Low-temperature complete combustion of volatile organic compounds over novel Pt/CeO2ZrO2SnO2/γ-Al 2O3 catalysts

Article abstract of DOI:10.1246/bcsj.20110382

Catalytic combustion of volatile organic compounds (VOCs) such as ethylene, toluene, and acetaldehyde over novel Pt/CeO₂ZrO₂SnO ₂/β-Al₂O₃ catalysts prepared by a coprecipitation method was investigated. The introduction of a small amount of SnO₂ within the CeO₂ZrO₂ lattice as a promoter was considerably effective to enhance the oxygen release and storage abilities of the catalysts, so that complete oxidation of VOCs was markedly activated. This improvement of the reducibility of the catalyst can be ascribed to the simultaneous reduction of Ce⁴⁺ and Sn⁴⁺ in the CeO ₂ZrO₂SnO₂ solid solutions. By the optimization of the composition and the Pt amount, complete oxidation of ethylene, toluene, and acetaldehyde was realized at temperatures as low as 55, 110, and 140°C over a Pt(10 wt%)/Ce_0.68Zr_0.17Sn_0.15-O _2.0(16 wt%)/β-Al₂O₃ catalyst, respectively.

Full text of DOI:10.1246/bcsj.20110382

522
Bull. Chem. Soc. Jpn. Vol. 85, No. 4, 522-526 (2012)
© 2012 The Chemical Society of Japan
Low-Temperature Complete Combustion of Volatile Organic Compounds
over Novel Pt/CeO2-ZrO2-SnO2/£-Al2O3 Catalysts
Keisuke Yasuda,1,2 Atsuki Yoshimura,1 Atsushi Katsuma,1 Toshiyuki Masui,1 and Nobuhito Imanaka*1
1Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
2Japan Society for the Promotion of Science, 8 Ichibancho, Chiyoda-ku, Tokyo 102-8472
Received December 6, 2011; E-mail: imanaka@chem.eng.osaka-u.ac.jp
Catalytic combustion of volatile organic compounds (VOCs) such as ethylene, toluene, and acetaldehyde over novel
Pt/CeO2-ZrO2-SnO2/£-Al2O3 catalysts prepared by a coprecipitation method was investigated. The introduction of
a small amount of SnO2 within the CeO2-ZrO2 lattice as a promoter was considerably effective to enhance the oxygen
release and storage abilities of the catalysts, so that complete oxidation of VOCs was markedly activated. This improve-
ment of the reducibility of the catalyst can be ascribed to the simultaneous reduction of Ce4+ and Sn4+ in the CeO2-ZrO2-
SnO2 solid solutions. By the optimization of the composition and the Pt amount, complete oxidation of ethylene, toluene,
and acetaldehyde was realized at temperatures as low as 55, 110, and 140 °C over a Pt(10 wt %)/Ce0.68Zr0.17Sn0.15
2.0(16 wt %)/£-Al2O3 catalyst, respectively.
-
O
Volatile organic compounds (VOCs) include a wide range
of organic chemical compounds such as alkenes, aldehydes,
ketones, aromatics, and other light-weight hydrocarbons. They
are emitted into the atmosphere from both anthropogenic
and biogenic sources (mainly vegetation). The anthropogenic
VOCs are released in a variety of processes such as exhaust
gases from the internal combustion engine, power generation
plants, and chemical processing plants. VOCs are recognized
as major sources of air pollution including photochemical
smog, ground-level ozone, suspended particulate matter, and
sick building syndrome, and are considered to be detrimental to
human health and the environment.1,2
and steam over the catalyst at moderate temperatures, leading
to minimal negative side effects such as NOx production.5
However, it is extremely difficult to accomplish complete
combustion of VOCs at low temperatures and, actually, the
catalyst has to be heated at least up to 150 °C.10-12 Thus, it is
considerably important to develop a novel active catalyst that
can realize complete oxidation of VOC at temperatures as low
as possible.
In our previous studies, we found that dibismuth trioxide
(Bi2O3) doping into the CeO2-ZrO2 lattice was significantly
effective to enhance the oxygen release and storage properties
of CeO2-ZrO2 solid solutions.13-15 In addition, a Ce0.64Zr0.16
-
Among a large number of VOCs, toluene is widely used
as an organic solvent for paints, printing inks, adhesives, and
antiseptics due to its excellent ability to dissolve substances,
and acetaldehyde is well-known as a raw material to produce
synthetic resins and rubbers. However, they have unpleasant
odors and cause sick building syndrome by evaporating into
the atmosphere. Ethylene is an important feedstock for the
manufacture of many organic products in the petrochemical
industry. Furthermore, it is a plant hormone released from fruits
to control physiological process such as seed germination and
the blooming of flowers. During postharvest storage of fruits
and vegetables, however, it induces negative effects enhancing
the senescence, ripening, and softening.3 To protect our health
and the environment and to maintain agricultural products fresh
away from such noxious influences, it is necessary to remove
the VOCs released into the atmosphere as much as possible.
Up to now, several technologies such as flame combustion,4
catalytic combustion,5 catalytic destruction using ozone and
plasma,6 photocatalytic decomposition,7 adsorption processes,8
and biological treatment9 have been developed to eliminate
VOCs. Among these methods, catalytic combustion is one of
the most promising processes for VOCs abatement because
VOCs can be completely oxidized to harmless carbon dioxide
Bi0.20O1.9 solid exhibits a remarkable low-temperature reduc-
tion behavior below 100 °C when it is supported on a high
surface area £-Al2O3 support.16,17 In order to establish complete
oxidation of VOCs at moderate temperature, novel catalysts,
in which CeO2-ZrO2-Bi2O3 is employed as a promoter in the
conventional Pt/£-Al2O3 catalyst, were prepared. As a result,
we demonstrated that the Pt/CeO2-ZrO2-Bi2O3/£-Al2O3 cata-
lysts can completely oxidize ethylene, toluene, and acetalde-
hyde at temperatures as low as 65,18-21 120,20-22 and 150 °C,21
which are dramatically lower than those of the conventional
catalysts, respectively. These significantly high catalytic activ-
ities of the Pt/CeO2-ZrO2-Bi2O3/£-Al2O3 catalysts will be
attributed to the appearance of electronic conduction associated
with the simultaneous reduction of both Ce4+ and Bi3+ ions.
In this study, we have focused on tin dioxide (SnO2) applied
for gas sensing element, which has been well-known as a typical
n-type semiconductor with electronic conduction.23 Tin ion has
both tetravalent (+4) and divalent (+2) states, and Ce1¹xSnxO2
solid solutions, in which Ce4+ sites in CeO2 were partially
substituted with Sn4+ ions, have been reported to possess
excellent oxygen storage and release properties compared with
those of CeO2 and SnO2.24-26 Therefore, introduction of a small
amount of SnO2 as the substitute for Bi2O3 into the CeO2-ZrO2
Published on the web March 24, 2012; doi:10.1246/bcsj.20110382

K. Yasuda et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
523
lattice will be reasonably effective in enhancing the reducibility
of the catalyst. Accordingly, Pt/CeO2-ZrO2-SnO2/£-Al2O3
catalysts were prepared by coprecipitation and the oxidation
activities for several VOCs were investigated.
Experimental
The CeO2-ZrO2-SnO2/£-Al2O3 supports were prepared by a
coprecipitation method. Tin(II) oxalate (Sn(C2O4)) was dis-
solved into a mixture of £-Al2O3 powder (DK Fine, AA-300),
¹1
¹1
aqueous solutions of 1.0 mol L Ce(NO3)3 and 0.1 mol L
ZrO(NO3)2 in a stoichiometric ratio. The CeO2-ZrO2-SnO2
content was adjusted to be 16 wt % of the total support and the
molar ratio of Ce:Zr:Sn were controlled to be 80:20:0, 76:19:5,
72:18:10, 68:17:15, and 64:16:20, respectively. The pH of the
aqueous mixture was adjusted to 11 by the dropwise addition
of aqueous ammonia (5%). After stirring for 12 h at room
temperature, the resulting precipitate was collected by filtration,
washed several times with deionized water, and then dried at
80 °C for 6 h. The sample was heated at 600 °C for 1 h in an
ambient atmosphere. Supported platinum catalysts (Pt/CeO2-
ZrO2-SnO2/£-Al2O3) were prepared by impregnating the
CeO2-ZrO2-SnO2/£-Al2O3 supports with a platinum colloid
stabilized with PVP (Tanaka Kikinzoku Kogyo Co., Ltd.).
After impregnation, the catalyst was dried at 80 °C for 12 h and
then finally calcined at 500 °C for 4 h. The amount of Pt in
the catalyst was adjusted in the range of 5-12 wt %. In addition,
the Pt(7 wt %)/Ce0.64Zr0.16Bi0.20O1.9(16 wt %)/£-Al2O3 catalyst
was also prepared by a similar procedure for comparison.22
The compositions of the catalysts were analyzed with an
X-ray fluorescence spectrometer (XRF; Rigaku, ZSX-100e).
The crystal structures of the catalysts were identified by X-ray
powder diffraction (XRD; Rigaku, SmartLab) analysis using
Cu K¡ radiation (40 kV, 30 mA). Brunauer-Emmett-Teller
(BET) specific surface area was measured using nitrogen
adsorption at ¹196 °C (Micromeritics Tristar 3000). X-ray
photoelectron spectroscopy (XPS; ULVAC 5500MT) was per-
formed at room temperature using Mg K¡ radiation (1253.6
eV). The effect of charging on the binding energies was cor-
rected with respect to the C 1s peak at 284.6 eV. The temper-
ature-programmed reduction (TPR) measurements were carried
out under a flow of pure H2 (80 mL min¹1) at a heating rate
Figure 1. XRD patterns of the Pt(7 wt %)/Ce0.8(1¹x)
Zr0.2(1¹x)SnxO2.0(16 wt %)/£-Al2O3 (0 ¯ x ¯ 0.20) cata-
lysts.
-
toluene (space velocity (S.V.) = 12000 L kg¹1 h¹1), 1 vol %
acetaldehyde (S.V. = 10000 L kg¹1 h¹1), or 1 vol % ethylene
(S.V. = 12000 L kg¹1 h¹1) in air balance. Prior to the measure-
ments, the catalysts were heated at 200 °C for 2 h in a flow of
Ar (20 mL min¹1) to remove water molecules adsorbed on the
surface of the catalyst. The catalytic activity was evaluated
in terms of VOCs conversion. The gas composition after the
reaction was analyzed using a gas chromatograph with TCD
(Shimadzu GC-8AIT) and a flame ionization detector (FID;
Shimadzu GC-8AIF).
Results and Discussion
Figure 1 shows XRD patterns of the Pt(7 wt %)/Ce0.8(1¹x)
-
Zr0.2(1¹x)SnxO2.0(16 wt %)/£-Al2O3 (0 ¯ x ¯ 0.20) catalysts.
The XRD patterns of the catalysts contain diffraction peaks
of only the cubic fluorite-type oxide, metallic platinum, and
£-Al2O3 and no crystalline impurities were observed. The inset
shows the enlargement of the pattern in the 2ª range from 54
to 60°. The diffraction peaks assigned to the cubic fluorite
structure slightly shifted to higher angles with increasing the
Sn4+ content, suggesting that Ce4+ (ionic radius: 0.097 nm)27
and Zr4+ (ionic radius: 0.084 nm)27 in CeO2-ZrO2 were par-
tially substituted with smaller Sn4+ (ionic radius: 0.081 nm)27
to form solid solutions.
The composition and BET specific surface area of the
Pt(7 wt %)/Ce0.8(1¹x)Zr0.2(1¹x)SnxO2.0(16 wt %)/£-Al2O3 (0 ¯
x ¯ 0.20) catalysts are summarized in Table 1. The composi-
tions of the catalysts were confirmed by the XRF analysis to be
in good agreement with their stoichiometric values within
the experimental errors. The BET specific surface area of the
catalysts decreased with increasing Sn content, probably due to
the lower melting point of SnO2 (1630 °C)28 compared to CeO2
(2445 °C)29 and ZrO2 (2710 °C).29
¹1
of 5 °C min using a gas chromatograph with a thermal
conductivity detector (TCD; Shimadzu GC-8AIT). Following
the TPR experiments, the total oxygen storage capacity (OSC)
was measured using a pulse-injection method at 427 °C
(700 K). The total OSC value was estimated as the uptake of
O2 from the O2 pulse by comparing the TCD signals obtained
with and without the catalyst. Dynamic OSC measurements
were also performed by increasing the temperature in a step-
wise manner between 50 and 300 °C. The catalyst was first
heated in a flow of He (100 mL min¹1) at each temperature, and
then pulses of H2 (0.2 mL) and O2 (0.2 mL) were alternately
injected over the catalyst. The average amount of oxygen
incorporated into the catalyst was evaluated as the dynamic
OSC by comparing the peak area detected by the TCD with that
obtained in the absence of the catalyst.
Figure 2 presents the temperature dependencies of tolu-
The oxidation activity for VOCs was tested in a conven-
tional fixed-bed flow reactor consisting of a 10 mm diameter
quartz glass tube. The feed gas was composed of 0.09 vol %
ene oxidation over the Pt(7 wt %)/Ce0.8(1¹x)Zr0.2(1¹x)SnxO2.0-
(16 wt %)/£-Al2O3 (0 ¯ x ¯ 0.20) catalysts. It was confirmed
that only CO2 and H2O were produced by the complete

524
Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
Low-Temperature Complete Combustion of VOCs
Table 1. Composition and BET Surface Area of the
Pt(7 wt %)/Ce0.8(1¹x)Zr0.2(1¹x)SnxO2.0(16 wt %)/£-Al2O3
(0 ¯ x ¯ 0.20) Catalysts
BET
Catalyst composition
surface area
¹1
/m2 g
Pt(7 wt %)/Ce0.79Zr0.21O2.0(17 wt %)/£-Al2O3
Pt(7 wt %)/Ce0.75Zr0.20Sn0.05O2.0(17 wt %)/£-Al2O3
Pt(7 wt %)/Ce0.71Zr0.19Sn0.10O2.0(16 wt %)/£-Al2O3
Pt(7 wt %)/Ce0.68Zr0.17Sn0.15O2.0(16 wt %)/£-Al2O3
Pt(7 wt %)/Ce0.64Zr0.16Sn0.20O2.0(17 wt %)/£-Al2O3
158
155
151
149
147
Figure 3. Temperature dependencies of toluene oxidation
over the Pt(5-12 wt %)/Ce0.68Zr0.17Sn0.15O2.0(16 wt %)/
£-Al2O3 catalysts.
optimized for the most effective toluene oxidation.22 The
toluene catalytic activity was remarkably promoted by the
Bi2O3 or SnO2 doping into the CeO2-ZrO2 lattice as a pro-
moter. Among these catalysts, the highest activity was obtained
for the Pt/CZS/Al2O3 catalyst, over which the toluene oxida-
tion activity appeared from 70 °C and the complete oxidation
of toluene was realized at a temperature as low as 110 °C,
which is the lowest temperature ever reported. The reason for
the composition dependence of the oxidation activity of the
catalyst can be ascribed to the different reducibilities of the
supports, as discussed in detail later. Furthermore, the tem-
perature dependencies of acetaldehyde and ethylene oxidation
over the Pt/CZ/Al2O3, Pt/CZB/Al2O3, and Pt/CZS/Al2O3
catalysts are shown in Figures 4b and 4c, respectively. Total
oxidation of acetaldehyde and ethylene into CO2 and steam was
confirmed by gas chromatography-mass spectrometry as in the
case of toluene. Also for acetaldehyde and ethylene oxidation,
the introduction of CeO2-ZrO2-SnO2 as the promoter was
highly effective to enhance the catalytic activity. As a result,
the complete oxidation of acetaldehyde and ethylene on the
Pt/CZS/Al2O3 catalyst was realized at temperatures as low as
140 and 55 °C, respectively.
Figure 2. Temperature dependencies of toluene oxidation
over the Pt(7 wt %)/Ce0.8(1¹x)Zr0.2(1¹x)SnxO2.0(16 wt %)/
£-Al2O3 (0 ¯ x ¯ 0.20) catalysts.
oxidation of toluene, and no toluene-derived compounds were
detected as by-products with a gas chromatography mass
spectrometer. The toluene oxidation activity was significantly
enhanced by the incorporation of SnO2 into the CeO2-ZrO2
lattice as a promoter, and the highest activity was obtained
for the Pt(7 wt %)/Ce0.68Zr0.17Sn0.15O2.0(16 wt %)/£-Al2O3 cat-
alyst, over which the complete oxidation of toluene to carbon
dioxide and steam was confirmed at 120 °C.
The temperature dependencies of the oxidation activity of
the catalysts with different amount of Pt in the Pt/Ce0.68Zr0.17
-
The oxidation state of Sn in the Pt/CZS/Al2O3 catalyst was
confirmed by XPS analysis. The XPS of Sn 3d core-level is
shown in Figure 5. The binding energies of Sn 3d3/2 (494.7 eV)
Sn0.15O2.0(16 wt %)/£-Al2O3 catalysts are shown in Figure 3.
The optimum amount of Pt to bring the highest activity to the
catalysts was approximately 10 wt %, and, as a result, toluene
was completely oxidized at a temperature as low as 110 °C.
For catalysts containing larger amounts of Pt, the efficacy of
the metal (Pt) dispersion was decreased by agglomeration
and particle growth of the Pt particles. As a result, it can be
concluded that excess amounts of Pt cause deterioration in the
catalytic activity in these systems. A similar result was observed
in the case of toluene oxidation over the Pt/CeO2-ZrO2-Bi2O3/
£-Al2O3 catalysts, and agglomeration of the platinum particles
was ascertained by the transmission electron micrographs.22
Figure 4a depicts the temperature dependencies of tolu-
ene oxidation over the Pt(10 wt %)/Ce0.79Zr0.21O2.0(16 wt %)/
30
and Sn 3d5/2 (486.3 eV) can be assigned to Sn4+ in SnO2
while no peaks that would suggest the presence of other
oxidation states of Sn were observed.
As mentioned above, the results in Figure 4 suggest that
the oxidation activity of the catalyst depends on the reduci-
bility of the catalyst support. Indeed, we demonstrated that the
CO oxidation activity is correlated with the reducibility of the
support.31 Temperature-programmed reduction profiles using
hydrogen (H2-TPR) were measured for comparison of the
reduction behavior of the Ce0.79Zr0.21O2.0(16 wt %)/£-Al2O3
(CZ/Al2O3), Ce0.64Zr0.16Bi0.20O1.9(16 wt %)/£-Al2O3 (CZB/
Al2O3), and Ce0.68Zr0.17Sn0.15O2.0(16 wt %)/£-Al2O3 (CZS/
Al2O3) supports as shown in Figure 6. The profile of CZ/
Al2O3 did not exhibit any noteworthy reduction peak in the
temperature range below 200 °C. In contrast for CZB/Al2O3,
the reduction initiated from 60 °C and a significant reduction
£-Al2O3 (Pt/CZ/Al2O3), Pt(7 wt %)/Ce0.64Zr0.16Bi0.20O1.9
(16 wt %)/£-Al2O3 (Pt/CZB/Al2O3), and Pt(10 wt %)/Ce0.68
Zr0.17Sn0.15O2.0(16 wt %)/£-Al2O3 (Pt/CZS/Al2O3) catalysts.
The amount of Pt(7 wt %) on the Pt/CZB/Al2O3 catalyst was
-
-

K. Yasuda et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
525
Figure 5. XPS of the Sn 3d core-level of the Pt/CZS/Al2O3
catalyst.
Figure 6. TPR profiles of the CZ/Al2O3, CZB/Al2O3, and
CZS/Al2O3 samples.
80 °C. In addition, the total oxygen storage capacity (OSC)
relevant to the overall reducibility of the catalyst was evaluated
by injecting oxygen into the sample reduced by the TPR
experiments. As a result, the total OSC value of CZS/Al2O3
was 198 ¯mol-O2 g¹1, which is significantly higher than those
of CZ/Al2O3 (107 ¯mol-O2 g¹1) and CZB/Al2O3 (130 ¯mol-
O2 g¹1). The improvement of the oxygen release and storage
abilities can be attributed to the enhancement of the redox
Figure 4. Temperature dependencies of (a) toluene, (b) acet-
aldehyde, and (c) ethylene oxidation over the Pt/CZ/
Al2O3, Pt/CZB/Al2O3, and Pt/CZS/Al2O3 catalysts.
activity by the synergistic effects of Ce4+/Ce3+ and Sn4+
Sn2+ 24-26
/
peak was observed at 97 °C. This promotion of the reduction
can be attributed to the high reducibility of Bi2O3 and the
formation of oxide anion vacancies by partially replacing the
Ce4+ and Zr4+ sites with Bi3+ ion to form solid solutions,
leading to the enhancement of the oxide anion conductivi-
ty.16,17 In the case of CZS/Al2O3, furthermore, the oxygen
release began at a temperature as low as 30 °C, and a specific
reduction peak was observed at 95 °C. Accordingly, the intro-
duction of SnO2 into the CeO2-ZrO2 lattice was remarkably
more effective than that of Bi2O3 to enhance the reduc-
ibility of the support at lower temperatures, corresponding to
high oxidation activity in the low-temperature region below
.
The total OSC expresses the maximum oxygen storage ca-
pacity, while the dynamic OSC is correlated to the most reactive
and readily available oxygen species on or near the surface of
the catalyst. The dynamic OSC, which is defined as the amount
of oxygen released and stored by every reduction and oxidation
cycle in a short time-scale, can be estimated by alternating pulse
injections of H2 and O2 every 20 s. As demonstrated in Figure 7,
the responses of the CZS/Al2O3 sample to rapid reduction and
oxidation was considerably higher than those of the CZ/Al2O3
and CZB/Al2O3 samples at each temperature. In particular, the
dynamic OSC value of the CZS/Al2O3 sample (0.565 ¯mol-

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Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
Low-Temperature Complete Combustion of VOCs
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Conclusion
Complete oxidation activities of VOCs such as ethylene,
toluene, and acetaldehyde were evaluated over the Pt/CeO2-
ZrO2-SnO2/£-Al2O3 catalysts prepared using a coprecipitation
method. These VOCs were completely oxidized into CO2 and
steam (H2O) over the Pt/CeO2-ZrO2-SnO2/£-Al2O3 catalysts
at low temperatures without the formation of by-products. The
oxidation activity is correlated with the reducibility and oxygen
storage capacity of the catalyst support, and these properties
are significantly enhanced by the introduction of SnO2 into the
CeO2-ZrO2 lattice. As a result, complete oxidation of ethylene,
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The authors thank Dr. Hirokazu Izumi (Hyogo Prefectural
Institute of Technology) for his assistance with the XPS
measurements. This study was partially supported by a Grant-
in-Aid for JSPS Fellows (No. 22526) from the Japan Society
for the Promotion of Science, the Industrial Technology
Research Grant Program ’08 (Project ID: 08B42001a) from
the New Energy and Industrial Technology Development
Organization (NEDO) of Japan, and the Environment Research
and Technology Development Fund (B-0907) of the Ministry
of the Environment, Japan.
31 N. Imanaka, T. Masui, H. Imadzu, K. Yasuda, Chem.
Commun. 2011, 47, 11032.