Article
pubs.acs.org/JPCC
Theoretical and Experimental Investigations on Single-Atom
Catalysis: Ir1/FeOx for CO Oxidation
Jin-Xia Liang,†,‡ Jian Lin,§ Xiao-Feng Yang,§ Ai-Qin Wang,§ Bo-Tao Qiao,§ Jingyue Liu,§,∥ Tao Zhang,*,§
and Jun Li*,‡
†
Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Normal College, Guiyang 550018, China
Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education,
Tsinghua University, Beijing 100084, China
§
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
∥
Department of Physics, Arizona State University, Tempe, Arizona 85287, United States
‡
ABSTRACT: Through periodic density functional theory (DFT) calculations we have investigated
the catalytic mechanism of CO oxidation on an Ir1/FeOx single-atom catalyst (SAC). The ratedetermining step in the catalytic cycle of CO oxidation is shown to be the formation of the second
CO2 between the adsorbed CO on the surface of Ir1/FeOx and the dissociated O atom from gas phase.
Comparing with Pt1/FeOx catalyst, the reaction activation barrier for CO oxidation is higher by 0.62
eV and the adsorption energy for CO molecule is larger by 0.69 eV on Ir1/FeOx. These results reveal
that Ir1/FeOx catalyst has a lower activity for CO oxidation than Pt1/FeOx, which is consistent with
our experimental results. The results can help to understand the fundamental mechanism of
monodispersed surface atoms and to design highly active single-atom catalysts.
1. INTRODUCTION
The concept of catalytic active site has been proposed by
Taylor more than 80 years ago1 and is now widely used in
biological enzyme2 as well as homogeneous3,4 and heterogeneous3,5−14 catalysis. In heterogeneous catalysis, one of the
important targets is to explore the relationship between the
catalytic behavior and the electronic structure of active sites,
thereby assisting in optimizing and designing new catalyst with
high activity and/or selectivity. Oxide-supported noble-metal
catalysts are extensively used as important catalysts in industry.
The sizes of metal nanoparticles7−10,13,15,16 and oxide
substrates15 are very important factors in determining the
activity of catalysts. As is well-known, the surface free energy of
metal species increases rapidly with dwindling the size of metal
particles, and especially it maximizes when the size is reduced
to single atoms.14 The size dwindling will facilitate the
activation of these metal species, thus generating more dangling
bonds and empty d atomic orbitals of metal species on the
surface. Accordingly, it is highly desirable for downsizing the
particles or clusters to single atoms in supported noble-metal
catalysts, which is the ultimate goal of heterogeneous catalysis.
However, stability of catalysts tends to decrease when the size
of metal particles is reduced. Recently, we have fabricated the
first practical single-atom catalyst (SAC) Pt1/FeOx,9 which
contains well-defined supported Pt single atoms on iron oxide
and exhibits high activity and significant stability for CO
oxidation. Our density functional theory (DFT) calculations
showed that the high catalytic activity of Pt1/FeOx correlates
with the partially vacant 5d orbitals of the positively charged,
© 2014 American Chemical Society
high-valent Pt atoms, which reduces the chance of CO
poisoning and facilitates the adsorption of oxygen. We also
prepared a Ir1/FeOx catalyst that shows remarkable performance in water-gas-shift (WGS) reaction.17 Since the finding of
Pt1/FeOx catalyst, significant progress has been made in the
research of single-atom catalysis.14,18−26
CO oxidation has been extensively investigated due to its
importance in the exhaust purification for motor vehicles,
environmental protection, gas purification for closed-cycle CO2
laser, and CO detectors.16,27 Selective oxidation of CO is also a
key step in fuel cell applications for removing CO from
reforming gas.28 Furthermore, CO oxidation is an elementary
step in the water-gas-shift (WGS) reaction.10,29 Accordingly,
CO oxidation is one of the most important model reactions in
catalysis science. In the past decades, a number of oxidesupported noble metals (Pt, Au, Ir, Pd, etc.)9,13,30−32 were
identified as active catalysts for CO oxidation. Comparing with
other 5d metals (such as Pt and Au), Ir has a higher melting
point and surface energy, leading to a good dispersion on the
surface and strong interactions of Ir particles with the
supports.5 Recently, highly active Ir−Fe/SiO2 catalysts have
been synthesized and characterized.33 The electronic states of
the Ir metal particles were modified by adding iron species,
resulting in a weaker CO adsorption on Ir with a red-shift of the
vibrational frequency of the adsorbed CO. Here the Fe2+ ion
Received: April 17, 2014
Revised: July 29, 2014
Published: September 4, 2014
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obtained by relaxing the force below 0.05 eV/Å via the dimer
method.41
2.2. Preparation of Ir1/FeOx and Pt1/FeOx SAC. The Ir1/
FeOx and Pt1/FeOx SAC were prepared by a coprecipitation
method as reported previously.10 Typically, an aqueous mixture
of H2IrCl6 or H2PtCl6 and Fe(NO3)3 was added dropwise to an
alkali solution with the pH controlled around 8. The resulting
precipitate was filtered and washed and then dried at 80 °C.
The Ir and Pt loading amounts were 0.22 and 0.17 wt %,
respectively, which were detected by ICP measurements. After
that, the as-synthesized samples were reduced in 10% H2/He at
200 °C for 0.5 h before further characterization and activity
test. The reduction temperature was lower than that reported
previously.16
2.3. HAADF-STEM Detection. High-angle annual darkfield scanning transmission electron microscopy (HAADFSTEM) images were obtained on a JEOL JEM-ARM200F
equipped with a CEOS probe corrector, with a guaranteed
resolution of 0.08 nm. Before microscopy examination, the Ir1/
FeOx and Pt1/FeOx samples were suspended in ethanol with an
ultrasonic dispersion for 5−10 min, and then a drop of the
resulting solution was dropped on a holey carbon film
supported by a copper TEM grid.
2.4. Catalytic Tests. The catalytic measurements of CO
oxidation in H2 rich stream were carried out in a fixed-bed
reactor. The feed gases were 1 vol % CO + 1 vol % O2 + 40 vol
% H2 balanced with He. The gas flow rate was 30 mL min−1,
which resulted in a space velocity of 18 000 mL gcat−1 h−1.
Before evaluation, the Ir1/FeOx and Pt1/FeOx samples were
reduced in a flow of 20 mL min−1 of 10 vol % H2/He at 200 °C
for 30 min. The concentration of CO in the effluent gas was
analyzed by an online gas chromatograph (Agilent 6890, TDX01 column) using He as carrier gas.
Specific reaction rates and TOFs of Ir1/FeOx and Pt1/FeOx
for CO oxidation with or without the presence of H2 at 80 °C
were obtained by keeping the CO conversion below 20%. The
TOFs were calculated based on the specific rates and the
dispersions of Ir1/FeOx and Pt1/FeOx catalysts, which were
measured by CO adsorption microcalorimetry at 40 °C with
the assumption of the stoichiometric ratio of adsorbed CO/
metal = 1.
becomes the active site for oxygen activation. For the highly
active Ir/Fe(OH)x catalyst,5 the support of Fe(OH)x or Fe3O4
played an important role in stabilizing the Ir particles with size
<2 nm.
To elucidate the nature of the binding of Ir1 single atoms to
FeOx support and the mechanism of CO oxidation on this Ir1/
FeOx SAC, we performed extensive theoretical investigations
using density functional theory (DFT) on the possible catalytic
reaction pathways of CO oxidation on the Ir1/FeOx SAC and
the electronic properties of the reactant, transition state, and
intermediate product. Bader charge analysis and the relative
local states of density (LDOS) were also calculated to evaluate
the performance of the Ir1/FeOx catalyst for CO oxidation.
Subsequently, the experiments were carried out to verify the
validity of the theoretical prediction. Based on these theoretical
and experimental results, a plausible catalytic mechanism has
been proposed. The diversity of the catalytic performances has
been discussed in detail for Ir1/FeOx and compared with the
previous Pt1/FeOx SAC.9
2. COMPUTATIONAL AND EXPERIMENTAL DETAILS
2.1. Computational Details. The (0001) surfaces of αFe2O3 were represented by a periodic slab model, constructed
using bulk cell dimensions: a = b = 5.04 Å and c = 13.72 Å.
Since α-Fe2O3 is antiferromagnetic and has atomic magnetic
moment on iron atoms, we used the primitive rhombohedral
unit cell of Fe2O3 with the magnetic configuration (+ − − +) to
build the surface slab, which was previously shown to be
energetically the most favored magnetic configuration for αFe2O3.34 A vacuum distance 12 Å width in the direction
perpendicular to the surface was set to eliminate the interaction
between layered nanostructures in the adjacent cells.
Considering the usually very large relaxations of the Fe2O3
surfaces,9,35 we chose the slabs containing 12 layers of Fe atoms
and 7 atomic layers of O3 (see below) to model the O3terminated surfaces.9 The 10 top-layer slabs of the surface were
allowed to relax while the other layers beneath the surface were
frozen during the geometry optimizations. On the basis of
previous studies,9,10 here the relatively favorable O3-terminated
surface of Fe2O3(001) has been selected to stabilize the Ir1
single atoms, where each Ir atom is coordinated by three
surface oxygen atoms (i.e., the O3 atoms), with the third-layer
Fe atoms below the Ir atoms. This location of Ir can be viewed
as the surface Fe atoms on the O3-terminated surface being
replaced by a single Ir1 atom.
The theoretical calculations were performed at the DFT level
using the Vienna Ab-initio Simulation Package (VASP).36,37
The core and valence electrons were represented by using the
projector augmented wave (PAW)38 method and plane-wave
basis functions with a kinetic energy cutoff of 400 eV.9
Inasmuch as Ir has significant relativistic effects, the mass−
velocity and Darwin relativistic effects were included through
the PAW potentials. The generalized gradient approximation
(GGA) with the Perdew−Burke−Ernzerhof (PBE)39 exchangecorrelation functional was used in the calculations. A
Monkhorst−Pack grid of size of 3 × 3 × 1 was used to sample
the surface Brillouin zone.9 Ground-state atomic geometries
were obtained by minimizing the forces on the atoms below
0.02 eV/Å. Because of the strong d-electron correlation effects
for Fe, the calculations were carried out with the DFT+U
method, using the formalism suggested by Liechtenstein and
Dudarev et al.40 The parameters were set at U = 4 eV and J = 1
eV according to previous study.9 The transition states were
3. RESULTS AND DISCUSSION
3.1. Catalytic CO Oxidation Cycle on Ir1/FeOx Catalyst.
3.1.1. Formation of the First CO2. The mechanism of catalytic
cycle of CO oxidation on the Ir1/FeOx SAC from the DFT
calculations is depicted in Figure 1 (top view), and the bond
lengths of these structures are summarized in Figure 2 (side
view). After prereduction by H2, the stoichiometric hematite
surface near the Ir atoms is reduced partially to form an oxygen
vacancy (step i), which facilitates the dissociative adsorption of
O2. In this model, an oxygen vacancy (Ovac) near the Ir atoms is
used to model the reduced FeOx surfaces, and the optimized
bond lengths of Ir−O (both Ir−OA1 and Ir−OA2, see Figure 1)
are 1.85 Å. In this case, the oxygen coordination number of Ir
changes from three to two. Here we first considered a
Langmuir−Hinshelwood (L−H) mechanism. In the dissociative adsorption of O2 molecule (step ii), one oxygen atom (OB)
adsorbs on the top of single Ir1 atom and the other (OC) heals
the Ovac. The OB−OC bond length is 3.18 Å, and the calculated
adsorption energy (3.85 eV) of dissociated O2 is higher than
the adsorbed O2 molecule (1.20 eV), indicating that adsorbed
oxygen molecule (O2, ad) is more easily dissociated on Ir1 single
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after the release of the first CO2 molecule from the Ir1 single
atoms, the remaining dissociated OC atom restores the Irsubstituted stoichiometric hemeatite surfaces (step iv).
Compared with the structure of the relatively high-valent Ir1
single atoms (Figure 1, step iii), we have also calculated the first
CO2 formation on the low-valent Ir single atoms, where the Ir
atom is connected to two surface oxygen atoms as shown in
Figure 3. Here the presumably high-valent and low-valent Ir
Figure 3. Proposed reaction pathway for the formation of the first
CO2 on the low-valent Ir single atoms catalyst. See caption of Figure 1.
atoms differ by the number of their coordinated O atoms,
although their calculated Bader charges are 1.57 and 1.54 |e|,
respectively. It turned out that the low-valent catalyst exhibits a
slightly higher reaction activation barrier (0.65 eV) than the
high-valent catalyst (0.59 eV) for the reaction of COad + OB →
CO2. It is therefore likely that the CO oxidation proceeds via
the high-valent Ir1 single atoms.
Considering the vertical adsorption of the dissociated Oad
atom on the single Ir1 atom, a Eley−Rideal (E−R) mechanism
cannot be naively excluded in the formation of the first CO2. In
the E−R mechanism, the CO molecules in gas phase directly
react with the dissociated Oad (OB) atom on the Ir1 single
atoms. The calculated reaction pathway for the formation of the
first CO2 molecule, including initial reactant (R), transition
state (TS), and final production (P), are displayed in Figure 4.
Figure 1. Proposed reaction pathway for CO oxidation on the Ir1/
FeOx catalyst (top view): OA1 (red) surface atom on the left of single
Ir, OA2 (red) surface atom on the right of single Ir, OB atom (dark
green) above the adsorption O2, OC atom (dark green) in the bottom
of the adsorption O2, OD atom (light green) in CO, C atom (pink) in
CO, and Ir atom (blue).
Figure 4. Proposed reaction pathway for the formation of the first
CO2 on the surface of the Ir1/FeOx catalyst via E−R mechanism. See
caption of Figure 1.
For initial reactant (R), the distance of carbon atom of
adsorbed CO and the vertically adsorbed OB atom from
dissociated O2 (C−OB) is 3.59 Å, indicating the weak
interaction between CO molecule and the adsorption of the
dissociated OB atom. In the transition state (TS), the distance
of carbon atom of adsorbed CO and the dissociated OB atom
decreases to 1.61 Å, and the bond length of the Ir atom and the
dissociated OB atom (Ir−OB) increases from 1.72 Å (as shown
in reactant) to 1.83 Å, suggesting formation of the CO2
molecule. After passing over the TS, a CO2 molecule is
generated completely and desorbed from the surface.
Comparing with the calculated activation energy barrier (0.65
eV) for reaction of CO + OB → CO2 in the L−H mechanism,
the barrier (1.01 eV) is much higher in the case of the E−R
mechanism. Therefore, the formation of the first CO2 is most
likely through the L−H mechanism.
3.1.2. Formation of the Second CO2. As seen from Figures
1 and 2, when the second CO molecule is adsorbed on Ir1
Figure 2. Proposed reaction pathway for CO oxidation on the Ir1/
FeOx catalyst (side view). See caption of Figure 1.
atoms and Ovac in Ir1/FeOx. Then CO and the dissociated OB
atom are coadsorbed on the single Ir1 atoms (step iii). The
binding energy of the CO adsorption on the Ir1 single atoms is
0.66 eV (step iii), which is much lower than that on the
calculated Ir8 model cluster. The OB atom starts to approach
the carbon atom of the adsorbed CO to reach the first
transition state, resulting in the distance of carbon atom of
adsorbed CO and the dissociated OB atom decreasing from
2.49 to 1.71 Å. The calculated activation barrier for the reaction
of COad + OB → CO2 (TS-1) is 0.59 eV. Passing on the TS-1,
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Table 1. Relative Binding Energies (eV) in the Proposed Reaction Pathway for CO Oxidation on Ir1/FeOx and Pt1/FeOx
Catalysts
a
SAC
i
ii
iii
TS-1
iv
v
Pta
Ir
0.0
0.0
−1.05
−3.85
−2.32
−4.51
−1.83
−3.92
−4.30
−5.17
−5.38
−6.96
TS-2
vi
TS-3
vi-final
−6.02
−6.29
−6.56
−5.50
−5.15
−6.49
−6.56
Reference 9.
Third, the more distinguished difference is the barrier
energies of the rate-determining step in the formation of the
second CO2. The calculated barrier of COad + OC → CO2 is
much higher on Ir1/FeOx (1.41 eV) than that on Pt1/FeOx
(0.79 eV), suggesting the lower activity for CO oxidation with
the former catalyst.
The calculated results show that the energy of the oxygen
vacancy (Ovac) formation is higher on the Ir-terminated surface
(∼2.07 eV) than that on the Pt-terminated surface (∼1.06 eV),
but both the energies are lower than the most stable Feterminated surface of the Fe2O3 support (∼2.99 eV). Indeed,
previous temperature-programmed reduction results also
showed that lower temperature was sufficient for the reduction
of Fe2O3 after depositing Ir1 and Pt1 single atoms, with Ir1/
FeOx requiring a relatively higher reduction temperature of 200
°C than the Pt1/FeOx (160 °C).9,17 Clearly, the presence of
individual Pt and Ir atoms can greatly improve the reducibility
of the Fe2O3 support, whereas Pt1 single atoms have a stronger
ability to facilitate the reduction of the Fe2O3 support than the
Ir1 single atoms. This result is consistent with the fact that the
energy barrier (1.41 eV) for the formation of the second CO2
(TS-2) is relatively higher for Ir1/FeOx than that (0.79 eV) for
Pt1/FeOx (Table 1).
To further understand the lower catalytic activity of Ir1/FeOx
for CO oxidation than that of Pt1/FeOx, and compare with the
difference of single atom Ir1 in the Fe2O3 support and the Ir
metal cluster, we considered two different Fe2O3 surfaces with
oxygen vacancy (Fe2O3−Ovac) and without oxygen vacancy
(Fe2O3) at Ir1 and Pt1 single atoms. Calculations were done on
the Bader charges, CO adsorption energies, and vibration
frequencies for CO adsorption on Ir1 and Pt1 single atoms on
the surfaces of Fe2O3−Ovac, Fe2O3, and a selected Ir8 model
cluster (Table 2). The relative local states of density (LDOS) of
Pt-5d (Ir-5d), C-2p, and O-2p (adsorbed CO) orbitals are
presented in Figure 5.
As seen from Table 2, the data of Pt1/Fe2O3-Ovac are very
close to the previous calculation results of *Pt1/Fe2O3-Ovac,
indicating the reasonable computational parameters in these
properties. The Bader charge analysis shows that the positive
charge carried by Ir atoms is more than that of Pt atoms either
in the case of Fe2O3-Ovac or Ovac-free Fe2O3 as the support.
single atom (step v), the CO molecule gradually comes close to
the OC atom on the Ovac, which forms a transition state with a
small imaginary vibration frequency (93i cm−1, TS-2) for the
CO adsorbed on Ir1 single atom. After the TS-2, CO molecule
is already near the OC atom (step vi), leading to a new reaction
of COad + OC → CO2 with an energy barrier of 1.41 eV (TS-3).
After releasing the second CO2, the Ir-embedded stoichiometric
surface is reduced again to form a new oxygen vacancy (step i).
This multistep procedure has thus completed the catalytic
cycle. The second CO oxidation reaction of COad + OC → CO2
also follows a L−H mechanism. The high activation barrier
(1.41 eV) on the Ir1 single atoms indicates the formation of TS3 is the rate-determining step in the whole catalytic cycle. This
activation barrier is remarkably higher than that of CO
oxidation at 300 K on bulk Ir(210) surface,42 indicating that
the formation of CO2 is not easy at low temperatures.
3.2. Activities of Ir1/FeOx and Pt1/FeOx Catalysts for
CO Oxidation. In our previous work,9 we investigated the CO
oxidation cycle on Pt1/FeOx catalyst using DFT calculations
and found that the formation of CO2 was facile from adsorbed
CO on Pt1 single atoms and adsorbed O on oxygen vacancy of
FeOx. However, comparing with the Pt1/FeOx catalyst, our
experiment shows that the Ir1/FeOx catalyst has lower catalytic
activity for CO oxidation at low temperatures (as shown in
Figure 6, Figure 7, and Table 3). To gain an insight into the
factors affecting the catalytic activities of Pt1/FeOx and Ir1/
FeOx, we compared the binding energies and the properties of
electronic structures in the proposed reaction pathway for CO
oxidation on these catalysts. There are several differences
between the Ir1/FeOx and Pt1/FeOx SACs for CO oxidation, as
shown in Figure 1 and Table 1. First, the adsorption states of
O2 on the Ir1 and Pt1 single atoms in Ir1/FeOx and Pt1/FeOx
SAC (step ii) are different. Our DFT calculations indicate that
O2 molecule readily dissociates on the single atom Ir1 and Ovac
of Ir1/FeOx catalyst, which is similar to that on the Ir(111)
surface.43 The energy of dissociative adsorption of O2 is 3.85 eV
on the surface of Ir1/FeOx catalyst, which is significantly higher
than the adsorption energy of O2 molecule on the surface of
Pt1/FeOx catalyst.
Second, there are difference in the structure and the energy
barrier of the first CO2 formation on Ir1 and Pt1 single atoms.
The distance of the dissociated OB atom vertically adsorbed on
the Ir atom and the other dissociated OC atom at the Ovac atom
is 3.46 Å, indicating that they interact weakly with each other
and the CO molecule has a relatively large degree of freedom
on the Ir1 single atoms. Compared to the structure of the step
iii of the Pt1/FeOx catalyst,9 the bond length of the Ir atom and
the dissociated OB atom (1.78 Å) is much shorter than the
distance (2.06 Å) of the Pt atom and the adjacent O atom of
adsorbed O2 molecule, showing that the formation of strong
Ir−O bond between the Ir atom and the dissociated OB atom.
Our calculations predict that Ir1/FeOx requires a slightly higher
activation energy (0.59 eV) than Pt1/FeOx (0.49 eV) in the
formation of the first CO2 (TS-1).
Table 2. Bader Charges, CO Adsorption Energies (Ead-CO),
and Vibration Frequencies for CO Adsorption (νC−O) at Ir1
and Pt1 Single Atoms on the Surfaces of Fe2O3 with Oxygen
Vacancy (Fe2O3-Ovac), the Surfaces of Fe2O3 without Oxygen
Vacancy, and Free Ir8 Cluster
charge (|e|)
νC−O (cm−1)
Ead-CO (eV)
a
21948
Ir1/
Fe2O3Ovac
Ir1/
Fe2O3
0.71
2074
−1.86
1.27
2037
−1.77
Ir8
Pt1/
Fe2O3Ovaca
Pt1/
Fe2O3Ovac
Pt1/
Fe2O3
1949
−2.60
0.45
2062
−1.96
0.53
2060
−1.92
1.04
2048
−1.08
Reference 9.
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Figure 5. Spin-polarized LDOSs projected on Pt-5d (Ir-5d), C-2p, and O-2p (adsorbed CO) orbitals. The Fermi level is set at zero.
These data indicate that there is more electron transfer from Ir
atoms than Pt atoms to the FeOx support either with or
without oxygen vacancy. The calculated adsorption energies of
CO are 1.86 and 1.77 eV on the Ir1 single atoms, respectively,
which while they are 1.92 and 1.08 eV on the Pt1 atom when
embedded in the Fe2O3-Ovac and Fe2O3 supports. Especially,
the calculated CO adsorption energy for the Ir1 single atoms of
Ir1/Fe2O3 is higher by 0.69 eV than that for the Pt1 single
atoms of Pt1/Fe2O3 (Table 2), indicating that there is a
stronger interaction between Ir atom and the CO molecule
adsorbed. This result agrees well with the spin-polarized
LDOSs in Figure 5 and shows that below the Fermi level (EF)
Ir 5d states mix better with C 2p and O 2p states than Pt 5d
states do.
The stretching vibration frequencies of CO adsorbed on Ir1
single atoms of the Ir1/Fe2O3-Ovac and the Ir1/Fe2O3 were
calculated as 2074 and 2037 cm−1, respectively, which are blueshifted by 125 and 88 cm−1 relative to the frequency of CO
adsorbed on the free Ir8 cluster, respectively. The calculated
adsorption energies of CO are 1.86 and 1.77 eV on the Ir1
single atoms, respectively, which are much lower than that on
the free Ir8 clusters (2.60 eV). All these results suggest that
single Ir1 atoms on the support are significantly different from
that of Ir atoms in metal clusters or metallic surface. All the
results suggest that Ir1 single atoms embedded into Fe2O3
surface have a higher oxidation state than that in nanoparticles
due to coordination by the three surface O atoms.
3.3. Experimental Results of CO Oxidation on Ir1/FeOx.
DFT calculation results predict the different performance of
CO oxidation on these two SACs, in which Ir1/FeOx exhibits
lower activity than Pt1/FeOx. This difference is verified by our
experimental results. The calculation model of Ir1/FeOx is
selected to be consistent with the HAADF images in Figure 6,
which clearly shows exclusively Ir1 single atoms on FeOx and
the individual Ir atoms occupies exactly a position of the Fe
atoms. Our experimental results reveal that with reduction
temperature of 200 °C the catalyst with Ir loading amount of
0.22 wt % shows homogeneously dispersed Ir1 single atoms for
this Ir1/FeOx catalyst. The synthesized Ir and Pt SACs virtually
have similar metal loading amounts.
It is interesting to evaluate the CO oxidation activities of
these two SACs. As shown in Figure 7, the CO conversions on
Ir1/FeOx are significantly lower than those on Pt1/FeOx. On
Ir1/FeOx the CO conversion is only around 30% even at 120
°C. In contrast, the CO conversion is nearly 100% at 80 °C on
Pt1/FeOx although the CO conversion decreases with the
Figure 6. HAADF-STEM images of Ir1/FeOx sample. In these images,
the Ir1 single atoms were uniformly dispersed on FeOx and exactly
occupied the positions of Fe atoms.
Figure 7. Conversion of CO as a function of the reaction temperature
on Ir1/FeOx and Pt1/FeOx SAC. Reaction conditions: 1 vol % CO, 1
vol % O2, 40 vol % H2, and balance He. Weight hourly space velocity
(WHSV): 18 000 mL h−1 gcat−1.
temperature becoming higher than 80 °C over the Pt1/FeOx
catalyst due to the competitive reaction between CO and H2
with O2. To show their different activities, we further measured
the specific rates and turnover frequencies (TOFs) at 80 °C
under differential conditions (the conversions of CO were
maintained below 20%). As shown in Table 3, either the
specific rate (0.418 molco h−1 gIr−1) or the TOF (0.0256 s−1) of
Ir1/FeOx is much lower than those on Pt1/FeOx (0.992 molco
h−1 gIr−1, 0.311 s−1). The activities for CO oxidation without H2
are also much lower on Ir1/FeOx than on Pt1/FeOx. All these
results indicate that the Ir1 single atoms are much less active
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for High Resolution Electron Microscopy at Arizona State
University was gratefully acknowledged. The calculations were
performed by using supercomputers at Tsinghua National
Laboratory for Information Science and Technology and the
Shanghai Supercomputing Center.
Table 3. Specific Rates and TOFs of Ir1 and Pt1 Single Atoms
at 80 °C
samples
metal
loadings
(wt %)
reactions
specific rates × 102
(molco h−1 gmetal−1)
TOF × 102
(s−1)
Ir1/FeOx
Pt1/FeOx
Ir1/FeOx
Pt1/FeOx
0.22
0.17
0.22
0.17
CO oxidation
CO oxidation
PROX
PROX
28.5
52.5
41.8
99.2
1.75
16.5
2.56
31.1
■
(1) Taylor, H. S. A Theory of Catalytic Surface. Proc. R. Soc. London,
A 2002, 108, 105−111.
(2) Woggon, W.-D. Metalloporphyrines as Active Site Analogues Lessons from Enzymes and Enzyme Models. Acc. Chem. Res. 2005, 38,
127−136.
(3) Ciardelli, F.; Altomare, A.; Michelotti, M. From Homogeneous to
Supported Metallocene Catalysts. Catal. Today 1998, 41, 149−157.
(4) Comas-Vives, A.; González-Arellano, C.; Corma, A.; Iglesias, M.;
Sánchez, F.; Ujaque, G. Single-Site Homogeneous and Heterogeneized
Gold(III) Hydrogenation Catalysts: Mechanistic Implications. J. Am.
Chem. Soc. 2006, 128, 4756−4765.
(5) Lin, J.; Qiao, B.; Liu, J.; Huang, Y.; Wang, A.; Li, L.; Zhang, W.;
Allard, L. F.; Wang, X.; Zhang, T. Design of a Highly Active Ir/
Fe(OH)x Catalyst: Versatile Application of Pt-Group Metals for the
Preferential Oxidation of Carbon Monoxide. Angew. Chem., Int. Ed.
2012, 51, 2920−2924.
(6) Liu, K.; Wang, A.; Zhang, W.; Wang, J.; Huang, Y.; Wang, X.;
Shen, J.; Zhang, T. Microkinetic Study of CO Oxidation and PROX on
Ir−Fe Catalyst. Ind. Eng. Chem. Res. 2010, 50, 758−766.
(7) Huang, Y.; Wang, A.; Li, L.; Wang, X.; Su, D.; Zhang, T. “Ir-inCeria”: A Highly Selective Catalyst for Preferential CO Oxidation. J.
Catal. 2008, 255, 144−152.
(8) Gómez-Cortćs, A.; Díaz, G.; Zanella, R.; Ramírez, H.; Santiago,
P.; Saniger, J. M. Au−Ir/TiO2 Prepared by Deposition Precipitation
with Urea: Improved Activity and Stability in CO Oxidation. J. Phys.
Chem. C 2009, 113, 9710−9720.
(9) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu,
J.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/
FeOx. Nat. Chem. 2011, 3, 634−641.
(10) Wang, L.; Zhang, S.; Zhu, Y.; Patlolla, A.; Shan, J.; Yoshida, H.;
Takeda, S.; Frenkel, A. I.; Tao, F. Catalysis and In Situ Studies of Rh1/
Co3O4 Nanorods in Reduction of NO with H2. ACS Catal. 2013, 3,
1011−1019.
(11) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.;
Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos,
M.; Sykes, E. C. H. Isolated Metal Atom Geometries as a Strategy for
Selective Heterogeneous Hydrogenations. Science 2012, 335, 1209−
1212.
(12) Fu, Q.; Luo, Y. Catalytic Activity of Single Transition-Metal
Atom Doped in Cu(111) Surface for Heterogeneous Hydrogenation. J.
Phys. Chem. C 2013, 117, 14618−14624.
(13) Wang, Y.-G.; Yoon, Y.; Glezakou, V.-A.; Li, J.; Rousseau, R. The
Role of Reducible Oxide−Metal Cluster Charge Transfer in Catalytic
Processes: New Insights on the Catalytic Mechanism of CO Oxidation
on Au/TiO2 from ab Initio Molecular Dynamics. J. Am. Chem. Soc.
2013, 135, 10673−10683.
(14) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. SingleAtom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc.
Chem. Res. 2013, 46, 1740−1748.
(15) Xu, B.-Q.; Wei, J.-M.; Yu, Y.-T.; Li, Y.; Li, J.-L.; Zhu, Q.-M. Size
Limit of Support Particles in an Oxide-Supported Metal Catalyst:
Nanocomposite Ni/ZrO2 for Utilization of Natural Gas. J. Phys. Chem.
B 2003, 107, 5203−5207.
(16) Haruta, M. Size- and Support-Dependency in the Catalysis of
Gold. Catal. Today 1997, 36, 153−166.
(17) Lin, J.; Wang, A.; Qiao, B.; Liu, X.; Yang, X.; Wang, X.; Liang, J.;
Li, J.; Liu, J.; Zhang, T. Remarkable Performance of Ir1/FeOx SingleAtom Catalyst in Water Gas Shift Reaction. J. Am. Chem. Soc. 2013,
135, 15314−15317.
than Pt1 single atoms for CO oxidation, which is in good
agreement with the DFT results.
One of the key points to achieve a highly effective catalyst for
CO oxidation is to decrease the adsorption strength of CO
because the strong CO adsorption on metal sites will greatly
prohibit the adsorption and activation of O2. A weaker
adsorption strength of CO at a lower temperature is more
favorable. In this work, the calculation results indicate that the
adsorption energy of CO on the Ir1 single atoms is higher than
that on the Pt1 single atoms, and the energy barrier for the
second CO2 formation on Ir1/FeOx is also higher than that on
the Pt1/FeOx catalyst. Accordingly, Ir1/FeOx is less active than
Pt1/FeOx for CO oxidation at lower temperatures. Our
experimental results have confirmed the difference in the CO
oxidation activity between these two types of SACs.
4. CONCLUSIONS
Theoretical calculations and experiments have been performed
to explore the catalytic mechanism and activity of the new Ir1/
FeOx single-atom catalyst for CO oxidation. We proposed a
plausible catalytic mechanism of CO oxidation on the Ir1/FeOx
catalyst using the DFT calculations. The calculation results
show that the formation energy of the second CO2 molecule is
much higher (1.41 eV) for Ir1/FeOx than for Pt1/FeOx (0.79
eV). The Ir1/FeOx catalyst has a higher energy barrier than Pt1/
FeOx for CO oxidation, which accounts for the lower activity of
the former. Our experimental results further confirm the
calculated results. The stabilization of the Ir1 single atoms on
the oxide support via a charge-transfer mechanism is similar to
those in Pt1/FeOx.9 The present theoretical investigation helps
to understand the differences in the specific rates and TOFs
between Ir1/FeOx and Pt1/FeOx for CO oxidation. The results
provide insight for design of new and active single-atom
catalysts.
■
REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (T.Z.).
*E-mail [email protected] (J.L.).
Author Contributions
J.-X.L. and J.L. contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
of China (Nos. 21203181, 21203182, and 21373206), the
Ch i n a P o s t d o c t o r a l S c i e n c e F o u n d a t i o n ( G r a n t
2013M540928), and NKBRSF (2011CB932400 and
2013CB834603). J. Liu acknowledges the start-up fund of the
College of Liberal Arts and Sciences of Arizona State
University. The use of facilities in the John M. Cowley Center
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The Journal of Physical Chemistry C
Article
(18) Huang, Z.; Gu, X.; Cao, Q.; Hu, P.; Hao, J.; Li, J.; Tang, X.
Catalytically Active Single-Atom Sites Fabricated from Silver Particles.
Angew. Chem. 2012, 124, 4274−4279.
(19) Kwak, J. H.; Kovarik, L.; Szanyi, J. Heterogeneous Catalysis on
Atomically Dispersed Supported Metals: CO2 Reduction on Multifunctional Pd Catalysts. ACS Catal. 2013, 3, 2094−2100.
(20) Chu, M.-W.; Chen, C. H. Chemical Mapping and Quantification
at the Atomic Scale by Scanning Transmission Electron Microscopy.
ACS Nano 2013, 7, 4700−4707.
(21) Li, L.; Wang, A.; Qiao, B.; Lin, J.; Huang, Y.; Wang, X.; Zhang,
T. Origin of the High Activity of Au/FeOx for Low-Temperature CO
oxidation: Direct Evidence for a Redox Mechanism. J. Catal. 2013,
299, 90−100.
(22) Sun, S.; Zhang, G.; Gauquelin, N.; Chen, N.; Zhou, J.; Yang, S.;
Chen, W.; Meng, X.; Geng, D.; Banis, M. N.; Li, R.; Ye, S.; Knights, S.;
Botton, G. A.; Sham, T.-K.; Sun, X. Single-Atom Catalysis Using Pt/
Graphene Achieved through Atomic Layer Deposition. Sci. Rep. 2013,
3.
(23) Moses-DeBusk, M.; Yoon, M.; Allard, L. F.; Mullins, D. R.; Wu,
Z.; Yang, X.; Veith, G.; Stocks, G. M.; Narula, C. K. CO Oxidation on
Supported Single Pt Atoms: Experimental and ab Initio Density
Functional Studies of CO Interaction with Pt Atom on θ-Al2O3(010)
Surface. J. Am. Chem. Soc. 2013, 135, 12634−12645.
(24) Guo, Z.; Liu, B.; Zhang, Q.; Deng, W.; Wang, Y.; Yang, Y.
Recent Advances in Heterogeneous Selective Oxidation Catalysis for
Sustainable Chemistry. Chem. Soc. Rev. 2014.
(25) Xing, J.; Chen, J. F.; Li, Y. H.; Yuan, W. T.; Zhou, Y.; Zheng, L.
R.; Wang, H. F.; Hu, P.; Wang, Y.; Zhao, H. J.; Wang, Y.; Yang, H. G.
Stable Isolated Metal Atoms as Active Sites for Photocatalytic
Hydrogen Evolution. Chem.Eur. J. 2014, 20, 2138−2144.
(26) Flytzani-Stephanopoulos, M. Gold Atoms Stabilized on Various
Supports Catalyze the Water−Gas Shift Reaction. Acc. Chem. Res.
2013, 47, 783−792.
(27) Gardner, S. D.; Hoflund, G. B.; Upchurch, B. T.; Schryer, D. R.;
Kielin, E. J.; Schryer, J. Comparison of the Performance Characteristics
of Pt/SnOx and Au/MnOx Catalysts for Low-Temperature CO
Oxidation. J. Catal. 1991, 129, 114−120.
(28) Song, C. Fuel Processing for Low-Temperature and HighTemperature Fuel Cells: Challenges, and Opportunities for Sustainable Development in the 21st Century. Catal. Today 2002, 77, 17−49.
(29) Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M. On the
Mechanism of Low-Temperature Water Gas Shift Reaction on
Copper. J. Am. Chem. Soc. 2008, 130, 1402−1414.
(30) Abbet, S.; Heiz, U.; Häkkinen, H.; Landman, U. CO Oxidation
on a Single Pd Atom Supported on Magnesia. Phys. Rev. Lett. 2001, 86,
5950−5953.
(31) Okumura, M.; Masuyama, N.; Konishi, E.; Ichikawa, S.; Akita, T.
CO Oxidation below Room Temperature over Ir/TiO2 Catalyst
Prepared by Deposition Precipitation Method. J. Catal. 2002, 208,
485−489.
(32) Wang, Y.-G.; Mei, D.; Li, J.; Rousseau, R. A DFT+U Study on
the Localized Electronic States and Their Potential Role During H2O
Dissociation and CO Oxidation Processes on CeO2(111) Surface. J.
Phys. Chem. C 2013.
(33) Liu, K.; Wang, A.; Zhang, W.; Wang, J.; Huang, Y.; Shen, J.;
Zhang, T. Quasi In Situ 57Fe Mössbauer Spectroscopic Study:
Quantitative Correlation between Fe2+ and H2 Concentration for
PROX over Ir−Fe/SiO2 Catalyst. J. Phys. Chem. C 2010, 114, 8533−
8541.
(34) Sandratskii, L. M.; Uhl, M.; Kübler, J. Band Theory for
Electronic and Magnetic Properties of a-Fe2O3. J. Phys.: Condens.
Matter 1996, 8, 983−989.
(35) Wang, X. G.; Weiss, W.; Shaikhutdinov, S. K.; Ritter, M.;
Petersen, M.; Wagner, F.; Schlögl, R.; Scheffler, M. The Hematite (αFe2O3) (0001) Surface: Evidence for Domains of Distinct Chemistry.
Phys. Rev. Lett. 1998, 81, 1038−1041.
(36) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid
Metals. Phys. Rev. B 1993, 47, 558−561.
(37) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the
Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758−
1775.
(38) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B
1994, 50, 17953−17979.
(39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient
Approximation Made Simple. Phys. Rev. Lett. 1997, 77, 3865−3868.
(40) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Szotek, Z.;
Temmerman, W. M.; Sutton, A. P. Electronic Structure and Elastic
Properties of Strongly Correlated Metal Oxides from First Principles:
LSDA + U, SIC-LSDA and EELS Study of UO2 and NiO. Phys. Status
Solidi A 1998, 166, 429−443.
(41) Henkelman, G.; Jonsson, H. A Dimer Method for Finding
Saddle Points on High Dimensional Potential Surfaces Using Only
First Derivatives. J. Chem. Phys. 1999, 111, 7010−7022.
(42) Chen, W.; Ermanoski, I.; Jacob, T.; Madey, T. E. Structure
Sensitivity in the Oxidation of CO on Ir Surfaces. Langmuir 2006, 22,
3166−3173.
(43) Xu, Y.; Mavrikakis, M. Adsorption and dissociation of O2 on
Ir(111). J. Chem. Phys. 2002, 116, 10846−10853.
21951
dx.doi.org/10.1021/jp503769d | J. Phys. Chem. C 2014, 118, 21945−21951