Hydride Transfer Limits Hydrogen Evolution Efficiency With Zn Porphyrin Photocatalysts

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 ZnCH P4 − 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 ZnCH P4 − 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.