| Supported gold catalysts have been extensively studied and developed as a new kind of catalytic material since the late 1980s, and have found applications in air purification, as sensor materials and in related areas in view of their excellent activities towards CO oxidation. Further prospects for extensive application of these materials have emerged as the research work has continued. In recent years, new methods for preparation of catalysts with low loadings, high activity and high stability, techniques for shaping the catalysts and research into the structure of the active phase in gold catalysts and the reaction mechanism have continued to be widely studied.Supported gold catalysts were prepared by co-precipitation (CP), organo-metal-complex grafting (OMCG), deposition-precipitation (DP) and photochemical deposition (FD) methods and characterized by BET, AAS, TEM, XRD, DTA-TG, TPR and XPS. The catalytic performance for low-temperature CO oxidation over these catalysts was evaluated and the effects of preparation methods and conditions on the structure and catalytic activities of the catalysts were studied in detail. The parameters governing the preparation of gold catalysts with low loadings, high activity and high stability were probed and the preparation conditions were optimized. The main conclusions are as follows:1. The best results were obtained for 3wt% Au/Fe2O3N Au/NiO and Au/Co3O4 prepared by CP and OMCG which exhibit very high catalytic activities for CO oxidation and can completely oxidize lvol%CO in air to CO2 at -23℃~ -22℃ and -5℃ respectively; 3wt%Au/TiO2 and lwt% Au/TiO2/shaped-oxide prepared by DP and sol-gel processing can transform lvol%CO in air into CO2 at -35 ℃ and -16℃ respectively. Gold catalysts prepared by FD show very poor catalytic activities for CO oxidation under the test conditions.2. The optimum preparation conditions are as follows:(1) CP method: The highest catalytic activities with Au/NiO and Au/Co3O4 canbe obtained by calcination at 3 00 ? and 200-3 00 ? respectively. Precipitation by addition of base to the mixture of gold and nickel or cobalt salts was found to give catalysts with higher activities than those obtained by addition of the salt mixture to the base. Compared with NiO, both Fe2O3 and Co3O4 were more effective supports. Chloride ions play the role of catalyst poison and must be removed.(2) OMCP method: Au(PPh3)NO3 was found to be the most suitable precursor amongst the organo-gold-complexes tested. The catalysts prepared by using K2CO3 as precipitant are more active than those obtained by using Na2CO3.(3) DP method: To reach a good catalytic activity for Au/TiO2 prepared by DP , the pH value should be controlled in the range of 6.5-7.0, 40ml water /(g support) added to the suspension, the temperature of the liquid maintained at 70 ? during the period of deposition-precipitation reaction, and the sample reduced finally at 85 ? for 60min.3. Compared with the OMCG method, the CP method gives a similar catalytic activity for low loadings of gold and higher activity for higher gold loadings.4. The shaped catalyst Au/TiO2/shaped-oxide prepared by sol-gel processing and DP is a promising candidate with high activity, and shows an excellent resistance to sulfur.5. Among the catalysts tested, Au/Fe2O3 and Au/TiO2 were shown to exhibit the best resistance to water vapor.6. Based on structural characterization, it is concluded that the effective utilization of gold with the catalysts prepared by CP and OMCG amounts to 75-88%. Combined with the previous results obtained in our laboratory, it can be concluded that the size of gold particles as active centers highly dispersed on the surface of a support is in the range of ten nanometers, with a certain fraction having a positive oxidation state, while the support shows a poorly crystalline nature. The temperature at which the supported catalysts form a suitable structure coincides with the optimum temperature at which the catalysts are pretreated. |
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