Study of the influence of the In2O3 loading on γ-alumina for the development of de-NOx catalysts

Article abstract of DOI:10.1016/j.jcat.2005.07.001

Alumina-supported indium oxide (In₂O₃) catalysts with In loadings between 2 and 22 wt% were prepared by impregnation and characterized for their main properties. Surface properties, such as the surface area; surface In content, measured by X-ray photoelectron spectroscopy (XPS); and acidity and basicity, measured both by adsorption microcalorimetry using ammonia and sulfur dioxide as probe molecules and by infrared spectroscopy of pyridine adsorption, were determined. Bulk properties, namely the crystallographic structure as determined by powder X-ray diffraction (XRD) analysis and the redox character of the In₂O₃-dispersed phase as determined by redox cycles performed both in a flow apparatus and in a thermobalance coupled with a differential scanning microcalorimeter (TG-DSC), were studied. The results obtained were interpreted in terms of In ₂O₃ surface dispersion or aggregation. The catalysts tested in the reduction of NO_x by ethene in an oxygen-rich atmosphere showed an interesting ability to selectively reduce NO_x to N ₂, independently of the In loading. The weakness of the oxidative properties of the In₂O₃ phases limited the ethene combustion and made the catalysts able to reduce NO_x even at high temperature (up to 550 °C). The rates of nitrogen formation depended strongly on the In₂O₃ aggregation state; the In centers were very active even at low amounts.

Full text of DOI:10.1016/j.jcat.2005.07.001

Journal of Catalysis 234 (2005) 421–430
www.elsevier.com/locate/jcat
Study of the influence of the In2O3 loading on γ -alumina
for the development of de-NOx catalysts
J.A. Perdigon-Melon a,b, A. Gervasini c, A. Auroux a,∗
a
Institut de Recherches sur la Catalyse, CNRS, 2 avenue Einstein, 69626 Villeurbanne cedex, France
b
Instituto de Química-Física “Rocasolano”, CSIC, c/ Serrano 119, 28006 Madrid, Spain
c
Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, via C. Golgi n.19, 20133 Milano, Italy
Received 16 May 2005; revised 25 June 2005; accepted 3 July 2005
Available online 11 August 2005
Abstract
Alumina-supported indium oxide (In O ) catalysts with In loadings between 2 and 22 wt% were prepared by impregnation and character-
2
3
ized for their main properties. Surface properties, such as the surface area; surface In content, measured by X-ray photoelectron spectroscopy
(XPS); and acidity and basicity, measured both by adsorption microcalorimetry using ammonia and sulfur dioxide as probe molecules and
by infrared spectroscopy of pyridine adsorption, were determined. Bulk properties, namely the crystallographic structure as determined by
powder X-ray diffraction (XRD) analysis and the redox character of the In O -dispersed phase as determined by redox cycles performed
2
3
both in a flow apparatus and in a thermobalance coupled with a differential scanning microcalorimeter (TG-DSC), were studied. The results
obtained were interpreted in terms of In O surface dispersion or aggregation. The catalysts tested in the reduction of NO by ethene in an
x
2
3
oxygen-rich atmosphere showed an interesting ability to selectively reduce NO to N , independently of the In loading. The weakness of the
x
2
oxidative properties of the In O phases limited the ethene combustion and made the catalysts able to reduce NO even at high temperature
x
2
3
◦
(up to 550 C). The rates of nitrogen formation depended strongly on the In O aggregation state; the In centers were very active even at low
2
3
amounts.
2005 Elsevier Inc. All rights reserved.
Keywords: Alumina-supported indium oxide; de-NO catalysts; Surface characterizations; Redox and acid/base sites
x
1. Introduction
can selectively reduce nitrogen oxides (NOx) in an oxidizing
environment has been underway for many years [8,9,14,15].
Since the first reports of the high selective catalytic reduction
and decomposition activity toward NOx of copper- [25,26]
and other cations (Fe, Co, Ga, and In) [24,27–30]-exchanged
zeolites, a large variety of nonzeolitic, practically usable,
catalysts have been tested for the NO abatement of emis-
sions from both motor vehicles and stationary sources under
net oxidizing conditions.
Gallium or indium supported on zeolites have been found
to be more suitable for the de-NOx reactions (i.e., selective
catalytic reduction of NOx by using hydrocarbons, HC-SCR
process) than copper or iron, which favor the undesired di-
rect complete oxidation of the used hydrocarbon species to
carbon dioxide rather than the hydrocarbon oxidation by
the NOx species [24,27,31]. Moreover, it has recently been
Catalysts containing group III elements (B, Al, Ga, and
In) have received much attention in recent years [1–13].
Besides their use as supports for active metal centers, par-
ticularly in Al2O3-containing materials, these elements play
an important role in novel catalytic materials such as pro-
moters, supported metal oxides and mixed oxides, supported
bimetallic catalysts, and modified zeolites [14–24]. These
catalysts have been tested in various catalytic reactions of
petrochemical (e.g., aromatization of light alkanes and de-
hydrogenation of long-chain alkanes) and environmental in-
terest. In this direction, the search for viable catalysts that
*
Corresponding author. Fax: +33-472445399.
E-mail address: auroux@catalyse.cnrs.fr (A. Auroux).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2005.07.001
2005 Elsevier Inc. All rights reserved.

422
J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
demonstrated that Ga2O3 and In2O3 oxides supported on
acidic supports, such as γ -Al2O3, TiO2, and SiO2–Al2O3,
present a remarkable de-NOx catalytic activity [13,32–36].
Low-loading In2O3 catalysts supported on alumina have
been reported to show not only high-activity NOx reduction,
but also high resistance to water and SO2 poisoning [12,37],
which makes them innovative and promising catalytic mate-
rials for de-NOx applications.
the adsorption of nitrogen at −196 ◦C. The crystallographic
structure was examined by XRD in a Bruker (Siemens)
D5005 apparatus (Cu-Kα radiation, 0.154 nm). The oxida-
tion state of indium was determined by XPS at room temper-
ature using an SSI 301 spectrometer.
The acidity and basicity of the supported indium sam-
ples and of the bare support were determined by adsorption
microcalorimetry, using NH3 and SO2 as probe molecules.
The measurements were carried out isothermically at 80 ◦C
in a heat flow calorimeter (Setaram C80) coupled with a
standard volumetric apparatus [39–42]. The samples were
pretreated at 400 ◦C in vacuum overnight before the mea-
surements were made. Successive doses of gas were sent to
the sample until a final pressure of 0.5 Torr was obtained.
The sample was then evacuated for 30 min at the same tem-
perature to remove the amount physically adsorbed, and a
second adsorption was performed. The quantity adsorbed at
0.2 Torr in the first adsorption is called VT (total adsorbed
amount). The difference between the amounts adsorbed in
the first and second adsorptions at 0.2 Torr is the irreversibly
chemisorbed amount (Virr).
FTIR measurements were carried out in self-supported
pellets in a Bruker Vector 22 spectrometer. Before these
measurements, the samples were pretreated in oxygen flow
at 400 ◦C overnight and pumped out for 1 h at the same tem-
perature. For each sample, one spectrum was recorded on
the clean sample, and a second spectrum was recorded after
pyridine adsorption at room temperature and evacuation at
150 ◦C for 30 min.
The redox properties of the supported indium samples
have been studied by TPR–TPO. The sample was held in a
U-shaped quartz reactor, allowing the reductant or oxidizing
gas stream to pass through the sample. In TPR experiments,
the temperature was increased by 5 ◦C min−1 from room
temperature up to 840 ◦C, under a reducing atmosphere of
5% H2/Ar (flux 20 cm3 min−1). The temperature was kept
at 840 ◦C for 1 h, then the sample was cooled under ar-
gon atmosphere. After TPR, a TPO experiment was carried
out using the same experimental conditions but increasing
the temperature only up to 800 ◦C, because volatilization of
In2O3 occurs at 850 ◦C [43]. The oxidizing atmosphere was
1% O2/He. The hydrogen or oxygen consumption was de-
termined by means of a TCD (Delsi Instruments DN11).
Before TPR experiments, the samples were reoxidized un-
der oxygen at 400 ◦C for 3 h and cooled down under argon
atmosphere.
The redox properties were also studied using a differen-
tial scanning calorimeter (DSC) coupled to a microbalance
(TG-DSC 111 from Setaram). This equipment makes it pos-
sible not only to calculate the reduced or oxidized amount
from the weight loss or gain of the sample, but also to si-
multaneously measure the evolved heat associated with the
reduction or oxidation process. Unlike in the TPR–TPO ex-
periments, the samples were held in a quartz crucible so
the reactant gas could not pass through the sample. As in
TPR–TPO, successive experiments of reduction and oxi-
Although promising catalytic results have been obtained
and in-depth characterization studies on In2O3 on alumina-
based catalysts have been performed in the last few years [5,
32,37,38], the detailed properties of such materials in rela-
tion to the presence of surface acidic and/or basic species
and indium redox character merit further attention due to
their high implication in the reactivity. In this paper, a series
of In2O3-oxide based catalysts with different loadings (3–
30 wt%) supported on a commercial γ -Al2O3 were prepared
and studied in terms of their surface and bulk properties and
catalytic activity. The acidic character of these catalysts and
the effect of acidity on their activity and selectivity have
been studied. The combined use of Fourier transform in-
frared (FTIR) spectroscopy and adsorption microcalorimetry
made it possible to characterize the acid–base properties of
the samples. FTIR spectroscopy of adsorbed pyridine gave
information on the types of acidic sites present, whereas ad-
sorption microcalorimetry of NH3 and SO2 probe molecules
gave the number and strength of the acidic and basic sites,
respectively. The understanding of the surface chemistry of
such systems also requires deeper insight into the redox
character of the active sites. The information derived from
the characterization techniques has been correlated with the
NOx reduction activity in catalytic tests carried out under
lean conditions and high space velocity. We conclude that
the In aggregation state at the surface and the relative redox
properties play a decisive role in the de-NOx reaction.
2. Experimental
2.1. Sample preparation
Samples with different amounts of indium oxide sup-
ported on γ -Al2O3 (oxide C from Degussa) have been pre-
pared by incipient wetness impregnation using appropriate
amounts of In(NO3)3·5H2O from Aldrich (99.9%). After
impregnation, the samples were dried at 120 ◦C overnight
and calcined at 500 ◦C in oxygen flow for 12 h. The amount
of indium oxide varied from 3 to 24 wt%. Bulk In2O3 was
prepared in the same way, by calcination of indium nitrate at
500 ◦C after drying at 120 ◦C overnight.
2.2. Sample characterization
The concentrations of the supported indium oxide were
determined by AES-ICP in a Spectroflame-ICP instrument.
Surface areas were determined by the BET method from

J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
423
dation were carried out. The temperature was increased at
5 ◦C min−1 up to 650 ◦C, then kept constant for 30 min be-
fore the samples were cooled under helium atmosphere. The
reduction experiments were carried out using a total flux of
30 cm3 min−1 with a mixture of 68% of hydrogen in helium.
For oxidation experiments, the flux was 15 cm3 min−1, with
a composition of 46% O2/He.
cific rates per unit mass of catalyst and per mol of In were
calculated. Selectivity to N2 was evaluated in terms of the
so-called “competitiveness factor” (SSCR, %), defined as the
ratio between the amount of C2H4 consumed to reduce NOx
to N2 and the total amount of C2H4 consumed [44,45].
3. Results and discussion
2.3. Catalytic activity experiments
3.1. Characterization analysis
The catalytic tests were carried out with powder samples
of mass around 0.1 g. The powders were introduced in a
quartz tubular microreactor (5 mm i.d.) between plugs of
quartz wool and pretreated in a flow containing 20% v/v
of O2/He while increasing the temperature in stages up to
350 ◦C and maintaining it for 4 h. The reactant stream was
obtained using a set of mass flow controllers (Bronkhorst,
Hi-Tec) supplying about 3000 ppm of NO and of C2H4,
and 40,000 ppm of O2 in helium at a total flow rate of
85 cm3 min−1, with the reactor at near-atmospheric pres-
sure. The simultaneous presence in the feed mixture of NO
with large amounts of O2 gave rise to homogeneous oxi-
dation of NO, leading to the presence of NO2 in the feed
(namely, NOx) as well. The amount of NO2 was typically
about 16% of the NO total feed. Contact time was main-
tained constant at 1.6 g s mmol−1 (GHSV = 50,000 h−1).
The temperature was increased from 250 to 550 ◦C with a
heating rate of 10 ◦C/min, and eight reaction temperatures
were investigated in this interval. Each temperature plateau
was maintained for 70 min. The attainment of steady-state
conditions was controlled by recording the concentration of
the different species as a function of time on stream (TOS).
As a general trend, 30 min was a sufficient time to obtain
stable concentrations of the different species.
The exit gas stream of the reactor flowed through a gas
cell (multiple reflection gas cell, path length 2.4 m) in the
beam of an FTIR spectrometer (Bio-Rad with a DTGS
detector). The spectrometer made it possible to individu-
ate and measure the concentrations of NO, N2O, and NO2
for N-containing species and of C2H4, CO, and CO2 for
C-containing species. The measurements were carried out
with a resolution of 0.50 cm−1, with an accumulation of
19 scans per spectrum. The gas cell presented an accuracy
of 10 ppm for NO and 4 ppm for N2O and for NO2 us-
ing lines at 1876, 2225, and 1619 cm−1, respectively. The
cumulative absorbance of all the IR active species flowing
from the reactor was monitored every 120 s. The absorbance
profile of each species detected as a function of time or
temperature of reaction was determined on the basis of the
chosen wavelength of absorption of the species, decompos-
ing the cumulative absorbance curve.
The indium oxide loading determined by chemical analy-
sis and the surface area are summarized in Table 1. A re-
markable decrease in the surface area occurred on increasing
the amount of indium oxide deposited. The variation of the
surface area of the different samples with the deposited in-
dium oxide amount is represented in Fig. 1. The data fit
very well with a straight line, indicating that the decrease
in the surface area can be due to the coverage of the sup-
port by a phase of much lower surface area, rather than to
a change of morphology or other modification of the sup-
port, due to blockage of its pores. Fig. 2 presents the XRD
spectra of all the samples. Whereas the chosen alumina was
an amorphous support, the bulk indium oxide presents very
well-developed crystalline phases. The peak intensities and
their 2θ angles were identified as characteristic of the cubic
structure of In2O3. In the supported samples, the crystalline
Table 1
Physico-chemical properties of the samples
Sample
Surface
area
(m g
Loading (wt%)
XPS
In
In O
2
BE
(eV)
In/Al molar
ratio
3
2
−1
)
Al O
InAl-3
InAl-6
InAl-10
InAl-13
InAl-17
InAl-27
112
113
106
103
96
90
82
21
0
2.64
5.5
0
3.19
6.65
10.01
13.03
17.25
26.76
2
3
0.02
444.9
445
444.9
444.8
444.7
444.5
0.052
0.064
0.033
0.034
0.047
8.28
10.78
14.27
22.13
82.71
In O
2
100
3
De-NOx activity in terms of NOx conversion was de-
termined by subtracting the total NO + NO2 concentration
flowing out from the reactor from the total NO + NO2 con-
centration fed into the reactor. The amount of NOx converted
corresponded to N2 formed. From this determination, spe-
Fig. 1. Relation between sample surface area and indium oxide loading.

424
J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
Fig. 2. XRD of the samples loaded with different amounts of indium oxide.
indium oxide phase was observed starting from 10% load-
ing. The position of the line and the relative intensities of
the signal were the same as for the bulk indium oxide, indi-
cating that the support did not influence the crystallization
of indium oxide. For the least-loaded samples (InAl-3 and
InAl-6), no signal was observed due to the small crystal
size or to the presence of amorphous indium oxide. In addi-
tion, sample InAl-17 displayed better dispersion than sample
InAl-13.
Fig. 3. FTIR spectra in the νOH region of the support and supported In O
samples after activation at 400 C.
2
3
◦
XPS experiments make it possible to determine not only
the oxidation state, but also the surface In/Al ratio. These
results are summarized in Table 1. In all cases, only one ox-
idation state was found, with binding energy for In 3d5/2
between 444.5 and 444.8 eV. These values are characteristic
of In2O3 [46]. Considering that XPS is a surface technique,
the In/Al ratio gives an idea of the dispersion of the de-
posited oxide. The higher the ratio, the higher the amount of
exposed indium oxide compared with the total amount of de-
posited indium oxide. Because the theoretical monolayer is
around 23 wt% of InO3 [8], increasing the In loading should
have given rise to an increasing or at least constant In/Al ra-
tio for a similar dispersion, but this was not the case. There
was no direct relationship between the amount of deposited
indium oxide and the In/Al ratio, because the maximum ra-
tio was reached for the InAl-10 sample. Above 10 wt% of
In2O3, the In/Al molar ratio decreased significantly without
a clear trend with In loading. This may be because the high
heterogeneity of the surfaces have more or less large zones
of uncovered support. Moreover, it is likely that only a lim-
ited amount of alumina centers can act as anchoring points
for bonding with indium oxide. Above 10 wt%, these cen-
ters appear to behave as nucleation or aggregation centers.
Similar results have been found by other authors [37].
Fig. 3 shows the FTIR spectra of the supported indium
samples and the bare alumina in the region of hydroxyl
groups after pretreatment in oxygen flow at 400 ◦C and fur-
ther outgassing at the same temperature. The typical OH
vibrations at 3672, 3723, and 3788 cm−1 found for γ -Al2O3
[47] were not altered by the indium oxide deposition. A new
signal not found in bare alumina appeared at 3766 cm−1 and
Fig. 4. Differential heats of ammonia adsorption vs coverage on indium
oxide samples supported on alumina.
became more intense as the indium content increased. High-
frequency OH groups are frequently attributed to basic sites.
The differential heats of ammonia adsorption versus cov-
erage at 80 ◦C are represented in Fig. 4. To compare the
ammonia-adsorbed amounts between the different samples,
these amounts are represented per square meter of surface
area. All the supported indium oxide samples present the
same profile as bare alumina. Only a slight difference can be
observed in the initial part of the curve, where the supported
indium oxide samples present higher adsorption heats than
the pure alumina, probably due to a charge imbalance along

J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
425
Table 2
Total and irreversibly adsorbed amounts of ammonia and sulfur dioxide at
◦
80 C
Sample
NH adsorbed amount
3
SO adsorbed amount
2
−2
−2
(µmol m
)
(µmol m
)
V
V
V
V
irr
T
T
irr
Al O
InAl-3
InAl-6
InAl-10
InAl-13
InAl-17
InAl-27
2.40
2.35
2.20
2.41
2.44
2.33
2.51
2.14
1.44
1.47
1.38
1.48
1.42
1.51
1.52
1.00
1.94
1.90
2.30
2.37
2.07
2.28
2.43
2.24
1.69
1.64
2.05
2.11
1.80
1.97
2.11
1.91
2
3
In O
2
3
Fig. 6. Differential heats of sulfur dioxide adsorption versus coverage on
indium oxide samples supported on alumina.
indicating the low acidity of this oxide. The doubled signal
of the ν8a band indicates the presence of two different types
of acidic centers on the alumina support. The supported in-
dium samples present the same behavior, but the relative area
of the second band becomes lower as the indium oxide con-
tent increases, suggesting that these centers could be blocked
by the indium oxide.
Fig. 5. FTIR spectra of pyridine after adsorption at room temperature and
evacuation at 150 C.
◦
Fig. 6 represents the adsorption heats of SO2 on all the
samples. In contrast to ammonia adsorption, marked differ-
ences are observed among the different samples. Bulk in-
dium oxide presents the highest adsorption heats of all the
samples, again suggesting the main basic character of this
oxide. A clearer idea can be obtained by observing the ir-
reversibly adsorbed amounts of SO2 (Table 2). No evident
relation can be established between Virr and the total indium
oxide loading, but the samples with the highest In/Al ratios
determined by XPS (Table 1)—namely InAl-10, InAl-6, and
InAl-27—are those with the highest irreversibly adsorbed
amounts, indicating a strong relation between surface in-
dium oxide content and adsorbed amount.
the Al–O–In linkages [48]. Bulk indium oxide is clearly
the sample with the lowest amount of adsorbed ammonia.
The total and irreversibly adsorbed amounts of ammonia
are summarized in Table 2. Bulk indium oxide has the low-
est irreversibly adsorbed amount, whereas the amounts for
the other samples are very similar. Because the surface area
decreases with increasing loading, a slight decrease in acid-
ity would be observed if these numbers were calculated in
−1
µmol g [8]. These results suggest that the amphoteric in-
cat
dium oxide can be considered more basic than acidic, and
that the acidity of the supported samples is due mainly to
alumina.
A similar trend is observed in the FTIR spectra of ad-
sorbed pyridine (Fig. 5). Only bands characteristic of pyri-
dine ads−or1bed on Lewis acid sites (1620–1600 cm−1 ν8a,
1450 cm ν19b) are observed. The position and multiplicity
of the ν8a band of adsorbed pyridine is related to the strength
and number of the different types of Lewis acid sites [6]. Al-
though a slight shift toward lower wave numbers is observed
after indium oxide deposition, no great differences in the
position of this band can be observed among the catalysts,
meaning that the strength of the Lewis acid sites is almost
always the same. Bulk indium oxide presents the smallest
band and the lowest wave number for pyridine adsorption,
The redox properties of the indium oxide materials sup-
ported on alumina were studied by TPR/TPO. The hydrogen
consumptions and temperatures of the maximum of the re-
duction peaks are given in Table 3. The reduction profile
of bulk indium oxide is represented in Fig. 7. A single and
broad peak centered at 755 ◦C, incompletely finished, is ob-
tained. The total reduction of In2O3 to In0 requires 3 mol
of hydrogen per mol of indium oxide. In the TPR exper-
iment, the hydrogen consumption was 2.8 molH mol−1
,
2
In2O3
indicating that almost all of the indium oxide has been re-
duced. The XRD spectra (not shown) recorded after TPR ex-

426
J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
Table 3
Hydrogen/oxygen consumptions and peak maximum temperatures in TPR
and TPO experiments
Sample
TPR
TPO
−1
−1
◦
◦
( C)
max
mol mol
H2
T
max
( C) mol mol
O2
T
In2O3
In2O3
Al O
InAl-3
InAl-6
InAl-10 2.8
InAl-13 2.9
InAl-17 2.8
InAl-27 2.6
–
–
2.2
–
–
–
–
–
–
n.d.
190 323
n.d.
200 345
200 337
–
2
3
295 560
244 594
280 659
275 699
230 713
755
n.d.
0.3
n.d.
0.8
0.9
–
In O
2
2.8
3
Fig. 8. TPO profiles of indium oxide samples supported on alumina.
TPO experiments were carried out on the reduced bulk
indium oxide, InAl-27, InAl-17, and InAl-10. Bulk indium
oxide did not show any oxygen consumption. Fig. 8 presents
the TPO profiles for the three other samples. The three sam-
ples present the same profile, a double peak with an onset
around 110 ◦C and the maximum around 320 ◦C (Table 3).
The oxidation temperature is clearly lower than the reduction
temperature. The amount of oxygen consumed per mol of in-
dium oxide is given in Table 3. Assuming that the complete
oxidation of In0 to In2O3 requires 1.5 mol of oxygen per mol
of indium, we can conclude that only part of the indium has
been oxidized. Nevertheless, the only detected phase after
TPO is indium oxide. One experiment of TPO on InAl-17
was stopped at 220 ◦C (before the main oxidation peaks) to
see whether the first oxidation peak was due to intermediate
oxidation states. The only phases found were In0 and In2O3,
suggesting that, as in the case of reduction, the presence of
different peaks is related to the size of the reduced particle
and not to oxidation in successive steps. The similitude of
InAl-27 and InAl-17 is probably due to the phenomenon of
sintering of the indium in the previous TPR experiment. The
oxidation peak on InAl-10 occurs at a lower temperature.
To complement the TPR/TPO experiments, the redox
properties of the supported indium oxide samples with the
highest loadings, as well as bulk indium oxide, were studied
by TG-DSC. Because of the different experimental condi-
tions (composition of reactive gases, total flow or flow path),
it is not possible to compare the two techniques quantita-
tively, but from a qualitative standpoint the results are simi-
lar. In reduction experiments (Fig. 9), only one endothermic
peak is observed, at a reduction temperature that increases
with increasing indium oxide loading. The experimental and
theoretical mass losses are compared in Table 4. Values close
to 100% are obtained, indicating that almost all of the indium
oxide has been reduced. Values slightly exceeding 100% can
be assigned to the presence of a small fraction of water or
hydroxide. The measured reduction heats are also summa-
rized in Table 4. Because these values are given per mol
of indium oxide, a constant value could be expected a pri-
ori. As can be seen, the higher the indium oxide loading,
the higher the measured heat, with the highest value for
Fig. 7. TPR profiles of indium oxide samples supported on alumina.
periments show only the signal corresponding to a metallic
indium phase. The supported indium samples with the high-
est indium oxide loadings (InAl-27, InAl-17, and InAl-13)
present the same trend (Fig. 7), but in these cases low hydro-
gen consumption starting around 200 ◦C is observed until
the main consumption begins, with this reduction amount at
lower temperature becoming larger as the indium oxide con-
tent decreases. For sample InAl-10, the TPR profile presents
the same trend, but the relative amount reduced at low tem-
perature is more important, presenting a well-marked peak
centered at 244 ◦C. For InAl-6, as in the case of InAl-10, the
relative amount reduced at low temperature is more impor-
tant than the amount reduced at high temperature. Because
the reduction peak at low temperature does not appear in the
TPR profile of bulk indium oxide, this peak can hardly be
attributed to a different oxidation step in the reduction from
In3+ to In0. The reduction of bulk indium oxide to metallic
indium seems to occur in one step [49], and the behavior ob-
served for the supported oxides is generally attributed to the
reduction of indium oxide particles of different sizes [37],
with the smallest particles being reduced at low tempera-
ture. This conclusion fits well with the dispersion suggested
by the XPS data. The reduced amount at low temperature is
more important for the samples with the highest In/Al ratio
(InAl-10 and InAl-6) than for the samples with poorer dis-
persion. In all cases, the reduced amount is nearly 100% of
the indium content.

J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
427
Fig. 10. Evolved heat in DSC reduction experiments versus indium oxide
loading.
Fig. 9. DSC heat flow signal in reduction experiments.
Table 4
Temperature of the maximum, evolved heat and mass variation in the re-
duction/oxidation experiments carried out by TG-DSC
Sample
Reduction
Heat
Oxidation
Heat
a
b
a
b
ꢀm
T
◦
ꢀm
T
◦
max
max
−1
−1
)
( C)
(kJ mol
)
(%)
( C)
(kJ mol
(%)
Al O
2
–
–
–
–
–
–
3
InAl-13
InAl-17
InAl-27
479
501
532
550
48.5
69.3
108.9
171.4
93
95
107
108
317
318
299
–
−149.7
−177.9
−203.8
113
81
107
–
In O
2
–
3
a
Heat evolved per mol of indium oxide.
Mass loss and gain relative to the theoretical one.
b
bulk indium oxide. In Fig. 10 the measured heat is repre-
sented versus the total amount of indium oxide deposited.
A good linear relationship is obtained, suggesting that this
variation of the measured heat with the loading is due to the
influence of the support. At low loadings, strong interactions
with the support lower the reduction heats. As in the TPR–
TPO experiments, no reoxidation of bulk metallic indium
is observed. Parallel to the TPR reoxidation experiments,
a double exothermic peak is observed in the reoxidation ex-
periments performed by DSC (Fig. 11). The maximum and
onset of the reoxidation peaks for the three samples are more
similar than they were in the reduction process, probably due
to the sintering of indium in the reduction process, leading
to a more similar dispersion of the three samples. The mass
gain (Table 4) suggests that the metallic indium is reversibly
reoxidized. The XRD spectra after oxidation do not show
any metallic phase lines.
Fig. 11. Evolved heat in DSC in reoxidation experiments.
indium and prevents it from being oxidized. This metallic
indium requires a higher temperature for oxidation.
3.2. De-NOx catalytic activity
The In-based catalysts prepared on amorphous alumina
were evaluated in the reduction of NOx by C2H4 in oxygen-
rich atmosphere at fixed contact time and variable temper-
ature in the 250–550 ◦C interval. The catalyst activity was
measured in terms of NOx (NO + NO2) conversion and se-
lectivity. Selectivity was expressed in terms of the compet-
itiveness factor, taking into account the catalyst’s ability to
selectively use the hydrocarbon to reduce the NOx species
rather than to be oxidized by O2. The experimental results,
in terms of reaction profiles of the main species involved in
the de-NOx reaction, are shown in Figs. 12a–12f for all of
the catalysts as a function of reaction temperature. The ad-
dition of a very low amount of In is sufficient to impart new
catalytic activity to the alumina support. Indeed, alumina is
completely unable to reduce any NOx under the same condi-
tions used for studying the catalysts in the present work, and
yields only a small amount of CO (about 10%) from ethene
An endothermic peak around 157 ◦C is observed for the
three samples, corresponding to the melting point of metallic
indium. The peak is not well developed, because the oxida-
tion occurs before it is finished. This can explain the two
peaks observed in the oxidation process. The oxidation peak
at low temperature is due to the oxidation of the melted in-
dium. The newly generated indium oxide at this temperature
is a solid that creates a layer over the rest of the metallic

428
J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 12. Percentages of conversion of NO (NO + NO ) and of C H , and percentages of formation of CO, and CO as a function of reaction temperature in
x
2
2
4
2
−1
the NO reduction by C H in O at 1.6 g s mmol of contact time: (a) InAl-3; (b) InAl-6; (c) InAl-10; (d) InAl-13; (e) InAl-17; and (f) InAl-27.
x
2
4
2
oxidation at higher temperatures. N2O was not detected
among the reaction products. This finding was expected, be-
cause it is known that the production of large amounts of
N2O in addition to N2 is a specific distinctive feature of
the reduction of NOx on noble metal phases [50]. As a gen-
eral trend, the onset temperature for the selective reduction
of NOx was around 400 ◦C. With the ability to detect the
NO2 species separately from NO, it was possible to observe
in every case a significant NO2 conversion at low tempera-
tures that could be due to nitrite/nitrate accumulation over
the surfaces. The subsequent decomposition of these pri-
mary formed species at higher temperatures could give rise
to N2 formation in some amount. The plots of the NOx con-
version versus temperature did not lead to volcano-shaped
curves, as are usually observed for most de-NOx catalysts.
This behavior is due to the presence of ethene even at the
highest reaction temperature tested. The incomplete ethene
conversion at 500 ◦C and above can be associated with the
weak oxidation properties of the In2O3 phase. This behavior
can be quantified by computing the competitiveness factor,
which represents the SCR selectivity (SSCR, %). During the
SCR reaction, the two oxidizing species, O2 and NOx, com-
pete for the small amount of ethene present; at the highest
reaction temperatures, O2 prevails over NOx, and SSCR de-
creases abruptly. The good selectivity of the In2O3 phases
is revealed by the SSCR values, which attained 8–10% at the

J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
429
trend followed by these calculated rates as a function of In
loading. It is more interesting to observe the values of the
rates expressed per mol of In, which show a regularly de-
creasing exponential trend. This corroborates our suggestion
concerning the role of In surface aggregation in de-NOx
activity. The addition of a very small amount of In2O3 to
alumina is sufficient to impart good activity to the sample,
whereas further In2O3 addition leads to samples with lower
activity. This means that only a limited amount of the indium
loaded over the alumina support can play the role of active
site in NOx reduction. This smaller amount could be asso-
ciated with the fraction present at the surface of the In2O3
aggregates, which became larger as the In loading increased
(see the XRD diffraction measurement results).
Fig. 13. Conversions of NO to N compared over the different In-loaded
samples at a reaction temperature of 500 C.
x
2
◦
4. Conclusions
The deposition of indium oxide slightly decreases the
acidic properties of alumina, but the amounts of ammonia
adsorbed per m2 of catalyst surface area do not vary signif-
icantly. Indium oxide deposition has a greater influence on
the adsorption of acidic molecules; the adsorbed amount of
SO2 is related to the indium oxide surface content, not to
the total deposited amount. The irreversible amount of SO2
adsorbed is superior to the irreversible amount of NH3 ad-
sorbed, suggesting the presence of stronger basic sites on the
indium-loaded samples. These findings suggest that indium
oxide can be considered more basic than acidic.
Fig. 14. Reaction rates in the C H -SCR of NO as a function of the
x
2
4
−1
−1
In-loading expressed as mmol mol
s
(rate/mol ), circle markers,
N2
In
In
−1
−1
◦
and in µmol
g
s
(rate/g ), square markers, calculated at 500 C.
cat
N2
cat
Indium oxide is easily reduced, at a reduction tempera-
ture related to particle size. For smaller particles, the reduc-
tion begins around 200 ◦C. The reoxidation process starts at
lower temperatures than the reduction process.
highest reaction temperatures. In terms of carbon selectiv-
ity, ethene was converted to CO2 for the most part, whereas
only a very limited amount of CO was detected for tem-
peratures around 450–500 ◦C (maximum yield, 3–4%). The
carbon balance was very satisfactory in any case.
It is possible to compare the NOx conversions over the
various In2O3-based catalysts by considering their activities
at a same reaction temperature. Fig. 13 shows the percentage
of NOx to N2 conversion obtained from each catalyst eval-
uated at 500 ◦C; the same trend was observed if different
temperatures were taken into account. The NOx conversion
increased with In2O3 loading up to 13%; from this point on,
further In addition caused a decrease in conversion. The con-
version of the InAl-27 sample was lower than that of the
InAl-17 and InAl-13 samples.
Balanced acidic and redox properties are indispensable
for an efficient conversion of NOx, that is, high activity
and selectivity (over a broad temperature window). This
evidence is in agreement with the conclusions drawn by
Park et al. [37] claiming a bifunctional mechanism of the
In2O3/Al2O3 systems to explain their high activity for NOx
reduction by propene. In this mechanism, the well-dispersed
In sites activate the hydrocarbons into partially oxygenated
compounds, and the acidic alumina sites readily use the oxy-
genated hydrocarbons to reduce NOx.
In2O3 catalysts demonstrate good NOx reduction per-
formance at high temperatures compared with other active
phases, such as CuO or SnO2 [44,51–53], and in this sense
they resemble Ga2O3-based catalysts [9,54]. With respect
to alumina-based catalysts, the hydrocarbon oxidation and
NO reduction activity have a strong mutual correlation, and,
moreover, the interaction between In2O3 and alumina is nec-
essary. The crystalline structure of the In2O3 phase observed
on the high-In-loading catalysts is not associated with high
de-NOx activity, whereas small In2O3 aggregates well dis-
persed over the alumina support are active and selective in
NOx reduction. A balanced presence of indium and alumina
sites indeed seems to be very important for achieving opti-
The observed behavior suggests a key influence of the In
surface aggregation on activity. Indeed, the sample with the
lowest In concentration (InAl-3) is very active, and further
addition of In resulted in a limited increase of the number
of active In sites on the alumina support, thus generating no
important variation in the de-NOx activity.
On the basis of the experimental data obtained, specific
integral reaction rates (µmolN g−ca1t s−1) were calculated at
2
each temperature. Moreover, from these values it is possible
to express the rates in mmolN molI−n1
s
−1, once the amount
2
of In per unit mass of catalyst is known. Fig. 14 shows the

430
J.A. Perdigon-Melon et al. / Journal of Catalysis 234 (2005) 421–430
mal catalytic performance over this type of catalyst, confirm-
ing the first results published in the literature concerning the
characterization and catalytic performance of In2O3/Al2O3
systems in NOx reduction [37].
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