N2O decomposition over Fe-zeolites: Structure of the active sites and the origin of the distinct reactivity of Fe-ferrierite, Fe-ZSM-5, and Fe-beta. A combined periodic DFT and multispectral study

• Published in: Journal of catalysis 2010-06, Vol.272 (2), p.262-274
• Main Authors: SKLENAK, Stepan, ANDRIKOPOULOS, Prokopis C, BOEKFA, Bundet, JANSANG, Bavornpon, NOVAKOVA, Jana, BENCO, Lubomir, BUCKO, Tomas, HAFNER, Juergen, DEDECEK, Jiří, SOBALIK, Zdeněk
• Format: Article
• Language: English
• Subjects: Catalysis; Chemistry; Decomposition; Exact sciences and technology; General and physical chemistry; Ion-exchange; Iron catalysts; Spectrum analysis; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Zeolites: preparations and properties
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2010.04.008

Two Fe(II) cations accommodated in two adjacent [beta] sites of Fe-ferrierite form the active site responsible for the superior activity of Fe-ferrierite in the N2 O decomposition in the absence of NO. The N2 O decomposition over Fe-ferrierite, Fe-beta, and Fe-ZSM-5 has been recently studied [K. Jisa, J. Novakova, M. Schwarze, A. Vondrova, S. Sklenak, Z. Sobalik, J. Catal. 262 (2009) 27] and a superior activity of Fe-ferrierite with respect to Fe-beta and Fe-ZSM-5 has been shown. In this study, we investigated (1) plausible active sites for the N2 O decomposition over Fe-ferrierite and (2) the origin of the distinct reactivity of Fe-ferrierite, Fe-ZSM-5 and Fe-beta employing a combined theoretical (periodic DFT) and experimental (UV-vis-NIR spectroscopy, IR spectroscopy, 29 Si MAS NMR spectroscopy and catalytic batch experiments) approach. We evidenced that two Fe(II) cations accommodated in two adjacent six-membered rings in the eight-membered ring channel ([beta] sites) of Fe-ferrierite (the calculated Fe-Fe distance is 7.4Å) form the active site responsible for the superior activity of this catalyst in the N2 O decomposition in the absence of NO. Similar structures can be formed in Fe-beta. However, the probability of their formation is very low. For Fe-ZSM-5, the geometrical arrangement of the cationic positions is far from that in Fe-ferrierite and it is not suitable for the N2 O decomposition. Therefore, the predicted order of the activity of the Fe(II) exchanged zeolites agrees with our experimental findings and it is: Fe-ferrieriteFe-beta>Fe-ZSM-5. We further showed that the accommodation of divalent cations in rings forming cationic sites can lead to significant rearrangements of the local structures of the zeolite framework, and therefore, the precise structure of sites binding a divalent cation cannot be derived from results of X-ray diffraction experiments, but can be inferred from theoretical calculations. [PUBLICATION ABSTRACT]