CO Oxidation And Low Temperature Water-gas Shift Reaction Over Supported Gold-base Catalysts

The supported gold-base catalysts are a kind of new materials developed recently and may be used widely in chemical industry, environmental protection and automobile industry because of their special property. Particularly, the supported gold-base catalysts exhibit excellent catalytic performance for CO oxidation and low temperature water-gas shift reaction. However, further work is still required in order to improve the catalytic performance of the catalysts, and to study the influence of catalyst structure, type of supports, reaction conditions, interaction between gold and supports etc on the catalytic property, and to clarify the nature of active sites for understanding the reaction mechanism. In this work, a series of supported gold-base catalysts were prepared by different methods, and the catalytic performance for CO oxidation and low temperature water-gas shift reaction was investigated for the purpose to search the new gold-base catalysts system, and to study the influence of various factors on the catalytic performance of CO oxidation and low temperature water-gas shift reaction. The physical and chemical natures of catalysts and the reaction mechanism were also discussed. The main experimental results and conclusions are as follows: 1. Catalytic performance of CO oxidation over novel gold-base catalysts supported on carbonate The alkaline-earth metal carbonate supported gold catalysts Au/MCO3 (M=Ca, Sr, Ba) were prepared by co-precipitation method and the catalytic performance of CO oxidation was investigated. The type of support affects the catalytic performance of these supported gold-base catalysts. The order of activity is Au/BaCO3>AuSrCO3>Au/CaCO3, the BaCO3 is the best effective support. The detailed investigation on the Au/BaCO3 catalyst shows that the calcination temperature and gold loading have considerable effect on the catalytic performance of CO oxidation reaction. The suitable calcination temperature is 200 and 300 oC for the catalysts with 1-4% and 8% gold loading, respectively. The 8%Au/BaCO3 catalyst calcined at 300 oC shows the highest stability for CO oxidation at ambient temperature and humidity, which can keep the 100% CO conversion for about 5500 min. Combined with the characterization results, we proposed that the presence of an optimal ratio of ionic to metallic gold as well suitable gold-support interaction might be responsible for the high activity of Au/BaCO3 catalyst calcined at the suitable temperature in CO oxidation. Generally, the catalytic performance of CO oxidation over all of the carbonate supported gold-base catalysts we investigated is higher in the presence of water inthe feed stream than that of absence of water. This indicate that the addition of water vapor in feed stream has positive influence on the activity of Au/MCO3 catalysts for CO oxidation, which might due to that water participate the activation of oxygen directly and then considerably enhances the reaction activity of Au/MCO3 catalysts. 2. Catalytic performance of CO oxidation over gold-base catalysts supported on metal oxide A series of Au/ZnO-Fe2O3 and Au-Pt/ZnO catalysts were prepared by different methods and the catalytic performance of CO oxidation was investigated. The variety of Zn/Fe ratio has great influence on the structure and redox property of the Au/ZnO-Fe2O3 catalysts, hence changing the stability of catalysts. The addition of platinum species into gold-base catalyst results in the decrease of the activity for CO oxidation. The influence of co-existed gases on the catalytic property is also studied. It is found that the existence of ethanol, benzene, SO2, H2S and CO2 lead to the decrease of the catalytic activity of Au/ZnO catalyst for CO oxidation. Similar results can also be observed in Au/Fe2O3 catalyst, the only difference is that the H2S has no effect on the performance of Au/Fe2O3. Combined with the related references, we suppose that the active sites for CO oxidation over these two catalysts are different. 3.Catalytic performance of low temperature water-gas shift reaction over Au/Fe2O3 catalystsThe influence of preparation method and gold loading on the catalytic performance of Au/Fe2O3 catalyst for low temperature water-gas shift reaction was investigated. The Au/Fe2O3 catalysts prepared by deposition-precipitation method exhibit relatively high activity and good stability and the catalytic activities increase with the gold loading. The presence of a great of surface hydroxyl groups and the large surface area has beneficial effect on the catalytic activity for low temperature WGS reaction. The catalysts pretreated at low temperature (140 oC) in hydrogen exhibit higher catalytic performance, while higher temperature (250 oC) lead to the activity decrease, which due to the aggregation of Au particles caused by the disruption of the interaction between gold and support. The high activity of Au/Fe2O3 catalysts for low temperature water-gas shift reaction is related to the high dispersion of gold species. The results of H2-TPR show that the presence of small gold species can improve the reducibility of iron oxide. The existence of strong interaction between gold species and the support could accelerate the redox process of Au/Fe2O3 catalysts, thus resulting in the improvement of the activity and stability of low temperature water-gas shift. 4. Catalytic performance of low temperature water-gas shift reaction over Au/ZnO and Au/ZnO-CuO catalysts The effects of preparation methods and conditions on the catalytic performance of Au/ZnO catalysts were investigated for the low temperature water-gas shift reaction. It was found that the catalytic performances of Au/ZnO catalysts could be considerably influenced by various factors, such as preparation...