Nanostructured platinum in ordered mesoporous silica as novel efficient catalyst for propane total oxidation

Article abstract of DOI:10.1039/C5RA27303J

Mesoporous SBA-15 and KIT-6 supporting platinum catalysts were prepared and subsequently used in propane total oxidation. The properties of the prepared catalysts were investigated by low-angle X-ray diffraction, nitrogen physisorption, transmission electron microscopy, CO chemisorption, X-ray photoelectron spectroscopy and H₂-titration methods. Significant reaction rate enhancements have been achieved via mesopore introduction, with turnover frequencies (TOF) in the sequence: Pt/KIT-6 (60.30 10³ s¹) [ Pt/SBA-15 (19.27 10³ s¹) > Pt/Al₂O₃ (5.11 10³ s¹). The results indicate that the synthesized catalysts have ordered mesoporous structures and well-dispersed Pt species. They exhibit superior catalytic activities for propane total oxidation in comparison to the conventional Pt/ Al₂O₃ catalyst. The 2-D hexagonal channels in SBA-15 and the 3-D cubic structure in KIT-6 can successfully control the particle sizes of platinum in the mesopores. By contrast, the platinum nanoparticles in KIT-6 are smaller than those in SBA-15, with homogeneous size distribution and exist in lower oxidation state, which are beneficial to the catalytic activity.

Full text of DOI:10.1039/C5RA27303J

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RSC Advances
ARTICLE
Nanostructured platinum in ordered mesoporous silica as novel
efficient catalyst for propane total oxidation
a
a,*
a
b
a,b,*
Yidan Cao, Rui Ran, Yusheng Chen, Xiaodong Wu and Duan Weng
Received 00th January 20xx,
Accepted 00th January 20xx
Mesoporous SBA-15 and KIT-6 supporting platinum catalysts were prepared and subsequently used in propane total
oxidation. The properties of the prepared catalysts were investigated by low-angle X-ray diffraction, nitrogen physisorption,
DOI: 10.1039/x0xx00000x
transmission electron microscopy, CO chemisorption, X-ray photoelectron spectroscopy and H
Significant reaction rate enhancements have been achieved via mesopore introduction with the turnover frequencies (TOF)
in the sequence of Pt/KIT-6 (60.30 × 10-3 s-1) >> Pt/SBA-15 (19.27 × 10-3 s-1) > Pt/Al (5.11 × 10-3 s-1). The results indicate
that the synthesized catalysts have orderd mesoporous structures and well-dispersed Pt species. They exhibit superior
catalytic activities for propane total oxidation in comparison to the conventional Pt/Al catalyst. The 2-D hexagonal
2
-titration methods.
www.rsc.org/
O
2 3
O
2 3
channels in SBA-15 and 3-D cubic structure in KIT-6 can successfully control the particle sizes of platinum in the mesopores.
By contrast, platinum nanoparticles in KIT-6 are smaller with homogeneous size distribution and exist in lower oxidation
state, which are beneficial to the catalytic activity.
pore diameters of 2.0-30.0 nm and high surface areas of 600-
2
-1
Introduction
Diesel engine vehicles have been rising rapidly in worldwide
due to their high fuel efficiency, reliability and durability
compared to gasoline cars. Consequently, diesel exhaust
pollutions are leading to increasingly stringent emission
standards. Besides particulate matter (PM) and nitrogen oxides
1000 m g provide an ideal scaffold for the synthesis of metal
nanoparticles. The disclosure of Santa Barbara amorphous-15
(
SBA-15) with 2-dimensional hexagonal pore structure
connected with small micropores, sparked considerable
interest for its large pore size, thicker wall and good stability 12
Korea Advanced Institute of Science and Technology-6 (KIT-6)
silica is another charming material with 3-dimensional cubic
pore structure which has already received attention of many
1
.
(NO
x
), diesel emissions also contain a large amount of unburned
2
hydrocarbons (HC) existing in both gas and particle phases. The
cold-start emissions account for 50–80% of the total HCs
13-17
researchers for various applications
. The combination of
noble metal nanoparticles and mesoporous materials has been
gaining increasing importance in heterogeneous catalysis,
exploiting the exquisite control over the surface, structural and
functional properties of the mesoporous supports, and high and
tailorable interface area between both components.18
Mesopores are indeed highly tuned nanoscopic environments
that can influence the catalytic behavior. Beyond the expected
effects of shape and size selectivity, nano-confinement effects
due to restricted volume and surface curvature result in a
3, 4
pollutions from automotives.
emission as well as CO and the soluble organic fraction (SOF) of
PM, Pt/Al system as a typical diesel oxidation catalyst (DOC)
For the reduction of HC
2 3
O
has been widely applied in current commercial oxidation
catalysts installed in diesel driven vehicles. However, the
efficient oxidation of saturated hydrocarbons on Pt/Al
catalyst generally requires relative high temperatures. It is
therefore important to find routes to further improve the low-
temperature oxidation activity of platinum catalysts.
5
2
O
3
6
18-20
14, 21, 22
lowering of kinetic barriers.
It has been reported
that
Mesoporous silica is an interesting catalyst support material
for active metal oxides and metals, owing to a combination of
good accessibility, uniform pore size and high surface area. The
discovery of highly ordered mesoporous silica has stimulated
extensive studies on the synthesis of silica-supported
nanostructured materials, which have potential applications in
catalysis, gas sensors, nano electronic/optical devices, medical
the mesoporous SBA-15 and KIT-6 acting as support materials
are beneficial to disperse and confine nanostructured noble
metals in the pores or channels of mesoporous silica. Yuranov
et al. reported that Pd nanoparticles with controlled size can
be synthesised within the mesopores of SBA-15 silicas. Parlett
et al. also found that improved mass transport and increased
Pd dispersion can be achieved by SBA-15 and KIT-6 support to
effectively catalyze the selective oxidation of allylic alcohols by
Pd. Similarly, mesopores in mesoporous silica such as SBA-15
has also been utilized to keep Pt nanoparticles in controlled size
22
2
3
7
-11
diagnosis, etc
. Mesoporous silica frameworks with uniform
a.State Key Laboratory of New Ceramics and Fine Processing, School of Materials
Science and Engineering, Tsinghua University, Beijing 100084, China.
2
4-28
and shape.
On the other hand, mesoporous silicas have
b.
Key Laboratory of Advanced Materials of Ministry of Education, School of
been investigated as catalytic supports for noble metals to
catalyse the combustion of HC such as methane 1
Materials Science and Engineering, Tsinghua University, Beijing 100084, China.
Corresponding author: ranr@tsinghua.edu.cn; duanweng@tsinghua.edu.cn
2,
29, 30
*
.
.
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However, according to our knowledge, SBA-15 and KIT-6 have Pt. Details of the procedure for CO chemisorption were
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3
3
DOI: 10.1039/C5RA27303J
not yet been investigated as support materials for platinum described elsewhere . The dispersion and particle size of
based catalysts in propane abatement.
platinum were calculated by the following equations in Ref. 34
.
In this work, SBA-15 and in particular KIT-6 have been
explored as supports for Pt catalysts for low temperature
abatement of propane emitted by diesel engines. Mesoporous
Pt/SBA-15 and Pt/KIT-6 catalysts were prepared and used in
DPt (%)  100   V / f / C W  22, 414  M 





s
s
s

9
dPt  6  C  M 10 /   N  D




a
a
Pt
where DPt is the metal dispersion of Pt (%),V
s
is the CO
propane total oxidation compared with convention Pt/Al
catalyst. The properties of the prepared catalysts were
investigated by low-angle X-ray diffraction, nitrogen is the Pt atomic mass (= 195.1 g/mol), dPt is the calculated
physisorption, transmission electron microscopy, CO particle size of Pt, C
is the concentration of surface metal atoms
(= 1.27×10 atoms/m2) 35, ρ is the volumic mass (= 12.02×106
2
O
3
volume adsorbed (mL at STP), f is the stoichiometric factor (= 1),
C
s
is Pt metal content (= 1 wt.%), W is the sample weight (g), M
s
a
1
9
chemisorption, X-ray photoelectron spectroscopy and H
titration methods.
2
-
3
35
is the Avogadro constant (=6.02×1023).
g/m ) and N
titration was performed on Micromeritics Auto Chem II
920 apparatus to characterize the stoichiometry of platinum
species. For each experiment, approximately 50mg of catalyst
was placed in a U-shaped quartz tube (i.d.=10 mm). The catalyst
was heated to 250 ºC and then held at for 10 min in flowing Ar
for proper degassing. After that the the loop gas of 10% H /Ar
2
was pulsed over the sample and the TCD signal was recorded
until the peak area became constant. The total consumption of
a
H
2
2
Experimental
SBA-15 and KIT-6 was obtained according to the procedure
reported by 3 and , respectively. Calcined SBA-15 and KIT-6
1
16
supported Pt catalysts were prepared by means of the incipient
wetness method, using a platinum nitrate precursor. The metal
loading of Pt is 1 wt. %. After impregnation, each catalyst was
dried in an oven at 100 ºC for 12 h. The catalysts were then
H
2
was integrated to deduce the stoichiometry of platinum
species.
X-ray photoelectron spectroscopy (XPS) experiments were
2 3
O (SBET= 150 m g , BASF).
calcined at 550 ºC for 3 h. Pt/Al
reference sample with commercial Al
2 3
O was also prepared as a
2
-1
performed on an ESCALAB 250Xi system equipped with
monochromatic Al Kα X-rays (1486.8 eV) and under ultrahigh
vacuum (~10− Pa). Prior to the measurements, samples were
degassed under vacuum at room temperature for 1 h. Sample
charging during the measurement was compensated by an
electron flood gun. The electron take-off angle was 45° with
respect to the sample surface. The analysis area was ~500 µm
in diameter. XPS data from the regions relating to Pt 4f core
levels were recorded. All binding energies were calibrated using
the C 1s carbon peak (284.6 eV).
The pore volumes and specific surface area of the catalysts
were measured by nitrogen physisorption-desorption at liquid
nitrogen temperature using a surface area and pore size
analyzer (JW-BK122F, Beijing JWGB Sci. &Tech., China). The
catalysts were degassed at 200 ºC for 2 h prior to the
measurements. The pore volumes for the samples were
calculatedby the BJH (Barrett–Joyner–Halenda) methods.
Low-angle X-ray diffraction (XRD) patterns were obtained on
9
a Rigaku D/Max-2500/PC diffractometer (Japan) with a Cu K
α
radiation (λ = 0.15418 nm) source operating at 30 kV and 40 mA.
The diffraction patterns were collected from 0.6 º to 10.0 º at
-
1
0
.02 º intervals with a rate of 1 ºmin . The crystal phase was
2
1, 32
identified according to the literatures
.
Transmission electron microscope (TEM) images were taken
using a JEOL 2100 transmission electron microscope (Japan). For
the sample preparation, the samples were crushed in an agate
mortar and suspended in ethanol. After ultrasonic dispersion, a
droplet was deposited on a copper grid supporting a perforated
carbon film. The mean Pt particle size was determined by
analyzing 200 particles for each sample.
The pulse CO chemisorption method (Autochem II 2920,
Micromeritics Co., USA) was used to measure the dispersion of
Table 1 Characteristic data of synthesized catalysts
2
-1
Sample
S
BET(m g )
Pore volume
Dispersion
of Pt (%)
dPt (nm)
valence
of Pt c
Pt species on
surface (%)
3
-1
a
d
(cm g )
Support Pt/support Support Pt/support
CO
chem. a
5.00 6.09
4.03 4.57
TEM b
Pt0
Pt2+ Pt4+
Pt/SBA15
Pt/KIT6
559
676
484
659
1.351
0.917
1.103
0.844
22.6
28.1
d
2.06
16.0 60.1 23.9
41.1 47.1 11.8
1.34
a
b
c
Dispersion of platinum and dPt were calculated from CO chemisorption;
Pt was calculated from TEM analysis; Valence of Pt was calculated
from H
2
-titration; d Ratios of Pt species were obtained from XPS analysis.
2
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For the catalytic activity test, 100 mg catalyst (50-80 mesh)
was diluted with 500 mg quartz sands to prevent a reaction
runaway. The mixture was loaded in a quartz reaction tube with
a diameter of 10 mm. The catalytic activity of the catalysts was
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DOI: 10.1039/C5RA27303J
evaluated by passing a gas mixture of 1000ppm C
3
H
8
, 2% O
in balance. The space velocity was 60000 h . The reactor was
heated from RT to 500 ºC at a heating rate of 10 ºC/min after N
2
and
-1
N
2
2
pretreatment at 500 ºC for 30 min. The outlet gas from the
reactor was analyzed by an FTIR gas analyzer (MKS MultiGas
2
030, USA). Turnover frequency (TOF) for propane oxidation on
the catalysts was calculated with the reaction rate for CO
2
formation at T=190 ºC giving C
3
H
8
-1
conversions <10% as follows:
min- Pt-atoms ) = (reaction rate for CO
1
TOF (mol-CO
formation (mol-CO
2
2
-1
-1
2
min gcat )) / (number of surface Pt atoms
-1
estimated by CO chemisorption (Pt-atoms g-cat )).
Results and Discussion
Low-angle XRD characterization
Low-angle XRD was used to study the ordered structure of
the prepared Pt/SBA-15 and Pt/KIT-6 catalysts (Fig. 1). Both
Pt/SBA-15 and Pt/KIT-6 displayed one major peak together with
two additional characteristic diffraction peaks corresponding to
the diffraction of mesoporous structure (Fig. 1a). It
demonstrates that the prepared catalyst has a well-ordered
mesoporous structure, which belongs to 2-D hexagonal p6mm
Fig. 2 (a) N adsorption/desorption isotherms and (b) pore size
distribution of Pt/SBA-15 and Pt/KIT-6 catalysts.
2
symmetry space group in SBA-15 36 and 3-D Ia3d bicontinuous
16, 37
cubic symmetry space group in KIT-6
. In both cases, the
addition of Pt does not influence the ordered structures of the
mesoporous silica in any way.
Pore structures
2
N adsorption-desorption isotherm curves at 77 K of the
samples are shown in Fig. 2a. The pore size distributions are
shown in Fig. 2b. Pt/SBA-15 displayed characteristic type IV
curves of the isotherms with a H1 type hysteresis loop and an
inflection at a relative pressure between 0.6 and 0.9, indicating
a high quality mesoporous material according to the IUPAC
38
definition . Uniform mesopores around 9.09nm were obtained
in the Pt/SBA-15 catalyst as shown in Fig. 2b. For Pt/KIT6
samples, the adsorption branch showed a sharp inflection at a
relative pressure in the range from 0.5 to 0.8, indicating a typical
capillary condensation within uniform pores with a typical H1
type hysteresis loop. Compared with Pt/SBA-15, Pt/KIT-6
possessed a narrower pore size distribution and smaller pore
size of 7.90nm, which can provide favorable conditions for
3
9
adsorption and activation of gas molecules . Both the
mesoporous materials showed large surface area and pore
volume, with the KIT-6 catalysts exhibiting higher surface area
2
-1
3 -1
(SBET = 676 m g ) and lower pore volume (PV = 0.917 cm g )
2
-1
3 -
values than the SBA-15 ones (SBET = 559 m g , PV = 1.351 cm g
. SBET and PV decreased significantly by 13% and 32%,
1
)
respectively, after Pt addition in the SBA-15 support.
Comparatively, the decreases of SBET and PV in Pt/KIT-6 are
smaller, with values of 2.5% and 7.9%, respectively. The severer
decrease in Pt/SBA-15 reveals possible incorporation of Pt
Fig. 1. Low-angle XRD characterization of (a) Pt/SBA-15 and (b) Pt/KIT-
6
catalysts.
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nanoparticles inside the hexagonal 2-dimensional mesoporous that in Pt/SBA-15 (+2.06), which indicates more reductive
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1
2
DOI: 10.1039/C5RA27303J
channels of SBA-15 , which may lead to a partial blocking of platinum species in Pt/KIT-6 catalyst.
the pores
40, 41
. On the other hand, Pt/KIT-6 with the 3-
The XPS measurements were employed to measure the
dimensional structures only shows a slight decrease of surface oxidation state of platinum species. The XPS spectra of Pt 4f
0
area after Pt addition, indicating a better diffusion and were fitted. The peaks of Pt 4f7/2 and 4f5/2 arise around 71.20
dispersion of platinum particles with respect to the SBA-15 eV and 74.55 eV, respectively, while the 4f7/2 and 4f5/2 peaks of
2
+
4+
supporting catalysts.
Pt and Pt arise around 72.40 eV, 75.75eV; 74.90 eV, 78.25 eV,
43, 44
respectively
. The relative percentage of Pt species can be
0
2+
4+
Dispersion of platinum by TEM and CO chemisorption
obtained by the area ratio of the 4f7/2 peaks of Pt , Pt and Pt .
Table 1 lists the integration results of peak fitting. The overall
oxidation state of Pt species can be calculated from the ratios
of each species. It gives an averaged oxidation state of +2.16
and +1.41 for SBA-15 and KIT-6 supporting Pt, respectively,
confirming that more Pt exsiting in reductive state in KIT-6 than
in SBA-15. The relative higer oxidation state from XPS analysis
TEM was carried out to analyze the mesoporous structure
of the prepared catalyst as well as the dispersion of Pt species.
As shown in Fig. 3, the well-ordered two-dimensional channel
structure in SBA-15 and three-dimensional cubic array of
mesopores in KIT-6 were conserved after Pt loading, which is
consistent with the physisorption and low-angle XRD results.
Platinum nanoparticles were highly dispersed in the support
materials. Most of the platinum species existed in the pores of
the SBA-15 and KIT-6 supports as shown in Fig. 3. For the
Pt/SBA-15 sample, nanowires as well as nanoparticles existed in
the channels of SBA-15 (Fig. 3a). The nanowires appeared as
2
compared with the results from H -titration may be related to
the limited detection depth (<10nm) of XPS analysis.
Catalytic performance and discussion
Catalytic performance of the supported Pt catalysts is
2 3
dark, necklace-like objects between the walls of SBA-15, which reported in Fig. 4. The catalytic activity of conventional Pt/Al O
leads to a broad distribution of Pt particle sizes. Comparatively, catalyst was characterized as a reference. The mesoporous silica
platinum nanoparticles in the Pt/KIT-6 sample had a narrow supported catalysts, especially the Pt/KIT-6 catalyst, exhibited
distribution of particle sizes with most of the nanoparticles superior catalytic performance compared with the conventional
existing in the 3-D mesopores in sphere shape. As listed in Table Pt/Al
1
2
O
3
catalyst. For all the mesoporous silica supported
, the average particle sizes of platinum in Pt/SBA-15 and catalysts, the combustion of C started at about 160 ºC. The
50 (50% conversion temperature) of the catalysts followed the
3 8
H
Pt/KIT-6 were determined to be 6.09 nm and 4.57 nm,
T
2 3
respectively, which were in agreement with the pore diameters sequence of Pt/KIT-6 (266 ºC) < Pt/SBA-15 (334 ºC) < Pt/Al O
of the SBA-15 and KIT-6 host. Such results indicate that the silica (351 ºC). Pt/KIT-6 with a faster reaction tendency reached
host can control the scale of nanoparticles formed inside the complete propane conversion at much lower temperatures
host effectively. The dispersion and particle sizes of platinum than the other two samples. Pt/SBA-15 lost its advantages at
characterized by CO chemisorption, as shown in Table 1, are high temperatures (>350 ºC), not succeeding in the complete
consistent with the TEM results. It demonstrates that the conversion of propane until 500 ºC, which may be related to the
27
platinum species are better dispersed in the Pt/KIT-6 sample, nonuniform Pt particle sizes . Also, the catalysts were
with a smaller particle size and narrower size distribution than subjected to the same temperature programmed reaction
nd
rd
that in Pt/SBA-15 sample.
atmosphere for a 2 and 3 round and the results were listed
in Table S2. It confirms that the Pt/KIT-6 possesses the best
catalytic performance. The turnover frequencies (TOF) of the
catalysts (Fig. 4b), which was measured at 190 ºC (at which
temperature the propane conversion is lower than 10%), were
2
Oxidation state of platinum by H -titration and XPS
H
2
titration method was used to determine the oxidation
state of platinum species, i.e. the x values in PtO . The titration
x
-
3
-1
in the following order: Pt/KIT-6 (60.30 × 10 s ) >> Pt/SBA-15
(19.27 × 10 s ) > Pt/Al O (5.11 × 10 s ).
2 3
temperature was 250 ºC, at which temperature the platinum
oxides can be totally reduced with prolonged time 42. The
calculated results are listed in Table 1. The average oxidation
state of platinum species in Pt/KIT-6 (+1.34) is much lower than
-3
-1
-3 -1
For the Pt catalyzed propane oxidation itself, the catalytic
performance of Pt-based catalysts is highly related to the
dispersion and oxidation state of platinum species, which can
be affected by the pore structures of support materials. It’s
generally accepted that catalysts with higher Pt dispersion with
uniform particle sizes and more metallic platinum species have
6
, 45
higher catalytic activity for propane oxidation
. KIT-6 with 3-
D mesopores ensures relatively smaller particle size with
homogeneous size distribution compared with SBA-15
supported catalyst. What’s more, Pt nanoparticles (NPs) in such
a 3-D mesopores of KIT-6 tend to be stabilized in more reductive
valence state. On the other hand, it should be noted that the
2 3
Pt/Al O catalyst possesses smaller Pt nanoparticles (Table S1
and Fig. S2) but higher oxidation states, resulting in lower
Fig. 3 TEM images of (a) Pt/SBA-15 and (b) Pt/KIT-6 catalysts.
4
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activity compared with Pt in mesoporous silica. Besides the
more reductive state of Pt in Pt/SBA-15 and Pt/KIT-6, the nano-
confinement effects of reactant molecules caused by
mesopores in mesoporous silica, which could affect the mass
Conclusions
View Article Online
DOI: 10.1039/C5RA27303J
SBA-15 and KIT-6 mesoporous silica were successfully
synthesized and supported with platinum to be used in the
catalytic oxidation of propane. Compared with the conventional
19, 46, 47
transport of reactants, may also be contributive
. It has
2 3
Pt/Al O catalyst, the prepared Pt/SBA-15 and Pt/KIT-6 catalysts,
1
8-20
been reported
that nano-confinement effects on reactant
with large specific surface area and ordered mesoporous
structures, exhibited superior performance for propane total
oxidation with lower light-off temperature and larger TOF. The
molecules due to restricted volume and surface curvature of
mesopores result in a lowering of kinetic barriers. A molecule in
nano-environment, like inside mesopores, behaves differently
from outside since the molecular orbitals are strongly perturbed
T
50 (50% conversion temperature) of the catalysts followed the
sequence of Pt/KIT-6 (266 ºC) < Pt/SBA-15 (334 ºC) < Pt/Al
351 ºC). The 2-D hexagonal channels in SBA-15 and 3-D cubic
2 3
O
48
by the solid walls and the density of confined reactants was
(
4
9
higher , which could affect the mass transport and facilitate
structure in KIT-6 can successfully control the particle sizes of
platinum existing in the mesopores. The average particle sizes
of platinum in Pt/SBA-15 and Pt/KIT-6 were determined to be
4
6
the reactivity . The ordered bicontinuous mesoporous
structure in mesoporous silica can provide favorable conditions
for adsorption, activation of gas molecules as well as the
diffusion and transfer of the reactant and product molecules in
6
.09 nm and 4.57 nm, respectively, which were in agreement
with the pore diameters of the SBA-15 and KIT-6 host. Platinum
nanoparticles with lower oxidation state were more
homogeneously dispersed within the pores of the KIT-6 support.
Such highly-dispersed platinum nanoparticles in KIT-6 would be
a promising catalyst candidate for further application study in
propane total oxidation.
39
the catalyst , resulting in the improvement of their catalytic
performance of Pt in mesoporous silica compared with the
2 3
conventional Pt/Al O catalyst. In this sense, homogeneous Pt
NPs confined in KIT-6 with smaller size and relative lower
valence as well as ordered 3-D mesoporous structure contribute
to the catalytic enhancement compared with Pt/SBA-15 and
2 3
Pt/Al O . It provides a potential candidate to be applied in
propane total oxidation.
Acknowledgements
The authors acknowledge financial support from the National
Natural Science Foundation of China (Project 51202116) and
the Ministry of Science and Technology of China (Project
2
015AA034603). We also thank the State Key Laboratory of
New Ceramics and Fine Process in Tsinghua University (China)
for the materials' characterizations.
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Fig. 4 Catalytic performance over Pt/Al
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O , Pt/SBA-15 and Pt/KIT-6
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N balance, flow rate-60,000 h .
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