A new synthesis of highly active Rh-Co alloy nanoparticles supported on N-doped porous carbon for catalytic C-Se cross-coupling and p -nitrophenol hydrogenation reactions

Article abstract of DOI:10.1039/d1nj00586c

Bimetallic Rh-Co nanoparticles supported on nitrogen-doped porous carbon (Rh-Co/NPC) were synthesized from metal precursors and urea through a simple thermal decomposition/reduction under a nitrogen flow. The Rh-Co/NPC nanocatalyst which contains highly dispersed alloy nanoparticles (～6 nm) showed high catalytic performance as well as good recyclability for the C-Se coupling reaction of diphenyl diselenide and aryl boronic acid and p-nitrophenol reduction.

Full text of DOI:10.1039/d1nj00586c

Showcasing research from Kang Hyun Park’s Lab
at Department of Chemistry, Pusan National University
and Ji Chan Park’s Lab at Clean Fuel Laboratory,
Korea Institute of Energy Research, Korea.
As featured in:
Volume 45
Number 18
14 May 2021
Pages 7903–8354
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A new synthesis of highly active Rh–Co alloy nanoparticles
supported on N-doped porous carbon for catalytic C–Se
cross-coupling and p-nitrophenol hydrogenation reactions
New Journal of Chemistry
A
journal for new directions in chemistry
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We reported new Rh–Co alloy nanoparticles supported on
nitrogen-doped porous carbon through a simple ball milling
method. The material exhibits high catalytic and recyclability
activity in the C–Se coupling reaction and hydrogenation of
p-nitrophenol.
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A new synthesis of highly active Rh–Co alloy
nanoparticles supported on N-doped porous
carbon for catalytic C–Se cross-coupling and
p-nitrophenol hydrogenation reactions†
b
Cite this: New J. Chem., 2021,
45, 7959
Dicky Annas,a Hack-Keun Lee,
Shamim Ahmed Hira,a Ji Chan Park *b and
a
Kang Hyun Park
*
Bimetallic Rh–Co nanoparticles supported on nitrogen-doped porous carbon (Rh–Co/NPC) were
synthesized from metal precursors and urea through a simple thermal decomposition/reduction under a
nitrogen flow. The Rh–Co/NPC nanocatalyst which contains highly dispersed alloy nanoparticles
(B6 nm) showed high catalytic performance as well as good recyclability for the C–Se coupling reaction
of diphenyl diselenide and aryl boronic acid and p-nitrophenol reduction.
Received 4th February 2021,
Accepted 22nd March 2021
DOI: 10.1039/d1nj00586c
rsc.li/njc
Rh–Pt bimetallic nanoparticles immobilized on polysilane for
the hydrogenation of arene under mild conditions.11 Thus,
Introduction
Recently, bimetallic nanoparticles have been developed and these reports demonstrate that a combination of two metals
have gained considerable attention because of their multiple can increase the catalytic activity compared to catalysts com-
unique properties compared to those of single metal nano- prising a single metal.
particles, such as high electronic and optical properties, selec-
In addition to the above-mentioned metals, Rh and Co nano-
tivity, and stability.1–3 These improved properties were particles have been reported as transition-metal catalysts for
attributed to the combination of properties arising from two organic reactions based on C–C and C–heteroatom bonds. Rho-
metals and the formation of new properties due to the syner- dium, as a single metal, is well known to have excellent catalytic
gistic effect of two metals.4,5 Owing to the advantageous proper- activity toward many reactions, particularly electro-oxidation reac-
ties, bimetallic nanoparticles have been used in many fields tions, hydrogenation, and cross-coupling reactions.12–14 Similarly,
such as the biomedical and food industries and as catalysts.6–8 cobalt also exhibits good catalytic activity toward cross-coupling
Many researchers have investigated the catalytic activity of reactions.15 However, because these metals easily aggregate and
bimetallic nanoparticles for many reactions. For example, exhibit a decrease in their catalytic activity, they are considered as
Wisniewska and Ziolek reported a combination of Pt and Ag unstable catalysts and, thus, have limited applications. Some
on a silica surface, which was applied for catalyzing the oxida- researchers reported that the use of carbon supports could resolve
tion of methanol; this catalyst exhibited good activity with high the problem of instability. Moreover, the construction of metal–
conversion.6 In addition, the other metal combinations such as support interactions can improve the stability and durability of
Ni and Pd have been reported to exhibit high electroactivity and materials.16 Park et al. immobilized Co nanoparticles on charcoal
serve as efficient catalysts for hydrogen production.9 Moreover, to develop a catalyst for the Pauson–Khand reactions; this catalyst
the Ni–Pd bimetallic nanoparticles have been reported as afforded a high product yield.17 Moreover, Rh deposited on
catalysts for C–C coupling reactions (e.g., Suzuki, Heck, and fullerene-C60 was recently reported and applied for the catalytic
Sonogashira reactions), and the rate of conversion of reactants reduction of 4-nitrophenol and Suzuki cross-coupling reactions.18
to products considerably increased using the Ni–Pd catalyst.10 Based on previous reports, the combinative deposition of these
Miyamura et al. also reported the high catalytic activity of the two transition metals (Rh and Co) on carbon materials is expected
to improve their catalytic activity.
Previously, Rh–Co bimetallic nanoparticles deposited on
carbon materials have been developed for several reactions
a Department of Chemistry, Pusan National University, Busan, 46241, South Korea.
E-mail: chemistry@pusan.ac.kr
b Clean Fuel Laboratory, Korea Institute of Energy Research, 152 Gajeong-Ro,
such as Pauson–Khand, hydroformylation, aminocarbonyla-
tion, reductive amination, and reductive cyclization.3,17,19–23
Daejeon 34129, Korea. E-mail: jcpark@kier.re.kr
Also, few researchers have reported that Rh–Co bimetals are
active for the C–Se coupling reaction.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00586c
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added to the extract, filtered, and evaporated using a rotary
evaporator. The product was analyzed by using GC-MS.
Hydrogenation of p-nitrophenol
A total of 5 mmol of 4-NP in 1.5 mL aqueous solution was mixed
with 40 mmol of fresh NaBH4 in 10 mL of aqueous solvent and
stirred using a magnetic bar. Then, 1 mg of catalyst was added
to the mixture. Next, 0.8 mL of solution was taken from the
mixture and diluted with distilled water and analyzed using
UV–vis spectroscopy. This step was continued until a complete
decrease in the 4-NP absorbance peak was observed.
Scheme 1 Brief scheme for the synthesis of the Rh–Co/NPC
nanocatalyst.
In the present work, we report new Rh–Co alloy nano-
particles supported on nitrogen-doped porous carbon (denoted
as Rh–Co/NPC) through a simple one-pot reaction based on
thermal treatment with a solid mixture of metal precursors and
urea under a nitrogen gas flow (Scheme 1). The Rh–Co/NPC
nanocatalyst was applied to a C–Se cross-coupling reaction for
organoselenide production, which is very useful in the bio-
chemistry field.24,25 Furthermore, the catalyst also showed high
performance with good recyclability in catalytic p-nitrophenol
hydrogenation. This result is supported by several reports
which mention that the interaction between metals and sup-
ports (such as carbon materials) can enhance selectivity and
yield in hydrogenation or reduction reactions.26–28
Material characterization
TEM images and HAADF-STEM images were obtained using a
Talos F200X operated at 200 kV. EDS and elemental analysis
and mapping were performed using a higher-efficiency detec-
tion system (Super X: 4 windowless SDD EDS system). The XRD
analysis was conducted on a high-power powder X-ray dif-
fractometer (Rigaku D/MAX-2500, 18 kW). The N2 sorption iso-
therms were measured at 77 K using a TriStar II 3020 surface area
analyzer. Before measurement, the samples were degassed in
vacuum at 573 K for 4 h. XPS was performed using a K-alpha
(TM) system with a micro-focused monochromator X-ray source
(Thermo VG Scientific, Inc.). Thermogravimetry (TG) measurement
was measured using a SETARAM apparatus with a heating rate of
10 1C minÀ1 under an air flow. The N content in the sample
was measured by using an Oxygen–Nitrogen–Hydrogen analyzer
(ONH-2000, ELTRA GmbH). Raman spectroscopy was performed
using a microscope (Horiba Lab RAM HR Evolution Visible NIR)
with a 514 nm laser. Gas chromatography-mass spectrometry
(GC-MS) was performed on a Shimadzu GC-1010 Plus GCMS-
QP2010 SE. UV-vis absorption spectroscopy was performed on the
Shimadzu UV-1800 spectrometer.
Experimental
Chemicals
Rhodium(III) acetylacetonate (Rh(acac)3, Rh(C5H7O2)3, 97%, Aldrich),
cobalt(II) acetylacetonate (Co(acac)2, Co(C5H7O2)2, 97%, Aldrich), and
urea (CH4N2O, 99.0–100.5%, Aldrich) were used as received without
further purification.
Synthesis of Rh–Co/NPC
For the synthesis of the Rh–Co/NPC nanocatalyst, 0.4 mmol of
Rh(acac)3 (0.16 g), 0.4 mmol of Co(acac)2 (0.10 g) and 20.8 mmol
of urea (1.25 g) were homogeneously mixed with methacrylate
beads (diameter: 3/8 inch), using a high-energy ball mill (SPEX
8000M Mixer/Mills) with 1725 rpm. After mixing for 5 min, the
dark brown powder sample was transferred to an alumina
boat in a tube furnace. The powder mixture was heated at a
ramping rate of 5.6 1C minÀ1 up to 700 1C under a flow of N2
(200 mL minÀ1) and thermally treated at the same temperature
for an additional 120 min under continuous N2 flow. After that,
the resulting black powder was cooled to room temperature
Results and discussion
Synthesis of the Rh–Co/NPC nanocatalyst
Scheme 1 illustrates the synthesis of the Rh–Co/NPC nano-
catalyst. First, Rh(acac)3 and Co(acac)2 as metal precursors and
urea as an N-doped carbon support source were mixed in the
solid state. Next, the thermal treatment of the solid mixture at
700 1C under an N2 gas flow was conducted. At high tempera-
tures, the Rh and Co precursors were decomposed and reduced
to Rh–Co alloy nanoparticles. At the same time, urea was
transformed to porous carbons with many defect sites which
can partly organize small pores in the structure.
and collected. Different Rh/Co nanoparticle ratios (Rh(1)–Co(3)
/
NPC and Rh(3)–Co(1)/NPC) were synthesized and compared
using the same method.
The transmission electron microscopy (TEM) image shows
an overall structure of the Rh–Co/NPC nanocatalyst (Fig. 1a).
C–Se coupling reactions
Diphenyl diselenide (1 mmol), arylboronic acid (1.1 mmol), The size of small nanoparticles dispersed on the porous carbon
base (2 equiv.), and catalyst (3 mol%) were mixed in round sheets was measured to be 6.2 Æ 1.3 nm, counting 200 particles
bottom tubes. Then, 2 mL of solvent was added to the mixture in the TEM images (Fig. 1b). The high-resolution TEM
using a syringe. The mixture was stirred at reflux under air for (HRTEM) image shows a spherical nanoparticle (Fig. 1c). The
24 h. After completion of the reaction, the mixture was cooled corresponding Fourier-transform (FT) pattern indicates that
to room temperature, diluted with dichloromethane, and alloy formation of Rh–Co crystals with a distance of 0.21 nm
extracted with Na2CO3 solution several times. MgSO4 was between adjacent fringes well matched with the (111) plane of
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carbon (Fig. 1e and f). The X-ray diffraction (XRD) spectrum of
the Rh–Co/NPC nanocatalyst shows representative peaks at
2y = 33 and 361, which are assigned to the reflections of (111)
and (200) planes in the face-centered-cubic (fcc) phase CoRh
(Fig. 1g). Using the Debye–Scherrer equation, the average size of
Rh–Co alloy is estimated to be 6.3 nm from the broadness of
the (111) peak; this value is well-matched with the particle size
determined from the TEM images.
From the energy-dispersive X-ray spectroscopy (EDS) data,
the atomic ratio of Rh and Co was measured to be 1.0
(Rh: 49.8% and Co: 50.2%) (Fig. S2 in the ESI†). In the Rh–
Co/NPC nanocatalyst, the content of carbon support was esti-
mated to be 36.2 wt% by thermogravimetric analysis (TGA) (Fig.
S3 in the ESI†).
The N2 sorption experiments for the Rh–Co/NPC nanocata-
lyst exhibited a type IV adsorption–desorption hysteresis
(Fig. 2a). The Brunauer–Emmett–Teller (BET) surface area and
the total pore volume were calculated to be 385 m2 gÀ1 and
0.26 cm3 gÀ1
, respectively. Applying the Barrett–Joyner–
Halenda (BJH) method, the small pore size was found to be
4.1 nm by the desorption branch (Fig. 2b).
The X-ray photoelectron spectroscopy (XPS) spectrum was
measured to confirm the surface states of the Rh–Co/NPC
nanocatalyst. The dominant signals in the XPS spectrum were
originated from Co, O, N, C, and Rh (Fig. 3a). The XPS spectrum
of the energy region of the Rh bands shows one set of peaks
(at 307.1 and 311.9 eV) in the range of the Rh 3d5/2 and 3d3/2
orbital signals, corresponding to the Rh0 state (Fig. 3b). The N
1s spectrum of the Rh–Co/NPC nanocatalyst revealed two split
peaks at 398.2 and 400.8 eV corresponding to pyridinic N and
pyrrolic N (Fig. 3c). The total N content in the Rh–Co/NPC
Fig. 1 (a) TEM image, (b) size distribution histogram, (c) HRTEM image
with the corresponding Fourier-transform pattern (inset of c), (d) HAADF-
TEM image, (e and f) elemental (Rh and Co) mapping images, and (g) XRD
spectrum of the Rh–Co/NPC nanocatalyst. The bars represent 40 nm (a),
4 nm (c), and 10 nm (d–f).
RhCo alloy (inset of Fig. 1c). From the JCPDS data, the lattice
distances for (111) planes are reported as 0.2119 nm at CoRh
alloy (JCPDS No. 01-071-7421), 0.2047 nm at Co (JCPDS No. 15-
0806), and 0.2196 nm at Rh (JCPDS No. 05-0685), respectively.
The high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) image clearly shows alloy
metal regions as bright dots and carbon support regions as
muddy clouds (Fig. 1d and Fig. S1 in the ESI†). The elemental
mapping of Rh (green) and Co (red) also demonstrates the
Fig. 2 (a) N2 adsorption/desorption isotherms and (b) pore size distribu-
formation of well-dispersed alloy nanoparticles supported on tion diagram of the Rh–Co/NPC nanocatalyst.
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Table 1 Optimization of the C–Se coupling reactions using the Rh–Co/
NPC nanocatalyst
Amount
catalyst
Time
(h)
Yielda
(%)
Entry
Solvent
Base
1
2
3
4
5
6
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO : H2O
(1 : 1)
KOH
24
24
24
24
24
24
61
33
28
41
78
37
NaOH
Na2CO3
Cs2CO3
K2CO3
K2CO3
7
8
9
10
11
12
13
14
1 mol%
1 mol%
1 mol%
2 mol%
3 mol%
3 mol%
3 mol%
3 mol%
Co(acac)2
3 mol%
Rh(acac)3
3 mol% urea
3 mol% Rh/
NPC
Water
DMF
THF
DMSO
DMSO
DMSO
DMSO
DMSO
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
24
24
24
24
24
18
36
24
Trace
57
Trace
87
92
66
Fig. 3 (a) XPS survey spectra of the Rh–Co/NPC nanocatalyst and XPS
spectra in the energy regions of (b) Rh 3d and (c) N 1s. (d) Raman spectra of
the Rh–Co/NPC nanocatalyst.
83
Trace
15
DMSO
K2CO3
24
Trace
sample was found to be 4.2 wt% by using a nitrogen analyzer
(OHN-2000).
16
17
DMSO
DMSO
K2CO3
K2CO3
24
24
Trace
49
In the Rh–Co/NPC nanocatalyst, the carbon crystallinity and
disorder in sp2 hybridized carbon were monitored by using
Raman spectroscopy (Fig. 3d). The Raman spectrum shows a
strong peak around 1357 cmÀ1 (D-band) that was caused by the
disordered structure and defect sites associated with dangling
bonds for the in-plane termination of carbon atoms. The G
band at B1584 cmÀ1 corresponds to the E2g phonon of the
bonded carbon atoms in a 2D hexagonal lattice. In the spec-
trum, the high ID/IG value (1.1) was observed, indicating the
presence of many disordered sites such as small sp2 domains
and defective carbon pores.
18
19
3 mol% Co/
NPC
3 mol%
(14 + 15 +
16)
3 mol%
(17 + 18)
DMSO
DMSO
K2CO3
K2CO3
24
24
36
Trace
20
DMSO
K2CO3
24
62
Reaction conditions: diphenyl diselenide (1 mmol), phenyl boronic acid
(1.1 mmol), base (2 equiv.), and solvent (2 mL). a Yields determined by
using GC-MS.
Furthermore, we screened the optimal catalyst amount.
When the amount was increased from 1 mol% to 2 mol%
and 3 mol%, the yield significantly increased to 87% (2 mol%
Catalytic performance of Rh–Co/NPC
The catalytic activity of Rh–Co/NPC for C–Se cross-coupling catalyst) and 92% (3 mol% catalyst) (Table 1, entries 10 and 11,
reactions using diphenyl diselenide and phenyl boronic acid as respectively). This shows that the catalyst amount influences
substrates was evaluated to optimize the reaction conditions. the conversion of reactants to the product. Furthermore, the
Initially, diphenyl diselenide and phenyl boronic acid reactions reaction was conducted over different reaction times. The yield
in DMSO at 110 1C for 24 h were completed in the presence of decreased to 66% when the reaction was conducted for 18 h
various bases. Compared to other bases, the presence of K2CO3 (Table 1, entry 12). Interestingly, when the reaction was con-
afforded the highest product yield (78%; Table 1, entry 5). In ducted for 36 h, the yield decreased to 83% (Table 1, entry 13).
addition, in the presence of KOH as the base, a moderate yield Thus, for further experiments, we used the following optimized
of 61% was obtained (Table 1, entry 1). However, NaOH, conditions: 3 mol% as the catalyst amount, K2CO3 as the base,
Na2CO3, and Cs2CO3 did not give good results (33, 28, and DMSO as the solvent, 110 1C as the reaction temperature, and
41% yields, respectively) because of the low substrate–product 24 h as the reaction time. For comparison, we examined other
conversion rate. Based on this result, K2CO3 was selected as the catalysts (3 mol% Co(acac)2, Rh(acac)3, and urea as raw materi-
optimal base. Next, we proceeded to screen the optimal solvent als for the synthesis of Rh–Co/NPC) for the C–Se cross-coupling
for this reaction. Several solvents including protic and aprotic reactions under the optimized conditions. However, no product
were used. No product was afforded when water or THF was was formed when these compounds were individually used as
used as the solvent (Table 1, entries 7 and 9, respectively). The the catalyst (Table 1, entries 14–16). The same result also
DMSO : H2O mixture and DMF afforded low yields (37% and occurred when a mixture of these three compounds was used
57%; Table 1, entries 6 and 8, respectively). Therefore, DMSO as the catalyst (Table 1, entry 19). In addition, low conversion
was selected as the optimal reaction solvent.
rates were achieved when only Rh (particle size: 4.7 nm) and Co
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Table 2 Reactant scope C–Se coupling reactions using the Rh–Co/NPC
nanocatalyst
(particle size: 6.7 nm) nanoparticles were used as catalysts
(49% and 36%; Table 1, entries 17 and 18, respectively). The
yield increased moderately (62%) when a mixture of Rh and Co
nanoparticles was used (Table 1, entry 20). Moreover, different
Entry Arylboronic acid
1
Product
Yield (%)
92
Rh/Co nanoparticle ratios (Rh(1)–Co(3)/NPC and Rh(3)–Co(1)
/
NPC) show lower yield compared to Rh–Co/NPC with 72 and
79% yields, respectively (Table 3). This shows that the ratio of
metal nanoparticles has an effect on the catalysis process to
influence the conversion.
Furthermore, we checked the C–Se cross-coupling reactions
using various phenylboronic acids with diphenyl diselenide
under the optimized conditions. Generally, the target products
were produced in good to excellent yields. Electron-donating
groups, such as p-OCH3, which attach to phenylboronic acid,
afforded a high yield of 91% (Table 2, entry 2). In addition, the
two methoxy groups at the ortho position showed a high
conversion rate and afforded 92% yield (Table 2, entry 3).
Electron-withdrawing groups such as p-CF3, p-CO2Et, o-CHO,
and p-F afforded a high yield of approximately 92%, 88%, 94%,
and 93%, respectively (Table 2, entries 4–7). Various heteroa-
tomic boronic acids were used to react with diphenyl disele-
nide, resulting in good yields (Table 2, entries 8 and 9). These
results showed that this catalyst under the optimized condi-
tions could be used with various reactants. In addition, the
comparison of the C–Se cross-coupling reactions with other
catalysts was displayed in Table 3 which shows that Rh–Co/NPC
has good catalytic activity.
2
3
4
91
92
92
5
88
The catalytic activity of Rh–Co/NPC was also investigated for
the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol
(4-AP) using NaBH4 as the reducing agent. Initially, 4-NP in an
aqueous medium exhibited UV-vis absorption at 317 nm. How-
ever, this absorption peak shifted to approximately 400 nm
when NaBH4 was added to the solution. This changed the color
from light yellow to bright yellow. This shows the formation of
4-nitrophenol ions (4-NPI) in the solution (Fig. 4a). The max-
imum absorption peak of 4-NPI remained unchanged when
Rh–Co/NPC was not used for the reaction, which indicated that
the hydrogenation of 4-NP to 4-AP did not occur. A new
absorption peak was observed at around 300 nm that can be
ascribed to the formation of 4-AP, followed by a decrease in the
absorption peak of 4-NP when a small amount of the catalyst
was used (Fig. 4b). In addition, the solution became colorless.
In this reaction, different conditions such as 4-NP concen-
tration, NaBH4 concentration, and catalyst amount were inves-
6
7
94
93
8
9
90
82
tigated at room temperature. The ln(C/C0) versus time plot in Therefore, 1 mg of catalyst was used for further investigation
Fig. S5 (ESI†) was determined from the absorbance peak, and because there was no noticeable reaction time difference when
the slopes were used to obtain rate constant k.
1 and 1.5 mg catalysts were used.
First, the catalyst amount was studied for 4-NP hydrogena-
The effect of the 4-NP concentration was also explored, while
tion using 5 mmol of 4-NP and 40 mmol of NaBH4. Fig. S6 the other conditions were constant (1 mg of catalyst and
(ESI†) shows the UV-vis absorption spectra of 4-NP in the 40 mmol of NaBH4). Fig. 4d shows a decrease in the rate
presence of various catalyst amounts. The spectra showed that constant with increasing 4-NP concentration. Increasing the
as the catalyst amount increased, the 4-NP hydrogenation rate 4-NP concentration from 3 to 7 mmol induced the conversion of
also increased. When 0.5 mg of catalyst was used, a complete 4-NP to 4-AP (Fig. S7, ESI†). This could happen because the
hydrogenation of 4-NP occurred after 14 min. Increasing the catalyst can absorb 4-NP and compete with BH4À, thereby
catalyst amount to 1 and 1.5 mg decreased the reaction time to decreasing the number of sites on the catalyst surface; this
À
6 and 5 min, respectively, and increased the k value Fig. 4c. facilitated the interaction between the catalyst and BH4 and
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Table 3 The comparison of the C–Se cross-coupling reactions with
Table 4 The comparison of hydrogenation of p-nitrophenol reactions
other catalysts
with other catalysts
Amount of
Reaction
Reaction
time
catalyst (mmol) (min)
Catalyst
Fe, FeCl2 or FeCl3 10
catalyst (mol%) time (min) Yield (%) Ref.
Amount of Dyes
Degradation
rate (%)
Catalyst
Ref.
20
8
30
12
24
24
24
91
14
94
93
72
79
92
29
30
31
32
Nano CoFe2O4
Cu2O-bpy, Mg
CuI-bpy
Rh(1)–Co(3)/NPC
Rh(3)–Co(1)/NPC
Rh–Co/NPC
5
5
5
3
3
3
RGO–Pd1(Co)15
0.12 mol% 1
5
80
7
99
95
99
99
33
34
35
36
Ag/ZnO flower like 150 mg
AgPd@UiO-66-NH2 1.5 mg
0.01
5
5
This work NaxWO3/Ag2O
This work nanocomposite
This work Fe3O4@COF–Au
Rh–Co2O3/Al2O3
1.5 mg
21
3 mg
100 mg
1 mg
108
7.2
5
20
240
4
99
93
99
37
38
This
work
Rh–Co/NPC
addition, the comparison of hydrogenation of p-nitrophenol
reaction with other catalysts was displayed in Table 4 which
shows that Rh–Co/NPC has good catalytic activity with a short
reaction time.
These two catalytic performance activities show that there
are synergistic effects between Rh, Co, N, and C in C–Se
coupling and p-nitrophenol hydrogenation reactions. The key
factor that is affecting the success of a catalytic process is the
electronic system interaction between metals and supporting
materials.40–42 Moreover, the porous material and N dopant can
also accelerate the catalytic process. This nitrogen dopant plays
an important role in the synthesis process of small nano-
particles due to the coordination between the lone pair electron
of N atoms and metal empty orbitals to form a strong
interaction.43 In addition, the lone pair electrons of N dopants
can conjugate with carbon materials by activating the deloca-
lized p electron systems of a graphene framework. As a result of
this electron conjugation, it can increase the active sites of the
catalytic system and the electron transfer process. Furthermore,
because the pyridinic N and pyrrolic N lone pair electrons
cannot delocalize to the p electron systems of a graphene
framework, they can coordinate with metal nanoparticle empty
orbitals to increase the metal and support electronic interac-
tions and the stability of the catalyst.40,41,44–46
Fig. 4 (a) UV–vis spectra of hydrogenation of 4-NP and 4-NPI. (b) UV–vis
spectra of hydrogenation of 4-NP using the Rh–Co/NPC nanocatalyst,
(c–e) rate constant under different catalyst amounts, concentration of
4-NP, and concentration of NaBH4.
promoted the transfer of H to 4-NP. Moreover, we investigated
the effect of the NaBH4 concentration, while the other condi-
tions were constant (1 mg of catalyst and 5 mmol of 4-NP)
(Fig. S8, ESI†). With the increase in NaBH4 concentration, the
rate constant also increased gradually (Fig. 4e). This is becausÀe
at high NaBH4 concentration, the amount of formed BH4
increases, which facilitates more electron transfer in this reac-
tion. In contrast, low NaBH4 concentration causes slow electron
transfer to 4-NP. Based on these results, it can be concluded that
catalyst amount, dye concentration, temperature, and NaBH4
concentration affect the hydrogenation of p-nitrophenol due to
the synergistic effect between all of these parameters.5,39 In Fig. 5 The recyclability of Rh–Co/NPC for C–Se coupling reactions.
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The recyclability of the Rh–Co/NPC nanocatalyst was explored for
Paper
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the C–Se coupling reaction using phenyl boronic acid and diphenyl
diselenide under the optimized conditions. After the reaction, the
nanocatalyst was separated and collected by centrifugation and
reused for five cycles. Fig. 5 shows that the Rh–Co/NPC catalytic
activity is still high without any significant decrease in yield. To
verify the stability and durability of Rh–Co/NPC, the catalyst was
recovered after the catalytic reaction and analyzed using TEM, as
shown in Fig. S9 (ESI†). After the reaction, Rh–Co/NPC main-
tained its original structure and did not undergo severe particle
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18 M. Gopiraman, S. Saravanamoorthy, S. Ullah, A. Ilangovan,
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Conclusions
The Rh–Co/NPC nanocatalyst (ca. 6 nm) was simply synthesized
through a one-pot thermal decomposition method. The high
catalytic activity was investigated for the C–Se coupling reaction
with various arylboronic acid reactants. In addition, this nano-
catalyst was active toward the hydrogenation of p-nitrophenol
over a short reaction time (B6 min). The catalyst was used for
5 cycles of the C–Se coupling reaction without any noticeable
decrease in product yields.
21 J. H. Park, E. Kim and Y. K. Chung, Org. Lett., 2008, 10,
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Conflicts of interest
22 J. H. Park, J. C. Yoon and Y. K. Chung, Adv. Synth. Catal.,
2009, 351, 1233–1237.
There are no conflicts to declare.
23 I. Choi, H. Chung, J. W. Park and Y. K. Chung, Org. Lett.,
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Acknowledgements
24 E. Y. Kim, T. W. Chung, C. W. Han, S. Y. Park, K. H. Park,
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27 H. Zhang, S. Gao, N. Shang, C. Wang and Z. Wang, RSC Adv.,
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This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) grant
funded by the Korea Government (MSIP) (NRF-2020R1I1A3067208).
H.-K. L. and J. C. P. at KIER were supported by the Research and
Development Program of the Korea Institute of Energy Research
(KIER) (No. C1-2478).
28 J. Li, L. Zhang, X. Liu, N. Shang, S. Gao, C. Feng, C. Wang
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