Preparation and electrochemical characterization of low-index rhodium single crystal electrodes in sulfuric acid

Abstract

The electrochemical properties of low-index phase Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) single crystal bead electrodes, prepared by a novel technique combining electron beam heating with inductive annealing in a controlled atmosphere, have been characterized in 0.1 M H 2SO 4 by cyclic voltammetry and chronoamperometry. Hydrogen and sulfate adsorption as well as surface oxidation depend strongly on the crystallographic orientation of the surface. The potentials of zero total charge ( E pztc) of all three Rh electrodes in 0.1 M H 2SO 4 were determined by the combination of charge displacement and voltammetric experiments. The charge balance reveals unambiguousely that the (√3 × √7) adlayer on Rh(1 1 1) is composed of specifically adsorbed sulfate ions eventually coadsorbed with water molecules. Hydrogen-sulfate coadsorbed with hydronium ions could be excluded. The kinetics of sulfate ion desorption followed by the adsorption of hydrogen at less positive potentials could be represented by a nucleation and growth mechanism coupled with a parallel first order process. The electro-oxidation of irreversibly adsorbed carbon monoxide monolayers was also investigated and revealed distinct structure sensitivity. The reaction pathway on all three low-index phases of Rh proceeds according to a Langmuir–Hinshelwood mechanism and is controlled by nucleation of OH ads at steps and other defect sites followed by a complex growth process on terrace sites. The low surface mobility of CO ads leads to a slow and incomplete CO monolayer electro-oxidation on Rh(1 1 1). The high density of step sites on Rh(1 1 0) and the reversible formation of oxygenated species on Rh(1 0 0) at rather low potentials significantly enhance the electro-oxidation activity leading to the following reactivity sequence: Rh(1 1 1) ≪ Rh(1 1 0) ∼ Rh(1 0 0). The shape of the experimental transients and attempts to model them demonstrate the occurrence of at least two processes occurring in parallel. The long-term response represents clearly a process involving a slow surface-diffusion step.