Study On The Synthesis And Performance Of Au-Based Nanocomposites

Au nanoparticles have received considerable attention due to their applications in diverse fields such as catalysis, sensor, biomedicine and so on. However, the unstability of the Au nanoparticles caused by large surface free energy leads to agglomeration and the decrease of activity. Core-shell nanoparticle is a new-type nanomaterials. Because of the protection of the shell, the intrinsic dispersity and activity of the Au core can be maintained. The structure of the core-shell nanoparticles can be tuned by varying the amount of the reactant, and the strong metal-support interaction (SMSI) between the core and the shell is beneficial to the improvement of their activity. Currently, the synthesis method of the core-shell nanoparticles is deficiency and the investigation of the relationship of structure-property still remains challenging. The performance of the core-shell nanocatalysts differ from their corresponding supported catalysts due to its unique structure and properties. Catalytic activity of core-shell nanomaterials is related to the size, pore structure, specific area, contact area between noble metal and support and so on. Synthesis of core-shell nanomaterials with high activity and revealing the catalytic mechanism are significant. This thesis focuses on designing Au-based nanocatalysts and gas sensor with high activity and stability by the guidance of the performance, and developing different method to prepare core-shell nanoparticles with multi-component and controllable structure. The composition, structure and morphology were regulated in order to investigate the properties in catalytic and sensing properties. The relationship of preparation, micro-structure, composition and performance was investigated, and the catalysis mechanism was explained on atomic level. The main contents were summarized as follows:1. Preparation and catalytic performance for the reduction of 4-nitrophenol of porous Au@SiO2 core-shell nanoparticles. Porous Au@SiO2 (Au@pSiO2) was prepared using a typical Stober method combined with hydrothermal etching. The effects of the hydrothermal time, hydrothermal temperature, surfactant polyvinylpyrrolidone (PVP) and Au species on the pore structure and activity for 4-nitrophenol were studied. It is indicated that the Au@pSiO2 possessed high sinter-resistant performance. Cationic Au species was favorable to the activity, and the existence of PVP led to the decrease of the activity. With the increasing of the annealing temperature, the amount of cationic Au species was decreased gradually, and PVP was removed gradually from the surface of the Au. Because of the combined effect of PVP and cationic Au species on catalytic performance, the order of activity for the Au@SiO2 is as follows:Au@pSiO2 annealed at 500 ℃>Au@SiO2 annealed at 350℃> Au@pSiO2 annealed at 700℃> Au@pSiO2 without annealing. The reaction rate of Au@pSiO2 annealed at 500℃ is 14*10-3 s-1 μmolAu-1. In addition, induction period related to the existence PVP. For the Au@pSiO2 without annealing, PVP blocked the diffusion and migration of 4-nitrophenol, resulting in the appearance of induction period. The removal of PVP for the Au@pSiO2 annealed at high temperature led to the disappearance of induction period.2. Preparation and catalytic performance of flower-like Au@NiSiO yolk-shell nanoparticles for the reduction of 4-nitrophenol. Flower-like Au@NiSiO yolk-shell nanoparticles were prepared by using SiO2 template combining hydrothermal etching. Taking Au@NiSiO as an example, the effects of reagent amounts on the morphology and catalytic performance were studied. The concentrations of reagents affect the thickness and cavity volume, and the size of the Au@NiSiO depends on the concentration of tetraethoxysilane (TEOS). The main factor to catalytic property is the flower-like structure. The denser flower promoted the adsorption of 4-nitrophenol, resulting in the enhancement of the activity. The catalysts had the better activities for Au@NiSiO prepared by using 50 and 100 μL TEOS when the amount of NiCl2 (HMTA) is 0.1 mmol. The reaction rates were 47× 10-3 and 29×10-3 s-1 μmolAu-1, k1 values are 0.358 and 0.245 min-1, and induction periods are<3 min and o min, respectively. The time of completely conversion of 4-nitrophenol is 13 min. Induction period related to the amount of Au and flower-like structure. Larger amount of Au and denser flower-like NiSiO resulted in the shorter Induction period. Due to the protection of NiSiO shell, Au@NiSiO processed good stability, and no deactivation was observed after used for 6 cycles.3. Preparation and catalytic performance for CO oxidation of different Au@CeO2 core-shell nanoparticles. Hollow CeO2 nanomaterials (H-CeO2), Au@CeO2 yolk-shell nanomaterials (Y-Au@CeO2) and Au@CeO2 core-shell nanomaterials with decoration of Au on the inner surface of the CeO2 (E-Au@CeO2) were prepared by using SiO2 template. The effect of pretreatment atmosphere on the catalytic activity for CO oxidation was investigated, and the catalytic mechanism was explained in atom level. The relationship of Au species, Ce species, AuCe alloy, SMSI between Au and CeO2 and catalytic performance was analyzed deeply. It is found that the main factors to the catalytic activity of H-CeO2 are adsorbed oxygen and oxygen vacancy. However, Au species and SMSI between Au and CeO2 play an important role in the improvement of the activity. The annealing in O2 facilitated the formation of cationic Au species, which improved the activity. The annealing in H2 may caused the formation of AuCe alloy, which lowered the activity. Because of the smaller size of Au and larger contact area between Au and CeO2, SMSI for E-Au@CeO2 annealed in O2 and H2 is stronger than that of Y-Au@CeO2. The order of activity for the Au@CeO2 is as follows:E-Au@CeO2 annealed in O2>Y-Au@CeO2 annealed in O2> Y-Au@CeO2 annealed in H2> E-Au@CeO2 annealed in H2. For E-Au@CeO2 annealed in O2, T90 is 74℃, the reaction rate 1.06 molco h-1 g-1Au. In addition, Y-Au@CeO2 和 E-Au@CeO2 had good stability, and no obvious deactivation were observed even after 60 h test.4. Rod-like In(OH)3 and Au/In2O3 were prepared by using bathoiling method with In(NO3)3 and HMTA, and the preparation method had many advantages, such as facile operation procedure, environmental-friendly, energy saving and so on. The relationship among preparation parameter, microstructure and gas-sensing properties was build. Experimental results indicated that the response of In2O3 to CO was very low, but the addition of Au improved the response of In2O3 greatly because of the increasing of the amount of adsorbed oxygen and the thickness of electron depletion layer and the activation of Au for CO at room temperature. Due to the collapse of the rod structure and the growth of the Au, the response of Au/In2O3 decreased with the increasing of the annealing temperature, and the Au/In2O3 annealed at 300℃ had the best gas sensing performance. The response of Au/In2O3 annealed at 300℃ to 100 ppm CO at room temperature is 9, and the response/recovery time is about 30 s.