Ethane Activation by Nb-Doped NiO - The Journal of Physical

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Ethane Activation by Nb-Doped NiO XiaoYing Sun,‡ Bo Li,‡ and Horia Metiu*,† †

Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States ‡ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China ABSTRACT: We use density functional theory to examine the dissociative adsorption of ethane on the surface of Nbdoped NiO. We find that the Nb dopant that substitutes a Ni atom in the surface layer of NiO adsorbs O2 from gas phase and binds it so strongly that the two oxygen atoms are no longer good oxidants. It is more reasonable to consider that the dopant is the NbO2 group. We show that this group acts in many ways like a lower-valence dopant and activates the surface oxygen atoms near it.

1. INTRODUCTION Ethylene is produced by steam cracking of naphtha or of natural gas condensates. This is a demanding process taking place at high temperature (between 750 and 875 °C) and short contact time, resulting in products that are difficult to separate. The preparation of ethylene by catalytic oxidative dehydrogenation (ODH) is an appealing alternative.1−66 The topic has been reviewed in several articles67−71 and an excellent monograph,72 which conclude that in spite of much research and some progress, no ethane ODH catalyst is ready for commercial application. Recent publications examined ethane ODH catalyzed by NiO doped with Nb,46,50,62,66,73 or with Zr,74 or with Ce,65 or with Li, Mg, Al, Ga, Ti, Nb, or Ta;63 the Nb-doped NiO appears to be the more promising. In this article we present results of calculations, using density functional theory (DFT), whose aim is to gain some understanding of how the Nb dopant affects the reactivity of NiO. It is widely believed that the rate-limiting step in ethane ODH is the breaking of the C− H bond, and for this reason one can learn much by examining how the presence of Nb affects the activation energy for this process. In our calculations we use a slab of NiO with a Nb atom substituting a Ni atom in the surface. It is impossible to reliably compare the results of such calculations to experiments since we do not know the morphology or the composition of the surface under the reaction conditions. We have no experimental methods that can test whether the prepared catalysts are similar to the model used in the computations. In particular, we have no probes that will guarantee that a doped oxide with the dopant in the surface layer has been synthesized. Given the uncertain connection between the model and the prepared catalysts, we confine ourselves to asking qualitative, general questions. (1) Does a higher-valence catalyst adsorb and activate oxygen? Our calculations say yes, and the margin of error is so large (i.e., the effect of the dopant is so large) that © 2013 American Chemical Society

this qualitative statement is correct no matter what the DFT errors are or whether we include zero-point energies. (2) We also discovered that the adsorption of the oxygen on the dopant is so large that the adsorbed O2 is very stable and will not adsorb dissociatively ethane to form a hydroxide and an ethoxide with the adsorbed O atoms. Because of this, one has to consider that the dopant is NbO2, not Nb. Since the excess electrons of Nb are not tied up in bonds with oxygen, the NbO2 becomes a lower-valence dopant. This is an unexpected qualitative conclusion that is independent of the accuracy of DFT and should be considered whenever higher-valence dopants are examined. (3) Every ODH experiment has found that all steps following the breaking of the C−H bond are so fast that none of their rates can be measured. The number of such steps is very large (we need to consider every possible oxidation product), and DFT does not have the accuracy needed to give a credible conclusion regarding the mechanism, nor is the slab model realistic enough to justify such an extensive computational effort. Such calculations will be of no help to the experimentalists since they do not give any hint as to how to improve performance. Because of all these considerations, it is much easier and more efficient to study selectivity experimentally, and we will not perform such studies here. Previous calculations suggest that some of the chemistry of the doped surface depends on the difference between the valence of the dopant and that of the cation it substitutes. Nb makes three stable oxides75NbO, NbO2, and Nb2O5 among which Nb2O5 is the most stable. We expect that two of Nb’s electrons are likely to replace the two electrons that the Ni atom, which Nb substituted, supplied to the oxide. This leaves three valence electrons on Nb, since neither the Ni cations nor Received: March 26, 2013 Revised: September 30, 2013 Published: November 4, 2013 23597

dx.doi.org/10.1021/jp402980d | J. Phys. Chem. C 2013, 117, 23597−23608

The Journal of Physical Chemistry C

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the O anions will take them. As a result, Nb is a strong Lewis base, and we expect it to react strongly with any Lewis acid.76 In particular, it is likely to bind oxygen very strongly. There are then two possibilities. (1) The adsorbed O2 received electrons from the Nb, and this weakens the O−O bond, making the molecule reactive. Ethane might bind dissociatively to these O atoms. (2) The O2 binds so strongly to Nb that it becomes an unreactive intermediate (the Sabatier principle). In this case, it is better to regard the NbO2 group as the real dopant. This group, which substitutes a Ni atom, has only one electron available for bonding to the oxide instead of the two that the Ni provided. Therefore, the formation of an NbO2 group creates a deficit of electrons in the oxide: the NbO2 is a lower-valence dopant (LVD). We know from previous work77−97 that an LVD affects the oxygen atoms nearby and makes them more reactive; in particular, they will adsorb ethane and break the C− H bond. The density functional calculations presented here tell us that the second mechanism is correct.

2. COMPUTATIONAL METHOD All calculations were performed by using periodic, spinpolarized, density-functional theory (DFT) with the Hubbard U correction, as implemented in Vienna ab initio program package (VASP). We used the value of U = 6.3 eV proposed by Rohrbach, Hafner, and Kresse,98,99 who showed that the method gives correct values for the binding energies of CO and NO measured by Freund and Kuhlenbeck.100,101 The use of GGA+U is necessary for removing some of the self-interaction error in DFT which causes the d-orbitals on Ni to be too delocalized.98,99 The electron−ion interactions are described by the projector augmented wave (PAW) method proposed by and implemented by Kresse. We used the PBE functional and a plane wave basis set with an energy cutoff of 330 eV. The NiO(011) slab had four atomic layers and a 15 Å vacuum region. A 4 × 4 surface cell was employed throughout the calculations, and each atomic layer contains 16 Ni atoms and 16 O atoms. Only the gamma point was used in the k-mesh sampling. During structure optimization, all ions in the unit cell were allowed to relax, except for the bottom layer in which the atoms were fixed at bulk positions. No symmetry was imposed during optimization, which was stopped when the force on the atoms was smaller than 0.02 eV/Å. The activation energy was calculated with the nudged elastic band (NEB) method. During the calculation of the reaction path the spin polarization of the system was held constant, following a suggestion made in previous work.102

Figure 1. Structure of the NiO(011) surface with a dopant in the top Ni layer, seen (a) from above and (b) from the side. The oxygen in the top layer is red, Ni is green, the oxygen in the second layer is yelloworange, and the dopant is gray. For later reference we label with B the oxygen atom bound to the dopant and with A the oxygen atom one site away.

small. Calculations in Pacchioni’s group106 showed that the removal of a Ni atom from the NiO(100) surface creates a hole localized on an oxygen atom, converting it (formally) from O2− to O−. It is likely that these holes (which are Lewis acids) are chemically compensated92 by adsorption of a Lewis base (such as H), but this possibility has not been examined. In the absence of such compensation, Ni vacancies act as lowervalence dopants and will increase the reactivity of the oxygen atoms near them. We have ignored the possibility that Ni vacancies (compensated or not) are present on the surface and calculated the energy of the dissociative adsorption of ethane (DAE) on vacancy free NiO(011). We did that because the Nb is likely to fill the Ni vacancies and also because Nb will donate two electrons to the holes created by the remaining vacancies (unfilled by Nb), neutralizing thus their acidity and diminishing substantially their chemical activity. We hope to study the chemistry of NiO with Ni vacancies in future work. We examined the (011) face because NiO cleaves extremely easily to form the (001) face,107 which suggests that (001) is not a reactive surface. We calculated the energy of the DAE reaction for four final states: both fragments bind to O atoms (Figure 2a); the ethyl binds to Ni and H to O (Figure 2b); the ethyl binds to O and H to Ni (Figure 2c); both fragments bind to Ni. The reaction energies and the distance between the C atom in ethyl and the H atom bound to the surface are given in Table 1. We have examined four different binding sites for the dissociation fragments. The dissociative adsorption energy to produce O−C2H5 and O−H is equal to that for producing Ni− C2H5 and O−H. The formation of Ni−C2H5 and Ni−H is very much uphill. This emphasizes the fact that the chemistry of Ni ions in the oxide is very different from the chemistry on metallic Ni; the latter binds strongly both H and C2H5.

3. PROPERTIES OF NiO If calcined at ∼700 °C in oxygen, during preparation, NiO is green; if the calcination temperature is ∼450 °C, NiO is black and has a NaCl structure (see Figure 1) with a deficit of Ni atoms.103 It is the black NiO that has been used so far as a catalyst. The absence of one Ni atom (i.e., the presence of one Ni vacancy) deprives the oxide of two electrons, which means that two holes exist in the electronic structure. As a result, the material is a pconductor.104,105 It has been suggested103 that these holes are either localized on Ni and convert a Ni2+ into a Ni3+ or localized on oxygen and convert it to O−. While Ni2O3 exists, it is a very strong oxidant,75 and the driving force to make Ni3+ is 23598



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The activation energy for the DAE reaction leading to the lowest energy final state (Figure 2a) is ∼2.00 eV, and the reaction path is shown in Figure 3.

Figure 3. Reaction path for the dissociative adsorption of ethane on NiO(011) leading to the final state shown below the NEB graph (same as Figure 2a). The initial state is ethane in the vacuum, far enough from the surface that it does not interact with it.

This is not consistent with experiments50,63 that find that the conversion of ethane, in the presence of oxygen, starts at ∼300 °C. Comparison with experiment is uncertain because the experiments find that ethane conversion in ODH catalyzed by NiO depends on the method of preparation.46,50,62,63,66,74 Possible reasons for discrepancy are the difference in surface morphology between experiments and the model, possible influence of adsorbates, and possible contamination of the surface with dopants present as impurities in the precursors. We are interested here in NiO mostly to compare it with Nbdoped NiO in the hope that this will help us understand the role played by the dopant.

Figure 2. Configuration for the binding sites of the fragments of ethane dissociation on NiO(011). (a) The most stable configuration: a hydroxyl and an ethoxide are formed on the oxygen atoms in the same row on the surface. (b) The second most stable configuration: H makes a hydroxyl and the ethyl is bound to a Ni atom. (c) The third most stable configuration: an ethoxide and a hydrogen bridging two Ni atoms. The oxygen atoms are red, the Ni atoms in the top layer are green, and the Ni atoms in the second layer are blue.

Table 1. Energy of Dissociative Adsorption of Ethane on NiO(011) in eVa dissociative fragment

4. Nb-DOPED NiO 4.1. Valence of the Nb Dopant. We give first a few definitions. In what follows the valence of a dopant is the valence the dopant has in its own oxide. For example, Li is monovalent because it has only one stable oxide, namely Li2O (the peroxide Li2O2 exist, but Li is monovalent in it). Al is trivalent because the only stable oxide is Al2O3. Li in Li-doped NiO is a lower-valence dopant (LVD) since a monovalent cation (Li) replaces a divalent one (Ni). Al in Al-doped NiO is a higher-valence dopant (HVD). Dopants having a different valence from the cation they replace change the Lewis acid− base properties of the doped surface, and this has a strong influence on the chemistry of the surface.76 The LVDs make the surface a Lewis acid, and this means that the doped oxide strongly adsorbs a Lewis base. The HVDs make the surface a

NiO(011)

C2H5 site

H site

RC−H

ΔEd

O O Ni Ni

O Ni O Ni

3.22 3.02 2.63 4.37

−0.51 −0.20 −0.50 1.99

Each row corresponds to a different pair of binding sites. In the first row, the ethyl binds to a surface oxygen atom and the hydrogen binds to another surface oxygen. In the second row, the ethyl binds to oxygen and H binds to a Ni atom, etc. RC−H is the distance (in Å) between the carbon atom in the adsorbed ethyl and the adsorbed hydrogen atom given in Å. a

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doping with Nb lowers the ability of NiO to act as an oxidation catalyst through a Mars−van Krevelen mechanism.110−112 4.3. O2 Adsorption on the Nb Dopant. In previous work,113 we have shown that in the case of Al-doped or Tidoped ZnO, these higher-valence dopants adsorb an O2 molecule from gas phase and activate it. Since the Nb dopant has three “unused” electrons localized in its vicinity, it is a very strong Lewis base, and we expect it to bind gas-phase oxygen very strongly.76 The calculations that follow address two questions: how strongly is O2 bonded to the dopant, and what is the reactivity of the adsorbed oxygen? The calculations show that O2 binds to the doped surface as shown in Figure 4.

Lewis base, and this means that the doped surface strongly adsorbs a Lewis acid.76 It is not a simple matter to apply these concepts to a Nb dopant because Nb makes three stable oxides:75 NbO, NbO2, and Nb2O5. Therefore, it is not clear what valence to attribute to the Nb dopant in the NiO host. The enthalpy of formation for various Nb oxides is (per mole of Nb) −950.5 kJ/mol (for Nb2O5), −797 kJ/mol (for NbO2), and −406 kJ/mol (for NbO). These values suggest that Nb2O5 is the more stable oxide. Two other facts reinforce the assumption that Nb prefers to be pentavalent. Hydrogen reduces108 Nb2O5 to NbO2 at a temperature between 800 and 1000 °C and to NbO at 1200 °C. Similarly, the reduction of Nb2O5 with carbon108 produces NbO2 at 900 °C and NbO at 1200 °C. The need for such a high temperature for these reduction reactions suggests that Nb prefers to be pentavalent. Therefore, the Nb dopant in NiO is able to provide five electrons for making chemical bonds. Formally, the host nickel oxide uses two Nb electrons to replace the two electrons that the substituted Ni atom contributed. This means that the Nb dopant has three electrons available for making additional chemical bonds, which makes Nb a very strong Lewis base. This explains many of the qualitative results of the calculations that follow. 4.2. Effect of Doping on the Energy of OxygenVacancy Formation. The energy of oxygen-vacancy formation, ΔEv, is defined by 1 ΔEv = E(Sv ) + E(O2 (g)) − E(S) 2 where E(Sv) is the energy of the surface having an oxygen vacancy per supercell, E(S) is the energy of the stoichiometric surface calculated with the same supercell, and E(O2(g)) is the energy of a gaseous O2 molecule. In all calculations reported here the vacancy is made by removing from the surface layer one oxygen atom per supercell. There are two kinds of oxygen atoms in the top layer: the one labeled A is not a neighbor of the dopant, and the one labeled B is bound to the dopant. We expect that the energy needed for removing the oxygen atom A differs from that needed for removing B. The calculated values of ΔEv are given in Table 2. We are unable to explain

Figure 4. Structure of a Nb-doped NiO(011) surface with an O2 molecule adsorbed and dissociated to form bonds with Nb and neighboring Ni atoms. The labels 1, 2, A, B, and C are for reference in the text. The oxygen atoms are red, the Ni atoms in the top layer are green, the Ni atoms in the second layer are blue, the two adsorbed oxygen atoms are dark blue, and the dopant is gray.

The distance between the two adsorbed O atoms (blue in Figure 4) is 3.02 Å, which indicates that the chemisorbed O2 molecule is dissociated. The O−Nb distance is 1.85 Å and the O−Ni distance is 1.85 Å, which means that the oxygen atoms formed by the dissociation of the O2 molecule are bonded to Nb and to Ni. The adsorption energy is −8.40 eV, which is an extremely large value for oxygen adsorption. It is conceivable that other more complex geometries are formed upon O2 adsorption, but we did not perform a thorough search for them. To determine qualitatively whether one of these adsorbed oxygen atoms can function as an oxidant, we calculated the energy of the reaction 1 NbO2 NiO → NbONiO + O2 (g) 2 where NbO2NiO(011) is the system shown in Figure 4 and NbONiO(011) is the system shown in Figure 5. The energy of this reaction is 4.12 eV. This means that any oxidation reaction that uses one of the adsorbed oxygen atoms shown in blue in Figure 5 must be exothermic by about 4.12 eV. It is therefore unlikely that these atoms will function as oxidants. 4.4. NbO2 as a Dopant. While the oxygen atoms bound to Nb are not good oxidants, they still have a strong influence on the chemistry of the surface. In the absence of the adsorbed

Table 2. Calculated Values of ΔEva O site

ΔEv (eV)

ΔEH (eV)

OA OB O

2.31 2.67 3.28

−0.93 −0.47 −0.78

The first column gives the energy ΔEv of oxygen-vacancy formation by removing an oxygen atom from the sites OA or OB (defined in Figure 4) for NiO doped with NbO2 (in rows 2 and 3). The last row gives ΔEv for the undoped NiO. The last column gives the energy ΔEH of the reaction 1/2H2(g) + S → H/S, where S is the surface (doped or undoped) and H/S is the surface with a H atom adsorbed to make a hydroxyl. a

qualitatively the dependence of ΔEv on the nature of the dopant. We note that Li, which is a lower-valence dopant, lowers ΔEv for both oxygen atoms while Nb increases ΔEv(OB) and leaves ΔEv(OA) practically unchanged. This seems to be a general phenomenon: an LVD decreases the energy of vacancy formation for both oxygen atoms, and an HVD increases the energy of oxygen-vacancy formation next to the dopant if the host oxide is irreducible.78,109 An increase in ΔEv indicates that 23600



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site B on NbO2-doped NiO even though it is easier to make an oxygen vacancy at B. We did not expect that. The rules on which these expectations are based assume that the chemistry of the system is dominated by acid−base interactions (the interaction between an electron donor and an electron acceptor). Figure 6 shows that the binding of H to an oxygen is concomitant with breaking a bond between that oxygen atom and a cation (Ni in the case of oxygen A and Nb in the case of oxygen B), and this results in a fairly large deformation of the structure. The effect of the relaxation energy is not included in arguments based on charge transfer. Furthermore, the Ni atom

Figure 5. Structure of a Nb-doped NiO(011) surface with an O atom adsorbed and forming bonds with Nb and a neighboring Ni atom. The labels are for reference in the text. The oxygen atoms are red, the Ni atoms in the top layer are green, the Ni atoms in the second layer are blue, and the adsorbed oxygen atom is dark blue.

oxygen, Nb has three valence electrons that are not used to form chemical bonds. If we assume that each oxygen atom bonded to Nb ties up two electrons, the NbO2 group has only one electron to offer to the oxide. Therefore, if we think that NbO2 is the dopant (rather than Nb), then we suspect that it acts as lower-valence dopant (LVD): it replaces a Ni atom that can donate two electrons to the oxide (formally), with a NbO2 group that donates only one (formally). We can verify whether the conclusion reached by this simple argument is valid as follows. If NbO2 is an LVD, then it should lower the energy of oxygen-vacancy formation. Our calculations show that this is the case (Table 2). The energy required for removing the oxygen atom marked B in Figure 4 is 2.67 eV, and that for removing atom A is 2.31 eV. Compare this to Ev for the Ni(011) surface doped with a typical LVD such as Li, for which Ev(OA) is 2.65 eV and Ev(OB) is 2.24 eV. The inequivalence between the two oxygen atoms in this system is also reflected by the Bader charges on them. The oxygen atom B gains 1.17 electron (compared to a neutral O atom), A gains 1.08 electron, and one of the adsorbed atoms (blue in Figure 4) gains 0.95 electron. NbO2 affects the oxygen A more strongly than B, while the trend is reversed for Li-doped surface. This is likely to happen because atom B is nearest to the Nb dopant and Nb makes strong bonds with oxygen. Based on these results, it seems reasonable to accept the suggestion that NbO2 is an LVD. We caution however that because of the complexity of the system, the NbO2 dopant does not behave in all respects as a typical LVD (Li, for example). For most LVDs studied so far, there is a correlation between the energy needed for removal of an oxygen atom and the ability of that atom to bind a hydrogen atom. Our calculations (see Table 2) show that the situation is more complicated in the system studied here. The binding energy of H to the oxygen atom B is smaller than the binding energy of H to the atom A, which is what we expect since it is easier to make an oxygen vacancy on A. The fact that H binds more strongly to site A (on NbO2-doped NiO) than on NiO is also what we expect since it is easier to make a vacancy on A. However, the binding energy of H to the oxygen atom B does not follow the rules: H binds more strongly to NiO than to the

Figure 6. On the NiO(011) surface, hydroxyl formation causes the breaking of the Ni−O bond and a large displacement of the Ni atom which is no longer bonded to one of the oxygen atoms. (a) The structure of the NiO(011) surface after the adsorption of a hydrogen atom to form a hydroxyl. (b) The structure of the NiO(011) surface doped with a NbO2 group, after the adsorption of a hydrogen atom to form a hydroxyl on the oxygen atom A. (c) The structure of the NiO(011) surface doped with a NbO2 group, after the adsorption of a hydrogen atom to form a hydroxyl on the oxygen atom B. The oxygen atoms are red, the Ni atoms in the top layer are green, the Ni atoms in the second layer are light blue, the two O atoms in the NbO2 group are dark blue, and the hydrogen atom is white. 23601



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bound to the oxygen atom A is also bound to one of the blue oxygen atoms that forms the NbO2 group. The presence of such a bond weakens the bond of the Ni with oxygen atom A, and this is why that oxygen atom is more reactive than oxygen atom B that is bonded to Nb. An unexpected behavior (as compared to a surface doped with an atomic LVD) is also observed for O2 adsorption energy on the NbO2-doped NiO, which is larger than the binding energy of O2 to the undoped oxide (−0.97 eV for the doped oxide and −0.70 eV for the undoped one). The geometry of the O2 adsorbates in these systems is very similar (Figure 7), so the

Figure 7. Structure obtained when O2 (black) is adsorbed on NiO(011) doped with NbO2. The O2 molecule binds to two Ni atoms. The color code is the same as in the previous figures. Figure 8. One of the final states for the fragments of ethane dissociative adsorption on NiO(011) doped with NbO2: (a) side view; (b) top view. The labels are the same as in Figure 4. The two carbons in ethyl are gray, the hydrogen atoms are white, the two oxygen atoms from NbO2 are dark blue, the surface oxygen atoms are red, and the surface Ni atoms are green. The dissociation forms an ethoxide with an oxygen atom of NbO2 and a hydroxyl with the surface oxygen atom B.

difference in the binding energy does not come from a different binding scheme. This is not what we expect for an ordinary LVD, which turns the surface into a Lewis acid and therefore will not increase the binding energy a Lewis acid such as O2. The qualitative conclusion is that the NbO2 dopant behaves like an LVD in some respects but not in others. In what follows we denote this surface by NbO2NiO, and we study its ability to adsorb ethane dissociatively. If NbO2 behaves like an LVD, as far as the dissociative adsorption of ethane is concerned, then the energy of this reaction will be larger on NbO2NiO than on NiO.

Table 3. Dissociation Adsorption of Ethane on NiO(011) Doped with NbO2a 1 2 3 4 5 6 7 8 9 10

5. ETHANE ACTIVATION BY NiO(011) DOPED WITH NbO2 To find the energy of the dissociative adsorption of ethane on the Nb-doped NiO surface, we need to examine a large number of possible locations of the fragments (H and C2H5). One such location is shown in Figure 8, where H is bound to surface oxygen atom B and the ethyl is bound to the oxygen atom 2 (that belongs to the NbO2 dopant). We have examined ten such structures, which differ through the binding sites of C2H5 and H. Table 3 gives the energy of the dissociative adsorption, ΔEd, and the distance RC−H between the carbon atom in the ethyl and the hydrogen atom in the hydroxyl, for all binding sites of the fragments. For example, the information in row 5 of Table 3 refers to a final state in which the ethyl binds to the surface-oxygen atom B (see Figure 8) and the hydrogen binds to the oxygen atom labeled 2 (the labels are defined in Figure 4). The distance RC−H between the C atom in the ethyl and the hydrogen atom in the hydroxyl is 2.90 Å, and the energy ΔEd of the dissociative adsorption reaction to reach this state (i.e., the reaction NbO2NiO + C2H6(g) → the dissociated ethane shown in Figure 8) is −0.06 eV. In the absence of the adsorbates, the pair of sites (O1, OA) is identical to the pair (O2, OC) because of the symmetry of the surface (see Figure 4). Therefore, the energy of the reaction to form C2H5−O1 and H−OA should be equal to the energy of the

C2H5 site

H site

RC−H (Å)

ΔEd (eV)

O2 O2 OC O2 OB OC OB OA O Ni

OA OC O2 OB O2 OB OA OB Ni O

5.21 2.93 2.71 3.36 2.90 3.05 2.79 3.51 2.71 2.98

−1.06 −0.94 −0.72 −0.36 −0.06 −0.70 −1.01 −0.88 0.11 −0.59

The first column numbers the rows for easy reference. The second and the third columns specify the binding sites of C2H5 and H. The subscripts on the oxygen atoms correspond to the sites defined in Figure 4. ΔEd is the dissociation energy of ethane, leading to the configuration defined in the second and third columns. RC−H is the distance between the H in the hydroxyl and the carbon atom in the ethoxide. a

reaction to form C2H5−O2 and H−OC. There is however a small difference (ΔEd = −1.06 eV versus −0.94 eV). This difference exists because there are several local minima corresponding to various positions that the ethyl can have with respect to the surface atoms. Such variations are small and within the error of DFT. The activation energies for reaching four of the final states are collected in Table 4. The reaction paths calculated by the nudged elastic band (NEB) method are shown in Figures 9−12. 23602



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Table 4. Activation Energies for Reaching Four of the Final Statesa final state Figure

C2H5 site

H site

Ea (eV)

ΔEd (eV)

9 10 11 12

OB O2 OC OA

OA OC O2 OB

1.08 0.66 1.75 1.28

−1.01 −0.94 −0.72 −0.88

a Ea is activation energy for dissociative adsorption of ethane on NbO2/ NiO. The “final state” columns indicate the surface sites to which the fragments are bound. For example, in the row for Figure 11, OC indicates that C2H5 binds to the surface-oxygen atom labeled C in Figure 4 and the hydrogen atom binds to the oxygen atom 2 (an atom adsorbed on Nb). ΔEd is the energy of the dissociative chemisorption (reproduced from Table 3).

Figure 10. Reaction path obtained with the NEB method, leading to the final state shown: ethoxide on the O atom 1 (of NbO2) and hydroxyl on the surface oxygen atom A. The initial state is ethane in the vacuum, far enough that it does not interact with the NbO2/NiO surface.

6. DISCUSSION The present calculations conclude that Nb-doped NiO will adsorb very strongly O2 from gas and dissociate it to create two chemisorbed O atoms, each making Nb−O−Ni bonds. During the preparation of the Nb-doped NiO, the material is calcined at fairly high temperature. If Nb ions are mobile at the calcination temperature, they will segregate at the surface because their reaction with gaseous O2 will lower the energy substantially. Therefore, we expect that most Nb atoms in the precursor solution will end up in the surface layer in the form of NbO2 groups, each substituting a surface Ni atom. We have found that the ethane dissociates fastest by using either two oxygen atoms in the neighborhood of NbO2 (namely OA and OB) or by using one of the oxygen atoms that belongs to NbO2 and a surface oxygen (namely OC and O2). This suggests that it is essential that the atomic ratio of Nb to Ni in the precursor solution should be small. Otherwise, the surface will consist essentially of a very thin film of NbOx. Experiments in Millet’s group62 have shown that depositing a submonolayer of NbOx on the NiO surface lowers the activity substantially. We have not studied the possibility that the affinity of Nb for oxygen is so great that the Nb will move on top of the surface and form NbOx clusters supported on NiO. There are ten possible binding sites for the dissociation fragments (H and C2H5), and the dissociative adsorption reaction is exoergic for nine of them. The activation energies

Figure 9. Reaction path obtained with the NEB method, leading to the final state shown: ethoxide on the surface oxygen atom B and hydroxyl on the surface oxygen atom A. The initial state is ethane in the vacuum, far enough that it does not interact with the NbO2/NiO surface.

In many cases the activation energy is related to the binding energy in the final state by a linear relation (the Brønstead− Evans−Polanyi (BEP) rule).114−122 We have tested whether this is the case for the four activation energies calculated here, and the results are shown in Figure 13. The largest error between the linear fit and the “data points” is 9%. BEP is not a bad rule, if one wants to estimate activation energies from binding energies. 23603



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Figure 11. Reaction path obtained with the NEB method, leading to the final state shown: hydroxide on the O atom 1 (of NbO2) and ethoxide on the surface oxygen atom A. The initial state is ethane in the vacuum, far enough that it does not interact with the NbO2/NiO surface.

Figure 12. Reaction path obtained with the NEB method, leading to the final state shown: ethoxide on the surface oxygen atom A and hydroxyl on the surface oxygen atom B. The initial state is ethane in the vacuum, far enough that it does not interact with the NbO2/NiO surface.

for the dissociative chemisorption leading to four of these final states (see Table 4) are substantially lower than the activation energy for the dissociative adsorption on undoped NiO. This suggests that a Nb-doped surface prepared to resemble the model used here is more reactive than the undoped NiO. This conclusion assumes that the NbO2-doped oxide is stable under the reaction conditions (that the NbO2 group does not leave the Ni site) and that the amount of Nb at the surface is not too large. Ethane ODH by Nb-doped NiO has been studied in several excellent papers from the groups of Lemonidou,46,50,63,73 Millet,62 and Caps.66 As explained below, attempts to compare calculations with experiment face many difficulties. The experiments lack tools that could prove beyond doubt that a doped oxide has been prepared and that the dopant is in the surface layer. In addition, we do not know the morphology and the composition of the surface under reaction conditions. Therefore, one can only hope that the qualitative conclusions of the calculations are supported by experiments. In particular, we predict that the NbO2-doped surface is more effective in breaking the C−H bond than undoped NiO. Experiments often report the amount of ethane converted, by measuring the ethane concentration in the effluent and comparing it to the amount going into the reactor. Such measurements conclude that the doped oxide converts more ethane (per gram of catalyst) than the undoped one. Therefore, it appears that these

Figure 13. Least-squares fit with a linear function of the dependence of the activation energy for the dissociative adsorption reaction versus the energy of the dissociative adsorption in the final state of the reaction.

measurements agree with the conclusions of the calculations. However, such measurements cannot be used to compare NiO to NbO2-doped NiO because the presence of very small amounts of Nb increases substantially the area of a gram of catalyst. For this reason we prefer to use data that report the rate of ethane conversion (defined as the amount of ethane converted per unit area of catalyst, per unit time). Even this 23604



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quantity is not a property of the catalyst unless the reactor is differential (the measurements are made at very low conversion). Two papers have measured this rate for both NiO and Nb-doped NiO.62,66 Both found that even a small amount of Nb decreases the ethane conversion per unit area and unit time. The reasons for the difference between predictions and experiments are not clear. In some cases one detects the presence of an amorphous Nb2O5 phase, and/or of NiNb2O6, and of a carbonate. Nb2O5 and NiNb2O6 are not catalytically active, and their presence on the surface would explain why the doped oxide is less efficient than the undoped one. As we have mentioned, it is possible that there is too much Nb on the surface and not enough NiO material between the Nb dopants. All in all, we are inclined to believe that either the experiments have not prepared a system similar to the model used in calculations or that (more likely, in our opinion) during the calcination in oxygen (when the catalyst is prepared) the Nb atoms form NbOx clusters on the surface or small amounts of NiNb2O6 (instead of forming NbO2 that substitutes a Ni atom in the surface of NiO). Experiments found that the catalyst deactivates slowly under reaction conditions, and this is attributed to the formation of NiNb2O6.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel 805-893-2256 (H.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was supplied by the Air Force Office of Scientific Research (FA9550-12-1-0147), the National Science Foundation (EFRI-1038234), and the U.S. Department of Energy (DE-FG02-89ER140048). We acknowledge support from the Center for Scientific Computing at the California NanoSystems Institute and the UCSB Materials Research Laboratory (an NSF MRSEC, DMR-1121053) funded in part by NSF CNS-0960316 and Hewlett-Packard. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.

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