Structural Features Of Hydrothermal Synthesis Titanate And A Pipe Mechanism

Recently, the special microstructure, relatively higher acidity and the potential nano confinement effect make nanotubes a very promisiy metal-supported bi-functional catalyst for various reactions. Anyway the available preparation methods, the titanate nanotubes synthesized by the hydrothermal route are characterized in uniform morphology, small diameter, and high surface area. Thus, much attentions been drawn on the hydrothermal synthesis and catalytic application of titanate nanotubes. However, analysis of the reported results indicates that TiO2is exclusively used as the precursor for the hydrothermal synthesis of titanate nanotubes. Moreover, the mechanism for the formation of titanate nanotubes is still controvercial.As a result of the special microstructure, high acidity, and potential nano confinement effect, titanate nanotubes are very promising for the catalytic application of metal-supported bi-functional catalyst. Among the available methods, titanate nanotubes prepared by the hydrothermal method have the advantage of uniform morphology and large specific surface area. Thus, much work has been done on the hydrothermal synthesis of titanate nanotubes. However, to the best of our knowledge, the precursor used for the hydrothermal synthesis of titanate nanotube is exclusively Tio2. Moreover, the formation mechanism of the nanotubes is still controversial. Based on these understandings, effect of precursors (anatase Tio2, Ti(OH)4, tetra-n-butyl titanate (TBOT), and the product of the hydrolyzed Ticl4) and hydrothermal conditions on the properties of titanate nano structure is systematically investigated in this thesis. The morphology and structural characteristics are studied by using TEM and XRD techniques, and are correlated with the synthetic parameters, i.e., the precursor and the hydrothermal conditions. The experimental and main conclusions are as follows:(1) A series of titanates were prepared by using the precursor of anatase Tio2, Ti(oH)4, TBOT, and the hydrolyzed TiCl4, respectively, under the hydrothermal conditions of1～10M NaOH solution,90-200℃, and0-7D. The hydrothermal products were characterized by TEM and XRD methods. Results indicate that titanate nanotubes can be easily formed when anatase Tio2or the hydrolyzed Ticl4was used asd the precursor, respectively. However, if the hydrolyzed TiCl4was aged for30D under atmospheric condition, and then applied as precursor for the hydrothermal synthesis, the morphology of the product was significantly varied with the NaOH concentration, i,e., similar morphology to the precursor at the NaOH concentration lower than3M, flake apperence at3-8M NaOH, mainly flake plus small amount of tubes at10M NaOH. Under the hydrothermal conditions applied, titanate nanotubes cannot be obtained when Ti(OH)4or TBOT was used as a precursor, respectively.(2) By using anatase TiO2as a precursor, the formation of titanate nanotubes were strongly dependent on the hydrothermal parameters. Titanate nanotubes cannot be formed if the hydrothermal period was too short (<1h) or unsuitable NaOH concentrations (>10M or<1M). Irrespective other hydrothermal parameters, bar-shaped product was obtained when temperature of200℃or greater than12M NaOH solution was applied during the hydrothermal process. Moreover, flake-like tritanate was obtained at a hydrothermal temperature of90℃. In the case of the hydrolyzed TiCl4as a precursor, uniform titanate nanotubes were obtained under the hydrothermal conditions of110℃and10M NaOH solution, in which the temperature is lower than that by using anatase TiO2as a precursor while the NaOH concentration is higher.(3) The morphology variation of the titanate with prolonged time was studied by usng anatase TiO2as a precursor under the hydrothermal conditions of8M NaOH and165℃. TEM observations indicate that the shape of the hydrothermal time was increased to1h. Moreover, the direct TEM observation of the raw product under the hydrothermal condition of3M NaOH and165℃indicates that they are nanotubes. The well structured nanotubes were still observed with TEM when the raw product was dispersed either in ethanol or distilled water. However, the raw product cannot be dispersed in acetone and no tubes were found in TEM. Thus, it is conclusive that the titanate nanotubes are formed during hydrothermal process other than during the washing step. Based on these results, apossible mechanism for the formation of titanate nanotubes is reasoned by proposed.