Nickel-Gallium intermetallic nanocrystal catalysts in the semihydrogenation of phenylacetylene

Article abstract of DOI:10.1002/cctc.201300813

The chemoselective hydrogenation of alkyne is of great importance in the chemical industry, in which intermetallic compounds (IMCs) have attracted extensive interest as efficient catalysts. Herein, we demonstrate the preparation of several supported Ni-Ga

Full text of DOI:10.1002/cctc.201300813

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DOI: 10.1002/cctc.201300813
Nickel–Gallium Intermetallic Nanocrystal Catalysts in the
Semihydrogenation of Phenylacetylene
Changming Li, Yudi Chen, Shitong Zhang, Junyao Zhou, Fei Wang, Shan He, Min Wei,*
[a]
David G. Evans, and Xue Duan
The chemoselective hydrogenation of alkyne is of great impor-
tance in the chemical industry, in which intermetallic com-
pounds (IMCs) have attracted extensive interest as efficient cat-
alysts. Herein, we demonstrate the preparation of several sup-
ported Ni–Ga IMCs (Ni Ga, Ni Ga , and NiGa) via a facile in situ
the best catalytic behavior can be obtained over the Ni Ga IMC
3
with a styrene yield of 87.7% (particle size=7.2 nm at 408C
and 0.3 MPa), which is better than that of most of the reported
Ni catalysts. The X-ray absorption fine-structure characteriza-
tion and DFT calculations reveal the electron transfer from Ga
to Ni and active-site isolation by Ga in Ni–Ga IMCs, which ac-
count for the excellent hydrogenation selectivity. The signifi-
cantly improved catalytic performance makes Ni–Ga IMC cata-
lysts promising candidates for the selective hydrogenation of
alkyne.
3
5
3
reduction of layered double hydroxide (LDH) precursors, which
demonstrate significantly improved catalytic activity and selec-
tivity for the selective hydrogenation of phenylacetylene to
styrene. The composition and particle size of Ni–Ga IMCs can
be tuned by adjusting the Ni/Ga ratio or reduction tempera-
ture during the topotactic transformation process of LDHs, and
Introduction
[
9]
Phenylacetylene removal in styrene feedstocks through semi-
hydrogenation is an important industrial process owing to its
serious poisoning effect on styrene polymerization catalysts,
and the selective hydrogenation of phenylacetylene to styrene
Al Fe IMC ) as effective catalysts for the selective hydrogena-
13 4
tion of acetylene to ethylene, the improved catalytic selectivity
of which is attributed to the combination of the electronic
structure and site-isolation effect. Although considerable effort
has been devoted to the preparation of IMCs by using chemi-
[
1]
has been commonly used. The main challenge in this process
is to develop effective catalysts with satisfactory activity and
high selectivity to avoid the complete hydrogenation of phe-
nylacetylene to ethylbenzene. Palladium-based catalysts have
received considerable attention owing to their good hydroge-
nation selectivity toward alkene (e.g., in the dispersed col-
[10]
cal colloidal methods, and their desirable catalytic behavior
has been demonstrated, they still inevitably require expensive
noble metals and costly and toxic organic reagents, damaged
activity by capping reagents, or serious self-agglomeration in
the reaction process. Therefore, it is highly essential to develop
facile and green strategies for the synthesis of IMCs serving as
efficient catalysts.
[
2]
[3]
loids and supported systems ). Although great progress has
been made, the limited resource and high cost of noble Pd
limit their practical applications in the industry. Therefore, the
design and fabrication of efficient and cost-effective substi-
tutes are desirable and remain a challenging goal.
Layered double hydroxides (LDHs) are a class of naturally oc-
curring and synthetic materials generally expressed by the for-
2
Àx
+
3+
nÀ
2+
3+
mula [M1
Mx (OH) ](A ) ·mH O, in which M and M
2 x/n 2
Intermetallic compounds (IMCs) with their unique electronic
cations disperse in an ordered and uniform manner in brucite-
[
4]
[11]
structures and geometries have attracted extensive research
interest in catalysis, for instance, hydrogenation of unsaturated
like layers. Considerable interest has been focused on LDH
materials as heterogeneous catalysts on the basis of their ver-
[
5]
[12]
aldehydes on RuTi or NiSn IMCs, methanol synthesis and
satility in chemical composition and structural architecture.
[
6]
methanol steam reforming on PdGa and PdZn IMCs, and fuel
A topotactic transformation of LDH materials to uniformly dis-
[
7]
electrocatalytic oxidation over Pt Ti or PtPb IMCs. Armbrꢀster
persed metal nanoparticles (NPs) supported on metal oxide
3
[8]
[13]
et al. recently developed a series of IMCs (e.g., PdGa IMC and
matrix occurs under reducing conditions.
Inspired by the
structural merits of LDH materials, we explored the idea of in-
corporating highly active but lowly selective Ni and inactive
but more electropositive Ga species into the LDH precursors
followed by a reduction process to fabricate non-noble Ni–Ga
IMC (e.g., Ni Ga, Ni Ga , and NiGa) catalysts for the selective
[
a] Dr. C. Li, Y. Chen, Dr. S. Zhang, J. Zhou, Dr. F. Wang, Dr. S. He, Prof. M. Wei,
Prof. D. G. Evans, Prof. X. Duan
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Beijing 100029 (P.R. China)
3
5
3
hydrogenation of phenylacetylene. This strategy would possess
the following desirable features: 1) the atomic scale intersper-
sion of Ni and Ga species in the LDH precursors would facili-
tate the formation of highly dispersed Ni–Ga IMCs under mod-
Fax: (+86)10-64425385
E-mail: weimin@mail.buct.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300813.
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erate reduction conditions, and 2) the catalytic performance of
Ni–Ga IMCs can be tuned by changing the Ni/Ga ratio in LDH
precursors.
due to their different metal proportion and ionic radius. The in-
itial and the resulting metal ratios of the products determined
from inductively coupled plasma atomic emission spectroscopy
analysis are summarized in Table S1, which indicates that the
metal ratios in these products are similar to those in the feed
intake. The SEM images (Figure 2) of Ni Mg Ga LDHs show
Herein, we report the fabrication of several supported Ni–Ga
IMCs (Ni Ga, Ni Ga , and NiGa) with tunable particle size by
3
5
3
using the LDH precursor method and demonstrate their appli-
cation as efficient catalysts for the selective hydrogenation of
phenylacetylene. The composition and particle size of the sup-
ported Ni–Ga IMCs can be tuned by changing the Ni/Ga ratio
or reduction temperature during the topotactic transformation
of LDHs. The X-ray absorption fine-structure (XAFS) characteri-
zation demonstrates the relatively low Ni K-edge X-ray absorp-
tion near-edge structure (XANES) absorption edge and low co-
ordination number in Ni–Ga IMCs compared with the Ni foil,
implying the charge transfer from Ga to Ni and active-site iso-
lation by Ga, which were confirmed by DFT calculations. The
resulting materials demonstrate significantly improved catalytic
selectivity for the hydrogenation of phenylacetylene to styrene,
x
y
z
that all the LDH precursors demonstrate a typical nanoplatelet
morphology with a diameter of 30–100 nm, which indicates
their proper crystallization.
and the best catalytic behavior was obtained over the Ni Ga
3
IMC with a styrene yield of 87.7% (particle size=7.2 nm at
408C and 0.3 MPa), which is better than that of most of the re-
ported Ni catalysts. The remarkably increased hydrogenation
selectivity can be attributed to the observed Ni–Ga synergistic
effect. Our approach holds significant promise for Ni–Ga IMCs
as efficient catalysts for the selective hydrogenation of phenyl-
acetylene.
Results and Discussion
Structure and morphology of Ni Mg Ga LDH precursors
x
y
z
Ni Mg Ga LDH precursors were prepared by using the copreci-
x
y
z
x y z
Figure 2. SEM images of the as-synthesized Ni Mg Ga LDH precursors with
pitation method. The XRD patterns of Ni Mg Ga LDHs with
various x(Ni)/y(Mg)/z(Ga) ratios: A) NiMg Ga -LDHs, B) NiMg Ga-LDHs,
x
y
z
3
4
3
various Ni/Mg/Ga ratios, all of which can be indexed as a rhom-
bohedral structure with the typical (003), (006), (012), (013),
C) Ni
5
Mg
4
Ga
3
-LDHs, and D) Ni
3
Mg
2
Ga-LDHs.
(
110), and (113) reflections at 2qꢀ11.2, 22.7, 34.2, 38.5, 59.8,
and 61.28 for LDH materials, respectively, are shown in
Figure 1. No other crystalline phase was detected, which indi-
cated the high purity of these LDH materials. The small differ-
ence in the 2q reflections among the four LDH precursors is
Topotactic transformation from LDH materials to supported
Ni–Ga IMCs
To study the topotactic transformation of Ni Mg Ga LDH mate-
x
y
z
rials to Ni–Ga IMCs, the temperature-programmed reduction
by H (H -TPR) measurements was performed with Ni Al LDHs
2
2
2
as reference samples (Figure 3). The peak below 4008C is as-
signed to the reduction of Ni species in Ni Mg Ga LDHs (Fig-
x
y
z
ure 3b–e) compared with the reduction peak at 3618C for
Ni Al-LDHs (Figure 3a). The peak above 4008C can be assigned
2
to the reduction of Ga species owing to the high temperature
[14]
required for reduction. Three main peaks were observed in
the range 400–9008C for the four Ni Mg Ga LDH samples (Fig-
x
y
z
ure 3b–e), which indicated the gradual reduction of Ga species
3
+
2+
0
[14]
(
Ga /Ga /Ga ) with the increase in reduction temperature.
With the increase in Ni/Ga ratio in Ni Mg Ga LDHs (from Fig-
x
y
z
ure 3b to Figure 3e), the reduction temperature of Ga decreas-
es significantly, indicating a Ni-promoted reduction of Ga spe-
cies to produce Ni–Ga IMCs, which has also been reported in
Figure 1. XRD patterns of the as-synthesized Ni
various x(Ni)/y(Mg)/z(Ga) ratios: a) NiMg Ga -LDHs, b) NiMg
c) Ni Mg Ga - LDHs, and d) Ni Mg Ga-LDHs.
x
Mg
y
Ga
z
LDH precursors with
[6a]
other bimetal reduction processes (e.g., Pa–Ga system ). The
observed Ni-promoted reduction of Ga species may offer an
3
4
3
Ga-LDHs,
5
4
3
3
2
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Figure 3. H
2
-TPR profiles of a) Ni
2
Al-LDHs, b) NiMg
3 4 3
Ga -LDHs, c) NiMg Ga-
LDHs, d) Ni
detector.
5
Mg Ga -LDHs, and e) Ni
4
3
3
Mg Ga-LDHs. TCD=thermal conductivity
2
opportunity for the controlled synthesis of Ni–Ga IMCs with
tunable composition.
We first performed the reduction of Ni Mg Ga LDHs at
x
y
z
5
008C for 4 h (labeled as Ni Mg Ga -500) to obtain Ni–Ga IMCs;
x y z
the XRD patterns of the resulting products are shown in
Figure 4. The MgO phase at 2qꢀ36.7, 43.1, and 62.38 is ob-
Figure 5. TEM images of the as-synthesized Ni Ga NPs derived from the
3
in situ reduction of Ni
B) NiMg Ga-500, C) Ni
x
Mg
Mg
y
Ga
z
LDH precursors at 5008C: A) NiMg
3 4
Ga -500,
3
5
4
Ga
3
-500, and D) Ni Mg Ga-500. The insets show
3
2
the high-resolution TEM lattice fringe images assigned to the Ni
phase.
3
Ga IMC
lattice spacing of 0.207 nm for all the four samples. Only few
Ni Ga NPs were found for the NiMg Ga -500 sample (Figure 5a)
3
3
4
owing to the low reduction degree of Ga species. With the in-
crease in Ni/Ga ratio, both the particle size and the density in-
crease significantly, which can be attributed to the improve-
ment in the reduction degree of Ga species. The size distribu-
tion and mean particle size were also statistically analyzed by
measurement over 100 particles (Figure S2). The average size
Figure 4. XRD patterns of the as-synthesized Ni–Ga IMCs derived from the
in situ reduction of Ni
b) NiMg Ga-500, c) Ni
x
Mg
Mg
y
Ga
Ga
z
LDH precursors at 5008C: a) NiMg
-500, and d) Ni Mg Ga-500.
3 4
Ga -500,
of Ni Ga NPs was calculated to be 4.8, 7.2, and 10.9 nm for
3
3
5
4
3
3
2
NiMg Ga-500, Ni Mg Ga -500, and Ni Mg Ga-500, respectively.
3
5
4
3
3
2
In brief, Ni Ga IMCs with various particle sizes and loading den-
3
served for all the four samples, which is derived from the calci-
sities can be obtained by controlling the reduction degree of
Ga species via different Ni/Ga ratios in Ni Mg Ga LDH precur-
nation of a Mg source in the LDH precursors. The 2q at
x
y
z
ꢀ
43.88, 50.94, and 75.258 can be indexed to a Ni Ga IMC
sors.
3
[15]
phase (PDF65-0141). For the NiMg Ga -500 sample, a weak re-
In the phase diagram of the Ni–Ga system, a series of Ni–
Ga IMCs can be formed, such as Ni Ga , NiGa, and NiGa . Ac-
3
4
flection intensity of Ni Ga was observed (Figure 4a). With the
3
5
3
4
increase in Ni/Ga ratio in Ni Mg Ga LDHs, the intensity of
cording to the H -TPR results (Figure 3), more Ga species in
x
y
z
2
Ni Ga IMC increases in the following order: NiMg Ga -500<
LDHs would be reduced at a high temperature, which may
result in other Ni–Ga IMCs with large Ni/Ga ratios. To obtain
Ni Ga IMCs with various Ni/Ga ratios, we further explored the
3
3
4
NiMg Ga-500<Ni Mg Ga -500<Ni Mg Ga-500 (Figure 4a–d).
3
5
4
3
3
2
This result suggests that a higher Ni/Ga ratio facilitates the re-
x
y
duction of Ga and the formation of Ni Ga species with larger
calcination of LDH precursors at a higher temperature (7008C;
3
crystal size, which is responsible for the better defined XRD
pattern. This observation will be further demonstrated by TEM
images.
labeled as Ni Mg Ga -700) in H atmosphere; the XRD patterns
x
y
z
2
of the resulting products are shown in Figure 6. For the
NiMg Ga -700 sample, a series of reflections at 2qꢀ44.3, 64.6,
3
4
The SEM images in Figure S1 reveal that the morphology of
the reduced products Ni Mg Ga -500 inherits the original plate-
and 81.68 are observed (Figure 6a), which correspond to the
formation of NiGa IMC (PDF65-6413). The NiMg Ga-700 sample
x
y
z
3
like morphology of LDH precursors. The detailed structural fea-
demonstrates 2q at ꢀ43.3, 48.3, 54.6, and 86.88, which can be
assigned to the typical (221), (002), (040), and (223) reflec-
tions of a Ni Ga IMC (Figure 6b). In the cases of Ni Mg Ga -
ture of the obtained Ni Ga IMC was further revealed by TEM
3
and high-resolution TEM characterizations (Figure 5). Notably,
only Ni Ga phase was obtained according to the Ni Ga (111)
5
3
5
4
3
700 and Ni Mg Ga-700 (Figure 6c and d), (111), (200), and
3
3
3
2
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Figure 6. XRD patterns of the as-synthesized Ni–Ga IMCs derived from the
in situ reduction of Ni
b) NiMg Ga-700, c) Ni
patterns for the corresponding Ni–Ga IMCs and MgO phase are shown in
a’) NiGa IMC (PDF65-6413), b’) Ni Ga IMC (PDF43-1376), c’) Ni Ga IMC
PDF65-0141), and e’) MgO (PDF65-0476).
x
Mg
y
Ga
z
LDH precursors at 7008C: a) NiMg
3 4
Ga -700,
3
5
Mg
4
Ga
3
-700, and d) Ni Mg Ga-700. The XRD standard
3
2
5
3
3
(
(
220) reflections of a Ni Ga phase are obtained, which indicate
3
that both the calcination products are Ni Ga IMCs. As a result,
3
two new Ni–Ga IMCs (NiGa and Ni Ga ) were obtained at an in-
5
3
creased reduction temperature.
After the reduction at 7008C, the resulting Ni Mg Ga -700
x
y
z
samples lost the plate-like morphology of their LDH precursors
Figure S3). The supported Ni–Ga IMC NPs on the matrix can
(
be recognized through the TEM images in Figure 7. According
to the lattice spacings, the observed NPs can be assigned to
NiGa IMC (d110 =0.205 nm), Ni Ga IMC (d =0.209 nm), and
5
3
221
Ni Ga IMC (d =0.207 nm) for NiMg Ga -700, NiMg Ga-700,
3
111
3
4
2
and both Ni Mg Ga -700 and Ni Mg Ga -700, respectively. In
5
4
3
3
2
1
comparison with the Ni Mg Ga -500 samples, in the Ni Mg Ga -
x
y
z
x
y
z
700 samples the particle size of Ni–Ga IMCs increases with the
increase in the mean size from 16.9, 18.8, 17.5 to 25.8 nm. In
general, IMCs obtained through conventional solid phase reac-
tions demonstrate large particle size (>100 nm); in contrast,
Ni–Ga IMCs obtained herein possess significantly decreased
particle size in spite of a high treatment temperature (7008C)
[
13]
owing to the anchoring effect of the MgO matrix. In conclu-
sion, by changing the Ni/Ga ratio in LDH precursors or reduc-
tion temperature, three types of supported Ni–Ga IMCs (NiGa,
Ni Ga , and Ni Ga) as well as Ni Ga IMCs with tunable particle
Figure 7. TEM images of the as-synthesized Ni–Ga IMCs NPs derived from
5
3
3
3
the in situ reduction of Ni
x
Mg
y
Ga
z
LDH precursors at 7008C: A1) NiMg
3 4
Ga -
size (from 4.9 to 25.8 nm) can be obtained.
7
00, B1) NiMg Ga-700, C1) Ni
3
5
Mg
4
Ga -700, and D1) Ni Mg Ga-700. The corre-
3
3
2
sponding high-resolution TEM lattice fringe images are shown in panels A2,
B2, C2, and D2, respectively. The insets show the size distribution of Ni–Ga
IMC NPs with a mean value of 16.9, 18.8, 17.5, and 25.8 nm, respectively.
Catalytic selective hydrogenation of phenylacetylene over
Ni–Ga IMCs
The semihydrogenation of phenylacetylene was used as
a model reaction for the evaluation of selective hydrogenation
performance of these supported Ni–Ga IMC catalysts. As a refer-
ence sample, the supported Ni/MgO catalyst was synthesized
by using the wet impregnation method (Figure S4). Because Ni
NPs could promote the transfer hydrogenation of C–C multiple
bonds by using alcohols (e.g., 2-propanol) as a H source, a con-
ure 8A and B, curve e). Thus, the Ni Mg Ga -700 (Ni Ga),
5
4
3
3
NiMg Ga-700 (Ni Ga ), and NiMg Ga -700 (NiGa) samples with
3
5
3
3
4
similar particle size are chosen to study the effect of the Ni/Ga
ratio. The detailed information for these catalysts is summar-
ized in Table S2. The catalytic conversion and the correspond-
ing selectivity versus reaction time over Ni/MgO and
Ni Mg Ga -700 catalysts are plotted in Figure 8. The hydrogena-
trol experiment in the absence of H was performed over the
2
x
y
z
Ni/MgO sample. No hydrogenation reaction was observed (Fig-
tion activity increases in the following order: Ni/MgO>
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lyst (Ni Ga, particle size=7.2 nm) were also studied (Figure S6).
3
Both the increased temperature and pressure hamper the sty-
rene selectivity. After the optimization of reaction conditions,
the best catalytic performance can be obtained over the
Ni Mg Ga -500 catalyst (Ni Ga, particle size=7.2 nm) with the
5
4
3
3
styrene yield of 87.7% (conversion=95.1% and selectivity=
2.2% at 408C and 0.3 MPa for 5 h). For comparison purposes,
9
we calculated the value of styrene evolution rate
À1 À1
[
molstyrene h g ]. The highest styrene evolution rate was
Ni
À1
À1
found to be 0.084 molstyrene
h m ) for the Ni Mg Ga -500
5 4 3
Ni
sample, which was higher than that of most of the reported
[1–3,16]
Ni-based catalysts.
The results demonstrate that the excel-
lent catalytic performance of Ni–Ga IMC catalysts make them
promising substitutes for noble metals for the selective hydro-
genation of alkyne.
Electron and geometry structure of Ni–Ga IMCs
To gain a deep insight into the structure–function correlation,
the XAFS characterization and DFT calculations were performed
to elucidate the electronic structure and the atomic configura-
tion in their local environment for the three Ni–Ga IMCs (Ni Ga,
3
Ni Ga , and NiGa). The normalized Ni K-edge XANES spectra of
5
3
supported Ni–Ga IMCs as well as the reference sample (Ni foil)
are shown in Figure 9a. The apparent shift of the absorption
edge toward low photon energy relative to the Ni foil reveals
dÀ
the enrichment of electrons on the Ni atom (labeled as Ni ) in
[17]
Ni–Ga IMCs. This result could be attributed to the electron
transfer from the Ga atom to the Ni atom, which is in accord-
ance with the electronegativity values of Ga (1.6) and Ni (1.9).
The Fourier transform of Ni K-edge extended XAFS (EXAFS) os-
cillations in R space is shown in Figure 9b. The first nearest-
neighbor distance in Ni–Ga IMCs increases to high R value
compared with that in the Ni foil: NiꢀNiGa<Ni Ga <Ni Ga,
Figure 8. A) The catalytic conversion and B) the corresponding selectivity
versus reaction time for the hydrogenation of phenylacetylene to styrene
over different catalysts: a) Ni/MgO, b) Ni
Ni Ga ), d) NiMg Ga -700 (NiGa), and e) Ni/MgO tested for comparison pur-
pose under 0.5 MPa of N pressure. Reaction conditions: phenylacetylene/Ni
ratio=15; 1.0 mL of phenylacetylene; 30 mL of 2-propanol; 508C; 0.5 MPa
of H pressure (except curve e).
5 4 3 3 3
Mg Ga -700 (Ni Ga), c) NiMg Ga-700
(
5
3
3
4
2
5
3
3
2
which indicates a strong Ni–Ga interaction in Ni–Ga IMCs
through chemical bonding. The curve fitting results of these
samples are listed in Table 1, which indicates that the specific
Ni–Ga bond length increases from 2.50 (NiGa) to 2.52 ꢂ
Ni Mg Ga -700 (Ni Ga)>NiMg Ga-700 (Ni Ga )>NiMg Ga -700
5
4
3
3
3
5
3
3
4
(NiGa). The Ni/MgO catalyst demonstrates the best hydrogena-
(
Ni Ga ) and finally to 2.53 ꢂ (Ni Ga). The decreased coordina-
5
3
3
tion activity but worst selectivity (Figure 8A and B, curve a),
tion number of Ni–Ga IMCs relative to that of the Ni foil (12)
indicates the interspersion of Ni and Ga elements in Ni–Ga
[
1e,16]
which is consistent with the previous report.
In contrast,
the selectivity toward styrene improves dramatically over Ni–
Ga IMC catalysts (Figure 8B). The Ni Mg Ga -700 catalyst
5
4
3
(
Ni Ga) with the particle size of 16.9 nm demonstrates the opti-
3
Table 1. Curve fitting results of Ni K-edge EXAFS spectra of supported
mal catalytic selectivity with the styrene yield of 70.2% (con-
version=96.3% and selectivity=72.9% at 508C and 0.5 MPa
for 3.5 h; Figure 8B, curve b). To study the effect of the particle
3 5 3
Ni Ga, Ni Ga , and NiGa catalysts.
[
a]
[b]
2
[c]
Sample
Ni foil
Shell
CN
12
R [ꢂ]
Ds [ꢂ]
size of the Ni Ga IMC on its catalytic behavior, evaluations over
Ni–Ni
Ni–Ga
Ga–Ni
Ni–Ga
Ga–Ni
Ga–Ni
Ga–Ni
2.49
2.53
2.53
2.52
2.52
2.50
2.50
0.0012
0.0066
0.0051
0.0023
0.0035
0.0011
0.0011
3
7.6Æ1.2
7.4Æ1.5
6.6Æ1.0
5.8Æ0.9
7.5Æ1.4
7.5Æ1.4
Ni Mg Ga -500 samples with various particle sizes (from 4.8 to
Ni Ga
x
y
z
3
10.9 nm) were performed (Figure S5). Notably, the catalytic ac-
Ni
5
Ga
3
tivity increases with the decreased particle size and the selec-
tivity remains the same. The styrene yield increased to 79.2%
NiGa
(conversion= 96.7% and selectivity=81.9% at 508C and
0
.5 MPa for 2.5 h) over the Ni Mg Ga -500 (Ni Ga) catalyst with
[a] Coordination number; [b] Distance between absorber and backscatter
atoms; [c] Change in the Debye–Waller factor value relative to that of the
reference sample.
5
4
3
3
the particle size of 7.2 nm. The effects of temperature and H2
pressure on the catalytic behavior of the Ni Mg Ga -500 cata-
5
4
3
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Figure 10. Result of DFT calculations for the geometry structure and the
atomic bonding in a single unit cell for A) Ni
3
Ga, B) Ni
5
Ga
Ga
3
, and C) NiGa.
(221), and
Their preferential crystal face for A’) Ni Ga(111), B’) Ni
3
5
3
C’) NiGa(110). The partial interatomic bond length in these Ni–Ga IMCs is
also labeled.
(
110) face. In the case of body-centered cubic NiGa IMC, the
Ni atom is located at the body-centered location and com-
pletely coordinated by the Ga atom with the same Ni–Ga
bond length of 2.50 ꢂ. The results agree well with the XAFS
data in Table 1. Moreover, the site isolation of active Ni by inac-
tive Ga in Ni–Ga IMCs can be seen in Figure 10a’–c’. With the
decrease in Ni/Ga ratio, the Ni–Ni coordination decreases grad-
ually whereas the Ni–Ga coordination increases. The Ni atom is
completely separated by Ga atoms at the (110) face of the
NiGa IMC.
Figure 9. A) The normalized intensity of Ni K-edge XANES spectra of sup-
ported Ni–Ga IMCs and Ni foil. B) The corresponding Fourier transforms of
The observed electron transfer and active-site isolation in
Ni–Ga IMCs have a great effect on their catalytic performance
for the selective hydrogenation of alkyne, which can be under-
stood from the following two aspects. First, the electrons
transferred from Ga to Ni occupy part vacant d-electron orbit
3
the k -weighted EXAFS spectra in R space and that of Ni foil for comparison.
IMCs, which is the unique structural feature of IMCs and is
[
4a,b]
[18]
termed active-site isolation.
of the Ni atom, which may decrease the adsorption capacity
Then, DFT calculations were performed to understand the
electron and geometry structure of Ni–Ga IMCs. The structural
models of Ni–Ga IMCs are shown in Figure S7, which were
chosen according to the XRD patterns. The total electron den-
sity contour maps in Figure S8 demonstrate the partial overlap
between Ni and Ga atoms, which indicates the strong Ni–Ga
electron interaction. The electron density difference contour
maps (Figure S9) demonstrate the polarization of Ni–Ga bond-
ing, resulting from the electron transfer from the Ga atom to
the Ni atom, which is consistent with the results of XAFS ob-
servations. The calculated specific charge quantity of the Ni
of the active H atom and thus inhibit the deep hydrogenation
of styrene over Ni–Ga IMC catalysts. Second, it has been re-
ported that the sequential hydrogenation of alkyne to alkene
via vinyl and vinylidene intermediates requires a decreased
[4a,b]
active site size,
which is the same as in the Ni–Ga IMC
system for the selective hydrogenation of phenylacetylene,
and the significantly improved selectivity can be attributed to
the charge transfer and active-site isolation in Ni–Ga IMCs.
Moreover, the hydrogenation selectivity does not increase
along with the improvement in electron transfer and active-
site isolation in Ni–Ga IMCs. The unique structural feature of
each Ni–Ga IMC, particle size, or even the support effect may
also impose effects on catalytic behavior, the study of which is
underway in our laboratory for further understanding.
atom increased from À0.069e (Ni Ga) to À0.406e (NiGa) with
3
the decrease in Ni/Ga ratio in Ni–Ga IMCs.
Another unique feature of IMCs is the active-site isolation,
[
4a,b]
proposed by the Schlçgl group.
For Ni–Ga IMCs, Ni is the
active component whereas Ga is inert in the catalytic hydroge-
nation reaction. The geometry and bonding structure in the
single unit cell of Ni–Ga IMCs with the marked calculated bond
length are shown in Figure 10a–c. For the face-centered cubic
Conclusions
We have developed a facile methodology for the preparation
of supported Ni–Ga intermetallic compounds (IMCs; Ni Ga,
3
Ni Ga IMC, the Ni atom is located at the face-centered position
3
Ni Ga , and NiGa) NPs with tunable size by using the layered
5
3
and both Ni–Ni and Ni–Ga bonds have the same bond length
double hydroxide (LDH) precursor method and demonstrated
their excellent catalytic behavior for the selective hydrogena-
tion of phenylacetylene. The temperature-programmed reduc-
(
2.53 ꢂ). The structure for Ni Ga is much complicated, and the
5 3
bond length for Ni–Ni and Ni–Ga bonds is mainly 2.52 ꢂ at the
ꢁ
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tion by H measurements reveal the Ni-promoted gradual re-
sulting sample was calcined in air at 4008C for 4 h and then re-
2
duced by flowing H at 7008C for 5 h. The Ni content was adjusted
to ·20 wt%.
duction mechanism of Ga species; the particle size or type of
Ni–Ga IMCs can be tuned by modulating the Ni/Ga ratio in
LDH precursors or the reduction temperature during the topo-
tactic transformation of LDHs to Ni–Ga IMCs. The resulting Ni–
Ga IMCs demonstrate significantly improved catalytic selectivi-
ty for the hydrogenation of phenylacetylene to styrene, and
2
Catalytic evaluation for the selective hydrogenation of
phenylacetylene
the best catalytic activity can be obtained over the Ni Ga IMC
3
In a typical method, the catalyst (0.05 g), phenylacetylene (1 mL),
and 2-propanol solution (30 mL) were placed into a stainless steel
reaction reactor, which was fitted inside a Teflon tank. To compare
these Ni-based catalysts, the total Ni content of each catalyst was
maintained at the same level by changing the catalyst consump-
tion in the catalytic test according to the inductively coupled
plasma results (see Table S2). The air in the vessel was replaced
with a particle size of 7.2 nm (styrene yield=87.7% at 408C
and 0.3 MPa), which is higher than that of most of the report-
ed Ni catalysts. The remarkable increase in hydrogenation se-
lectivity can be attributed to the charge transfer and active-
site isolation in Ni–Ga IMCs, which is confirmed by X-ray ab-
sorption fine-structure characterization and DFT calculations.
The excellent catalytic performance of Ni–Ga IMCs demonstrat-
ed herein makes them promising low-cost catalysts for the
chemoselective hydrogenation of phenylacetylene.
thrice with H with a pressure of 3.5 MPa, vented, and sealed. After
2
the reactor temperature was increased to the target temperature
(
e.g., 508C), H was introduced into the reactor with an initial pres-
2
sure of 0.5 MPa. After a given reaction time, the reaction product
was analyzed off-line by using GC (Shimadzu GC-2014C equipped
with a flame ionization detector) or by GC–MS (Shimadzu GC-
Experimental Section
2
010).
Materials
Ga(NO ) ·xH O was purchased from Sigma–Aldrich. The following
analytical grade chemicals were used without further purification:
3
3
2
Characterization
H -TPR was performed in a quartz tube reactor on a Micromeritics
2
NaOH, Na CO , Ni(NO ) ·6H O, and Mg(NO ) ·6H O. Deionized H O
2
3
3 2
2
3 2
2
2
was used in all the experimental processes.
ChemiSorb 2720 equipped with a thermal conductivity detector. In
each case, the sample (100 mg) was sealed and pretreated at
2
008C in N atmosphere for 2 h in the reactor and then a gaseous
2
Synthesis of LDH precursors and supported Ni–Ga IMCs
mixture of H2 and Ar (1:9, v/v) was fed to the reactor at
À1
4
0 mLmin . The temperature was increased to 10008C (heating
Synthesis of Ni Mg Ga LDH precursors
À1
x
y
z
rate=108Cmin ). The powder XRD measurements were per-
formed on a Rigaku XRD-6000 diffractometer using CuK radiation
Ni Mg Ga LDHs with different Ni/Mg/Ga molar ratios (labeled as
a
x
y
z
À1
(l=0.15418 nm) at 40 kV, 40 mA, a scanning rate of 108min , and
NiMg Ga -LDHs, NiMg Ga-LDHs, Ni Mg Ga -LDHs, and Ni Mg Ga-
3
4
3
5
4
3
3
2
2
1
q=3–908. The Ni XAFS measurements were performed with the
W1B-XAFS beam line at the Beijing Synchrotron Radiation Facility,
LDHs) were synthesized by using a coprecipitation method. Typi-
cally, Ni(NO ) ·6H O, Mg(NO ) ·6H O, and Ga(NO ) with a given
3
+
2
2
2+
3 2
2
3 3
2
3+
Institute of High Energy Physics, Chinese Academy of Sciences. The
element content in the samples was determined from inductively
coupled plasma atomic emission spectroscopy analysis (Shimadzu
ICPS-7500). The morphology of the samples was investigated by
using a Zeiss Supra 55 scanning electron microscope operating at
an accelerating voltage of 20 kV in combination with energy dis-
persive X-ray spectroscopy for the determination of metal compo-
sition. The TEM images were recorded with JEOL JEM-2010 high-
resolution transmission electron microscopes operating at an accel-
erating voltage of 200 kV.
ratio of Ni /Mg /Ga were dissolved in deionized H O (100 mL)
2
to obtain a solution with a total cationic concentration (0.15m, so-
lution A). A certain amount of NaOH and Na CO were dissolved
2
3
2À
together to give a base solution (100 mL, solution B, [CO ]=
.0[M ], [OH ]=1.8([M ]+[M ]). Solutions A and B were then
3
3
+
À
2+
3+
2
mixed together at a steady rate of 3000 rpm for 1 min. The result-
ing suspension was aged in a sealed Teflon autoclave at 1258C for
2
4 h. All the obtained precipitate was washed thoroughly with H O
2
and dried in an oven at 608C overnight.
Synthesis of supported Ni–Ga IMCs
Computational methods
Various supported Ni–Ga IMCs were obtained by using an in situ
reduction process of the LDH precursors. In a typical method,
LDHs (1.0 g) were reduced in a H /N (50:50, v/v) stream at 500 or
All calculations were performed with the periodic DFT method
using Dmol3 module in Material Studio 5.5 software package (Ac-
celrys Inc., San, Diego, CA).
2
2
À1
7
008C for 5 h (initial heating rate=28Cmin ). The reduction pro-
[19]
The single crystal cell structural
cess results in the phase transformation from LDHs to Ni–Ga IMCs.
The resulting product was slowly cooled to the reaction tempera-
models for Ni Ga, Ni Ga , and NiGa were built according to the pre-
3
5
3
[20]
vious reports, in which the crystal structure is consistent with
ture in a N stream for the subsequent catalytic evaluation.
2
our experimental results: Ni Ga, P m¯ 3m(221); Ni Ga , Cmmm(65);
3
5
3
NiGa, P m¯ 3m(221) (see details in the Supporting Information). The
generalized gradient approximation with the Perdew–Burke–Ern-
Synthesis of Ni/MgO
[21]
zerhof functional and effective core potentials with double-nu-
meric quality basis were used for the geometric optimization and
single-point energy calculations. For the calculations, the conver-
The Ni/MgO sample was prepared by using the conventional im-
pregnation method for comparison purposes. MgO (1.0 g, J&K
Chemicals Ltd, ꢀ50 nm) was added in a Ni(NO ) ·6H O solution
À6
gence tolerance was set as follows: energy=1.0ꢃ10 Ha, force=
3
2
2
À3
À1
À3
(
10 mL, 0.34m) for 24 h and then dried at 608C overnight. The re-
1.0ꢃ10 Haꢂ , and displacement=1.0ꢃ10 ꢂ.
ꢁ
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Acknowledgements
This work was supported by the 973 Program (grant no.
2
011CBA00504), the National Natural Science Foundation of
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Received: September 24, 2013
Revised: December 8, 2013
2
Sci. Technol. 2012, 2, 2139.
Published online on February 12, 2014
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 824 – 831 831