Heterometal incorporation in metal-exchanged zeolites enables low-temperature catalytic activity of NO _x reduction

A series of new heterobimetallic zeolites has been synthesized by incorporating a secondary metal cation M (Sc ³⁺, Fe ³⁺, In ³⁺, and La ³⁺) in Cu-exchanged ZSM-5, zeolite, and SSZ-13 zeolites under carefully controlled experimental conditions. Characterization by diffuse-reflectance ultraviolet-visible spectroscopy (UV-vis), X-ray powder diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS), and electron paramagnetic resonance spectroscopy (EPR) does not permit conclusive structural determination but supports the proposal that M ³⁺ is hosted in zeolite structures in the vicinity of Cu(II), resulting in high NO _x conversion activity at 150 °C. Among various zeolites reported here, CuFe-SSZ-13 offers the best NO _x conversion activity in the 150-650 °C range and is hydrothermally stable when tested under accelerated aging conditions. Mechanistic studies employing stopped-flow diffuse reflectance FT-IR spectroscopy (DRIFTS) suggest that the high concentration of NO ⁺ generated by heterobimetallic zeolites is probably responsible for their superior low-temperature NO _x activity.