Nitrogen and sulfur co-doped carbon nanospheres for highly efficient oxidation of ethylbenzene

Article abstract of DOI:10.1039/C8NJ02948B

Nitrogen and sulfur co-doped carbon nanospheres (NS-CNs) have been found to have broad applications in a large variety of areas due to their well tailored physicochemical properties. To achieve an efficient co-doping effect, the synthesis protocols usuall

Full text of DOI:10.1039/C8NJ02948B

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Nitrogen and Sulfur Co-Doped Carbon Nanospheres for Highly
Efficient Oxidation of Ethylbenzene
Received 00th January 20xx,
Accepted 00th January 20xx
Minghui Liu, Yingcen Liu, Zhanming Gao, Cui Wang, Wanyue Ye, Rongwen Lu*, Shufen Zhang
DOI: 10.1039/x0xx00000x
Nitrogen and sulfur co-doped carbon nanospheres (NS-CNs) have found broad applications in a large variety of areas due
to their well tailed physicochemical properties. To achieve an efficient co-doping effect, the synthesis protocols usually
rely on a stepwise procedure which particularly includes a post treatment of the N-doped precursors by using a sulfur-
containing material such as H2S and thiourea, accordingly making the process time-consuming and complicate in doping
control. Here, we demonstrated an effective synthesis strategy to achieve a designated co-doping effect of carbon
nanospheres through a molecular design of the precursors used for the formation of carbon nanospheres. Specifically, we
identified an efficient co-polymerization process of both diaminobenzenesulfonic acid and melamine with formaldehyde,
which forms uniform resin nanospheres at room temperature with both N and S inherited from the two reactants. After a
carbonization process, carbon nanospheres were achieved with co-doped elements favorable for their applications. For
example, high performance catalyst was demonstrated from this synthesis protocol with promising potential for the
catalytic ethylbenzene oxidation.
www.rsc.org/
Na2S, and thiourea have been used to achieve a favorable sulfur
doping after the high temperature treatment11-13. In addition to
Introduction
such
a , typically
step-wise route, other synthetic routes14
Compositional doping of carbon materials by different elements,
particularly nitrogen and sulfur, have drawn increasing research
attention. The existence of doping elements were able to endow
the carbon materials interesting benefits such as good electrical
conductivity, favorable chemical inertness, and high catalytic
activity, thereby showing promising potentials in a wide range of
research fields1-5. Moreover, co-doping of carbon materials by two
or three elements with different electronegativities gives rise to a
unique electron distribution and then results in a synergistic effects
hydrothermal based ones, have also found to be successful to
simultaneously introduce different elements into the carbon
structure. Despite of the success in composition control, it is noted
that the current synthesis routes usually involve complicated
process which are usually time and energy consuming with the use
of expensive starting reagents. From the synthesis point of view, it
is highly favorable that simple and economic methods be developed
for the co-doping process with the use of environmental benign
materials. It is equally important that the synthesis be efficient for
compositional control with the potential for scale up production.
Here, we demonstrated that an effective synthesis strategy to
achieve a designated co-doping effect of carbon nanospheres
when used as catalyst6-8
.
Zheng et al.9 indicated that the
incorporation of atoms into N-doped graphene created
B
a
synergistic coupling and significantly enhanced the activity of
pyridinic N through a B–C–N bridge. Duan et al.10 incorporated
sulfur and nitrogen atoms into graphene sheets and discovered that
co-dopants (S and N) presented synergistically catalytic activity to
enhance the peroxymonosulfate activation.
through
a molecular design of the precursors used for the
formation of carbon nanospheres. Specifically, 2,4-diamino-
benzenesulfonic acid was used as the sulfur-containing reactant,
which is a water soluble aromatic amine widely used in the
production of dyes. The two imine groups formed by the
Although a co-doping effect has been highly apprecied by the
research community, the synthesis for a successful doping effect
with the capability to simultaneously introduce different elements
into the carbon matrix has become a challenging issue. For an
effective doping of both N and S elements, the pre-formation of a
condensation
of
2,4-diamino-benzenesulfonic
acid
and
formaldehyde, could crosslink with the nitrogen-containing species
of melamine via the addition reaction, and later on nitrogen and
sulfur containing colloidal spheres can form after the further
nitrogen-containing carbon precursor together with
a post-
polymerization.
A following carbonization process at high
treatment procedure to introduce sulfur is usually adopted.
Different sulfur-containing precursors including element sulfur,
temperature can produce carbon nanospheres with a uniformly-
dispersed both nitrogen and sulfur. Our preliminary test showed
that the co-doped carbon nanospheres are highly active for the
catalytic oxidation of ethylbenzene, which is a crucial process for
converting ethylbenzene to acetophenone.
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obtained on an Elementar Vario Macro EL Cube microanalyzer.
Thermogravimetric (TG) analysis was recorded on TGR Q 500 with a
heating rate of 10 oC⋅ min-1 under a nitrogen flow (100 mL⋅ min-1) or
in air.
Experimental details
Materials. 2,4-diaminobenzenesulfonic acid (≥98.0%) was
purchased from Aldrich. Tert-butyl hydroperoxide (70% aqueous
solution) was acquired from J&K scientific Ltd. Melamine,
formaldehyde (37 wt% - 40 wt%), ammonium hydroxide (NH3·H2O,
25% - 28%), and ethanol were from Sinopharm Chemical Reagent
Co., Ltd. All chemicals were AR-grade and used without further
purification. Deionized water (18.2 MΩ) was used in all
experimental processes as needed.
Catalytic Reaction. Ethylbenzene (1.0 mmol), catalyst (0.01 g), tert-
butyl hydroperoxide (413 μL, 3.0 mmol, 70 wt% in water), and
water (1 mL) were introduced into a 10 mL glass reactor sealed with
o
Teflon lid. The reaction mixture was stirred in a preheated 80 C oil
bath for 10 h. After that, 80 μL of anisole was added to the system
as an internal standard. Then CH2Cl2 (3×15 mL) was added to
extract organic compounds from the reaction system. The organic
phase was filtrated with a 220 μm size of oil film and analyzed by
Agilent 6820 GC with a HP-5 capillary column (30 m×0.25 mm×0.25
μm) and an Agilent GC-MS, with He gas as carrier gas. The
temperature program was set from 120 oC (maintained for 2 min) to
280 oC by 10 oC/min for the detection of oxidation of ethylbenzene.
For the recovery of catalyst, the catalyst was separated by
centrifugation and washed with ethanol three times and dried
under 50 oC for 10 h.
Preparation of the Nitrogen and Sulfur Co-Doped Colloidal
Spheres.
The nitrogen and sulfur co-doped colloidal spheres were
synthesized through a one-pot method. Typically, melamine (0.19 g,
1.5 mmol), 2,4-diaminobenzenesulfonic acid (0.057 g, 0.3 mmol),
and ammonium hydroxide (0.1 mL) were added into deionized
water (50 mL) in
a 100 mL two-necked round-bottom flask
equipped with a magnetic stirrer and a thermometer. Then, the
mixture was heated to 25 °C under stirring to obtain a transparent
solution. Afterward, formaldehyde solution (1.2 mL, 15 mmol) was
added to the system. After 1 h of stirring, a suspension of particles
was obtained. The suspension was centrifuged at 6000 rpm for 10
min and the products were washed by ethanol (30%) three times to
remove the remaining unreacted materials. After drying at 100 °C
for 10 h, the light yellow product was obtained.
Results and Discussion
The preparation of the monodisperse nitrogen and sulfur co-
doped carbon spheres is illustrated in Figure 1. Briefly, the
formation of nanospheres is initiated with the addition of
formaldehyde to the solution of 2,4-diaminobenzenesulfonic acid
and melamine at room temperature. A Schiff base was formed by
the reaction between 2,4-diaminobenzenesulfonic acid and
formaldehyde. Then the subsequent addition of amino groups on
melamine to the newly formed imine groups (-C=N-) induced the
cross-linking of 2,4-diaminobenzenesulfonic acid and melamine and
hence the formation of small clusters (Figure S1a). Afterwards, the
small clusters self-aggregated to form colloidal nanospheres in
order to minimize the total surface energy (Figure S1b)17 and grew
to nearly monodisperse colloidal nanospheres within a matter of a
few minutes (Figure S1c). Finally, nitrogen and sulfur co-doped
carbon nanospheres (denote as NS-CSs) were obtained by direct
carbonization of the colloidal spheres.
For comparison, resorcinol-formaldehyde spheres were
prepared using resorcinol/formaldehyde as polymer precursor by
using the reported method15. Melamine-formaldehyde spheres
were synthesized from melamine and formaldehyde according to
the method reported16
.
Carbonization. The synthesized colloidal sphere was carbonized at
500 °C under N2 flow for 2 h with a heating rate of 2.5 °C⋅min-1. The
product
was
denoted
as
NS-CSs.
The
carbonized
resorcinol/formaldehyde spheres were denoted as CSs, and the
carbonized melamine-formaldehyde colloidal spheres were
denoted as N-CSs.
Characterization. Field emission scanning electron microscopy
(FESEM) images were recorded on
a
Nova Nano-SEM 450
kV. Transmission
microscope at an acceleration voltage of
5
electron microscopy (TEM) images were obtained using an FEI-TF30
transmission electron microscope at 300 kV. Fourier transform
infrared FT-IR spectroscopy was performed on a NICOLET 6700
spectrometer, equipped with an attenuated total reflection (ATR)
setup. 13C and 15N cross-polarization magic angle spinning (CP-MAS)
NMR spectrum were obtained on an Agilent DD2-500 MHz at 125.7
MHz and 50.6 MHz, spining rate of 11 kHz and 7 kHz, respectively.
13C and 15N cross-polarization were performed both using a contact
time of 4 s. X-ray photoelectron spectroscopy (XPS) was acquired by
Thermo VG ESCALAB 250 with a Al-Kα X-ray source operating at 150
W (15 kV).The nitrogen adsorption/desorption isotherms were
measured using a 3H-2000PS1, Beishide Instrument Technology Co.,
Ltd., China, and the surface area and pore size found using the
Brunauer-Emmett-Teller (BET) method. The binding energies were
calibrated using the C 1s peak at 284.6 eV, and the software XPS
PEAK 4.1 was used for Curve fitting. Elemental analysis was
Figure 1. Schematic representation of the preparation of nitrogen
and sulfur co-doped carbon spheres from 2,4-diaminobenzenesul-
fonic acid, melamine, and formaldehyde.
Figure 2 shows the TEM image and corresponding elemental
mapping of NS-CSs. Figure 2a shows the spherical morphology of
the NS-CSs, which were obtained from the carbonize of the colloidal
spheres that synthesized by stirring the reaction mixtures for only 1
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melamine at 3469 and 3422 cm-1 (NH2 stretching vibration) and
1620 cm-1 (NH2 deformation vibration) are absent in the spectrum
h. Figure 2b and 2c confirm the presence and homogenous
distribution of N and S in NS-CSs. The content of N, S was 53.8% and
0.3%, respectively. Importantly, the elemental mapping images of the colloidal spheres. The distinct bands corresponding to
asymmetric stretching vibration (1169 cm-1) of S=O, stretching
reflect the fact that the in situ incorporation of a N and S in colloidal
vibration of C-N (1335 cm-1) and the quadrant (1552 cm-1) and
spheres can obtain well-distributed N and S co-doped carbon
semicircle stretching (1489 cm-1) vibrations of the triazine ring are
present in the spectrum of the colloidal spheres20, indicating the
successful incorporation of 2,4-diaminobenzenesulfonic acid and
melamine into the network. While, no bands that may be attributed
to imine linkages such as the C=N stretching vibration around 1600
cm-1 are found in the FTIR spectrum of colloidal spheres, implying
the subsequent attacking of the imine double bond, formed from
reaction of 2,4-diaminobenzenesulfonic acid and formaldehyde, by
the primary amine groups of melamine and the generation of an
aminal.
spheres, which is benefit for using as a catalyst18, 19
.
To further confirm the successful incorporation of 2,4-
diaminobenzenesulfonic acid and melamine into the network, a
detailed analysis of the materials chemical structure was performed
by 13C and 15N solid-state NMR spectroscopy (Figure 4). The 13C
cross-polarization magic angle spinning (CP-MAS) NMR spectrum of
colloidal spheres shows eight resonances at 167, 156, 145, 130, 103,
119, 96, and 60 ppm, respectively. The signal at 167 ppm can be
ascribed to the carbon atoms in the triazine ring of the melamine,
whereas the signals at 156, 145, 130, 103, 119, 96 ppm originates
from the CH aromatic carbons of the benzene ring of 2,4-
diaminobenzenesulfonic acid.
Figure 2. (a) BF-STEM image, (b) elemental mapping images and (c)
EDS of NS-CSs.
The rapid formation of colloidal spheres in solution was
evidenced by the corresponding photographs of the solutions with
time after the addition of formaldehyde. The solutions were left for
1 min at room temperature, during which they turned to turbid and
the turbidity of the solution increased quickly within 1 hour (Figure
S2). In addition, amorphous solids were observed in the image
(Figure S2 A) after drying of a drop of the reaction mixtures left for
30 sec, a very clear solution (Figure S2 a) on the silicon substrate.
The conversion of clear solution to amorphous solids further
reflected the rapid reaction of the mixture. After 3 min reaction, a
small amount of microspheres were found in the image (Figure S2
B). After 10 min reaction, microspheres can be clearly seen,
showing the rapid formation of colloidal spheres (Figure S2 c).
Figure 4. 13C CP-MAS (a) and 15N CP-MAS (b) NMR spectrum of the
synthesized colloidal spheres. The asterisk denotes spinning side
bands.
The resonance at 60 ppm can be assigned to the secondary
carbon atoms formed upon addition of the primary amine groups of
melamine to the newly formed imine double bond. Moreover, 15N
CP-MAS NMR spectrum of colloidal spheres shows two major
resonances at -207 and -285 ppm, respectively. The signal at -207
ppm can be ascribed to the nitrogen atoms in the triazine ring,
whereas the signal at -285 ppm can be attributed to the secondary
amine present in the aminal. Importantly, no signals from
unreacted primary amine groups are detected in the 13C and 15N CP-
MAS NMR spectra of colloidal spheres, which is in accordance with
the results from the FTIR analysis. In addition, no resonance at
around -55 ppm corresponding to an imine nitrogen is found.
Hence, the formation of the aminal in the cross-linking step
mentioned above is further confirmed.
Figure 3. FTIR spectra of 2,4-diaminobenzenesulfonic acid (a),
melamine (b), and the synthesized colloidal spheres (c)
The successful incorporation of 2,4-diaminobenzenesulfonic
acid in the build-up of the colloidal spheres as well as high
conversion of the functional groups initially present in the
starting materials was confirmed by Fourier transform infrared
(FTIR) spectroscopy (Figure 3). Bands which can be attributed to the
amino group of 2,4-diaminobenzenesulfonic acid at 3384 and 3322
cm-1 (NH2 stretching vibration), as well as to the amino group of
TG analysis (under nitrogen with a heating rate of 10 °C⋅min-1)
also shows the structural difference between synthesized colloidal
spheres, melamine-formaldehyde spheres, and mixture of
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acts as a crosslinker, a stabilizer and also a source of N and S during
the synthesis of colloidal spheres like cysteine did in the synthesis
of phenolic resin based polymer spheres23. Thus, a molecule
containing both diamino and sulfonic acid group is an indispensable
factor for achieving this colloidal spheres (see Figure S6 in
supporting information).
melamine-formaldehyde spheres and 2,4-diaminobenzenesulfonic
acid. As shown in Figure S3, the TG curve of melamine-
formaldehyde spheres exhibits an obvious weight loss at 400 °C and
mixture of melamine-formaldehyde spheres and 2,4-diamino-
benzenesulfonic acid shows
a two steps weight loss in the
temperature range of 255 °C to 400 °C, which can be attributed to
the gradual decomposition of 2,4-diaminobenzenesulfonic acid and
the decomposition of melamine-formaldehyde spheres. While, the
TG curve of the synthesized colloidal spheres shows an intricate
thermal decomposition process. This continuous, slow weight loss
can be ascribed to the transformation of a thermally unstable
polymer (containing C, H, O, and S) to a stable nitrogen and sulfur
co-doped carbonaceous materials21. This further proves that 2,4-
diaminobenzenesulfonic acid was not simply embedded inside the
synthesized colloidal spheres.
The use of other aromatic polyamines such as 2,4,6-triamino-
pyrimidine, 4,4’,4’’-triaminotriphenylamine, 1,2,4-benzenetriamine,
and 1,5-diaminonaphthalene to replace melamine succeeded in
generating nitrogen and sulfur containing colloidal spheres also.
The corresponding FESEM images of the products are shown in
Figure 5. Hence, this is a general route toward the synthesis of
nitrogen and sulfur co-doped carbon spheres from 2,4-diamino-
benzenesulfonic acid and aromatic polyamines.
To better understand the nature of 2,4-diaminobenzenesul-
fonic acid, electrospray ionization mass spectrometry was used for
the analysis of mixture of 2,4-diaminobenzenesulfonic acid,
formaldehyde and ammonia that stirred at 25 °C for 3 min. As
shown in Figure S4, the spectrum of the three-component mixture
contains peaks at m/z 423.1 in the m/z range from 100 to 800,
which corresponds to the species [2M-H]- of the Schiff base formed
from 2,4-diaminobenzenesulfonic acid and formaldehyde. We thus
propose that imine groups were formed via Schiff base reaction.
This is consist with the fact that the formation of Schiff base from
aromatic amine and aldehyde could conduct under room
temperature22. Besides, mixture of melamine, formaldehyde and
ammonia was also analyzed by LC-MS and no other products were
detected (Figure S5 ). Although melamine is also a primary amine, in
fact, the reaction of -NH2 groups of melamine with formaldehyde
should be reliably conducted at a temperature of 50 °C16, reflecting
that 2,4-diaminobenzenesulfonic acid is more active than
melamine. So, it is reasonable to say that in the reaction mixtures,
2,4-diaminobenzenesulfonic acid convert to Schiff base
preferentially and then melamine, a primary amine with higher
concentration in the mixtures, add to the newly formed imine (-
C=N-) groups and form polymeric colloidal spheres.
Figure 5. FESEM images of the products obtained from the reaction
of (a) 2,4,6-triaminopyrimidine, (b) 4,4’,4’’-triaminotriphenylamine,
(c) 1,2,4-benzenetriamine, and (d) 1,5-diaminonaphthalene with
2,4-diaminobenzenesulfonic acid and formaldehyde, respectively.
It is worthy to note that this general method enables nitrogen
To further illustrate the role of 2,4-diaminobenzenesulfonic and sulfur co-doped carbon spheres to be easily scaled up to
acid, supplementary experiment was carried out in the absence of
2,4-diaminobenzenesulfonic acid while keeping the other reaction
conditions the same as those of this synthesized colloidal spheres.
As a consequence, the reaction solution kept clear and no solid
spheres obtained. This indicates that the 2,4-diaminobenzenesulfo-
nic acid is indeed necessary to generate nanospheres. Besides 2,4-
diamino-benzenesulfonic acid, two other aromatic compounds,
namely, an aromatic sulfonic acid without amino group (benzene-
sulfonic acid), and a diaminobenzene without sulfonic acid group
(1,3-diaminobenzene), were employed to synthesize colloidal
spheres under the same reaction conditions. Even by prolonging the
reaction time to 375 min, the reaction solution kept clear when
benzenesulfonic acid presented. Despite the reaction solution
turned to turbid after 28 min when 1,3-diaminobenzene employed,
but the obtained nanoparticles were non-spherical and were agg-
lomerated. While, the particles are spherical and in nonagglomer-
ated state with the presence of 2,4-diaminobenzenesulfonic acid as
motioned above. These are clear indication that amino groups on
benzenesulfonic acid are essential for the formation of colloidal
particles and the sulfonic acid group could inhibit the aggregation of
the particles. In another words, 2,4-diaminobenzenesulfonic acid
multigram level with yield up to 90.3% with no special apparatus
(Figure S7). X-ray photoelectron spectroscopy (XPS) analysis was
employed to investigate the electric states of the surface elements
of NS-CSs and the results are shown in Figure 6. The full spectrum
of NS-CSs shows four typical peaks of C 1s, N 1s, O 1s and S 2p24
.
The C 1s peak of the NS-CSs can be split into two peaks including C-
C (ca. 284.6 eV), C-O/C-N/C-S (ca. 286.3 eV, 288.5 eV) 24, 25. The N 1s
spectrum of the NS-CSs shows peaks centered at 398.4 eV and
399.9 eV, which can be assigned to pyrrole N and pyridinic N,
respectively25, 26. Meanwhile, the S 2p spectrum of the NS-CSs
shows peaks centered at 163.9 eV, 164.9 eV, and 167.8 eV, which
corresponds to C–S, C=S, and C-SOx bonds18, 25. These results
demonstrate that nitrogen and sulfur atoms have successfully
incorporated into the NS-CSs. Besides, XRD and Raman analyses of
the material were also conducted and the results indicated that
carbon of the material is amorphous (see Figure S8 in supporting
information).
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94.2% selectivity with an ethylbenzene conversion of 93.9%. While,
the conversion of ethylbenzene was only 16.1% with the selectivity
85.0% for acetophenone when CSs used, which was almost the
same with control experiment. The conversion reached to 72.2%
with the selectivity of 90.3% for acetophenone when N-CSs used,
reflecting the enhancement of catalytic activity via N-doped. The
94.2% selectivity and 93.9% conversion, a much better than those
of the test samples, catalyzed by NS-CSs proved the further
improvement of catalytic activity via N and S co-doped (see Table
S2 in supporting information for the detailed results of the
oxidation). This result is comparable to or even better than those of
nitrogen-doped graphene or nitrogen, phosphorus, and sulfur co-
28
doped carbon catalysts27,
. Besides, the catalyst stability was
tested with five cycles, during which the selectivity of the catalyst
didn’t change, although the conversion rate decreased gradually
Figure 6. (a) Survey XPS spectra of NS-CSs, and high resolution (see Figure S10 in supporting information).
spectra for the elements of (b) C, (c) N, and (d) S.
Conclusions
The porous properties of CSs, N-CSs and NS-CSs were
characterized by nitrogen adsorption−desorption experiments. The
NS-CSs exhibit typical I type adsorption desorption isothermal,
indicating the porous structure of NS-CSs (Figure S9 and Table S1).
The porous structures are beneficial for catalysis.
In summary, we developed a newly designed strategy to synthesize
nitrogen and sulfur containing colloidal spheres from diamino-
benzenesulfonic acid and aromatic polyamines by a facile Schiff
base formation and addition of amino group to the imine group (-
C=N-) of Schiff base under ambient condition, avoiding high
reaction temperature, complicated time-consuming processes, or
expensive starting reagents. The well-distributed N and S co-doped
carbon spheres were obtained after carbonization of the formed
colloidal spheres. When used as a catalyst for selective oxidation of
ethylbenzene in aqueous solution, the prepared NS-CSs showed
comparable activity and selectivity compared with other nitrogen-
doped graphene layered carbon or nitrogen, phosphorus, and sulfur
co-doped hollow carbon shell catalysts, reflecting the co-doping
effect achieved through the successful co-doping of both N and S in
the carbon nanospheres. These findings provide a new strategy for
the design and synthesis of well-distributed N and S co-doped
carbon materials from large-scale industrial chemicals for a variety
of fundamental studies and applications, including catalysts, sorbing
materials, and high-performance electronics.
To test the catalytic performance of this NS-CSs for the
selective oxidation of C-H bonds, ethylbenzene was chosen as a
substrate to explore the efficiency. For comparison, control samples
including the carbonized resorcinol/formaldehyde spheres (denote
as CSs) and the carbonized melamine-formaldehyde colloidal
spheres (denote as N-CSs), were also tested.
OH
O
O
O
Cat, TBHP
H2O
H
OH
+
+
+
80 o
C
Conflicts of interest
There are no conflicts to declare
Acknowledgements
This work is supported by the Joint Funds of the National Natural
Science Foundation of China (U1608223), National Natural Science
Foundation of China (21576044, 21536002, 21576039), the Science
Fund for Creative Research Groups of the National Natural Science
Foundation of China (21421005), and Program for Innovative
Research Team in University (IRT_13R06).
Figure 7. Catalytic activity of different carbon materials for the
oxidation of ethylbenzene in aqueous phase.
We firstly examined the oxidation of ethylbenzene in aqueous
phase with tert-butyl hydroperoxide (TBHP) as the oxidant without
using catalyst. The conversion of ethylbenzene was only 15.7% with
the selectivity 87.1% for acetophenone (Figure 7). Then we used
NS-CSs as the catalyst for this reaction. As expected, NS-CSs
activated ethylbenzene at 353 K to generate acetophenone in
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New Journal of Chemistry
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DOI: 10.1039/C8NJ02948B
TOC:
A facile synthesis of well-distributed N and S co-doped carbon spheres and its
enhanced activity on selective oxidation of ethylbenzene.