Hierarchically porous silica supported ceria and platinum nanoparticles for catalytic combustion of toluene

Article abstract of DOI:10.1016/j.jallcom.2021.159030

The hierarchically porous silica (NKM-5) was synthesized by the polyelectrolyte-surfactant mesomorphous complex templating method, and the NKM-5 was used as the carrier to load different amounts of ceria to prepare xCe/NKM-5 samples (x represents the CeO₂ loading amount) by the impregnation method, and then supported Pt to prepare Pt/xCe/NKM-5 catalyst for toluene catalytic combustion. NKM-5 sample has excellent specific surface area and pore structure, with the increase of CeO₂ loading amount, the specific surface area and pore volume of xCe/NKM-5 sample decreased gradually. As the CeO₂ loading amount increased, XRD and TEM characterization showed that the Pt dispersion of Pt/xCe/NKM-5 catalyst increased; XPS results showed that the proportion of Pt⁰ decreased and the proportion of Ce³⁺ increased; H₂-TPR characterization showed that the reducibility increased; and the interaction between Pt and CeO₂ increased gradually. The toluene catalytic combustion activity of Pt/xCe/NKM-5 catalyst first increased and then decreased with the increase of CeO₂ loading amount. Pt/30Ce/NKM-5 catalyst has the most excellent catalytic activity with 50% toluene conversion at 174 °C and 90% conversion at 195 °C, which is the result of the synergy effect of the hierarchical pore structure of the support and the metal-support interaction between Pt and CeO₂.

Full text of DOI:10.1016/j.jallcom.2021.159030

Journal of Alloys and Compounds 867 (2021) 159030
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Hierarchically porous silica supported ceria and platinum nanoparticles
for catalytic combustion of toluene
a
a
a
a
b
a,⁎
Yu Hao , Shaohua Chen , Luming Wu , Rui Chen , Pingchuan Sun , Tiehong Chen
a
Institute of New Catalytic Materials Science, School of Materials Science and Engineering, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai
University, Tianjin 300350, PR China
Key Laboratory of Functional Polymer Materials (MOE), College of Chemistry, Nankai University, Tianjin 300071, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The hierarchically porous silica (NKM-5) was synthesized by the polyelectrolyte-surfactant mesomorphous
complex templating method, and the NKM-5 was used as the carrier to load different amounts of ceria to
Received 17 November 2020
Received in revised form 28 January 2021
Accepted 31 January 2021
prepare xCe/NKM-5 samples (x represents the CeO
supported Pt to prepare Pt/xCe/NKM-5 catalyst for toluene catalytic combustion. NKM-5 sample has ex-
cellent specific surface area and pore structure, with the increase of CeO loading amount, the specific
surface area and pore volume of xCe/NKM-5 sample decreased gradually. As the CeO loading amount
increased, XRD and TEM characterization showed that the Pt dispersion of Pt/xCe/NKM-5 catalyst increased;
2
loading amount) by the impregnation method, and then
Available online 3 February 2021
2
2
Keywords:
Hierarchically porous
Toluene catalytic combustion
Pt nanoparticles
0
3+
XPS results showed that the proportion of Pt decreased and the proportion of Ce increased; H
2
-TPR
characterization showed that the reducibility increased; and the interaction between Pt and CeO increased
2
CeO
2
loading amount
gradually. The toluene catalytic combustion activity of Pt/xCe/NKM-5 catalyst first increased and then de-
creased with the increase of CeO2 loading amount. Pt/30Ce/NKM-5 catalyst has the most excellent catalytic
activity with 50% toluene conversion at 174 °C and 90% conversion at 195 °C, which is the result of the
synergy effect of the hierarchical pore structure of the support and the metal-support interaction between
Metal-support interaction
2
Pt and CeO .
©
2021 Elsevier B.V. All rights reserved.
1
. Introduction
Noble metal catalysts, especially Pt-based catalysts are widely used
because they have excellent catalytic activity and durability [14–16].
Zeolite and mesoporous silica are widely used as the catalyst sup-
ports due to their excellent specific surface area and uniform pore
size [17–20], but their small and single pore size limits the transport
of reactants, so it is necessary to synthesize the hierarchical pore
material that combines the advantages of various pore materials.
When hierarchical pore materials are used as the catalyst supports,
the diffusion efficiency of reactive molecules in the pores can be
improved, thereby improving the transfer efficiency of substances in
the catalytic process; in addition, the active species also can be well
dispersed [21–23]. However, the support is generally an inert carrier
and there is no interaction between the noble metal and the support,
the dispersion of the precious metal is relatively low, and the pre-
cious metal nanoparticles are easy to agglomerate, resulting in re-
latively low catalytic activity, so it is necessary to introduce a
substance that interacts with Pt to stabilize the Pt nano-
particles [24,25].
Volatile organic compounds (VOCs) are those compounds with
the melting point lower than room temperature and the boiling
point between 50 and 260 °C under the normal temperature and
pressure condition [1]. VOCs are considered to be the precursors of
ozone, photochemical smog, secondary aerosols, PM 2.5, and other
pollutants, which have a serious impact on global air pollution; in
addition, it also has physiological toxicity, carcinogenesis, and ter-
atogenic effects to the human body. Therefore, it is essential to re-
duce VOC emissions, among the available technologies, there are
already adsorption [2], photocatalysis [3,4], plasma catalysis [5],
catalytic combustion [6,7], and other technologies; catalytic com-
bustion is the most promising technology due to its high efficiency,
low energy consumption, and no secondary pollution [8].
The catalysts for catalytic combustion are mainly divided into
noble metal catalysts [9–12] and non-noble metal catalysts [8,13].
The ceria support has been widely used in various catalytic re-
actions due to its oxygen storage capacity, redox capacity, and strong
metal-support interaction [26,27]. However, the specific surface area
⁎
Corresponding author.
E-mail address: chenth@nankai.edu.cn (T. Chen).
https://doi.org/10.1016/j.jallcom.2021.159030
0925-8388/© 2021 Elsevier B.V. All rights reserved.

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
and pore volume of ceria are very low [28,29], which limits the
transfer of reactants and products; the particle size of ceria also
affects its activity, by reducing the particle size of ceria, the contact
area between noble metal and ceria can be increased, thereby pro-
moting the interaction between them. One of the methods to reduce
ceria is to load ceria on a carrier with large specific surface area
The catalysts supported Pt (1 wt%) were prepared by impregna-
tion method. 0.3 g NKM-5, xCe/NKM-5, or CeO
ethanol solution, and stirred at room temperature for 5 h, then an
appropriate amount of H PtCl .6H O was added, followed by stirring
2
was dissolved in
2
6
2
at room temperature for 5 h. Finally, the solvent was evaporated
under 60 °C water bath, the product was dried at 80 °C for 12 h,
calcined in a muffle furnace at 400 °C for 2 h, and reduced at 300 °C
[28,30–32], and then load the noble metal to achieve strong metal-
support interaction.
for 2 h under 10% H
Pt/xCe/NKM-5, and Pt/CeO
The SBA-15 sample was prepared under strong acidic conditions
[36]. 2.0 g of P123 was dissolved in the mixed solution of 15 g of H
and 60 g of 2 mol/L of HCl, and heated in a water bath at 35 °C. After
the solution was uniformly mixed, 4.25 g of TEOS was added drop-
wise under stirring. After stirring for 20 h, the white emulsion was
transferred into a Teflon-lined steel autoclave and then reacted in an
oven at 80 °C for 2 days. The obtained product was filtered, washed
to neutral, dried at 60 °C and calcined at 550 °C for 6 h. The obtained
material was denoted as SBA-15.
2
/Ar. The catalysts were denoted as Pt/NKM-5,
The hierarchically porous silica NKM-5 was synthesized by the
polyelectrolyte-surfactant mesomorphous complex templating
method, the poly(acrylic acid) (PAA) as the anionic polyelectrolyte
and hexadecyl pyridinium chloride (CPC) as the cationic surfactant;
2
.
2
O
and the xCe/NKM-5 (x represents the CeO
2
loading amount) samples
with different CeO loadings were synthesized by the impregnation
2
method. The Pt/xCe/NKM-5 catalyst was synthesized by the im-
pregnation method and used for the catalytic combustion of toluene,
the effects of different CeO loading amounts on the physical prop-
2
erties, dispersity of Pt, valence state, and reduction ability were
studied. In addition, the metal-support interaction between Pt and
The MCM-41 sample was synthesized based on the reference
[37], 0.85 g of CTAB was dissolved in 200 mL of deionized water in a
2
CeO , and the relationship with catalytic activity were also explored.
3 2
60 °C oven, and then 17.2 g of NH ·H O was added at room tem-
perature under rapid stirring, 4.37 g of TEOS was added to the above
homogeneous solution, after stirring for 10 min, the white emulsion
was transferred into a Teflon-lined steel autoclave for hydrothermal
synthesis at 80 °C for 2 h. The reaction product was separated by
filtration, washed with water, dried at 120 °C and calcined at 550 °C
for 6 h. The sample was denoted as MCM-41.
2
. Experimental section
2.1. Materials
Hexadecyl pyridinium chloride (CPC, 99%), Tetraethyls orthosili-
cate (TEOS, >99%), Ethanol (CH
ahydrate (Ce(NO .6H O, 99.5% metal basis), Manganese nitrate
tetrahydrate (Mn(NO .4H O, 98%), tetrabutyl titanate (TBT, ≥99%)
hexahydrate (H PtCl .6H O, Pt≥37.5%),
, ACS), Nitric acid (HNO , 70%), Hydrofluoric
3 2
CH OH, >99%), Cerium nitrate hex-
The preparation method of xMn/NKM-5 (x represents the loading
)
3 3
2
of MnO
persed and stirred, Ce(NO
NO .4H O. After stirred, evaporated, and dried, the product cal-
cined in a muffle furnace at 400 °C for 5 h. The Mn(NO .4H O was
calcined in a muffle furnace at 400 °C for 5 h to prepare MnO
The preparation method of xTi/NKM-5 (x represents the loading
amount of TiO ) is similar to that of xCe/NKM-5, the difference is
that the Ce(NO .6H O was replaced by tetrabutyl titanate (TBT),
and then 2 mL H O was added, then stirred, evaporated, dried, and
calcined in a muffle furnace at 550 °C for 6 h. A certain amount of
TBT was added to the ethanol, stirred for 5 h, then 2 mL H O was
added, then stirred, evaporated, dried, and finally calcined in a
muffle furnace at 550 °C for 6 h. The sample was denoted as TiO
The preparation method of Pt/MCM-41, Pt/SBA-15, Pt/NKM-5-0.3,
Pt/xMn/NKM-5, Pt/MnO , Pt/xTi/NKM-5, and Pt/TiO catalysts was
2
) was similar to that of xCe/NKM-5. After NKM-5 was dis-
)
3 2
2
3 3
)
.6H was replaced with Mn
2
O
Chloroplatinic
acid
2
6
2
(
)
3 2
2
Orthoboric acid (H
3
BO
3
3
)
3 2
2
acid (HF, ≥40%), and Cetyltrimethylammonium bromide (CTAB, 99%)
were obtained from Aladdin. Poly(acrylic acid) (PAA) (average mo-
lecular weight 240,000, 25 wt% solution in water) was purchased
2
.
2
3 2
from Alfa Aesar. Ammonium hydroxide solution (NH ·H O, 25–28%)
)
3 3
2
was purchased from Macklin. Hydrochloric acid (HCl, 36–38%) was
purchased from Tianjin Chemical Reagent Wholesale Company,
China. Pluronic P123 (EO20PO70EO20, average molecular weight
2
2
5
800) was purchased from Sigma Aldrich. All the reagents were used
without any further purification.
2
.
2.2. Catalyst preparation
2
2
the same as that of Pt/NKM-5. The loading amounts of Pt were 1%.
Hierarchically porous silica (NKM-5) was synthesized under al-
kaline condition [33,34]. 1.08 g of CPC was dissolved in 50 mL of
deionized water, and then 6.0 g of PAA was added at room tem-
2.3. Characterization
perature. After stirring for 40 min, 8.0 g of NH
above solution under vigorous stirring, after further stirring for
0 min, 4.16 g of TEOS was added. The mixture was stirred for 1 h,
and finally transferred into a Teflon-lined steel autoclave for hy-
drothermal synthesis at 80 °C for 48 h. The product was centrifuged
and washed with deionized water, dried at 60 °C for 12 h, and cal-
cined in a muffle furnace at 550 °C for 6 h. The sample was denoted
as NKM-5 [35]. NKM-5-0.3 was synthesized with 0.3 g PAA and other
reaction conditions are the same.
3
·H
2
O was added to the
X-ray diffraction (XRD) and small-angle XRD were performed
with powder sample on the Rigaku Smart Lab 3 kW diffractometer
3
(Cu K
is 40 mA.
adsorption-desorption isotherms were measured on the
α
radiation), the tube voltage is 40 kV and the tube current
N
2
Micromeritics ASAP 2020 physisorption instrument. The specific
surface area was calculated by the BET (Brunauer-Emmett-Teller)
method, the pore size distribution and the pore volume were cal-
culated by the BJH (Barrett-Joyner-Halenda) method.
The CeO
.3 g NKM-5 was dissolved in ethanol solution, and stirred at room
temperature for 5 h, then an appropriate amount of Ce(NO .6H O was
added, followed by stirring at room temperature for 5 h. Finally, the
solvent was evaporated under 60 °C water bath, the product was dried
at 80 °C for 12 h and calcined in a muffle furnace at 500 °C for 4 h. The
samples were designated as xCe/NKM-5 (x represents the loading of
CeO2, x = 20, 30, 40, and 50 wt%).
2
-loaded samples were prepared by impregnation method.
The morphology of the carrier was observed by field emission
scanning electron microscopy (FE-SEM, JSM-7800). The high re-
solution morphology and the elemental distribution of the catalysts
were obtained by transmission electron microscope (TEM, JEM-
2800) equipped with energy dispersive spectrdmeter (EDS).
The loading amount of Pt was detected by inductively coupled
plasma atomic emission spectrometry (ICP-AES) on the SpectroBlue
instrument. A certain amount of sample was put into the sample
tube, then a certain amount of aqua regia and HF were added
dropwise successively, and then heated to dissolve. After dissolution,
0
3
)
3
2
The Ce(NO
3 3 2
) .6H O was calcined in a muffle furnace at 500 °C for
4
h to prepare CeO .
2
2

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
the solution was poured into a volumetric flask, the remaining so-
lution was washed with deionized water several times and added to
the volumetric flask. Then a certain amount of 4% H
and finally the volume was fixed in the volumetric flask. Aqua regia
and 4% H BO were prepared in advance.
3 3
BO was added,
3
3
The valence state of the catalysts were determined by X-ray
photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi)
with Al K X-ray source, the binding energy (BE) was calibrated using
α
the C 1s peak (BE = 284.8 eV) of carbon contaminants.
The hydrogen temperature programmed reduction (H
2
-TPR),
hydrogen chemisorption, and toluene temperature programmed
desorption (toluene-TPD) were tested by the Auto Chemisorb II 2920
instrument. In the H
00 °C for 1 h under 5% O
perature cooled down to 30 °C, the residual O
flow of 10% H /Ar (30 mL/min) gas for 30 min, then the H
2
-TPR experiment, the catalyst was pretreated at
/He (30 mL/min) gas. After the tem-
was removed at the
-TPR curve
3
2
2
2
2
was obtained by raising the temperature to 600 °C at the heating
rate of 10 °C/min. In the hydrogen chemisorption experiment,
the catalyst was pretreated at 400 °C for 1 h under 10% H
30 mL/min) gas, then the gas was switched to argon gas to purge
the material. After the temperature cooled down to 30 °C and the
baseline was flattened, pulses of 10% H /Ar gas were introduced to
2
/Ar
(
2
the sample until the peak area unchanged. In the toluene-TPD
experiment, the catalyst was pretreated with 30 mL/min of 10%
H
2
/Ar gas at 300 °C for 1 h. Then, purged with helium gas to cool
down to 50 °C, the gas was switched to 500 ppm toluene/N gas for
0 min until the adsorption was saturated. Subsequently, the re-
sidual toluene gas was removed by purging with helium gas for
0 min. After the baseline was stabilized, the temperature was raised
to 200 °C at the rate of 10 °C/min.
Fig. 1. XRD patterns of the samples: (a) NKM-5, (b) 20Ce/NKM-5, (c) 30Ce/NKM-5,
2
(
2
d) 40Ce/NKM-5, (e) 50Ce/NKM-5, and (f) CeO .
6
3
catalyst was monitored by the thermocouple placed in the center of
the reaction bed. Before the test, the catalyst was pretreated in a N
flow (50 mL/min) at 200 °C for 1 h. By bubbling synthetic air (21%
+ 79% N ) through the carrier tank containing pure toluene and
2
2.4. Catalytic combustion of toluene
O
2
2
then further diluting it with another air stream to generate the inlet
gas containing 1000 ppm toluene. The temperature of the carrier
tank containing toluene was controlled by a condensing pump to
stabilize the saturated vapor pressure of toluene. The total flow rate
A fixed bed reactor consisting of a quartz tube (inner diameter =
mm) was used to evaluate the toluene catalytic combustion ac-
tivity. 0.1 g of the catalyst with the particle size of 40–60 mesh
obtained by extrusion and sieving was placed in the middle of the
tube reactor and fixed by quartz wool. The temperature of the
6
was kept at 100 mL/min, corresponding to the weight hourly space
velocity (WHSV) was 60,000 mL g−1
h . The toluene catalytic
−1
Fig. 2. (A) Nitrogen adsorption-desorption isotherms and (B) Pore size distribution curves of the samples: (a) NKM-5, (b) 20Ce/NKM-5, (c) 30Ce/NKM-5, (d) 40Ce/NKM-5,
e) 50Ce/NKM-5, and (f) CeO (the adsorption-desorption isotherms for NKM-5, 20Ce/NKM-5, 30Ce/NKM-5, 40Ce/NKM-5, 50Ce/NKM-5, and CeO were vertically shifted upward
by 0, 400, 800, 1200, 1600, and 2000 cm
(
2
2
3
−1
g
, respectively).
3

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
Table 1
The specific surface area, pore size, and pore volume of NKM-5, xCe/NKM-5, and CeO
2
samples.
Sample
Specific surface
area (m /g)
Mesopore
size (nm)
Nanopore size (nm)
Pore volume
(cm /g)
V
meso
3
V
nano
3
2
3
(cm /g)
(cm /g)
NKM-5
618
505
459
407
367
64
2.7
2.7
2.7
2.7
2.7
–
11–59
13–52
13–49
18–47
20–40
8–28
0.95
0.77
0.69
0.61
0.51
0.23
0.45
0.37
0.35
0.32
0.28
–
0.50
0.40
0.34
0.29
0.23
0.23
2
3
4
5
0Ce/NKM-5
0Ce/NKM-5
0Ce/NKM-5
0Ce/NKM-5
CeO2
−
1
g
Pt−1), Ea is the ap-
activity tests were tested at the temperatures between 100 and
00 °C. The conversion of toluene was calculated based on the inlet
and outlet concentration of toluene, CO and H O were the only
where r is the toluene oxidation rate (mol s
−1
3
parent activation energy (J mol ), R is the universal gas constant
(J mol−1
K ), T is the catalyst temperature (K).
−1
2
2
products in the catalytic combustion of toluene and the carbon
content was basically the same for inlet and outlet gas.
3. Results and discussion
The concentrations of toluene and CO
890 gas chromatograph (China) equipped with two Flame
Ionization Detectors (FID), where the concentration of toluene
was tested using FID with SE-30 capillary column
30 m × 0.32 mm × 0.25 µm), the CO concentration was measured
by a FID with a TDX-01 column, a methanation furnace in front of the
FID was installed. The conversion of toluene was calculated by the
following Eq. (1):
2
were analyzed by a SP-
7
3.1. Structural and morphological characterization
a
a
Fig. S1 shows the small-angle XRD of the NKM-5 sample, three
obvious diffraction peaks of (200), (210), and (211) are attributed to
the highly ordered cubic mesostructure of Pm3n [38]. Fig. 1 shows
(
2
the wide-angle XRD of NKM-5, xCe/NKM-5, and CeO
22° corresponds to the amorphous silica peak of NKM-5, as the CeO
loading amount increased, the diffraction peak decreased gradually.
The peak intensity of CeO (JCPDS No.43-1002) increased gradually
with the increase of CeO loading amount, indicating that the CeO
nanoparticles has been successfully loaded on the NKM-5 support,
and the CeO crystal particles gradually grew up with the increase of
loading amount. The CeO crystallite sizes of 20Ce/NKM-5, 30Ce/
NKM-5, 40Ce/NKM-5, 50Ce/NKM-5, and CeO samples calculated by
Scherrer equation are 4.3, 5.7, 6.5, 7.2, and 8.9 nm, respectively.
Fig. 2 shows the N adsorption-desorption isotherms and pore
size distribution curves of NKM-5, xCe/NKM-5, and CeO samples. It
2
samples, 2θ at
2
CTol,in
CTol,out
Toluene Conversion(%) =
× 100%
2
CTol,in
(1)
2
2
where CTol, in and CTol, out represent the concentration of toluene in
the inlet gas and outlet gas, respectively.
2
2
In the test, the catalyst could be maintained at a stable tem-
perature, each temperature point was maintained for about 1 h, the
concentration of reactants and products were collected for several
times at a certain temperature, and the average value was taken as
the final result. In the repeatability test, the results of the catalyst
could be repeated accurately.
The activation energy of the reaction was calculated at the con-
version rate of toluene was less than 20% according to the following
Arrhenius formula:
2
2
2
can be seen from Fig. 2A that the NKM-5 and xCe/NKM-5 samples
show type IV adsorption-desorption isotherms [39], it can be seen
from Fig. 2B that the NKM-5 and xCe/NKM-5 samples have meso-
pores concentrated at 2.7 nm and secondary pores distributed at
2
10–60 nm, the CeO sample have stacked pores in the range of
Ea
RT
8–28 nm. Table 1 shows the specific surface area, pore size dis-
tribution, and pore volume of NKM-5, xCe/NKM-5, and CeO2
lnr =
+ lnA
(2)
2
Fig. 3. SEM images of the samples: (a) NKM-5, (b) 20Ce/NKM-5, (c) 30Ce/NKM-5, (d) 40Ce/NKM-5, (e) 50Ce/NKM-5, and (f) CeO .
4

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
2
Fig. 4. STEM images of the samples: (a) NKM-5, (b) 20Ce/NKM-5, (c) 30Ce/NKM-5, (d) 40Ce/NKM-5, (e) 50Ce/NKM-5, and (f) CeO .
samples. It can be seen from Table 1 that the NKM-5 and xCe/NKM-5
samples have excellent specific surface area and pore volume. With
the increase of CeO loading amount, the specific surface area, total
2
pore volume, mesopore volume Vmeso, and secondary nanopore
volume Vnano all decreased gradually. Table S1 shows the specific
surface area and pore volume of xCe/NKM-5 samples normalized to
gram of NKM-5, it can be seen from Table S1 that with the increase
not destroy the morphology of NKM-5; the hierarchical pore struc-
ture became not obvious, which mainly due to the CeO loaded
blocked the pores. It can be seen from Fig. 3f and Fig. 4f that the pure
CeO sample has no regular morphology and the particles are also
2
2
relatively large.
2
Using NKM-5, xCe/NKM-5, and CeO as supports, the Pt/NKM-5,
Pt/xCe/NKM-5, and Pt/CeO catalysts were synthesized by impregnation
2
of CeO
xCe/NKM-5 decreased gradually. From the CeO
available volume, the pore volume decreased due to the CeO
2
loading amount, the specific surface area and pore volume of
method. Scheme 1 shows the synthesis process of Pt/xCe/NKM-5
catalyst, the actual Pt loading amounts of Pt/NKM-5, Pt/20Ce/NKM-5,
Pt/30Ce/NKM-5, Pt/40Ce/NKM-5, Pt/50Ce/NKM-5, and Pt/CeO are
2
2
volume and un-
2
blocked the pore volume and the pore volume accessibility de-
creased. It can also be seen from the Table 1 that the specific surface
1.07, 1.04, 1.08, 1.03, 1.04, and 1.00 wt%, respectively. The actual Pt
loading amounts of these catalysts are similar and consistent with the
theoretical loadings. Fig. 5 shows the XRD patterns of Pt/NKM-5,
2
area and pore volume of the CeO sample calcined by cerium nitrate
are very low.
2
Pt/xCe/NKM-5, and Pt/CeO catalysts. It can be seen from Fig. 5 that
Fig. 3 and Fig. 4 show the SEM and STEM images of NKM-5, xCe/
NKM-5, and CeO samples, it can be seen from Fig. 3a and Fig. 4a that
the NKM-5 has the spherical morphology with the particle diameter
of 200–500 nm and the hierarchical pore structure. After loading
with the increase of CeO loading amount, the diffraction peaks of Pt
2
2
(2θ at 39.8°, 46.2°, and 67.5°) weakened gradually, indicating that the
dispersion of Pt increased gradually [40]. The Pt particle sizes of
Pt/NKM-5, Pt/20Ce/NKM-5, Pt/30Ce/NKM-5, and Pt/40Ce/NKM-5 cat-
alysts calculated by Scherrer equation are 5.8, 4.9, 3.9, and 2.7 nm,
CeO
2
(Fig. 3b-e and Fig. 4b-e), the morphology of xCe/NKM-5 sample
did
is similar to that of NKM-5, indicating that the addition of CeO
2
respectively. The Pt diffraction peaks of Pt/50Ce/NKM-5 and Pt/CeO
2
Scheme 1. The preparation procedure of Pt/xCe/NKM-5 catalyst.
5

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
corresponds to the 4f7/2 peak of Pt4+ [25,41]. The ratios of Pt /Pt
0
0
2
+
4+
+
Pt +Pt are listed in the Table 2, the Pt mainly existed in the form
0
2
of Pt in the Pt/NKM-5 catalyst, with the increase of CeO loading
0
amount, the proportion of Pt decreased gradually, and the propor-
tion of Pt2++Pt increased gradually, which is because Pt loaded on
4+
the CeO
loading, the proportion of Pt loaded on the CeO
metal-support interaction between Pt and CeO
2
formed the Pt-O-Ce bond, with the increase of CeO
increased, and the
increased [42]. Ac-
2
2
2
0
cording to the literature, Pt has better catalytic combustion per-
formance than Pt2+ and Pt [43–45]. Fig. 7B shows the Ce 3d spectra
of Pt/xCe/NKM-5 catalyst, the u''' (916.9), v''' (898.6), u'' (907.7), v''
4+
(
(
889.9), u (901.1), and v (882.7) are classified as Ce4+ species, the u′
3+
0 0
903.7), v′ (885.8), u (898.4), and v (880.8) are classified as Ce
3+
3+
4+
species [15,46,47], the proportions of Ce /Ce +Ce are listed in the
3+
Table 2. It can be seen from Table 2 that the ratio of Ce increased
gradually with the increase of CeO
loading, the generation of Ce3+ is
attributed to the strong interaction between Pt and CeO ; with the
increase of CeO loading amount, the Pt tended to be dispersed on
the CeO , and more proportion of CeO was loaded with Pt, and the
2
2
2
2
2
3+
higher concentration of Ce indicates a higher concentration of
oxygen vacancies [15,48–50].
Fig. 8 shows the H
catalysts, Fig. S4 shows the H
the Pt/CeO catalyst. It can be seen from Fig. S4 that the CeO
2
-TPR curves of the Pt/NKM-5 and Pt/xCe/NKM-
-TPR curves of the CeO sample and
sample
5
2
2
2
2
has two peaks at 380 °C and 487 °C, which are attributed to the re-
duction of surface oxygen and subsurface oxygen, respectively. After
Fig. 5. XRD patterns of the catalysts: (a) Pt/NKM-5, (b) Pt/20Ce/NKM-5, (c) Pt/30Ce/
NKM-5, (d) Pt/40Ce/NKM-5, (e) Pt/50Ce/NKM-5, and (f) Pt/CeO .
2
loading Pt, the Pt/CeO
the reduction temperature shifted to lower temperature, which is
mainly because the interaction between Pt and CeO lead to the
2
catalyst has two peaks at 200 °C and 253 °C,
2
formation of Pt-O-Ce bond, which could promote the low-tem-
perature reducibility [51,52]. Fig. 8 shows that the Pt/NKM-5 catalyst
catalysts are too weak to be calculated, which means that they have
more higher dispersion. The addition of CeO could promote the
2
2
has no reduction peak, after CeO loaded, the Pt/xCe/NKM-5 catalyst
dispersity of Pt and reduce the particle size of Pt nanoparticles.
Fig. 6 and Fig. S2 show the STEM and EDS elemental mapping of
has a peak at 210 °C; the reduction peak shifted to lower tempera-
ture relative to the Pt/CeO
support dispersed the CeO
Pt and CeO
easier to break, which enhanced the overflow of oxygen on the CeO
carrier surface and improved the reducibility of the catalyst. In ad-
dition, with the increase of CeO loading amount, the hydrogen
2
catalysts, which is because the NKM-5
Pt/NKM-5, Pt/xCe/NKM-5, and Pt/CeO
2
catalysts, it can be seen
2
nanoparticles, the interaction between
from Fig. 6a-c that the Pt/NKM-5 catalyst still keep the spherical
morphology and the hierarchical pore structure, which indicates
that loading Pt did not cause obvious damage to the NKM-5.
However, the Pt nanoparticles are very obvious, and the Pt ag-
glomeration is relatively serious, which is because the NKM-5 is an
inert carrier and there is no interaction between Pt and NKM-5, the
NKM-5 support can not stabilize and disperse Pt [24,25]. The
morphology of Pt/xCe/NKM-5 catalysts also did not change sig-
nificantly after loading Pt, and there are no obvious Pt particles.
EDS elemental mapping also shows that the Pt distribution is re-
2
was enhanced, and the Ce-O bond adjacent to Pt was
2
2
consumption increased gradually, and the reducibility increased
gradually.
3.3. Catalytic activity
To investigate the hierarchical porosity of NKM-5, the commonly
used mesoporous materials MCM-41 and SBA-15, and the NKM-5-0.3
sample with few secondary pores were used for comparison. The
detailed preparation methods are shown in the Catalyst preparation.
latively uniform (Fig. 6d-f and Fig. S2), indicating that adding CeO
2
could promote the dispersion of Pt, which is consistent with the
result of XRD. Fig. 6g-i shows the STEM and EDS elemental map-
ping of Pt/CeO
2
catalyst, there is no regular morphology, which is
2
Fig. S5 shows the N adsorption-desorption isotherms and pore size
consistent with the SEM images. The Pt dispersities of Pt/NKM-5,
Pt/20Ce/NKM-5, Pt/30Ce/NKM-5, Pt/40Ce/NKM-5, Pt/50Ce/NKM-5,
distribution of MCM-41, SBA-15, and NKM-5-0.3 samples, Table S2
shows the specific surface area, pore size distribution, and pore
volume of the samples. It can be seen from Fig. S5 and Table S2 that
MCM-41 and SBA-15 only have mesopores and no secondary pores,
NKM-5-0.3 has mesopores and few secondary pores. The specific
surface area and pore volume of the three samples are relatively
excellent. Using MCM-41, SBA-15, and NKM-5-0.3 samples as car-
riers, Pt/MCM-41, Pt/SBA-15, and Pt/NKM-5-0.3 catalysts were syn-
thesized by impregnation method. The Pt loadings of Pt/MCM-41,
Pt/SBA-15 and Pt/NKM-5–0.3 catalysts are 1.03, 1.03, and 1.05 wt%,
respectively. The Pt loading of the three catalysts is similar to that of
Pt/NKM-5. Fig. S6 shows the XRD patterns of Pt/MCM-41, Pt/SBA-15,
and Pt/NKM-5-0.3 catalysts. From the Fig. S6, it can be seen that the
three catalysts have obvious Pt diffraction peaks. The Pt particle sizes
of Pt/MCM-41, Pt/SBA-15, and Pt/NKM-5-0.3 catalysts calculated by
and Pt/CeO
2
catalysts determined by hydrogen chemisorption are
2
6.3%, 43.6%, 48.7%, 51.5%, 74.8%, and 86.9%, respectively; the Pt
2
surface areas are 0.65, 1.07, 1.20, 1.27, 1.84, and 2.14 m /g catalyst,
respectively.
3.2. Chemical states and reduction property
Fig. 7 and Fig. S3 show the Pt 4f and Ce 3d XPS spectra of the
Pt/NKM-5, Pt/xCe/NKM-5, and Pt/CeO
2
catalysts. Fig. 7A shows the Pt
4
f spectra of Pt/NKM-5 and Pt/xCe/NKM-5 catalysts, Fig. S3A shows
2
the Pt 4f spectra of Pt/CeO catalyst, the peak at around 71.2 eV cor-
0
responds to the 4f7/2 peak of Pt , the peak at around 72.4 eV corre-
sponds to the 4f7/2 peak of Pt2+, and the peak at around 74.9 eV
6

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
Fig. 6. STEM of the catalysts: (a) (b) Pt/NKM-5, (d) (e) Pt/30Ce/NKM-5, and (g) (h) Pt/CeO
2
. EDS elemental mapping of the catalysts: (c) Pt/NKM-5, (f) Pt/30Ce/NKM-5, and
(
2
i) Pt/CeO .
Scherrer equation are 5.4, 4.8, and 6.0 nm, respectively, and the Pt
particle size is similar to that of Pt/NKM-5. Fig. 9 shows the toluene
catalytic combustion activity of Pt/NKM-5, Pt/MCM-41, Pt/SBA-15,
and Pt/NKM-5–0.3 catalysts, Table S3 shows the T50 (the tempera-
ture at 50% conversion) and T90 (the temperature at 90% conversion)
of the catalysts. From the Fig. 9 and Table S3, it can be seen that the
Pt/NKM-5 catalyst has better catalytic activity than Pt/MCM-41,
Pt/SBA-15, and Pt/NKM-5–0.3 catalysts, which is mainly due to the
hierarchical pore structure of the NKM-5 sample, the hierarchical
pore could promote the mass transport, reduce the diffusion path of
reactants, improve the accessibility of active sites, and thus improve
the catalytic performance [23,53,54].
seen that both T50 and T90 first decreased and then increased with the
increase of CeO loading amount. The Pt/30Ce/NKM-5 catalyst has the
lowest T50 and T90 temperatures, of which the T50 is 174 °C and the T90
is 195 °C. The activity first increased may be due to the addition of CeO
could enhance the metal-support interaction between Pt and CeO
promote the dispersion of Pt, and increase the reduction ability. When
CeO loading amount increased to a certain extent, the activity de-
creased may be due to the CeO added reduced the specific surface area
and pore volume, hindering the transfer of reactants and products, in
addition, the active species Pt0 also decreased. The excellent catalytic
activity of Pt/30Ce/NKM-5 is attributed to the hierarchical pore struc-
ture of the support and the metal-support interaction between Pt and
2
2
2
,
2
2
Fig. 10A shows the toluene catalytic combustion activity of
2
CeO . Besides, the activity of the catalysts in this work were compared
Pt/NKM-5, Pt/xCe/NKM-5, and Pt/CeO
activities of the catalysts first increased and then decreased with
the increase of CeO loading amount. The Pt/30Ce/NKM-5 catalyst has
the most excellent catalytic activity. Fig. 10B and Table 3 shows the T50
and T90 of the Pt/NKM-5, Pt/xCe/NKM-5, and Pt/CeO catalysts. It can be
2
catalysts. It can be seen that the
with other catalysts in the literature, the results are listed in the Table S3,
It can be seen from the Table S3 that the Pt/30Ce/NKM-5 catalyst has
excellent catalytic activity. The reaction space velocity and particle
size were adjusted to eliminate internal and external diffusion, and
the kinetic measurement was performed under the conditions of
2
2
7

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
Fig. 7. (A) Pt 4f XPS spectra of the catalysts: (a) Pt/NKM-5, (b) Pt/20Ce/NKM-5, (c) Pt/30Ce/NKM-5, (d) Pt/40Ce/NKM-5, and (e) Pt/50Ce/NKM-5. (B) Ce 3d XPS spectra of the
catalysts: (a) Pt/20Ce/NKM-5, (b) Pt/30Ce/NKM-5, (c) Pt/40Ce/NKM-5, and (d) Pt/50Ce/NKM-5.
Table 2
The ratios of Pt /Pt +Pt2++Pt4+ and Ce3+/Ce3++Ce4+ of Pt/NKM-5, Pt/xCe/NKM-5, and
Pt/CeO catalysts.
WHSV = 120,000 mL g−1 h−1 and size = 40–60 mesh, while keeping
the conversion rate below 20% [55,56]. Fig. 10C shows the Arrhenius
plots of toluene conversion reaction rate vs 1/T of Pt/NKM-5 and
Pt/30Ce/NKM-5 catalysts. The activation energy of the Pt/NKM-5 and
Pt/30Ce/NKM-5 catalysts are 149 and 104 kJ mol−1, respectively. The
activation energy of Pt/30Ce/NKM-5 is lower than that of Pt/NKM-5,
which indicates that the toluene catalytic combustion reaction oc-
curred more easily on the Pt/30Ce/NKM-5 catalyst. The stability test
0
0
2
0
0
Ce3+/Ce3++Ce4+
Catalyst
Pt /Pt +Pt2++Pt4+
Pt/NKM-5
0.64
0.56
0.54
0.52
0.46
0.25
–
Pt/20Ce/NKM-5
Pt/30Ce/NKM-5
Pt/40Ce/NKM-5
Pt/50Ce/NKM-5
Pt/CeO2
0.32
0.34
0.35
0.36
0.37
2
of Pt/30Ce/NKM-5 catalyst was performed at 190 °C, CO was the
only product detected and no other by-products were produced. The
relationship between the catalytic activity of toluene and the reac-
tion time is shown in Fig. 10D. It can be seen from Fig. 10D that the
catalyst could maintain 80% conversion during 30 h of catalytic re-
action, indicating that the catalyst has excellent stability.
2
Fig. 8. H -TPR curves of the catalysts: (a) Pt/NKM-5, (b) Pt/20Ce/NKM-5, (c) Pt/30Ce/
Fig. 9. Toluene catalytic combustion activity of Pt/NKM-5, Pt/MCM-41, Pt/SBA-15, and
NKM-5, (d) Pt/40Ce/NKM-5, and (e) Pt/50Ce/NKM-5.
Pt/NKM-5-0.3 catalysts.
8

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
2 2
Fig. 10. (A) Toluene catalytic combustion activity of Pt/NKM-5, Pt/xCe/NKM-5, and Pt/CeO catalysts. (B) The T50 and T90 of the Pt/NKM-5, Pt/xCe/NKM-5, and Pt/CeO catalysts.
(
C) Arrhenius plots of reaction rate vs 1/T of Pt/NKM-5 and Pt/30Ce/NKM-5 catalysts. (D) Dependence of toluene catalytic combustion activity on reaction time over the
−1
−1
Pt/30Ce/NKM-5 catalyst under the conditions of toluene concentration = 1000 ppm, mass space velocity = 60,000 mL g
h , and reaction temperature = 190 °C.
Table 3
prepare Pt/xMn/NKM-5 and Pt/xTi/NKM-5 catalyst by the im-
pregnation method, the detailed preparation process of the cata-
lysts are shown in the Catalyst preparation. Fig. S8 shows the XRD
patterns of Pt/xMn/NKM-5 and Pt/xTi/NKM-5 catalysts. It can be
The Pt loading amount, Pt surface area, T50, and T90 of Pt/NKM-5, Pt/xCe/NKM-5, and
Pt/CeO catalysts.
2
Catalyst
Pt loading
amount (wt%)
Pt surface area
(m /g catalyst)
T
50 (°C)
T90 (°C)
2
seen that the diffraction peaks of MnO
TiO (JCPDS No.21-1272) gradually increased, the Pt diffraction
peaks decreased, indicating that the addition of MnO and TiO
2
(JCPDS No.24-0735) and
Pt/NKM-5
1.07
1.04
1.08
1.03
1.04
1.00
0.65
1.07
1.20
1.27
1.84
2.14
202
191
174
187
221
291
217
2
Pt/20Ce/NKM-5
Pt/30Ce/NKM-5
Pt/40Ce/NKM-5
Pt/50Ce/NKM-5
Pt/CeO2
208
195
206
236
315
2
2
could promote the distribution of Pt, which is consistent with the
results of the Pt/xCe/NKM-5 catalyst. Fig. 12 and Table S4 show the
toluene catalytic combustion activity of Pt/xMn/NKM-5 and
Pt/xTi/NKM-5 catalysts. It can be seen from Fig. 12 and Table S4
that the Pt/40Mn/NKM-5 and Pt/30Ti/NKM-5 catalyst have the
most excellent catalytic activity. From the results of Pt/xCe/NKM-5,
Pt/xMn/NKM-5 and Pt/xTi/NKM-5 catalysts, it could be concluded that
Fig. 11 shows the toluene-TPD curves of Pt/NKM-5 and Pt/30Ce/
NKM-5 catalysts. It can be seen that the desorption temperature of
Pt/30Ce/NKM-5 catalyst is higher than Pt/NKM-5 catalyst, indicating
that toluene bond more closely with Pt/30Ce/NKM-5, and the des-
orption peak area of Pt/30Ce/NKM-5 catalyst is larger than Pt/NKM-
2 2 2
adding a certain content of CeO , MnO , or TiO could improve the
activity of the Pt/NKM-5 catalyst.
4. Conclusions
5
, indicating that Pt/30Ce/NKM-5 catalyst has stronger toluene ad-
sorption capacity than Pt/NKM-5 catalyst [57–59].
In addition to CeO , other oxides such as MnO
also been studied, the NKM-5 was used as the carrier to load
different amounts of MnO and TiO to prepare xMn/NKM-5 and
xTi/NKM-5 samples (x represents the MnO or TiO loading
amount) by the impregnation method, and then supported Pt to
The Pt/xCe/NKM-5 catalyst was prepared by using the hier-
archically porous silica NKM-5 as support and successively loading
CeO and Pt nanoparticles by the impregnation method. The NKM-5
2
support has mesopores (2.7 nm) and secondary pores (10–60 nm),
and the specific surface area and pore volume are excellent. After
2
2
and TiO
2
have
2
2
2
2
2
CeO loaded, the specific surface area and pore volume decreased
9

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
δ+
gradually, the proportion of Pt increased gradually, the proportion
of Ce3+ increased gradually, and the reducibility increased gradually,
indicating that the metal-support interaction between Pt and CeO
increased gradually. As the CeO loading amount increased, the
2
2
metal-support interaction increased gradually, the dispersion of Pt
and the reducibility of the catalyst increased gradually, which is
beneficial to the catalytic reaction; but the specific surface area, pore
0
volume, and the proportion of Pt decreased, which is not conducive
to catalytic reaction. The results of toluene catalytic combustion
show that the Pt/30Ce/NKM-5 catalyst with a suitable loading has
the most excellent catalytic performance.
CRediT authorship contribution statement
Yu Hao: Experiments, data analysis and paper writing.
Shaohua Chen: Experiments and data analysis. Luming Wu:
Experiments and data analysis. Rui Chen: XPS measurements and
data analysis. Pingchuan Sun: Discussion and data analysis.
Tiehong Chen: Discussion and paper writing.
Fig. 11. Toluene-TPD curves of the Pt/NKM-5 and Pt/30Ce/NKM-5 catalysts.
gradually with the increase of CeO
of Pt nanoparticles is very poor in the Pt/NKM-5 catalyst, the Pt
nanoparticles have a serious agglomeration phenomenon, after CeO
loaded, the dispersity of Pt nanoparticles was improved. With the
increase of CeO
loading amount, the proportion of Pt0 decreased
2
loading amount. The dispersity
Declaration of Competing Interest
2
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
2
Fig. 12. (A) Toluene catalytic combustion activity of Pt/NKM-5, Pt/xMn/NKM-5, and Pt/MnO
2
catalysts. (B) The T50 and T90 of the Pt/NKM-5, Pt/xMn/NKM-5, and Pt/MnO
2
catalysts.
(
C) Toluene catalytic combustion activity of Pt/NKM-5, Pt/xTi/NKM-5, and Pt/TiO catalysts. (D) The T50 and T90 of the Pt/NKM-5, Pt/xTi/NKM-5, and Pt/TiO
2
2
catalysts.
10

Y. Hao, S. Chen, L. Wu et al.
Journal of Alloys and Compounds 867 (2021) 159030
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