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

Cu/Zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction steps remain unknown. Herein, using combined spectroscopic, catalytic and DFT approach we show that nitrosyl ions (NO+) in the zeolitic micropores are the key intermediates for NO reduction. Remarkably, they react with ammonia even below room temperature producing molecular nitrogen (the reaction central to turning NO pollutant to benign nitrogen) through the intermediacy of the diazo N2H+ cation. Experiments with isotopically labeled N-compounds confirm our proposed reaction path. No copper is required for N2 formation to occur during this step. However, at temperatures below 100 ⁰C, when NO+ reacts with NH3, the bare Brønsted acid site becomes occupied by NH3 to form strongly bound NH4+ and, consequently, this stops the catalytic cycle, because NO+ cannot form on NH4-Zeolites when their H+ sites are already occupied by NH4+. On the other hand, we show the reaction becomes catalytic on H-zeolites at temperatures when some ammonia desorption can occur (>120 ⁰C). We suggest that the role of Cu(II) ions in Cu/Zeolite catalysts for low-temperature NO reduction is to produce abundant NO+ by the reaction: Cu(II) + NO  Cu(I)---NO+. NO+ then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Our findings provide novel understanding of the hitherto unknown steps of the SCR mechanism pertinent to N-N coupling. The observed chemistry of Cu ions in zeolites bears striking resemblance to the copper-containing denitrification and annamox enzymes, which catalyze transformation of NOx species to N2, via di-azo compounds.