Transformation of Active Sites in Fe/SSZ-13 SCR Catalysts during Hydrothermal Aging: A Spectroscopic, Microscopic, and Kinetics Study

Fe/SSZ-13 catalysts (Si/Al = 12, Fe loadings of 0.37% and 1.20%) were prepared via solution ion-exchange, and they were hydrothermally aged at 600, 700, and 800 °C. The fresh and aged catalysts were characterized with surface area/pore volume analysis, Mössbauer, solid-state MAS NMR, NO titration FTIR spectroscopies, and TEM and APT imaging. Hydrothermal aging causes dealumination of the catalysts and transformation of various Fe sites. The latter include conversion of free Fe2+ ions to dimeric Fe(III) species, the agglomeration of isolated Fe-ions to Fe-oxide clusters, and incorporation of Al into the Fe-oxide species. These changes result in complex influences on standard SCR and NO/NH3 oxidation reactions. In brief, mild aging causes catalyst performance enhancement for SCR, whereas harsh aging at 800 °C deteriorates SCR performance. In comparison to Fe/zeolites more prone to hydrothermal degradation, this study demonstrates that via the utilization of highly hydrothermally stable Fe/SSZ-13 catalysts, more accurate correlations between various Fe species and their roles in SCR related chemistries can be made.