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<inf>P4</inf>⁻ 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<inf>P4</inf>⁻ 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.

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