EQCM Study of Rhodium Anodic Dissolution in Sulfuric Acid

Rh anodic dissolution is studied in 0.5 M H₂SO₄ solution in the E range from 0.2 to 1.2 V (RHE) by means of EQCM, cyclic voltammetry, photometry, and XPS. Bright pure Rh electroplate 0.5 m thick on a gold sputtered quartz crystal electrode is used for electrochemical and microgravimetrical studies. It is found that the increase in Rh electrode weight during the anodic process is lesser than its decrease during the cathodic one. The difference is 120 ± 60 ng cm^–2. The electrode weight also decreases under open-circuit conditions, i.e. at E _{I = 0}. A linear relationship between the weight change and the charge exists for the anodic process. The presence of Rh(III) compounds in the solution and on the electrode surface is confirmed by a photometrical analysis and XPS measurements. It is assumed that the formation and reduction of Rh(OH)₃ phase on Rh electrode surface within E range investigated proceed according to equation Rh + 3H₂O Rh(OH)₃]_s + 3H⁺ + 3e, where Rh(OH)₃]_s is a surface layer of Rh(OH)₃ phase. is evaluated to be 0.6 V. Rh(OH)₃ partly dissolves in the electrolyte.