Facile, surfactant-free synthesis of Pd nanoparticles for heterogeneous catalysts

Article abstract of DOI:10.1016/j.jcat.2011.03.022

A simple route to a highly active and selective Pd/C heterogeneous hydrogenation catalyst has been developed. The 1.5 nm Pd⁰ nanoparticles (NPs) were synthesized at room temperature from the reduction of palladium acetate in methanol under anhydrous conditions. A powdered carbon support was added during the synthesis and dried to produce an active catalyst that did not require any additional treatment. This technique alleviates many of the difficulties reported in using colloidal NPs, such as residues left from the removal of capping agents or agglomeration during ligand removal.

Full text of DOI:10.1016/j.jcat.2011.03.022

Journal of Catalysis 280 (2011) 145–149
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Priority Communication
Facile, surfactant-free synthesis of Pd nanoparticles for heterogeneous catalysts
a
b
a,
⇑
Patrick D. Burton , Timothy J. Boyle , Abhaya K. Datye
a
Department of Chemical and Nuclear Engineering and Center for Micro-Engineered Materials, University of New Mexico, Albuquerque, NM 87131, United States
Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, NM 87106, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple route to a highly active and selective Pd/C heterogeneous hydrogenation catalyst has been
developed. The 1.5 nm Pd nanoparticles (NPs) were synthesized at room temperature from the reduction
of palladium acetate in methanol under anhydrous conditions. A powdered carbon support was added
during the synthesis and dried to produce an active catalyst that did not require any additional treatment.
This technique alleviates many of the difficulties reported in using colloidal NPs, such as residues left
from the removal of capping agents or agglomeration during ligand removal.
Received 13 January 2011
Revised 21 March 2011
Accepted 25 March 2011
Available online 23 April 2011
0
Keywords:
Ó 2011 Elsevier Inc. All rights reserved.
Palladium nanoparticles
Acetylene hydrogenation
Catalyst pre-treatment
Surfactant-free nanoparticles
1
. Introduction
Supported Pd nanoparticles constitute the active phase in cata-
adsorbing probe molecules (i.e., CO) [12] and to be active in a vari-
ety of liquid-phase reactions, such as olefin hydrogenation [3] as
well as alcohol oxidation [15]. However, when the polymer-capped
nanoparticles are deposited on a support for gas-phase reactions,
the capping agent must be removed to achieve catalytic activity
[1,16,17]. The high-temperature oxidation and reduction treat-
ments used can lead to particle growth and loss of monodispersity.
Therefore, there is a need to develop novel routes that can provide
metal nanoparticles without protective polymers or capping
ligands.
In solvothermal synthesis, precursors, such as palladium chlo-
ride or palladium (bis acetyl acetonate), are added to high boiling
solvents such as bromobenzene, toluene, or methyl isobutyl ketone
in the presence of a surfactant to achieve reduction of the metal
[18]. A more easily reduced precursor, such as palladium acetate
lysts used for energy conversion, chemical synthesis, and pollution
abatement. To achieve the highest selectivity and reactivity, it is
desirable to have well-dispersed nanoparticles (NPs) that have
identical properties and distribution of active sites [1]. Unfortu-
nately, conventional synthesis routes do not provide the requisite
degree of control, since they start from Pd salts that are first depos-
ited on a support by impregnation or precipitation. Reduction is
achieved by high-temperature treatments involving calcination
and H
2
reduction, or chemical reduction by sodium borohydride
[
2]. The resulting broad distribution of particle size, shape, and
composition is detrimental to catalyst performance. Hence, there
has been considerable interest in developing colloidal routes to
synthesize well-defined nanoparticles that could be used to pre-
pare heterogeneous catalysts [3–5]. Typically, solution routes re-
quire various reducing agents such as hydrazine [6], alkaline
borohydrides, [7], or amine groups [8] where the particles are pro-
tected by polymer groups, surfactants or ligands to prevent
agglomeration and growth [7,9].
Polymer protecting agents such as poly(vinyl pyrrolidone) (PVP)
and polyvinyl alcohol (PVA) allow preparation of metal colloids
that can be stable for months with reasonable control over size
as well as shape [10–14]. The synthesis involves addition of poly-
mer to the metal salt followed by chemical or thermal reduction
2
(noted as Pd(OAc) ) allows for colloidal synthesis at lower temper-
ature using simple alcohols as reducing agents [4,5,19]. However,
literature reports that utilize methanol (MeOH) without a capping
agent indicate that large aggregates will form [19]. These aggre-
gates can reach diameters of 50 nm and are not suited for catalytic
applications. The uncontrolled reduction in Pd complexes at ele-
vated temperatures has been described in the homogeneous catal-
ysis literature as a nuisance [20]. These studies all suggest that
capping agents or ligands are essential for the synthesis of nano-
particles in solution at elevated temperatures.
Recent work by Chen et al [21] has shown that graphene oxide
can directly reduce K PdCl to produce NPs. While this is an effec-
2 4
0
to produce a stable black suspension of Pd particles. These poly-
mer-capped nanoparticles have been shown to be capable of
tive technique and yields Pd nanoparticles of about 3 nm in diam-
eter, it is limited to reactions using graphene oxide as a support.
We describe here a more general route for synthesizing nanoparti-
cles that involves the reduction of the precursor by the solvent. In
⇑
Corresponding author. Fax: +1 505 272 1024.
E-mail address: datye@unm.edu (A.K. Datye).
0
021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.03.022

1
46
P.D. Burton et al. / Journal of Catalysis 280 (2011) 145–149
order to avoid capping agents, a mild reducing agent must be em-
ployed to limit rapid particle formation, growth, and coalescence.
2.3. Acetylene hydrogenation
We have found that the room temperature reduction of Pd(OAc)
2
The catalyst was granulated by pressing the dried powder under
ꢀ7 metric tons with a Carver hydraulic press to produce a pellet.
The pellet was ground and the powder sieved to between 106
in MeOH is slow enough to produce a suspension of Pd NPs. These
NPs can then be used as the metal phase of a heterogeneous cata-
lyst without the need for thermal treatment prior to reaction. A Pd-
NP/C catalyst was prepared by mixing the carbon support into the
suspension of Pd NPs and evaporating the solvent. Aggregate for-
mation was a concern, as there were no capping agents to prevent
particle growth. Therefore, the nanoparticles were collected
quickly before substantial aggregation could occur. As the reduc-
tion was not caused by the support, this technique is general and
can be extended to other powder supports. The catalyst was active
as-prepared and was found to be active and very selective for acet-
ylene hydrogenation in the presence of excess ethylene.
and 250
VWR. A sample of 15 mg of powder was mixed with 400 mg SiC
(350 m average grain size, Washington Mills) as an inert to min-
lm using # 140 and # 60 US standard testing sieves from
l
imize temperature non-uniformity. The mixed powder was packed
in a 0.25 in. quartz tube between two plugs of quartz wool. A mix-
ture of acetylene (0.5%) and ethylene (35%) in a balance of nitrogen
was passed over the powder at a flowrate of 66 mL/min. Hydrogen
was passed at 1.4 mL/min to produce a 5:1 ratio of hydrogen to
acetylene. Product gases were sampled at temperature intervals
of 10 °C by a Varian 3800 gas chromatograph equipped with a
CP-PoraBOND U column and an FID detector.
2
. Methods
2.4. Characterization
2.1. Chemicals
ꢁ1
FTIR (Nicolet 6700, 32 scans, 4 cm resolution) was used to
All chemicals were used as received and stored under ambi-
probe for the presence of hydroxyl groups on each precursor. The
prepared colloids were examined by transmission electron micros-
copy (TEM, JEOL 2010F) operated at 200 kV in scanning (STEM) and
high- resolution (HRTEM) modes to determine the size and compo-
sition of the particles. X-ray diffraction (XRD, Panalytical X’Pert
Pro, 45 kV 40 mA) was used as a complementary technique. An ali-
quot of the colloid was pipetted onto a zero-background Si wafer
and allowed to dry at room temperature. As the solvent evapo-
rated, the particles coalesced and produced a film, which was then
analyzed to verify production of a metallic phase.
ent conditions unless otherwise noted. Two separate containers
of Pd(OAc) were used, one stored and used in an argon-filled
2
glovebox and the other stored and used under bench-top condi-
tions. Anhydrous MeOH and bench-top MeOH, ethanol, 2-propa-
nol, toluene, and acetone were reagent grade and purchased
from Sigma Aldrich. Vulcan XC72R was obtained from Cabot
Corporation. Hydrogen, nitrogen, and a mixture of 0.5% acety-
lene and 35% ethylene (balance nitrogen) were UHP grade from
Matheson Trigas.
2
2
.2. Synthesis
3
. Results and discussion
.2.1. Preparation of colloidal nanoparticles
For a typical sample, Pd nanoparticles were synthesized by mix-
2
The initial investigation involved the reduction of Pd(OAc) in
MeOH to produce a suspension of Pd NPs (Fig. 1). A systematic
study of precursor conditions, light exposure, and solvent was
undertaken. In the initial study, using an air-exposed precursor
2
ing Pd(OAc) (5 mg) in MeOH (15 mL) in a scintillation vial and
stirring for approximately 5 min with unobstructed exposure to
room lighting. The vial was placed on an elevated stir plate for
observation and allowed to react undisturbed for 20 min. After
2
and solvent, the dissolution of the red–orange Pd(OAc) in MeOH
yielded a pale yellow solution that progressively darkened to a
black solution after 60 min (Fig. 1a). Subsequent attempts to repro-
duce the reaction resulted in longer reaction times (up to days) and
moderate variation in particle size. Under rigorously dry condi-
tions, the color change was found to be both faster (dark green
in 20 min, black in 30 min) and reproducible. A TEM grid was
dipped into the solution at 20 min and a dark field STEM image
is shown in Fig. 1b. The mean diameter of 1.5 nm (Fig. 1d) is
remarkable in view of the simplicity of the preparation. Multiple
tests were performed and it was found that the size distribution
was very similar in each case. The Pd particles were so small that
they were very difficult to detect via HRTEM on the carbon film
and could only be imaged via dark field imaging in a STEM. Our
estimate of precision in size measurement is based on the probe
size used for our STEM imaging, 0.2 nm. The electron diffraction
pattern is very diffuse in agreement with the small size and the
low loading of the Pd NPs on the carbon film.
2
0 min, a TEM grid was dipped into the vial and allowed to dry.
Variations on this method were conducted with air-exposed pre-
cursors and solvents, including acetone, toluene, ethanol, and 2-
propanol. The dependence upon light was evaluated by isolating
a vial in an insulated container placed over a stir plate. The solution
was stirred for 5 min in the dark and allowed to sit undisturbed
over a 2 h period. Observations were conducted at 10 min intervals
to minimize light exposure.
2.2.2. Preparation of supported catalyst
A supported catalyst was prepared by dissolving anhydrous
2
Pd(OAc) (20 mg) in anhydrous MeOH (30 mL) and stirring con-
tinuously for 10 min in a Schlenk flask under inert atmosphere.
The flask was removed to ambient atmosphere and Vulcan
XC72R carbon (1.0 g) was added. The slurry was mixed for an
additional 10 min and subsequently attached to a rotovap, using
the bath to maintain the flask at ambient temperature. The sol-
vent was removed while the reaction continued to progress.
After 30 min, the reaction was complete and the slightly damp
powder was allowed to air dry prior to characterization. No
additional treatments were performed prior to catalytic activity
measurements. This sample will be referred throughout the
manuscript as Pd-NP/C. A similar sample was prepared under
identical conditions, except toluene was used as the solvent. As
this sample did not form nanoparticles in solution, it will be re-
ferred to simply as Pd/C.
2
A sample of Pd(OAc) in MeOH that was stored in a ‘black box’
was found to yield no color change after 2 h. Exposure to UV light
only led to a similar reaction rate as the sample under ambient
light. Larger chain alcohols (ethanol and 2-propanol) produced a
color change in several hours, but non-alcohol solvents, such as
acetone took one week to react. TEM samples prepared from the
acetone preparation revealed that large Pd aggregates had formed.
These observations indicated that alcohols were effective reducing
agents, as noted by Hirai et al. [3]. Exposing a solution of Pd(OAc)
in anhydrous MeOH to ambient room light yielded optimal results.
2

P.D. Burton et al. / Journal of Catalysis 280 (2011) 145–149
147
1
Dried Pd film
Pd (OAc)2
0
0
.75
0.5
(
a)
.25
30
40
50
60
70
80
90
100
Fig. 2. XRD pattern of dried film colloid on a Si substrate matches Pd. The precursor,
Pd(OAc) , shows a very different pattern, indicating that the organic precursor was
2
reduced to fcc metal.
(b)
(c)
+
+
1
+
+
4
4
3
3
2
2
1
1
5
0
5
0
5
0
5
0
+
Pd-NP/C
+
+
0
.1
+
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
+
+
+
0
.01
0.001
+
+
+
5
0
+
+
Pd/C
+
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0
.1
Particle diameter [nm]
(
d)
0
.01
Fig. 1. (a) Time-progression (in min) of formation of air-exposed Pd NP solution in
MeOH, (b) STEM image of colloid deposited on carbon film, (c) Pd-NP/C before
reaction, (d) particle size distribution of Pd NP from MeOH.
0
.001
2.4 2.5 2.6 2.7 2.8 2.9
3
3.1 3.2 3.3
1000/Temperature [K−1]
Following these reaction conditions, a catalyst was prepared by
mixing Vulcan XC72R carbon powder into the solution after
Fig. 3. Graph of fraction of acetylene generated over the reaction temperature
range for Pd-NP/C (top graph) and Pd/C (bottom graph). Five repeat trials are shown
for each catalyst.
1
1
0 min of reaction. The suspension was stirred for an additional
0 min followed by drying in vacuo at room temperature. The pow-
der was analyzed by TEM (Fig. 1c) and the nanoparticles were
found to be similar in size to those noted in the previous synthesis
(
Fig. 1b). The Pd NPs in this colloid were confirmed to be metallic
other metals when used for selective hydrogenation of acetylene
[22,23]. The Pd-NP/C catalyst yielded selectivity equivalent to the
performance of the best catalyst reported in the literature, Pd Ga
3 7
based on the XRD analysis of the dried colloid, which yielded the
expected fcc Pd pattern. An XRD sample of the colloid was pre-
pared by pipetting an aliquot of the suspension on a silicon wafer
and allowing it to dry prior to analysis. As there were no surfac-
tants, the metal dried to a thin film which produced peaks corre-
sponding to metallic Pd. These peaks were indexed to ICCD card
[24]. The Pd/C catalyst exhibited an activation process where sub-
sequent trials showed much greater activity in comparison with
the initial experiment (Fig. 3, activation energies tabulated in Ta-
ble 1). Each trial was significantly less selective (Fig. 4) at 100%
conversion of acetylene. Both catalysts demonstrated activation
energies within the reported values of 40–50 kJ/mol [25,23], ex-
cept for trial 1 of Pd/C.
Selectivity was defined as the moles of ethylene generated per
mole of acetylene consumed, as shown in Eq. (1). In the notation
used, f denotes feed and e denotes effluent.
8
9-4897 (fcc Pd) while the precursor did not display any metallic
phase, shown in Fig. 2. The Pd-NP/C powder did not feature any
prominent Pd peaks, which was expected due to the small crystal-
lite size and low weight loading. EDS indicated that ꢀ1 wt% load-
ings had been achieved.
Each as-prepared catalyst was subsequently tested for acety-
lene hydrogenation with a 70:1 mixture of ethylene to acetylene
in a packed bed reactor. Interestingly, the Pd-NP/C catalyst was
found to be selective (Eq. (1)) toward ethylene production at high
Ethylene
Ce
Ethylene
ꢁ Cf
Acetylene
Selectivity ¼
ꢂ 100%
ð1Þ
Acetylene
Cf
ꢁ Ce
conversion (Fig. 4) and active (Fig. 3, E
pre-treatment. Four subsequent trials showed similar results
Figs. 3 and 4, Table 1). The catalyst was therefore active without
a
= 49 kJ/mol) without any
The change
D
CEthylene is caused by production of ethylene from
(
acetylene hydrogenation and consumption to ethane. Therefore, a
positive selectivity indicates a net gain in ethylene compared with
the feed. A negative value represents a loss due to consumption of
pre-treatment, and stable over the time interval studied. Pd is a
well-known hydrogenation catalyst and is commonly diluted with

1
48
P.D. Burton et al. / Journal of Catalysis 280 (2011) 145–149
1
00 + ++ + +
0.05
Pd-NP/C Trial 1
Pd-NP/C Trial 2
Pd-NP/C Trial 3
Pd-NP/C Trial 4
Pd-NP/C Trial 5
Studt et al. [23]
8
6
4
2
0
0
0
0
+
+
+
0
.04
+
+
+
0.03
Pd-NP/C
0
0
.02
.01
0
+
+
1
-
00
+
+
+
+
+
0
++ + +
5
0
0
Pd/C
25
50
75
100
+
+
+
+
% Acetylene Converted
50
Trial 1
+
Trial 2
Trial 3
Trial 4
Trial 5
+
Fig. 5. Ethane content of outlet gas in the present work normalized to inlet
ethylene for comparison to the literature [23]. Lines added as a visual aid.
-
-
-
-
100
150
200
250
support, or possible carbonaceous species derived from the catalyst
precursor and is under investigation. As the nanoparticles formed in
solution, any carbonaceous impurities present could have deposited
onto the particle as the sample dried. The presence of carbonaceous
layers may have resulted in improved selectivity, as reported previ-
ously in the literature [26].
0
20
40
60
80
100
%
Acetylene Conversion
2 4
Fig. 4. Graph of ethylene selectivity showing excess C H production as positive
selectivity (consumption as negative values). As in Fig. 3, five repeat trials are
shown for both Pd-NP/C (top) and Pd/C (bottom). Lines are added as a visual aid.
4
. Conclusions
Table 1
Comparison of the performance of the Pd catalysts using selectivity as defined by Eq.
In summary, a simple route to a highly active and selective Pd-
(2).
NP/C heterogeneous hydrogenation catalyst has been developed.
Sample
E
a
kJ/mol
Conversion (%)
Selectivity (%)
The Pd0 NPs were synthesized from the ambient temperature
2
reduction of Pd(OAc) in MeOH under anhydrous conditions. The
Pd-NP/C
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
49
47
47
45
42
76
100
100
98
+72
+52
+43
+44
+40
inclusion of Vulcan XC72R during the synthesis led to isolation of
the active catalyst that did not require any additional treatment.
This technique alleviated the need to pre-treat the catalyst and
provided a substantial improvement in selectivity. Additional work
on understanding the performance of these catalysts, and Pd NPs
on other supports, is being undertaken to explore the broader util-
96
Pd/C
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
98
37
39
42
42
100
100
100
100
100
99
ꢁ105
ꢁ224
ꢁ203
ꢁ76
ꢁ89
+71
0
ity of this simple route to Pd nanoparticles.
Acknowledgments
Pd
Pd/Al
3
Ga
7
[24]
[24]
2
O
3
43
+17
This work has been supported by the United States Department
of Energy, Office of Basic Energy Sciences under contract number
DE-FG02-05ER15712 (University of New Mexico) and DE-
AC0494AL85000 (Sandia National Laboratories). Sandia is a multi-
program laboratory operated by Sandia Corporation, a Lockheed
Martin Company for the United States Department of Energy. The
authors thank Dr. C.A. Stewart and Ms. L.A. Ottley for technical
assistance.
the feed ethylene in addition to total hydrogenation of acetylene to
ethane. This definition of selectivity differs from an equation used
by Osswald et al. [24] who consider the change in ethylene concen-
Ethylene
tration
D
C
to be unreliable, and use Eq. (2) instead.
Acetylene
Acetylene
Cf
ꢁ Ce
Selecti
v
ity ¼
ð2Þ
Acetylene
Cf
Acetylene
Ethane
þ Ce
other
þ 2Ce
ꢁ Ce
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