Pd-Cu2O and Ag-Cu2O hybrid concave nanomaterials for an effective synergistic catalyst

Article abstract of DOI:10.1002/anie.201303912

Palladium and silver salts were combined with Cu₂O octadecahedra in concave heterostructures. The formation of concave faces involved selective oxidative etching of Cu₂O on the {100} faces and in situ growth of Pd/Ag on different sites. The structures showed superior catalytic activities to both single domains and their mixtures in a model Sonogashira-type organic reaction. Copyright

Full text of DOI:10.1002/anie.201303912

Angewandte
Chemie
DOI: 10.1002/anie.201303912
Concave Hybrid Materials
Pd-Cu2O and Ag-Cu2O Hybrid Concave Nanomaterials for an
Effective Synergistic Catalyst**
Lingling Li, Xiaobin Chen, Yuen Wu, Dingsheng Wang, Qing Peng, Gang Zhou, and Yadong Li*
Dedicated to Professor Xiao-Zeng You on the occasion of his 80th birthday
Metal–semiconductor or metal–metal-oxide heterostructures
have been of great interest for their distinguished properties,
such as catalysis,[1] supercapacitor electrodes,[2] and multi-
functional probes.[3] A single heterostructure is expected to
have a large diversity of shapes, compositions, and especially
interfaces, as each of its domains can be tuned, whereby its
combinational functionality changes accordingly.[4] For heter-
ostructures, the interface is believed to play a vital role in
many cases owing to its distinguished charge state, atomic
arrangement, and so on.[5] Rational design of the metal–
semiconductor interface might thus help us to obtain catalysts
with good activity, selectivity, and/or durability for an idio-
graphic reaction. Admittedly, site-selective growth has pro-
vided an efficient way to prepare delicate hybrid architectures
with controlled interfaces by regulating one domain to
selectively nucleate and grow on specific sites of other
domains.[6]
Recently, nanoscale heterostructures were reported to
display advantages in organic catalysis, especially as tandem
catalysts, owing to their flexibility of structures and compo-
sitions.[7] CuI compounds are among the leading catalysts in
organic reactions, and some are lately introduced into the
field of nanocatalysis.[8] Cu2O is an effective catalyst for
oxidative arylation of phenylacetylene,[9] based on which
Cu2O nanocrystal was designated as one domain of the
designed heterostructure. Additionally, Leiꢀs group proved
that transmetalation is the rate-limiting step in the Sonoga-
shira coupling reaction with palladium complexes and cop-
per(I) as synergistic catalysts by in situ IR spectroscopy.[10] So
Pd0 was chosen as the other domain to test its synergistic
effect with Cu2O in the oxidative arylation of phenylacety-
lene. To date, several metal–Cu2O heterostructures have been
constructed by wet chemistry routes.[11] However, few Pd-
Cu2O heterostructures were reported, which can possibly be
ascribed to the comparative large lattice mismatches between
their usually available low-index facets. Great effort needs to
be put into developing new heterostructures for Pd-Cu2O to
explore its application as organic catalyst.
We therefore tried to cooperate the two parts with
controlled architecture to study its structure-dependent
properties. Herein we present a seed-based strategy to
construct a site-selective growth of noble metals on Cu2O
nanopolyhedra, the {100} faces of which were more vulner-
able than {110} and were gradually oxidatively etched to
concaves. PdNPs and Ag nanodisks preferred different sites
of Cu2O concaves in formation of metal-Cu2O hybrid concave
nanomaterials (“nanoconcaves”). Thereafter, Pd- and Ag-
Cu2O hybrid nanoconcaves were tested and found to have
superior catalytic activities to both of the single domains and
their mixtures for the Sonogashira-type oxidative arylation of
phenylacetylene. This synergistic effect might be the result of
electron transfer between metal and Cu2O, as validated by
shifts in XPS spectra and related density functional theory
(DFT) calculations, which led to the weakening of interfacial
ꢀ
Cu O bonds and concomitant electron rich of Cu2O.
Representative Pd-Cu2O nanocrystals are compiled in
Figure 1. Almost a full yield of nanoconcaves was obtained
with several NPs of 15 nm on the concavities of Cu2O
octadecahedra. The novel architecture of nanoconcaves was
further investigated using its high-magnification SEM and
TEM together with high-resolution TEM (HRTEM) images
(Figure 1b,c, and e). The Cu2O octadecahedron, which is
composed of six {100} and twelve {110} facets, was excavated
on {100} facets and converted into its concave counterpart.[9]
EDX analyses confirmed the existence of Pd (Supporting
Information, Figure S2a). Additionally, both mapping and
line-scanning indicated the concavity and homogeneous
distribution of Pd on the concavities (Figure 1d; Supporting
Information, Figure S2b). However, no metallic Pd could be
traced in the XRD data of Pd-Cu2O hybrid nanoconcaves
(Supporting Information, Figure S3), probably on account of
the small ratio of Pd in the hybrids that might go beyond the
detection limit of the powder diffractormeter. The later ICP-
MS data showed the atomic ratio of Pd to Cu was 1.6: 100
(approximately 2.3 wt% of Pd).
[*] L. L. Li, Y. E. Wu, Dr. D. S. Wang, Dr. Q. Peng, Prof. Y. D. Li
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
E-mail: ydli@mail.tsinghua.edu.cn
X. B. Chen
Department of Physics, Tsinghua University (China)
Prof. G. Zhou
State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, Beijing 100029 (China)
[**] This work was supported by the State Key Project of Fundamental
Research for Nanoscience and Nanotechnology (2011CB932401
and 2011CBA00500), National key Basic Research Program of China
(2012CB224802), and the National Natural Science Foundation of
China (Grant No. 21221062, 21171105 and 21131004).
The formation process was investigated by sampling at
different reaction stages (Supporting Information, Figure S4).
Etching of Cu2O seeds became more severe, as indicated in
SEM images and successively weakening of (200) peak in
XRD patterns with time. The newly formed Pd0 atoms
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303912.
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consequently affected the binding sites for O2. Such facet-
selective capping has been previously reported.[12] This
speculation was later verified (Supporting Information, Fig-
ure S8). In absence of OAm, Cu2O octadecahedra were
etched mainly on corners and edges in low coordination
states. This preliminarily demonstrated that OAm could
promote the selective corrosion of {100}. Herein, reducibility
and Pd precursor also played important roles in controlling
the reaction kinetics and finally the products. We tried
elevating the reducibility of the system to reduce dimensions
of Pd in the Pd-Cu2O nanoconcave by introducing borane–
tert-butylamine complex (TBAB, 0.01 mmol) as a strong
reductant. PdNPs were obtained scattering off Cu2O with
much smaller sizes (Supporting Information, Figure S9).
[Pd(OAm)x]2+ stripped electrons from TBAB rather than
Cu+ species, and thus they were unable to grow on Cu2O.
Pd(OAc)2 provided similar structure to [Pd(acac)2] as OAcꢀ
and acacꢀ are both bidentate ligands. For PdCl2, Cu2O were
completely etched within 36 h, presumably promoted by the
Clꢀ/O2 etching (Supporting Information, Figure S10).[16]
We further demonstrated this strategy was applicable for
Ag-Cu2O hybrid nanoconcaves. Though flat Cu2O octadeca-
hedra were also subject to etching and converted into their
concave counterparts, the growth of Ag was distinct from that
of Pd. As shown in Figure 2, AgNPs had a disk-like shape,
Figure 1. a) SEM image of Pd-Cu2O nanoconcaves and b) its magnified
counterpart; c) high-magnification TEM image and e) HRTEM image
recorded in the direction of one concavity of an individual Pd-Cu2O
nanoconcave; d) EDX-mapping with the HAADF-STEM image of the
hybrid.
inclined to grow on the concavities driven by thermodynamics
since the chemical potential is lower when an atom is added
on the concavity than on the flat surfaces.[12] We noticed that
noteworthy PdNPs emerged mainly after the etching of Cu2O
(Supporting Information, Figure S4a,b). [Pd(OAm)x]2+ was
supposed to be reduced by the fresh Cu2O species adjacent to
those were just etched,[13] which in turn significantly promoted
the etching of Cu2O octadecahedra. Indeed, the amount of
[Pd(acac)2] had a big impact on the final product of hybrids.
Formation of octadecahedral Cu2O concaves took almost 60 h
in absence of [Pd(acac)2] (Supporting Information, Figure S5
and Figure S6a–d). When plentiful [Pd(acac)2] was supplied,
the vast majority of Cu2O seeds were excessively etched in
formation of 4 nm sized tetrahedral PdNPs. PdNPs were
overgrown into dendrites after 9 h (Supporting Information,
Figure S6). As such, [Pd(acac)2] was believed to have
accelerated the etching of Cu2O octadecahedra, but not as
a catalyst as was previously reported.[14]
The composition and morphology of the final product
were demonstrated to be strictly affected by reaction
conditions. No depression could be perceived on any faces
after 60 h under Ar purge, and PdNPs scattered randomly
around flat faces of Cu2O. When the reaction system was
aerated with an O2 balloon (ca. 500 mL of pure O2), Cu2O
octadecahedra were pronouncedly pitted on {100} faces after
12 h, and later subject to severe corrosion at 24 h (Supporting
Information, Figure S7). The above results strongly identified
the etching of Cu2O was an O2-participated oxidative
reaction. Some previous reports found {100} were the most
stable faces.[15] We speculated that the absorption capability of
OAm might vary on different facets of Cu2O seeds, and
Figure 2. a) SEM image of Ag-Cu2O nanoconcaves and b) its magni-
fied counterpart; c) high-magnification TEM image and e) HRTEM
image of an edge of an individual Ag-Cu2O nanoconcave; d) EDX-
mapping with the HAADF-STEM image.
and their average diameter and height were measured as D =
23 nm and h = 10 nm, respectively. Ag nanodisks had a pro-
pensity to grow on edges and vertices of Cu2O nanoconcaves
(Figure 2a–c). As depicted in Figure 2e, Ag0 directly inter-
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faced with Cu2O through epitaxial growth owing to their
moderate lattice mismatch of {200} faces (4.2%). The two
discrete sets of XRD patterns of Ag-Cu2O could be separately
indexed to cubic Ag and Cu2O (Supporting Information,
Figure S3). Ag was also confirmed by EDX analyses (Fig-
ure 2d; Supporting Information, Figure S11). Based on ICP-
MS analysis, the amount of Ag reached up to 9 mol% (Ag/
Cu = 4.9:100 in mole) in the hybrid nanoconcaves.
In terms of Ag-Cu2O nanoconcaves, Ag+ could also be
reduced by Cu2O according to the different electrochemical
redox potentials.[13] In the present system, a mass of AgNPs
formed immediately when the temperature reached 1208C,
indicating the reduction of [Ag(OAm)x]+ was much easier
than [Pd(OAm)x]+ by Cu+ species (Supporting Information,
Figure S12a). Consequently, Ag0 nucleated preferentially at
edges and vertices of Cu2O seeds which were shown to be
more active than faces,[17] together with a small fraction on
{100} faces at the initial stage. Then more Ag grew with the
h100i orientation in formation of disks (Supporting Informa-
tion, Figure S12b,c). Afterwards, with the unceasing oxidative
etching of Cu2O on its {100} facets by O2, most Ag nanodisks
were finally on the edges and vertices of concave Cu2O, and
phase evolution was revealed (Supporting Information, Fig-
ure S12).Accordingly, the proposed formation of Cu2O, Pd-
Cu2O and Ag-Cu2O nanoconcaves is illustrated in the
Supporting Information, Figure S13.
In our previous work, Cu2O octadecahedra provided good
activity in the aerobic oxidative arylation of phenylacety-
lene.[9] This reaction was herein introduced to test properties
of Pd-Cu2O and Ag-Cu2O hybrid nanoconcaves (Supporting
Information, Scheme S1). Pd/Ag-Cu2O provided excellent
yields of 95% and 91% after 20 h, and their time-dependent
catalytic activity is shown in the Supporting Information,
Table S1. As the common domain of the hybrids, Cu2O
nanoconcaves were less active in ca. 72% yield (Supporting
Information, Table S2, entry 1). So it is supposed that the
enhanced activity of hybrids might have association with Pd
and Ag counterparts. Pd NPs/C and commercial Pd/C were
inactive in the reaction during the whole process, while the
activity of Ag nanodisks and Ag NPs/C began to be detectable
by 12 h in 37% and 41% yields (Supporting Information,
Figure S14; Table S2, entries 2, 3, and 6). Besides, Pd NPs/C +
Cu2O nanoconcaves and Ag nanodisks + Cu2O nanoconcaves
were less active compared with their hybrid counterparts
(Supporting Information, Table S2, entries 4 and 5). The
integrate comparisons of activities among the catalysts were
illustrated in Figure 3. The hybrids had a higher rate than
Cu2O at the initial stage of the catalysis (Supporting
Information, Figure S15). After catalysis, the structure and
composition were basically maintained for Pd/Ag-Cu2O
hybrid nanoconcave, and no evident oxidation was observed
(Supporting Information, Figure S16–18). The hybrids could
be recycled without noteworthy decrease of activity, 92% for
Pd-Cu2O and 89% for Ag-Cu2O after 20 h in the second run.
Afterwards the hot filtration was performed, which is one of
the valid heterogeneity tests.[18] GC showed filtrate 1 and 2
proceeded to 45% and 41% respectively for a total time of
20 h, hardly any increase in yield compared with 41% at 1 h
(see the Supporting Information). Notably, in present catal-
Figure 3. Catalytic activities as a function of time in the model reaction
for catalysts: a) Pd-Cu2O hybrid nanoconcave, the single domains
(PdNPs/C, concave Cu2O) and its mixtures as PdNPs/C+concave
Cu2O; b) Ag-Cu2O hybrid nanoconcave, the single domains (Ag nano-
disks, concave Cu2O) and its mixtures as Ag nanodisks +concave
Cu2O.
ysis, phenylboronic acid was adopted as the substrate instead
of the commonly used aryl halides, and thus PdNPs could be
prevented from leaching.[19]
We referred to X-ray photoelectron spectroscopy (XPS)
to inspect the interaction between noble metals and the host,
because coupling will cause changes of spectroscopic proper-
ties of the hybrid.[20] As shown in Figure 4a, the peaks at 932.7
and 952.5 eV could be assigned to Cu2p3/2 and Cu2p1/2 of
Cu2O nanoconcaves with binding energy calibrated with
C1s = 284.8 eV. XPS spectrum of Pd-Cu2O hybrids depicted
that the binding energy of Cu2p3/2 decreased by 0.2 eV
relative to that of Cu2O nanoconcaves. Moreover, the binding
energies of Pd and Ag in hybrid nanoconcaves were slightly
shifted up when compared with their single domains as Pd
NPs and Ag nanodisks (Supporting Information, Figure S19
and 20). The above shifts could probably be ascribed to
electron transfer from the noble metal domain to Cu2O
host.[20b,c]
To elucidate, the nature of synergistic effects in the Pd/
Ag-Cu2O nanoconcaves, we performed extensive theoretical
investigations using DFT. The calculated work functions of Pd
and Ag were smaller than that of {100} faces of Cu2O with the
O-terminated surface (7.27 eV) which was regarded as the
normal state (Supporting Information, Table S3).[21] This
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a result, the band gap of Ag-Cu2O nanoconcaves increased,
corresponding to the blue-shifted peak of UV/Vis spectrum
relative to Cu2O nanoconcaves, which is consistent with the
measured UV/Vis spectra (Supporting Information, Fig-
ure S21). It was the same case for Pd-Cu2O hybrids. Lei
showed that transmetalation of the Sonogashira reaction is
the rate-limiting step by in situ IR spectroscopy.[10] The
electron-rich alkyne–copper species with enhanced nucleo-
philicity would facilitate the transmetalation reaction with Pd
intermediate and further the whole reaction. As a result, Pd-
Cu2O hybrid nanoconcaves showed superior catalytic activ-
ities to their mixture of single domains in the Sonogashira-
type reaction. It should be noted that Ag cannot participate
the oxidative addition and reductive elimination reaction as
Pd did, and thus should not keep to such a catalytic cycle. The
specific relationship of charge transfer with enhanced cata-
lytic properties for Ag-Cu2O hybrid nanoconcaves is now
under investigation.
In summary, PdNP-Cu2O and Ag nanodisk-Cu2O hybrid
nanoconcaves were prepared through site-selective growth of
noble metals on Cu2O seeds. Pd could only nucleate on the
cavities of Cu2O, while Ag0 principally grew on edges and
vertices of concave Cu2O. The hybrid nanoconcaves exhibited
superior catalytic activities to the single domains and their
mixtures for the model organic reaction. Shifts of Cu2p and
Pd (or Ag) 3d in the XPS spectra of the hybrids, together with
DFTresults, preliminarily revealed the electron transfer from
the noble metal part to the Cu2O host, which could be the
cause of synergistic effect of the hybrid nanonconcaves. From
the synergistic effects, correlated with the electronic com-
pensation mechanism of metal oxides, we could propose the
similar metal/oxide heterostructures as a potential candidate
for a catalyst if following the two fundamental principles: 1) a
good lattice match between metal and oxide, ensuring
potential metal–oxide interaction; and 2) a pronounced
work function difference, corresponding to a steady electron
transfer between domains.
Figure 4. a) Cu2p XPS spectra of Pd-Cu2O, Ag-Cu2O, and Cu2O nano-
concaves. b) Difference charge density in Ag-Cu2O (left) and Pd-Cu2O
(right) nanoconcaves with the Ag/O- and Pd/O-terminal interfaces,
respectively, referred from the experimental observations. Blue and
yellow areas represent the charge accumulation and depletion, respec-
tively. The isosurface value is 0.03 eꢀ3. O Red, Cu brick-red, Ag light
blue, Pd cyan.
indicated the possibility of charge transfer between two
domains of the hybrids. The electron transfer associated with
metal–oxide interactions was irrespective of the morpholo-
gies of metal and oxide,[22] so we proposed a straightforward
interface model for the hybrid nanoconcaves to directly track
the charge transfer in the interface region, and analyzed its
effects on the interface property from and the domains from
electronic contributions. In what follows, the Ag-Cu2O hybrid
nanoconcave was taken as an example because its epitaxial
Received: May 7, 2013
Revised: July 3, 2013
Published online: August 26, 2013
Keywords: concave nanohybrids ·
growth on {100} of Cu O was clearly observed (Figure 2e).
.
2
density functional calculations · heterogeneous catalysis ·
site-selective growth · synergistic effects
More precisely, difference charge density (Figure 4b) and
Bader charge analysis showed that in addition to the electron
transfer from Pd/Ag to Cu2O (0.25/0.36e), part of donated
electrons of interfacial O were steady transferred back to the
neighboring Cu+ (0.17/0.2e), indicating an electronic com-
pensation mechanism of metal oxides. This process made the
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