B. Shi et al.
Molecular Catalysis 456 (2018) 31–37
by the Fe-active sites, and the actual amount of Fe in the catalysts and
the microsphere size may influence the number of active sites and the
CO conversion.
Table 1
FTO performance over Fe-M catalysts.
Catalyst
Fe
3
O
4
Fe-Mn
5.4
Fe-Co
Fe-Ni
5.3
To obtain more in-depth information with respect to the local
structure, the Raman spectra were recorded at room temperature
CO conversion (%)
Selectivity, carbon based (%)
7.0
(
Fig. 3). Three obvious first-order Raman modes—E
g
, T2g, and A1g—are
shown at approximately 300, 460, and 670 cm , respectively, which
closely match those reported earlier for Fe [27]. The peaks centered
) represent symmetric and asymmetric bending of Fe-O
CH
4
17.3
5.4
9.4
15.6
4.3
6.5
12.7
10.2
4.5
18.8
3.1
27.3
3.4
14.6
12.9
6.1
−1
=
C
C
C
C
C
C
C
C
2
2
3
3
4
4
2
5
°
3 4
O
=
−1
at 334 cm (E
bonds, that at 473 cm (T2g) is due to the asymmetric stretching of Fe-
g
°
−1
=
11.7
3.8
4.8
8.5
−1
O bonds, and that at 635 cm−
stretching of O atoms along Fe-O bonds [28]. The strongest peaks re-
lated to the A1g mode of Fe , Fe-Mn, Fe-Co, and Fe-Ni were centered
at 684, 611, 677, and 674 cm , respectively. There is an obvious shift
to lower wavenumber for other ferrites compared with Fe because
(A1g) is attributed to the symmetric
°
9.2
=
=
-C
4
27.5
15.6
16.7
33.0
1.2
40.7
10.0
25.2
54.5
3.6
21.0
9.7
10.2
23.4
0.7
+
3 4
O
−
1
CO
-C
2
C
2
4
Without CO
-C
2
3 4
O
O/P (C
2
4
)
of the M-O vibrations (M=Mn,Co,Ni). These results further indicate
that the second metal is incorporated into the lattice matrix to form M-
O bonds with short-range order. These effects are commonly ascribed to
the decrease of particle size, which directly affects the force constants
and vibration amplitudes of the nearest-neighboring bonds [29].
Catalysts: 25 mg sample + 75 mg SiC.
Reaction conditions: 533 K; 2.0 MPa; GHSV, 4000 h−1; H
2
/CO, 1:1.
Meanwhile, the molar ratio of olefin to paraffin (denoted O/P) in the
-C range hydrocarbons is as low as 1.2. For Fe-Mn catalyst, the CH
selectivity decreases to 12.7%, while C -C olefin selectivity increases
to 40.7% and CO selectivity increases to 25.2%. This means that Fe-Mn
enables suppression of the formation of CH , while increasing the se-
lectivity of light olefin and CO . In contrast, for Fe-Co and Fe-Ni, the
CH selectivity increased to 27.3% and 32.8%, respectively, and the C
olefin selectivity dropped to 21.0% and 7.4%, respectively. The se-
lectivity to CO decreased to 10.2% and 8.0%, respectively. The sys-
C
2
4
4
3.3. Reduction behavior of Fe-M ferrite
2
4
2
The reduction behavior was investigated by H
shown in Fig. 4, and in situ XRD, with the results shown in Fig. 5. H
TPR revealed that Fe-M (M=Mn,Co,Ni) ferrites were easier to reduce
by H than pure Fe . The doping of Mn slightly favors a decrease of
the initial reduction temperature; nevertheless, the catalyst was not
completely reduced at 1073 K. However, in contrast, pure Fe was
2
-TPR, with the results
4
2
-
2
4
2
-
C
4
2
3 4
O
2
tematic characterizations are provided to clarify the difference of cat-
3 4
O
alytic performance and structure evolution in the following text.
completely reduced. Moreover, the reduction process of Fe-Mn at
temperatures higher than 900 K occurred much more slowly than for
the other three catalysts, because Fe and Mn can form a Mn
x
Fe1−xO
3.2. Structure of fresh Fe-M ferrite
solid-solution phase that can suppress the reduction of Mn Fe1−xO to
x
the Fe° phase. It is obvious that Fe-Co and Fe-Ni were much easier to
reduce. Fe-Co ferrite was totally reduced at 887 K, while Fe-Ni ferrite
was totally reduced at 822 K. While the doping of a second metal can
Fig. 1 displays the SEM images of the fresh ferrite. Generally, as can
be seen, the Fe O ferrite exhibits a spherical morphology that is uni-
3 4
form and monodisperse and composed of many tiny nanocrystals. For
other Fe-M (M=Mn,Co,Ni) ferrites, the particles still maintain spherical
morphology, but the microspheres are no longer monodisperse, parti-
cularly for Fe-Ni ferrite. In addition, the particle size of Fe-M
3 4
increase the dispersion of Fe O and decrease the grain size, the cata-
lysts can form Fe-Co or Fe-Ni alloys, which can decrease the reduction
temperature [30]. The reduction process was further studied by in situ
XRD under a H
that Mn could suppress the reduction of the Fe
To further investigate the phase evolution during the reduction
process, the Fe-M ferrite was characterized by in situ XRD under a H
atmosphere (Fig. 5). For all the catalysts, the spinel structure
2
atmosphere. Some previous studies [31] have reported
(
M=Mn,Co,Ni) ferrites (120–250 nm) were smaller than that of Fe
3
O
4
3 4
O
catalyst.
(
390 nm). The ICP results (Table 2) indicate the actual doping content
of elemental Mn was only 10.4% for Fe-Mn ferrite, while the doping
contents of Co and Ni are 29.4% and 32.8%, respectively. The actual
doping content for the second metal (M=Mn,Co,Ni) is lower than the
nominal content (33.3%). Mn atoms were found to have more difficulty
in penetrating the spinel structure compared with other metals. By ICP,
we also obtained the content of remnant Na in the samples and found
that the remanent content of Na was so little that the influence of
remnant Na on the catalytic performance can be ignored in this work.
XRD patterns of the as-prepared ferrite are shown in Fig. 2. The
diffraction peaks at 18.3°, 30.1°, 35.5°, 43.1°, 53.5°, 57.0°, and 62.6°
2
II
M
x
Fe3−x
O
4
(M=Fe,Mn,Co,Ni) was observed at room temperature.
With increasing temperature, the diffraction peaks became sharper and
narrower, indicating the growth of nanocrystals in the H atmosphere.
However, the phase transformation was quite different among these
3 4
catalysts. There was no new phase emergent below 350 °C for Fe O
2
[Fig. 5(a)]. However, after reduction at 350 °C for 1 h, the Fe° phase
appeared. With increasing time, the amount of Fe° also increased, while
no FeO peak could be observed. It seems that Fe O could be directly
3 4
[
Fig. 2(b)] could be assigned to the cubic inverse spinel Fe
Card No. 75-1609). The other three patterns of Fe-M (M=Mn,Co,Ni)
were similar to those of Fe , indicating the successful synthesis of
(M=Mn,Co,Ni) by the solvothermal method. The
3
O
4
(JCPDS
reduced to Fe°, and the formation of FeO phase was not observed.
Another possibility is that the transformation rate of intermediate FeO
to Fe° is too fast to be detected [32]. The result suggests that the phase
3 4
O
II
spinel M
x
Fe3−xO
4
transformation in the bulk regions for the Fe
3 4 2
O catalyst in a H at-
XRD peaks were collected at a slow scanning rate until the highest peak
counts were above 10,000 cps. Using the TOPAS software, the crystal
size and lattice parameters were calculated, and are listed in Table 2.
mosphere follows the order Fe →FeO→Fe°. For an Fe-Mn ferrite
catalyst [Fig. 5(b)], similar to Fe-Mn ferrite, no significant structural
change could be initially found with increasing reduction temperature.
3 4
O
Compared to pure Fe
shows a slight change, indicating that the second metal was successfully
doped into the lattice structure. The crystal size of pure Fe was
5.8 nm, whereas the mean crystal size was between 6.4 and 10.5 nm
calculated by Scherrer’s formula, smaller than that of Fe ferrite.
3
O
4
, the lattice parameter of Fe-M (M=Mn,Co,Ni)
After reduction at 350 °C for 2 h, the diffraction peaks of Mn
emerged, and, after reduction for 5 h, the catalyst was composed of
Mn and Mn Fe1−xO phases. Meanwhile, no Fe° phase was
detected, which could be caused by the doping of Mn, which can inhibit
Mn Fe1−xO phase, further reducing to Fe° and MnO, and thereby sta-
bilizing the active phase [33]. The reduction tendency of the Fe-Co
3 4
catalyst [Fig. 5(c)] was found to be quite different from that of Fe O
x
Fe1−xO
3
O
4
x
Fe3−x
O
4
x
1
3
O
4
x
As stated above, the CO conversion decreased with the addition of
the second metal. Moreover, the catalytic performance may be affected
33