| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 96 === Au/Y catalysts, with high catalytic activity for CO oxidation at 0℃, were prepared in chloroauric acid solutions (HAuCl4). A possible deposition mechanism of gold supported on Y-type zeolite (Au/Y) was developed, according to the experimental results obtained from this research. The effects of water vapor on catalytic activity of Au/Y and the reason for deactivation during the reaction were studied. The catalysts were characterized by AA, ICP, temperature-programmed reduction (TPR), XRD, UV-Vis, HRTEM.
It was found that an optimum ratio of H+ and Na+ in zeolite Y, from the pretreatment process of HY and the pretreatment temperature, caused high gold loading. However, more H+ on zeolite Y (HY) would enhance the aggregation of gold species during deposition on Au/Y and result the lower gold loading and the lower catalytic activity. The effects of preparation conditions (i.e. chloroauric acid concentration, NaOH(aq) addition process, pH of gold solution, mixing time, deposition temperature and drying temperature) on gold loading and catalytic activity of Au/Y were significant. In a solution with higher pH, the gold species contained less chlorine, which would generate more gold polymers for deposition. Therefore, Au particles on Au/Y were larger. Gold loading and catalytic activities were lower. In a solution with higher gold concentration, more gold species were deposited on the surface of Y, which caused dramatic deactivation during CO oxidation. Drying temperature of Au/Y catalysts can highly affect the activity of Au/Y for CO oxidation. Some gold species can be reduced during drying at 60℃, which causes the sintering of gold particles and the decreasing of catalytic activity. The main reason for deactivation of Au/Y during CO oxidation were CO2 poison and increasing the gold particle size.
It is proposed in this study that during the deposition process, chloride dissociated from AuCl(OH)2 to form [Au(OH)2]+, which then reacted with the zeolite as follows:
[Au(OH)2]+ + Hx(Na)1-xY
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