Chemistry Letters Vol.32, No.6 (2003)
557
Figure 2. Effect of reduction temperature on the sur-
face areas of H1:55MoO3 (ꢃ), MoO3 (4), and Pt/MoO3
(Ã). Reduction period: 12 h.
Figure 3. Relationship between the pantane isomerization ac-
tivity and the average valence of Mo. H1:55MoO3 (ꢃ), MoO3
(4), and Pt/MoO3 (Ã) were reduced at 573–773 K for 12 h.
H1:55MoO3 (l) was reduced at 773 K for 6 h. Reaction condi-
tions: temperature, 523 K; H2/C5, 10; W/F, 10 g-cat./h mol.
face area very little. In the cases of H1:55MoO3 and Pt/MoO3,
the surface area increased in proportion to reduction tempera-
ture, and the largest surface area was obtained after reduction
at 673 K. H2-reduced Pt/MoO3 with large surface area was re-
ported to possess pores with the diameter of 0.6–3.0 nm.3;5 H2
reduction of H1:55MoO3 was also accompanied by the formation
of micropores. If one compares the surface area at reduction
temperatures above 673 K, H1:55MoO3 and Pt/MoO3 exhibited
much larger surface areas than MoO3. The average valence of
Mo in H1:55MoO3, Pt/MoO3, and MoO3 which were reduced
at 673 K for 12 h were 2.64, 2.04, and 2.65, respectively. So,
the extent of reduction can not account for the difference in
the surface area among these materials.
the relationship between the isomerization activity and the aver-
age valence of Mo. Here, the isomerization activity was evalu-
ated by taking the surface area into consideration, because the
surface area differed markedly for the different catalysts. No ap-
preciable difference appeared in the isomerization activity
among the catalysts tested when their average valences of Mo
were in the range of 5–3. The isomerization activity of H2-re-
duced MoO3 was lowered by further reduction. In contrast,
the isomerization activity of 0.01 mol%Pt/MoO3 was enlarged
by a decrease in the average valence of Mo, and the highest ac-
tivity appeared at a Mo average valence of 1.4. The isomeriza-
tion activity of H2-reduced H1:55MoO3 depended on the average
valence of Mo similarly to that of H2-reduced Pt/MoO3.
As shown in Figure 1, H1:55MoO3 reduced at 573 K for
12 h, of which the average valence of Mo was 3.97, gave no dif-
fraction lines due to a MoO2 phase, and the diffraction lines ap-
peared at 2ꢀ ¼ 38:1ꢂ and 44.1ꢂ. These lines seem to reflect the
formation of the MoOxHy phase, which is analogous to the mo-
lybdenum oxycarbide, MoOxCy phase.6 Reduction at 673 K
strengthened the intensity of the diffraction lines at
2ꢀ ¼ 38:1ꢂ and 44.1ꢂ. The diffraction line due to the Mo metal
phase was detected at 2ꢀ ¼ 40:6ꢂ after reduction at 773 K.
These phenomena are almost the same as those observed in
Pt/MoO3:Pt/MoO3 was reduced to MoOxHy and Mo metal with-
out the formation of MoO2.3 We reported in previous papers7;8
that reduction of MoO3 at 623 K involved the formation of
HxMoO3, while MoO3 was directly reduced to MoO2 at
673 K. Pt/MoO3 was converted to HxMoO3 during heating to re-
duction temperature so that reduction proceeded through the
formation of HxMoO3, irrespective of reduction temperature.
This phenomenon can easily be understood by taking hydrogen
spillover into consideration. H1:55MoO3 exhibited a large sur-
face area even after reduction at temperatures above 673 K.
Furthermore, there was no appreciable difference in the surface
area between H2-reduced H1:55MoO3 and H2-reduced Pt/MoO3.
We conclude from these results that molybdenum oxide with a
large surface area can be formed by reduction of hydrogen mo-
lybdenum bronze, and this reduction process cannot be affected
by the presence of Pt.
H
1:55MoO3 reduced at 773 K for 6 h, of which the Mo average
valence was 1.5 exhibited a lower isomerization activity than
H2-reduced Pt/MoO3 with a Mo average valence of 1.4. How-
ever, difference in the activity between H1:55MoO3 and Pt/
MoO3 was very small except for these two catalysts. These re-
sults clearly show that reduction of HxMoO3 can generate the
active sites for pentane isomerization. In the case of Pt/MoO3,
the major role of Pt will be to promote the formation of
HxMoO3.
This work was supported in part by a Grant-in-aid for
Scientific Research (C) from Japanese Society for the Promo-
tion of Science.
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Conversion of pentane was carried out at 523 K. Pentane
was selectively isomerized to 2-methylbutane on all of the cat-
alysts tested. Hence, the isomerization activity was estimated
using the formation rate of 2-methylbutane. Figure 3 displays
Published on the web (Advance View) May 27, 2003; DOI 10.1246/cl.2003.556