| Summary: | 碩士 === 國立彰化師範大學 === 化學系 === 92 === Niobium oxide nanorods and nanotubes of high purity were prepared in gram quantities by treating Nb2O5 powder with NaOH solution at 150℃ without utilizing template molecule. These niobium oxide nanomaterials became acidic after being rinsed with diluted sulfuric solution or phosphoric solution.
SEM indicated majority of theses sodium hexaniobate nanorods formed in 10M of NaOH had a diameter distribution of 200-450 nm and a length distribution of 40-60 μm, though finer rods with a diameter of 50-100 nm could also be found in the product mixture. The BET surface area of these nanorods was only 4 m2/g. On the other hand, sodium hexaniobate nanotube bundle formed in 1M NaOH had a diameter distribution of 50-250 nm. TEM image of a signal sodium hexaniobate nanotube had an outer diameter of 15-20 nm, an inner pore diameter of 3-4 nm and a length of several microns. The BET surface area of the nanotubes was increased to 70 m2/g. And after being rinsed with diluted sulfuric solution or phosphoric solution, they became niobic acid nanotubes and had an outer diameter of 30-50 nm, an inner pore diameter of 10-30 nm and a length of several microns. The BET surface area of theses niobic acid nanotubes was in the range of 70-110 m2/g.
The molecular composition of the nanorods was identified as Na6H2Nb6O19•2H2O with EDX, XRD, XPS, TGA/DTA, TPD/MS and ICP-AES. Sodium hexaniobate nanotubes had a similar molecular composition with the sodium hexaniobate nanorods, and might be written as Na6-xH2+xNb6O19•nH2O. After being rinsed with diluted sulfuric solution or phosphoric solution, they became niobic acid nanotubes with a composition of H8Nb6O19•nH2O.
TPD/NH3 coupling with a mass spectrometer indiacted that the number of the strong acid site in niobic acid nanotubes rinsed with 0.3M H3PO4(aq) catalyst was higher than that rinsed with 0.1M H2SO4(aq). However, the activity in catalyzing the formation of cyclic acetals from aldehyde and ethylene glycol indicated the opposite. These results implied that acid site on the surface of phosphated niobic acid nanotubes had severe steric effect and only permit the adsorption of small molecule like ammonia. On the other hand, large molecule such as heptaldehyde might have difficulty to be adsorbed on the surface of phosphated niobic acid nanotubes and resulted in a lower catalytic activity than that of the less hindered sulfated niobic acid nanotubes.
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