In Situ Studies of the Electrochemical Reduction of a Supported Ultrathin Single-Crystalline RuO2(110) Layer in an Acidic Environment

With in situ surface X-ray diffraction (SXRD) and X-ray reflectivity (XRR) in combination with ex situ characterization by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry, the electrochemical reduction of an ultrathin (1.66 nm thick) single-crystalline RuO2(110) layer supported on Ru(0001) is studied in an acidic environment, providing clear-cut evidence and mechanistic details for the transformation of RuO2 to hydrous RuO2 and metallic Ru. The reduction process proceeds via proton insertion into the RuO2(110) lattice. For electrode potentials (0 to -50 mV vs standard hydrogen electrode), the layer spacing of RuO2(110) increased, maintaining the octahedral coordination of Ru (SXRD). Continuous proton insertion at -100 to -150 mV leads to the transformation of the lattice oxygen of RuO2 to OH and water, which destroys the connectivity among the Ru-O-6 octahedrons and eventually leads to the loss of crystallinity (SXRD) in the RuO2(110) film at -200 mV accompanied by a swelling of the layer with a well-defined thickness (XRR). During the protonation process, soluble Ru complexes may form. With XPS the transformation of RuO2(110) to a hydrous RuO2 layer is followed, a process that proceeds first homogeneously and at higher cathodic potentials heterogeneously by re-deposition of previously electrochemically dissolved Ru complexes.