The Enhancement of the Catalytic Oxidation of CO on Ir/CeO2 Nanojunctions

Article abstract of DOI:10.1021/acs.inorgchem.9b02356

Heterogeneous catalysts facilitate various chemical reactions through the changing of their surface charge density. Herein, we demonstrate an interface engineering strategy to enhance the Ir catalytic performance for oxidation of CO by constructing Ir/CeO₂ nanojunctions using a wet chemical reduction process. The as-prepared Ir/CeO₂ nanojunctions show a complete CO conversion temperature (T₁₀₀) of 110 °C under 1 vol % CO and retains long-term durability even after 24 h. The Ir atoms and CeO₂ supports were connected by O atoms, which changes the surface electronic structure of Ir atoms. DFT calculations demonstrate that the significant increase in electron density at the interface between Ir and CeO₂ plays a pivotal role in the enhancement of CO oxidation.

Full text of DOI:10.1021/acs.inorgchem.9b02356

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
The Enhancement of the Catalytic Oxidation of CO on Ir/CeO2
Nanojunctions
Cichang Zong,†,‡,∥ Changlai Wang,§,∥ Lin Hu,‡ Ruirui Zhang,§ Peng Jiang,§ Jing Chen,§
Lingzhi Wei,† and Qianwang Chen*,‡,§
†Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China
‡Anhui Province Key Laboratory of Condensed Matter Physics at Extreme Conditions, High Magnetic Field Laboratory of the
Chinese Academy of Sciences, Hefei 230031, China
§Hefei National Laboratory for Physical Science at Microscale and Department of Materials Science & Engineering, University of
Science and Technology of China, Hefei 230026, China
S
* Supporting Information
ABSTRACT: Heterogeneous catalysts facilitate various chemical reac-
tions through the changing of their surface charge density. Herein, we
demonstrate an interface engineering strategy to enhance the Ir
catalytic performance for oxidation of CO by constructing Ir/CeO2
nanojunctions using a wet chemical reduction process. The as-prepared
Ir/CeO2 nanojunctions show a complete CO conversion temperature
(T100) of 110 °C under 1 vol % CO and retains long-term durability
even after 24 h. The Ir atoms and CeO2 supports were connected by
O atoms, which changes the surface electronic structure of Ir atoms. DFT calculations demonstrate that the significant increase
in electron density at the interface between Ir and CeO2 plays a pivotal role in the enhancement of CO oxidation.
INTRODUCTION
Scheme 1. Schematic Illustration of the Synthesis Procedure
of the Ir/CeO2 Interfacial Structure
■
a
Fossil energy has been widely used in the 21st century due to
rapid developments of science and technology.1 Limited by
current technology, there always exist small amounts of CO
(0.3−1%) in the product of fossil energy after combustion.2 Due
to the damage it causes to human health, the residual CO must
be absolutely removed through the effect of the catalyst. In this
regard, CO oxidation aiming to reduce the shift temperature and
improve the stability of the catalyst in the reaction is one of the
most important directions in current research.3
According to a previous study, a noble metal is usually applied
in the catalytic reaction because of its high activity and good
stability.4 However, some noble metals such as Ir do not show
good performance in CO oxidation. Ir has a higher surface
energy than other metals with a 5f orbital, such as Au and Pt, and
Ir has good dispersing ability5 and interaction with the support.6
However, compared to Pt- and Au-based catalysts, an Ir-based
catalyst is rarely reported in CO oxidation,7 most probably
because of its poor absorption ability for CO. Therefore, the
construction of a nanojunction structure which could make Ir
combine with CO more strongly is a way to improve the
performance of Ir in CO oxidation.
For multicomponent catalytic materials, the interface between
two components in hybrid structures is an important parameter
for determining the catalytic performance. For example, Ha et al.
built a model of Au nanoparticles supported on CeO2 nano-
crystals and found an enhancement of the interface on CO oxida-
tion.8 Somorjai’s group demonstrated that the CO oxidation rate
at the interface of mesoporous oxides and Pt nanoparticles could
a
(a) CeO2 nanoparticles, (b) Ir-incorporated CeO2, and (c) CO
oxidation at the interface between Ir and CeO2.
be enhanced compared with pure Pt nanoparticles and pure
mesoporous oxides.9 Recently, Chen et al. presented a Pt−
FeNi(OH)x interfacial structure as a new promising catalyst
system for CO oxidation.10 Liu et al. characterized oxygen
vacancy during the CO oxidation process on ZnO-supported
gold catalysts.11 According to the result of our DFT calculations
in this paper, the formation of an interface between Ir and the
metal-oxide changes the electronic structure of the atoms. The
Received: August 8, 2019
© XXXX American Chemical Society
A
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Figure 1. (a) TEM image of CeO2 before reduction; (b) TEM image of Ir/CeO2 after reduction; and (c) HRTEM images of Ir/CeO2 after reduction.
Figure 2. Scanning TEM image of Ir/CeO2 nanocomposites and energy dispersive X-ray spectrometry elemental maps of Ce, Ir, and O, respectively.
electrons transferred from the interface to the antibonding
orbital of the CO molecule, which decreased the bond order of
the CO molecule and stretched the C−O bond length, thus
making the CO oxidation happen more easily.
Inspired by this, we propose here a strategy to construct an
Ir/CeO2 interface using a simple reduction process. CeO2 is a
good transmission channel for oxygen and provides O atoms for
CO oxidation. During the process of reduction in material
synthesis, the iridium ion was reverted on the surface of CeO2.
The iridium clusters distributed on the surface greatly increase
the contact area between Ce and Ir, which is beneficial to CO
oxidation.
RESULTS AND DISCUSSION
■
Here, we developed a mild method to synthesize iridium highly
dispersed Ir/CeO2 nanoparticles at room temperature. Scheme 1
Figure 3. XRD patterns of the samples before and after reduction.
shows the way to prepare the Ir/CeO2 nanoparticles.
Figure 4. (a) Nitrogen adsorption isotherm at 77 K for CeO2. Inset: pore size distribution calculated using the BJH equation from the N2 adsorption for CeO2.
(b) Nitrogen adsorption isotherm at 77 K for Ir/CeO2. Inset: pore size distribution calculated using the BJH equation from the N2 adsorption for Ir/CeO2.
B
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Figure 5. XPS spectra of the as-prepared Ir/CeO2 nanojunctions: (a) Ce 3d binding energy spectrum, (b) O 1s binding energy spectrum, (c) Ir 4f
binding energy spectrum. XPS spectra of the as-prepared Ir in the Supporting Information: (d) Ir 4f binding energy spectrum.
Figure 6. (a) Temperature-programmed profiles of the 1 vol % CO oxidation for the prepared samples annealed at different temperatures. (b) Stability
of Ir/CeO2 under 1 vol % CO with a space velocity of 36000 mL g−1 h−1 at a temperature of 110 °C.
Figure 1 shows TEM and HRTEM images of the prepared
Ir/CeO2 before and after loading. The iridium clusters were
deposited on the surface of CeO2 nanoparticles owing to in situ
chemical reduction by NaBH4. The CeO2 nanoparticles can
retain their previous spherical-like morphology as before due to
the chemical stability, thus providing the support for the deposi-
tion ofan iridium cluster. There are many black spots in Figure 1b,
which indicates the formation of iridium−cerium load structures
after the reaction is completed. Figure 1c and Figures s1−s3 show
the HRTEM image of the heterojunctions of Ir and CeO2. The
interlaced boundaries marked with white dashed lines demon-
strate the generation of the heterojunctions of Ir and CeO2 in
Figure 1c, and the distribution of repetitive elements is shown in
Figure 2. Obviously, we can find that the iridium clusters with a
size on the order of 10 nm distributed on the surface of CeO2
nanoparticles. The interplanar spacing of exposed CeO2 planes
is 3.354 Å, corresponding to the (111) surface of CeO2.
The XRD patterns of Ir/Ce nanocomposites taken before and
after the deposition of Ir are shown in Figure 3. There are six
sharp diffraction peaks which could be indexed by CeO2 (JCPDS
card no. 78-0694). According to the image in Figure 2 and the
results of inductively coupled plasma−atomic emission spectros-
copy (ICP), the existence of iridium has been proved, and the
mass percentage of Ir in the catalyst is 2.588%. However, the
XRD pattern did not change after the deposition of Ir due to the
small amount of Ir present.
Figure 4 shows the N2 sorption isotherms at 77 K of the
samples before and after reduction, and its inset shows the pore
C
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the metal oxides. The other peak located at 531.4 eV is ascribed
to the oxygen in hydroxyl groups.13 Figure 5c and 5d show
the XPS spectrum of the Ir 4f track. In Figure 5c, it can be
deconvolved into six peaks, on which are marked two peaks as P1
and Q1. The peaks centered at 63.9 and 60.9 eV stand for the Ir0
states, and the others stand for the Ir4+ states.14 In Figure 5d, the
spectrum can only be deconvolved into two peaks, which stand
for the Ir0 states. Based on the results of the HRTEM and XPS
spectra, we can find that two different valence values of iridium
exist in the sample, i.e., the Ir4+ state atoms located at the
interface of CeO2 nanoparticles and iridium clusters and the Ir0
state atoms attributed to the iridium clusters. The Ce atoms and Ir
atoms were connected by O atoms, forming the heterojunction
structures.
Table 1. Comparison between Different Kinds of
Heterojunction Structures of Traditional Noble Metal and
Oxide Basements for CO Conversation Temperature
catalyst
Ir/CeO2
T
100/°C
ref
110
110
220
250
100
120
130
110
200
this paper
Co−Pd
Pd−Ag
Ce−Pd
Au−FeOx
Au−NiO
Au/Mg(OH)2
Ag−Co3O4
Pd−La/Al2O3
15
16
17
18
19
20
21
22
To study the catalytic performance of Ir/CeO2 nano-
composites and comparative samples, CO oxidation as a typical
probe reaction was carried out using 1 vol % CO in air. We evalu-
ated the effect of iridium on the catalytic activity by using 1 vol %
with a space velocity of 36000 mL g−1 h−1. Figure 6a shows the
temperature-programmed profiles of the CO oxidation for the
prepared samples. It is found that the temperatures for CO 100%
conversation (T100) for CeO2, Ir-SiO2, Ir/CeO2, and iridium
powder are 400, 170, 110, and 160 °C, respectively. The com-
plete CO oxidation temperature for a sample of Ir/CeO2 is
much lower than for pure Ir powder and Ir−SiO2 samples.
In order to know the stability of the compound, we also carried
out the experiment at 110 °C for 24 h (Figure 6b). The results
show that the conversion percentage of CO is 100% during
size distributions. The BET specific surface area of Ir/CeO2
decreased by 7 m2/g compared to CeO2, but there is an obvious
fall at 10−50 Å for the pore size distributions. This could be
attributed to the reduction of IrCl3, where the formation of
iridium clusters blocked the pores of CeO2 nanoparticles.
In order to investigate the surface composition and chemical
state of the iridium and cerium cations on the catalyst surface,
XPS and X-ray induced Auger spectra (XAES) of the samples
were measured. As shown in Figure 5a, the Ce 3d XPS spectrum,
which can be deconvolved into eight peaks, marked two peaks as
M1 and N1. They are the characteristic peaks of Ce3+ states, while
others are the characteristic peaks of Ce4+ states.12 The XPS
spectrum of O 1s can be divided into two peaks in Figure 5b. The
peak centered at 529.3 eV stands for the surface lattice oxygen in
Figure 7. (a) The interfacial model of Ir/CeO2. (b) The structure of CO adsorbed at the Ir/CeO2 interface. (c) The charge density difference of the
CO molecule adsorbed at the Ir/CeO2 interface. The olive and cyan colors illustrate the increase and decrease of electron distributions, respectively.
(d) The structure of CO adsorbed on the CeO2 (111) surface.
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the whole experiment, indicating that the heterojunction struc-
ture between Ir and CeO2 is very stable and protects the iridium
cluster from poisoning. We compared the performance of
our catalyst with the results of other reports. As shown in
Table 1, it is found that the performance of Ir/CeO2 nano-
junctions is better than that of supported traditional noble metal
catalysts.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02356.
■
S
Experimental section, including catalysis preparation,
sample preparation, characterization, catalysis perform-
ance measurements, and DFT computational methods
(PDF)
In order to understand the nature of the high catalytic activity
of the Ir/CeO2 catalyst for CO oxidation, we carried out density
function theory (DFT) calculations; the calculation details can
be seen in the Supporting Information.23 According to our
experimental HRTEM results, we constructed the interfacial
structure of Ir clusters supported on the CeO2 (111) surface,
which is shown in Figure 7a. As the adsorption process is essential
and taken as the starting step for many chemical reactions, we
calculated the adsorption of a CO molecule on the Ir/CeO2
interfacial structure. The optimized geometry configuration of
the Ir/CeO2 interfacial structure after adsorption of a CO
molecule is shown in Figure 7b. It can be seen from Figure 7b
that the CO molecule adsorbs at iridium atoms at the Ir/CeO2
interface with an adsorption energy of −2.08 eV, indicating
that the CO molecule chemisorbs at the Ir/CeO2 interface.
The bond length of the CO molecule after adsorption was
elongated from 1.143 Å in the gas phase to 1.173 Å, indicating
that the CO molecule had been activated by Ir/CeO2 interface
structures.
To obtain deeper insight into the electronic structure, we per-
formed charge density difference and Bader charge calculations.
Figure 7c shows the charge density difference map of a CO mol-
ecule adsorbed at the Ir/CeO2 interface. As shown in Figure 7c,
electrons transfer from the Ir/CeO2 interface to the CO mole-
cule. According to Bader charge analysis, the Ir/CeO2 interface
structure transfers 0.1142 electrons to the antibonding orbital of
the CO molecule, which decreases the bond order of the CO
molecule and stretches the C−O bond length, thus activating
the CO molecules.
For comparison, we also constructed a CeO2 (111) surface
structure and calculated the adsorption of a CO molecule on the
CeO2 (111) surface. Figure 7d shows the optimized geometry
configuration of the CeO2 (111) surface after adsorption of CO
molecules. As shown in Figure 7d, a CO molecule adsorbs on the
top of the Ce atom with a distance of 2.878 Å, suggesting that the
adsorption of a CO molecule on the CeO2 (111) surface is
very weak. Further calculations showed that the adsorption
energy of CO on the CeO2 (111) surface was −0.18 eV, indi-
cating physisorption, which was consistent with a previous
report.2 Further, the C−O bond length is unchanged with
respect to the calculated value for a free CO molecule, which is
1.143 Å, indicating that CO molecules cannot be activated on
the CeO2 (111) surface. This is the reason why the CO catalytic
oxidation performance of CeO2 is lower than for Ir/CeO2.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cqw@ustc.edu.cn.
ORCID
Cichang Zong: 0000-0002-3819-5216
Ruirui Zhang: 0000-0003-1543-6846
Peng Jiang: 0000-0001-8975-5706
■
Author Contributions
∥These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by the National Natural Science
Foundation (51772283, 21271163, 21972145) and the National
Key R&D Program of China (2016YFA0401801). The calcu-
lations were completed on the supercomputing system at the
Supercomputing Center of USTC.
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In summary, the Ir/CeO2 nanojunction for CO oxidation has
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