Selectivity-determining role of C3H8/NO ratio in the reduction of nitric oxide by propane in presence of oxygen over ZSM5 zeolites

The reaction of (NO + C3H8 + O2) can result in selective formation of NO2 over H-ZSM5, Cu,H-ZSM5, Ag,H-ZSM5, and Li,H-ZSM5 catalysts when the concentrations of NO and O2 are 0.1 and 9%, SV > 60,000 h−1 (typical for automotive exhausts), and C3H8/NO > 1. Despite stoichiometric excess of reductant hydrocarbon below this limit, the in situ formed NO2 does not react with C3H8, thus conversion of NO to N2 is negligible. NO can be reduced by C3H8 selectively to N2 only when C3H8/NO ≧ 1. Contrary to many suggestions the reaction temperature, concentration of oxygen, space velocity, and type of exchange ions have minor influence on the selectivity for N2. These parameters affect the rates of reactions (NO + 2), (C3H8 + NOx) and (C3H8 + O2), therefore they also affect the production of N2 in the HC-SCR process, but only when the ratio of C3H8/NO permits. The metal-exchanged zeolites were prepared in situ by solid-state ion exchange from H-ZSM5. Despite the low degree of copper exchange (63%), Cu,H-ZSM5 produces substantially more N2 than H-ZSM5, Ag,H-ZSM5, or Li,H-ZSM5. However, the selectivity for N2 is lowest over Cu,H-ZSM5, which also produces considerable NO2 in the reaction of (NO + C3H8 + O2) even at C3H8/NO ≧ 1. Contrary to prior findings, the catalytic activity of Cu,H-ZSM5 for the oxidation of NO by O2 to NO2 in absence of hydrocarbon was comparable to that of H-ZSM5 at high space velocities (2.3 l g−1 min−1). By replacing 30 and 40% of the protons of H-ZSM5 by Ag+ and Li+ ions in Ag,H-ZSM5 and Li,H-ZSM5, respectively, the catalytic activity for this reaction becomes negligible at temperatures ≧100°C. Some mechanistic consequences of these experimental observations are discussed.