Cobalt catalysts embedded in N-doped carbon derived from cobalt porphyrin via a one-pot method for ethylbenzene oxidation

Article abstract of DOI:10.1016/j.molcata.2015.07.011

Abstract A template-free method was employed to synthesize cobalt catalysts embedded in N-doped carbon (Co-N-C) through heating cobalt(II) meso-tetraphenyl porphyrin (CoTPP) precursor at different temperature (600°C, 700°C, 800°C). These catalysts were characterized by techniques such as BET, TG-DTA, TEM, STEM, XRD, Raman and XPS. The selective oxidation of ethylbenzene under solvent-free condition with molecular oxygen was carried out to explore the catalytic performance of Co-N-C. 13.4% conversion of ethylbenzene with 73.3% selectivity to acetophenone was achieved over Co-N-C and the catalytic performance of the catalyst was similar to the fresh even after six runs (i.e., 12.5% of ethylbenzene conversion and 72.2% of selectivity to acetophenone). The results show that the stable catalytic performance of the catalysts may be attributed to the stable N-doped graphitic carbon, the stability of carbon-encapsulated Co nanoparticles and the synergistic effect between N-C structure and cobalt oxide.

Full text of DOI:10.1016/j.molcata.2015.07.011

Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Cobalt catalysts embedded in N-doped carbon derived from cobalt
porphyrin via a one-pot method for ethylbenzene oxidation
Lingling Fu, Yuan Chen, Zhigang Liu∗
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan
410082, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2015
Received in revised form 7 July 2015
Accepted 10 July 2015
Available online 21 July 2015
A template-free method was employed to synthesize cobalt catalysts embedded in N-doped carbon
(Co–N–C) through heating cobalt(II) meso-tetraphenyl porphyrin (CoTPP) precursor at different tem-
perature (600 ◦C, 700 ◦C, 800 ◦C). These catalysts were characterized by techniques such as BET, TG-DTA,
TEM, STEM, XRD, Raman and XPS. The selective oxidation of ethylbenzene under solvent-free condition
with molecular oxygen was carried out to explore the catalytic performance of Co–N–C. 13.4% conversion
of ethylbenzene with 73.3% selectivity to acetophenone was achieved over Co–N–C and the catalytic per-
formance of the catalyst was similar to the fresh even after six runs (i.e., 12.5% of ethylbenzene conversion
and 72.2% of selectivity to acetophenone). The results show that the stable catalytic performance of the
catalysts may be attributed to the stable N-doped graphitic carbon, the stability of carbon-encapsulated
Co nanoparticles and the synergistic effect between N–C structure and cobalt oxide.
Keywords:
N-doped carbon
One-pot method
Cobalt porphyrin
Synergistic effect
Selective oxidation of ethylbenzene
© 2015 Published by Elsevier B.V.
1. Introduction
any assisted reaction condition or catalyst pretreatment. Ma et al.
[13] synthesized N-doped partially graphitic mesoporous carbon
Traditional carbon materials, such as activated carbon, carbon
black, and many nano-carbon materials like carbon nanofibers, car-
bon nanotubes, and graphene are not only employed as supports in
heterogeneous catalysis [1,2] but also utilized as catalysts in many
catalytic reactions, including the selective oxidation of benzene [3]
and the reduction of nitrobenzene [4]. In these catalytic systems, it
was observed that doping with heteroatoms such as N [5,6], B [7,8],
P [8], S [9] could improve the catalytic activity of the carbon mate-
rials. Among these, N-doped carbon materials have attracted much
interest due to their unexpected catalytic performances. There are
three common bonding configurations of N when a nitrogen atom is
framed within a carbon lattice, i.e., quaternary (or graphitic), pyri-
dinic and pyrrolic [10]. The spin density and the charge distribution
of the carbon atoms in a given N-doped carbon framework are
influenced by the neighbor nitrogen dopants, which induce acti-
vated regions on the surface [11]. Besides, Yu et al. [12] reported
that N-doping could increase the zigzag edges/defects of carbon
materials which may act as catalytic active sites to the reaction.
Xu et al. [6] prepared microporous graphitic carbon nitride mate-
rial for Knoevenagel reactions which can be used readily without
for nitrobenzene reduction which shows excellent catalytic per-
formance for nitrobenzene reduction reaction with 88.6% yield to
aniline.
Transition metal (Co, Fe, Mn)–nitrogen–carbon hybrids, which
are regarded as low-cost, efficient and durable alternatives for the
replacement of Pt-based electrocatalysts, have attracted intensive
research interests in last decades. However, most of the appli-
cations of M–N–C catalysts are focused on the oxygen reduction
reaction [14,15] or oxygen evolution reaction [16] in which M–N–C
catalysts show almost no catalytic effect on methanol.
Metalloporphyrins are well known for their high performance
in many biological functions because they can be employed to
mimic natural enzymes as active reaction sites [17]. However, the
direct application of metalloporphyrin has encountered great chal-
lenge due to the difficult recovery and easy degradation caused
by self-destruction [18]. Heat treatment of metalloporphyrins has
been proved to be an effective approach to enhance the catalytic
performance as well as improve the recyclability [19]. Therefore,
with unique carbon-rich macrocycle and inherent metal-nitrogen
coordination, metalloporphyrins, especially cobalt porphyrin, iron
porphyrin, and manganese porphyrin, are attractive precursors
for one-step synthesis of M–N–C catalysts. What’s more, catalytic
performance of M–N–C catalysts in selective oxidation of ary-
lalkanes has been explored. In our previous work, Liu et al. [19]
∗
Corresponding author.
E-mail address: liuzhigang@hnu.edu.cn (Z. Liu).
http://dx.doi.org/10.1016/j.molcata.2015.07.011
1381-1169/© 2015 Published by Elsevier B.V.

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L. Fu et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
reported that Co–N–C/SiO2, which was prepared through heating of
supported cobalt(II) 5-(4-carboxyphenyl)-10,15,20-triphenyl por-
phyrin, exhibited high catalytic performance for ethylbenzene
oxidation. Moreover, the influence of heat treatment on catalytic
performance of Co–N–C/SiO2 has been explored. However, the cat-
alytic activity decreased sharply after the first run of use which
may be attributed to the easy decomposition of active sites such as
Co–N4–C at oxidation atmosphere [20].
In order to improve the stability of the Co–N–C catalysts, herein,
we synthesized cobalt catalysts embedded in N-doped carbon
through the pyrolysis of cobalt(II) meso-tetraphenyl porphyrin.
Herein, N–C is designed to not only prevent the growth of Co
particles but also stabilize Co valence via the ability of N–C in
donating and accepting electrons. Techniques such as BET, TG-
DTA, TEM, STEM, XRD, Raman and XPS are employed to investigate
their physicochemical properties. Furthermore, the selective oxida-
tion of ethylbenzene under solvent-free condition with molecular
oxygen as oxidant is carried out to investigate the catalytic perfor-
mance of Co–N–C catalysts.
Fig. 1. N2 adsorption–desorption isotherm plot of Co–N–C-800. Inset on top left:
BJH pore size distribution of Co–N–C-800.
2. Experimental
X-ray diffractometer D8-Advance (Bruker AXS, Germany). Raman
measurements were performed under ambient conditions using
a 532 nm (2.33 eV) laser in the back-scattering configuration on a
Mono Vista 2560 spectrometer from Princeton Instruments. X-ray
photoelectron spectroscopy (XPS) measurements were evaluated
on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg
K␣ radiation (h␯ = 1253.6 eV) or Al K␣ radiation (h␯ = 1486.6 eV).
The obtained binding energies were calibrated using the C 1s peak
at 284.6 eV as reference.
2.1. Preparation of catalysts
The compound cobalt(II) meso-tetraphenyl porphyrin (CoTPP)
was synthesized according to the literature [21]. Typically, distilled
pyrrole (4.69 g, 70.0 mmol) was added dropwise into a three-neck
flask containing a mixture of propanoic acid (250 mL), benzalde-
hyde (5.56 g, 52.5 mmol) and 4-carboxy benzaldehyde (2.62 g,
17.5 mmol), and then heated to reflux for 1 h. The obtained product
was cooled overnight, then filtered and purified. meso-Tetraphenyl
porphyrin was obtained. 1.0 g (1.6 mmol) of the as-synthesized
sample was dissolved in 100 mL N,N-dimethylformamide (DMF).
After loading 2.5 g (10.5 mmol) of CoCl2·6H2O in batches, the
mixture was heated to reflux under stirring until meso-triphenyl
porphyrin was exhausted. Cooling overnight, the achieved mixture
was filtered and washed repeatedly with deionized water, and the
product cobalt(II) meso-tetraphenyl porphyrin, denoted as CoTPP,
was yielded. CoTPP was loaded in a quartz boat and pyrolyzed
at different temperatures (namely, 600 ◦C, 700 ◦C and 800 ◦C) in
nitrogen atmosphere with the heating rate of 10 ◦C/min. The result-
ing cobalt-nitrogen doped carbons were denoted as Co–N–C-600,
Co–N–C-700 and Co–N–C-800. N–C-800 and CoOx-800, derived
from meso-tetraphenyl porphyrin and CoCl2·6H2O, respectively,
were prepared for comparison. Co/N–C-800, prepared by imprega-
nation method and Co–N–C/SiO2-800, with silica as support, were
synthesized for comparison as well.
2.3. Catalytic performance test
The selective oxidation of ethylbenzene with molecular oxygen
as oxidant was conducted in a 50 mL autoclave. 10 mL of ethylben-
zene and 30 mg catalyst were loaded in the reactor and then sealed
and raised pressure to 0.8 MPa with O2. Following that, temperature
was elevated to 120 ◦C and kept for 5 h.
The products were analyzed by gas chromatography gas chro-
matography (Shimadzu GC-2014 equipped with a capillary column
(RTX-5)) using a flame ionization detector with internal standard
method using bromobenzene and 1,4-dichlorobenzene as refer-
ence. The recovered catalyst was obtained by centrifugation, then
washed with ethanol and dried at 80 ◦C in air.
3. Results and discussion
3.1. Catalyst characterization
2.2. Catalysts characterizations
3.1.1. BET
Surface area was measured by nitrogen adsorption/desorption
at 77 K on a NOVA 1000e apparatus from Quantachrome Instru-
ments. The samples were degassed at 300 ◦C for 3 h prior to the
adsorption experiments. The Brunauer–Emmett–Teller (BET) and
Barrett–Joyner–Halenda (BJH) models were used to determine the
specific surface areas, pore volume, and pore sizes of the sam-
ples, respectively. TG-DTA of the sample was carried out on a
Shimadzu DTG-60(H) thermogravimetric analyzer, and the sam-
ple was heated from 20 ◦C to 850 ◦C at 10 ◦C/min in N2 atmosphere
with flowing rate of 30 mL/min. Transmission electron microscopy
(TEM) studies were obtained on a JEM-3010 high-resolution
transmission electron microscope operating at 200 kV. Scanning
transmission electron microscopy (STEM) images were recorded
on Titan G2 60-300 with image corrector operating at 300 kV.
X-ray diffraction (XRD) patterns were recorded with a powder
N2 adsorption–desorption isotherms and the corresponding
pore size distribution curves of Co–N–C-800 are shown in Fig. 1.
As shown in Fig. 1, the amount of absorbed N2 increases/decreases
monotonically at a high P/P0 (P/P0 > 0.9) without an adsorption
limit for Co–N–C-800. According to the International Union of Pure
and Applied Chemistry classification, the N2 sorption isotherms of
Co–N–C-800 belong to type IV shape which indicates the presence
of mesopores. There is a relatively large adsorption–desorption
hysteresis loop of H3 type at a relative pressure above 0.4, which
is because of the capillary condensation of N2 in the slit-shaped
nanopores [22]. The BJH pore size distribution illustrates that meso-
pores are non-uniform and the content is low when the pyrolysis
temperature is 600 ◦C. However, when the temperature rises to
700 ◦C and 800 ◦C, pore diameters are uniform and mainly dis-
tributed around 3.7 nm (Table 1). It is worth noting that the specific

L. Fu et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
93
Table 1
Physical properties of Co–N–C-600, Co–N–C-700, Co–N–C-800.
Table 2
I(D)/I(G) analysis of Co–N–C-600, Co–N–C-700, Co–N–C-800.
a
b
c
Samples
SBET (m2 g−1
)
Vtot (cm3 g−1
)
Dp (nm)
dV(d) (cc/nm/g)
Catalyst
I(D)
I(G)
I(D)/I(G)
Co–N–C-600
Co–N–C-700
Co–N–C-800
61
219
232
0.07
0.21
0.25
4.15
3.72
3.73
0.03
0.31
0.27
Co–N–C-600
Co–N–C-700
Co–N–C-800
536.0
639.5
783.8
551.5
638.0
771.5
0.9719
1.0024
1.0159
a
Surface areas calculated using the BET method.
Total pore volumes estimated from the N2 sorption isotherms at P/P0 = 0.98.
Mesopore diameters calculated from the N2 desorption branches using the BJH
b
c
generation of graphitic structures is heavily dependent on pyrol-
ysis temperature. In addition, cobalt nanoparticles can be seen
obviously in TEM images of 600 ◦C, 700 ◦C and 800 ◦C. Their par-
ticle sizes follow the order: 6 nm (600 ◦C) < 20 nm (700 ◦C) < 43 nm
(800 ◦C), which shows that the particle sizes of cobalt nanoparti-
cles become larger with the increasing of treating temperatures. It
can be concluded that high temperature may cause the aggregation
of cobalt nanoparticles. It is worth noting that cobalt nanoparticle
is embedded in N-doped carbon architecture and surrounded by
a few graphitic carbon layers (Fig. 3g), which is favorable to the
stability of catalysts [25].
method.
STEM is carried out to further demonstrate the element distribu-
tion in Co–N–C structures with Co–N–C-800 as the example (Fig. 3h
and i). The elemental mapping analysis of Co–N–C-800 (Fig. 3i) sug-
gests the presence of Co, C, N, and O components in the product
and the cobalt signal along with oxygen which indicates the exis-
tence of cobalt oxides. It has been reported that when exposed to
air under ambient conditions, the top surface layers of Co can be
easily oxidized by oxygen [26]. Therefore, we argue that the metal-
lic Co first forms during the pyrolysis process and then is oxidized
when exposed to air.
Fig. 2. TG-DTA curve of CoTPP.
surface area and total volume tend to increase with the elevated
temperature.
3.1.4. XRD
Fig. 4 presents the X-ray diffraction (XRD) patterns of Co–N–C-
600, Co–N–C-700 and Co–N–C-800. The wide and weak diffraction
peaks at 26◦ and 43◦ are ascribed to the (0 0 2) and (1 0 0) plane
of carbon. Relatively higher peak intensity of Co–N–C-700 and
Co–N–C-800 indicates a higher graphitization degree [27]. The
Co–N–C-700 and Co–N–C-800 display well defined peaks corre-
sponding to JCPDS card 43-1003, which suggests the existing of
Co3O4. The XRD patterns of Co–N–C-700 and Co–N–C-800 include
three characteristic peaks at approximately 44.3◦, 51.6◦, and 75.8◦,
which are attributed to metallic cobalt. However, peaks of Co3O4
and metallic cobalt have not been detected in XRD patterns of
Co–N–C-600 which is probably due to the small size of cobalt
nanoparticles and high dispersion in nitrogen-doped carbon [27].
As temperature increases, small nanoparticles tend to sinter to
grow up and exposed to be more detectable.
3.1.2. TG-DTA
TG-DTA analysis is conducted to further investigate the struc-
ture evolution of Co–N–C with increasing temperature (Fig. 2).
Despite decades of work in this field, there is still not fully under-
stood about this process. Initially, CoTPP losses crystal water at
around 95 ◦C and shows mass loss up to 8% as shown by broad
endothermic peak given in Fig. 2. Further, it presents total mass
loss up to 68% as measured according to the TG curve. The DTA
curve shows four decomposition temperatures at 445, 482, 545 and
738 ◦C, respectively, which indicates that CoTPP decomposes in four
stages [23] and the different stages are associated to the sequen-
tial thermal degradation of the porphyrin macrocycles [24]. These
exothermic peaks may be because of detachment of phenyl rings
from pyrole rings, rupture and detachment of cobalt from pyrole
rings, cleavage of phenyl rings and the decomposition of remaining
organic fragments. Finally, Co–N–C structure, i.e., N-doped carbon
with Co embedded in, comes into formation.
3.1.5. Raman
Raman spectroscopy is used to illustrate the graphitization pro-
cess and chemical bonding state of pyrolyzed CoTPP. All these
Co–N–C materials have an obvious D band at 1338 cm−1 which
is associated with defects, and a G band at 1567 cm−1 which is
corresponding to graphitic carbon (Fig. 5) [5]. All of the spectra
3.1.3. TEM
The morphology and structure of the Co–N–C composites
pyrolyzed under different temperatures (600 ◦C, 700 ◦C and 800 ◦C)
are examined by transmission electron microscopic (TEM) images
as presented in Fig. 3. As shown in Fig. 3a, the TEM image of
Co–N–C-600 shows that Co nanoparticles (6 nm) are embedded in
a flat sheet-like structure which is composed of amorphous carbon
(Fig. 3d). As for Co–N–C-700, the structure is twisted and spiral,
which features ringed graphitic layers is formed (Fig. 3b and e). As
the temperature increases to 800 ◦C (Fig. 3c and f), little change
in the overall structure of Co–N–C-800 is observed compared to
Co–N–C-700. Evidently, graphite carbon and amorphous carbon
are coexisted in Co–N–C-700 and Co–N–C-800. It can be clearly
seen that the graphitic spiral structures are not found at 600 ◦C
but appear at 700 ◦C and 800 ◦C. This phenomenon reveals that the
exhibit very weak peaks at about 452 cm−1, 499 cm−1, 595 cm−1
,
and 662 cm−1, corresponding respectively to the Eg, F22g, F32g, and
A1g modes of surface dispersed cobalt oxide [28]. In particular, the
broad Raman band near 600 cm−1 is characteristic of dispersed
cobalt oxide species (i.e., CoO), as reported in literature [29,30].
Increasing of I(D) and I(G) with rising temperature reveals the
enhanced graphitization degree of Co–N–C. The increased I(D)/I(G)
with the rising temperature (Table 2) indicates that the graphiti-
zation degree of carbon in the Co–N–C samples increases with the
elevated pyrolysis temperature. Among all these Co–N–C materi-
als, Co–N–C-800 possesses the highest I(D)/I(G) indicating that this
sample has the highest graphitization degree which may simulta-
neously produce more defects than the other samples [31]. Because

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L. Fu et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
Fig. 3. TEM and HRTEM images of Co–N–C catalysts. (a, d) Co–N–C-600, (b, e) Co–N–C-700, (c, f, g) Co–N–C-800, (h) STEM image and (i) elemental mapping of Co–N–C-800.
Fig. 5. Raman spectroscopy of Co–N–C-600, Co–N–C-700 and Co–N–C-800.
Fig. 4. XRD patterns of Co–N–C samples and JCPDS card #43-1003, #15-0806 for
the corresponding Co3O4 and metallic cobalt, respectively.

L. Fu et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
95
Fig. 6. XPS spectra of Co–N–C catalysts. (a) survey, (b) C 1s, (c) N 1s and (d) Co 2p spectra of Co–N–C-600, Co–N–C-700, Co–N–C-800.
Table 3
peaks: pyridinic N (398.6 eV), pyrrolic N (399.8 eV) and graphitic N
(401.3 eV), respectively [34].
The ratio analysis of the peaks in XPS spectra of the Co–N–C catalysts.
In the cobalt region (Fig. 6d), the band around 778.5 eV is
attributed to CoO [25]. Besides, peaks characteristic for oxidic Co
with the typical binding energy of 780.2 eV of the Co 2p3/2 and
796.2 eV of the Co 2p1/2 electrons are also found. Generally, most
cobalt oxides and hydroxides (e.g., CoO, Co2O3, Co3O4 and CoOOH)
have similar values. It is difficult to determine the oxidation states
of cobalt cations merely through Co 2p main lines. Therefore, satel-
lite peak information should be considered [35–39]. The weak
shake-up satellite peaks at 784.9 eV and 802.0 eV suggest that the
oxidic Co is mainly composed of Co3O4 [39], agreeing with the XRD
and Raman results. The low energy bands can be deconvoluted into
two peaks with the binding energies of 781.2 eV and 779.6 eV which
represent Co2+ and Co3+, respectively [40]. This agrees very well
with TEM results, which means that the very small particles are
oxidic and the bigger particles contain a Co core and a cobalt oxide
shell. In the literature [41], it is discussed that CoO on the surface
is not stable and therefore oxidized to Co3O4. Thus, it can be con-
cluded that the surface of all Co-containing particles in the catalyst
consists of Co3O4 while CoO and Co (reflected by XRD) might be
enriched in the bulk.
Catalysts
C (at%)
O (at%)
Co (at%)
N (at%)
Co–N–C-600
Co–N–C-700
Co–N–C-800
88.00
90.20
95.60
10.01
6.08
0.41
0.74
0.86
0.84
1.25
2.86
3.15
of rupture of N-doped carbon skeleton of CoTPP, the N-doped meso-
porous graphitic structure shows a gradual overlap of the D and G
bands [32]. The overlap is most obvious for the sample Co–N–C-
800 due to the further decomposition degree. The changing trends
of the Raman spectra agreed with the conclusion of the TEM images
and XRD analysis.
3.1.6. XPS
The X-ray photoelectron spectroscopy (XPS) analysis is per-
formed to gain further information on surface compositions and
surface chemical states of the catalysts (Fig. 6). The full XPS spec-
trum of Co–N–C catalysts is presented in Fig. 6a. Co, C, N, and O
elements can be found in the products, in consistent with the ele-
mental mapping results (Fig. 3i).
Among the surface composition in Co–N–C as depicted in
Table 3, the content of C is the largest part. Moreover, with the
heat temperature increases, the content of C in Co–N–C increases.
Apparently, heat is beneficial to produce more carbon [20]. The con-
tent of N follows the same trend with C while O follows the opposite
one. In the meantime, the content of Co increases slightly from
0.74% (600 ◦C) to 0.86% (700 ◦C) and remains similar with 800 ◦C
(0.84%).
All of these above observations manifest that Co–N–C catalysts
with cobalt nanoparticles embedded in high specific surface area
N-C structure have been prepared successfully through annealing
of cobalt(II) meso-tetraphenyl porphyrin (CoTPP).
3.2. Activity measurement
The C 1s spectrum is fitted according to the following car-
bon bonding environments: 284.6 eV (C C), 285.7 eV (C N), and
286.6 eV (C N) (Fig. 6b), which suggests the successfully doping of
N atoms in Co–N–C catalysts [33].
The high-resolution N1s peaks in Co–N–C catalysts are shown
in Fig. 6c. The N 1s XPS spectrum can be deconvoluted into three
The selective oxidation of ethylbenzene is chosen as the probe
reaction to measure the catalytic performance of as prepared
Co–N–C. The catalytic performances of catalysts estimated by TON
of cobalt are listed in Table 4. From Table 4 (Entries 1–3), we can see
that Co–N–C-800 show slightly higher conversions of ethylbenzene
and selectivity to acetophenone for ethylbenzene oxidation com-

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L. Fu et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
Table 4
Catalytic performance of Co–N–C catalysts for ethylbenzene selective oxidation.a
Entry
Catalyst
Conversion of ethylbenzene (%)
TON
Selectivity (%)
Acetophenone
Phenethyl alcohol
Benzaldehyde
1b
2b
3b
4b
5c
6b
7
Co–N–C-600
Co–N–C-700
Co–N–C-800
N–C-800
CoOx-800
Co/N–C-800
SN [19]
11.8
11.6
13.4
9.0
9.6
9.8
3.5
6.2
15.7
14.1
5.3
87.9
86.4
99.8
–
71.5
73.0
–
72.5
72.2
73.3
73.0
74.6
67.6
63.9
68.3
76.5
73.3
54.7
55.9
21.8
23.9
23.7
17.9
17.4
28.1
30.6
31.7
21.3
24.4
22.8
24.0
5.7
3.9
3.0
9.1
8.0
4.3
–
–
–
2.3
22.5
20.1
8
9
Co/SN [19]
–
–
Co–N–C/SN [19]
Co–N–C/SiO2-800
Co–N–C-800
Co–N–C/SiO2-800
10d
11e
12f
105.0
39.5
32.8
4.4
a
Reaction conditions: ethylbenzene 10 mL, O2 pressure 0.8 MPa, reaction time 5 h.
Catalyst 30 mg, temperature 120 ◦C.
b
c
d
e
f
Catalyst 8 mg, temperature 120 ◦C.
Catalyst 52 mg, temperature 120 ◦C.
Catalyst 30 mg, temperature 80 ◦C.
Catalyst 52 mg, temperature 80 ◦C.
Table 5
The ratio of Co3+/Co2+ of Co–N–C catalysts.
Catalysts
Co3+/Co2+
Co–N–C-600
1.14
Co–N–C-700
1.16
Co–N–C-800
1.28
pared with Co–N–C-600 and Co–N–C-700. The TON exhibits the
same order with conversion of ethylbenzene which illustrates that
Co–N–C-800 possesses better catalytic activity.
In the reported literature, probably attributed to the fact that the
redox potential of Co3+/Co2+ (1.808 V) is more positive than that of
the other metals, the amount of Co3+ on the surface of cobalt oxides
has a positive influence in catalytic activity [42,43]. Therefore we
conclude that the similar oxidation results may be attributed to the
similar content of Co and ratio of Co3+/Co2+ in these Co–N–C cata-
lysts. The ratio of Co3+/Co2+ of Co–N–C catalysts is shown in Table 5.
What’s more, the ethylbenzene conversion of Co–N–C-800 is a little
higher than the others which may be ascribed to the larger specific
surface area and more defects caused by graphitization from the
analysis of BET and Raman results.
In order to demonstrate the synergistic effect between N-C
structure and cobalt oxide, taken Co–N–C-800 as representative, we
compare the catalytic activity of Co–N–C-800, N–C–800, CoOx-800
and Co/N–C-800 (Table 4, Entries 3–6). Obviously, the ethylbenzene
conversion of Co–N–C-800 (13.4%) is higher than that of N–C-800
(9.0%), CoOx-800 (9.6%), and Co/N–C-800 (9.8%) which means there
is synergistic effect between N–C structure and cobalt oxide in
Co–N–C-800 to some extent.
Compared with SN mentioned in literature, N–C displayed
higher ethylbenzene conversion (9.0%) and selectivity to acetophe-
none (73.0%) than SN (solid silica nanoparticles) [19], ethylbenzene
conversion (3.5%) and selectivity to acetophenone (63.9%). And
Co–N–C-800 showed higher catalytic activity than Co/SN as well
which illustrates that the hydrophobic nature of supports may
facilitate the reaction. In the reported literature [44], the hydropho-
bic carrier is beneficial to the adsorption of nonpolar substrate,
ethylbenzene, and desorption of polar products of the reaction.
However, the catalytic performance of Co–N–C-800 was not as
good as catalysts Co–N–C with silica as support, i.e., Co–N–C/SN,
Co–N–C/SiO2-800 (Table 4, Entry 9 and 10). This result was consis-
Fig. 7. Reusability of Co–N–C-800 for ethylbenzene selective oxidation. Reaction
conditions: catalyst 30 mg, ethylbenzene 10 mL, O2 pressure 0.8 MPa, temperature
120 ◦C, reaction time 5 h.
tent with the phenomenon in literature [19]. That may be owing
to the entrapped role of N–C structure on cobalt nanoparticles in
Co–N–C as can be seen in TEM and HRTEM images listed above
(Fig. 3c and g), which may hinder the contact between the sub-
strate and cobalt nanoparticles while preventing the loss of cobalt
nanoparticles. But catalysts Co–N–C with silica as support possess
better exposure of cobalt nanoparticles [19].
The catalytic performances of Co–N–C-800 and Co–N–C/SiO2-
800 under milder conditions, mainly at low temperature, were
tested. As can be seen in Table 4 (Entry 11 and 12), the low conver-
sion of ethylbenzene and selectivity of acetophenone reveal that
the selective oxidation of ethylbenzene is difficult to react under
relative low temperature even with the existence of Co–N–C cata-
lysts. This suggests that the catalytic activities of Co–N–C catalysts
under milder conditions remain to be improved.
Additionally, in order to demonstrate the stability and reusabil-
ity of the catalyst material, six consecutive oxidation experiments
are performed (Fig. 7). After being reused up to 6 times, the cat-
alytic performance of Co–N–C-800 has only slightly changed, which
presents the Co–N–C structure is stable in the heterogeneous selec-
tive oxidation of ethylbenzene.

L. Fu et al. / Journal of Molecular Catalysis A: Chemical 408 (2015) 91–97
97
4. Conclusions
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In conclusion, we have demonstrated the synthesis of
a
novel mesoporous graphitic Co–N–C materials with high specific
surface area and large pore volume derived from cobalt(II) meso-
tetraphenyl porphyrin (CoTPP) via template-free approach, which
can be expanded to produce metal or metal oxide nanoparticles
embedded in N-doped carbon materials in one-step. The Co–N–C
catalysts exhibit remarkable catalytic activity and in the solvent-
free selective oxidation of ethylbenzene which may be ascribed to
the stable N-doped graphitic carbon, the synergistic effect of N–C
structure and cobalt oxide and the stability of carbon-encapsulated
Co nanoparticles.
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