Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium catalyst supported on H3PW12O 40-incorporated MCF silica

Article abstract of DOI:10.1016/j.molcata.2010.12.012

Palladium catalysts supported on H₃PW₁₂O₄₀ heteropolyacid incorporated into MCF silica (Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9)) were prepared with a variation of H ₃PW₁₂O₄₀ content (X, wt.%). The prepared catalysts were then applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Conversion of hydrogen over Pd/HPW-MCF-X catalysts showed no great difference, while selectivity for hydrogen peroxide and yield for hydrogen peroxide over the catalysts showed volcano-shaped curves with respect to H₃PW₁₂O₄₀ content. Acidity of Pd/HPW-MCF-X catalysts also showed a volcano-shaped trend with respect to H ₃PW₁₂O₄₀ content. It was observed that yield for hydrogen peroxide increased with increasing acidity of Pd/HPW-MCF-X catalyst. Thus, acidity of Pd/HPW-MCF-X catalyst played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. HPW-MCF-X support efficiently served as an alternate acid source in the direct synthesis of hydrogen peroxide.

Full text of DOI:10.1016/j.molcata.2010.12.012

Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium
catalyst supported on H3PW12O40-incorporated MCF silica
Sunyoung Parka, Dong Ryul Parka, Jung Ho Choia, Tae Jin Kimb, Young-Min Chungb,
Seung-Hoon Ohb, In Kyu Songa,∗
a School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
b SK Energy Corporation, Yuseong-ku, Daejeon 305-712, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium catalysts supported on H3PW12O40 heteropolyacid incorporated into MCF silica (Pd/HPW-
MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9)) were prepared with a variation of H3PW12O40
content (X, wt.%). The prepared catalysts were then applied to the direct synthesis of hydrogen peroxide
from hydrogen and oxygen. Conversion of hydrogen over Pd/HPW-MCF-X catalysts showed no great
difference, while selectivity for hydrogen peroxide and yield for hydrogen peroxide over the catalysts
showed volcano-shaped curves with respect to H3PW12O40 content. Acidity of Pd/HPW-MCF-X catalysts
also showed a volcano-shaped trend with respect to H3PW12O40 content. It was observed that yield for
hydrogen peroxide increased with increasing acidity of Pd/HPW-MCF-X catalyst. Thus, acidity of Pd/HPW-
MCF-X catalyst played an important role in determining the catalytic performance in the direct synthesis
of hydrogen peroxide. HPW-MCF-X support efficiently served as an alternate acid source in the direct
synthesis of hydrogen peroxide.
Received 30 September 2010
Received in revised form
17 December 2010
Accepted 21 December 2010
Available online 12 January 2011
Keywords:
Hydrogen peroxide
Palladium
Heteropolyacid
MCF silica
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
ꢀG ◦ 298 K = − 354.0 kJ/mol), and decomposition of hydrogen
peroxide
(H2O2 → H2O + 0.5O2,
ꢀH ◦ 298 K = − 105.8 kJ/mol,
Hydrogen peroxide (H2O2) has been widely used as a clean
and strong oxidant in pulp industry, textile industry, and green
chemical synthesis such as epoxidation of olefins and hydroxyla-
tion of benzene [1–4]. Hydrogen peroxide currently available in the
industrial market is mostly produced through the anthraquinone
auto-oxidation process [1,2]. However, this process uses toxic
compounds and requires many energy intensive steps for separa-
tion and purification of hydrogen peroxide [1,2]. Therefore, direct
synthesis of hydrogen peroxide from hydrogen and oxygen has
attracted much attention as an economical and environmentally
benign process [5–21].
ꢀG ◦ 298 K = − 116.8 kJ/mol) [2]. All these reactions are thermo-
dynamically favorable and highly exothermic. In particular,
formation of water and hydrogenation of hydrogen peroxide are
thermodynamically more favorable than selective oxidation of
hydrogen to hydrogen peroxide. Consequently, selectivity for
hydrogen peroxide in the direct synthesis of hydrogen peroxide
is limited by these undesired reactions. Therefore, many attempts
have been made to increase the selectivity for hydrogen peroxide
in the direct synthesis of hydrogen peroxide [9–13].
Various noble metals such as palladium [5–13], palladium–gold
[14–16], and palladium–platinum [17] are known to be efficient
catalysts in the direct synthesis of hydrogen peroxide from hydro-
gen and oxygen. These noble metals have been supported on
various materials such as silica, alumina, titania, zirconia, and car-
bon for effective dispersion of active metal component [8–17].
Acids and halides have been used as additives to enhance the
selectivity for hydrogen peroxide in the direct synthesis of hydro-
gen peroxide from hydrogen and oxygen [1,2,9–13]. It has been
reported that acids prevent the decomposition of hydrogen perox-
ide and halides inhibit the formation of water [1,2,9–13]. However,
acid additives cause the corrosion of reactor as well as the dis-
solution of active metal component from the supported catalyst.
Therefore, solid acid supports have been investigated in the direct
In the direct synthesis of hydrogen peroxide from hydrogen
and oxygen, several undesired reactions occur simultaneously
together with selective oxidation of hydrogen to hydro-
gen
peroxide
(H2 + O2 → H2O2,
ꢀH ◦ 298 K = − 135.8 kJ/mol,
ꢀG ◦ 298 K = − 120.4 kJ/mol) [2]. These undesired reactions include
formation of water (H2 + 0.5O2 → H2O, ꢀH ◦ 298 K = − 241.6 kJ/mol,
ꢀG ◦ 298 K = − 237.2 kJ/mol),
hydrogenation
of
hydrogen
peroxide
(H2O2 + H2 → 2H2O,
ꢀH ◦ 298 K = − 211.5 kJ/mol,
∗
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.12.012

S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
79
synthesis of hydrogen peroxide as an alternate acid source [18–21].
Heteropolyacids (HPAs) are inorganic acids. It has been reported
that acid strength of HPAs is stronger than that of conventional
solid acids [22–27]. Therefore, HPAs have been utilized as solid
acid catalysts in several acid-catalyzed reactions [22,23]. However,
HPAs are highly soluble in polar solvents and have low surface area
(<10 m2/g) [22]. In order to solve these problems, insoluble HPAs
have been prepared by exchanging protons with certain cations
[26,27]. It has been reported that HPA salts with large cations such
as NH4+, K+, Rb+, and Cs+ are insoluble, and have high surface area
(>100 m2/g) and porous structure by forming a tertiary structure
[26,27]. In our previous work [20], palladium-exchanged insolu-
ble HPA catalysts showed high catalytic performance in the direct
synthesis of hydrogen peroxide from hydrogen and oxygen. How-
ever, it was difficult to separate palladium-exchanged insoluble
HPA catalyst from reaction medium because insoluble HPA was
composed of very fine particles with an average size of ca. 10 nm
[26,27]. To overcome this problem, insoluble HPAs have been sup-
ported on porous materials such as zeolites and mesoporous silicas
[21,28,29].
Mesoporous silicas have uniform pore size, high surface area,
and large pore volume. Therefore, they have been used in many
areas of science and engineering such as catalysis, adsorption, and
separation [30–33]. Especially, it has been reported that mesostruc-
tured cellular foam (MCF) silica exhibits a 3-dimensional pore
structure with large pores in the range of 10–50 nm [31–33]. Due
to its unique pore characteristics, MCF silica has been used as an
efficient support for immobilization of large molecules such as
enzymes [33]. In our previous work [21], it was observed that
insoluble Cs2.5H0.5PW12O40 heteropolyacid supported on Pd/MCF
catalyst showed high catalytic performance in the direct synthe-
sis of hydrogen peroxide from hydrogen and oxygen. However,
insoluble Cs2.5H0.5PW12O40 heteropolyacid supported on Pd/MCF
catalyst required many preparation steps.
added into the solution. The resulting mixture was stirred at 40 ◦C
for 20 h, and it was maintained at 130 ◦C for 20 h under static con-
dition. After filtering and washing a solid product with distilled
water, the solid was dried at room temperature. The solid product
was then calcined in air at 500 ◦C for 5 h to yield H3PW12O40-MCF
support. Palladium nitrate (Pd(NO3)2, Sigma–Aldrich) was sup-
ported onto H3PW12O40-MCF. The impregnated solid was dried
overnight at 80 ◦C, and calcined at 500 ◦C for 3 h. The palladium
loading was fixed at 0.5 wt.%. The calcined catalyst was charged
into a tubular quartz reactor, and then it was reduced with a
mixed stream of H2 (5 ml/min) and N2 (20 ml/min) at 200 ◦C for 2 h
to yield Pd/H3PW12O40-MCF catalyst. H3PW12O40 content in the
Pd/H3PW12O40-MCF catalysts was adjusted to be 1.0, 4.8, 9.1, 13.0,
16.7, 20.0, 23.1, and 25.9 wt.%. Pd/H3PW12O40-MCF catalysts were
denoted as Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1,
and 25.9), where X represented weight percentage of H3PW12O40
incorporated into MCF silica.
For comparison, palladium catalyst supported on MCF silica
(Pd/MCF) was prepared by an incipient wetness method. MCF sil-
ica was synthesized according to the reported method [31]. The
preparation procedures for Pd/MCF were almost identical to those
for Pd/H3PW12O40-MCF, except that H3PW12O40 was not employed
for the preparation of Pd/MCF. The palladium loading was also fixed
at 0.5 wt.%.
2.2. Catalyst characterization
H3PW12O40 content in the catalyst was measured by ICP-
AES analysis (Shimadzu, ICPS-7500). Pore structure, pore size,
and palladium dispersion of the catalyst were examined by TEM
analysis (Jeol, JEM-3000F). N2 adsorption–desorption isotherm
of the catalyst was obtained with an ASAP-2010 instrument
(Micromeritics), and pore size distribution was determined by the
BJH (Barret–Joyner–Hallender) method applied to the desorption
branch of the isotherm. X-ray diffraction (XRD) pattern of the cata-
lyst was confirmed by XRD measurement (Rigaku, D-Max2500-PC)
using CuK␣ radiation operated at 50 kV and 100 mA. Chemical state
of H3PW12O40 heteropolyacid incorporated into MCF silica was
examined by 31P MAS NMR analysis (Bruker, AVANCE 400 WB).
NH3-TPD (temperature-programmed desorption) experiment was
carried out in order to measure the acidity of the catalyst. 0.05 g
of each catalyst charged into the TPD apparatus was pretreated at
200 ◦C for 1 h with a stream of helium (20 ml/min). After cooling
the catalyst to room temperature, 20 ml of NH3 was pulsed into the
reactor every minute under a flow of helium (5 ml/min) until the
acid sites were saturated with NH3. Physisorbed NH3 was removed
by evacuating the catalyst sample at 100 ◦C for 1 h. Furnace tem-
perature was then increased from room temperature to 900 ◦C at a
heating rate of 5 ◦C/min under a flow of helium (10 ml/min). Des-
orbed NH3 was detected using a GC-MSD (Agilent, MSD-6890N
GC).
In this work, a series of H3PW12O40 heteropolyacid incorporated
into MCF silica (H3PW12O40-MCF) were prepared with a variation
of H3PW12O40 content for use as a solid acid support. Palladium
catalysts supported on H3PW12O40 heteropolyacid incorporated
into MCF silica (Pd/H3PW12O40-MCF) were then applied to the
direct synthesis of hydrogen peroxide from hydrogen and oxy-
gen. The effect of H3PW12O40 content on the catalytic performance
of Pd/H3PW12O40-MCF catalysts in the direct synthesis of hydro-
gen peroxide was examined. A correlation between acidity and
catalytic performance of Pd/H3PW12O40-MCF catalysts was then
established.
2. Experimental
2.1. Catalyst preparation
A series of H3PW12O40 heteropolyacid incorporated into MCF
silica (H3PW12O40-MCF) were prepared with
a
variation of
2.3. Direct synthesis of hydrogen peroxide
H3PW12O40 content. Palladium catalysts supported on H3PW12O40
heteropolyacid incorporated into MCF silica (Pd/H3PW12O40-MCF)
were then prepared by an incipient wetness impregnation method.
Typical procedures for the preparation of Pd/H3PW12O40-MCF
catalyst are as follows. 10 g of PEO-PPO-PEO triblock copoly-
mer (Pluronic P123, BASF), an organic template, was dissolved in
350 ml of 1.6 M HCl aqueous solution at 40 ◦C. 25 ml of H3PW12O40
(HPW) (Sigma–Aldrich) aqueous solution was added dropwise
into the solution under vigorous stirring, and the mixed solution
was stirred for 6 h. 4.6 ml of 1,3,5-trimethylbenzene (Mesitylene,
Sigma–Aldrich), a swelling agent, was then added into the mixed
solution. After stirring the solution at 40 ◦C for 1 h, 24.1 ml of
tetraethyl orthosilicate (TEOS, Sigma–Aldrich), a silica source, was
Direct synthesis of hydrogen peroxide from hydrogen and oxy-
gen was carried out in an autoclave reactor in the absence of acid
additive. 80 ml of methanol and 6.32 mg of sodium bromide were
charged into the reactor. 1 g of each catalyst was then added into the
reactor. H2/N2 (25 mol% H2) and O2/N2 (50 mol% O2) were bubbled
through the reaction medium under vigorous stirring (1000 rpm).
H2/O2 ratio in the feed stream was fixed at 0.4, and total feed rate
was maintained at 44 ml/min. Catalytic reaction was carried out
at 28 ◦C and 10 atm for 6 h. In the catalytic reaction, mixed gases
diluted with an inert gas (H2/N2 (25 mol% H2) and O2/N2 (50 mol%
O2)) and an autoclave reactor equipped with a flashback arrestor
as well as a safety valve were used in order to solve the safety

80
S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
Fig. 1. TEM images of MCF silica and HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) supports.
problem. Unreacted hydrogen was analyzed using a gas chromato-
graph (Younglin, ACME 6000) equipped with a TCD. Concentration
of hydrogen peroxide was determined by an iodometric titration
method [34]. Conversion of hydrogen and selectivity for hydro-
gen peroxide were calculated according to the following equations.
Yield for hydrogen peroxide was calculated by multiplying conver-
sion of hydrogen and selectivity for hydrogen peroxide.
2.4. Hydrogenation of hydrogen peroxide
80 ml of methanol, 6.32 mg of sodium bromide, and 1 g of
Pd/HPW-MCF-20.0 catalyst were charged into the reactor. 3 ml of
30 wt.% hydrogen peroxide was then added. H2/N2 (25 mol% H2)
and N2 were bubbled through the reaction medium under vigor-
ous stirring (1000 rpm). H2/N2 ratio in the feed stream was fixed at
0.1, and total feed rate was maintained at 44 ml/min. The reaction
was carried out at 28 ◦C and 10 atm for 6 h. Concentration of hydro-
gen peroxide was determined by an iodometric titration method
[34]. Degree of hydrogenation of hydrogen peroxide was calculated
according to the following equation. For comparison, hydrogena-
tion of hydrogen peroxide was also carried out using 1 g of Pd/MCF
under the same reaction conditions.
Conversion of hydrogen
moles of hydrogen reacted
=
moles of hydrogen supplied
Degree of hydrogenation of H2O2
Selectivity for hydrogen peroxide
moles of hydrogen peroxide hydrogenated
moles of hydrogen peroxide formed
=
=
moles of hydrogen peroxide supplied
moles of hydrogen reacted
Fig. 2. TEM images of Pd/MCF and Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts.

S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
81
Pd/MCF
1000
800
600
400
200
0
Pd/HPW-MCF-9.1
Pd/HPW-MCF-20.0
Pd/MCF
Pd/HPW-MCF-1.0
Pd/HPW-MCF-4.8
Pd/HPW-MCF-9.1
Pd/HPW-MCF-13.0
Pd/HPW-MCF-16.7
Pd/HPW-MCF-20.0
Pd/HPW-MCF-23.1
Pd/HPW-MCF-25.9
HPW
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
0
20
60
80
100
Pore4s0ize (nm)
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 3. N2 adsorption–desorption isotherms and pore size distributions of Pd/MCF,
Pd/HPW-MCF-9.1, and Pd/HPW-MCF-20.0.
2.5. Decomposition of hydrogen peroxide
80 ml of methanol, 6.32 mg of sodium bromide, and 1 g of
Pd/HPW-MCF-20.0 catalyst were charged into the reactor. 3 ml
of 30 wt.% hydrogen peroxide was then added. N2 (44 ml/min)
was bubbled through the reaction medium under vigorous stirring
(1000 rpm). The reaction was carried out at 28 ◦C and 10 atm for 6 h.
Concentration of hydrogen peroxide was determined by an iodo-
metric titration method [34]. Degree of decomposition of hydrogen
peroxide was calculated according to the following equation. For
comparison, decomposition of hydrogen peroxide was also carried
out using 1 g of Pd/MCF under the same reaction conditions.
10
20
30
40
50
60
70
80
2 Theta (Degree)
Fig. 4. XRD patterns of Pd/MCF, H3PW12O40 (HPW), and Pd/HPW-MCF-X (X = 1.0,
4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts.
Degree of decomposition of H2O2
moles of hydrogen peroxide decomposed
moles of hydrogen peroxide supplied
=
Fig. 3 shows the N2 adsorption–desorption isotherms and pore
size distributions of Pd/MCF, Pd/HPW-MCF-9.1, and Pd/HPW-MCF-
20.0. N2 adsorption–desorption isotherm and pore size distribution
of Pd/HPW-MCF-X catalysts were similar to those of Pd/MCF.
Pd/HPW-MCF-X catalysts showed IV-type isotherms with H1-type
hysteresis loops, as reported in the literature [31]. This result also
supports that pore structure of MCF silica was still maintained
even after the incorporation of H3PW12O40 and the loading of
palladium, as also evidenced by TEM images. Detailed textural
properties of Pd/HPW-MCF-X catalysts are summarized in Table 1.
Surface area of Pd/HPW-MCF-X catalysts decreased with increasing
H3PW12O40 content. This is attributed to the increased incorpora-
tion of H3PW12O40 into MCF silica. Pore volume of Pd/HPW-MCF-X
catalysts also decreased with increasing H3PW12O40 content. Fur-
thermore, it was observed that average pore size of Pd/HPW-MCF-X
catalysts slightly increased with increasing H3PW12O40 content.
However, all the Pd/HPW-MCF-X catalysts still exhibited the unique
pore characteristics of MCF silica.
Fig. 4 shows the XRD patterns of Pd/MCF, H3PW12O40 (HPW),
and Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9)
catalysts. Pd/MCF showed no diffraction peaks due to an amor-
phous nature of MCF silica [32]. No diffraction peaks for H3PW12O40
were also observed in the Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0,
16.7, and 20.0) catalysts. This indicates that H3PW12O40 was not
in a crystal state but in an amorphous-like state, demonstrating
that H3PW12O40 was finely and molecularly incorporated into MCF
silica. However, Pd/HPW-MCF-23.1 and Pd/HPW-MCF-25.9 cata-
lysts exhibited weak diffraction peaks for H3PW12O40. This implies
that H3PW12O40 was aggregated by the excessive incorporation of
H3PW12O40 into MCF silica.
3. Results and discussion
3.1. Catalyst characterization
H3PW12O40 (HPW) content in the Pd/HPW-MCF-X (X = 1.0, 4.8,
9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts determined by ICP-AES
analyses is listed in Table 1. H3PW12O40 content in the Pd/HPW-
MCF-X catalysts increased with increasing designed value. This
indicates that Pd/HPW-MCF-X catalysts were successfully prepared
as attempted in this work.
Fig. 1 shows the TEM images of MCF and HPW-MCF-X (X = 1.0,
4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) supports. Pore structure and
pore size of HPW-MCF-X were almost identical to those of MCF sil-
ica. HPW-MCF-X exhibited a disordered pore structure with large
pores in the range of 8–10 nm. This indicates that HPW-MCF-X sup-
ports were successfully prepared and pore structure of MCF silica
was still maintained even after the incorporation of H3PW12O40
.
Fig. 2 shows the TEM images of Pd/MCF and Pd/HPW-MCF-X
(X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts. Pd/HPW-
MCF-X catalysts exhibited almost the same pore structure and pore
size as HPW-MCF-X supports, indicating that pore structure of the
supports was still maintained even after the palladium loading.
Furthermore, small palladium particles with a size of 10–40 nm
were observed in the TEM images of the catalysts. Palladium dis-
persion determined by hydrogen chemisorption was in the range
of 2.7–7.9%.

82
S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
-15.6
3.0
2.5
2.0
1.5
1.0
0.5
0
100
Pd/HPW-MCF-1.0
80
-15.6
-15.6
Pd/HPW-MCF-4.8
60
Pd/HPW-MCF-9.1
40
-15.6
Pd/HPW-MCF-13.0
20
-15.8
-15.9
0
Pd/HPW-MCF-16.7
2
4
6
Reaction time (h)
Pd/HPW-MCF-20.0
Pd/HPW-MCF-23.1
Pd/HPW-MCF-25.9
HPW
Fig. 6. Catalytic performance of Pd/HPW-MCF-20.0 catalyst in the direct synthesis
of hydrogen peroxide from hydrogen and oxygen with time on stream.
-15.9
-16.3
-16.0
3.2. Catalytic performance in the direct synthesis of hydrogen
peroxide
Fig. 6 shows the catalytic performance of Pd/HPW-MCF-20.0
catalyst in the direct synthesis of hydrogen peroxide from hydro-
gen and oxygen with time on stream. Conversion of hydrogen
was almost constant during a 6 h-reaction, while concentration of
hydrogen peroxide continuously increased with increasing reac-
tion time. This indicates that the catalyst deactivation did not occur
during the reaction extending over 6 h. Furthermore, it is expected
that decomposition of hydrogen peroxide during the reaction was
negligible.
150
100
50
0
-50
-100
-150
δ (ppm)
Fig. 5. 31P MAS NMR spectra of H3PW12O40 (HPW) and Pd/HPW-MCF-X (X = 1.0, 4.8,
9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts.
Fig. 7 shows the catalytic performance of Pd/HPW-MCF-X
(X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts in the
direct synthesis of hydrogen peroxide from hydrogen and oxy-
gen after a 6-h reaction, plotted as a function of H3PW12O40
content. Conversion of hydrogen over the catalysts showed
no great difference, while selectivity for hydrogen peroxide
exhibited a volcano-shaped curve with respect to H3PW12O40
content. As a consequence, yield for hydrogen peroxide showed
a volcano-shaped curve with respect to H3PW12O40 content.
Final concentration of hydrogen peroxide after a 6 h-reaction
also showed a volcano-shaped curve with respect to H3PW12O40
content. Among the catalysts tested, Pd/HPW-MCF-20.0 catalyst
showed the best catalytic performance in terms of selectivity for
hydrogen peroxide, yield for hydrogen peroxide, and final concen-
tration of hydrogen peroxide.
Fig. 5 shows the 31P MAS NMR spectra of H3PW12O40 (HPW) and
Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9)
catalysts. It has been reported that H3PW12O40 exhibited a reso-
nance peak at around −15 or −16 ppm [35–37]. It is known that
small difference in chemical shift was attributed to the difference
in the degree of hydration [35,36]. All the Pd/HPW-MCF-X catalysts
showed a resonance peak in the range from −15.6 to −16.3 ppm.
This chemical shift was similar to that of H3PW12O40 (−16.0 ppm),
indicating that the chemical interaction between H3PW12O40 and
MCF silica in Pd/HPW-MCF-X catalysts was negligible. Therefore, it
is expected that H3PW12O40 was incorporated into MCF silica by
being embedded in the silica framework. Furthermore, any peaks
were not detected at −30 ppm. This implies that no phosphorous
oxide was formed under our preparation conditions [37]. This result
indicates that the primary structure of H3PW12O40 was still main-
tained even after the incorporation into MCF silica.
It is noteworthy that no significant dissolution of H3PW12O40
was observed in the Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7,
20.0, 23.1, and 25.9) catalysts before and after the direct synthesis of
Table 1
HPW content, surface area, pore volume, average pore size, and acidity of Pd/MCF and Pd/HPW-MCF-X catalysts.
Catalyst
HPW content (wt.%)
Surface area
(m2/g)a
Pore volume
(cm3/g)b
Average pore
size (nm)c
Acidity
(mmol-NH3/g)
Before reaction
After reaction
Pd/MCF
–
0.7
4.8
7.5
10.0
12.1
17.2
17.5
22.8
–
0.7
4.4
6.5
561.8
538.1
524.1
468.0
403.0
365.4
358.1
348.2
347.7
1.6
1.6
1.4
1.4
1.5
1.5
1.4
1.1
1.1
8.0
8.3
7.1
8.0
8.8
8.9
9.8
9.6
9.2
26.4
106.3
130.0
131.7
156.6
161.8
170.9
167.7
127.2
Pd/HPW-MCF-1.0
Pd/HPW-MCF-4.8
Pd/HPW-MCF-9.1
Pd/HPW-MCF-13.0
Pd/HPW-MCF-16.7
Pd/HPW-MCF-20.0
Pd/HPW-MCF-23.1
Pd/HPW-MCF-25.9
8.7
10.1
14.4
15.8
17.9
a
Calculated by the BET (Brunauer–Emmett–Teller) equation.
BJH (Barret–Joyner–Hallender) desorption pore volume.
BJH (Barret–Joyner–Hallender) desorption average pore diameter.
b
c

S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
83
90
80
70
60
50
40
30
20
2.4
2.2
Conversion of H2
Selectivity for H2O2
Yield for H2O2
2.0
1.8
1.6
1.4
1.2
0
5
10
15
20
25
0
5
10
15
20
25
HPW content in Pd/HPW-MCF (wt%)
HPW content in Pd/HPW-MCF (wt%)
Fig. 7. Catalytic performance of Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts in the direct synthesis of hydrogen peroxide from hydrogen and
oxygen after a 6 h-reaction.
hydrogen peroxide from hydrogen and oxygen, as listed in Table 1.
This indicates that H3PW12O40 was well incorporated into the pores
of MCF silica, as attempted in this work.
different concentration of H3PO4. Conversion of hydrogen in the
presence of H3PO4 was similar to that in the absence of H3PO4,
while selectivity for hydrogen peroxide in the presence of H3PO4
was much higher than that in the absence of H3PO4. Consequently,
yield for hydrogen peroxide in the presence of H3PO4 was consider-
ably higher than that in the absence of H3PO4. Final concentration
of hydrogen peroxide in the presence of H3PO4 after a 6 h-reaction
was also much higher than that in the absence of H3PO4. This is
in good agreement with the fact that acid additives enhanced the
selectivity for hydrogen peroxide by preventing the decomposition
of hydrogen peroxide [2].
Direct comparison of Figs. 9 and 10 revealed that the catalytic
performance of Pd/MCF was more sensitive to the amount of H3PO4
additive than that of Pd/HPW-MCF-20.0 catalyst. Furthermore, the
catalytic performance of Pd/HPW-MCF-20.0 catalyst even in the
absence of H3PO4 was much higher than that of Pd/MCF in the pres-
ence of H3PO4. This result implies that H3PW12O40-incorporated
MCF silica served as an efficient alternate acid source in the direct
synthesis of hydrogen peroxide from hydrogen and oxygen.
Direct comparison of catalytic performance of Pd/HPW-
MCF-20.0 with that of Pd0.15Cs2.5H0.2PW12O40 (the optimum
palladium-exchanged insoluble HPA catalyst in the literature [20])
revealed that yield for hydrogen peroxide over Pd/HPW-MCF-20.0
catalyst (51.6%) was higher than that over Pd0.15Cs2.5H0.2PW12O40
catalyst (40.3%). Final concentration of hydrogen peroxide after a
6 h-reaction over Pd/HPW-MCF-20.0 catalyst (2.29 wt.%) was also
higher than that over Pd0.15Cs2.5H0.2PW12O40 catalyst (1.77 wt.%).
For comparison, Pd/MCF was applied to the direct synthe-
sis of hydrogen peroxide from hydrogen and oxygen. Fig. 8
shows the catalytic performance of Pd/MCF and Pd/HPW-MCF-
20.0 in the direct synthesis of hydrogen peroxide. Conversion
of hydrogen over Pd/HPW-MCF-20.0 catalyst was almost identi-
cal to that over Pd/MCF, while selectivity for hydrogen peroxide
over Pd/HPW-MCF-20.0 catalyst was much higher than that over
Pd/MCF. Consequently, yield for hydrogen peroxide over Pd/HPW-
MCF-20.0 catalyst was much higher than that over Pd/MCF. Final
concentration of hydrogen peroxide after a 6 h-reaction over
Pd/HPW-MCF-20.0 catalyst was also much higher than that over
Pd/MCF. It has been reported that acid additives increase the selec-
tivity for hydrogen peroxide by preventing the decomposition of
hydrogen peroxide, because acid additives inhibited the dissoci-
ation of hydrogen peroxide (H2O2 ⇔ HO2− + H+) by surrounding
hydrogen peroxide with protons [2]. Therefore, it can be inferred
that the improved selectivity for hydrogen peroxide over Pd/HPW-
MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts
was attributed to the enhanced acid property of Pd/HPW-MCF-X
catalysts.
3.4. Catalytic performance in the hydrogenation of hydrogen
peroxide and the decomposition of hydrogen peroxide
Fig. 11 shows the catalytic performance in the hydrogena-
tion of hydrogen peroxide and the decomposition of hydrogen
peroxide over Pd/MCF and Pd/HPW-MCF-20.0. Both Pd/MCF and
Pd/HPW-MCF-20.0 showed high activity for hydrogenation of
hydrogen peroxide. This indicates that the enhanced acid prop-
erty of Pd/HPW-MCF-20.0 catalyst showed no significant effect on
the prevention of hydrogenation of hydrogen peroxide. However,
activity for decomposition of hydrogen peroxide over Pd/HPW-
MCF-20.0 catalyst was much lower than that over Pd/MCF. This
implies that the enhanced acid property of Pd/HPW-MCF-20.0 cat-
alyst inhibited the decomposition of hydrogen peroxide. Therefore,
it can be said that Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0,
23.1, and 25.9) catalysts increased the selectivity for hydrogen per-
oxide by preventing the decomposition of hydrogen peroxide.
3.3. Effect of acid additive on the catalytic performance
Fig. 9 shows the catalytic performance of Pd/HPW-MCF-20.0
catalyst in the direct synthesis of hydrogen peroxide from hydro-
gen and oxygen at different concentration of H3PO4. 0.645 ml of
1 M H3PO4 solution was added to the reaction medium in order to
make the concentration of H3PO4 become 1000 ppm. Conversion
of hydrogen and selectivity for hydrogen peroxide in the presence
of H3PO4 were almost identical to those in the absence of H3PO4.
Consequently, yield for hydrogen peroxide in the presence of H3PO4
was similar to that in the absence of H3PO4. Final concentration of
hydrogen peroxide in the presence of H3PO4 after a 6 h-reaction
was also almost identical to that in the absence of H3PO4. This indi-
catesthat H3PO4 additivehad noeffect onthecatalyticperformance
of Pd/HPW-MCF-20.0 catalyst.
3.5. Acidity of Pd/H3PW12O40-MCF catalysts
Fig. 12 shows the NH3-TPD profiles of Pd/MCF and Pd/HPW-
MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9) catalysts.
Acidity of Pd/HPW-MCF-X catalysts measured from the peak area
is summarized in Table 1. Acidity of the catalysts showed a volcano-
shaped trend with respect to H3PW12O40 content. Among the
Pd/HPW-MCF-X-X catalysts, Pd/HPW-MCF-20.0 showed the largest
acidity. It has been reported that acidity of H3PW12O40-SBA-15
did not increase in proportion to H3PW12O40 content, because
H3PW12O40 was covered with SBA-15 silica walls [37]. In this
Fig. 10 shows the catalytic performance of Pd/MCF in the direct
synthesis of hydrogen peroxide from hydrogen and oxygen at

84
S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
Fig. 8. Catalytic performance of Pd/MCF and Pd/HPW-MCF-20.0 in the direct synthesis of hydrogen peroxide from hydrogen and oxygen after a 6 h-reaction.
Fig. 9. Catalytic performance of Pd/HPW-MCF-20.0 catalyst in the direct synthesis of hydrogen peroxide from hydrogen and oxygen after a 6 h-reaction at different
concentration of H3PO4.
Fig. 10. Catalytic performance of Pd/MCF in the direct synthesis of hydrogen peroxide from hydrogen and oxygen after a 6 h-reaction at different concentration of H3PO4.
100
90
80
70
60
50
40
30
20
10
0
45
40
35
30
25
20
15
10
5
0
Pd/MCF
Pd/HPW-MCF-20.0
Pd/MCF
Pd/HPW-MCF-20.0
Fig. 11. Catalytic performance in the hydrogenation of hydrogen peroxide and the decomposition of hydrogen peroxide over Pd/MCF and Pd/HPW-MCF-20.0.

S. Park et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 78–86
85
3.6. Effect of acidity on the catalytic performance
Pd/MCF
Fig. 13 shows the correlation between yield for hydrogen per-
oxide and acidity of Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7,
20.0, 23.1, and 25.9) catalysts. The correlation clearly shows that
yield for hydrogen peroxide over Pd/HPW-MCF-X catalysts was
closely related to the acidity of the catalysts. Yield for hydrogen
peroxide increased with increasing acidity of Pd/HPW-MCF-X cat-
alyst. Among the catalysts tested, Pd/HPW-MCF-20.0 catalyst with
the largest acidity showed the highest yield for hydrogen perox-
ide. It has been reported that acidity on the surface of the catalyst
is an important factor determining the catalytic performance in
the direct synthesis of hydrogen peroxide from hydrogen and oxy-
gen [19–21]. Therefore, it is concluded that the improved yield for
hydrogen peroxide over Pd/HPW-MCF-X catalysts was attributed
to the enhanced acidity of the catalysts. Thus, Pd/HPW-MCF-X effi-
ciently served as an alternate acid source and as an active metal
catalyst in the direct synthesis of hydrogen peroxide.
Pd/HPW-MCF-1.0
Pd/HPW-MCF-4.8
Pd/HPW-MCF-9.1
Pd/HPW-MCF-13.0
Pd/HPW-MCF-16.7
Pd/HPW-MCF-20.0
Pd/HPW-MCF-23.1
4. Conclusions
A series of H3PW12O40 heteropolyacid incorporated into MCF
silica (HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9))
were prepared with a variation of H3PW12O40 content (X, wt.%). Pal-
ladium catalysts supported on H3PW12O40-incorporated MCF silica
(Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9))
were then prepared for use in the direct synthesis of hydrogen
peroxide from hydrogen and oxygen. High catalytic performance
of Pd/HPW-MCF-X catalysts compared to Pd/MCF was attributed
to the enhanced acid property of Pd/HPW-MCF-X catalysts. Con-
version of hydrogen over Pd/HPW-MCF-X catalysts showed no
great difference, while selectivity for hydrogen peroxide, yield for
hydrogen peroxide, and final concentration of hydrogen peroxide
over the catalysts showed volcano-shaped curves with respect to
H3PW12O40 content. Acidity of the catalysts also showed a volcano-
shaped trend with respect to H3PW12O40 content. It was revealed
that yield for hydrogen peroxide increased with increasing acidity
of Pd/HPW-MCF-X catalyst. Among the catalysts tested, Pd/HPW-
MCF-20.0 catalyst with the largest acidity showed the highest yield
for hydrogen peroxide. It is concluded that acidity of Pd/HPW-
MCF-X catalyst played a crucial role in determining the catalytic
performance in the direct synthesis of hydrogen peroxide.
Pd/HPW-MCF-25.9
0
100 200 300 400 500 600 700 800 900
Temperature (ºC)
Fig. 12. NH3-TPD profiles of Pd/MCF and Pd/HPW-MCF-X (X = 1.0, 4.8, 9.1, 13.0, 16.7,
20.0, 23.1, and 25.9) catalysts.
work, the aggregation of H3PW12O40 was observed, when a large
amount of H3PW12O40 was added into MCF silica during the
preparation step. Therefore, it can be inferred that the amount of
H3PW12O40 exposed on the surface of H3PW12O40-incorporated
MCF silica decreased because H3PW12O40 agglomerates were caged
in the silica framework, when a large amount of H3PW12O40 was
incorporated into MCF silica. This result indicates that acidity of
Pd/HPW-MCF-X decreased due to the restriction of effective expo-
sure of H3PW12O40, when an excess amount of H3PW12O40 more
than 20.0 wt.% was employed for incorporation.
Acknowledgements
This work was financially supported by the grant from the Indus-
trial Source Technology Development Programs (10033093) of the
Ministry of Knowledge Economy (MKE) of Korea.
180
Pd/HPW-MCF-20.0
Pd/HPW-MCF-23.1
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