Synergetic Effects in Oxygen Evolution on Ru-Ir-O Solid Solutions: A DFT and In-Situ Soft X-ray Absorption Spectroscopy Study

Nanocrystalline Ru1−xIrxO2 catalysts with particle size between 4 and 10 nm were prepared by microwave-assisted hydrothermal reaction from ruthenium chloride and iridium acetate. Prepared materials were single-phase solid solutions conforming to the rutile structural type with random mixing of both cations in the lattice positions. Mixing Ru and Ir in the rutile structure leads to synergetic improvement of the catalyst activity in oxygen evolution in acid media with a distinct maximum of the activity at a composition conforming to Ru0.5Ir0.5O2. The quantum chemical characterization of the prepared materials attributes the observed activity improvement to the ability of Ru-Ir-O catalysts to activate the bridge position, breaking the scaling relationships. This allows to optimize the OER catalytic activity beyond the top of the volcano. The DFT calculations also suggest that the activity improvement is caused by a minor fraction of the active sites at the catalyst surface, while the majority of the surface sites (∼50 %) remains inactive even at 1.7 V vs. RHE. The viability of the DFT proposed reaction mechanism is confirmed by in-situ soft X-ray absorption spectroscopy, which identifies Ru in cus positions as the primary site starting the oxygen evolution. The catalytic cycle starting on cus Ir sets in at ca. 100 mV higher potentials. The activation of the bridge positions presents itself as a weak negative signal at 531 eV observed in differential spectra under oxygen evolution conditions.