Lean NOx 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 NOx 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 operando 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 operando 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-NOx system under cyclic lean/rich conditions, accompanied by NOx reduction under periodic lean (0.1% NO + 2% O₂)/rich (0.1% NO + 2% H₂) cycles. The NOx 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.