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Mechanistic insights into the light-driven hydrogen evolution reaction from formic acid mediated by an iridium photocatalyst

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Abstract

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.

Graphical abstract: Mechanistic insights into the light-driven hydrogen evolution reaction from formic acid mediated by an iridium photocatalyst

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Publication details

The article was received on 21 Apr 2017, accepted on 17 May 2017 and first published on 17 May 2017


Article type: Paper
DOI: 10.1039/C7CY00785J
Citation: Catal. Sci. Technol., 2017, Advance Article
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    Mechanistic insights into the light-driven hydrogen evolution reaction from formic acid mediated by an iridium photocatalyst

    P. Xiao, D. Wu, W. Fang and G. Cui, Catal. Sci. Technol., 2017, Advance Article , DOI: 10.1039/C7CY00785J

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