Discrete Polyoxopalladates as Molecular Precursors for Supported Palladium Metal Nanoparticles as Hydrogenation Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.8b03513

We have used discrete polyoxopalladates(II) (POPs) of the MPd₁₂X₈ nanocube- and Pd₁₅X₁₀ nanostar-types (M = central metal ion, X = capping group) as molecular precursors (diameter ca. 1 nm) for the formation of supported (SBA-15) metallic nanoparticles. These materials proved to be highly active in the hydrogenation of o-xylene. The characterization of such hydrogenation catalysts revealed that the average size of the resulting alloy particles is quite uniform with diameters ranging from 1 to 3 nm (indicating little to no agglomeration). The central transition-metal ion Mⁿ⁺ (Mn^II, Fe^III, Co^II, Ni^II, Cu^II, Zn^II, Pd^II) in the POP structure and also the nature of the capping group (AsO₄^3-, SeO₃^2-, PO₄^3-, phenyl-AsO₃^2-) influence the resulting catalytic performance.

Full text of DOI:10.1021/acs.inorgchem.8b03513

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Discrete Polyoxopalladates as Molecular Precursors for Supported
Palladium Metal Nanoparticles as Hydrogenation Catalysts
†,∥
Wassim W. Ayass,† Juan F. Minambres, Peng Yang,†,⊥ Tian Ma,† Zhengguo Lin,†,△
̃
Randall Meyer,§ Helge Jaensch,‡ Anton-Jan Bons,‡ and Ulrich Kortz*,†
†Department of Life Sciences and Chemistry, Jacobs University, Campus Ring 1, 28759 Bremen, Germany
§Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, New Jersey 08801, United States
‡Global Chemical Research, ExxonMobil Chemical Europe Inc., 1831 Machelen, Belgium
S
* Supporting Information
ABSTRACT: We have used discrete polyoxopalladates(II) (POPs) of the
MPd12X8 nanocube- and Pd15X10 nanostar-types (M = central metal ion, X =
capping group) as molecular precursors (diameter ca. 1 nm) for the formation
of supported (SBA-15) metallic nanoparticles. These materials proved to be
highly active in the hydrogenation of o-xylene. The characterization of such
hydrogenation catalysts revealed that the average size of the resulting alloy
particles is quite uniform with diameters ranging from 1 to 3 nm (indicating
little to no agglomeration). The central transition-metal ion Mn+ (MnII, FeIII,
CoII, NiII, CuII, ZnII, PdII) in the POP structure and also the nature of the
capping group (AsO43−, SeO32−, PO43−, phenyl-AsO32−) influence the resulting
catalytic performance.
10 years, >70 derivatives have been reported. This young field
of research has been reviewed twice in recent years.4 In the
very first reported POP, [PdII13O8(AsO4)8H6]8− (Pd13As8),
the 13 PdII ions exhibit square-planar coordination geometry
and form a distorted cuboid structure with eight external
AsO43− capping groups.3 In 2009, it was demonstrated that the
arsenate capping groups in Pd13As8 can be replaced by selenite
or phenylarsonate, resulting in [PdII13O8(O3SeIV)8]6−
(Pd13Se8, with 6-coordinated central PdII ion) and
[PdII13O8(O3AsPh)8]6− (Pd13(AsPh)8, with 8-coordinated
central PdII ion), respectively.5 In the following years, a
procedure was developed allowing to replace the central PdII
ion in the Pd13L8 (L = capping group) nanocube by lanthanide
ions,6a and also by transition-metal ions,6b−e and eventually
even by alkaline earths metal ions.6f In 2010, the first
polyoxoaurate(III) was reported,6g and in 2012 the first
mixed palladium-gold polyoxo-noble-metalate.6h In 2016, the
AgI-capped palladate cube was reported.6i This shows that to
date the class of cuboid POPs is large, with different capping
groups, central metal ion guests, and d8 metal addenda.
1. INTRODUCTION
Key challenges in designing the “perfect” heterogeneous metal
catalyst for a specific reaction include metal utilization
(dispersion) and control of selectivity through tuning of
both the physical and electronic structures of metal clusters
and support. Therefore, there is much scientific interest in
investigating the effect of composition and cluster size on the
activity and selectivity of metal catalysts.1a,b The reactivity of a
metal catalyst can sometimes be changed by varying only one
single atom. As an example, this was seen for icosahedral Au
and Pt clusters with dopants where the conversion of styrene
oxidation increased from 59 to 91% by just replacing one gold
atom with platinum.1c Another important factor is the exact
metal surface structure, which has motivated researchers to
investigate varying/developing catalyst synthetic methods, thus
influencing the structure of the catalyst to form core−shell
structures, homogeneously alloyed structures (solid solutions),
porous structures, or other unique structures which may give
rise to specific reactivity.2 Finally, the post-synthetic treatment
(activation) will depend on the nature and composition of the
materials used and will further significantly influence the
catalytic performance. From this knowledge of design
principles, it can be expected that the ability to broadly vary
polyoxometalate (POM) composition and to prepare
supported metal clusters with well-defined stoichiometry and
size may allow to prepare closely related systems and to study
their activity and/or selectivity.
The first POP with a structure different from the nanocube
w a s t h e p h o s p h a t e - c a p p e d P d 1 5 n a n o s t a r ,
[Pd0.4Na0.6⊂Pd15O40(PO)10H6.6]12− (Pd15.4P10), which is a
co-crystallized, nonseparable mixture of [NaPd15P10O50]19−
(Pd15P10) and [PdPd15P10O50]18− (Pd16P10) in a ratio of
3:2.7a The selenite-capped analogue [Pd15Se10O40]10−
POMs comprising exclusively palladium(II) addenda (poly-
Received: December 17, 2018
oxopalladates, POPs) were discovered in 2008,3 and in the last
© XXXX American Chemical Society
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k2, and k3-weighted Fourier transform for the fits. See Supporting
Information for more information.
(Pd15Se10) was later synthesized by Predieri’s and Hu’s groups,
respectively.6d,7b A barium(II)-centered POP nanostar6f as well
as a silver(I)-centered and capped POP nanostar are also
known.6i
All of the above-mentioned discrete POPs could potentially
be considered as molecular precursors for the formation of
monodisperse noble-metal nanoparticle catalysts. Metal
catalysts displaying activity in the metallic state (e.g., transition
metals in groups VIIIB and IB) are useful for chemical
transformations such as hydrogenations, dehydrogenations,
and naphtha reforming.8 Heterogeneously catalyzed hydro-
genations are common in industry.
There are a few reports on using POPs as homogeneous
catalysts.3,6b However, until very recently, no heterogeneous
catalysts based on POPs have been reported.6j Hence, we
decided to investigate the immobilization of POPs on a
mesoporous support, followed by chemical reduction leading
to discrete metal nanoparticles. These prepared heterogeneous
catalysts were then used for the hydrogenation of organic
substrates.
2.4. Catalytic Hydrogenation. A model substrate should provide
useful information about the catalytic system in terms of reactivity,
stereochemistry, isomerization, possible transalkylation, cracking
propensity, etc. Therefore, o-xylene hydrogenation to cis/trans-1,2-
dimethylcyclohexane (1,2-DMCH) was selected as the reaction
model to investigate and rationalize the key features (shape, size,
composition) of POPs as noble-metal-based heterogeneous hydro-
genation precatalysts. The reaction was carried out in a 100 mL
stainless steel, high-pressure Parr Compact reactor equipped with a
magnetically coupled stirrer drive, ensuring a well-mixed environment
of reactants (Figure S2). The reaction mixture containing 3.5 mL of o-
xylene (0.029 mmol, 0.569 M) in 47.5 mL hexane and 50 mg of
activated supported POP (5 wt %) was stirred at 950 rpm at 180 °C.
The absence of diffusion limitations was confirmed by the fact that a
similar reaction rate was observed when stirring at 1500 rpm at 180
°C. The same reaction at 300 °C was found to be diffusion-controlled
(Figure S3). The autoclave was purged with H2 and then heated and
pressurized to the desired temperature set point (180 or 300 °C) and
pressure (80 or 90 bar), respectively. After calcination, the precatalyst
was reduced in situ in the reactor. For the reaction at 180 °C or (300
°C), 50 bar of H2 was introduced in the reactor at 140 °C or (250
°C), and then the mixture was stirred for 1 min to reduce the catalyst.
This allows for a temperature increase, since this process is
exothermic. Once the temperature set point is reached, the needed
pressure is introduced, and the reaction starts once the stirring is
turned on. Milder reaction conditions (i.e., stoichiometric amounts of
H2 around 40 bar) resulting in long reaction times could have been
used,9 but a higher pressure was preferred in order to push the
2. EXPERIMENTAL SECTION
2.1. Materials. All reagents and chemicals were of high-purity
grade and were used as purchased from Sigma-Aldrich without further
purification. The commercial palladium 5 wt % (dry basis) on wet
activated carbon support Pd/C was used as is (product number:
330116).
̂
reaction forward according to Le Chatelier’s principle. In order to
2.2. Synthesis of Supported POP Precatalysts on SBA-15-
apts. All POPs were prepared following published procedures (see
Supporting Information). The respective POP was dissolved in 100
mL of water, resulting in a colored solution. While stirring, the
mesoporous support (SBA-15) modified by 3-aminopropyltriethox-
ysilane (apts) was slowly added to the POP solution so that the
respective amounts were 95 and 5 wt %. The pH of the POP solution
was ca. 6, which is sufficient to protonate the amino group in SBA-15-
apts (see Figure S1). The exact quantities depend on the desired
amount of supported catalyst to be prepared. For example, to a
solution of 25 mg of POP dissolved in 100 mL water, 475 mg of SBA-
15-apts was added to yield ca. 500 mg of POP@SBA-15-apts. The
mixture was stirred for 24 h at 40 °C and then filtered and washed
three times with water. The filtrate was colorless, indicating that the
POP was quantitatively loaded onto the SBA-15-apts support. The
product was air-dried and then collected (yield: 95−100%). Elemental
analysis confirmed the expected percentage of POP loading. See
Supporting Information for details regarding the synthesis procedures
used for SBA-15-apts support and POPs.
2.3. Characterization. The surface area of the supported catalysts
was determined with a gas adsorption analyzer for surface area
measurements. Nitrogen physisorption isotherms were measured at
77 K using a Quantachrome AUTOSORB 1 apparatus, and this was
used to determine the Brunauer−Emmett−Teller (BET) surface area.
The samples were degassed at 70 °C for 24 h. The specific surface
areas were evaluated with the BET method in the P/P0 range of 0.05−
0.35. Elemental analyses of the prepared supported POPs were
performed at ExxonMobil Chemical Europe Inc., Belgium. A thermal
analysis instrument SDT Q600 was used to determine the thermal
stability of the materials under N2 flow (100 mL/min) up to 800 °C.
Transmission electron microscopy (TEM) images were collected on
an FEI Tecnai G2 F20ST TEM/STEM operated at 200 kV. Images
were obtained in scanning (STEM) mode using a high-angle annular
dark-field (HAADF) detector. Energy dispersive X-ray spectroscopy
(TEM-EDS) spectra and hyperspectral element maps were acquired
using an EDAX Inc. detector. X-ray absorption spectroscopy (XAS)
measurements were performed at Argonne National Laboratory at the
20 BM and 9 BM beamlines. In all experiments, a metal foil was used
for calibration. The Demeter program was used to fit the data to
obtain coordination numbers and bond distances using a combined k,
ensure catalyst recyclability, a new portion of substrate (3.5 mL) was
added into the reactor following all catalytic runs (i.e., running more
than one cycle). The blank reaction on the SBA-15-apts support alone
at both temperatures did not show any hydrogenation activity. The
reaction performed in the compact reactor was followed by H2
consumption and gas chromatography (GC) analysis (Figures S4−
S5). A Shimadzu GC-2010 equipped with a flame ionization detector
(FID) was used to measure substrate conversion and selectivity of the
obtained products via a HP-5 column (15 m × 0.25 mm, I.D. 0.25
μm), providing a good separation of the reaction products. The
carrier gas was He. This overall procedure ensured good
reproducibility of the catalytic experiments. Hot filtration experiments
indicated that the reaction is heterogeneous (Figure S6).10
3. RESULTS AND DISCUSSION
The structures of the POP nanocubes Pd13As8 and MPd12Se8
as well as the nanostar Pd15P10 are shown in Figure 1. In the
course of this work, we first synthesized several POP
nanocubes with various capping groups and central metal ion
guests, MPd12X8 (M = MnII, FeIII, CoII, NiII, CuII, ZnII, PdII; X
= Se, P, As, AsPh) as well as the two POP nanostars Pd15X10
(X = Se, P), and also SBA-15-apts, in all cases following
published procedures (for details see Experimental Section and
Figure 1. Ball-and-stick representation of the POP nanocubes
Pd13As8 (left) and MPd12Se8 (middle) and the nanostar Pd15P10
(right). Color code: Pd (blue), As (light green), O (red), central
metal guest M (dark green), Se (orange), and P (pink).
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Supporting Information). Then we immobilized the POPs on
the support (at ca. 5 wt %) and tested their activity as catalysts
for the hydrogenation of o-xylene. We discovered that
appropriate pretreatments were extremely important (see
Supporting Information) and also that the nature and
composition of the capping groups as well as the central
metal ion guest are essential. We used extended X-ray
absorption fine structure (EXAFS) and X-ray absorption near
edge structure (XANES) spectroscopic techniques to inves-
tigate the structure−activity relationships for this class of
catalysts and to characterize the resulting Pd metallic clusters.
3.1. Immobilization of POPs on SBA-15-apts. Ele-
mental analysis of the supported POPs on SBA-15-apts
in situ at 250 °C (similar to the reaction scenario at 300 °C),
followed by a reaction temperature of 180 °C. Reduction of
the central metal atom (Cu, Ni) was somewhat retarded
compared to reduction of Pd (particularly in the case of Ni, see
Figure S8). Reducing the catalyst at a higher temperature
improved the reaction time at 180 °C by 26%, hinting at the
formation of a more active catalyst structure at a higher
reduction temperature, thus also explaining the higher
reactivity at 300 °C. The same experiment was also performed
for CuPd12Se8 and an improvement of 31% was observed
(Figure S9). X-ray absorption spectroscopy (XAS) was used to
characterize the structure of such more active catalysts (vide
infra). In order to find out if the chemical nature of the
capping group on the POP has an influence on the resulting
catalytic activity, we decided to study this phenomenon in
some detail.
confirmed a loading of 4.9
1 wt %. Elemental analysis
calculated (found): Pd, 2.27 (2.23 wt %). The as-synthesized
materials showed no catalytic activity, and we revealed that a
thermal pretreatment or a calcination step was required before
observing any catalytic activity.
3.2. Catalytic Activity of Supported MPd12Se8 Nano-
cubes (M = MnII, FeIII, CoII, NiII, CuII, ZnII, PdII). Initially, we
decided to investigate the catalytic activity (o-xylene hydro-
genation) of the selenite-capped nanocubes MPd12Se8 (M =
MnII, FeIII, CoII, NiII, CuII, ZnII, and PdII) on the support (5 wt
%), abbreviated as MPd12Se8-SBA-15-apts. All precatalysts
were air-calcined at 550 °C for 4.5 h prior to their in situ
reduction (XANES data verifying the reduction is shown in
Figure S7). Figure 2 shows the catalytic results at two different
3.3. Role of Capping Groups for Supported MPd12X8
Nanocubes. 3.3.1. Pd13X8 and MnPd12X8. The ease of
removal of capping groups and the effect of the remaining
capping group elements on the catalytic activity were
investigated for the Pd13X8 and MnPd12X8 POP nanocubes.
Table 1 shows the optimal catalytic activities achieved using
Table 1. o-Xylene Hydrogenation Results at 300 °C, 90 bar,
1500 rpm Using Activated POP Nanocube Catalysts
(Loading 5 wt %) Pd13X8 (X = Se, PhAs, As) and MnPd12X8
(X = Se, PhAs, P)
a
supported catalyst treatment method conv. (%) time (min)
Sc/t
Pd13(PhAs)8
Pd13As8
1
2
36
97
∼100
∼100
63
98
∼100
∼100
2700
1072
960
50
1000
32
40/60
38/62
40/60
43/57
40/60
42/58
43/57
43/57
b
3 + 2
Pd13Se8
MnPd12(PhAs)8
MnPd12Se8
1
3 + 1
1
4
1
32
38
MnPd12P8
a
Method 1: Air calcination at 550 °C, 4.5 h. Method 2: Air
calcination at 650 °C, 4.5 h. Method 3: Hydrazine chemical reduction
(see Supporting Information). Method 4: Air calcination at 300 °C, 3
b
h. Error bar 2 min. This reaction was performed at 120 bar.
Figure 2. Reaction times for o-xylene hydrogenation at 180 °C, 80 bar
(error bar 20 min) and 300 °C, 90 bar (error bar 2 min) using
MPd12Se8-SBA-15-apts (5 wt %) calcined at 550 °C for 4.5 h. The
average cis/trans selectivity ratio (Sc/t) of 1,2-DMCH is 45/55 at 180
°C and 43/57 at 300 °C (see also Table S1, which summarizes
measured conversions and cis/trans selectivity ratios).
these POPs with arsenate, phenyl-arsonate, and phosphate
capping groups activated under different conditions. The
selenite- and phosphate-capped POPs showed the fastest
reaction rates and exhibited even the same rate for MnPd12X8.
Replacing the central PdII ion by MnII in the case of Pd13Se8
resulted in an almost 50% improvement of activity (reduction
of reaction time) at 300 °C, which highlights the important
role of the 3d central metal ion guest. The phenyl-arsonate-
containing Pd13(AsPh)8 and MnPd12(AsPh)8 showed a slow
reaction rate compared to the selenite/phosphate-capped POP
analogues, with MnPd12(PhAs)8 (63% conversion after 1000
min) performing better than Pd13(PhAs)8 (36% conversion
after 2700 min). The arsenate-capped Pd13As8 achieved
complete conversion, but only after ∼1000 min.
reaction temperatures, indicating the time to completion of the
reaction (shorter time = higher activity). A higher catalytic
efficiency at 180 °C was observed for Ni, Mn, Fe, and Pd, as
compared to Co, Cu, and Zn, with Ni resulting in the shortest
reaction time (highest activity). At 300 °C, the large reaction
time differences level out, leading to similar reaction times
between 25 and 33 min, with Pd and Fe being the exception
(∼50 min).
Interestingly, a pronounced effect of the central metal ion on
catalyst performance is present at 180 °C, unlike at 300 °C.
This behavior may be attributed to the fact that the reaction is
kinetically controlled at 180 °C, whereas at 300 °C it is
diffusion-controlled (Figure S3). In order to better understand
this phenomenon, the role of the pre-reduction temperature
was studied by reducing the NiPd12Se8 or CuPd12Se8 sample
It turns out that the selenite and phosphate capping groups
can essentially be eliminated completely (>99%), unlike
arsenate. Elemental analysis of MnPd12P8 calcined at 300 °C
showed a P content decrease by 50% (elemental analysis
found: P, 0.22 wt %). However, washing the sample further
with water showed no remaining P (<0.1%; elemental analysis
(found): P, 0.091 wt %), suggesting the presence of P as a
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water-soluble oxide after oxidative treatment. In contrast, for
Pd13As8, it is estimated based on XANES that around 15% of
As remained present after calcination at 650 °C for 4.5 h
(Figure 3). EXAFS on Pd13As8 showed a Pd coordination
4.5 h (Figure S11). Elemental analysis of the pure POP
calcined in air at 550 °C for 4.5 h showed that ∼7.7 equiv of
SeO2 out of 8 were eliminated (elemental analysis found: Se ∼
1 wt %). The XANES spectra of the Se K edge on the
supported sample NiPd12Se8 (5 wt %), see Figure 3, are in
agreement with the latter observation, as they exhibit a large
drop in the absolute magnitude after calcination, correspond-
ing to efficient removal of Se (<1% of Se remaining).
Table 2 shows the catalytic results of NiPd12Se8-SBA-15-
apts (5 wt %) calcined under these conditions (N2 vs air). An
Table 2. Catalytic Activity of NiPd12Se8-SBA-15-apts (5 wt
%) Treated under Different Conditions
treatment
a
BET surface
area (m2/g)
conv.
(%)
reaction time
reaction time
method
(min) at 180 °C (min) at 300 °C
as-made
1
2
288
378
415
0
∼100
∼100
−
−
57
25
555 20
310 20
2
2
a
Method 1: Heating under N2 at 550 °C, 4.5 h. Method 2: Air
calcination at 550 °C, 4.5 h.
increase of the BET surface area after calcination suggests an
increased accessible pore volume after calcination (the
multipoint BET plot is shown in Figure S12). The air-
calcination method at 550 °C (also used in the previous
section) was more efficient than heating under N2, as shown by
the higher reaction rates at both reaction temperatures. This is
most likely due to the more efficient removal of Se under air,
which seems to be essential for achieving high activity.
The average cis/trans selectivity ratio (Sc/t) of 1,2-DMCH
was 45/55 at 180 °C and 40/60 at 300 °C. As observed by
others in the past for Pd-based catalysts,11 our system shows a
marked kinetic effect in approaching the expected thermody-
namic cis/trans ratio for 1,2-dimethylcyclohexane. At low
conversion, the less stable cis isomer dominates the product
mixture, but as the reaction proceeds to completion, the more
stable trans isomer becomes dominant. The approach to
thermodynamic equilibrium continues, albeit at a lower rate,
after full conversion is attained (see Figure S13 and Table S1).
3.4. Role of Capping Groups for Supported Pd15.4P10
and Pd15Se10 Nanostars. Further investigations of the role of
Se vs P capping groups led to similar trends when comparing
Figure 3. (Top) XANES spectra of the Se K edge on supported
NiPd12Se8 (5 wt %) sample (in blue) calcined at 550 °C for 4.5 h,
showing that no Se is present (<1%) after calcination. (Bottom)
XANES spectra of the As K edge on supported {Pd13As8} (5 wt %)
sample (in blue) calcined at 650 °C for 4.5 h, showing that around
15% of As remains after calcination.
number CN(Pd−Pd) of 8 (an example fit is shown in Figure S10)
with a diameter d of about 1.3 nm (the {Pd13As8} nanocube
has d ca. 1 nm), indicating a smaller particle size than for the
selenite or phosphate-capped POPs (vide infra). These
findings point out (i) the advantageous inherent nature of
phosphate-capped POPs over the selenite-capped POPs due to
the lower temperature needed for capping group removal (300
°C for P vs 550 °C for Se), while keeping a comparable
activity, and (ii) the slower reaction rate resulting from the As-
capped POP due to insufficient removal of As under these
conditions. Additionally, the retardation of mobility by the
arsenate capping ligand may hint at the ability of the arsenate
system to resist sintering to a greater extent than either the
selenite or phosphate-based systems.
7a
the phosphate-capped Pd15.4P10 and the selenite-capped
6d
Pd15Se10 nanostars. Figure 4 shows a decrease in reaction
3.3.2. NiPd12Se8 Nanocube. For the nickel(II)-centered
POP nanocube with selenite caps NiPd12Se8, we discovered
that partial or complete removal of the selenite capping groups
was required to activate the catalyst. The optimum conditions
for this elimination was found by inspection of the
thermogravimetric analysis (TGA) curve of the pure POP
measured under N2. The latter indicated a loss of 5.6 equiv of
SeO2 after ramping the temperature to a plateau at 550 °C for
Figure 4. Reaction times for o-xylene hydrogenations at 300 °C, 90
bar for supported (5 wt %) nanostars Pd15.4
P10 and Pd15Se10 activated
at different calcination temperatures (see Table S1).
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time for Pd15Se10 when increasing the calcination temperature
from 300 to 550 °C, as more Se is removed with increasing
temperature. On the other hand, Pd15.4P10 showed already a
high activity (reaction time of 15 min) at a calcination
temperature of 300 °C. By increasing the calcination
temperature, the reaction time for Pd15.4P10 increases to
ultimately reach a similar reaction time within error bars of
Pd15Se10 for calcination at 550 °C. Such an increase in the
reaction time for Pd15.4
particle size with calcination temperature. EXAFS showed a
CN(Pd−Pd) of 8 and a d of about 1.6 nm for Pd15.4 10 (fit shown
P10 may be attributed to an increase in
P
in Figure S10). At a reaction temperature of 180 °C, the
performance of Pd15.4P10 calcined at 300 °C outperformed
Pd15Se10 (315 vs 450 min reaction time).
3.5. Characterization of the Formed Catalysts. XAS
revealed the formation of bimetallic alloy nanoparticles. This
alloying effect suggests that the catalytic activity may directly
depend on how the 3d metal M in the MPd12Se8 precursors
alloys to Pd. EXAFS spectra of the supported catalysts
CuPd12Se8 and NiPd12Se8 reduced at 300 °C were obtained
at both the Pd K edge and the K edge of the 3d metals (Cu or
Ni). From this data, the CNs of Pd−Pd, M−Pd and M−M
were extracted (no attempt was made to include Pd−M in the
fit of the Pd K edge). Based on the CN for Pd−Pd (∼9), both
clusters have a particle diameter d of about 2 nm (see
Supporting Information for the derived relationship between d
and CN). Figure 5 depicts the EXAFS for the 3d metal K edge
Fourier transformed spectra of both the Ni and Cu-containing
MPd12Se8 catalysts. In both cases, a large perturbation from
bulk behavior can be observed, clearly indicating that a
bimetallic alloy cluster is formed. Fits performed for these
spectra indicate the presence of M−Pd bonds, but cannot
conclusively identify M−M bonds (fits were of a lower quality
when M−M bonds were included due to the high number of
correlated parameters resulting in large uncertainties). This
result indicates that the metal clusters did not experience
significant sintering and that segregation of the heterometal
atoms (Cu, Ni) was limited, as it appears no M−M bonds were
formed. When looking at the EXAFS spectra, the nickel in
NiPd12Se8 has no preference for the surface or the bulk and
forms a random arrangement (CN(Pd−Pd) ≈ CN(Ni−Pd) ≈ 9).
On the other hand, the copper in CuPd12Se8 segregates to the
surface (CN(Cu−Pd) ≈ 7.4 < CN(Pd−Pd) ≈ 9.3) (Figure S14).
This is not surprising, given the relative surface energies of Ni,
Pd, and Cu.12
The TEM image of the as-prepared NiPd12Se8 supported on
SBA-15-apts shows particles of ca. 0.5−1 nm (Figure 6a),
which is compatible with individual or clustered POP units
(note, the size of a discrete POP nanocube NiPd12Se8 is ca. 1
nm). TEM-EDS spectra and element mapping confirm that
these nm-sized bright particles are rich in Pd and hence
derived from POP units (see Figures S15 and S16 for more
details). After calcination at 550 °C, the size of the clusters
increases to ca. 1−2 nm (Figure 6b), possibly due to fusion of
adjacent POPs. The TEM results are consistent with the
particle sizes estimated by EXAFS (see Table S2).
3.6. Catalytic Performance Assessment. The hydro-
genation activity of the 5 wt % supported NiPd12Se8 and
Pd15.4P10 catalysts was higher than that of the reference
material potassium tetrachloropalladate(II) K2[PdCl4] sup-
ported on SBA-15-apts (Table 3). K2[PdCl4] was chosen due
to the ease of attaching the [PdCl4]2− anion to the positively
charged modified SBA-15-apts following the same method-
Figure 5. (Top) Ni K edge Fourier transformed EXAFS spectra of
supported NiPd12Se8 catalyst (5 wt %) after reduction at 300 °C. A
Ni metal foil is shown for comparison. (Bottom) Cu K edge Fourier
transformed EXAFS spectra of supported CuPd12Se8 catalyst (5 wt
%) after reduction at 300 °C. A Cu metal foil is shown for
comparison.
ology and wt % amounts mentioned in Section 2.2.
Furthermore, the commercial standard Pd/C (5 wt % Pd
loading, 0.024 Pd mmol) was also compared to our POP
catalysts, showing a similar performance at 300 °C and a
slightly lower performance at 180 °C even at higher Pd loading
(0.024 Pd mmol instead of 0.01 Pd mmol); see Figures S17
and 18. Multiple cycles were also performed indicating that the
POP catalysts are recyclable (Figures S19 and S20). Both TEM
(Figure 6c,d) and EXAFS (Table S2) techniques show only
minor particle growth after the hydrogenation experiment.
4. CONCLUSIONS
In summary, we have used discrete polyoxopalladates (POPs)
as molecular precursors for the formation of supported metal
nanoparticles upon chemical reduction. Different POP
structure types (MPd12X8 nanocubes and Pd15X10 nanostars)
have been immobilized on apts-modified SBA-15 (POP-SBA-
15-apts), and then the catalytic activity of such formed
supported POP materials for the hydrogenation of o-xylene as
a test substrate has been investigated as a function of shape,
size, and composition of the POPs. We systematically varied
the central metal ion guest and the capping group of the POP
nancubes MPd12X8 and discovered that pretreatment was
extremely important in order to achieve catalytic activity.
Overall, the Se-capped nanocube family MPd12Se8 turned out
to be the most reactive, after calcination at 550 °C for 4.5 h. At
E
DOI: 10.1021/acs.inorgchem.8b03513
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cube NiPd12Se8 and the nanostar Pd15.4P10 were both highly
active for o-xylene hydrogenation and therefore were compared
to commercially supported palladium on mesoporous silica and
carbon, respectively. The POPs showed stability across the
reaction cycles accompanied by little particle growth as shown
by EXAFS and TEM. This work demonstrates for the first time
the bottom up approach using discrete POPs as molecular
precursors for supported noble-metal nanoparticles as highly
dispersed hydrogenation catalysts. After this initial proof-of-
principle study, we will now investigate POPs as precatalysts
for other substrates in terms of catalyst reactivity and
selectivity.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03513.
Synthetic procedures for SBA-15-apts support and
POPs, catalyst activation methods, details on X-ray
absorption spectroscopy, XANES and EXAFS spectra,
optimization of stirring speeds; hydrogen consumption
profiles, reaction times, conversions, and product
selectivity for the o-xylene hydrogenation reaction; gas
chromatogram, TGA, BET plot, TEM-EDS spectrum,
drift-corrected TEM-EDS element maps, and EXAFS
data. Figures S1−S20 and Tables S1−S2 (PDF)
Figure 6. HAADF-STEM images of NiPd12Se8 supported on SBA-15-
apts. The SBA-15-apts lattice can be seen as broad bands with a
spacing of ca. 10 nm. The POP or metal particles are visible as bright
dots: (a) as-prepared; (b) calcined at 550 °C; (c) after o-xylene
hydrogenation at 180 °C; and (d) after o-xylene hydrogenation at 300
°C.
AUTHOR INFORMATION
Corresponding Author
*E-mail: u.kortz@jacobs-university.de.
ORCID
■
Table 3. o-Xylene Hydrogenation at 300 °C and 90 bar
Using PdCl4-SBA-15-apts (0.01 Pd mmol) (K2PdCl4-1 and
K2PdCl4-2) Compared to NiPd12Se8-SBA-15-apts (0.01 Pd
a
mmol) Treated by Air Calcination at 550 °C for 4.5 h
Wassim W. Ayass: 0000-0003-2902-2142
Zhengguo Lin: 0000-0002-9628-6888
Randall Meyer: 0000-0002-0679-0029
Ulrich Kortz: 0000-0002-5472-3058
sample
reaction time (min)
conv. (%)
Sc/t
K2PdCl4-1
K2PdCl4-2
NiPd12Se8
Pd15.4P10
1000
170
25
70
≈100
≈100
≈100
40/60
40/60
40/60
40/60
Present Addresses
15
∥J.F.M.: Institut fu
̈
r Physikalische Chemie, TU Bergakademie
a
The sample K2PdCl4-1 was air calcined at 550 °C for 4.5 h, whereas
Freiberg, Leipziger Straße 29, 09599 Freiberg, Germany
⊥P.Y.: Smart Hybrid Materials Research Group (SHMs),
Advanced Membranes and Porous Materials Center
(AMPMC), King Abdullah University of Science and
Technology (KAUST), Thuwal 23955, Saudi Arabia
△Z.L.: Key Laboratory of Cluster Science, Ministry of
Education of China, School of Chemistry and Chemical
Engineering, Beijing Institute of Technology, 100081 Beijing,
P.R. China
the sample K2PdCl4-2 was chemically reduced (see Supporting
Information) followed by air calcination at 550 °C for 4.5 h. All
reactions were performed as described in the experimental procedure.
a 180 °C reaction temperature, the highest catalytic efficiency
for MPd12Se8 was observed for M = Ni, Mn, Fe, and Pd, as
compared to Co, Cu, and Zn, with the nickel-derivative
NiPd12Se8, resulting in the shortest reaction time (highest
activity). Overall, only the selenite- and phosphate-capped
palladate nanocubes MPd12Se8 and MPd12P8 could be
efficiently activated, in contrast to the arsenate- and phenyl-
arsonate-capped analogues MPd12As8 and MPd12(AsPh)8,
respectively. The inherent lability of the selenite and phosphate
capping groups presents an advantage, allowing for lower
activation temperatures. A similar trend was also found for the
POP nanostar Pd15X10, confirming the applicability of the
developed activation methods for Se vs P capping groups. The
catalyst materials were characterized by IR, XANES, EXAFS,
TEM, and N2 physisorption. XAS revealed an alloying effect,
suggesting that the catalytic activity may directly depend on
how the 3d metal alloys to Pd in the resulting mixed
transition/noble-metal(0) clusters. The supported POP nano-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
U.K. acknowledges support by ExxonMobil and Jacobs
University. W.W.A. thanks Deutscher Akademischer Aus-
tauschdienst (DAAD) for a doctoral fellowship, and P.Y.
acknowledges CSC for a doctoral fellowship. We also thank
̈
Talha Nisar, Jonas Kohling, and Prof. Veit Wagner (all Jacobs
University) for using a furnace, allowing heating under N2. We
also acknowledge Lana Yassine for designing the TOC graphic.
Figure 1 was generated using Diamond version 3.2 software
(copyright, Crystal Impact GbR).
F
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Article
M.; Graiff, C.; Elviri, L.; Predieri, G. Dalton Trans. 2010, 39, 4479−
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