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.