Iron Loading Effects in Fe/SSZ-13 NH3-SCR Catalysts: Nature of the Fe Ions and Structure–Function Relationships

Using a traditional aqueous solution ion exchange method under a protecting atmosphere of N 2, a series of Fe/SSZ-13 catalysts with various Fe loadings were synthesized. UV–vis, EPR, and Mössbauer spectroscopic methods, coupled with temperature-programmed reduction and desorption techniques, were used to probe the nature of the Fe sites. The major Fe species are extraframework Fe(III) species: Fe(OH) 2 + (monomeric) and HO–Fe–O–Fe–OH 2+ (dimeric). Larger oligomers with unknown nuclearity, poorly crystallized Fe oxide particles, together with isolated Fe 2+ ions, are minor Fe-containing moieties. Reaction rate and Fe loading correlations, and temperature and Fe loading effects on SCR selectivities, suggest that isolated Fe 3+ ions are the active sites for low-temperature standard SCR, and dimeric sites provide the majority of reactivity at higher temperatures. For NO oxidation, dimeric sites are the active centers. NH 3 oxidation, on the other hand, is catalyzed by sites with higher nuclearity.