Synthesis of Ru/CoNi crystals with different morphologies for catalytic hydrogenation

Article abstract of DOI:10.1039/c7ce00702g

Cobalt-nickel alloy crystals with different morphologies, such as flower-like, column-like, mushroom-like and dendrite-like, were prepared by a facile hydrothermal or solvothermal reduction approach without the addition of any surfactant, using hydrazine

Full text of DOI:10.1039/c7ce00702g

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HIGHLIGHT
Tiddo J. Mooibroek, Antonio Frontera et al.
Towards design strategies for anion–π interactions in crystal engineering
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Synthesis of Ru/CoNi crystals with different morphologies for
catalytic hydrogenation
Received 00th January 20xx,
Accepted 00th January 20xx
*
ab
a
b
a
b
a
a
Lihua Zhu, Tuo Zheng, Jinbao Zheng, Changlin Yu, Nuowei Zhang, Qi Liao, Qing Shu and Bing
*b
H Chen
DOI: 10.1039/x0xx00000x
Cobalt-nickel alloy crystals with different morphologies, such as flower-like, column-like, mushroom-like and dendrite-like,
were prepared by a facile hydrothermal or solvothermal reduction approach without the addition of any surfactant, using
hydrazine hydrate as reducing agent, ethanediamine as capping agent and the mixture of nickel(II) chloride hexahydrate
www.rsc.org/
(
NiCl
2 2 2 2
·6H O) and cobalt(II) chloride hexahydrate (CoCl ·6H O) as precursor. The effect of hydrothermal temperature (120,
1
50 or 180 °C) and solvent (water or ethanol) on the shape of the CoNi crystal was investigated in this work. The
corresponding Ru/CoNi catalysts (Ru-on-CoNi nanocrystal) were obtained via galvanic replacement reaction. The size,
element chemical state, morphologies and structures of the CoNi and Ru/CoNi samples were characterized by X-ray
diffraction (XRD), scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron
spectroscopy (XPS), high-sensitivity low-energy ion scattering (HS-LEIS) techniques. The catalytic performance of the as-
synthesized catalysts was evaluated by using benzene hydrogenation reaction. And the Ru/CoNi catalyst with dendrite-like
morphology exhibited the highest catalytic hydrogenation activity among the Ru/CoNi catalysts with different shapes. It
was mainly due to its high Ru dispersion, many defect sites and positive synergistic effect among ruthenium, nickel and
cobalt related species. Importantly, the cost of recycling the Ru/CoNi catalysts was relatively low because they could be
recycled by magnetic separation.
or microꢀparticles. Currently, some approaches have been
applied to obtain CoNi nanomaterials with different
morphologies, such as liquid chemical reduction and
hydrothermal synthesis. The reported shapes of the CoNi
1
Introduction
Generally, multiꢀmetallic (such as bimetallic or triꢀmetallic)
nanomaterials take on more excellent properties (e.g. catalytic
performance) than the corresponding monoꢀmetallic
nanomaterials, and the multiꢀmetallic nanomaterials even show
unique behavior. Their properties are quite dependent on the
morphologies, size, composition and structures of the
nanomaterials. The size, morphology, element chemical state,
composition and nanostructure of the bimetallic nanocatalyst
play an important role in determining its selectivity, activity
and stability. Hence, the controlled synthesis of the multiꢀ
metallic nanocatalysts has attracted more and more interests
18
crystal mainly consist of CoNi alloy particles, Ni@Co coreꢀ
19
20,21
22
shell, CoNi nanowires,
CoNi nanofibers, roseꢀlike NiCo
23
24,25
nanoflowers, chainꢀlike CoNi crystal,
nanowires and hollow microspheres, CoNi nanorings, CoNi
nanoneedles,
handkerchiefꢀlike NiCo nanostructures, rodꢀlike, wheatꢀlike
and leafꢀlike CoNi structures. For instance, Li et al. prepared
CoNi nanourchins,
20
26
1
ꢀ5
27
13
28
CoNi films,
CoNi nanodumbbells,
29
6
ꢀ8
30
the leafꢀlike CoNi nanomaterial at 180 °C via wetꢀchemical
reduction method without adding any surfactant or template,
using sodium citrate as complexing agent. They also
investigated the effect of synthesis temperature on the
morphology of the products, and found that low temperature
6
ꢀ8
9
from chemists and material scientists.
Cobaltꢀnickel bimetallic nanomaterials are extensively
applied in the fields of memory storage, magnetic recording and
magnetic resonance imaging, biomedical, microwave absorbers,
microelectromechanical systems and catalysis.
significance to prepare highꢀquality cobaltꢀnickel nanomaterials
30
was favorable to form CoNi microspheres. CoNi nanourchins
and nanowires were obtained by using microwaveꢀassisted
20
polyolꢀsynthesis method in Shen’ group. And it was found
1
0ꢀ17
So it is of
that the catalytic activity and selectivity of the CoNi catalyst in
glycerol hydrogenolysis reaction were strongly affected by its
shape and size. Cobaltꢀnickel alloy flowerꢀlike superstructures
were synthesized at 150 °C via a simple hydrothermal reduction
3
1
method with the addition of potassium sodium tartrate.
However, to the best of our knowledge, the synthesis of
mushroomꢀlike and dendriteꢀlike CoNi crystal and the effect of
solvent on the shape of CoNi were hardly reported. In our
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study, focusing on the above two topics, the CoNi integral forming the ethanol mixture of NiCl
Journal Name
2
2
and CoCl solution. And
structure with different morphologies (such as flowerꢀlike, other experimental conditions were unchanged (synthesis
columnꢀlike, mushroomꢀlike and dendriteꢀlike) were acquired temperatureꢀ120 °C and reaction timeꢀ1 h). The obtained CoNi
by adjusting the synthesis temperature and solvent.
For aromatics hydrogenation reactions, the Niꢀbased, Ptꢀ
sample was signed as CoNiꢀ120ꢀethanol.
The Ru/CoNi triꢀmetallic catalysts (Ruthenium loaded on the
3
2
3
3
34,35
36
37
38
based, Ruꢀbased,
multiꢀmetallic nanocatalysts were widely used, and with reaction method.
excellent catalytic performance. But these catalysts were of g of the asꢀprepared CoNi crystal was dispersed in a certain
Pdꢀbased, Rhꢀbased, Irꢀbased and CoNi crystal) were obtained by the galvanic replacement
3
9
40ꢀ45
The main steps were described below. 0.1
−
3
−1
high recycling cost due to their nanoscale. In addition, much amount of RuCl
3
aqueous solution (9.64×10 mol L ),
heat was generated in arene hydrogenation reactions and ultrasoundꢀassisted synthesis at RT for 2 h. The Ru/CoNi
needed to be removed. To solve the above two problems, the catalysts were collected by filtration, repeatedly washed with
CoNi alloy crystal with excellent magnetic property, deionized water and anhydrous ethanol, and then dried in a
outstanding heatꢀconducting property and monolith were vacuum oven at 60 °C for 6 h. The Ru content in the catalysts
designed and prepared, and it was used as the catalyst support was analyzed by energy dispersive Xꢀray spectroscopy (EDS).
for depositing ruthenium. Herein, the CoNi alloy samples were The Ru loading is 1.10 wt% in the Ru/CoNi catalysts, very
synthesized by a simple hydrothermal or solvothermal method. consistent with the results obtained from inductively coupled
And the morphologies of CoNi were successfully controlled by plasma atomic mass spectrometry (ICPꢀMS) tests.
varying the synthesis temperature or solvent. A proposed
Characterizations for the samples: Powder Xꢀray diffraction
growth mechanism of various morphologies of CoNi alloy (XRD) patterns for the samples were recorded on a Rigaku
crystal was also provided. The Ru/CoNi catalysts were gained Ultima IV Xꢀray diffractometer equipped with highꢀspeed array
by galvanic replacement reaction. Benzene hydrogenation was detection system. Cu Ka radiation was used as the Xꢀray
used as a model reaction, in which the catalytic performance of source, operating at 40 kV and 30 mA. The 2θ scanning range
the Ru/CoNi catalysts with different shapes was evaluated. The was 10°~90°. The scanning electron microscope (SEM) and
reasons for the difference in their catalytic activity were also EDS analysis of the samples were conducted on a fieldꢀ
investigated.
emission scanning electron microscope (MLA650F). XPS
measurements for the catalysts were run on PHI Quantum 2000
Scanning ESCA Microprobe with monochromatic Al Ka
radiation (1486.6 eV). The highꢀsensitivity lowꢀenergy ion
scattering (HSꢀLEIS) tests for the catalysts were performed on
IonTOF Qtac100 lowꢀenergy ion scattering analyzer, to gain
2
Experimental
The chemicals with analytic grade were purchased from
Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). They
were used as received. Deionized water was prepared in our
group by using MilliꢀQ system.
4
+
the atomic composition of outer surface of the catalysts. He
ions with a kinetic energy of 3 keV were applied at a low ion
ꢀ2
20
+
flux equal to 1325 pA cm . Ne ions with a kinetic energy of
ꢀ2
5
keV were applied at a low ion flux of 1600 pA cm . The
The CoNi samples were prepared through the following
scattering angle was 145°.
steps. NiCl
dissolved in deionized water (30 mL), forming the aqueous
mixture of NiCl and CoCl solution. 5 mL of anhydrous
2 2 2 2
·6H O (0.6500 g) and CoCl ·6H O (0.6500 g) were
Catalytic measurements of the hydrogenation of benzene
were carried out in a stainless steel autoclave (Parr 4848).
Catalystꢀ0.0500 g and benzeneꢀ10 mL were mixed evenly, and
then taken into the reactor. The closed reactor was firstly
2
2
ethanol was added to the above solution under magnetic stirring
for 10 min, followed by the addition of anhydrous
ethanediamine (EDAꢀ5 mL). The aqueous NaOH solution (5
mL) (1.8130 g NaOH dissolved in 5 mL deionized water) was
added into the aboveꢀobtained solution with agitation for 10
min. Then the addition of reducing agent (85 wt% hydrazine
2
purged with hydrogen for 1 min to remove air, and then H was
continuously added into the reactor until the initial pressure
increased to 4.8 MPa. The reagents were quickly heated to the
desired reaction temperature (100 °C), and maintained for 1 h
under vigorous stirring (500 rpm). After the reaction was over,
the reaction system was stopped and quickly cooled down to
RT. Quantitative analysis of the products were conducted with
a gas chromatography (Shimadzu GC 2010) equipped with a
DBꢀ35 60 m×0.32 mm capillary column and a flame ionization
detector (FID). And the products were qualitatively analyzed by
using gas chromatography~mass spectrometry (Shimadzu
GC~MS 2010). The catalytic activity of the catalysts was
2 4 2
hydrateꢀN H ·H O) (5 mL) was performed. After stirring for 10
min, the mixture liquid was transferred into a 100 mL Teflonꢀ
lined stainlessꢀsteel autoclave and sealed. The hydrothermal
synthesis process was maintained at X °C for 1 h; where X =
120, 150 or 180 °C. After the reaction completed, the autoclave
was naturally cooled down to room temperature (RT). The solid
product was washed thoroughly with deionized water and
anhydrous ethanol, and then dried in vacuum at 60 °C for 6 h,
the asꢀobtained CoNi samples were denoted as CoNiꢀX, where
X represented the synthesis temperature (120, 150 or 180 °C).
With the purpose of investigating the effect of solvent on the
morphology of CoNi nanostructures, the mixture of
obtained by calculating the rate of benzene hydrogenation (
according to the following equation:
r)
r
= n conversion of benzene / (n total ruthenium×t)
where n conversion of benzene, t and n total ruthenium represent mols of
conversing benzene, reaction time (h) and total mols of
2 2 2 2
NiCl ·6H O (0.6500 g) and CoCl ·6H O (0.6500 g) were
ꢀ1
ꢀ1
ruthenium in the catalysts. The unit of r is mol molRu h .
dissolved in 30 mL of ethanol instead of deionized water,
2
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increased up to 180 °C, the CoNi crystal became mushroomꢀlike
with the diameter of ~1 µm. The surface of each mushroom is
circular, looking like the surface of a microꢀsphere. The reason is
that the growth rate of three different crystal planes is almost the
3
Results and discussion
3
.1 Influence of hydrothermal temperature on morphology
of CoNi
47
same at such high temperature.
XRD patterns for the asꢀsynthesized CoNi structures under different
hydrothermal temperatures (120 °C, 150 °C or 180 °C) are shown in
Fig. 1. The peaks at 2θ = 44.4°, 51.7° and 76.1° can be attributed to
CoNi(111), CoNi(200) and CoNi(220) planes of the faceꢀcentered
3
0
cubic (fcc) phase of the CoNi alloy. It indicates that the CoNi alloy
materials have formed. And the peak with quite low intensity located
at 2θ = 47.6° is ascribed to Co(101) plane of hexagonal close packed
(
hcp) cobalt (JCPDS card No. 05ꢀ0727). The standard peaks of fcc
cobalt are described below: Co(111)ꢀ44.2°, Co(200)ꢀ51.5° and
Co(220)ꢀ75.9° (JCPDS card No. 15ꢀ0806). The standard peaks of fcc
nickel are described below: Ni(111)ꢀ44.5°, Ni(200)ꢀ51.8° and
Co(220)ꢀ76.4° (JCPDS card No. 04ꢀ0850). Although ruthenium is
supported on the CoNi crystal, there is no any diffraction peak
corresponding to reduced or oxidized state ruthenium crystallite
phase. It suggests that ruthenium maybe in the form of single atoms
or nanoclusters is well dispersed. And XRD patterns of the CoNiꢀ
1
20ꢀethanol and the corresponding Ru/CoNiꢀ120ꢀethanol catalyst are
very similar to those of the CoNiꢀX samples and Ru/CoNiꢀX
catalysts (figure not given). Based on the XRD results, it can be
inferred that the CoNi alloy formed when the ratio of Ni to Co was
1
:1.
Fig. 1. (A) Powder XRD patterns for the CoNi crystals synthesized
at different temperatures: (a) 120 °C, (b) 150 °C and (c) 180 °C; (B)
Powder XRD patterns for the Ru/CoNi catalysts: (a) Ru/CoNiꢀ120,
(b) Ru/CoNiꢀ150 and (c) Ru/CoNiꢀ180.
Fig. 2 shows the SEM images of the CoNi materials synthesized
at different hydrothermal temperatures (Fig. 2aꢀd~120 °C, Fig. 2eꢀ
h~150 °C and Fig. 2iꢀl~180 °C). As shown in Fig. 2aꢀh, the flowerꢀ
like CoNi alloy structure with the size of 2~6 µm can be clearly seen.
And some flowerꢀlike CoNi crystals stick together. The flowerꢀlike
CoNi consists of a flower core and some petals. Most petals in the
CoNiꢀ120 sample look like flat leaves, but in the CoNiꢀ150 sample
they begin to grow columnꢀlike. The formation of this flowerꢀlike
CoNi probably goes through two steps: the first step is the producing
of the flower core, and the second step is the growing of petals. The
morphologies of the crystals depend on the nucleation and growth
rate to some extent. Our previous works reported that the Ni(111)
planes grow preferably, due to the high surface energy of different
46
planes in the order of (111) > (100) > (110). Herein, the growing
regularity of CoNi alloy is similar to that of monometallic Ni crystal.
So with the hydrothermal temperature increasing to 150 °C, the
Fig. 2. SEM images of the CoNi crystal prepared at hydrothermal
temperature of (aꢀd) 120 °C, (eꢀh) 150 °C and (iꢀl) 180 °C observed
petals would grow columnꢀlike. When the temperature successively under different magnifications.
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SEM images of the Ru/CoNiꢀ120 catalyst are given in Fig. 3.
Although ruthenium was loaded on the surface of the CoNiꢀ120
sample, its structure and morphology were without any change. It
suggests that the structure of flowerꢀlike CoNi is very stable.
Therefore, a conclusion can be drawn that the morphology of CoNi
crystal remains unchanged despite a small quantity of ruthenium
atoms on it.
Fig. 3. SEM images of the Ru/CoNiꢀ120 catalyst observed under
different magnifications.
SEMꢀEDS analysis results of CoNiꢀ120, CoNiꢀ150, CoNiꢀ180 and
Ru/CoNiꢀ120 samples are provided in Fig. 4. According to the EDS
spectrum, oxygen, nickel and cobalt are detected in the selected
black spot region in the image, including the contaminated carbon. It
demonstrates that one CoNi crystal unit consists of Ni and Co
element, implying the production of CoNi bimetallic particles.
Combined with the above XRD characterization results, it is certain
that the CoNi alloy structure has formed and the Ni/Co ratio is close
to the added theoretical Ni /Co ratioꢀ1:1. EDS spectra of the
Ru/CoNiꢀ120 catalyst proves that ruthenium has been successfully
supported on the CoNi crystal. And the Ru loading is 1.10 wt%, in
good agreement with ICPꢀMS analysis results.
Fig. 4. SEM images of the CoNi crystal prepared at hydrothermal
temperature of (a) 120 °C, (c) 150 °C, (e) 180 °C and (g) Ru/CoNiꢀ
120; EDS spectrums for the selected black spot region in (b) image
a. (d) image c, (f) image e and (h) image g.
2
+
2+
3
.2 Influence of solvent on morphology of CoNi
Fig. 5 shows the XRD patterns of the CoNi crystal prepared by using
ethanol as solvent, with other synthesis conditions identical with
those of preparing above CoNiꢀ120. It is revealed that the diffraction
peaks are also with the CoNi alloy characteristic, indicating the
successful preparation of CoNi alloy structure.
4
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According to the above analysis results, a growth mechanism of
different shapes of CoNi alloy crystal is proposed in Fig. 7. As
2+
2+
depicted, firstly, the [Ni(EDA)
2
]
and [Co(EDA)
2
]
complexes are
reduced by hydrazine hydrate, forming the CoNi alloy nanospheres.
Secondly, the produced CoNi alloy nanoparticles serve as a center,
which provide new nucleation sites favorable for continuous
nucleation and growth of cobaltꢀnickel nanocrystals with anisotropic
3
0,49,50
morphologies.
particles, the magnetic field strength at both ends of the pole magnet
And due to the magnetic property of CoNi
5
1
is larger than at other positions. And the CoNi nuclei forming
during later synthesis process tend to be attracted by the protruding
parts of the core via the magnetic dipole interactions and added to
the tip of protruding parts. As the growth proceeds, more and more
petals will be produced on the surface of the cores, finally forming
flowerꢀlike CoNi crystal. The probable growth mechanism given in
Fig. 5. Powder XRD patterns for (a) CoNi synthesized with ethanol
as solvent (CoNiꢀ120ꢀethanol) and (b) corresponding Ru/CoNi
catalyst (Ru/CoNiꢀ120ꢀethanol). Other synthesis conditions:
hydrothermal temperature, 120 °C; hydrothermal time, 1 h; EDA, 5
mL; hydrazine hydrate, 5 mL.
this investigation is similar to that of the flowerꢀlike nickel and
49,52ꢀ55
cobalt structure.
Therefore, the flowerꢀlike CoNi alloy
structure form at relatively low temperature of 120 °C by
hydrothermal approach. But with the hydrothermal temperature
increasing to 150 °C or 180 °C, the flowerꢀlike morphology is
transformed into the shape of columnꢀlike or mushroomꢀlike,
respectively. The reason is that high temperature increases the
nucleation and growth rates of crystal, resulting in more nucleuses
and growth in each orientation. It is beneficial to generate the CoNi
structures with smooth surface, such as columnꢀlike or mushroomꢀ
like CoNi (containing CoNi nanospheres). Interestingly, the
dendriteꢀlike CoNi alloy microstructure appears when the synthesis
process of CoNi is performed with ethanol as solvent. This is
because the dielectric constant of ethanol is distinct from that of
To investigate the effect of solvent on the shape of CoNi crystal,
the ethanol mixture of NiCl
solution substituted for the aqueous mixture of NiCl
CoCl ·6H O solution while other synthesis parameters remained
unchanged (hydrothermal time 1 h, hydrothermal temperature
20 °C, EDA, 5 mL, NaOH 1.8130 g, hydrazine hydrate 5 mL). The
2
(0.6500 g) and CoCl
2
(0.6500 g)
2
·6H O and
2
2
2
1
images of the asꢀobtained CoNiꢀ120ꢀethanol sample are shown in
Fig. 6. All of the CoNi crystals are with uniform dendriteꢀlike shape
in micrometer scale. Each dendrite is composed of many small subꢀ
branches in nanometer scale, trunks and branches, which possess
much more defect sites. It is probably beneficial for the
improvement of its catalytic performance. The large difference in the
morphology between the CoNiꢀ120 and CoNiꢀ120ꢀethanol samples
may be attributable to different solvents, because the dielectric
constant is different for various solvents, resulting in distinct surface
2
+
2+
water, resulting in different solubility of Ni and Co ions in the
solvent, as well as the rates of nucleation and growth of CoNi.
4
7,48
energy of newly generated cobaltꢀnickel.
Based on the above
results, a conclusion that the morphology of CoNi remained after a
small amount of Ru supported on it has been obtained, so the shape
of Ru/CoNiꢀ120ꢀethanol is identical to CoNiꢀ120ꢀethanol.
Fig. 7. Schematic illustration of the possible formation mechanism
for the flowerꢀlike and dendriteꢀlike CoNi alloy microstructure.
3
.3 XPS characterization for the Ru/CoNi catalysts
With the purpose of obtaining the chemical state and general element
composition on the surface of the catalysts, XPS analysis was
performed on the typical catalysts (Ru/CoNiꢀ120ꢀethanol and
Ru/CoNiꢀ150) and the results are shown in Fig. 8. In the Ni 2p XPS
spectra of these two catalysts (Fig. 8A,B), the four doublets peaks
located at the binding energies of 852.7 eV and 870 eV, 853.9 eV
and 871.2 eV, 855.4 eV and 872.9 eV, 857.1 eV and 874.4 eV, can
be ascribed to related Ni species in Ni(0), NiO, Ni(OH) and NiOOH,
2
respectively. The other two peaks with the binding energies of 861.6
56,57
eV and 879.7 eV may be assigned to shakeꢀup satellite.
By
calculating the peak area corresponding to different species, it can be
obviously found that the oxidation state nickel (such as nickel oxide,
nickel hydroxide and hydroxy nickel oxide) is predominant. The
abundant existence of oxidized state nickel is mainly due to Ni(0)
Fig. 6. SEM images of the CoNi crystal (CoNiꢀ120ꢀethanol) atoms on the surface easily oxidized in air and moisture environment
3
2
prior to XPS tests. In the Co 2p XPS spectra (Fig. 8C,D), the peaks
centered at the binding energies of 778.2 eV (793.4 eV), 780.2 eV
(
prepared by using ethanol as solvent observed under different
magnifications.
795.4 eV) and 781.1 eV (796.3 eV) correspond to Co(0), CoO and
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58,59
Co(OH) species, also with two ‘shakeꢀup’ peaks.
The surface cobalt and ruthenium coꢀexist on the outer surface of the catalysts.
2
composition of oxidized state cobalt is much higher than Co(0). By Another obvious characteristic is that the ratio of Ru atoms signal
comparing the peak area corresponding to each specie in the Ni 2p intensity/Ni(Co) atoms signal intensity in Ru/CoNiꢀ120ꢀethanol is
XPS spectra of these two catalysts, it can be deduced that Ni(0) higher than that in Ru/CoNiꢀ150. And we can come to the same
20
+
atoms on the surface of Ru/CoNiꢀ120ꢀethanol are more easily conclusion from the 5 keV Ne HSꢀLEIS spectra for these two
oxidized than those on the surface of Ru/CoNiꢀ150. Similar catalysts (Fig. 9B). Therefore, on the basis of the same Ru loading in
conclusion can be made based on the Co 2p XPS spectra. This is the Ru/CoNi catalysts, the HSꢀLEIS results indicate that Ru atoms
probably because nickel and cobalt atoms on the edges of the are supported on CoNi with better dispersion in Ru/CoNiꢀ120ꢀ
branches of dendriteꢀlike CoNi, due to their lower coordination ethanol than Ru/CoNiꢀ150.
number, are more active than those on the surface of columnꢀlike
CoNi.
The XPS signals in Fig. 8E,F are the Ru 3p XPS spectra of the
Ru/CoNiꢀ120ꢀethanol and Ru/CoNiꢀ150 catalysts. The peaks at the
binding energy values of 462 eV and 484 eV are attributed to Ru(0),
4
+
while 464.2 eV and 486.2 eV are related to Ru in the form of
60,61
4+
RuO
2
.
It can be obviously found that the ratio of surface Ru to
Ru(0) of the Ru/CoNiꢀ120ꢀethanol catalyst is slightly higher than
that of the Ru/CoNiꢀ150 catalyst, maybe owing to higher Ru
dispersion on CoNi for the Ru/CoNiꢀ120ꢀethanol catalyst, which is
more easily oxidized in air.
4
+
20
+
Fig. 9. (A) 3 keV He and (B) 5 keV Ne HSꢀLEIS spectra for (a)
Ru/CoNiꢀ120ꢀethanol and (b) Ru/CoNiꢀ150.
3
.5 Catalytic performance of the catalysts for benzene
hydrogenation
The Ru/CoNi and CoNiꢀ120ꢀethanol samples described above were
applied in benzene hydrogenation. And the reaction results are listed
in Table 1. Benzene was hardly converted to any product within 1
hour reaction time at 100 °C under the hydrogen pressure of 5.3 MPa
(
(
Table 1, Entry 1). For the Ru/CoNiꢀ120 catalyst (flowerꢀlike)
Table 1, Entry 2), benzene conversion was 65% and the selectivity
ꢀ
1
ꢀ1
to cyclohexane was 100%, with a r of 13474.1 mol molRu h . The
yield to cyclohexane and r decreased when the hydrothermal
synthesis temperature of CoNi increased to 150 °C (columnꢀlike)
and 180 °C (mushroomꢀlike) (Ru/CoNiꢀ150~yield = 22% and r =
ꢀ1
ꢀ1
4
560.4 mol molRu h , Ru/CoNiꢀ180~yield = 55% and r = 11400.8
ꢀ1 ꢀ1
mol molRu h ). The Ru/CoNiꢀ120ꢀethanol catalyst (dendriteꢀlike)
gave the highest product yield~81% within the reaction time of 1 h.
With the reaction time increasing up to 1.5 h, benzene was totally
converted to cyclohexane. The most excellent catalytic activity of
the Ru/CoNiꢀ120ꢀethanol catalyst is mainly due to the high Ru
dispersion and the synergistic effects of Ru, Ni and Co related
species in the catalyst. And the accurate mechanism of this synergy
effect will be illustrated in details later in our group.
Table 1. Catalytic performance of different catalysts for benzene
a
hydrogenation.
Fig. 8. Ni 2p XPS spectra of (A) Ru/CoNiꢀ120ꢀethanol and (B)
Ru/CoNiꢀ150; Co 2p XPS spectra of (C) Ru/CoNiꢀ120ꢀethanol and
b
ꢀ1 ꢀ1
Entry Catalyst (0.05 g)
Yield
ꢀ
r (mol molRu h )
(
D) Ru/CoNiꢀ150; and Ru 3p XPS spectra of (E) Ru/CoNiꢀ120ꢀ
1
2
3
4
5
6
CoNiꢀ120ꢀethanol
Ru/CoNiꢀ120
Ru/CoNiꢀ150
Ru/CoNiꢀ180
ꢀ
ethanol and (F) Ru/CoNiꢀ150.
65%
22%
55%
13474.1
4560.4
11400.8
16791.0
3
.4 HS-LEIS results of the Ru/CoNi catalysts
The highꢀsensitivity lowꢀenergy ion scattering (HSꢀLEIS) technique
has been considered as a powerful method for providing the atomic
Ru/CoNiꢀ120ꢀethanol 81%
62
c
composition of the outmost surface. HSꢀLEIS measurements for
the catalysts were carried out to further confirm their different Ru
Ru/CoNiꢀ120ꢀethanol 100% 13819.8
a
Reaction conditions: benzene (10 mL), reaction time (1 h),
reaction temperature (100 °C), reaction pressure (5.3 MPa) ; Yield
4
+
dispersion and to obtain the atomic composition. The 3 keV He
b
HSꢀLEIS spectra for the Ru/CoNiꢀ120ꢀethanol and Ru/CoNiꢀ150
samples is displayed in Fig. 9A, which reveals that oxygen, nickel,
c
to cyclohexane; reaction time (1.5 h).
6
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Additionally, the Ru/CoNiꢀ120ꢀethanol catalyst exhibited jxxjbs15008), and National Natural Science Foundation of China
relatively high catalytic activity compared to most of the reported (Grant No. 21466013).
32ꢀ34,44,61,63ꢀ66
catalysts under similar reaction conditions.
For example,
it showed much better catalytic performance than the monolithic
Ni@Pd catalyst for benzene hydrogenation under milder reaction
Notes and references
4
4
conditions. And the stability of the Ru/CoNiꢀ120ꢀethanol catalyst
in benzene hydrogenation reaction is shown in Fig. 10A. The results
demonstrate that there is no obvious deactivation of the catalyst after
recycled for five times. SEM measurement for the recycled
Ru/CoNiꢀ120ꢀethanol catalyst was also performed, and SEM image
is displayed in Fig. 10B. The dendriteꢀlike crystals can be clearly
seen. The morphology of the catalyst was without change before and
after catalytic reaction. Therefore, we can infer that the Ru/CoNi
catalysts are relatively stable during reaction.
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