Nanocage-Shaped Co3?xZrxO4 Solid-Solution Supports Loaded with Pt Nanoparticles as Effective Catalysts for the Enhancement of Toluene Oxidation

Article abstract of DOI:10.1002/smll.202005715

Nanocage-shaped Co_3?xZr_xO₄ solid-solution supports and the corresponding platinum loaded nanocomposites, yPt/Co_3?xZr_xO₄ (x?=0.27, 0.50, 0.69; y = 0.5, 1.0, 2.0 wt.%), are successfully fabricated via a Cu₂O nanocube hard template method and a glycol reduction method, respectively. The hollow nanocage structures obviously improve surface areas; moreover, the Zr doping forms the Co_3?xZr_xO₄ solid-solution supports, and the corresponding yPt/Co_3?xZr_xO₄ catalysts promote the enhancement of catalytic performance. Catalytic activity toward toluene combustion is enhanced for the 2.0 wt% Pt/Co_2.73Zr_0.27O₄ catalyst. The catalysts are characterized using multiple techniques. Pt nanoparticles are uniformly dispersed across the Co_2.73Zr_0.27O₄ nanocage surface. The 2.0 wt% Pt/Co_2.73Zr_0.27O₄ catalyst exhibits the highest catalytic activity among all the samples and demonstrates good stability, with 90% toluene conversion obtained at a temperature of 165 °C. The same catalyst accomplishes full toluene oxidation at 180 °C, at a weight hourly space velocity of 36 000 mL h^?1 g^?1. The apparent activation energy (E_a) over the yPt/Co_2.73Zr_0.27O₄ samples are significantly lower than those over the Co_3?xZr_xO₄ supports, with the 2.0 wt% Pt/Co_2.73Zr_0.27O₄ catalyst exhibiting the lowest E_a value. These findings demonstrate the potential of the 2.0 wt% Pt/Co_2.73Zr_0.27O₄ catalyst as a promising catalyst toward atmospheric toluene removal.

Full text of DOI:10.1002/smll.202005715

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Nanocage-Shaped Co Zr O Solid-Solution Supports
3
−x x 4
Loaded with Pt Nanoparticles as Effective Catalysts
for the Enhancement of Toluene Oxidation
Mengmeng Wang, Dongyun Chen,* Najun Li, Qingfeng Xu, Hua Li, Jinghui He,
and Jianmei Lu*
removal, as this strategy has demonstrated
to be effective and economical.
[
1–3]
Nanocage-shaped Co3−xZr O solid-solution supports and the corresponding
x
4
platinum loaded nanocomposites, yPt/Co3−xZr O (x =0.27, 0.50, 0.69;
Noble metal and transition metal oxide-
based supported catalysts are hitherto, the
principal catalyst systems for the catalytic
oxidation of toluene. Among the various
noble metal-based catalysts, Pt-based cata-
lysts have attracted significant attention
because of their excellent catalytic activity
toward toluene. However, the zero-valent
atoms comprising the Pt nanoparticles
tend to migrate across the support under
oxidizing atmospheres, and thereafter,
Pt particle aggregation occurs leading to
x
4
y = 0.5, 1.0, 2.0 wt.%), are successfully fabricated via a Cu O nanocube hard
2
template method and a glycol reduction method, respectively. The hollow
nanocage structures obviously improve surface areas; moreover, the Zr doping
forms the Co3 Zr O solid-solution supports, and the corresponding
−x
x
4
yPt/Co3−xZr O catalysts promote the enhancement of catalytic performance.
x
4
Catalytic activity toward toluene combustion is enhanced for the
.0 wt% Pt/Co2.73Zr0.27O catalyst. The catalysts are characterized using multiple
2
4
techniques. Pt nanoparticles are uniformly dispersed across the Co2.73Zr0.27O4
nanocage surface. The 2.0 wt% Pt/Co2.73Zr0.27O catalyst exhibits the highest
4
[4]
catalytic activity among all the samples and demonstrates good stability, with
the loss of catalytic performance. Thus,
numerous nanostructured catalysts have
been designed to try and circumvent the
issue of aggregation, such as core-shell
90% toluene conversion obtained at a temperature of 165 °C. The same catalyst
accomplishes full toluene oxidation at 180 °C, at a weight hourly space velocity of
−1
−1
3
6 000 mL h g . The apparent activation energy (E ) over the yPt/Co2.73Zr0.27O4
a
structures and hollow porous materials
samples are significantly lower than those over the Co3−xZr O supports,
[5,6]
x
4
having high surface areas.
Additionally,
with the 2.0 wt% Pt/Co2.73Zr0.27O catalyst exhibiting the lowest E value. These
4
a
the charges generated at the metal oxide–
metal interface are known to enhance
the catalytic activity toward toluene oxida-
findings demonstrate the potential of the 2.0 wt% Pt/Co2.73Zr0.27O catalyst
4
as a promising catalyst toward atmospheric toluene removal.
[
7,8]
[9]
tion. For instance, MnO ,
Co O , and
2
3 4
[10]
CeO2 having redox characteristics have
been used as supports for Pt nanoparticles.
1
. Introduction
Co3O4 exhibits a high oxygen storage capacity as a
typical transition metal oxide and can easily interconvert
between Co2 and Co , and therefore Co O4 is regarded
+
3+
Driven by environmental pollution as a result of long-term
emissions of volatile organic compounds, especially toluene,
efficient technologies are required to address the issue and to
limit, or reverse, the damage to the environment and human
health. Low temperature catalytic oxidation has received a
wealth of interdependent research for atmospheric toluene
3
as a promising support to load Pt particles. However, there
are numerous drawbacks in using pure Co O nanocrystals,
3
4
one of which is poor thermal stability. Nevertheless, doping
foreign transition elements (such as Zr and La) into the
Co O lattice to construct solid-solution materials has been
3
4
[
11]
reported to overcome the aforementioned problems.
M. Wang, Prof. D. Chen, Prof. N. Li, Prof. Q. Xu, Prof. H. Li,
Prof. J. He, Prof. J. Lu
Furthermore, the redox properties, oxygen storage capacity,
and the degree of dispersion of the obtained mixed oxides
are significantly improved. Additionally, the catalytic perfor-
mance toward toluene oxidation also significantly improved.
Therefore, Pt nanoparticles loaded onto Co3−xZrxO4 solid-
solution supports is expected to increase the oxidation
activity toward toluene.
College of Chemistry
Chemical Engineering and Materials Science
Collaborative Innovation Center of Suzhou Nano Science
and Technology
Soochow University
Suzhou 215123, P. R. China
E-mail: dychen@suda.edu.cn; lujm@suda.edu.cn
Compared with traditional catalytic supports, hollow
mesoporous metal oxides possess higher surface areas,
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202005715.
which provide
a significantly enhanced accessible inter-
DOI: 10.1002/smll.202005715
face to uniformly load and disperse active species such as
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Scheme 1. Schematic illustration for preparing mesoporous yPt/Co3−xZr O catalysts.
x
4
Pt nanoparticles.[12–16] Herein, a series of hollow nanocage-
shaped yPt/Co3−xZr O catalysts were synthesized, as shown
solid-solution supports (Figure 1f–h,j–l). The cubic morphology
is retained; however, the surface of the nanosheets on sup-
ports became rough. In comparison, the Co3−xZr O supports
x
4
in Scheme 1. As for the Co3−xZr O solid-solution support, in
x
4
x
4
comparison, Co2.73Zr0.27O exhibits improved catalytic activity.
Furthermore, after Pt nanoparticle loading, the 2.0 wt% Pt/
are observed to contain additional folds due to the doping of
Zr. Compared with the Co3−xZr O solid-solution supports, the
4
x
4
Co2.73Zr0.27O catalyst exhibited superior catalytic performance
surface of the 2.0 wt% Pt/Co3−xZr O catalysts (Figure 1m–p) is
4
x 4
and achieved complete toluene catalytic oxidation at 180 °C.
rougher although the same cubic morphology is also retained.
TEM micrographs of the samples are shown in Figure 2.
Co(OH)2 (Figure 2a,b) and Zr (OH) /Co(OH) (Figure 2e,f)
Additionally, the 2.0 wt% Pt/Co2.73Zr0.27O catalyst was observed
4
to possess the lowest apparent activation energy (E ) and also
a
x
4x
2
demonstrated acceptable catalytic durability during toluene oxi-
dation. The excellent performance is suggested to derive from
the uniform distribution of small Pt nanoparticles, large sur-
nanocages with hollow structures and randomly orientated sur-
face nanosheets are observed. After calcination, the obtained
Co O4 and the Co2.73Zr0.27O4 solid-solution support maintain
3
2+
face areas, a higher Co /Co
ratio, and the higher number
the cubic and hollow structures, as shown in Figure 2c,d and
Figure 2g,h, respectively. After loading Pt nanoparticles onto
the Co2.73Zr0.27O4 solid-solution support (Figure 2i,j), the Pt
nanoparticles are observed to be uniformly dispersed across the
total
of oxygen vacancies generated. Therefore, the catalysts were
subjected to a plethora of characterization analyses, such
as scanning electron microscopy (SEM), transmission elec-
tron microscopy (TEM), X-ray diffraction (XRD), Brunauer–
Emmett–Teller (BET) surface area measurements, and X-ray
photoelectron spectroscopy (XPS).
surface of the hollow Co2.73Zr0.27O support. To further confirm
4
the crystallographic features of the 2.0 wt% Pt/Co2.73Zr0.27O4
catalyst, high resolution TEM micrographs were obtained
(
Figure 2k). The lattice fringes of 0.230 and 0.250 nm are attrib-
uted to the (111) crystal planes of Pt and the (311) crystal planes
[
21–23]
of the Co2.73Zr0.27O4 solid-solution support, respectively.
2
. Results and Discussion
Furthermore, the high-angle annular dark-field scanning trans-
mission electron microscopy (HAADF-STEM) image of the
2
.1. Morphology and Structure
2
.0 wt% Pt/Co2.73Zr0.27O catalyst confirms the presence and
4
The yPt/Co3−xZr O catalysts were successfully synthesized
homogeneous distribution of the 2.0 nm Pt nanoparticles, as
shown in Figure S1, Supporting Information. Additionally,
the energy-dispersive X-ray spectroscopy elemental mappings
(Figure 2k) also show the uniform dispersions of Co, O, Zr, and
x
4
via chemically etching Cu O nanocube templates, followed by
2
[17–19]
thermal and impregnation treatments.
First, the clear and
homogeneous nanocube morphology of Cu O template can be
2
observed by SEM, with an average diameter size of ≈650 nm
Pt elements in the 2.0 wt% Pt/Co2.73Zr0.27O catalyst.
4
(
Figure 1a–d). After a solution-phase cation exchange method,
To further evaluate the microstructure, N2 adsorption–
desorption isotherms were performed. The pore size distribu-
tion of the Co O , Co2.73Zr0.27O , and 2.0 wt% Pt/Co2.73Zr0.27O4
reported elsewhere,[
20]
Co(OH)2 and Zr (OH) /Co(OH)
x 4x 2
nanosheets grew into hollow nanocages, as shown in
Figures 1e,i, respectively. Thereafter, the nanosheets were sub-
jected to thermal treatment to obtain Co O and Co3−xZr O4
3
4
4
samples were obtained from the Barrett–Joyner–Halenda
model, as shown in Figure 3. The textural properties to include
3
4
x
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Figure 1. SEM images of: a,b,c) the Cu O nanocube template, e) Co(OH)2 nanocages, f,g) Co O4 nanocages, i) Zr (OH) /Co(OH) nanocages,
2
3
x
4x
2
j,k) Co2.73Zr0.27O4 solid-solution supports, and m,n,o) 2.0 wt% Pt/Co2.73Zr0.27O4 catalysts; models of: d) the Cu O template, h) Co O nanocages,
2
3
4
l) Co2.73Zr0.27O solid-solution supports, and p) 2.0 wt% Pt/Co2.73Zr0.27O catalysts.
4
4
Figure 2. TEM images of pre-calcined a,b) Co(OH) and e,f) Zr (OH) /Co(OH) nanocages, and the respective samples obtained after calcination c,d) Co O
4
2
x
4x
2
3
and g,h) Co2.73Zr0.27O solid-solution support; magnified TEM micrographs i,j) of the 2.0 wt% Pt/Co2.73Zr0.27O catalyst, and k) HAADF-STEM and high
4
4
resolution TEM images of the 2.0 wt% Pt/Co2.73Zr0.27O catalyst and STEM energy-dispersive X-ray spectroscopy elemental mappings of Co, O, Zr, and Pt.
4
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peaks assigned to the (220), (311), (400), (511), and (440) planes of
[27]
the spinel phase (JCPDS card no. 42-1467). As the zirconium
content increases from Co2.73Zr0.27O4 to Co2.50Zr0.50O4 and to
Co2.31Zr0.69O , the obtained Co Zr O samples exhibit broader
4
3−x
x
4
peaks, shifting to lower diffraction angles. This suggests that Zr
is successfully doped into the Co lattice structure, indicating the
[4,28,29]
formation of the Co3−xZr O solid-solution supports.
Fur-
x
4
thermore, the lattice parameter of the solid-solution supports
increases as a function of Zr content because the ionic radius of
Zr is larger than that of Co. The ZrO sample is relatively well
2
crystalline and displays the typical tetragonal phase (JCPDS no.
[30]
7
9-1769). Additionally, no significant changes to the XRD pat-
terns after Pt loading (Figure S3, Supporting Information) were
observed, which confirms the uniform dispersion of small Pt nan-
[31]
oparticles and the retention of the Co3−xZr O crystal structure.
x
4
The Raman spectra shown in Figure S4, Supporting Infor-
mation, further provide evidence for the formation of the
Co3−xZr O4 solid-solution support. The strong bands of the
x
−
1
(1)
pure Co O sample at 193 and 683 cm correspond to the F2g
3
4
symmetry of the tetrahedral sites (CoO ) and the A symmetry
Figure 3. N adsorption-desorption isotherms (inset: pore size distribu-
4
1g
2
[
32]
tions) of Co O , Co2.73Zr0.27O , and 2.0 wt% Pt/Co2.73Zr0.27O4.
of the octahedral sites (CoO ), respectively.
6
As for the
3
4
4
Co3−xZr O materials, the bands gradually shift to lower ener-
x
4
the BET surface area and pore information are presented in
Table S1, Supporting Information. The isotherms are type IV
gies, resulting from the increased substitution of Co with Zr,
which increases the average length of the CoO bond. There
[24,25]
adsorption isotherms with H3-type hysteresis loops.
Obvi-
are no Raman shifts observed that are associated with ZrO
2
ously, the surface area of Co2.73Zr0.27O is significantly larger
in the Co3−xZr O materials, suggesting that ZrO and Co O
4
x
4
2
3
4
than the other samples. Additionally, the pore size distributions
form a solid-solution consistent with the XRD results. Further-
(
Figure 3 inset) show mode pore sizes of 1–3 nm, which indi-
more, with respect to the phase-pure ZrO , the obvious bands
2
−
1
cate the presence of mesopores in the catalysts. The mesopo-
rosity will enhance toluene diffusion within the catalysts, and
thus, improve the catalytic activity.[
at 148, 266, 319, 461, and 643 cm are assigned to tetragonal
[
4,11,33]
ZrO2.
The above data evidence the high degree of crystal-
26]
linity and the excellent structure of the nanocomposites.
2
.2. XRD and Raman Analysis
2.3. Analyses of the Surface Oxygen Vacancies and Chemical
States of the Catalysts
XRD patterns of the prepared samples are shown in Figure 4.
The phase-pure Co O catalyst displays well-resolved diffraction
XPS analysis was performed to explore the surface ele-
ment compositions and the chemical status of the catalysts.
Figure 5 displays the high-resolution Co 2p, O 1s, Zr 3d, and
Pt 4f XPS spectra of Co O , Co2.73Zr0.27O , ZrO , and 2.0 wt%
3
4
3
4
4
2
Pt/Co2.73Zr0.27O . Additionally, the XPS survey scan spectra
4
(
Figure S5, Supporting Information) of the above catalysts
clearly present elemental differences. As shown in Figure 5a,
the Co 2p spectrum includes two main peaks, the two fitted
peaks of Co2.73Zr0.27O at 796.4 and 781.3 eV are assigned to
4
2+
2+
Co 2p1/2 and Co 2p3/2, and the other peaks at 794.7 and
79.6 eV are indexed to Co 2p1/2 and Co 2p3/2, respectively.
3+
3+
[34]
7
Importantly, compared with phase-pure Co O , the binding
3
4
energies of Co2.73Zr0.27O4 and 2.0 wt% Pt/Co2.73Zr0.27O4 dis-
play a negative shift. Furthermore, the Zr 3d spectrum of ZrO2
(
Figure 5c) comprises two main peaks at 184.2 and 181.8 eV,
which are assigned to the Zr 3d and Zr 3d5/2 spin-orbit
3
/2
[
29]
splitting.
In comparison, the peaks observed in the Zr 3d
spectra of the Co2.73Zr0.27O and 2.0 wt% Pt/Co2.73Zr0.27O sam-
4
4
ples shift to high binding energies. These findings suggest the
presence of electronic coupling and electron transfer between
ZrO and Co O , which could further evidence the formation
2
3
4
of a Co3−xZr O solid-solution nanocomposite and an abundant
Figure 4. XRD patterns of Co O , Co2.73Zr0.27O4, Co2.50Zr0.50O4,
x
4
3
4
[
4,11,28]
Co2.31Zr0.69O , and ZrO .
number of oxygen vacancies.
Additionally, the 2.0 wt%
4
2
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Figure 5. High-resolution XPS spectra of Co O , Co2.73Zr0.27O , ZrO , and 2.0 wt% Pt/Co2.73Zr0.27O . a) Co 2p, b) O 1s, c) Zr 3d, and d) Pt 4f.
3
4
4
2
4
2+
Pt/Co2.73Zr0.27O catalyst possess the highest Co /Co
ratio
As presented in Table 1, the Ov content is observed to
4
total
when compared with the other catalysts (Table S2, Supporting
Information), which indicates that this catalyst possesses the
highest concentration of oxygen vacancies.[
decrease in the order of 2.0 wt% Pt/Co2.73Zr0.27O > 1.5 wt%
4
Pt/Co2.73Zr0.27O > 0.5 wt% Pt/Co2.73Zr0.27O > Co2.73Zr0.27O >
4
4
4
35]
Co O , which further reveals that the formation of surface
3
4
Furthermore, the O 1s signal of the catalysts, as shown in
oxygen vacancies could be enhanced through platinum loading.
The concentration of oxygen vacancies reaches 48.1% for the
1
Figure 5b, includes three fitted peaks. The peaks labeled as O ,
2
3
O , and O , centered at 529.7–529.9, 531.5–531.6, and 533.0–
2.0 wt% Pt/Co2.73Zr0.27O catalyst, which can provide the suf-
4
2−
5
33.1 eV, are indexed to latticed oxygen (O ), surface oxygen
ficient active centers or an oxygen adsorption sites for the
catalysis reaction. Additionally, the calculated electron spin
resonance (ESR) g value of ≈2.002 from the ESR spectra of the
samples (shown in Figure S6, Supporting Information) fur-
ther shows the presence of oxygen vacancies. Comparison of
vacancies (O ), and surface adsorbed hydroxyl species, respec-
v
tively.[ In addition, the oxygen content in O of the catalysts are
1]
v
evaluated by the integrated peak areas using Equation (1):
AO2
the O density agrees well with the findings of the O 1s spectra.
v
C(O
v
) =
× 100%
(1)
AO
1
+ AO
2
+ AO
3
Notably, the higher the concentration of surface oxygen vacan-
cies in the catalyst leads to enhanced adsorption of oxygen
molecules, and a concomitant higher catalytic activity toward
toluene oxidation.
AO
1
, AO
2
, and AO are the photoelectron peak areas of latticed
3
oxygen, O , and surface-adsorbed hydroxyl species, respectively.
v
Table 1. Oxygen vacancy concentration and Pt content determined by XPS and inductively coupled plasma atomic emission spectroscopy (ICP-AES)
analyses, respectively.
Catalyst
Co3O4
36.2
0:1
Co2.73Zr0.27O4
45.3
0.5 wt% Pt/Co2.73Zr0.27O4
1.0 wt% Pt/Co2.73Zr0.27O4
2.0 wt% Pt/Co2.73Zr0.27O4
Surface content of Ov [%]a)
Zr/Co atomic molar ratiob)
Actual Pt content [wt%]b)
46.4
0.07:1
0.43
47.3
0.08:1
0.89
48.1
0.09:1
1.68
0.08:1
—
—
a)Determined by XPS; b)Determined by ICP.
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The Pt 4f spectra presented in Figure 5d show the Pt 4f5/2 and
Pt 4f7/2 two spin-orbit splitting. The two weak-fitted peaks at 76.0
in good agreement with the XPS data. The observed enhanced
catalytic activity of 2.0 wt% Pt/Co2.73Zr0.27O4 is suggested to
result from the formation of abundant oxygen vacancies and
2+
2+
and 72.0 eV are related to Pt 4f5/2 and Pt 4f7/2, with the other
two peaks at 74.6 and 71.2 eV, being indexed to Pt 4f and Pt0
0
higher Co /Co ratios. Figure 6d presents the Arrhenius
2+
[37]
5/2
total
[36]
2+
4f7/2
.
The cationic Pt is suggested to result from Pt(acac) not
plots for toluene combustion over the Co O , Co2.73Zr0.27O4,
3 4
2
being fully reduced by ethylene glycol (EG), or the Pt nanopar-
ticles are slightly oxidized if exposed to air during the synthesis
steps.[ Importantly, metallic Pt is the major Pt phase in the
catalysts, which plays a role in the oxidation of toluene. Further-
more, the actual Zr/Co atomic molar ratio and Pt content were
obtained by inductively coupled plasma atomic emission spec-
troscopy and are presented in Table 1. The product compositions
are observed to be slightly lower than the theoretical values, likely
caused through inevitable losses during the preparation steps.
0.5 wt% Pt/Co2.73Zr0.27O4, 1.0 wt% Pt/Co2.73Zr0.27O4, and
2.0 wt% Pt/Co2.73Zr0.27 samples at a weight hourly space
O
4
18]
−1 −1
velocity of 36 000 mL h g . Comparison of the apparent E
a
−
1
values shows Co2.73Zr0.27O (E = 98.69 kJ mol ) and 2.0 wt%
4
a
−
1
Pt/Co2.73Zr0.27O (E = 66.18 kJ mol ) to have lower E values
compared with Co O , which further implies enhanced perfor-
mance toward the catalytic oxidation of toluene.
4
a
a
3
4
[
38,39]
Further-
more, the comparison of the temperature at which 90% toluene
is converted is presented in Figure 6c. The findings demon-
strate that Zr doping to form solid-solution supports and sub-
sequent loading of Pt nanoparticles could enhance the catalytic
behaviors. Additionally, the catalytic activities toward toluene
oxidation of the catalysts developed herein are compared with
the other catalysts previously reported in the literature and are
2
.4. Catalytic Performance and Kinetic Parameters
As shown in Figure 6, a comparison of the catalytic perfor-
mance for the samples is presented. From Figure 6a, among the
Co3−xZr O samples, Co2.73Zr0.27O exhibits optimized catalytic
listed in Table 2. Notably, the 2.0 wt% Pt/Co2.73Zr0.27O catalyst
4
exhibits the highest catalytic activity toward toluene oxidation.
x
4
4
activity, and achieves complete catalytic toluene oxidation at
2
85 °C. Furthermore, after loading with the noble metal Pt, the
catalytic performance of the yPt/Co2.73Zr0.27O samples is fur-
2.5. Stability and Water Resistance Test
4
ther improved. The 2.0 wt% Pt/Co2.73Zr0.27O catalyst evidently
4
expresses optimized performance and provides full toluene con-
version at 180 °C, as shown in Figure 6b. The above results are
To further investigate the stability of the 2.0 wt% Pt/Co2.73Zr0.27O4
catalyst, experiments were performed to investigate the influence
Figure 6. a,b) Catalytic performance of different Co3−xZr O samples and the corresponding toluene conversion as a function of reaction temperature
x
4
over Co2.73Zr0.27O with different Pt loadings. c) Temperatures for 90% toluene oxidation (T90%). d) Toluene catalytic oxidation Arrhenius plots over the
4
Co O , Co2.73Zr0.27O , 0.5 wt% Pt/Co2.73Zr0.27O , 1.0 wt% Pt/Co2.73Zr0.27O , and 2.0 wt% Pt/Co2.73Zr0.27O catalysts.
3
4
4
4
4
4
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Table 2. A comparison of catalytic activities toward toluene oxidation among the prepared catalysts with previously reported catalysts.
WHSV [mL h− g−1
1
]
Ea [kJ mol−1]
66.18
Sample
.0 wt% Pt/Co2.73Zr0.27O4
.0 wt% Pt/Co2.73Zr0.27O4
.5 wt% Pt/Co2.73Zr0.27O4
Loading [wt%]
1.68
Mass [mg]
CToluene [ppm]
T90% [°C]
165
Ref.
This work
This work
This work
[40]
2
50
50
50
50
36 000
1
0.89
36 000
71.33
175
0
0.43
50
50
36 000
77.32
190
Au/3DOM Co3O4
Pt-Pd-SBA-15-PVP
Pt/γ-Al2O3
6.50
50
1000
1000
1000
1000
40 000
74.00
256
0.50
150
30
26 000
83.50
179
[41]
0.72
60 000
88.00
205
[42]
Pt-Pd/HMS
0.50
150
26 000
80.23
197
[43]
of time-on-stream and water vapor addition on the catalytic per-
formance at 180 °C. As shown in Figure 7, toluene conversion
is observed to slightly decrease; however, the conversion level
is maintained >94% during the continuous 12-h reaction pro-
cess. Furthermore, the addition of water vapor results in a small
loss of conversion (≈2.5%); however, when the supply of water
vapor is cut off, the catalytic activity almost returned to the initial
value. Additionally, toluene conversion was maintained at >96%.
This phenomenon is as a result of the competitive adsorption of
water and toluene, as well as the presence of oxygen molecules
on the sample surface.[ In short, the 2.0 wt% Pt/Co2.73Zr0.27O4
catalyst exhibits excellent stability and durability during toluene
oxidation.
The toluene molecule is preferentially adsorbed onto the more
active Pt metal on the catalyst surface and is activated as a dehy-
drogenation intermediate. Thereafter, this intermediate reacts
with active oxygen species to form key intermediates, such as
benzaldehyde, benzoate, and finally completely oxidizes to CO2
and H O. Furthermore, the active oxygen species are form on
2
the generated oxygen vacancies that are beneficial to the rapid
activation of O molecules. Moreover, the Pt metallic atoms also
2
could active O2 molecules which further accelerate the rapid
activation of O molecules. Facilitating the rapid activation of O
2
2
39]
molecules, these oxygen species migrate from the support sur-
face to the 2.0 wt% Pt/Co2.73Zr0.27O interface and react with the
4
[
9]
intermediates to produce non-toxic carbon dioxide. According
[
7,36]
to previously reported literature,
the catalytic redox perfor-
mance is significantly influenced by the surface oxygen species
2
.6. Proposed Catalytic Mechanism
of the catalyst. First, O atoms, obtained from O , are adsorbed
2
onto oxygen vacancies. A small portion of the adsorbed oxygen
atoms can react directly with the reactants. Lattice oxygen par-
ticipates in the reaction in the form of active oxygen and is
supplemented by adsorbed oxygen introduced into the lattice
through vacancies. Migration and transformation cycles of the
Scheme 2 shows the proposed mechanisms of toluene decom-
position and catalytic oxidation over 2.0 wt% Pt/Co2.73Zr0.27O4.
The reaction mechanism of toluene oxidation is understood
to follow successive steps to form various intermediates.
Common pathways for the Pt-based catalysts developed herein
[
44]
oxygen species during toluene oxidation occur. The forma-
tion of additional surface oxygen vacancies improves oxygen
transfer, which directly provides active oxygen species for the
[
21]
are observed, which have been reported in the literature.
[
21]
reaction, thereby improving the efficiency toward the cata-
lytic oxidation of toluene. Additionally, oxygen migration pro-
motes additional surface oxygen vacancies during the reaction.
3
. Conclusion
Co3−xZr O4 solid-solution supports and yPt/Co2.73Zr0.27O4
x
catalysts were synthesized through a solution-phase cation
exchange method using Cu O nanocubes as a template and by
2
a glycol reduction method, respectively. The yPt/Co2.73Zr0.27O4
catalysts possess high surface areas because of the hollow
nanocage structures formed. Furthermore, Pt nanoparticles
are uniformly dispersed across the surface of the Co2.73Zr0.27O4
solid-solution support with an average size of ≈2.0 nm. From
the characterization analyses, Zr doping significantly improved
the catalytic performance with the Co2.73Zr0.27O support exhib-
4
iting optimized performance. The 2.0 wt% Pt/Co2.73Zr0.27O4
catalyst presents the highest catalytic activity and achieves
full toluene oxidation at 180 °C, the lowest E value and good
Figure 7. Time-on-stream and the effect of water vapor addition on the
catalytic performance toward toluene oxidation at 180 °C, over 2.0 wt%
a
stability. Hence, doping Zr into the Co O lattice to construct
3
4
−
1
−1
Pt/Co2.73Zr0.27O at a weight hourly space velocity = 36 000 mL h g .
solid-solution supports, then subsequent loading of Pt, which
4
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Scheme 2. Proposed mechanism for enhanced catalytic oxidation toward toluene over 2.0 wt% Pt/Co2.73Zr0.27O4.
further effectively promotes the catalytic activity toward toluene
oxidation. Therefore, the 2.0 wt% Pt/Co2.73Zr0.27O4 catalyst
demonstrates promise as an effective catalyst for atmospheric
toluene removal.
CoCl2∙6H2O were first dispersed into 40 mL of ethanol/H2O solvent
volume ratio = 1:1) in the presence of 1.33 g of PVP (Mw = 40 000) by
sonication for 20 min. Then, 25 mL 1 ꢀ Na S O solution was slowly
(
2
2
3
dropped into the above system under stirring. After the addition of
Na S O solution, the color of the solution turned from yellow to light
2
2
3
green, indicating the successful synthesis of Zr (OH) /Co(OH) hollow
x
4x
2
nanocages. The product was collected by centrifugation, washed by
ultra-pure water and ethanol twice, and eventually dried in a vacuum at
50 °C overnight. Through adjusting the addition ratio of ZrOCl2·8H2O
to CoCl ·6H O, the corresponding Co(OH) hollow nanocages and
4
. Experimental Section
Materials: Copper(II) sulfate pentahydrate (CuSO ∙5H O), sodium
2
2
2
4
2
Zr (OH) /Co(OH) hollow nanocages with different Co /Zr (x = 0.27,
x 4x 2 3−x x
citrate, dimethylformamide (DMF), EG, and absolute ethanol
C H OH) were purchased from Sinopharm Chemical Reagent Co.
0
.50, 0.69) molar ratio can be prepared.
Preparation of Nanocrystalline Co3−xZr O Solid Solutions: The obtained
(
2
5
x
4
Ltd. (China). Zirconium oxychloride octahydrate (ZrOCl ·8H O),
2
2
Zr (OH) /Co(OH) hollow nanocages sample was heated to 400 °C
x 4x 2
sodium thiosulfate pentahydrate (Na S O ), and sodium hydroxide
−1
2
2
3
for 2 h with a heating rate of 5 °C min under air atmosphere. Then,
(
(
NaOH) were bought from Sigma-Aldrich. Cobaltous chloride
CoCl ∙6H O), ascorbic acid, platinum(II) acetylacetonate (Pt(acac) ),
the Co3−xZr O4 solid solution was collected after cooling to ambient
x
2
2
2
temperature under air atmosphere.
and polyvinylpyrrolidone (PVP (Mw = 40 000)) were purchased from
Tokyo Chemical Industry Co. Ltd. The used ultra-pure water in the
process was purified by a Millipore system (18.2 MΩ cm). Toluene was
bought from Messer Air Liquide, Ltd. (China). All reagents were used
as received without further purification.
Preparation of yPt/Co3−xZr O4 Catalysts: Pt nanoparticles were
x
successfully prepared to the support by a glycol reduction method.
Typically, the certain amount Pt(acac)2 and 200 mg Co3−xZr O support
x
4
were added into a mixed solvent containing 30 mL of DMF and 20 mL of
EG, then dispersed by the ultrasonic treatment for 20 min and magnetic
stirring for 4 h. Then the obtained suspension was transferred into Teflon
autoclave container, and heated to 160 °C for 12 h. The powder was
collected by centrifugation, washed by ethanol several times followed by
drying in a vacuum at 60 °C overnight. The yPt/Co3−xZr O catalysts with
Characterization: Physicochemical properties of the Co3 Zr O4
−x
x
solid solutions and yPt/Co3 Zr O catalysts were characterized
−x
x
4
using diverse techniques, such as X-ray diffraction (XRD, D8
QUEST, Germany) carried on a X'Pert-Pro MPD diffractometer. X-ray
photoelectron spectroscopy (XPS, ESCALAB 250Xi), scanning electron
microscopy (SEM, Hitachi S-4700, Japan), transmission electron
microscopy (TEM, FEI Tecnai G-20), and high-angle annular dark-
field (HAADF–STEM, FEI Tecnai F-20) were utilized to characterize
the morphologies and microstructures of the samples. Energy-
dispersive X-ray spectroscopy (EDX, FEI Tecnai F-20) showed the
element distribution and content in catalysts. Inductively coupled
plasma mass spectrometer (ICP-AES, Varian 710-ES) was carried out
to analyze the actual metal contents in samples, N2 adsorption–
desorption (BET) with a Micromeritics ASAP 2010 system to examine
the specific surface area.
x
4
theoretical Pt loading of y = 0.5, 1.0, 2.0 wt% were prepared.
Catalytic Activity Tests: Catalytic activities for the toluene oxidation
of the all catalysts were carried out in a continuous flow fixed-bed
reactor. 50 mg of the sample was loaded in the U-shaped tube of the
reactor for the test. The inlet concentration toluene concentration
−1
was controlled at 50 ppm, and the total flow rate was 30 mL min ,
which correspond to the weight hourly space velocity (WHSV) of
−1
−1
3
6000 mL h g . The reaction temperature was maintained until
obtaining stable data and then continued the next designed temperature.
The inlet and outlet concentrations of in the reaction were determined
through gas chromatograph-mass spectrometer (GCMS-QP2020). The
toluene conversion (ηtoluene) was calculated by the following equation:
Preparation of Cu O Nanocubes Template: In a typical synthesis,
2
CuSO ∙5H O (1.5 mmol) was dissolved into 80 mL of ultra-pure water,
4
2
then sodium citrate (0.5 mmol) was added to the above solution under
stirring. Afterward, 20 mL NaOH (1.25 ꢀ) was added to the obtained
solution under stirring for 15 min, and 50 mL ascorbic acid (0.03 ꢀ) was
put into the suspension under stirring for 15 min. Finally, the obtained
mixed solution was aged for 1 h at 50 °C. The precipitate was collected
by centrifugation, washed with ultra-pure water and ethanol for three
times.
CInlet − COutlet
ηtoluene =
× 100%
(2)
CInlet
where CInlet and COutlet represent the inlet and outlet concentration of
toluene at steady state, respectively.
Preparation of Zr (OH) /Co(OH) Nanocages: The Zr (OH) /
x
4x
2
x
4x
Supporting Information
Co(OH) hollow nanocages were synthesized by using a solution-phase
2
cation exchange method from Cu O nanocube templates. A quarter of
Supporting Information is available from the Wiley Online Library or
from the author.
2
the as-prepared Cu O nanocubes, 1.73 mg ZrOCl ·8H O and 12 mg of
2
2
2
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[
16] Y. Zou, X. Zhou, J. Ma, X. Yang, Y. Deng, Chem. Soc. Rev. 2020, 49,
Acknowledgements
The authors gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (21938006, 51973148,
1173.
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