Ferromagnetic Behaviour in Nanoscale Cobalt Particles Dispersed by Zeolite Na-X.
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I. HUSSAIN, I. GAMESON, P.A. ANDERSON AND P. P. EDWARDS. The School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B 15 2TT, UK.
ABSTRACT This investigation has looked at the preparation of nanoscale cobalt particles by a simple solid state reaction involving cobalt (II) nitrate and zeolite Na-X under vacuum conditions followed by reduction in an hydrogen atmosphere. Samples were characterised by powder x-ray diffraction and scanning/transmission electron microscopy (TEM). Magnetic measurements were performed on the samples below 300 K using a SQUID magnetometer.
INTRODUCTION Zeolites are a class of crystalline aluminosilicates whose structures are formed by comer linking of A104 and Si04 tetrahedra. A very important property of zeolites is that the internal surface consists of interconnecting channels and cavities which can be sufficiently large to allow the entry of small atomic, molecular and metallic clusters of metal. Zeolite Na-X is the synthetic structural analogue of the mineral faujasite. In this zeolite the sodalite cages are joined together by
hexagonal prisms or double six rings, resulting in the formation of a supercage which has a central diameter of -12.5 A. These supercages are accessible through four 12 membered rings which have a free diameter of-7.4 A.
Occlusion In Zeolites The process in which the host zeolite is penetrated by atoms or salt molecules of another substance is generally known as occlusion. Interestingly, the pore space within a dehydrated zeolitic framework may be filled from the vapour phase, via salt melts or by heating the zeolite with salt powders1 -4 . The ultimate aim of this research is to successfully insert cobalt atoms, clusters and particles into the supercages of various zeolites and examine the magnetic properties of these new materials. A wide variety of synthetic conditions were used and the resulting materials were investigated using a combination of structural and magnetic techniques.
529 Mat. Res. Soc. Symp. Proc. Vol. 384 ©1995 Materials Research Society
Experimental In order for non-framework species to be introduced into the zeolite, its water of crystallisation must first be removed. This is best achieved by heating under vacuum and for this reason the occlusion of cobalt into zeolites must take place under vacuum conditions. A high vacuum system has been designed and constructed for this purpose. Stoichiometric amounts of Co(N0 3 )2 .6H 2 0 (pink powder) and zeolite Na-X (white powder), supplied by Laporte Inorganics , composition Na86 (AlO 2 )86(SiO 2)106 .264H 2 0 were intimately ground together using a mortar and pestle. The mixture was heated to a temperature of 550°C for 12 hours under vacuum conditions, followed by reduction with 35%H 2/Ar gas mixture for 4 hours at 550*C, to yield the occluded and reduced zeolites. On heating, most anhydrous metal nitrates decompose to the metal oxide, NO 2 and 02. However, two possible oxides can be formed when cobalt nitrate decomposes, these are CoO and Co 3 04. When heated under vacuum, t
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