Preparation of Hierarchically Porous Niobium(V) Oxide and Alkaline Niobate Monoliths via Sol-Gel Accompanied by Phase Separation

Hierarchically porous niobium(V)oxide monoliths were preparedthrough the sol-gel process accompanied by phase separationand subsequent heat treatment. Macroporous and mesoporous characteristicsas well as crystal structures were investigated in detail. Macroporousstructures were controlled by the starting composition to manipulatephase separation, whereas mesoporous structures were modified by heattreatment in air to crystallize niobium(V) oxide. The results showedthat by heat treating the as-dried gel at 550 & DEG;C in air, a hierarchicallyordered macro/mesoporous material with BET surface area of 54 m(2) g(-1) was obtained. With an increase of theheat treatment temperature, an increase in the mesopore size accompaniedby a decrease in the specific surface area was observed as a resultof crystallite growth. Heat treatment under a N-2 atmosphereresulted in the formation of phases with lower-valence niobium suchas NbO2 and Nb12O29, which were derivedfrom the reduction by the carbothermal reaction due to the presenceof carbonized organic compounds. The conversion of niobium(V) oxideinto niobium-containing complex oxides was also examined. Alkalinemetals, namely, Li, Na, and K, were incorporated by postgelation treatments.Niobium-rich alkaline niobate phases were obtained when alkaline chloridewas incorporated, whereas the majority of lithium niobate (LiNbO3) phases were obtained when samples were treated with lithiumhydroxide.