Atypical high selectivity for the conversion of CO₂ to methanol has been reported when pyridine is dissolved in the electrolyte of a photoelectrochemical cell employing a p-type gallium phosphide (GaP) photocathode.[1] Despite considerable electrochemical characterization and theoretical consideration, there is no consensus on the mechanism by which pyridine catalyzes CO₂-to-methanol conversion. A fundamental understanding of the origin of high selectivity in this system is important for continued optimization of solar-driven CO₂ reduction. Our research combines a surface science approach with in situ and operando spectroscopy to investigate GaP surface chemistry and determine the role of heterogeneous processes in this catalysis.

Ambient pressure photoemission spectroscopy (APPES) was used to study the adsorption of H₂O [2] on GaP(110) and pyridine (Py) on clean and hydrided GaP(110). The stable adsorption of both H and pyridine on GaP(110) characterizes the precursor state for the formation of adsorbed dihydropyridine (DHP), which could be a key hydride-shuttling catalyst for heterogeneous CO₂ reduction. Using APPES we also observed dissociative adsorption of methanol on GaP(111), which leads to the formation of surface-bound methoxy (CH₃O) species, while simultaneous formation of surface-bound formate (HCOO) and methoxy occurs on the more reactive GaP(110) surface. Low temperature scanning tunneling microscopy (LT-STM) was used to observe orbital-resolved bonding of adsorbed pyridine on the GaP(110) surface.[3] By examining the distribution of unoccupied molecular orbitals with high spatial and energy resolution, we have identified the sites on pyridine susceptible to nucleophilic attack that lead to the formation of hydrogenated species such as dihydropyridine. Operando FTIR studies of pyridine hydrogenation at a Pt electrode under relevant electrochemical conditions was used to observe the formation of a hydrogenated piperidinium (PyH₇⁺) near-surface species, which implies that dihydropyridinium (PyH₃⁺), the protonated form of dihydropyridine, must exist transiently during CO₂ reduction.[4]

1. Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B., J. Am. Chem. Soc. 130, 6342 (2008)

2. Kronawitter, C. X.; Lessio, M.; Zhao, P.; Riplinger, C.; Boscoboinik, A.; Starr, D. E.; Sutter, P.; Carter, E. A.; Koel, B. E., J. Phys. Chem. C 119, 17762 (2015)

3. Kronawitter, C. X.; Lessio, M.; Zahl, P.; Muñoz-García, A. B.; Sutter, P.; Carter, E. A.; Koel, B. E., J. Phys. Chem. C 119, 28917 (2015)

4. Kronawitter, C.; Chen, Z.; Zhao, P.; Yang, X.; Koel, B. E., Catal. Sci. Tech., 7, 831 (2017)