Synthesis And Catalytic Properties Of Ceria Supported Nano-Gold Catalyst

Au NPs have attracted considerable attention in catalysis. Especially, supported Au catalysts which the gold nanoparticles are supported on the metallic oxide, have been applied to catalyze many different types of reactions. Such as hydrocarbon combustion, methanol synthesis, water gas shift reaction and so oa Among the several active oxides that have been studied, CeO2 has been received increasing attention. It is also has been recognized as a prevalent supporting material for catalysis at supported Au nanoparticles. Au-CeO2 systems have drawn continuous attention because of its high oxygen storage capacity, abundant oxygen vacancy defects between the III and IV oxidation states, giving rise to the enhanced rates of the reaction Gold catalysts with various morphologies support will have different active. At the same time, due to the high surface energy, nanosized gold particles are relatively unstable and tend to rapidly aggregate into larger clusters during the processes of calcination treatment and catalytic reaction, which results in the rapid deactivation. In order to synthesize new topography catalytic support and stable gold particles, we prepared the special morphology of ceria as gold carrier, and then build core-shell structure to maintain gold particles size. The main conclusions are described as follows:1. Uniform ceria hollow nanospheres composed of ceria nanocrystals have been synthesized via a simple one-step hydrothermal method without using any template. It was found that the obtained porous CeO2 hollow nanospheres were morphologically uniform, with an average diameter of 210 nm and high specific surface area of 167 m2/g. According to the basis of a time-dependent experiment, a self-assembly process coupled with an Ostwald ripening mechanism was proposed to explain the evolution of CeO2 hollow nanospheres. Afterwards, these hollow materials are used as support to prepare the Au/CeO2 catalyst for the reduction of 4-nitrophenol (4-NP). The synthesized hollow Au/CeO2 nanospheres catalyst exhibited significantly enhanced catalytic activity in comparison with the commercial CeO2 powder supported sample. In addition, the results of cyclic stability of the catalyst indicated that similar catalytic performance without visible reduction could be found after 7 repeated cycles. As for this catalyst system, the unique porosity structures and high surface area were responsible for the improved reaction properties.2. To development of Au nanocatalyst possessing high reaction stability and anti-sinter capacity, we designed a structure of hollow CeO2/Au@mSiO2 core-shell hierarchical structure with decoration amount of Au nanoparpticles (Au NPs) from sintering and detaching by a simple way. Porous hollow CeO2 spheres were selected as cores for they have high specific surface area and unique porosity structures. Besides, mesoporous SiO2 has been chosen as shell to prevent the coalescence of Au nanoparticles and it can provide channels for chemical species to reach the surface of the Au-CeO2 hybrid. Every core-shell particle has a large hole in the center, which favors the transfer reaction mass. The results of reduction of 4-nitrophenol (4-NP) indicates that the synthesized hollow hierarchical catalysts exhibit superior catalytic performance than traditional core-shell structure and can be easily recycled without a decrease of the catalytic activities in the reaction. Moreover, the obtained hollow spheres show a superior thermal stability, as it resists both Au and ceria sintering during the calcinations progress even at 750℃. Comparing with this, the samples prepared by deposited Au nanoparticles on CeO2 sphere without the protection of silica were found to sinter severely although calcinated at lower temperatures.3. Uniform hierarchical SiO2/Au/CeO2 rod-like nanostructures were fabricated by combining three individual synthesis steps. SiO2/Au/CeO2 was possessed integral core shell structure including encapsulated Au NPs as core and mesoporous CeO2 as shell. Meantime, the inner silica plays an important role on the morphology control and improvement of the catalyst mechanical strength. The obtained nanocapsules were characterized by several techniques, including XRD, BET, TEM and so oaThe sample shows unique features such as uniform rod-like morphology, well dispersed of Au NPs, and large specific surface area. The results of reaction performance indicate that the synthesized SiO2/Au/CeO2 catalysts exhibit significantly enhanced catalytic activity. Moreover, the catalytic activity of our as-prepared nanocomposite catalysts well maintained even after 8 repeated cycles.