Figure 2 . SEM and TEM graphs of (a, d, g) 24PCNFs, (b, e, h)
MnOx /24PCNFs and (c, f, i)
Zn@MnOx /24PCNFs. (j) EDX graphs of elements C, O,
Mn and Zn within sorbent Zn@MnOx /24PCNFs.
The morphology of the modified CNFs and the supported sorbents after
electrospinning, heat treatment and etching was characterized via SEM
(Figures. 2 and S2). As shown in
Figure. 2a, 24PCNFs exhibit the
characteristics of smooth surface and 3D continuously interconnected
networks with hollow carbon shells uniformly embedded. In addition, as
displayed in Figure. S2, supporters
CNFs, 0PCNFs, 8PCNFs 16PCNFs and
32PCNFs were also prepared to uncover the effect of the compositions of
spinning solution on the morphology of carbon nanofibers. It can be seen
that the morphology of CNFs (Figure. S2a) is smooth surface and no
significant pore structure have been observed, which is similar to that
of 0PCNFs (Figure. S2b). When the content of SiO2increased from 0 to 8 wt.%, the content of embedded hollow carbon
shells in 8PCNFs was less and still retained most of the morphology of
0PCNFs (Figure. S2c). With the increase of the content of hollow carbon
shells embedded on porous carbon nanofibers, the original
micro-structure of 0PCNFs was completely changed. However, the content
of hollow carbon shells embedded on 16PCNFs was significantly lower than
that of 24PCNFs, which show little potential for the purpose of
enlarging the surface area of CNFs (Figures. S2d and e). The content of
carbon shells embedded on carbon nanofibers is limited when the polymer
content in the electrospinning solution is certain. Due to the excessive
SiO2 content, the hollow carbon shells on 32PCNFs were
no longer distributed uniformly along the fibers, the aggregation
phenomenon increased, and the fiber
fracture was aggravated
(Figure.
S2f). In conclusion, the morphology of 24PCNFs were consistent with the
preset expectation of the optimal supporter.
Due to the surface hydrophobicity and chemical inertness,
KMnO4 oxidation was adopted in order to anchor the
manganese oxide uniformly on the carbon nanofibers based on the
reaction: MnO4- + C +
H2O → MnO2 +
CO32- +
HCO3- .19 As show in
Figure. 2b, the morphology of
MnOx /24PCNFs
oxidized under 30 °C is similar to that of 24PCNFs, and no obvious
observations involved manganese oxide particles have been found upon the
supporter 24PCNFs, indicating that the structure of porous carbon
nanofibers after the oxidation process is well preserved under 30 °C
(Figure. S3a). 24PCNFs were also
oxidized at 45 and 60 °C for increasing the loading of manganese oxides,
however, compared with MnOx /24PCNFs oxidized at
30 °C, fractures and cracks have been observed within the 24PCNFs
processed at 45 and 60 °C (Figures. S3b and c). Therefore, 30 °C is
selected as the optimal activation temperature of 24PCNFs, which is to
maintain the completeness and continuous characteristics of activated
MnOx /24PCNFs
sorbent.
MnOx /CNFs and
MnOx /XPCNFs were prepared under the same
oxidation parameters to investigate the effect of different structure
and morphology of carbon nanofibers on the loading content of manganese
oxide anchored on them. As displayed in the SEM images of sorbents
MnOx /CNFs and MnOx /XPCNFs
(Figures. S4a-f), the activated sorbents remained structural integrity,
except the sorbent supported by 32PCNFs, which appeared fractures
within the nanofibers
(Figure. S4f). The loading content
of element Mn in the sorbent was tested by ICP. The order of
MnOx /32PCNFs >
MnOx /24PCNFs >
MnOx /16PCNFs
> MnOx /8PCNFs >
MnOx /CNFs >
MnOx /0PCNFs based on the manganese oxide content
loaded is proposed
(Table S1). The results show that
the embedded hollow carbon shells greatly increase the number of anchor
sites and improve the loading of the active components. Furthermore, by
comparing the integrity of the oxidized CNFs and desulfurization
performance of the corresponding sorbents, 24PCNFs was seemed as the
optimal supporter and the sorbent MnOx /24PCNFs
was selected as ‘seed’ for further investigation for the bi-oxides
desulfurization sorbents.20
As shown in Figure. 2c, namely the
SEM graph of sorbent Zn@MnOx /24PCNFs, nano
particles with uniform size and evenly distribution appeared on the
surface of porous carbon nanofibers, of which the structure completeness
did not change obviously. In addition, sorbents
Zn0@MnOx /24PCNFs,
Zn2@MnOx /24PCNFs and
Zn3@MnOx /24PCNFs were also prepared to
investigate the effect of Zn2+ concentration in
hydrothermal solution on the morphology and performance of the
composited sorbents. As shown in Figure. S5a, manganese oxides were
found to be randomly scattered on supporters with bulk crystals in large
particle size when Zn2+ was not introduced in the
hydrothermal process. While for that of the sorbent
Zn@MnOx /24PCNFs, it was suggested that the
addition of Zn2+ could inhibit the formation of bulk
crystals, thus forming uniformly dispersed particles on 24PCNFs
(Figure. S5b). As far as the
concentration of Zn2+ reached up to 0.02 M, in
addition to nano particles anchored on the carbon nanofibers, nanowires
with length of 50-80 nm appeared within the nanofibers have been
observed in the sorbent Zn2@MnOx /24PCNFs (Figure.
S5c). Besides, the quantity of nanowire increased when the content of Zn
went up to 0.03 M (Figure. S5d). By analyzing the elemental mapping
graphs of sorbents
ZnY@MnOx /24PCNFs
in Figure. S5, the nanoparticles
anchored on porous carbon nanofibers could be attributed to zinc oxide
and manganese oxides, while the
nanowires between the fibers may be
the excessive zinc oxide that restricted by the attachment sites on
CNFs. The presence of nanowires can improve the sulfur capacity of the
composite sorbents only through the way of increasing the content of
active components (Table S1), but its utilization rate is low according
to their large size and the uneven distribution. Therefore, based on the
above analyses, sorbent Zn@MnOx /24PCNFs was
selected as the optimal composited sorbents, in which the active
components were uniformly anchored on porous carbon nanofibers, and the
active components that in the shape of nanowires and bulk crystals were
absent.
Figure. 2d shows the TEM image of PCNFs, the structure of nanofibers
embedded with hollow carbon shells can be clearly observed. The
structures of MnOx /24PCNFs
(Figure. 2e) and
Zn@MnOx /24PCNFs
(Figure. 2f) were similar to the
original PCNFs, verifying that the results from SEM. Figures. 2g-i
represent the corresponding enlarged TEM images of Figures. 2d-f,
respectively, showing that the carbon layer thickness of the hollow
carbon shells was about 12 nm. Compared with 24PCNFs, the surface of
MnOx /24PCNFs after oxidation treatment was darker
in many regions (Figure. 2h), which could be the anchored ‘seed layer’
of manganese oxides, as verified by elemental mapping images of
MnOx /24PCNFs
(Figure. S6). Evenly distributed
granular active components with diameter around 3.1-13.0 nm have been
observed in sorbent Zn@MnOx /24PCNFs. The EDX
elemental mapping images in Figure. 2j also prove the even distributions
of elements O, Zn and Mn.
Structural performance of various CNFs-derived supporters and loaded
sorbents has been investigated to reveal the variations under different
compositions of the spinning solution as well as the changes before and
after oxidation and hydrothermal processes. As shown in Figure. S7 and
Table S2, the smooth surface and absence of pores led to low specific
surface area (51.765 m2 g-1) of
CNFs, which is not conducive to gas adsorption. Therefore, PVP was added
in the spinning solution to modify the structural parameters due to its
low carbon yield after decomposition that increasing the
micro-pores.21-23 As a result, 0PCNFs exhibited
specific surface area of 275.077 m2g-1 and pore volume of 0.169 cm3g-1.
Further, SiO2 was also introduced to increase the pore
volume. After the introduction of SiO2,
the specific surface area of 8PCNFs
was 145.193 m2 g-1, the reduction
from 0PCNFs was due to the fact that part of micro-pores was destroyed
and only a small count of hollow carbon shells was generated in 8PCNFs.
With the increase of SiO2, the specific surface area of
16PCNFs, 24PCNFs and 32PCNFs was similar to that of 0PCNFs and
pore volume values increased
largely, which were 0.200, 0.228 and 0.247 cm3g-1, respectively (Figure. 3a ). The
adsorption-desorption isotherm curves of 16PCNFs, 24PNCFs and 32PCNFs
all showed type IV isotherms with a weak H4-type hysteresis loop (P
P0-1 > 0.5) according to
the IUPAC classification, suggesting the presence of slit shaped
mesopores.24,25 These results indicate that with the
increase of the content of hollow carbon shells embedded on porous
carbon nanofibers, the structure of pristine CNFs was completely changed
and exhibited positive effect of SiO2 on the enlargement
of surface area and pore volume.