Heterometal Incorporation in Metal-Exchanged Zeolites Enables Low-Temperature Catalytic Activity of NO_<i>x</i>Reduction

Abstract

A series of new heterobimetallic zeolites has been synthesized by incorporating a secondary metal cation M (Sc3+, Fe3+, In3+, and La3+) in Cu-exchanged ZSM-5, zeolite-β, 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 M3+ is hosted in zeolite structures in the vicinity of Cu(II), resulting in high NOx conversion activity at 150 °C. Among various zeolites reported here, CuFe-SSZ-13 offers the best NOx 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 NOx activity.