Improvement of toluene oxidation catalysis by Cu doping into Co3O4in Pt/Co3O4/CeO2-ZrO2-SnO2/γ-Al2O3 catalysts

Article abstract of DOI:10.1246/bcsj.20140331

1 wt % Pt/11 wt % (Co_1-xCu_x)₃O_4-δ/16 wt % Ce_0.62Zr_0.20Sn_0.18O_2.0/γ-Al₂O₃ catalysts were prepared to realize complete combustion of toluene at lower temperatures than that of a 1 wt % Pt/11 wt % Co₃O₄/16 wt % Ce_0.62Zr_0.20Sn_0.18O_2.0/γ-Al₂O₃ catalyst previously reported by our group without increasing the amount of platinum. Addition of copper into the Co₃O₄ lattice was effective to facilitate oxygen release and storage, and thereby, the toluene oxidation activity was enhanced. The highest activity for the combustion of toluene was observed using a 1 wt % Pt/13 wt % (Co_0.97Cu_0.03)₃O_4-δ/16 wt % Ce_0.62Zr_0.20Sn_0.18O_2.0/γ-Al₂O₃ catalyst, and despite a smaller platinum loading in the present catalyst, toluene was completely oxidized to carbon dioxide and water vapor at the lower temperature of 140°C compared to that (160°C) using 1 wt % Pt/11 wt % Co₃O₄/16 wt % Ce_0.62Zr_0.20Sn_0.18O_2.0/γ-Al₂O₃.

Full text of DOI:10.1246/bcsj.20140331

Improvement of Toluene Oxidation Catalysis by Cu Doping into
Co3O4 in Pt/Co3O4/CeO2-ZrO2-SnO2/γ-Al2O3 Catalysts
Toshiyuki Masui, Tomoya Kamata, Nashito Fukuhara, and Nobuhito Imanaka*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
E-mail: imanaka@chem.eng.osaka-u.ac.jp
Received: October 30, 2014; Accepted: February 17, 2015; Web Released: February 25, 2015
1 wt % Pt/11 wt % (Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18
O2.0/γ-Al2O3 catalysts were prepared to realize com-
plete combustion of toluene at lower temperatures than that of a 1 wt % Pt/11 wt % Co3O4/16 wt % Ce0.62Zr0.20Sn0.18O2.0
/
γ-Al2O3 catalyst previously reported by our group without increasing the amount of platinum. Addition of copper into
the Co3O4 lattice was effective to facilitate oxygen release and storage, and thereby, the toluene oxidation activity was
enhanced. The highest activity for the combustion of toluene was observed using a 1 wt % Pt/13 wt % (Co0.97Cu0.03)3O4¹¤
16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 catalyst, and despite a smaller platinum loading in the present catalyst, toluene
was completely oxidized to carbon dioxide and water vapor at the lower temperature of 140 °C compared to that (160 °C)
/
O
using 1 wt % Pt/11 wt % Co3O4/16 wt % Ce0.62Zr0.20Sn0.18
O2.0/γ-Al2O3.
Toluene is a well-known and popular organic solvent for
paints and adhesives. However, this compound is also a
principal chemical substance responsible for sick building
syndrome. Therefore, it is necessary to reduce the amount of
toluene released into the atmosphere as much as possible to
avoid the harmful effect. For effective reduction, complete
catalytic combustion of toluene into carbon dioxide and water
vapor is an ecologically simple and clean technology.1
Although a number of combustion catalysts have been reported
until recently for the reduction of toluene,2-9 it is difficult to
accomplish complete combustion of toluene at low temper-
atures.
of toluene was realized at 160 °C without excessive use of
¹1
platinum at S.V. = 12000 L kg¹1 h
.
In this study, copper ion was doped into the Co3O4 lattice
to realize complete oxidation at lower temperatures. Copper
ion can take both mono and divalent states. When copper ions
are doped into the Co3+ and Co2+ sites in Co3O4, oxide anion
vacancies have to be produced by the charge compensation
mechanism. Consequently, it is expected that diffusion of O2¹
through the Co3O4 lattice is enhanced to facilitate toluene oxi-
dation. Therefore, 1 wt % Pt/y wt % (Co1¹xCux)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 catalysts were prepared and the
effect of copper doping on the activity of toluene oxidation was
investigated. Furthermore, the catalyst composition was opti-
mized to give the highest activity.
In contrast, we found in our previous studies that a
combination of platinum and a solid that can supply reactive
oxygen molecules from the inside of the bulk below 100 °C
was significantly effective in facilitating the oxidation of
Experimental
toluene.10,11 In fact, a 7 wt % Pt/16 wt % Ce0.64Zr0.16Bi0.20O1.90
/
Catalyst Preparation.
A Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3
γ-Al2O3 catalyst can completely oxidize toluene at 120 °C,
where the feed gas was composed of 0.09 vol % toluene in an
air balance and the flow rate was 20 mL min¹1 over 0.1 g of the
catalyst [space velocity (S.V.) = 12000 L kg¹1 h¹1].12 Further-
support was synthesized by coprecipitation and impregnation
methods: the Ce0.62Zr0.20Sn0.18O2.0 was synthesized by copre-
cipitation and the subsequent deposition on γ-Al2O3 was carried
out by impregnation. SnC2O4 (7.75 © 10¹2 g) was dissolved in a
mixture of 1.0 mol L Ce(NO3)3 (1.70 mL), 0.1 mol L ZrO-
(NO3)2 (4.25 mL), and 3.0 mol L¹1 nitric acid (15 mL) aqueous
solutions in a stoichiometric ratio, and then the mixture was
impregnated on commercially available γ-Al2O3 (DK Fine, AA-
300). The Ce0.62Zr0.20Sn0.18O2.0 content was adjusted to 16 wt %
of the total support to optimize the oxygen release ability.13,14
The pH of the aqueous mixture was adjusted to 11 by dropwise
¹1
¹1
more, we demonstrated that a 10 wt % Pt/16 wt % Ce0.68Zr0.17
-
Sn0.15 2.0/γ-Al2O3 catalyst can completely oxidize toluene at a
O
temperature as low as 110 °C at the same S.V.13 Unfortunately,
a relatively large amount of platinum was supported on these
catalysts, leading to the use of a scarce resource and expensive
production, and these are not favorable from a practical appli-
cation standpoint. Therefore, it is necessary to reduce the
precious metal content in the catalysts as much as possible with
no significant decrease in the catalytic activity.
addition of aqueous ammonia (5%), whereby the Ce0.62Zr0.20
-
Sn0.18O2.0 particles were precipitated on the surface of γ-Al2O3.
After stirring for 12 h at room temperature, the resulting
To realize such an advanced catalyst, we prepared a 1 wt %
Pt/11 wt % Co3O4/16 wt % Ce0.62Zr0.20Sn0.18
O
2.0/γ-Al2O3 cat-
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 support was collected by filtra-
alyst.14 In this catalyst, cobalt oxide (Co3O4) was employed
as a promoter to facilitate the oxidation of toluene and to
reduce the amount of platinum. As a result, complete oxidation
tion, washed several times with deionized water, and then dried
at 80 °C for 6 h. The sample was ground in an agate mortar and
calcined at 600 °C for 1 h in an ambient atmosphere.
746 | Bull. Chem. Soc. Jpn. 2015, 88, 746–751 | doi:10.1246/bcsj.20140331
© 2015 The Chemical Society of Japan

Copper-doped cobalt oxide promoters were deposited on the
surface of the Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 support by mixing
0.1 mol L¹1 Co(NO3)3 and 0.1 mol L¹1 Cu(NO3)2 aqueous solu-
tions with the Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 support (0.60 g)
dispersed in dionized water (20 mL) in stoichiometric ratios.
After mixing, the homogeneous samples were evaporated to
dryness at 80 °C for 12 h, and then calcined at 500 °C for 1 h in
an ambient atmosphere. The composition was adjusted initially
(FID; Shimadzu GC-8AIF) and a gas chromatograph-mass
spectrometer (GC-Mass; Shimadzu GCMS-QP2010 Plus). The
catalytic test for toluene oxidation was also conducted at
150 °C using a feed gas composed of toluene (0.09 vol %), N2
(79.91 vol %), and 18O2 (20.0 vol %) to confirm the presence or
absence of the contribution of lattice oxygen. Reaction time
dependence of the masses of C16O2, C16O18O, and C18O2
produced by toluene oxidation were analyzed by GC-Mass.
O
O
to 11 wt % (Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18O2.0
/
Results and Discussion
γ-Al2O3 (x = 0, 0.01, 0.03, 0.05, and 0.10), where the amount
of 11 wt % was based on the results for Co3O4 to give the high-
est toluene oxidation activity.14 After the optimization of the
copper content, the amount of the Co3O4-based promoter was
then optimized to give the highest toluene oxidation activity.
Finally, a supported platinum catalyst (1 wt % Pt/y wt %
Figure 1 shows XRD patterns of 1 wt % Pt/11 wt %
(Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18
0, 0.01, 0.03, 0.05, and 0.10). Only cubic fluorite-type Ce0.62
O2.0/γ-Al2O3 (x =
-
Zr0.20Sn0.18O2.0, cubic spinel-type (Co1¹xCux)3O4¹¤, and γ-
Al2O3 were observed in the XRD patterns and no crystalline
impurities were observed. No peaks corresponding to platinum
appeared probably due to the small platinum particles that are
highly dispersed on the surface of the catalysts. The XRD
peaks corresponding to spinel-type (Co1¹xCux)3O4¹¤ shifted to
lower angle with increasing copper content. The cubic lattice
constants of the (Co1¹xCux)3O4¹¤ promoters increased with
increasing copper content, as summarized in Table 1. In Co3O4,
Co2+ and Co3+ are located in tetrahedral (4-coordination) and
octahedral (6-coordinate) sites, and their ionic radii are 0.058
and 0.061 nm, respectively.15 The ionic radii of Cu+ for 4- and
6-coordination are 0.060 and 0.077 nm and those of Cu2+ are
0.057 and 0.073 nm, respectively.15 As we will discuss later on
the XPS results, Cu+ is dissolved into the tetrahedral Co2+ site,
while Cu2+ is doped into the octahedral Co3+ site to form solid
solutions. As a result, the cubic lattice constant monotonically
increases with increasing the Cu+ and Cu2+ contents. BET spe-
(Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18
6, 11, 13, and 17) was prepared by impregnating the y wt %
(Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 solid
O2.0/γ-Al2O3, y =
O
with a commercially available 4 wt % platinum colloid stabi-
lized with poly(vinylpyrrolidone) in a water solvent. After
impregnation, the sample was dried at 80 °C for 6 h, and then
calcined at 500 °C for 4 h. For reference, the 1 wt % Pt/11 wt %
Co3O4/16 wt % Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 catalyst without
the copper doping was also prepared using the same procedure.
The sample composition of each sample was confirmed using
an X-ray fluorescence spectrometer (XRF; Rigaku, ZSX-100e).
Catalyst Characterization. The crystal structures of the
catalysts were identified by X-ray powder diffraction (XRD;
Rigaku, SmartLab) using Cu Kα radiation (40 kV, 30 mA). The
cubic lattice parameter of (Co1¹xCux)3O4¹¤ was calculated from
the XRD peak angles, which were refined using α-Al2O3 as a
standard. The Brunauer-Emmett-Teller (BET) specific surface
area was measured by nitrogen adsorption at ¹196 °C (Micro-
meritics Tristar 3000). X-ray photoelectron spectroscopy (XPS;
ULVAC 5500MT) measurement was performed at room tem-
perature using Mg Kα1 radiation (1253.6 eV). The effect of
charging on the binding energies was corrected with respect to
the C1s peak at 284.6 eV. Transmission electron microscopic
images were also taken with an accelerating voltage of 300 kV
(TEM; Hitachi H-9000NAR). Temperature-programmed reduc-
tion (TPR) measurements were carried out under a flow of
5 vol % H2-95 vol % Ar (50 mL min¹1) at a heating rate of
cific surface areas of the 1 wt % Pt/11 wt % (Co1¹xCux)3O4¹¤
/
¹1
5 °C min (BEL JAPAN, BELCAT-B). Following the TPR
experiments, the total oxygen storage capacity (OSC) was
measured using a pulse-injection method at 427 °C. The metal
dispersion analysis was carried out using a pulse method at
¹50 °C with 10% CO-He (0.03 mL) using the same apparatus
(BELCAT-B).
Catalytic Test for Toluene Oxidation.
The oxidation
activity for toluene was tested in a conventional fixed-bed flow
reactor consisting of a 10-mm-diameter quartz glass tube. The
feed gas was composed of 0.09 vol % toluene in an air balance
and the rate was 20 mL min¹1 over 0.1 g of the catalyst (S.V. =
12000 L kg¹1 h¹1). Prior to the measurements, the catalyst was
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 toluene con-
version. The gas composition after the reaction was analyzed
using a gas chromatograph with a flame ionization detector
θ
2 /degree
Figure 1. XRD patterns of the 1 wt % Pt/11 wt %
(Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18
(x = 0, 0.01, 0.03, 0.05, and 0.10) catalysts ( : Ce0.62
Zr0.20Sn0.18O2.0 : γ-Al2O3, : (Co1¹xCux)3O4¹¤).
O2.0/γ-Al2O3
-
,
Bull. Chem. Soc. Jpn. 2015, 88, 746–751 | doi:10.1246/bcsj.20140331
© 2015 The Chemical Society of Japan | 747

Table 1. Cubic Spinel Lattice Parameter of the
(Co1¹xCux)3O4¹¤ Promoters
Lattice parameter of
x in (Co1¹xCux)3O4¹¤
(Co1¹xCux)3O4¹¤/nm
0
0.8084
0.8088
0.8093
0.8101
0.8114
0.01
0.03
0.05
0.10
Table 2. BET Surface Area of the 1 wt % Pt/11 wt %
(Co1¹xCux)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18
Catalysts
O2.0/γ-Al2O3
¹1
x in (Co1¹xCux)3O4¹¤
BET surface area/m2 g
Figure 3. Temperature dependencies of toluene oxidation
on the 1 wt % Pt/y wt % (Co0.97Cu0.03)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 (y = 6, 11, 13, and 17)
0
131
122
120
116
109
0.01
0.03
0.05
0.10
catalysts.
x = 0.03. Despite a small amount of platinum loading in the
present catalyst, toluene was completely oxidized at a lower
temperature of 150 °C compared to that using 1 wt % Pt/11
wt % Co3O4/16 wt % Ce0.62Zr0.20Sn0.18O
2.0/γ-Al2O3 (160 °C),14
indicating the advantage of copper addition to Co3O4.
To further reduce the complete toluene oxidation temper-
ature, then the amount of (Co0.97Cu0.03)3O4¹¤ was optimized.
Figure 3 shows the temperature dependencies of toluene oxida-
tion over the 1 wt % Pt/y wt % (Co0.97Cu0.03)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 catalysts (y = 6, 11, 13, and 17).
The toluene oxidation activity also depended on the amount
of (Co0.97Cu0.03)3O4¹¤, and the highest activity was obtained
for y =13. However, the activity decreased when the (Co0.97
-
Cu0.03)3O4¹¤ content was beyond the optimum amount, prob-
ably due to agglomeration and particle growth.14 The toluene
oxidation activity of the optimum catalyst (y = 13) was initially
observed at 110 °C, and complete oxidation was confirmed at
140 °C.
Figure 2. Temperature dependencies of toluene oxida-
tion on the 1 wt % Pt/11 wt % (Co1¹xCux)3O4¹¤/16 wt %
TEM images of the samples corresponding to y = 11 and
13 are depicted in Figure 4. There is no clear difference in the
size and dispersion state of platinum particles in these catalysts.
The particle size of platinum was smaller than 20 nm for
both catalysts. The platinum dispersion for the 1Pt/11Co100/
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 (x = 0, 0.01, 0.03, 0.05, and
0.10) catalysts.
16 wt % Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 catalysts are summariz-
ed in Table 2. The surface areas decreased with increasing Cu
content. This might be just the result of the larger atomic
weight of Cu (63.55) than that of Co (58.93), because the
specific surface area is expressed per unit of mass (m2 g¹1) and
it tends to decrease for heavy samples.
16CZS/Al2O3,
1Pt/11Co97Cu3/16CZS/Al2O3,
1Pt/
13Co97Cu3/16CZS/Al2O3, and 1Pt/17Co97Cu3/16CZS/
Al2O3 catalysts were 47%, 40%, 43%, and 40%, respectively.
The Pt dispersion was slightly decreased by the Cu doping, but
Figure 2 depicts temperature dependencies of toluene oxi-
dation over the 1 wt % Pt/11 wt % (Co1¹xCux)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 catalysts. Toluene was com-
it was not independent of the amount of (Co0.97Cu0.03)3O4¹¤.
Furthermore, it is also found that Co3O4 is present as nee-
dle crystals, as is the case with Co3O4 on 1 wt % Pt/11 wt %
pletely oxidized into CO2 and water vapor, and no CO or
toluene-derived compounds were detected as by-products with
a gas chromatograph. Accordingly, the carbon balance was
100%. The toluene oxidation activity depended on the catalyst
composition. The Cu doping showed a negative effect below
120 °C. This is probably due to the decrease of the Pt disper-
sion by the Cu doping into the Co3O4 lattice, as we will discuss
later. Focusing on the temperature at which 100% toluene
conversion was achieved, the highest activity was obtained for
Co3O4/16 wt % Ce0.62Zr0.20Sn0.18
surface area of the 1 wt % Pt/y wt % (Co0.97Cu0.03)3O4¹¤
16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 catalysts is summa-
O
2.0/γ-Al2O3.14 BET specific
/
O
rized in Table 3. The surface area decreased with increasing
(Co0.97Cu0.03)3O4¹¤ content, suggesting that some of the
(Co0.97Cu0.03)3O4¹¤ particles are supported in the pores of
Ce0.62Zr0.20Sn0.18
O2.0/γ-Al2O3.
TPR profiles of 1 wt % Pt/11 wt % Co3O4/16 wt %
Ce0.62Zr0.20Sn0.18
O
2.0/γ-Al2O3 (1Pt/11Co100/16CZS/Al2O3),
748 | Bull. Chem. Soc. Jpn. 2015, 88, 746–751 | doi:10.1246/bcsj.20140331
© 2015 The Chemical Society of Japan

Figure 4. TEM photographs of the 1 wt % Pt/y wt %
(Co0.97Cu0.03)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-
Al2O3 catalysts; (a) y = 11 and (b) y = 13.
O
Table 3. BET Surface Area of the 1 wt % Pt/y wt %
Figure 5. TPR profiles of the 1 wt % Pt/11 wt % Co3O4/
16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 (1Pt/11Co100/
16CZS/Al2O3), 1 wt % Pt/11 wt % (Co0.97Cu0.03)3O4¹¤
16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 (1Pt/11Co97Cu3/
16CZS/Al2O3), 1 wt % Pt/13 wt % (Co0.97Cu0.03)3O4¹¤
16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 (1Pt/13Co97Cu3/
16CZS/Al2O3), and 1 wt % Pt/17 wt % (Co0.97-Cu0.03)3-
4¹¤/16 wt % Ce0.62Zr0.20Sn0.18 2.0/γ-Al2O3 (1Pt/
17Co97Cu3/16CZS/Al2O3) catalysts.
(Co0.97Cu0.03)3O4¹¤/16 wt %
γ-Al2O3 Catalysts
Ce0.62Zr0.20Sn0.18O2.0/
O
/
¹1
y
BET surface area/m2 g
O
/
6
11
13
17
125
120
118
117
O
O
O
1 wt % Pt/11 wt % (Co0.97Cu0.03)3O4¹¤/16 wt % Ce0.62Zr0.20
Sn0.18 2.0/γ-Al2O3 (1Pt/11Co97Cu3/16CZS/Al2O3), 1 wt %
Pt/13 wt % (Co0.97Cu0.03)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18O2.0
γ-Al2O3 (1Pt/13Co97Cu3/16CZS/Al2O3), and 1 wt % Pt/
17 wt % (Co0.97Cu0.03)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18O2.0
-
O
/
/
γ-Al2O3 (1Pt/17Co97Cu3/16CZS/Al2O3) were measured and
their oxygen release behaviors were compared to identify the
cause for the positive effects produced by the copper doping and
to optimize the amount of the promoter, as shown in Figure 5.
In the case of 1Pt/11Co100/16CZS/Al2O3, the oxygen release
peak was observed at 183 °C, while the peaks appeared at lower
temperatures of 165, 152, and 156 for 1Pt/11Co97Cu3/16CZS/
Al2O3, 1Pt/13Co97Cu3/16CZS/Al2O3, and 1Pt/17Co97Cu3/
16CZS/Al2O3, respectively. Accordingly, the introduction of
Cu into the Co3O4 lattice and optimization of the amount of
the (Co0.97Cu0.03)3O4¹¤ promoter were remarkably effective in
enhancing the oxygen supply capability at low temperatures.
In the case of 1Pt/17Co97Cu3/16CZS/Al2O3, the reduction of
the sample initiated from the lowest temperature, but the reduc-
tion peak was slightly shifted to higher temperature, which was
consistent with the decrease in the toluene oxidation activity
mentioned above (Figure 3). Furthermore, the total oxygen
storage capacity of 1Pt/13Co97Cu3/16CZS/Al2O3 was 591
¯mol O2 g¹1, which was about 1.3 times larger than that of
1Pt/11Co100/16CZS/Al2O3 (461 ¯mol O2 g¹1). Therefore, the
increase in the oxygen release and storage abilities is thought to
be the cause of the enhancement of the catalytic activity.
Figure 6 illustrates reaction time dependence of CO2 species
produced by toluene oxidation on 1Pt/13Co97Cu3/16CZS/
Al2O3 at 150 °C using a feed gas composed of toluene, N2, and
18O2. At the initial stage of the toluene oxidation, C16O2 and
C16O18O were detected, but C18O2 was not observed. Almost
all CO2 species were C16O2 and a small amount of C16O18O
species were produced. With the reaction time, the ratio of the
Figure 6. Reaction time dependence of the CO2 species
produced by toluene oxidation on 1 wt % Pt/13 wt %
(Co0.97Cu0.03)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/
γ-Al2O3 at 150 °C using a feed gas composed of toluene,
N2, and 18O2.
former decreased, while that of the latter increased. From these
results, it is evidenced that the lattice oxygen of the catalyst
significantly contributes to the toluene oxidation.
The oxidation states of Ce, Sn, Co, Cu, and Pt in 1Pt/
11Co100/16CZS/Al2O3 and 1Pt/11Co97Cu3/16CZS/Al2O3
were analyzed by XPS. Cerium existed both in the trivalent
and tetravalent states,16-18 while tin remained in the tetravalent
state and no peaks of the Sn2+ species were observed19 and
platinum existed in the metallic state (Pt0).20 There were no
differences on the XPS spectra for these elements between 1Pt/
11Co97Cu3/16CZS/Al2O3 and 1Pt/11Co100/16CZS/Al2O3
(data are not shown).
The XPS results of Co2p and Cu2p core-levels are shown in
Figure 7. In the case of cobalt, the Co2p peaks in Figure 7a are
Bull. Chem. Soc. Jpn. 2015, 88, 746–751 | doi:10.1246/bcsj.20140331
© 2015 The Chemical Society of Japan | 749

/eV
/eV
Figure 7. XPS results of the 1 wt % Pt/11 wt % Co3O4/16 wt % Ce0.62Zr0.20Sn0.18
O
2.0/γ-Al2O3 (1Pt/11Co100/16CZS/Al2O3)
and 1 wt % Pt/11 wt % (Co0.97Cu0.03)3O4¹¤/16 wt % Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 (1Pt/11Co97Cu3/16CZS/Al2O3) catalysts:
(a) Co2p and (b) Cu2p core-levels.
deconvoluted into five peaks. The Co2p3/2 and Co2p1/2 peaks
can be attributed to the divalent (Co2+: 781.6 and 797.9 eV)
and trivalent (Co3+: 780.0 and 796.0 eV) oxidation states, and
the broad one at 786.0 eV corresponds to a satellite peak of
Co2+ 21,22 As seen in these spectra, the relative ratio of Co2+
.
was increased, while that of Co3+ was decreased after Cu
doping.
The Cu2p peaks for 1Pt/11Co97Cu3/16CZS/Al2O3 are
shown in Figure 7b. The Cu2p3/2 peak was deconvoluted into
two components at 930.9 and 933.5 eV, which correspond to
Cu+ in tetrahedral sites (4-coordination) and Cu2+ in octa-
hedral sites (6-coordination), respectively.21,23,24 The two satel-
lite peaks at 939.0 and 942.2 eV also indicate the presence of
Cu2+ 21,25,26
Taking into account of the XPS results, it is
.
/°C
considered that Cu+ was preferentially dissolved into the Co2+
site (4-coordination), while Cu2+ substituted for the Co3+ site
(6-coordination). In fact, the Cu+:Cu2+ ratio in (Co0.97Cu0.03)3-
O4¹¤ estimated from the XPS peak areas at 930.9 eV for Cu+
and 933.5 eV for Cu2+ was 19:81, and this ratio was in good
agreement with 20:80 calculated using the cubic lattice con-
stant in Table 1, on the assumption that Co2+ and Co3+ were
substituted with Cu+ and Cu2+, respectively. This result was
consistent with the fact that the relative Co3+ ratio was
decreased after Cu doping, as mentioned above. As a result of
these substitutions, oxide anion vacancies were produced in
the Co3O4 lattice due to the charge compensation mechanism.
Therefore, it is surmised that these vacancies facilitated oxide
anion mobility to enhance the toluene oxidation activity.
For practical applications of the toluene oxidation catalysts,
it is important to investigate the impact of water vapor (steam)
on the catalytic activities. Steam is a product of toluene oxi-
dation and usually acts as a catalyst poison, because water
molecules are adsorbed on the surface of the catalyst. There-
fore, the catalytic performance of toluene oxidation in the
presence of moisture must be evaluated. The temperature
dependencies of toluene oxidation on the 1 wt % Pt/13 wt %
Figure 8. Temperature dependencies of toluene combustion
on the 1 wt % Pt/13 wt % (Co0.97Cu0.03)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3. Open circle ( ), open
square ( ), and closed circle ( ) symbols correspond to
data obtained in saturated water vapor at 0 °C (0.60 vol %),
at 25 °C (3.13 vol %), and dry atmospheres, respectively.
dioxide and steam. As evidenced by these results, the toluene
oxidation activity of the 16.9Co/CZS catalyst was almost
unaffected in the presence of saturated water vapor at 0 °C
(0.60 vol %) and 25 °C (3.13 vol %), and complete oxidation of
toluene was observed at 150 °C in both cases. It is significantly
important for the catalysts to maintain their oxidation activity
in the presence of moisture for them to be put to practical use.
Conclusion
1 wt % Pt/y wt % (Co1¹xCux)3O4¹¤/Ce0.62Zr0.20Sn0.18O2.0
/
γ-Al2O3 catalysts were successfully prepared. The catalytic
tests for toluene oxidation on these materials showed that
addition of Cu into the Co3O4 lattice was significantly effective
in increasing the catalytic activity without increase in the
platinum content in the catalysts. In fact, complete oxida-
tion of toluene was realized using the 1 wt % Pt/13 wt %
(Co0.97Cu0.03)3O4¹¤/16 wt %
Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3
catalyst under dry and moist conditions are shown in Figure 8.
Also in the moist condition, the reaction products confirmed
by gas chromatography-mass spectrometry were only carbon
(Co0.97Cu0.03)3O4/16 wt % Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 cata-
lyst at 140 °C, which was lower than that using the conven-
750 | Bull. Chem. Soc. Jpn. 2015, 88, 746–751 | doi:10.1246/bcsj.20140331
© 2015 The Chemical Society of Japan

10 N. Imanaka, T. Masui, Chem. Rec. 2009, 9, 40.
tional 1 wt % Pt/11 wt % Co3O4/16 wt % Ce0.62Zr0.20Sn0.18O2.0
/
11 N. Imanaka, T. Masui, K. Yasuda, Chem. Lett. 2011, 40,
780.
γ-Al2O3 (160 °C). The cause for the high toluene oxida-
tion activity observed in the optimized 1 wt % Pt/13 wt %
12 T. Masui, H. Imadzu, N. Matusyama, N. Imanaka,
J. Hazard. Mater. 2010, 176, 1106.
13 K. Yasuda, A. Yoshimura, A. Katsuma, T. Masui, N.
Iamanaka, Bull. Chem. Soc. Jpn. 2012, 85, 522.
14 M. Y. Kim, T. Kamata, T. Masui, N. Imanaka, J. Mater. Sci.
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15 R. D. Shannon, Acta Crystallogr., Sect. A 1976, 32, 751.
16 P. Burroughs, A. Hamnett, A. F. Orchard, G. Thornton,
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(Co0.97Cu0.03)3O4/16 wt % Ce0.62Zr0.20Sn0.18O2.0/γ-Al2O3 cata-
lyst was attributed to the increase in the oxygen release and
storage abilities of the Co3O4-based promoter, in which oxide
anion vacancies to enhance the oxygen mobility were produced
by Cu+ doping on the Co2+ site and Cu2+ doping on the Co3+
site simultaneously, accompanying with the lattice expansion.
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Bull. Chem. Soc. Jpn. 2015, 88, 746–751 | doi:10.1246/bcsj.20140331
© 2015 The Chemical Society of Japan | 751