J. Kubo, W. Ueda / Materials Research Bulletin 44 (2009) 906–912
911
6+
gaseous oxygen, resulting in the formation of distorted Mo
octahedra species, which exist as small-sized, island-like aggre-
gates on the surface of the BaMoO . Redox progresses at the
3
6
+
4+
interface of these distorted Mo octahedra aggregates and Mo
octahedra, and oxidative dehydrogenation is promoted. The
aggregates of Mo octahedra grow gradually to form BaMoO
crystallites. Because the interface between Mo octahedra and
Mo octahedra increases as the number of Mo octahedra
increases, oxidative dehydrogenation is further promoted. The
6
+
4
4
+
6
+
6+
3
BaMoO catalyst can thus control and maintain an adequate
oxidation state at its active sites, enhancing its own gaseous
oxygen reactivity, which leads to an increase in oxidative
4
dehydrogenation above 523 K, as seen in Fig. 5. Although AMoO ,
6
+
like MoO
have an acidic character, AMoO
behavior because of the change in oxidation state of its Mo.
3
, possesses high oxidation state Mo and is considered to
3
shows more complicated catalytic
4. Conclusions
3 3
Perovskite-type oxide catalysts BaMoO and SrMoO were
prepared by reduction of corresponding scheelite-type oxide
catalysts BaMoO and SrMoO at 873 K for 16 h. These catalysts
were tested for 2-propanol oxidation. Both the BaMoO and the
SrMoO catalysts promoted mainly dehydration, and the amount
of acidic sites on each was about 1.20
after catalytic reaction. In the case of the BaMoO
4
4
4
4
ꢀ1
m
mol g both before and
and SrMoO
3
3
4 3
Fig. 7. UV–vis DR spectra of BaMoO , BaMoO and reference compounds.
catalysts, however, selectivity to acetone by oxidative dehydro-
genation increased with time over the course of the catalytic
reaction. After catalytic reaction, the generation of acidic sites on
these catalysts was confirmed, though acidic sites had not been
observed in either catalyst before the catalytic reaction. Selectivity
to the product of oxidative dehydrogenation was higher over
2
ꢀ
6+
3
20 nm for MoO
MoO . The spectrum of the BaMoO
absorption band, from 400 to 600 nm, similar to that of MoO
reference compound. The absorption peak of the BaMoO catalyst
after 1 h of 2-propanol oxidation appeared at 260 nm, similar in
position to that of BaMoO having the tetrahedral structure of
3
, in both cases also due to the LMCT: O ! Mo of
catalyst showed a broader
as a
6
3
2
3
the BaMoO
of acidic sites on the SrMoO
3
catalyst than over the SrMoO
catalyst after catalytic reaction
close to that of the SrMoO catalyst
mmol g ). On the other hand, the amount of acidic sites
3
catalyst. The amount
4
3
6
+
6+
ꢀ
1
Mo (T
the surface of the BaMoO
d
). This indicates that Mo (T
d
) species formed promptly on
was 1.05
1.20
on the BaMoO
that of the SrMoO
m
mol g
,
4
ꢀ
1
3
catalyst on exposure to gaseous oxygen
under reaction conditions. In addition, a second absorption peak
appeared at 296 nm after 13 h. This peak position is close to that of
(
ꢀ
1
3
catalyst was 0.12
m
mol g , only 1/10 as large as
catalyst, the
3
catalyst. In the case of the BaMoO
3
the peak for (NH
distorted Mo (O
4
)
6
Mo
), its peak position appears at a slightly lower
wavelength than that of MoO . The new absorption peak which
appeared for BaMoO at 296 nm thus suggested that the Mo ion is
surroundedby a distortedMo (O
7
O
24ꢁ4H
2
O. Since (NH
4
)
6
Mo
7
O
24ꢁ4H
2
O has a
fact that the amount of acidic sites formed after catalytic oxidation
remained suppressed by the reductive effect of 2-propanol
contributed to selectivity to the product of oxidative dehydro-
genation. The results of UV spectra and catalytic testing also
6+
h
3
3
6+
6+
h
) polyanion-like structure. Weber
indicated that redox progresses at the interface of distorted Mo
4
+
has reported that the energy of the UV absorption edge reflects the
local structure of the Mo center, that is, that energy increases as
octahedra and Mo octahedra on the perovskite phase in the
BaMoO catalyst, promoting oxidative dehydrogenation catalysis.
The catalytic behavior of BaMoO can thus be classified into the
following three types according to reaction conditions. BaMoO
3
6+
the aggregation size of Mo (O
BaMoO catalyst after 13 h catalytic reaction, it can be concluded
that a small-sized aggregate of distorted Mo (O
h
) decreases [34]. In the case of the
3
3
3
6+
h
) formed on
) by gaseous oxygen, because the BaMoO
peak of 296 nm was at a slightly lower wavelength than that of
promotes dehydrogenation, like a metallic catalyst, in the absence
of gaseous oxygen. In the presence of a small amount of gaseous
oxygen, it promotes dehydration, as an acid catalyst. When
4
+
oxidation of Mo (O
h
3
6
+
(
NH
gradually rearranged to Mo (T
cubicBaMoO ischangedintotetragonalBaMoO
the small aggregate of distorted Mo (O
4
)
6
Mo
7
O
24ꢁ4H
2
O. This suggests that distorted Mo (O
h
) is
) during catalytic reaction and that
. However, because
) formed in the case of
3 h
BaMoO is surrounded by Mo (O ), it is readily reduced again by
3
gaseous oxygen is abundant, the BaMoO catalyst promotes
6+
d
oxidative dehydrogenation, as a redox-type catalyst.
3
4
6+
h
References
4+
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´
s, J. Catal. 231 (2005) 232.
2
-propanol. It is likely that the transition between distorted
6+
4+
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Mo (O
h
) $ Mo (O
h
) in the perovskite phase promotes oxidative
catalyst, while selectivity to
catalyst islowerthanthatoverthe BaMoO
is not readily reduced again.
We postulate that the catalytic oxidation of 2-propanol over the
BaMoO catalyst progresses as follows: acidic sites are formed on
the surface of BaMoO as soon as the BaMoO catalyst is exposed to
gaseous oxygen, and dehydration progresses on these acidic sites.
In this process, the surface of the BaMoO becomes oxidized by the
[
dehydrogenation over the BaMoO
3
acetoneoverthe SrMoO
catalyst because SrMoO
3
3
[
[
[
3
3
[
[
[
3
3
12] T. Nitadori, M. Muramatsu, M. Misono, Bull. Chem. Soc. Jpn. 61 (1988) 3831.
[13] T. Nitadori, S. Kurihama, M. Misono, J. Catal. 98 (1986) 221.
3