Does Halogen Adsorption Activate the Oxygen Atom on an Oxide

Article pubs.acs.org/JPCC

Does Halogen Adsorption Activate the Oxygen Atom on an Oxide Surface? I. A Study of Br2 and HBr Adsorption on La2O3 and La2O3 Doped with Mg or Zr Bo Li and Horia Metiu* Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States ABSTRACT: Experiments suggest that adsorbing halogen atoms on an oxide surface can improve its catalytic properties. We use density functional theory to examine the dissociative adsorption of Br2 and of HBr on La2O3(001) and on La2O3(001) doped with Mg or Zr. We find that the presence of Br on the surface makes it much easier to make oxygen vacancies. In addition, there is a very strong interaction between the fragments made by the dissociative adsorption of Br2 or HBr: the presence of one fragment increases substantially the binding energy of the other one. We propose that this is a general behavior whenever one fragment is a Lewis acid and the other is a Lewis base, while the oxide is neither.

1. INTRODUCTION There are many articles reporting the use of halogen additives to improve catalytic activity of oxides1−19 for alkane activation or for hydrocarbon halogenation.20−23 Some of the catalysts used in those works were prepared by mixing halides with the oxide catalyst during the synthesis of the solid. Others halogenated the surface by exposing a halide, an oxide, or an oxihalide to a gas containing the reactant of interest, oxygen, and a halogen source. The halogen source tends to convert the oxide into a halide, and the oxygen converts the halide into an oxide. As a result, the surface layer is a mixture of oxygen and halogen in a proportion that depends on the ratio of the oxygen/halogen source in the gas, the chemistry performed by the reactant, and the temperature. A similar situation is encountered when oxidation, catalyzed by oxides, is used to destroy unwanted halogenated hydrocarbons.24−27 Of particular interest to us are cases in which a very small amount of halogen has caused substantial modifications of catalytic activity. A classic example is ethylene epoxidation when trace amounts of compounds containing Br or Cl change the selectivity,28 inhibiting the formation of water and CO2. Klabunde’s group29 has shown that the addition of very small amounts of I2 in the feed affects substantially the oxidative dehydrogenation of butane to butadiene by an oxide catalyst. Similarly, Li et al.30 reported that addition of small amounts of Br2 to a feed of CH4 + O2 in the presence of a mercury oxide catalyst allows the formation of formaldehyde at a remarkably low temperature. It appears that in some cases, adding small amounts of halogen to an oxide surface can improve its catalytic activity. This information prompted us to perform density functional calculations that examine the adsorption of Br2 and HBr on three oxides: La2O3(001), Mg-doped La2O3(001), and © 2012 American Chemical Society

Zr-doped La2O3(001). The main question in this work is whether the presence of the halogen on the oxide surface facilitates the formation of oxygen vacancies. It is often assumed that an oxide that easily makes oxygen vacancies is a better oxidant than one that would not let go of oxygen. We find that the dissociative adsorption of Br2 or HBr causes a substantial lowering of the energy of oxygen vacancy formation on the three oxides we studied. We have chosen to look at La2O3 because it has been used for oxidative halogenation and the destruction of halogencontaining compounds. It is also representative of very stable, irreducible oxides. We examine La2O3 doped with Mg or Zr because the former is typical of low-valence dopants, and the latter, of high-valence dopants. We decided to study bromination because using bromine is thermodynamically most favorable31 for methane activation by a halogenated oxide: chlorine is too reactive and iodine is not reactive enough. Another interesting aspect revealed by our calculations is the strong interaction between the adsorbed Br atoms formed by the dissociative adsorption of Br2 or between the H and Br atoms formed by the dissociation of HBr. The calculations show that the energy released by the dissociative adsorption of Br2 or HBr on one of the oxides mentioned above exceeds by a large amount the sum of the energies of adsorbing the atoms on separate (but identical) surfaces. We show that this happens because one of the fragments formed by dissociation acts as a Lewis acid and the other as a Lewis base. We suggest that this strong stabilization takes place whenever a Lewis acid is coadsorbed with a Lewis base on the surface of an oxide that is Received: October 13, 2011 Revised: December 29, 2011 Published: January 5, 2012 4137

dx.doi.org/10.1021/jp209857s | J. Phys. Chem. C 2012, 116, 4137−4148

The Journal of Physical Chemistry C

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neither a strong Lewis acid nor a strong Lewis base. If an oxide is a strong Lewis base, it will adsorb strongly a Lewis acid and vice versa.

between the energy of dissociative adsorption of XY, denoted by ΔE[(X, Y)/Ox] (the two fragments are next to each other on the same oxide surface, but not bonded to each other) and ΔEiso[X/Ox, Y/Ox] (the fragments are adsorbed on different, but identical, surfaces) gives a measure of the extent of the fragment−fragment “interaction” during dissociative adsorption. This interaction may be caused by competition between the fragments for the electrons of the oxide or by the fact that one fragment donates electrons and the other accepts them.

2. COMPUTATIONAL METHODOLOGY The calculations reported here were performed by using periodic, spin-polarized, density functional theory (DFT) as implemented in the Vienna ab initio program package (VASP).32−35 The electron−ion interactions are described by the projector augmented wave (PAW) method proposed by Blöchl36 and implemented by Kresse.37 We used the rPBE functional and a plane-wave basis set with an energy cutoff of 400 eV. The undoped La2O3 and the doped La2O3 slabs had 10 atomic layers (four lanthanum and six oxygen layers) and a 15 Å vacuum region. A 3 × 3 surface cell, with nine oxygen atoms in the top layer, was employed in all calculations. Due to the large supercell, all energies were calculated at the Γ-point of the Brillouin zone. We have performed one calculation with a grid of 6 k-points for the dissociative adsorption of Br2: the reaction energy changed from 2.67 to 2.58 eV. During structure optimization, all ions in the unit cell were allowed to relax, and no symmetry was imposed. A variety of initial configurations were used when the optimization of the geometry was started. The search for minimum energy was stopped when the force acting on each atom was smaller than 0.02 eV/Å. In all cases, we optimized several spin states but report here the results for the spin state having the lowest energy. Because of the large number of reactions studied, we define the adsorption energy and the energy of oxygen vacancy formation in the text when we report the results. We have found that when Br2 or HBr adsorb dissociatively, there is a strong interaction between the fragments. To quantify this interaction we use a sequence of calculations based on the following reactions: XY(g) → (1/2)X2(g) + (1/2)Y2(g)

ΔE[XY(g)]

3. BR2 ADSORPTION 3.1. Br2 Dissociation on La2O3. The energy of the dissociative-adsorption reaction Br2 + La2O3(001) → (Br, Br)/La2O3

ΔE[X/Ox]

(2)

(1/2)Y2 + Ox → Y/Ox

ΔE[Y/Ox]

(3)

Table 1. Reaction energies and Bader Chargesa

ΔE iso[X/Ox, Y/Ox]

ΔE[Br/Ox]

QBr1[(Br, Br)/Ox]

QBr2[(Br, Br)/Ox]

QBr[Br/ Ox]

La2O3 MgLa2O3 ZrLa2O3

−0.40 (0) −0.89 (1) −2.67 (1)

+0.10 (1) −0.87 (0) −3.54 (0)

+0.21 +0.16 −0.68

−0.76 −0.53 −0.49

−0.40 +0.16 −0.67

is shown in Figure 1a. In this figure, the Br atoms formed by the dissociation of Br2 are labeled 1 and 2; in what follows, we call them Br1 and Br2. These atoms are not equivalent. Br1 binds to a surface-oxygen atom (see Figure 1a). The Br1−O distance is 1.86 Å, which is close to the bond length of the negative BrO ion (1.83 Å) in the gas phase.38 In what follows, we refer to this geometry as the “BrO group”. The electron localization function, shown in Figure 2a, indicates the formation of a covalent Br−O bond. The charge density plot (Figure 2b) of the bonding orbital also shows a σ bond between Br and O. The Bader charge on Br1 is +0.21 electron (see Table 1); Br1 is an electron donor. The distance between Br2 and the three nearest La atoms is 3.4 Å; this is close to the bond distance in LaBr3, which is 3.1 Å.39 Therefore, we suggest that Br2 is bonded to these three La atoms, and we refer to this structure as the “BrLa3 group”. To make bonds with the La atoms, Br2 pushes the surface oxygen atom labeled “s” (in Figure 1a) toward the bulk, 1.2 Å below the plane of the surface-oxygen atoms; this is a very large displacement from the site normally occupied by O (in the absence of adsorbed Br atoms). The distance between the oxygen atom labeled “s” and Br2 is 3.12 Å; these atoms are not bonded to each other. The Bader charge on the Br2 atom is −0.76 electron; Br2 is an electron acceptor, as expected from its similarity to a Br atom in LaBr3. All other binding sites of the two Br atoms to the surface have substantially higher energy than the binding scheme shown in Figure 1a. It is interesting to compare the binding of the Br atoms formed by the dissociative adsorption of Br2 with the binding of a single Br atom on the same surface. In the lowest-energy

(4)

ΔE iso[X/Ox, Y/Ox] (5)

The energy ΔEiso[X/Ox, Y/Ox] is what the dissociative adsorption energy would be if the presence of X on the surface has no influence on the binding energy of Y and vice versa. The difference,

ΔE int[(X, Y)/Ox] = ΔE[(X, Y)/Ox] − ΔE iso[X/Ox, Y/Ox]

ΔE[(Br, Br)/Ox]

ΔE[(Br, Br)/Ox] is the energy of the dissociative adsorption reaction Br2 + Ox → (Br, Br)/Ox, where (Br, Br)/Ox represents the two Br atoms formed by dissociation and bound to the oxide Ox. ΔE[Br/Ox] is the energy of the reaction (1/2)Br2 + O2 → Br/Ox, where Br/Ox represents a Br atom bound to Ox. QBrj[(Br, Br)/Ox], j = 1, 2, is the Bader charge of the Br atom labeled Brj in Figure 1. QBr[Br/Ox] is the Bader charge on an isolated Br atom bound to Ox

ΔEiso[X/Ox, Y/Ox] is the energy of a reaction in which the initial state consists of two independent, identical oxide surfaces Ox, and an XY molecule in the gas phase, and the final state consists of X adsorbed on one oxide surface and Y adsorbed on the other. Since the first three reactions add to the fourth one, we have

= ΔE[XY(g)] + ΔE[X/Ox] + ΔE[Y/Ox]

Ox

a

Here, XY(g) is a gas phase molecule (Br2 or HBr), Ox is the oxide (i.e., La2O3, or MgLa2O3, or ZrLa2O3), and X/Ox is the oxide with the X atom adsorbed on the surface. The symbols ΔE in these equations are the energies of the reactions. The sum of these reactions is XY(g) + 2Ox → X/Ox + Y/Ox

(7)

is ΔE[(Br, Br)/La2O3] = −0.4 eV (exothermic) (see Table 1). The lowest-energy structure of Br2 dissociated on La2O3(001)

(1)

(1/2)X2 + Ox → X/Ox

ΔE[(Br, Br)/La2O3]

(6) 4138



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Figure 2. (a) The electron localization function and (b) the partial charge density of the bonding orbital of the Br2 adsorption on La2O3(001) (Figure 1a). The isodensity surface value is 0.02 e/Å3.

Figure 1. The optimized structures of the adsorbed, dissociated Br2 molecule on three surfaces: (a) La2O3(001), (b) MgLa2O3(001), and (c) ZrLa2O3(001). The adsorption energy ΔE[(Br, Br)/surface] (eV) and selected bond distances (Å) are listed under the figures. For later reference, we label the Br atoms as Br1 and Br2. The oxygen atom labeled “s” is located underneath Br2 (for La2O3(001) and for MgLa2O3(001)).

structure (see Figure 3), an isolated Br atom forms a LaBr3 group, similar to the Br2 atom formed by Br2 dissociation (Figure 1a). The distances between the single Br and the three nearest La atoms are close to the Br−La bond length in LaBr3. The Bader charge on Br is −0.40 electron (see QB(Br) in Table 1); this is much smaller than the Bader charge on Br2 (formed by Br2 dissociation), which also forms a BrLa3 group, but whose charge is −0.76 electron (see Q(Br2) in Table 1). When Br2 dissociates, there are two Br atoms on the surface, and Br1, which forms the BrO group, is an electron donor that feeds electron charge into Br2 (which is part of the BrLa3 group). We generalize this observation to suggest that the coadsorption of one Br atom with any charge donor (e.g., alkali, H, CH3) will have the effect of transferring electrons to the BrLa3 group, increasing the Bader charge on Br and strengthening the bond of Br with the surface. One might expect that if we dissociate Br2, both atoms will bind to the location preferred by a single atom: in other words, both will form a BrLa3 complex. This is not the case. The presence of the BrLa3 complex on the surface induces the other Br atom (Br1 in Figure 1a) to bind to a surface-oxygen atom and form a BrO group. Thus, the presence of the BrLa3 complex activates surface oxygen to bind Br.

Figure 3. The geometry of the Br atom adsorbed on (a) La2O3 and (b) La2MgO3. The La atoms are blue, the oxygen atoms in the top layer are red, the oxygen atoms in the second layer are yellow. The oxygen atom to which Br binds is dark blue. This is pushed toward the bulk when the Br−O bond is formed.

To calculate the energy of interaction between Br1 and Br2, we use the energy of the reaction (1/2)Br2(g) + La2O3(001) → Br/La2O3(001)

ΔE(Br/La2O3)

(8)

which is ΔE(Br/La2O3) = 0.10 eV (see Table 1) and the energy of the reaction Br2(g) → Br + Br, which is ΔE[Br2(g)] = 2.36 eV (DFT calculation). If we use eq 5 to calculate ΔEiso[Br/La2O3, 4139



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Br/La2O3], we obtain ΔEiso[Br/La2O3, Br/La2O3] = 2.36 + 0.1 + 0.1 = 2.56 eV. This reaction, in which we dissociate one Br2 molecule and bind the resulting atoms on two disconnected La2 O3(001) surfaces, is endoergic by 2.56 eV. The energy to dissociate a Br2 molecule and bind the two Br atoms on the same La2O3 surface, in the positions indicated in Figure 1a, is ΔE[(Br, Br)/ La2O3(001) = −0.40 eV (see Table 1). Thus, the “interaction energy” between the two atoms is:

formed a BrLa3 complex. (3) The Bader charge of the single Br atom changes from −0.40 electron (on La2O3) to 0.16 electron on MgLa2O3. In the case of MgLa2O3, the Br donates electrons, despite its very large electronegativity. This happens because the presence of Mg creates a hole,44,45,47 and this is an electron acceptor. The interaction energy between the Br atoms formed by dissociating Br2 on MgLa2O3 is (see eq 6) ΔE int[(Br, Br)/MgLa 2O3] = ΔE[(Br, Br)/La2O3]

ΔE int[(Br, Br)/La2O3(001)] = ΔE[(Br, Br)/La2O3] − ΔE iso [Br/La2O3 , Br/La2O3] = − 0.40 − 2.56 = − 2.96 eV

− ΔE iso[Br/La2O3 , Br/La2O3]

(9)

with

Clearly, the two Br atoms formed by dissociative adsorption on La2O3(001) surface help each other to form bonds to the surface. They do this because one of them donates electrons and the other accepts them. The energy gain due to this “interaction” is very large. The effect is qualitatively robust and does not rely on the accuracy of DFT. These calculations do not predict that Br atoms will not bind to the La2O3(001) surface. If one exposes the surface to a beam of Br atoms, they will bind to the surface because of the van der Waals interactions. Then they will move along the surface until two Br atoms are sufficiently close to each other to exchange electrons and form the (Br, Br)/La2O3 complex shown in Figure 1a. The net result is that the surface will end up looking as if dissociative chemisorption of Br2 took place (even though the gas consisted of Br atoms). 3.2. Br2 Dissociation on MgLa2O3. According to our classification,40−46 Mg is a low-valence dopant (LVD), and its presence will activate the surface-oxygen atoms next to it. According to the rules formulated in previous work, we expect the oxygen atoms on the MgLa2O3 surface to be more reactive than those on La2O3(001), and indeed, they are. The energy ΔE[(Br, Br)/MgLa2O3] of the dissociative adsorption reaction,

ΔE iso[Br/La2O3 , Br/La2O3] =ΔE[Br2(g)] + 2ΔE[Br/MgLa 2O3] =2 × 1.18 + 2 × ( − 0.87) = 0.62 eV

The value of ΔE[Br] is taken from Table 1, and 2 × 1.18 is twice the energy of the reaction (1/2)Br2(g) → Br(g). Using ΔE[(Br, Br)/La2O3] = −0.89 eV, from Table 1, we obtain for the interaction energy,

ΔE int[(Br, Br)/MgLa 2O3] = − 0.89 − 0.62 = − 1.51 eV Again, we find that the two Br atoms help each other to bind to the surface. The interaction energy between the two Br atoms on MgLa2O3 (which is −1.51 eV) is smaller than on La2O3 (which is −2.96 eV). Mathematically, this happens because the binding energy of an isolated Br atom on MgLa2O3 is much higher (−0.87 eV) than the binding energy on La2O3 (0.10 eV). It is likely that the interaction between the Br atoms is weaker on MgLa2O3 because of the presence of a hole in the valence band of MgLa2O3. Therefore, the electron-accepting Br atom must compete for electrons with the hole, and this diminishes the Br-to-Br charge exchange to which we attribute the high interaction between the Br atoms. 3.3. Br2 Adsorption on ZrLa2O3(001). In our past work, we introduced the concept of high-valence dopant (HVD), which is a dopant whose valence is higher than that of the cation that it replaces. For example, in Ti-doped ZnO, Ti is a high-valence dopant,44 and so is Zr in La2O3, which we consider here. In addition, we restrict this classification to irreducible oxides. Thus, we do not consider Ta dopant in CeO2 to be a HVD because Ta donates one electron to reduce a Ce atom (formally, from Ce4+ to Ce3+);46 therefore, Ta becomes tetravalent. After this electron transfer, Ta has the same valence as Ce and is no longer a HVD. We have found that the HVDs share a number of properties.42−44 Since the dopant has more valence electrons than the cation it replaces, it is electron-rich and undercoordinated. Because of this, it will cause the surface oxygen atoms in its neighborhood to bind more strongly to the oxide; this is opposite to the effect of an LVD. In addition, the dopant will have a strong tendency to bind electrophiles. For example, HVDs bind O2, transfer electrons to it, weaken the O−O bond, and make this adsorbed O2 reactive.44 We have postulated a general rule: if the valence of the HVD is not much higher than that of the cation it replaces, the dopant will adsorb electrophiles and make them more reactive. If the HVD has a very high valence (e.g., Nb dopant in NiO) and makes a stable

Br2 + MgLa 2O3(001) → (Br, Br)/MgLa 2O3(001) ΔE[(Br, Br)/MgLa 2O3]

(10)

is ΔE[(Br, Br)/MgLa2O3] = −0.89 eV (exothermic), which is roughly double the energy of the same process on La2O3(001) (see Table 1). The structure of the (Br, Br)/MgLa2O3(001) surface is shown in Figure 1b. The Br1 atom (the Br atom labeled 1 in Figure 1b) forms a covalent bond with a surfaceoxygen atom, with a bond length of 1.87 Å. This is very similar to the Br−O group formed by Br1 on La2O3(001) (Figure 1a). The Br2 atom (Figure 1b) binds to the Mg dopant and two La atoms; the Mg−Br bond length is 2.71 Å, and the two La−Br bonds have a length of 3.4 Å. We refer to this as a BrLa2Mg complex. As in the case of La2O3, the Br1 (in the BrO group) is an electron donor (its Bader charge in Table 1 is +0.16 electron), and Br2 (in the BrMgLa2 group) is an electron acceptor (its Bader charge is −0.53 electron). The fact that an LVD in the surface layer of an oxide activates the oxygen atoms40−43,45,47,48 manifests itself here in three ways: (1) Table 1 shows that the energy of the reaction, (1/2)Br2 + MgLa 2O3 → Br/MgLa 2O3

ΔE[Br/MgLa 2O3]

(11)

is ΔE[Br/MgLa2O3] = −0.87 eV, which is very large compared with the energy of the same reaction on La2O3 (which is 0.10 eV). (2) The presence of the LVD changes the binding site of a single Br atom: now Br binds on MgLa2O3(001) to an oxygen atom (see the structure in Figure 3b), whereas on La2O3, Br 4140



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oxide, it will bind O2 too strongly and the dopant−O2 complex is no longer a good oxidant. The results obtained for the interaction of Br2 with ZrLa2O3(001) are consistent with these rules. After replacing a La atom in the surface layer, Zr has (formally) one “unused valence electron”. This is available to bind Br2 and transfer electronic charge to it. This electronic charge populates an antibonding orbital of Br2 and weakens the Br−Br bond considerably (in this particular case it causes Br2 to dissociate (see Figure 1c)). The energy of the reaction Br2 + ZrLa2O3(001) → (Br, Br)/ZrLa2O3(001) ΔE[(Br, Br)/ZrLa2O3]

is ΔE[(Br, Br)/ZrLa2O3] = −2.67 eV (see Table 1), which is much larger than the dissociative adsorption energy on the other surfaces investigated here (Table 1). The two Br atoms formed by the dissociation of Br2 are not equivalent, even though both bind to Zr. The gain in the Bader charge on the Br1 atom (the atom labeled 1 in Figure 1c) is 0.68 electron; the Br2 atom (the atom labeled 2 in Figure 1c) gains only 0.49 electron. The same asymmetry is observed in the Zr−Br bond lengths: the bond of Zr with Br1 is longer than that with Br2 (see Figure 1c). To determine the “interaction energy” between the two Br atoms, we examine the adsorption of one Br atom on the Zr dopant. The energy of the reaction (1/2)Br2 + ZrLa2O3 → Br/ZrLa2O3

ΔE[Br/ZrLa2O3]

is ΔE[Br/ZrLa2O3] = −3.54 eV. Here, Br/ZrLa2O3 is the slab of ZrLa2O3 with one Br atom adsorbed on the Zr dopant. The energy of dissociating the Br2 molecule and adsorbing each Br atom on a different Zr/La2O3 is given by eq 5. For this particular system, this gives ΔEiso[Br/ZrLa2O3, Br/ZrLa2O3] = 2 × 1.18 − 3.54 − 3.54 = −4.72 eV. The “interaction energy”, given by eq 6, is ΔEint[Br/ZrLa2O3, Br/ZrLa2O3] = (−2.67) − (−4.72) = 2.05 eV. This is a very large “repulsion energy” as opposed to the dissociative adsorption on La2O3 or MgLa2O3, where this interaction energy is attractive. Intuitively, and very qualitatively, this is easy to understand: the Zr dopant has one “extra electron” (the tetravalent Zr replaces a trivalent La), and this is used by the first Br atom to make a very strong bond. The second atom has to share this extra electron with the first and as a result the binding energy of two atoms is much less than that of binding each Br atom to two separate ZrLa2O3 surfaces. This means that if the surface has Zr atoms that have not adsorbed Br, there is a thermodynamic driving force for some Br atoms to leave the Br2Zr complex and form a Br/Zr complex. At very low pressure of Br2, it is likely that the surface will have many BrZr groups and very few Br2Zr. The precise conditions when this happens cannot be determined without a thermodynamic equilibrium calculation, which is beyond the scope of this article. We found that Br2 will not bind to the oxygen atoms near the dopant. This, too, is consistent with our previous observations regarding the effect of a HVD on the oxygen atoms adjacent to it: they are less reactive and are harder to remove to make oxygen vacancies.

Figure 4. The optimized structure of HBr adsorbed dissociatively on (a) La2O3(001), (b) MgLa2O3(001), and (c) ZrLa2O3(001). The dissociative adsorption energies ΔE[(H, Br)/Ox] (eV) and selected bond distances (Å) are listed for three structures.

very similar to that of the Br2 atom formed (Figure 1a) by the dissociation of Br2 on La2O3(001). The hydrogen atom binds to a surface-oxygen atom to form a hydroxyl. The energy of the reaction HBr + La2O3 → (H, Br)/La2O3

ΔE[(H, Br)/La2O3]

is ΔE[(H, Br)/La2O3] = −2.04 eV (exothermic). This is larger than the binding energy of Br2 on the same surface, mainly because the H−O bond, formed on (H, Br)/La2O3, is stronger than the Br−O bond formed on (Br, Br)/La2O3. The Br atom formed by the dissociation of HBr binds at a location similar to the Br2 atom formed by the dissociation of Br2. Moreover, the two atoms (i.e., Br2 originating from Br2 and the Br originating from HBr) have the same Bader charge (see Tables 1 and 2). The Bader charge of the H atom (formed by the dissociation of HBr) is +0.65 electron. Compare this with the Bader charge of Br1 (which is 0.21 electron). Obviously, the H atom is a much better charge donor than the Br1 atom. This also helps to make the dissociative adsorption energy of HBr larger than that of Br2. It is interesting to note that the reactions (1/2)H2 + La2O3 → H/La2O3 and (1/2)Br2 + La2O3 → Br/La2O3 are both uphill (see Tables 1 and 2). However, if adsorbed together, H and Br manage to form a strong bond with the surface. We quantify next the interaction between H and Br when (H, Br)/La2O3(001) is formed. For this, we need to calculate the energy ΔEiso[H/La2O3, Br/La2O3] of the reaction

4. HBR ADSORPTION 4.1. HBr Adsorption on La2O3(001). HBr adsorbs and dissociates on La2O3(001). The most stable geometry is shown in Figure 4a. In what follows, we call the surface produced by this dissociative adsorption (H, Br)/La2O3. The Br atom binds to the La atoms to form a BrLa3 group, with a local geometry 4141



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Table 2. Energies of Reaction and Bader Chargesa Ox

ΔE[(HBr)/Ox]

ΔE[H/Ox]

QB(BrHBr)

QB(HHBr)

QB(Br)

QB(H)


−1.78 (0) −2.04 (1) −1.17 (1)

+1.53 (1) −2.35 (0) +1.09 (0)

−0.77 −0.74 −0.70

+0.65 +0.57 +0.62

−0.40 +0.16 −0.67

+0.59 +0.56 +0.54

a The energy ΔE[(HBr)/Ox] of reaction HBr(g) + Ox → (H, Br)/Ox (dissociative adsorption of HBr on the oxide Ox) and the energy ΔE[H/Ox] of the reaction 1/2H2 + Ox → H/Ox. QB(BrHBr) and QB(HHBr), are the Bader charges on the H and Br atoms formed by the dissociative adsorption of HBr. QB(Br) and QB(H) are the Bader charges on a single Br or a single H atom adsorbed on the oxide. The numbers in parentheses in the first two columns give the spin polarization (the number of spins up minus the number of spins down) of the lowest-energy state.

HBr(g) + 2La2O3 → H/La2O3 + Br/La2O3, where the products consist of two separate oxide surfaces: one adsorbs a H atom and the other a Br atom. For this specific example, eq 5 becomes ΔEiso[H/La2O3, Br/La2O3] = ΔE[HBr(g)] + ΔE[H/ La2O3] + ΔE[Br/La2O3]. The energy to dissociate HBr in the gas phase is ΔE[HBr(g)] = 0.66 eV (from DFT). From Table 1 we have ΔE[Br/La2O3] = 0.10 eV, and from Table 2, ΔE[H/La2O3] = 1.53 eV. With these values, ΔEiso[H/La2O3, Br/ La2O3] = 0.66 + 1.53 + 0.10 = 2.29 eV. We can now calculate ΔEint[(H, Br)/La2O3] from eq 6 and obtain ΔEint[(H, Br)/ La2O3] = −1.78 − 2.29 = −4.07 eV. H and Br interact very strongly: the energy of dissociating HBr and placing H and Br as in Figure 4a is 4.07 eV larger than that of dissociating HBr and placing the H atom on one La2O3 surface and the Br atom on another La2O3 surface. 4.2. Dissociation of HBr on MgLa2O3(001). The structure formed by dissociating HBr on MgLa2O3(001) is shown in Figure 4b. We denote the system created by this dissociation by (H, Br)/MgLa2O3. The H atom binds to an oxygen atom and the Br atom binds to two La atoms and to the Mg dopant. The energy of the reaction, HBr(g) + MgLa 2O3 → (H, Br)/MgLa 2O3

in which the interaction energy between the adsorbed Br and H is −4.07 eV. The difference between MgLa2O3 and La2O3 is caused, mathematically, by the fact that the isolated H and the isolated Br bind much more strongly to the surface of MgLa2O3 than on La2O3. This happens because Mg, being an LVD, creates an electron deficit in the surface, weakening the binding of the O atoms to the oxide and inducing them to make stronger bonds with either Br or H. The difference between the dissociation of HBr on La2O3 and MgLa2O3 can be understood in terms of the concept of chemical compensation.47 An LVD activates the oxygen atoms on the surface by creating a hole in the valence band. Adsorbing a charge donor, such as H, tends to cancel the effect of the dopant, making the oxygen atoms near the dopant less reactive than they would be in the absence of H. This is why even though the H and Br atoms bind strongly when they are adsorbed separately on MgLa2O3, the presence of H on the surface makes it less reactive for Br. Note that the location where the Br atom formed by dissociating HBr binds to the surface is similar to that of the Br2 atom formed by dissociating Br2. However, this is where the similarity ends. The Bader charge of the Br2 atom, in the (Br, Br)/MgLa2O3 compound, is −0.53 electron (Table 1). The Bader charge of the Br atom in the (H, Br)/MgLa2O3 compound is −0.74 electron (Table 2). The presence of H on the surface pushes more electron charge onto Br. 4.3. Dissociation of HBr on ZrLa2O3. The geometry of the lowest-energy state for the dissociated HBr on ZrLa2O3(001) is shown in Figure 4c. Br binds to Zr and H binds to a surface oxygen atom. The Br−H distance in Figure 4c is 2.44 Å, which is much larger than the H−Br distance in the HBr molecule (1.41 Å). The binding energy is −1.17 eV, which is smaller than the binding energy on La2O3 or on MgLa2O3. The energy of the reaction

ΔE[(H, Br)/MgLa 2O3]

is ΔE[(H, Br)/MgLa2O3] = −2.04 eV. This is larger than the energy of dissociative adsorption on La2O3, a behavior similar to that observed for the dissociation of Br2. The Bader charges of Br and H are about the same as on La2O3. There is, however, a substantial difference between La2O3 and MgLa2O3. The energy of the reaction (1/2)H2 + MgLa 2O3 → H/MgLa 2O

ΔE[H/MgLa 2O3]

is ΔE[H/MgLa2O3] = −2.35 eV, whereas the same reaction on La2O3 has an energy of +1.53 eV. This difference arises because the oxygen atoms on MgLa 2O3 are dramatically more reactive than those of La2O 3. The presence of the Mg dopant also activates the binding of a Br atom. The energy of the reaction (1/2)Br2 + MgLa 2O3 → Br/MgLa 2O

(1/2)Br2 + ZrLa2O3 → Br/ZrLa2O3

ΔE[Br/ZrLa2O3]

is ΔE[Br/ZrLa2O3] = −3.54 eV. This is a very large binding energy, much higher than that for La2O3 or MgLa2O3. This happens because Br binds to the tetravalent Zr dopant, which sits at a site previously occupied by a trivalent La atom; the dopant site has an excess of electrons, and this helps Br bond to it. On the other hand, the energy of the reaction

ΔE[Br/MgLa 2O3]

is ΔE[Br/MgLa2O3] = −0.87 eV, compared with 0.1 eV on La2O3. We expect this, since Mg is a LVD and it activates the oxygen atoms neighboring it. We can use these results to calculate the energy of the isolated fragments (eq 5 and data from Tables 1 and 2), ΔEiso[H/MgLa2O3, Br/MgLa2O3] = 0.66 − 0.87 − 2.35 = −2.56 eV. Compare this to the case of La2O3, for which ΔEiso = 2.29 eV. The interaction energy ΔEint[(H, Br)/MgLa2O3] = −2.04 − (−2.56) = 0.52 eV is repulsive. Dissociating HBr and adsorbing H on one MgLa2O3 surface and Br on another MgLa2O3 surface produces more energy than dissociating HBr and placing H and Br on the same MgLa2O3 surface, in the positions shown in Figure 4b. This is very different from La2O3,

(1/2)H2 + ZrLa2O3 → H/ZrLa2O3

ΔE[H/ZrLa2O3]

is ΔE[H/ZrLa2O3] = 1.09 eV. This is high because Zr already creates an excess of electrons in the system and H is an electron donor that has difficulty binding to an electron-rich surface. We can calculate ΔEiso[H/ZrLa2O3, Br/ZrLa2O3] = 0.66 − 3.54 + 1.09 = −1.79 eV and ΔEint[(H, Br)/ZrLa2O3] = −1.17 − (−1.79) = 0.62 eV. The H and the Br atoms “repel” each other 4142



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The structure of the surface when a vacancy is created on (Br, Br)/La2O3 is shown in Figure 5. Br2 binds to three La

when HBr dissociates on the ZrLa2O3(001) surface. This is again consistent with the view that electron transfer is an important part of the interaction between adsorbates. Placing H on the system costs energy, but it does not help in binding more strongly the Br atoms, which can take from Zr all the electrons it needs to form the bond with the surface.

5. OXYGEN VACANCY FORMATION ON SURFACES ON WHICH BR2 WAS PREADSORBED One of the questions we address here is whether brominating an oxide surface (e.g., La2O3, MgLa2O3 or ZrLa2O3) affects the reactivity of the oxygen atoms in the surface layer. We use as a predictor of this reactivity the energy of the reaction Ox → (1/2)O2(g) + ROx, where Ox is one of the oxides we study here and ROx is the reduced oxide Ox (ROx is Ox after removing an oxygen atom from the surface layer of the supercell). It turns out that the energy of oxygen vacancy formation on (Br, Br)/Ox or on (H, Br)/Ox is much smaller than the energy of oxygen vacancy formation on the clean oxide (with nothing adsorbed on the surface). 5.1. Oxygen Vacancy Formation on the (Br, Br)/La2O3 Surface. It is very difficult to remove an oxygen atom from the pure La2O3(001) surface: the energy of vacancy formation is ΔEv[La2O3(001)] = 6.33 eV. Brominating the surface helps. After the dissociative adsorption of Br2, the energy of the reaction (Br, Br)/La2O3(001) → (1/2)O2(g) + (Br, Br)/RLa2O3 ΔE v [(Br, Br)/La2O3]

is ΔEv[(Br, Br)/La2O3] = −0.63 eV (see Table 3). Here, RLa2O3 is the reduced lanthanum oxide (with one oxygen atom Table 3. Energy of Reaction and Bader Chargesa surface

ΔEv[(Br, Br)/Ox] (eV)

QB(Br1)

QB(Br2)


−0.63 −0.24 +2.16

−0.75 −0.8 −0.75

−0.76 −0.46 −0.71

Figure 5. The optimized structures formed after an oxygen vacancy is formed on (H, Br)/La2O3(001), (H, Br)/MgLa2O3(001), and (H, Br)ZrLa2O3(001). The energy of vacancy formation, ΔEv[(H, Br)/ Ox], is the energy of the reaction (H, Br)/Ox → (Br, Br)/ROx + (1/2)O2, where Ox is one of the three oxides and ROx is Ox after one oxygen atom has been removed. The position that had been occupied by the removed oxygen atom is marked by a blue circle. That site is now occupied by a Br atom.

The first column gives the energy of the reaction (Br, Br)/Ox → (1/2)O2 + (Br, Br)/ROx; this forms an oxygen vacancy in the top layer of the supercell of the surface (Br, Br)/Ox consisting of two adsorbed Br atoms on the surface of the oxide Ox. ROx is the reduced oxide surface created by the reaction. QB(Br1) and QB(Br2) are the Bader charges on the bromine atoms labeled 1 and 2 in Figure 5. a

atoms, and Br1, which used to bind to an oxygen atom, fills the site left vacant by the removal of the oxygen atom. This has a dramatic effect on the Bader charge of Br1. The Bader charge was equal to +0.21 electron (Table 1) for Br1 in (Br, Br)/ La2O3 and is −0.75 electron in (Br, Br)/RLa2O3 (the reduced (Br, Br)/La2O3 surface). It is easy to understand why. Forming an oxygen vacancy breaks the bonds of the removed oxygen atom with the surface and leaves behind two unpaired electrons. In an irreducible oxide such as La2O3, these electrons have no place to go and are localized at the vacancy site; the vacancy is a strong Lewis base, and this forces the Br atom that fills the vacancy to act as a Lewis acid. The formation of this acid−base pair contributes substantially to the lowering of the energy of vacancy formation. A similar interpretation is suggested by the partial density of states shown in Figure 6. The top graph in Figure 6a shows the density of states of RLa2O3. Removing an oxygen atom has produced two filled orbitals in the gap, below the conduction

removed from the surface layer), and (Br, Br)/RLa2O3 is the compound formed when an oxygen vacancy is created on (Br, Br)/La2O3. This is not helpful for oxidation catalysis by a Mars−van Krevelen mechanism. In that mechanism, the reductant (e.g., CH4) reacts with the oxygen atoms in the surface and creates one or more oxygen vacancies; the gas-phase oxygen fills the vacancy, reoxidizing the catalyst. The oxide must be a good oxidant but not so good that it cannot be reoxidized. As far as oxidation catalysis is concerned, there is a “sweet spot” for the energy of vacancy formation: it should be small but not too small. We call this statement the “moderation principle”. According to this, it is unlikely that (Br, Br)/La2O3(001) is a good oxidation catalyst. Moreover, because the reduction of the brominated oxides is exoergic, it is possible that an oxygen atom will be lost from the surface by the time the reaction temperature is reached. 4143



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Figure 6. The partial density of states of (a) the reduced La2O3(001), reduced MgLa2O3(001), and reduced ZrLa2O3(001) surface (one oxygen vacancy per supercell) and (b) the reduced Br2/La2O3(001), Br2/MgLa2O3(001), and Br2/ZrLa2O3(001) (one vacancy per unit cell).

band. The top panel in Figure 6b shows the density of states of (Br, Br)/RLa2O3. There are no filled states in the gap, and Br states appear in the valence band. Although the total energy is not the sum of the orbital energies, a substantial lowering of the orbital energies is part of the reason for the observed lowering of the total energy of vacancy formation. 5.2. Oxygen Vacancy Formation on (Br, Br)/MgLa2O3. Since Mg is a LVD, its presence lowers the energy of vacancy formation45 from 6.33 to 2.59 eV. This happens because the replacement of a trivalent La atom with divalent Mg creates an electron deficit in the system. Creating a vacancy provides unpaired electrons to heal the deficit, and this lowers the energy. Another way of saying this is to note that doping with Mg creates a hole in the valence band, and when a vacancy is created, one of the unpaired electrons “falls” into the hole, a process that lowers the energy of vacancy formation. Yet another way of saying the same thing is that the doped oxide is a Lewis acid (the hole will take electrons) and the vacancy is a Lewis base (it tends to donate the unpaired electrons). Forming a vacancy amounts to creating a Lewis base on a surface that is a Lewis acid. The acid−base interaction accounts for the low energy of vacancy formation.

The energy of vacancy formation on (Br, Br)/MgLa2O3 is even lower than on MgLa2O3: −0.24 eV (see Table 3) versus 2.59 eV.45 This happens because the presence of the adsorbed Br atoms makes the surface (Br, Br)/MgLa2O3 more acidic than MgLa2O3. This will lower the energy of vacancy formation because the vacancy is a Lewis base. This can be seen in the Bader charges on the Br atoms (−0.8 electron for Br1 and −0.46 electron for Br2 (Table 3)) on the reduced (Br, Br)/ MgLa2O3 compared with the Bader charges of the same atoms on (Br, Br)/La2O3 (0.16 and −0.53 electron (Table 1)). The Br atoms accept some of the electronic charge produced when the vacancy is formed. The density of states in Figure 6 (middle panels) tells the same story. A filled state in the gap (in the middle panel in Figure 6a) disappears, and Br-related states appear in the valence band. Transferring electrons on Br atoms lowers the energy of the accepting orbitals. Because the energy of vacancy formation is “downhill”, (Br, Br)/ MgLa2O3 is not a good candidate for Mars−van Krevelen chemistry because the reoxidation of the catalyst is problematic. We have not looked at the energy needed for removing a second oxygen atom (making a second vacancy), and there is a 4144



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chance that this process may be favorable for an oxidation reaction. 5.3. Oxygen Vacancy Formation on (Br, Br)/ZrLa2O3. The energy of oxygen vacancy formation on (Br, Br)/ ZrLa2O3(001) is 2.16 eV (Table 3). This system is more promising for Mars−van Krevelen catalysis because the surface is easy to reduce, but not too easy, so that reoxidation is possible. Of course an oxidation reaction on this catalyst might have to compete with a possible bromination reaction. Our goal is to study how bromination affects the chemistry of surface oxygen and studying bromination reactions are not within the scope of this article. As for the other oxides studied here, the Br1 atom fills the oxygen vacancy, and the Bader electron charge on both Br atoms is increased (−0.75 on Br1 and −0.71 on Br2; see Table 3), when compared with the charges on the Br atoms prior to making the vacancy (−0.68 on Br1 and −0.49 on Br2; see Table 1). The ZrLa2O3 surface is a Lewis base, since the replacement of La with Zr brings an additional electron in the system. The two Br atoms in (Br, Br)/ZrLa2O3 gain charge from Zr; they could use two electrons (formally), but only one is available. Therefore, (Br, Br)/ZrLa2O3 is an acid since the Br atoms are still capable of accepting electrons. Making an oxygen vacancy on this system requires less energy because the vacancy is a Lewis base and the surface is an acid. This is reflected in the densities of state in Figure 6, where the ZrLa2O3 system has three states in the gap (see the bottom panel in Figure 6a) and the reduce (Br, Br)/RZrLa2O3 has only one state in the gap and has Br states in the valence band. This lowering of the orbital energies when the vacancy is formed mirrors the lowering of the total energy of vacancy formation.

6. OXYGEN VACANCY FORMATION ON (H, BR)/OX 6.1. Oxygen Vacancy Formation on (H, Br)/La2O3. The geometry of the lowest-energy state for this system is rather surprising: the hydrogen atom penetrates in the vacancy site, below the outermost La layer, and the Br atom is bound above it (see Figure 7 a). The Bader charge on the Br atom is practically the same as prior to the formation of the oxygen vacancy, however, the charge on the H atom is changed dramatically, from +0.65 electron for (H, Br)/La2O3 (Table 2) to −0.65 electron for (H, Br)/RLa2O3. When an oxygen vacancy is made in (H, Br)/La2O3, the H atom, which is normally a Lewis base, is forced to act as a Lewis acid . Such behavior is not uncommon in acid−base chemistry: a very strong base can force a weak base to act as an acid. Note that the Br atom, which is a Lewis acid, has taken all the electron density it needs before the vacancy is formed; its Bader charge is practically unaffected by the formation of the vacancy. The energy to make an oxygen vacancy on (H, Br)/La2O3 is 3.91 eV, which is substantially smaller than the energy to make a vacancy on a clean La2O3 surface, and larger than on the (Br, Br)/La2O3 surface. We understand this qualitatively: when a vacancy is made on clean La2O3, the unpaired electrons have nowhere to go, and they are stuck in the vacancy, in a highenergy orbital. Although the formation of the vacancy creates a strong base, there is nothing in the system that can act as an acid to lower the energy of vacancy formation. When a vacancy is made on (Br, Br)/La2O3, one Br atom acts as an acid and lowers the energy more than on (H, Br)/La2O3, where the H atom acts as an acid; this happens because Br is a better electron acceptor than H.

Figure 7. The optimized structures after an oxygen vacancy was made on La2O3(001), MgLa2O3(001), or ZrLa2O3(001). The site where the O atom was removed is indicated by a blue circle. The Br atom moved at the oxygen vacancy site after the O atom was removed. ΔEv[X] is the energy of oxygen vacancy formation on the surface of the compound X.

6.2. Oxygen Vacancy Formation on (H, Br)/MgLa2O3. The lowest-energy structure of (H, Br)/RMgLa2O3 is shown in Figure 7b. Prior to the formation of the vacancy, the H atom was bound to a surface oxygen atom, and the Br atom was bonded to Mg and La atoms. When an oxygen vacancy is made, the H atom remains bonded on the oxygen site, and the Br atom “falls” into the vacancy. The energy of vacancy formation is 2.50 eV, higher than the energy of making a vacancy on (Br, Br)/La2O3 but substantially lower than the energy of making a vacancy on La2O3. The energy of oxygen vacancy formation on MgLa2O3 is45 2.59 eV. This is practically equal to the energy of forming an oxygen vacancy on (H, Br)/MgLa2O3; the presence of H and Br on the MgLa2O3 surface makes no difference as far as the energy of oxygen vacancy formation is concerned. This is consistent with the fact that making an oxygen vacancy causes practically no change in the Bader charges on Br and H (see Tables 2 and 4). The charge on the H atom is 0.57 electron for (H, Br)/MgLa2O3 and 0.60 electron for (H, Br)/RMgLa2O3; the charge on the Br atom is −0.74 electron for (H, Br)/MgLa2O3 and −0.8 electron for (H, Br)/RMgLa2O3. No charge redistribution takes place when the oxygen vacancy is formed on (H, Br)/MgLa2O3. A plausible explanation is that Br acts as a 4145



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Table 4. Energy of Reaction and Bader Chargesa Ox

ΔEv[(H, Br)/Ox] (eV)

QB(Br)

QB(H)


+3.91 +2.50 +3.06

−0.75 −0.8 −0.76

−0.65 +0.6 −0.63

It is interesting to note that introducing oxygen into the feed and allowing the formation of water makes the products CH3Br and H2O thermodynamically favorable, regardless of the state of the dissociated CH4. This is why oxihalogenation is used in practice. However, one should be mindful that the presence of oxygen can cause additional reactions and cause low selectivity. As we have emphasized, these methane dissociation calculations are exploratory: their purpose is to test whether the presence of Br on the surface facilitates the breaking of the C− H bond in methane, and we find that it does. Methane activation on unbrominated La2O3 is possible only at very high temperature (700 °C or above) in the presence of oxygen, and it is suspected that the reaction is partly taking place in the gas phase. In fact, La2O3 is used for the oxidative coupling of methane (to ethylene) precisely because it is inactive and can be used at high temperature without causing the combustion of the methane. Brominating the La2O3 surface does facilitate the breaking of the C−H bond, which should happen at lower temperature than on pure La2O3.

a ΔEv[(H, Br)/Ox] is the energy of the reaction (H, Br)/Ox → (1/2)O2 + (H, Br)/ROx, where Ox is one of the three oxides and ROx is the oxide after one O atom was removed (the reduced surface). QB(Br) and QB(H) are the Bader charges of the H or Br atoms after an oxygen vacancy was formed on the surface.

base and H acts as an acid, and they “neutralize” each other so that forming a vacancy on (H, Br)/MgLa2O3 requires the same energy as forming one on MgLa2O3. 6.3. Oxygen Vacancy Formation on (H, Br)/ZrLa2O3. The energy of vacancy formation on this surface is 3.06 eV, which is much lower than the energy of forming a vacancy on ZrLa2O3. The presence of H and Br on the surface of ZrLa2O3 makes the oxide more reducible. The structure of the system after the formation of the oxygen vacancy is shown in Figure 7. As in the case of the other surfaces, the Br atom takes the place of the removed oxygen atom. The formation of the vacancy leaves the Bader charge on Br unchanged but changes the Bader charge of H from +0.62 electron (Table 2) to −0.63 electron (Table 4). The formation of the vacancy converts the H atom from a Lewis base (in (H, Br)/ZrLa2O3) to a Lewis acid (in (H, Br)/RZrLa2O3). On all three surfaces, the presence of dissociated HBr makes it easier to make oxygen vacancies. This makes these surfaces better oxidants. However, this does not necessarily mean that they are better oxidation catalysts because the presence of Br may mean that the bromination reactions are more efficient than the oxidation reactions.

8. SUMMARY We have found that the dissociative adsorption of Br2 and HBr on La2O3, MgLa2O3, and ZrLa2O3 is exothermic. An interesting and unexpected outcome of the calculations is the observation that the fragments made by dissociation interact very strongly with each other by exchanging electrons. This interaction and the trends in the energy of dissociative adsorption can be rationalized in terms of the Lewis acid−base properties of the reaction participants. La2O3 is neither a Lewis acid nor base: the adsorption of H (which tends to be a Lewis base) and the adsorption of Br (which tends to be a Lewis acid) are both endoergic. It is therefore surprising that the adsorption of two Br atoms (when Br2 adsorbs dissociatively) or of a H and Br atom (when HBr adsorbs dissociatively) are exoergic. In the case of Br2 dissociation, the two Br atoms bind on two different surface sites and manage to stabilize each other because one Br becomes a Lewis acid, and the other, a Lewis base. The same is true for the dissociation of HBr: the binding energy is increased substantially, compared with the binding of the isolated fragments, because H acts as a Lewis base, and Br, as a Lewis acid. Unlike La2O3, MgLa2O3 is a strong Lewis acid because replacing a trivalent La with a divalent Mg causes an electron deficit in the system. The density of states of MgLa2O3 shows the presence of a hole in the valence band, which is not present in La2O3. Because of this, MgLa2O3 binds H (which is a Lewis base) very strongly, unlike La2O3. In addition, it binds Br strongly, and it forces it to be a Lewis base, even though normally Br is an electron acceptor (Lewis acid). However, when two Br atoms are adsorbed, as a result of Br2 dissociation, the energy is substantially lowered because one Br atom becomes a Lewis acid and the other a Lewis base; their “interaction energy” is attractive but not as attractive as in the case of La2O3, because the acidity of MgLa2O3 (as compared with La2O3) interferes with the ability of the adsorbed atoms to exchange electrons. ZrLa2O3 has very different properties. Zr has an excess of electrons (tetravalent Zr replaces a trivalent La), and the surface is a strong Lewis base. Among the surfaces studied here, this binds Br2 (dissociatively) most strongly because both Br atoms act as acids. This is the only surface on which both Br atoms formed by dissociation gain electron charge. The energy

7. METHANE DISSOCIATION ON (BR, BR)/LA2O3 In this section, we present very preliminary calculations regarding the reactivity of the (Br, Br)/Ox surface with methane. The dissociation of methane on this surface is exoergic. The energies of several dissociation products are shown in Figure 8. The zero of energy is the energy of Br2(g), CH4(g), and La2O3. Br2 adsorption is exoergic by 0.40 eV (Table 1). We found three states for the dissociated methane on the (Br, Br)/La2O3 surface. The one in which the dissociation forms adsorbed HBr and CH3Br (see Figure 8a) has the highest energy, and it is slightly endothermic (compared with CH4 in the gas and (Br, Br)/La2O3). With a small energy uptake, this state will decompose to form CH3Br and HBr in gas and a clean La2O3 surface. This seems to be a perfect system for making CH3Br. Unfortunately, other dissociative adsorption states are possible. Figure 8b shows a state in which CH3 binds to Br and H forms a hydroxyl. This intermediate is much too stable to produce CH3Br(g) plus HBr(g). It could evolve easily into CH3Br(g) and adsorbed HBr; however, this evolution is not catalytic, and it is likely that the surface will end up covered with HBr and be poisoned by this. A third state formed by the dissociative adsorption of methane is shown in Figure 8c. In this the dissociation makes a hydroxyl and a methoxide, and the two Br atoms are spectators; however, they are not passive spectators, since their presence activates the oxygen atoms. The state in Figure 8c has the lowest binding energy. 4146



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Figure 8. The dissociation of CH4 on (Br, Br)La2O3. The energy level diagram gives the three states of the dissociated methane together with the energy of three possible final states: (a) the dissociation of CH4 with the formation of adsorbed HBr and CH3Br, (b) the dissociation of CH4 with the formation of adsorbed CH3Br and a hydroxyl, and (c) the dissociation of CH4 with the formation of a hydroxyl and a methoxide. The diagram shows the energies of these three states and the energies of several possible final states (CH3Br and HBr in the gas phase, CH3Br in the gas and adsorbed HBr, CH3Br and water in the gas phase).

transferred to another part of the surface. Two Br atoms are better electron acceptors than a Br atom and a hydroxyl. Again, the formation of a Lewis base (the vacancy) is facilitated by the presence of Lewis acid sites on the surface. The formation of a Lewis acid−base pair stabilizes strongly the dissociative adsorption. We have observed this behavior in our previous work49 on TiO2 and postulate now that this is a general feature of dissociative adsorption on oxides: if the oxide is neither a Lewis acid nor a Lewis base and if one fragment produced by dissociative adsorption is a Lewis acid and the

of the dissociative adsorption of HBr on this surface is smaller than on the other oxides studied here, because the adsorption of H is very endoergic: H is a base, and it will not bind well on ZrLa2O3, which is a strong Lewis base. The bromination of the surface by the dissociative adsorption of either Br2 or HBr facilitates the formation of an oxygen vacancy. Brominating with Br2 lowers the energy of vacancy formation more than brominating with HBr (see Tables 3 and 4). A simple explanation is that making a vacancy creates two unpaired electrons whose energy is lowered if they can be 4147



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other one is a Lewis base, the energy of dissociative adsorption is much larger than the sum of the energies of adsorbing each fragment in isolation. The same rule holds for the coadsorption of a Lewis base with a Lewis acid on an oxide that is neither an acid nor a base: the two adsorbates bind much more strongly together than the sum of their binding energy alone on the surface. Moreover, any modification that makes an oxide surface a base will increase the binding energy of a Lewis acid and decrease the binding energy of a Lewis base. The opposite is true for modifications that turn the surface into a Lewis acid.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (805) 893-2256. Fax: (805) 893-4120. E-mail: [email protected].

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ACKNOWLEDGMENTS This research was supported in part by the U.S. Department of Energy (DE-FG02-89ER140048) and the Air Force Office of Scientific Research (FA9550-09-1-0333), and by the National Science Foundation through TeraGrid resources provided by Ranger@TACC under Grant No. TG-ASC090080. We made use of the computer facility of the California NanoSystems Institute funded in part by the National Science Foundation. 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 No. DE-AC0206CH11357. We thank Zhenpeng Hu for stimulating discussions.

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