Ag-Pd and CuO-Pd nanoparticles in a hydroxyl-group functionalized ionic liquid: Synthesis, characterization and catalytic performance

Article abstract of DOI:10.1039/c4cy01382d

Heteronuclear Ag-Pd and CuO-Pd nanoparticles with a controllable Ag : Pd or Cu : Pd ratio were easily synthesized through thermal decomposition of their acetate salts in a functionalized ionic liquid, [C₂OHmim][NTf₂]. The structural and electronic properties of these particles were characterized by ICP-OES, TEM, XRD and XPS techniques. The Ag-Pd nanoparticles have a silver enriched core covered by a Pd enriched shell. The addition of Ag clearly enhanced the catalytic activity of Pd in the hydrogenation of -NO₂ and -CC- functionalities. Interestingly, in the CuO-Pd nanoparticles, Pd tends to stay in the inner part of the particle, leaving the outer layer rich in CuO, which exhibits less activity than Pd NPs.

Full text of DOI:10.1039/c4cy01382d

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Ag–Pd and CuO–Pd nanoparticles in a hydroxyl-
group functionalized ionic liquid: synthesis,
characterization and catalytic performance
a
Cite this: DOI: 10.1039/c4cy01382d
Bin Zhang,a Yuan Yuan,a Karine Philippotbc and Ning Yan
*
Heteronuclear Ag–Pd and CuO–Pd nanoparticles with a controllable Ag: Pd or Cu : Pd ratio were easily
synthesized through thermal decomposition of their acetate salts in
a functionalized ionic liquid,
Received 22nd October 2014,
Accepted 6th December 2014
ijC2OHmim]ijNTf2]. The structural and electronic properties of these particles were characterized by ICP-OES,
TEM, XRD and XPS techniques. The Ag–Pd nanoparticles have a silver enriched core covered by a Pd
enriched shell. The addition of Ag clearly enhanced the catalytic activity of Pd in the hydrogenation of
–NO2 and –CC– functionalities. Interestingly, in the CuO–Pd nanoparticles, Pd tends to stay in the inner
part of the particle, leaving the outer layer rich in CuO, which exhibits less activity than Pd NPs.
DOI: 10.1039/c4cy01382d
www.rsc.org/catalysis
liquids (ILs) are attractive since they can act as both stabi-
lizers for NPs and reaction media.31–36 Furthermore, the
Introduction
Metal nanoparticles (NPs) that are composed of two different
metal elements, namely, bimetallic NPs have long been receiv-
ing considerable attention due to their distinct physico-
chemical properties compared to monometallic counterparts.
Indeed, new properties can be derived from electronic and
steric interactions between the two metal components.1–4 In
catalysis, this can lead to improvement of performance both
in terms of activity and selectivity. There are generally three
types of structures for bimetallic NPs,5,6 including nano-alloy,7,8
heterodimer9 and core–shell (or partial core–shell)10 struc-
tures. The core–shell configuration is featured by one metal
constituting (or dominating) the core of particles, whereas the
second one is taking up (or enriching in) the outer layer.11 This
peculiar structure sometimes endows NPs an outstanding
catalytic activity over those of alloyed NPs or heterodimers.12–15
For instance, core–shell Rh–Pt NPs outperform equivalent
Pt1−xRhx alloys and monometallic mixtures.16 Currently, a variety
of bimetallic NPs with core–shell structures have been investi-
gated, such as Au–Pd,17–19 Au–Ag,20,21 Pt–Rh,2,16,22 Pt–Ru,23–25
Pt–Pd,26–28 and Ni–M NPs (M = Au, Rh, Ru, Pd).29,30
properties of ILs can be tailored by modifying their chemical
structure through incorporation of different functional
groups,37–39 such as hydroxyl,40–42 nitrile,43,44 thiol45,46 or car-
boxyl47,48 groups into the cation or the anion. These func-
tional groups may interact with the NP's metallic surface and
further provide higher stability to metal NPs as well as influ-
ence their performance in catalytic reactions.40,46,47,49–52
Recently, a number of bimetallic NPs synthesized in ILs
have been reported. Au–M (M = Pd, Ag, Pt) NPs were investi-
gated intensively with either non-functionalized and func-
tionalized ILs or ionic liquid composites.53–57 Besides, Pt–M
(M = Co, Ru, Pd),58–60 Ru–Cu61 and Pd–Ni62 bimetallic
NP–ionic liquid systems were also studied. Previously, we
prepared core–shell Au–Pd bimetallic NPs displaying
a
Pd-enriched surface around an Au-core in 1-ethyl-3-methyl-
imidazolium bis-IJtrifluoromethylsulfonyl) imide IJ[C2OHmim]-
ijNTf2]).63 This IL was selected after screening a series of
different ILs to provide the NPs with the highest stability as
well as a well-controlled size.64 In hydroxyl-functionalized
imidazolium based ILs, the hydroxyl group may both acceler-
ate the formation of NPs and protect them from oxidation.
The so-obtained Au–Pd NPs exhibited higher selectivity for
the dehalogenation of aryl halides than pure Pd NPs, while
pure Au NPs were inactive. The enhanced selectivity of Au–Pd
bimetallic NPs was attributed to the enrichment of Pd on the
surface and the charge transfer from Pd to Au. It is worth
noting that the synthetic method applied for synthesizing
these Au–Pd NPs, i.e., thermal-decomposition of the two metal
acetates under vacuum,64 is simpler compared with other
approaches, such as decomposition through thermal treatment
For stabilization of metal NPs in the liquid phase, suitable
solvents and/or stabilizers are often employed to prevent
them from agglomerating.4 Among various solvents, ionic
a Department of Chemical and Biomolecular Engineering, National University of
Singapore, 4 Engineering Drive 4, Singapore. E-mail: ning.yan@nus.edu.sg
b Laboratoire de Chimie de Coordination, CNRS, BP 44099, 205 Route de Narbonne,
F-31077 Toulouse Cedex 4, France
c Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
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with H2,65,66 reduction with a chemical or electrical agent,38,67
bombardment of large particles through irradiation68 or trans-
fer of pre-synthesized NPs into ionic liquids,69 for example.
Motivated by the effectiveness and the simplicity of the
thermal-decomposition under vacuum method for Au–Pd NPs,
we envisaged to extend this methodology to the synthesis
of other bimetallic NPs in ILs. Considering that both Pd
and Au are noble metals, it is economically attractive to
replace the core metal with another metal having higher
natural abundance and much cheaper price.70,71 For this
purpose, Ag and Cu, staying in the same group as Au in the
elemental table, appeared as ideal. So far, only limited
research on Ag–Pd and Cu–Pd bimetallic systems in ILs has
been reported. Tai et al. prepared a series of Ag–Pd alloys
(larger than 100 nm) via electrodeposition.72 Safavi et al.
obtained Ag–Pd nanoalloys through microwave-assisted
decomposition and used them in electrocatalysis.73,74 Thus,
it is challenging to synthesize Ag–Pd and Cu–Pd bimetallic
NPs of smaller size via simple methods and to study their
catalytic properties. In this context, we report here the syn-
thesis of Ag–Pd and CuO–Pd NPs by thermal co-decomposition
under vacuum of acetate salts in ijC2OHmim]ijNTf2]. The cata-
lytic performance of the so-obtained Ag–Pd and CuO–Pd NPs
was also studied in detail, namely, in hydrogenation and
hydrodehalogenation reactions of model substrates.
Results and discussion
Synthesis of Ag–Pd NPs in ijC2OHmim]ijNTf2]
A series of three bimetallic Ag–Pd NPs was synthesized through
thermal decomposition (393 K, 2 h) under vacuum of the two
metal acetates in ijC2OHmim]ijNTf2]. Monometallic Pd and Ag
NPs were also prepared in the same manner, as control sam-
ples. After two hours of thermal decomposition, the reaction
medium formed a stable colloidal solution, making it possible
to conduct catalytic reactions without further treatment. The IL
thus functions as both the solvent and the stabilizer. According
to the ratio of the two starting metal precursors, the resulting
NPs are denoted as Pd, Ag0.2Pd0.8, Ag0.5Pd0.5, Ag0.8Pd0.2 and Ag.
Characterization of Ag–Pd bimetallic NPs
The as-prepared Ag–Pd NPs were first characterized by Trans-
mission Electron Microscopy (TEM). The representative images
and size distributions obtained for Ag, Ag–Pd and Pd NPs
are presented in Fig. 1. Most of the Ag–Pd NPs are spherical
and have narrow size distributions, with average diameters
between 3.4 and 3.7 nm, showing that the incorporation of
Ag to Pd does not change the size of the NPs significantly.
Ag–Pd bimetallic NPs with similar size and morphology
were previously obtained through an ion-exchange method
on ultrathin TiO2-gel films,75 and through supporting two
nitrates on reduced graphene oxide.76 Compared with these
previous methods, the NPs described here are more uni-
form and monodisperse in size, indicating that the func-
tionalized ionic liquid may play an important role. To obtain
a thorough understanding of the structural features of the
Fig. 1 TEM images and corresponding size distributions of Pd NPs,
Ag–Pd NPs and Ag NPs synthesized in ijC2OHmim]ijNTf2].
Ag–Pd NPs, ICP-OES, XPS, XRD analyses were subsequently
conducted.
The metal composition of the Ag–Pd NPs was calculated
from the ICP-OES analysis (see Table 1). The atomic composi-
tion appeared to be close to the ratio of the two metal precur-
sors introduced for the NP synthesis. This result implies that
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Table 1 Elemental composition of Ag–Pd NPs calculated from ICP analy-
sis and the near-surface composition calculated from XPS analysis
the TEM results, further confirming that the particle sizes of
the Ag–Pd NPs prepared in ijC2OHmim]ijNTf2] are very small.
XPS was employed to investigate the elemental composition
and electronic state of the NP surface. The XPS spectra of Pd
3d and Ag 3d electrons for the whole series of NPs are shown
in Fig. 3. The near-surface elemental ratio was calculated
based on the fitted curves of XPS data, revealing that Pd has
a higher tendency to locate near the surface than Ag (Table 1).
From XRD and XPS results, it is highly plausible that the
Ag–Pd NPs adopt a core (Ag)–shell (Pd) structure, as similarly
observed in a thermodynamic study of the surface segregation
of Ag3Pd under an environment rich in oxygen.82 In this pub-
lished study, DFT results showed that the segregation energy
of the AgPd alloy decreases with the increase in Ag percentage
of the topmost layer, while the oxygen binding energy increases
with more Pd on the surface.82 Therefore, the formation of
the Pd enriched shell in vacuum suggests that the hydroxyl
functionalized ionic liquid ijC2OHmim]ijNTf2] may play a
dominant role in controlling the structure of the Ag–Pd NPs.
A close inspection of the Pd 3d XPS spectra reveals that
the peaks ranging from 335.1–335.4 eV and 341.1–341.4 eV
can be attributed to the 3d5/2 and 3d3/2 spin–orbit states of
zero-valent Pd, whereas the peaks ranging from 338.0–338.5 eV
and 343.7–344.2 eV correspond to the states of PdIJII). It is
obvious that when the concentration of Ag increases, the Pd
species tends to stay at a higher valence state. Furthermore,
the binding energy increases with increasing Ag content.
Ag0.2Pd0.8 NPs contain both Pd(0) and PdIJII) with a ratio of
1 : 1.2 for near surface atoms, according to the XPS curve
fitting. From the positions of Ag 3d5/2 (368.0–368.5 eV) and
3d3/2 (374.0–374.5 eV) peaks in the XPS spectra, the valence
state of Ag in Ag–Pd NPs is always zero, regardless of the Pd
content. The binding energies of Ag 3d electrons tend to
decrease with increasing Pd in the NPs, thus Ag becomes
electronically negative. This reveals that Pd donates electrons
to Ag in the Ag–Pd NPs, resulting in negatively charged Ag
and positively charged Pd. Another reason accounting for the
positively charged Pd may be that Pd preferentially locates at
the outer shell and therefore can have more interaction with
electronegative elements such as oxygen from the hydroxyl-
functionalized IL.
NPs
Ag/Pd ratio (ICP)
Ag/Pd ratio (XPS)
Ag–Pd (4 : 1)a
Ag–Pd (1 : 1)
Ag–Pd (0.25 : 1)
3.06 : 1
0.92 : 1
0.26 : 1
1.2 : 1
0.48 : 1
0.22 : 1
a
This ratio corresponds to the initial molar amounts of the two
acetate precursors.
the Ag/Pd ratio can be easily tuned by adjusting the ratio of
the precursors during preparation.
XRD is an efficient tool to identify the structure of NPs,
especially for bimetallic NPs.73,77,78 The XRD spectra of all NP
samples are compiled in Fig. 2. For monometallic Pd NPs,
the observed intense peaks at 40°, 47°, 68° and 81° represent
the (111), (200), (220), and (311) Bragg reflections of Pd cubic
(fcc) crystalline, respectively, which is in excellent agreement
with literature data. For Ag NPs, these four peaks are centred
at 38°, 44°, 64° and 77°. Interestingly, in the XRD patterns of
all the Ag–Pd bimetallic NPs, the diffraction peaks are located
at 38°, 44°, 64°, 77° and 82°, matching exactly the reflections
of metallic Ag in the (111), (200), (220), (311) and (222)
planes. No other peaks were observed. This suggests that the
Ag–Pd NPs do not form alloy structures and only the silver
atoms constitute the crystalline structure. Previous XRD stud-
ies on Ag–Pd NPs from the literature76,79–81 show that the
observed diffraction peaks are between those of monometal-
lic Ag and Pd, suggesting the formation of alloy structures.
The Ag–Pd NPs we here describe appear thus structurally dif-
ferent from those in previous reports (vide infra). The crystal-
line size of the NPs was also calculated using the peak broad-
ening profile of the (111) peak at 38° and 40° through
Scherrer's equation. The calculated values (ca. 3–5 nm) match
Application of Ag–Pd NPs in catalysis
Four catalytic reactions were conducted to evaluate the cata-
lytic performance of the series of as-prepared Ag–Pd NPs in
Fig. 2 X-ray diffraction patterns of the Ag, Ag–Pd and Pd NPs. The
predicted diffraction peaks for Ag metal are indicated. From top to
bottom: Pd, Ag0.2Pd0.8, Ag0.5Pd0.5, Ag0.8Pd0.2, and Ag NPs.
Fig. 3 XPS spectra of Pd 3d peak (a) and Ag 3d peak (b) for Ag–Pd NPs.
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ijC2OHmim][NTf2]. These reactions include: hydrogenation
of 4-nitrophenol, nitrobenzene, and styrene, and hydro-
dehalogenation of 4-bromotoluene. As a control, the catalytic
performance of single component Ag and Pd nanocatalysts
was also evaluated.
activity of Ag0.5Pd0.5 NPs increased ca. 3 times compared with
those of Ag and Pd NPs under the same reaction conditions.
In all the cases, hydrogenation of the benzene ring was not
observed under the applied reaction conditions, providing
100% selectivity for ethyl benzene.
The hydrogenation of 4-nitrophenol over Pd, Ag–Pd and
Ag NPs was carried out in the presence of H2 (3 MPa) at
353 K for 2 h. The results are compiled in Table 2 (entries 1–5);
activity is calculated based on the Pd content determined by
ICP. The results indicate that the Ag–Pd bimetallic NPs are
significantly more active in hydrogenation of 4-nitrophenol
compared with the monometallic Pd and Ag NPs. Indeed, Ag
NPs were observed to be inactive, whereas Pd NPs exhibited
very low activity (0.7 h−1) under the conditions applied. Incor-
porating 20% Ag into Pd enabled the improvement of the
activity to one order of magnitude higher (4.8 h−1), and the
activity kept increasing when the Ag content was further
increased. The activity reached 55 h−1 for Pd0.2Ag0.8 NPs,
ca. 80 times more active than single component Pd NPs.
Aminophenol is the only product, suggesting that this is a
highly selective system for nitro group reduction without fur-
ther hydrogenation.
The hydrogenation of nitrobenzene is known to be easier
than that of 4-nitrophenol,83–85 due to the fact that the –OH
group affects the electronic state of the substrate. As such,
the hydrogenation of nitrobenzene was carried out under
milder reaction conditions (2 MPa H2, 333 K, see Table 3,
entries 1–5). It is worth noting that the promotional effect of
Ag to Pd was observed in Ag–Pd NPs until a certain point.
Indeed, the activity of Pd did not exhibit a monotonic trend
with increasing content of Ag. Instead, Ag0.5Pd0.5 NPs appeared
to be the most effective nanocatalyst and further increasing
of Ag to 80% induced undesired side reactions. Interest-
ingly, the selectivity for aminocyclohexane was enhanced over
Ag enriched catalysts, especially Ag0.8Pd0.2. Considering that
electrons were transferred from Pd to Ag, positively charged
Pd surface species may act as Lewis acids, which can activate
the benzene ring for hydrogenation.86
Considering that all the NPs in the series have similar
mean particle size, the difference observed in catalytic activity
cannot be attributed to the size effect. Thus, the increase in
catalytic activity with increasing Ag content in the bimetallic
Ag–Pd NPs may be attributed to both the enrichment of Pd
on the surface and the electronic modifications of the Pd
atoms induced by the presence of electronegative Ag atoms.
The positive charged Pd atoms could facilitate the adsorption
of the substrates on the metallic surface which would favour
higher conversion.
Previously, Au–Pd bimetallic NPs were found to be
much more effective than Au and Pd NP counterparts in
hydro-dehalogenation reactions.63 Thus, dehalogenation of
4-bromotoluene was also studied over Ag, Pd and Ag–Pd NPs
(Table 5, entries 1–5). Ag was inactive for the reaction but
the addition of Ag enhanced the catalytic performance of
Pd. However, the enhancement in activity is not as strong as
in the case of Au–Pd NPs.63 Two reasons may account for
this: a) the electronic interaction between Ag and Pd is differ-
ent from that between Au and Pd, resulting in different cata-
lytic properties; b) although the NPs have a Pd-enriched shell,
a small percentage of Ag atoms may still exist on the surface
and bind with Br−, which could poison the catalysts.
Synthesis and characterization of CuO–Pd NPs
Following a similar protocol to that described above for
AgPd NPs, thermal decomposition of PdIJOAc)2 and CuIJOAc)2
(with Pd : Cu ratio of 10 : 0, 8 : 2, 5 : 5, 2 : 8 and 0 : 10) was
carried out in ijC2OHmim]ijNTf2]. The resulting NPs were sys-
tematically investigated by ICP-OES, TEM, XRD and XPS anal-
ysis. Since previous reports claimed that the decomposed
products of either CuIJOAc)2 or CuIJOAc)2ĴH2O contained a large
percentage of CuOx,87–89 the presence of Cu or CuO was finely
studied. Interestingly, the NPs containing Cu and Pd are
drastically different from Ag–Pd NPs prepared by the same
method, in terms of both the metal valence state and the NP
structure.
Hydrogenation of styrene also revealed the superior cata-
lytic activity of Ag–Pd NPs (Table 4, entries 1–5). Since both
Pd and Ag can catalyse the reaction to a similar degree, activ-
ity was calculated based on the total amount of catalysts. The
Table 2 Hydrogenation of 4-nitrophenol catalysed by Ag, Pd, CuO, Ag–Pd and CuO–Pd NPs in ijC2OHmim]ijNTf2]a
Entry
NPs
Conversion [%]
Activityb [h−1
]
Selectivity for 4-aminophenol [%]
Selectivity for 4-aminocyclohexanol [%]
1
2
3
4
5
6
7
8
9
Pd
1.4
7.8
23
11
0
1.1
0.8
0.3
0
1.2
4.8
23
30
—
0.8
0.8
0.7
—
100
100
100
100
—
100
100
100
—
0
0
0
0
—
0
0
0
Ag0.2Pd0.8
Ag0.5Pd0.5
Ag0.8Pd0.2
Ag
IJCuO)0.2Pd0.8
IJCuO)0.5Pd0.5
IJCuO)0.8Pd0.2
CuO/Cu2O
—
a
Reaction conditions: 4-nitrophenol (0.5 mmol), NP-IL (0.05 mL), IL (0.45 mL), K2CO3 (0.05 g) H2 (3 MPa), 353 K, 2 h (IL = ijC2OHmim]ijNTf2]).
Activity = moles of converted substrate/moles of Pd.
b
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Table 3 Hydrogenation of nitrobenzene catalysed by Ag, Pd, CuO, Ag–Pd and CuO–Pd NPs in ijC2OHmim]ijNTf2]a
Entry
NPs
Conversion [%]
Activityb [h−1
]
Selectivity for aniline [%]
Selectivity for aminocyclohexane [%]
1
2
3
4
5
6
7
8
9
Pd
52
67
77
25
9
13
5.1
3.4
0.4
52
84
99.7
99.8
99.7
89.1
92.4
100
100
100
100
0.3
0.2
0.3
10.9
7.6
0
0
0
0
Ag0.2Pd0.8
Ag0.5Pd0.5
Ag0.8Pd0.2
Ag
IJCuO)0.2Pd0.8
IJCuO)0.5Pd0.5
IJCuO)0.8Pd0.2
CuO/Cu2O
154
100
—
9.5
5.1
7.8
—
a
b
Reaction conditions: nitrobenzene (1 mmol), NP-IL (0.05 mL), IL (0.45 mL), H2 (2 MPa), 333 K, 2 h (IL = ijC2OHmim]ijNTf2]). Activity = moles
of converted substrate/moles of Pd.
Table 4 Hydrogenation of styrene catalysed by Ag, Pd, CuO, Ag–Pd and CuO–Pd NPs in ijC2OHmim]ijNTf2]a
Entry
NPs
Conversion [%]
Activity [h−1
]
Selectivity for ethylbenzene [%]
Selectivity for ethylcyclohexane [%]
1
2
3
4
5
6
7
8
9
Pd
27
41
67
35
20
15
5.9
3.3
0.8
81b
100
100
100
100
100
100
100
100
100
0
0
0
0
0
0
0
0
0
Ag0.2Pd0.8
Ag0.5Pd0.5
Ag0.8Pd0.2
Ag
IJCuO)0.2Pd0.8
IJCuO)0.5Pd0.5
IJCuO)0.8Pd0.2
CuO/Cu2O
123b
201b
105b
60b
11c
5.9c
7.6c
—
a
b
Reaction conditions: styrene (3 mmol), NP-IL (0.05 mL), IL (0.45 mL), H2 (2 MPa), 353 K, 1 h (IL = ijC2OHmim]ijNTf2]). Activity = moles of
converted substrate/moles of Ag and Pd. c Activity = moles of converted substrate/moles of Pd.
Table 5 Hydrogenation of 4-bromotoluene catalysed by Ag, Pd, CuO, Ag–Pd and CuO–Pd NPs in ijC2OHmim]ijNTf2]a
Entry
NPs
Conversion [%]
Activityb [h−1
]
Selectivity for toluene [%]
Selectivity for 1-bromo-4-methylcyclohexane [%]
1
2
3
4
5
6
7
8
9
Pd
36
60
42
23
0
9.9
1.2
1.1
0
18
37
42
58
—
7.2
1.2
2.5
—
100
100
100
100
—
100
100
100
—
0
0
0
0
—
0
0
0
Ag0.2Pd0.8
Ag0.5Pd0.5
Ag0.8Pd0.2
Ag
IJCuO)0.2Pd0.8
IJCuO)0.5Pd0.5
IJCuO)0.8Pd0.2
CuO/Cu2O
—
a
Reaction conditions: 4-bromotoluene (0.5 mmol), NP-IL (0.05 mL), IL (0.5 mL), K2CO3 (0.069 g), H2 (3 MPa), 343 K, 2 h (IL = ijC2OHmim]ijNTf2]).
Activity = moles of converted substrate/moles of Pd.
b
As presented in Fig. 4, TEM images showed that the
Pd reacted precursors. Nonetheless, the trend is clear that
with increasing amount of Cu precursor, the Cu content in
the resulting NPs increases accordingly. XPS was employed to
investigate the electronic state of Cu and Pd in the NPs. As
can be seen in Fig. 5(b), the satellite peak of CuIJII) at around
943.5 eV exists for all the NPs.90 Based on the ICP result that
most of the precursor has been consumed, the CuIJII) should
be attributed to CuO, which is later confirmed by XRD analy-
sis. In Fig. 5(b), two sets of binding energies of the Cu 2p
electrons were obtained for NPs prepared from CuIJOAc)2. The
lower one (around 931.3 eV) may correspond to Cu2O or even
Cu, while the larger one (around 933.4 eV) can be assigned to
Cu-based NPs are well dispersed with a mean size of ca.
3.9 nm, whereas the size of bimetallic CuO–Pd NPs is esti-
mated to be in the range 7–10 nm. The elemental ratio of
total CuO–Pd NPs calculated based on ICP-OES analysis and
the surface composition calculated using XPS analysis are
given in Table 6. It is clear that the near surface Cu : Pd is
much larger than the total elemental ratio of Cu : Pd in NPs,
which implies that the Cu–Pd NPs may also adopt a core–
shell type structure, with Pd atoms in the core.
ICP-OES results in Table 6 showed that the Cu to Pd ratio
has some derivation from the stoichiometric ratio of Cu and
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CuO. The two types of Cu species have a 2 : 3 ratio. Auger
spectra (Fig. 6) were employed to further clarify the electronic
state of the unknown Cu species. The kinetic energy obtained
from Fig. 6a is 916.8 eV, close to that of Cu2O standard
(916.2–917.2 eV).91 Then the Auger parameter and Wagner
plots were employed to confirm the existence of CuIJI).92 Fur-
ther, Auger Cu LMM spectra (Fig. 6b) corroborate the coexis-
tence of Cu+ at BE ca. 569.6 eV and Cu2+ at BE ca. 568.5 eV.93
For the other three NP samples, only a peak corresponding to
CuO was detected, suggesting that CuO is the predominant
Cu species on the surface. In Fig. 5(a), the binding energy of
the Pd 3d5/2 electron stayed at 335.1–335.4 eV (corresponding
to metallic Pd) regardless of CuO content. As such, the NPs
appear to be metal–metal oxide nanocomposites, consisting
of CuO and Pd. A close inspection of the XPS spectra indi-
cates that the binding energy of Cu increases with more
Pd incorporation, while the tendency is the reverse for Pd.
Therefore, charge transfer occurred from Cu to Pd in the
CuO–Pd NPs. The valence state of the CuO–Pd NPs obtained
here appears to be different from that observed for colloids
formed by thermal decomposition of the two acetates in
some organic solvents, namely, Cu–Pd and Cu2O–Pd.87 Such
a difference may be due to the fact that the hydroxyl-
functionalized ionic liquid employed in this study has a
high affinity for water, which can facilitate the formation of
oxide during the decomposition process. Decomposition of
CuIJOAc)2ĴH2O in an imidazole-based ionic liquid [C4mim]
[OAc] was also studied and a mixture of Cu2O and CuO was
obtained.89
Fig. 4 TEM images and size distribution of CuO–Pd NPs (the IJCuO)0.5Pd0.5
and IJCuO)0.8Pd0.2 NPs are not well separated, therefore their size
distribution is not provided).
XRD was employed to determine the crystalline structure
of the NPs. As shown in Fig. 7, only reflections for CuO
(peaks centred at 36° and 38°) were found for the NP sample
obtained from decomposition of CuIJOAc)2, in agreement with
XPS analysis.
Table 6 Elemental composition of CuO–Pd NPs calculated from ICP
analysis and the surface composition calculated from XPS analysis
The XRD patterns of both IJCuO)0.2Pd0.8 and IJCuO)0.5Pd0.5
bimetallic NPs show only four predominant peaks centred at
40°, 47°, 68° and 82°, matching the characteristic (111),
(200), (300) and (220) reflections of Pd(0), while no crystalline
domains of CuOx or Cu were observed. This phenomenon
further corroborates the fact that the structure of these NPs
is a palladium core covered by a CuO enriched shell, which
could be expected as copper is more oxophilic and thus will
be attracted outside in the presence of oxygen atoms of the
NPsa
Cu/Pd ratio
Surface Cu/Pd ratio
CuO–Pd (4 : 1)
CuO–Pd (1 : 1)
CuO–Pd (0.25 : 1)
2 : 1
0.74 : 1
0.22 : 1
7.52 : 1
7.38 : 1
1.42 : 1
a
This ratio corresponds to the initial molar amounts of two acetates.
Fig.
CuO–Pd NPs.
5
XPS spectra of Pd 3d peak (a) and Cu 2p peak (b) for
Fig. 6 Cu KLL (a) and Cu LMM (b) XPS spectra of the decomposed
products of CuIJOAc)2.
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evaporator to remove the solvents before dissolving) were
digested in aqua regia (8 mL, HCl/HNO3 = 3 : 1) at 353 K for
4 h and then diluted with deionized water to 100 mL before
analysis.
TEM
Transmission electron microscopy (TEM) measurements
were carried out with a JEOL JEM-2010 electron microscope
operated at 200 kV. After isolation by centrifugation (vide
infra), the NPs were re-dispersed in acetonitrile and sonicated
for 5 seconds. One drop of the NP solution was placed onto a
copper grid coated with carbon film and dried overnight at
room temperature (r.t.). The average diameter of the NPs was
determined through analysis of 150–200 NPs with Adobe
Acrobat 9 Pro.
Fig.
7 X-ray diffraction patterns of CuO–Pd NPs. The predicted
diffraction peaks for Pd metal are indicated. From top to bottom:
IJCuO)0.2Pd0.8, IJCuO)0.5Pd0.5, IJCuO)0.8Pd0.2, Pd and CuO.
XRD
X-ray diffraction (XRD) analysis was carried out using a
Bruker D8 Advance X-Ray diffractometer with Cu Kα radia-
tion, at a scan rate of 8° min−1 with a scanning angle from
30° to 85° (Pd from 36° to 85°). It was operated at 40 kV with
a potential current of 30 mA.
hydroxyl groups.94,95 For IJCuO)0.8Pd0.2 NPs, the diffraction
peaks for Pd(0) were not observed, which may be due to the
fact that CuO takes up most of the NPs and the crystalline
size of the Pd core is too small to be detected by XRD. The
four reactions used to evaluate Ag–Pd NPs were conducted
under the same conditions over these CuO–Pd NPs (Tables 2–5,
entries 6–9). The CuO–Pd NPs are more active than pure CuO
NPs but much less active than pure Pd NPs in all cases. The
catalytic performance of CuO–Pd NPs correlates with the
structural features of these NPs, i.e., the catalytically active
Pd is mostly buried inside the particle.
XPS
X-ray photoelectron spectroscopy (XPS) was performed under
Al Kα radiation (hν = 1486.71 eV) at 5 × 10−9 Torr. The model
is Kratos AXIS Ultra DLD. Binding energies (BE) are cali-
brated by the C (1 s) binding energy of adventitious carbon
contamination (set at 284.6 eV). The analysis of Pd, Ag and
Cu was based on Pd 3d, Ag 3d and Cu 2p peaks, respectively.
Experimental
Materials and methods
GC
Materials. Materials were purchased as follows: palladiumIJII)
acetate (Pd 48%), copperIJII) acetate (Cu 34%), silver acetate
(≥99.0%) from Sinopharm Chemical Reagent; ijC2OH]ijNTf2]
from Centre for Green Chemistry and Catalysis, Lanzhou Insti-
tute of Chemical Physics, Chinese Academy of Science; ethyl
acetate (AR) from Merck Pte Ltd.; acetonitrile from Tedia Com.
Inc.; ethanol (analytical reagent grade) and diethyl ether (analyt-
ical reagent grade) from Fisher Scientific, UK; styrene (≥99%),
nitrobenzene (99%), 4-nitrophenol (≥99.5%), 4-bromotoluene
(98%) and cyclohexanone (≥99.0%) from Sigma Aldrich.
For all catalytic reactions, an Agilent 7890A gas chromatogra-
phy (GC) system equipped with a flame ionization detector
and a HP-5 capillary column was employed following the
internal standard method to determine reactivity (conversion
and selectivity).
Preparation of the bimetallic NPs
The synthetic procedure applied for preparing Ag0.5Pd0.5
NPs is given as a typical experiment: PdIJOAc)2 (11.2 mg,
0.05 mmol) and Ag(OAc) (8.3 mg, 0.05 mmol) were dissolved
in acetonitrile (4 mL) under vigorous stirring and then
mixed with ijC2OHmim]ijNTf2] (1 mL) in a round bottom flask
(10 mL) covered with an aluminium foil. Acetonitrile was
evaporated under vacuum at 310 K for 30 min. Kept under
vacuum (to remove of the volatile compounds during the
decompensation), the system was heated at 393 K in a pre-
heated oil bath for 2 h with intensive stirring (800 r min−1),
during which co-decomposition of PdIJOAc)2 and Ag(OAc)
occurred, affording a dark brown colloidal suspension. After
cooling to r.t., two different post treatment methods were per-
formed for characterization.
ICP analysis
Metal content in Ag–Pd NP series was determined using an
iCAP 6000 series inductively coupled plasma optical emission
spectrometer (ICP-OES). For Cu–Pd NPs, it was determined
by using an Agilent Technologies 7500 series inductively
coupled plasma mass spectrometer (ICP-MS). Both the NPs
and the supernatants obtained from the purification step
were analysed. The supernatant was transferred to another
flask, whereas all the NPs were re-dispersed in acetonitrile
(2 ml, solution A). 1/4 of the total volume of solution A
and 1/3 of supernatants (both were treated with a rotary
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For XRD and XPS analysis, the NP/IL suspension was
part and Pd in the outer layer. Charge transfer from Pd to Ag
was observed by XPS, which endows the NPs with special
electrical properties and promotes their catalytic performance
in hydrogenation. For the CuO–Pd NPs, Pd has a high ten-
dency to stay in the core while CuO prefers to be located on
the surface, as expected given its oxophilic character. This
study reveals that thermal decomposition of metal acetates is
a convenient method to prepare various bimetallic NPs in ILs
with a core–shell structure for applications in catalysis and
potentially beyond.
centrifuged at 8000 r min−1 for 4 min. The supernatant
was removed and the remaining NPs were collected for
measurement.
For ICP-OES and TEM analysis, the colloidal suspension
was diluted with acetone/ethanol (10 mL in total, v/v = 7 : 3).
Then the organic phase and the NPs were separated through
centrifugation (8400 r min−1, 4 min) at r.t. Staying at the
bottom, the NPs were easily separated, whereas the superna-
tant contained a mixture of the washing solvent, IL and
unreacted precursors.
The Pd NP suspension was treated in the same way, except
that the suspension was diluted with ethanol/water (10 mL
in total, v/v = 7 : 3) instead of acetone/ethanol for easier
separation.
For the synthesis of CuO and CuO–Pd NPs, the procedure
was slightly modified. 413 K was selected as the decomposi-
tion temperature and aluminium foil was not used. After
cooling to r.t., a mixture of ethanol/water (10 ml in total,
v/v = 7 : 3) was added to the suspension of NPs for their isola-
tion by centrifugation.
Acknowledgements
The authors thank the A*Star, PSF funding (WBS: R-279-000-
403-305), the Merlion project (WBS: R-279-000-421-133) and
CNRS for financial support.
Notes and references
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