The selective reduction of NOx with propene on Pt-beta catalyst: a transient study

Abstract

The mechanism of the NO/C3H6/O2 reaction has been studied on a Pt-beta catalyst using transient analysis techniques. This work has been designed to provide answers to the volcano-type activity behaviour of the catalytic system, for that reason, steady state transient switch (C3H6/NO/O2 → C3H6/Ar/O2, C3H6/Ar/O2 → C3H6/NO/O2, C3H6/NO/O2 → Ar/NO/O2, Ar/NO/O2 → C3H6/NO/O2, C3H6/NO/O2 → C3H6/NO/Ar and C3H6/NO/Ar → C3H6/NO/O2) and thermal programmed desorption (TPD) experiments were conducted below and above the temperature of the maximum activity (Tmax). Below Tmax, at 200 °C, a high proportion of adsorbed hydrocarbon exists on the catalyst surface. There exists a direct competition between NO and O2 for Pt free sites which is very much in favour of NO, and therefore, NO reduction selectively takes place over hydrocarbon combustion. NO and C3H6 are involved in the generation of partially oxidised hydrocarbon species. O2 is essential for the oxidation of these intermediates closing the catalytic cycle. NO2 is not observed in the gas phase. Above Tmax, at 230 °C, C3H6 ads coverage is negligible and the surface is mainly covered by Oads produced by the dissociative adsorption of O2. NO2 is observed in gas phase and carbon deposits are formed at the catalyst surface. From these results, the state of Pt-beta catalyst at Tmax is inferred. The reaction proceeds through the formation of partially oxidised active intermediates (CxHyOzNw) from C3H6 ads and NOads. The combustion of the intermediates with O2(g) frees the Pt active sites so the reaction can continue. Temperature has a positive effect on the surface reaction producing active intermediates. On the contrary, formation of NOads and C3H6 ads are not favoured by an increase in temperature. Temperature has also a positive effect on the dissociation of O2 to form Oads, consequently, the formation of NO2 is favoured by temperature through the oxygen dissociation. NO2 is very reactive and produces the propene combustion without NO reduction. These facts will determine the maximum concentration of active intermediates and consequently the maximum of activity.