多型態銠金屬奈米晶體之合成與其在二氧化碳氫化上之應用

Abstract

銠金屬奈米晶體因其良好的活性使其在催化反應的研究上備受矚目。在本論文中，我們描述如何利用一步一鍋化的水相合成法，藉由調控反應劑的使用量，成功合成了具有不同表面晶面的銠金屬奈米晶體，並將其應用至二氧化碳氫化反應上，探討生成甲烷的選擇性與晶體結構之關係。我們探討的銠金屬奈米結構包含凹面、鑿面與雙晶相奈米晶體，其中凹面晶體為具有高均一性的四面體結構。此三種形態可藉由在反應條件中，調整界面活性劑—溴化十六烷基三甲铵的濃度高低而互相轉變。以凹面四面體的條件為核心，溴化十六烷基三甲铵濃度較低時可形成鑿面奈米晶體，然而較高時可得到具雙晶相的奈米晶體。此三種結構經由電子顯微鏡、X光電子能譜儀等分析可被證實為面心立方的純銠金屬，但在表面有因暴露空氣下而產生的氧化銠層。在二氧化碳氫化的催化反應中，此三種結構皆被分散在氧化鋯上製備成非勻相觸媒。在條件測試中，三者皆在溫度高於攝氏400度才有明顯二氧化碳轉化率，且轉化率隨溫度上升。然而，甲烷的生成只有在凹面四面體的結構上甚為明顯，其餘皆產生高比率的一氧化碳。經參考文獻結果與推測探討，此凹面四面體銠奈米觸媒因具有110的晶面存在於凹面中，而改變了催化反應的途徑，提高了甲烷生成的選擇性，而存在於表面的111與100晶面則趨向於一氧化碳反應途徑的生成。此項發現不僅證實了觸媒表面結構與產物選擇性的關係，更提供一個極重要的方法，將二氧化碳高選擇地轉化成高經濟價值的甲烷。

Rhodium (Rh) is a widely used metal by virtue of its superior performance in hydrogenation. In this thesis, the one-pot aqueous synthesis was developed to synthesize shaped Rh nanocrystals as catalysts and applied to CO2 hydrogenation for investigating the relation the between crystal morphology and the selectivity in methane formation. The three kinds of Rh catalysts we investigated were concaved, excavated and twinned nanocrystals, among which the concaved nanocrystals were in highly monodispersive size and tetrahedral shapes. The three nanocrystals could be obtained in the same reaction condition except for the concentration of capping agent, CTAB. Compared to the use amount of CTAB in the synthesis of concaved tetrahedra, the lower CTAB amount induced the formation of excavated nanocrystals while the higher resulted in the twinned ones. Characterized by SEM, TEM, and XPS, all shaped nanocrystals were confirmed the pure Rh with the Rh2O3 layers over their surfaces because of the exposure in the air. In CO2 hydrogenation, they exhibited significant activity in CO2 conversion when the temperature was over 400˚C. Notably, a relatively high yield ratio of methane was observed taking place when the concaved Rh tetrahedra were used as catalysts. In contrast, the other two gave high yield ratio of CO. Referring to former literature and the results of structural analysis. The high selectivity of methane formation possibly came from the existence of 110 crystal faces over the concaved surfaces of a tetrahedron; however, the 111 and 100 crystal faces typically caused the favored formation of CO. This is undoubtedly an exciting discovery which not only validates the relation between crystal structure and product selectivity in CO2 hydrogenation, but also provides a promising road leading to efficient conversion of CO2 to the value-added chemicals, such as methane.