Thermal Stability Of Gold Nanoparticles Based On Confinement Effect And Its Application For Selective Oxidation Of Alcohols

Gold nanoparticles, typically with a size of1～10nm, spreading over the internal surface of porous materials have great promise for industrial catalysis because of their high catalytic activities in hydrocarbon reactions. However, sintering (growth of MNPs that lowers surface metal atoms and metal-to-support interfaces) and coking (deposition of carbon species on MNPs (Metal Nanoparticles) that blocks their surface active sites) are two major causes that will lead to the deactivation of MNP catalysts under realistic process conditions. In this thesis, we propose a new strategy that multiple AuNPs can be encapsulated in every extra-large mesopore, leading to a successful sintering-resistant AuNPs/mesoporous silica system. We can also get a sintering-and coking-resistant catalyst system through adjusting the meso-structure and the finely controlled AuNP loading concentration.1) AuNPs supported on different silica carriers were prepared by colloid deposition in non-aqueous solution. By selecting ordered mesoporous SiO2support with different pore size and controlling the calcination temperatures from723to1073K, a series of different-sized gold nanoparticles (i.e.3.5nm) were obtained.2) We demonstrated a new sintering-and coking-resistant catalyst system using mesoporous EP-FDU-12as a suitable support with finely controlled AuNP loading concentration. Meanwhile, this new catalyst system exhibits dramatically increased lifetime (>500h) during gas-phase, cyclohexanol selective aerobic oxidation than those located within relatively small mesopores, most likely due to much less coke formation (<4wt%).3) We carefully explored the sintering resistant mechanism. It is demonstrated that the unique three-dimensional porous structure of EP-FDU-12makes particle-migration difficult to occur and thus prevents direct particle-particle aggregation. This further allows two or more AuNPs to be encapsulated in every extra-large cage at high particle concentrations (10～35wt%), which enables inter-particle interactions via significant overlapping of the diffusion-spheres of AuNPs. As a result, atom-migration via vapour from cage to cage is largely shut off and local vapour-particle equilibrium within each cage is possible, leading to a successful stable AuNPs/mesoporous silica system.4) We investigated the effect of particle size on the aerobic gas-phase oxidation of cyclohexanol to cyclohexanone. The meso-structure of ordered mesoporous SiO2support and calcination temperature of catalyst is employed to vary the size of the gold nanoparticles in the range of3.3-14nm. The gold catalysts show promising activity at relatively low reaction temperature (180～250℃), and they are all highly selective for cyclohexanol-to-cyclohexanone transformations (selectivity>99.5%).Importantly, the optimal size for gold nano-catalyst loaded on large-pore mesoporous silica is in the range of5～6nm, smaller or larger will both lead to worse catalytic activity. In addition,gold nanoparticles within the extra-large pores exhibit unexpected long lifetimes without evident activity decrease after more than550h reaction.