Mechanistic Aspects of the Selectivity of RuO₂ and Ru–Mn–O Catalysts in Parallel Chlorine and Oxygen Evolutions Reactions

Mechanistic differences between oxygen evolution and chlorine evolution on Ru-based oxide electrodes in acidic media were assessed by in situ soft X-ray absorption spectroscopy. O K-edge spectra recorded under cyclic polarization in 0.1 M HClO4 containing variable concentrations of chlorides in energy dispersion mode were used to follow the potential dependence of the key intermediates in the oxygen evolution process as a function of the electrode potential and chloride concentration. In the absence of chlorides, the oxygen evolution reaction (OER) process is characterized by the formation of surface-confined OOH at potentials preceding the onset of the bulk oxygen evolution. In the presence of chlorides, the spectral behavior reflects a change of the electrode selectivity towards chlorine evolution, which shows a suppression of the absorption of the gaseous oxygen as well as intermediates of the OER process. The observed behavior is compliant with a mechanism assuming chlorine binding via surface-confined oxygen atoms. The Ru–Mn–O phases, which are selective for oxygen evolution even in the presence of chlorides, show a different behavior. The sites involved in the OER retain their selectivity to the OER even in the presence of chlorides. The actual mechanism of the OER changes in the presence of chlorides. While in the absence of chlorides one observes accumulation of surface OOH groups only at potentials positive to the onset of the bulk oxygen evolution, the presence of chlorides enables the formation of surface-confined OOH at potentials negative to the onset of the OER. The onset of the actual oxygen evolution, on the other hand, leads to removal of the surface-confined OOH, suggesting that the OER is controlled by the third electron transfer in the presence of chlorides.