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

Xu, Shenzhen, Carter, Emily A.
Journal of the American Chemical Society 2018 v.140 no.28 pp. 8732-8738
activation energy, carbon dioxide, formates, hydrides, hydrogen, hydrogen production, pyridines
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.