Time-resolved in situ/operando infrared spectroscopy of Cu-supported zeolite X catalyst during NH3-SCR of NO: A modulated excitation study

Zeolite X was used as support of copper catalyst (Cu/X) for selective catalytic reduction (SCR) of nitrogen oxide (NO) with ammonia (NH3). Modulated excitation in situ/operando infrared (IR) spectroscopy and phase-sensitive detection (PSD) were applied to study the reaction intermediates that formed on the Cu/X during transient experiments. Changing the gas concentration to periodically create unsteady conditions enabled the selective extraction of the dynamic species that were directly involved in the reaction. The IR results showed that only Lewis acid sites (LX) existed on zeolite X,and Bronsted acid sites (B) and extra Lewis acid sites (LCu) were generated after Cu loading. The transient experiment results indicated that the SCR reactions on Cu/X obeys the Eley - Rideal (E - R) mechanism, in which NH3 is first adsorbed on the LX and LCu sites (LX-NH3, LCu-NH3) and then moved to the B sites (B-NH3), where it reacted with gas-phase NO to form NH2NO intermediate, which finally decomposed into N2 and H2O. Moreover, it is found that the NH3 adsorbed on LCu and B sites exhibits stronger reactivity than that on the LX sites.