Rhodium-nickel bimetallic nanocatalysts: High performance of room-temperature hydrogenation

Article abstract of DOI:10.1039/c2cc37668g

Rhodium-nickel bimetallic nanocrystals were fabricated with high activity in hydrogenation of olefins, nitroarenes and arenes at room temperature, indicating that bimetallic nanocrystals of noble and non-noble metals represent a novel kind of nanocatalyst. This journal is

Full text of DOI:10.1039/c2cc37668g

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Rhodium–nickel bimetallic nanocatalysts: high
performance of room-temperature hydrogenation†
Cite this: DOI: 10.1039/c2cc37668g
Haohong Duan,a Dingsheng Wang,a Yuan Koub and Yadong Li*a
Received 22nd October 2012,
Accepted 16th November 2012
DOI: 10.1039/c2cc37668g
www.rsc.org/chemcomm
Rhodium–nickel bimetallic nanocrystals were fabricated with high non-noble metals may offer opportunities for developing effec-
activity in hydrogenation of olefins, nitroarenes and arenes at tive catalysts that can reduce Rh usage while retaining or even
room temperature, indicating that bimetallic nanocrystals of noble enhancing the catalytic properties. In this work we focus on
and non-noble metals represent a novel kind of nanocatalyst.
rhodium–nickel NCs because, in addition to its lower cost,
nickel (Ni) offers an additional advantage of imparting ferro-
The selective hydrogenation of arenes is one of the most magnetic behavior to the NCs. Such magnetic catalysts can be
important organic reactions since it is used to prepare a wide reclaimed by magnetic separation after use, which allows the
variety of compounds which find applications as industrial catalyst to be easily reused and minimizes loss of Rh during
feedstocks, such as cyclohexane, and in the production of catalyst recycling. Since the nanoparticles can be easily sepa-
low-aromatic diesel fuels.1–5 Hydrogenation of arenes is tradi- rated in this way, it is not necessary to support them on
tionally carried out using heterogeneous noble metal catalysts materials such as Al2O3, silica, SBA-15, and carbon nanotubes
under forced reaction conditions (high temperature and/or (CNTs) to facilitate their recycling.8,10,24 The main drawback of
high pressure)6–9 and developing effective catalysts that can such heterogeneous supported catalysts is the loss of activity
catalyze hydrogenation of arenes under milder conditions and selectivity because the active sites are confined in two-
remains a significant challenge.
dimensional space which limits access of the reactants. Metal
Rhodium nanocrystals (Rh NCs) are commonly used as NCs can generally be well dispersed in non-polar solvents due
catalysts for the hydrogenation of arenes in the laboratory.10–13 to their hydrophobic surfaces, and so they should also be well
However, their use on an industrial scale is restricted by the dispersed in an arene hydrogenation reaction system. This type of
limited reserves and high price of Rh. Many efforts have been catalyst, which combines the advantages of both homogeneous and
made by scientists worldwide to solve this problem. One heterogeneous catalysts, can be regarded as a ‘‘semi-homogeneous
approach has focused on increasing the efficiency of the catalyst’’: the NCs are heterogeneous in nature but, similar to
utilization of Rh by preparing NCs with selective exposure of homogeneous catalysts, their high degree of dispersion in the
well-defined crystal facets that show high activity and high solvent facilitates the access of reactant molecules.
selectivity.14–16 Another potential solution is the partial replace-
Although many methods for preparation of monometallic
ment of Rh with a non-noble metal. In addition to the Rh NCs have been developed in recent years,25–27 the control-
economic benefits of this substitution, the resulting bimetallic lable synthesis of bimetallic NCs of Rh and non-noble metals is
NCs do not always possess a simple combination of the proper- much more difficult because non-noble metals are much less
ties of their individual constituents, and may also show new readily reduced than noble metals like Rh.28–30 Herein, we
properties and capabilities due to a synergy between the demonstrate the synthesis of RhxNi1Àx bimetallic NCs using a
two metals.17–23 Therefore, bimetallic NCs containing Rh and simple one-pot procedure. The composition of the bimetallic
NCs can be tailored over a broad range of values of x. We have
evaluated the catalytic performance of Rh–Ni NCs with different
compositions in the room-temperature hydrogenation of
arenes and found that they exhibit higher activity than pure
Rh NCs. Furthermore, the bimetallic nanocatalysts exhibit high
performance in hydrogenation of olefins and nitroarenes. These
exciting preliminary results are an indication that bimetallic
NCs of noble and non-noble metals represent a new kind of
nanocatalysts that may replace noble metals.
a Department of Chemistry and the State Key Laboratory of Low-Dimensional
Quantum Physics, Tsinghua University, Beijing, China.
E-mail: ydli@mail.tsinghua.edu.cn; Tel: +86-10-62772350
b PKU Green Chemistry Center, Beijing National Laboratory for Molecular Sciences,
College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China. E-mail: yuankou@pku.edu.cn
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, further TEM images, XRD patterns, and catalytic data. See DOI:
10.1039/c2cc37668g
c
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Table 1 Hydrogenation of phenol catalyzed by RhxNi1Àx NCsa
Conv.b
(%)
Cyclohexanoneb
(%)
Cyclohexanolb
(%)
TOFc
Catalyst
(hÀ1
)
Rh
42.5
54.4
22.3
N.D.
38.8
28.8
38.5
N.D.
61.2
71.2
61.5
N.D.
283
363
149
0
Rh0.67Ni0.33
Rh0.5Ni0.5
Ni
a
64 mmol phenol substrate, 0.025 mol% catalyst (based on Rh), 6 mL
cyclohexane under H2 (40 atm) at room temperature (25 1C) for 6 hours.
b
c
Fig. 1 (a) TEM image of Rh–Ni nanocrystals, (b) HRTEM image of an individual
Rh–Ni nanocrystal (inset, up-right: enlarged HRTEM image, down-right:
FFT pattern). (c–e) HAADF-STEM image together with the EDX mapping of an
individual Rh–Ni nanocrystal (yellow = Rh–L, red = Ni–K).
Determined by GC-MS. Turnover frequency (TOF) measured in
[mol product][mol metal]À1 hÀ1
.
corresponding variation of lattice parameters (Fig. S3†) also
show the expected decrease in unit cell size with increasing
values of x. It should be noted that monometallic Ni NCs (x = 0)
could also be obtained but the process demands more harsh
conditions and the methods changed a little (see ESI†).
As a probe reaction to evaluate the catalytic performance of
RhxNi1Àx nanoparticles, phenol hydrogenation was carried out
at room temperature (25 1C) and 40 bar hydrogen pressure for
6 h. We choose Rh, Rh0.67Ni0.33, Rh0.5Ni0.5 and Ni nanoparticles
(Fig. S4 and S5, ESI†) to preliminarily evaluate the catalysis for
their obviously different Rh/Ni ratio and similar size distribu-
tion (Fig. S6†). The results are summarized in Table 1. The
observed hydrogenation products of phenol were cyclohexa-
none and cyclohexanol. Compared with the monometallic Rh,
the Rh0.66Ni0.33 nanocatalyst showed both higher activity and
higher selectivity for cyclohexanol. However, when the Ni con-
tent was further increased, the resulting Rh0.5Ni0.5 NCs showed
a much lower activity, and comparable selectivity, to the
monometallic Rh nanocatalyst. Because the catalytic reactions
using RhxNi1Àx NCs with similar size and morphology were
performed under the same conditions, the very different per-
formances of the three catalysts must arise from the different
Ni content in the NCs as the influence of other parameters can
be eliminated. The molar ratio of Rh to Ni can greatly influence
the surface structure, electron density distribution, and the
effective electronegativity of bimetallic NCs, all of which
might play a key role in determining the catalytic activity and
selectivity.
In the synthesis of RhxNi1Àx bimetallic NCs, RhCl3Á3H2O
and Ni(acac)2 (acac = acetylacetonate) were used as the metal
precursors while octadecylamine (ODA) was employed as
simultaneously the solvent, stabilizing surfactant, and reducing
agent. The reaction was performed at 230 1C and required only
2 min (the detailed procedure is described in the ESI†). Using
the synthesis result of Rh0.67Ni0.33 NCs as an example (Fig. 1a),
Rh0.66Ni0.33 NCs with an average size of 10 nm are successfully
produced in high yield. Fig. 1b presents a representative
enlarged HRTEM image of an individual Rh0.66Ni0.33 NC. The
well-resolved fringes and fast Fourier-transform (FFT) pattern
shown in the insets of Fig. 1b reveal clear lattice fringes with a
regular distance of 0.218 nm, indicating the {111} facets of
bimetallic Rh–Ni NCs with face-centered cubic (fcc) structure.
The powder X-ray diffraction (XRD) analysis also determines
the structural nature of the NCs (Fig. S1, ESI†). In the XRD
pattern of Rh0.67Ni0.33, the peaks corresponding to the (111),
(200), and (220) reflections appeared between those of Rh
(JCPDS 05-0685) and Ni (JCPDS-04-0850) standard peaks,
demonstrating the bimetallic structure of obtained NCs. The
(111) peak has a much higher intensity than the others due to
the exposure of (111) planes. To further confirm the bimetallic
structure, the elemental distributions of these two metals were
studied by a high-angle annular dark-field scanning transmis-
sion electron microscope (HAADF-STEM). Fig. 1c–e show a
representative STEM image and its corresponding Rh and Ni
elemental maps, clearly revealing that both Rh and Ni were
distributed evenly in each individual NC. The compositions of
the Rh0.66Ni0.33 NCs – as determined by inductively coupled
plasma atomic emission spectrometry (ICP-AES) – were close to
the expected stoichiometries, with the measured results for
Room-temperature hydrogenation of benzene catalyzed by
Rh, Rh0.67Ni0.33, and Rh0.5Ni0.5 NCs was also studied, and the
results are given in Table 2. Hydrogenation of benzene cata-
lyzed by RhxNi1Àx nanocatalysts afforded the completely
saturated product cyclohexane. In this case, the activity of the
catalysts increased with increasing nickel content, with the TOF
Rh0.66Ni0.33 being Rh0.69Ni0.31
.
It was also possible to prepare RhxNi1Àx NCs with other
compositions in a broad range due to the high miscibility of Rh
and Ni and the high versatility of the procedure. XRD patterns
of the as-obtained products are also shown in Fig. S1 (ESI†). For
all the RhxNi1Àx, the corresponding peaks appeared between
those of Rh and Ni standard peaks. More importantly, with the
increasing content of Ni, the peaks continuously shifted from
the Rh standard peaks to Ni standard ones, demonstrating that
the cubic unit cell parameter decreases when Rh atoms are
progressively replaced by smaller nickel atoms. The HRTEM of
Table 2 Hydrogenation of benzene catalyzed by RhxNi1Àx NCsa
Catalyst
Conv.b (%)
TOFc (hÀ1
)
Rh
28.6
45.7
50.8
N.D.
164
261
290
0
Rh0.67Ni0.33
Rh0.5Ni0.5
Ni
a
64 mmol benzene substrate, 0.025 mol% catalyst (based on Rh),
6 mL cyclohexane under H2 (40 atm) at room temperature (25 1C)
b
c
for 7 hours. Determined by GC-MS. TOF measured in
RhxNi1Àx with different composition (Fig. S2, ESI†) and the [mol product][mol metal]À1 hÀ1
.
c
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for Rh0.67Ni0.33 and Rh0.5Ni0.5 nanocatalysts being, respectively, incorporation of Ni into Rh NCs can endow them with superior
1.6 and 1.8 times higher than that of the pure Rh catalyst. It is catalytic properties compared to pure Rh. Therefore these novel
interesting that for hydrogenation of phenol the catalytic Rh–Ni bimetallic nanocatalysts have potential applications in
activity increases in the order Rh0.5Ni0.5 o Rh o Rh0.67Ni0.33
while for hydrogenation of benzene the catalytic activity
increases in the order Ni o Rh o Rh0.67Ni0.33 o Rh0.5Ni0.5
;
industrial hydrogenation reactions.
Financial support for this work by the State Key Project of
Fundamental Research for Nanoscience and Nanotechnology
.
By virtue of the differences in steric effects and electron density (2011CB932401), NSFC (20921001, 21051001) and China
distributions between benzene and arene derivatives, hydro- Postdoctoral Science Foundation (20100470335) is gratefully
genation catalysts often show different performance. Clarifica- acknowledged.
tion of the influence of the composition of RhxNi1Àx NCs on
their catalytic properties in different reactions requires further
detailed investigation which is currently underway in our
laboratory.
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c
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