Dual kinetic effect from confined iron nanoparticles in zeolite modulates high-temperature catalytic NO reduction and NH3 oxidation

Abstract

The selective catalytic reduction of ammonia (NH₃-SCR) is a promising technology for abating nitrogen oxides (NO_x), yet its application at high temperatures is severely hampered by the over-oxidation of ammonia, leading to a trade-off between NO_x conversion and N₂ selectivity. Herein, we construct a series of Fe-exchanged ZSM-5 catalysts with controlled Fe loadings (0.05–0.5 wt%) to decouple the competing reaction pathways. The optimized 0.1Fe@ZSM-5 catalyst achieves 83.0% NO_x conversion at 700 °C and maintains exceptional stability for over 120 h under harsh conditions, representing a significant performance enhancement. Mechanistic investigations combining kinetic modeling and in situ spectroscopy reveal a dual kinetic regime, governed by the size of the Fe species. Catalysts with low Fe loadings favor the standard SCR pathway via stable NH₄⁺ and intermediates, whereas catalysts with higher loadings and larger Fe nanoparticles promote the undesirable oxidation of ammonia to NO_x. The result identifies that the optimal catalytic sites for high-temperature SCR rely on a delicate balance, activating ammonia for the desired reaction while suppressing its subsequent over-oxidation. These findings provide new implications for advanced catalyst design by tuning the active site structure to navigate competing reaction kinetics.

Keywords: Selective catalytic reduction; NH₃ oxidation; Kinetics modeling; Brønsted acid site; N₂ selectivity.