An overview of the mechanisms of NO_x reduction with oxygenates over zeolite and γ-Al₂O₃ catalysts

The selective reduction of NO_x with added oxygenates over BaNa/Y and Ag/Y zeolites and Ag/γ-Al₂O₃ takes place via complex reaction pathways with a number of common crucial intermediates. Acetate ions are formed by the oxidation of acetaldehyde over these catalysts. These acetate ions react with NO₂ to form nitromethane which decomposes to HNCO via a dinitro-C₁ intermediate. HNCO hydrolyzes to form NH₃ which can react with HONO to form ammonium nitrite. This NH₄NO₂ efficiently decomposes to N₂ and H₂O at 100 °C, and at even lower temperatures in an acidic environment. The neutral surface species are expected to be in equilibrium with their ions. The rate-limiting step in these reaction sequences is the reaction of acetate ions to form nitromethane. When nitromethane is directly added to a NO_x stream over Ag/Y, ∼100% conversion of NO_x to N₂ is achieved at temperatures as low as 140 °C. In such schemes, NO acts as a reductant of nitrate ions, ammonium nitrate and nitric acid. The benefits of isotopically labeled compounds in the elucidation of such reaction mechanisms and for providing insights into reaction dynamics are also discussed.