Controllable Photochemical Deposition Synthesis And Catalytic Applications Of Supported Metal Nanoparticles

With the development of nano-chemistry, metal nanoparticles have received more and more attention. Especially the multi-metallic nanoparticles supported on porous materials, which have great prospects of development in industrial catalysis. However, precise composition and thermal stability of metal nanoparticles are two main problems in the practical applications. Many methods have been developed to prepare metal nanoparticles, while most of them could not achieve a tighter control over the composition from particle-to-particle. Moreover, phase separation or sintering is likely to occur during the subsequent thermal treatment. In this thesis, we propose a simple and general synthesis strategy for various of supported metal nanoparticles, which based on an in situ photochemical deposition process. Within ordered mesoporous confinement space, this approach utilizes the one step simultaneous photo-deposition of multiple metal precursors, and obtain metal nanoparticles with high dispersion, high activity and high thermal stability. On one hand, the composition of single nanoparticle could be readily controlled through changing the initial concentration of metal precursors. On the other hand, homogeneous alloy nanoparticles are prepared through metal alloying process, which possess considerably small size and excellent anti-sintering property. We also investigated the catalytic applications of metal nanoparticles, such as photocatalytic hydrogen evolution, high-temperature catalytic combustion, etc. In addition, preliminary result has been obtained in the study of nano-alloy phase diagram. The main content of research are listing as follows. 1) On the basis of in situ photochemical deposition approach, Cu nanoparticles were uniformly supported on mesoporous TiO2. The relationship between the concentration of Cu precursor and the size of Cu nanoparticle were explored. We investigated the catalytic activity and reaction mechanism of Cu-TiO2 under ultraviolet light, using the photocatalytic hydrogen evolution from formaldehyde solution as the probe reaction. It suggests that low concentration of Cu precursor generates Cu particles with small size, which favors the photocatalysis and improves hydrogen production.2) Within three-dimentional cages of extra-large pore mesoporous TiO2 (EP-TiO2), we prepared Au/Pt/Pd monometallic nanoparticles using photo-deposition method. Through the control of annealing temperature, the size of Pt nanoparticles could be changed. Moreover, by regulating the concentration of metal precursors, the relationship between metal loading amount and catalytic properties of n-hexane combustion were explored. Large specific surface area and ordered confinement structure of the cage-like mesoporous pores effectively enhance the thermal stability of metal nanoparticles, and make contribution to the excellent catalytic reaction.3) According to the strategy of one step photosynthesis, a variety of metal precursors were co-deposited within mesopores of EP-TiO2, obtaining a series of supported metal nanoparticles (AuPt/AuPd/PtPd). Taking AuPt as an example, we examined the changes of size, composition and structure of nanoparticles under low-temperature annealing. AuPt particles are highly dispersed throughout the mesoporous EP-TiO2, with core-shell structure changing to mono-phase alloy structure simply through annealing in air. Due to the synergistic effect between multiple metals, AuPt alloy shows better catalytic performance in catalytic combustion reaction.4) In our further study of AuPtPd trimetallic nanoparticles, we investigated their changes of size, composition and structure during high-temperature annealing. Owing to the in situ co-deposition process, the content of each component in one single nanoparticle could be precisely controlled. More importantly, AuPtPd trimetallic system displays ultra-high thermal stability and oxidation-resistant property. With respect to other Pd-containing monometallic or bimetallic nanoparticles, trimetallic nanoparticles possess homogeneous alloy structure during high temperature (700-900 ℃) annealing, as well as a high proportion of metallic Pd. The composition of trimetallic-nanoparticles and confinement space of mesoporous support play key roles for the obove properties. In addition, AuPtPd catalyst showed highly stable catalytic performance in n-hexane combustion.5) Taking AuPtPd system as an example, we demonstrated the high temperature nano-alloy phase diagram of trimetallic materials. From the point of experimental data, the transition process of trimetallic nanoparticles after 800 ℃ annealing was revealed. For AuPtPd nanoparticles with different composition, the peak position and type of XRD patterns are different, as well as element distribution from EDS results. Based on the above results, we drew the immiscible area isotherm of AuPtPd nano-material at 800 ℃ for the first time. On the nano scale, the alloy phase diagram is not only related with the size effect, but also affected by the composition effect. Different composition of samples in the phase diagram show different catalytic performance of n-hexane combustion, which is a preliminary study of the guidance of nano phase diagram to the catalysis.