Metallic cobalt and iron particles in large and medium pore zeolites: Methods of generation and ferromagnetic resonance characterization

Abstract

Spectroscopic studies utilizing photoacoustic electronic spectroscopy, electron paramagnetic resonance and ferromagnetic resonance have been made on the formation of (1) Co metal particles from Co(NH 3) 3+ 6 exchanged into Y-zeolite and (2) Fe metal particles from non-framework Fe 3+ ions in an aluminoferrisilicate analogue of ZSM-5 zeolite. Decomposition of the Co(NH 3) 3+ 6 complex is accompanied by autoreduction to the Co 2+ state, observed by EPR at 7 K, but no Co metal is formed. The Co 2+ ions migrate from the supercage locations of the parent complexes into the hexagonal prisms and are subsequently very difficult to reduce, even with H 2 at 823 K. High-temperature FMR data suggest that the small amount of Co metal particles which are formed exist with the f.c.c. crystal structure yielding g = 2.17 at 508 K, consistent with an empirically calculated value for f.c.c. Co. Hexagonal close-packed Co particles are expected to exhibit much larger magnetic anisotropy than was observed. The Fe 3+ ions in the mediumpore pentasil aluminoferrisilicate zeolite can be reduced to the metallic state following either (a) precipitation of the ‘inclusion’ compound, Fe III[Fe II(CN) 5(NO)], in the zeolite, or (b) generation of superparamagnetic oxidic ferric ion clusters. High temperature FMR data establish that smaller Fe particles can be obtained by the latter method.