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F. Zhang et al. / Journal of Catalysis 348 (2017) 212–222
2.6. Characterization
HRTEM images of Fig. 1g and h reveal that the Co NPs are well-
embedded in a few layers of N-doped amorphous carbon and no
aggregation phenomenon takes place for Co NPs, which is well con-
sistent with the literature reported results [44,45]. A representa-
tive high-angle annular dark-field scanning TEM (HAADF-STEM)
image further confirms a homogeneous distribution of C, N and
Co elements at Co@NMC-800 catalyst surface, and the Co atoms
are completely surrounded by a large amount of C and N atoms.
It should be mentioned that the difference between the elemental
mapping of cobalt and STEM results is probably due to the pres-
ence of tiny Co NPs or CoNx species.
Transmission electron microscope (TEM) was carried out on a
FEI Tecnai G2 F20S-Twin using an accelerating voltage of 200 kV.
For sample preparation, the powders were dispersed in ethanol
with the assistance of sonication, and one drop of the solution
was dropped onto a micro grid. XRD measurements were per-
formed on a Rigaku Ultima IV diffractometer using Cu-Ka radiation
as the X-ray source in the 2h range of 10–80°. The metal dispersion
was carried out using a Builder PCA-1200 chemical adsorption
instrument. Prior to testing, Co@NMC-800 catalyst was pretreated
in argon at 423 K for 60 min. The N2 adsorption-desorption iso-
therms were obtained on an ASAP2020 analyzer. Before measure-
ment, samples were degassed under vacuum at 393 K for 6 h.
Surface area of the samples was calculated by the Brunauer-
Emmet-Teller (BET) method, and pore volume and pore size distri-
bution were calculated using the Barrett-Joyner-Halenda (BJH)
model. Thermogravimetric analysis (TGA) was determined on a
Setaram Labsys Evo apparatus. The samples were heated in an alu-
mina pan from 30 °C to 1200 °C at a heating rate of 10 °C/min
under argon atmosphere. Fourier transform infrared (FTIR) spectra
were obtained using a Nicolet iS5 spectrophotometer (frequency
range from 4000 to 400 cmꢁ1) with KBr pellet. The metal Co load-
ing amount of Co@NMC-800 catalyst was determined by NexION
350 inductively coupled plasma mass spectrometry (ICP-MS). The
X-ray photoelectron spectra (XPS) was analyzed on the PHI-5702
instrument and the C1s line at 284.5 eV was used as the binding
energy reference.
The FTIR spectra prove that Co@NMC-600 has a weaker, but
similar characteristics of diffraction peaks with CoPc precursor,
such as 745 cmꢁ1 (CAH out-of-plane bending vibration),
1096 cmꢁ1 (CAH in-plane bending vibration) and 1321 cmꢁ1
(CAN stretching vibration), indicating CoPc begins to decompose
and porous polymer gradually formed at this stage (Fig. 2a). The
phenomenon is in accordance with the TGA and elemental analysis
results (Table S1). In contrast, other samples only with 1617 cmꢁ1
and 3432 cmꢁ1 two peaks appear in the catalyst surface, which are
corresponding to N-H deformation and stretching vibration, sug-
gesting the quantitative graphitization of CoPc complex. TGA anal-
ysis is also devoted to confirming the structure evolution of the
catalyst during thermolysis process (Fig. 2b). In terms of CoPc com-
plex, the weight loss of first step is approximately 0.5 wt% below
200 °C due to the physically adsorbed water molecules. The second
step is between 200 and 500 °C with a weight loss of 5.1 wt%,
which is ascribed to the decomposition of organic small molecular
impurities. And the weight loss of third step reaches up to 23.4 wt%
from 500 to 900 °C, which suggests the self-polycondensation and
graphitization of CoPc complex and the Co NPs are in situ gener-
ated during the process. The tendency curve of Co@NMC-600 is
analogous to CoPc complex except for the lower weight loss
(18.8 wt%). In contrast, the weight losses of rest catalysts are less
than 10 wt% even if the temperature rises to 800 °C under N2 atmo-
sphere, implying the excellent thermal stability. The textural prop-
erties of these catalysts are evaluated by N2 adsorption-desorption
analysis. As shown in Fig. 2c, the isotherms of all the catalysts are
of type IV in the IUPAC classification, which is characteristic of
mesoporous materials. The steep and high capillary condensation
steps (P/P0 = 0.6–0.9) demonstrate the uniform and well-
developed mesoporosity with large pore volume, which is in line
with the TEM observations. The pore size distribution curves dis-
play that the average pore size of all the catalysts is about
6.8 nm, which is 2 times higher than that of Co@NC-800 catalyst
(Table S2). The specific surface area and pore volume of
Co@NMC-800 (631 m2 gꢁ1, 0.878 cm3 gꢁ1) are almost 4 and 7 times
of Co@NC-800 catalyst (148 m2 gꢁ1, 0.128 cm3 gꢁ1). It is obvious
that the huge specific surface area and large pore size will benefit
chemical accessibility, promoting the adsorption/diffusion process
of substrate molecules during the reaction.
3. Results and discussion
3.1. Preparation and characterization of various Co-based N-doped
mesoporous carbon catalysts
The detailed preparation route of Co@NMC-T (T = 600–900 °C)
catalyst is illustrated in Scheme 1. Herein, the commercially avail-
able cobalt phthalocyanine is served as carbon, nitrogen and cobalt
sources, while the Ludox HS-40 colloidal silica as a hard template.
Firstly, the aforementioned two ingredients are assembled
together via physical adsorption and hydrogen bonding interac-
tions in EtOH-CHCl3 mixture. Secondly, the resulting composite is
subjected to a thermal treatment at various temperatures under
flowing nitrogen, and finally the silica hard template is removed
through HF or NaOH etching. For comparison, Co@NC@SiO2-800
and Co@NC-800 catalysts are prepared in a similar way, just with-
out the removal of silica template or direct thermolysis of cobalt
phthalocyanine precursor in the absence of silica colloid (the cor-
responding TEM and N2 adsorption measurements are shown in
Figs. S1 and S2). Besides that, our developed synthetic method is
highly reproducible and easy to scale up through the combination
of CoPc with other abundant and low-cost carbon precursors.
The morphology and microstructure of Co@NMC-T (T = 600–
900 °C) catalyst are investigated by transmission electron
microscopy (TEM). The TEM images in Fig. 1a–d clearly show the
generation of typical three-dimensional (3D) network structure
and a plentiful interconnected mesoporous channels are presented
in the as-prepared N-doped carbon catalysts. For Co@NMC-600
catalyst, one hardly observes the existence of Co NPs except for
N-doped mesoporous carbon polymer. As the temperature rises
to 700 °C, the Co NPs with less than 3.0 nm are gradually produced.
With further increasing the temperature from 800 to 900 °C, the Co
NPs with an average diameter of 16 3 nm and 27 5 nm can be
seen clearly for Co@NMC-800 and Co@NMC-900 catalysts. Thus it
can be seen that the thermolysis temperature has a significant
impact on the formation and size distribution of Co NPs. The
The surface constituents and chemical state of CoPc and
Co@NMC-T catalyst are further investigated by X-ray photoelec-
tron spectroscopy (XPS) and X-ray diffraction (XRD). As depicted
in XPS survey spectrum, the C, N, O and Co peaks are clearly pre-
sented in Co@NMC-T catalyst. The oxygen element can be classi-
fied into N-oxide and template residual oxygen species (Fig. 3a)
[46–48]. XPS detailed scan in the Co region of Co@NMC-800 and
Co@NMC-900 catalysts shows a weak peak at the binding energy
of 778.3 eV, corresponding to 2p3/2 level of Co (0). It is demon-
strated that the external surface exposed cobalt oxides or Co NPs
are successfully removed by HF etching. In terms of Co@NMC-
600 and Co@NMC-700 catalysts, which show two broad peaks at
binding energy of 780.4 and 796.0 eV, corresponding to Co 2p3/2
and 2p1/2 levels, respectively (Fig. 3b). The peak positions and the
separation of 15.6 eV between these two peaks indicate the