J. Phys. Chem. C 2008, 112, 4257-4266
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Density Functional Theory Study of Methanol Decomposition on the CeO2(110) Surface Donghai Mei,*,† N. Aaron Deskins,† Michel Dupuis,† and Qingfeng Ge*,‡ Chemical and Materials Sciences DiVision, Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, and Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: October 30, 2007; In Final Form: December 22, 2007
Methanol decomposition on the stoichiometric CeO2(110) surface has been investigated using density functional theory slab calculations. Three possible initial steps to decompose methanol by breaking one of three bonds (O-H, C-O, and C-H) of methanol were examined. The relative order of thermodynamic stability for the three possible bond scission steps is C-H > O-H > C-O. We further isolated transition states and determined activation energies for each of the bond-breaking modes using the nudged elastic method. The activation barrier for the most favorable dissociation mode, the O-H bond scission, is 0.3 eV on the (110) surface. An even lower activation barrier ( C-O > C-H. Our results are consistent with experimental observation that methoxy is the dominant surface species after the stoichiometric CeO2 surface was exposed to methanol. The experimentally observed methanol chemistry was determined by the kinetics of the initial dissociation steps rather than the thermodynamic stability of product states. The surface coverage of methanol was found to affect the relative stability between molecular and dissociative adsorption modes: Dissociative adsorption modes are preferred for methanol coverages up to 0.5 ML but only molecular adsorption was found to be stable at a full monolayer coverage.
1. Introduction Ceria-based catalysts have been widely used in steam reforming of methanol for hydrogen production.1-8 In these catalysts, ceria not only is used as a support to stabilize the transition metal nanoparticles but also plays some direct role in the catalytic reactions. A detailed characterization of the interactions between methanol and the well-defined singlecrystal ceria surface is an important step toward understanding the chemistry of methanol on ceria-based catalysts.9,10 The adsorption and reaction of oxygenates such as methanol on a metal oxide surface are believed to depend on the redox properties of the oxide as well as on the extent of coordinative unsaturation of the surface metal atoms.11,12 As such, experimental investigations of methanol adsorption on supported unreduced and reduced CeO2(111) surfaces have been performed.13-17 Recently, we reported a DFT study of methanol adsorption on the CeO2(111) surface.18 We found that molecular adsorption and dissociative adsorption via the O-H bond breaking were almost equally stable at 0.25 ML methanol coverage. As the coverage increases, the molecularly adsorbed state became energetically more favorable due in part to the H-bond formation between the coadsorbed methanol molecules. We also studied other dissociative adsorption modes on CeO2(111) but did not search for transition states. * Corresponding authors. (D.M.) E-mail:
[email protected]. Tel: (509) 375-6303. Fax: (509) 375-4381. (Q.G.) E-mail:
[email protected]. Tel: (618) 453-6406. Fax: (618) 453-6408. † Pacific Northwest National Laboratory. ‡ Southern Illinois University
The first step for methanol decomposition can be achieved by breaking one of three bonds: O-H, C-O, and O-H. Many previous theoretical and experimental studies over metal and metal oxide surfaces have suggested that the initial decomposition step proceeds by breaking the O-H bond, forming coadsorbed methoxy and a hydrogen adatom (or a hydroxyl group by combining with a surface oxygen site).11,12,19 De la Fuente et al. reported methanol decomposition to form carbonaceous species CHx (x ) 0-3) via the C-O bond scission on Pd(111) using X-ray photoelectron spectroscopy (XPS).20 A theoretical study of methanol decomposition on the Pt(111) and Cu(111) surfaces using density functional theory (DFT) calculations indicated that the O-H bond and the C-H bond scissions were competitive.21-23 Our previous theoretical study had found that the dissociative adsorption of methanol via C-H bond scission on the CeO2(111) surface was energetically more favorable than the dissociative adsorption via either O-H or C-O bond scission.18 Most of previous experimental investigations indicated that the initial step for methanol decomposition on unreduced ceria surfaces was the O-H bond breaking. The dissociation of methanol on the ceria surface was considered heterolytic and described as an acid-base reaction process.13 The adsorbed methoxy (CH3O) was the only observed stable surface species upon methanol dissociation on ceria surfaces.14-17,24 Based on their infrared spectroscopy results, Badri et al. suggested three types of adsorption modes for methoxy species on ceria surfaces.24 Recently, Mullins et al. investigated the reactivity of methanol on a Ru(0001) supported fully oxidized CeO2(111) film using soft X-ray photoelectron spectroscopy (SXPS) and
10.1021/jp710484b CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008
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Mei et al. TABLE 1: Adsorption Energies, Reaction Barriers, and Overall Reaction Energies for O-H and C-H Bond Scissions on CeO2(110) with Different U Parameters O-H bond scission molecularly adsorbed methanol (eV) activation barrier (eV) reaction energy (eV) C-H bond scission molecularly adsorbed methanol (eV) activation barrier (eV) reaction energy (eV)
Figure 1. Optimized structure of CeO2(110) surface. Surface O atom (O3C) in red; second layer O atom (O4C) in dark red; surface Ce atom (Ce6C) in blue; and second layer Ce atom (Ce8C) in light blue.
near-edge X-ray absorption fine structure (NEXAFS) as well as temperature programmed desorption (TPD).10,14 Their results from SXPS and NEXAFS suggested that methoxy was the only surface intermediate for methanol decomposition on the fully oxidized CeO2(111) surface.14 On the other hand, their TPD data indicated that there was a possibility of C-H bond breaking during temperature ramping.14 More open and corrugated surfaces have been demonstrated to be more active for reactions catalyzed by metal surfaces, especially for the bond breaking steps.25,26 Although the (111) surface is the most stable CeO2 surface, the more open (110) surface has been shown to be more active.27,28 In the present study, we focused on searching for the most favorable initial reaction steps for methanol decomposition on the (110) surface and compare it with that on the (111) surface. The overall chemistry of methanol on CeO2(110), as well as the comparison with that on the (111) surface, will provide a better understanding of methanol chemistry on the ceria surfaces. Both molecular and dissociative adsorption of methanol on the CeO2(110) surface with coverages ranging from 0.25 ML to a full monolayer were investigated. The three methanol decomposition pathways via O-H, C-O, or C-H bond scission have been examined. 2. Computational Details We used the same approach as we previously applied for methanol adsorption on CeO2(111).18 In brief, DFT slab calculations were performed using the Vienna ab initio simulation package (VASP).29-32 The projector augmented wave (PAW) method combined with a plane wave basis and cutoff energy of 500 eV was used to describe core and valence electrons.33,34 The Perdew-Burke-Ernzehof (PBE) form of generalized gradient approximation (GGA)35 was used in the calculations. The ground-state atomic geometries of bulk and surfaces were obtained by minimizing the forces on each atom to below 0.05 eV/Å. Spin-polarization had been applied in all calculations. The relaxed bulk structure of CeO2 with a lattice parameter of 5.47 Å was used to construct the slab for methanol adsorption. The CeO2(110) surface was treated by a slab model with three-dimensional boundary conditions, as shown in Figure 1. Different number of atomic layers had been tested in our calculations to properly model the (110) and (111) surfaces. Surface energies calculated using nine and twelve atomic layers for the (111) surface slab as well as five, seven, and nine atomic layers for the (110) surface slab were found converged within 0.005 J/m2. We therefore chose a slab consisting of five atomic layers, separated from their images in the z direction by a 15 Å vacuum layer, to simulate CeO2(110). For each calculation, the bottom three layers were fixed in their corresponding bulk
U)0
U)5
U)7
0.39 0.30 -0.77
0.43 0.52 -1.04
0.44 0.44 -1.63
0.42 2.05 -0.94
0.48 2.31 -1.73
0.41 1.96 -2.47
positions while all the other atoms were allowed to relax. The (110) surface is terminated by six-coordinated Ce atoms (Ce6C) and three-coordinated O atoms (O3C). Bulk O and Ce atoms are four-coordinated and eight-coordinated, respectively. As shown in Figure 1, the (110) structure surface of CeO2 consists of neutral (O3C-Ce6C-O3C) repeating units. We also tested different surface unit cell sizes, number of atomic layers in the slab, and k-point sampling methods of the surface Brillouin zone in order to obtain reliable computational results. As a result, we chose the CeO2(110)-(2 × 2) surface with five layers (10.93 Å × 7.73 Å × 7.88 Å) as our computational model of the CeO2(110) surface slab. Different k-point grids ranging from (1 × 1 × 1) to (3 × 3 × 1) were tested for both the bare (110) surface slabs and the surface with adsorbed methanol. We found the difference between adsorption energies using (1 × 1 × 1) and (2 × 2 × 1) k-point grids were negligible ( O-H bond scission > C-O bond scission. Dissociative adsorption via the C-H bond-breaking was again found to be the most favorable bond dissociation mode. 3.1.3. CoVerage Effects on Molecular and DissociatiVe Adsorption. We investigated molecular and dissociative adsorption at high methanol coverages: 0.5 ML and a full monolayer. The optimized structures for the molecular adsorptions at 0.5 ML are shown in Figure 4a-c. Two H-O3C configurations for molecular adsorption are examined: one has both methanol molecules binding to two surface O3C atoms (shown in Figure 4a) and the other has two methanol molecules binding to two O3C atoms across the trough of the (110) surface (shown in Figure 4b). The adsorption energies for these two H-O3C configurations are 0.66 and 0.61 eV/molecule, respectively. Two methanol molecules can also adsorb via O-Ce6C connections on the (110) surface. This O-Ce6C configuration (shown in Figure 4c) is less stable than the H-O3C configurations, with an adsorption energy of 0.37 eV/molecule. Our results show the adsorption energy per methanol molecule is nearly the same as those at 0.25 and 0.5 ML, indicating a negligible interaction between the coadsorbed methanol molecules. This is in direct contrast to molecular adsorption on the (111) surface. In our previous study,18 we showed that an attractive interaction between two coadsorbed methanol molecules on the (111) surface originated from hydrogen bonding resulting in an overall
stronger molecularly adsorbed state at 0.5 ML. In the hydrogenbonded configuration on the (111) surface, the distance between the hydroxyl hydrogen of one methanol molecule and the O atom of another coadsorbed methanol molecule is 1.65 Å. The distance became 3.12 Å on the (110) surface at 0.5 ML. The large separation between the two adsorbed methanol molecules on the (110) surface prevents the hydrogen bonding interactions and results in negligible changes in the adsorption energy in the coadsorbed systems. The stable dissociated configurations at 0.5 ML coverage are given in Figure 4d-f. Similar to the dissociative adsorption of methanol at 0.25 ML, we studied three bond-breaking modes. The adsorption geometries for all of the dissociation fragments at 0.5 ML are similar to those at 0.25 ML coverage. However, the adsorption energies per methanol molecule for all three dissociative adsorption modes were decreased from the corresponding values at 0.5 ML. The adsorption energy is 1.02 eV/ molecule for the O-H bond scission, 0.83 eV/molecule for the C-H bond scission, and 0.59 eV/molecule for C-O bond scission. Although dissociative adsorption is still favorable over molecular adsorption at 0.5 ML coverage, repulsive interactions between dissociated products destabilize the product states significantly. Furthermore, the order of the stability at 0.5 ML coverage for dissociative adsorption is different from that at 0.25 ML. At 0.5 ML coverage, the stability order follows: O-H bond scission > C-H bond scission > C-O bond scission. At a full monolayer coverage, four methanol molecules are initially arranged on the CeO2(110)-2×2 surface. Only the molecular adsorption of methanol via the H-O3C and the O-Ce6C connections was found to be stable, as shown in Figure 5, panels a and b. The adsorption energies for these configurations are 0.61 and 0.64 eV/molecule. This further suggests that coverage effects for molecular adsorption appear to be negligible on the (110) surface. No stable structures for dissociative adsorption through any one of the three bond-breaking modes have been found at the full monolayer coverage. Likely, there is simply no room to accommodate the dissociated fragments on the surface at this coverage. Our results indicate that the C-H bond scission step will result in the thermodynamically most stable product states on the CeO2 surfaces. However, hydroxymethyl species have not been experimentally observed during methanol decomposition on the CeO2 surfaces. Kinetic factors were believed to play an important role in determining along which pathway the adsorbed methanol on the CeO2 surfaces decomposes. In the following sections, we will present details about each of the three initial decomposition steps and the corresponding activation barriers. The overall reaction energies reported here are the total energy differences between the molecularly adsorbed methanol (initial
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Figure 6. O-H bond breaking step to form adsorbed methoxy and hydrogen on the CeO2(110) surface. The initial O-H bond length in the adsorbed methanol is 0.97 Å.
state) and the coadsorbed dissociative products (final state) whereas the activation barrier reported in the following sections is the total energy difference between the transition state (TS) and the corresponding initial state. The molecular and dissociative states were redrawn in the description of the reaction pathway, and linked by the transition state. 3.1.4. O-H Bond Scission. We first studied methanol decomposition through O-H scission on the (110) surface. The initial state of this decomposition pathway is the molecularly adsorbed methanol at the Ce6C site shown in Figure 6a (OCe6C configuration in Figure 2c). The final state has coadsorbed methoxy and hydrogen, as shown in Figure 6c (and Figure 2d). In the initial state, the distance between the hydroxyl hydrogen and the surface O3C atom is 3.38 Å and the O-H bond length of methanol is 0.97 Å. When the O-H bond is activated, the hydroxyl hydrogen atom of methanol was displaced from the oxygen atom toward the closest surface O3C atom. At the transition state, the O-H bond length of methanol was stretched to 1.12 Å, whereas the distance between the hydroxyl hydrogen atom and the surface O3C atom was shortened to 2.52 Å. The C-O bond of methanol was shifted toward the (110) surface plane during the O-H bond activation. The shift made the O atom of methanol closer to the Ce6C atom on the (110) surface, decreasing the distance between the O atom of methanol and the surface Ce6C atom from 2.67 Å at the initial state to 2.47 Å at the transition state (Figure 6b). This O-Ce6C bond length was further decreased to 2.14 Å in the final state. The dissociated hydrogen atom combined with a surface O3C atom, forming a surface hydroxyl group on the surface. In the final state, the surface hydroxyl was tilted toward another O3C atom across the (110) surface trough shown in Figure 6c. The overall reaction of initial decomposition step via the O-H scission is exothermic with reaction energy of 0.57 eV. The activation barrier is 0.30 eV. 3.1.5. C-H Bond Scission. Methanol decomposition via the C-H bond scission produces coadsorbed hydroxymethyl and hydroxyl group, formed by the combination of hydrogen with the surface O3C atom. The corresponding initial state is the molecularly adsorbed methanol at an O3C site in the H-O3C configuration (B) (Figures 7a and 2b). Prior to the C-H bond activation, the methanol molecule was rotated on-top the O3C site. The energy cost for this rotation is 0.36 eV. After the
rotation, the distance between the C atom of methanol and the surface O3C atom decreases from 3.23 to 3.06 Å. The distance between the hydroxyl hydrogen of methanol and the surface O3C across the trough also decreases from 2.38 to 2.22 Å. The hydroxyl hydrogen of methanol plays a key role in stabilizing the adsorbed methanol molecule during the rotation. Approaching the transition state, shown in Figure 7b, one of the three hydrogen atoms from the methyl group became detached from the C atom. Meanwhile, the C atom moved closer to the surface. At the transition state, the distances of C-O3C and H-O3C are 2.96 and 2.02 Å, respectively. The distance between the C atom and the dissociated hydrogen atom is 1.58 Å. In the final state shown in Figures 7c and 2e, the dissociated hydrogen atom was further displaced from the C atom and combined with a surface O3C atom across the trough to form a surface hydroxyl group. In the final state, the C-O3C bond length of the adsorbed hydroxymethyl is 1.40 Å. The overall reaction energy for the C-H bond scission step is -0.94 eV (exothermic). The activation barrier is 2.05 eV. 3.1.6. C-O Bond Scission. The initial state we used for C-O bond scission is the state with adsorbed methanol in the O-Ce6C configuration. This initial state is the same as that used for the O-H bond scission (Figures 6a and 2c). As shown in Figure 8a, the distance between the C atom and the closest surface O3C atom is 3.67 Å, and the O-Ce6C bond length is 2.67 Å in the initial state. The structure of the transition state is shown in Figure 8b. At the transition state, the C-O distance was stretched to 2.29 Å from 1.44 Å of the initial state. This stretched C-O distance indicates that the C-O bond has already been broken in the transition state. The distances for the C-O3C and O-Ce6C bonds are 1.58 and 2.21 Å, respectively. The dissociated methyl group binds to a surface O3C atom with the C-O3C bond axis almost normal to the surface plane. We note that the O3C atom bound to the methyl group at the transition state was pulled out of the surface plane by 0.59 Å, indicating a strong interaction between methyl and the surface O3C atom. In the final state, the C-O3C and O-Ce6C distances were further decreased to 1.42 and 2.16 Å, respectively, and the coadsorbed product fragments move away from each other. As shown in Figure 8c, the C-O distance in the final state is 3.16 Å. The overall reaction for C-O bond scission on the (110) surface is
DFT Study of Methanol Decomposition
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Figure 7. C-H bond breaking step to form adsorbed hydroxymethyl and hydrogen on the CeO2(110) surface. The initial C-H bond length in the adsorbed methanol is 1.12 Å.
Figure 8. C-O bond breaking step to form adsorbed methyl and hydroxyl on the CeO2(110) surface. The initial C-O bond length in the adsorbed methanol is 1.44 Å.
Figure 9. Comparison of activation barriers of O-H, C-O, and C-H bond scission of methanol on the CeO2 surfaces.
exothermic with a reaction energy of 0.36 eV. The calculated activation barrier is 1.08 eV. Figure 9 compares the overall reaction energies and the activation barriers of the three initial bond-breaking steps of methanol decomposition on the (110) surface. It is clear that
the elementary step via the O-H bond-breaking to form the coadsorbed methoxy and hydrogen is kinetically preferred on the CeO2(110) surface. The activation barrier for the O-H bond scission is only 0.30 eV, much lower than the barriers of 1.08 and 2.05 eV for the C-O and the C-H bond scissions. In particular, the significantly high barrier of the C-H bond scission indicates that methanol decomposition is unlikely to take place although the elementary step of the C-H bond breakage produces the most stable product states. One possible reason for the high activation barrier of the C-H bond scission is steric hindrance. The transition state for the C-H bond breaking is highly strained. The methanol molecule has to be rotated prior to the C-H bond activation and is almost 3 Å above the surface plane during the activation. In contrast, the transition states of the O-H and the C-O bond scissions remain close to the surface. Furthermore, the iconicity, i.e., the charge polarization of the surface makes the C-H bond scission less surface-assisted as compared with other decomposition modes. For example, at the transition state for the O-H bond scission, the two fragments (hydroxyl and methoxy) are polarized with opposite charges on H atom and on methoxy fragments. These two fragments are stabilized by interacting with surface ions,
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Figure 10. O-H bond breaking to form adsorbed methoxy and hydrogen on the CeO2(111) surface. The initial O-H bond length in the adsorbed methanol is 0.99 Å.
thereby leading to a relatively low-energy transition barrier of 0.30 eV. In contrast, the C-H bond is less polarized. In order to reach a transition state that eventually breaks into charged fragments on the surface, a charge separation between the H atom and hydroxymethyl has to take place. The two fragments (H and hydroxymethyl) at the transition state are not sufficiently stabilized by the surface leading to a much higher activation barrier. Both steric effect and charge redistribution contribute to the high energy of the transition state for the C-H bond scission. 3.2. Methanol Decomposition via the O-H Bond Scission on the (111) Surface. Although the C-H bond scission is thermodynamically most favorable, the O-H bond scission turned out to have the lowest barrier for methanol decomposition on the (110) surface. This motivated us to examine methanol decomposition on the CeO2(111) surface, in particular, via the O-H bond scission. We selected the initial and final state structures for the O-H bond scission on the (111) surface in our previous study.18 In the initial structure, methanol is adsorbed above a Ce7C through an O-Ce7C connection, as shown in Figure 10a. The O-Ce7C distance is 2.57 Å. The adsorbed methanol was tilted toward the closest surface O3C atom. The distance between the hydroxyl hydrogen of methanol to the surface O3C atom is 1.92 Å. The adsorption energy of this O-Ce7C configuration is 0.48 eV. As the O-H bond was activated, the hydroxyl hydrogen was displaced from the methanol O atom. At the transition state shown in Figure 10b, the O-H distance was stretched to 1.06 from 0.99 Å of the molecularly adsorbed methanol. The small change of the O-H bond distance (0.07 Å) indicates the transition state of the O-H bond scission of methanol on the (111) surface is an early transition state. The imaginary frequency at the transition state of is 192 cm-1, which is close to the imaginary frequency of O-H bond scission transition state on the (110) surface (186 cm-1). The calculated activation barrier for the O-H bond breaking is only 0.08 eV. In the final state, as shown in Figure 10c, the distance between the O atom of the coadsorbed methoxy and the dissociated hydrogen is 1.72 Å. The O-Ce7C distance is 2.20 Å, decreased from 2.57 Å of the initial state. The overall reaction for the O-H bond scission step on the (111) surface is slightly exothermic with a reaction energy of 0.12 eV. Our results demonstrated that the C-H and the C-O bond scissions have significantly higher barriers than the O-H bond breaking step on the CeO2(110) surface. We expect that the
C-H and C-O scission pathways will also have high activation barriers on the (111) surface. These results and expectations lead us to conclude that the first step for methanol decomposition on the clean CeO2(110) and CeO2(111) surfaces is likely to occur via the O-H bond scission to form an adsorbed methoxy species. This conclusion is consistent with the experimental observations that methoxy was the only surface intermediate during methanol decomposition on the (111) surface.14-16,60 The O-H bond of methanol is readily broken on the clean (111) surface and the reaction is slightly exothermic. Consequently the reversal reaction that coadsorbed methoxy and hydrogen atom of the surface hydroxyl to form methanol on the (111) surface also has a low activation barrier (0.20 eV). This indicates that methanol decomposition via the O-H bond scission on the clean (111) surface is highly reversible and equilibrium will be established between the adsorbed molecular methanol and coadsorbed methoxy and hydroxyl. On the other hand, the activation barrier for the O-H bond breaking on the (110) surface is 0.30 eV. But the activation barrier for the reversal reaction, i.e., forming methanol from the recombination of the adsorbed methoxy and hydrogen, has a relatively high activation barrier of 0.87 eV. Consequently, the formation of methanol from the recombination of methoxy and hydrogen (in the form of the hydroxyl on the oxygen site of the surface) is not favored on the (110) surface as compared with that on the (111) surface. Our calculations indicate that the molecular adsorption and the dissociative adsorption of methanol are almost equally stable on the (111) surface because the activation barriers for the forward (dissociation) and the reverse (recombination) reactions are comparable. In our previous study of methanol adsorption on the (111) surface,18 we found that adsorbed methanol molecules are thermodynamically favorable in a molecular state at high coverage while being in a dissociative state at low coverage. This suggests that the state of adsorbed methanol on the (111) surface depends upon the surface coverage. The state of the methanol also depends on the surface temperature, which can affect methanol decomposition rates. Mullins et al. found that methanol could molecularly adsorb on the defect-free CeO2 (111) surface up to 0.5 ML, corresponding to ∼2-4 molecule/ nm2,14 which is much higher than that of 0.6 molecule/nm2 reported by Ferrizz et al.15 The differences in the initial temperatures of dosing methanol molecules during the TPD experiments might affect the measured methanol coverage. At
DFT Study of Methanol Decomposition
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TABLE 2: Kinetic Parameters at 300 and 500 K for Methanol Decomposition on CeO2(110) and CeO2(111) reactions O-H bond scission on CeO2(110) C-O bond scission on CeO2(110) C-H bond scission on CeO2(110) O-H bond scission on CeO2(111)
ν (s-1)
Eact (eV)
k (300 K)
k (500 K)
2.40 × 1012 0.30 2.19 × 107
2.27 × 109
7.73 × 1011 1.08 5.54 × 10-7
1.00 × 101
5.10 × 1012 2.05 1.85 × 10-22 1.11 × 10-8 2.73 × 1011 0.08 1.24 × 1010
4.26 × 1010
low temperature, i.e., 100 K by Mullins et al.,14 the initial coverage of methanol is high. The adsorbed methanol prefers to be in a molecular state rather than in a dissociative state. Consequently, relatively high saturation coverage of methanol was measured by Mullins et al. In contrast, Ferrizz et al. studied methanol adsorption at a high starting temperature.15 At 300 K, few molecular methanol adsorbed on the surface because of a weak adsorption energy (lower than 0.5 eV18), resulting in a very low methanol coverage reported in their study due to quick methanol decomposition.15 3.3. Rate Constants. We have determined the energetics of methanol adsorption and dissociation on the CeO2(110) surface. We also mapped out the reaction pathways that connect the adsorbed methanol molecules to different dissociative product states and isolated the corresponding transition states along each pathway. We note that the rate of a reaction is not determined by the activation barrier alone. According to Arrhenius relationship, the rate constant (k) of an elementary reaction step is related to the activation barrier (Eact) through the following formula:
( )
k ) ν exp -
Eact RT
(2)
was found to be the most thermodynamically stable on the (110) surface. The thermodynamic stability order on the (110) surface at 0.25 ML is the same as that on the (111) surface: C-H bond scission > O-H bond scission > C-O bond scission. When the coverage is increased to 0.5 ML on the (110) surface, the O-H bond scission becomes thermodynamically more favorable than the C-H bond scission. Kinetically, the O-H bond scission is preferred due to a low activation barrier of 0.30 eV. The barriers for the C-O bond scission and the C-H bond scission of methanol are 1.08 and 2.05 eV, respectively. The activation barrier for breaking the O-H bond on the more stable CeO2(111) surface was found to be only 0.08 eV. Our results are consistent with the previous experimental observations that methoxy is the only abundant and detectable surface intermediate on the fully oxidized (111) and (110) ceria surfaces upon exposure of methanol. The adsorption of methanol on CeO2(110) is also coverage dependent. At 0.25 ML, dissociative adsorption is preferred over the molecular adsorption. As the methanol coverage increases to 0.5 ML, the dissociative adsorption becomes less stable whereas the molecular adsorption was affected little as compared with those at 0.25 ML. At a full monolayer, only molecular adsorption was found possible. Acknowledgment. This work was supported by a Laboratory Directed Research and Development (LDRD) project of the Pacific Northwest National Laboratory (PNNL). The computations were performed using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), which is a U.S. Department of Energy national scientific user facility located at PNNL in Richland, Washington. Computing time was made under a Computational Grand Challenge “Computational Catalysis”. Part of the computing time was also granted by the National Energy Research Scientific Computing Center (NERSC). References and Notes
where ν is the pre-exponential factor, R is the gas constant, and T is the reaction temperature. Once the activation barrier was determined, we only need the pre-exponential factor to calculate the rate constant at a certain temperature. Assuming that the harmonic transition state theory is applicable to all three pathways of methanol dissociation studied here, the preexponential factor can be estimated from the normal-mode frequencies, νi, at the initial (IS) and transition (TS) states according to the following equation:61
ν)
∏3N1 νISi νTS ∏3N-1 1 i
(3)
By selectively perturbing the structures, we used a finite difference scheme to construct the Hessian matrix and obtain the vibrational frequencies at the initial and transition states. In Table 2, we summarize the calculated pre-exponential factors together with the activation barriers for the reaction pathways on CeO2(110). The activation barriers and rate constants calculated at 300 and 500 K indicate that the O-H bond scission is still the most favorable reaction path for methanol decomposition on the CeO2(110) surface. 4. Conclusions First-principles density functional theory has been used to investigate methanol adsorption and decomposition on the stoichiometric CeO2(110) surface. Of three possible dissociation modes, the dissociative adsorption via the C-H bond breaking
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