Acid–Base Interaction and Its Role in Alkane Dissociative

Article pubs.acs.org/JPCC

Acid−Base Interaction and Its Role in Alkane Dissociative Chemisorption on Oxide Surfaces Steeve Chrétien and Horia Metiu* Department of Chemistry & Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States ABSTRACT: We use density functional theory to examine methane dissociative adsorption on lanthana to produce H and CH3 bonded to the surface. Previous work suggested that the binding energy of a Lewis base and a Lewis acid to an oxide exceeds by far the value expected from adding the binding energies of the same compounds chemisorbed alone. This rule suggests that the fragments (H and CH3) formed by the dissociative adsorption of methane on an oxide will adsorb so that they could benefit from this strong acid−base interaction. To do this, one fragment must bind to oxygen and the other to a cation. Such a configuration should have lower energy than the one obtained by binding both fragments to oxygen, even though each fragment prefers to bind to oxygen when it binds alone. We suggest that this behavior is encountered in most systems.

1. INTRODUCTION The conversion of the alkanes from natural gas to higher value chemicals is an important goal of industrial organic chemistry.1,2 Much work has been devoted to the use of oxide surfaces or modified oxide surfaces as catalysts for alkane activation.3−5 The rate-limiting step for alkane conversion is the breaking of the C−H bond by dissociative chemisorption of the alkane:

surface, both halogen atoms will bind to oxygen. This is not what happens: in the state having the lowest energy, one halogen atom binds to oxygen and the other to cerium. In this configuration the halogen atom bound to oxygen donates electrons to the halogen atom bonded to cerium, and this Lewis acid−base interaction lowers the energy. A similar observation was made by Schneider,13,14 who showed that when one NOx molecule adsorbs on CaO, or BaO, or MgO, its presence will induce the other NOx molecule to bind more strongly to the cation. In this article we investigate whether this type of behavior is observed for the dissociative adsorption of an alkane on an oxide. Each dissociation fragment (the hydrogen atom and the alkyl) binds to the oxygen when binding alone to the surface, and each acts as a Lewis base (electron donor). However, hydrogen is amphoteric: when it forms hydroxyls, it is a Lewis base, but if it forms a hydride, it is a Lewis acid (in a hydride H has a formal charge of −1). It is therefore possible that the adsorption of the alkyl to an oxygen atom will force H to bind to the cation since in this configuration the alkyl is a Lewis base and the hydrogen in a Lewis acid. This would be unexpected since neither H nor CH3 would bind to the cation if each is alone on the surface. The formation of an acid−base pair is expected to lower the energy of coadsorption much below the sum of the energies of binding the fragments alone.5,11 This article presents density functional theory (DFT) calculations that show that this conjecture is correct in the case of the dissociative chemisorption of methane on several irreducible oxides. Reducible oxides may behave differently because their cations act as acids, and this acidity competes with the hydride for the electron donated by the alkyl.

R−H → R/S1 + H/S2

Here R−H is an alkane and S1 and S2 are the surface sites to which the fragments produced by dissociation adsorb. Many computations have shown that, when chemisorbed, a hydrogen atom prefers to bind to an oxide surface by making a hydroxyl, and an alkyl radical binds to a surface oxygen atom to form an alkoxide; neither H nor the alkyl binds to the cation. These conclusions are valid when each fragment is chemisorbed alone on a clean oxide surface (no dopants, or defects, or hydroxyls). Given this information, it was natural to assume that when an alkane chemisorbs dissociatively, the fragments (H and alkyl) will bind to the sites they prefer when they are alone on the surface. This means that both will bind to the oxygen atoms. We show here that, for subtle reasons, this assumption if completely off the mark. The importance of Lewis acid−base interactions for chemisorption on oxide surfaces has been pointed out for many specific examples.6−10 These results were generalized to suggest5,11 that the presence of a Lewis base on an oxide surface enhances the adsorption energy of a Lewis acid, and vice versa. Of interest to the present article is the observation that if two amphoteric species are coadsorbed, one will bind to a site where it can function as a Lewis base and will force the other to adsorb to a site where it can act as a Lewis acid.7,10,12 For example,7 a halogen atom will bind to an oxygen atom on the surface of CeO2; therefore, one would expect (perhaps naively) that when a halogen molecule adsorbs dissociatively on the © 2014 American Chemical Society

Received: July 18, 2014 Revised: October 28, 2014 Published: November 13, 2014 27336

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2. COMPUTATIONAL METHOD The Vienna Ab Initio Package (VASP)15−18 was used to perform spin-polarized DFT calculations with the Perdew− Burke−Ernzerhof (PBE)19 exchange and correlation functional. The number of unpaired electrons (Ns) was kept constant during the geometry optimization. The ionic cores were described by scalar relativistic projected augmented wave (PAW) pseudopotentials.20,21 12 (Ce), 11 (La), 10 (Ca and Ti), 8 (Mg), 6 (O), 4 (C), and 1 (H) valence electrons were explicitly taken into account using a plane wave basis set with an energy cutoff of 400 eV. The effect of spin−orbit coupling was ignored. Fractional occupancies of the Kohn−Sham molecular orbitals were allowed using the Gaussian smearing method and a window of 0.05 eV. The Brillouin zone was sampled at the Γ-point only. The electrostatic interaction between the slab and its periodic images in the direction perpendicular to the slab was canceled by applying monopole, dipole, and quadrupole corrections to the energy.22 The La2O3(001) surface was modeled using an hexagonal [3 × 3] supercell and a slab composed of 3 stoichiometric (15 atomic) layers (see Figure 1). Previous work6 showed that GGA+U is not needed for the description of this system.

difference between the parameters U and J) on the decomposition of methane on the TiO2(110) surface has been addressed by considering the values Ueff = 3.0 eV and Ueff = 4.5 eV. The parameter Ueff = 4.5 eV has been used for Ce. A vacuum space of 15 Å was inserted between the slab and its periodic replicas along the direction perpendicular to the surface. The atoms in the bottom stoichiometric layer were held fixed at their bulk positions during the geometry optimization. The positions of the remaining atoms were optimized to give the minimum energy. The optimization was carried out until the forces on the atoms were smaller than 0.01 eV/Å. The optimization of geometry was performed by using the limited-memory Broyden−Fletcher−Goldfarb−Shanno algorithm implemented in the VASP Transition State Tool package (this toolkit can be downloaded without cost at http:// theory.cm.utexas.edu/vtsttools/). The electronic energy was considered self-consistent when the energy change was 10−5 eV. The molecular symmetry was not imposed during optimization. The dissociative adsorption of methane took place on only one side of the slab.

3. DISSOCIATIVE ADSORPTION OF CH4 Figure 2 shows the lowest-energy structure when H or CH3 binds alone to the surface. Both fragments, when adsorbed

Figure 2. Lowest-energy structure formed by adsorbing H or CH3 alone on the clean stoichiometric La2O3(001) surface. (a) The geometry in which a H atom forms the strongest bond to the La2O3(001) surface. The figure at left is a side view; the one at the right is the surface seen from above. The surface oxygen atom to which the hydrogen binds (to form a hydroxyl) is colored blue. ΔE[CH4] is the energy of the reaction CH4(g) + La2O3 → (H−O)/La2O3 + CH3(g). The symbol (H−O)/La2O3 indicates that H binds to a surface oxygen atom. Q[H] is the Bader charge on the chemisorbed H atom, which donates 0.65 electrons to the oxide. (b) Similar results for the lowest-energy structure of a methyl radical bound to the surface. ΔE[CH4] is the energy of the reaction CH4 + La2O3 → (CH3−O)/ La2O3 + H(g). CH3 donates 0.53 electron to the oxide when it binds to form a methoxide. The structures are doublet (Ns = 1).

Figure 1. Clean stoichiometric La2O3(001) surface. (a) Side view showing one stoichiometric (five atomic) layer. (b) Top view showing the [3 × 3] supercell used in calculations. The labels L1 to L5 indicate the position of the layers relative to the vacuum. L1 is closest to and L5 is farthest from the vacuum.

A [3 × 3] supercell was used to mimic the infinite rock-salt MgO(001) and CaO(001) surfaces. The slab was composed of 3 stoichiometric (3 atomic) layers. The rutile TiO2(110) surface was modeled using a [4 × 2] supercell. The slab was made of 4 stoichiometric (12 atomic) layers. A [4 × 3] supercell and a slab composed of 3 stoichiometric (9 atomic) layers were used to model the fluorite CeO2(111). The approach due to Dudarev et al.23 was used to include the on-site correction to the 3d electron of Ti atoms and to the 4f electron of Ce atoms. The effect of the parameter Ueff (the

alone on the surface, bind exclusively to an oxygen atom. All attempts to adsorb a fragment on a La atom failed: during the optimization the fragment moves on top of a nearby oxygen atom or moves into the vacuum. In Figure 2a, the O atom to which H is bound is colored blue and in Figure 2b the O atom to which CH3 is bound is colored pink. In what follows we use the notation (H−O)/La2O3 to denote a structure in which one H atom per supercell is bound to a surface oxygen atom. The notation (CH3−O)/La2O3 has a similar meaning. 27337

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It was therefore assumed, quite reasonably, that when CH4 adsorbs dissociatively, it will make the structure shown in Figure 3a and denoted by (H−O, CH3−O)/La2O3. In other

Table 1. Three Possible Binding Schemes for the Fragments Produced by the Dissociative Adsorption of CH4 on La2O3(001)a reaction 1 2 3

La2O3 + CH4(g) → (H−O,CH3− O)/La2O3 La2O3 + CH4(g) → (H−La,CH3− O)/La2O3 La2O3 + CH4(g) → (H−O,CH3− La)/La2O3

ΔE[CH4] (eV)

Q[H] (e)

Q[CH3] (e)

4.18

0.57

0.47

2.11

−0.61

0.47

1.62

0.58

−0.65

The first column specifies the dissociative adsorption reaction to different final structures. For example, (H−O,CH3−O)/La2 O3 specifies that H binds to an oxygen atom and CH3 to another oxygen atom. ΔE[CH4] is the reaction energy. All reactions are endothermic, but the final states are metastable. Q[H] is the Bader charge on the chemisorbed H atom, and Q[CH3] is that on the chemisorbed CH3 fragment. Q[H] = 0.57 e means that the chemisorbed H atom lost 0.57 electron as compared to the H atom in the gas. Q[CH3] is reference with respect to the electron charge on gaseous CH3. a

charge of a gas phase H atom and the charge on the H atom in one of the final states of dissociative adsorption of methane (for example, in (H−O, CH3−O)/La2O3). A positive value indicates that H donates electron charge to the oxide. Q[CH3] is the difference between the total electron charge on gaseous CH3 and the electron charge on CH3 in one of the final dissociative adsorption states (for example, (H−La, CH3− O)/La2O3). In the compound (H−O, CH3−O)/La2O3 + Q[H] = 0.57 and Q[CH3] = 0.47, which indicates that both fragments are Lewis bases. In this geometry, no acid−base interaction lowers the energy of the system. The situation is dramatically different when the dissociative chemisorption forms (H−La, CH3−O)/La2O3 (see Figure 3b), with CH3 bonded to a surface oxygen atom and H bonded to La. To be more precise, the H atom is located in the middle of a triangle formed by three La atoms in the surface (see the right-hand-side panel in Figure 3b), rather than being bonded to one La atom. In what follows, whenever we say that a fragment is bonded to La, we mean that it is bonded to three La atoms. The Bader charges of the chemisorbed fragments are now very different. Q[H] = −0.61 e indicates that the H atom in (H−La, CH3−O)/La2O3 gains electron charge (as compared to a gas phase H atom) when it adsorbs on La, which means that it is now a Lewis acid. Moreover Q[CH3] = 0.47 e, which shows that CH3 is a Lewis base. According to the rules proposed previously, there should be a strong energy lowering interaction between these fragments because they form an acid−base pair. Therefore, the energy of the reaction CH4(g) + La2O3(001) → (H−La, CH3−O)/La2O3 should be lower than the energy of CH4(g) + La2O3(001) → (H−O, CH3−O)/ La2O3 since in the latter both fragments are bases. The calculations show that this is the case. The energy of the reaction CH4(g) + La2O3(001) → (H−La, CH3−O)/La2O3 is 2.11 eV (see Table 1), substantially lower than the energy of the reaction CH4(g) + La2O3(001) → (H−O, CH3−O)/La2O, which is 4.18 eV. Changing the final state of the dissociative adsorption reaction from one in which both fragments are bases to one in which one fragment is a base and the other is an acid results in an energy gain of 4.18 − 2.11 = 2.07 eV. If the acid−base interaction is responsible for this energy lowering, then the effect should be also observed when the dissociative adsorption forms (H−O, CH3−La)/La2O3. The structure of this compound is shown in Figure 3c. The Bader

Figure 3. Three structures generated by the dissociative CH4 chemisorption onto the clean, stoichiometric La2O3(001) surface. (a) Structure of (H−O, CH3−O)/La2O3. Both fragments are bonded to surface oxygen atoms forming a hydroxyl (OH) and a methoxy (OCH3) compound. Left panel is a side view; right panel is the view looking toward the surface. (b) Structure of (H−La, CH3−O)/La2O3. The hydrogen atom is adsorbed between three lanthanum atoms to form a hydride (H is charged negatively), and CH3 is adsorbed on an oxygen atom. (c) Structure of (H−O, CH3−La)/La2O3. The methyl radical is adsorbed between three lanthanum atoms, forming a negatively charged species, and H is bound to an oxygen atom. In all figures ΔE[CH4] is the energy of the dissociative chemisorption reaction. The values are positive, indicating an endoergic transformation. ΔE is the relative energy of the final states referenced to structure (c). Q[X] is the Bader charge of the fragment X, indicating by how much the charge on the chemisorbed fragment differs from the charge of the same fragment in gas phase. The structures are singlet (Ns = 0).

words, it was assumed that when they are together on the surface, the fragments will bind to the sites they prefer when they are alone. It is this assumption that is challenged here. The energy of the dissociative adsorption reaction, CH4(g) + La2O3(001) → (H−O, CH3O)/La2O3, is 4.18 eV (see Table 1). The reaction is strongly endothermic, but the state (H−O, CH3−O)/La2O3 is metastable: energy barriers prevent the spontaneous desorption. The energy of the dissociation reaction CH4(g) → CH3(g) + H(g) in a gas phase is 4.72 eV (calculated by DFT). A relatively small energy is gained (4.72 − 4.18 = 0.54 eV) when the dissociation fragments bind to the oxygen atoms on the surface. We use the Bader charges24−27 to determine whether a chemisorbed fragment is a Lewis acid (takes electrons) or a Lewis base (donates electrons). Table 1 gives the Bader charges Q[H] and Q[CH3] of the chemisorbed H and CH3, respectively. Q[H] is the difference between the electron 27338

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charges (see Table 1, row 3) are Q[H] = 0.58 e and Q[CH3] = −0.65 e. The roles of H and CH3 are now reversed: H is a Lewis base, and CH3 is a Lewis acid. The energy of the reaction CH4(g) + La2O3(001) → (H−O, CH3−La)/La2O3 is 1.62 eV. The energy of this reaction is the lowest, consistent with more charge being exchanged between the fragments. This also shows that H is a stronger base than CH3. The Lewis acid−base interaction does not require that the acid is in contact with the base: they do interact even when the distance between them is large (several lattice sites).28 Here, we have considered the adsorption of the two fragments adsorbed far from each other to prevent any direct interaction between them. Bringing the fragments closer or pushing them further apart changes the decomposition energies ΔE[CH4] reported in Table 1 by less than 0.1 eV. This is negligible compared to the accuracy of the calculation and to the increase in the binding energy of the fragments due to the formation of the acid−base pair.

atom to one of the oxygen atoms of the surface (see Figure 2a), not with the hydroxyls in which HO− binds to a cation. This section shows that the presence of the hydroxyl, which is a Lewis base, facilitates the dissociation of the methane to form a hydride and a CH3 radical in the gas-phase. In the case of La2O3 the process CH4(g) + La2O3 → (H− O)/La2O3 + CH3(g) is endoergic by 3.95 eV. The geometry of the final state is shown in Figure 2a. If a hydroxyl is present on the surface, there are two possible final states for the above process. In one the H atom binds to an oxygen atom to form a hydroxyl, a process that is symbolized by CH4(g) + (HO)/ La2O3 → (HO, HO)/La2O3 + CH3(g). In the other, the hydrogen shed by the methane forms a hydride, as symbolized by CH4(g) + (HO)/La2O3 → (HO, H−La)/La2O3 + CH3(g). Note that in both cases the CH3 radical is gaseous. The second process creates a Lewis acid−base pair and is expected to be favored over the first one (which creates two bases). This anticipation, based solely on the acid−base rules, is supported by DFT calculations. Figure 4 shows the two structures formed by the transfer of one hydrogen atom of methane to the OH/La2O3(001) surface (La2O3 on which a hydroxyl was present prior to CH4 dissociation).

4. ENERGY TO DESORB FRAGMENTS FROM THE SURFACE The oxidative coupling of methane to make ethane and ethylene is performed at very high temperature (≥700 °C). The oxides used to catalyze this reaction must be very poor oxidants (hence very stable) to avoid the oxidation of methane to carbon oxides. This is why irreducible oxides such as La2O3, MgO, etc., were used as catalysts for such reactions. There is experimental evidence that at these high temperatures the surface helps produce CH3 radicals in the gas phase, and these play an important role in forming the products. It is therefore of interest to examine how the acid−base interaction between the chemisorbed fragments affects CH3 desorption. The results are given in Table 2, and they show a dramatic difference between Table 2. Energy for Desorbing a CH3 Radical from the Systems Created by Dissociative Chemisorption of CH4 on La2O3(001) reaction

ΔE (eV)

(H−O, CH3−O)/La2O3 → (H−O)/La2O3 + CH3(g) (H−O, CH3−La)/La2O3 → (H−O)/La2O3 + CH3(g) (H−La, CH3−O)/La2O3 → (H−O)/La2O3 + CH3(g)

−0.23 2.33 1.84

various structures. The desorption of CH3 from (H−O, CH3− O)/La2O3 is energetically downhill, but the presence of (H−O, CH3−O)/La2O3 on the surface is very unlikely since (H−O, CH3−La)/La2O3 has much lower energy. The desorption of a CH3 radical from the latter species requires 2.33 eV. The lowenergy states formed by dissociative adsorption of CH4 are much less likely to release CH3 into the gas. This is consistent with the high temperature required for methane activation in this system. While we have not performed detailed thermodynamic calculations, the fact that TS (T = temperature, S = entropy) for CH3 in gas phase at 1100 K is equal to 2.86 eV indicates that the 2.33 eV desorption energy does not prevent (thermodynamically) the formation of CH3 radicals in the gas.

Figure 4. Two structures formed by transferring a hydrogen atom of CH4 to the La2O3(001) surface precovered with one hydroxyl and moving the CH3 fragment into the vacuum. (a) The hydrogen atom of CH4 is bonded to a surface oxygen atom forming a hydroxyl. ΔE[CH4] is the energy for the transformation CH4(g) + (H−O)/ La2O3 → (H−O, H−O)/La2O3 + CH3(g). (b) Hydrogen atom of CH4 is adsorbed between 3 lanthanum atoms to form a hydride (the hydrogen is negatively charged). ΔE[CH4] is the energy for the transformation CH4(g) + (H−O)/La2O3 → (H−La, H−O)/La2O3 + CH3(g). The values of ΔE[CH4] are positive, indicating endoergic transformations. ΔE is the reaction energy in (a) minus that in (b). Q[H(La2O3)] is the Bader charge on the hydrogen atom of the hydroxyl preadsorbed on the surface. Q[H(CH4)] is the Bader charge on the hydrogen atom that was transferred from CH4 to the surface. The structures are doublet (Ns = 1).

5. CH4 DECOMPOSITION ON A PARTIALLY HYDROXYLATED La2O3 SURFACE Hydroxyls are a common contaminant on oxide surfaces. It is of some interest to determine how their presence may affect the dissociative adsorption of methane. We emphasize that we are concerned with hydroxyls formed by attaching a hydrogen 27339

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base interaction in which the hydroxyl is a base and CH3 is the acid. The acid−base behavior of the adsorbates on these oxide surfaces is not due to a similarity between the cations: La has incomplete d- and f-shells while Mg and Ca atoms have two 2selectrons outside a noble gas shell.

In Figure 4a the hydrogen atom shed by CH4 binds to a surface oxygen atom, forming a second hydroxyl. The Bader charges are 0.6 e on both hydrogen atoms, confirming that both hydrogen atoms donate electrons. The transformation CH4(g) + (H−O)La2O3 → (H−O, H−O)/La2O3 + CH3(g) is endoergic by 3.94 eV. This is similar to the value of 3.95 eV obtained for the dissociation on the clean, stoichiometric La2O3 surface. As expected, the formation of a Lewis base−base pair does not significantly affect the reactivity of the surface. In Figure 4b, the hydrogen atom formed by the dissociation of CH4 is bonded with 3 La atoms, to form a “hydride”. This process is symbolized by CH4(g) + (H−O)/La2O3 → (H−La, H−O)/La2O3. The Bader charge on the hydrogen atom forming the hydride (i.e., H bonded to 3 La atoms) is −0.61 e compared to 0.66 e for the hydrogen atom of the original hydroxyl (present on the surface before CH4 dissociation). This indicates that the final state of the process CH4(g) + (H−O)/ La2O3 → (H−La, H−O)/La2O3 creates a Lewis acid−base pair on the surface. The energy of the reaction CH4(g) + (H− O)La2O3 → (H−La, H−O)/La2O3 + CH3(g) is uphill by 1.62 eV. Contrast this with the energy of CH4(g) + (H−O)La2O3 → (H−O, H−O)/La2O3 + CH3(g), which is 3.94 eV; the formation of a Lewis acid−base pair (i.e., a hydride and a hydroxyl) reduces the decomposition energy of CH4 by (3.94 eV − 1.62 eV =) 2.32 eV. The overall conclusion is that the presence of a preadsorbed hydroxyl facilitates the reaction of methane with the surface to form CH3(g) and a “surface hydride”.

7. DISCUSSION The results presented here show that knowing how CH3 and H bind to an oxide surface, when they bind alone, can lead to an erroneous guess about the binding sites when they bind together. Since CH3 and H are amphoteric (they could be acids or bases, depending on partner), they will have a strong preference for binding sites that allow one of them to be an acid and the other one to be a base. In most cases these binding sites would not be the same as when each fragment binds alone to the surface. In the case of methane dissociative adsorption on La2O3, CaO, or MgO, this means that placing one fragment on oxygen and the other on the cation leads to a lower energy than binding both fragments to oxygen (where they prefer to bind when they are alone on the surface). We see no reason why this would not be the case for any dissociative adsorption resulting in amphoteric fragments and for any irreducible oxide surface. Other examples7,12 of this kind of behavior have been reported. The dissociation of a halogen molecule results in one atom bound to oxygen and another bound to the cation, even though when a halogen atom binds alone to the surface it would bind to oxygen. In another example10 the lowest energy structure of a Au4 or Au6 cluster adsorbed on TiO2 is changed by coadsorption with O2 so that the cluster takes a structure in which it donates electrons to oxygen. A possible exception to this rule are reducible oxides, such as TiO2, CeO2, etc., whose cations are Lewis acids: they have a propensity of accepting electrons29−33 to convert to Ti3+ or Ce3+. This means that H and CH3 can bind to the oxygen atoms at the surface of the oxide and donate electrons to Ce4+ or Ti4+ rather than one binding to oxygen and the other to Ti so that they can exchange electrons with each other (which is the mechanism proposed here). The results of a few exploratory calculations for TiO2 and CeO2 are shown in Table 4: the final state (H−O, CH3−O)/TiO2 is slightly more

6. CH4 DECOMPOSITION ON OTHER IRREDUCIBLE OXIDES The conclusions obtained so far are based on acid−base behavior of the adsorbates rather than on specific properties of lanthanum oxide. One is tempted to assume that this behavior is general for all irreducible oxides, or at least for those that are not radically different from lanthana. To test this possibility, we examined the decomposition of CH4 on two other irreducible oxides: MgO(001) and CaO(001). Table 3 shows the energies associated with the dissociative adsorption of methane on MgO(001) and CaO(001) surfaces. Table 3. CH4 Dissociative Adsorption Energy (in eV) on Various Sites on the MgO(001) and CaO(001) Surfacea 1 2 3 4 a

CH4(g) CH4(g) CH4(g) CH4(g)

+ + + +

MO MO MO MO

→ → → →

(H−O)MO + CH3(g) (H−O, CH3−O)MO (H−O, CH3−M)MO (H−M, CH3−O)MO

MgO(001)

CaO(001)

3.84 4.27 2.40 3.10

3.37 3.46 1.95 2.50

Table 4. Energy of Dissociative Adsorption of Methane on the Reducible Rutile TiO2(110) and Fluorite CeO2(111) Surface ΔE[CH4] (eV) TiO2(110)

Here M represents either Mg or Ca.

All reactions described in the table are endothermic, but CH4(g) + MO → (H−O, CH3−M)/MO, in which H makes a hydroxyl and CH3 binds to the cation, requires the lowest energy (the third line in the table). The H atom in the hydroxyl donates electrons to the CH3 bound to the metal. The behavior is very similar to what we found for La2O3. The adsorption of a CH3 radical on MgO or CaO is endothermic. However, the energy of the reaction (H−O)MO + CH3(g) → (H−O, CH3−M)MO is exothermic by 1.4 eV on both MgO and CaO. In this reaction the CH3 radical adsorbs on a cation site of a surface on which hydrogen has been preadsorbed to form a hydroxyl. Again, this is due to an acid−

CeO2(111)

reaction

U = 3.0 eV

U = 4.2 eV

U = 4.5 eV

CH4 + MO2 → (H−M, CH3−O)/ MO2 CH4 + MO2 → (H−O, CH3−M)/ MO2 CH4 + MO2 →(H−O, CH3−O)/ MO2

1.49

1.31

2.28

0.67

0.50

1.32

0.61

0.11

−1.02

stable than (H−O, CH3−Ti)/TiO2. This example does not violates the rules proposed here. The rule works, but the outcome is determined by the fact that the Ti4+ cations are stronger acids that either H or CH3 would be when they bind to Ti. It is widely believed that breaking the C−H bond is the ratelimiting step in alkane activation. Calculations have shown that 27340

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the dissociative adsorption of CH4 on an oxide surface proceeds in two steps.6,34−39 The first is the breaking of a C−H bond to form a hydroxyl. The second is the formation of a bond between CH3 and the surface or the expulsion of CH3 as a gasphase radical. The first step, the transfer of the hydrogen atom from methane to the oxide, has the highest activation energy. Because the structure of the transition state associated with the hydrogen transfer is similar to that of the “final state” (hydrogen bonded to the surface), the activation energy is likely to satisfy the Brønstead−Evans−Polanyi (BEP) rule,40 which states that the lower the energy of the final state, the lower the activation energy. The calculations presented here show that the presence of a preadsorbed hydroxyl (i.e., a hydrogen bound to a surface oxygen atom, not a hydroxyl bound to a cation) will lower substantially the energy of the reaction CH4(g) + (HO)/La2O3 → CH3(g) + (H−La, HO)/ La2O3. The BEP rule suggests that preadsorption of a hydroxyl on the oxide surface also lowers the activation energy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided the Air Force Office of Scientific Research FA9550-12-1-0147 and the Department of Energy, Office of Science, Office of Basic Energy Sciences DEFG03-89ER14048. 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 CNS0960316 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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