J. Cao et al.
Molecular Catalysis 505 (2021) 111507
and Co/MoS2-x showed a typical flower structure with size about
150ꢀ 200 nm. These nanoflowers are aggregated by MoS2 nanosheets in
order to reduce the total surface energy. By increasing Co content, the
surface of MoS2 nanosheets gradually becomes rough due to the inter-
action between Co and MoS2, thus resulting in a weakened flower-like
structure (Fig. 4c). For Co/MoS2-0.5, Co9S8 particles with large size
were found to deposit on the MoS2 surface because of excessive Co
addition (Fig. 4d). HRTEM images of MoS2 catalyst show a typical
lamellar structure with layer spacing of 0.65 nm (Fig. 5a and Fig. S2a).
In addition, it is obvious that most MoS2 particles have slab length of
10ꢀ 20 nm and layer number of 2–4 layer. A statistic distribution was
conducted by counting 200 MoS2 particles and the result is shown in
Fig. S3. The calculated average slab length and layer number is 13.1 nm
and 2.87 layer. This indicates the MoS2 catalyst is high dispersed, which
is advantageous to expose more active sites. Meanwhile, some irregular
fringes such as basal curvature and defects were also found. It has been
previously reported that the curved or defect-rich MoS2 slabs could
generate new active sites on the original inert basal planes, which is
beneficial to improve its catalytic performance in hyrodesulfurization,
hydrodeoxygenation and electro-catalysis [24,37,38]. For Co-doped
catalysts, the statistical MoS2 average size of Co/MoS2-0.1,
Co/MoS2-0.3 and Co/MoS2-0.5 was 14.2, 13.8 and 13.5 nm, respectively
(Table 3). The decreased size was caused by the inhibition effect of Co to
the growth of MoS2 crystallites during the high temperature sulfidation
process. Besides, no lattice fringes assigned to any CoxSy phase could be
observed for Co/MoS2-0.1 and Co/MoS2-0.3. Although the diffraction
peaks of Co9S8 appears on the XRD pattern of Co/MoS2-0.3, the particles
are too small to be observed. However, another group of lattice fringes
with interlayer spacing of 0.58 nm corresponding to (111) planes of
Co9S8 were observed for Co/MoS2-0.5 (Fig. 5d and Fig. S2b). The
approximate size of Co9S8 particles is 8ꢀ 10 nm, which is consistent with
the results of large particles observed by XRD and SEM. Elemental
analysis results of Co/MoS2-0.3 catalyst in Fig. S4 display a homoge-
neously distribution of cobalt, molybdenum and sulfur elements, which
further proves the formation of “Co-Mo-S” active phase.
Fig. 3. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of
MoS2 and Co/MoS2-x catalysts.
The surface valence states and species distribution of the catalysts
were analyzed by XPS. Fig. 6a and b show the Mo3d and S2p spectra of
MoS2 catalyst. Two main peaks located at binding energy of 229.0 and
232.2 eV are attributed to Mo3d5/2 and Mo3d3/2 of Mo(IV) species [39].
Besides, very few amount of Mo(V) and Mo(VI) species at 233.3 eV and
235.2 eV were found after carefully deconvoluting the Mo3d spectra.
This means that the Mo precursors are highly sulfided and most of the
Mo species exist in Mo(IV) active phase. As for the sulfur species, the
doublet at 161.9 eV and 163.0 eV are ascribed to S2p3/2 and 2p1/2 or-
bitals of S2ꢀ . No corresponding peaks of S22- species were detected on the
surface of MoS2. For Co-doped catalysts, Fig. 6c exhibits that a small
amount of Mo5+ and Mo6+ species still exists after the Co incorporation
and sulfidation process. Detailed fitting of Co2p3/2 envelope can be
conducted as follows: Co9S8 (778.0 eV), Co-Mo-S (778.7 eV), CoOx
(781.6 eV) and their corresponding satellite peaks (Fig. 6d) [40]. The
presence of CoOx for all Co-doped catalysts indicates not all of the Co2+
has been sulfided to cobalt sulfides. Only one major peak assigned to
Co-Mo-S was observed for Co/MoS2-0.1, while another cobalt sulfide
attributed to Co9S8 besides Co-Mo-S was found for Co/MoS2-0.3 and
Co/MoS2-0.5. This phenomenon is the same as XRD results and could be
understood as follows. When a small amount of Co is introduced, the Co
atoms are preferentially located at the edge site of MoS2 catalyst to form
the Co-Mo-S phase [41]. With the Co content reaching a threshold, the
edge sites of MoS2 particles are fully covered by Co. If further increasing
the Co content, the excess Co species will aggregate to form separated
Co9S8 particles.
Table 3
Physical properties of MoS2 and Co/MoS2-x catalysts.
Sample
Surface area
Pore volume
Pore size
(nm)
n(Co)/n
(Co +
Mo)a
MoS2 size
(nm)b
(m2/g)
(cm3/g)
MoS2
Co/
131
98
0.55
0.30
2.5, 9.4
2.5, 6.7
/
13.1
14.2
0.1
MoS2-
0.1
Co/
77
32
0.24
0.11
2.3, 8.6
2.3
0.3
0.5
13.8
13.5
MoS2-
0.3
Co/
MoS2-
0.5
a
Determined by ICP analysis.
b
Determined by HRTEM statistical analysis.
pore volume of MoS2 are 131 m2/g and 0.55 cm3/g, which are relatively
high values for reported unsupported MoS2 materials synthesized by
hydrothermal method. With the introduction and increase of Co, the
specific surface area and pore volume of catalysts decreased gradually,
indicating that Co species had successfully filled the pore channels
(Table 3). Moreover, the bimodal pore structure of MoS2 catalyst grad-
ually changed into single small pores after adding a large amount of Co,
which was probably caused by the pore blockage of large Co9S8
particles.
Fig. 7 shows the TPR profiles of MoS2 and Co/MoS2-x catalysts,
which is important to well comprehend the information about active
centers. For non-promoted MoS2, a sharp peak at 285 ◦C was attributed
to the reduction of non-stoichiometric sulfur at edge planes [42], thus
producing coordinative unsaturated sites (CUS). The broad reduction
The morphologies of MoS2 and Co/MoS2-x catalysts were observed
by SEM and HRTEM characterization. As shown in Fig. 4a and b, MoS2
4