INCAPE   05401
Unidad Ejecutora - UE
congresos y reuniones científicas
Simultaneous removal of soot and nitrogen oxides. Effect of rhodium on lanthanum supported catalysts
Río de Janeiro, Brasil
Congreso; 15to Congreso Brasilero de Catálisis- 5to Congreso de Catálisis del MERCOSUR; 2009
Introduction The main contaminants present in diesel exhaust gases are soot particles and nitrogen oxides. The combination of traps and catalysts appears as the most suitable after treatment to remove soot particles [1]; while NOx adsorbers are a developing technology for the elimination of nitrogen oxides in diesel engines (oxigen excess conditions) [2].   We have previously shown that La2O3 is capable of adsorbing NOx [3]; and that K/La2O3  catalyst is very active for soot oxidation [4]. In this work, different lanthanum supported catalysts were evaluated for the simultaneous removal of diesel contaminants, using rhodium in catalyst formulation to enhance NOx adsorption and decomposition.   Materials and Methods The catalysts were prepared by wet impregnation. To evaluate their capacity to remove soot and NOx, catalysts were pre-treated in [NO(4%) + O2(18%)] flow. It was previously found that during this treatment, nitrate species are formed over the catalyst surface, which are then released while heating in helium atmosphere. The extent of the decomposition and the temperature at which it begins depends on the catalyst formulation. The soot combustion experiments were carried out mixing the nitrated catalysts with soot (20:1 mass ratio, tight contact) and heating them in inert atmosphere (N2), or in oxidative atmosphere (O2(6%)/ N2). The CO2 generated was continuously monitored using a FID detector. For comparative reasons, the same experiments were performed using fresh catalysts (not pre-treated in NO) mixed with soot.  Soot-catalyst mixtures were characterized by  FTIR after being used in combustion experiments.   Results and Discussion The catalyst performance for NOx adsorption-desorption is shown in Figure 1. It shows that Rh containing catalysts adsorbed similar amount of NOx as K/La2O3 catalyst (first part of this figure).  Rhodium containing catalysts enhance the decomposition of the nitrates species adsorbed during the NO+O2 treatment. Moreover, this effect was also observed even when rhodium was impregnated on SiO2 forming a mechanical mixture with K/La2O3 catalysts. On the contrary, the nitrates formed over the K/La2O3  are very stable, and remain on the catalyst surface up to 490 ºC, both during adsorption on NO+O2, and during the heating in He. The adsorbed compounds start decomposing below 350°C when Rh is present, both in NO or in He flow.  This is a very important result regarding the use of these materials for NOx traps.   Figure 2 shows soot gasification in N2 flow for different catalysts formulations, after the catalysts have been treated in NO. It should be noticed that the temperature at which CO2 peaks are observed are similar among them. According to this, the activity for soot combustion of the nitrated catalysts does not depend on their ability to decompose adsorbed nitrates. Consequently, carbonaceous materials is probably oxidized directly with the nitrates  species adsorbed on the catalysts surface, not being necessary a previous decomposition of such species. This is an important point regarding the mechanism involved in the simoultaneous soot+ NOx removal.     Figure 1. NO adsorption (left) and decomposition                   Figure 2. Soot gasification in N2 flow. in He (right)                                                                               Catalysts pre-treated in NO+O2   Potassium is an active material in the soot combustion process. The nitrated Rh/La2O3 catalyst burns soot at almost the same temperatures than the K containing catalysts, but to a much lesser extent (see %C, calculated integrating the areas under the CO2 peaks). It has been verified that catalyst not treated with NO is not active for soot oxidation under an inert gas.   FTIR results showed the evolution of the nitrate species on the catalyst before and after the soot gasification. It was found that the nitrated compounds are not fully removed during the reaction with soot.   References 1.     R.M.  Heck, R.J. Farrauto, Cat.  Air Pollution Control, Van Nostrad Reinhold Ed. (1995) 2.     W. Addy Majewski, NOx adsorbers, Diesel Net Technology Guide (2001) 3.     V.G. Milt, M.L. Pisarello, E.E. Miró, C.A. Querini, Appl. Catal. B: Environ. 41 (2003) 4.     M.L.Pisarello, V. Milt, M.A. Peralta, C.A. Querini, E.E. Miró, Cat. Today 2751 (2002)