Abstract
Understanding the interaction of CO2 with perovskite metal oxide surfaces is crucial for the design of various perovskite (electro)chemical functionalities, such as solid oxide fuel cells, catalytic oxidation reactions, and gas sensing. In this study, we experimentally investigated the reactivity of CO2 with a series of cobalt-based perovskites (i.e., LaCoO3, La0.4Sr0.6CoO3, SrCoO2.5, and Pr0.5Ba0.5CoO3−δ) by a combined ambient-pressure XPS (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) approach. Isobaric measurements by AP-XPS on epitaxial pulsed laser deposition-grown (100)-oriented thin films under 1 mTorr CO2 showed the formation and uptake of adsorbed adventitious-like C–C/C–H, −CO species, monodentate carbonate, and bidentate (bi)carbonates. DRIFTS measurements on powder samples under CO2 atmosphere revealed the presence of multiple configurations of carbonate in the asymmetric O–C–O stretching region with peak splittings of ∼100 and ∼300 cm–1 correlated to the monodentate- and bidentate-bound carbonate adsorbates, respectively. The synergy between chemical state identification by AP-XPS and vibrational state detection by DRIFTS allows both the carbonaceous species type and the configuration to be identified. We further demonstrate that the surface chemistry of the A-site cation strongly influences CO2 reactivity; the La, Sr, and Ba cations in the LaCoO3, La0.4Sr0.6CoO3, SrCoO2.5, and Pr0.5Ba0.5CoO3 thin films showed significant carbon adsorbate speciation. Additionally, we link the La0.4Sr0.6CoO3 surface chemistry to its surface reactivity toward formation of bidentate (bi)carbonate species via exchange of lattice oxygen with carbonate oxygen. In conclusion, we show that the perovskite electronic structure ultimately dictates the driving force for formation of oxidized oxo-carbonaceous species (CO3) versus reduced species (C–C/C–H). A higher O 2p-band center relative to the Fermi level was correlated with a higher degree of (bi)carbonate formation relative to the other carbonaceous species observed (C–C/C–H and −CO) due to a more facile charge transfer from oxygen states at the Fermi level to free CO2 gas.
