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.目前，针对p型磷化镓（p-GaP）光电极上吡啶（Py）催化CO₂还原的活性中间体，学界仍存在争议。对所提出中间体的后续反应路径展开探究，可深化对CO₂还原机制的理解，而该机制对于优化该光电化学体系中的助催化剂设计至关重要。此前，Carter团队提出吸附态2-吡啶负离子（2-PyH–*）是介导氢负离子转移（HT）至CO₂以生成甲酸盐的关键中间体。然而，2-PyH–*的寿命——其大概率由2-PyH–*质子化生成吸附态二氢吡啶（DHP*）的速率所决定——仍有待明确。本研究通过对比氢负离子向CO₂转移的活化能，与2-PyH–*质子化生成吸附态二氢吡啶（DHP*）或通过析氢反应（HER）生成Py* + H₂的活化能，为2-PyH–*在p-GaP表面的瞬态存在提供了实验证据。我们预测，紧邻相邻表面羟基（OH–*）的2-PyH–*是介导p-GaP表面CO₂还原的最具活性的中间体。此外，经由2-PyH–*或氢负离子（H–*）发生的析氢反应（HER）均具有较高的反应势垒，这一结果也解释了实验中观测到的CO₂还原高选择性现象。