Synthesis And Characterization Of Titania Nanomaterials For Support Of Gold Catalyst

Supported gold catalysts have been extensively studied during the past 20 years. Au/TiO₂ is one typical type of supported Au catalysts and has been extensively investigated. However, it is not known well how the catalytic behavior of Au/TiO₂ changes if we use different kinds of TiO₂, especially one-dimensional (1D) TiO₂ as supports. On the other hand, Au/TiO₂ may deteriorate when storing at room temperature, which has hindered the future applications of Au/TiO₂. In this paper, we show two new methods to synthesize 1D-TiO₂. We also compared the catalytic activity of Au supported on 1D-TiO₂, P25-TiO₂ and anatase TiO₂ nanoparticles (TiO₂-A), for the CO oxidation and selective hydrogenation of 1,3-butadiene. It is shown that the crystal phase and morphology of TiO₂ strongly affect the catalytic activity of Au/TiO₂ catalysts. The investigation on storage of Au/TiO₂ showed the first time that the influence of Au3+ percentage and water amount on the deactivation behavior of catalysts stored at room temperature.A novel method to synthesize single-phase titania nanomaterials was shown by autoclaving titanium tetrachloride in acetone at 80-140 oC. Depending on the molar ratio of TiCl4 to acetone (TiCl4/Ac), TiO₂ materials with different phases and morphologies were obtained. When the TiCl4 concentration was no higher than TiCl4/Ac = 1/15, single-phase anatase TiO₂ nanocrystals in sizes ranging from 4 nm to 10 nm were prepared by tuning TiCl4/Ac ratios from 1/90 to 1/15. On the other hand, when the TiCl4 concentration was high enough (e.g., TiCl4/Ac≥1/10), single-phase rutile TiO₂ nanofibres were obtained selectively. With the aid of GC-MS analysis of organic products in the liquid phase, it is shown that the controlled hydrolysis of TiCl4 with water, which was in-situ generated from the TiCl4 catalyzed aldol-condensation reactions of acetones, played an important role in the formation of the titania nanomaterials. This mechanism was also supported by our success in using other ketones as the alternatives of acetone in the synthesis. We also autoclaved anatase titania in NaOH-containing ethanol-water solutions to make controlling-synthesis of titanate nanotubes and nanowires. Nanotubes turned to be anatase nanorobs (NR-TiO₂-A) with calcination at 400 oC, while nanowires changed to TiO₂(B) nanowires (NW-TiO₂(B)).The synthesized 1D-TiO₂ was then used to prepare supported Au catalysts with deposition-precipitation method. CO oxidation data show that the activity of Au/NR-TiO₂-A is 1.6 times that of Au/P25-TiO₂, 4 time that of Au/TiO₂-A, and 8 times that of Au/NW-TiO₂(B). While conversion of 1,3-butadiene on Au/NW-TiO₂(B) is comparable to that on Au/P25-TiO₂, and 3 times that on Au/NR-TiO₂-A. These data suggest that 1D-TiO₂ is a kind of promising support of Au catalysts, which may provide opportunities for discovering and inventing high-performance Au/TiO₂ catalysts.In order to improve the stability of Au/TiO₂ during storage at ambient temperature, we controlled the syntheses of Au/P25-TiO₂ catalyst with different percentage of Au3+ and water amount. It was found that maximizing the percentage of Au3+ and minimizing the phy-sisorbed water on Au/TiO₂ surface could slowdown the deactivation during storage. Basing on the above results, A Au/P25-TiO₂ containing 100% Au3+ and less amount of physi-sorbed water could effectively preserve its activity for CO oxidation after storing at room temperature for 150 days. These data show that the main point to preserve activity of Au/TiO₂ during storage is to keep more percentage of Au3+ and less amount of physic-sorbed water.