Phenylacetylene hydrogenation on Au@Ni bimetallic core-shell nanoparticles synthesized under mild conditions

Article abstract of DOI:10.1039/c5cy02005k

The synthesis of Au@Ni bimetallic core-shell nanoparticles through an energy efficient (lower temperature) route in oleylamine following a sequential reduction strategy is reported. The method is found to be useful for the synthesis of a very thin nickel

Full text of DOI:10.1039/c5cy02005k

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Phenylacetylene hydrogenation on Au@Ni
bimetallic core–shell nanoparticles synthesized
under mild conditions†
Cite this: DOI: 10.1039/c5cy02005k
Received 24th November 2015,
Accepted 5th January 2016
ab
ab
a
a
abc
A. B. Vysakh, Anish Lazar, V. Yadukiran, A. P. Singh and C. P. Vinod*
DOI: 10.1039/c5cy02005k
www.rsc.org/catalysis
2
0,21
The synthesis of Au@Ni bimetallic core–shell nanoparticles
through an energy efficient (lower temperature) route in
oleylamine following a sequential reduction strategy is reported.
The method is found to be useful for the synthesis of a very thin
nickel shell (2 nm) over a gold core (15 nm). Synergistic effects are
observed in catalyzing phenylacetylene hydrogenation under
different solvent conditions.
composition and relative sizes of the core and shell.
Here,
instead of supported catalysts, colloidal nanoparticles func-
tion as a true model, and they bridge the properties of homo-
geneous catalysis and heterogeneous catalysis. The catalytic
efficiency is largely governed by the Sabatier principle,
whereby the interactions of adsorbate molecules are largely
2
2
23
controlled by the electronic structure of the catalyst surface.
The combination of metals like gold (Au) and nickel (Ni) has
been less explored due to their large lattice mismatch but are
the ones which can demonstrate fascinating catalytic proper-
Metals and metal oxides have been widely used in their
“
nano” form for diverse physical, biological and chemical
1–3
applications. Nanocatalysis is a rapidly growing field in
heterogeneous catalysis which involves the use of metal/metal
oxide nanoparticles for tailoring the catalytic activity and
24
ties. Gold nanoparticles are widely known for oxidation
25–27
reactions,
whereas nickel acts as a good hydrogenation
28
catalyst. In this report, we show that Au@Ni core–shell
4
–8
selectivity towards the optimum.
In the case of metal
nanoparticles can be synthesized at a relatively milder tem-
nanoparticles, the quantum size effects are found to have
profound effects in their physical and chemical properties
29,30
perature than the ones reported previously.
Further, they
are shown to synergistically catalyze phenylacetylene (PA) hy-
drogenation reaction to styrene which is an industrially im-
portant commodity for polymer synthesis. It should be noted
that a commercially more viable process for the production of
9
,10
compared to bulk materials.
Vast improvements in the
characterization tools have led to a better understanding of
the atomic details of the materials, especially catalytic
materials, which has led to rapid developments in tailor-
styrene is through the dehydrogenation route of ethylbenzene
1
1–13
made heterogeneous catalyst design.
In this regard, de-
31
(
EB). The importance of finding an optimum catalyst for PA
signing catalysts through surface modification via a bimetallic
hydrogenation stems from the fact that any trace amount of
PA in the styrene feedstock is found to adversely affect the
1
4–16
route has shown a lot of promise.
Bimetallic nano-
particles, especially in the form of core–shell morphology, can
32
polystyrene product distribution.
be considered as a model system where the role of core mate-
The synthesis of a Au core nanoparticle and the growth of
Ni as a shell over the core can be monitored by the UV-vis
spectroscopy technique. The wine red colour of the colloidal
solution after reduction of gold ions due to the surface
plasmon resonance (SPR) indicates the formation of gold
1
7–19
rial, shell thickness, and interface
can be fine tuned for
desired applications. This core–shell morphology also extends
the possibility of tuning various properties especially catalytic
activity and selectivity by controlling their chemical
3
3
(
Au) nanoparticles. The surface plasmon resonance appears
a
as a band in the UV-vis spectrum. Here, the preformed gold
seeds act as nucleation sites for the adsorption of the nickel
ions in the second stage which subsequently undergo reduc-
tion upon addition of a hydrazine/NaOH mixture at 80 °C to
form the bimetallic nanoparticles (see the ESI† for the syn-
thesis procedure). The previous literature analysis on the syn-
thesis of Au–Ni or Ni nanoparticles using oleylamine as the
solvent shows that the reduction of Ni ions take place above
Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune, India. E-mail: cp.vinod@ncl.res.in; Fax: +91 20 25902633;
Tel: +91 20 25902086
b
Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2
Rafi Marg, New Delhi, Delhi 110001, India
c
Center of Excellence on Surface Science, CSIR-National Chemical Laboratory,
Pune, India
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy02005k
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3
4,35
2
00 °C resulting in the formation of Ni nanoparticles.
minutes. HR-TEM analysis of Au@Ni bimetallic nanoparticles
confirmed the core–shell morphology of nanoparticles
through lattice fringe analysis and contrast difference be-
tween a heavier noble metal (Au) and a lighter non-noble (Ni)
metal. The large area TEM analysis (Fig. 1a and S3, ESI†)
showed the size distribution of nanoparticles (∼20 nm), and
EDX (energy dispersive X-ray) analysis (Fig. 1e) indicated the
elemental composition between both metals. From
Fig. 1b and c, it is evident that the overall size of the core–
shell nanoparticle is on average around 20 nm with a core
size of around 15 nm and a shell thickness of approximately
2 nm. The HR-TEM image of a single particle in Fig. 1b
shows a clear contrast difference between the two metals in
an isolated particle indicating the formation of a core–shell
Here, we are able to bring down the reduction temperature
below 100 °C with the successful generation of Au@Ni bime-
tallic nanoparticles. The complete damping of the gold
plasmon band in the later stage of nickel reduction shows
the successive formation of the nickel shell over the gold
core. The UV-vis spectra obtained at different stages of the
synthesis are shown in Fig. S1.† Curve a in the UV-vis spectra
indicates the first stage of gold reduction which shows
plasmon absorbance around 515 nm. Curve b in Fig. S1† cor-
responds to the final stage of nickel reduction over the gold
nanoparticles which confirms the complete damping of gold
plasmon. Powder XRD analysis of Au@Ni bimetallic nano-
structures are shown in Fig. S2.† The diffraction from the
gold and nickel planes indicates that both metals exist in the
FCC lattice without any alloying effects. The fairly large
FWHM (full width at half maximum) values of the XRD peaks
indicate the small size of the nanoparticles. The particle size
analysis using the Scherrer equation corroborated with the
TEM analysis and confirms the total size of Au@Ni core–shell
nanoparticles to be around 20 nm in average. The diffrac-
tions in Fig. S2† correspond to gold alone (curve b), nickel al-
one (curve a) and Au@Ni (curve c) core–shell nanoparticles.
The diffractions at 2θ values of 38.2, 44.3 and 64.6 corre-
2
0
3
7
morphology. The lattice fringe analysis at the core and the
shell (Fig. 1d) gave values of 2.38 Å and 2.04 Å, respectively,
confirming that Ni is present as the shell and Au as the
3
5
core. The FFT (fast Fourier transform) pattern (inset of
Fig. 1d) also indicated the presence of gold and nickel
d-spacing values. Despite the large lattice mismatch between
the Au and Ni systems, there have been recent reports that it
3
8
is indeed possible to epitaxially grow Ni on Au. Fig. 1c
shows moiré fringes which arise due to the mismatch of lat-
38
tice parameters of gold and nickel. The moiré fringe values
(1.5 nm) were found to be in agreement with the literature
3
6
spond to Au(111), Au(200) and Au(220), respectively. Simi-
larly, Ni(111), Ni(200) and Ni(220) are diffracted at 2θ values
3
5
values for gold and nickel. Thus, the microscopy observa-
tions provided here gave clear evidence for the formation of a
Au@Ni core–shell system under milder conditions reported
3
4
of 44.5, 51.8 and 76.4, respectively. Since there are strong
overlap of diffractions from Au(200) and Ni(111), the most
discernable peak for the identification of Ni is at a 2θ value
of 51.8. The less intense and very broad peak at 51.8 in the
enlarged Fig. S2(II)† (shaded region) of the Au@Ni core–shell
nanoparticle gives a clear confirmation of the thin nickel lat-
tice on the Au core. It is understandable that the diffractions
from the nickel lattice in the case of Au@Ni are not clearly
visible because of the extremely thin nickel shell on the gold
surface. However, in the case of nickel nanoparticles alone,
the reflections are clearly visible and they confirm the forma-
3
9
so far with oleylamine as the solvent and reducing agent.
The EDX analysis (Fig. 1e) showed the elemental composition
of approximately 1 : 0.25 (see Fig. S3† for quantification re-
sults) for gold to nickel which is in good agreement with the
metal precursor ratios used for the synthesis. Our previous
report demonstrated the need for an extremely thin nickel
shell for the demonstration of synergistic effects in cataly-
2
1
sis. To understand the electronic state of the metals espe-
cially at the surface, we carried out an XPS study by drop cast-
ing the sample on a silicon wafer. Fig. S4† shows the wide
area scan of the as-synthesized samples. It shows the pres-
ence of characteristic signature peaks from gold, nickel, oxy-
gen and carbon. The peak positions were calibrated by con-
sidering a carbon standard value at 284.6 eV. Fig. 2a and b
are the individual scans performed for gold and nickel core
levels, respectively. The Ni 2p3/2 spectra (Fig. 2b) from the bi-
metallic Au@Ni core–shell provide evidence for the metallic
feature (852.6 eV) along with hydroxide features at 855.8
3
4
tion of a pure FCC phase of the nickel metal. Fig. 1 shows
the TEM image of the as-synthesized bimetallic Au@Ni nano-
structures drop cast on the Cu grid after sonication for 5
2
9
eV. The solution phase synthesis of nickel nanoparticles al-
ways produces some amount of hydroxides on the surface
which is unavoidable under such synthesis conditions. It
must be noted that even though all the syntheses were car-
ried out in ambient atmosphere with samples dried in the
open atmosphere, the material managed to prevent the com-
plete aerial oxidation of nickel nanoparticles. The absence of
a NiO peak at 854 eV in the nickel spectra and the presence
of more than 50% of metallic nickel species even after expo-
sure to the atmosphere proved the strong surface
Fig. 1 (a) TEM, (b–d) HR-TEM images and (e) EDX data of Au@Ni core–
shell nanoparticles. The inset of (d) shows the FFT pattern of an indi-
vidual Au@Ni nanoparticle.
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4
hydrogenation product of phenylacetylene. The reaction
was carried out using a stirred type Parr reactor which can go
up to 300 °C in temperature and up to 50 bar pressure. We
optimized the reaction conditions by carrying out a range of
experiments by varying the pressure and temperature condi-
tions (conditions shown in the figure caption). The reactions
were carried out in different solvents like methanol, tetrahy-
drofuran (THF) and dichloromethane (DCM). These solvents
were chosen as it was previously reported that the selectivity
towards the desired product (styrene) crucially depends on
Fig. 2 XPS core-level spectra of Au 4f (a) and Ni 2p (b) obtained for
Au@Ni core–shell nanoparticles. The violet dotted line in (b) indicates
the nickel metallic feature, whereas green corresponds to the hydrox-
ide feature.
28
the solvent. The reaction mixture was analyzed with the
help of an offline GC. Initially, we tested the core–shell sys-
tems with their monometallic counterparts for understanding
the enhancement in catalytic activity. To exclude the influ-
ence of quantum size effects in catalytic activity, we com-
pared the activity of bare Au and Ni nanoparticles having a
similar size (around 15 and 20 nm, respectively) with Au@Ni
core–shell nanoparticles. Fig. 3 shows the phenylacetylene
(PA) hydrogenation for Au nanoparticles, Ni nanoparticles
and Au@Ni core–shell nanoparticles in methanol (MeOH)
modification in the Au–Ni bimetallic combination. This ob-
servation is a direct demonstration of the gold core-induced
surface modification of the ultrathin nickel nanoshell which
4
0
shows resistance towards surface oxidation. These types of
bimetallic systems which are geometrically and electronically
modified due to the core–shell morphology are well known in
the literature for enhanced physical and chemical properties
1
9,41
including good catalytic performances.
gold scan in XPS (Fig. 2a) showed binding energy values of
4 eV (4f7/2) and 87.6 eV (4f5/2) which are typical of metallic
gold. These two peaks arise due to the spin orbit splitting of
f energy levels and the ejection of 4f core level photoelec-
trons. The values obtained for the Au(4f) core level also prove
The individual
8
4
4
2
the absence of any unreduced Au(3+) in the final catalyst.
Nickel (in a face-centred cubic structure) is well known for its
ferromagnetic behaviour, and we explored the magnetic prop-
erties of these Au@Ni core–shell nanoparticles by measuring
the hysteresis loop using SQUID. In Fig. S5,† we show the
magnetic behaviour of Au@Ni core–shell nanoparticles which
demonstrate typical hysteric behaviour of a ferromagnetic Ni
(
ref. 34) with a very low saturation magnetization of 2 emu
−
1
gm in an applied field of 6 kOe. The inset of the image
Fig. S5, ESI†) shows the remanent magnetisation of the ma-
Fig. 3 Phenylacetylene conversion (brown) and ethylbenzene selectivity
(
blue) in methanol solvent obtained for Au@Ni core–shell catalysts and
their monometallic counterparts (gold and nickel). The reaction
conditions are given as follows: 50 °C and 50 psi H pressure [catalyst
(
terial along with very low coercivity values. The synthesized
core–shell nanomaterials were tested for potential applica-
tions in catalysis by carrying out hydrogenation reactions.
The first row transition elements are prone to oxidation, even
under ambient conditions as they are known to form surface
oxides which poison the catalytic activity of the materials.
2
weight – 5 mg, substrate – 5 mmol and reaction time – 4 hours].
4
3
The formation of oxide layers is found to limit the hydroge-
nation pathway; hence, the need of retaining of the metallic
feature is necessary for efficient catalytic activity. Our previ-
ous reports showed that synergistic effects are observed in
core–shell bimetallic combinations up to around 2 nm shell
thickness compared to the individual monometallic counter-
2
1
parts. To test the efficiency of the synthesized Au@Ni bime-
tallic system, we checked the catalytic activity of core–shell
particles for phenylacetylene (PA) hydrogenation reaction.
This is an industrially important reaction where the partially
hydrogenated product styrene is an important starting mate-
rial for many industrially relevant polymers. In addition,
the reaction is a testing ground for activity and selectivity cor-
relations where ethylbenzene is also a widely reported
Fig. 4 Selectivity (red and yellow) obtained for complete conversion
of PA in various solvents by using Au@Ni core–shell catalysts. The blue
bar represents the time taken for complete conversion [reaction
3
2
conditions: for MeOH and THF: 50 °C and 50 psi H
5 mg; substrate – 5 mmol. For DCM: 40 °C and 50 psi H
catalyst – 10 mg; substrate – 1 mmol].
2
pressure; catalyst
–
2
pressure;
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Table 1 Hydrogenation results obtained for PA by using Au@Ni catalysts and Ni nanoparticles in MeOH (50 ml) and DCM (50 ml) under various temper-
atures [in MeOH and DCM, the substrate is 5 and 1 mmol, respectively]
Serial no.
Catalyst
Amount (mg)
Time (h)
Solvent
Temp. (°C)
Press. (psi)
Conversion
Selectivity (%) EB/styrene
1
2
3
4
5
6
7
8
Ni alone
Au@Ni
Ni alone
Au@Ni
Ni alone
Au@Ni
Ni alone
Au@Ni
5
5
5
4
4
4
MeOH
MeOH
MeOH
MeOH
DCM
DCM
DCM
DCM
50
50
30
30
40
40
30
30
50
50
50
50
50
50
50
50
60%
100%
35%
60%
55%
100%
30%
65%
65/35
100/0
50/50
60/40
25/75
20/80
20/80
15/85
5
4
10
10
10
10
10
10
10
10
solvent carried out at 50 °C and 50 psi pressure. From the re-
activity analysis in Fig. 3, it can be seen that Au@Ni core–
shell nanoparticles showed excellent reactivity and selectivity
for ethylbenzene (EB). Under similar conditions, we could
achieve only 60% conversion and 65% selectivity to EB on Ni
nanoparticles with Au nanoparticles being negligibly active.
The poor activity on gold can be attributed to the large size
of Au nanoparticles in our case (∼15 nm), while a recent re-
port has suggested the particle size of ∼5 nm for Au nano-
particles to be optimum for selective conversion towards sty-
shell nanoparticles. From the conversion and selectivity anal-
ysis, we found that methanol is more suitable for ethylben-
zene production via the complete hydrogenation and DCM is
suitable for the partial hydrogenation to styrene. To explore
further on the temperature and solvent effects, we carried out
a series of hydrogenation reactions and the results are tabu-
lated in Table 1. A decrease in temperature from 50 °C to
room temperature (30 °C) marked a decrease of ∼40% in the
conversion rates of both solvents (MeOH and DCM). Interest-
ingly, the selectivity towards styrene is higher while lowering
the temperature which shows that the complete hydrogena-
tion pathway goes through styrene. From these results, we
can conclude that the selectivity and rate towards a desired
product can be controlled by temperature and solvent.
4
5
rene.
In the case of Ni nanoparticles, most of the
phenylacetylene hydrogenation has been carried out using in-
termetallics of nickel to tune the selectivity towards
4
6–48
styrene.
The activity of Au@Ni nanoparticles reported
here shows that the catalyst is extremely active where the tri-
ple bond in PA can be totally saturated under moderate
conditions.
In conclusion, we report here a facile energy efficient
route for the synthesis of highly lattice mismatched gold–
nickel core–shell bimetallic nanoparticles. The nickel shell
thickness over the gold core is fine tuned to obtain maximum
catalytic efficiency of the Au@Ni core–shell catalysts. The
bare nanoparticles without any support showed good catalytic
enhancement towards hydrogenation of PA with high selec-
tivity towards styrene or ethylbenzene with respect to the sol-
vent and temperature. The catalysts were found to show the
activity under mild conditions of temperature and pressure.
The effect of solvent on PA hydrogenation is shown in
Fig. 4 and Table 1, respectively. The reaction results show
that there is a strong dependence of solvent and temperature
on the conversion rates. We found that using methanol and
THF as solvent, the optimum conditions for hydrogenation
of carbon–carbon triple bonds are 50 °C and 50 psi pressure
with 5 mg of the catalysts with 100% conversion and selectiv-
ity towards ethylbenzene. We observed that the rate was
lower in THF which took almost 7 hours for complete conver-
sion compared to 4 hours in methanol. In DCM solvent also,
we achieved similar conversions, whereas the hydrogenation
results obtained for phenylacetylene in dichloromethane as
solvent showed 80% selectivity towards styrene (Fig. 4; see
also Fig. S6, ESI† for GC results), which has a high commer-
cial value in the polymer industry, and the remaining product
was ethylbenzene. The effect of solvent plays a crucial role in
deciding the selectivity which is known for some other bime-
Acknowledgements
CPV thanks Dr. C. S. Gopinath for constant support and en-
couragement. The authors thank Dr. P. A. Joy and Govind K.
Raj for magnetic measurements. ABV thanks UGC for the re-
search fellowship. This work was supported by CSIR through
the project CSC0404 (CoE on Surface Science) for CPV.
2
8
tallic combination like Fe–Ni reported by Polshettiwar et al.
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We found that our catalyst is highly active under these mild
reaction conditions of temperature and pressure. In DCM sol-
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not go beyond 40 °C as DCM starts to boil beyond that tem-
perature. For complete conversion in DCM solvent, it took 10
hours but with good selectivity towards styrene. Under simi-
lar conditions, nickel nanoparticles showed only 55% conver-
sion demonstrating the synergistic effects in Au@Ni core–
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