Identification of the mechanism of NO reduction with ammonia (SCR) on zeolite catalysts

Abstract

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 a combined spectroscopic, catalytic and DFT approach, we show that nitrosyl ions (NO⁺) in 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 the 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 °C, 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 that the reaction becomes catalytic on H–zeolites at temperatures when some ammonia desorption can occur (>120 °C). 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 can then 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 NO_x species to N₂, via di-azo compounds.