單一粒徑分佈二氧化矽膠體和銀-二氧化矽核-殼結構奈米粒子的製備及應用

Preparation and applications of mono-sized distribution silica colloids and Ag-SiO2 core-shell structural nanoparticles

陳貞志(Chen-Chih Chen)

指導教授 ： 周更生

國立清華大學/工學院/化學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00433

摘要

本研究成功製備出單一分佈的二氧化矽膠體和銀-二氧化矽核-殼結構的奈米粒子。藉由無機二氧化矽耐高溫的特性，銀-二氧化矽核-殼結構的奈米粒子可應用於須經過高溫加工程序的抗菌領域和甲醇部份氧化成甲醛的觸媒催化反應上。
在單一分佈二氧化矽微粒的製備上，藉由一系列批次程序建立了次微米二氧化矽之粒徑及粒徑分佈的製程視窗，並整理得一經驗式。混合兩種尺寸的二氧化矽種子成長實驗，證明二氧化矽顆粒各自獨立成長與顆粒之尺寸大小無關。而粒徑分佈自我窄化之現象乃是由於整體粒徑變大所造成的。在種子成長製程中為了避免新的成核產生而破壞原本的單一分佈，當使用的種子尺寸較大時則所需之溶液中種子總表面積也較大。種子成長過程中，新成核的發生是由於反應中間物還沒接觸到種子的表面之前，反應中間物已經互相碰撞形成新的成核點。而新成核的發生與否則是決定於溶液中種子與種子之間的距離，與種子尺寸大小無關。
在製備銀-二氧化矽核-殼結構奈米粒子上。利用溶膠-凝膠法，二氧化矽可均勻被覆在奈米銀顆粒的表面。TEM結果顯示二氧化矽殼層厚度均一並不會受到奈米銀顆粒尺寸大小的影響。而二氧化矽殼層的厚度則會影響奈米銀的表面電漿共振波長。此外結構分析顯示聚乙烯吡咯酮高分子(PVP)不僅包覆在奈米銀的表面，同時也分散到二氧化矽殼層裡面。因此經過高溫熱處理將PVP燒除後，可獲得多孔的二氧化矽殼層。高分子PVP的含量與銀-二氧化矽核-殼奈米粒子的比表面積之間的關係也成功地建立。在熱穩定性方面，二氧化矽殼層可維持銀-二氧化矽核-殼奈米粒子的整體外觀形狀高達1000℃的高溫，即使二氧化矽的殼層厚度只有25奈米的厚度。
奈米銀已知為一廣效型抗菌劑。在二氧化矽殼層的保護下，銀-二氧化矽核-殼奈米粒子可作為須經過高溫加工程序之產品的抗菌添加劑。抗菌實驗顯示對於革蘭氏陰性菌(大腸桿菌)和革蘭氏陽性菌(金黃色葡萄球菌)都具有抗菌效果。
此外銀-二氧化矽核-殼結構奈米粒子也可作為觸媒使用，應用在甲醇部份氧化成甲醛的觸媒催化反應上。催化反應是部份氧化反應(CH3OH + 1/2 O2 → HCHO + H2O)與直接脫氫反應(CH3OH → HCHO + H2)同時進行。在無氧環境下觸媒可催化甲醇進行直接脫氫反應。反應溫度升高則可增加甲醇轉化率，同時提高甲醛選擇率並降低二氧化碳選擇率。然而太高的反應溫度或太厚的二氧化矽殼層會面臨到質傳限制，導致甲醛進一步分解成一氧化碳和氫氣。降低氧氣濃度則可提高甲醛的選擇率，但同時也降低了甲醇的轉化率。在最佳的反應條件下，甲醛產率可達91%。與工業上的銀觸媒比較，反應溫度可下降約100℃，具有節省反應器材料成本的優點。

關鍵字

銀 ； 二氧化矽 ； 核－殼結構

並列摘要

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.

並列關鍵字

silver ； silica ； core-shell structure

參考文獻

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