Experimental studies to aid in establishing the role of the GaP electrode surface in pyridine-catalyzed CO2 reduction, reported to give high yields for methanol and formic acid, will be discussed. Ambient pressure photoelectron spectroscopy (APPES) was used to spectroscopically identify in situ surface-bound species on GaP(110), the most stable surface of GaP, at pressures up to 1 Torr. The data show that the interaction with water is characterized by the presence of a partially dissociated adlayer, with Ga-OH, P-H, and adsorbed molecular H2O species detected on the surface, consistent with theoretical calculations. Isobaric APPES measurements at elevated pressures were used to probe the thermal stabilities of the P-hydride species as well as the oxidation of surface Ga and P. We observe the surface hydride to be remarkably stable in the presence of water, which is notable given the critical role of hydride transfer to catalysts and CO2 during chemical fuel synthesis reactions in aqueous environments. This high stability may contribute to the remarkable near-100% faradaic efficiency for methanol generation by solar-driven CO2 reduction by this system. We have also reported the orbital-resolved adsorption state of pyridine on GaP(110) using scanning tunneling microscopy (STM). By examining the distribution of unoccupied molecular orbitals with high spatial and energetic resolution, we showed that scanning probe techniques could be used to positively identify the sites on pyridine susceptible to nucleophilic attack, consistent with frontier orbital theory. This technique can be used to explore the local reaction centers of adsorbed catalysts relevant to artificial photosynthesis. Our observations of the stable adsorption of both H and pyridine on this surface is notable, because it characterizes the proposed precursor state for the formation of adsorbed dihydropyridine, which could be a key hydride-shuttling catalyst for heterogeneous CO2 reduction.

Surface bonding and chemistry for pyridine-catalyzed CO2 reduction on GaP

253rd National Meeting (2017)

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