将铂(Ⅱ)-四(4-羧基苯基)卟啉组装到光捕获金属有机框架中增强可见光驱动的产氢效能

氢气析出反应的分子催化剂因能够将其整合到用于光催化水分解的光捕集复合物中而受到广泛关注.研究者期望通过构建吸光网络,提高分子催化剂的光催化产氢效能.本文报道了以(TCPP)Pt^Ⅱ]TCPP=meso-四(4-羧基苯基)卟啉]络合物作为光催化产氢的分子催化剂.采用氯冉酸(CA)作为电子牺牲剂可以很好地稳定光催化剂,使CA被氧化为CA-H·]自由基.当使用三乙醇胺作为电子牺牲剂时,(TCPP)Pt^Ⅱ]分解形成Pt纳米颗粒.电化学循环伏安实验结果表明,光催化产氢的第一步是质子偶联电子转移,以获得(TCPP)Pt+H]0.然而,第二个电子转移-1.02 V的氧化还原电位不随添加三氟乙酸而位移,表明该电子转移未与质子转移耦合,得到(TCPP)Pt+H]-.此外,第二次电子转移的峰值处产生催化波,表明氢气是由(TCPP)Pt+H]-的质子化生成,然后再生(TCPP)Pt^Ⅱ]并释放氢气.密度泛函理论计算结果表明,(TCPP)Pt^Ⅱ]分子催化剂光催化产氢的机理可能先经过质子耦合电子转移反应,形成(TCPP)Pt^Ⅰ]-NH,然后依次经过电子注入和质子化形成(TCPP)Pt^Ⅱ-H]-NH中间体,最终释放H2.由于整个催化循环过程涉及多个电子的注入,光捕获网络的引入有助于提供多个光电子.因此,本文通过将(TCPP)Pt^Ⅱ]掺杂生长到主要由(TCPP)Zn^Ⅱ]构筑的金属有机框架中,构筑了与分子催化剂连接的光捕获网络,从而将其活性提高了约830倍.纳秒瞬态吸收光谱和时间分辨的磷光光谱表明,向(TCPP)Pt^Ⅱ]均相溶液中加入氯冉酸会因电荷转移而缩短3(TCPP)Pt^Ⅱ]*寿命.同样现象在金属有机框架体系中也被观察到.然而,在磷光猝灭后,瞬态吸收光谱观察到均相溶液中(TCPP)Pt+H]0及CA-H·]自由基信号迅速衰减,在微秒时间尺度上衰减为0,表明大部分还原的(TCPP)Pt+H]0迅速与氧化后的CA-H·]复合,限制了光催化氢气析出的光量子效率.然而,在金属有机框架体系中,磷光猝灭后纳秒瞬态吸收光谱在较长时间尺度观察到残留吸收带,表明随后消耗CA,向反应体系中注入电子,推动了反应的完成.本文研究了能量转移对光催化H2析出的影响,并强调了光捕获网络在多电子注入中的重要性.

Incorporating porphyrin-Pt in light-harvesting metal-organic frameworks for enhanced visible light-driven hydrogen production

Molecular catalysts for H2-evolution are of interest for their integration into light-harvesting complexes for photocatalytic water splitting. Here, we report the meso-tetra (4-carboxyphenyl) porphine (TCPP)PtⅡ] complex as a molecular H2-evolving photocatalyst using chloranilic acid (CA) as a sacrificial electron donor, the choice of which is critical to the stability of the photocatalyst. When triethanolamine was used, (TCPP)PtⅡ] decomposed to form Pt nanoparticles. Density functional theory calculations together with evidence from electrochemical and spectroscopic analyses suggested that the catalysis was possibly initiated by a proton-coupled electron transfer (PCET) to form (TCPP)PtⅠ]-N-H, followed by another electron injection and protonation to form a (TCPP)PtⅡ-hydride]-N-H intermediate that can release H2. As the whole catalytic cycle involves the injection of multiple electrons, a light-harvesting network should be helpful by providing multiple photo-induced electrons. Thus, we integrated this molecular catalyst into a light-harvesting metal- organic framework to boost its activity by ～830 times. This work presents a mechanistic study of the photocatalytic H2 evolution and energy transfer and highlights the importance of a light-harvesting network for multiple electron injections.