The TPO of ethanol was observed using the Ce /NaZSM-5
uptake and release. The greater capacity in the O uptake of
the Rb added catalyst than the one without Rb would con-
tribute to benzaldehyde formation. The similar dependence
H
2
and the Rb added Ce /NaZSM-5 (Rb/Ce /NaZSM-5) cata-
H
H
lysts (Ce, 3.15 mmol (g-NaZSM-5)~1; Rb, 0.90 mmol (g-
NaZSM-5)~1), as shown in Fig. 14. Instead of benzyl alcohol,
ethanol was used as a reactant because its vapor pressure is
relatively low. An exothermic broad peak with weight loss was
observed at 473È697 K over both catalysts. The area of the
between the benzaldehyde yield (Fig. 3) and the amount of O
uptake (Fig. 6) on the Rb : Ce atomic ratio, of which depen-
2
dence both the maximum values were observed at the atomic
ratio of around 4, suggests that the O species sorbed on the
2
exothermic peak over the Ce /NaZSM-5 catalyst was calcu-
catalyst have a strong correlation with the formation of benz-
H
lated to be 1.38 times larger than that over the
aldehyde, and one of the important roles of the added alkali
Rb/Ce /NaZSM-5 catalyst. Particularly the addition of Rb to
such as Rb is to increase the amount of the sorbed O species.
H
2
the Ce /NaZSM-5 catalyst caused a decrease in the area of
H
the exothermic peak at the region of lower temperature.
High Ce impregnated NaZSM-5 (Ce /NaZSM-5) catalysts
H
(Ce, ¿1 mmol (g-NaZSM-5)—1) and the alkali added
counterparts
Discussion
The addition of alkali metal to the Ce /NaZSM-5 catalyst
H
Cerium species supported on NaZSM-5 zeolite were found to
be active for the gas-phase catalytic oxidation of benzyl
alcohol. The chemical and physical properties of Ce species on
decreased the yield of benzaldehyde as shown in Table 4, in
contrast to the behavior of the added alkali metal to the
Ce /NaZSM-5 catalyst (Table 3). The XRD patterns of both
L
both the low Ce ion-exchanged/impregnated NaZSM-5 (Ce -,
the Ce /NaZSM-5 (Ce, 3.15 mmol (g-NaZSM-5)~1) and the
L
H
and Ce /NaZSM-5) and the high Ce impregnated NaZSM-5
Rb added counterpart clearly indicated the presence of CeO .
L
2
(Ce /NaZSM-5) catalysts are discussed separately, hereafter,
because the e†ect of alkali added to the low and high Ce sup-
However, no clear peaks based on CeO were observed in the
H
2
Ce /NaZSM-5 catalyst. The di†erence in the structure of the
L
ported catalysts was found, through this study, to be quite
di†erent from each other.
Ce species on the Ce / and Ce /NaZSM-5 catalysts will be
H
L
attributed to the reverse e†ect of the added alkali. The larger
peaks based on the CeO on the Rb added Ce /NaZSM-5
catalyst would suggest that the added Rb causes an increase in
2
H
Low Ce supported NaZSM-5 (Ce -, and Ce /NaZSM-5)
L
L
the size of CeO particle, followed by the decrease in the benz-
catalysts (Ce, ¿10—2 mmol (g-NaZSM-5)—1) and the alkali
2
aldehyde formation.
added counterparts
The reaction results (Fig. 7) that benzaldehyde was formed
The addition of alkali metal to Ce ion-exchanged (Ce -
NaZSM-5) and/or relatively low Ce impregnated
even in the absence of gaseous O , in contrast to the imme-
L
2
diate disappearance of CO , indicate that the O species
2
2
(Ce /NaZSM-5) zeolite catalysts was found to increase the
sorbed on the Ce /NaZSM-5 catalyst during the steady-state
L
H
catalytic activity for benzyl alcohol (Tables 1 and 3). Among
oxidation are responsible for the formation of benzaldehyde.
the added alkali metals, Rb had a maximum e†ect for benz-
aldehyde formation under the present reaction conditions
(Table 1). It is of interest to note that the added alkali kept the
high yield of benzaldehyde and also the high selectivity for
benzaldehyde at a higher reaction temperature. Thus the
added alkali is considered to promote partial oxidation activ-
ity, rather than the perfect oxidation, even at high tem-
perature. Because Rb impregnated NaZSM-5 (Rb/NaZSM-5)
without Ce had only low oxidation activity, the added Rb
species are suggested to be promoters for the Ce supported
NaZSM-5 catalyst, rather than active species. The direct
contact of the added alkali metal with active Ce species was
conÐrmed to be important for the e†ective promotive role
However, the enlargement of the size of the CeO particle by
the addition of an alkali metal such as Rb would decrease the
2
capacity of the sorbed O species. A decrease in the amount of
2
O
uptake by the addition of Rb to the Ce /NaZSM-5 cata-
2
H
lyst, which di†ered from the result of Ce /NaZSM-5 (Fig. 5a,
b and 6), was in fact observed (Fig. 8), though the di†erence in
L
the measurement temperature could not be ignored.
The DR spectra of high Ce impregnated NaZSM-5
(Ce /NaZSM-5) catalysts with and without the added Rb
H
were observed under various treatment conditions to study
the redox behavior of the Ce species. The large peak at 350
nm observed for bulk CeO has been reported to shift to
2
around 280 nm by supporting the Ce species on silica, and the
because the physical mixture of Ce /NaZSM-5 and Rb/
NaZSM-5 did not have the same high oxidation activity as
band centered at around 280 nm has been assigned as a CeIV-
L
oxygen charged transfer.18 The decreases in the KM function
the Rb impregnated Ce /NaZSM-5 (Rb/Ce /NaZSM-5) cata-
at the bands (280È320 nm) of both Ce /NaZSM-5 catalysts by
L
L
H
lyst (Table 2). The addition of too much alkali metal will
cover the active Ce species followed by a decrease in the yield
of benzaldehyde as evidenced from having a maximum of
benzaldehyde yield at the variation of the Rb : Ce atomic ratio
(Fig. 2 and 3). The oxidation results over both the Rb and Ce
supported NaZSM-5 catalysts prepared by varying the sup-
porting method and the supporting order showed that the
reduction treatment (Fig. 9) will indicate a decrease in the CeIV
species in the Ce /NaZSM-5. The DR spectra of CeO
H
2
reduced at 473È973 K under H have been reported to show a
2
band at ca. 588 nm, which has been attributed to the reduced
Ce species:
2 CeO ] H ] Ce O ] H O.20 The CO
2
2
2
3
2
pretreatment of the Ce /NaZSM-5 catalyst caused the
appearance of a broad band at around 600 nm, in contrast to
H
catalyst (Ce /Rb/NaZSM-5) prepared (a consecutive impreg-
nation method) by impregnating Rb followed by the impreg-
no appearance of this band for the calcined Ce /NaZSM-5
catalyst. This will be due to the formation of reduced Ce
L
H
nation of Ce had the largest oxidation activity among the
prepared catalysts (Fig. 4). The results would also imply that
(Ce ) species by the pretreatment under CO. The increase in
the broad band at around 600 nm by increases in the Ce
red
the higher oxidation activity for the Ce /Rb/NaZSM-5 cata-
lyst is due to the exposure of the active Ce species, which are
promoted by the added Rb, to the outer surface.
content (Fig. 10) and/or the CO pretreatment temperature
(Fig. 11) also supported that the band at ca. 600 nm is assign-
ed as reduced Ce species. This variation of the DR spectra
L
The amount of O uptake of the low Ce supported
(Fig. 12) that the addition of Rb to the Ce /NaZSM-5 catalyst
inversely brought the decrease in the intensity of the band
attributed to the reduced Ce species is compatible with the
decreases in both the benzaldehyde yield (Table 4) and the
2
H
NaZSM-5 (Ce /NaZSM-5) catalysts, which catalysts were
L
pretreated under CO and degassed, were found to increase by
added Rb to the Ce /NaZSM-5 catalysts (Fig. 5a and b). The
L
added Rb is thus supposed to promote the redox properties of
amount of
O
uptake (Fig. 8) by adding Rb to the
2
Ce species supported on the NaZSM-5 through the ease of O
Ce /NaZSM-5 catalyst. The TPO of ethanol over the Rb
2
H
380
Phys. Chem. Chem. Phys., 1999, 1, 373È381