Journal of Catalysis 206, 143–154 (2002) doi:10.1006/jcat.2001.3475, available online at http://www.idealibrary.com on Environmental Catalysis on Iron Oxide–Silica Aerogels: Selective Oxidation of NH 3 and Reduction of NO by NH 3 P. Fabrizioli, T. B¨ urgi, and A. Baiker 1 Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH H¨ onggerberg-HCI, CH-8093 Z ¨ urich, Switzerland Received September 13, 2001; revised November 15, 2001; accepted November 19, 2001 The catalytic properties of mesoporous iron oxide–silica aerogels prepared by a sol–gel process combined with ensuing supercriti- cal extraction with CO 2 was investigated in the selective oxidation (SCO) of ammonia and the selective reduction (SCR) of NO by ammonia. The main parameters changed in the aerogel prepara- tion were the type of base used as gelation agent, the iron content, and the calcination temperature. The aerogels differed significantly in acidity and iron dispersion. Diffuse reflectance infrared Fourier transform spectroscopy studies of ammonia adsorption at differ- ent temperatures revealed that ammonia was bound to Brønsted- and Lewis-type sites, the latter being dominant at 300 ◦ C. A frac- tion of low coordinated Fe 2+ sites were probed by NO adsorption measurements. Lewis-type sites were found to be associated with low-coordinated iron sites. Catalytic tests were performed in a con- tinuous fixed-bed reactor in the temperature 210–550 ◦ C range and at ambient pressure. The catalytic activity of the aerogels in SCO correlated with the abundance of more strongly bound ammonia adsorbed on Lewis sites (low coordinated iron). High selectivity to nitrogen (97%) could be reached up to 500 ◦ C, whereas at higher temperature the formation of N 2 O and NO became significant. The apparent activation energy of N 2 formation ranged from 69 to 94 kJ/mol, whereby catalysts with higher selectivity and activity showed lower activation energy. In SCR, selectivity to nitrogen was for all aerogels > 98% at T < 460 ◦ C, and activation energies varied from 38 to 53 kJ/mol. The catalytic activity for SCR did not cor- relate with the population density of Lewis sites. We propose that SCO predominantly occurs on Lewis sites consisting of highly dis- persed iron atoms of low coordination, whereas in SCR these sites do not play an important role. c  2002 Elsevier Science (USA) Key Words: iron oxide–silica aerogels; selective oxidation of ammonia; selective reduction of NO; supercritical drying; Brønsted and Lewis acidity. 1. INTRODUCTION Iron–silicon oxide-based materials have received consid- erable attention as potential catalysts for reactions relevant to environmental and petrochemical catalysis. This is most prominently demonstrated by the family of crystalline iron 1 To whom correspondence should be addressed. Fax: 41 1 632 11 63. E-mail: baiker@tech.chem.ethz.ch. silicalites (1), which have been reported to possess inter- esting catalytic potential in a variety of reactions. Indeed, the acid properties of the incorporated Fe 3+ and the re- dox properties of finely dispersed iron oxide particles, to- gether with shape selectivity of the zeolite, are attractive properties for catalytic application (1). Comparatively lit- tle attention has been given to supported iron oxide (2–5) and to iron–silicon oxide aerogels: Wang and Willey (6) synthesized high-temperature Fe 2 O 3 –SiO 2 aerogels for the oxidation of methanol. Willey et al. prepared iron oxide– chromia–alumina high-temperature aerogels (7), and mag- nesium oxide–iron oxide spinel aerogels (8) for the selective catalytic reduction of NO by ammonia. No report deals with iron oxide–silica aerogels, prepared by low-temperature extraction with supercritical CO 2 . Aerogels can offer high flexibility in the tuning of the textural properties and chemical composition, as has been exemplified for the titania–silica-mixed oxides used for the epoxidation of olefins (9). A problem inherent to the sol– gel-derived materials is, however, that mixing of the con- stituents on the molecular scale is often not perfect, and real “single-site” catalysts are difficult to achieve. Never- theless comparative studies of titania silicalites and titania– silica aerogels proved that the catalytic properties of the amorphous aerogels are in some cases outperforming those of the crystalline materials (10). This prompted us to ex- plore the potential of iron oxide–silica aerogels in cata- lysis. In a preceding study (11) we reported on the synthesis and chemical and structural properties of iron oxide–silica aerogels. Here we address the catalytic properties of these materials. Two environmentally relevant reactions are the focus of our study: the selective catalytic oxidation (SCO) of ammonia and the selective catalytic reduction (SCR) of NO x by ammonia. The latter is the most frequently used technique to eliminate NO x from oxygen-containing waste gases of stationary combustion sources. Although various catalysts were applied for this reaction (12), most attention has been given to vanadia- on titania-based catalysts. Com- paratively little is known about the catalytic performance of iron-containing catalysts, such as iron-exchanged zeolites 143 0021-9517/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved.