奈米金觸媒(Au/TiO2)近室溫之製備程序及儲存方式開發研究

Abstract

The main objective of this research was to improve the practical applications of Au/TiO2 catalysts, and it was achieved by two approaches; developing processes for preparing Au/TiO2 catalysts at room temperature, and investigating conditions for storing Au/TiO2 catalysts. Manufacturing Au/TiO2 catalysts at room temperature could avoid temperature- control at a large scale process, and save the energy consumption. However, it was discovered that Au/TiO2 catalysts prepared at room temperature contained residual chloride, which would poison the catalytic activity. In this research, it was found that 80°C water treatment, NaOH(aq) treatment and increasing pH of gold solution all could enhance the catalytic activity of Au/TiO2 catalysts prepared at room temperature. However, both NaOH(aq) treatment and increasing pH of gold solution would also result in lower gold loading on TiO2. Moreover, 80°C water treatment was only effective while it was carried out after drying, in contrast to NaOH(aq) treatment, which would be only effective before drying. In addition, the step of washing would significantly affect the catalytic activity, both to 80°C water treatment and NaOH(aq) treatment. Therefore, 80°C water treatment followed by washing with room temperature de-ionized water could enhance the catalytic activity of Au/TiO2 catalysts prepared at room temperature to the performance as high as Au/TiO2 catalysts prepared at high temperature. As to the research of investigating conditions for storing Au/TiO2 catalysts, literatures accumulated so far show that storing supported gold catalysts would decay unless it were stored in the dark, freezing and vacuum conditions. However, neither activity decay nor rigorous storing condition would limit its practicability. In this research, it was found that UV light, vacuum condition and storing container would all influence the catalytic activity of Au/TiO2 catalysts during storage. The results indicated that UV light (wavelength less than 400 nm) would cause the growth of gold particle size slightly, the reduction of nano-gold cluster mostly and the decrease of gold content on the surface substantially. And then, the catalytic activity Au/TiO2 catalysts would decay. Hence, storing Au/TiO2 catalysts in UV-unpenetrable containers (for instant HDPE bottles and glass vials) would be able to avoid the activity decay during storage. Moreover, the catalytic activity of catalysts stored under vacuum exhibited substantial decay. The results indicated that the decrease of gold content on the surface and the growth of gold particle for catalysts stored under vacuum were more serious than that in atmosphere. Nevertheless, storing containers would influence the catalysts during storage by not only the penetration of UV light, but also the stability of materials. Catalysts stored in PE bag would be covered hydrocarbon species (a layer of organic-like species) on the surface. It was suggest that Au/TiO2 catalysts would react with PE bag and then cause the dramatically activity decay. To summarize, the growth of gold particle was not the major cause of activity decay for Au/TiO2 catalysts during storage. In contrast, it was suggested that decrease of the amount of gold on the surface was the major cause of activity decay. Moreover, XPS results indicated that the amount of gold on the surface would not decrease during storage while the catalysts were reduced beforehand. Therefore, the activity decay of reduced Au/TiO2 catalysts was successfully avoided during storage for up to 9 months in atmosphere. However, the reduced Au/TiO2 catalysts would still substantial decay while it stored under vacuum or stored in PE bag. Hence, the most effective conditions for storing Au/TiO2 catalysts suggested herein was that storing reduced Au/TiO2 catalysts in a UV-unpenetrable container and in atmosphere.