Gold Catalysts Preparation, Characterization And Catalytic Performance In Co Oxidation And Water Vapor Transform

Gold catalyst, as a novel catalytic material, has been taken great progress for its excellent activity in chemical industrial and environmental applications. In this paper, a series of gold catalysts supported on different M_xO_y oxides ( M_xO_y=Al₂O₃, SiO₂, TiO₂, ZrO₂, CeO₂, CeO₂-Al₂O₃, UOx, TiO₂-h) were prepared by anionic impregnation with the alkaline HAuCl4 solution. Their catalytic performance for carbon monoxide oxidation and water-gas shift reaction was accessed systematically. N2-physical adsorption, XRD, TEM, H₂-TPR and XPS, FE-SEM characterization techniques were employed to study the structure, chemical state and the reduction/oxidation behavior of these catalysts. The main results summarizes as follows:1. Remarkable influence of catalyst support on the catalyst activity is found. The activities order is Au/SiO₂～Au/Al₂O₃～Au/CeO₂﹥Au/ZrO₂﹥Au/TiO₂ for CO oxidation and Au/CeO₂ > Au/ZrO₂ > Au/TiO₂～Au/SiO₂～Au/Al₂O₃ for water-gas shift reaction. The results from XRD and TEM confirm gold particles with about 10 nm size are dispersed on the surface. The gold catalyst with higher surface area and larger pore size is favor for CO oxidation and the reducibility and oxygen storage capacity of catalyst support are crucial for water-gas shift reaction, as shown by N2-physical adsorption, H2-TPR and XPS characterization. It has been suggested that both gold zero valence and partly oxidized gold species are with responsibility for good activity though comparative study of Au/SiO₂ and Au/CeO₂ catalysts.2. For water-gas shift reaction over Au/CeO₂ catalyst, effects of pretreatment way of the catalyst, calcination temperature of ceria support, and gold loading on the catalytic activity were systematically studied. The reaction conditions of CO flow rate and water volume are optimized. The catalyst calcined in air at 300℃for 3h and then reduced in hydrogen gas at 200℃for 2h gives higher activity. Further characterization makes us presume that the high activity results from oxygen species adsorbed and hydroxyl groups on ceria surface and the presence of gold can promote the reduction of ceria and create more oxygen species.3. Considering the surface area and mechanism intensity, CeO₂-Al₂O₃ support was prepared by impregnation method and deposition-precipitation method, respectively. It was found that CeO₂-Al₂O₃ support has larger specific surface area and pore size compared with pure CeO₂. The support prepared by impregnation method represents obvious CeO₂ characteristics and supported gold catalysts exhibits higher water-gas shift reaction activity than gold catalyst supported on CeO₂ or CeO₂-Al₂O₃ support prepared by deposition-precipitation method. The activity is also affected by calcination temperature and ceria loadings of CeO₂-Al₂O₃ support.4. Various uranium oxides were prepared through thermal decomposition of uranyl nitrate hexahydrate. It is found that there is orange yellow UO3 phase at 300℃-450℃, in bottle green U3O8 phase at 500℃-800℃, and in dark brown UO₂ phase at 800℃or higher. Its application as support of gold catalyst is first investigated. The catalytic activity in water-gas shift reaction is found to depend on the phases of uranium oxide. Gold catalyst supported on UO₃ exhibits comparative high activity while the catalyst supported on U3O8 gives almost no activity under the same conditions. More active oxygen species are found by various characterization of Au/UO3 beside reducibility and oxygen storage capacity of the uranium oxide support.5. Titania microsphere with hollow structure was prepared tentatively through interfacial reaction assistant template method. It was found that the microsphere has larger specific surface area and pore size compared with general TiO₂ support. Gold catalyst supported on the microsphere can give higher activity in water-gas shift reaction.