Anodic Dissolution of Palladium in Sulfuric Acid: An Electrochemical Quartz Crystal Microbalance Study

Abstract

Regularities of formation of a palladium oxide layer and its cathodic reduction in 0.5 M H2SO4 at 0.5–1.3 V (SHE) are studied by cyclic voltammetry, x-ray photoelectron spectroscopy, and electrochemical quartz crystal microbalance. A pure Pd plate and a ∼0.5-μm-thick Pd coating on gold-sputtered quartz crystal is used for electrochemical and microgravimetric studies. It is shown that a Pd electrode dissolves electrochemically in 0.5 M H2SO4 when its potential is cycled between 0.5 and 1.3 V. In the case of ∼0.5-μm-thick Pd coating on the gold substrate, the decrease in the electrode weight during one anodic–cathodic cycle is 1.0–1.5 μg/cm2. It is suggested that anodic process at 0.5–1.3 V (SHE) represents electrochemical oxidation of palladium, yielding a surface layer of poorly soluble Pd(OH)2 and/or PdO phases, as expressed by the equation Pd + 2H2O ⇄ (Pd(OH)2/PdO)s + 2H+ + 2e. This surface layer, (Pd(OH)2/PdO)s, undergoes reduction during the cathodic process. About 5% of the total amount of ionized palladium dissolve in electrolyte.