奈米金觸媒(Au/TiO2)近室溫之製備程序開發研究

Abstract

本研究主旨為增進Au/TiO2觸媒的實際應用性，其研究內容為為開發在近室溫(30℃)下製備Au/TiO2的程序。 在近室溫下製備Au/TiO2與在高溫下製備相比，可減少溫度控制的困難，減少能源的消耗，降低生產成本，進而提高未來奈米金觸媒在實際應用上的潛力。然而，先前研究結果顯示，在近室溫下製備的奈米金觸媒，會有殘留氯毒化觸媒而降低觸媒活性的問題。本研究中發現在50 °C~95 °C的飽和水蒸汽壓下乾燥，並且以少量室溫水洗的後處理程序，可以提高近室溫製備之Au/TiO2的觸媒活性，且所製備出來的金觸媒不會因這些後處理程序而減少其金的附載量。 此外，由於近年來金原物料的持續上漲，在本研究中也開發直接由王水溶解較低價格的金片以及金粉，以做為製備Au/TiO2的原料。研究結果顯示，以該原料經由傳統方式在80 °C金溶液中所製備之Au/TiO2，其催化活性已近似於以Merck公司提供的四氯化金酸在同樣條件下所製備出來的金觸媒活性。然而，此金溶液原料在近室溫下所製備之Au/TiO2觸媒活性，則尚不及以Merck之四氯化金酸在近室溫所製備出來的金觸媒活性，活性較差的原因很可能是因為在觸媒表面上尚有來自於王水的硝酸根殘留物。

In the past the processes for the deposition of gold species on TiO2 surface developed in different research laboratories were usually carried out at solution temperatures higher than 60°C in a state of good mixing. However, when it is applied in the large scale production, the good mixing in order to maintain uniform temperature in the whole solution system is a challenge in engineering. Moreover, rising the solution temperature consumes energy. Therefore, there is a need to develop the deposition process around room temperature. Although the room temperature prepared catalysts contain substantial amount of residual chloride, which poisons the active sites for the catalytic activity, the drying under saturated water vapor pressure process can not only improved the catalytic activity but also maintained the gold loading. It has been found in this research that the drying at different saturated steam temperatures influences the catalytic activity. Furthermore, drying under steam at the temperature above 50°C was more effective, in contrast to drying below 50°C. The catalyst dried under steam at 80°C and followed by the washing process possessed the highest CO conversion among all the RT-prepared Au/TiO2 in this research. The resulting Au/TiO2 possessed activity as high as those from the deposition processes in the solution at 80°C. On the other hand, it is confirmed from the analyses by using ionic chromatography that drying under steam at 80°C can break Au and Cl bond of gold species remaining on Au/TiO2 prepared at room temperature. The chloride on catalyst surface can be removed easily by the later washing process. Nevertheless, the catalyst which was dried below 50°C can not break the bond of Au-Cl effectively. The remaining chloride still causes the lower catalytic activity of Au/TiO2. In order to cost down the price of raw materials of gold for making Au/TiO2, the gold plate or the gold powder was dissolved in aqua regia. The solution was used for deposition of gold species on TiO2 at room temperature. The post treatments under steam and of washing for removing chloride were also applied. It is found in this research that the as-prepared catalysts possess lower catalytic activities than that prepared from gold solution, which was dissolved from HAuCl4 purchased from Merck. The residual nitrate on the catalysts from aqua regia solution may be the main cause for the lower catalytic activity.