Pd-Au bimetal supported on rutile-TiO2 for selective synthesis of hydrogen peroxide by oxidation of H2 with O2 under atmospheric pressure

Article abstract of DOI:10.1246/cl.2007.878

Rutile TiO₂ support is effective for increasing the formation rate as well as selectivity to H₂O₂. A great enhancement in the H₂O₂ formation rate and selectivity is achieved by controlling the particle size of Pd-Au (7:3 in weight ratio) alloy, and the average size of 18 nm exhibits the high selectivity to H₂O ₂ under atmospheric pressure. Copyright

Full text of DOI:10.1246/cl.2007.878

878
Chemistry Letters Vol.36, No.7 (2007)
Pd–Au Bimetal Supported on Rutile–TiO2 for Selective Synthesis of Hydrogen Peroxide
by Oxidation of H2 with O2 under Atmospheric Pressure
Tatsumi Ishihara,Ã1 Yuiko Hata,1 Yohei Nomura,1 Kenji Kaneko,2 and Hiroshige Matsumoto1
1Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
2Department of Materials Sciences and Engineering, Faculty of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
(Received April 11, 2007; CL-070394; E-mail: ishihara@cstf.kyushu-u.ac.jp)
Rutile TiO2 support is effective for increasing the formation
Palladium–gold catalyst supported on TiO2 was prepared by
using the conventional incipient wetness techniques using
HAuCl4 and PdCl2. The amount of loaded Pd–Au metal was
always 1.7 wt %, and the composition of Pd:Au is 82:18 in
molar ratio, if not mentioned. Thus, obtained catalyst (1 g, metal
content: 25.5 mg) was suspended in deionized water (pH 6,
100 mL), and the gaseous mixture of H2 and O2 at each feed rate
of 50 mL/min was fed into catalyst suspending water at 283 K.
Before addition of catalyst, HCl of 84 mmol/L and H2SO4 of
0.368 mol/L were always added for controlling pH and amount
of ClÀ ion. The amount of formed H2O2 was analyzed by the UV
absorption method which is used TiO(SO4) as the pigment. The
selectivity to H2O2 is defined as H2O2 formation rate divided by
H2 consumption rate. Metal dispersion was measured with the
H2 adsorption isotherm (up to 90 kPa) by using a conventional
volumetric gas adsorption system (Nippon Bell, Belsorb SP-
18) at 298 K. Chemisorption amount was estimated by subtract-
ing the second adsorption amount from that of the initial one. It
is noted that absorption of H2 into Pd bulk is negligibly small at
298 K and even though it occurs, it can be neglected by the com-
pensation for the first adsorption amount with that of second one.
The support oxides play important roles for the formation
rate of H2O2, in particular, TiO2 showed a positive effect among
the examined support oxides. Since the morphology and surface
composition of Pd–Au particles could easily be affected by
the crystal structure of TiO2 owing to the lattice mismatch and
the surface area, the effects of crystal phases of TiO2 on the
H2O2 synthesis were investigated. Table 1 summarizes the for-
mation rate and the selectivity of H2O2 using the Pd–Au bimet-
allic alloy on TiO2 support. It was seen that the formation rate
and the selectivity were strongly dependent on the crystal phase
of TiO2. The selectivity reached 78% in the case of Pd–Au
alloy supported on P-25 TiO2, which is the mixed phase of
anatase and rutile with the large surface area 40.2 m2/g. On
the other hand, the high formation rate of 22.1 mmol LÀ1 hÀ1
is achieved in spite of the small surface area (2.0 m2/g) when
rutile TiO2 was used for the support, whereas the low formation
rates, 16.5 mmol LÀ1 hÀ1, with similar BET surface area (2 m2/
g) when anatase was used. XPS measurement suggests that the
rate as well as selectivity to H2O2. A great enhancement in the
H2O2 formation rate and selectivity is achieved by controlling
the particle size of Pd–Au (7:3 in weight ratio) alloy, and the
average size of 18 nm exhibits the high selectivity to H2O2 under
atmospheric pressure.
Hydrogen peroxide (H2O2) is currently synthesized by using
anthraquinone as an intermediate from hydrogen and oxygen.1
Since this process is composed of multistep reactions, it requires
a relatively large energy input to produce H2O2 so that the
production costs of H2O2 become high as an oxidant for the
industrial processes. In addition, H2O2 is unstable and easily
decomposes, and so this reagent is not suitable for transporta-
tion. Therefore, there are strong demands for the development
of the on-site H2O2 synthesis process by the direct oxidation
of H2 with gaseous oxygen. It is reported that Pd catalyst is
highly active in the direct synthesis of H2O2 from H2.2–4 On
the other hand, Au is also reported as the active in this direct syn-
5–7
thesis of H2O2 from H2 (in CH3OH/H2O liquid), albeit the
low selectivity. Furthermore, Au–Pd alloy supported on Al2O3
or TiO2 could be another candidate for the catalyst of H2O2 for-
mation,6,8 and it was reported that TiO2 exhibits positive support
effects on H2O2 formation.6 However, the effects of crystal
phase of TiO2 are not studied in details. It is also reported that
the addition of Pd to Au is effective for increasing the H2O2 for-
mation rate.9 In addition to these, there are several numbers of
reports on the direct H2O2 synthesis from H2,10,11 however, large
part of the conventional studies on direct synthesis of H2O2 were
performed under pressurized conditions. H2O2 selectivity is not
high enough (< ca. 80%) so far. In addition, accumulation of
H2O2 to high concentration by the partial oxidation of H2 with
O2 is difficult because of the high activity of H2O2 against H2.
Therefore, in order to apply H2O2 synthesis from H2 to the in-
dustrial process, high selectivity to H2O2 as well as high concen-
tration of H2O2 is strongly demanded. In this study, the selective
synthesis of H2O2 by oxidation of H2 is achieved by choosing a
suitable TiO2 support for the Pd–Au particle sizes.
Table 1. Effects of crystal phase of TiO2 support on H2O2 formation rate
H2 Conversion
/%
Selectivity
/%
H2O2 formation rate
/mmol LÀ1 hÀ1
BET surface area
/m2 gÀ1
Surface Au/Pd
ratio in weight
Support
P-25a
1.7
3.9
2.6
78.4
51.1
62.3
14.7
16.5
22.1
40.2
1.6
1.076
0.521
1.191
Anatase
Rutile
2.0
aMixed phase of rutile and anatase, Catalysis Society of Japan reference catalyst, Pd:Au = 44:56 (molar ratio), NaCl: 84 mmol/L,
H2SO4: 0.368 mol/L, H2 feed rate: 50 mL/min, O2 feed rate: 50 mL/min, Metal loading; 1.7 wt %.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.7 (2007)
879
Table 2. Effects of average particle size on H2O2 formation rate
and selectivitya
a
b
0.1993
Average particle BET Surface H2O2 formation Selectivity
area/m2 gÀ1 rate/mmol LÀ1 hÀ1
0.2830
size/nm
/%
17.4
18.3
21.7
31.1
76.0
45.5
29.8
2.0
49.2
71.0
64.0
62.8
95.7
99.7
90.8
97.2
aW/F = 6 g-cat.h/mol, values at 2 h after reaction started,
Pd:Au = 82:18 (molar ratio), Total metal loading: 1.7 wt %,
HCl: 84 mmol/L, and H2SO4: 0.368 mol/L.
Figure 1. TEM images of Pd–Au supported on rutile TiO2: (a)
Low magnification image, (b) high resolution image. Metal is
pointed by arrows.
study under atmospheric pressure is 189.8 molH2O2/h kgcat.
,
surface composition was also sensitively varied with the crystal
phase of TiO2 support, and the surface enrichment of Au was
observed in case of rutile TiO2 or P-25 and Pd in case of anatase
TiO2 for support.
which is almost the same with that (202 mol H2O2/h kgcat.) of
the reported maximum value6 under pressurized condition
(14 MPa); however, selectivity to H2O2 is much higher in this
study. Since the H2O2 is quite unstable and easy to decompose,
this ca. 100% selectivity to H2O2 is highly interesting and this
high selectivity can be sustained over a few hours after reaction
started (up to 0.5 wt %). In addition, 80% selectivity is sustained
up to 0.8 wt % after 6 h. The amount of H2O2 concentration
monotonically increased with the reaction time and it attained
a value of 0.9 wt % after 8 h. This study demonstrated that the
high formation rate and the selectivity could be achieved under
the atmospheric pressure by controlling the particle size and the
surface structure of Pd–Au by lattice matching with rutile TiO2.
This size controlled Pd–Au alloy on rutile TiO2 may lead to a
new on-site H2O2 synthesis process.
Metal particle size is another important factor for increasing
the formation rate; however, up to now, there is no detail study
on the particle size effects on the H2O2 synthesis. In this study,
particle size of Pd–Au was controlled by changing the surface
area of rutile support. Figure 1 shows the TEM images of Pd–
Au particles supported on rutile. Although some distribution in
particle sizes was observed, Pd–Au particles distributes between
few nm to 30 nm. The estimated particle size by TEM observa-
tion was almost the same with that estimated by H2 chemisorp-
tion amount. High-resolution image of Pd–Au particle is also
shown in Figure 1b. From the lattice image of TiO2 support,
the observed layer distance is 0.293 nm, which is close to
c lattice length of rutile (0.29592 nm). Therefore, the electron
beam is inserted from [001] direction of rutile TiO2. On the other
hand, the Pd–Au particles tends to be a rectangular shape which
reflects on the fcc lattice. The plane distance estimated from
this image is 0.199 nm, which is larger than that of (002) plane
distance of Pd (0.1941 nm) but smaller than that of Au (0.2039
nm). Therefore, obviously, Au and Pd forms alloy with fcc
structure. In addition, as shown in Figure 1b, clear lattice image
suggests that the particle is oriented to (002) plane on c lattice of
rutile TiO2. Since the always same lattice image is observed on
rutile TiO2, rutile may be effective for exposing {102} Pd–Au or
{010} Pd–Au plane for the surface by minimizing the lattice
mismatch. This seems to be the origin of positive effects on
H2O2 formation rate from H2 and O2.
Part of this study was financially supported by R & D of
novel interdisciplinary fields based on nanotechnology and
materials from MEXT.
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Published on the web (Advance View) June 9, 2007; doi:10.1246/cl.2007.878