Synthesis and catalytic properties of nanoparticulate intermetallic Ga-Pd compounds

Article abstract of DOI:10.1021/ja202869d

A two-step synthesis for the preparation of single-phase and nanoparticulate GaPd and GaPd₂ by coreduction of ionic metal-precursors with LiHBEt₃ in THF without additional stabilizers is described. The coreduction leads initially to the formation of Pd nanoparticles followed by a Pd-mediated reduction of Ga³⁺ on their surfaces, requiring an additional annealing step. The majority of the intermetallic particles have diameters of 3 and 7 nm for GaPd and GaPd ₂, respectively, and unexpected narrow size distributions as determined by disk centrifuge measurements. The nanoparticles have been characterized by XRD, TEM, and chemical analysis to ensure the formation of the intermetallic compounds. Unsupported nanoparticles possess high catalytic activity while maintaining the excellent selectivity of the ground bulk materials in the semihydrogenation of acetylene. The activity could be further increased by depositing the particles on α-Al₂O₃.

Full text of DOI:10.1021/ja202869d

ARTICLE
pubs.acs.org/JACS
Synthesis and Catalytic Properties of Nanoparticulate Intermetallic
GaꢀPd Compounds
Marc Armbr€uster,*,† Gregor Wowsnick,† Matthias Friedrich,† Marc Heggen,‡ and Raul Cardoso-Gil†
†Max-Planck-Institut f€ur Chemische Physik fester Stoffe, N€othnitzer Strasse 40, 01187 Dresden, Germany
‡Institut f€ur Festk€orperforschung, Forschungszentrum J€ulich GmbH, 52425 J€ulich, Germany
ABSTRACT: A two-step synthesis for the preparation of single-phase and
nanoparticulate GaPd and GaPd2 by coreduction of ionic metal-precursors with
LiHBEt3 in THF without additional stabilizers is described. The coreduction
leads initially to the formation of Pd nanoparticles followed by a Pd-mediated
reduction of Ga3þ on their surfaces, requiring an additional annealing step. The
majority of the intermetallic particles have diameters of 3 and 7 nm for GaPd
and GaPd2, respectively, and unexpected narrow size distributions as deter-
mined by disk centrifuge measurements. The nanoparticles have been char-
acterized by XRD, TEM, and chemical analysis to ensure the formation of the intermetallic compounds. Unsupported nanoparticles
possess high catalytic activity while maintaining the excellent selectivity of the ground bulk materials in the semihydrogenation of
acetylene. The activity could be further increased by depositing the particles on R-Al2O3.
’ INTRODUCTION
reduction of GaCl3 with anthracene-activated Mg, and Cokoja
et al.8 obtained Ga2Cu by reaction of a copper-mesitylene
complex with quinuclidine-stabilized GaH3. The products were
characterized by XRD and no information about particle sizes
was given. Several groups attempted the synthesis of supported
nanoparticles of Ga-based intermetallic compounds by reductive
treatment of Ga2O3-supported transition metals like Pd or Co.
Precursors of the metals were reduced in a hydrogen-atmosphere
at elevated temperatures leading to a reductive metalꢀsupport
interaction (RMSI) and the subsequent formation of intermetallic
compounds.9ꢀ11 These approaches resulted in mixtures of different
phases, often still containing elemental transition metals.
The semihydrogenation of acetylene is an important reaction
for the industrial production of polyethylene. Traces of acetylene
(∼1%) in the ethylene stream produced by naphtha cracking
have to be diminished to the low ppm regime since acetylene
poisons the polymerization catalyst.1 Usually Pd or substitutional
AgꢀPd alloys2 supported on R-alumina are applied for the
selective hydrogenation of acetylene to ethylene. These catalysts
show a high hydrogenation activity but only a limited selectivity
to ethylene.3 In addition, improving the longtime stability is a
challenge for future catalytic systems.
The intermetallic compounds GaPd2, GaPd, and Ga7Pd3 were
found to be excellent catalysts for the selective semihydrogena-
tion of acetylene.4ꢀ6 The ground or milled bulk materials show a
high long-term stability and an excellent selectivity to ethylene of
70ꢀ78%. This originates from the isolation of the active sites in
the crystal structure as well as from the strong alteration of the
electronic structure in comparison to elemental Pd. In situ XRD,
in situ EXAFS and in situ XPS proved the stability of these
compounds under reaction conditions. The aim of these studies
was to explore the intrinsic catalytic properties of these well-
defined compounds by avoiding additional influences, for exam-
ple, particle size effects or influences from the support. For this
reason, low specific surface areas and hence a low specific activity
were tolerated. Industrial systems on the other hand have to
make best use of the noble metal. To enhance the catalytic
activity of the GaꢀPd intermetallic compounds, we are inter-
ested in the synthesis of well-defined nanoparticles of these
materials and their catalytic properties for the semihydrogena-
tion of acetylene.
For the synthesis of unsupported nanoparticulate intermetallic
compounds either laser ablation12 or solution-based synthesis
procedures may be applied. A common solution-based method
to generate metallic nanoparticles is the (co)reduction of metal-
salts in an organic solvent with a suitable reducing agent, for
example, hydrogen or borohydrides. Sometimes the solvent is
used simultaneously as a reducing agent like in the case of
ethylene glycol or tetraethylene glycol. It was shown in particular
by Schaak that the use of high boiling alcohols, known as “polyol
process”, as mild reducing agents allows the preparation of a wide
range of intermetallic compounds.13ꢀ15 In the case of bimetallic
nanoparticles containing a metal with an unfavorable reduction
potential for the polyol process, for example, Al or Zn, thermal
decomposition reactions, hydrogenolysis or combinations of
decomposition and hydrogenolysis of adequate metalꢀorganic
precursors can be applied.8,16,17
The challenge in the preparation of intermetallic GaꢀPd
compounds is the difference in redox potentials of Ga3þ and
Until now little has been known about successful syntheses of
single-phase nanoparticulate intermetallic compounds of Ga.
Abraham et al.7 produced finely divided powder of Mg2Ga5 by
Received: March 29, 2011
Published: May 09, 2011
r
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Pd2þ. This complicates a conucleation, which is usually desired
for synthesis of bimetallic nanoparticles. The strong Lewis acidity
of Ga3þ-salts, which tend to form stable complexes with electron
donors, also hinders the reduction of Ga3þ. In the case of Ga, the
situation is even more difficult in comparison to other hard-to-
reduce metals because the low melting point of 29.77 °C18 of
elemental Ga impedes a controlled synthesis of nanostructured
Ga-materials, due to the high tendency to coagulation of the
liquid metal.
Recently, a procedure for the synthesis of intermetallic
CuꢀPd nanoparticles was developed.19 In the following we
describe the adoption of this process resulting in a two-step
synthetic route giving access to nanoparticulate GaPd2 and GaPd
using LiHBEt3 in THF (Superhydride) as reducing agent.
Emphasis is placed on the absence of elemental Pd as well as
surfactants, which would block or modify the catalytically active
surface.17 Another requirement is the absence of byproducts in
the samples, which could modify the catalytic properties in the
semihydrogenation of acetylene. Since the development of a
reproducible synthesis including the mentioned requirements is
not straightforward, special care was taken to also investigate
mechanistic aspects of the synthesis. The syntheses described
below were developed for unsupported single-phase materials.
Modifying the synthesis protocol also allowed the preparation of
particles deposited on R-Al2O3.
solution, resulting in a black suspension instantly. The suspension was
stirred for further 4 h under reflux and subsequently separated by
centrifugation at 6000 rpm. The black precipitate was washed three
times with THF and dried in dynamic vacuum at 100 mbar for 20 min.
The dried particles were resuspended in dioctylether, heated to 185 °C
and stirred for another 4 h. After the suspension was centrifuged at
6000 rpm to separate the solid from the liquid, the black precipitate was
washed with THF three times to remove the high-boiling dioctylether
and dried for 20 min at 100 mbar in dynamic vacuum.
GaPd was synthesized by dissolving 0.1386 g (0.455 mmol) Pd-
(acac)2 in 15 mL THF in a 100 mL 3-neck round-bottom flask and
stirring at ambient temperature; 0.107 g (0.608 mmol) of GaCl3 were
added to 5.1 mL Superhydride, filled in a syringe, placed in a perfusor
and connected to the 3-neck flask as described above. In contrast to the
synthesis of GaPd2, the GaCl3 solution was added to the THF solution
with 150 mL/h at room temperature resulting in a black suspension
instantly. The black suspension was subsequently heated to reflux and
stirred for 4 h. The obtained products were separated, cleaned and
thermally treated in dioctylether as described above.
For the synthesis of supported particles, coreduction was performed
following the above protocols. After washing the particles, they were
resuspended in 10 mL THF. Two milliliters of this suspension were filled
in a 200 mL 2-neck round-bottom flask filled with 21 g of R-Al2O3 as
support (20 mL bulk volume, cylindrical pellets treated with 40 mL
acetone, subsequently shaken to replace Al2O3 powder, then dried at
80 °C for 2 h and at last moistened with 10 mL THF before use). During
careful shaking, the THF was slowly evaporated by applying vacuum
(150 mbar), which was slowly decreased to 20 mbar during 20 min
allowing a slow and homogeneous deposition of the particles on the
Al2O3. The catalysts were then dried at 1 mbar for 2 h. Subsequently,
50 mL of dioctylether were added and the deposited particles were
treated in suspension at 250 °C for 4 h. The dioctylether was decanted
and the catalyst was washed four times with 50 mL of THF under careful
shaking, causing some loss of particles due to the mechanical treatment.
In the last step, the pellets were dried at 1 mbar for 2 h.
X-ray powder diffraction (XRD) was performed on an image plate
Guinier camera (Huber G670, CuꢀKR1 radiation, λ = 1.54056 Å, quartz
monochromator, 3° e 2θ e 100°). To prevent contact with air, the
powders were mounted between two 7.5 μm Kapton foils. Temperature-
dependent X-ray powder diffraction was performed on a STOE-STA-
DIP-MP diffractometer (CuꢀKR1 radiation, λ = 1.54056 Å, Ge-mono-
chromator) using a high-temperature setup and placing the samples in
Ar-filled quartz glass capillaries.
’ EXPERIMENTAL SECTION
The following reagents were used: GaCl3 (ultra dry, 99.999%,
Aldrich), Pd(acac)2 (acac = acetylacetonate, purum, Fluka), LiHBEt3
(Superhydride, 1 M in tetrahydrofuran (THF), Aldrich), NaBH4
(ABCR, powder 98%), Pd/Al2O3 (5 wt.%, Aldrich), Pd (ChemPur,
99.95%), Ga (ChemPur, 99.99%), BN (hexagonal, 99.95%, 325 mesh,
Aldrich), THF (99.9%, Roth), acetone (Overlack, 99.7%) and diocty-
lether (99%, Aldrich). Solvents, except acetone, were distilled over CaH2
under argon before use to remove water and oxygen. Other chemicals
were used without further purification. All procedures were carried out
under protective argon atmosphere in a glovebox with oxygen and water
concentrations below 0.1 ppm. Bulk samples of GaPd and GaPd2 were
prepared by melting stoichiometric amounts of the elements under Ar in
a high-frequency furnace and subsequent annealing in vacuum as
described earlier.5
TEM images were taken at a Tecnai 10 (LaB6 cathode) at 100 kV.
High-resolution TEM images were taken at a spherical-aberration
corrected FEI TITAN 80ꢀ300 transmission electron microscope
operated at 300 kV. Negative spherical aberration imaging (NCSI)
was used, providing optimum bright atom contrast up to the informa-
tion limit.20,21 Samples for TEM analysis were prepared from a
methanol suspension on a carbon-coated copper grid (300 mesh, SPI
supplies).
Disk centrifuge (DC) measurements on a CPS DC24000 were used
to determine the particle size distribution of the unsupported samples.
The dried samples were suspended in cyclohexane before being
injected into the rotating disk. The disk speed was set to 24 000 rpm
and the density gradient was built up from cyclohexane and halocarbon
with a density of 1.8 g/cm3. Elemental compositions of the samples
were determined by ICP-OES (Varian, Vista RL). Nanoparticulate
samples were dissolved in aqua regia diluted with deionized water and
measured applying a commercial standard for calibration. Metal con-
tents of organic solutions were determined using a matrix adjusted
standard.
For the investigation of the formation of the nanoparticles, LiHBEt3
in THF was also transferred to dioctylether. Fifteen milliliters of
Superhydride were filled in a 3-neck round-bottom flask and 15 mL of
dioctylether were added. The solution was allowed to stir for 1.5 h at 1
mbar at 90 °C to remove THF from the mixture. The final solution had a
volume of 16.5 mL, indicating that some THF was still present. After
adding 0.3 g (1.704 mmol) of GaCl3 to 5.5 mL of LiHBEt3 in
dioctylether, a white precipitate of LiCl was formed immediately,
followed by an exothermic reaction under formation of gray elemental
liquid Ga, which coated the glass wall easily. The suspension was
separated by centrifugation (EBA 20, Hettich), washed four times with
THF to remove the LiCl and dried at 1 mbar for 20 min to result in
35 mg of elemental Ga.
For the synthesis of nanoparticulate GaPd2, 0.1384 g (0.454 mmol) of
Pd(acac)2 were solved in 15 mL THF in a 100 mL 3-neck round-bottom
flask. The yellow clear solution was heated to reflux temperature while
stirring; 0.1000 g (0.568 mmol) GaCl3 were added to 5.1 mL Super-
hydride in a separate vessel accompanied by an exothermic reaction
leading to a colorless clear solution. This solution was filled in a syringe,
placed in a perfusor and connected to the 3-neck flask by a cannula
through a septum and pumped with 150 mL/h to the refluxing THF
The catalytic measurements were performed in a plug-flow reactor at
473 K; a detailed description of the reaction setup has been given
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elsewhere.5 Deposited samples were crushed and diluted with 150 mg of
inert BN. The feed composition was 0.5% acetylene (solvent free, 99.5%,
5% in He, 99.999%), 5% hydrogen (99.999%), 50% ethylene (99.95%)
in helium (99.999%), and the total flow was 30 mL/min. The gases were
supplied by Praxair (C2H2, He and H2) and Westfalen Gas (C2H4). The
product gas composition was determined by a Varian CP 4900 MicroGC
gas chromatograph. Conversion and selectivity to ethylene were calcu-
lated as stated in ref 5.
Scheme 1. Proposed Mechanism for the Formation of the
Nanoparticulate GaꢀPd Precursora
a Stable Ga3þ complex is reduced by palladium-activated hydrogen. The
exclusive formation of Ga on the Pd surface inhibits the coagulation of
the elemental Ga.
’ RESULTS AND DISCUSSION
The First Step—Coreduction. The challenge for the synthesis
of intermetallic GaꢀPd compounds is to find the optimum
reducing conditions for Ga. On the one hand, mild reducing
agents are not capable to reduce the Ga3þ with its low redox
potential and its high Lewis-acidity. On the other hand, a very fast
reduction of Ga3þ will most probably cause coagulation of the
low-melting metal, handicapping the formation of nanoparticulate
materials. Because Pd2þ is easier to reduce to the elemental state,
a coreduction of Pd2þ and Ga3þ is likely to produce first Pd
nanoparticles, which then can act as nucleation centers for the
Ga atoms.
Single-phase nanoparticulate GaPd2 and GaPd can indeed be
obtained by coreduction of Pd(acac)2 and GaCl3 with LiHBEt3
in THF (“coreduction”) and subsequent annealing in diocty-
lether at high temperatures (“annealing”). In the first step, an
excess of GaCl3 is necessary for the formation of GaPd and
GaPd2 since the reduction of the Ga precursor is not quantitative
and high temperatures and 4 h reaction time with a large excess of
Superhydride are required to reduce Ga3þ. The reaction para-
meters are crucial. The success of the coreduction strongly
depends on the reduction temperature, the reducing agent and
the solvent as well as on the counterion of Ga3þ. The initial
reduction temperature is important to obtain reproducible
single-phase intermetallic GaPd or GaPd2, and experiments
revealed that using NaBH4 instead of LiHBEt3 as reducing agent
was not powerful enough to reduce the GaCl3 and hence to
obtain any intermetallic compounds. Replacing GaCl3 by Ga-
(acac)3 hindered a successful reduction, even when Superhydride
was used as the reducing agent. The strong dependence of the
system on the solvent can be seen by changing from THF to
dioctylether as solvent for the reduction of GaCl3 in the absence
of Pd2þ. While no reaction occurs in THF, most likely because of
a better stabilization of Ga3þ by O-donor bonds, it is possible to
reduce Ga3þ to the elemental state in dioctylether, using
LiHBEt3 as reducing agent in both cases. These observations
show that the synthesis does not follow the intuitive reaction
equation given below for GaPd:
reduction of Pd(acac)2 to Pd is fast and quantitative at ambient
temperature under evolution of H2. Therefore, the reaction
scheme can be written as
PdðacacÞ2 þ 2LiHBEt3 f Pd þ H2 þ 2LiðacacÞ þ 2BEt3
In the presence of unconsumed LiHBEt3, the complex
Et3BHBEt3 is formed,23 which might still be able to reduce
ꢀ
Pd2þ to the elemental state.
Looking at the second reduction separately, that is, GaCl3 to
elemental Ga, an interesting observation is made. If only GaCl3 is
reacted with LiHBEt3, no significant amount of Ga is formed,
even at reflux temperature. Adding GaCl3 to Superhydride at
ambient temperature causes an exothermic reaction and results
in a colorless solution. The reason is most probably the high
Lewis-acidity of Ga3þ and the electron-donor activity of THF
leading to stable complexes, for example, GaCl3 THF,24 of GaCl3
3
with this ligand, which are resistant against further reduction.
Furthermore, ether cleavages of THF forming GaꢀO bonds or
the formation of lower valent Ga species, for example, Ga2Cl4-
(THF)2,25 being themselves strong reducing agents are possible
additional reactions. Evidence for the responsibility of the electron
donor activity of THF as cause for the difficulty of the reduction
can be collected by exchanging THF for dioctylether. In this more
unpolar solvent the reduction of GaCl3 by LiBEtH3 takes place
immediately. The yield is about 30% but this might be due to
residual THF in the mixture (see Experimental section). Although
dioctylether seems to lead to higher yields of Ga, such fast
reductions do not seem to be suitable for the preparation of
nanoparticulate intermetallic Ga compounds since the low melting
point of Ga causes a strong coagulation of the liquid metal under
these conditions. In addition, Pd(acac)2 being less soluble in
dioctylether is the reason why the optimized protocols were
performed using THF as solvent.
From these experiments follows that the reduction of Ga3þ
occurs in THF only in the presence of Pd. The following scenario
is suggested: During the coreduction of Pd2þ and Ga3þ, Pd2þ
will be reduced immediately and then mediates the reduction of
Ga3þ. The role of Pd can be understood by considering its ability
to activate hydrogen by forming very active Pd hydrides,26 which
in turn reduce the Ga3þ to elemental Ga. In addition, this
explains the homogeneity of the products as well as why
elemental Ga is never observed in the coreduction, since Ga
would only be reduced on the surface of the Pd particles and does
not form particles of its own.
2PdðacacÞ2 þ 2GaCl3 þ 10LiHBEt3 f 2GaPd
þ 4LiðacacÞ þ 6LiCl þ 10BEt3 þ 5H2
To gain a deeper understanding of the processes during the
coreduction, we fragmented this process into simpler reaction
systems and investigated them separately.
Even if the reduction of Pd2þ to the elemental state by
Superhydride should be straightforward, the details of this
reaction will elucidate important findings for the complete
synthesis. Superhydride (LiHBEt3 in THF) is a strong reducing
agent, powerful enough to reduce even such hard-to-reduce
metal ions in metal halogenides like ZnCl2 to result in finely
dispersed metal powders.22 Besides being a solvent, THF acts as
stabilizer and inhibits the growth of the particles. As expected, the
To summarize these results: A controlled reduction of Ga3þ to
a nanoparticulate Ga compound in THF is hindered by the high
stability of the immediately formed Ga3þ complexes. Using an
unpolar solvent diminishes the formation of complexes but
causes a fast uncontrolled reduction followed by coagulation.
The presence of elemental Pd mediates the reduction of Ga3þ in
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Figure 1. X-ray diffraction patterns of the products of the coreduction
for (a) GaPd2 and (b) GaPd after the first reaction step. Calculated
patterns are shown in the lower parts of the panels.
Figure 3. Temperature dependent X-ray diffraction patterns of particles
obtained after the coreduction step during the synthesis of GaPd2. The
formation of the intermetallic compound GaPd2 starts at around 150 °C.
The fwhm of the diffraction lines decreases with increasing temperature
due to sintering of the particles.
Figure 1 shows the X-ray powder diffraction data of the
products after the first step of the synthesis. The product after
the coreduction again depends on the initial reduction tempera-
ture. For GaPd2 only nanocrystalline Pd can be observed by
X-ray diffraction. In the case u GaPd, a mixture of GaPd2 and
GaPd is present. Elemental Ga, liquid due to the low melting
point and small size, was not detected by XRD in either case. HR-
TEM images (Figure 2) of the particles after the coreduc-
tion show small crystalline particles of Pd surrounded by a
2 nm thin layer which most probably consists of Ga, corroborat-
ing the suggested formation mechanism. EDX measurements
on the particles resulted in a composition of Pd48(1)Ga52(1) and
Pd61(1)Ga39(1) for GaPd and GaPd2, respectively.
During the synthesis, special care has to be taken with the
drying procedure after the first and the second step. Drying the
particles means to remove the stabilizer THF and to cause
some unavoidable agglomeration. If the THF is completely
removed by applying harsh conditions (e.g., <100 mbar at
ambient temperature), the obtained particles will start to sinter
spontaneously, leading to a strong energy release, that is, the
sample is heated to 300ꢀ400 °C. This process was also observed
under inert conditions within the glovebox, since the driving
force is the minimization of surface energy and not a chemical
reaction.
The Second Step—Annealing. As in the case of the CuꢀPd
nanoparticles,19 a treatment at elevated temperatures is necessary
to obtain the desired materials. In the present study, annealing
in high-boiling dioctylether at 185 °C for 4 h proved to be a
suitable procedure to obtain single-phase products. To elucidate
the processes in the second step of the synthesis, a powder
sample for the preparation of GaPd2 was subjected to tempera-
ture dependent X-ray diffraction in Ar atmosphere after the first
step. The formation of the intermetallic compound by diffusion
of gallium starts around 150 °C (Figure 3). The fwhm of the
diffraction lines decreases with increasing temperature due to
sintering of the particles. As calculated with the Scherrer
equation,27 the crystallite size of GaPd2 is 10 nm at 150 °C,
12 nm at 200 °C and 25 nm at 400 °C.
Figure 2. Aberration-corrected HR-TEM of nanoparticles after the
coreduction step during preparation of (a) GaPd2 and (b) GaPd.
Crystalline particles are surrounded by an additional layer consisting
of elemental Ga according to the suggested formation mechanism.
THF by forming active Pd hydrides. From this follows that Ga is
formed exclusively on the surface of Pd, thus inhibiting the
uncontrolled coagulation (Scheme 1). It also becomes clear why
the reaction conditions at the beginning of the reaction, when Pd
is formed, are critical for the complex synthesis and determine
the products obtained.
After the coreduction, which is the first step in the synthesis,
no single-phase intermetallic compounds could be obtained. As
discussed in detail in the following, Ga does not (GaPd2
synthesis) or only partially (GaPd synthesis) diffuse from the
Pd surface into the Pd particles during the coreduction and the
formation of Pd or a mixture of GaPd2 and GaPd, respectively.
This, as well as the yield of Ga, is more dependent on the initial
reduction temperature rather than the Ga:Pd ratio used. Ele-
mental analysis of the solid phase after the coreduction shows
that only 40 mol-% of Ga3þ is reduced when GaPd2 should be
synthesized, where the initial reduction temperature is 68 °C
(reflux temperature); 75 mol-% of Ga are reduced in the case of
GaPd, where the initial temperature is 23 °C. Chemical analyses
proved in both cases that the missing Ga3þ fraction remained in
the liquid, which excludes the formation of any gaseous Ga-
species. Two reasons may be responsible for the partial reduction
of Ga3þ: First, it could be caused by the resulting Ga layer on the
Pd particles, which hinder the reformation of the Pd hydride.
Second, the incomplete reduction could be due to the formation
of highly stable Ga complexes (e.g., with acetylacetonate).
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Figure 6. Surface weighted size distribution of GaPd2 and GaPd nano-
particles. GaPd2 nanoparticles have a maximum size at 7 nm and GaPd at
3 nm. The distribution is sharper for GaPd2.
Figure 4. X-ray diffraction patterns of nanoparticulate (a) GaPd and
(b) GaPd2 obtained after the second step of the synthesis. Calculated
patterns of GaPd and GaPd2 are given for comparison.
Figure 7. TEM images of (a) GaPd2@Al2O3 and (b) GaPd@Al2O3.
The particles are deposited in form of small agglomerates.
Figure 5. TEM images of the nanoparticles after the annealing step: (a)
GaPd2 agglomerate, with primary particles of 3ꢀ7 nm; (b) GaPd nano-
particles with diameters of 1ꢀ5 nm.
XRD results. Considering the absence of stabilizers, the particle
size distribution is surprisingly sharp in both cases.
Depositing the Nanoparticles. To further enhance the acti-
vity of the catalysts, the particles can be deposited on R-Al2O3.
In addition, a comparison of the catalytic properties of unsup-
ported and deposited particles allows directly observing influences
of the support on the catalytic properties. Since the nanoparticles
can be suspended during the first synthesis step (coreduction) and
before drying, an easy way to deposit the particles on R-Al2O3 is by
evaporation of the THF. For this purpose, it is sufficient to wet the
support with THF and add a suspension of the nanoparticles. By
shaking the pellets during the evaporation of THF, larger agglom-
erates are broken up, allowing a homogeneous distribution of the
particles on the support. The annealing step of the synthesis can be
performed by adding dioctylether after evaporation of the THF
and treating the deposited particles at 250 °C.
Figure 7 shows TEM images of the deposited particles
after reduction and annealing. The intermetallic compounds are
deposited as small agglomerates on the Al2O3. The Pd content of
the materials as determined by chemical analyses is 55 ppm and
123 ppm for GaPd2@Al2O3 and GaPd@Al2O3, respectively.
Catalytic Performance of the Nanoparticles. Figures 8 and
9 show the conversion of acetylene during the reaction over
unsupported and deposited nanoparticles of GaPd2 and GaPd in
comparison with ground bulk material and a commercial 5 wt.-%
Pd/Al2O3. Pd shows a strong deactivation (more than 50% loss)
within 20 h, which is typical for this catalyst. This is caused by the
Even if the annealing after the coreduction is performed in
suspension, the resulting nanoparticles show a strong agglom-
eration because the product was dried beforehand and no stable
suspensions are formed. The reflections of GaPd in the powder
pattern obtained after annealing (see Figure 4) are sharper in
comparison with the reflections of GaPd2. A crystallite size of
11 nm was calculated for GaPd and 7 nm for GaPd2, which is
smaller than detected during the temperature dependent XRD,
probably due to the stabilizing influence of the solvent and the
lower temperature. Figure 5 shows TEM images of the dried
particles of GaPd and GaPd2. They are nearly spherical and have
diameters of around 3ꢀ7 nm. Nearly all particles form large
agglomerates, which is hardly avoidable because of the absence of
additional stabilizers. The surface weighted size distribution
relevant for the catalytic properties of the GaPd and GaPd2
nanoparticles was determined by fractional sedimentation in the
disk centrifuge after dispersing the primary particles of the
agglomerates (Figure 6). The majority of the GaPd nanoparticles
possess diameters around 3 nm, but particles with somewhatlarger
diameters are also present. The discrepancy between the crystallite
size of 11 nm and the most abundant particle size of 3 nm can be
explained by the small contribution of crystallites <5 nm to the
XRD signals. In the case of GaPd2 nanoparticles, the maximum in
the size distribution was obtained at 7 nm in accordance with the
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Figure 8. Comparison of (a) conversion and (b) selectivity to ethylene
of bulk GaPd2 (10 mg), unsupported nanoparticulate GaPd2 (0.1 mg)
and nanoparticulate GaPd2@Al2O3 (100 mg). In contrast to a com-
mercial 5% Pd/Al2O3 (0.15 mg), the intermetallic compounds exhibit a
high long-time stability and excellent selectivities for the semihydro-
genation of acetylene.
Figure 9. Catalytic properties, (a) conversion and (b) selectivity to
ethylene, of bulk GaPd (400 mg), unsupported nanoparticulate GaPd
(2.5 mg) and nanoparticulate GaPd@Al2O3 (75 mg) in comparison to
5% Pd/Al2O3. While the stability of all intermetallic compounds is
similar, the selectivity of supported GaPd nanoparticles even exceeds the
high selectivity of bulk GaPd.
low selectivity of Pd leading not only to ethylene or ethane but
also to carbon and various oligomers which quickly deposit on
the Pd surface.3 The ground bulk-materials of GaPd and GaPd2
show no deactivation during 20 h.26 The reason for the high
stability is the high selectivity, which avoids the deposition of
carbon or oligomers by the site-isolated Pd atoms and by the high
in situ stability of the compounds.4,5
The nanoparticulate materials show only a slight deactivation
during the reaction, which is most likely caused by sintering of the
particles. GaPd2 shows a slight deactivation during the first 5 h and
the selectivity is about 13% lower compared with the bulk material.
Nanoparticulate GaPd also deactivates with time and reaches the
selectivity of bulk GaPd. The mass of unsupported intermetallic
material necessary to reach >90% conversion is significantly reduced
by the nanostructuring. Nanoparticulate GaPd2 shows a 90 times
higher activity as compared to the ground bulk material, while the
activity of nanoparticulate GaPd is enhanced by a factor of more
than 180. Table 1 summarizes the activity of the different catalysts.
By depositing the nanoparticles on the support, the activity could
be significantly further enhanced, which is expected since the
particles are finely dispersed on the support, reducing agglomera-
tion and resulting in a higher accessible active surface area.
The situation is somewhat different when the activity per
surface area for the different materials is compared. Here, the
activity of the nanoparticulate GaPd materials is about 1 order of
magnitude higher compared to bulk-GaPd, which most likely can
be assigned to the higher surface curvature of the nanoparticles.
In line with this is the slightly stronger deactivation with time of
the nanoparticulate GaPd as well as the increase of the selectivity,
which corresponds to a structural healing of the material.
Deposition of nanoparticulate GaPd on Al2O3 results in higher
stability and selectivity, which could be a consequence of the
higher annealing temperature of 250 °C, leading to a more
ordered surface. Bulk-GaPd2, on the other hand, shows a specific
activity that is about 1 order of magnitude higher than for the
nanostructured materials. This counterintuitive phenomenon
can either be caused by structural or electronic variations, and
further investigations are in progress.
In summary, we could show that the selectivity and stability of
the intermetallic compounds in a nanoparticulate state—also in
presence of a support—are only slightly affected in comparison
to the ground bulk materials, but the specific activity per mass is
significantly enhanced. An increase by a factor 90 and more than
180 for GaPd2 and GaPd, respectively, could be obtained for the
unsupported particles. Deposition of the particles results in an
activity increase by a factor of nearly 35 000 for GaPd and 1 300
in the case of GaPd2 compared to the ground bulk material.
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Journal of the American Chemical Society
ARTICLE
Table 1. Catalytic Activities of GaꢀPd Catalysts in Comparison to 5% Pd/Al2O3
catalyst
nPd/mmol
conversion/%
activity/molacetylene (molPd h)ꢀ1
activity/molacetylene (ametal h)ꢀ1
3
3
bulk-GaPd
2.27
65.5
78.1
83.9
94.7
86.5
87.6
44.9
1.21 ꢁ 10ꢀ1
2.31 ꢁ 101
4.14 ꢁ 103
5.44
5.03 ꢁ 102
7.12 ꢁ 103
2.45 ꢁ 103
8.49 ꢁ 10ꢀ3
6.31 ꢁ 10ꢀ2
7.26 ꢁ 10ꢀ2
1.93
2.89 ꢁ 10ꢀ1
2.99 ꢁ 10ꢀ1
2.06 ꢁ 10ꢀ1
nano-GaPd
1.42 ꢁ 10ꢀ2
8.67 ꢁ 10ꢀ5
7.08 ꢁ 10ꢀ2
7.08 ꢁ 10ꢀ4
5.17 ꢁ 10ꢀ5
7.05 ꢁ 10ꢀ5
nano-GaPd@Al2O3
bulk-GaPd2
nano-GaPd2
nano-GaPd2@Al2O3
5% Pd/Al2O3
’ CONCLUSION
Wrackmeyer, Yu. Grin as well as R. Schl€ogl on the underlying
processes proved very valuable.
In this work, the first synthetic route to single-phase nanoparti-
culate GaPd2 and GaPd is developed. Two steps are necessary: the
first step includes the coreduction of the ionic precursors by
Superhydride in THF. Annealing the particles in dioctylether at
185 °C in a second step leads to a complete diffusion of Ga into Pd
and gives the desired intermetallic compounds. The resulting
agglomerated spherical particles have a relatively small size dis-
tribution with a maximum of 3 nm for GaPd and 7 nm for GaPd2.
The reaction parameters are crucial for the success of the reaction,
and the mechanism involved is not as simple as might be assumed.
The reduction of Ga3þ is not quantitative and is mediated by
elemental Pd during the first step when THF is used as solvent.
Stable complexes due to the high Lewis-acidity inhibit the reduction
of Ga3þ by LiHBEt3 in absence of Pd in THF. The formation of
elemental Ga at the surface of Pd seems to be an important
requirement for the success of the reaction and allows the formation
of intermetallic nanoparticles with homogeneous composition.
The catalytic properties are nearly maintained when compared
to the ground bulk material. Nanoparticulate GaPd and GaPd2
show a high selectivity (77 and 60%) and a good long-time
stability for the semihydrogenation of acetylene. The overall
activity in comparison with ground bulk material is enhanced by a
factor of 90 in the case of GaPd2 and more than 180 in the case of
GaPd. The nanoparticles could be successfully deposited on
Al2O3, leading to slightly higher selectivities and to much higher
activities, which are enhanced by a factor of 1 300 in the case of
GaPd2 and 35 000 in the case of GaPd. On the basis of the total
Pd content, the materials possess activities comparable to a com-
mercial Pd/Al2O3 catalyst, while outperforming its selectivity.
This work presents a successful strategy for the development
of a novel catalyst: Starting from a model catalyst system using a
knowledge-based approach and taking into account fundamental
aspects, it was possible to gradually create a high performance
catalyst. This alternative way of catalyst development demon-
strates the superiority and relevance of the intermetallic com-
pounds in selective hydrogenation catalysis as well as their
potential for industrial applications.
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’ AUTHOR INFORMATION
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Corresponding Author
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(28) Kovnir, K.; Armbr€uster, M.; Teschner, D.; Venkov, T. V.;
Szentmiklꢀosi, L.; Jentoft, F. C.; Knop-Gericke, A.; Grin, Yu.; Schl€ogl,
R. Surf. Sci. 2009, 603, 1784.
’ ACKNOWLEDGMENT
We thank H. Lichte and F. R€oder from the Triebenberg
Laboratory for TEM measurement time. Discussions with B.
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dx.doi.org/10.1021/ja202869d |J. Am. Chem. Soc. 2011, 133, 9112–9118