Role of Zeolitic Oxygens During the Decomposition of 15 N 2 18 O Over Fe-ferrierite
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Catalysis Letters Vol. 104, Nos. 3–4, November 2005 (Ó 2005) DOI: 10.1007/s10562-005-7945-9
Role of zeolitic oxygens during the decomposition of 15N218O over Fe-ferrierite Jana Nova´kova´, Michael Schwarze, and Zdeneˇk Sobalı´ k* J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic
Received 23 June 2005; accepted 24 June 2005
Isotopic species of dioxygen released during the decomposition of 15N218O over Fe-ferrierite show that the zeolite oxygens participate in the reaction. While Fe-ferrierite alone does not exchange its oxygens with 18O2 below 400 °C, this exchange is very rapid in the mixture of 18O2+N2O. The amount of participating zeolite oxygen (ca. 1–6 per iron atom) is practically the same in the latter case as in the decomposition of 15N218O. The time dependence of individual dioxygen isotope species released during the 15 N218O decomposition points to the primary release of 18O2 which is very rapidly exchanged for the zeolite oxygen by a single-step mechanism. KEY WORDS: Fe-ferrierite; decomposition of
15
N218O; participation of zeolite oxygens.
1. Introduction The involvement of oxygens from Fe-Y, Fe-mordenite and Fe-ZSM-5 during the decomposition of nitrous oxide above 350 °C was reported by Hall and co-workers using either 18O exchanged zeolite or 18O labeled N2O [1–3]. These authors assume that ca. 10% of all zeolitic oxygens can participate in the reaction which is enabled by their high mobility; the desorption of dioxygen can proceed from sites not identical with those on which the oxygen from nitrous oxide was captured. Similarly, the participation of Fe-ferrierite oxygens was reported using the desorption of dioxygen after the decomposition of N2O and subsequent exchange of captured oxygen for 18O2 at ambient temperature [4,5]. Pirngruber enriched Fe-ZSM-5 by the exchange with 18O2 and analyzed the isotopic composition of dioxygen formed during the decomposition of nitrous oxide. He assumes an equilibrated ‘‘oxygen pool’’ of ca. one zeolite oxygen per Fe atom with low reactivity of these oxygens [6]. The present paper concerns the decomposition of 18O labeled nitrous oxide on Fe-ferrierite and is focused on the route of the involvement of zeolitic oxygens into the reaction products. 2. Experimental Fe-ferrierite further Fe/FER (Si/Al 8.5; 0.55 wt% of Fe; Fe/Al 0.055) was prepared according to patent [7]. Forty to 200 mg of the sample was placed in a quartz reactor (reaction volume 165 mL) and pretreated in situ at 450 °C in vacuum, followed by treatment in oxygen (2 kPa), and cooled to reaction temperature in vacuum.
* To whom correspondence should be addressed.
Nitrous oxide 15N218O (supplied by Medical Isotopes Inc., USA, 99 atom%) was allowed to react with Fe/ FER at 280 °C (in some cases at 300, 350 and 400 °C) under the pressure of 120–1500 Pa. The simultaneous reaction of unlabeled nitrous oxide mixed with 18O2 (Technabsexport, Russia, 87 atom%) was also measured. Temperature programmed desorption (TP
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