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[
31]
ous study. Therefore, the XAFS method, which is very suita-
ble to be used for the characterization of such amorphous or
microcrystalline materials with no long-range order, was used
to elucidate the microstructure of iron titanate crystallite in
FeTiOx catalyst in detail. Using X-ray absorption near-edge
structure (XANES) and extended X-ray absorption fine structure
(EXAFS) data in combination with DFT calculations to simulate
the electronic density, we concluded that, different from the
3
+
crystal structure of Fe species in hematite Fe O and Fe O /
2
3
3
2
3
4+
+
TiO supported-type catalyst, a homogeneous Fe ꢀ(O) ꢀTi
2
2
structure was clearly formed in the FeTiO catalyst, in which
x
3
+
4+
the Fe species and Ti species were strongly linked in an
edge-shared fashion (as shown by the structural model in Fig-
[31]
Figure 3. H
2 x 2 3 2
-TPR profiles of FeTiO catalyst, hematite Fe O , anatase TiO ,
[
34]
and ilmenite FeTiO .
3
ure 2D). The electronic inductive effect was confirmed to be
3
+
4+
present between Fe species and Ti species by XANES and
[
31,32,34]
X-ray photoelectron spectroscopy (XPS) results,
effective-
extent from the reduction processes observed for g-Fe O
2 3
3
+
ly reducing the electron density around Fe species thus lead-
ing to the enhancement of NO adsorption, oxidation ability
nanorods enclosed by abundant reactive {110} and {100} crys-
[27]
tal facets. However, we could still calculate the H /Fe molar
2
and finally the low temperature NH -SCR performance. Further-
ratio for the reduction peak at low temperature to confirm the
reduction process of Fe species in hematite Fe O below
3
3
+
4+
more, the Fe ꢀ(O) ꢀTi structure also showed high durability
2
2
3
to SO poisoning in the NH -SCR reaction mainly attributable
5008C, which is typically the maximum reaction temperature
2
3
to the close and homogeneous combination of Fe species and
Ti species as well as the easy decomposition of sulfate on Ti
for the NH -SCR reaction. As shown in Figure 3, the H /Fe
3
2
molar ratio for the reduction peak of Fe O at 4058C was calcu-
2
3
[
36]
sites. In previous studies by other researchers, the impor-
tance of the complex generated between the active phase and
catalyst support was also emphasized, such as the AgꢀOꢀAl
lated to be 0.17, which was exactly consistent with the reduc-
tion process from Fe O to Fe O . Further raising the reduction
2
3
3
4
temperature could lead to the formation of FeO or metallic
[
37]
0
species in Ag/Al O catalysts for the CH -SCR of NO
x
and the
Fe , yet different reduction mechanisms have been proposed
2
3
4
[
38]
CeꢀOꢀTi species in CeTiO catalysts for the NH -SCR of NO .
by former researchers including the two-stage reduction of
x
3
x
[
39,40]
Therefore, the study on the microstructure and chemical prop-
erties of such catalytically active species is an important and
universal issue in the field of heterogeneous catalysis.
3Fe O !2Fe O !6Fe
and the three-stage reduction of
2
3
3
4
[27,41,42]
3Fe O !2Fe O !6FeO!6Fe.
XANES measurements
2
3
3
4
could clearly discriminate the possible mixed Fe species in re-
duced Fe O , which will be discussed in detail below.
2
3
Based on the H -TPR results of TiO and Fe O , the H reduc-
2
2
2
3
2
H -TPR results of FeTiO catalyst and reference samples
2
x
tion peaks observed for the FeTiO catalyst could be only as-
x
Although the microstructure of iron titanate crystallites in the
FeTiOx catalyst has been clearly elucidated in our previous
study, the reducibility of the Fe species has not been well rec-
ognized although it is actually very important for the comple-
cribed to the reduction of Fe species, because the Ti species
were barely reduced under these conditions. In the FeTiO cat-
x
alyst with much higher NH -SCR performance than TiO and
3
2
Fe O , the Fe species started to be reduced at approximately
2
3
tion of the redox cycle in the NH -SCR reaction. Therefore, an
1508C, 508C lower than pristine Fe O . This result clearly
2 3
3
H -TPR experiment for the FeTiOx catalyst was performed,
2
shows that the oxygen species (mainly surface-adsorbed
using hematite Fe O , anatase TiO , and ilmenite FeTiO as ref-
oxygen reduced at low temperatures) in the FeTiO catalyst is
2
3
2
3
x
erences, and the results are shown in Figure 3.
more labile probably owing to the strong interaction between
Fe species and Ti species, leading to rich surface defects for
the activation of oxygen. Interestingly, totally different from
the reduction processes of Fe O , only one composite reduc-
As we can clearly see, no notable reduction peaks were pres-
ent for pristine TiO , which indicates that the anatase support
2
did not participate in the redox cycle for the NH -SCR reaction,
3
2
3
owing to its lack of reducibility, only supplying adsorption sites
for the reactants in the deNO process. As for pristine Fe O ,
tion peak was observed for the FeTiO catalyst with the maxi-
x
mum H consumption rate at 4278C. According to the calculat-
x
2
3
2
3
+
there were two H2 consumption peaks located at 405 and
258C, respectively. According to the baseline of the H signal,
ed H /Fe molar ratio of 0.52, the Fe species in the FeTiO cat-
2 x
2
+
7
alyst could be totally reduced to Fe species below 5008C,
2
the Fe species in hematite Fe O started to be reduced at ap-
probably resulting in the transformation of iron titanate crys-
2
3
3
+
2+
proximately 2008C, similar to the initial reduction process of
tallite Fe TiO to ilmenite Fe TiO . Although the temperature
x 3
[
27]
g-Fe O nanorods prepared by Mou et al.. Possibly owing to
point for the maximum H consumption rate of the FeTiO cat-
2 x
2
3
the relatively large particle sizes and difficult H diffusion, the
alyst (4278C) was higher than that of hematite Fe O (4058C),
2 3
2
first reduction peak at 4058C did not exhibit Gaussian symme-
try, and the second reduction peak at 7258C, overlapping with
a shoulder reduction peak above 8008C, did not return to the
baseline until as high as 9008C, which was different to a certain
the reducibility of Fe species in the former catalyst was actually
much higher than that in the latter sample. Therefore, the syn-
ergistic combination of Fe species and Ti species in the FeTiO
x
3
+
catalyst not only produced a unique edge-shared Fe ꢀ(O) ꢀ
2
ꢀ
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ChemCatChem 2013, 5, 3760 – 3769 3762