Mechanism of selective reduction of N2O by CO over Fe-β catalysts studied by in-situ/operando spectroscopy

Abstract

Selective reduction of N₂O by CO under excess O₂ was effectively catalyzed by Fe(0.9 wt%)-exchanged β zeolite (Fe0.9β) in the temperature range of 250–500 °C. Kinetic experiments showed that the apparent activation energy for N₂O reduction with CO was lower than that for the direct N₂O decomposition, and the rate of N₂O reduction with CO at 300 °C was 16 times higher than that for direct N₂O decomposition. Reaction order analyses showed that CO and N₂O were involved in the kinetically important step, while O₂ was not involved in the important step. At 300 °C, the rate of CO oxidation with 0.1% N₂O was two times higher than that of CO oxidation with 10% O₂. This quantitatively demonstrates the preferential oxidation of CO by N₂O under excess O₂ over Fe0.9β. Operando/in-situ diffuse reflectance ultraviolet-visible spectroscopy showed a redox-based catalytic cycle; α-Fe-O species are reduced by CO to give CO₂ and reduced Fe species, which are then re-oxidized by N₂O to regenerate the α-Fe-O species. The initial rate for the regeneration of α-Fe-O species under 0.1% N₂O was four times higher than that under 10% O₂. This result shows quantitative evidence on the higher reactivity of N₂O than O₂ for the regeneration of α-Fe-O intermediates, providing a fundamental reason why the Fe0.9β catalyst selectively promotes the CO + N₂O reaction under excess O₂ rather than the undesired side reaction of CO + O₂. The mechanistic model was verified by the results of in-situ Fe K-edge X-ray absorption spectroscopy.