Figure 4 . (a) XRD patterns of MnS/24PCNFs and (Zn, Mn)S/24PCNFs. (b) Fitted XPS spectra S 2p in (Zn, Mn)S/24PCNFs. (c) N2 adsorption-desorption curves and structural parameters, TEM graphs of (d ,e) MnS/24PCNFs and (f, g) (Zn, Mn)S/24PCNFs. (h) high revolution TEM graphs and lattice fringe analyses, (i) elemental mapping graphs of elements Zn, Mn and S of (Zn, Mn)S/24PCNFs.
Chemical compositions and states of elements in the desulfurized sorbents MnS/24PCNFs and (Zn, Mn)S/24PCNFs were characterized using XPS technology. As shown in Figure. S17a, the desulfurization process introduced element S into sorbents MnOx /24PCNFs and Zn@MnOx /24PCNFs. High-resolution XPS spectra of the S 2p in MnS/24PCNFs showed characteristic peaks at 161.90 and 163.11 eV, which could be derived from the bond of S-Mn-S, while the peaks at 163.73 and 164.91 eV were attributed to the bond Mn-S, implying that the main sulfide products were manganese sulfides (Figure. S18b).49-51 While for the XPS spectra of S 2p in sorbent (Zn, Mn)S/24PCNFs, the peaks at 161.25, 162.44 eV and 163.70, 164.85 eV should be attributed to the bonds of Zn-S and Mn-S, respectively.52 Besides, the proportion of bond Zn-S was much higher than that belonged to MnS when referring to the multiples fitting of the S 2p peaks (Figure. 4b). In addition, the characteristic peaks belonged to SO42-may be the result of the adsorption of water by SO2generated from the oxidation of H2S.53Moreover, by comparing the O 1s spectra of MnOx /24PCNFs and Zn@MnOx /24PCNFs before and after desulfurization, it was found that the relative content of Mn-O decreased, and the fitting peak of Zn-O disappeared in the O 1s spectra of (Zn, Mn)S/24PCNFs. These also indicate that desulfurization reaction is a replacement process between O2- and S2- (Figure. S17c-d).
Structural performance of desulfurized MnS/24PCNFs and (Zn, Mn)S/24PCNFs has been investigated and represented in Figure. 4c and Table S2. The pore volume and pore size increased from 0.140 cm3g-1, 7.957 nm of MnOx /24PCNFs to 0.164 cm3 g-1, 12.621 nm of MnS/24PCNFs, respectively, while the specific surface area decreased from 70.280 m2 g-1 of MnOx /24PCNFs to 52.119 m2g-1 of MnS/24PCNFs. By combining the TEM graphs of sorbent MnOx /24PCNFs displayed in Figure. 4d and e, it could be seen that the nanosized particles MnOx transferred into MnS particles with diameter about 8-12 nm, meaning that the pore structure of MnOx /24PCNFs before and after desulfurization changed obviously. The larger particle volume of the desulfurized products generated new structures with larger pore size and reduced the proportion of micro-pores that contributed a lot to the specific surface area. In the contrast, opposite trend in structural parameters of sorbents Zn@MnOx /24PCNFs and (Zn, Mn)S/24PCNFs has been observed. Specifically, the specific surface area and pore volume increased of from 113.854 m2 g-1, 0.180 cm3 g-1 of Zn@MnOx /24PCNFs to 198.300 m2g-1, 0.196 cm3 g-1of (Zn, Mn)S/24PCNFs, respectively (Table S2). The N2adsorption-desorption isotherm type (IV) and hysteresis ring type (H4) of sorbent (Zn, Mn)S/24PCNFs were similar to that of Zn@MnOx /24PCNFs, except for the sharp increasing in the region 0 <P P0-1< 0.1, representing the existence of massive micro-pores instead of macro- or meso-pores (Figure. 4c). The structural variations between sorbents before and after desulfurization were mainly attributed to the fact that O2- in manganese oxide was replaced by S2- in H2S during desulfurization, which increased the size of active components and thus blocked meso-pores into micro-pores, and this is why the specific surface area increased and pore size decreased from 6.336 to 3.949 nm in the meantime.54 TEM analysis was also performed to observe the morphology changes of (Zn, Mn)S/24PCNFs. The particle size of desulfurization products was found to be in range of 9.1-23.0 nm, which was increased as compared to that of fresh sorbent due to the fact that the MnS and ZnS are much larger than MnOx and ZnO in molecular size (Figure. 4f and g). High-resolution TEM images of (Zn, Mn)S/24PCNFs in Figure. 4h implied the presence of zinc sulfide and manganese sulfide according to the lattice fringes with spacing distances of 0.3345 and 0.2964 nm, respectively. In addition, elemental mapping graphs in Figure. 4i indicated the even distribution of the elements Zn and Mn after desulfurization, meaning the high stability of the as-prepared sorbent.
Furthermore, deconvoluted peaks of Mn 2p3/2 spectrum of MnS/24PCNFs were resolved into three parts, namely 640.73 and 642.03 eV, attributing to Mn2+ of MnS and MnO, respectively, while the peak located at 645.32 eV could be attributed to Mn4+ (Figure. S17e). However, the Mn 2p3/2 peak of used (Zn, Mn)S/24PCNFs was consisted of four separated peaks (Figure. S17f), the difference from MnS/24PCNFs was that the peak represented Mn4+ at 641.10 eV and the peak located at binding energies of 644.87 corresponded to the shake-up satellite. Moreover, as shown in Figure. S18g, the two peaks at binding energies of 1021.51 and 1044.56 eV were consistent with Zn2p3/2 and Zn2p1/2, respectively, indicating the oxidation state of Zn was Zn2+.55