Support effect in the gas phase oxidation of ethanol over nanoparticulate gold catalysts

Article abstract of DOI:10.1039/c1nj20297a

Twenty three kinds of metal oxides were screened as supports for Au nanoparticles (Au NPs) in the gas phase oxidation of ethanol. Mild oxidation to acetaldehyde, which is economically preferable, is catalyzed by Au NPs deposited on catalytically inert metal oxides, in particular, strongly acidic MoO ₃ or weakly basic La₂O₃. Deep oxidation to acetic acid takes place over Au NPs deposited on n-type semiconductive metal oxides such as ZnO and V₂O₅, which exhibit a little catalytic activity for ethanol oxidation at 200 °C. Complete oxidation to CO₂ and H₂O preferentially takes place over p-type semiconductive metal oxides such as MnO₂ and Co₃O ₄, and CeO₂ which has oxygen-storage and discharge capability. These metal oxides show catalytic activity for ethanol oxidation even at 100 °C producing mainly acetaldehyde, and their catalytic activity is noticeably enhanced in the conversion of ethanol and the selectivity to CO₂ by the deposition of Au NPs. The wide range of product tunability can be explained by the adsorption structures of ethanol and by the reactivities of oxygen species on the metal oxide supports.

Full text of DOI:10.1039/c1nj20297a

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PAPER
Support effect in the gas phase oxidation of ethanol over nanoparticulate
gold catalystsw
Takashi Takei,*ab Norihiko Iguchiab and Masatake Harutaab
Received (in Montpellier, France) 5th April 2011, Accepted 25th May 2011
DOI: 10.1039/c1nj20297a
Twenty three kinds of metal oxides were screened as supports for Au nanoparticles (Au NPs)
in the gas phase oxidation of ethanol. Mild oxidation to acetaldehyde, which is economically
preferable, is catalyzed by Au NPs deposited on catalytically inert metal oxides, in particular,
strongly acidic MoO
Au NPs deposited on n-type semiconductive metal oxides such as ZnO and V
little catalytic activity for ethanol oxidation at 200 1C. Complete oxidation to CO
preferentially takes place over p-type semiconductive metal oxides such as MnO and Co O , and
3
or weakly basic La
2
O
3
. Deep oxidation to acetic acid takes place over
, which exhibit a
and H
2 5
O
2
2
O
2
3
4
CeO which has oxygen-storage and discharge capability. These metal oxides show catalytic
2
activity for ethanol oxidation even at 100 1C producing mainly acetaldehyde, and their catalytic
activity is noticeably enhanced in the conversion of ethanol and the selectivity to CO by the
2
deposition of Au NPs. The wide range of product tunability can be explained by the adsorption
structures of ethanol and by the reactivities of oxygen species on the metal oxide supports.
Introduction
Catalytic transformation of ethanol has recently been
3
reviewed for the last ten years. Acetaldehyde can be selectively
The chemical industry is gradually shifting its primary resources
from petroleum towards renewable ones. Biomass-based ethanol
produced in the gas phase by dehydrogenation of ethanol over
4
supported Cu catalysts and by oxidation with O over V and
2
(
bioethanol) has been attracting growing interest as a fuel
5,6
Mo based oxides. Acetic acid can be produced in water
to blend into gasoline and as a chemical feedstock to
replace ethylene. Ethanol production all over the world has
continuously expanded from 2000 (6.4 million kL a year) up to
now (34 million kL a year in 2008). Bioethanol is currently
made from cornstarch in USA, from wheat-, barley-, and
rye-starch in Europe, and from sugarcane-derived sucrose in
Brazil. It can also be produced from the stalks left over from
corn harvesting or from other cellulosic agricultural materials
7
solvent over Au catalysts supported on MgAl
2
O
4
or on Cu
8
doped NiO and in the gas phase over Mo–V–Nb mixed oxides
9
2
combined with TiO colloids.
This work explores new heterogeneous catalytic systems
composed of Au for gas phase ethanol oxidation with molecular
oxygen. As in other reactions Au NPs can provide a wide
range of tunability depending on metal oxide supports and
cover mild, deep, and complete oxidations.
1
with little commercial value such as waste wood and switchgrass.
Ethanol can be transformed into valuable chemicals such as
acetaldehyde, acetic acid, ethyl acetate, diethyl ether, and ethylene
oxide. Current market prices of these derivatives in Japan
Experimental
À1
motivate ethanol (52 yen kg ) transformation into acetaldehyde
À1
À1
2
88 yen kg ) rather than into acetic acid (47 yen kg ). This is
(
1. Catalyst preparation
probably because major industrial processes for producing acetic
acid are based on the carbonylation of cheap methanol produced
Supports. The majority of metal oxide supports used were
commercially available metal oxide powders: MoO
3
(Kanto
2
from synthetic gases (CO, H ) with a Rh complex catalyst,
Chemical, specific surface area, abbreviated as SSA,
whereas acetaldehyde needs ethylene as a source and corrosive
PdCl –CuCl as a catalyst.
2
À1
.9 m g ), Al O (Catalysis Society of Japan, JRC-ALO-2,
2 3
1
2
2
2
À1
2
À1
SSA 285 m g ), TiO (Nippon Aerosil, P 25, SSA 50 m g ),
2
a
SnO (Sigma-Aldrich, nanopowder, particle size o100 nm),
2
Department of Applied Chemistry, Graduate School of Urban
Environmental Sciences, Tokyo Metropolitan University, 1-1
Minami-osawa, Hachioji, Tokyo 192-0397, Japan. E-mail: takei-
takashi@tmu.ac.jp
2
À1
SiO
WO
61 m
2
(Fuji Silysia Chemical, CARiACT Q-10, SSA 300 m g ),
3
2
(propriety material), CeO (Shin-etsu Chemical, SSA
2
À1
1
g
), and ZrO
2
2
(Daiichi Kigenso Kagaku Kogyo,
À1
b
JST, CREST, 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012, Japan
w Dedicated to Prof. Didier Astruc on the occasion of his 65th
birthday.
RC-100, SSA 80–120 m g ). In addition, the following metal
oxides were prepared as supports for Au catalysts.
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 2227–2233 2227

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(
a) ZnO and La
À1
2
O
3
. An aqueous solution of metal nitrate
The X-ray powder diffraction measurements were conducted
by using a Rigaku RINT-TTR III diffractometer using CuKa
radiation (l = 0.154056 nm) in the transmission mode
radiation in an operating mode of 50 kV and 300 mA.
Transmission electron microscopic (TEM) observations were
carried out by using a JEOL JEM-3000F operating at 300 kV
and a JEM-2100F operating at 200 kV.
À1
(
1 Â 10 M L ) was heated to 70 1C and poured under
agitation into an aqueous solution of Na CO (1.2 times of the
2
3
À1
À1
stoichiometric amount, 1 Â 10 M L ) heated at 70 1C to
obtain hydroxide or carbonate precipitate. Then the suspension
was kept stirred at 70 1C for 1 h. The suspension was
centrifuged and the precipitate was repeatedly washed to
remove sodium and chloride ions until the pH reached a
steady value of around 6. The solid precursor was filtrated
and dried at 80 1C overnight and finally calcined in air at
3
.
Catalytic tests for gas phase oxidation of ethanol and CO
oxidation
3
00 1C for 4 h.
Catalytic activities were measured by a continuous-flow
fixed-bed quartz reactor with an inner diameter of 6 mm.
Prior to the measurements, catalyst samples were heated in an
air stream at 250 1C for 30 min. The reactant gas was passed
through the catalyst bed (150 mg) under a total pressure of
(
b) V and Nb
5
2
O
2
O
5
. NH VO
4
3
(Wako Pure Chemical
NNbO
ÁxH
Industries, Co., Ltd., purity 99.0%) or C
Sigma-Aldrich Co. Ltd., purity 99.99%) were calcined in air
at 300 1C for 4 h.
4
H
4
9
2
O
(
À1
À1
1
atm and at a space velocity of 20 000 mL h
g
cat
at
À1
(
c) MnO . An aqueous solution of KMnO (0.4 M L
4
)
temperatures of 100–280 1C. Ethanol solution was used as a
feedstock and supplied by a plunger pump. The liquid was
continuously evaporated and mixed with a feed gas (2.3% O2
2
À1
À1
and NaOH (1.2 M L ) was poured into an aqueous solution
of Mn(NO
)
3 2
(0.6 M L ) under agitation at room temperature
1
precipitate. Then the suspension was stirred
0
to obtain MnO
2
in N ) in an evaporator heated to 180 1C. Reactant gas feed
2
at room temperature for 1 h. The suspension was centrifuged
and the precipitate was repeatedly washed to remove potassium
and sodium ions until the pH reached a steady value of around
was in a molecular composition of C H OH/O /N = 1/3/126,
2
2
5
2
containing a stoichiometric amount of molecular oxygen for
the complete oxidation of ethanol. The effluent gas was
analyzed by using a FID gas chromatograph (GC-14A/
Shimadzu, column Gaskuropack 54) and a TCD gas chromato-
graph (GC-8A/ Shimadzu, column Porapak Q and MS-5A)
equipped with an automatic gas sampling system.
9
. The solid precursor was treated in a similar manner to those
of ZnO and La O .
2
3
Gold catalysts
(
a) Coprecipitation (CP) method. An aqueous solution of
À1
Catalytic activity measurements for CO oxidation were also
carried out by using a fixed-bed reactor with an inner diameter
of 6 mm. A reactant feed gas containing 1 vol% CO in air was
passed through a catalyst bed under the same conditions as in
ethanol oxidation by using a mass flow controller. The inlet
and outlet gases were analyzed with an on-line gas chromato-
graphy (GC-8A/ Shimadzu, activated carbon and molecular
sieve 13X were used as column packing agents) to obtain both
À1
HAuC1 and a metal nitrate (1 Â 10 M L ) was heated
4
at 70 1C and poured into an aqueous solution of Na
2
CO
3
À1
À1
(
1.2 times of the stoichiometric amount, 1 Â 10 M L
)
heated at 70 1C to obtain hydroxide or carbonate coprecipitate.
Then the suspension was stirred at 70 1C for 1 h and was
centrifuged and washed to remove chloride and sodium ions
until the pH reached a steady value of around 6. The solid
precursor was filtrated and dried at 80 1C overnight, and
finally calcined in air at 300 1C for 4 h. The Au loading was
adjusted to 5 atom% [100Au/(Au + Metal)] in the starting
solutions.
2
the conversion of CO and the formation of CO .
Results
(
b) Deposition precipitation (DP) method. The pH of
1
Characterization of catalysts
1
the aqueous solution of HAuCl
À3
4 2 8 2 2 3
or Au(C H N ) Cl
À1
The preparation methods of Au catalysts, DP, CP and SG
methods were chosen depending on the type and surface
properties of metal oxide supports so that Au was mostly
(
1 Â 10 M L ) was adjusted to 7 by adding aqueous NaOH
solution and heated to 70 1C. Then the support was dispersed
and stirred at 70 1C for 1 h. The precursor was treated in a
similar manner to that for coprecipitation. The concentration
1
2–15
deposited as nanoparticles smaller than 10 nm.
Some Au
catalysts which showed interesting catalytic behavior were
characterized in detail.
in the starting solution of HAuCl
4 2 8 2 2 3
or Au(C H N ) Cl was
adjusted to a Au loading of 1 wt%.
A Au/La O catalyst prepared by coprecipitation (CP) was
2
3
(
c) Solid grinding (SG) method. A mixture of an organo-
active in mild oxidation to selectively produce acetaldehyde.
metallic complex (CH ) Au(C H O ) (Tri Chemical Laboratories
3
Our previous work showed that when Au/La(OH) was prepared
3
2
5
7
2
Inc.) and supports was mechanically ground in a mortar for
0 min. The sample was calcined in air at 300 1C for 4 h or
with a molar ratio of La/Au = 2 and by calcination at 150 1C,
the mean diameter of Au particles was very small and was
2
1
1.5 nm. In this study the molar ratio of La/Au was 19 times
6
reduced in a stream of 10 vol% H
Au loading was 1 wt%.
2
in N
2
at 300 1C for 2 h. The
higher and calcination temperature was 300 1C to transform
3 2 3
La(OH) into La O . The larger ratio of La/Au is favorable to
2
.
Catalyst characterization
obtain highly dispersed smaller Au particles, while the higher
calcination temperature causes the aggregation of Au NPs to
larger ones. The catalytic activity of this Au/La O catalyst
The specific surface area of catalysts was calculated by the
BET method from the nitrogen adsorption isotherms obtained
by using a Micromeritics Tristar (Shimadzu) apparatus.
2
3
for CO oxidation was moderately high and provided 100%
2
228 New J. Chem., 2011, 35, 2227–2233
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oxide supports in each group of Table 1 are listed in the order of
higher main product yields at lower temperatures.
Many acidic or basic or neutral metal oxides make Au active
for mild oxidation leading to the preferential formation of
acetaldehyde. In particular, MoO , La O , Bi O , SrO, and
3
2
3
2
3
SnO2 can provide yields higher than 50% at temperatures
below 280 1C without the formation of acetic acid. Gold on
La O , Bi O , Al O , TiO , and Y O produces CO in yields
2 3 2 3 2 3 2 2 3 2
of 20–70% at temperatures above 280 1C. It is another
characteristic feature that other compounds such as ethylene,
acetone, ethylacetate, and diethyl ether are also byproduced at
2 3 2 3 3
high temperatures over Al O , Y O , and WO supports. The
byproduction of these compounds can be ascribed to the surface
properties of the support metal oxides as described later.
Fig. 3 and 4 show the temperature dependence of ethanol
conversion and product yields over MoO which is strongly
3
Fig. 1 TEM image of Au/ZnO prepared by CP method.
acidic and over Au/MoO . Over MoO without Au NPs no
3
3
oxidation took place at temperatures below 200 1C but at
higher temperatures acetaldehyde is gradually produced being
accompanied by ethylene formation (Fig. 3). Iwasawa et al.
conversion of CO at 80 1C. These results suggest that Au
particles were deposited on La O as NPs below 10 nm. It was
2
3
difficult to observe small Au NPs over La O by TEM with a
2
3
reported that
formed acetaldehyde during ethanol oxidation. The deposition
of Au NPs on MoO shifted the temperature of oxidation
2
a SiO —attached Mo(VI)—dimer catalyst
sharp contrast.
17
The TEM photograph of Au/ZnO prepared by CP is
presented in Fig. 1. It contains Au NPs of a size of 3 nm
and tiny Au clusters of around 1 nm. The mean diameter of
Au nanoparticles was 2.6 nm.
3
toward lower temperatures by about 90 1C and markedly
enhanced acetaldehyde formation. A maximum acetaldehyde
yield of 94% was obtained at 240 1C (Fig. 4). The formation of
2
Fig. 2 shows the XRD pattern of MnO which is active for
the complete oxidation without Au deposition. Although the
XRD pattern was very broad, meaning low crystallinity,
MnO prepared by the reaction of Mn(NO ) with KMnO
2
3 2
4
in aqueous alkaline solution was identified as d- or g-MnO2,
which was identical with the main phase of electrolyte
manganese dioxide.
Mild oxidation of ethanol
The results of catalytic tests for Au NPs supported on 23 kinds of
metal oxides are summarized in Table 1. Acetaldehyde, acetic
acid, and carbon dioxide are the major products. Metal oxide
supports, in principle, define the types of products. The first
group leads to mild oxidation to produce acetaldehyde with high
selectivities, often above 90%. The second proceeds reaction
towards deep oxidation which produces both acetaldehyde and
acetic acid. The selectivities to acetic acid remain at around 50%.
Fig. 3 Ethanol conversion (K), yields of acetaldehyde (J), ethylene
(
,) as a function of reaction temperature over MoO
3
. Reaction
À1 catÀ1
2 5 2 2
, C H OH/O /N = 1/3/126.
conditions: SV = 20 000 mL h
g
The third group enables complete oxidation to produce CO
2
and
2
H O at relatively low temperatures below 200 1C. The metal
Fig. 4 Ethanol conversion (K), yields of acetaldehyde (J), acetic
acid (n), and ethylene(,) as a function of reaction temperature over
À1
catÀ1
Fig. 2 XRD pattern of MnO
2
obtained by the reaction of KMnO
4
Au/MoO
3
.
Reaction conditions: SV
/N = 1/3/126.
=
20 000 mL
h
g
,
with Mn(NO in aqueous solution.
2
3
)
C
2
H
5
OH/O
2
2
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New J. Chem., 2011, 35, 2227–2233 2229

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Table 1 Support effect on product selectivity in gas phase oxidation of ethanol over gold catalysts
Yield (%)
a
Others (mainly )
Group
Support Preparation method Au loading/wt% Temperature/1C CH
3
CHO CH COOH CO
3
2
Mild oxidation
MoO
3
SG
CP
CP
CP
DP
DP
DP
DP
CP
CP
SG
CP
SG
1.0
6.0
4.3
9.1
1.0
1.0
1.0
1.0
8.4
20.6
1.0
6.3
1.0
180
27
94
82
15
81
75
18
68
8
0
0
7
0
0
0
0
0
0
0
0
0
0
0
0
0
1
6
0
0
2
3
8
22
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
2
40
80
5 (E)
10 (E)
0
La
2
O
3
180
2
2
60
80
1 (A)
20 2 (A)
0
0
Bi
2
O
3
100
0
1 (EA)
1
2
80
80
72 7 (A)
SrO
100
1
0
0
0
0
0
0
1
2
80
80
16
67
14
65
35
20
62
49
23
47
64
52
57
50
27
57
35
1
6
34
1
30
25
1
8
27
2
18
1
0
1 (EA)
Al
2
O
3
120
2
2
20
80
24 11 (DE)
50 15 (DE)
0
22 3 (EA)
41 1 (EA)
0
0
0
0
0
9
0
9
TiO
2
180
1 (EA)
2
2
60
80
SnO
2
160
3 (EA)
8 (EA)
3 (EA)
1 (EA)
1 (EA)
1 (EA)
0
2
2
40
80
SiO
2
220
2
2
40
80
Y
2
O
3
180
2
2
40
80
6 (A)
46 13 (A)
MgO
100
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
80
80
2
Nb O
5
100
0
3 (E)
10 (E)
0
0
0
2
2
60
80
BaO
100
1
2
80
80
WO
3
100
0
1
2
80
40
36 (E)
65 (E)
Deep oxidation
ZnO
CP
CP
SG
11.3
6.9
180
74
44
11
45
23
14
67
29
7
15
46
25
14
40
0
3
36
28
0
0
61
0
2 (EA)
6 (EA)
0
2
2
20
60
In
2
O
3
140
3 (EA)
1
2
80
20
12 3 (EA)
86
0
14 21 (E)
29 36 (E)
0
0
V
O
2 5
1.0
200
2
2
40
80
Complete oxidation MnO
2
DP
DP
CP
CP
CP
DP
CP
1.0
1.0
100
10
9
0
22
49
0
10
80
0
38
3
0
21
18
0
38
30
0
0
0
0
0
0
0
0
4
0
0
0
0
0
0
0
12
5
0
0
0
0
0
79
100
0
0
0
1
1
40
60
0
4 (EA)
CeO
2
100
1
1
40
80
14 7 (EA)
0
100
CuO
11.5
11.4
12.2
1.0
100
0
5
100
0
41
100
0
27
100
0
0
0
0
0
0
0
0
0
1
2
60
00
Co
3
O
4
140
1
2
60
00
NiO
160
1
2
80
40
0
20 (EA)
ZrO
2
180
2
2
00
60
39 12 (EA)
100
0
14 6 (EA)
100
0
6 (EA)
Fe O
2 3
11.5
180
25
35
0
2
2
00
80
0
a
Alphabetical characters denote A = acetone, DE = diethyl ether, E = ethylene, and EA = ethyl acetate.
2
230 New J. Chem., 2011, 35, 2227–2233
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Deep oxidation
Deep oxidation of ethanol to form acetic acid takes place over
Au NPs deposited on ZnO, In , and V , which are n-type
2
O
3
2 5
O
semiconductors (Table 1). The Au catalysts of this group show
high yields of acetaldehyde at temperatures below 200 1C.
Even at 100 1C yields of acetaldehyde over 20% were obtained
on Au/ZnO and Au/In
the major product shifted to acetic acid and then to CO
Au/V O at above 200 1C ethylene yield was comparable to
2
O
3
. With an increase in temperature
2
. On
2
5
those of acetic acid and CO . Formation of ethylene is
2
attributed to the acidic property of the V O support.
2
5
Fig. 5 Ethanol conversion (K), yields of acetaldehyde (J) as a
The best support for the deep oxidation of ethanol is ZnO
which gives a high yield of acetic acid of 46% at 220 1C with a
selectivity of 48%. The conversion and yields for ZnO and
Au/ZnO as a function of temperature are shown in Fig. 7 and 8,
respectively. The ZnO support was almost inert as a catalyst at
and below 280 1C for ethanol oxidation (Fig. 7). The deposition
of Au NPs dramatically changed the catalytic activity, giving
function of reaction temperature over La
2
O
3
. Reaction conditions:
À1 catÀ1
2 5 2 2
, C H OH/O /N = 1/3/126. Acetaldehyde
SV = 20 000 mL h
g
yield was the same as ethanol conversion, meaning selectivity is 100%.
9
0% conversion at 200 1C (Fig. 8). In the low temperature
region (o200 1C) a main product was acetaldehyde and the
highest yield of acetaldehyde was 74% at 180 1C. Formation
of acetic acid started at around 180 1C and a maximum yield
of acetic acid of 46% was obtained at 220 1C. Over 250 1C
2
a main product was CO . These changes in the products
distribution as a function of temperature suggest that the
Fig. 6 Ethanol conversion (K), yields of acetaldehyde (J), CO
2
(
&), and others (Â) as a function of reaction temperature over Au/
À1
À1
cat , C
La
2
O
3
. Reaction conditions: SV = 20 000 mL h
= 1/3/126.
g
2
H
5
OH/
O
2
/N
2
ethylene was observed over Au NPs deposited on acidic metal
oxides (in principle, those denoted as Me , MeO , Me
and MeO , Me: metal) at higher temperatures. This is because
2
O
3
2
2 5
O ,
3
acid sites promoted dehydration when reaction temperature
was raised to above 180 1C.
As shown in Fig. 5, La
basic, was almost inert at and below 280 1C. The deposition of
Au NPs on La also dramatically changed the catalytic
2 3
O without Au NPs, which is weakly
Fig. 7 Ethanol conversion (K), yields of acetaldehyde (J) and ethyl
2
O
3
acetate (
conditions: SV = 20 000 mL h gcat , C H OH/O /N = 1/3/126.
}
) as a function of reaction temperature over ZnO. Reaction
À1
À1
activity (Fig. 6). At temperatures above 200 1C the yield of
acetaldehyde remarkably increased reaching a maximum yield
of 81% at 260 1C without the formation of acetic acid. Above
2
5
2
2
Acetaldehyde yield was the same as ethanol conversion up to 260 1C.
2
60 1C the evolution of CO was observed. Other basic metal
2
oxides (MeO: SrO, MgO, and BaO) showed similar catalytic
properties; very high selectivities to acetaldehyde without
byproducing ethylene and ethylacetate. The nature of the Au
catalysts in this group is that the next oxidation step, formation
of acetic acid, hardly takes place. The moderate reactivity of
oxygen species at around 200 1C and the moderate stability of
surface adsorbed species of ethanol are required for the
selective formation of acetic acid. The basic supports of Au
catalysts firmly stabilize the surface ethoxide groups and the
reactivity of oxygen species is too low to form acetic acid. Such
mild oxidation capability is one of the important characteristic
features of Au catalysts. It is also noteworthy that the high
selectivity to acetaldehyde is characteristic to Au catalysts in
gas phase oxidation and is not obtained in the oxidation in
liquid water.
Fig. 8 Ethanol conversion (K), yields of acetaldehyde (J), acetic
acid (n), ethyl acetate ( ), CO
}
2
(&), and others (Â) as a function
of reaction temperature over Au/ZnO. Reaction conditions:
À1 catÀ1
SV = 20 000 mL h
g
2 5 2 2
, C H OH/O /N = 1/3/126.
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formation of acetic acid over Au/ZnO proceeds stepwise;
CH CH OH - CH CHO - CH COOH.
Christensen et al. reported that Au catalysts were more
2
of acetic acid (Fig. 9). This indicates that over MnO the
3
2
3
3
oxidation of acetaldehyde to acetic acid is slower than the
oxidation of acetic acid to CO at temperatures below 130 1C.
2
selective to acetic acid over 80% than Pd and Pt catalysts in
7
liquid phase oxidation of ethanol in water as a solvent. The
Deposition of Au NPs lowers the complete oxidation
temperature to reduce the yield of acetaldehyde (Fig. 10).
Complete oxidation was attained at 160 1C. Apparent decrease
in the yield of acetaldehyde means that acetaldehyde oxidation
best performance was a selectivity of 86% to acetic acid when
the conversion was 97%. Until now there is no report
concerning selective acetic acid formation by the oxidation
of gaseous ethanol over Au catalysts. The best supports in
2
to CO is markedly accelerated by the deposition of Au NPs.
These catalysts are useful not only to decompose ethanol as
one of volatile organic compounds (VOCs) in air at low
temperature but also to apply to ethanol fuel cells.
7
8
liquid phase oxidation (MgAl
2
O
4
and Cu-doped NiO ) were
). In
not the same as those in gas phase oxidation (ZnO, In
2 3
O
the case of liquid phase oxidation water might be involved in
the oxidation mechanism of ethanol.
Discussion
Complete oxidation
Mechanism of ethanol oxidation over supported Au catalysts
Oxide supports which enable complete oxidation of ethanol to
In ethanol oxidation over supported Au catalysts, the formation
of surface ethoxide groups on the surfaces of metal oxide
supports appears to be one of the most important processes.
As shown in Fig. 11 surface ethoxide groups can be formed by
the reaction of ethanol molecules with hydroxyl groups or
metal cations of (a) basic metal oxide, (b) acidic metal oxide,
and (c) n-type semiconductive metal oxide supports.
CO
Co
2
and H
2
O at lower temperatures are MnO
2 2
, CeO , CuO,
3
O
4
, NiO, ZrO
2
2 3
and Fe O . Most of these supports are
p-type semiconductors in contrast to an n-type semiconductor
which is preferable for deep oxidation. Conversion and yields
in ethanol oxidation over MnO and Au/MnO as a function
2
2
of temperature are shown in Fig. 9 and 10, respectively. Metal
oxides by themselves exhibited catalytic activity at a temperature
as low as 100 1C mainly producing acetaldehyde. Complete
The next reaction step is the formation of acetaldehyde.
The surface metal ethoxide is oxidized at C–H bonding to
lose H to form acetaldehyde. Gold NPs deposited on metal
2
oxidation to produce CO took place over 130 1C and 100%
conversion of ethanol was achieved at 180 1C without formation
À
À
oxides activate molecular oxygen to O2 or O depending on
reaction temperature, leading to the formation of acetaldehyde.
Fig. 9 Ethanol conversion (K), yields of acetaldehyde (J), and CO
2
(
&) as a function of reaction temperature over MnO
2
. Reaction
À1 catÀ1
2 5 2 2
, C H OH/O /N = 1/3/126.
conditions: SV = 20 000 mL h
g
Fig. 10 Ethanol conversion (K), yields of acetaldehyde (J), and
CO
2
(&) as a function of reaction temperature over Au/MnO
À1 catÀ1
2
.
Fig. 11 Probable routes for the formation of surface ethoxide from
Reaction conditions: SV = 20 000 mL h
= 1/3/126.
g
, C
2
H
5
OH/O
2
/
ethanol and its transformation to acetaldehyde, acetic acid, and CO
over supported gold catalysts.
2
N
2
2
232 New J. Chem., 2011, 35, 2227–2233
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These active oxygen species might be formed on the corners or
edges of Au NPs and at the perimeter interfaces between Au
NPs and the metal oxide supports.
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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
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