Anodic oxidation of Pd alloys with Pt and Rh

Anodic oxidation of Pd alloys with Pt and Rh has been studied in acidic solution (0.5 M H2SO4) using cyclic voltammetry coupled with the electrochemical quartz crystal microbalance (EQCM). The influence of electrode potential and alloy composition on the processes of surface oxide formation/reduction and electrochemical dissolution of metals was investigated. For Pd-Rh alloys containing up to ca. 85-90% Pd on the surface the oxidation of electrode material starts at potentials 200-300 mV lower than on pure Pd or Pd-Pt alloys and 50-100 mV higher than on pure Rh. The formation of a monolayer of surface oxide is completed at ca. 1.36 V for Rh-rich Pd-Rh alloys, and ca. 1.46 V for Pd-Pt and Pd-rich Pd-Rh alloys. The EQCM results indicate that surface oxide formed on Pd alloys has MO stoichiometry. Both Pd and Rh dissolve during alloy polarization to the potential region of surface oxidation. The amount of dissolved metal is inversely proportional to scan rate and increases with the increase in electrode potential and Pd surface content in alloys. Electrochemical dissolution of metals from the alloys proceeds differently than from pure metal electrodes.