Clarification of the NOx chemistry and mechanism of NO reduction with ammonia (SCR) on zeolite catalysts
Konstantin Khivantsev, Pacific Northwest National Laboratory
Cu/Zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction chemistry remains unknown. Herein, we clarify different mechansims of various NOx species’ formation in zeolites and show that nitrosyl ions (NO+) in the zeolitic micropores are the key intermediates for NO reduction. Remarkably, they react with ammonia even at or 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 even needed for this reaction to occur. 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, 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 ammonia desorption can occur (>120 C). We postulate 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 critical missing understanding of the complete SCR mechanism. 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.