A sinter-resistant catalytic system based on platinum nanoparticles supported on TiO2 nanofibers and covered by porous silica

Article abstract of DOI:10.1002/anie.201001839

Holey support: The generation of a porous coating of SiO₂ on Pt-decorated TiO₂ nanofibers enables the preparation of a sinter-resistant catalytic system (see picture). The Pt nanoparticles could resist sintering at temperatures up to 750C in air, as the SiO₂ coating acts as a physical barrier that slows down surface migration, but the system remained catalytically active because of the porous nature of the coating.

Full text of DOI:10.1002/anie.201001839

Angewandte
Chemie
DOI: 10.1002/anie.201001839
Nanostructures
A Sinter-Resistant Catalytic System Based on Platinum Nanoparticles
Supported on TiO2 Nanofibers and Covered by Porous Silica**
Yunqian Dai, Byungkwon Lim, Yong Yang, Claire M. Cobley, Weiyang Li, Eun Chul Cho,
Benjamin Grayson, Paul T. Fanson, Charles T. Campbell, Yueming Sun, and Younan Xia*
Platinum is a key catalyst that is invaluable in many important
industrial processes such as CO oxidation in catalytic
converters, oxidation and reduction reactions in fuel cells,
nitric acid production, and petroleum cracking.[1] Many of
these applications utilize Pt nanoparticles supported on
oxides or porous carbon.[2] However, in practical applications
that involve high temperatures (typically higher than 3008C),
the Pt nanoparticles tend to lose their specific surface area
and thus catalytic activity during operation because of
sintering. Recent studies have shown that a porous oxide
shell can act as a physical barrier to prevent sintering of
unsupported metal nanoparticles and, at the same time,
provide channels for chemical species to reach the surface of
the nanoparticles, thus allowing the catalytic reaction to
occur. This concept has been demonstrated in several systems,
transition metals.[2f,5] Herein, we demonstrate a thermally
stable catalytic system consisting of Pt nanoparticles that are
supported on a TiO2 nanofiber and coated with a porous SiO2
sheath. In this system, the porous SiO2 coating offers an
energy barrier to prevent the migration of individual Pt atoms
or nanoparticles because of its weak interaction with late
transition metals, including Pt. The porous-SiO2/Pt/TiO2
catalytic system was prepared in three steps (Figure 1):
including
Pt@SiO2,[3]
Pt@CoO,[4]
Pt/CeO2@SiO2,[5]
Pd@SiO2,[6] Au@SiO2,[7] Au@SnO2 and Au@ZrO2 core–
shell nanostructures. Despite these results, a sinter-resistant
system has not been realized in supported Pt nanoparticle
catalysts.
[8]
[9]
Figure 1. Preparation of the catalytic system based on Pt nanoparticles
that are supported on TiO2 nanofibers and then covered by porous
sheaths of SiO2: 1) deposition of PVP-stabilized Pt nanoparticles onto
the surface of TiO2 nanofibers; 2) formation of SiO2 coating by the
Stꢀber method; and 3) removal of CTAB and PVP by calcination in air
to generate a porous sheath of SiO2.
Improved catalytic or photocatalytic properties are often
achieved when metal nanoparticles are supported on oxides
such as TiO2 and CeO2 that interact strongly with late
1) deposition of Pt nanoparticles onto the surface of TiO2
nanofibers; 2) coating of SiO2 with cetyltrimethylammonium
bromide (CTAB) as a pore-generating agent; and 3) calcina-
tion in air to generate a porous sheath of SiO2. By using this
approach, we were able to produce a platinum-based catalytic
system that can resist sintering up to 7508C in air, while
retaining the catalytic activity of the Pt nanoparticles.
[*] Y. Dai, Dr. B. Lim, C. M. Cobley, W. Li, Dr. E. C. Cho, Prof. Y. Xia
Department of Biomedical Engineering
Washington University, St. Louis, MO 63130 (USA)
E-mail: xia@biomed.wustl.edu
Y. Dai, Prof. Y. Sun
School of Chemistry and Chemical Engineering
Southeast University, Nanjing, Jiangsu 211189 (P. R. China)
Dr. Y. Yang, Prof. C. T. Campbell
Department of Chemistry, University of Washington
Seattle, WA 98195 (USA)
The TiO2 nanofibers were prepared by electrospinning
and subsequent calcination in air at 7508C for 2 hours.[10] The
as-prepared nanofibers had a rough surface and a polycrys-
talline structure that contained both anatase and rutile phases
(69% anatase and 31% rutile; Figure S1 in the Supporting
Information). Poly(vinyl pyrrolidone) (PVP) stabilized Pt
nanoparticles were prepared by using the polyol method.[11]
The as-synthesized Pt nanoparticles were uniform in size, with
an average size of (3.1 Æ 0.5) nm (Figure 2a,b). These Pt
nanoparticles were deposited onto the TiO2 nanofibers by
immersing the sample in a suspension of the Pt nanoparticles,
which was prepared by a 10-fold dilution of the as-prepared Pt
sample with ethanol. As shown in Figure 2c, the Pt nano-
particles were well dispersed on the surface of each TiO2
nanofiber, without significant aggregation. The Pt loading in
the Pt/TiO2 nanofibers was 1.3 wt%, as determined by
inductively coupled plasma mass spectrometry (ICP-MS)
measurements. Figure 2d shows a high-resolution TEM
Dr. B. Grayson, Dr. P. T. Fanson
Toyota Motor Engineering & Manufacturing North America, Inc.
Ann Arbor, MI 48105 (USA)
[**] This work was supported by the Toyota Motor Engineering &
Manufacturing North America, Inc. Part of the research was
performed at the Nano Research Facility (NRF), a member of the
National Nanotechnology Infrastructure Network (NNIN) that is
funded by the National Science Foundation under award no. ECS-
0335765. As a visiting graduate student from Southeast University,
Y.D. was also partially supported by the National Basic Research
Program (973 program, 2007CB936300), the Innovation Program
for Graduate Students in Jiangsu Province (CX08B-051Z), and the
China Scholarship Council. We thank Prof. Jimmy Liu at the
University of Missouri (St. Louis) for helping us with the sintering
experiments under hydrogen gas.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001839.
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each Pt/TiO2 nanofiber (Figure 2e). The HRTEM image in
Figure 2 f clearly shows that the thin SiO2 sheath covered both
the TiO2 nanofiber and the Pt nanoparticles and that its
thickness was uniform. We did not observe the formation of
isolated SiO2 particles in the product in both TEM and
HRTEM analyses.
The as-prepared SiO2/Pt/TiO2 sample contains CTAB
within the SiO2 sheaths and the Pt nanoparticles are
surrounded by PVP. These organic species can be removed
from the SiO2/Pt/TiO2 nanofibers by calcination in air at
3508C to generate porous SiO2 sheaths.[2g,13] The TEM image
and particle size analyses of the calcined porous-SiO2/Pt/TiO2
nanofibers revealed that the Pt nanoparticles did not form
aggregates and retained small sizes with an average size of
(3.5 Æ 0.8) nm (Figure 3a). We also proved that the Pt surface
in these materials was accessible to reactants by measuring
the Pt surface area using selective chemisorption (CO
titration of O adatoms; reference [14]). For several samples
before (Figure 2c) and after (Figure 3a) silica coating, the
average Pt dispersions were measured to be 42% and 29%,
respectively. These dispersions are very close to the value of
39%, which was estimated using the average diameter of
3.5 nm from TEM, assuming that the spherical particles have
the same surface packing density as Pt(111), that is, 1.5 ꢂ 1015
Pt atoms per cm2 (representative titration curves and surface
areas are shown in Figure S2 in the Supporting Information).
Notably, the Pt nanoparticles did not exhibit a morpho-
logical change upon calcination at temperatures up to 7508C
(Figure 3b,c). However, in sharp contrast, the Pt nanoparti-
cles were observed to aggregate in the absence of SiO2 sheaths
at all temperatures in the range 350–7508C (Figure 3d–f). At
7508C, the Pt nanoparticles aggregated extensively to form
particles larger than 20 nm in size (Figure 3 f). These results
Figure 2. a) TEM image of PVP-stabilized Pt nanoparticles synthesized
using the polyol method and b) the corresponding particle size
distribution obtained by counting 150 particles in the TEM images.
c) TEM and d) HRTEM images taken from the Pt/TiO2 nanofiber
sample prepared by immersing the calcined TiO2 nanofibers in a
suspension of the Pt nanoparticles. The inset in (d) shows a schematic
model of a Pt truncated octahedron on the TiO2 surface. e) TEM and
f) HRTEM images taken from the sample that was obtained by coating
the Pt/TiO2 nanofibers shown in (c) with SiO2 sheaths 4–6 nm thick.
(HRTEM) image of
a
single Pt nanoparticle on
a TiO2 nanofiber recorded
along the [011] zone axis.
The
exhibited
Pt
nanoparticle
truncated
a
octahedral shape with
exposed {111} and {100}
facets. The fringe spacing
of 1.9 ꢀ and 2.3 ꢀ corre-
spond to the {200} and
{111} planes of face-cen-
tered cubic Pt, respective-
ly.[1b] The Pt/TiO2 nano-
fibers were then coated
with SiO2 sheaths by
using a modified Stꢁber
method,[12]
CTAB was used as
in
which
a
pore-generating
The TEM
showed that
agent.
analysis
uniform
a
SiO2 coating with a thick-
ness of 4–6 nm was
Figure 3. Comparison of thermal stability of the SiO2/Pt/TiO2 nanofibers (a–c) and Pt/TiO2 nanofibers without
formed on the surface of SiO2 coating (d–f) at three different calcination temperatures in air for 2 h.
8166
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demonstrate that the porous SiO2 sheath could serve as an
effective barrier to prevent the migration of Pt atoms and/or
nanoparticles during calcination, thus enabling the Pt nano-
particles to become sinter-resistant. This new catalytic system
also exhibited good resistance against sintering in a reducing
gas environment such as hydrogen gas (Figure S3 in the
Supporting Information).
The effect of Pt loading on the sinter resistance of the
porous-SiO2/Pt/TiO2 nanofibers was also investigated. In this
case, the Pt/TiO2 nanofibers with a relatively higher Pt
loading (ca. 3.6wt%) were prepared by immersing the
calcined TiO2 nanofibers in the as-prepared suspension of
Pt nanoparticles (Figure S4a in the Supporting Information).
These nanofibers were coated with porous SiO2 sheaths using
the same procedure as in Figure 2e (Figure S4b in the
Supporting Information). This sample exhibited good sinter
resistance upon calcination in air up to 5508C, but sintering
started to occur at 7508C (Figure S5 in the Supporting
Information). In this case, sintering could be facilitated by
the significant reduction in the nearest-neighbor distance
between the Pt nanoparticles.
hydrogen molecules were able to diffuse through the porous
SiO2 sheaths to the surface of the embedded Pt nanoparticles.
The highest catalytic activity was achieved from the porous-
SiO2/Pt/TiO2 nanofiber catalyst prepared by calcination at
3508C, with a conversion of 87%. The conversion dropped
slightly to 81% for the sample calcined at 5508C and to 61%
for the sample calcined at 7508C. These reductions in catalytic
activity with increasing calcination temperature might also be
caused by surface reconstruction of the Pt nanoparticles,[3] as
well as densification of the silica coating.[16] We also con-
firmed the absence of homogeneous catalysis that might arise
from partial dissolution of the Pt into the solution by
conducting a control experiment in which the methyl red
solution was not bubbled with H2 gas until it was stirred with
the catalyst for 2 minutes and then separated from the catalyst
by centrifugation. In these experiments, there was essentially
no change to the absorption peak (Figure S6 in the Support-
ing Information). Taken together, these results clearly show
that the porous SiO2 coating was permeable toward chemical
species, and that the porous-SiO2/Pt/TiO2 nanofibers retained
their catalytic properties.
We also investigated the catalytic property of the Pt
nanoparticles embedded in the porous SiO2 sheaths by
employing the hydrogenation of methyl red (an azo dye) as
In summary, we have demonstrated a sinter-resistant
catalytic system based on TiO2-supported Pt nanoparticles
coated by a porous sheath of SiO2. The porous-SiO2/Pt/TiO2
system could resist sintering up to a temperature of 7508C in
air, thus demonstrating the excellent sinter resistance of the
catalyst. The porous SiO2 sheath provides a physical barrier to
slow down surface migration for the Pt atoms and/or nano-
particles and thus effectively prevent sintering. Significantly,
the porous-SiO2/Pt/TiO2 nanofibers were still catalytically
active towards hydrogenation of methyl red even after high-
temperature calcination, thus demonstrating accessibility of
the Pt nanoparticleꢃs surface because of the porous nature of
the SiO2 sheath. The synthetic strategy presented herein
should be extendible to other catalytic systems with different
compositions.
=
a model reaction (Figure 4a). The hydrogenation of the N N
Received: March 29, 2010
Revised: August 24, 2010
Published online: September 24, 2010
Keywords: nanostructures · platinum · silica · sinter resistance ·
.
supported catalysts
Figure 4. a) Hydrogenation of methyl red. b) UV/Vis spectra of a
methyl red solution before and after the hydrogenation reaction with
porous-SiO2/Pt/TiO2 nanofibers. The conversions were 87%, 81%, and
61% for the porous-SiO2/Pt/TiO2 nanofibers obtained by calcination in
air for 2 h at 350, 550, and 7508C, respectively.
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