Ambient-Pressure X-ray Photoelectron Spectroscopy of the Water Interface of Lanthanum-Modified Cobalt Oxide Surfaces

Abstract Metal oxide surface properties, including surface reactivity, deconstruction, and electrostatics, all impact catalytic activity, but characterization of the metal oxide surface under hydrated conditions is notoriously challenging. Here, we modify a spinel cobalt oxide (Co3O4) lattice with La3+, systematically varying both the catalyst composition and the water vapor pressure to reveal how the ancillary cation changes the catalyst–water interface with ambient-pressure X-ray photoelectron spectroscopy (APXPS). Together with impedance spectroscopy and titrations, characterization of the interfacial surface reveals that increased La3+ incorporation increases metal oxide surface hydroxylation with higher water pressures (from 0 to 10 to 17 Torr). This increased hydroxylation of the unbiased metal oxide surface increases the formation of active sites that promote the oxygen evolution reaction (OER) by proton-coupled electron transfer. The correlation of increased hydroxylated active sites with La3+ ion incorporation is consistent with the previously established role of rare-earth cations in deconstructing Co3O4 to result in higher OER activity arising from more hydroxylated active sites in the amorphous overlayer of the Co3O4 spinel oxide lattice.