Development of palladium surface-enriched heteronuclear Au-Pd nanoparticle dehalogenation catalysts in an ionic liquid

Article abstract of DOI:10.1002/chem.201203605

Heteronuclear Au-Pd nanoparticles were prepared and immobilized in the functionalized ionic liquid [C₂OHmim][NTf₂]. The structural and electronic properties of the nanoparticles were characterized by a range of techniques and the surface of the nanoparticles was found to be enriched in Pd. Moreover, the extent of Pd enrichment is easily controlled by varying the ratio of Au and Pd salts used in the synthesis. The heteronuclear nanoparticles were found to be effective catalysts in dehalogenation reactions with no activity observed for the pure Au nanoparticles and only limited activity for the pure Pd nanoparticles. The activity of the heteronuclear nanoparticles may be attributed to charge transfer from Pd to Au and consequently to more efficient reductive elimination. Copyright

Full text of DOI:10.1002/chem.201203605

FULL PAPER
DOI: 10.1002/chem.201203605
Development of Palladium Surface-Enriched Heteronuclear Au–Pd
Nanoparticle Dehalogenation Catalysts in an Ionic Liquid
Xiao Yuan,[a, b] Geng Sun,[a] Hiroyuki Asakura,[c] Tsunehiro Tanaka,[c] Xi Chen,[d]
Yuan Yuan,[b] Gabor Laurenczy,[b] Yuan Kou,[a] Paul J. Dyson,[b] and Ning Yan*[b, d]
Abstract: Heteronuclear Au–Pd nano-
particles were prepared and immobi-
lized in the functionalized ionic liquid
ment is easily controlled by varying the
ratio of Au and Pd salts used in the
synthesis. The heteronuclear nanoparti-
cles were found to be effective catalysts
in dehalogenation reactions with no ac-
tivity observed for the pure Au nano-
particles and only limited activity for
the pure Pd nanoparticles. The activity
of the heteronuclear nanoparticles may
be attributed to charge transfer from
Pd to Au and consequently to more ef-
ficient reductive elimination.
[C2OHmim]ACHTUNGTRENNUNG[NTf2]. The structural and
electronic properties of the nanoparti-
cles were characterized by a range of
techniques and the surface of the nano-
particles was found to be enriched in
Pd. Moreover, the extent of Pd enrich-
Keywords: dehalogenation · gold ·
ionic liquids · nanoparticles · palla-
dium
Introduction
unique physicochemical properties.[15] Surprisingly, investiga-
tions on bimetallic NPs in ILs are rather limited. The first
report in 2008 described the use of simultaneous sputter
deposition techniques to immobilize Au–Ag NPs alloys in
The first description of metal nanoparticles (NPs) prepared
and immobilized in an ionic liquid (IL) was reported in
2002.[1] This landmark paper was the forerunner to many ex-
citing discoveries concerning the synthesis and application
of NPs in ILs with more than one thousand publications
now in the domain.[2] IL-facilitated NP-formation and stabi-
lization and their application has become an important part
of nanoscience. To date, a predominant fraction of research
has focused on single metal (monometallic) NPs with many
examples based on Rh,[3] Ru,[4] Pd,[5] Pt,[6] Au,[7] Ag,[8] Ir,[9]
Cu,[10] Fe,[11] Co,[9e,11a,12] and Ni[4h,11a,13] and the synthesis of
Cr, Mo, W, and Os NPs in ILs were also reported.[14]
[C4mim]ACHTNUTRGENNG[U PF6] that exhibit a composition-sensitive surface
plasmon resonance.[16] Subsequently, further examples of
Au–Ag NPs,[17] and other bimetallic Au–Pd,[18] Au–Pt,[19] Cu–
Ru[20] and Fe–Ru[21] NPs, were reported, some of which were
used as catalysts[21–22] or biosensors.[19,23]
Amongst bimetallic NPs, those based on Au–Pd are par-
ticularly intriguing because some exhibit improved catalytic
activities and increased resistance to poisoning in certain re-
actions (note that most studies on Au–Pd NPs were not per-
formed in ILs).[24] Au–Pd NPs in pure, non-functionalized
ILs,[18,22a,d] and more recently Au–Pd NPs prepared by using
Despite the emphasis on monometallic NP systems, bi-
metallic NPs are equally, if not more, attractive materials
for catalytic and other applications due to their distinct and
[C4mim]
plate[25] or an ionic polymer as a support,[22b] or [C4mim]-
[BF4] as an electrolyte,[26] have been reported. Functional-
ACHTUGNTRNEN[UNG PF6]/Triton X-100/H2O microemulsions as a tem-
AHCTUNGTRENNUNG
[a] Dr. X. Yuan, G. Sun, Prof. Y. Kou
ized ILs, which have been shown to provide additional bene-
fits in monometallic NP catalysis,[5c,6d,27] have not been used
to immobilize Au–Pd NPs, and have only been sparsely used
for other bimetallic NPs.[6e] The catalytic applications of Au–
Pd NPs in ILs also remain limited—only C=C bond hydro-
genation[22b,d] and alcohol oxidation[22a] reactions have been
evaluated.
PKU Green Chemistry Center
Beijing National Laboratory for Molecular Sciences
College of Chemistry and Molecular Engineering
Peking University, Beijing (P.R. China)
[b] Dr. X. Yuan, Dr. Y. Yuan, Prof. G. Laurenczy, Prof. P. J. Dyson,
Prof. N. Yan
Institut des Sciences et Ingenierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
Lausanne (Switzerland)
We previously described the preparation of Pd NPs with a
small size and narrow size distribution from the thermal de-
[c] H. Asakura, Prof. T. Tanaka
composition of PdACTHNUTRGNEUNG(OAc)2 in a series of hydroxyl-functional-
Department of Molecular Engineering
Kyoto University, Nishikyo, Kyoto (Japan)
ized ILs comprising the 1-(2’-hydroxylethyl)-3-methylimida-
zolium, [C2OHmim]+, cation and various anions.[28] The hy-
droxyl group leads to the accelerated formation of NPs and
also helps to protect the NPs from oxidation once formed
(compared with Pd NPs isolated from the non-functional-
[d] X. Chen, Prof. N. Yan
Department of Chemical & Biomolecular Engineering
National University of Singapore, 10 Kent Ridge Crescent
Singapore 119260 (Singapore)
E-mail: cheyann@nus.edu.sg
ized IL, [C4mim]ACHTNUTRGNEUNG[NTf2]). Based on these promising studies
we decided to extend this approach to the synthesis of bi-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203605.
Chem. Eur. J. 2013, 19, 1227 – 1234
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metallic Pd–Au NPs. The elec-
tronic and structural features of
the NPs were thoroughly inves-
tigated and were shown to be
excellent catalysts in dehaloge-
nation reactions, presumably
due to the charge transfer from
Pd to Au, which results in
slightly electron deficient Pd
atoms and consequently more
efficient reductive elimination.
Results and Discussion
The hydroxyl-functionalized IL
[C2OHmim]ACHTUNGTRENNUNG[NTf2] was used as
a solvent for the preparation of
bimetallic Au–Pd NPs from the
co-decomposition of Pd
and Au(OAc)3. The two salts
were mixed in different ratios
in [C2OHmim][NTf2] and
ACHTUNGERTN(NUNG OAc)2
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
heated at 393 K for 2 h under
vacuum to remove volatiles
generated in the decomposition
reaction. Monometallic Au and
Pd NPs, employed as controls,
were prepared in the same
manner. The obtained NPs
were characterized by TEM
and HRTEM (see Figure 1 for
representative images and size
distributions,
of
the
Au,
Au0.8Pd0.2, Au0.5Pd0.5, Au0.2Pd0.8
and Pd NPs). All the NPs are
spherical and of similar size, ap-
proximately 4 nm. An average
size range of 3–5 nm was also
determined from XRD patterns
(see below).
The HRTEM image of the
Au NPs shows lattice fringes
corresponding to (111) and
(200) planes indicative of
a
crystalline material with a face-
centered cubic structure (simi-
lar exposed planes are present
in the HRTEM images of the
Pd and bimetallic Au–Pd NPs).
Since Au and Pd have the same
crystal structure and similar lat-
tice parameters (Au=4.08,
Pd=3.89 ꢂ, D=4.6%), TEM/
Figure 1. TEM and HRTEM images and size distribution histogram of the Au, Pd, and Au–Pd NPs obtained
in [C2OHmim][NTf2].
AHCTUNGTRENNUNG
HRTEM does not provide further information regarding the
composition and structure (see Scheme 1 for a few possible
structures) of the bimetallic NPs. Therefore, STEM-EDX,
XRD, XPS, UV/Vis and XAS analysis were performed.
Note, Au–Pd NPs prepared in molecular solvents with alloy
structures[29] and core-shell structures[30] have been observed
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Chem. Eur. J. 2013, 19, 1227 – 1234

Au–Pd Nanoparticle Dehalogenation Catalysts
FULL PAPER
in their synthesis. The line-scan EDX analysis indicates that
the Au and Pd atoms are well mixed in all three samples as
the counting curves for Au and Pd along the scanned lines
share essentially the same variations.
XRD can be used to confirm the alloy structure of bimet-
allic NPs: the d-spacing, determined by deconvolution of a
certain diffraction peak (see below) in the XRD pattern fol-
lows Vegardꢃs law, which predicts a linear relationship be-
tween the crystal lattice parameter and the concentration of
the constituent elements. For example, the diffraction peak
Scheme 1. The possible structures of the Au–Pd NPs. Left: alloy struc-
ture; middle: core-shell structure (Au atoms as the core; however, Pd
atoms could in principle also occupy the core); and right: partial core-
shell structure (Pd-enriched surface layers).
of the PdACTHUNRTGENNUG(111) plane and the AuACHTUNTGREN(NUNG 111) plane is located at
2q=40o and 2q=38o, respectively, and therefore for Au–Pd
NPs in which the Au atoms and the Pd atoms are well
mixed (see the alloy structure in Scheme 1, left), the diffrac-
tion peak for the (111) plane should appear between 38 and
40o. For the Au0.8Pd0.2, Au0.5Pd0.5 and Au0.2Pd0.8 NPs this is
not the case; all three samples exhibited a (111) diffraction
peak at 2q=38o, matching that of the Au NPs (Figure 3).
Analysis of diffraction peaks from other planes, including
the (200), (220), and (311) planes, suggests that the positions
of all diffraction peaks in the Au–Pd NPs are the same as
those observed in the Au NPs and no crystalline Pd domains
were detected. These data suggest that the Au–Pd NPs may
have a core-shell (or partial core-shell) structure—with Au
atoms in the core and a Pd-enriched shell.
and the structure of Au–Pd NPs prepared in ILs has not
been thoroughly investigated.[22a,d]
STEM-EDX was used to measure the atomic percentage
of Au and Pd in the Au–Pd NPs. Two scanning modes, in-
cluding selected area scan (see the Supporting Information,
Figure S1) and selected line scan (see Figure 2), were ap-
plied. For the Au0.8Pd0.2, Au0.5Pd0.5, and Au0.2Pd0.8 NPs, the
EDX analysis of the selected area gave the atomic composi-
tion close to the expected values (Au: 82.4 vs. Pd: 17.6%,
Au: 46.9 vs. Pd: 53.1%, and Au: 15.7 vs. Pd: 84.3%, respec-
tively), thus confirming that the Au/Pd ratio can be control-
led by simply adjusting the ratio of the two precursors used
Based on the full widths at half maximum (FWHM) of
the Au–Pd NP (111) reflection
in the diffraction pattern the
average size of the NPs can be
estimated by the Scherrer equa-
tion. For the calculation of the
crystallite size the instrument
contribution on the peak width
was taken in account (Si
powder was used, FWHM Si-
AHCTUNGTER(UNNGN 111)=0.58). For the Au0.2Pd0.8
NPs, an average particle size of
3.9 nm was estimated, which is
in excellent agreement with the
data obtained from TEM analy-
sis ((4.0Æ0.8) nm). The average
size of the Au0.5Pd0.5 NPs
(5.4 nm) and Au0.8Pd0.2 NPs
(4.5 nm) calculated by XRD is
slightly larger than that ob-
tained with TEM, but overall
the values are in good agree-
ment.
UV/Vis spectra were record-
ed of the Au and Pd NPs and
their physical mixtures at the
ratios corresponding to those of
the
bimetallic
systems
(Figure 4, top) and on the ac-
tually bimetallic Au–Pd NPs
(Figure 4, bottom). The purple
solution of the Au NPs in
Figure 2. STEM-EDX images of the Au–Pd NPs in selected line mode (indicated by the line) using a HAADF
detector: a): counts for Au atoms; b): counts for Pd atoms.
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1229

N. Yan et al.
and a pronounced SPR peak is not observed. Note that a
dampened plasmon band has been observed for both core-
shell[31] and alloy[32] NPs and therefore UV/Vis spectra could
not be used to reveal the structural information of the Au–
Pd NPs.
The surface composition and the electronic state of the
Au–Pd NPs was studied by XPS spectroscopy directly on
NP suspensions in [C2OHmim]ACHTNUTRGNENUG[NTf2]. Although the signal-
to-noise ratio was reduced for the Au 4 f and Pd 3d peaks,
due to shielding from the ionic liquid on the NP surface, this
method avoids any structural or valent state variations ema-
nating from post-sample treatment. The XPS spectra of the
Au 4 f and Pd 3d regions of the Au, Pd, and bimetallic Au–
Pd NPs are shown in Figure 5. The Au NPs exhibit two pho-
toemission peaks at binding energies (BEs) of 83–85 and
87–89 eV, which can be assigned, respectively, to Au 4 f7/2
and Au 4 f5/2 spin-orbit states of zero-valent Au. No other
peaks were detected indicating that the metallic Au0 domi-
nates. Similarly, the Pd species exhibit photoemission peaks
at 334–336 eV and 340–342 eV, both of which suggest the
presence of metallic Pd0. However, a third peak at 338–
339 eV corresponding to PdII is also present in the Pd0.2Au0.8
and Pd0.5Au0.5 NP solutions, suggesting that Au increases the
percentage of charged Pd in the bimetallic NPs.
Figure 3. X-ray diffraction patterns of the Au, Pd, and Au–Pd NPs
(dashed line: predicted diffraction peaks for the Au metal, dotted line:
predicted diffraction peaks for the Pd metal).
According to the intensity of the XPS peaks, the elemen-
tal ratios of Au/Pd in the outer layer of the bimetallic NPs
could be estimated (see Table 1). The surface region of
Au0.8Pd0.2 and Au0.5Pd0.5 NPs is enriched with Pd atoms rela-
Table 1. XPS analysis of the NPs.
Sample
Au/Pd ratio
Peak position
Pd0 3d5/2
[eV]
PdII [%]
Au0 4f7/2
[eV]
PdII 3d5/2
[eV]
Au
–
84.9
84.2
84.3
83.6
–
–
32.2
34.5
0
Au0.8Pd0.2
Au0.5Pd0.5
Au0.2Pd0.8
Pd
56.4:43.6
33.7:66.3
25.2:74.8
–
335.8
336.1
335.2
335.3
338.7
338.9
–
Figure 4. UV/Vis spectra of (top) mixtures of Au and Pd NPs and
(bottom) the bimetallic Au–Pd NPs. Spectra of pure Au and pure Pd NPs
are shown in both (top and bottom).
0
[C2OHmim]
ized by
ACHTUNGTRENNUNG
a
band in the visible region with
a peak maxima at 550 nm and
may be ascribed to the surface
plasmon resonance (SPR) of
the Au NPs. The Pd NPs absorb
in the UV region with virtually
no structure. For the three
physically mixed Au and Pd NP
solutions the peak at around
550 nm is present and its inten-
sity decreases, as expected, at
lower Au/Pd ratios. The bimet-
allic Au0.8Pd0.2, Au0.5Pd0.5, and
Au0.2Pd0.8 NPs give distinct
Figure 5. Fine XPS spectra of the Au, Pd, and bimetallic Au–Pd NPs in Pd 3d5/2 and Au 4f7/2. The arrow on the
left represents the variation trend of the Pd 3d5/2 peak and one on the right represents the variation trend of
spectra to those of the mixtures the Au 4f7/2 peak.
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Au–Pd Nanoparticle Dehalogenation Catalysts
FULL PAPER
tive to the Au/Pd ratio employed in the synthesis. This ob-
servation is consistent with the XRD patterns and UV/Vis
spectra, and in accordance with some of the previous studies
on Au–Pd NPs prepared in molecular solvents.[30b,33] It is in-
teresting to note that there is no appreciable surface enrich-
ment of Pd in the Au0.2Pd0.8 NPs—the Au/Pd atomic ratio
being 25.2:74.8, which is close to the ratio of the two precur-
sors used in synthesis. XPS provides further evidence that
the AuPd NPs have a partial core-shell structure.
XAS has previously been used to probe structural and
electronic properties of bimetallic Au–Pd NPs[33–34] and con-
sequently we used this technique to study the Au–Pd NPs
dispersed in [C2OHmim]ACHTNUTRGNE[UNG NTf2]. Pd K-edge and Au L3-edge
XAS spectra of the NPs suspensions were recorded and in
Figure 6 (top) the Pd K-edge XANES spectra of the Au–Pd
NPs is compared with that of Pd foil. The absorption edge
energy of the Au–Pd NPs is slightly higher than that of Pd
foil—this trend being consistent with the peak position shift
observed in the Pd 3d5/2 XPS spectra indicating that the Pd
atoms are electropositive. There is no significant difference
in the Au L3-edge XANES spectra, except a damping in os-
cillation is observed for the Au–Pd NPs relative to the Au
foil, due to the small size of the Au–Pd NPs.
The Au0 3 f7/2 peak moves to lower BEs when Pd is incor-
porated into the NP, with the energy difference becoming
more pronounced as the percentage of Pd increases
(Figure 5, right, the arrow indicates the trend). The Pd BEs
A simple EXAFS curve fitting procedure for bimetallic
NPs[30b,35] was applied to both the Pd K-edge and the Au L3-
edge EXAFS spectra of the Au–Pd NPs (the essence of the
approach is to employ constraints such as the use of the
same atomic distance and the estimation of the XAFS
Debye-Waller factor of two metal shells). The quality of the
Au L3-edge EXAFS spectrum of the Au0.2Pd0.8 NPs was in-
sufficient for the fitting procedure, but it was possible to fit
the EXAFS spectra of the two other bimetallic NPs, see
Figure 7 and Tables 2 and 3. The sum of coordination num-
Figure 6. Pd K-edge XANES (top) and Au L3-edge XANES spectra
(bottom) of the bimetallic Au–Pd NPs and reference samples.
Figure 7. Pd K-edge and Au L3-edge Fourier-transformed EXAFS spectra
of the Au–Pd NPs (black) and the fitting (red).
exhibit the opposite trend: for the Pd0 3d5/2 peak, the pure
Pd NPs have the lowest BE (335.5 eV) whereas the
Au0.8Pd0.2 NPs have the highest (335.8 eV, Figure 5, left, the
arrow indicates the trend). The electronic properties of both
the Pd and Au metals are modified in the bimetallic NPs,
with the Pd atom denoting electron density to the Au atoms,
consistent with the higher electronegativity of Au (2.54) rel-
ative to Pd (2.20).
bers of Au–Au and Au–Pd shells are 13.3+/À5.2 (Au0.5Pd0.5)
and 10.9+/À3.0 (Au0.8Pd0.2), which are larger than that of
the Pd–Pd and Pd—Au shells in both the Au0.5Pd0.5
(9.1+/À2.8) and the Au0.8Pd0.2 (3.8+/À1.8) NPs. Thus, the
Au atoms have a larger coordination number than the Pd
atoms in the bimetallic Au–Pd NPs, as expected for NPs
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1231

N. Yan et al.
Table 2. Hydrogenation of 2-chloronitrobenzene by Au, Pd, and Au–Pd NPs.[a]
slightly lower than that of the
pure Pd NPs in [C2OHmim]-
AHCTUNRTGEG[NUNN NTf2] and an order of magni-
tude lower than the Au0.8Pd0.2
NPs.
Cat.
Conv. [%]
Selectivity [%]
The increase in catalytic ac-
tivity with increasing Au con-
tent in the bimetallic Au–Pd
NPs may be attributed to both
the enrichment of Pd at the NP
surface and the electronic
changes to the Pd atoms in-
duced by the electronegative
Au atoms. A graph of the per-
centage of Pd in the Au–Pd
NPs versus the calculated TOFs
Pd
99.0
96.1
91.2
82.3
0
85.4
17.9
14.6
12.1
–
6.5
23.4
9.6
7.7
–
8.1
58.8
76.0
80.6
–
13.6
82.2
85.6
88.3
–
Au0.2Pd0.8
Au0.5Pd0.5
Au0.8Pd0.2
Au
[a] Reaction conditions: 2-chloronitrobenzene (0.1 g), NP-IL (0.1 mL), IL (1 mL), H2 (30 bar), 353 K, 4 h (IL=
[C2OHmim][NTf2]).
ACHTUNGTRENNUNG
with an Au-rich core and Pd-rich shell structure. From kinet-
ic considerations, it is not unreasonable to speculate that
AuIII reduction occurs prior to PdII reduction in Au–Pd NP
formation and therefore is enriched in the core. Further in-
situ XAS investigation is currently ongoing to reveal the for-
mation mechanism of AuPd NPs in ILs.
Table 3. Hydrogenation of 4-bromotoluene and 3,5-dibromotoluene cata-
lyzed by the Au, Pd, and Au–Pd NPs in [C2OHmim]ACHTUNGTRNEUNG
[NTf2].[a]
Cat.
Substrate
Conv.
[%]
TOF
Selectivity [%]
[b]
[hÀ1
]
toluene
The bimetallic Au–Pd NPs have structural and electronic
properties that could facilitate certain catalytic reactions.
Consequently, the Au–Pd NPs were initially evaluated in the
hydrogenation of 2-chloronitrobenzene as this compound
can potentially undergo reduction of the nitro group, the ar-
omatic ring and/or undergo dechlorination. The Pd NPs lead
to the highest conversions whereas the Au NPs are com-
pletely inactive (Table 2). However, the bimetallic Au–Pd
NPs also result in high conversions, but compared with the
pure Pd NPs dechlorination and hydrogenation of the aro-
matic ring takes place much more efficiently. Indeed, amino-
cyclohexane is the major product from the hydrogenation of
2-chloronitrobenzene with the bimetallic NPs. Compared
with the Pd NPs, Au0.8Pd0.2 NPs are at least five times more
active in arene ring hydrogenation and dechlorination reac-
tions. Arene hydrogenation over NPs in ILs has been exten-
sively studied, but previous reports are more focused on
Pd
32.4
37.7
53.8
70.8
–
16
24
–
–
–
–
–
100
100
100
100
–
Au0.2Pd0.8
Au0.5Pd0.5
Au0.8Pd0.2
Au
54
177
–
Pd
55.0
55.8
95.2
98.5
–
28
35
62.1
56.1
21.6
7.1
37.9
homoACHTUNGTRENNUNG
nuclear Rh,[36] Pt,[37] Ir[38], and Ru NPs.[39] Dechlorina-
Au0.2Pd0.8
Au0.5Pd0.5
Au0.8Pd0.2
Au
43.9
78.4
92.8
–
tion reaction in ILs are less commonly reported and previ-
ous studies did not employ metal NPs as catalysts.[40]
Due to the efficacy of the Au–Pd NPs in the dechlorina-
tion of 2-chloronitrobenzene, debromination was also evalu-
ated. Two substrates were explored, 4-bromotoluene and 3,
5-dibromotoluene (Table 3), and in all cases the bimetallic
Au–Pd NPs were superior to the Pd NPs and the Au NPs
were completely inactive. It should be noted that arene ring
hydrogenation is not observed due to the presence of base
that suppresses this reaction.[41] As the amount of Au in the
bimetallic NPs increases, there is a considerable increase in
the reaction rate; the Au0.8Pd0.2 NPs are about ten times
more active than the Pd NPs. Pd/C was previously employed
95
246
–
–
in dehalogenation reactions in [C2mim]ACTHNUTGRNEUGN[BF4] (C2mim=the
[a] Reaction conditions: 4-bromotoluene or 3,5-dibromo toluene
(0.125 g), NP-IL (0.05 mL), IL (1 mL), K2CO3 (138 mg), H2 (30 bar),
343 K, 2 h (IL=[C2OHmim]ACHTUGNTRENNNUG
1-methyl-3-ethyl-imidazolium cation) under similar reaction
conditions with TOFs for bromobenzene of 10–15 hÀ1.[40c]
The turnover frequency (TOF) for the Pd/C catalyst is
total Pd content.
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Au–Pd Nanoparticle Dehalogenation Catalysts
FULL PAPER
Catalysis: Hydrogenation/dechlorination of 2-chloronitrobenzene was
carried out in a multicell autoclave system equipped with quartz reaction
tubes (10ꢄ10 mL). Typically, the substrate (0.1 g, 0.63 mmol), NP/IL sol-
ution (0.1 mL, total amount of metal: 0.01 mmol) and pure IL (1 mL)
were added into the reaction tube, and then the autoclave was sealed,
purged and vent with N2 (1.0 MPa) three times. After purging with H2
(3.0 MPa), the autoclave was sealed and the system heated at 353 K for
2 h. After reaction the products were extracted with diethyl ether (3ꢄ
5 mL) and analyzed by GC and GC-MS.
Dehalogenation reactions: The dehalogenation reactions were performed
at 343 K under H2 (3 MPa) by using the same multicell autoclave system.
The appropriate substrate (0.5 mmol), NP/IL suspension (0.1 mL, total
amount of metal: 0.01 mmol) and IL (1 mL) were added to the autoclave,
and afterwards the procedure is exactly the same as hydrogenation/de-
AHCTUNGTRENNcGUN hlorination of 2-chloronitrobenzene.
Figure 8. TOFs of 4-bromotoluene and 3,5-dibromotoluene as a function
of the Pd percentage in the Au–Pd NPs. Fitting curve for 4-bromotoluene
(a), fitting curve for 3,5-dibromotoluene (c).
Acknowledgements
(see Figure 8) reveals an exponential trend (TOF=AeÀBx
,
We thank the National Basic Research Project of China
(2011CB201402), the Swiss National Science Foundation, EPFL and a
start-up grant from NUS (WBS: R-279-000-368-133) for financial sup-
port.
in which A and B are constants, and x corresponds to the
fraction of Pd in the Au–Pd NPs) for both substrates. The
catalytic promotion by the Au–Pd NPs may be attributed to
the presence of electropositive Pd atoms enriched on the
NP surface. Such atoms should facilitate reductive elimina-
tion of the product that is presumably the rate-determining
mechanistic step. Moreover, it is also possible that the IL
cation interacts preferentially with the electron-rich Au sur-
face atoms allowing more facile interactions between the
catalytically active Pd atoms and the substrates.
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Conclusion
A
series of bimetallic Au–Pd NPs were prepared in
[C2OHmim][NTf2] by the thermal decomposition of Pd-
(OAc)2 and Au(OAc)3 affording NPs enriched at the surface
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
with Pd. The Au atoms are mostly located in the NP core
leaving the Pd atoms enriched at the surface. Moreover, the
Pd atoms are electropositive, which leads to high activity in
catalytic dehalogenation reactions. Compared with previous
studies employing Pd/C as the catalyst, the Au0.8Pd0.2 NPs
described herein are an order of magnitude more active
(based on the same Pd content).
Experimental Section
Preparation of the bimetallic Au–Pd NPs—procedure for Pd0.8Au0.2 NPs:
Pd
(OAc)2 (0.08 mmol) and Au
E
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tonitrile (5 mL) and then mixed with [C2OHmim]AHCNUTGTRENNUNG
round bottom flask (10 mL) at room temperature. The acetonitrile was
removed under vacuum with vigorous stirring for 60 min. Under vacuum,
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ACHUTNGTRNENUG(OAc)2 to AuACHTUGNTERN(NUNG OAc)3
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Received: October 9, 2012
Published online: January 4, 2013
1234
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