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A full monolayer of superoxide: oxygen activation on the unmodified Ca₃Ru₂O₇(001) surface.

ABSTRACT: Activating the O₂ molecule is at the heart of a variety of technological applications, most prominently in energy conversion schemes including solid oxide fuel cells, electrolysis, and catalysis. Perovskite oxides, both traditionally-used and novel formulations, are the prime candidates in established and emerging energy devices. This work shows that the as-cleaved and unmodified CaO-terminated (001) surface of Ca₃Ru₂O₇, a Ruddlesden-Popper perovskite, supports a full monolayer of superoxide ions, O₂^-, when exposed to molecular O₂. The electrons for activating the molecule are transferred from the subsurface RuO₂ layer. Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA), predict the adsorption of O₂^- with E_ads = 0.72 eV and provide a thorough analysis of the charge transfer. Non-contact atomic force microscopy (nc-AFM) and scanning tunnelling microscopy (STM) are used to resolve single molecules and confirm the predicted adsorption structures. Local contact potential difference (LCPD) and X-ray photoelectron spectroscopy (XPS) measurements on the full monolayer of O₂^- confirm the negative charge state of the molecules. The present study reports the rare case of an oxide surface without dopants, defects, or low-coordinated sites readily activating molecular O₂.

SUBMITTER: Halwidl D

PROVIDER: S-EPMC6003542 | biostudies-literature | 2018 Apr

REPOSITORIES: biostudies-literature

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A full monolayer of superoxide: oxygen activation on the unmodified Ca3Ru2O7(001) surface.

Halwidl Daniel D Mayr-Schmölzer Wernfried W Setvin Martin M Fobes David D Peng Jin J Mao Zhiqiang Z Schmid Michael M Mittendorfer Florian F Redinger Josef J Diebold Ulrike U

Journal of materials chemistry. A 20180305 14

Activating the O2 molecule is at the heart of a variety of technological applications, most prominently in energy conversion schemes including solid oxide fuel cells, electrolysis, and catalysis. Perovskite oxides, both traditionally-used and novel formulations, are the prime candidates in established and emerging energy devices. This work shows that the as-cleaved and unmodified CaO-terminated (001) surface of Ca3Ru2O7, a Ruddlesden-Popper perovskite, ...[more]

PMID: 30009023

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Project description:Improved oxygen evolution reaction (OER) electrocatalysts based on an additional understanding of surface changes that occur upon metal dissolution are needed to enable the efficient use of electrochemical water splitting. This work integrates theoretical and experimental studies of the effects of metal dissolution from the RuO2 and Ru1-x Ti x O2 surfaces on the OER activity and electrochemical stability. Our computational analysis shows that the energetic barriers for metal dissolution depend highly on the surface site and Ti-substituent location. Metal dissolution induces the formation of new active surface sites with different electronic density distributions. In addition to dissolution-induced changes to the surface composition, electron density changes occur in the interfacial electrolyte components. Surface reconstruction changes the activation barriers for the OER steps. Our experimental analysis of RuO2 and Ru0.8Ti0.2O2 using a two-step durability test in acidic electrolytes shows that the OER activity, surface, and metal dissolution change over the durability tests. Ti-substitution exhibits improved electrochemical stability with cycling. For RuO2, changes in the mass activity of RuO2 with cycling are directly correlated with Ru dissolution and lowering of the electrochemical surface area (ECSA). In contrast, Ru0.8Ti0.2O2 showed a 19 times lower Ru dissolution rate, and metal dissolution results in increasing the ECSA and new active sites. Our STEM and EELS analysis supports that repeated cycling under OER conditions results in surface reconstruction for both RuO2 and Ru0.8Ti0.2O2, with the formation of a disordered RuO2 surface and changes to the distribution of Ru and Ti at the Ru0.8Ti0.2O2 surface. The experimentally observed changes in activity and surface structure after cycling are consistent with computational analysis, which shows how metal dissolution may alter the OER activation barriers. Combining experimental and computational insights, this work reveals the effects of metal dissolution on the surface atomic and electronic structure and OER activity and advances our comprehension of metal dissolution dynamics and surface reconstruction, which may have implications for other catalytic processes.

Dataset Information

A full monolayer of superoxide: oxygen activation on the unmodified Ca₃Ru₂O₇(001) surface.

Publications

A full monolayer of superoxide: oxygen activation on the unmodified Ca<sub>3</sub>Ru<sub>2</sub>O<sub>7</sub>(001) surface.

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