A theoretical study on the complete dehydrogenation of methanol on Pd (100) surface

Applied Surface Science 364 (2016) 613–619

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

A theoretical study on the complete dehydrogenation of methanol on Pd (100) surface Zhao Jiang, Bin Wang, Tao Fang ∗ School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, PR China

a r t i c l e

i n f o

Article history: Received 10 December 2015 Received in revised form 23 December 2015 Accepted 24 December 2015 Available online 29 December 2015 Keywords: Density functional theory Pd (100) surface Methanol Adsorption Decomposition

a b s t r a c t Density functional theory (DFT) method was employed to investigate the adsorption and decomposition mechanisms of CH3 OH on Pd (100) surface. Different kinds of possible adsorption modes of relevant intermediates on the surface were identified. It was found that CH3 OH and CH2 OH prefers to adsorb on the top site, CH3 O, CHOH and CO occupy preferentially on the bridge site, while CH2 O, CHO, COH and H species adsorb on the hollow site. The adsorption energies of all species exhibit the following trend: CH3 OH < CH2 O < CH3 O < CO < CH2 OH < H < CHO < CHOH < COH. Subsequently, four possible dissociation pathways of CH3 OH via initial O H and C H bond scissions were proposed and studied systematically. The transition states, energy barriers and reaction energies were calculated to explore the dehydrogenation mechanisms of CH3 OH on Pd (100) surface. It was indicated that the scission of C H bond is more favorable for CH3 OH and CH2 OH and the H O bond cleavage is easier for CHOH. The path 2 (CH3 OH CH2 OH CHOH CHO CO) is the most possible dehydrogenation pathway, where the highest energy barrier of CH3 OH dissociation makes it to be the rate-determining step of the whole dehydrogenation reaction. © 2015 Elsevier B.V. All rights reserved.

1. Introduction A fundamental research of the adsorption and decomposition of methanol with metal surface is considerable primarily for a couple of reasons. Firstly, hydrogen is considered to be one of the most promising energy carriers that have attracted a lot of interest owing to its high power density and environmental friendly nature [1,2]. As a hydrogen-rich compound, methanol has been regarded as a potential and convenient hydrogen carrier in fuel cell applications due to the abundant source as well as the high hydrogen-to-carbon ratio [3,4]. Hydrogen can be produced directly from methanol through the following different ways: methanol decomposition [5] (CH3 OH = CO + 2H2 ), methanol oxidation [6] (CH3 OH + 1/2O2 = CO2 + 2H2 ) and methanol steam reforming [7] (CH3 OH + H2 O = CO2 + 3H2 ). Additionally, methanol is one of the most important synthetic chemicals and can provide more chemical information for more complex carbohydrates. Therefore, understanding of the interaction between methanol and metal surfaces has attracted widespread attention. Experimentally, it was indicated that methanol was adsorbed on the Pd (111) surface molecularly and the main

∗ Corresponding author. E-mail address: [email protected] (T. Fang). http://dx.doi.org/10.1016/j.apsusc.2015.12.204 0169-4332/© 2015 Elsevier B.V. All rights reserved.

decomposition pathway includes stepwise dehydrogenation to CO through initial C H and O H bond scission [8]. By vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS), Morkel et al. [9] studied the decomposition on Pd (111) surface at 300 and 400 K in situ from 5 × 10−7 to 0.1 mbar. Quantification of carbon-containing species was performed by XPS, while the preferred binding site of CHx was determined by SFG using CO as probe molecule for postreaction adsorption. It was found that the CHx formation must be suppressed or CHx must be selectively removed/oxidized without total oxidation of methanol to produce CO and H2 from CH3 OH on Pd catalysts. Using high-resolution electron energy loss spectroscopy (HREELS), the CH2 O species had been observed around 170 K, which finally lead to adsorbed CO and H on Pd (111) surface at 300 K [10]. Also, HREELS study had observed HCO at 110 K during CH3 OH decomposition on Pd (110) surface [11]. Christmann and Demuth [12] investigated the dissociation of CH3 OH on Pd (100) surface and found that most of methanol adsorbed molecularly at 77 K with relatively little dissociative adsorption as methoxy. Recently, theoretical computation methods have been a powerful research tool for understanding the chemical reactions at the molecular level. Especially, periodic DFT calculations using the slab model have become a powerful approach to study the adsorption of atoms and small molecules on the metal surfaces. Many theoretical calculations have been employed to ascertain the interaction

614

Z. Jiang et al. / Applied Surface Science 364 (2016) 613–619

of CH3 OH with the surfaces of pure metals, such as Ni [13–15], Rh [16], Pd [17–19], Pt [20–23], Au [24,25], Ru [26,27], Cu [28–35], Ag [36]. Due to its high hydrogen permeability and selectivity, palladium is an effective industrial catalyst that is employed in numerous commercial processes. Pd has been used as the basis of highly selective hydrogen separation membranes and membrane reactors. Yang et al. [17] studied the decomposition pathways of methanol on Pd (111) surface in both neutral and alkaline medium by DFT. It was found that the energy barrier of H O bond scission is greatly decreased with the assistance of the hydroxyl adsorbed on the surface. Jiang et al. [18,19] investigated the dissociation of methanol to CO and H on Pd surface using self-consistent period DFT. It was found that the most possible dehydrogenation pathway on Pd (111) surface is CH3 OH CH2 OH CH2 O CHO CO. These studies were based on density functional theory (DFT) calculations. However, the key factors which determine whether methanol will dissociate, what the final products of methanol dissociation on different surfaces, and the pathways of producing hydrogen are the controversial thesis. In this work, density functional theory (DFT) method with period slab model are employed to investigate the energy barriers and thermochemistry of elementary reactions involved in the methanol dehydrogenation on Pd (100) surface. Besides, four reaction pathways (the scission of H O and C H bonds) and transition states are considered. This work will provide a better understanding the reaction network of methanol dehydrogenation and is significant to predict for the selective modification of Pd-based catalysts. In the following parts, our computational details are described in Section 2. Then, the most stable adsorption geometry structures of relevant species on Pd (100) surface are presented. Finally, the different reaction pathways, the transition states and the dehydrogenation mechanisms of methanol are discussed in Section 3 and the conclusions are summarized in Section 4.

The transition states (TS) were searched by means of complete LST/QST method for four elementary reactions [46], starting from reactants and products. The LST (Linear Synchronous Transit) method performed a single interpolation to a maximum energy, and the QST (Quadratic Synchronous Transit) method searched for an energy maximum with constrained minimizations in order to refine the transition state to a high degree. In addition, for validating the transition states involved in the reaction, Dmol3 program was employed to calculate the frequency of transition state since the CASTEP program cannot generate the frequency information. Additionally, it was confirmed that every transition state could lead to the desired reactants and products [40–44]. The adsorption energy for atoms or molecules on Pd (100) was calculated as follows: Eads = Emolecule + EPd(100) − Emolecule/Pd(100)

(1)

where Emolecule/Pd (100) is the total energy of the Pd (100) surface together with the adsorbed molecule, Emolecule is the total energy of the free molecule, and EPd (100) is the total energy of the clean Pd (100) surface. For a reaction such as AB → A + B on Pd (100) surface, the reaction energy (H) and activation barrier (Eb ) are calculated on the basis of the following formulas: H = E(A+B)/Pd(100) − EAB/Pd(100)

(2)

Eb = ETS/Pd(100) − EAB/Pd(100)

(3)

where EAB/Pd (100) is the total energy of the adsorbed AB, E(A + B)/Pd (100) is the total energy of the co-adsorbed A/B on Pd (100) surface, and the ETS/Pd (100) is the total energy of transition state on Pd (100) surface. In all calculations, Pd atoms of the top three layers and the adsorbed species were allowed to relax, while the other layers were fixed. 3. Results and discussion

2. Computational details The DFT calculations were conducted using the program package Cambridge Sequential Total Energy Package (CASTEP) with a plane wave basis set in the Materials Studio software (Accelrys, Inc.) [37,38]. In this research, the Perdew-Wang-91 (PW91) functional with the generalized gradient approximation (GGA) was applied to calculate the exchange–correlation energy [39]. A plane wave cut-off energy of 400 eV was used in the calculation of the compact convergence. Brillouin zone sampling was calculated using a Monkhorst-Pack grid with respect to the symmetry of the system and the electronic occupancies were determined according to a Methfessel-Paxton scheme with an energy smearing of 0.1 eV. The meshes were set to k-points 4 × 4 × 1 for the Pd (100) surfaces. Because of the nonmagnetic properties of Pd, spin polarization on Pd (100) was not considered. The convergence criteria for configuration optimization was set to the tolerance for SCF, energy, maximum force, with a maximum displacement of ˚ 2.0 × 10−6 eV/atom, 2.0 × 10−5 eV/atom, 0.05 eV/A˚ and 2.0 × 10−3 A, respectively. Based on the optimized Pd bulk structure, the four-layered Pd (100) surface model was built, and a p (3 × 3) super-cell was employed (36 Pd atoms in a cell), with a coverage of adsorbates of 1/9 ML. This approach has been widely used in previous theoretical studies dealing with molecular interactions with metal surfaces [40–45]. Simultaneously, three adsorption sites were calculated for the Pd (100) surface: top, bridge and hollow site. The Pd Pd ˚ The interatomic equilibrium distance was calculated to be 3.954 A. vacuum space of 15 A˚ was added perpendicular to the surface, in order to avoid the interactions between periodic configurations.

In this section, the adsorption geometries and energies for all species involved in the dehydrogenation reaction were investigated. Then, the possible reaction pathways of methanol decomposition on Pd (100) surface were considered and the related transition states were determined. 3.1. Structures and energetics of adsorbed intermediates 3.1.1. Calculation of CH3 OH and bulk Pd The calculated results can be applied to verify the credibility of the selected calculation methods. Firstly, the calculated ˚ bond lengths for CH3 OH in gas phase are r(C H) = 1.098 A, ˚ which are in fairly consistent r(C O) = 1.435 A˚ and r(O H) = 0.973 A, ˚ 1.43 A˚ and 0.95 A˚ with the reported experimental values of 1.09 A, [47], respectively. Our next validation test is to predict the lattice constant of bulk Pd. The calculated value for the lattice constant is ˚ being close to the experimental value of 3.89 A˚ [47]. 3.954 A, 3.1.2. Adsorption geometries of related species on Pd (100) surface For each adsorbate, three typical adsorption sites, including top, bridge and hollow sites, were considered. The adsorption energies were calculated according to formula (1) and the results are listed in Table 1, only the most stable adsorption configurations are summarized. Also, the corresponding key geometric parameters for all the species at their favoring positions are listed in Table 1. The most stable structures of all intermediates adsorbed on Pd (100) surface were shown in Fig. 1. For CH3 OH, similar to the situation of Pd (111) and (110) surfaces [18], it prefers to adsorb on the top site through the oxygen

Z. Jiang et al. / Applied Surface Science 364 (2016) 613–619 Table 1 Stable adsorption sites, adsorption energies and structural parameters for related species on Pd (100) surface. Species

Adsorption sites

Configurations

Bond ˚ length (A)

Eads (eV)

CH3 OH

Top

O-bound

0.42

CH3 O

Bridge

O-bound

CH2 OH

Top

C-bound

CH2 O

Hollow

O-bound and C-bound

CHOH

Bridge

C-bound

CHO

Hollow

O-bound and C-bound

COH

Hollow

C-bound

CO

Bridge

C-bound

H

Hollow

H-bound

2.29 (O Pd) 2.08 (O Pd) 2.06 (C Pd) 2.24 (C Pd) 2.15 (O Pd) 2.04 (C Pd) 2.05 (C Pd) 2.24 (O Pd) 2.10 (C Pd) 2.01 (C Pd) 1.94 (H Pd)

1.96 2.13 0.90

3.58 3.23

5.31 2.09 2.79

Fig. 1. The most stable configurations of dissociated intermediates on Pd (100) surface.

atom. Compared with previous reports, it is generally believed that methanol adsorbs on the metallic surface via donation of the lone pair of oxygen. The result of C O Pd bond angle of 21.81◦ supports this conjecture, due to that the C O bond will tilt an angle if methanol is bound to the surface via the oxygen lone pair. Any attempt to find a minimum of energy in the other symmetric sites leads to the top site after complete optimization. In the most stable configuration, the distance between oxygen atom and the near˚ The bond lengths for C O, C H and H O est Pd atom is 2.29 A. ˚ which are well in agreeare calculated to be 1.46, 1.10 and 0.97 A, ment with previous experimental and calculation values [19]. The adsorption energy is 0.42 eV, which is close to that on Pd (111) [18], Ni (100) [15], Au (100) [25]. For CH3 O, previous experiments and calculations revealed that methoxy is upright adsorbed via the oxygen atom on Pd (111) surface [18]. To obtain the most stable adsorption configuration,

615

the optimization on top, bridge and hollow sites were considered. It was found that methoxy prefers to adsorb on the bridge site through oxygen atom with the highest adsorption energy of 1.87 eV, slightly higher than that on Pd (111) surface. In the optimized configuration, the C O bond axis is almost perpendicular to the Pd (100) surface, the CH3 O species maintains its local C3v symmetry and the three hydrogen atoms keep in a plane paralleling to the metal surface. The calculated bond length of C O and O Pd are ˚ which are in good agreement with the results of 1.42 A˚ and 2.08 A, previous work on Pd (111) surface and experiments [18]. For CH2 OH, interestingly, it was found that CH2 OH prefers to bind at the top site on Pd (100) surface via the carbon atom, which is consistent with the previous studies on other metals [13,25]. The adsorption energy is 2.48 eV, slightly higher than that on Pd (111) surface (1.90 eV). In the most stable configuration, the bond lengths ˚ of C O and C Pd are 1.38 and 2.06 A. For CH2 O, it is an important intermediate for the synthesis and decomposition of methanol, and can be adsorbed on metal surfaces through two ways: (1) the oxygen lone pair electrons in an upright structure; (2) both the carbon and oxygen atoms interact with the metal surface atoms. Davis and Barteau [48] examined the adsorption of CH2 O on group VIII metal surfaces. It was found that the preferred mode is the latter and that formaldehyde binds to the surface through the carbon ␲ orbital and simultaneously through overlap between the metal d state and the carbonyl ␲* orbital. Through optimization, the stable configurations on the bridge and hollow sites were obtained. The higher adsorption energy of 0.90 eV on the hollow site indicates that this configuration can be considered as the most stable structure. CH2 O adsorbs on Pd (100) surface with the ␩1-C ␩1-O mode, where the distances of C Pd, O Pd, and ˚ respectively. The calculated C O bonds are 2.24, 2.15 and 1.34 A, results are accordance with the previous reports [48]. For CHOH, the calculated results suggest that CHOH group prefers to adsorb on the bridge site with the adsorption energy of 3.58 eV, which is larger than that on Pd (111) surface by Jiang et al. [18]. The reason may be that the lower coordination number of metal atoms on Pd (100) surface leads to a stronger back-bonding as compared with Pd (111) surface. On the bridge site, the C atom in the CHOH species binds to Pd (100) surface with the C Pd bond ˚ length of 2.04 A. For CHO, only the hollow configuration was obtained with the adsorption energy of 3.23 eV. The H C O bond angle in adsorbed state is calculated to be 114.4◦ , reduced from the value of 121.9◦ in gas phase. The bond lengths of C Pd, O Pd and C O are 2.05, ˚ suggesting that the formation of C O bond on the 2.04 and 1.30 A, ˚ and strengths the Pd (100) surface weakens the C H bond (1.13 A) ˚ C O bond (1.30 A). For COH, it is preferentially adsorbed on the hollow site on Pd (100) surface, which is similar to the report on Pd (111) surface [18]. On the hollow site, the calculated adsorption energy is 5.31 eV, bigger than CHO group on Pd (100) surface. The bond axis of C O is perpendicular to Pd (100) surface with the C O bond length of ˚ The distance between C atom and the nearest Pd atom is 1.35 A. ˚ and the C O H bond angle is 111.5◦ . 2.10 A, For CO, there are a few experimental and theoretical studies of interaction between CO and transition metal surfaces [13–25]. It was indicated that CO favors an upright adsorption at the bridge sites with the C O bond length of 1.15 A˚ using LEED experiments [49]. On the bridge site of Pd (100) surface, the adsorption energy is 2.07 eV, higher than that on the top (1.58 eV) and hollow (1.96 eV) sites. In the most stable configuration, the bond lengths of C Pd ˚ Our calculated results and C O bond length are 2.01 and 1.18 A. are excellent accordance with the previous reports [49]. For H, it can be adsorbed on all possible sites of Pd (100) surface. The corresponding adsorption energies for the most stable structures are 2.30 (top), 2.75 (bridge), 2.79 eV (hollow), respectively.

616

Z. Jiang et al. / Applied Surface Science 364 (2016) 613–619

Fig. 2. The optimized co-adsorption structures on Pd (100) surface. Table 2 The most stable co-adsorption structures, co-adsorption energies and the sum of separated adsorption energies (eV). Species

Corresponding adsorption sites

Coadsorption energies

The sum of separated adsorption energies

CH3 O + H CH2 OH + H CH2 O + H CHOH + H CHO + H COH + H CO + H

Bridge + hollow Top + hollow Hollow + bridge Bridge + hollow Hollow + bridge Hollow + hollow Bridge + bridge

4.65 5.14 3.62 6.30 5.99 8.04 4.80

4.75 4.92 3.69 6.37 6.02 8.10 4.88

It was found that the single H atom prefers to adsorb at the hollow ˚ The calculated results are site with the Pd H bond length of 1.94 A. in line with the earlier calculations [40,41]. The significant distinction between the adsorption energies may be due to the different atoms arrangement of Pd (100) and Pd (111) surfaces, leading the change of electronic configuration and active sites of surface. 3.1.3. The co-adsorption of relevant species on Pd (100) surface In order to explore the dehydrogenation mechanism of CH3 OH on the Pd (100) surface, the most stable co-adsorbed configurations of CH3 O + H, CH2 OH + H, CH2 O + H, CHOH + H, CHO + H, COH + H and CO + H on the Pd (100) surface have been investigated. All the optimized co-adsorbed configurations are illustrated in Fig. 2. The co-adsorption energies, corresponding adsorption sites and the sum of separated adsorption energies are listed in Table 2. The co-adsorption energies between correlative species on Pd (100) surface can be defined as follows: Eco-ads = EA + EB + Eslab − E(A+B)/slab

(4)

where EA , EB , Eslab and E(A + B)/slab are the total energy for the correlative free molecule of A and B, the Pd slab with a (3 × 3) supercell and the co-adsorbed (A + B)/Pd slab systems, respectively. In all calculations, for the initial co-adsorption configuration on Pd (100) surface, the corresponding species are placed at the adjacent and the most stable adsorption sites. For example, for the co-adsorption of CH3 O/H on Pd (100) surface, the initial structure is CH3 O positioned on the bridge site and H atom on the adjacent hollow site. After geometry optimization, it is found that most

Fig. 3. Schematic of the optimal reaction pathway for the dissociation of CH3 OH on Pd (100) surface.

of the optimized co-adsorption configurations maintain their initial states. However, for the co-adsorption systems of CH2 O + H, CHO + H and CO + H, the H atom in the initial states moves to the bridge site (Fig. 2), which maybe due to the relatively strong repulsion between them. The co-adsorption energies calculated for CH3 O + H, CH2 OH + H, CH2 O + H, CHOH + H, CHO + H, COH + H, and CO + H are 4.65, 5.04, 3.62, 6.30, 5.99, 8.04 and 4.80 eV, comparable to the sum of the separated adsorption energies 4.75, 4.92, 3.69, 6.37, 6.02, 8.10 and 4.88 eV, respectively. The difference between the co-adsorption energies and the sum of separated energies indicates that there exists interaction energy between the co-adsorbed species on Pd (100) surface. 3.2. Dehydrogenation paths of CH3 OH on Pd (100) surface The characteristic of the reaction paths and the dissociation mechanism of chemisorption CH3 OH on Pd (100) surface were investigated in this section. Two possible ways for the dehydrogenation of CH3 OH to CO are considered: one starts with the breakage of H O bond, the other goes through the C H bond scission. Considering the sequential dehydrogenation of other intermediates, four pathways are proposed and analyzed in Fig. 3. The energy barriers and the reaction energies of each elementary reaction were calculated, and the results are listed in Table 3. The related transition states (TS) of elementary reactions are shown in Fig. 4. The potential energy profiles are illustrated in Fig. 5. The most stable adsorption structure of CH3 OH/Pd (100) was chosen to be the initial reactant. Therefore, all the reaction pathways begin with CH3 OH adsorbed on the top site, pass the co-adsorption configurations occupying the reasonable positions, and end up with CO + H holding the adjacent bridge sites. The corresponding reactions are listed as follows: CH3 OH → CH3 O + H;

(R1)

CH3 O → CH2 O + H;

(R2)

CH2 O → CHO + H;

(R3)

CHO → CO + H;

(R4)

Z. Jiang et al. / Applied Surface Science 364 (2016) 613–619

617

Table 3 Energies of the transition states of all dehydrogenation reactions from CH3 OH to CO + H on Pd (100) surface (eV). Surface

Reactions

TS

E(b)f

E(b)b

H

Clean Pd (100)

CH3 OH → CH3 O + H CH3 O → CH2 O + H CH2 O → CHO + H CHO → CO + H CH3 OH → CH2 OH + H CH2 OH → CH2 O + H CH2 OH → CHOH + H CHOH → CHO + H CHOH → COH + H COH → CO + H

TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 TS9 TS10

1.92 1.79 0.46 0.96 1.79 1.18 1.08 0.63 1.56 1.37

1.71 2.26 1.10 1.73 1.86 1.24 1.52 0.98 2.01 1.98

0.21 −0.47 −0.64 −0.77 −0.07 −0.06 −0.44 −0.35 −0.45 −0.61

E(b)f is the energy barrier of the forward reaction; E(b)b is the energy barrier of the backward reaction; H is the reaction energy.

Fig. 4. The configurations of transition states in the dehydrogenation reactions of CH3 OH on Pd (100) surface.

CH3 OH → CH2 OH + H;

(R5)

CH2 OH → CHOH + H;

(R6)

CH2 OH → CH2 O + H;

(R7)

CHOH → CHO + H;

(R8)

CHOH → COH + H;

(R9)

COH → CO + H

(R10)

3.2.1. The O H bond of initial scission (path 1) For the scission of H O bond in CH3 OH on Pd (100) surface (reaction (R1)), CH3 OH adsorbs on the top site as the initial configuration of reactant. Then CH3 OH molecule dehydrogenates into CH3 O and H with the breakage of O H bond. In transition state (TS1) (Fig. 4), the distance between the dissociated H atom and O atom increases ˚ which is longer than the initial O H bond length (0.973 A). ˚ to 2.29 A, After TS1, CH3 O group and the dissociated H atom shift to the bridge and the adjacent hollow sites. This elementary reaction is endothermic by 0.21 eV and needs to overcome the energy barrier of 1.92 eV for the forward reaction. Compared with the adsorption energy, it was found that the energy barrier of dissociation is bigger, implying that CH3 OH prefers to desorb rather than dissociate. For the scission of H O bond in CH3 O on Pd (100) surface (reaction (R2)), the most initial structure is that CH3 O adsorbs on the bridge site. Then CH3 O is dissociated into CH2 O and H with the

Fig. 5. Schematic potential energy profiles for the dissociation of CH3 OH on Pd (100) surface.

breakage of O H bond. The C O bond axis of CH3 O turns to the parallel direction of metal surface gradually. In TS2 (Fig. 4), the distance between the dissociated H atom and C atom increases to ˚ The new formation H Pd bond length is 1.60 A. ˚ After TS2, 3.13 A.

618

Z. Jiang et al. / Applied Surface Science 364 (2016) 613–619

CH2 O and the dissociated H atom move to the hollow and the adjacent bridge sites. This reaction has an energy barrier of 1.92 eV for the forward reaction, with the exothermic energy of 0.47 eV. Compared with CH3 OH, it was found that CH3 O prefers to dissociate rather than desorb. For the scission of H O bond in CH2 O (reaction (R3)), the initial configuration is that CH2 O adsorbs on the hollow site. Then CH2 O dissociates into CHO and H with the breakage of O H bond. In TS3 (Fig. 4), the distance between the dissociated H atom and C atom ˚ After TS2, CHO and the dissociated H increases to 1.60 A˚ from 1.15 A. atom move to the hollow and the adjacent bridge sites. This reaction needs to overcome a relatively small energy barrier of 0.46 eV, with the exothermic energy of 0.64 eV. For the reaction (R4) (the scission of H O bond in CHO), the most initial structure is that CHO adsorbs on the hollow site. Then CHO is dissociated into CO and H with the breakage of O H bond. In TS4 (Fig. 4), the bond lengths of C H, C H and H Pd are 1.32, ˚ After TS4, CO and the dissociated H atom move to 1.28 and 1.87 A. two adjacent bridge sites. Reaction has an energy barrier of 0.96 eV for the forward reaction, with the exothermic energy of 0.77 eV. 3.2.2. The C H bond of initial scission (path 2–4) From path 2 to 4 (Fig. 3), the C H bond scission was considered, similar to the previous H O bond breakage. For the scission of C H bond in CH3 OH on Pd (100) surface (reaction (5)), the top bound CH3 OH is chosen as the initial state of reactant. Then CH3 OH dissociates and generates CH2 OH and H with the breakage of C H bond, where CH2 OH and H adsorb on the top and hollow sites on Pd (100) surface. In TS5 (Fig. 4), the ˚ distance between the dissociated H atom and C atom is 1.76 A, ˚ This reaction needs longer than the initial C H bond length (1.09 A). to overcome the energy barrier of 1.79 eV for the forward reaction. Compared with the results of H O bond scission, the similar condition is that CH3 OH prefers to desorb rather than dissociate, however, the energy barrier of C H dissociation is smaller than that of H O bond, implying that the scission of C H bond is easier than that of H O bond, which is in agreement with the experimental results [18]. Besides, the adsorption energy of CH2 OH is bigger than that of CH3 O, which maybe confirm this conjecture. Two paths exist in the dehydrogenation of CH2 OH on Pd (100) surface (reactions (R6) and (R7)). For the scission of H O bond (reaction (R6)), the most initial state is that CH2 OH occupies the top site. Then it is dissociated into CH2 O and H with the breakage of H O bond, where CH2 O adsorbs on the hollow site and H occupies the adjacent bridge position. In TS6 (Fig. 4), The C O bond axis of CH2 O is nearly parallel to Pd (100) surface. The distance between ˚ This elethe dissociated H atom and O atom is elongated to 1.82 A. mentary reaction needs to conquer the energy barrier of 1.18 eV for the forward reaction, with the exothermic energy of 0.06 eV. For the scission of C H bond (reaction (R7)), the most initial state is similar. However, in the next step it will be dissociated into CHOH and H with the breakage of C H bond, where CHOH and H adsorbs on the bridge and the adjacent hollow sites. In TS7 (Fig. 4), the dissociated H atom shares the same Pd atom with the C atom in CHOH ˚ group. The bond lengths of H Pd and C Pd are 1.70 and 2.00 A. This reaction has an energy barrier of 1.08 eV for the forward reaction, with the exothermic energy of 0.44 eV. Therefore, due to the lower energy barrier and the higher reaction energy, the C H bond cleavage of CH2 OH is slightly favorable, that is, CH2 OH prefers C H bond cleavage to produce CHOH rather than O H bond scission to CH2 O. For the dissociation of CHOH, two possible reactions on Pd (100) surface are examined: C H bond scission to CHO and H O bond cleavage to COH. For the reaction (R8), CHO formation starts from the initial state, where CHOH occupies the bridge site. This elementary reaction has an activation barrier of

0.63 eV and is found to be exothermic by 0.35 eV. In TS8 (Fig. 4), the C atom adsorbs on the bridge site of Pd (100) surface. The distance between the dissociated H atom and the O atom is elongated to ˚ For the final state, CHO and the dissociated H atom shift to a 1.74 A. hollow site and the adjacent bridge site. For the reaction (R9), COH is obtained from the C H bond scission. In TS9 (Fig. 4), the length ˚ For the between the C atom and the dissociated H atom is 1.71 A. final state, COH and the dissociated H atom move to two adjacent hollow sites. This reaction has the energy barrier of 1.56 eV, with the exothermic energy of 0.45 eV. For the dehydrogenation of COH (reaction (R10)), the most favorable adsorption configuration (hollow) is chosen to be the initial state. Then COH dissociates into CO and H with the breakage of O H bond. In TS10 (Fig. 4), the distance between the dissociated ˚ Subsequently, CO and H H atom and O atom increases to 1.24 A. occupy two adjacent bridge sites. This elementary reaction needs to overcome an energy barrier of 1.37 eV, with the exothermic energy of 0.61 eV. According to our calculations on Pd (100) surface, it is found that the scission of C H bond is more favorable for CH3 OH and CH2 OH in kinetics. However, for CHOH group, the H O bond cleavage is easier. Furthermore, the energy barrier of CHO dehydrogenation is lower than that of COH (0.96 < 1.37). Thus, it is proposed that the path 2 (CH3 OH CH2 OH CHOH CHO CO) is the most possible pathway. In path 2, the highest energy barrier of CH3 OH dehydrogenation inhibits the cleavage of the C H bond and thus makes it to be the rate-determining step of the whole dehydrogenation reaction of CH3 OH. Besides, it is predicted that CH2 OH to be the most abundant species in the path 2 due to the higher energy barrier. Compared with the results on Pd (111) surface [17], it was reported that the initial H O bond scission on Pd (111) surface is favored for both CH2 OH and CHOH, which is different from our results. This may be attributed to the discrepant atoms arrangement in them, leading the change of electronic configuration and active sites of surfaces. The different reactivity and selectivity on Pd (100) and Pd (111) surfaces suggest that the dehydrogenation reaction of CH3 OH might be structure-sensitive.

4. Conclusions In this study, a systematic investigation of CH3 OH dehydrogenation mechanism on Pd (100) surface has been investigated by using DFT calculations at the molecular level. The most stable adsorption configurations for CH3 OH, CH3 O, CH2 OH, CH2 O, CHOH, CHO, COH, CO, and H species were obtained. It is indicated that CH3 OH and CH2 OH prefers to adsorb on the top site, CH3 O, CHOH and CO occupy preferentially on the bridge site, while CH2 O, CHO, COH and H species adsorb on the hollow site. The adsorption energies of all species exhibit the following trend: CH3 OH < CH2 O < CH3 O < CO < CH2 OH < H < CHO < CHOH < COH. Furthermore, the minimum energy path for the dissociation of CH3 OH into adsorbed CO and H is identified to explore the dehydrogenation mechanism. It is revealed that CH3 OH prefers to desorb rather than dissociate on Pd (100) surface, nevertheless, CH2 OH and CHOH maybe dissociated preferentially to desorb. Besides, our results suggest that the scission of C H bond is more favorable for CH3 OH and CH2 OH and the H O bond cleavage is easier for CHOH. It is proposed that the path 2 (CH3 OH CH2 OH CHOH CHO CO) is the most possible pathway, in which the highest energy barrier of CH3 OH dehydrogenation inhibits the cleavage of the C H bond and thus makes it to be the rate-determining step of the whole dehydrogenation reaction of CH3 OH. It is hoped that the information provided by this work might be useful for the future design of catalytic materials.

Z. Jiang et al. / Applied Surface Science 364 (2016) 613–619

Acknowledgements The authors greatly appreciate Prof. B.J. Wang (Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan Univ. of Tech.), who help us get access to the software of Materials Studio. In addition, The authors would like to acknowledge the following financial supports: PetroChina Innovation Foundation (2014D-5006-0401), National Natural Science Foundation of China (No. 21376186), the Ministry of Education (Doctoral Special Research Foundation No. 20110201110032) China, Fundamental Research Funds for the Central Universities (New Teacher Research Support Plan No. 08141002, International Cooperation Project No. 2011jdhz37 and Integrated Cross Project xjj2014136 in Xi’an Jiaotong University), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2012JM2010) and Sci. & Tech. Project for Overseas Scholars (the Ministry of Human Resources and Social Security of China, No. 19900001). References [1] T.N. Veziroglu, Hydrogen energy system as a permanent solution to global energy environmental problems, Chem. Ind. 53 (1999) 383–393. [2] M. Momirlan, T.N. Veziroglu, Current status of hydrogen energy, Renew. Sustain. Energy Rev. 6 (2002) 141–179. [3] G.W. Huber, J.W. Shabaker, J.A. Dumesic, Raney Ni-Sn catalyst for H2 production from biomass-derived hydrocarbons, Science 300 (2003) 2075–2077. [4] I.J. Drake, K.L. Fujdala, A.T. Bell, T.D. Tilley, Dimethyl carbonate production via the oxidative carbonylation of methanol over Cu/SiO2 catalysts prepared via molecular precursor grafting and chemical vapor deposition approaches, J. Catal. 230 (2005) 14–27. [5] M.S. Wilson, Methanol decomposition fuel processor for portable power applications, Int. J. Hydrogen Energy 34 (2009) 2955–2964. [6] R.M. Navarro, M.A. Pena, J.L.G. Fierro, Production of hydrogen by partial oxidation of methanol over a Cu/ZnO/Al2 O3 catalyst: influence of the initial state of the catalyst on the start-up behaviour of the reformer, J. Catal. 212 (2002) 112–118. [7] E. Swaramoorthi, A.K. Dalai, A comparative study on the performance of mesoporous SBA-15 supported Pd–Zn catalysts in partial oxidation and steam reforming of methanol for hydrogen production, Int. J. Hydrogen Energy 34 (2009) 2580–2590. [8] R. Schennach, A. Eichler, K.D. Rendulic, Adsorption and desorption of methanol on Pd (111) and on a Pd/V surface alloy, J. Phys. Chem. B 107 (2003) 2552–2558. [9] M. Morkel, V.V. Kaichev, G. Rupprechter, H.J. Freund, Methanol dehydrogenation and formation of carbonaceous overlayers on Pd (111) studied by high-pressure SFG and XPS spectroscopy, J. Phys. Chem. B 108 (2004) 12955–12961. [10] J.L. Davis, M.A. Barteau, Spectroscopic identification of alkoxide, aldehyde, and acyl intermediates in alcohol decomposition on Pd(111), Surf. Sci. 235 (1990) 235–248. [11] A.K. Bhattacharya, M.A. Chesters, M.E. Pemble, N. Sheppard, The decomposition of methanol on Pd (110) as studied by electron energy loss spectroscopy: evidence for the formation of CH3 O and HCO surface species, Surf. Sci. 206 (1988) 845–850. [12] K. Christmann, J.E. Demuth, The adsorption and reaction of methanol on Pd (100). I. Chemisorption and condensation, J. Chem. Phys. 76 (1982) 6308–6317. [13] Y.H. Zhou, P.H. Lv, G.C. Wang, DFT studies of methanol decomposition on Ni (100) surface: compared with Ni(111) surface, J. Mol. Catal. A: Chem. 258 (2006) 203–215. [14] G.C. Wang, Y.H. Zhou, Y. Morikawa, J. Nakamura, Z.S. Cai, X.Z. Zhao, Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study, J. Phys. Chem. B 109 (2005) 12431–12442. [15] T.H. Upton, Theoretical studies of the decomposition of methanol on Ni(100), J. Vac. Sci. Technol. 20 (1982) 527–531. [16] R. Jiang, W. Guo, M.M. Li, H. Zhu, L. Zhao, X. Lu, H. Shan, Methanol dehydrogenation on Rh(111): a density functional and microkinetic modeling study, J. Mol. Catal. A: Chem. 344 (2011) 99–110. [17] J. Yang, Y. Zhou, H. Su, S.J. Jiang, Theoretical study on the effective methanol decomposition on Pd (111) surface facilitated in alkaline medium, Electroanal. Chem. 662 (2011) 251–256. [18] R. Jiang, W. Guo, M. Li, D. Fu, H.J. Shan, Density functional investigation of methanol dehydrogenation on Pd(111), J. Phys. Chem. C 113 (2009) 4188–4197.

619

[19] R. Jiang, W. Guo, M. Li, X. Lu, J. Yuan, H. Shan, Dehydrogenation of methanol on Pd(100): comparison with the results of Pd(111), Phys. Chem. Chem. Phys. 12 (2010) 7794–7803. [20] J. Greeley, M. Mavrikakis, Competitive paths for methanol decomposition on Pt(111), J. Am. Chem. Soc. 126 (2004) 3910–3919. [21] A.V. Miller, V.V. Kaichev, I.P. Prosvirin, J. Bukhtiyarov, Mechanistic study of methanol decomposition and oxidation on Pt(111), J. Phys. Chem. C 117 (2013) 8189–8197. [22] E.M. Karp, T.L. Silbaugh, M.C. Crowe, C.T. Campbell, Energetics of adsorbed methanol and methoxy on Pt(111) by microcalorimetry, J. Am. Chem. Soc. 134 (2012) 20388–20395. [23] S.K. Desai, M. Neurock, K. Kourtakis, A periodic density functional theory study of the dehydrogenation of methanol over Pt(111), J. Phys. Chem. B 106 (2002) 2559–2568. [24] W.K. Chen, S.H. Liu, M.J. Cao, Q.G. Yan, C.H. Lu, Adsorption and dissociation of methanol on Au(111) surface: a first-principles periodic density functional study, J. Mol. Struct. Theochem. 770 (2006) 87–91. [25] A. Hussain, S.H. Shah, MMENT>Computational study of complete methanol dehydrogenation on Au(100) and Au(310) surfaces: dominant role of atomic oxygen, Surf. Sci. 620 (2014) 30–37. [26] P. Gazdzicki, P. Jako, Methanol oxidation on monolayer Cu/Ru(0001), J. Phys. Chem. C 115 (2011) 16555–16566. [27] R.B. Barros, A.R. Garcia, L.M. Ilharco, Effect of oxygen precoverage on the reactivity of methanol on Ru(001) surfaces, J. Phys. Chem. B 108 (2004) 4831–4839. [28] S. Sakong, A. Gross, Total oxidation of methanol on Cu(110): a density functional theory study, J. Phys. Chem. A 111 (2007) 8814–8822. [29] D. Mei, L. Xu, G. Henkelman, Potential energy surface of methanol decomposition on Cu(110), J. Phys. Chem. C 113 (2009) 4522–4537. [30] X.K. Gu, W.X. Li, First-principles study on the origin of the different selectivities for methanol steam reforming on Cu(111) and Pd(111), J. Phys. Chem. C 114 (2010) 21539–21547. [31] J. Greeley, M. Mavrikakis, Methanol decomposition on Cu(111): a DFT study, J. Catal. 208 (2002) 291–300. [32] J.R.B. Gomes, J.A.N.F. Gome, A DFT study of the methanol oxidation catalyzed by a copper surface, Surf. Sci. 471 (2001) 59–70. [33] X. Sun, R. Zhang, B. Wang, Insights into the preference of CHx (x = 1–3) formation from CO hydrogenation on Cu(111) surface, Appl. Surf. Sci. 265 (2013) 720–730. [34] R. Zhang, H. Liu, L. Ling, Z. Li, B. Wang, A DFT study on the formation of CH3 O on Cu2 O(111) surface by CH3 OH decomposition in the absence or presence of oxygen, Appl. Surf. Sci. 257 (2011) 4232–4238. [35] Z. Zuo, L. Wang, P. Han, W. Huang, Insights into the reaction mechanisms of methanol decomposition, methanol oxidation and steam reforming of methanol on Cu(111): a density functional theory study, Int. J. Hydrogen Energy 39 (2014) 1664–1679. [36] A. Montoya, B.S. Haynes, Methanol and methoxide decomposition on silver, J. Phys. Chem. C 111 (2007) 9867–9876. [37] B. Delly, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1990) 508–517. [38] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, First-principles simulation: ideas, illustrations and the CASTEP code, J. Phys. Condens. Matter 14 (2002) 2717–2744. [39] J.P. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy, Phys. Rev. B 45 (1992) 13244–13249. [40] Z. Jiang, L. Li, J. Xu, T. Fang, Density functional periodic study of the dehydrogenation of methane on Pd (111) surface, Appl. Surf. Sci. 286 (2012) 115–120. [41] Z. Jiang, B. Wang, T. Fang, Adsorption and dehydrogenation mechanism of methane on clean and oxygen-covered Pd (100) surfaces: a DFT study, Appl. Surf. Sci. 320 (2014) 256–262. [42] B.J. Wang, L.Z. Song, R.G. Zhang, The dehydrogenation of CH4 on Rh(111), Rh(110) and Rh(100) surfaces: a density functional theory study, Appl. Surf. Sci. 258 (2012) 3714–3722. [43] R. Zhang, L. Song, Y. Wang, Insight into the adsorption and dissociation of CH4 on Pt(h k l) surfaces: a theoretical study, Appl. Surf. Sci. 258 (2012) 7154–7160. [44] R. Zhang, T. Duan, L. Ling, B. Wang, CH4 dehydrogenation on Cu(111), Cu@Cu(111), Rh@Cu(111) and RhCu(111) surfaces: a comparison studies of catalytic activity, Appl. Surf. Sci. 341 (2015) 100–108. [45] B. Xing, X.Y. Pang, G.C. Wang, C H bond activation of methane on clean and oxygen pre-covered metals: a systematic theoretical study, J. Catal. 282 (2013) 74–82. [46] T.A. Halgren, W.N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chem. Phys. Lett. 49 (1977) 225–232. [47] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 76th ed., CRC Press, Boca Raton, FL, 1996. [48] J.L. Davis, M.A. Barteau, Polymerization and decarbonylation reactions of aldehydes on the Pd(111) surface, J. Am. Chem. Soc. 111 (1989) 1782–1792. [49] R.J. Behm, K. Christmann, G. Ertl, M.A. Van Hove, Adsorption of CO on Pd(100), J. Chem. Phys. 73 (1980) 2984–2995.