Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH₃)₂ with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO⁺ in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N₂H⁺ cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO⁺ reacts, “freed up” Bronsted acid site gets occupied by NH₃ to form NH₄⁺ – and so the catalytic cycle stops because NO⁺ does not form on NH₄-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO⁺ by the reaction: Cu(II) + NO à Cu(I) + NO⁺ which we further confirm spectroscopically. The NO⁺ then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.