(121d) A Perovskite-like Catalyst for the Simultaneous Removal of Soot and NO: Effect of the Synthesis Method

Due to the environmental
problems associated with the use of diesel engines, and the strengthening of
the regulations concerning pollutant emissions in the transport field. The
study of the catalytic simultaneous removal of soot and NO at relatively low
temperatures (within the range of diesel engine exhaust: 150-450°C) has taken a
paramount importance in pollution control area.

Mixed oxides coming from
perovskite-like materials, seems to be a promising catalyst for such purpose.
In this way, the present work tested the La_0.7Ag_0.3MnO₃
perovskite-like material, prepared for different synthetic routes, for the
simultaneous removal of soot and NO. Three different synthetic routes are
compared: microwave-assisted hydrothermal (HMW), microwave-assisted (MW) and
solid state (SS).

The catalytic activity of
the prepared catalysts was tested in a temperature-controlled furnace inside
which a soot-catalyst-sand fixed bed was placed. The mixture of carbonaceous
material and catalysts was carried out with spatula (i.e. loose contact),
keeping always a 1:9 weight ratio. The soot model used in this study consisted
of a carbon black, FW200, from Orion Engineering. The soot-catalyst-sand mixture
was exposed at a constant heating rate of 10°C/min up to 600°C, under a 2200
ppm NO/ 10% vol.O2/ He atmosphere. The evolution of CO₂, CO, NO,
NOx, N₂,O₂ gases with temperature were followed by mass
spectrometry and a KIGAZ 310 gas analyzer. The catalyst materials were also
characterized by SEM, TEM, XRD, H₂-TPR and O₂-TPD.

The results show a good
catalytic behavior of the materials tested for the oxidation of soot. Figure 1
and Table 1 present the temperature at which soot ignition begins (T_ig),
and the temperatures at which the conversion of soot to CO₂ was 50%,
90% and 100%, named as T50, T90 and T100, respectively. When it is compared the
catalytic activity results with the non-catalytic one; it was found an ignition
temperature shift of 278°C when the HMW-synthesized catalyst was used, and a
shift of 241°C for the MW-synthesized catalyst. Also, the maximum soot
oxidation peak, (T50), for all the catalytic systems evaluated in this study
was shifted to lower temperatures according to the following decreasing order:
non-catalytic > SS > HMW > MW. Another important result regarding to
catalytic soot oxidation, it is that no CO emission was found in the gas
stream, indicating 100% selectivity toward CO₂. In contrast, for the
non-catalytic one, the selectivity CO₂/CO ratio was near one.

Figure 1. CO₂ Yield as a function of
soot oxidation temperature over La_0.7Ag_0.3MnO₃
perovskite-like material synthesized by different methodologies (MW, HMW, SS).

Table 1. Ignition temperature (Tig);
temperatures at which the soot conversion was 50%, 90% and 100%, respectively;
and CO₂ selectivity for catalytic and the non-catalytic test.

Regarding NO removal,
Figure 2 shows the NO evolution with temperature, for the experiments carried
out with La_0.7Ag_0.3MnO₃ synthesized by
methodologies. Figure 2 shows that during soot oxidation, the catalysts tested
presented some NO adsorption/desorption events followed by a NO reduction
toward N₂ around the soot oxidation peak. The catalyst synthesized
through MW route shows a prominent desorption event between 210°C and 350°C
compared with the other two materials. Nevertheless, its adsorption or
reduction capacity was not as good as the one obtained from the HMW-synthesized
catalyst.

Figure 2. NO evolution as a function of
temperature for each of the synthesized catalysts tested.

Figure 3 shows the CO₂
and NO/NOx profiles as function of temperature when the HMW-synthesized
catalyst was used. From figure, it is possible to see that this catalyst was
able to remove 45% of the initial NO concentration as N2. Also, it is possible
to see that the temperature for the NO/NOx peak reduction was found around 400
°C matching the temperature at which the evolution of CO₂ wasmaximum.
This result shows the ability for the simultaneous removal of soot and NO of
the La_0.7Ag_0.3MnO₃ perovskite-like material synthesized
by the HMW method.

Figure 3. CO₂, NO/NOx and N₂ profiles
as a function of temperature when the HMW-synthesized solid was used as
catalyst.

The results presented above
demonstrate that the selection of the synthetic route has a significant effect
on the catalytic performance of a material. Also, it was evidenced that the
perovskite-like material (La_0.7Ag_0.3MnO₃)
synthesized by microwave-assisted hydrothermal (HMW) method has an excellent
catalytic activity for the simultaneous removal of soot and NO by shifting the
soot oxidation peak toward lower temperature and removing up to 45% of the
initial NO as N2.