Supermicroporous silica-based SiO2–Al2O3–NiO materials: Solid-state NMR, NMR relaxation and magnetic susceptibility

Microporous materials SiO2–Al2O3–NiO with a pore size of 8–20 Å, prepared by the sol–gel method at wide variation in Ni2+ concentrations, have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, magnetic susceptibility measurements and multinuclear NMR. It has been shown that the MAS NMR spectra and particularly NMR relaxation in amorphous paramagnetic solids can be successfully used for their characterizations. The 1H and 29Si spin-lattice NMR relaxation is always non-exponential and governed by direct dipolar interactions with paramagnetic centers while the spin-diffusion mechanism is negligible. The 29Si relaxation rates, obtained in the limits of the stretched exponential, reflect distribution of paramagnetic centers. It has been found that aluminum atoms are incorporated into the silica matrix of the materials while nickel centers are accumulated within pores. The nature of the nickel centers has been studied by magnetic susceptibility measurements supported by the XPS and X-ray experiments. These centers represent NiO and Ni0 (observed at high nickel loadings) aggregated into nanoparticles. The Ni0 nanoparticles are responsible for the room-temperature ferromagnetic behavior of the materials prepared with high nickel loadings.