Methanol oxidative decomposition over zirconia supported silver catalyst and its reaction mechanism

Article abstract of DOI:10.1016/j.apcata.2015.09.017

To develop a new catalyst for catalytic decomposition of volatile organic compounds (VOCs), the activity of various oxide supported silver (Ag) based catalysts for methanol (MeOH) oxidation reaction have been evaluated. Based on the activity evaluation, zirconia (ZrO₂) is considered to be a substitute to ceria (CeO₂) as a support material. The ZrO₂ supported catalyst loading Ag component can oxidize MeOH to CO₂ completely, while the main product is CO for MeOH oxidation over pure ZrO₂. In the present work, 2.0 wt.% Ag/ZrO₂ exhibits excellent activity comparable to Ag/CeO₂. Furthermore, according to in situ FT-IR analysis over Ag/ZrO₂ and pure ZrO₂, it is considered that the methoxy, formate, and bicarbonate species adsorbed on the ZrO₂ surface are intermediate species. We thus deduce that Ag component significantly enhances the oxidation step of methoxy species to CO₂ via formate species, leading to the complete oxidation of MeOH to CO₂ over Ag/ZrO₂ catalyst.

Full text of DOI:10.1016/j.apcata.2015.09.017

Applied Catalysis A: General 507 (2015) 56–64
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Methanol oxidative decomposition over zirconia supported silver
catalyst and its reaction mechanism
Naohiro Shimodaa, Shota Umeharaa, Masaki Kasaharaa, Teruhisa Hongoa,1
,
Atsushi Yamazakib, Shigeo Satokawaa,∗
a Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino-shi, Tokyo 180-8633,
Japan
b Development of Resources and Envirnmental Engineering, School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinju-ku, Tokyo
169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
To develop a new catalyst for catalytic decomposition of volatile organic compounds (VOCs), the activity
of various oxide supported silver (Ag) based catalysts for methanol (MeOH) oxidation reaction have
been evaluated. Based on the activity evaluation, zirconia (ZrO2) is considered to be a substitute to ceria
(CeO2) as a support material. The ZrO2 supported catalyst loading Ag component can oxidize MeOH to
CO2 completely, while the main product is CO for MeOH oxidation over pure ZrO2. In the present work,
2.0 wt.% Ag/ZrO2 exhibits excellent activity comparable to Ag/CeO2. Furthermore, according to in situ
FT-IR analysis over Ag/ZrO2 and pure ZrO2, it is considered that the methoxy, formate, and bicarbonate
species adsorbed on the ZrO2 surface are intermediate species. We thus deduce that Ag component
significantly enhances the oxidation step of methoxy species to CO2 via formate species, leading to the
complete oxidation of MeOH to CO2 over Ag/ZrO2 catalyst.
Received 12 June 2015
Received in revised form
10 September 2015
Accepted 11 September 2015
Available online 25 September 2015
Keywords:
Methanol oxidation
Ag catalyst
VOCs
Zirconia
FTIR
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
more efficient on a cost basis as compared with precious metal
ones; however, the former exhibits lower activity for VOCs com-
Volatile organic compounds (VOCs) are recognized as air pol-
lutants that cause photochemical smog, ground-level ozone, sick
house syndrome, and chemical sensitivity [1–4]. Thus, detoxifica-
tion treatment of VOCs is required and the emission regulation
of VOCs is established by Air Pollution Control Act in any coun-
tries. Among VOCs, formaldehyde is produced by partial oxidation
of methanol (MeOH) and used as organic solvents, coating mate-
rials, and resinous raw materials [5]. Various methods: flame
combustion, catalytic decomposition using plasma, photocatalytic
decomposition, adsorbent-based methods, have been intensively
investigated for the effective abatement of VOCs. The catalytic com-
bustion process has advantageous features for VOCs removal as
compared with other methods [6]. In the catalytic combustion pro-
cess, the selection of active metal materials is an important factor
for the development of catalysts. Transition metal catalysts are
bustion [7–9]. On the other hand, precious metal catalysts can
achieve the complete oxidation at low temperature due to their
high activity [10–13]. For the MeOH oxidative decomposition pro-
cess, Pt [14], Ru [15,16], Au [17–19], and Ag [20–25] metals have
been well researched as active metals. In addition, transition metal
based catalysts using the Cu [26–29], Mo [30], Ni [31,32], and
V [33–36] metals have been also reported. Cerium oxide (ceria,
CeO2) is considered an attractive support due to its high oxy-
gen storage capacity (OSC) [37,38]. Actually, many researchers
reported that ceria supported precious metal catalysts exhibit high
activity for formaldehyde, MeOH, and other VOCs oxidation reac-
tions [7,8,13,17,19,24,37,39]. However, the cost of precious metal
catalysts and their catalytic degradation will become serious prob-
lems for the practical application. Additionally, the supply risk
of precious metals and rare-earth metals including cerium is a
serious international problem. Thus, several oxides such as TiO2
[18,40], SiO2 [26], Al2O3 [23,41], ZnO [18,28], FeOx [42], ZrO2
[15,16,26,29,30], CeO2–ZrO2 [14,24], and mesoporous SiO2 [19,27]
have been investigated as support materials and a novel catalyst
∗
Corresponding author. Fax: +81 422 37 3871.
E-mail address: satokawa@st.seikei.ac.jp (S. Satokawa).
1
Present address: Work Environment Research Group, National Institute of Occu-
pational Safety and Health, 6-21-1 Nagao, Tama-Ku, Kawasaki 214-8585, Japan.
http://dx.doi.org/10.1016/j.apcata.2015.09.017
0926-860X/© 2015 Elsevier B.V. All rights reserved.

N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
57
100
80
60
40
20
0
for the oxidation process of MeOH as well as other VOCs have been
studied [43].
Based on this background, in the present study, we have focused
on a supported silver (Ag) catalyst for the oxidation of MeOH which
convert to formaldehyde easily. In particular, we have studied zir-
conium oxide (zirconia, ZrO2) supported Ag (Ag/ZrO2) catalysts,
because the ZrO2 support is considered to be a suitable alterna-
tive candidate to CeO2 and also there is no report of the MeOH
oxidative decomposition over Ag/ZrO2 catalyst. Specifically, the
comparative study of the catalytic activity of Ag/ZrO2 with other
several oxide supported metal catalysts for MeOH oxidation has
been conducted. Furthermore, in-situ Fourier transform infrared
spectroscopy (FT-IR) analysis of the catalyst have been undertaken
to obtain the insight on the reaction mechanism for MeOH oxida-
tion over Ag/ZrO2 catalyst.
0
100
200
300
400
Temperature / ºC
2. Experimental
Fig. 1. Catalytic activity of various oxides for MeOH oxidation: (᭹) ZrO2 (JRC-
ZRO-3), (᭿) CeO2, (JRC-CEO-3), (ꢀ) MnO2 (MN-280); reaction conditions: 700 ppm
2.1. Catalyst preparation
MeOH/20% O2/He balance; W/F: 0.04 g h cm−3
.
The various metal oxide supported metal catalysts were pre-
pared by the impregnation method [44]. The following metal oxides
were used as support materials: ZrO2 (JRC-ZRO-3 and JRC-ZRO-2),
CeO2 (JRC-CEO-3), MgO (JRC-MGO-4 500A) provided by the Cataly-
sis Society of Japan; MnO2 (MN-280) provided by Clariant Catalysts
(Japan) K.K.; Al2O3 prepared from aluminum hydroxides (Catapal
B) provided by Sasol Ltd. by the heat treatment in air at 600 ◦C
for 2 h. An aqueous solution of AgNO3 provided by Wako Pure
Chemical Industries, Ltd. was used as the Ag source. In addition,
Pt(NH3)2(NO2)2 nitric acid solution provided by Kojima Chemical
Co., Ltd. and Co(NO3)2 hexahydrate, Cu(CH3COO)2 monohydrate,
and SnCl2 dihydrate provided by Wako Pure Chemical Industries,
Ltd., were used as each metal source. The loading of Ag metal was
adjusted to 0.5–5.0 wt.% in the metallic form. Each metal oxide
powder was impregnated with an aqueous solution of the metal
component of interest. The mixture was stirred at room temper-
ature under 13 kPa for 2 h and then the solution was evaporated
sufficiently on a water bath at 80 ◦C under 40 kPa. The obtained
samples were dried in air at 110 ◦C and then calcined in air at 500 ◦C
for 2 h. Subsequently, the obtained powders were pressed, crushed,
and sieved to particle size of 150–250 ␮m for the catalytic activity
tests of MeOH oxidation.
To identify the crystalline structure of each support oxide and
each prepared catalyst, the powder X-ray diffraction (XRD) pattern
was measured using a Rigaku Ultima IV instrument equipped with
a Cu K␣ radiation source (ꢀ = 0.154 nm). The typical working con-
ditions such as an acceleration voltage and current were 40 kV and
20 mA with a scanning speed of 1◦ min−1
.
Transmission electron microscopy (TEM) of 2.0 wt.% Ag/ZrO2
and 2.0 wt.% Ag/CeO2 was performed using a JEOL JEM-2100F oper-
ated at 200 kV. The samples were dispersed by ultrasonic in ethanol
followed by deposition of the resultant suspension onto a standard
Cu grid covered with a holey carbon film.
To elucidate the adsorbed species and intermediates during
MeOH oxidation over pure ZrO2 and Ag/ZrO2, in-situ Fourier
transform infrared spectroscopy (FT-IR) analysis was conducted
using a SHIMADZU FTIR-8300 equipped with a glass reaction cell
(Makuhari Rikagaku Garasu Inc.). Thin disks of each sample were
made and placed in the FT-IR cell. For the measurement of Ag/ZrO2,
the spectrum was recorded under 20% O2/He at the temperature
range of room temperature to 125 ◦C after the pre-treatments as
follows: heat-treatment in 20% O2/He at 200 ◦C for 2 h, and then
cooling to room temperature. Subsequently, a gaseous mixture
of 1% MeOH/20% O2/He balance was fed the samples to adsorb
the MeOH-derived species on the sample surface. The interior of
reaction cell was then purged with 20% O2/He for 15 min. For the
measurement of pure ZrO2, the adsorption step of MeOH was car-
ried out at 200 ◦C and the spectrum was recorded under 20% O2/He
gaseous condition at the temperature range of 200–300 ◦C after
the pretreatment as mentioned above. For the both measurements,
prior to the MeOH adsorption step, back ground spectrum was
recorded in 20% O2/He at room temperature or 200 ◦C, respectively.
The IR measurement was operated at a resolution of 4 cm−1, and
45 scans were collected for each spectrum.
2. 2 Catalytic activity test
The evaluation of catalytic performance for MeOH oxidation was
conducted using a fixed-bed flow reactor. The prepared catalyst
(0.2 g) was housed in the quartz tube reactor with a diameter of
6 mm and a reaction gas mixture (700 ppm MeOH/20% O2/He bal-
ance) was fed to the catalyst bed at a total flow rate of 300 mL/min
(gas hourly space velocity: 100,000 L kg−1 h−1). The compositions of
the inlet and outlet gases were analyzed using a gas chromatograph
equipped with a flame ionization detector (Shimadzu, GC-8A). A
SHINCARBON ST 50–80 column (Shinwa Chem. Ind.) and a metha-
nizer accessory were used to determine the amount of MeOH, CO,
and CO2.
3. Results and discussion
2.3. Catalyst characterization
3.1. Catalytic activity for MeOH oxidation
The specific surface area of each support oxide without and with
the heat treatments and each prepared catalyst was determined
by N2 adsorption measurement at 77 K by the conventional BET
method using a Microtrac BEL BELSORP-mini II instrument. Prior to
N2 adsorption, the sample was degassed at 200 ◦C for 2 h in order to
remove the moisture adsorbed on the surface and inside the porous
network.
3.1.1. Activity of various oxides
Firstly, the catalytic activity of the representative pure oxides
for MeOH oxidative decomposition has been evaluated. Fig. 1
shows the temperature dependence of MeOH conversion for MeOH
oxidation over ZrO2 (JRC-ZRO-3), CeO2, (JRC-CEO-3), and MnO2
(MN-280) oxides as-obtained. Among them, pure MnO2 exhibited
much higher activity than ZrO2 and CeO2. As shown in Table 1,

58
N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
Table 1
BET surface areaa and crystalline phase of various metal oxides.
Metal oxide
Abbreviation
Heat treatment
BET surface area (m2 g−1
86
)
Crystalline phase
CeO2 (JRC-CEO-3)
MnO2 (MN-280)
CeO2
MnO2
MgO
Al2O3
As-obtained
As-obtained
As-obtained
Cubic (fluorite)
Tetragonal (rutile)
Cubic
Cubic (gamma)
Monoclinic
Amorphus
Amorphus
Monoclinic and tetragonal
Monoclinic and tetragonal
222
MgO (JRC-MGO-4 500A)
36
178
99
227
191
102
13
b
Al2O3
600 ◦
C
ZrO2 (JRC-ZRO-3)
ZrO2 (JRC-ZRO-2)
ZrO2 (JRC-ZRO-2)
ZrO2 (JRC-ZRO-2)
ZrO2 (JRC-ZRO-2)
ZrO2
As-obtained
As-obtained
ZrO2(2)
ZrO2(2) 320
ZrO2(2) 400
ZrO2(2) 800
320 ◦
400 ◦
800 ◦
C
C
C
a
Heat-treated at 200 ◦C for 2 h in vacuum prior to N2 adsorption and desorption measurement.
Alumina was prepared by the calcination of AlOOH in air at 600 ◦C for 2 h.
b
Table 2
BET surface areaa of various metal oxide supported Ag catalysts.b
100
80
60
40
20
0
Metal oxide
Final calcination temperature
BET surface area (m2 g−1
)
CeO2
MnO2
MgO
Al2O3
500 ◦
500 ◦
500 ◦
500 ◦
500 ◦
110 ◦
320 ◦
400 ◦
500 ◦
C
C
C
C
C
C
C
C
C
74
104
104
176
62
191
178
87
ZrO2
ZrO2(2)
ZrO2(2) 320
ZrO2(2) 400
ZrO2(2) 800
15
a
Heat-treated at 200 ◦C for 2 h in vacuum prior to N2 adsorption and desorption
measurement.
b
Ag loading: 2.0 wt.%.
0
50
100
150
200
250
300
230 ◦C, 220 ◦C, 160 ◦C, 140 ◦C, and 135 ◦C for the MgO, Al2O3, MnO2,
ZrO2 (JRC-ZRO-3), and CeO2 supported Ag catalysts, respectively.
The Ag/ZrO2, Ag/CeO2 and Ag/MnO2 exhibited higher activity than
those of the other catalysts. In particular, 2.0 wt.% Ag/ZrO2 exhib-
ited excellent activity comparable to 2.0 wt.% Ag/CeO2. At the low
conversion region, 2.0 wt.% Ag/MnO2 exhibited high conversion of
MeOH; however, Ag loading negatively affected MeOH oxidation
compared with the catalytic activity of pure MnO2. In contrast, for
the ZrO2 and CeO2 catalysts, the loading of Ag component on each
oxide was significantly improved catalytic activity. Furthermore, it
was found that the product was only CO2 and H2O over all Ag-based
catalysts. This indicates that the oxide supported Ag catalysts can
oxidize MeOH to CO2 completely due to the facilitative effect of Ag
element for CO oxidation step. As shown in Table 2, the BET specific
Temperature / ºC
Fig. 2. Catalytic activity of various oxide supported Ag catalysts for MeOH oxida-
tion: (ꢀ) Ag/ZrO2, (ᮀ) Ag/CeO2, (ꢁ) Ag/MnO2, (♦) Ag/MgO, (×) Ag/Al2O3; reaction
conditions: 700 ppm MeOH/20% O2/He balance; W/F: 0.04 g h cm−3; metal loading:
2.0 wt.%.
the BET specific surface area of MnO2 was 104 m2 g−1 which was
higher than those of ZrO2 (99 m2 g−1) and CeO2 (86 m2 g−1). We
have also examined the activity of gamma-Al2O3 (178 m2 g−1) and
MgO (36 m2 g−1) forMeOHoxidation(resultsarenotshown), which
resulted in the lower activity. For the MnO2, ZrO2, and CeO2, the
complete conversion of MeOH was achieved at 125 ◦C, 300 ◦C, and
300 ◦C, respectively. For the MnO2 and CeO2, MeOH was mainly
converted to CO2, meaning the low CO selectivity at the temper-
ature rages where MeOH was oxidized. In particular, MnO2 can
oxidize MeOH to CO2 completely, whilst main product for MeOH
oxidation over ZrO2 was found to be CO and H2O as shown in Fig.
S1. In the literature, Delimaris and Ioannides reported high CO2
selectivity for oxidation of ethanol, ethyl acetate, and toluene over
MnOX , CeO2, and MnOX –CeO2 catalysts. It may be considered that
MnO2 and CeO2 exhibit high activity and high selectivity of CO2 for
the MeOH oxidation reaction. In addition, ZrO2 showed relatively
high performance for the MeOH oxidative decomposition to CO. We
thus consider that the important factor contributing to the catalytic
performance of metal oxides is not only their surface area but also
other physicochemical properties.
100
80
60
40
20
0
Supplementary material related to this article found, in the
online version, at http://dx.doi.org/10.1016/j.apcata.2015.09.017.
0
50
100
150
200
250
300
3.1.2. Effect of the precious metal loading upon the various oxides
To improve the catalytic activity accompanying complete oxi-
dation for MeOH oxidation, we have studied the effect of Ag
loading. Fig. 2 shows the MeOH conversions for MeOH oxidation
over various oxide supported Ag catalysts with 2.0 wt.% loading.
The temperatures at which MeOH was converted completely were
Temperature / ºC
Fig. 3. Catalytic activity of zirconia (JRC-ZRO-3) supported various metal cata-
lysts for MeOH oxidation: (ꢀ) Ag/ZrO2, (᭹) Pt/ZrO2, (᭿) Co/ZrO2, (ᮀ) Cu/ZrO2,
(ꢀ) Sn/ZrO2; reaction conditions: 700 ppm MeOH/20% O2/He balance; W/F:
0.04 g h cm−3; metal loading: 2.0 wt.%.

N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
59
(A)
(g)
(d)
(b)
(a)
20
30
40
50
60
70
2θ / degree
(B)
(g)
(f)
(e)
(d)
(c)
(b)
(a)
20
30
40
50
60
70
2θ / degree
Fig. 5. XRD patterns of (A) zirconia and (B) ceria supported Ag catalysts with various
loading: (a) 0 wt.%, (b) 0.5 wt.%, (c) 1.0 wt.%, (d) 2.0 wt.%, (e) 3.0 wt.%, (f) 4.0 wt.%, (g)
5.0 wt.%; (ᮀ) ZrO2-monoclinic, (᭹) ZrO2-tetragonal, (ꢁ) Ag, (ꢀ) CeO2-cubic.
MeOH oxidation are shown in Fig. 3. Over Pt/ZrO2, MeOH oxidation
abruptly occurred at 75 ◦C. It is well known that CO can be strongly
adsorbed on Pt. The adsorption amount of CO on Pt is higher than
those on other metal catalysts and may be considered dependent
upon the temperature. Therefore, it is presumed that the adsorption
of formed CO is more dominant than the progress of CO oxidation
reaction at low temperature. We suppose that this may be one of
the reasons that the steep activation behavior for MeOH oxidation
over Pt/ZrO2. In addition, Ag/ZrO2 catalyst exhibited the compara-
ble high activity. However, Co, Cu, and Sn/ZrO2 catalysts showed
low activity. We consider that this reason may be due to their low
activity for the CO oxidation step.
Fig. 4. Effect of Ag loading of (A) zirconia and (B) ceria supported Ag catalysts on the
activity for MeOH oxidation: (closed symbol) reaction temperatures to achieve the
conversion of 20% (T20, ᭹), 50% (T50, ꢀ), 80% (T80, ᭿); (open symbol, ᮀ) conversion at
100 ◦C; reaction conditions: 700 ppm MeOH/20% O2/He balance; W/F: 0.04 g h cm−3
.
surface areas of the supported Ag catalysts were lower than those of
all metal oxides except MgO (104 m2 g−1). The difference between
pure MnO2 (222 m2 g−1) and Ag/MnO2 (104 m2 g−1) was particu-
larly large among the catalysts. This may be the reason why the
final calcination during the Ag loading decreased the BET specific
surface area of MnO2. Based on the XRD analysis, the pure MgO as-
obtained (JRC-MGO-4 500 A) included a fixed content of Mg(OH)2.
We consider that the heat treatment in air at 500 ◦C in the proce-
dure of Ag loading resulted in the formation of pure MgO, leading
to an increase in the BET specific surface area of Ag/MgO.
3.2. Comparison of the activity of Ag/ZrO2 and Ag/CeO2 catalysts
As mentioned above, 2.0 wt.% Ag/ZrO2 exhibits an excellent
activity which is comparable to 2.0 wt.% Ag/CeO2. Hence, we have
studied the effect of loadings of Ag on Ag/ZrO2 and Ag/CeO2 for
MeOH oxidation. Fig. 4 presents the activity of both catalysts with
various loadings of Ag. The reaction temperature to achieve a MeOH
conversion of 20%, 50%, and 80% for MeOH oxidation and the MeOH
conversion at 100 ◦C are plotted in Fig. 4. From these results, the
optimum amounts of silver loading for zirconia and ceria catalysts
were found to be 5.0 wt.% and 2.0 wt.%, respectively.
3.1.3. Zirconia supported precious metal catalysts
Next, we have studied the effect of active metal species for
MeOH oxidation over zirconia supported catalysts. The MeOH
conversion over zirconia supported 2.0 wt.% metal catalysts for

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N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
Fig. 6. TEM images and Ag particle size distribution of (a, c) 2.0 wt.% Ag/ZrO2 (JRC-ZRO-3) and (b, d) 2.0 wt.% Ag/CeO2 (JRC-CEO-3) catalysts after the activity tests of MeOH
oxidation shown in Fig. 2.
Fig. 5 shows the XRD patterns of Ag/ZrO2 and Ag/CeO2 catalysts
with various loadings. The main crystalline phase of support oxide
for both catalysts were monoclinic zirconia and cubic ceria, respec-
tively. The diffraction peaks attributed to Ag component were
observed for the catalyst with the loading amount above 2.0 wt.%.
The crystallinity of the Ag phase for Ag/CeO2 were higher than for
Ag/ZrO2. Furthermore, TEM observation of 2.0 wt.% Ag/ZrO2 and
2.0 wt.% Ag/CeO2 was conducted as shown in Fig. 6. The average
particle sizes of silver metal for both catalysts were approximately
12.6 nm and 14.6 nm, respectively. As shown in Table 2, the BET sur-
face areas of 2.0 wt.% Ag/ZrO2 and 2.0 wt.% Ag/CeO2 were 62 m2 g−1
and 74 m2 g−1, respectively. We consider that the loading of Ag on
both oxides enhances the catalytic activity for MeOH oxidation with
the same level of positive effect. For the catalytic combustion reac-
tion, the BET surface area of the oxide is an important factor for the
activity of catalyst [6–8]. In the present work, the difference of BET
surface area between the 2.0 wt.% Ag/ZrO2 and 2.0 wt.% Ag/CeO2
was 12 m2 g−1. Thus, we consider that this difference is negligible
for the comparison of both catalysts.
3.3. Effect of ZrO2 type on the catalytic activity of Ag/ZrO2
catalyst for MeOH oxidation
Next, we have examined the effect of zirconia surface area and
crystallinity on the catalytic activity for various Ag/ZrO2 materials.
The physical properties of various ZrO2 and ZrO2 supported Ag cat-
alysts are summarized in Tables 1 and 2, respectively. Two types
ZrO2 were used in the present work and abbreviated as ZrO2(2)
and ZrO2 which is the ZrO2 (JRC-ZRO-3). The crystalline phases
of pure ZrO2(2) and ZrO2 samples as obtained were amorphous
and monoclinic and their BET surface areas were 227 m2 g−1 and
99 m2 g−1, respectively. Additionally, ZrO2(2)X samples produced

N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
61
appeared at 2870 cm−1, 1575 cm−1, 1385 cm−1, and 1360 cm−1
;
100
80
60
40
20
0
they were assigned to methoxy (CH3) groups and formate (HCOO)
groups, respectively. The bands associated with bicarbonate (HCO3)
groups was also observed at 1421 cm−1. These results indicate a
decrease of the hydroxyl groups due to the adsorption of MeOH
with the replacement of the hydroxyl species on the ZrO2 surface.
When the measurement temperature was increased, the adsorp-
tion bands of methoxy species decreased but those of formate
and bicarbonate bands increased. We consider that this proves
the progress of transformation of methoxy species to formate and
bicarbonate species. After the further increases in the measurement
temperature, a decrease in the inverse bands of the hydroxyl groups
was apparently observed. We consider that this implies the regen-
eration of the hydroxyl groups on the ZrO2 surface, indicating the
oxidation of formate species to CO and the hydroxyl species. Dur-
0
50
100
150
200
Temperature / ºC
Fig. 7. Catalytic activity of various zirconia supported Ag catalysts for MeOH oxi-
dation: (᭹) ZrO2, (ꢀ) ZrO2(2), (ᮀ) ZrO2(2) 320, (♦) ZrO2(2) 400, (ꢁ) ZrO2(2) 800;
reaction conditions: 700 ppm MeOH/20% O2/He balance; W/F: 0.04 g h cm−3; metal
loading: 2.0 wt.%.
by the heat treatment in air at 320 ◦C, 400 ◦C, 800 ◦C (X denotes the
temperature of treatment) were used as support oxides and the
final calcination temperatures after the loading of Ag were same as
each heat treatment temperature. With an increase in the temper-
ature of heat treatment, the BET surface area of ZrO2(2) decreased
and the crystallite phase varied from amorphous to polymorphism
phase of monoclinic and tetragonal.
Fig. 7 shows the activity of these Ag/ZrO2 catalysts for MeOH oxi-
dation. Among the ZrO2(2) supported Ag catalysts, the Ag/ZrO2(2)
exhibited no activity for oxidation below 200 ◦C. In contrast, the
activity of Ag/ZrO2(2) 400 exhibited high activity, which was com-
parable to the activity for Ag/ZrO2. In addition, we unexpectedly
found that the Ag/ZrO2(2) 800 possessed higher activity than
Ag/ZrO2(2) 320. The BET surface areas of Ag/ZrO2(2)X with the heat
treatment at 320 ◦C, 400 ◦C, and 800 ◦C were 178 m2 g−1, 87 m2 g−1
,
15 m2 g−1, respectively. Considering the difference of the activity
between the catalysts using ZrO2(2) 320 and ZrO2(2) 400 of which
crystalline phases were amorphous as shown in Table 1, it is found
that the crystallization of zirconia affect the activity of catalyst sig-
nificantly. This may be because the low mobility of lattice oxygen
in the zirconia with the low crystallinity results in the low activ-
ity of zirconia for MeOH oxidation. Indeed, in the present work,
the monoclinic ZrO2 supported Ag catalysts exhibited high activity.
In addition, we consider that the larger BET surface area of zirco-
nia leads to the higher catalytic activity of Ag/ZrO2 by comparing
the activity of the catalysts with the heat treatment at 400 ◦C and
800 ◦C. However, it is worth noting that the effect of the BET surface
area of support oxides was less important than that of crystallinity
of support oxides for MeOH oxidation over Ag/ZrO2.
Fig. 8. In situ FT-IR spectra of adsorbed species on pure ZrO2 (JRC-ZRO-3) for MeOH
oxidation upon increasing temperature: (a) 200 ◦C, (b) 225 ◦C, (c) 250 ◦C, (d) 275 ◦C,
(e) 300 ◦C.
3.4. FT-IR analysis on MeOH oxidation over pure ZrO2 and
Ag/ZrO2 catalysts
To obtain insight into the reaction mechanism of MeOH oxi-
dation over Ag/ZrO2, we have conducted in situ FT-IR analysis.
Firstly, we have examined the adsorbed species on the pure ZrO2
(JRC-ZRO-3), as shown in Fig. 8. It should be noted that the FT-
IR analysis was carried out from 200 ◦C to 300 ◦C for the pure
ZrO2. After MeOH was adsorbed at 200 ◦C, two inverse bands
at 3725 cm−1 and 3670 cm−1 were observed. These bands are
assigned to hydroxyl (OH) groups on ZrO2, including terminal
(higher) and bridging (lower) types. In addition, several adsorption
bands appeared at 2920 cm−1, 2810 cm−1, and 1195 cm−1, and also
Fig. 9. In situ FT-IR spectra of adsorbed species on 2.0 wt.% Ag/ZrO2 (JRC-ZRO-3) for
MeOH oxidation upon increasing temperature: (a) room temperature, (b) 50 ◦C, (c)
75 ◦C, (d) 100 ◦C, (e) 125 ◦C.

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N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
Fig. 10. Reaction mechanism over pure ZrO2 for MeOH oxidation: (a) terminal type mechanism, (b) bridging type mechanism.
ing the measurement temperature increased from 200 ◦C to 300 ◦C,
the methoxy species almost disappeared and the formate species
maximized at 275 ◦C and then decreased at 300 ◦C. And noted that
the large inverse bands appeared at 1150 cm−1 with increasing
in the temperature to 300 ◦C. We suppose that this is attributed
to desorption of some organic species contaminated in pure ZrO2
as-obtained.
ature range of room temperature to 125 ◦C was also examined,
as shown in Fig. 9. As a result, we observed one inverse band at
around 3670 cm−1 associated with the bidentate hydroxyl groups,
the adsorption bands attributed to methoxy species appeared at
2925 cm−1, 2810 cm−1, 1457 cm−1, and 1195 cm−1, and the bands
to formate species appeared at 2870 cm−1, 1573 cm−1, 1384 cm−1
,
and 1359 cm−1. These results prove the adsorption of methoxy
species on the ZrO2 surface as with the case of pure ZrO2. With
an increase in the temperature, the inverse band assigned to
Next, in situ FT-IR analysis of the adsorption species during the
MeOH oxidation over 2.0 wt% Ag/ZrO2 (JRC-ZRO-3) at the temper-

N. Shimoda et al. / Applied Catalysis A: General 507 (2015) 56–64
63
can be completely oxidized to CO2 by the catalytic effect of the Ag
component in a manner similar to the soot oxidation over Ag/ZrO2
as mentioned above. In addition, Ag component can enhances the
oxidation rate of methoxy species to formate species. Namely, we
deduce that these assist of Ag component for two steps leads to the
improvement of catalytic activity of Ag/ZrO2 for MeOH oxidation.
It is well known that the CeO2 based noble metal catalyst pos-
sesses the large OSC and its large OSC leads to the high catalytic
activity for several oxidation reaction [38,48,49]. For the MeOH
oxidation process, the Ag/ZrO2 catalyst exhibits excellent activity
comparable to the Ag/CeO2 catalyst due to the significant effect
of Ag component as an activator of oxygen molecular as revealed
in the present work. Therefore, there is a possibility that Ag/ZrO2
becomes a substitute material to CeO2 supported various metal cat-
alysts and it is also considerably attractive from the perspective of
material cost.
4. Conclusions
Ag/ZrO2 prepared by the impregnation method exhibited
excellent activity comparable to Ag/CeO2 for MeOH oxidative
decomposition. The most active catalyst was 5.0 wt.% Ag catalyst
supported on tetragonal ZrO2 (JRC-ZRO-3) which was calcined at
500 ◦C. According to in-situ FT-IR study, we found that the adsorp-
tion sites of methanol are hydroxyl groups on the ZrO2 surface. In
addition, the formations of methoxy and formate and bicarbonate
groups were observed as intermediate species. We thus conclude
that Ag component enhances the oxidation step of methoxy species
to formate species and CO formed by decomposition of formate
species to CO2, leading to the complete oxidation of MeOH over
the Ag-based catalyst. The oxygen species mainly and effectively
used for MeOH oxidation is considered an active oxygen species
adsorbed and activated by the Ag component.
Fig. 11. Reaction mechanism over Ag/ZrO2 for MeOH oxidation.
hydroxyl groups decreased gradually, indicating the regeneration
of the hydroxyl groups by the progress of MeOH oxidation. In
addition, it was observed that the formate species increased along
with a decrease in the methoxy species. When the measurement
temperature increased furthermore, the methoxy species almost
disappeared at 125 ◦C and the formate species increased at once
and then decreased slightly.
3.5. Proposing the reaction mechanism of MeOH oxidation over
Ag/ZrO2
Acknowledgment
Based on the FT-IR analysis and the activity test results, we
have deduced the reaction mechanism of the MeOH oxidation on
the pure ZrO2 and Ag/ZrO2. In the literature, the reaction mecha-
nism of MeOH oxidation on CeO2 and also Au/CeO2 was reported
by Rousseau et al. [17]. In addition, that of MeOH decomposition
over Cu/ZrO2 was also reported by Tsoncheva et al. [29]. We thus
consider that the similar reaction occurs on ZrO2 and Ag/ZrO2.
In the case of pure ZrO2, two types mechanism are considered
as shown in Fig. 10. This is because that terminal and bridging
hydroxyl groups were consumed by the adsorption of MeOH as
shown in Fig. 8. With an increase in the reaction temperature,
methoxy species are considered to be oxidized to formate species. It
should be noted that this formate species can be easily transformed
to bicarbonate species. When the reaction temperature increases
furthermore, formate and bicarbonate species are decomposed to
CO and the hydroxyl species for each type mechanism. The oxi-
dation rate of methoxy species to formate species are slow below
250 ◦C, leading to the low activity of pure ZrO2 for MeOH oxidation
as shown in Fig. 1.
Regarding the effect of Ag on the catalytic oxidation reaction,
several studies were reported, especially for the soot oxida-
tion [45–47]. Aneggi et al. reported the high catalytic activity
of Ag/ZrO2 comparable to Ag/CeO2 for soot oxidation, and also
Nanba et al. revealed the presence of active oxygen species on Ag
species for soot oxidation over Ag/ZrO2 by analysis of temperature-
programmed reduction by NH3 (NH3-TPR). For the case of MeOH
oxidation over Ag/ZrO2, the adsorption site of MeOH is considered
to be only the bidentate hydroxyl groups on the ZrO2 surface as
shown in Fig. 11. The adsorbed methoxy species are easily oxi-
dized to only the formate species, and then the formate species are
decomposed to CO and the hydroxyl species. Here, the formed CO
This study was supported by the Cooperative Research Program
of Catalysis Research Center, Hokkaido University (Grant 14B1009).
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