Investigations of Surface Chemistry for Pyridine-Catalyzed CO2 Reduction on Gallium Phosphide

The surface chemistry of N-containing heteroaromatics, molecular co-catalysts that enable the selective electrochemical reduction of CO₂ to fuels, is discussed. The presented experimental results focus on elucidating the role of the electrode surface in CO₂ reduction reactions that are co-catalyzed by pyridine. For this catalysis, exceptionally high selectivity for reduced fuels has been reported when the reaction occurs at the surface of a GaP photocathode. For this reason, experimental emphasis is placed on assessing preferential adsorption sites and bonding interactions of adsorbates on surfaces of GaP. A surface science approach is used, whereby ultrahigh vacuum conditions facilitate the fabrication of highly characterizable electrode-adsorbate systems. The use of single crystal surfaces permits analysis of surface chemistry independent of complicating factors such as grain boundaries and morphology. Surface-sensitive core-level and vibrational spectroscopy techniques, including high-resolution X-ray photoelectron spectroscopy, synchrotron-based photoemission, and high-resolution electron energy loss spectroscopy, are used to probe adsorbate-substrate and adsorbate-adsorbate interactions for pyridine, water, hydrogen, and carbon dioxide on GaP. Scanning tunneling microscopy (STM) was used to obtain molecular orbital-resolved images of adsorbed molecules. Conclusions from experimental results on these model systems are supported by calculations using density functional theory. This work assists in generating a molecular-level understanding of the heterogeneous processes important to the reaction mechanisms involved in the efficient photoelectrocatalytic generation of carbon-containing fuels with high energy densities.

Two recent highlights from these studies will be discussed. First, we report on a combined investigation through experiment and theory on the surface-bound species on GaP(110) formed upon interaction with water. Experimentally, surface-bound species over 10 orders of magnitude of pressure were spectroscopically identified in situ using synchrotron-based ambient pressure photoelectron spectroscopy (APPES). Ga3d and O1s core-level spectra indicate that the interaction of GaP(110) with H₂O induces formation of a partially dissociated adlayer, characterized by the presence of both Ga−OH and molecular H₂O species. Measurements of the P2p core level indicate formation of a negatively charged hydride that irreversibly bonds to surface P in vacuum. The surface densities of the hydroxide and hydride species increase with increasing pressure (surface coverage) of water. Isobaric measurements at elevated pressures were used to probe the thermal stabilities of adsorbed species as well as the oxidation of surface Ga and P. The observation of stable surface hydride formation induced by interaction with water is especially notable given the critical role of hydride transfer to catalysts and CO₂ during chemical fuel synthesis reactions in aqueous environments. Secondly, we describe a scanning tunneling microscopy (STM) and density functional theory investigation of the orbital-resolved adsorption state defining the dative bonding interaction between a GaP(110) surface and a N-containing heteroaromatic (pyridine). By examining the distribution of unoccupied molecular orbitals, we show that STM images can be used to positively identify the sites on pyridine susceptible to nucleophilic attack, consistent with frontier orbital theory. This indicates that STM can be used to explore the local reaction centers of adsorbed ambidentate electrophiles and nucleophiles relevant to artificial photosynthesis, and more broadly to generate critical mechanistic information for various heterogeneous acid−base reactions.