A redox reaction between fine particulate Al and TiO₂·nH₂O during milling

A process of complex formation between fine particulate Al and titania hydrogel during milling is examined with reference to a similar process between two hydrogels, Al(OH)₃ and TiO₂ H₂O. One of the most typical features for the present system is the redox reaction, where metallic Al atoms are coupled with oxygen to form ordinary AlO₆ units via an intermediate 5-coordinated states, as detected by high resolution ²⁷Al solid state NMR. Ti³⁺ ions were detected by ESR. The amount of these bivalent ions increased with milling time but leveled off on prolonged milling. A thermit reaction was detected by thermal analyses of the mixture after prolonged milling. At the same time, a highly polarized OH bond was formed during milling, as detected by solid state proton NMR, serving as a hinge to link aluminum and titanium atoms, leading to a precursor of Al₂TiO₅. Local inhomogeneity in an atomic scale, as detected from a detailed analysis of ²⁷Al is typical and unavoidable for materials prepared via a mechanical route.