Crystallization of highly ordered mesoporous niobium and tantalum mixed oxide

Abstract

A highly ordered (2D-hexagonal) mesoporous Nb-Ta mixed oxide was synthesized as a amorphous precursor for crystal. It showed 184 m 2 g −1 BET surface area and 5.6 nm pore size, and TEM images and electron diffraction patterns verified the 2D-hexagonal mesoporous structure. Crystallization was performed by calcination at 650°C for 1 h under various conditions. When Nb-Ta oxide was calcined in air, single crystal particles of wormhole mesoporous material with 66 m 2 g −1 surface area and 13 nm pore size were obtained due to a drastic rearrangement of mesostructure accompanied by crystallization. Next, carbon deposition inside the pores and over the particles was attempted followed by crystallization in He. The deposited carbon was removed by calcination in air after crystallization. BET surface area decreased to 93 m 2 g −1 , but preservation of porosity was improved compared with the sample crystallized in air. 20–30% of the original ordered mesopores were maintained and coexisted with the original structure. The rest resulted in the rearrangement to 13 nm pores. In addition, crystallization in UHV chamber for TEM observation preserved the original ordered mesoporous structure, while the amorphous wall was converted into polycrystalline domains.