Studies of Ru(001) electrodes in aqueous electrolytes containing silver ions and methane: LEED, HREELS, Auger spectroscopy and electrochemistry

Abstract

Potent catalysis by Ru electrodes has been reported by various workers. Reported here are studies of chemisorption, surface vibrational spectroscope and electrochemical reactivity at Ru(001) single-crystal electrode surfaces. Electrochemical oxidation of methane on these Ru electrode surfaces in aqueous electrolytes was investigated. The influence of surface oxide and electrodeposited silver on methane oxidation were explored. Immersion of Ru(001) into pure water at open circuit forms a layer of adsorbed hydrous oxides with an ordered (2 × 2) structure as measured by Auger spectroscopy and low energy electron diffraction (LEED). Anodization of Ru(001) in 1 M HClO 4 produces a disordered Ru O/OH film consisting of several atomic layers. The high resolution energy electron loss spectrum of this O/OH layer exhibits RuO and OH stretching bands, and the layer is not removed by subsequent electrolysis at negative potentials. Various submonolayer and multiple-layer amounts of silver were electrodeposited on Ru(001). A continuous film is formed, based upon attenuation of the substrate Auger signal. The silver layer lacks long-range order, as judged by LEED. Under the present conditions, namely Ru(001) single-crystal surfaces with or without the O/OH and/or silver layers in aqueous electrolytes, the faradaic current due to oxidation of methane is generally less than 1 μA cm −2. Experiments were performed with Ru(001) surfaces brought to an atomically clean ordered state by Ar + ion bombardment and annealing in UHV. Immersion of Ru(001) into water at open circuit formed a submonolayer of oxide/hydroxide with an ordered (2 × 2) structure and HREELS vibrational bands attributable to RuO and OH libration and traces of adsorbed CO. Cyclic voltammetry of Ru(001) in aqueous KF and HClO 4 electrolytes illustrated the substantial irreversibility of oxide formation—reduction and the near reversibility of hydrogen adsorption—desorption. Electrochemical oxidation of CH 4 in 10 mM KF electrolyte at pH 3 produced an increase in current density equal to 1 μA cm −2 at potentials between − 0.1 and 0.2 V vs. Ag/AgCl. Electrodeposition of Ag onto the bare Ru(001) surface did not alter the current density for CH 4 oxidation significantly. Electrochemical oxidation of the Ru(001) surface to form a (1 × 1) oxide/hydroxide layer decreased the activity of the surface for anodic oxidation of CH 4 in aqueous KF electrolyte. Electrodeposition of a fractional monolayer or multilayer of Ag onto the electrochemically pre-oxidized surface did not detectably influence the rate of CH 4 electrochemical oxidation. The clearly demonstrated catalytic activity of metal-doped Ru oxide electrodes [1–4] is all the more intriguing in the light of the present results which suggest that the observed catalytic behavior is not due primarily to the metallic Ru surface, layers of adsorbed oxide on Ru or electrodeposited metallic Ag.