Au/xCeO2/Al2O3 catalysts for VOC elimination: Oxidation of 2-propanol

Article abstract of DOI:10.1039/c3cy00238a

2-Propanol oxidation (1 mol% in air) was investigated over 1%Au/CeO ₂, 1%Au/Al₂O₃ and 1%Au/xCeO₂/Al ₂O₃ (x = 1.5, 3, 5 and 10 wt%) catalysts and the corresponding pure supports. The effects of the catalyst activation procedure (calcination or reduction), cerium oxide morphology and loading, the presence/absence of O₂ in the feed, and stability of activity were studied. The nature of the support strongly influences the 2-propanol oxidation pathway, and one can distinguish three reaction temperature ranges: low (50-150 °C), intermediate (175-250°C) and high (>300°C). In the low temperature range, only acetone is formed whatever the nature of the support. On Au/Al₂O₃ and Au/xCeO₂/Al₂O ₃ catalysts propene becomes the main reaction product by increasing the temperature, followed by CO₂ formation at higher temperature. The acidic character of Al₂O₃ and CeO₂/Al ₂O₃ promotes the propene formation whereas the basic one of CeO₂ favors the formation of acetone, and the gold catalysts also present an acidic-basic behavior, which depends largely on the reaction temperature. Replacing air with He during the catalytic run over Au/Al ₂O₃ and Au/CeO₂ provides evidence of how O ₂ is activated by Au, revealing, as expected, that dehydrogenation to form acetone and total oxidation to CO₂ require both O₂ and Au as the catalyst, whereas dehydration to form propene does not require O₂. For both calcined and reduced catalysts, the catalytic activity towards total oxidation varies in the order: Au/CeO₂ > Au/xCeO₂/Al₂O₃ > Au/Al₂O ₃.

Full text of DOI:10.1039/c3cy00238a

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Au/xCeO2/Al2O3 catalysts for VOC elimination:
oxidation of 2-propanol
Cite this: DOI: 10.1039/c3cy00238a
Pandian Lakshmanan,a Laurent Delannoy,b Catherine Louis,b Nicolas Biona and
Jean-Michel Tatibouet*a
¨
2-Propanol oxidation (1 mol% in air) was investigated over 1%Au/CeO2, 1%Au/Al2O3 and 1%Au/
xCeO2/Al2O3 (x = 1.5, 3, 5 and 10 wt%) catalysts and the corresponding pure supports. The effects of
the catalyst activation procedure (calcination or reduction), cerium oxide morphology and loading, the
presence/absence of O2 in the feed, and stability of activity were studied. The nature of the support
strongly influences the 2-propanol oxidation pathway, and one can distinguish three reaction
temperature ranges: low (50–150 1C), intermediate (175–250 1C) and high (4300 1C). In the low
temperature range, only acetone is formed whatever the nature of the support. On Au/Al2O3 and
Au/xCeO2/Al2O3 catalysts propene becomes the main reaction product by increasing the temperature,
followed by CO2 formation at higher temperature. The acidic character of Al2O3 and CeO2/Al2O3 pro-
motes the propene formation whereas the basic one of CeO2 favors the formation of acetone, and the
gold catalysts also present an acidic–basic behavior, which depends largely on the reaction temperature.
Replacing air with He during the catalytic run over Au/Al2O3 and Au/CeO2 provides evidence of how
O2 is activated by Au, revealing, as expected, that dehydrogenation to form acetone and total oxidation
to CO2 require both O2 and Au as the catalyst, whereas dehydration to form propene does not require
O2. For both calcined and reduced catalysts, the catalytic activity towards total oxidation varies in the
order: Au/CeO2 4 Au/xCeO2/Al2O3 4 Au/Al2O3.
Received 12th April 2013,
Accepted 5th July 2013
DOI: 10.1039/c3cy00238a
www.rsc.org/catalysis
At higher Au loadings, nanosized CeO2 particles stabilize small
gold clusters or gold ions, which may even be incorporated into
1. Introduction
the cerium oxide nanoparticles at higher temperatures.16 The
incorporation of gold ions into the ceria lattice can generate
both electronic and geometric effects. Cerium oxide crystallizes
in the fluorite structure with a face-centered cubic (fcc) unit
cell.11 In this structure, each cerium cation (Ce4+, ionic radii
0.097 nm)17 is coordinated to the 8 nearest oxygen anions at
the corner of a cube. If a trivalent cation (in the present study:
Au3+ ionic radii 0.085 nm)17,18 is introduced into the ceria
lattice, extrinsic oxygen vacancies can be formed by a charge-
compensating mechanism.11,17 The difference between the
cationic radii of Ce4+ and Au3+ eventually allows the compensa-
tion of the volume increase when the Ce4+ cation is reduced to
Ce3+ (0.114 nm)17 thus facilitating this reduction, as already
observed for Zr4+ in the ceria lattice.11,19 In fact, the presence of
gold enhances the reducibility of ceria surface oxygen8,20,21 by
weakening the Ce–O bonds adjacent to gold atoms, leading to
the activation of surface capping oxygen.2,3,8,22–24 Therefore, the
nature of the ceria surface with which gold interacts can greatly
influence the oxidation state, size, and distribution of gold in
the catalysts.
Au nanoparticles supported on cerium oxide surfaces present
interesting catalytic applications derived from the versatile
redox properties of cerium oxide and its interaction with gold.
Important applications include catalytic combustion of volatile
organic compounds (VOCs),1–7 low-temperature water-gas shift
(LT-WGS) reaction8,9 and preferential oxidation of CO (PROX).10
CeO2 is well known for its unique property, namely its ability to
shift between two oxidized states (Ce3+ 2 Ce4+) and to accom-
modate variable levels of bulk and surface oxygen vacancies.11
The interaction between Au and CeO2 is intimate and the
electronic state of gold is critically influenced by the structure
and morphology of cerium oxide.6–10,12–15 In 1%Au/CeO2 (SBET
200 m2 gÀ1) gold exists in oxidized form (Au3+) after calcination
and in reduced form (Au0) after reduction with hydrogen.6
a
´
Institut de Chimie des Milieux et Materiaux de Poitiers (IC2MP) UMR CNRS 7285,
´
´
´
Universite de Poitiers, Ecole Nationale Superieure d’Ingenieurs de Poitiers,
´
1, rue Marcel Dore, 86022 Poitiers cedex, France.
E-mail: jean.michel.tatibouet@univ-poitiers.fr
b
´
´
Laboratoire de Reactivite de Surface (LRS), UMR CNRS 7197,
´
Universite Pierre et Marie Curie, 4 place Jussieu, 75252 Paris cedex 5, France
c
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Although the effects of morphology of ceria for gold catalysts the catalyst. The 2-propanol concentration was 1 mol% in air
have been already studied,13 ceria is usually not used as a (total gas flow rate of 50 mL minÀ1). The reaction temperature
support due to its poor resistance against thermal sintering. was increased stepwise from room temperature to 400 1C at a
CeO2 is often supported on an oxide support, such as Al2O3, to heating rate of 1 1C minÀ1 with a 2 h temperature plateau every
obtain CeO2–Al2O3 mixed oxides.25 Depending upon the ceria 25 or 50 1C. Catalytic activity at each temperature was measured
loading on alumina, three-dimensional particles (3D-CeO2) and after the steady-state was reached (30–40 min). No deactivation
two-dimensional ceria layers (2D-CeO2) can be formed, their was observed during the measurement at each plateau of
redox properties differing from that of unsupported ceria.26,27 temperature (2 h). After obtaining three consistent analyses,
Such variations in the morphology of alumina-supported ceria the temperature was increased to the next plateau.
affected the distribution of gold between alumina and ceria
surfaces, the redox properties of ceria and then the catalytic propene, CO, CO2 and ethane) was performed using a gas
activity of gold based catalysts.7
chromatograph (Perkin-Elmer AutoSystem) equipped with a
The analysis of 2-propanol and the various products (acetone,
Based on this background, the present study was undertaken Porapak-R column (1.8 m, 3.17 mm o.d), and a Carboxen-1000
to study the 2-propanol oxidation reaction on various gold based column (4.5 m, 3.1 o.d).
catalysts containing 1 wt% of Au loaded on unsupported ceria,
For the purpose of comparison, pure supports without gold
pure alumina and alumina-supported ceria with different ceria loading were also tested under similar reaction conditions. The
loadings. The reaction conditions are close to those of VOC reproducibility of the catalytic results was carefully checked.
elimination, i.e., 2-propanol is highly diluted in air. The catalytic Effects of the catalyst activation method, ceria loading, and the
results are discussed in terms of mechanistic formation of presence/absence of O2 in the feed were investigated. The effect
acetone and propene related to the oxidation state of gold.28–32 of the presence/absence of O2 in the reactant flow was investi-
gated at a temperature at which both acetone and propene
form. During the catalytic run, air was replaced by He at steady
2. Experimental
state under similar reaction conditions, and the gas composition
exiting the reactor was analyzed as a function of time-on-stream.
Pure alumina (AluC Degussa, 110 m2 gÀ1) was used as a support
After running reaction under He, again He was replaced by air to
to load cerium oxide (1.5, 3, 5 and 10 wt% with respect to
verify the reversibility.
alumina) by impregnation in excess of an aqueous solution of
Ce(NO3)3,Á6H2O (Aldrich, 99.9%). 1 wt% of gold was loaded on
3. Results and discussion
the various CeO2–Al2O3 samples by deposition–precipitation
with urea. The details of experimental procedures regarding
catalyst preparation, and characterization by elemental analyses,
BET surface area analysis, XRD, XPS and combined TEM-EFTEM
techniques were largely described in our previous study.7
Catalysts are named Au/CeO2 and Au/Al2O3 for 1 wt% Au
supported on pure CeO2 and Al2O3, and Au/xCeO2/Al2O3 for
1 wt% Au on Al2O3 support containing x wt% of CeO2 (Table 1).
Before use, catalysts were either activated under an air flow
at 400 1C (calcination) or under pure H2 at 300 1C (reduction)
(100 mL minÀ1, 2 1C minÀ1 from room temperature to the final
temperature then 2 h at the final temperature). Note that for the
sake of brevity, these treatments are called calcinations and
reductions, respectively, in the following.
3.1. Summary of the characterization study of the catalysts
First, a summary of the characterization study performed earlier
on these catalysts7 is presented here. The morphology of
alumina-supported ceria varied according to the ceria loading.
In agreement with Arias’s results,27 it was proposed that up to
3 wt% CeO2 on alumina, ceria clusters are mainly as 2-D
patches and that above this loading, 3-D CeO2 nanoparticles
begin to appear, as shown by the results of XRD and TEM.
In 5CeO2/Al2O3 and 10CeO2/Al2O3 samples, 3-D CeO2 nano-
particles (B8 nm) are present. These variations in the
morphology of ceria affected the characteristics of the gold
nanoparticles in terms of their distribution between alumina
and ceria, oxidation state, and average particle size. As the ceria
loading increased (1.5, 3, 5, and 10 wt% CeO2), the percentage
of gold particles visible in the TEM micrographs decreased
while the particle size did not change, and since the gold
particles are visible on alumina and not on ceria because of
the poor contrast between gold and ceria, it was deduced that
the percentage of gold particles on ceria increased as the ceria
loading increased. Energy filtered TEM (EFTEM) confirmed
that gold particles were in close contact with ceria particles.
The XPS analyses of Au/CeO2/Al2O3 catalysts carried out at
liquid nitrogen temperature revealed that the oxidation state
of gold depended upon the ceria loading and the type of
catalyst activation (calcination or reduction). Gold on pure
Al2O3 was in metallic state, whatever the activation treatment,
whereas gold on pure ceria was metallic only after reduction,
2-Propanol oxidation was carried out in a flow-type packed
bed tubular micro-reactor (inner diameter 3 mm) with 10 mg of
Table 1 Elemental analyses and the average gold particle size for various
catalysts
Elemental analysis (wt%) Au particle size (nm)
Au
CeO2 Calcination Reduction
Catalyst
Au/CeO2
0.97
98.3 n.m
n.m
Au/1.5CeO2/Al2O3 0.93
1.41 2.6 (0.55)a
2.64 2.7 (0.53)
4.26 2.6 (0.43)
8.06 2.6 (0.57)
2.3 (0.52)
2.3 (0.45)
2.2 (0.47)
2.0 (0.45)
2.0 (0.49)
Au/3CeO2/Al2O3
Au/5CeO2/Al2O3
0.99
0.89
Au/10CeO2/Al2O3 0.89
Au/Al2O3 0.88
—
2.4 (0.56)
n.m – not measurable.a Standard deviation.
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most of the gold remaining unreduced after calcination; inter-
action between gold and ceria stabilizes gold in a higher
oxidation state. Consistently, the XPS results also showed that
in the reduced Au/xCeO2/Al2O3 samples, gold was metallic as
on pure alumina and ceria whereas in calcined samples the
results depended on the CeO2 loading.
Finally, the average size of the gold particles visible by TEM is
in general smaller after reduction treatment than after calcina-
tion (Table 1).
3.2. Oxidation of 2-propanol
As a blank experiment, 2-propanol oxidation was conducted in
the empty reactor. The thermal decomposition of 2-propanol
begins around 325 1C. At 400 1C, 23% of 2-propanol was converted
into acetone (94%), propene (4%) and carbon dioxide (2%).
Fig. 1 compares the catalytic performances of calcined
(Fig. 1B) and reduced (Fig. 1C) Au/CeO2 along with those of
the pure CeO2 support (Fig. 1A). Acetone forms at low tempera-
ture over pure CeO2 due to the basic nature of ceria. Ceria is
also active for the total oxidation of 2-propanol to CO2 at higher
temperature since at 200 1C, 2-propanol is completely converted
into CO2 (Fig. 1A). The addition of gold decreases the temperature
at which 2-propanol is totally converted: B150 1C for calcined or
reduced Au/CeO2 (Fig. 1B and C) compared to 200 1C for pure
ceria. One can note that at low temperature, reduced Au/CeO2 is
more active than calcined one.
Fig. 2 compares the catalytic performances of Au/Al2O3
samples after calcination (Fig. 2B), reduction (Fig. 2C) and
the pure calcined Al2O3 support (Fig. 2A). On pure alumina,
2-propanol undergoes dehydration and forms propene due to
its acidic character. 2-Propanol is completely converted into
propene around 200 1C, and the CO2 formation at higher
temperature is very low: 6% at 400 1C with 2% CO. By contrast,
the presence of gold on Al2O3 changes completely the catalytic
behavior of 2-propanol transformation. Whatever the catalyst
treatment (calcination or reduction), acetone instead of propene
is formed on Au/Al2O3 catalysts at low temperature (Fig. 2B
and C). At temperatures higher than 175 1C, and at about 100%
of 2-propanol conversion, propene gradually appears at the
expense of acetone and reaches a maximum at about 350 1C,
while the CO2 formation remains low. For the calcined sample,
Fig. 1 Conversion of 2-propanol (J) and products distribution (m – acetone;
K – CO2; ’ – propene) on a pure CeO2 support (A); calcined (B) and reduced
Au/CeO2 (C).
the propene starts to form at lower temperature and reaches about 200 1C (Fig. 3A) without any formation of acetone
higher selectivity than for the reduced sample. These results although ceria particles are present.
show the poor performance of the Au/Al2O3 catalyst compared
The calcined Au/10CeO2/Al2O3 catalyst exhibits a catalytic
to Au/CeO2 towards the total oxidation of 2-propanol, but behavior rather similar to that of Au/Al2O3 (Fig. 2B and C)
reveal the capacity of Au/Al2O3 to act as a selective oxida- except that it is more active, and the curves are drastically
tion catalyst. One can distinguish three reaction temperature shifted to lower temperatures. In the low temperature region,
regions: dehydrogenation of 2-propanol to acetone (50–150 1C), acetone is formed exclusively up to 125 1C, followed by a
dehydration to propene (175–250 1C) and total oxidation to progressive decrease in the selectivity to acetone and an
CO2 (4350 1C).
increase in the selectivity to propene, which passes through a
Fig. 3 reports the catalytic results obtained for the pure maximum at 225 1C instead of 350 1C for calcined Au/Al2O3.
calcined 10CeO2/Al2O3 support (Fig. 3A), calcined (Fig. 3B) and The reduced Au/10CeO2/Al2O3 sample behaves very differently
reduced (Fig. 3C) Au/10CeO2/Al2O3 samples. The pure 10CeO2/ from the calcined one, and rather behaves like the reduced
Al2O3 support exhibits roughly the same behavior as Al2O3 Au/CeO2 catalyst (Fig. 1C) The formation of propene is very
(Fig. 2A). 2-Propanol undergoes dehydration to form propene weak between 175 and 225 1C, and CO2 becomes the main
above 150 1C. Complete conversion to propene is reached at product at temperature as low as 225 1C.
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Fig. 2 Conversion of 2-propanol (J) and products distribution (m – acetone;
K – CO2; ’ – propene, Â-CO) on a pure Al2O3 support (A); calcined (B) and
reduced Au/Al2O3 (C).
Fig. 3 Conversion of 2-propanol (J) and products distribution (m – acetone;
K – CO2; ’ – propene; Â-CO) on a pure 10CeO2/Al2O3 support (A); calcined (B)
and reduced Au/10CeO2/Al2O3 (C).
If the nature of the pretreatment does not modify the
temperature dependence of 2-propanol conversion to a large
The catalytic behavior of the Au/1.5CeO2/Al2O3 catalyst
extent, the reduced Au/10CeO2/Al2O3 catalyst is much more resembles closely the one of Au/10CeO2/Al2O3 with however a
active for the total oxidation of 2-propanol. Indeed, at B225 1C, lower selectivity to propene in the intermediate temperature range
100% conversion is obtained for both catalysts, but at this for the calcined sample. The two other samples of intermediate
temperature, the CO2 selectivity is close to 100% for the ceria loadings, Au/3CeO2/Al2O3 and Au/5CeO2/Al2O3 catalysts,
reduced one whereas for the calcined one, selectivities to CO2 behave in between. The product distribution of Au/3CeO2/Al2O3
and propene are 7% and 85%, respectively, and 100% of CO2 resembles closely that of Au/1.5CeO2/Al2O3 whereas the behavior
selectivity is reached at 325 1C, only. Moreover, the formation of of Au/5CeO2/Al2O3 is close to that of Au/10CeO2/Al2O3.
propene on reduced Au/10CeO2/Al2O3 is very weak between 175
and 225 1C, and acetone is the main product.
As can be seen from the figures of the calcined Au/CeO2 and
Au/10CeO2/Al2O3 samples (Fig. 1B and 3B), the fall of selectivity
Hence, the catalytic results obtained with Au/10CeO2/Al2O3 to acetone when the temperature is increased corresponds to
indicate that gold on ceria is more active in total oxidation of the maximum selectivity to propene. One can therefore wonder
2-propanol when gold is in the metallic state than when it is whether acetone is an intermediate for the formation of propene.
unreduced (calcined), which is in agreement with our previous To verify that, the reaction of acetone oxidation was conducted
studies on CO and propene oxidation.7
under similar reaction conditions. Whatever the catalyst, CO2 was
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Fig. 4 Conversion of acetone on various catalysts (K – Au/CeO2; ’ – Au/10CeO2/Al2O3; m – Au/1.5CeO2/Al2O3;
~ – Au/Al2O3).
the only product obtained and propene was not formed. Hence,
one can conclude that propene directly arises from 2-propanol
oxidation and not from acetone; this demonstrates the existence of
two different reaction pathways for the formation of propene and
acetone. The light-off curves for acetone oxidation (Fig. 4) show that
the activity order determined at 50% conversion is the following:
Au/CeO2 4 Au/10CeO2/Al2O3 4 Au/1.5CeO2/Al2O3 4 Au/Al2O3.
3.3. Effect of the presence/absence of O2 in the feed
It has been shown above that, when they are calcined in air, the
pure Al2O3 and CeO2/Al2O3 supports (Fig. 2A and 3A) mainly
transform 2-propanol into propene, while the pure CeO2 and all
the gold catalysts exclusively form acetone, suggesting a crucial
role of Au in the dehydrogenation reaction of 2-propanol into
acetone.
In order to better understand how the supports interact with
gold or participate in the catalytic process, we studied the
catalytic 2-propanol transformation in the absence of oxygen
for reduced Au/CeO2 and Au/Al2O3. Reduction pre-treatment
was selected based on the fact that Au0 is expected to be the
catalytically active species for the CO oxidation reaction,6 so the
replacement of air by helium should make it possible to
determine if the oxygen species needed for the reaction come
from the gas phase or via the support. The reaction temperature
was chosen to correspond to the one at which the selective
Fig. 5 Effect of replacement of air by He during 2-propanol oxidation with
reduced Au/CeO2 at 125 1C (A); reduced Au/Al2O3 at 225 1C (B). The changes in
the distribution of products: J-2-propanol; m – acetone; ’ – propene; K – CO2.
formation of acetone begins to shift to CO2 (on Au/CeO2) or to
propene (on Au/Al2O3) with air, i.e. 125 1C for reduced Au/CeO2
and 225 1C for reduced Au/Al2O3.
Fig. 5A shows the evolution of the product distribution
during 2-propanol conversion at 125 1C over reduced Au/CeO2 If the remaining air (in large excess compared to 2-propanol)
as a function of the nature of carrier gas, air or He. After resulting from the slow purge of the whole experimental set-up
2.5 hours during which 2-propanol in air and at 125 1C converts certainly contributes to the slow decrease of conversion, the
into acetone and into CO2 to a very small extent, in agreement fact that CO2 formation collapses much more rapidly than
with the results presented in Fig. 1C, the air flow is replaced acetone formation allows us to conclude that the reaction of
by He. Gradually, the conversion of 2-propanol falls down and oxidative dehydrogenation of 2-propanol into acetone involves the
reaches zero after 4 hours. Meanwhile, the CO2 production stops contribution of oxygen released from the readily reducible ceria,
within 2 hours and the acetone formation within 4 hours, (Fig. 5A). indicating a Mars and van Krevelen-type reaction mechanism;
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Scheme 1 Reaction scheme for the Au/Al2O3 catalyst.
the oxygen storage/release capacity (OSC) of ceria is well
known,11 and the occurrence of the Mars and van Krevelen
mechanism was proposed by Nieuwenhuys et al.15 to explain
the catalytic activity in propene total oxidation of Au/CeO2–
Al2O3 catalysts under an Ar flow. Conversely, when He is
replaced by air flow after 6.5 hours (Fig. 5A), the initial conver-
sion is recovered much faster within around 2 hours with the
same proportion of acetone and CO2 as initially because of the
easy replenishment of oxygen-depleted ceria by oxygen of the
gas phase.
When the same experiment is performed over the reduced
Au/Al2O3 catalyst at 225 1C, acetone is the main product in the
presence of air, along with very small amounts of propene and
CO2 (Fig. 5B), in agreement with the results in Fig. 2C. When air
is replaced by He, there is no significant change in the con-
version of 2-propanol but selectivity drastically changes, and
acetone and CO2 formation is replaced by propene formation.
Again these changes are totally reversible when air is reintro-
duced. This behavior indicates that in the absence of oxygen,
the catalyst behaves like pure alumina (Fig. 2A), i.e. as a
dehydrating catalyst, and that gold does not play any role. In
contrast, the different selectivity in the presence of O2 in the gas
phase indicates that activation of oxygen requires the presence
of Au0 nanoparticles. Moreover, the same 2-propanol conver-
sion with or without oxygen in the gas phase, but with a totally
different selectivity (acetone with oxygen, propene without
oxygen) could indicate that the rate limiting step for these
two reactions is the formation of a common intermediate
species, this intermediate species being transformed into
Scheme 2 General reaction scheme for Au/CeO2/Al2O3 reduced and calcined
catalysts.
acetone in the presence of oxygen and into propene in the product is acetone over Au/CeO2/Al2O3 although in the reduced
absence of oxygen (Scheme 1).
sample gold is in metallic form (Au0) while in the calcined
According to this interpretation, the selectivity change from sample, gold is metallic (Au0) on alumina but mainly in ionic
acetone to propene when the reaction temperature increases in form (Aud+) on ceria particles (see Section 3.1). This behavior is
the presence of oxygen on Au/Al2O3 catalysts (Fig. 2B and C) the same as the one observed on Au/Al2O3 and Au/CeO2
could be explained by the decreasing ability of gold particles catalysts in the same temperature domain (Fig. 1B and C and
to participate in the activation of oxygen when the reaction 2B and C). However if one examines more closely the activities
temperature increases, thus favoring propene formation at the at 125 1C for the set of Au/CeO2/Al2O3 catalysts (Fig. 6), one can
expense of acetone formation.
distinguish differences between the reduced and calcined
samples, and according to the ceria loading: the reduced
samples are more active than the calcined ones, and the activity
of the reduced ones increases as the ceria loading increases
3.4. General reaction scheme of 2-propanol oxidation over
Au/CeO2/Al2O3 catalysts
A model describing the catalytic behavior of the Au/CeO2–Al2O3 while this follows the opposite trend for the calcined samples.
catalysts can be proposed on the basis of the respective Since when the ceria loading increases, the amount of gold
behavior of the Au/CeO2 and Au/Al2O3 catalysts (Scheme 2). interacting with ceria also increases (see Section 3.1), leading to
We have to distinguish three reaction temperature domains: a decreasing amount of metallic Au interacting with alumina,
low (50–150 1C), intermediate (175–250 1C) and high (4250 1C). the lower catalytic activity of the calcined samples indicates
In the low temperature domain, and whatever the catalyst that the presence of Aud+ in the samples (on the ceria particles)
pretreatment (calcination or reduction), the single reaction is detrimental for the activity. Moreover, one can also deduce
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Table 2 Catalytic results
Activitya (Â104 mol sÀ1 gAu
)
À1
Calcination
Reduction
Catalyst
Au/CeO2
10.14b
9.85
9.78
9.44
9.13
8.9b
11.22b
10.22
10.54
11.07
12.04
9.1b
Au/1.5CeO2/Al2O3
Au/3CeO2/Al2O3
Au/5CeO2/Al2O3
Au/10CeO2/Al2O3
Au/Al2O3
a
b
Activity measured at 125 1C. After subtracting the contribution of
the pure support.
4. Conclusions
Oxidation of 2-propanol over 1%Au/CeO2, 1%Au/Al2O3 and
1%Au/xCeO2/Al2O3 catalysts has been investigated. The total
oxidation into CO2 pathway involved the formation of acetone
and propene as intermediates. Except for Au/CeO2, three different
reaction temperature regions can be distinguished. The low
temperature region (50–150 1C) showed the exclusive 2-propanol
dehydrogenation into acetone. Above 175 1C, the propene for-
mation appeared to be the main reaction product at 225 1C on
Au/Al2O3 and on calcined Au/CeO2/Al2O3 catalysts. For Au/Al2O3,
the temperature range for dehydration expanded up to 400 1C,
due to the low activity of gold on alumina towards total oxidation.
The effects of activation treatment, ceria loading and the
presence/absence of O2 were studied. Whatever the catalyst
activation procedure (calcination or reduction) the catalytic activity
followed the order: Au/CeO2 4 Au/xCeO2/Al2O3 4 Au/Al2O3. For
the Au/xCeO2/Al2O3 catalysts, with increasing ceria loading, the
catalytic activity decreased for calcination pre-treatment and
increased for reduction pre-treatment likely due to the low
activity of oxidized gold on ceria particles. Another interesting
result is that although the Au/Al2O3 and Au/CeO2/Al2O3 catalysts
are not very good for total oxidation reaction, they are efficient
towards the low temperature selective oxidation, i.e., dehydro-
genation of 2-propanol into acetone. Au on both supported and
unsupported ceria samples exhibited stable catalytic activity for
more than 24 hours without any deactivation at 125 1C.
Fig. 6 Variation of activity measured at 125 1C over Au/CeO2/Al2O3, as a
function of the ceria loading and catalyst activation method for various catalysts:
J – calcination; K – reduction.
from Fig. 6 that Au0 on ceria particles is more catalytically active
than Au0 on alumina since the catalytic activity of the reduced
samples, where all the gold is metallic, increases with the ceria
content.
In the intermediate temperature domain, the reduced
Au/CeO2/Al2O3 catalysts lead mainly to CO2 formation and the
calcined ones lead to propene formation with a maximum of
selectivity in propene around 225 1C. The only difference
between the reduced and calcined Au/CeO2/Al2O3 catalysts that
can explain the different selectivities in this intermediate tem-
perature domain is the oxidation state of the gold interacting
with the ceria particles. Moreover, the similarity of catalytic
behavior between calcined Au/CeO2/Al2O3 and Au/Al2O3 indicates
that Aud+/CeO2 is poorly active in 2-propanol transformation.
In the reduced Au/CeO2–Al2O3 catalyst, gold is metallic (Au0)
both on alumina and ceria, and the selective formation of CO2
could result from the high oxidation activity of Au0 on ceria
particles, as observed on the reduced Au/CeO2 catalyst, which
would be high enough to fully oxidize the propene formed on
the Au0/Al2O3 active phase.
In the high temperature range, both the calcined and reduced
Au/CeO2/Al2O3 catalysts selectively form CO2. This may suggest
that, above 225 1C, the redox reaction on Aud+ on ceria particles
is able to gradually reduce Aud+ to Au0 and then to suppress the
difference between the reduced and calcined catalyst, as attested
by the drop in selectivity into propene at the benefit of CO2
observed above 225 1C. Such gold reduction has been already
observed during propene oxidation6 with reduction starting
around 100 1C and completed around 200 1C. We can then
assume that the same behavior occurs with 2-propanol since we
reasonably expect that the reducing power of 2-propanol is
close to that of propene.
Acknowledgements
The authors acknowledge the ANR (Agence Nationale pour
la Recherche), which sponsored this work (ANR-BLANC07-2
183612).
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