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.
© 2016 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
Published: February 9, 2016
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DOI: 10.1021/acs.jpcc.5b09799
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The Journal of Physical Chemistry C
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
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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
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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.
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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.
■
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)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: +86 551 63600029.
Notes
The authors declare no competing financial interest.
■
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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