Preparation, Characterization, and Performance of Fe−ZSM-5 Catalysts

Abstract

A number of iron−ZSM-5 catalysts have been prepared and characterized by X-ray absorption spectroscopy using fluorescence detection, electron spectroscopy, temperature-programmed reduction, infrared spectroscopy, and electron microscopy. Iron has been introduced by aqueous exchange, by a novel method recently proposed by Feng and Hall ( Catal . Lett . 1996, 41, 45), by exchange from a rigorously dried methanolic solution accompanied by agitation with ultrasound, and by a method intended to promote solid-state exchange. The degree of interaction with the zeolite framework has been probed by examining the effect on the zeolite proton OH band in the infrared spectrum. Less than 30% of the protons were exchanged from aqueous solution, but almost 80% exchange was achieved using ultrasound, as well as by the method reported by Feng and Hall (FH). Initially, both methods exhibited mainly isolated metal ions; however, calcination of the samples prepared according to FH exhibited rather large oxide clusters. After aqueous exchange and activation, most of the iron is present in the form of small oxygen-containing nanoclusters within the zeolite matrix, with EXAFS measurements indicating an average composition of Fe 4 O 4, although electron microscopy identifies some larger particles at the external surface of the zeolite. Depending on the preparation methods, isolated cationic species within the zeolite matrix were also found. The small Fe 4 O 4 type clusters cannot be reduced to the metallic state, even by hydrogen at 1100 K, although interconversion between Fe(II) and Fe(III) is facile. When the zeolite was exposed to nitric oxide, stretching vibrations corresponding to adsorption on the different iron species present could be identified by infrared spectroscopy. It is proposed that the ultrastable iron−oxygen nanoclusters have structures similar either to the iron−sulfur compounds ferredoxin II of desulfovibrio Gigas or to the cubanes of high-potential iron protein (HIPIP). Reactivity of these Fe−ZSM-5 materials in the selective catalytic reduction of NO x by propene in oxygen/helium differs significantly, depending irreversibly on whether they are initially activated in oxygen or in an inert atmosphere. Correlations between catalytic activity and the infrared spectroscopy results for adsorbed NO indicate that the nanoclusters are more active (per iron atom) in the SCR reaction than the isolated cations.