Abstract
In this study, the monodispersed silica colloids and silver-silica core-shell structural nanoparticles have been successfully prepared. In particular, the high temperature resistant nature of inorganic silica shell enables the silver-silica core-shell nanoparticles applicable to antibacterial field where high temperature treatment procedure is required and to formaldehyde synthesis from partial oxidation of methanol as catalyst. In the preparation of monodispersed silica colloids, the process windows of submicron silica colloids have been established and were expressed as an empirical equation. The mixed silica seeded growth experiments with two particle sizes show that silica seeds grow independently and irrelevant to seed size. The self-sharpening effect of particle size distribution is resulted from the increasing of average diameter. In the seeded growth process, in order to avoid the new nucleation, which would destroy the original monodispersity, when larger silica seeds were used, it requires large total seed surface area in the solution. During the seed growth, new nucleation occurs via aggregation of reaction intermediates to form nuclei before these intermediates diffuse to the surface of seeds. New nucleation is determined by the distance between seeds and irrelevant to seed size. In the preparation of silver-silica core-shell structural nanoparticles (Ag@SiO2), using sol-gel method, silica can coat uniformly on the surface of silver nanoparticles. TEM results indicate that the thickness of silica shell was uniform and not affected by the particle size of silver nanoparticles. The wavelength of surface plasomn resonance of silver nanoparticles was influenced by silica shell thickness. Moreover, structural analysis indicate that polyvinyl pyrrolidone (PVP) polymer was not only surrounding on silver surface but also dispersed into silica shell. As a result, when PVP is burned off during calcination, a corresponding porous structure will be obtained. The correlation between the specific surface area of Ag@SiO2 particles and PVP quantity in the original silver colloids was established. The silica shell, even at a thickness of 25nm, can maintain the original shape of Ag@SiO2 particles up to 1000℃. Nano silver particles were well known as an antibacterial agent. Under the protection of silica shell, Ag@SiO2 particles can be utilized as antibacterial agent for high temperature process without worrying about sintering effect. The antibacterial tests exhibited antibacterial efficiency against both Gram-negative bacterium E. coli and Gram-positive bacterium S. aureus. Furthermore, Ag@SiO2 particles can also be utilized as catalyst for formaldehyde synthesis from partial oxidation of methanol. Both partial oxidation reaction (CH3OH + 1/2 O2 → HCHO + H2O) and direct dehydrogenation reaction (CH3OH → HCHO + H2) take place simultaneously. In the oxygen-free condition, Ag@SiO2 catalyst can catalyse the direct dehydrogenation of methanol. Increasing reaction temperature could increase the methanol conversion and formaldehyde selectivity as well as decrease carbon dioxide selectivity. However, higher reaction temperature or thicker silica shell would exhibit mass transfer limitation leading to formaldehyde decomposition to carbon monoxide and hydrogen. Decreasing oxygen concentration can increase formaldehyde selectivity, but also decrease methanol conversion. At the optimal reaction condition, the yield of formaldehyde can reach 91%. Compared with current industrial silver catalyst, the reaction temperature can decrease by about 100℃, which is very beneficial to reactor cost saving.