2-Pyridinide as an Active Catalytic Intermediate for CO₂ Reduction on p-GaP Photoelectrodes: Lifetime and Selectivity

The active intermediate responsible for pyridine (Py)-catalyzed reduction of CO₂ 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 CO₂ 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 CO₂ 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 CO₂ to those predicted for 2-PyH^-* being protonated to form either DHP* or Py* + H₂ 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 CO₂ reduction on p-GaP. Predicted high barriers of HER (via either 2-PyH^-* or H^-*) also explain the high selectivity toward CO₂ reduction observed in experiments.