Mechanistic insights into the light-driven hydrogen evolution reaction from formic acid mediated by an iridium photocatalyst
Mechanistic insights into the light-driven hydrogen evolution reaction from formic acid mediated by an iridium photocatalyst
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DOI：
10.1039/c7cy00785j
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发表时间：
2017-07
影响因子：
5
通讯作者：
Pin Xiao;Dan Wu;W. Fang;G. Cui
中科院分区：
文献类型：
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作者：
Pin Xiao;Dan Wu;W. Fang;G. Cui
最近通过实验报道了一种由 Ir(III) 光催化剂介导的新型光触发甲酸析氢反应。然而，其反应机制仍不清楚。在这里，我们采用密度泛函理论（DFT）方法来详细探索这种光催化反应。根据这些结果，我们提出了可能的光催化反应机理。在金属氢化物 [Cp*Ir(bpy)(H)]+ (5) 的形成过程中，甲酸充当协助质子穿梭的桥梁。辐照后，两个非绝热激发态衰变路径迅速从最初填充的激发单线态 S1 态填充到金属氢化物的最低三线态 T1 态。在 T1 态，水和甲酸促进激发态氢化物/质子从 Ir 中心转移到 Cp* 和 bpy 配体，产生几种能量较低的三重态异构体，表明三重态金属氢化物 5* 不可能是光催化的唯一前体。 T1 状态下的绝热 H2 演化在能量上是不利的。这些T1异构体通过T1/S0交叉点通过无辐射的T1→S0系间交叉跳跃到发生H2演化的S0状态。在这些反应中，溶剂充当助剂和催化剂，减少反应势垒，从而加速氢气的释放并增强整体光催化性能。我们目前的工作为含铱光催化剂的光诱导析氢反应提供了重要的机理见解。
A novel light-triggered hydrogen evolution reaction from formic acid mediated by an Ir(III) photocatalyst has been experimentally reported recently. However, its reaction mechanism remains elusive. Herein, we have employed the density functional theory (DFT) method to explore this photocatalytic reaction in detail. On the basis of the results, we have proposed a possible photocatalytic reaction mechanism. In the formation of the metal hydride [Cp*Ir(bpy)(H)]+ (5), formic acid acts as a bridge assisting proton shuttling. Upon irradiation, two nonadiabatic excited-state decay pathways quickly populate the lowest triplet T1 state of the metal hydride from its initially populated excited singlet S1 state. In the T1 state, water and formic acid facilitate excited-state hydride/proton transfers from the Ir center to Cp* and bpy ligands producing several energetically lower triplet-state isomers demonstrating that the triplet-state metal hydride 5* could not be the only precursor for the photocatalysis. Adiabatic H2 evolution in the T1 state is energetically unfavorable. These T1 isomers hop, through radiationless T1 → S0 intersystem crossings via T1/S0 crossing points, to the S0 state in which H2 evolution takes place. In these reactions, solvents acting as assistants and catalysts reduce reaction barriers, thereby accelerating H2 release and enhancing the overall photocatalytic performance. Our current work provides significant mechanistic insights into light-induced hydrogen-evolution reactions of iridium-containing photocatalysts.