Preparation And Catalytic Performances Of Supported Gold Catalysts In CO Preferential Oxidation

The proton exchange membrane fuel cell (PEMFC) uses hydrogen as a fuel, and hydrogen is usually produced by autothermal reforming of hydrocarbons. However, the trace levels of CO (about 10 ppm) in the reformed H2-rich stream will severely poison Pt-based electrode and cause a deactivation of electrocatalyst. Therefore, selective and efficient elimination of CO while minimizing the H2 consumption processes in H2-rich stream is highly desirable. At present, preferential oxidation of CO (CO-PROX) has been seen as one of the most simple and effective approaches, and gold nanoparticles show very promising low-temperature catalytic activity. To our knowledge, how to controllably develop one suitable Au catalyst for improved catalytic performance under the operating temperature of the PEMFC (80-120?), especially their catalytic stability, remains a big challenge.In this thesis, a series of Au/Al2O3 catalyst with greatly enhanced low-temperature CO oxidation activity and stability have been synthesized. On the basis of Au/Al2O3 catalyst, Au-Cu/Al2O3, Au/CeO2/Al2O3, and Au/MnO2/Al2O3 catalysts were prepared and those catalysts showed much higher activity for CO-PROX. Furthermore, the influence of preparation conditions on the catalyst structure and surface chemical property as well as the CO oxidation activity were systematically studied. Meanwhile, the relationship between structure of Au catalyst and their catalytic performances have been fundamentally investigated.(1) Alumina support was prepared by a hydrothermal method and the corresponding Ai/Al2O3 catalyst was prepared by the deposition precipitation method. The results showed that the activity of CO oxidation strongly depended on the pretreatment atmosphere. A 100% CO conversion at 60? can be reached and was stable in the co-present of H2O and CO2. It was found that the H2-pretreated catalyst greatly enhanced the CO adsorption capacity and facilitated the oxygen activation.(2) A new type of Au-Cu/Al2O3 bimetallic catalyst was synthesized by taking the advantages of CO/O2 adsorption and activation on Cu. It was found that the uniform Au-Cu particles basically maintained the highly dispersed with a smaller size and the average metal particle size of the Au-Cu/Al2O3 catalyst was approximately 1.8±0.2nm. Compared with Au/Al2O3 catalyst, the as-prepared Ai-Cu/Al2O3 catalyst has higher catalytic activity for CO-PROX that are able to reach a 100% CO conversion in the temperature range of 50-100 ?. In order to further enhance the catalytic selectivity, the tiny amount of potassium was added to the Au-Cu/Al2O3 catalyst. The results showed that 2 wt.% of K doping can effectively improve the CO conversion and selectivity. A 100% CO conversion at 60-110? and 60% selectivity of CO2 at 80? can be obtained. In situ DRIFTS have demonstrated that the introduction of potassium could strengthen the CO adsorption on active Cu+ species at higher temperature which improved the catalytic activity and selectivity for the CO-PROX reaction.(3) Considering the effect of the strong metal-support interaction (SMSI), a series of Au/CeO2/Al2O3 and Au-Cu/CeO2/Al2O3 catalysts were synthesized. The influences of the amount of CeO2 on the SMSI and reducibility as well as catalytic performance for CO-PROX were investigated. The Raman and H2-TPR results indicated that the improved catalytic performance of the Au catalysts can be ascribed to the enhanced SMSI and reducibility. Moreover, the formation of oxygen vacancy on the catalysts can increased the activity of CO-PROX reaction. These catalysts exhibited a 100% CO conversion in the temperature range of 30-70? which are broader than Au/Al2O3 catalyst.(4) The MnO2-AlO3 composite support was prepared by a redox method. One novel Au/MnO2/Al2O3 catalyst with highly dispersed Au nanoparticles was synthesized on this support. The obtained results showed that the structure and physical property of Au/MnO2/Al2O3 catalyst have been little affected by manganese doped. It was found that the gold nanoparticles were highly dispersed with narrow size distribution and the average metal particle size of the as-prepared catalyst was about 4.8 nm. The Au/MnO2/Al2O3 catalyst is not only highly active (100%) and selective (80%) for the CO-PROX reaction in the temperature range of 80-120?, but also shows stable catalytic performance with 97% conversion of CO and 55% selectivity of CO2 in the co-presence of H2O and CO2 at 80? after run 40 h. This is the most impressive result reported in literatures. The characterization results indicated that highly activity can be ascribed to the valence changes of gold, which suggested that the introduction of manganese could greatly enhance the Au-support interaction and reducibility of Au/MnO2/Al2O3 catalyst.