中孔洞鈷觸媒應用於產氫及單壁奈米碳管生成

Abstract

本篇論文利用各種覆載於中孔洞二氧化矽材料MCM-41的鈷觸媒進行催化反應之探討，共分成兩章，第一章是硼烷氨錯合物(ammonia-borane, NH3BH3)的水解產氫反應研究，具有酸性的中孔洞二氧化矽作為載體之鈷催化劑在水解反應前與還原劑作用容易形成鈷硼化物，對於原本在空氣中容易氧化的鈷金屬來說形成了一層保護，而有利於產氫反應的進行，並藉由X光吸收模擬三種合成方式的鈷催化劑，得到載體酸性對於還原程度具有影響，也證明了此材料在NH3BH3的水解產氫反應中具有較好催化活性的原因。 第二章是單壁奈米碳管(single-walled carbon nanotube, SWCNTs)的生成反應，分別使用表面修飾法、共沉澱法及離子交換法將鈷覆載於MCM-41，並比較了在MCM-41中加入beta-zeolite晶種以提高載體的結構熱穩定性對於SWCNTs純度的影響。藉由調控觸媒的金屬含量、單壁奈米碳管的生長時間、預處理條件、等等，以生成高純度及高產量的SWCNTs。在純化後的產物中，利用光激發螢光儀測量SWCNT的旋光性(chirality)。對於不同製備方式的鈷觸媒，也利用X光吸收光譜進行原位分析，於不同氫氣還原溫度下的結果進行模擬，得到其配位數及價態對於生成單壁奈米碳管的影響。

In Chapter 1, mesoporous silica supported cobalt boride (Co-B) catalysts are rationally designed for hydrogen generation in ammonia-borane hydrolysis reactions under ambient conditions. Cobalt boride catalysts are supported on three different mesoporous silica, including beta-zeolite seeded MCM-41 (Co@M41S) and traditional MCM-41 (Co@M41T) via chemical adsorption onto functionalized surface with 3-trihydroxysilylpropylmethylphosphonate (THPMP), and one-step co-precipitation into mesoporous silica framework (Co@M41C). Our preparation strategies provide two insights to the reactions: first, cobalt oxide species are intrinsically deposited as ultra-small nanoparticles (<2 nm) on mesoporous silica supports; subsequently the nanoparticles are converted to active Co-B catalysts by reduction with sodium borohydride (SB). Three catalysts exhibit significant differences in catalytic reactivities with hydrogen production rates ranked in an order of Co@M41S > Co@M41T > Co@M41C. Detailed analysis of the coordination environments from in situ X-ray absorption spectroscopy (XAS) results confirm reducibility in SB. Amorphous nature of Co-B catalysts are responsible for efficient catalytic activity in Co@M41S and Co@M41T. Ammonia temperature programmed desorption (NH3-TPD) demonstrates support acidity that correlates to the degree of high dispersity and effective reducibility to Co-B. Effects from catalyst sizes, reducibility in SB treatment and surface acidity are studied in detail to compare catalytic reactivities among three types of supports. In Chapter 2, mesoporous silica-supported cobalt catalysts have been developed by ion-exchanged and co-precipitation method for synthesis of single-walled carbon nanotubes (SWCNTs). In this work, we use CO disproportion reaction at high pressure for production of SWCNTs. The influence of different synthesis conditions and reaction temperature have been investigated with respect to the yield and diameter uniformity of SWCNTs. MCM-41 was used as support. All catalysts were characterized by powder X-ray diffraction (XRD), and N2 adsorption–desorption isotherm, which shown well-ordered nanochannels and high surface area of the silica mesoporous structures. The metal loading amounts were determined by inductively coupled plasma-mass spectroscopy (ICP-MS). SWCNTs were characterized by Raman spectroscopy and transmission electron microscopy (TEM). In Raman spectra, the ratio between signal intensities of G and D-bands (G/D) was used as a purity index to qualitatively estimate the graphitization and the amount of defective carbon in samples. In this study, the effect of metal loading amount to SWCNT have been studied, and the results shown that increasing the metal loading to about 1 wt% could improve G/D values. In early studies, researchers found that cobalt catalysts could synthesize smaller diameter size of SWCNTs than nickel catalysts. However, it seems to have a wider distribution of diameters and lower SWCNTs selectivity. In order to increase the purity of SWCNTs, four kinds of catalyst preparation strategies have been developed for SWCNTs synthesis. We varied the cobalt loading amounts, growth temperature, H2 prereduction temperature, CO duration time and purified the synthesized SWCNTs. A high selective SWCNTs chirality took from our synthesis process.