Carbon black oxidation mechanism in loose and tight contacts with Al 2O3 and CeO2 catalysts

Article abstract of DOI:10.1134/S0023158407060134

The catalytic combustion of carbon black was investigated in the presence of CeO₂ and Al₂O₃. The influence of contact type between carbon particles and these oxides was examined by thermal analysis, the BET specific area, and EPR spectroscopy. For tight contact carbon black-catalyst mixtures, a new paramagnetic species is observed and can be considered as a fingerprint of the contact between the two solids. These new paramagnetic species increase the reactivity of the catalytic reaction of carbon black (CB) combustion and take part in the oxidation mechanism of CB.

Full text of DOI:10.1134/S0023158407060134

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 6, pp. 841–846. © MAIK “Nauka /Interperiodica” (Russia), 2007.
Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 6, pp. 899–904.
MECHANISMS
OF CATALYTIC REACTIONS
Carbon Black Oxidation Mechanism in Loose
1
and Tight Contacts with Al O and CeO Catalysts
2
3
2
E. Saab, S. Aouad, E. Abi-Aad*, M. N. Bokova, E. A. Zhilinskaya, and A. Aboukaïs
Laboratoire de Catalyse et Environnement E.A. 2598, Université du Littoral—Côte d’Opale,
MREI-1, 145 av. Maurice Schumann, Dunkerque, 59140 France
*
e-mail: abiaad@univ-littoral.fr
Received June 30, 2006
Abstract—The catalytic combustion of carbon black was investigated in the presence of CeO and Al O . The
2
2
3
influence of contact type between carbon particles and these oxides was examined by thermal analysis, the BET
specific area, and EPR spectroscopy. For tight contact carbon black–catalyst mixtures, a new paramagnetic spe-
cies is observed and can be considered as a fingerprint of the contact between the two solids. These new para-
magnetic species increase the reactivity of the catalytic reaction of carbon black (CB) combustion and take part
in the oxidation mechanism of CB.
DOI: 10.1134/S0023158407060134
1
1
. INTRODUCTION
results [3, 5]. In the same order, synthetic soot (carbon
black, Printex U, fullerenes, etc.) is used as a model
The particulate matter (soot) amount emitted by die- compound for diesel soot during simulated laboratory
sel engines is much larger than that emitted by these oxidation experiments [6, 7] since the chemical compo-
engines equipped with catalytic converters. This pollut- sition of real soot is variable and depends on several
ant is hazardous to human health due to its potential parameters (fuel composition, kind and state of engine,
mutagenic and carcinogenic activity and to their small etc.).
size, which can penetrate into the lungs [1]. In order to
Ceria (CeO ) is widely used as a promoter in cur-
2
reduce soot emissions, it is possible to use alternative
rent-based automobile catalysts. In fact, one current
combustibles or special devices to reduce particulates
application is the presence of cerium additives in diesel
of exhaust gases [2]. As the particulate matter combus-
fuel which favors the reactivity of the doped soot and
tion temperature is much higher than the operational
shifts the combustion reaction towards low tempera-
temperature found in the exhaust pipe, the combustion
tures [1]. Three main properties make ceria an essential
temperature must be decreased, and, consequently,
component in such redox catalysts: its oxygen storage
active catalysts must be developed in such temperature
levels [1, 2]. Catalysts can be used in a number of ways
to accelerate the combustion of soot. It can be added
either through the fuel, in which case it is incorporated
into the soot spherules as they form, or physically
mixed in after collection of soot. 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. The contact
between soot and catalyst is a very important parameter
in the catalytic oxidation of soot. Only limited informa-
tion is available on the intimacy of the solid–solid con-
tact between the catalyst (solid) and the soot (solid)
4+
3+
capacity (OSC) [1], its redox properties (Ce /Ce ),
and its thermal stability influence on alumina [3, 8].
Moreover, the literature data show that ceria is very
active for catalytic soot combustion [4, 5]. According to
the literature [6], ceria shows high activity due to the
total oxidation of CB into CeO . It is related to the
2
capacity of ceria to be easily reduced during the com-
bustion reaction into nonstoichiometric oxides
(
CeO2 − x), which contribute to the formation of anionic
vacancies on the surface. These vacancies are the active
sites in the oxidation catalysis [7, 9, 10].
Al O is often used as a support in the oxidation cat-
2 3
under real conditions [3–5]. However, it was shown that alysts due to its thermal stability and significant specific
under practical conditions the contact between the soot area surface [10]. In addition, alumina and silica are the
and the catalyst is rather poor [4]. Nevertheless, the use major components of the ceramic honeycomb monolith
of a mechanical mill is often chosen to establish close filters. Moreover, in a previous work [11] we studied
contact between the soot and the catalyst in order to the impact of Al O and CeO supports on manganese-
2 3 2
define an intrinsic catalytic activity under optimal con- based catalysts. It was clearly shown that for Mn/CeO
2
2
+
ditions and to obtain reproducible and comparable with CB mixtures, the Mn content considerably
increases consequently to the contact between the two
1
This article was submitted by the authors in English.
solids, indicating the reduction of some manganese
8
41

8
42
SAAB et al.
2
+
species with higher oxidation states into Mn ions. The
reduction of the manganese species, in these condi-
tions, is considered as an important factor in the carbon
black oxidation mechanism. In parallel, the oxidation
states of manganese, in Mn/Al O catalysts, are not
2.4. Specific Area
The BET surface areas of CB and mixtures were
measured by a Quantasorb Junior (Ankerscmidt),
through physisorption at liquid nitrogen temperature
2
3
(
77 K). This method consists of the measurement of the
affected by such a treatment. These results have permit-
quantity of inert gas (N ) physisorbed on the solid sur-
2
ted us to explain the better reactivity of Mn/CeO sam-
2
face. The sample deposed in a U form cell was degased
first at 130°ë for thirty minutes under pure nitrogen
N60. After cooling down to room temperature, a mix-
ples compared to Mn/Al O catalysts.
2
3
Owing to the important role played by the catalytic
support on the carbon black oxidation process, we
believed that it was necessary to study the reactivity of
CB combustion in the presence of CeO and Al O in
ture of 30% of adsorbant gas (pure N ) and 70% of por-
2
ter inert gas (pure He) are physisorbed. The adsorption
and desorption are produced respectively where the U
cell containing the sample was immerged and then
retired from a dewar of liquid nitrogen. With this
method, alumina is characterized with a specific area of
2
2
3
both loose and tight contact conditions. To elucidate the
contribution of cerium and aluminium oxides in the
carbon black oxidation process, thermal analysis, BET,
and EPR techniques are used in this work.
2
–1
2
–1
3
60 m g . The specific area of ceria is 91 m g , and
2
–1
that of CB is 76 m g .
2
. EXPERIMENTAL
2
.5. Electron Paramagnetic Resonance
The electron paramagnetic resonance measure-
ments are performed at room temperature on an EMX
2
.1. Preparation of Solids
Alumina is synthesized by the sol-gel method. Sec- BRUKER spectrometer with a cavity operating at a fre-
ondary aluminum butylate (Al(OC H ) , Fluka, quency of ~9.5 GHz (X band). The magnetic field was
11.0 wt % Al) is dissolved in butan–2-ol (Fluka, modulated at 100 kHz, and the power supply was suffi-
4
9 3
~
purity ≥99.5%). Then, a complexing agent (Butan–1,3- ciently small to avoid the saturation effect. Modulation
diol, Fluka, purity ≥98%) is added before hydrolysis. amplitudes (Mod Am) from 0.5 to 10 G were used. The
Alumina is calcined at 500°ë before use as a support g-values were determined using the following relation:
for preparing supported catalysts.
hν = gβH where h is the Planck constant, β is the Bohr
magneton, H the magnetic field, and ν is the microwave
frequency measured with high precision using a fre-
quency-meter. All the thermal treatments (between 100
and 600°C) of the samples are carried out in a microf-
low reactor, which is assembled with a quartz EPR tube
to allow the introduction of the sample into the reso-
nance cavity.
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°ë, and cal-
cined at 600°ë for 4 h with a temperature rate of
0.5 K/min.
3
. RESULTS AND DISCUSSION
.1. Carbon Black Oxidation
The TG curves of carbon black combustion in the
presence of Al O and CeO are presented in Fig. 1. The
2
.2. Preparation of Mixtures
3
Commercially available carbon black (CB) (N330
Degussa: 97.23 wt % C; 0.73 wt % H; 1.16 wt % O;
.19 wt % N; 0.45 wt % S) is used as a model soot. Two
types of mixtures of CB with the catalyst are prepared:
i) with a spatula—loose contact mixture and (ii) mixed
in an Al O ball miller for 40 min—tight contact mix-
0
2
3
2
observed weight loss corresponds to the combustion of
the total mass of carbon black. The TG curve gives the
(
temperature values T (beginning of the carbon black
i
2
3
combustion) and T (complete conversion of carbon
ture. Different weight percentages of CB and the cata-
lyst are used: 1% CB + 99% Cat, 3% CB + 97% Cat,
% CB + 95% Cat and 6% CB + 94% Cat.
f
black). The T shows the maximum temperature on the
m
DTA curve and corresponds to the temperature of the
highest combustion velocity. These specific tempera-
tures (T , T , and T ) are reported in Table 1 for CB
5
i
m
f
without catalysts and in the presence of Al O and
CeO in loose and tight contacts.
2
3
2
.3. Combustion Tests
2
The tests of the combustion of CB are studied by
The carbon black combustion, in the absence of a
simultaneous gravimetric and differential thermal anal- catalyst, occurs at T = 632°ë. An insignificant differ-
m
ysis (TG–DTA) with a NETZSCH STA 409 apparatus. ence is observed when the reaction is performed in the
Twenty milligrams of the sample is loaded in an alu- presence of Al O . The drastic weight loss at low tem-
2
3
mina crucible and heated from room temperature to peratures for Al O + CB mixtures is usually attributed
2
3
7
50°C (5 K/min) in air flow (75 ml/min).
to water desorption. The entire quantity of CB (6%) is
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

CARBON BLACK OXIDATION MECHANISM IN LOOSE AND TIGHT CONTACTS
843
oxidized from T = 577°ë up to T = 628°ë with T = Weight loss, %
i
f
m
6
6
25°ë (loose contact) and from T = 567°ë up to T =
i f
29°ë with T = 607°ë (tight contact) (see Fig. 1 and
m
Table 1). From these data, one can conclude that the
reactivity is higher in tight contact conditions than in
1
2
T
i
loose conditions. In fact, the T values are shifted to
0.6%
m
5
%
lower temperatures from 7 to 25°C when the reaction is
performed in the presence of alumina in loose and tight
contact, respectively (Table 1). However, in both cases,
the reactivity (T , T , and T ) seems not to be notably
5
%
T
f
1
.7%
3
5
%
360°C
i
m
f
affected by the presence of alumina. These data are in
accordance with literature showing that alumina is a
nonactive catalyst in such a reaction [4, 10, 12–14]. The
characteristic temperatures of CB combustion in the
presence of cerium oxide depend on the nature of con-
tact between the solids. For the loose contact mixture,
Tm is practically the same as for noncatalytic CB com-
bustion. On the contrary, for the tight contact mixture,
the characteristic temperatures are shifted to the low-
temperature region. The temperature T for the CeO +
4
5
6
%
6
%
m
2
CB mixture in tight contact (CeT) is 272°C lower than
2
0
120
220
320
420
520
620
720
that for the initial CB. Thus, CeO is an active catalyst
2
Temperature, °C
only in tight contact with carbon black. This result is in
accordance with previous studies and data presented in
the literature [4, 11, 15, 16].
Fig. 1. TG curves of carbon black combustion in the pres-
ence of Al O and CeO . (2, 4) Loose contact, (3, 5) tight
contact.
2
3
2
3
.2. Specific Surface Area (Ssp)
Table 2 summarizes the BET specific area of the CB to the catalytic oxidation of the adsorbed hydrocarbons
and CeT after different thermal treatments. At 25°ë, and then to CB combustion leading to the increase in
2
–1
CB S was about 76 m g . This S is approximately the number of pores on the CB surface. In parallel, the
sp
sp
2
–1
the same between 25 and 300°C (~80 m g ). At TG curve on Fig. 1 clearly shows that the CB combus-
00°C, the S of CB (133 m g ) increases, which can tion takes place in this range of temperature (329–
2
–1
4
sp
be related to the formation of pores with the increase of 392°C; Table 1). At 500°C, the catalytic oxidation of
2
–1
temperature. In fact, adsorbed hydrocarbons on the CB CB is finished and the measured S (54 m g ) is that
sp
surface can be oxidized at this temperature in the pres-
of the CeO catalyst. The decrease in S revealed after
2
sp
ence of oxygen. At 450°C, this phenomenon is more
the catalytic test may be related to
2
–1
pronounced (S = 311 m g ) and reached the highest
sp
2
–1
(i) the effect on CeO of the mechanical mill used to
values of S = 595 m g for a calcination temperature
2
sp
establish the tight contact,
of 500°C. From the chemical composition of the CB
used for these experiments (97.23 wt % C) and the TG
curve for the mixture 5% CB + 95% SiC (Fig. 1) show-
ing a negligible variation of the weight loss at 500°C,
one can conclude that the first step of the CB oxidation
process is the oxidation of the adsorbed hydrocarbons
leading to the increase of the specific area, which facil-
(
ii) the increase of the local temperature after the
combustion of the CB particulate,
Table 1. Characteristic temperatures of carbon black com-
bustion
itates the adsorption of O in order to oxidize the pure
2
CB particulate.
Sample
Ti, °C
Tm, °C
Tf, °C
The same study was carried out on the CeO + CB
2
5% CB + 95% SiC
580
571
632
618
640
629
mixture in tight contact (CeT). The specific area of the
mixture CeT is constant between room temperature and
5
% CB + 95% CeO2
(
loose contact)
2
–1
2
00°ë (83 m g ). For calcination temperatures of 300
5
% CB + 95% CeO2
329
577
567
360
625
607
392
628
629
and 400°ë, a slight increase of the Ssp is observed
(
tight contact)
(
Table 2). However, this increase should be compared
6
% CB + 94% Al O
2
to the 5% weight of CB in the mixture since the S of
3
sp
(
loose contact)
calcined ceria could be considered as a constant until
5
00°ë. This increase reported to CB would represent a 6% CB + 94% Al O
2
3
2
–1
(tight contact)
variation of 80 m g . This behavior could be attributed
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

8
44
SAAB et al.
Table 2. S of CB and CeT treated at different temperatures
sp
Temperature, °C
25
76
83
100
76
200
84
300
81
400
133
86
450
311
–
500
595
54
2
–1
CB S , m g
sp
2
–1
CeT S , m g
83
83
87
sp
(
iii) the catalytic effect of CeO on this mixture after CB radicals consistent with intrinsic paramagnetic spe-
2
thermal treatment.
cies of CB [11, 17]. It was shown that these paramag-
netic species (S1 signal) correspond to weakly interact-
ing Curie spins but they are movable and obviously
interact with the paramagnetic oxygen molecules
inducing EPR line-width broadening [11].
Since the S of the milled CeO treated at 600°C is
sp
–1
2
2
about 62 m g , the difference between the two values
is probably related to the two last effects. In fact, due to
its oxygen storage capacity, and under tight contact
conditions, CeO can accelerate CB combustion by
2
Figure 2a shows the evolution of EPR spectra of the
mixture CeL in the presence of air versus the heating
temperature. The CB signal (S1) is present until 500°ë,
and its intensity decreases as calcination temperatures
increase. S1 is not detected at 600°ë, showing the total
combustion of the CB. Another signal was detected
from 200 until 600°ë. This signal is characterized by
g = 1.965, g = 1.942, and g = 1.957 and was widely
using its own oxygen species. By comparison with the
2
–
4+
literature results, O species bound to Ce ions can
4+
react with CB to form CO and consequently Ce ions
2
3
+
are reduced to Ce species [17, 18].
3
.3. EPR Study
⊥
||
iso
3
+
investigated and attributed to Ce ions or to the inter-
Figures 2a and 2b show the evolution of the X-band
action between conduction electrons and the 4f orbital
EPR spectra of respectively mixtures 95% CeO +
2
4
+
of Ce ions in the CeO matrix [19].
5
% CB in loose contact (CeL) and 95% CeO + 5% CB
2
2
in tight contact (CeT) recorded at 25°C.
Figure 2b shows the EPR spectra of the mixture CeT
In previous works, EPR analysis of the initial carbon treated at different temperatures and recorded at room
black showed an S1 signal attributed to intrinsic para- temperature. For this mixture, the S1 signal attributed to
magnetic centers on CB. The S1 signal presents a slight intrinsic paramagnetic centers on CB is detectable from
axial distortion with a giso value of ≈2.000 and a line- room temperature until 300°C. At 400°C this signal is
width of ∆H = 63 ± 4 G. This signal has been attrib- not observable. The disappearance of this signal is in


uted to unpaired electrons in the CB structure or to free accordance with the weight loss on the TG curve
(
‡)
CeT (b)
S1
S2
S1
2
5°C
CB
2
5°C
CeL
2
2
5°C
1
2
00°C
00°C
00°C
3
4
00°C
00°C
5
6
00°C
00°C
300°C
Ce3+
Ce3+ 400°C
3
000
3200
3400
3600
3800
4000 3000
3200
3400
3600
3800
4000
Magnetic field, G
Magnetic field, G
Fig. 2. EPR spectra recorded at 25°C, in the presence of air, for 5% CB + 95% CeO samples: (a) in loose contact; (b) in tight contact.
2
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

CARBON BLACK OXIDATION MECHANISM IN LOOSE AND TIGHT CONTACTS
845
4
+
3+
(
Fig. 1). At 392°ë total CB combustion is observed reduction of Ce ions into Ce ions with the reaction
during the catalytic test.
advance. Thus, the increase of the EPR intensity rela-
3
+
Another signal denoted S2 is observed. The S2 sig- tive to the Ce signal is more pronounced between
nal is isotropic with giso = 2.003 and a line width of 500–600°ë for loose contact and 300–400°ë for tight
∆
H = 3.5 G. The appearance of the S2 signal indicates contact mixtures, respectively (Fig. 1 and Table 1), in

the formation of new paramagnetic species consistent accordance with the temperature ranges for catalytic
with localized paramagnetic spins on the carbon parti- oxidation of CB in the presence of CeO in loose and
2
cles and catalyst interface and can be considered as a tight contacts.
fingerprint of the contact between the two solids [11]. It
is important to note that neither compressed carbon
CONCLUSIONS
black nor the catalyst, treated alone in the mixing con-
ditions, provides the S2 signal. This result supports the
above attribution of the S2 signal. The S2 signal is
observed only for the samples heated up to 100°ë. For
the mixture calcined at 200°ë, the S2 signal, attributed
to CB–catalyst contact, is not detectable. In parallel, on
the TG curve and in the range of 100–200°ë, we
observe a weight loss equal to 1.7%, whereas, in the
loose contact condition (absence of the S2 signal), this
weight loss is equal to 0.6%. Thus, the difference in the
weight losses at low temperature, due to contact condi-
tions, is 1.1% (corresponding to 22% of the total CB
weight in the sample). This latter can be related first to
The catalytic oxidation of carbon black was investi-
gated in the presence of CeO and Al O . The rate of
2
2
3
carbon black combustion increases considerably in
tight contact conditions. The CB oxidation process
starts by the elimination of the adsorbed hydrocarbons
leading to the formation of pores and to the increase of
the specific area with the calcination temperature. This
phenomenon would facilitate the adsorption of O mol-
2
ecules in order to oxidize the CB particulate. For tight
contact CB–catalyst mixtures, a new EPR signal, desig-
nated by S2, is observed. The appearance of the S2 sig-
nal indicates the formation of new paramagnetic spe-
cies consistent with localized paramagnetic spins on
the carbon particles and catalyst interface and can be
considered as a fingerprint of the contact between the
two solids. These new species increase the reactivity of
the effect of CeO as an oxygen storage catalyst and
2
second to the tight contact between CeO and CB sug-
2
gesting the first step in the mechanism of CB oxidation
in the presence of cerium oxide. Similar data have been
observed in such conditions and attributed to the high
reactivity of ceria–CB interface species [17]. In addi-
tion, the catalytic reaction continues at higher tempera-
tures taking advantage of heat diffusion related to the
partial oxidation of CB in the range of 100 and 200°C.
Therefore, the catalytic proprieties of cerium oxide
play an important role leading to an important shift in
the CB combustion temperature particularly in tight
contact conditions [17, 18]. Two general mechanisms
have been proposed to account for the diversified cata-
lytic effects of metals and oxides in carbon oxidation:
electron-transfer and oxygen-transfer mechanisms [19,
the tight contact CB + CeO mixtures in the catalytic
2
reaction of CB combustion. In addition, reduction of
4+
3+
Ce ions into Ce is evidenced simultaneously in the
catalytic oxidation process showing the participation of
ceria by means of its catalytic properties in the combus-
tion reaction of CB. These results may be considered an
important step in the elaboration of a general mecha-
nism explaining the catalytic oxidation process of die-
sel soot.
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1
2
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