Journal of Colloid and Interface Science 259 (2003) 331–337 www.elsevier.com/locate/jcis Comparison of the structural properties of isomorphously substituted Fe in mordenite zeolites prepared by different methods Mohamed M. Mohamed, a,∗ N.S. Gomaa, b M. El-Moselhy, c and N.A. Eissa b a Chemistry Department, Faculty of Science, Benha University, Benha, Egypt b Mössbauer Laboratory, Physics Department, Faculty of Science, Al-Azhar University, Cairo, Egypt c Chemistry Department, Faculty of Science, Al-Azhar University, Cairo, Egypt Received 28 June 2002; accepted 19 December 2002 Abstract Fe was introduced in mordenite zeolite by means of ion exchange either in solid or in liquid state. The iron loading (50–200 wt%), iron precursor (FeSO 4 ·7H 2 O and FeCl 3 ), and mordenite starting material (NH 4 M, HM, and NaM) were varied during the exchange processes. The Fe species were characterized by N 2 adsorption measurements as well as by XRD and Mössbauer spectroscopies. The Fe–mordenite samples prepared by liquid-state ion exchange attained remarkable Fe dispersion and surface areas higher than those of the parent. It was found that Fe 3+ ions, which substituted the framework Al and accordingly occupied tetrahedral sites, were decreased with Fe loadings with concomitant increase in Fe 3+ -occupied octahedral sites. The latter sites disappeared at 20 K to provoke the superparamagnetic α-Fe 2 O 3 in different particles size. The acid leaching (0.1 M HCl, 333 K, 3 h) of the samples showed the disappearance of the most highly distorted extra-framework Fe 3+ species, providing an indication of their presence on the external surface. On the other hand, a hematite phase was detected in the solid-state ion exchange of FeCl 3 with either HM or NH 4 M at the loading of 100% Fe. More correlations between Mössbauer data on one hand and XRD and texturing properties on the other hand were evaluated and discussed.  2003 Elsevier Science (USA). All rights reserved. Keywords: Iron–mordenites; Ion exchange; Acid leaching; XRD; Texturing; Mössbauer spectroscopy 1. Introduction Selective catalytic reduction (SCR) of NO x to N 2 has attracted much interest at the academic level looking for an effective catalyst that practically can be utilized. This is due to the fast growing of NO x , which arises as a component of the exhausts of automobiles and of the emissions of various chemical processes. The impact of NO x on the atmosphere involves its sharing in the destruction of the ozone layer in the stratosphere. Sincere efforts have been paid to efficiently convert NO x into N 2 , or to the direct decomposition of NO to N 2 and O 2 , to protect our global environment. So far, two main catalysts were employed for SCR of NO x to N 2 . The first catalyst was Cu–ZSM-5, which showed high activity under dry reaction conditions. Whereas, in a wet environment, which is inevitably produced in the combustion process within the engine, the catalyst un- * Corresponding author. E-mail address: mohmok2000@yahoo.com (M.M. Mohamed). dergoes poisoning followed by dealumination and physi- cal breakdown [1,2]. The second catalyst was Fe–ZSM-5, which showed activity toward NO x removal greater than that of Cu–ZSM-5, especially in the presence of reducing agents [3]. In addition, the former catalyst showed greater resistance to poisoning by water vapor and further exhibited greater durability [4,5]. Identification of Fe species, their locations, and coordina- tion are prerequisites to understanding their high activities in zeolites following calcination temperatures. The presence of low coordinated Fe 2+ or Fe 3+ cations in zeolite makes them much more susceptible to adsorb NO than those of highly coordinated ones [6,7]. This can be obtained by using the ion exchange process for preventing the formation of highly coordinated Fe 3+ species, which were capable of forming aggregated oxide clusters. The latter clusters were found to be inactive for either the decomposition of nitrogen oxides or the SCR of NO [8–10]. Unfortunately, it has been reported that the majority of Fe 3+ ions incorporated in pentasil ze- olites (especially, ZSM-5 and mordenite) showed relatively 0021-9797/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9797(02)00221-7