Density Functional Investigation of Methanol Dehydrogenation on Pd

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Density Functional Investigation of Methanol Dehydrogenation on Pd(111) Ruibin Jiang,† Wenyue Guo,*,† Ming Li,† Dianling Fu,† and Honghong Shan*,‡ College of Physics Science and Technology, and College of Chemistry and Chemical Engineering, China UniVersity of Petroleum, Dongying, Shandong 257061, P. R. China ReceiVed: December 9, 2008; ReVised Manuscript ReceiVed: January 7, 2009

Methanol dehydrogenation to CO and H on Pd(111) is systematically investigated using self-consistent periodic density functional theory (DFT). All possible intermediates involved are calculated. Methanol and formaldehyde adsorb weakly on the Pd(111) surface because they are saturated molecules. CO and H prefer 3-fold sites with the adsorption energies of 41.6 and 64.4 kcal/mol. CH3O binds stably at 3-fold and bridge sites. Most of the other intermediates are inclined to adsorb to the surface with the sp3 configuration of the carbon atom and a hydroxyl-like configuration for O, i.e., top (η1-C) for CH2OH, bridge (η2-C) for CHOH, 3-fold (η3-C) for COH, bridge (η1-C-η1-O) for CH2O, and 3-fold (η2-C-η1-O) for CHO. All possible dehydrogenation pathways are calculated and four different routes via initial O-H and C-H bond scissions are found. The theoretical calculations indicate the initial C-H bond scission is more favorable for methanol decomposition, while O-H bond scission is preferable to C-H bond scission for CH2OH and CHOH, and the most possible dehydrogenation pathway on Pd(111) thus takes place via CH3OH f CH2OH f CH2O f CHO f CO. 1. Introduction Methanol decomposition and synthesis have attracted widespread attention because of their importance in direct methanol fuel cells (DMFC), as well as in petroleum industry. Methanol steam re-forming based on the decomposition has been suggested as an efficient way to generate hydrogen in the context of fuel-cell technology.1,2 Moreover, methanol is one of the most important synthetic chemicals. In order to fully clarify the catalysis mechanism for the decomposition and synthesis, there have appeared a large number of publications concerning reaction of methanol with various pure metal surfaces, such as Ni(100),3,4 Ni(110),5 Ni(111),6,7 Cu(100),8-11 Cu(110),12-15 Cu(111),16-19 Ag(110),20,21 Ag(111),22 Au(110),23 Pd(110),24,25 Pd(111),26-37 Pd(100),38 Pt(100),39 Pt(110),40 Pt(111),30,41-43 Rh(111),44-46 Rh(100),47 Rh(110),48 and Al(111).49,50 These investigations have demonstrated that the low-temperature surface chemistry of methanol depends on both the metal substrates and on the temperatures of adsorption. Palladium is an effective catalyst for methanol synthesis, decomposition, and steam re-forming.51-53 Christman and Demuth38 found that on Pd(100) 20% of methanol molecules in the first monolayer decompose into CH3O and most of the CH3O dehydrogenate to CO and H between 280 and 300 K based on thermal desorption spectroscopy (TDS), electron energy loss spectroscopy (EELS), ultraviolet photoemission spectroscopy (UPS), and work function measurements. The decomposition of CH3OH on Pd(110) was investigated by Sheppard et al. using EELS.25 They found that CH3OH adsorbed molecularly at 110 K and CH3O* was produced after annealing to 200 K, annealing to 300 K led to formation of surface species (HCdO)Pd, and further heating of the sample resulted in CO* formation. In addition to these works, methanol adsorption and decomposition on Pd(111) have also been experimentally extensively investigated.26-33,54 High-resolution electron energy loss spectroscopy * Corresponding authors. E-mail: [email protected] (W.E.) and [email protected] (H.S.). † College of Physics Science and Technology. ‡ College of Chemistry and Chemical Engineering.

(HREELS) showed that methanol adsorbs molecularly at 140 K, while at 300 K it decomposes into CO; methoxy was not detected.26 Temperature-programmed desorption (TPD) and EELS experiments30,31,54 indicated that methanol decomposes into CO and H2 via η1-C-η1-O formaldehyde intermediate at 170-250 K. C-O bond scission in methanol was first investigated by Levis et al. using X-ray photoelectron spectroscopy (XPS) and second ion mass spectrometry (SIMS).29 Further studies indicated that the C-O bond scission occurs at high coverages of methanol.27 Furthermore, XPS data suggested that CH3O* is a principal intermediate for the methanol decomposition,27 while SIMS test found prominent peaks associated with CH2OH+, CH3+, CHO+, etc.27,29 The adsorption and decomposition of methanol on Pd(111) were also studied by Kruse et al. using static secondary ion mass spectrometry (SSIMS), XPS, and pulsed field desorption mass spectrometry (PFDMS).33 They found the PdCH3O+ peak in SSIMS and CH2O+ and CHO+ ions in PFDMS. On the basis of the experimental findings, a pathway of CH3OH f CH3O f CH2O f CHO f CO was suggested for the methanol decomposition.33 From the theoretical point of view, the adsorption and decomposition of methanol on Ni(111) have been investigated using density functional theory (DFT) slab calculations.7 This work showed that the preferable pathway of methanol decomposition involves initial O-H bond scission, followed by sequential hydrogen abstractions to form intermediates species formaldehyde and formyl and final products CO and H. On Pt(111), all possible methanol dehydrogenation pathways have been investigated using the DFT-GGA method through the slab model.43 The initial C-H bond scission was found to be preferable, and the route CH3OH f CH2OH f CHOH f CO was suggested as the main pathway under a wide range of reaction conditions. For the adsorption and decomposition of methanol on Pd(111), Schennach et al.34 have studied the initial C-H, C-O, and O-H bond scissions by use of DFT. They concluded that cleavage of methanolic C-H bond is preferred. Zhang and Hu36 have investigated the initial O-H and C-O bond scission and found that the O-H bond scission is

10.1021/jp810811b CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009

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TABLE 1: Adsorption Sites, Adsorption Energies (in kcal/mol), and Structural Parameters (in angstroms and degrees) for Intermediates Involved in Methanol Dehydrogenation over Pd(111) species

sitesa

Eadsb

dC-O

CH3OH* CH3O*

top fcc hcp bridge top bridge bridge top fcc hcp top bridge fcc hcp hcp fcc top fcc hcp

9.1 (8.1) 39.9 (35.9) 38.3 (34.5) 38.2 (34.4) 43.8 (41.2) 12.5 (10.7) 72.0 (70.8) 63.9 (62.4) 58.5 (55.6) 57.8 (55.0) 51.2 (48.5) 50.4 (47.5) 98.2 (94.9) 98.1 (94.9) 41.6 (40.0) 41.3 (39.7) 31.6 (30.2) 64.4 (60.7) 63.9 (60.1)

1.427 1.407 1.402 1.413 1.372 1.290 1.337 1.295 1.250 1.250 1.193 1.209 1.317 1.317 1.172 1.171 1.143

CH2OH* CH2O* CHOH* CHO*

COH* CO* H*

dO-Pd

anglesc

2.509 2.187, 2.218, 2.192 2.335, 2.178, 2.177 2.151, 2.149

61 11 15 40 68 85 46 56 72 72 58 74 4 1 3 4 3

dC/H-Pd

2.052 2.179 2.041, 2.051 1.899 2.073, 2.073 2.077, 2.078 1.949 1.942 1.996, 1.981,1.981 1.988, 1.985, 1.971 2.062, 2.099,2.095 2.115, 2.116, 2.040 1.868 1.783, 1.788, 1.792 1.785, 1.790, 1.794

2.135 2.198 2.212 2.479

a Abbreviations: fcc, face-centered cubic; hcp, hexagonal close-packed. b Parameters in parentheses are adsorption energies after zero-point energy corrections. c Values are angles between the surface normal and the C-O axis in the corresponding species.

energetically more favorable. Furthermore, the adsorption and decomposition of methoxy on Pd(111) surface were investigated by Chen et al. using DFT,35,37 where the C-O bond scission and dehydrogenation were studied. In spite of these theoretical works, only the pathway associated with intermediate CH3O* via initial O-H bond scission has been studied in detail for methanol dehydrogenation on Pd(111). However, intermediate CH2OH through initial C-H bond activation has also been detected in some experiments.27,29 Both facts motivate us to study whether the C-H bond of methanol could be activated on Pd(111) and what the exact dehydrogenation mechanism is. The goal of this paper is to understand the dehydrogenation mechanism of methanol on Pd(111) by using the periodic, selfconsistent DFT. All possible reaction pathways through both the initial C-H and O-H bond scissions are considered. Since experiments have suggested that methanolic C-O cleavage occurs only at nearly one monolayer of the initial coverage of methanol27,34 and our calculation model corresponds only to onequarter monolayer of the initial coverage, the initial C-O bond scission pathway is not considered in the present paper. We address both the structures and energies of the involved intermediates and present a detailed potential energy surface (PES) for the title reaction. 2. Computational Details DFT calculations were performed with the program package DMol3 in Materials Studio of Accelrys Inc.55-57 The exchangecorrelation energy was calculated within the generalized gradient approximation (GGA) using the form of functional proposed by Perdew and Wang,58,59 usually referred to as Perdew-Wang 91 (PW91). To take the relativity effect into account, the density functional semicore pseudopotential (DSPP)60 method was employed for the Pd atoms, and the carbon, oxygen, and hydrogen atoms were treated with an all-electron basis set. The valence electron functions were expanded into a set of numerical atomic orbitals by a double-numerical basis with polarization functions (DNP). A Fermi smearing of 0.005 hartree and a realspace cutoff of 4.5 Å were used to improve the computational performance. All computations were performed with spinpolarization.

The Pd(111) surface was modeled with a three-layer slab model with four palladium atoms per layer representing a (2 × 2) surface unit cell and with a 10 Å vacuum region. The reciprocal space of the (2 × 2) unit cell was sampled with a (6 × 6 × 2) k-points grid generated automatically using the Monkhorst-Pack method.61 A single adsorbate was allowed to adsorb on one side of the (2 × 2) unit cell, which corresponds to a surface coverage of 25%. Full geometry optimization was performed for all relevant adsorbates and the uppermost two layers without symmetry restriction, while the bottom layer Pd atoms were fixed at the bulk-truncated positions at the experimentally determined lattice constant of 3.891 Å. The tolerances of energy, gradient, and displacement convergence were 1 × 10-5 hartree, 2 × 10-3 hartree/Å, and 5 × 10-3 Å, respectively. Nonperiodical structures were fully optimized at the same theoretical level for the isolated atoms, radicals, and molecules involved in the title reaction, i.e., CO, H, H2, CHO, CHOH, CH2O, CH3O. Under the current computational conditions, the bond length of the free CO molecule was calculated to be 1.131 Å, in excellent agreement with the experimental value (1.13 Å).62 Transition state (TS) searches were performed at the same theoretical level with the complete LST/QST method.63 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. The convergence criterion for the TS searches was set to 0.01 hartree/Å for the root-mean-square of atomic forces. The adsorption energies reported herein were calculated using the equation

Eads ) Eadsorbate + EM - Eadsorbate/M

(1)

where Eads is the adsorption energy of the adsorbate on metal surfaces, Eadsorbate/M is the energy of the adsorbate-M adsorption

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system, and Eadsorbate and EM are the energies of the free adsorbate and the clean slab, respectively. By this definition, stable adsorption will have a positive adsorption energy. At the present theoretical level, adsorption energy for CO on Pd(111) is calculated to be 41.6 kcal/mol, compared to the values of 42.3 kcal/mol calculated using GGA-PW91 functional and projector augmented wave (PAW) potential implemented in the VASP code,35 and 45.7 kcal/mol calculated using GGA-PW91 functional and ultrasoft plane-wave (USPP) potential in the VASP code.42 For a reaction like AB f A + B occurring on metal surfaces, the total adsorption energy of coadsorbed A and B in the 64 involved transition state (ETS A+B) can be represented as follows: TS TS EA+B ) EATS + EBTS - Eint

(2)

where EATS (ETS B ) is the adsorption energy of A (B) at the TS geometry without B (A), and ETS int is a quantitative measurement of interaction between A and B in the TS. The corresponding energy barrier was thus divided into five terms:65

Figure 1. Side and top view of the adsorption structure of methanol on Pd(111). The value (in kcal/mol) in parentheses is the adsorption energy without zero-point energy correction for the corresponding species.

(g) IS TS (g) IS TS Ea ) ∆EAB + EAB - EA+B ) ∆EAB + EAB + Eint -

EATS - EBTS

(3)

(g) IS where ∆EAB is the bond energy of the gas-phase AB and EAB is the adsorption energy of AB in the initial state (IS).

3. Results and Discussion For clarity, this section is organized as follows. First, we present structures and energies for all the adsorbed intermediates involved in the title reaction. Then, all the possible elementary reactions of the methanol dehydrogenation are presented. Last, the dehydrogenation PES is discussed. 3.1. Adsorbed Intermediates. In this subsection, we present a systematic investigation of structures and energies for all the involved intermediates. Table 1 tabulates information for all the possible adsorption modes. For direct comparison with the previous theoretical results, values of adsorption energies without zero-point energy (ZPE) corrections (-1 to -4 kcal/ mol; see Table 1) are reported in this subsection. 3.1.1. Structures and Energies of Adsorbed Intermediates. Methanol. For the gas-phase methanol, bond lengths are calculated to be 1.086 (1.09) Å for C-H, 1.411 (1.43) Å for C-O, and 0.954 (0.95) Å for O-H. The values in parentheses are experimental values;62 excellent agreements between calculation and experiment are observed. It is generally believed that methanol adsorbs via donation of the lone pair of oxygen to metallic surfaces.66-68 Our result (see Figure 1 and Table 1), showing that methanol prefers a top site of Pd(111) through oxygen with the C-O-surface angle of 39° and the O-Pd distance of 2.509 Å, supports this point, because the C-O bond must tilt an angle if methanol is bound to the surface via the oxygen lone pair orbital.69 The calculated adsorption energy is small, being only 9.1 kcal/mol (see Table 1). TPD studies by Davis and Barteau30 have indicated that methanol desorbs at 200 K over Pd(111). Assuming a preexponential factor of 1013/s and a first-order desorption kinetics, Redhead analysis estimates the adsorption energy of methanol over the Pd surface to be 11.8 kcal/mol,70 in reasonable agreement with our result. Previous DFT slab calculations also presented comparable adsorption energies for CH3OH/Pd(111) (7.4, 8.5, and 7.1 kcal/mol).34,36,42 This relatively weak adsorption is consistent

Figure 2. Side and top view of the stable adsorption structures of methoxy and hydroxymethyl on Pd(111). Parameters follow the same notation as in Figure 1.

with the large O-surface distance as well as no obvious changes in the structure of methanol upon the adsorption. Previous theoretical calculations have given almost the same adsorption geometries of methanol on Ni(111)7 and Pt(111).43 Methoxy. Methoxy prefers the 3-fold fcc site, which has been well-established by the analysis of low-temperature intermediates from methanol using low-energy electron diffraction (LEED) and infrared reflection absorption spectroscopy (IRAS).26,71 The adsorption of methoxy on Pd(111) has also been systematically investigated using the DFT method.35,42 These theoretical works indicated that the fcc adsorption energy is 38.7 kcal/ mol, which is as much as 16.7 kcal/mol higher than that of the top adsorption. In the present work, methoxy is found to be stably adsorbed at the fcc, hcp, and bridge sites; the top site is indeed unstable, which would move to the fcc configuration during optimization. The fcc and hcp adsorptions account for a nearly similar configuration, in which the O-C axis tends to orient along the surface normal with the methyl group directed away from the surface. In contrast, the bridge site favors to some extent the tilt of the C-O bond toward the surface (see Figure 2). One effect of the adsorption is a substantial stretching of the C-O bond (∼1.4 Å versus 1.34 Å) of methoxy (especially in the bridge case) due to electron donation from metallic 4d orbitals to the π*C-O orbital in CH3O. The bond energy of methoxy at the fcc site is 39.9 kcal/mol, which is at most 1.7 kcal/mol higher than those at the hcp and bridge sites. This minor variation in adsorption energies suggests coexistence of

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Figure 3. Side and top view of the most stable adsorption structures of formaldehyde and hydroxymethylene on Pd(111). Parameters follow the same notation as in Figure 1.

Figure 4. Side and top view of the most stable adsorption structures of formyl and hydroxymethylidyne on Pd(111). Parameters follow the same notation as in Figure 1.

these adsorbed states. A similar situation has also been found for methoxy on Ni(100).3 Hydroxymethyl. Hydroxymethyl has been detected in methanol decomposition using SIMS,27,29 suggesting it is really an intermediate. Similar to the analogous adsorption configuration found on Ni(111)7 and Pt(111),42,43 hydroxymethyl prefers to locate with the carbon atom atop a Pd atom with the C-Pd distance of 2.052 Å (see Figure 2 and Table 1), favoring the formation of a σ-type bond between the sp3(C) hybridized orbital and 4dz2(Pd). The angle between the O-C axis and the surface normal is 68°. Different from the above-mentioned surface methoxy, this configuration is featured by both the methyleneand hydroxyl-H close to the surface, which is expected to favor the scission of the corresponding bonds, giving different dehydrogenation products. This configuration affords an adsorption energy of 43.8 kcal/ mol (see Table 1), in good agreement with the result calculated by Desai et al. using a DFT slab model (42.8 kcal/mol),42 but obviously lower than that of Kua et al. using a DFT cluster model (53.5 kcal/mol)72 due to the fundamental differences between cluster and periodic calculations. Note that CH2OH accounts for a stronger bonding to Pd(111) than CH3O (by about 5 kcal/mol). In the gas phase, CH2OH is calculated to be energetically more stable than CH3O by 5.6 kcal/mol. Taking these two aspects into account, the adsorbed CH2OH is expected to be more stable than the adsorbed CH3O. This fact indicates that, on the Pd(111) surface, methanol should dissociate preferably into CH2OH via C-H bond scission, which will be discussed below. Formaldehyde. Formaldehyde is an important intermediate in methanol decomposition and synthesis.73,74 Adsorption of the molecule on Pd(111) has been studied using DFT,34,35,42 and theη1-C-η1-Oadsorptionmode(di-σ)isingeneralaccepted.34,35,42,66 Our calculated result concerts with the point that formaldehyde is stabilized by that mode over a bridge site (see Figure 3), with the C-Pd and O-Pd distances of 2.179 and 2.135 Å, respectively. The adsorption energy is calculated to be 12.5 kcal/ mol (see Table 1), in perfect agreement with those determined using the TPD method (∼12.0 kcal/mol)70,75 as well as in DFT slab calculations (10.3-14.5 kcal/mol).34,35,42 Hydroxymethylene. To our knowledge, no experimental information on the structures and adsorption energies can be found for CHOH on Pd(111). Present calculations suggest that CHOH interacts with the surface with the carbon atom at bridge or top site. The adsorption energies are calculated to be 72.0 (bridge) and 63.9 (top) kcal/mol; the relatively large variation in the values indicates obvious corrugation of the adsorption PES. Note that, for the bridge state, previous DFT cluster

calculation has found a relatively high binding energy (83.0 kcal/ mol),72 perhaps because of the finite size effects of the cluster used. As shown in Figure 3, the carbon atom in the bridgebound CHOH restores its tetravalency, which binds simultaneously to O, H, and two Pd atoms via sp3 hybridization, stabilizing the system largely. Note that the same site preference has also been found for CHOH on Ni(111)76 and Pt(111).43,72 Formyl. In free formyl radical, the HCO angle is calculated to be 123° and the C-O bond length is 1.177 Å. These values agree well with the experiment values of 127° and 1.17 Å.62 Formyl can stably adsorb on all possible sites of the surface, i.e., fcc, hcp, bridge, and top sites, with the adsorption energies of 58.5, 57.8, 50.4, and 51.2 kcal/mol, respectively. The most stable (fcc) adsorption favors an η2-C-η1-O configuration, where the carbon atom sits over a Pd-Pd bridge site and the oxygen atom is oriented on top of an adjacent Pd atom (see Figure 4). The C-Pd and O-Pd distances are 2.073 and 2.198 Å. The formation of bonds of C and O with the surface Pd atoms weakens the C-O bond (R ) 1.250 Å). For formyl on Pd(111), Desai et al. found an adsorption energy of 56.6 kcal/ mol for the η2-C-η1-O configuration using the DFT slab model,42 compared to our results; Lim et al. found the top site has the largest bond energy of 51.1 kcal/mol,37 where, although the bond energy agrees well with ours, the adsorption preference is different maybe due to the fact that all substrate metal atoms in their study are frozen; Kua et al. found a bond energy of 64.7 kcal/mol for the most stable (top) adsorption on a Pd8 cluster,72 higher obviously than ours because of the finite cluster effects. Hydroxymethylidyne. COH binds at fcc and hcp sites on Pd(111) with the same preference (adsorption energy, 98 kcal/ mol). In both states, C sits on a 3-fold site and O locates directly above it. The C-Pd lengths vary from 1.981 to 1.996 Å, the C-O bond is 1.317 Å long, and the C-O-H angle is 110° (see Figure 4 and Table 1). We cannot find any configurations associated with the top and bridge adsorption in spite of careful searches. Kua et al.72 found an adsorption energy of 114.0 kcal/ mol for the 3-fold adsorption of COH on Pd(111) using a DFT cluster model, higher obviously than ours due to the fundamental difference between cluster and periodic calculations. On Ni(111)76 and Pt(111),72,77 COH was also found to be adsorbed at 3-fold sites. Carbon Monoxide. Scanning tunneling microscopy (STM) indicated that CO occupies 3-fold sites on Pd(111).78 Orita et al. found that 3-fold sites are the most stable for the CO adsorption; the bond energies are 41.5 (fcc) and 41.3 (hcp) kcal/ mol.79 Comparable values [41.3 (fcc) and 41.6 (hcp) kcal/mol] are also obtained in the present work. In both 3-fold states, CO

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Figure 5. Side and top view of the most stable adsorption structures of carbon monoxide and hydrogen atom on Pd(111). Parameters follow the same notation as in Figure 1.

TABLE 2: Calculated Reaction Energies ∆H and Energy Barriers Ea (in kcal/mol) for All the Elementary Reactions Involved in Methanol Dehydrogenation on Pd(111)a reactions

∆Hb

Ea

CH3OH(g) + * f CH3OH* CH3OH* f [CH3O + H]* CH3O* f [CH2O + H]* CH2O* f [CHO + H]* CHO* f [CO + H]* CH3OH* f [CH2OH + H]* CH2OH* f [CHOH + H]* CH2OH* f [CH2O + H]* CHOH* f [COH + H]* CHOH* f [CHO + H]* COH* f [CO + H]* CO* f CO(g) + * H* f 1/2H2(g) + *

-9.2 (-8.1) 18.0 (15.0) -3.5 (-10.6) -7.5 (-10.8) -17.0 (-19.9) 3.8 (0.5) 2.4 (-1.2) 7.3 (-1.0) -4.5 (-7.8) -3.7 (-7.8) -22.2 (-25.2) 41.6 (40.1) 11.3 (10.2)

33.5 (28.5) 17.3 (12.7) 14.1 (10.4) 13.5 (9.9) 28.2 (24.3) 26.5 (21.8) 22.0 (16.2) 17.4 (13.2) 15.4 (10.7) 21.9 (17.6)

a

Values in parentheses are energies after zero-point energy corrections. b Values are obtained by subtracting the energies of the IS from those of the FS.

binds vertically to the metal surface via the carbon end (see Figure 5). In addition, a top configuration is also found, but it is about 10 kcal/mol less stable than the 3-fold states. Atomic Hydrogen. LEED study showed that on the Pd(111) surface H atoms reside in fcc sites.80 We examine the adsorption of atomic hydrogen in the 3-fold sites. For the fcc and hcp sites, the adsorption energies are calculated to be 64.4 and 63.9 kcal/ mol, respectively, compared to the experiment value of 62.0 kcal/mol.81 A rather flat PES for H on Pd(111) found by Chen et al.35 implies a significant mobility of the adsorbed H, which favors stepwise dehydrogenation of surface intermediates. 3.1.2. Summary of Adsorptions of Intermediates CHxOHy (x ) 0-2; y ) 0, 1). After getting the structures of CHxOHy (x ) 0-2; y ) 0, 1) on Pd(111), we can give a general relationship between the preferred adsorption and the bonding ability of the involved C and O atoms by summarizing the structural features. It can be found from Figures 1-5 and Table 1 that the adsorptions tend to make these species form saturated-type structures by bonding with the surface metal atom(s), i.e., C is almost tetrahedral via sp3 hybridization and O has the tendency to bond to two atoms with the missing H atoms replaced by the metal atoms. For example, CH2OH, CHOH, and COH prefer η1-C top, η2-C bridge, and η3-C fcc or hcp sites; CH2O and CHO prefer η1-C-η1-O bridge and η2-C-η1-O (fcc or hcp) hollow sites; and CO prefers η3-C (fcc or hcp) hollow sites. Note that this relation can also be applied to other group VIII metal surfaces, such as Pt(111)43 and Ni(111),76 suggesting the ability of the metals to form bonds with these adsorbates due to the d8 electronic configuration.

Figure 6. Dehydrogenation of methanol via initial O-H bond scission on Pd(111). Energies (in kcal/mol) of the initial, transition, and final states are relative to gas-phase methanol plus the clean slab (with zeropoint energy corrections). [A + H]* denotes the coadsorbed A and H species.

3.2. Reaction Pathways. In this subsection, we present all possible pathways involved in the title reaction. Generally, the dehydrogenation reaction can be classified into two modes. One starts with O-H bond scission, followed by sequential C-H bond scissions to produce CO. The other takes place via initial C-H bond scission, followed by sequential dehydrogenations to produce CO. For simplicity, the IS’s involved are chosen as the most stable adsorption configurations of the corresponding species, and the final states (FS) are taken to be coadsorptions of atomic H and corresponding product species in their most favorable sites, respectively. Reaction energies as well as energy barriers for all the elementary reactions are listed in Table 2. 3.2.1. Initial O-H Bond ActiWation. This reaction pathway involves initial O-H bond activation followed by stepwise H-abstraction to form adsorbed CO and H. Calculated structures and energies for the IS, TS, and FS involved are shown in Figure 6. O-H Bond Scission in Methanol. As shown in Figure 6a, this step involves rotation of methanol for the C-O axis directed along the surface normal, such that the hydroxyl H can be close to a surface Pd atom and forms bond with it. In the TS of this step (TS1), the O-H bond has been ruptured and the Pd-H bond has been formed as mirrored by the corresponding bond lengths, the methoxy binds atop the same Pd atom as in the IS

Methanol Dehydrogenation on Pd(111) through also the O atom with the O-C axis approximately vertical to the surface, and the atomic H stays at an adjacent bridge site. As has been discussed in section 3.1, both the bridge and top sites are unstable for the corresponding species (atomic H and methoxy); they would eventually move to their most stable sites, forming the FS. Taking into account the binding energies of methanol-Pd(111) (9.1 kcal/mol), CH3O-H (104.1 kcal/mol),82 methoxy-Pd(111) (39.9 kcal/mol), and H-Pd(111) (64.4 kcal/mol), we can estimate that methanol dissociation into the infinitely separated methoxy and H on the surface is endothermic by 8.9 kcal/mol. Considering indirect bonding competition effect between the coadsorbed methoxy and atomic H in the FS (destabilizing the system by 6.2 kcal/mol), because they bond to the same Pd atom, the process is expected to be endothermic by 15.1 kcal/ mol, consistent with our calculated results (15.0 kcal/mol). This step involves a very high-energy barrier (28.5 kcal/mol), which is indeed 20.4 kcal/mol higher in energy than the gas-phase methanol plus the clean slab. The latter value is higher than the result (13.1 kcal/mol) calculated previously for the same reaction at the initial coverage of one-sixth using a four-layer slab model.34 We also calculate the process by considering the same IS in conjunction with other possible FS’s, i.e., coadsorbed Hfcc + CH3Obridge and Hfcc + CH3Ohcp; no obvious decreases in energy barriers are found. Stepwise C-H ActiVation from Methoxy. The dehydrogenation of methoxyl on Pd(111) has been studied by Chen and coworkers using DFT.35,37 In order to get a complete picture for the methanol dehydrogenation, we also perform a study of this pathway. Methyl H abstraction from the adsorbed methoxy could afford coadsorbed formaldehyde and H. As shown in Figure 6b, this process involves an intrarotation of the C-O bond from the surface normal to a nearly parallel position, such that the shifted hydrogen atom could be close to a surface Pd atom and form a covalent bond with it. The involved TS2 is structurally similar to the analogous TS found on Ni(111),7 i.e., formaldehyde-like, in which the C-O axis tilts to the surface by about 28°, and the respective distances of H* to C and Pd are 1.703 and 1.576 Å, indicating rupture of the C-H bond and formation of the Pd-H bond. Subsequently, arrangement of the surface species to their most stable sites accounts for the FS. This step affords an energy barrier of 12.7 kcal/mol, higher obviously than the value of 7.9 kcal/mol calculated by Chen et al.35 The difference may be due to the fact that all substrate metal atoms were frozen, and the interaction between atomic cores and electrons was described by the PAW method in ref 35. It should be pointed out that choosing the bridge-bound methoxy as IS does not decrease the energy barrier. C-H bond scission of formaldehyde would give formyl and H with an energy loss of 10.8 kcal/mol (see Figure 6c). This process involves a C-H bond activation by the Pd atom anchored by the methylene C in the IS, such that in the TS the involved C-H bond is elongated to 1.579 Å and both the atomic H and the C atom of the formyl bond to the same Pd atom. The calculated energy barrier of this step is 10.4 kcal/mol, compared to a value of 5.4 kcal/mol in the previous calculation.37 This discrepancy may be caused by the same reasons as for the methoxy dehydrogenation as mentioned above. In the final step (Figure 6d), a swag vibration in the adsorbed CHO makes O apart from the surface and the H atom bond to a surface atom, such that, in TS4, CO sits still at a bridge site with the molecular axis tilting from the surface normal by 31° and the departing H sits atop a Pd atom (RPdH ) 1.610 Å). This

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Figure 7. Dehydrogenation of methanol via initial C-H bond scission on Pd(111). Parameters follow the same notations as in Figure 6.

reaction is strongly exothermic (by 19.9 kcal/mol). It should be pointed out that, although indirect bonding competition effect in the relevant TS is expected to be large (because the two entities interact with the same Pd atom), and the corresponding energy barrier is relatively low (9.9 kcal/mol). Note that the calculated energy barrier agrees well with that from the previous calculations (9.8 kcal/mol).37 3.2.2. Initial C-H Bond ActiWation. This reaction pathway involves initial C-H bond activation followed by stepwise H-abstraction to form adsorbed CO and H. Calculated structures and energies for the IS, TS, and FS involved are shown in Figure 7. C-H Bond Scission in Methanol. Some experiments have found hydroxymethyl species in methanol decomposition on

4194 J. Phys. Chem. C, Vol. 113, No. 10, 2009 Pd(111),27,29 demonstrating that initial C-H bond scission is indeed possible. This possibility involves a rotation of the adsorbed methanol, such that the methyl end moves toward the surface, leading to insertion of a Pd atom into a C-H bond (see Figure 7a). In TS1′, both the carbon and leaving H atoms are bonding at the same Pd atom with the C-H* and C-Pd distances of 1.534 and 2.326 Å, respectively; the CH2OH entity is almost at its stable site. After the TS1′, the atomic H moves to a nearly fcc site because of strong repulsion of CH2OH, and the C-Pd distance is further shortened (2.079 Å). In free methanol, the C-H bond (bond energy, 94.6 kcal/ mol)83 is weaker than the O-H bond. As mentioned above, CH2OH interacts with Pd(111) more strongly than CH3O. These two aspects indicate that initial C-H bond scission accounting for CH2OH should be more favorable for the methanol dehydrogenation. Indeed, the energies of the TS and FS for the C-H bond scission are calculated to be lower than those for the alternative initial O-H bond scission (by 4.2 and 14.5 kcal/ mol). A similar trend has also been found in the previous theoretical works.34,84 Dehydrogenation of CH2OH. We can imagine CH2OH would be followed by C-H and O-H bond scissions, giving CHOH and CH2O, respectively. To our knowledge, there are no experimental and computational studies concerning the CH2OH dehydrogenation on Pd(111). Figure 7b gives information for the C-H bond scission. This step involves simultaneous movements of CHOH and H toward adjacent fcc sites, giving the nearly thermoneutral FS. The involved TS2′ is featured by both the leaving H* and C atoms coordinating with the same Pd atom with an elongated C-H* distance (1.516 Å) and shortened C-Pd (1.978 Å) and H*-Pd (1.689 Å) bonds; indirect bonding competition effect accounts for a rather high energy barrier (21.8 kcal/mol). Alternatively, O-H bond scission of CH2OH would result in intermediate formaldehyde. In the course of the reaction, the hydroxyl H transfers from CH2OH to surface Pd atom due to O-H stretch vibration. As shown in Figure 7c, TSa for this process has a geometry in which the bond between the atomic H* and O is almost broken; the atomic H and CH2O (with the C-O bond nearly parallel to the surface) bind at adjacent bridge sites. The calculated energy barrier for this step (16.2 kcal/mol) suggests that CH2OH dehydrogenate preferably to formaldehyde rather than CHOH. This situation is different from that on Pt(111), in which C-H bond scission of CH2OH is more favorable.42 Following TSa, movements of the two new entities to adjacent hcp sites would account for the stable FS, which is almost isoenergetic with the IS. Dehydrogenation of CHOH. CHOH has also two possible dehydrogenation channels (producing COH or CHO). For the COH-forming channel, methylidyne H abstraction is facilitated by the movement of CHOH from the bridge site to adjacent hcp site (see Figure 7d). In TS3′, COH sits nearly at the hcp site through the C end with the C-O axis pointed approximately along the surface normal (68°), while the atomic H sits atop one of the three Pd atoms bound to COH. The corresponding energy barrier is calculated to be 13.2 kcal/mol. Passing through the TS, COH* would rotate to its favorable configuration, while the atomic H moves to the neighbor fcc site. This step is exothermic by 7.8 kcal/mol. The IS, TS, and FS for forming CHO from CHOH are shown in Figure 7e. This reaction is excited by abstraction of the hydroxyl H with the help of O-H bond stretch vibration. The involved TSb is product-like, in which the new CHO is still at the bridge site and the atomic H at top site. TSb is followed by

Jiang et al. TABLE 3: Energy Barriers and Contribution Factors (in kcal/mol) for All the Elementary Reactions Involved in Methanol Dehydrogenation on Pd(111)a reactions

(g) ∆EAH

CH3OH* f CH3O* + H* 107.8 CH3OH* f CH2OH* + H* 101.7 CH2OH* f CH2O* + H* 36.2 CH2OH* f CHOH* + H* 92.7 CHOH* f CHO* + H* 37.8 CHOH* f COH* + H* 82.7 CH3O* f CH2O* + H* 30.1 CH2O* f CHO* + H* 94.3 CHO* f CO* + H* 28.7 COH* f CO* + H* -16.2

IS EAH

TS Eint

9.1 3.3 9.1 5.2b 43.8 3.3 43.8 8.3b 72.0 6.2 72.0 8.4b 39.9 3.5 12.5 6.9b 58.5 10.6b 98.2 14.0b

EATS

EHTS

Ea

28.8 35.0 5.9 61.7 48.4 92.0 1.0 44.5 32.0 27.8

56.0 53.1 55.5 55.5 52.5 52.4 53.5 53.7 52.0 45.6

33.5 28.2 22.0 26.5 15.4 17.4 17.3 14.1 13.5 21.9

a Without zero-point energy corrections. b Indirect bonding competition effect is involved in the corresponding TS.

movement of the atomic H to its stable fcc site, forming the FS, which is 7.8 kcal/mol more stable than its IS. Compared to the alternative C-H bond scission in the CHOH dehydrogenation, the O-H bond scission is energetically more favorable, as mirrored by the values of the involved energy barriers (see Table 2). H Abstraction from COH. For the surface COH, the incline of the C-O bond would bring the hydrogen atom close to a surface atom and forms a bond with it (see Figure 7f). In TS4′, CO still sits at the 3-fold site with the C-O bond tilting at 27° from the surface normal; the abstracted H* sits atop one of the Pd atoms bound also to CO. Due to the strong indirect bonding competition effect and the large binding energy of CO-H, this process involves a relatively high energy barrier (17.6 kcal/ mol). After the TS, the C-O bond gradually rotates to the surface normal and the atomic H moves to the adjacent hcp site, forming the strongly exothermic FS (by 25.2 kcal/mol). 3.2.3. Determining Factors for the Dehydrogenation Energy Barriers. As shown in eq 3, the energy barrier of reaction AH f A + H on a metal surface can be divided into five terms: the gas-phase A-H bond energy (∆E(g) AH), the adsorption energy of AH in the relevant IS (EIS AH), the repulsive interaction between TS ), the adsorption energy of A at the TS A and H in the TS (Eint geometry without H (EATS), and the adsorption energy of H at the TS geometry without A (ETS H ). The first three terms contribute positively to the energy barrier, while the situation is the reverse for the last two factors. In order to understand the determining factors for the energy barriers, all these terms are calculated for the elementary steps involved in the methanol dehydrogenation on Pd(111). The resulting parameters are given in Table 3. First of all, we can find that the adsorption energy of atomic H in the TS’s (EHTS) does not change obviously across all the steps except for COH dehydrogenation, which has a somewhat smaller value. This fact suggests that this term has less effect on the energy barriers, and the dehydrogenation barriers are indeed determined by the other four factors. Also, dehydrogenations via C-H and O-H bond scission from CH3OH*, IS because they CH2OH*, or CHOH** have a same value of EAH involve the same IS. Furthermore, as expected, we can find that TS ) is indeed large in the TS’s with the repulsive interaction (Eint one metal atom shared by two separated adsorbates. Compared to the initial O-H bond scission, although binding of the two separated species to the same metal atom in the alternative initial C-H scission TS (TS1′; see Figure 7a) results in a larger indirect bonding competition effect, the weaker C-H bond of methanol and the stronger adsorption of CH2OH at the TS geometry (EATS) explain the lower energy barrier and thus

Methanol Dehydrogenation on Pd(111)

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Figure 8. Potential energy surface of methanol dehydrogenation on Pd(111). Energies (in kcal/mol) are relative to gas-phase methanol plus the clean slab with zero-point energy corrections: 1, CH3OH*; 2, [CH3O + H]*; 2′, CH3O* + H*; 3, [CH2OH + H]*; 3′, CH2OH* + H*; 4, [CH2O + H]* + H*; 4′, CH2O* + 2H*; 5, [CHOH + H]* + H*; 5′, CHOH* + 2H*; 6, [CHO + H]* + 2H*; 6′, CHO* + 3H*; 7, [COH + H]* + 2H*; 7′, COH* + 3H*; 8, [CO + H]* + 3H*; 8′, CO* + 4H*. A* + nH* represents respective adsorptions of A and n H atoms on (n + 1) separated slabs.

the preference for the process. For the C-H and O-H bond scission from CH2OH or CHOH, the contributions of the A-H bond energies and the adsorption energies of A at the TS geometry (EATS) almost counteract each other, the difference in the energy barriers indeed being determined by indirect bonding TS ). because the O-H bond scission competition effects (Eint process always involves a TS with relatively weaker indirect bonding competition effect, the energy barrier involved is lower than that for the alternative C-H bond scission. The weak H-CH2O bond as well as the rather weak adsorption of CH2O in the involved TS account for the intermediate value of energy barrier for the CH3O dehydrogenation. The relatively low energy barrier for CH2O dehydrogenation is mainly due to the weak adsorption of formaldehyde. In the cases of CHO and COH dehydrogenation, although the CO-H bond is obviously weaker than the H-CO bond, the differences in the other four factors account for the relatively high energy barrier for the COH dehydrogenation. 3.3. Dehydrogenation PES. A detailed PES for methanol dehydrogenation on Pd(111) is presented in Figure 8. The energy reference used in the figure corresponds to the total energy of one gaseous molecule of CH3OH and the clean slab. The much weaker interaction of methanol with the surface suggests that desorption rather than dehydrogenation would be preferable for the adsorbed methanol. For the adsorbed formaldehyde, the comparable values of the dehydrogenation energy barrier and adsorption energy indicate comparable possibilities for the two processes, agreeing with the experimental finding that desorption of formaldehyde was observed in the formaldehyde decomposition on clean Pd(111) surface.75 Furthermore, dehydrogenation is expected to be more favorable for the other species due to the strong adsorptions. As shown in Figure 8, along the whole dehydrogenation pathway, the first step of the methanol dehydrogenation, i.e., initial C-H or O-H bond scission, is the most energetically demanding, which makes the C-H bond scission energetically more favorable, concerting with the previous theoretical34,43,84 and experimental results.27,29 However, experiments also showed that methoxy is an abundant intermediate, which indicates the possibility of the initial O-H bond rupture.27,30,32 Several factors could be responsible for this phenomenon. First, the surface coverage of methanol is relatively high in the experiments, thus hydrogen bonds could be formed between the adsorbed molecules, weakening the O-H bond. Then, the residual surface

oxygen atoms, which are unavoidable in experiments, could lower the energy barrier of the O-H bond scission but have less influence on the C-H bond scission.11 Last, some other factors, such as defects, steps, and edges, may also change the dehydrogenation PES. Because there are four steps of the dehydrogenation and the O-H bond scission could take place in any one of them, the methanol dehydrogenation may involve four different pathways. Along the initial C-H bond scission pathways, O-H bond scission is preferred to C-H bond scission in both CH2OH and CHOH, as mirrored by the corresponding energy barriers, suggesting O-H bond scission in the second step (CH3OH f CH2OH f CH2O f CHO f CO) is indeed the main reaction channel, in agreement with the experimental facts that methanol dehydrogenation on Pd(111) always accounts for intermediates CH2O and CHO but does not afford intermediates CHOH and COH.25,27,30,31 ThedehydrogenationofCH3OH/Ni(111)7 andCH3OH/Pt(111)43,77 has been theoretically investigated, and several points can be drawn by comparing these two systems with CH3OH/Pd(111). First, initial C-H bond scission is more facile on Pd(111) and Pt(111),43,77 while on Ni(111) the more favorable process is initial O-H bond scission due to the stronger adsorption of methoxy than hydroxymethyl.7 Then, along the initial C-H scission pathway, dehydrogenation of CH2OH to CH2O via O-H bond rupture is more favorable on Pd(111), while on Pt(111) C-H bond scission accounting for CHOH is preferable. Last, on Pt(111) and Pd(111), methanol dehydrogenation involves different pathways, i.e., CH3OH f CH2OH f CHOH f CO43 and CH3OH f CH2OH f CH2O f CHO f CO, respectively. 4. Conclusions Methanol dehydrogenation to carbon monoxide and hydrogen on Pd(111) has been investigated by DFT. We can now conclude by summarizing a number of the main points below. (1) The saturated molecules of methanol and formaldehyde adsorb weakly over the Pd(111) surface with the adsorption energies of 9.1 and 12.5 kcal/mol, respectively. CH3O binds stably at 3-fold and bridge sites. Most intermediates involved [CHxOHy (x ) 0-2; y ) 0, 1)] prefer to adsorb to the surface with a saturated sp3 configuration of carbon atom and a hydroxyl-like configuration for O; i.e., CH2OH, CHOH, and

4196 J. Phys. Chem. C, Vol. 113, No. 10, 2009 COH prefer η1-C top, η2-C bridge, and η3-C fcc or hcp sites; CH2O and CHO prefer η1-C-η1-O bridge and η2-C-η1-O hollow (fcc or hcp) sites; and CO prefers η3-C hollow (fcc or hcp) sites. The adsorption energies of the most stable adsorptions are calculated to be 39.9 kcal/mol for methoxy, 43.8 kcal/mol for hydroxymethyl, 72.0 kcal/mol for hydroxymethylene, 58.5 kcal/mol for formyl, 98.2 kcal/mol for hydroxymethylidyne, 41.6 kcal/mol for carbon monoxide, and 64.4 kcal/mol for atomic hydrogen. (2) Desorption rather than dehydrogenation is preferable for the adsorbed methanol due to the weak adsorption. For the adsorbed formaldehyde, because the dehydrogenation energy barrier is comparable with the adsorption energy, these two processes are expected to have a comparable possibility. For the other species, however, dehydrogenation is expected to be more favorable due to the strong adsorptions. (3) For the methanol dehydrogenation on Pd(111), the preference of initial C-H bond scission (to the alternative O-H bond scission) is mainly due to the C-H and O-H bond strengthens in the gas-phase methanol as well as the adsorption energies of CH2OH and CH3O in the involved TS’s. For the CH2OH or CHOH dehydrogenation, the O-H bond scission process always involves a TS with relatively weak indirect bonding competition effect; thus, it has the lower energy barrier than the alternative C-H bond scission. The intermediate value of energy barrier for the CH3O dehydrogenation is caused by the weak H-CH2O bond as well as the rather weak adsorption of CH2O in the TS. The energy barrier for CH2O dehydrogenation is relatively low due mainly to the weak adsorption of formaldehyde. In the cases of CHO and COH dehydrogenation, although the CO-H bond is obviously weaker than the H-CO bond, the adsorption energies of the reactant species, the adsorption energies of the coadsorbed species CO and H in the relevant TS’s, and the indirect bonding competition effects account for the relatively high energy barrier for the COH dehydrogenation. (4) There are four steps of the methanol dehydrogenation on Pd(111), which may involve four different pathways because the O-H bond scission could take place in any one of these steps. In all cases, the first step demands the most energy. Because the energetically more favorable pathway is the C-H bond scission in the adsorbed methanol, while for the adsorbed CH2OH and CHOH it involves the O-H bond scission, the most likely reaction channel of methanol dehydrogenation on Pd(111) is thus CH3OH f CH2OH f CH2O f CHO f CO. Acknowledgment. This work was supported by NCET-050608, and Program for Changjiang Scholars and Innovative Research Team in University (IRT0759) of MOE, PRC, NSFC (20476061), and State Key Basic Research Program of China (2006CB202505). References and Notes (1) Peppley, B.; Amphlett, J.; Kearns, L.; Mann, R. Appl. Catal., A 1999, 179, 21. (2) Peppley, B.; Amphlett, J.; Kearns, L.; Mann, R. Appl. Catal., A 1999, 179, 31. (3) Huberty, J. S.; Madix, R. J. Surf. Sci. 1996, 360, 144. (4) Zhou, Y. H.; Lv, P. H.; Wang, G. C. J. Mol. Catal. A: Chem. 2006, 258, 203. (5) Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sci. 1985, 150, 399. (6) Zenobi, R.; Xu, J.Jr.; Persson, B. N. J.; Volotkin, A. I. Chem. Phys. Lett. 1993, 208, 414. (7) Wang, G. C.; Zhou, Y. H.; Morikawa, Y.; Nakamura, J.; Cai, Z. S.; Zhao, X. Z. J. Phys. Chem. B 2005, 109, 12431. (8) Karolewski, M. A.; Cavell, R. G. Surf. Sci. 1995, 344, 74. (9) Ellis, T. H.; Wang, H. Langmuir 1994, 10, 4083.

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