Nitrogen and sulfur co-doped cobalt carbon catalysts for ethylbenzene oxidation with synergistically enhanced performance

Article abstract of DOI:10.1039/C9RA00672A

Heteroatom doping has been demonstrated to be an effective strategy for improving the performance of catalysts. In this paper, cobalt carbon catalysts co-doped with nitrogen and sulfur (N and S) were synthesized through a hydrothermal method with chelate composites involving melamine, thioglycolic acid (C₂H₄O₂S), and tetrahydrate cobalt acetate (Co(OAc)₂·4H₂O). In addition, the selective oxidation of ethylbenzene under solvent-free conditions with molecular oxygen was used as a probe reaction to evaluate the activity of the catalysts. The optimized catalyst shows an ethylbenzene conversion of 48% with an acetophenone selectivity of 85%. Furthermore, the catalysts were systematically characterized by techniques such as TEM, SEM, XRD, Raman, and XPS. The results reveal that the species of cobalt sulfides and synergistic effects between N and S has inserted a key influence on their catalytic performance.

Full text of DOI:10.1039/C9RA00672A

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Nitrogen and sulfur co-doped cobalt carbon
catalysts for ethylbenzene oxidation with
Cite this: RSC Adv., 2019, 9, 9462
synergistically enhanced performance†
Sheng Chen, Yujie Wu, Shanshan Jie, Chak Tong Au and Zhigang Liu *a
a
a
a
b
Heteroatom doping has been demonstrated to be an effective strategy for improving the performance of
catalysts. In this paper, cobalt carbon catalysts co-doped with nitrogen and sulfur (N and S) were
synthesized through a hydrothermal method with chelate composites involving melamine, thioglycolic
2 4 2 2 2
acid (C H O S), and tetrahydrate cobalt acetate (Co(OAc) $4H O). In addition, the selective oxidation of
ethylbenzene under solvent-free conditions with molecular oxygen was used as a probe reaction to
evaluate the activity of the catalysts. The optimized catalyst shows an ethylbenzene conversion of 48%
with an acetophenone selectivity of 85%. Furthermore, the catalysts were systematically characterized by
techniques such as TEM, SEM, XRD, Raman, and XPS. The results reveal that the species of cobalt
sulfides and synergistic effects between N and S has inserted a key influence on their catalytic performance.
Received 25th January 2019
Accepted 10th March 2019
DOI: 10.1039/c9ra00672a
rsc.li/rsc-advances
active sites while the carbon-based material serves as support.
The goal is to prevent aggregation and loss of metal particles,
1
. Introduction
Heterogeneous catalytic oxidation reactions are essential thus ensuring catalytic performance superior to that of the
processes in chemical industries because the products are carbon-based material. For example, iron nanoparticles
widely applied for the manufacture of perfumes, resins, phar- enclosed in graphitic shells are known for catalytic interactions
1
6
maceuticals, and so on. The selection of catalysts determines since the last century. However, despite the research conducted
the reaction rate as well as the type and yield of products. For on NMCs, their performance is far short from industrial appli-
excellent performance, noble metals (Pd, Pt, Au, etc.) and strong cations. Since 2009, Dai and coworkers reported that as a metal-
2
oxidizers were used. However, the high cost and environmental free electrocatalyst for oxygen reduction reaction, nitrogen-
limitation of these materials limits the development of this kind containing carbon nanotubes worked signicantly better than
of catalytic system. Recently, there is an increasing interest in carbon nanotubes, in which N facilitates a four-electron
7
the use of catalysts that are based on non-noble transition pathway for the reaction. Among heteroatom dopants, N is
metals due to their low cost and high availability. Nonetheless, most frequently used to enhance electron transfer ability and
3
8
the performance of these catalysts needs further improvement.
defect generation. Beside N, heteroatoms such as B and P can
Nowadays, carbon based materials, such as graphene, also improve the catalytic performance of carbon-based mate-
carbon nanotubes, activated carbon, and carbon spheres etc., rials. In recent years, the co-doping of carbon materials with two
4
are essential materials in many domains. With in-depth different heteroatoms is considered promising for the
9
researches, the excellent performance of carbon-based mate- enhancement of catalytic performance. For instance, meso-
5
rials has been repeatedly demonstrated. It is noted that pure porous carbon materials doped with N and S are promising
carbon materials without modication display poor perfor- catalysts for water remediation where the dopants in the active
10
mance in heterogeneous catalysis. Researchers combined non- phase play a key role.
noble transition metals and carbon-based materials for the
Traditionally, catalytic oxidation processes are conducted in
formation of “non-noble transition metal carbon-based mate- liquid phase, leading to high capital cost as a result of catalyst
11
rials” (denoted herein as NMCs), in which the metal provides separation for recycling. The oxidation of substrates under
solvent-free conditions using O (or air) as oxidant is ideal, and
2
efforts were put in to explore green protocols, and signicant
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry progresses have been achieved. In this article, we report N and
a
12
and Chemical Engineering, Hunan University, Changsha 410082, China. E-mail:
S co-doped cobalt carbon catalysts synthesized by a hydro-
thermal method, using melamine as nitrogen and carbon
source, C H O S as sulfur source, and Co(OAc) $4H O as cobalt
liuzhigang@hnu.edu.cn
b
College of Chemistry and Chemical Engineering, Hunan Institute of Engineering,
2
4
2
2
2
Xiangtan, 411104, China
precursor. The catalysts were characterized by TEM, SEM,
Raman, XRD, and XPS techniques. The catalytic performance of
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra00672a
1
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the catalysts was evaluated using the selective oxidation of 2.3. Catalytic activity test
ethylbenzene as model reaction.
In the present study, the oxidation of ethylbenzene with
molecular oxygen (O ) under solvent-free conditions was used to
2
test the catalytic performance of the prepared samples. Quin-
tessentially, 28 mg of catalysts and 10 ml of ethylbenzene were
added into a Teon-lined autoclave, and aer being well sealed,
the system was heated to a designated temperature (e.g., 120
2
. Experimental section
2.1. Catalyst preparation
In the present study, the catalysts were synthesized hydrother-
mally. In a 100 ml round-bottom ask, 3 mmol of melamine was
dissolved in 50 ml of deionized water at 100 C to form
a homogeneous solution, which was heated to reux for 15 min
aer adding 1 mmol of Co(OAc) . Then 2 mmol of C H O S was
added into the resulted solution, and the mixture was contin-
uously stirred for 1 h. Aer that, the obtained mixture was
transferred to a 75 ml stainless-steel autoclave for hydrothermal
ꢀ
C). Then, the oxygen valve was adjusted to keep the system
ꢀ
under 0.8 MPa of O , and the reaction was maintained at 120 C
ꢀ
2
for 5 h with continuous magnetic stirring. At length, ethyl-
benzene conversion and product selectivity were measured by
gas chromatography over an Echrom A90 instrument equipped
with a HP-5 ms capillary column, employing bromobenzene (0.2
g) and p-dichlorobenzene (0.02 g) as internal standard.
2
2
4 2
Aer the oxidation of ethylbenzene, the reaction solution
was centrifuged and the obtained black powder was washed
with anhydrous ethanol for three times and then dried in an
ꢀ
reaction at 200 C for 18 h in a blast oven. Aer cooling down to
room temperature, the mixture was subject to ltration and the
as-obtained solid substance was washed several times with
abundant amount of deionized water. The collected black
precipitate was dried overnight at 80 C in an oven. Finally, the
solid was nely ground and transferred into a small quartz boat
and heated to 700 C at a heating rate of 5 C min in a tube
furnace and then calcined at 700 C for 90 min in a nitrogen
atmosphere. The substance was well ground and is herein
denoted as Co-N-S-C-700.
ꢀ
oven at 80 C overnight; the as-resulted dry black powder is
herein denoted as Co-N-S-C-700-R. The Co-N-S-C-700-R sample
ꢀ
ꢀ
was calcined at 700 C in nitrogen once again, and the obtained
sample was denoted as r-Co-N-S-C-700-2. Accordingly, the
sample aer another cycle of reaction is denoted as r-Co-N-S-C-
ꢀ
ꢀ
ꢁ1
ꢀ
700-3. In this paper, all the characterizations for the re-activated
catalyst were based on r-Co-N-S-C-700-3.
For comparison purposes, samples denoted as Co-N-S-C-600,
Co-N-S-C-800 and Co-N-S-C-900 were prepared similarly but
with calcination conducted at 600, 800, and 900 C, respectively.
ꢀ
3. Results and discussion
For the control experiments, Co-N-C-700, Co-S-C-700 and Co-N-
S-C-700 catalysts were prepared. For the synthesis of Co-N-C-
3
.1. Morphology and structure analysis
Scheme 1 illustrates the general procedure for the synthesis of
the Co-N-S-C-T (T stands for the temperature adopted for cata-
lyst calcination) catalysts. Generally speaking, during the one-
pot pyrolysis of precursor molecules involving melamine,
Co(OAc) $4H O and C H O S, there is the gradual generation of
brownish black precipitates as a result of homogeneous chela-
tion. Aer hydrothermal carbonization at 200 C and calcina-
tion at a designated temperature (600, 700, 800 or 900 C), the
7
1
00, 3 mmol of melamine, 50 ml of distilled water and
mmol of Co(OAc) $4H O were mixed and stirred for 1 h, and
2
2
the resulted mixture was processed as described in the case of
Co-N-S-C-700. Similarly, the Co-S-C-700 sample was prepared
having melamine replaced by 3 mmol of glucose monohydrate.
As for the generation of N-S-C-700, the Co-N-S-C-700 sample was
2
2
2
4 2
ꢀ
ꢀ
treated with 50 ml of 4 M hydrochloric acid at 90 C for 4 h.
ꢀ
targeted samples were nally obtained. Fig. 1a displays the
2.2. Catalyst characterization
Transmission electron microscopy (TEM) was executed on
a Tecnai G2 F20 S-TWIN implement operated at 120 kV and
a Philips CM200 FEG implement operated at 200 kV to visualize
the size and morphology of the samples and probe the effect of
cobalt, nitrogen and sulfur on sample structure and
morphology. Field emission scanning electronic microscopy
(
SEM) operated on a JSM-6700F microscope and scanning
transmission electron microscopy (STEM) performed on JEM-
100F for elemental mapping were conducted for further anal-
2
ysis of sample morphology. X-ray diffraction (XRD) investiga-
tion was carried out on a Japan XRD-6100 diffractometer using
Ni-ltered Cu Ka radiation (50 kV, 10 mA). The surface
elemental compositions of samples were measured by X-ray
photoelectron spectroscopy (XPS) conducted on a PHI 5000C
ESCA system (PerkinElmer). Raman spectra were recorded with
a 632.8 nm laser on a LabRAM Aramis micro Raman spec-
trometer at ambient temperature.
Scheme 1 Schematic illustrations of the synthesis process of the
representative Co-N-C-X catalysts and the catalytic reaction for
ethylbenzene.
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Fig. 2 (a) XRD patterns of Co-N-S-C-600, Co-N-S-C-700, and Co-
Fig. 1 (a) SEM image of Co-N-S-C-700; TEM images of (b) Co-N-S- N-S-C-800; (b) Raman spectra of Co-N-C-700, Co-S-C-700, Co-N-
C-700 and (c) Co-N-C-700 (inset is the size distribution of Co-N-C- S-C-700, and N-S-C-700.
700 particles); (d) high-resolution TEM image of Co-N-S-C-700; (e)
STEM image and elemental mappings of Co-N-S-C-700.
can be detected over Co-N-S-C-600 and Co-N-S-C-700 but not
structure and morphology of Co-N-S-C-700 as obtained in over Co-N-S-C-800. It is apparent that the diffraction peaks of
scanning electron microscopy (SEM). It is clear that a portion of Co-N-S-C-700 are sharper and bigger than those of Co-N-S-C-600
the N,S co-doped cobalt carbon materials are with rod-like and Co-N-S-C-800, indicating higher crystallinity of the
13
19
rather than hollow structure, and they are essentially irreg- former. What is more, Co-N-S-C-600 with the lowest calcina-
ular in size. It can be observed that these rod-like structures are tion temperature is the lowest in crystallinity. There is no
different in length and width (Fig. S1a†), similar phenomena detection of signals ascribable to impure phases, demon-
20
were reported before, indicating the effectively of this method strating the high purity of samples. It is worth pointing out
14
in forming different rod structures. As depicted in the trans- that there are no characteristic peaks of cobalt nitride over the
mission electron microscopy (TEM) images of Fig. 1b, c and samples, which could be due to the low content and/or high
2
1
S1b,† the Co-N-S-C-700, Co-N-C-700 and Co-S-C-700 samples are dispersion of nitride compounds.
15
different in morphology and structure. In Fig. 1b, a rod-like
The Raman spectra of Co-N-C-700, Co-S-C-700, Co-N-S-C-700,
structure of Co-N-S-C-700 is displayed, and the coarse struc- and N-S-C-700 are showed in Fig. 2b. There is the obvious
14b,16
ꢁ1
ture is estimated to be 0.36 mm in width.
The lattice spacing presence of D and G band at 1339 and 1594 cm , respectively.
of particles marked in red lines in Fig. 1d measured to be 0.3 nm The former is related to lattice defects while the latter to in-
2
22
9 8
is corresponding to the (311) plane of Co S . Moreover, the plane stretching vibration of sp carbon atoms. The D band
23
elemental mapping images of Co-N-S-C-700 demonstrate the is more intense than the G band of the four samples. The ID/IG
existence of C, N, O, Co and S (Fig. 1e), which indicates the value of Co-N-S-C-700, Co-S-C-700, Co-N-C-700, N-S-C-700 is
successful synthesis of the catalyst. Also, there is uniform 1.91, 1.32, 1.20, and 1.17, respectively. The remarkably high ID/
distribution of N in the Co-N-S-C-700 catalyst. As showed in IG value of Co-N-S-C-700 suggests that the presence of Co and
Fig. 1c, Co-N-C-700 is in the form of particles with size ranging the co-doping of N and S promote the formation of defective
from 19 to 59 nm (inset), in a way similar to that reported by sites and vacancies in the graphite lattice. In other words, the
17
Wang et al., and a small portion of the particles are with size of amount of structural defects in graphite increases due to the
around 100 nm. In contrast to Co-N-C-700, the Co-S-C-700 presence of transition metals and the co-doping of hetero-
24
sample has rod-like structures with little detection of particles atoms. Moreover, it was observed that the I /I value decreases
D
G
(
Fig. S1b†), indicating that the C H O S precursor has an effect with the increase of calcination temperature, indicating that the
2 4 2
on the formation of rod-like entities. Moreover, the entities of degree of graphitization increases with the rise of calcination
Co-S-C-700 are closely aggregated, while those of the other temperature (Fig. S2†).
samples are more dispersed, indicating the N atoms can act as
X-ray photoelectron spectroscopy (XPS) analysis was carried
anchoring sites for better dispersion of metal particles as sug- out to obtain information about the compositions and chemical
18
25
gested by Chen et al.
states of surface elements. The survey spectra indicate the
The X-ray diffraction (XRD) patterns of the Co-N-S-C-T presence of C, N, O, S, and Co (Fig. S3a†). The O 1s signals could
samples are depicted in Fig. 2a. The broad peak at 2q ¼ 23.3ꢀ be due to adsorbed water and/or oxygen species. As indicated
belongs to the (002) planes of graphitic carbon. Nearly all the in Table S1,† there is obvious difference in N-S-Co composition
peaks of the samples can be assigned to crystals of cobalt across the Co-N-S-C-T samples. Due to the lack of N-S-Co regu-
suldes. There are eight peaks that are assignable to face- larity, it is deduced that not only the relative content of cobalt
26
centered cubic Co
9
S
8
(JCPDS no. 19-0364). Five of the Co
9
S
8
and heteroatoms but also their bonding mode with the
diffraction peaks (indicated by purple dashed lines) can be graphitic structure play an essential role in catalytic
detected over Co-N-S-C-700 and Co-N-S-C-800 but not over Co-N- performance.
S-C-600. Beside the peaks of Co
9
S
8
ꢀ
, there are three intense
In Fig. 3a, the two sets of peaks at ca. 780 eV and 797 eV
and Co 2p1/2 signals, respec-
ꢀ
ꢀ
diffraction peaks at 2q ¼ 35.3 , 46.8 and 54.2 attributable to binding energies are Co 2p
3
/2
27
the (101), (102), and (110) reections of Co S (JCPDS no. 02- tively. The Co 2p3/2 peaks can be deconvoluted into two
4
3
1458). Two of these three peaks (marked by red dashed lines) components at 777.9 and 781.3 eV. The former is due to metallic
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Table 1 Catalytic activity of different catalysts for ethylbenzene
a
oxidation with O
2
c
Sel. (%)
b
Entry
Catalyst
Conv. (%)
AP
PA
BA
1
2
3
4
5
6
7
8
—
6
17
48
25
14
11
13
14
79
82
85
83
81
82
84
86
19
17
14
16
18
17
14
13
2
1
1
1
1
1
2
1
Co-N-S-C-600
Co-N-S-C-700
Co-N-S-C-800
Co-N-S-C-900
Co-N-C-700
Co-S-C-700
N-S-C-700
a
Reaction conditions: ethylbenzene (10 ml), catalyst (28 mg), O
2
(0.8
ꢀ
b
c
Fig. 3 High-resolution XPS spectra of Co-N-S-C-600, Co-N-S-C- MPa), 120 C and 5 h. Conversion of ethylbenzene. Selectivity to
00 and Co-N-S-C-800: (a) Co 2p and (b) S 2p.
acetophenone (AP), phenethyl alcohol (PA) and benzaldehyde (BA).
7
3
+
2+ 28
Co while the latter Co and Co . Similar phenomena can be catalytic activity. Among the Co-N-S-C-T catalysts, Co-N-S-C-700
observed with the Co 2p1/2 peaks. It is noted that there are is the highest in ethylbenzene conversion (48%), followed by
3
+
features of Co 2p3/2 satellite at ca. 786 eV corresponding to Co
Co-N-S-C-800 (25%), Co-N-S-C-600 (17%) and Co-N-S-C-900
2
+
ꢀ
and/or Co species. The acquired S 2p spectra are illustrated in (14%) (Table 1, entries 2–5), indicating that 700 C is the most
Fig. 3b. The S 2p spectra can be deconvoluted into four peaks at suitable temperature for catalyst calcination. It is apparent that
1
61.4, 162.3, 163.3 and 168.7 eV, with the last one in line with there are overt differences in ethylbenzene conversion but the
that of oxidized sulfur. The two peaks at around 161.4 and variation in product distribution is less signicant. The sharp
62.3 eV are S 2p3/2 and S 2p1/2 signals, respectively, of Co–S variation of ethylbenzene conversion reiterates that a proper
entities in Co-N-S-C-T. As for the S 2p peak at 163.3 eV, it is calcination temperature is a key factor in the synthesis efficient
1
29
attributed to the C–S–C and C]S bonds of thiophenic S.
As displayed in Fig. S3b,† the C 1s peaks at 284.4, 283.9,
85.8 and 288.9 eV can be ascribed to C–C & C]C, C–S, C–N, performance, a number of control experiments were conducted
catalyst.
In order to elucidate the effect of dopant on catalytic
2
and C–O, respectively. The detection of C 1s signals assignable under the optimized conditions. It has been generally accepted
to C–N and C–S suggests the successful doping of N and S atoms that the doping of heteroatoms into the carbon materials may
into the Co-N-S-C-T catalysts, in consistent with the results of result in the changes in charge and spin densities of adjacent
30
elemental mappings (Fig. 1e). The N 1s proles can be carbon atoms. Moreover, the kind of doped heteroatoms, the
deconvoluted into four peaks (Fig. S3c†) attributable to doping level and type of bonds formed between dopants and
pyridinic N (398.4 eV), pyrrolic N (399.2 eV), graphitic N (401.0 carbon atoms can also exert an key inuence on the changes in
31
35
eV) and oxidized N (403.9 eV). The N 1s signals of Co-N-S-C-600 charge and spin density. As we all know, N atoms have
and Co-N-S-C-700 are nearly undetectable whereas that of Co-N- a stronger electronegativity and S atoms have a weaker elec-
S-C-800 is relatively strong. It is apparent that when the catalyst tronegativity, compared with C atoms. Hereaer, the changes in
ꢀ
calcination temperature is 800 C, there is the presence of charge and spin density of carbon atoms can be tailored by
a high proportion of pyridinic N which may serve as coordina- controlling the doping of N and S, which may enhance the
tion sites with cobalt ions, leading to high dispersion of active activity of the as-prepared catalysts via the synergistic effect N
3
2
sites apt for catalytic reaction.
and S. Thus, the role of nitrogen and sulfur atoms in catalytic
activity was investigated over the Co-N-S-C-700, Co-S-C-700 and
Co-N-C-700 catalysts. It can be seen that Co-N-S-C-700 is supe-
rior to the other two in catalytic activity (Table 1, entries 3, 6 and
3
.2. Catalytic performance analysis
Aerobic oxidation of ethylbenzene is an environmentally-
friendly process widely used for the manufacture of chemical
intermediates. Thus, the selective oxidation of ethylbenzene
under solvent-free conditions was chosen as probe reaction to
evaluate the performance of the as-prepared catalysts. The
intrinsic nature of the catalysts in terms of catalyst recovery and
7), suggesting the existence of synergistic effect between N and S
for the better performance of the former. As shown in Table S2,†
the effect of dopant on catalytic performance can also be
observed in other reported works. To study the inuence of
cobalt nanoparticles on catalytic activity, Co-N-S-C-700 has
33
ꢀ
treated in 4 M hydrochloric acid at 90 C for 4 h. Over the as-
34
product separation was also considered. The catalytic perfor-
mance of Co-N-S-C-T catalysts was tested and showed in Table 1.
In the absence of a catalyst, ethylbenzene conversion is low and
selectivity to acetophenone is 79% (Table 1, entry 1). On the
basis of the results in Table 1, it is obvious that the temperature
adopted for catalyst calcination has a signicant inuence on
obtained N-S-C-700A sample, ethylbenzene conversion (14%)
is low, indicating the need of cobalt for high catalytic perfor-
mance. According to the Raman spectra, Co-N-S-C-700 has the
highest I /I value that corresponds to the presence of defects,
D G
which should be a combined effect of nitrogen, sulfur and
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cobalt. It is hence deduced that nitrogen, sulfur, and cobalt Co
species are essential for the carbon-based system.
4
S
3
and Co
9
S
8
has a positive effect on catalytic performance.
Also, by comparing the XRD patterns of high-performance Co-
It is undeniable that catalyst stability is a key factor in the N-S-C-700 with the low-performance Co-N-S-C-600 and Co-N-S-
judging of catalyst efficiency. Unfortunately, the Co-N-S-C-700 C-800, the importance of Co4S3 and Co9S8 in the catalytic
catalyst shows a sharp decrease in catalytic activity in the reaction can also be dened.
second reaction cycle (Table S3,† entry 1). Nonetheless, the used
catalyst can be re-activated by calcination at 700 C for 90 min in there are strong and weak shake-up satellite features at around
In the Co 2p spectra of r-Co-N-S-C-700 and Co-N-S-C-700-R,
ꢀ
a nitrogen atmosphere. It can be seen from entries 2, 3 of Table 793.3 eV ascribable to spin–orbit components as previous
30
S3† that r-Co-S-N-C-700-2 and r-Co-S-N-C-700-3 have ethyl- literature reported before (Fig. 4b), conrming the presence of
benzene conversion of 49% and 53%, respectively. To explain surface Co–O components. Compared to r-Co-S-N-C-700, Co-N-
the high catalytic activity of the re-activated catalyst, we had the S-C-700-R shows peaks assignable to S–O –S and C–O bonds of
x
r-Co-S-N-C-700 and Co-S-N-C-700-R samples characterized. higher intensity (Fig. 4c and S3b†), illustrating the enrichment
From the TEM images of r-Co-N-S-C-700, it is clear that aer the of inactive oxidative groups during the ethylbenzene oxidation
ethylbenzene oxidation reaction and being subjected to calci- reaction in the expanse of the active groups as reected in the
nation, the catalyst breaks down from the rod-like structure to high oxygen content of the Co-N-S-C-700-R sample (Table S1†).
particles without any specic morphology (Fig. S1c†). As On the contrary, the content of active groups such as thiophenic
revealed in the XRD patterns of Fig. 4a, beside the ten peaks of S is relatively higher in the re-activated r-Co-S-N-C-700 sample.
ꢀ
Fig. 2a, a weak diffraction peak at 54.2 is detected over r-Co-N- In addition, it is noted that there is the presence of uorine in
S-C-700, which is attributable to the (110) planes of Co S . Three Co-N-S-C-700-R, which might be resulted from the polytetra-
4
3
ꢀ
ꢀ
ꢀ
new peaks at 38 , 50.3 , and 55 ascribable to the (400), (511), uoroethylene lining (Table S1†). The sharp decrease of cata-
and (440) planes of Co (JCPDS no. 42-1448) are observed over lytic activity could be ascribed to uorine, which veried in
the Co-N-S-C-700-R sample; which are not detected over the previous work of our group. According to previously studies,
other catalysts in the present study. What is more, the peaks at the oxidation of ethylbenzene with O is a free radical reac-
Being strongly electron-withdrawing, uorine is
3 4
S
36
2
ꢀ ꢀ
11,36
7
3.3 and 76.7 attributed to Co S are absent in the case of Co- tion.
9 8
N-S-C-700-R. It is clear that in comparison to Co-N-S-C-700-R, r- a winner in capturing the unpaired electrons of free radicals.
Co-N-S-C-700 shows Co S peaks that are signicantly higher in
9
8
intensity. It is observed that there is no detection of Co S peaks
4
3
4. Conclusion
over Co-N-S-C-700-R but rather those of Co S , suggesting that
3
4
4 3
Co S can be generated during calcination of Co-N-S-C-700-R at We have successfully synthesized the N,S co-doped cobalt
ꢀ
7
00 C in nitrogen. It can hence be stated that the presence of carbon catalysts by hydrothermal carbonization process with
precursor molecules involving melamine, S, and
Co(OAc) $4H O. Among the Co-N-S-C-T samples, Co-N-S-C-700
exhibits the best catalytic performance for the selective oxida-
tion of ethylbenzene using O as oxidant under solvent-free
2 4 2
C H O
2
2
2
conditions. Combining the results of controlled experiments
and those of TEM, SEM, XRD, Raman, and XPS analyses, it is
deduced that the outstanding performance of Co-N-S-C-700 is
ascribable to Co S and Co S active sites as well as the syner-
9 8 4 3
gistic effect between N and S.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the State Key Laboratory of Heavy
Oil Processing in China (No. SKCHOP201504) and the National
Natural Science Foundation of China (No. 21872045).
Notes and references
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9466 | RSC Adv., 2019, 9, 9462–9467
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