Decomposition of Ethanol on Pd(111): A Density Functional Theory

Dec 15, 2009 - Ethanol decomposition over Pd(111) has been systematically investigated using self-consistent periodic density functional theory, and t...
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Decomposition of Ethanol on Pd(111): A Density Functional Theory Study Ming Li,† Wenyue Guo,*,† Ruibin Jiang,† Lianming Zhao,† and Honghong Shan*,‡ †

College of Physics Science and Technology, China University of Petroleum Dongying, Shandong 257061, PR China and ‡State Key Laboratory for Heavy Oil Processing, China University of Petroleum Dongying, Shandong 257061, PR China Received July 19, 2009. Revised Manuscript Received October 27, 2009 Ethanol decomposition over Pd(111) has been systematically investigated using self-consistent periodic density functional theory, and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to follow the gas-phase bond order rules, wherein C is tetravalent and O is divalent with the missing H atoms replaced by metal atoms. Desorption is preferable for adsorbed ethanol, methane, and CO, while for the other species decomposition is preferred. For intermediates going along the decomposition pathways, energy barriers for the C-C, CR-H, and O-H scissions are decreased, while it is increased for the C-O path or changes less for the Cβ-H path. For each of the C-C, C-O, and C-H paths, the Broensted-Evans-Polanyi relation holds roughly. The most likely decomposition path is CH3CH2OH f CH3CHOH f CH3CHO f CH3CO f CH2CO f CHCO f CH þ CO f CO þ H þ CH4 þ C.

1. Introduction Ethanol, one of the most renewable resources that can be easily obtained in large amounts from biomass by fermentation, has a number of important applications in the chemical and fuel industries, including hydrogen production in steam reforming1 and direct alcohol fuel cells (DAFCs)2 as well as the synthesis of various organic products such as aldehydes and ketones.3 The study of ethanol adsorption, decomposition, and oxidation on catalytically active surfaces is of considerable importance for developing catalysts for these applications as well as for clarifying the Fischer-Tropsch synthesis mechanism of alcohols from syngas (CO and H2).4 Hydrogen production from ethanol requires selective cleavage of bonds in ethanol. In order to clarify the performance of the applied catalysts, ethanol decomposition and oxidation were carried out on various pure metal surfaces using surface science techniques, including Ni(111),5 Rh(111),6 Pt(111),7 Cu(110),8 Ag(110),9 Pd(111), and Pd(110).12-14 Also, ethanol chemistry *Corrsponding author. E-mail address: [email protected] (W.G.); [email protected] (H.S.). Telephone: 86-546-839-6634. Fax number: 86546-839-7511. (1) Louz, V.; Fierro, V.; Denton, P.; Katz, H.; Lisse, J. P.; Mauduit, S. B.; Mirodatos, C. J. Power Sources 2002, 105, 26. (2) Zhou, W. J.; Zhou, Z. H.; Song, S. Q.; Li, W. Z.; Tsiakaras, P.; Xin, Q. Appl. Catal., B 2003, 46, 273. (3) Dijksman, A.; M.Gonzalez, A.; Payeras, A. M.; Arends, I. W. C. E.; Sheldon, R. A. J. Am. Chem. Soc. 2001, 123, 6826. (4) Van Der Laan, G. P.; Beenackers., A. A. C. M. Catal. Rev. 1999, 41, 255. (5) Xu, J.; Zhang, X.; Zerobi, R.; Yoshihobu, J.; Xu, Z.; Yates, J. T. Surf. Sci. 1991, 256, 288. (6) Papageorgopoulos, D. C.; Ge, Q.; King, D. A. J. Phys. Chem. 1995, 99, 17645. (7) Lee, A. F.; Gawthrope, D. E.; Hart, N. J.; Wilson, K. Sur. Sci. 2004, 548, 200. (8) Sexton, B. A.; Rendulic, K. D.; Hughes, A. Z. Surf. Sci. 1982, 121, 181. (9) Zhang, R.; Gellman, A. J. J. Phys. Chem. 1991, 95, 7433. (10) Davis, J. L.; Barteau, M. A. Surf. Sci. 1987, 187, 387. (11) Davis, J. L.; Barteau, M. A. Surf. Sci. 1988, 197, 123. (12) Davis, J. L.; Barteau, M. A. Surf. Sci. 1990, 235, 235. (13) Shekhar, R.; Barteau, M. A. Catal. Lett. 1995, 31, 221. (14) Bowker, M.; Holroyd, R. P.; Sharpe, R. G.; Corneille, J. S.; Francis, S. M.; Goodman, D. W. Surf. Sci. 1997, 370, 113. (15) Alcala, R.; Mavrikakis, M.; Dumesic, J. A. J. Catal. 2003, 218, 178. (16) Wang, H. F.; Liu, Z. P. J. Phys. Chem. C 2007, 111, 12157.

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on Pt(111),15-17 Rh(111),18 Rh/CeO2(111),19 and Ru/ZrO2(111)20 has been theoretically investigated using the self-consistent density functional theory (DFT) slab calculations. These investigations have demonstrated that the low-temperature surface chemistry of ethanol strongly depends on the identity of the metals. Palladium is an effective catalyst for ethanol synthesis and decomposition as well as a good electrocatalyst for ethanol oxidation in alkaline media.21,22 On the Pd(111) surface, temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) experiments indicated that most of the adsorbed ethanol desorb at 200 K; the remainder decomposes into H2, CO, and CH4 without desorption of any partial dehydrogenation products via a decarbonylation pathway through the acetaldehyde and acyl species; decomposition of acyl requires temperatures above 260 K, which is the rate-limiting step in the ethanol decomposition.10-12 On the Pd(110) surface, Shekhar and Barteau found that ethanol decomposes via the same dehydrogenation and decarbonylation steps as on Pd(111) without any C-O scission involved;13 however, Bowker et al. suggested that a competing path involving the C-O bond scission of ethoxy also exists.14 Despite these experimental studies, to our knowledge, there are no any theoretical works relevant to ethanol reaction on pure Pd surfaces. In this paper, we present a theoretical investigation of ethanol decomposition on Pd(111) based on the periodic DFT calculations. Our main purpose is to map out the decomposition network including the structures and energies of the intermediates involved as well as the decomposition potential energy surface (PES). (17) Wang, H. F.; Liu, Z. P. J. Am. Chem. Soc. 2008, 130, 10996. (18) Choi, Y. M.; Liu, P. J. Am. Chem. Soc. 2009, 131, 13054. (19) Chen, H. L.; Liu, S. H.; Ho, J. J. J. Phys. Chem. B 2006, 110, 14816. (20) Chen, Y. W.; Ho, J. J. J. Phys. Chem. C 2009, 113, 6132. (21) Yee, A.; Morrison, S. J.; Idriss, H. J. Catal. 1999, 186, 279. (22) Xu, C. W.; Cheng, L. Q.; Shen, P. K.; Liu, Y. L. Electrochem. Commun. 2007, 9, 997.

Published on Web 12/15/2009

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2. Computational Details The DFT calculations were performed with the program package of DMol3 in the Materials Studio of Accelrys, Inc.23-25 The exchange-correlation energy was calculated within GGAPW91 approximation.26,27 The low coveraged Pd(111) surface was modeled by a three-layer slab with√four√Pd atoms per layer representing a p(2  2) unit cell, and a ( 3  3)R30° three-layer slab unit cell was used to model the relatively high coverage surface. A vacuum region of 10 A˚ thickness was used to separate the surface from its periodic image in the direction along the surface normal. The reciprocal space was sampled by a grid of (6  6  2) k-points generated automatically using the Monkhorst-Pack method.28 A single adsorbate was allowed to adsorb on one side of the unit cells, corresponding to the surface coverages of 1/4 and 1/3 ML. The adsorption energies reported herein were calculated using the equation ΔE ¼ Eads þ EPdð111Þ -Eads=Pdð111Þ where ΔE is the adsorption energy of the adsorbate on Pd(111), Eads is the total energy of the adsorbate, EPd(111) is the total energy of the clean Pd(111) slab, and Eads/Pd(111) is the total energy of the adsorbate on Pd(111). By this definition, a positive ΔE implies a stable adsorption. This three-layer model has been extensively used in the calculation for ethanol oxidation and synthesis.16,18 Furthermore, the adsorption energies of ethanol on larger models (four-layer, p(2  2) and p(3  3)) were calculated, and the discrepancies between the results of the four- and three-layer slabs were found to be less than 0.5 kcal/mol. For a reaction such as AB f A þ B on Pd(111), the reaction energy was calculated on the basis of the following formula: ΔH ¼ EðA þ BÞ=Pdð111Þ -EAB=Pdð111Þ where E(AþB)/Pd(111) is the total energy for the coadsorbed A and B on Pd(111). Transition state (TS) searches were performed at the same theoretical level with the complete LST/QST method.23-25,29 In this method, the linear synchronous transit (LST) maximization was performed, followed by an energy minimization in directions conjugating to the reaction pathway to obtain an approximated TS. The approximated TS was used to perform quadratic synchronous transit (QST) maximization and then another conjugated gradient minimization was performed. The cycle was repeated until a stationary point was located. Each TS structure was characterized by a vibrational analysis with exactly one imaginary frequency. Further computational details can be referred to our previous study.30 Vibrational frequencies were calculated for all the initial (IS) and final (FS) states, as well as the TSs from the Hessian matrix with the harmonic approximation, and the zero-point energy (ZPE) was calculated from the resulting frequencies. Rate constant k for unimolecular decomposition of an adsorbed species on a surface was calculated using conventional (23) Delley, B. J. Chem. Phys. 1990, 92, 508. (24) Delley, B. J. Chem. Phys. 1996, 100, 6107. (25) Delley, B. J. Chem. Phys. 2000, 113, 7756. (26) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (27) Perdew, J. P.; Wang, Y. Phys. Rev. B 1986, 33, 8800. (28) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (29) Halgren, T. A.; Lipscomb, W. N. Chem. Phys. Lett. 1977, 49, 225. (30) Jiang, R. B.; Guo, W. Y.; Li, M.; Fu, D. L.; Shan, H. H. J. Phys. Chem. C 2009, 113, 4188. (31) Hill, T. L. An Introduction to Statistical Thermodynamics; Dover: New York, 1986.

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Figure 1. The most stable adsorption structures of intermediates involved in the most likely pathways of ethanol decomposition on Pd(111).

TS theory:31 k ¼

kB T QTS -ðEact 0 0 =kB TÞ e ¼ Ae -ðEact =kB TÞ h QIS

kB is the Boltzmann constant, and h is the Planck constant; QIS and QTS are the partition functions at the IS and TS, respectively; E0act is the ZPE-corrected energy barrier; T is the temperature, and A is the pre-exponential factor.

3. Results This part is divided into two sections. In section 3.1, we give structures and energies for the most stable adsorption configurations involved in the most likely pathway of ethanol decomposition. In section 3.2, we investigate the most likely reaction steps in order to gather a general view of the reaction process, including thermodynamics and kinetics data. For clarification, all energies reported herein are after ZPE corrections. 3.1. Adsorbed Intermediates. In this section, we present a detailed investigation of all stable adsorptions of intermediates involved in the most likely pathways of ethanol decomposition at 1/4 surface coverage. The corresponding configurations are shown in Figure 1, and some important parameters for the intermediates are listed in Table 1. Details of intermediates involved in the excluded steps are given in Figure S1 and Table S1, Supporting Information. Ethanol. Analogous to the experimental results for Ni(111)5 and Rh(111)6 as well as the DFT result of Pt(111),15 ethanol prefers to adsorb at the top site on Pd(111) through the oxygen atom with the O-H bond parallel to the surface and the C-O axis tilted by 56° from the surface normal (see Figure 1 and Table 1) due to interaction via the lone pair electrons of oxygen.32 (32) Rubloff, G. W.; Demuth, J. E.; Vac, J. Sci. Technol. 1977, 14, 419.

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Table 1. Adsorption Sites, Adsorption Energies (kcal mol-1), and Structural Parameters (A˚ and °) for Intermediates Involved in Ethanol Decomposition on Pd(111) species

sites

ΔEadsa

dC-Pd

dO-Pd anglesb

CH3CH2OH* CH3CHOH* CH3CHO* CH3CO*

top 11.7 (10.7) 2.46 56 top 42.5 (40.8) 2.09 75 bridge 9.0 (8.0) 2.23 2.14 84 top 50.7 (48.7) 1.95 79 hcp 50.2 (48.3) 2.06, 2.22 2.25 74 fcc 50.1 (48.1) 2.03, 2.29 2.26 74 bridge 26.6 (24.8) 2.00, 2.06 49 CH2CO* CHCO* hcp 68.6 (66.2) 2.05, 2.07, 2.04 C* hcp 143.2 1.88, 1.90, 1.90 fcc 142.9 1.87, 1.90, 1.91 CH* fcc 138.0 (133.4) 1.94, 1.97, 1.97 hcp 137.7 (133.0) 1.95, 1.96, 1.96 bridge 87.5 (83.3) 2.01, 2.04 CH2* top 43.2 (39.5) 2.04 CH3* hcp 3.6 (2.6) 3.76, 3.83, 3.85 CH4* fcc 3.3 (2.3) 3.77, 3.77, 3.77 top 3.1 (3.1) 3.49 bridge 3.0 (2.8) 3.48, 3.60 a Parameters in parentheses are adsorption energies after ZPE corrections. b Values are angles between the surface normal and the C-O axis in the corresponding species.

The binding energy is calculated to be 10.6 kcal mol-1, in good agreement with the estimated value of 11.8 kcal mol-1 by Redhead analysis of the TPD data.33 This relatively weak adsorption is consistent with the large O-Pd distance (2.46 A˚), suggesting that the adsorbed ethanol is an unstable precursor for further decomposition, especially at relatively high temperatures. Furthermore, the calculated vibrational frequencies of the adsorbate also agree well with the experimental values (see Table S2).12 1-Hydroxyethyl. Alcohol decomposition on the Pt-group metals is generally believed to start with the hydroxyl-H elimination forming an alkoxide intermediate.34 However, on Pd(111), ethoxide has not been directly experimentally detected.12 The present DFT slab calculations indicate that initial CR-H bond scission is indeed preferred (see section 3.2), thus we focus our study on adsorbed CH3CHOH. To our knowledge, no experimental or theoretical structural information can be obtained for the adsorbate on Pd surfaces. Similar to the situation for Pt(111),15 CH3CHOH is only stably adsorbed at the top site on Pd(111) with the CR-Pd distance at 2.09 A˚ due to formation of a σ-type bond between the sp3(CR) hybridized and 4dz2 (Pd) orbitals (see Figure 1 and Table 1). The C-C bond is upright, resulting in relatively weak interaction between the terminal methyl and the surface, while the hydroxyl H points to the surface favoring the scission of the related bond. This configuration accounts for an adsorption energy of 40.8 kcal mol-1. Acetaldehyde. Acetaldehyde is an important intermediate in ethanol decomposition and synthesis.35,36 It was reported that acetaldehyde could be adsorbed on metal surfaces in η1(O) or η1(CR)-η1(O) configuration.37 In the present calculations, we find that, similar to the situation of formaldehyde on Pd(111),38 the η1(O) configuration is unstable, while the η1(CR)-η1(O) configuration over a bridge site with the CR-Pd and O-Pd distances at 2.23 and 2.14 A˚, respectively, is indeed located for acetaldehyde (33) (34) (35) (36) (37) (38) 8068.

Redhead, P. A. Vacuum 1962, 12, 203. Christmann, K.; Demuth, J. E. J. Chem. Phys. 1982, 79, 6308. Inui, T.; Yamamoto, T. Catal. Today 1998, 45, 9. Wang, Y.; Luo, H. Y.; Liang, D. B.; Bao, X. H. J. Catal. 2000, 196, 46. Davis, J. L.; Barteau, M. A. J. Am. Chem. Soc. 1989, 111, 1782. Chen, Z. X.; K. Neyman, M. K.; Lim, H.; Rosch, N. Langmuir 2004, 20,

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(see Figure 1). The adsorption energy, ca. 8.0 kcal mol-1, is rather low because of the closed-shell electronic configuration of the adsorbate. Vibrational frequencies for the adsorbed acetaldehyde listed in Table S2 suggest satisfactory agreement between theory and experiment.37 Acetyl. Consistent with the experimental results,37 acetyl could adsorb stably at the top site through the CR atom with the C-Pd axis (RCPd = 1.95 A˚) inclined by 17° from the surface normal and the C-O axis almost parallel to the surface (see Figure 1 and Table 1). In addition, the η2(CR)-η1(O) configurations over hexagonal close-packed (hcp) and face-centered cubic (fcc) sites are also found (see Table 1). The almost equivalent adsorption energies (48.7 (top), 48.3 (hcp), and 48.1 (fcc) kcal mol-1) suggest the comparable preference of these configurations. The bridge site is indeed unstable, which would migrate to the top site during optimization. The calculated vibrational frequencies of the adsorbed CH3CO are consistent with the experimental values (see Table S2).37 Ketene. Further dehydrogenation of acetyl accounts for ketene, which emerges as an important intermediate for its relevance to catalytic C1 chemistry.39 As shown in Figure 1, similar to the situation for Pt(111) and Ru(001),15,39 ketene only binds to Pd(111) in a top-bridge-top manner (η1(CR)-η1(Cβ)), in which the two C atoms lie above adjacent top sites with comparable C-Pd bond lengths (2.00 and 2.06 A˚) and the C-C bond is elongated to 1.45 A˚ from 1.30 A˚ (gas-phase). This configuration accounts for the binding energy of 24.8 kcal mol-1. Ketenyl. Ketenyl comes from the dehydrogenation of ketene, which is a critical intermediate in a variety of reactions, such as combustion. However, the spectroscopic information of the radical is scarce.40 Our calculation shows ketenyl is a linear radical, in which both C atoms are sp hybridized; the calculated bond lengths are 1.19, 1.25, and 1.05 A˚ for C-O, C-C, and CR-H, respectively, in good agreement with the results of the previous quantum chemical coupled cluster study (1.172, 1.297, and 1.066 A˚).40 Analogous to the same adsorbate on Pt(111) in the previous DFT slab calculations,15 CHCO interacts with the Pd surface forming the η1(CR)-η2(Cβ) configuration (see Figure 1), in which the CR atom sits at the top site through the sp2 hybridized orbital (RCPd = 2.05 A˚) and Cβ sits at the bridge site (RCPd = 2.04 and 2.07 A˚) via the sp3 hybridization. The adsorption energy is calculated to be 66.2 kcal mol-1. Carbon Monoxide and Hydrogen. The adsorption of carbon monoxide and hydrogen over the Pd(111) surface has been extensively investigated by others36,41 and us at the present theoretical level.30 Our calculation indicates that CO binds to the Pd surface through the carbon end with chemisorption energies of 40.0 (fcc) and 39.7 (hcp) kcal mol-1. For atomic hydrogen, we found that both fcc and hcp are the most stable sites with adsorption energies of 60.7 and 60.1 kcal mol-1, respectively. One important point that should be noted is that the significant mobility of adsorbed H favors the dehydrogenation process.30 Carbon. As is well-known, coke formation is a severe problem to be overcome in the DAFC and steam reforming applications. Although a significant number of studies have been performed on this topic,42,43 it is necessary to understand the formation and distribution of carbon and relevant intermediates from an atomic (39) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744. (40) Szalay, P. G.; Fogarasi, G.; Nemes, L. Chem. Phys. Lett. 1996, 263, 91. (41) Sautet, P.; Rose, M. K.; Dunphy, J. C.; Behler, S.; Salmeron, M. Surf. Sci. 2000, 453, 25. (42) Watwe, R. M.; Bengaard, H. S.; Rostrup-Nielsen, J. R.; Dumesic, J. A.; Noerskov, J. K. J. Catal. 2000, 189, 16. (43) Paul, J. F.; Sautet, P. J. Phys. Chem. B 1998, 102, 1578.

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Figure 2. Calculated reaction network and energy barriers (kcal mol-1) for ethanol decomposition on Pd(111). H atoms produced are omitted for clarity. The data in parentheses are barriers after the ZPE corrections. The (g) represents gas-phase product, and the corresponding data are desorption energies.

scale. Similar to the situation of Ni(111),42 C is stably adsorbed at the hollow sites on Pd(111) (RCPd ∼ 1.9 A˚) (see Figure 1) with binding energies of 143.2 (hcp) and 142.9 (fcc) kcal mol-1, in agreement with the previous theoretical results.43 Top and bridge sites are indeed unstable, which would move to the hcp site during optimization, indicating low mobility of C on Pd(111). Methyne. CH prefers the hollow sites on the Pd(111) surface via the C end in an upright configuration (see Figure 1), in line with the previous DFT finding.43 For both the fcc and hcp adsorptions, the C atom is about 1.14 A˚ above the metal surface, and the C-Pd bonds are 1.94-1.97 A˚ long. These configurations account for almost equivalent binding energies (∼133 kcal mol-1). Methylene. Methylene is stably adsorbed at the bridge site on Pd(111) in a tetrahedral configuration with a binding energy of 83.3 kcal mol-1 (see Figure 1), which is in good agreement with the previous DFT findings.43 In this mode, the H-C-H angle is 112°, and the two C-Pd bonds are 2.01 and 2.04 A˚ long. Methyl. Methyl is only adsorbed at the top site through the C atom with the C3 axis normal to the metal surface, accounting for an adsorption energy of 39.5 kcal mol-1. The C-Pd distance is 2.04 A˚, and the H-C-H angle is 112°. The optimal structure shows an out of plane bending of H atoms (16°) (see Figure 1), in agreement with the previous DFT results (18°).43 Methane. Methane does not exhibit any site preference on Pd(111) with very low adsorption energy (about 3 kcal mol-1) and a rather large distance between C and the surface (about 3.46 A˚) at the four different sites (see Table 1). 3.2. Reaction Pathways. In this section, we present the ethanol decomposition network. For simplicity, we choose the adsorption/coadsorption of the involved intermediate(s) at the most stable site(s) as IS, and the corresponding FS are taken to be the coadsorption/adsorption of the product species at the most stable site(s). Our strategy is to build the reaction network by iterating the procedures of searching all possible reaction channels for a given adsorbed species obtained from the previous step and only walking along the channel that has a relatively low reaction barrier and a large rate constant until the final products are reached. The calculated decomposition network is schematically shown in Figure 2. Calculated TS configurations involved in the most likely steps are shown in Figure 3, and the corresponding FS and IS geometries are shown in Figures S2 and S3. Table 2 lists calculated reaction energies, energy barriers, rate constants, as well as imaginary frequencies for all steps considered. Structures involved in the abandoned processes are shown in Figures S4-S9. Bond Cleavage of Ethanol. As the starting process, we identify all possible bond scission paths involving CR-H, Cβ-H, O-H, C-O, and C-C in ethanol. As shown in Figure 2, the C-C path 1882 DOI: 10.1021/la902641t

Figure 3. The TS structures involved in the most likely pathway of the decomposition of ethanol on Pd(111).

is the most impossible at decomposition conditions due to the extremely high energy barrier involved (77.8 kcal mol-1); the next most unfavorable path is the C-O cleavage because it also involves a high energy barrier (48.9 kcal mol-1); O-H bondbreaking also has a high energy barrier (32.3 kcal mol-1), so the other two paths (CR-H and Cβ-H) could serve as the candidates of ethanol decomposition for their comparable low energy barriers (21.3 and 21.9 kcal mol-1). In order to determine which one is the most possible, rate constants are estimated for these paths (see Table 2). The CR-H path has the largest rate constant at 300 K, which is about 44 times larger than the Cβ-H paths, suggesting that initial CR-H bond scission is indeed the main channel of ethanol decomposition. In this path, a rotation of the adsorbed ethanol results in activation of CR-H bond by a Pd atom. In TS1 (see Figure 3), the top-site bound CH3CHOH is more close to the metal surface than in the IS, while the departing H* atom stays nearly at the bridge site; the C-H*, H*-Pd, and C-Pd distances are 1.84, 1.65, and 2.71 A˚, respectively. The imaginary frequency is 908i cm-1 (see Table 2), and the corresponding normal mode just describes the stretch vibration of the Langmuir 2010, 26(3), 1879–1888

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Table 2. Calculated Reaction Energies ΔH, Energy Brriers Ea (kcal mol-1), Rate Constants k (s-1), and Imaginary Frequency ν (cm-1) of TS for the Elementary Reactions Involved in Ethanol Decomposition on Pd(111)a reaction

ΔH

-11.7 (-10.6) CH3CH2OHþ* f CH3CH2OH* 6.6 (2.3) CH3CH2OH* f CH3CHOH*þH* 15.0 (11.0) CH3CH2OH* f CH2CH2OH*þH* 14.5 (11.0) CH3CH2OH* f CH3CH2O*þH* 34.7 (33.2) CH3CH2OH* f CH3*þCH2OH* 21.9 (20.5) CH3CH2OH* f CH3CH2*þOH* 5.1 (1.1) CH3CHOH* f CH3CHO*þH* 0.6 (-3.0) CH3CHOH* f CH3COH*þH* CH3CHOH* f CH2CHOH*þH* 7.7 (4.7) 34.6 (33.1) CH3CHOH* f CH3*þCHOH* 26.9 (28.3) CH3CHOH* f CH3CH*þOH* -7.2 (-10.3) CH3CHO* f CH3CO*þH* 9.4 (6.5) CH3CHO* f CH2CHO*þH* 18.7 (17.7) CH3CHO* f CH3*þCHO* 39.5 (38.9) CH3CHO* f CH3CH*þO* 11.2 (7.8) CH3CO* f CH2CO*þH* 0.43 (-0.8) CH3CO* f CH3*þCO* 37.9 (37.3) CH3CO* f CH3C*þO* 8.4 (4.8) CH2CO* f CHCO*þH* 1.1 (0.1) CH2CO* f CH2*þCO* 44.8 (43.7) CH2CO* f CH2C*þO* CHCO* f CH* þCO* -0.14 (-0.8) CHCO* f CCO*þH* 11.2 (9.1) CHCO* f CHC*þO* 67.8 (66.3) -0.3 (1.9) CH*þH* f CH2* CH* f C*þH* 20.9 (18.4) -13.9 (-10.6) CH2* þ H* f CH3* 0.3 (-1.9) CH2* f CH* þ H* -10.6 (-7.9) CH3* þ H* f CH4* 13.9 (10.6) CH3* f CH2* þ H* 10.6 (7.9) CH4* f CH3* þ H* a Values in parentheses are energies after ZPE corrections.

CR-H bond. After the TS, the atomic H* moves to the adjacent fcc site, and the distance between CR and Pd is further shortened to 2.10 A˚, forming the FS with a reaction energy of only 2.3 kcal mol-1. Bond Scission in 1-Hydroxyethyl. As long as CH3CHOH is formed, it would be possible to further degrade through rupture of the CR-H, Cβ-H, O-H, C-O, or C-C bond. The C-C and C-O paths can be discarded because they involve high energy barriers (63.2 and 45.1 kcal mol-1). For the O-H, CR-H, and Cβ-H paths, the lowest energy barrier (12.4 kcal mol-1) accounts for the largest rate constant for the O-H path, which is about 103 and 107 times larger than those for the CR-H and Cβ-H paths. In the O-H path, the hydroxyl H of CH3CHOH is transferred to a Pd atom as a result of the O-H stretching vibration. In TS2 (see Figure 3), the bond between the atomic H* and O is already broken (ROH* = 1.54 A˚), and both the atomic H* and acetaldehyde sit at bridge sites with the C-O bond nearly parallel to the surface. An imaginary frequency of 620i cm-1 in the TS is associated with mode of the expected bond-breaking. Following TS2, movements of the new entities to the fcc and bridge sites account for the stable FS, an almost thermoneutral product with a reaction energy of 1.1 kcal mol-1. Decomposition of Acetaldehyde. For acetaldehyde decomposition, four different bond-breaking paths involving CR-H, Cβ-H, C-C, and C-O can be envisaged (see Figure 2). In this case, the C-O path is the most unfavorable with an energy barrier of 56.8 kcal mol-1, higher than those for the same bond scission in ethanol and 1-hydroxyethyl; the C-C path becomes the next unfavorable with an energy barrier of 33.1 kcal mol-1; the Cβ-H path holds an energy barrier of 18.6 kcal mol-1. The most favorable channel seems to be the CR-H path, giving coadsorbed CH3CO and H as the FS with an energy barrier of only 6.8 kcal mol-1. This path starts with insertion of Pd into the CR-H bond. Langmuir 2010, 26(3), 1879–1888

Ea 27.1 (21.3) 27.0 (21.9) 38.3 (32.3) 82.5 (77.8) 53.8 (48.9) 18.1 (12.4) 21.3 (16.4) 25.8 (21.5) 67.0 (63.2) 47.7 (45.1) 11.2 (6.8) 23.4 (18.6) 35.8 (33.1) 59.1 (56.8) 29.8 (25.5) 33.5 (31.8) 57.3 (55.4) 27.3 (22.5) 30.8 (28.9) 63.0 (60.9) 22.5 (21.0) 27.2 (23.0) 92.2 (90.0) 15.3 (13.6) 38.6 (34.6) 18.5 (16.9) 15.7 (11.7) 15.2 (12.9) 28.4 (23.5) 25.8 (20.8)

k 91.1 2.08

2.96  107 1.39  104 5.42  10-1 4.20  1012 4.07  103 1.75  10-1 2.36  10-11 1.95 2.65  10-7 5.49  10-2 1.31  10-3

ν 908i 1115i 611i 642i 488i 620i 1019i 1133i 517i 442i 1014i 1139i 459i 480i 1096i 531i 541i 1163i 444i 501i 544i 1132i 469i 1065i 1029i 1125i 1065i 1008i 1125i 1008i

In TS3 (see Figure 3), because of the lost of H, interactions of CR with both O and Pd atoms are strengthened, but the bond between O and Pd is weakened as reflected by the corresponding bond lengths (varieties: 0.37 A˚ for CR-H, -0.07 A˚ for C-O, -0.17 A˚ for C-Pd, and 0.26 A˚ for O-Pd). TS3 has an imaginary frequency of 1014i cm-1, which accounts for the corresponding bond scission. After TS3, the atomic H continues to go down to the fcc site, while the CR-Pd and CR-O bonds are further shortened to 2.00 and 1.20 A˚, forming a stable FS with an energy fall of 10.3 kcal mol-1. Bond Cleavage of Acetyl. We investigate all the three possible bond scissions (Cβ-H, C-C, and C-O) for adsorbed acetyl (see Figure 2). Similar to acetaldehyde, the C-O path is the most unfavorable because it has the highest energy barrier (55.4 kcal mol-1). The energy barrier for the C-C path continues to drop down to 31.8 kcal mol-1; Cβ-H has the lowest energy barrier (25.5 kcal mol-1), and its rate constant is 1010 times larger than that of the C-C path, so that the Cβ-H path accounting for coadsorbed CH2CO (bridge) and H (fcc) is the most favorable. The Cβ-H activation results from the incline of methyl in CH3CO toward the surface so that a methyl H could migrate to an adjacent top site. When TS4 (see Figure 3) is located, the leaving H sits at the top site, and CH2CO sits at the bridge site forming two C-Pd bonds (2.0 and 2.3 A˚). TS4 is characterized by an imaginary frequency of 1096i cm-1, which corresponds to just the bond-breaking movement. After the TS, the C-C bond is further inclined, favoring the top-bridge-top configuration, and the atomic H diffuses to the adjacent fcc site, giving the FS. The reaction energy of this process is 7.8 kcal mol-1. Reaction of Ketene. Three kinds of bonds may be activated for adsorbed CH2CO. As shown in Figure 2, the energy barrier for the C-O scission is increased to be as high as 60.9 kcal mol-1. The energy barrier for the C-C path continues to decrease to 28.9 kcal DOI: 10.1021/la902641t

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mol-1, but is still 6.4 kcal mol-1 higher than that for the Cβ-H path (22.5 kcal mol-1). Obviously, the Cβ-H path is the most favorable, which gives the coadsorbed ketenyl in an fcc-η2(Cβ)-η1(CR) configuration and atomic H at the fcc site adjacent to the FS. In this case, intrarotation of the CH2 group along the C-C axis brings the methylene H close to the surface so that the Cβ-H bond could be activated. In TS5 (see Figure 3), CHCO remains in the top-bridge-top configuration, and the leaving H shares a Pd atom with Cβ; the Cβ-H*, C-C, and H*-Pd bonds are, respectively, 1.61, 1.41, and 1.66 A˚ long. An imaginary frequency of 1163i cm-1 is associated with TS5 to characterize the bond-breaking. This path is completed by movements of the new entities to adjacent fcc sites with an endothermicity of 4.8 kcal mol-1. Decomposition of Ketenyl. As long as ketenyl is produced, its C-H, C-C, and C-O bonds may be ruptured. The C-O path is still the most unfavorable owing to the very high energy barrier involved (90.0 kcal mol-1). For the other two paths, although the energy barriers are close in value, i.e., 20.1 (C-C) and 23.0 (Cβ-H) kcal mol-1, the C-C path would occur preferably because it has a rate constant 42 times larger than that of the Cβ-H path (see Table 2). A similar result has also been found for Pt(111).15 In the path, the C-C stretching vibration results in the scission of the bond, and an imaginary frequency of 544i cm-1 corresponds to the TS. In TS6 (see Figure 3), CO lies at the top site with C-O tilted from the surface normal by 20°, and CH locates at the adjacent bridge site with the C-H axis tilted by 22°; the C-C distance is 2.07 A˚. After TS6, rearrangements of CO to the exact top site and CH to the adjacent fcc site afford the thermoneutral FS. Reaction of Adsorbed CO. CO is one product of ketenyl decomposition. We can imagine two possible exits of adsorbed CO, desorption and decomposition. The decomposition energy barrier (100.3 kcal mol-1) is much larger than the adsorption energy of CO (40.0 kcal mol-1), thus desorption of CO is more favorable, confirming the experimental result that no C-O scission on Pd(111) was observed.10,12 Reaction of Methyne. Methyne is the other product of ketenyl decomposition. Once methyne is produced, it may further decompose into atomic carbon and hydrogen or hydrogenate to methylene. Both possibilities are checked. For the decomposition, the coadsorbed C and H at adjacent fcc sites are taken as the FS. This process involves the wiggle of the adsorbed CH so that the C-H bond could be activated. In TS7 (see Figure 3), the atomic C still sits at the same fcc site as in the IS, and the H atom sits at adjacent bridge site sharing one Pd atom with C; the C-H*, C-Pd, and H-Pd distances are 1.64, 1.96, and 1.65 A˚, respectively. This TS configuration accounts for an energy barrier of 34.6 kcal mol-1. An imaginary frequency of 1029i cm-1 is associated with the C-H bond-breaking. After TS7, the atomic C preserves the fcc site with the C-Pd distance shortened further (1.86 A˚), and the atomic H moves to the adjacent fcc site, accounting for the FS being exothermic by 18.4 kcal mol-1. Alternatively, for the hydrogenation process, we take the coadsorbed CH and H at adjacent fcc sites as the IS and the adsorbed methylene at bridge site as the FS. In TS8 (see Figure 3), the adsorbed CH entity still lies at its stable fcc site; both the atomic H* and C share a Pd atom with the C-H* and C-Pd distances being 1.51 and 2.01 A˚, respectively. TS8 is characterized by an imaginary frequency of 1065i cm-1, which just depicts the bond-breaking movement. Following TS8, the atomic H* approaches CH further to form the adsorbed CH2. This process accounts for an energy rise of 1.9 kcal mol-1 and an energy barrier of 13.6 kcal mol-1, the latter of which is 20.9 kcal mol-1 lower 1884 DOI: 10.1021/la902641t

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Figure 4. Curves of energy barriers for the scissions of the C-C, C-O, C-H, and O-H bonds in the intermediates going along the decomposition pathways of ethanol on Pd(111). The square, circle, triangle, pentacle and open-square represent the C-C, C-O, Cβ-H, CR-H, and O-H bond scissions, respectively.

than that for the alternative dehydrogenation process. Therefore, hydrogenation is more favorable for adsorbed methyne on Pd(111). Reaction of Methylene. We also consider both dehydrogenation and hydrogenation processes for adsorbed methylene, which possess quite different thermochemistry, i.e., -1.9 (dehydrogenation) and -10.6 (hydrogenation) kcal mol-1. The dehydrogenation process is just the reverse process of the methyne hydrogenation as mentioned above, affording an energy barrier of 11.7 kcal mol-1. Comparable energy barrier (12.9 kcal mol-1) is found for the hydrogenation path. Therefore, both paths would be possible at real conditions. Methylene hydrogenation involves coadsorbed CH2 (bridge) and H (fcc) as the IS; In TS9 (see Figure 3), CH2 sits still at the bridge site, and the atomic H locates at the adjacent bridge site close to CH2; an imaginary frequency of 1125i cm-1 corresponds to the TS. After TS9, the atomic H approaches C, and the formed CH3 migrates to the top site. Reaction of Methyl. For adsorbed methyl, both hydrogenation and dehydrogenation processes are considered. As shown in Figure 2, the hydrogenation to adsorbed CH4 with an energy fall of 7.9 kcal mol-1 seems to be more favorable because of the relatively low energy barrier (12.9 kcal mol-1). The IS is the coadsorbed CH3 and H at their most stable sites. This process involves the movement of the atomic H to the hcp site sharing a Pd atom with CH3. In TS10 (see Figure 3), the C-Pd axis is tilted by 14° from the surface normal with the C-H* and C-Pd distances being 1.63 and 2.22 A˚; an imaginary frequency of 1008i cm-1 is involved to identify this bond-breaking. After the TS, the bond between C and H is formed, giving adsorbed CH4. Reaction of Adsorbed Methane. As long as the adsorbed methane is produced, it may desorb as gas-phase CH4 or decompose into coadsorbed CH3 and H. The fact that the calculated decomposition energy barrier (20.8 kcal mol-1) is 17.7 kcal mol-1 higher than the desorption energy suggests methane prefers to desorb once formed.

4. Discussion In this section, we discuss some important points based on the above calculated results. General Structural Features for the Involved Intermediates. In the previous paper,30 we gave a general structural feature Langmuir 2010, 26(3), 1879–1888

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Table 3. Energy Barriers and Contribution Factors (kcal mol-1) for the Elementary Reactions Involved in Ethanol Decomposition on Pd(111)a reactions

ΔEgAB

EIS AB

ETS int

ETS A

ETS B

Ea

CH3CH2OH* f CH3CH2* þ OH* CH3CHOH* f CH3CH* þ OH* CH3CHO* f CH3CH* þ O* CH3CO* f CH3C* þ O* CH2CO* f CH2C* þ O* CHCO* f CHC* þ O* CO* f C* þ O* CH3CH2OH* f CH3* þ CH2OH* CH3CHOH* f CH3* þ CHOH* CH3CHO* f CH3* þ CHO* CH3CO* f CH3* þ CO* CH2CO* f CH2* þ CO* CHCO* f CH* þ CO* CH3CH2OH* f CH2CH2OH* þ H* CH3CHOH* f CH2CHOH * þ H* CH3CHO* f CH2CHO* þ H* CH3CO* f CH2CO* þ H* CH2CO* f CHCO* þ H* CHCO* f CCO* þ H* CH3CH2OH* f CH3CHOH* þ H* CH3CHOH* f CH3COH * þ H* CH3CHO* f CH3CO* þ H* CH3CH2OH* f CH3CH2O* þ H* CH3CHOH* f CH3CHO * þ H* a Values without ZPE corrections.

101.8 126.3 206.5 199.6 191.3 175.5 266.4 91.8 65.9 91.5 25.0 95.2 96.8 109.4 44.1 99.9 47.6 110.1 108.8 98.9 88.3 95.0 107.3 32.1

11.7 42.5 9.0 50.7 26.6 68.6 41.8 11.7 42.5 9.0 50.7 26.6 68.6 11.7 42.5 9.0 50.7 26.6 68.6 11.7 42.5 9.0 11.7 42.5

2.6 0.9 3.1 8.7 11.0 13.2 14.8 -2.1 -0.1 4.8 7.5 13.0 7.5 -2.2 6.4 6.5 3.1 4.9 9 -2.3 5.6 6.2 6.1 5.4

12.9 75.1 71.5 114.7 80.7 88.8 138.0 6.3 8.0 27.4 25.1 76.4 123.9 37.1 11.5 34.4 18.3 57.7 101.2 25.4 59.1 45.1 32.7 3.4

49.4 46.9 88.0 87.0 85.2 76.3 83.2 12.6 33.3 42.1 24.6 27.6 26.5 54.8 55.7 57.6 53.3 56.6 58.0 55.8 56.0 53.9 54.1 58.4

53.8 47.7 59.1 57.3 63.0 92.2 101.8 82.5 67.0 35.8 33.5 30.8 22.5 27.0 25.8 23.4 29.8 27.3 27.2 27.1 21.3 11.2 38.3 18.2

for intermediates involved in methanol dehydrogenation on group VIII transition metal surfaces, i.e., the most stable adsorption of the involved species tends to follow the gas-phase bond order rules by bonding with the surface metal atom(s), wherein C is tetravalent and O is divalent with the missing H atoms replaced by the metal atoms. For intermediates relevant to ethanol decomposition on the Pd(111) surface, this relationship is once again highlighted, i.e., η1(C) (top) for CH3CHOH, CH3CO, and CH3; η2(C) (bridge) for CH2; η3(C) (hollow) for CH and CO; η1(C)-η1(O) and η1(C)-η1(C) (bridge) for CH3CHO and CH2CO; η2(C)-η1(C) (hollow) for CHCO. Note that this relation can also be applied to other group VIII metal surfaces, such as Pt(111) and Ni(111),44,45 suggesting the ability of the metals to form bonds with these adsorbates due to the d8 electronic configuration. Trends of Energy Barriers for Bond Cleavages in Relevant Intermediates. It is interesting to give the general trends of energy barriers for different bond scissions in the C-, O-, and Hcontaining species, and the results are shown in Figure 4. On going from the reactant side to the product side, for the C-O bond scission, the energy barrier is increased from 49 kcal mol-1 for ethanol to 90 kcal mol-1 for CHCO, and last to 100 kcal mol-1 for CO; energy barriers for the C-C, CR-H, and O-H bond scission are gradually decreased; while energy barriers for the Cβ-H path do not change obviously across all intermediates. For a reaction such as AB f A þ B on metal surfaces, the relevant energy barrier Ea can be divided into five terms:30,46,47 g IS TS Ea ¼ΔEAB þ EAB þ Eint -EATS -EBTS

ð1Þ

where ΔEgAB is the bond energy of the gas-phase AB; EIS AB is the adsorption energy of AB in the IS; ETS int is a quantitative measureTS ment of interaction between A and B in the TS; and ETS A (EB ) is the adsorption energy of A (B) at the TS geometry without B (A). (44) Greeley, J.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 3910. (45) Remediakis, I. N.; Pedersen, F. A.; Noerskov, J. K. J. Phys. Chem. B 2004, 108, 14535. (46) Liu, Z. P.; Hu, P. J. Chem. Phys. 2003, 119, 6282. (47) Liu, Z. P.; Hu, P. J. Am. Chem. Soc. 2003, 125, 1958.

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Obviously, the first three terms contribute positively to the energy barrier, while the last two factors have the reverse effect. In order to obtain the determined factors for the changes of the energy barriers, these terms are calculated for all elementary steps involved. The resulting parameters are given in Table 3. For the C-O bond scission, we can divide these intermediates into three categories: (a) single C-O bond adsorbates, CH3CH2OH and CH3CHOH, with C-O bond energies in the range of 102-126 kcal mol-1 and energy barriers of about 50 kcal mol-1; (b) double C-O bond adsorbates, CH3CHO, CH3CO, CH2CO, and CHCO, with C-O bond energies of 175-206 kcal mol-1 and energy barriers of about 60-90 kcal mol-1; (c) triple C-O bond adsorbates, CO, with a C-O bond energy of 266 kcal mol-1 and an energy barrier of 102 kcal mol-1. We can find that, totally, the change of the energy barriers is mainly determined by the relevant bond strengths. In the first class, it is the very strong adsorption of CH3CH at TS (ETS A ) that makes the energy barrier for CH3CHOH slightly lower than that for CH3CH2OH. In the second class, the change of the energy barriers is also controlled by the factors of adsorption energy of the adsorbates (EIS AB), repulsion between fragments in the TS (ETS int ), and adsorption energy of C2Hx (x = 1-4) at the TS geometry (ETS A ); for CHCO, the surface corrugation (see S-TS16 in Figure S9), which weakens the adsorption of O at the TS geometry (ETS B ), increases the relevant energy barrier. For the C-C bond scission, we divide the involved intermediates into two categories: (a) CH3CH2OH, CH3CHOH, CH3CHO, and CH3CO, which give adsorbed CH3 as one of the decomposition products; (b) CH3CO, CH2CO, and CHCO, which produce adsorbed CO. For the first class, as shown in Table 3, compared to the situation of CH3CHO and CH3CO, the obvious higher energy barrier for CH3CH2OH and CH3CHOH is mainly controlled by the relatively weak adsorption of CH3 at the TS geometry (ETS A ) due to the steric effect (see S-TS3 in Figure S4 and S-TS7 in Figure S5). Furthermore, for the first three intermediates in this category, the change of the energy barriers also depends remarkably on the adsorption energy of CHxO* (x = 3-1) at the relevant TS geometry (ETS B ); for CH3CO, the very weak strength of the bond (ΔEgAB) has an obvious effect on the energy barrier. For the second class, the adsorption energy of CO DOI: 10.1021/la902641t

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at the TS geometry (ETS B ) does not change obviously; the significant increase of the adsorption strength of CHx* (x = 3-1) at the TS geometry (ETS A ) is the main factor for the decrease of energy barrier. By the way, a similar trend that lower activation energy comes with a smaller number of hydrogen atoms has also been found for the C-C path of CH3C, CH3CH, and CH3CH2 in ethane decomposition.48 In the case of Cβ-H scission, the adsorption energy of H* at the TS geometry (ETS B ) has less effect on the variety of the energy barriers because it does not change obviously. The cooperation of the other four factors accounts for the almost equivalent energy barriers for CH3CH2OH, CH3CHOH, CH3CHO, CH2CO, and CHCO. It is noted that at the TS of CH3CO, the indirect bonding competition between the H and C atoms weak the binding TS (ETS A and EB ), and further accounts for a little higher energy barrier than others. For the CR-H bond scission, the decrease of the energy TS barriers is mainly due to the cooperation of ΔEgAB, EIS AB, and EA , since the other two factors account for the reverse variety of the energy barriers (see Table 3). For the O-H bond scission, the bond energy (ΔEgAB) is the dominant factor for the drop of the energy barriers, as shown in Table 3 and Figure 4, because the adsorption energy of H* at TS IS TS geometry (ETS B ) does not change obviously, and EAB and EA favor the reverse change of the energy barriers. In order to understand the determining factors for the energy barriers deeply, we analyze the partial density of states (PDOS) for the five initial bond scission TSs; the PDOS spectra are given in Figures S10 and S11 in the Supporting Information. For the C-C bond-breaking (see Figure S10a), the broadening of the PDOS spectra of both C atoms at TS are negligible, thus the carbon atoms interact weakly with surface in the TS (see Table 3), and a rather high energy barrier is afforded. For the C-O bond scission (Figure S10b), although the PDOS of O atom is obviously broadened, the PDOS of C is still narrow, accouting for a lower energy barrier compared to the C-C bond-breaking. In the case of O-H, Cβ-H, and CR-H bond scission (see Figure S11), similar broadenings of PDOS for the fragment H at TSs are in line with the same ETS H values listed in Table 3, whereas strikingly different broadenings are found for the PDOS spectra of O, Cβ, and CR (following the order of Cβ > O > CR), consistent with the binding energies of these species at TS (ETS A ; see Table 3). Thermodynamical Factors in Ethanol Decomposition. In order to further investigate the decomposition process, we make a comparison of the activation barrier and the heat of reaction, known as the Broensted-Evans-Polanyi (BEP) behavior,46 for each elementary step to look into whether there is a clear relationship for different bond cleavages. Following the framework suggested by Alcala et al.,15 the elementary reactions are written as exothermic steps, and the IS and FS are defined accordingly. The energy reference for each step is the clean slab plus gas-phase energy of the corresponding reactants. As shown in Figure 5, we can identify three distinct beelines for different bond scission reactions, which is similar to the results for ethanol oxidation on platinum surface.17 This feature simply reflects that no single BEP relation can be established for all reactions on the same surface, whereas in each bond scission class, a linear relationship holds roughly for most reactions. The upperleft line represents the C-O bond scission, the linear regression equation is ETS = 0.94EFS þ 44.3 (kcal mol-1), the square of (48) Zeigarnik, A. V.; Valdes-Perez, R. E.; Myatkovskaya, O. N. J. Phys. Chem. B 2000, 104, 10578.

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Figure 5. Plot of the TS energy against the FS energy for all the bond-scission reactions involved in ethanol decomposition on Pd(111). The pentacle, triangle, and circle symbols denote the C-O, C-C, and C-H bond-breaking reactions, respectively. The points with the largest deviation correspond to the reactions CHCO f CHC þ O (labeled as 1), CH3CH2OH f CH3 þ CH2OH (labeled as 2), and CH3CHOH f CH3 þ CHOH (labeled as 3).

the correlation coefficient is 0.99, and the standard error is 5.5 kcal mol-1; the bottom-right is for the C-H scission, in which ETS = 0.96EFS þ 22.7 (kcal mol-1), the corresponding square of the correlation coefficient is 0.99, and the standard error is 5.4 kcal mol-1; the line in between is for the C-C bond scission, and the linear regression equation is ETS = 0.90EFS þ 20.9 (kcal mol-1) with the square of the correlation coefficient and the standard error being 0.95 and 6.3 kcal mol-1, respectively. It can be found that a few data points obviously deviate from the lines, e.g., point 1 (for the C-O path of CHCO), and points 2 and 3 (for the C-C paths of CH3CH2OH and CH3CHOH). For the C-O path of CHCO, as mentioned above, the obvious corrugation of the metal surface destabilizes the TS (see Figure S9a) and thus increases the energy barrier. For the C-C path of both CH3CH2OH and CH3CHOH, the steric effect in the TSs as mentioned above (see Figures S4 and S5) accounts for the relatively high energy barriers. Checking the structures involved in all reactions, we found that, unlike other steps, the TS structures corresponding to these deviated points do not resemble the FS structures, in line with the previous results that the surface reaction following the general BET principle generally possesses a “late” TS.15,17 Decomposition PES. The DFT calculations suggest ethanol interacts weakly with Pd(111), thus desorption of adsorbed ethanol is energetically preferable, which is confirmed by the experimental finding that most (60%) of the adsorbed ethanol desorbed at 200 K.10 For the adsorbed CO and CH4, because of the weak adsorptions and relatively high decomposition barriers, desorption is also preferred. For other species involved, however, decomposition is expected to be more favorable. A detailed PES for ethanol decomposition on Pd(111) is presented in Figure 6. The energy reference used here corresponds to the total energy of one gaseous ethanol molecule plus one clean Pd(111) slab. As shown in Figure 6, the overall pathway of ethanol decomposition on Pd(111) can be described as follows: ethanol adsorbes initially at the top site and dehydrogenates into CH3CHOH with a barrier of 21.3 kcal mol-1, then the O-H bond scission of CH3CHOH affords adsorbed CH3CHO with a barrier of 12.4 kcal mol-1. Adsorbed CH3CHO prefers to dehydrogenate Langmuir 2010, 26(3), 1879–1888

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Figure 6. PES of ethanol decomposition on Pd(111). All energies (kcal mol-1) are relative to the energy of the gas-phase CH3CH2OH plus the clean slab with ZPE corrections. [AþB]* denotes the coadsorbed A and B, and A*þB* represents respective adsorptions of A and B on two separated slabs. 1, CH3CH2OH*; 2, [CH3CHOHþH]*; 20 , CH3CHOH*þH*; 3, [CH3CHOþH]*þH*; 30 , CH3CHO*þ2H*; 4, [CH3COþH]*þ2H*; 40 , CH3CO*þ3H*; 5, [CH2COþH]*þ3H*; 50 , CH2CO*þ4H*; 6, [CHCOþH]*þ4H*; 60 , CHCO*þ5H*; 7, [CHþCO]*þ5H*; 70 , CH*þCO*þ5H*; 8, [CþH]*þCO*þ5H*; 9, CH2*þCO*þ4H*; 10, CH3*þCO*þ3H*; 11, CH4*þCO*þ2H*.

to CH3CO with a low activation barrier (6.8 kcal mol-1) and a large rate constant (4.20  1012 s-1), consistent with the TPD results that no desorption of CH3CHO was observed.10 CH3CO dehydrogenates to CHCO step by step, in which the most energetic demand as well as the lowest rate constant of the first step agree well with the experimental results that dehydrogenation of CH3CO is the rate-limiting step in the ethanol decomposition.12 CHCO then decomposes through C-C bond cleavage yielding CH and CO with a reasonable barrier (21.0 kcal mol-1). Finally, the CO fragment desorbs intactly as a result of the very high C-O cleavage barrier (100.3 kcal mol-1), consistent with the experimental result that 100% of the original C-O bonds are preserved in product CO;10,12 CH could either decompose to C and H or hydrogenate to adsorbed CH4. Obviously, hydrogenation of CH is more favorable because of the relatively high barrier for the alternative decomposition (13.6 vs 34.5 kcal mol-1). Adsorbed CH4 could desorb readily with a very low desorption energy (3.1 kcal mol-1), in keeping with the experimental results that methane is one of main products and only small carbon deposition is observed.10,12 The whole reaction pathway can be written as CH3CH2OH f CH3CHOH f CH3CHO f CH3CO f CH2CO f CHCO f CH þ CO f CO þ H þ CH4 þ C. Energy barriers are also calculated for different paths having close energy barriers from the same intermediate at a surface coverage of 1/3 ML, e.g., CR-H, Cβ-H, and O-H bond scission of ethanol, and C-H and C-C bond scission of acetyl, ketene, and ketenyl. The results (see Table S3) indicate that the surface coverage effect indeed does not change the decomposition network, although energy barriers are increased at the higher coverage due to the stronger steric exclusion. (49) Mavrikakis, M.; Barteau, M. A. J. Mol. Catal. A: Chem. 1998, 131, 135.

Langmuir 2010, 26(3), 1879–1888

On the basis of the previous experimental works, a decomposition pathway of ethanol on Pd(111) was proposed:49 CH3CH2OH f CH3CH2O f CH3CHO f CH3CO f CH2CO f CH2 þ CO f CO þ H þ CH4 þ C. Compared with ours, two obvious differences can be found, i.e., initial bond activation and position of the C-C bond scission. For the initial bond activation, experiments have indicated that O-H bond scission on Ni(111), Rh(111), and Cu(110) is possible because ethoxy is commonly an abundant intermediate.5,6,8 However, for Pd(111), no definite evidence about the presence of ethoxy was given by HREELS.12 The present calculations indicate that the CR-H activation of ethanol is preferable, in accordance with the fact that, in free ethanol, the CR-H bond (bond energy: 94.8 kcal mol-1)39 is weaker than both the O-H and Cβ-H bonds (bond energies: 104.6 and 100.8 kcal mol-1).50,51 A similar situation has also been found for methanol dehydrogenation on Pd(111)30 and ethanol decomposition on Pt(111).15 For ethanol decomposition on Pd(111), it was proposed that the C-C bond scission occurs at intermediate CH2CO, which was inferred from the fact that ketene desorption on Pt(111) is accompanied by the decomposition of acetyl species.12,37 As discussed above, for CH2CO, the C-H path is indeed the most favorable, and the C-C bond scission indeed occurs at CHCO on Pd(111). It should be pointed out that Wang and Liu found an unique partial oxidation path for ethanol oxidation on Pt(111) at 1/4 ML using the DFT-slab calculation, which produces adsorbed (50) Holmes, J. L.; Lossing, F. P.; Mayer, P. M. J. Am. Chem. Soc. 1991, 113, 9723. (51) Mallard, W. G., Ed. NIST Chemistry Webbook; http://webbook.nist.gov/, 1998-2004.

DOI: 10.1021/la902641t

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acetaldehyde via simultaneous loss of two hydrogens (CH3CH2OH f CH3CHO þ 2H).16 We also pursued such a path on Pd(111), but our efforts failed. Checking the structures involved, we find that on Pd(111) the stable adsorption sites for the H atoms in the FS are fcc, whereas, on Pt(111), the H atoms in the FS of the concerted path are at top sites, which reduces repulsion between the leaving H atoms and CH3CHO and favors the concerted loss of two hydrogens.

5. Conclusions This work represents the first theoretical attempt to establish a comprehensive mechanism for ethanol decomposition on Pd(111). The decomposition network has been mapped out. Both the structures and energetics involved have been obtained. The most stable adsorption of the involved species tends to follow the gas-phase bond order rules by bonding with the surface metal atom(s), wherein C and O are tetravalent and divalent with the missing H atoms replaced by the metal atoms. Desorption rather than decomposition is preferable for adsorbed ethanol, methane and CO because of the weak adsorptions or the relatively high decomposition energy barriers, whereas decomposition is expected to be more favorable for the other species involved. The decomposition reaction

1888 DOI: 10.1021/la902641t

Li et al.

proceeds through the route CH3CH2OH f CH3CHOH f CH3CHO f CH3CO f CH2CO f CHCO f CH þ CO f CO þ H þ CH4 þ C. For intermediates going along the decomposition pathway, energy barriers for the scissions of the C-C, CR-H, and O-H bonds are gradually decreased, while it is gradually increased for the C-O bond cleavage or does no obviously change for the Cβ-H bond cleavage except CH3CO. Although there are some obviously deviated points corresponding to the C-O path of CHCO and the C-C path of both CH3CH2OH and CH3CHOH, the BEP relation holds roughly for each of the C-C, C-O, and C-H paths. Acknowledgment. This work was supported by NCET-050608 and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0759) of MOE, PRC, NSFC (20476061), the State Key Basic Research Program of China (2006CB202505), and the Graduate Innovative Foundation of the China University of Petroleum (S2009-12). Supporting Information Available: Additional details as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(3), 1879–1888