Rotation-Assisted Hydrothermal Synthesis of Thermally Stable Multiwalled Titanate Nanotubes and Their Application to Selective Catalytic Reduction of NO with NH3.
Titanate nanotubes are widely applied in various fields, including photocatalysts and electronic devices, but their weak thermal stability limits their application for catalyst support. Here, we found that titanate nanotubes with a thick multiwalled structure of 15 layers or more can be prepared by using rotation-assisted hydrothermal synthesis. The porous structure of conventional nanotubes synthesized without rotation collapsed easily after thermal treatment, whereas the nanotubes having a thick multiwalled structure retained their pore structure and the specific surface area (∼300 m2/g) even after calcination at 400 °C in air. Systematic variation of rotation speed suggested that rotation in the synthesis process accelerated the stacking of layered titanate nanosheets, which are known to be intermediates of nanotubes. Thus, the rapid assembly of titanate nanosheets facilitated by rotation led to the formation of nanotubes with a multiwalled structure. Overly fast rotation, however, caused excessive stacking and created a thicker structure that cannot be easily wrapped into nanotubes. Therefore, it is essential to maintain the optimum rotation speed to obtain both the nanotube morphology and the thick multiwalled structure. Vanadium-tungsten-oxide catalyst supported on the multiwalled titanate nanotubes was used in NH3-selective catalytic reduction, which showed stable NO x reduction performance with high selectivity to N2, which may originate from the suppressed sintering of VO x on multiwalled nanotubes. This study demonstrates that the morphology of nanotubes can be tuned by controlling the degree of interaction supplied by external forces.