Propene oxidation over palladium catalysts supported on zirconium rich ceria-zirconia

Article abstract of DOI:10.1016/j.cattod.2014.04.004

Ceria-zirconia solid solution with the Ce/Zr ratio of 1/3 was prepared and used as support material for palladium catalysts. The palladium catalysts supported on the zirconium rich ceria-zirconia (Pd/CZ) were studied for propene oxidation reaction. In the

Full text of DOI:10.1016/j.cattod.2014.04.004

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Propene oxidation over palladium catalysts supported on zirconium
rich ceria–zirconia
Naoto Kamiuchia,b, Masaaki Hanedaa, Masakuni Ozawaa,c,∗
a Advanced Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu 507-0071, Japan
b The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Ceria–zirconia solid solution with the Ce/Zr ratio of 1/3 was prepared and used as support material for
palladium catalysts. The palladium catalysts supported on the zirconium rich ceria–zirconia (Pd/CZ) were
studied for propene oxidation reaction. In the activity tests, the influences of palladium loadings, pre-
treatment conditions, and reaction conditions were investigated. The catalytic activities of Pd/CZ with
small amount of palladium component were strongly affected by the pretreatment conditions such as
oxidative or reductive atmosphere. From IR measurements, it was suggested that the palladium species
with high reducibility should be active for the complete oxidation of propene. The initial activities of
Pd/CZ catalysts under the insufficient oxygen condition were equivalent to the activities under the stoi-
chiometric condition due to the property of high oxygen storage capacity (OSC). It was revealed that CZ
support helped the high dispersion of palladium oxide particles and its OSC property assisted the propene
oxidation reaction.
Received 31 January 2014
Received in revised form 27 March 2014
Accepted 2 April 2014
Available online xxx
Keywords:
Palladium catalyst
Ceria–zirconia
Propene oxidation
Oxygen storage capacity
OSC
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
of high thermal stability and high oxygen storage capacity (OSC)
[1–3], and it has already been put into practical use as the pro-
Automobile has been an indispensable means of transportation
for human life over the decades. Recently, some electric vehicles are
available in several countries, whereas the automobile that runs on
gasoline still remains the majority. In the case of the gasoline-fueled
automobile, the treatment of exhaust gases as carbon monoxide
(CO), hydrocarbon (HC), nitrogen oxide (NOx), and sulfur oxide
(SOx) is a significant technology, because the gases have a large
impact on human body and the global environment. For exam-
ple, it is believed that the photochemical oxidants are produced
by the photochemical reaction of hydrocarbons and NOx, and the
oxidants induce the air pollution such as photochemical smog.
The gases of NOx and SOx would be the main reason of acid rain
and air pollution. Therefore, various types of automotive catalysts
have been developed to remove their harmful gases. One of the
most famous catalysts is three-way catalyst (TWC), which is com-
posed of precious metal, ceria–zirconia, and alumina. Especially,
the ceria–zirconia (CZ) solid solution has the superior characters
moter of automotive catalysts. Furthermore, it was reported that
the growth of platinum particles was inhibited by the strong inter-
action with ceria–zirconia system under oxidative condition. The
unique effect of CZ solid solution was named as “anchor effect”
[4,5]. The catalysts with CZ mixed oxide were investigated for not
only the purification of exhaust gases [6,7] but also some kinds
of oxidation reaction. For instance, CeO2–ZrO2 doped with tran-
sition metals and rare earth elements [8] was studied for soot
combustion, and Co3O4/CeO2–ZrO2 composite oxide catalyst [9,10]
and CeO2–ZrO2 [11] were investigated on total methane oxidation
at low temperature. Methane combustion [12,13], toluene com-
bustion [14], and methanol decomposition to synthesis gases [15]
over palladium catalysts supported on ceria–zirconia have also
been reported by several researchers. Almost all researches, how-
ever, were conducted on catalysts with cerium rich ceria–zirconia.
There are only a few researches about Zr rich ceria–zirconia sup-
ported palladium catalysts for total oxidation of hydrocarbon such
as methane [16,17]. Ce element is classified with the rare earth
element, and the price of precious metals is extremely expen-
sive. Therefore, the decreases of precious metals and cerium
usage are desired in addition to the achievement of high catalytic
performance for the strict regulation of exhaust gases such as
EURO6.
∗
Corresponding author at: EcoTopia Science Institute, Nagoya University, Furo-
cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel.: +81 52 789 5863;
fax: +81 52 789 5863.
E-mail address: ozawa@numse.nagoya-u.ac.jp (M. Ozawa).
http://dx.doi.org/10.1016/j.cattod.2014.04.004
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: N. Kamiuchi, et al., Propene oxidation over palladium catalysts supported on zirconium rich
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(micromeritics®), after the heat treatment at 300 ◦C for 3 h under
vacuum. X-ray diffraction (XRD) analysis and Raman spectroscopy
were conducted with MiniFlex II (Rigaku) and NRS-3100 (JASCO),
respectively, in order to investigate the crystal phase and struc-
ture. The XRD patterns were recorded by using Cu K˛ radiation.
In the Raman spectroscopy, the Nd: YVO4 laser with a 532 nm line
was used at around 10 mW power. In addition, the O2–CO2–H2–CO
pulse method [18] was performed to estimate the more precise
dispersion of palladium particles. The microstructures of catalysts
were observed by transmission electron microscopy equipped with
field emission gun (FE-TEM) (JEM2100F, JEOL) at the accelerating
voltage of 200 kV. TEM images were recorded by CCD camera.
Fourier transform infrared (FT-IR) spectroscopy, temperature-
programmed reduction (TPR) measurement and evaluation of
oxygen storage capacity (OSC) were also carried out to investi-
gate the chemical properties of prepared catalysts. FT-IR spectra
of adsorbed CO were acquired with FT/IR-4200 (JASCO) at a reso-
lution of 4 cm−1. After a sample disk was pretreated at 400 ◦C for
1 h in 26.7 kPa (200 Torr) of oxygen or hydrogen, it was evacuated
at 400 ◦C for 1 h. Then, IR cell with pretreated catalyst was filled
with 6.7 kPa (50 Torr) of CO gas at room temperature. After the cell
was evacuated, IR spectra were carefully recorded. The reducibility
of Pd/CZ catalysts, in other words, the reactivity of oxygen species
in catalysts was investigated by TPR measurement. The as-calcined
catalyst was heated in 5% H2/Ar at a heating rate of 10 ◦C min−1, and
the outlet gas was recorded by TCD detector (BP-1, HEMMI Slide
Rule). After H2-TPR measurement, the amount of adsorbed oxygen
was evaluated by the pulse method of oxygen gas. The as-calcined
catalysts were pretreated at 300 or 400 ◦C before the several char-
acterizations. The chemical state of catalysts would not change due
to the pretreatments, because the catalysts were calcined at higher
temperature of 800 ◦C.
In this study, ceria–zirconia solid solution with 25% of cerium
and 75% of zirconium were used for the support material, and
the influence of palladium amount was carefully investigated for
propene (C3H6) oxidation as the fundamental reaction of exhaust
gas treatment. In addition, the effects of reaction conditions and
pretreatment to catalysts were researched by using the different
atmosphere, because the automotive catalysts were exposed to
both lean and rich conditions.
2. Experimental
2.1. Preparation of CZ support and Pd/CZ catalysts
Palladium catalysts supported on ceria–zirconia were prepared
according to the following procedure. First, ceria–zirconia powder
was synthesized by the coprecipitaion method. Cerium ammo-
nium nitrate (Wako Pure Chemical) and zirconyl nitrate (Wako
Pure Chemical) were sufficiently mixed with pure water, and excess
ammonium hydroxide was added to the mixed solution. The pre-
cipitated powder was washed in pure water and dried at 110 ◦C for
1 day. The resultant powder was preheated at 600 ◦C for 3 h in air
to remove the residual nitrate compounds, and calcined at 800 ◦C
for 3 h in air for thermal stability improvement. By this process, the
CZ(1/3) support with the chemical formula of Ce0.25Zr0.75Ox was
obtained.
Secondly, Pd component was loaded on the CZ(1/3) support by
the conventional impregnation method. The powder of CZ sup-
port was impregnated with a nitrate solution of Pd(NO2)2(NH3)2
(Tanaka Kikinzoku Kogyo), and it was dried at 110 ◦C for 1 day.
After the heat-treatment at 600 ◦C for 3 h, the catalyst was also cal-
cined at 800 ◦C for 3 h in air as well as CZ support. The loadings of
palladium in the samples were set at 0.04, 0.2, 1.0 and 5.0 wt.%. The
y wt.% Pd/CZ(1/3) catalysts were hereinafter called “yPd/CZ”.
3. Results and discussion
2.2. Propene oxidation tests
3.1. Catalytic activities on propene oxidation
The activities of Pd/CZ catalysts for propene oxidation were
measured with a typical fixed-bed flow reactor. The as-calcined
catalysts with the amount of 100 mg were pretreated at 600 ◦C in
a flow of 10% O2/N2 (50 ml min−1) in the reactor, and then the
catalytic reaction tests were carried out by passing gaseous mix-
tures composed of propene (3345 ppmC), O2 (0.5% or 0.4%), and
N2 gas as diluted gas with a total flow rate of 500 ml min−1 (space
velocity: 300,000 ml g−1 h−1). The oxygen concentration of 0.5% and
0.4% corresponds to the stoichiometric condition and the oxygen
deficient condition of 20%, respectively, when the catalytic reac-
tion proceeds as the total oxidation (C3H6 + 9/2O2 → 3CO2 + 3H2O).
The reacted gases were analyzed by a flame ionization detector
(FID) (Shimadzu, VMS-1000F) and a portable gas analyzer (HORIBA,
PG-230). The propene conversion to CO2 was calculated with CO2
concentration at reactor outlet. The temperature was increased
from 50 to 600 ◦C at a rate of 10 ◦C min−1. After the reaction test,
the catalyst was heat-treated at 600 ◦C under a flow of 10% H2/N2
(50 ml min−1), and the same reaction test was conducted for the
investigation of the effect of pretreatment condition. The amount
of catalyst was set at 100 mg, and the thickness of catalytic layer
in the reactor was about 2 mm. Therefore, it is considered that the
temperature around samples must be homogeneous.
The results of propene oxidation tests under stoichiometric
condition (O2: 0.5%) over Pd/CZ catalysts after the oxidation and
reduction treatment are shown in Fig. 1. A reaction test without
catalyst was performed, and the curve of CO2 yield was added
to Fig. 1(a). After the oxidation treatment (Fig. 1(a)), the propene
oxidation reaction was initiated at 200 ◦C over all Pd/CZ, and the
ignition temperature was ca. 250 ◦C over CZ support. In the case of
the catalysts with Pd loadings of 0.2, 1.0, and 5.0 wt.%, the total
oxidation of propene was achieved under 500 ◦C. On the other
hand, the propene conversion over 0.04Pd/CZ was 70% even at
600 ◦C, whereas the light off temperature was 200 ◦C and its activ-
ity was higher than that of CZ. The order of catalytic performance
(5.0 > 1.0 > 0.2 > 0.04Pd/CZ > CZ) after the oxidation treatment was
same with the order after the reduction, as can be seen in Fig. 1(b).
The activities of 0.04, 0.2 and 1.0Pd/CZ, however, were improved
by the reduction treatment, and the total oxidation of propene was
achieved at 450 ◦C over 0.04Pd/CZ reduced in hydrogen. This result
indicated that the chemical state of palladium species should be
significant, as discussed in Section 3.5.
Fig. 2 shows the results of propene oxidation tests under insuf-
ficient oxygen condition (O2: 0.4%). After the oxidation at 600 ◦C
(Fig. 2(a)), the ignition temperatures on Pd/CZ catalysts were
around 200 ◦C, and the light off curves below the conversion of
20% were similar with the curves of Pd/CZ under stoichiomet-
ric condition (Fig. 1(a)). In the conversion over 20%, however, the
catalytic performances under insufficient oxygen conditions were
inferior to the performances of Pd/CZ under stoichiometric con-
dition. The improvements of catalytic activities by the reduction
2.3. Characterization of Pd/CZ(1/3) catalysts
Several kinds of characterization of as-calcined catalysts were
carried out so as to reveal the correlation between catalytic activ-
ities and the loading of palladium, and to realize the variation
of activities due to pretreatment and reaction conditions. The
BET surface area of the catalysts was estimated by TriStar II
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Fig. 2. CO2 yield over CZ(1/3) and Pd/CZ(1/3) catalysts after heat-treatment in (a)
oxidation and (b) reduction atmosphere. (×) CZ, (ꢀ) 0.04Pd/CZ, (ꢁ) 0.2Pd/CZ, (ꢀ)
1.0Pd/CZ, (♦) 5.0Pd/CZ. Reaction conditions: propene, 3345 ppmC; O2, 0.4%; N2, bal-
Fig. 1. CO2 yield over CZ(1/3) and Pd/CZ(1/3) catalysts after heat-treatment in (a)
oxidation and (b) reduction atmosphere. (×) CZ, (ꢀ) 0.04Pd/CZ, (ꢁ) 0.2Pd/CZ, (ꢀ)
1.0Pd/CZ, (♦) 5.0Pd/CZ, (ꢁ) no catalyst. Reaction conditions: propene, 3345 ppmC;
ance; total flow rate = 500 ml min−1; S.V. = 300,000 ml g−1 h−1
.
O2, 0.5%; N2, balance; total flow rate = 500 ml min−1; S.V. = 300,000 ml g−1 h−1
.
treatment were confirmed in the reduced samples (Fig. 2(b)). The
total decomposition of propene was achieved due to the formation
of by-products such as CO, while the yields of CO2 did not reach to
100% even at high temperature such as 500 ◦C. This is because O2
concentration was set as 80% (O2: 0.4%) for total propene oxidation
(O2: 0.5%).
of CZ support, only a fluorite type structure was observed, though
the peaks were a little broad. Therefore, the CZ powder prepared
by the coprecipitation method seems to be composed of the homo-
geneous solid solution of ceria and zirconia despite a large amount
of zirconium component. The crystal phases derived from palla-
dium species were not detected in all Pd/CZ catalysts. This might be
due to the small amount and low crystallinity of palladium compo-
nent. The difference of diffraction patterns was not shown by XRD
analysis, whereas the Raman spectra of the as-calcined catalysts
were gradually changed with the increase of Pd loadings (Fig. 4).
The Raman spectra of CZ support, 0.04Pd/CZ and 0.2Pd/CZ were
assigned to a tetragonal fluorite structure, as reported by Hirata
[19]. Meanwhile, in the case of 1.0 and 5.0Pd/CZ, the Raman spec-
3.2. Structure of CZ support and Pd/CZ catalysts
The Pd/CZ catalysts were characterized to investigate the cause
of difference in catalytic activities. The fundamental properties of
BET surface areas, dispersion and diameter of Pd particles, and OSC
values were summarized in Table 1. The surface area of CZ sup-
port was 64.2 m2 g−1, and those of Pd/CZ catalysts were more than
45 m2 g−1, even after the heat treatment at 800 ◦C. The high ther-
mal stability of CZ powder was confirmed by the measurements.
Fig. 3 shows XRD patterns of as-calcined catalysts. In the pattern
tra were totally broad, and the peaks of F2g band (ca. 460 cm−1
)
were shifted to a lower frequency. Therefore, it was indicated that
the Raman spectra were weakened by the palladium species at top
surface of CZ support. Furthermore, the variation of lattice structure
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Table 1
BET surface area, dispersion, diameter of palladium particles and OSC value of Pd/CZ catalysts.
Sample
BET surface area (m2 g−1
)
CO/Pd (%)a
Diameter of palladium particles (nm)
OSC (␮mol g−1
)
CZ(1/3)
64.2
55.8
59.8
55.0
46.3
–
–
400.4
400.2
411.3
446.1
620.8
0.04Pd/CZ
0.2Pd/CZ
1.0Pd/CZ
5.0Pd/CZ
31.7
37.4
35.9
11.2
4.7
3.9
4.1
13.2
aDispersion of Pd particles was estimated by the O2–CO2–H2–CO adsorption method at 50 ◦C [18].
[20–23], a B1g vibration mode of palladium oxide was clearly mea-
sured at 650 cm−1, and the weak Raman peaks were also observed
at 445 and 1,120 cm−1. In this study, however, the peaks assigned
as PdO were not identified due to the strong spectra of CZ support.
TEM observation was carried out in order to reveal the morphol-
ogy and state of palladium species in Pd/CZ catalysts. TEM image of
1.0Pd/CZ calcined at 800 ◦C for 3 h are shown in Fig. 5(a). The fine
particles with the size of ca. 2 nm were clearly observed on the sur-
face of CZ support, as indicated by white arrows. It was confirmed
that the palladium species were highly dispersed on CZ support. The
Fig. 3. XRD patterns of CZ(1/3) and Pd/CZ(1/3) catalysts.
was induced by the loading of palladium component and calcina-
tion at high temperature. In other words, a part of palladium species
might dissolve into the lattice of CZ support. This would be the rea-
son of the shift of F2g band from 460 to 430 cm−1. In several reports
(a)
(b)
(c)
(d)
(e)
100
200
300
400
500
600
700
800
Raman shift / cm-1
Fig. 4. Raman spectra of CZ(1/3) and Pd/CZ(1/3) catalysts. (a) CZ support, (b)
Fig. 5. TEM images of 1.0Pd/CZ catalyst: (a) after calcination at 800 ◦C for 3 h, and
0.04Pd/CZ, (c) 0.2Pd/CZ, (d) 1.0Pd/CZ, (e) 5.0Pd/CZ.
(b) after propene oxidation test.
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5
2023
2029
1956
2136
(a)
(b)
(c)
2063
(a)
(b)
1971
1958
1969
1973
1972
2137
2067
2068
2083
(c)
(d)
2141
2145
1957
1974
2048
2082
(d)
2200
2100
2000
1900
1800
2200
2100
2000
1900
1800
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 7. FT-IR spectra of CO species adsorbed on Pd/CZ with various Pd loadings after
reduction treatment at 400 ◦C. (a) 0.04Pd/CZ, (b) 0.2Pd/CZ, (c) 1.0Pd/CZ (d) 5.0Pd/CZ.
Fig. 6. FT-IR spectra of CO species adsorbed on Pd/CZ with various Pd loadings after
oxidation treatment at 400 ◦C. (a) 0.04Pd/CZ, (b) 0.2Pd/CZ, (c) 1.0Pd/CZ (d) 5.0Pd/CZ.
metallic Pd was confirmed by FT-IR spectroscopy. It was suggested
that the reducibility of palladium oxide supported on CZ should be
very high. The reducibility was examined by TPR measurement in
the next section. The wide IR bands from 1200 to 1700 cm−1 were
detected and identified as carbonate species adsorbed on CZ sup-
port [25]. After the reduction treatment at 400 ◦C, on the other hand,
IR bands assigned as bridged and linear CO on Pd0 were obtained
around 1960 and 2025 cm−1, as shown in Fig. 7. In 0.04Pd/CZ cat-
alyst, the IR band of linearly adsorbed CO was relatively stronger
than that of bridged CO. This result indicates that palladium species
in Pd/CZ catalysts with low Pd loading were highly dispersed in
atomic state.
catalyst of 1.0Pd/CZ after propene oxidation test under insufficient
oxygen was also observed by FE-TEM. As can be seen in Fig. 5(b),
the nano-sized particles existed on CZ support. The small parti-
cles in Fig. 5 must be the palladium oxide with the amorphous
state, because the lattice images were not observed. TEM obser-
vation revealed that the palladium particles maintained the highly
dispersed state, even after the propene oxidation test at 600 ◦C.
Thereby, the strong interaction between palladium and CZ support
seems to inhibit the particle growth of palladium.
In the CO chemisorption experiments (O2–CO2–H2–CO pulse
method), the mean diameters of Pd particles in Pd/CZ catalysts were
calculated as 4–13 nm by the assumption of linear CO adsorption, as
summarized in Table 1. On the contrary, the size of palladium oxide
particles in TEM images was ca. 2 nm. The discrepancy between the
diameters calculated from CO chemisorption and the size obtained
from TEM observation could be explained by the underestima-
tion of the CO adsorption amount. In fact, not only linear but also
bridged CO adsorption was confirmed in FT-IR measurements, as
mentioned in next section (Figs. 6 and 7). The bridged CO adsorp-
tion causes the underestimation of the amount of CO adsorption.
Therefore, the calculated diameters via CO chemisorption experi-
ments were larger than the diameters of PdOx particles observed
by TEM.
3.4. TPR measurement
The reducibility of catalyst is important for oxidation reaction,
especially under insufficient oxygen condition, because the oxy-
gen species in catalysts assist the chemical reaction with oxygen.
So, the reducibility was examined by H2-TPR measurements, and
the profiles are summarized in Fig. 8. As can be seen in Fig. 8, a
broad peak of oxygen release was observed at 300–700 ◦C in CZ
support without palladium component. In the case of 0.04 and
0.2Pd/CZ, on the contrary, the consumption of hydrogen gas was
recorded as a sharp peak at 50–140 ◦C. It was revealed that the
oxygen release was drastically accelerated by a tiny amount of pal-
ladium, whereas the OSC values of 0.04 and 0.2Pd/CZ were almost
same with CZ material (Table 1). The reduction temperature of PdOx
should be strongly affected by particle size and morphology. In
addition, palladium could assist the dissociation and spillover of
H2, resulting in the decrease of reduction temperature. A weak and
broaden peaks at 90–140 ◦C were confirmed in 1.0 and 5.0Pd/CZ,
and a peak at 80 ◦C was reversely recorded in 5.0Pd/CZ. The neg-
ative peak should be assigned to the hydrogen desorption from
palladium hydride, because the metallic palladium has the prop-
erty of hydrogen storage capacity. The similar results were reported
for PdO/SnO2 catalysts by Takeguchi et al. [26]. The hydrogen con-
sumption due to OSC property of Pd/CZ catalyst simultaneously
proceeded with the hydrogen release from metallic palladium par-
ticles. This is why the strong peak was not detected at around 80 ◦C
in 1.0Pd/CZ. The OSC value of 5.0Pd/CZ was much larger than the
3.3. FT-IR spectroscopy
The chemical states of palladium species in Pd/CZ catalysts were
investigated by FT-IR measurements. FT-IR spectra of CO species
adsorbed on Pd/CZ catalysts heat-treated at 400 ◦C in O2 and H2
gas atmosphere are shown in Figs. 6 and 7, respectively. After the
oxidation treatment (Fig. 6), three kinds of IR bands (1970, 2065
and 2140 cm−1) were ascribed to bridged CO adsorbed to Pd0, lin-
ear CO on Pd0, and linear CO on Pd2+ [24]. Although palladium
species must be in oxidized state after the oxidation treatment,
a part of palladium oxide seems to be easily reduced by CO gas
below 50 ◦C. When IR cell was filled with CO gas at room tempera-
ture, the yellow–brown color of sample disks was quickly changed
to dark gray. The change in color also implicated that the top surface
of Pd/CZ catalysts should be easily reduced. Thus, the existence of
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Ce1−x−yZrxPdyO2−ı enhanced the catalytic activities in methane
oxidation and CO/NO reaction [27,28]. In this study, the dissolu-
tion of palladium into the lattice of CZ support was intimated by
Raman spectroscopy. However, the enhancement of activities by
the formation of Ce1−x−yZrxPdyO2−ı was not verified for propene
oxidation reaction.
572
CZ
112
The oxygen concentration in propene oxidation reaction test
also affected the catalytic performance. However, the initial
activities below 20% of CO2 yields were not changed, whether the
oxygen concentration was set at stoichiometric (0.5%) or insuffi-
cient condition (0.4%). The reducibility of Pd/CZ catalyst, which was
investigated by TPR measurements (Fig. 8), would play an impor-
tant role for the oxidation of propene under the insufficient oxygen
condition. In other words, the property of oxygen storage capacity
(OSC) of Pd/CZ catalysts should assist the complete oxidation of
propene. The temperature, at which the OSC property of catalysts
works, depends on the amount of palladium. As mentioned in
Section 3.4, H2-TPR profiles indicated that the order of reduc-
tion temperature was 5.0, 1.0Pd/CZ < 0.2Pd/CZ < 0.04Pd/CZ « CZ.
Therefore, the order of catalytic performances was also
5.0Pd/CZ > 1.0Pd/CZ > 0.2Pd/CZ > 0.04Pd/CZ » CZ. In the activity
tests of propene oxidation, the reducing agent is not H2 gas but
propene, and the oxygen in samples should be released at higher
temperature. The variations in slopes of light off curves were
confirmed in the insufficient oxygen condition (Fig. 2(a) and (b)).
The variation was clearly observed in 5.0Pd/CZ under the 0.4%
of oxygen concentration, as can be seen in Fig. 2(a). The results
implicated that the propene oxidation reaction proceed with two
reaction mechanisms. As discussed above, the OSC property of
CZ support helps the propene oxidation at low temperature and
under 0.4% of oxygen concentration. At high temperature, on the
contrary, the amount of lattice oxygen from CZ material would not
be enough to decompose propene gas. Thereby, the rate of total
oxidation of propene reduced over the 40% of CO2 yields.
0.04Pd/CZ
82
0.2Pd/CZ
1.0Pd/CZ
5.0Pd/CZ
114
118
76
0
100
200
300
400
500
600
700
800
Temperature / ºC
Fig. 8. H2-TPR profiles of CZ and Pd/CZ catalysts.
others. Therefore, the palladium component in Pd/CZ should be
easily reduced and oxidized as well as CZ material.
3.5. Discussion
The difference of catalytic activities due to Pd loading, pretreat-
ment conditions, and oxygen concentration should be carefully
discussed on the basis of the results of characterization. With the
increase of Pd loading, Pd/CZ catalysts exhibited higher activities,
regardless of pretreatment and reaction conditions. Therefore, it
was suggested the palladium species must be the active site of
propene oxidation. After the reduction treatment (Fig. 1(b) and
Fig. 2(b)), the catalytic performance of 1.0Pd/CZ was almost equiv-
alent to that of 5.0Pd/CZ. The Pd loading amount of 5.0 wt.% was
enough large to decompose propene gas, and some part of pal-
ladium component seems not to play a role as active site. This
assumption was supported by the low dispersion of palladium and
calculated diameter of palladium particles in 5.0Pd/CZ, as can be
seen in Table 1.
The effects of pretreatment conditions were explained by the
results of IR measurement with CO adsorption (Figs. 6 and 7).
The catalysts of Pd/CZ reduced at 600 ◦C in hydrogen exhibited
higher activities than the oxidized Pd/CZ catalysts (Figs. 1 and 2).
The palladium species in reduced catalysts existed in metallic
state, and the chemical state of palladium in the catalysts after
oxidation treatment was metallic and oxidized state. This means
that metallic palladium (Pd0) shows higher activities for propene
oxidation than oxidized palladium (Pd2+). In addition, the exist-
ence of metallic palladium was confirmed even after the heat
treatment at 600 ◦C in O2 gas. This is because a part of oxidized
palladium was rapidly reduced in CO atmosphere. The reactive
palladium species should be the active site. Thereby, the perfor-
mance of 0.04Pd/CZ after the oxidation treatment was extremely
low, because the IR bands ascribed as metallic palladium (1971 and
2063 cm−1) were weaker than the other Pd/CZ catalysts, as shown
in Fig. 6. In the case of 5.0Pd/CZ, the existence of metallic palladium
was confirmed by the negative peak derived from the desorption
of hydrogen gas. So, the light off curves of 5.0Pd/CZ in Fig. 1(a)
was similar to the curves of reduced 5.0Pd/CZ (Fig. 1(b)) with
no relation to the pretreatment conditions. Several researchers
have reported that the complex oxides such as Ce1−xPdxO2−ı or
4. Conclusions
The propene oxidation tests over CZ(1/3) and Pd/CZ(1/3) treated
in oxidation and reduction atmosphere were conducted, and the
effects of Pd loadings, pretreatment conditions, and oxygen con-
centration were investigated. The Pd/CZ catalysts exhibited higher
activities, with the increase of Pd loadings, after the reduction
treatment in hydrogen, and under 0.5% of oxygen concentration
(stoichiometric condition). Consequently, it was concluded that
the reactive palladium species with high reducibility were active
for propene oxidation. The high dispersion of PdOx particles was
achieved due to the strong interaction between PdOx particles and
CZ support, and the dispersion was maintained after the propene
oxidation tests. Furthermore, the lattice oxygen of CZ material
assists the complete oxidation of propene, especially under the
insufficient oxygen condition. In the future, it is necessary to inves-
tigate the activities of Pd/CZ(1/3) catalysts in the other purification
reaction of hydrocarbons and CO, or three way catalytic reaction.
Acknowledgment
A part of this work was supported by a project of the Gifu Tech-
nical Innovation Program from the Ministry of Education, Culture,
Sports, Science and Technology in Japan.
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