2007_6_Nr6_EEMJ

Syntheis, characterization and catalytic reduction of NOx emissions over LaMnO 3 perovskite
60
(c)
50
(b)
(a)
0 200 400 600 800 1000
Conversia NO x
[%]
40
30
20
10
600 °C
800 °C
1000 °C
Temperature (°C)
0
150 200 250 300 350 400 450 500 550 600 650
Fig. 2. TPR curves for three LaMnO 3 perovskite samples
calcined at 600, 800 and 1000 o C, respectively
100
Temperatura [°C]
The first region corresponds to manganese
oxides reduction, already discussed, reduction that
gradually takes place as the corresponding
temperature peaks show. The first peak corresponds
to Mn 4+ la Mn 3+ reduction when reduction performs
between 78 331-351 o C, followed by Mn 3+ la Mn 2+
reduction ate temperatures comprised between 421-
471 o C. The high consuming oh hydrogen happens in
the second region of reduction curves due to both,
Mn 3+ reduction to Mn 2+ in LaMnO 3 perovskite and
carbonate species such as La 2 O 2 CO 3 those reduction
corresponds to this temperature interval. Considering
that these perovskites were prepared by citric method
it is very plausible that carbonates traces are in their
structure (Hackenberger, 1998; Stephan et. al., 2002).
3.2. Catalitic activity testing SCR-C 3 H 6
The perovskite samples were tested in nitrogen
oxides removing by SCR-C 3 H 6 in presence and also
in absence of oxygen atmosphere. In oxygen
atmosphere (5%), the experimental results indicated
of 100% propene oxidation activity between 300 and
450 o C for the samples obtained at 600 and 1000 o C
while for sample obtained at 800 o C, the maxim
activity was around 78% after which the oxidation
activity had kept just below this value.
The maxim point of propene conversion
corresponded to the temperature interval in which
propene could decompose to carbon dioxide and
water. The perovskite synthesized sample at 1000 o C
achieved maximal activity at 300 o C then it sharply
deactivated. Regarding nitrogen oxides reduction as it
can be seen in the experimental data processing (Fig.
3) all three LaMnO 3 perovskite samples practically
showed negligible conversion values.
The tests performed on synthetic gas mixture
without oxygen have shown high activity for propene
oxidation process but these were moved on high
temperature range, over 400 o C.
Conversia C 3
H 6
[%]
90
80
70
60
50
40
30
20
10
0
600 °C
800 °C
1000 °C
150 200 250 300 350 400 450 500 550 600 650
Temperatura [°C]
Fig. 3. Nitrogen oxides (a) and propene (b) conversin for
the LaMnO 3 perovskite samples (propene 600 ppm, NOx
600 ppm, 5% O 2 )
For LaMnO 3 perovskite sample obtained at
600 o C the catalytic activity test was carried out on a
large temperatures range between 150 and 600 o C.
Therefore, it can be observed for this sample that
oxidation activity becomes maxim over 500 o C and it
remains constant until 600 o C, experimental limit
temperature. The other two perovskites samples,
synthesized at 800 and 1000 o C, the values of
nitrogen oxides conversion was increasing until
ending of experiment, 450 o C. At this temperature,
LaMnO3 sample calcined at 800 o C presents the
higher value (100%) while sample calcined at 1000 o C
has the lower catalytic activity (65.24%) (Fig. 4).
3.3. Catalytic activity testing SCR-C 3 H 8
Data processing obtained in selective catalytic
reduction of nitrogen oxides using propane as
reduction agent with 5% oxygen in synthetic gas
mixture has led to the curves grouped in Fig.5. In this
case, oxidation reaction reaches maximal values at
high temperatures interval (350-450 o C). As it is
shown in Fig. 5 diagram b, the LaMnO 3 perovskite
samples achieve maximal hydrocarbon conversion
point at 450 o C.
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