Search Results

Ammonia/urea-SCR is a mature technology, applied worldwide for the control of NOx emissions in combustion exhausts from thermal power plants, cogeneration units, incinerators and stationary diesel engines and more recently also from mobile sources. However a greater DeNOx activity at low temperatures is desired in order to meet more and more restrictive legislations. In this paper we report transient and steady state data collected over commercial Fe-ZSM-5 and V₂O₅-WO₃/TiO₂ catalysts showing high NOx reduction efficiencies in the 200 - 350°C T-range when NO and ammonia react with nitrates, e.g., in the form of an aqueous solution of ammonium nitrate. Under such conditions a new reaction occurs, the so-called "Enhanced SCR" reaction, 2 NH₃ + 2 NO + NH₄NO₃ → 3 N₂ + 5 H₂O.

In this paper the low temperature reduction process of nitrates stored at high temperatures over model Pt-Ba/Al2O3 LNT catalysts using both H2 and C3H6 is analyzed. The results indicate that over the Pt-Ba/Al2O3 catalyst the reduction of stored NOx with both H2 and C3H6 occurs at temperature below those corresponding to their thermal stability. Accordingly, the reduction process occurs through a Pt-catalyzed surface reaction, which does not involve, as a preliminary step, the thermal decomposition of the adsorbed NOx species. The occurrence of such a pathway also requires the co-presence of the storage element and of the noble metal on the same support.

In a previous systematic investigation on the adsorption of NOx/O2 over Pt/Al2O3, Ba/Al2O3 and Pt-Ba/Al2O3 samples, a novel pathway for NOx storage was proposed that implies the direct uptake of NO in the presence of oxygen in the form of nitrites at Ba sites nearby Pt. In order to better elucidate the role of the Pt-Ba interaction on NOx storage and reduction, in the present study the adsorption of NOx/O2 and their subsequent reduction was further investigated over Pt-Ba/Al2O3 samples having different Ba loadings; a physical mixture Pt/Al2O3+Ba/Al2O3 was also considered for comparison purposes. A kinetic literature model has been adapted to our catalytic system in order to simulate the transient NOx adsorption over a wide range of operating conditions.