Article pubs.acs.org/JPCC
Microscopic Insight into the Activation of O2 by Au Nanoparticles on ZnO(101) Support Chuanyi Jia,†,‡ Wenhui Zhong,†,‡ Mingsen Deng,†,‡ and Jun Jiang*,‡,§ †
Guizhou Provincial Key Laboratory of Computational Nano-material Science, Institute of Applied Physics, and ‡Guizhou Synergetic Innovation Center of Scientific Big Data for Advance Manufacturing Technology, Guizhou Normal College, Guiyang 550018, China § School of Chemistry and Materials Science, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China (USTC), Hefei 230026, China S Supporting Information *
ABSTRACT: We carry out density functional theory calculations to cast insight on the microscopic mechanism of the activation of O2 by Au7 cluster on ZnO(101)-O support. The excellent catalytic activity of Au/ZnO catalyst was ascribed to the distribution of polarized surface charge associated with interface structure. It is found the stoichiometric ZnO(101)-O easily adsorbs and dissociates O2 to form very stable oxygen-saturated surface. For Au7 on stoichiometric ZnO(101)-O surface, the two Au atoms neighboring to O could accumulate positive charges, which then upshift the d-band centers toward the Fermi level. These favor the adsorption and dissociation of O2, providing two Au activation sites. In contrast, for the Au7 on the oxygen-saturated ZnO(101)-O, all Au atoms become neighboring to O and consequently provide seven activation sites. The workfunction difference between the Au7 and support induces effective polarized surface charges, substantially promoting O2 adsorption and dissociation both dynamically and thermodynamically. Further analysis on the effect of different Au positions demonstrates the polarized charge as the microscopic driving force for catalysis. These results would help design of better metal/oxide catalysts by providing important implications for the role of atomic and electronic structures.
1. INTRODUCTION Gold-based catalysts have received considerable attention during the past decades because of remarkable electrical, optical, and catalytic properties.1−5 Since the pioneering work by Haruta,6 gold-based hybrid nanoparticles were proved to exhibit good activity and selectivity in various oxidation reactions, particularly in catalytic reactions involving oxygen molecules.7−12 They hold a great potential in the field of automotive emission control, CO removal in enclosed atmospheres, and selective oxidation of organic compounds.13−20 A general consensus is that among all the oxidation reactions on gold, O2 activation is one of the most important elementary steps with respect to the catalytic activity.21−26 Since pure gold is one of the most inert transition metals with low catalytic activity, the choice of suitable support for gold nanoparticles becomes vital for O2 activation, which has been a hot topic of many experimental and theoretical studies.27−31 Those studies proposed various reaction mechanisms to understand the reaction process and revealed many factors that could help the optimization of catalysts performances. On Au/SiO2, Zheng et al. conducted temporal analysis of products kinetic to show that O2 is activated to form more reactive O adatoms prior to further oxidation reactions.27 On Au/Fe2O3, Daniells’ experiments suggested that the oxygen defects on the metal oxide surface play a very important role in the catalytic activity of gold catalysts for O2 activation.28 Additional weight has also been leant to this argument by theoretical studies.29−31 Their results indicated that the sizes, steps, and edges of gold clusters all have significant impact on the dissociation of O2. © XXXX American Chemical Society
Among many Au/oxides hybrid catalysts, Au/ZnO nanocomposites with unique physical and chemical properties have been extensively studied.32−35 Most previous theoretical studies on ZnO focused on two special polar surfaces of (0001) and (000−1),36−40 while the more common nonpolar surfaces such as (101) surface were barely investigated. Note that a recent advance by He et al. demonstrated that small Au(111) dots supported on ZnO(101) surface could form very efficient catalyst for O2 activation.41 A theoretical study on the mechanism of O2 activation by Au/ZnO(101) thus becomes necessary to examine those main influencing factors for designing highly efficient Au/ZnO catalysts. In this contribution, we have chosen an Au7 cluster42 on the O-terminated ZnO(101) surface (ZnO(101)-O) as the model system and performed first-principles simulations at the density functional theory (DFT) level to explore the catalytic roles of surface atomic and electronic structure from a microscopic view. Our recent work on catalytic materials has revealed that the surface polarized charge often serves as the driving force for many catalytic reactions.43,44 Following this track, we have shown that the best sites for O2 activation are the Au atoms neighboring to surface O atoms. The presaturation of O atoms on the ZnO(101)-O surface not only increases the number of active Au sites but also enhances the surface workfunction. The latter change induces polarized positive charge on each Au site Received: October 7, 2015 Revised: January 19, 2016
A
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adsorbate species. In this definition of Eb, negative values of adsorption energy correspond to an exothermic process, whereas positive values correspond to an endothermic process.
and consequently promotes the catalytic activity greatly. The analysis of electronic structures of gold nanoparticles on stoichiometric ZnO(101)-O and oxygen-saturated ZnO(101)-O, as well as the discussion on the effect of different Au sites, revealed the important role of surface polarized charge in O2 adsorption and dissociation.
3. RESULTS AND DISCUSSION Oxygen Saturation of Bare Stoichiometric ZnO(101)-O Surface. From the optimized structure of the bare stoichiometric ZnO(101)-O surface in Figure 1 (Per-Suf in the top
2. CALCULATION DETAILS All of DFT calculations were carried out using Vienna Ab Initio Simulation Package (VASP).45 The Perdew, Burke, and Ernzerhof (PBE)46 functional and periodic boundary conditions were employed for the exchange-correlation interactions. The valence electrons were treated using a plane-wave basis set with energy cutoff of 400 eV. The projector augmented wave (PAW) method was used to describe the interactions between the ions and the electrons with the frozen-core approximation.47−49 Fully structural optimizations were performed until the force on any atom was below 0.02 eV/Å. The Monkhorst− Pack grids of 3 × 3 × 1 and 9 × 9 × 1 κ-point were used for geometry optimization and density of states (DOS) calculation, respectively. The minimum energy paths of O2 dissociation reactions were searched by climbing image nudged elastic band (CI-NEB) method integrated in VASP.50,51 The bulk crystal structure of ZnO was modeled using a κ-point mesh of 6 × 6 × 6. The obtained optimal crystallographic parameters are a = b = 3.259 Å and c = 5.222 Å, which are in good agreement with experiment at room temperature (a = b = 3.250 Å, c = 5.207 Å52). There are two kinds of terminations for the (101) surface. It can either be terminated with threefold-coordinated O (O-terminated) or with threefold-coordinated Zn (Zn-terminated), and the O-terminated surface is chosen in this study (see reasons in Supporting Information Section 1). The bare ZnO(101)-O surface was modeled using a 3 × 3 unit cell. Fifty-four ZnO molecular units in each slab were distributed in 12 layers (108 atoms in total). A 15 Å vacuum gap was introduced along the c-direction to screen the self-interaction effects of the periodic boundary conditions. A cluster of seven Au atoms (Au7) with two-dimensional plane structure (2D) taken from the Au(111) surface41 was built to model the gold dots on ZnO surface, so as to examine the effect of polarized charge on catalytic performance (see details in section 2 of Supporting Information). Here the Au7 model was optimized in a 15 × 15 × 15 Å unit cell, while only the Γ-point was used in all directions. For the Au7/ZnO(101)-O system, the adsorbates (Au7 cluster and O2) and all of the atoms in the six topmost layers of the ZnO(101)-O surface were allowed to relax, whereas the rest of the six layers at the bottom were fixed to simulate the bulk effects. Meanwhile, the model of Au10 (10 Au atoms in a nonplanar form) on ZnO(101)-O has also been built and tested (see details in section 2 of Supporting Information). It should be noted that simulations with even larger Au clusters to represent the real gold dots are often prohibitively expensive in terms of computational costs. Our investigations thus focus on Au7 (and Au10) to examine the dependence of catalytic activities on polarized charge. The binding energies of the adsorbates are calculated according to the following equation E b = Esur + ad − Esur − Ead
Figure 1. Optimized structure of the stoichiometric ZnO(101)-O surface (Per-Suf) and the molecular adsorption (Per-O2-mol) and dissociative adsorption (Per-O2-dis) of O2 on stoichiometric ZnO(101)-O surface. Color coding: red, O atoms; gray, Zn atoms.
panel), one can see that the surface layer is composed of threecoordinated O atoms (O3c) and the trough site holds threecoordinated Zn atoms (Zn3c). The strong repulsive forces between O3c atoms and O2 molecule can prevent spontaneous adsorption on these sites, as indicated by our calculations. On the other hand, the trough sites with Zn3c are suitable for O2 anchoring, and a binding energies of −3.35 eV indicates that the O2 adsorption on ZnO(101)-O surface is stable (the PerO2-mol in Figure 1). The bond length of O2 is stretched from 1.21 to 1.51 Å, suggesting that O2 is already slightly activated. It is interesting to find that the adsorbed O2 can be further dissociated. The reaction path from CI-NEB calculations in Figure 2 shows that the O2 dissociation process is exothermic by −0.45 eV, with an energy barrier of 0.55 eV which is much smaller than the binding energy of O2 (−3.35 eV). This means that the dissociation of O2 on the stoichiometric ZnO(101)-O surface can be easily achieved. Meanwhile, a binding energy of −3.80 eV larger than that of Per-O2-mol suggests that the binding of two O atoms in the dissociated state in Figure 1 (Per-O2-dis) is more favorable thermodynamically. However, the binding of these O atoms on the trough sites is very strong, making further oxidation reactions extremely hard to realize (see details in Figure S5 of Supporting Information). One can expect that the ZnO(101)-O surface exposed in reality can be easily covered by O atoms (surface passivation) and generates an oxygen-saturated surface. Moreover, since oxygen vacancy
where Esur+ad is the total energy of the surface and adsorbate species in their optimized reference states, Esur is the total energy of the bare surface, and Ead is the total energy of the B
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contrast, the Au atoms carrying negative charges would repel the O atoms in O2. Then, four stable adsorption structures of O2 were found by simulations, as shown in Figure 4. As expected, the computed
Figure 2. Potential energy profile for the dissociation of O2 by the stoichiometric ZnO(101)-O surface.
often appears as intrinsic defects in metal oxides, we performed simulations demonstrating the easy saturation of oxygen defective sites in the ZnO(101)-O (see details in Supporting Information Figures S6, S7, and S8). Therefore, the dissociation of O2 on the Au/ZnO(101) surface is more inclined to take place on the Au-NPs, and the ZnO(101) substrate plays a secondary/indirect role. O2 Adsorption and Dissociation on the Stoichiometric Au7/ZnO(101)-O System. Geometry optimizations have been performed on the composite configuration of the Au7 on the stoichiometric ZnO(101)-O surface. Among all the possible configurations (Figure S9), the most stable structure in Figure 3
Figure 4. Four optimized stable structures and binding energies of O2 adsorbed to the Au7 supported on stoichiometric ZnO(101)-O surface. See Figure 3 for color coding.
binding energies suggest that the states Com-O2-3-mol and Com-O2-4-mol for the O2 absorbed by Au-2 and Au-5 with positive polarized charge are exothermic, while the states Com-O2-1-mol and Com-O2-2-mol for absorption to other negative charged Au atoms are endothermic (among which the state Com-O2-2-mol is even less stable than the adsorption of O2 on pure Au(111) surface in Figure S12). Naturally, the surface polarized charge would change effectively the d-band electronic states in gold cluster, which are responsible for most catalytic activities. In Figure 5, the projected d-band partial density of states (pDOS) of the seven Au atoms in the hybrid shows that the d-band center of Au-2 and Au-5 was significantly upshifted toward the Fermi level. In this case, the antibonding states of Au atom would be pushed above the Fermi level and decrease the Pauli repulsion.38 Such a response increases the bonding strength between Au and O atoms and further enhances the stability of the adsorption state. After elucidating the molecular adsorption, we now focus on O2 dissociation. As depicted in Figure 6, an energy barrier of 1.39 eV was found for the O2 dissociation on the Com-O2-4mol Au7/ZnO(101)-O system, which is much larger than the binding energy (−0.36 eV) encountered. This means that the dissociation of O2 requires additional energy to overcome the barrier. Moreover, the dissociative adsorption state (Com-O24-dis) has lower adsorption energy than the molecular adsorption
Figure 3. Optimized structure of Au7 on the stoichiometric ZnO(101)-O surface. The Bader charges carried by Au atoms are shown at the right side. Color coding: yellow, Au atoms; others are the same as in Figure 1.
shows that the central Au atom was popped up from the Au7 plane, leaving only two Au atoms (labeled as Au-2 and Au-5) in close contact with O atoms on the ZnO(101)-O surface. The computed workfunction of bare stoichiometric ZnO(101)-O surface with 4.41 eV (Figure S10) agrees well with experimental values of 4.45 eV53,54 which is smaller than that of pure gold (5.10 eV). Therefore, one can expect that ZnO donates electrons to the Au7, as confirmed by the accumulation of −0.89 e charges in the Au7 cluster (Figure 3). Normally, the negative charge disfavors the adsorption of O2. However, Bader charge analysis55,56 in Figure 3 shows that the charges carried by these seven Au atoms are different from each other (charges on surface O atoms are given in Figure S11). It is found that the Au-2 and Au-5 atoms in contact with surface O atoms still hold positive charges. These two Au atoms can act as Lewis acids57 to attract the O atoms in O2, which can stabilize the adsorption state and provide two sites for O2 activation. In C
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Figure 7. Optimized structure of Au7 cluster on the oxygen-saturated ZnO(101)-O surface. The Bader charges carried by Au atoms are shown at the right side. See Figure 3 for color coding.
Figure 5. Projected d-band partial density of states (pDOs) of Au atoms on the stoichiometric ZnO(101)-O surface. The d-band center is marked by green arrow. The Fermi level is set to 0.
Figure 6. Potential energy profile for the dissociation of O2 by Au7 on stoichiometric ZnO(101)-O surface.
state (Com-O2-4-mol), meaning the dissociation of O2 is a thermodynamically disadvantageous case. O2 Adsorption and Dissociation on the OxygenSaturated Au7/ZnO(101)-O System. Since the stoichiometric surface is easily oxidized in the air to generate an oxygensaturated surface, the effect of O atom fully covered surface need to be investigated. Different from the stoichiometric surface, the oxygen-saturated ZnO(101)-O surface induces less distortion to the Au7 cluster and the seven Au atoms are all neighboring the surface O atoms, as shown in Figure 7. More importantly, the workfunction of oxygen-saturated ZnO(101)-O is 6.59 eV (Figure S10), becoming larger than that of gold. This would drive the flow of electrons from gold to ZnO. As a result of the interfacial bonding, all of the seven Au atoms become positively charged. Overall, the Au7 cluster lost 2.15 e (charges on surface O atoms are given in Figure S13). Consequently, the molecular adsorptions of O2 on these seven Au atoms (Figure 8) are more stable than that of the stoichiometric Au7/ZnO(101)-O surface, and the number of active sites is increased from two to seven. It is interesting to find that the stability of the adsorption states relies on the polarized charges on different Au atoms. The positive charge carried by Au-7 (0.05 |e|) is much less than
Figure 8. Four optimized stable structures and binding energies of O2 at Au7 cluster supported on oxygen-saturated ZnO(101)-O surface. See Figure 3 for color coding.
Au-6 (0.30 |e|), making the electrostatic attraction of Au-7-OII weaker than that of Au-6-OII. As a result, the state Satu-O2-1mol has much higher binding energy than the Satu-O2-2-mol in Figure 8. As for the state Satu-O2-4-mol, although the charge carried by Au-3 and Au-4 (0.50 |e|) is more than that by Au-1 and Au-6 (0.30 |e|), the binding energy of Satu-O2-4-mol is 0.44 eV lower than that of Satu-O2-1-mol. This is because the Au-3 and Au-4 are both at the bridge site of surface O atoms and hence are much closer to the surface O than others. The O2 adsorbed on Au-3 and Au-4 is too close to the surface O atoms (distance less than 3.00 Å), in which negative charges would repel the O2 molecule. Thus, the polarized charge on ZnO surface also plays an important role in the adsorption capacity of O2. D
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trough sites are too stable to be activated for further oxidation reaction. Comparing the parameters of O2 activation on the stoichiometric and oxygen-saturated Au7/ZnO(101)-O surfaces in Table 1, the oxygen-saturated surface could donate more
Based on the most stable adsorption structure, we now turn to explore the dissociative mechanism of O2. We first confirmed that the positive charges on Au atoms effectively upshift their dband centers toward the Fermi level (Figure 9) to promote the catalytic activity. As illustrated in Figure 10, the energy barrier
Table 1. Most Stable Molecular and Dissociated Adsorption Energies, Reaction Energies, and Reaction Barriers of O2 on the Bare Stoichiometric ZnO(101)-O Surface (Per-ZnO(101)-O), Stoichiometric Au7/ZnO(101)-O Surface (Sto-Au7/ZnO(101)-O), and Oxygen-Saturated Au7/ZnO(101)-O Surface (Sato-Au7/ZnO(101)-O)a surface
molecular adsorption (eV)
dissociated adsorption (eV)
reaction energies (eV)
reaction barriers (eV)
Per-ZnO(101)-O Sto-Au7/ZnO(101)-O Sato-Au7/ZnO(101)-O
−3.35 −0.36 −1.18
−3.80 −0.31 −1.91
−0.45 0.05 −0.73
0.55 1.39 0.88
The reaction energy is defined as Edis − Emol, and negative value means exothermic process. a
positive charges to the Au7 cluster, which promotes the adsorption of O2 and makes its dissociation process more favorable both thermodynamically and kinetically. Thus, the presaturation of the ZnO(101) support is helpful for improving the activity of Au/ZnO(101) catalyst.
4. CONCLUSIONS Through a comprehensive DFT study of O2 activation by the Au7 cluster on ZnO(101)-O surface, we have examined the influence of the atomic and electronic structure of Au cluster and oxygen saturation of the support on the catalytic activity of Au/ZnO catalyst in O2 activation. It is found that O2 can be easily adsorbed and dissociated on the stoichiometric and oxygen defective ZnO(101)-O surface, resulting in stable oxygensaturated ZnO(101)-O surface. The fully coverage of O substantially increases the workfunction of ZnO(101)-O surface, inducing positive polarized charges on the Au7 cluster in the Au7/ZnO(101)-O hybrid. The positive charges on Au atoms not only substantially promote the adsorption of O2 molecule but also lower down the dissociation barrier of O2 by upshifting the Au d-band center. Therefore, one should let the support presaturated by O atoms before Au cluster deposition, as long as we synthesize an effective Au/ZnO(101) catalyst for O2 activation. More importantly, among various structural and electronic factors that could affect the catalytic performance but are unfortunately difficult to simultaneously evaluate, these findings pointed out that workfunction and polarized charges can be chosen as simple yet effective target parameters for the optimization of metal/semiconductor hybrid catalyst.
Figure 9. Projected d-band states of Au atoms on the oxygen-saturated ZnO(101)-O surface. The d-band center is marked by green arrow. The Fermi level is set to 0.
Figure 10. Potential energy profile for the dissociation of O2 by Au7 cluster on the oxygen-saturated ZnO(101)-O surface.
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for O2 dissociation on the Satu-O2-1-mol state is 0.88 eV. Compared to the Au7 cluster on stoichiometric ZnO(101)-O surface (energy barrier of 1.39 eV), the dissociation of O2 by the Au7 cluster on oxygen-saturated ZnO(101)-O surface is more favorable dynamically. Moreover, since the product (Satu-O2-1dis) is more stable than the reactant (Satu-O2-1-mol) (0.73 eV lower), the dissociation of O2 will be thermodynamically more favorable. From the above discussions, although the dissociation of O2 on the trough sites of the bare stoichiometric ZnO(101)-O surface is an exothermic process and has a very low energy barrier (as shown in Table 1), the dissociated O atoms on the
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09799. The reason for choosing the O-terminated ZnO(101) surface as the support for Au7 cluster, the potential energy profiles for O2 dissociation on the oxygen defective ZnO(101) surface, the less stable composite configurations of Au7/ZnO(101)-O, the potential surface and workfunction of ZnO(101)-O surface, and the computed stable adsorption structure (energy) of O2 on pure Au(111) surface (PDF) E
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86 551 63600029. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (NSFC 21303027, 21473166), the Natural Science Foundation of Guizhou Province (no. QKJ[2013]2254 and QKJ[2015]2129), the Program for Innovative Research Team of Guizhou Province of China (QKTD-[2012]4009), the Construction Project for Guizhou Provincial Key Laboratories (Z[2013]4009), and the GZNC startup package (no. 14BS022). We thank Prof. T. J. Xiao for helpful discussions.
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DOI: 10.1021/acs.jpcc.5b09799 J. Phys. Chem. C XXXX, XXX, XXX−XXX
