Disclosure of the hydrogen evolution mechanism on [FeFe]-hydrogenases-inspired molecular catalysts – a DFT study

The FeFe bio-inspired molecular catalysts that mimic the [FeFe] hydrogenases have been widely studied. However, the hydrogen evolution mechanism on the molecular catalysts is still not fully understood. In this work, the theoretical calculation was linked with experimental catalytic performance to reveal the possible reaction mechanism of FeFe molecular catalysts. The Density Functional Theory (DFT) calculations on the FeFe molecular catalysts exhibited a good match with the experimental overpotential data, with a R2 of 0.592. The detailed DFT study indicated that the first H+/e− injection was the largest thermodynamic impediment in the whole hydrogen evolution reaction (HER) cycle which follows a proton transfer – electron transfer (PT-ET) mechanism. The injected hydrogen binds to the bridging position between FeFe centre (µ-H) and then transfers to a terminal hydrogen on Fe (t-H). Later, the t-H combines with the second injected hydrogen to form a H2 molecule which is then released from the catalyst. The effect of different ligands on HER was also studied. It was found that different ligands around the FeFe centre could significantly change the PT and ET energy, and some could provide additional binding sites for protons.