A simple method for preparing highly active palladium catalysts loaded on various carbon supports for liquid-phase oxidation and hydrogenation reactions

Article abstract of DOI:10.1016/j.molcata.2006.12.010

Preparation of nanosized palladium (Pd) catalysts supported on various carbon supports using a simple liquid-phase reduction of aqueous Pd complexes with potassium borohydride (KBH₄) was investigated. We found that addition of appropriate amounts of sodium hydroxide (NaOH) into aqueous solutions of sodium tetrachloropalladate (Na₂PdCl₄) followed by reduction with KBH₄ produced highly dispersed Pd nanoparticles less than 5 nm in diameter on any carbon supports. Ultraviolet-visible light (UV-vis) absorption spectra of aqueous Na₂PdCl₄ solutions in the presence of various amounts of NaOH clarified the occurrence of partial or complete ligand exchange of the Pd complex from chloride ions (Cl^-) to hydroxide ions (OH^-). Hence, the production of small Pd nanoparticles on carbon supports is likely to be induced by structural change in the precursor complex prior to the reduction with KBH₄. Moreover, evaluation of catalytic activities for liquid-phase oxidation of benzyl alcohol into benzaldehyde using oxygen and hydrogenation of the C{double bond, long}C bond in cinnamaldehyde with hydrogen revealed that catalytic functions were significantly dependent on the size and the dispersion of Pd particles; i.e., both reactions proceeded efficiently on small Pd catalysts highly dispersed on carbon supports but did not occur efficiently on Pd particles of relatively large sizes or with poor dispersion on those supports.

Full text of DOI:10.1016/j.molcata.2006.12.010

Journal of Molecular Catalysis A: Chemical 268 (2007) 59–64
A simple method for preparing highly active palladium catalysts
loaded on various carbon supports for liquid-phase
oxidation and hydrogenation reactions
Takashi Haradaa, Shigeru Ikedaa,∗, Mayu Miyazakia, Takao Sakatab,
Hirotaro Morib, Michio Matsumuraa
a Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka 560-8531, Japan
b Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1 Mihogaoka, Ibaraki 567-0047, Japan
Received 19 June 2006; received in revised form 1 December 2006; accepted 6 December 2006
Available online 10 December 2006
Abstract
Preparation of nanosized palladium (Pd) catalysts supported on various carbon supports using a simple liquid-phase reduction of aqueous Pd
complexes with potassium borohydride (KBH4) was investigated. We found that addition of appropriate amounts of sodium hydroxide (NaOH)
into aqueous solutions of sodium tetrachloropalladate (Na2PdCl4) followed by reduction with KBH4 produced highly dispersed Pd nanoparticles
less than 5 nm in diameter on any carbon supports. Ultraviolet–visible light (UV–vis) absorption spectra of aqueous Na2PdCl4 solutions in the
presence of various amounts of NaOH clarified the occurrence of partial or complete ligand exchange of the Pd complex from chloride ions (Cl−)
to hydroxide ions (OH−). Hence, the production of small Pd nanoparticles on carbon supports is likely to be induced by structural change in
the precursor complex prior to the reduction with KBH4. Moreover, evaluation of catalytic activities for liquid-phase oxidation of benzyl alcohol
into benzaldehyde using oxygen and hydrogenation of the C C bond in cinnamaldehyde with hydrogen revealed that catalytic functions were
significantly dependent on the size and the dispersion of Pd particles; i.e., both reactions proceeded efficiently on small Pd catalysts highly dispersed
on carbon supports but did not occur efficiently on Pd particles of relatively large sizes or with poor dispersion on those supports.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Palladium nanoparticles; Carbon supports; Effect of sodium hydroxide addition; Liquid-phase catalytic reactions; Ligand exchange
1. Introduction
Pd(OAc)2 [14], N-heterocyclic carbene complexes [15], and a
triphenylphosphine complex (Pd(Ph3P)4) [16].
Pd catalysts supported on carbon materials such as acti-
vated carbon (AC) and carbon nanofibers have been extensively
employed as catalysts not only for hydrogenation and dehydro-
genation in industrial reactions but also for oxidation in the area
of fine chemicals [1–4]. Use of these catalysts has many advan-
tages from a practical point of view, e.g., chemical stability,
harmlessness, availability of carbon supports, and simplicity of
recovery of Pd metal by just burning off carbon components.
Moreover, these catalysts are particularly attractive in applica-
tions in various kinds of C–C bond formation reactions [5–13]
because of their expediency of re-use and sufficient catalytic
activities in comparison with homogeneous Pd catalysts such as
It is well known that the catalytic activity of such Pd-
supported carbons strongly depends on the physicochemical
properties of Pd particles and carbons, such as size and dis-
persion of loaded Pd particles, porous structures and surface
properties of carbon supports [17]. Hence, effective control
of these properties by modifying surface functional groups on
carbon supports [18,19] or by the use of carbons with well-
controlled nanostructure such as mesoporous carbons [20,21]
and carbon nanotubes [22,23] has been studied in order to con-
struct efficient catalytic systems. However, the former requires
multiple and/or elaborate preparation steps and the latter repre-
sents an incoming research at present.
On the basis of these facts, our interest has been focused on
the development of a simple method for controlling size, disper-
sion, and catalytic functions of Pd nanoparticles on any carbon
supports. Among various possible methods, we paid particular
∗
Corresponding author. Tel.: +81 6 6850 6696; fax: +81 6 6850 6699.
E-mail address: sikeda@chem.es.osaka-u.ac.jp (S. Ikeda).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.12.010

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T. Harada et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 59–64
attention to a standard liquid-phase reduction of an aqueous pal-
ladium complex as a candidate because of its simplicity and mild
operation temperature. We found that addition of an appropriate
amount of NaOH to an aqueous solution containing Na2PdCl4
followed by reduction with KBH4 produced highly dispersed Pd
nanoparticles with a narrow size distribution on both commer-
cial and synthesized carbon supports. Moreover, thus-obtained
Pd nanoparticles on carbon supports were found to exhibit a high
level of catalytic activity for liquid-phase oxidation of benzyl
alcohol (BA) using molecular oxygen (O2) and hydrogenation
of the C C bond of cinnamaldehyde (CA) under mild condi-
tions. In this paper, we describe in detail the effects of NaOH
addition on structural characteristics and catalytic activity of Pd
nanoparticles loaded on some representative carbon supports,
carbon black (CB), AC, and a mesoporous carbon (CMK-3)
[24].
Fig. 1. XRD patterns of Pd/CMK-3 prepared with various NaOH contents: (a)
γ = 0, (b) γ = 2, (c) γ = 4, (d) γ = 8, (e) γ = 10, and (f) γ = 20.
2. Experimental
2.1. Preparation of catalysts
2.2. Characterization
CB (80 m2 g−1) and AC (1500 m2 g−1) were purchased from
Wako Pure Chemical and were used as received. CMK-3
(1200 m2 g−1, 3.3 nm in average pore diameter) was synthesized
by using mesoporous silica (SBA-15) as a template [24,25].
Typically, 4 g of SBA-15 (700 m2 g−1, 6.7 nm in average pore
diameter) was added to 0.29 mmol dm−3 of aqueous sulfuric
acid containing 5 g of sucrose. The slurry was placed in a dry-
ing oven at 373 K for 6 h. Then the temperature of the oven
was increased to 433 K and maintained at that temperature for
6 h. A brown lump thus-obtained was ground into a powder and
was again impregnated with carbon sources by the same treat-
ment. After carbonization of the powder as-obtained at 1173 K
under vacuum, the SBA-15 template was removed by soaking
in 10 vol% hydrofluoric acid for 3 h followed by repeated wash-
ing with distilled water. The structure of thus-obtained CMK-3
was confirmed by powder X-ray diffraction (XRD) and nitrogen
adsorption/desorption measurement.
Pd particles loaded on various carbon supports were pre-
pared as follows. 0.2 g of CMK-3, CB, or AC was added to
5 cm3 distilled water containing NaOH. After agitation of the
slurry for a few minutes, 10 cm3 of aqueous Na2PdCl4 solution
(10.1 mmol dm−3, corresponding to 5 wt% of Pd) was added
dropwise and the mixture was stirred for 48 h at room tem-
perature. Subsequently, 5 cm3 of aqueous KBH4 solution was
added and the mixture was further stirred for 0.5 h. The result-
ing suspension was filtered, washed with distilled water several
times, and dried at 353 K overnight. Thus-obtained powders of
Pd nanoparticles loaded on CMK-3, CB, and AC are labeled
Pd/CMK-3, Pd/CB, and Pd/AC, respectively. Note that the
amount of NaOH in the solution was varied to study the effect
on the structure and catalytic performance of Pd nanoparticles.
For a convenient abbreviation of the NaOH content during the
above-described procedure, γ, defined as specific molar content
of NaOH to that of Na2PdCl4 ([NaOH]/[Na2PdCl4]) was intro-
duced, and six kinds of samples on each support were prepared
with different γ values (γ = 0, 2, 4, 8, 10, and 20).
XRD patterns were recorded using a Rigaku MiniFlex X-ray
diffractometer (Cu K␣, Ni filter). Transmission electron micro-
scope (TEM) images were obtained using a Hitachi H-9000
TEM at a voltage of 300 kV. UV–vis absorption spectra were
recorded using a Shimadzu UV-2500PC spectrophotometer.
2.3. Catalytic reactions
Catalytic activities of various Pd-supported carbons were
evaluated by liquid-phase oxidation of BA into benzalde-
hyde (BAH) and liquid-phase hydrogenation of CA into
3-phenylpropionaldehyde (PPA). To a cylindrical reaction ves-
sel (2 cm in diameter) with a reflux condenser, 10 mg of catalyst
powder (corresponding to 4.7 ␮mol of Pd) and 5 cm3 of 1,4-
dioxane were added. For oxidation of BA, 2 mmol of BA was
subsequentlyadded, andtheheterogeneousreactionmixturewas
stirred at 353 K under atmospheric pressure of O2. Hydrogena-
tion of CA was conducted using 1.5 mmol of CA at 303 K under
continuous hydrogen (H2) bubbling (20 cm3 min−1) through the
liquid phase. Amounts of products were determined using a
Shimadzu GC2010 gas chromatograph equipped with a flame
ionization detector and a TC-FFAP capillary column.
3. Results and discussion
3.1. Characterization of catalysts
Fig. 1 shows XRD patterns of Pd/CMK-3 prepared with var-
ious NaOH contents (γ = 0–20). All of the samples exhibited
three peaks at 2θ of 40◦, 46◦ and 68◦, ascribed, respectively, to
(1 1 1), (2 0 0), and (2 2 0) reflections of Pd metal with a face-
centered cubic (fcc) structure. Noticeable changes observed in
these XRD patterns are their peak width and height depending on
the value of γ: a large decrease in peak height accompanied by
broadening of width was observed in the case of γ values rang-

T. Harada et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 59–64
61
ing from 0 to 10, and vice versa at γ = 20. These results imply
that addition of NaOH changes crystallite sizes of loaded Pd
nanoparticles. Similar tendencies were also observed on Pd/CB
and Pd/AC (data not shown).
For the evaluation of these changes quantitatively, crystallite
sizes of Pd particles were calculated from the peak width at
half height of the Pd(1 1 1) reflection by applying the Scherrer
equation, and results are shown in Fig. 2. Despite significant
differences in crystallite sizes ranging from ca. 8 nm (for Pd/CB)
to ca. 80 nm (for Pd/AC) without addition of NaOH (γ = 0), these
sizes decreased when NaOH was present in the solution during
the synthesis. They were drastically reduced to less than 6 nm
in the case of γ values of 4–10 on all carbon supports.
Fig. 3 shows TEM images of Pd/CMK-3 prepared with
various NaOH contents. When Pd particles were loaded with-
out NaOH (γ = 0), Pd particles of various sizes were observed
(Fig. 3a). The average particle size was estimated to be 10 nm in
diameter. A significant deviation from the crystallite size (25 nm
at γ = 0, see Fig. 2) determined by the above XRD analysis
is due to the presence of much larger Pd particles (arrows in
Fig. 3a), which probably lead to overestimation of the crystal-
lite size using the Scherrer equation [26]. On the other hand,
well-dispersed Pd nanoparticles with a narrow particle-size dis-
Fig. 2. Plots of crystallite sizes of Pd particles on various carbon supports ver-
sus γ value. Diamonds, squares, and triangles denote Pd/CMK-3, Pd/CB, and
Pd/AC, respectively.
tribution centered at around 5 nm in diameter were observed at
the γ value of 4 (Fig. 3b). This value is consistent with the above-
described results of XRD analysis, indicating a single crystalline
nature of these nanoparticles. It should be noted that the size of
Fig. 3. TEM images of Pd particles supported on CMK-3 prepared with various NaOH contents: (a) γ = 0, (b) γ = 4, (c) γ = 10, and (d) γ = 20. Scale bars correspond
to 50 nm. Insets of figures (a) and (b) represent size distribution of Pd particles determined from TEM images by measuring sizes of more than 200 particles.

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T. Harada et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 59–64
Fig. 4. TEM images of Pd particles supported on AC (a–c) and CB (d–f) prepared at γ = 0 (a and d), γ = 4 (b and e), and γ = 20 (c and f). Scale bars correspond to
50 nm. Particle size distributions of Pd particles are also shown in insets of figures (a), (b), (d), and (e).
these Pd nanoparticles is larger than the pore diameter of CMK-3
used (3.3 nm in average diameter). Thus, Pd particles are dis-
tributed on the external surface of CMK-3 rather than inside
the pore system of CMK-3. As shown in Fig. 3c, an increase
in the NaOH content (γ = 10) led to poor dispersion of Pd on
CMK-3, while the sizes of individual particles seemed to be
smaller than the sizes of particles prepared at the γ value of 4
as expected from the results of XRD analysis. Aggregation of
particles became intense when the content of NaOH increased to
the γ value of 20; production of aggregates of Pd nanoparticles
of several hundreds of nanometers in diameter was confirmed
from the TEM image shown in Fig. 3d. On AC and CB supports,
strikingly similar changes in particle size, size distribution, and
dispersibility of Pd particles depending on the NaOH content
were observed, as shown in Fig. 4; inhomogeneous sizes and
size-distributions of Pd particles at γ = 0 (Fig. 4a and d) became
uniform (5.4 at γ = 4), and further addition of NaOH (γ = 20)
induced aggregation (Fig. 4c and f). Hence, we clearly demon-
strated the controllability of particle sizes and dispersibilities of
Pd nanoparticles by just adding appropriate amounts of NaOH
regardless of the kind of carbon support.
In order to investigate the above-described structural changes
of Pd particles induced by NaOH addition, we tried to identify
the structure of Na2PdCl4, the precursor of Pd nanoparticles,
in the solution in the absence and presence of NaOH by
using UV–vis absorption spectroscopy. Representative results
are shown in Fig. 5. The spectrum of the sample without NaOH
(γ = 0) exhibitstwoabsorption bandscentered at207 and237 nm
(Fig. 5a). These are assigned to ligand-to-metal charge transfer
(LMCT) bands of a [PdCl3(H2O)]− complex [27]. Addition of
a small amount of NaOH, corresponding to the γ value of 0.5,
induces a decrease in these two absorption bands accompanied
by the appearance of a broad absorption band centered at ca.
280 nm, which reached a maximum at the γ value of 1 (Fig. 5b
and c). According to the literature [28–30], this absorption band
can be assigned to the LMCT band of mixed chlorohydroxypal-
ladium(II) species, e.g., [PdCl2(OH)2]2− and [PdCl3(OH)]2−
Other noticeable changes observed in the spectrum (γ = 1) are
.

T. Harada et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 59–64
63
Fig. 6. Oxidation of benzyl alcohol into benzaldehyde by Pd particles loaded
on various carbon supports and by a commercial catalyst (c-Pd/C) under atmo-
spheric pressure of O2 at 353 K for 1 h. Inset shows conversion data obtained on
Pd/CB for a shorter period (0.5 h).
Fig. 5. UV–vis spectra of aqueous Na2PdCl4 solutions in the absence and pres-
ence of NaOH: (a) γ = 0, (b) γ = 0.5, (c) γ = 1, (d) γ = 4, and (e) γ = 10.
catalytic function of present materials. Since the selectivity of
the aldehyde production, defined as the percentage of the BAH
yield to the total amount of detected products, was ca. 80% on all
of the catalysts shown in this figure, we used conversion of BA
as a measure of their catalytic performance. On all the catalysts,
enhancement of conversion was clearly observed when cata-
lysts prepared by addition of an appropriate amount of NaOH
(2 ≤ γ ≤ 10) wereused. Maximumefficienciesmuchhigherthan
that of c-Pd/C were achieved on catalysts prepared at the γ value
of 4 (CMK-3, AC) or 8 (CB). In accordance with the results of
structural analyses described above, such an increase in catalytic
activity is mainly due to the small size of Pd nanoparticles, lead-
ing to an increment in active sites for the reaction. On the other
hand, there are apparent decreases in activity of all of the cata-
lysts prepared at the γ value of 20. As was confirmed by TEM
observation (Figs. 3d and 4c,f), this is due to the formation of
large aggregates of Pd nanoparticles; a limited number of active
sites can be utilized for the catalytic cycle on such aggregates.
Another point worth noting is that the Pd/CB catalyst gave
higher conversion than did Pd/CMK-3 and Pd/AC. As described
below, a similar trend was also observed in the hydrogenation of
CA. Based on the XRD and TEM results, this trend is probably
attributable to the relatively small crystallite (particle) size of
Pd nanoparticles on CB compared to those on other supports,
leading to utilization of relatively large number of active sites.
Table 1 summarizes the results of hydrogenation of CA on
Pd/CMK-3 and Pd/AC as well as on the commercial c-Pd/C cat-
alyst. All of the catalysts used in this study showed activity in
the production of PPA with 3-phenylpropanol (PPOH), a by-
product formed by hydrogenation of both C C and C O bonds
in CA. PPA obtained by the catalysts prepared without NaOH
(γ = 0) are comparable (Pd/AC) or less than (Pd/CMK-3 and
Pd/CB) that obtained by c-Pd/C. On the other hand, the CA con-
version of samples prepared in the presence of NaOH (γ = 2–10)
exceeded that of c-Pd/C. It is notable that the conversion reached
a maximum at the γ value of around 4 in the same manner as
the above-described BA oxidation. Thus, both particle size and
dispersion of Pd nanoparticles remarkably affect the conversion
in this reaction; i.e., the larger the surface area of Pd particles,
the higher the conversion of CA.
disappearance of absorption bands of the [PdCl3(H2O)]− com-
plexandanincrementinthebackgroundintensityatca.<240 nm.
Since the background intensity increased at a higher content of
NaOH (γ > 1, Fig. 5d and e), this is probably attributed to the
scattering induced by the formation of colloidal Pd(OH)2 par-
ticles. Actually, visual observation indicated the formation of
a colloidal sol in the solution when the content of NaOH was
further increased (γ = 20).
Consequently, addition of NaOH with γ values between 0
and 20 in the present study induced partial or complete ligand
exchange of Cl− for OH−. The change in particle size from a
few tens nm (γ = 0) to ca. 5 nm (γ ≤ 10) shown by the XRD and
TEM results corresponds to such ligand exchange prior to the
reduction with KBH4. Namely, precursor complexes containing
OH− ligand(s) are much more favorable for the formation small
Pd nanoparticles on carbon supports than those without an OH−
ligand. On the other hand, due to the formation of Pd(OH)2
particles before the KBH4 reduction in the presence of a rela-
tively large amount of NaOH (γ ≥ 10), monodispersibility of Pd
nanoparticles was reduced by the formation of large aggregates
as shown by TEM findings (see Figs. 3 and 4).
3.2. Catalytic activities
Fig. 6 shows catalytic properties of Pd/CMK-3, Pd/CB, and
Pd/AC prepared with various contents of NaOH for oxidation of
BA using atmospheric pressure of O2 at 353 K for 1 h, together
with those of a commercial carbon-supported Pd catalyst (c-
Pd/C, Pd: 5 wt%) supplied by N.E. Chemcat [31]. For Pd/CB
samples, conversion of BA for a shorter reaction period (0.5 h)
is also shown in the inset of this figure in order to evaluate
the dependence on the γ value. All of the catalysts suspended
and agitated in 1,4-dioxane containing BA gave BAH with pro-
duction of other by-products, mainly benzoic acid and benzyl
benzoate. No leaching of palladium species into the solvent was
confirmed by inductively coupled plasma (ICP) analysis (under
the detection limit (<5 ppm)) for some selected samples, indicat-
ing that the observed activity is derived from the heterogeneous

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T. Harada et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 59–64
Table 1
be prepared by a simple liquid-phase reduction from available
sources. Although we have evaluated catalytic functions only
using two substrates as model compounds for oxidation and
hydrogenation in the present study, these catalysts might have
the potential for inducing these reactions of a variety of sub-
strates. Moreover, these catalysts are expected to be applicable
to various kinds of reactions for C–C bond formation. Hence,
further studies along these lines are now in progress.
Hydrogenation of cinnamaldehyde into 3-phenylpropionaldehyde by Pd parti-
cles on various carbon supports
Catalyst
γ
Conversion (%)a
Selectivity (%)b
Pd/CMK-3
0
2
4
8
10
20
13
56
84
81
81
61
52
75
78
89
86
69
Acknowledgements
Pd/CB
Pd/AC
c-Pd/Cc
0
4
20
36
>99
34
89
95
85
The authors thank Professors K. Kaneda and T. Hirai (Osaka
University) for their stimulating suggestions and discussion.
This research was partially supported by a Grant-in-Aid for
Young Scientists (A) (No. 16685020) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology, and Iketani
Science and Technology Foundation.
0
4
20
53
52
22
95
90
89
–
45
92
Catalytic reactions were carried out at 303 K for 3 h with cinnamaldehyde
(1.5 mmol), 1,4-dioxane (5 cm3), and catalyst powder (10 mg, corresponding
to 4.7 ␮mol of Pd) under continuous bubbling with H2 (20 cm3 min−1).
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We have developed a method for synthesizing highly dis-
persed Pd nanoparticles on various carbon supports. It was
confirmed that these materials worked as efficient catalysts for
liquid-phase oxidation of benzyl alcohol using O2 and for C C
bond hydrogenation of cinnamaldehyde with H2 under mild
conditions. It is advantageous that the present catalysts can
[31] The c-Pd/C was activated by H2 reduction at 473 K just before the reaction.