Reduction of NOx over zeolite MFI supported iron catalysts: Nature of active sites

Abstract

FTIR spectroscopy, catalytic reduction of NO_x with iso-C₄H₁₀, and oxidation of C¹⁶O with ¹⁸O₂ have been combined to characterize the Fe entities in Fe/MFI catalysts prepared by a variety of methods, including sublimation of FeCl₃ vapor onto the H-form of MFI. Iron hydroxo and oxo ions in exchange positions, iron oxide clusters inside zeolite channels, and Fe₂O₃ particles at the external surface have been identified. Of special interest are IR bands in the 830–1000 cm⁻¹ region. They are assigned to the perturbations by Fe ions of zeolite lattice vibrations and discriminate between hydroxo(oxo) ions and oxide clusters. Two bands in this region are assigned to Fe³⁺ and Fe²⁺; their respective intensities follow the changes in valency of the Fe ions. When MFI containing Fe ions are treated with aqueous NaOH, small iron oxide clusters are generated. This process is largely reversible: Fe ions are regenerated upon replacing the Na⁺ by H⁺ ions and initiating protonolysis at high temperature. The high CO/CO₂ ratio in the product of NO_x reduction with alkanes is consistent with a dominant role of hydroxo(oxo) ions, not oxide particles, as active sites. Oxidation of C¹⁶O with ¹⁸O₂ over Fe/MFI results in a remarkably high concentration of C¹⁸O₂ molecules. The superiority of the sublimation method in preparing Fe/MFI catalysts for NO_x reduction is mainly due to the high dispersion of the Fe. Immediately after subliming FeCl₃ onto H-MFI, the 3610 cm⁻¹ band is completely wiped out, indicating that all protons have been replaced by [FeCl₂]⁺ ions. Terminal OH groups and internal SiOH groups in MFI with high density of lattice effects are also destroyed, but these are completely regenerated in a subsequent hydrolysis step.