Evaluation of Polycrystalline Platinum and Rhodium Surfaces for the Electro-Oxidation of Aqueous Sulfur Dioxide

Abstract

Polycrystalline Rh and Pt were studied to ascertain their electrocatalytic activity for the electro-oxidation of SO₂, an important reaction in sulfur dioxide depolarized electrolyzers used to produce hydrogen. Cyclic voltammetry and linear polarization methods were employed to evaluate the catalytic activity of these surfaces. Rh exhibited 25-fold lower catalytic activity than Pt and was more susceptible to poisoning by adsorbed intermediate sulfur species. Koutecky-Levich analysis indicated a two-electron transfer reaction on the Pt surface, which corresponded to the most commonly accepted SO₂ electro-oxidation reaction mechanism. The Tafel slopes in the low potential region (near the onset potential), in conjunction with an analysis of well-known reaction mechanisms, suggested that the step leading to the oxidation of water to form adsorbed hydroxyl species was the rate-determining step (RDS). This mechanistic model predicts a decrease in Tafel slope with increasing coverage of catalyst active surface sites by adsorbed sulfur species. For Pt, we estimate a surface sulfur coverage of 4 % based on the experimentally measured Tafel slope. In the case of Rh, the sulfur coverage was calculated to be approximately 1 %. The Tafel slopes obtained changed from 106 mV decade⁻¹ for Rh and 80 mV decade⁻¹ for Pt at potentials below 0.7 V vs. standard hydrogen electrode (SHE) to 210 mV decade⁻¹ for Rh and 162 mV decade⁻¹ for Pt at potentials above 0.7 V vs. SHE, suggesting a change in the reaction mechanism corresponding to a change in the surface of the electrocatalyst.