The Role of Lewis AcidâBase Pairing - American Chemical Society

Article pubs.acs.org/JPCC

Halogen Adsorption on CeO2: The Role of Lewis Acid−Base Pairing Zhenpeng Hu† and Horia Metiu* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States

ABSTRACT: We use density functional theory to examine the dissociation of halogen molecules on CeO2(111). We are interested in this because oxides are used to catalyze the oxihalogenation of alkanes. Our calculations show that the exothermicity of the dissociative adsorption is increased substantially if one of the dissociation fragments is a Lewis acid and the other is a Lewis base. Doping the CeO2 surface to turn it into an acid or a base can be used to influence strongly halogen dissociation. Finally, we show that the presence of a halogen on the oxide surface facilitates the breaking of the C−H bond in methane.

1. INTRODUCTION Numerous papers have pointed out that adding halogens to an oxide surface often (but not always) improves the catalytic activity of the oxide toward alkane activation.1 Halogenated oxide surfaces are also of interest in oxyhalogenation reactions,2−10 and in the destruction of unwanted halogenated side products.7,11−18 In all of these systems, an oxide catalyst is exposed to a gas that contains oxygen and a halogen source. The halogen source converts the oxide into the halide, and oxygen converts the latter into the oxide. Depending on the ratio of the halogen source to oxygen in the gas, and on the temperature of the surface, the oxide surface contains a certain amount of halogen. We call this a halogenated oxide surface. In this Article, we use density functional theory to perform a preliminary investigation of the manner in which halogens adsorb on CeO2 surface and the way in which they influence the chemistry of the surface. Our most interesting finding is that the chemisorption of halogens on ceria is controlled by the ability of the halogen atoms (formed by dissociative adsorption) to function as a Lewis acid−base pair. A halogen atom can adsorb on ceria on either an oxygen site (we denote the system formed in this way by (X−O/CeO2(111)) or a Ce site (denoted (X−Ce)/ CeO2(111)). Here, X denotes a halogen atom. The reactions 1/2X2 + CeO2(111) → (X−O)/CeO2(111) and 1/2X2 + CeO2(111) → (X−Ce)/CeO2(111) are both endoergic. One would therefore be inclined to predict that the reaction X2 + CeO2(111) → (X,X)/CeO2(111), which forms two halogen atoms adsorbed on the surface, would be even more endoergic. This is indeed the case if both halogens adsorb on a Ce site, or © 2012 American Chemical Society

both adsorb on an oxygen site. However, the reaction X2 + CeO2(111) → (X−O, X−Ce)/CeO2(111), in which one X formed by dissociation binds on O and the other X binds on Ce, is exoergic. Our calculations show that this strong interaction between the dissociation fragments occurs because the X atom bound to O is a Lewis base (electron donor) and the X atom bound to Ce is a Lewis acid (electron acceptor). We have seen similar behavior in the coadsorption of a Lewis acid with a Lewis base on rutile TiO2(110).19 Therefore, we postulate that this is a general behavior: the coadsorption of a Lewis base with a Lewis acid, on an oxide surface that is not a strong acid or base, results in a large stabilization energy (“attractive” interaction energy between the acid and the base). This stabilization does not take place because the acid binds chemically to the base: it is caused by an indirect, through the oxide, interaction. To test this propensity rule, we have studied the coadsorption of halogens with other acids or bases. We found that the presence of a hydroxyl (a Lewis base) forces the halogen to bind to the Ce site where it is a Lewis acid, and stabilizes considerably the bond of the halogen to the oxide. La-doped CeO2 is a Lewis acid, and on this surface a halogen atom binds to the O site, on which it is a base. The bond of the halogen atom to an O site on the surface of the doped oxide is considerably stronger than the same bond to the undoped oxide because the halogen in X−O is a base and the doped oxide is a much stronger Lewis acid than the undoped one. Moreover, binding the halogen atom Received: December 5, 2011 Revised: February 9, 2012 Published: February 16, 2012 6664

dx.doi.org/10.1021/jp211693v | J. Phys. Chem. C 2012, 116, 6664−6671

The Journal of Physical Chemistry C

Article

Table 2. Energy of the Reaction 1/2Cl2 + MCeO2 → Cl/ MCeO2, Where M Is a Substitutional Dopant (M = La or Ta)a

to the Ce site (of the La-doped ceria) is energetically uphill because O−Ce is a Lewis acid and so is the doped oxide. The acidity of the surface discourages the adsorption of another acid. Ta-doped ceria surface is a Lewis base, because the pentavalent Ta replaces a tetravalent Ce atom. On this basic surface, the halogen binds to the Ce site where it is an acid, and its bond is substantially stronger than on the pure ceria. The adsorption on the site where the halogen is a base is uphill.

Ead[Cl]/eV dopant

Cl is donor

Cl is acceptor

La Ta

−1.46 +0.10

not observed −1.09

a

Cl donor means that the Cl atom binds to O, and acceptor means that Cl binds to Ce.

2. COMPUTATIONAL METHODOLOGY All calculations reported here were performed with the LDA+U method,20 with a U value of 5.5 eV for Ce. This method has been used in previous work to study various properties of ceria.21−33 We are interested here only in showing that the interaction between Lewis acids and bases, through the surface, is very strong. This conclusion is robust because this interaction is much stronger than the errors expected from the DFT+U method. As in previous work,25,32−34 we use a p(3 × 3) slab to simulate the CeO2(111) surface; this has nine atomic layers (three CeO2 layers) with a total of 81 atoms. The vacuum layer has 15 Å, and the volume of the supercell is 11.45 × (11.45 × (√3)/(2)) × 22.79 Å3. All calculations were performed with the VASP35−39 package with PAW40,41 pseudopotential. We used a 2 × 2 × 1 k-point mesh and a 400 eV energy-cutoff for plane wave basis set. In geometry optimization, all atoms except the oxygen atoms in the bottom layer were fully relaxed until the force on each atom was less than 0.02 eV/Å. The total energy was considered converged when the energy difference between two successive iterations was less than 10−4 eV, for geometry optimization, or 10−5 eV for a single-point energy calculation.

the atom in the chemical compound of interest. A positive value means that when forming the compound the atom donates electrons (it is a Lewis base); negative values mean that the atom gains electrons (it is a Lewis acid). The number of electrons on an atom within a compound is calculated by the Bader method42 with the algorithm proposed by Henkelman and Jonsson.43,44 The Cl−O bond distance is 1.67 Å, which is the same as the Cl−O bond distance in the gas-phase HClO molecule. In what follows, we will regard the adsorption of a Cl atom on an O site as the replacement of a surface oxygen atom with a Cl−O group. Formally, this is a substitutional anion doping: O is replaced with Cl−O. From the point of view of formal charges, we replace an O2− anion with a ClO− anion, and this creates an electron “excess” in the oxide because ClO only “binds” one electron instead of two. Thus, adsorbing Cl on an oxygen atom turns the oxide into a Lewis base. We will show later that the extra electron reduces a Ce4+ ion to Ce3+. We have used here formal charges to describe the system, with the understanding that in no system do the ions have the formal charges. To substantiate the formal picture used above, we have calculated the BV of the atoms involved in the discussion. The BV of Cl (in Cl−O) is 0.15 electron. The Cl atom loses electron charge when it makes the bond, despite its high electronegativity. The BV on the oxygen atom in the Cl−O group is −0.91 electron. Thus, the net BV on the Cl−O group is −0.91 + 0.15 = −0.76 electron. This is very different from the BV of −1.20 electron, on an oxygen atom in the surface layer of the clean CeO2 surface. Thus, replacing an O atom with a Cl−O group does create an excess of electrons in the rest of the doped oxide as compared to the case in which no Cl atom is adsorbed. The upshot is that creating a Cl−O group on the surface causes an electron excess in the oxide. To find out where the excess electron goes, we need to examine the partial density of states (PDOS) of the (Cl−O)/ CeO2 system (Figure 1a). The state in the gap is a Ce 4f state (see Figure 1b), which appears because one Ce atom is reduced when Cl is adsorbed on O. We can view the adsorption of Cl to form Cl−O as a two-step process: first, one forms an oxygen vacancy, and then one adsorbs an O−Cl group at the vacancy site. Forming the vacancy leaves behind two unpaired electrons; adsorbing O−Cl at the vacancy site uses one of them (to make O−Cl−). The bonding orbital of this group is shown in Figure 1c. The other unpaired electron reduces a Ce4+ ion to Ce3+; the orbital in the gap is the one occupied by this electron, and its shape and location in the oxide are shown in Figure 1b. Such electrons form polarons,21,27,45−48 and the energy of the system depends on the location of the polaron with respect to the Cl−O site. Calculating the energy of the system for different polaron locations is outside the scope of this Article. On the basis of previous work, we know that finding the best location

3. RESULTS AND DISCUSSION 3.1. The Adsorption of a Single Cl Atom. We are interested in the dissociative adsorption of Cl2, and it is instructive to compare the energy of this process with the energy of adsorbing two noninteracting Cl atoms. A Cl atom could bind on the CeO2(111) surface either on top of an oxygen atom or on top of a Ce atom. We denote by Cl−O the Cl atom bound to oxygen and by Cl−Ce the Cl atom bound to Ce. The energy of the reaction 1/2Cl2(g) + CeO2 → (Cl−O)/ CeO2 is +0.24 eV (see Tables 1 and 2). The adsorbed Cl atom Table 1. Results for Cl Adsorption on CeO2(111)a model

ΔEad/eV

(Cl−O)−CeO2 (Cl−Ce)−CeO2 (Cl−O, Cl−Ce)−CeO2 (Cl−Ce, Cl−Ce)−CeO2

+0.24 +0.18 −0.58 0.43

ΔEint/eV

Cl Bader valence

−1.01 0.07

+0.15/D −0.33/A −0.62/A, +0.18/D −0.31, −0.32/A, A

Ead is the energy of the reaction 1/2Cl2 + CeO2(111) → (Cl−A)/ CeO2(111). Here, A indicates the binding site (O or Ce). ΔEint is the interaction energy defined in the text. The Bader valence (BV) indicates how much electronic charge is lost (positive value) or gained (negative value) by an atom. A indicates that the halogen is an electron acceptor (Lewis acid), and D indicates that it is a donor (Lewis base). a

is metastable with respect to the recombination of two adsorbed Cl atoms to form Cl2 in the gas phase. In this Article, we use what we call the Bader valence (BV): the number of valence electrons (i.e., those not included in the core) of the neutral atom in gas, minus the number of electrons on 6665



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Figure 2. (a) PDOS of CeO2(111) with a Cl atom adsorbed at a Ce site. (b) Top view of the structure and the charge-density of the nonbonding orbital. (c) Side view of the structure and the charge density of nonbonding orbital. The color scheme and the iso-surface used for calculating the charge density are the same as in Figure 1.

Figure 1. (a) The partial density of states (PDOS) for the (Cl−O)/ CeO2(111) (a Cl atom bound to an O atom on the surface of CeO2(111). (b) Charge density of a localized Ce 4f state whose energy is in the gap; the state is viewed from above the surface. (c) Charge density of Cl−O σ-bonding state (side view). Black dashed lines represent the unit cell, red balls are oxygen, yellow ball is Cl, gray balls are Ce, and the blue surface is an iso-surface of charge density (the charge density value on the surface is 0.02 e/Å3).

that is 2.28 Å away; the BV on this oxygen atom is −0.97, which is smaller that the BV of −1.20 electron on the oxygen atoms in the surface of pure ceria. Figure 2a gives the PDOS of (Cl−Ce)/CeO2(111). The system has two spin-orbitals in the gap, formed by a combination of atomic orbitals of Cl and oxygen (in the figure, the Cl and O densities of states are equal, and because of this only one of them is seen in the graph). Only one of these two states is occupied. No Ce atom is reduced when Cl−Ce is formed. The fact that the Cl atom in the Cl−Ce group is a Lewis acid and the Cl atom in the Cl−O group is a Lewis base plays an important role in what follows. 3.2. The Dissociative Adsorption of Cl2. Because there are two adsorption sites for each Cl atom, there are three pairs

of the polaron will lower the energy of the final state. For ceria, this lowering is approximately 0.3 eV. The energy of the reaction 1/2Cl2 + CeO2(111) → (Cl−Ce)/ CeO2(111) is +0.18 eV (Table 1). This state is also metastable with respect to the recombination of adsorbed Cl and formation of a gas-phase Cl2 molecule. The BV of Cl in Cl−Ce is −0.33 electron (compare this to +0.15 for Cl in Cl−O); the Cl atom in the Cl−Ce group is a Lewis acid. The presence of the Cl atom affects an oxygen atom 6666



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bases, and the dissociation is no longer helped by the formation of a Lewis acid−base pair. Figure 4 gives some information about the (Cl−Ce, Cl−Ce)/ CeO2(111) system. The PDOS in Figure 4 are very similar to the PDOS in Figure 1a (for Cl−Ce/CeO2(111)). The only difference is that when there are two Cl−Ce groups on the surface, we have twice as many states in the gap. The iso-density plot of the K−S orbitals in the gap is similar to the one shown in Figure 1b. The BVs of −0.31 and −0.32 electron, for the two Cl atoms in (Cl−Ce, Cl−Ce)/SiO2(111), are almost the same as the value of −0.33 electron obtained in the case of (Cl−Ce)/CeO2(111). This is consistent with the fact that the interaction energy of the two Cl atoms formed by the dissociation of Cl2 to form (Cl−Ce, Cl−Ce)/CeO2 is practically zero. 3.3. Cl Coadsorption with a Lewis Acid or a Lewis Base. We have argued so far that (Cl−O, Cl−Ce)/CeO2(111) is stable because one Cl atom is a Lewis acid and the other is a Lewis base. If this is true, the adsorption of Cl on the ceria surface can be manipulated by coadsorption of Cl with a Lewis acid or a Lewis base. We expect that the presence of a Lewis acid on the surface will induce the Cl atom to bind strongly to an oxygen atom where it is a Lewis base; similarly, the presence of a Lewis base on the surface should drive the Cl atom to bind strongly to a Ce atom to form a Lewis acid. In this section, we perform calculations to test these expectations. To create a ceria surface that is a Lewis acid, we dope it substitutionally with a La atom. This LaCeO2(111) system is created by removing a Ce atom in the top surface layer and replacing it with a La atom. Because the trivalent La atom replaces a tetravalent Ce atom, the doped oxide has a “deficit” of electrons; therefore, this system is “inclined” to gain electrons, and it is therefore a Lewis acid. In previous work,32,33 we have shown that doping with La creates a hole in the valence band. This lowering of the LUMO of the oxide (which is now at the top of the valence band instead of the bottom of the conduction band) means an increase in the strength of the Lewis acidity of the system. The undoped CeO2 is a weak Lewis acid because of the tendency21,27 of Ce4+ to be reduced to Ce3+; doping with La makes CeO2 a much stronger Lewis acid. We have placed a Cl atom on all possible binding sites on the LaCeO2 surface and found that the highest energy for the reaction:

of binding sites when a Cl2 molecule adsorbs dissociatively. The reaction: Cl2 + CeO2 (111) → (Cl−Ce, Cl−O)/CeO2 (111)

has the highest dissociative adsorption energy, equal to ΔEad = −0.58 eV (see Table 1). Here (Cl−Ce, Cl−O)/ CeO2(111) represents the CeO2 slab with a Cl adsorbed on the oxygen site and another Cl adsorbed on a Ce site. This fairly high exothermicity is unexpected, because the adsorption of a single Cl atom is endoergic regardless of the adsorption site (see Table 1). Therefore, there is a strong interaction between the two Cl atoms formed by the dissociation of Cl2. We have pointed out in work on other systems19 that this also happens when a Lewis acid is coadsorbed on TiO2 with a Lewis base. We can calculate the interaction energy between the fragments A and B, formed by the dissociative adsorption of the molecule AB, by using: E int[AB] = Ead[(A, B)/CeO2 ] − Ead[A/CeO2 ] − Ead[B/CeO2 ]

Here, Ead[(A,B)/CeO2] is the energy of the CeO2 slab with both fragments produced by the dissociation adsorbed on it, and Ead[A/CeO2] is the energy of the CeO2 slab with the fragment A adsorbed on it (in the absence of B). The meaning of Ead[B/CeO2] is similar. The interaction energy for the case when A = Cl−O and B = Cl−Ce is −1.01 eV (Table 1). Other definitions of the interaction energy are possible, and they will differ by an energy that is constant throughout this Article; therefore, any one of these definitions is acceptable because our purpose is to compare different dissociation reactions on ceria. The BV for the Cl atom bound at the Ce site, in (Cl−O, Cl− Ce)/CeO2, is −0.62 (Table 1), which is nearly twice the value of the BV for a Cl atom adsorbed alone on the Ce site (which is −0.33 (Table 1)). The BV value on the Cl atom adsorbed on an oxygen site, in the compound (Cl−O, Cl−Ce)/CeO2(111), is the same as that obtained when Cl adsorbed alone on the oxygen site. There is, however, a fundamental difference between (Cl−O)/CeO2 and (Cl−O, Cl−Ce)/CeO2. When Cl adsorbs alone on O, the chlorine atom loses charge and a Ce4+ ion is reduced; Ce4+ is a Lewis acid, and it forces the Cl atom to behave like a Lewis base. However, on the (Cl−Ce, Cl−O)/ CeO2 surface, the Cl atom in the Cl−Ce group is a strong Lewis acid and takes the electron that would have otherwise gone to reduce Ce4+; therefore, the Cl−Ce group is a stronger Lewis acid than is the Ce4+ ion. The PDOS of (Cl−Ce, Cl−O)/CeO2(111) are shown in Figure 3. The energies of four spin−orbitals are located in the gap, just above the valence band and below the Fermi level. Isodensity surfaces of the orbitals having energies above the valence band are shown in Figure 3b and c; these orbitals are essentially p-electrons localized on the Cl atoms. The new states formed by Cl2 dissociation located below the valence band are all related to the Cl−O bonds. We have also examined the possibility that Cl2 dissociates with both Cl atoms bound at the Ce site. In this case, both atoms are Lewis bases (if adsorbed alone). The energy of the reaction:

1 Cl2 + LaCeO2 (111) → (Cl−O)/LaCeO2 (111) 2

is −1.46 eV. In this final state, the Cl atom binds to an oxygen atom to form a [Cl−O]− anion, and the Cl atom is an electron donor (Lewis base). Contrast this with the binding energy of +0.24 eV when Cl binds to an oxygen atom on the surface of the undoped ceria. Clearly, the presence of the La dopant increases dramatically the binding energy of the Cl atom and also “guides” the atom to bind to an oxygen site where it functions as a Lewis base. We have also found that Cl does not bind to any site on the LaCeO2(111) surface in which it would be an electron acceptor (Lewis acid). Thus, the acidity of the oxide (created by doping with La) prevents the adsorption of Cl as a Lewis acid and stimulates the adsorption of Cl as a Lewis base. Because Cl is a very electronegative element, one would expect it to act as an acid (acceptor), but apparently LaCeO2 is a stronger acid than Cl and Cl is forced to act as a base. Doping Ce with Ta creates an excess of electrons in the oxide (Ta is pentavalent), and we expect TaCeO2(111) to be a strong

Cl2 + CeO2 (111) → (Cl−Ce, Cl−Ce)/CeO2 (111)

is 0.43 eV, and the interaction energy between the two dissociation fragments is 0.07 eV. In forming the product (Cl−Ce, Cl−Ce)/CeO2(111), we are trying to coadsorb two Lewis 6667



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Figure 3. (a) PDOS of CeO2(111) with two adsorbed Cl atoms, one on an oxygen site and the other on a Ce site (a Lewis acid−base pair). (b,c) Cl 2p states for the Cl that accepts electrons (Cl in Cl−Ce); (d−f) Cl−O σ and π bonding states for Cl that donates electrons. Color scheme and isosurface setting are as in Figure 1.

In this case, Ce4+ acts as an acid. However, when Cl is adsorbed on the TaCeO2 surface, Cl is a stronger acid than Ce4+, and as a result the electron provided by Ta is used to bind more strongly the Cl atom; we see no reduction of a Ce ion when Cl is adsorbed on a Ce site. A further test of the idea that a Lewis acid−base interaction plays an important role in chemisorption on oxides was performed as follows. A H atom tends to donate electrons to an oxide when it adsorbs to make a hydroxyl,19,49 and it is therefore a Lewis base. The energy of the reaction 1/2H2 + CeO2(111) → H/CeO2(111) is −1.24 eV (Table 3). The energy of the reaction 1/2H2 + 1/2Cl2 + CeO2(111)→ (H−O, Cl−O)/CeO2(111) is −1.16 eV (Table 3). In the final state, Cl adsorbs on the O atom and is a Lewis base; the BV for the Cl

Lewis base. According to the rule formulated here, this dopantinduced basicity will force an adsorbed Cl atom to act as a Lewis acid (electron acceptor). Our calculations show that Cl adsorbs on TaCeO2(111) on two sites. On one, Cl adsorbs on an oxygen atom and it is a Lewis base (donor); the binding energy (i.e., the energy of the reaction 1/2Cl2 + TaCeO2 → (Cl−O)/LaCeO2) is +0.10 eV. When Cl binds to a Ce atom, it is a Lewis acid, and the energy of the reaction 1/2Cl2 + TaCeO2 → (Cl−Ce)/LaCeO2) is −1.09 eV. As expected, a modification that increases the Lewis basicity of the surface will help a Lewis acid to bind more strongly. It is interesting to note that in previous work32 we have shown that in Ta-doped ceria the Ta dopant loses an electron, which is transferred to a Ce4+ ion (which is reduced to Ce3+). 6668



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Figure 4. PDOS and charge density for two adsorbed Cl atoms on the Ce sites (Lewis acids). Four nonbonding states appear in the band gap, and two of them are occupied; (b) top view and (c) side view of the electron density surface of the nonbonding state on one Cl; (d) top view and (e) side view of the electron density surface of the nonbonding state on another Cl. The color scheme and iso-surface value are the same as in Figure 1.

Table 3. Energy of Reactionsa model

Ead/eV

Eint/eV

Cl Bader valence/Cl type

(H−O)−CeO2 (H−O, Cl−O)−CeO2 (H−O, Cl−Ce)−CeO2

−1.24 −1.16 −2.01

−0.16 −0.96

+0.11/D −0.64/A

as a Lewis acid, increases the binding energy of Cl, and dictates the binding site (Cl binds to Ce not to O).

4. BR ADSORPTION ON CEO2(111) If the model proposed here is correct, we should observe the same behavior when we adsorb Br. Indeed, we do. The results of the calculations are collected in Table 4. The trends and their interpretation are the same as for Cl, but the numerical values of the reaction energies and the BVs of the adsorbates are different, as expected. The Lewis acid−base interaction in this system is weaker than in the case of chlorine, because Br is a weaker Lewis acid than Cl.

a

In the column with the heading Ead, we show the energy of the reaction 1/2H2 + CeO2(111) → (H−O)/CeO2(111), the energy of the reaction 1/2H2 + 1/2Cl2 → (H−O, Cl−O)/CeO2(111) in which Cl is a bound to an O site (Lewis base), and the energy of the reaction 1/2H2 + 1/2Cl2 → (H−O, Cl−Ce)/CeO2(111), where Cl is bound to a Ce site (Lewis acid). Eint is the interaction energy between H and Cl defined in the text. The last column gives the Bader valence for Cl, and the letters D or A indicate whether Cl is an electron donor or acceptor (base or acid), respectively.

5. POSSIBLE APPLICATION TO ALKANE COUPLING

atom is 0.11 electron, just like the case when the Cl is adsorbed alone. Coadsorbing two Lewis bases does not lead to any substantial interaction between adsorbates. However, the energy of the reaction 1/2Cl2 + 1/2H2 → (H−O, Cl−Ce)/ CeO2(111) is −2.01 eV; the interaction energy between H and Cl is −0.96 eV; and the BV of Cl is −0.64 electron. Again, the presence of H, which is a Lewis base, induces the Cl atom to act

This acid−base model suggests that a halogenated ceria surface (i.e., a ceria surface on which we have adsorbed a halogen at low coverage) may be more favorable for alkane activation than the clean ceria. We consider here the possibility that methane interacts with a halogenated surface and produces a CH3 radical in the gas phase and an adsorbed H atom. Because the H atom 6669



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“manipulate” the chemistry of the surface. A halogen atom can bind to a Ce atom or an O atom at the surface of Ce(111), and either process is energetically uphill. However, if a halogen atom adsorbs on a Ce site and the other on an O site, the adsorption energy is downhill, because of a strong interaction between the Lewis acid (Cl−Ce) and the Lewis base (Cl−O). If we adsorb a H atom to form a hydroxyl, this acts as a Lewis base and forces Cl to bind to the Ce site where it is a Lewis acid. Doping the surface with La turns it into a Lewis acid, and this forces the halogen atom to bind to the oxygen site on which it is a Lewis base. Doping with Ta has the opposite effect because the presence of Ta turns the surface into a strong Lewis base, and this forces Cl to bind to a site where it is a Lewis acid. We suggest that halogenated surfaces may break the C−H bond in methane easier than does the clean oxide surface.

Table 4. Computational Results for Br Adsorption, Br2 Dissociation, and the Interaction of Br with Coadsorbed Ha

a

model

Ead/eV

(Br−O)−CeO2 (Br−Ce)−CeO2 (Br−O, Br−Ce)−CeO2 (Br−Ce, Br−Ce)−CeO2 (H−O)−CeO2 (H−O, Br−O)−CeO2 (H−O, Br−Ce)−CeO2

+0.23 +0.19 −0.27 0.43 −1.24 −1.14 −1.75

Eint/eV

Br Bader valence/Br type

−0.69 0.06

+0.26/D −0.28/A −0.60/A, +0.28/D −0.23/A, −0.23/A

−0.14 −0.70

+0.25/D −0.60/A

The notation is similar to that in Tables 1−3.

is a Lewis base and Cl can function as a Lewis acid, the presence of the halogen will cause the H atom to bind more strongly and therefore shifts the reaction energy favorably. The energy of the reaction:

■

Corresponding Author

CH 4(g) + (Cl−Ce)/CeO2 (111)

*E-mail: [email protected].

→ CH3(g) + (H−O, Cl−Ce)/CeO2 (111)

Present Address †

School of Physics, Nankai University, Tianjin 300 071, People’s Republic of China.

is 0.71 eV, if Cl is adsorbed on Ce. This is much lower than the energy of reaction on pure ceria, which is 1.67 eV (Table 5). If we preadsorb Br2 to form Br−Ce on the surface, the energy of the methane dissociation reaction is +0.97 eV (Table 5). As we have already mentioned, the reaction energy is reduced because H is a Lewis base and the halogen is a Lewis acid. A more dramatic reduction of the reaction energy occurs on La-doped CeO2(111), where the reaction energy is 0.01 eV (Table 5).

Notes

The authors declare no competing financial interest.

■

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 number TG-ASC090080. We acknowledge support from the Center for Scientific Computing from the CNSI, MRL: an NSF MRSEC (DMR-1121053) and 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 no. DE-AC02-06CH11357.

Table 5. Reaction Energy of Dissociative Adsorption of CH4 with the Formation of a CH3 Radical in the Gas Phase (CH4(g) + surface → CH3(g) + H/surface)a CH4(g) + surface → CH3(g) + H/surface surface: ΔE/eV:

CeO2(111) +1.67

(Cl−Ce)/ CeO2(111) +0.71

(Br−Ce)/ CeO2(111) +0.97

AUTHOR INFORMATION

LaCeO2(111) +0.01

■

a

The second column gives the energy when the reaction takes place on CeO2(111), the third column gives the reaction energy on (Cl− Ce)/CeO2(111) (Cl is a Lewis acid), the fourth column gives the reaction energy on (Br−Ce)/CeO2(111) (Br is a Lewis acid), and the fifth column gives the reaction energy when the surface is La-doped CeO2(111) (which is a Lewis acid).

REFERENCES

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This happens because La-doped CeO2(111) is a much stronger Lewis acid then (Cl−Ce)/CeO2(111). However, the formation of adsorbed H, which is a Lewis base, will neutralize the Lewis acidity of the surface through what we called previously a chemical compensation effect.33 The surface will be reactivated only if the adsorbed hydrogen is removed by a reaction or by desorption. If a halogen is used in the gas feed, to activate the surface, the hydrogen may be removed by desorption as a halogen hydride. Predicting the evolution of the system after the breaking of the C−H bond is beyond the scope of this Article. The recombination of the CH3 radicals to form methane is a possibility. It is also possible to form a methyl halide, which would be beneficial because this can be converted to useful products easier than can methane.3,4,8,50−52

6. CONCLUSIONS These calculations show that adsorbed Lewis acids and Lewis bases interact strongly, and this interaction can be used to 6670



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