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.