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

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.