奈米金觸媒(Au/TiO2)近室溫之製備程序及儲存方式開發研究

Developing processes for preparing and storing nano-gold catalysts (Au/TiO2) at room temperature

郭建男(Chien-Nan Kuo)

指導教授 ： 萬本儒

國立臺灣大學/工學院/化學工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.02599

摘要

本研究主旨為增進Au/TiO2觸媒的實際應用性，分兩部分進行探討，分別為：開發近室溫的Au/TiO2製備程序，與研發有效的Au/TiO2長時間儲存方式。
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氯殘留毒化觸媒而降低觸媒活性的問題。本研究中發現以80°C熱水後處理、NaOH鹼液後處理與提高金溶液pH值三種方式都可提高近室溫製備之Au/TiO2的觸媒活性，但NaOH鹼液後處理與提高金溶液pH值兩種方式都會附帶造成觸媒金載附量的下降。此外，研究過程也發現80°C熱水後處理方式只在觸媒經乾燥程序之後才有效，且不論是在80°C熱水後處理後，或在NaOH鹼液後處理後，水洗步驟都會明顯影響奈米金觸媒活性。研究結果顯示在經80°C熱水後處理後，再以室溫之去離子水沖洗的方式，可以更有效提高近室溫製備之Au/TiO2的觸媒活性。
在研發有效的Au/TiO2長時間儲存方式部份，過去研究顯示不同儲存條件會影響觸媒活性衰退的幅度，並建議將奈米金觸媒儲存在低溫、低壓與隔絕光等嚴苛的儲存條件下，然而觸媒活性衰退與嚴苛的儲存條件都會限制其實際應用性。本研究發現UV光、真空環境與儲存容器等因素都會不同程度地影響觸媒活性衰退。研究結果顯示波長400 nm以下的UV光會影響奈米金觸媒活性衰退，觸媒暴露在UV光下，會造成金顆粒些微成長，表面大部分金都還原為金屬金，以及表面金含量明顯減少，進而導致觸媒活性衰退。因此，將奈米金觸媒儲存於可阻絕UV光的環境或容器（如HDPE瓶或普通玻璃瓶），可大幅降低觸媒活性衰退幅度。此外，相對於一般大氣環境，奈米金觸媒存放在真空環境時，其觸媒活性衰退的幅度更大，且表面金含量下降比例與金顆粒成長幅度也都比存放在一般大氣環境下的觸媒顯著。另，儲存容器除了對UV光的穿透度外，本身材料性質的差異也會影響觸媒活性衰退的幅度，當觸媒儲存於PE夾鍊袋時，觸媒表面會明顯覆蓋一層有機物，表示觸媒會與PE夾鍊袋作用，進而導致觸媒活性更大幅度的衰退。綜合以上，金顆粒的成長並非影響觸媒活性衰退的主因，相對地，表面金含量下降才是造成觸媒活性衰退的主要因素，且表面金含量下降會伴隨著金還原成金屬金的現象。研究結果顯示經200°C緞燒還原過後的Au/TiO2觸媒，其表面金含量在儲存過程中即沒有顯著減少，且在儲存於一般大氣環境下時，其觸媒活性可維持長達9個月不會衰退，表示還原程序可避免儲存過程中表面金含量的下降。然而，還原後的Au/TiO2觸媒儲存於真空環境與PE夾鍊袋下時，其觸媒活性仍會明顯衰退。總結以上，本研究建議有效避免觸媒活性衰退的方式為將還原後的奈米金觸媒儲存於可阻絕UV光的容器中，並避免存放於真空環境與PE夾鍊袋下。

關鍵字

奈米金 ； 二氧化態 ； 低溫CO氧化反應 ； 近室溫製備 ； 活性衰退

並列摘要

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.

並列關鍵字

nano-gold catalysts ； TiO2 ； CO oxidation at low temoerature ； room temperature procedure ； storage effect

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