Generation of magnetic metal particles in zeolite by borohydride reduction at ambient temperature

Abstract

Amorphous metallic particles containing some boron have been generated in a mordenite matrix by reducing the ion-exchanged M²⁺-mordenite (M = Fe, Co or Ni) at ambient temperature with sodium borohydride. The products have been characterized by X-ray powder diffraction, chemical analysis, transmission electron microscopy and SQUID magnetometer measurements. The Ni–B particles have a smaller particle size (20–40 nm) compared to the Co–B (70–120 nm) and Fe–B particles (140–200 nm). The Ni–B particles are deposited in defect pores within the mordenite framework while the Co–B and Fe–B particles are deposited on the external surfaces. The differences seem to arise owing to an easier reducibility of Ni²⁺ compared to Co²⁺ or Fe²⁺, and a reduction of Ni²⁺ in solid but Co²⁺ and Fe²⁺ in solution. All three systems exhibit superparamagnetic behaviour with the blocking temperature increasing from the Ni system to the Fe system. The differences in magnetic properties among the three systems are explained on the basis of the changes in magnetic anisotropic energy. The decreasing reducibility from Ni²⁺ to Fe²⁺ as well as the deposition of Co and Fe on the external surfaces seem to increase the compositional inhomogeneities and hence the magnetic anisotropic energy from the Ni system to the Fe system.