface ions together with the corresponding bond lengths and bond angles, altered by surface reconstruction, are shown. In the case of the (100)-O termination (Figure 1a), the cationic composition of the (1 × 1) surface element consists exclusively of the pentacoordinated CoO5c ions (coded blue, the nc subscript denotes their coordination number). The two concealed tetrahedral cobalt ions of full coordination (CoT4c, green) are located in the first sublayer. The surface anionic composition (oxygen ions are marked in red) includes six truncated 3-fold coordinated oxygen ions (denoted as O2O,1T)

4.1. Accuracy of the Calculations. For modeling of the complex transition metal oxides, a good compromise between the accuracy and computational cost is provided by the GGA +U scheme. In the particular case of cobalt spinel, the choice of the U value has been systematically investigated,45,46 and it has been shown that computationally expensive hybrid functionals exhibit irrelevant difference compared to the properly selected GGA+U approach.20,47 The benchmarking structural, electronic, and magnetic properties of the cobalt spinel bulk phase were then computed and compared to the experimental data to verify the adequacy of the employed computational approach (Table 1). The lattice parameter, a, and the oxygen parameter, u, were chosen for structure validation, since they allow for complete determination of the atomic positions within the unit cell. The optimized lattice constant of the Co3O4 spinel, a = 8.149 Å, and the u parameter of 0.2638 compare well with the experimental XRD values of 8.084 Å and 0.2632, respectively (see Table 1). The calculated CoO−O and CoT−O bond lengths are equal to dCoO−O = 1.932 Å and dCoT−O = 1.960 Å, respectively, again in C


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Figure 1. Structure of the (a) (100)-O, (b) (100)-S, and (c) (100)-R terminations of Co3O4. Color coding: CoO, blue; protruding CoT, purple; subsurface CoT, green; and O, red.

Figure 2. Coordination environment of the cationic and ionic surface sites on the relaxed (a) (100)-O, (b) (100)-S, and (c) (100)-R termination of the cobalt spinel. Color coding: CoO, blue; protruding CoT, purple; recessed CoT, green; and O, red.

that are linked to two CoO and to one CoT cation, and two 3fold coordinated anions (O3O) that are linked to the CoO ions exclusively. Thus, the surface composition of the (100)-O termination can be described as {4CoO5c, 2CoT4c, 6O2O,1T, 2O3O}. The aerial concentration of CoT4c equals 0.03 ion Å−2, being two times lower than that of the CoO5c ions (0.06 ion Å−2). Bader population analysis showed that for the surface oxygen ions, the partial charges are nearly the same as in the bulk, whereas for the surface cobalt ions they are systematically higher. More pronounced changes were noted for CoO than for CoT cations, with the difference with respect to the bulk value reaching 0.15|e|. For the CoO5c ions, a pronounced change in the magnetic moment, from 0.0μB to 1.9μB, with respect to the bulk value was observed, whereas in the case of the CoT4c and O ions, the changes in μ were practically irrelevant. This fact is well-illustrated by spin density repartition, where all the CoT and CoO surface cobalt cations exhibit clearly a nonzero spin density, in contrast to the bulk of the spinel, where only the CoT ions are magnetic apparently (see Figure 5a below). These effects can be accounted for by surface crystal field of the local C4v symmetry that give rise to the (dxz,dyz)4(dxy)1(dz2)1 configuration with S = 1 in the case of B sites, whereas for the surface A sites, the local C2v symmetry implies a (dz2,dz2−y2)4(dxy)1(dxz)1(dyz)1 configuration, preserving the parent S = 3/2 state. As implied by Figure 2a, reconstruction of the surface is rather small. Both CoT−O and CoO−O bonds are slightly shorter than in bulk (1.91 vs 1.96 Å and 1.91 vs 1.93 Å, respectively), whereas the tetrahedral angle of the recessed CoT4c is considerably larger than in the bulk (117° vs 109°). The stoichiometric (100)-S termination (Figure 1b) differs from the previously described surface as the protruding CoT2c

ions (coded purple) with two dangling bonds are present. Since in the (1 × 1) surface element only every second site is occupied (with respect to the bulk), the composition of the corresponding slab model, Co48O64, is equal to the bulk stoichiometry. The presence of the CoT2c ions changes the anionic coordination, as two O3O ions transform into 4-fold coordinated O3O,1T species. Thus, the composition of the surface element can be expressed as {1CoT2c, 4CoO5c, 2CoT4c, 4O2O,1T, 2O3O, 2O3O,1T}. The surface concentration of the CoT2c sites (0.015 ion Å−2) is four times lower than that of the CoO5c ions (0.06 ion Å−2). Similarly to the previous (100)-O termination, only the surface CoO ions change their magnetization (from 0.0 to 2.0) upon surface cleaving. The Bader charge analysis showed minute changes in the anion and cation oxidation states compared to the bulk values (the differences do not exceed −0.02|e|). Surface reconstruction led to noticeable recession of the CoT2c ions toward the bulk by about 0.41 Å, whereas an analogous shift of CoO5c is much smaller (0.09 Å). Those changes are accompanied by reduction of the CoT−O and CoO−O bond lengths (by 0.14 and 0.03 Å, respectively), and substantial increase in the tetrahedral O3O,1T−CoT2c− O3O,1T angle, from 109° to 120° (see Figure 2b). In the case of the reconstructed Co3O4 (100)-R termination (Figure 1c), there are three types of oxygen anions and two types of CoT cations per (1 × 1) unit cell. In the unrelaxed geometry (see Figure S1b), the surface composition can be described as {2CoT2c, 4CoO5c, 2CoT4c, 2O2O,1T, 2O3O, 4O3O,1T}. However, contrary to the previous terminations, not only the vertical but also the lateral relaxation of the surface CoT2c ions was observed. It involved displacement of the CoT2c ions (indicated by the purple arrow in Figure 1c) to the nearest D


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Figure 3. Surface energies (γ) for different terminations of the (100) facet of the cobalt spinel plotted against temperature and oxygen pressure (see main text for coding): (a) 2-dimensional plot as a function of O2 chemical potential, (b) as a function of temperature (for pO2 = 0.1 atm), and (c) 3dimensional γ(T, pO2) plot with the projection of the lowest surface free energies onto the (T, pO2) plane (d).

observed.53 In the case of stoichiometric surface, such reconstruction is disfavored since it would lead to local accumulation of the excessive positive charge due to the presence of trivalent cobalt ions in one place, and the resultant energy rise cannot be counterbalanced by the configurational entropy gain within the temperature window of cobalt spinel stability.54 For the sake of comprehensiveness, we evaluated the off-stoichiometric models also for the two other terminations, and assessed the significance of the configurational entropy contribution explicitly. Construction of the off-stoichiometric (100)-S and (100)-O slabs is described in Figure S3. The configurational entropy per surface (1 × 1) element was calculated as

empty truncated octahedral site (B′) to produce defect species labeled as CoB5c′ hereafter. This shift is accompanied by only slight changes in the qB (from 1.31|e| to 1.28|e|) and the magnetic moment (from 2.60μB to 2.58μB) of the migrating cobalt ion. The resultant local symmetry alteration from C2v to C4v imparts a new configuration of the d 7 manifold, [(dz2,dz2‑y2)4(dxy)1(dxz)1(dyz)1 → (dxz,dyz)4(dxy)1(dz2)1(dx2‑y2)1], which preserving the number of the unpaired electrons, speaking qualitatively, changes the SOC contribution to the magnetic moment. The qB of the adjacent CoO5c cations decreased from 1.45|e| to 1.21|e|, and the magnetic moment increased from 2.0μB to 2.7μB. Such changes can be interpreted as one-electron reduction of the CoO5c cations that assume a (dz2,dz2‑y2)4(dxy)1(dxz)1(dyz)1 configuration with three unpaired electrons. It should be mentioned that the CoB5c′ sites differ from those denoted by CoO5c since instead the corners they share a face with the neighboring occupied tetrahedral CoT4c site. Columbic repulsion between the positively charged CoT4c and CoB5c′ ions (the Co−Co distance is equal to 2.44 Å) results in asymmetric coordination of the CoB5c′ species (Figure 2c). Moreover, after reconstruction, all the oxygen ions on the surface have the same 4-fold O3O,T1 coordination (Figure 1c). The {2CoB5c′, 4CoO5c, 2CoT4c, 8O3O,1T} composition of such termination leads to the following areal concentration of the cations: 0.03 /Å−2 for CoB5c′ and 0.06 /Å−2 for CoO5c. The shift of the CoT2c ions toward B′ sites is facilitated by reduction of the adjacent trivalent Co O cations, as experimentally

⎧ Off Sconfig = kB⎨(1 − x)ln(1 − x) + x ln x + (4 − x) ⎩ ⎛4 − x⎞ x⎫ ⎟ + x ln ⎬ ln⎜ ⎝ 4 ⎠ 4⎭

(eq 6)

where x refers to the fraction of the cobalt ions moved from CoT2c or CoT4c positions to the adjacent hollow interstitial sites B′. The results shown in Figure S4 reveal that regardless the offstoichiometry level even at the highest temperatures of the cobalt spinel phase stability ( −1.10 eV. The slopes of these lines are directly related to the Co/O ratio in the slab model, which for the reduced facet (Co50O64) is equal to 0.781 and for the oxidized one (Co46O64) to 0.719. However, between the threshold values of ΔμO2 the most stable termination is the (100)-S facet with Co/O ratio of 0.75 (blue horizontal line). Stability of the off-stoichiometric, (100)-SOff and (100)-OOff, terminations with CoT ions moved to intertitial B′ positions is distinctly lower, in comparison to the corresponding parent (100)-S and (100)-O planes, in the whole range of the thermodynamic conditions, as indicated by the dotted lines (Figure 3a), even upon inclusion of configurational entropy term. Figure 3b shows how temperature influences the stability of those surfaces at pO2/p° = 0.1. For T < ∼ 100 °C, the (100)-O and (100)-S terminations coexist, whereas above this threshold the stoichiometric facet prevails. The three-dimensional plot (Figure 3c) summarizes the stabilities of the thermodynamically most preferred regular (100)-S, (100)-O, and (100)-R terminations as a function of T and pO2. Although at high pressures and low temperatures, the oxidized termination is apparently more stable than the stoichiometric one, since the difference in their surface energies is not very significant (Δγ < 0.05 J m−2) and both types of facets may simultaneously appear. However, at high temperatures and low O2 pressures, the surface energy difference between the reduced and the stoichiometric terminations is large enough (Δγ > 0.4 J m−2) to make the former the only present. The stability regions of the cobalt spinel of various stoichiometry, Co(2.87−3.12)O4, are better revealed in the surface phase diagram obtained by 2D projection of the bottom surface free energies onto the (T, pO2) plane (Figure 3d). The persistence of the stoichiometric (100)-S termination extends over the wide range of temperatures and oxygen partial pressures, typical for most catalytic processes. Among the nonstoichiometric terminations, the (100)-O surface can be prepared experimentally quite readily (vide infra), in contrast to the (100)-R surface (featured by the excess of cations, Co/O > 0.75). Indeed, in the reducing conditions (high T, low pO2), the Co/O ratio >0.75 is preferably achieved by formation of the oxygen vacancies, as discussed below in more detail. Therefore, for discussion of the cationic and anionic vacancies, only the (100)-S and (100)-O terminations were considered. 4.3. Surface Oxidation and Reduction via Vacancy Formation. 100-S Termination. In the conceptually simplest way, oxidation of the cobalt spinel can be achieved via generation of the cationic vacancies (VCoT and VCoO). For the (100)-S termination, the cations of the lowest stability are obviously the protruding CoT2c. Removal of these species leads to the (100)-O termination, which was already analyzed in detail (vide supra). The (100)-S termination can be reduced upon release of the lattice oxygen (at high T and low pO2). The corresponding

O2 vac Evac = Eslab +

1 EO − Eslab 2 2

(eq 7)

Evac slab

In this equation, indicates the energy of the defected slab with the oxygen vacancy, (1/2)EO2 refers to the half of the gas phase oxygen electronic energy, whereas Eslab accounts for the energy of the parent slab. For the most stable stoichiometric termination of the (100) facet, two kinds of anionic vacancies produced by extraction of O2O,1T (VO2O,1T) and O3O (VO3O) surface oxygen ions were considered. Since such defects may possibly diffuse into the bulk of the cobalt spinel, formation energies of the oxygen vacancies in the first (VO(layer1)) and the second sublayers (VO(layer2)) as well as in the spinel bulk, VO(bulk), were also calculated. The obtained results are summarized in Table 3. Table 3. Formation Energies of Oxygen Vacancies on the (100)-S Termination with Respect to Gas Phase O2 Molecule E (eV)

VO2O,1T

VO3O

VO(layer1)

VO(layer2)

VO(bulk)

2·VO

0.91

1.90

0.8

0.77

0.76

1.67

As can be inferred from Table 3, removal of an oxygen atom to form a VO2O,1T vacancy requires the energy expenditure of O Evac2 = 0.91 eV. The resultant atomic configuration of the relaxed surface is shown in Figure 4a, where the vacancy position is indicated as a yellow cube. In the vicinity of the created defect, shortening of the Co−O bonds from 1.98 to 1.89 Å took place. The structure relaxation caused a shift of the exposed CoT2c cation toward the neighboring empty surface B′ site, similarly to that observed for the off-stoichiometric (100)R termination (Figure 1c). The direction of this displacement is opposite to the vacancy position. These alternations of the geometry can be attributed to localization of the two electrons left by the removed oxygen on a pair of the adjacent octahedral CoO4c ions, revealed by the changes in the Bader charges (from 1.46|e| to 1.23|e|) and the magnetic moments (from 2.0 to 2.75). The migrating divalent tetrahedral cation (CoT2c → CoB5c′) that accompanies the reduction preserves its valence and spin states, as previously observed for the (100)-R termination. This is well-illustrated in Figure 5b, where the corresponding spin density repartition (isosurface value of 0.3|e| A−3) of the relevant surface cobalt ions are shown. The shapes of the contours for the reduced divalent CoO4c ions (pale green) are distinctly different than that of the intact trivalent CoO5c cation (dark green). Interestingly, due to the asymmetric position of the interstitial CoB′5c (pale blue), the proximal CoO5c cation (3.12 Å) assumes the low spin configuration in contrast to the distal one (3.94 Å), which preserves its surface high spin state. As a result, formation of the oxygen vacancy leads not only to reduction of octahedral cobalt but also is accompanied by pronounced structure (migration of CoT into interstitial B′ positions) and uneven spin relaxation of the adjacent CoO5c ions. As expected, formation of the VO3O vacancies is much more O energetically demanding (Evac2 of 1.90 eV), and it also influences T the position of the Co 2c ion. In this case, the doubly unsaturated CoT2c preserves its location in the site A, but the symmetry of the Co−O bonds is broken, as revealed in Figure 4b. Pronounced difference in the formation energy with respect F


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Figure 4. Perspective view of the relaxed spinel (100)-S termination after formation of the anionic (a) VO2O,1T, and (b) VO3O vacancies, with the most important bond lengths indicated explicitly. Color coding: CoO, blue; CoT, purple; spinel O, red; VO, yellow cube.

the electrons left by the released O atom are accommodated by the CoO5c ions. As can be learned from Table 3, the calculated energy of the O-vacancy formation in the first and second sublayer and in the bulk are lower than that of the surface VO2O,1T. This provides a driving force for migration of the surface-formed vacancies toward the bulk. In our modeling, we considered also the presence of more involved defects, by taking into account simultaneous presence of the VO2O,1T and VO(layer2) vacancies. The calculated energies showed that such vacancies can be treated as virtually noninteracting, since the energy of the double vacancy formation (1.67 eV) is close to the sum of the single vacancy energies (0.91 eV + 0.77 eV = 1.68 eV). The results of the thermodynamic calculations for the different anion depleted (100)-S terminations are presented in Figure 6. The panel shows their stabilities as a function of temperature for oxygen pressures corresponding to (i) reduction conditions, pO2/p° = 10−8 (achievable, for example, by pCO/pCO2 ∼ 102.5 buffer), (ii) oxygen pressures of typical catalytic oxidation reactions (pO2/p° = 10−2), and (iii) strong oxidation conditions (pO2/p° = 104). For the sake of completeness, we also added the (100)-R facet, slightly reduced by the excess of the CoT2c cations (vide supra). Since for all those surfaces, the Co/O ratio >0.75 at fixed pO2 then the higher the temperature the more stable they are. In the low oxygen pressure conditions (Figure 6a), the reference line (blue) of the (100)-S termination is crossed by that of the VO2O,1T one (dark green) at about 300 °C. Two lines referring

Figure 5. Spin density contours for the (a) bulk and surface cobalt ions and for the (b) (100)-S-VO2O,1T termination showing the reduction of the cobalt ions (light green) in the vicinity of the oxygen vacancy (yellow cube) and spin relaxation of CoO5c (dark green) ions upon CoT2c (light blue) migration into the B′ site.

to the VO2O,1T defect (0.91 eV) can be associated also with the fact that no strong geometric relaxation of the surface takes place in this case. Furthermore, whereas the change in the electronic and magnetic states of the CoT2c ions was negligible, significant drop in the qB (from 1.45|e| to 1.21|e|) and the concomitant increase of the magnetic moment (from 2.0μB to 2.7μB) was revealed for the cobalt cations located in the adjacent octahedral sites. These results imply that, once again,

Figure 6. Surface energy (Δγ) of the oxygen depleted (100)-S termination as a function of temperature for three different oxygen pO2/p° values: (a) 1.0 × 10−8, (b) 1.0 × 10−2, and (c) 1.0 × 104. Minimum energy lines define the most stable terminations, whereas the intersection areas depict the transition regions. G


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Figure 7. Perspective view of the relaxed (100)-O termination after formation of cationic (a) CoT4c and (b) CoO5c vacancies, with the indicated most important bond lengths. Color coding: CoO, blue; CoT, purple; spinel O, red; VCo, pink cube.

Figure 8. Surface energy (Δγ) of the oxygen rich (100)-O termination depleted of Co or oxygen ions as a function of temperature for three different oxygen pO2/p° values: (a) 1.0 × 10−8, (b) 1.0 × 10−2, and (c) 1.0 × 104. Minimum energy lines represent the most stable termination, whereas the intersection areas depict the transition regions.

to the stability of the sublayer vacancies (VO(layer1), dark gray and VO(layer2), light gray) are lying lower, thus their crossing with the reference bare surface line is shifted toward lower temperatures. It accounts well for the thermodynamic incentive for diffusion of the surface formed VO vacancies into the bulk, where they are stable already at 250 °C. The steep olive green line corresponding to the double vacancy (2VO) becomes the lowest at T > 350 °C, indicating that for pO2/p° = 10−8 above this temperature threshold, enhanced reduction of the cobalt spinel is thermodynamically favored. The cobalt rich (100)-R termination (dotted green) is less stable than oxygen depleted surfaces below 700 °C, whereas due to the high-formation energy of the VO3O vacancies, the (100)-S-VO3O facet (dashed orange line) does not appear even at elevated temperatures. The overall diagram for oxygen pressures corresponding to typical conditions of catalytic oxidation processes is similar to the previous case (Figure 6b), but the crossing of the thermodynamic lines is shifted toward higher temperature by about 200 °C. The (100)-S surface in such conditions is the most stable until 425 °C. Above this temperature, the regions of single (VO2O,1T, VO(layer1), and VO(layer2)) and then double (2V O ) vacancy formation appear. However, since the corresponding curves are quite close to each other, they may actually coexist in the region of 600 °C. As the temperature further increases, the double vacancies dominate. Finally, for high oxygen pressure conditions (Figure 6c) in the whole range of the considered temperatures, the (100)-S termination remains the most stable, indicating that for obtaining a

stoichiometric cobalt spinel calcination at high oxygen pressures is recommended. 100-O Termination. Oxidation of the 100-O surface can conceivably be realized by extraction of CoT4c or CoO5c cations. The relaxed surface after VCoT vacancy formation (pink cube) is shown in Figure 7a. The resultant geometrical rearrangement of the spinel structure is quite small despite the significant change in the Bader charges from 1.45 to 1.63, consistent with increase in the oxidation state of the adjacent CoO5c cations. It is also reflected in drop of their magnetic moments from 2.0μB to 1.1μB. This corresponds nicely to the hole (dxz,dyz)4(dxy)1(dz2)0(dx2‑y2)0 configuration of the tetravalent cobalt. Analogous extraction of the CoO5c cation gave similar results (Figure 7b) but is energetically much more costly (by 1.6 eV), so such disorder is less favored. The excessive positive charge is similarly accommodated on the nearest CoO5c cations (their charge increased from 1.45|e| to 1.67|e| and the magnetic moment decreased from 2.0μB to 1.2μB). Geometrical relaxation of the surface lattice, again, is small. As a result, the observed changes in the redox state of the octahedral cobalt centers are not accompanied by substantial surface relaxation, which otherwise will enhance the activation energy of the redox processes. This may be associated with the fact that they involve a largely nonbonding “t2g” manifold of the octahedral cobalt, so alternations in the electron count do not entail any significant bond length modifications, in contrast to the antibonding “eg” level. Upon formation of oxygen vacancies, the (100)-O termination is progressively reduced. As expected, the H


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The Journal of Physical Chemistry C corresponding energies (equal to 0.76 and 1.35 eV for VO2O,1T and for VO3O, respectively) are smaller than the analogous values for the (100)-S termination. Indeed, due to the presence of the CoT vacancies, part of the CoO ions is oxidized, lowering thereby the Fermi level, which favors accommodation of two electrons left by the released oxygen. In line with previously described reduction of the (100)-S surface, we considered the model with two coexisting vacancies (VO2O,1T and VO(layer2)). The calculated energy of the double vacancy formation (1.44 eV) is close to the sum of the single vacancy energies (0.76 + 0.69 eV), indicating a negligible mutual interaction between them. The results of thermodynamic modeling for the defected (100)-O terminations are presented in Figure 8, for the three different oxygen pressures. For the cation depleted facets at fixed pO2, the lower the temperature the more stable are the investigated terminations, in contrast to surfaces reduced by oxygen vacancies. In the low pressure regime (Figure 8a), the (100)-O termination (orange solid line) is the most stable until ∼200 °C, then upon oxygen release it is transformed into the (100)-O-Vo surface (green line). The second vacancy appears around ∼425 °C, and above this temperature threshold the (100)-O-2 Vo facet is the most stable (navy blue line). Positions of the lines corresponding to the cationic vacancies indicate that the cation-depleted (100)-O-VCoT and (100)-O-VCoO terminations are highly disfavored in the whole range of the investigated temperatures. Moreover, the results show that formation of the VCoT vacancies is much more favored in comparison to VCoO, therefore, while analyzing defect equilibria of cobalt spinel the latter can safely be neglected. Upon passing to higher oxygen pressures (Figure 8b), the stability of the (100)-O facet extends to higher temperatures (375 °C at pO2/ p° = 10−2), and finally at high oxygen pressure, it becomes the most stable in the whole investigated temperature range (Figure 8c). At medium pO2 values, in the temperature window of 370 to 500 °C, the (100)-O-Vo facet is the most stable. However, since the difference in the surface energies between the (100)-O and both reduced facets is not significant, all terminations may actually coexist. Above 500 °C, the spinel surface with two oxygen vacancies becomes the most stable. To summarize the obtained results in a concise way, for the (100) surface reduced by formation of the oxygen vacancies or oxidized by extraction of the cobalt cations, the most stable terminations are presented in the form of 3D plots, γ vs T and pO2 (Figure 9, panels a and b, respectively). In the case of oxygen vacancies (Figure 9a), at low O2 pressures and in high temperatures the reduced terminations are situated below the (100)-S surface, and since their slopes are quite steep, they shall not coexist. The lowest γ value in these regions corresponds to the double O vacancy plane. In the case of the Co depleted surfaces (Figure 9b) in the whole range of the examined T, and pO2 values, the octahedral vacancies are thermodynamically disfavored. The (100)-O and (100)-O-VCoT surfaces are more stable than the (100)-S plane in the more oxidizing environment only. Moreover, since their slopes are similar, they may concur in such conditions. The redox state diagram of the (100) surface can be at best examined using the joint 2D projection of the lowest surface free energies, taken from Figure 9 (panels a and b) on the (T, p) plane (Figure 9c). The largest area in the center is covered by the preponderant (100)-S termination. Moving along the diagonal direction toward reducing conditions (lower pO2 and higher T), we pass a sequence of terminations with the gradually increasing number

Figure 9. Collective 3D diagram of the surface free energies versus T and pO2 for the (100) terminations of Co3O4 depleted of (a) oxygen anions and (b) cobalt cations together with the two-dimensional resultant surface redox state diagram for the (100) facets (c).

of anionic vacancies, corresponding to Co3O3.88 ((100)-SVO2O,1T) and then to Co3O3.75 ((100)-S-2 VO) nonstoichiometry. The black dotted lines show the shift of the transition boundaries upon incorporation of the surface created vacancies into the bulk. In the opposite direction, the (100)-S termination is replaced by the (100)-O termination (Co2.87O4) and in the most oxidizing conditions, the (100)O-VCoT facet (Co2.75O4) becomes the most stable. As implied by the plot, reduction of the (100)-S surfaces via lattice oxygen release at pO2/p° < 0.01 occurs above 550 °C, in a good agreement with the experiment (vide infra Figure 10). At lower temperatures and medium O 2 pressures, the revealed coexistence of the (100)-S and (100)-O termination implies the presence of the cobalt vacancies in tetrahedral sites. This is in line with experimentally found Co/O ratio >1.33 for the samples calcined at temperatures below 450 °C (see Figure 11). 4.4. Comparison with Experimental Data. The calculated free enthalpies of the lattice oxygen extraction from the cobalt spinel can be translated into the continuous changes of its stoichiometry, Co/O = 3/(4 − x), as a function of temperature, where x is described by the equilibrium: Co3O4 ↔ Co3O4−x + (VO)x + (x/2)O2. Figure 10 shows the plot of the corresponding Co/O ratio versus T for pO2 = 0.01 atm, calculated using the data derived from the analysis of Figure 6b. The values of the equilibrium constants, Ki, describing the sequential generation of given type of oxygen vacancies and their coexistence, were calculated as Ki = exp[(ΔGi − ΔGi+1)/ I


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not only the temperature threshold of the lattice oxygen liberation (∼450 °C) remains in excellent accordance with the theoretical predictions but also the overall profile is quite wellreproduced. In Figure 11, thermogravimetric profiles used for determination of the cobalt spinel stoichiometry are shown for the samples calcined at 400 and 700 °C at pO2/p° ∼ 0.2. The nonstoichiometry of both samples was determined upon the weight-loss measurement assuming that above 875 °C, a stoichiometric CoO is produced in both cases. For the sample calcined at 700 °C, the cobalt spinel exhibits a stoichiometric composition (i.e., the O/Co ratio of 1.333) until 800 °C where dramatic decomposition into CoO takes place. For the sample calcined at 400 °C, the TG profile is more involved, and three regions can be distinguished (blue line). The region (a) corresponds to the desorption of the oxygen and hydroxyl adspecies (accomplished at ∼300 °C), leaving a bare 100-O termination [region (b)]. The latter is stable until 400 °C, where release of the lattice oxygen begins [region (c)] and continues until the stoichiometric composition of cobalt spinel is reached at ∼650 °C. This indicates clearly that sample calcined at 400 °C is nonstoichiometric and exhibits an excess of oxygen. Although, strictly speaking, this experiment probes the whole spinel sample, it is worth noting that the results remain in nice agreement with the predictions implied by Figure 9c, where at ambient pressures, low temperatures (below 300 °C) favor a nonstoichiometric spinel with cationic vacancies Co2+(1−x)Vx[Co3+2]O4 (the dark red area). Calcination at higher temperatures (600−800 °C) results in the formation of a stoichiometric spinel (dark green area), as it was actually observed (orange line). Summarizing, the obtained results reveal that the redox chemistry of cobalt spinel is a complex phenomenon, where the octahedral trivalent cations (CoO) play a key role as electron shuttling centers, as it has been proposed already based on experimental data.13 They also provide a suitable, comprehensive background for molecular in-depth understanding of redox properties in relation to defect structure of the Co3O4 catalyst.

Figure 10. Evolution of Co/O ratio as a function of temperature for cobalt spinel calculated for pO2/p° = 0.01 on the basis of the presence and coexistence of the most stable terminations shown in Figure 6b. The inset shows the corresponding experimental TPD-O2 profile. The asterisk shows the desorption peak of the oxygen associated with the CoT2c sites not discussed in this work.

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CONCLUSIONS Surface redox state diagram for various terminations of the (100) facet of cobalt spinel was constructed for the stoichiometry range of Co2.75O4 to Co3O3.75, using spin unrestricted periodic DFT-PW91+U calculations joint with first-principles thermodynamics. It was shown that the stoichiometric, (100)-S, surface is the most stable one in the temperatures and pressures typical for the catalytic processes (T ∼ 200 °C to ∼500 °C, pO2/p° ∼ 0.001 to ∼1). In the reducing conditions (T > 600 °C and pO2/p° < 0.0001), the (100)-S termination becomes defected by lattice oxygen release, whereas in the oxidizing conditions (T < 200 °C and pO2/p° > 10), coexistence of the (100)-S and (100)-O terminations was revealed. With the rising oxygen pressure the (100)-O termination becomes defected due to the presence of the cationic vacancies in the tetrahedral sites. The stability of the off-stoichiometric terminations, (100)-SOff and (100)-OOff, with the surface tetrahedral cobalt ion that moved into the adjacent empty octahedral 16c positions (B′) is distinctly lower in comparison with the parent (100)-S and (100)-O planes, in the whole range of the thermodynamic conditions, even upon inclusion of the configurational entropy contribution. Formation of the oxygen vacancy leads to reduction of two

Figure 11. Thermogravimetric profiles of two Co3O4 samples calcined at 400 and 700 °C used to determine the spinel stoichiometry by the weight-loss analysis.

(RT)]. In this equation, the subscript “i” refers to the number of oxygen vacancies present in the relevant (100)-S, (100)-SVO2O,1T, and (100)-S-2 VO terminations (0, 1, and 2, respectively). The actual concentration of the oxygen vacancies generated at given temperature was calculated as a weighted average of the vacancies corresponding to each of the considered terminations. As can be seen in Figure 10, the onset of the lattice oxygen desorption from the (100) plane appears at ∼450 °C. From this point on, the Co/O ratio rises quite steeply to reach the value of 0.775 at 500 °C, which corresponds to dominance of the (100)-S-VO2O,1T surface with one oxygen vacancy. This facet is stable to 600 °C, where the generation of the next vacancy, associated with the appearance of the (100)-S-2 VO termination, gradually develops, raising the Co/O ratio to 0.800. The dotted line shows the extrapolation of this process to higher levels of the cobalt spinel reduction (not accounted for by our calculations explicitly). The shape of the calculated profile remains in remarkable agreement with the experimental profile of oxygen release with the rising temperature. As can be inferred from the inset of Figure 10, J


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(4) Zasada, F.; Gryboś, J.; Indyka, P.; Piskorz, W.; Kaczmarczyk, J.; Sojka, Z. Surface Structure and Morphology of M[CoM′]O4 (M = Mg, Zn, Fe, Co and M′ = Ni, Al, Mn, Co) Spinel Nanocrystals − DFT+U and TEM Screening Investigations. J. Phys. Chem. C 2014, 118, 19085−19097. (5) Xie, X.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W. Low-temperature Oxidation of CO Catalysed by Co3O4 nanorods. Nature 2009, 458, 746−749. (6) Grillo, F.; Natile, M. M.; Glisenti, A. Low Temperature Oxidation of Carbon Monoxide: The Influence of Water and Oxygen on the Reactivity of a Co3O4 Powder Surface. Appl. Catal., B 2004, 48, 267− 274. (7) Petryk, J.; Kolakowska, E. Cobalt Oxide Catalysts for Ammonia Oxidation Activated with Cerium and Lanthanum. Appl. Catal., B 2000, 24, 121−128. (8) Schmidt-Szałowski, K.; Krawczyk, K.; Petryk, J. The Properties of Cobalt Oxide Catalyst for Ammonia Oxidation. Appl. Catal., A 1998, 175, 147−157. (9) Finocchio, E.; Busca, G.; Lorenzelli, V.; Escribano, V. S. FTIR Studies on the Selective Oxidation and Combustion of Light Hydrocarbons at Metal Oxide Surfaces. Part 2. - Propane and Propene Oxidation on Co3O4. J. Chem. Soc., Faraday Trans. 1996, 92, 1587− 1593. (10) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Deganello, G. Catalytic Performance of Co3O4/CeO2 and Co3O4/CeO2-ZrO2 Composite Oxides for Methane Combustion: Influence of Catalyst Pretreatment Temperature and Oxygen Concentration in the Reaction Mixture. Appl. Catal., B 2007, 70, 314−322. (11) Tyo, E. C.; Yin, C. R.; Di Vece, M.; Qian, Q.; Kwon, G.; Lee, S.; Lee, B.; DeBartolo, J. E.; Seifert, S.; Winans, R. E.; et al. Oxidative Dehydrogenation of Cyclohexane on Cobalt Oxide (Co3O4) Nanoparticles: The Effect of Particle Size on Activity and Selectivity. ACS Catal. 2012, 2, 2409−2423. (12) Liu, Y.; Higgins, D. C.; Wu, J.; Fowler, M.; Chen, Z. Cubic Spinel Cobalt Oxide/multi-Walled Carbon Nanotube Composites as an Efficient Bifunctional Electrocatalyst for Oxygen Reaction. Electrochem. Commun. 2013, 34, 125−129. (13) Stelmachowski, P.; Maniak, G.; Kaczmarczyk, J.; Zasada, F.; Piskorz, W.; Kotarba, A.; Sojka, Z. Mg and Al Substituted Cobalt Spinels as Catalysts for Low Temperature deN2O-Evidence for Octahedral Cobalt Active Sites. Appl. Catal., B 2014, 146, 105−111. (14) Maniak, G.; Stelmachowski, P.; Kotarba, A.; Sojka, Z.; RicoPérez, V.; Bueno-López, A. Rationales for the Selection of the Best Precursor for Potassium Doping of Cobalt Spinel Based deN2O Catalyst. Appl. Catal., B 2013, 136−137, 302−307. (15) Inger, M.; Kowalik, P.; Saramok, M.; Wilk, M.; Stelmachowski, P.; Maniak, G.; Granger, P.; Kotarba, A.; Sojka, Z. Laboratory and Pilot Scale Synthesis, Characterization and Reactivity of Multicomponent Cobalt Spinel Catalyst for Low Temperature Removal of N2O From Nitric Acid Plant Tail Gases. Catal. Today 2011, 176, 365−368. (16) Ding, Y.; Xu, L.; Chen, C.; Shen, X.; Suib, S. L. Syntheses of Nanostructures of Cobalt Hydrotalcite Like Compounds and Co3O4 Via a Microwave-assisted Reflux Method. J. Phys. Chem. C 2008, 112, 8177−8183. (17) Feng, J. H.; Zeng, C. Size-controlled Growth of Co3O4 nanocubes. Chem. Mater. 2003, 15, 2829−2835. (18) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Heterogeneous Catalytic Decomposition of Nitrous oxide. Appl. Catal., B 1996, 9, 25− 64. (19) Liu, X.; Qiu, G.; Li, X. Shape-controlled Synthesis and Properties of Uniform Spinel Cobalt Oxide Nanocubes. Nanotechnology 2005, 16, 3035−3040. (20) Montoya, A.; Haynes, B. S. Periodic Density Functional Study of Co3O4 Surfaces. Chem. Phys. Lett. 2011, 502, 63−68. (21) Zasada, F.; Piskorz, W.; Cristol, S.; Paul, J.-F.; Kotarba, A.; Sojka, Z. Periodic Density Functional Theory and Atomistic Thermodynamic Studies of Cobalt Spinel Nanocrystals in Wet Environment: Molecular Interpretation of Water Adsorption Equilibria. J. Phys. Chem. C 2010, 114, 22245−22253.

octahedral cobalt cations. It is accompanied by migration of CoT into interstitial octahedral positions B′ and uneven spin relaxation of the adjacent CoO5c ions. The CoO5c cation proximal to the interstitial CoB′ adopts a low spin configuration in contrast to the distal one which preserves its surface high spin state. In the case of the Co depleted surfaces, the octahedral vacancies are thermodynamically disfavored with respect to tetrahedral ones in the whole range of the examined T and pO2 values. The obtained results correlate well with the experimental TPD-O2 and thermogravimetric data and provide a comprehensive molecular background for interpretation of redox properties in relation to defect structure of the cobalt spinel catalyst.

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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.5b05136. Cobalt spinel terminations nomenclature, detailed picture of the employed slab models; spinel sample characteristic: Raman spectra and XRD diffraction patterns; off-stoichiometric (100)-SOff and (100)-OOff models; and configurational entropy plot (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +48 12 663 20 73. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the financial support of Polish National Science Center Grant DEC-2011/03/B/ST5/01564. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract POIG.02.01.00-12023/08). The authors thank Dr. Janusz Janas and Mr. Jan Kaczmarczyk for their help in the experiments.

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ABBREVIATIONS (100)-O, oxidized termination; (100)-S, stoichiometric termination; (100)-R, reduced termination; BG, band gap; DFT+U, density functional theory with Hubbard corrected functionals; GGA, generalized gradient aproximation; TPD-O2, temperature-programmed oxygen desorption; TG, thermogravimetric studies; VO, oxygen vacancy; VCo, cobalt vacancy

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Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into

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