CATALYTIC BEHAVIOR OF Pt/SbOx FOR SELECTIVE OXIDATION
189
Similarly Pd, Rh, and Ir were expected to work as pro-
moters for the i-C4H10-selective oxidation. However, Pt was
the only metal that can promote the selective oxidation of
i-C4H10 to MAL as shown in Table 2. Rh in Rh/SbOx was
transformed to the oxide under the catalytic reaction con-
ditions, which would be the reason why Rh did not pro-
mote the dehydrogenation of i-C4H10 (24, 25). The differ-
ence between Pt and Pd in the promoter effect is not clear
at present, but the subtle difference of the two metals may
be attributed to different modification of the metal surface
by Sb. Ag/SbOx was also tested as it has been reported that
SCHEME 1. The reaction pathway of i-C4H10 oxidation on Pt/SbOx
catalysts.
tion of the Pt particles by Sb is induced by reduction of the
Pt/SbOx catalyst under the reaction conditions. Thus the re-
action pathway for the i-C4H10 selective oxidation may be
shown in Scheme 1.
The complete oxidation of i-C4 hydrocarbons to CO2 was
remarkably reduced by the prereduction of the Pt/SbOx
catalyst with H2 at 473–623 K, but the deeper reduction
spoiled the activity of Pt/SbOx. The selectivity to MAL in
the i-C4H8 oxidation wasdeteriorated under the high i-C4H8
concentration condition in Fig. 9. In this case, the structural
change occurred due to too much loss of lattice oxygen
of SbOx as was determined by XRD: -Sb2O4 was formed
under the reductive conditions (24, 25). The Pt/SbOx sam-
ple treated under flowing He at 773 K promoted complete
oxidation to CO2 and no MAL synthesis. Under this treat-
ment -Sb2O4 was observed to be formed. On the other
hand, only Sb6O13 was observed when the selective oxida-
tion of i-C4H10 and i-C4H8 to MAL was successful (24, 25).
The Sb6O13 structure may be responsible for the selective
oxidation to MAL (Scheme 1). Pt promotes the selective
catalysis of the Sb6O13 under favorable reaction conditions,
where Sb-modified Pt particles on the Sb6O13 support are
produced, which are active for selective oxidation. Too se-
vere reductive conditions deteriorated the MAL yield as
well as the MAL selectivity, and it may be due to the loss of
“selective” lattice oxygen supplied from the Sb6O13 surface.
Generally, the sites for hydrocarbon activation and allyl
intermediate oxidation in selective oxidation catalysts are
different and the sites for reoxidation of the catalysts are
separated from these sites, leading to the high selectivities
(27). In the Pt/SbOx catalyst, the Pt metal is a promoter
for the selective oxidation catalysis of the SbOx, which is
contrasted to typical oxide-supported metal catalysts where
metal oxide promotes the supported metal particles. The Pt
particles modified by Sb enhance i-C4H10 dehydrogenation
to the methallyl intermediate, which moves to the SbOx
surface and reacts with the active lattice oxygen to form
MAL. Pt also influences the reactivity of the lattice oxygen
of SbOx, which will be reported in a separate paper (24).
– –
Ag worked as a good dopant for Bi V O scheelite-type ox-
ides for propane ammoxidation to acrylonitrile (3). Table 2
shows no promotion effect of Ag for the i-C4H10 selective
oxidation on SbOx. In addition, Cu/SbOx was investigated
–
in relation to the effect and role of the Cu SbOx junction
due to the band-bending effect between metal and oxide
(21). The Cu/SbOx system was inactive for the selective ox-
idation (Table 2).
Propene oxidation has been reported to take place on Sb
oxide (Sb6O13) (10, 11). In the i-C4H10 oxidation the i-C4H8
intermediate should be further oxidized to MAL on the
SbOx surface in Pt/SbOx. It is to be noted that Pt also pro-
moted the selective oxidation of i-C4H8 to MAL, and the
selectivity to MAL was as high as 90% , as shown in Fig. 4.
The addition of a small amount (0.5 wt% ) of Pt to SbOx
increased the MAL yield from i-C4H8 to five times of what
it was on SbOx alone, while retaining the high selectivity of
the SbOx itself (Fig. 4). The yield to MAL was increased
seven times for 1.0 wt% Pt. Further, the activation energy
drastically decreased from 150 kJ mol 1 for SbOx to 60 kJ
mol 1 for Pt (0.5 wt% )/SbOx (Fig. 8). As mentioned above,
the formation of MAL from i-C4H8 occurs through methal-
lyl intermediate (i-C4H7) (27, 28). These results indicate
that the dehydrogenation of i-C4H8 to i-C4H7 preferentially
proceeds on the Pt surface and the methallyl species may
migrate to the SbOx surface on which MAL is produced.
The complete oxidation of i-C4H8 to CO2 preferentially oc-
curred in the low temperature range below 623 K, while
above 650 K CO2 formation was remarkably suppressed
and MAL selectivity dramatically increased as shown in
Fig. 4. This phenomenon was irreversible as shown in Fig. 6.
Once an active phase at the Pt/SbOx catalyst surface was
formed under the reaction conditions, typically at 773 K,
the high selectivity to MAL is maintained when the tem-
perature is lowered.
XRD, analytical TEM, and EXAFS reveal that modifi-
cation of the Pt metallic particles on SbOx takes place dur-
ing the selective oxidation reactions of i-C4H10 (20% ) and
i-C4H8 (1.7% ) at 773 K, and this modification is caused by
SbOy (y < x) (24, 25). The oxidation of i-C4H10 and i-C4H8
to CO2 which readily takes place on unmodified Pt metal-
lic particles is suppressed by the Sb modification, leading
to enhancement of the selectivity to MAL. The modifica-
CONCLUSIONS
(i) We have found that the Pt/SbOx catalyst showed good
performance for the selective oxidation of i-C4H10 and
i-C4H8 to MAL with 57 and 90% selectivities, respectively.
The selectivities to MAL and i-C4H8 in the i-C4H10 oxida-
tion reaction were 80–90% . The reaction of i-C4H10 did not
proceed on the SbOx without Pt.