同步輻射X光光源應用於奈米材料之光/電催化水分解反應研究

Abstract

過去數十年間研究人員致力於研究高性能之水分解觸媒材料，分別應用於催化產氧與產氧反應，其利用光能或電能驅動繼之以化學燃料的形式儲存，滿足外來潔淨能源的需求，但水分解效能之核心關鍵在於熱力學、反應動力學、觸媒內部電荷的傳遞路徑，與催化同時所涉及的相轉變進而伴隨著晶格之結構變化，皆需要克服此些問題以發展出高效率、低成本且穩定之水分解材料。本研究計畫可分為三個主題，分別利用同步輻射X光光源技術對觸媒材料之物化特性與其水分解產氣反應進行即時檢測分析，深入瞭解其反應機制，並加以改進、提升催化效能。第一部分，首次利用同步輻射光源之X光繞射與X光吸收分析技術，建立光催化效率與材料結構的關聯性。我們發現Zn1−xCdxS固溶體之鋅、鎘含量於臨界比例時(x = 0.45)產生相變，同時存在閃鋅礦(Zinc blende)相與纖鋅礦(Wurtzite)相，並於介面處生成異質接面(Heterojunction)，此接面有效傳導和分離光激發載子，大幅提升其觸媒之光催化效能。於第二部分，藉由發展原位(In-situ)X光繞射與X光吸收分析技術，於電催化分解水反應當下臨場分析水相與固相介面材料結構與化學性質之變化。結果顯示鈷金屬觸媒中分子幾何結構將主導產生具水解產氧活性之羥基氧化物(Oxyhydroxide)之反應活性位置。承上研究，第三部分以尖晶石(Spinel)結構之MFe2O4 材料為基礎分別置換四種不同的二價過渡金屬，如錳、鈷、鎳和鋅，合成均勻之單晶奈米粒子觸媒，並藉由各種原位分析技術建立不同過渡金屬對水分解產氧活性之影響機制，以作為未來催化觸媒修飾改質的依據。由結果可知，以二價金屬鈷、鎳所取代的金屬氧化物於水解產氧實驗中，反應介面之羥基氧化物是否產生是決定催化活性的重要因素。隨著電壓上升進行產氧反應時，利用X光分析技術可即時偵測鈷鐵及鎳鐵氧化物表面之相轉變與其生成金屬羥基氧化物。

Over the past few years, a great effort has been devoted to searching for high performance of catalysts in water splitting reaction. Solar/electricity-driven conversion from water to O2 and H2 is a sustainable approach for efficient energy conversion and storage to substitute fossil fuels. However, there are still many challenges to be overcome before it is widespread implementation. Herein, three approaches are employed to improve the performance of water splitting reaction all the characterization and mechanism in these three systems have been studied by synchrotron X-ray light source. In the first part, we have demonstrated the correlation between the photocatalytic activity and structural property that has been studied firstly through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The heterojunction formed due to the coexistence of zinc blende and wurtzite phases in Zn1−xCdxS solid solution at a critical point (x = 0.45) can significantly improve the separation and migration of photoinduced electron-hole pairs. Second part, an in-situ X-ray measurement method has been developed to observe a strong correlation between the initialization of the O2 evolution and the formation of active metal oxyhydroxide phase. In addition, it has been revealed that the OER activity of cobalt-based electrocatalysts is geometrical-site-dependent. Following the second part, we utilize several in-situ analyses to conclude a universal rule for the OER activity of spinel-type ferrite MFe2O4 (M = Mn, Fe, Co Ni, Zn). It has been suggested that the divalent metal ions (M) in MFe2O4 system significantly dominate the formation of oxyhydroxide through an epitaxial relationship based on the atomic arrangement in the interface between spinel and metal oxyhydroxide. X-ray characterization for all the samples has provided direct evidence that the superior OER activities of CoFe2O4 and NiFe2O4 sample could be attributed to a remarkable structural transformation.