Sequential and simultaneous electrodeposition of Pt-Ru electrocatalysts on a HOPG substrate and the electro-oxidation of methanol in aqueous sulfuric acid

The catalytic activity for methanol electro-oxidation of Pt-Ru electrodeposits produced either sequentially or simultaneously on activated highly ordered pyrolytic graphite (HOPG) substrates using linear potential electrodeposition techniques was investigated. The voltammetric behavior of electrodeposits in 0.5 M aqueous sulfuric acid was compared to that of Pt electrodeposits on HOPG in the hydrogen electroadsorption-desorption and early stages of electro-oxidation potential regions. Morphological and microscopic surface characterization carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM), showed that the deposits consist of a number of aggregates. Scanning tunneling microscopy (STM) showed rounded clusters from 5 to 10 nm in diameter. For sequentially produced Pt-Ru electrodeposits, XPS and voltammetry data indicate an irregular distribution of the active materials and a non homogeneous surface consisting of Ru islands on Pt. The electrocatalytic activity of these electrodes for the oxidation of methanol in 0.5 M aqueous sulfuric acid increases by a factor of about 5 as compared to Pt. A lower electrocatalytic efficiency was found for Pt-Ru electrodes produced by simultaneous electrodeposition. For both types of electrodes, the Ru content decreased after methanol electro-oxidation, but for simultaneously formed Pt-Ru electrodes the reactivation of methanol electro-oxidation, observed in the negative potential scan, increased as the Ru content on the Pt surface decreased. (C) 2004 Elsevier B.V. All rights reserved.