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 + O-2) and rich (NO + H-2) cycle conditions, we studied the reactions of adsorbed NO and NH3 on Rh-exchanged Al-rich (Si/Al = 5) beta zeolites (Rh4 beta 5) 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), H-2-temperature-programed reduction (TPR), and operando IR spectroscopy. Rh-0 metal clusters, Rh+, and Rh3+ species were copresent in the catalyst after H-2 reduction at 500 degrees C. Under NO or NO + O-2 flow, the Rh+ site (NO storage site) in the reduced Rh4 beta 5 captured NO in the form of [Rh(NO)(2)]+, which was stable under oxidative (lean) conditions. The captured NO was selectively reduced by H-2 to NH3. The in-situ-generated NH3 was captured by a Bronsted acid site (NH3 storage site). The captured NH3 reduced NO to N-2 in the next lean (NO + O-2) period. Finally, Rh4 beta 5 was applied to the lean de-NOx system under cyclic lean/rich conditions, accompanied by NOx reduction under periodic lean (0.1% NO + 2% O-2)/rich (0.1% NO + 2% H-2) cycles. The NOx trapped in the lean period is reduced to adsorbed NH3 in the rich period, which subsequently reduces NO to N-2 in the next lean period.

Automated summary

The study focused on the rational design of new lean NOx reduction catalysts under periodic lean and rich conditions. By investigating the reactions on Rh-exchanged Al-rich beta zeolites, it was found that NO was captured by Rh+ sites and selectively reduced to NH3, which in turn reduced NO to N-2. This cyclic process demonstrated effective NOx reduction in the lean de-NOx system.