Hydride Transfer Limits Hydrogen Evolution Efficiency With Zn Porphyrin Photocatalysts

TL;DR

This study explains why a zinc-based metalloporphyrin photocatalyst is inefficient at producing hydrogen, due to a key hydride transfer step with a high energy barrier.

Contribution

The paper identifies the hydride transfer step as the main efficiency-limiting factor in Zn porphyrin photocatalysts for hydrogen evolution.

Findings

The EPEH photocatalytic cycle is favored, but hydride transfer has a high kinetic barrier.

Hydride donation by ZnCHP4− is inefficient due to loss of aromaticity.

Hydrogen generation competes with porphyrin hydrogenation, reducing catalytic efficiency.

Abstract

We performed a computational study on the photocatalytic hydrogen evolution mechanism using a Zn‐based metalloporphyrin (ZnP), water, and a cheap sacrificial donor. Based on previous experiments, the active species is a Zn chlorin (ZnC), formed by photohydrogenation of ZnP. Our calculations favor an electron‐proton‐electron‐hydride (EPEH) photocatalytic cycle that consists of one‐electron photoreduction of ZnC followed by protonation of a bridge carbon and a second photoreduction, leading to a key ZnCHP4
− intermediate. One‐electron photoreduction increases the aromaticity of the porphyrin rings, which explains the favorable photoreduction steps. The final step is a hydride transfer from ZnCH− to a proton donor like an ammonium cation or water, resulting in hydrogen generation. Although this process is thermodynamically allowed, it has a high kinetic barrier and leads to loss of aromaticity, which limits catalytic efficiency. Hydrogen generation competes with ZnCH− protonation and photohydrogenation. The poor activity of ZnCHP4
− as a hydride donor may be related to the loss of aromaticity associated with the hydride donation. The results have implications for electrocatalytic hydrogen production using porphyrins, which share a similar common intermediate. Therefore, our work will be useful to improve the molecular design of porphyrin‐based photo‐ and electrocatalysts for hydrogen generation.

The mechanism of photocatalytic hydrogen evolution catalyzed by a Zn‐based metalloporphyrin, using water and a cheap sacrificial donor, is studied computationally. The efficiency of hydrogen formation is limited by the competition with hydrogenation of the porphyrin, which leads to a reduced macrocycle.

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