Highly active and stable Au@Cu:XO core-shell nanoparticles supported on alumina for carbon monoxide oxidation at low temperature

Article abstract of DOI:10.1039/c6ra07358a

Au@Cu_xO (x = 1 or 2) core-shell heterostructure nanoparticles (NPs) are synthesized by a facile aqueous solution approach. γ-Alumina support Au@Cu_xO (Au@Cu_xO/γ-Al₂O₃) catalysts used for CO oxidation at low temperature are prepared by dispersing the core-shell NPs on the surface of γ-alumina. It proves that the formation of the core-shell structure has led to the interaction between Au and Cu_xO and the co-existence of Au^δ+ and Au⁰, which accounts for the improvement in catalytic activity. It can even catalyze CO oxidation at room temperature and the conversion of CO can reach to 38%. In the same case, Au/γ-Al₂O₃ does not have any catalytic activity. In particular, the embedding of Au NPs into Cu_xO shells has also improved the catalytic stability of Au based catalysts remarkably, which remains unchanged after 108 hours of reaction. To the best of our knowledge, Au@Cu_xO/γ-Al₂O₃ is really one of the most stable catalysts with adequate activity at room temperature.

Full text of DOI:10.1039/c6ra07358a

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Highly Active and Stable Au@CuxO Core-Shell Nanoparticles
Supported on Alumina for Carbon Monoxide Oxidation at Low
Temperature
，
Weining Zhanga,b 1, Qingguo Zhaoa,b,1, Xiaohong Wanga,b,*, Xiaoxia Yana,b, Sheng
Received 00th January 20xx,
Accepted 00th January 20xx
Hanc,*,Zhigang Zengd
DOI: 10.1039/x0xx00000x
Au@CuxO (x=1 or 2) core-shell heterostructure nanoparticles (NPs) are synthesized by a facile aqueous solution approach.
γ-alumina support Au@CuxO (Au@CuxO/γ-Al2O3) catalysts used for CO oxidation at low temperature are prepared by
dispersing the core-shell NPs on the surface of γ-alumina. It proves that the formation of the core-shell structure has led to
the interaction between Au and CuxO and the co-existence of Auδ+ and Au0, which accounts for the catalytic activity
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improvement. It can even catalyze CO oxidation at room temperature and the conversion of CO can reach to 38%. In the
same case , Au /γ-Al2O3 has not any catalytic activity. In particular, the embedding Au NPs into CuxO shell has also
improved the catalytic stability of Au based catalysts remarkably, which remains unchanged after 108 hours reaction. To
our knowledge, Au@CuxO/γ-Al2O3 is really one of the most stable catalyst with adequate activity at room temperature.
the role of additives to improve the physical and chemical activities
26
Introduction
of the metal particles by interacting with them within nanoscale
.
It has been extensively demonstrated that the strong metal-support
interaction (SMSI) plays a significant role in affecting catalytic
performance27-34. However, the SMSI between Au and support can’t
More and more studies and applications focus on Au NPs as
catalysts have been reported since the earliest report on the
outstanding catalytic activity for oxidizing CO at low temperature by
Haruta in 1987 1. CO oxidation on Au based catalyst at room
temperature is generally following the size-dependent reaction
normally exist because Au has a lower work function and surface
35, 36
energy
. The encapsulation of Au Nps by metal oxide can
2, 3
4-6
strengthen the SMSI thus lead to the electron transfer between Au
and support 37
pathways
and Au NPs tend to sinter to larger particles
or
6-
.
change to bicarbonate-like species under the reaction conditions
13
and thus leads to a substantial loss of activity 14, which is a major
drawback of Au based catalysts and this undesirable characteristic
Cu2O is an important p-type semiconductor with the direct band
gap of 2.17 eV, which could find practical applications in many
has reduced their potential for industrial applications 8, 15
.
aspects, such as solar energy conversion 38, hydrogen production by
39, 40
photocatalytic decomposition of water
, and lithium-ion
Up to now, many strategies have been proposed to solve the
batteries as electrode material 41. It is also a promising alternative
to expensive noble metals as the catalyst for CO oxidation at
moderate temperatures, owing to its higher natural abundance and
lower price. It has been reported that Cu2O layers can grow
epitaxially on the surface of Au-Cu particles with small lattice
deactivation problem of Au based catalysts, which includes: (1) the
16-18
replacement of Au particles with alloy or bimetallic particles
,
19, 20
(2) the modification of the catalysts with other additives
and
(3) the fabrication of core-shell type structure particles using Au as
core 21-24. Actually, the first two methods can really improve the
catalytic stability to some extent, but cannot completely avoid the
deactivation caused by sintering or poisoning. Compared with the
first 2 methods, the encapsulation of Au NPs as a core inside of an
oxide as a shell is more practicable to maintain the size and shape
of Au NPs as well as to protect them from poisoning, which leads to
the stability and compatibility enhancement of Au based catalysts
mismatch and the CO oxidation reactions can occur on the Cu2O-Au
43
interfaces 42. CuO is also a good catalyst for CO oxidation
.
Consequently, CuxO (x=1 or 2) is a reasonable choice as the shell
material to improve the catalytic performance of Au NPs.
Herein we report an entirely new design of Au based catalyst for
CO oxidation at low temperature. Instead of pure Au particles,
Au@CuxO core-shell NPs are fabricated and dispersed on the
surface of γ-alumina and a new kind of CO oxidation catalyst
(Au@CuxO/γ-Al2O3) is synthesized. By coating Au NPs with CuxO,
both the catalytic activity and stability of Au particles have been
improved. It not only can catalyze CO oxidation at room
temperature with 38% CO conversion, but also can maintain the
activity. The stability is tested in 108 hours and after the 108 hours
25
.
Among
the
various
types
of
core-shell
NPs,
metal@semiconductor particles represent a significant class of
core-shell structure. The semiconductor shell plays both the role of
masker to protect metal particles from sintering and poisoning and
a Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, China
b Shanghai Key Laboratory of High Temperature Superconductors, Shanghai 200444, China
cNew Energy Material Lab, Shanghai Institute of Technology, Shanghai 201418, China
d Department of Physics, College of Science, Shanghai University, Shanghai 200444, China*
This journal is © The Royal Society of Chemistry 20xx
*Corresponding author. E-mail address: xiaohongwang@shu.edu.cn (Xiaohong Wang),
hansheng654321@sina.com (Sheng Han)
1 These authors contributed equally to this work and should be considered co-first authors.
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application, the catalytic activity has not any loss. As the Catalytic activity Test
comparison, the general Au based catalyst without CuxO has not
any catalytic activity at room temperature.
For CO activity measurement, about 0.1 g catalyst specimen (40 ~
60 mesh) is placed into a stainless flow reactor (Ф 8 × 150 mm) with
a fixed bed. Then, the gas mixture consisted of 2 vol% CO and 98
vol% air are introduced into the reactor at a flow rate of 50 ml·min-1,
and the GHSV (Gaseous Hourly Space Velocity) is 30,600 mL·g-1·h-1.
The activity tests of the samples are carried out in the continuous
flow reactor from room temperature to the temperature that the
CO had been completely converted into CO2 with the heat-up
period of 1 °C·min-1 rising and analyzed by the gas chromatograph
(GC).
Experimental section
Materials preparation
Preparation of Au NPs: Au NPs are synthesized following the
previously reported traditional Frens’s citrate reduction synthesis
method. Briefly, 150 mL of aqueous HAuCl4 (0.25 mM) is prepared
in a 250-mL beaker and brought to boil in a 100 °C water bath under
magnetic stir. Then, 7.5 mL of 0.1 M sodium citrate solution is
added rapidly. When the reaction mixture begins to change color
and finally reaches a wine red color, it meants the end of the Characterization
reaction. The solution is kept stirring for 30 min, cooled to room
temperature, and then Au colloids are collected by centrifugation,
washed with anhydrous ethanol and water for 3 times , and finally
redispersed in 7 mL of water.
Nitrogen adsorption-desorption isotherms using the Barrett-
Emmett-Teller (BET) technique on a Micromeritics ASAP2020 V4.00
surface areas and porosity analyzer is used to detect the specific
surface areas of the samples. X-ray diffraction (XRD) patterns are
Preparation of Au@CuxO Core-Shell NPs: Au@CuxO is prepared collected on a Dandong Haoyuan Instrument Co., Ltd. DX-2200V/PC
26
using the method reported by Zhang et al
. 1.0 g of diffractometer with Cu Kα radiation (λ=0.154056 nm). Transmission
polyvinylpyrrolidone (PVP, average M.W. 40000, TCI Shanghai) is electron microscope (TEM), high-resolution transmission electron
dissolved in 50 mL of 0.01 M Cu(NO3)2 (Sinopharm Chemical microscopy (HRTEM), energy dispersive spectroscopy (EDS) and
Reagent Co., Ltd.) aqueous solution under constant magnetic stir high angle annular dark field scanning transmission electron
and the solution is kept stirring for another 10 min to ensure the microscopy (HAADF-STEM) are conducted on JEM-2010F
PVP solution uniformly. Subsequently, 17 μL of N2H4•H2O solution transmission electron microscope with an acceleration voltage of
(85 wt %, Sinopharm Chemical Reagent Co., Ltd.) is immediately 200 kV. The surface morphology of the samples is observed with a
introduced into the reaction mixtures after 6 mL of fresh prepared ULTRA55-36-69 Zeiss field-emission scanning electron microscopy
Au colloid solution is added. The colloid solution turns dark green (FESEM). Au content is measured by a Perkin-Elmer Optima 7300DV
within 10 s, which is the characteristic color of Au@CuxO core-shell inductively coupled plasma-atomic emission spectrometry (ICP-AES).
NPs typically. The reaction liquid is kept stirring for 2 min, and then X-ray photoelectron spectroscopy (XPS) measurements are carried
the Au@CuxO core-shell NPs are washed with anhydrous ethanol out on a Thermo ESCALAB 250 spectrometer using monochromatic
and water for 3 times and finally redispersed in anhydrous ethanol Al Ka (ht =1486.6 eV). All binding energies (BE) are calibrated by the
and kept in freezer at 6 ℃.
C 1 s signal (BE = 284.8 eV).
Preparation of γ-Al2O3: The amorphous Al2O3 (Sinopharm Chemical Results and Discussion
Reagent Co., Ltd.) are calcined from room temperature to 550 °C
with the heating rate of 5 °C·min-1 in the air and then maintains 6
Characterization of Au@CuxO Core-shell NPs and Au@CuxO/γ-
hours and cools to room temperature naturally.
Al2O3 Catalysts
Preparation of Au@CuxO/γ-Al2O3: Au@CuxO ethanol solution is
taken to the beaker with a certain amount of γ-Al2O3 and stirred at
room temperature until the ethanol is entirely volatilized. Then
Au@CuxO/γ-Al2O3 is got. Au contents are measured by the
Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-
AES).
Au@CuxO core-shell structure NPs are fabricated and dispersed on
γ-Al2O3. Herein, γ-Al2O3 is used as the catalyst support to anchor
Au@CuxO particles so as to increase the surface area and decrease
the Au use to reduce costs. Au content in the catalysts is measured
by the ICP-AES. As reference, Au /γ-Al2O3 and CuxO/γ-Al2O3 are also
prepared with the same method and same Au or CuxO content.
Preparation of Au/γ-Al2O3: Au colloid is added to the beaker with a
certain amount of γ-Al2O3 and stirred at room temperature until the
solution is completely volatilized.
Figure 1 shows the structural characterizations of Au@CuxO core-
shell NPs, Au /γ-Al2O3 and Au@CuxO/γ-Al2O3 catalyst. As shown in
Figure 1a, Au@CuxO core-shell Nps with multiple Au cores are
26
formed. According the report of Zhang et al
,
reducing
Preparation of CuxO/γ-Al2O3: CuxO solution is prepared by the HAuCl4•3H2O with sodium citrate leads to the formation of quasi-
method of Au@CuxO preparation where the adding of Au is omitted spherical Au NPs and the subsequent reduction of Cu(NO3)2 with
and then the solution is added to the beaker with a certain amount N2H4•H2O results in the formation of CuxO shell in a self-assemble
of γ-Al2O3 and stirred at room temperature until the solution is process, which deposits on the Au surface to form core-shell like
completely volatilized.
particles with multiple cores. The Au particle size is mainly
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concentrated at 20 nm (Figure 1d) and the distribution interval is low (0.11 and 0.088 wt%), their diffraction peaks could not clearly
narrow. The diameter of Au@CuxO nanospheres varies within the made out, which suggests a well dispersed metal phase 46
range of 60-120 nm and mainly concentrates at 100 nm (Figure 1e).
The average shell thickness is about 40 nm and the core-shell
structure is not uniform. The greater details of the Au@CuxO core-
shell structures can be observed by HAADF-STEM (Figure 1b) and
HR-TEM (Figure 1c) image. The HR-TEM image recorded on the Au
core region indicates that the interplanar spacing is about 0.235 nm
corresponding to the (111) planes of face-centered cubic Au 44. The
(111) planes of Cu2O with a spacing of 0.248 nm are found to grow
epitaxially on the facets of Au 45. EDS line scan of an individual
Au@CuxO core-shell heterostructure reveals strong Cu and O signals
across the entire particle. The signal for Au appears only in the
central region of the particle, where the Au core is located (Figure
1b). It confirms further the formation of core-shell structure.
.
Figure 2. XRD patterms of Au@CuxO/γ-Al2O3 with 0.11 wt% Au and 0.088 wt%
CuxO, Au/γ-Al2O3 with 0.11 wt% Au , CuxO/γ-Al2O3 with 0.088 wt% CuxO and γ-
Al2O3. The intensity scale for all the samples are the same.
Figure 3 shows the SEM images of Au@CuxO/γ-Al2O3, Au /γ-Al2O3,
CuxO/γ-Al2O3 and pure γ-Al2O3. The crystals surface of all the
samples is rough and their shapes show a perceptible similarity to
each other, which are stacked loosely by irregular alumina particles
and we could not clearly find out the Au, CuxO or Au@CuxO core-
shell NPs. Otherwise, the addition of a little amount of Au, CuxO or
Au@CuxO has no effect on the morphology of alumina support.
Figure 1. Structural characterization of Au@CuxO and the catalysts. a) Bright-field
TEM image of Au@CuxO core-shell NPs, b) High-magnification HAADF-STEM
image of an individual Au@CuxO core-shell NPs with spatial elemental
distribution analysis by EDS line-scan, c) HR-TEM image of the Au core and CuxO
shell, d) The particle size distribution of pure Au NPs, e) The particle size
distribution of Au@CuxO core-shell structure particles, f) HAADF-STEM image of
Au@CuxO/γ-Al2O3, g) TEM image of Au/γ-Al2O3, h) The particle size distribution of
Au in Au/γ-Al2O3.
Figure 3. The SEM image of a) Au@CuxO/γ-Al2O3 with 0.11 wt% Au and 0.088 wt%
CuxO, b) Au/γ-Al2O3 with 0.11 wt% Au , c) CuxO/γ-Al2O3 with 0.088 wt% CuxO and
d) γ-Al2O3. All SEM images share the same scale-bar in panel a).
To further study the physical properties of Au@CuxO/γ-Al2O3
catalyst, Brunauer-Emmett-Teller (BET) surface area and pore
structure of the catalyst is measured by nitrogen adsorption at 77 K
and is compared with that of pure γ-Al2O3. Figure 4 displays a type
of IV isotherms with a hysteresis loop at relative pressure (P/P0)
between 0.5 and 1, indicating the presence of mesopores. BET
surface area is calculated to be 118 m2•g-1 for Au@CuxO/γ-Al2O3,
which is almost same to that of pure γ-Al2O3. In the Barret-Joyner-
Halenda (BJH) pore size distribution analysis, the sharp peak at
about 4.6 nm and 4.9 nm for pure γ-Al2O3 and Au@CuxO/γ-Al2O3
inset in Figure 4 suggests a relatively narrow pore size distribution
Figure 1f and 1g are the HAADF-STEM image of Au@CuxO /γ-
Al2O3 and TEM image of Au /γ-Al2O3 catalyst. It shows that the core
shell structure of Au@CuxO and the diameter of Au particles and
Au@CuxO nanospheres haven’t changed after dispersing on γ-Al2O3.
The particle size of Au is mainly concentrated at 20 nm (Figure 1h).
The XRD patterns of Au@CuxO/γ-Al2O3, Au/γ-Al2O3, CuxO/γ-Al2O3
and pure γ-Al2O3 are shown in Figure 2. The peaks at 37.6, 45.8 and
67.0° of all the samples are due to the (311), (400), (440) reflections
of the γ-Al2O3. As the content of Au and CuxO in the samples is very
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iterval. It shows that Au@CuxO has little effect on the pore T90% of Au@Cu2O/γ-Al2O3 with 1.24 wt% Au is only 144 °C. The most
structure of alumina support, while the relatively high surface area important thing is that CO conversion on this kind of catalyst can
of alumina will provide abundant active sites for catalytic reaction reach to 18% at room temperature (Figure 5c).
47
.
Figure 5. Catalytic activity of the catalysts. a) The 90% CO conversion
Figure 4. Nitrogen adsorption isotherms and pore size distribution of a) pure γ-
temperature of Au@CuxO/γ-Al2O3 with 0.11 wt% Au and 0.088 wt% CuxO, Au/γ-
Al2O3, and b) Au@CuxO/γ-Al2O3 with 0.11 wt% Au and 0.088 wt% CuxO.
Al2O3 with 0.11 wt% Au , CuxO/γ-Al2O3 with 0.088 wt% CuxO and γ-Al2O3, b) The
influence of Au content on the 90% CO conversion temperature of Au@CuxO/γ-
CO Oxidation on Au@CuxO/γ-Al2O3 Catalysts
Al2O3, c) the CO conversion at different temperature on Au@CuxO/γ-Al2O3 with
1.24 wt% Au.
Figure 5a shows the catalytic activity of Au@CuxO/γ-Al2O3, Au/γ-
Al2O3, CuxO/γ-Al2O3 and pure γ-Al2O3. Here, the 90% CO conversion
For CO oxidation at low temperature, the catalytic activity of Au
temperature (T90%) is used to estimate the catalytic activity of the
based catalyst is high enough but the stability is still lower. Wang et
catalysts. The low T90% indicates the high activity of the catalyst. T90%
of pure γ-Al2O3 is about 216 oC, and the introduction of a little
amount Au (0.11 wt%) makes the T90% decrease to 203 oC, which
indicates a little improvement of the catalytic activity. Generally,
the Au content in the Au based catalysts is equal to or greater than
1 wt %, and it even reaches to 5 wt% in some catalysts 48-50, the low
catalytic activity of Au/γ-Al2O3 here can be ascribed to the low Au
content. On the other hand, the large Au particle size (~20 nm) is
also a cause leads to the low catalytic activity of Au/γ-Al2O3 because
that CO oxidation on the simple supported Au catalyst at low
temperature is generally following the size-dependent reaction
pathways 2, 3. For this kind of catalysts, 3~5 nm Au particles are
more active. The introduction of only a little amount CuxO (0.088
wt%) has not obvious effect on the catalytic activity of pure γ-Al2O3.
T90% of CuxO/γ-Al2O3 is about 220 oC, which is even a little higher
than that of pure γ-Al2O3. In brief, the effect of a little (0.11 wt%)
pure Au or pure CuxO (0.088 wt%) alone on the catalytic activity of
γ-Al2O3 is weak. However, the combination of these two kinds of
material and the fabrication of Au@CuxO core-shell structure
generate a material more suitable to be used as CO oxidation
catalyst. Although the content and particles size of Au in
Au@CuxO/γ-Al2O3 is same to Au/γ-Al2O3, the catalytic activity has
been improved a lot. T90% of Au@Cu2O/γ-Al2O3 is only 158 oC, which
18
al have improved the catalytic stability of Au/Al2O3 by adding Rh
into the catalyst and Lin et al have improved the catalytic stability of
Au/Al2O3 by adding LaFeO3 into alumina 50. However, the stability of
the catalyst is still limited. To study the catalytic performance of
Au@CuxO/γ-Al2O3 deeply, Au@CuxO/γ-Al2O3 with 1.24 wt% Au is
chosen as example to study the catalytic stability at both 144 oC and
room temperature. It can be seen from Figure 6a that, after 72 h
reaction at 144 oC, the catalytic activity has not any loss, which
means that the high temperature stability of the catalyst is quite
high.
The catalytic stability test of the catalyst at room temperature in
Figure 6b shows the catalytic activity increase rather than decrease
before 30 hours. CO conversion on this catalyst can reach to 38 % at
room temperature after 30 hours’ application and then hold the
line to 108 h. To our knowledge, this is one of the most stable Au
based catalysts with high catalytic activity. The catalytic
performance improvement of the catalyst may be ascribed to the
reason that the interaction between Au and CuxO has led to the
high catalytic activity and the protection of Au by CuxO has led to
the high stability.
o
is 58 C lower than that of pure γ-Al2O3. It indicates that Au@CuxO
hybrid NPs exhibit good catalytic activity due to the combination of
the properties from the individual components and nanoscale
interactions between the disparate metal and semiconductor
components.
To improve the catalytic activity further, Au@CuxO/γ-Al2O3
catalysts with different Au content are prepared and the influence
of Au content on the catalytic activity for CO oxidation is studied.
Figure 5b shows that T90% decreases with Au content, which means
that the catalytic activity can be improved by increasing Au content.
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Figure 6. Catalytic stability of Au@CuxO/γ-Al2O3 with 1.24 wt% Au. a) At 144 o
C
the same reaction mechanism may also occur via a Cu2+
mechanism.
⇌
Cu+ redox
and b) At room temperature.
CO Oxidation Mechanism on Au@CuxO/γ-Al2O3
In spite of a great number of contributions to reveal the origin of
the high CO oxidation activity of Au catalysts, the reaction
mechanism is still debated. The main debatable topics include the
active Au species, the mechanism of activation of oxygen molecules,
the role of the support and the Au-support interface, the
interaction between supports and Au particles and the possible CO
oxidation pathway. The difference of the catalysts system and the
reaction conditions may be the mandatory reason leads to the
controversies.
Figure 7. The XPS spectra of different catalysts. a) Au 4f, b) Cu 3d and c) O 1s.
Figure 8 is the diagrammatic sketch for CO oxidation mechanism
on Au@CuxO/γ-Al2O3. To sum up, the pairs of Au atoms and Au
cations are active sites. Au clusters act as a reservoir of charge
through the catalytic cycle facilitating both reduction processes
involving the O2 molecule and oxidation process such as the
transformation of CO to CO2. The detail process is that CuxO donate
and withdraw charge from Au and build up the possible charge at
the metal/oxide interface. CO molecules quickly attach to the
oxygen species bond at the interface. It has been proved that CO
Generally speaking, the size of the Au particles, the valence state
of Au and the interaction between Au and support are main factors
influence dramatically the activity of the catalysts, which are
difficult to isolate because they are normally intertwined together
51. To study the possible reaction mechanism, XPS is employed to
detect the surface elements and their valence states. The XPS
profile of Au 4f region is shown in Figure 7a. The main peaks near
83.8 eV and 87.5 eV are corresponded to Au 4f7/2 and Au 4f5/2 of
metal Au and the peaks at 84.3 eV and 88.0 eV are corresponded to
Au 4f7/2 and Au 4f5/2 of Auδ+. The Au signal of Au@CuxO/γ-Al2O3 is
weaker than that of Au/γ-Al2O3 due to the encapsulation of Au into
CuxO. However, the catalytic activity of the former is higher. It may
be ascribed to the reason that the Au species on the 2 samples are
quite different. All the surface Au of Au/γ-Al2O3 are Au0, and there
are 2 kinds of Au species (Au0 and Auδ+ ) on the surface of
Au@CuxO/γ-Al2O3. It indicates that the interaction between Au and
CuxO has led to the co-existence of Auδ+ and Au0. The ratio of Auδ+/
Au0 is 1.1 and the co-existence of the 2 kinds of Au species makes
can react with the bonded oxygen and release CO2 within 4 Ps67
.
the catalyst more active. Au cations 52, and pair of Au atoms and Au
53
cations
have also been proposed as active sites by other
researchers.
Figure 7b shows the XPS profile of Cu 3d region of CuxO/γ-Al2O3
and Au@CuxO/γ-Al2O3. The peaks at 932.3, 933.4 and 935.5 eV are
ascribed to Cu+ of Cu2O, Cu2+ of CuO and Cu2+ of CuCO3 and Cu(OH)2
respectively. As the catalytic activity of the sample without CuxO is
lower, the function of Cu species may be ascribed to the interaction
between Au and CuxO, which plays a crucial role in the electronic
properties of Au particles and strongly affects the catalytic activity.
Figure 8. The diagrammatic sketch for CO oxidation mechanism on Au@CuxO/γ-
Al2O3.
59-
The reducing nature of CuxO can donate 54-58and withdraw
Conclusion
65charge from Au and build up the possible charge at the
metal/oxide interface.
In summary, Au /γ-Al2O3 and CuxO/γ-Al2O3 haven’t any catalytic
activity at room temperature and their 90% CO conversion
temperature (T90%) are almost same to pure γ-Al2O3. Au@CuxO/γ-
Al2O3 not only can decrease the T90% but also can catalyze CO
oxidation at room temperature and the stability is very high. The
high catalytic activity of Au@CuxO/γ-Al2O3 can be ascribed to the
reason that the formation of Au@CuxO core-shell structure has led
to the interaction between Au and CuxO, which generates the Auδ+
and Au0 pairs and the co-existence of the 2 kinds of Au species
makes the catalyst more active. The protective function of CuxO
As shown in Figure 7c, the O 1s XPS spectra are fitted with two
peaks contributions. One peak at BE = 531.8–531.7 eV can be
ascribed to adsorbed oxygen. The other peak at BE = 530.7–530.8
66
eV may belong to lattice oxygen O2- of CuxO. Li et al have shown
unambiguously that with the promotion of Au NPs, the surface
lattice oxygen of the FeOx participates directly in the CO oxidation
⇌
via an Fe3+ Fe2+ redox mechanism even well below the room
temperature, and this redox mechanism predominates in low
temperature CO oxidation on Au/FeOx. As CuxO is similar to FeOx,
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Catalysis a-General, 2007, 327, 226-237.
Z. Wang, L. Li, D. Han and F. Gu, Materials Letters, 2014,
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C. Chen, C. Nan, D. Wang, Q. Su, H. Duan, X. Liu, L. Zhang,
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shell may be one reason accounts for the catalytic stability
improvement.
21.
22.
Acknowledgments
This research is supported by National Natural Science Foundation of China
(21103104). We also acknowledge the Instrumental Analysis and Research Center
of Shanghai University for providing measurement services.
23.
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