Synthesis of efficient Co and N co-doped carbon catalysts with high surface areas for selective oxidation of ethylbenzene

Article abstract of DOI:10.1039/c8nj01402g

In this manuscript, Co and N co-doped carbon catalysts with high surface areas were prepared via the pyrolysis of cobalt nitrate and 1,10-phenanthroline monohydrate, using Mg(OH)₂ as a pore former, followed by acid etching. Techniques such as B

Full text of DOI:10.1039/c8nj01402g

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Synthesis of efficient Co and N co-doped carbon
catalysts with high surface areas for selective
oxidation of ethylbenzene†
Cite this:DOI: 10.1039/c8nj01402g
Yuan Li, Shanshan Jie,‡ Kun Li and Zhigang Liu
*
In this manuscript, Co and N co-doped carbon catalysts with high surface areas were prepared via the pyrolysis
of cobalt nitrate and 1,10-phenanthroline monohydrate, using Mg(OH)2 as a pore former, followed by acid
etching. Techniques such as BET analysis, TEM, HRTEM, XPS, and Raman and XRD spectroscopy were employed
to characterize the as-prepared catalysts. The results revealed that the pore formers were beneficial for
improving the physicochemical properties of the catalysts with higher specific surface areas, larger pore volumes
and more surface active sites in comparison with those of the sample without the addition of Mg(OH)2.
Moreover, the as-prepared catalysts show a remarkable catalytic performance for selective oxidation of
ethylbenzene with 99.8% ethylbenzene conversion and 99.9% selectivity to acetophenone. In addition, the
efficient catalytic performance of the catalysts was ascribed to the improved structure–activity relationship,
suitable ratio of Co2+ to Co3+ and the synergistic effect between Co and N-C.
Received 22nd March 2018,
Accepted 11th June 2018
DOI: 10.1039/c8nj01402g
rsc.li/njc
oxidation of hydrocarbons.20,21 However, the complex preparation
process of cobalt porphyrin generally prevents the wide use of these
1. Introduction
Selective oxidation of hydrocarbons which serves to activate catalysts. Mu’s group have prepared a kind of transition metal and
C–H bonds to give high value-added products such as alcohols, nitrogen dual-doped mesoporous graphene-like carbon nanosheet
aldehydes and ketones has attracted a lot of attention using an acid etching method22 and the desirable catalytic activity
recently.1–4 However, traditional catalysts for the selective oxidation of the catalyst was attributed to the introduction of transition metal
of aliphatic hydrocarbons face challenges such as limited conversion species and few-layer mesoporous carbon structures. Generally, a
of the substrates, low selectivity to the target products and being larger surface area is helpful to improve the activity of the catalyst,
difficult to reuse.5–8 Normally, the catalytic activity of the catalysts as the porous structures may achieve higher surface areas and
can be improved by the addition of noble metals such as Ag and Pt expose more active sites.23,24
or their oxides.2,9–11 However, due to the high cost of noble metals,
catalysts with non-noble transition metals such as Fe, Co, Ni and Mn high surface areas of cobalt and nitrogen co-doped carbon catalysts,
have been designed to replace them.12–17
a one-step synthesis strategy has been reported to prepare a porous
Herein, in order to simplify the synthesis method and achieve
In the past decade, carbon materials have become a promising cobalt and nitrogen co-doped carbon catalyst (p-Co–N-C), as shown
kind of catalyst owing to their unique physical and chemical in Scheme 1, using cobalt nitrate and 1,10-phenanthroline
properties, such as low cost and controllable structure and monohydrate as metal and nitrogen precursors and Mg(OH)2
characteristics.17,18 According to density functional theory (DFT), as the pore inducer, respectively. The selective oxidation of
nitrogen doping can adjust the charge density, spin density and ethylbenzene is used to investigate the catalytic performance of
energy bandgap.19 Therefore, nitrogen doping is a novel strategy to the as-prepared catalysts with tert-butyl hydroperoxide as the
modify the physical and chemical properties of carbon materials. oxidant. Furthermore, the as-prepared catalysts have been tested
Our previous work demonstrated that cobalt sites embedded in using techniques such as nitrogen adsorption–desorption iso-
N-doped carbon are highly active and stable for selective therms, TEM, HRTEM, XPS, and XRD and Raman spectroscopy.
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China.
2. Experimental
2.1 Materials
E-mail: liuzhigang@hnu.edu.cn; Tel: +86-731-8882-3327
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj01402g
Cobalt nitrate, 1,10-phenanthroline monohydrate, Mg(OH)2,
‡ These authors contributed equally to this work and should be considered
co-first authors.
carbon black, tert-butyl hydroperoxide (TBHP) and all of the
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electron microscopy (HRTEM) were carried out to investigate
the morphologies of the catalysts, which were obtained using a
Tecnai G2 F20 S-TWIN instrument operated at 120 kV and a
Philips CM200 FEG instrument operated at 200 kV. Powder
X-ray diffraction (XRD) patterns of the products were obtained
using a Japan XRD-6100 diffractometer with Ni-filtered Cu Ka
radiation (50 kV and 10 mA) in the 2y range from 101 to 801.
Raman measurements were performed under ambient conditions
using a 532 nm laser in a backscattering configuration on a Mono
Vista 2560 spectrometer from Princeton Instruments. X-ray photo-
electron spectroscopy (XPS) measurements were conducted on an
RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Ka
radiation (hn = 1253.6 eV) or Al Ka radiation (hn = 1486.6 eV).
The obtained binding energies were calibrated using the C 1s
peak at 284.6 eV as a reference. In addition, elemental analyses
were carried out using a Vario EL III instrument. The Co and Mg
content of the sample was analyzed using a simultaneous ICP-AES
allied analytical system (Optima5300DV, Perkin Elmer).
Scheme 1 The preparation of p-Co–N-C catalysts.
solvents are commercially available and of analytical grade and
can be used without further purification.
2.4 Activity measurement
2.2 Synthesis of Co–N-C and related composites
Selective oxidation of hydrocarbons was carried out in a 25 mL
Schlenk tube (Beijing Synthware Glass, Inc. Pressure Vessel, Heavy
Wall). At first, 15 mg of Co–N-C catalyst was added to the reactor
and then 3.0 mL of water, 1.0 mmol of ethylbenzene and 3.0 mmol
of TBHP were sequentially added. Then, the glass tube was sealed
and heated to 80 1C under magnetic stirring, and kept at 80 1C for
12 h. The reaction was cooled to room temperature, and 100 mL of
n-dodecane and 9 mL of ethyl acetate were added as an extracting
agent and internal standard, respectively. Finally, the catalyst was
filtered, and the sample was analyzed by gas chromatography
(Shimadzu GC-2014 equipped with a capillary column (RTX-5)).
After the reaction, the used catalyst was filtrated, washed with
ethanol, and then dried overnight at 80 1C under vacuum and
the recovered catalysts were reused for the next run.
A mixture including cobalt nitrate (0.29 g), 1,10-phenanthroline
monohydrate (0.36 g) and ethanol (30 mL) was heated to 60 1C
and stirred for 6 hours. Then after cooling down to room
temperature, the obtained khaki precipitate was recovered
and dried overnight at 80 1C in air. Following that, the solid
was ground into powder with Mg(OH)2 (0.36 g) and transferred
to a quartz boat, then heated at 800 1C and kept for 2 h at a
heating rate of 5 1C minÀ1 under a nitrogen atmosphere. The
sample was denoted p-Co–N-C-800. For comparison, the sample
heated at different temperatures was denoted p-Co–N-C-t
(t means the heating temperature such as 700 1C, 800 1C and
900 1C). The above samples were etched with 2 mol LÀ1 HCl for
4 h under stirring, and then filtrated and rinsed with water
before being dried at 80 1C in air for 12 h. Finally, the products
obtained were denoted p-Co–N-C-700H, p-Co–N-C-800H and
p-Co–N-C-900H, respectively.
The catalyst was synthesized under similar conditions without
the addition of Mg(OH)2 and denoted Co–N-C-700H. Moreover,
N-doped carbon without cobalt salt was also prepared under
similar conditions and denoted p-N-C-700H, and p-Co–C-700H
was prepared via a similar method but 1,10-phenanthroline mono-
hydrate was replaced with carbon black. Furthermore, the reused
p-Co–N-C-700H was recovered by filtration and washed with
ethanol before being dried at 80 1C overnight. After being reused
for four runs, the sample was denoted p-Co–N-C-700H-r4.
3. Results and discussion
3.1 BET analysis
The textural properties of the as-prepared catalysts were measured
by N2 adsorption–desorption analysis. As displayed in Fig. 1a, the
nitrogen sorption isotherms of the catalysts showed a typical type
IV isotherm with a hysteresis loop, indicating a mesoporous
structure, and the pore size distribution was centered at about
4 nm (Fig. 1b).25 The total surface area, pore volume and pore
diameter were calculated and the results are shown in Table 1.
Compared to p-Co–N-C-700, the surface areas of p-Co–N-C-700H
and p-N-C-700H were 578 m2 gÀ1 and 531 m2 gÀ1, respectively, and
2.3 Characterization of the catalysts
The surface areas of the samples were measured using a much larger than that of p-Co–N-C-700 (232 m2 gÀ1). In addition,
nitrogen adsorption/desorption technique at 77 K on NOVA the pore volume of p-Co–N-C-700H also increased to 1.976 cm3 gÀ1
1000e apparatus which was from Quantachrome Instruments. in comparison with that of p-Co–N-C-700 (0.812 cm3 gÀ1). Moreover,
The as-prepared catalysts were degassed at 300 1C for 3 h prior the surface area of Co–N-C-700H was 267 m2 gÀ1 with a pore volume
to the adsorption experiments. The Brunauer–Emmett–Teller of 0.994 cm3 gÀ1, which was a little smaller than those of p-Co–N-C-
(BET) method was used to determine the specific surface areas 700H due to the addition of Mg(OH)2. It was evident that Mg(OH)2,
and pore sizes of the samples, respectively. Transmission as a pore inducer, played a key role in the surface areas and pore
electron microscopy (TEM) and high resolution transmission sizes of the catalysts after acid etching. However, p-Co–C-700H
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Fig. 1 The nitrogen adsorption–desorption isotherms (a) and pore size distribution (b) of p-Co–N-C-700H, Co–N-C-700H, p-Co–N-C-700, p-N-C-
700H and p-Co–C-700H.
Table 1 The structural properties of Co–N-C catalysts
a
SBET
Pore volumeb
Pore diameter
Dv (nm)
Sample
(m2 gÀ1
)
(cm3 gÀ1
)
p-Co–N-C-700
p-N-C-700H
p-Co–N-C-700H
p-Co–C-700H
Co–N-C-700H
232
531
578
62
0.812
2.675
1.976
0.503
0.994
3.80
5.48
3.71
8.80
4.03
267
a
b
Surface areas calculated using the BET method. Total pore volumes
estimated from the N2 sorption isotherms at P/P0 = 0.98.
almost retained a comparably small surface area and pore
volume even after the addition of Mg(OH)2 and acid etching,
indicating that surface area and pore volume were determined
by the properties of the support, i.e. black carbon. Generally,
high surface areas would be favorable to uncovering more active
sites and enhancing the catalytic performance of the as-prepared
catalysts.24,26
3.2 TEM and SEM analysis
TEM and HRTEM techniques were employed to investigate the
morphology of the as-prepared catalysts. As shown in Fig. 2a,
cobalt nanoparticles with particle sizes of 12.8 nm can be
observed in p-Co–N-C-700. However, as shown in Fig. 2b, the
TEM image of p-Co–N-C-700H illustrated that there were many
cavities which were achieved by acid etching. Moreover, cobalt
nanoparticles with particle sizes of 4.9 nm appeared, indicating
that acid etching eliminated the large cobalt nanoparticles and
exposed comparably small cobalt nanoparticles, which may
enhance the catalytic performance of the as-prepared catalysts.
With respect to the HRTEM images of p-Co–N-C-700, the lattice
Fig. 2 TEM images of p-Co–N-C-700 (a) and p-Co–N-C-700H (b); the
insets (a and b) correspond to the particle size distribution histograms.
HRTEM images of p-Co–N-C-700 (c) and p-Co–N-C-700H (d). An SEM
surface morphology image of p-Co–N-C-700H (e), and the EDS spectra
of p-Co–N-C-700H along with elemental mapping of p-Co–N-C-700H
for carbon (purple), nitrogen (green), oxygen (red) and cobalt (blue).
spaces of the cobalt oxides and magnesium oxides were calcu- the elemental mapping images of p-Co–N-C-700H illustrated
lated to be about 0.16 nm and 0.13 nm, corresponding to the the existence of C, N, O and Co, which indicates the successful
interplanar distances of the cubic crystal cobalt oxide (220) doping of Co and N and the presence of well-dispersed active
facet and cubic crystal magnesia (620) facet, respectively. How- sites in p-Co–N-C as displayed in Fig. 2e.
ever, as shown in Fig. 2d, only two cubic crystals, a metallic
3.3 XRD analysis
cobalt (103) facet and cobalt oxide (311) facet, can be observed,
respectively. It was evident that the MgO on the surface had The crystalline structures of the catalysts were investigated by
been removed by acid etching. In addition, the EDS spectra and X-ray diffraction (XRD). As seen in Fig. 3, the broad peak at
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Fig. 4 Raman spectra of p-N-C-700H, p-Co–N-C-800H, p-Co–N-C-
700H, p-Co–C-700H and Co–N-C-700H.
Fig. 3 XRD patterns of p-Co–N-C-700, p-Co–N-C-700H, p-N-C-700H
and Co–N-C-700H.
3.5 XPS analysis
26.41 in the XRD patterns of the catalysts was ascribed to the
(002) plane of graphitic carbon.27 There were no other peaks
that appeared in the pattern of p-N-C-700H except for the peak
for graphitic carbon at 26.41. However, for p-Co–N-C-700,
characteristic peaks for CoO can be found at 36.51 and 77.61,
which were indexed to the (111) and (222) plane of CoO,
respectively (JCPDS 43-1004),28 and the peaks at 44.21, 51.51,
and 75.91 were attributed to the (111), (220) and (220) reflec-
tions of cubic Co.15 Moreover, a distinct diffraction peak for
MgO was found at 42.91, which was indexed to the (200) plane
of MgO, meaning that MgO had been mixed in the catalysts.
With respect to p-Co–N-C-700H and Co–N-C-700H, no peaks
can be observed except for the peaks for Co at 44.21 and 51.51.
This suggests that MgO, and some CoO, had been removed
after acid etching. The rest of the cobalt species in the catalysts
were fairly stable and could not be eliminated easily by acid
etching, which was consistent with the results of TEM and BET
analysis.
X-ray photoelectron spectroscopy (XPS) was carried out to acquire
more information about the surface compositions and the surface
chemical states of the catalysts. As illustrated in Fig. 5a, the full
survey spectra displayed the peaks of elements such as C, N, O and
Co, which were consistent with the results of elemental mapping
and the peaks of Co and N implied that N and Co were successfully
introduced. In the surface composition of p-Co–N-C shown in
Table 2 the largest content was of C. It was interesting to find that
the N content of p-Co–N-C-700H (3.89%) was higher than that of
p-Co–N-C-700 (2.73%) but p-Co–N-C-700H had a lower Co content
(0.86%) than p-Co–N-C-700 (1.96%). This may be due to acid etching,
which removed some unstable Co particles and led to the exposure
of N-functionalities.31 To verify the results more precise measure-
ments have been performed. As displayed in Table S2 (ESI†), although
an augmentation of the N content from 0.8 wt% to 5.7 wt%
was observed after etching, there is a significant decrease in the
cobalt content. Moreover, the Mg content ranged from 21.8 wt% to
0.2 wt%, indicating that Mg was almost removed after acid etching.
In order to investigate the binding states of the atoms in the
3.4 Raman spectroscopy
The graphitization process and chemical bonding state of as-prepared catalysts, high resolution XPS spectra were recorded and
cobalt catalysts can be illustrated by Raman spectroscopy. As are shown in Fig. 5b–d. As shown in Fig. 5b, the C 1s spectrum of
shown in Fig. 4, the peak at 1350 cmÀ1 is attributed to the D Co–N-C was fitted according to the following carbon bonding
band and originates from the disorder caused by lattice defects, environments: 284.6 eV (CQC), 285.7 eV (CQN) and 286.6 eV
and the G band at 1587 cmÀ1 represents the in-plane vibrational (C–N).32,33 The existence of CQN and C–N bonds indicated that N
modes of sp2 carbon atoms in graphene.28 Generally, the intensity atoms were successfully doped in the hybrid matrix.32 Additionally,
ratio of the peaks from carbon vibrations at the edges (D) and as displayed in Fig. S2 (ESI†), no obvious peaks were observed at the
from within the graphene layer (G) provided information about corresponding binding energies of the Mg 1s spectrum, which
the extent of planarity of the graphene layer.29 The values of further verified that the templates were removed during acid etching.
the ID/IG ratio of p-Co–N-C-700H, p-Co–N-C-800H, p-Co-C-700H, The high-resolution N 1s peaks of the Co–N-C catalysts are displayed
p-N-C-700H and Co–N-C-700H were 1.11, 1.13, 1.47, 1.05 and in Fig. 5c. The N 1s XPS spectrum can be deconvoluted into three
1.07, respectively. By comparing the ratios of p-Co–N-C-700H and peaks, i.e., pyridinc N (398.9 eV), pyrrolic N (399.5 eV) and graphitic
p-Co–N-C-800H it was determined that the value of ID/IG N (400.8 eV).34–37 Among the N species, pyridinic N may be able to
increased with increasing heating temperature, indicating that coordinate with the cobalt ions to form Co–N.38
the degree of graphitization of Co–N-C was elevated, which
The high resolution XPS spectra of Co are shown in Fig. 5d.
illustrated that the degree of graphitization of carbon in the Characteristic peaks of Co can be observed with the typical binding
Co–N-C samples increased as the heating temperature rose. In energies of 795.5 eV for Co 2p1/2 and 780.2 eV for Co 2p3/2. Generally,
comparison to p-N-C-700H, there was an obvious increase in most cobalt oxides and hydroxides (e.g. CoO, Co2O3, Co3O4 and
graphitization among the Co–N-C catalysts, which is attributed CoOOH) had similar values, and thus it was difficult to determine
to the incorporation of metallic atoms in the matrix.30
the oxidation states of the cobalt cations merely by using the Co 2p
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Fig. 5 XPS spectra of the as-prepared catalysts: (a) survey, (b) C 1s, (c) N 1s and (d) Co 2p spectra of p-Co–N-C-700 and p-Co–N-C-700H.
active sites.42 The as-prepared p-Co–N-C-700H may have superior
Table 2 Ratio analysis of the peaks in the XPS spectra of p-Co–N-C-700
and p-Co–N-C-700H
catalytic performance due to its well-dispersed cobalt nanoparticles,
suitable ratio of Co2+ to Co3+, and synergistic effect between Co and
Element content
p-Co–N-C-700
p-Co–N-C-700H
N-C.
C (at%)
O (at%)
Co (at%)
78.47
16.90
1.90
10.99
2.73
89.71
3.29
7.00
86.79
8.46
0.86
1.93
3.89
93.78
1.68
4.54
3.6 Measurement of catalytic performance
Co3+/Co2+ (%)
N (at%) total
Pyridinic N (%)
Pyrrolic N (%)
Graphitic N (%)
In this study, the selective oxidation of ethylbenzene was used
as the probe reaction to measure the catalytic performance of
the as-prepared catalysts, and the results are listed in Table 3.
From Table 3 (entries 1–3), it can be found that the catalysts
without acid etching have comparably low activity and selectivity
main peaks. Therefore, information about the satellite peaks to acetophenone. For example, the conversion of ethylbenzene
should be taken into consideration. The low energy bands can be over p-Co–N-C-700 was lowered to 62.9%, which was lower than
deconvoluted into two peaks with binding energies of 779.7 eV and that over p-Co–N-C-800 and p-Co–N-C-900, namely, 64.5% and
780.4 eV, which represent Co3+ and Co2+, respectively.39 The weak 67.7%, respectively. However, as seen in Table 3 (entries 4–6),
shake-up satellite peaks at 784.9 eV and 802.0 eV suggested that the the catalysts with acid etching achieved much higher activity
oxidic Co was composed of Co3O4.40 In the Co 2p spectra of Co–N-C than those without acid etching and their conversions of ethyl-
the band around 778.5 eV was attributed to Co0.41 As shown in benzene were up to 99.8%. In addition, p-Co–N-C-700H had a
Table 2, the amount of Co decreased from 1.90% to 0.86% after higher selectivity to acetophenone in comparison with that of
acid etching, but the amount of Co2+ increased from 6.35% to p-Co–N-C-800H and p-Co–N-C-900H. The selectivity to acetophenone
11.13% and the amount of Co3+ decreased from 69.50% to 21.44%, over p-Co–N-C-700H was as high as 99.9% and much higher than
suggesting that Co3+ was easier to remove by acid etching than the 95.3% and 91.7% selectivity over p-Co–N-C-800H and p-Co–
Co2+. The ratio of Co3+/Co2+ ranged from 10.99% to 1.93% after N-C-900H, respectively. Obviously, compared with p-Co–N-C-700,
acid etching. Previous work confirmed that the synergistic effect the conversion of ethylbenzene over p-Co–N-C-700H greatly
between the N-C and M moieties in M–N-C led to them acting as improved from 62.9% to 99.8%, and the selectivity to acetophenone
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Table 3 Catalytic performance of Co–N-C catalysts for the selective
oxidation of ethylbenzene
Select. (%)
Entry
Catalyst
Conv. (%)
1
2
3
4
5
6
7
8
9
p-Co–N-C-700
p-Co–N-C-800
p-Co–N-C-900
p-Co–N-C-700H
p-Co–N-C-800H
p-Co–N-C-900H
Co–N-C-700H
p-N-C-700H
62.9
64.5
67.7
99.9
99.8
99.3
47.0
77.8
64.4
77.7
72.8
80.1
99.9
95.3
91.7
—
22.3
27.2
17.4
0.1
4.7
8.07
91.3
17.4
17.5
Fig. 6 The reusability of p-Co–N-C-700H for the selective oxidation of
ethylbenzene. Reaction conditions: 15.0 mg catalyst, 1.0 mmol substrate,
3.0 mL H2O and 3.0 mmol TBHP (70 wt% in water) at 80 1C for 12 h.
81.2
82.5
p-Co–C-700H
increased from 77.7% to 99.9%. The XPS spectra showed that the shown in Fig. S3 and Table S1 (ESI†), but the total N content
total nitrogen and pyridinic-N content increased, while the peaks increased compared to that of the fresh catalyst. In addition,
for the pyrrolic N, graphitic N and cobalt content decreased compared to that of the fresh catalyst, the ratio of Co3+/Co2+
(Table 2 and Fig. 5c, d). Generally, Co nanoparticles, without being further decreased from 1.93% to 0.31%. Based on the experiments
fully wrapped by a carbon layer, were removed, leaving mesopores and characterization, the excellent catalytic activity and stability of
in the carbon materials. However, some Co nanoparticles inside p-Co–N-C-700H can be ascribed to its stable mesoporous structure
the N-doped carbon matrix were stable in acidic solution and and the synergistic effect between N-C and Co-N species.
remained in the carbon matrix after acid leaching.27,38 As a result,
the superior catalytic performance can be attributed to the acid
etching that removed magnesium oxides and surface metal oxides,
4. Conclusions
which may lead to more exposed active sites and enlarged specific
In conclusion, we have successfully synthesized Co and N co-doped
surface areas.30
porous carbon catalysts with high surface areas via the pyrolysis
of cobalt nitrate and 1,10-phenanthroline monohydrate, using
Moreover, as shown in Table 3 (entries 4 and 7), it can be
observed that the catalyst without adding Mg(OH)2, namely,
Mg(OH)2 as a pore former, followed by acid etching. Among the
Co–N-C-700H, showed poorer ethylbenzene conversion (47.0%)
and lower selectivity (almost zero) in comparison with p-Co–N-
activity for the selective oxidation of ethylbenzene using TBHP,
C-700H. It is apparent that Mg(OH)2, acting as a pore inducer,
played an important role in the catalytic performance of the
acetophenone. It was revealed that the pore former played an
catalysts. However, p-N-C-700H was found to possess a large
as-prepared catalysts, p-Co–N-C-700H exhibits the best catalytic
namely, 99.8% ethylbenzene conversion and 99.9% selectivity to
important role in the improvement of the catalytic performance
pore size and surface area but had poor catalytic activity, which
of p-Co–N-C-700H. By combining the results, the outstanding
suggested that pore size and surface area were not key factors
catalytic performance of p-Co–N-C-700H is ascribed to the
that influenced catalytic performance. In addition, ethylbenzene
improved structure–activity relationship, the suitable ratio of
Co2+ to Co3+ and the synergistic effect between Co and N-C.
conversation over p-Co–N-C-700H was 99.8%, much higher than
those over p-Co–C-700H (64.4%, table entry 9) and p-N-C-700H
(77.8%, table entry 8), respectively. It was interesting to find that
Conflicts of interest
either N or Co doping may promote the catalytic performance of the
catalysts. Moreover, N or Co co-doping may cause the synergistic
effect between the N-C and Co–N species in p-Co–N-C-700H, which
There are no conflicts to declare.
may lead to remarkable improvements to the catalytic performance
of the catalysts.
Acknowledgements
Additionally, it was worth mentioning that p-Co–N-C-700H
demonstrated attractive stability and reusability for the selective
the State Key Laboratory of Heavy Oil Processing in China (no.
oxidation of ethylbenzene. As seen in Fig. 6, even after four runs,
the catalytic activity of p-Co–N-C-700H-r4 remained very similar
(i.e. ethylbenzene conversion of 98.8% with a selectivity to aceto-
The authors gratefully acknowledge the financial support from
SKCHOP201504) and the Key Laboratory of Mineralogy and Metal-
logeny in Chinese Academy of Sciences (no. KLMM20150103).
phenone of 88.4%) to that of the fresh catalyst (i.e. ethylbenzene
conversation of 99.8% with a selectivity to acetophenone of 95.3%).
TEM and HRTEM images showed that p-Co–N-C-700H-r4 had a
References
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