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Dehydrogenation of Ethanol on a 2Ru/ZrO2(111) Surface: Density Functional Computations Yu-Wei Chen and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal UniVersity, 88, Section 4, Tingchow Road, Taipei 116, Taiwan ReceiVed: December 3, 2008; ReVised Manuscript ReceiVed: January 22, 2009
We applied periodic density functional theory to investigate the dehydrogenation of ethanol on a 2Ru/ZrO2 (111) surface. A structure with ethanol adsorbed with its O atom attached to a Ru atom is calculated to exhibit the largest energy of adsorption; it reacts via an O-Ru path: the sequence of bond scission is O-H f βC-H f C-O that eventually forms ethene and coke. Another structure adsorbed via the R-C atom onto Ru that exhibits the second largest adsorption energy dissociates via an RC-Ru path. The sequence of bond scission is RC-H f O-H f RC-H f (βC-H f) C-C, and eventually forms H2. Possible surfaces of potential energy to form H2 from a combination of adsorbed H atoms were calculated at the final stage, subject to a barrier about 20-30 kcal/mol, were also calculated. These results indicate that a Ru-doped ZrO2 surface might be a fairly effective catalyst to dehydrogenate ethanol. 1. Introduction
2. Computational Methods
Hydrogen is considered a desirable fuel for several reasons: it is the least polluting, reactive in a highly efficient hydrogen/ oxygen fuel cell to produce electricity,1-3 and increasingly in demand in oil-refining and coal-processing industries to diminish impurities such as nitrogen, sulfur, and metals at high levels.4 Because hydrogen in its elemental form (H2) is scarce, its synthesis becomes essential. Reforming methanol to produce H2 has been considered extensively,5-8 but its toxicity is a main drawback. Ethanol is superior to methanol in being nontoxic, safely handled, and produced from sugar- or starch-containing crops.9-16 The use of ethanol derived from biomass to produce hydrogen and energy makes no contribution to global warming because biomass absorbs carbon dioxide from the atmosphere for its growth.17 Hydrogen produced from ethanol via steam reforming or catalytic partial oxidation has been investigated extensively.18-22 Our interest is the dehydrogenation of ethanol on catalytic metaldoped metal oxide surfaces. Our calculations show that the formation of H2 depends on the nature of the support: for instance, the acidic nature of Al2O3 favors dehydration, whereas the basic nature of MgO favors dehydrogenation and condensation,23,24 and the reducible nature of CeO2 and ZrO2 provides a high selectivity for H2 relative to undesirable byproduct.25-28 For this investigation of the dehydrogenation of ethanol, we choose ZrO2 as a support. Apart from the support, the effect of doped metals is important for ethanol dehydrogenation. Noblemetal catalysts such as Ru, Rh, Pd, and Pt show a catalytic action in terms of conversion of ethanol and production of hydrogen.29 Goerke et al. found that the water-gas shift reaction at 250-300 °C with a Ru/ZrO2 catalyst decreased the CO content by more than 95%,30 which stimulated our investigation of a Ru-doped ZrO2 surface for ethanol dehydrogenation; we found that scission of the C-C bond is favorable for the formation of ethene. We present here the results of a systematic, periodic, self-consistent, density functional theory (DFT) calculation of the dehydrogenation of ethanol on a Ru/ZrO2(111)(3 × 3) surface, and we address in particular the chemical behavior of the adsorbed intermediates, the reaction barriers, and the potential-energy surfaces.
All present calculations were performed with the DFT planewave method utilizing the Vienna simulation package (VASP).31-34 To calculate the total energy, we solved the Kohn-Sham equations in a self-consistent manner under a generalized gradient approximation35 with the Perdew-Wang 1991 exchangecorrelation formulation,36 which has been shown to work well for surfaces. We applied the projector-augmented wave method,37 an all-electron method combining the accuracy of augmented plane waves with cost-effective pseudopotentials implemented in VASP. The Brillouin zone was sampled with the Monkhorst-Pack grid.38 Calculations were performed with the (3 × 3 × 3) and (3 × 3 × 1) Monkhorst-Pack mesh k-points for bulk and surface calculations, respectively, with truncation energy 400 eV that allows convergence to 1 × 10-4 eV in total energy (the differences of relative energy corresponding to different convergence criterion and KPONITS chosen are described in S-Table 1 of the Supporting Information). Use of the spin-polarization method yielded a proper description of the magnetic property of the Ru/ZrO2(111) surface model. Side and top views of our model of the ZrO2(111) and Ru/ZrO2(111) surfaces appear in Figure 1. The p(3 × 3) lateral cell of ZrO2(111) surface is modeled as a periodically repeating slab with six layers, as shown in Figure 1a. The supercell is fully relaxed under the restriction of fixed cell parameters and the lower three layers, whereas the remaining layers are fully relaxed during the calculations. Grau-Crespo et al.39 discussed the influence of a varied number of layers and reported that a model with nine atomic layers is sufficient to represent the geometric relaxation of zirconia surfaces; because the difference of adsorption energy between six and nine atomic layers is only about 1 kcal/mol, we constructed the slab model with six atomic layers. The lateral cell has dimensions a ) b ) 8.02 Å and c ) 20.01 Å, which includes a vacuum region of thickness greater than 15 Å to ensure no interaction between the slabs. We calculated adsorption energies according to the following equation:
* Corresponding author. E-mail:
[email protected].
∆Eads ) E[slab + adsorbate]-(E[slab] + E[adsorbate]) in which E[slab + adsorbate] is the electronic energy calculated for an adsorbed species on a Ru/ZrO2(111) surface, E[slab] for a
10.1021/jp810624g CCC: $40.75 2009 American Chemical Society Published on Web 03/20/2009
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Figure 2. The averaged adsorption energy that corresponds to different numbers of doped Ru atoms on the ZrO2 surface.
The transition state refers to an image for which the tangential force to the reaction path tends to zero. A frequency calculation is also applied to this transition structure, and only one imaginary frequency has to be obtained to confirm the transition state. The pathways of minimum energy (MEP) were constructed accordingly.
Figure 1. (a) Top view and (b) side view of the optimized ZrO2 surface. (c) Top view and (d) side view of the optimized 2Ru/ZrO2 surface. The slab is repeated in the z-direction with a 15-Å-thick vacuum layer between slabs. Od represents the oxygen atom locating at a downsite position whereas Ou is at an up-site position of the supercell surfaces. Green, red, and blue spheres correspond to Zr, O, and Ru atoms, respectively.
clean Ru/ZrO2(111) surface, and E[adsorbate] for a molecule in the gaseous phase. Vibrational wavenumbers of the adsorbed structures were analyzed on diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudged elasticband (NEB) method40-42 was applied to locate transition structures. According to this method, we created a linear path between the reactant and product states of each elementary process for use as an initial search coordinate and then divided the path into a series of images. Each image is optimized with respect to all nuclear degrees of freedom, except the one along the reaction pathway.
3. Results and Discussion 3.1. Bulk. Zirconia has a fluorite structure, with a facecentered cubic unit cell. Each zirconium cation is surrounded by eight equivalent oxygen anions, and each oxygen anion is surrounded by four equivalent zirconium cations. With an initial fluorite structure, we performed a full optimization of the cell parameters, which involves calculating forces and a stress tensor, relaxing ions, and altering the shape and volume of the unit cell. The resulting cell parameter is 5.100 Å, consistent with an experimental value of 5.090 Å43,44 and a calculated value of 5.130 Å.39 Our calculated Zr-O distance is 2.21 Å, also near a calculated value of 2.22 Å.39 3.2. The Clean Surface of Ru/ZrO2(111)-(3 × 3). We constructed a slab model of Ru/ZrO2(111)-(3 × 3) from an initial bulk structure. Surface (111) of cubic zirconia is the most stable termination, according to both experimental observations and calculations. Experiments with low-energy electron diffraction show well-defined (111) surfaces,45-48 and calculations with both interatomic potentials and quantum-mechanical methods yielded the same results.49,50 The supercell that we constructed comprised six atomic layers, fully relaxed under a restriction of fixed cell parameters and the lower three layers. The metal-zirconia interface is reported to play an important role in heterogeneous catalysis,51 such that dehydrogenation on Rh/CeO2 occurs almost entirely at the interface sites.52 Dehydrogenation on Ru/ZrO2 is likely to occur at the interface sites, because both Rh/CeO2 and Ru/ZrO2 are metal-fluorite structure surfaces. The average adsorption energy that corresponds to Ru doped to a varied extent is shown in Figure 2, and its corresponding structure in S-Figure 1 of the Supporting Information. We calculate all the possible Ru doping structures with different arrangements and obtain the structures that exhibit the biggest adsorption energies. The average adsorption energy increases with an increasing number of Ru atoms, and the adsorbed structures that correspond to varied numbers of Ru atoms are all gathered together. The greater the extent of doping with Ru, the more interfaces are generated. To provide enough
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TABLE 1: The Averaged Adsorption Energy (kcal/mol) of (a) One Ru and (b) Two Ru Atoms on ZrO2(111) Surface at Different Adsorbed Sites
atop-Zr atop-Ou atop-Od Zr-O bridge
atop-Zr
atop-Ou
atop-Od
Zr-O bridge
-41.10
(a) -33.54
-62.48
-48.79
-47.97
(b) -73.52 -43.34
-70.92 -73.31 -63.07
-51.83 -60.10 -64.01 -80.23
Ru/ZrO2 interface sites for ethanol dehydrogenation, we added only two Ru atoms, the minimum number that can generate a three-boundary phase, to the clean ZrO2(111)-(3 × 3) surface. The supercell was designed to consist of an array of 3 × 3 of surface unit cells for the purpose of a small surface coverage (2/9 ML). The preferred location for one ruthenium is atop an Od site (in which Od represents a subsurface oxygen atom in the ZrO2 third layer, whereas Ou is the outermost oxygen). Top and side views of the highly symmetric surface sites for adsorption appear in Figure 1, and the corresponding adsorption energies of ruthenium at various sites are listed in Table 1. Although the adsorption energy of one ruthenium atom atop an Od site is about 14 kcal/mol larger than at a ZrO-bridge site, the preferred locations for two ruthenium atoms are both at ZrObridge sites, in which both Ru atoms gather side-by-side on the ZrO2(111) surface and possess the largest mean adsorption energy because of the strong adhesion of ruthenium. The calculated distance between the closest Ru atoms in two neighboring cells is around 9.38 Å. 3.3. Dehydrogenation of Ethanol on the Ru/ZrO2 Surface. 3.3.1. Adsorption Geometries and Energies of Ethanol. We constructed the ethanol adsorbate on adding an ethanol molecule to the surface. Of several locations tested, we found two adsorption geometries with ethanol attached through either the O atom to the Ru atom (O-Ru, 2.10 Å) or the R-C atom onto Ru (RC-Ru, 2.32 Å) to be stable. Of these two, the more stable structure is shown in Figure 3a (O-Ru, 2.10 Å), with the corresponding adsorption energy of 32.10 kcal/mol. The O-H bond length of this adsorbed configuration is 1.03 Å, extended slightly relative to ethanol, whereas the lengths of other bonds, including C-H, C-O, and C-C, remain constant. We found also that the barrier for O-H scission is only 3.00 kcal/mol. A second stable structure for adsorption is shown in Figure 3b (RC-Ru, 2.32 Å), with an adsorption energy of 23.66 kcal/ mol. The RC-H bond length of this adsorbed conformation is 1.21 Å, so again, it is extended slightly relative to ethanol, with other bonds, including O-H, βC-H, C-O, and C-C, remaining intact. Our calculated barrier for RC-H scission is only 2.20 kcal/mol. These two adsorbed structures yield similar adsorption energies and barriers in the first bond scission, but they differ considerably in the consecutive steps. Before dehydrogenation, we investigated the adsorption energy of an H atom onto the surface. For three structures with the strongest adsorption (H-Zr, H-O and H-Ru), the corresponding adsorption energies and bond lengths are calculated to be 12.22 kcal/mol and 1.89 Å (H-Zr), 48.01 kcal/mol and 0.98 Å (H-O), and 65.14 kcal/mol and 1.67 Å (H-Ru), respectively; a hydrogen atom detached from ethanol is thus favored to be adsorbed at a ruthenium atom (H-Ru). To distinguish the two reaction paths originating from distinct adsorbed structures, we name the reaction path from the (O-Ru) adsorption structure the O-Ru path; and that from (RC-Ru), the RC-Ru path. They are described as follows.
Figure 3. Calculated optimized structures of ethanol adsorbate onto the 2Ru/ ZrO2(111) surface with (a) the largest adsorption energy, (b) the second-largest adsorption energy. Green, red, blue, gray, and white spheres correspond to Zr, O, Ru, C, and H atoms, respectively.
3.3.2. Potential-Energy Surfaces. O-Ru Path. The potentialenergy surface for the dehydrogenation of ethanol on a Ru/ ZrO2 (111) surface is drawn in Figure 4; the structures of possible intermediates are depicted in Figure 5. The transition structures and the imaginary wavenumbers are shown in S-Figure 2 of the Supporting Information. Ethanol adsorbed from the gaseous phase onto the Ru/ZrO2 surface releases 32.10 kcal/mol and then passes a barrier of 3.00
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Figure 4. The calculated potential energy surfaces for the ethanol dehydrogenation on the 2Ru/ZrO2 surface (a) following the O-Ru pathway and (b) following the RC-Ru pathway.
kcal/mol to break the O-H bond, forming adsorbed species (2) CH3CH2O-Ru(a) (exothermic by 1.98 kcal/mol). In this transition structure (TS1), the calculated O-H bond is elongated to 1.28 Å. Because other scissions (RC-H, βC-H etc.) are subject to a larger barrier at the first step, scission of O-H to yield CH3CH2O-Ru(a) becomes the most probable path at the first step. Next, in adsorbed species (2) CH3CH2O-Ru undergoes β C-H scission with a barrier 23.02 kcal/mol to form another adsorbed species (3) Ru-CH2CH2O-Ru(a) (endothermic by 10.60 kcal/mol) in the favored sequence. In this transition structure (TS2), the βC-H bond is elongated to 1.58Å. Because the scission of RC-H is subject to a barrier of 45.31 kcal/mol and endothermic by 40.91 kcal/mol, this occurrence is unlikely. Adsorbed species (3) Ru-CH2CH2O-Ru(a) undergoes C-O scission with a barrier of 25.98 kcal/mol to form species (4) Ru-CH2CH2(a) and Ru-OH(a) (exothermic by 31.68 kcal/mol). In this transition structure (TS3), the calculated C-O bond is elongated to 2.12 Å, and the breaking of RC-H, Cβ-H, and C-C bonds over barriers 49.76, 32.51, and 39.98 kcal/mol, respectively, are all highly endothermic. Because these bond scissions are thus unlikely to occur relative to the C-O counterpart, the dehydrogenation sequence for ethanol in the O-Ru reaction path is scission first of O-H, then Cβ-H,
followed by C-O, resulting in formation of species Ru-CH2CH2, which is classified eventually to form coke on the surface. R C-Ru Path. Ethanol first adsorbs from the gaseous phase onto the Ru/ZrO2, releasing 23.66 kcal/mol, but a reaction barrier of 2.20 kcal/mol is calculated for RC-H scission to form adsorbed species (6) CH3CH(Ru)OH(a) that is exothermic by 2.99 kcal/mol; the RC-H bond in the transition structure (TS4) is calculated to extend to 1.53Å. Other scissions (O-H, βC-H, etc.) are subject to larger barriers, making RC-H scission the most likely path at the first step. The resulting adsorbed species (6) CH3CH(Ru)OH(a) further undergoes O-H scission with a barrier of 3.08 kcal/mol to form adsorbed species (7) CH3CHO-Ru(a) (endothermic by 0.69 kcal/mol). In this transition structure (TS5), the O-H bond is stretched to 1.36 Å. The scission of a second RC-H bond over a barrier of 14.70 kcal/ mol and endothermic by 9.22 kcal/mol is less likely to occur than the above-mentioned O-H bond scission. Adsorbed species (7) CH3CHO-Ru(a) undergoes RC-H scission with a barrier of 6.62 kcal/mol to form adsorbed species (8) CH3CO-Ru(a) (exothermic by 8.28 kcal/mol). In this transition structure (TS6), the RC-H bond is elongated to 1.31 Å. Scission of other bonds (βC -H, C-C and C-O) that are subject to larger barriers are
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Figure 5. Calculated optimized configurations of adsorbed ethanol and its dissociated fragments on the 2Ru/ZrO2(111) surface. (a) Side view and (b)top view (some H(a) connect to Ru and some H(a) connect to O on the surface). The bottom part of the surface has been omitted, which is similar to the one shown in Figure 1.
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TABLE 2: The Calculated Forward and Backward Reaction Activation Energy (kcal/mol) for the Absorbing Species at Each Local Minimum on the Dehydrogenation Potential Energy Surfaces of Ethanol on 2Ru/ZrO2(111) Surface Eforward a
Ebackward a
∆Ea
rf/rba
79 (1) CH3CH2OH(a) 98
reaction path desorption
TS1
3.00
32.10
-29.10
3.98 × 1010
TS1
TS2
79 (2) CH3CH2O–Ru(a) 98
23.02
4.98
18.04
2.68 × 10-7
79 (3) Ru–CH2CH2O–Ru(a) 98
11.69
12.42
-0.73
1.84 × 100
TS2
TS3
TS3
45.15
79 (4) Ru–CH2CH2(a) + Ru–OH(a) 79 (5) CH3CH2OH(a) 98
2.20
23.66
-21.46
6.56 × 107
79 (6) CH3CH(Ru)OH(a) 98
3.08
5.19
-2.11
5.87 × 10 °
79 (7) CH3CHORu(a) 98
6.62
2.39
4.23
2.88 × 10-2
21.69
12.90
8.79
6.28 × 10-4
15.19
12.90
2.29
1.47 × 10-1
17.60
3.88 × 10-7
desorption
TS4
TS4
TS5
TS5
TS6
TS6
TS7
TS6
TS9
79 (8) CH3CORu(a) 98 79 (8) CH3CORu(a) 98 TS7
TS8
25.20
79 (9) Ru–CH3(a) + Ru–CO(a) 98 TS9
TS10
79 (11) RuCH2CORu(a) 98 TS10
26.94 TS11
79 (12) RuCH2 + RuCO(a) 98
9.34 40.30
a
rf/rb is the ratio of forward rate constant and backward rate constant calculated at 600 K, assuming the same pre-exponential factors, where ∆Ea ) Eforward - Ebackward . a a
Figure 6. Proposed possible dehydrogenation reaction schemes of the adsorbed ethanol on the 2Ru/ZrO2(111) surface.
less likely to occur. Moreover, adsorbed species (8) CH3CORu(a) might undergo separately one of two reactions: either C-C scission with a barrier of 21.69 kcal/mol to form adsorbed species (9) Ru-CO(a) and Ru-CH3(a) (exothermic by 3.51 kcal/ mol) in which the C-C bond in the transition structure (TS7) is elongated to 1.98Å, or βC-H scission with a barrier of 15.19 kcal/mol to form adsorbed species (11) Ru-CH2CO-Ru(a) (with a four-membered ring) and endothermic by 5.85 kcal/mol. In this transition structure (TS9), the calculated βC-H bond is extended to 1.99Å. In comparing the barriers of these two paths, we know that adsorbed species (8) CH3CO-Ru might undergo β C-H scission at lower temperature because of its smaller barrier, or it might undergo C-C scission at higher temperature resulting from its slightly larger barrier but greater exothermicity. Furthermore, adsorbed species (11) Ru-CH2CO-Ru(a) might undergo C-C scission, with a barrier of 26.94 kcal/mol, to form species (12) CH2-Ru(a) and CO-Ru(a) (exothermic by 13.36 kcal/mol), with the C-C bond extended to 1.82Å in the transition structure (TS10). In addition, adsorbed species CH2-Ru(a) and CO-Ru(a) might undergo steam-reforming and water-gas-shift reactions, respectively, if sufficient steam is provided. In sum, the ethanol dehydrogenation sequence in this R C-Ru path is scission first of RC-H; then O-H; then RC-H; and eventually, C-C or βC-H, followed by C-C. Adsorbed structure 1 that exhibits the greatest adsorption energy to undergo the O-Ru path must overcome a major barrier in the βC-H scission, finally to form Ru-CH2CH2; a trace of coke might thereby be formed. After forming adsorbed species 2, the forward path might be hindered so that species 2
might more readily react reversely (overcoming a smaller barrier of 4.98 kcal/mol; the ratio of forward and reverse rate coefficients is 2.68 × 10-7, in Table 2); thus the reaction might be trapped between the adsorbed ethanol form, 1, and an O-H dissociated conformation, 2. The conversion of ethanol via this O-Ru path is clearly less likely than through another adsorbed structure that exhibits the second largest adsorption energy and undergoes the RC-Ru path. The proposed schematic path is displayed in Figure 6. 3.3.3. Formation of H2. During dehydrogenation, H atoms become detached from ethanol and adsorbed on Ru or O atoms on the surface. Some such adsorbed H atoms eventually combine to form H2. A large entropy term at elevated temperatures is known to drive this desorption.52 Accordingly, we present possible surfaces of potential energy for the desorption H(a) + H(a) f H2(g) at the final step. The final adsorbed species 9, 12, and 13 that exhibit more than one adsorbed H atom suffer barriers of 21.22, 17.19, and 29.03 kcal/mol in forming H2(g) and are endothermic by 6.72, 10.35, and 23.37 kcal/mol, respectively, because of the varied locations of adsorbed H atoms on the surface. The barrier and the endothermic term from an original adsorbed species 13 are larger than for the other two adsorbed species, 9 and 12. The formation of a second mole of H2(g) is more difficult than that of the first mole of H2(g) because of the abruptly decreased coordination number on the Ru atom, which rapidly makes the system unstable. All transition structures are depicted in S-Figure 2 of the Supporting Information.
6138 J. Phys. Chem. C, Vol. 113, No. 15, 2009 3.4. Dehydrogenation of Ethanol on a Clean ZrO2 Surface. In investigating also the dehydrogenation of ethanol on a clean ZrO2 surface, we began the construction of the ethanol adsorbate on adding an ethanol molecule onto the surface. Of several possible locations, the adsorption geometry with ethanol attached through O onto the Zr atom (O-Zr) and H onto O (H-O) is the most stable, which releases 29.61 kcal/mol. A barrier of 1.62 kcal/mol is involved in breaking the O-H bond, forming adsorbed species CH3CH2O-Zr(a) (exothermic by 2.23 kcal/mol). (Our result of O-H scission mechanism is in good agreement with experimental observations by Lavalley et al.,53-55 which suggests that the surface oxygen anion interacts with the OH group of alcohol by means of a hydrogen bond and then dissociates on the Zr-O site.) This species might undergo βC-H scission with a large barrier of 59.07 kcal/mol, forming Ru-CH2CH2O-Zr(a) (endothermic by 37.64 kcal/mol). Despite this large barrier, βC-H at this stage has the smallest barrier relative to other scissions. Because of this large barrier (59.07 kcal/mol) for βC-H scission, this process might be hampered, with trapping in a deep well to form adsorbed species CH3CH2O-Zr. This result indicates that clean ZrO2 would not effectively catalyze the dehydrogenation of ethanol, which might well be improved by doping ZrO2 with Ru. 4. Conclusion Calculations with density functionals have been used to investigate the dehydrogenation of ethanol on a Ru/ZrO2 surface. Adsorbed structure 1 that exhibits the greatest adsorption energy reacts via an O-Ru path; a sequence of bond scissions O-H f βC-H f C-O eventually yields ethene, but the conversion of ethanol to ethene might be inefficient as trapped adsorbed species 1 and 2 hardly move further because of the large barrier difference of 22.66 kcal/mol between the forward and reverse reactions. Adsorbed structure 5, which exhibits the second largest adsorption energy, reacts via an RC-Ru path; the sequence of bond scission is RC-H f O-H f RC-H f (βC-H f) C-C, eventually forming H2, CH2, and CO, of which CH2 might further convert to carbon oxide and hydrogen via steam reforming or carbon oxide to carbon dioxide and hydrogen via the water-gas shift reaction25 if sufficient steam is provided. The RC-Ru pathway produces the major ethanol dehydrogenation, in contrast to the O-Ru path with the minor formation of ethene and eventually coke. All these results are based on the limited size of the metal particles (Ru dimer) on the ZrO2 surface. Acknowledgment. This research was supported by the Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan (ROC), under Contract No. NL940251 and National Science Council, Republic of China, NSC 96-2113-M-003-007MY3. We are deeply indebted to Professor M. C. Lin for his persistent encouragement and instruction. We are also grateful to the National Center for High-Performance Computing where the computer time was provided. We want to thank Professor J. C. Jiang and Professor M. Hayashi, as well as H. L. Chen for their useful discussion. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Whittingham, M. S.; Savinell, R. F.; Zawodzinski, T. Chem. ReV. 2004, 104, 4243.
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