Low-silica chabazite zeolites for NOx reduction: Tailoring the active center of Cu2+-2Z and [Cu(OH)]+-Z through the regulation of CHA-cages opening

Abstract

Low-silica chabazite (CHA) catalysts with varying configurations of Cu2+-2Z and [Cu(OH)]+-Z active centers are developed through the regulation of the opening of CHA-cages. The catalysts are subsequently applied to the selective catalytic reduction of NOx by NH3 (NH3-SCR) reactions. A series of characterizations and performance evaluations indicate that the density of [Cu(OH)]+-Z in eight-member rings (8MRs) is positively correlated with low-temperature performance. Moreover, the presence of partial Cu2+-2Z in six-member rings (6MRs) enhances activity and tolerance at high temperatures. The residual K+ ions in the CHA-cages after NH4+ exchange are retained in the 6MRs. Density functional theory calculations demonstrate that the residual K+ ions stabilize the 6MRs structure, ensuring the entry and stable presence of the active Cu2+-2Z centers. In situ analysis reveals the reaction intermediates involved in the NH3-SCR process on representative catalysts. These findings can further elucidate the active centers of CHA-type catalysts and guide the design of commercial catalysts.