Liberating N-CNTs Confined Highly Dispersed Co?Nx Sites for Selective Hydrogenation of Quinolines

Article abstract of DOI:10.1002/adma.201906051

Selective hydrogenation of quinoline and its derivatives is an important means to produce corresponding 1,2,3,4-tetrahydroquinolines for a wide spectrum of applications. A facile and efficient “laser irradiation in liquid” technique to liberate the inaccessible highly dispersed Co?N_x active sites confined inside N-doped carbon nanotubes is demonstrated. The liberated Co?N_x sites possess generic catalytic activities toward selective hydrogenation of quinoline and its hydroxyl, methyl, and halogen substituted derivatives into corresponding 1,2,3,4-tetrahydroquinolines with almost 100% conversion efficiency and selectivity. This laser irradiation treatment approach should be widely applicable to unlock the catalytic powers of inaccessible catalytic active sites confined by other materials.

Full text of DOI:10.1002/adma.201906051

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
Liberating N-CNTs Confined Highly Dispersed Co Nx Sites
for Selective Hydrogenation of Quinolines
Wanbing Gong, Qinglin Yuan, Chun Chen, Yang Lv, Yue Lin,* Changhao Liang,
Guozhong Wang, Haimin Zhang,* and Huijun Zhao*
important for the future of pharmaceutical
Selective hydrogenation of quinoline and its derivatives is an important
means to produce corresponding 1,2,3,4-tetrahydroquinolines for a wide
spectrum of applications. A facile and efficient “laser irradiation in liquid”
and agrochemical synthetic industries.
Recently, the N-doped carbon sup-
ported transition metal (e.g., Fe, Co, and
Ni) based catalysts have demonstrated to
be a new class of efficient heterogeneous
catalysts for SH of quinoline and its
derivatives.[1a,c,5,6] Beller’s group success-
fully demonstrated for the first time that
the N-doped graphene supported cobalt
oxide/cobalt nanoparticles (NPs) possess
high activity and selectivity toward hydro-
genation of quinolines.[1a] The same group
recently reported the use of an easily syn-
thesizable N-doped carbon supported Co
NPs catalyst to achieve efficient SH of

technique to liberate the inaccessible highly dispersed Co Nx active sites
confined inside N-doped carbon nanotubes is demonstrated. The liberated

Co Nx sites possess generic catalytic activities toward selective hydrogena-
tion of quinoline and its hydroxyl, methyl, and halogen substituted derivatives
into corresponding 1,2,3,4-tetrahydroquinolines with almost 100% conversion
efficiency and selectivity. This laser irradiation treatment approach should be
widely applicable to unlock the catalytic powers of inaccessible catalytic active
sites confined by other materials.
The selective hydrogenation (SH) of quinoline and its deriva-
tives is an important means to produce corresponding
1,2,3,4-tetrahydroquinolines for pharmaceuticals, agrochemi-
cals, and other applications.[1] Most of the high-performance
heterogeneous catalysts for hydrogenation of quinolines are
precious metal-based catalysts (e.g., Pd,[2] Pt,[3] Ru,[1b] and
Rh[4]); however, their selectivities are insufficient. Addition-
ally, the scarcity and high cost of such precious metal catalysts
limit their practical applications.[5] The development of cheap
and earth abundant nonprecious metal-based catalysts to
realize efficient SH of quinoline and its derivatives is critically
quinolines.[6a] The N-doped graphene encapsulated Co[1c] and
NPs based catalysts were also successfully applied
for SH of quinolines.
[6b]
CoOx
The highly dispersed catalysts such as single-atoms (SAs) and
atom-clusters (ACs) catalysts have emerged as a new form of
catalysts with enormous potentials.[7] In particular, the N-doped
carbon supported highly dispersed transition metal (e.g., Fe, Co,
and Ni) catalysts have shown superior electrocatalytic activities
toward a wide range of reactions, including the hydrogenation
of CO2 and N2 into hydrocarbons and ammonia.[8] However,
their potential as thermal catalysts for organic synthesis have
not yet been well explored.[9] To date, only one published litera-
ture reports the use of highly dispersed single-atom catalyst for
transfer hydrogenation of quinolines.[10]
Dr. W. Gong, Dr. Q. Yuan, Dr. C. Chen, Y. Lv, Prof. C. Liang,
Prof. G. Wang, Prof. H. Zhang, Prof. H. Zhao
Key Laboratory of Materials Physics
Centre for Environmental and Energy Nanomaterials
Anhui Key Laboratory of Nanomaterials and Nanotechnology
CAS Center for Excellence in Nanoscience
Institute of Solid State Physics
Chinese Academy of Sciences
Hefei 230031, P. R. China
E-mail: zhanghm@issp.ac.cn; h.zhao@griffith.edu.au

We recently reported the use of Co NPs with Co Nx active
sites confined on the tips of the ZIF-67 derived N-doped carbon
nanotubes (N-CNTs) as a highly active and stable catalyst for
efficient SH of various biomass-derived organic compounds.[11]
Our subsequent studies revealed the presence of rich highly
dispersed Co SAs/ACs sites confined inside these N-CNTs. In
an attempt to investigate the catalytic properties of such con-
fined Co SAs/ACs sites, we removed the Co NPs on the tips of
N-CNTs by acid leaching. Although the confined Co SAs/ACs
sites inside N-CNTs are well retained, the hydrogenation perfor-
mance of the acid-leached sample was found to be dramatically
decreased. We speculated at the time that the poor catalytic
performance of such confined Co SAs/ACs sites could be due
to that only few exposed Co SAs/ACs sites are available for
reaction, while the vast majority of the confined Co SAs/ACs
sites inside N-CNTs are inaccessible to the reactant. This work
reports an effective approach to unlock the catalytic powers
of the confined Co SAs/ACs sites. For the first time, a laser
Dr. Y. Lin
Hefei National Laboratory for Physical Sciences at the Microscale
University of Science and Technology of China
Hefei 230026, P. R. China
E-mail: linyue@ustc.edu.cn
Prof. H. Zhao
Centre for Clean Environment and Energy
Griffith University
Gold Coast Campus, Queensland, Southport 4222, Australia
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201906051.
DOI: 10.1002/adma.201906051
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Figure 1. a) Typical SEM images (inset: high magnification image), b) TEM image, c) STEM-EDX mapping, and d) AC HAADF-STEM image of the
Co-SA/AC@N-CNTs. The Co clusters and single atoms are marked with yellow and red circles, respectively.
irradiation in liquid (LIL) technique was employed to liberate
the N-CNTs confined Co SAs/ACs sites for efficient SH of qui-
noline and its derivatives with hydroxyl, methyl, and halogen
groups into corresponding 1,2,3,4-tetrahydroquinolines. The
reported approach would be widely applicable to liberate other
carbon materials confined catalytic active sites for catalysis
applications.
The N-CNTs with confined Co NPs (Co@N-CNTs; Figure S1,
Supporting Information) were synthesized according to our
previously reported method.[11] An acid-leaching approach[12] was
employed to remove the Co NPs confined on the tips of N-CNTs.
The field emission scanning electron microscopy (FESEM)
and the transmission electron microscopy (TEM) images of
the acid-leached sample (denoted as Co-SA/AC@N-CNTs)
show the well-retained original dodecahedral shape while
confirm the removal of the Co NPs on the tips of N-CNTs
(Figure 1a,b). The elemental mapping from high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) images (Figure 1c) displays homogeneously
distributed Co, N, and C. The aberration-corrected HAADF-
STEM (AC HAADF-STEM) image (Figure 1d) confirms the
presence of highly dispersed Co SAs (red circle spots) and ACs
(yellow circle spots) in the acid-leached N-CNTs. The induc-
tively coupled plasma (ICP) analysis reveals the presence of
1.6 wt% Co in Co-SA/AC@N-CNTs (Table S1, Supporting
Information). However, Co-SA/AC@N-CNTs possesses a very
poor catalytic activity toward SH of quinoline (entry 3, Table 1)
due to the difficulties for the reactant in accessing the highly
dispersed Co SAs/ACs active sites confined inside N-CNTs. An
initial attempt to expose highly dispersed Co SAs/ACs sites
inside N-CNTs via ultrasonic and plasma treatments failed
because of the insufficient power to break the CNT structures
(Figure S2, Supporting Information).
An LIL technique[13] was then employed to break CNT struc-
tures, which could liberate the confined Co SAs/ACs sites
(Figure S3, Supporting Information). The resultant sample
is denoted as Co-SA/AC@N-CNTs-L. Figure 2a confirms that
LIL treatment is capable of not only completely destroying
the dodecahedral architecture but also breaking N-CNTs
structures. The HAADF-STEM image of Co-SA/AC@N-CNTs-L
(Figure 2b) shows that the LIL-treated N-CNTs consist of a
large number of broken opening structures. If highly dis-
persed Co SAs/ACs sites still remain, these opening structures
will make them readily accessible by the reactants. The
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average graphene layers.[15] This is reasonable
as the CNTs in Co-SA/AC@N-CNTs-L
are multiwall CNTs and the reduced gra-
phene layers can occur at the LIL treatment
induced opening structural sites. X-ray photo-
electron spectroscopy (XPS) was used to
identify the chemical states of the embedded
highly dispersed Co sites. Although the
observed Co peak intensity is low (due to the
Table 1. SH of quinoline catalyzed by different catalysts.
Entrya)
Catalyst
ZIF-67
Co/substrateb)
0.045
Conv. [%]
–
Sel. [%]
–
TOFc) [h−1
]
1
2
3
4
5
–
Co@N-CNTs
0.072
91.9
10.5
97.4
17.2
99.0
99.4
99.1
99.3
4.2
Co-SA/AC@N-CNTs
Co-SA/AC@N-CNTs-L
Co-SA/AC@N-CNTs-L-SCN−
0.0027
12.9
0.0025
127.7
22.6
0.0025

low Co content), the Co N is still observable
a)Catalyst: 5 mg; quinoline: 0.5 mmol; ethanol: 5 mL; reaction temperature: 100 °C; reaction time: 3 h; and
H2 pressure: 2 MPa; b)The molar ratio of Co/substrate; c)TOF = molconverted quinoline/(molCo ∙h).
(Figure S5, Supporting Information).[11,16]

The increased Co N peak intensity after
LIL treatment suggests an increased

AC HAADF-STEM image confirms the well-preserved highly
dispersed Co SAs/ACs sites in Co-S@N-CNTs-L. Importantly,
Co-SA/AC@N-CNTs-L retains 1.5 wt% of the Co content, sug-
gesting that the LIL treatment only reduces less than 1% of
the Co content in the original Co-SA/AC@N-CNTs (1.6 wt%;
Table S1, Supporting Information). No obvious characteristic
crystalline Co peaks observed from the X-ray diffraction (XRD)
patterns (Figure S4a, Supporting Information) further suggests
the retention of the highly dispersed forms of Co species in
Co-SA/AC@N-CNTs-L. The Raman spectra (Figure S4b, Sup-
porting Information) reveal the characteristic D, G, and 2D
bands, respectively. An essentially unchanged D and G bands
intensity ratio (ID/IG) before and after LIL treatment suggests
that the sp2 carbon structures remain largely intact, despite
the presence of LIL treatment induced opening structures.[14]
Nevertheless, the observed increase in the 2D and G bands
intensity ratio (I2D/IG) after LIL treatment implies a decrease in
Co N contents exposed to the surface. Fourier transform
infrared spectroscopy confirms that the chemical functional
groups remain unchanged before and after LIL treatment
(Figure S6, Supporting Information). The synchrotron X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) were conducted to investi-
gate the local chemical states and coordination environments of
the highly dispersed Co SAs/ACs sites. The Co K-edge XANES
spectra of Co-SA/AC@N-CNTs-L, Co-SA/AC@N-CNTs, and
various referencing samples of Co foil, CoO, Co2O3, and Co3O4
(Figure 2c) suggest that the Co absorption edge of Co-SA/
AC@N-CNTs-L is located between the metallic Co and CoOx,
due possibly to the formation of Co–Nx. The Fourier-transform
EXAFS spectrum observed from Co-SA/AC@N-CNTs-L
reveals a metallic Co–Co bonding peak at 2.08 Å, indicating
the presence of ACs (Figure 2d). Importantly, a peak at

1.40 Å corresponding to Co N bonding can be clearly observed
Figure 2. a) Typical SEM image (inset: TEM image), b) AC HAADF-STEM image (inset: HAADF-STEM image) of the Co-SA/AC@N-CNTs-L;
c) Co K-edge XANES spectra, and d) comparative k3-weighted x(k)-function of EXAFS spectra of Co foil, CoO, Co2O3, Co3O4, Co-SA/AC@N-CNTs, and
Co-SA/AC@N-CNTs-L. The Co clusters and single atoms are marked with yellow and red circles, respectively.
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from both Co-SA/AC@N-CNTs-L and Co-SA/AC@N-CNTs.
Compared to the Co-SA/AC@N-CNTs, an obviously decreased
efficiency and selectivity under 100 °C and 2 MPa H2. An
increased reaction time leads to an increase in quinoline con-
version and 97.4% and 100% conversions can be, respectively,
achieved within 3 and 4 h. Again, the selectivity is found to be
independent of reaction time. The catalyst stability is a critical
performance indicator and was therefore investigated. No
noticeable decay of the conversion efficiency and selectivity can
be observed from a reusability test that involves five consecutive
runs (Figure 3d). No detectable Co in the reaction mixture can
be found after five cycle runs, suggesting the leached Co in the
reaction mixture is below the ICP detection limit. It can be attrib-
uted to the superior structural stability of Co-SA/AC@N-CNTs-L
as confirmed by TEM and AC HAADF-STEM images (Figure S8,
Supporting Information), XRD patterns (Figure S9, Supporting
Information), and XANES and EXAFS spectra (Figure S10, Sup-
porting Information) of the catalyst after five consecutive runs,
where no obvious damage of carbon structures and aggregation
of Co SAs/ACs sites are observable; more importantly, the cata-

metallic Co Co bonding peak intensity accompanying an

increased Co N bonding peak intensity for Co-SA/AC@N-CNTs
can be observed. Considering the Co contents in both samples
are almost the same (1.5 wt% and 1.6 wt%), the observed peak
intensity changes suggest that some of the ACs are converted
into SAs during LIL treatment. As expected, the LIL treatment
increases the specific surface area from 371 to 403 m2 g−1 due
to the formation of opening structures on CNTs. Accordingly,
the total pore volume increases from 0.956 to 1.249 cm3 g−1,
which favors the adsorption and the hydrogenation of macro-
molecular compounds (Figure S7 and Table S1, Supporting
Information).
The SH of quinoline into 1,2,3,4-tetrahydroquinoline using
Co-SA/AC@N-CNTs-L was first conducted. The effects of reac-
tion conditions including reaction temperature, H2 pressure,
and reaction time were investigated. The influence of reaction
temperature on SH of quinoline was carried out under a con-
stant H2 pressure of 2 MPa and 3 h reaction time (Figure 3a).
The results indicate that the quinoline conversion is increased
from 12.3% to 76.8% and 97.4% with the reaction temperature
increased from 70 to 90 and 100 °C, and further increased to
100% when the reaction temperature is increased to 110 °C.
In contrast, the selectivity toward 1,2,3,4-tetrahydroquinoline
(≈99%) is found to be reaction temperature independent. As
can be seen from Figure 3b, with 3 h reaction time and 100 °C
reaction temperature, 97.4% and 100% quinoline conversions
can be, respectively, achieved with 2 and 3 MPa H2 pressures.
Figure 3c shows the effect of reaction time on the conversion

lytically active Co Nx sites are well preserved.
The SH of quinoline using Co-SA/AC@N-CNTs-L,
Co-SA/AC@N-CNTs, and various referencing materials as
the catalysts was also conducted to confirm the effectiveness
of LIL treatment approach and the catalytic activity origin.
Co-SA/AC@N-CNTs-L exhibits the best catalytic performance
among all catalysts investigated (Table 1). The conversion
rate and turnover frequency (TOF) of Co-SA/AC@N-CNTs-L
are remarkably increased to 97.4% and 127.7 h−1 from 10.5%
and 12.9 h−1 for Co-SA/AC@N-CNTs, respectively, confirming
the effectiveness of LIL treatment to liberate the highly dis-
persed Co SAs/ACs active sites (entries 3 and 4, Table 1).
Figure 3. a) Effect of reaction temperature (reaction time: 3 h; H2 pressure: 2 MPa), b) H2 pressure (reaction temperature: 100 °C; reaction time: 3 h), and
c) reaction time (reaction temperature: 100 °C; H2 pressure: 2 MPa) on conversion and selectivity. d) Reusability of Co-SA/AC@N-CNTs-L (reaction
temperature: 100 °C; reaction time: 0.5 h; H2 pressure: 2 MPa). Other reactions conditions—catalyst: 5 mg; quinoline: 0.5 mmol; ethanol: 5 mL.
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Table 2. SH of various substrates catalyzed by Co-SA/AC@N-CNTs-L.
Table 1 also shows that SH of quinoline can only be observed

from those catalysts containing Co Nx, indicating the cata-
Entrya)
1
Substrate
Product
Time [h]
3
Conv. [%] Sel. [%]

lytic origin of Co Nx. Excellent selectivities (>99%) toward

1,2,3,4-tetrahydroquinoline were observed from all Co Nx
97.4
96.9
99.1
99.2
containing catalysts. The increased TOF follows a trend of
Co-SA/AC@N-CNTs-L > Co-SA/AC@N-CNTs > Co@N-CNTs,
indicating the liberated Co SAs/ACs sites are mainly respon-
sible for the achieved SH performance. Comparatively, the
performance achieved by Co-SA/AC@N-CNTs-L is equal to or
better than the reported high-performance catalysts, especially
under moderate reaction temperatures (Table S2, Supporting
Information). To further confirm if the catalytic activity is orig-
2
3
3
4
97.2
99.0

inated from the highly dispersed Co Nx sites, we conducted
a thiocyanate (SCN−) poisoning experiment (the SCN− deac-
tivates the catalytic activity of the metal-centered complex
4
5
4
4
98.1
97.0
99.1
99.1

catalysts, i.e., Co Nx, but does not affect the catalytic activity
of NPs).[17] The energy-dispersive X-ray (EDX) mapping and
spectrum of the catalyst poisoned by SCN− reveal a uniformly
distributed S without observable agglomeration of Co species
(Figure S11, Supporting Information). As shown in Table 1
(entry 5), the conversion efficiency of Co-SA/AC@N-CNTs-L
in the presence of SCN− (Co-SA/AC@N-CNTs-L-SCN−) is
drastically decreased from 97.4% to 17.2%. This indicates
6
3
96.2
99.0

a loss of ≈80.2% catalytic activity when the Co Nx sites are
7
8
3
5
96.1
98.6
99.3
99.2
−

inactivated by SCN , categorically confirming that the Co Nx
sites are the major form of catalytically active sites of Co-SA/
AC@N-CNTs-L.
The performance of Co-SA/AC@N-CNTs-L to convert
hydroxyl, methyl, and halogen functional groups substituted
quinolines into corresponding 1,2,3,4-tetrahydroquinolines
was subsequently evaluated. Remarkably, under the mild reac-
tion conditions, over 96% conversion and 99% selectivity are
obtained from all cases investigated regardless of the quinolines
substituted with electron-donating or electron-withdrawing
groups (Table 2), confirming the superior catalytic performance
of Co-SA/AC@N-CNTs-L for SH of all important quinolines.
Interestingly, nearly 100% conversion and selectivity toward
the hydrogenation of methyl substituted quinolines on either
the benzene or the heteroarene ring (entries 3–5, Table 2) can
9
6
99.7
99.8
a)Catalyst: 5 mg; substrate: 0.5 mmol; ethanol: 5 mL; reaction temperature: 100 °C;
and H2 pressure: 2 MPa.
widely applicable approach to unlock the catalytic powers
of the inaccessible catalytic active sites confined by other
materials.


be achieved. The SH of halogen (e.g., Cl and Br) substituted
quinolines to produce corresponding halogenated 1,2,3,4-tet-
rahydroquinolines is challenging due to the easy formation
of the dehalogenated and over-hydrogenated byproducts.[3,18]
Impressively, such substrates can be completely converted
into the halogenated 1,2,3,4-tetrahydroquinolines without pro-
ducing byproducts (entries 6−8, Table 2). In addition, the steri-
cally hindered phenyl group substituted quinoline can also be
completely converted into the desired product with almost
100% selectivity (entry 9, Table 2). The above results revealed
that neither the electronic nor the steric hindrance effects of
the substituted functional groups have noticeable influences
on SH performance, demonstrating the generic applicability of
Co-SA/AC@N-CNTs-L for efficient SH of quinolines.
Experimental Section
Chemicals: 2-methylimidazole was purchased from Aladdin
Reagent Company. Methanol, ethanol, and cobalt nitrate hexahydrate
(Co(NO3)2 •6H2O) were purchased from Sinopharm Chemical Reagent
Co. Ltd. Unless otherwise stated, all other solvents and chemicals were
of commercially available analytical grade and used without any further
treatment.
Synthesis of ZIF-67: ZIF-67 was synthesized according to the reported
method.[19] In a typical synthesis, 2-methylimidazole (1.97 g) was dissolved
in a mixed solution of 20 mL methanol and 20 mL ethanol. Co(NO3)2•6H2O
(1.746 g) was dissolved in another mixed solution of 20 mL methanol and
20 mL ethanol. The above two solutions were then mixed under continuous
stirring for 10 s, and the resultant solution was kept for 20 h at room
temperature. The purple precipitate was collected by centrifugation, washed
in ethanol several times, and dried at 70 °C in air for 12 h. The obtained
ZIF-67 was used as the precursor to synthesize Co@N-CNTs.
In summary, we have demonstrated a simple and effec-
tive LIL treatment approach to liberate the highly dispersed
inaccessible Co SAs/ACs sites confined inside N-CNTs. The
liberated highly dispersed Co Nx sites possess high catalytic
activity and selectivity toward SH of quinoline and a wide
range of its derivatives. The findings of this work provide a
Synthesis of Co@N-CNTs and Co-SA/AC @N-CNTs: The ZIF-derived
cobalt catalyst (Co@N-CNTs) was synthesized according to the reported
method.[11] Typically, 1.0 g ZIF-67 was mounted in a ceramic boat in a tube
furnace and heated to 440 °C with the heating ramp of 2 °C min−1 and
held for 8 h under continuous flow of 10 vol% H2/Ar. The temperature

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was then raised to 900 °C at a heating rate of 2 °C min−1 and maintained
for 2 h at 900 °C before naturally cooling down to room temperature.
The resultant Co@N-CNTs was treated with 6.0 m HNO3 at 90 °C
overnight to remove Co NPs, followed by washing with deionized water
and ethanol, then dried at 70 °C to obtain Co-SA/AC@N-CNTs.
Synthesis of Co-SA/AC@N-CNTs-L: Co-SA/AC@N-CNTs-L was
obtained by an LIL treatment of Co-SA/AC@N-CNTs. Typically, 10 mg
Co-SA/AC@N-CNTs was placed in a vessel filled with 10 mL of ethanol
and subjected to LIL treatment using a focused 355 nm, 10 Hz, and
60 mJ pulsed Nd:YAG laser beam (Innolas, Compact 400) for 20 min
under constant stirring. The resultant Co-SA/AC@N-CNTs-L was dried
at 70 °C for use.
Co-SA/AC@N-CNTs-Ultrasound and Co-SA/AC@N-CNTs-Plasma Samples:
Co-SA/AC@N-CNTs-ultrasound sample was obtained by ultrasonic
treatment of 10 mg Co-SA/AC@N-CNTs in a vessel containing 10 mL
of ethanol for 3 h using an ultrasonic cell disruptor (Ningbo Scientz
Biotechnology Co., Ltd, JY98-111DN).
Co-SA/AC@N-CNTs-plasma sample was obtained by N2 plasma
treatment of 10 mg Co-SA/AC@N-CNTs in a plasma reactor (CIF
International Group Co., Ltd, CPC-H) for 60 s under a power of 300 W.
Co-SA/AC@N-CNTs-L-SCN−: In the case of potassium thiocyanate
(KSCN) as a poison, 20 mmol KSCN was added into the reactants to
conduct the reaction.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
W.G. and Q.Y. contributed equally to this work. This work was supported
by the Natural Science Foundation of China (Grant Nos. 51872292,
51432009, and 51902311) and the Postdoctoral Science Foundation of
China (Grant No. 2019M652223). The authors thank the 1W1B station
for XAFS measurement in Beijing Synchrotron Radiation Facility (BSRF).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
Characterization: FESEM images of the samples were taken on
an FESEM (SU8020) operated at an accelerating voltage of 10.0 kV.
TEM (JEOL-2010) images were operated at an acceleration voltage
of 200 kV. STEM images of the samples were recorded by a high-
resolution TEM (Philips TecnaiG2 F20) operated at an acceleration
voltage of 200 kV. AC HAADF-STEM images were recorded by a JEOL
JEM-ARM 200F field-emission transmission electron microscope with
a spherical aberration corrector. Powder XRD patterns were analyzed
on a Philips X-Pert Pro X-ray diffractometer with using the Ni-filtered
monochromatic Cu Kα radiation (λKα1 = 1.5418 Å) at 40 keV and
40 mA. The surface area and porosity of samples were measured at 77 K
using a Surface Area and Porosity Analyzer (Autosorb iQ Station 2).
XPS analysis was performed on an ESCALAB 250 X-ray photoelectron
spectrometer (Thermo, USA) equipped with Al Kα1,2 monochromatized
radiation at 1486.6 eV X-ray source. Raman spectra of the samples
were recorded on a LabRAM HR800 confocal microscope Raman
system (Horiba Jobin Yvon) using an Ar ion laser operating at 532 nm.
The metal content in the composite structure was determined by the
inductively coupled plasma spectroscopy after microwave digestion of
the samples (ICP 6300, Thermo Fisher Scientific). The X-ray absorption
fine structure spectra data (Co K-edge) were collected at 1W1B station
in Beijing Synchrotron Radiation Facility (BSRF, operated at 2.5 GeV
with a maximum current of 250 mA). The data were collected in
fluorescence excitation mode using a Lytle detector. All samples were
pelletized as disks of 13 mm diameter with 1 mm thickness using
graphite powder as a binder.

confined single atoms, Co Nx, laser irradiation, quinoline, selective
hydrogenation
Received: September 16, 2019
Revised: October 2, 2019
Published online:
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