2-Pyridinide as an Active Catalytic Intermediate for CO ₂ Reduction on p-GaP Photoelectrodes: Lifetime and Selectivity

The active intermediate responsible for pyridine (Py)-catalyzed reduction of CO ₂ on a p-GaP photoelectrode is currently under debate. Exploration of the proposed intermediates' available pathways for further reaction may yield a deeper understanding of the CO ₂ reduction mechanism that will be essential to designing better cocatalysts in such photoelectrochemical systems. Adsorbed 2-pyridinide (2-PyH ^- ∗) was recently proposed by Carter and co-workers to be an intermediate that facilitates hydride transfer (HT) to CO ₂ to produce formate. However, the lifetime of 2-PyH ^- ∗, most likely controlled by the rate of 2-PyH ^- ∗ protonation to form adsorbed dihydropyridine (DHP∗), is still in question. In this work, we provide evidence for the transient existence of 2-PyH ^- ∗ on a p-GaP surface by comparing the activation energy for HT to CO ₂ to those predicted for 2-PyH ^- ∗ being protonated to form either DHP∗ or Py∗ + H ₂ via a hydrogen evolution reaction (HER). We predict that 2-PyH ^- ∗ situated next to an adjacent surface hydroxide (OH ^- ∗) will be the most effective intermediate leading to CO ₂ reduction on p-GaP. Predicted high barriers of HER (via either 2-PyH ^- ∗ or H ^- ∗) also explain the high selectivity toward CO ₂ reduction observed in experiments.