Size Regulation of Platinum Nanoparticles by Using Confined Spaces for the Low-Temperature Oxidation of Ethylene

Article abstract of DOI:10.1021/acs.inorgchem.7b02988

Noble-metal nanoparticles have attracted great attention because of their excellent catalytic activity. However, the activity of noble-metal nanoparticles is severely dependent on the size of the metal nanoparticles. Therefore, regulation of the nanoparticle size is of great importance. Herein we report an efficient strategy for the first time to regulate the size of platinum (Pt) nanoparticles in a typical mesoporous silica SBA-15 by using the confined spaces. The Pt-containing precursor is introduced to the confined spaces between the template and silica walls in as-prepared SBA-15 by grinding. Subsequent calcination allows template removal and precursor conversion in one step and avoids repeated calcination in conventional modification processes. This leads to the formation of Pt nanoparticles with smaller size, making Pt metal catalysts highly active. Hence, the catalytic activity of the oxidation of trace ethylene is very superior to that of those prepared by the conventional method. For a typical material, PtAS-5, containing 5.0 wt % Pt, the complete conversion of ethylene was achieved at 40 °C, which is lower than that on the catalyst PtCS-5 prepared by the conventional method (50 °C) as well as that on a series of catalysts reported previously.

Full text of DOI:10.1021/acs.inorgchem.7b02988

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Size Regulation of Platinum Nanoparticles by Using Confined Spaces
for the Low-Temperature Oxidation of Ethylene
Yu Kou and Lin-Bing Sun*
State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced
Materials, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
S
* Supporting Information
ABSTRACT: Noble-metal nanoparticles have attracted great attention
because of their excellent catalytic activity. However, the activity of noble-
metal nanoparticles is severely dependent on the size of the metal
nanoparticles. Therefore, regulation of the nanoparticle size is of great
importance. Herein we report an efficient strategy for the first time to
regulate the size of platinum (Pt) nanoparticles in a typical mesoporous
silica SBA-15 by using the confined spaces. The Pt-containing precursor is
introduced to the confined spaces between the template and silica walls in
as-prepared SBA-15 by grinding. Subsequent calcination allows template
removal and precursor conversion in one step and avoids repeated
calcination in conventional modification processes. This leads to the
formation of Pt nanoparticles with smaller size, making Pt metal catalysts
highly active. Hence, the catalytic activity of the oxidation of trace
ethylene is very superior to that of those prepared by the conventional
method. For a typical material, PtAS-5, containing 5.0 wt % Pt, the complete conversion of ethylene was achieved at 40 °C, which
is lower than that on the catalyst PtCS-5 prepared by the conventional method (50 °C) as well as that on a series of catalysts
reported previously.
such as oxidation and hydrogenation. Researchers have
reported the use of Pt as active sites to catalyze the oxidation
of hydrocarbons.9,10 Until now, some researchers have studied
the catalytic performance and catalytic mechanism of Pt metal
catalysts for the oxidation of ethylene. Ethylene reacts with
oxygen to generate formaldehyde, which adsorbed on Pt. Then
formaldehyde decomposes into carbon monoxide and hydro-
gen species, and these two intermediate species react with
oxygen to generate the final products carbon dioxide and
water.11 It has been discovered that the size of Pt nanoparticles
(Pt NPs) has a direct impact on the activity of Pt catalysts,
while it is not easy to fabricate Pt NPs with smaller size.
Mesoporous silica, a kind of material with high surface area
and large pore volume, is an ideal support to regulate the size of
metal nanoparticles. Many attempts have been made to
produce a small size of metal nanoparticles on mesoporous
silica by various methods including ion implantation,12 sol−gel
processing,13 and sputtering. Functional groups (e.g., NH2,14
SH,15 and SO3H16) are usually grafted onto mesoporous silica
to promote regulation of the size of metal nanoparticles. This
traditional method is useful; however, the complicated surface
functionalization is tedious. Meanwhile, this traditional method
usually ignored the nature of as-prepared mesoporous
silica.17,18 Actually, between the template and silica walls in
INTRODUCTION
■
Ethylene, a low-molecular-weight gaseous volatile organic
compound, is an important industrial chemical; meanwhile, it
has some negative effects such as photochemical pollution.1
Therefore, eliminating ethylene gas from some environments is
of great necessity. For example, the removal of ethylene can
decelerate the maturation of fruits and vegetables and keep
them fresh for longer time. By using catalytic oxidation
technology, the trace ethylene can be transformed completely
to carbon dioxide and water. Until now, a collection of catalysts
were attempted for the oxidation of ethylene, such as Au/
2
Co3O4 and copper manganese oxides.3 However, most of
those catalysts show relatively low catalytic activities for the
oxidation of ethylene, and the temperature for the complete
conversion of ethylene is usually higher than 100 °C. For
example, the temperature for 100% conversion of ethylene over
the catalyst MCS/Co-30 is 185 °C,4 and the catalyst Au/Co3O4
can convert ethylene completely at 150 °C.2 Despite great
efforts, the catalytic conversion of ethylene at low temperatures
remains a great challenge.
Noble metals have attracted wide interest as catalysts in the
past few years because of their superior activity.5 Compared
with pure noble-metal films, noble-metal nanoparticles are
extensively studied.6,7 It is worth noting that the size of noble-
metal nanoparticles can largely influence the catalytic activity.8
Among many noble metals, platinum (Pt) is widely used in
catalysis, which has superior activity in some kinds of reactions,
Received: November 29, 2017
© XXXX American Chemical Society
A
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as-prepared mesoporous silica, there is a special microenviron-
ment, which can provide confined spaces and abundant silanol
groups; these are beneficial to promoting the size regulation of
noble-metal nanoparticles. Hence, the fabrication of a small size
of noble-metal nanoparticles and high-activity metal catalysts by
using the confined space is extremely desirable.
Here, we report an efficient strategy to regulate the size of Pt
NPs by using confined spaces. The Pt-containing precursor is
introduced to the confined spaces between the template and
silica walls in as-prepared SBA-15 (denoted as AS) by grinding.
Decomposition of the Pt-containing precursor occurs within
the confined spaces between the template and silica walls
(Scheme 1). During calcination, the size of Pt NPs can be well
Scheme 1. Formation of (A) Small-Size Pt NPs in Confined
Spaces and (B) Large-Size Pt NPs in Conventional
Mesoporous Silica
Figure 1. (A) Low-angle XRD patterns, (B) wide-angle XRD patterns,
(C) N2 adsorption−desorption isotherms, and (D) pore-size
distributions of CS, PtAS-5, and PtCS-5. Curves are plotted offset
for clarity.
Compared with the PtCS, the PtAS samples exhibit relatively
broader diffraction peaks of metallic Pt, which suggest smaller
size Pt NPs in the PtAS samples.25 According to the Scherrer
equation, the size of the Pt NPs was calculated by use of the
Pt(111) diffraction peak. The size of the Pt NPs in PtAS-5 is
3.0 nm, which is obviously smaller than that in PtCS-5 (13.1
nm; Table 1). What is more, the size of the Pt NPs in all of the
PtAS samples is smaller compared with that of the PtCS
samples. According to the above results, it is reasonable to say
that using the AS as the support is obviously beneficial for the
formation of small-size Pt NPs.
The surface components of Pt NPs are measured by X-ray
photoeectron microscopy (XPS). It is apparently seen from
Figures 2 and S3 that both the PtAS and PtCS samples show
double peaks of zero-valence Pt 4f7/2 and 4f5/2 centered at 74.2
and 71.0 eV, which correspond to metallic Pt NPs.26 This result
indicates that there is no existence of other valence Pt ions
(such as Pt4+ and Pt2+) but only the metallic Pt in the samples.
Meanwhile, this also indicates that PtO could be reduced
absolutely to metallic Pt by calcination at 200 °C under a H2
atmosphere.
The N2 adsorption−desorption isotherms of all samples are
shown in Figures 1C and S4. This reflects that the samples are
all of type IV isotherms with an H1-type hysteresis loop, which
is the same as CS and is attributed to the characteristics of
materials with regular mesoporous structure.21 Compared with
the PtAS samples, the PtCS samples show a “tail” during the
desorption stage, which becomes clear gradually with an
increase of the Pt loading. Compared to the samples with the
same noble-metal content, the hysteresis loop of PtCS moves
to the area of lower relative pressure. These results illustrate
that Pt NPs prepared using the confined spaces have small size.
Further calculation shows that the samples with the same
noble-metal content, all samples prepared by our strategy, have
higher surface areas and pore volumes, in line with the sample
using the conventional method (Table 1), which provides
evidence for the small size of Pt NPs in the pores of the
controlled by the confined effect. It is simple and easy for our
strategy, which allows template removal and precursor
conversion in one step and avoids repeated calcination in a
conventional modification process. It can not only save time
but also reduce energy consumption.19 The results show that
noble-metal Pt NPs prepared by our strategy have smaller size
than that using the conventional method.20 The size of Pt NPs
in PtAS-1 (1.0 wt %) prepared by our strategy is only 0.5 nm.
Also, the typical material PtAS-5 containing 5.0 wt % Pt has a
size of 3.0 nm. The size of Pt NPs in PtAS-5 is much smaller
than that in the sample PtCS-5 prepared by the conventional
method (13.2 nm). Meanwhile, the Pt NPs supported on SBA-
15 prepared by our strategy showed higher activity in the
catalytic oxidation of ethylene.
RESULTS AND DISCUSSION
■
Structural and Surface Properties. Characterization of
the samples was carried out by different physicochemical
methods. The low-angle X-ray diffraction (XRD) patterns
(Figures 1A and S1) show that the Pt-containing samples have
a strong diffraction peak and two weak ones, which is similar to
the support CS; this could correspond to (100), (110), and
(200) reflections and is attributed to a two-dimensional
hexagonal pore symmetry.21,22 Figure 1A reflects that the
samples PtAS-5 and PtCS-5 retain the ordered mesoporous
structure after the loading of Pt NPs. Compared with the
support CS, the Pt-containing samples possess an obviously
weaker intensity of the diffraction peaks, which can be
attributed to the decrease of the scatter contrast between the
pore volume and pore space after the introduction of Pt NPs.
All of the samples show a broad diffraction peak centered at
23° in the wide-angle XRD patterns, which corresponds to
amorphous silica walls (Figures 1B and S2).23 Pt-containing
samples have three characteristic XRD peaks at 40°, 46°, and
67° derived from metallic Pt NPs (JCPDS 04-0802).24
B
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Table 1. Physicochemical Properties and Catalytic Performance of CS and Modified Samples
a
sample
SBET (m2·g−1
)
Vp (cm3·g−1
)
Dp (nm)
dPt (nm)
conversion at 25 °C (%)
temperature for 100% conversion (°C)
CS
905
779
742
721
748
722
504
1.147
1.062
1.038
1.006
0.904
0.856
0.572
8.0
8.0
7.9
7.6
8.0
7.8
7.4
0
PtAS-1
PtAS-3
PtAS-5
PtCS-1
PtCS-3
PtCS-5
0.5
2.1
38.6
39.0
52.0
22.3
19.1
30.8
66.8
52.0
39.9
82.1
58.7
49.9
3.0
1.2
5.4
13.1
a
Estimated from XRD using the Scherrer equation.
Figure 2. XPS peak fitting of Pt 4f7/2 and 4f5/2 spectra of (A) PtAS-5
and (B) PtCS-5.
support. Furthermore, it is reasonable to believe that the large
size of Pt NPs and the use of CS change the shape of the
adsorption−desorption isotherm and the physicochemical
properties for PtCS.
It is of great importance to characterize the structure of the
support SBA-15 and the size of Pt NPs by transmission electron
microscopy (TEM). For both PtAS and PtCS, the periodic
mesopores with uniform pore size and wall thickness are
obviously seen from the bright−dark TEM images (Figure 3),27
which shows that the ordered pore structure of the supports is
not destroyed after the introduction of Pt NPs. The bright-field
TEM images also provide powerful evidence of the presence of
Pt NPs on the supports. It can be observed from the bright-field
image that the size of the Pt NPs in the PtAS-5 sample is
smaller and loaded uniformly. In contrast, the bright-field image
of PtCS-5 shows some large size of the Pt NPs. The dark-field
images of PtAS-5 and PtCS-5 further proved the smaller Pt NPs
in PtAS-5 than in PtCS-5. The average size of the Pt NPs in
PtAS-5 is 2.8 nm by further analysis of the TEM image.
However, the size of Pt in the sample PtCS-5 is 13.2 nm. These
results fit well with the wide-angle XRD patterns. The size of
the Pt NPs in PtAS is smaller than that in PtCS. According to
the above analysis results, it is safe to say that the use of the as-
prepared SBA-15 existing confined space could decrease the
size of the Pt NPs to a great extent.
Figure 3. Bright-field TEM images of (A) PtAS-5 and (B) PtCS-5.
Dark-field TEM images of (C) PtAS-5 and (D) PtCS-5. Correspond-
ing particle-size distributions of (E) PtAS-5 and (F) PtCS-5.
Catalytic Performance on Ethylene Oxidation. An
ethylene concentration of 0.32 vol % in the initial gas was
used to research the activity of Pt catalysts in this work. Figures
4A and S5 and S6 show the curves of ethylene conversion over
different samples. Complete oxidation of trace ethylene was
measured on samples PtAS and PtCS over a wide range of
reaction temperatures (25−150 °C). The conversion of
ethylene over pure mesoporous silica CS was extremely low,
which suggests that the oxidation of ethylene cannot proceed at
all in the absence of Pt NPs. After the introduction of Pt NPs
into the support, the catalytic activities of PtAS and PtCS were
obviously improved. Meanwhile, the catalytic activity of PtAS
was obviously superior to that of PtCS, which can give evidence
that the preparation method using the template-occluded AS
Figure 4. (A) Conversion profiles for ethylene oxidation over different
samples and (B) reaction tests at 40 °C for ethylene oxidation with
time-on-stream over PtAS-5.
has a significant influence on the catalytic performance. Table 1
also shows the catalytic conversion of trace ethylene over PtAS
C
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and PtCS. The catalyst PtAS-1 containing 1.0 wt % Pt exhibits a
conversion of 38.6% at 25 °C and such a catalytic activity is
higher than that of PtCS-1 (22.3%) possessing the same Pt
content prepared by the conventional method. PtAS-1 can
catalyze the conversion of ethylene completely at 66.8 °C, while
that over PtCS-1 requires a higher reaction temperature of 82.0
°C. With an increase of the loading of Pt, the activity of the
catalysts increases. For PtAS-3, the ethylene conversion reaches
39.0% at 25 °C, and the temperature for complete oxidation is
52.0 °C. In contrast to the catalytic performance of PtCS-3, the
sample PtAS-3 shows a lower temperature for complete
oxidation and higher catalytic activity. When the loading of
Pt is enhanced to 5.0 wt %, the catalytic activity of the sample is
further improved. Typically, the ethylene conversion is as high
as 52.0% at 25 °C by using PtAS-5 as the catalyst, which is
higher than that over PtCS-5 (30.8%) and other Pt-containing
samples under the same reaction conditions. What is more, the
conversion of ethylene over PtAS-5 remains approximately
100% at a pretty low temperature of 40 °C. Among these
samples, there is no doubt that PtAS-5 has the best catalytic
activity for the oxidation of ethylene. In contrast to some
reported catalysts in the literature, the catalytic activity of the
sample PtAS-5 is better (Table S1). No particle size change was
observed for the spent catalyst from wide-angle XRD patterns
(Figures 5 and S7). Meanwhile, the recyclability of the catalyst
conducted at 40 °C even as the reaction time is increased to
150 h. To study the deactivation behavior of catalysts, a high
reaction temperature of 90 °C was employed. As shown in
Figure S8, the conversion of ethylene over PtAS-5 at 90 °C is
100% for more than 24 h. With an increase of the reaction time,
the catalytic activity of PtAS-5 decreases gradually. The catalyst
PtAS-5 shows no activity upon the oxidation of ethylene when
the reaction time exceeds 34 h. The wide-angle XRD pattern of
PtAS-5 after deactivation was measured (Figure S9). It is
obvious that the Pt NPs in PtAS-5 seriously aggregated. In
short, the present catalysts show high stability upon ethylene
oxidation at a low temperature of 40 °C, which is promising for
applications in storehouse storage.
Proposed Mechanism. On the basis of the aforemen-
tioned results, it is safe to say that noble-metal Pt NPs can be
introduced to the support with a small size by using our
method and the samples show better catalytic activity than
those prepared using the conventional method. It is reasonable
to believe that the confined space between the template and
silica walls is of great importance in regulation of the size of
noble-metal nanoparticles. As shown in Figure 7, the weight
Figure 7. (A) TGA and (B) differential thermal gravimetry curves of
AS, PtAS-5, and PtCS-5.
Figure 5. Wide-angle XRD patterns of PtAS-5 (a) before and (b) after
reaction at 40 °C.
loss of the sample AS is 55% after reaching 600 °C, and the
decomposition temperature of the template P123 in the
support AS is about 170 °C, which is obviously lower than
that of pure P123 (210 °C).28 This is because silica walls can
promote decomposition of the block copolymer. Meanwhile, it
is worth noting that decomposition of the template occurs at
188 °C in the sample PtAS-5, which is also higher than the
decomposition temperature of P123 in AS. The decomposition
temperature of the template in other Pt-containing samples is
also higher than that in the support AS (Figure S10). This
result indicates that the noble-metal precursor has been
introduced to the confined spaces successfully. Therefore, the
silica walls cannot act as catalysts in decomposition of the
template because of separation of the silica walls and template.
As a result, the confined spaces constructed between the
template and silica walls in mesoporous silica provide a desired
platform for the fabrication of noble-metal nanoparticles.
Besides the confined-space effect, the rich silanol groups (Si−
OH) existing in the support AS could also be beneficial to
controlling the size of the nanoparticles. The IR spectra of the
supports and Pt-containing samples before and after calcination
are shown in Figures 8 and S11 and S12. The support AS shows
IR bands in the range 2850−3000 cm−1, which can be
attributed to the bending vibrations of P123. It is worth noting
that the IR band appeared at 960 cm−1, which is caused by the
was examined (Figure 6). The sample PtAS-5 can still convert
ethylene completely after running for five cycles, which suggests
better recyclability of the catalyst.
The durability of Pt-containing catalysts was investigated. As
shown in Figure 4B, the catalytic conversion of ethylene over
PtAS-5 at 40 °C remains at approximately 100% for more than
12 h. Moreover, no loss in activity is observed for the reaction
Figure 6. Recyclability of PtAS-5 as the catalyst for the oxidation of
ethylene at 40 °C..
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Figure 8. IR spectra of SBA-15, PtAS-5, and PtCS-5 (A) before and (B) after calcination.
ORCID
stretching vibration of silanol groups. It is clear that the PtAS
samples had more silanol groups than PtCS before calcination.
During calcination, the silanol groups on the silica walls can
interact with metal nanoparticles, which benefits the prepara-
tion of small-size Pt NPs. Therefore, a higher hydroxyl density
can promote regulation of the size of noble-metal nanoparticles
when metal nanoparticles are introduced to the support.29
Lin-Bing Sun: 0000-0002-6395-312X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21722606 and 21576137) and
the Project of Priority Academic Program Development of
Jiangsu Higher Education Institutions.
CONCLUSIONS
■
In summary, a facile, efficient strategy has been developed to
regulate the noble-metal nanoparticles in confined spaces. A
smaller size of noble-metal Pt NPs was achieved by using as-
prepared mesoporous silica AS; in contrast, large-size Pt NPs
produce conventional calcined mesoporous silica CS. The small
size of the metal nanoparticles is attributed to the confined
spaces between the template and silica walls in the support AS
and the strong interaction with the support because of the
abundant silanol groups on the silica walls of AS. Meanwhile, in
comparison with the samples by the conventional method, the
samples prepared by our strategy exhibit better catalytic activity
of ethylene oxidation. What is more, the catalytic activity of the
sample PtAS-5 is obviously superior to that of some samples
reported in the literature. On the one hand, the strategy of
confined space might provide a new way to regulate the size of
metal nanoparticles with small size, which should enable one to
incorporate metals, bimetals, and even composites into
mesoporous silica to prepare new functional materials. On
the other hand, it is meaningful to further decrease the size of
the Pt NPs on the basis of our work. In addition to the use of
confined spaces, work is in progress to enhance the interaction
between metal nanoparticles and the surface of supports via
surface modification (e.g., doping heteroatoms). As a result,
catalysts with even smaller size of metal nanoparticles, which
are highly expected in various applications, are supposed to be
prepared.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02988.
Experimental details and additional characterizations
(PDF)
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: lbsun@njtech.edu.cn (L.-B.S.).
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DOI: 10.1021/acs.inorgchem.7b02988
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