Effect of ruthenium nickel bimetallic composition on the catalytic performance for benzene hydrogenation to cyclohexane

Article abstract of DOI:10.1016/j.apcata.2015.04.016

For bimetallic catalysts, the composition of each metal component has a crucial effect on their catalytic performance. In this work, the Ni/Ni(OH)₂/C sample is synthesized at room temperature (RT) by a simple hydrazine hydrate reduction method

Full text of DOI:10.1016/j.apcata.2015.04.016

Accepted Manuscript
Title: Effect of ruthenium nickel bimetallic composition on the
catalytic performance for benzene hydrogenation to
cyclohexane
Author: Lihua Zhu Hanlei Sun Hao Fu Jinbao Zheng Nuowei
Zhang Yunhua Li Bing H. Chen
PII:
S0926-860X(15)00249-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.04.016
APCATA 15353
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
4-2-2015
3-4-2015
9-4-2015
Please cite this article as: L. Zhu, H. Sun, H. Fu, J. Zheng, N. Zhang, Y. Li, B.H.
Chen, Effect of ruthenium nickel bimetallic composition on the catalytic performance
for benzene hydrogenation to cyclohexane, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.04.016
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Graphical Abstract
1
Page 1 of 30

Highlights
•
•
•
•
The Ru Ni /C catalysts with different Ru/Ni atomic ratios are prepared
x y
The property of catalysts are in using benzene hydrogenation
Catalytic activity of the catalyst relevant to Ru/Ni atomic ratio
The Ru0.56Ni0.44/C catalyst has the highest catalytic performance
2
Page 2 of 30

Effect of ruthenium nickel bimetallic composition on the catalytic
performance for benzene hydrogenation to cyclohexane
*

Lihua Zhu, Hanlei Sun, Hao Fu, Jinbao Zheng, Nuowei Zhang, Yunhua Li and Bing H Chen

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

Corresponding authors: Tel: +86-592-2185253, Fax: +86-592-2184822
Email addresses: cezhouh@xmu.edu.cn (H Zhou), chenbh@xmu.edu.cn (B. H. Chen)
3
Page 3 of 30

Abstract
For bimetallic catalysts, the composition of each metal component has a crucial effect
on their catalytic performance. In this work, the Ni/Ni(OH) /C sample is synthesized
2
at room temperature (RT) by a simple hydrazine hydrate reduction method using
carbon black as support, and then Ru-Ni/Ni(OH) /C is prepared via galvanic
2
replacement reaction based on Ni/Ni(OH) /C. The Ru-Ni/C catalyst can be obtained
2
when Ru-Ni/Ni(OH) /C is annealed in N at 380 °C. The different compositions of
2
2
Ru-Ni bimetallic catalysts are successfully controlled by adjusting the addition
amount of aqueous RuCl solution during the galvanic replacement process. The
3
Ru Ni /C catalysts (the subscripts represent atomic ratios) are evaluated by the
x
y
reaction of the hydrogenation of benzene to cyclohexane and the experimental results
reveal a strong correlation between the catalytic activity for benzene hydrogenation
and Ru/Ni atomic ratios. The Ru0.56Ni0.44/C catalyst exhibits the highest catalytic
performance among the Ru Ni /C catalysts in this work. The characterizations of
x
y
X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission
electron microscope (TEM), high resolution transmission electron microscope
(HRTEM) and EDS-elemental mapping for the Ru0.56Ni0.44/C catalyst demonstrate
that the nanostructure of Ru-on-Ni/NiO nanoparticles can improve its catalytic
properties.
Keywords: Ruthenium-nickel bimetallic nanocatalysts, composition-effect, benzene
hydrogenation
4
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1
. Introduction
Bimetallic nanocatalysts have attracted increasing and considerable research
attention in the field of heterogeneous catalysis [1,2]. Bimetallic nanocatalysts exhibit
distinct catalytic activity, selectivity, and stability in catalytic processes in comparison
with their corresponding monometallic nanocatalysts [3,4]. Besides, the shape [5],
size [6,7], composition [8,9] and nanostructure [10-15] of bimetallic nanocatalysts
play crucial roles in their catalytic performance. Focusing on composition, Li’s group
[16] synthesized many carbon supported Pt-Fe bimetallic nanocatalysts (Pt-Fe/C) with
various Pt/Fe ratios, and examined their catalytic performance in oxygen reduction
reaction (ORR). It was found that the Pt-Fe/C catalyst with a Pt:Fe ratio of 1.2:1
showed the highest mass activity and specific activity in ORR. The reason for this
was considered to be smaller Pt-Fe particle size and better Pt-Fe alloy structure in this
Pt-Fe/C catalyst (Pt:Fe = 1.2:1). The Pt-Ni/graphene nanocatalysts with Pt/Ni molar
ratio of 1:0.5, 1:1 and 1:2 were prepared via a one-step hydrazine reduction method in
Cai’s team [17], and the optimal Pt/Ni molar ratio was found to be 1:1 with the
highest catalytic activity for methanol electro-oxidation reaction. Based on these, it is
clear that the catalytic properties of bimetallic nanocatalysts are also highly dependent
on the composition of their components in addition to other factors such as
morphology, size and nanostructure.
Cyclohexane is a significant and imperative chemical intermediate for the synthesis
of ε-caprolactone, caprolactam, nylon-66 and nylon 6 [18,19], as well as an excellent
and nontoxic solvent. Till now, cyclohexane is mainly produced by the method of
catalytic benzene hydrogenation in chemical industry. Many homogeneous complexes
have been used as the catalysts for this reaction [20-23], but the separation turned out
to be a common problem for homogeneous catalysis. Regarding the heterogeneous
catalysts for benzene hydrogenation to cyclohexane, they might be categorized as
nickel-based [24,25], platinum-based [26-32], palladium-based [33], rhodium-based
5
Page 5 of 30

[34-38], iridium-based [39] and ruthenium-based [40-47] catalysts. Among them, the
Rh-based and Ru-based catalysts are currently considered as the highly efficient
catalysts for benzene hydrogenation to cyclohexane and widely investigated. Likewise,
bimetallic nanocatalysts have been also applied in this reaction, such as Pd-Rh [48],
La-Ni [49], Pt-Pd [50], Pt-Rh [51], Pt-Co [52], Pt-Ni [52], Rh-Ni [53], and Au-Pd [54]
catalysts. However, to the best of our knowledge, the composition-effect of these
bimetallic nanocatalysts on the catalytic performance for benzene hydrogenation has
not been reported.
Herein, carbon supported Ru-Ni bimetallic nanocatalysts with various Ru/Ni
atomic ratios (Ru Ni /C) were synthesized via hydrazine hydrate reduction and
x
y
galvanic replacement methods, and evaluated by the reaction of benzene
hydrogenation to cyclohexane. And Ru0.56Ni0.44/C exhibited the best performance for
this reaction. The fundamentals for the optimum Ru/Ni atomic ratio (0.56/0.44) have
been revealed by XRD, XPS, TEM, HRTEM and EDS-elemental mapping
characterizations in this study.
2. Experimental Section
2
.1 Materials
Carbon black (BLACK PEARLS 2000 LOT-1366221) was obtained from Cabot
Corporation and used as catalyst support. All other reagents, nickel(II) chloride
hexahydrate (NiCl ·6H O), anhydrous ethanol, sodium hydroxide (NaOH), anhydrous
2
2
ruthenium chloride (RuCl ), polyvinyl pyrrolidone (PVP), 85 wt% (weight percent)
3
hydrazine hydrate solution and benzene were purchased from Sinopharm Chemical
Reagent Co. Ltd. (Shanghai, China). Water used in all experiments was deionized
water. All reagents were used as received.
2
.2 Preparation of Ni/Ni(OH) /C
2
6
Page 6 of 30

Ni/Ni(OH) /C was synthesized at RT by hydrazine hydrate reduction method
2
reported in our previous works [11,55], An appropriate amount of nickel(II) chloride
hexahydrate (0.225 g) was dissolved in the mixture of 82.5 mL of aqueous PVP
solution and 12.5 mL of anhydrous ethanol. The as-obtained green transparent
solution was vigorously stirred at RT for ten minutes. Then, aqueous NaOH solution
(1.813 g of NaOH dissolved in 12.5 mL of deionized water) was added into the above
solution, vigorously magnetic agitation for ten minutes. Subsequently, 25 mL of 85
wt% aqueous hydrazine hydrate solution was placed into the above mixture. Ten
minutes later, the addition of 1.250 g of carbon black was performed. After stirred for
another ten minutes, the mixture was transferred into a Teflon cup and sealed, and the
reaction maintained at RT for about 18 hours. The solid sample was collected by
-
filtration, repeatedly washed with deionized water and anhydrous ethanol until no Cl
ions were detected by aqueous AgNO solution. Finally, Ni/Ni(OH) /C was obtained
3
2
after dried in vacuum at 60 °C for 6 h, with Ni (Ni element) loading of 2.48 wt%
determined by inductively coupled plasma atomic mass spectrometry (ICP-MS).
2
.3 Synthesis of the Ru Ni /C catalysts
x y
Ni/Ni(OH) /C was introduced into an appropriate amount of aqueous RuCl
2
3
-3
-1
solution with a certain concentration (9.64×10 mol L ) at RT for 12 h under strong
magnetic stirring. Then Ru-Ni/Ni(OH) /C could be obtained by galvanic replacement
2
reaction [15,56,57]. The solid was obtained by filtration, thoroughly washed with
-
deionized water and anhydrous ethanol until no Cl ions were detected by aqueous
AgNO solution. After dried in a vacuum oven at 60 °C for 6 h, the as-obtained solid
3
-1
was calcined in flowing 99.999%N of 80 mL min at 380 °C for 3 h. The Ru loading
2
could be adjusted by changing the addition amount of aqueous RuCl solution. Finally,
3
the catalysts with accurate Ru and Ni loadings determined by ICP-MS were gained,
signed as Ru Ni /C, where x and y represent Ru and Ni atomic ratios (as shown in
x
y
Table 1), respectively. Carbon~380 was gained after carbon black annealed in N at
2
7
Page 7 of 30

3
80 °C for 3 h.
2
.4 Catalyst characterization
X-ray diffraction (XRD) patterns for the catalysts were collected on a Rigaku X-ray
diffractometer equipped with high-speed array detection system. Cu Kα radiation (35
kV and 20 mA) was used as the X-ray source. The samples were scanned in the range
-1
of 20° to 90° and with a scanning rate of 10° min . X-ray photoelectron spectra (XPS)
for the catalysts were recorded on the PHI Quantum 2000 Scanning ESCA
Microprobe equipment using monochromatic Al Kα radiation (1846.6 eV) as the
X-ray source. Transmission electron microscope (TEM), high resolution transmission
electron microscope (HRTEM), high angle annular dark field-scanning transmission
electron microscopy (HAADF-STEM) and EDS-elemental mapping tests were
performed on TECNAI F30-HRTEM with a field emission source, operated at the
accelerating voltage of 300 kV. Before the tests, the samples were dispersed into
ethanol to make a suspension. Several drops of the suspension were dripped on a
copper grid, ethanol evaporated in air at RT. The contents of Ru and Ni in each
sample were measured by inductively coupled plasma atomic mass spectrometry on
an Agilent ICP-MS 4500-300.
2
.5 Catalytic performance measurements
The hydrogenation of benzene was performed in a stainless steel autoclave (Parr
848) at 60 °C. The catalyst (50 mg) and benzene (10 mL) were together placed into
4
the reactor and quickly sealed. The reactor was firstly flushed with nitrogen for 1 min
to remove air, then residual nitrogen replaced with hydrogen for 1 min. Hydrogen was
continuously added up to the pressure of 4.8 MPa. The reactants were then heated to
6
0 °C and pressurized to 5.3 MPa, with the stirring rate of 500 rpm. After the desired
reaction time reaching, the mixture was quickly cooled down to about 5 °C using an
8
Page 8 of 30

ice-water bath and then H inside the reactor was evacuated. The liquid was separated
2
from the solid by centrifugation. The liquid products were analyzed using gas
chromatography (GC) (Shimadzu GC 2010), equipped with a DB-35 60 m×0.32 mm
capillary column and a flame ionization detector (FID) as well as identified by gas
chromatography~mass spectrometry (GC~MS) (Shimadzu GC~MS 2010). TOF
(
turnover frequency) of the Ru Ni /C catalysts was calculated on the basis of the mols
x y
of total ruthenium metal because nickel was mainly in the form of oxidation state.
Additionally, the metal dispersion was not measured for most of the supported
catalysts for liquid benzene hydrogenation in the reported references (as shown in
Table 3). For comparison, the metal dispersion of the Ru Ni /C catalysts was not
x
y
considered in the calculation of TOF.
3. Results and discussion
3
.1 Catalytic activity for benzene hydrogenation
In order to definitely distinguish the catalysts with different Ru/Ni atomic ratios,
the nomenclature of the catalysts is listed in Table 1. The catalytic performance of the
as-prepared Ru Ni /C catalysts for benzene hydrogenation is listed in Table 2. When
x
y
this reaction was conducted under the conditions (5.3 MPa H , 60 °C, 1 h), the
2
Ru0.08Ni0.92/C catalyst showed relatively low catalytic activity, with a 3.99% yield to
-1
cyclohexane and TOF of 726.6 h (Table 2, entry 1). With the increase of Ru/Ni
atomic ratio, the catalytic property gradually increased (Table 2, entries 2-4), and the
Ru0.56Ni0.44/C catalyst provided the highest catalytic performance for benzene
hydrogenation to cyclohexane, giving a 99.8% yield to cyclohexane within 0.5 h and
-1
an exceptionally high TOF of 24167.4 h (Table 2, entry 4). Moreover, Ru0.56Ni0.44/C
was a relatively efficient catalyst for the hydrogenation of benzene to cyclohexane
relative to other catalysts reported in the literatures (as shown in Table 3). Therefore,
this catalyst has huge potential of the industrial application. As Ru/Ni atomic ratio
9
Page 9 of 30

continuously was elevated to 0.83/0.17 (Ru0.83Ni0.17/C), the catalytic activity sharply
-1
declined, with a 41.3% yield to cyclohexane and a low TOF of 7520.9 h under the
reaction conditions (5.3 MPa H , 60 °C and 1 h) (Table 2, entry 5). Based on the
2
reaction results listed in Table 2, it can be drawn that the optimum Ru/Ni atomic ratio
is 0.56/0.44 with the maximum catalytic activity for benzene hydrogenation. When
Ru/Ni atomic ratio of the catalyst was lower or higher than 0.56/0.44, the catalytic
activity for benzene hydrogenation decreased. The detailed explanations for this
phenomenon will be provided by the results of characterizations, as described in the
following subsection.
3
.2 XRD studies
To obtain the crystallite phase structure and dispersion of Ru-Ni BNPs in the
as-prepared catalysts, X-ray diffraction (XRD) tests for carbon~380 and Ru Ni /C
x
y
samples were carried out. For carbon~380, the diffraction peaks of carbon (002) and
carbon (100) [65,66] can be clearly seen in Fig. 1Aa. No any diffraction peak for Ru
metal can be observed in the XRD patterns for the Ru0.08Ni0.92/C, Ru0.16Ni0.84/C and
Ru0.34Ni0.66/C catalysts, indicating that the Ru nanoparticles are either very small or
amorphous with a good dispersion. The XRD peak occurring at 2θ = 43.3 can be
attributed to NiO(111) or carbon(100) (2θ = 43.1) [67,68]. Two weak, even
ambiguous peaks at 2θ = 35.0° and 54.3° could be grudgingly observed in the XRD
patterns for the Ru0.56Ni0.44/C catalyst, demonstrating some oxidation state Ru (RuO2)
in the Ru-Ni BNPs [56]. Additionally, the characteristic peak of NiO(111) also
appeared in the patterns for the Ru0.56Ni0.44/C sample. However, with the continuous
increase of Ru loading and reduction of Ni content-Ru0.83Ni0.17/C, the two more
characteristic peaks of 2θ = 35.0° and 2θ = 54.3° can be ascribed to RuO (101) and
2
RuO (211) [56]. And the observed XRD feature at 2θ = 43.3 was due to the
2
diffraction peak of (111) facets of face centered cubic (fcc) NiO. According to the
above XRD results, with the Ru/Ni atomic ratio increasing, the proportion of RuO2
crystallite phase in Ru species increased. The reason for this would be discussed in
1
0
Page 10 of 30

the following section. Also note that the Ru0.83Ni0.17/C catalyst has lower catalytic
activity for benzene hydrogenation than Ru0.56Ni0.44/C as shown in Table 2.
3
.3 XPS analysis
To further get the chemical states of Ru and Ni in the Ru0.56Ni0.44/C and
Ru0.83Ni0.17/C samples, X-ray photoelectron spectroscopy (XPS) measurements were
performed on them. As present in Fig. 2a, the Ni 2p peaks are located at 862 and
8
82.1 eV due to multielectron excitation [69,70]. The Ni 2p3/2 peaks which appear at
binding energies of 852.7, 853.9, 855.4 and 857.1 eV are attributed to Ni(0), NiO,
Ni(OH) and NiOOH species, respectively. The binding energies of 870, 871.2, 872.9
2
and 874.4 eV are corresponding to Ni(0), NiO, Ni(OH) and NiOOH in Ni 2p XPS
2
1/2
spectra, respectively [69,71-73]. By comparing the intensity of each peak, it could be
clearly found that the major Ni-species in these two catalysts was oxidation state
nickel (NiO, Ni(OH) and NiOOH), with a small amount of reduced state nickel
2
(Ni(0)). Unfortunately, no significant Ni signals were detected for the Ru0.83Ni0.17/C
sample because of the low loading of Ni in this sample (Fig. 2b).
Ru 3d XPS spectrum signal was not distinct for our catalysts because Ru 3d XPS
spectra is often obscured by the strong C 1s signal from carbon [74]. Therefore, XPS
measurement for Ru was performed in the range of 458 eV~472 eV where Ru 3p
peaks are expected to be observed (Fig. 2c,d). As shown in Fig. 2c, the peaks at the
binding energies of 462 eV and 464.2 eV in the Ru 3p3/2 spectrum are assigned to
4+
Ru(0) and Ru (RuO ), respectively [75]. The other two peaks (484 eV and 486.2 eV)
2
in Ru 3p XPS spectra could be ascribed to metallic ruthenium (Ru(0)) and oxidation
1/2
state ruthenium (RuO ), respectively [76,77]. It suggests that Ru(0) and RuO co-exist
2
2
on the surface of the Ru0.56Ni0.44/C sample. The presence of RuO was resulted from
2
the surface oxidation of Ru(0) nanoparticles [42]. But for Ru0.83Ni0.17/C, the Ru
element on the surface is mainly in the form of RuO (Fig. 2d). XPS binding energies
2
and relative composition (mol%) of different Ru species (Ru(0) and RuO ) on the
2
surface of the catalysts are shown in Table 4.
1
1
Page 11 of 30

The existence of so much RuO in Ru0.83Ni0.17/C is mainly attributed to the
2
following reasons. Ru nanoparticles are easily oxidized in air [42]. More importantly,
Ru-Ni/Ni(OH) /C was synthesized via galvanic replacement method (as depicted in
2
Scheme 1A below). But due to the limited amount of Ni(0) in Ni/Ni(OH) and
2
solubility product constant of Ni(OH) being greater than that of Ru(OH)
2
3
(
K (Ni(OH) ) > K (Ru(OH) ), Ru(OH) may form based on Scheme 1B below.
sp 2 sp 3 3
Then Ru(OH) could decompose into RuO and H O after annealed in N at 380 °C.
3
2
2
2
In addition, Scheme 1A preferentially takes place because the reaction rate of process
A is much greater than that of process B (r >> r ) in Scheme 1. Hence, if the addition
A
B
amount of RuCl is excessive during the catalyst preparation process, the formation of
3
much RuO will be inevitable.
2
(A): 3Ni+2RuCl →2Ru+3NiCl
3 2
(
B): 3Ni(OH) +2RuCl →2Ru(OH) +3NiCl
2
2
3
3
Scheme 1. (A) The chemical equation for the main galvanic replacement process between
3
+
Ni/Ni(OH) and Ru . (B) The chemical equation for the production of Ru(OH) by Ni(OH)
2
2
3
3
+
reacting with Ru .
3
.4 TEM and HRTEM studies
TEM studies: The representative TEM images and Ru-Ni BNPs size distribution
for the Ru0.56Ni0.44/C and Ru0.83Ni0.17/C catalysts are present in Fig. 3. The size
distributions were obtained by counting more than 100 Ru-Ni BNPs. The average
Ru-Ni BNPs size in Ru0.56Ni0.44/C is 4.38±4.19 nm with a wide size distribution (Fig.
3
a-c), almost without agglomeration. The mean Ru-Ni BNPs size in Ru0.83Ni0.17/C is
controlled to be 8.40±5.24 nm with a relatively wide size distribution (Fig. 4a,b), with
serious BNPs agglomerating. This is an important reason for the low catalytic
performance of the Ru0.83Ni0.17/C catalyst for benzene hydrogenation.
HRTEM studies: Fig. 5 displays the HRTEM images for the Ru0.56Ni0.44/C and
Ru0.83Ni0.17/C samples. As present in Fig. 5a (Ru0.56Ni0.44/C), lattice fringes with
1
2
Page 12 of 30

spacing of 0.205 nm are assigned to the (101) planes of hexagonal close packed (hcp)
78
Ru. It suggests that Ru species are mainly in the form of reduced state, which is
beneficial to the absorption and dissociation of hydrogen molecular. It is very
advantageous to increase the catalytic performance of the Ru0.56Ni0.44/C catalyst for
benzene hydrogenation. This conclusion is in a good agreement with that the
Rh-based and Ru-based catalysts are believed to be highly efficient for the
hydrogenation of benzene to cyclohexane [34-38,40-47,79].
As for Ru0.83Ni0.17/C (Fig. 5b), the HRTEM image clearly reveal lattice fringes with
a regular spacing of 0.318 nm and 0.255 nm, due to the (110) and (101) facets of
RuO , respectively. And the lattice fringes with a regular spacing of 0.241 nm could
2
be ascribed to the (111) facets of fcc NiO. In Fig. 5b, it clearly indicates that NiO was
surrounded by RuO . HRTEM test was also carried out in another area of the
2
Ru0.83Ni0.17/C sample (Fig. 5c). The lattice fringes with a regular spacing of 0.318 nm
attributed to RuO (110) are obviously observed. It further suggests that Ni was mainly
2
in the form of NiO and Ru was mainly in the form of RuO coated on NiO for the
2
Ru0.83Ni0.17/C catalyst. But RuO on the bulk phase can only be reduced at about 120
2
°
C [56], which is higher than the reaction temperature of the hydrogenation of
benzene in this study (60 °C). Therefore, it can be believed that the chemical state of
Ru in Ru0.83Ni0.17/C was mainly RuO during the reaction process. RuO is
2
2
disadvantaged for the adsorption and activation of hydrogen, resulting in the low
catalytic activity for benzene hydrogenation (entry 5 in Table 2).
3
.5 HAADF-STEM images and EDS-elemental mapping
HAADF-STEM and EDS-elemental mapping tests for the Ru0.42Ni0.58/C and
Ru0.83Ni0.17/C catalysts were performed to further investigate the nanostructures and
elemental distributions of Ru-Ni BNPs. Fig. 6 shows typical HAADF-STEM image
and corresponding Ru and Ni elemental maps for Ru0.56Ni0.44/C. It can be found that
most of BNPs are with a relatively narrow size distribution and size of 3~4 nm, as
shown in the white square region in Fig. 6a and its enlarged view (Fig. 6b). A small
1
3
Page 13 of 30

amount of large Ru-Ni BNPs in the red and blue squares region in Fig. 6a are with the
size of about 10 nm or 20 nm. The elemental mapping results for the Ru0.83Ni0.17
BNPs in the red square region clearly indicate that the signal of Ru element is slightly
stronger than that of Ni element (Fig. 6c,d). Together with the above XRD, XPS and
HRTEM results for this catalyst, it could be considered that Ru is supported on the
surface of Ni/NiO nanoparticles. This nanostructure of Ru-on-Ni/NiO nanoparticles
provides multiple and synergetic active sites (Ru, Ni and NiO) for the hydrogenation
of benzene to cyclohexane, where hydrogen can be adsorbed and activated on the Ru
sites, benzene can adsorbed and activated on the NiO sites via the interaction of its
pi-electrons with the positively-charged sites of nickel oxide (P-type semiconductor
﹡
with holes) and Ni sites function as a “bridge” for transferring the activated H
species to the activated benzene via spillover process. Finally, the activated benzene
can be hydrogenated by spilled-over hydrogen, producing cyclohexane [80,81]. Such
synergetic effect of these active sites is very beneficial for increasing the catalytic
performance in this reaction. This conclusion will be proved in our follow-up works.
However, the size of many Ru-Ni BNPs in the Ru0.83Ni0.17/C sample is larger than
1
0 nm, with obvious particle agglomeration (Fig. 7a). Fig. 7b shows the EDS
spectrum of this catalyst. On the basis of elemental maps (Fig. 7c,d), it can be clearly
seen that the signal of Ni element is much weaker than that of Ru and distribution
scope of Ni element is much smaller than that of Ru. The qualitative compositions of
Ru and Ni in the catalysts obtained from the EDS-elemental mapping analysis are
3
+
2+
very consistent with the addition amount of Ru and Ni ions during the catalyst
preparation process.
Based on the above experimental results, it can be concluded that one reason for
high Ru/Ni atomic ratio leading to low catalytic performance is that the catalyst was
with the large Ru-Ni BNPs and aggregation of BNPs. Another reason is that Ru was
mainly in the form of RuO crystallite phase and NiO was surrounded by RuO .
2
2
Therefore, the activation and dissociation of hydrogen molecular were limited to some
extent. While the Ru/Ni atomic ratio was too low, the strong synergistic
effect-specialization and cooperation of the Ru, Ni and NiO sites cannot be well
1
4
Page 14 of 30

developed.
4
. Conclusions
In summary, the catalysts with different Ru/Ni atomic ratios were successfully
synthesized and characterized. The composition effect of the Ru Ni /C catalysts on
x
y
the catalytic activity for the hydrogenation of benzene to cyclohexane was
investigated in this work. It was found that the optimized Ru/Ni atomic ratio was
0
.56/0.44, which provided the highest catalytic performance among the Ru Ni /C
x y
catalysts for benzene hydrogenation to cyclohexane due to the highly efficient
synergistic effect of the Ru, Ni and NiO sites stemming from the nanostructure of
Ru-on-Ni/NiO nanoparticles. Based on the characterization results, the main reasons
for lower catalytic activity of the Ru0.83Ni0.17/C catalyst were that the major Ru
chemical state was ruthenium oxide (RuO ), RuO coated on NiO surface and large
2
2
Ru-Ni BNPs. These factors were not favourable for the activation and dissociation of
hydrogen. The Ru Ni /C catalysts with low Ru/Ni atomic ratio (≤ 0.56/0.44) also
x
y
showed low catalytic performance because the synergistic effect of the Ru, Ni and
NiO sites has not been well developed. This investigation not only discovers the
relationship between Ru and Ni atomic composition and catalytic activity of the
Ru Ni /C catalyst for benzene hydrogenation, but also provides a simple method to
x
y
make effective use of noble metal in bimetallic catalysts contained precious metal.
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central
Universities (Grant No. ZK1013), National Natural Science Foundation of China
(Grant No. 201106118) and Fujian Province Natural Science Foundation (Grant No.
2
012J01052).
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Page 18 of 30

List of Tables
Table 1 Nomenclature of the catalysts with various Ru/Ni atomic ratios
Table 2 Catalytic performance of the Ru Ni /C catalysts under the similar reaction conditions for
x
y
a
the hydrogenation of benzene to cyclohexane
Table 3 Catalytic performance of other metallic catalysts for benzene hydrogenation to
cyclohexane reported in the literatures
Table 4 Chemical states, binding energies (BE) and ratios of integrated intensities (atomic ratios;
AR) of Ru element in the fresh Ru0.56Ni0.44/C and Ru0.83Ni0.17/C catalysts
1
9
Page 19 of 30

List of Figs
Fig. 1. X-ray diffraction (XRD) patterns for the samples. (A), (a) carbon~380, (b)
Ru0.08Ni0.92/C, (c) Ru0.16Ni0.84/C, (d) Ru0.34Ni0.66/C. (B), (a) Ru0.56Ni0.44/C and (b)
Ru0.83Ni0.17/C. The main XRD peaks of Ni and Ru oxidation states are marked with
black and blue dash-dot lines, respectively. These patterns results are the raw data
recorded on the Rigaku Ultima IV X-ray diffractometer, without any other treatment
(such as background subtraction and smoothing).
Fig. 2. Ni 2p XPS spectrum for (a) Ru0.56Ni0.44/C and (b) Ru0.83Ni0.17/C; Ru 3p XPS
spectrum for (c) Ru0.56Ni0.44/C and (d) Ru0.83Ni0.17/C.
Fig. 3. (a,b) Typical TEM images and (c) Ru-Ni BNPs size distribution for the
Ru0.56Ni0.44/C sample. Scale bar, 20 nm.
Fig. 4. (a) Typical TEM image and (b) Ru-Ni BNPs size distribution for the
Ru0.83Ni0.17/C sample. Scale bar, 20 nm.
Fig. 5. HRTEM images for (a) Ru0.56Ni0.44/C sample, the indicated lattice fringes,
0
.205 nm, corresponding to Ru(101) planes. (b,c) Ru0.83Ni0.17/C sample, the indicated
lattice fringes, 0.318, 0.255 and 0.241 nm, corresponding to RuO (110), RuO (101)
2
2
and NiO(111) planes. Where the insets show their respective fast Fourier transform
FFT) patterns results. Scale bar, 2 nm.
(
Fig. 6 (a) HAADF-STEM image for the Ru0.56Ni0.44/C sample, (b) Enlarged view of
the selected white square region in image a, EDS-elemental mapping of (c) Ru (green)
and (d) Ni (orange) from the selected red square region in mage a, respectively. Scale
bar, 50 nm.
Fig. 7. (a) HAADF-STEM image for the Ru0.83Ni0.17/C sample, (b) an EDS spectrum
of the Ru0.83Ni0.17/C sample, EDS-elemental mapping of (c) Ru (cyan) and (d) Ni
(
yellow) from the selected red square region in image a. Scale bar, 50 nm.
2
0
Page 20 of 30

Table 1
Different Ru and Ni loading
Supported catalysts
Ru0.08Ni0.92/C
Ru0.16Ni0.84/C
Ru0.34Ni0.66/C
Ru0.56Ni0.44/C
Ru0.83Ni0.17/C
Ru-0.32 wt%, Ni-2.22 wt%
Ru-0.63 wt%, Ni-1.94 wt%
Ru-1.25 wt%, Ni-1.40 wt%
Ru-1.88 wt%, Ni-0.85 wt%
Ru-2.50 wt%, Ni-0.30 wt%
Table 2
-1
4
b
Entry Catalyst (0.05 g) t (h)
P (H ) (MPa) T (°C) TOF (h )х10 Yield
2
1
2
3
4
Ru0.08Ni0.92/C
Ru0.16Ni0.84/C
Ru0.34Ni0.66/C
Ru0.56Ni0.44/C
Ru0.83Ni0.17/C
1
5.3
5.3
5.3
5.3
5.3
60
60
60
60
60
0.72
1.50
1.82
2.42
0.75
3.99%
8.26%
100%
1
1
0.5
1
99.80%
41.30%
5
a
All of entries were conducted under the following reaction conditions: benzene (10 mL).
Yield to cyclohexane
b
2
2
Page 21 of 30

Table 3
-1
f
Ref
Catalyst
t (h) P (H ) (MPa) T (℃) TOF (h ) Yield
2
a
4
5
4
4
4
4
4
4
5
6
4
6
3
3
6
6
3
3
6
4
5
5
0
8
1
2
3
4
4
5
9
0
7
1
9
3
2
3
7
8
4
8
3
1
2mol%Perfluoro-tagged Ru [40]
0.5
2.5
0.1
6.0
60
29
34%
b
6
[H Ru (h -C H ) ][BF ] [58]
90
364
100%
99.8%
100%
100%
19.8%
61.3%
99.97%
100%
96%
4
4
6
6 4
4 2
4.2wt%Ru/C-silica [41]
3.3wt%Ru/MMT [42]
1.4%Ru(0)-Zeolite-Y [43]
4.6wt%Ru/MCF-C [44]
4.6%Ru/rGO [44]
Ru/CNTs [45]
0.53 8.0
110
110
22
37700
4000
1040
17000
38000
6983
85.3
2.5
1
8
0.28
4.0
4.0
4.0
1.0
6.0
1
130
130
80
1
0.5
-
Ru/SBA-15 [59]
20
Ru/MOF [60]
1.5
60
3200
5430
375
0.27%Ru/NFS [47]
Ir NPs [61]
2.25 0.29
25
100%
100%
100%
14%
c
c
0.8
8
4.0
0.3
0.83
0.1
1
20
Ir NPs@zeolite [39]
25
3190
10000
91
0.1wt%Pd/SiO [33]
2
140
20
2
Rh/HEA-C16 [62]
6.6
3
100%
80%
11.2wt%Rh/MWNTs [63]
20
1038
33300
33333
1241
592.6
290
d
d
e
Rh colloids@TiO [37]
0.01 3.0
0.01 3.0
RT
RT
40
100%
100%
52%
2
TiO -supported Rh NPs [38]
2
0.7wt%Rh(cod)-9.86wt%Pd/SiO [64]
2
3
2
6.3wt%Pd-6.9wt%Rh/CNT [48]
Rh0.5Ni0.5 [53]
24
7
1
20
100%
50.8%
4.0
25
3
1
20
60
100%
Pt-Rh/M W NTs[51]
1953
24167.4
This work Ru0.56Ni0.44/C
0.5
5.3
99.80%
a
b
c
d
2
-propanol as solvent. Ionic liquid and water as solvent. surfactant and water as solvent. water as solvent.
e
f
n-pentane as solvent. Yield to cyclohexane
2
3
Page 22 of 30

Table 4
Ru (3p )
3
/2
Samples
Ru(0)
462
RuO2
464.2
53.1
Ru0.56Ni0.44/C
BE (eV)
AR (%)
BE (eV)
AR (%)
46.9
462
Ru0.83Ni0.17/C
464.2
73.1
26.9
2
4
Page 23 of 30

Figure 1
Page 24 of 30

Figure 2
Page 25 of 30

Figure 3
Page 26 of 30

Figure 4
Page 27 of 30

Figure 5
Page 28 of 30

Figure 6
Page 29 of 30