Kinetic and Spectroscopic Characterization of Fe-SSZ-13 NH3 SCR Catalysts

Janos  Szanyi, Institute for Integrated Catalysis, Pacific Northwest National Laboratory

Reduction of environmentally harmful NOx gases from automotive exhausts in the presence of excess O2 has been one of the most challenging problems in heterogeneous catalysis. Zeolite-based selective catalytic reduction (SCR) (either by NH3 or hydrocarbons) showed initial promise, however that hydrothermal stabilities of potential zeolite frameworks (ZSM-5 and Beta) proved insufficient for practical applications. With the introduction of small pore CHA-based catalysts (SSZ-13 and SAPO-34) there has been a renewed interest in NH3 SCR processes for automotive applications. In particular Cu-SSZ-13 has shown excellent NO conversion and N2 selectivity in a wide operating temperature window. Our recent work [1] has shown that Fe-SSZ-13 is also a very efficient NH3 SCR catalyst, especially at higher temperatures. Extensive spectroscopic characterization of the active centers in Cu-SSZ-13 has been carried out by a number of groups [2 and references therein] that greatly enhanced our understanding of the working of this material in the NH3 SCR process. Here we present the results of our combined activity and spectroscopy (FTIR and Mössbauer) study aimed at understanding the nature of ionic Fe sites in the SSZ-13 framework, and the properties of the differently coordinated Fe sites. To this end we prepared two series of Fe-SSZ-13 catalysts with different Si/Al ratios, and for each Si/Al ratio and varied the Fe ion exchange levels. The catalytic activities of these materials were tested in the standard NH3 SCR of NO, N2O decomposition and the NH3 SCR with N2O.
All the parent Na-SSZ-13 zeolite samples with Si/Al ratios of 6 and 12 were hydrothermally synthesized. Fe-SSZ-13 catalysts with different Fe ion exchange levels were prepared using a two-step solution ion exchange protocol. First, Na-SSZ-13 was exchanged to NH4-SSZ-13 from a NH4NO3 solution. Next, under protection of an N2 atmosphere, FeSO47H2O powder was added into the zeolite/water (pH~3) suspension to carry out ion exchange at 80 C for 1 h. Fe-SSZ-13 was washed multiple times with deionized water, dried at 120 C in flowing N2, and finally calcined in air at 550 C for 5 h. All the activity measurements were carried out in a plug-flow reaction system, utilizing IR gas analysis. The FTIR characterization of adsorbates (NOx and CO) was carried out under static conditions in a homebuilt IR cell using a Bruker Vertex 80 spectrometer. The Mössbauer measurements were done on either a WissEl Elektronik or Web Research Company instrument that included a closed-cycle cryostat, an Ar-Kr proportional counter detector with WissEl setup or a Ritverc NaI detection system. The Mössbauer data were modeled with RecoilTM software using a Voigt-based structural fitting routine.
Ten samples, two Si/Al ratios and five Fe ion exchange levels each, were tested under standard NH3 SCR of NO, N2O decomposition and NH3 SCR of N2O form 323 to 873 K in our flow reactor system. All the Fe-SSZ-13 samples showed god NH3 SCR of NO activity, primarily at temperatures higher than 573 K. N2O decomposition activity was high on these catalysts at temperatures above 673 K, similar to NH3 SCR of N2O. This latter reaction selectively produced N2. FTIR characterization of adsorbed NOx and CO was conducted on annealed, oxidized (with NO2 and N2O) and reduced (with H2) samples (3 different ion exchange level for each Si/Al ratio) at 150 and 300 K. In the IR spectra of adsorbed NO band characteristic of N-O vibrations of adsorbed (mono-, di-, and tri) nitrosyls on Fe2+ sites (bands between 1770 and 1900 cm-1), NO+ species (2162 cm-1) and nitrates on Fe3+ sites (1450-1650 cm-1) are observed. The intensity ratios of the mono-, di- and tri-nitrosyl species vary with the pretreatment condition, showing (1) that the number of Fe2+ sites available for NO adsorption is highest after reduction, and lowest after oxidation with NO2 and (2) some Fe2+ ions are located in spatially confined sites, likely faces of 6-membered rings and formation of di- and tri-nitrosyls is not allowed.
Mössbauer spectrum of the same sample shows that the majority of Fe ions after calcination are present in the 3+ oxidation state. In this presentation we will provide results of the detailed IR/Mössbauer spectroscopy study on all the Fe-SSZ-13 zeolites, and correlate them with their catalytic activities in the above mentioned reactions.
1. F. Gao, M. Kollár, R.K. Kukkadapu, N.M. Washton, Y. Wang, J. Szanyi, C.H.F. Peden, Appl.Catal.B:Environ., 164 (2015) 407-419.
2. J.H. Kwak, T. Varga, C.H.F. Peden, F.Gao, J.C. Hanson, J.s Szanyi, J.Catal. 314 (2014) 83–93