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 RuO 2 (110) layer supported on Ru(0001) is studied in an acidic environment, providing clear-cut evidence and mechanistic details for the transformation of RuO 2 to hydrous RuO 2 and metallic Ru. The reduction process proceeds via proton insertion into the RuO 2 (110) lattice. For electrode potentials (0 to −50 mV vs standard hydrogen electrode), the layer spacing of RuO 2 (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 RuO 2 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 RuO 2 (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 RuO 2 (110) to a hydrous RuO 2 layer is followed, a process that proceeds first homogeneously and at higher cathodic potentials heterogeneously by re-deposition of previously electrochemically dissolved Ru complexes.