Catalytic Activity and Selectivity of H-ZSM5 for the Reduction of Nitric Oxide by Propane in the Presence of Oxygen

Abstract

The reactions of NO, C3H8, and O2were studied over a H-ZSM5 catalyst at various conditions. The reaction can result in the formation of either NO2or N2. The selectivity for N2mainly depends on the ratio of NO/C3H8in the feedstock; the reaction temperature and the concentration of O2have only minor influence. When the concentration of NO is 1000 ppm (which is typical for automotive exhausts) and NO/C3H8>1, the oxidation of NO to NO2dominates, but the NO2does not react significantly with C3H8at these reaction conditions. NO can be reduced by C3H8selectively to N2at NO/C3H8≤1 when oxygen is present. Results suggest that the selective catalytic reduction (SCR) of NO can proceed via two different reaction pathways below and above 500°C. At low temperatures, the oxidation of NO by O2to NO2might be the initial reaction step. Below 500°C, this reaction is much faster than other possible reactions in the mono- or bimolecular mixtures of the three reactants. At 500°C, the reaction rate of the oxidation of C3H8by O2is comparable to that of the NO oxidation. At 600°C, the oxidation of C3H8is faster than other reactions in the mono- or bimolecular mixtures of reactants. Thus, probably the combustion of propane initiates the SCR process above 500°C. The rates of both initial reactions depend on the concentration of oxygen and the reaction temperatures, but contrary to common belief these parameters have little effect on the yield of N2in the SCR process. It appears that the active sites for the reactions of NO+O2, C3H8+O2, and C3H8+NO2are similar to each other, probably involving the Brønsted acidic bridging hydroxyls of H-ZSM5. When NO/C3H8>1, adsorbed NOxspecies may prevent the adsorption of C3H8since propane barely reacts with the other reactants. At NO/C3H8≤1, the SCR of NO to N2probably proceeds via the secondary reactions of NO2+C3H8(below 500°C) or CxHyOz+NOx(above 500°C). Radicals are likely involved in both the initial and the secondary reaction steps. At 300°C, the probable overall stoichiometry of the SCR process is 2C3H8+6NO+4O2→6CO+8H2O+3N2. At higher temperatures, CO2also appears in the products and the efficiency of propane to form N2decreases.