Large-Pore Mesoporous CeO2–ZrO2 Solid Solutions with In-Pore Confined Pt Nanoparticles for Enhanced CO Oxidation

Article abstract of DOI:10.1002/smll.201903058

Active and stable catalysts are highly desired for converting harmful substances (e.g., CO, NO_x) in exhaust gases of vehicles into safe gases at low exhaust temperatures. Here, a solvent evaporation–induced co-assembly process is employed to design ordered mesoporous Ce_xZr_{1? x}O₂ (0 ≤ x ≤ 1) solid solutions by using high-molecular-weight poly(ethylene oxide)-block-polystyrene as the template. The obtained mesoporous Ce_xZr_{1? x}O₂ possesses high surface area (60–100 m² g^?1) and large pore size (12–15 nm), enabling its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without blocking pore channels. The obtained mesoporous Pt/Ce_0.8Zr_0.2O₂ catalyst exhibits superior CO oxidation activity with a very low T₁₀₀ value of 130 °C (temperature of 100% CO conversion) and excellent stability due to the rich lattice oxygen vacancies in the Ce_0.8Zr_0.2O₂ framework. The simulated catalytic evaluations of CO oxidation combined with various characterizations reveal that the intrinsic high surface oxygen mobility and well-interconnected pore structure of the mesoporous Pt/Ce_0.8Zr_0.2O₂ catalyst are responsible for the remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/Ce_xZr_{1? x}O₂-s with small pore size (3.8 nm), ordered mesoporous Pt/Ce_xZr_{1? x}O₂ not only facilitates the mass diffusion of reactants and products, but also provides abundant anchoring sites for Pt nanoparticles and numerous exposed catalytically active interfaces for efficient heterogeneous catalysis.

Full text of DOI:10.1002/smll.201903058

FULL PAPER
Mesoporous Materials
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Large-Pore Mesoporous CeO –ZrO Solid Solutions with
2
2
In-Pore Confined Pt Nanoparticles for Enhanced CO Oxidation
Xuanyu Yang, Xiaowei Cheng,* Junhao Ma, Yidong Zou, Wei Luo,* and Yonghui Deng*
have attracted increasing concerns. The
Active and stable catalysts are highly desired for converting harmful sub-
catalytic converter in combustion engines
is one of the most efficient methods to
abate vehicle emission, in which CO cata-
stances (e.g., CO, NO ) in exhaust gases of vehicles into safe gases at low
x
exhaust temperatures. Here, a solvent evaporation–induced co-assembly
[1,2]
lytic oxidation to CO is a key reaction.
2
process is employed to design ordered mesoporous Ce Zr O (0 ≤ x ≤ 1)
x
1−x
2
Therefore, to meet increasingly strict emis-
sion standards, high-performance and
stable catalysts in advanced engines are
in urgent demand to work at the exhaust
solid solutions by using high-molecular-weight poly(ethylene oxide)-block-
polystyrene as the template. The obtained mesoporous Ce Zr O possesses
x
1−x
2
2
−1
high surface area (60–100 m g ) and large pore size (12–15 nm), enabling
its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without
blocking pore channels. The obtained mesoporous Pt/Ce Zr O catalyst
[1]
temperatures below 150 °C. During the
past decade, the platinum (Pt)-based cata-
lysts have been widely explored for CO
0
.8 0.2
2
exhibits superior CO oxidation activity with a very low T100 value of 130 °C
temperature of 100% CO conversion) and excellent stability due to the rich
lattice oxygen vacancies in the Ce Zr O framework. The simulated catalytic
[2b]
low-temperature oxidation. However, Pt
(
nanoparticles containing zero-valent atoms
tend to migrate on the support in oxidizing
atmosphere and aggregate or sinter into
large particles, resulting in loss of catalytic
0.8 0.2
2
evaluations of CO oxidation combined with various characterizations reveal
that the intrinsic high surface oxygen mobility and well-interconnected pore
structure of the mesoporous Pt/Ce Zr O catalyst are responsible for the
[3]
activity. Therefore, considerable efforts
have been made to rationally design the
sinter-resistant catalysts with different
nanostructures and high surface area,
0.8 0.2
2
remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/
Ce Zr O -s with small pore size (3.8 nm), ordered mesoporous Pt/Ce Zr O2
x
1−x
2
x
1−x
[
3]
[4]
not only facilitates the mass diffusion of reactants and products, but also pro-
vides abundant anchoring sites for Pt nanoparticles and numerous exposed
catalytically active interfaces for efficient heterogeneous catalysis.
such as core–shell structure, nanotube,
[5]
porous materials, and so on. Besides, the
metal oxide–metal interface can easily pro-
duce charges that help to enhance catalytic
[
5]
efficiency in CO oxidation, so various
metal oxides with redox properties, such
[5]
[6]
[7a]
[8]
[1]
as Co O , TiO2, FeOx,
MnO2, and CeO2, have been
1
. Introduction
3
4
explored as supporting matrix for Pt nanoparticles.
Nowadays, severe effects of automobile exhaust gas on environ-
ment and human health, caused by the major harmful pollut-
As a typical transition metal oxide, CeO2 owns excellent
3
+
oxygen storage capacity and facile interconversion between Ce
4+
ants such as CO, NO , hydrocarbons, and particulate matter,
and Ce , which has been regarded as one of the particularly
x
[
9]
promising candidates for loading Pt nanoparticles. However,
pure CeO nanocrystals usually suffer several major drawbacks
2
Dr. X. Yang, Prof. X. Cheng, J. Ma, Y. Zou, Prof. Y. Deng
Department of Chemistry
State Key Laboratory of Molecular Engineering of Polymers
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
iChEM
Fudan University
Shanghai 200433, China
E-mail: xwcheng@fudan.edu.cn; yhdeng@fudan.edu.cn
such as poor thermal stability and severe deactivation due to
[
10]
rapid sintering.
introducing foreign elements (e.g., Zr,
These problems can be well overcome by
[
11]
[12]
[13]
La,
and Pr
)
into CeO2 lattice to construct the so-called solid solutions, and
they show increasing redox properties and catalytic perfor-
mance in CO oxidation due to the significantly enhanced gen-
[
14]
eration of lattice oxygen vacancies. Particularly, CeO –ZrO
2
2
Prof. W. Luo
4
+
(
Ce Zr O ) solid solutions formed by introducing Zr in
x 1−x 2
State Key Laboratory for Modification of Chemical
Fibers and Polymer Materials
College of Materials Science and Engineering
Donghua University
Shanghai 201620, China
E-mail: wluo@dhu.edu.cn
2−
CeO2 lattice possess distorted O sublattices and easy mobility
of lattice oxygen from bulk phase to surface, which endows
them with excellent thermal stability against high-temperature
[
15]
sintering and improved oxygen storage capacity.
Ordered mesoporous metal oxides with high porosity and
surface area have received great attention, because they not
only provide enormous uniform pore channels beneficial to
mass diffusion, but also offer huge accessible interface for
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201903058.
DOI: 10.1002/smll.201903058
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homogeneous loading and dispersion of active species such
as Pt nanoparticles. These diverse features enable them to
own great potential for applications in catalysis, adsorption
Herein, we report a solvent evaporation–induced co-assembly
strategy to controllably synthesize highly ordered mesoporous
Ce Zr O (0 < x < 1) solid solutions with large pores, highly
x
1−x
2
[
15b,16]
and separation, energy storage, and chemical sensors.
Although porous Ce Zr O solid solutions have been reported
crystalline pore walls, and tunable Ce/Zr molar ratios, using
lab-made PEO-b-PS of high molecular weight as the template.
After further loading ultrafine Pt nanoparticles (≈4.0 nm), one
x
1−x
2
previously by using different templates, including colloidal
crystals,[
13,14]
amphiphilic cationic surfactant (cetyltrimethyl-
of the obtained catalysts, mesoporous Pt/Ce0.8Zr0.2O , shows
2
ammonium bromide),[
17]
and nonionic block copolymers
superior catalytic performance in CO oxidation with a low light-
(
poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene
off temperature of 130 °C and long-term stability over 50 cycles
[18,19]
oxide)
and
poly(ethylene-co-butylene)-b-poly(ethylene
compared with other mesoporous Pt/Ce Zr O materials and
x 1−x 2
oxide)),[ the obtained mesoporous Ce Zr O materials still
20]
the mesoporous Pt/Ce0.8Zr0.2O -s with small pore size. The
x
1−x
2
2
suffer the disadvantages of poor thermal stability (<500 °C), low
crystallinity, and small mesopores (<10 nm), which are unfa-
vorable to load metals or metal oxides in mesopores as active
species for catalysis. To date, it still remains a great challenge
enhanced catalytic performance can be attributed to the intrinsic
catalytic properties and the interconnected large-pore structures
of mesoporous Pt/Ce0.8Zr0.2O catalyst, which promise the high
2
surface oxygen mobility during the reaction and provide abun-
dant catalytic interfaces, respectively. The results could give a
great opportunity for large-pore mesoporous Pt/Ce Zr O cat-
to synthesize highly ordered mesoporous Ce Zr1−xO2 solid
x
solutions with well-connected large pores (>10 nm) and fully
crystalline framework, which are desirable to stably load Pt
nanoparticles and improve catalytic efficiency for CO oxidation
as well. In recent years, our group has succeeded in designing
new amphiphilic block copolymer templates of poly(ethylene
oxide)-b-polystyrene (PEO-b-PS), which have hydrophobic PS
segments of high molecular weights and relatively higher ther-
modegradation temperatures than most reported copolymers.
These novel templates were demonstrated to be powerful for
synthesis of mesoporous metal oxides of single component,
x
1−x
2
alysts used as the commercial catalysts in catalytic convertor of
advanced engines and other catalytic reactions.
2
. Results and Discussion
Scheme
1 illustrates the synthesis strategy of ordered
mesoporous Ce Zr1−xO2 (0 < x < 1) solid solutions and Pt/
x
Ce Zr1−xO2 catalysts. Typically, the water-insoluble diblock
x
such as TiO2,[ WO3,
21]
[22]
Nb O ,
[23]
and Al O ,
[24]
with large
copolymer of PEO-b-PS was dissolved in tetrahydrofuran, and
then the solution was mixed with an ethanolic solution con-
taining Zr(acac) and CeCl ·7H O to form the homogeneous
2
3
2
3
mesopores, high crystallinity, and enhanced mass transport.
They possessed high surface area with numerous exposed and
accessible active sites, and thus showed advantages in creating
rich metal oxide–metal interfaces for catalysis.[
4
3
2
solution. Because the hydrolyzed hydrophilic inorganic zirco-
nium and cerium species can interact with the PEO segments
22b]
Scheme 1. The synthesis process of ordered mesoporous Ce Zr O solid solutions and Pt/Ce Zr O catalysts. Step 1: the spherical PEO-b-PS/
x
1−x
2
x
1−x
2
inorganic sol composite micelles with PS blocks as the core and PEO segments as the shell were formed and packed into ordered fcc mesostructure
with the evaporation of solvent. Step 2: ordered mesoporous Ce Zr O (0 < x < 1) solid solutions with large mesopores and tunable Ce/Zr ratios were
x
1−x
2
obtained by calcination in air at 600 °C. Step 3: the functional Pt/Ce Zr O catalysts were synthesized through in situ reduction of H PtCl aqueous
x
1−x
2
2
6
solution containing Ce Zr O powders by NaBH .
x
1−x
2
4
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by hydrogen bonding,[18–20] during the solvent evaporation pro-
cess, PEO-b-PS copolymers tend to form spherical composite
micelles composed of hydrophobic PS blocks as the core and
inorganic species associated PEO segments as the shell. The
spherical composite micelles gradually pack into face-centered
cubic (fcc) ordered mesostructures to achieve a minimum
interface free energy (step 1). The as-formed inorganic–
polymer composites were further calcined in air at 600 °C to
remove the template and promote the crystallization of CeO2–
ZrO framework, and then the ordered mesoporous Ce Zr1−xO2
phenomenon is attributed to the slight lattice shrinkage
4+
induced by partial replacement of Ce cations (0.97 Å) with
smaller radius Zr4 cations (0.84 Å),
+
[14,25]
which indicates the
uniform formation of CeO –ZrO2 solid solutions. The pure
2
mesoporous ZrO2 shows well-resolved diffraction peaks in
range of 10–70°/2θ (Figure 1A, a), corresponding to the typical
tetragonal phase of ZrO (JCPDS Card No. 50-1089). It should
2
be mentioned that, in comparison with pure mesoporous CeO2
and ZrO with relatively sharp diffraction peaks (Figure 1A, e
2
and a), the samples of mesoporous Ce0.8Zr0.2O , Ce0.5Zr0.5O ,
2
x
2
2
(
0 < x < 1) solid solutions with large mesopores and tunable Ce/
and Ce0.2Zr0.8O2 exhibit broadened peaks (Figure 1A, d–b),
implying the formation of smaller Ce Zr O crystallites in the
framework. This result clearly indicates that the cohydrolysis
of cerium with zirconium precursors and their co-assembly
Zr ratios were obtained (step 2). For comparison, in this work
mesoporous CeO , Ce0.8Zr0.2O , Ce0.5Zr0.5O , Ce0.2Zr0.8O , and
ZrO were synthesized and studied in detail. Finally, through
x
1−x
2
2
2
2
2
2
in situ reduction of H PtCl ·6H O aqueous solution by NaBH ,
with block copolymers tend to form Ce Zr O crystallites at
2
6
2
4
x
1−x
2
[
10a]
Pt nanoparticles were deposited within the pore channels of the
ordered mesoporous Ce Zr O solid solutions (step 3), and
nanoscale with enhanced thermal stability.
Ce Zr O solid
x 1−x 2
solutions have three stable phases, i.e., monoclinic, tetragonal,
x
1−x
2
[26]
the Pt/Ce Zr O materials were used as catalysts in CO low-
and cubic, but it is difficult to identify their crystallographic
structure simply according to the broadened diffraction peaks
in XRD patterns. Raman spectroscopy is very sensitive to rec-
ognize both MO (M represents metal) bond arrangement and
lattice defects, which can be used to further determine the exact
x
1−x
2
temperature oxidation.
The wide-angle X-ray diffraction (XRD) pattern of pure
mesoporous CeO displays four well-resolved diffraction peaks
2
(Figure 1A, e) indexed to (111), (200), (220), and (311) reflec-
tions of cubic fluorite-type structure (JCPDS Card No. 34-0394),
respectively. With the introduction of zirconium of increasing
contents from Ce0.8Zr0.2O to Ce0.5Zr0.5O and to Ce0.2Zr0.8O ,
phases of our mesoporous Ce Zr O samples. Generally, there
is only one F2g Raman active mode centered at around 470 cm
x 1−x 2
−1
in cubic fluorite CeO , while tetragonal ZrO exhibits six Raman
8
2
2
2
2
[14,19]
the obtained mesoporous Ce Zr O materials exhibit peaks
active modes (A1g + 2B1g + 3Eg).
In this study, the samples
x
1−x
2
shifting to higher diffraction angles (Figure 1A, d–b). Such
of mesoporous CeO , Ce0.8Zr0.2O , and Ce0.5Zr0.5O show one
2 2 2
Figure 1. A) XRD patterns, B) Raman spectra, C) SAXS patterns, and D) N sorption isotherms of the ordered mesoporous Ce Zr O samples
2
x
1−x
2
3
−1
(
0 ≤ x ≤ 1) (a, ZrO ; b, Ce0.2Zr0.8O ; c, Ce0.5Zr0.5O ; d, Ce0.8Zr0.2O ; e, CeO ). The sorption isotherms are vertically offset by 50, 100, 150, and 200 cm g
from (b) to (e), respectively, for clarity. STP: standard temperature and pressure.
2 2 2 2 2
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−
1
strong Raman band at 470 cm , whose intensity diminishes
with the increase of zirconium content (Figure 1B, e–c). It
confirms their structure of cubic fluorite phase, in good agree-
ment with the corresponding XRD patterns (Figure 1A, e–c).
For the zirconium-rich samples (Figure 1B, b and a), the
appearance of six Raman bands located at 150, 268, 320, 465,
high-resolution TEM image (Figure 2F), and the d-spacing of
0.309 nm can be well indexed to the (111) lattice plane. The cor-
responding selected area electron diffraction (SAED) pattern
(Figure 2F, inset) with clear spotted diffraction rings further
confirms the polycrystalline walls of mesoporous Ce0.8Zr0.2O2.
The other samples of CeO , Ce0.5Zr0.5O , Ce0.2Zr0.8O , and
ZrO2 also possess highly ordered fcc mesoporous structure
with uniform spherical mesopores and highly crystalline walls
(Figures S2–S5, Supporting Information). As zirconium content
2
2
2
−1
5
60, and 630 cm illustrates the formation of Ce0.2Zr0.8O solid
solutions or ZrO with stable tetragonal phase.
2
2
Small-angle X-ray scattering (SAXS) patterns of all Ce Zr1−xO2
x
samples exhibit three well-resolved scattering peaks (Figure 1C),
which can be exactly indexed to the 111, 311, and 400 reflec-
tions of the ordered fcc mesostructure with the space group of
Fm3m.[ Compared to the mesoporous Ce Zr O solid solu-
increases in mesoporous Ce Zr O , the d-spacing of (111) lat-
x 1−x 2
tice plane gradually decreases from 0.312 to 0.294 nm (Table S1,
Supporting Information), showing the lattice shrinkage, which
agrees well with XRD measurement results.
24]
x
1−x
2
tions and ZrO samples (Figure 1C, d–a), the scattering peaks
Because the crystalline mesoporous Ce Zr O solid solu-
2
x 1−x 2
of mesoporous CeO shift to higher q values (Figure 1C, e). The
tions present the well-connected porous structures with high
surface area, large pores, intrinsic reducibility, and high oxygen
storage capacity, they can provide the enhanced target–receptor
interface, making them an ideal carrier for supported catalysts
in various heterogeneous catalytic reactions. In this study,
2
unit cell parameter is calculated to be 27.2 nm, smaller than that
of the mesoporous zirconium-containing samples (Table S1,
Supporting Information), implying the mesoporous CeO2
underwent a distinct structural shrinkage and large grains were
formed during high-temperature crystallization because of its
relatively poor thermal stability. These results further confirm
mesoporous Pt/Ce Zr1−xO2 samples were obtained through
x
decorating Pt nanoparticles, which were further utilized as
the catalysts for CO oxidation, a crucial reaction in catalytic
converter of automobile exhaust. A series of mesoporous Pt/
that the introduction of zirconium in mesoporous CeO can
2
significantly improve the thermal stability by preventing the
structural shrinkage and generation of large grains, in good
accordance with XRD patterns. Nitrogen adsorption–desorp-
tion isotherms of mesoporous Ce Zr O samples (0 ≤ x ≤ 1)
Ce Zr1−xO2 samples were synthesized by impregnating the
x
mesoporous Ce Zr1−xO2 samples (e.g., Ce0.8Zr0.2O2) with
x
H PtCl6 solution followed by subsequent in situ reduction
x
1−x
2
2
[
27]
show the typical type-IV curves with H -type hysteresis loops
in NaBH solution. Their surface area, pore volumes, and
1
4
(
Figure 1D). The sharp capillary condensation step occurs in the
pore sizes slightly decrease due to the deposition of Pt nano-
particles in the mesopores (Table S1, Supporting Informa-
tion). The mesoporous Pt/Ce0.8Zr0.2O2 sample with about
0.4 wt% Pt loading measured by inductively coupled plasma
(ICP) (Table S2, Supporting Information) exhibits the same
relative pressure (P/P ) range of 0.80–0.90, implying the forma-
0
tion of large uniform mesopores in each sample. Based on the
Barrett–Joyner–Halenda model, the average pore sizes derived
from the adsorption branch of the isotherms are calculated to
be 10.6, 12.9, 13.3, 14.9, and 15.1 nm for mesoporous CeO2,
Ce0.8Zr0.2O , Ce0.5Zr0.5O , Ce0.2Zr0.8O , and ZrO , respectively
XRD pattern as the parent mesoporous Ce0.8Zr0.2O , and
2
no signal about Pt nanoparticles is observed in the pattern
(Figure S6, Supporting Information). High-angle annular dark-
field scanning transmission electron microscopy (HAADF-
STEM) image of Pt/Ce0.8Zr0.2O2 (Figure 3A) confirms the
ordered mesoporous structure and homogeneous dispersion of
ultrafine Pt nanoparticles in size of around 4.0 nm. The cor-
responding energy-dispersive X-ray (EDX) element mappings
(Figure 3C–F) also show the uniform distributions of O, Ce, Zr,
2
2
2
2
(Figure S1 and Table S1, Supporting Information). It provides an
additional evidence that doping zirconium in mesoporous CeO2
can effectively prevent the framework shrinkage during calcina-
tion and benefit the formation of larger unit cells and mesopores,
in good consistence with SAXS results. The Brunauer–Emmett–
Teller (BET) surface area and pore volume of mesoporous
Ce0.8Zr0.2O , Ce0.5Zr0.5O , and Ce0.2Zr0.8O2 solid solutions
2
2
2
−1
3
−1
are in the range of 65.8–73.7 m g and 0.121–0.136 cm g
Table S1, Supporting Information), respectively, which are
noticeably high for mesoporous metal oxides considering the
high density of Ce Zr O solid solutions and their large pore
and Pt elements in mesoporous Pt/Ce0.8Zr0.2O . The Pt loading
2
(
measured by energy-dispersive spectrometer (EDS; Table S2,
Supporting Information) is 0.48 wt%, which is quite close to
the ICP result. The combined TEM images with different rota-
x
1−x
2
size (>10 nm). The mesoporous CeO has a BET surface area of
tion angles (Figure S15, Supporting Information), N adsorp-
tion, EDX mapping, and EDS results suggest that ultrafine Pt
nanoparticles were uniformly dispersed within the crystalline
2
2
2
−1
3
−1
4
8.7 m g and a pore volume of 0.112 cm g , lower than those
of mesoporous zirconium-containing samples, also proving that
Ce Zr O solid solutions can improve the framework thermal
mesoporous Ce0.8Zr0.2O matrix.
x
1−x
2
2
stability, and then prohibit the formation of large grains.
Field-emission scanning electron microscope (FESEM) char-
acterizations (Figure 2A,B) from both the surface and the cross-
In order to study the catalytic performance of our mesoporous
Pt/Ce Zr1−xO2 materials in CO oxidation, the gas mixture of
x
1 vol% CO, 21 vol% O , and 78 vol% N was prepared and
2
2
[1]
sections of Ce0.8Zr0.2O solid solutions exhibit the highly ordered
used as the reaction atmosphere. The catalytic activities of the
mesoporous Pt/Ce Zr O catalysts and their parent Ce Zr1−xO2
2
and well-interconnected mesostructure over a large domain.
Transmission electron microscopy (TEM) images taken along
x
1−x
2
x
matrix for CO oxidation are summarized in Figure 4 and Figure S7
[100], [110], and [211] directions (Figure 2C–E) further confirm
(Supporting Information), respectively. The pure mesoporous
the ordered fcc mesostructure (Fm3m symmetry) with large
CeO exhibits the light-off temperature (temperature of 100% CO
2
pore size and high degree of periodicity over large domains.
conversion, T100 value) as high as 403 °C (Table S3, Supporting
Information), and this value increases to 470 °C for mesoporous
The visible crystal lattice of Ce0.8Zr0.2O was observed from the
2
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Figure 2. FESEM images of the ordered mesoporous Ce0.8Zr0.2O : A) surface and B) cross-section; and their TEM images taken along the directions
2
of C) [100], D) [110], and E) [211]. F) High-resolution TEM image; the inset in the panel is the corresponding SAED pattern.
Ce0.8Zr0.2O2 and to 582 °C for mesoporous Ce0.5Zr0.5O2.
mesoporous Pt/CeO are lowered to 120 and 150 °C, respectively.
2
Such lower catalytic activity of Ce0.8Zr0.2O and Ce0.5Zr0.5O in
Noticeably, the mesoporous Pt/Ce0.8Zr0.2O exhibits better cata-
2
2
2
CO oxidation is mainly due to their lower content of catalytically
active cerium species and weaker CO adsorbability. After loading
lytic performance than mesoporous Pt/CeO , with the onset tem-
perature of 100 °C and T100 value of 130 °C. The mesoporous
Pt/Ce0.5Zr0.5O2 shows the comparable catalytic reactivity
2
about 0.4 wt% Pt nanoparticles, all the mesoporous Pt/Ce Zr1−xO2
x
catalysts exhibit significantly improved low-temperature reac-
tivity with T100 values lower than 200 °C (Figure 4A). Compared
to mesoporous CeO , the onset temperature and T value on
(T100 = 153 °C) with mesoporous Pt/CeO , while as zirconium
2
content further increases, T100 values increase to 170 and 198 °C
for mesoporous Pt/Ce0.2Zr0.8O and Pt/ZrO , respectively. The
2
100
2
2
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Figure 3. A) HAADF-STEM image, B) high-resolution TEM image, and energy-dispersive X-ray element mapping of C) O, D) Zr, E) Ce, and F) Pt
elements in the ordered mesoporous Pt/Ce0.8Zr0.2O ; the inset in panel (A) is the size distribution of Pt nanoparticles.
2
Zr-content-dependent catalytic performance can be explained
by considering the porosity and oxygen vacancies of Ce Zr
dramatically, which led to a low concentration of active cerium
species and inferior catalytic performance as well. To evaluate the
O
2
x
1−x
4
+
solid solutions. On one hand, the partial replacement of Ce
durability of the best catalyst of mesoporous Pt/Ce0.8Zr0.2O , the
2
4+
cations with Zr cations prevents the formation of large grains;
thus, the Ce Zr O catalyst exhibits much higher surface area
dynamic continuous CO conversion test was performed at 160 °C
for 72 h with the gas hourly space velocity of 60 000 mL g− h ,
and the results show that 100% CO conversion can be main-
tained during the whole period (Figure S7, Supporting Informa-
tion). In addition, the recycling experiment running at 20–200 °C
indicates that the mesoporous Pt/Ce0.8Zr0.2O2 catalyst has a
stable performance with T100 value remaining at around 130 °C
even after 50 cycles (Figure 4B). In order to investigate the
effect of pore size on catalytic performance, mesoporous Pt/
1
−1
x
1−x
2
and larger pore volume with more exposed active sites and
enhanced mass diffusion. On the other hand, zirconium intro-
duced into CeO lattice with proper content (Zr/Ce < 0.2) can
2
promote the formation of surface oxygen vacancies, proved by
X-ray photoelectron spectroscopy (XPS) spectra in the following
section, which are beneficial for the enhancement of catalytic
efficiency. However, when too many Zr atoms entered the lat-
tice (Zr/Ce>0.5), the Ce content in the solid solutions decreased
Ce0.8Zr0.2O -s with smaller pores of 3.8 nm was synthesized by
2
Figure 4. A) Light-off curves of CO oxidation on the catalysts of ordered mesoporous Pt/Ce Zr O (0 ≤ x ≤ 1). B) CO conversion from 20 to 200 °C
x
1−x
2
with 1st to 50th cycles by using the catalyst of ordered mesoporous Pt/Ce0.8Zr O . [CO] = 1 vol%, [O ] = 21 vol%, and balanced with N (78 vol%) at
0.2
2
2
2
−
1
−1
−1
a gas hourly space velocity of 60 000 mL g h . Temperature ramp: 1 °C min ; pressure: 1 atm.
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using Pluronic P123 as a template for comparison. Its light-off
curve of CO oxidation shows T100 value at 150 °C (Figure S8 and
Table S4, Supporting Information), higher than 130 °C for Pt/
presents the lowest reduction temperatures (350 and 600 °C)
−
1
and the highest H consumption (0.87 mmol g ; Table S3,
2
Supporting Information), in good agreement with its best cata-
lytic activity in CO oxidation.
Ce0.8Zr0.2O with a large pore size of 12.3 nm. In addition, we
2
plot an Arrhenius profile, and give the pre-exponential factors
In addition, XPS was performed to acquire more detailed
information about surface chemical compositions and element
valence states in our mesoporous samples. The XPS spectra of
(
Γ) of the reaction to estimate the number of possible active
[28a]
5
centers.
The Γ value of Pt/Ce0.8Zr0.2O (≈1.5 × 10 ) is much
2
4
larger than that of Pt/Ce0.8Zr0.2O -s (≈6.9 × 10 ), implying that the
Pt/Ce0.8Zr0.2O has more available active sites for CO oxidation
Pt 4f, Ce 3d, and O 1s of mesoporous Pt/Ce0.8Zr0.2O and Pt/
2
2
CeO catalysts are shown in Figure 5, and their corresponding
2
2
than Pt/Ce0.8Zr0.2O -s (Figure S9 and Table S4, Supporting Infor-
mation). Despite the fact that the values of apparent activation
element compositions calculated by the normalized peak areas
are listed in Table S3 (Supporting Information). As shown in
Figure 5A,D, the fitted binding energy peaks at 70.8 and 74.1 eV
2
energy (E ) of Pt/Ce0.8Zr0.2O and Pt/Ce0.8Zr0.2O -s are very close,
a
2
2
0
0
the turnover frequency of Pt/Ce0.8Zr0.2O is nearly twice that of
can be assigned to Pt 4f and Pt 4f of metallic Pt, while
2
7/2 5/2
2
+
Pt/Ce0.8Zr0.2O -s. The results indicate that the mesoporous Pt/
the signals at 72.7 and 76.0 eV are associated with Pt 4f7/2 and
2
2+
2+
[27,30a]
Ce0.8Zr0.2O with large pore size and interconnected pore struc-
Pt 4f5/2 of cationic Pt , respectively.
Both metallic Pt and
2
2+
ture possesses more available anchoring sites for Pt nanoparticles
within the ordered channels, thus providing abundant catalytic
interfaces. However, much smaller pore size (3.8 nm) of Pt/
cationic Pt species are present in mesoporous Pt/Ce0.8Zr0.2O2
and Pt/CeO2 samples, indicating that H PtCl6 was not fully
2
reduced by NaBH4 or metallic Pt was partially oxidized by
2+
0
Ce0.8Zr0.2O -s is adverse to load Pt nanoparticles (≈4 nm) in the
oxygen during preparation. The Pt /Pt atomic ratios calculated
by the normalized peak areas of Pt 4f are 0.29 for mesoporous
2
pore channels, resulting in poor dispersion of Pt nanoparticles in
the matrix surface. It is clearly demonstrated that much larger Pt
nanoparticles (≈10 nm) are formed on the surface of mesoporous
Pt/Ce0.8Zr0.2O2 and 1.02 for mesoporous Pt/CeO , demon-
2
strating the quite different electronic environment of Pt species
over the two mesoporous matrices due to the strong metal–sup-
Ce0.8Zr0.2O -s, which are much larger than the pore size of
2
[30a]
Ce0.8Zr0.2O -s (Figure S10, Supporting Information). Although Pt
port interaction effect.
This result also indicates that metallic
2
loading amount of Pt/Ce0.8Zr0.2O -s is approximately the same as
Pt species are the major form in mesoporous Pt/Ce0.8Zr0.2
catalyst, which exhibit superior activity toward CO oxidation.
O
2
2
[
2b]
that of Pt/Ce0.8Zr0.2O , the Pt nanoparticles are hardly dispersed
2
inside the bulk mesostructure framework, and they are mainly
anchored on the surface of matrix, resulting in fewer catalytic
interfaces and poorer dispersion of Pt nanoparticles. In addition,
much smaller Pt nanoparticles (≈4 nm) facilitate CO oxidation
The valence state of Ce in mesoporous Pt/Ce Zr1−xO2 and
x
mesoporous Pt/CeO samples can be obtained by analyzing the
2
Ce 3d XPS spectra (Figure 5B,E). The peaks labeled as v, v , v
2
3
(3d5/2) and u, u , u (3d ) are assigned to Ce4 3d, while the
+
2
3
3/2
compared with larger ones (≈10 nm) due to more interfacial Pt
peaks labeled as v (3d ) and u (3d3/2) are characteristic of
1 5/2 1
atoms on the metal oxide–metal interfaces.[
28b]
Hence, the large-
Ce 3d.
3+
[27]
These results indicate the coexistence of trivalent
pore mesoporous Pt/Ce0.8Zr0.2O exhibits much higher catalytic
and tetravalent cerium species in both the pristine mesoporous
Ce0.8Zr0.2O and CeO (Figure S12A,C, Supporting Information)
2
activity than the corresponding mesoporous Pt/Ce0.8Zr0.2O -s
2
2
2
with small pore size.
and their Pt-loaded samples (Figure 5B,E, Supporting Informa-
3+
3+
4+
H2 temperature-programmed reduction (TPR) measure-
ment was conducted to assess the intrinsic reducibility of the
obtained samples, and the hydrogen consumption profiles as
a function of temperature were displayed in Figure S11 (Sup-
porting Information). The pristine mesoporous CeO exhibits
two reduction peaks at around 550 and 850 °C, ascribed to the
surface and bulk reduction of Ce , respectively.
TPR profile of mesoporous Ce0.8Zr0.2O solid solutions also pre-
sents two reduction peaks centered at 450 and 650 °C, much
lower than those of mesoporous CeO , which is mainly due to
tion). The atomic ratio of Ce /(Ce + Ce ) in all samples was
calculated to be around 0.2 (Table S3, Supporting Information),
implying that the introduction of zirconium and loading Pt
nanoparticles produce a negligible effect on the valence states
of Ce species. The XPS spectra of O 1s were used to identify the
oxygen valence states in the samples (Figure 5C,F). The broad
peaks could be fitted into two components at higher binding
energy of 533–535 eV and lower binding energy of 530–532 eV,
2
4+
[10a]
The H2-
2
α
attributed to the surface chemisorbed oxygen (denoted as O )
and lattice O2 species (denoted as O ), respectively.
−
β
[10a]
The
2
α
α
α
β
the increased mobility of lattice oxygen from bulk to surface
content of surface O from calculation of O /(O + O ) on
mesoporous CeO , Ce0.8Zr0.2O , Pt/CeO , and Pt/Ce0.8Zr0.2O2
after introduction of Zr4 into CeO2 lattice.
+
[29]
Furthermore,
2
2
2
the supported Pt nanoparticles can dramatically enhance the
reduction capacity of the surface oxygen in CeO or Ce Zr1−xO2
Therefore, the mesoporous Pt/Ce0.8Zr0.2O2
catalyst displays reduction peaks at much lower temperatures
is 0.16, 0.27, 0.26, and 0.32 (Table S3, Supporting Information),
respectively, revealing that both zirconium doping and platinum
loading could promote the surface oxygen production with the
2
x
solid solutions.[
10c]
α
increase of oxygen vacancies. Actually, surface O species show
of 350 and 600 °C and shows much higher H consumption
higher mobility during catalytic reactions, and the fast diffusion
of oxygen adspecies endows the mesoporous Pt/Ce0.8Zr0.2O2
with high catalytic efficiency at low temperature, resulting in its
enhanced catalytic activity in CO oxidation.
In order to further investigate the mechanistic comprehen-
sion of the enhanced low-temperature CO oxidation reactivity
2
than the parent mesoporous Ce0.8Zr0.2O2.[ This phenomenon
10c]
can be attributed to the dissociated H on Pt surfaces and the
2
formed hydrogen spillover onto the vicinity of metal oxide sup-
port, which can promote the reduction of metal oxide support
[
10,29,30]
at low temperature.
Similar results were also observed
in other Pt supported catalysts (Figure S11B, Supporting Infor-
mation). It can be seen that the mesoporous Pt/Ce0.8Zr0.2O2
of mesoporous Pt/Ce0.8Zr0.2O , in situ CO diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS) was carried
2
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Figure 5. A) Pt 4f, B) Ce 3d, and C) O 1s XPS spectra of ordered mesoporous Pt/Ce0.8Zr0.2O . D) Pt 4f, E) Ce 3d, and F) O 1s XPS spectra of ordered
2
mesoporous Pt/CeO2.
out on mesoporous Pt/Ce0.8Zr0.2O and Pt/CeO for comparison
However, CO on cationic Pt is not reactive at low temperature,
which becomes more active only at higher temperature.
2
2
−1
[2d]
(Figure 6). The isolated peak at around 1825 cm is assigned
to the bridge bonded CO on Pt (Figure 6A,C), which is consid-
Through calculating the integral area of the two resolved
DRIFTS bands, the fraction of reactive CO molecules adsorbed
[12a]
ered as a characteristic of Pt nanoparticles in both samples.
The broad peaks in the range of 1950–2150 cm− can be well
1
on metallic Pt nanoparticles in mesoporous Pt/Ce0.8Zr0.2O is
2
−1
resolved into two components centered at 2057 and 2089 cm ,
respectively. The shoulder peak at about 2057 cm− is attributed
to CO molecules linearly adsorbed on metallic Pt nanoparticles,
0.63, much higher than 0.48 in mesoporous Pt/CeO , mainly
2
1
because of the synergistic effects between Pt nanoparticles and
Ce0.8Zr O2 solid solution matrix with more surface defect
0.2
−1
[31,32]
and the narrow peak at 2089 cm is due to the IR absorption
sites.
The results are quite consistent with Pt 4f XPS anal-
of CO on ionic Pt,[
2b,d,12a]
further confirming the coexistence of
ysis, and the superior activity of mesoporous Pt/Ce0.8Zr0.2O for
2
metallic and cationic Pt species in our Pt-loading mesoporous
catalysts. As reported earlier, only the CO molecules adsorbed
on metallic Pt nanoparticles can be oxidized and then desorbed
CO oxidation in comparison with mesoporous Pt/CeO2.
It is widely accepted that CO oxidation over Pt/CeO catalysts
2
follows the Mars–van Krevelen reaction mechanism, namely,
the CO molecules bonded on Pt are oxidized by the surface
[2b]
as CO from the adsorption sites at low reaction temperature.
2
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Figure 6. Selected DRIFT spectra of CO adsorbed on the ordered mesoporous A,B) Pt/Ce0.8Zr0.2O and C,D) Pt/CeO under different conditions.
2
2
A,C) Adsorption of CO for 30 min; B,D) desorption with flushing in N at different temperatures for different times.
2
lattice oxygen in CeO and the reduced ceria is subsequently
mesoporous Pt/Ce0.8Zr0.2O shows more visible catalysis hys-
2
2
[1,2a,27]
reoxidized by gas-phase oxygen.
However, in our work
teresis phenomenon in CO oxidation, presenting a much lower
T100 value of 90 °C and a broader hysteresis loop (ΔT = 40 °C)
during the extinction process (Figure 7A; Table S5, Supporting
Information). However, the mesoporous Pt/Al O , Pt/ZrO ,
the surface chemisorbed oxygen species with facile mobility
and activity in mesoporous Pt/Ce0.8Zr0.2O play a key role in
2
enhancing the catalytic performance for CO oxidation, and thus
the reaction mechanism can be interpreted as follows. First, the
2
3
2
and Pt/TiO catalysts show higher light-off and light-out tem-
2
strong interaction between Ce0.8Zr0.2O matrix and Pt species
peratures as well as narrower hysteresis loops than mesoporous
Pt/CeO and Pt/Ce0.8Zr0.2O , indicating their superior catalytic
2
facilitates the formation of metallic Pt nanoparticles at the inter-
face, where active CO molecules are selectively adsorbed and the
chemisorbed oxygen species can be easily spilled over to neigh-
boring Pt. Second, due to the lattice distortion and production
of surface defect sites caused by zirconium doping, the surface
oxygen mobility can be significantly enhanced, which facilitates
the formation of surface oxygen vacancies and plays a pivotal
role in enhancing the catalytic activity for CO oxidation. Third,
the intrinsic porous structures with interconnected large pores
2
2
activity and enhanced catalysis hysteresis (ECH) behavior for
CO oxidation. To the best of our knowledge, few reports have
been focused on this ECH phenomenon; even the normal or
inverse hysteresis in CO oxidation was studied on Pt/Al O
2
3
[
33–35]
catalysts,
which was usually explained as a combination of
three possible reasons, such as inherent kinetic bistability, inter-
action between reaction kinetics and diffusion phenomena, and
locally high temperatures on the catalyst surface. The intrinsic
and high surface areas of mesoporous Pt/Ce0.8Zr0.2O provide
surface properties of mesoporous Ce0.8Zr0.2O solid solutions
2
2
abundant anchoring sites for Pt nanoparticles and numerous
catalytically active interfaces within the mesostructure frame-
and interfacial interactions with Pt nanoparticles are quite dif-
ferent from the matrices of mesoporous Al O , ZrO , TiO ,
2
3
2
2
work compared with the mesoporous Pt/Ce0.8Zr0.2O -s.
and CeO . On one hand, CO adsorption on both metallic and
2
2
In order to test and verify this mechanism, an additional igni-
tion/extinction experiment was conducted to compare the catal-
ysis hysteresis behavior of various catalysts in CO oxidation.
The different catalytic activity during the light-off and light-out
procedures was defined as the catalysis hysteresis behavior,
which was usually found on Pt/Al O catalysts with a narrow
cationic Pt species is favored at low temperatures, resulting in
the predominant covering and self-poisoning of Pt surface by
CO and quite low probability of oxygen species adsorbed on
Pt surface. Therefore, a high temperature is needed to remove
some weakly adsorbed CO molecules, evidenced by the gradual
decrease of CO covered on metallic Pt sites (Figure 6B). Simul-
taneously, the chemisorbed oxygen species are quickly spilled
2
3
[33–35]
hysteresis loop (ΔT) in previous reports.
Interestingly, our
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Figure 7. A) CO oxidation light-off (solid lines, indicated by arrows) and light-out (dotted lines) over different catalysts with large ordered mesopores.
B) Successive evaluation of the ordered mesoporous Pt/Ce0.8Zr0.2O catalyst for CO oxidation maintained at 100 °C for 24 h in the extinction stage. [CO] =
2
−1
−1
−1
1
vol%, [O ] = 21 vol%, and balanced with N (78 vol%) at a gas hourly space velocity of 60 000 mL g h . Temperature ramp: 1 °C min ; pressure: 1 atm.
2 2
over to the vacant Pt sites in competition with CO, and react 3. Conclusion
[36]
with the surface-adsorbed CO to form CO2.
On the other
hand, as discussed in O 1s XPS analysis (Figure 5), both zirco-
nium doping and platinum loading can promote the formation
of chemisorbed oxygen adspecies and a high density of defect
sites, which are shown to accelerate the diffusion and mobility
of surface oxygen at the interface between Pt and Ce0.8Zr0.2O2.
Once the reaction occurs, the active oxygen species are strongly
adsorbed on Pt surface even at the decreased reaction tempera-
ture, which can be continuously supplied by the Ce0.8Zr0.2O2
solid solution matrix, resulting in the lowered light-out tempera-
ture of 90 °C during the extinction process and the ECH pheno-
menon with ΔT = 40 °C. The XPS results of mesoporous Pt/
In summary, highly ordered mesoporous Ce Zr O (0 < x < 1)
1−x 2
x
solid solutions with high surface area, large pore size, crystal-
line walls, and surface reducibility have been rationally and con-
veniently synthesized through a solvent evaporation–induced
co-assembly process by using PEO-b-PS diblock copolymer as a
template, and Zr(acac) and CeCl ·7H O as the inorganic source.
4
3
2
By employing the obtained mesoporous Ce Zr O solid solu-
x
1−x
2
tions as a support, ultrafine Pt nanoparticles (about 4.0 nm) were
in situ confined inside the mesopores without pore blocking,
forming the mesoporous composite nanocatalysts with highly dis-
persed Pt species for CO catalytic oxidation at very low tempera-
tures. Compared with other mesoporous Pt/Ce Zr O materials
α
α
β
Ce0.8Zr0.2O -a after the stability test show that the O /(O + O )
2
x
1−x
2
2+
0
ratio was decreased from 0.32 to 0.22 and the ratio of Pt /Pt
and the mesoporous Pt/Ce Zr O -s with small pore size, the
0.8 0.2 2
was increased from 0.29 to 0.35. These results imply that the
surface chemisorbed oxygen species can immigrate from the
mesoporous Ce0.8Zr0.2O2 matrix to the immobilized Pt spe-
cies and react with CO adsorbed in Pt nanoparticles’ surface.
Notably, the catalytic activity of mesoporous Pt/Ce0.8Zr0.2O2
can be maintained at 100 °C over 24 h in the extinction stage
large-pore mesoporous Pt/Ce Zr O catalyst shows superior
0.8 0.2 2
catalytic performance with T100 value of 130 °C and long-term cat-
alytic stability over 50 cycles. The detailed investigation results of
H -TPR, XPS, in situ CO DRIFTS, and the light-out experiments
2
revealed that its outstanding CO oxidation behavior is mainly
attributed to the synergistic interface effect between Pt nano-
particles and the highly interconnected large-pore structures of
mesoporous Pt/Ce Zr O catalyst. The lattice distortion and for-
(
Figure 7B), suggesting that the high mobility of oxygen adspe-
cies endows the catalyst with a high reaction rate and long sta-
0.8 0.2
2
bility. Mesoporous Pt/CeO has a comparable ECH behavior
mation of rich surface defect sites induced by zirconium doping
were found to be responsible for the increased surface oxygen
mobility that facilitates the formation of surface oxygen vacan-
cies and enhances the catalytic activity for CO oxidation. Apart
from the Ce Zr O support, the highly dispersed and accessible
2
(ΔT = 40 °C) but higher light-out temperature (110 °C), mainly
due to the combined effects of facile formation and low mobility
of surface chemisorbed oxygen species (Figure 5F). It should
be mentioned that the effect of reaction exothermicity has been
considered as the reason of catalysis hysteresis, which can
result in the local overheating of catalyst surface. In order to
prohibit the local overheating, a comparison test was conducted
0.8 0.2
2
metallic Pt nanoparticles within the mesoporous Ce Zr O
0
.8 0.2
2
matrix provide abundant active sites for CO adsorption and oxida-
tion, which can also enhance the catalytic efficiency and thermal
stability. Considering their facile synthesis and excellent catalytic
to dilute the catalyst of mesoporous Pt/Ce0.8Zr0.2O with the
2
same volume of quartz sands, and the same ECH behavior was
also observed with ΔT = 40 °C, which is much broader than
those in mesoporous Pt/Al O , Pt/ZrO , and Pt/TiO catalysts
performance, the ordered mesoporous Pt/Ce Zr O materials
hold a great promise as the high-efficiency catalysts for vehicle
emission abatement and other complex heterogeneous catalysis.
x 1−x 2
2
3
2
2
(ΔT = 15, 23, and 20 °C, respectively) under the same light-out
conditions, further proving that the ECH behavior is mainly
due to the combination effects of reaction exothermicity and
Supporting Information
high mobility of oxygen adspecies of the Pt/Ce0.8Zr0.2O and Pt/
2
Supporting Information is available from the Wiley Online Library or
from the author.
CeO catalysts.
2
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Keywords
catalysis hysteresis, CeO –ZrO solid solutions, co-assembly, CO
2
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oxidation, mesoporous materials
Received: June 12, 2019
Revised: July 12, 2019
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