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.