異質結光觸媒以模擬太陽光照射分解海水並同步在雙胞光反應器分離H2和O2

Abstract

利用太陽能驅動的海水光催化在近期受到極大的關注，光催化可使海水同時分解生成氫氣與氧氣，具有低成本、環境友善等優點，特別是太陽能與海水的豐富資源，使得光催化分解海水具有環境永續性，在未來製造綠色化學燃料方面具有廣闊的前景。而如何提高陽光使用率、增進光催化穩定性，以及如何同步分離產物中的氫氣與氧氣成為了發展的重要課題。 新型雙胞光反應器可以使產物中的氫氣與氧氣分離到不同腔室中，從而降低了分離氫氣與氧氣的成本、減少氧氣與氫氣混合的爆炸風險，同時防止逆反應發生。雙胞反應器由兩個部分組成，一側為還原側，裝有產H2光觸媒，另一側氧化側，裝有產O2光觸媒，兩個反應室中間以陽離子或陰離子交換膜作為間隔，兩邊以離子交互擴散，傳遞兩側的電荷。 在單反應器實驗上，使用異質結構光觸媒，其中產H2光觸媒合成Pt/GaP-C3N4 (PGC)、Pt/GaP-TiO2-SiO2:Rh (PGTSR)、Rh@Cr2O3/SrTiO3:Rh (RCSTOA)，產O2光觸媒則採用Cl-BiVO4-SiO2 (CBS)。在水分解實驗，比較去離子水、人造海水和天然海水在模擬陽光照射下的光催化性能。運用XRD、FTIR、UV-Vis、BET、SEM、TEM、HRTEM、EDS、XPS、PL、TRPL和UPS進行觸媒鑑定。其中光觸媒PGTSR具有均勻圓型外貌(粒徑20 nm)、擁有高數量Ti3+的缺陷和氧空缺(VO)以致有很多的電洞儲存量在複合型的TiO2-SiO2:Rh、可密切接觸GaP形成Z-Scheme的異質結構光觸媒。為了要達到高光催化效率，表面異質結光觸媒CBS的曝露晶面主要是{010}-{101}反應基，發現是在Cl-在水熱法合成時形成的。另一方面核殼結構Rh@Cr2O3的薄層約5-20 nm厚度，是和SrTiO3:Al密切地接觸在一起。這個核殼結構是在RCSTOA展現光催化很重要的角色，導致即使沒有犧牲劑，也有最高的光催化水分解效能，不過在酸性水溶液，卻穩定性不佳。 光催化水分解分離H2和O2的實驗是用雙胞光反應器，以模擬太陽光照射進行。Z-Secheme光催化在HEP-OEP-Mediator-Membrane 的系統，能有效的光催化分解水和同步分離H2和O2，在Z-Secheme的光催化會受到反應溶液pH值的影響很大。在PGTSR-CBS-Fe3+/2+-Nafion系統，以pH=2.40呈現最高的光催化活性，達到產H2率68/35 µmol.g-1，但在RCSTOA-CBS-IO3-/I--(Neosepta or FAA)系統，需要還原端在pH=10.0和產氧端在pH=4.0，才能達到分別是544/183 µmol.g-1 和721/280 µmol.g-1。綜上結果顯示異質結構光觸媒有發展潛力，也增進人工光合作用在光催化水分解產氫的效能。

關鍵詞：Z-Scheme異質結光觸媒；表面異質結光觸媒；核-殼結構；雙胞反應器；光催壞海水分解

Using solar-driven photocatalytic seawater splitting process to produce simultaneously hydrogen and oxygen gas has attracted enormous attention as the promising prospects in the future to create green chemical fuels due to its low-cost, friendly environment and specially taking advantage abundant unlimited resources from sunlight and seawater. A perfect engineering for improving this process efficiency should fulfill some requirements including high photoactivity, photostability for long-term usages as well as simultaneous separation of hydrogen and oxygen gas from photocatalytic seawater splitting. A novel twin photoreactor can split and separate hydrogen and oxygen into each compartment lead to depressing explosion risk and lowering separation cost of H2-O2 mixture as well as preventing the backward reaction. The major components in twin photoreactor include H2-photocatalyst and O2-photocatalyst placed in reduction side and oxidation side, respectively, and shuttle mediators circulated through cation or anion exchanged membrane. In this study, heterojunction photocatalysts Pt/GaP-C3N4 (PGC), Pt/GaP-TiO2-SiO2:Rh (PGTSR), Rh@Cr2O3/SrTiO3:Rh (RCSTOA) for H2-photocatalysts and Cl-BiVO4-SiO2 (CBS) for O2-photocatalyst were prepared and applied in photocatalytic DI H2O, artificial and natural seawater splitting under simulated sunlight illumination. Based on the characteristic analysis results of X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-Visible light (UV-Vis), Specific surface area (BET), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), High-resolution transmission electron microscopy (HRTEM), Energy-dispersive X-ray spectroscopy (EDS), Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL), Ultraviolet photoelectron spectroscopy (UPS) measurements, heterojunction photocatalyst PGTSR with the uniform spherical morphology (20 nm) possessed the high Ti3+ defects and oxygen vacancy (VO) sites with high rich-hole reservoir in TiO2-SiO2:Rh composite which intimately contacted with high-electron reservoir of GaP in Z-scheme heterojunction photocatalyst. In order to achieve a high photoactivity efficiency, the surface heterojunction photocatalyst CBS with dominantly exposed {010}-{101} reactive facets was formed through Cl- ion hydrothermal treatment. On the other hand, the thin layer of Rh@Cr2O3 core-shell structure with the thickness about 5-20 nm present strong intimate contact with SrTiO3:Al photocatalyst. This core-shell structure displayed the important role for RCSTOA photoactivity lead to its highest photocatalytic water splitting performance without using artificial agents, but this photocatalyst has low stability in acidic condition. The separated H2 evolution from O2 through the photocatalytic seawater splitting was also carried out in a twin photoreactor under simulated sunlight irradiation. The Z-scheme photocatalysis including HEP-OEP-Mediator-Membrane system can work and be effective to split and separate hydrogen from oxygen from photocatalytic water-splitting performance. The photoactivity of Z-scheme photocatalysis system strongly depended on pH of the reactant solution. In details PGTSR-CBS-Fe3+/2+-Nafion system displayed the highest photoactivity in seawater splitting at pH 2.40 reached HER/OER of 68/35 µmol.g-1, while the highest performance RCSTOA-CBS-IO3-/I--(Neosepta or FAA) system exhibited at pH 10.0 for reduction side and pH 4.0 for oxidation side achieved of 544/183 µmol.g-1 and 721/280 µmol.g-1, respectively. The results indicate a promising construct of heterojunction photocatalyst as well as the improvement of artificial photosynthesis system for overall photocatalytic seawater splitting.