Active site design in a core-shell nanostructured catalyst for a one-pot oxidation reaction

Article abstract of DOI:10.1002/chem.201101241

Designing an active site: A new type of core-shell structured catalyst was prepared, with a uniform SiO₂ core supporting Pd nanoparticles (NPs) covered with a Ti-containing mesoporous silica shell. Generated H ₂O₂ on the i

Full text of DOI:10.1002/chem.201101241

COMMUNICATION
DOI: 10.1002/chem.201101241
Active Site Design in a Core–Shell Nanostructured Catalyst
for a One-Pot Oxidation Reaction
Shusuke Okada,[a] Kohsuke Mori,[a] Takashi Kamegawa,[a] Michel Che,[b] and
Hiromi Yamashita*[a]
Hydrogen peroxide (H2O2) is not only an environmentally
friendly oxidizing agent, but also a highly selective oxidant
used in the manufacturing of numerous organic and inorgan-
ic compounds. The commercial production of H2O2 by the
sequential hydrogenation and oxidation of an alkyl anthra-
quinone requires a great deal of toxic solvent and a large
energy input due to the multistep reactions. The direct syn-
thesis of H2O2 from H2 and O2 over supported Pd-based cat-
alysts has attracted significant attention recently, because it
would minimize adverse environmental effects and result in
lower cost.[1] The key problem is to stabilize the resulting
H2O2, because H2O2 simultaneously undergoes hydrogena-
tion and/or decomposition to water by the same catalysts
employed for its formation. This undesirable feature causes
a slow H2O2 formation rate and prevents the accumulation
of high H2O2 concentration. The addition of an acid or
halide to the reaction medium is useful to achieve high se-
lectivity;[2] however, the additional operation for removal of
the acid or halide becomes an even more troublesome issue.
To overcome this problem, the utilization of in situ generat-
ed H2O2 from H2 and O2 to subsequently oxidize organic re-
actants in the same reaction vessel (one-pot oxidation reac-
tion) could be an alternative strategy. It is possible to use
the unstable H2O2 immediately without isolation/purification
steps, which would contribute to saving energy and time, as
well as avoid the risk of transportation of the concentrated
H2O2.
have been recent reports on one-pot oxidation using in situ
generated H2O2. The most convenient method is a physical
mixture of both components.[3] A combination of Ti-contain-
ing zeolite (TS-1) and supported Pd NPs has also been de-
veloped;[4] however, all such catalyst systems still suffer
from low utilization efficiency of diluted H2O2, because the
random location of both types of active sites resulted in un-
desirable H2O2 decomposition.
Considering this, we present a new type of core–shell
structured catalyst to enable the one-pot oxidation of sulfide
to sulfoxide with high efficiency and selectivity, in which a
uniform SiO2 core supporting Pd NPs was covered with a
Ti-containing mesoporous silica shell (Pd/SiO2@TiMSS). Al-
though the inner Pd NPs are located at the boundary of the
core–shell structure, reactants can penetrate to the Pd NPs
through the mesoporous shell.[5] The Ti-oxide moieties locat-
ed within the mesoporous region are catalytically active
toward sulfide oxidation with H2O2 and are considered to be
highly dispersed at the atomic level.[6] It is expected that the
H2O2 generated on the inner Pd NP sites could selectively
interact with the Ti-oxide moieties within the mesopores
before dispersion to the solvent can occur, which would ulti-
mately enhance the oxidation activity.
The procedure for the synthesis of Pd/SiO2@TiMSS is
schematically illustrated in Figure 1. Colloidal SiO2 NPs
(average diameter: 280 nm: see the Supporting Information,
Figure S1) were firstly prepared by the Stçber method.[7]
The Pd NPs were then successfully deposited on the surface
of SiO2 spherical NPs by reduction of a PdCl2 precursor
using SnII anchored to the hydroxyl groups of SiO2 as reduc-
ing agent in the presence of sodium formate.[8] The Pd NPs
supported on SiO2 were further coated with Ti-containing
mesoporous silica shell using cetyltrimethyl ammonium bro-
mide (CTAB) as structure directing agent (SDA), tetrae-
thoxysilane (TEOS) as a silica source, and tetrapropyl or-
thotitanate (TPOT) as a Ti source.[9] The final core–shell
structure, Pd/SiO2@TiMSS (Pd: 1.0 wt%, Ti: 0.6 wt% deter-
mined by ICP analysis), was obtained after calcination at
823 K in air to remove the SDA (Figure 2).
We predict that the key to construct a one-pot oxidation
system is the efficient dispersion of H2O2 generated on Pd
nanoparticle (NP) sites to the neighboring catalytically
active oxidation sites before any undesirable conversion to
water can occur. Therefore, the primary requirement is the
precise architecture of the two types of active sites. There
[a] S. Okada, Dr. K. Mori, Dr. T. Kamegawa, Prof. Dr. H. Yamashita
Graduate School of Engineering
Osaka University, 1-2 Yamadaoka, Suita
Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7457
E-mail: yamashita@mat.eng.osaka-u.ac.jp
To elucidate the effect of the Pd NPs location on en-
hancement of one-pot oxidation using in situ generated
H2O2, two reference samples, SiO2@Pd(R)/TiMSS and
SiO2@Pd(S)/TiMSS, were synthesized. In the case of the
SiO2@Pd(R)/TiMSS, in which (R) indicates random, the Pd
NPs were deposited after calcination, which allowed the
[b] Prof. Dr. M. Che
Institut Universitaire de France
and Laboratoire de Rꢀactivitꢀ de Surface
Universitꢀ Pierre et Marie Curie-Paris 6 (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101241.
Chem. Eur. J. 2011, 17, 9047 – 9051
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9047

NPs (Pd: 1.0 wt%, Ti: 0.6 wt% determined by ICP analy-
sis). The SDA was removed at the final step, so that most of
the Pd NPs are thought to be deposited on the outer surface
of the mesopores.
The HR-TEM images of all samples show that the spheri-
cal NPs consist of nonporous SiO2 cores, Ti-containing mes-
oporous shells with channels oriented perpendicular to the
core surface, and Pd NPs (Figure 2). The thickness of the
shell was determined to be approximately 30 nm, which cor-
responds to the value estimated from SEM images (see the
Supporting Information, Figure S1). The Pd NPs were appa-
rently present at the intended sites; the boundary between
the SiO2 core and mesoporous silica shell, randomly distrib-
uted within the mesopores, and the surface of mesoporous
shell in Pd/SiO2@TiMSS, SiO2@Pd(R)/TiMSS, and
SiO2@Pd(S)/TiMSS, respectively. The average size of Pd
NPs is 3.5 nm and no significant differences in the Pd NP
size are observed in the three samples.
The mesoporous structure was characterized by small
angle XRD and N2 adsorption–desorption measurements.
All the samples exhibit sharp diffraction peak centered
around 2q=2.58 (see the Supporting Information, Figure S2)
in the small angle XRD measurements, whereas the nonpo-
rous SiO2 nanosphere used as the core does not show any
peaks.
Figure 1. Schematic illustration of the procedure used to prepare Pd/
SiO2@TiMSS, SiO2/@Pd(R)/TiMSS and SiO2/@Pd(S)/TiMSS.
Typical type IV N2 adsorption–desorption isotherms are
observed without significant hysteresis, which is characteris-
tic of micelle-templated materials (Figure 3). In the relative
pressure range from 0.2 to 0.3, the isotherm exhibits a sharp
inflection attributed to capillary condensation inside the
mesopores. The average pore diameter is 2.1 nm calculated
by the Barrett–Joyner–Halenda (BJH) method. The Bruna-
uer–Emmett–Teller (BET) surface area and total pore
volume of Pd/SiO2@TiMSS, SiO2@Pd(R)/TiMSS, and
SiO2@Pd(S)/TiMSS were estimated to be 473 m2 gÀ1 and
0.30 cm3 gÀ1, 272 m2 gÀ1 and 0.15 cm3 gÀ1, and 240 m2 gÀ1 and
0.21 cm3 gÀ1, respectively, which is clear evidence for the for-
mation of a mesoporous structure with uniform size. UV/Vis
spectroscopy and Ti K-edge X-ray absorption fine structure
(XAFS) measurements were performed to investigate the
local structure of the Ti species in the Ti-containing mesopo-
rous silica shell. It is well accepted that the formation of tet-
rahedral Ti species is crucial to achieve selective oxidation
using H2O2, whereas the octahedral Ti species and bulk
Figure 2. HR-TEM images of (a) and (b) Pd/SiO2@TiMSS; (c) and (d)
SiO2/@Pd(R)/TiMSS; and (e) and (f) SiO2/@Pd(S)/TiMSS.
random distribution of Pd NPs within the mesoporous struc-
ture. SiO2@Pd(S)/TiMSS, where (S) indicates the surface,
was obtained by firstly coating the SiO2 core with the Ti-
containing mesoporous shell, followed by loading of the Pd
Figure 3. (a) N2 adsorption-desorption isotherms and (b) BJH pore size
~
*
distribution. Pd/SiO2@TiMSS ( ); SiO2@Pd(R)/TiMSS ( ); SiO2@Pd(S)/
^
TiMSS (&); SiO2 ( ).
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Chem. Eur. J. 2011, 17, 9047 – 9051

Active Site Design in a Core–Shell Nanostructured Catalyst
COMMUNICATION
TiO2 promote the decomposition of H2O2.[10] In the UV/Vis
spectra of the Pd/SiO2@TiMSS, a ligand-to-metal charge-
transfer (LMCT) band for O2À to Ti4+ attributed to the site-
isolated Ti species was observed in the range 210–230 nm
(see the Supporting Information, Figure S3).[11] The TiO2
powder exhibited a wide absorption ranging from 400 to
À À
200 nm, which suggests the presence of extended Ti O Ti
connectivity. The Ti K-edge XANES spectra of the TiO2
powder showed several well defined pre-edge peaks attrib-
utable to Ti in a symmetric octahedral environment. In con-
trast, Pd/SiO2@TiMSS exhibit an intense single pre-edge
peak at 4967 eV, which indicates that the local coordination
of the Ti is tetrahedral[12] and suitable for oxidation with
H2O2 as oxidizing agent (see the Supporting Information,
Figure S4).
Figure 4. (a) Kinetics of methyl phenyl sulfide (1) oxidation using in situ
generated H2O2, and (b) dependence of the conversion level on the selec-
*
tivity. Selectivity=[2/
(2+3)]ꢂ100. Pd/SiO2@TiMSS
(
); SiO2@Pd(R)/
~
&
TiMSS ( ); SiO2@Pd(S)/TiMSS ( ).
The potential catalytic activities of these samples were
first evaluated in the direct synthesis of H2O2 from atmos-
pheric H2 and O2 gases on Pd NP sites. The H2O2 concentra-
tion in the reaction medium after 3 h was almost independ-
ent on the catalysts employed (Table 1). The position of the
at the same conversion level, respectively. Similar phenom-
ena were also observed in the case of diphenyl sulfide (4)
with high selectivity toward the corresponding sulfoxide 5,
as shown in Figure 5.
Table 1. Effect of catalyst on the concentration of H2O2.
Pd/
SiO2@Pd(R)/
SiO2@Pd(S)/
TiMSS
SiO2@TiMSS TiMSS
H2O2 concentration
3.8 3.0
3.9
[mm]
Pd NP site does not influence the catalytic activity, and thus
the smooth transfer of H2 and O2 gases is not circumvented,
even in the internal Pd NPs of the Pd/SiO2@TiMSS core–
shell structure.
Figure 5. (a) Kinetics of diphenyl sulfide (1) oxidation using in situ gener-
ated H2O2, and (b) dependence of the conversion level on the selectivity.
~
&
*
Pd/SiO2@TiMSS ( ); SiO2@Pd(R)/TiMSS ( ); SiO2@Pd(S)/TiMSS ( ).
The catalysts were then investigated for the one-pot oxi-
dation of methyl phenyl sulfide (1) into methyl phenyl sulf-
oxide (2), catalyzed by isolated Ti oxide moieties using in
situ generated H2O2. Several reagents and catalytic systems
have been extensively studied for this transformation be-
cause sulfoxide is a very useful intermediate for the prepara-
tion of pharmaceuticals and agrochemicals; however, the
suppression of over-oxidation toward the corresponding sul-
fone 3 still remains a challenging task.[13] Figure 4 shows that
Pd/SiO2@TiMSS is the most effective catalyst among those
investigated; the reaction rate for Pd/SiO2@TiMSS is almost
three times higher than those for SiO2@Pd(S)/TiMSS and
SiO2@Pd(R)/TiMSS. Both H2 and O2 were necessary compo-
nents to achieve a one-pot oxidation reaction. No reaction
was observed for either SiO2@TiMSS without Pd NPs or Pd/
SiO2@MSS without Ti. It should be emphasized that, as ex-
pected, the relative position between the H2O2 generation
sites and the oxidation sites has a significant effect on the
catalytic activity. More significantly, a high selectivity of
The high catalytic activity of Pd/SiO2@TiMSS can be as-
cribed to the efficient utilization of H2O2 for the subsequent
sulfide oxidation, in which the in situ generated H2O2 within
the mesoporous channels can promptly react at the neigh-
boring Ti sites before H2O2 can be dispersed into the sol-
vent. Therefore, H2O2 is mainly used for the oxidation reac-
tion and undesirable reactions are prevented. On the contra-
ry, H2O2 is mainly produced at the outer surface of
SiO2@Pd(S)/TiMSS and it is difficult for the generated H2O2
to enter the mesoporous channels to interact with the Ti
species; therefore, most of the H2O2 is decomposed by the
Pd catalyst used in its formation. In the case of
SiO2@Pd(R)/TiMSS, the random distribution of the Pd NPs
within the mesoporous channels may inhibit the diffusion of
reactants to both types of active site.
To confirm this assumption, the oxidation of 1 was per-
formed using a commercially available aqueous H2O2 solu-
tion (3 equiv to 1) instead of the in situ generated H2O2.
Figure 6 shows that Pd/SiO2@TiMSS exhibits the highest ac-
91% toward
2 is attained in the presence of Pd/
SiO2@TiMSS, which is superior to the 81 and 79% selectivi-
ty obtained for SiO2@Pd(R)/TiMSS and SiO2@Pd(S)/TiMSS
Chem. Eur. J. 2011, 17, 9047 – 9051
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www.chemeurj.org
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H. Yamashita et al.
Synthesis of SiO2 core: TEOS was added to the mixture of ethanol and
aqueous ammonia. The molar ratio of solution was TEOS/EtOH/NH3/
H2O=1:70.1:2.1:19.5. The suspension was stirred for 6 h. The precipitate
was centrifuged and washed with ethanol three times, and dried under
vacuum overnight at room temperature.
Synthesis of Pd/SiO2@TiMSS: To deposit Pd NPs on SiO2 core, the pre-
pared SiO2 core (500 mg) was dispersed in water (250 mL) under ultra-
sonic irradiation. Then, a solution of SnCl2 2H2O (500 mg) was added
into HCl aqueous solution (2ꢂ10À2 m, 100 mL) and stirred for 10 min.
The suspension was centrifuged and washed with water several times.
The Sn2+ adsorbed SiO2 core was redispersed in water (246 mL) and
mixed with PdCl2 acidic solution (11.64mm, 4.08 mL). After 10 min,
sodium formate (0.15m) centrifuged and washed with water several
times, resulting in the Pd/SiO2. In this step, we confirm that no Pd precur-
sor remains in the filtrate. To encapsulate the Pd/SiO2 with Ti-containing
mesoporous silica shell, the Pd/SiO2 was dispersed in a solution made of
water (200 mL), ethanol (150 mL), CTAB (0.75 g), and ammonia solution
(2.84 mL). Then the mixture of TEOS (750 mL), TPOT (20.3 mL), and
acetylacetone (14.3 mL) was added and stirred overnight. The precipitate
was isolated by centrifugation and washed with ethanol, then dried at
373 K in air overnight. The dried composite was calcined at 823 K in air
for 6 h to remove the structure-directing agent (CTAB). The synthesized
sample was treated with H2 (20 mLminÀ1) at 473 K for 1 h before testing
its catalytic performance.
Figure 6. (a) Kinetics of methyl phenyl sulphide (1) oxidation using aque-
ous H2O2 solution and (b) dependence of the conversion level on the se-
~
*
lectivity. Pd/SiO2@TiMSS
( ); SiO2@Pd(R)/TiMSS ( ); SiO2@Pd(S)/
&
TiMSS ( ).
tivity, whereas SiO2@Pd(S)/TiMSS significantly slows down
the reaction. This tendency is consistent with the results for
the one-pot oxidation. It is noteworthy that Pd/SiO2@TiMSS
gives a high H2O2 production efficiency of 65% after com-
pletion of the reaction, because the decomposition of H2O2
can be kept to a minimum by the location of Pd NPs at the
inner surface of the mesoporous channels. In contrast, the
undesirable decomposition of H2O2 occurred in the case of
SiO2@Pd(S)/TiMSS (H2O2 production efficiency of 35%),
due to the exposure of bare Pd NPs on the surface of the
catalyst. The high catalytic activity and H2O2 production ef-
ficiency of Pd/SiO2@TiMSS supports the assumption for a
successful one-pot oxidation. SiO2@Pd(R)/TiMSS exhibits
moderate catalytic activity and H2O2 production efficiency,
which can be explained by the inhibited diffusion of reac-
tants to both types of active site, due to the random distribu-
tion of Pd NPs within the mesoporous channels.
Synthesis of SiO2@Pd(S)/TiMSS: The Ti-containing mesoporous shell
was formed on a SiO2 core and then the Pd NPs were loaded on the Ti-
containing mesoporous silica surface as described above. The precipitate
was calcined to remove CTAB and heated in H2 (20 mL minÀ1) at 473 K
for 1 h before testing its catalytic performance.
Synthesis of SiO2@Pd(R)/TiMSS: The Ti-containing mesoporous shell
was formed on SiO2 core and then it was calcined in air to remove
CTAB. Pd NPs were loaded on the Ti-containing mesoporous silica as de-
scribed above. The precipitate was calcined at 823 K in air for 6 h again
and treated by H2 (20 mLminÀ1) at 473 K for 1 h before testing its cata-
lytic performance.
Direct synthesis of H2O2 from H2 and O2: The catalyst (50 mg) was
placed in the reaction vessel (50 mL) and HCl aqueous solution (1ꢂ
10À2 m, 20 mL) was added. The resulting mixture was stirred under bub-
bling of gaseous of H2 and O2 (20 mLminÀ1, H2/O2 =1:1) at 303 K for 3 h
with magnetic stirring. The amount of generated H2O2 was monitored
using a hydrogen peroxide counter HP-300 (Hiranuma).
In summary, a new type of core–shell structure catalyst,
Pd/SiO2@TiMSS, consisting of Pd NP-supported uniform
SiO2 core and a Ti-containing mesoporous silica shell was
prepared. The designed architecture offers a simple and effi-
cient catalyst system to enable the one-pot oxidation of sul-
fide in the presence of H2 and O2, in which both Pd NPs
and isolated Ti oxide moieties within the frameworks are
active sites for the formation of H2O2 and the oxidation of
sulfide, respectively. The present design strategy provides
significant flexibility for the selection of metal NPs and cata-
lytically active sites, and could be further applied to a wide
range of functionalized catalysts aimed at developing envi-
ronmentally benign chemical processes.
One-pot oxidation of methyl phenyl sulfide using in situ generated H2O2:
Catalyst (50 mg), methyl phenyl sulfide (0.3 mmol) and acetonitrile
(5 mL) were placed in the reaction vessel (50 mL) fitted with a reflux
condenser. The mixture was reacted at 303 K in a flow of H2 and O2
(20 mLminÀ1, H2/O2 =1:1) with magnetic stirring. The amounts of prod-
uct and reactant were analyzed by GC-mass (Shimadzu CMS-2010 plus)
equipped with TC-5HT columns using an internal standard technique (bi-
phenyl).
Oxidation of methyl phenyl sulfide using commercially-available aqueous
H2O2 solution: Catalyst (30 mg), methyl phenyl sulfide (0.3 mmol), H2O2
solution (30 wt%, 0.9 mmol), and acetonitrile (5 mL) were placed in the
reaction vessel (50 mL). The mixture was reacted at 303 K in ambient
with magnetic stirring. The amounts of product and reactant were detect-
ed by an internal standard technique (biphenyl) using GC-mass (Shi-
madzu GCMS-2010 plus) equipped with TC-5HT columns.
Experimental Section
Acknowledgements
Materials: Tetraethoxysilane (TEOS), tetrapropyl orthotitanate (TPOT),
SnCl2·2H2O, PdCl2, ammonia solution (28 wt%), sodium formate, etha-
nol, acetonitrile, methyl phenyl sulfide, and biphenyl were purchased
from Nakarai Tesque. Cetyltrimethylammonium bromide was obtained
from Wako Pure Chemical Ind., Ltd. Acetylacetone was purchased from
Tokyo Chemical Ind. Co., Ltd. All chemicals were used as received with-
out further purification.
The authors express special thanks for Priority Assistance for the Forma-
tion of Worldwide Renowned Centers of Research–The Global COE
Program (Project: Center of Excellence for Advanced Structural and
Functional Materials Design) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. The authors appreciate
Dr. Eiji Taguchi and Prof. Hirotaro Mori at the Research Center for
Ultra-High Voltage Electron Microscopy, Osaka University for assistance
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Active Site Design in a Core–Shell Nanostructured Catalyst
COMMUNICATION
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Keywords: core–shell catalyst
oxidation · palladium · mesoporous silica
·
hydrogen peroxide
·
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Received: April 22, 2011
Published online: July 8, 2011
Chem. Eur. J. 2011, 17, 9047 – 9051
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9051