NH3-SCR on fresh and hydrothermally aged Fe-SSZ-13 catalysts

Feng  Gao, PNNL

With the successful implementation of Cu-SSZ-13 (a small pore zeolite with Chabazite (CHA) structure) as part of the emission control systems for diesel passenger vehicles, and light- and medium-duty trucks in the U.S. and Europe, there has been a new surge of research interest in NH3-SCR over Cu-Chabazite. While Cu-SSZ-13 has been extensively studied, systematic investigations for Fe-SSZ-13 as NH3-SCR catalysts are still lacking. In this study, we synthesized Fe-SSZ-13 with various Si/Al ratios and Fe loadings by ion-exchanging NH4-SSZ-13 with FeSO4 solution at 80 °C and pH ~3.0 for 1h, under a N2 atmosphere. The powder catalysts were further hydrothermally aged at 800 °C for 16 h in the presence of 20% O2 and 10% H2O. Routine characterizations of the catalysts included BET surface area/pore volume measurements, X-ray diffraction (XRD) for crystallinity, and ICP analyses for Fe loadings and Si/Al ratios. To elucidate the nature of the Fe species, catalysts were further characterized with H2 temperature programmed reduction (H2-TPR), infrared (FTIR), 57Fe Mössbauer, and nuclear magnetic resonance (NMR) spectroscopies. Standard NH3-SCR and NO/NH3 oxidation reactions were measured using a plug-flow reaction system. These studies show that Fe stays predominately as Fe3+ ions (monomers and dimers) while Fe2O3 clusters (< 10%) and Fe2+ ions (~10%) are minor species in freshly prepared catalysts. The freshly prepared catalysts are highly active and selective in NH3-SCR. After hydrothermal aging, the activity largely maintains above 350 °C. More importantly, high SCR selectivity also maintains, in dramatic contrast to Cu-SSZ-13, which typically loses high-temperature selectivity upon hydrothermal aging. Mössbauer measurements show that Fe3+ ions are highly stable and do not convert to Fe2O3 clusters during aging, consistent with the maintenance of high SCR selectivity.