Figure 3 . (a) N2 adsorption-desorption curves and structural parameters, (b) XRD patterns and (c) FT-IR spectra of 24PCNFs, MnOx /24PCNFs and Zn@MnOx /24PCNFs. (d) XPS spectra of MnOx /24PCNFs and Zn@MnOx /PCNFs. Fitted XPS spectra of elements (e) Zn, (f) Mn, (g) high revolution TEM graphs and lattice fringe analyses of sorbent Zn@MnOx /24PCNFs.
After activation, the specific surface area and pore volume of sorbents MnOx /CNFs and MnOx /XCNFs decreased sharply and displayed a unified trend with the increased content of SiO2. Taking MnOx /24PCNFs as a sample, whose specific surface area and pore volume decreased of from 250.684 m2g-1 and 0.228 cm3g-1 of 24PCNFs to 70.280 m2g-1 and 0.140 cm3g-1 of MnOx /24PCNFs, respectively, which may be due to the blocking of micro-pores by the anchored manganese oxides (Figures. 3a and S8, Table S2).26 Interestingly, an increase in the pore size of MnOx /XCNFs sorbents have been observed, which is beneficial to alleviate the reduction of specific surface area after oxide loading to improve the gas adsorption performance. During the hydrothermal process, the attachment of zinc oxide generates new micro structures on the original basis, which is the reason why the specific surface area and pore volume (113.854 m2g-1 and 0.180 cm3g-1) of Zn@MnOx /24PCNFs are better than MnOx /24PCNFs. BET characterizations were also performed on ZnY@MnOx /24PCNFs sorbents to reveal the effect of loading content of ZnO on pore structures of sorbents. As shown in Figure. S9 and Table S2, the pore volume values of ZnY@MnOx /24PCNFs sorbents with different Zn2+ content were similar but the specific surface area decreased slightly with the increase of Zn2+contents in hydrothermal solution, which might be caused by the aggregation of excessive ZnO particles and the increase of ZnO nanowires.
Figure. 3b shows the XRD patterns of 24PCNFs, MnOx /24PCNFs and Zn@MnOx /24PCNFs. 24PCNFs possessed a relatively broad diffraction peak at around 25° for the presence of C, which corresponded to the (002) planes of graphite carbon.27,28 Besides, no added diffraction peaks of XPCNFs were observed to that of CNFs, indicating that no impurities were introduced in the etching process (Figure. S10). The broad diffraction peaks appeared at 25.2°, 37.3° and 65.6° of sorbent MnOx /24PCNFs could be attributed to the (002), (-111) and (020) planes of MnO2.29Characteristic peaks in the same location but with different intensities were found in the XRD patterns of sorbents MnOx /CNFs and MnOx /XPCNFs, implying that nanosized MnO2 was successfully loaded on carbon nanofibers with different structures but there were more manganese oxides loaded on sorbent with XPCNFs as supporter, which was influenced by their bigger surface area (Figure. S11). After hydrothermal growth of ZnO, weak diffraction peaks at 2θ = 34.4°, 36.2° and 56.6° were assigned to the (002), (101) and (110) planes of the ZnO (No. JCPDS 36-1451).30 Besides, the strong diffraction peaks at 2θ = 30.2° and 35.6° were assigned to the (220), and (311) planes of the ZnMnO3(No. JCPDS 19-1461).31 The above phenomenon indicated that the particles anchored on 24PCNFs were the composited oxides of Zn and Mn, in which the part of Zn2+ was directly loaded on manganese oxide as ZnO, while the other part was combined with manganese oxide to form ZnMnO3 within sorbent Zn@MnOx /24PCNFs. In addition, the trace of Mn3O4(No. JCPDS 13-0162) was also found after hydrothermal according to the strong diffraction peaks at 2θ = 29.9°, 35.3°, 12.7°, 43.0°, 53.2°, 56.7° and 62.3°, which could be assigned to the (220), (311), (400), (422), (511) and (440) planes of Mn3O4.32 The XRD patterns of Zn0@MnOx /24PCNFs, Zn2@MnOx /24PCNFs and Zn3@MnOx /24PCNFs were also showed in Figure. S12. It was found that MnO2 was easily reduced to Mn3O4 (No. JCPDS 80-0382) with better crystalline when there was no Zn2+ in the hydrothermal solution, which was verified by the sharp peaks of at 2θ = 18.0°, 28.9°, 30.9°, 32.3°, 36.0°, 44.4°, 50.8° and 58.4° within sorbent Zn0@MnOx /24PCNFs.33 While in contrast to sorbent Zn@MnOx /24PCNFs, strong and sharp diffraction peaks of ZnO appeared upon sorbent Zn2@MnOx /24PCNFs. With the further increase of Zn2+ concentration in hydrothermal solution, the diffraction peaks of ZnO in sorbent Zn3@MnOx /24PCNFs were much stronger, indicating the existence of ZnO nanowires with complete crystalline between carbon nanofibers, which was basically consistent with the results from SEM and elemental mapping images.
The FT-IR spectra of 24PCNFs, MnOx /24PCNFs and Zn@MnOx /24PCNFs are exhibited in Figure. 3c. Characteristic peaks at 3414, 1617 and 1278 cm-1 were similar in all the three samples, relating to the stretching vibrations of -OH, C=O and C-C, respectively.34 The carboxylic groups in the samples were mainly originated from the etching treatment. Si containing peaks were observed at 623 and 477 cm-1of 24PCNFs that belong to the stretching vibration of Si-O and O-Si-O from silicate, suggesting the completely removing of SiO2.35 After oxidation and hydrothermal processes, peaks that corresponding to Mn-O (613, 483 cm-1) and Zn-O (524 cm-1) were recorded, implying that the sorbents MnOx /24PCNFs and Zn@MnOx /24PCNFs have been successfully synthesized.36,37
The chemical composition and state of elements in sorbents MnOx /24PCNFs and Zn@MnOx /24PCNFs composites were examined by XPS, as shown in Figure. 3d, in which elements C, N, O and Mn were identified, while element Zn was traced in spectrum of Zn@MnOx /24PCNFs. High resolution spectrum of Mn 2p was presented to investigate the properties of Mn in the as-prepared sorbent MnOx /24PCNFs. As seen from Figure. S13a, the Mn 2p3/2 peak and Mn 2P1/2 peak could be decomposed to three parts, namely, Mn2+(641.40 eV), Mn4+ (642.13 eV) and Mn3+ (644.92 eV), respectively.38C 1s spectrum in Figure. S13b could be deconvoluted into three peaks, C-C (284.66 eV), C=N (285.42eV) and O-C=O (288.14 eV), and the presents of O-C=O bonds representing the partial oxidation of the surface carbon layer.39While the spectrum of N 1s could be fitted into four peaks, which corresponding to pyridinic N (398.28 eV), pyrrolic N (400.10 eV), graphitic N (402.25 eV) and oxidized N (406.60 eV), respectively (Figure. S13c).40,41 Deconvoluted peaks of the O 1s spectrum were centered at 529.58, 531.11, 532.59 and 533.97 eV, respectively (Figure. S13d). The lower binding energy located at 529.58 and 531.11 eV could be attributed to the O2- forming oxide with manganese (Mn-O-Mn, Mn-O-C) and the hydroxide (Mn-O-H), respectively.42,43While the latter two peaks were assigned to O-C=O and H2O.44The high resolution XPS spectra of the C 1s, N 1s, O 1s, Mn 2p and Zn 2p of Zn@MnOx /24PCNFs were presented in Figures. 3e and f, S18e-g. The Mn 2p3/2 peak were consisted of three separated peaks at 640.95, 642.32 and 644.68 eV, which were in accordance with Mn2+, Mn3+ and Mn4+, respectively.45It could be inferred that element manganese was mainly in the state of Mn2+ and Mn3+, and the relative content ratio of Mn2+/Mn3+ was close to 1:2, indicating that manganese oxide was partially in the form of Mn3O4. Figure. 3f displays two sharp peaks of Zn 2p3/2 and Zn 2p1/2 at binding energy of 1021.55 and 1044.64 eV, implying the oxidation status of ZnO and ZnMnO3 in Zn@MnOx /24PCNFs.46,47 The C 1s and N 1s spectra of sorbent Zn@MnOx /24PCNFs were almost the same as that in the sorbent MnOx /24PCNFs, which were both deconvoluted into three peaks, respectively (Figure. S13e and f). Deconvoluted peaks of O 1s spectrum were resolved into, 529.76, 530.89, 531.58, 532.47 and 533.57 eV, attributing to Mn-O-Mn (or Mn-O-C), Zn-O, Mn-O-H, O-C=O and H2O, respectively (Figure. S13g).47 Based on the above analyses, both the ‘seed layer’ of MnOx and Zn@MnOx nanoparticles were successfully anchored at 24CPNFs through strong chemical bonds.
High-resolution TEM images of sorbent Zn@MnOx /24PCNFs in Figure. 3g indicated the presence of ZnO, Mn3O4 and ZnMnO3. Specifically, the lattice fringes with spacing distances of 0.2474, 0.2138, 0.2981 and 0.2947 nm could be recognized, corresponding to the planes of (101) in ZnO, (400) and (220) in Mn3O4, and (220) in ZnMnO3, respectively. The above results are consistent with those observed by XRD and XPS. Besides, the HRTEM image of MnOx /24PCNFs was displayed in Figure. S14, no obvious lattice fringes were found, indicating that manganese oxides were existed in grains with small size and high dispersion.
Furthermore, the as-prepared MnOx /24PCNFs and Zn@MnOx /24PCNFs sorbents were applied to remove H2S at 500 °C for practical application. In Figures. 4a , S15 and S16, it was found that the active component MnOx and Zn@MnOx transferred to the products of MnS (No. JCPDS 88-2223) and (Zn, Mn)S (No. JCPDS 11-0513) after the desulfurization reaction, respectively. The representative peaks at 34.3° and 49.3° corresponded to the (200) and (220) planes of MnS, besides, the strong diffraction peaks at 26.5°, 28.2°, 30.1°, 46.8°, 51.1° and 55.6° were assigned to the (100), (002), (101), (110), (103) and (112) planes of the (Zn, Mn)S.48 However, XRD diffraction peaks of ZnY@MnOx /24PCNFs sorbents with different contents of active components were different after desulfurization (Figure. S16). It can be seen that the used Zn0@MnOx /24PCNFs sorbent exhibited diffraction peaks of MnS and MnO (No. JCPDS 78-0424), suggesting that Mn3O4 was transformed in the presence of H2S to MnS and MnO with poor utilization rate of Mn2+. XRD patterns of Zn2@MnOx /24PCNFs and Zn3@MnOx /24PCNFs after desulfurization showed that they were mainly consisted of (Zn, Mn)S and ZnO crystals. The strong ZnO diffraction peaks came from ZnO nanowires that were not fully involved in the desulfurization reaction. In conclusion, this is the reason for the much higher utilization rate of active components of sorbent Zn@MnOx /24PCNFs than that of the sorbents contained ZnO nanowires.