| Summary: | 碩士 === 元智大學 === 化學工程學系 === 90 === The effect of base treatment on Au/γ-Al2O3 prepared by either deposition precipitation or impregnation methods was studied. EXAFS results showed that the pH of γ-Al2O3 suspension during deposition didn’t affect the major Au-O coordination in deposit prepared at pH4.4 and 10.5. However, the second shell coordination was altered. An Au-O-Al bonding environment was suggested by EXAFS of the catalyst prepared at pH4.4 while Au-O-Au was formed for the catalyst prepared at pH10.5. The pH10.5 catalyst exhibited a much higher CO oxidation activity than the pH4.4 catalyst, which suggests the specific Au-O morphology of Au-O-Au resulted in a higher activity than Au-O-Al morphology.
O2 calcination enhanced the CO oxidation activity over the pH10.5 catalyst, where the degree of activity enhancement increased as the calcination temperature was raised from 373K to 473K. XANES spectrum showed the white line intensity diminished with 373K calcination and completely disappeared after 473K calcination, which indicates an increase in extent of Au reduction. After 4h CO oxidation, all Au species develop to be Au-Au metallic-like morphology with or without calcination, suggesting the reduced form as the active morphology. Nevertheless, such an Au reduction is partially suppressed by the presence of moisture. Moisture also appears to temporarily suppress the CO oxidation activity over the pH 10.5 sample. The changes in the degree of Au reduction may be the main reason that leads to activity development. However, either the partial reduced Au or Au-Au metallic-like morphology can catalyze 100% CO conversion under reaction condition in this study.
EXAFS results showed that fresh Au/γ-Al2O3 prepared by impregnation under low vacuum drying at 298K contains abundant Cl, although Au-O is still the major coordination. A thermal treatment or an oven drying resulted in the formation of Au-Cl bond, which suggests that Cl can migrate to re-bond to Au species. Au species are mainly in Au-O Au-Cl, and Au---Al (Au-O-Al) bonding morphology environment. After another 25% NH3 solution impregnation and oven drying, Au-O and Au-Au contribution appeared while Au-Cl and Au---Al shells disappeared, suggesting that Cl-ligand of surface Au species exchanged with OH-ligand and that the surface Au species were partially reduced. In addition, the Au-O bonding distance increased from 1.99Å to 2.04Å, which indicates a change in Au-O morphology through the procedure. This post-impregnation base treatment is concluded to have a similar effect as pH adjustment in Au/γ-Al2O3 prepared by deposition.
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