Effects of Ozone on Catalytic and Physicochemical Properties of Cu-Ce-Al-O Catalysts for Soot Combustion

Article abstract of DOI:10.1023/A:1026102408243

The catalytic combustion of carbon particles was studied in the presence of Cu-Ce-Al oxides. The influence of ozonation on the catalytic and physicochemical properties of Cu-Ce-Al oxides at the reaction of diesel soot combustion was examined by thermal analysis and EPR methods. Al₂O₃ did not influence CB combustion. Ceria and copper oxide were the most reactive samples but ceria presented better reactivity than copper oxide. For the Cu-Ce-Al oxides, reactivity increased with increasing ceria content. Copper oxide favored the reactivity when supported on alumina but had little effect on ceria reactivity. Due to the ozone pretreatment, the catalytic combustion of carbon black passed to lower temperatures. This phenomenon was observed for all samples, but the most important decrease of temperature was evidenced for copper-based catalysts with the lowest ceria content. Ozonation did not change the catalytic properties of solids. Physicochemical characterization showed that ozonation influences the nature of carbon particles increasing the concentration and stability of free radicals. However, ozone pretreatment of carbon black in the mixture with a Cu-Al₂O₃ catalyst gave a better result, evidencing a synergistic effect between carbon black and catalyst.

Full text of DOI:10.1023/A:1026102408243

Kinetics and Catalysis, Vol. 44, No. 5, 2003, pp. 677–681. From Kinetika i Kataliz, Vol. 44, No. 5, 2003, pp. 739–743.
Original English Text Copyright © 2003 by Decarne, Bokova, Abi-Aad, Pryakhin, Lunin, Aboukaís.
CATALYTIC REACTION
MECHANISMS
Effects of Ozone on Catalytic and Physicochemical Properties
of Cu–Ce–Al–O Catalysts for Soot Combustion1
C. Decarne*, M. N. Bokova**, E. Abi-Aad*, A. N. Pryakhin**, V. V. Lunin**, and A. Aboukaís*
*
Laboratoire de Catalyse et Environnement E.A., 2598, MREID, avenue Maurice Schumann 145, 59140 Dunkerque, France
*
*Catalysis and Gas Electrochemistry Laboratory, Chemistry Department, Moscow State University,
Vorob’evy gory, Moscow, 119899 Russia
e-mail: abiaad@univ-littoral.fr, bokova@kge.msu.ru
Received December 23, 2002
Abstract—The elimination of carbon particles at the level of the exhaust line is one of the present challenges
of the car manufacturer. In this study, the catalytic combustion of carbon particles has been investigated in the
presence of Cu–Ce–Al oxides. The influence of ozonation on the catalytic and physicochemical properties of Cu–
Ce–Al oxides at the reaction of diesel soot combustion was examined by thermal analysis and EPR methods.
1
INTRODUCTION
(Al(OC H ) , Fluka, ~11.0 wt % Al) is dissolved in
4 9 3
butan-2-ol (Fluka, purity ≥99.5%). Then, a complexing
agent (Butan-1,3-diol, Fluka, purity ≥98%) is added
before hydrolysis [11]. Alumina is calcined at 500°C
before being used as a support for preparing supported
catalysts. With this method, the alumina is character-
The elimination of carbon particles emitted into air
from diesel engines is one of the main environmental
problems. The elimination or, at least, the reduction of
carbon particle emissions may be obtained by two dis-
tinct and complementary ways: the reduction of its for-
mation during the combustion process and its removal
downstream of the combustor. The latter way can be
reached by impregnation of porous ceramic filters, such
as ceramic honeycomb monoliths, placed through the
exhaust stream with an oxidation catalyst. Ceria (CeO2)
is widely used as a promoter in current based automo-
bile catalysts. Three main properties make ceria an
essential component in such redox catalysts: its oxygen
storage capacity (OSC) [1], its redox properties
2
ized by a specific area of 420 m /g and a pore volume
of 2.3 ml/g.
Ceria is prepared by precipitation of Ce(NO ) ·
2
9.5%) in an ammonia aqueous solution (0.7 mol/L).
3
3
6
9
H O (Prolabo, total amount of rare earth oxides is
The solid is filtered, washed, dried at 100°C, and cal-
cined at 600°C for 4 h with a temperature rate of
0
.5°C/min.
Different Ce–Al oxides are prepared by incipient
4
+
3+
wetness of cerium nitrate solution onto alumina precal-
cined at 500°C. Then, the sample is dried at 100°C and
calcined for 4 h in a flow of dry air at 500°C. Cerium
containing solids are denoted 1Ce10Al, 3Ce10Al, and
(
Ce /Ce ), and its thermal stabilizing influence on
alumina [2–4]. Moreover, literature data show that ceria
is very active for catalytic soot combustion [5–7]. Cop-
per oxide is considered as a possible substitute for
noble metal in automobile exhaust converters. Copper
oxide has already studied for catalytic combustion of
soot [5, 6, 8–10]. The catalytic system may be active
1
0Ce10Al, where the number before the chemical sym-
bol represents the atomic content in the solid.
Copper containing solids are prepared by incipient
even in low oxygen concentration conditions. Indeed, wetness of a support calcined at 500°C (alumina, ceria,
an exhaust gases medium is sometimes poor in oxygen. or Ce–Al oxides) with aqueous solution of copper
But ozone, a high oxidation reagent, is also present in nitrate (Cu(NO ) · 3H O, Prolabo, purity ≥99.0%). The
3
2
2
an exhaust gases medium and thus can enhance cata- mixture is dried at 100°C and calcined at 600°C for 4 h
lytic properties of solids. The purpose of this work is to in a flow of dry air. Copper containing solids are
investigate the influence of ozonation at the catalytic denoted as supports: 1Cu10Al, 1Cu1Ce10Al,
and physicochemical properties of Cu–Ce–Al oxides 1Cu3Ce10Al, 1Cu10Ce10Al, 5Cu3Ce10Al, and
on the reaction of diesel soot combustion.
1Cu10Ce.
The table summarizes the composition of supports
and copper containing catalysts performed in the cen-
tral elementary analysis office of the CNRS (Vernai-
son).
Electronic paramagnetic resonance. The electron
paramagnetic resonance measurements are performed
at –196°C on a EMX Bruker spectrometer with a cavity
EXPERIMENTAL
Preparation of catalysts. Alumina is synthesized
by the sol-gel method. Secondary aluminum butylate
1
This article was submitted by the authors in English.
0
023-1584/03/4405-0677$25.00 © 2003 MAIK “Nauka /Interperiodica”

6
78
DECARNE et al.
Composition of solids
out a catalyst in the same conditions. For high reactive
catalysts and 40 mg of analyzed mixture, TG–DTA
curves reveal a fast oxidation of carbon black followed
wt %
wt %
Catalyst
Atomic composition
Al O
of CuO of CeO2
by a slower one. Thus, two temperatures í
are
max
observed. This phenomenon can be explained by mass
and heat transfer limitation [6, 12, 13]. The analysis of
combustion products is performed evaluating the
Al O
–
–
100
–
2
3
2
3
CeO2
–
CeO2
1
1
1
5
1
1
Cu10Al
16.4
10.8
6.6
25.6
3.6
4.4
Cu1.26Al O
10
16.26
CO/(CO + CO ) molar ratio from a Varian 3600 chro-
2
Cu1Ce10Al
Cu3Ce10Al
Cu3Ce10Al
Cu10Ce10Al
Cu10Ce
23.3 Cu1.05Ce1.05Al O
matograph.
10
18.15
47.6 Cu0.92Ce Al O
3.08
10 22.08
Ozonation. Ozonation of catalysts/carbon black
samples is performed in the continuous gas flow reac-
tor; the laboratory ozonizer produces approximately
38.1 Cu4.53Ce3.11Al O
10
25.75
34.54
73.1 Cu0.98Ce9.28Al O
10
2
vol % ozone in oxygen. Ozone concentration is deter-
95.6 Cu0.99Ce O
10
20.99
mined in the gas phase inlet and outlet by UV-spectros-
copy. The samples are allowed to react for 60 min in an
oxygen/ozone flow of 75 ml/min at room temperature.
operating at a frequency of ~9.5 GHz (X band). The
magnetic field was modulated at 100 kHz. The g values
are determined from precise frequency and magnetic
field values. EPR intensity is given by the double inte-
gration of the EPR signal.
RESULTS AND DISCUSSION
1
. Carbon black combustion before ozonation.
Figure 2 shows the reactivity of different catalysts
before and after ozonation. The reactivity tests are per-
formed on 20 mg of mixture. Combustion of carbon
Activity tests. Commercially available carbon
black (CB) (N330 Degussa: 97.23 wt % C, 0.73 wt %
H, 1.16 wt % O, 0.19 wt % N, 0.45 wt % S) is used as
a model soot. The catalytic tests towards the combus-
tion of CB are studied by simultaneous gravimetric and
differential thermal analysis (TG–DTA) with a Netzsch
STA 409 apparatus. Before the catalytic test, CB (20 wt %)
and catalyst (80 wt %) are mixed in an Al O ball miller
for 40 min; 40 or 20 mg of this mixture is loaded in an
alumina crucible and heated from room temperature to
black alone takes place at high temperature (ímax
=
CB
609
612
Before ozonation
After ozonation
2
3
592
85
Al O + CB
2
3
5
7
00°C (5°C/min) in air flow (75 ml/min). Figure 1
shows an example of TG–DTA curves. A weight loss of
0 wt % is observed, corresponding to the combustion
CeO + CB
380
62
416
06
2
2
3
of the total mass of carbon black. The TG curve allows
one to obtain the temperature values T (beginning of CuO + CB
i
the carbon black combustion) and T (complete conver-
sion of carbon black). ímax is the temperature of the
f
4
maximum of the DTA curve and corresponds to the
temperature of the highest combustion velocity. Cata-
lytic properties of solids are evaluated by comparing
these temperatures with those of CB combustion with-
1
1
1
1
Cu1Ce10Al + CB
556
4
45
Cu3Ce10Al + CB
538
4
31
Cu10Ce10Al + CB 412
96
Weight loss, %
0
–
–
–
–
2
4
6
8
3
T
max
T
i
Cu10Ce + CB
383
Exo
–
–
–
–
–
–
–
10
12
14
16
18
20
22
364
5Cu3Ce10Al + CB
514
4
12
T
f
200
300
400
500
600
700
T, °C
1
00
200
300
400
500
600
T, °C
Fig. 2. Reactivity of solids for (20 wt % carbon + 80 wt %
catalysts) before and after ozonation.
Fig. 1. Example of TG–DTA curve.
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

EFFECTS OF OZONE ON CATALYTIC AND PHYSICOCHEMICAL PROPERTIES
679
6
09°C). An insignificant difference is observed when
513
the reaction is performed in the presence of Al O . For
2
3
the other samples, combustion temperature decreases,
showing the reactivity of copper- or ceria-based cata-
lysts. Ceria and copper oxide are the most reactive sam-
ples with a combustion temperature of, respectively,
529
1
Cu10Al + 20%CB
3
80 and 416°C. For the Cu–Ce–Al oxides, reactivity
increases with increasing ceria content. Thus, the tem-
perature í decreases from 556°C for the
Cu1Ce10Al sample to 538°C for the 1Cu3Ce10Al
3
88
max
1
sample and to 412°C for the 1Cu10Ce10Al sample. For
the 1Cu10Ce sample, the reactivity (ímax = 383°C) and
4
97
17
(1Cu10Al + 20%CB) + O3
427
CeO (í
= 380°C). Moreover, increasing copper
2
max
content leads to higher reactivity. For the same support
Ce10Al, the temperature í decreases from 538°C
5
3
max
for 1Cu3Ce10Al to 514°C for 5Cu3Ce10Al. Thus, it
seems that copper oxide favors the reactivity when sup-
ported on alumina but has little effect on ceria reactiv-
ity, since this latter presents better reactivity than cop-
per oxide.
5
31
(
(
1Cu10Al + O ) + 20%CB
3
2
. Carbon black combustion after ozonation.
4
84
Compared to untreated carbon black combustion, the
ozonized carbon black combustion occurs at lower tem-
perature (16°C) and with higher reaction velocity (∆T =
T – T ). Indeed, ∆T decreases from 92°C for untreated
429
1Cu10Al + 20%(CB + O3)
4
88
f
i
CB combustion to 60°C for ozonized CB combustion.
For the Al O , CeO , and CuO catalysts, small differ-
(1Cu10Al + O ) + 20%(CB + O )
3 3
2
3
2
430
ences for the combustion temperature (about 20–30°C)
are observed. When the reaction is performed in the
presence of Cu-based catalysts, the temperature of CB
combustion is significantly lowered. Concerning the
Cu–Ce–Al oxides, the increase of ceria content in the
sample decreases the ozone influence on the catalysts’
50
150
250
350
450
550
650
T, °C
Fig. 3. Reactivity of 1Cu10Al for (20 wt % carbon + 80 wt %
catalysts) with or without ozonation step.
reactivity. Thus, for the 1Cu1Ce10Al catalyst, the í
max
temperature of the mixture after ozonation is 111°C
lyst and carbon black. Without any ozone treatment, the
carbon black combustion in the presence of 1Cu10Al
shows a fast and slow oxidation process with a ímax
lower than an untreated mixture, but for the
1
Cu10Ce10Al system the temperature difference is
only 16°C. On the other hand, the addition of copper
does not affect the ozone influence on the catalyst reac-
tivity: for the mixture of carbon black with
temperature of 513 and 529°C, respectively. When ozo-
nation is carried out on a CB/catalyst mixture, the first
ímax is at 388°C, which is 135°C lower than the ímax of
1
Cu3Ce10Al and 5Cu3Ce10Al the difference in í
max
before and after ozonation is almost the same (~100°C). an untreated mixture. The DTA-curve of ozonized
Thus, ozonation decreases the temperature í of car- 1Cu10Al catalyst with untreated carbon black has the
max
bon black combustion. This effect is stable for all sam- same characteristic temperatures as the curve obtained
ples and the difference in í for ozonized and initial for the untreated mixture. It means that the ozonation of
max
CB is not less than 20°C. But the most important 1Cu10Al does not influence its activity. Nevertheless,
decrease of temperature is observed for the copper- for the mixture of untreated catalyst/ozonized carbon
based catalyst with the lowest ceria content black, the DTA-curve is significantly different: the
(
1Cu1Ce10Al). Thus, it seems important to study the decreases of T and í are, respectively, 70 and 33°C.
i max
influence of ozone on a copper-based catalyst without Finally, the combustion of the mixture of separately
ceria.
Figure 3 presents the reactivity of the 1Cu10Al cat-
ozonized catalyst and ozonized carbon black gives sim-
ilar results to those of the mixture of untreated cata-
alyst depending on ozone pretreatment. Five mixtures lyst/ozonized carbon black. These data allow one to
were studied: a mixture with CB/catalyst without any conclude that, when ozonized separately, O influences
3
ozonation, an ozonized mixture, a mixture with ozo- the nature of carbon particles but does not affect the cat-
nized catalyst and untreated CB, a mixture with alytic properties of the solids. Such an effect may be
untreated catalyst and ozonized CB, and a mixture with due to the formation of several oxygen complexes on
both separately ozonized catalyst and CB. These tests the surface of carbon black [14, 15], which favors the
are expected to point out the role of ozonation on cata- catalytic oxidation of CB. Thus, due to the ozone pre-
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

6
80
DECARNE et al.
(
a)
(b)
Recording
temperature
Intensity
Recording
temperature
Intensity
Signal S2
–
–
–
170°C
150°C
100°C
–170°C
–150°C
–100°C
–50°C
1
4.0
22.8
11.0
24.1
23.0
21.7
19.8
9.6
9.3
8.2
–
50°C
1
0°C
10°C
3200
Signal S1
Signal S1
3400
3
000
3200
3400
3600
3800
40003000
3600
3800
4000
δ, G
Fig. 4. EPR spectra of the CB recorded at different temperatures: (a) initial CB, (b) ozonized CB.
treatment of CB, the catalytic combustion of CB passes
The S1 signal presents a slight axial symmetry with
iso
at lower temperatures. However, ozone pretreatment of a g value of ~2.000 and line width of 46 ± 1 G. Owing
carbon black/catalyst mixture gives a better result, to the landscape and line width this signal can be attrib-
showing a synergistic effect between CB and the cata- uted to free electrons in interaction with oxygen species
lyst.
Concerning the CO selectivity of the reaction, non-
catalytic combustion of CB presents a CO selectivity of
despite the fact that the g-factor value is lower than
those for free electrons (g = 2.0023). Moreover, the S1
e
intensity for CB before ozonation decreases with
increasing recording temperature, while for CB after
ozonation, this intensity is considerably higher and sta-
ble versus recording temperature. This phenomenon
shows that these free electrons in interaction with oxy-
gen species are more numerous and more stable for CB
after ozonation.
1
4% (CO/(CO + CO )). The same value is obtained for
2
a pure alumina-catalyzed reaction. However, pure ceria
is totally selective in CO . Whatever the support is, the
2
introduction of copper allows for total CO selectivity.
2
Finally, ozonation does not influence the selectivity of
the CB combustion process.
3
. Characterization of carbon black before and
The S2 signal is isotropic and sharp with g ~ 2.003
iso
after ozonation. Untreated and ozonized carbon black and a line width of ~3.5 G. The EPR parameters of the
are studied by EPR. Samples are subjected to vacuum S2 signal are consistent with free hydrocarbon radicals
5.2 × 10 mbar, 3 h) treatment and EPR spectra are strongly adsorbed on the carbon particles [16]. The
–
5
(
recorded at different temperatures from –170 to 10°C. interaction between free radicals and carbon gets
Figure 4 shows the EPR spectra of initial and ozonized weaker with an increase in temperature and the degree
CB versus the recording temperature. From –170°C of liberty of these radicals increases. The intensity of
until 10°C for untreated CB only one signal S1 is evi- the S2 signal decreases with an increase in recording
denced but for ozonized CB two EPR signals S1 and S2 temperature, and at 10°C this signal becomes slightly
are detected.
detectable. Thus, it can be assumed that at higher tem-
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

EFFECTS OF OZONE ON CATALYTIC AND PHYSICOCHEMICAL PROPERTIES
681
peratures the mobility of these species is improved.
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of its combustion.
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KINETICS AND CATALYSIS Vol. 44 No. 5 2003