Aligning Electronic Energy Levels in Pyridine-Assisted CO₂ Activation at the GaP(110)/Water Interface Using Ab Initio Molecular Dynamics

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Photoelectrochemical CO₂ reduction has attracted considerable attention as a route to convert CO₂ into value-added products. Pyridine (Py)-catalyzed CO₂ reduction on a GaP photoelectrode has been shown to be a promising photoelectrochemical system to produce methanol under the underpotential condition. However, whether the dramatic decrease in overpotential can be attributed to the CO₂ activation by the formation of the zwitterionic complex PyCO₂ is currently under debate. Because the alignment between the band edge positions of photoelectrodes and the redox potentials of species determines the desired redox reactions, calculations have been performed to evaluate the band edge positions of GaP and the redox potentials of relevant reactions. In these works, the water effect has been either neglected or approximated by using the dielectric continuum or a few explicit water molecules, which may not be enough to determine the accurate energy level alignment in realistic chemical environments. Moreover, calculations performed in conventional implicit solvation models suggested that PyCO₂ is unstable in homogeneous aqueous, while the bonding interactions between CO₂ and N species have been experimentally detected. Thus, we performed ab initio molecular dynamics to investigate the band alignment of GaP, as well as the stability and the reducibility of PyCO₂ in more realistic chemical environments. Our results showed that the solvation effect and the pyridine adsorption could shift up the band edge positions of GaP significantly, and neglecting such effects could result in a serious underestimation of the activity of the photocatalysts. More importantly, we found that the interaction between pyridine and CO₂ at the GaP(110)/water interface is strong due to the synergetic stabilization effect, which leads to an about 0.6 V less negative redox potential of PyCO₂/PyCO₂^– than that of CO₂/CO₂^– in the homogeneous aqueous. Furthermore, we compared the redox potential of PyCO₂/PyCO₂^– at the GaP(110)/water interface with the conduction band minimum of GaP, which showed that the reduction of the adsorbed PyCO₂ is thermodynamically feasible. Our results suggested that the CO₂ activation by the formation of PyCO₂ at the GaP(110)/water interface could be responsible for the low overpotential. This work provides valuable insights into the mechanism of pyridine-catalyzed CO₂ reduction on GaP and could pave the way for the development of efficient catalysts for CO₂ reduction.