Tailoring the oxidation activity of Pt nanoclusters via encapsulation

Article abstract of DOI:10.1021/cs501763k

To tailor the catalytic activities of metal catalysts at will to achieve efficient conversion in chemical processes remains a challenge, particularly for noble metals, such as Pt. We demonstrate herein that encapsulation within the carbon nanotube (CNT) channels with a diameter of 1.0-1.5 nm not only allows restriction of the size of Pt nanoclusters around 1.0 nm but also enables modulating of the Pt species at the active reduced states through host-guest interaction. The encapsulated Pt is protected from oxygen under reaction conditions in toluene oxidation up to 200 °C, as unveiled by in situ X-ray absorption spectroscopy and density functional theory calculations. As a result, the encapsulated Pt clusters deliver a remarkably higher activity and stability than the clusters located on the open surfaces of the CNT exterior walls and carbon black support, although the latter are much more accessible to reactants. This characteristic of the CNT channels can be explored to tune the properties of other metal catalysts for oxidation reactions. (Chemical Equation Presented).

Full text of DOI:10.1021/cs501763k

Research Article
pubs.acs.org/acscatalysis
Tailoring the Oxidation Activity of Pt Nanoclusters via Encapsulation
Fan Zhang,† Feng Jiao,† Xiulian Pan,*,† Kang Gao,‡ Jianping Xiao,† Shuo Zhang,∥ and Xinhe Bao*,†
†State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
‡School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China
∥Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204,
China
S
* Supporting Information
ABSTRACT: To tailor the catalytic activities of metal catalysts at will to
achieve efficient conversion in chemical processes remains a challenge,
particularly for noble metals, such as Pt. We demonstrate herein that
encapsulation within the carbon nanotube (CNT) channels with a diameter of
1.0−1.5 nm not only allows restriction of the size of Pt nanoclusters around 1.0
nm but also enables modulating of the Pt species at the active reduced states
through host−guest interaction. The encapsulated Pt is protected from oxygen
under reaction conditions in toluene oxidation up to 200 °C, as unveiled by in
situ X-ray absorption spectroscopy and density functional theory calculations.
As a result, the encapsulated Pt clusters deliver a remarkably higher activity and
stability than the clusters located on the open surfaces of the CNT exterior
walls and carbon black support, although the latter are much more accessible to
reactants. This characteristic of the CNT channels can be explored to tune the
properties of other metal catalysts for oxidation reactions.
KEYWORDS: platinum, cluster, confined catalysis, carbon nanotube, toluene oxidation
porous carbon support, which exhibited a high activity in
oxygen reduction8 and hydrogenation of low-reactive olefins.11
Li et al. and Witham et al. used hydroxyl-terminated
polyamidoamine dendrimers to encapsulate Pt40 clusters on a
silica support, SBA-15.12,13 The resulting catalysts gave a high
selectivity and exceptional recyclability for multiple uses in a
series of solution-phase electrophilic π-bond activation
reactions.12,13 However, the choice of surfactants and
dendrimers is reaction-specific, which may have steric effects
shielding the active sites from access by certain reactants and
may also modify the electronic environment of the metal
centers and, hence, their catalytic activity.14 On the other hand,
the relatively poor thermal stability of the organic capping agent
could hinder their utilization in higher temperature reactions.
Furthermore, fine metal clusters are prone to oxidation in an
oxygen atmosphere, particularly at higher temperatures, which
frequently leads to deactivation. An elegant approach reported
recently is to confine metal centers within the lattice of the
support materials, which can stabilize the under-coordinated
metal sites and even isolated atoms.15 Alternatively, support
materials with well-defined pore structures, such as zeolite,16
metal organic frameworks,17 and carbon nanotubes
(CNTs),18−20 have also been used to encapsulate metal
clusters. For example, a CNT can be considered as a graphene
INTRODUCTION
■
Pt is a versatile noble metal catalyst for a wide range of
industrial processes, including catalytic oxidation of volatile
organic compounds (VOCs) in industrial gaseous pollutants.1,2
The high price and scarce reserves of noble metals have
impelled wide efforts to improve the utilization efficiency. In
particular, it has been demonstrated that reducing the size of
metal nanoparticles down to a few nanometers, and even
subnanometer scale, is an efficient approach because they
exhibit significantly higher activities than larger nanopar-
ticles.3−9 For instance, it was reported that Pt clusters with a
size around 1.0 nm supported on interconnected porous
CaMnO3 nanoparticles give a much higher mass and specific
activity in oxygen reduction reaction relative to commercial
carbon-supported Pt nanoparticles with a larger size.3
Subnanometer Pt clusters distributed on a graphene nanosheet
showed an unusually high activity for methanol oxidation
compared with conventional Pt nanoparticles (2−3 nm)
supported on carbon black.6 However, these fine nanoclusters
are unsaturatedly coordinated and prone to aggregation,
especially under reaction conditions. Therefore, surfactants,
polymers, and dendrimers are frequently indispensable to
stabilize them and to control their size.
Hossain et al. synthesized Pt nanoparticles with an average
diameter of 1.4 0.3 nm using poly(N-vinyl-2-pyrrolidone) as
a stabilizer.10 Yamamoto et al. developed a method to prepare
Pt subnanometer-sized nanoclusters, Pt12, with a fourth-
generation pheylazomethine dendrimer loaded onto a meso-
Received: November 7, 2014
Revised: December 19, 2014
© XXXX American Chemical Society
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Figure 1. (a) HRTEM image with the inset showing discrete clusters encapsulated within the CNT channels; (b) HAADF-STEM image with the
size distribution of Pt clusters in the inset showing that most particles fall in a range of 0.7−1.2 nm.
Figure 2. Activities and stability of Pt@CNT in comparison with Pt clusters exposed on open surfaces of the CNT exterior walls and CB (Pt/CNT
and Pt/CB): (a) toluene conversion as a function of reaction temperatures; (b) the stability test.
layer rolling up to form the tubular morphology, where a
variety of nanostructured materials have been entrapped.21,22
The confined metal nanoparticles showed intriguing catalytic
activity in various reactions, for example, Fischer−Tropsch
synthesis,23 asymmetric hydrogenation,24 and hydrogenation
reactions in the supercritical CO2 phase.25 More interestingly,
within the confined environment of CNTs, oxidation of
metallic iron was hindered compared with those located on
the exterior walls.26 We wonder if CNTs will exert a similar
protection effect on the extremely active Pt nanoclusters.
Therefore, we introduced Pt nanoclusters with a size around 1
nm within the CNT channels with an inner diameter of 1.0−1.5
nm, and investigated their catalytic activity and stability, taking
the oxidation of toluene as a model. This is an important
industrial process to remove VOC and has been widely studied
as a prototype reaction. The encapsulated Pt clusters deliver a
much higher activity with a stable performance than the clusters
distributed on the open surfaces of carbon black and the CNT
exterior walls. In situ X-ray absorption near edge spectroscopy
(XANES) reveals that encapsulation allows tailoring of the Pt
clusters at the active metallic states throughout the reaction, in
contrast to those easily oxidized on the open surface, which is
confirmed by density functional theory (DFT) calculations.
the CNTs were opened during purification and cut into
sections shorter than 1 μm. Volatile (trimethyl)-
cyclopentadienyl platinum was used as the precursor, following
a procedure similar to that of our previous study.27 The Pt
complex on the exterior walls of CNTs was removed by excess
ethanol. The sample was reduced in situ in H2 prior to reaction
(denoted as Pt@CNT) (see Supporting Information for more
details). The HRTEM image in Figure 1a shows that discrete
nanoclusters are distributed within the channels. The high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) image of Figure 1b reveals that
these nanoclusters are homogeneously distributed. Statistical
analysis shows that more than 80% of the clusters fall in a size
range of 0.7−1.2 nm. Because of the spatial restriction of the
CNT channels, particles larger than 2 nm were scarcely
observed. Further CO chemisorption shows that the average
size of the encapsulated Pt nanoclusters is 1.3 nm. The
dispersity of Pt reaches as high as 87%.
The reaction was carried out under atmospheric pressure in a
quartz fixed-bed reactor in a temperature- programmed mode,
from room temperature to 200 °C. An Ar flow containing 1000
ppm toluene and 20% oxygen was fed into the reactor. The
effluents were analyzed by an online GC when the temperature
had stabilized for at least 30 min. Complete oxidation to CO2 is
confirmed by an online GC equipped with TCD and FID
(Figure S1) and a time-of-flight mass spectrometer. Figure 2a
shows that the toluene starts to be converted by the confined Pt
clusters at 110 °C. The conversion shoots up with an increase
RESULTS AND DISCUSSION
■
Highly purified thin-walled CNTs with an inner diameter of
1.0−1.5 nm were purchased from TimesNano Inc., which were
produced by catalytic decomposition of methane. The ends of
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Figure 3. In situ XAFS of Pt catalysts during toluene oxidation. (a) The Pt LIII edge XANES spectra of reduced Pt@CNT with respect to those of
Pt/CNT, Pt foil, and PtO2 references. The inset shows the whiteline intensity of Pt@CNT is similar to that of Pt/CNT and Pt foil. (b) k2-weighted
EXAFS spectra with the inset showing those of reference samples Pt foil and PtO2, which show their characteristic Pt−Pt and Pt−O bonds,
respectively. (c) k2-weighted EXAFS spectra of Pt/CNT during toluene oxidation at 100 and 200 °C with the inset showing the whiteline of the Pt
LIII edge spectra along with the reduced Pt/CNT. (d) k2-weighted EXAFS spectra of Pt@CNT during reaction under the same conditions as c, with
the inset showing the whiteline of the Pt LIII edge spectra.
in the temperature to 130 °C, and it reaches 100% at 145 °C. In
contrast, Pt nanoparticles dispersed on a commercially
available, well graphitized carbon black (designed as Pt/CB)
with 70% Pt clusters falling in a size range of 1.0−2.5 nm gives
a lower activity (Figure S2), as toluene starts to be converted
only above 120 °C. It does not reach a complete conversion
until 185 °C, which is similar to other reported Pt
catalysts.28−31 Pt clusters with an average size of 1.0−2.0 nm
were also distributed on the exterior walls of the same batch of
CNTs as that of Pt@CNT (Figure S3) using the same
precursor for comparison, denoted as Pt/CNT. This catalyst is
obviously less active than Pt@CNT, similar to Pt/CB. The
higher activity of Pt@CNT than Pt clusters exposed on the
open surfaces of the exterior walls of CNTs and CB indicates
the importance of encapsulation. However, further comparison
with the zeolite H-mordernite HMOR reveals that it is more
than just spatial restriction. Note that the loadings of the Pt of
these four catalysts are close, in a range of 1.96−2.14% (weight
percentage), as examined by inductively coupled plasma−
atomic emission spectrometer (ICP−AES), as listed in
Supporting Information Table S1.
Pt clusters were introduced into the pores of HMOR zeolite
(with a pore size of 0.70 nm) in the parallel channel of the same
reactor for Pt@CNT following the same procedure and
conditions. The resulting catalyst, denoted as Pt@HMOR,
exhibits very small clusters. The size of the discernible particles
falls in a range of 0.4−0.7 nm (Figure S4). Figure 2a shows that
this catalyst initiates toluene conversion around 110 °C, similar
to Pt@CNT; however, toluene conversion is only 20%, even at
the relatively high temperature of 200 °C, exhibiting a far lower
activity than that over Pt@CNT. Obviously, solely spatial
restriction and small particle size are not the key for the high
activity observed for Pt@CNT.
For better comparison, the temperatures at which toluene
conversion reaches 30% (T30), and 100% (T100) are listed in
Table S2. The conversion rate in terms of converted toluene
per second per gram of platinum (μmol·s−1·g−1 Pt) is also
displayed. One sees that the activity of Pt@CNT is remarkably
higher than other experimentally reported Pt catalysts,
including Pt/Al2O3 with an averaged Pt particle size of 1.0
nm.32 It was reported that the turnover frequency of Pt was
found to decrease with a smaller particle size in oxidation of
VOCs32,33 as a result of the tendency of smaller Pt clusters to
be oxidized in the presence of oxygen, as evidenced by a higher
heat of oxygen chemisorption than that on larger Pt
clusters.33−35 Therefore, the confined environment of the
CNT channels likely has modulated the states of the
encapsulated Pt clusters.
This is confirmed by in situ XAFS, which allows monitoring
of the structure evolution of Pt clusters under reaction
conditions.36−38 One sees that the XANES of the Pt LIII edge
of the reduced Pt@CNT and Pt/CNT catalyst exhibit a feature
similar to those of Pt foil (Figure 3a). Further comparison of
the radial structural function (RSF) curves obtained from
Fourier transformation of the EXAFS function, λ(k), of
momentum space (Figure 3b) also suggests the contribution
of Pt−Pt interaction and the absence of Pt−O interaction. This
confirms that Pt clusters in both Pt@CNT and Pt/CNT have
been reduced fully prior to reaction and do not exhibit obvious
differences.
The in situ RSF and Pt LIII edge of Pt/CNT (Figure 3c)
show that the Pt−O scattering paths appear during oxidation of
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toluene and become more prominent with the increasing
temperature. Meanwhile, the whiteline also becomes more
intense (inset of Figure 3c). For noble metals such as Pt, the
LIII edge XAS is derived from the electronic transition from
2p3/2 to 5d states. The whiteline intensity directly reflects the
unoccupancy of 5d states and, thus, is a good indicator for
oxidation state variation, as shown in previous studies.12,39,40
This indicates that Pt species over the CNT exterior walls are
oxidized during reaction. In contrast, the Pt−O scattering is
hardly observable for Pt@CNTs (Figure 3d) under reaction
conditions even up to 200 °C (in the presence of oxygen) with
no significant change of the whiteline intensity (inset of Figure
3d). Furthermore, a lower coordination number, 0.5(7), and
oxidize the encapsulated Pt clusters than those on the open
surface, which is consistent with in situ EXAFS.
The stability test over a longer period of time under reaction
conditions further confirms the beneficial effects of confine-
ment. A more severe testing condition that is, 2-fold weight
hour space velocity (WHSV) was employed for Pt@CNT
compared with Pt/CB and Pt/CNT. Figure 2b shows that the
activity of Pt@CNT increases in the first 2 h and levels off in
the following 7 h. In contrast, the activities of both Pt/CB and
Pt/CNT decrease steadily and drop by 30% within 9 h.
HRTEM image (Figure S7) shows that Pt clusters of Pt@CNT
after reaction are limited to the range of 0.8−2 nm inside the
channels. In comparison, without the spatial restriction, the
particles of Pt/CNT grow larger, to 2−6 nm. Furthermore,
Pt@HMOR loses activity even more quickly, and the used
catalyst turns from the yellow of the fresh catalyst to dark
brown. This is likely attributed to coke formation, similar to the
previously reported Pt/HFAU41 and Pt/HBEA,42 leading to
deactivation with time on stream. In this respect, CNTs have
additional advantages of being devoid of strong acidic sites,
consequently avoiding coke deposition and pore blocking
during reaction. This demonstrates that encapsulation within
CNTs enables limiting the aggregation of Pt nanoclusters and
tailoring the Pt species at the active reduced states, even in the
presence of oxygen up to 200 °C. Consequently, the
encapsulated Pt delivers a much higher activity and more
stable performance.
In summary, encapsulation within the CNT channels allows
one to tailor the oxidation activity of Pt nanoclusters, not only
by restricting the cluster size down to around 1.0 nm but, more
importantly, by retaining the Pt species at the active reduced
state, even in the presence of oxygen up to 200 °C. In contrast,
clusters of a similar size dispersed on more open surfaces, such
as commercial carbon black and the CNT exterior walls, are
partially oxidized under the same conditions, leading to a
significantly lower activity and stability in toluene oxidation,
although the latter are much more accessible to reactants. The
capability of the CNT channels’ stabilizing reduced state metal
clusters is expected to be a general feature, which can be
explored to tailor the properties of other nanocatalysts for
oxidation.
longer Pt−O bond, 2.13
0.02 Å, are found for the
encapsulated Pt clusters with respect to the coordination
number of 1.2(5) and Pt−O bonds of 2.06 0.02 Å for the
outside Pt clusters from the first shell EXAFS analysis (Figure
S5 and Table S3). Although it is difficult to discriminate
between Pt−O and Pt−C observed for Pt/CNT at this stage,
the results clearly demonstrate that encapsulation protects the
fine Pt clusters, hindering its poisoning by either oxygen or
other organic species under reaction conditions. Therefore, the
confined catalyst is able to deliver a higher activity and stability
in the oxidation of toluene, which requires reduced Pt as the
active species. In contrast, the partially oxidized Pt species
either because of oxygen or organic species on open surfaces,
such as CB, and the CNT exterior walls are much less active,
although they are much more accessible to reactants.
The capability of the CNT channels to protect the reduced
metal species in the presence of oxygen is further validated by
DFT calculations (see Supporting Information for details).
CNT (18, 0) with a diameter of ∼1.5 nm and Pt43 clusters with
a size of 0.96 nm encapsulated inside the channel were taken as
models (Figure S6). We compared the oxidation reaction
energy of Pt43 clusters inside and outside of the CNT channels
because this reflects the affinity to oxygen, that is, the tendency
to oxidation. A variety of adsorption sites and coverage of
oxygen atoms were considered. Figure 4 shows that the
ASSOCIATED CONTENT
* Supporting Information
The following file is available free of charge on the ACS
■
S
Publications website at DOI: 10.1021/cs501763k
Experimental and calculation details; TEM and HAADF-
STEM images of Pt/CB, Pt/CNT, and Pt@HMOR;
TEM and HAADF-STEM images of Pt@CNT and Pt/
CNT after the reaction; Tables S1−S3 (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: panxl@dicp.ac.cn.
*E-mail: xhbao@dicp.ac.cn.
Figure 4. Averaged oxidation reaction energies per oxygen for Pt@
CNT in comparison with Pt/CNT as a function of different oxygen
coverages.
Notes
The authors declare no competing financial interest.
averaged oxidation reaction energy is about −1.72 eV/O, with
four oxygen atoms on Pt/CNT. It decreases to −1.63 eV/O
within the confined space of CNTs. The oxidation reaction
energy further drops to −1.30 eV/O for Pt@CNT at the higher
coverage with 10 oxygen atoms, corresponding to a higher
extent of oxidation. This indicates that it is more difficult to
ACKNOWLEDGMENTS
■
P.X. and B.X. gratefully acknowledge Prof Y. Hu for kind help
on discussions on XAFS results. This work was financially
supported by the Natural Science Foundation of China (Grant
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