Lean NO<i>x</i> Reduction by In-Situ-Formed NH₃ under Periodic Lean/Rich Conditions over Rhodium-Loaded Al-Rich Beta Zeolites

Toward the rational design of new lean NO<i>x</i> reduction catalysts under periodic lean (NO + O₂) and rich (NO + H₂) cycle conditions, we studied the reactions of adsorbed NO and NH₃ on Rh-exchanged Al-rich (Si/Al = 5) beta zeolites (Rh4β5) under transient (lean ↔ rich) and temperature ramping conditions. In situ infrared (IR) spectra of adsorbed species were collected while monitoring the outlet gas by mass spectrometry (MS) and another IR gas cell, enabling an <i>operando</i> analysis of the surface reactions. Dynamic changes in the catalyst structure were studied by X-ray absorption spectroscopy (XAS), H₂-temperature-programed reduction (TPR), and <i>operando</i> IR spectroscopy. Rh⁰ metal clusters, Rh⁺, and Rh³⁺ species were copresent in the catalyst after H₂ reduction at 500 °C. Under NO or NO + O₂ flow, the Rh⁺ site (NO storage site) in the reduced Rh4β5 captured NO in the form of [Rh­(NO)₂]⁺, which was stable under oxidative (lean) conditions. The captured NO was selectively reduced by H₂ to NH₃. The in-situ-generated NH₃ was captured by a Brønsted acid site (NH₃ storage site). The captured NH₃ reduced NO to N₂ in the next lean (NO + O₂) period. Finally, Rh4β5 was applied to the lean de-NO<i>x</i> system under cyclic lean/rich conditions, accompanied by NO<i>x</i> reduction under periodic lean (0.1% NO + 2% O₂)/rich (0.1% NO + 2% H₂) cycles. The NO<i>x</i> trapped in the lean period is reduced to adsorbed NH₃ in the rich period, which subsequently reduces NO to N₂ in the next lean period.