Lab-in-a-shell: Encapsulating metal clusters for size sieving catalysis

Article abstract of DOI:10.1021/ja505903r

Here we describe a lab-in-a-shell strategy for the preparation of multifunctional core-shell nanospheres consisting of a core of metal clusters and an outer microporous silica shell. Various metal clusters (e.g., Pd and Pt) were encapsulated and confined in the void space mediated by the entrapped polymer dots inside hollow silica nanospheres acting first as complexing agent for metal ions and additionally as encapsulator for clusters, limiting growth and suppressing the sintering. The Pd clusters encapsulated in hybrid core-shell structures exhibit exceptional size-selective catalysis in allylic oxidations of substrates with the same reactive site but different molecular size (cyclohexene ～0.5 nm, cholesteryl acetate ～1.91 nm). The solvent-free aerobic oxidation of diverse hydrocarbons and alcohols was further carried out to illustrate the benefits of such an architecture in catalysis. High activity, outstanding thermal stability and good recyclability were observed over the core-shell nanocatalyst.

Full text of DOI:10.1021/ja505903r

Communication
pubs.acs.org/JACS
Lab-in-a-Shell: Encapsulating Metal Clusters for Size Sieving Catalysis
Zhen-An Qiao,† Pengfei Zhang,*,† Song-Hai Chai,† Miaofang Chi,§ Gabriel M. Veith,# Nidia C. Gallego,#
Michelle Kidder,† and Sheng Dai*,†,‡
†Chemical Sciences Division, §Center for Nanophase Material Sciences, and #Materials Science and Technology Division, Oak Ridge
National Laboratory, Oak Ridge, Tennessee 37831, United States
‡Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States
S
* Supporting Information
aggregation and detachment from the support.6 One important
ABSTRACT: Here we describe a lab-in-a-shell strategy
for the preparation of multifunctional core−shell nano-
spheres consisting of a core of metal clusters and an outer
microporous silica shell. Various metal clusters (e.g., Pd
and Pt) were encapsulated and confined in the void space
mediated by the entrapped polymer dots inside hollow
silica nanospheres acting first as complexing agent for
metal ions and additionally as encapsulator for clusters,
limiting growth and suppressing the sintering. The Pd
clusters encapsulated in hybrid core−shell structures
exhibit exceptional size-selective catalysis in allylic
oxidations of substrates with the same reactive site but
different molecular size (cyclohexene ∼0.5 nm, cholesteryl
acetate ∼1.91 nm). The solvent-free aerobic oxidation of
diverse hydrocarbons and alcohols was further carried out
to illustrate the benefits of such an architecture in catalysis.
High activity, outstanding thermal stability and good
recyclability were observed over the core−shell nano-
catalyst.
role of this strategy is to hinder the interaction/contact
between metal particles and thereby keep active metal cores
stable even under harsh conditions, meanwhile allowing the
smooth diffusion of small reactant molecules in and out of the
shells. Moreover, the enclosed void space of core−shell
nanoparticles is expected to be useful for chemical storage,
compartmentation, and confinement of host−guest interactions
in the catalysis processes.7
Several approaches have been developed for the synthesis of
core−shell nanostructures, such as soft template, ship-in-a-
bottle, Kirkendall effect, galvanic replacement, inside-out
Ostwald ripening process, and so forth.8 Although some
progress in the synthesis of core−shell structures has been
achieved, most of these methods still suffer from some
disadvantages, such as low efficiency and tedious processing
steps, which mainly result in a low yield. In addition, there are
only a limited set of reports that can stabilize or entrap almost
the smallest nanoparticle clusters (≤2 nm), with extremely high
activity, into the interior of the hollow structures.9
Herein, we demonstrate a facile, efficient, yet unexplored
approach, referred to as a lab-in-a-shell strategy, for the
fabrication of a new type of core−shell nanospheres with a
hollow microporous silica shell, encapsulating and immobilizing
various metal clusters inside the cavity. The essence of the lab-
in-a-shell strategy lies in the use of functionalized hollow shell
as a microlaboratory integrating the laboratory functions and
providing a unique microenvironment on a single hollow
sphere of only several nanometers in size.10 Our core−shell
structured metal cluster@silica nanospheres were prepared by
this approach in three steps (Figure 1a): (1) polymerization of
ethylenediamine and carbon tetrachloride inside the hollow
silica nanospheres to form polymer dot@silica core−shell
nanoreactors;11 (2) adsorbing and fixing target metal ions by
the amine groups at the surface of polymer dots; and (3) H2 in
situ reduction of metal ions to metal clusters, removal of the
polymers by calcination in the air and H2 reduction of metal
oxide to produce the metal cluster@silica core−shell nano-
spheres. As a result, the metal clusters were successfully
encapsulated with both super-fine particle size (0.8−4.5 nm),
which is much smaller than those of nanoparticles prepared by
the conventional methods,12 and uniform size distribution. In
this approach, the polymer dot@silica core−shell structures
etal catalysts are playing a vital role in petroleum
Mrefining, fine chemical production, and environmental
protection, such as automobile exhaust treatment and selective
oxidation.1 In general, metal particles, typically with a size of 1−
10 nm, are dispersed on support materials of high surface area
to expose a high fraction of accessible atoms on the surface of
the particles.2 During the past few decades, extensive efforts
have focused on the control of particle size of metals, the nature
of the supports, and the surface and interfacial effects to
improve the performance of these catalysts.3 However, many
intractable problems that hinder the development of this field
(one of them is the thermally induced sintering) still remain.4
The supported metal particles tend to grow or aggregate during
the catalysis process, accompanied by a distinct loss of catalytic
activity. Additionally, metal particles usually detach from the
supports when the corresponding catalyst is rubbed recip-
rocally, which results in a sharp decrease of the activity. Hence
the design of an ideal nanostructure for metal supported
catalysts that can overcome the above-mentioned limits, at the
same time displaying high stability, is a great challenge in this
field.5
Encapsulation of metal particles into a nanoporous shell to
form a core−shell nanostructure is a straightforward and
effective strategy to address the problems of particle
Received: June 12, 2014
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
some of the Pd0 surface atoms have been oxidized to Pd2+ by
O2 in the air, owing to their extremely high activity. The
formation of Pd metallic clusters was also further confirmed by
the wide-angle X-ray powder diffraction (XRD) pattern (Figure
1e). From the patterns of Pd cluster@silica nanospheres, only
one broad Pd diffraction peaks can be seen at 2θ = 38.15°
assigned to (111) reflections of the cubic (fcc) palladium lattice
(JCPDS no. 46-1043) as well as a very broad signal at 2θ =
22.31° ascribed to amorphous silica, indicating the presence of
ultrasmall crystalline Pd inside the hollow silica nanospheres.
The average size of the Pd clusters is 1.6 nm calculated by the
Scherrer equation, which is in agreement with the value
obtained from TEM images. The FTIR spectrum of Pd
cluster@silica spheres shows that the interior polymers have
been completely burned off in the air (Figure S2). After
calcinations in air at 400 °C for removal of the polymer, the Pd
clusters were actually oxidized to obtain PdO clusters with a
cluster size of 2.6 nm as shown by the XRD pattern.
Interestingly, the entrapped Pd clusters are very stable. The
size only increases from 1.6 to 2.5 nm as the temperature
increasing from 300 to 600 °C in the H2. These results indicate
that the entrapped Pd clusters are prevented from sintering and
aggregation even at a high reaction temperature.
The entrapment of Pt clusters was also confirmed by TEM
and XPS (Figure 2). The size of Pt clusters assembled inside
Figure 1. Procedure for the preparation of metal cluster@silica core−
shell nanospheres (a). TEM images of Pd cluster@silica core−shell
nanospheres: (b) Z-contrast and (c) bright-field. (d) Pd 3d XPS
spectra for Pd cluster@silica nanospheres. (e) XRD patterns for Pd
cluster/polymer@silica nanospheres (A), PdO cluster@silica nano-
spheres (B), and Pd cluster@silica nanospheres (C).
have a profound effect on the formation of the metal clusters.
The large amount of residual amine groups on the polymer
dots surface acts as a coordinating agent for the target metal
ions and additionally as an encapsulator for the metal clusters,
limiting growth and aggregation, especially when used at high
temperature (up to 600 °C) in the synthesis.
Transmission electron microscopic (TEM) images of the as-
prepared Pd cluster@silica core−shell nanospheres are
presented in Figure 1. These images clearly reveal that Pd
clusters are successfully encapsulated inside the silica hollow
spheres, and most of the core clusters are attached on the
interior wall of the silica shell due to the gravity. The entrapped
Pd clusters have a narrow size distribution in the range of 1.1−
2.3 nm as estimated from the TEM images (Figure 1b, c). Low-
magnification TEM and elemental mapping images (Figure S1)
show that the distribution of Pd clusters are evenly dispersed
within the hollow silica nanospheres. The dispersion of 66.7%
and particle size of 1.7 nm were also measured by pulse H2
chemisorption of Pd in this sample. In addition, the amount of
entrapped Pd clusters could be easily controlled from 0.1 to 15
wt % by adjusting the concentration of PdCl2 precursor
solution.
Figure 2. TEM images of Pt cluster@silica core−shell nanospheres: (a,
b) Z-contrast and (c) bright field. Pt 4f XPS (d) and EDX (e) spectra
for Pt cluster@silica nanospheres.
the silica spheres ranges from 10 to 50 atoms (Figure 2a−c).
The diameter of ∼0.8 nm was averaged from 100 randomly
selected clusters. The Pt0 4f7/2 and 4f5/2 peaks are centered at
71.6 and 75.1 eV, while the peaks around 73.4 and 76.8 eV
correspond to Pt2+ species, similar to the Pd samples (Figure
2d). No peaks of Pt4+ were observed, which suggested that all
of Pt4+ precursor had been reduced in situ by H2 treatment.
The energy dispersive X-ray spectroscopy (EDX) spectrum of
composite nanomaterials clearly identifies the peaks of element
Pt (Figure 2e). Additionally, we could not observe any Pt and
PtO2 peaks in the XRD patterns synthesis (Figure S3). The size
of entrapped Pt clusters can be readily controlled from 0.8 to
4.2 nm by varying the calcination temperature of 400−500 °C
in the air (Figure S4).
X-ray photoelectron spectroscopy (XPS) reveals that the Pd
cluster@silica nanospheres exhibit two energy bands at 339.7
and 334.7 eV, which are typical values for the Pd 3d3/2 and
3d5/2 electrons of metallic Pd0 (Figure 1d). The peaks around
336.3 and 341.3 eV correspond to Pd2+ species, suggesting that
Actually, core−shell structures are powerful platforms for
confined nanocatalysis, especially an ideal matrixes for “dream”
B
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size-selective catalysis, which is common in nature (e.g., size
recognition in biological enzymes), yet seldom realized by
heterogeneous organocatalysis.13 Considering the molecular
sieving capability of the microporous silica shell (Figure S5)
and the ultrasmall size of entrapped Pd clusters, molecular-size
recognition of our core−shell catalyst was probed by the allylic
oxidations of cyclohexene versus cholesteryl acetate (CA),
substrates with the same reactive site but different molecular
size (cyclohexene ∼0.5 nm, CA ∼1.91 nm). For comparison, 25
nm solid silica nanoparticles loading 0.9 wt % Pd on their
surface were synthesized by wetness impregnation method, and
size of Pd particles was in the range of 4−8 nm (Figure S6). In
the presence of this Pd/silica catalyst, cyclohexene and CA were
oxidized in 60 min with 2-cyclohexen-1-one, 2-cyclohexenol,
cyclohexene oxide, and 7-ketocholesteryl acetate as main
products, giving the conversions of 23.1% and 9.8%,
respectively (Figure 3). Generally, the metal nanoparticles
alcohols by atmospheric O2 were studied over Pd cluster@
silica nanospheres (Pd loading of 0.9 wt %). The core−shell
nanocatalyst was very active for hydrocarbons (Table 1, entries
Table 1. Oxidation of Hydrocarbons and Alcohols Catalyzed
a
by Pd Cluster@Silica Core−Shell Nanospheres
Figure 3. Size-selective catalysis of Pd cluster@silica core−shell
a
Reaction conditions: hydrocarbon 4 mL or alkanol 4 mL or aromatic
nanospheres.
alcohol 40 mL, 0.9 wt % Pd cluster@silica 20 mg, O2 1 atm, 20 h.
b
TOF = [reacted mol substrate]/[(total mol metal) × (reaction
entrapped in the zeolite or MOF materials have a relatively low
catalytic activity, because of large core particles prepared by
traditional methods and slow mass diffusion through the thick
microporous shells (hundreds of nanometers).13 However, for
our Pd cluster@silica catalyst (Pd loading of 0.9 wt %), around
4-fold cyclohexene conversion (93.7%) was reached with high
selectivity for dehydrogenation product benzene (97.7%). The
dehydrogenation of saturated C−C bond to CC bond should
be induced by the astonishingly fast hydride elimination of
active Pd clusters, therefore suppressing the O2-addition
process (Supporting Information).14 In sharp contrast, no
detectable oxidation products of CA were observed by the Pd
clusters−core@silica−shell catalyst. The size-sensitive perform-
ance of our core−shell catalyst can be directly ascribed to the
molecular sieving characteristics of ultrathin microporous silica
layer (∼5 nm), which facilitates mass transfer of cyclohexene in
liquid-phase oxidation but in turn lays a barrier between the big
CA molecule and active Pd clusters inside (Supporting
Information). The ultrasmall entrapped Pd clusters enable
higher catalytic properties as compared to Pd nanoparticles
synthesized by traditional approaches. The combination of the
two factors results in the excellent size-selectivity and high
catalytic activity of the core−shell catalyst.
c
time)]. The TOFs were measured after 0.5 h of reaction. Controlled
oxidation without catalyst.
1−3), representative alcohols (entries 4−7) with remarkable
turnover frequencies (tetralin, 469 h−1; benzyl alcohol, 40910
h−1). During the oxidation of chemically inert aliphatic octan-2-
ol, a record TOF of 417 h−1 was achieved, which is significantly
higher than previous results reported by other active catalysts,
such as Au-Pd@TiO2 (0 h−1),15a Pd@N-Carbon (TOF: 86
h−1),15b Pd@Hydroxyapatite (TOF: 7 h−1),1c Ru(OH)x@
Fe3O4 (TOF: 4 h−1).15c Besides the high catalytic activity, the
core−shell nanospheres also have a good thermal stability
under high reduction temperature. The catalytic activity of Pd
cluster@silica nanospheres was well retained after temperatures
up to 600 °C under H2 atmosphere, indicating that the
morphology and cluster size are not essentially changed after
the high-temperature treatment. The core−shell catalyst could
be reused at least 20 times without any loss of activity for
benzyl alcohol oxidation (120 °C, O2 1 atm), and the
corresponding turnover number was estimated to be
∼280,000 (Figure S7). In consideration of catalytic results
and the size of the Pd clusters after reusing 20 times, it is
reasonable to confirm the fully recyclable character of current
Pd catalyst, a prerequisite for industrial application. The
outstanding catalytic performance of the core−shell catalyst
in the aerobic oxidation demonstrates that the unique structure
in deed endows the entrapped ultrasmall clusters with high
catalytic activity, high thermal stability, and favorable reusability
The selective oxidation of organic compounds, such as
hydrocarbons or alcohols, is a primary and essential trans-
formation in organic synthesis and industrial chemistry.15 The
as-made core−shell nanospheres with multi unique characters
in principle could also afford some exceptional performance,
and therefore solvent-free oxidation of hydrocarbons or
C
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Journal of the American Chemical Society
Communication
(5) (a) Lu, J. L.; Fu, B. S.; Kung, M. C.; Xiao, G. M.; Elam, J. W.;
Kung, H. H.; Stair, P. C. Science 2012, 335, 1205. (b) Chan-Thaw, C.
E.; Villa, A.; Katekomol, P.; Su, D. S.; Thomas, A.; Prati, L. Nano Lett.
2010, 10, 537.
(6) (a) Arnal, P. M.; Comotti, M.; Schuth, F. Angew. Chem., Int. Ed.
2006, 45, 8224. (b) Liu, J.; Yang, H. Q.; Kleitz, F.; Chen, Z. G.; Yang,
T. Y.; Strounina, E.; Lu, G. Q.; Qiao, S. Z. Adv. Funct. Mater. 2012, 22,
591. (c) Huang, X. Q.; Tang, S. H.; Liu, B. J.; Ren, B.; Zheng, N. F.
Adv. Mater. 2011, 23, 3420.
(7) Liu, R.; Mahurin, S. M.; Li, C.; Unocic, R. R.; Idrobo, J. C.; Gao,
H. J.; Pennycook, S. J.; Dai, S. Angew. Chem., Int. Ed. 2011, 50, 6799.
(8) (a) Liu, J.; Qiao, S. Z.; Hartono, S. B.; Lu, G. Q. Angew. Chem.,
Int. Ed. 2010, 49, 4981. (b) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.;
Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711.
(c) Liu, J.; Qiao, S. Z.; Chen, J. S.; Lou, X. W.; Xing, X. R.; Lu, G. Q.
Chem. Commun. 2011, 47, 12578.
(9) (a) Nie, X. T.; Qian, H. F.; Ge, Q. J.; Xu, H. Y.; Jin, R. C. ACS
Nano 2012, 6, 6014. (b) Ma, G. C.; Binder, A.; Chi, M. F.; Liu, C.; Jin,
R. C.; Jiang, D. E.; Fan, J.; Dai, S. Chem. Commun. 2012, 48, 11413.
(10) (a) Duan, R. X.; Zuo, X. L.; Wang, S. T.; Quan, X. Y.; Chen, D.
L.; Chen, Z. F.; Jiang, L.; Fan, C. H.; Xia, F. J. Am. Chem. Soc. 2013,
135, 4604. (b) Schmit, V. L.; Martoglio, R.; Scott, B.; Strickland, A. D.;
Carron, K. T. J. Am. Chem. Soc. 2012, 134, 59.
during the heterogeneous catalysis, while the microporous shell
around the encapsulated metal clusters allows the small size
reactant to freely access to the encapsulated metal sites.
Moreover, the protective microporous shells could prevent
aggregation of the metal nanoparticles.
In conclusion, we have demonstrated a versatile lab-in-a-shell
strategy to encapsulate various clusters within hollow silica
nanospheres in a controllable way. This strategy is applicable to
a broad range of clusters in non-agglomerated fashion and is
capable of controlling the spatial distribution of clusters within
the hollow silica matrix. On the basis of the characterization of
their core−shell structure, the catalytic properties were
explored in the solvent-free aerobic oxidation of hydrocarbons
and alcohols, and the core−shell nanospheres were shown to
have excellent catalytic activity with unexpected size selectivity,
high thermal stability, and good recyclability. It is expected that
such metal cluster@silica core−shell nanoreactors will serve as
a useful platform to control substrate orientation and reactivity
in heterogeneous catalysis as the diffusion of reactants and
morphology of active catalysts can readily be manipulated
through rational catalyst design. We believe the synthesis
strategy reported here may open up new opportunities for
preparing core−shell structured nanomaterials with various
compositions.
(11) Qiao, Z. A.; Huo, Q. S.; Chi, M. F.; Veith, G. M.; Binder, A. J.;
Dai, S. Adv. Mater. 2012, 24, 6017.
(12) (a) Schalow, T.; Brandt, B.; Starr, D. E.; Laurin, M.;
Shaikhutdinov, S. K.; Schauermann, S.; Libuda, J.; Freund, H. J.
Angew. Chem., Int. Ed. 2006, 45, 3693. (b) Davis, S. E.; Ide, M. S.;
Davis, R. J. Green Chem. 2013, 15, 17. (c) Johnston, E. V.; Verho, O.;
Karkas, M. D.; Shakeri, M.; Tai, C. W.; Palmgren, P.; Eriksson, K.;
Oscarsson, S.; Backvall, J. E. Chem.Eur. J. 2012, 18, 12202.
(13) (a) Lu, G.; Li, S. Z.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X.
Y.; Wang, Y.; Wang, X.; Han, S. Y.; Liu, X. G.; DuChene, J. S.; Zhang,
H.; Zhang, Q. C.; Chen, X. D.; Ma, J.; Loo, S. C. J.; Wei, W. D.; Yang,
Y. H.; Hupp, J. T.; Huo, F. W. Nat. Chem. 2012, 4, 310. (b) Shang, Z.
Y.; Patel, R. L.; Evanko, B. W.; Liang, X. H. Chem. Commun. 2013, 49,
10067. (c) Rajagopalan, R.; Ponnaiyan, A.; Mankidy, P. J.; Brooks, A.
W.; Yi, B.; Foley, H. C. Chem. Commun. 2004, 2498.
(14) Campbell, A. N.; Stahl, S. S. Acc. Chem. Res. 2012, 45, 851.
(15) (a) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.;
Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.;
Hutchings, G. J. Science 2006, 311, 362. (b) Zhang, P. F.; Gong, Y. T.;
Li, H. R.; Chen, Z. R.; Wang, Y. Nat. Commun. 2013, 4, 1593.
(c) Kotani, M.; Koike, T.; Yamaguchia, K.; Mizuno, N. Green Chem.
2006, 8, 735. (d) Prati, L.; Porta, F.; Wang, D.; Villa, A. Catal. Sci.
Technol. 2011, 1, 1624. (e) Chen, Y.; Wang, H.; Liu, C.; Zeng, Z.;
Zhang, H.; Zhou, C.; Jia, X.; Yang, Y. J. Catal. 2012, 289, 105.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional figures. This material is
■
S
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
dais@ornl.gov; chemistryzpf@163.com
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences, Chemical Sciences,
Geosciences, and Biosciences Division. A portion of this
research was conducted at the Center for Nanophase Materials
Sciences, which is sponsored at Oak Ridge National Laboratory
by the Division of Scientific User Facilities, Office of Basic
Energy Sciences, U.S.
REFERENCES
■
(1) (a) Abad, A.; Concepcion, P.; Corma, A.; Garcia, H. Angew.
Chem., Int. Ed. 2005, 44, 4066. (b) Kesavan, L.; Tiruvalam, R.; Ab
Rahim, M. H.; bin Saiman, M. I.; Enache, D. I.; Jenkins, R. L.;
Dimitratos, N.; Lopez-Sanchez, J. A.; Taylor, S. H.; Knight, D. W.;
Kiely, C. J.; Hutchings, G. J. Science 2011, 331, 195. (c) Mori, K.; Hara,
T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004, 126,
10657.
(2) (a) Bell, A. T. Science 2003, 299, 1688. (b) Zheng, N. F.; Stucky,
G. D. J. Am. Chem. Soc. 2006, 128, 14278. (c) Corma, A. Chem. Rev.
1997, 97, 2373.
(3) (a) Chen, C.; Nan, C. Y.; Wang, D. S.; Su, Q. A.; Duan, H. H.;
Liu, X. W.; Zhang, L. S.; Chu, D. R.; Song, W. G.; Peng, Q.; Li, Y. D.
Angew. Chem., Int. Ed. 2011, 50, 3725. (b) Do, P. T. M.; Foster, A. J.;
Chen, J. G.; Lobo, R. F. Green Chem. 2012, 14, 1388.
(4) (a) Joo, S. H.; Park, J. Y.; Tsung, C. K.; Yamada, Y.; Yang, P. D.;
Somorjai, G. A. Nat. Mater. 2009, 8, 126. (b) Datta, K. K. R.; Reddy, B.
V. S.; Ariga, K.; Vinu, A. Angew. Chem., Int. Ed. 2010, 49, 5961.
D
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