Brønsted-to-Lewis NH3 migration: An elemental step during selective catalytic reduction of nitric oxide

The key to optimizing cerium-based mid-low temperature NH 3 –SCR catalysts lies in elucidating the respective roles of Lewis acid sites (LAS) and Brønsted acid sites (BAS). Herein, a highly dispersed cerium-loaded ZrO 2 catalyst (Ce/Zr) with well-defined LAS and BAS was prepared to investigate their roles in the NH 3 –SCR reaction using in situ DRIFTS, transient DRIFTS coupled with mass spectrometry, operando modulation excitation (ME) DRIFTS, and isotopic 18 O 2 labeling. Experimental results demonstrate that the NH 3 –SCR reaction obeys Eley–Rideal mechanism, in which gas-phase NO reacts with surface adsorbed NH 3 . Both NH 3 adsorbed on LAS (L−NH 3 ) and NH 3 adsorbed on BAS (B−NH 3 ) can be consumed during the reaction. L−NH 3 directly reacts with NO to form N 2 , whereas BAS participates indirectly by adsorbing and transferring NH 3 to LAS, thereby acting as an NH 3 reservoir. Transient half-cycles experiments combined with 18 O 2 labeling reveal that O 2 reoxidizes Ce and drives NH 3 migration from BAS to LAS, regenerating reactive L–NH 3 . Density functional theory calculations further confirm that NH 3 transfer from BAS to LAS proceeds without an energy barrier. These findings reveal a redox-coupled NH 3 transfer mechanism and provide theoretical guidance for the design of efficient Ce-loaded SCR catalysts.