Ruthenium-nickel-nickel hydroxide nanoparticles for room temperature catalytic hydrogenation

Article abstract of DOI:10.1039/c7ta01437f

Improving the utilization of metals in heterogeneous catalysts with excellent catalytic performance, high selectivity and good stability represents a major challenge. Herein a new strategy is disclosed by enabling a nanoscale synergy between a transition metal and a noble metal. A novel Ru/Ni/Ni(OH)₂/C catalyst, which is a hybrid of Ru nanoclusters anchored on Ni/Ni(OH)₂ nanoparticles (NPs), was designed, prepared and characterized. The Ru/Ni/Ni(OH)₂/C catalyst exhibited a remarkable catalytic activity for naphthalene hydrogenation in comparison with existing Ru/C, Ni/Ni(OH)₂/C and Ru-Ni alloy/C catalysts. This is mainly attributed to the interfacial Ru, Ni and Ni(OH)₂ sites of Ru/Ni/Ni(OH)₂/C, where hydrogen is adsorbed and activated on Ru while Ni transfers the activated hydrogen species (as a "bridge") to the activated naphthalene on Ni(OH)₂ sites, producing decalin through a highly effective pathway.

Full text of DOI:10.1039/c7ta01437f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Ruthenium-nickel-nickel hydroxide nanoparticles for room
temperature catalytic hydrogenation
Lihua Zhu,*ab Shiyao Shan,d Valeri Petkov,f Weiwei Hu,c Anna Kroner,e Jinbao Zheng,b Changlin Yu,a
Nuowei Zhang,b Yunhua Li,b Rafael Luque,*bg Chuan-Jian Zhong,d Hengqiang Ye,c Zhiqing Yang,*c and
Bing H Chen*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
It represents a major challenge to improve the utilization of metals in heterogeneous catalysts with excellent catalytic
performance, high selectivity and good stability. Herein a new strategy is disclosed by enabling a nanoscale synergy
between a transition metal and a noble metal. A novel Ru/Ni/Ni(OH)2/C catalyst with the hybrid of Ru nanoclusters
anchored on Ni/Ni(OH)2 nanoparticles (NPs) was designed, prepared and characterized. The Ru/Ni/Ni(OH)2/C catalyst
exhibited a remarkable catalytic activity in naphthalene hydrogenation in comparison with existing Ru/C, Ni/Ni(OH)2/C and
Ru-Ni alloy/C catalysts. This is mainly attributed to the interfacial Ru, Ni and Ni(OH)2 sites in Ru/Ni/Ni(OH)2/C, where
hydrogen is adsorbed and activated on Ru whereas Ni transfers the activated hydrogen species (as a “bridge”) to the
activated naphthalene on Ni(OH)2 sites, producing decalin in a highly-effective pathway.
removal.14 A number of heterogeneous catalysts employed in
catalytic hydrogenation include nonꢀnoble metal catalysts
(nickelꢀbased),15 precious metal catalyst (platinumꢀbased,16
1 Introduction
palladiumꢀbased,17
rhodiumꢀbased,18
rutheniumꢀbased,19
Aromatic compounds are important platform chemicals in
chemical industries, with extended products from the
hydrogenation of aromatic ring being relevant for a number of
applications.1ꢀ7 For example, tetralin, octahydronaphthalene and
decalin can be generated from naphthalene hydrogenation.1
Tetralin and decalin are mainly used as grease, resin and rubber
dissolving agents as well as varnish remover, additive in
lubricants, dyes, pesticides and pharmaceuticals.8–10 Despite
their extensive use in the petrochemical industry, the removal
of aromatics is of great practical significance due to their
inherent toxicity and carcinogenic effects.11–13 Catalytic
hydrogenation has been extensively employed for aromatics
bimetallicꢀbased (PtꢀPd20) and some novel types of catalystsꢀ
transition metal carbides, nitrides and phosphides.9 To the best
of our knowledge, there is no reported catalysts with superior
catalytic activity, high stability and low cost for arene
hydrogenation under mild reaction conditions (ca. room
temperature). An optimum way to design and develop lowꢀcost,
highly efficient and selective nanocatalysts for naphthalene
hydrogenation at room temperature is significant not only in
fundamental researches but also for potential industrial
applications.
Synergistic effect can significantly enhance the catalytic
activity, selectivity and stability of multiꢀmetallic catalysts.21–24
Taking catalytic oxidation reaction as an example, Yates et al.25
found that CO oxidation could be promoted efficiently using
Au/TiO2 with excellent catalytic performance attributed to the
synergetic effect of AuꢀTi interface and TiO2. CO and O2 were
more easily activated on TiO2 and AuꢀTi interfaces,
respectively. This synergistic effect of multiple active sites was
also present in various multiꢀmetallic nanocatalysts for
hydrogenation reactions such as tetralin hydrogenation over Ptꢀ
Ni/AC26 and benzene hydrogenation over Pd/C and [bmim]Clꢀ
AlCl3.27 Additionally, the hybrid nanomaterials have been
widely used in the field of catalysis, energy conversion and
storage, and etc. For example, Schaak’ group successfully
prepared AgꢀPtꢀFe3O4 heterotrimer isomer and further
investigated the formation mechanism of AgꢀPtꢀFe3O4
heterotrimer.28,29 In this study, the Ru/Ni/Ni(OH)2/C
nanocatalyst with the hybrid nanostructure of rutheniumꢀonꢀ
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nickelꢀonꢀnickel hydroxide NPs was designed and prepared wt% Ni loading. The calculated amount of an aqueous solution
with synergetic multiple catalytic sites (Scheme 1). This of RuCl3 (9.64×10ꢀ3 mol Lꢀ1) was added to the above asꢀ
nanostructure was proved by a variety of techniques including prepared solid sample (Ni/Ni(OH)2/C ꢀ 0.3000 g) under
highꢀresolution transmission electron microscope (HRTEM), vigorous stirring for 6 h. And then the black powder was
high angle annular dark fieldꢀscanning transmission electron collected by filtration, thoroughly washed with deionized water
microscopy (HAADFꢀSTEM), EDSꢀelemental mapping and and anhydrous ethanol. The fresh catalyst (Ru/Ni/Ni(OH)2/C)
lineꢀscans, highꢀsensitivity lowꢀenergy ion scattering (HSꢀ was obtained after dried in a vacuum oven at 60 °C for 6 h.
LEIS), synchrotron highꢀenergy XRD (HEꢀXRD) and Xꢀray Ni/NiO/C was obtained after Ni/Ni(OH)2/C was calcined in N2
absorption spectroscopy (XAS). The catalytic performance of of 80 mL minꢀ1 at 380 °C for 2 h, with the heating speed of 2 °C
Ru/Ni/Ni(OH)2/C was tested in the selective hydrogenation of minꢀ1. Ru/Ni/NiO/C with 7.5 wt% Ru content was formed by
naphthalene, and it exhibited high catalytic activity and ~100% galvanic replacement with the Ni/NiO/C sample ꢀ a certain
selectivity to decalin. These structural investigations provide amount of an aqueous solution of RuCl3 (9.64×10ꢀ3 mol Lꢀ1)
important information for understanding the origin of the was added to 0.3000 g of Ni/NiO/C. For comparison, 7.5
excellent performance of the Ru/Ni/Ni(OH)2 nanocatalyst.
wt%Ru/C was synthesized via incipient wetness impregnation
and then annealed in N2+10%H2 of 80 mL minꢀ1 at 380 °C for 3
h, with the heating rate of 2 °C minꢀ1. The chemical
compositions of the catalysts were analyzed by inductively
coupled plasma atomic mass spectrometry by using an Agilent
ICPꢀMS 4500ꢀ300.
The crystallite phase of Ni in the catalysts were analyzed by
powder Xꢀray diffraction (XRD) technique by using Rigaku Xꢀ
ray diffractometer equipped with highꢀspeed array detection
system. The Cu Kα radiation (40 kV and 30 mA) was used as
Scheme 1. Design of ruthenium nanoclustersꢀonꢀnickelꢀonꢀnickel the Xꢀray source. The scanning 2θ range is 10°~90°. The
hydroxide nanoparticles hybrid (Ru/Ni/Ni(OH)2/C).
chemical state and surface composition of Ru and Ni species
were measured by Xꢀray photoelectron spectra (XPS) on the
PHI Quantum 2000 Scanning ESCA Microprobe, with
monochromatic Al Kα radiation (1846.6 eV) as the Xꢀray
source. Transmission electron microscope (TEM) and highꢀ
resolution TEM (HRTEM) measurements for the samples were
performed on a Tecnai F30 electron microscope with an
accelerating voltage of 300 kV. HAADFꢀSTEM images, EDS
elemental mapping and lineꢀscan profiles results were obtained
by using Tecnai F30 electron microscope. The electron beam
size is below 2.0 nm, which was used for EDS elemental
mapping and lineꢀscan. The EDS spectrum was usually
acquired during above 10 s, while each step time of lineꢀscan
and twoꢀdimensional mapping is 1.0 s. The EDS spectra
provided the chemical composition of Ru and Ni in the catalyst,
while the EDS lineꢀscan profiles and elemental mapping were
used to analyze the variation of Ru or Ni element. Because the
RuꢀNi bimetallic nanoparticles (BNPs) were very small and the
EDS counts were relatively low, all the EDS elemental tests
results are qualitative, rather than quantitative. HSꢀLEIS tests
for the samples were conducted on IonTOF Qtac100 lowꢀ
energy ion scattering analyzer, with a 20Ne+ beam energy of 5
keV and a sample current of 1.6 nA.
2 Experimental
The catalyst supportꢀcarbon black (BLACK PEARLS 2000
LOTꢀ1366221) was purchased from Cabot Corporation. All
other reagents, including nickel(II) chloride hexahydrate
(NiCl2·6H2O), anhydrous ethanol, sodium hydroxide (NaOH),
anhydrous ruthenium chloride (RuCl3), 85 wt% (weight
percent) hydrazine hydrate solution, tetralin, decalin and
naphthalene were obtained from Sinopharm Chemical Reagent
Co. Ltd. (Shanghai, China). Water used in this study was
deionized water. All chemicals were used as received.
Ni/Ni(OH)2/C was prepared at room temperature (RT) via
hydrazine hydrate reduction method, with Ni (total Ni element)
loading of 12.3 wt%. The synthesis details were as follows: a
certain amount of nickel chloride hexahydrate (NiCl2·6H2O ꢀ
0.9000 g) was dissolved in the mixture of 82.5 mL of deionized
water and 12.5 mL of anhydrous ethanol (adjust the dielectric
constant of solution), magnetically stirred at RT for ten
minutes. And then NaOH (1.813 g) was dissolved in 12.5 mL
of deionized water, forming aqueous NaOH solution. Aqueous
NaOH solution was immediately added into the above solution.
Ten minutes later, the addition of 25 mL of 85 wt% hydrazine
hydrate solution was performed. Subsequently, 1.250 g of
carbon black was added in the above mixture after ten minutes,
and transferred into the autoclave (Parr 4848). The mixture was
magnetically stirred at RT for 18 hours. The solid sample was
collected and washed with deionized water and anhydrous
ethanol, then dried in a vacuum oven at 60 °C for 6 h. The
Ru/Ni/Ni(OH)2/C catalyst was obtained by galvanic
replacement with Ni/Ni(OH)2/C,30–37 with 7.5 wt% Ru and 6.0
HighꢀEnergy XRD (HEꢀXRD) measurements were carried
out at beamline11ꢀIDꢀB at the Advanced Photon Source,
Argonne National Laboratory, using Xꢀrays with an energy of
115 keV (wavelength, λ = 0.1080 Å). Samples were measured
at room temperature in ambient atmosphere. HEꢀXRD patterns
were reduced to the soꢀcalled atomic PDFs G(r). By their
nature, PDFs oscillate around zero and show positive peaks at
real space distances, r, where the local atomic density ρ(r)
exceeds the average one ρo. More precisely, G(r)
=
4πrρo[ρ(r)/ρo−1], where ρ(r) and ρo are the local and average
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atomic number density, respectively. Highꢀenergy XRD and in an iceꢀwater bath. Subsequently, hydrogen was released and
atomic PDFs have already proven to be very efficient in the reaction liquid was obtained by centrifugation. The products
studying the atomicꢀscale structure of nanosized materials.38
were analyzed by gas chromatography (Shimadzu GC 2010)
The XAS experiment was carried out on beamline B18 at equipped with a DBꢀ35 60 m×0.32 mm capillary column and a
Diamond Light Source. B18 is a bending magnet beamline flame ionization detector (FID), and gas chromatography~mass
which has been designed to deliver monochromatic Xꢀrays in spectrometry (Shimadzu GC~MS 2010). The turnover
the energy range of 2 to 35 keV. The beamline was set up on frequency (TOF) of the catalyst was calculated on the basis of
the Ptꢀcoated branch and with Si(311) monochromator. The the mols of total Ru metal. The measurement of metal
beam was defocused up to a dimension slightly lower than 1x1 dispersion was not conducted for most of the unsupported or
mm. All of the samples were measured as received without supported catalysts for the liquid naphthalene hydrogenation in
mixing with any diluting material. Prior to the measurement the the reported references (as shown in Table S1), and the Ni
samples were pressed into 12 mm diameter pellets.
element in the catalysts was mainly in the oxidized state.
The Ru XAS measurements were performed in transmission Therefore, with the purpose of comparing the results of this
mode using ionization chambers: I0 that measures intensity of work with the references, the metal dispersion (D) of the
the beam before the sample and It that records the intensity of catalysts was not investigated in this work.
the beam after Xꢀray penetration through the sample. The Ni Kꢀ
edge XAS measurements were performed in fluorescence mode
3 Results and discussion
using a high count rate fluorescence 36 element Germanium
detector. For each sample, a scan was performed to optimize
the position of the sample in the beam and then six XAS scans
were acquired. The acquisition time of a single XAS spectrum
was ca. 5 minutes. Ru Kꢀedge XAS spectra were collected in
the energy range of 21910~23100 eV which corresponds to
about ꢀ220~970 eV with respect to the edge. Ni Kꢀedge XAS
spectra were collected in the energy range of 8130~9335 eV
which corresponds to about ꢀ215~990 eV with respect to the
edge. The energy step in the whole XAS region was 0.5 eV.
The energy was calibrated at the Ru Kꢀedge and Ni Kꢀedge by
measuring metal foils of Ru and Ni along the XAS
measurement on the sample, respectively.
All of the data reduction and XANES analysis was carried
out by using ATHENA, a program for XAS data processing.39
During the data processing, two scans from the same sample
were merged and used for the analysis. The data processing was
done in a standard manner: the raw data were calibrated, the
bestꢀfit preꢀedge and postꢀedge background removed, summed
and normalized by setting the edge step to 1. The edge position
was determined from the derivative of the normalized data,
with the first maximum peak after the preꢀedge region in this
spectrum considered to be the main absorption energy.
Selectively catalytic hydrogenation of naphthalene
The catalytic activity of the catalysts was tested in the selective
hydrogenation of naphthalene. Table 1 summarizes the catalytic
properties of the synthesized catalytic systems. Ru/Ni/Ni(OH)2/C
efficiently catalyzed naphthalene hydrogenation (>99% conversion,
158.8 hꢀ1 TOF) with a high selectivity to decalin (>90%) at 100 °C
for 0.5 h (Table 1, entry 1). Naphthalene was totally hydrogenated to
decalin within 1 h (TOF = 80 hꢀ1) (Table 1, entry 2). As the reaction
temperature decreased to 80 °C, quantitative decalin yields could be
obtained within 2 h (entry 3 in Table 1). Ru/Ni/Ni(OH)2/C also
provided 100% selectivity to decalin complete naphthalene
conversion at 40 °C within 11 h (entry 4 in Table 1). An increased
weight of Ru/Ni/Ni(OH)2/C (0.3 g) could provide quantitative
decalin yield at RT after 13 h reaction (or at 15 °C under 1.45 MPa
hydrogen and within 75 h) (Table 1, entries 5,6). Importantly,
virtually no conversion in the systems was observed for RuꢀNi
alloy/C, Ni/Ni(OH)2/C and Ru/C catalysts forcing the reaction
conditions (4.48 MPa, 100 °C and 0.5 h, Table 1, entries 7 to 9).
Based on the catalytic performance of all asꢀsynthesized catalysts,
an unprecedented catalytic activity was observed for
Ru/Ni/Ni(OH)2/C, remarkably improved in comparison to Ru/C,
Ni/Ni(OH)2/C) and RuꢀNi alloy catalysts. The catalytic performance
of Ru/Ni/Ni(OH)2/C was further found to be unprecedented under
similar reaction conditions to that of most reported supported and
unsupported catalysts (Table S1). This includes excellent catalytic
activities for naphthalene hydrogenation at 15 °C, which to the best
of our knowledge is the lowest reaction temperature reported in
heterogeneous naphthalene hydrogenation. These results can be
attributed to the synergy effect of multiple catalytic active sites (Ru,
The program Artemis40 was used to model the oscillations in
the Extended Xꢀray Absorption Fine Structure (EXAFS)
spectra and to determine structural parameters for the Ru and
Ni coordination environment. The phase shifts corresponding to
the photoelectron wave associated with the central atom and
potential surrounding atoms were calculated by ab initio
methods, using a muffin tin approximation..
Typically, naphthalene hydrogenation reaction was
conducted in the autoclave (Parr 4848). Firstly, the mixture of Ni and Ni(OH)2). Detailed characterization of the designed catalyst
catalyst (0.050 g) and cyclohexane naphthalene solution (30 was able to prove the proposed Ru/Ni synergy.
mL) (wt% = 3.02%) was fed into the reactor. And then air in
the reactor was replaced with N2 and H2 for 1 min, sequentially.
Secondly, hydrogen was introduced into the reactor until the
pressure reaching 4.8 MPa at RT. The reactor was quickly
heated to desired reaction temperature. After the reaction kept
for indicated time with a mechanical agitation speed of 500
rpm, the reactor was stopped and quickly cooled to about 5 °C
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Table 1. Catalytic performance of the catalysts for naphthalene
hydrogenation.a
Yieldb Yieldc
T
t
TOF
(hꢀ1)
Entry
Catalyst (0.1 g)
(°C) (h)
1
2
Ru/Ni/Ni(OH)2/C
Ru/Ni/Ni(OH)2/C
Ru/Ni/Ni(OH)2/C
Ru/Ni/Ni(OH)2/C
Ru/Ni/Ni(OH)2/Cd
Ru/Ni/Ni(OH)2/Cd
RuꢀNi alloy/C
Ru/C
100 0.5 158.9 <10%
90%
>99%
>99%
>99%
>99%
>99%
ꢀ
100 1.0
80.0
40.0
7.3
2.1
0.4
ꢀ
ꢀ
3
80
40
25
15
2
ꢀ
4
11
13
75
ꢀ
5
6e
ꢀ
ꢀ
7
100 0.5
100 0.5
ꢀ
<15%
0
8
9e
ꢀ
ꢀ
Ru/Cd
15
75
ꢀ
ꢀ
a
Reaction conditions: cyclohexane naphthalene solution (30 mL,
3.02% weight percent naphthalene), initial H2 pressure (4.48 MPa).
Yields for Ni/Ni(OH)2/C and carbon were < 0.1%. bYield to tetralin.
cYield to decalin. dcatalyst (0.3 g). einitial H2 pressure (1.45 MPa)
Fig. 1. HRTEM images for a) Ni/Ni(OH)2/C b) Ru/C and c, d)
Ru/Ni/Ni(OH)2/C. Scale bar: 2 nm. Insets: fast Fourier transform
(FFT) patterns.
TEM and HRTEM tests of the nanocatalysts
In addition, HAADFꢀSTEM images and elemental analysis results
for Ru/Ni/Ni(OH)2/C are present in Fig. S5,6. As depicted,
Ru/Ni/Ni(OH)2 NPs feature isolated Ru and Niꢀcontaining NPs
(RuꢀonꢀNi).
Fig. S1 shows TEM image and NPs size distribution for different
samples, pointing out the presence of nicely dispersed nanoparticles
(NPs) in the materials. To investigate the nanostructure of NPs in the
catalysts, HRTEM experiments were subsequently conducted. As
shown in Fig. 1a, a lattice fringe with an interplanar spacing of 0.232
nm corresponded to (101) facets of Ni(OH)2. Lattice fringe with a
spacing of 0.235 nm ascribed to (100) facets of hexagonal close
packed (hcp) Ru can be clearly seen in HRTEM images for Ru/C
(Fig. 1b). HRTEM images for Ru/Ni/Ni(OH)2/C displayed lattice
fringes of 0.235 nm and 0.232 nm (Fig. 1c), assigned to (100) facets
of hcp Ru and Ni(OH)2(101) planes, respectively. Fig. 1d also
depicts one HRTEM image for another nanoparticle in
Ru/Ni/Ni(OH)2/C, the indicated lattice fringesꢀ0.206 and 0.232 nm
corresponding to Ru(101) and Ni(OH)2(101) planes. These results
demonstrate that Ni is mainly present in the form of Ni(OH)2 in
Ru/Ni/Ni(OH)2/C, fully consistent with XRD and XPS
characterization results of the catalysts (Fig. S2,3 and Table S2,3). It
also suggests that Ru particles could be in intimate contact with
Ni/Ni(OH)2 NPs (RuꢀonꢀNi/Ni(OH)2 hybrid) in Ru/Ni/Ni(OH)2/C.
Additionally, TEM and HRTEM tests for the Ru/Ni/Ni(OH)2/C
catalyst were also performed, and the images are shown in Fig. S4. It
could be clearly found that the size distribution and nanostructure of
the Ru/Ni/Ni(OH)2/C catalyst was without change, suggesting that
Ru/Ni/Ni(OH)2/C was relatively stable during the reaction at mild
conditions.
Synchrotron high energy X-ray diffraction (HE-XRD) date of
the nanocatalysts
To gain further insights into the atomic structure of
Ru/Ni/Ni(OH)2/C, synchrotron high energy Xꢀray diffraction
(HEꢀXRD) coupled to atomic pair distribution function (PDF)
analysis was conducted. A representative set of experimental (PDF)
data is shown in Fig. 2 and Fig. S7, with PDF data of Ru/Ni/NiO/C
also given for comparative purposes. Control samples such as Ru/C,
Ni/NiO/C and Ni/Ni(OH)2/C were also analyzed (see Fig. S8).
Analysis of the experimental PDFs revealed that Ru/Ni/Ni(OH)2/C is
a multiꢀphase mixture of metal oxides, metals and metal hydroxides
including 25%NiO, 34%(Ni
23%RuO2. Ru/Ni/NiO/C appeared as
+
Ru) metals, 18%Ni(OH)2 and
mixture of 72%NiO,
a
6%Ni(OH)2 and 20%(Ni + Ru) metals. Notably, the percentage of
RuO2 in the catalyst decreased dramatically to 2%, while the relative
amount of NiO increased. The reason is possibly related to a much
higher dispersion of Ru species on the surface of Ru/Ni/Ni(OH)2/C
as compared to that on the surface of Ru/Ni/NiO/C, resulting in Ru(0)
more easily oxidized in air. The surface RuO2 with high dispersion
could be however reduced to Ru(0) during the hydrogenation
reaction at relatively low temperature in the RuꢀNi bimetallic
catalyst. These results clearly confirmed the presence of
nanostructured Ru nanoclusters supported on Ni/Ni(OH)2 NPs in
Ru/Ni/Ni(OH)2/C.
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edge XAS (Xꢀray Absorption Spectroscopy) tests were carried out
for Ni/Ni(OH)2/C and Ru/Ni/Ni(OH)2/C, with XANES results
shown in Fig. 4a. Combining with the reference compoundsꢀNiO
and Ni(OH)2, these spectra suggest that the average oxidation state
of Ni atoms on Ni/Ni(OH)2/C is close to +2. Based on preliminary
visual analysis, the absorption edge for Ru/Ni/Ni(OH)2/C is shifted
towards higher energies, and an increased intensity in white line
(~8350 eV) in comparison with Ni/Ni(OH)2/C. These features could
indicate that Ni species in Ru/Ni/Ni(OH)2/C are also present at
higher oxidation state when compared to nonꢀRuꢀbased catalyst.
This could be explained by the fact that a certain amount of Ni(0)
atoms was replaced with Ru(0) atoms after Ru/Ni/Ni(OH)2/C was
obtained via galvanic replacement. Fig. 4b displays the Ru
Kꢀedge
XANES spectra for Ru/C and Ru/Ni/Ni(OH)2/C. The results
suggested that the Ru oxidation stateꢀRuO2 and Ru reduced stateꢀ
Ru(0) coꢀexist in these two samples after comparison to the Ru
Kꢀ
edge XANES spectra of reference compounds (RuO2 and Ru foil).
However, the average oxidation number for Ru atoms in
Ru/Ni/Ni(OH)2/C is clearly much larger to that of Ru/C. This can
further indicate that Ru atoms in Ru/Ni/Ni(OH)2/C are mainly
supported on the surface and easily oxidized in air due to higher
dispersion with respect to those on Ru/C.
Fig. 2. a) Atomic PDFs for a) Ru/Ni/Ni(OH)2/C and b)
Ru/Ni/NiO/C. Black curves: experimental PDFs; Red curves:
theoretical PDFs fitting the experimental PDFs.
Fig. 4c shows Fourier Transform of EXAFS (Extended Xꢀray
Absorption Fine Structure) spectra at Ni
Kꢀedge for Ni/Ni(OH)2/C
HS-LEIS analysis of the nanocatalysts
and Ru/Ni/Ni(OH)2/C along with Ru foil. Due to a relatively large
portion of surface atoms with unsaturated coordination, nanoꢀscaled
particles always possess a smaller number of neighboring atoms in
average so that Ni/Ni(OH)2/C and Ru/Ni/Ni(OH)2/C exhibited much
weaker EXAFS oscillations (and hence weaker FT peaks) with
respect to bulk structure of reference compounds. For Ni/Ni(OH)2/C
and Ru/Ni/Ni(OH)2/C, except for the first and second coordination
shell, the EXAFS contributions from higher shells are almost absent,
indicating a very high dispersion of Ni atoms. Most Ni atoms in
these two samples remained in the form of oxidized state. Ni2+/Ni(0)
atomic ratio in Ru/Ni/Ni(OH)2/C was found to be obviously larger
than that in Ni/Ni(OH)2/C, which is consistent with the normalized
To obtain the outmost surface atomic composition in the catalysts,
HSꢀLEIS tests were performed with results displayed in Fig. 3. The
signal of Ru atoms was detected on the outmost surface of Ru/C and
Ni atoms on Ni/Ni(OH)2/C in 5 keV 20Ne+ HSꢀLEIS spectra. For
Ru/Ni/Ni(OH)2/C, Ru and Ni atoms are present on the outmost
surface. Obviously, the amount of Ni atoms on the outer surface of
Ni/Ni(OH)2/C is much larger with respect to those measured for
Ru/Ni/Ni(OH)2/C. We speculate that the main reason is that some Ni
atoms have been fully covered by Ru nanoclusters in
Ru/Ni/Ni(OH)2/C.
XANES spectra at Ni Kꢀedge obtained for these two samples.
Fig. 3.
5
keV 20Ne+ HSꢀLEIS spectra of a) Ru/C, b)
Ru/Ni/Ni(OH)2/C and c) Ni/Ni(OH)2/C.
Fig. 4. Xꢀray absorption studies of the catalysts in ambient
condition. a) XANES and c) Fourierꢀtransformed EXAFS spectra for
Ni foil, Ni/Ni(OH)2/C and Ru/Ni/Ni(OH)2/C catalysts recorded at Ni
X-ray absorption spectroscopic results of the nanocatalysts
With the purpose of investigating the relationship between the
catalytic performance and the surface structure as well as the atomicꢀ
scale coordination structures of Ru and Niꢀbased catalysts, Ni
K
ꢀedge; b) XANES and d) Fourierꢀtransformed EXAFS spectra for
Kꢀ
Ru foil, Ru/C and Ru/Ni/Ni(OH)2/C catalysts recorded at Ru ꢀedge.
K
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Fourier transformation of k3ꢀweighted EXAFS oscillations at Ru as 1) naphthalene preferential adsorption and activation on
Kꢀedge for Ni/Ni(OH)2/C, Ru/Ni/Ni(OH)2/C and Ru foil are also accessible Ni(OH)2 sites. 2) Hydrogen is easily adsorbed and
shown in Fig. 4d. According to the strength of EXAFS oscillations, dissociates at Ru sites, forming activated H* species due to the
it can be found that the size of Ru particles in Ru/Ni/Ni(OH)2/C is effectiveness of noble metals in hydrogen activation. 3) Ni sites
much smaller than that in Ru/C and Ru dispersion in located between Ru and Ni(OH)2 play an vital role in transferring H*
Ru/Ni/Ni(OH)2/C is much better than that in Ru/C.
to Ni(OH)2 sites (as a “bridge”) by hydrogen spillover.41–43 4) the
activated naphthalene is hydrogenated by the activated H* species on
Ru sites, forming decalin.
The structural and statistical information derived from each
sample is listed in Table 2 (see detailed analysis in Supporting
Information). For Ni/Ni(OH)2/C, a distance of 2.34 Å and 3.08 Å
(shorter than NiꢀNi bonding distance in Ni foil) represented NiꢀNi
contributions with an average coordination number (CN) of 0.5 and
2.9, respectively. The presence of NiꢀO contributions was detected
with an average coordination number of 2.1. The average CN of Niꢀ
Ni/NiꢀO ratio in Ni/Ni(OH)2/C was larger than that in
Ru/Ni/Ni(OH)2/C, implying that Ni(0) nanoclusters were probably
loaded on Ni(OH)2 NPs in these two samples. For the Ru/C catalyst,
the average CN of RuꢀRu bonds with a distance of 2.68 Å and 3.78
Å was 5.3 and 2.4, respectively. A distance of 1.98 Å was attributed
to RuꢀO bonding with CNꢀ2.8. However, a much larger RuꢀO
coordination number (4.8) was observed for Ru/Ni/Ni(OH)2/C as
compared to Ru/C, suggesting that the major Ru species are present
in the oxidized form under ambient condition. Most importantly, the
observed distance of 2.54 Å could be assigned to RuꢀNi bonds with
CN of 0.8. These findings support the deposition of Ru nanoclusters
on the surface of Ni/Ni(OH)2 NPs (Ru nanoclustersꢀonꢀNi/Ni(OH)2
NPs) in Ru/Ni/Ni(OH)2/C, which is in good agreement with other
characterization results described in this work.
4 Conclusions
In conclusion, nanostructured Ru nanoclusters supported on the
surface of Ni/Ni(OH)2 NPs and loaded on carbon was designed and
prepared via hydrazine hydrate reduction and galvanic replacement.
An interface synergetic effect of Ru, Ni and Ni(OH)2 sites largely
improved the catalytic activity of Ru/Ni/Ni(OH)2/C for naphthalene
hydrogenation, comparably higher to that of Ru/C, Ni/Ni(OH)2/C
and RuꢀNi alloy/C. This study is a classic example to illustrate the
concept of designing a highly efficient nanocatalyst ꢀ noble metal
(NM) nanoclustersꢀonꢀtransition metal (TM)ꢀonꢀtransition metal
oxide (TMO) or transition metal hydroxide (TMOH) NPs hybrid for
hydrogenation reactions. The proposed approach to design multiꢀ
metallic nanocatalysts can take full advantage of metal atoms
(especially precious metal atoms) towards advancing the
development of multifunctional materials for catalytic applications.
Acknowledgements
Table 2. Best-fit EXAFS parameters of the Ru/Ni/Ni(OH)2/C,
Ni/Ni(OH)2/C and Ru/C samples
Catalyst
Ru/Ni/Ni(OH)2/C
Ru Kꢀedge
Scattering
path
Ru – O
r (Å)
CN
E
F (eV)
Rf (Å)
k range (Åꢀ1
)
R range (Å)
k: 3 – 11
2σ2/Å2
0.0075 (1)
0.0030 (2)
0.0046 (1)
0.0050
The authors would like to thank the Natural Science Foundation of
Jiangxi Province of China (Grant No. 20161BAB213083), Research
Foundation of Education Bureau of Jiangxi Province of China
(GJJ160666), National Natural Science Foundation of China (Grant
No. 21303140, 21466013, 51371178 and 51390473). Diamond Light
Source is gratefully acknowledged for a rapid access to the facility
for EXAFS measurement on B18.
2.00 (2)
2.54 (5)
2.56 (4)
3.10 (4)
4.8 (5)
0.8 (4)
1.5 (4)
0.8 (2)
ꢀ3.7
0.0057
Ru – Ni
Ru – Ru
Ru – Ru
R: 1.2 – 3.4
Ru/Ni/Ni(OH)2/C
Ni – O
Ni – Ni
2.04 (1)
2.99 (1)
3.9 (5)
4.8 (7)
0.0060 (1)
0.0120 (3)
ꢀ5.6
ꢀ1.4
0.0045
0.0046
k: 3 – 11
Ni Kꢀedge
R: 1 – 3.6
Ni/Ni(OH)2/C
Ni – O
Ni – Ni
Ni – Ni
2.04 (1)
2.34 (3)
3.08 (1)
2.1 (2)
0.0048 (1)
0.010 (2)
0.011 (1)
k: 3 – 12
Notes and references
Ni Kꢀedge
R: 1 – 3.0
0.5 (1)
2.9 (5)
1
2
3
4
5
6
7
8
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Table of Contents Entry
Ru nanoclusters-on Ni/Ni(OH)2 nanoparticles (Ru/Ni/Ni(OH)2/C) supported on
carbon were successfully prepared and characterized. Ru/Ni/Ni(OH)2/C exhibited an
unprecedented catalytic activity for naphthalene hydrogenation at room temperature
due to synergetic catalysis.