Decomposition of Nitrous Oxide over Fe-Ferrierites. Effect of High-Temperature Pretreatment on the Formation of Deposite
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Catalysis Letters Vol. 98, Nos. 2–3, November 2004 (Ó 2004)
Decomposition of nitrous oxide over Fe-ferrierites. Effect of high-temperature pretreatment on the formation of deposited oxygen Jana Nova´kova´, Michael Schwarze, Zdena Tvaru˚zˇkova´ and Zdeneˇk Sobalı´ k J. Heyrovskyˇ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic
Reiceived 16 July 2004; accepted 20 July 2004
The dehydration of Fe-ferrierites doubled the decomposition of N2 O at 200 C and corresponding deposition of oxygen. The rehydration resulted in a reversible process. Samples with iron located predominantly in cationic positions (Fe/Al 0.05–0.25) exhibited the Odep /Fe ratio of ca. 0.5 and 1 after pretreatment at 450 and 700 C, respectively. KEY WORDS: Fe-Ferrierite; high-temperature treatment; effect on N2 O decomposition.
1. Introduction There is a general agreement concerning the dominant role of iron in the decomposition of nitrous oxide to nitrogen and dioxygen over zeolites, e.g., [1–5]. The function of Bro¨nsted and/or Lewis acid sites alone in this reaction has been excluded [4]. Since the pioneering work of Panovs´ group [6], most efforts have been focused on iron in MFI structures. The N2 O decomposition was examined to a substantially lesser extent over other zeolites, e.g., Fe–silicalites, Fe–Bea and Fe-Ferrierite [7– 12]. In spite of many sophisticated experimental as well as theoretical methods used, the mechanisms of nitrous oxide decomposition as well as the nature of the active site have not been unambiguously established. Iron active sites were proposed as mono- or bi-nuclear Fe complexes, or as enzyme-like species bonded to one or two Al atoms (see e.g. refs. [13–23]). The nature of oxygen atoms captured during the decomposition of nitrous oxide, denoted by Panov as a-oxygen, has been widely discussed. The fraction of active iron sites participating in oxygen atoms bonding, i.e. the formal Odep /Fe ratio, has been found in a surprisingly broad scale, ranging over various zeolites between 0.1 and 1, and even above the value of 1 [16– 24]. Different preparation and pretreatment as well as different experimental methods used make the explanation of this broad interval difficult. The values of Odep /Fe ratio can be employed to rationalize the nuclearity of iron complexes:Odep / Fe = 0.5 agrees with a binuclear Fe complex bridged by a single deposited oxygen, while Odep /Fe = 1 is in line with mononuclear isolated Fe–Odep complex. The Odep /Fe ratios can be obtained at low temperatures of the N2 O decomposition, when only N2 is released, and oxygen (Odep ) remains captured in the zeolite. *To whom correspondence should be addressed. E-mail: [email protected]
Captured oxygens are known to react with many reductants at mild temperature, and thus TPR has become a suitable method for the quantification of Odep . The reduction of any iron oxide starts at substantially higher temperature [13, 18, 25, 26]. Determination of captured oxygen also can be realiz
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