DFT Study on the Synergistic Effect of PdâCu Bimetal on the

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DFT Study on the Synergistic Effect of Pd-Cu BiMetal on the Adsorption and Dissociation of H2O Zhao Jiang, and Tao Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05274 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

DFT Study on the Synergistic Effect of Pd-Cu Bi-Metal on the Adsorption and Dissociation of H2O Zhao Jiang, Tao Fang∗ School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, PR China

Aiming at exploring the effect of surface composition on the catalytic activity, by density functional theory, the adsorption and complete dehydrogenation mechanisms of H2O on five Pd-Cu (100) surfaces have been investigated. About adsorption, it is found that the adsorption OH, O and H on Pd-Cu0-4, Pd-Cu1-3, Pd-Cu2-2, Pd-Cu3-1 and Pd-Cu4-0 surfaces are chemisorption, and H2O may be considered as physisorption. Interestingly, it is revealed that the difference of Pd-Cu doping ratios has no effect on the adsorption configurations of all intermediates, nevertheless, it has an evident influence on the interaction between adsorbates and substrates. In addition, the energy pathways for the complete dehydrogenation of H2O on five Pd-Cu (100) surfaces are analyzed to explore the dissociation mechanisms. It is found that the doped Pd atoms on the first layer of Cu (100) surface are beneficial for the scission of H-O bond of H2O except for the

Pd-Cu4-0 and Pd-Cu3-1 surfaces and can promote the scission of H-O bond of OH except for the Pd-Cu4-0 surface. Compared with that on other Pd-Cu (100) surfaces, it can be proposed that the Pd-Cu2-2 surface is the optimal surface for H2O dissociation in kinetic and thermodynamic aspects. Therefore, by the way of regulating the doped ratios of metal surfaces, the activity and selectivity of catalysts may be modulated effectively.

∗

Corresponding author. E-mail address: [email protected] (T. Fang) 1

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1. Introduction Water is one of the most plentiful and essential compounds in nature. The adsorption of water on the surfaces of transition metals is one of the important subjects in surface science. Due to the particular relevance to heterogeneous catalysis, corrosion of materials, membrane science, hydrogen production and electrochemistry, it is significant to expound of the surface phenomena about water molecule 1-2. For example, in the pathway of partial oxidation of methane, the reaction CH4+H2O=CO+3H2 is considered to be the key step to produce syngas. The knowledge about how water participates in the reactions is helpful to clarify the corresponding reaction mechanisms. Therefore, it is necessary to explore the interaction mechanisms between H2O and metal surfaces in the micro level. Using various surface analytical technologies, the interaction of H2O with metal surfaces has been investigated experimentally. Using the technologies of electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED), Nyberg et al.

3

studied the adsorption of

water on Pd (100), and revealed that water occupies the top site preferentially about 10 K and that the molecule axis of water is tilted relative to the surface normal. By means of HREELS and TPD technologies, Xu et al. 4 found that at 110 K H2O binds on Pd (110) surface and desorption from the corresponding adsorption sites occurs (about 200 K). By scanning tunneling microscopy (STM), on Pd (111) surface, Mitsui et al. 5 reported that the icelike puckered hexagonal units are generated with the continuous adding of water monomer. For the adsorption of H2O on Cu surfaces, Andersson et al. 6 studied the H2O decomposition on Cu (110) surface under near normal conditions by X-ray photoelectron spectroscopy (XPS). They found that the autocatalytic reaction of H2O is contributed by the strong hydrogen bond in H2O-OH complex. Besides, Nyberg et al.7 2


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found that H2O molecule binds at the top site on Cu (100) surface with its molecule axis being tilted to the meal surface. Theoretically, DFT (Density Functional Theory) methods have been proved an efficiency tool for exploring the chemical reaction mechanism at the atomic level. Numerous efforts have been focused on clarifying the mechanisms of H2O adsorption and dissociation on different metal surfaces, such as Pd

8-12

, Cu

13-15

, Ru

16-17

, Pt

18

, Au

19

and Ni

20-21

. For example, Cao et al.

8-9

investigated the sequential decomposition of H2O on the clean and X/Pd (111) surfaces. They revealed that the doped C, N and O atoms can promote the scission of H-O bond in H2O molecule. Li et al.

12

studied the adsorption mechanism of water molecule on clean Pd (100) surface,

indicating that d and 5s orbits of Pd play key roles in the adsorption. Wang et al. 14 investigated the adsorption and dissociation of H2O molecule on various Cu surfaces and found that the joining of atomic oxygen enhances the water dissociation markedly, which is due to the acid-base pair interactions. In our previous reports, we have studied the adsorption and dissociation mechanisms of H2O on clean and O-assisted Pd (100), Cu (111) and Au (100) surfaces 6, 22, 23, revealing that the existence of atomic oxygen can decrease the activation energy of H2O decomposition. In addition, it is reported that the surfaces catalytic activity can be regulated by doping small amounts of promoters24-25. Wang et al.

26-27

have investigated the doping effect of Mn on the formation

mechanisms of ethanol from syngas on Cu (211) and Rh (211) surfaces, indicating that the two bi-metal surfaces (MnCu (211) and MnRh (211)) show the higher catalytic performance. Wang et al.

28

and Ishikawa et al.

29

have studied the adsorption and dissociation of H2O on bimetallic

clusters PtM and found that the catalytic activity for water dissociation is improved with the doping of the second metal. ” 3



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To the best of our knowledge, the previous research are mainly focused on the adsorption and dissociation of water on the clean metal surfaces. However, there are very few studies to study the effect of the different doped ratio of atoms on metal surfaces. Therefore, it’s crucial to study the structure-activity relationship between the surface composition and catalytic activity. In our study, aiming at illuminating the effect of surface composition by regulating the doped ratio of Pd and Cu atoms, DFT is used to study the adsorption and sequential dissociation of water on five Pd-Cu (100) surfaces,

2. Computational Methodology 2.1 Computational method All the calculations are carried out using the code Cambridge Sequential Total Energy Package (CASTEP) in Materials Studio30-31. The exchange and correlation are conducted by the generalized gradient approximation (GGA) with the Perdew-Wang-91 (PW91)

32

functional. The

plane-wave cutoff energy is set at 400 eV to describe the electronic wave functions

22-23

. The

Brillouin zone is sampled using a 4×4×1 Monkhorst-Pack k-point grid with a Methfessel-Paxton smearing of 0.1eV. The convergence criteria for configuration optimization is 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/ Å and 2.0×10−3 Å , respectively. The transition states (TS) are searched with complete LST/QST method for the elementary reactions 33. Additionally, frequency analysis and TS confirmation are performed to verify the transition state 34-35. 2.2 Surface model In this study, Pd-Cu (100) surfaces are modeled by a (2×2) super-cell with five-layer slab, and a vacuum region of 12 Å is added in order to avoid the image interactions. This method has been 4


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used in previous theoretical research treating the interactions between water and metal surfaces 8-10, 22-23

. During all the geometry optimizations, the atoms in top three layers together with the

adsorbates are allowed to relax, whereas other layers are fixed. To investigate the effect of Pd and Cu doped ratio on the catalytic activity of metal surface for the dissociation of H2O, five Pd-Cu (100) surfaces have been built: 1) the ratio of Pd and Cu atoms in the first layer is 0:4, denoted as Pd-Cu0-4 surface; 2) the ratio of Pd and Cu atoms in the first layer is 1:3, denoted as Pd-Cu1-3 surface; 3) the ratio of Pd and Cu atoms in the first layer is 2:2, denoted as Pd-Cu2-2 surface; 4) the ratio of Pd and Cu atoms in the first layer is 3:1, denoted as Pd-Cu3-1 surface; 5) the ratio of Pd and Cu atoms in the first layer is 4:0, denoted as Pd-Cu4-0 surface. All the corresponding surfaces are shown in Fig.1.

3. Results and Discussion 3.1 Evaluation of calculation method and model To check the reliability of the selected method and model, two tests have been conducted before calculations. Firstly, the structure of free H2O molecule is optimized and the geometrical parameters are estimated. The H-O bond length and H-O-H bond angle are 0.978 Å and 103.75°, in agreement with the experimental values of 0.96 Å and 104.4° 36. The next test is to calculate the lattice constant of bulk Cu. After calculations, the lattice constant is 3.65 Å, close to experimental value (3.62 Å) 36 and other theoretical results 35. These tests indicate that the method employed is reliable.

3.2 The adsorption of intermediates on five Pd-Cu (100) surfaces The adsorption energies for all intermediates on five surfaces are calculated as follows:

Eads = Emolecule + EPd − Cu (100) − Emolecule / Pd −Cu (100) 5


(1)


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in which E (molecule/Pd-Cu(100)) is the total energy of the adsorbate and Pd-Cu (100) surface, E (molecule) is the energy of adsorbate in vacuum, and E (Pd-Cu(100)) is the energy of Pd-Cu (100) surface. In this section, for all possible reactants, intermediates and products, the adsorption configurations have been optimized at three adsorption sites (top, bridge and hollow sites). The most stable adsorption configurations on different Pd-Cu (100) surfaces are demonstrated in Fig.2. According to the calculated results, adsorption energies and the pivotal geometric parameters at the preferred sites are shown in Table 1. 3.2.1 Adsorption of H2O, OH, O and H on Pd-Cu (100) surfaces with different doped ratios For the adsorption of H2O on five Pd-Cu (100) surfaces, it is revealed that H2O binds at the top position preferentially and locates on the surface in a nearly parallel way (Fig. 2). At the top position, the distances between the atomic O and the nearest metal atom are in the range of 2.48-2.51 Å. The corresponding adsorption energies are 0.27, 0.21, 0.20, 0.19 and 0.62 eV for

Pd-Cu0-4, Pd-Cu1-3, Pd-Cu2-2, Pd-Cu3-1 and Pd-Cu4-0, respectively, which are close to the previous reports14, 37-38 except for that on Pd-Cu4-0 surface. Besides, the corresponding adsorption energies are slightly changed when the number of doped Pd atoms is lower than three. However, the interaction between H2O and the Pd-Cu (100) surface with the uppermost layer being substituted by Pd atoms (Pd-Cu4-0 surface) is promoted observably, indicating that the method of using the doped atoms to regulate the interaction between adsorbates and metal surfaces may be feasible. In addition, the effect on the structure of H2O molecule on five doped Pd-Cu (100)

surfaces is considered, it is fund that the lengths of O-H bonds are 0.98 Å and the H-O-H bond angles are 104.3°, close to that of H2O molecule in vacuum. 6


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For the adsorption of OH on five Pd-Cu (100) surfaces, different configurations of OH are optimized at the corresponding adsorption sites. Our calculations indicate that the OH group prefers occupying the hollow site on five Pd-Cu (100) surfaces and that the doped Pd atoms

make no effect on the stable adsorption structures of OH. At the hollow sites, the corresponding adsorption energies are 2.95, 3.00, 2.85, 2.61 and 2.36 eV for Pd-Cu0-4, Pd-Cu1-3, Pd-Cu2-2, Pd-Cu3-1, Pd-Cu4-0 (100) surfaces, respectively. The adsorption energy of OH on Pd-Cu0-4 surface agrees with the previous reports

14,38

. Compared to the corresponding

energies, it can be indicated that the interaction between OH and the Pd-Cu (100) surfaces are weakened except for the Pd-Cu1-3 surface. Compared with our previous calculations on the clean Pd (100) surface 10, we can find that the adsorption energy of OH on Pd-Cu4-0 surface decreases, suggesting that the second layer (Cu atoms) of metal surface has a negative effect on the adsorption of OH species. For atomic oxygen adsorption on five Pd-Cu (100) surfaces, different initial structures are also optimized at the corresponding adsorption position. It is found that oxygen atom favors to adsorption at the hollow site on the five doped Pd-Cu (100) surfaces (Fig.2). The corresponding adsorption energies of atomic oxygen are calculated to be 4.81, 4.70, 4.48, 4.13 and 4.14 eV on

Pd-Cu0-4, Pd-Cu1-3, Pd-Cu2-2, Pd-Cu3-1 and Pd-Cu4-0 surfaces, indicating that the joining of Pd atoms impair the interaction between oxygen atom and Pd-Cu (100) surfaces. The distances between the atomic oxygen and the nearest metal atom (Pd or Cu) are in the range of 1.98-2.23 Å. For atomic H adsorption on five doped ratio Pd-Cu (100) surfaces, the optimized configurations are depicted in Fig. 2, where the H atom is favored at the hollow site. The adsorption energies are 2.36, 2.64, 2.55, 3.02 and 3.10 eV for for Pd-Cu0-4, Pd-Cu1-3, Pd-Cu2-2, 7



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Pd-Cu3-1 and Pd-Cu4-0 (100) surfaces. The trend of adsorption energies is Pd-Cu0-4 < Pd-Cu2-2 < Pd-Cu1-3

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