Preparation of a PdRuNi/C tri-metallic nanocatalyst and its excellent catalytic performance for ethylbenzene hydrogenation reaction

Article abstract of DOI:10.1039/c9nj04083h

Herein, Ni/C was prepared at room temperature (RT) by liquid chemical reduction by using hydrazine hydrate as the reducing agent. The impregnation method was adopted to deposit precious metals on carbon black (C) to obtain the Pd/C, Ru/C monometallic and PdRu/C bimetallic catalysts. The PdNi/C, RuNi/C bimetallic and PdRuNi/C tri-metallic catalysts were obtained through a chemical replacement reaction method. The as-prepared catalysts were characterized by inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM) and STEM energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping and line-scanning, scanning electron microscopy (SEM) and SEM-EDS, X-ray photoelectron spectroscopy (XPS) and high-sensitivity low-energy ion scattering spectroscopy (HS-LEIS) techniques, which proved that PdRu islands were supported on the Ni/Ni(OH)₂ nanoparticles (NPs) (PdRu-on-Ni/Ni(OH)₂ NPs) in PdRuNi/C. The results revealed that the PdRuNi/C catalyst showed the most excellent catalytic performance for ethylbenzene hydrogenation under mild reaction conditions mainly owing to the synergistic effect among the Pd, Ru and Ni related species. The PdRuNi/C catalyst also exhibited good stability in ethylbenzene hydrogenation.

Full text of DOI:10.1039/c9nj04083h

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Preparation of a PdRuNi/C tri-metallic nanocatalyst
and its excellent catalytic performance for
ethylbenzene hydrogenation reaction†
Cite this: New J. Chem., 2019,
43, 17306
a
a
b
a
c
a
An Pei, Luna Ruan, Jianhua Liao, Huan Zhang, Jiexiang Wang, Kai Yang,
a
ad
d
Zhiping Liu, Lihua Zhu
*
and Bing Hui Chen
Herein, Ni/C was prepared at room temperature (RT) by liquid chemical reduction by using hydrazine
hydrate as the reducing agent. The impregnation method was adopted to deposit precious metals on
carbon black (C) to obtain the Pd/C, Ru/C monometallic and PdRu/C bimetallic catalysts. The PdNi/C,
RuNi/C bimetallic and PdRuNi/C tri-metallic catalysts were obtained through a chemical replacement
reaction method. The as-prepared catalysts were characterized by inductively coupled plasma mass
spectrometry (ICP-MS), X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution
TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM) and STEM energy dispersive
X-ray spectroscopy (STEM-EDS) elemental mapping and line-scanning, scanning electron microscopy
(
SEM) and SEM-EDS, X-ray photoelectron spectroscopy (XPS) and high-sensitivity low-energy ion
scattering spectroscopy (HS-LEIS) techniques, which proved that PdRu islands were supported on the
Ni/Ni(OH) nanoparticles (NPs) (PdRu-on-Ni/Ni(OH) NPs) in PdRuNi/C. The results revealed that the
Received 6th August 2019,
Accepted 17th October 2019
2
2
PdRuNi/C catalyst showed the most excellent catalytic performance for ethylbenzene hydrogenation
under mild reaction conditions mainly owing to the synergistic effect among the Pd, Ru and Ni related
species. The PdRuNi/C catalyst also exhibited good stability in ethylbenzene hydrogenation.
DOI: 10.1039/c9nj04083h
rsc.li/njc
Because of the synergistic effect between different metals,
the bimetallic nanocatalysts were reported to have better catalytic
It is well known that metal supported catalysts are widely used in the activity, stability and selectivity than monometallic catalysts.
1
Introduction
27,28
1–3
catalysis field, such as in heterogeneous hydrogenation reactions. For instance, He et al. prepared the Cu -Fe /SiO bimetallic catalysts
x
y
2
More and more attention from researchers has focused on how to via a sol–gel method. They found that the addition of Fe promoted
2+
increase their catalytic properties (including activity, selectivity and the reduction of Cu , and the synergistic effect between copper
4–7
stability). Their catalytic behaviors can be adjusted by changing and iron improved the conversion of diethyl malonate (up to
8,9
the distribution of the active component on the support, the 96.3%) and the selectivity to 1,3-propanediol (93.3%) for diethyl
1
0–13
29
properties of the support,
adding a second metallic component, malonate selective hydrogenation (P = 5.0 MPa, and T = 210 1C).
1
4–16
17
18
19
etc.
The nickel-based, copper-based, platinum-based,
palladium-based, ruthenium-based,
Besides, in Li’s group, the addition of Ag to palladium-based
catalysts caused a much higher catalytic activity and selectivity to
2
0
21,22
23
and iridium-based
monometallic catalysts and multi-metallic (bi- and tri-) hydrocinnamaldehyde (98.6%) than those catalysts without Ag for
2
4–26
nanocatalysts
have been widely investigated in recent years. selective hydrogenation of cinnamaldehyde (1 MPa H , and
2
30
110 1C). Additionally, the PtSn bimetallic catalyst with a particle
size of 2–3 nm synthesized by Dietrich was found to have 100%
a
College of Chemistry and Chemical Engineering, Faculty of Materials Metallurgy
and Chemistry, Jiangxi University of Science and Technology, Ganzhou 341000,
Jiang Xi, China. E-mail: zhulihua156@163.com; Fax: +86-797-8312204
School of Pharmaceutical Sciences, Gannan Medical University, Ganzhou 341000,
Jiangxi, China
selectivity to a,b-unsaturated cinnamyl alcohol when it catalyzed
31
2
cinnamaldehyde hydrogenation at 80 1C under 1 MPa H . According
b
c
to the reported references, the catalytic properties of the tri-metallic
catalysts were generally superior to those of their corres-
ponding monometallic or bimetallic nanocatalysts. For instance,
Hou et al. adopted the impregnation method to prepare a series of
Fine Chemical Industry Research Institute, School of Chemistry,
Sun Yat-Sen University, Guangzhou 510275, China
d
Department of Chemical and Biochemical Engineering, National Engineering
Laboratory for Green Productions of Alcohols-Ethers-Esters, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005, China
monometallic (Ru/Al
Al , Ru Sn /Al
2
O
3
, Pt/Al
2
O
3
, and Sn/Al
2
O
3
), bimetallic (Ru
) and tri-metallic sup-
) for bis(2-hydroxy ethylene
4
Pt/
2
O
3
4
4
2
O
3
, and PtSn
4
/Al
2
O
3
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj04083h ported nanocatalysts (RuPtSn/Al
2
O
3
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terephthalate) (BHET) selective hydrogenation. The Ru
6
PtSn
4
/
ethylbenzene-C
ÁH O (85 wt%) were purchased from Shanghai Aladdin Bio-
excellent catalytic performance, with 100% conversion of BHET Chem Technology Co., Ltd. High purity hydrogen-H (99.999%),
8 8
H10, ethylcyclohexane-C H18, and hydrazine hydrate-
Al
2
O
3
(Ru/Sn = 1.5) tri-metallic nanocatalyst provided the most
N H
2 4
2
2
3
2
and 100% selectivity to 1,4-cyclohexanedimethanol (CHDM).
N2 (99.999%) and the mixture gas of 90%N2 + 10%H2 were
Xiang and his co-workers designed and synthesized a novel obtained from Linde. Additionally, n-hexane-C H , ethanol-
6
14
Co
1
Cu
2
Nb0.2 tri-metallic catalyst with nanosized particles
C
2
H
5
OH, NiCl
2
Á6H
2
O (nickel chloride hexahydrate) and NaOH
for producing higher alcohols by CO hydrogenation. The CO (sodium hydroxide) used in the experiments were analytical
conversion rate ranged from B5% to 20%, and the selectivity to reagents and provided by Shanghai Sinopharm Chemical
3
3
1-alcohol/1-olefin combinations reached up to B73%.
Reagent Co., Ltd. The carbon black (BP-2000) was obtained from
The monometallic and bimetallic catalysts used for catalytic Cabot Corporation.
hydrogenation of ethylbenzene were widely studied. For example,
the DyNi/MPS catalyst (MPS: unmodified mesoporous silica) was
synthesized by Shafigulin for ethylbenzene hydrogenation. The
2
.2 Preparation of the catalysts
34
Synthesis of Pd/C and Ru/C. Pd/C and Ru/C were prepared by
3
9–42
catalyst exhibited 100% selectivity to ethylcyclohexane. In addition, the impregnation method.
the conversion of ethylbenzene over D Ni/MPS reached 100% and aqueous (NH PdCl O solution and carbon black was
only 80% over Ni/MPS (150 1C, 0.3 MPa). The NiMo0.16 (nMo/nNi evaporated at 80 1C in a water bath with vigorous stirring.
.16) bimetallic catalyst was prepared by Sheng’s group and the Then, it was placed in a blast oven at 110 1C for 3 h. After
The turbid liquid containing
y
4
)
2
4 2
H
=
0
catalyst was used for the hydrogenation of ethylbenzene at 150 1C drying, the sample was ground and weighed. Finally, the
under 1.0 MPa H , the yield of ethylcyclohexane was 100% and the sample (0.2000 g) was treated in a quartz tube furnace (2 h,
2
À1
Ni monometallic catalyst provided a 76.5% yield of ethylcyclo- 400 1C) with a heating rate of 5 1C min in the mixture flow
hexane under the same reaction conditions. However, the tri- (20 mL min 90%N + 10%H ). Then, the catalyst was weighed
3
5
À1
2
2
metallic nanocatalysts catalyzing hydrogenation of ethylbenzene and recorded as the Pd/C catalyst. The synthesis method of
have not been reported. Ru/C was the same as the above process, but the addition of
According to the literature, generally, the synergistic effect in aqueous solution was aqueous RuCl solution.
Synthesis of Ni/C. The Ni/C sample was synthesized by the
3
the tri-metallic catalysts is much stronger than that in the
bimetallic catalysts, which is mainly because the addition of hydrazine hydrate reduction method at RT.
4
3,44
The whole
another metal probably changed the crystallite phase of the synthesis process was conducted at ambient temperature with
catalyst, or increased the metal dispersion, and it could exhibit mechanical stirring at a speed of 500 rpm for 18.5 h. The
36–38
much higher catalytic activity than the bimetallic catalysts.
catalyst was collected via filtration, and washed with absolute
In this work, the PdRuNi/C tri-metallic supported nanocatalyst was ethanol and deionized water, respectively, several times. Ni/C
prepared and used for ethylbenzene hydrogenation. The synergistic was crushed and weighed after drying in a vacuum oven at
effect among the Ru, Pd and Ni related species was also discussed. 60 1C for six hours.
Herein, a novel and highly efficient PdRuNi/C catalyst for
Synthesis of PdNi/C and RuNi/C. The galvanic replacement
ethylbenzene hydrogenation was designed and constructed. method was applied to deposit palladium and ruthenium on
The influence of the introduction of Pd and Ru on the nano- the Ni/C sample, finally obtaining the nanocatalysts of PdNi/C
4
5
structure and catalytic hydrogenation behaviors of the PdRuNi/C and RuNi/C. 0.3000 g of Ni/C was suspended in a certain
catalyst was investigated. Characterization techniques of XRD, amount of aqueous (NH ) PdCl ÁH O solution under magnetic
4
2
4
2
TEM, HRTEM, HAADF-STEM, STEM-EDS elemental mapping stirring at RT for 4 h. Then, deionized water and anhydrous
and line-scanning analysis, SEM-EDS, XPS, and HS-LEIS proved alcohol were used to wash the black solid repeatedly. After-
the nanostructures of the as-obtained catalysts, e.g. PdRuNi/C with wards, it was dried under vacuum at 60 1C for six hours, and
2
the nanostructure of PdRu island-on-Ni/Ni(OH) nanoparticles. The marked as PdNi/C. RuNi/C was synthesized by the same pre-
catalytic properties of the catalysts for ethylbenzene hydrogenation paration process as PdNi/C, but the addition solution was
at different temperatures (100 1C, 80 1C, 60 1C, and 40 1C) were aqueous RuCl solution.
3
investigated. The reasons for the difference in the catalytic
performance of the PdRuNi/C tri-metallic catalyst, bimetallic through the impregnation method. The turbid liquid contain-
catalysts (RuNi/C, PdNi/C, and PdRu/C) and monometallic ing aqueous (NH PdCl O and RuCl solution and carbon
catalysts (Ru/C, Pd/C, and Ni/C) for ethylbenzene hydrogena- black was evaporated by vigorous stirring in a water bath at
Synthesis of PdRu/C. The PdRu/C sample was obtained
4
6
4
)
2
4
H
2
3
tion were analyzed.
80 1C, and the sample was dried at 110 1C for three hours. The
as-obtained sample was weighed to be about 0.2000 g and
placed in a quartz tube furnace to be annealed at 400 1C for
two hours in 90%N + 10%H mixture flowing gas. After the
tube heater was naturally cooled to room temperature, the
calcined catalyst was taken out, and denoted as PdRu/C.
Synthesis of PdRuNi/C. The galvanic replacement reaction
2
2
2
Experimental
2
.1 Reagents
Tetraammine dichloropalladium(II) monohydrate-(NH
ÁH
1.000 g of (NH PdCl O in 100 mL of H
2
ruthenium chloride solution (1.0000 g of RuCl in 250 mL of H O), temperature, gaining the PdRuNi/C tri-metallic catalyst.
4
)
2
PdCl
4
ÁH
2
O
(
)
4 2
4
2
2
O), aqueous method was used to deposit Pd and Ru on Ni/C at ambient
4
7–49
3
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Table 1 Metal contents in different catalysts measured by ICP-MS
to 5.0 MPa. The stirring speed was set at 870 rpm and the
reactor was persistently heated to reach the desired temperature
Metal loading
(e.g. 100 1C). After reacting for a certain time, the reactor was
Entry
Catalyst
Pd/wt%
Ru/wt%
Ni/wt%
quickly cooled to 5 1C. Afterward, 1 mL of hexane was mixed into
1
2
3
4
5
6
7
Ni/C
Pd/C
Ru/C
PdNi/C
RuNi/C
PdRu/C
PdRuNi/C
—
5.82
—
5.79
—
2.94
3.10
—
—
5.94
—
5.73
3.08
3.06
11.78
—
—
5.96
6.02
—
the reactor with a liquid transfer gun. After mingling evenly,
3–4 mL of the upper liquid was removed to the centrifuge tube,
and then centrifuged with a speed of 10000 rpm for 10 min.
Ultimately, the liquid products were analyzed via gas chromato-
graphy (a Shimadzu GC 2010 instrument, Japan Shimadzu Co.
Ltd) with a DB-35 (60 m  0.32 mm) capillary column and a
flame ionization detector (FID), and qualitative analysis was
5.60
The aqueous solution of (NH ) PdCl ÁH O was added to a performed by using gas chromatography-mass spectrometry
4
2
4
2
2
50 mL conical bottle, followed by the addition of 200 mL of (Shimadzu GCBMS 2010).
deionized water. Then, Ni/C (0.3000 g) was mixed with the
Herein, the catalytic performance of the catalysts was indicated
above solution. After magnetically stirring at RT for two hours, by the turnover frequency (TOF), which represents the number of
the aqueous RuCl
solution was injected and continuously active sites required to convert each mole of furfural per unit time.
stirred for 2 h. The solid was placed in a vacuum dryer The following equation was used to calculate the dispersion:
60 1C, 6 h) and crushed and weighed to acquire a black
3
50,51
(
D = (6n M)/(rNd )
s
p
powder, which was named PdRuNi/C.
The actual loadings of metal components (Pd, Ru, and Ni) in
the as-prepared catalysts were measured by inductively coupled
plasma mass spectrometry by using an Agilent ICP-MS 4500-
The turnover frequency (TOF) was calculated as:
TOF = nconversion of ethylbenzene/(ntotal palladium and ruthenium  t  D)
300, as shown in Table 1.
The yield of ethylcyclohexane was determined by the equation
described below:
2.3 Characterization
The X-ray diffraction patterns of the samples were recorded on
a Rigaku Ultima IV X-ray diffractometer; Cu Ka radiation was
Y = nethylcyclohexane/ntotal ethylbenzene  100%
s
used as the X-ray resource and the diffraction angle range was where n is the number of metal atoms contained on the
1
9
À2
1
01–901. X-ray photoelectron spectroscopy (XPS) spectra of the surface of the catalyst per unit area (Pd: 1.49 Â 10
m , and
1
9
À2
catalysts were obtained on a PHI Quantum 2000 Scanning ESCA Ru: 1.63 Â 10
m
), M is the relative atomic mass of metal
À1
À1
microprobe with the corresponding monochromatic Al Ka radiation (Pd: 106.42 g mol , and Ru: 101.07 g mol ), r is the metal
À3
À3
(
1486.6 eV). The determination of the morphology, particle size and density (Pd: 12.02 g cm , and Ru: 12.10 g cm ), N is the
2
3
À1
nanostructures of the catalysts could be achieved by using Avogadro constant (6.023 Â 10 at. mol ), and d
represents
p
scanning electron microscopy (SEM), transmission electron micro- the average size of the metal particles measured by TEM
À9
scopy (TEM), and high resolution transmission electron micro- (8.16 Â 10 m).
scopy (HRTEM). A TECNAI F30 transmission electron microscope
(
300 kV) was used to obtain TEM and HRTEM images, HAADF-
3
Results and discussion
STEM images and STEM-EDS elemental mapping and line-
scanning results. The analysis of HS-LEIS spectra of the samples
3.1 Catalyst characterization results
4
20
(3 keV He and 5 keV Ne spectra) was conducted by using a low
XRD patterns of Ni/C, Ru/C, Pd/C, RuNi/C, PdNi/C, PdRu/C, and
PdRuNi/C are given in Fig. 1. The diffraction peaks at 2y = 19.21,
energy ion scattering analyzer.
3
3.11, 38.11, 52.11, 59.11 and 62.71 in the XRD patterns of Ni/C
2.4 Catalytic ethylbenzene hydrogenation
(Fig. 1Aa) could be attributed to Ni(OH) (001), Ni(OH) (100),
2
2
The hydrogenation reactor was an MS-50-316L autoclave (Anhui Ni(OH) (101), Ni(OH) (102), Ni(OH) (110) and Ni(OH) (111)
2
2
2
2
5
2
Kemi Machinery Technology Co., Ltd), equipped with a tem- planes (JCPDS card No. 04-0117), respectively. Furthermore,
perature controlling instrument and DF101S constant tempera- a weak peak with a 2y angle of 44.41 can be observed, which was
5
3
ture heating magnetic stirring. A series of catalysts (Ni/C, Pd/C, deemed to be the Ni(111) facet. The XRD results indicate that
Ru/C, PdRu/C, PdNi/C, RuNi/C, and PdRuNi/C) for catalytic the main crystalline phases in Ni/C are Ni and Ni(OH) . There
hydrogenation of ethylbenzene at different temperatures are no characteristic diffraction peaks related to the Ru or Ni
40 1C, 60 1C, 80 1C, and 100 1C) was investigated. The catalytic phase in the XRD patterns of Ru/C and RuNi/C (Fig. 1Ab and c).
2
(
reaction of ethylbenzene hydrogenation was also carried out for The possible reason is that Ru in high dispersion was sup-
different reaction times (1 h, 2 h or 6 h). After the reactor was ported on carbon black or Ni related nanoparticles in these
installed, 0.0500 g of catalyst and 10 mL of ethylbenzene were catalysts. At the same time, the dispersions of Ni and Ni(OH)
mixed and added into it. The residual air in the reactor were increased by chemical replacement and the presence of
was blown away by H , and the reactor pressure was increased Ru during the preparation process of RuNi/C. Both Ru and Ni
2
2
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Fig. 2 (a–d) TEM images of PdRuNi/C, (e) PdRuNi nanoparticle size
distribution for the PdRuNi/C catalyst and (f) selected area electron
diffraction pattern of PdRuNi/C.
Fig. 1 XRD patterns for (A) (a) Ni/C, (b) Ru/C, and (c) RuNi/C; (B) (a) Pd/C
and (b) PdNi/C and (C) (a) PdRu/C and (b) PdRuNi/C.
the selected area electron diffraction pattern of PdRuNi/C
related particles were loaded on carbon black in the form of tiny simultaneously.
or amorphous particles. Therefore, the accurate nanostructures
The nanostructure of PdRuNi/C was further confirmed by
of metallic particles in the catalysts need to be proved by a series HRTEM (Fig. 3). Most metallic particles are uniformly dispersed
of other characterization techniques.
with a size of less than 10 nm. The crystallite plane spacing
Fig. 1Bb and c illustrate that the Pd loading in the Pd-containing distance of the measured particle is 0.225 nm, which corresponds
catalysts is identical, but the half-width of the Pd characteristic to the Pd(110) plane. The HRTEM results are very consistent with
diffraction peak in the PdNi/C bimetallic catalyst is obviously much the XRD pattern. They also confirm that Pd element exists with the
smaller than that in Pd/C. This demonstrates that the grain size of isolated phase in the catalyst.
Pd in PdNi/C is much smaller than that in Pd/C. Simultaneously,
For the sake of further investigating the morphology and
there are no characteristic diffraction peaks related to Ni species elements of the catalysts, SEM and SEM-EDS analyses were
because Ni is highly dispersed and some Ni atoms are consumed performed on RuNi/C and PdRuNi/C (Fig. S1, ESI†). The EDS
2+
by the reduction of Pd . In addition, no characteristic peaks of spectrum of RuNi/C verifies the presence of Ni and Ru in the
Ni(OH)
are seen, which is possibly due to the fact that the RuNi/C catalyst. After Ru replaces Ni by the galvanic replace-
2
reduction process also promotes the dispersion of Ni(OH)
makes it highly dispersed.
2
and ment reaction method, Ni is still present in the catalyst. The
EDS spectrum of PdRuNi/C also confirms the presence of Ni, Pd
In the XRD pattern of the PdRu/C catalyst (Fig. 1Ca), the and Ru in PdRuNi/C.
diffraction peaks with 2y angles of 44.01, 46.71, 68.21 and 82.21
To visually observe the elemental distribution in the sample,
are assigned to Ru(101), Pd(200), Pd(220) and Pd(311) planes, HAADF-STEM was performed on the PdRuNi/C sample. The
5
4
respectively, and a clear peak with a two theta value of 40.11 is bright particles can be attributed to the metal-containing nano-
assigned to the Pd(111) crystallite plane in Fig. 1Cb (XRD particles on carbon black without agglomeration (Fig. 4a), which
pattern of PdRuNi/C) (JCPDS card No. 65-2867). Contrasting is very consistent with the TEM results. STEM-EDS elemental
with the PdRu/C bimetallic catalyst, the characteristic peaks of mapping analysis well confirms that Pd, Ru, and Ni are together
Ru and Pd related phases cannot be observed in the XRD present in the PdRuNi nanoparticles. In Fig. 4b–d, it can be seen
pattern of the PdRuNi/C tri-metallic catalyst. This can perhaps
be attributed to the existence of Ni, which promoted Ru and Pd
dispersion, leading to their particle size being outside of the
scope of XRD detection. Therefore, the unique nanostructure of
the PdRuNi/C catalyst further demands to be determined by
other analytical methods as shown below.
Fig. 2 displays the TEM images of PdRuNi/C and its corres-
ponding PdRuNi nanoparticle size distribution. It is obvious
that there are many black dots distributed on carbon black.
These small black spots are supposed to be metallic nano-
particles. This illustrates that the PdRuNi nanoparticles are
dispersed on the catalyst support, with a mean size of 8.16 nm.
The crystallite planes of Pd(311), Pd(220), Pd(200), Pd(111) and
Ni(OH)
(101) and the existence of Pd element are revealed by ponding fast Fourier transform (FFT) pattern.
Fig. 3 (a and b) HRTEM image of the PdRuNi/C catalyst and corres-
2
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Fig. 5 Ni 2p XPS spectra of (a) Ni/C, (b) PdNi/C, (c) RuNi/C and
(
d) PdRuNi/C catalysts.
XPS of the monometallic, bimetallic and tri-metallic catalysts
was carried out to reveal the characteristics of electron transfer
among Ru, Pd, and Ni elements, as well as to measure the
chemical states and compositions of different elements on the
surface. The XPS spectra of Ni 2p in the Ni/C, PdNi/C, RuNi/C,
and PdRuNi/C catalysts are shown in Fig. 5. The peaks with the
binding energies of 852.7 eV, 853.9 eV, 855.4 eV and 857.1 eV in
the Ni 2p3/2 XPS spectra are consistent with Ni(0), NiO, Ni(OH)
2
5
5,56
and NiOOH species, respectively.
The nickel hydroxide and
nickel oxyhydroxide are found to be dominant by analyzing
the peak area of each species. Moreover, the nickel hydroxide/
nickel oxyhydroxide ratio on the surface of Ni/C is obviously
much higher than that on the surfaces of RuNi/C, PdNi/C, and
PdRuNi/C. The binding energy positions of different Ni species
are identical for various catalysts containing nickel element,
which further reveals that PdRuNi alloy is not present in the
PdRuNi/C catalyst.
Fig. 4 (a) HAADF-STEM image of PdRuNi/C, STEM-EDS mapping of the
selected region in the red square of PdRuNi/C: (b) Ni (red), (c) Pd (dark
cyan) and (d) Ru (yellow), (e) HAADF-STEM image of PdRuNi/C, and (f)
STEM-EDS elemental line-scanning of a single PdRuNi nanoparticle along
the red arrow in (e), black (Ni), red (Pd) and blue (Ru).
that the distribution region of Pd can overlap that of Ru, but the
The Pd 3d XPS spectra for the Pd/C, PdRu/C, PdNi/C, and
location region of Ni is obviously much larger than that of Pd or PdRuNi/C samples are displayed in Fig. 6. The peaks at 335.6 eV
n+
Ru. Moreover, combining with the above other characterization (Pd(0)-3d5/2) and 337.5 eV (Pd -3d5/2) manifest that the Pd(0)
n+
57,58
results, it is very clear that islands in the form of PdRu tiny and Pd species together exist on the surface.
nanoclusters or PdRu binary atoms are supported on the surface area of different Pd species in Pd 3d5/2 XPS spectra was
of the nanoparticles containing Ni element. calculated to get a conclusion that the palladium element is
The peak
n+
For an in-depth understanding of the nanostructure and mainly in the form of oxidized palladium (Pd ). This is mainly
elemental distribution of the catalyst, STEM-EDS elemental line- because most palladium atoms on the surface are easily oxi-
scanning measurement was also performed on it. The brighter dized in air before measurements. Fig. 6a illustrates that the
n+
particles in the STEM image of PdRuNi/C may be metallic Pd/C catalyst has a small portion of Pd , which is much lower
nanoparticles supported on carbon (Fig. 4e). Fig. 4f shows the than those of PdRu/C, PdNi/C and PdRuNi/C, and the order of
n+
STEM-EDS elemental line-scanning of a single PdRuNi particle the Pd /Pd(0) ratio is PdRuNi/C c PdNi/C c PdRu/C c Pd/C.
along the red arrow in Fig. 4e. It indicates that the content of Pd To the best of our knowledge, highly dispersed Pd atoms are
is higher than Ru, and the distribution position of high intensity more easily oxidized in air. Therefore, the Pd atoms on the
of the Pd element is the same as that of Ru element. Moreover, it surface of the PdRuNi/C tri-metallic catalyst have a much better
is obvious that the increase or decrease of Pd and Ru elements dispersion than the Pd/C monometallic catalyst and the PdRu/
does not synchronize with that of the Ni element, indicating no C and PdNi/C bimetallic catalysts. This is the key reason for the
formation of PdRuNi alloy, probably with phase segregation. catalytic performance of PdRuNi/C for ethylbenzene hydro-
This demonstrates that the nanostructure of PdRu island-on-Ni/ genation being much more excellent than those of Pd/C,
Ni(OH)
2
nanoparticles is created in PdRuNi/C.
PdRu/C, and PdNi/C (as listed Table 2).
17310 | New J. Chem., 2019, 43, 17306--17314
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Table 2 Catalytic performance of different catalysts for ethylbenzene
a
hydrogenation in the reported literature and this work
Temperature
(1C)
Yield of
ethylcyclohexane (%)
TOF
(h
À1
Entry
Catalyst
)
b
1
Ni/MPS
DyNi/MPS
Ni
NiMo0.16
Ni/C
150
150
150
150
100
40
60
80
100
40
60
80
100
40
60
80
100
100
100
100
60
80
100
60
80.0
100.0
76.5
100.0
o0.1
14.0
16.9
19.5
23.1
12.5
15.6
18.8
20.1
25.0
30.1
32.0
35.7
12.1
15.8
17.6
28.4
32.7
39.9
26.5
29.6
36.8
51.6
54.7
58.9
100
—
—
—
—
—
b
2
c
3
c
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
RuNi/C
RuNi/C
RuNi/C
RuNi/C
PdNi/C
PdNi/C
PdNi/C
PdNi/C
PdRuNi/C
PdRuNi/C
PdRuNi/C
PdRuNi/C
Pd/C
2501.5
3019.7
3484.3
4127.5
2331.9
2910.3
3507.3
3749.8
4271.5
5142.9
5467.5
6099.7
2245.9
2727.1
3074.4
2537.3
2921.4
3564.7
2472.0
2761.1
3432.8
4408.2
4673.0
5031.8
2847.7
0
1
2
3
4
5
6
7
8
9
0
1
Fig. 6 Pd 3d XPS spectra of (a) Pd/C, (b) PdRu/C, (c) PdNi/C and
(d) PdRuNi/C catalysts.
Ru/C
PdRu/C
RuNi/C
RuNi/C
RuNi/C
PdNi/C
PdNi/C
PdNi/C
PdRuNi/C
PdRuNi/C
PdRuNi/C
PdRuNi/C
Fig. 7 shows the Ru 3p XPS spectra of Ru/C, PdRu/C, RuNi/C,
d
d
d
d
d
d
d
d
d
e
and PdRuNi/C. The binding energies of 462 eV and 464.2 eV are 22
4
+ 59–61
23
correspondingly assigned to Ru(0) and Ru .
Comparing the
2
2
4
5
4
+
peak areas corresponding to Ru(0) and Ru in the Ru 3p3/2 XPS
80
100
60
80
100
100
4
+
spectrum, the contents of Ru on the surface of the Ru/C and 26
2
2
2
7
8
9
PdRu/C catalysts are close to that of Ru(0). The order of oxidation
state ruthenium/reduction state ruthenium ratio on the surfaces
of the catalysts is: PdRuNi/C 4 RuNi/C 4 PdRu/C 4 Ru/C, 30
which is very consistent with the regulation of Pd element. As a
result of most ruthenium atoms being rapidly oxidized in air, the catalyst (0.0500 g), and reaction pressure (5.0 MPa). Ref. 34. Reaction
a
Reaction conditions: ethylbenzene (10 mL), reaction time (1 h),
b
conditions: m(catalyst) = 0.3000 g, P = 0.3 MPa, T = 150 1C, and t = 10 min.
ruthenium atoms on the surface of the RuNi/C and PdRuNi/C
c
À1
Ref. 35. Reaction conditions: m(catalyst) = 0.1500 g, LHSV = 10.0 h
,
catalysts were dominated by the form of RuO
2
.
d
P = 1.0 MPa, T = 150 1C, and n(H
Fig. 8A and D shows the 3 keV He (5 keV Ne ) HS-LEIS time (2 h), other reaction conditions were unchanged. Reaction time
6 h), other reaction conditions were unchanged.
2
)/n(ethylbenzene) = 16 : 1. Reaction
4
+
20
+
e
(
spectra of Ni/C, Pd/C and PdNi/C; Fig. 8B shows that of E-Ni/C,
Ru/C, and RuNi/C; and Fig. 8C shows that of F-Ni/C, PdRu/C,
and PdRuNi/C. The carbon, oxygen, palladium, ruthenium and
nickel atoms on the outermost layer of the PdRuNi/C catalyst
come to light in the HS-LEIS spectra (Fig. 8C and F). This
confirms that ruthenium and palladium are successfully loaded
on the surface of the catalyst. The composition of Ni on the
outer surface of the PdRuNi/C catalyst is much larger than
those of Pd and Ru (Fig. 8Fc). This proves that some of the
nickel atoms are occupied by Ru and Pd atoms, but most nickel
atoms are not occupied. It can be observed that the palladium
content on the outer surface of PdNi/C is much larger than that
on the outmost surface of Pd/C due to the higher Pd dispersion
in PdNi/C (Fig. 8D). A similar conclusion can be drawn from the
2
0
+
5
keV Ne HS-LEIS spectra for Ru/C and RuNi/C (Fig. 8E). Note
that the total intensity of Ru and Pd atoms on the outer surface
of PdRuNi/C is much stronger than PdRu/C, indicating better
Ru and Pd dispersion in PdRuNi/C (Fig. 8F). This is supposed to
be beneficial to enhance the catalytic performance of the
catalysts for ethylbenzene hydrogenation.
Fig. 7 Ru 3p XPS spectra of (a) Ru/C, (b) PdRu/C, (c) RuNi/C and
d) PdRuNi/C catalysts.
(
3.2 Catalytic hydrogenation performance of the catalysts
The hydrogenation reactions were performed under the reaction
Table 2 shows the comparison of the catalytic performance of the conditions below: reaction time (1 h), hydrogen pressure (5.0 MPa),
as-obtained and reported catalysts for ethylbenzene hydrogenation. ethylbenzene (10 mL) and catalyst (0.0500 g). It could be found
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sites because of the small size of Ru and Pd particles and the
capacity of activating hydrogen of precious metals, forming activated
hydrogen species (H*); Ni(OH) sites adsorb and activate ethyl-
2
benzene; activated H* is transferred to the Ni(OH) surface via
2
4
4,62
the spill-over effect of Ni sites,
reacting with the activated
ethylbenzene to produce ethylcyclohexane.
In order to investigate the effect of reaction time on the
catalytic properties of the catalysts, the reaction time was
extended to two hours. We found that the RuNi/C catalyst
is slightly more active than the PdNi/C catalyst under the same
reaction parameters. The yield of the product within the
reaction time of 2 h was much higher than within 1 h, and
the yield of ethylcyclohexane of the PdRuNi/C catalyst reached
up to 58.9% (entry 29 in Table 2) under the same reaction
conditions. Ethylbenzene was totally hydrogenated to ethyl-
cyclohexane with the reaction time increasing to 6 h, with a
À1
TOF of 2847.7 h (Table 2, entry 30). This indicates that a
prolonged catalytic reaction can increase the yield of ethyl-
cyclohexane to some extent.
3
.3 Stability of PdRuNi/C for ethylbenzene hydrogenation
The stability test of PdRuNi/C was carried out by reusing the
Fig. 8 (A) 3 keV He and (D) 5 keV Ne HS-LEIS spectra for (a) Ni/C, catalyst five times at the reaction temperature of 100 1C, reac-
4
+
20
+
4
+
20
+
(
b) Pd/C and (c) PdNi/C; (B) 3 keV He and (E) 5 keV Ne HS-LEIS tion pressure of 5.0 MPa, catalyst amount of 0.0500 g and
4 +
spectra for (a) Ni/C, (b) Ru/C and (c) RuNi/C; (C) 3 keV He and (F) 5 keV
reaction time of 1 h. The recycle hydrogenation reaction results
are displayed in Fig. 9, which reveals that the catalytic activity of
the catalyst decreases slightly (the yield of ethylcyclohexane
2
0
+
Ne HS-LEIS spectra for (a) Ni/C, (b) PdRu/C and (c) PdRuNi/C.
that all the as-obtained catalysts showed 100% selectivity to ethyl- decreased by less than 1.2%), and the yield of ethylcyclohexane
cyclohexane at various reaction temperatures. The catalytic activity and TOF of the PdRuNi/C catalyst almost remained unchanged.
of Ni/C was below 0.1% yield of ethylcyclohexane when the reaction This indicates that the stability of the catalyst is fine, and the
temperature was 100 1C (Table 2 entry 5). When the hydrogenation PdRuNi/C catalyst is relatively stable, which can be proved by
reaction was catalyzed by the RuNi/C or PdNi/C catalyst, the the XPS and TEM test results of the recycled PdRuNi/C catalyst
reaction temperature changed from 40 1C to 100 1C and thus (Fig. S2 and S3, ESI†). This is probably because the Ni/Ni(OH)2
the yield of ethylcyclohexane was enhanced from 14.0% to 23.1% nanoparticles stabilize the Pd and Ru related species, which
À1
À1
(
TOF from 2501.5 h to 4127.5 h ) (Table 2 entries 6–9) (RuNi/C), lessens the loss of the active components. The small decline in
À1
PdNi/C (40 1C-yield = 12.5% and TOF = 2331.9 h , 100 1C-yield = catalytic activity is perhaps due to the slight weight loss during
0.1% and TOF = 3749.8 h ) (Table 2 entries 10–13). The Pd/C, the separation process of catalyst and liquid.
À1
2
Ru/C, and PdRu/C catalysts provided a 12.1% yield of ethylcyclo-
À1
hexane and TOF of 2245.9 h , a 15.8% yield of ethylcyclohexane
À1
and TOF = 2727.1 h , and a 17.6% yield of ethylcyclohexane and
À1
TOF of 3074.4 h
at 100 1C, respectively (entries 18–20 in
Table 2). Under the same reaction conditions, the catalytic
activities of RuNi/C and PdNi/C were higher than those of the
Pd/C, Ru/C and PdRu/C catalysts due to the nano-synergistic effect
between the noble and transition metal related species. When
ethylbenzene hydrogenation was conducted over the PdRuNi/C
tri-metallic catalyst, its catalytic performance was improved
(25.0% to 35.7%) with the increase of reaction temperature
(40 1C to 100 1C) (entries 14–17 in Table 2). More importantly, the
PdRuNi/C tri-metallic nanocatalyst gave much more outstanding
catalytic activity than the Ni/C, Pd/C, and Ru/C monometallic and
PdNi/C, RuNi/C, and PdRu/C bimetallic nanocatalysts. The main
reason is that the novel nanostructure of RuPd islands (nanoclusters
2
or binary atoms)-on-Ni/Ni(OH) NPs produces the synergistic effect
Fig. 9 Stability of PdRuNi/C for ethylbenzene hydrogenation. Reaction
among Ru, Pd and Ni species in PdRuNi/C. This synergistic effect is
conditions: ethylbenzene (10 mL), reaction time (1 h), reaction temperature
as follows: hydrogen is easily adsorbed and activated at the Ru or Pd (100 1C), reaction pressure (5.0 MPa), and catalyst amount (0.0500 g).
17312 | New J. Chem., 2019, 43, 17306--17314
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4
Conclusions
7 M. Hu, Z. Yao, L. Li, Y. H. Tsou, L. Kuang, X. Xu, W. Zhang
and X. Wang, Nanoscale, 2018, 10, 10203–10212.
In summary, the PdRuNi/C tri-metallic catalyst with good stability
and high catalytic activity for ethylbenzene hydrogenation was
synthesized at ambient temperature via hydrazine hydrate reduction
and chemical replacement methods. The PdRuNi/C sample was
characterized by XRD, SEM-EDS, TEM, HRTEM, HAADF-STEM,
STEM-EDS elemental mapping and line-scanning, XPS and
HS-LEIS methods, and PdRuNi/C with a PdRu island-on-Ni/
Ni(OH)2 nanoparticle nanostructure and 10 nm size of the
PdRuNi particles were found. PdRuNi/C showed the highest
catalytic activity for ethylbenzene hydrogenation reaction
among all the as-synthesized Ni/C, Pd/C, and Ru/C monometallic,
PdNi/C, RuNi/C, and PdRu/C bimetallic and PdRuNi/C tri-metallic
nanocatalysts. When the reaction was continued for one hour and
the reaction temperature was 100 1C, the yield of ethylcyclohexane
was ranked as PdRuNi/C (35.7%) 4 RuNi/C (23.1%) 4 PdNi/C
8
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Acknowledgements
The funding support of this research by the National Natural
Science Foundation of China (Grant No. 21763011), the Natural
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