製備程序(後處理、pH值、載負溫度)對Au/TiO2及Au/Y之影響

Abstract

本研究探討氯離子對金觸媒催化活性之影響，以擔體不同分為兩部分(TiO2與Y-type zeolite)，前者是以室溫下製備之觸媒，針對不同後處理程序對於觸媒上元素以及觸媒催化0°C CO氧化反應之活性之影響進行探討。而後者是利用沸石Y特殊的孔洞結構，製備具催化0°C CO氧化反應之活性的Au/Y觸媒，藉由改變Au/Y觸媒製備程序，瞭解各製程參數對金觸媒載負量與催化CO氧化反應活性的影響。觸媒性質的鑑定包含：原子吸收光譜儀、離子層析儀、高解析穿透式電子顯微鏡。 Au/TiO2觸媒以氯化金酸(HAuCl4)為前驅物，在室溫下以沈澱沈積法製備之金觸媒含有大量的氯，因而毒化觸媒，導致催化活性下降。本研究發現經由60°C乾燥或80°C高溫水洗後處理程序皆可以有效去除氯，使催化活性提升。然而，在80°C高溫水洗去除氯的同時，會導致金載負量下降，但是若在80°C高溫水洗前先進行60°C乾燥，則可以稍微減少載負量降低的問題。另外，60°C的乾燥會產生大顆粒的金，若在飽和水蒸氣壓下進行，則可減少較大顆粒的金，而且氯殘留量較少，因此導致反應活性較高。若在飽和水蒸氣壓下，用60°C乾燥觸媒，再經過80°C水洗的步驟，可以有效的將氯去除，提高CO氧化活性，是本實驗中最有效的後處理程序。而且使用'酸溶液'溶金，自行製備之金母液，於近室溫下製備觸媒，再經過上述後處理程序，即可以有效提升CO氧化反應活性，符合商用需求。 Au/Y觸媒製備程序中，金溶液調整後的pH值、載負溫度和去離子水沖洗等步驟是影響氯離子殘留、金顆粒載負和反應活性的主要製備變因。金溶液pH值為決定金錯離子載負比例的因素，pH值越高溶液中的OH基取代氯化金酸上Cl數目越多，但殘留於觸媒上Cl則差異不大，而且於pH為5與6製備之觸媒都有不錯的初始催化活性，只是pH為5所製備出來的觸媒，其金載負量較高，金顆粒距離較近，使得金易聚集而催化活性降低，pH為6之觸媒則維持活性。另一方面，載負溫度上升會使得觸媒上殘留較少的氯，金顆粒上的氯含量大幅下降，而且在高載負溫度製備之觸媒，其金載負率較高，推測為高溫時溶液中存在更小的金錯合物易進入孔洞當中，使金不易離開沸石Y，進一步導致觸媒催化活性增加。

The effects of residual chloride on supported gold for CO oxidation were studied, and the supports of the gold load on were TiO2 or Y-type zeolite in this research. The effects of post-treatment conditions on Au/TiO2 (gold supported on titania and prepared at room temperature) catalysts for CO oxidation were studied, and the effects of preparation conditions (i.e., pH of gold solution, solution temperature, and de-ionized water flushing ) on Au/Y (gold supported on Y-type zeolite) catalysts for CO oxidation were studied. The catalysts were characterized by AA, IC and HRTEM. Supported Au catalysts are generally prepared from chloride-containing Au precursors, and Au/TiO2 catalyst prepared at room temperature contains a substantial amount of residual chloride, which can poison active site and reduce the catalytic activity. For the catalysts dried at 60°C and washed by hot water at 80°C, the residual chloride can be removed effectively and the catalytic activity can be improved. However, Au loading on TiO2 would drop when the catalyst was washed in hot water first. The drop of gold loading can be reduced, if the drying process at 60°C (either with steam or not) can be carried out first. However, the catalyst which was dried at 60°C contained large gold particles. If the catalyst was dried under saturated vapor pressure, the particle growing can be limited and more chloride can be removed effectively; therefore, the resulting catalyst possessed the higher activity for CO oxidation. For the catalysts dried under saturated vapor pressure and washed by hot water at 80°C, the residual chloride can be removed effectively and the catalytic activity can be enhance the most. The gold plate was dissolved in acidic solution. The solution was used for the deposition of gold species on TiO2 at room temperature. The post treatment which dried under saturated vapor pressure and washed by hot water at 80°C were also applied. It is found that the catalytic activity can be effectively improved by the post treatment. The effects of preparation conditions (i.e., pH of gold solution, deposition temperature and de-ionized water flushing) on residual chloride, gold loading and catalytic activity of Au/Y were significant. The results show there are similar residual chloride on Au/Y catalysts with pH value of 5 and 6. Also there are similar initial CO oxidation conversion. However, the gold loading and the gold recovery percentage decreased as the pH value of the solution was increased. For the samples from the solutions at pH 5, there was higher gold loading which could cause agglomeration of gold particles and decreasing of catalytic activity during reaction. On the other hand, it was found that higher residual chloride remained on Au/Y after the lower deposit temperature (i.e., 30°C) deposition. And the sample prepared at lower temperature causes not only a decrease of gold loading but also a reduction of CO conversion, in contrast to both higher gold loading and CO conversion observed from that prepared at 80°C.